ALKALINE INTRUSIVES AND WHAT THEY TELL US ABOUT THE UPLIFT OF THE MENDERES MASSIF, W. ANATOLIA, TURKEY

One of the most prominent features of Western Anatolia is the Menderes Massif. It is regarded as a section of deeper levels of continental crust exhumed in the Late Miocene during a period of extension that affected the entire Aegean province. The Menderes Massif is widely interpreted as a core complex bordered by low angle detachment faults. More recent studies claim, that the detachment faults were originally steep and experienced subsequent back-tilting. Shallow subduction of Neotethyan oceanic lithosphere and erosion of overlying strata instead of low angle normal faulting were suggested as explanation for the uplift history deduced by thermochronology. Current questions concerning the history of the Menderes Massif pertain to the timing of the onset of uplift, the rate of uplift, the depth at which the magmas intruded and the characteristics of their mantle-source region.

This abstract presents an update on the scientific achievements of an ongoing project, aimed to put more constraints on the origin of the Menderes Massif. Here we present the preliminary geochemical results of investigation of plutonism that cuts the core complex. We are currently investigating mafic intrusive rocks from the northern part of the Menderes massif, which occur mainly as concordant intrusions, with the thickness of individual sills ranging from one to ten metres. The sills extend laterally from a few tens to a few hundreds of metres. Most rocks are porphyritic with phenocrysts of mica, amphibole, clinopyroxene and/or calcite within a fine grained groundmass of plagioclase with or without amphibole and mica. According to their mineralogy, the rocks can be classified as kersantites and andesites, the latter containing mafic enclaves.

Geochemically, the rocks are high-K alkaline and of intermediate (basaltic andesite to andesite) composition. Whole rock SiO₂ contents range from 49 to 60 wt.%, MgO from 3.2 to 8.4 wt.%, Cr 100 – 360 ppm and Ni 20 – 250 ppm. Whole rock Mg# (100 * molar Mg/(Mg + Fe²⁺)) ranges from 37 up to 50. The larger sills show internal geochemical zoning from more primitive (high MgO, Mg# and K₂O, low SiO₂) near the contact to more evolved (lower MgO, Mg# and K₂O, higher SiO₂) towards the centre. Trace element variation diagrams (’spidergrams’) of whole rock analyses show a positive Pb and a negative Nb anomaly, which is less pronounced in the most primitive samples, suggesting clear orogenic affinity. Geochemistry of the kersantites demonstrates clear resemblance with ultrapotassic volcanic rocks from the area.

A number of geochemical analyses are in progress, including mineral major and trace elements as well as radiogenic isotope systematics (Sr, Nd and Pb) on the selected samples. Additionally, U/Pb dating of zircon by laser-ablation ICP-MS and fission track dating of apatite will be conducted to investigate the rate of uplift. The new data will be presented at the forthcoming GSA conference, and will provide time-constrained information on mantle dynamics under the Menderes Massif, a P-T history for the mantle-derived magmas, and their relation to the known history of uplifting.