贵州省黔西县大寨地区中三叠统关岭组绿豆岩锆石U-Pb年代学、地球化学特征及地质意义

针对贵州省黔西县大寨地区中三叠统关岭组底部绿豆岩,利用LA-ICPMS锆石U-Pb测年手段,明确贵州黔西县大寨地区早—中三叠世界线年龄。对贵州省黔西县大寨地区中三叠统关岭组底部绿豆岩1件U-Pb年龄样品、 8件地球化学样品的测试分析,结果表明,锆石U-Pb加权年龄为248.6 Ma±2.7 Ma,谐和年龄为248.7 Ma±1.1 Ma;绿豆岩属于高硅(w_SiO2均值为62.40%)、铝(w_Al2O_3均值为14.61%)、钾(w_K2O均值为7.22%),低铁(w_Fe2O_3均值为0.94%,w_FeO均值为0.51%)、钛(w_TiO2均值为0.24%)的中酸性火山物质,具有稀土元素总量相对较低、相对富集轻稀土元素和轻重稀土元素分馏程度低的特征,负Eu异常显著,δEu=0.17～0.22。综合分析,该绿豆岩是由火山活动喷发的火山灰经风化、搬运、沉积改造而形成,为下三叠统嘉陵江组与中三叠统关岭组松子坎段界线岩性标志,同时为测区早—中三叠世界线年龄提供了准确的年代学证据。

ZIRCON U-Pb GEOCHRONOLOGY AND GEOCHEMISTRY OF THE MUNG BEAN ROCKS OF THE MIDDLE TRIASSIC GUANLING FORMATION IN DAZHAI AREA OF QIANXI COUNTY,GUIZHOU PROVINCE

Zircon U-Pb LA-ICP-MS dating and geochemical analysis on the mung bean rocks occurred on the bottom of middle Triassic Guanling Formation in Qianxi County,Guizhou Province are carried out with one geochronology sample and 7 geochemical samples.It shows that mung the bean rock has weighted mean age of 248.6 Ma±2.7 Ma and concordant age of 248.7 Ma±1.1 Ma.Geochemically,the mung bean rocks are rich in Si,Al,K(the average value of wSiO2 is 62.40%,wAl2O3 is 14.61% and wK2O is 7.22%) and depleting in Fe,Ti(the average value of wFe2O3 is 0.94%,wFeO is 0.51% and wTiO2 is 0.24%),characteristic of moderate-acidic volcanic material.Total amount REE of the mung bean rocks is low,LREE is relatively rich and the REE fractionation is low with obvious negative Eu anomaly(δEu of 0.17~0.22).Comprehensive analysis indicates that the mung bean rocks are formed by volcanic ash which is subsequently experienced the processes of weathering,transporting and deposition.The mung bean rock is the demarcation marker layer for the lower Triassic Jialingjiang Formation and middle Triassic Guanling Formation.The zircon U-Pb LA-ICP-MS dating age of mung the bean rock provides geochronologcal evidence for the demarcation of early-middle Triassic boundary.