Abstract

Giant plagioclase basalts, rich in plagioclase megacrysts commonly 5 cm long, form widespread lava flows and dikes in the Deccan Traps continental flood basalt province. They also occur in the Scottish Hebrides, Columbia River, and Emeishan continental flood basalt provinces, and closely similar rocks are known from Archean and Proterozoic terrains and oceanic settings. The giant plagioclase basalts have usually been interpreted as end products of magmatic cycles operating in high-level chambers, and taken as the boundaries of stratigraphic formations. A new model is proposed in which the plagioclase megacrysts grow over long (15 k.y.) time periods in a deep crustal (possibly Moho-level) sill complex, which forms tens to hundreds of thousands of years before the continental flood basalt eruptions. Megacryst flotation produces anorthositic mushes containing ferrodioritic interstitial liquids. Giant plagioclase basalts form when these components (and any melt from the crust heated by the sill complex) are remobilized by a newly injected tholeiitic magma batch, and brought to the surface or high crustal levels by dikes. Giant plagioclase basalts are thus mixed, heterogeneous rocks from the deep continental crust. The new model explains a range of geological, petrological, mineralogical, and geochemical data on the giant plagioclase basalts. Formation and remobilization of anorthositic mushes appear to be fundamental processes producing highly feldspar-phyric basalts over a wide range of tectonic settings. An important corollary of the new model is that anorthosites have continued to form well after the Precambrian, on a smaller scale, during Phanerozoic continental flood basalt events. Therefore, continental flood basalt provinces should be recognized as sites of anorthosite formation in the deep continental crust.

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