胶北粉子山群石榴云母片岩的三叠纪独居石年龄及其地质意义  

Triassic monazite ages and its geological significance of garnet-micas chist in Fenzishan Group,Jiaobei Massif

作　　者：曹汇[1,2] VERVOORT Jeff 王达[2] 李广旭[1,3] NEILL Owen 

机构地区：[1]中国地质调查局大陆构造与动力学创新研究中心,中国地质科学院地质研究所,北京100037 [2]美国华盛顿州立大学,普尔曼wa99163 [3]中国地质大学,北京100083

出　　处：《岩石学报》2016年第12期3800-3816,共17页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41202153;41472198;41430212);国土资源部公益性行业科研专项经费(201211093);中国地质调查局地质调查项目(2015020401003;20160102002);中国地质科学院地质研究所中央级公益性基本业务费专项基金(J1402);大陆构造与动力学国家重点实验室自主基金(Z1401)联合资助

摘　　要：胶北地块粉子山群石榴云母片岩中石榴石变斑晶内包裹物迹线明显,保留了岩石形成过程中的多期变质变形信息。电子探针成分面扫描图显示石榴石成分环带明显,可分为核部、幔部和边部。石榴石中MgO、FeO、MnO和CaO含量变化特征表明其核部到边部温度先升高后降低,对应进变质及退变质过程。根据原位独居石Y元素成分面扫描图显示,部分独居石颗粒由核部到边部Y含量呈现逐渐降低趋势,说明测得的232.6±1.1Ma^229.5±3.7Ma的独居石U-Pb年龄,对应石榴石的进变质生长过程。结合1869±72Ma的锆石U-Pb年龄数据,可推断粉子山群石榴云母片岩至少经历了古元古代及三叠纪两期变质事件的改造。粉子山群石榴云母片岩卷入了苏鲁超高压变质带的俯冲碰撞造山事件。电子探针成分分析结果表明粉子山群石榴云母片岩中的石榴石属于铁铝榴石,反映出经受中级区域变质作用的特征。说明粉子山群石榴云母片岩虽然参与了三叠纪苏鲁超高压变质带的俯冲碰撞造山过程,但俯冲深度较浅。这可用大陆俯冲过程中上盘的俯冲剥蚀来解释,并可为陆-陆碰撞俯冲剥蚀模式提出的扬子板片在240~220Ma的深俯冲作用过程中拽动胶北地块向下俯冲又折返的运动过程提供佐证,但胶北地块是否经历了深俯冲超高压变质作用,还需要进一步验证。The inclusion trail of garnet porphyroblasts in garnet-mica schists of Fenzishan Group, Jiaobei Massif record the multi- stage deformation and metamorphism history during rock formation. Garnets with clear zonation structures can be divided into core, mantle, and rim based on electron microprobe compositional mapping. The content variation of MgO, FeO, MnO, and CaO in garnet shows that the temperature of core to rim first increased then decreased, indicating prograde to retrograde metamorphism. The Y content in monazite decreases from the core to rim based on the compositional mapping of monazite grain. The 232. 6 ＋ 1.1Ma ~ 229. 5 ＋ 3.7Ma monazite U-Pb ages correspond to the prograde metamorphic growth process of garnet. Combined with 1869 ：t： 72Ma zircon U-Pb age, it can be inferred that the garnet-mica schist in the Fengzishan Group has undergone at least two metamorphic events in the Paleoproterozoic and Triassic. The garnet-mica schist in the Fenzishan Group records the subduction-eollision orogenic process of the Snlu UHP metamorphic belt in Triassic. The electron microprobe analysis of garnet shows an almandine composition, and indicates it had experienced mid-level regional metamorphism. It further indicates that although the garnet-mica schist in the Fenzishan Group was involved in the Triassic subduction-collision orogenic process of the Sulu UHP metamorphic belt, the subduetion depth it reached was not very deep. This can be explained by the subduction erosion model for continental subduction process, and provides evidence for the hypothesis of deep subduction erosion model of continent-continent collision for the formation of the Sulu HP-UHP metamorphic terrain, which mentioned that part of Jiaobei Massif was partially dragged down during the 240 - 220Ma Yangtze plate subduction process. Further study is required to demonstrate if Jiaobei Massif experienced deep subduction UHP metamorphism.

关 键 词：粉子山群 石榴石变斑晶 变质作用 独居石U—Pb定年 

分 类 号：P588.344[天文地球—岩石学] P597.3[天文地球—地质学]