Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera
Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera
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DOI：
10.1130/b25443.1
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发表时间：
2005-05
影响因子：
4.9
通讯作者：
J. Shervais;B. Murchey;D. Kimbrough;P. Renne;B. Hanan
中科院分区：
文献类型：
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作者：
J. Shervais;B. Murchey;D. Kimbrough;P. Renne;B. Hanan
加利福尼亚州北部的海岸山脉蛇绿岩 (CRO) 包括两个不同的遗迹。 Elder Creek 蛇绿岩是典型的超俯冲带蛇绿岩，具有三个连续的深成岩套件（层状辉长岩、韦尔岩-辉石岩、石英闪长岩）、镁铁质至长英质岩脉杂岩和镁铁质长英质火山岩；整个套房被晚期洋中脊玄武岩（MORB）岩墙切割，并被蛇绿角砾岩覆盖。斯托尼福德火山群 (SFVC) 由三个火山系列组成，这些火山系列具有夹层燧石层，形成封闭在剪切蛇纹岩中的海底火山。这座海山下方的结构是与埃尔德溪类似的 CRO 混杂岩块。 Elder Creek 斜长花岗岩和石英闪长岩的 U/Pb 锆石年龄范围为 165 Ma 至 172 Ma。从 SFVC 以下的 CRO 混杂岩块获得的 U/Pb 锆石年龄相似（166-172 Ma）。上层 SFVC 碱玄武岩玻璃的 40 Ar- 39 Ar 年龄均较年轻，约为 164 Ma。从 SFVC 中夹有玄武岩的燧石透镜体中提取的放射虫表明，沉积地层的年龄范围从接近杂岩体底部的巴通阶（统一伴生带 6-6，Baumgartner 等人，1995a）到上部卡洛世晚期到早启莫里阶（单一伴生带 8-10）。 SFVC沉积记录保留了动物区系发生重大变化的证据，其中相对较小规模的多栖放射虫动物群被非常强大的、寡聚的、以纳氏菌为主的动物群所取代，其中包括Praeparvicingula spp。我们认为，CRO 的形成始于中侏罗世早期（172-180 Ma），外来弧或边缘弧与北美碰撞，并开始形成新的或重新配置的东倾俯冲带。数据显示，CRO 形成于晚侏罗世内华达造山运动之前，可能是由于弧前在新生俯冲带上方快速延伸而形成。我们推断，CRO 扩散随着海洋扩散中心的碰撞而结束。 164 Ma，与方济各会组合中结构底层中最古老的高品位区块重合。我们进一步认为，“经典”内华达造山运动代表了对扩张中心碰撞的响应，年轻岩石圈的浅俯冲导致了最初的压缩变形，随后北美板块运动变化为快速向北漂移（J2尖点），导致了内华达山麓的左旋压裂和张拉。这些数据与 Sierra 山麓晚侏罗世弧碰撞模型或 CRO 弧后起源不一致。
The Coast Range ophiolite (CRO) in northern California includes two distinct remnants. The Elder Creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites (layered gabbro, wehrlite-pyroxenite, quartz diorite), a mafi c to felsic dike complex, and mafi c-felsic volcanic rocks; the entire suite is cut by late mid-oceanic-ridge basalt (MORB) dikes and overlain by ophiolitic breccia. The Stonyford volcanic complex (SFVC) comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite. Structurally below this seamount are melange blocks of CRO similar to Elder Creek. U/Pb zircon ages from plagiogranite and quartz diorites at Elder Creek range in age from 165 Ma to 172 Ma. U/Pb zircon ages obtained from CRO melange blocks below the SFVC are similar (166‐172 Ma). 40 Ar- 39 Ar ages of alkali basalt glass in the upper SFVC are all younger at ≈164 Ma. Radiolarians extracted from chert lenses intercalated with basalt in the SFVC indicate that the sedimentary strata range in age from Bathonian (Unitary Association Zone 6‐6 of Baumgartner et al., 1995a) near the base of the complex to late Callovian to early Kimmeridgian (Unitary Association Zones 8‐10) in the upper part. The SFVC sedimentary record preserves evidence of a major faunal change wherein relatively small-sized, polytaxic radiolarian faunas were replaced by very robust, oligotaxic, nassellarian-dominated faunas that included Praeparvicingula spp. We suggest that CRO formation began after the early Middle Jurassic (172‐180 Ma) collision of an exotic or fringing arc with North America and initiation of a new or reconfi gured east-dipping subduction zone. The data show that the CRO formed prior to the Late Jurassic Nevadan orogeny, probably by rapid forearc extension above a nascent subduction zone. We infer that CRO spreading ended with the collision of an oceanic spreading center ca. 164 Ma, coincident with the oldest high-grade blocks in the structurally underlying Franciscan assemblage. We further suggest that the “classic” Nevadan orogeny represents a response to spreading center collision, with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in North American plate motion to rapid northward drift (J2 cusp) causing sinistral transpression and transtension in the Sierra foothills. These data are not consistent with models for Late Jurassic arc collision in the Sierra foothills or a back-arc origin for the CRO.