高黎贡西北缘早白垩世火山活动与怒江洋俯冲:来自流纹岩岩石地球化学、锆石U-Pb定年和Hf同位素的证据

Early Cretaceous volcanism in the northwestern Gaoligong orogen and its relationship with subduction of the Nujiang Ocean:Evidence from geochemistry,zircon U-Pb dating and Hf isotopic compositions of rhyolites

高黎贡构造带西北缘早白垩世火山岩的厘定将为揭示腾冲与保山地块之间的关系补充强有力的证据。本文通过对构造带内高家寨和濮家寨一带的流纹岩进行岩石学、地球化学、锆石U-Pb定年和Hf同位素地质学等方面的研究,并结合前人对同时代侵入岩的研究成果,分析其形成的构造背景。锆石U-Pb定年结果表明高家寨和濮家寨流纹岩的形成时代相同,分别为122±2.2Ma和121±1.9Ma。岩石学和地球化学分析结果说明高家寨和濮家寨流纹岩都属于科迪勒拉型、镁质、弱过铝质-过铝质高钾钙碱性岩类,具有轻稀土富集、分馏程度高、重稀土相对亏损、Eu中度亏损及大离子亲石元素相对原始地幔强烈富集,高场强元素(HSFE) Nb、Ta、P、Ti及Sr强烈负异常的特点;流纹岩的Mg~#值较高,在SiO_2-Mg~#图解上分布于壳源物质部分熔融区上方;锆石ε_(Hf)(t)值变化范围大(-9.6^-2.6)和t_(DM)~C值高(1189~1572Ma);在构造环境判别图解上所有样品落在大陆边缘弧区;流纹岩斑晶中普遍存在黑云母。这些特征与高黎贡构造带早白垩世岩浆岩带内壳幔混源的中酸性侵入岩一致,说明其岩浆主要来源于壳源物质的部分熔融,同时有少量幔源岩浆的混入,是怒江洋壳向腾冲地块下俯冲的结果。

It is a strong evidence to show the relationship between the Tengcong and Baoshan blocks by identifying the geochemical and isotopic characteristics of the Early Cretaceous rhyolites in the northwestern Gaoligong orogen.Here we discuss the tectonic setting by studying on the petrological,geochemical and chronological results for these Early Cretaceous rhyolites in Gaojiazhai and Pujiazhai.The zircon U-Pb dating on the rhyolite samples yielded weighted mean206Pb/238U ages of 122±2.2 Ma and 121±1.9 Ma,representing the ages of their generation,respectively.The zirconεHf(t)values range from-9.6 to-2.6 with tDMC of 1189~1572 Ma.The studied rocks have geochemical characteristics including high-K calc-alkaline affinity,metaluminous to peraluminous compositions,and Cordilleran-type magmatic features,enrichment of LILE and LREE,strong negative Nb,Ta,Sr,P and Ti anomalies,high values of Mg#,indicating a continental arc affinity.Furthermore,on the diagram of SiO2 vs.Mg#,all samples fall in the region above the field of crustal partial melts,and on the tectonic discrimination diagrams,where all samples fall in the continental margin-arc field,and the presence of biotite,reflects the contribution of mantle-derived material in the rhyolitic parent magma.These features are coherent with coeval granites in Gaoligong orogen,suggesting that the volcanic magmas originated mainly from the partial melting of ancient upper crustal materials,and these melts were mixed with small amounts of mantle-derived magma in active continental margin,reflecting its tectonic setting related with subduction of Nujiang oceanic crust.