Separating regional metamorphic and metasomatic assemblages and events in the northern Khetri complex, NW India: Evidence from mineralogy, whole-rock geochemistry and U-Pb monazite chronology

The distinction between regional metamorphic and metasomatic mineral assemblages may be occasionally challenging when the two phenomena show a very close spatial and temporal association. The Khetri complex in NW India, provides an excellent example of a geological terrane that has been significantly affected by metamorphism followed by extensive metasomatism. The albitic quartzite, orthoamphibole-cordierite quartzite and chlorite metapelite in northern Khetri complex are the outcome of Na-Cl-Fe-Mg metasomatism. The andalusite metapelite and associated scapolite-bearing metasediments, however, do not show any significant metasomatic overprint, and the former rock unit has been used to deduce the metamorphic conditions, nature of provenance and weathering processes. The peak P-T conditions inferred from the conventional geothermobarometry and pseudosection modelling are consistent at 550°C and 3.5 kbar. The geochemical data suggest that the source rocks of the metapelites were compositionally mature and experienced intense chemical weathering in a passive margin setting. The metasomatic overprint postdates the regional metamorphism and, the new monazite U-Th-Pb age data constrains the age of metasomatism at 900-850 Ma and that of the regional metamorphism at ca. 975 Ma in the Khetri complex.