Jabuka Shoal , a New Location with Igneous Rocks in the Adriatic Sea

The central part of the Adriatic Sea is situated between the western coast of Croatia and the eastern Italian coast, and its shape is characterised by the surrounding Alpine mountains. Two mountain chains, the Apennines and the Dinarides, mark its edges, predetermine its nature, and separate it from the western Mediterranean, and from the Eastern Alps and the Pannonian Basin, respectively. In the Adriatic Sea area, occurrences of igneous rocks are exceptionally rare at the surface but have been observed since the 18th century (DONATI, 1750) and recognised since the end of the 19th century and later (HAUER, 1882; KIŠPATIĆ, 1892; TUĆAN, 1953; BARIĆ, 1961; GOLUB & VRAGOVIĆ, 1975; BALOGH et al., 1994). The igneous rocks from the western part of Vis Island were classified as augiteporphyry and melaphyre (HAUER & STACHE, 1862). CARELLA (1961) named them augite-andesite and augite basalt. GOLUB & VRAGOVIĆ (1975) determined them as extrusive types with augite-andesite, spilite-keratophyre and spilite. In contrast, the rocks that form the entire Brusnik and Jabuka islets, to the west of Vis Island are regarded as intrusive rocks because of their texture and were classified as augitediorite (FOULLON, 1883), and diorite (PELLERI, 1942). KIŠPATIĆ (1892) and RAFFAELLI (1977) Jabuka Shoal, a New Location with Igneous Rocks in the Adriatic Sea


Jabuka Shoal, a New Location with Igneous Rocks in the Adriatic Sea
Mladen JURAČIĆ 1 , Anđelko NOVOSEL 2 , Darko TIBLJAŠ 1 and Dražen BALEN 1 found diabasic texture in these rocks and named them diabase and quartz-diabase.GOLUB & VRAGOVIĆ (1975) considered them to be subvolcanic augite-diabase, VRKLJAN (1979) named the Jabuka Islet rocks as augite diabase and spilitized augite diabase, whereas BALOGH et al. (1994) as medium-grained gabbro.Recently, GARAŠIĆ et al. (2001), described dolerite from the islet of Brusnik.The main difference between rocks found on the Jabuka and Brusnik islets is in their appearance.Jabuka Islet is probably a core of a small magmatic body while Brusnik Islet is partially built up of conglomerate with pebbles up to 50 cm in diameter (CRNJAKOVIĆ, 1998).
The radiometric ages (K-Ar 215-200 Ma, and Ar-Ar 200 Ma) measured on gabbro from Jabuka Islet reflects the latest stage of crystallization after the intrusion, since negligible rejuvenating processes due to syn-and postmagmatic hot fluids circulation have been inferred (BALOGH et al., 1994).This age corresponds to the Triassic-Jurassic boundary and could be interpreted as a late stage of rift-related magmatism.
The aim of this paper is to present a new locality with igneous rocks in the Adriatic Sea, provide basic petrographic and geochemical data, and discuss their significance.

RESULTS AND DISCUSSION
Nautical maps reveal few shoals in the region near to the Jabuka and Brusnik islets and we presumed that some of the shoals might also have igneous rocks.Research in the summer of 2001 concentrated on the shoal approximately 2300 m WNW of Jabuka Islet, as indicated/mapped on the coastal chart (HHI, 2000) 100-22 Jabuka-Vis, Scale 1:100,000.The location of the Jabuka Shoal is 43°06'06"N, 15°26'21"E.The top of the shoal is located 6 m below sea level.Its location and relationship to other islands in the region is presented in Fig. 1 (after HHI, 1980).No precise map of the Jabuka Shoal exists.
In order to locate sampling points the Jabuka Shoal was measured and a sketch of the top of the shoal was produced (Fig. 2a and b).Transects where rock sampling was performed, at different depths on the flanks ("cliffs") are also presented.A large part of the shoal is covered with algae (fucus) and it was rather difficult to sample the source rock.In appearance it is interesting that at the Jabuka Shoal a massive (monolithic rock) along with cobbles and pebbles (conglomerate) is present, a combination of the way the rock appears at Brusnik Islet.

Petrography
Samples from Jabuka Shoal (Fig. 3) are mostly mediumgrained rocks of hypidiomorphic holocrystalline texture having a massive structure.However, some portions of rock characteristically resemble ophitic texture while other parts rarely contain spherulites, amygdales and fissures, filled with non-oriented needles of prehnite.
The most common mafic minerals are clinopyroxenes (pale green and brown monoclinic pyroxene).The first one is slightly zoned and shows weak pale green pleochroism.The distribution of fine-grained opâque minerals marks zoning in the clinopyroxene.Uralitization altered clinopyroxene and produced secondary green amphibole.In these mineral aggregates chlorite is also present as a product of a later alteration process.The plagioclase commonly displays weak zoning with a more basic core and an acidic rim.Usually plagioclase contains sericite flakes and non-oriented opâque mineral inclusions (poikilitic texture).Also it shows polysynthetic twins.Accessories are opâque mineral(s) and apatite.
All of the primary minerals are altered to various degrees.In spite of the fact that the rock suffered changes up to prehnite-pumpellyite facies, the presence of small amounts of newly-formed minerals mean that the rock is still determined as gabbro.
Petrographic data reveal that the samples from Jabuka Shoal have similar structural, textural and compositional features compared to the ~2 km distant Jabuka Islet as described in detail in BALOGH et al. (1994).

Geochemistry
Major and trace element contents of rock sample PJ 2/18 from the Jabuka Shoal, analyzed by the XRF method in the Activation Laboratories Ltd., Canada, along with a sample from Jabuka Islet (JA01/4) and data from BALOGH et al. (1994) and from GOLUB & VRAGOVIĆ (1975) are given in Table 1.The low content of MgO (3.49 wt.%), low magnesium number (41.0) and also the low content of Ni and Cr (8, <5 ppm, respectively) show that the magma was not primitive in its nature.The major element content of Jabuka Shoal rocks is similar with those of the nearby Jabuka Islet magmatic rocks.Figure 4 -TAS classification diagram (LE BAS et al., 1992) shows that both the Jabuka Shoal and the Jabuka Islet rock samples plot in a relatively narrow area overlapping the basaltic andesite and basaltic trachyandesite fields.The trace elements distribution reflects the major element content.
A diagram after WINCHESTER & FLOYD (1977) based on immobile trace elements (Fig. 5) shows that the samples chemically correspond to the basalt/ andesite field.
The presented results indicate that magmatic melts moved upwards along fractures into the overlying continental crust.In the newly formed "magma chambers", various differentiation and crystallization processes took place and finally produced gabbro.
The position of three known localities (Jabuka, Brusnik and Komiža) together with the new Jabuka Shoal occurrence, 2300 m west of the Jabuka Islet, show a linear arrangement that can be set up as a superficial marker of fault system(s) which predisposed the occurrences of the magmatic rocks.

CONCLUSION
The main contribution of this research and the discovery of magmatic rocks on the Jabuka Shoal is that the linear arrangement indicates a structure connecting the Jabuka Shoal, Jabuka and Brusnik islets and Komiža.They may be regarded as an important superficial marker of a fault system(s) which predisposed the occurrences of magmatic rocks.

Fig. 2
Fig. 2 (a) Sketch of the top of the Jabuka Shoal with transect locations PJ1 and PJ2; depths in m; (b) schematic transects with sampling locations.