Geochemistry of the Neoproterozoic Volcanic Rocks of the Nakora Area of Malani Igneous Suite, Barmer District, Western Rajasthan, India

The Neoproterozoic rocks of the Nakora Ring Complex (NRC) consist of three phases (Extrusive, Intrusive and Dyke) that are based on the detailed geological mapping (contact relationship, mode of occurrence, position of xenolith, flows, dykes and veins) and their stratigraphic position. NRC consists mainly of acid volcanic rocks besides minor amount of basic rocks with intermediate calc-alkaline to tholeiitic affinities and occurs in the form of ring structures. The Nakora basaltic rocks show LREE enriched nature and they have consistent negative Nb, Ta, Sr and Zr anomalies. The HREE pattern is showing parallel arrangement with HREE pattern of other basic rocks. The Nakora acid volcanic rocks exhibit high LREE enrichment than the HREE with negative Ba, Sr, Eu and Ti anomalies in primitive mantle normalized multi-element diagrams. All the samples show negative Ba, Sr, Eu and Ti anomalies. The diminution in amounts of Sr and Eu is apparently related to the fractionation of feldspars or their retention in the refractory minerals resistant to melting in the lower crust. As compared to trachytes, the rhyolites show high SiO2, high Al2O3, low total alkalis, low total iron, low TiO2, high CaO and high MgO. The petro-mineralogical and geo-chemical data specifies that the NRC rocks are generated from a co-magmatic source through a co-genetic process in a rift tectonic context. Petrogenetic modeling indicates that both the basic rocks and acid volcanic rocks of Nakora may have been derived from rocks akin to Bhilwara mafic metavolcanic/mixed Nakora gabbros and Siwana rhyolite/banded gneiss from Kolar Schist Belt by different degrees of partial melting respectively.

Geochemistry and petrogenesis of Neoproterozoic A-type granites at Nakora in the Malani Igneous Suite,Western Rajasthan,India

The Nakora Ring Complex(NRC)(732 Ma) occurs as a part of Malani Igneous Suite(MIS) in the West-ern Rajasthan,India.This complex consists of three phases(volcanic,plutonic and dyke).Geochemically,the Na-kora granites are peralkaline,metaluminous and slightly peraluminous.They display geochemical characteristics of A-type granites and distinct variation trends with increasing silica content.The peralkaline granites show higher concentrations of SiO2,total alkalies,TiO2,MgO,Ni,Rb,Sr,Y,Zr,Th,U,La,Ce,Nd,Eu and Yb and lower concen-trations of Al2O3,total iron,Cu and Zn than metaluminous granites.AI content is ≥1 for peralkaline granites and <1 for peraluminous and metaluminous granites.Nakora peralkaline granites are plotted between 4 to 7 kb in pressure and are emplaced at greater depths(16-28 km and 480-840℃) as compared to metaluminous granites which indicate the high fluorine content in peralkaline granites.The primitive mantle normalized multi-element profiles suggest that Nakora granites(peralkaline,metaluminous and peraluminous) are characterized by low La,Sr and Eu and relatively less minima of Ba,Nb and Ti which suggests the aspects related to crustal origin for Nakora magma.The Nakora granites are characterized as A-type granites(Whalen et al.,1987) and correspond to the field of "Within Plate Gran-ite"(Pearce et al.,1984).Geochemical,field and petrological data suggest that Nakora granites are the product of partial melting of rocks similar to Banded Gneiss from Kolar Schist Belt of India.