Fluid inclusions as metamorphic process indicators in the Southern Aravalli Mountain Belt (India)

Abstract

Fluid inclusions from a biotite-garnet schist in the Southern Aravalli Mountain Belt (India) give information on both peak metamorphic conditions and post-peak metamorphic processes during uplift. A combination of careful petrography, microthermometry and Raman spectroscopy reveals the presence of at least ®ve generations of enclosed¯uids. Lower amphibolitefacies pressure-temperature conditions of the growth of garnet rims are reproduced by the highest¯uid density of the relatively oldest inclusion type of CO 2 (N 2 )-rich compositions. A calculated¯uid composition in the COH system, in equilibrium with the graphite buer corresponds to a CO 2 -rich¯uid at metamorphic conditions. However, the results of these calculations are very sensitive to small¯uctuations in oxygen fugacity and the accuracy of thermodynamic properties of mineral equilibria. Re-equilibration, conceived by speci®c sizedensity distribution and the absence of an aqueous phase in inclusions that contain nahcolite crystals, is monitored in these inclusions as post-peak metamorphic processes, like partial decrepitation and preferential leakage. The other¯uid types represent heterogeneous uid trapping of coexisting aqueous NaCl-bearing solutions with CO 2 -CH 4 -rich vapour bubbles in healed cracks, and probably the introduction of external¯uids containing high salinity aqueous CaCl 2 -rich solutions in nearly pure N 2 vapour bubbles, at lower P-T conditions. This study illustrates that¯uid inclusions remain a valuable database of peak metamorphic conditions, moreover, alterations of the entrapped¯uids and surrounding crystals are illustrative for speci®c exhumation evolutions.

Nahcolite in fluid inclusions from the Ryoke metamorphic rocks and its implication for fluid genesis

“…In this case, of course, the temperature of the inclusion entrapment was high as described in the above studies. Although some nahcolite daughter minerals coexist with nearly pure CO 2 fluids, the fluids might have contained some H 2 O in the past (Olsen, 1987;Bakker and Mamtami, 2000).…”

“…On the other hand, nahcolite solids often show highly variable volume ratios of the total inclusion volumes even in a single host mineral (e.g., Bühn et al, 1999;Bakker and Mamtami, 2000). They might have been accidentally trapped during inclusion formation below the melting temperature of nahcolite (270 °C).…”

Phase relations of nahcolite and trona at high P-T conditions

“…These trapping P -T conditions were established by microthermometry Bakker and Mamtani, 2000;Moroz et al, 2001;Vapnik and Moroz, 2002), mineralogical geothermometry -geobarometry (Olsen, 1987) or stratigraphic reconstruction combined with fluid inclusion isochors . According to our experimentally determined univariant curve of nahcolite + trona + vapour + fluid, the mostly likely stable sodium carbonate -sodium hydrogen carbonate phase in these fluid inclusions at the trapping P -T conditions appears to be trona rather than nahcolite, and any nahcolite grains accidentally trapped by the fluids in excess Bakker and Mamtani, 2000) probably would be dissolved in the fluids due to the high temperature. Indeed trona has been occasionally documented in the field (Markl and Baumgartner, 2002).…”

“…It occurs as massive evaporite ore beds in the Piceance Creek Basin of Colorado (e.g., Dyni, 1996) and the Anpeng deposit in Henan Province of China (e.g., Wang et al, 1991), and as smallsized ore beds, pockets, lenses or concretions in some other geological basins (e.g., Foshag, 1940;Mees et al, 1998;Sugitani et al, 2003). In addition, nahcolite has been reported as a mineral hosted in fluid inclusions in quartz (Larsen et al, 1998;Barrie and Touret, 1999;Bakker and Mamtani, 2000;Ram Mohan and Prasad, 2002) and emerald Vapnik and Moroz, 2002) from metamorphic rocks. Nahcolite is also closely related to carbonatitic volcanoes, where it crystallizes from the orthomagmatic carbonatitic fluids and is preserved within fluid inclusions in quartz and apatite (e.g., Rankin and Le Bas, 1974;Vard and Williams -Jones, 1993;Samson et al, 1995).…”

High density carbonic fluids in a slab window: Evidence from the Gangdese charnockite, Lhasa terrane, southern Tibet

“…On the basis of the data from exhumed ultrahigh-pressure (UHP) metamorphic rocks and mantle-derived magmas and xenoliths, the upper crustal fluids are generally considered to be dominated by H 2 O, with subordinate CO 2 , CH 4 , and N 2 , whereas the lower crustal fluids are mostly CO 2 -rich (e.g., Touret, 2009;. The growing importance of CO 2 -rich fluids associated with major orogenic cycles is reinforced by the numerous works in this topic including those of Harlov (2000), Agrad et al (2000), Bakker and Mamtani (2000), Bolder-Schrijver et al (2000), Touret (2001Touret ( , 2009, Tsunogae et al (2002), Mohan et al (2003), Cuney et al (2007), Santosh and Omori (2008) and Santosh and Kusky (2009), among others. Some workers have attempted a combination of mineralogic thermobarometry with microthermometric data of high density carbonic fluid inclusions in anhydrous granulites and charnockites to substantiate the model that CO 2 has been instrumental in the formation and stabilization of the mineral assemblages in these rocks (e.g., Mohan et al, 2003;Santosh and Tsunogae, 2003;Cuney et al, 2007;Santosh et al, 2011;Tsunogae and Santosh, 2011).…”