Metamorphism Evolution And Tectonic Implications Of Clinopyroxene-bearing Amphibolite In The Xigaze Ophiolite,Southern Tibet

Amphibolite developed in an ophiolite suite is a high-temperature metamorphic rock that is commonly formed due to its interior high temperature and structural friction during ophiolite emplacement.It is also called as metamorphic floor,and is a major hallmark to recognize early-stage subduction of an ocean crust.Therefore,a proper knowledge on the formation and metamorphic evolution of amphibolite in an ophiolite can provide us an important clue to better understand oceanic subduction along convergent plate margins.Clinopyroxene-bearing amphibolites are commonly exposed as lens within ophiolitic melange in Xigaze area of the central Yarlung Zangbo Suture Zone.Previous studies have shown that the amphibolites are remnants of the New-Tethys oceanic crust.Therefore,a further detailed study on the metamorphic process of amphibolites can provide essential quantitative constraints concerning the metamorphic and tectonic evolution of the Yarlung Zangbo Suture Zone.In this dissertation,we present the results of combined petrography,mineral geochemistry and phase equilibrium modeling on a suite of amphibolites in the Xigaze ophiolite massif,in order to determine the metamorphic P-T conditions and metamorphic evolution.In addition,we compare the pyroxene amphibolite and garnet amphibolite by whole rock geochemistry and Sr Nd isotopes.The metamorphic mineral assemblages of the clinopyroxene-bearing amphibolite include amphibole,clinopyroxene,epidote,plagioclase and sphene.We carried out phase equilibrium modeling of clinopyroxene-bearing amphibolite,and the results showed that the metamorphic evolution process of calcium amphibolite was limited to four stages at the intersection area of Ti isoline in amphibole and Ca isoline in plagioclase within the amphibolite:which are(?)initial stage of metamorphism(8.6kbar,880?),(?)temperature peak stage(10.8-13.4kbar,800-840?),(?)pressure peak stage(12.7-13.2kbar,650-660?),and(IV)retrograde metamorphic stage(<11.2kbar,<640?).Mineral compositional characteristics indicate that the P-T conditions in the initial stage of metamorphism of the clinopyroxene-bearing amphibolite may reach up to granulite facies metamorphic conditions.It can be seen that the P-T conditions of the clinopyroxene-bearing amphibolite which exposed in the central Yarlung Zangbo Suture Zone reached high amphibolite to granulite facies conditions,indicating a subduction depth of 41-44 km,belonging to medium-pressure facies series.Whole-rock geochemical analysis shows that the clinopyroxene-bearing amphibolite and garnet-bearing amphibolite in the Xigaze area have similar compositional features,and they exhibit trace element distribution patterns that are generally similar to N-MORB,except for their positive K,U,Pb and negative Nb,Ta.Ti anomalies.They also have Sr contents that are different from normal N-MORB.The two types of amphibolite samples have consistent ENd(t)values(+7.95?+11.18),which are comparable tothose of the Luqu and Jiding basic rocks.These Nd isotope values also coincide with the respective values of the Indian Ocean MORB.Based on all the above geochemical investigations,it can be inferred that both types of amphibolite from the Xigaze area have experienced metasomatism by subduction-related fluids,although both of them still preserved MORB type source characteristics.The similar geochemical characteristics of Tethyan Ocean represented by the Xigaze ophiolite with those of Indian Ocean mantle indicate that the Indian Ocean inherited the geochemical characteristics peculiar to the Tethyan Ocean.Based on previous research,this dissertation considers that the source of clinopyroxene-bearing amphibolite in the xigaze area is similar to that of garnet-bearing amphibolite.Both of two kingds of amphibolite were formed in a mid-ocean ridge environment,and they were successively subducted into the mantle as a part of the Neo-Tethys oceanic crust.