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Xerox University Microfilms 300 North Zeeb Road Ann Arbor, Michigan 48100 76- 24,576 CARNEIN, Carl Robert, 1943- GEOLOGY OF THE SUNCOOK 15-MINUTE QUADRANGLE NEW HAMPSHIRE. The Ohio State University, Ph.D., 1976 Geology

Xerox University Microfilmst Ann Arbor, Michigan 48106

i GEOLOGY OF THE SUNCOOK 15-MINUTE QUADRANGLE

NEW HAMPSHIRE

DISSERTATION

Presented in Partial Fulfillment of the Requirements for

the Degree Doctor of Philosophy in the Graduate

School of The Ohio State University

Carl Robert Carnein, B.S., M,Sc.

# * * * *

The Ohio State University

1976

Reading Committee: Approved By

Ernest G. Ehlers

George E. Moore, Jr.

Douglas E. Pride (/Adviser V Department of Geology and jlineralogy ACKNOWLEDGMENTS

The author is indebted to Professor George E. Moore, Jr., of The Ohio State University for his assistance both in the field and in the laboratory# Professor Glenn W. Stewart of the

University of New Hampshire provided helpful suggestions in the field on two occasions# Financial assistance was provided by the Orton Fund of The Ohio State University and by a grant from the Penrose Bequest of The Geological Society of America. VITA

November 2, 1943 ...... Born - Philadelphia, Pennsylvania

1964 ...... B.S., The Ohio State University, Columbus, Ohio

1965-1967...... Teaching Assistant, Department of Geology, The Ohio State University, Columbus, Ohio

1967 ...... M.Sc., The Ohio State University, Columbus, Ohio

1967-1969* ...... Teaching Assistant, Department of Geology, The Ohio State University, Columbus, Ohio

1970-1975...... Assistant Professor, Department of Geology, Waynesburg College, Waynes- burg, Pennsylvania

PUBLICATION

Bull, C., and C„ R. Carnein, 1968, The mass balance of a cold glaciers Meserve Glacier, south Victoria Land, Antarctica: I.S.A.G.B. Symposium, Hanover, N. H., 3-7 September, 1968, p. 429-446.

FIELDS OF STUDY

Major Field: Geology'

Studies in Glaciology. Professor Colin Bull

Studies in Structural Geology, Petrology, and Field Geology. Professor George E. Moore, Jr. TABLE OF CONTENTS

Page ACKNOWLEDGMENTS...... '...... ii

VITA ...... iii

LIST OF TABLES ......

LIST OF FIGURES ...... viii

Chapter

I. INTRODUCTION ...... I

Purpose and Scope...... I Location and Geography ...... 2 Methods of Investigation...... 4 Previous Work ...... 4

II. STRATIGRAPHY...... 7

Berwick Formation...... 7 Distribution...... 7 Lithology ...... 8 Metamorphism...... T h i c k n e s s ...... 15 Correlation and A g e ...... 16

V Littleton Formation...... 19 Distribution ...... 19 L i t h o l o g y ...... 21 Pittsfield Member ...... 21 Jcnness Pond Member...... 30 Metamorphism...... 40 Thickness* ...... 46 Correlation and Age ...... •••••• 4 7

Massabesic Gneiss ...... 51 Distribution ...... 51 L i t h o l o g y ...... 52 Foliated Microcline Granite ...... 62 Origin of the Massabesic Gneiss ••••••• 57

iv Page Previous W o r k ...... 67 Mesoscopic Features ...... 72 Small-Scale Compositional Banding...... 72 Schollen Structure •••••••• ...... 73 Folded Structure ...... 74 Schlieren Structure...... 77 Ptygmatic Structure • •..»•••••••• 78 Relations of Massabesic Gneiss with* • • • • 84 Pegmatite and Foliated Granite •••••• Broad Compositional Variations ...... 89 Contact Relations •• 97 Microscopic Features ...... 99 Relations with Q-Ab-Or System ...... 104 Summary and Conclusion * 110

III, PLUTONIC ROCKS ...... 113

Concord Granite ...... 113 Distribution...... * ...... 113 Lithology...... 115 Age Relations ...... 118

Microcline-Oligoclase Pegmatite ...... 121 Distribution ...... 1 2 1 L i t h o l o g y ...... * ...... 121 Age Relations ...... * ...... 123

Post-Orogenic Dikes and Sills ...... 125 Distribution ...... 125 L i t h o l o g y ...... 127 B a s a l t ...... 127 Spessartite ...... 131 D i a b a s e ...... 133 Andesite ...... 1 3 3 Age Relations ...... 133 Structural Control ...... 134

IV. STRUCTURE ...... 138

Introduction ...... 138 Mesoscopic Structures ...... 140 B e d d i n g ...... 140 Rock Cleavage and Schistosity...... 141 Lineations...... * ...... 149 Minor Folds ...... 150 Joints ...... 152

v Page Silicified Fault Zones • ...... 154 Distribution...... 154 Lithology ...... 1 5 7 Attitude and Displacement of Faults • • • • * 159 Age Relations ...... j.61 Other Faults ...... 1 6 3

V. SUMMARY AND CONCLUSIONS...... 165

Environment of Deposition of the Metasediments . . . 165 Source of the Metasediments ...... 166 Geologic History and Regional Synthesis. • ...... 172 Previous W o r k ...... * 172 Application to the Suncook Quadrangle...... 181

LIST OF REFERENCES...... 187

vi LIST OF TABLES

Page Table 1 Approximate Modes of the Upper Member of the 9 Berwick Formation

Table 2 Approximate Modes, Pittsfield Member of the 22 Littleton Formation

Table 3 Approximate Modes, Jenness Pond Member of the 35 Littleton Formation

Table 4 Approximate Modes of the Massabesic Gneiss 53

Table 5 Approximate Modes of the Foliated Microcline 63 Granite

Table 6 Approximate Modes for Lime-Silicate Rocks of 94 the Massabesic Gneiss

Table 7 Petrographic Characteristics of Typical 111 Migmatites

Table 8 Approximate Modes of the Concord Granite 116

Table 9 Approximate Modes of Post-Orogenic Dikes 128 and Sills

vii LIST OF FIGURES

Page Figure 1. Index map showing the Suncook quadrangle 3 (ruled) and other areas referred to in this report: Manchester quadrangle (M), Haverhill quadrangle (H), Mount Pawtuckaway quadrangle (MP), Alton quadrangle (A), and Gilroanton quadrangle (G).

Figure 2 . Locations and trends of glacial striae in the 5 Suncook quadrangle.

Figure 3 Typical thin-bedded schist of the Jenness Pond 31 memberf 1.1 miles east of Epsom Mt., Epsom.

Figure 4. Crenulated silvery medium-grained mica schist 32 of the Jenness Pond member, about 1.25 miles east of Epsom Mt., in the town of Northwood.

Figure 5 . Alternating fine-grained schistose gneiss and 33 medium-grained mica schist of the Jenness Pond member. The schist contains "knots" composed of quartz, muscovite, and sillimanite. Location: in town of Northwood, about 1.2 miles east of Epsom Mt., Epsom.

Figure 6 . Fine-grained mica gneiss of the Jenness Pond 37 member, showing nearly horizontal bedding, and axial-plane cleavage dipping steeply to the west. Brunton compass points north. Location: on hill southeast of Pleasant Lake and just north of Route 107, where the latter leaves the east edge of the Suncook quadrangle.

Figure 7 . Small lenses of lime-silicate rock in medium- 38 grained mica-sillimanite schist of the Jenness Pond member. Location: in the town of Northwood, about 1.1 miles east of Epsom Mt,, Epsom.

Figure 8 . Phase relations, sillimanite-rouscovite zone, 42 for the Suncook quadrangle, using Thompson's AFM projection. Quartz and muscovite present.

• • * V 1 1 X Page Figure 9. Complex twinning in plagioclase from the 57 Massabesic Gneiss (sample 7-25-3a). Grains show combinations of albite, pericline, and Carlsbad twinning.

Figure 10. White oligoclase gneiss of the Massabesic 59 Gneiss (sample 7-14-lf) showing relations between grains of quartz (q), plagioclase (p), biotite (b), and microcline (m).

Figure 11. Sketch showing relations between Massabesic 65 Gneiss and foliated microcline granite. Foliation in granite shown by short dashes; that in gneiss shown by longer lines. Location: 0.8 mile N. 80° E. of Four Corners, Candia.

Figure 12. Microcline pegmatite intruding foliated micro- 66 cline granite in the Massabesic Gneiss, Chester Turnpike overpass, Route 101, Candia.

Figure 13. Temperatures in basement during sedimentation. 71 Sedimentary thickness indicated by horizontal scale,increasing toward the right. Initial temperature corresponds to zero depth of sediment; curves for 30 m. y. and 50 m.y. after beginning of deposition at rate of 0.3 km/m.y. Parts of curves above 650 C are meaningful only in the absence of melting and movement of. magma. From Birch, Roy, and Decker, 1968, p. 441.

Figure 14. Schollen structure, in which white oligoclase 75 gneiss occurs as inclusions in intermediate or pink Massabesic Gneiss. Location: National Guard firing range, Mt. Miner, Auburn.

Figure 15, Folded structure in white oligoclase Massabesic 76 Gneiss, Chester Turnpike overpass on Route 101, Candia.

Figure 16. Ptygmatic structure in the Massabesic Gneiss. 79 View is perpendicular to fold axes and to foliation. Dashed lines show orientation of biotite flakes in the schist and of elongate quartz aggregates in the granitoid layer. Sample 97-1-la, from Dcrryfield Park, east side of Manchester, N. H.

ix Page Figure 17. Sketch showing variation of wavelength of 80 ptygmatic folds in the Massabesic Gneiss with thickness of folded layer. The folded layers are granite (stippled), pegmatite (checked), and quartz (dashed); the enclosing rock is fine-grained gray biotite schist. Location: in quarry on northwest side of 520- foot hill, east edge of Manchester, N. H.

Figure 18. Ptygmatic structure in Massabesic Gneiss, 82 Derryfield Park, Manchester, New Hampshire.

Figure 19. Pink microcline pegmatite with twisted 86 inclusions of white oligoclase Massabesic Gneiss, Chester Turnpike overpass on Route 101, Candia.

Figure 20. Pegmatite dikes occupying shear zones in the 87 Massabesic Gneiss. Location:north side of Route 101, 0.9 mile southwest of Chester Turnpike, Auburn.

Figure 21. Massabesic Gneiss of intermediate composition 91 containing hypidioblastic magnetite grains, north side of Route 101, 1300 feet northeast of its intersection with Smith Road, Candia.

Figure 22. The system quartz-albite-orthoclase-water 105 showing the effect of water-vapor pressure (0.5-10kb) on the isobaric minimum (m^ ^ 3 ) and the "ternary" eutectic (m^ j q )« After Tuttle and Bowen, 1958, p. 75; Luth, et al.. 1964, p. 765-766; Mehncrt, 1968, p. 85.

Figure 23. The plane Ab/An =3.8 projected from the 106 anorthite apex onto the Q-Ab-Or base at a water pressure of 2000 bars. The dashed line applies to a system containing no anorthite; the heavy curves apply to a system in which Ab/An = 3.8. Lines QX and QY are discussed in the text. After von Platen (1965), Winkler (1967), and Ehlers (1972).

Figure 24. Modal composition of 8 samples of Massabesic 109 Gneiss with more than 80 percent quartz and feldspar, in weight percent. See text for discussion. x Page Figure 25. Locations and strikes of nineteen mafic 126 dikes and'sills in the Suncook quadrangle. All are vertical or nearly vertical; lengths are not to scale.

Figure 26, Index map showing major structures of New 139 Hampshire and surrounding areas.

Figure 27, Synoptic pi diagram of poles to S2 schist- 142 osity in the Littleton Formation and upper member of the Berwick Formation, Suncook quadrangle, based on 164 readings.

Figure 28. Pi diagram of poles to S„ schistosity in the 144 Jenness Pond member of the Littleton Formation.

Figure 29. Pi diagram for foliation in the Concord Granite 146 and associated pegmatites, Suncook quadrangle (47 readings).

Figure 30. Pi diagram for foliation in the Massabesic 148 Gneiss, Suncook quadrangle (67 readings).

Figure 31, Pi diagram for joints in the Suncook quadrangle 153 (122 readings).

Figure 32. Lithofacies and paleogeography of the middle 169 and upper Helderberg Stage in the northern Appalachians (Boucot, 1968, p. 92).

Figure 33. Lithofacies and paleogeography of the 170 Oriskany Stage in the northern Appalachians (Boucot, 1968, p. 93).

Figure 34. Sketch showing history of proto-Atlantic Ocean 176 and surrounding continents from late Pre- cambrian through Late Devonian time (Bird and Dewey, 1970, Figure 9).

Figure 35. Approximate boundaries between major tectonic 178 zones in New England, as defined by Bird and Dewey (1970, p. 1033). INTRODUCTION

Purpose and Scope

A regional approach characterizes much of the recent geo logical work in the northern Appalachians. Without the know ledge gathered from a large number of small detailed studies,

such works as those by Zen, et al. (1968), Cady (1969), Rodgers

(1970), and Bird and Dewey (1970) would not have been possible.

One area in which detailed studies have only lately been made

includes that part of the Merrimack synclinorium extending across central New Hampshire. This mass of metasediments and

intrusive rocks lies in what may prove to be the source area of an extensive belt of penninic nappes identified by Thompson, et

al. (1968), in eastern Vermont, western New Hampshire, and central Massachusetts. Therefore, study of the Merrimack syn clinorium may provide critical information in the search for a

unified regional approach to the geology of the northern Appa

lachians.

The Suncook quadrangle was chosen for study because of its position on the southeastern limb of the Merrimack synclinorium

and because almost nothing has been published on the area since

C. H. Hitchock's small-scale atlas and accompanying text appeared

in 1877. The largest part of the present report is devoted to a

description of the stratigraphy, petrology, and structure of the metasedimentary and igneous rocks of the quadrangle. An effort is also made to fit this small area into the broad framework provided by recent regional studies, including those relying on the new global tectonics.

Location and Geography

o * o ’ The Suncook quadrangle lies between 40 00 and 43 15 north o * o ® latitude and 71 15 and 71 30 west longitude (Figure 1). It straddles the boundary between the seaboard lowland and New

England upland sections of the New England physiographic pro vince. The total relief of the area is 1370 feet; the lowest point is 140 feet above sea level, where the Merrimack River crosses the southern border of the quadrangle; the highest point has an elevation of 1410 feet and is on the summit of Fort

Mountain in Epsom. Local relief averages approximately 250 feet.

The quadrangle encompasses about 217 square miles of mature ly dissected hills modified by the erosional and depositional effects of continental glaciation. Two major streams, the Merri mack River and its tributary, the Suncook River, drain southward.

Small streams in the southeastern part of the area drain eastward into the Lamprey River. Outwash and kame-terrace deposits are abundant along the Merrimack and Suncook rivers and are econom ically important in the town of Hooksett. A few small cskers occur in the towns of Auburn and Chester, near the southern bor der of the quadrangle* Till covers much of the rest of the area, making outcrops scarce and widely spaced in many places. Glacial 3

r__J

Figure 1. Index map showing the Suncook quadrangle (ruled) and other areas referred to in this report: Manchester quadrangle (M), Haverhill quadrangle (H), Mount Pawtuckaway quadrangle (MP), Alton quadrangle (A), and Gilmanton quadrangle (G). striae trend between S. 5° E. and S. 52° E., the average being

S. 20° to 30° E. (Figure 2).

Methods of Investigation

Field work in the Suncook quadrangle took a total of six months during the summers of 1968 and 1969* Outcrops were located on an enlarged copy of the 1957 edition of the topo graphic map by the pace and compass method. Structural features find lithology were described at each outcrop, and several hundred samples were collected for later study. Short excursions into the surrounding quadrangles aided lithologic correlation with rock units described there. Laboratory work included a study of

92 thin sections of the rocks, and X-ray diffraction and index of refraction studies of several of the minerals.

Previous Work

C. T. Jackson, director of the first geological and rain- eralogical survey of New Hampshire, described deposits of mag netite, arsenopyrite, and "diluvial granitic sand" in the Sun cook quadrangle in his Final Report, published in 1844. Until recently, most work in the area was of a cursory nature. C. H.

Hitchcock (1877) reported briefly on some specific features of

the area in his survey of the state. F. J. Katz (1917) describ

ed the stratigraphy to the east and northeast of the Suncook

quadrangle. Broad studies of the surficial geology and geo morphology of New Hampshire were published by J. W. Goldthwait s

- '10 *

20 '

Figure 2. Locations and trends of glacial striae in the Suncook quadrangle. (1925) and by Goldthwait et al. (1951), said Billings (1956) gives a general account of the bedrock geology of the state.

i T. R. Meyers (1941) reported on the silicified fault zones of the towns of Hooksett and Candia in his survey of quartz de posits of New Hampshire. More detailed studies of surrounding quadrangles were published by Freedman (1950a, b) and

Sriramadas (1966), who worked in the Mt. Pawtuckaway and

Manchester quadrangles, respectively (Figure 1). Geologic maps of the Gilmanton and Alton quadrangles were published by

Heald (1955) and Stewart (1961). STRATIGRAPHY

Berwick Formation Distribution Katz (1917, p. 167), following work in southeastern New

Hampshire and southwestern Maine, described a sequence of quartz-

feldspar-biotite gneiss, micaceous quartzite (in places anti-

nolitic), mica schist, "argillite schist," and slate. He named

these rocks the Berwick gneiss, the type locality being near

Berwick, Maine. Later workers noted that these rocks normally

show well-developed bedding rather than gneissic structure,

and Freedman (1950a, p. 456) introduced the name Berwick Form

ation in his report on the Mount Pawtuckaway quadrangle. Freed man (1950a, b) mapped a one- to three-mile-wide band southeast of the Fitchburg pluton and northwest of the Berwick Formation,

in the Mount Pawtuckaway quadrangle, as a part of the Littleton

Formation. However, Billings (1952, p. 44) showed that on the

bases of mineralogy and lithology these strata should be in

cluded in the Berwick Formation, and he so designated them on

the Geologic Map of New Hampshire (Billings, 1955).

Sriramadas (1966, p. 9, 18) divided the Berwick Formation

of the Manchester quadrangle into a lower less calcareous member

and an upper more calcareous member consisting of schist and

conspicuous, though not abundant, lime-silicate rocks. The upper

7 member approximates that portion of the Berwick Formation that

Freedman mapped as> Littleton Formation* It crops out in a band that crosses the extreme southeastern corner of the Suncook quadrangle, where it occupies an area of about 2 square miles in the towns of Raymond and Chester (Plate 1) • The rocks in this area are continuous with the Berwick Formation of the

Manchester and Mount Pawtuckaway quadrangles in a belt that can be traced to the type locality (Katz, 1917, Plate 61). The

Berwick Formation is overlain on the northwest by hornblende- rich gneisses that Sriramadas (1966, p. 32) included in the

Massabesic Gneiss. It is also locally overlain by small amounts of infolded Littleton Formation, as, for example, in the north ern part of the Manchester quadrangle and southern part of the

Suncook quadrangle, east of Massabesic Lake (Plate 1).

Lithology

The Berwick Formation of the Suncook quadrangle consists of biotite schist and lime-silicate granulite. The lime-sili cate granulite, which locally comprises as much as 30 percent of the formation, is a fine-grained, light-gray to greenish- gray resistant rock that lacks well-developed schistosity but does show alternating light and dark beds that range from 0.5 to 3.0 cm thick. Units of this banded rock are as much as

30 cm thick but are more commonly about 10 cm thick. The lime- silicate granulite alternates with fine- to medium-grained dark purplish-gray quartz-feldspar-biotite schist. Layering in the 9

TABLE 1

Approximate Modes 6 f the Upper Member of the Berwick Formation

Sample No. 6-24-4b 6-25-lb 6-25-ld(l) 6-25-ld(2) 6—25—,

Quartz 39.2 38.1 40.2 43.0 36.5

Plagioclasea 29.8 32.2 38.2 30.6 35.5

Biotite 29.0 - - 24,4 -

Amphibole - 12.2 10.6 1.6 9.9 00 Clinopyroxene - 12.6 * - 13.8

Sphene - 1.1 2.2 0.4 1.7

Apatite 0.6 0.4 0.4 - 0.1

Zoisite - 0.6 -- 0.3

Carbonate - 2,3 - - 1.9

Pyrite 0.6 tr. - 0.3

Leucoxene - 0.8 - - -

Zircon 0.2 tr. _

^Composition of ^ ^ 5 ^65 ^60 ^**35 ^50 plagioclase

Description and Location of Samples Sample No.: 6-24-4b Fine-grained gray biotite schist, on road just south of Norton Pond, Raymond* 6-25-lb Greenish-gray lime-silicate gneiss, on hill 670 feet north of intersection of Murphy and Raymond roads, Raymond. 6-25-ld Greenish-gray lime-silicate gneiss (1), and fine-grained purplish-gray biotite schist (2), on hill 670 feet north of intersection of Murphy and Raymond roads, Raymond. 6-25-le Greenish-gray lime-silicate gneiss, on hill 670 feet north of intersection of Murphy and Raymond roads, Raymond. 10 schist parallels that in the granulite. The schist commonly shows a faint to obvious lineation caused by parallel arrange ment of elongate biotite flakes. The lineation parallels the strike of the bedding and cleavage.

Estimated modes for the upper member of the Berwick

Formation) based on.counts of 500 points, are given in Table 1.

Microscopic examination shows the lime-silicate granulite to be granoblastic, with poikiloblastic diopside and (or) hornblende, * » and quartz and calcic plagioclase (An^g to An^5) as xenoblastic grains averaging 0.1 to 0.5 mm in diameter but ranging up to about 2.0 mm. Quartz commonly exhibits slight undulatory extinction and contains abundant tiny solid and liquid in clusions. The plagioclase in most of the lime-silicate rocks is slightly sericitized. Faint continuous zoning is common, as are various combinations of albite,!pericline, and Carlsbad twinning.

Hornblende occurs in two forms: first, as separate idiobiastic to xenoblastic grains and very irregular poikiloblastic grains containing most of the other minerals present; and second, as a patchy, ragged, or bladed alteration product derived from clinopyroxene. Amphibole derived from alteration of pyroxene is faintly colored as compared with that in separate crystals, which is pleochroic in shades of pale brownish-green and bright bluish-green. Diopside was identified on the basis of 2V and indices of refraction. It is generally less abundant than amphibole, occurring as small xenoblastic grains and irregular poikiloblastic grains. The poikiloblastic grains of amphibole 11 and clinopyroxene are coarser-grained than the plagioclase and quartz; otherwise, all of the minerals have about the same average grain size.

Sphene and apatite, in small hypidioblastic or xenoblastic grains, occur in varying amounts in the lime-silicate granulite.

Sericite, pyrite, chlorite, and patchy calcite are relatively abundant in the more altered rocks. The calcite occurs as small irregular untwinned aggregates between plagioclase grains and is therefore most likely an alteration product. Traces of epidote occur locally and zircon produces pleochroic halos in some of the amphibole.

The biotite schist of the Berwick Formation is lepido- blastic to granoblastic, the grain size ranging from 0.05 mm to

L.O mm and averaging about 0.2 mm. The rock consists mainly of biotite, quartz, and plagioclase, with somewhat less amphibole and pyroxene, and minor orthoclase. Accessory minerals include sphene, monazite, apatite, epidote, allanite, chlorite, and pyrite.

Pleochroism in the biotite ranges from pale tan to bright reddish-brown. The flat surfaces of the majority of the biotite flakes appear to lie within a few degrees of parallelism with compositional banding. However, in thin sections cut normal to the lineation, two sets of biotite flakes are seen to intersect at about a 90-degree angle. One of these sets parallels tex tural and compositional layering in the rock, and in fine grained layers, this set predominates. Quartz and feldspars 12 also tend to be flattened parallel to bedding in the fine-grained layers* In coarser-grained layers, both sets tend to be equally well developed, but in a few cases, the set perpendicular to bedding predominates. There is no systematic cross-cutting relation between the two sets of flakes, and it is therefore probable that both sets formed at the same time.

Metamorphism

The mineralogy of the biotite schist and lime-silicate granulite of the upper member of the Berwick Formation indicates derivation from calcareous siltstone and shale. The mineralogy does not clearly indicate the metamorphic facies to which these rocks should be assigned. Turner and Verhoogen (1960, p. 494) list the index minerals useful in correlating zones of regional metamorphism from one rock type to another. They correlate the first appearance of hornblende in a metamorphosed calcareous sandstone with that of kyanite or staurolite in pelitic meta sediments. Thompson and Norton (1968, p. 321) correlate the first appearance of diopside in calc-silicate rocks with that of staurolite in pelitic rocks.

Freedman (1950a, p. 463) maps metamorphic facies in the •■

Mount Pawtuckaway quadrangle in such a way.that, if his zones were continuous albng strike to the southwest, all of the Berwick

Formation in the Suncook quadrangle would occur in the muscovite- billimanite zone of the amphibolite facies as defined by Turner

(1968, p. 311). Because Freedman omits locality information in \

his discussion of the sillimanite-bearing schists southeast of

the Fitchburg pluton, it is difficult to evaluate how accurately

his sillimanite isograd has been drawn. Billings' Geologic Map of New Hampshire (1955) agrees with Freedman's location of the

sillimanite isograd.

Sriramadas (1966, p. 18) states that lime-silicate rocks are especially abundant in the "garnet" zone of the Berwick

Formation in the Manchester quadrangle. He draws the garnet

zone in such a way that it should continue along strike across

the southeastern corner of the Suncook quadrangle, thus dis

agreeing with the interpretations of Freedman and Billings.

However, the minerals present in the lime-silicate rocks of

Sriramadas* garnet zone are essentially the same as those in

lime-silicate rocks of his microcline-sillimanite zone

(Sriramadas, 1966, p. 56). Garnet was identified in only two described samples from the upper member of the Berwick Formation

in the Manchester quadrangle, a schist and a spotted gneiss

(Sriramadas, 1966, p. 19). The spotted gneiss also contains

diopside. Both of these samples came from localities well with

in the garnet zone as drawn by Billings (1955). Sriramadas

apparently did not recognize a sillimanite zone in the upper

member of the Berwick Formation because he found no sillimanite

in that unit. However, the absence of sillimanite does not

prove that the rocks were not exposed to temperatures and pres

sures capable of producing sillimanite in rocks of favorable

composition. As shown by Hyndman (1972, p. 354), mineral assemblages like those found in the upper member of the Berwick Formation could form in any of the zones of the amphibolite facies of metaroorphism. The rocks of the Berwick Formation in the Suncook quadrangle contain neither garnet nor sillimanite.' However, the presence of diopside in the lime-silicate granulites indicates that they have probably undergone staurolite- or higher-grade metamorphism. The absence of characteristic index minerals in both the Manchester and the Suncook quadrangles probably re flects a deficiency of alumina in the sediments from which the

Berwick Formation was derived. Considering all of the above information, the Berwick Formation in the Suncook quadrangle is tentatively placed in the muscovite-sillimanite zone of the amphibolite facies of metamorphism. This agrees with Freedman's and Billings' work in the surrounding areas and also makes sense when related to the origin of the Massabesic Gneiss, de scribed below.

Minor chloritization of biotite, slight sericitization of plagioclase, and production of hornblende at the expense of clinopyroxene all indicate that the Berwick Formation underwent some degree of retrograde metamorphism. This may be related to the intrusion of numerous small pegmatites or to the intrusion of late synkinematic and postkinematic granites of the Fitchburg pluton. The Concord Granite and some parts of the Massabesic

Gneiss and related rocks might have supplied slowly decreasing 15

temperatures and fluids to produce diaphthoresis in the last stages of deformation.

Thickness

Scarcity of outcrops and abundance of glacial cover com plicate the estimation of thickness of the upper member of the Berwick Formation in the Suncook quadrangle. The contact with the Massabesic Gneiss is not exposed* and its position must therefore be estimated. Additional difficulties result from the lack of primary sedimentary structures* drag folds* and other structures that might ordinarily be used to deter mine tops of beds. However* cleavage in the schists strikes uniformly N. 40° - 65^ E. and dips 55° - 85° southeast. Be cause cleavage generally parallels bedding in the Suncook quadrangle* the thickness of the Berwick Formation can be es timated using the following formula developed by Billings and first published by Freedman (1950a* p. 464):

n n t = b • — • r — b • — • r n w n e

t = thickness of formation

b - breadth of outcrop of formation measured perpendicular to strike

nw = number of outcrops with tops to west

ne * number of outcrops with tops to east

n = number of outcrops for which data on direction of tops are available 16

r » average ratio of breadth of outcrop to thickness w in observed outcrops with top to west

r a average ratio of breadth of outcrop to thickness e in observed outcrops with top to east

Because all exposures of the Berwick Formation in the Suncook quadrangle dip uniformly to the southeast, this formula can be simplified thus:

n t = b •— f • r n e

The breadth of outcrop must be estimated because the contact with the Massabesic Gneiss is not exposed. Assuming an average strike of N. 55°E,, b is approximately 5500 feet. The function ne is 1.0 in this case because all beds dip in the same direc- n tion. The average dip of bedding is 78 . Therefore, r^, is sin 78° = 0.98. Thus, the thickness of the upper member of the

Berwick Formation in the Suncook quadrangle is approximately

5390 feet, assuming a uniform dip to the southeast and assuming that the contact with the Massabesic Gneiss is correctly located on Plate 1. Billings (1956, p. 42) estimates that the total thickness of the Berwick Formation is about 10,000 feet.

Correlation and Age

The upper member of the Berwick Formation is stratigraph- ically the topmost unit of the Merrimack Group, which was named by Hitchcock (1870, p. 34) and revised by Billings (1956, p. 43) and Sriramadas (1966, p. 10). The type locality of the Merrimack t

Group is along the valley of the Merrimack River in Massa

chusetts. The group includes the Bliot and Berwick Form

ations, each of which is divided into a lower lime-poor and

an upper lime-rich member. The two formations are grada

tional with one another, and the Berwick is conformably over-

lain by the Lower Devonian Littleton Formation at a number of

localities southeast' of the Fitchburg pluton.

Correlations of the Berwick Formation with other rock

units in New England are based on a combination of lithologic

similarities, thickness, and meager paleontologic data.

Freedman (1950a, p. 488) suggested a Silurian age for the

Berwick Formation on the basis of indirect correlations with

rocks in western Maine. A Silurian age was also suggested by

Billings (1956, p. 103), who correlated the Eliot and Berwick formations with the Vassalboro and Waterville formations near

Waterville, Maine. The Waterville Formation contains a "middle"

Silurian graptolite. Billings also believed that the Merrimack

Group is a more clastic easterly facies of the Fitch Formation.

On the basis of fossils found near Littleton, N. H., Billings

(1937, p. 486) assigned a middle Silurian age to the Fitch

Formation. Boucot and Thompson (1963) suggested a Ludlow (middle

Late Silurian) age for fossils collected at Fitch Farm. Pavlides, et. al. (1968, p. 71) proposed a Wenlock (?) (earliest Late

Silurian) age for the Fitch Formation of the Bronson Hill anti cline, where its maximum thickness is only 400 feet. Osberg, et al. (1968, p. 245) correlated the Madrid Formation (west-

central Maine), the Hardwood Mountain Formation (northwestern

Maine), and the Fitch formation (New Hampshire), The Madrid,

Hardwood Mountain, and Fitch formations are all calcareous,

as are the Bliot and Berwick formations. All are overlain by

the Lower Devonian Littleton Formation and its equivalents,

and all are commonly underlain by Lower Silurian and (or)

Ordovician rocks. Therefore, although no fossils have been

found in the Berwick Formation, a Late Silurian (Wenlock or

Ludlow) age apparently fits it best. Littleton Formation

\ Distribution

C. P. Ross (1923) made a detailed study of a small area near

Littleton, New Hampshire, that was known as the Amonoosuc Mining

District. There he found a series of black argillites and

lighter-colored quartzites that had been called "clay slates" by

Hitchcock (1874, p. 474) and that, in part, closely resemble rocks

of the upper part of Laliee's Blueberry Mountain Series (Lahee,

1913, p. 242). Ross (1923, p. 284) proposed the name Littleton

Argillite for these rocks, which are well exposed in the township

of Littleton. Because of the variable character of equivalent

rocks elsewhere in New Hampshire, later workers changed the name

to Littleton Formation.

The Littleton Formation is the most widespread of all form

ations exposed in New Hampshire, the largest area of outcrop be

ing in the Merrimack synclinorium (Billings, 1956, p. 27).

Billings (1945, p. 42) traced the Littleton Formation southeast

from the type locality to the southern edge of the Winnipesaukee

and Wolfeboro quadrangles. Stewart (1961) mapped the Littleton

Formation in the Alton quadrangle, and Heald (1955) mapped the

Littleton of the Gilmanton quadrangle. Freedman (1950a, b), in mapping the Mount Pawtuckaway quadrangle, included in the

Littleton Formation areas of schist both northwest and southeast

19 20 of the Fitchburg pluton. Those southeast of the Fitchburg pluton were included in the Berwick Formation by Billings (1955). In the Manchester quadrangle, Sriramadas (1966) mapped Littleton schists in two narrow bands just southeast of the Fitchburg pluton. Billings (1$55), in the Geologic Map of New Hampshire, included these and similar rocks further southwest in the

Merrimack Group. 0n6 of the bands of schist mapped by Sriramadas continues northeastward into the southern part of the Suncook quadrangle. The rocks in this band closely resemble schists of the Littleton Formation exposed in the northern half of the quad rangle.

In the Suncook quadrangle, the Littleton Formation crops out in an irregular area of about 125 square miles that covers most of the northern half of the map (Plate 1) and in an area of about

0.5 square mile on the south edge of the map, east of Auburn.

The larger area is interrupted, from the center of the quadrangle to the southern part of.the western edge, by a 4-mile-wide pluton of massive binary granite (Concord Granite). A small area of

Littleton Formation occurs in the southwestern part of this body of Concord Granite. Faults separate the Littleton Formation and binary granite along at least a part of the contact on both the northwest and southeast sides of the pluton. The contact between the Littleton Formation and the Massabesic Gneiss is gradational.

Near the contact, the Littleton Formation contains much pegmatite in concordant lenses and layers. The pegmatite may have been 21 emplaced by permissive intrusion of mobilized material derived from the Massabesic Gneiss, or it may represent material derived from the schist itself.

Lithology

The Littleton Formation of the Suncook quadrangle has been subdivided into two members. The Jenness Pond and the Pittsfield members, first described by Heald (1955) in the Gilraanton quad rangle, are easily recognized and hence are described separately below.

Pittsfield Member

The Pittsfield member contains several distinct rock types, the. commonest being medium-grained quartz-mica schist and gneiss.

Medium-grained schist containing biotite, muscovite, plagioclase, and quartz constitutes about 35 percent of the Pittsfield member

(Table 2). The rock is generally rusty brown to almost black because of the presence of iron oxides derived from the decom position of pyrite, Schistosity results from parallel arrange ment of biotite and chlorite, and, where present, sillimanite.

Muscovite flakes are parallel to the foliation in some places and cut across it in others. The schistosity parallels bedding in all but a few places; rarely there is an axial-plane cleavage that intersects bedding. However, the schistosity is commonly crinkled or bent by small open folds.

Under the microscope, the medium-grained mica schist is seen to consist of biotite, muscovite, quartz, and plagioclase. Table 2. Approximate Modes, Pittsfield Member of the Littleton Formation

Sample No. 8-1-la 8-6-4b 8-ll-2d 8-20-2c(l) 8-20-2c(2) 96-19-ld 97-17-2a 97-27-la 98-l-6a 98-7-0a

Quartz 64 51 15 24 21 66 38 62 54 59 Microcline 14 Plagioclase3 19 37 26 46 43 25 27 24 29 7 Biotite 6 15 10 16 12 Chlorite 4 1 2 Muscovite 17 21 Garnet tr. 1 Sillimanite tr. tr. Hornblende 13 11 32 Diopside 9 26 Apatite tr. tr. tr. tr. tr. Sphene 1 1 tr. Soricite 12 tr. Calcite Epidote tr. tr. Zoisite tr. Zircon tr. tr. tr. tr. tr. tr. Magnetite tr. tr. 1 Pyrite tr. tr. tr. Pyrrhotite tr. Ilmenitc

Plagioclase Composition An^5 An An An An An, An AnS5 An. 30 55 50 '45 2 0 45 20 Tj 23

Table , continued. Description and Location of Samples

Sample No.

8-1-la Fine-grained massive gray hornblende granulite, 1.1 mile N.66° E. of Clay Fond, Hooksett.

8-6-4b Fine-grained massive pinkish-gray lime-silicate granulite, south side of Route 202, at Route 106 overpass, Concord.

8-ll-2d Medium-grained gray diopside-hornblende gneiss, west side of Route 3A, 0.7 mile south of Bow- Hooksett town line.

8—20—2c (1) Medium-grained greenish-gray diopside gneiss

8-20-2c (2) Medium-grained dark green hornblende gneiss, on west side of Interstate Route 93, 0.25 mile northwest of WG1R radio towers, Manchester.

96-19-ld Fine-grained greenish-gray mica gneiss, just southeast of Beaver Pond, Bear Brook State Forest.

97-17-2a Medium-grained rusty gray mica schist, on northeast side of road, 1.2 miles southeast of North Pembroke, Pembroke.

97-27-la Fine-grained gray quartz-plagioclase-biotite schistose gneiss, just southeast of Beaver Pond, Bear Brook State Forest.

98-l-6a Fine-grained gray quartz-plagioclase-biotite schist, on power line, 1 mile S. 45° E. of Beaver Pond, Candia.

98-7-6a Medium-grained gray quartz-mica schist, 0.5 mile S. 30° w. of Plausawa Hill, Pembroke. 24

Accessory minerals include garnet, sillimanite, and chlorite.

Chlorite is an important constituent only within a few thousand feet of the two major faults of the quadrangle or along their projected trends. In a few cases, sillimanite and chlorite occur together in a single thin section, and chlorite may have been produced by late retrograde metamorphism or by solutions travel ing along the faults. Quartz occurs as equigranular or flat tened grains averaging 0.5 mm across and ranging up to 2.0 mm.

It commonly contains fine dusty inclusions, and thin needle-like inclusions of sillimanite are locally abundant. Plagioclase, which is commonly untwinned, was identified on the basis of index of refraction. Its composition ranges from about AniQ to An25.

Plagioclase grains are generally equigranular, xenoblastic, are

t slightly smaller than quartz and locally show minor sericitization. Quartz and plagioclase form thin granulose layers separ-

t ating mica-rich layers. Biotite flakes generally are parallel to compositional banding in the schist. They average 0.5 mm and range up to 5 mm in length. Common associates include zircon and apatite. Muscovite occurs as coarsely intergrown aggregates of flakes or as individual flakes averaging 0.5 mm to 1 mm long, intergrown with biotite and chlorite. Most of the muscovite cuts across the biotite and the schistosity at various angles and is therefore assumed to be a late mineral. Garnet occurs as xenoblastic or hypidioblastic porphyroblasts that average 1.5 mm across. Most are poikiloblastic with quartz and feldspar, and a 25 few contain abundant small quartz inclusions concentrated near their centers. The inclusions average less than 0.1 mm across and show no preferred orientation. '

Medium- to coarse-grained biotite-muscovite gneiss makes up about 55 percent of the Pittsfield member in the Suncook quad rangle. The gneiss is gray or purplish gray on fresh surfaces but weathers rusty brown to dark gray. The cleavage results from parallel grains of muscovite, biotite, and sillimanite.

Unlike that of the schist, the cleavage of the gneiss generally shows small-scale undulations but no crinkling. Cleavage and bedding are parallel in all outcrops examined. Bedding is de fined by granulose layers averaging 1 to 3 mm thick and schistose layers ranging from a fraction of a millimeter to 10 mm thick.

Gradation between granulose and schistose layers results in the rock being classed as a schistose gneiss. In some places, micas and sillimanite form a- or b-lineations in the gneiss. Tour maline crystals are parallel to the lineation at a few localities.

In thin section, the schistose gneiss consists essentially of quartz, biotite, plagioclase, and muscovite. Quartz occurs as xenoblastic grains ranging from 0,1 mm to 4 mm-long and is coarsest in the granulose layers. The grains are equigranular or are flattened parallel to the foliation. Flattened quartz grains exhibit moderately well-developed undulatory extinction, and all of the quartz is at least slightly strained. Grain broundaries are rounded where quartz and plagioclase are in contact and are 26 irregular elsewhere. Quartz also occurs poikiloblastically in plagioclase. Xenomorphic plagioclase grains average 0.3 nun to

0.5 mm across. Some exhibit albite twinning, but most are un twinned. The plagioclase was identified by index of refraction as ranging from about An10 to An^. The plagioclase is commonly fresh, but slight to moderate sericitization occurs in gneisses that are highly deformed. Continuous zoning and myrmekite were identified in a few thin sections. Biotite flakes average about

0.5 mm long and are generally aligned parallel to the composi tional layering of the gneiss* Partial alteration of biotite to chlorite and an opaque mineral occurs locally, and small zircon crystals are present in nearly all the samples examined microscopically. Muscovite, as 0.5 mm to 1.0 mm flakes, is commonly intergrown with and cuts across biotite. Accessory minerals include perthitic microcline, garnet, sillimanite, tourmaline, ilmenite, and magnetite. Microcline occurs in gneisses whose granulose layers are coarser and thicker than normal. In some cases, these coarse layers, which are commonest within one to two miles of the contact with Massabesic Gneiss, grade into thin pegmatoid layers or lenses. Grain size averages

0.5 cm to 2 cm but ranges up to 10 cm. The microcline exhibits various kinds of perthitic intergrowths, including microperthite and string, braid, and patch perthites. Separate plagioclase grains exhibiting myrmekite and albitic rims also occur .in the unusually coarse layers. Sillimanite is a local constituent of 27 all the Pittsfield gneisses, occurring as slender needles in

1 quartz and other minerals and as coarser aggregates associated with ragged biotite flakes. Individual sillimanite crystals range from 0.05 ram to 5 ram long.

In addition to the mica schist and schistose gneiss, the

Pittsfield member contains several common minor rock types.

At many outcrops, the mica schist is interbedded with fine grained light-gray or purplish-gray quartz-plagioclase-biotite schistose gneiss. The rock occurs as beds from a few millimeters

to several meters thick. Biotite, making up 10 to 25 percent of the rock, is evenly distributed within the individual beds, giv ing the layers a massive appearance. The biotite flakes are oriented parallel to foliation in adjacent layers of schist. One peculiarity of this rock is the smoothness of the foliation.

Even where foliation in nearby schist or gneiss is crinkly and finely folded, that of the fine schistose gneiss is smooth and only broadly folded. In thin section, the rock is seen to consist essentially of quartz, plagioclase, and biotite (Table 2). Quartz grains are strained and flattened, averaging 0.2 mm to 0.4 mm across. Plagioclase is more calcic than that in the more common rocks of the Pittsfield member, ranging from about A n ^ to An55.

It is fresh in some thin sections and sericitized in others.

Biotite flakes average 0.4 mm long and exhibit leochroic halos around small zircon crystals. Apatite is a common minor associate of the biotite. Other minor minerals include magnetite, pyrite, 28 sphene, and chlorite. Neither muscovite nor sillimanite was identified in the fine schistose gneiss.

Lime-silicate granulite and diopside-hornblende gneiss make up small amounts of the Pittsfield member. The granulite is fine-grained, massive, pinkish-gray with pink and dark green spots, and it occurs in beds several centimeters to a meter thick.

Essential minerals are quartz, calcic plagioclase (An^^), garnet, and hornblende. Accessories include sphene, zoisite, apatite, magnetite, and pyrite. Grain size averages 0.05 mm to 0.2 mm for the quartz and feldspar, and 1 mm to 2 mm for the poikiloblastic garnet and hornblende. Quartz is unstrained and plagio clase is unaltered.

The diopside-hornblende gneiss is essentially two rock types in alternating beds a few millimeters to several centimeters thick.

Diopside-rich layers are massive, greenish-gray, contain no horn blende, and are generally several centimeters thick. The hornblende-rich gneiss occurs as thin discontinuous layers and lenses within the diopsidic rock, and is generally devoid of diopside.

In thin section, the diopside-rich rock is seen to be essentially a granulite consisting of quartz, plagioclasej diopside, and garnet (sample 8-20-2c(l), Table 2). The quartz and plagioclase average 0.3 mm across, and diopside averages 0.5 mm to 1.0 mm, but there are differences in grain size across the bedding. Quartz occurs as small slightly strained equigranular xenoblastic grains,

Plagioclase is generally partly altered to sericite and zoisite. 29

Diopside is fresh to partly altered to amphibole and occurs mainly as xenoblastic poikiloblastic grains containing randomly oriented quartz and feldspar. Garnet averaging 0.5 mm across is also xenoblastic and poikiloblastic. Accessory minerals in clude sphene, calcite, and pyrite.

The hornblende-rich layers in the diopside-hornblende gneiss contain about the same percentages of quartz and plagioclase as the diopsidic bands (sample 8-20-2c(2)f Table 2). Hornblende grains are poikiloblastic and average 0.5 mm to 1.0 mm long.

They generally form discontinuous aggregates of intergrown crystals strung out parallel to the compositional layering.

In addition to the rock types described above, the Pittsfield member contains a relatively small amount of gray thin-bedded to massive quartzite with a few percent of biotite or muscovite so oriented to produce a schistosity parallel to bedding. The quartzite is generally not highly deformed, having apparently behaved competently during the deformation. The quartzite con sists of quartz with differing amounts of accessory minerals.

Fine-grained quartzite in which quartz grains average 0.1 to

0.4 mm generally contains small amounts of plagioclase and biotite, the biotite giving the rock a purplish color on a fresh surface. The quartz is moderately strained and occurs as xeno blastic interlocking grains. 30

Jenness Pond Member

The Jenness Pond member of the Littleton Formation crops out over an area of about 17 square miles in the northeastern and eastern part of the Suncook quadrangle, and in a small area on the west edge of the quadrangle 2 miles north of Manchester. A variety of rock types occurs in this member, and nearly all are characterized by a predominance of muscovite over biotite. Thin- bedded mica schist makes up about 90 percent of the member (Fig ure 3). The schist is mostly medium-grained and silvery-gray, with fine disseminated pyrite that oxidizes to produce a rusty- brown stain on weathered outcrops. The schist is commonly crenulated, and slip cleavage occurs at a few places (Figure 4).

Sillimanite is common at many exposures and forms thin gray fibrous layers intergrown with micas, and coarse white or gray aggregates intergrown with quartz. The latter resist weathering more than the enclosing schist and so stand out on weathered surfaces, giving the rock a "knotty" appearance (Figure 5).

Minor rock types within the Jenness Pond member include fine-grained silvery to rusty crinkled mica schist, fine-grained silvery mica gneiss, quartzite, and lime-silicate rock. The fine-grained crinkled mica schist is similar to the medium-grained schist described above, but it does not contain sillimanite or granulose .layers. It is silvery gray when fresh, weathers rusty, and commonly exhibits well-developed crenulations and, locally, slip cleavage. The fine-grained mica gneiss is generally silvery Figure 3. Typical thin-bedded schist of the Jenness Pond member, 1.1 miles east of Epsom Mt., Epsom. Figure 4. Crenulated silvery medium-grained mica schist of the Jenness Pond member, about 1.25 miles east of Epsom Mt., in the town of Northwood. 33

Figure 5. Alternating fine-grained schistose gneiss and medium- grained mica schist of the Jenness Pond member. The schist - contains "knots" composed of quartz, muscovite, and .silli manite. Location: in town of Northwood, about 1.2 miles east of Epso.m Mt., Epsom. 34 light-gray on a fresh surface and it weathers medium gray with local rusty spots. Quartz and feldspar make up most of this rock; both biotite and muscovite are present, the muscovite predominating, and locally it contains garnet. No sillimanite was identified. Axial-plane cleavage is well shown on several outcrops (Figure 6). The quartzite is thin-bedded, phyllitic, and contorted by small-scale folds. Lime-silicate granulite is relatively uncommon in the Jenness Pond member, occurring as small isolated lenses elongate parallel to the foliation in the medium-grained mica schist (Figure 7).

Table 3 gives approximate modes for rocks of the Jenness

Pond member. In thin section, the medium-grained mica schist

(sample 98-21-3b) is seen to consist essentially of quartz, biotite, and muscovite. Other minerals commonly present include plagioclase, garnet, sillimanite, chlorite, apatite, zircon, magnetite, and pyrite. Quartz occurs as equidimensional or flattened xenomorphic grains, averaging 0.5 mm to 1.0 mm long, most of which are slightly strained. The relations between biotite and muscovite vary from place to place. Muscovite gen erally cuts biotite, but either mineral may be parallel to the schistosity. Both occur as thin intergrown flakes averaging

0.5 mm to 2.0 mm long* In a few samples, biotite forms relatively thick (1.0 mm) porphyroblasts flattened parallel to the schist osity. In sample 9B-21-la, muscovite flakes are parallel to schistosity and compositional banding. Biotite occurs as rel atively large (1.0 mm by 2.0 mm) lens-shaped porphyroblasts Table 3. Approximate Modes, Jenness Pond Member of the Littleton Formation

Sample No. 98-15-9a 98-21-la 98-21-3a 98-21-3b 98-21-3c 98-21-4a 99-11-1

Quartz 87 43 61 71 26 81 39 Plagioclase 8 2 3 - Biotite 1 26 7 8 19 4 Chlorite 3 1 tr. tr. - 1 9 Muscovite 8 21 22 18 1 12 49 Sillimanite tr. 50 - Garnet 8 2 tr. tr. - Apatite - tr. tr. tr. - - Epidote tr. tr. tr. Limonite 3 Zircon tr. tr. tr. tr, tr. Magnetite 1 1 tr. Ill 36

Table 3 continued. Description and Location of Samples

Sample No.

98-15-9a Fine-grained silvery gray phyllitic quartzite, 0.85 mile north of Meetinghouse Hill, Deerfield.

98-21-la Medium-grained silvery to rusty gray garnet-mica schigt, on southwest side of dirt road 1.2 miles N.20 W. of intersection of Routes 202 and 107, Northwood.

98-21-3a Medium-grained crinkled silvery to rusty gray Q garnet-mica schist, on dirt road 1.6 miles N.20 W. of intersection of Routes 202 and 107, Northwood.

98-21-3b Medium-grained silvery gray quartz-mica schist, same location as 98-21-3a.

98-21-3c Quartz-mica sillimanite lens from medium-grained mica schist, same location as 98-21-3a.

98—21—4a Fine-grained silvery gray quartz-mica gneiss, east edge of quadrangle, 0.25 miles north of Route 107, Deerfield.

99-11-1 Fine-grained silvery to rusty gray muscovite schist, on east edge of quadrangle, 0.5 mile N.36 B. of intersection of Routes 107 and 202, Northwood. 37

Figure 6* Fine-grained mica gneiss of the Jenness Pond member, showing nearly horizontal bedding, and axial-plane cleavage dipping steeply to the west. Brunton compass points north. Location: on hill southeast of Pleasant Lake and just north of Route 107, where the latter leaves the east edge of the Suncook quadrangle. Figure 7. Small lenses of lime-silicate rock in medium-grained mica-sillimanite schist of the Jenness Pond member. Location; in the town of Northwood, about 1.1 miles east of Epsom Mt., Epsom. 39 flattened parallel to the foliation. The c axes of the porphyroblasts lie in the plane of foliation rather than being perpendicular to it. In a few samples, biotite is partly al tered to chlorite. Plagioclase occurs as partly sericitized xenomorphic grains that average 0.3 mm in diameter. Its com position ranges from Anls to An ^ , as determined by refractive indices. Garnet occurs as xenoblastic or hypidioblastic grains averaging 0.5 mm to 1.5 mm across. It commonly contains quartz, and a few crystals contain small quartz grains of varied orien tation concentrated in the center of the garnet crystal. The garnet of the Jenness Pond member generally cuts biotite or muscovite rather than being enveloped in them, Sillimanite forms ragged aggregates of fine needle-like crystals intergrown with quartz, muscovite, and biotite. The largest grains occur in elongate flattened white or gray quartz-mica-sillimanite aggregates averaging 5 to 10 cm long and 1 to 2 cm thick. These crystals generally form a lineation and range from 0.05 mm to

0.30 mm thick. However, the finer needles bend and swirl and do not show a preferred orientation. Some of the sillimanite grows in and around ragged biotite flakes in such a way as to suggest replacement. The only minerals not penetrated by sillimanite are garnet, magnetite, and pyrite.

In thin section, the finer-grained mica schist and gneiss of this member exhibit many of the same characteristics as the more abundant medium

40 in abundance and commonly cuts it at a low angle* The biotite is oriented so as to produce a foliation parallel to compositional layering, and locally is partially altered to chlorite. Both biotite and muscovite occur as flakes averaging 0.5 mm across.

Quartz and plagioclase occur as xenoblastic grains averaging 0.1 to 0.4 mm across. The quartz is strained and, in rocks showing crenulation or axial-plane cleavage, somewhat crushed. The plagioclase occurs as fresh or slightly sericitized grains with an average composition of A n ^ to A n ^ . Sillimanite, compara tively rare in the fine-grained rocks, occurs mainly as slender needles in quartz.

Quartzite is relatively uncommon in the Jenness Pond member.

Most of it is similar to that of the Pittsfield member, but at a location one mile north of Meetinghouse Hill, it is thin-bedded and intensely folded on a small scale. The quartz grains are crushed and show well-developed undulose extinction. Thin folia of muscovite wrap around aggregates of quartz and are parallel to the contortions of the folded layers.

Metamorphism

The determination of metamorphic grade for rocks of the

Littleton Formation is complicated because the rocks vary in com position from impure quartzites to aluminous schists. In the

Suncook quadrangle, the only Al^SiO^ polymorph recognized, either in thin section or in powdered samples used for refractive-index I

41 determinations, was sillimanite. Northwest of the Hall Mountain-

Campbell Hill fault, sillimanite occurs with muscovite at widely scattered locations over a large outcrop area. Staurolite, kyanite, and andalusite are absent there* At many outcrops, the rocks contain biotite or biotite and garnet with no Al^SiO^.

One could map the metamorphic grade in this area in either of two ways. First, one could assume that only those outcrops with identifiable sillimanite should be included in the sillimanite- muscovite metamorphic zone. The result of this approach would be a very complex isograd system separating the sillimanite- muscovite zone from the almandine and biotite zones. The absence of staurolite, andalusite, or kyanite would be difficult to ex plain. A second alternative is to assume that all of the rocks in the area containing scattered sillimanite-bearing schists and gneisses should be mapped together in the sillimanite zone.

The absence of sillimanite in some rocks would simply reflect their chemical composition.

The writer is inclined to accept the second of the two alternatives. The AFM projection (Figure 8) shows that there is a wide range of bulk compositions in which no sillimanite could be expected, even in rocks that have undergone high-grade metamorphism. The difficulties involved in trying to break the

Littleton Formation down into a number of metamorphic zones are illustrated by attempts to correlate zones mapped by Freedman

(1950a, p. 454) in the Mount Pawtuckaway quadrangle across the A l-04 SilliBanite

MgO

Blotit#

K - P • Ida a r

Figure 8, Phase relations, silliraanite-muscovite zone, for the Suncook quadrangle, using Thompson's AFM projection. Quartz and muscovite present. border into the northeastern Suncook quadrangle. According to

Freedman*s map, the Jenness Pond member in the northwestern Mount

Pawtuckaway quadrangle is all in the biotite zone. However, out crops less than one-half mile west of the boundary between the quadrangles contain garnet-mica schist with abundant sillimanite.

There is no evidence for faulting between the two areas, and the structures in both areas are similar. There is also no gradation through garnet, staurolite, and kyanite zones. Freedman (1950a, p. 460) states that the rocks of the biotite zone of the Mount

Pawtuckaway quadrangle are about 50 percent quartz-mica schists and quartzites, 25 percent schists consisting of muscovite, quartz, biotite and chlorite, and 25 percent crinkled silvery- gray quartz-sericite phyllites. The quartz-mica schists and quartzites probably contain insufficient alumina to produce sillimanite on metamorphism. Medium-grained mica schists are described by Freedman as occurring in all of the metamorphic zones he recognized, and they are therefore not distinctive of the biotite zone. The quartz-sericite phyllites contain no biotite. They are apparently similar to crinkled fine-grained quartz-muscovite-chlorite schist of the Jenness Pond member in the

Suncook quadrangle. G. W. Stewart (personal communication) noted that this rock resembles schist that he has seen interbedded with andalusite schists in the Gilmanton and Alton quadrangles. Hence, the phyllite of the Mount Pawtuckaway quadrangle does not prove biotite-grade metamorphism there. The fact that biotite is the 44 highest-grade index mineral recognized in the northwestern corner of the Mount Pawtuckaway quadrangle does not necessarily mean that the rocks exposed there did not undergo higher-grade meta morphism. The same thing can be said for the Littleton Formation in the Suncook quadtangle.

All samples of the Littleton Formation containing raega- scopically identifiable sillimanite were examined in thin section or were powdered and examined in index oils. In no'case was potash feldspar associated with sillimanite in the common schists and gneisses. The two minerals are associated only where schists and gneisses of the Littleton Formation contain large pegmatite pods, as on the west side of Route 1-93, west of the village of

Suncook. At this locality, pegmatite pods measuring as much as

1 meter long and 10 cm to 20 cm thick contain coarse microcline, quartz, and oligoclase with accessory apatite, garnet, dumortierite and tourmaline. Sillimanite occurs at the boundary between the schist and pegmatite and within the pegmatite. The sillim anite here may have been introduced into the pegmatite from the adjacent gneiss and schist. If the pegmatite formed from magma generated elsewhere and injected into the Littleton Formation, the occurrence of sillimanite and microcline together would not necessarily mean that the rock should be mapped in the sillimanite-microcline metamorphic zone. However, if the pegmatite formed in place by partial mobilization of schist and gneiss, the rocks have probably undergone sillimanite-microcline-grade metamorphism* At this particular locality, the presence of

large masses of discordant pegmatite suggest that the first

explanation is the most likely. There are numerous places in

the Littleton Formation where small layers and lenses of pegmatoid material probably formed by partial mobilization, but

these contain neither microcline nor sillimanite.

The situation in the band of Littleton Formation south of

the Hall Mountain-Campbell Hill fault is similar to that to

the north. Sillimanite and muscovite occur together at scat

tered outcrops of the Pittsfield schist and gneiss. In addi

tion, chlorite is relatively common in the vicinity of the

fault and along its trend to the northeast of the last outcrop of silicified rock. Some of the rocks near the fault are basic

ally chlorite-muscovite schist with little fresh biotite (for

example, southeast of Beaver Pond, in Bear Brook State Forest).

Occurrences of this sort probably resulted from alteration of

the rocks by fluids associated with those that deposited quart2

along the fault.

Chlorite is common in small amounts in most of the rocks of

the Suncook quadrangle. In the Littleton Formation, it partly

replaces biotite and locally occurs along fractures in garnet.

Because it is associated with minerals representing higher-grade

metamorphism, including sillimanite and muscovite, the chlorite

is most likely a result of retrograde metamorphism, perhaps re

lated to the late-tectonic and post-tectonic intrusion and cool

ing of the Concord Granite and associated pegmatites. 46

Thickness

A calculation of the true thickness of the Littleton Form ation in the Suncook quadrangle is not possible. Outcrops are widely distributed and discontinuous, the structure is very com plex, and there are virtually no marker horizons that one can use for reference. In addition, the Campbell Hill-Hall Mountain and Pinnacle faults cut the formation. However, the thickness and the strike and dip of bedding were measured at numerous exposures, and some outcrops exhibit channels, cross bedding, and drag folds, which allow determination of tops of beds. The data available allow estimation of thickness by the method de scribed by Freedman (1950a, p. 464) and discussed in detail on pages 15-16 of this paper. The thicknesses so calculated are considered to be reasonable estimates given the impossibility of direct measurements.

Thickness calculations were made for both the Jenness Pond and the Pittsfield members of the Littleton Formation. About 38 separate outcrops of the Jenness Pond member were examined, of which 26 supplied data usable for calculating thickness. Eigh teen outcrops exhibited tops to the west. The average dip for these is 52°, and the ratio of thickness to width of outcrop is

0.79. Eight outcrops showed tops to the east. Their average dip is 70° and the thickness to width ratio is 0.94. The width of outcrop measured normal to the strike is about 14,800 feet.

Using this data and the formula given on page 16, the thickness 47 of the Jenness Pond member in the Suncook quadrangle is approxi mately 3,800 feet.

Structures useful in determining tops of beds are less abundant in the Pittsfield member than in the Jenness Pond member.

Of the approximately 170 exposures of the Pittsfield member visited in the course of this study, only 25 showed clear indica tions of tops of beds. Of these, 18 have tops to the west and 7 have tops to the east. The average dip of bedding where tops are to the west is 34°; that of the outcrops with tops to the east is

42°. The ratio of thickness to width of outcrop is thus 0.56 for the former and 0.67 for the latter. The breadth of outcrop of the Pittsfield member, measured perpendicular to the strike, is about 57,000 feet. The thickness calculated on the bases of these figures and the formula previously described is 12,312 feet*

Thus, the total calculated thickness of the Littleton Formation in the Suncook quadrangle is approximately 16,000 feet.

Correlation and Age

Because no fossils were found in the Littleton Formation of the Suncook quadrangle, correlation of these rocks is based on lithology and stratigraphic position. The rusty-weathering garnet-sillimanite-mica schist, biotite-sillimanite gneiss, and quartjsite are much like those described by Freedman (1950a),

Sriramadas (1966), and other workers. Rocks mapped as Littleton

Formation in the Gilmanton, Alton, Mount Pawtuckaway, and Man chester quadrangles are continuous with those of the Suncook 48 quadrangle and thus provide a sound basis for lithologic cor relation* Correlations to the west and southwest are not so easy to establish because of a scarcity of detailed information in those areas. However, Greene (1970) described the Littleton

Formation and estimated its thickness as 24,000 to 25,000 feet in the Peterborough quadrangle.

Heald (1955, p. 10) divided the Littleton Formation of the

Gilmanton quadrangle into three members, and Stewart (1961) recognized two of these three members in the Alton quadrangle.

The Durgin Brook member is confined to the northwest corner of the Gilmanton quadrangle and was not recognized in the Suncook quadrangle. The Jenness Pond member occurs at the type locality in the southeast corner of the Gilmanton quadrangle and also occurs in the southwest corner of the Alton quadrangle. Rocks of the Jenness Pond member typically consist of thin-bedded pseudo-andalusite and mica-sillimanite schists, and the second of these rocks is easily recognized and mapped in the Suncook quadrangle. The Pittsfield member is far more extensive than the others and was mapped by Heald and Stewart and by the writer.

Included in it are mica-sillimanite schist, in places rich in pyrite, and sillimanite gneiss. Of the three members, the Pitts field is the oldest and the Durgin Brook the youngest.

On a regional scale, the Littleton Formation of New Hampshire has been correlated with cyclically banded slates and argillites of Oriskany age in eastern Connecticut, central Massachusetts, and central, western, and northern Maine (Boucot, 1968, p. 89).

Included among units correlated with the Littleton Formation are the Fortin, Temiscouata, Seboomook, Meetinghouse, and Gile Moun tain formations. Thompson, et al. (1968, p. 207), believe the

Gile Mountain Formation is younger than the Littleton Formation in the Bronson Hill anticlinorium, but Boucot (1968, p. 90) and

Green and Guidotti (1968, p. 260) consider them to be of the same age west of the Connecticut Valley. The Gile Mountain

Formation can be correlated with the Compton, Frontenac, and

Tarratine formations and with the St. Francis and St. Juste groups in Quebec (Green and Guidotti, 1968, p. 260-262). How ever, Boucot (1968, p. 92) places the St. Juste Group in the

Middle Devonian (Eifelian), making it younger than the Littleton

Formation. The Seboomook Formation overlies the Hardwood Moun tain Formation in the Somerset geanticline of northwestern Maine and adjacent Quebec and the Madrid Formation in south-central

Maine. These two formations correlate with the Berwick Formation, which underlies the Littleton Formation in southeastern New

Hampshire. The Littleton Formation is continuous with the

Shapleigh Group of southwestern Maine (Hussey, 1968, p. 299).

The Shapleigh Group includes the Gonic, Rindgemere, and Towow formations. Thompson, et al. (1968, p. 208), suggest that the

Kinsman quartz monzonite and the Bethlehem Gneiss may represent ignimbrite masses in the Littleton Formation mobilized by partial fusion. They note similarities in extent and chemical composition between the Kinsman and Bethlehem rocks and ignimbrites in the 50

Littleton Formation in north-central Maine. The Kinsman quartz monzonite and Bethlehem Gneiss were included by Billings

(1956, p. 53-61) in the Upper Devonian (?) New Hampshire Plutonic

Series. Other workers (Page, 1968) consider the New Hampshire series to be Middle Devonian in age.

Lahee (1913, p. 242) identified marine Early Devonian fossils from banded argillites in the Blueberry Mountain Series near Littleton, New Hampshire. These rocks are now included in the Littleton Formation. Later workers (Billings and Cleaves,

1934; Boucot and Arndt, 1960) have also assigned the Littleton

Formation to the Lower Devonian, According to Boucot (1968, p. 90), the Littleton Formation at the type locality is Schoharie in age, whereas elsewhere east of the Connecticut Valley it ranges from Lower Helderberg through Esopus in age. MassabesAc Gneiss

Distribution

Pink and white gneisses composed mainly of microcline, oligoclase, quartz, and biotite occupy a seven-mile band extend ing northeast across the southeastern portion of the Suncook quadrangle, and similar rocks crop out in the Manchester, Mount

Pawtuckaway and Peterborough quadrangles* Billings (1952, p. 29} included these rocks in the Fitchburg pluton (Billings, 1955,

Geologic Map of New Hampshire)* Freedman (1950a, p. 464-467) subdivided the rocks of the New Hampshire magma series, which includes the Fitchburg pluton, into a number of separate units.

The microcline granite and quartz monzonite described by Freedman are similar to rocks that Sriramadas (1966, p. 32) named

Massabesic Gneiss, which is well-exposed around Massabesic Lake, on the border between the Suncook and Manchester quadrangles.

Sriramadas recognized three main rock types in the Massabesic

Gneiss of the Manchester quadrangle. These are, in order of decreasing abundance, pink microcline gneiss, white oligoclase gneiss, and amphibolite.

The Massabesic Gneiss is well-exposed at various localities in the Suncook quadrangle. White oligoclase gneiss crops out over a large area 0.8 mile N. 80° E. of Four Corners, Candia; and also where Route 101 passes under the Chester Turnpike, in

51 52

Candia. Pink microcline gneiss is well-exposed along the south

shore of Tower Hill Pond, Auburn. Good exposures of both rock

types occur on Mount Miner and along the eastern shore of

Massabesic bake, both in Auburn. White oligoclase gneiss con

taining fairly abundant hornblende occurs in a road cut on the

east side of Route 28 bypass, 1.2 miles north of its intersection

with Wellington Road, in Hooksett.

Lithology

The Massabesic Gneiss contains rocks of varied mineralogy.

Sriramadas (1966, p. 32) used the presence or absence of pink microcline to subdivide the unit into pink microcline gneiss

and white oligoclase gneiss. In the Suncook quadrangle, roost of

the gneiss that looks white in outcrop contains small amounts of microcline, although some is without microcline. Among the gneisses containing microcline, there is a wide range in the proportion of microcline to plagioclase (Table 4). Therefore the white oligoclase gneiss, as defined by Sriramadas, probably represents one end member of a gradational series of rocks rang

ing from those containing little or no oligoclase to those con

taining little or no microcline. In the Suncook quadrangle, most outcrops of Massabesic Gneiss are of intermediate composition.

Those containing more microcline than oligoclase are pinkish whereas those in which oligoclase predominates are generally whitish. The two types are commonly interlayered in a given out

crop and so were not mapped separately. Tabic 4. Approximate Modes of the Massabesic Gneiss

Samole No.: 6-29-la 6-29-lb 6-30-la 7-5-2a 7-8-lc 7-15-4a 7-17-4a 7-18-la 7-25-3a

Microcline 55 tr. 21 25 - 22 37 8 9

Plagioclase* 9 25 33 32 51 37 34 12 42

Quartz 24 58 36 38 44 37 28 71 27

Biotite 10 11 10 3 3 3 1 2 5

Chlorite tr. tr. tr. tr. tr. tr. tr. - tr.

Muscovite tr. - 1 1 1 1 tr. - tr.

Hornblende - 5 - -- - - 5 15b

Garnet - - --- tr. - - tr.

Apatite 1 tr. tr. tr. - - tr. tr. tr.

Zircon tr. tr. tr. tr. - tr. tr. tr. -

Sphene - tr. - - - - - 1 1

Allanite tr. - - tr. - -- tr. tr.

Epidote -- tr. - - tr. - tr. tr.

Carbonate - - - - tr. --- -

Magnetite tr. - tr. tr. - - tr. - -

Pyrite - - -- tr. - - tr. -

*Average Compo An25 A n , ? An__ to25 *"25 *"25 An,5 2a ^ 3 2 ^ 3 6 sition of plagioc]Lasc

bThe anphibole in sample 7-25-3a in hastingsite 54

Table 4 (continued)

Description and Location of Samples

Sample No*

6-29-la Pink microcline gneiss, on eastern knob of Mt. Pisgah, Auburn.

6-29-lb White microcline-oligoclase gneiss, same location as 6-29-la.

6-30-la Intermediate microcline-oligoclase gneiss, on west- facing slope 700 feet east of Spruce Lake, Auburn.

7-5-2a Intermediate microcline-oligoclase gneiss, 2000 ft. N. 20° W. of intersection of Main: Street, Candia (P0) and Route 101.

7-8-lc White oligoclase gneiss, south side of Route 101, 750 ft. west of intersection with Langford Road, Candia.

7-15-4a Intermediate microcline-oligoclase gneiss, on north side of east-west road, 1.22 miles east of village of Candia.

7-17-4a Intermediate microcline-oligoclase gneiss, on east side of Route 28 bypass, 1.05 miles southeast of intersection with Auburn Road, Hooksett.

7-18-la White hornblende-oligoclase gneiss, on east side of Route 28 bypass, 1.55 miles southeast of intersection with Auburn Road, Hooksett.

7-25-3a White hastingsite-oligoclase gneiss, on south side of Wellington Road, 1.6 miles west-southwest of intersection with Route 28 bypass, Manchester. 55

In hand sample, the Massabesic Gneiss is a pink to white medium- to-coarse-grained, hypidioblastic-granular gneiss that weathers pink, buff, or dark gray, with intercalated thin black schistose layers. Granulose layers consist of xenoblastic or hypidioblastic white oligoclase surrounded by generally xenoblastic pink microcline and irregular grains of gray quartz.

The white gneiss commonly contains hypidioblastic oligoclase porphyroblasts up to several centimeters long surrounded by oligoclase, microcline, and quartz grains averaging 2 to 4 mm across. Locally, the oligoclase porphyroblasts are elongate parallel to the foliation. Grain size commonly varies widely in adjacent granulose layers.

Schistose layers average 1 to 5 mm thick and consist of biotite, in flakes from 0.5 to 5 mm across, surrounded by fine grained quartz, microcline, and oligoclase. Some schistose layers are nearly all biotite, but most contain 20 to 50 percent biotite. Schistose layers are fairly continuous in outcrops not interrupted by dikes of pegmatite, aplite, or foliated microcline granite. Biotite flakes in a few outcrops are elongate and are oriented so that they form a lineation in the plane of foliation.

Hornblende occurs with biotite in about 5 percent of the outcrops examined. Banding is generally not as well-developed in the biotite-hornblende gneisses as it is in those containing only biotite. The white oligoclase gneiss also contains isolated pods of lime-silicate rock containing hornblende, diopside, garnet, and 56

plagioclase. These probably represent calcareous concretions

that were metamorphosed during recrystallization of the gneiss.

Both the pink and white gneisses commonly contain hypidioblastic

magnetite in grains ranging from 2 to 15 mm across. Magnetite

tends to occur in the gneisses in which the foliation is poorly developed.

Twenty-six thin sections of Massabesic Gneiss were ex

amined. Table 4 gives approximate modes for 9 of them based on counts of 500 to 1000 points. The following describes some of the typical features seen.

The essential minerals in the gneiss are plagioclase, quartz, and biotite. Microcline is an important constituent in most of this rock but is absent in some thin sections. Horn blende is also locally abundant. Common accessory minerals are listed in Table 4.

Plagioclase in most of the gneiss is oligoclase, An2,.; locally the plagioclase is as calcic as An^g, andesine. The more calcic plagioclase is most common in hornblende-rich gneiss. The plagioclase occurs as blocky or elongate hypidioblastic or xeno blastic grains averaging 2 to 3 mm across and ranging from 0.1 to

10 mm across. Well-developed albite twinning is common, and a i few samples show complex combinations of several types of twin ning (Figure 9). Alteration ranges from very minor to nearly complete sericitization, and is commonly confined to one of the sets of twin lamellae. Carbonates and epidote are locally 57

- j 1mm

Figure 9. Complex twinning in plagioclase from the Massabesic Gneiss (sample 7-25-3a). Grains show combinations of albitc, pericline, and Carlsbad twinning. 58 associated with the sericite. Albitic reaction rims, generally unaltered, and myrmekite are common at boundaries between oligoclase and microcline. Most of the larger oligoclase grains contain small rounded quartz inclusions that are commonly in optical continuity with one another. Nearly rectangular to irregular-shaped inclusions of untwinned potash feldspar about

0.05 mm in diameter are also common. In some of the plagioclase grains, these inclusions all have the same crystallographic i orientation; in others, they do not.

Quartz occurs as very irregular xenoblastic grains aver aging 2 to 4 mm across. Quartz grains in the granulose layers are as much as 20 mm long; those in the schistose layers are generally smaller. The boundaries between adjacent quartz grains interlock irregularly. Where quartz exists side-by-side with feldspars, the feldspar is embayed (Figure 10); some biotite is also embayed by quartz, but, in most cases, biotite flakes cut across quartz. Quartz grains all show varying degrees of undulatory extinction. Lines of tiny liquid inclusions in the quartz in some casgs extend entirely across the slide, and they generally cut across the foliation shown by nearby micas. The lines appear to be traces of planes, and where two sets of lines are present, they commonly intersect at nearly right angles. Some individual inclusions are elongate parallel to the line of inclusions. Fine anastomosing lamellae consisting of tiny inclusions were also observed in a few quartz grains. Some 59

j 1mm

Figure 10. White oligoclase gneiss of the Massabesic Gneiss (sample 7-14-lf) showing relations between grains of quartz (q)» plagioclase (p), biotite (b), and microcline (m). 1

60 of the quartz shows a dimensional orientation of irregular elongate grains parallel to the foliation; these grains are cbmmonly fractured) and a few have crushed zones around their margins.

Biotite is concentrated in schistose layers in the gneiss . and occurs as 0.5 to 2 mm flakes, most of which are parallel to the foliation. Although most of the biotite is fresh, most thin sections show a few flakes partly altered to chlorite in which there are very fine nematoblastic crystals of rutile.

Small crystals of allanite or zircon, surrounded by pleochroic halos, are commonly included in the biotite. Apatite and sphene are most abundant in the biotite-rich bands, the sphene usually being partly altered to leucoxene.

Microcline occurs as xenoblastic grains averaging 2 to 3 mm and ranging from 0.5 to 6 mm across. It generally shows well- developed "plaid" twinning and, irf suitably oriented grains, fine string perthite. The microcline occurs both as distinct nearly equidimensional grains and as interstitial irregular aggregates. It is embayed by quartz, and boundaries with plagioclase are commonly myrmekitic. Inclusions of quartz and plagioclase are common, the plagioclase inclusions having fresh albitic reaction rims and sericitized center. Quartz inclusions are rounded or, in a few cases, idioblastic. Most microcline is slightly altered to sericite.

Hornblende occurs locally in both the white gneiss and in gneisses of intermediate composition. It is xenoblastic or 61 hypidioblastic and contains quartz and plagioclase poikiloblastically. Pleochroic halos occur around included grains of allanite. In sample no. 7-25-3a, the amphibole is hastingsite; in the others it is common hornblende. In all thin sections, biotite appears to partially replace the amphibole.

Partial alternation to chlorite is also common. Foliated Microcline Granite

Several types of rocks are closely associated with the

Massabesic Gneiss and are probably related to it in origin. One

of these is a fine- to medium-grained pinkish-gray or pink

faintly to moderately well-foliated microcline granite. Good

exposures occur on the south side of Route lOl, 750 feet west of

its junction with Langford Road, in Candia; on Route 101 at the

Chester Turnpike overpass, in Hooksett; and on Route 101, 0.9 mile southwest of the Chester Turnpike overpass, also in

Hooksett.

The composition of this granite is very similar to that of

the intermediate Massabesic Gneiss. The essential minerals are

plagioclase (An15 to An^), microcline, quartz, and biotite

(Table 5). Plagioclase occurs as anhedral or subhedral grains averaging 1.0 mm long. These are slightly to moderately

sericitized, the amount of alteration being greatest where they

are enclosed by microcline. Myrmekite and albitic reaction rims commonly occur along the borders between plagioclase and micro-

clinc, and plagioclase commonly includes small uniformly oriented

grains of quartz or potash feldspar. Microcline grains are

anhedral or subhedral, average 1 mm across, and, in places, appear

to fill spaces between plagioclase and quartz. The microcline is

generally unaltered and contains fine string perthite and,

62 Table 5

Approximate Modes of the Foliated Microcline Granite

Sample No. 6-3Q-3a 7-8-le 7-14-le

Microcline 26 36 44

Plagioclase^ 33 33 29

Quartz 36 24 20

Biotite 4 7 5

Chlorite tr. tr. 1

Muscovite 1 tr. 1

Zircon tr.

Apatite tr. tr.

Magnetite tr. tr. tr.

^Plagioclase An An An 30 25 15 Composition

Locations of samples:

6-30-3a 0.2 mile east of Spruce Lake, on Chester-Auburn border.

7-8-le South side of Route 101, 750 feet west of its junction with Langford Road, Candia.

7-14-le On Route 101 at Chester Turnpike overpass, Candia, 64 occasionally, fine braid perthite. Quartz occurs poikilitically in the microcline, and very small fresh inclusions of plagioclase show both preferred and random orientation. Quartz, in anhedral grains averaging 0.5 to 1.0 mm across, commonly embays the feldspars. Quartz shows undulatory extinction in all thin sections. Biotite flakes averaging 0.5 to 1.0 mm long show moderately well-developed preferred orientation. Most of the biotite is partly altered to chlorite and rutile. Muscovite and apatite commonly occur in small amounts associated with biotite, and accessory minerals include sphene, magnetite, epidote, and, in one sample, garnet.

The foliated microcline granite occurs only in association with Massabesic Gneiss and microcline pegmatite. In most places the granite forms irregular masses and dikes surrounded by gneiss and microcline pegmatite. At a few localities, in clusions of microcline granite are elongate parallel to the foliation in the surrounding gneiss. However, the foliation in the inclusions is neither parallel to that of the gneiss nor to the length of the inclusions (Figure 11). The granite also occurs as irregular dikes, usually a few centimeters thick, cut ting the gneiss. Microcline pegmatite, described below, commonly cuts the granite (Figure 12) but the granite locally cuts the pegmatite. 65

Figure 11* Sketch showing relations between Massabesic Gneiss and foliated microcline granite* Foliation in granite shown by short dashes; that inQgneiss shown by longer lines. Location: O.S mile N.80 E. of Four Corners, Candia. 1 66

Figure 12. Microcline pegmatite intruding foliated microcline granite in the Massabesic Gneiss, Chester Turnpike overpass, Route 101, Candia. Origin of the Massabesic Gneiss

Previous Work

Several workers have commented on the origin of rocks that are now included in the Massabesic Gneiss. Freedman (1950a, p.

477) noted that the southeastern contact of quartz monzonite in

the Mount Pawtuckaway quadrangle clearly cuts across meta sediments now included in the upper member of the Berwick Form ation. He believed that forceful injection was not a plausible mechanism for emplacement of the quartz monzonite and implied

that schlieren of coarse-grained biotite in the quartz monzonite and microcline granite may represent layers of country rock engulfed in magma and recrystallized. These and other features led Freedman (1950a, p. 479) to conclude that the plutonic rocks of the Mount Pawtuckaway quadrangle were emplaced by piecemeal stoping with local permissive intrusion. Freedman's quartz monzonite and microcline granite are continuous with rocks here included in the Massabesic Gneiss.

Billings states that the origin of the magmas of the New

Hampshire series is problematical. "Although they may be differentiates from basalt, they may equally well be melted up older rocks or granitized sediments that moved up from greater depths" (Billings, 1956, p. 147). Billings does not specifically deal with the origin of the Massabesic Gneiss*

67 68

Sriramadas (1966, p* 67) believed that the pink microcline gneiss of the Massabesic Gneiss in the Manchester quadrangle formed by permissive intrusion of microcline granite into schists of the upper member of the Berwick Formation. He notes that the contact between the microcline granite and Berwick

Formation on Rattlesnake Hill, Auburn, is a transition zone composed of Massabesic Gneiss. The pink microcline gneiss would therefore be a migmatite as defined by Sederholm (1913).

Sriramadas hypothesized that the white oligoclase gneiss of the

Massabesic Gneiss formed by partial fusion ("palingenesis" of

Sederholm, 1923) of schists of the upper member of the Berwick

Formation. The coarse-grained granitic bands represent fused material, and biotite-rich bands are recrystallized country rock.

He notes that "A mode of the white oligoclase gneiss, including both the light colored and dark colored bands, is similar to the average mode of the schists of the upper member of the

Berwick Formation" (Sriramadas, 1966, p. 67). Sriramadas states that the low-grade metamorphism in the center of the Manchester quadrangle is not directly connected with plutonism but that the higher-grade rocks to the northwest are invaded by numerous plutonic lenses and bodies, "suggesting that plutonism might have caused this high-grade metamorphism" (Sriramadas, 1966, p. 71).

Thompson and Norton also note a striking correlation between the distribution of plutonic rocks of the New Hampshire Plutonic 69

Series and inetamosphic grade* They state that a genetic relationship is highly likely but that "it is by no means clear which is the primary and which the secondary feature"

(Thompson and Norton, 1968, p. 325). They note that:

(1) The rock types are on the whole, highly siliceous and also characterized by an excess of alumina over the sum of the alkalis and lime. This is commonly expressed mineralog- ically by the presence of muscovite or garnet* These are compositional characteristics that would be expected if the magmas were formed by partial fusion of eugeosynclinal sediments* (2) Migmatites and abundant pegmatites are characteristic of the sillimanite and sillimanite-K feldspar zones. Recent studies of phase equilibria pertinent to reactions at this metamorphic grade suggest temperatures at or near thfe melting intervals, for many common rocks, particularly if the activity of H^O is high. (3) The near absence of mafic rocks that can be assigned un ambiguously to the New Hampshire Plutonic Series is difficult to explain if the magmas originated in the mantle or in the deepest part of the crust. (Thompson and Norton, 1968, p. 325).

These observations and the fact that contact aureoles are obscure or absent around New Hampshire series plutons in south western New England suggest that rocks of the series might have resulted from anatexis of common eugeosynclinal sediments. The magma thus produced by metamorphic processes might in some cases escape from the zone where it originated and intrude overlying rocks, forming sharp contact aureoles such as those associated with plutons of the New Hampshire series in northeastern Vermont and northern New Hampshire.

The granites of the New Hampshire series generally have compositions close to that of the ternary eutectic in the Q-Ab-Or 70

system. Birch, Roy, and Decker note that, in central New

Hampshire, the total thickness of rocks deposited prior to in

trusion of the New Hampshire Plutonic Series was on the order of

25 km (Birch, et al.. 1968, p. 439). This includes about 10 km

of Ordovician and Silurian sediments and some 15 km of Devonian

rocks now represented mainly by the Littleton Formation. Using

various assumptions and measurements of heat flow from 22 places

in New York and New England, Birch, et al.. computed temperatures

that could be expected in sediments in that area during de

position. They found that, after 50 million years, a mass of

sediment 12 km thick could theoretically reach a temperature

near 650°C (Birch, et al., 1968, p. 441) (Figure 13). This is

' probably close to the temperature at which average eugeosynclinal

sediments would begin to melt (Winkler, 1967, p. 207-208).

Similarly, the base of a mass of sediment 15 km thick would reach

such a temperature in about 45 million years. Birch, et al.,

conclude that, because orogenic activity accompanied by uplift

and erosion immediately followed deposition of the Littleton Form

ation, that formation, despite its great thickness, was probably

too cool for general fusion at the time of New Hampshire series

plutonism. However, the volcanics and older sediments under

lying the Littleton Formation "may have been adequate as the

source of magma" (Birch, et al., 1968, p. 443). These under

lying formations include, among others, the Berwick Formation

in southeastern New Hampshire. Thus, partial melting of 71 aoo

soo SEDIMENT BASEMENT

700

000

500

400

300

200

too

20 10 0 10 20 30 40 50 60 70 km 00

Figure 13. Temperatures in basement during sedimentation. Sedi mentary thickness indicated by horizontal scale, increasing toward the right. Initial temperature corresponds to zero depth of sediment; curves for 30 m.y. and 50 m.y. after begin ning of deposition at rate of 0.3 km/m.y. Parts of curves above 650° C are meaningful only in the absence of melting and movement of magma. From Birch, Roy, and Decker, 1968, p. 441. 72 sediments might have begun producing New Hampshire series magma during the deposition of the Littleton Formation.

Mesoscopic Features

A variety of large-scale features must be considered be fore attempting to explain the origin of the Massabesic Gneiss.

Those features visible on outcrop include the following:

(1) Small-scale compositional banding, resembling bedding,

between the white oligoclase gneiss and pink micro-

cline-oligoclase gneiss.

(2) Small-scale structures resembling those described

by Mehnert (1968) in migmatite complexes. These in

clude schollen or raft structure, folded structure,

schlieren structure, and ptygmatic structure.

(3) Complex relations between the Massabesic Gneiss and

associated pegmatites and foliated microcline granite.

(4) Broad compositional variations, including the presence

or absence of hornblende and (or) magnetite.

(5) Relations between structural features in the Massabesic

Gneiss and those of the surrounding rocks.

(6) Contact relations.

Small-Scale Compositional Banding

An attempt was made to map the various lithologies of the

Massabesic Gneiss separately. However, this proved to be im possible because of small-scale interlayering between the main rock types. As was mentioned earlier, the pink and white 73 gneisses recognized by Sriramadas in the Manchester quadrangle are not well defined in the Suncook quadrangle. In fact, the two types are apparently end members of a gradational series, and the gradation in composition is well shown on outcrops at various places (for example, on the south side of Route 101,

750 feet west of its intersection with Langford Road, Candia; and on the north side of Route 101, 1300 feet northeast of its intersection with Smith Road, Candia). Where white and pink geniss are interlayered, the foliation shown by biotite is parallel the the compositional banding. At such places, the

Massabesic Gneiss has the appearance of a coarse paragneiss.

In places, gneissic bands rich in hornblende and (or) magnetite also parallel the foliation and appear to be related to com positional variations in the rocks from which the gneiss was derived (see Hyndman, 1972, p. 295).

Schollen Structure

Although many outcrops of the Massabesic Gneiss show features like those described above, others exhibit more com plex relationships between the various rock types. For example, at several outcrops pink gneiss that is otherwise typical contains 0.5 to 3.0 meter oval or irregular masses of white gneiss whose compositional banding and foliation are contorted and rotated relative to banding and foliation in the pink gneiss.

Some of these masses of white gneiss have indistinct borders and appear to have been partly dissolved in the pink gneiss; others 74 have sharply defined borders. Such features are shown at a large outcrop 4250 feet N. 77° E. of Four Corners, Candia; and on the south end of Mount Miner, Auburn (Figure 14).

These features resemble schollen or raft structure, described by Mehncrt (1968, p. 15) as typical of migmatites. If they are schollen structures, this would mean that the white gneiss is analogous to Mehnert's paleosome, the "parent rock" of a migmatite; and the pink gneiss would correspond with a neosome, the newly formed part of a migmatite (Mehnert, 1968, p. 356). Schollen structure is formed in much the same way as agmatic (breccia) structure and diktyonitic (net-like) structure, but represents a more advanced stage of migmatization. Here, the paleosome is altered and deformed so that the pieces no longer fit together.

Folded Structure

Schollen-liko structure commonly accompanies other structures typical of migmatite massifs. Folded structures are especially common in the white gneiss (Figure 15). These are mostly shear folds showing the characteristic thinning of limbs and thickening of crests. Such structures commonly exist in the fragments of white paleosome included in pink neosome in schollen structures. The pink gneiss in such cases is usually faintly banded and shows little evidence of complex folding.

Thus, the folds in the white gneiss are apparently relatively old features produced before the formation of the pink gneiss. 75

pink giniti

Figure 14. Schollen structure, in which white oligoclase gneiss occurs as inclusions in intermediate or pink Massabesic Gneiss. Location: National Guard firing range, Mt. Miner, Auburn. 76

Figure 15. Folded structure in white oligoclase Massabesic Gneiss, Chester Turnpike overpass on Route 101, Candia. 77

Schlieren Structure

In many places, both the pink and the white gneiss contain

wispy discontinuous biotite-rich layers rather than a continuous

banding or foliation. These wispy layers are commonly a few millimeters wide and up to a meter in length. They show gentle

undulations rather than tight folds and contortions, and they

taper at their ends except where cut by younger pegmatites. At

a few localities, wispy biotite-rich bands wrap around more or

less spindle-shaped masses of other rocks, including pegmatite,

white gneiss, and lime-silicate rock, and they may also surround

larger irregular masses of fine-grained pink foliated microcline

granite. Where such features occur in the white gneiss, they

are locally accompanied by large (1 to 2 cm) rectangular plagioclase crystals showing a rough alignment parallel to the biotite-rich streaks. Most of the features described are well

shown on the large outcrop 4,250 feet N. 77° E. of Four Corners,

Candia, and at other exposures of Massabesic Gneiss in the central part of its outcrop area. The wispy biotite-rich bands and aligned plagioclase crystals are interpreted as a flow fabric analogous to schlieren structure. Mehnert (1968, p. 39) describes similar structures in migmatites and interprets them

as indicating a high degree of mechanical mobility at the time of their formation. 78

Ptygmatic Structure

At a few localities in the Massabesic Gneiss large masses of fine-grained quartz-feldspar-biotite schist contain thin ptygmatic folds of fine-grained faintly foliated light gray granitoid material (Figure 16). The cleavage in the schist generally parallels the axial planes of folds in the ••granite."

However, immediately adjacent to the granitoid layers, the biotite flakes are aligned in such a way that the foliation wraps around the folds. The thickness of the granitoid layers varies greatly along their lengths, and there is no obvious relationship between thickening or thinning and location on the axes or on the limbs of the folds. A faint foliation is shown by biotite flakes and elongate aggregates of gray quartz grains in the granitoid layers. This foliation consistently follows the contortions in the folds and could represent either a flow structure or an older foliation parallel to original compositional banding in the rock. There appears to be a relationship between widths of the veins and wavelengths of the folds even where the folds occur in adjacent veins

(Figure 17), Thin veins show shorter wavelengths than thicker ones. The relationship between vein width and fold wavelength was demonstrated by Ramberg (1959, p. 148-149), who experimented with the production of ptygmatic folds under controlled con ditions. Ramberg found that ptygmatic structures and various pinch-and-swell structures can be produced where a layer of 79

Quarts-Paldapar-Biotlta Schiat

Granitoid Uy i r

(XI)

Figure 16. Ptygmatic structure in the Massabesic Gneiss. View is perpendicular to fold axes and to foliation. Dashed lines show orientation of biotite flakes in the schist and of elongate quartz aggregates in the granitoid layer. Sample 97-l-laf from Derryfield Park, east side of Manchester, N. H. 80

Figure 17* Sketch showing variation of wavelength of ptygmatic folds in the Massabesic Gneiss with thickness of folded layer. The folded layers are granite (stippled), pegmatite (checked), and quartz (dashed); the enclosing rock is fine-grained gray biotite schist. Location: in quarry on northwest side of 520-foot hill, east edge of Manchester, N. H. 81 higher competence is surrounded by slightly less competent material and the whole is subjected to pure-shear deformation.

The geometry of the features produced depends on the initial orientation of the more competent layer relative to the stress system (Ramberg, 1959, p. 110). Thus, boundinage or pinch-and- swell structures may accompany ptygmatic folding where the initially flat layer was oriented in the plane including the compressive and extensive strains. A layer inclined to the maximum and minimum stress directions and parallel to the intermediate stress may initially develop ptygmatic folds and then rotate in such a way that the layer is stretched and broken into several folded segments. Features of this type are shown on outcrops at Derryfield Park, on the east edge of

Manchester (Figure 18).

Some other features are typically associated with the ptygmatic structure of the Massabesic Gneiss. Foliation in the schistose host rock generally parallels the axial planes of the folds. One would expect this to be true if the folds are a result of buckling caused by compression in a direction per pendicular to the axial planes. The foliation of the host rock results from crystallization of biotite flakes per pendicular to the maximum compressive stress. The only area in which schistosity does not parallel the axial planes of the folds is immediately adjacent to the granitic layers, where biotite flakes parallel the contortions of the layers (Figure

16). This may result from drag effects on "skin friction" 82

Figure 18. Ptygmatic structure in Massabesic Gneiss, Derryfield Park, Manchester, New Hampshire. 83

adjacent to the granitic layer which buckled while it and the

adjacent host rock were both in a relatively plastic condition.

The lack of joints, cracks, or cataclasis indicates that the

folded granitic material was itself plastic when it buckled or

that it recrystallized afterwards. However, a complete recrys

tallization would probably have destroyed the faint foliation or flow structure still visible in some of the ptygmas.

Although the mechanics of the production of ptygmatic

structure are relatively simple, the problem of origin of the folded granitoid layers is as yet not solved* The various

theories have been summarized by Mehnert (1968, p. 30-35) and can be divided into two groups: those that assume that the

folding and production of the vein occurred simultaneously, and those that assume that the vein formed first, followed after some interval by the folding. The second explanation seems most

reasonable for the ptygmatic structures of Massabesic Gneiss.

The granitic material itself was most likely derived from the

surrounding rocks by partial melting near the peak of their

thermal metamorphism. Directed stresses responsible for buck

ling of the veins and development of foliation in the surrounding

schist came later.

The presence of ptygmatic structures, though characteristic of migmatites, does not conclusively demonstrate a particular mode of origin for the rocks containing them. Ptygmatic

structures generally consist of aplo-granitic or nearly 84

pegmatitic veins in more or less migmatic gneisses. However,

ptygmatic structures can also occur in a variety of other

metamorphic and even sedimentary rock types (Mehnert, 1968,

p. 27).

Relations of Massabesic Gneiss with Pegmatite and Foliated Granite

The pink and white gneiss of the Massabesic in the Sun-

cook quadrangle are intimately associated with pink micro

cline pegmatite and pinkish-gray foliated microcline granite.

Both of these rocks appear to be genetically related to the

Massabesic Gneiss, as indicated by the complex relations seen

on many of the outcrops.

The pink microcline pegmatite resembles pink Massabesic

Gneiss in composition. Both contain pink microcline and white

oligoclase, and they commonly contain both muscovite and biotite.

Some outcrops exhibit graphic intergrowths of quartz and micro

cline. Where oligoclase is especially abundant, the pegmatite

commonly contains tourmaline and occasionally contains apatite

and garnet. Pyrite and magnetite also occur at a few localities,

but the two minerals were not observed together. Sillimanite is

a rare constituent of the pegmatite, occurring with muscovite,

mainly along small shear zones.

The pinkish-gray foliated microcline granite also appears

to be related to the pink Massabesic Gneiss. It consists of 85 microcline, oligoclase - An^), quartz, biotite, and muscovite. Evenly distributed biotite flakes make up 5 to

10 percent of the rock and are generally aligned, forming a regular foliation or flow structure. Poikilitic or subhedral muscovite has an irregular orientation. Some of the foliated granite contains small garnet crystals.

Contact relations between the Massabesic Gneiss, micro cline pegmatite, and foliated microcline granite are very com plex. In places, the pegmatite forms concordant masses, lenses, or layers within the pink or white gneiss. These bodies are generally no more than a few feet long, and the foliation in the surrounding gneiss wraps around them. Locally, as on the northeast side of Massabesic Lake, the pink gneiss grades laterally into pink pegmatite. At other localities, masses of pink pegmatite engulf twisted inclusions of white gneiss (Figure

19) or cut across pink and white gneiss along what appear to be shear zones (Figure 20). Such relations are shown on the hilltop

1500 feet south of the intersection of Auburn Road and the Boston and Maine Railroad, in Auburn. The foliated microcline granite occurs in two types of bodies; most commonly it forms dikes or small irregular intrusive bodies that cut the gneiss, and in some cases include xenoliths of the gneiss. These bodies in trude pegmatite in some places but are intruded by pegmatite at others. A few outcrops exhibit both relations, and pegmatite may even contain inclusions of foliated granite. The second 86

Figure 19. Pink microcline pegmatite with twisted inclusions of white oligoclase Massabesic Gneiss, Chester Turnpike overpass on Route 101, Candia. 10c*

Figure 20. Pegmatite dikes occupying shear zones in the Massabesic Gneiss. Location: north side of Route 101, 0.9 mile southwest of Chester Turnpike, Auburn. 88 node of occurrence is as inclusions in the Massabesic Gneiss; these are commonly but not invariably elongate parallel to the foliation in the gneiss. Biotite may be concentrated in the gneiss around their edges, or they may themselves contain biotite flakes oriented parallel with their contacts with surrounding rocks. Foliation in the inclusions most commonly cuts across that of the gneiss (Figure 11). However, even this relationship is not shown in all cases; the foliation occasion ally parallels that of the gneiss. Foliated microcline granite occurs at most of the large outcrops of Massabesic Gneiss. Good exposures occur near the Chester Turnpike overpass, on Rout 101,

Candia, and at the large outcrop 4250 feet N. 77°E. of Four

Corners, Candia.

The complex relations between foliated granite, pegmatite, and gneiss can be explained in a number of ways. First, there may be several generations of pegmatite and foliated granite in the Fitchburg pluton. One could assume several separate periods of intrusion and metamorphism as outlined below:

(1) Metamorphism of Massabesic Gneiss parent rock.

(2) Intrusion of fine-grained granite and development of

foliation in it.

(3) Mobilization of Massabesic Gneiss with development of

pegmatoid mobilizates, some of which form concordant

bodies deformed during their intrusion. Pegmatoid

mobilizates and pink gneiss formed at the same time,

probably by partial melting of older material. 89

i (4) Late shearing of previously formed rocks with

continued intrusion of pegmatoid mobilizates across

the gneissic foliation.

(5) Late intrusion of fine-grained granite.

Considering the complex relations between the different rock

types, all of the events probably occurred within a relatively short period of time. They probably represent responses of a mass of sediments to a single episode of high-grade meta morphism. The intrusions of granite and pegmatite were most likely syntectonic and very early post-tectonic in age. Con sidering the small size of the intrusive bodies and their com positional similarities to the pink Massabesic Gneiss, the magmas that produced them were probably produced by partial melting of the gneiss.

Broad Compositional Variations

Several compositional variants are conspicuous at some outcrops of Massabesic Gneiss, where the common banded pink and white gneisses are interbedded with layers rich in amphibole or magnetite and with layers and small lenses of lime-silicate rock.

Magnetite is a minor accessory mineral in most outcrops of

Massabesic Gneiss and in some of the pink pegmatites associated with it. However, in places magnetite is a conspicuous com-

| ponent of the rock, making up as much as 5 percent of its

j volume. It is especially abundant in the southeastern part of

j the outcrop belt, where it occurs evenly distributed and with 90 equal frequency in both the pink and the white gneiss. Typical magnetite-rich pink gneiss occurs on the south side of Route 101,

750 feet west of its junction with Langford Road, Candia (Figure

2 1 ).

The individual magnetite grains are generally hypidioblastic and range up to 4 cm across, the commonest size being about 0*5 cm. In a few specimens, the grains are somewhat rounded and are poikiloblastic with feldspars and quartz. The magnetite-rich gneisses generally contain biotite in normal abundance, and the magnetite occurs in both biotite-rich and biotite-poor bands. The magnetite cuts across biotite flakes but does not include or displace them.

The high magnetite content of some parts of the Massabesic

Gneiss may result from processes similar to those that occur in magnetite-bearing igneous rocks, where, as the Q also in- 2 creases during fractional crystallization of a magma, the P_ 2 also increases. A high Pn results in oxidation of iron and 2 continual crystallization of magnetite (Hyndman, 1972, p. 81).

The presence of magnetite in the Massabesic Gneiss may reflect an initially high water content for the sediments from which the gneiss was derived. The magnetite is definitely not dctrital, as indicated by its hypidioblastic and poikiloblastic habits.

Amphibole occurs mainly in the white facies of the

Massabesic Gneiss. It forms equidimensional to slightly xenoblastic elongate to hypidioblastic grains averaging 0.5 cm 91

■ ■ 1 1

Figure 21. Massabesic Gneiss of intermediate composition con taining hypidioblastic magnetite grains, north side of Route 101, 1300 feet northeast of its intersection with Smith Road, Candia. 92 long and ranging from 0.1 to 1.0 cm long, the long axes of the grains are parallel to the foliation of the gneiss. The amphibole is common hornblende in most samples, but sample

7-25-3a, collected on the south side of Wellington Road, 1.6 miles west-southwest of its intersection with Bypass Route 28,

Manchester, contains hastingsite. The rock there is a white gneiss containing sodio andesine and large scattered magnetite crystals (Table 4).

The amphibole-rich gneiss, like that rich in magnetite, forms distinct layers in the Massabesic Gneiss and probably reflects original compositional layering in the sediments.

Amphibole generally constitutes 3 to 10 percent of the rock, and layers rich in hornblende are generally a few centimeters thick.

The plagioclase is generally more calcic than that in normal

Massabesic Gneiss, and so the most likely parent rock is an impure slightly calcareous siltstone. Although the original rock might have been volcanic, the relatively thin layers, small volume, and discontinuous distribution of hornblende-rich gneiss suggest a sedimentary origin.

Another compositional variant common in the Massabesic

Gneiss is lime-silicate gneiss of three basic types. One type is fine grained, light pinkish- or greenish-gray, and occurs as discontinuous to fairly continuous layers that show a faint gnuissic foliation, and have a spotted appearance on the outcrop.

A second type occurs mainly as small rounded masses or lenses of 93 fine-to medium-grained rock containing coarse prophyroblasts of hornblende as much as 2.5 cm long, and commonly containing diopside. Compositional layering is distinct in this rock. The third type occurs as layers of amphibolite or actinolite gneiss.

Actinolite grains are flattened in the plane of foliation and, in a few cases show an obvious lineation.

Table 6 gives approximate modes for four samples of lime- silicate rock from the Massabesic Gneiss. The specimens ex amined are of the first and third types described above. The modes for these samples do not differ much from those for lime- silicate rocks in the upper member of the Berwick Formation as published by Freedman (1950a, p. 461) and Sriramadas (1966, p.

19), and in the Massabesic Gneiss (Sriramadas, 1966, p. 37).

The quartz content of rocks in this study is generally higher than that for the similar rocks of the Mount Pawtuckaway and

Manchester quadrangles. The plagioclase content is comparable with that reported by Sriramadas for amphibolite in the

Massabesic Gneiss and for lime-silicate gneiss in the upper member of the Berwick Formation. It is also similar to that reported by Freedman for actinolite granulite in the Littleton

Formation.

The difference in quartz content might be explained in either of two ways. First, it could reflect a difference in the

The lower part of the Littleton Formation, as mapped by Freedman in the Mount pawtuckaway quadrangle is the same as the upper member of the Berwick Formation, as mapped by Sriramadas in the Manchester quadrangle. 94

Table 6. Approximate Modes for Lime-Silicate Rocks

of the Massabesic Gneiss

Sample No.: 7-25-Sb 8—1—la 96-30-2a 96-30-3a

Plagioclasea 29 19 27 29

Quartz 58 64 36 56

Amphibole 10 13 35 12

Sphene 2 1 1 1

Apatite tr. tr. - 1

Garnet tr. tr. - -

Biotite - - tr. tr.

Chlorite - - - tr.

Epidote - tr. - -

Clinozoisite tr. - --

Carbonate - - tr. -

Zircon - tr. - -

Magnetite - 2 - -

Pyrite tr. - - - a Plagioclase Composition: ^ 7 0 ^ 6 5 ^ 5 0 to6S •

Description and location of samples: 7-25-5b Spotted gneiss, east side of Route 28, 115 feet north of its intersection with Route 101B, Hooksett. 8-1-la Lime-silicate granulite, north side of dirt road, 3300 feet S, 52° W. of Hall Mountain, Hooksett. 96-30-2a Amphibolite, northwest flank of north peak of Mount Miner, Auburn. 96-30-3a Actinolite gneiss, north peak of Mount Miner, Auburn. 95

composition of the sediments from which the various rocks were

derived. The samples used to make Table 6 might have originated

as impure calcareous siltstones or sandstone with a high quartz

content. However, it is also possible that quartz was added to

the limc-silicate rdcks during metamorphism and mobilization of

the Massabesic Gneiss. If the quartz were introduced late, one

would expect to find evidence that quartz has replaced other

minerals. Examination of the thin sections suggests just the

opposite situation. In samples 96-30-2a and 96-30-3a, quartz

occurs as oriented inclusions in poikiloblastic plagioclase.

The inclusions occupy as much as 30 percent of the volume of the

plagioclase grains. In all samples, quartz occurs poikiloblastically in amphibole. If anything, it appears that quartz was replaced and that there has been little or not late addition

of silica in these samples. Therefore, the present composition probably either reflects that of the parent rocks or resulted from the addition of silica to more calcareous rocks before maximum metamorphic grade was reached.

The small pods and lenses of hornblende-diopside gneiss contain quartz and labradorite and are commonly surrounded by biotite. Although they most commonly occur singly, 2 or more lenses may be found aligned with each other and with the foliation of surrounding gneiss. In no case do they form continuous or even discontinuous layers. The amphibolite-gneiss pods are interpreted as highly metamorphosed calcareous concretions that, i because of their relatively refractory nature, recrystallized but 96

were not assimilated into the mobilized Massabesic Gneiss.

As described in earlier sections, schistosity in the

Massabesic Gneiss is shown by parallel arrangement of biotite

flakes, by flattened and elongate amphibole grains, and by

flattened aggregates of quartz and feldspars* In almost all places where compositional layering occurs in the gneiss, the compositional layering parallels schistosity. The only ex

ceptions are the rare ptygmatic folds. No outcrops of

Massabesic Gneiss show axial-plane cleavage, shear cleavage, or slip cleavage.

Though it is very irregular on a small scale, the

schistosity of the Massabesic Gneiss generally strikes north east and dips moderately to steeply to the northwest or south east. Strikes of east-west, northwest, and north-south occur on the plunging noses of folds (Plate 1). The structural geology of the Massabesic Gneiss and other formations in the

Suncook quadrangle is described elsewhere in this report.

Suffice it to say here that the schistosity in the gneiss de spite many small-scale irregularities, is generally parallel to

the bedding and schistosity in metasedimentary rocks to the northwest and southeast. Local exceptions occur where other evidence indicates extreme mobilization of the gneiss.

Parallelism of schistosity in the Massabesic Gneiss with

that of the surrounding rocks could, on its own merits, be

interpreted in three ways. First, the foliation might post-date 97 the intrusion of a shefet-like New Hampshire magma series pluton that is now Massabesic Gneiss. Second, the gneiss might be a syntectonic intrusive. Or, third, the gneiss might represent a highly metamorphosed sequence of sediments that began to melt before deformation ceased. Considering the variety of features described in previous sections of this report, the author prefers the third interpretation.

Contact Relations

The last large-scale characteristic to be taken into account is the nature of the contact between the Massabesic

Gneiss and the adjacent formations. If the gneiss is a pre tec tonic intrusive, some remnant of a contact aureole might be present. If the gneiss is a syntectonic intrusive, little or no contact metamorphism would be expected. If the gneiss resulted from regional metamorphism and partial melting of a thick sedimentary sequence, one might expect either sharp or gradational contacts and migmatization of the metasediments adjacent to the anatexite, depending on how far the partial melt moved.

Unfortunately, outcrops in the Suncook quadrangle are so widely scattered that contacts between the Massabesic Gneiss and the Berwick and Littleton Formations cannot be followed for any great distance. Most outcrops are so small that there is no way to determine whether the rocks in contact with

Massabesic Gneiss are in place or are large xenoliths. In most places where the gneiss is in contact with the meta sediments | the boundary is gradational and concordant on a large scale* No contact-metamorphic effects can be discerned*

The metasediments, particularly those of the Littleton Formation* contain abundant lenses and layers of pegmatite near the contact.

These pegmatites might result from metasomatism* from forceful intrusion of the surrounding schists by pegmatitic fluids coming from the Massabesic Gneiss, or from "sweating off" of such fluids during partial mobilization of the schists them selves* The fact that pegmatites near the contact in the

Littleton Formation occur mainly as small isolated lenses and thin layers suggests the third mode of origin. The pegmatites are massive and show little evidence of shearing, but they are concordant and hence arc probably syntectonic. Thus, the small pegmatite bodies in metasediments near the contact zone between the Massabesic Gneiss and the Littleton and Berwick formations might correspond to the pegmatoid loucosoraes described by

Mehnert (1968, p. 56-63) for typical migmatites. Of course, this does not really solve the problem of whether migmatization occurred as a result of heating up of sediments during the intrusion of a New Hampshire series pluton or whether both the gneiss and the migmatic pegmatites resulted from highest grade regional metamorphism. However, the writer prefers the latter interpretation because there is no large mass of definitely plutonic rock exposed in direct contact with the Massabesic 99

Gneiss. The relatively small bodies of Concord Granite in the Fitchburg pluton might indicate the presence of a larger pluton buried beneath the metasediments. However, it is questionable whether such a body could supply sufficient heat localized in a narrow linear belt to produce the migmatization of the Massabesic Gneiss.

Microscopic Features

The main purpose of the examination of thin sections of

Massabesic Gneiss was to obtain additional information regard ing the origin of the gneiss and associated foliated microcline granite and pegmatite. The field evidence reviewed above suggests a complex origin for these rocks; one that involves high-grade metamorphism and partial mobilization or migmatiz ation of a sequence of sediments. Whether mobilization can occur without partial fusion is a question that has caused much debate among petrologists. Some of those who accept granitization as a valid process minimize the importance of liquid magma and maximize that of mobile fluids traveling along grain boundaries and transforming one rock to another in the solid state. However, as Moorhouse (1959, p. 275) notes in his discussion on the origin of granite, "there are no petrographic criteria that can be said to point exclusively to the operation of solid diffusion, except for such relatively localized phenomena as the exsolution of alkali feldspars to form perthites." Thus, there is little point in considering 100 granitization by solid-state diffusion here. Its operation is purely theoretical and is not based on recognizable petrographic features. Most recent writers (e.g. Turner, 1968,

Mehnert, 1968, Raguin, 1965) prefer a broader view in which granitization of pre-existing rocks takes place by a variety of processes, including metaraorphism, metasomatism, and partial melting. The processes involved in granitization represent a continuum and vary with place and time in a given granite massif. According to this view, most large granitic massifs should exhibit features indicating the mixed origin of the rock and the operation of a number of processes active both simul taneously and consecutively over an extended period of time.

Very few small-scale features are useful in distinguishing granitic rocks of magroatic origin from those of metamorphic or metasomatic origin. The typical feldspar of magmatic granites is microperthite (Williams, et al,, 1954, p. 143). Broad, irregularly formed perthites indicate a relatively low tem perature of formation (Mehnert, 1969, p. 102). However, moderately coarse but regular string and vein perthites like those of the Massabesic Gneiss can form at various temperatures, and their formation may even be promoted by shearing stress

(Chayes, 1952),

i Microcline of the Massabesic Gneiss commonly contains in clusions of quartz and plagioclase. Quartz inclusions are either round and "drop-like" or are idiomorphic dihexahedra. 101

The latter are generally regarded as indicating formation at temperatures above 573°C (Mehnert, 1969, p. 192). The quartz inclusions presumably pre-date the microcline. Plagioclase inclusions in microcline are squarish to round in shape. Some exhibit albitic rims and highly altered centers, and a few themselves include rounded quartz. These probably also pre date the microcline.

The relationships between microcline and its included quartz and plagioclase are most easily explained by crystallization from a magma. The occasional euhedral quartz inclusions would be difficult to explain if the rock was produced by purely metamorphic processes, and the plagioclase inclusions that them selves contain quartz inclusions are difficult to explain under any circumstances. They might represent grains derived from a pre-existing rock.

The plagioclase in Massabesic Gneiss commonly shows complex combinations of several types of twinning. In sample

7-5-3a (white oligoclase gneiss), several grains exhibit com binations of albite, Manebach, and Carlsbad twins* Similar combinations can be seen in a number of thin sections.

According to Williams, et al. (1954, p. 144), such combinations characterize plagioclase of magmatic origin. Another character istic of plagioclase in the Massabesic Gneiss is the presence of inclusions of quartz and potash feldspar. The quartz in clusions in a given grain are round and show no consistent orientation relative to one another. The potash feldspar in clusions arc round to squarish in shape and generally show a preferred orientation within the host grain. Quartz inclusions in plagioclase would be inconsistent with the normal sequence of crystallization for magmatic granites, but the "inclusions" may

i actually be secondary quartz formed in the same way as myrmekitc but on a coarser scale (see Moorhouse, 1959, p. 453). Myrmekite of the usual type is common in the Massabesic Gneiss. Although abundant myrmekite Suggests a replacement origin in granitic rocks (Moorhouse, 1959, p. 279), its presence does not prove either a metamorphiC or a magmatic origin for the gneiss. Quartz inclusions in sodic plagioclase might also result from replacement of an originally siliceous rock. The optical continuity of potash 103 feldspar inclusions in plagioclase suggests that plagioclase replaced K-feldspar. Abundant albitic reaction rims and occasional "chessboard1* albite suggest a replacement origin for the gneiss, but all of these features might also have been produced during the late stages of crystallization of magmatic material. Evidence of late-stage alteration of the gneiss is abundant, and its presence only serves to confuse the issue of how the gneiss originated.

The features described thus far suggest but do not prove that the Massabesic Gneiss originated at least in part from a liquid melt. Certain textural relations between minerals in the gneiss are also consistent with the idea of a partly magmatic origin. Plagioclase is the only mineral that commonly shows well- developed crystal faces, generally being hypidioblastic. Micro cline fills in spaces between plagioclase and quartz and only rarely exhibits any well-formed crystal faces. Quartz occurs in the interstices between the other minerals or embays the feldspars and biotite. Zircon occurs as idioblastic or round crystals that are invariabley included in biotite or hornblende.

Apatite is like zircon in both form and associations, although some apatite crystals are not directly associated with ferromagnesian minerals. Biotite cuts across and in some cases appears to replace hornblende. 104

Relations with the Q-Ab-Or System

Another approach to the problem of the origin of the gneiss involved plotting of modal compositions, as determined by thin- section analysis, on the ternary diagram Q-Ab-Or. This procedure follows that used by Chayes (1951, 1952) in studies of granites in the eastern United States. The position of the ternary minimum for the Q-Ab-Or system depends on a number of variables, the most important being water pressure. Tuttle and

Bowen (1958) and Luth, et al. (1964) determined the fractionation curves and position of the ternary minimum for water pressures ranging from 0,5 to 10 kb (Figure 22). In addition, few granites contain pure albite, and the position of the minimum migrates toward the Q-Or join with increasing anorthite content in the plagioclase. Winkler (1967, p. 203) showed that, for an Ab/An ratio of 3.8 and a water pressure of 2000 bars, the ternary minimum for the Q-Ab-Or system occupies the position shown in

Figure 23. Mehnert (1968, p. 85) considers a water pressure, of 2 to 4 kb to be representative for the formation of most migmatites. Therefore, Figure 23 should predict the behavior of a rock of granitic composition and with a plagioclase of composition AngQ undergoing partial melting at pressures com parable to those that occur where migmatites form. Figure 22 indicates that, under a water pressure of 4 kb, the ternary minimum in Figure 23 would shift slightly toward the Ab corner of the diagram. The plagioclase in the Massabesic Gneiss has 105

Figure 22. The system quartz-albite-orthoclase-water showing the effect of water-vapor pressure (0.5-lOkb) on the isobaric mini mum (mQ 5_3) and the "ternary" eutectic After Tuttle and Bowen, 1958, p. 75; Luth, et al., 1964, p. 765-766; Mehnert, 1968, p. 85. Figure 23, The plane Ab/An = 3 . 8 projected from the anorthite apex onto the Q-Ab-Or base at a water pressure of 2000 bars. The dashed line applies to a system containing no anorthite; the heavy curves apply to a system in which Ab/An = 3.8. Lines QX and QY are discussed in the text. After von Platen (1965), Winkler (1967), and Ehlers (1972). an average composition of A n » and so Figure 23 approximates

the phase relations that would be expected if the gneiss formed either by partial melting of a pre-existing rock or by crystal lization from a magirta that originated elsewhere. The composition of a granite or gneiss produced by strictly magmatic processes

should plot either in the thermal valley separating the quartz and feldspar fields or outside of it on the side nearest the

initial composition of the magma. If equilibrium conditions pre vailed, one would expect the rocks formed to have a bulk com position equal to that of the original magma,and the individual rock types should all lie on the path followed by this magma as it crystallized from a primary magma or from magma derived from complete or nearly complete melting ("diatexis" of Mehnert,

1968, p. 253) of pre-existing rocks. Magma generated by partial melting (called "metatexis" by Mehnert) of a pre-existing rock will follow one of several paths on the Q-Ab-Or diagram. If the composition of the pre-existing rock falls roughly between lines

QX and QY in Figure 23, the magma formed first will have a com position corresponding to that shown by the ternary minimum.

(The positions of QX and QY would be determined by the composition of the feldspar generated during crystallization of a liquid of composition m. The actual tie lines have not been determined for this particular diagram, and the lines are therefore approx imated. ) In this situation, partial melting of rocks of varied composition might generate magmas whose compositions were all 108

nearly the sane. However, where the rocks undergoing metatexis

fall outside the area bounded by QX and QY, the first liquids

formed are likely to fall on the subtraction curves separating

the quartz and feldspar fields. In this case, rocks of different

composition produce, through metatexis, magmas of different com

position, and the magmas themselves will crystallize to form different rock types under equilibrium conditions. Crystal

lizing magmas do not invariably reach the minimum point in the

Q-Ab-Or system, nor does melting invariably begin there (see discussion in Ehlers, 1972, p. 89).

The modal compositions of 8 samples of Massabesic Gneiss

containing more than 80 percent quartz and feldspar are shown

in Figure 24. Sample 1 is pink microcline gneiss, samples 2,

3, 5, and 7 are gneiss of intermediate composition, and samples

4, 6, and 8 are white oligoclasc gneiss. None of the samples plot close to the ternary minimum, and so they were not likely

to have been derived from partial melting of rocks in the region

between QX and QY in Figure 23. Samples 2, 5, and 7 fall close

enough to the subtraction curve separating quartz and feldspar

so that they might have been produced by partial melting of

rocks whose initial composition was to the left of QX. The

boundary curve would shift toward the Ab-Or join if the water

pressure was greater than 2000 bars. Sample 3 cannot be ex

plained as resulting from partial melting. Samples 1, 4, 6

and 8 are widely scattered and may represent diatexites or the Q

Figure 24. Modal composition of 8 samples of Massabesic Gneiss with more than 80 per cent quartz and feldspar, in weight percent. See text for discussion. 109 110

unmelted portion of rocks of metatectic origin. An alternative

interpretation is that all of the rocks analyzed are metamorphic

in origin. However, the petrography and megascopic features described above suggest that this is not the case and that the

Massabesic Gneiss contains some material that crystallized from a melt.

Summary and Conclusion

None of the individual bits of information discussed above proves unequivocally a particular origin for the Massabesic

Gneiss. However, numerous features suggest that at least a part of the gneiss crystallized from liquid magma. Other features suggest a metamorphic origin for parts of the gneiss.

Hocks showing this combination of magmatic and metamorphic features are generally called migmatites. Table 7, modified after Mehnert (1968, p. 255), lists the characteristic features of migmatites produced by both metatexis and diatexis. Features exhibited by the Massabesic Gneiss are preceded by an asterisk.

Features of both metatexites and diatexites are present, and in many cases both types of feature occur on the same outcrop. This is especially so in the central part of the outcrop belt. The writer believes that this central portion of the Massabesic

Gneiss is a migmatite formed under conditions intermediate between those that would produce minor partial melting and those that would produce complete melting of a sedimentary or meta morphic sequence. The border zone of the gneiss and the Berwick Table 7. Pctrographic Characteristics ol Typical Migmatites (After Mehnert, 1968, p. 235)

Metatexites Diatexites

Pegmatoid mctatects Mafic restitcs No distinction between netatects and resites

Typical penetration structures of Phlebites Mafic rims accompanying both •Schlieren around deformed in nigmatitcs Agnatites sides of netatects clusions of metatexites Diktyonites •Nebulites *Ptygmatites •Hooophanous diatexites

Microfabric •Generally coarse-grained, •Generally gneissose or schis Generally medium-grained, pegmatoid texture tose texture. In parts blastic plutonitic texture "Criss-cross"texture

Quartz •Abundant drop-like quartz No quartz3 •Xenomorphic quartz inclusions in feldspar Potash feldspar •Xenomorphic or porphyro- No potash feldspar3 •Xenomorphic or porphyro- blas tic crystals blastic crystals Plagioclase •Large hypidionorphic crystals No plagioclase •Medium sized, idionorphic c without zoning crystals with inverse zoning Biotite No biotite** •Xenomorphic aggregates often •Idionorphic to hypidionorphic coarsened by recrystallization crystals C jrdiertie Idionorphic crystals, often Foliated aggregates formed by Idionorphic porphyroblasts, with inclusions of sillimanite the reaction biotite-* cordierite strongly poikiloblastic Apatite Xenomorphic aggregates •Small inclusions in biotite •Idionorphic prismatic crystals Perthitc No perthitc No perthitc •Perthite present in snail amounts Antipcrthite •No antiperthitc •no antiperthite Antiperthite present in small amounts

Occurrence Outer parts of anatectic Outer parts of anatectic Inner parts of anatectic massifs massifs massifs aOn principle not present in restitcs but locally occurring as the non-nobilized surplus over the ratio of quartz + feldspar mobilized within motatects.

^On principle not present in pegmatoid actatects but locally occurring in small amounts mechanically introduced from the adjacent parent Ill rock or restite. Q Inverse zoning was not observed in plagioclascs of the Massabesic Gneiss. and Littleton formations near the border are metatexites. They

contain abundant pegmatoid metatects in restites that are

basically schist and gneiss of obviously metamorphic origin.

The composition of the gneiss indicates that it was probably derived from a series of impure shales or siltstones that were)

in places, somewhat calcareous. There was appraently very little quartz sandstone and no limestone in this sedimentary sequence.

The writer believes the Massabesic Gneiss to have been derived from the Littleton Formation, rather than from the Berwick

Formation, as suggested by Sriramadas (1966, p. 67). Sriramadas based his conclusion on the supposed similarity in modal com position between the Massabesic white oligoclasc gneiss and the

schists of the upper member of the Berwick Formation. The writer bases his conclusion on the initimate association of

Massabesic Gneiss and Littleton Formation along the northern and western boundaries between the two formations. In addition, the small beds and lenses of pinkish-gray lime-silicate granulite

in the Massabesic Gneiss closely resemble similar material from

the Littleton Formation. It is of course possible that the

Massabesic Gneiss might be derived in part from both formations.

The original formational boundary between the Berwick and the

Littleton formations is obscured by the presence of the gneiss.

However, until detailed chemical analyses are available for all

3 formations, the problem of what served as the parent rock for

the gneiss will remain unproven. PLUTONIC ROCKS

Conorcd Granite

Distribution

Massive or faintly foliated two-mica granites and quartz monzonites have been described at numerous localities in New

Hampshire (Billings, 1956, p. 62-63). Hitchcock (1877, p. 112) called them Concord Granite because of excellent exposures in quarries around the capital city.- The names Bickford and

Fitzwilliam have been used for apparently contemporaneous granitic rocks of similar lithology in other parts of the state.

Billings (1956, p. 62) suggests that they might all be called

Concord Granite.

Massive to faintly foliated binary granite crops out over large areas in the Suncook quadrangle. Excellent exposures occur in quarries 1.2 miles east of Suncook, 2 miles south of

Hooksett, and between Manchester Road and Route 1-93, about 2 miles north of Manchester. Comparison between massive binary granite from the Suncook quadrangle and granites from the area around Concord shows them to be nearly identical. Therefore the massive binary granite is here assumed to be correlative with the Concord Granite of the type area.

Rocks assigned to the Concord Granite occur mainly in a large body in the west-central part of the Suncook quadrangle

113 114

(Plate 1). This body, measuring about 4 by 9 miles, is bounded on the northwest by the Pinnacle fault zone and on the southeast by the Hall Mountain-Campbell Hill fault zone. To the northeast and southwest, the granite intrudes and is commonly intimately mixed with the Littleton Formation. Lack of continuous outcrops obscures the details of these contacts, and they are therefore highly generalized on the geologic map.

Concord Granite also occurs in an area immediately southeast of the Hall Mountain-Campbell Hill fault. The binary granite here intimately intrudes rocks of the Littleton Formation. The contact with the Massabesic Gneiss to the southeast is not expo sed, but rocks in the area of the contact are unlike those in either formation and may be a result of partial assimilation or re-heating of the Massabesic Gneiss along the contact,

A small mass of binary granite shown by Billings (1955) as occurring on the southeast flank of Pembroke Hill, Pembroke, could not be found. However, boulders of binary granite are abundant in this area, which is about 4 miles east and southeast of the extensive outcrops around Concord. Glacial striae in the northern part of the Suncook quadrangle generally trend S20° -

30°E (see Figure 2, p. 5 ). Therefore, it is not likely that the boulders around Pembroke Hill came from the Concord area.

The nearest outcrops in line with the boulders along the line of glacial transport are those in the southwest corner of the

Gilmanton Quadrangle, at Oak Hill straddling the northeast 115 boundary of the city of Concord (Heald, 1955, plate). These outcrops lie 5 to 7 miles north-northwest of the boulders in the Pembroke Hill area and are probably the source of those boulders.

Lithology

Rocks assigned to the Concord Granite are generally gray to white on fresh siirfaces and gray to tan or buff when weathered. They are fine- to medium-grained (average 0.5 to

2.0 mm), hypidiomorphic, subporphyritic, and faintly foliated.

The essential minerals include quartz, potash feldspar (mainly microcline), plagioclase (^20-30^* biotite. The feldspars at some localities occur both in the groundmass and as scattered, generally subhedral, phenocrysts. Muscovite, commonly subhedral or euhedral, is one of the distinctive accessory minerals of the

Concord Granite. Other common accessory minerals include tourmaline, chlorite, magnetite, apatite, and zircon.

Table 8 gives estimated modes for five representatives samples of the Concord Granite, based on counts of 500 points per thin section. These are all granite according to the classification scheme adopted by the I.U.G.S. (1973, p. 26). In thin section the granite is hypidiomorphic-granular to cataclastic. Quartz, making up 26 to 36 percent of the rocks, occurs as distinct interstitial grains between feldspar and as both inyrmckitic and i>oikilitic intergrowths in plagioclase and micro cline. In places, the quartz is strained. Potash feldspar, 116

Table 8

Approximate Modes of the Concord Granite

Samole No.: 7-30-3a 8-19-lb 9-5-2a 97-16-2a 97-23-lc

Quartz 27 36 31 26 26

K-feldspara 42 27 33 42 39 U Plagioclase 26 27 30 23 25

Biotitc 2 5 2 5 3

Chlorite tr. - tr. tr. 1

Muscovite 3 5 4 3 5

Magnetite tr. -- - -

Tourmaline - - - 1

Apatite tr. tr. tr. tr. tr.

Zircon tr. tr. tr. tr. tr.

0^ mostly microcline

oligoclase ranging from An2Q -

Location of samples:

7-30-3a New Boston Road, 0.15 mile east-southeast of its intersection with Currier Road, Candia.

8-19-lb Route 1-93, 1.1 miles west of River Road School, Bow.

9-5-2a Quarry 1.0 mile southeast of intersection of Hackett Hill Road and Manchester Road, llooksett.

97-16-2a Under power line 1.0 mile south-southwest of New Rye, Allenstown.

97-23-lc Bailey Quarry, 1.3 miles east of junction of Routes 28 and 3, Allenstown. making up 27 to 42 percent of the rocks, commonly exhibits the

"plaid" twinning of microcline. Subhedral phenocrysts of microcline occur in some samples, and these commonly contain quartz poikilitically. String perthite occurs in some of the non-porphyritic samples. Plagioclase (oligoclase, An2q_3q) slightly to moderately sericitized. Brown biotite occurs as anhedral flakes, some of which are altered to chlorite and magnetite* Many biotite flakes exhibit pleochroic halos around small included zircon crystals. The biotite commonly shows a preferred orientation, the flakes being sub-parallel and forming.the faint foliation seen at many places. Muscovite, in subhedral or euhedral crystals, shows no such preferred orientation and, in fact, commonly cuts across biotite. Apatite occurs as small grains associated with the biotite. Tourmaline locally forms poikilitic masses as much as an inch across, but also occurs as small individual crystals. Tourmaline is espec ially common where microcline-albite pegmatite intrudes the

Concord Granite.

The granite locally exhibits a cataclastic texture. In

sample 8-19-lb, grains of quartz, microcline, plagioclase, biotite, and muscovite, averaging 0.5 - 2.0 mm across, are

surrounded by fine-grained (average 0.05 mm) micas and crushed quartz and feldspar. Some of the larger quartz grains are elongate parallel to the foliation* 118

Age Relations

The Concord Granite of the Suncook Quadrangle intrudes the

Littleton Formation, the Massabesic Gneiss, and the microcline pegmatite* The contacts with the Littleton Formation are

locally concordant, but on a large scale the contacts are discordant. Partially assimilated inclusions of gneiss and

schist of the Littleton Formation can be recognized at local

ities near the contact. Relations between the binary granite

and the Massabesic Gneiss are unclear because of poor exposures

along the contact. Foliated pinkish microcline granite and

quartz monzonite containing biotite but very little muscovite occur at several localities in the contact area. These rocks

resemble the Massabesic Gneiss more closely than they do the

Concord Granite, but they are not typical of either rock unit.

The Concord Granite is itself cut by the late dark-colored dikes, by the massive quartz of the fault zones, and by microcline-albite pegmatite and associated aplite. Lamprophyre dikes

intrude Concord Granite at an outcrop 0.15 mile east-southeast of the intersection of New Boston Road and Currier Road, in

the town of Candia. Massive quartz veins cut the granite at

many places along the fault zones, and quartz locally replaces

the Concord Granite along the faults. Near the faults, the

granite commonly contains pyrite and chlorite. White micro-

cline-albite pegmatite, usually containing muscovite and biotite

or muscovite and tourmaline, intrudes the Concord Granite at many 119 localities. This is especially well shown in the quarries near

Hooksett, Suncook, and Manchester.

Several lines of evidence suggest that the Concord Granite was emplaced during the late stages of the main orogenic episode

(Acadian) of the area. Although well-developed foliation is rare, faint alignment of biotite flakes characterizes the

Concord Granite at many places. The orientation of this align ment varies widely, but it is generally nearly parallel to the * regional foliation (Figure 29). There is no evidence for chilled contacts or contact metamorphism where the granite in trudes the Littleton Formation and Massabesic Gneiss. Therefore, these formations must have been hot at the time of intrusion.

Regionally, the highest grade of metamorphism in the Merrimack synclinorium occurs where the rocks are intruded by the more massive circular bodies of the New Hampshire Plutonic Series.

It is therefore likely that the metamorphism of the sediments and the intrusion of the New Hampshire Plutonic Series are closely related. The older members of the series (Bethlehem

Gneiss and Kinsman Quartz Monzonite), concentrated along the outer edges of the Merrimack synclinorium in areas of lower metamorphic grade, are elongate, gneissoid, generally con cordant bodies (Page, 1968, p. 380; Cady, 1969, p. 52). These older units are most likely synkinematic intrusives; the Concord

Granite is late synkinematic or early postkinematic.

Radiometric dating of rocks of the New Hampshire Plutonic

Series is hampered by a widespread late Paleozoic overprint. 120

For exampley the Kinsman Quartz Monzonite and Bethlehem Gneiss and a "binary granite from the Fitchburg pluton" (probably

Massabesic Gneiss or microcline granite) were dated by Faul, et al, (1963! p. 15) and by Zartmanf et al. (1970, p. 3362) respectively. The rocks all gave Permian ages. Wetherill! et al. (1965, p. 196) give Sr8 7 /Rb87 dates of 304 and 318 million years for biotite and muscovite from the Concord

Granite. Cady (1969, p. 112) thinks the late Paleozoic dates may reflect delayed cooling of the metamorphic and igneous rocks of the Merrimack synclinorium after the Acadian orogeny.

Page (1968, p. 380) notes that the Concord Granite and associated pegmatite is the youngest member of the New Hampshire Plutonic

Series, which is younger than the Littleton Formation and older than Page*s Late Devonian Plutonic Series. Therefore the age of the Concord Granite is most likely Middle Devonian. Microcline-Oligoclase Pegmatite

Distribution

White microcline-oligoclase pegmatite is widely distrubuted

in the Suncook quadrangle and is especially common in areas

underlain by the Littleton Formation and Concord Granite. The pegmatite commonly occupies hilltops and crops out in areas where no other bedrock is exposed. Individual bodies range in size from small pods and dikes a few inches to a lew feet thick to masses several hundred feet across. A mass about 600 feet long

is exposed at Rimmon Park, on the west edge of the city of

Manchester.

Lithology

The microcline-oligoclase pegmatite generally consists of microcline and oligoclase (either of which may predominate), quartz, muscovite, and either biotite or tourmaline, or both.

Microcline grains, averaging 5 cm and up to 0.5 m across, are buff, white, or light gray, perthitic, and commonly exhibit graphic intergrowths with quartz. Oligoclase occurs as fresh white finely-striated grains averaging 1 to 5 cm long. The average composition is AnjL5-20* Gray quartz forms graphic intergrowths with microcline and irregular masses averaging

1 to 2 cm across. Milky quartz and, rarely, pale rose quartz may substitute for the smoky variety. Muscovite is present in

121 almost all of the microcline-oligoclase pegmatite, occurring as plates from 1 to 10 cm across and, at two localities, as feather-like aggregates. It also forms fine-grained coatings along partings and on feldspars where the pegmatite has been sheared. Muscovite flakes are commonly subhedral or euhedral.

Biotite, in anhedral plates that locally are altered to chlorite, is generally not as common as muscovite; some pegmatite contains no biotite. Although tourmaline and biotite cam and do occur together, biotite is less abundamt in those outcrops containing tourmaline than in those that do not. The tourmaline occurs as poikilitic masses, radiating groups of crystals, amd as single crystals and parallel growths.

A number of other minerals are fairly common in the pegmatite. Garnet ramges from small euhedral crystals averaging

2 to 3 mm across to irregular granular masses 3.0 cm across.

Apatite is common as small pale green prismatic crystals.

Crystals of yellow, pale-green, or grayish-white beryl were identified at five localities. Arsenopyrite was identified in five outcrops, dumortierite in two, autunite in one, amd graphite in one. Fluorite, chalcopyrite, galena, dolomite, siderite, and sphalerite occur along joints in the pegmatites, but these min erals were probably carried in by mineralizing fluids related to the faulting that occurred some time arfter intrusion of the pegmatites (see p. 158). 123

The pegmatite is commonly associated with massive or faintly foliated gray to white aplite. The aplite consists mainly of oligoclase, microcline, and quartz. Accessory minerals are muscovite, biotite, tourmaline, and garnet.

Tourmaline and garnet, occurring as small crystals, form gneissoid bands in some aplites associated with pegmatite; this is most common near contacts with surrounding metasediments and associated with inclusions of schist or gneiss in the pegmatite.

In many places, aplitic rock occurs at the border separating pegmatite from the wall rock. Aplite dikes also cut and are cut by pegmatite.

Age Relations

The microcline-oligoclase pegmatite and associated aplite intrudes all rock units except the late dark-colored dikes and sills and the quartz masses of the fault zones. Pegmatite in the

Littleton Formation forms spindle- or pod-shaped bodies from a few feet to tens of feet long. These bodies are discordant in detail but are elongate and flattened in the plane of foliation.

Larger masses of pegmatite clearly cut across the schistosity of the Littleton Formation. Microcline-oligoclase pegmatite in the

Concord Granite forms dikes from a few inches to several feet thick that apparently intrude joints formed as the granite cooled.

However, both gradational and sharp contacts were observed. The dikes cut the faint foliation of the granite. 124

Pegmatite in the Concord Granite is generally whiter and more uniform in texture than that in the Littleton Formation.

However, the mineralogy of the two types of occurrence is

nearly the same. Pegmatites in both rock types contain garnet

and tourmaline, and beryl and arsenopyrite were observed in both

associations. Post-Oroqenic Dikes and Sills

Distribution

Twenty-two late, mostly mesocxatic dikes and sills were

mapped in the Suncook quadrangle. Nineteen of these are shown

in Figure 25; the other three are parallel to and at the same

localities as dikes shown in the figure. They are similar to dikes and sills elsewhere in New Hampshire that have been assigned to the White Mountain plutonic-volcanic series

(Billings, 1928, p. 125; Chapman and Williams, 1935, p. 504-

506; Quinn, 1937, p. 390-394; Fowler-Billings, 1944; and

Billings, 1956, p. 86). Rock types described by these authors

include olivine diabase, kersantite, diorite, camptonite, spessartite, augite syenite porphyry, porphyritic syenite, syenite porphyry, bostonite, hastingsite solvsbergite, paisanite, hastingsite granite, biotite granite, aplite, and quartz porphyry. Freedman (1950a, p. 469-472) and Sriramadas (1966, p. 32-34) found about forty such dikes in the Mount Pawtuckaway and Manchester quadrangles. Sriramadas (1966, p. 32) classified

the rocks of these dikes as diabase, but Freedman described

trachyte, hornblende diorite, augite diorite, camptonite, ouachitite, and diabase. In both quadrangles, the majority of

the dikes strike northeast and dip at very high angles.

125 ;26

os’

zs* ZD'

Figure 25. Locations and strikes of nineteen mafic dikes and sills in the Suncook quadrangle. All are vertical or nearly vertical; lengths are not to scale. 127

Late dikes and sills occur only in the southern half of

the Suncook quadrangle. Most are thin and poorly exposed, but

a few are as much as 10 feet thick and at least one can be

traced for 1500 feet along strike. The strikes of the dikes

and sills range from N. 70° W. to N. 60° E., but more than half

strike between N. 20° E. and N. 30° E. Dips range from 60° NW

to 85° SE; most of the dikes are about vertical.

Lithology

Microscopic examination of fifteen dike rocks resulted in

identification of five rock types. Estimated modes for nine of

these are given in Table 9. Six of the fifteen dike rocks are much-altered aphanitic rocks best classified as basalt, two of

which are porphyritic. Two other porphyritic dikes are atypical

in that they have a glassy groundmass. These would probably best

be named vitrophyre. Five dike rocks are spessaxtites (Williams,

et al., 1954, p. 88), two of them much altered but the other

three comparatively fresh. A single sample of diabase and one

of highly altered andesite were identified. The following are detailed descriptions of rocks in these dikes.

Basalt

The altered basalts consist of fine-grained (0.05 mm±)

amphibole, chlorite, and altered plagioclase, commonly with a

few euhedral phcnocrysts of relatively fresh pigconitc and an

altered pyroxene that has been replaced by chlorite and

serpentine. Magnetite is abundant in the groundmass. 1 2 8

Table 9. Approximate Modes of Post-Orogenic Dikes and Sills

Sample No. 6-24-4a 6-25-lc 6-25-lf 7-14-lb 7-14-Id 7-16-la 7-22-6a 7-25-2b 8-15-13b

Altered glass - - 49 ------

Plagioclase 43. 5 41 30 45.5 83.5 42 37.5 35 5.5

Augito 3 - 14.3 - - 3.5 0.5 --

Pigeon ito - 18 7 - - - - - 11

Qxyhornblende - - - 32.5 - 36 33.5 41.5 29.5

Hornblende 29 16.5 ------

Biotite 13.5 13.5 ------

Chlorite - 2.5 tr. 10 6.2 3 2.5 - 9

Serpentine - tr. ------

Talc - 3 14 - - - - - tr.

Magnetite 6 5.5 - 4.5 - 6 6 7.5 19

Pyrite tr. 0.5 - 0.5 - - tr. tr. -

Carbonsto tr. - - 7 - 9.5 19.5 15 26

Sericite tr. ------tr. Antbophyllito 0.5 - - tr. - tr. - tr. -

Chalcedony ------1 tr. - Leucoxeno tr. tr. - tr. - tr. tr. tr. tr.

Zoisito - - tr. ------Hematite - - - 9.7 -- - -

Coethite - - - - 1.6 - •- - -

Composition of 7 Plagioclase ^ 3 5 A*170 ^ 5 0 *"55 to35 *>50 * ’50 **50 129

Table 9, continued. Description and Location of Samples.

Sample No.

6-24-4a Porphyritic Basalt, on road just south of Norton Pond, Raymond.

6-25-lc Diabase, 670 feet north-northwest of intersection of Murphy and Raymond roads, Raymond.

6-25-lf Vitrophyre, same locality as 6-25-lc.

7-14-lb Spessartite, at Chester Turnpike overpass on Route 101, Candia.

7-14-Id Andesite, same locality as 7-14-lb.

7-16-la Spessartite, north side of Route 101, 1300 feet north east of its intersection with Smith Road, Candia.

7-22-6a Hornblende basalt, on Route 101B, 0.42 mile west of Four Corners, Candia.

7-25-2b Spessartite, on north side of Old Candia Road, 250 feet southwest of its intersection with Tower Hill Road.

8-15-l3b Altered spessartite, on west side of Route 93, just west of south end of Pinnacle Pond, Hooksett. 130

Oxyhornblende or biotite also occur in the groundmass of some

samples, and carbonate is very common in those that are much

altered or those that contain, amygdules. Leucoxene is abundant,

especially along cracks in altered pyroxene phenocrysts, which may have been titanaugite before alteration* Trachytic, diabasic, and felted textures all occur, the last being most

common. Plagioclase and oxyhornblende in the groundmass are elongate but roost do not show sharp straight edges*

Biotite is abundant as 0*5 mm flakes in sample 6-24-4a

(Table 9)* Some of the biotite appears to replace amphibole, which in turn has replaced clinopyroxene. Although magnetite is common in most of the dark dikes, it occurs as skeletal crystals and dendritic aggregates only in this sample. Labradorite (An^), in phenocrysts as much as 5.0 mm long, exhibits oscillatory zoning, saussuritization, and braid antiperthite. Plagioclase

in the groundmass exhibits subparallel to random orientation. A few small (0.5 - 1.0 mm) twinned augite crystals have much- altered edges, and fine-grained (0.3 mm + long) acicular antho- phyllite occurs in unusual abundance. The rock is probably an altered porphyritic basalt.

The glassy dike rocks contain, in addition to plagioclase, fresh polysynthetically twinned euhedral pigeonite from 0.2 to

2.0 mm across. Most of the pigeonite occurs as single grains, but some occurs as glomeroporphs. Highly altered pyroxene phenocrysts are distributed among the fresh ones. Small 131 unaltered patches in the former have the same optical properties as the latter, and so these probably represent two generations of phenocrysts. Alteration occurs in the form of concentrically arranged talc, chlorite, and opaques. Labradorite (An^) occurs as small (0.2 mm) unaltered grains showing trachytic texture and as larger (2.0 mm) somewhat altered zoned euhedral grains. The groundmass of these rocks is a very fine-grained dark-colored material that looks like devitrified glass, and in which no minerals can be identified. Dikes exhibiting this type of groundmass are very narrow and presumably were chilled quickly when intruded. Sample 6-25-lf (Table 9) cuts across a pegmatite exhibiting sheared zones parallel to the contact. The contact is sharp and there is surprisingly little evidence of alteration of the pegmatite.

The presence of two generations of ferromagnesian pheno crysts is a typical feature of lamprophyres (Williams, et al.,

1954, p. 85), However, feldspar is almost invariably restricted to the groundmass in lamprophyres, and the glassy dike rocks of

Suncook quadrangle, which contain abundant plagioclase pheno crysts, are probably best classified as vitrophyres.

Spessartite

Spessartite are "hornblende-or augite-rich lamprophyres with plagioclase as their typical feldspar" (Williams, et al.. 1954, p. 88). The spessartites of Suncook quadrangle contain oxyhorn blende both as euhedral phenocrysts (1 to 5 mm long) and as a | 132

prominent constituent of the groundmass. The phenocrysts are

bright reddish-brown, commonly with lighter-colored rims. The

amount of alteration varies widely irom sample to sample and

within individual specimens. In one thin section (sample

7-25-2b), several twinned oxyhornblende phenocrysts are altered !i in such a way that one twin is fresh, whereas the other is

altered to chlorite, carbonate, magnetite, and leucoxene.

Clinopyroxene, commonly pigeonite, occurs as phenocrysts in

three of the five spessartites studied. Alteration of the

clinopyroxene varies widely, and some euhedral phenocrysts are

completely altered to talc, chlorite, and other minerals. One

sample (7-16-la) contains altered phenocrysts of plagioclase,

oxyhornblende, and clinopyroxene set in a groundmass composed

mainly of the same three minerals in a nearly unaltered state.

Plagioclase is normally altered, both as phenocrysts and as a

constituent of the groundmass. Although phenocrysts are

euhedral, plagioclase in the groundmass commonly has poorly de- j fined altered borders gradational with surrounding altered material. Carbonate, clay minerals^ and zoisite occur in j altered plagioclase. Much-altered plagioclase, in both pheno

crysts and groundmass, exists sidej-by-side with fresh oxyhorn

blende and fresh pigeonite, or both, in several,thin sections.

In two specimens, euhedral oxyhornblende phenocrysts contain cores I of plagioclase. Oxyhornblende in the groundmass cuts across,

and so is younger than, plagioclase. All of these features i 133 suggest that the spessartites originated from xnagma that was partially crystallized before injection. Deuteric alteration of the phenocrysts followed injection and crystallization of the groundmass.

Diabase

Sample 6-25-lc is a typical diabase. Most of the minerals are not greatly altered, but a few phenocrysts, probably olivine originally, have been converted to chlorite and talc or ser pentine. Magnetite is also abundant. Anhedral pigeonite is surrounded by thin lath-shaped crystals of labradorite (An^Q), giving the rock a typical diabasic texture.

Andesite

A single dike of altered andesite (sample 7-14-ld, Table 9) was found in close proximity to three spessartite dikes on

Route 101 at the Chester Turnpike overpass. The rock consists almost wholly of a felted mass of plagioclase laths averaging about 0.5 mm long. Ferromagnesian minerals are represented by hematite and chlorite, which are evenly distributed throughout the rock. Hematite also occurs in concentrations suggesting former euhedral amphibole or pyroxene crystals that were as much as 1,0 mm long. A few of the plagioclase grains are relatively unaltered and have a composition near An35 (andesine).

Age Relations

The dark basaltic dikes and sills in the Suncook quadrangle cut across all of the other bedrock units, including massive silicified rock along faults. There is no evidence of regional

metamorphism of any of the dikes. Although there are no pub

lished radiometric dates for any of the basaltic dikes in south

eastern New Hampshire, they are probably structurally and com-

positionally related to the Early Jurassic White Mountain

plutonic-volcanic series. Billings (1956, p. 88) considers

camptonite and related dikes as the latest representatives of

the series. A K-Ar date published by Zartman, et al. (1967,

p. 862) on a lamprophyre dike in western Vermont indicates an

age of 136 + 7 million years. Although this is younger than the

average 180 million-year age usually given for the White Moun

tain series (Lyons and Faul, 1968, p. 312), it agrees well with

some determinations. For example, Faul, et al. (1963, p. 10) give an age of 130 million years for a hornfels of the Ascutney,

Vermont, stock, which is apparently a part of the White Mountain series. Wanlcss, et al. (1966) dated lamprophyre dikes in

Newfoundland at between 115 and 144 million years.

Structural Control

The generally north-northeasterly trend of the late dikes and sills exposed in the Suncook quadrangle corresponds with

trends reported by Sriramadas (1966, p. 33) in the Manchester quadrangle and by Freedman (1950a, p. 470) in the Mount

Pawtuckaway quadrangle. It also corresponds with trends of the

78 larger basaltic dikes in Connecticut and Rhode Island de

scribed by King (1961, p. B95) and DeBoer (1967, p. 2238-2239). 135

Dikes in the Manchester and Mount Pawtuckaway quadrangles gen erally parallel the strike but cut across the dip of bedding in surrounding metasediments. In the Suncook quadrangle, dikes and sills were found in definitely bedded metasediments at only two places. At a locality 0.2 mile east-northeast of Norton

Pond, a diabase sill occurs in the schists of the upper member of the Berwick Formation that strike N. 60° E. and dip 85° SE.

A basaltic dike at the same locality cuts across both the strike and dip of bedding. At a locality 0.1 mile south of i Norton Pond, a basalt dike trends parallel to the strike of bedding in the upper member of the Berwick Formation but cuts across the dip at a low angle. Many of the other dikes intrude the Massabesic Gneiss, in which compositional banding and foliation probably parallel bedding. Although one dike, 1.2 mile northeast of Four Corners, nearly parallels the foliation in the Massabesic Gneiss, all others cut across foliation at various angles. The strike and dip of foliation in the gneiss is so variable that any concordant contacts between late dikes and Massabesic Gneiss are probably coincidental.

At several localities, the trends of the late dikes correspond well with trends of joints in the surrounding rock.

A set of vertical joints striking on the average N. 20° E. to

N. 35° E. occurs in many places in the Massabesic Gneiss and in granites and pegmatites associated with it, and many of the dikes in southeastern New Hampshire follow this trend. The joints are 136

nearly parallel to the trend of fold axes and the strike of

foliation and are probably either a result of the Devonian

compression that produced the major structures in the area or

of Late Triassic or later arching. DeBoer (1967, p. 2246)

related the systematic orientation of the large Jurassic

basaltic dikes of New England to tensional fractures of

primary origin formed during an episode of tectonic activity

following and separate from the Late Triassic arching. Wilson

(1962 a,b) suggested the existence of a major sinistral trans

current fault, the Cabot fault, extending southwestward from

Newfoundland to Massachusetts. DeBoer suggested that the dikes

occupy features related to this shear zone. A simple-shear

system with sinistral polarity, striking northeast-southwest, would produce primary tension fractures striking about north-

northeast to north-northwest. It should be noted in this

connection that evidence for major strike-slip displacements

is lacking in eastern Connecticut, where most of the dikes

used in DeBoer's study are located. Instead, the major faults

in this area, the Lake Char and Honey Hill faults, are apparently

thrust faults dipping, respectively, west and north (Dixon and

Lundgrcn, 1968, p. 225). In addition, other workers (Belt, 1968,

p. 101-102, 112-113; Lock, 1969, p. 790; Webb, 1969, p. 757;

Pitcher, 1969, p. 731) have found extensive evidence for dextral

movements on the proposed Cabot fault and other faults related to it. First-order tension fractures related to a northeast- southwest- trending simple-shear system with dextral polarity should strike between ENE and WNW. Because of the hypo thetical nature of discussions of the Cabot fault and other features indicating large-scale strike-slip movements in New

England, it is not possible at this time to relate the dark- colored dikes and sills of the White Mountain plutonic vol canic series to such features. 138

STRUCTURE

Introduction

The Suncook quadrangle lies on the southeast limb of the

Merrimack synclinorium (Figure 26). The synclinorium extends south across central Massachusetts and eastern Connecticut and is cut off by the Honey Hill fault about 15 miles north of Long

Island Sound (Dixon and Lundgreny 1968, p. 219-220). It widens and plunges toward the northeast (Skehan, 1969, p. 795), con tinuing at least into central Maine (Osberg, et al.. 1968, p. 241). The synclinorium is bordered on the southeast by the

Rockingham anticlinorium, the axis of which plunges southwest across southeastern New Hampshire (Sundeen, 1971, p. 79-82).

To the northwest, the Bronson Hill-Boundary Mountain anti clinorium plunges southward over most of its length. The south east limb of the Merrimack synclinorium in New Hampshire is cut by several lineaments defined by large outcrops of massive quartz.

Various workers have interpreted these lineaments as faults, and two of them, herein named the Pinnacle and Hall Mountain-Campbell

Hill faults, extend across the Suncook quadrangle (Plate 1),

Large-scale structures such as the Merrimack synclinorium and the faults shown by outcrops of silicified rock cannot be observed directly in the field and so must be reconstructed on the basis of detailed mapping of a large area. Such structures * . 139

Me.

VI.

Conn. JU

Itlio Chir - Fault Honey Hill Fault

Figure 26. Index map showing major structures of New Hampshire and surrounding areas. 140

are macroscopic in scale, according to the terminology of

Turner and Weiss (1963, p. 16), as are the individual anti

clines and synclines making up the synclinorium. These in dividual anticlines and synclines are indicated by smaller-scale

structures (termed mesoscopic by Turner and Weiss) that one can

study in hand specimens or continuous exposures. Among the mesoscopic structures of the metasediments of the Suncook

quadrangle are bedding, rock cleavage and schistosity, lineations, minor folds, and joints.

Mesoscopic Structures

Bedding

Despite the high grade of metamorphism, sedimentary bedding

is well-defined at many places in the Suncook quadrangle. In the

upper member of the Berwick Formation, layers of massive lime-

silicate rock and occasional changes in mineral composition' reveal

the bedding in the mica schist. Bedding, shown by changes in mineral composition, is better developed in the Littleton Form

ation than in the upper member of the Berwick Formation. Medium-

grained crinkled rusty mica schist contains widespread interbeds

of fine-grained gray schistose gneiss. Graded-bedding and cross bedding can be seen on a few outcrops, and interbeds or lenses

of quartzite and lime-silicate rock also help to define bedding.

Only rarely are bedding and schistosity not parallel in the

Littleton Formation. 141

Rock Cleavage and Schistosity

Sevecal types of rock cleavage and schistosity occur in the metasedimentary rocks of the Suncook quadrangle. In the upper member of the Berwick Formation, the mica schist exhibits a schistosity (S2) parallel to the compositional layering that defines bedding (S^). In addition, a few thin-sections show a poorly-developed schistosity that intersects the more prominent one. Unfortunately, this second schistosity was not recognized in the field, and not enough information is available to relate it to other structures in the Berwick Formation. The pre dominant schistosity of the Littleton Formation is also S^, but, in addition, a few outcrops exhibit a continuous axial-plane cleavage (S^) or a spaced crenulation or slip cleavage (S^) that is well defined in schistose layers but dies out in the more granulose layers. is rarely shown, but S2 is commonly crenulated.

Figure 27 is a synoptic pi diagram showing the orientation of schistosity in the metasedimentary rocks of the Suncook quadrangle. The diagram shows concentrations of poles normal to schistosity for 164 readings. The prominent concentration of points in the lower right sector of the diagram indicates that the schistosity has an average strike of N. 25° B. and an average dip of 20° northwest. This agrees with regional studies of southeastern New Hampshire showing that the rocks in this area lie on the southeastern limb of the Merrimack synclinorium. The scatter ■ I 1-3 1-3 3* 4 t II 1-7 M PtKCtNT

Figure 27. Synoptic pi diagram of poles, to schistosity in the Littleton Formation and upper member or the Berwick Form ation, Suncook quadrangle, based on 164 readings. 143 of points around the maximum is caused by the presence of smaller-scale folds on the limb of the synclinorium. A second concentration of points is present near the right edge of the diagram. In order to determine the reason for this concentration, an attempt was made to divide the quadrangle into distinct structural domains. One such domain includes the area underlain by the Jenness Pond member of the Littleton Formation, in the northeastern portion of the quadrangle. Figure 28 is a pi diagram of schistosity as measured in that area. This diagram shows that most of the concentration of points in the eastern portion of Figure 27 results from measurements from the Jenness

Pond member. The concentration shown in Figure 28 indicates that the average strike of in this area is N. 8° W., and the average dip is about 70° southwest. None of the possible struct ural domains in the Pittsfield member show this maximum. Field observations indicate that the rocks are overturned at several localities along the western side of the Jenness Pond domain

(Plate 1) and the stratigraphy and structure in the area suggest that the Jenness Pond member occupies a syncline. The domain widens to the north-northeast, suggesting that the syncline plunges in that direction; b lineations in the Jenness Pond member plunge both to the north and to the south.

Axial-plane cleavage (S^) and crenulation of slip cleavage

(S^) were positively identified at only a few localities in the

Littleton Formation. Axial-plane cleavage is very difficult to 1-4 4 1 0-12 11-11 11-20 20-24 24-20 PERCEHT

Figure 28. Pi diagram of poles to S. schistosity in the Jenness Pond member of the Littleton Formation. identify in the montonous mica schists that characterize parts of this formation, but is easily seen at the few localities where it occurs in interbedded mica schist and fine-grained granulose schistose gneiss. A good example of axial-plane cleavage can be seen at a folded outcrop on the 740-foot hill on the east edge of the quadrangle at 43° 10* 18" N. and 71°

1 5 * 3*« w. On one part of this outcrop, the foliation dips steeply west-northwest but the bedding is nearly horizontal

(see Figure 6). Unfortunately, well-defined examples of S3 are so rare and scattered that they are seldom useful in clarifying the large-scale structures in the Suncook quadrangle.

Examples of crenulation or slip cleavage are even less common.

At a few places in the crinkled schist of the Jenness Pond member, there is evidence of recrystallization of micas along spaced planes of slippage. This is not shear cleavage, because nearby schist shows no evidence of spaced cleavage despite the fact that the cleavage is highly crenulated.

The Concord Granite is massive at most places, but biotite flakes exhibit a faint parallel alignment in many outcrops. A pi diagram (Figure 29) shows that the foliation has an average strike of N. 65° E. and a dip of about 36° W. The well-defined maximum shown in Figure 29 could be explained in two ways. First, the foliation may be a primary flow structure produced during intrusion of the elongate granite stock. This interpretation is supported by the fact that the orientation of the foliation 1-3 J-B S-7 7 1 M l 11-13 MRCINT

Figure 29. Pi diagram for foliation in the Concord Granite and associated pegmatites, Suncook quadrangle (47 readings). 147 generally follows the elongation of the stock* A second possibility is that the foliation is secondary in origin and

resulted from recrystallization during the final phases of the

Acadian orogeny, or, probably less likely, during the rel atively weak late Paleozoic Alleghenian orogeny. The lack of foliation at most outcrops strongly suggests that the first

interpretation is correct. Additional support for this inter pretation is provided by the observation that muscovite, which is present in large subhedral or euhedral grains, never shows a preferred orientation. The muscovite apparently crystallized late and probably post-dates the emplacement of the granitic

Figure 30 is a pi diagram of 67 measurements of foliation in the Massabesic Gneiss. Most of the data was collected in the white oligoclase gneiss, which generally shows the foliation better than the pink gneiss. The average strike of the foliation, as shown by the diagram, is about N. 66° E. A broad girdle convex toward the southwest suggests folds plunging about 20° northeast. The marked concentrations in the northwest and south sectors of the diagram indicate that the folds have straight limbs and narrow hinges. This observation was confirmed in the field, and field observations also indicate that the concentra tions shown by the pi diagram result from numerous small-scale folds having similar trends rather than a single large fold. 148

I'M Z.i-4 Ml U'T Til fllCIMT

Figure 30. Pi diagram for foliation in the Massabesic Gneiss, Suncook quadrangle (67 readings). 149

Lineations

Lineations of three basic types occur in the metamorphic rocks of the Suncook quadrangle. The three types are crinkles, a lineations shown by elongate minerals and slickensides, and b lineations shown by elongate minerals. The Littleton Formation contains the most lineations of all three types, the commonest being a crinkle lineation shown by incompetent schist within the formation. The crinkles measure a few millimeters in amplitude and 1 to 2 cm in wavelength. Although they plunge across the foliation at some places, most of the crinkle axes are parallel to the local strike of foliation. No crinkles were observed in the Berwick Formation.

Two types of lineation are shown by minerals in the meta sediments. The first is an a lineation, in which minerals in are elongated parallel to the direction in which slippage oc curred during folding. The a lineation is generally perpen dicular to hinges of local folds. The elongate minerals showing the lineation are muscovite, sillimanite, and, more rarely, tourmaline and biotite. At a few localities, faint slickensides can be discerned parallel to the a lineation. The second type of mineral lineation consists of alignment of sillimanite and

(or) elongate flakes of muscovite and occasionally tourmaline parallel to local fold axes. This b lineation is parallel to

the axes of any crinkles present. The development of b linea

tions may be related in part to stretching parallel to the axes of folds having variable directions and amounts of plunge. 150

Most of the rocks exhibiting a and b lineations have an

S2 foliation, that is schistosity is parallel to bedding. The

S2 foliation is presumed to have resulted from nearly hori zontal compression coupled with heating from below. The a and b lineations are closely related to folding that occurred during the same compressive episode. Because the a and b lineations are both commonly shown by sillimanite and muscovite, and be cause sillimanite and muscovite also occur in rocks having no lineation, conditions producing high-grade metamorphism ap parently persisted through the folding of the metasediments.

This implies that the rocks of the Suncook quadrangle were deeply buried during the Acadian deformation, a conclusion reached independently in the discussion of the origin of the

Massabesic Gneiss.

Minor Folds

Folds small enough to be seen in a single outcrop occur at many locations in the Littleton Formation and in the

Massabesic Gneiss. In the latter, the folds are generally dis- harmonic, having flat limbs and narrow hinges and lacking con tinuity even on small outcrops. Amplitude and wavelength of folds in the Massabesic Gneiss can be measured in dimensions of a few inches or feet. Ptygmatic folds also occur locally, as described in the section on the Massabesic Gneiss and shown in

Figure 17. Minor folds in the Littleton Formation were produced during a single episode of compression and include upright, gentle folds having wavelengths of one to fifty feet and amplitudes on the same order, small chevron folds with wave lengths and amplitudes of a few inches, and overturned to recumbent folds with wavelengths and amplitudes of a few feet.

Good examples of the first type can be seen in Deerfield, along the power line extending east-southeast across the Allenstown-

Deerfield town line. Chevron folds and small recumbent folds occur in the same area. The large outcrop on the 460-foot hill near the west edge of the quadrangle just north of

Dunbarton Road, Manchester, contains good exposures of over turned folds. Drag folds occur at a number of localities and indicate the presence of larger-scale folds that can not be recognized on a single outcrop.

Most of the minor folds in the Littleton Formation plunge gently toward the northeast, but a few plunge to the southwest.

In addition, there are a moderately large number of folds whose orientations are completely at odds with the regional structure.

Because these folds are not systematically distributed or orien ted, they are believed to have resulted from local hetero geneities in .the rocks involved. Small bodies of pegmatite are abundant in the Littleton Formation, and these could act as buttresses against which the forces were deflected during folding. 152

Joints

The strike and dip of joints were measured at numerous outcrops in the Concord Granite and in the Massabesic Gneiss and associated pegmatite and foliated microdine granite. In addition, a relatively small number of measurements were made in the Littleton Formation (which generally does not exhibit good jointing), and in the silicifled rock along the Pinnacle and Hall Mountain-Campbell Hill faults. Two major concentrations can be recognized on the pi diagram plot of the joints (Figure

31). Most of the joints strike between N. 20° E. and N. 35° E. and dip to the northwest or southeast at angles greater than

45°. A second set, less intense than the first, strikes about

N. 35° W. and dips steeply to the southwest or northeast. Joints in the first set are probably release joints resulting from re laxation of the northwesterly-directed compressive stress that produced the folding. The significance of the second set is not known. In addition.to the two main joint sets, most outcrops of Concord Granite display moderately well-developed sheeting approximately parallel to the surface of the ground. The com bination of sheeting and the steeply dipping joints was used to facilitate quarrying operations at a number of places. Examples can be seen at the Baily quarry, about 1 mile east of Suncook, and at several other places between Suncook and Manchester. 02 1-4 4 8 • I PlnC EN T

Figure 31. Pi diagram for joints in the Suncook quadrangle (122 readings). Silicified Fault Zones

Distribution

Silicified rock occurs along several northeast-trending

zones in southeastern New Hampshire, Meyers (1941), Freedman

(1950a} b), Heald (1955), and Billings (1956), have described

some of these silicified zones and suggested that they occur

along faults. The silicified rock occurs as discontinuous

bodies along the faults, a result, according to Meyers (1941, p, 5), of variations in permeability along the fracture zones.

Meyers (1941, p. 5) and Freedman (1950a, p. 479-480) found the

silicified zones to be nearly vertical, whereas Sriramadas

(1966, p. 38) found that silicified zones in the Manchester quadrangle dip 50° SE. He states (p. 74) that they probably follow faults and gives several possible explanations for the mode of emplacement and origin of the silica (p. 68). Thompson et al. (1968, p. 215) described massive quartz in Triassic fault zones in western New Hampshire, and Osberg et al. (1968, p. 245) described silicified zones along faults in the Merrimack

synclinorium of south-central Maine. Cox (1970) described mineralised and silicified fualt zones in a north-trending belt coinciding roughly with the belt of post-tectonic White Mountain

plutons in New Hampshire and Maine.

154 In the Suncook quadrangle, silicified rock occurs in two zones, one striking about N. 55° E., the other N. 45° E. The first zone extends southwest for 10.4 miles from a point one- half mile south-southwest of Spruce Pond (Plate 1). This zone includes prominent exposures of silicified rock at Hall Mountain and Campbell Hill and is here referred to as the Hall Mountain-

Campbell Hill fault zone. Billings' Geologic Map of New

Hampshire (1955) shows this silicified zone to continue for twelve miles southwest of the Suncook quadrangle, ending about

1.5 miles north of Amherst. The second zone extends from the large hill on the west side of Route 28, just south of Suncook, to a point 3 miles to the southwest and includes the prominent hill known as the Pinnacle in Hooksett; this second zone is referred to as the Pinnacle fault zone. The Geologic Map of

New Hampshire shows this zone to continue for at least 15 miles southwest of the Suncook quadrangle.

Silicified rock along both fault zones in the Suncook quadrangle is discontinuous. Eight separate areas of silicified rock occur along the Hall Mountain-Campbell Hill fault; these range from scattered outcrops occurring over a small area to large masses of quartz surrounded by country rock that is criss crossed b y 'innumerable small quartz veinlets. The large masses of quartz range in surface dimensions from 50 by 50 feet to 500 by 1000 feet, the largest being 0.7 miles east-northeast of

Clay Pond, Hooksett. Five large masses o f ‘silicified rock occur 156

along the Pinnacle fault zone in the Suncook quadrangle. The

largest of these, known as the Pinnacle, occurs on the west

side of the Merrimack River in Hboksett, The Pinnacle stands

180 feet above the surrounding outwash deposits of the river

valley and is visible for several miles in most directions.

Silicified rock is the main indicator of both the Hall

Mountain-Campbell Hill and the Pinnacle fault zones. In addition,

chlorite is abundant in the normally high-grade schists of the

Littleton Formation along the Hall Mountain-Campbell Hill fault.

The northeasternmoSt outcrop of silicified rock on the latter is

0.5 miles west-southwest of Beaver Pond. Chloritized schists of

the Littleton Formation occur along the fault trace as pro

jected northeastward through Willow Hill and Meetinghouse Hill.

The presence of chlorite in these areas suggests that the Hall

Mountain-Campbell Hill fault zone extends to or beyond the

eastern edge of the quadrangle, 5*5 miles northeast of the out

crop of silicified rock near Beaver Pond. Continued in this

way, the fault lines up with the Saddleback Mountain fault in

the northwestern part of the Mount Pawtuckaway quadrangle, which

Freedman (1950a, p. 480) proposed to explain changes in lithology, structure and metamorphic grade in schist of the Little

ton Formation just south of Saddleback Mountain. The fault neither crops out nor contains silicified rock. Freedman

traced the Saddleback Mountain fault to a point 1.4 miles cast

of the boundary between the Suncook and Mount Pawtuckaway 157

quadrangles. Further study is needed to determine whether it

extends into the Hall Mountain-Campbell Hill fault.

Shearing and other effects related to the two major faults

in the Suncook quadrangle can be seen in outcrops as much as

0.4 miles on either side of the zone of silicification. The

effects shown depend upon the nature of the country rock. Thus,

sheared pegmatite and binary granite commonly contain bands or

lens-shaped aggregates of quartz and feldspar surrounded by thin

envelopes of fine-grained ragged muscovite. Slickensided joint

surfaces occur locally, and small quartz veins are abundant.

Schists of the Littleton Formation are also sheared and altered

along the fault lines. Chlorite, which is uncommon elsewhere,

is locally a major constituent of such schists. Muscovite and

chlorite form thin coatings around quartz lenses or rods that may represent sheared out recrystallized quartzose layers.

The schists are commonly contorted and exhibit more crinkles along the faults than elsewhere.

Lithology

The quartz masses in the fault zones are generally of

uniform lithology, consisting predominantly of massive milky quartz with local vugs lined with quartz crystals. At many

localities the massive quartz contains angular patches of yellow

ish altered material composed of broken and altered feldspar,

patchy sericite, fine-grained strained quartz, and, locally, large 158

(1 mm) discreet muscovite flakes. The altered material probably represents granitic country rock (pegmatite or massive binary granite) that was sheared and then partly replaced as silica- bearing fluids invaded the faults along the permeable zones.

Locally, as along the west side of Route 93 west of

Pinnacle Pond, fluids invading the faults deposited small quantities of sulfides and other minerals as well as quartz.

Sphalerite, galena, chalcopyrite, and pyrite fill vugs as much as 2 inches across in the quartz. Fluorite and arsenopyrite occur along joints in a few places. The mass of silicified rock on Campbell Hill was prospected for gold, but the only metallic mineral found was a small amount of sphalerite.

A thin section of typical silicified rock from an outcrop on Podunk Road, between Hall Mountain and Spruce Pond (Sample

No. 8-29-la), contains more than 90 percent quartz as uncrushed but strained grains that average 1 mm in diameter, as clusters of finer grains, and as fine (average 0.007 mm) grains in ir regular or streaky patches averaging 0.5 mm across that surround the larger grains. The streaky patches of fine grains are more or less parallel to one another. The large quartz grains show no preferred crystallographic orientation but are commonly elongate parallel to the fine streaky patches. They are anhedral to subhedral, show undulatory extinction, and contain one or two sets of tiny aligned liquid inclusions. Where two sets of inclusions are present, they intersect each other at nearly right angles. 159

Lines of inclusions in a given set are nearly parallel over large areas of the slide, and they generally intersect the streaky patches at angles of 30 to 50 degrees. Feldspar, making up about 3 percent of the sample,is broken and strained, but unaltered. A few grains show distorted "plaid" twinning of microcline. Sericite occurs as patchy concentrations associ ated with muscovite flakes averaging 1 mm across. The mus covite has random orientation. A little fine-grained epidote is also present.

Sample 8-29-la probably represents a granite or pegmatite

that has been sheared and nearly completely replaced by quartz.

The presence of undulatory extinction in the coarse quartz grains indicates that movement continued after deposition of the quartz. Streaky patches of fine-grained quartz may represent zones along which shear was concentrated. Lines of liquid in clusions in the coarse quartz grains may be analogous to Boehm lamellae which "have been attributed to intracrystalline gliding on surfaces inclined at low angles to 0001" (Williams, et al.,

1954, p. 201).

Attitude and Displacement of Faults

Neither the Hall Mountain-Campbell Hill nor the Pinnacle fault is exposed, but the outcrop patterns of the quartz masses

indicate that both faults must be nearly vertical. Where quartz outcrops are closely spaced, topography exerts no obvious in

fluence on their positions. 160

Attempts to determine the sense and amount of displacement on the faults were unsuccessful. Bodies of silicified rock were examined for slickensides and zones of mylonite, but none were found. Silicification apparently occurred in the latest stages of faulting or after the movement stopped. Slickensided joints near the faults are rare and show no systematic orien tation or direction of movement. There are no coherent in dications of drag folding in the bedded rocks of the Littleton

Formation adjacent to either fault. Although crinkling is un usually common in the schists of the Littleton Formation near the faults, the orientations of crinkle axes vary widely. Most of them plunge from N. 55° W. to S. 50° W. at an angle of 30 to

80 degrees. The Littleton Formation is exposed on both sides of the northeast end of the Hall Mountain-Campbell Hill fault zone.

Careful comparison of rocks on both sides revealed no obvious differences in lithology or metamorphic grade.

Faults similar to those in the Suncook quadrangle have been described by Freedman (1950a, p. 479-480) in the Mount Pawtuckaway quadrangle, and by Sriramadas (1966, p. 43) in the Manchester quadrangle. Freedman postulated that the Saddleback Mountain fault, which may be continuous with the Hall Mountain-Campbell

Hill fault, is a reverse fault that is vertical or dips steeply to the northwest, lie suggested that the throw is considerable, bused on a change from biotite-to staurolite-grade motamorphism across the fault (Freedman, 1950a, p. 480)■• Sriramadas noted 161 that the dip of a mass of silicified rock in the Manchester quadrangle coincides with that of axial planes of tight over turned folds in adjacent schists of the Berwick Formation.

He postulated that a couple produced both folding and faulting but neglected to discuss the location, extent, find orientation of the features described (Sriramadas, 1966, p. 43).

Age Relations

The two faults in the Suncook quadrangle coincide with the northern and southern boundaries between the Littleton Form ation and the massive binary Concord Granite. The Hall Mountain-

Campbell Hill fault forms the southeastern boundary of the massive binary granite, and, although there is no direct evi dence for the Pinnacle fault northeast of Suncook, this fault may continue to the northeast in the Suncook River Valley, thus bounding the massive binary granite on the northwest. The faults do not cut the Berwick Formation or the Massabesic Gneiss, but these are both older than the massive binary granite. The age of the massive binary granite is most likely Middle Devonian.

The only rocks in the quadrangle younger than the faults are the mafic dikes and sills. At one locality, on the west side of Route 93 directly west of Pinnacle Pond, Hooksett, two spessartite dikes cut quartz veins in a typical silicified zone. The dikes themselves cross each other and are offset about 15 inches by a small fault at their north end. They are, however, definitely younger than the main faulting. The mafic 162 dikes and sills exposed in the Suncook quadrangle are probably related to other dikes considered by Billings (1956, p. 86, 88) to represent the last phase of the White Mountain plutonic- volcanic series. The White Mountain series has been dated by various authors (Tilton, et. al., 1957;- Lyons, et al,, 1957;

Aldrich, et al.. 1958, Hurley, et al.. 1958; Toulmin, 1961;

Poland, et al,, 1970), and most of the reported dates fall around 180 million years. Thus, direct evidence preserved in the Suncook quadrangle indicates that faulting occurred sometime between the Middle Devonian and Early Jurassic.

At several places, most notably west of Pinnacle Pond and on Campbell Hill, silicified rock in the Suncook quadrangle contains sulfide mineralization. Small amounts of sphalerite, galena, chalcopyrite, arsenopyrite and pyrite as well as calcite and fluorite occur in or are associated with massive or vuggy quartz, Cox (1970, p. D17) described similar deposits in the

Gilmanton quadrangle near Websters Mill, Merrimack County.

Northeast of Hooksett, the Suncook River follows a relatively straight northeasterly course aligned with the trend of the

Pinnacle fault zone. If this trend is projected into the southern part of the Gilmanton quadrangle, it includes silici fied rock mapped by Heald (1955) at Nudds Hill and Websters

Mill. Thus, the Pinnacle fault zone may continue through and include the mineralized silicified rock at Websters Mill. Cox relates the Websters Mill and similar deposits to lead-zinc- silver veins that occur within the Conway granite at Iron 163

Mountain, New Hampshire (Cox, 1970, p. D4). The Conway granite is the youngest of the White Mountain series plutons but is older than the camptonite and other lamprophyre dikes included in the series by Billings (1956, p. 88). Assuming that the

Pinnacle fault zone is related to the other mineralized silicified faults described by Cox, and remembering that the silicified rock west of Pinnacle Pond is intruded by spessartite dikes, the Pinnacle fault zone can be tentatively assigned an early Jurassic age. The Pinnacle fault, and probably also the

Hall Mountain-Campbell Hill fault, can therefore be related to the latest phases of intrusive and extrusive activity that pro duced the White Mountain plutonic-volcanic series.

Other Faults

A possible third fault zone, between and approximately parallel to the Hall Mountain-Campbell Hill and Pinnacle faults and about 1,1 miles northwest of the former, is shown on Plate 1.

Along the line shown there are a number of outcrops of sheared pegmatite and massive binary granite similar to those occurring near the Hall Mountain-Campbell Hill fault zone. Slickensided joints striking between N. 50° E. and N. 83° E. and dipping from

27° to 40° northwest occur in these outcrops. In all of the small faults examined, the northwest side moved down relative to the southeast side. Evidence indicates that this possible fault may extend northeast to a point 0.8 mile northeast of

Bear Hill Pond, whore an outcrop of sheared massive binary granite occurs. Evidence for the fault is confined to the area of outcrop of massive binary granite, and, because of widely- separated and poorly-exposed outcrops, the existence of the fault is questionable. 165

SUMMARY AND CONCLUSIONS

Environment of Deposition of the Metasediments

The metasediments in the Suncook quadrangle belong to the

Berwick Formation or the Littleton Formation. These two form

ations have a combined thickness of at least 26,000 feet in some

parts of New Hampshire and about 21,300 feet in the Suncook

quadrangle. The age of the Berwick Formation is probably Late

Silurian; that of the Littleton Formations is probably Early

Devonian. Considering the thickness of these formations and

the relatively short time in which they were deposited, it is

apparent that southeastern New Hampshire was a site of very

rapid sedimentation in the period after the Taconic orogeny.

Both the Littleton Formation and the Berwick Formation

consist of a monotonous sequence of thin-bedded schist contain

ing no traceable marker horizons and apparently deposited as

flysch in a eugeosynclinal environment, Graded-bedding, cross bedding, and channels occur locally in the Littleton Formation, and thin beds or lenses of lime-silicate rock and quartzite consititue only a small percentage of the total volume of meta-

sediments, Although recognizable turbiditcs and volcanic

sediments are absent in the Suncook quadrangle, Billings (1956,

p, 26-31) described a number of localities elsewhere in New

Hampshire where the Littleton Formation contains fairly abundant 166 volcanics. Considering the great thickness and uniform char acter of the sediments, the writer believes that the Silurian and Devonian rocks of the Suncook quadrangle were laid down in a deep-water environment. Rapid deposition suggests that the source areas underwent active deformation and uplift as the sediments were being deposited. The relatively fine texture suggests either that the sediment source was distant from the basin of deposition or that the source area was itself under lain by large quantities of fine-grained material that did not resist mechanical breakup during weathering and erosion. The sediments were apparently dumped fairly rapidly into the area now occupied by the Merrimack synclinorium and the Rockingham anticlinorium.

Source of the Metasediments

Billings (1956, p. 151) believed that the Ordovician,

Silurian, and Devonian metasediments of New Hampshire were derived from "... a large landmass, Appalachia, lying in what is now the Gulf of Maine. . ." . Later workers (especially

Berry, 1968, and Boucot, 1968) synthesized large quantities of new stratigraphic data for the northern Appalachians demon strating complex spatial and temporal relationships between land and water where a single "Appalachia" once sufficed to explain everything.

Boucot (1968, p. 86-88) described Silurian lithofacies and paleogeography for the northern and maritime Appalachians. The part of southeastern New Hampshire that includes the Suncook quadrangle was in a belt of thick marine clastic sedimentation throughout the Silurian. The main source for these sediments was a land area that covered northwestern Maine, western New

Hampshire, Vermont* western Connecticut and Massachusetts* and eastern and central New York during the lower Llandovery Stage.

This land area was uplifted during the Taconic orogeny. By the Ludlow Stage, the Taconic land mass had shrunk to a com paratively narrow belt in western Connecticut, Massachusetts, and Vermont, extreme eastern New York, and eastern Quebec.

Most of southeastern New Hampshire remained a site of marine clastic sedimentation, although an area of combined volcanic activity and marine deposition followed the present New England coast. Thus, at the time of deposition of the Berwick Formation, the nearest substantial source area for clastic sediments was the Taconic belt. Although volcanic islands occupied the coast of New England, there was no great ’'Appalachia" in the Gulf of

Maine.

Boucot (1968, p. 92-93) believes that, by Helderberg and

Oriskany times, the Taconic land area had been completely eroded away in the northern Appalachians. Thus, there would be no nearby westerly source for the thick flysch-type sediments of the Littleton Formation. Boucot suggests that a belt of highlands extended through eastern Rhode Island and Massachusetts, extreme southeastern New Hampshire, southeastern Maine and New 168

Brunswick, Prince Edward Island, and northern Nova Scotia*

This landmass extended to some undefined point far off the present coast of New Hampshire (Figures 32, 33)* Its western margin included the part of southeastern New Hampshire now occupied by Silurian metasediments and a portion of the Fitch burg pluton. To the west of Boucot1s Devonian landmass is an area of slate, siltstone, and argillaceous sandstone that in cludes the Littleton Formation of the Suncook quadrangle (Fig ures 32, 33). Thus, Boucot*s reconstruction postulates a southeasterly source area for the Littleton Formation* The

Acadian orogeny, which Boucot (1968, p. 92) dates as mid-

Middle Devonian, ended marine sedimentation in most of the northern Appalachians.

Other workers dispute Boucot*s interpretation of Appa lachian Paleozoic paleogeography. Bird and Dewey (1970), in particular, suggest that the Lower Devonian clastic sediments that include the Littleton Formation came from the Taconic orogenic belt to the west. The Bird and Dewey hypothesis is described in detail in the following section. Suffice it to say here that Bird and Dewey do not recognize the existence of a major landmass in what is now coastal New England until the

Middle Devonian, when Africa approached and then collided with

North America. This collision supposedly produced the Acadian orogeny.

It is not possible at this time to reconcile reconstructions like that of Boucot with the Bird and Dewey hypothesis, at least 169

■S f Siltstene

Figure 32. Lithofacies and paleogeography of the middle and upper Helderberg Stage in the northern Appalachians (Boucot, 1968, p. 92). 170

Ar**>

M t * \ir'Cen3lomtr*tC

Figure 33. Lithofacies and paleogeography of the Oriskany Stage in the northern Appalachians {Boucot, 1968, p. 93). for events that occurred in the northern Appalachians after the

Taconic orogeny. Regional sedimentation patterns are too poorly known to clearly support one hypothesis over the other.

Sedimentary structures indicating a consistent direction of transport are not present in the post-Taconic metasediments of the Merrimack synclinorium and Rockingham anticlinorium. Facies relations are also confusing. Although rocks of the eugeosyncline apparently grade westward through the Connecticut

Valley-Gaspe synclinorium into miogeosynclinal rocks (Cady, 1969, p. 110), there is also evidence for a second miogeosyncline southeast of the eugeosyncline. Cady (1969, p. 30) noted that the existence of Precambrian (?) and Cambrian quartzite and limestone in eastern Massachusetts and Rhode Island suggests

Early Paleozoic miogeosynclinal deposition in that area, but the areal and temporal extent of this southeasterly zone has not yet been fully defined. Even if a southeasterly sequence of miogeosynclinal sediments is eventually well established, it will be difficult to prove that this sequence was attached to North

America at the time of its deposition. Considering that mio geosynclinal sediments are most likely deposited on continental crust, the southeasterly shelf sequence might overlie a fragment of African continental crust added to North America in the

Acadian orogeny and left behind during the re-opening of the

Atlantic in the Mesozoic Era. Geologic History and Regional Synthesis

Previous Work

Since the development of the geosynclinal concept in the middle nineteenth century, there has been general agreement

that at least some parts of the Appalachians owe their origin

to the deformation of a geosyncline. However, workers in

North America and Europe developed divergent viewpoints on the nature of geosynclines. The issue of what constitutes a geo syncline became so muddled by individual prejudice that some workers (e.g. Evans, 1926, p. lxxv) suggested that the word might best be abandoned. The basic problem was that most of

the early efforts were concentrated on relatively small parts of geosynclines. Until a large number of such studies had been produced, it was not possible to recognize the whole range of (9 characteristics to be found in a single geosyncline. Stille

(1936, 1941J helped to unify the geosynclinal theory by re cognizing that orthogeosynclines consist of a miogeosyncline- eugeosyncline couplet. However, at that time no such couplets were recognized in the process of formation.

The inablility of early workers to recognize modern geo synclines resulted in part from misconceptic about what a modern geosyncline would look like and in part from a lack of data on the thickness and character of sediments along 172 continental margins. Drake, Ewing, and Sutton provided new insight into the problem in 1959. Using oceanographic data collected in the 1940's and 1950's, Drake, et al. (1959) demonstrated the existence of a sedimentary complex comparable with Stille's orthogeosynclines on the eastern margin of North

America. They attempted to show that the sediments accumulating on the present continental shelf and rise are analogous to those in the Appalachian miogeosyncline and eugeosyncline respectively, as defined by Kay (1951). Drake, Ewing, and Sutton recognized that, although they had solved the problem of where to find a modern geosyncline, there remained the even more puzzling questions of how and why such structures become deformed to produce mountain systems like the Alps and Appalachians. They noted that "Although the continental margin described. . . bears many of the attributes of earlier geosynclines, it cannot become a mountain system unless sufficient energy can be supplied to convert it", and "The key to the causes of orogensis lies in the heat problem. Once the sources of the heat are determined and the method by which it is focused on small areas established, the nature of orogenic movements should be revealed" (Drake, et al., 1959, p. 191, 189).

One possible energy source for orogensis became apparent shortly after Hess (1962) published his hypothesis of sea-floor spreading. Dietz (1963) and Dietz and Holden (1966) modified

Drake, Ewing, and Sutton's concept of the nature of the eastern continental margin of North America. Dietz (1963, p. 322)

suggested that the continental-rise prism is ensimatic and that

the boundary between continental and oceanic crust occurs some

where beneath the continental slope. Sinking of the continental

margin results mainly from the deposition of thick continental-

rise sediments on thin oceanic crust. Because the oceanic and

continental crust both overlie a rigid layer of upper mantle

(lower lithosphere), loading of the oceanic crust with sediments

causes a regional depression of the lithosphere which includes depression of the continental margin. The result is deposition of an ensialic wedge of shallow-water sediments (miogeocline)

thickening toward the ocean and an ensimatic wedge of deep-water sediments (eugeocline) thickening toward the continent. In the case of deformed geosynclines, such as the Appalachians, the continental-rise sediments are deformed and plutonized as a result of sea-floor spreading and are shoved against the mio geocline, which undergoes relatively loose folding and thrust faulting (Dietz and Holden, 1966, p. 568).

Dietz and Holden did not attempt a detailed explanation of how sea-floor spreading may have activated the deformation of geosynclinal sequences in the past. However, in a series of papers beginning in 1969, J. M. Bird and J. F. Dewey, among others, have dealt with this problem in detail. Bird and

Dewey believe that energy is concentrated in the relatively narrow orogenic belts through processes related to subduction. 175

They accept the Drake, et al., model of geosynclinal development on stable continental margins, as modified by Dietz. They also believe that areas of geosynclinal subsidence on stable margins are prone to become areas of subduction. The reason for the development of subduction zones is not known. However, sediment loading on the relatively thick dense oceanic lithosphere adjacent to the continent may be partly responsible (Dewey and

Horsfield, 1970, p. 523-524). It is also likely that some stable continental margins, including parts of the east coast of North

America, were at one time sites of subduction zones that have since healed or become inactive. Such areas might remain weak despite their lack of activity.

The hypotheses of Bird and Dewey that relate sea-floor spreading and mountain building are summarized in a number of papers (Dewey, 1969a, Dewey and Bird, 1970). The same authors have also described the evolution of the Appalachians in terms of sea-floor spreading (Dewey, 1969b, Bird and Dewey, 1970,

Bird, 1974). Their work, which concentrates on the evolution of the northwestern Appalachians, suggests that after a period of sedimentation on a stable continental margin similar to the present east coast, a subduction zone developed on the seaward edge of the pre-Taconian continental rise (Bird and Dewey, 1970, p. 1045; Figure 34). Deformation and metamorphism of the early

Paleozoic sequence began with the Late Cambrian and Barly

Ordovician Penobscot orogeny and culminated in the Taconic 176

Z o n * fl Ton*. C z

✓« N —✓

\ r«e oni 'ao^ H uni ^ «r i «n J(^ u en e4l

LateVV Ord,-E. —U| WAAWASilur. • s ' V". X' " " "nV

Mid.-Ord.

> '**-/-/ - N % “ (it(io»/>(*«r«

Late € /- T / " ''-','7 iVj/ ^ >*• / I

\ cU»tie», j \ M(e«ntd 1 / Late Pre ■€ Vf 'A v ,/,'-// ’Ver«»t ' x' " -/ - -/ s s * ^ ~ r s - ^ -i //»f*

Figure 34. Sketch showing history of proto-Atlantic Ocean and surrounding continents from late Precambrian through Late Devonian time (Bird and Dewey, 1970, Figure 9). 177

orogeny of Middle and Late Ordovician age. The Taconic de

formation was concentrated in what is now the northwestern part

of the northern Appalachians, in a mobile belt that Dewey

(1969b, p. 124) called Zone A (Figures 34, 35). Deformation,

uplift and metamorphism of the Zone A continental rise pro

duced an orthotectonic orogen that shed sediments and gravity

slides toward the west. The Taconic allochthons consist of

Ordovician continental-slope and -rise sediments overlying

continental-shelf (miogeocline) sediments of Logan's zone.

The gravity slides probably originated in the area now occupied

by Grenville basement blocks that were thrust up in Zone A

during the Taconic orogeny. Zen (1968, p. 136) divides the

emplacement of Taconic allochtons into two stages. The materials of the first slices were apparently still soft when emplaced, but

those of the second group behaved in a brittle manner.

During and after the Taconic deformation, sediments derived

from the orthotectonic orogen of Zone A spread both toward the

west and toward the Ccist* The Taconic and post-Taconic sediments

to the east of the orogen are ensimatic deep-water flysch de

posited on older sediments in a trench. This trench represents

an area where consumption of oceanic lithosphere occurred during

the Taconic orogeny and continued until the Acadian orogeny. It

contained Cambrian to Lower Devonian sediments that were sub

sequently deformed during the Acadian orogeny. This second

mobile belt, east-southeast of Zone A was called Zone B by 178

Figure 35. Approximate boundaries between major tectonic zones in New England, as defined by Bird and Dewey (1970, p. 1033). 179

Dewey (1969b, p. 124). The boundary between Zone A and Zone B in New England is difficult to locate accurately because Acadian deformation affected both zones. The boundary probably follows a supposed volcanic island arc that includes the gneiss domes of the Bronson Hill anticlinorium in western New Hampshire, Massa chusetts, and Connecticut (Thompson, et al., 1968, p. 217; Bird and Dewey, 1970, p. 1033) (Figure 35).

The proto-Atlantic Ocean continued to contract during the

Silurian and Early Devonian, eventually closing in the Middle

Devonian. The closure brought northwestern Africa against north eastern North America, resulting in deformation of the sediments of Zone B and re-deformation of the sediments of Zone A (Bird and Dewey, 1970, p. 1048). The Devonian deformation, the

Acadian orogeny, is the main metamorphic and tectonic event affecting the rocks of Zone B.

Dewey (1969b, p. 124-125) recognized a third mobile belt southeast of Zone B; this he called Zone C (Figure 35). This belt underwent widespread deformation just before the Cambrian

(Hadrynian) and again during the Acadian orogeny. Bird and

Dewey (1970, p. 1049-1050, and Figure 9B) suggest that Zone C is a fragment of African sial welded onto the North American con tinent during the final Acadian contraction of the proto-

Atlantic and left attached to North America when the Atlantic

re-opened in the Mesozoic. Wilson (1966) suggested essentially

the same thing, based on the distribution of Lower Paleozoic 180

faunal realms in the northern Appalachians, western Europe, and northwestern Africa. He placed the boundary between the

African and North American parts of the northern Appalachians along a system of faults that include the Honey Hill fault of eastern Connecticut, a fault in east-central Massachusetts and southern New Hampshire (mapped by Novotny, 1961, p. D48), and the zone of silicified faults in southeastern New Hampshire and southwestern Maine that'includes the Pinnacle and Hall

Mountain-Campbell Hill faults of this report. Dewey (1969, p. 125) places the boundary between Zones B and C several tens of km east-southeast of this group of faults.

The culminating deformation of the rocks of Zone B occurred during the Acadian orogeny of Middle or Late Devonian age. The

Bird and Dewey hypothesis suggests a collision between Africa and North America as the cause of this deformation. However, there is widespread evidence forlater, comparatively minor, activity in Zone B and elsewhere in the northern Appalachians

(see, for example, Lyons and Faul, 1968). Bird and Dewey (1970, p. 1051) relate evidence for Carboniferous, Permian, and Triassic activity to the initiation of processes that eventually caused the opening of the present Atlantic Ocean. They suggest that a new spreading center developed under the lower and middle Paleozoic orogenic belt near the junction between the African and North

American plates sometime after the Acadian orogeny. The

Alleghenian deformation resulted from heat and mechanical energy 181

given off during the development of this spreading center. The

Triassic basins of the eastern seaboard also owe their origin

to tensile stresses that caused a nocking of the crust prior

to initial opening of the present Atlantic Ocean.

Application to the Suncook Quadrangle

According to the Bird and Dewey hypothesis, all of south

eastern New Hampshire, including the Suncook quadrangle, lies

in the Zone B orthotectonic orogen defined by Dewey. The

structure and stratigraphy of the Suncook quadrangle support

this view.

The oldest rock unit exposed in the Suncook quadrangle is

the upper member of the Berwick Formation. Structural and

stratigraphic relations in other parts of New Hampshire and

New England suggest that older units probably underlie the

Berwick Formation but are buried under it and the thick sequence of Lower Devonian Littleton Formation of the Merrimack syncli- orium. Rodgers (1970, p. 107) noted that the Ordovician-

Silurian contact may be exposed in the Rockingham anti-

clinorium southeast of the synclinorium. The lowest strati

graphic unit of southeastern New Hampshire and soutlwestern

Maine is the Rye Formation, which Billings (1956, p. 105, 150)

correlated with the Ordovician Ammonoosuc volcanics of western

New Hampshire. The Rye Formation is overlain by the Kittery,

Eliot, and Berwick formations, which constitute the Merrimack 182

Group of Billings (1956, p. 43). The contact between the Rye

Formation and the Merrimack Group is probably a disconformity

(Hussey, 1968, p. 294), but the stratigraphic and age relations

are uncertain because of the high degree of deformation and

metamorphism and lack of fossils. There is no direct evidence

that the Rye, Kittery, and Eliot formations occur in the subsur

face of the Suncook quadrangle, but the structural relations in

southeastern New Hampshire make such a conclusion plausible.

The upper member of the Rye Formation consists of felsic

metavolcanics and metagraywackes. Thus, in the Ordovician,

the Suncook quadrangle was probably in or close to the site of

extrusive activity related to the Taconic orogeny. Volcanism

occurred in a northeast-trending belt from northern Massachusetts

to New Brunswick and Quebec during most of the Middle Ordovician

(Berry, 1968, p. 29-31). The Rye Formation may represent part of

a volcanic island-arc system formed by subduction of proto-

Atlantic lithosphere near the edge of the continent. While vol

canic activity migrated landward from the distal part of the

Ordovician continental rise, the continental slope area was up

lifted, eventually producing the gravity slides, thrusts, and

sediments of the main belt of Taconic deformation. Sediments shed

from the Taconic belt were deposited in the eugeosynclinal region

to the east. In addition, the belt of volcanic islands to the

southeast was apparently uplifted to produce some of the quartz-

ites and graywackes that are common in the lower units of the Merrimack Group. The sediments of the upper part of the

Merrimack Group and the Littleton Formation were deposited in deep water in an area of subsidence southeast of the Taconic belt. The great thickness of these sediments indicates that the Taconic source areas continued to supply large volumes of sediments through the Early Devonian. The Suncook quadrangle was probably near the site of deepest subsidence and thickest sedimentation. The writer believes that the most likely source of the Upper Silurian and Lower Devonian sediments was to the northwest because no large potential source of sediment is likely to have existed to the southeast prior to the approach of Africa in the Middle Devonian.

In the Middle Devonian, deposition ceased in the Suncook quadrangle and elsewhere in New Hampshire. The Merrimack synclinorium and Rockingham anticlinorium formed at the site of post-Taconic eugeosynclinal sedimentation. During the

Acadian orogeny, the thick sedimentary sequence of the syn clinorium was folded, metamorphosed to the sillimanite grade, and partially migmatized, probably as a result of closing of the proto-Atlantic. The orogenic belts to the west-northwest and southeast (Dewey's Zones A and C) were affected to a lesser degree. Syntectonic intrusives include foliated microcline granite and, in other parts of southeastern New Hampshire, foliated binary granite.

Immediately after the main Acadian deformation, the Concord

Granite intruded metasediments and other rocks over a large area 184 of eastern New Hampshire. Intrusion of pegmatite continued dur

ing this time. Although the foliated and non-foliated granites

arc generally all included in the New Hampshire plutonic series,

their ages and origins differ. Both the late granite and pegmatite may have formed by crystallization of melts derived from older rocks during the Acadian orogeny.

The post-Acadian history of southeastern New Hampshire probably included minor metamorphism during the late Paleozoic, resetting the radiometric ages of rocks deformed or emplaced during the Acadian orogeny. Although structural and stratigraphic evidence suggest a Middle Devonian age for the New

Hampshire plutonic series, Middle Devonian ages are themselves uncommon because the rocks cooled slowly after the orogeny.

Faulting and minor intrusive activity characterized the Late

Triassic and Barly Jurassic of the Suncook quadrangle. All of the late Paleozoic and early Mesozoic activity probably reflects the breakup of the middle and late Paleozoic North

American-African continent and opening of the present spreading- expanding Atlantic Ocean. The faulting and associated minor hydrothermal activity are most likely related to the Palisades tensile episode that affected much of eastern North America in the Late Triassic, The basalt and lamprophyre dikes probably belong to the Early Jurassic White Mountain plutonic-volcanic series. Actual opening of the north Atlantic Ocean is now believed to have occurred between Early Jurassic (about 180 mil.lion years ago) (LePlchon and I'ox, 1971, p. 6302; Pitman, 185

et al., 1971, p. 199) and Middle Cretaceous times (McConnell,

1969, p. 1779).

As described earlier, various workers have suggested that

the Atlantic Oecan opened in such a way that a fragment of pre-

Acadian African crust was left welded to North America after

the breakup. Wilson (1966, p. 679) clains that the fault

system that he used as the boundary between the African and

North American parts of the northern Appalachians separates

faunal realms and areas of grossly different lithology. He

notes that the faults "... lie along the line which separates

those formations which underlie the greater part of New Hampshire

from a suite of completely different formations, underlying south

eastern New Hampshire." However, in the Suncook quadrangle, rocks of the Pittsfield member of the Littleton Formation are easily

recognizable on both sides of the Pinnacle and Hall Mountain-

Campbell Hill faults. Therefore, on a strictly lithologic basis,

Wilson’s location of the boundary must bo rejected, at least in

southeastern New Hampshire. Dewey placed the line of junction between his Zone B and Zone C in such a way that it would nearly correspond with Wilson's boundary in Connecticut. However, the

line swings to the east of Wilson’s boundary in Massachusetts and the rest of New England. In fact, the boundary between

Dewey's Zone B and Zone C crosses a part of the present contin

ental shelf off the coast of New Hampshire and Maine. Therefore,

in Dewey's (1969, p. 125) and Bird and Dewey's (1970, p. 1033) 186 reconstructions, all of southeastern New Hampshire is a part of the paratectonic orogen of Zone B. It is interesting to note that this reconstruction does not conflict with the faunal evidence used by Wilson in his reconstruction. All of the

"Atlantic-realm" fo&sil localities used by Wilson (1966, p. 680) are in eastern Massachusetts, in an area included in Dewey's

Zone C. Therefore, the writer is inclined to accept the Bird and Dewey version of the separation of Africa from North America. 187

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t : )

1 fv £ 1*1 T4. a

y y y S >\ f '•/? - k .?M v>. / V.y

^ 0 / Y { > x C'-Vv «3 w m m ^ m

\ 43°15 *

EXPLANATION

u VtM tc< a< tc w Mafic Dikes * Small, discontinuous, dark^cDlorad dlhat of basalt, diabast, andistt*. and ipassartita. M oit a rt aphanitlc and many a n porphyrltlc.

uuM K UJ v» u Pegmatite A' While pegmatita composed ol microcllne, oligoclate. and quarti with variable amounti of mutcovite. biotiia. tourmaline, and othar acceitory minaralt.

uiK I VI Q. s x< ui> 04 UI£ X Concord Granila While to buir.m aiiive or 21 ' foliated, medium-grained binary granite, commonly with aubhedral or ouhadral m uicoviti.

ft 10 ’ ^ 1 - mgn \ ft

Maiiabaiic Gnalii White.pink, or gray coarsa-gtainod gnain containing microclina, oligo- claia, q u a rti. biotite. and muacovila. Magnatlta is a common accassory mineral, Diacontinuoua sills and dikes of pink, fine-grained foliated microclina granite and pink microclina pegmatita are common. V. r

Jenntas Pond Mombar

Chiefly ailvery-gray to rutty, medium-grained, thin-bedded muscovite- 07 a schist, commonly with silllmnnite and almandits. Minor rock < typos include silvery or rutty, fine-grained,crinkled mica aehist, sil S cc very, lint-grained mica gneiss, and lim s-silicata granulite. r V i •

w v & L H. / '-I 'H- •'V

. rxS i S3P»(M; A,

nTSii i. ' I -Ls- .*( 1 f S*./ * '■**. /

3t& >. * * m

N i—j. v Ax ^ . yo,rS.-^%;v v**»7'i — : is *\

j ^ v k < f ':■ • ' H ^ ' -i*r v**v , • • jobsfi .• **1 y T f t f

4 !!■

■fs . V *5 * SSP V f t sds&i M ^ 4 ' « \>-

V i r r ^

St(i*$f5$\r .jr * 5 ^ «p. ' 1 . T felcXlPrhUOrtK i ofVL 5

■ i :

k i

i 3?

^ K : i 1 F ^ T 3

a & u i««^tif v^MhCf m data, quarti, blotita, and muacovlta. Magnetite la a common accattory mineral. Ditcontinuott* cilia and dlfcet of pink. fine-grainad foliatad microclina granlta and pink microclina pegmatita are common.

Jennoit Pond Mamfaar

Chiefly ailvary-gray to rutty, medium-grained. thin-baddad muscovite- biotita schist, commonly with ailllmanita and almandita. Minor rock typaa include silvery or rutty, finu-gralnad,crinkled mica schist, sit- very. line-grained mica gnaitt. and lim e -iilic a tt granuiito.

Dip

PittaUald Member Chiefly gray to rutty-brown medium-grainad quartz-mica ichiit and gnaitt with tmail pod* and thin tayart of quarti and faldtpar. Com monly contain* almandita and tillim an ita. Minor rock typaa Include light gray fine-grained quartz-plagioclaie-biotita tchittota gnaiii, lime- tillcata granuiito, dioptida-hornblende gnaitt, and quartiito. Miitd tym- bol rtpratantt gradation between Olp and mgn.

H's'',''

Barwick Formation. Upper Member

Purplith-gray fine- to medium-grained quartz-plagioclata-biotita schist with fight gray to groan fine-grained lim e -tific a tt granuiito.

SYMBOLS

Contact approaimataly located

Inferred fault

ti ’inferred fault with aillcifitd tone

Strike and dip of btdding

X Strike of vertical bedding

& Strike and dip of overturned bedding

Horiiontal bedding

p , Strike and dip of claavaga 71 ° 30 ^ SCALE 1:62.500 ^

Approximate mun Dielinition. 1957

S.L.

GEOLOGIC MAP AND STRUCTURE SECTIONS ' - < j r

tHhfhh

B 71 15 SCALE 1:62.500 MILES

Ouadranglo Location

mgn

/ / ^ /-?&✓

URE SECTIONS OF THE SUNCOOK 15' QUADRANGLE. m m * ,

i t Inlarrad fault with illicifiad ton*

Strika and dip of bidding

X Sltika of vortical badding

& Strika and dip of ovarturnad badding

-f* Horiiontal bidding

60, y Strika and dip of claavaga

_ 4 3 ° 0 0 ' 71° 1B' y Strika ol vortical claavaga

* Strike and dip ol claavaga with bearing and plunga ol linaalion

'y Claavaga parallal to badding

Quarriei. m oitly abandonid

Sbu — VV.l'VA'Y S.L.

VAA'Afl B'

GEOLOGY SURVEYED BY C.R.CARNEtN.I96B-E9. RANGLE. NEW HAMPSHIRE DRAFTED BY C.R. CARNEIN. 1976.