NEOPROTEROZOIC ACID MAGMATISM IN WESTERN INDIA
MANOJ K. PANDIT
Department of Geology, University of Rajasthan, Jaipur 302004 - India
Presented by ALCIDES N. SIAL
'Neoproterozoic' signifies an eventful period in the global tectonics as it coincides with amalgamation of different continental fragments to form a supercontinent 'Rodinia' and its subsequent break-up. It is essential to evaluate the interrelationship of different events in individual continental blocks of the Gondwana and to synthesize the pieces of evidence to reconstruct the Gondwna assembly. Although the geological evidence provide a basis for relative timing of events, the absolute ages are fundamental in attempting any meaningful correlation of the events that are now represented in widely spaced continents. The 770-750 Ma Malani Igneous Suite (MIS) in western India represents the Indian examples with analogues identified in Seychelles and Madagascar as coeval magmatic events. Some salient characteristics of the MIS are presented here.
Spread over an area of > 51, 000 km2 in the state of Rajasthan (NW India), the MIS is considered to be the third largest felsic volcanic province of the world. However, the magmatism is polyphase in nature with predominant felsic volcanics (at places with minor basic flows at the base) with granites and a wide array of dyke rocks (rhyolite, trachyte, dolerite/gabbroic) that intrude the earlier lithologies. Geochemically the felsic rocks (volcanics and granites) are peraluminous and peralkaline, both the varieties defining a systematic spatial relationship with each other, however, the former is the predominant facies. Geochemical features are typical of A-type granites, such as high abundances of silica, alkalis, LREE, HFSE, LILE and a high Fe/Mg ratio, with extreme enrichment of such elements in the peralkaline rocks. The geochemical features, in concert with non-orogenic emplacement, indicate derivation from high temperature melting of a lower-crustal source under thinning crust, induced by an underplating basic magma. The 'peralkalinity' was introduced by some mantle flux. Available geochronologic information (mainly Rb-Sr whole rock) is contentious as one group infers a protracted magmatic history (680-780 Ma) whereas another proposes a very short event at 730 Ma. Our U-Pb zircon data indicating 770-750 Ma for peraluminous rhyolites and granites supports a rather limited time span and coevality between volcanic and plutonic episodes. The geochronologic and geophysical evidence suggest a spatial contiguity of MIS with Seychelles and Madagascar, possibly representing the western margin of Rodinia. ( May 18, 2001 )
Publication Dates
-
Publication in this collection
08 Oct 2001 -
Date of issue
Sept 2001
