PETROLOGY OF MALANI VOLCANIC ROCKS WITH SPECIAL EMPHASIS ON THE GENESIS OF RHYOLITES

Abstract

The late Proterozoic rocks of Malani Igneous suite are spread for about 51,000 km2 inseven districts of western Rajasthan and extend further in oind province of Pakistan. Malani is the largest anorogenic acid volcanism in India and perhaps in the world also, showing little metamorphic effect. The hrinpura granite (850 m.y.) is the dominant basement rock of the complex in southern peripheries. The volcanism is associated with post mt. Abu granitic activity and pre-Parwar sedimentation covering a span of 8D-0 to 550 m.y. and some of them were emplacecl at least 745 ± 10 m.y. ago (Crawford and Cornpston, 197Q). Based on the extensive field studies of the area following stratigraphic sequence is suggested by this author. Piarwar oupergroup ———— . Nonconformity • Phase III. Acid and basic dykes Phase II. Alkali feldspar and aegrine/riebeckite bearing granites occurring as batholiths, plutons and ring dykes Phase I. Acia pyroclostics: Acid lava flows 3asic lava flows —— Unconformity Erinpura and mt. Abu batholiths.

On tiie basis of field evidences, mode and type of magmatism,texture and composition, three stages of igneous activity nave been recognised during phase-I Malani volcanism. The volcanics associatea with the first stage of activity are mostly basalt with occasional andesite or trachybasalt flows. Out of the 12 representative mafic roc^s it is noted that seven are tnoleiitic and the rest are alKalic. The mafic rocKs are subsequently covered by the voluminous outpouring of peralxaline and peraluminous rhyolite, dacite and trachyte flows during the second stage. The third stage ceased with the outburst of ash flows ae^.osits. The dominant felsic volcanics of Malanis (second stage) 2 are rhyolites and rhyodacites spread for about 31,000 km . The other rock types associated v/ith rhyolite are trachytes, dacites, pitchstone, welded tuff, vitric, lithic and crystal ash, ignimbrite, obsidian, pyroclastic slates, agglomerate, volcanic breccia and volcanic conglomerates. The cumulative thiCKness of felsic volcanics exceeos 3.5 Km in 95 cumulative flows. On the basis of 70 analyses it is found that 96/. of the acid vol canics are high potassic uid 2/. each as calcalicaline or low potassic.

The final eruptive stage witnessed ash flow 2 deposits for about 350 km near Pokaran. These ash beds are more than 50 m thick composed essentially of light yellow and light brown lithic, crystal and vitric ash having microscopic xenoliths of basalt and rhyolite embeded in it. The tuffs are generally of rhyolitic c opposition. The EPPA of clinOjjyroxene from the basalts plot in the augite field. The analysis show up to 6.75 wty. of CaTiA^Cv molecule is contained in pyroxene as solid solution, The probe analysis of K-feldspar in welded tuffs show that sanidine contains 10 to 13;4 albite and 2/ anorthite. The muscovite of welded tuff contains about 10 wt/o phlogopite and annite in solid selution. The feldspars phenocrysts in peralkaline rhyolite plot near the two end members of th-e join KAloi^Op and K-feldspar are seen to coexist with pure albite. The composition of alkali feldspar in trachyte vary between anorthcclase and sanidine. The significant variation in feldspar indicate that they were not in equilibrium with the liquid. The clinopyroxene has up to 44/* hedenbergite and 6/ diopside. Determination of the composition of ampnibole shows that it is a solid solution of riebeckite and arfvedsonite. The feldspar geothermometry suggests the temperature of equilibrium to be 650°C. Similar results wore obtained by magnotite-ulvospinel geothermometry. The oxygen fugacity

is estimated to be about 10"18 under FMQ-NNO buffer conditions. Based en 89 major oxide analysis end 120 trace element data, e typical bimodal distribution is noted with a single cycle of mafic volcanics followed by mother cycle of felsic volcanics* Mo differentiation trend could be seen, Malenis show e close similarity with the bimodal distribution involing rhyollte-basalt association, as noted in SW Australia. Malani volcanics occur towards the western side of tightly compressed Delhi-Arcvalli folded belt. A possible sudCi&n Release of pressure, causing rifting may have triggered the eruption. N7-SE trading fissures and several calderas and cones are the possible mode of eruption. The eruptions were essentially under terrestrial conditions, although depositssunder aqueous conditions are seen at two localities. The initial basaltic magmr was possibly generated at deeper depth. This magma while migration upwr.rds supplied additional heat for the partial melting of lower sialic crust resulting in the generation of felsic magmas. The peralkaline rhyclites are generally seen associated with the central type of eruption is Malani volcanism he increr.se in volalite content during the period of quiescence increased the alkalinity of the liquid-a fact substantiated by the shift in the position of the granite minimum under higher pressure in presence of water. solid/liquid Using partion coefficient (KwholG rock )data for the trace elements (Mn, Ba and Sr), a thermodynamic calculation was m de to see i^ partial inciting of Erinpur; pluton could produce a liquid with trace element content similar so that of Malani rhyolite?. It is noted that Erinpura pluton does not have the requisite chemistry tc become a source rock for the acid volcanics.