5:15 PM - 7:15 PM
[SMP27-P07] Precambrian mantle dynamics of the Western Dharwar Craton, India: Comprehensive petrogeochemical, multi-isotope, and paleomagnetic evidence from mafic dyke swarms.
Keywords:Precambrain, Mafic dykes, Dolerite, Mineral chemistry, Mantle dynamics
Archean cratons which are the remnants of Earth's earliest formed continents, reflect a complex interplay of mantle dynamics, geochemical differentiation, and tectonic history. Multiple episodes of magmatism and tectonism characterize these cratons and the sub-continental lithospheric mantle (SCLM) beneath undergoes continuous geochemical modification, resulting in either depletion or enrichment through interactions with ascending magmas, subduction, recycling, and volatile-induced reactions. These terranes are typically composed of tonalite-trondhjemite-granodiorite (TTG) gneisses, greenstone belts with ultramafic to felsic volcanic sequences including komatiites, basaltic volcanics, and dolerite dykes, banded iron formations (BIFs), metasedimentary rocks, and felsic intrusives reflecting the complex mantle dynamics. Several generations of Precambrian dolerite dykes are common in these cratons, evealing critical details about the composition and dynamics of their mantle source at the time of formation.The current study examines Precambrian mafic dykes from the Western Dharwar Craton, southern India, to explore their connection to mantle processes and the craton's tectonic evolution. Multiple generations of mafic dykes are exposed all along the Dharwar Craton with or without a clear cross cutting relation with the basement gneisses. Western Dharwar Craton is the oldest continental nucleus and is distinct from the Eastern Dharwar craton in terms of lithology, age and tectonics. The reported ages of the mafic dykes are also older in the Western Dharwar craton (~3.0 Ga to 1.4 Ga?) and younger in the Eastern Dharwar craton (2.37~1.89 Ga).
In the Western Dharwar Craton, the dykes predominantly intrude the basement gneiss, trending NE-SW and NW-SE, with a few occurring along E-W and N-S orientations. These dykes are mainly composed of dolerite, gabbro, and olivine norite. Their mineral assemblages consist largely of olivine, plagioclase, clinopyroxene, and orthopyroxene, with many showing characteristic ophitic or poikilitic textures. The oldest generation of dolerites (~3.0 Ga metadolerite dykes) exhibits signs of metamorphic alteration while still preserving its original mineralogy and texture. Remnants of ophitic textures, including partially preserved lath-shaped plagioclase and amphibole needles growing along the rim of clinopyroxene, are observable. These metadolerites are considered the oldest intrusives, originating from the most primitive mantle source. Their rare earth element patterns and Sr-Nd isotope compositions indicate a depleted source, while the sulfur isotope data, showing a mixture of mass-dependent and mass-independent fractionation, suggesting the addition of sulfur from recycled oceanic sediments. The paleomagnetic record of these dykes shows evidence of a metamorphic overprint. Younger dolerite dykes are classified into three groups based on mineral composition and geochemical characteristics. A gradual enrichment of the subcontinental lithospheric mantle, from a depleted MORB source mantle to an enriched mantle due to subduction and recycling processes, is observed from the oldest olivine dolerite dykes (~2.7 Ga) to the younger enriched dolerite dykes. The paleomagnetic data for the enriched dolerite dykes overlap with those of the oldest 2.3 Ga dykes found in the Eastern Dharwar Craton. Furthermore, the nature and composition of the source mantle for each group of dykes are determined, with the degree of partial melting and fractional crystallization being characterized using mineral trace element and rare earth element concentrations.
In the Western Dharwar Craton, the dykes predominantly intrude the basement gneiss, trending NE-SW and NW-SE, with a few occurring along E-W and N-S orientations. These dykes are mainly composed of dolerite, gabbro, and olivine norite. Their mineral assemblages consist largely of olivine, plagioclase, clinopyroxene, and orthopyroxene, with many showing characteristic ophitic or poikilitic textures. The oldest generation of dolerites (~3.0 Ga metadolerite dykes) exhibits signs of metamorphic alteration while still preserving its original mineralogy and texture. Remnants of ophitic textures, including partially preserved lath-shaped plagioclase and amphibole needles growing along the rim of clinopyroxene, are observable. These metadolerites are considered the oldest intrusives, originating from the most primitive mantle source. Their rare earth element patterns and Sr-Nd isotope compositions indicate a depleted source, while the sulfur isotope data, showing a mixture of mass-dependent and mass-independent fractionation, suggesting the addition of sulfur from recycled oceanic sediments. The paleomagnetic record of these dykes shows evidence of a metamorphic overprint. Younger dolerite dykes are classified into three groups based on mineral composition and geochemical characteristics. A gradual enrichment of the subcontinental lithospheric mantle, from a depleted MORB source mantle to an enriched mantle due to subduction and recycling processes, is observed from the oldest olivine dolerite dykes (~2.7 Ga) to the younger enriched dolerite dykes. The paleomagnetic data for the enriched dolerite dykes overlap with those of the oldest 2.3 Ga dykes found in the Eastern Dharwar Craton. Furthermore, the nature and composition of the source mantle for each group of dykes are determined, with the degree of partial melting and fractional crystallization being characterized using mineral trace element and rare earth element concentrations.