Zoning and resorption of plagioclase in a layered gabbro, as a petrographic indicator of magmatic differentiation

Abstract

The Murotomisaki Gabbroic Intrusion is a sill-like layered gabbro emplaced in sedimentary strata of Tertiary age in southwest Japan. The zoning (including resorption structures) and the compositional variations of plagioclase from throughout the intrusion were studied, and it was found that the zoning pattern may be classified into four types, which may well correlated with the hosting rock types, the mode of occurrences and their stratigraphic positions in the intrusion. The plagioclase zoning was successfully decoded, and the sequence of events that took place during the magmatic differentiation was deduced and further interpreted in the context of a stratified basal boundary layer slowly ascending in a solidifying magma body. It was shown that various layered structures – modal layering, podiform gabbroic pegmatites and anorthositic layers – observed in the Murotomisaki Gabbro were formed within the moving basal boundary layer by flushing of H2O-rich fluid and fractionated silicate melts from below. By the fluxing of hydrous fluids, plagioclase crystals preferentially dissolved and then melt fraction increased in the basal boundary layer. Under these circumstances, plagioclase-rich fractionated melts diapirically segregated from the crystal pile. Calcic plagioclases, which are out of equilibrium in the central part of the intrusion, may have originated from the basal boundary layer in this manner.

Petrological Constraints on the Recycling of Mafic Crystal Mushes and Intrusion of Braided Sills in the Torres del Paine Mafic Complex (Patagonia)

“…A variety of parameters including temperature, pressure, melt water content and melt compositional variations have been proposed to cause complex plagioclase zoning (e.g. Blundy & Shimizu, 1991;Kuritani, 1998;Berlo et al, 2007;Ginibre et al, 2007;Streck, 2008;Hoshide & Obata, 2010).…”

“…Plagioclase primocrysts will eventually be concentrated in the more H 2 O-rich liquid (e.g. Hoshide & Obata, 2010). The estimated liquid fraction responsible for the layered gabbronorite bulk-rock Zr concentration (CLF-Zr;Meurer & Boudreau, 1998) is only about 20%.…”

“…It often displays complex textures such as normal, reverse, oscillatory or patchy zoning, associated with crystal chemistry variations (e.g. Blundy & Shimizu, 1991;Kuritani, 1998;Wallace & Bergantz, 2002;Berlo et al, 2007;Ginibre et al, 2007;Streck, 2008;Hoshide & Obata, 2010). Owing to slow element diffusion (Costa et al, 2003), plagioclase may preserve a chronological record of the physico-chemical variations in the magmatic system.…”

Amphibole-bearing Multiphase Solid Inclusions in Olivine and Plagioclase from a Layered Gabbro: Origin of the Trapped Melts

“…A brief summary of the characteristics of each zone is given below. For further petrographic details the reader is referred to Hoshide et al (2006) and Hoshide & Obata (2009).…”

“…The AC subzone looks homogeneous and monotonous in the field except for some gradual upward coarsening of the rocks. The GR subzone, on the other hand, is more inhomogeneous and contains conspicuous compositional banding or layering as well as abundant pods and veins of gabbroic pegmatite (a few centimetres to 2 m in thickness) and layers of anorthosite (Hoshide & Obata, 2009).…”