Abstract

Lower Ordovician marine strata, formed in an ensialic marginal basin environment, record early episodes of distant explosive silicic volcanism and a later volcanotectonic episode which was a precursor to the development of a wide variety of near-to-source submarine silicic volcanic phenomena. The early episodes are represented by well-bedded turbiditic tuffs. These and older strata, at least 770 m thick in normal stratigraphical succession, are allochthonous, and large-scale wet-sediment sliding with reworking of strata in cohesive debris flows reflects major disturbance of the sea floor during the emplacement of silicic magmas. Volcanotectonic uplift along a N-S fault exceeded 1 km and led to the emergence and erosion of a rhyolitic volcanic island. This was followed closely by submergence, and in water depths perhaps of about 500 m there accumulated welded and non-welded rhyolitic ash-flow tuffs, rhyolitic turbiditic tuffs, rhyolite lavas and a wide variety of debris flow deposits. Shallow intrusions of 'low'-viscosity rhyolite were emplaced contemporaneously. The more or less simultaneous emplacement of rhyolitic tuff with both 'low'-viscosity and high-viscosity rhyolites is attributed to variable water content related perhaps to volatile fluxing in a magma body. The location of the near-to-source extrusive rocks and consanguineous intrusions along the contemporary N-S fault is considered to reflect channelling of magma, initially along a deep crustal fracture.

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