东天山哈尔里克地区早石炭世A型花岗岩年代学、地球化学及地质意义

Geochronology,Geochemistry and Geological Implications of Early Carboniferous A-Type Granites in Harlik Area from East Tianshan

A型花岗岩对研究天山造山带壳幔相互作用和构造演化具有重要意义.对东天山哈尔里克地区的碱长花岗岩和正长花岗岩进行了详细的岩石学、地球化学和年代学研究,旨在阐明其成因及构造意义.其中碱长花岗岩的LA⁃ICP⁃MS锆石U⁃Pb年龄为350.7±2.0 Ma和351.8±2.0 Ma,表明该花岗岩体形成于早石炭世早期.岩石含大量碱性长石,暗色矿物以黑云母为主,见钠铁闪石等碱性暗色矿物.岩石高硅、富碱、贫钙镁,富集Rb、Th、K等大离子亲石元素和Zr、Hf等高场强元素,而强烈亏损元素Ba、Sr、Eu,具弱右倾“V”字型的稀土分配曲线((La/Yb)_(N)=3.23~5.55,δEu=0.19~0.28).这些矿物学和地球化学特征表明哈尔里克早石炭世花岗岩属高钾准铝质-弱过铝质花岗岩,为典型的A型花岗岩.花岗岩正的ε_(Nd)(t)值(+4.2~+4.8)和新元古代的二阶段Nd模式年龄(t_(DM2)=0.71~0.75 Ga),表明其源区可能为新生年轻地壳,源岩可能是亏损地幔来源的下地壳中基性岩和少量大洋沉积物.结合前人对东天山岩浆活动和构造环境的研究,认为早石炭世哈尔里克与博格达处于同一构造背景下,早石炭世早期A型花岗岩可能形成于博格达弧后裂谷的伸展早期阶段.

Study of A-type granites is critical for understanding the crust-mantle interaction and tectonic evolution in the Tianshan orogenic belt.Here it reports a detailed petrologic,geochemical and geochronologic data for the alkali-feldspar granite and syenogranite from Harlik in East Tianshan,to constrain their petrogenesis and tectonic setting.LA–ICP–MS zircon U–Pb ages indicate that the granites were emplaced at early Early Carboniferous(350.7±2.0 Ma and 351.8±2.0 Ma).The rocks contain abundant alkali feldspar and biotite with little sodium amphibole.The granites are characterized by high silicon and alkali,low CaO and MgO contents,and enriched in Rb,Th,K(LILE),and Zr,Hf(HFSE),but depleted in Ba,Sr,and Eu.The chondritenormalized REE patterns show slight enrichment in LREE with(La/Yb)_(N)from 3.23 to 5.55,and great negative Eu anomalies(δEu=0.19-0.28).These feactures indicate the geochemical characteristics of high-K meta-to per-aluminous A-type granite.They have positive ε_(Nd)(t)(+4.2 to +4.8) values,and the Neoproterozoic two stage Nd model ages(t_(DM2)=0.71-0.75 Ga),suggesting that the A-type granitic magma was probably generated from partial melting of a juvenile crustal source produced by depleted mantle-derived basaltic magmas and a few marine sediments.Combining with the previous works in this region,our study reveals that Harlik and Bogda are constrained by the same tectonic setting in Early Carboniferous,and the A-type granites from Harlik were genetically linked to the initial extensional stage of the Bogda back-arc rift.