新疆谢米斯台地区晚古生代俯冲作用:来自塞勒肯特岩体的岩石学、年代学及地球化学证据

Late Paleozoic subduction in Xiemisitai:Evidence from the petrology,geochronology and geochemistry of Sailekente pluton,Xinjiang

塞勒肯特岩体出露于谢米斯台中东部,岩性主要为二长花岗岩、石英闪长岩及花岗闪长岩。本文通过锆石U-Pb年代学、岩石地球化学和Sr-Nd-Hf同位素等研究,探讨其形成的构造环境及岩石成因。结果表明,二长花岗岩(400.9±4.3Ma)和石英闪长岩(398.1±4.5Ma)形成于早泥盆世,花岗闪长岩(381.7±2.9Ma)形成于晚泥盆世。岩体整体富碱,属于准铝质-弱过铝质高钾钙碱性花岗岩类。轻重稀土分馏较明显且富集轻稀土((La/Yb)_(N)=5.09~9.22),Eu异常不明显,相对富集Rb、Th、U、K等元素,亏损Nb、Ta、Ti等高场强元素;二长花岗岩和石英闪长岩具有低的(^(87)Sr/^(86)Sr)_(i)值(0.7040~0.7043),正ε_(Nd)(t)值(+4.85~+6.18),年轻的t_(DM1)(Nd)年龄(663~732Ma),二长花岗岩锆石εHf(t)值为+7.94~+12.12,t_(DM2)(Hf)=648~889Ma;花岗闪长岩也具有低的(^(87)Sr/^(86)Sr)_(i)值(0.7045~0.7046),正ε_(Nd)(t)值(+4.61~+4.80),年轻的t_(DM1)(Nd)年龄(731~749Ma),花岗闪长岩锆石ε_(Hf)(t)值为+4.26~+11.69,t_(DM2)(Hf)=631~1103Ma。综合研究表明,塞勒肯特岩体形成于俯冲背景下的大陆边缘弧环境,可能是俯冲板片脱水交代地幔楔产生的玄武质岩浆上涌,导致新生下地壳发生部分熔融。二长花岗岩及石英闪长岩均来源于新生下地壳的部分熔融;花岗闪长岩主要来源于新生下地壳的部分熔融,并有少量幔源物质的加入,花岗闪长岩中的暗色微粒包体可能是幔源物质与新生下地壳部分熔融的岩浆未发生完全混合,最终冷凝结晶的产物。谢米斯台地区与俯冲相关的中酸性岩浆活动至少从晚奥陶世一直延续至晚泥盆世。

The Sailekente pluton occurs in the middle-east part of Xiemisitai area.It is mainly composed of the monzonitic granite,quartz diorite and granodiorite.We carried out integrated studies of geology,geochronology,geochemistry and whole rock Sr-Nd and zircon Hf isotopes of the Sailekente pluton,and discuss their petrogenesis and implications on the tectonic evolution.The results show that monzonitic granite(400.9±4.3 Ma)and quartz diorite(398.1±4.5 Ma)are formed in Early Devonian,and granodiorite(381.7±2.9 Ma)is formed in Late Devonian.The whole pluton belongs to metaluminous-weakly peraluminous high-K calc-alkaline granite,and which is characterized by high silicon and enrich of alkali.The pluton has obvious fractionation of LREE and HREE,and is enriched in LREE((La/Yb)N=5.09~9.22).The REE patterns show negative Eu anomaly.They are also enriched in LILE and depleted in Nb,Ta and Ti.The monzonitic granite and quartz diorite are characterized by positive ε_(Nd)(t)(+4.85~+6.18),low(^(87)Sr/^(86)Sr)_(i)(0.7040~0.7043)and relatively young model ages t_(DM1)(Nd)ranging from 663 Ma to 732 Ma,the monzonitic granite has ε_(Hf)(t)values ranging from +7.94~+12.12 and t_(DM2)(Hf)model ages of zircon between 648 Ma and 889 Ma,respectively;granodiorite is characterized by positive ε_(Nd)(t)(+4.61~+4.80),low(^(87)Sr/^(86)Sr)_(i)(0.7045~0.7046)and relatively young model ages t_(DM1)(Nd)ranging from 731 Ma to 749 Ma,and the ε_(Hf)(t)values and t_(DM2)(Hf)model ages of zircon are ranging from +4.26 to+11.69,and between 631 Ma and 1103 Ma,respectively.In conclusion,Sailekente pluton is formed in the environment of continental margin arc under the subductional background,which could be a product of emplacement and condensation crystallization of magma which is derived from partial melting of the juvenile crust.Both monzonitic granite and quartz diorite originate from the partial melting of the juvenile crust;the granodiorite mainly comes from the partial melting of the juvenile crust with the addition of a small amount of mantle-derived materials.The dark microgranular enclaves may be the product of the mantle-derived material and the partially molten magma of the juvenile crust not being completely mixed,and finally condensed and crystallized.The medium-acid magmatism is associated with subduction continuing at least from the Late Ordovician to the Late Devonian in the Xiemisitai area.