Slowly ascending magmas in long-lived accretionary orogens: unraveling temporal variations in the Cordilleran-style Sveconorwegian Orogeny

Abstract:

Southwestern Norway is host to spatially associated magmatic provinces that have been interpreted as magmatic products of the Sveconorwegian Orogeny. The Sirdal Magmatic Belt (SMB) and the Hornblende-Biotite Granite (HBG) Suite were emplaced between 1050-1020 Ma and 980-930 Ma, respectively. Geochronology of the Rogaland Anorthosite Province (RAP) indicates that magmatism began at 1041 Ma (high pressure, cogenetic megacrysts) and culminated in anorthosite emplacement at ±930 Ma. Decompression exsolution ages for these high-P megacrysts indicate that decompression during anorthosite ascent took place ±80-100 m.y. after crystallization at the Moho. The contact aureole of the RAP shows concordant arrays of zircon ages between 1050-930 Ma, recording continuous, long-lived high temperature magmatic events in southwestern Norway. Zircon ages from outside the contact aureole show a metamorphic event at ±1035 Ma, but show no younger concordant arrays. Metapelites from the contact aureole of the RAP also show a spread of monazite ages, where monazite inclusions in garnet record ages of 1038-992 Ma, while groundmass monazite preserve ages of 952±10 Ma, indicative of multiple and/or long-lived thermal events. Age coincidence for several events, including matrix monazite formation, megacryst decompression (and anorthosite emplacement) and HBG granite crystallization suggest a dynamic system with multiple feedback loops. Varying isotopic signatures recorded in the region hint at changing sources and processes associated with the genesis of the magmas. These observations indicate that the Sveconorwegian orogeny was a long-lived magmatic system that featured slowly ascending magmas, punctuated periods of magmatism and temporal geochemical variation – features analogous to younger Cordilleran systems. When estimating magmatic tempos in arc environments, factors such as large degrees of differentiation in the lower crust, polybaric magma evolution and slow magma ascent rates should be considered.