华北东南缘徐-淮地区下地壳包体Pb-Nd-Hf-O-Mg同位素地球化学研究

The ancient lower continental crustal xenoliths entrained by Early Cretaceous dioritic intrusions from the Xu-Huai area on the southeastern margin of the North China Craton have two important features that distinctly differ from ancient lower crustal xenoliths from other regions worldwide: (1) highly radiogenic Pb isotopic compositions and (2) prominent Nd-Hf isotope decoupling. Previous studies attributed the radiogenic Pb as derivation from the Triassic subducted Yangtze crust and the Nd-Hf decoupling to the Early Cretaceous partial melting of the deep crustal rocks and extraction of felsic melts. However,the Dabie-Sulu ultrahigh pressure metamorphic rocks generally have unradiogenic Pb and thus can not be possible protoliths of the Xu-Huai xenoliths. On the other hand,the Early Cretaceous melt extraction event is not long enough to yield the present-day extremely radiogenic Hf of the Xu-Huai xenoliths because of the limited radiogenic accumulation over the limited timescale. Therefore,origins of both the radiogenic Pb and Nd-Hf decoupling of the Xu-Huai xenoliths were not well contrained yet. This current application will aim to study Pb-O-Mg isotopic compositions of the Xu-Huai xenoliths and LCC metamorphic rocks from Bengbu area to reply on the first question. With respect to the second question, we will analyze Nd and Hf isotopic compositions of the Xu-Huai xenoliths and their host diorite rocks as well as the paleoproterozoic granites from the Bengbu area. This will provide constraints on the origin of Nd-Hf isotope decoupling of the Xu-Huai xenoliths and the time since which the decoupling happened.

华北克拉通东南缘徐-淮地区中生代侵入体中古老下地壳包体具有不同于全球其它地区大多数古老下地壳包体的两个重要特征: (1)高放射成因Pb和(2)显著的Nd-Hf同位素解耦。前人研究解释其高放射成因Pb来自三叠纪深俯冲的扬子陆壳物质，而Nd-Hf解耦与早白垩纪华北古老下地壳岩石部分熔融和熔体提取有关。然而，代表俯冲扬子陆壳物质的大别-苏鲁超高压变质岩具有低放射成因Pb，不能作为徐-淮包体高放射成因Pb的来源；早白垩纪的熔体提取事件不足以产生足够的放射成因Hf积累，无法解释目前观察到的Nd-Hf解耦程度。本申请拟对徐-淮地区下地壳包体及蚌埠地区出露的五河群基底变质岩及沉积岩进行Pb-O-Mg同位素联合研究，查明高放射成因Pb的起源；通过对徐-淮包体的寄主岩石（中生代侵入岩）和蚌埠地区古元古代花岗岩进行Nd-Hf同位素研究，查明Nd-Hf解耦的成因及其发生的时代。

按照申请书计划，对华北克拉通东南缘徐-淮深源包体和凤阳地区出露的麻粒岩地体以及碳酸岩围岩进行了全岩氧同位素、Mg同位素、Sr-Nd-Pb-Hf同位素及单矿物Mg同位素分析，取得重要进展，主要结论如下：.（1）包体具有很大的氧同位素变化范围，全岩Pb同位素与O同位素有很好的正相关性，表明这些下地壳包体经历了再循环表壳物质的交代作用,这一交代结果是导致古老的镁铁质下地壳具有高放射成因Pb的主要原因。.（2）Mg同位素结果表明：中地壳包体具有和地幔岩石相似或略重的Mg同位素组成；下地壳包体的Mg同位素组成变化很大，轻Mg同位素组成的样品普遍富集CaO含量，表明岩石遭受了碳酸盐流体的交代，与这些岩石的高放射性Pb同位素组成一致；（3）岩石圈地幔包体具有异常轻的Mg同位素组成，这些岩石的CaO含量很高，是地幔受碳酸盐熔体/流体交代的直接产物，交代地幔的碳酸盐熔体/流体可能来自俯冲向北俯冲古特斯洋壳。本研究另外的一个重要意义是证明华北克拉通深部岩石圈的Mg同位素不均一性，并且表现出从中地壳到下地壳，再到岩石圈地幔Mg同位素逐渐变轻的趋势，这反映了熔体/流体中的碳酸盐溶解度随岩石圈深度变深而升高，与高温高压下碳酸盐溶解实验的结果一致，即流体中碳酸盐的溶解度随压力的增大而升高，这也能解释为什么中地壳的基性岩往往具有正常的Mg同位素组成，而下地壳基性岩石会出现轻Mg同位素组成的现象。.（3）部分下地壳岩石（包体和地体）发生了显著地Hf-Nd同位素解耦现象。虽然Hf-Nd同位素发生了解耦，但是Lu/Hf比却没有显著变化，绝大多数落在了正常的地壳岩石范围内，这表明该地区下地壳岩石Hf-Nd解耦的原因的并不是熔体抽取后石榴石残留所造成的，这也与徐-淮地区埃达克岩的Hf-Nd同位素没有发生解耦的观察一致。因此，我们认为研究区域下地壳岩石发生Hf-Nd解耦的原因可能是受高Hf熔体/流体交代的影响。