40Ar/39Ar dating of Giant Plagioclase Basalts of the Deccan Traps
Abstract
So-called Giant Plagioclase Basalts (GPBs) are sparsely intercalated in the Deccan Traps (DT) lava pile. GPBs are defined by >30 volume-% weakly zoned plagioclase phenocrysts as large as 5 cm, typically with aphanitic groundmass and only clinopyroxene as a typically minor second phenocryst phase. GPBs are used to define stratigraphic boundaries between different formations of the DT, particularly in the Kalsubai Subgroup of the Western Ghats (WG). Individual GPBs have been correlated laterally over distances of 100s of km. The petrogenesis of GPBs is debated. They tend to be more differentiated than typical DT tholeiites, and some have postulated that they represent end-stage differentiates of episodically generated batch melts. Whether the plagioclase megacrysts accumulate by buoyant flotation, or by mobilization of crystal mushes, is unclear. REE patterns of GPBs reveal no consistent Eu anomalies, suggesting that neither fractionation nor accumulation of plagioclase has occurred. Plagioclase in Deccan GPBs is strikingly less calcic than in most other DT basalts, with a narrow range of compositions (An55-An65) and Ca/K of 35-70 compared with values of 100-300 typifying the latter. Accordingly, and also due to their grain size and general freedom from melt inclusions, plagioclase separates from GPBs are relatively easy to prepare and are proving to yield the most reliably accurate and precise 40Ar/39Ar ages obtainable for DT lavas. Step-heating invariably produces plateaux over >90% of the 39Ar released, and results from multiple aliquots yield relative age uncertainties as low as 0.05%. Dating of GPB's is providing critical data for evaluating (i) eruptive volume vs time for the DT, and (ii) applicability of WG-based chemical stratigraphy to other subprovinces of the DT.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V24A..03R
- Keywords:
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- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCES;
- 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHY;
- 8137 Hotspots;
- large igneous provinces;
- and flood basalt volcanism;
- TECTONOPHYSICS;
- 8408 Volcano/climate interactions;
- VOLCANOLOGY