Age and setting of the Bronson Hill magmatic arc: a re-evaluation based on U-Pb zircon ages in southern New England
Age and setting of the Bronson Hill magmatic arc: a re-evaluation based on U-Pb zircon ages in southern New England
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布朗森山岩浆弧的年龄和背景：基于新英格兰南部 U-Pb 锆石年龄的重新评估
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发表时间：
1990
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通讯作者：
P. Robinson
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作者：
R. Tucker;P. Robinson
新英格兰南部布朗森山复向斜中部分岩石的锆石U-Pb年龄如下：Pelham穹隆Dry Hill片麻岩中的微斜长石片麻岩，被认为是变质碱性流纹岩，确定为晚元古代，年龄为613 ± 3 Ma。另一个样品显示了锆石继承的有力证据，最小平均年龄为1402 Ma。富含斜长岩的片麻岩与相关的斜长角闪岩（Swanzey、Pauchaug、蒙森和Fourmile片麻岩），现在被认为是变质侵入火成岩的代表，其年龄为454 ~ 442 + 3/-2 Ma，范围从晚奥陶世早期到晚奥陶世，与该地区Oliverian深成岩系的其他绝对年龄一致。AmmonooooOstanic火山岩上段的石英斑状流纹岩和ParmonoOstanic组的流纹岩凝灰岩的年龄分别为453 ± 2 Ma和449 ± 3/-2Ma，表明上覆火山岩段中长英质熔岩的喷发时间约为400万年。在Swanzey片麻岩（381 ± 3 Ma）中的一个横切但变形的英云闪长岩脉和从Prescott侵入杂岩（407 + 3/-2 Ma）侵入Parnesian组的一个粗粒辉长岩中报告了额外的锆石年龄。普雷斯科特半岛英云闪长片麻岩和干山片麻岩的U-Pb年龄分别为357 ± 5 Ma和292 ± 5 Ma，表明该地区在北西向造山和阿勒格尼造山过程中的变质叠加作用。
上述年龄和其他锆石年龄，以及岩石学和地球化学，Fourmile，蒙森，Swanzey，Pauchaug和其他富含斜长石的片麻岩的布朗森山复向斜表明，它们代表了至少250公里长的钙碱性岩浆弧的深成根源，该岩浆弧部分或全部产生于晚奥陶纪时期（约200年）的大陆边缘。454-442 Ma）。化学性质不同的拉斑玄武岩弧火山岩和沉积岩的Ammonooooeastic火山岩和Parkanic形成的物理覆盖序列，以前认为休息不整合的斜长石丰富的深成片麻岩，现在被证明有一个岩浆的历史与他们重叠。深成片麻岩和盖层序列中的锆石年龄表明，两者都是相同的年龄或略年轻于晚奥陶世（卡拉多纪）的Taconian外来体的Giddings Brook切片的侵位，太年轻，已经形成以上的早，中奥陶世俯冲带。
New U-Pb zircon ages of selected rocks from the Bronson Hill anticlinorium in southern New England are as follows: Microcline gneiss of the Dry Hill Gneiss in the Pelham dome, believed to be metamorphosed alkali rhyolite, is confirmed as Late Proterozoic, yielding an age of 613 ± 3 Ma. Another sample shows strong evidence of zircon inheritance with a minimum mean age of 1402 Ma. Plagioclase-rich gneisses with associated amphibolites (Swanzey, Pauchaug, Monson, and Fourmile gneisses), now believed to represent metamorphosed intrusive igneous rocks, yield ages from 454 to 442 +3/-2 Ma, thus ranging from early Late through latest Ordovician, consistent with other absolute ages of the Oliverian plutonic series in the region. Quartz-phyric rhyolite from the upper member of the Ammonoosuc Volcanics and rhyolite tuff from the Partridge Formation are dated at 453 ± 2 Ma and 449 +3/-2 Ma, respectively, indicating that eruption of felsic lavas in the overlying volcanic section spanned an interval of approximately 4 m.y. Additional zircon ages are reported from a crosscutting but deformed tonalite dike in the Swanzey Gneiss (381 ± 3 Ma) and a coarse-grained gabbro from the Prescott Intrusive Complex (407 +3/-2 Ma) intrusive into the Partridge Formation. Metamorphic overprinting of the region during both the Acadian and Alleghanian orogenies is indicated by U-Pb titanite ages of 357 ± 5 Ma and 292 ± 5 Ma from a tonalite gneiss on Prescott Peninsula and the Dry Hill Gneiss, respectively.
The above ages and other zircon ages, as well as petrography and geochemistry, of the Fourmile, Monson, Swanzey, Pauchaug, and other plagioclase-rich gneisses of the Bronson Hill anticlinorium suggest that they represent the plutonic root of a calc-alkaline magmatic arc at least 250 km long that was produced partly or entirely on a continental margin in Late Ordovician time (ca. 454-442 Ma). The physically overlying cover sequence of chemically different tholeiitic arc-volcanic and sedimentary rocks of the Ammonoosuc Volcanics and Partridge Formation, previously thought to rest unconformably on the plagioclase-rich plutonic gneisses, is now shown to have had a magmatic history overlapping with them. The zircon ages in the plutonic gneisses and the cover sequence indicate that both are the same age or slightly younger than the Late Ordovician (Caradocian) emplacement of the Giddings Brook slice of the Taconian allochthons, and too young to have formed above an early and medial Ordovician subduction zone.