Experimental Study Of The Blueschis And Marble-bearing High Pressure Metamorphic Belt Of Rongma Area In The Central Qiangtang, Tibet

A500-km-belt of metamorphic exposures in the central QiangTang block, Thismetamorphic belt is part of the Longmu Co-Shuanghu suture. Lanling（Rongma area）high pressure metamorphic zone is located in the core position of qiangtang highpressure metamorphic belt, which provides an opportunity for us to study subductionprocess of Paleo-Tethys Ocean.The predecessors have systematicly studied on Blue ridge high pressuremetamorphic, However, the maximum metamorphism of this belt is highly debated,because the different type of research methods and the object of study. Accordingly,several models have been proposed to explain this formation of this belt.The experimental results show that the total Fe content of barroisite has asignificant positive correlation with pressure, Mg²⁺+^ViAl has a obvious negativecorrelation with pressure; glaucophane also has this tendency.In order to constrain this behaviour, a series of piston–cylinder experiments wereperformed at pressures from1.2to2.3GPa and temperatures from500to800°C. Thestarting composition representative of Rongma high pressure metamorphic belt（synthesised with93%basalt,7%limestone and excess H2O） was run for hundreds ofhours.The first experiment demonstrates that clinopyroxene+plagioclase+barroisitebecome the main mineral assemblage under the condition of1.2GPa and800℃, whilesmall amount of water was found in clinopyroxene, whose rim is barroisite andplagioclase, suggesting the trend of retrograde metamorphism. The second experimentwas run for168hours at1.2GPa and600°C, producing a clinopyroxene+plagioclase+barroisite（few） assemblage, this condition may be the initial stage of blueschistmetamorphism. The third experiment was run for168hours at1.8GPa and800°C,producing a barroisite+clinopyroxene+plagioclase+glaucophane（few）, the total Fecontent of glaucophane and barroisite is smaller than these in blueschist, the numberof plagioclase is greater than field observations. The fourth experiment was run for168hours at2.3GPa and600°C, producing a barroisite+glaucophane+quartz+ clinopyroxene, the number of barroisite is greater than glaucophane, the total Fecontent of two types amphibole mineral is consistent with these in blueschist, most ofthe plagioclase is decomposed The fifth experiments was performed at2.2GPa and500°C for168hours, producing a glaucophane+clinopyroxene+plagioclase+barroisite（few）, the number of glaucophane is greater than barroisite, However, thetotal Fe content of glaucophane and barroisite is smaller than these in blueschist andthe decomposition of plagioclase is not existence. Consequently, the maximumcondition of Lanling high pressure metamorphic belt is2.2².3GPa500⁶00°C.Lanling high pressure metamorphic belt is the product of the Paleo-Tethys subductionand the part of Palaeo-Tethyan suture zone...