Stable association wollastonite-anorthite, and other cale-silicate assemblages in amphibolite-facies crystalline schists of Nanga Parbat, Northwest Himalayas

Abstract

Progressive regional metamorphism and granitization of a dominantly pelitic sequence was essentially isobaric and proceeded on a level of rather high P within the field of amphibolite facies. The transition from kyanite to sillimanite zone does not coincide with the boundary between medium- and high-grade rocks but is displaced toward higher T. Many of the Al-Si-rich high-grade rocks carry stable muscovite. Staurolite, andalusite, and cordierite are characteristically absent. Green hornblende occurs throughout the high-grade zone in rocks of appropriate composition. Calcareous intercalations permit recognition of three distinct zones of high-grade metamorphism. They, and the reactions defining their boundaries, are Reaction (b) is sensitive to PCO2. It is suggested that most of the rocks, especially those with excess silica, were open systems for CO2, so that PCO2≪ Ptotal. Locally, as in some tight marbles, CO2 was retained, and the wollastonite reaction was arrested or altogether prevented, in zone (2) and even in zone (1). Reaction (a), involving no gas phase, is dependent only on P, T. Under the prevailing high pressures, the temperatures required to make reaction (a) proceed to the right, must have been rather high. Quartz-excess and calcite-excess, diopside-bearing assemblages of the three zones are described. Associated K-feldspar is orthoclase with very small — 2V. Metamorphism of argillaceous dolomitic limestones illustrates the principle of preferential decarbonation of Mg. Assemblages include (omitting dominant calcite) : forsterite-spinel-phlogopite, found only in zone (1) ; phlogopite-diopside-(tremolite); diopside-(tremolite)-orthoclase; diopside-(tremolite)-anorthite-(orthoclase); diopside-wollastonite-grossularite (zone 2); diopside-grossularite-quartz (zone 3). Relations of mineral assemblages found in the three talc-silicate zones to bulk compositions are discussed. Boundary layers between diopsidic tale-silicate rocks and associated pelitic schists are described. As a rule, a thin band of para-amphibolite is developed at the contact, characterized by the garnet-free assemblage, green hornblende-anorthite (bytownite)-quartz. In adjacent, partly almandine-bearing biotite-quartz-plagioclase schists, plagioclase ranges from sodic andesine to bytownite. Widespread reverse zoning of this plagioclase is attributed to slow, short-range (a few centimeters) diffusion of anorthite-building substance from the adjacent Ca-richer rock into the schist. Locally, the amphibolite band is lacking, and diopside and biotite mingle in the boundary layer. Although this local association recalls pyroxene hornfels facies, nearby rocks have amphibolite facies mineralogy. Part of the diopsidic tale-silicate rocks of zone (1) show incipient retrogression under conditions of zone (2) or (3), the pair, wollastonite-anorthite (bytownite), being “parasitically” replaced by grossularite. Retrogression below the limits of zone (3) locally displays the assemblage, zoisite-labradorite-grossularite. Calcic scapolite found in some of the high-grade tale-silicate rocks is retrogressive, replacing anorthite (bytownite). Cale-silicate mica schists of higher-medium grade carry mutually stable calcic scapolite, andesine and zoisite, associated with calcite, quartz, local epidote, biotite, muscovite, and garnet. Much of their plagioclase is zoned. Reverse followed by normal zoning is interpreted as a record of the thermal history of metamorphism. Associated non-calcareous mica schists, commonly with almandine and kyanite, carry more sodic plagioclase which is unzoned. Garnet amphibolites of this zone have andesine plus zoisite or epidote, and some contain diopside. Physical conditions of the metamorphism of the high-grade tale-silicate rocks are discussed further, in the light of available experimental data.