PETROGRAPHY AND PETMGCHEMISTEY OF IMPACT MELTS FROM LONAR CRATER, BULDANA DISTRICT, MAHARASHTRA, INDIA

摘要

The Lonar crater in Buldana district, Maharashtra has long been established as the largest hypervelocity impact crater of the world on Deccan basalt flows, although no extraterrestrial fragment could be located from geological, geophysical, geochemical and drilling approach. Considering the impact origin, the crater was revisited with an aim to throw light on the nature of target rocks and its impact melts. The country rock, prior to the extra-terrestrial impact, has been distinguished as a slightly oversaturated continental tboleiitic basalt with a distinct chemical signature. It is low in iroe and alkali and relatively high in magnesia compared to mean composition of Deccan basalt. Some amount of variation has been indicated by Cr, Ni, V, TH and Ta in relation to Mg No.. Petrogenetically, it has been identified as a secondary basaltic (less evolved) magma (Mg No. 54.7) of more subalkaline nature (alkalinity=1.39). Neither pyroxene-nor fractionation has fractionation has played any significant role in the magmatic evolution as indicated from texture and petrochemistry including the REE fractionation pattern. Petrochemistry of the glass ejecta (impact melt) and throw-out basaltic fragments do not reveal much significant change in their bulk composition from that of basalts of the area except some shock-induced optical features. Lonar impact melt exhibits flow banding and contortions along with vesicles of varying size, shape and proportions. Evidence recorded in the target rock as well as in glass indicates both equilibrium and non-equilibrium shock pressure upto more than 1000 kb. Available major minor- and trace-element chemistry of the impact melt when compared to that of target rock reveals that the release of high-magnitude volatile flux in plasma state from the projectile during the impact has caused heterogeneities in melt composition and dispersal of the plagioclase fraction-enriched component of the impact melt.