Age and emplacement conditions of the Chalmers mafic intrusion deduced from contact melts

Abstract

Mineral textures, thermobarometry and ion probe U–Pb dating were used to determine the age and emplacement conditions of a gabbro intrusion in Göteborg, southwest Sweden. Granitic diapirlike structures intruding the gabbro are interpreted as contact-metamorphic melts of ∼1.6 Ga country rock gneiss which back-intruded the mafic magma. Metasomatising fluids from these diapirs triggered garnet growth and hornblende alteration at a late stage of magmatism. Xenocrystic zircons in the diapirs were first partly resorbed, then experienced new growth at the time of the intrusion. Thermobarometric calculations show equilibrium conditions of 800[ddot]C and 10 kbar, reflecting an emplacement depth of 33 km. Zircon and titanite U–Pb analyses were made by secondary ionisation mass spectrometry (SIMS). Zircon cores and rims give a coherent concordia age of 1333±8 Ma (2σ), interpreted as the age of intrusion, at which time most xenocrystic zircon cores were totally reset. Titanite gives an age of 991±16 Ma (2σ) with a trend toward the formation age, reflecting incomplete Sveconorwegian resetting. The Chalmers intrusion was emplaced in a lower crustal environment in a block of ∼1.6 Ga arc-related country rock. The tectonic setting at 1.3 Ga was either anorogenic or a pre-Sveconorwegian rift environment. Subsequent amphibolite grade Sveconorwegian regional metamorphism largely reset the U–Pb system in titanites but did not cause either re-equilibration of the thermobarometric minerals or lead-loss in zircon.