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Cretaceous alkali magmatism within Slavonian Mts. (Mts. Požeška Gora and Papuk)

Progressive closure of the Neotethys Ocean followed by the intra-oceanic subduction in the Jurassic, finally ended with the Late Cretaceous – Paleogene continent-continent collision between tectonic Tisia Mega-Unit (part of the European plate) as the upper plate and Inner Dinnarides (Adria microplate) as the subducting plate (SCHMIDT et al., 2008). This collision formed a large suture zone named Sava(-Vardar) Zone (PAMIĆ, 2002) ; a belt with different type of rocks that bare evidences of this geodynamic event(s). Among them, rocks investigated in this study are rhyolite from the Rupnica locality (Voćin) and red granite from Mt. Požeška Gora (Gradski Vrhovci). The former one is also known for the well exposed phenomenon of columnar jointing which is interpreted as a consequence of a rapid cooling (BALEN & PETRINEC, 2014). The rocks from this two localities, although presently 35 km apart, have similar geochemical characteristics and contemporaneous ages, suggesting the same source of alkali magmatism, as it was previously proposed by PAMIĆ (1987), PAMIĆ & LANPHERE (1992) and PAMIĆ et al. (2000). Rupnica rhyolite is mainly composed of albite and quartz with minor clinopyroxene and amphibole. Zircon, apatite, anatase and Fe- oxides are accessory phases. Mt. Požeška gora granite is mainly composed of alkali feldspar (perthite) and quartz with minor albite. Hematite (responsible for the rock colour), zircon, apatite and monazite are accessory phases. Both rocks have highly-siliceous composition (66-76 wt.% SiO2), are enriched in alkalies (8.1−9.2 wt.% K2O+Na2O, high-K calc- alkaline series) and classify as an alkali rhyolite i.e. alkali granite. They both belong to a group of peraluminous, oxidized and ferroan rocks with low CaO, MgO, MnO and FeOT contents, but with high FeOT/(FeOT+MgO) ratios (MI=0.87−0.98), which is similar to magmas derived from the lower crust and are typical for an A-type of granitic rocks (WHALEN et al., 1987). Some specific elemental concentrations combined with specific elemental ratios (such as K/Ba, K/Rb, Rb/Sr, Zr/Hf and Th/U) and zircon geochemistry imply crustal but also mantle signature in the melt, where rhyolite might have had a slightly more mantle contribution than the granite. The investigation of zircon morphology after PUPIN (1980) resulted in the dominance of the G1-type for the rhyolite and D-type for the granite. Both types are ascribed to an alkali and dry A-type of magma with the origin in the lower crust or even upper mantle, where G1-type is characteristic for somewhat lower temperatures compared to D-type. Calculated Zr- saturation temperatures are high for both rocks (840−870°C for rhyolite ; 860−950°C for granite) compatible with rhyolite high Ti-in-zircon and apatite saturation temperatures (930°C and 900°C, respectively). Geochemical discrimination diagrams place the investigated rocks into a within-plate setting and to a lesser extent to a volcanic arc setting as a post-collision rhyolite/granite. Moreover, Rupnica rhyolite and Mt. Požeška Gora granite both show the geochemical signature of an A2- type (EBY, 1992), derived by melting of subcontinental lithosphere or lower continental crust with or without mantle input. This type of rocks can be found in the post-collisional i.e. post-orogenic settings, originally formed by subduction or continent-continent collision. Zircon ages of Rupnica rhyolite obtained with LA-ICP-MS and determined from 206Pb/238U, 207Pb/235U and 208Pb/232Th ratios are 81.2±1.0, 82.9±0.9 and 81.6±0.9 Ma, respectively. Those ages are contemporaneous with the recently determined zircon ages from Mt. Požeška Gora granite, which are 85.9±1.4, 86.3±1.0 and 86.1±0.8, respectively (BALEN et al., in review). Studied igneous rocks therefore indicate the local transition from compressional to extensional tectonic regime at about 86–82 Ma.

alkali magmatism ; extension ; Cretaceous ; zircon ; Europe-Adria collision

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