埃达克质岩-I-A型花岗岩类复合岩体成因研究——以浙江舟山岩体为例

Study on the petrogenesis of complexes consisting of adakitic, I- and A-type granitoids, generally could shed new insights into the regional crust-mantle interaction processes and tectonic evolution. The geology of coastal Fujian and Zhejiang is characterized by widespread Cretaceous high-K calc-alkaline I-type granite with minor A-type granite, a few scattered Jurassic adakitic rocks however are also identified. Based on our previous accumulation for I- and A-type composite granite pluton studies in SE China Coast, this project plans to deal with the Zhoushan adakitic, I- and A-type composite granitoid pluton in coastal Zhejiang, brings into full play the superiority of this rock association on combined tracing, and conducts an integrated study including zircon U-Pb dating and in-situ Hf-O isotope measurements, mineral chemistry, and whole-rock geochemical analyses. The aim of this project is to focus emphatically on the following issues: (1) to identify the nature of crustal and mantle components involved in rock-formation; (2) to reveal magma evolution processes; (3) to unravel geochemical evolution features of the composite pluton and combined with related simulation results, finally to comprehensively reveal the regional late Mesozoic processes of continental crust evolution and crust-mantle interaction. The scientific importance of the study depends on their facilitation of a better understanding of the Mesozoic magmatism mechanism, as well as their further petrologic evidence to constraints the Late Mesozoic tectonic evolution of SE China

针对埃达克质岩石与I-A型花岗岩构成的复合岩体开展成因研究，可为揭示区域壳幔相互作用过程与构造演化提供重要信息。浙闽沿海以广泛发育白垩纪高钾钙碱性I型花岗岩及少量A型花岗岩为特色，近来发现区内还零星出露有侏罗纪埃达克质岩石。本项申请拟在浙闽沿海I-A型复合花岗岩体研究积累的基础上，选择浙江舟山侏罗纪埃达克质岩石与白垩纪I-A型花岗岩构成的复合岩体为对象，充分发挥该岩石组合的联合示踪优势，进行锆石U-Pb年代学和Hf-O同位素、矿物化学及全岩地球化学组成的系统研究，着重探索参与成岩的壳幔组分性状，查明岩浆的演化历程，明确复合岩体中各类岩石地球化学组成的时序演变特点，并结合相关模拟计算，以期揭示研究区晚中生代大陆地壳演化与壳幔相互作用过程。本项目的研究既有助于深化对中国东南部晚中生代岩浆活动过程的认识，也可为揭示区内晚中生代的构造演化提供进一步的岩石学证据，因而具有重要科学意义。

前人针对浙闽沿海广泛发育的晚中生代中酸性火成岩进行了较深入研究，但对幔源组分参与成岩的方式及岩石形成的动力学过程尚有不同认识。本项目以浙江舟山由埃达克质石英闪长玢岩、I型碱长花岗岩和A型碱性花岗岩构成的复合岩体为对象，进行了系统的锆石U-Pb年代学和Hf同位素矿物化学及全岩地球化学组成的系统研究。定年结果表明石英闪长玢岩岩、I型和A型花岗岩分别侵位于170Ma、98-96Ma和89-86Ma。石英闪长玢岩显示中硅、准铝以及高Sr/Y和La/Yb比值的特征，指示其属埃达克质岩石。I型花岗岩具有亚碱、准铝-弱过铝及低104Ga/Al比值的特征，而A型花岗岩显示过碱性、富F及高场强元素和重稀土元素的特征及高104Ga/Al比值。埃达克质岩具强烈富集的Sr-Nd-Hf同位素组成（ISr =0.7184–0.7211，εNd(t)=-18.1~ -17.7，εHf (t)=-17.9~ -24.2），二阶段 Nd-Hf 模式年龄变化于2.35–2.75 Ga，这在浙闽沿海尚属首次发现。结合华夏地块基底数据，这些表明舟山埃达克质岩应起源于八都杂岩中基性火成岩的部分熔融，并有少量斜长石和石榴石残留于源区。舟山A型花岗岩的同位素特征（εNd(t)=-6.5~ -5.7，εHf (t)=-7.5~ -1.6）较之较早侵位的I型花岗岩（εNd(t)=-7.5~ -7.0，εHf (t)=-12.3~ -4.8）略显亏损。两类花岗岩的锆石Hf同位素组成均显示分散的特征，同时岩相上有暗色包体出现，且存在具反环带特征的斜长石，表明二类岩石起源于壳幔岩浆混合。二端元混合模拟计算表明在舟山I-A型花岗岩成岩过程中应至少有50%幔源岩浆混入，这与岩石的高硅特征明显不符。这进一步表明区域古老基底可能受到岩浆作用的强烈改造，并为后期中酸性火成岩提供了具亏损特征的源区。同位素较为亏损的源区组成和幔源岩浆混入共同导致了舟山I-A型花岗岩亏损的同位素特征。对中国东南部岩浆活动的综合分析表明，在浙闽沿海存在一条NNE向的早-中侏罗世弧岩浆岩带，指示古太平洋板块的俯冲可能发生于200Ma。早期受板片俯冲作用的持续影响，浙闽沿海大陆地壳明显加厚，并形成了中侏罗世舟山埃达克质岩。晚期受俯冲板片折返影响，诱发了浙闽沿海强烈的构造伸展，进而形成了广泛分布的晚白垩世I-A型花岗岩。