40AR-39AR GEOCHRONOLOGY OF NARCONDAM ISLAND VOLCANO, ANDAMAN-INDONESIAN ISLAND ARC, ANDAMAN SEA: VOLUMINOUS ANDESITIC ERUPTION AT 0.56 MA AND PRESERVATION OF >1.8MA OLD PLAGIOCLASE XENOCRYSTS

Narcondam Island, a dormant volcano in Andaman Sea, is a part of Andaman-Indonesian island arc related to oblique subduction of Indian plate beneath the Burma microplate. The Narcondam volcanics are largely andesitic in nature while the only other volcano in the Andaman Sea, an active Barren Island volcano, situated ~140 km southwest of Narcondam Island has mainly basaltic flows. Narcondam andesites are proposed to be either a result of magma mixing or crystal fractionation. These have plagioclase as a dominant phenocryst with amphibole, pyroxenes and rarely biotite being other phenocryst-sized phases.

We report results from ⁴⁰Ar-³⁹Ar step-heating dating of plagioclase grains and groundmass separated from two samples collected from two different parts of the island. Sample NCI09/05 was collected from northern part of the island at ~ 100 m from the sea level, while sample NCI09/19 was collected from the southern flank at sea-level. These samples have 58.1% and 60 .4% SiO₂ and 4.52 % and 5.20% total alkali, respectively. Groundmass of NCI09/05 yielded an age of 0.56 ± 0.1 Ma (2σ) for 96. 8% of Ar released. Plagioclase separated from this sample yielded an age of 1.8 ± 0.3 Ma (2σ) for 66.1 % of Ar released. The groundmass of other sample NCI09/19 yielded plateau age of 0.56 ± 0.01 Ma (2σ) for 66.3 % Ar released and plagioclase grains yielded plateau age 3.46 ± 0.15 Ma (2σ) for 53.3% Ar released.

The indistinguishable ages yielded by the groundmass of both the samples record timing of a voluminous subaerial eruption of andesite at 0.56 Ma. This is consistent with an earlier study (Streck et al. 2011) which reported whole rock age of 548 ± 22 ka. The much older ages of the plagioclase grains separated from these samples clearly reveal the presence of xenocrystic grains entrained from an earlier crystallized magma probably at shallow crustal level. The time difference between the host lava flow and the xenocrysts, which is more than a million year, indicates longevity and protracted nature of shallow crustal level storage and ascent of magma prior to eruption. This might be an important factor in understanding the generation of Narcondam andesites.

Reference:

Streck M.J., Ramos F, Gillam A, Haldar D, Duncan R (2011) in J. Ray et al. (eds), Topics in Igneous Petrology, Springer Sci, 241-273.