The Characteristics And Origin Of Rapakivi Granites,in Cu-Ni Sulfide Deposits,Xiaonanshan Of Siziwangqi,Inner Mangolia Province

Siziwangqi area of Inner Mongolia is between the North China plate and the Central Asian Orogenic Belt in construction,owing to the strata outcropped in Siziwangqi area is very limited,so study the outcropped granitic rocks for understanding the evolutionary history of the Central Asian Orogenic Belt and large-scale mineralization in the area are of great significance.This article choose the granite of Siziwangqi area,study the characteristics of the petrography,mineralogy and geochemistry.To sum up,I come to these conclusions:1.Rapakivi granites are vein form with porphyritic structure,phenocrysts are feldspar,hornblende and quartz,matrix are mainly feldspar and quartz.2.Geochemical characteristics of the samples are rich in Mg O,TFe₂O₃ and Ca O,samples are metaluminous,middle k-high k calcium-alkali series.Samples in the study have little Eu negative anomaly,characterized by LREE enrichment and HREE loss,and they are all relatively enriched in Rb,Ba and other LILE,depleted in Nb,Ta,Ce and high field strength elements.3.The feldspars in phenocrysts are mainly albite,oligoclase and anorthoclase.The rapakivi texture of alkali feldspar have two side parts which are oligoclase and the value of An are 12.7 and 11.8,two curtain which are albite and the value of An are10 and 7,the center is anorthoclase which the value of Or is 39.9.There are mainly albite and oligoclase in four plagioclase phenocrysts.Two show edge-core-edge firstly increase and then decrease,the other two show firstly decrease and then increase,amphiboles are mainly edenite and pargasite.Feldspars of matrix are mainly albite,oligoclase anorthoclase and sanidine.4.LA-ICP-MS zircon U-Pb dating results show that the ages of granites are about:280 Ma.According to the geochemical characteristics of rocks and the features of mineralogy and chronology,we know the formation mechanism of the magma is magmatic underplating+lower crust melting+crust-mantle migmatization.