FLUID EVOLUTION DURING UPLIFT OF THE ANNAPURNA HIMAL, CENTRAL NEPAL

Fluid inclusions from post-metamorphic quartz show that there has been considerable CO2-rich fluid flux during uplift of greenschist-amphibolite facies rocks along the Main Central Thrust (MCT). Maximum fluid CO2 content increased with metamorphic grade of the host rock, ranging from ∼ 20-30 mole% in the greenschist facies to both the north and south of the MCT, to 70 mole% in the amphibolite facies. The highest CO2 content is in late-stage veins which cross-cut uplift-related schistosity in the sillimanite zone. This contrasts with the syn-metamorphic fluid which was probably dominantly aqueous. Fluid immiscibility occurred in quartz veins near the brittle-ductile transition, at 280-310°C and 700±300 bar fluid pressure. An upper limit of 1600 bar lithostatic pressure was determined from a CO2-rich isochore. The high thermal gradient implied by P-T data is due to rapid uplift along the MCT. MCT movement ceased in the Miocene, and the rocks have experienced slower uplift along a lower thermal gradient. During this slower uplift CaNa brines, some with freezing point depression below -20°C, have traversed the rock. At first the brines mixed with CO2, then brines dominated until diluted by incursion of lowsalinity, presumably meteoric water near the surface. Brines and CO2 may be reaching the surface today, in diluted from as numerous warm and ambient springs. Brines were probably derived from connate fluids in the Tibetan carbonate/clastic sedimentary succession to the north of the MCT. CO2 was derived from metamorphosed equivalents of these sediments, and perhaps from similar sediments beneath the MCT. © 1990.