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

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Bedrock Geologic Map of the Grafton Quadrangle, Worcester County, Massachusetts 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 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit http://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. 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
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