新疆北天山沙湾地区晚石炭世火山岩地球化学特征及地质意义

摘要

天山北部沙湾地区火山岩由火山碎屑岩和火山熔岩组成,主要包括安山质(岩屑)晶屑凝灰岩、英安质熔结凝灰岩、玄武安山岩和流纹岩,为钙碱性系列.玄武安山岩和流纹岩主量元素平均含量分别为:SiO2＝51.03％、77.33％,TiO2＝0.76％、0.16％,CaO＝10.30％、0.43％,MgO＝5.19％、0.89％,Na2O＝2.10％、1.27％,K2O＝ 1.28％、2.54％；稀土元素特征分别为:∑REE＝369.1 ×10-6、729.0× 10-6,(La/Yb)N＝5.0、3.5,(La/Sm)N＝2.7、2.4,δ Eu＝0.82、0.54；Sr、Nd、Pb同位素特征分别为:(87Sr/86Sr)t＝0.70680和0.70476,(143Nd/144Nd)t＝0.52224和0.51225,ε Nd (t)＝0.2和0.1,(206Pb/204Pb)t＝17.775和17.720,(207Pb/204Pb)t＝15.790和15.826,(28pb/204Pb)t＝38.240和38.876.火山熔岩亏损Nb、Ta、Ti和P,富集K、Rb、U、Th、Zr和Hf,玄武安山岩的Zr为117×10-6～121×10-6,Zr/Y比值为5.63～5.82.流纹岩是玄武安山质岩浆结晶分异的产物,流纹岩LA-ICPMS锆石的U-Pb年龄为(310±2) Ma.本区火山岩原始岩浆产出于大陆板内拉张环境,是源自软流圈地幔的大陆玄武岩浆受到岩石圈地幔和大陆上、下地壳混染的结果.玄武安山岩中辉石斑晶的结晶温度和压力分别为1176.4C,4.13 kpa,相当于16.7 km的深度,反映了混染的大陆玄武岩浆在中、上地壳缓慢结晶形成单斜辉石斑晶后,因陆壳的拉张、快速上升至地表的过程.%Anjihai volcanic rocks in northern Tianshan are composed of pyroclastic rocks and lavas, which include andesite crystal tuff, andesite detritus crystal tuff, dacite welded tuff, basaltic andesite and rhyolite, thus being of calc- alkaline series. Major and rare earth element compositions of basaltic andesite and rhyolite are as follows; SiO2=51.03%,77.33%, TiO2=0.76%,0.16%, CaO=10.30%,0.43%, MgO=5.19%,0.89%, Na2O=2.10%,1.27%, K2O= 1.28%,2.54%,∑REE=369.1ppm,729.0ppm, (La/Yb)N=5.0,3.5, (La/Sm)N=2.7,2.4 and 5 Eu =0.82,0.54. The primitive mantle normalized spider diagrams of volcanic rocks are characterized by depletion of HFSE (Nb, Ta, Ti and P) and enrichment of K, R.b, U, Th, Zr and Hf. One significant difference is that Sr and Ba display insignificant positive anomalies in basaltic andesite and distinct negative anomalies in rhyolite. Isotope geochemical characteristics of basaltic andesite and rhyolite show the following values: ￡ Nd (t) =0.2,0.1, (87Sr/86Sr)t = 0.70680,0.70476, (143iNd/144Nd)t =0.52224,0.51225, (206Pb/204Pb), =17.775,17.720, (207b/204b), =15.790,15.826 and (208Pb/204Pb)t=38.240,38.876. LA-ICPMS zircon U-Pb dating of rhyolite yielded the age of 310±2 Ma. Rhyolite was fractionated from the magma of basaltic andesite. The primary magma of volcanic rocks occurred in continental intraplate stretching setting, derived from asthenosphere (or plume) -generated continental basaltic magma, which was contaminated by lithosphere as well as upper and lower continental crust. Crystal temperature and pressure of phenocryst clinopyroxene are 1176.4℃ and 4.13 kpa respectively, equivalent to the depth of 16.7km. It is thus inferred mat pyroxene phenocrysts were crystallized slowly from contaminated continental basaltic magma in middle -upper crust, and subsequently moved quickly to the surface of the earth, due to continental crust stretching.