Precambrian Rocks of the Lake Hopatcong Area, New Jersey

DAVIS A. YOUNG Department of Geology, Washington Square College of Arts and Science, New University, New Yor{, New Yor\ 10003 Precambrian Rocks of the Lake Hopatcong Area, New Jersey

ABSTRACT

Precambrian rocks near Lake Hopatcong, New Jersey, form a part of the intensely deformed and metamorphosed Reading Prong. The Lake Hopatcong area is divisible into several northeast-trending fault blocks, each of which contains a mappable stratigraphic sequence of paragneisses and granitic or syenitic rocks. The paragneisses generally are well foliated and well layered. They consist chiefly of biotite- feldspar-quartz gneisses and quartz-oligoclase Figure 1. Geographic setting of the Reading Prong. leucogneisses that are interpreted as meta- Stippled area represents Precambrian crystalline rocks of morphosed potassium-rich sandstones and the Prong. Dashed area locates the Lake Hopatcong quartz keratophyres, respectively. A thin well- region of New Jersey. foliated unit of biotite-plagioclase gneiss is line rocks that extends from the Connecticut- thought to be a metamorphosed sill of gabbroic New York boundary to the vicinity of Reading, anorthosite. Pennsylvania (Fig. 1). To the northwest of the The granitic and syenitic rocks generally Prong are complexly refolded lower Paleozoic form thick, regionally concordant sheets. They sedimentary rocks (Drake, 1969; Epstein and are typically foliated and are composed chiefly Epstein, 1969), and to the southeast are chiefly of microcline microperthite and plagioclase (or Triassic sedimentary rocks of the Newark mesoperthite), quartz, and iron-rich hornblende Group, basalts, and diabase sheets and dikes. and clinopyroxene. These foliated granitic and Comparatively little is known of the geology syenitic rocks are viewed as syntectonic of the Prong, although several recent studies magmatic intrusives. One regionally discordant, have commented on the gross structure and unfoliated sheet of clinopyroxene quartz major rock types of various parts of the prov- syenite is probably a late tectonic magmatic ince. The work of Hague and others (1956), intrusive. Sims (1958), Buddington and Baker (1961), Mineral assemblages in Lake Hopatcong Drake (1969), and Young (1969) in New paragneisses may be assigned to the hornblende Jersey, of Buckwalter (1959, 1962) and Drake granulite subfacies of metamorphism. The (1969) in Pennsylvania, and of Hotz (1953), presence of Ca-bearing mesoperthite in biotite- Dodd (1965), Helenek (1965), Omeld (1967), feldspar-quartz gneiss indicates that meta- and Murray (1968) in New York has demon- morphic temperatures exceeded 700° C, and strated that the Prong consists of several the assemblage garnet-sillimanite-quartz with- parallel, northeast-trending fault blocks, each out cordierite indicates that load pressure was of which contains a stratigraphic sequence of greater than 2.5 kb. The rocks have thus prob- gneissic units that characteristically have been ably been buried to depths in excess of 10 km. deformed into northeast-plunging isoclinal to open folds. The gneissic sequences commonly INTRODUCTION include thick, homogeneous sheets of foliated The New Jersey Highlands form a part of the granitic and syenitic gneiss of probable in- Reading Prong, a belt of Precambrian crystal- trusive origin. The general geology of the

Geological Society of America Bulletin, v. 82, p. 143-158, 7 figs., January 1971 143

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northern part of the New Jersey Highlands well-foliated, well-lineated, and generally lay- recently has been summarized by Smith (1969). ered feldspar-quartz paragneisses and horn- The present paper presents new field, blende- or biotite-plagioclase gneisses that form chemical, and mineralogical data from the Lake open folds plunging about 22° N., 36° E. The Hopatcong, New Jersey, region and evaluates gneissic sequence overlies hornblende-bearing metamorphism and magmatic activity in the granitic gneiss and is partly transected by a Reading Prong from these data. broadly folded sheet of clinopyroxene quartz syenite. GEOLOGIC SUMMARY OF THE LAKE Although numerous stratigraphic units con- HOPATCONG AREA tinue along strike for several miles within each The Lake Hopatcong area of New Jersey block, it has not yet been possible to correlate (Fig. 1) is located about 40 mi west-northwest units from one block to another or to determine of New York City. It is situated just west of the top and bottom in any one block (Fig. 3). Green Pond syncline, a strip of Silurian and Devonian sedimentary rocks that is infolded PETROLOGY OF THE GNEISSES and infaulted into the Precambrian Highlands and divides them into two roughly equal parts. Biotite-Feldspar-Quartz Gneisses Major geological features are portrayed in The Lake Hopatcong area contains several Figure 2, and stratigraphic columns from lithologically heterogeneous units that consist each of the four major fault blocks in the area chiefly of well-layered, distinctly foliated are presented in Figure 3. biotite-feldspar-quartz gneiss interlayered with Block A contains a group of paragneisses lenses of amphibolite, hornblende-quartz- forming isoclinal folds that are overturned to feldspar gneiss, and biotite-quartz-plagioclase the southwest and plunge about 33° S., 64° E. gneiss. The units range from 100 to 1500 ft in Interlayered with these rocks are thick, homo- thickness, and individual layers typically are geneous, concordant sheets of hornblende- 0.5 to 4 in. thick. The dominant rock in these bearing granitic gneiss and hornblende-bearing units is xenoblastic or lepidoblastic gneiss that syenitic gneiss. contains, in rough order of decreasing abun- Block B contains two units of the eastern dance, quartz, microcline microperthite, plagi- limb of the Beaver Lake antiform that was oclase, and biotite. Common variations of this mapped in detail to the north by Buddington mineral assemblage and typical modal analyses and Baker (1961). The antiform, a large are listed in Table 1. isoclinal fold that plunges 25° N., 35° E. and is About 5 percent of well-oriented biotite slightly overturned toward the northwest, is flakes are randomly distributed throughout defined in the Franklin and Hamburg quad- biotite-feldspar-quartz gneiss, although locally rangles by a series of quartz-microcline and the biotite, as well as garnet, blocky sillimanite, quartz-plagioclase gneisses that is regionally and graphite, is concentrated into 1-mm-thick conformable with sheets of hornblende-bearing seams parallel to compositional layering and granitic gneiss and clinopyroxene-bearing sy- mineral foliation. Mesoperthite, with host enitic gneiss. In the Lake Hopatcong area, a oligoclase (AnirAn22), is quite common, but body of indistinctly foliated quartz-oligoclase discrete grains of oligoclase and highly perthitic leucogneiss that may exceed 2000 ft in thick- microcline coexist in most rocks. Minor ilmenite ness is exposed in the apparent core of the fold. is present locally, and zircon forms exceedingly Overlying the leucogneiss unit, and locally small (0.01-0.05 mm), nearly spherical grains. transecting foliation in it at a low angle, is a The pronounced layering, wide lithologic sheet of clinopyroxene-bearing syenitic gneiss. variations, high quartz content, abundance of Block C consists of clinopyroxene-bearing potassium feldspar, low mafic mineral content, amphibolite and biotite-feldspar-quartz gneiss and local occurrence of sillimanite and garnet units interlayered with thick sheets of clino- in the predominant gneisses of the units similar pyroxene-bearing granitic gneiss. The rocks to A-2 and A-3 (Table 1) generally are sug- are cast into a series of northeast-trending gestive of derivation from potassium-rich sand- isoclinal folds that alternately plunge to the stone parent rocks. Such biotite-feldspar-quartz northeast and southwest and are overturned gneisses comprise about 40 percent of the to the northwest. Compositional layering in all paragneisses of the Lake Hopatcong area. units is transected by axial-plane foliation near Similar rocks are very common elsewhere the noses of minor folds. throughout the Reading Prong. Field and Block D consists of more than 5000 ft of petrographic data suggest that microcline

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/ hg /'hqo/ hg / bfq y^r

LEGEND

RWAGNEISSES

DioptidB-scapolte-feldapar-quartz

bfq Biotite-feldipar- quartz gntie

Biotite-plagioctot* gntitt

am Amphibolit*

hqo | Hornblende- qjartz-otifoclas*

'qo "I Quartz-oligoclas* lBucogn«l«t

boqp Bkitite-orthopyroxena-quartz-plagiodaM gn*it»

i u Undlfftrcntiatad gn*iss«t

GRANITIC ANDSYENITIC ROCKS

Hornbltnd* cycnitic gntiss

> / pg Clinowraxtnt granitic gnttu / P19SJS. i / / X P'Xl Oinopyroxcnc syenitic gneiss

~ — "-H pq> ainopyroxtnt quartz ty«nit«

x-.-'' E3./ ^ x X^'T

Gneisilc foliation \ - -/" X.X Lineation Overturned plunging anti form

Overturned plunging synform

Plunging •ynform

Figure 2. Geologic map of the Lake Hopatcong area. Numbered crosses indicate locations of samples discussed in text.

YOUNG, FIGURE 2 Geological Society of America Bulletin, v. 82, no. 1

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/143/3428234/i0016-7606-82-1-143.pdf by guest on 26 September 2021 — «— hg 800 am (basalt) dsfq 1200 (calc sandstone) hs x -l_^~"r- x x . 600 p„g 20O-600 bqo X * bfq (feldspathic » x sandstone) x •"••- :'-'••: * X - " | .*.',. bfq (feldspathic boqp X X : . 1400.;, 400 (groywacke, basalt) 1200 esq * X sandstone) x 1600 x pg •".'.•' ; :.".- hs _^^ X * ji bp x (anorthosite) qo * * ** - 850 (quartz keratophyre, 1000 qm basalt) r-^-rs, •• .•.-•-.'• '. • bfq (feldspathic .'.• 1400' »• I4QQS sandstone) » j am (basalt) •.-.•.^gr^ X * * » * ^ 1IOO VAJvOl pncs am .^ ^M^£ X /^ X X n"/"20(jr^ bfq (sandstone) *" X x K hqo ~ — * X 1000 X (acid volcanics) . — - qo X * X X _^^=^ - 2000 (quartz keratophyre, * 2800 pg X » hg • basalt) X ',., , .

K X X * * X * X

BLOCK A BLOCK B BLOCK C BLOCK 0 Figure 3. Stratigraphic columns of the four major inferred parent materials. Tops and bottoms are no- taken from Buddington and Baker (1961); bqo = fault blocks of the Lake Hopatcong region with ap- where known. Symbols of units are identical to those in biotite-quartz-oligoclase gneiss; esq = epidote-scapolitc- proximate apparent thicknesses (in ft) of units and Figure 2. The four uppermost units in Block B are quartz gneiss; and qm = quartz-microcline gneiss.

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TABLE 1. MODAL ANALYSES (VOLUME PERCENT) AND MINERAL ASSEMBLAGES OF BlOTITE-FELDSPAR-QuARTZ GNEISS*

A-1 A-2 A-3 A-4 Quartz 8.0 63.5 51.3 65.6 Microcline microperthite 70.6 11.9 40.9 2.0 Plagioclase 12.3 15.4 6.0 17.4 Biotite 5.1 8.4 0.4 7.6 Orthopyroxene 2.5 Garnet 0.5 1.3 3.4 Sillimanite 3.4 Ilmenite 6.7 0.1 0.6 Graphite 0.2 Zircon Tr Tr Apatite Tr Tr 99.4 99.7 100.0

" See Figure 2 for sample locations Some common mineral assemblages in'biotite-feldspar-quartz gneiss: 1. quartz-microcline microperthite-plagioclase-biotite-ilmenite-zircon 2. quartz-microcline microperthite-plagioclase-gafnet-biotite-ilmenite-zircon (A-3) 3. quartz-plagioclase-microcline microperthite-biotite-garnet-zircon (A-2) 4. mesoperthite-biotite-quartz-sillimanite-ilmenite-zircon 5. mesoperthite-biotite-quartz-garnet-sillimanite-ilmenite-zircon 6. microcline microperthite-plagioclase-quartz-biotite-orthopyroxene-ilmenite-graphite (A-1) 7. microcline microperthite-quartz-biotite-plagioclase-garnet-sillimanite-graphite-ihiienite-zircon 8. quartz-plagioclase-biotite-garnet-sillimanite-microcline microperthite-ilmenite (A-4)

microperthite - rich biotite - feldspar - quartz where amphibolite is present. Leucogneiss units gneiss predominates in the western half of the may be as thick as 2000 ft. Prong, whereas plagioclase-rich types are more Oligoclase (Ann-Anig) forms irregular an- common in the eastern half. Hotz (1953), hedra 1 to 3 mm in diameter with indistinct Hague and others (1956), Buddington and albite twinning. Quartz occurs both as small Baker (1961), and Drake (1969) have reported round inclusions in oligoclase or as much larger garnet- or sillimanite-bearing quartz-microcline lenticular grains. Randomly distributed biotite gneisses that were variously interpreted as flakes generally are shredded and replaced by siliceous rocks, sandy shales, rhyolitic tuffs, or chlorite and epidote. Small round garnet potassium-rich sandstones. crystals occur sporadically (Figs. 4A and 4B). Prucha (1956), Sims (1958), Dodd (1965), Modal data are presented in Table 2. Offield (1967), Murray (1968), and Helenek Leucogneiss units form about 30 percent of (1969, oral commun.) have described biotite- the paragneisses in the Lake Hopatcong region. quartz-plagioclase gneisses that contain variable Similar material has been reported elsewhere amounts of garnet, sillimanite, graphite, and throughout the province by Hotz (1953, pyrite in the Hudson Highlands of New York. quartz-oligoclase gneiss), Hague and others They considered most of these rocks to be (1956, Losee gneiss), Sims (1958, albite- metamorphosed carbonaceous shales of gray- oligoclase granite and oligoclase-quartz-biotite wacke-like sandstones. gneiss), Buddington and Baker (1961, quartz- oligoclase gneiss), Dodd (1965, quartz-plagi- Quartz-Oligoclase Leucogneiss oclase leucogneiss), Murray (1968, garnet-bio- Two units that are composed of about 90 tite leucogneiss), and Drake (1969, oligoclase- percent quartz-oligoclase leucogneiss and 10 quartz gneiss and albite-oligoclase granite). All percent amphibolite occur within the Lake these occurrences of leucogneiss contain inter- Hopatcong area (Fig. 2). Minerals in leucogneiss layered amphibolite, but virtually no other are, in order of decreasing abundance, oligo- kinds of rock occur within bodies of leucogneiss. clase, quartz, microcline microperthite, biotite, A satisfactory theory of the premetamorphic garnet, ilmenite, and apatite. The rock is parentage of the leucogneiss must account for indistinctly foliated and displays virtually no the following: (1) widespread occurrence of the textural or compositional layering, except rock as stratigraphic units in several fault

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Figure 4A. Quartz-oligoclase leucogneiss. Hypauto Figure 4B. Quartz-oligoclase leucogneiss. Photo- morphic-granula(lorphic-granular texture. Enlargement directly fromn micrograph (x-nicols) showing details of mineralogy. thin section with partly crossed polarizers; therefore, Minerals are identified as follows; hi = biotite; p = garnet appears white. plagioclase; q = quartz; gt = garnet. Scale in millimeters. blocks throughout the Prong, (2) homogeneity and lack of layering in amphibolite-free portions (Drake, 1969), and the Lake Hopatcong area. of the units, (3) paucity of mafic minerals in the Most specimens of leucogneiss from the Lake leucogneiss, (4) extremely high ratio of oligo- Hopatcong area have higher oligoclase to clase to microcline and therefore of Na2O to microcline microperthite ratios and so probably K.2O, and (5) invariable presence of amphibolite have higher Na2O/K2O ratios than that of the layers and absence of other lithologies. one chemically analyzed specimen (B-l). The The hypothesis of magmatic injection has major discrepancies between the leucogneiss little to recommend it. Although soda-rich and many of the quartz keratophyres are the magmas possibly could be derived by partial higher CaO and A^Os and lower Na2O contents fusion of hornblende- or biotite-quartz-plagio- in the leucogneiss. The occurrence of oligoclase clase gneisses at depth, it is more likely that rather than albite in the leucogneiss accounts granitic or granodioritic melts would have for these chemical differences. Some Puerto formed in view of the abundance of potassium- Rican keratophyres contain calcic plagioclase rich gneisses in the terrane. Moreover, intrusion that is partly replaced by albite (Lidiak, 1965). of a leucogneiss "magma" from below fails The partial chemical data from these rocks to explain why leucogneiss is associated only indicate compositions quite similar to that of with amphibolite. the leucogneiss. Although chemical data for the The association of leucogneiss and amphib- amphibolite layers in the leucogneiss are not olite tentatively is explained as the meta- yet available, it is tempting to speculate that morphosed equivalent of an originally volcanic they originally were of spilitic composition. association. The mineralogy and chemistry of the leucogneiss somewhat resemble those of Biotite-Plagioclase Gneiss quartz keratophyres. Dodd (1965), in his Block D contains a unit of very well-foliated discussion of quartz-plagioclase leucogneiss, quartz-free biotite-plagioclase gneiss that is also favored an hypothesis involving altered about 200 ft thick. The gneiss is characterized sodic volcanic rocks, and Drake (1969) sug- by granoblastic texture (Fig. 4C) and is com- gested that sodic tuff or keratophyre was the posed largely of andesine to labradorite and likely parent rock. Table 2 compares selected biotite as well as variable amounts of horn- chemical analyses of quartz keratophyres with blende, clinopyroxene, orthopyroxene, garnet, chemical analyses of leucogneiss from the Dover magnetite, ilmenite, and apatite. Common district (Sims, 1958), the Delaware River area mineral assemblages are listed in Table 3 as are

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TABLE 2. MODAL AND CHEMICAL DATA FROM QUARTZ-OLIGOCLASE LEUCOGNEISS AND QUARTZ KERATOPHYRES

Modal analyses (volume percent)*

B-l B-2 B-3 B-4 Quartz 33.2 33.7 35.7 26.7 Microcline microperthite 10.1 2.7 2.2 Plagioclase 52.8 59.8 58.4 67.9 Biotite 3.0 1.7 1.4 2.7 Garnet 0.8 1.2 1.2 Ilmenite Tr 0.3 Aptite Tr Tr Tr 0.1 Alteration 0.1 0.8 1.1 2.2 100.0 99.9 100.0 99.9

Chemical analyses of leucogneiss and quartz keratophyres

B-l 1 2 3 4 5 6 7*

SiO2 71.9 68.9 72.4 68.81 75.04 75.10 74.9 TiO2 0.22 0.30 0.38 0.69 0.10 0.22 0.33 A1203 15.3 16.0 13.8 17.65 13.39 12.84 13.7 Fe2O3 0.21 0.9 0.5 0.85 1.61 0.70 1.0 4.89 2.90 FeO 1.5 1.9 1.2 0.36 0.37 1.36 0.31 MnO 0.07 0.05 0.08 Tr 0.05 0.04 0.02 MgO 0.45 0.57 0.97 0.20 0.18 0.30 0.16 1.18 1.21 CaO 2.1 3.9 3.4 0.71 0.40 0.32 0.68 2.73 3.22 Na2O 4.8 4.6 5.6 7.77 6.36 5.12 7.4 6.54 3.17 K2O 1.8 1.6 1.0 1.83 0.83 2.39 0.21 1.23 2.46 + H2O 0.35 0.84 0.48 0.66 1.07 1.22 0.76 98.70 99.56 99.81 99.53 99.40 99.61 99.47 16.57 12.96

* See Figure 2 for sample locations t FeO + FejOs given as Fe2O3 B-l, leucogneiss from Lake Hopatcong area; see Figure 2 for sample location; rapid rock analysis by A. G. Loomis, Berkeley, California; 1, quartz-oligoclase gneiss from Delaware River area (Drake, 1969); 2, quartz-oligoclase gneiss from Delaware River area (Drake, 1969); 3, albite-oligoclase granite from Dover district (Sims, 1958); 4, quartz keratophyre from eastern Oregon (Gilluly, 1935); 5, quartz keratophyre from Great King Island, New Zealand (Bartrum, 1936); 6, quartz keratophyre from central Oregon (Dickinson, 1962); 7 and 8, quartz keratophyres from Puerto Rico (Lidiak, 1965)

some modal analyses. Similar units have not or biotite-feldspar-quartz gneiss cast doubt on been reported from elsewhere in the Reading this hypothesis. Both the mineralogy and Prong. chemistry of the rock are suggestive of anor- The biotite-plagioclase gneiss is totally lack- thosite or gabbroic anorthosite. The chemical ing in quartz, alkali feldspar, and zircon. The analysis presented in Table 4 is from a specimen rock contains between 65 and 95 percent (G-1) that is exceptionally rich in relatively plagioclase that ranges in composition from calcic plagioclase (Ansv); hence, the rock is Anas to Ansv. Locally, plagioclase is replaced by correspondingly rich in CaO and A^Os. In clusters of epidote. Biotite, the most widespread contrast, specimen G-5, for which a chemical mafic mineral, has Y-indices of refraction rang- composition was calculated from the modal ing between 1.654 ± 0.002 and 1.668 + 0.002, analysis, contains relatively low amounts of suggestive of relatively high Fe++ content plagioclase of composition Arui. The chemical (Wones, 1963). Some flakes are replaced by composition calculated for G-5 yields relatively chlorite and epidote. small amounts of Al2Os and CaO and relatively That the biotite-plagioclase gneiss occurs as a high Na2O content. Whereas most specimens of continuous stratigraphic unit suggests meta- biotite-plagioclase gneiss contain plagioclase in sedimentary origin, but the chemical composi- the range An4s to An62, that is, more calcic tion (Table 4) and lack of interbedded meta- than that in G-5, it is likely that most specimens sedimentary rocks, such as marble, quartzite, of the gneiss have AUOs, CaO, and Na2O con-

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tents between those of G-1 and C-5, and, therefore, are quite similar to those of the anortho- sitic rocks listed in Table 4. The rock is completely devoid of cumulate and cataclastic textures so typical of the great masses of anorthosite in the Adirondacks, and the shape of the unit does not resemble that of other known Precambrian anorthosites in granulite-facies terranes. Therefore, the writer tentatively suggests that the unit represents a 200-ft-thick sill of gabbroic anorthosite that has been thoroughly reconstituted during granulite-facies metamorphism. Other Gneisses A variety of other gneisses occurs in the Lake Hopatcong area. Some modal data and common mineral assemblages of these rocks are Figure 4C. Biotite-plagioclase gneiss. Granoblastic listed in Table 5. Block D contains a unit of texture. Enlargement directly from thin section with partly crossed polarizers; therefore, most biotite flakes well-layered feldspar-quartz gneiss character- appear whitish. Scale in millimeters. ized by the widespread occurrence of diopside. As much as 15 percent scapolite (ManMeig) quite similar to the epidote-scapolite-quartz commonly coexists with both microcline and gneiss of Buddington and Baker (1961). plagioclase (An4s) in the unit. Sphene is an Also present in Block D is a unit of thinly abundant accessory. Some of the more cal- layered hornblende-quartz-oligoclase gneiss that careous layers are virtually devoid of feldspar, is 1400 ft thick. Pronounced mineralogical and but contain abundant quartz, scapolite, trem- modal variations occur across the layering, and olite, diopside, brown garnet, and sphene. The the unit is migmatitic. The granitic portions of mineral content is suggestive of metamorphosed the migmatite are identical to the rock in the calcareous or dolomitic sandstones. The unit is larger masses of granitic gneiss, and the amount

TABLE 3. MINERALOGICAL DATA FOR BIOTITE-PLAGIOCLASE GNEISS

Modal analyses (volume percent)*

C-l C-2 C-3 C-4 C-5 Plagioclase 93.2 68.4 89.0 80.2 75.3 Biotite 6.4 2.2 7.0 5.7 5.9 Hornblende Tr 15.4 1.3 16.7 Clinopyroxene 0.3 13.5 1.4 Orthopyroxene 13.2 Garnet 1.3 Magnetite and ilmenite 0.2 0.5 Apatite 0.1 0.1 0.2 0.2 0.7 Alteration 0.2 1.2 100.0 100.0 100.0 99.8 100.0

* See Figure 2 for sample locations Typical mineral assemblages: 1. plagioclase-biotite-apatite-ilmenite 2. plagioclase-biotite-clinopyroxene-apatite-ilmenite 3. plagioclase-biotite-hornblende-apatite-ilmenite 4. plagioclase-biotite-hornblende-clinopyroxene-apatite-ilmenite 5. plagioclase-biotite-hornblende-clinopyroxene-orthopyroxene-apatite-ilmenite 6. plagioclase-biotite-hornblende-garnet-apatite-ilmenite 7. plagioclase-biotite-orthopyroxene-apatite-ilmenite

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C-l C-5 A B C D

SiO2 50.3 54.6 53.40 53.34 54.54 49.68 TiO2 0.62 0.3 0.77 0.72 0.67 0.40 A120, 26.6 21.5 23.96 22.50 25.61 22.60 Fe20, 0.39 0.7 0.91 1.26 1.00 2.31 FeO 3.3 4.4 3.02 4.14 1.26 4.10 MnO 0.08 0.07 0.06 MgO 1.8 2.8 1.88 2.21 1.03 6.41 CaO 9.6 8.1 9.85 10.12 9.92 9.91 Na2O 4.2 5.8 4.17 3.79 4.58 3.30 K20 1.9 0.8 0.80 1.19 1.01 0.61 P206 0.02 0.3 0.18 0.13 0.09 H O+ 0.25 0.5 2 | 0.69 I 0.55 0.57 H2Cr 0.12 0.18 99.18 99.8 99.63 99.47 100.17 100.22

G-1, biotite-plagioclase gneiss, Lake Hopatcong; rapid rock analysis by A. G. Loomis, Berkeley, California; C-5, chemical composition calculated from mode of biotite-plagioclase gneiss, Lake Hopatcong area; A, gabbroic anorthosite, Adirondacks, postulated as similar to primary Adirondack anorthosite magma (Buddington, 1939); B, average of 7 analyses of Whiteface type Adirondack gabbroic anorthosite (Buddington, 1939); C, average of 4 analyses of Marcy type Adirondack anorthosite (Buddington, 1939); D, noritic anorthosite, Morin series, Quebec (Philpotts, 1966)

TABLE 5. MODAL ANALYSES (VOLUME PERCENT) AND MINERAL ASSEMBLAGES OF VARIOUS GNEISSES*

D-l" D-2* D-3* D-4*

Quartz 63.5 23.2 15.7 Microcline microperthite 1.5 8.7 Plagioclase 6.4 54.3 60.5 43.2 Scapolite 14.8 Biotite 2.3 12.1 Hornblende 4.6 6.9 36.3 Clinopyroxene 0.4 18.7 Orthopyroxene 11.2 Magnetite and ilmenite 0.2 1.5 1.1 Zircon Tr Tr Apatite 0.2 0.7 0.5 0.2 Sphene 0.8 Epidote 7.0 Sericite 0.4 99.8 97.6 100.0 99.5

"See Figure 2 for sample locations; D-l, diopside-scapolite-feldspar-quartz gneiss; D-2, hornblende-quartz-oligoclase gneiss; D-3, biotite-orthopyroxene-quartz-plagioclase gneiss; D-4, amphibolite Common mineral assemblages: Diopside-scapolite-feldspar-quartz gneiss 1. quartz-plagioclase-microcline-scapolite-diopside-hornblende-sphene-ilmenite-apatite-zircon 2. quartz-scapolite-diopside-plagioclase-tremolite-garnet-sphene-apatite Hornblende-quartz-oligoclase gneiss 1. plagioclase-quartz-microcline microperthite-hornblende-biotite-magnetite-ilmenite-apatite-zircon 2. plagioclase-quartz-hornblende-chnopyroxene-orthopyroxene-magnctite-ilmenite-apatite-zircon Biotite-orthopyroxene-quartz-plagioclase gneiss 1. plagioclase-quartz-biotite-orthopyroxene-apatite-ilmenite 2. plagioclase-hornblende-orthopyroxene-quartz-magnetite-ilmenite-apatite Amphibolite 1. plagioclase-hornblende-clmopyroxene-magnetite-apatite 2. plagioclase-hornblende-clinopyroxene-orthopyroxene-biotite-magnetite-apatite

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and composition of the granitic layers is independent of the variations in composition of the surrounding rock. The granitic rock, therefore, is regarded as intrusive. The mineral content and heterogeneous layering of the non- granitic parts of the unit suggest derivation from acid to intermediate volcanic parent rocks. Well-foliated, homogeneous biotite-orthopyroxene- quartz -plagioclase gneiss also occurs in Block A as a unit that is about 800 ft thick. The plagioclase typically is andesine. Alkali feldspar, hornblende, garnet, and sillimanite are not present as in many of the other biotitic gneisses. The rock grades along strike into well-layered hornblende-quartz- plagioclase gneiss that locally contains orthopyroxene. These rocks may represent metamorphosed intermediate volcanic rocks and possibly graywacke. The unit bears some resemblance to the quartz diorite of Sims (1958), the hypersthene-quartz-oligoclase gneiss Figure 5A. Hornblende syenitic gneiss. Hypauto- of Buddington and Baker (1961), the hyper- morphic-granular texture. Enlargement directly from sthene-quartz-oligoclase gneiss of Dodd (1965), thin section with partly crossed polarizers; therefore, and the charnockitic quartz diorite of Drake hornblende appears white. (1969). Amphibolites, containing the assemblage clinopyroxene-hornblende-plagioclase, are not so abundant in the Lake Hopatcong area as elsewhere in the Reading Prong, and marble is virtually absent.

PETROLOGY OF THE GRANITIC AND SYENITIC ROCKS Syenitic Gneisses Two large sheets of well-foliated syenitic gneiss occur in the Lake Hopatcong region. Block A contains a gently folded sheet of homogeneous hornblende syenitic gneiss that is about 2400 ft thick and is conformable with the surrounding paragneisses. The rock is devoid of compositional layering, but a very pronounced foliation is defined by streaks of aligned hornblende crystals. The rock is characterized by medium-grained (1 to 3 mm), hypautomorphic-granular texture (Figs. 5A Figure 5B. Hornblende syenitic gneiss. Photomicro- graph (x-nicols) showing details of mineralogy. Miner- and 5B). The minerals are microcline micro- als are identified as follows; mm = microcline microperthite, oligoclase (An2o), hastingsitic horn- perthite; p = plagioclase; q = quartz; h = hornblende, quartz, clinopyroxene, magnetite, blende ; cp = clinopyroxene. Scale in millimeters. ilmenite, apatite, and zircon. Mesoperthite commonly occurs in place of coexisting micro- syenitic gneiss that is about 1000 ft thick and is cline microperthite and plagioclase. Modal regionally conformable, but locally discordant, analyses of the gneiss are listed in Table 6 (E-l with the surrounding paragneisses occurs in and E-2); a chemical analysis with CIPW norm Block B. It can be traced along strike for is presented in Table 7 (E-l). several miles north of the Lake Hopatcong A sheet of well-foliated clinopyroxene area around the nose of the Beaver Lake anti-

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transected both regionally and locally by a nearly concordant sheet of clinopyroxene quartz syenite that is 800 ft thick and at least 4 mi long. The rock is internally homogeneous and lacks layering or foliation. Figures 6A and 6B show the characteristic hypautomorphic- granular texture of the syenite. The minerals are mesoperthite that has exceedingly fine exsolution lamellae, quartz, ferrohedenbergitic clinopyroxene, fayalitic olivine that is partly altered to iddingsite, hornblende, magnetite, ilmenite, plagioclase, zircon, and apatite. Modal (E-4 and E-5) and chemical (E-4) data are presented in Tables 6 and 7, respectively. A detailed report of the petrology and mineralogy of this body is currently in preparation.

Figure 5C. Pyroxene syenitic gneiss. Granoblastic Granitic Gneisses texture with indistinct foliation. Enlargement directly from thin section with partly crossed polarizers; there- Block C consists largely of clinopyroxene fore, pyroxene appears white. granitic gneiss that contains isoclinally folded amphibolitic layers and hornblende-rich streaks. form (Buddington and Baker, 1961). The unit The amphibolite layers are transected by axial- is devoid of compositional and textural layering, plane foliation that is defined by parallel and a distinct foliation is defined by preferred alignment of quartz and hornblende grains. alignment of elongated clinopyroxene crystals. Syenitic facies of the gneiss are widespread and The gneiss has a medium-grained (1 to 2 mm), commonly are associated with the amphibolitic granoblastic texture (Fig. 5C). The minerals rocks. The granitic gneiss continues northward are oligoclase, slightly perthitic microcline, into the area investigated by Buddington and clinopyroxene, hornblende, magnetite, il- Baker (1961). The rock has medium-grained, menite, apatite, sphene, quartz, and zircon. hypautomorphic-granular texture and contains A modal analysis (E-3) is listed in Table 6. patchy mesoperthite, quartz, ferrohedenbergitic clinopyroxene, hornblende, microcline, Clinopyroxene Quartz Syenite plagioclase, magnetite and ilmenite, apatite, Folds in some of the gneisses of Block D are sphene, and zircon. Large equant grains of

TABLE 6. MODAL ANALYSES (VOLUME PERCENT) OF GRANITIC AND SYENITIC ROCKS*

E-l E-2 E-3 E-4 E-5 F-l F-2 F-3 F-4 Quartz 0.6 5.8 0.4 15.4 12.4 25.1 25.1 34.7 31.4 Microcline microperthite 45.2 46.7 12.1 12.3 38.8 41.8 Mesoperthite 67.9 73.4 60.7 8.7 Plagioclase 34.5 31.7 76.2 5.1 0.3 8.0 45.1 25^8 22.7 Hornblende 18.1 12.2 0.3 3.3 5.2 Tr 1.5 Clinopyroxene 5.4 4.7 9.4 0.8 Olivine 2.4 Magnetite and ilmenite 1.2 1.7 3.2 3.3 1.7 0.7 5.6 Tr 1.3 Zircon Tr Tr Tr 0.2 0.3 0.4 Tr 0.4 Apatite 0.2 0.4 0.6 0.1 0.1 0.2 Tr Sphene 1.6 0.4 99.8 98.5 99.8 100.0 100.0 99.7 98.6 99.3 99.1

* See Figure 2 for sample locations E-l, hornblende syenitic gneiss, Block A; E-2, hornblende syenitic gneiss, Block A; E-3, clinopyroxene syenitic gneiss, Block B; E-4, clinopyroxene quartz syenite, Block D; E-5, clinopyroxene quartz syenite, Block D; F-l, hornblende granitic gneiss from clinopyroxene granitic gneiss unit, Block C; F-2, clinopyroxene granitic gneiss, alaskitic facies, Block C; F-3, hornblende granitic gneiss, alaskitic facies, Block A; F-4, hornblende granitic gneiss, Block D

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Figure 6C. Hornblende granitic gneiss. Hypauto- morphic-granular texture. Enlargement directly from Figure 6A. Clinopyroxcne syenite. Hypautomor- thin section with partly crossed polarizers. Minerals are phic-granular texture. Enlargement directly from thin identified as follows; m = mesoperthite; mm = section with partly crossed polarizers; therefore, microcline microperthite; p = plagioclase; q = quartz; pyroxene, olivine, and opaque oxides appear whitish. cp = clinopyroxene; ol — olivine; opq = opaque oxide; z = zircon. Scale in millimeters.

quartz-plagioclase gneiss are present in both Blocks A and D. Units in contact with the granitic gneisses commonly display a migmatitic character. The hornblende granitic gneiss is homogeneous, and foliation is defined by aligned quartz lenticles and hornblende prisms. The granitic gneiss possesses medium-grained, hypautomorphic-granular texture (Fig. 6C) and contains microcline microperthite, quartz, plagioclase, hornblende, magnetite and ilmenite, apatite, and zircon. Alaskitic facies are quite common. Modal analyses (F-3 and F-4) are given in Table 6. Hornblende granitic gneisses differ from clinopyroxene granitic gneisses in higher quartz content and in lack of mesoperthite. The content of free plagioclase in the hornblende granitic gneisses is far higher than it is in clinopyroxene granitic gneiss; the content of plagioclase lamellae in the perthites in Figure 6B. Clinopyroxene syenite. Photomicro- the hornblende granitic gneisses is correspond- graph (x-nicols) showing details of mineralogy. ingly reduced.

mesoperthite commonly are surrounded in Origin of the Granitic and Syenitic Rocks part by fine-grained granoblastic mosaics of Despite the paucity of crosscutting relation- oligoclase and perthitic microcline that evident- ships in the majority of the granitic and syenitic ly were produced by granulation and recrys- rock bodies, the over-all homogeneity and tallization of the mesoperthite. Modal analyses great thickness (800 to 2400 ft) of the units and (F-l and F-2) are presented in Table 6. the chemistry and mineralogy of the rocks is Thick masses of distinctly foliated horn- consistent with intrusion of granitic and blende granitic gneiss containing as much as 10 syenitic magmas. The abundant transgressive percent of interlayered hornblende- or biotite- field relations and lack of foliation in the

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TABLE 7. CHEMICAL ANALYSES OF SYENITIC ROCKS Figure 7 diagrammatically portrays some of FROM LAKE HOPATCONG AREA* the mineral equilibria that are relevant to estimation of the pressure-temperature condi- E-l E-4 tions of metamorphism. The lower limit of temperature attained during the metamorphism SiO2 62.3 63.5 may be estimated from the locus of the crest TiO2 0.88 0.84 A1203 15.8 14.4 of the alkali feldspar solvus (Tuttle and Bowen, Fe203 4.1 3.9 1958; Orville, 1963; Morse, 1968). The pres- FeO 3.1 4.6 ence of mesoperthite in Lake Hopatcong MnO 0.14 0.15 MgO 0.27 0.05 paragneisses suggests that temperature probably CaO 2.3 1.8 was at least close to, if not in excess of, the Na2O 4.5 3.9 critical temperature of the alkali feldspar solvus. K2O 5.0 5.0 Morse (1968), moreover, has demonstrated H20+ 0.39 0.85 that addition of the anorthite molecule to feld- H20- 0.14 0.07 P205 0.19 0.11 spar drastically raises the temperature of the C02 0.20 0.13 ternary feldspar solvus crest. For example, he 99.31 99.30 located the crest of the solvus of a natural mesoperthite of composition Or29.eAb62.2Ans.2 Norms at about 920° C and 500 bars PHJO- Extinction angle measurements by the Michel-Levy Quartz 14.4 19.1 Orthoclase 29.2 29.2 method on coarser parts of Lake Hopatcong Albite 39.7 34.7 mesoperthites indicate plagioclase compositions Anorthite 8.3 6.9 of about An2o. The anorthite content of these Diopside 0.8 0.3 mesoperthites, therefore, insures that tem- Hypersthene 1.5 4.3 Magnetite 3.6 3.3 perature likely exceeded by several degrees that Ilmenite 1.5 1.5 of the calcium-free alkali feldspar solvus crest. Apatite 0.3 0.3 Calcite 0.7 0.4 12 100.0 100.0 II * See Figure 2 for sample locations; rapid rock analyses 10 by A. G. Loomis, Berkeley, California; E-l, hornblende syenitic gneiss, Block A; E-4, clinopyroxene quartz 9 syenite, Block D £ 8

GO O 7 clinopyroxene quartz syenite sheet render the _J ' magmatic hypothesis especially attractive for 6 that body. All granitic and syenitic rocks in z

the Lake Hopatcong area presently are regarded u. 5 as syntectonic or late tectonic magmatic c = 4 intrusive bodies, although much more work »

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Hence, the minimum temperature of metamor- alent to a depth of about 10 km. phism was about 700° C. Another assemblage that may ultimately be The upper temperature limit of meta- useful in estimating the lower limit of pressure morphism can be estimated from the decom- is biotite-sillimanite-quartz. Schreyer and position curve of tremolite, a constituent of Seifert (1969) pointed out that the univariant some layers of diopside-scapolite-feldspar- curve phlogopite + sillimanite + quartz = quartz gneiss. Although the decomposition of Mg cordierite + K feldspar + H2O terminates tremolite has been investigated experimentally at an invariant point located at 695° C and 5 kb. only to 2 kb pressure (Boyd, 1959), extrapola- Furthermore the curve has a positive slope, so tion of the decomposition curve to higher that higher pressure is indicated for higher pressures suggests that tremolite would not be termperatures. The natural assemblage, how- stable above temperatures on the order of 950° ever, contains Fe, which, as Schreyer and C. If pressure on the pore fluid phase were less Seifert (1969) indicated, would lower the than load pressure, tremolite would have univariant curve with respect to pressure. The decomposed at still lower temperatures. magnitude of the shift has not been determined Sillimanite occurs in some biotite-feldspar- experimentally, so that the biotite-sillimanite- quartz gneisses, but kyanite has not been found quartz assemblage is at best a crude indicator in the Lake Hopatcong area. Hence, the upper of the lower limits of pressure. limit of load pressure may be estimated from The mineral assemblages reported by nearly the kyanite-sillimanite curve. There is, of all other workers in the Reading Prong suggest course, considerable uncertainty in exper- that a similar range of pressure and temperature imental location of the Kyanite-sillimanite obtained during a metamorphism that pro- curve (Zen, 1969), but the most recent results duced rocks assignable to the hornblende (Newton, 1966; Matsushima and others, 1967: granulite subfacies. Richardson and others, 1968) have indicated that the curve may extend from about 700° C SUMMARY AND CONCLUSIONS at 7.5 kb to 900° C at 12 kb. Given the un- Paragneisses of the Lake Hopatcong area con- certainty in maximum temperature attained sist chiefly of layered biotite-feldspar-quartz during peak metamorphism as well as the gneiss and quartz-oligoclase leucogneiss that uncertainty in the experimental data, it is have been interpreted as metamorphosed feld- probable that load pressure on Lake Hopatcong spathic sandstone and quartz keratophyre, rocks did not exceed about 12 kb. respectively. Less abundant rock types have The absence of cordierite in rocks of ap- been interpreted as metamorphosed acid and propriate bulk composition permits estimation intermediate volcanics and calcareous sand- of a lower pressure limit. Occurring in the Lake stones. A unit of biotite-plagioclase gneiss Hopatcong area is the subassemblage garnet- strongly resembles gabbroic anorthosite both sillimanite-quartz. The location of the univar- chemically and mineralogically. These units iant curve almandine + sillimanite + quartz form complexly folded layered sequences that = Fe cordierite has been determined exper- have been invaded by sheets of syntectonic and imentally by Richardson (1968) on the fayalite late-tectonic granitic and syenitic rocks. + quartz + magnetite oxygen buffer. The Mineral assemblages of the hornblende curve terminates at an invariant point located granulite subfacies indicate that these para- at 3.5 kb and 680° C. At 800° C, the curve is gneisses have been exposed to load pressures in located at 2.5 kb. Richardson (1968) pointed excess of 2.5 kb and temperatures above 700° C. out that addition of Mg to the system, as in Knowledge of the time relationships of this the natural assemblage garnet-sillimanite- granulite-facies metamorphism with the various quartz, would raise the curve with respect to magmatic events is limited. Field relations pressure. He further noted that oxygen fugacity demonstrate that the transgressive clino- in natural rocks generally exceeds that of the pyroxene quartz syenite is clearly a younger FQM buffer and that an increase in oxygen intrusive body than the hornblende granitic fugacity over that of FQM also will raise the gneiss. Thus there are indications within the curve with respect to pressure. It is safe, there- Reading Prong of at least two magmatic events fore, to assume that load pressure must have or of successive stages of one event. The solu- been greater than that denned by the curve tion to problems of this type, however, depends almandine + sillimanite + quartz = Fe largely upon the radiometric methods of age cordierite, that is, greater than 2.5 kb, equiv- determination. Thus while there is still need for

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detailed structural and petrologic work within of selected areas in New Jersey and eastern the Reading Prong, significant advance in our Pennsylvania and guidebook of excursions: knowledge of the history of the terrane is Rutgers Univ. Press, New Brunswick, N. J., strongly dependent upon both U-Th-Pb data 382 p., 1969. Epstein, J. B., and Epstein, A. G., Geology of the from zircons and Rb-Sr whole-rock and valley and ridge province between Delaware mineral isochrons. Water Gap and Lehigh Gap, Pennsylvania: in ACKNOWLEDGMENTS Subitzky, S. (ed.), Geology of selected areas in New Jersey and eastern Pennsylvania and Field work during the summers of 1966 and guidebook of excursions: Rutgers Univ. Press, 1967 was made possible through the generous New Brunswick, N. J., 382 p., 1969. support of grants 1098-66 and 1153-67 from Gilluly, J., Keratophy res of eastern Oregon and the the Geological Society of America. Thin sec- spilite problem: Amer. J. Sci., Vol. 29, 5th tions were made available through funds of the ser., p. 225-252, 336-352, 1935. Hague, J. M., Baum, J. L., Herrman, L. A., and Department of Geological Sciences, Brown Pickering, R. J., Geology and structure of the University. The Arts and Science Research Franklin-Sterling area, New Jersey: Geol. Soc. Fund of New York University supported the Amer., Bull., Vol. 67, p. 453-474, 1956. cost of the chemical analyses. Helenek, H. L., An investigation of the origin, The writer wishes to acknowledge his structure, and metamorphic evolution of major indebtedness to P. C. Franks of New York rock units in the Hudson Highlands: Sc. M. University, Y. W. Isachsen of the Geological thesis, Brown Univ., Providence, R. I., 1965. Survey of the New York State Museum and Hotz, P. E., Magnetite deposits of the Sterling Lake, Science Service, and A. A. Drake, Jr. and P. K. N.Y.—Ringwood, NJ. area: U.S. Geol. Surv., Bull. 982-F, p. 153-244, 1953. Sims of the U.S. Geological Survey. They have Lidiak, E.G., Petrology of andesitic, spilitic, and graciously provided many critical comments keratophyric flow rock, north-central Puerto and helpful suggestions which have led to con- Rico: Geol. Soc. Amer., Bull, Vol. 76, p. 57- siderable improvement of the manuscript. 88, 1965. Matsushima, S., Kennedy, G. C., Akella, J., and REFERENCES CITED Haygarth, J., A study of equilibrium relations Bartrum, J. A., Spilitic rocks in New Zealand: Geol. in the systems Al2O8-SiO2-H2O and A12O3- Mag., Vol. 73, p. 414-423, 1936. H2O: Amer.'j. Sci., Vol. 265, p. 28-44, 1967. Boyd, F. R., Hydrothermal investigations of Morse, S. A., Feldspars: Carnegie Inst. Washington, amphiboles: in Abelson, P. H. (ed.), Researches Yr. Bk. 67, p. 120-126, 1968. in geochemistry: John Wiley St Sons, New York, Murray, D. P., An investigation of origin, structure, 511 p., 1959. and metamorphic evolution of the Precam- Buckwalter, T. V., Geology of the Precambrian brian rocks of the West Point Quadrangle, rocks and Hardyston Formation of the Boyer- New York: Sc.M. thesis, Brown Univ., town quadrangle: Pennsylvania Geol. Sur., Providence, R. I., 69 p., 1968. 4th ser., Geol. Atlas 197, 15 p., 1959. Newton, R. C., Kyanite-sillimanite equilibrium at Buckwalter, T. V., The Precambrian geology of the 750° C: Science, Vol. 151, p. 1222-1225, 1966. Reading 15-minute quadrangle: Pennsylvania Offield, T. W., Bedrock geology of the Goshen- Geol. Surv., 4th ser., Prog. Rept. 161, 49 p., Greenwood Lake area, N.Y.: New York State 1962. Mus. and Sci. Serv., Map and Chart No. 9, Buddington, A. F., Adirondack igneous rocks and 78 p., 1967. their metamorphism: Geol. Soc. Amer., Mem. Orville, P. M., Alkali ion exchange between vapor 7, 354 p., 1939. and feldspar phases: Amer. J. Sci., Vol. 261, p. Buddington, A. F., and Baker, D. R., Geology of 201-237, 1963. the Franklin and part of the Hamburg Philpotts, A. R., Origin of the anorthosite-mangerite Quadrangles, New Jersey: U.S. Geol. Surv., rocks in southern Quebec: J. Petrology, Vol. 7, Misc. Geol. Invest. Map 1-346, 1961. p. 1-64, 1966. Dickinson, W. R., Metasomatic quartz keratophyre Prucha, J. J., Geology of the Brewster magnetite in central Oregon: Amer. J. Sci., Vol. 260, p. distinct of southeastern New York: New York 249-266, 1962. State Mus. and Sci. Serv., Circ. 43, 48 p., 1956. Dodd, R. T., Precambrian geology of the Popolopen Richardson, S. W., Staurolite stability in a part of Lake Quadrangle, southeastern New York: the system Fe-Al-Si-O-H: J. Petrology, Vol. New York State Mus. and Sci. Serv., Map and 9, p. 467-488, 1968. Chart No. 6, 39 p., 1965. Richardson, S. W., Bell, P. M., and Gilbert, M. C., Drake, A. A., Precambrian and lower Paleozoic Kyanite-sillimanite equilibrium between 700° geology of the Delaware Valley, New Jersey— C and 1500° C: Amer. J. Sci., Vol. 266, p. Pennsylvania: in Subitzky, S. (ed.), Geology 513-541, 1968.

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