Petrology,Geochemistry And Chronology Of The Intermediate-acid Dykes In Luodian,Southern Guizhou Province

Luodian,located in the southern Guizhou Province,belongs to the outer zone of Emeishan Large Igneous Province.Numerous diabase of the same period and origin as the Emeishan Large Igneous Province are exposed in the area.The diabase in the area has been systematically studied,and its formation age is between 263.0 and255.0 Ma,which is the product of Emeishan mantle plume magmatism.The research object of this paper is intermediate-acid dykes invade into these diabase.The age of these dykes is still controversial,and there is little research on the genesis of these dykes at present.The study of intermediate-acid dykes is profitable to enrich the understanding of the law of lithology combination and magmatism in Emeishan Large Igneous Province at the end of Permian.Therefore,Petrology,SIMS Chronology and Geochemistry are performed systematically on the intermediate-acid rock in Luodian in this paper.Petrography and Geochemical analysis show that intermediate-acid rock is Quartz Monzonite Porphyry.The sample shows a Porphyritic-Texture under the microscope,and the phenocryst mineral are mainly Plagioclase,Potash feldspar and Quartz.The result of isotope geochronology of Quartz Monzonite Porphyry is258.5±2.1Ma（n=6,MSWD=1.08）by SIMS.In addition,the large number of inherited zircons in the samples are found.There is a certain regularity in the age distribution of ancient zircons,which can be roughly divided into three groups:（1）2688±9～2180±29 Ma;（2）1142±48 Ma～965±21 Ma;（3）500±13 Ma～434±10 Ma.Average percentage content of major element oxides are Si O₂ 67.99%,Cao1.79%,Al₂O₃ 14.99%,Mg O 0.77%,TFe₂O₃ 4.56%,K₂O 3.74%,Na₂O 5.33%,and average value of A/CNK is 0.93,display the features of high silicon,rich alkali and quasi-aluminous.（La/Yb）_N ratios of 7.71 to 10.60 displays apparent differentiation between LREE and HREE with an averageδEu of 0.91.The chondrite-normalized curve of REE is right inclined,and have ocean island basalts（OIB）signature.The dykes relatively enrich Rb,Ba,Th,K,Nd and relatively depleted Nb,Ta,P,Ti.The values of(⁸⁷Sr/⁸⁶Sr)_i=0.704247～0.705292 andε_Nd（t）=1.08～1.54 are similar with diabase,and both Sr-Nd isotopic compositions are in the range of OIB and Emeishan high-Ti basalt,which suggest that their parental magmas were derived from a common mantle reservoir.The Pb isotopic compositions of the samples are²⁰⁶Pb/²⁰⁴Pb=18.6639～20.1667,with an average of 19.2455;²⁰⁷Pb/²⁰⁴Pb=15.6094～15.6818,with an average of 15.6394;²⁰⁸Pb/²⁰⁴Pb=39.0356～41.3995,with an average of 39.8890.The variation of Pb isotopic composition indicates that the source of Pb in Quartz Monzonite Porphyry is complex.Taking Emeishan high-Ti basaltic magma as parental magma,the result of Rhyolite-MELTS simulation shows that the parental magma experienced～72%separation crystallization at～980℃,including～18%Spinel,～20%Clinopyroxene,～30%Plagioclase,～2%Clinopyroxene,～1%Ferrotitanium-Oxide and～0.5%Apatite,which can form a magma composition very similar to Luodian Quartz Monzonite Porphyry.The simulation temperature is very close to the crystallization temperature of～950℃calculated by calculating the saturation temperature of zircon in previous study.Based on the petrological,geochemical and zircon U-Pb isotopic characteristics of the Quartz Monzonite Porphyry,it is considered that the age of the intermediate-acid dykes is 258.5±2.1Ma,which is consistent with the magmatism time of the Emeishan Igneous Province and is the product of magmatism in the Emeishan Igneous Province.The ancient zircons in samples might be the response of activity at different tectonic stages in the Yangtze block.The Quartz Monzonite Porphyry is the offspring of fractional crystallization of basaltic magma produced by partial melting of mantle plume,and contaminated with a small amount of upper crust material.