The Late Cretaceous A-type alkali-feldspar granite from Mt. Požeška Gora (N Croatia): Potential marker of fast magma ascent in the Europe–Adria suture zone

Abstract

An alkali-feldspar granite (Požega granite) of reddish colour occurs in northern Croatia in the Cretaceous suture zone (Sava Zone) between the collided plates of Europe and Adria (Africa). This granite is mainly composed of alkali feldspar (perthite) and quartz, with small amounts of albite. Accessories are hematite with ilmenite exsolution, zircon, apatite and monazite. Anatase, rutile (?), kokchetavite, and kumdykolite are found only as inclusions in zircon. The granite shows a geochemical signature typical for an A2-subtype granite, characterized by a peraluminous, highly siliceous and alkaline composition, and belongs to the group of oxidized and ferroan granites with low CaO, MgO, and MnO contents and high FeOT / FeOT + MgO ratios. Trace element contents plotted in chondrite and primitive mantle normalized element diagrams show positive anomalies of K, Pb, and Zr and negative anomalies of Ba, Nb, P, Eu, and Ti. Based on whole-rock geochemical data, the magma originated mainly from melting of lower continental crust. According to the zircon typology (D and J5 types prevail), zircon and whole-rock chemistry, and high Zr-saturation temperatures (T=860–950 °C), the melting process at high temperature and dry conditions could have been triggered by upwelling hot mantle. The ascent of the thus produced A-type granitic magma into the Europe–Adria suture was fast. The Požega granite indicates the transition from compression to extension accompanied by opening of a sedimentary basin. According to the 206Pb/238U versus 207Pb/235U concordia age determined on zircon, this event occurred 83.6±1.5 Ma ago.