Abstract

Petrological and geochemical data denote that the Late Arenig–Early Llanvirn Hølonda Porphyrites belong to the subduction-related shoshonite association. Original compositions ranged from absarokite through shoshonite to latite, and their very evolved nature signifies a mature arc aYnity in an active continental margin setting. Much of the geochemical variation can be interpreted in terms of fractional crystallization followed by a varying admixture of cumulus phases to the ascending melt and accumulation of phenocrysts within the magmatic feeder system. Their eruptive products covered an area which was at least 20–25 km across, forming local islands with carbonate-rich shelves in a shallow to deeper marine environment.

The immediate substrate to the volcanosedimentary ‘Hølonda basin’ is the Løkken Ophiolite, which formed in a primitive oceanic arc/marginal basin setting some 10–20 Ma prior to the shoshonitic magmatic event. Arc–continent collision led to emplacement of the oceanic, arc and marginal basin crust onto the continental margin in pre-Late Arenig times. Strong uplift of the obducted oceanic plate occurred in response to the buoyancy effect of the underthrusting, less dense, continental margin. This is interpreted to have been followed by a reversal of subduction polarity and formation of a steeply dipping subduction zone against the continental margin lithosphere, and was succeeded by the very evolved Hølonda Porphyrite shoshonitic magmatism in volcanic islands along this submerged continental margin.

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