扬子板块西缘石棉安顺场新元古代钾长花岗岩地球化学特征及其地质意义

摘要

扬子板块西缘新元古代岩浆活动强烈,其成因研究对探讨Rodinia超大陆的演化有着重要意义.本文对石棉安顺场钾长花岗岩形成年龄及其地球化学特征研究,结果表明锆石206 pb/238U年龄加权平均值为777.3±4.8 Ma(MSWD =O.23,2σ),代表花岗岩的结晶年龄.岩石SiO2含量高(72.64％～76.27％),铝饱和指数A/CNK大于1(1.06 ～ 1.24),K2O/Na2O在1.40 ～2.22之间,里特曼指数σ小于3.3(2.08 ～2.74).岩石轻稀土元素(LREE)富集,Nb、Ta轻微亏损,Eu负异常明显(δEu =018～0.23).岩石的εNd(t)为0.5～3.3(平均值为2.1),TDM在1.19～1.61Ga之间,反映其源区以古老地壳物质为主.铅的初始同位素比值分别为[n(206pb)/n(204pb)]i=15.4103 ～17.2707,[n(207pb)/n(204pb)]i=15.4265～ 15.5479,[n(208pb)/n(204Pb)]i=33.3518 ～ 35.8641.此外,岩石具有高的Rb/Sr比值和低的CaO/Na2O比值、较低的Al2O3/TiO2比值和低的Rb/Ba比值,表明其起源于泥质岩石的部分熔融.综合地球化学、同位素特征和区域地质资料,我们认为石棉安顺场钾长花岗岩为过铝质高钾钙碱性S型花岗岩,它是地壳泥质源岩部分熔融的产物,形成于挤压的构造环境中.%Objectives:Neoproterozoic magmatic rocks are widespread in western margin of the Yangtze block,Petrogenesis of the Neoproterozoic granitic rocks has potential significance to the tectonic evolution of supercontinent Rodinia.Methods:In this paper,we present zircon U-Pb age,and whole-rock major and trace elements,Sr—Nd isotopic compositions of the K-feldspar granites from the Anshunchang,Shimian county.Results:K-feldspar granites zircon U-Pb result show that the crystallization age of 777.3 ± 4.8Ma (MSWD =0.23,2σ),which belongs to Neproterozoic granites.The results indicate that the K-feldspar granites are peraluminous,high-K calc-alkaline S-type granites,with SiO2content ranging from 72.64％ to 76.27％,A/CNK =1.06 ～ 1.24,K2O/Na2O =1.40 ～ 2.22,σ =2.08 ～2.74.The granites are enriched in LREE and depleted in Nb and Ta,with significant negative Eu anomalies (δEu =018 ～ 0.23).The εNd (t) vary from 0.5 to 3.3 (average value =2.1),and TDM =1.19 ～ 1.61Ga,suggesting Neoproterozoic crust source region.The K-feldspar granites have [n (206 Pb)/n (204 Pb)]i =15.4103 ～ 17.2707,[n (207 Pb)/n (204 Pb)]i =15.4265 ～ 15.5479,and [n (208 Pb)/n (204 Pb)]i =33.3518 ～ 35.8641,respectively.Conclusion:In combination with geochemistry,isotopic signatures and the regional geology in the western of the Yangtze block,we argued that K-feldspar granites from the Anshunchang are peraluminous,high-K calcalkaline S-type granites,these K-feldspar granites were derived from partial melting of the pelite in middle—lower crust.They were formed in an compressive tectonic setting.