DATING OF NEOTETHYAN INCIPIENT RIFTING MAGMATISM on THE MID-ADRIATIC ISLANDS JABUKA, BRUSNIK AND VIS, CROATIA

The Mid-Adriatic island Jabuka, Brusnik and western part of the Vis island, are built of basic rocks placed within the Mesozoic successions of the Adriatic carbonate platform. Jabuka and Brusnik islands are built exclusively of igneous rocks, of two-clinopyroxene gabbros, derived from crystallization of differentiated subalkaline magma. The late stage dykes, with higher content of potassium cross-cut the main magmatic bodies in ductile and brittle manner. The trace elements and the MORB-normalized, spider-diagram support similarity of the gabbros with continental tholeiitic magmas, excluding MORB type affinity, widespread in ophiolitic sequences. The high content of Al₂O₃, alkali elements, major and trace elements are comparable with Triassic magmatic rocks of Alps, Dinarides and Hellenides, developed in the extentional rift environments. Triassic platform carbonates and evaporates on the Vis island are associated with volcanic rocks, which are formed as lava littoral cone. The parent magma is, however, the same. The first radiometric dating, and detailed mineralogical and geochemical study was done on the Jabuka by Balogh et al. (1994). The K/Ar ages obtained on the whole rock and mineral fractions differ insignificantly. The avergage K/Ar age is 205 ± 6.9 Ma, and two isochrones gave 200.3 ± 7.9 and 199.5 ± 11.9 Ma with little amount of excess radiogenic Ar. The dating, however, suggests uppermost Triassic, what does not correspond with the major phase of Triassic rifting, although calc-alkaline character excludes MORB type lithotypes (Balogh et al., 1994). De Min et al. (2008) obtained, by analyzing plagioclase separates on Jabuka and Brusnik, a new Ar/Ar plateau ages of 227 ± 5 Ma and an isochrom 226 ± 3 Ma for Jabuka, and plateau age 226 ± 3 Ma for Brusnik. The age is shifted toward the Middle Triassic time what is a better fit to the general rifting conditions in the area. Recent research on the Jabuka and Brusnik intrusives (Palinkaš et al., 2010) revealed a plateau age of 273 ± 1.1 Ma on biotite, from the Jabuka gabbro, with loss of Ar on the 77.2 ± 2.4 Ma. K-feldspar, from the Jabuka gabbro, shows similar value of 269.4 ± 2.3 Ma, but K-feldspar from the dyke on the Jabuka is dated 254.5 ± 2.0 Ma. Biotite in the Brusnik dolerite is dated at 265 ± 1.1 Ma, while K-feldspar has plateau age 214 ± 0.9, 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 coherent age with the other magmatic representatives of 276.6 ± 1.7 Ma. The ages determined on biotites are more consistant, and differ from those obtained on K-feldspars, which was the object of the previous dating. This explains why the former K/Ar and Ar/Ar dating gave ages not compatible with the geotectonic events in the early history of the Tethyan evolution. Upper Permian ages obtained recently correspond to the early intra-continental rifting and the geochemistry of the within plate magmatism.

Balogh, K., Colantoni, P., Guerrera, F., Majer, V., Ravasz-Baranyai, L., Renzulli, A., Veneri, F. & Alberini, C. (1994): The medium-grained gabbro of the Jabuka islet (“Scoglio del Pomo”, Adriatic Sea). Geologia (Bologna), 56 (2): 13–25.

De Min A., Jourdan, F., Marzoli, A., Renne, P.R., & Juraèiæ, M. (2008): The tholeiitic Magmatism of Jabuka, Vis and Brusnik Islands: A Carnian magmatism in the Adria Plate. Rendiconti online Soc. Geol. It., 2, 1/3.

Palinkaš, A.L., Borojeviæ Šoštariæ, S., Neubauer, F. Strmiæ Palinkaš, S., Razum, I. & Molnár, F. (2010): Ar/Ar dating of magmatism on the Mid-Adriatic islands Jabuka, Brusnik and Vis, Croatia. 4th Croatian Geological Congress, Šibenik, submitted.