增生造山过程中大陆地壳的净生长——来自中亚造山带中段南缘岩浆岩的约束

Net growth of the continental crust during the process of accretionary orogeny:Constraints from igneous rocks,southern margin of the middle section of the Central Asian Orogenic Belt

软流圈地幔上涌是大陆地壳净生长的重要方式,这种壳幔作用在增生造山作用过程的地质表现如何有待深入研究。内蒙古西部的居延海地区位于中亚造山带中段南缘,处于北山造山带与阿拉善地块北缘造山带的构造衔接部位。通过对居延海地区4口井所钻获的具壳幔作用特征的花岗质岩石的研究,本文深入剖析了增生造山过程中软流圈地幔物质贡献及其具体地质表现。首先,花岗质岩石中的闪长质暗色微粒包体(富含针状磷灰石微晶)指示了壳幔岩浆混合特征。再者,花岗质岩浆锆石结晶年龄为306~313Ma,具有正的ε_(Hf)(t)值,平均值介于+13.0~+14.3之间,二阶段模式年龄t_(DM2)平均值介于410~588Ma之间,揭示一次显著的大陆地壳净生长事件,锆石Hf同位素表明花岗质岩浆源区具有初生陆壳重熔和亏损幔源岩浆混合的特征。以上地质事实暗示居延海所在的中亚造山带中段南缘晚石炭世处于洋中脊俯冲或俯冲板片回卷的地球动力学过程中。

The upwelling of the asthenosphere mantle is an important way for the net growth of the continental crust.While the geological performance of crust-mantle interaction in the process of accretion and orogeny still needs further research.The Juyanhai area in western Inner Mongolia locates in the southern margin of the middle section of the Central Asian Orogenic Belt(CAOB),where it bridges the northern Beishan Orogenic Belt and the northern margin of the Alxa block.Based on the granitoids with crust-mantle interaction characteristics encountered in four wells of Juyanhai,this paper systemically analyzes the contribution of the asthenosphere mantle material and its specific geological performance during the accretion and orogeny.We have determined the dioritic microgranular enclaves of granodiorite rich in needle-like apatite.Furthermore,the granitoids yield ^(206)Pb/^(238)U magmatic crystallization ages of 306~313 Ma,which display extremely depleted Hf isotopic signatures(ε_(Hf)(t)=+13.0~+14.3)and show relatively young two-stage Hf model age(t_(DM2)=410~588 Ma),reflecting a remarkable continental crustal growth event.The zircon ε_(Hf)(t)value reveals that the granitic magma source includes remelting of juvenile continental crust and injection of depleted mantle.The corresponding Late Carboniferous crustal growth implies geodynamics of mid-ocean ridge subduction or slab roll-back in Juyanhai area,in the southern margin of middle part of CAOB.