大别造山带碰撞后埃达克质花岗岩地球化学研究

发布时间：2019-01-12 13:07

【摘要】：埃达克岩最初是用来定义那些富硅、高Sr/Y和La/Yb的源于俯冲带玄武质洋壳部分熔融形成的火山岩和侵入岩。埃达克岩当初被认为仅形成于岛弧背景。然而近些年来的研究表明,非岛弧环境也可以形成埃达克特征的岩石,形成机制包括:镁铁质下地壳底侵部分熔融、玄武质岩浆结晶分异、高Sr/Y比源区部分熔融、加厚下地壳部分熔融或拆沉等。大陆背景下的埃达克质岩通常被认为是加厚下地壳部分熔融的产物。然而最近的实验岩石学和地球化学模拟计算表明,埃达克质熔体可以形成于正常下地壳深度(30-40km),埃达克质特征也可以继承自源岩,加厚下地壳不是必须的。这些结果已被用于解释克拉通内部产出的埃达克质岩。大陆碰撞造山带通常产出碰撞后埃达克质岩,它们是否是碰撞加厚造山带地壳部分熔融的产物?本文选择大别造山带碰撞后埃达克质花岗岩作为研究对象,对其进行了系统的锆石U-Pb定年和微量元素分析、全岩主微量元素和放射成因Sr-Nd-Hf同位素分析,相关结果与伴生的普通花岗岩一并进行对比研究。结果表明,大别碰撞后花岗岩可以分为三个阶段:低镁埃达克质岩阶段、高镁埃达克质岩阶段和普通花岗岩阶段。埃达克质岩浆作用(143-130Ma)系统早于普通花岗质岩浆作用(133-112Ma),130Ma左右的高镁埃达克质岩承前启后。从130Ma以前多为埃达克质到130Ma以后多为普通花岗质,表明部分熔融深度在130Ma左右发生了显著改变。大别碰撞后花岗岩普遍含有丰富的新元古代和变质成因的三叠纪继承锆石,表明它们是俯冲华南陆壳物质再造的产物。少量的古元古代-太古代年龄的继承锆石以及富集的Sr-Nd-Hf同位素组成和古老的Nd-Hf模式年龄(古元古代-太古代)表明,它们的源区含有古老的扬子基底地壳物质。与普通花岗岩相比,大别埃达克质岩显著亏损HREE,Eu负异常不明显,具有高Sr/Y和La/Yb比。另外,埃达克质花岗岩同岩浆锆石HREE含量和Eu负异常程度总体低于普通花岗岩。这些结果表明埃达克质岩的母岩浆亏损HREE,缺乏明显的Eu负异常。再者,在同一颗锆石上,核部继承锆石Eu负异常明显,HREE含量明显高于边部的同岩浆锆石,表明大别花岗岩的埃达克质特征并非继承自源岩。因此,大别造山带碰撞后埃达克质岩是由加厚下地壳部分熔融的产物。
[Abstract]:Adakite was originally used to define silicon-rich, high-Sr/Y and La/Yb volcanic rocks and intrusive rocks derived from the partial melting of Black Tortoise oceanic crust in the subduction zone. Adakite was originally thought to have been formed only in the island arc background. However, recent studies have shown that non-island-arc environments can also form adake-specific rocks. The formation mechanisms include partial melting of the lower crust of magnesia-ferrite, crystallization differentiation of Black Tortoise magma, partial melting of high Sr/Y ratio source region. Thickening of the lower crust, partial melting or delamination, etc. Adakite in the continental setting is generally considered to be the product of partial melting of the thickened lower crust. However recent experimental petrological and geochemical simulations show that adakitic melts can be formed at normal lower crustal depth (30-40km) and that adakitic characteristics can inherit from the source rocks and thickening the lower crust is not necessary. These results have been used to explain the adakite produced within the craton. Continental collision orogenic belts usually produce post-collision adakitic rocks. Are they the product of partial melting of the crust of the collisional thickening orogenic belt? In this paper, the adakitic granite after collision in the Dabie orogenic belt is selected as the research object. The zircon U-Pb dating and trace element analysis, the whole rock main trace element and radiogenic Sr-Nd-Hf isotopic analysis are carried out. The correlation results were compared with the associated common granite. The results show that the granite after Dabie collision can be divided into three stages: low magnesium adakite stage, high magnesium adakitic stage and ordinary granite stage. The adakitic magmatism (143-130Ma) system is earlier than the ordinary granitic magmatism (133-112Ma), and the high magnesium adakitic rocks about 130Ma carry on the past and the future. Most of the samples from 130Ma were adakitic to normal granitic after 130Ma, indicating that the depth of partial melting changed significantly around 130Ma. The granites after Dabie collision generally contain abundant Neoproterozoic and metamorphic Triassic inherited zircon which indicates that they are the product of subducting southern China continental crust material reconstruction. A small amount of inherited zircon from Paleoproterozoic to Archean ages, enriched Sr-Nd-Hf isotopic compositions and ancient Nd-Hf model ages (Paleoproterozoic to Archean) indicate that their source regions contain ancient Yangtze basement crustal material. Compared with ordinary granite, Dabie adakitic rocks show significant depletion of HREE,Eu negative anomaly is not obvious, with high Sr/Y and La/Yb ratio. In addition, the HREE content of zircon and the negative anomaly degree of Eu of adakitic granite are lower than those of ordinary granite. These results suggest that the parent magma depletion HREE, of the adakitic rocks lacks obvious negative Eu anomalies. Furthermore, on the same zircon, the negative Eu anomaly of the inherited zircon in the core is obvious, and the content of HREE is obviously higher than that of the synmagmatic zircon at the edge, indicating that the adakitic characteristics of the Dabie granite are not inherited from the source rocks. Therefore, the adakitic rocks after the collision of the Dabie orogenic belt are the product of partial melting of the thickened lower crust.
【学位授予单位】：中国科学技术大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P588.121

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