The Prescott Peninsula contains two intrusive bodies: the Cooleyville Gneiss (449 Ma) and the Packard Gabbro (407 Ma), which cuts the Cooleyville. These are centrally located in the Bronson Hill Anticlinorium, a complex structural belt in western New England that includes metamorphosed rocks of the Ordovician Taconian arc plutonic complex (TAPC), and associated metamorphosed volcanic rocks. TPAC gneisses (locally 454–442 Ma) are overlain by stratified units including the Ordovician Ammonoosuc Volcanics (locally 453 Ma), Partridge Formation (locally 449 Ma), and several Silurian and Devonian units. The Cooleyville Gneiss and Packard Gabbro are both calc-alkaline and have similar, arc-associated geochemical anomalies expressed in multi-element diagrams. These units are chemically distinct from several regionally exposed Devonian plutons, but are almost identical to plutonic rocks of the TAPC. The Cooleyville Gneiss cuts the Ammonoosuc Volcanics and Partridge Formation, but the TAPC does not, at least over extensive regions. Additionally, the Ammonoosuc Volcanics and Partridge Formation are each in contact with the TAPC in different areas: Partridge Formation locally to the east and west, and Ammonoosuc Volcanics in a central belt. A multi-part model explains these relationships: 1) The Cooleyville Gneiss is part of the TAPC, having intruded the Ammonoosuc Volcanics and Partridge Formation east of the main arc axis. 2) Following arc collision with Laurentia, extensive detachment faulting decapitated the TAPC and Cooleyville pluton, sliding them and adjacent units west and down. 3) A second detachment carried part of the Ammonoosuc and Partridge farther west, completing the observed contact geographic pattern. 4) Following isostatic readjustment, erosion, and cooling, Silurian and Devonian units were deposited, in part unconformably on the TAPC. 5) Early Acadian (Devonian) subduction beneath composite Laurentia produced extensive central New England magmatism, and reactivated remnant Taconian lithospheric mantle beneath the TAPC, producing the Packard Gabbro and age-equivalent volcanics in the nearby Erving Formation. Detachment faulting is common in arc and collisional settings. This model of late Taconian detachment faulting appears to solve a long-standing problem understanding the TAPC contact with overlying Ordovician units.

中文翻译：

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美国马萨诸塞州中部普雷斯科特半岛侵入岩的地球化学：奥陶纪 Taconian 火山弧内晚期拆离断层的意义

普雷斯科特半岛包含两个侵入天体：库利维尔片麻岩（449 Ma）和帕卡德辉长岩（407 Ma），它们切割库利维尔。它们位于 Bronson Hill Anticlinorium 的中心，这是新英格兰西部的一个复杂构造带，包括奥陶纪 Taconian 弧深成复合体 (TAPC) 的变质岩和相关的变质火山岩。TPAC 片麻岩（当地 454-442 Ma）被分层单元覆盖，包括奥陶系 Ammonoosuc 火山（当地 453 Ma）、鹧鸪组（当地 449 Ma）和几个志留系和泥盆纪单元。Cooleyville 片麻岩和 Packard Gabbro 都是钙碱性的，并且在多元素图中具有相似的、与弧相关的地球化学异常。这些单元在化学上与几个区域暴露的泥盆纪岩体不同，但几乎与 TAPC 的深成岩相同。Cooleyville 片麻岩切割 Ammonoosuc 火山和鹧鸪组，但 TAPC 不切割，至少在大片区域切割。此外，Ammonoosuc 火山和鹧鸪组分别在不同区域与 TAPC 接触：东部和西部局部的鹧鸪组，中央带的 Ammonoosuc 火山。一个多部分模型解释了这些关系：1) Cooleyville 片麻岩是 TAPC 的一部分，侵入了主弧轴以东的 Ammonoosuc 火山和鹧鸪组。2) 在与 Laurentia 的弧形碰撞之后，广泛的分离断层使 TAPC 和 Cooleyville 岩体斩首，将它们和相邻单元向西和向下滑动。3) 第二支分队运载部分 Ammonoosuc 和鹧鸪更西边，完成观察到的接触地理模式。4) 在等压调整、侵蚀和冷却之后，志留系和泥盆系单元沉积在 TAPC 上，部分不整合。5) 复合 Laurentia 之下的早期阿卡迪 (泥盆纪) 俯冲产生了广泛的新英格兰中部岩浆作用，并重新激活了 TAPC 下方的残余 Taconian 岩石圈地幔，在附近的 Erving 组中产生了 Packard Gabbro 和年龄等效的火山岩。分离故障在电弧和碰撞环境中很常见。这种晚期 Taconian 拆离断层模型似乎解决了一个长期存在的问题，即理解 TAPC 与上覆奥陶纪单元的接触。5) 复合 Laurentia 之下的早期阿卡迪 (泥盆纪) 俯冲产生了广泛的新英格兰中部岩浆作用，并重新激活了 TAPC 下方的残余 Taconian 岩石圈地幔，在附近的 Erving 组中产生了 Packard Gabbro 和年龄等效的火山岩。分离故障在电弧和碰撞环境中很常见。这种晚期 Taconian 拆离断层模型似乎解决了一个长期存在的问题，即理解 TAPC 与上覆奥陶纪单元的接触。5) 复合 Laurentia 之下的早期阿卡迪 (泥盆纪) 俯冲产生了广泛的新英格兰中部岩浆作用，并重新激活了 TAPC 下方的残余 Taconian 岩石圈地幔，在附近的 Erving 组中产生了 Packard Gabbro 和年龄相当的火山岩。分离故障在电弧和碰撞环境中很常见。这种晚期 Taconian 拆离断层模型似乎解决了一个长期存在的问题，即理解 TAPC 与上覆奥陶纪单元的接触。