Deccan Basalts,
India
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Sub-volcanic magma chambers may be a widespread component of flood
basalt provinces, and their presence may be revealed in some cases by
plagioclase megacrystic basalts. In at least four levels within the Deccan
group the generally low abundance of small plagioclase crystals increases
to 5-25% with some as large as 30 mm long. These giant plagioclase basalts
flows (GPB) were formed by mixing of megacryst-rich and megacryst-poor
magmas. The crystal size distributions (CSD) of these megacrysts mostly
plot as almost straight lines on a classic CSD diagram. For a plagioclase
growth rate of 10-10 mm.s-1 steady-state magma
chamber models and simple continuous growth suggest residence times of
500-1500 yrs. However, the lack of crystals
smaller than 2 mm suggests that coarsening may have been involved and
crystal shape can help define the environment where this happened.
Plagioclase megacrysts are very tabular and commonly form clusters of
sub-parallel crystals, characteristics that are also found in the
plagioclase of anorthosites formed by floatation at the top of shallow
magma chambers and crystallisation in a high Peclet number regime (e.g.
Skaergaard; Sept Īles). A possible history is: 1) Crystallisation of
plagioclase megacrysts in a convecting magma chamber just below the lava
pile. 2) Currents sweep the crystals to the top of the chamber where they
accumulate due to their buoyancy. The crystals coarsen in response to the
continuous supply of hot magma. 3) New magma sweeps through the
plagioclase mush, mingles and mixes, then erupts to form the GPBs. The
residence time recorded by the megacrysts in the GPBs is that of the magma
chamber where the megacrysts formed, not that of the magmas that make up
the megacryst-poor part of the GPBs or the other megacryst-poor lavas.
Lavas with megacrysts similar to the GPBs are uncommon but widespread
(Galapagos; Surtsey; etc) suggesting the presence of sub-volcanic magma
chambers elsewhere.
Higgins, M. D., and Chandrasekharam, D., 2007, Nature of sub-volcanic
magma chambers, Deccan province, India: Evidence from quantitative
textural analysis of plagioclase megacrysts in the Giant Plagioclase
Basalts: Journal of Petrology, v. 48, p. 885-900. https://doi.org/10.1093/petrology/egm005