青藏高原北羌塘南缘俯冲型石英正长斑岩的发现:来自地球化学分析证据  

Subduction-related Quartz Syenite Porphyries in the Eastern Qiangtang Terrane, Qinghai—Xizang Plateau:Constraints from geochemical analyses

作　　者：张洪瑞[1] 侯增谦[1] 杨天南[1] 宋玉财[1] 李政[1] 王召林[1] 王晓虎[1] 汪元奎 刘群 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]青海地质调查院,西宁810012

出　　处：《地质论评》2010年第3期403-412,共10页Geological Review

基　　金：国家科技支撑计划项目(编号2006BAB01A08);国家自然科学重点资助项目(编号40730419);中国地质调查项目(编号1212010818096)的成果

摘　　要：北(东)羌塘地块的二叠纪构造演化久存争议。笔者等在沱沱河地区发现了一套晚二叠世石英正长斑岩,其大地构造属性对北羌塘地块及青藏高原北缘古特提斯构造演化具有重要意义。该石英正长斑岩侵位于早二叠世火山岩和灰岩中,属钙碱性系列,富集轻稀土元素(LREE)和大离子亲石元素(LILE)、亏损高场强元素(HFSE),具明显Eu负异常,εNd(t)值在2.1～4.1之间,n(87Sr)/n(86Sr)初始比值为0.70065～0.70566。岩石地球化学特征显示岩浆起源于俯冲板片或板片流体交代的地幔楔部分熔融,具有岛弧岩浆典型特征,岩浆在上升过程中经历了同化混染与分离结晶作用,最终高温条件下浅成侵位,由此反映北羌塘地块在晚二叠世还存在一定规模的消减作用。It is chronically debated on the Permian tectonic evolution of the northern Qiangtang block, Qinghai—Xizang（Tibet） Plateau. Newly identification of the Late Permian quartz syenitic porphyries in the Tuotuo River area may provide some constraints on the Paleo-Tethyan tectonic evolution of the northern Qiangtang, even of the whole northern Qinghai—Xizang（Tibet） Plateau. The Later Permian porphyries, containing numerous basaltic and limestone xenoliths, intrude in the Early Permian andesites and limestones. They are calc-alkaline, characterized by enrichment of light rare earth elements （LREE） and large ion lithophile elements （LILE） with depleted high field strength elements （HFSE） and significantly negative Eu anomalies. Sr—Nd isotopic data of the porphyries exhibit ／[n（^87Sr）/n（^86Sr）／]ⅰ=0.70065～0.70566 and εNd（t）= 2.1～4.1 with TDM（Ga） = 0.679～0.828, suggesting the magma might have resulted from partial melting of the mantle wedge induced by dehydration of a subducted slab, e.g., the Late Permian quartz syenitic porphyries are arc-like. The magma crystallized at shallow level under the surface with high crystallization temperature. During the ascending and intrusion, fractional crystallization and crustal contamination may have modified the rock＇s chemistry.Obviously, there must be a subduction of oceanic slab during the Late Permian along one margin of the northern Qiangtang block.

关 键 词：石英正长斑岩 地球化学 洋壳俯冲 二叠纪 沱沱河 羌塘 青藏高原 

分 类 号：P584[天文地球—岩石学]