Magmatic nanogranitoids in the Late Cretacous zircon from Slavonian Mts. (Croatia)

Micro-Raman spectroscopy was used to determine the inclusions in early-crystallised magmatic zircon from the Late Cretaceous (~82 Ma on zircon) acidic igneous rocks in the Slavonian Mts. (Mt. Papuk and Mt. Požeška Gora), located in the southwestern part of the Pannonian Basin (Croatia). The host rocks (granite and rhyolite) are predominantly peraluminous and alkali to calcic-alkali and were formed from high-temperature (up to 950 °C), ferroan and oxidised, dry A₂-type of magma.

The mineral inclusions detected in the zircon from granite and rhyolites are anatase, apatite, hematite, ilmenite and possibly magnetite. Although anatase is generally considered to form in low-temperature hydrothermal environments and is regarded as metastable, experimental studies in the field of materials science have shown that anatase, compared to rutile, is being stabilised in the systems under a high cooling rate.

Numerous multiphase solid inclusions contain various polymorphs of feldspars (albite, K-feldspar, kokchetavite and kumdykolite) and SiO₂ (quartz or cristobalite), hematite and phyllosilicates (kaolinite and muscovite). Such mineral association confirms the existence of an early granitic melt; therefore, this type of inclusion can be regarded as melt inclusions. Melt inclusions with such composition represent nanogranitoids, commonly found in peritectic garnets from high grade metamorphic rocks. Compared to the nanogranitoids of anatectic origin previously reported in partially melted rocks, here they represent primary inclusions of parental magma, i.e. magmatic nanogranitoids, that were protected from later equilibration with the melt or alteration by fluids. Furthermore, here we present the first finding of kokchetavite and kumdykolite in a magmatic zircon. This finding is a strong evidence that kumdykolite and kokchetavite do not require ultra-high pressure (UHP) to form and therefore should not be considered as exclusively UHP phases. Instead, together with anatase, these polymorphs are likely evidence of rapid uplift and consequent rapid cooling of hot oxidised magma generated in the deep (lower) crustal level. The rapid uplift was most likely aided by the weak zones in the continental crust, such as deep faults in a local back-arc extensional (half-)graben in the course of the Late Cretaceous regional geological event associated with the closure of the Neotethys Ocean in the area of the present-day Slavonian Mts.