The Characteristics And Petrogenesis Of Porphyritic Granite In Gejiu,Yunnan

Gejiu is located in the western part of South China,and also a well-known city as the“tin capital”.The granites are widely distributed with various lithologies in the area.Based on previous studies,this article selects the Longchahe and Beipaotai porphyritic biotite granite in Gejiu as research objects.And the petrographic characteristics,mineralogical characteristics,geochemical characteristics,genesis,physicochemical conditions and tectonic background of porphyritic granites of the two plutons are systematically studied.The main achievements of this study are shown as followings:Both plutons are porphyritic biotite granite.The Longchahe pluton has medium-grained structure,and the Beipaotai pluton has fine-grained structure.The phenocryst is mainly K-feldspar.From the Longchahe pluton to the Beipaotai pluton,the former has more phenocryst and they are in larger sizes.The contents of some minerals gradually increased:such as quartz and K-feldspar,but the contents of other minerals gradually decreased:such as biotite and plagioclase.The biotite of both two plutons are belong to iron biotite;the plagioclase is mainly andesine-oligoclase,Some plagioclase can see the"purification edge"of albite;the K-feldspar is orthoclase.K-feldspar phenocrysts were formed in the middle and later stages of magma crystallization due to the enrichment of potassium in magma and temperature,resulting in structural thickening.At this time,the magma has not been completely consolidated,and earlier minerals such as biotite,plagioclase,and potash feldspar have crystallized,while late minerals such as quartz were still crystallizing.The characteristics of the major and trace elements of the two rocks are similar.They are characterized by calc-alkaline,high-potassium-potassium and quasi-aluminum-peraluminous granite.The two plutons are both rich in Rb,Th,U,La,Nd and Sm,and depleted in Ba,Sr,Nb,P,Ti and so on.The chondrite-normalized REE patterns of two plutons shows the right type,and the characteristic of LREE is more richer than HREE.The Longchahe pluton is rich in potassium,alkali and aluminum compared with the Beipaotai pluton,while the latter is relatively acid.At the same time,compared with the former,the total amount of rare earths is relatively low,negative Eu anomalies slightly stronger.The mean crystallization temperatures of magma in Longchaghe and Beipaotai pluton are calculated by using zirconium saturation thermometer?>716.35?,<813.45??.The ratio of Al₂O₃/TiO₂ also has showed that the partial melting temperature of the two plutons were>875?,and the partial melting temperature of Longchahe pluton was higher than Beipaotai pluton.In terms of diagenetic pressure,the average pressure calculated by biotite pressure gauge was 1.65Kbar and 1.07Kbar.The corresponding average diagenetic depth is about 6.24km and 4.04km respectively,and the diagenetic depth of the Beipaotai pluton is relatively shallow.The porphyritic granites of the two plutons is an A-type granite that reflects some of the characteristics of the I-type granite,which has formed by the partial melting of the ancient igneous and sedimentary rocks of the crust origin,and it have similar source areas and origin.However,the porphyritic biotite granite of the Beipaotai pluton has a higher degree of separation and crystallization evolution.Both of two plutons were formed in the extension tectonic environment after the orogeny period.