Petrogenesis of a post-collisional, shallow crustal mafic complex: A case study of the Yıldız Dağı gabbroic intrusion (Northern Türkiye)

Abstract

The Yıldız Dağı gabbroic intrusion and associated dykes intruded the southern part of the Pontides, along the İzmir-Ankara-Erzincan suture zone (central-northern-Türkiye). The intrusion has an elongated shape and is exposed over 2 km². It consists of melagabbro, leucogabbro, troctolitic enclaves, hornblende gabbro, and pegmatite. Melagabbro is the most common rock type and displays complex internal structures such as crossbedding and faint layering. Leucogabbro is situated at the upper level of the intrusion. Whole-rock geochemical data suggest that the gabbroic units of the pluton are cumulates primarily controlled by the fractionation of ferro-magnesian minerals such as olivine, clinopyroxene, and magnetite. All rocks display negative NbTa and HfZr anomalies, LILE enrichment, and slightly elevated potassium contents similar to those of other post-collisional early Cenozoic magmatic units in northern Türkiye. The leucogabbro (44.1 ± 3.2 Ma) and hornblende gabbro (42.9 ± 0.2 Ma) have Eocene UPb zircon and ArAr hornblende ages. Whole rock SrNd isotopic data are clustered within the mantle array, which is typical of primitive Eocene post-collisional igneous rocks of northern Türkiye. Olivine, clinopyroxene, high-An plagioclase, and magnetite are early cumulus minerals, while brown hornblende, low-An plagioclase, biotite, ilmenite, apatite, and quartz crystallized from the intercumulus liquid. Simulated trapped melt fractions range from 10 to 12 %, resulting in a tenfold increase of REE contents in clinopyroxene from core to rims. High anorthite plagioclase (up to 90) and widespread interstitial amphibole indicate moderately hydrous source regions for the gabbros. P-T calculations based on different mineral pairs and single clinopyroxene geothermobarometry indicate upper-crustal intrusive conditions. As a well-preserved example of small-scale post-collisional gabbroic magmatism within the İzmir–Ankara–Erzincan suture zone, the Yıldız Dağı gabbroic intrusion serves as a valuable analogue for mafic intrusions in similar tectonic settings worldwide and exemplifies the diversification of shallow-level differentiated plutons across the Alpine–Himalayan orogenic belt.