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 Gneiss 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 gneisses, 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 Ordovician 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 amphibolite (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 Tonalite (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 schist (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 amphibolites 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-epidote-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 Connecticut 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 Vermont 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 u includes late Precambrian rocks Dpg: Massive, medium- to e i Southern domes, MA, CT a r coarse-grained, weakly foliated, Partridge Formation n 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 n i i N Syncline Dpgb Dpgb gabbro Avalon block, includes late Pre- 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 New Hampshire Magma Highlandcroft plutons, northern New Hampshire Magma s s Other plutonic units Sc Faults G H 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 quartz- 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-feldspar 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.