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Complex Cretaceous evolution of the Moslavačka Gora crystalline: different aspects from various types of “foreign” and “cognate” enclaves inside granites

The Moslavačka Gora (MG), together with the surrounding basement, is mainly made up of crystalline rocks showing mostly Cretaceous age which can be divided into three groups: (1) granitoids, (2) migmatites, and (3) amphibolite facies metamorphic rocks. The granitoid pluton itself comprises about 110 km2 of the exposed crystalline hosting several types of granitic rocks, as inferred from field, petrographic and geochemical studies. All types of granites show similar chemical signatures defining them as S-type granites. On the basis of trace element distribution (especially REE’s) a clear distinction has been made between the two most abundant granite types: two-mica (predominantly biotite) granite, and two subtypes of leucogranites. The first type, two-mica granite regularly hosts different types of enclaves. The two subtypes of pegmatitic and aplitic leucogranites both penetrate two-mica granite and are not devoid of enclaves, although they are less abundant than in the previous type. “Foreign” enclaves are characterized by the composition and origin that is not related to that of their host rock. Various types of xenoliths, representing mostly parts of the MG metamorphic complex, have been ascribed to this category: surmicaceous and/or biotite-rich xenoliths, gneissic xenoliths, cordierite xenoliths, amphibolite xenoliths and quartz xenoliths/xenocrysts. Contrarily, “enclaves homoéogènes” or “cognate” enclaves show evidence of mineralogical or genetic links to their host rock. They resulted from the processes that took place in the magmatic body itself i.e. granitoid pluton. These are, for example, dark granular enclaves called mafic microgranular / microgranitoid / magmatic enclaves (MME) and tourmaline nodules. Another feature can be also regarded as a type of “cognate” enclaves – miaroles. Inside the MG magmatic system there are also numerous “inhomogenities” like pegmatitic and aplitic bodies, tourmaline “suns” and K-feldspar megacrysts. From their characteristics, it is evident that “foreign”enclaves (xenoliths) give information on the undoubted continental crust contribution to the studied magmatic system, mode of emplacement of granitoid magmas, nature of country rocks and the relative age of the granitoid pluton. Preliminary geochemical model for production of melts inside MG magmatic system indicates muscovite dehydration melting at low aH2O of a (meta)pelitic source which is, at least partly, consistent with field evidence and observations made on enclaves. “Cognate” enclaves, on the other hand, hold a different type of information – primarily that concerning the evolution of the MG igneous body. Being found inside the host granite, they support a scenario according to which magma escaped from the plutonic depths, reached shallow level, where it stalled in a low-pressure crustal setting. This was already inferred from the field occurrences of miarolitic cavities and the presence of magmatic andalusite inside the host granite. P-T conditions at the crystallization level of the host granite have been obtained from the zircon saturation temperature and REE thermometry (ca. 720–730°C). From these temperatures, pressure values have been estimated to 70–270 MPa (on average ~ 5 km depth) and water content of the melt up to 4.2 wt%. Decreasing pressure and related cooling were major factors controlling the melt differentiation, liquid immiscibility, fluid behaviour, partitioning of elements and the crystallization path of the two-mica granite as an integral part of the main plutonic body. Present geological relations inside the Moslavačka Gora crystalline reflect a complex history of the metamorphic and igneous evolution during multiple, closely related, Cretaceous events. Metamorphic evolution that preceded igneous events remained recorded in the metamorphic part of the complex that became the source for the “foreign” enclaves encapsulated inside the intruding granite magma. The granite body itself shows evidence of at least two subsequent igneous events – their signature was left behind in the form of various types of “cognate” enclaves, rock textures and relation among known rock types, their mineral assemblages and geochemical features.

granite; enclave; xenoliths; Cretaceous; Moslavačka Gora

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