Prepared in cooperation with the Commonwealth of Massachusetts, Massachusetts Geological Survey and the State of New Hampshire, New Hampshire Geological Survey Bedrock Geologic Map of the Nashua South Quadrangle, Hillsborough County, New Hampshire, and Middlesex County, Massachusetts By Gregory J. Walsh, Richard H. Jahns, and John N. Aleinikoff Pamphlet to accompany Scientific Investigations Map 3200 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: 2013 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. 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Suggested citation: Walsh, G.J., Jahns, R.H., and Aleinikoff, J.N., 2013, Bedrock geologic map of the Nashua South quadrangle, Hillsbor- ough County, New Hampshire, and Middlesex County, Massachusetts: U.S. Geological Survey Scientific Investigations Map 3200, 1 sheet, scale 1:24,000, 31-p. pamphlet. iii Contents Introduction.....................................................................................................................................................1 Previous Work .......................................................................................................................................1 Age of the Rocks ............................................................................................................................................1 Stratigraphy ....................................................................................................................................................2 Igneous Rocks ................................................................................................................................................2 Diorite-Gabbro Suite.............................................................................................................................4 Ayer Granodiorite—Devens-Long Pond Facies ..............................................................................5 New Hampshire Plutonic Suite ..........................................................................................................8 Chelmsford Granite ......................................................................................................................8 Pegmatite ....................................................................................................................................14 Jurassic Dikes .....................................................................................................................................14 Geochronology ....................................................................................................................................14 Methods.......................................................................................................................................14 Results .........................................................................................................................................15 Ayer Granodiorite ..............................................................................................................15 Chelmsford Granite ...........................................................................................................15 Structural Geology .......................................................................................................................................18 Ductile Structures ...............................................................................................................................18 Brittle Structures.................................................................................................................................20 Glacial Features ..................................................................................................................................24 Metamorphism .............................................................................................................................................27 Discussion .....................................................................................................................................................27 References Cited..........................................................................................................................................27 Figures 1. Photographs of the Berwick Formation ....................................................................................3 2. Photographs of migmatitic Berwick Formation .......................................................................4 3. Photographs of the diorite-gabbro suite...................................................................................5 4. Photographs of the Ayer Granodiorite ......................................................................................7 5. International Union of the Geological Sciences (IUGS) rock classification of samples of Ayer Granodiorite and Chelmsford Granite .........................................................9 6. Major-, trace-, and rare-earth-element discrimination diagrams for the dated samples of the Ayer Granodiorite and Chelmsford Granite .................................................11 7. Photographs of the Chelmsford Granite .................................................................................12 8. Lower hemisphere equal-area projection (stereonet) and azimuth-frequency (rose) diagram of pegmatite dikes ...........................................................................................15 9. Cathodoluminescence and transmitted light images of representative zircons and concordia plots of dated rocks from the Nashua South quadrangle .........................16 10. Photographs and companion sketches of relict D1 structure in the Berwick Formation .....................................................................................................................................19 11. Photographs and companion sketches of D2 and D3 structures .........................................20 iv 12. Lower hemisphere equal-area projections (stereonets) of ductile fabrics in the Nashua South quadrangle ........................................................................................................21 13. Photographs of major fracture types ......................................................................................22 14. Lower hemisphere equal-area projections (stereonets) and azimuth-frequency (rose) diagrams of fractures .....................................................................................................23 15. Map of brittle faults in the Nashua South quadrangle .........................................................25 16. Lower hemisphere equal-area projections (stereonets) azimuth-frequency (rose) diagram of brittle faults ..................................................................................................26 17. Rose diagram of nine glacial striations in the Nashua South quadrangle .......................26 Tables 1. Modal compositions and rock classifications of selected rocks from the diorite- gabbro suite ...................................................................................................................................6 2. Modal compositions and rock classifications of selected rocks from the Ayer Granodiorite ...................................................................................................................................8 3. Major, trace, and rare-earth element whole-rock geochemistry of the dated samples of the Ayer Granodiorite and Chelmsford Granite .................................................10 4. Modal compositions and rock classifications of selected rocks from the Chelmsford Granite .....................................................................................................................13 5. SHRIMP U-Th-Pb data for zircons from igneous rocks of the Nashua South quadrangle, N.H.-Mass ..............................................................................................................17 Conversion Factors SI to Inch/Pound Multiply By To obtain Length centimeter (cm) 0.3937 inch (in.) millimeter (mm) 0.03937 inch (in.) meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) Mass gram (g) 0.03527 ounce, avoirdupois (oz) kilogram (kg) 2.205 pound, avoirdupois (lb) Bedrock Geologic Map of the Nashua South Quadrangle, Hillsborough County, New Hampshire, and Middlesex County, Massachusetts By Gregory J. Walsh,1 Richard H. Jahns,2 and John N. Aleinikoff3 Introduction Previous Work The bedrock of the Nashua South quadrangle consists The 7.5-minute Nashua South quadrangle was formerly primarily of Silurian metasedimentary rocks of the Berwick named the Tyngsborough quadrangle. Jahns mapped the New Formation. Regionally, the Berwick Formation is part