Age and petrogenesis of plagiogranite intrusions in the Ankara melange, central Turkey

摘要

The Ankara melange within the Izmir-Ankara-Erzincan suture zone in north-central Turkey includes ophiolitic fragments that represent the remnants of an oceanic basement evolved between the Sakarya and Kirsehir continental blocks in the early Mesozoic. The serpentinized upper mantle peridotites and lower crustal rocks in these ophiolites are cross-cut by dolerite and plagiogranite dykes, which show mutually intrusive relations indicating their synchronous emplacement into the pre-existing oceanic lithosphere. Zircon dating (U-Pb) of a plagiogranite dyke has revealed a concordia age of similar to 179 +/- 15 Ma that is interpreted here as the crystallization age of this differentiated rock. A fourth fraction of the zircon separates from this rock has also shown an inherited component greater than 1.7 Ga, possibly derived from the Precambrian core of the Rhodope-Strandja Metamorphic Massif in the Balkan Peninsula. Models for plagiogranite formation were tested and it is concluded that a high extent (< 70%) of anhydrous or water-undersaturated, early amphibole-free fractionation of a basaltic melt source may have readily produced the observed REE concentrations for the Ankara melange plagiogranites. The trace element abundances and other geochemical features of the coeval dolerite dykes are similar to those of the plagiogranites, suggesting a common melt source. The Ta-Nb patterns shown by both the plagiogranite and dolerite dykes are typical of arc-related petrogenesis and can be explained by the addition of slab-derived components to a depleted mantle wedge. The Early Jurassic ophiolitic basement and the dyke intrusions were formed in a back-arc setting between the Paleo- and Neo-Tethyan domains in the eastern Mediterranean region. The Izmir-Ankara-Erzincan Sea developed in this back-arc environment and the related suture zone had a diachronous evolutionary history.