Short-lived alkaline magmatism related to the Reunion plume in the Deccan Large Igneous Province: inferences from petrology, 40Ar/39Ar geochronology and palaeomagnetism of lamprophyre from the Sarnu-Dandali Alkaline Igneous Complex

Available geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Reunion mantle plume based on the 40Ar/39Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali Complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40Ar/39Ar age determinations of the phlogopites from this lamprophyre reveal two distinct ages of 65.44 +/- 1.5 Ma and 68.17 +/- 1 Ma. However, palaeomagnetic results show a VGP at 32.31 degrees N and 298.52 degrees E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at c. 65.44 +/- 1.5 Ma. Analysed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of c. 68.17 +/- 1 Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an 01B -type enriched (metasomatized) mantle source with an involvement of phlogopite. This finding suggests that the pre-Deccan ages of c. 68-69 Ma reported previously may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at c. 68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short duration (between 67-65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Reunion (Deccan) plume. Available geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Reunion mantle plume based on the 40Ar/39Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali Complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40Ar/39Ar age determinations of the phlogopites from this lamprophyre reveal two distinct ages of 65.44 +/- 1.5 Ma and 68.17 +/- 1 Ma. However, palaeomagnetic results show a VGP at 32.31 degrees N and 298.52 degrees E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at c. 65.44 +/- 1.5 Ma. Analysed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of c. 68.17 +/- 1 Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an 01B -type enriched (metasomatized) mantle source with an involvement of phlogopite. This finding suggests that the pre-Deccan ages of c. 68-69 Ma reported previously may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at c. 68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short duration (between 67-65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Reunion (Deccan) plume.