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南阿尔金木纳布拉克地区高压泥质麻粒岩的确定及其地质意义

南阿尔金木纳布拉克地区高压泥质麻粒岩的确定及其地质意义

ISSN:1000-0569
2013年第29卷第5期
曹玉亭1,刘良1,,王超1,2,康磊1,杨文强1,梁莎1,廖小莹1,王亚伟1 CAO YuTing1, LIU Liang1, WANG Chao1,2, KANG Lei1, YANG WenQiang1, LIANG Sha1, LIAO XiaoYing1, WANG YaWei1
1. 大陆动力学国家重点实验室, 西北大学地质学系, 西安 710069; 2. 西安地质矿产研究所, 西安 710054 1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China; 2. Xi'an Institute of Geology and Mineral Resource, Xi'an 710054, China

南阿尔金木纳布拉克地区出露一套典型的高压泥质麻粒岩, 其峰期特征矿物组合为Grt+Ky+Kfs+Qz+Ilm。根据矿物内部一致性热力学数据和Thermocalc3.33程序计算, 确定其峰期变质温压条件为T>850℃和P>11kbar。结合岩相学研究和P-T视剖面图计算, 可识别出该岩石经历了3个阶段的变质演化, 构成了一个早期降温降压, 后期近等压降温的顺时针型的退变质P-T演化轨迹。该岩石锆石阴极发光图像显示其内部具有明显的核-边结构, 核部为残留的原岩碎屑锆石, 边部则表现为面状生长的变质锆石的特征。微区原位LA-ICP-MS微量元素分析和锆石U-Pb定年表明, 该岩石原岩的形成时代上限值约为579Ma, 变质年龄为486±5Ma。该麻粒岩与南阿尔金淡水泉地区的高压麻粒岩具有相似变质演化轨迹和一致的峰期变质年龄, 亦与南阿尔金其它超高压岩石的峰期变质年龄一致, 表明它们都是南阿尔金陆壳深俯冲作用引发的高压-超高压变质事件的产物, 它们共同构成南阿尔金高压-超高压变质带。同时代的UHP榴辉岩和高压麻粒岩共存的现象, 可以很好地利用“俯冲隧道模型”来解释, 即可能是由于陆壳在深俯冲过程中不同深度不同热状态下发生拆离作用后折返引起的。另外, 该麻粒岩的原岩形成时代(约为579Ma), 可能为新元古代晚期, 与南阿尔金高压-超高压岩石的原岩形成时代基本一致或稍晚, 因此不应再作为岩石地层单元划归为“长城系”, 而应归属为南阿尔金高压-超高压变质岩带的一部分。


A suit of high pressure (HP) pelitic granulite was identified in the Munabulake area in the South Altyn Tagh. The peak mineral assemblage of the rock is Grt+Ky+Kfs+Qz+Ilm, and the peak P-T condition is estimated to be T>850℃ and P>11kbar according to the THERMOCALC 3.33. Three stages of metamorphic evolutions are described by means of P-T pseudosections for the selected rock, showing a clockwise retrograde metamorphic P-T path with early decompression and late isobaric cooling. Zircon CL images show the internal structure of core-rim microstructure, the cores preserve residual detrital zircon, and the rims display metamorphic zircons with sector and planar structure. LA-ICP MS in situ trace element analyses and U-Pb dating yielded the toplimit protolith age of 579Ma and the peak metamorphic age of 486±5Ma. The granulite has similar metamorphic evolution and peak metamorphic age with the high-pressure (HP) granulite from Danshuiquan area, and has identical metamorphic age with the ultrahigh-pressure (UHP) rock from other areas in the South Altyn, indicating the granulite in this study was a production of the South Altyn continental deep subduction. The coexistent of the contemporary UHP eclogite and the HP granulite might be accounted for by the crustal detachment of subducting continental lithosphere at different depths with different thermal conditions in subduction channels. Moreover, the protolith age of the granulite is in the Late Neoproterozoic Period, which is identical to those of the HP-UHP rocks from the South Altyn. Therefore, the rock should be a part of the South Altyn Tagh HP-UHP metamorphic belt rather than a lithostratigraphic unit belonging to "Changcheng System".

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ISSN:1000-0569
2013年第29卷第5期

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