豫西济源盆地中三叠世-中侏罗世碎屑锆石年代学及其对秦岭造山带造山过程的启示

Geochronology from Middle Triassic to Middle Jurassic Detrital Zircons in Jiyuan Basin and Its Implications for the Qinling Orogen

豫西济源盆地位于秦岭造山带北部,是三叠纪-侏罗纪秦岭造山带的同造山盆地.对济源盆地中三叠世-中侏罗世6个组的碎屑锆石样品进行了LA-ICP-MS的U-Pb年龄分析.结果表明,碎屑锆石年龄主要分布于以下区间:2.9～1.7Ga、1.6～1.0Ga、1.0～0.8Ga、800～650Ma、520～380Ma、350～245Ma和～220Ma,其中除了2.9～1.7Ga的碎屑锆石主要来自于华北克拉通基底外,其他几个年龄段的锆石则主要来自于秦岭造山带,并且显示出随着地层年龄的逐渐变新,碎屑锆石年龄有逐渐变老的趋势.在中三叠世-晚三叠世早期样品中,主要年龄是350～245Ma,在晚三叠世晚期-中侏罗世早期样品中,1.6～1.0Ga、1.0～0.8Ga、800～650Ma和520～380Ma则逐渐增多,到了中侏罗世晚期,1.6～1.0Ga和520～380Ma依然存在于样品中,并且还发现了～220Ma年龄.碎屑锆石年龄结构指示了秦岭造山带印支期经历了由年轻的盖层到较老的基底的去顶过程.并与合肥、黄石盆地的碎屑锆石数据对比发现,早侏罗世时,去顶强度东强西弱;东部高压-超高压变质岩或晚三叠世岩体可能暴露于早侏罗世,而西部则到了中侏罗世.

Jiyuan basin is located in the north of Qinling orogen and regarded as a Triassic-Jurassic synorogenic basin. This paper presents a great number of detrital zircon U-Pb ages for synorogenic sediments in age ranging from the Middle Triassic to Middle Jurassic in the Jiyuan basin. The results show seven major age groups at 2. 9--1. 7 Ca, 1. 6--1.0 Ca, 1.0--0. 8 Ga, 800--650 Ma, 520-- 380 Ma, 350--245 Ma and -220 Ma It is obvious that age group of 2. 9--1.7 Ca is directly derived from the Precambrian basement of North China Craton whereas other groups are typical characteristics of the Qinling orogenic belt. An important observation is that the Qinling-sourced detrital zircons become older from the Middle Triassic to the Early Jurassic sedimentary sequences, which is character ized by the main age clusters at 350--245 Ma occurring in the T2-T3^1 strata while age groups at 1.6--1.0 Ca, 1.0--0. 8 Ca, 800-- 650 Ma and 520--380 Ma increase successively in the T3^2 j2^1 samples. In addition, little grains of age groups at 1.6--1.0 Ca, 520-- 380 Ma and -220 Ma are presented in the J2^2 sample. The above-mentioned findings indicate that unroofing pattern of Qinling orogenic belt developed by the denudation of sendiments from young covers to old basements in order, then to the Camian plutons. Integrated with the data reported from Hefei and Huangshi basins, it is well established that the intensity of unroofing increased from the west to the east in the Early Jurassic, and the denudation timing of the ultra high pressure （UHP）-high pressure （HP） rocks or Carnian plutons changed successively from the Early Jurassic in the east to the Middle Jurassic in the west.