ABSTRACT

New field and petrographic observations, whole-rock major- and trace-element geochemical data, Nd and Sr isotope systematics and U-Pb SHRIMP zircon ages of the Peltetec metaophiolite and related rocks (Ecuador) allow the characterization of the igneous and sedimentary protoliths and the evaluation of the geodynamic evolution of the active western margin of Gondwana during the Mesozoic. The Peltetec ophiolitic belt, exposed along the western margin of the Cordillera Real (Ecuador) and previously considered of Cretaceous age, comprises slices of metamorphosed (greenschist facies) and variably deformed peridotites, gabbros (locally cumulate), basaltic lavas and dykes, hydrothermal breccias and sedimentary rocks. The geochemical characteristics of the tholeiitic basaltic rocks indicate a Sr-Nd isotope depleted mantle source with variable imprint of subduction zone-derived or continental crust-assimilated components, as indicated by enrichment in Th, negative anomaly of Nb and relative depletion in high field strength elements (HFSE). Geochemical diversity allows defining different groups of noncumulate igneous rocks with oceanic island arc and NMORB-like back-arc affinities. U-Pb zircon ages from a metabasite yielded 228.1 ± 1.7 Ma, interpreted as the crystallization age of the magma. These data allow proposing the generation of the Peltetec ophiolite in a back-arc setting as a consequence of late Triassic extension of the western continental margin of Gondwana. Granites located at the western margin of the Peltetec Unit with S-type affinity and equal Triassic age (U-Pb age of 228.2 ± 0.8 Ma) document coeval anatexis of the continental crust as a consequence of heating associated with the same extensional event that originated the ophiolite. Geologic correlation allows enlarging such tectonic scenario to include ophiolitic units of similar age in Colombia.

摘要

Peltetec 变绿岩和相关岩石（厄瓜多尔）的新野外和岩相观察、全岩主要和微量元素地球化学数据、Nd 和 Sr 同位素系统学和 U-Pb SHRIMP 锆石年龄允许表征火成岩和沉积原岩和冈瓦纳纪活动西缘中生代地球动力学演化评价. Peltetec 蛇绿岩带沿 Cordillera Real（厄瓜多尔）西缘出露，以前被认为是白垩纪时代，由变质（绿片岩相）和变形的橄榄岩、辉长岩（局部堆积）、玄武岩熔岩和岩脉、热液角砾岩组成和沉积岩。拉斑玄武岩的地球化学特征表明，Sr-Nd 同位素贫化地幔源具有俯冲带衍生或大陆地壳同化组分的可变印记，如 Th 富集、Nb 负异常和高场强相对贫化所示元素 (HFSE)。地球化学多样性允许定义具有大洋岛弧和类似 NMORB 的弧后亲和力的不同组的非堆积火成岩。来自变碱矿的 U-Pb 锆石年龄为 228.1 ± 1.7 Ma，被解释为岩浆的结晶年龄。这些数据允许提出作为冈瓦纳大陆西部大陆边缘晚三叠世延伸的结果在弧后环境中生成 Peltetec 蛇绿岩。位于 Peltetec 单元西缘的具有 S 型亲和力和相等三叠纪时代（U-Pb 年龄为 228.2 ± 0.8 Ma）的花岗岩记录了大陆地壳的同代深渊，这是由于与起源的同一拉伸事件相关的加热结果蛇绿岩。地质相关性允许扩大这种构造场景，以包括哥伦比亚类似年龄的蛇绿岩单元。