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Home , Ammonoosuc Volcanics

1Geology Department, Union College, Schenectady, NY 12308, [email protected], [email protected] Prescott Intrusive Complex, central Massachusetts, geochemistry and structural reinterpretation 2Geological Survey of Norway, Trondheim, NO-7491, Norway, [email protected] 1 1 2 3 3Massachusetts Geological Survey, U. Mass., Amherst, MA 01003, Benjamin Lucas , Kurt Hollocher , Peter Robinson , Stephen B. Mabee [email protected]

Rb Rb Abstract These diagrams help Rb Ba Th U Nb' K La Ce Pb Pr Sr Nd Zr'SmEuGd Ti Tb Dy Li Ho Er TmYb Ba Th U Nb' K La Ce Pb Pr Sr Nd Zr'SmEuGd Ti Tb Dy Li Ho Er TmYb Rb Ba Th U Nb' K La Ce Pb Pr Sr Nd Zr'SmEuGd Ti Tb Dy Li Ho Er TmYb Ba Th U Nb' K La Ce Pb Pr Sr Nd Zr'SmEuGd Ti Tb Dy Li Ho Er TmYb compare Prescott rocks (red Cooleyville Cooleyville Gneiss Prescott Gabbro Prescott Gabbro The Prescott Complex of metamorphosed igneous intrusions in the central Bronson Hill Anticlinorium, is predominated by Cooleyville Gneiss (32 samples; A 6D6 B A B squares) with other southern 100 La <163 ppm La >163 ppm 100 Lan/Smn <1.395 Lan/Smn >1.395 449 Ma, Tucker and Robinson, 1990), with lesser Prescott Gabbro, in one large mass and smaller bodies enclosed in the Gneiss (29 samples, 407 Ma). The New England igneous suites, 64 Gneiss is weakly to strongly foliated and lineated, with ~1 to 4 mm grain size and 62-75% SiO . The Gabbro is massive to weakly foliated, fine-grained (<0.5 2 shown in the key. Both mm) to very coarse (~15 mm) with 47-53% SiO2. The wide composition gap implies no genetic relationship, consistent with the ~42 Ma age difference. The Cooleyville Gneiss and 10 10 rocks are calc-alkaline and metaluminous to weakly peraluminous, and all are LREE-enriched with Lan/Yb ratios of 2.3-17 for , 1.5-4.6 for gabbros. n Prescott Gabbro are M82 OI6 Multi-element diagrams show strong positive anomalies for Pb, Li, and other LILEs, and negative anomalies for Nb, Ta, and, to a lesser extent, Zr and Hf, consistently more similar to 5D2 consistent with arc-related sources. Compositions are like those of the nearby Late Monson and Fourmile Gneisses, plutonic roots to the Bronson 1 1 Taconian (Ordovician) rocks 69 Hill Arc, but differ from other local igneous rocks: Early Devonian Erving Formation (REE like N-MORB), Middle Devonian Belchertown Intrusion B57, age-dated sample, 159A 6A Concentration/MORB in the Bronson Hill Concentration/MORB 449 Ma (dashed line, Age-dated coarse Garnet-bearing gabbros (380 Ma, strange chemistry), Late Devonian Hardwick (360 Ma, mildly alkaline). The complex is exposed within the Late Devonian Prescott Syncline, anticlinorium than to the other hidden by other samples) 6B gabbro, 407 Ma on the east limb apparently intruding the Late Ordovician Partridge Formation, based on near-contact rock types contaminated with (including gabbro 0.1 0.1 suites. Monson and Fourmile Gneisses, Taconian Monson with 5 cm garnets), and inclusions of Partridge in Gneiss and Gabbro. On the west limb it is in apparent intrusive contact with Partridge and the Ammonoosuc 6D6 Unusual Cooleyville Gneiss, not Prescott Gabbro C D C Taconian plutonic rocks D and Fourmile gneisses 100 including in the gray average band 100 Prescott gabbro average, +_1 , Volcanics, and fault contact with Fourmile Gneiss of the Kempfield anticline. The north end was originally interpreted as intruding Silurian Clough Quartzite excluding these 5 samples and Devonian Littleton Formation, but the 449 Ma radiometric age contradicts this. The north contact is now interpreted as a southeast-dipping Mesozoic M82 Rare Nb-rich normal fault, connecting the previously mapped Ludlow-Lighthouse Hill fault on the west with the New Salem faults to the northeast. We correlate the Typical gneisses 64 Cooleyville geochemically with the plutonic Monson Gneiss, but likely a hypabyssal intrusion into Partridge at a high level in the Taconic arc. The much Cooleyville Gneiss, dated at 449 Ma (Tucker and 10 10 5D2 Robinson, 1990). This is a biotite--titanite OI6 granodiorite gneiss, with darker inclusions (top). 6A 1 1

Omos 69 159A 6B Concentration/MORB

Preliminary Planimetric Bedrock Geologic Map Concentration/MORB Oaf Oaa Cooleyville Gneiss, excluding Mesozoic rocks, Oaa Omo Adakitic gneisses Most amphibolites CANADA Valley basin of the Prescott Intrusive Complex and the 6 samples here, +_ 45°N Undifferentiated Ordovician, Dl Oaa Silurian, Devonian stratified rocks Omog Surroundings, Prescott Peninsula, Quabbin 0.1 0.1 Upper Ordovician to lower Silurian Kempfield m Anticline Reservoir Area, Massachusetts Other nearby units Erving Formation Partridge Formation, u Highlandcroft plutonic series Other Taconian plutonic E F E F ri Late Ordovician intrusives, amphibolites (Devonian) o Bears Omos Geology by Peter Robinson (1959-2016) and Kurt Hollocher (1979, 2016) 100 Taconian volcanics n southwestern Connecticut Den 100 rocks in the Bronson Hill belt li Wendell with compilations from the M.S. Thesis of Jordan Makower,1964 and c Dome gneisses in Bronson Hill Syncline Fault 202 Dry Hill Gneiss, Pelham n and Shelburne Falls belts results from students in Advanced Mapping Classes 1980-1985. y Dea Sf Oamc dome (Late Proterozoc) s Accretionary wedge and forearc é sediments, volcanics p Omo Northern domes, NH s Taconic sequence slope Main body Dated samples (Tucker and Robinson, 1990) a and rise sediments of 10 10 G Oaf Monson Gneiss - m Parautochthonous carbonate bank Ops Opsm y u e i and continental rise sediments ll r Mesozoic brittle faults New a o Ops Autochthonous shelf sediments Ofm Sc V n i York t l Nashoba block, includes late u c Early Devonian Packard Gabbro of Prescott Complex c Precambrian to Devonian rocks i i 1 1 t t m Massabesic Gneiss complex, c n i u includes late Precambrian rocks Dpg: Massive, medium- to e r Southern domes, MA, CT Partridge Formation n a coarse-grained, weakly foliated, Late Precambrian rocks, Pelham Oaa Concentration/MORB l o 43° Concentration/MORB n l dome and in Connecticut Prescott faintly lineated hornblende Taconian volcanics o i n i N Avalon block, includes late Pre- Syncline Dpgb Dpgb gabbro C l H cambrian to Pennsylvanian rocks Oaf Dpgb: Western border facies, c 0.1 n n very coarse, mottled dark-green 0.1 Mid-Proterozoic (Grenville) rocks Pelham Dome o Dl and white hornblende gabbro. y New Salem Magma Highlandcroft plutons, northern New Hampshire Magma s s Sc Other plutonic units G H Faults G H n New Hampshire (Late e Maine 70°W Series (Devonian) Series (Devonian) New 100 100 o n Dl Ordovician-Early Silurian) r i Opce Lower Devonian Erving Formation (Wendell Syncline) Hampshire Oph B a Rochester pluton, NH, Oa Notice the wide, completely Kinsman type M Ops Dea possibly similar to Hardwick

Ops l Oa Prescott Gabbro, dated at 407 Ma (Tucker and empty 54-63% SiO2 Kinsman type a 10 r Robinson, 1990). Originally interpreted as a gabbro 10 Opcg Opsm Omo composition gap between the t Lower Devonian Littleton Formation (Prescott Syncline) 43°N n Opcf pegmatite, its composition suggests it’s a cumulate. Prescott Gabbro and e Dl

Ops C Cooleyville Gneiss (red - Atlantic Ocean Ops k 1 Upper Silurian Fitch Formation (Prescott Syncline) 1 c Dpgb Opcg squares). The gap is Opcw Hardwick pluton (latest Devonian) a Dpg Packard Sf Concentration/MORB Spaulding type Ops consistent with the 449 Ma Concentration/MORB m Gabbro

i Massachusetts Lighthouse S F B r Hill Fault (gneiss) vs. 407 Ma (gabbro) Spaulding type r Ops Highlandcroft plutons, northern NH (Late Ordovician) Lower Silurian Clough Quartzite (Prescott Syncline) Dea 0.1 0.1 e age difference, indicating that Rb Ba Th U Nb' K La Ce Pb Pr Sr Nd Zr'SmEuGd Ti Tb Dy Li Ho Er TmRb Ba Th U Nb' K La Ce Pb Pr Sr Nd Zr'SmEuGd Ti Tb Dy Li Ho Er TmYb Rb Ba Th U Nb' K La Ce Pb Pr Sr Nd Zr'SmEuGd Ti Tb Dy Li Ho Er TmYb Ba Th U Nb' K La Ce Pb Pr Sr Nd Zr'SmEuGd Ti Tb Dy Li Ho Er TmYb Oph M Sc Yb Rb 42°N Ofm Opcg Dpg Element Element Element Element Rhode Opcf the plutons are unrelated. alley Basin Cooleyville Gneiss comparisons Prescott Gabbro comparisons Dpgb Opcw Opce Ops Island Dpgb Omoa Late Ordovician Shallow Intrusive Dl Cooleyville Gneiss of Prescott Complex N Opcw WD Opcg: Well lineated medium-grained gneiss. Opcf Opcf Opce Opce: Coarse-grained, micaceous gneiss. Opcf: Fine-grained gneiss. Connecticut V Ops Opcg Opcw: Coarse-grained micaceous gneiss with Previous interpretation New interpretation Omo Opcw plagioclase megacrysts. Cooleyville Gneiss, having Gabbro Conclusions 0 10 20 30 40 50 km Opce the assemblage - Connecticut 407 Ma 1) Similar geochemistry and age indicate that the Cooleyville Gneiss is a set of Taconian intrusions, Late Ordovician Partridge Formation Ops plagioclase-microcline- Early 73°W 71°W 70°W Oph Wendell Prescott like the Bronson Hill dome gneisses. Cooleyville Intrusion Syncline Syncline Ops: Rusty-weathering garnet-biotite- Devonian muscovite schist. biotite-epidote-titanite, here 72°30’ 72°15’ Opsm: Well-layered, rusty-weathering garnet- 2) Intrusive contacts with the Partridge Formation indicate that the Cooleyville intruded at shallow Opcf Opf Opa biotite-muscovite-sillimanite schist. showing a large allanite Gneiss Ops levels. Oph Opa: Amphibolite. 449 Ma Oph Opsm Opf: Layered gray- to brown-weathering grain (brown) that Omo quartz- gneiss. Earliest 3) With its Late Ordovician age (449 Ma), the Cooleyville Gneiss can’t have intruded Silurian and Dea Ops Oph: Hornblendite with amphibolite. apparently grew interstitial Silurian? Devonian units at its north end. This contact has been reinterpreted as a brittle fault, supported by Sedimentary and Opsm Mesozoic volcanic rocks Prescott Gabbro: hornblende-plagioclase-titanite- to quartz and plagioclase. Mt. Hermon Gabbro Main body Late Ordovician Ammonoosuc Volcanics careful re-working of field notes. Oph (latest Silurian- of Prescott Syncline Oaf: Upper Member. magnetite. It is somewhat deformed and also has a The mineralogy of the Interpretive sequence early Devonian) Monson Gneiss Oafs: White schist member. 4) The Prescott Gabbro is Lower Devonian (407 Ma), younger than local 454-442 Ma Taconian Oaf Gile Mtn. Formation/ Prescott Peninsula Oaf Oaau: Uppermost part of the mafic lower composition indicative of cumulate origin. dark-orange parts in the Erving Formation. Oafs member. Taconic arc igneous rocks, but similar in age (but not composition) to mafic portions of the New Hampshire Ops Oaau Oa Oaab: Biotite gneiss in middle of mafic lower Ordovician Omo Oaf Oaa member. lower-right and lower-left of plutonic complex Devonian Oafs Oaab Magma Series. Oaau Oaal Oaau Oamc Oaal: Lowermost part of the mafic lower Oaa the allanite grain are Littleton Formation Oafs member. 5) The Prescott Gabbro is 13-33 Ma older than the nearby Belchertown Complex and Hardwick Oaab Oaa: Mafic lower member undivided. Oaal Oamc: Moosehorn Conglomerate Member. uncertain. Plutonic rocks Oaf Oa: Ammonoosuc undivided. 1 mm Plutons respectively. Prescott Opa Complex: 6) Though the Prescott Gabbro is more LREE-enriched than amphibolites in the Lower Devonian Fitch Formation Late Ordovician Deep Intrusive Rocks gneiss 449 Ma Opcg

42°30’ Monson Gneiss Erving Formation, their ages are similar and they share arc-related geochemical anomalies, gabbro 407 Ma Fourmile Gneiss Ofm: Fourmile Gneiss, coarse- Silurian Main Body and Pelham Dome Kempfield Anticline to fine-grained gneiss with minor Relatively undeformed Ground level Mt. Mineral Formation/ Lighthouse amphibolite. possibly inherited from residual Taconian lithospheric mantle. Hill Fault Omos Clough Quartzite Wendell Omo: Monson Gneiss, coarse- Prescott Gabbro showing Syncline grained, layered to massive, Simplified and otherwise and amphibolite. Quabbin Reservoir Omoa (left) a large, poikilitic, modified cross sections Ofm Omos: Rusty-weathering gneiss from the Massachusetts Omo containing pyrite, muscovite and optically continuous State Map, showing post-early

Partridge Formation Ofm Kempfield rare sillimanite. State map cross section Anticline Omoa: Interlayered amphibolite Devonian structures Hardwick Omog hornblende grain, which is Quabbin Quabbin Reservoir and gneiss, separately mapped. Line of section Pluton Partial references Reservoir Omog: Areas where massive Simplified map patterns (360 Ma) loaded with Fe-Ti oxide Hollocher K., Bull, J., Robinson P., 2002, Geochemistry of the metamorphosed Ordovician Taconian magmatic arc, Bronson Hill anticlinorium, western New England: Physics and Chemistry of the Earth v. 27, p. 5-45. Ammonoosuc Opf granitoid gneiss predominates. Dorais, M.J. and Paige, M.L., 2000, Regional geochemical and isotopic variations of northern New England plutons: implications for magma sources and for Grenville and Avalon basement-terrane boundaries. Geological Society Ordovician Volcanics of America Bulletin, v. 112, p. 900-914. Opsm dust. To the right is Silurian and Devonian Dorais M.J., Watts B.G., Wintsch R.P., 2000. Geochemistry of Early Devonian calc-alkaline plutons in the Merrimack Belt: implications for mid-Paleozoic terrane relationships in the New England Appalachians. Atlantic Geology 0 1 2 3 4 km stratified units 36, 79-102. Prescott polycrystalline hornblende Ordovician Hodgkins, C.E., 1985, Geochemistry and petrology of the Dry Hill gneiss and related gneisses, Pelham dome, central Massachusetts: Contribution No. 48 (M.S. Thesis), Department of Geology and Geography, University of Fourmile Gneiss, Oafs Syncline stratified units Massachusetts, Amherst, 137 p. Monson Gneiss, etc. + plagioclase. The large Hollocher, Kurt, Bull, Jon, and Robinson, Peter, 2002, Geochemistry of the Metamorphosed Ordovician Taconian Magmatic Arc, Bronson Hill Anticlinorium, western New England: Physics and Chemistry of the Earth, v. 27, p. 5- Oph Monson Gneiss etc./ 45. Oaal N grain was probably was an Cooleyville Gneiss Leo, G.W., 1991, Oliverian domes, related plutonic rocks, and mantling Ammonoosuc Volcanics of the Bronson Hill anticlinorium, New England Appalachians: U.S. Geological Survey, Professional Paper 1516, 92 p. Late Dry Hill Gneiss, 0 5 10 km Maczuga, D.E., 1981, The petrology and geochemistry of the Fitchburg plutonic complex, central Massachusetts: Contribution No. 36 (M.S. Thesis), Department of Geology and Geography, University of Massachusetts, Amherst, 128 p. Poplar Mtn. Gneiss, Metamorphosed sedimentary igneous hornblende or Mesozoic fault N Opf Makower J., 1964, Geology of the Prescott intrusive complex [Ph.D. thesis]: University of Massachusetts, 106 p. Proterozoic Simplified map and related rocks rocks not differentiated pyroxene oikocryst, which Brenan Hill Thrust Robinson P., 2008, Bedrock geologic map and cross sections of the Orange area, Massachusetts. Office of the Massachusetts State Geologist Open File Report 09-01, scale 1:24,000, 5 sheets.Robinson P., Tucker P., 1990, Age Coys Hill Pluton Dea Prescott Gabbro, close to a contact with Partridge and setting of the Bronson Hill magmatic arc: a re-evaluation based on U-Pb zircon ages in southern New England: Geological Society of America Bulletin, v. 102, p. 1404-1419. Unconformity Shearer, C.K. and Robinson, Peter, 1988, Petrogenesis of metaluminous and peraluminous within the Merrimack synclinorium: Hardwick tonalite, central Massachusetts: American Journal of Science, v. 288-A, p. 148- (396 Ma) Ops Oaau grew interstitially. 42°15’ Fault at base of Ordo- 195. Belchertown Omo Formation schist. Schist contamination allowed 1 mm Complex (380 Ma) vician stratified units Schumacher, John, 1988, Stratigraphy and geochemistry of the Ammonoosuc Volcanics, central Massachusetts and southwestern New Hampshire: American Journal of Science, v. 288, p. 619-663. Oaab growth of metamorphic garnets. Shumway, D.O., 1984, Geochemistry of the Ammonoosuc Volcanics; tectonic setting of massive sulfide ore deposits: M.S. Thesis, Dartmouth College, Hanover, N.H., 102 p.