Ca-Amphibole Paragenesis in the Shirataki District, Central Shikoku, Japan

Abstract

Calcic amphibole and coexisting albite 4-white mica + chlorite -I-epidote from 11 metabasaltic and 2 metasedimentary schists have been investigated from a portion of the relatively high pressure, low temperature (blueschist type) Sanbagawa terrane. The host rocks are representative of two adjacent and intergradational parts of a progressive metamorphic series.Although Ca-amphibole, white mica, and, to some extent, chlorite show enrichments in A1 with progressive metamorphism, the plagioclases change only from an average of An (l0 3 to An 017 . The Ca/Na ratios of calcic amphiboles do not appear to be related simply to metamorphic grade or to the An contents of coexisting plagioclases. The Ti0 2 contents of the mafic phases and white micas increase with grade. The distributions of the large Mn 2+ and small Ti 4+ ions between coexisting phases are: for manganese, chlorite > Ca-amphibole > epidote > white mica; and for titanium, Ca-amphibole = white mica > epidote > chlorite. These fractionations reflect in part the different sizes of the structural sites in the competing minerals. Although the paragenesis is roughly similar to those of other, higher temperature type terranes, there is a slight tendency for Shirataki Ca-amphibole (Si0 2 +Al 2 0 3 ) sums and soda contents to be higher and (total iron + magnesia) sums, as well as titania and lime contents, to be lower than values for calcic amphiboles from non-blueschist terranes.In contrast to higher temperature terranes elsewhere, the continuous change in composition of Shirataki Ca-amphiboles from actinolite at lower metamorphic grade through barroisitic amphibole to hornblende at higher grade argues against the presence of a solvus in this series under relatively high pressure, low temperature (glaucophane schist type) conditions. 73 on June 23, 2015 memoirs.gsapubs.org Downloaded from 74 W. G. ERNST CONTENTS

Devolatilization of metabasic rocks during greenschist–amphibolite facies metamorphism

“…Studies of these processes in metabasic rocks have been limited even though such rocks are common in orogenic belts. Many field‐based studies of greenschist and amphibolite facies metabasic rocks (Cooper & Lovering, 1970; Ernst, 1972; Graham, 1974; Misch & Rice, 1975; Tagiri, 1977; Otsuki & Banno, 1990; Terabayashi, 1993) were hampered by the fine‐grained nature of the rocks and general lack of variety in mineral assemblages. Unlike pelitic rocks, where the mineral assemblage changes significantly as a function of pressure and temperature, metabasic rocks are distinguished more by continuous changes in mineral compositions and modes, making it difficult to characterize the P – T conditions precisely from mineral assemblages alone.…”