Study Of KMnFMASH Metamorphic Phases System About Progressive Metamorphic Zones In Aletai Area, Xingjiang

The zonal progressive metamorphic zones are widely distributed in Aletai area, typical zonal metamorphic zones which have been recorded are biotite zone, garnet zone, staurolite zone, staurolite - andalusite zone, staurolite– kyanite zone, sillimanite zone.Author quantitatively estimate the P-T evolution path and metamorphic peak condition of biotite– garnet zone, garnet– staurolite zone, staurolite– andalusite zone on the basis of using THERMOCALC 3.30 software fluently to calculate the KMnFMASH P-T Pseudosection for each metamorphic zone and using the Fe,Mn content isopleths: x(g),x(m) in garnet and H2O content isopleths in the paragenetic mineral.The main characteristic metamorphic minerals in rocks that form the biotite– garnet zone are biotite, garnet. The Fe,Mg,Mn content of biotite do not change severely, but are similar to the biotite in other two metamorphic zones. The Fe, Mg content of garnet are 1/3-1/2 compared with other two metamorphic zones, by contrast, the Mn content are 2-3 times larger than other two metamorphic zones.The main characteristic metamorphic minerals in rocks that form the garnet– staurolite zone are chlorite, muscovite, biotite, garnet and staurolite. Author chose two typical specimens to study the garnet– staurolite zone. The first one represents the initial stage of staurolite zone, the second one represents later stage of staurolite zone, in this specimen, the staurolite will be consumed away quickly. Garnet in the rocks forming the metamorphic zone has no composition zone.The main characteristic metamorphic minerals in rocks that form the staurolite– andalusite zone are chlorite, muscovite, biotite, garnet, staurolite and andalusite. The Fe content of this zone is more than the other two metamorphic zones, but Mg,Mn content are less than the other two metamorphic zones. The representative specimens 112-4 for garnet– staurolite zone and 104-9 for staurolite– andalusite zone have clear composition zone, especially the Fe, Mn elements zone. So, the P– T path defined by Fe, Mn content in garnet, of these two metamorphic zone are more accurately. If we match these P– T paths to metamorphic ages, we will get the P– T– t path of these metamorphic zones.By calculating the P– T Pseudosection for zone, garnet– staurolite zone, staurolite– andalusite zone in zonal progressive metamorphic zones of Aletai, Xingjiang, Athor limited the metamorphic peak condition of garnet– staurolite zone, staurolite– andalusite zone. The metamorphic peak condition of garnet - staurolite zone is P=6.3 kbar -7.8kbar,T=585℃-600℃, after the peak condition, garnet - staurolite zone experienced a retrograde metamorphic point P=6.5kbar-6.8kbar,T=585℃-595℃; The metamorphic peak condition of staurolite - andalusite zone is P=6.8kbar-7kbar,T=660℃-670℃。Andalusite in rocks forming the staurolite– andalusite zone locate in the area in which kyanite is stable, Wei et al.(2006)has putted forth that the andalusite-type metamorphic zones in Aletai zonal progressive zones are the production of retrograde of kyanite-type metamorphic zone during the exhuming and decompression process. Now, author gets the same conclusion from calculated P-T pseudosection, which gives more evidence to the conclusion.The metamorphic peak pressure of zonal progressive metamorphic zones in Aletai mainly around 6 kbar -8 kbar, are not typical low-pressure -type metamorphic zones, but medium type metamorphic zones; peak temperature mostly during 600℃-670℃. So these zonal progressive metamorphic zones in Aletai are not strict low-pressure heating metamorphic zones, but the medium pressure metamorphic zones.