贵东岩体与下庄铀矿田年代学特征

Geochronological characteristics of the Guidong pluton and the Xiazhuang uranium ore-field

贵东岩体是华南南岭地区EW向大东山—贵东—五里亭岩浆岩带的重要组成部分。下庄铀矿田位于贵东岩体东部。前人针对贵东岩体、下庄铀矿田以及发育其中的岩脉开展了大量的年代学测试工作,获取了丰富的年龄数据。本文总结了该区年代学研究进展与特征,结果表明:1)贵东岩体是由加里东—燕山期岩浆组成的复式岩体,其形成年龄为450~151 Ma,自东向西可划分为加里东期岩体(450~418 Ma)、印支期岩体(246~214 Ma)和燕山期岩体(189~151 Ma),其分布具有自西向东逐渐变老的规律。下庄矿田发育NWW、NEE和NNE向3组基性岩脉,岩脉形成年龄为211~91 Ma,按年龄段可进一步划分为5组:211~185 Ma、~180 Ma、145~139 Ma、110~100 Ma和93~91 Ma,为燕山期岩浆活动产物。下庄矿田铀成矿作用自中侏罗世已经开始,一直持续到中新世,年龄为175~20 Ma,铀成矿作用可划分为5期6阶段:中侏罗世铀成矿作用(175~162 Ma)、晚侏罗世—早白垩世铀成矿作用(138~123 Ma、113~100 Ma)、晚白垩世铀成矿作用(96~66 Ma)、古新世铀成矿作用(65~54 Ma)、始新世—中新世铀成矿改造作用(52~20 Ma),其中138~123 Ma、96~54 Ma为该区铀主成矿期。2)根据年代学数据和区域地质资料,可将贵东岩体与铀成矿作用划分为8期、12个阶段:①加里东中晚期岩浆活动(450~418 Ma),主要在贵东岩体北部与东部外围发育次火山岩和花岗岩体;②印支期花岗岩浆活动(246~214 Ma),以中三叠世—晚三叠世早期花岗岩浆活动为主;③燕山早期基性岩浆活动与铀成矿作用(211~162 Ma),进一步划分为3个阶段:三叠世末—早侏罗世基性岩脉活动(211~200 Ma)、早侏罗世花岗岩浆活动(~180 Ma)、中侏罗世基性岩脉与晶质铀成矿作用(179~162 Ma);④燕山期岩浆活动(163~139 Ma),该时期形成了贵东西部的燕山期岩体,可划分为中侏罗世晚期花岗岩浆活动(163~151 Ma)和晚侏罗世岩浆活动(145~139 Ma)2个阶段;⑤燕山晚期早白垩世铀成矿作用及基性岩脉活动(138~100 Ma),进一步划分为2个阶段:早白垩世早期铀成矿作用(138~123 Ma)和早白垩世晚期基性岩脉与铀成矿作用(113~100 Ma);⑥燕山晚期晚白垩世铀成矿作用及基性岩脉活动(96~66 Ma),为主成矿期之一;⑦喜山期古新世铀成矿作用(65~54 Ma);⑧喜山期始新世—中新世铀成矿改造作用(52~20 Ma)。3)区内岩体和铀成矿作用的形成时间均具有相对集中的特点,如加里东期岩体(450~418 Ma)、印支期花岗岩体(238~220 Ma)、燕山期岩体(163~151 Ma)、基性脉岩活动(211~91 Ma)、铀成矿作用(138~123 Ma、96~54 Ma),铀成矿作用与花岗岩体存在一定的时间差,说明铀成矿并非岩体同期作用的产物。每期铀成矿作用之前都发育一期基性岩脉活动,显示铀成矿作用与基性岩脉活动关系密切。

The Guidong plutonis a crucial part of the EW-directed Dadongshan-Guidong-Wuliting magmatic rock belt in the Nanling region of South China.The Xiazhuang uranium orefield resides in the eastern portion of the Guidong pluton.Researchers have conducted extensive chronological testing on the Guidong pluton,the Xiazhuang uranium orefield,and their veins,obtaining abundant age data.This study synthesized the advances and characteristics of chronological research in the study area.The results show that:(1)The Guidong pluton is a complex pluton formed by Caledonian-Yanshanian magma at 450~151 Ma.From east to west,it can be divided into the Caledonian(450~418 Ma),the Indosinian(246~214 Ma),and the Yanshanian(189~151 Ma)plutons,manifesting a gradually aging pattern from west to east.The Xiazhuang uranium orefield hosts NWW-,NEE-,and NNE-directedmafic veins form eddue to Yanshanian magmatism at 211~91 Ma.These veins can be categorized into five sets according to their ages:211~185 Ma,ca.180 Ma,145~139 Ma,110~100 Ma,and 93~91 Ma.The uranium mineralization in the orefield lasted from the middle Jurassic to the Miocene with an age of 175~20 Ma,involving five phases and six stages:The Middle Jurassic uranium mineralization(175~162 Ma),the late Jurassic-early Cretaceous uranium mineralization(138~123 Ma,113~100 Ma),the late Cretaceous uranium mineralization(96~66 Ma),the Paleocene uranium mineralization(65~54 Ma),and the Eocene-Miocene uranium mineralization and modification(52~20 Ma).Among them,138~123 Ma and 96~54 Ma constituted the primary mineralization epochs.(2)According to the chronological and geological data of the study area,the formation of the Guidong pluton and its uranium mineralization can be divided into the following eight phases and 12 stages.The first phase is the middle-late Caledonian magmatism(450~418Ma),forming subvolcanic and granitic rocks primarily in the northern and eastern peripheries of the Guidong pluton.The second phase is the Indosinian granitic magmatism(246~214 Ma),which was dominated by the granitic magmatism from the middle Triassic to the early stage of the late Triassic.The third phase is the early Yanshanian mafic magmatism and uranium mineralization(211~162 Ma),including three stages:The mafic vein activity from the late stage of the Triassic to the early Jurassic(211~200 Ma),the early Jurassic granitic magmatism(ca.180 Ma),and the middle Jurassic mafic vein activity and uraninite mineralization(179~162 Ma).The fourth phase is the Yanshanian magmatism(163~139 Ma),which formed the Yanshanian pluton in the west of the Guidong pluton,consists of the granitic magmatism during the late stage of the middle Jurassic(163~151 Ma)and the late Jurassic magmatism(145~39 Ma).The fifth phase is the late Yanshanian early Cretaceousuranium mineralization and mafic vein activity(138~100 Ma),which comprises the uranium mineralization at the early stage of the early Cretaceous(138~123 Ma)and the mafic vein activity and uraninite mineralization at the late stage of the early Cretaceous(113~100 Ma).The sixth phase is the late Yanshanian late Cretaceous uranium mineralization and mafic vein activity(96~66 Ma),serving as one primary mineralization phase.The seventh phase is the Himalayan Paleocene uranium mineralization(65~54 Ma).The eighth phase is the Himalayan Eocene-Miocene uranium mineralization and modification(52~20 Ma).(3)The pluton formation and uranium mineralization are relatively concentratedin the study area,such as the Caledonian pluton(450~418 Ma),the Indosinian granitic pluton(238~220 Ma),the Yanshanian pluton(163~151 Ma),mafic vein activity(211~91 Ma),and uranium mineralization(138~123 Ma,96~54 Ma).There exists a certain time difference between uranium mineralization and granitic pluton formation,indicating that they are non-contemporaneous products.Each uranium mineralization was preceded by mafic vein activity,suggesting an intimate relationship between uranium mineralization and mafic vein activity.