Orocopia Schist in the northern Plomosa Mountains, west-central Arizona: A Laramide subduction complex exhumed in a Miocene metamorphic core complex
Orocopia Schist in the northern Plomosa Mountains, west-central Arizona: A Laramide subduction complex exhumed in a Miocene metamorphic core complex
复制标题
亚利桑那州中西部普洛莫萨山脉北部的 Orocopia 片岩：在中新世变质核杂岩中挖掘出的拉拉米德俯冲杂岩
作者：
Evan D. Strickland;J. Singleton;G. Haxel
我们的文件领域的关系，岩相学和地球化学的一个新发现的暴露Orocopia片岩，Laramide俯冲复合体，在北方Plomosa山脉变质核复杂的中西部亚利桑那州（美国）。这个核心复杂的特点是无处不在的糜棱岩组构与早中新世侵入。石英长石质Orocopia片岩在其整个约2-3 km的构造厚度和10 km 2的顶向NE的Plomosa拆离断层下盘暴露范围内记录了顶向NE的糜棱岩化。北方普洛莫萨山脉的片岩局部含有石墨质斜长石变粒岩和分散的粗粒阳起石豆荚，这两者都是奥罗科普亚和相关片岩的特征。阳起石豆荚镁，镍，铬含量高，被解释为交代橄榄岩-在附近的公墓岭Orocopia片岩中观察到的协会。一个3.5公里长的斜长角闪岩单元，其内层有少量含铁锰石英岩，沿着奥罗莫皮亚片岩和片麻岩之间的SE倾斜接触面。根据其岩性和地球化学特征，我们将角闪岩和石英岩分别解释为变玄武岩和Meta硅质岩。Orocopia片岩的顶部仅低于约3-4公里。21 Ma凝灰岩在下盘的Plomosa拆离断层，这表明，一个主要的古近纪折返事件带来了上地壳深处的片岩后，它是在最后的白垩纪俯冲，但在大多数中新世核心复杂的折返。普罗莫萨山脉北方的奥罗科皮亚片岩位于玛丽亚褶皱冲断带中心附近，可能代表了晚白垩世的地壳贴边。在流变性弱的奥罗莫皮亚片岩底侵之前，该地壳贴边的龙骨可能已被浅倾的法拉隆板片剪切掉。北方普洛莫萨山脉奥罗科皮亚片岩的古近纪折返与加州最东南部加维兰山奥罗科皮亚片岩中记录的伸展折返一致，后者较晚于片岩底侵作用，表明片岩的俯冲可能引发了该地区的古近纪伸展。LITHOSPHERE; v. 10; no. 6; p. 723-742; GSA数据储存库项目2018358|在线发布2018年10月18日https://doi.org/10.1130/L742.1
We document field relationships, petrography, and geochemistry of a newly identified exposure of Orocopia Schist, a Laramide subduction complex, in the northern Plomosa Mountains metamorphic core complex of west-central Arizona (USA). This core complex is characterized by pervasive mylonitic fabrics associated with early Miocene intrusions. The quartzofeldspathic Orocopia Schist records top-to-the-NE mylonitization throughout its entire ~2–3 km structural thickness and 10 km2 of exposure in the footwall of the top-to-the-NE Plomosa detachment fault. The schist of the northern Plomosa Mountains locally contains graphitic plagioclase poikiloblasts and scattered coarsegrained actinolitite pods, both of which are characteristic of the Orocopia and related schists. Actinolitite pods are high in Mg, Ni, and Cr, and are interpreted as metasomatized peridotite—an association observed in Orocopia Schist at nearby Cemetery Ridge. A 3.5-km-long unit of amphibolite with minor interlayered ferromanganiferous quartzite is localized along a SE-dipping contact between the Orocopia Schist and gneiss. Based on their lithologic and geochemical characteristics, we interpret the amphibolite and quartzite as metabasalt and meta chert, respectively. The top of the Orocopia Schist is only ~3–4 km below a ca. 21 Ma tuff in the footwall of the Plomosa detachment fault, suggesting that a major Paleogene exhumation event brought the schist to upper-crustal depths after it was subducted in the latest Cretaceous but before most Miocene core complex exhumation. The Orocopia Schist in the northern Plomosa Mountains is located near the center of the Maria fold-and-thrust belt, which likely represented a crustal welt in the Late Cretaceous. The keel of this crustal welt may have been sheared off by the shallowly dipping Farallon slab prior to underplating of rheologically weak Orocopia Schist. Paleogene exhumation of the Orocopia Schist in the northern Plomosa Mountains is consistent with extensional exhumation recorded in Orocopia Schist in the Gavilan Hills of southeasternmost California, which shortly postdated schist underplating, suggesting that subduction of schist may have triggered Paleogene extension in the region. LITHOSPHERE; v. 10; no. 6; p. 723–742; GSA Data Repository Item 2018358 | Published online 18 October 2018 https://doi.org/10.1130/L742.1