天山石炭—二叠纪大火成岩省裂谷火山作用与地幔柱

Carboniferous-Permian Rift-Related Volcanism and Mantle Plume in the Tianshan, Northwestern China

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。

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 C〉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 LT lavas can be luther divided into two subtypes. LT1 lavas exhibit lower Nb/Zr （〈0. 15） and higher εNd （t） （＋3. 1to＋9. 7）; LT2 lavas have higher Nb/Zr （〉 0. 16） and lower εNd （t） （=-0. 98to - 2. 91）. Elemental and isotopic data suggest that the chemical variations of the HT and LT lavas can not be explained by crystallization from a common parental magma. Instead, they most likely originated from an oceanic-island-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 and underwent distinct differentiation and contamination processes. The predominantly HT alkaline lavas were generated by low degrees of partial melting in the garnet stability field of the mantle source. The chemical variation of the HT lavas is controlled by a clinopyroxene （Cpx） [±olivine （O1）] fractionation. In contrast, parental magmas of the LT type were generated by lower degree of partial melting for the LT2 （alkaline） subtype and by higher degree of partial melting for the LT1 （predominantly tholeiitic） subtype of the mantle source around the spinel-garnet transition zone. These magmas were then subjected to shallow level gabbroic fractionation, which led to larger chemical variation. The Tianshan basalts may result from a starting mantle plume. The petrogenesis of both the HT and LT magmas was further complicated by contamination of crust and lithospheric mantle. Our data show that spatial petrogeochemical variations exist in the volcanic rocks of the Tianshan large igneous province. The location of the thickest LT1 volcanic succession, which may record the main episode of the basalt emplacement, in the eastern Tianshan may have been centered over the mantle plume or mantle melting anomaly. In contrast, the less abundant HT and LT2 basalts may imply a weakening of the influence of the mantle plume activity. In fact, the HT and LT2 basalts are the dominant magma type in the periphery of the province. The lower degrees of mantle melting of the HT and LT2 lavas may be the result of a relatively thicker lithosphere and lower geotherm.