Petrogenesis and U-Pb zircon dating of Chaitma Alkaline Complex from the southern margin of the Central Indian Tectonic Zone: geodynamic implications

In this study, we constrain the petrogenesis and U-Pb zircon age of a newly discovered alkaline complex, christened the Chaitma Alkaline Complex at the southern margin of the Central Indian Tectonic Zone in Central India. The Chaitma Alkaline Complex comprises syenites and gabbro, emplaced coevally and show features consistent with magma mixing. Geochemically, syenites are potassic to ultrapotassic (K2O/Na2O: 0.79-3.42) and contain high Ba (∼800 to 2700 ppm) and Sr (∼1400-3200 ppm). They show enrichment of the light rare earth elements (LREE) relative to the heavy rare earth elements (HREE) (La/Yb: 32-103) and do not display Eu-anomaly. Based on their geochemical signatures such as low MgO (<0.87 wt.%), Ni (8-16 ppm) and Cr (7-44 ppm) contents and prominent Zr-Hf negative anomaly, the syenites are inferred to have been derived by partial melting of a carbonated/metasomatised thickened lower crustal source. The coeval gabbros are undersaturated in silica (41-44 wt.%) with relatively high total alkalis (Na2O+K2O: 3.7-5.1 wt.%), Fe2O3 (17-19 wt.%), P2O5 (3.1-4.9 wt.%), Sr (1600-3400 ppm) and Ba (300-3500 ppm) contents. These have low MgO (<4.8 wt.%), Ni (13-30 ppm) and Cr (18-84 ppm). Their chemistry is interpreted to be the result of interaction with the syenitic magma. These geochemical characters along with high LREE/HREE ratio, negative trough in Nb-Ta, Zr-Hf, Ti, Sr and Rb and positive spike of Pb in spider diagram, and enrichment of LILE over HFSE indicate their derivation from metasomatised subduction modified garnet-peridotite mantle source. Our study indicates that syenites and gabbros of the Chaitma Alkaline Complex were formed from genetically unrelated parental magmas derived from distinct sources. U-Pb dating of zircon yielded magmatic emplacement age of 1626±15 Ma for the syenites. The Chaitma Alkaline Complex was presumably formed during a short period of crustal extension in the midst of a protracted period of continent-continent collision and granulite grade metamorphism (c. 1.71-1.58 Ga) at the southern margin of the Central Indian Tectonic Zone.

Tourmaline growth and evolution in S-type granites and pegmatites: constraints from textural, chemical and B-isotopic study from the Gangpur Schist Belt granitoids, eastern India

An extensive dataset of major- and trace-element and B-isotope composition related to the compositional variation of the tourmalines from the S-type granite and pegmatite from the Upper Bonai and Gangpur Group granitoids, eastern India, is provided. The Gangpur Schist Belt, consisting of the Upper Bonai and Gangpur Group, lies at the eastern end of the Proterozoic Central Indian Tectonic Zone. An attempt to constrain the source of the boron isotope and describe the magmatic–hydrothermal evolution of the granite–pegmatite system is made. The tourmalines record generally low trace-element concentrations, with the exception of some elements such as Zn, Li, Ga, Mn and Ti. The Zn and Li concentration increases from the tourmalines in the granites to the tourmalines in the pegmatite. Trace-element compositions also vary slightly from the core tourmaline to the rim tourmaline. Tourmalines in granites/pegmatites are mostly Fe-rich schorl, and have S-type B-isotopic signature (δ11B = −10‰ to −12‰). Their chemistries reflect changing salinities of the granitic melts as well as vapour exsolution during progressive crystallization. This study indicates that the tourmalines in the pegmatites crystallized from evolved flux-element-rich (F, B, Li) melt, probably after the crystallization of granite-forming melt.