北祁连东段冷龙岭地区毛藏寺岩体和黄羊河岩体的岩石成因及其构造意义

Petrogenesis and tectonic implications of the Maozangsi and Huangyanghe granitic intrusions in Lenglongling area,the eastern part of North Qilian Mountains,NW China

文中研究了北祁连东段冷龙岭地区毛藏寺岩体和黄羊河岩体的年代学、地球化学和Sr-Nd同位素组成。毛藏寺岩体主要岩石类型为花岗闪长岩。锆石U-Pb定年获得花岗闪长岩岩浆结晶年龄为(424±4)Ma。花岗闪长岩具有高的Mg#(约55),K2O/Na2O=0.77～0.91,A/CNK=0.92～0.94,表明岩石属准铝质。在微量元素组成上,花岗闪长岩富集LILE、亏损HFSE,轻重稀土分异明显[(La/Yb)N=16.9～19.5],具有弱的Eu负异常(Eu/Eu*=0.75～0.83);花岗闪长岩具有ISr=0.706 3～0.706 5,εNd(t)=-1.5～-1.1,TDM=1.10～1.16Ga。这些地球化学特征和Sr-Nd同位素组成表明,花岗闪长岩岩浆源区为基性下地壳变玄武质岩石,但在成岩过程中有少量幔源物质的加入。黄羊河岩体主要由钾长花岗岩组成,其岩浆结晶年龄为(402±4)Ma。岩石富碱(K2O+Na2O=6.91‰～7.66%),K2O/Na2O>1,A/CNK=0.97～1.05。钾长花岗岩富集LILE及HFSE,轻重稀土元素分馏中等[(La/Yb)N=10.6～17.8],并具有明显的负Eu异常(Eu/Eu*=0.43～0.68),表明钾长花岗岩具有铝质A型花岗岩的地球化学特征。钾长花岗岩具有ISr=0.710 3～0.711 3,εNd(t)=-6.7～-6.0,TDM=1.46～1.55Ga,反映岩浆主要来自地壳中长英质物质的部分熔融。冷龙岭地区花岗岩类的岩石成因及其岩浆演化揭示了北祁连山造山带从加里东早期的挤压构造体制向加里东晚期的伸展构造体制的演化。这些花岗岩类形成于碰撞后构造背景,岩浆的产生可能与俯冲的北祁连洋板片的断离作用有密切联系。

This paper reports a detailed study of U-Pb zircon dating,geochemical and Sr-Nd isotopic compositions for the Maozangshi granodiorite and Huangyanghe K-feldspar granite in Lenglongling area,North Qilian Mountains.U-Pb zircon dating by LA-ICP-MS shows magma crystallization ages of 424±4 Ma for the Maozangshi granodiorite and 402±4 Ma for the Huangyanghe K-feldspar granite.The Maozangshi granodiorite has high Mg#（~55）,K2O/Na2O=0.77-0.91,A/CNK=0.92-0.94.It is rich in LILE and depleted in HFSE.REE data for the granodiorite show strongly fractionated REE patterns,with（La/Yb）N = 16.9-19.5 and Eu/Eu＊=0.75-0.83.The granodiorite has ISr=0.7063-0.7065 and εNd（t） =-1.5——1.1,with TDM=1.10-1.16 Ga.The geochemical characteristics and Sr-Nd isotopic composition indicate that the magma of the granodiorite was derived from partial melting of basaltic rocks in lower crust,but with minor contribution from mantle material.The Huangyanghe K-feldspar granite has K2O＋Na2O=6.91%-7.66%,K2O/Na2O 1 and A/CNK=0.97-1.05.The K-feldspar granite is rich in LILE and HFSE.It has moderately fractionated REE patterns with（La/Yb）N =10.6-17.8 and Eu/Eu＊=0.43-0.68.These features show that the K-feldspar granite has an affinity with aluminous A-type granite.The K-feldspar granite shows ISr=0.7103-0.7113 and εNd（t）=-6.7——6.0,with TDM =1.46-1.55 Ga,suggesting that the magma resulted from partial melting of felsic materials in crust.The diverse petrogenesis and evolutionary history of the granitoids in Lenglongling area reveal a tectonic transformation from compression to extension for North Qilian orogen.Both the Maozangshi granodiorite and Huangyanghe K-feldspar granite formed in post-collisional tectonic setting.And the petrogenesis of the post-collisional granitoids in North Qilian Mountains may have close relation with the previously subducted North Qilian oceanic slab.