Geology and petrogenesis of Topuk granitoid (NW Anatolia)

In this study, geological, petrographic, and petrologic features of Topuk granitoid and the magmatic rocks associated with it has been studied. Topuk granitoid is located at the Northwest Anatolia, in Orhaneli county of Bursa province, covering 180 km2 of area. The study area consists of basement rocks, magmatic rocks, sedimentary rocks and alluvium. Petrographic studies and field studies showed that the basement rocks consist of ophiolitic rocks and metamorphic rocks. Kocasu formation, which creates the very base of the basement rocks in the area, are made out of metapelites and metavolcanites (metatuffs and metalavas), which has gone through metamorphism in blueschist facies. İnönü marble includes a massive marble with calcite crystals and occasional dolomite layers, which also have gone through blueschist metamorphism. Ophiolitic rocks, which are determined as Orhaneli ophiolite has overlain these metamorphic rocks with a tectonic contact (thrust fault). Orhaneli ophiolite consists of dunite, lherzolite and serpantinite, and is suggested to be the base of Neo Tethys ocean. Sedimentary rocks overlay the basement rocks and magmatic rocks in the study area, and made out of basal conglomerates with basement and magmatic rock pebbles, sandstones, and limestone-marn-claystone intercalations. All rock units that outcrop in the study area are overlain by the alluvium of Nilufer river that has carried rocks and pebbles of the units from the study area. Petrographic investigations has done on 50 samples that are collected from the field in a polarized optical microscope using modal analysis with point-counting methods. According to the petrographic investigations, the mineral paragenesis of Kocasu formation is chlorite + albite + epidote + glauchophane for metavolcanics and biotite/muscovite + albite + chlorite + quartz + calcite for metapelites with a lepidoblastic-microlepidoblastic texture. The mineral paragenesis of İnönü Marble is calcite ∓ dolomite with granoblastic texture. Orhaneli ophiolite consists of dunites, lherzolites and serpantinites. These rocks mainly include olivine, pyroxene, serpentine and chromite minerals. The magmatic rocks in the study area are represented by Topuk granitoid and hypabyssal rocks. The main plutonic body of Topuk granitoid is in granodiorite composition with holocrystalline porphyritic texture and occasionally granular texture. It consists of mafic microgranular enclaves (MME), and is cut by quartz porphyry and aplite dykes of granite composition. The composition of mafic microgranular enclaves is diorite/quartz diorite. Hypabyssal rocks show microcrystalline-cryptocrystalline porphyritic textures and have the composition of granodiorite/quartz monzonite. A contact metamorphism has occurred in the basement rocks with the intrusion of Topuk granitoid into the basement rocks in Middle Eocene, with a mineral paragenesis of calcite + garnet + tremolite + quartz and quartz + albite + epidote + muscovite + biotite, which indicates contact metamorphism conditions that changes between hornblend- hornfels and albite-epidote-hornfels metamorphic facies. This indicates a 500-600 oC temperature conditions for the contact metamorphism. From the magmatic rocks of the study area, major oxide and trace element analysis has done on 12 samples; 5 from Topuk granodiorite, 2 from mafic microgranular enclaves, 2 from aplite dykes and 3 from hypabyssal rocks. The geochemical features of Topuk granodiorite, mafic microgranular enclaves, aplite dykes and hypabyssal rocks are similar to each other, and are compatible with the petrographical investigations. The magmatic rocks in the study area are all subalkaline and calc-alkaline in character. They are mostly metaluminous and mid to high-K, I type. The SiO2 compositions of the magmatic rocks vary from 54,9% to 77%. Harker diagrams that show major oxides versus SiO2 present a systematical and regular trend from mafic microgranular enclaves, which are the least acidic end-members, to aplitic dykes, which are the most acidic end-members. According to the primitive mantle-normalized trace element patterns, the samples are enriched in large ion lithopile elements (LILE) and light rare earth elements (LREE). They show negative anomalies in Nb, Ti and Ta elements. Chondrite normalized spider diagrams showed that the rocks mostly show a moderate to high Eu anomalies and a convex structure in heavy rare earth elements (HREE). Sr-Nd-Pb isotopic analysis has done from 6 samples of magmatic rocks in the study area, including 2 Topuk granodiorite samples, 2 mafic microgranular enclave samples and 2 hypabyssal rock samples. The initial 87Sr/86Sr(48) isotope values of the magmatic rocks change between 0,706108-0,706705 with an exception of a hypabyssal rock sample, having the value of 0,709901. 143Nd/144Nd isotopic values are all similar with the interval of 0,51233-0,51262. The 206Pb/204Pb values change between 18,831- 19,046, 207Pb/204Pb values 15,673-15,748; 208Pb/204Pb values 38,951-39,269; 208Pb/206Pb values, 2,0618-2,0725; and 207Pb/206Pb values are 0,82686-0,83321. The major-trace geochemistry and Sr-Nd-Pb isotopical characteristics of magmatic rocks with different textures and compositions that outcrop in the study area has evaluated together with the specifications of previously studied Eocene plutons that show similarities in time and space with Topuk granitoid. These evaluations brought a conclusion that the magma that created the magmatic rocks in the region has originated from an enriched lithospheric mantle, however, it has a fair amount of contribution of astenospheric mantle as well. The trace element and isotope models show that evolution of the mantle derived magma has driven by assimilation fractional crystallization processes (AFC). Additionally, the magmatic rocks of different compositions show that fractional crystallization (plagioclase and amphibole) has acted a part in the development of these magmatic rocks when they are evaluated within themselves.