Abstract
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.
Highlights
The rock exposure revealed in the Nakora Ring Complex (NRC) is from the Trans-Aravalli Block’s (TAB) Neoproterozoic Malani Igneous Suite (MIS)
The selected acid volcanic rocks are lying in the field of rhyodacite/dacite and the Nakora basic rocks lie in the field of andesite/basalt (Figure 5)
The Nakora basalts show the close affinity to continental field in the MgO-Fe2O3-Al2O3 diagram (Pearce et al 1977) [60]
Summary
The rock exposure revealed in the Nakora Ring Complex (NRC) is from the Trans-Aravalli Block’s (TAB) Neoproterozoic Malani Igneous Suite (MIS). The rocks of MIS are granite (hypersolvus and subsolvus), rhyolite, dacite, trachyte, andesite, pyroclasts, basalt, gabbro and dolerite. They have been encountered in the form of ring structures, ring dykes, residual hills, inselbergs and scattered hummocks where the exposures are covered by sand dunes. La Touche (1902) [36] provided initial accounts on the geology of the suites of rocks belonging to MIS. He first sighted Malani Igneous Suite in which the rocks are well exposed in the erstwhile region of Malani in Western Rajasthan.
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