摘要：
长期的华南地块研究取得了一系列重要成果,但是古太平洋俯冲作用于华南的地质记录不是很清晰,尚存争论.一派认为始于二叠纪,另一派认为中生代.本文试图通过华南中生代EW向特提斯构造域和NE向古太平洋构造域的构造转换过程及转换时间入手,探讨古太平洋俯冲启动.雪峰山地区早侏罗世地层以及侏罗纪类磨拉石建造呈NE或NNE向展布,燕山期主要发育2期褶皱变形,早期褶皱轴向为NE NNE向,晚期为NNE或近南北向的隔槽式褶皱;而印支期也发育2期褶皱变形,D1期为EW NEE向,D2期为NNE向的紧闭褶皱.这些都说明晚三叠世雪峰山地区已经从EW向特提斯构造域向NE—NNE向太平洋构造域的构造方向转换.而南岭地区,以及更南部的南海北部海域,构造转换时间相对较晚,为早 中侏罗世.综合前人的测年数据,德兴斑岩型铜矿三个含矿斑岩体形成于大约172 Ma,形成于古太平洋板块俯冲所形成的活动大陆边缘环境,成矿物质来源于俯冲洋壳的部分熔融,与地幔楔发生混染,推测与该时期古太平洋板块的平板俯冲、板片撕裂、拆沉和俯冲后撤一系列过程密切相关.同期,华南东南部发育了NE NNE向、NW向和近EW向三组断裂,其中白垩纪(135～100 Ma)华南NE NNE向的走滑断裂强烈活动,从东向西依次为:滨海断裂、长乐 南澳断裂、政和 大埔、邵武 河源 阳江断裂、吴川 四会断裂和合浦 北流断裂,主要表现为右旋走滑作用,在东南沿海地区形成一系列的拉分盆地,并非典型的“盆岭构造”.根据群速度和S波速度层析成像,华南地块总体上从西向东地壳的厚度整体上逐渐减薄;华南地块东部发生两次壳幔相互作用,对应两次岩石圈拆沉,这两次拆沉都与古太平洋板块的俯冲有关,第一次拆沉为古太平洋板块平板俯冲时板片撕裂所致,第二次拆沉为俯冲板片俯冲后撤和高角度俯冲造成.%Long-term research of the South China Block (SCB) has made a series of important results,but the geological records in South China of the initiation of Palaeo-Pacific subduction are not very clear,and there is still controversy.One school emphasized the Permian subduction initiation,the other stressed it start in the Mesozoic.This paper discusses the Mesozoic transition from the E W-trending Tethys to the NE-trending Paleo-Pacific tectonic systems in the SCB to go further into the initiation of the Palaeo-Pacific subduction.The Early Jurassic formation and the Jurassic molasses like deposits in the Xuefengshan intercontinental tectonic system were distributed in NE-or NNE-trending.There are two-stages of the Yanshanian folding in the Xuefengshan tectonic system,the early-stage folds extend in the NE-NNE directions,and the late-stage folds in SN-NNE directions;there are two-stages of Indonesian folding too,the early-stage folds extend in the EW-ENE directions,and the late-stage fold in SN-NNE directions,indicating that this region was affected by the Triassic transition from the E-W-trending Tethys to the NE-trending Palaeo-Pacific tectonic systems.While the transition time in the Nanling region and the northern South China Sea was the Early-Middle Jurassic.The porphyries of the Dexing porphyry copper deposit were emplaced in an active continental margin setting coupled with the subduction of the Palaeo-Pacific Plate in about 172 Ma based on the previous dating data.The metallogenic material was from the partial melting of the subducted slab and mixed with the mantle wedge.We speculate that the Dexing porphyry copper deposit was associated with the flat-slab subduction,tearing,ddamination and roll-back of the Palaeo-Pacific Plate.Coevally there developed the NE-NNE-,NWand E-W-trending faults.The NE NNE trending faults reactivated intensively in SCB in the Cretaceous (135-100 Ma),from east to west,namely the Binhai Fault,the Changle-Nan'ao Fault,the Zhenghe-Dabu Fault,the Shaowu-Heyuan-Yangjiang Fault,the Wuchuan-Sihui Fault and the Hepu-Beiliu Fault,which are dextral strike-slip faults.A series of pull-apart basins formed in the southeast coastal area.Actually it is not the typical "basin and range region" in the SCB.The crustal thickness in the SCB thinned from west to east generally,according to the group velocity and S-wave velocity perturbations.Crust mantle interaction in the eastern SCB can be divided into two stages,corresponding to two-stage lithosphere delamination which both are related to the subduction of the Palaeo-Pacific Plate.The early-stage lithosphere delamination and lithospheric extension resulted from the Palaeo-Pacific slab subduction coupled with slab tearing,and the latestage lithosphere delamination,crustal activation and the enhanced extension were induced by sharp angle subduction and roll-back.
