Global Intracratonic Boninite-Norite Magmatism during the Neoarehean -- Paleoproterozoic: Evidence from the Central Indian Bastar Craton

摘要

Although boninites and boninite-like rocks formed throughout Earth history and have been emplaced in different tectonic settings, they are most common in Phanerozoic settings where they are associated with subduction-related magmatism. However, Neoarchean -- Paleoproterozoic noritic intrusions, which have bulk-rock geochemistry closely similar to boninites, occur in many Archean terrains, chiefly emplaced in intracratonic settings. Thus, noritic intrusions are probably derived from melts similar to those of boninites. The best examples of such boninite-norite associations occur in the Bushveld, southern West Greenland, East Antarctica, Wyoming, northwest Scotland, and the eastern Fennoscandian shield. A similar boninite-norite suite also is present in the central Indian Bastar craton. These high-Mg mafic igneous rocks were emplaced as dikes and volcanic rocks during Neoarchean -- Paleoproterozoic time in an intracratonic rift. They contain high Si (>52 wt percent), high Mg (>8 wt percent), and low Ti (mostly <0.5 wt percent) and are true boninites. Based on Al_2O_3/CaO ratios, these rocks may be classified as high-Ca and low-Ca boninites. The high-Ca rocks show typical boninitic geochemical characteristics, whereas the low-Ca variety shows geochemical characteristics similar to high-Mg norites. Bulk-rock compositions of these two varities of high-Mg igneous rocks suggest a comagmatic relationship through a high-magnesium mafic melt derived from a metasomatized, highly refractory mantle. Very close geochemical similarities exist between the Bastar and boninite-norite suites worldwide. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchean -- Paleoproterozoic. Many Archean terrains were united as a supercontinent as expanded Ur and Arctica at this time, and its rifting gave rise to numerous mafic dike swarms, including boninite-norite.