The Contrast Study Of Two Types Garnet Amphibolite In The Lahsa Block:Petrology And Phase Equilibrium

The garnet amphibolite is traditionally recognized as a kind of metamorphic rock that experienced amphibolite facies metamorphism, which is lower than the eclogite facies and granulite facies metamorphism. However, the discovery of ultrahigh pressure index mineral like coesite in the amphibolite indicates that the garnet amphibolite might also experience ultrahigh pressure metamorphism. The main mineral assemblage of garnet amphibolite is garnet and amphibolite with more or less plagioclase. The other minerals are depending on the protolith and the formation environment. We choose two type of garnet amphibolite(retrograde eclogite and staurolite-bearing garnet amphibolite) in the Lhasa terrane as the target to do the petrological investigation in order to provide further evidence to control the Paleo-Thethys suture inside the Lhasa terrane.Eclogite and its retrogress body occur as lens or layers near Sumdo and Jinda area with muscovite schist as the country rocks. The mineral assemblage of the eclogite is garnet, amphibolite, omphacite, phengite, rutile and epidote. Phase equilibrium calculation shows that the garnet amphibolite is the retrogress part of the eclogite during exhumation after subduction with ca. 32 kbar, 600℃ of the peak metamorphic condition. The staurolite-bearing garnet amphibolite occur in the strong formation zone in the Nyingchi complex with mineral assemblage of garnet, amphibolite, staurolite, muscovite, paragonite, chlorite, biotite etc, which is aluminum rich character for the whole rock composition. The peak metamorphic condition is 11-12 kar and 600-620℃ with mineral assemblage of garnet amphibolite, staurolite and muscovite. Comparison study show that two type of garnet amphibolite have different mineral assemblage, geochemical character and different metamorphic evolutionaries. The garnet amphibolite with eclogite signature preserved eclogite facies mineral like omphacite and rutile. The staurolite-bearing garnet amphibolite always preserve the lower metamorphic minerals. The garnet in it preserved good prograde zonation. Whereas the other type have phengite with higher Si content. Both type of garnet amphibolite have similar P-T path and also share similar age peaks of zircon at 268 Ma and 218 Ma.With detailed petrological and geochronological investigations on two types of eclogite, we conclude that they have experienced the same or similar orogenesis process during Paleo-Thythes period and further testified that the suture not only preserved in Sumdo area but also can extent to the Nyingchi area.