Characteristics of UHP Pelites, Gneisses, and Other Unusual Rocks

Abstract

The main felsic lithologies experiencing ultrahigh-pressure metamorphism (UHPM) are described, with a focus on rock association, nature, and composition of the peak minerals, as well as nature of the protolith. Felsic rocks usually preserve poor evidence of UHPM, being pervasively deformed and retrogressed during exhumation. Locally, however, unstrained lithologies are preserved that show UHPM minerals and even microstructures inherited from the protolith. Among the number of localities where UHPM has been reported, the following have been considered in detail— the southern Dora-Maira massif and Lago di Cignana in the Western Alps, the Dabie Shan-Sulu region in northeastern China, the Kokchetav massif in northern Kazakhstan, and the Western Gneiss Region of the Scandinavian Caledonides. At Lago di Cignana, metasediments of oceanic affinity are carbonate-bearing garnet-phengite-quartz schist and oxidized manganiferous quartz schists, all preserving rare coesite relics. In the Dora-Maira massif, the Brossasco-Isasca Unit includes coesite-bearing phengite-jadeite-almandine-quartz micaschist, pyrope whiteschist, jadeite-kyanite quartzite, sodic whiteschist, and jadeite-almandine orthogneiss. In the Dabie Shan-Sulu region, felsic rocks with clear evidence of UHPM include jadeite quartzite, talc-kyanite quartzite, and other kyanite-bearing quartzites. Moreover, other lithotypes like aegirine orthogneiss, paragneiss, and micaschist also may have been subjected to UHPM. In the Kokchetav massif, microdiamond-bearing rocks include garnet-biotite gneisses and schists, garnet-pyroxene and pyroxene-carbonate-garnet rocks, and possibly also whiteschists. Other localities where UHPM felsic rocks have been reported include the northern Tien Shan, northern Mali, the Maksyutov Complex in the southern Urals, and the Western Gneiss Region of the southwestern Caledonides. In felsic rocks, the most “common” UHP minerals are coesite, clinopyroxenes, garnets, epidotes, kyanite, phengite, talc, aragonite, dolomite, and accessory rutile, titanite, apatite, tourmaline, and zircon. Exceptionally, rare or unique phases also may be found as inclusions armored in more competent minerals. In the highest-P areas, microdiamond occurs, its growth being strongly controlled by the composition of the fluid phase. Mineral chemistry is sensitive to both bulk-rock chemical composition and P-T conditions: garnets are solid solutions of the pyralspite series, the Mn-rich members being high in calderite; pyroxenes are jadeite-diopside solid solutions; epidotes include allanite, zoisite, clinozoisite-epidote solid solutions, and piemontite; and micas are highly celadonitic phengites. In rocks with microdiamond, extreme UHPM conditions are further suggested by the unusual composition of some minerals, such as garnet high in Si, Y, and Yb, or Na-pyroxenes rich in K. Clear evidence of partial melting during UHMP is missing, although peak conditions are close to or above the minimum granite wet solidus. This may be due to either the aH2O < 1, or to melt extraction.