中亚造山带东南缘三叠纪解放营子岩体全岩Sr⁃Nd⁃Li同位素特征及成因

The Whole⁃Rock Sr⁃Nd⁃Li Isotopic Characteristics and Genesis of the Triassic Jiefangyingzi Pluton in the Southeastern Margin of the Central Asian Orogenic Belt

中亚造山带东南缘三叠纪花岗岩类型多样且成因复杂,为研究其物源及形成背景,本文选取解放营子岩体不同位置的二长花岗岩、片麻状似斑状花岗岩和闪长玢岩,并对其进行全岩地球化学、锆石年代学及全岩Sr-Nd-Li同位素分析.锆石U-Pb年代学结果显示解放营子岩体不同类型的岩石均形成于晚三叠世(234~226 Ma).地球化学特征表明岩体为经历不同程度结晶分异作用的I型花岗岩.略负到正的ɛ_(Nd)(t)值(-3.9~+1.5)指示它们主要来源于新生地壳,且源区有不同程度古老地壳物质的加入.为了进一步示踪再循环物质对岩浆源区的贡献,本文首次报到了该地区岩体的全岩Li同位素特征,δ^(7)Li值变化范围为+1.1~+6.6‰,平均值为+3.37‰,接近上地幔平均值,表明有显著幔源组分的贡献.结合区域地质特征,晚三叠世解放营子岩体为未明显受再循环物质作用的幔源组分在造山后伸展背景下发生部分熔融形成,且岩浆源区有古老陆壳组分的加入.

The Triassic granites in the southeastern margin of the Central Asian Orogenic Belt are of diverse types and complex genesis.In order to study their provenance and formation setting,we selected monzogranite,gneissic porphyritic granite and diorite porphyrite in the different locations of the Jiefangyingzi pluton,and performed the whole-rock elemental and Sr-Nd-Li isotope analyses,and zircon geochronology.Zircon U-Pb geochronology results show that the different rock types of the Jiefangyingzi pluton were similarly formed in the Late Triassic(234-226 Ma).Geochemical characteristics show that all of them are I-type granites undergoing different degrees of fractionation crystallization.Slightly negative to positive ɛ_(Nd)(t)values(-3.9-+1.5)indicate that they derived from juvenile materials with the addition of ancient crustal materials to varying degrees.The whole-rock Li isotopic characteristics of pluton in this region are reported for the first time.The range of δ^(7)Li value is+1.1-+6.6‰with an average value of+3.37‰,similar to the average value of upper mantle,indicating that there is a significant contribution of mantle-derived components.Combined with regional geology,the Jiefangyingzi pluton was formed in the postorogenic extension stage after the closure of the Paleo-Asian Ocean.Decompression melting induced the partial melting of the juvenile mantle-derived components without obvious water-rock interaction related to the early subduction process.