engleski

Lithospheric extension in the east Adria: evidence from ultraalkali volcanic rocks in the Pliocene sedimentary succession of Dugi Otok depression

The Dugi Otok depression located along the north Dalmatian archipelago in the east Adria is a remnant of Oligocene to Miocene foredeep basin developed in front of the External Dinaridic thrust belt. The depression records high-intensity Late Miocene transpressional tectonic deformation resulting in flower structure systems. During Messinian Time the whole area was emerged and eroded, as documented on seismic sections by clearly expressed angular unconformity. The rapid Pliocene transgression, which followed, cowered entire area with less than 200 m of sediments, consistent to low rate of sedimentation observed on basin highs and lows throughout the submerged Adria. Fragments of intraplate volcanic rocks have been collected from drilling fluids documenting 40-60 m thick intervals, which were not cored, within the Pliocene sedimentary succession in two exploration wells located around 50 km west from Zadar. One well contains only nephelinites whereas in another nephelinites superimpose leucitites suggesting younger age of the former. Volcanic pieces are angular (<0.6 mm), likely crushed short before being entrained into the drilling mud. In the area of Dugi Otok depression the onset of the Pliocene transgression over the Miocene sedimentary succession coincides with the first appearance of planktonic foraminifera Globorotalia margaritae and abundant glaucony in the alternating layers of marls and poorly consolidated clayey siltstones and sandstones. The greenish glaucony pelets show composition of K0.73(Fe1.38Al0.10Mg0.54)[Al0.34Si3.66O10|(OH)2] which is typical of evolved glauconitic mica, i.e. glauconite. K-Ar isotopic measurement performed on the separate of glauconite pelets with the highest magnetic susceptibility yielded the age 5.7± ; ; 0.3 Ma. Transformation of the glauconitic smectite to glauconite under favourable conditions in a starved sedimentary basin may take 0.1– 1.0 Ma. The volcanic material-bearing horizons contain fragments of melilite-bearing leucitites and nefelinites. The lowest horizon is marked by extinction of Gt. margaritae and occurrence of planktonic foraminifera Globorotalia puncticulata to Gt. bononiensis and the group of Globorotalia crassaformis, and thus the age of deposition can be constrained to the stratigraphic range of the Globorotalia puncticulata biozone reported between 4.52 and 3.57 Ma. Around 30% of foraminifera from this horizon have greyish pyritised sceletons suggesting that volcanic eruptions may have been coeval to the deposition. The uppermost level of the volcanic-pieces-bearing horizon is characterized by the first appearance of Globorotalia inflata. The appearance of Globorotalia inflata in the Periadriatic foredeeps is slightly younger than 2.13 Ma, as it may be inferred from the age of a volcanic ash layer intercalated in the basins sediments, suggesting this age as time of cessation of volcanic activity in the Dugi Otok depression. Leucitite fragments show porphyritic texture with leucite and pale green clinopyroxene set in seriate matrix composed of phenocrystic assemblage and minor olivine, melilite, nepheline, phlogopite, magnetite and apatite. Leucite has almost stoichometric composition. Clinopyroxene is diopside with range of Al abundances (2.3– 9.7 wt.% Al2O3) and relatively low Ti (<2.3 wt.% TiO2). Most clinopyroxenes have homogeneous Mg-core and thin Fe-rich rind whilst some are oscillatory zoned. Olivine has homogeneous core (Fo73.8-83.0) and thin normally zoned Fe-enriched rim (Fo46.6-72.8) with Mn and Ca continuously increasing to the grain periphery up to 2.3 wt.% MnO and 3.1 wt.% CaO. Nepheline is saturated (Ne69.2-63.1Ks34.7-30.3Q2.5-0.2) or slightly undersaturated. Melilite is enriched in å ; ; kermanite (65.4– 74.6%) and Na-melilite (28.1– 37.0%) with minor gehlenite and Na-ferrimelilite components. Flogopite is high in Mg and Ba (Mg# = 0.81– 0.88 ; 5.3– 9.4 wt.% BaO) and relatively low in Ti (1.1– 2.2 wt.% TiO2). Magnetite has very variable composition in the term of Ti, Al and Mg (0.6-9.9 wt.% TiO2, 1.1– 7.9 wt.% Al2O3 ; 0.04– 2.3 wt.% MgO). Nephelinite slivers are porphyric with phenocrysts of olivine, black clinopyroxene and magnetite in the hollocrystalline matrix constituted of clinopyroxene and interstitial nepheline, minor olivine, melilite, magnetite, and accessory perovskite and apatite. Olivine phenocrysts are normally zoned (Fo86.6-84.3 in core, Fo82.9-81.0 at rim). Matrix olivine composition is similar to the phenocrystic rim. Clinopyroxene is diopside showing homogeneous high Mg# core and complex zoning patterns towards periphery. They are enriched on Al and Ti (up to 12.2 wt.% Al2O3 and 7.7 wt.% TiO2). Nepheline is either saturated (Ne76.7-74.0Ks23.9-19.5Q2.4-0.9) or slightly undersaturated. Ti-magnetite shows enhanced abundances of Mg and Al (up to 7.1 wt.% MgO and 4.5% Al2O3). Melilite is enriched in å ; ; kermanite and Na-melilite (62.2-69.7% and 28.2-37.0%, respectively) and low in gehlenite and Na-ferrimelilite (<3.4% and 0.9%, respectively). The pre-Miocene anorogenic within-plate volcanism within the Adriatic segment of the Adria microplate is well documented for the Punta delle Pietre Nere on Gargano penninsula and Veneto Volcanic Province. The volcanism is interpreted as reaction to tensional tectonics affected these foreland provinces during the Apenninic and Alpine orogenesis, respectively. It seems that these volcanic provinces were not reactivated during post-Late Miocene tectonic deformations of Adria microplate. Post-Late Miocene deformation of the Adria resulted in pervasive internal deformations and development of major fractures in the northern Adriatic foreland. The distributions of the analysed volcanic rocks clearly trace the northern sector of proposed Nubia-Adria plate lithospheric boundary. Moreover, our data are perfectly consistent to the geodynamic model of intracontinental passive rifting wherein an asthenospheric ultraalkaline volcanism postdates cracking of the lithosphere by a time gap of 2-3 Ma.

Adriatic off-shore; Pliocene; leucitites; nephelinites; intraplate volcanism

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