The exsolution lamellae in basic granulite facies scapolite and its formation mechanism: An example from the granulite in the Ama Drime Massif, southern Tibet

Scapolite-group minerals are widespread volatile -bearing aluminosilicates in granulite facies metamorphic rocks, but the formation mechanism and the behavior of scapolite is poorly understood. EPMA and TEM analyses of the scapolite and its exsolution lamellae from a mafic granulite in Dinggye area show that: (1) the scapolite is rich in S02,-; (2) the lamellae are troilite with (001) parallel to the host crystal (100); and (3) its unit cell of the lamellae is: a = b =0. 5968nm, c =1. 174nm, a = p =90, -y =120. The scapolite before exsolution formed in eclogite facies conditions and its upper limit of pressure is strongly controlled by Fe content. During later granulite facies metamorphism, S042- with these scapolites experienced a self-redox reaction to produce the lamellae plus 02 in a H20 -free condition, the reaction equation is described as follows: Fee{+SO4- =FeS +202. Similar reactions could lead to an elevated oxygen fugacity in the lower crust than in the middle and upper crust. The formation and breakdown of scapolite provide a valuable record on the deep processes during the tectonic evolution of the Himalayan orogen. In addition, these processes might play an important role controlling the geochemical behaviors of volatile components during the subduction and exhumation of continental orogenic belts. This exploits a new tool for the investigation of deep -Earth oxygen fugacity adjustment.