Petrochemistry of the south Marmara granitoids, northwest Anatolia, Turkey

Abstract

Post-collision magmatic rocks are common in the southern portion of the Marmara region (Kapıdağ, Karabiga, Gönen, Yenice, Çan areas) and also on the small islands (Marmara, Avşa, Paşalimanı) in the Sea of Marmara. They are represented mainly by granitic plutons, stocks and sills within Triassic basement rocks. The granitoids have ages between Late Cretaceous and Miocene, but mainly belong to two groups: Eocene in the north and Miocene in the south. The Miocene granitoids have associated volcanic rocks; the Eocene granitoids do not display such associations. They are both granodioritic and granitic in composition, and are metaluminous, calc-alkaline, medium to high-K rocks. Their trace elements patterns are similar to both volcanic-arc and calc-alkaline post-collision intrusions, and the granitoids plot into the volcanic arc granite (VAG) and collision related granite areas (COLG) of discrimination diagrams. The have high ⁸⁷Sr/⁸⁶Sr (0.704–0.707) and low ¹⁴³Nd/¹⁴⁴Nd (0.5124–0.5128). During their evolution, the magma was affected by crustal assimilation and fractional crystallization (AFC). Nd and Sr isotopic compositions support an origin of derivation by combined continental crustal AFC from a basaltic parent magma. A slab breakoff model is consistent with the evolution of South Marmara Sea granitoids.

Appendix: Analytical techniques

Major and trace elements were determined at Cardiff University. Samples were ignited at 900°C in a muffle furnace to determine loss on ignition. 0.1 g of ignited powder were fused with 0.4 g of Li-metaborate, and the resulting melts were dissolved and taken up in 100 ml of 2% HNO₃. Sample solutions were analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES) using a JY Horiba Ultima 2 ICP-OES system. Calibration was performed using international reference materials BIR-1, W2, MRG-1, JA2, and JG3. Instrumental precision varied between 0.2 and 8.5% depending on the elemental concentration present. Accuracy was determined by repeat analysis of separate (user-prepared) solutions of W2 and JA2 and of the international granite standard GSP-1. Results were consistently within 5% of the accepted values.

For isotope analyses, Rb, Sr, and light rare-earth elements were isolated on quartz columns by conventional ion exchange chromatography with a 5 ml resin bed of Bio Rad AG 50W-X12, 200–400 mesh. Nd was separated from other rare-earth elements on quartz columns using 1.7 ml Teflon powder coated with HDEHP, di(2-ethylhexyl) orthophosphoric acid, as cation exchange medium. All isotopic measurements were made by Thermal Ionization Mass Spectrometry, on a Finnigan MAT 262 mass spectrometer at Tübingen University. Sr was loaded with a Ta-HF activator on pre-conditioned W filaments and was measured in single-filament mode. Nd was loaded as phosphate on pre-conditioned Re filaments, and measurements were performed in a Re double filament configuration. The ⁸⁷Sr/⁸⁶Sr isotope ratios were normalized to ⁸⁶Sr/⁸⁸Sr =  0.1194 and the ¹⁴³Nd/¹⁴⁴Nd isotope ratios to ¹⁴⁶Nd/¹⁴⁴Nd =  0.7219. Analyses of 21 separate loads of Ames metal, (Geological Survey of Canada; Roddick et al. 1992), during the course of this study (01-07/2003) gave a ¹⁴³Nd/¹⁴⁴Nd of 0.512141 ± 0.000020 (± errors are 2-sigma of the mean) and within the same period the NBS 987 Sr standard yielded a ⁸⁷Sr/⁸⁶Sr of 0.710248 ± 0.000021 (n = 29). Total procedural blanks (chemistry and loading) were <200 pg for Sr and <50 pg for Nd.

The K/Ar age determinations has been done at Activation (geochronology and isotopic geochemistry) laboratories. The K concentration was performed by ICP. The argon analysis was performed using the isotope dilution procedure on noble gas mass spectrometry. An aliquot of the sample is weighted into Al container, loaded into sample system of extraction unit, degassed at ∼100°C during 2 days to remove the surface gases. Argon is extracted from the sample in double vacuum furnace at 1,700°C. Argon concentration is determined using isotope dilution with ³⁸Ar spike, which is introduced to the sample system prior to each extraction. The extracted gas is cleaned up in two-step purification system. Then pure Ar is introduced into customer build magnetic sector mass spectrometer (Reinolds type) with Varian CH5 magnet. The ion source has an axial design (Baur-Signer source), which provides more than 90% transmission and extremely small isotopic mass-discrimination. Measurement Ar isotope ratios is corrected for mass-discrimination and then atmospheric argon is removed assuming that ³⁶Ar is only from the air. Concentration of ⁴⁰Ar radiogenic is calculated by using ³⁸Ar spike concentration. After each analysis the extraction temperature is elevated to 1,800°C for few minutes and furnace is prepared for next analysis. K-analysis; aliquot of the sample is weighted into graphite crucible with lithium metaborate/tetraborate flux and fussed using LECO induction furnace. The fusion bead is dissolved with acid. Standards, blanks, and sample are analyzed on Thermo Jarrell Ash Enviro II ICP Spectrometer.