Eocene Granitic Magmatism in NW Anatolia (Turkey) revisited: New implications from comparative zircon SHRIMP U-Pb and Ar-40-Ar-39 geochronology and isotope geochemistry on magma genesis and emplacement


Altunkaynak Ş., Sunal G., ALDANMAZ E. , Genc C. S. , Dilek Y., Furnes H., et al.

LITHOS, cilt.155, ss.289-309, 2012 (SCI İndekslerine Giren Dergi) identifier identifier

Özet

During the Eocene, the northern Neo-Tethys suture zone and immediately adjacent areas in NW Turkey were loci of emplacement of a number of granitoid plutons with age relations and geochemical signatures indicative of magma generation in association with plate convergence. Discrete granitoid plutons of Eocene age in this region are exposed mainly in two distinct belts within and north of the northern Neo-Tethys suture zone. We report new SHRIMP U-Pb zircon and Ar-40/Ar-39 geochronology along with Sr-Nd isotope data from the granitoids in order to constrain their melt source(s), cooling history, and tectono-magmatic evolution with respect to the Early Cenozoic collisional tectonics of the region. Ranging in composition from diorite to granite, the plutons are largely represented by medium- to high-K calc-alkaline, I-type granites accompanied by minor amounts of shoshonitic rocks with syenite compositions and mafic microgranular enclaves in some of the plutons. In terms of trace element systematics the granitoids and the associated enclaves from the entire Eocene suite display close similarities to magmas from subduction-related or active continental margin settings, characterized by significant enrichment in LILE/HFSE relative to MORB. The rocks display significantly heterogeneous distributions of radiogenic isotopes with Sr-87/Sr-86 and Nd-143/Nd-144 ranging from 0.705824 to 0.708363 and from 0.512384 to 0.512718 respectively, suggesting multi-component melt interaction in their genesis. The granitoids from the southern and northern belts yielded zircon SHRIMP U-Pb ages of 52.8 +/- 1.8 to 45.41 +/- 0.34 Ma and 47.02 +/- 0.82 to 36.7 +/- 0.67 Ma, respectively, indicating overlapping emplacement ages, with some relatively younger ages from the northern belt. Ar-40/Ar-39 dating of biotite and hornblende also yielded similarly overlapping cooling ages (51.8 +/- 0.1 to 44.9 +/- 0.2 Ma and 45.3 +/- 0.1 to 36.0 +/- 0.1 Ma, respectively). Our combined geochronological data from the co-genetic zircon, and hornblende and biotite separates reveals rapid cooling of the plutons immediately after emplacement, which can readily be attributed to rapid uplift starting at similar to 53 Ma. Unlike the intrusions in the southern belt, some of the granitoids in the northern belt contain inherited zircons of older ages, possibly reflecting more pronounced effects of crustal input in their genesis. On the basis of new geochemical and geochronological data, we infer that melt generation to produce the granitic plutons shortly after subduction waned in the region is most probably related to abrupt thermal weakening of the orogenic lithosphere due either to slab break-off or partial removal of a thermal boundary layer between lithosphere and asthenosphere that led to syn-convergent extension and collapse in the overriding plate, resulting in the onset of the development of the Aegean extensional province. (C) 2012 Elsevier B.V. All rights reserved.