特提斯喜马拉雅带中段东部三叠纪火山岩的地球化学和岩石成因

Geochemistry and petrogenesis of the Triassic volcanic rocks in the east-central segment of Tethyan Himalaya.

三叠纪火山岩分布于特提斯喜马拉雅带中段东部,古地理上位于大印度北东部。分别采用原子吸收分光度法、ICP-MS和全岩同位素稀释法对三叠纪火山岩的主量元素、微量元素和Sr-Nd同位素进行了分析,并用来研究其成分特征和成因。岩相学和地球化学指标显示三叠纪火山岩为玄武岩,MgO含量均很低(范围在4．27%～7．72%),属于演化岩浆。早三叠世热马组和中三叠世桑康组具有较高的TiO_2、TFeO和P_2O_5含量,稀土元素总量和轻重稀土元素的分馏程度均较高,Ti/V、Ti/Y、Zr/Y比值较高,富集高场强元素(如Nb、Zr等),地球化学特征与峨眉山高Ti玄武岩相似,以更富集La、Ce、Zr、Hf等元素而有别于夏威夷碱性玄武岩,它们属大陆边缘裂谷背景的具有OIB型地球化学特征的碱性玄武岩;晚三叠世涅如组玄武岩以较低TiO_2、TFeO和P_2O_5含量为特征,稀土元素总量和轻重稀土分馏程度均较低,高场强元素(如Nb、Zr等)丰度很低,为大陆边缘裂谷背景的具有E-MORB地球化学特征的拉斑玄武岩。三叠纪玄武岩Nd同位素组成随时间从轻度富集到亏损(ε_(Nd)(t)=-1．2→0．5→4．4),指示岩浆源区具有从轻度富集到亏损的演化趋势。不相容元素(如La/Sm、La/Ta以及(Th/Ta)_(PM)、(La/Nb)_(PM)等)指示热马组和桑康组玄武岩遭受了一定程度下地壳混染,而涅如组没有遭受地壳混染。热马组和桑康组玄武岩还显示岩石圈地幔物质的印记,并与石榴石相二辉橄榄岩的低度部分熔融有关,而涅如组玄武岩除了具有E-MORB特征外,还与尖晶石相二辉橄榄岩的较高程度部分熔融有关。它们被解释为上涌的软流圈地幔物质与岩石圈地幔物质混合后在不同程度的拉张背景下发生减压熔融的产物。特提斯喜马拉雅带的三叠纪裂谷作用很可能具有主动裂谷作用的性质,较小规模的火山岩浆作用可用裂谷带之下上涌的地幔物质由于传导作用而变冷或正在孕育的地幔柱成因模式来解释。

The Triassic volcanic rocks are distributed in the east-central Tethyan Himalaya as interbeds, and are paleogeographically belonged to the northeastern margin of Greater India. Major and trace element compositions and whole-rock isotopes for these volcanic rocks are systematically analyzed by atomic absorption spectrophotometry, ICP-MS and isotopic dilution using Finnigan MAT 262, respectively, to constrain their compositional variations with time and to infer their petrogenesis. The Triassic volcanic rocks are belonging to basalt based on the petrographical and geochemical indicators. Low contents of MgO （4,27% -7.72% ） for these basalts indicate an evolved magma. High TiO2, TFeO and P2O5 contents of the Early and Middle Triassic basalts, together with marked fractionation between LREE and HREE, enriched HFSE （e. g. Nb, Zr）, additionally high ratios of Ti/V, Ti/Y and Zr/Y, indicating a similarity with Emeishan high-Ti basalts and a difference with Hawaiian alkaline basalts for their higher La, Ce, Zr and Hf abundances. These basalts belonged to alkaline basalt with OIB-type features that formed in a rift setting of continental margin. However, the Late Triassic basalts, which belonged to tholeiitic basalt with E-MORB features that formed in the rift setting of continental margin, have low TiO2, TFeO and P2O5 contents, together with slight fractionation between LREE and HREE, low concentrations of HFSE （e. g. Nb, Zr）. Nd isotopic compositions of the Triassic basalts show a continuous change from enriched compositions to depleted compositions with time （e. g. averaging εNd（t）=-1.2→-0.5→4.4 for the Early, Middle and Late Triassic basalts respectively）, indicating that magmatic source changed from enriched to depleted with time. Trace element indicators （e. g. La/Sm, La/Ta, （Th/Ta）PM, （La/Nb）PM） show that the Early and Middle Triassic basalts were experienced variable contamination of lower crust while the Late Triassic basalts were not contaminated by crust. Trace element signatures and Nd isotopic compositions for the Early, Middle Triassic basahs provide the discernable contributions from both the OIB-type source and the lithospheric mantle source, in which the Early, Middle Triassic basalts are related to the low-degree partial melting of garnet lherzolite, and for the Late Triassic basalts are not only characterized by E-MORB, but also are associated with the larger-degree partial melting of spinel lherzolite. The Triassic basalts are interpreted as the products of variable degree of partial melting of the magmas that upwelling asthenosphere-derived melts mixed with lithospheric mantle-derived components at extensional settings with different extents. The Triassic rifting in Tethyan Himalaya is most probably related to the active rifting involving in the upwelling of asthenosphere, and the less Triassic melts could be attributed to the mantle rising beneath the rift may cool by conduction, or to the incubating plume beneath eastern Gondwana lithosphere at that time.