北祁连造山带石包城地区花岗岩年代学及成因

摘要

The Paleozoic granites in the North Qilian area are important for understanding the tectonic evolution of the orogenic belt.The granite in Shibaocheng area is studied by petrography,geochemistry,zircon U-Pb chronology and Hf isotopic analyses.The results show that:(1) The granite in Shibaocheng area has an emplacement age of 465 ～463Ma and belongs to calc-alkaline series with strongly peraluminous features.(2) The granite has low average temperatures of Tzr (760℃) and TREE (732℃),respectively.Meanwhile,the granite is characterized by positive εHf(t) values (+ 7.6 ～ + 13.5) and two-stage Hf model ages of 581 ～ 957Ma.(3) Geochemical and isotopic data exhibits that the granite is better considered as transitional between I-and S-type granites,and is most likely derived from the juvenile basic crustal materials by the dehydration from amphiboles with minor addition of Mesoproterozoic-Neoproterozoic sedimentary components.The magma was without apparent fractional crystallization process while the residual rocks may be the granulite mainly composed by garnet,amphibole and plagioclase.(4) The granite in Shibaocheng area may be formed under the active continental margin setting triggered by the north dipping subduction of the North Qilian Ocean,and underwent a crustal contamination during magma emplacement into sedimentary cover.%北祁连造山带内发育大量早古生代花岗质岩石,对其进行研究有助于加深对造山带构造演化的理解.本文以带内西段石包城地区的花岗岩为研究对象,对其进行锆石U-Pb年代学、Hf同位素以及地球化学方面的研究.结果表明:(1)该花岗岩形成时代为465-463 Ma,地球化学数据表明其属于钙碱性系列,并具强过铝质特征;(2)该花岗岩TZr和TEE平均温度分别为760℃和732℃,εHf(t)变化于+7.6～+13.5,二阶段模式年龄变化于581 ～ 957Ma;(3)可能属I-S过渡型花岗岩,是新生基性地壳低温条件下由于角闪石脱水分解而发生部分熔融而成,并混入了少量中-新元古代沉积组分,岩浆未发生明显的分离结晶作用,残留相为石榴石、角闪石和斜长石为主的麻粒岩;(4)该花岗岩形成于北祁连洋北向俯冲的活动大陆边缘背景之下,并在侵位上升过程中加入了部分沉积物质.