Research On Metamorphic Minerals And Rocks Of “wangyayan Formation”in Feng Qing County,western Yunnan

It is a hotspot always to research on geological evolutionary history of Changning-Menglian Suture Zone, including the stratigraphic division of “Wangyayan Formation" and its metamorphic rocks located in western edge of the southern section in the zone.The rocks of " Wangyayan Formation " are recongnized staurolite biotite schist con-taining kyanite and garnet, biotite-quartz schist, a small amount of felsic schist and granite from the two-period granite veins, with the typical symbiotic combinations of metamorphic minerals divided into:M1(staurolite+almandine+biotite+plagioclase+ quartz, M2(kyanite+almandine+biotite+k-feldspar, M3(almandine+biotite+quartz), based on field geological survey, lab testing and study. Then these rocks were tansfomred into tectonic schist and mylonite with the minerals combination:quartz+ sericite in the last tectonic deformation period.Crystal chemical formula of kyanite:(Fe0.0049-0.0189Al1.9311-1.9933)1.9453-1.9929 [(Si0.95-0.9956 Al0.0046-0.05)1O4]O, the AlⅣ/AlⅥvalue about 0.0251. And the staurolite:(Fe0.6503-0.7006Al0.024-0.5016)0.8-1.3Al4[(Al0.1136Si1.8864)O4]2O2(OH)2,AlⅣ/AlⅥ value about 0.0252, lower XMg and richer iron. The garnet, mainly contained of almandine, and pyrope percent content of 2.75%- 9.29%, the grossular of 6.64%- 19.63%, Ca0.2553-0.6032,Mg0.0851-0.2816,Fe1.8667-2.2495,Mn0.1370-0.4168)2.9334-2.9698 Al1.869-1.997 [(Al0.0008-0.0182Si0.9818-0.9992)1O4]3,AlⅣ/AlⅥvalue about 0.0091, its F value is between 88.87 ~ 95.97,and it is showed that the garnet was formed at various pressures with research on the metamorphic mineralogy and crystal chemistry. Biotite,iron-rich,high Al2O3(average value 20.72%),FeOT(20.16%),and low MgO(average 9.16%), MF values 0.408 ~ 0.540(higher), TiO2 percent content between 1.16%~2.32%,is recongnized three genetic periods: the biotite in M1(fiest metamorphic period), the same with staurolite and almandine,was reformed into palimpsest phenocrys in later deformation periods; biotite of M2 have long scaly crystals; M3’s was monolithic with non-directinal.Three metamorphic facies are suggested in this paper.The first happened in the forming stage of staurolite, garnet, with synchro-deformation(M1-D1), is the main period of metamorphism in this area with low pressure epidote amphibolite facies, Metamorphic temperature in range of 511℃~513℃ and the pressure 0.439 GPa. The second, synchro-deformation(M2-D2) and forming the muscovite reached middle pressure amphibolite facies with higher temperature(553℃) and pressure(0.551Gpa). The third, synchro-deformation(M3- D3) with the metamorphic mineral assemblages of biotite+garnet is low pressure greenschist facies with the temperature(303℃~310℃) and pressure(0.09 Gpa). Thereafter deformations are recongnized schistosity fold(D4) and later mylonitization(D5) with sericite+quartz in the area.Thus the time of main metamorphism(Jinnin Period) is earlier than the main deformation(Yanshan – Himalayan period).