Petrogenesis of the Late Cretaceous intrusive series in the Strandja Massif, Kırklareli-Türkiye

Abstract

The Strandja Massif is located in northwestern Türkiye and is intruded by the Late Cretaceous intrusives formed by the Apuseni-Banat-Timok-Srednogorie magmatic arc, which resulted from the closure of the Tethys Ocean. The Late Cretaceous Strandja Intrusives are generally divided into five subgroups: (I) granitoids (Demirköy Intrusion), (II) monzonites (Karacadağ Intrusion), (III) syenites (Dereköy Intrusion), (IV) diorite (Değirmentepe Intrusion), and (V) gabbro (İncesırt Intrusion). The felsic rocks consist of quartz, orthoclase, oligoclase-andesine (An_19-48Ab_50-78Or_1.1-2.1), Fe-biotite, Mg-hornblende, and augite in varying proportions. Geochemical and isotopic analyses indicate that their formation was influenced by fractional crystallization and crustal contamination resulting from crust-mantle interaction. The mafic rocks, which occur predominantly as diorite and gabbro intrusions are mainly composed of labradorite-bytownite (An_75-25Ab_71-24Or_4.5-0.5), Fe-biotite, tschermakite-pargasite, and augite (Wo_27-45En_36-46Fs_25-13), with the occasional presence of olivine or quartz in variable proportions. The Sr-Nd isotopic compositions of felsic rocks in the Strandja Intrusives show an enrichment trend, with granitoids having ⁸⁷Sr/⁸⁶Sr values from 0.705801 to 0.706319 and ¹⁴³Nd/¹⁴⁴Nd from 0.512506 to 0.512599, while syenites have ⁸⁷Sr/⁸⁶Sr values between 0.707632 and 0.708293 and ¹⁴³Nd/¹⁴⁴Nd between 0.512420 and 0.512558. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd values of the diorite and gabbro intrusions range from 0.704408 to 0.707518 and from 0.512486 to 0.512716, respectively. The geothermobarometric data from the Strandja Intrusives reveal a complex magmatic system with clinopyroxene crystallization occurring under variable high-pressure and high-temperature conditions indicative of deep lithospheric and arc-related magmatic processes. The U-Pb zircon ages obtained are 82.1 ± 1.4 Ma for granites, while for syenites, they are 79.03 ± 0.6 Ma. Meanwhile, the U-Pb zircon ages of mafic intrusions undergoing bimodal evolution vary from 85.6 ± 0.8 Ma to 81.80 ± 0.42 Ma. The variation in U-Pb age data from east to west in the felsic intrusive rocks suggests a general rejuvenation from granite-granodiorite to syenite. It confirms that the Strandja Intrusives formed due to the northwestward subduction of the Vardar Ocean due to the compression between Africa and Eurasia during the Late Cretaceous period.