Mass balance during gabbro-amphibolite transition, Bamble Sector, Norway: implications for petrogenesis and tectonic setting of the gabbros

Abstract

Mafic intrusions of mid-Proterozoic ages are widespread in the Bamble Sector, southern Norway, and elsewhere in the Baltic Shield. They vary from subordinate ultramafic rocks to troctolitic gabbros, olivine–ferrogabbro, olivine-free gabbros and norites. Ni–Cu sulfide ores locally occur in the marginal parts of the intrusions. Post-solidus retrograde reactions under prolonged high P– T conditions led to serial changes from corona development around ferromagnesian minerals, to replacement of olivine and pyroxenes by hornblende and to the formation of amphibolites. Variations of elements during the gabbro-amphibolite transformation are evaluated using a statistical approach that takes into consideration both initial magmatic differentiation effects, and metasomatic changes imprinted during metamorphism. This indicates that rare earth elements (REE), high field strength elements (HFSE), and transition metals were immobile. Large ion lithophile elements (LILE) and halogens were significantly introduced into the amphibolites. Sulfur, Se, Cu, Au, and As remained constant, or locally remobilized and reprecipitated; Sb was enriched in the amphibolites. The Bamble gabbros are tholeiitic, enriched in Fe (Mg#=35–70); on MORB-normalized plots, they show features typical of destructive margins setting (i.e. enriched LILE and LREE, low Th abundance, and Nb trough). They are further characterized by low Ce/Yb ratios (Ce/Yb<10) and nearly flat chondrite-normalized REE patterns (La N/Yb N=1–2). The absence of major Mg-rich cumulates, such as peridotites, argues against fractional crystallization as the main cause of the enrichment in LILE–LREE. Lack of crustal contamination is shown by the REE patterns as well as radiogenic and stable isotope data [Precambrian Research 64 (1993) 403; Chemical Geology 181/1–4 (2001) 23]. A model consistent with the geochemical data favors an early fractionation of olivine in a source area that had been metasomatized by LILE–LREE bearing fluids.