**7. Magmatism and tectonism**

MIS shows the within plate environment that is related to the activity of the hot spot and represents the tensional environment in its spot that is flanked by different elliptical/ring structures [17, 21]. About 700–900 m.y., the magma of the Trans Aravalli MIS erupted. Alkaline magmatism, like subalkaline subvolcanic complexes in lithospheric continents, represents the igneous activity, while the plume gives rise to basaltic oceanic chains/aseismic ridges in oceanic crustal regions. Intraplate, anorogenic existence and extensional tectonic environment are well known by MIS granites and Mahe (750 Ma) and Ste. Anne (764 Ma) Seychelles granites [32] show the similar environment. They are aligned with Siwana (732 Ma old) hypersolvus granites and subsolvus granites from Jalor (MIS).

The higher amount of Zr is reported by Kochhar et al., [33] in the Siwana rhyolites which formed during the Zr crystallization and Nakora area also exhibits the appreciable amount of Zr concentrations. Partial melting and enrichment of alkali elements initiate due to releasing of pressure at a depth which influx the volatiles in to the crust [20]. The peralkaline association of trachyte–rhyolite and basalt within–plate character denotes the zones of crustal extension [34]. The Arabian–Nubian shield shows the tensional tectonic environment which is formed by the bimodal magmatic activity during the uppermost Precambrian crustal evolution [35]. The Nakora area is characterized by the close association of trachyte, rhyolite, granite (peralkaline, peraluminous and metaluminous) with gabbro, basalt and dolerite. Close association of granite is observed with trachyte flows in Mokalsar [36] and Goliya Bhaylan area [37] of Siwana. Granites are associated with gabbro and basalt in Guru Nal area and Jalor area respectively [38]. In view of association of trachyte, rhyolite, basalt and gabbro, with the subsolvus and hypersolvus granite of the Malani area, it is suggested that the model of deep crustal hot zone which is suggested by Annen et al. [39] as outlined above can be applied to the area under study.

A deep crustal hot zone is created in the model by mantle-derived hydrous basalts accumulated as a series of sills into the lower crust. Melts are generated in Nakora area from two distinct sources; partial crystallization of basalt to produce residual H2O-rich melts; and partial melting of pre-existing crustal rocks. Partially melting of the underlying crust, which may involve meta-sedimentary and meta-igneous basement rocks, as well as earlier basalt intrusions, is aided by heat and H2O transfer from the crystallising basalt [39]. According to the evidence in the literature and the geochemical and petrological findings presented, MIS is the result of a complex series of geological processes that include partial melting, fractional crystallisation, magma mixing and assimilation, crustal contamination, and fluid-melt interactions. However, while Nakora is made up of common hydrous minerals such as biotite, amphiboles, apatite, and muscovite, it also contains volatile components in the form of halogens, indicating that the comagmatic rocks went through many phases during their evolution.

Kochhar [40] explained the similarities between Trans–Aravalli Block (TAB) and Arabian Nubian shield (ANS) in terms of granite emplacement, ring structures and cauldron subsidence. In both the terrines, the alkali granites mark the cratonisation of the shield and show evidences of Strutian glaciation. Western Central Medagascar granites near Ambistra were emplaced during extensional collapse of Pan–African orogen and indicate emplacement age of 804–775 Ma. They are *Petrology and Geochemistry of Nakora Ring Complex with Emphasis on Tectonics… DOI: http://dx.doi.org/10.5772/intechopen.98609*

very similar to Jalor granites in terms of gabbroic sleeves [41]. 750 Ma alkaline magmatism which is widespread and well developed in the continents viz., TAB, Central Iran, Arabian–Nubian shield, Medagascar and South China, Somalia, Seychelles [40] were characterized by common crustal stress pattern, rifting and thermal regime, shrutian glaciation and dessiciation and similar paleolatitudinal positions which could be attributed to the existence of a Supercontinent–the Malani Supercontinent. Rogers [42] has also suggested the similarities of their development including production of alkali granites, subsidence of thick partly deformed basin on recently formed crust and ultimate development of platform cover sediments. Hence in future, the significance of mantle plume and crust–mantle interaction should be studied for the emplacement of MIS.

A volcanic vent is observed in NRC with steep slope. It is characterized by elongated or semicircular depression with various volcanic flow products. Volcano– plutonic associations of NRC are located along the Luni River which flows 2 km Northeast of NRC. The Luni River takes sudden 'U' turn from West to South direction which represents the continental rift. Luni rift is an important tectonic lineament in TABLE [19]. This lineament is related to major crustal dislocations of continental rift type for the exrusions and intrusions of the magma [20]. Hence Luni rift served the way for magma rising through various major fractures, it is accompanied by anorogenic volcanism which may precede/follow the emplacement of ring complexes. The Nakora peralkaline granites support the view of association of rifting of crust in the area [43]. The semi–circular ring structure of Nakora area indicates the emplacement of magma into tensional environment. Hence, NRC indicates the presence of rift mechanism operated in the area in their emplacement vis–a–vis the tectonism and volcanism (**Figure 12**).

#### **Figure 12.**

*Schematic presentation of Nakora area (after Annen et al. [39]), showing magma chamber and intrusion basaltic magma producing a zone of melting in the continental crust.*
