Mineral chemistry and crystallization conditions of the Artfjället gabbro and dolerites (Central Swedish Caledonides)

Abstract

On the basis of plagioclase zoning and other considerations, the compositions of the silicate minerals in the Artfjället gabbro are judged to be close to their original magmatic compositions. They can be used, therefore, to estimate the crystallization conditions. Pyroxene temperatures are 1280-1210°C for picrites, 1240-1180°C for olivine gabbros, and 1180-1050°C for norites. Olivine-spinel-augite thermometry yields ca. 1200°C for the picrites. Comparison with data from melting experiments on basalts suggests that the Artfjället gabbro is one of the few intrusions where pyroxene compositions still record magmatic temperatures. The pressure is constrained between 6 and 10 kbar by the country-rock metamorphism and the presence of plagioclase in the gabbro. No combination of the anorthite-Ca-Tschermakite (An-CaTs) and forsterite-enstatite reaction formulations gives a correct pressure, most yielding overestimates. The An-CaTs reaction is unreliable, especially for low-alumina clinopyroxenes, because of the small CaTs activities and consequent large errors (∼ 5–10 kbar). During crystallization of the gabbro the silica activity probably was nearly constant and the oxygen fugacity near the QFM buffer. For the dolerites, which occur in and are related to the gabbro, temperatures are considered to reflect their liquidus-solidus interval: pyroxene ca. 1200-950°C, brown hornblende 1025-975°C, and biotite roughly 1000-900°C. In quartz-dolerites, temperatures for hornblende (820-660°C) and biotite (ca. 800°C) are lower. The primary water content of the gabbro and dolerite magmas is estimated to have been between 0.5 and 1.5 wt.%.