engleski

On granite emplacement mechanisms at Moslavačka Gora (Croatia): the role of the xenolith studies

Xenoliths are foreign fragments of sedimentary, metamorphic and igneous rocks, mainly angular in shape with sharp contacts toward the surrounding igneous host rock. They are predominantly found near pluton margins, especially in their apical parts. Fragmentation of country rock and entrainment of xenoliths by the intruding magma is directly related to the magma emplacement processes in the upper crust. One of the emplacement mechanisms, called stoping, involves fracturing of the wall rocks and floating or sinking of the detached blocks. The main field evidence for the stoping mechanism include: 1) sharp, discordant contacts between plutons and wall rocks, 2) a lack of pluton-related ductile deformation of wall rocks, 3) xenoliths in plutons, 4) mixed population of xenoliths in plutons, 5) evidence for rotation of xenoliths, and 6) geochemical evidence for magma contamination (GLAZNER & BARTELY, 2006). In the case of Moslavačka Gora (MG), granitoid rocks comprising a central part of the crystalline are associated with various types of medium- to high-grade metamorphic rocks that are also present throughout the pluton in form of xenoliths of different sizes (KOROLIJA & CRNKO, 1985 ; PAMIĆ, 1990). One of the best xenolith exposures can be found in an abandoned quarry of Pleterac. At the studied locality, a leucocratic host rock, typical MG two-mica granite, hosts various sizes of metamafic xenoliths (amphibolites). They are characterized by angular shapes and mainly show sharp contacts toward the granite. Leucogranite dykes of variable width (cm to m) crosscut the two-mica granite and sometimes xenoliths can also be observed throughout the quarry. Microstructurally, the studied amphibolite samples collected from xenoliths are divided into three groups. All of them are characterized by the well-developed foliation but variable mineral proportions resulting in more massive (Hbl>>Pl>Di), net-structured (Pl=Hbl>Di) or distinct stromatic appearance (Hbl>Di>Pl). Stromatic amphibolites contain significant amount of diopside following the preferred orientation of amphibole in the sample. Furthermore, three types of leucocratic rocks were sampled in the quarry, two of them corresponding to the already mentioned two-mica granites and leucogranites, while the third rock type was classified as trondhjemite. Clear chemical distinction can be made between the trondhjemite and the two granite types based on the REE trends, accompanied by high SiO2 (85.45 wt.%), low K2O (1.27 wt.%) and a pronounced positive europium anomaly (Eu/Eu*=4.05), high LaN/YbN (5.06) and low YbN (1.15) (Fig. 1a). The occurrence of trondhjemite and diopside in the metamafic xenoliths, together with the coarse equigranular nature of the leucocratic segregations and the development of accompanying Hbl-rich melanocratic envelopes, indicates a process of dehydration melting of amphibole through the reaction Hbl + Pl + Qtz = Di  Ttn  Grt + trondhjemite melt (HARTEL & PATTISON, 1996). Field evidence points to segregation of the produced trondhjemitic melt and formation of lenticular to vein-like leucosomes that are predominantly concordant with the main foliation in the xenoliths, whereas effects of melt migration can be observed only locally. Additionally, our observations, especially the uniformity of textural features throughout the blocks on the macro- and microscale, imply that the anatectic reaction mentioned above took place before the entrainment of xenoliths by the intruding magma, relating it to an earlier metamorphic episode of the MG crystalline evolution. The uniformity of textural features throughout the amphibolite xenoliths in Pleterac, sharp discordant contacts between the blocks and the host granite, and the relative rotation of the blocks point to brittle behavior of the country rock during the intrusion of the two-mica granite body. On the other hand, subordinate leucogranites cross-cutting the two-mica granite and the metamafic blocks were emplaced subsequently through dyking mechanism (Fig. 1b). Our observations thus show that the stoping process was one of the mechanisms involved in emplacement of granitoid pluton at MG.

Moslavačka Gora, stoping, pluton, trondhjemite, xenoliths

Rad je sufinanciran sredstvima Zaklade Hrvatske akademije znanosti i umjetnosti za projekt "Mehanizmi nastanka, smještavanja i evolucije ksenolita prilikom intruzije granita u stijene metamorfne podloge Panonskog bazena (Moslavačka gora, Hrvatska)" (nositeljica doc. dr. sc. Z. Petrinec), natječaj 02/2018.