Mantled feldspar from the Noring granite, Peninsular Malaysia: petrography, chemistry and petrogenesis

Abstract

Mantle feldspar found in the Noring granite, have been investigated in terms of their petrography and geochemistry. Petrographically the mantle feldspar consists of whitish plagioclase rim which mantle the pinkish K-feldspar core. The K-feldspar core consists of a single crystal or several crystals usually showing simple twinning. Plagioclase rim is generally composed of numerous euhedral to subhedral plagioclases with size usually less than 1 mm. In places the plagioclase rim consists of a single crystal about 2 mm thick. :irormation of the mantled feldspar can be explained in two stages, (1) magma mixing and (2) late stage replacement. The plagioclase rim probably precipitated from a different magma type to the Noring magma probably of andesitic composition. The K-feldspar that has crystallised in the Noring magma act as substrates for the growth of plagioclase, producing the mantled texture. The aggregates of individuals crystals that have floated together in the magma attached to the K-feldspar as in the synneusis model of Vance (1969). Once this nucleation takes place there is relatively the straight forward process of crystal growth yielding the mantled texture (Hibbard, 1981). Small irregular outline plagioclase inclusions occur in the K-feldspar core being most abundant near the contact with plagioclase rim suggesting that the K-feldspar core replacing and enclosing relicts of plagioclase rim at the very late stage.