南海北部晚中生代花岗岩类：古太平洋板块俯冲华南大陆的约束

The northern South China Sea area is spatially located between the continent of SE China and the South China Sea basin,where the Late-Mesozoic granitoids developed well particularly within the Pearl River Mouth Basin basement are considered to act as an important role in understanding and revealing the process of the Paleo-Pacific plate subduction underneath the South China plate.This reasearch project focuses on the northern South China Sea area, a conjugationsite from sea to land, and selects the Late-Mesozoic granitoids both from the borehole samples of the Pearl River Mouth basin and from the outcropped samples within the southeastern Guangdong province to carry out the research, using petrologic, geochemical and isotopic dating methods. In terms of zircon U-Pb dates,rock associations, granitoid compositions, geochemical characteristics and Sr-Nd isotopes, etc. that will be obtained in this study, we attempt to mainly analyses granitoid ages, rock types, magma differentiation, magma formation temperatures, magmatic sources, and the related tectonic settings. Their changing trends or possible relation spatially from the sea of Pearl River Mouth basin to the land of southern Guangdong province will be our further attention. The main purpose of this research is to combine our data and arguments with others available obtained from SE China, South China Sea basin and related areas even as well as geophysical interpretation results previously obtained in order to support or deny our assumed Late-Mesozoic tectonic model changing from Volcanic-Arc of the Pearl River Mouth basin to Back-arc Extension of the continent of SE China, in mechanism under the control of the paleo-Pacific slab subduction underneath the South China plate.

南海北部处于东南大陆与南海海盆的结合部，是揭示晚中生代古太平洋板块俯冲汇聚华南板块作用过程的重要地区。本项目拟从海陆结合角度，以珠江口盆地基底钻井花岗岩类为重点，同时结合粤东南露头区的花岗质侵入体，从区域上开展锆石U-Pb定年、地球化学和Sr、Nd同位素等研究；分析其岩浆岩组合与岩浆活动时代及其由海到陆的变化特点；分析岩浆源区、分异演化特点、形成温度及其由海到陆的变化特点；在此基础上，分析珠江口盆地和粤东南晚中生代花岗岩类形成的构造控制环境及其相互关系。最后综合东南大陆和南海地区的现有研究成果，以及我们已有的区域重磁资料，分析由珠江口盆地到东南大陆是否存在一个受控于晚中生代古太平洋板块俯冲华南大陆的、由海至陆变化的"火山弧-弧后伸展"的构造模式，或形成新的构造模式。

对南海北部岩浆岩开展了岩石学、年代学和球球化学等工作，对南海北部开展了构造和地球物理解释等工作，主要结论和认识包括：1) 南海北部古近系火山岩中继承岩浆锆石U-Pb年龄表明：太古代至元古代2.8~2.5Ga、1.9~1.7Ga和1.1~1.0Ga是南海北部基底演化的重要时期；9.3~7.8Ga年龄表明南海北部晚元古代经历了华夏与扬子拼合及超大陆裂解事件，440~437Ma年龄表明南海北部经历过加里东造山作用。印支、燕山期也是南海北部重要构造演化期，年龄记录包括267~240Ma和184~96Ma。南海北部与华南构造演化对应关系较好，是识别华夏地块向南海北部延伸的重要证据。2) 南海北部发现了早侏罗世低钾拉板玄武质镁质花岗岩和闪长岩, 锆石U-Pb年龄为198~195Ma, 锆石Ti温度为673~695 ºC, Sr、Nd同位素表明它们是幔源岩浆演化的结果，岩石中流体活动性元素富集，Nb-Ta-Ti显著亏损，具有岩浆弧特点。东海南部发现了镁质高钾钙花岗闪长岩，锆石U-Pb年龄为187Ma, 锆石Ti温度为675 ºC，具有活动性元素富集和Nb-Ta-Ti亏损岩浆弧的特点。东海南部碎屑岩中发现了大量侏罗纪(198~175Ma)低温(600~740 ºC)岩浆弧锆石。它们与台湾大仑花岗岩构成了东沙-雁荡岩浆弧带, 区域上与俯冲杂岩和关键断裂一道构成了侏罗纪早期板块俯冲及东亚大陆边缘的构造格局。3) 南海北部钻遇大量165~100Ma (锆石U-Pb年龄)花岗岩类, 这类岩石流体活动性元素富集，Nb-Ta-Ti亏损，属于I型岩浆弧型花岗岩类。闽粤同期花岗岩与此不同，以高分异I型或A型岩石为主，岩浆组分富硅、富碱和富铁以及Ba、Sr和Eu强烈亏损。将Palawan俯冲杂岩、南沙埃达克岩、南海北部岩浆弧及闽粤非造山花岗岩一道，可构成受古板块俯冲控制的165~100Ma活动大陆边缘模式。4) 分析了中生界构造变形特征, 识别追踪了不整合Tg界面, 认为白垩纪晚期区域上挤压变形不均一性明显, 应与南婆罗地块与巽他大陆碰撞有关。