Age and petrogenesis of the Çavuşbaşi Granodiorite

AGE AND PETROGENESIS OF THE ÇAVUŞBAŞI GRANODIORITE SUMMARY The Çavuşbaşı Granodiorite is in the east of the Istanbul Bosphorus. It is located at within The Istanbul Zone in the Western Pontides. It's a Late Cretaceous old magmatic intrusive rock (pluton), intrudes the Paleozoic units of the Istanbul Zone locally. It intrudes into Lower Ordovician sandstones and shales. Its emplacement gave rise to the development approximately 300 - 500 m wide contact metamorphic aureole. Our objective is to constrain the age and petrogenesis of the intrusion. The Çavuşbaşı granodiorite covers nearly 25 km2 area. The Çavuşbaşı pluton is a circular granodiorite sphere with a diameter of approximately 4,5 to 5,6 km. Geological units have been distinguished by microscopic and field observations of the samples taken from stream beds, outcrops in construction and roadside areas, although a topography which is almost entirely observed due to surface alteration is observed. The granitic pluton, located at the edge of the continental plate with subduction. It was characterized by calc-alcaline, hornblende-containing, magmatic-compound I-type granite. Mainly consists of granodiorite; become to on a small scale quartzdiorite porphyry. The country rock is composed of granodiorite and quartzdiorite porphyry. Main components of the granodiorite are plagioclase, quartz, K-feldspar, hornblend and biotite. Quartzdiorite consists of the some minerals but displays a marked porfiric texture. The Çavuşbaşı Granodiorite includes veins of aplite and pegmatite, mafic magmatic enclaves and dikes. Aplites are white colored, fine-grained, composed of feldspar and quartz-containing. They are granite-derived vein rocks. Aplite veins are composed of granules in mm size and their width varies between 0,5 - 40 cm. Pegmatites are pink-colored, coarse-grained veins with the same composition as aplite. Pegmatite veins are composed of granules in cm size and their width varies between 3 – 8 cm. Aplite and pegmatite veins can be watched horizontally in meters in the weathered granodiorite. The aplites in the study area are concentrated in Karanlıkdere, pegmatites are concentrated in Çiftlik district. Aplites and pegmatites are late stage product of granite magmatism. Mafic magmatic enclaves are fine-grained, dark colored, monzodioritic in compositions and precisely contacted. Enclaves are locally rounded, locally angular, and locally ellipsoidal in shape with dimensions ranging from 0,5 to 30 cm. Enclaves are microgranular texture and consist of hornblende and plagioclase minerals up to 2 mm in fine grained matrix (plagioclase, quartz, hornblende, biotite). Formation of flow banding by magma mingling of discrete magma compositions are visible. Dikes are fractured, hornblende and biotite containing dacite and andesite with locally dark green colored lamprophyre. In the matrix of fine grained feldspar group, hornblend, orthoclase and plagioclase minerals up to 6 mm are seen in the intermediate-composition dikes. The size of the dikes varies from 20 cm to 3,50 m. On very few of the intermediate-composition dikes have partly chill margin which formed at the edges of the hot dikes . The chill margins are thinly sliced in some places and some in melting. The surrounding rock (country rock) is affected by the heat transfer as the country rock heat up, and it has baked contact which is noticed by the color change seen along the chill margin. Adakitic rocks belong calc-alkaline series, they are related to subduction. SiO2 ≥ 56%; Al2O3 ≥ 15%; MgO < 3%; %3,5 ≤ Na2O ≤ %7,5; Sr > 300 ppm; Y < 15 ppm; Sr/Y > 20; Yb ≤ 1,9 ppm ve (La/Yb)N > 10. K2O/Na2O rate is low (~0,42), Mg number is 51. Zr and Ti anomalies are very small or lacking. They have negative Nb anomalies, negative Eu anomaly and positive Sr anomaly. The high content of Sr is typical of the adakitic magma. Geochemical composition of the Çavuşbaşı granodiorite (country rock composed of granodiorite and quartzdiorite porphyre) changes rapidly within short limits. The Çavuşbaşı Granodiorite belongs to medium-K calc-alkaline series. Aluminium saturation index ranges from 0,90 to 1,04; indicates metaluminous-peraluminous affinity. As in adakitic rocks SiO2 ≥ 56% (64,91 – 68,26), Al2O3 ≥ 15% (16,13 – 16,84), MgO < 3% (1,52 – 2,04), 3,5% ≤ Na2O ≤ 7,5% (4,80 – 5,20), Sr > 300 ppm (578,10 – 900,70 ppm), Y < 15 ppm (5,30 – 10,20 ppm), Sr/Y > 20 (70,2 – 112,49), Yb ≤ 1,9 ppm (0,56 – 0,95) ppm and (La/Yb)N > 10 (12,64 – 19,98 ppm). (In contrast to adakitic rocks K2O/Na2O rate is less than 0,42 (0,25 – 0,41); Mg number is greater than 51 (52,99 - 61,68); it has positive Zr anomaly and negative Ti anomaly). Çavuşbaşı Granodiorite has negative Nb anomaly; slightly evident no Eu anomaly and positive Sr anomaly. Overall, the Çavuşbaşı Granodiorite has with these geochemical characteristics, the Çavuşbaşı granodiorite formed by subduction, which resembles adacitic rocks. Aplites belong to high-K calc-alkaline series. Aluminium saturation index is 1,02; indicates metaluminous-peraluminous affinity. It is seperated from adakitic rocks with low Na2O (3,26%), low Sr/Y ratio (7,04), low Sr (72,6 ppm), and negative Eu anomaly (Eu/Eu*=0,50). Aplites are late stage processes of granite magmatism, with granitic composition. Aplites have the highest SiO2 (77,69%), the highest K2O (5,18%), the lowest Fe2O3 (%0,44), the lowest MgO (0,04%) and the lowest CaO (0,56%) in the study area. Therefore, it is similar to the magma derived from the upper crust. Enclaves belong to medium-K calc-alkaline series. Aluminium saturation index is 0,84; indicates metaluminous affinity. Enclave sample is the most mafic member in the study area with SiO2 (55,45%). Heavy Rare Elemenths are higher than adakitic rocks: Yb (2,48 ppm) and Y (25,7 ppm). It is seperated from adakitic rocks with Sr/Y ratio (27,99) and negative Eu anomaly (Eu/Eu*=0,62). Enclaves have the highest Fe2O3 (6,75%); the highest MgO (3,57%) ve the highest CaO (6,06%) in the study area. Therefore, it is similar to mantle-derived mafic magma. Dating of zircons by LA-ICP-MS U/Pb dating yielded on age of 68,32 ± 0,66 My (2σ) at the granodiorite sample. So, Çavuşbaşı granodiorite intruded during Maastrichtian. It is related with the loss of nortwards subduction of The Neotethyan Oceanic Lithosphere at the Late Cretaceous. The formation is probably related to the northward subduction of the oceanic lithosphere along either Intra-pontide or İzmir-Ankara-Erzincan Suture.