Multiple processes in the genesis of the Pohorje igneous complex: Evidence from petrology and geochemistry

Abstract

The present contribution reports new whole-rock major and trace element data, REE, and Sr–Nd isotopic ratios for the early Miocene Pohorje igneous complex (PIC) in Slovenia (Eastern Alps), which, along with published data, are used to investigate its genesis and evolution. The complex comprises three main rock-groups. The largest one is made of granodiorite and minor tonalite (GTD) enclosing mafic microgranular enclaves (MME), and a small Q-dioritic body. The second group prevailingly consists of dacite stocks and dykes, and porphyritic microgranodiorite (DAMG). Andesitic dykes (AD), intruding metamorphic rocks and less often the GDT, constitute the third group. Aplites and pegmatites intrude both the Pohorje igneous complex and the country rocks.Evidence of interaction between magmas has been observed only in the GDT rocks. In the southeastern part of the complex the GDT rocks are less evolved and emplaced at greater depth (ca. 16.5–17.6 km) than the rocks cropping out in the northwestern part (ca. 12.4–13.8 km). DAMG rocks can be divided geochemically into two groups, high-Ga DAMG and low-Ga DAMG, having similar chemical composition with the most evolved GDT, and the most evolved andesitic dykes, respectively. The andesitic dykes can be divided into three groups that differ in LILE contents from each other up to three orders of magnitude.A Mixing plus Fractional Crystallization (MFC) process between the least evolved MME and the most evolved DAMG rocks is considered responsible for the formation of the two diversely evolved GDT. Two of the three AD groups (AD1, AD3) originated in the mantle, whereas the third one (AD2) is considered as the result of an AFC process between the less enriched andesite (AD1) and continental crust (gneiss). The same process is considered responsible for the low-Ga DAMG genesis.The most mafic magma is presumably the result of melting of mantle wedge differently metasomatized and enriched in LILE due to fluids, sediment melts, and bulk sediments. The felsic end-member magma of the MFC process originated from partial melting of intermediate-lower crust having intermediate to basaltic composition. Geodynamic sequence for the formation of the PIC comprehend mantle metasomatism during Alpine subduction, thickening of crust during the collision that followed subduction, and production of mantle melts in response to delamination caused by opening of the Pannonian basin and asthenospheric upwelling. Crustal derived melts resulted by underplating of mantle-derived magmas.