天山东部麻粒岩定年与地球化学研究  

A STUDY OF ISOTOPIC DATING AND GEOCHEMISTRY OF GRANULITES IN THE EASTERN TIANSHAN BELT

作　　者：舒良树[1] 卢华复[1] CharvetJ LaurentCS 印栋浩[1] 

机构地区：[1]南京大学地球科学系,江苏南京210093 [2]法国奥尔良大学地球科学系,奥尔良45067

出　　处：《南京大学学报（自然科学版）》2000年第6期718-727,共10页Journal of Nanjing University（Natural Science）

基　　金：国家自然科学基金!(4 9772 15 1) ;重点基金!(4 9832 0 40 )

摘　　要：天山造山带的下地壳麻粒岩主要出露在哈密市尾亚和托克逊县榆树沟 .尾亚麻粒岩种类多 ,有基性岩类和中酸性岩类麻粒岩等 ,以推覆岩片或花岗岩顶垂体的面貌出现 ;榆树沟的则属基性岩类麻粒岩 ,呈构造岩片散布在蛇绿混杂岩中 .根据岩相学特征 ,尾亚麻粒岩中可分出3期变质矿物组合 ,即M1∶PI +Q +Mus +Bi;M2 :Cpx +Gt +Q +Antiper±Opx ;M3:Gt +Cd +Ep +Amp ;榆树沟麻粒岩中可识别出 2期变质矿物组合 ,M1:Cpx +Gt+PI±Opx ;M2 :Px +Amp +PI +Q±Bi.岩石化学数据表明 ,尾亚麻粒岩种类较多 ,基、中、酸性岩类俱全 .基性岩类麻粒岩富钙贫镁 ;中酸性岩类麻粒岩富钙碱 .基性岩类麻粒岩稀土元素配分曲线呈左倾斜的重稀土轻度富集型 ,稀土总量低 ,中酸性岩类麻粒岩则呈明显右倾的轻稀土富集型 ,稀土总量高 .榆树沟麻粒岩属基性岩类麻粒岩 ,以低硅贫碱高钙为特征 ,稀土总量低 ,配分曲线平坦型 .笔者对尾亚麻粒岩作斜长石 +辉石 +全岩Sm Nd等时线测年 ,获 ( 541± 11)Ma同位素年龄值 ,它代表麻粒岩相变质的峰期年龄 ;又对尾亚麻粒岩从辉石退变质形成的角闪石作4 0 Ar - 39Ar年龄测定 ,获主坪年龄值 ( 4 32 .0± 0 .72 )Ma和等时线年龄值 ( 4 34 .67± 2 .4 5)Ma ,它们代表麻粒岩形成的退变质年龄 ,反映加里东期构造?Basic rock type granulitic slabs exist in Weiya and Yushugou areas in the Tianshan Paleozoic belt, NW China. Some obvious lithological differences and geological attitudes exist between the Weiya and the Yushugou granulites. The Weiya granulite was derived from protolites of basic and intermediate acid rocks contains three phases of metamorphic mineral assemblages. M 1=plagioclase+quartz+muscovite + biotite, M 2=clinopyroxene+garnet+quartz+antiperthite±orthopyroxene, M 3=garnet+cordierite+epidote+amphibole. These granulitic rocks are outcropped as the fault bounded slabs and as the roof pendant blocks of the foliated Paleozoic granite. The Yushugou granulitic rocks are basic type and occurred as thrust slabs in the allochthonous ophiolitic melange. Two phases of metamorphic mineral assemblages were observed, namely, M 1= clinopyroxene+ garnet+ plagioclase±orthopyroxene, M 2=pyroxene+amphibole+plagioclase+quartz±biotite. We obtained a Sm Nd isochron age of (540.8±11.3) Ma for the Weiya granulite (pligioclase +pyroxene+ whole rock), corresponding to a peak metamorphic period during Early Cambrian. This paper reported also the 40 Ar/ 39 Ar ages of (432.1±0.7) Ma (flat age) and of (434.7±2.5) Ma (isochron age) for the amphibole mineral of Weiya granulite, corresponding to a retrograde metamorphic age, suggesting a strong Caledonian tectonothermal event.

关 键 词：麻粒岩 地球化学 同位素测年 天山 地质构造 

分 类 号：P588.3[天文地球—岩石学] P597[天文地球—地质学]