摘要:
中国西北部石炭纪—早二叠世喷发的天山裂谷火山岩系构成了一个大火成岩省.该火山岩系的组成以玄武质熔岩为主,其次有中性和酸性熔岩及火山碎屑岩.根据岩石学、主元素、微量元素和Sr-Nd-Pb同位素数据,天山玄武岩可分为两个主要岩浆类型:1高Ti/Y(HT)类型,以高Ti/Y(>500)、高Ce/Y(>3)和相对低Nb/Zr(<0.11)、低εNd(t)为特征;2低Ti/Y(LT)类型,以低Ti/Y(<500)为特征.LT熔岩又可以进一步分为两个亚类:LT1熔岩以低Nb/Zr(<0.15)和高εNd(t)(+3.1~+9.7)为特征;LT2熔岩具有较高的Nb/Zr值(>0.16)和较低的εNd(t)值(-0.98~-2.91).元素和同位素数据表明,HT和LT熔岩的化学变异不是由一个共同母岩浆的结晶分异作用所产生.它们极有可能是源于一种似洋岛玄武岩源的幔源(87Sr/86Sr(t)≈0.7045,εNd(t)≈+4,206Pb/204Pb(t)≈18.35,207Pb/204Pb(t)≈15.66,208Pb/204Pb(t)≈38.25,La/Nb≈0.7),且具有不同的熔融条件和经受了不同的分异和混染.以碱性熔岩为主的HT熔岩是产生于幔源石榴子石稳定区的低度部分熔融,其化学变异受控于单斜辉石(Cpx)[±橄榄石(Ol)]分离作用.相反,LT类型的母岩浆则是形成于幔源的尖晶石—石榴子石过渡带:碱性LT2亚类的母岩浆是产生于部分熔融程度较低的条件下;而以拉斑玄武质为主的LT1亚类的母岩浆则是产生于部分熔融条件较高的条件下.它们经受了浅层辉长岩质分离作用,化学变异较大.天山玄武岩可能是产生于地幔柱头.HT和LT岩浆的岩石成因又进一步为地壳和岩石圈地幔的混染作用所复杂化.我们的研究揭示,天山大火成岩省的火山岩中存在空间上的岩石地球化学变化.天山东段的LT1火山岩系的厚度最大,它们记录了玄武岩侵位的主幕,该处可能是地幔柱或地幔熔融异常的中心位置.相反,厚度较小的HT和LT2玄武岩则可能是意味着地幔柱活动影响的减弱.事实上,HT和LT2玄武岩也是该大火成岩省边缘部分的主要岩浆类型.HT和LT2熔岩的地幔熔融程度较低,可能是与地幔柱边部的岩石圈相对较厚和地热较低有关.
Abstract:
The Tianshan rift-related volcanic rocks make up a large igneous province erupted during the Carboniferous-Early Permian period in northwestern China.The volcanic successions comprise thick piles of basaltic lavas and subordinate intermediate and silicic lavas and pyroclastics.Based on petrographic,major and trace element,and Sr-Nd-Pb isotope data,the Tianshan basalts can be classified into two major magma types.These are:(1) a high-Ti/Y(HT) type that exhibits high Ti/Y(>500),Ce/Y(>3),and relatively low Nb/Zr(< 0.11) and εNd(t)(-1.15);(2) a low-Ti Y(LT) that has low Ti/Y(<500).The L T lavas can be futher divided in to two sub types.LT 1 lavas exhibit lower Nb/Zr(< 0.15) and higher εNd(t)(+3.1 to + 9.7);LT 2 lavas have higher Nb/Zr(> 0.16) and lower εNd(t)(=-0.98 to -2.91).Elemental and isotopic data suggest that the chemical variations of the H T and L T lavas can not be explained by crystallization from a common parental magma.In stead,they most likely originatedfroman oceanic-sland-basalt-like mantle source(87Sr/86Sr(t)≈0.7045,εNd(t)≈+4,206Pb/204Pb(t)≈18.35,207Pb/204Pb(t)≈15.66,208Pb/204Pb(t)≈38.25,La/Nb≈0.7) under various melting conditions andunderwent distinct differentiation and contamination processes.
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