The Nea Santa submarine rhyolite dome of the Triassic silicic volcano-sedimentary succession, Circum-Rhodope Belt, northern Greece

A Triassic Silicic Volcano-Sedimentary (SVS) succession is part of the Circum-Rhodopes Belt in Northern Greece. It records the voluminous rhyolitic activity that occurred on a Paleozoic composite basement (united Vertiscos plus Pelagonia terranes) at the early stages of extension that ended in continental break-up, separation of the above two terranes and creation of the Almopias Ocean. The SVS succession stratigraphically overlies the alluvial fan deposits of the Permo-Triassic Examili Formation, sourced from the eroded Vertiscos terrane, and is overlain by a Neritic Carbonate Formation of Triassic age. It comprises pyroclastic rocks, lava flows and quartz-feldspar-phyric intrusions, as well as epiclastic volcanic, non-volcanic and mixed volcanic – non-volcanic sediments, all now metamorphosed in low greenschist facies.
   The Nea Santa rhyolite dome is part of the SVS succession and is exposed in the Xiropotamos Creek between Nea Santa and Krithia villages. The dome is ~1000 m across and includes four facies recognizable despite their metamorphism and deformation. These are: (a) the “coherent rhyolite facies”, representing the core of the dome and consisting of massive, non-vesicular quartz-feldspar porphyry, locally flow-banded; (b) the “lithophysal rhyolite facies”, occurring in parts of the periphery of the dome. It is perlitic rhyolite porphyry containing spherulites weathered out from the host rock. Each spherulite contains a quartzfilled, star-shaped internal cavity (lithophysa); (c) the gradational “carbonate sediment matrix – sericite-altered rhyolite breccia facies”, defining the original contacts of the dome with carbonate sediments of the Neritic Carbonate Formation. It is composed of fluidal, ragged clasts and stringers of sericite-altered pumiceous rhyolite enclosed in bio-calcirudite host sediment (reef-debris). It is interpreted as intrusive hyaloclastite or fluidal peperite, based on criteria like: hydrothermal metamorphism of the host sediment adjacent to rhyolite clasts (bleaching, silicification and calcite recrystallization) and fluidization of the host sediment (calcite-filled vesicles in rhyolite clasts); (d) the “carbonate sediment matrix – quartz-feldspar porphyry breccia facies”, occurring as dyke-like breccia zones that range from 5 mm to 50 cm in width and penetrating the western part of the dome. It comprises blocky, angular, in places jigsaw-fitted porphyry clasts enclosed in carbonate host sediment. It is interpreted as blocky peperite intruded into dome’s open fractures formed at its last, brittle stage solidification. Some clasts were also spalled from the sides of the fractures. A relatively younger facies, named “mixed rhyolite – carbonate epiclastic sedimentary facies” was formed adjacent to the dome. It consists of rounded quartz-feldspar porphyry and carbonate clasts (granular siltstone, pebbly granular siltstone and pebble conglomerate). It is interpreted as mixed provenance mass- and debris-flow deposits.
   The Nea Santa dome displays typical characteristics of domes formed in submarine successions. During emplacement, its margins were quench-fragmented and mingled with wet unconsolidated carbonate sediment forming intrusive hyaloclastite (fluidal pepperite). The pumiceous nature of the fluidal hyaloclasts and the lithophysal nature of the periphery of the dome imply volatile exsolution not inhibited by the confining pressure, implying further that the sediment cover above the dome was thin and the water depth probably less than 200 m. The host carbonate sediment composed of reef-debris indicates that the dome intruded in a shallow submarine environment, below wave-base. The dome finally reached above storm wave-base level and was at least partly extrusive. Its fragmented margins were subjected to reworking and were syn-deposited with carbonate clasts on its flanks below wave-base as mixed provenance gravity-driven debris- and mass-flows. The identification of peperitic or intrusive hyaloclastite margins of the Nea Santa dome within the SVS succession is decisive for the relative chronology, facies architecture and palaeoenvironmental reconstruction because its presence demonstrates approximate contemporaneity of rift magmatism and sedimentation.