华南云开地区幕式岩浆作用与大陆再造

Episodic Magmatism and Continental Reworking in the Yunkai Domain,South China

云开地区是华南古老基底出露的关键地带,记录了华南多期次复杂的地壳再造过程.庞西垌地区位于云开地块的南西缘,区内出露古老基底残片的云开群石英云母片岩.本研究通过详细的野外地质、岩石学和年代学分析,获得该地区花岗片麻岩、片麻状花岗岩和块状黑云母花岗岩高精度锆石U-Pb年龄分别为~450 Ma、~440 Ma和~105 Ma,指示该区经历了加里东期和燕山期幕式岩浆作用.其中,早古生代花岗质岩石中存在多组锆石年龄(~470 Ma、~450~430 Ma和~390 Ma),暗示其经历了长期、多幕次的岩浆过程.此外,在锆石边部还获得了228~219 Ma(低Th/U=0.01~0.03)和101~99 Ma(高Th/U=0.30~0.78)的年龄组,是成岩后印支期变质流体和燕山期岩浆流体的改造印迹.研究区燕山期花岗岩(~770℃)较加里东期(~740~720℃)具有更高的锆石饱和温度,可能是由于地壳经过多次熔融后易熔组分消耗、成熟度较高,发生部分熔融所需的温度更高.它们的年龄谱图与华南古老基底一致,且具有S型花岗岩特征,是缺乏幔源物质加入的古老地壳重熔改造的产物.结合区域地质、年代学和地球化学资料,华南显生宙以来至少经历了加里东期(470~390 Ma)、印支期(~248~200 Ma)和燕山期(~180~90 Ma)的大陆地壳再造/变质‒岩浆流体改造过程.加里东期和印支期主要发生古老地壳物质的部分熔融,极少幔源岩浆参与;燕山期改造过程则主要是地幔热源和物质主导的地壳重熔和壳‒幔混合的结果.

The Yunkai domain is a key area where the ancient basement of the South China Block is exposed,and it records multistage,complex crustal reworking processes of the South China Block.The Pangxidong area is located in the southwestern margin of Yunkai,where the ancient basement fragments of the Yunkai group quartz mica schist are outcropped.In this study,detailed field geological,petrological and chronological analyses were conducted and the high-precision zircon U-Pb ages of granitic gneiss,gneissic granite and massive biotite granite in this area were obtained at~450 Ma,~440 Ma and~105 Ma,respectively,indicating that this area has experienced episodic magmatism in the Caledonian and Yanshanian.There are several sets of ages(~470 Ma,~450-430 Ma and~390 Ma)found in the Early Paleozoic granitic rocks,suggesting that multiple episodes of magmatic processes may have occurred during the Caledonian.In addition,zircon ages of 228-219 Ma with low Th/U(0.01-0.03)and 101-99 Ma with high Th/U(0.30-0.78)were obtained in their dark rims,recording the overprints of the Indosinian metamorphic fluids and Yanshanian magmatic fluids.The zircon saturation temperature of the Yanshanian granite(~770℃)in the study area is higher than that of the Caledonian granite(~740-720℃),which may be due to the consumption of fusible components after multiple melting of the crust,and thus,higher temperatures are required for crustal partial melting.Their age spectrum of the granitoids is consistent with that of the ancient basement of South China,and they have the characteristics of S-type granite.These indicate the granitoids are mainly the products of ancient crustal remelting with insignificant mantlederived material.Combined with regional geological,geochronological and geochemical data,the South China Block has at least experienced Caledonian(~470-390 Ma),Indosinian(~248-200 Ma)and Yanshanian(~180-90 Ma)crustal reconstruction/overprint of metamorphic and magmatic fluids since the Phanerozoic.The partial melting of ancient crustal materials mainly occurred in the Caledonian and Indosinian with few mantle-derived magma inputs,while the crustal melting in the Yanshanian was induced by adding mantle-derived heat with or without mantle materials input.