浙江中生代二长质-正长质侵入体成因及其对壳幔相互作用深部过程的启示

Monzonitic-syenitic intrusions commonly have higher concentrations of alkalis, and therefore can roughly be ascribed to generalized alkaline rocks. These rocks, usually with a relatively deep source, are explicit indicative of an extension tectonic regime, and contain more abundant information of crust-mantle interaction than that of the typical granitic rocks. Apart from widespread of Late Mesozoic granitic rocks, there are also some small-scale monzonitic-syenitc stocks developed in SE China, to which little attentions have been paid in previous research. Moreover, the dynamic mechanism of crust-mantle interaction beneath SE China still remains unclear. As a consequence, this project plans to deal with the monzonitic-syenitic intrusions and their enclosed mafic enclaves distributed with a roughly EW-trending in the south of the Jiangshan-Shaoxing deep fault, and conduct an integrated study including zircon U-Pb dating and in-situ Hf-O isotpe measurements, mineral chemistry, and whole-rock elemental and Sr-Nd-Pb isotopic analyses. The aim of this project is to focus emphatically on the following issues: (1) to unravel the deep dynamic process of the interaction between the crustal- and mantle-derived magmas; (2) to identify the nature of crustal and mantle components involved in rock-formation; (3) to understand the detailed ways controlling the formation and evolution of magmas and finally to comprehensively reveal the petrogenesis of the monzonitic-syenitic plutons. This study are helpful not only in deepen and enrich the understanding of the Mesozoic magmatism mechanism, but also can provide further petrologic evidence in ascertaining the Mesozoic tectonic evolution in SE China. Therefore, it has important scientific significances.

二长质-正长质岩石常具较高的碱含量，基本可归为广义的碱性岩石，它们常来源较深，并具有明确的指示拉张构造背景的意义，较之典型花岗质岩石蕴含更丰富的壳幔深部作用过程信息。中国东南部除广泛发育晚中生代花岗岩外，也分布有一些二长质-正长质的偏碱性侵入体，但以往对这类岩石研究明显不足，对区内壳幔岩浆相互作用的动力学过程也缺乏深入认识。为此，本项目选择浙江江绍断裂以南近东西向展布的二长质-正长质侵入体及其中的镁铁质包体为对象，进行锆石U-Pb年代学和Hf-O同位素、矿物化学及全岩元素与Sr-Nd-Pb同位素组成的系统研究，着重探索壳幔岩浆相互作用的深部动力学过程，揭示参与成岩的壳幔组分的性状，查明岩浆形成与演化的具体途径，进而全面阐明区内二长质-正长质岩体的成因。本项目的研究既有助于深化并丰富中国东南部中生代岩浆作用的认识，也可为揭示区内中生代的构造演化提供进一步的岩石学证据，因而具有重要的科学意义。

二长质-正长质岩石常具较高的碱含量，多形成于拉张的构造背景。它们常来源较深，较之典型花岗质岩石蕴含有更丰富的壳幔深部作用过程信息。中国东南部除广泛发育晚中生代花岗岩外，也分布有一些二长质-正长质的偏碱性侵入体，但以往对这类岩石研究明显不足。为此，本次研究选择浙江江绍断裂以南近东西向展布的二长质-正长质侵入体为对象，进行了锆石U-Pb年龄与Hf同位素组成、矿物化学及全岩元素与Sr-Nd同位素组成的系统研究，据此阐明了它们的成因，进而为深入认识中国东南部晚中生代壳幔岩浆相互作用过程提供了新的线索。这些二长质-正长质侵入体的主体岩性为石英二长岩或石英正长岩，多数二长质侵入体中发育有塑变形态的暗色镁铁质微粒包体。二长质侵入体主要形成于112~100 Ma，镁铁质包体与其寄主二长质岩石具有一致的年龄；正长质侵入体的年龄相对年轻，主要变化于100~90Ma。这些岩石具有中性-中酸性、富钾、弱碱性、弱过铝至准铝质的化学组成特征。它们均富集轻稀土和大离子亲石元素（如Rb、Th、U、Pb），亏损高场强元素（如Nb、Ta、P、Ti），总体具有偏低的Sr初始值 (ISr = 0.7058~0.7090)、偏高的εNd(t)值（-8.06~-2.29）及较年轻的二阶段Nd模式年龄（0.86~1.57 Ga）。它们的锆石εHf(t)值具有较大的变化范围大，但大部分样品点均位于华夏地块基底岩石Hf同位素演化域之上。综合年代学、岩石学及元素与Sr-Nd-Hf同位素组成资料，表明浙江晚中生代二长质-正长质岩石是在弧后伸展引张背景下，由底侵的镁铁质岩浆与其诱发古元古代地壳物质部分熔融形成的长英质岩浆通过混合作用，并经进一步分异演化后结晶的产物。本项研究一方面深化并丰富了中国东南部中生代岩浆作用的认识，另一方面也为揭示区内中生代的构造演化提供了进一步的岩石学证据，因而具有重要的科学意义。