Metamorphic Characteristics And Prospecting Significance Of The Upper Triassic Rock Strata In The Jiajika Area,Western Sichuan

The Jiajika area,Western Sichuan,is one of the regions with the richest lithium resources in China,which is located in the middle part of Yajiang fold-nappe belt in the central Songpan-Ganzi orogenic belt.The research object of this study is the Triassic system of Jiajika area,combinn g with previous research data,we conducted this study from the perspective of the combination characteristics of the Upper Triassic metamorphic rocks in this area.By using multi-methods as field investigation,cross-section measurement,sample testing,as well as theories and methods of petrology,petrography and structural geology,the types,characteristics and stages of metamorphism in this area were explored,and the relationship between the metamorphism characteristics and the rare metal mineralization was discussed.Through the field geological survey of the Jiajika area,the indoor microscopic observation on rock slices and the sample composition test,the study concluded that the main metamorphic rocks in this area are the cordierite staurolite two mica schist,the staurolite andalusite two mica schist,the staurolite two mica schist,the andalusite two mica schist and the biotite schist;Their primary rocks are upper Triassic sandy-muddy flysch,which is influenced by granite magma intrusion and formed by regional dynamothermal metamorphism.In this study,we selected the garnet and biotite minerals in the Upper Triassic schist in the methyl card area that are symbiotic with the crucisite and andalusite,and explored the temperature and pressure of metamorphism through formula.The results showed there were two formation stages of the metamorphic rock in the Jiajika area,with the temperature ranges of 454 ? ? 478 ?;492 ? ? 496 ? and the pressures from 0.450 Gpa to 0.745 Gpa.The area has experienced four stages of metamorphism.The first stage was the shallow metamorphism with the sericite-chlorite low-green schist facies,which was related to the intra-continental orogeny of Songpan-Ganzi,but with no direct relationship to the rare metal mineralization in this area.The second period was in the late Triassic period.Accompaning by the Songpan-Ganzi intra-continental orogeny in the late Indosinian period,the north-south and east-west bi-directional compression thickened the Late Triassic thick flysch strata in the Yajiang-Kangding area,and thus caused the thickening of the crust and the emplacement of the granite diapir,formed the amphibolite facies incremental dynamic thermal metamorphic belt.The third stage was the thermal contact metamorphism caused by the intrusion of the two-mica granite in the late stage of the granite diapir emplacement and the penetration of the pegmatite vein along the fissure.The fourth stage was the late stage accompaning by the retrograde metamorphism of the thermal relaxation stage.Among them,contact metamorphism of the third stage was most closely related to rare mineralization.The cordierite zone in this area not only plays an indicating role on the formation of the blind granitic pegmatite vein,but also can accurately predict the typies,occurrence,scale and the level of its mineralization of the granitic pegmatite combining the characteristics of the cordierite zone,provide evidence for finding blind granitic pegmatite.