Geochemistry and Sm-Nd geochronology of the metasomatised mafic rocks in the Khetri complex, Rajasthan, NW India: Evidence of an Early Cryogenian metasomatic event in the northern Aravalli orogen

The mafic magmatic rocks associated with 1720-1700 Ma albitised A-type granites in the northern segment of the Aravalli orogen, NW India show evidence of metasomatism. It is, however, not clear whether the metasomatism of mafic rocks is related to the cooling of these associated granites or whether it took place much later after the emplacement of the granites on a regional scale. For this, we have investigated the mafic magmatic rocks, which occur in close association with these granites. In the Biharipur intrusive, the mafic rocks are intensely commingled with the A-type granites, whereas in the vicinity of the Dosi intrusive, the mafic rocks (clinopyroxenite) do not show any evidence of granite mingling. The commingled and metasomatised Biharipur mafics occur in contact with the albitised granites instead of original granite, indicating that the mafics were metasomatised along with the granites. This is supported by the similarity in REE and spider patterns of the intermixed mafic rocks and the albite granites. On the other hand, the Dosi mafic rocks, free from granite commingling, are scapolitised where the original diopside has been partly transformed to chlorine-rich marialites with a meionite component ranging from Me14.0 to Me16.0. The scapolite, occurring as anastomosing veins, within these rocks is also of similar composition, and the undeformed nature of these veins suggests that the scapolitisation postdates regional metamorphism in the region. Mineralogical, geochemical and Nd isotopic characteristics of the mafic rocks indicate that originally, these were clinopyroxenites, which have been altered to a monomineralic actinolite-bearing rock. The immobile incompatible trace element ratios indicate a continental tholeiite affinity for the mafics, which is in consonance with the A-type nature of the associated granites. During this metasomatic event, the mafic magmatic rocks experienced albitisation and scapolitisation, although the dominance of these processes varied on a local scale depending on the fluid composition. A whole-rock-mineral (clinopyroxene and scapolite) Sm-Nd isochron of the scapolitised clinopyroxenite at Dosi yields an age of 831 15 Ma. Synthesis of this age data along with previously published geochronological data indicate an important Early Cryogenian (850-830 Ma) metasomatic event in the northern Aravalli orogen, which is also synchronous with the Erinpura granite event in the southern Aravalli orogen. (C) 2012 Elsevier Ltd. All rights reserved.