阿拉善北部亚干地区花岗岩锆石年代学、地球化学特征及其构造意义

Zircon Geochronology and Geochemistry of the Granitoids in Yagan Area,Northern Alxa and Their Tectonic Implications

由于缺少可靠的年代学资料和系统研究,阿拉善北部亚干地区的基底时代和性质尚不清楚,制约了对本区构造属性及造山带结构的进一步认识.利用亚干地区广泛出露的花岗岩锆石U-Pb年代学和Hf同位素研究,揭示源区深部物质组成特征,对探讨该地区的基底性质具有重要意义.LA-ICP-MS锆石U-Pb分析结果表明,切刀黑云母二长花岗岩体侵位于380±1Ma,亚东花岗闪长岩体侵位于271±2 Ma,同时,原北山群中识别出大量的三叠纪花岗岩(亚干片麻状花岗岩,228±2 Ma;都热糜棱岩化花岗岩,214±2 Ma).地球化学分析表明,切刀花岗岩和都热糜棱岩化花岗岩为准铝质-弱过铝质的A型花岗岩,亚东花岗岩为钙碱性I型花岗岩.锆石Hf同位素分析显示主要的花岗岩体εHf(t)值为-2.8^+4.3,地壳模式年龄为1.0~1.5 Ga,表明源区可能以中元古代地壳物质为主.结合前人获取的前寒武纪岩石年龄,亚干地区花岗岩Hf同位素特征,以及花岗岩出现中-新元古代继承锆石等证据,亚干地区深部应具有中-新元古代基底,南戈壁微陆块范围可以延伸到阿拉善北部边境地区.

Due to the lack of detailed field geological and high-precision geochronological studies,the basement nature and tectonic attribute of the Yagan area have been poorly understood so far,which prevent the further understanding of the basic architecture and tectonic evolution of the southern Central Asian orogenic belt.Revealing the age and origin of the granitoids is essential for the understanding of the basement characteristics and tectonic architecture.In this paper presents systematic petrology,zircon U-Pb and Hf isotopic studies for the granitoids in the Yagan area.LA-ICP-MS zircon U-Pb isotopic dating results indicate that the biotitemonzonitic granite of Qiedao pluton and granodiorite of Yadong pluton emplaced at 380±1 Ma and 271±2 Ma,respectively.Meanwhile,the zircon U-Pb ages of 228±2 Ma and 214±2 Ma were obtained from the gneissic granite and mylonitic granite(their protoliths are granite rocks)in Paleoproterozic Beishan Group.Mineralogical and geochemical features suggest that the Qiedao and Dure granites are high-K calc-alkaline and metaluminous to slightly peraluminous,belonging to the A-type granites,and the Yadong granodiorites are calc-alkaline I-type granites.The Qiedao biotite-monzonitic granite and Yadong granodiorite have zirconεHf(t)values ranging from-2.8 to+4.3 and the crustal model ages of 1.5 to 1.0 Ga,suggesting that the magma sources were mainly derived from the Meso-proterozoic crustal material.Inherited zircon ages within the gneissic granite are mainly around0.8-1.0 Ga.These observations,in combination with previous work,indicate that the basement of Yagan area is composed of Meso-Neoproterozoic rocks,and the South Gobi microcontinent might extend into the northern Alxa.