海南岛元古宙变质基底性质和地壳增生的Nd、Pb同位素制约

摘要

The metamorphic basement in the Hainan Island is mainly composed of Baoban and Shilu Groups. The Baoban Group is dominated by amphibolite-facies gneisses, metasedimentary rocks (quartz muscovie-biotite schists), whereas the Shilu Group consists mainly of metamorphic volcano-sedimentary associations. There also occur voluminous granitic plutons of different ages from the Mid-Proterozoic to the Late Mesozoic, indicating multi-stage modification and complicated accretion history of the crust. Nd and Pb isotope data of 63 samples in different lithological associations in Hainan Island demonstrate that the major crustal accretion age ranged from the Early Proterozoic to the Late Proterozoic, with three peak stages of 2.0Ga, 1.7Ga and 1.2Ga. The regional crust is characterized by low degree of maturation, and the mafic metamorphic basement was mainly derived from a chronically depleted mantle source. The εNd(t) values for the Baoban Group vary from 2.2 to 2.8, with the TDM mainly about 2.00Ga and εNd(t) values for Proterozoic metabasic rocks vary from 4.7 to 8.1. The granitic plutons of different ages generally show higher εNd(t) values (-19.1～+3.5 for the Proterozoic granitic plutons and -6.3～-0.1 for the Phanerozoic granitic plutons) and lower TDM values (1.68～2.02Ga for the Proterozoic granitic plutons and 0.99～2.34Ga for the Phanerozoic granitic plutons ) than those in southeast China, suggesting their origin from a younger lower/middle crust with involvement of depleted mantle-derived materials. The Pb and Nd isotope data of the metamorphic basement in Hainan Island are somewhat different from those in the Cathaysia Block and Yangtz Block, indicating that the metamorphic basement of these three blocks might be different in formation ages and evolution histories. The metamorphic basement beneath the Hainan Island should not be regarded as the southern extended segment of the Cathaysia Block.%基于海南地块各类型岩石的63个样品Nd和Pb同位素分析数据，研究了海南地块元古宙地壳变质基底的时代、特征和演化。研究结果表明，海南岛元古宙变质基底成熟度低，基底变质岩系的母岩物质来源于长期亏损的地幔源区，主要形成时代为古元古宙晚期-新元古宙；不同时代花岗岩具有较高的εNd(t)值和较低的Nd模式年龄，主要形成于幔源物质参与下的或含地幔成分较多的初生地壳再循环。地壳增生具幕式增生的特点，并在2.0Ga、1.7Ga、1.2Ga 出现高峰；Pb同位素组成既不同于扬子地块又不同于华夏地块，介于两地块之间，和Nd同位素特征具有一致或耦合关系。结合海南岛地质特征，初步认为不能单纯地将海南岛基底理解为华南地块统一南延部分或是华夏古陆的部分，可能为不同的构造块体。