The Origin of Garnets in Anatectic Rocks from the Eastern Himalayan Syntaxis, Southeastern Tibet: Constraints from Major and Trace element zoning and phase Equilibrium Relationships
The Origin of Garnets in Anatectic Rocks from the Eastern Himalayan Syntaxis, Southeastern Tibet: Constraints from Major and Trace element zoning and phase Equilibrium Relationships
复制标题
西藏东南喜马拉雅构造系东深熔岩中石榴石的成因：主微量元素分带及相平衡关系的制约
DOI：
10.1093/petrology/egaa009
复制
发表时间：
2019
影响因子：
3.9
通讯作者：
Xu Luo
中科院分区：
文献类型：
--
作者：
Qiong-Xia Xia;Peng Gao;Guang Yang;Yong-Fei Zheng;Zi-Fu Zhao;Wan-Cai Li;Xu Luo
这些岩石主要由片麻岩、角闪岩和片岩组成，它们经历了不同程度的岩浆岩作用，形成了白花岗、伟晶岩和长英质脉。锆石U-Pb测年结果表明，合成带的黑云母片麻岩、白斑脉状花岗岩和脉状花岗岩的年龄一致，为~ 49 Ma，表明印度和亚洲大陆碰撞期间发生了地壳深熔。根据石榴石的赋存方式、矿物包裹体和主微量元素分带，可将其分为包岩型和隐晶型。包晶石榴石主要产于黑云母片麻岩（中粒层）和白岩脉中。它们呈倒面体，含有丰富的高钛黑云母和石英等矿物包裹体，主要元素组成（除Ca外）几乎均匀，从岩心到岩缘的HREE含量逐渐降低，表明在p -和t -递增的深熔过程中生长。在760-800°C和9-10·5 kbar的峰间溶出条件很好地受到相平衡计算、矿物组合和石榴石等长体的约束。相反，隐晶石榴石只出现在脉状花岗岩中。它们呈圆形或半面体，含有石英包裹体，从岩心到岩缘的辉晶石和微量元素含量逐渐增加。石榴石-黑云母地球温度测量和石榴石-黑云母-斜长石-石英地球温度测量表明，深熔型石榴石在~ 620 ~ 650℃和4 ~ 5kbar下结晶。部分石榴石颗粒在主微量元素上呈两阶段分带，其核心类似于包晶石榴石，边缘类似于隐晶石榴石。矿物学、全岩主微量元素组成和锆石O同位素表明，两种白色体分别由水合（有水）熔融和脱水（无水）熔融形成。白晶脉中含包晶石榴石，不含钾长石，全岩K2O含量和Rb/Sr比值较低，全岩CaO含量和Sr/Ba比值较高，δ18O值均低于残余锆石，表明白晶脉是水化熔融形成的。脉状花岗岩含包晶石榴石和钾长石，全岩K2O含量和Rb/Sr比值较高，全岩CaO含量和Sr/Ba比值较低，δ18O值与残余锆石相当，属于脱水熔融花岗岩。因此，喜马拉雅构造合带东部同时存在水化和脱水融化机制。
Amphibolite- and granulite-facies metamorphic rocks are common in the eastern Himalayan syntaxis of southeastern Tibet. These rocks are composed mainly of gneiss, amphibolite and schist that underwent various degrees of migmatization to produce leucogranites, pegmatites and felsic veins. Zircon U–Pb dating of biotite gneiss, leucocratic vein and vein granite from the syntaxis yields consistent ages of ∼49 Ma, indicating crustal anatexis during continental collision between India and Asia. Garnets in these rocks are categorized into peritecitc and anatectic varieties based on their mode of occurrence, mineral inclusions and major- and trace-element zoning. The peritectic garnets mainly occur in the biotite gneiss (mesosome layer) and leucocratic veins. They are anhedral and contain abundant mineral inclusions such as high-Ti biotites and quartz, and show almost homogeneous major-element compositions (except Ca) and decreasing HREE contents from core to rim, indicating growth during theP- andT-increasing anatexis. Peak anatectic conditions at 760–800°C and 9–10·5 kbar are well constrained by phase equilibrium calculations, mineral assemblages, and garnet isopleths. In contrast, anatectic garnets only occur in the vein granite. They are round or subhedral, contain quartz inclusions, and exhibit increasing spessartine and trace-element contents from core to rim. The garnet–biotite geothermometry and the garnet–biotite–plagioclase–quartz geobarometry suggest that the anatectic garnets crystallized at ∼620–650°C and 4–5 kbar. Some garnet grains show two-stage zoning in major and trace elements, with the core similar to the peritectic garnet but the rim similar to the anatectic garnet. Mineralogy, whole-rock major- and trace-element compositions and zircon O isotopes indicate that the two types of leucosomes were produced by hydration (water-present) melting and dehydration (water-absent) melting, respectively. The leucocratic veins contain peritectic garnet but no K-feldspar, have lower whole-rock K2O contents and Rb/Sr ratios, higher whole-rock CaO contents and Sr/Ba ratios, and show homogeneous δ18O values that are lower than those of relict zircons, indicating that such veins were produced by the hydration melting. In contrast, the vein granite contains peritectic garnet and K-feldspar, has higher whole-rock K2O contents and Rb/Sr ratios, lower whole-rock CaO contents and Sr/Ba ratios, and shows comparable δ18O values with those of relict zircons, suggesting that this granite were generated by the dehydration melting. Accordingly, both hydration and dehydration melting mechanisms have occurred in the eastern Himalayan syntaxis.