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Hydrothermal processes related to some Triassic and Jurassic submarine basaltic complexes in northeastern Hungary, the Dinarides and Hellenides

Comparative studies on hydrothermal alteration of submarine peperitic basalt occurrences related to the Triassic early rifting of the Neotethys were carried out in various parts of the Dinarides and Hellenides. The study areas included the displaced fragments of the Dinarides in the Darnó Unit, NE Hungary, the Kalnik Mts. in Croatia and the Vares-Šmreka area in Bosnia and Herzegovina. In the Hellenides, similar environments were studied in the Stragopetra Mts., Greece. Jurassic pillow basalts formed in a back-arc-basin of the Neotethys were also studied in the Szarvaskő Unit, NE Hungary, which also represents a displaced unit of Dinaridic origin. Within the submarine basaltic lava flows, six volcanic facies were distinguished. The hydrothermal alteration was characterised according to those facies. The first process was the albitisation of the rock-forming plagioclase at ~300°C in all localities. During the higher temperature stage of the subsequent cooling, chloritisation in the ground mass is typical for all types of basalts, however chlorite and rarely quartz formed in the fractures and amygdales of the Triassic basalts, while chlorite, quartz and prehnite precipitated in the fractures of the Jurassic rocks. At lower temperatures of this cooling- related process, calcite is a common mineral filling up the larger amygdales, jig-saw type fractures and other open spaces, but some epidote, pumpellyite, prehnite and laumontite also occur in the Triassic basalts. The late stage alteration (occurring at the lowest temperature) is characterised by argillitisation at every locality. The observed hydrothermal alteration patterns also show slight differences according to the volcanic facies as a function of the distal/proximal setting in relation to the eruptive centres and the presence/absence of water-saturated and unconsolidated carbonate or siliciclastic sediments at the time of the emplacement of lava flows. The study revealed that the most important factors influencing mineralogy and zoning of hydrothermal alteration in these short lived local hydrothermal systems are the rapid cooling of the hydrothermal fluid, the dominance of the poorly evolved seawater as the source of hydrothermal fluid and the local, i.e. effective water/rock ratio, determined by the degree of fracturing in the rock. The mineralogical-textural peculiarities of the highly localised hydrothermal fluid/rock interaction in the studied submarine seamount type volcanoes are clearly different from the products of the large-scale hydrothermal processes occurring at mid-oceanic ridges. Recognition of these differences is important in the evaluation of ore potential in the Neotethyan realm or other areas with occurrences of submarine basaltic units.

submarine basaltic volcanism ; submarine hydrothermal processes ; mineralogy and geochemistry of submarine fluid/ rock interaction ; fluid inclusions ; chlorite thermometry

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