青藏高原拉萨地块西部亚热南复式岩体年代学与地球化学

The petrogenesis of southern Yare composite pluton in western Lhasa terrane: Constraints from zircon U-Pb geochronology and geochemistry

青藏高原拉萨地块发育着中新世斑岩,该后碰撞时期的斑岩因与斑岩型Cu-Mo矿床有着密切关系已被前人做过大量研究。然而对于拉萨地块西部未成矿斑岩岩体的年代学、地球化学报道研究较少。本文对拉萨地块西部亚热南复式岩体识别出的两类侵入岩进行了锆石LA-ICP-MS U-Pb定年和岩石地球化学研究表明,亚热南复式岩体主要由始新世黑云母二长花岗岩（年龄49.4±0.9Ma）和中新世花岗斑岩（年龄16.3～16.5Ma）组成。始新世黑云母二长花岗岩属钾玄岩系列准铝质-弱过铝质,具有低的Sr/Y、（La/Yb）N值高Y、YbN值具有典型的岛弧岩浆岩性质;中新世花岗斑岩为钾玄岩系列、准铝质,具有类埃达克质特征。这两种岩性所反映的源区亦存在差别,始新世黑云母二长花岗岩源区为地幔楔混染过的中下地壳;而具有埃达克性质的花岗斑岩则可能是源于古老地壳部分熔融。结合年代学及构造背景,推论出始新世黑云母二长花岗岩的岩石成因为新特提斯洋板片断离引发混染过的中下地壳发生熔融并结晶分异形成;而中新世花岗斑岩则形成于某种动力学机制引发的古老下地壳熔融后侵位于上地壳。

Miocene porphyry systems,which are closely associated with porphyry Cu-Mo deposit,have been found widely spreading in Lhasa terrane,Tibetan Plateau and already well studied. However,the study of barren porphyry in western Lhasa terrane is rare.This paper,base on the LA-ICP-MS U-Pb dating for southern Yare composite pluton,presents that this pluton is composited from Eocene biotite monzonitic granite（ age 49. 4 ± 0. 9Ma） and Miocene porphyry（ age 16. 3 ～ 16. 5Ma）. The Eocene biotite monzonitic granite are shoshonite series,metaluminous-weakly peraluminous,low Sr / Y and（ La / Yb）N,high Y and YbNrocks,shows typically arc magma. Meanwhile,the Miocene porphyry and shows typical shoshonite series,metaluminous and like-adakitic feature. The source magma of those two kinds of rocks are also different： the Eocene biotite monzonitic granite＇s magma come from the middle-lower crust which has been contaminated by the mantle wedge; the Miocene porphyry is the product of the partial melting of ancient crust.Combined with geochronology and tectonic setting,this paper infers that petrogenesis of the Eocene biotite monzonitic granite is crystal fractionation of the melt which come from the partial melting of the contaminated middle-lower crust caused by Neo-Tethys slab breakoff; petrogenesis of Miocene porphyry is magma from ancient lower crustal melting triggered by some dynamic mechanism invaded to the upper crust.