International Geologiical Congress - Oslo 2008

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MPI-01 General contributions to igneous petrology

 

Petrogenetic characterization of low-SiO2 and high-SiO2 charnockites

 

Rajesh HM, University of Johannesburg (South Africa)
 

 

Charnockites characterized by orthopyroxene-bearing granitic mineral assemblages are important components of the lower continental crust in many high-grade terrains. A compilation of geochemical data from selected charnockite occurrences from different parts of the world showed the possibility of two main compositional groups, low-SiO2 charnockites and high-SiO2 charnockites. Low-SiO2 charnockites are dominantly calc-alkalic to calcic (in terms of modified alkali-lime index) and predominantly magnesian (in terms of Fe-number), while silicic (high-SiO2) charnockites are alkalic to alkali-calcic to calc-alkalic and predominantly ferroan. Here mangerites (opx-bearing monzonite) and quartz mangerites are an exception. In spite of their low-SiO2, mangerites and quartz mangerites are alkali-calcic to alkalic and predominantly ferroan similar to silicic (high-SiO2) charnockites.
A charnockite massif from southern India, showing spatial association of older low-SiO2 and younger high-SiO2 charnockites, was investigated to evaluate the difference (if any) in their petrogenetic scenarios. Petrographically the low-SiO2 charnockite is distinguished from the silicic type by its relatively low K-feldspar content, greater mafic mineral content and the occurrence of clinopyroxene. Although their close spatial association and near continuous variations in major and trace element compositions point to a genetic link, separate petrogenetic scenarios are suggested for the older low-SiO2 charnockites and younger silicic charnockites. Within the constraints imposed by near basaltic composition of the most mafic samples and their relatively high concentrations of both compatible and incompatible elements, comparison with recent experimental studies on various source compositions, and trace- and rare-earth-element modeling, the distinctive features of the low-SiO2 charnockites can be best explained in terms of assimilation-fractional crystallization (AFC) models involving interaction between a mantle-derived basaltic magma and lower crustal materials. Silicic charnockites on the other hand are high temperature melts of moderately hydrous basaltic magmas. A two-stage model which involves an initial partial melting of hydrous basaltic magma and later fractionation explains the geochemical features of the silicic charnockites, with the fractionation stage most probably an open system (AFC). It is suggested that for massifs showing spatial association of low-SiO2 and high-SiO2 charnockites, a model taking into account of their compositional difference in terms of the effect of variations in the conditions (e.g., temperature, water fugacity) that prevailed, can account for plausible petrogenetic scenarios. Here assimilation fractional crystallization process forms an important mechanism to account for the compositional features of both low-SiO2 and high-SiO2 charnockites.

 

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