金沙江缝合带西段蛇绿岩与弧火山岩成对性关系--来自地球化学和LA-ICP-MS锆石U-Pb年龄证据

摘要

A study of the field section of Duocai ophiolites mélange and Dangjiangrong volcanic rocks in Zhiduo area of western Jin-sha River suture zone shows that ophiolites are mainly composed of gabbro, cumulate gabbro and basalt with the lack of the mantle peridotite unit. XRF tests of fine-grained gabbro and basic lava and LA-ICP-MS zircon U-Pb dating of cumulate gabbro from sys-tematic sampling of ophiolite mélange show that basic lavas can be divided into two series, i.e., OIB type and MORB-IAT transi-tional type. The first series is not the member of ophiolite, and was involved in the mélange belt by tectonic emplacement. The sec-ond series is a transitional type and has the special geochemical characteristics of MORB-IAT ophiolites. The gabbros have obvious"TNT"slot in primitive mantle-normalized distribution patterns, evidencing that it has the"signal of island arc"feature;the gabbros, together with the second series of basic lava, belong to the ophiolite. The single zircon U-Pb age of cumulate gabbro is 252.5Ma ± 0.58Ma, which can be interpreted as the formation age of ophiolites. Duocai ophiolites and Dangjiangrong arc volcanic rocks are c on-sistent well in geochemistry with each other, their formation age and a comparative study of their other aspects indicate that they have the paring relationship. Combined with the trench-arc-basin tectonic framework in the orogenic belt, the authors come to the conclusion that the Duocai ophiolites might have been formed in a fore-arc tectonic setting apart from the arc and close to the trench, resulting from the continual subduction of the Jinsha Ocean at the end of Permian period.%对金沙江缝合带西段青海治多地区的多彩蛇绿混杂岩和当江荣中酸性岛弧火山岩进行了研究，野外地质剖面显示，蛇绿岩主要由辉长岩、堆晶辉长岩和玄武岩组成，缺少地幔橄榄岩单元。通过对蛇绿岩内部细粒辉长岩、基性熔岩的地球化学测试及堆晶辉长岩的LA-ICP-MS锆石U-Pb测年发现，基性熔岩可分为2种类型，即洋岛玄武岩OIB型和MORB-IAT型。前者并非蛇绿岩组分，为构造就位时带入；后者为过渡类型，具有典型洋中脊-岛弧蛇绿岩地球化学特征。辉长岩具有明显的TNT槽等岛弧信号，与类型二均属于蛇绿岩成分。测得的堆晶辉长岩中锆石U-Pb年龄为252.50Ma±0.58Ma（MSWD=0.95），是蛇绿岩的形成年龄。研究认为，多彩蛇绿岩与当江荣火山岩具有成对性关系，结合造山带沟-弧-盆体系构造格局，认为前者形成于岛弧偏海沟的弧前构造背景，是晚二叠世金沙江洋持续俯冲的产物。