The Chogat-Chamardi subvolcanic complex, Saurashtra, northwestern Deccan Traps: Geology, petrochemistry, and petrogenetic evolution

The Saurashtra region in the northwestern part of the Deccan continental flood basalt province (India) is notable for its many volcano-plutonic complexes, compositional diversity, and the abundance of rhyolite and granophyre. In this first detailed study of the Chogat-Chamardi subvolcanic complex in eastern Sau- rashtra, we report microgranite and granophyre plutons, gabbro plutons cut by basaltic andesite dykes, as well as rhyolite plugs. Bulk-rock geochemical (including Nd–Sr isotopic) data suggest a strong contribu- tion from ancient, Rb-rich basement crust to the silicic magmas. The mafic rocks are also crustally con- taminated but less than the silicic rocks, in conformity with observations elsewhere (e.g., the Skye and Skaergaard granophyres). The Chogat-Chamardi silicic rocks have initial 87Sr/86Sr (for t = 65 Ma) as high as 0.72914, and initial eNd values as low as -13.9. Trends defined by the Chogat-Chamardi and other Dec- can rhyolitic suites on plots of Sr concentration vs. Sr isotopic ratio are modeled with concurrent assim- ilation and fractional crystallization (AFC) processes involving a basaltic magma and granitic basement rocks. Considering both Nd and Sr isotopic variations, the Chogat-Chamardi silicic rocks notably require crustal end members very different from those inferred in most previous isotopic studies of Deccan rock

Geology, petrochemistry, and genesis of the bimodal lavas of Osham Hill, Saurashtra, northwestern Deccan Traps

The Saurashtra region in the northwestern Deccan continental flood basalt province (India) is notable for compositionally diverse volcano-plutonic complexes and abundant rhyolites and granophyres. A lava flow sequence of rhyolite–pitchstone–basaltic andesite is exposed in Osham Hill in western Saurashtra. The Osham silicic lavas are Ba-poor and with intermediate Zr contents compared to other Deccan rhyo- lites. The Osham silicic lavas are enriched in the light rare earth elements, and have eNd (t = 65 Ma) values between -3.1 and -6.5 and initial 87Sr/86Sr ratios of 0.70709–0.70927. The Osham basaltic andesites have initial eNd values between +2.2 and -1.3, and initial 87Sr/86Sr ratios of 0.70729–0.70887. Large- ion-lithophile element concentrations and Sr isotopic ratios may have been affected somewhat by weath- ering; notably, the Sr isotopic ratios of the silicic and mafic rocks overlap. However, the Nd isotopic data indicate that the silicic lavas are significantly more contaminated by continental lithosphere than the mafic lavas. We suggest that the Osham basaltic andesites were derived by olivine gabbro fractionation from low-Ti picritic rocks of the type found throughout Saurashtra. The isotopic compositions, and the similar Al2O3 contents of the Osham silicic and mafic lavas, rule out an origin of the silicic lavas by frac- tional crystallization of mafic liquids, with or without crustal assimilation. As previously proposed for some Icelandic rhyolites, and supported here by MELTS modelling, the Osham silicic lavas may have been derived by partial melting of hot mafic intrusions emplaced at various crustal depths, due to heating by repetitively injected basalts. The absence of mixing or mingling between the rhyolitic and basaltic andes- ite lavas of Osham Hill suggests that they reached the surface via separate pathways

Correlations between silicic volcanic rocks of the St Mary's Islands (southwestern India) and eastern Madagascar:implications for Late Cretaceous India-Madagascar reconstructions

The St Mary's Islands (southwestern India) expose silicic volcanic and sub-volcanic rocks (rhyolites and granophyric dacites) emplaced contemporaneously with the Cretaceous igneous province of Madagascar, roughly 88-90 Ma ago. The St Mary's Islands rocks have phenocrysts of plagioclase, clinopyroxene, orthopyroxene and opaque oxide, moderate enrichment in the incompatible elements (e.g. Zr = 580-720 ppm, Nb = 43-53 ppm, La/Ybn = 6.9-7.2), relatively low initial 87Sr/86Sr (0.7052-0.7055) and near-chondritic initial 143Nd/144Nd (0.51248-0.51249). They have mineral chemical, whole-rock chemical and isotopic compositions very close to those of rhyolites exposed between Vatomandry-Ilaka and Mananjary in eastern Madagascar, and are distinctly different from rhyolites from other sectors of the Madagascan province. We therefore postulate that the St Mary's and the Vatomandry-Ilaka-Mananjary silicic rock outcrops were adjacent before the Late Cretaceous rifting that split Madagascar from India. If so, they provide a valuable tool to check and aid traditional Cretaceous India-Madagascar reconstructions based on palaeomagnetism, matching Precambrian geological features, and geometric fitting of continental shelves. Supplementary materialMineral analyses, mass-balance calculations and locality information are available at http://www.geolsoc.org.uk/SUP18332

Polychronous (Early Cretaceous to Palaeogene) emplacement of the Mundwara alkaline complex, Rajasthan, India: 40Ar/39Ar geochronology, petrochemistry and geodynamics

The Mundwara alkaline plutonic complex (Rajasthan, north-western India) is considered a part of the Late Cretaceous–Palaeogene Deccan Traps flood basalt province, based on geochronological data (mainly 40Ar/39Ar, on whole rocks, biotite and hornblende). We have studied the petrology and mineral chemistry of some Mundwara mafic rocks containing mica and amphibole. Geothermobarometry indicates emplacement of the complex at middle to upper crustal levels. We have obtained new 40Ar/39Ar ages of 80–84 Ma on biotite separates from mafic rocks and 102–110 Ma on whole-rock nepheline syenites. There is no evidence for excess 40Ar. The combined results show that some of the constituent intrusions of the Mundwara complex are of Deccan age, but others are older and unrelated to the Deccan Traps. The Mundwara alkaline complex is thus polychronous and similar to many alkaline complexes around the world that show recurrent magmatism, sometimes over hundreds of millions of years. The primary biotite and amphibole in Mundwara mafic rocks indicate hydrous parental magmas, derived from hydrated mantle peridotite at relatively low temperatures, thus ruling out a mantle plume. This hydration and metasomatism of the Rajasthan lithospheric mantle may have occurred during Jurassic subduction under Gondwanaland, or Precambrian subduction events. Low-degree decompression melting of this old, enriched lithospheric mantle, due to periodic diffuse lithospheric extension, gradually built the Mundwara complex from the Early Cretaceous to Palaeogene time

Correlations between silicic volcanic rocks of the St Mary's Islands (southwestern India) and eastern Madagascar: implications for Late Cretaceous India–Madagascar reconstructions

<p>The St Mary's Islands (southwestern India) expose silicic volcanic and sub-volcanic rocks (rhyolites and granophyric dacites) emplaced contemporaneously with the Cretaceous igneous province of Madagascar, roughly 88–90 Ma ago. The St Mary's Islands rocks have phenocrysts of plagioclase, clinopyroxene, orthopyroxene and opaque oxide, moderate enrichment in the incompatible elements (e.g. Zr = 580–720 ppm, Nb = 43–53 ppm, La/Yb_n = 6.9–7.2), relatively low initial ⁸⁷Sr/⁸⁶Sr (0.7052–0.7055) and near-chondritic initial ¹⁴³Nd/¹⁴⁴Nd (0.51248–0.51249). They have mineral chemical, whole-rock chemical and isotopic compositions very close to those of rhyolites exposed between Vatomandry–Ilaka and Mananjary in eastern Madagascar, and are distinctly different from rhyolites from other sectors of the Madagascan province. We therefore postulate that the St Mary's and the Vatomandry–Ilaka–Mananjary silicic rock outcrops were adjacent before the Late Cretaceous rifting that split Madagascar from India. If so, they provide a valuable tool to check and aid traditional Cretaceous India–Madagascar reconstructions based on palaeomagnetism, matching Precambrian geological features, and geometric fitting of continental shelves. </p