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Dating of rifting magmatizm and evaporites within the Adria Plate, on Jabuka, Brusnik and Vis islands, Croatia

Permo-Triassic rift-related magmatism in the Dinarides produced gabbro-diorite-syenite-granite formation and an extrusive basalt-andesite-dacite formation with abundant pyroclastic rocks. Wide range of igneous rock ages from Permian to Triassic times show two stage evolution from the early intra-continental rifting to the advanced rifting and oceanization. The igneous rocks from Vis, Jabuka and Brusnik islands consist of the two main groups: a) sul-bvolcanic types with augite-diabases ; b) extrusive types with augite-andasite, spilite- keratophyres and spilites. The first group characterizes Jabuka and Brusnik, whereas the second one is on the Vis. The parent magma is, however, the same. The gabbro of the Jabuka is a phaneritic, medium-grained rock with sub-hypidiomorphic texture. Plagioclase and two pyroxenes (augite and pigeonite) represent the main crystallization phase with subordinate amphibole (uralite), biotite, Ti-magnetite and ilmenite. The trace elements and a normalized-MORB incompatible element spider-diagram support similarity of the Jabuka gabbro with continental tholeiitic magmas, excluding MORB type affinity. The gabbro intrusive-body was dissected by a swarm of dyke-veins of pinkish, coarse-grained gabbro. Amphibols, selected from the dykes, contain measurable fluid inclusion (FIs). Liquid-rich inclusions yielded homogenizations (Th) between 420° and 505°C, and salinities around 25 wt % NaCl equ. Minimum pressure was 0.6 kbar, indicating minimum 2 km of overburden under lithostatic conditions. The dyke swarm is perpendicular to the pseudo-layering and Th suggests brittle intrusion of fractionated, alkali enriched magma. The dolerite of the Brusnik Island has phaneritic, holocrystalline, intergranular diabase texture with ophitic to subophitic groundmass. Mineral composition is the same as in the Jabuka gabbro. Major and trace elements point their calc-alkaline character. Quartz from coarse grained gabbroid dykes contains well developed fluid inclusions. Petrographic examination shows coexistence of vapour rich FIs and high salinity liquid rich FIs containing halite and silvite daughter minerals. High salinity liquid-rich FIs melted silvine at T between 80° to 150°C, followed by halite melting at T range from 345° to >600°C, priori to or synchronous with the vapour disappearance. Their total salinities are between 42 to >70 wt % NaCl equ. Total homogenization to a liquid phase was in the range from 415° to >600°C under the minimum pressure of 0.3 to 1.2 kbars, indicating minimum 1 to 4 km of overburden under lithostatic conditions.Vapour-rich inclusions show presence of CO2±CH4 within the vapour phase, but the homogenization attempts failed, and further FIs measurements are needed. The winding, wavy shape of dykes and high Th of FIs in magmatic quartz suggest ductile conditions during intrusion of dykes. Volcanic rocks on the island of Vis are situated around a small village Komiža. One can distinguish sub-volcanic types represented by augite-andesite, spilite-keratophyres and spilites. At the coast a submarine lava flow is made of rare pillow lava lobs, volcanic agglomerates, lapillistones, lapilli tuffs, tuffs and hyaloclastite, formed by autobrecciation. Amygdales and vesicles filled with chlorite, prehnite and calcite, in radial patterns are commonly less than 1 mm, but rarely exceed 0.5 cm. Prehnite from the Komi`a Bay, Vis Island, fills amygdales and vein lets within pillow lavas and represents the product of hydrothermal alteration of basalt under seawater-dominant conditions. Petrographic observations and microthermometric measurements of prehnite distinguish three types of primary FIs. Low saline (5.2–6.3 wt% NaCl equ.) aqueous two-phase (L+V) FIs with Th between 110°–125°C represent seawater relicts. Interaction with basaltic lava slightly increased salinity. The coexistence of aqueous three-phase (L+V+Siso ; salinity = 29.3–30.5 wt % NaCl equ. ; Th = 135°–180°C) and vapour-rich (V+L ; Th=135°–180°C) FIs points to precipitation from boiling fluid. Assuming modified seawater salinity around 5.5 wt%NaCl equ., incipient boiling (135°C) took place at a depth of 20 m. Recent research of the Jabuka and Brusnik intrusives revealed their age determined by Ar/Ar method on the gabbro, felsic dykes, and a coherent basalt fragment from Komi`a on the Vis Island. Biotite from the Jabuka brittle dyke gives a plateau age of 273±1.1 Ma, with loss of Ar around 77.2±2.4Ma. K-feldspar from the Jabuka brittle dyke is unaffected and shows similar plateau age of 269.4±2.3 Ma, but K-feldspar from the additional dyke on the Jabuka is dated at 254.5±2.0 Ma (Fig. 1a). Biotite in the Brusnik dolerite is dated at 265±1.1 Ma, while K-feldspar has a plateau age of 214±0.9, obviously lowered due to release of Ar, during some later thermal episode. K-feldspar separated from the basalt-andesitic fragment in the volcanics from Komiža, Vis Island, gave a plateau age of 276.6±1.7 Ma coherent with the other magmatic representatives. The ages determined on biotites are more consistent, and differ from those obtained on K-feldspars, which was the object of the previous dating. This is explanation why the former K/Ar and Ar/Ar dating gave ages which are not completely compatible with the geotectonic events in the early history of Tethyan evolution. Upper Permian ages correspond to the early intra-continental rifting and the geochemistry to the within plate magmatism. Evaporites on the Vis Island, near Komiža are in tectonic contact with volcanic rocks. Sulphur isotope values, d34S, 16.1, 16.3, 16.6, 17.1, 17.1, (average 16.6) might be attributed to the secular variation of the Lower and Upper Triassic sea water, what is confirmed by polen analysis.

basalt; gabbro; rifting; Ar/Ar date; evaporites; sulphur isotopes; Adria plate; fluid inclusions

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