ALBERT

Description: Massif-type charnockites of the Eastern Ghats granulite terrain, India, abound in mafic enclaves, which are generally hornblende mafic granulites with relatively minor occurrences of pyroxenite enclaves in the marginal segments only. The mafic granulite enclaves may be interpreted as earlier mafic melts within plutonic charnockite, where prograde heating in the hornblende in these mafic granulite enclaves was probably due to the host charnockite crystallization. Pyroxenite enclaves, on the other hand, are likely to be cumulates from an episode of mafic magmatism. The trace-element characteristics of hornblende-mafic granulite xenoliths are akin to arc-derived basalt, indicating a tectonic setting of subduction and slab melting. Further, low values of primitive mantle-normalized Nb/U ratios and enriched radiogenic isotopic compositions in the mafic xenoliths clearly indicate recycled continental crust in the mantle source region. While most of the internal segments of the Eastern Ghats mobile belt are Paleoproterozoic domains, the marginal (cratonic) segments in the north and west are Archean domains. The average Nd model age of ca. 2.5 Ga for the protoliths of hornblende-mafic granulite xenoliths for the Paleoproterozoic domains may be interpreted as the age of arc magmatism. For the marginal segments, the average Nd model age of ca. 3.3 Ga probably represents earlier Archean arc magmatism. Initial 87Sr/86Sr ratios calculated at these periods of mafic magmatism are high, which, together with negative {varepsilon}Nd values calculated for 2.5 and 3.3 Ga mafic magmatism, indicate recycled continental crust in their mantle source region. However, juvenile crustal addition seems to have been significant at 2.5 Ga, as is evident from the positive {varepsilon}Nd values for the majority of samples representing 2.5 Ga magmatism.

Description: Charnockite is considered to be generated either through the dehydration of granitic magma by CO 2 purging or by solid-state dehydration through CO 2 metasomatism during granulite facies metamorphism. To understand the extent of dehydration, CO 2 migration is quantitatively modeled in silicate melt and metasomatic fluid as a function of temperature, H 2 O wt%, pressure, basal CO 2 flux and dynamic viscosity. Numerical simulations show that CO 2 advection through porous and permeable high-grade metamorphic rocks can generate dehydrated patches close to the CO 2 flow path, as illustrated by the occurrences of "incipient charnockites." CO 2 reaction-front velocity constrained by field observations is 0.69 km/m.y., a reasonable value, which matches well with other studies. On the other hand, temperature, rate of cooling, and basal CO 2 flux are the critical parameters affecting CO 2 diffusion through a silicate melt. CO 2 diffusion through silicate melt can only occur at temperature greater than 840 °C and during slow cooling (≤3.7 x 10 –5 °C/yr), features that are typical of magma emplacement in the lower crust. Stalling of CO 2 fluxing at ~840 °C explains why some deep-level plutons contain both hydrous and anhydrous (charnockitic) mineral assemblages. CO 2 diffusion through silicate melt is virtually insensitive to pressure. Addition of CO 2 basal flux facilitates episodic dehydrated melt migration by generating fracture pathways.