HR: 0800h
AN: V11B-0589    [Abstracts]
TI: Sr Isotope Zoning in Plagioclase Phenocrysts from Giant Plagioclase Basalts: Mixing Processes in the Oldest Deccan Formations.
AU: * Borges, M R
EM: melroy.borges@fiu.edu
AF: Florida International University, Department of Earth Sciences, 11200 SW 8th Street, PC 344, University Park, Miami, FL 33199, United States
AU: Sen, G
EM: seng@fiu.edu
AF: Florida International University, Department of Earth Sciences, 11200 SW 8th Street, PC 344, University Park, Miami, FL 33199, United States
AU: Hart, G L
EM: ghart@wsu.edu
AF: Washington State University, School of Earth & Environmental Sciences, P.O. Box 642812, Pullman, WA 99164, United States
AU: Wolff, J A
EM: jawolff@mail.wsu.edu
AF: Washington State University, School of Earth & Environmental Sciences, P.O. Box 642812, Pullman, WA 99164, United States
AB: The older Deccan Formations of the Western Ghats are each capped by highly evolved ultraphyric flows that contain "giant" (up to 5 cm long) plagioclase phenocrysts, termed Giant Plagioclase basalts (GPB). We investigate the origin of GPB from the two lowermost Deccan formations and are able to deduce magmatic events that occurred during plagioclase growth. We carried out in situ LA-ICPMS analysis of selected trace elements and 87Sr/86Sr on giant plagioclase crystals in the Thalghat GPB from the Jawhar Formation (oldest) and Keshele GPB from the Igatpuri Formation (second oldest). The Thalghat GPB has two populations of crystals, indistinguishable with respect to size: unzoned crystals and a few zoned crystals with sharp Sr isotopic zoning between core and rim. In these, the ⁸⁷Sr/⁸⁶Sr of the cores is 0.70960, while the rims are isotopically similar to the unzoned crystals with ratios of 0.71060. Neither population shows significant zoning in An or trace element contents other, and they are devoid of visible (optically or with BSE) resorption features. The zoned crystals record a single mixing event between two magma compositions, and two lava flows that underlie the Thalghat GPB have MgO contents and ⁸⁷Sr/⁸⁶Sr that would be appropriate for the mixing end-members. Furthermore, mixing between the same end-members (followed by variable fractionation) can also account for the compositions of the lava flows that comprise the overlying Igatpuri Formation. Keshele GPB crystals show multiple ⁸⁷Sr/⁸⁶Sr zones, and a core to rim transect encompasses the variation in the underlying Igatpuri flows: 0.71049 - 0.71120. The zone boundaries in Thalghat crystals are sharp, about 500 microns, whereas in Keshele GPB, ratios vary gradually over 3 mm in some cases. These variations are not due to diffusion, but by growth from varying mixed melt compositions. Based on growth rate calculations, we can assign a growth timescale of 780 years to the largest Keshele crystals. If the crystals have "seen" magmas represented by all the underlying lavas, then the same timescale can be applied to the 220m thick Igatpuri Formation. This lends further credence to a short duration of Western Ghats Deccan activity. The large plagioclase phenocrysts of the Deccan could therefore have originated by growth during rapid mixing events, rather than from a dormant magma and the growth probably took place in a feeder conduit that was constantly flushed with mixed magma.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3618 Magma chamber processes (1036)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting