Prepared in cooperation with the Commonwealth of Massachusetts, Massachusetts Geological Survey

Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

By Gregory J. Walsh, John N. Aleinikoff, and Michael J. Dorais

Pamphlet to accompany Scientific Investigations Map 3171

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation: Walsh, G.J., Aleinikoff, J.N., and Dorais, M.J., 2011, Bedrock geologic map of the Grafton quadrangle, Worcester County, Massachusetts: U.S. Geological Survey Scientific Investigations Map 3171, 1 sheet, scale 1:24,000, 39-p. text.

The text and map of this report are part A, and the database is part B. Both parts are available online at http://pubs.usgs.gov/sim/3171/.

ISBN 978–1–4113–3289–8 iii

Contents

Introduction...... 1 Stratigraphy...... 1 U-Pb Geochronology...... 3 Methods...... 3 Results...... 7 Ponaganset Gneiss (Zpg)...... 7 Northbridge Granite Gneiss (Zncg)...... 7 Hope Valley Alaskite Gneiss (Zhvb and Zhv)...... 11 Grafton Gneiss (_gg)...... 11 Marlboro Formation—Amphibolite (_m)...... 13 Geochemistry...... 13 Methods...... 17 Results...... 17 Structural Geology...... 17 Ductile Structures...... 17

First-Generation Foliation—D1 Deformation...... 17

Second-Generation Foliation—D2 Deformation...... 19

Third-Generation Foliation—D3 Deformation...... 19 Ductile Faults...... 23 Spencer Brook and Assabet River Faults...... 23 Bloody Bluff Fault...... 23 Burlington Mylonite Zone...... 24 Fisherville Pond Fault...... 26 Comment on the Hope Valley Shear Zone...... 26 Brittle Structures...... 26 Brittle Faults...... 27 Metamorphism...... 30 Acknowledgments...... 34 References Cited...... 34

Figures

1. Simplified geologic map of the Grafton quadrangle showing the surficial deposits on bedrock...... 2 2. Photographs of rocks representative of the dated samples in this study...... 6 3. Cathodoluminescence and transmitted light images of representative zircons and concordia plots of dated rocks from the Avalon zone in the Grafton quadrangle...... 10 4. Photograph of light-colored alaskite sill in the Ponaganset Gneiss at point GR–141 along State Route 122 at the junction with State Route 122A in Farnumsville...... 11 5. Cathodoluminescence and transmitted light images of representative zircons and concordia plots of dated rocks from the Nashoba zone in the Grafton quadrangle...... 12 6. Major, trace, and rare earth element discrimination diagrams for four granitic rock samples from the Grafton quadrangle...... 15 iv

7. Trace element and isotope discrimination diagrams for four granitic rock samples from the Grafton quadrangle...... 16 8. Photographs of dominant foliations in the Nashoba and Westboro Formations...... 18

9. Summary diagrams of D2 and D3 ductile structures, including veins and dikes...... 20 10. Formline map of the dominant foliation showing modeled strike and dip...... 22 11. Photograph of en echelon extensional veins associated with shear bands and faults showing normal displacement near the Bloody Bluff fault...... 23 12. Photographs of mylonitic foliation in the footwall of the Bloody Bluff fault near Wesson Road...... 24 13. Photographs and map showing mylonitic foliation in the Burlington mylonite zone...... 25 14. Summary diagrams of brittle structures including joints, faults, and parting fractures along ductile structures...... 28 15. Block diagrams and photograph of the most conspicuous fractures in the Grafton quadrangle...... 29 16. Summary histogram of foliation-parallel fractures, or parting, by formation...... 30 17. Map of brittle faults in the Grafton quadrangle and lower hemisphere equal-area projections...... 31 18. Photograph of a parting fracture along a thin pegmatite dike at “Fat Man’s Misery” in Purgatory Chasm State Park...... 32 19. Simplified map of the Grafton quadrangle showing peak metamorphic conditions...... 33 20. Photograph of mylonitic fabric parallel to the dominant foliation in the Nashoba Formation...... 34 21. Photograph showing late-forming hornblende porphyroblasts oriented randomly

in the plane of the S2 foliation in the Westboro Formation...... 34

Tables

1. Modes, in percent, of selected rocks in the Grafton 1:24,000-scale quadrangle, Massachusetts...... 4 2. Sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb data for zircon from rocks of the Grafton 1:24,000-scale quadrangle, Massachusetts...... 8 3. Major and trace element whole-rock geochemistry from the Grafton 1:24,000-scale quadrangle, Massachusetts...... 13 4. Summary of foliation-parallel parting fractures, by formation...... 30

Conversion Factors

Multiply By To obtain Length centimeter (cm) 0.3937 inch (in) meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

By Gregory J. Walsh,1 John N. Aleinikoff,2 and Michael J. Dorais3

Introduction that followed the publication of the State map (Goldsmith, 1991a,b,c; Wones and Goldsmith, 1991). Recent 1:24,000-scale mapping in adjacent areas includes The bedrock geology of the Grafton quadrangle, Worces- work in the Oxford (Barosh, 2005), Milford (Kopera and ter County, Mass., consists of metamorphic and igneous rocks. others, 2007), Marlborough (Kopera and others, 2006), and Neoproterozoic intrusive, metasedimentary, and metavolcanic Shrewsbury quadrangles (Markwort, 2007). This report pre­ rocks crop out in the Avalon zone, and Cambrian to Silurian sents mapping by G.J. Walsh, geochronology by J.N. Aleini- intrusive, metasedimentary, and metavolcanic rocks crop out in koff, and geochemistry by M.J. Dorais. the Nashoba zone (Zen and others, 1983). Rocks of the Avalon and Nashoba zones, or terranes, are separated by the Bloody Bluff fault. Pegmatite dikes of Permian age occur throughout the quadrangle but are more abundant in the Avalon zone. One Stratigraphy diabase dike of Jurassic age was observed in the Avalon zone. The Avalon zone contains the Westboro Formation, which The metamorphic and igneous rocks in the Grafton represents a Neoproterozoic peri-Gondwanan marginal shelf quadrangle range in age from Neoproterozoic to Jurassic. Ages sequence, intruded by Neoproterozoic arc-related plutonic rocks in this report conform to the time scale of Gradstein and others of the Rhode Island batholith (Zen and others, 1983; Zartman (2004). and Naylor, 1984; Bailey and others, 1989; Rast and Skehan, Rocks of the Avalon zone are Neoproterozoic and occur 1990; Skehan and Rast, 1990, 1995; Goldsmith, 1991a; Hermes in two distinct belts (fig. 1): and Zartman, 1992; Hibbard and others, 2006). The Nashoba • Metasedimentary and metavolcanic rocks of the West- zone contains lower Paleozoic peri-Gondwanan arc-related boro Formation located between the Bloody Bluff and metavolcanic rocks of the Marlboro Formation and metasedi- Fisherville Pond faults mentary and metavolcanic rocks of the Nashoba Formation (Zen and others, 1983; Goldsmith, 1991b; Hepburn and others, • Intrusive rocks of the Rhode Island batholith (Gold- 1995; Hepburn, 2004; Hibbard and others, 2006). Rocks of the smith, 1991a) in the Milford antiform (Goldsmith, Marlboro Formation were intruded by the Grafton Gneiss. 1991c) located southeast of the Fisherville Pond fault The bedrock is overlain by Pleistocene to Quaternary allu- The oldest rocks in the quadrangle presumably belong to vial deposits, stratified glacial deposits, areas of thick till in drum- the Westboro Formation. The age is considered Neoproterozoic lins or drumlinoids, and thin till (fig. 1; Stone and Stone, 2006). on the basis of crosscutting Neoproterozoic intrusive rocks in Southeast of the Fisherville Pond fault in the Milford antiform, the Avalon zone (Zen and others, 1983; Zartman and Naylor, the bedrock of the Rhode Island batholith is well exposed. North- 1984; Bailey and others, 1989; Goldsmith, 1991a; Skehan and west of the fault, however, the bedrock exposures are limited due Rast, 1990, 1995). Recent preliminary results suggest that the to low topographic relief and extensive surficial deposits. Westboro Formation at its type locality may have a maximum age Previous mapping in the area is limited, and unpublished of 600±3 Ma (mega-annum) on the basis of U-Pb detrital zircon reconnaissance mapping at 1:24,000 scale by H.R. Dixon and analyses (Hepburn and others, 2008). This age is slightly younger Richard Goldsmith was the primary source of geologic data than the dated intrusive rocks in this study (approximately compiled on the 1:250,000-scale geologic map of Massachu- 612–606 Ma), but Hepburn and others (2008) claim that setts (Zen and others, 1983). In addition to the information on definitive crosscutting relations are absent from the Westboro the State map (Zen and others, 1983), much of the work by Formation at the type locality. In the Grafton quadrangle, the Goldsmith is contained in several important regional papers Westboro Formation is cut by intrusive rocks of the Rhode Island batholith (Zpg and Zhv), and xenoliths of greenstone (Zwa) occur 1U.S. Geological Survey, Montpelier, VT 05601. in rocks informally named the granodiorite gneiss at Nourse Farm 2U.S. Geological Survey, Denver, CO 80225. (Zgdn) at roadcuts along the Massachusetts Turnpike. In this map, 3Brigham Young University, Provo, UT 84602. the intrusive rocks that cut the Westboro Formation have not been 2 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

Nashoba zone Avalon zone 71°45' 71°37'30" 42°15'

T L T U L A U F A F K O F O F U R L B B R E Y C D N O E O P L S B

T L U A F

T L U A F

R D E V N I O P R E T L E IL B RV A E S SH S FI A

B LA C K S T ON E V A L LE Y FA U LT

PCF 42°07'30" Base map from MassGIS 1:5,000-scale 0 1 2 MILES shaded-relief digital terrain model http://www.mass.gov/mgis/img_shdrel5k.htm 0 1 2 KILOMETERS EXPLANATION Surficial geology Nashoba zone Brittle fault Alluvial deposits Nashoba structural domain Mylonitic thrust fault Stratified glacial Marlboro structural domain Burlington mylonite zone deposits Milford antiform Thick till Avalon zone Westboro structural domain Outcrop (this study) MASSACHUSETTS Milford structural domain Map area

Figure 1. Simplified geologic map of the Grafton quadrangle showing the surficial deposits on bedrock. The two terranes, the Nashoba zone and the Avalon zone, are each divided into two structural domains. The four structural domains are fault bounded. Surficial geology from Stone and Stone (2006); areas of thin till show the color of the underlying bedrock zone. PCF, Purgatory Chasm fault. U-Pb Geochronology 3 dated; thus the slight discrepancy in ages is unresolved. Modes of and poorly constrained whole-rock Rb/Sr ages (Zartman and selected samples from the Avalon zone are shown in table 1. Naylor, 1984; Goldsmith, 1991a) from the Andover Granite in The rocks of the Nashoba zone are Cambrian to Silurian the Nashoba Formation are probably meaningless. On our map, and also occur in two distinct belts (fig. 1): Silurian metadiorite (Sd) intruded the Marlboro Formation and • Mafic metavolcanic rocks of the Marlboro Formation the Grafton Gneiss, and, although undated, the metadiorite is and the intrusive Grafton Gneiss located between the probably correlative with the Sharpners Pond Diorite. Modes of Bloody Bluff and Assabet River faults selected samples from the Nashoba zone are shown in table 1. • Metasedimentary and minor metavolcanic rocks of the Nashoba Formation located northwest of the Assabet River fault U-Pb Geochronology Igneous zircon cores from a sample of the volcaniclastic Here we report the U-Pb zircon geochronology results granofels in the Marlboro Formation (_mg) yield a Cambrian U-Pb SHRIMP4 age of 501±3 Ma (Walsh and others, 2011). The of five samples collected from the Grafton quadrangle; three Grafton Gneiss, which intrudes part of the Marlboro Formation, samples of granite gneiss (GR–23, GR–352, and GR–512) yields a slightly older U-Pb SHRIMP age of 515±4 Ma (see were collected from the Avalon zone, and two samples were below). We assign a Cambrian age to the Marlboro Formation collected from the Nashoba zone including the Grafton Gneiss on the basis of these two ages. Loan and others (2011) report (GR–203) and a sample of amphibolite of the Marlboro provisional detrital zircon LA-ICP-MS5 U-Pb ages of about Formation (GR–32) (fig. 2). The results are presented in table 480 to 470 Ma, suggesting that parts of the Marlboro Forma- 2. Geochronology sample locations are shown on the map tion range into the Early Ordovician. Acaster and Bickford and in the accompanying GIS database. A description of the (1999) reported ages of 473 to 430 Ma by conventional U-Pb geochronology sample locations follows table 2. zircon analyses from what they interpret as volcanic rocks in the Marlboro Formation, but it is unclear if the rocks are volcanic or Methods intrusive, and the older conventional ages may not be reliable. The age of the Nashoba Formation is not well-constrained, Rock samples weighing about 10 kilograms (kg) were and current data suggest an age range between Cambrian and collected from outcrops in the Grafton quadrangle. After the rocks Silurian. The Cambrian age of the Nashoba Formation is based on were crushed and pulverized, the material was processed over a a conventional U-Pb igneous zircon age of 499 +6/-3 Ma by Hep- Wilfley table to concentrate heavy minerals. Further processing burn and others (1995) from the Fish Brook Gneiss from North included separating iron-bearing minerals on a magnetic separator Andover, Mass., approximately 70 km to the northeast. Bell and and concentrating the minerals with densities greater than about others (1977) and Bell and Alvord (1976) interpreted the Fish 3.3 grams/cubic centimeter (g/cm3) by sinking in methylene Brook Gneiss as a metavolcanic rock on the basis of the presence iodide. Individual grains were handpicked using a binocular of ripple beds. Their interpretation is problematic, however, as microscope, mounted in epoxy, ground to nearly half-thickness to their ripples are defined by two foliations that exhibit a tectonic expose internal zones, and polished by using 6 µm (micron) and “shearband boudin” fabric (Passchier and Trouw, 2005, p. 152). 1 µm diamond suspension. All grains were imaged in transmitted Castle (1965) suggested that the protolith of the Fish Brook and reflected light using a petrographic microscope and in Gneiss was either an intrusive rock or an ancient igneous or sedi- cathodoluminescence (CL) using a scanning electron microscope. mentary rock now exposed in the core of a dome. The problems U-Pb geochronology of zircon was performed using the U.S. associated with the interpretation of the Fish Brook Gneiss are Geological Survey/Stanford University sensitive high resolution discussed in Goldsmith (1991a), Hepburn and others (1995), ion microprobe-reverse geometry (SHRIMP–RG) or SHRIMP and Hepburn (2004). In Massachusetts, a possible maximum II at the Research School of Earth Sciences, Australian National age for the Nashoba Formation is represented by the crosscut- University, Canberra. SHRIMP analysis, following the methods ting Sharpners Pond Diorite dated at 430±5 Ma (Zartman and of Williams (1998), consisted of excavating a pit about 25 to Naylor, 1984). Wintsch and others (2007) report U-Pb SHRIMP 35 µm in diameter and about 1 µm in depth, using a primary detrital and metamorphic zircon ages that bracket deposition of oxygen beam at a current of about 4 to 6 nanoamps (nA). The the potentially correlative Tatnic Hill Formation in Connecticut magnet cycled through the mass stations five times per analysis. to between about 425 and 418 Ma. Loan and others (2011) report Raw data were reduced by using Squid 1 (Ludwig, 2001) or provisional detrital zircon LA-ICP-MS U-Pb ages of about 450 to Squid 2 (K.R. Ludwig, written commun., 2008) and plotted by 440 Ma from the Nashoba Formation and Tadmuck Brook Schist, using Isoplot 3 (Ludwig, 2003). Measured 206Pb/238U ages were suggesting deposition in the Ordovician. Less well-constrained referenced to zircon standard R33 (419±1 Ma; Black and others, detrital zircon ages (Olszewski, 1980), Nd isotope model ages 2004). Uranium concentrations are believed to be accurate to ±20 (DiNitto and others, 1984) (see discussion in Hepburn, 2004), percent. The U-Pb data are plotted on Tera-Wasserburg concordia plots to visually identify coherent age groups. Weighted averages 206 238 4Sensitive high-resolution ion microprobe. of individual Pb/ U ages (shown as insets with the concordia 5Laser ablation-inductively coupled plasma-mass spectrometry. plots) were calculated to obtain an age for each sample. The time 4 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts 0.4 0.3 0.7 0 4.2 1 0 0 0.3 0.1 0 0 0 0 0 gg 27.2 32.8 33.1 _ 1,012 GR–203 515±4 Nashoba Grafton Gneiss 1.4 1.1 0 0 0.3 0 0 1.2 0.7 2.8 0 0 0 0 11.1 11.6 48.9 20.9 — Zgdn 1,053 GR–301 8.1 5.5 4.3 0 0.7 0 0 0 2 0.2 3.3 0 0 0 0 30.9 29.7 15.2 — Zgdn 1,093 Granodiorite gneiss at Nourse Farm GR–299 2.4 1.5 0.3 0.6 0 0 0.6 0.3 0.4 0 0 0 0 30.3 23.5 30.1 10.2 — Zpgh 1,057 GR–14B 5.4 0.6 0.4 0 0 1 0 0 0 0.4 0 0 0 0 0 32.7 32.9 26.6 Zpg GR–23 Ponaganset Gneiss 1,109 612±5 Avalon 2.8 0.5 0 0 0 0.9 0 0 0.1 0.1 0 0.1 0 0.1 0 37.7 23.1 34.6 Zncg GR–352 1,055 607±5 Northbridge Granite Gneiss 0.5 1.3 0.1 0 0 0.8 0 0 0 0.2 0 0 0 0 0 33 25.2 38.9 — Zhv 1,062 GR–55 , Modes of plutonic rocks based on a minimum 1,000 point counts A 0.2 1.9 0 0 0 0.6 0 0 0 0 0 0 0 0 0 41.3 23.2 32.7 1,269 606±5 GR–512 Zhvb 1.6 0.6 0.1 0 3 0.6 0 0.1 0 0.1 0.1 0 0 0 0 33.2 26.1 34.5 — Hope Valley Alaskite Gneiss Hope Valley 1,285 GR–466 Zone Field no. Modes, in percent, of selected rocks the Grafton 1:24,000-scale quadrangle, Massachusetts. Map unit label Map unit name

Points counted U-Pb zircon age this study (Ma) Quartz Plagioclase K-feldspar Biotite Muscovite Epidote Hornblende Chlorite Magnetite Opaque Hematite Titanite Apatite Clinozoisite Allanite Garnet Zircon Calcite Table 1. Table [Abbreviations used: Ma, mega-annum; —, no data] U-Pb Geochronology 5 0.5 0 3.8 3.3 0 0 0 0 0 3.3 1.9 2.4 0 0 0 0 13.3 49.5 21.9 210 GR–123 Sd Diorite 6.6 0 0 1 0 0 0 0 0 1.5 0 0.5 0 0 0 0 37.9 14.6 37.9 198 GR–120 Nashoba 2.3 0.6 0 4.6 0 0 0 2.9 0 0 1.7 0 0 0 0 0 Zd 10.4 36.4 41 173 GR–272A Metadiorite 6.4 6.4 2.8 0.9 0 0 0.9 0 0 0 0 0.5 0 0 0 0 0 42.2 39.9 Zpg 218 GR–53A Ponaganset Gneiss Avalon 5.8 2.2 1.8 0 0 0.4 0 0.4 1.3 0.9 6.7 0 0 0 0 11.2 38.6 17 13.5 Zngd 223 GR–45B Granodiorite gneiss at Nourse Farm m 0 2.6 0 4.7 0 9.1 9.1 0.9 0 0.9 0 0 0 0 0 0 11.7 37 23.9 _ 230 GR–244 Calc-silicate gneiss Marlboro Formation m 0 0 2 0 1.5 0 2 0 2.4 0 0 0 0 0 0 Nashoba 11.2 40.5 13.2 27.3 _ , Estimated modes of igneous and calc-silicate rocks based on a minimum 170 point counts 205 GR–205 B Dioritic gneiss gg 4.2 0 3.3 0 0 2.3 0.5 0 0 0.5 0.5 0 0 0 0 0 22.5 39.4 26.8 _ 213 GR–225 Grafton Gneiss Modes, in percent, of selected rocks the Grafton 1:24,000-scale quadrangle, Massachusetts.—Continued

Zone Field no. Map unit label Map unit name Points counted Quartz Plagioclase K-feldspar Biotite Muscovite Epidote Hornblende Actinolite Chlorite Magnetite Opaque Hematite Titanite Apatite Clinozoisite Allanite Garnet Zircon Calcite Table 1. Table [Abbreviations used: Ma, mega-annum; —, no data] 6 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

A B

Mylonite 612±5 Ma

C D

Mylonite 515±4 Ma

Vein Mylonite

606±5 Ma

E 339±17 Ma 356±4 Ma (metamorphic) Figure 2. Photographs of rocks representative of the dated samples in this study. Photographs are from the sampled outcrop or from nearby outcrops. Ages discussed in text. A, Thin mylonite zone in megacrystic biotite granite gneiss of the Ponaganset Gneiss (Zpg), point GR–23. B, Coarse-grained biotite granite gneiss of the Northbridge Granite Gneiss (Znclg), point GR–385. C, Medium-grained alaskite gneiss of the Hope Valley Alaskite Gneiss (Zhvb), point GR–485. Outcrop shows thin dark-weathering mylonite zone, and crosscutting, deformed quartz vein. D, Medium-grained biotite granite gneiss of the Grafton Gneiss (_gg), point GR–202. E, Amphibolite of the Marlboro Formation (_m), point GR–32. Ma, mega-annum. U-Pb Geochronology 7 of igneous crystallization was determined for four granite gneiss Northbridge Granite Gneiss (Zncg) samples, whereas metamorphic zircons were dated from an amphibolite of the Marlboro Formation. The dated rock is a coarse-grained biotite granite gneiss (fig. 2B, table 1A) that is similar to the biotite granite gneiss in the Ponaganset Gneiss; however, it contains less biotite, lacks Results the characteristic microcline augen, and is coarser grained. The rock is equigranular, but locally it contains trace amounts Ponaganset Gneiss (Zpg) of anhedral K-feldspar megacrysts, as much as 1 cm across, that lack augen shapes. Outcrops of the biotite granite gneiss This dated rock is a medium- to coarse-grained unit (Zpg) of the Ponaganset Gneiss locally contain layers megacrystic biotite granite gneiss with microcline augen and lenses of very coarse grained biotite granite gneiss that (fig. 2A, table 1A). The deformation of the unit varies. In most resemble the Northbridge Granite Gneiss. Although definitive places, microcline occurs as deformed augen, although in some crosscutting relations were not observed, we interpret the places the megacrysts are rectangular, and, locally, the augen are layers of coarse-grained biotite granite gneiss as Northbridge- destroyed and the rock contains thin centimeter-scale mylonite like rock that intruded the Ponaganset, although the two rocks zones (fig. 2A). Wayne and others (1992) dated a hornblende- yield similar ages with overlapping uncertainty. bearing phase of the Ponaganset Gneiss (which we map as Zpgh) On the State map, our sample location was called the from a roadcut on State Route 146, and reported conventional Scituate Granite Gneiss, and the description matches the rock U-Pb zircon ages of 635±50 and 621±27 Ma. The hornblende- we map here, but the name “Scituate,” as mentioned above, bearing phase forms a small percentage of the Ponaganset no longer applies. Wones and Goldsmith (1991) noted the Gneiss, and we focused our efforts on the more widespread and problems associated with the name “Scituate” and suggested representative biotite granite gneiss. reviving the name “Northbridge Granite Gneiss” of Emerson The sampled rock matches the description of Ponaganset (1917), and we agree. Gneiss on the State map (unit Zpg of Zen and others, 1983), but Goldsmith and others (1982) suggested the abandonment the location coincides with the Scituate Granite Gneiss (unit of the name “Northbridge Granite Gneiss” because they thought Zsg) on the State map. While the Scituate Granite Gneiss was the rock was part of the Ponaganset Gneiss or the Scituate considered Neoproterozoic on the State map (Zen and others, Granite Gneiss. In the adjacent Oxford quadrangle, Barosh 1983), Hermes and others (1981) and Hermes and Zartman (2005) used the name “Northbridge Granite” instead of “Scituate (1985, 1992) demonstrated that the Scituate Granite Gneiss at Granite Gneiss,” but he did not apply the name “Ponaganset its type locality in Rhode Island (Quinn, 1951), approximately Gneiss” to any rocks nor did he separate the equigranular versus 40 km to the south-southeast, is Devonian, yielding a inequigranular megacrystic rocks on his map. Our mapping and conventional U-Pb zircon age of 373±3 Ma. In part due to the geochemistry show that the name “Northbridge Granite Gneiss” discrepancy between the Neoproterozoic and the Devonian is valid and should be applied to the coarse-grained, equigranular ages, Wones and Goldsmith (1991) recognized that the name biotite granite gneiss. It is distinct from other units and can be “Scituate Granite Gneiss” should not apply to the rocks shown mapped separately from the Ponaganset Gneiss. The chemistry on the State map of Zen and others (1983) and further stated that and age are more similar to the Hope Valley Alaskite Gneiss it was unknown if true Devonian Scituate rocks even existed in than to the Ponaganset. Wones and Goldsmith (1991) noted the Massachusetts. Because the megacrystic biotite granite gneiss chemical similarity between rocks mapped as Scituate Granite matches the description of Ponaganset Gneiss on the State map Gneiss on the State map (Zen and others, 1983), which we now (Zen and others, 1983) and matches the description of the major map as Northbridge Granite Gneiss, and rocks of the Hope Valley rock type at the type locality in the Chepachet quadrangle, Alaskite Gneiss, and our data agree (see section on geochemistry Rhode Island (Quinn, 1967), approximately 35 km to the south, below). In addition, the name “Scituate” is not appropriate we use the name here and apply it only to the megacrystic because the Scituate Granite Gneiss is Devonian in age. biotite granite gneisses (Zpg and Zpgh). The type locality of the Northbridge Granite Gneiss has not Zircons from the Ponaganset megacrystic biotite granite been described in detail. Emerson (1917, p. 155) first used the gneiss (sample GR–23) are medium dark brown, euhedral, and name “Northbridge granite gneiss” and applied the name to a have length-to-width (l/w) ratios of about 1 to 4 (fig. A3 ). The wide expanse of undivided granitoid gneiss that included coarse, grains are somewhat deformed with pitted and bent crystal faces. porphyritic, and muscovitic gneiss. Emerson (1917, p. 156) stated Most grains contain dark (in CL) interior zones, mantled by fine, that the rock was “named for its occurrence at Northbridge” concentric oscillatory zoning, which is indicative of an igneous and cited Emerson and Perry (1907). Emerson and Perry (1907, origin. Twenty of 21 analyses yield a coherent age population p. 9–10) used the name “Northbridge gneiss” and stated that it with a weighted average of 612±5 Ma (fig. B3 ), interpreted as the was a “name given elsewhere” and cited Emerson (1898). Despite time of crystallization of the protolith of the megacrystic granite the claim of Emerson and Perry (1907) that the name is given gneiss. Analysis 23–5.1 yielded discordant ages of about 525 Ma elsewhere, the type locality is not described by Emerson (1898). (table 2), possibly due to inadvertent placement of the spot on a Emerson and Perry (1907, p. 9) noted that the dominant phase crack in the grain; this area probably lost some radiogenic Pb. was a medium- to coarse-grained granitoid gneiss with “regularly 8 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

Table 2. Sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb data for zircon from rocks of the Grafton 1:24,000-scale quadrangle, Massachusetts.

[Abbreviations used: Ma, mega-annum; ppm, parts per million; —, no data]

Measured Measured % common U 206Pb/238U1 err2 err2 err2 Sample Th/U 238U/206Pb1 207Pb/206Pb1 204Pb/206Pb 207Pb/206Pb 206Pb (ppm) (Ma) (Ma) (%) (%) Zpg—Ponaganset Gneiss (GR–23) 23–1.1 0.000337 0.0604 0.02 73 0.54 612.4 8.1 10.09 1.4 .0555 4.3 23–2.1 0.000118 0.0604 0.00 154 0.90 619.7 7.1 9.93 1.2 .0587 2.0 23–3.1 0.000163 0.0597 –0.07 205 0.84 615.2 7.0 10.02 1.2 .0574 2.2 23–4.1 0.000171 0.0612 0.14 87 0.53 602.7 7.6 10.22 1.3 .0587 2.9 23–5.1 0.000048 0.0629 0.62 104 0.44 525.3 6.5 11.72 1.3 .0622 2.2 23–6.1 0.000359 0.0633 0.40 74 0.86 602.6 7.9 10.23 1.4 .0581 4.1 23–7.1 0.000451 0.0608 0.08 73 0.64 608.4 8.1 10.18 1.4 .0542 5.2 23–8.1 — 0.0605 0.10 66 0.45 593.8 8.2 10.35 1.4 .0605 2.3 23–9.1 0.001060 0.0624 0.28 42 0.91 608.6 9.7 10.27 1.8 .0468 12.7 23–10.1 0.000120 0.0602 –0.01 126 0.83 616.0 7.6 10.00 1.3 .0585 2.4 23–11.1 0.000243 0.0614 0.17 114 0.56 605.8 7.8 10.18 1.3 .0579 3.3 23–12.1 — 0.0581 –0.23 61 0.84 605.2 9.2 10.18 1.5 .0581 2.8 23–13.1 0.000176 0.0621 0.24 124 0.91 609.5 8.2 10.09 1.4 .0596 3.1 23–14.1 0.000122 0.0609 0.07 152 0.92 617.1 7.9 9.97 1.3 .0592 2.5 23–15.1 0.000226 0.0587 –0.16 81 0.52 603.3 9.1 10.25 1.6 .0554 5.0 23–16.1 0.000123 0.0617 0.16 279 0.66 617.0 7.5 9.96 1.2 .0599 2.3 23–17.1 0.000196 0.0612 0.12 174 0.46 612.3 8.3 10.06 1.4 .0584 3.4 23–18.1 — 0.0589 –0.14 235 0.58 604.8 7.7 10.18 1.3 .0589 2.3 23–19.1 0.000150 0.0613 0.14 132 0.54 609.7 9.3 10.09 1.6 .0591 4.0 23–20.1 0.000144 0.0590 –0.21 321 0.94 629.7 7.7 9.79 1.3 .0569 2.5 23–21.1 0.000156 0.0603 –0.04 223 0.57 626.1 7.6 9.84 1.2 .0580 2.7 Zncg—Northbridge Granite Gneiss (GR–352) 352–1.1 –0.000019 0.0603 0.01 359 0.59 611.9 6.3 10.04 1.1 .0606 1.0 352–2.1 — 0.0588 –0.15 256 0.65 604.9 6.4 10.18 1.1 .0588 1.1 352–3.1 0.000087 0.0589 –0.18 240 0.51 614.9 6.6 10.02 1.1 .0576 1.5 352–4.1 0.000118 0.0603 0.02 315 0.79 608.7 6.3 10.12 1.1 .0586 1.5 352–5.1 0.000056 0.0591 –0.15 177 0.64 613.3 7.2 10.04 1.2 .0582 1.7 352–6.1 0.000054 0.0600 –0.04 490 0.58 616.3 6.2 9.98 1.0 .0592 1.0 352–7.1 0.000063 0.0603 0.04 328 0.50 603.6 6.3 10.19 1.1 .0594 1.2 352–8.1 0.000086 0.0600 –0.01 236 0.54 604.1 6.5 10.20 1.1 .0587 1.5 352–9.1 0.000043 0.0606 0.08 114 0.74 604.1 7.2 10.18 1.2 .0600 1.9 352–10.1 0.000043 0.0600 0.09 559 0.48 577.7 5.8 10.66 1.0 .0594 0.9 352–11.1 –0.000019 0.0603 0.04 442 0.53 603.9 6.2 10.17 1.1 .0606 1.0 352–12.1 0.000134 0.0601 0.01 211 0.64 605.0 6.8 10.19 1.2 .0581 2.1 352–13.1 0.000040 0.0622 0.29 283 0.57 596.6 6.6 10.29 1.1 .0616 1.4 352–14.1 0.000050 0.0592 –0.14 161 0.44 614.8 7.6 10.02 1.3 .0585 2.0 352–15.1 0.000118 0.0608 0.10 523 0.56 605.8 6.3 10.16 1.1 .0591 1.5 352–16.1 — 0.0615 0.17 142 0.61 607.4 8.3 10.10 1.4 .0615 2.3 352–17.1 0.000022 0.0600 –0.02 1136 0.36 610.7 7.8 10.07 1.3 .0597 0.9 352–18.1 0.000099 0.0614 0.21 491 0.37 592.4 6.7 10.39 1.2 .0600 1.6 352–19.1 — 0.0592 –0.15 222 0.70 618.8 7.6 9.94 1.3 .0592 1.8 352–20.1 0.000123 0.0595 –0.07 272 0.51 608.4 7.2 10.13 1.2 .0578 2.3 Zhvb—Hope Valley Alaskite Gneiss (GR–512) 512–1.1 — 0.0612 0.14 97 0.88 606.7 7.5 10.12 1.3 .0612 1.8 512–2.1 0.000344 0.0622 0.24 105 0.71 612.0 7.5 10.08 1.3 .0572 3.8 512–3.1 0.000031 0.0597 –0.07 167 0.43 615.3 6.9 10.00 1.2 .0593 1.7 512–4.1 0.000117 0.0599 –0.05 192 0.71 616.7 6.8 9.99 1.1 .0582 1.9 512–5.1 0.000156 0.0594 –0.08 161 0.56 604.5 6.8 10.21 1.2 .0571 2.3 512–6.1 0.000024 0.0595 –0.12 293 0.75 622.0 6.5 9.89 1.1 .0592 1.1 512–7.1 — 0.0613 0.13 120 0.88 613.2 7.2 10.01 1.2 .0613 1.6 512–8.1 –0.000047 0.0601 0.07 160 0.55 586.1 6.7 10.49 1.2 .0608 1.6 512–9.1 –0.000145 0.0611 0.17 57 0.72 592.9 8.3 10.34 1.4 .0632 2.9 512–10.1 0.000035 0.0606 0.05 451 1.12 613.5 6.3 10.02 1.0 .0601 1.0 512–11.1 0.000128 0.0585 –0.19 134 0.65 607.3 7.2 10.17 1.2 .0567 2.4 512–12.1 0.000100 0.0602 0.08 143 0.55 587.0 7.0 10.50 1.2 .0587 2.1 512–13.1 0.000085 0.0632 0.32 83 0.74 624.1 8.7 9.82 1.4 .0619 2.7 512–14.1 0.000073 0.0596 –0.04 143 0.57 601.8 7.5 10.24 1.3 .0586 2.1 512–15.1 –0.000192 0.0599 0.01 98 0.88 597.2 8.2 10.27 1.4 .0627 3.1 512–16.1 0.000069 0.0611 0.18 146 0.84 589.5 7.8 10.44 1.4 .0601 2.4 512–17.1 — 0.0617 0.24 42 0.63 593.9 11.8 10.34 2.0 .0617 4.0 512–18.1 0.000059 0.0615 0.16 355 0.81 610.3 7.2 10.07 1.2 .0606 2.1 512–19.1 0.000217 0.0601 0.04 247 0.46 595.0 7.2 10.38 1.2 .0570 2.8 U-Pb Geochronology 9

Table 2. Sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb data for zircon from rocks of the Grafton 1:24,000-scale quadrangle, Massachusetts.—Continued

[Abbreviations used: Ma, mega-annum; ppm, parts per million ; —, no data]

Measured Measured % common U 206Pb/238U1 err2 err2 err2 Sample Th/U 238U/206Pb1 207Pb/206Pb1 204Pb/206Pb 207Pb/206Pb 206Pb (ppm) (Ma) (Ma) (%) (%) _gg—Grafton Gneiss (GR–203) 203–20.1 — 0.0563 0.000 361 0.541 514 6.4 12.1 1.3 0.0563 2.0 203–21.1 0.000049 0.0576 0.088 299 0.673 505 6.7 12.3 1.3 0.0569 2.6 203–22.1 0.000009 0.0571 0.016 403 0.606 518 6.3 12.0 1.2 0.0570 2.0 203–23.1 — 0.0571 0.000 287 0.606 508 6.6 12.2 1.3 0.0571 2.2 203–24.1 — 0.0592 0.000 573 0.685 511 5.9 12.1 1.2 0.0592 1.6 203–25.1 0.000064 0.0569 0.114 319 0.722 518 6.6 12.0 1.3 0.0560 2.4 203–26.1 — 0.0585 0.000 351 0.641 514 6.4 12.0 1.3 0.0585 2.0 203–27.1 — 0.0572 0.000 473 0.687 519 6.2 11.9 1.2 0.0572 1.7 203–28.1 — 0.0576 0.000 347 0.666 508 6.4 12.2 1.3 0.0576 2.0 203–29.1 0.000009 0.0585 0.016 385 0.838 522 6.4 11.8 1.3 0.0583 2.0 203–29.2 — 0.0577 0.000 2298 0.286 522 5.4 11.9 1.0 0.0577 0.8 203–30.1 — 0.0578 0.000 267 0.632 514 6.8 12.0 1.3 0.0578 2.2 203–31.1 — 0.0568 0.000 372 0.841 514 6.3 12.1 1.3 0.0568 2.0 _m—Amphibolite of the Marlboro Formation (GR–32) 32–1.1m3 0.001947 0.0654 1.52 27 0.18 336.6 12.3 18.37 3.6 .0654 10.3 32–1.2m3 0.004671 0.0572 0.44 8 0.10 358.9 22.8 17.39 6.3 .0572 20.0 32–2.1m3 — 0.0582 0.59 151 0.30 349.0 6.2 17.87 1.8 .0582 4.5 32–2.2m3 0.005169 0.0763 2.88 7 0.14 332.3 22.6 18.36 6.7 .0763 18.6 32–3.1m3 0.000172 0.0531 –0.06 217 0.41 353.7 6.0 17.74 1.7 .0531 3.9 32–4.1m3 — 0.0536 –0.01 196 0.30 358.2 5.8 17.50 1.6 .0536 4.0 32–4.2m3 — 0.0535 –0.01 8 0.14 355.3 22.1 17.65 6.2 .0535 19.3 32–5.1m3 — 0.0654 1.58 6 0.08 318.0 23.5 19.46 7.3 .0654 21.8 32–6.1m3 0.001515 0.0586 0.64 233 0.51 347.7 5.4 17.93 1.6 .0586 4.5 32–7.1m3 0.008847 0.1434 11.25 165 0.31 322.9 19.9 17.28 1.9 .1434 29.8 32–8.1m3 0.000503 0.0559 0.28 188 0.45 355.6 5.9 17.59 1.7 .0559 4.0 32–9.1m3 0.000371 0.0555 0.22 193 0.26 358.9 6.3 17.43 1.8 .0555 4.1 32–10.1m3 — 0.0559 0.27 298 0.48 359.3 5.2 17.40 1.5 .0559 3.2 32–11.1m3 — 0.0566 0.45 209 0.30 329.8 5.4 18.96 1.6 .0566 4.0 32–12.1m3 0.000201 0.0530 –0.09 289 0.37 360.0 5.2 17.43 1.5 .0530 3.3 32–13.1m3 0.000119 0.0525 –0.14 296 0.42 357.6 5.2 17.56 1.5 .0525 3.3 32–14.1m3 0.000248 0.0510 –0.31 141 0.39 350.8 6.3 17.94 1.8 .0510 4.8 32–15.1m3 0.000144 0.0558 0.26 417 0.45 361.6 4.9 17.29 1.4 .0558 2.7 1Corrected for common Pb. 206Pb/238U ages corrected for common Pb using the 207Pb-correction method; 207Pb/206Pb and 238U/206Pb corrected for common Pb using the 204Pb-correction method. 21-sigma error. 3Abbreviation used: m, metamorphic grain. Description of Geochronology and Geochemistry Sample Locations Zpg—Ponaganset Gneiss (GR–23) Roadcut located on the northern side of Purgatory Road in Northbridge, 0.1 mile east of the junction with State Route 146, across from the entrance to the West End Creamery ice cream stand. Sample was collected about 2.0 meters west of a 0.7-meter-thick pegmatite dike on the western end of the outcrop. Coordinates (NAD27) in south-central 1/9 of quadrangle: 42º07′35.7″ N., 71º42′05.9″ W. Zncg—Northbridge Granite Gneiss (GR–352) Roadcut located on the eastern side of School Street in Northbridge, 0.7 mile southeast of the junction of State Route 122 in downtown Northbridge. The roadcut is located just north of the “Richard T. Larkin” trailhead to Shining Rock. Coordinates (NAD27) in southeast 1/9 of quadrangle: 42º09′02.0″ N., 71º38′30.2″ W. Zhvb—Hope Valley Alaskite Gneiss (GR–512) Large roadcut located on the eastern side of the northbound off-ramp at the junction of Hartford Turnpike and State Route 146 in Sutton. From the Hartford Turnpike, proceed south on the off-ramp (walk the “Wrong Way”) about 0.1 mile from turnpike junction. Sample location is about 8 paces south of the blue “Attractions” sign. Coordinates (NAD27) in southwest 1/9 of quadrangle: 42º08′55.8″ N., 71º43′14.9″ W. _gg—Grafton Gneiss (GR–203) Roadcut located at lot 8 on Autumn Harvest Court, in a new development, on the southeastern flank of Potter Hill in Grafton. The cut is located on the driveway to lot 8 on the northern side of the cul-de-sac. There are two roadcuts along the driveway; the sample came from the outcrop on the northern side of the driveway. Coordinates (NAD27) in west-central 1/9 of quadrangle: 42º12′06.8″ N., 71º42′45.5″ W. _m—Amphibolite of the Marlboro Formation (GR–32) Roadcut on Brigham Hill Road (labeled “Lordvale Avenue” on the base map) in Grafton, on the northwestern side of Brigham Hill. Coordinates (NAD27): 42º12′51.2″ N., 71º43′06.4″ W. 10 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

Ponaganset Gneiss (GR–23) 0.070 A B

0.066 650 610

U age 570 238 Age=612±5 Ma 0.062 700 Pb/ 530 (MSWD=1.18)

660 206

Pb 490

206 Analyses 580

Pb/ 0.058 540 207 500 460 0.054 420 Ma 100 µm

0.050 8 9 10 11 12 13 14 15 16

Northbridge Granite Gneiss (GR–352) 0.070 Age=607±5 Ma C D 650 (MSWD=0.95) 630 0.066 610 U age

238 590

Pb/ 570 206 0.062 700 550 Pb Analyses

206 660

Pb/ 580

207 0.058 540 500 460 0.054 420 100 µm

0.050 8 9 10 11 12 13 14 15 16

Hope Valley Alaskite Gneiss (GR–512)

0.070 Age=606±5 Ma E F 660 (MSWD=1.39) 640

0.066 U age 620 238 600 Pb/ 580 206 0.062 700 560

Pb Analyses 660 206

Pb/ 580

207 0.058 540 500 460 0.054 420

0.050 8 9 10 11 12 13 14 15 16 100 µm 238U/206Pb

Figure 3. Cathodoluminescence (CL) and transmitted light images of emplacement of igneous protoliths. White error ellipses are data of representative zircons and concordia plots of dated rocks from the that are interpreted to be younger than the age of the rock, due to a Avalon zone in the Grafton quadrangle. A and B, Ponaganset Gneiss. variety of causes (see text for further explanation). Dark-gray error C and D, Northbridge Granite Gneiss. E and F, Hope Valley Alaskite ellipses are data that are interpreted to be older than the age of the Gneiss. Medium-gray error ellipses are data used in weighted rock, probably due to inheritance. Ma, mega-annum; MSWD, mean averages of selected 206Pb/238U ages (see insets) for determining time square weighted deviation; µm, micrometer. U-Pb Geochronology 11 and evenly granular” feldspar. The correlation and extent of the equigranular gneiss at Northbridge and porphyritic varieties elsewhere had “not been definitely traced in the field” (Emerson and Perry, 1907, p. 10). Here, we define the type locality for exposures at Shining Rock in Northbridge. Very good exposures also occur in Northbridge along School Street where the dated sample was collected and in Grafton along Milford Road, especially at the junction with Bay Farm Lane (road is not on the base map), approximately 0.9 km east of the junction of Milford Road and State Route 122. Zircons from the Northbridge Granite Gneiss (sample GR–352) are medium brown and euhedral and have l/w ratios of 2 to 4 (fig. C3 ). The crystal faces are significantly pitted, and most grains contain numerous inclusions. Most grains have fine concentric oscillatory zoning in CL, which is indicative of an igneous origin. Nineteen of 20 analyses yield a coherent Figure 4. Photograph of light-colored alaskite sill in the age population with a weighted average of 607±5 Ma (fig. D3 ), Ponaganset Gneiss at point GR–141 along State Route 122 at interpreted as the time of crystallization of the protolith of the the junction with State Route 122A in Farnumsville. The sill is Northbridge Granite Gneiss. Analysis 352–10.1 yielded a younger correlated with the Hope Valley Alaskite Gneiss. Strike and dip age of about 578 Ma (table 2). The location of this analysis symbols on the map show the location of similar sills. overlapped a relatively light-colored (in CL), finely oscillatory- zoned mantle and a darker, broadly zoned outer rim. Although crystal faces. Because it is possible that some of these zircons this rim was originally thought to be the outer part of the igneous are xenocrystic (in other words, inherited from the parent rock mantle, it may be a younger metamorphic overgrowth. The age of the alaskite), we mostly analyzed fine oscillatory zones near of this 578 Ma analysis, therefore, is geologically meaningless the tips of relatively pristine grains. Fifteen of 19 analyses yield because it is a mixture of two growth components and was, a coherent age population with a weighted average of 606±5 Ma therefore, discarded from the age calculation. (fig. 3F), interpreted as the time of crystallization of the alaskite. Four analyses were excluded from the age calculation, including (1) 512–6.1 and 512–13.1 with slightly older ages of 622 and Hope Valley Alaskite Gneiss (Zhvb and Zhv) 624 Ma, respectively, and (2) 512–8.1 and 512–12.1 with slightly This rock is a medium-grained alaskite gneiss with trace younger ages of 586 and 587 Ma, respectively (table 2). The amounts to a few percent biotite (fig. C2 , table 1A). The sample older ages were obtained on interiors of zircons and, thus, are was collected from a large roadcut that consists mainly of interpreted as inherited cores. The younger ages are from areas alaskite gneiss with accessory magnetite. Within the roadcut that appear to be pristine and have similar CL zonations as those there are layers of granite gneiss that contain as much as 3 locations that yielded the ages used in the weighted average percent biotite, which appears as small pepper-like flakes, calculation. The cause of the younger ages is undetermined but alternating with layers that contain only trace amounts of biotite; may be due to undetected imperfections in the zircons. Our age is thus we map the unit as Zhvb. The map shows the distribution similar to, within uncertainty, an earlier conventional U-Pb zircon of two units: an alaskite gneiss with trace amounts to no biotite age of 601±5 Ma obtained by Hermes and Zartman (1985). (Zhv) and an alaskite gneiss with trace amounts to 3 percent biotite (Zhvb). The two units appear to be coeval and part of the Grafton Gneiss (_gg) same intrusive event and, thus, part of the same magma. On the State map (Zen and others, 1983), the rock was mapped as Hope This rock is a biotite granite gneiss (fig. D2 , table 1A) Valley Alaskite Gneiss (unit Zhv), and we agree. The alaskitic in the Nashoba zone, shown as an unnamed feldspathic rocks intruded the units of the Northbridge Granite Gneiss and gneiss (unit OZmg) on the State map (Zen and others, 1983). the Ponaganset Gneiss (fig. 4), and our observations support the Goldsmith (1991b, p. F5) called the unit a biotite granodiorite findings that the Hope Valley Alaskite Gneiss is the youngest gneiss and stated that the unit was “informally named the intrusive rock in the Milford antiform (Hermes and Zartman, Grafton gneiss” on the basis of a written communication with 1985; Wones and Goldsmith, 1991). H.R. Dixon in 1977. In this report we propose the formal name The dated sample of Hope Valley Alaskite Gneiss is a “Grafton Gneiss.” The type locality is in the town of Grafton, biotite-poor variety within the Zhvb unit (sample GR–512, table and typical exposures occur on Brigham Hill, on Potter Hill, 1A). The rock yielded very few zircons, which allowed for along Snow Road, and on the Massachusetts Turnpike east nearly all to be mounted and imaged. The zircon population of Snow Road. Other good exposures occur on an unnamed is heterogeneous; the grains range from light to dark brown, hill northwest of the Grafton train station on Westboro Road subhedral to euhedral, and stubby to prismatic (l/w ratios of 1–4; and on unnamed hills east and west of Willard Street on the fig. 3E). Many of the grains have pitted and partially resorbed campus of Tufts University, south of the Westboro Road. 12 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

Our mapping shows that the rock is intrusive into the the poorly documented sample location provided by Acaster Marlboro Formation. It contains xenoliths of amphibolite, and and Bickford (1999, sample GG–1), and our work supersedes the margin of the Grafton Gneiss is migmatitic, exhibiting lit- their results. par-lit sills of granite in amphibolite, dioritic gneiss, and calc- Zircons from the Grafton Gneiss (sample GR–203) silicate gneiss of the Marlboro Formation. Hepburn (1978) are light brown and euhedral and have l/w ratios of 2 to 4. interpreted the Grafton Gneiss as plutonic, and we agree. Most grains show very fine, fairly dark-colored concentric Goldsmith (1991b) suggested that the Grafton Gneiss could oscillatory zoning in CL (fig. 5A), which is indicative of be a metamorphosed felsic volcanic rock, and we disagree. an igneous origin. Thirteen analyses yield a coherent age Acaster and Bickford (1999) dated a rock that they called the population with a weighted average of 515±4 Ma (fig. B5 ). “Grafton Gneiss” and obtained a conventional U-Pb zircon This date is a minimum age for the part of the Marlboro age of 584±8 Ma. Considerable uncertainty exists regarding Formation into which the Grafton Gneiss intrudes.

Grafton Gneiss (GR–203)

0.064 Age=515±4 Ma A B 545 (MSWD=0.75) 0.062 535 525 U age 515 0.060 238 505 Pb/ 495 206

Pb 0.058 560 485 Analyses 206 520

Pb/ 0.056 480 207 440 0.054 400

0.052

100 µm 0.050 10 11 12 13 14 15 16 17 238U/206Pb

Amphibolite of the Marlboro Formation (GR–32)

0.08

Age=356±4 Ma C D 390 (MSWD=0.99) 0.07 370 350 U age

238 330

Pb/ 310

0.06 206 290 Cores Pb 270 550 206 Age=339±17 Ma 450 250 (MSWD=0.56) Pb/ 350 410 200

207 0.05 370 U age

238 330 100 µm 0.04 Pb/ 290

206 Rims 250 Analyses 0.03 8 10 12 14 16 18 20 22 24 26 238U/206Pb

Figure 5. Cathodoluminescence (CL) and transmitted light images the Marlboro Formation. Concordia plot uncorrected for common of representative zircons and concordia plots of dated rocks from Pb. Medium-gray error ellipses are data from cores of metamorphic the Nashoba zone in the Grafton quadrangle. A and B, Grafton zircons. Light-gray error ellipses are data from rims of metamorphic Gneiss. Medium-gray error ellipses are data used in weighted zircon containing very low U content. White error ellipse is from averages of selected 206Pb/238U ages (see insets) for determining a slightly younger core. Ma, mega-annum; MSWD, mean square time of emplacement of igneous protolith. C and D, Amphibolite of weighted deviation; µm, micrometer. Geochemistry 13

Marlboro Formation—Amphibolite (_m) many grains contain dark-colored sector-zoned, irregular cores that are overgrown by very faint sector-zoned, or unzoned, The sampled rock is an amphibolite typical of the major rims (fig. C5 ). The weighted average age of the dark cores is rock found throughout the Marlboro Formation (fig. E2 ). It 356±4 Ma; five analyses of rims have very large uncertainties is a medium-grained, plagioclase-hornblende amphibolite (shown as light-gray error ellipses in figure D5 ) due to very gneiss with millimeter-scale epidote laminations. The rock low U contents (6–27 ppm; table 2) and yield an imprecise age was collected to determine the age of metamorphism in the of 339±17 Ma. Because most of the zircons show evidence Nashoba zone. We presume that zirconium in the protolith for two growth events (in other words, dark cores overgrown resided in pyroxenes; during high-grade metamorphism, the and crosscut by light rims), we suggest that these two ages are igneous pyroxenes reacted to form amphibole, which cannot indicative of two metamorphic events in the Nashoba zone. accommodate Zr, which then formed metamorphic zircon. Because metamorphic zircons form under conditions that are very different from igneous crystallization, they usually have different external morphologies and internal zoning. Geochemistry Zircons from the sample of the amphibolite of the The four dated samples of granitic rocks were analyzed Marlboro Formation (sample GR–32) are colorless to light for major, trace and rare earth elements, and isotopes of Sm, tan and equant to prismatic (l/w ratios of 1–3) and contain Nd, and Pb—three samples are from the Avalon zone, and one very few inclusions. The equant grains are multifaceted and is from the Nashoba zone. Results are presented in table 3 and exhibit the so-called “soccer ball” morphology (Schaltegger in figures 6 and 7. and others, 1999). Cathodoluminescence imaging shows that

Table 3. Major and trace element whole-rock geochemistry from the Grafton 1:24,000- scale quadrangle, Massachusetts.

[Methods and error limits described in the text. Abbreviations used: LOI, loss on ignition; Ma, mega- annum]

Sample no. GR–23 GR–352 GR–512 GR–203 Map unit label Zpg Zncg Zhvb _gg U-Pb zircon age this study (Ma) 612±5 607±5 606±5 515±4 Major elements (percent)

SiO2 75.869 75.001 76.894 72.603

TiO2 0.137 0.187 0.09 0.213

Al2O3 12.748 12.467 12.078 13.544 1 Fe2O3 1.5 1.507 1.232 2.501 MnO 0.047 0 0.027 0 MgO 0.156 0.218 0.078 0.245 CaO 0.882 0.529 0.304 1.557

Na2O 3.12 2.588 3.495 3.872

K2O 5.308 6.081 4.812 3.723

P2O5 0.014 0.045 0.005 0.033 LOI 0.24 0.44 0.38 0.32 Total 100.021 99.063 99.395 98.611 Trace and rare earth elements (parts per million) Ba 463 423 181 447 Ce 83.9 85.2 70.9 73.3 Co 22.6 13.3 31.7 10.2 Cs 0.98 1.07 1.7 4.5 Dy 5.64 3.54 5.38 5.65 Er 2.57 2.5 3.67 3.25 Eu 1.26 0.6 0.49 0.89 Ga 20 14.6 15.1 15 Gd 7.49 3.67 5.89 6.23 Hf 7.4 5 3.8 3.7 Ho 0.97 0.73 1.13 1.11 14 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

Table 3. Major and trace element whole-rock geochemistry from the Grafton 1:24,000- scale quadrangle, Massachusetts.—Continued

[Methods and error limits described in the text. Abbreviations used: LOI, loss on ignition; Ma, mega- annum]

Sample no. GR–23 GR–352 GR–512 GR–203 Map unit label Zpg Zncg Zhvb _gg U-Pb zircon age this study (Ma) 612±5 607±5 606±5 515±4 Trace and rare earth elements (parts per million)—Continued La 39.3 21.1 33.2 34.6 Lu 0.29 0.42 0.61 0.43 Nb 9.6 12.8 15.2 9.9 Nd 37.6 18.8 30.3 30.6 Pb 18 13 16 15 Pr 9.77 5.27 8.17 8.16 Rb 107.5 168 182 172 Sm 7.73 3.43 5.92 5.97 Sr 145.5 62.1 41.5 94.6 Ta 0.6 1 2.2 0.9 Tb 1.05 0.57 0.93 0.95 Th 19.6 15.15 19.7 11.4 Tm 0.34 0.39 0.56 0.48 U 2.37 2.67 4.76 1.68 V 8 7 <5 5 W 410 220 288 160 Y 24.7 21.3 34.1 29.5 Yb 2.09 2.89 4.08 3.12 Zn 49 33 20 22 Zr 278 162.5 94.4 108 Isotopes (parts per million) and isotopic ratios Sm 6.395 3.485 5.91 8.01 Nd 32.11 18.77 30.03 38.4 147Sm/144Nd 0.1205 0.1123 0.1191 0.1262 143Nd/144Nd 0.512312 0.512297 0.512301 0.512343 206Pb/204Pb 18.629 19.305 19.301 19.311 207Pb/204Pb 15.607 15.578 15.544 15.623 208Pb/204Pb 39.423 40.070 38.992 39.753 -0.40 -0.10 -0.56 -1.09 Initial εNd CIPW norms2 Qtz 35.4 35.8 37.8 32.8 Or 31.4 36.4 28.7 22.4 Ab 26.5 22.2 29.9 33.4 An 4.3 2.4 1.5 7.6 Crn 0.3 0.8 0.6 0.4 Di 0.0 0.0 0.0 0.0 Hyp 0.4 0.6 0.2 0.6 Mt 0.0 0.0 0.0 1.7 Ilm 0.1 0.0 0.1 0.4 Hem 1.5 1.5 1.2 0.6 Ap 0.0 0.1 0.0 0.1 Ru 0.1 0.2 0.1 0.0 1 Total iron as Fe2O3. 2CIPW norms calculated using Igpet software. U-Pb Geochronology 15

A FeOT B An

EXPLANATION Graphs A–C and E Graph D Grafton Gneiss Zhvb (GR–512) Tholeiitic Rocks of the _gg (GR–203) Milford antiform Zncg (GR–352) Zpg (GR–23)

Tonalite

Granodiorite

Calc-alkaline Trondhjemite Granite

Alk MgO Ab Or

C D 3 Metaluminous Peraluminous

100

2 Al/(Na+K) Rock/chondrites 10 1 Peralkaline

0 0.5 1.0 1.5 2.0 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Al/(Ca+Na+K) Rare earth elements

E 1,000

100

10 Figure 6. Major, trace, and rare earth element discrimination diagrams for four granitic rock samples from the Grafton

quadrangle. A, Na2O + K2O (Alk)-FeOT-MgO (AFM) diagram after

Rock/primitive mantle Irvine and Baragar (1971). B, Normative An-Ab-Or classification 1 diagram of O’Connor (1965) and Barker (1979). C, Alumina saturation diagram after Shand (1951) and Maniar and Piccoli (1989). D, Rock- chondrite normalized rare earth element diagram after Sun and McDonough (1989). E, Rock-primitive mantle normalized multi- 0.1 Ba Rb Th K Nb La Ce Sr Nd P Sm Zr Ti Tb Y Tm Yb element “spider” diagram after Thompson and others (1984). See Rare earth elements text for discussion. An, anorthite; Ab, albite; Or, orthoclase. 16 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

A B EXPLANATION 100 1,000 This study Syn-collisional granite Grafton Gneiss Within plate Rocks of the granite Milford antiform 10 100

Volcanic arc From Wones and Goldsmith (1991) granite 1 Rb, in parts per million 10 Ocean ridge Th, in parts per million Neoproterozoic intrusive rocks granite in the Milford antiform Neoproterozoic intrusive rocks in the Milford-Dedham zone

1 0.1 2 10 100 1,000 0 1 2 3 4 5 6 7 8 9 10 Y+Nb, in parts per million U, in parts per million C D 8.0 Middletown complex 16.0 + Depleted mantle 15.9 6.0 Killingworth complex High µ Avalon zone mantle Z-age rocks 15.8 Enriched 4.0 + Neoproterozoic gneiss in mantle II Clinton and Lyme domes, Conn. + 15.7 Bulk silicate earth 2.0 Neoproterozoic gneiss in +

+ Selden Neck block, Conn. Pb 15.6 Enriched 0 Nd

0.0 20 4 ε mantle I 800 Pb / 15.5 Prevalent mantle

–2.0 Laurentian Gander 207 Initial (Grenville) zone 15.4 Z-age Y-age rocks Depleted mantle source –4.0 rocks 1,600 15.3 of E-MORB –6.0 15.2

–8.0 Depleted mantle source 15.1 of N-MORB –10.0 15.0 800 700 600 500 400 300 200 15 16 17 18 19 20 21 22 Age, in Ma 206Pb/204Pb E 15.85

15.80 Figure 7. Trace element and isotope discrimination diagrams 15.75 for four granitic rock samples from the Grafton quadrangle. Gander A, Tectonic discrimination diagram after Pearce and others (1984). 15.70 Miramichi B, Th versus U plot showing data from Wones and Goldsmith

Pb Avalon

204 15.65 (1991) for comparison. C, Initial εNd versus age modified from Pb/ Aleinikoff and others (2007). D, Lead isotope ratios for different 207 15.60 mantle sources after Wilson (1989). Average growth curve from Stacey and Kramers (1975). E, Lead isotope ratio versus ε at 15.55 Nd Laurentian 515 Ma. Fields compiled from Whalen and others (1994), Kerr and crust 15.50 others (1995), Barr and others (1998), and Sampson and others (2000). See text for discussion. Abbreviations used: E-MORB, 15.45 –9.0 –7.0 –5.0 –3.0 –1.0 1.0 3.0 5.0 7.0 9.0 enriched mid-ocean ridge basalt; N-MORB, normal mid-ocean at 515 Ma ridge basalt; Ma, mega-annum. εNd Structural Geology 17

Methods Structural Geology Major elements were analyzed by X-ray fluorescence (XRF) using fused disks on a Siemens SR 303 at the Ductile Structures Department of Geological Sciences at Brigham Young University, Provo, Utah. Trace elements were analyzed by The rocks of the Grafton quadrangle occur in four main inductively coupled plasma-mass spectrometry (ICP–MS) structural domains. From northwest to southeast, they are the at ALS Chemex, Reno, Nev. Isotopic measurements of Nd Nashoba, Marlboro, Westboro, and Milford domains (fig. 1). and Pb were conducted at the Department of Geological Each structural domain is bounded by a fault. At least three Sciences, University of Colorado, Boulder, Colo. Errors are generations of ductile deformation and associated structures 143 144 2σ of mean. Measured Nd/ Nd values were normalized to are recognized and are designated by their relative ages as D1 146 144 Nd/ Nd = 0.7219. Analyses were dynamic mode, three- to D3. The oldest structures in the Avalon and Nashoba zones collector measurements. Thirty-three measurements of the (D1) are probably not correlative across the terrane-bounding La Jolla Nd standard during the study period yielded a mean Bloody Bluff fault, and the relative ages of structures do not 143Nd/144Nd = 0.511838±8 (2σ mean). Isotopic analyses of correlate to the absolute ages of the structures (Goldsmith, Pb were four-collector, static mode measurements. Sixteen 1991c). Thus, D1 in the Nashoba zone did not develop at measurements of standard SRM–981 during the study period the same time or under the same conditions as D1 in the yielded 208Pb/204Pb = 36.56±0.03, 207Pb/204Pb = 15.449±0.008, Avalon zone. In addition, it appears unlikely that the second- 206 204 and Pb/ Pb = 16.905±0.007 (2σ mean). Measured Pb isotope generation structures (D2) correlate across the Bloody Bluff ratios were corrected to SRM–981 values (208Pb/204Pb = 36.721, fault as well, because the metamorphic histories are different. 207Pb/204Pb = 15.491, 206Pb/204Pb = 16.937). Total procedural In the following discussion, we present the ductile structures blanks averaged ~1 nanogram (ng) for Pb and Sr, and in order of their relative ages, irrespective of their absolute 100 picogram (pg) for Nd, during the study period. ages.

Results First-Generation Foliation—D1 Deformation The oldest foliation is a relict layer-parallel schistosity All four rocks are calc-alkaline granites (fig. A6 , B). They (S ) observed in the Nashoba, Marlboro, and Westboro are mildly metaluminous with the Grafton Gneiss sample 1 domains (fig. 8). In the Nashoba zone, the relict foliation plotting along the metaluminous-peraluminous boundary is found in rootless isoclinal F folds (fig. 8A, B). The (fig. 6C). The Rb versus Y+Nb tectonic discrimination 2 relict foliation is characterized by a lepidoblastic texture diagram (fig. A7 ) shows the rocks as volcanic arc granites, of aligned micas or leucosomes. The relation of this relict consistent with the chondrite-normalized rare earth foliation to bedding is unclear as bedding was not observed element patterns and multi-element “spider diagrams” in the Nashoba Formation and only rarely observed in the (fig. 6D, E, respectively). All samples have fairly steep light Marlboro Formation. Definitive folds associated with this rare earth element abundances with normalized La values oldest relict foliation were not seen in outcrop but appear to of approximately 100 and relatively flat heavy rare earth be responsible in part for the map pattern between the Spencer elements. Prominent negative Eu anomalies are present Brook and Assabet River faults. There, the sulfidic schist in all four samples (fig. D6 ). The spider diagrams for the of the Nashoba Formation (S_ns) is interpreted to define a four samples are similar, showing enrichment in Rb, Th, truncated F fold whose axial surface is deformed and aligned and K with negative Sr, Nb, P, and Ti anomalies that are 1 with the second deformation (D2). In the Nashoba zone, this characteristic of continental arc granites (fig. E6 ). Uranium/ relict deformation and metamorphism occurred at a high thorium ratios (fig. B7 ) are similar to those reported by Wones enough grade to produce leucosomes. In the Nashoba zone, and Goldsmith (1991) for similar rocks in the Avalon zone. D1 predates the peak metamorphic assemblages associated Age-corrected εNd values are plotted in figure C7 and E with D2. Hepburn and others (1995) report a U-Pb monazite along with fields for rocks from the Avalon and Gander terranes age of approximately 425 Ma for metamorphism of the Fish and Laurentian rocks. There is considerable overlap in the Brook Gneiss, and if this correlates with the D1 event it could fields for Avalon and Gander, and theε Nd values for the Milford be attributed to the Salinic orogeny or an early phase of the antiform and the Grafton Gneiss plot in the Gander zone (fig. Acadian orogeny. 7C), in the overlap zone (fig.7E ), or just below the overlap zone In the Westboro Formation, the relict S1 foliation is found (fig. 7E) but well above the Laurentian field. The at 515 εNd parallel to bedding in tight to isoclinal F2 folds (fig. 8C–E). Ma versus 207Pb/204Pb diagram (fig. E7 ) also shows that the four The relict foliation is characterized by a lepidoblastic texture samples plot in the overlap zone between the Gander and the of aligned micas or amphiboles. Definitive 1F folds associated Avalon fields. Lead isotope ratios (fig.D 7 ) indicate the rocks with this oldest foliation were not seen in outcrop. In one came from enriched mantle sources derived from mixing with sample of calc-silicate phyllite from the Westboro Formation, continental crust, ocean island crust, or sediments. the relict S1 foliation occurs as aligned tremolite in thin 18 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

A B

S2 S1

S1

S2

WEST EAST

C S1 D

Area of E

S3

S2 S2 S3

5 mm

Figure 8. A and B, Photographs of rootless F2 folds in the plane

of the dominant foliation S2, showing leucosomes parallel to a

relict S1 foliation in the Nashoba Formation (points GR–039 (A) E and GR–109 (B)). C, Photograph showing three generations of foliation in the Westboro Formation: A relict bed-parallel S1, a

dominant S2 foliation associated with isoclinal folds, and late open

folds with S3 cleavage and kink bands showing west-side-down rotation sense. Arrow points to quarter for scale (point GR–067). S 1 D, Photomicrograph of a thin section showing the dominant S2

foliation deformed by S3 cleavage and F3 folds in calc-silicate

phyllite of the Westboro Formation. The dominant S2 foliation is penetrative and defined by aligned tremolite. Field of view is 21 x 44 mm. E, Photomicrograph of area shown in figure D8 . The relict

S1 foliation was not seen in outcrop or hand sample but is visible microscopically in polarized light as an isoclinally folded foliation S2 in microlithons between S . Field of view is 5 mm wide (point S 2 3 GR–247). Structural Geology 19

section (fig. D8 ) but was not seen in the field because it was changes in orientation of S2 and L2 across the four structural transposed by the younger S2 fabric, and it is too fine grained domains. West of the Fisherville Pond fault, the L2 mineral to see without the aid of the microscope. This relict foliation lineations show northwestern to northeastern trends. L2 is reported to be a deformational and metamorphic fabric that mineral lineations show a very consistent northeastern trend predates the intrusion of the plutonic rocks in the Avalon zone, east of the Fisherville Pond fault in the Milford domain, indicating that it is Neoproterozoic (Bailey and others, 1989; except in the southwestern part. In the three domains west

Goldsmith, 1991c). In the Grafton quadrangle, it is unclear of the Fisherville Pond fault, the L2 mineral lineation data if this relict S1 foliation is bedding or is related to an older are sparse but generally show a preferred northeastern- deformational event. northwestern trend in the Westboro and Marlboro domains The plutonic rocks in the Avalon zone locally contain and north-northwestern and minor southwestern trends in the a deformed foliation that predates the dominant S2 foliation Nashoba zone. in the rocks. This rare, older foliation appears along and is parallel to the contacts between rocks of the Ponaganset Gneiss and Northbridge Granite Gneiss. It is interpreted as an Third-Generation Foliation—D3 Deformation igneous flow foliation. The third generation of ductile deformation produced

open to tight folds (F3) of the dominant foliation (S2) Second-Generation Foliation—D Deformation (fig. 9D). The folds are associated with a crenulation 2 cleavage in the fine-grained rocks of the Westboro Formation

The second generation foliation (S2) is the dominant (fig. 8C–E, 9D). In the coarser grained rocks, the cleavage planar fabric in the Grafton quadrangle. In the Avalon is very weakly developed or not recognized. In the granitic zone, S2 is a schistosity in the Westboro Formation and a rocks of the Milford antiform, outcrop-scale F3 folds or S3 gneissosity in the plutonic rocks of the Milford domain. cleavage were rarely observed. The Milford antiform is

In the Nashoba zone, S2 varies from a schistosity to a a D3 structure, as the dominant S2 foliation is folded. The gneissosity in the metasedimentary and metavolcanic trends of F3 fold axes and L3 lineations plunge to the north- rocks depending on the grain size and is a gneissosity northeast (fig. D9 ). The strike and dip of F3 axial surfaces in the deformed intrusive rocks. The S2 foliation is axial are variable, but the general strike is northwest with a planar to tight to isoclinal folds of relict S1 foliation in the northeastern dip (fig. D9 ). This direction is at a high angle to metasedimentary and metavolcanic rocks (fig. 8) and rare the northeastern trend of the Milford antiform but matches folds of igneous flow foliation in the intrusive rocks. The the trend of the Oxford anticline (Goldsmith, 1991c; Barosh, orientation of S2 is generally northeast striking and northwest 2005). In the Milford antiform, an L3 grain lineation defined dipping in the Nashoba, Marlboro, and Westboro domains, by aligned hornblende was observed in three places. The and in the northwestern part of the Milford domain (fig. L3 lineation trends to the northeast or to the west (fig. E9 ),

9A). Steeply dipping, subvertical, northeast-striking S2 and it postdates the L2 lineation. The aligned hornblende foliation is common in the Burlington mylonite zone in the lineation indicates formation during lower amphibolite-

Westboro domain. In the Milford domain, the S2 foliation facies metamorphic conditions. In addition to F3 folds, a is deformed by the Milford antiform. F2 fold axes in the small number of late kink bands, locally associated with Nashoba zone, west of the Bloody Bluff fault, show average veins filled with greenschist-facies minerals (largely epidote, north to northwest moderate plunges (fig. C9 ). East of the chlorite, and quartz), postdate the dominant foliation (S2)

Bloody Bluff fault, the F2 fold axes and L2 lineations show and appear to be related to D3, or younger, deformation. variable, moderate to steep plunges to the north-northeast The relative displacement sense and orientation of the kink and southwest (fig. 9B, C). bands are variable, but locally near the Bloody Bluff fault,

Second-generation mineral lineations (L2) form extensional veins, shear bands, and thin mylonitic fault zones “aggregate lineations” and “grain lineations” (Piazolo show normal west-side-down displacement (fig. 11). This and Passchier, 2002; Passchier and Trouw, 2005). In the late faulting is consistent with observations that the Milford Nashoba zone, the mineral lineations consist of quartz antiform is truncated by late movement along the Bloody rods, and aligned biotite flakes, amphibole, sillimanite, Bluff fault (Goldsmith, 1991c). In the Grafton quadrangle, and K-feldspar, consistent with formation during upper the trace of the antiform, while not obviously truncated, amphibolite-facies metamorphic conditions. In the Avalon is oblique to the trace of the poorly exposed Fisherville zone, the mineral lineations consist of quartz rods, and Pond fault, and it is unclear if the fault experienced post-S2 aligned biotite, amphibole, and deformed and recrystallized reactivation.

K-feldspar tails around microcline augen consistent with Quartz veins and pegmatite dikes are younger than the D3 formation during upper greenschist- to lower amphibolite- fabrics. The quartz veins show random orientations (fig. E9 ). facies metamorphic conditions. Formline analysis The pegmatite dikes strike northeast and dip subvertically summarizes the distribution of the dominant S2 foliation and (fig. 9E). Quartz veins may contain small amounts of alkali

L2 mineral lineation across the map (fig. 10). The formline feldspar and locally grade into pegmatite. Thicker veins are map, combined with the stereonet data (fig. A9 ), shows the locally cored by pegmatite. 20 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

Dominant foliation (S2) by structural domain Equivalent to the following map symbols:

Strike and dip of dominant foliation (S2) Inclined Vertical

Strike and dip of mylonitic or phyllonitic S2 foliation

Nashoba Marlboro Westboro Milford N N N N

Pi pole Pi pole Pi pole Pi pole 328°, 48° 343°, 49° 10°, 50° 23°, 20°

113°, 70°

58°, 42° 73°, 41° 100°, 40°

(n=60, 2%) (n=59, 2%) (n=42, 2%) (n=340, 2%)

Mineral lineation (L2) by structural domain Equivalent to the following map symbol:

Bearing and plunge of L2 mineral lineation

N N N N

Mean 28°, 17°

(n=19) (n=7) (n=8) (n=203)

Figure 9. Summary diagrams of D2 and D3 ductile structures, lineations, L3 intersection lineations, and late kink bands. Late kink including veins and dikes. A, Lower hemisphere equal-area bands may be related to D3 or younger deformation. E, Normalized projections (stereonets) of dominant foliation (S2) separated into azimuth-frequency (rose) diagrams of quartz veins (left) and the four structural domains from west to east: Nashoba, Marlboro, pegmatite dikes (right). Plotted data include dips >60º. For all Westboro, and Milford (see fig. 1 for map of domains). The four diagrams, the number of points and the contour interval (where stereonets show the poles to S2, contoured poles to S2, best-fit plotted) are shown in parentheses at the bottom of each diagram. great circle to poles, and the Pi pole to a best-fit great circle. Map symbols corresponding to plotted data are shown above

B, Stereonets of L2 mineral lineations. C, Stereonets showing poles diagrams. Stereonets and rose diagrams are plotted using the to F2 axial surfaces, F2 fold axes, and L2 intersection lineations. Structural Data Integrated System Analyser (DAISY, version 4.65b)

D, Stereonets for poles to F3 axial surfaces, F3 fold axes, L3 mineral software by Salvini (2007). Structural Geology 21

F2 folds associated with the dominant foliation Folds that postdate the dominant foliation Equivalent to the following map symbols: Equivalent to the following map symbols:

Bearing and plunge of F2 minor fold axis Strike and dip of axial surface of F3 minor fold Strike and dip of kink band Bearing and plunge of L2 intersection lineation Inclined Vertical Inclined Strike and dip of axial surface of F2 minor fold Bearing and plunge of F3 minor fold axis Vertical parallel to S2 foliation, inclined Bearing and plunge of L3 mineral lineation Nashoba and Marlboro domains Westboro and Milford domains Entire quadrangle—All domains N N N N

Pi pole 16°, 45°

106°, 45°

(n=6)

F2 fold axis from Marlboro F2 fold axis from Milford (n=1) Pole to F3 axial surface from Pole to late kink band (n=7) or Nashoba (n=7) domain or Westboro (n=2) domain Nashoba (n=11), Westboro (n=15), Marlboro (n=4), or Milford (n=1) domain L2 intersection lineation from L2 intersection lineation from Nashoba (n=1) domain Westboro (n=3) domain F3 fold axis from Nashoba (n=11), Westboro (n=16), Marlboro (n=4), Pole to F2 axial surface from Pole to F2 axial surface from or Milford (n=1) domain Marlboro (n=7) or Nashoba Milford (n=1) or Westboro (n=6) domain (n=4) domain L3 mineral lineations from Milford (n=3) domain

L3 intersection lineation from Nashoba (n=4) or Marlboro (n=1) domain

Quartz veins and pegmatite dikes Equivalent to the following map symbols: Strike and dip of quartz vein Strike and dip of pegmatite dike (Permian) Inclined Vertical Inclined Vertical Entire quadrangle—All domains Milford domain N N

(n=55) (n=11)

Figure 9.—Continued 22 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

71°45' 71°37'30" 42°15'

T L U A F

F F U T L L B U

A F Y

D T R O L E O U V L I A B F R

K T E OO B A BR T S Burlington mylonite zoneL R S U E A A Area of map in figure 13C F C N

E D N SP O P LE IL RV HE IS F

B LA C K S TO N E VA L LE Y FA U LT

PCF

42°07'30"

0 1 2 MILES Figure 10. Formline map 0 1 2 KILOMETERS of the dominant foliation (S2) EXPLANATION showing modeled strike and dip (red symbols). Calculated Nashoba zone Upright folds of the Milford antiform strike and dip values were Nashoba structural domain showing plunge direction interpolated by inverse Marlboro structural domain Antiform distance-weighting analyses of Avalon zone Synform 490 S2 measurements in ArcGIS Westboro structural domain Calculated formline of dominant foliation (S2) spatial analyst. PCF, Purgatory Milford structural domain Measured L2 mineral lineation Chasm fault. Structural Geology 23

of the Nashoba Formation (S_n) in the eastern part of the formation, and the rusty schist defines a map pattern, which is

interpreted as a truncated F1 fold. We place the Spencer Brook and Assabet River faults as bounding faults in the eastern part of the Nashoba Formation. Our mapping is consistent with Hepburn (1978), Hepburn and DiNitto (1978), Kopera and others (2006), and Markwort (2007), which places the Assabet River fault between the Nashoba and Marlboro Formations in this area. Displacement and sense of motion are unknown, so we show them as thrust faults on the basis of Goldsmith’s (1991c, p. H9) interpretation that motion during the late stage of faulting on these and similar faults in the Nashoba zone was, ”…east directed, west side up, and right lateral.”

WEST EAST Bloody Bluff Fault The Bloody Bluff fault crosses the map from northeast to Figure 11. Photograph of en echelon extensional veins southwest and represents the boundary between the Nashoba (white) associated with shear bands and faults showing normal and Avalon zones (Bell and Alvord, 1976; Zen and others, 1983; displacement near the Bloody Bluff fault. Faulted surfaces locally Goldsmith, 1991c). The fault has been extensively studied, and contain veins of very fine grained quartz, chlorite, and calcite. it is a complex boundary with a prolonged Neoproterozoic to Veins and faults developed after the formation of porphyroblasts Mesozoic tectonic history (Cuppels, 1961; Skehan, 1968; Castle of biotite, epidote, clinozoisite, and apatite that postdate the S 2 and others, 1976; Nelson, 1976; Barosh, 1984; Hepburn and dominant foliation. Rock is a dark-green sphene-epidote-biotite others, 1987a; Nelson, 1987; Goldstein, 1989; Goldsmith, 1991c; schist in the Marlboro Formation (unit _m, point GR–244). Dime Grimes and Skehan, 1995; Rast and Skehan, 1995; Goldstein shown for scale. and Hepburn, 1999; Kohut and Hepburn, 2004; Roden-Tice and Wintsch, 2007). Early motion on the fault is presumed to be northwest-side-up thrust motion of higher grade Nashoba zone Ductile Faults rocks over Avalon zone rocks (for example, Zen and others, 1983; Goldsmith, 1991c) with later motion involving right- Five significant ductile faults or fault zones occur in the lateral (Cuppels, 1961), left-lateral (Kohut and Hepburn, 2004), map area. From west to east, they are the Spencer Brook fault, or normal displacement (Goldstein, 1989, 1998; Goldstein and the Assabet River fault, the Bloody Bluff fault, the Burlington Hepburn, 1999; Roden-Tice and Wintsch, 2007). On the basis of mylonite zone, and the Fisherville Pond fault (fig. 1). the trend of the L2 mineral lineations in the Avalon zone in the

map area, the thrust motion during D2 was oblique left-lateral Spencer Brook and Assabet River Faults motion with northwest-side up and to the south-southwest. Unfortunately, like elsewhere in Massachusetts The Spencer Brook and Assabet River faults (Alvord and (Goldsmith, 1991c), the fault is not well exposed in the Grafton others, 1976; Bell and Alvord, 1976) occur in the northwestern quadrangle. We show the fault as an early thrust fault, and on part of the map. Their locations are inferred because neither is cross section A–A′ we show younger normal displacement as exposed in the quadrangle. Goldsmith (1991c) described the reactivation along or across the fault. The normal displacement history and extent of the faults in the Nashoba zone, described is consistent with our northwest-side-down observations of the the problems with identifying their locations southwest of youngest mylonitic fabrics in the Burlington mylonite zone, the town of Marlborough, and explained how the location rotation sense of younger F3 folds, displacement sense of nearby of the faults was extrapolated on the State map through the veins and shear bands, and with the overall relative motion Grafton quadrangle. Goldsmith (1991c, p. H9) explained that sense of brittle faults (see below). Our observations confirm the the Assabet River fault in the Grafton quadrangle was placed complicated and protracted history of the fault. structurally above, or west of, a rusty schist in the Nashoba Outcrops closest to the fault show well-developed Formation because unpublished reconnaissance mapping by mylonitic fabric that is parallel to the dominant foliation H.R. Dixon did not identify a fault below the rusty schist. (fig. 12). This foliation is parallel to at least the second- Despite the limited exposure, several belts of rusty schist generation foliation in both the Avalon and Nashoba zones. (S_ns) occur in the eastern part of the Nashoba Formation, The mylonitic foliation strikes northeast and dips moderately and widespread mylonitic foliation occurs along the eastern northwest near Millbury Street and dips subvertically near edge of the Nashoba Formation. Rusty sulfidic schist S( _ns) Wesson Road (fig. 12). The change in orientation is related to and amphibolite (S_na) are interlayered with the gray schist deformation by younger F3 folds and crosscutting veins and shear bands (figs. 11, 12). 24 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

A B S2

NORTHWEST SOUTHEAST

F3 Area of B

C Alaskite gneiss

Figure 12. A, Photograph of mylonitic foliation in the footwall of the Bloody Bluff fault near Wesson Road (point GR–276). Mylonitic

foliation is parallel to the regional dominant foliation (S2, solid

magenta line), and it is deformed by younger folds (F3, dashed lines), which plunge to the north and show west-side-down asymmetry in this photograph. Arrow points to penny for scale. B, Close-up photograph of area shown in figure 12A. C, The rock is predominantly alaskite gneiss (light-colored rock, unit Zhvb), but locally it is interlayered with megacrystic biotite granite gneiss Megacrystic (gray-colored rock, unit Zpg, point GR–275). Outcrops to the west biotite granite of the fault, in the upper plate, are migmatitic amphibolite of the gneiss Marlboro Formation.

Burlington Mylonite Zone two younger generations of high-strain deformation showing west-side-down displacement. The youngest generation Sheared rocks in the Burlington mylonite zone (BMZ) of mylonitization is characterized by steeply dipping to occur in a belt as much as 5 km wide in the footwall of the subvertical foliation. Kinematic analysis of two oriented thin Bloody Bluff fault (Castle and others, 1976; Zen and others, sections collected from roadcuts along the Massachusetts 1983; Goldsmith, 1991c; Skehan and others, 1998). Goldsmith Turnpike shows mylonites with C′-type shear bands (1991c) suggested that the southward extent of the BMZ was (Passchier and Trouw, 2005) showing northwest-side-down limited by the Weston fault on the basis of work by Nelson to the north-northwest motion for mylonitization along the (1976), approximately 15 km to the northeast. Skehan and mapped faults. This motion is consistent with regional north- others (1998) noted that the zone was complex and contained northwestern–south-southeastern extension and dextral, top to many internal zones with various names and an extended the north-northeast, motion at the end of Alleghanian ductile Neoproterozoic to late Paleozoic tectonic history. Here, we use deformation in the Permian (fig. 13) (Goldstein, 1994, 1998; the term for an approximately 2-km-wide zone in the footwall Gromet and others, 1998; Goldstein and Hepburn, 1999). of the Bloody Bluff fault that contains abundant mylonitic Kohut and Hepburn (2004) reported oblique sinistral motion foliation and local discrete faults. The mylonite zone is in the BMZ in Lincoln, Mass., and that motion matches the exposed along the Massachusetts Turnpike (fig. 13). Multiple relative sense seen along the central part of the BMZ in the generations of mylonitic to phyllonitic foliation are present in Grafton quadrangle (fig. 13). the zone (fig. 13). The oldest mylonites are sheared by at least Structural Geology 25

EAST WEST

Z Qtz

Kfs Y

(

( ( ( X FAULT ( ( ( ( D U ( D ( ( Zgdn ( U ( Zwa D GR–45 ( ( FAULT

( ( U

( ( GR–52 GR–53 ( (

( POND ( ( EXPLANATION ( ( ( ( ( Structural domain Older mylonitic thrust fault—

( Sawteeth on upper plate ( Marlboro BLOODY BLUFF N ( Younger mylonitic fault—Bar and ( Westboro FISHERVILLE ball show direction of dip; Burlington mylonite zone ( ( Milford U, upthrown side; D, downthrown ( side. Arrows show relative movement GR–52 Sample location for kinematics Inclined 0 0.5 1 MILE and identifier Vertical 0 0.5 1 KILOMETER Overall relative motion of block

Figure 13. A, Photograph of mylonitic foliation in the Burlington side-down motion that separates epidote granodiorite (Zgdn) mylonite zone. Mylonitized biotite granite gneiss (unit Zpg, point from Westboro Formation (Zwa) at point GR–45. View is looking GR–52) shows highly recrystallized microcline exhibiting grain south. C, Map showing faults in the Burlington mylonite zone size reduction of white and light-pink augen (Kfs) alternating in the northeastern part of the quadrangle. Relative motion for with gray quartz ribbons (Qtz). B, Photograph showing multiple the youngest generation of steeply dipping faults shows overall fault generations in the Burlington mylonite zone. Older mylonitic oblique dextral offset with the northwestern side down and quartzite (Z) is truncated by narrow shear zones (Y) showing southeastern side up. Offset at point GR–45 shows sinistral normal displacement. Both are truncated along a fault zone motion, similar to motion in the Lincoln, Mass., area (Kohut and (X) containing phyllonitic to mylonitic foliation showing west- Hepburn, 2004). 26 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

Fisherville Pond Fault Gromet and O’Hara (1985) noted correctly that the mylonites and foliation at the roadcut strike at a right angle to the The Fisherville Pond fault, herein named, occurs in the inferred trend of the HVSZ, but Wayne and others (1992) Avalon zone and separates the structurally higher rocks of incorrectly stated that the mylonites had a north-south strike the Westboro Formation in the upper plate from the intrusive and dipped gently to the west, parallel to the HVSZ. rocks of the Milford antiform in the lower plate. The fault Our mapping in the Grafton quadrangle found no is not exposed in this quadrangle, and exposure north of the evidence for the proposed HVSZ for the following reasons: fault is quite poor; thus its presence is conjectural. On the • Numerous thin (<10-cm-thick) mylonite zones are vis- State map (Zen and others, 1983), an unnamed fault, internal ible at the State Route 146 roadcut and at nearby road- to the intrusive rocks of the Milford antiform, is shown in this cuts (fig. 2A, C), but no single zone is more significant position. Goldsmith (1991c) explained that the fault was an than the other. Each thin mylonite zone represents unnamed fault splay of the Bloody Bluff fault that extended internal deformation parallel to the dominant foliation from Millbury to Framingham to connect to a fault mapped within the Ponaganset Gneiss. by Nelson (1975). Goldsmith (1991c, p. H44, H45) gave apparently contradictory explanations for the fault saying both • The Hope Valley Alaskite Gneiss intrudes both the that it was “interpreted from discordance of foliation pattern” Ponaganset and Northbridge Granite Gneisses, and we and located “…along contacts of units aligned parallel to its did not recognize mylonitic fabric between the Hope possible projected trace and parallel to the gneissic foliation Valley Alaskite Gneiss and the Ponaganset Gneiss. in the plutonic rocks in this area.” Kopera and others (2007), • There is no fabric in the rock in this quadrangle that who mapped in the Milford quadrangle between the Grafton strikes north-south and dips subvertically, parallel to quadrangle and the Framingham area, did not recognize this the proposed HVSZ. The dominant foliation and the fault, but they did map a zone of mylonite folded around the thin mylonite zones near the proposed location of the Milford antiform in the southwestern corner of their map. Our HVSZ strike east-west and dip gently north. Thus, mapping suggests the presence of a fault for the following the dominant foliation is virtually at a right angle to reasons: the proposed location of the shear zone (recognized • Near Fisherville Pond in Grafton, the outcrops of the by Gromet and O’Hara, 1985). Ongoing mapping in Hope Valley Alaskite Gneiss closest to the inferred the Uxbridge quadrangle to the south by G.J. Walsh fault contain a mylonitic foliation. (unpublished data) has identified north-south striking, steeply dipping shear zones and thin mylonites, which • The trace of the fault is parallel to the dominant folia- represent internal deformation within the Ponaganset tion on both sides of the fault. and Northbridge Granite Gneisses, but they do not • South of the fault, we map no rocks like the Westboro continue north into the Grafton quadrangle. Formation. • O’Hara and Gromet (1985) and Gromet and O’Hara • There is an apparent truncation of units in the lower (1985) stated that considerable differences existed plate within the Hope Valley Alaskite Gneiss. between the rocks of the Hope Valley and Esmond- Dedham terranes on either side of the HVSZ. In contrast, our geochemical and geochronologic data for Comment on the Hope Valley Shear Zone the Hope Valley Alaskite Gneiss and the Northbridge An Alleghanian fault called the Hope Valley shear zone Granite Gneiss are quite similar and demonstrate that (HVSZ) has been interpreted to both pass through the Grafton these rocks, located on either side of the proposed quadrangle and represent a regionally significant terrane HVSZ, cannot be separated into two terranes. Our boundary between the Hope Valley terrane to the west and chemical data support earlier work by Wones and the Esmond-Dedham terrane to the east (O’Hara and Gromet, Goldsmith (1991), suggesting that the Hope Valley and 1985). The northern part of the shear zone is inferred to be at Northbridge (formerly Scituate) are similar. the contact between the Hope Valley Alaskite Gneiss and the Ponaganset Gneiss (Gromet and O’Hara, 1985; O’Hara and Brittle Structures Gromet, 1985; Hermes and others, 1994). Mylonitic foliation found in the Ponaganset Gneiss on State Route 146 in the The planar brittle structures in this study are also called town of Sutton was attributed to southward thrusting at a fractures. Fractures with slickensides or visible displacement high angle to the overall right-lateral offset along the HVSZ are brittle faults. Fractures without displacement are separated (Gromet and O’Hara, 1985; O’Hara and Gromet, 1985). on the basis of whether or not they are related to an older Wayne and others (1992) obtained conventional U-Pb ages fabric in the metamorphic and igneous rocks: of 285±25 and 270±92 Ma from metamorphic zircons in a • Parting fractures where the rocks break, or part, along mylonite at this location indicating Alleghanian deformation preexisting fabric. Parting fractures are related to older and metamorphism of the Neoproterozoic plutonic rocks. sedimentary, igneous, or metamorphic fabrics due to Structural Geology 27

structural inheritance and occur along axial surfaces to the foliation. In the two Milford domains, such fractures are of folds, schistosity, gneissosity, mylonitic foliation, rare because the gently dipping fractures usually occur along contacts, dikes, and veins. the foliation. • Fractures that cross the metamorphic or igneous fabric The spacing of fractures, although not quantified in this of the rocks include brittle faults, joints, and joint sets. study, is generally quite variable in the metasedimentary and metavolcanic rocks of the Nashoba, Marlboro, and Westboro The orientation of measured joints includes those with domains but rarely exceeds a few meters. Spacing of steeply trace lengths greater than 20 cm (Barton and others, 1993). dipping crossing and orthogonal joints in the two Milford Joints were measured subjectively using methods described domains locally exceeds several meters. The spacing and by Spencer and Kozak (1976) and Walsh and Clark (2000), abundance of foliation-parallel fractures, or parting fractures, and the dataset includes only the most conspicuous joints in the Milford antiform are related to rock type; finer grained observed in a given outcrop. Fracture data (fig. 14) are plotted rocks exhibit more parting fractures and smaller spacing on rose diagrams (azimuth-frequency) and stereonets (lower (fig. 16, table 4). The Hope Valley Alaskite Gneiss exhibits hemisphere equal-area projections) using Structural Data abundant parting fractures with spacing around 0.2 to 1 meter Integrated System Analyser (DAISY, version 4.65b) software (m), and the coarser Ponaganset and Northbridge Granite by Salvini (2007). The software generates rose diagrams Gneisses exhibit less abundant parting fractures with spacing using a Gaussian curve-fitting routine for determining peaks around 0.3 to 2.5 m (fig. 15). Strongly lineated rocks of the in directional data (Salvini and others, 1999) that was first Northbridge Granite Gneiss (Znclg) show less parting along described by Wise and others (1985). The rose diagrams the foliation than the moderately to weakly lineated rock include strike data for steeply dipping fractures (dips ≥60°, (Zncg) because the lineated rock is generally not as strongly after Mabee and others, 1994). foliated (fig. 16, table 4). The Grafton Gneiss shows the least- For the purpose of fracture analysis, fracture data were developed parting (fig. 16, table 4). divided into the four main structural domains: Nashoba, Marlboro, Westboro, and Milford (fig. 14). The rocks in the Milford domain in the Milford antiform were further Brittle Faults subdivided by the Blackstone Valley fault into north and south subdomains (fig. 14). In general, the most conspicuous Thirty two brittle faults were identified in outcrop by fractures occur in four categories (fig. 15): Parting fractures slickensides or visible offsets (fig. 17A), and two brittle parallel to the foliation (A); steeply dipping fractures that faults are shown on the map. The outcrop-scale faults cross the foliation at a high angle (B); steeply dipping include 19 normal faults, 12 reverse faults, and one left- fractures that are orthogonal to the foliation (C); and gently lateral, strike-slip fault (fig. 17). The faults have a preferred dipping sheeting fractures (D). Not all four categories show northeast strike and dip steeply, similar to the principal trend principal trends in all domains. All five domains show of crossing joints (figs. 14, 17B). Although it is unknown if prominent foliation-parallel fractures (fig. 14), with the all these faults are coeval, a calculation of the mean P (σ1) most consistent trends occurring in both the north and south and T (σ3) axes (fig. 17) shows a calculated average stress parts of the Milford domain and the least consistent trends field that is consistent with Late Triassic to Early Jurassic occurring in the Westboro domain. Steeply dipping joints northwest-southeast extension associated with rifting of the are dominated by throughgoing fractures that cross entire New England crust during the initial opening of the Atlantic outcrops with trace lengths on the order of meters, and they basin (Kaye, 1983; de Boer and Clifford, 1988; Manning tend to cross the foliation at a high angle (fig. 15); these joints and de Boer, 1989; Goldsmith, 1991c). Extension may have are called “crossing” fractures (Barton and others, 1993). started during the late Paleozoic (Goldstein, 1994; Goldstein These steeply dipping crossing fractures show preferred and Hepburn, 1999). northeastern trends (26°–32°) in all but the Nashoba domain. The mapped brittle faults include the Blackstone The northeastern trend of steeply dipping crossing fractures Valley fault and the Purgatory Chasm fault. The unexposed is similar to the trend of planar pegmatite dikes and mineral Blackstone Valley fault is inferred from northwest-trending lineations and suggests that brittle failure was inherited and outcrop-scale faults and displaced units. The fault was shown occurred not only along igneous planar features but also on the State map (Zen and others, 1983) and discussed briefly along ductile linear features in the rock. The Nashoba domain by Goldsmith (1991c, p. H23) where he mentioned offset of shows steeply dipping crossing fractures with a preferred ductile fabrics “…along the Blackstone River valley...” The northwestern trend. The steeply dipping northwestern trend fault is interpreted as a steeply southwest-dipping normal fault also occurs in the Milford north domain and, to a limited with minor displacement. The Purgatory Chasm fault is well extent, in the Milford south domain. In these two areas, these exposed at Purgatory Chasm State Park. It forms a straight- trends represent crossing joints orthogonal to the foliation. walled canyon approximately 350 m long, 20 m wide, and 30 Gently to moderately dipping sheeting fractures that cross the m deep. Outcrop-scale faults, en echelon fractures, and Reidel foliation-related fractures occur in the Nashoba, Marlboro, and shears show that the fault is steeply dipping with west-side-up Westboro domains, and, locally, these fractures are orthogonal reverse displacement. Structural inheritance is illustrated by 28 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts (n=302) N ±5° 29 ° (n=386) N ±5° 304 ° diagram. Stereonets show contoured poles to all fractures, and a subset of the data showing poles to parting fractures along foliation (black symbols). Rose diagrams include a subset of the data shown in the corresponding stereonet for dips >60º. Principal peaks on the rose diagrams are shown with 1 standard For all diagrams, the deviation error. number of points and the contour interval (where plotted) are shown in parentheses at the bottom of each diagram. (n=430, 1%) Milford structural domain—North N (n=575, 1%) Pole to parting fracture (n=78) Milford structural domain—South N Summary diagrams of

Pole to parting fracture (n=133) North Figure 14. brittle structures including joints, faults, and parting fractures along ductile structures. Map data are plotted separately for the four structural domains shown on the small map of the quadrangle in center of figure (see 1 for explanation of structural domains), and data for the Milford antiform are further subdivided into areas northeast and southwest fault. Pairs of of the Blackstone Valley diagrams include a stereonet and rose South ±6° ±6° 29 ° 26 ° (n=110) (n=55) N N (n=94) N ±4° (n=89, 1%) Marlboro structural domain (n=154, 1%) Westboro structural domain Westboro 291 ° N N Nashoba structural domain (n=163, 1%) Pole to parting fracture (n=22) Pole to parting fracture (n=25) N Pole to parting fracture (n=27) Structural Geology 29

NW NE

C

D

C

B A A

B

Gently to moderately dipping dominant foliation strikes to the northeast, east, or northwest and dips to the northwest, north, or northeast. Arrows show linear valleys or troughs created by the intersection of fracture surfaces A and C and linear valleys parallel to fracture surface B. Point GR–286 in the Northbridge Granite Gneiss.

EXPLANATION Plane of joint C A Parting along dominant foliation A B Northeast or northwest throughgoing joints C Crossing and abutting joints orthogonal to foliation D B Trace of joint Parting along dominant foliation Northeast or northwest throughgoing joints Crossing and abutting joints orthogonal to foliation

D Gently dipping sheeting fractures

Steeply to moderately dipping dominant foliation A strikes to the northeast and dips to the northwest.

Figure 15. Block diagrams and photograph of the most Note the variable spacing between parting fractures. C, Block conspicuous fractures (categories A–D) in the Grafton diagram of fractures in the Nashoba domain. Category B fractures quadrangle. A, Block diagram and B, photograph of fractures in show a principal trend to the northwest. Principal fractures in the Milford domain. Category B fractures show a principal trend to the Marlboro and Westboro domains are similar to those in the the northeast, locally parallel to pegmatite dikes and the dominant Nashoba domain, but category B fractures show a preferred mineral lineation. Photograph at point GR–286 in the Northbridge northeastern orientation as seen in the Milford domain. Gently Granite Gneiss. Double arrows show the relations between dipping sheeting fractures are most common in the Nashoba and valleys and fractures. These valleys are common in areas of Marlboro domains. shallow bedrock, especially in the southwestern part of the map. 30 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

100 Table 4. Summary of foliation-parallel parting fractures, by formation. 90 [Data are plotted in figure 16] 80 Zhv Zhvb 70 Zncg Number of 60 measure- ments that Total number Percentage 50 Formation exhibit of foliation of parting parting measurements fractures 40 Znclg along the Parting, in percent 30 foliation

20 Grafton Gneiss 6 24 25 Northbridge Granite 10 Gneiss (Znclg) 12 33 36 0 Nashoba Formation 23 50 46 Marlboro Formation 11 20 55 Westboro Formation 12 21 57 Grafton Gneiss

Nashoba FormationMarlboro FormationWestboro FormationPonaganset Gneiss Ponaganset Gneiss (Zpg + Zpgh) 75 129 58 Northbridge Granite Gneiss Northbridge GraniteHope Gneiss Valley Alaskite Gneiss Northbridge Granite Formation Gneiss (Zncg) 42 66 64 Hope Valley Alaskite Gneiss Figure 16. A summary histogram of foliation-parallel fractures, or (Zhvb) 20 29 69 parting, by formation shows that the Grafton Gneiss is least likely to fracture along the foliation and the Hope Valley Alaskite Gneiss Hope Valley Alaskite Gneiss is most likely to fracture along the foliation. Data for map units (Zhv) 43 57 75 in the Grafton Gneiss, Nashoba Formation, Marlboro Formation, Westboro Formation, and Ponaganset Gneiss are combined into a single data column each. Data for the Northbridge Granite Gneiss and Hope Valley Alaskite Gneiss are separated by map units, labeled at the top of each column. Percentage parting is a ratio of In the Nashoba zone, sillimanite-orthoclase assemblages the number of foliation measurements that exhibit parting to the are pre- to syntectonic relative to the second-generation (D ) total number of foliation measurements for a given unit. See table 2 deformation. Mylonitic foliation subparallel to the S foliation 4 for data. 2 in the Nashoba Formation locally postdates K-feldspar porphyroblast growth, and many feldspars exhibit grain-size reduction in the mylonite zones (fig. 20). Limited replacement of sillimanite and K-feldspar by muscovite in samples from the Grafton quadrangle indicates only minor retrograde metamorphism of the Nashoba zone, in agreement with earlier parting fractures along Permian pegmatite dikes and quartz observations (Abu-Moustafa, 1976; Goldsmith, 1991c). In veins in the State park at places such as “Fat Man’s Misery” Massachusetts, Abu-Moustafa and Skehan (1976) estimated (fig. 18) and “The Devil’s Corn Crib.” peak metamorphic conditions of 650ºC to 625ºC at 6 kilobars (kb), whereas in Connecticut, conditions reached 700ºC at 5.7 kb (Moecher and Wintsch, 1994). The assemblage Metamorphism K-feldspar-kyanite-garnet-quartz-sillimanite-biotite seen in the garnetiferous schist (S_ng) of the Nashoba Formation The upper and lower plate rocks on opposite sides of suggests that even higher peak conditions, as much as 700ºC the Bloody Bluff fault experienced different metamorphic at 8 kb, may have been reached in the western part of the histories (for example, Wintsch and Aleinikoff, 1987; Grafton quadrangle. In the garnetiferous schist, garnet and

Goldsmith, 1991c) (fig. 19). The upper plate rocks in kyanite postdate syn-S2 sillimanite, suggesting increasing the Nashoba zone experienced upper amphibolite-facies metamorphic conditions along a counterclockwise P-T path metamorphic conditions, whereas the rocks in the lower during D2. The peak assemblages and the metamorphic plate Avalon zone experienced upper greenschist- to lower isograds are cut by the faults bounding the Nashoba zone (Zen amphibolite-facies metamorphic conditions. and others, 1983; Goldsmith, 1991c). Metamorphism 31

71°45' 71°37'30" 42°15' A B 71 R N

48 N 60 36 N R 30 88 60 53 30 38 70 54 R 88 65 38 N 70 N R

N 83 83 R 58 N N 87 (n=19) N 41 40 81 58 N Normal faults 58 55 58 38 N 48 N 51

U D 0 B LL LA C K 14 S TO R 79 R N 78 21 N E VA 74 21 LL EY F AU LT

19 84 84 N 80 65 R R 82 (n=12) 83 82 83 Reverse faults N 16 N 74 75 78 N 78 N N EXPLANATION R 74 89 54 Slickenlines 89 76 U 86 R Calculated principal stress axes 79 D R PURGATORY CHASM FAULT N σ1 σ2 σ3 86 42°07'30" 45 R 68 64 0 1 2 MILES 68

0 1 2 KILOMETERS

EXPLANATION Nashoba zone Mapped brittle fault—U, upthrown side; D, downthrown side Nashoba structural domain Mylonitic fault Marlboro structural domain Strike and dip of outcrop-scale brittle fault— Avalon zone 68 Movement sense indicated by N (normal), Westboro structural domain R (reverse), or LL (left lateral) Milford structural domain 68 Trend and plunge of slickenlines

Figure 17. A, Map of brittle faults in the Grafton quadrangle. Outcrop-scale faults are shown with symbols. B, Lower hemisphere equal-area projections showing fault planes, slickenlines, and calculated mean P and T axes for normal and reverse faults, n = number of faults. Shortening (P) axis = σ1 , and extension (T) axis = σ3. 32 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

unfoliated phase of the pluton. Our data from metamorphic zircons in the Marlboro Formation indicate a significant thermal event in the Early Mississippian at around 354 Ma. The 354-Ma age should reflect the time when zirconium was released from primary pyroxene in the reaction to form amphibole, and the zirconium went into metamorphic zircon (Hoskin and Schaltegger, 2003; Aleinikoff and others, 2006). Our zircon data from both the Marlboro Formation and the Grafton Gneiss record no evidence of the earlier Silurian event (430 to ~425 Ma). Hepburn and others (1987b) report Carboniferous 40Ar/39Ar cooling ages from amphibole ranging from 354 to 325 Ma. More recent 40Ar/39Ar data show only uplift and cooling through the amphibole closure temperature (500ºC) in the Mississippian and cooling through the muscovite closure temperature (~350ºC) in the Pennsylvanian (Attenoukon and others, 2005; Attenoukon, 2008). Both closure temperatures confirm minimal high-temperature thermal resetting of the Nashoba zone in the Alleghanian (Goldsmith, 1991c; Wintsch and others 1992, 1993, 1998; Hepburn, 2004). In contrast to the Nashoba zone, early metamorphism in the Avalon zone is considered Neoproterozoic (Goldsmith 1991c; Hepburn and Bailey, 1998; Attenoukon and others, 2005; Attenoukon, 2008), and peak metamorphism is late Paleozoic, related to Alleghanian orogenesis (Wintsch and Sutter, 1986; Wintsch and Aleinikoff, 1987; Zartman and others, 1988; Gromet, 1989; Wintsch and others 1991, 1992, 1993, 1998; Getty and Gromet, 1992a,b; Gromet and others, 1998). Figure 18. Photograph of a parting fracture along a thin The timing of Neoproterozoic metamorphism in relation

(10-cm-thick) pegmatite dike at “Fat Man’s Misery” in Purgatory to the earliest episode of deformation (D1) is not constrained. Chasm State Park. The Purgatory Chasm fault is parallel to the Goldsmith (1991c) reported that the metamorphism predated dike and the principal fracture trend in the park. Unit Zpg at point the Neoproterozoic plutonic events and that metamorphism GR–30. may have been related to subduction. Hepburn and Bailey (1998) attributed the early thermal event to contact

metamorphism. It is possible that the early D1 fabric, locally observed as aligned tremolite in the Westboro Formation (fig. 8D, E), is a relict of the Neoproterozoic event. The most The timing of peak metamorphism in relation to dominant event produced the penetrative gneissic fabric (S2) in the two early episodes of deformation (D1 and D2) is not the rocks of the Milford antiform. well constrained but is considered to be early Paleozoic The timing of D2 deformation in the Avalon zone is (Goldsmith, 1991c; Wintsch and others, 1991, 1992, 1993; constrained by the age of syn-D2 mylonites in the Grafton Hepburn and others, 1995). In the Grafton quadrangle, quadrangle and syntectonic development of L2 hornblende leucosomes associated with migmatization of the Nashoba lineations and lower amphibolite-facies mineral assemblages Formation formed during both the D and D events and, 1 2 in S2. Wayne and others (1992) report conventional U-Pb regionally, ages from migmatites range from Devonian metamorphic zircon ages of 285±25 and 270±92 Ma from (Hepburn and others, 1995) to Carboniferous (Acaster and mylonitic rocks exposed on State Route 146 in the Ponaganset Bickford, 1999). A U-Pb monazite age of 425±3 Ma for the Gneiss; these mylonites developed during D2 (fig. 2A). metamorphism of the Fish Brook Gneiss (Hepburn and others, Amphibole cooling ages in the quadrangle range from 268 to 1995) and a U-Pb zircon age of 430±5 Ma from the Sharpners 260 Ma, and K-feldspar cooling ages range from 254 to 227 Pond Diorite (Zartman and Naylor, 1984) provide evidence Ma (Attenoukon and others, 2005; Attenoukon, 2008). Thus, of Silurian metamorphism and intrusion, which might be peak Alleghanian metamorphism probably occurred in the attributed to the Salinic or Acadian orogenies. Hepburn and late Carboniferous to Early Permian, followed by continuous others (1995) and Hepburn (2004), however, point out that the cooling associated with uplift. In the Westboro Formation, dated Sharpners Pond Diorite is unfoliated, implying that the randomly oriented hornblende porphyroblasts in the plane of deformation either predates the intrusion or wraps around the S2 indicate that metamorphic heat continued after D2 (fig. 21). Metamorphism 33

71°45' 71°37'30" 42°15'

T L U A F

F F U L B

Y D O O L B

Figure 19. Simplified map of the Grafton quadrangle showing peak metamorphic conditions. Peak conditions 0 1 2 MILES were achieved in the Silurian

0 1 2 KILOMETERS to Devonian in the Nashoba zone and in the Pennsylvanian 42°07'30" EXPLANATION to Permian in the Avalon zone (see text for discussion). The Nashoba zone—Silurian-Devonian upper amphibolite facies (sil-kfs) higher grade rocks of the Metasedimentary and metavolcanic rocks Nashoba zone were thrust Intrusive rocks over the lower grade rocks of Avalon zone—Pennsylvanian-Permian upper greenschist to lower amphibolite facies the Avalon zone perhaps as Metasedimentary and metavolcanic rocks the result of tectonic wedging Intrusive rocks (Wintsch and others, 2005a,b; Brittle fault Walsh and others, 2007). sil-kfs, Mylonitic thrust fault sillimanite–K-feldspar. 34 Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts

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Wintsch, R.P., Aleinikoff, J.N., Webster, J.R., and Unruh, Williams, I.S., 1998, U-Th-Pb geochronology by ion D.M., 2005b, The Killingworth complex; A middle and late microprobe, in McKibben, M.A., Shanks, W.C., III, Paleozoic magmatic and structural dome, in New England and Ridley, W.I., eds., Applications of microanalytical Intercollegiate Geological Conference, 97th Annual Meeting, techniques to understanding mineralizing processes: New Haven, Conn., Sept. 30–Oct. 2, 2005, Guidebook for Reviews in Economic Geology, v. 7, p. 1–35. field trips in Connecticut: Hartford, Conn., State Geological Wilson, Marjorie, 1989, Igneous petrogenesis; A global and Natural History Survey of Connecticut, Guidebook no. 8, tectonic approach: Norwell, Mass., Kluwer Academic p. 305–324. (Edited by N.W. McHone and M.J. Peterson.) Publishers, 466 p. 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Wintsch, R.P., Coleman, M., Pulver, M., Byrne, T., Aleinikoff, Wise, D.U., Funiciello, Renato, Parotto, Maurizio, and Salvini, J., Kunk, M., and Roden-Tice, M., 1998, Late Paleozoic Francesco, 1985, Topographic lineament swarms; Clues deformation, metamorphism, and exhumation in central to their origin from domain analysis of Italy: Geological Connecticut, in New England Intercollegiate Geological Society of America Bulletin, v. 96, no. 7, p. 952–967. Conference, 90th Annual Meeting, Kingston, R.I., Oct. Wones, D.R., and Goldsmith, Richard, 1991, Intrusive rocks 9–11, 1998, Guidebook to field trips in Rhode Island of eastern Massachusetts, in Hatch, N.L., Jr., ed., The and adjacent regions of Connecticut and Massachusetts: bedrock geology of Massachusetts: U.S. Geological Survey Kingston, R.I., University of Rhode Island, p. C2-1–C2-21. Professional Paper 1366 E–J, p. I1–I61. (Edited by D.P. Murray.) Wintsch, R.P., Kunk, M.J., Cortesini, Henry, Jr., and Sutter, Zartman, R.E., Hermes, O.D., and Pease, M.H., Jr., 1988, J.F., 1991, 40Ar/ 39Ar age-spectrum data for the Avalon and Zircon crystallization ages, and subsequent isotopic Putnam-Nashoba lithotectonic zones, eastern Connecticut disturbance events, in gneissic rocks of eastern Connecticut and western Rhode Island: U.S. Geological Survey Open- and western Rhode Island: American Journal of Science, File Report 91–399, 133 p. v. 288, no. 4, p. 376–402. Wintsch, R.P., and Sutter, J.F., 1986, A tectonic model for the Zartman, R.E., and Naylor, R.S., 1984, Structural implications late Paleozoic of southeastern New England: Journal of of some radiometric ages of igneous rocks in southeastern Geology, v. 94, no. 4, p. 459–472. New England: Geological Society of America Bulletin, v. 95, no. 5, p. 522–539. Wintsch, R.P., Sutter, J.F., Kunk, M.J., Aleinikoff, J.N., and Boyd, J.L., 1993, Alleghanian assembly of Proterozoic Zartman, R.E., and Marvin, R.F., 1991, Radiometric ages and Paleozoic lithotectonic terranes in south central of rocks in Massachusetts, in Hatch, N.L., Jr., ed., The New England; New constraints from geochronology and bedrock geology of Massachusetts: U.S. Geological Survey petrology, in Cheney, J.T., and Hepburn, J.C., eds., Field trip Professional Paper 1366 E–J, p. J1–J19. guidebook for the Northeastern United States: University Zen, E-an, ed., and Goldsmith, Richard, Ratcliffe, N.M., of Massachusetts, Department of Geosciences, Geosciences Robinson, Peter, and Stanley, R.S., comps., 1983, Bedrock Contribution 67, v. 1, p. H–1 to H–30. (Guidebook for geologic map of Massachusetts: Reston, Va., U.S. the 1993 meeting in Boston of the Geological Society of Geological Survey, 3 sheets, scale 1:250,000. Manuscript approved on July 19, 2011.

Prepared by the Reston Publishing Service Center. Edited by Katharine S. Schindler. Cartography by Donald P. Mathieux. Graphics by Caryl J. Wipperfurth. Design and typography by Anna N. Glover. Web support by Angela E. Hall. Printing support by Gary A. Nobles.

For more information concerning the research in this report, contact Gregory J. Walsh U.S. Geological Survey P.O. Box 628 Montpelier, VT 05601 Walsh, Aleinikoff, and Dorais—Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts—Scientific Investigations Map 3171