GEOCHEMISTRY AND EVOLUTION OF PAVAGARH LAVA FLOWS, GUJARAT

Abstract

Volcanic rock series of Pavagarh area, covering 2 an area of 16 km and involving seventeen flows (total thickness 829 m.) have been the subject of a great deal of controversies in relation to their genesis. Pavagarh lavas, which are considered to be an outlier of Deccan Traps, are underlain by the Nimar sandstone belonging to the Bagh beds (inf'ratrappeans) . The Bagh formation in turn is underlain by the Champaner group of rocks (s Delhi?) which overlie the Aravallis. Study pertaining to lineament and fracture patterns of this area reveals that there are two major trends (NW-SE and NE-SW) along which the faults and major fractures are aligned. They are also found to be subparallel to the one present in the nearby Precambrian rocks of Champaner group. These pre-existing fractures or zone of weaknesses apparently guided the formation of the deep-seated fracture in the vicinity of Pavagarh during the Deccan tectonism (Upper Cretaceous) and the ascent of the magma took place through them. The outpouring of lavas through these fissures later developed a cone at Pavagarh. Two cycles of magmatism could only be established with the help of vertical variation diagrams and each cycle commenced with a picritic flow. The volcanic rocks of Pavagarh exhibit following two cycles of magmatic (iv) episodes in order of their stratigraphic sequence: Flow No. H O >> o o o <D m H O >> o -p 03 U •H 17 16 14-15 13 12 11 8-10a 4-6 Thickness 69 m 50 m 15 m 40 m 30 m 25 m 40 m 5 m 155 m 20 m 130 m 35 m 40 m Rock type Rhyolite, pitchstone and obsedian Pyroclastite Tholeiitic andesite Tholeiites Olivine tholeiite Tholeiite Tholeiitic picrite Pyroclastite Tholeiites Olivine tholeiite Tholeiites Oligoclase tholeiite Olivine tholeiite Tholeiitic nicrite Based on fortythree chemical analyses it is established that Pavagarh lava flows exhibit an iron depletion tholeiitic trend similar to the Thingmuli volcanic suite of Iceland. The differentiation seauence for the first cycle is. hypersthene normative picritic —> olivine tholeiite -> oligoclase tholeiite —^ tholeiite. Whereas, (v) for the second cycle it is hypersthene normative picrite —> olivine tholeiite —> tholeiite —¥ tholeiitic andesite —> rhyolite. Lavas with compositional range between 54*/. and 65'/. SiO? are missing. This can be attributed to a period of queisoence of eruption in the second cycle, during which gas pressure presumably was built up followed by outburst of nyrocla sties and acidic magma, (rhyolites) . The oogenetic relationship h^s been established with the help of numerous variation diagrams. It shows a gradual change of magma composition as depicted by enrichment (e.g. KpO, Na„0, and SiC-) and depletion (e.g. MgO, CaO, and total iron) of major element concentrations with advancing fractionation. Trace element distribution pattern also subscribes to a similar conclusion. Reference to modified alkali-silica diagram of Irvin and Baragar, it is noted that they plot within the tholeiitic field. The coexistance of calcic and calcium-poor pyroxenes in the mode as well as in the norm, occurrence of acidic differentiates, presence of interstitial acidic glass (70-74*/. Si02 established by EPMA.) and reaction relation of olivine and liquid (to produce orthopyroxene) provide compelling evidence for the tholeiitic descent. A typical tholeiitic trend is also confirmed by AFM, Na„P - KpO - CaO, Si02 " ^ Fe2°3^ 21 Fe2fS + Mg0 Pnd ^L" Fe2°3 " Mg0 di3Srams- EPMA data of the analysed pyroxenes reveal that they are mostly calcic, plotting in the augite and endiopside (vi) fields. Three calcium-poor pyroxenes having intermediate pigeonite composition is also encountered in pitchstone. Orthopyroxenes from the basic members are identified petrographically only. The plagioclases are essentially labradorite or bytownite. However, oligoclase phenocryst is noted in the third flow and the plagiocla.se composi tion of tholeiitic andesite (Sixteenth flow) is andesine.Due to lack of sufficient geochemical data the tholeiitic lineage of the third flow was not established earlier and these were misrepresented as mugearites. The olivines of the first picrite is relatively more magnesium-rich and calcium-poor (Fa-,,-, Mo^ ) compared to those of the second picrite (Fa,,-, Mo 1) . The process of magma generation is considered to be related to the partial melting of garnet peridotite or pyrolite. On the basis of thermodynamic calculations, it is estimated that the observed concentration of K20 and Po0c in the first oicrite would require ^s 23*/. melting of 2 5 the ultramafic source rock (pyrolite) whereas ,-'20*/. fusion of the rejuvenated upper mantle is necessary to account for the known abundance of potassium and phosphorous in the second picrite flow. The oxygen fugacity (fn ) condition at the beginning L2 of the magmatism is found to be extremely reducing corres- -9 ponding to the WM buffer (10 bars) and the estimated temperature and silica activity is established to be around (vii) 1200°C and lo"° 'respectively. The initial f condition of the second cycle is determined to be 10 4 - 1 bar. The pressure-temperature condition of crystallization of magnetite in equilibrium with olivine for the flow is estimated to be 16.34 Kb and 840° + 30°C respectively. The silica activity of the same lava is calculated to be 10 'a"t 16 Kb, which is high enough to plot in the tholeiitic field of £-agifl diagram. The degrees of fractionation for successive flows have been estimated with the help of potassium and rubidium abundances. It is noted that the first batch of magma possibly fractionated upto 78/. whereas the second cycle of magmatism could achieve 9o*/. fractionation to produce rhyolites as their end products.