Fluid evolution during uplift of the Annapurna Himal, central Nepal

Abstract

Fluid inclusions from post-metamorphic quartz show that there has been considerable CO 2-rich fluid flux during uplift of greenschist-amphibolite facies rocks along the Main Central Thrust (MCT). Maximum fluid CO 2 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 CO 2 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 CO 2-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 CaNa brines, some with freezing point depression below −20°C, have traversed the rock. At first the brines mixed with CO 2, then brines dominated until diluted by incursion of lowsalinity, presumably meteoric water near the surface. Brines and CO 2 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. CO 2 was derived from metamorphosed equivalents of these sediments, and perhaps from similar sediments beneath the MCT.