基性麻粒岩造岩矿物微量元素再分配特征及其对变质历史的指示作用：以胶北地体高压麻粒岩为例

Trace element redistributed patterns in rock-forming minerals of mafic granulite as indicators for metamorphic history: A case study of high-pressure granulite from the Jiaobei terrane,North China,Craton

高级变质岩的变质历史是反演地壳构造-热事件的重要依据,然而高温扩散和重结晶作用能够改造造岩矿物中的主量元素分布,这对峰期变质温压条件的反演产生很不利的影响。相对于主量元素,微量元素,尤其是离子半径较大的REE,由于其在晶格中的扩散速率远小于主量元素,在高级叠加变质过程有可能记录前期变质作用。本文以胶北地体的高压基性麻粒岩为研究对象,通过详细的岩相学和矿物化学分析,初步解析了变质重结晶过程中的矿物微量元素再分配特征及其对变质作用的指示意义。岩相学上的证据表明这些样品经历了麻粒岩相变质和后期重结晶作用。单矿物的原位化学成分分析,峰期矿物石榴石、单斜辉石的主量元素Mg、Fe、Ca等二价阳离子分布均一,但部分稀土元素及微量元素则表现出钟形剖面环带分布,暗示主量元素遭受到成份扩散及重结晶所致的元素再分配,微量元素可记录峰期历史。结合主、微量元素温压计,我们分别估算了胶东基性高压麻粒岩的峰期（〉828℃、〉1.27GPa）和中压麻粒岩相退变质温压条件（810~840℃、0.6~1.0GPa）,并推测其后期经历过角闪岩相退变质叠加。结合前人的年代学工作,我们认为该基性麻粒岩经历了近等温快速减压的变质历史。

Metamorphism history of high-grade metamorphic rocks is an important clue to shed light on crust tectono-thermal events.However,intra-crystalline diffusion and recrystallization induced element re-distribution between rock-forming minerals,which have important influence on inversion of peak metamorphic history. On the contrary,intra-crystalline diffusion rates of trace elements are relatively slower than those of the major elements. Therefore,trace elements distribution and partition patterns in rock-forming minerals might be useful tools to shed light on metamorphic history of mafic high-pressure granulites. Major and trace elements of major rockforming minerals in mafic high-pressure granulite are analyzed by in-situ LA-ICP-MS measurement to learn about trace element redistribution patterns between minerals and their indication of metamorphism. Our results show major elements（ Mg,Fe,Ca） in peak stage minerals including garnet,clinopyroxene and plagioclase are homogenized by diffusion and recrystallization distribution,so P-T condition in peak stage can ＇t get good inversion through minerals thermobarometry,but rare earth elements and trace elements distributed in garnet and clinopyroxene as bell shape to infer peak history recorded by trace elements and REE. Compiling major element thermobarometry and trace element thermometer results,we estimate P-T conditions of the peak and retrograde metamorphism of 828℃ and 1. 27 GPa,810 ~ 840℃ and 0. 6 ~ 1. 0GPa,respectively. Deduced amphibolite facies metamorphism took place after retrograde medium pressure granulite metamorphism. Integreting with previous data,we suggest that our mafic granulite experienced an ITD metamorphic history.