冈底斯中段阿木雄杂岩体岩石成因及地质意义

The petrogenesis of Amuxiong gabbro-granite complex in the middle segment of Gangdise belt

冈底斯南缘发育大量的岩浆岩,前人对这条岩浆岩带东段的研究完善了冈底斯南缘的大地构造框架和岩石圈结构及演化理论体系,而关于冈底斯南缘中段岩浆作用的研究相对较少。本文对中段阿木雄杂岩体的角闪辉长岩和黑云二长花岗岩进行了详细的岩石学、年代学及地球化学研究,以期阐明岩浆成因并探究岩浆源区深部地质过程。阿木雄杂岩体中黑云二长花岗岩和角闪辉长岩锆石U-Pb年龄均为49 Ma,属于始新世同期岩浆作用产物。地球化学特征显示黑云二长花岗岩属于I型高钾钙碱性花岗岩,富集大离子亲石元素,亏损高场强元素,具低正的εHf(t)值(-0.2^+2.6),Hf同位素地壳模式年龄tDMC为1136~393 Ma;角闪辉长岩主量元素特征属于典型的辉长岩,Mg#值接近于原始的玄武质岩浆,稀土元素配分曲线平缓右倾,无Eu亏损,富集大离子亲石元素,亏损高场强元素,具有高正的εHf(t)值(+5.1^+16.8)和年轻的tDMC年龄(798~46 Ma)。分析后认为角闪辉长岩源于受过板片流体交代的亏损地幔,而黑云二长花岗岩源于新生地壳的部分熔融并有少量古老地壳贡献,且岩浆经过一定程度的分异作用。结合前人和本文研究认为该区的古老地壳物质来自于古老的拉萨地块,并通过对比同时期冈底斯岩浆岩带的岩浆作用,认为阿木雄杂岩体岩浆作用的主要诱发机制可能为北向俯冲的新特提斯洋板片断离。

Large quantities of magmatic rocks are developed on the southern margin of Gangdise belt.Previous studies mainly focused on the eastern part of this magmatic rock belt,which completed the theory of the tectonic framework and lithospheric structure as well as the evolution of the southern margin of the Gangdise belt.The Amuxiong complex is in the middle part of the magmatic rock belt on the southern margin of the Gangdise belt.The authors described zircon U-Pb geochronology,zircon Hf isotope and whole-rock geochemical analyses of hornblende gabbro and biotite monzonitic granite of the Amuxiong complex,discussed the emplacement order of these two types of magma,and clarified the genesis of magma and the hypogene geological process of the magma source area.The zircon U-Pb dating results of the biotite monzonitic granite and the hornblende gabbro are both at 49 Ma,suggesting magmatism of the Eocene period.The biotite monzonitic granite is I-type calc-alkaline granite,characterized by enrichment of the large ion lithophile elements(LILE),and depletion of the high field strength elements(HFSE),with the small positiveεHf(t)value of-0.2^+2.6 and the Hf isotopic crustal model age tDMC of 1136~393 Ma,suggesting that this hornblende gabbro belongs to typical gabbro.The rare earth element patterns show a gentle right wing,with very slight negative Eu anomalies,whereas the trace element pattern shows the enrichment of LILE and depletion HFSE,with the high positiveεHf(t)value of+5.1^+16.8 and the young tDMC age of 798~46 Ma.The analytical results suggest that the hornblende gabbro was derived from the depleted mantle that had undergone lamellar fluid replacement,and that,after some differentiation,the magma of monzonite was derived from partial melting of juvenile lower crust and a small amount of old continental crust.There have been some disputes about the origin of the ancient materials in this area.Based on the analysis of previous results and the characteristics of magmatic rocks in this study,the authors hold that the contribution of the ancient materials came from the old Central Lhasa subterrane.By comparing the magmatism of the Gangdise magmatic belt in the same period,it is concluded that the main inducing mechanism of the magmatism of the Amuxiong complex might have been plate break off.