藏南亚东地区早古生代花岗质片麻岩的成因与构造意义

Petrogenesis and tectonic significance of the Early Paleozoic granitic gneisses from the Yadong area,southern Tibet

早古生代的花岗质片麻岩产出在青藏高原南部亚东地区的高喜马拉雅结晶岩系中。岩石学研究表明,这些岩石主要由斜长石、钾长石、石英和黑云母组成,普遍发育片麻状构造。它们的SiO_2含量为68.04%~76.55%,铝饱和指数(A/CNK)为1.04~1.14,属钙性-钙碱性弱过铝-过铝质岩石。它们的稀土总量较高,轻稀土富集,Eu负异常,富集大离子亲石元素Rb,相对亏损Nb、Ta、P和Ti等高场强元素。三个样品获得的岩浆锆石U-Pb年龄分别为491Ma、512Ma和501Ma,代表花岗质片麻岩的原岩年龄。它们的锆石εHf(t)值分别为-8.1^+0.1、-5.9^-2.1和-4.8^+6.7,对应的二阶段模式年龄分别为1440~1959Ma、1585~1826Ma和1028~1762Ma。本研究表明,亚东地区的早古生代花岗岩包括I型和S型花岗岩。I型花岗岩以K_2O/Na_2O<1、弱过铝质和刚玉标准矿物分子数<1为特征,具较高且正的锆石εHf(t)值,或相对集中的负的εHf(t)值。I型花岗岩显示岩浆弧花岗岩的特征。S型花岗岩以K_2O/Na_2O>1、过铝质和刚玉标准矿物分子数>1为特征,锆石εHf(t)值均为低的负值,且变化范围较大。本文和现有的研究成果表明,I型和S型花岗岩在喜马拉雅地体广泛存在,这为冈瓦纳超大陆北缘经历了早古生代的安第斯型造山作用提供了重要证据。

Early Paleozoic granitic gneisses occur in the Higher Himalayan Crystallization Sequence of Yadong area,southern Tibet.Petrological study shows that the granitic gneisses are mainly composed of quartz,plagioclase,K-feldspar and biotite,and show distinct gneissic structure. They have variable SiO_2 contents ranging from 68. 04% to 76. 55%,and alumina saturation index（ A / CNK） of1. 04 ~ 1. 14,indicative of calcic to calc-alkaline and weakly peraluminous to peraluminous. They show distinct fractionated REE patterns with negative Eu anomalies,and are characterized by the enrichment of LILEs（ such as Rb） and the depletion of HFSEs（ such as Nb,Ta,P and Ti）. LA-ICP-MS U-Pb dating of zircons from the three granitic gneisses indicates that their protoliths were formed at 491 Ma,501Ma and 512 Ma,respectively. The zircon εHf（ t） values range from- 8. 1 to ＋ 0. 1,- 5. 9 to- 2. 1 and- 4. 8to ＋ 6. 7,with corresponding Hf two-stage model ages ranging from 1440 Ma to 1959 Ma,1585Ma to 1826 Ma and 1028 Ma to 1762 Ma,respectively. This study shows that the Early Paleozoic granites of Yadong area include I-type and S-type granitoids. The I-type granitoids are characterized by K_2O/Na_2O（ %） 〉1,weakly peraluminous,positive zircon εHf（ t） values or negative but narrowεHf（ t） values range,and show typical geochemical features of magmatic arcs. In contrast,minor S-type granites are characterized by K_2O / Na_2O（ %）〈 1,peraluminous,negative and variable zircon εHf（ t） values. The coexistence of I-type and S-type granitoids in the Himalayas indicates that the northern margin of the Gondwana supercontinent experienced an Andean-type orogeny during the Early Paleozoic.