The Paleo-Mesoproterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex is exposed in parts of Northeast India. The studied metadolerites are from the volcano-sedimentary sequence of Shillong Basin from the Borjuri area in the Mikir Massif. This episode of mafic magmatism can be correlated with the Columbia supercontinent formation and bears significance to its reconstruction. The present work discusses the field, petrography and geochemical characteristics of the metadolerites, which occur in close association with the quartzites of the Shillong Group of rocks (metasedimentary rocks of the Shillong Basin). Our data show distinctive characteristics of subduction-related magmatism exhibiting high LREE/HREE, large ion lithophile element/high field strength element ratios and pronounced negative Nb anomaly. Elemental ratios such as Zr/Ba (0.21–0.46), La/Nb (1.23–2.32) and Ba/Nb (30.08–56.90) point to a fluid-enriched lithospheric mantle source in a subduction regime. Metadolerites plot in the field of ‘back-arc basin basalts’ in tectonic discrimination diagrams reinforcing a subduction zone tectonic setting. The mafic rocks correspond to a 6–10 % partial melting of a mantle source incorporating spinel+garnet lherzolite. The metamorphic P-T of the metadolerites estimated from plagioclase-hornblende geothermobarometer (7–8 kbar, 664 °C) is indicative of amphibolite facies metamorphism in a medium P-T zone. Based on the comparative analysis of field observation, petrography, geochemistry and geological ages given by previous workers, we infer that the Shillong Basin represents a back-arc rift region and is the eastern continuation of the Bathani volcano-sedimentary sequence of the Chotanagpur Granite Gneiss Complex marking continuation of the Central Indian Tectonic Zone to the Mikir Massif.

The Central Indian Tectonic Zone demarcates the zone of amalgamation between the North Indian Craton and the South Indian Craton. Presently, the major controversies in the existing tectonic models of the Central Indian Tectonic Zone revolve around the direction of subduction and the precise timing of accretion between the North Indian Craton and the South Indian Craton. A new model for the tectonic evolution of the Central Indian Tectonic Zone is postulated in this contribution, based on recent geological and geophysical evidence, combined with previously documented tectonic configurations. The present study employs the slab break-off hypothesis and subsequent polarity reversal to explain the tectonic processes involved in the evolution of the Central Indian Tectonic Zone. We propose that the subduction initiated (c. 2.5 Ga) in a S-directed system producing island-arc sequences on the South Indian Craton. The southward subduction regime culminated with slab break-off underneath the South Indian Craton between c. 1.65 Ga and 1.55 Ga, which subsequently induced subduction polarity reversal and set the course for N-directed subduction (<1.55 Ga). The final closure along the Central Indian Tectonic Zone is governed by the collisional regime during the Sausar Orogeny (1.0–0.9 Ga).