华夏陆块东北部古元古代八都杂岩多期变质作用及其地质意义

Badu Complex is by far the only basement rock unit that recorded multiple early Precambrian and Phanerozoic metamorphism (with the highest metamorphic grade up to granulite facies) in northeastern Cathaysia block and is thus the ideal research subject for the deciphering of the geological evolutionary processes of the Cathaysia block. However, the following several urgent problems have greatly hampered further interpretations about the tectonothermal evolutionary histories of the Cathaysia block. 1) Recognition of the granulite facies rock types, their field occurrence, their distribution patterns and so on is quite incomplete; 2) Seldom any studies about metamorphic evolution have been done on the newly discovered granulites; 3) There is a lack of precise geochronological data to evidence the specific metamorphic time of the granulites. This research plans to focus on the following three rock types: pelitic high-pressure granulite, mafic granulite and pelitic low- to medium-pressure granulite. And through 1) detailed field work, 2) calculating metamorphic P and T conditions using both traditional thermobarometry method and thermodynamic modeling method and 3) detailed zircon U-Pb geochronological studies, we hope to comprehensively identify granulite facies rock types and their distribution patterns, further constrain the metamorphic P-T path of each granulite and specify geological significance of the multiple metamorphism of the Badu Complex by correlating each episode of metamorphism with geochronological data. These studies can provide strong metamorphic and geochronological evidence for the proper interpretation of the evolutionary processes of the Cathaysia block.

八都杂岩是华夏陆块东北部已知的、唯一记录了早前寒武纪和显生宙多期变质作用并且级别达麻粒岩相的基底岩系，是通过变质作用探究华夏陆块地质演化的理想对象。八都杂岩的研究中存在以下几个亟待解决的问题：1）麻粒岩的类型、野外产状、分布规律等认识不足；2）麻粒岩变质作用的研究程度较低；3）缺乏对变质作用和地质年代学的综合研究。本申请将以在八都杂岩中新发现的泥质高压麻粒岩、基性麻粒岩和中低压泥质麻粒岩为重点研究对象：1）通过详细野外地质工作，进一步查明八都杂岩中麻粒岩的种类、产状和分布规律；2）进行详细的变质矿物组合、结构和叠加变质反应的研究，综合使用矿物温压计和基于内部一致性热力学数据库的相平衡模拟软件，准确限定其变质演化P-T轨迹；3）结合锆石成因分析，确立麻粒岩变质作用的精细年代学过程并探讨八都杂岩多期变质作用的具体地质意义，为华夏陆块的形成和多期热演化提供麻粒岩相变质作用和年代学方面的有力证据。

在该项目资助下，研究者对华夏地块东北部八都杂岩进行了系统的野外地质考察，在归纳总结前人数据的基础上，厘定出八都杂岩分布区主要记录了印支期高级变质，而八都杂岩分布区的西北方向，加里东期高级变质以北东-南西向分布于更加广泛的范围内。八都杂岩中，印支期基性麻粒岩、退变榴辉岩、泥质麻粒岩、中性麻粒岩共同出露，其中泥质麻粒岩为基性和中性麻粒岩的围岩。基性麻粒岩、退变榴辉岩、中性麻粒岩的产状多为透镜状，尺度可达几十上百米。变质作用研究结果显示，退变榴辉岩的变质演化P-T轨迹呈现顺时针样式，峰期压力可达23-24kbar，对应变质温度为500-560℃，伴随着降压，该岩石记录的变质温度持续升高，峰期变质温度达到730-750℃。基性麻粒岩麻粒岩相阶段变质温压条件为860-890℃、5-6kbar。退变榴辉岩的直接围岩：石榴黑云片麻岩，经历了类似的变质演化过程。变质锆石时代集中在245-251 Ma，并且所有变质锆石均显示重稀土稍微平坦的特征，按时变质锆石与变质成因石榴石的化学平衡。这些发现性的报道，证实华夏地块存在印支期高压变质作用，这为理解华夏地块印支期再造，提供了最直接信息，并且为解释华夏，乃至整个华南板块的印支期强烈的构造变形，提供了新的研究思路。