HIGH-PRESSURE METAMORPHISM of SERPENTINIZED CHROMITITE at LUOBUSHA (SOUTHERN TIBET)

Abstract

Podiform chromitite with a dunite envelope occurs in harzburgite in the Luobusha ophiolite in the Yarlung-Zangbo suture zone, Tibet, SW China. The chromite and olivine grains come in two main forms in the chromitite: larger sphericallyshaped (nodular), and dispersed (disseminated) smaller-grains. The nodular chromite and olivine grains, and their "melt inclusions," indicate that their formation was via a magmatic process, most likely in the upper mantle. The disseminated chromite and olivine are fragments of the nodular grains that were recrystallized in a metamorphic process. In contrast to the magmatic grains, these metamorphic (that is non-magmatic) chromite and olivine grains are characterized by the presence of inclusions of lizardite, brucite, chlorite, talc, actinolite, uvarovitic garnet, and magnetite. This occurrence suggests that the high-pressure metamorphism was not coeval with the chromitite formation but occurred later as an independent event, with chromitite formation and high-pressure metamorphism separated by passage to near-surface conditions. There, lizardite grew, now occurring as inclusions in the preserved high-pressure metamorphic minerals. Serpentinization took place in the shallow crust, prior to the highpressure metamorphism.