Petrology of the shoshonitic Çambaşı pluton in NE Turkey and implications for the closure of the Neo-Tethys Ocean: Insights from geochemistry, geochronology and Sr–Nd isotopes

Highlights

• The Çambaşı pluton cooled during Lutetian time.

• Shoshonitic and I-type magmatism was produced in a post-collision extensional tectonic regime.

• Chemical characteristics of the pluton were controlled by fractional crystallization.

• It has initial Sr ratios varying between 0.70470 and 0.70478 and initial Nd ratios between 0.51262 and 0.51263.

Abstract

The Eastern Pontide Orogenic Belt (EPOB) is one of the well-preserved continental magmatic arcs in the Alpine–Himalayan Belt and consists of Paleozoic to Miocene plutonic bodies. This paper presents new geological, bulk-rock major and trace element analyses, biotite Ar–Ar ages and Sr–Nd isotope data on representative samples from the Çambaşı granitic body exposed in the northwestern part of the EPOB. Although the majority of the plutons in the EPOB show calc-alkaline affinity and I-type character, the Çambaşı pluton has a shoshonitic affinity and an I-type, metaluminous signature. The pluton is composed of mainly monzonite, quartz monzonite and granite bodies including monzo-dioritic mafic microgranular enclaves (MMEs). Excluding the MMEs, these rocks have 57.7–70.2 wt.% SiO₂, 13.1–17.5 wt.% Al₂O₃, 3.06–3.64 wt.% Na₂O, 3.66–5.92 wt.% K₂O, 0.33–3.3 wt.% MgO and Mg# < 30, indicating intermediate to evolved compositions. They are characterized by light rare earth element (LREE)-enrichment, relatively flat heavy rare earth element (HREE) patterns with small negative Eu anomalies and moderately fractioned REE patterns [average (La/Yb)_N = 11.32]. Decreasing Fe₂O₃^T, MgO, CaO, TiO₂, Ba, Eu, and Sr with increasing SiO₂ contents are consistent with fractional crystallization and can be related to fractionation of plagioclase, clinopyroxene, hornblende, apatite and Fe–Ti oxides. Initial ⁸⁷Sr/⁸⁶Sr ratios vary between 0.70470 and 0.70478, while those of ¹⁴³Nd/¹⁴⁴Nd lie between 0.51262 and 0.51263. Major, trace element and initial Sr–Nd homogeneity show that fractional crystallization played a key role in the evolution of the pluton. ⁴⁰Ar/³⁹Ar dating of four biotite samples from the pluton gave plateau ages of between 44.62 ± 0.1 Ma and 44.10 ± 0.0 Ma, which are interpreted as cooling ages of the plutonic rocks. We suggest that the Çambaşı pluton formed in a post-collisional extensional setting where partial melting of chemically enriched lithospheric mantle was controlled by lithospheric thinning and upwelling of asthenosphere that followed the closure of the northern branch of Neo-Tethys Ocean in Lutetian time on the northern margin of Gondwanaland.

Introduction

Paleozoic to Miocene magmatism led to the emplacement of a number of granitic plutons with convergent margin geochemical signatures in the Eastern Pontide Orogenic Belt (EPOB). Excluding the Sisdağı pluton (Karslı et al., 2012), these plutons have calc-alkaline I-type geochemical signatures. In this study, we focus on the Çambaşı pluton which has shoshonitic I-type characteristics. Many petrologists are interested in shoshonitic plutons because these magmas are thought to be transitional between calc-alkaline and alkaline magmas (Bonin, 2004, Liegeois et al., 1998). The term ‘shoshonite’ was proposed by Iddings (1895) for orthoclase-bearing basalts from Yellowstone Park, USA. Also, the term ‘shoshonite magma series’ was used by Joplin (1965) for a suite of basaltic to trachytic rocks that were considered as the K-rich counterpart of the alkali basalt magma series. Jakes and Gill (1970) recognized three rock associations in young island arcs as the shoshonite association, tholeiitic series, and calc-alkaline series. Jakes and White (1972) reported rocks with similar chemical characteristics also occur along continental margins. Morrison (1980) redefined the shoshonitic rock association as a group of potassium-rich, near-saturated rocks with both calc-alkaline and alkaline affinities and reclassified two groups of rocks that were formerly classified as shoshonitic: the undersaturated potassium-rich group was reclassified as the potassic series of alkali olivine basalts and includes some leucititic rocks, while rocks with low total alkalis and high K₂O/Na₂O were reclassified as potassium-rich calc-alkaline rocks. The remaining rocks classified as shoshonitic mainly occur in three tectonic settings: 1) propagating rifts in intra-oceanic island arcs and back-arc basins (Gill and Whelan, 1989, Sun and Stern, 2001); 2) rifts in continental magmatic arcs (Bacon, 1990, Conrey et al., 1997) and post-subduction continental rifting areas (Davis et al., 1993); and 3) in post-collisional orogenic settings (Eklund et al., 1998, Turner et al., 1996).

The E–W trending the Sakarya Zone of northern Turkey, is separated into three parts: İstanbul Zone, Central Pontides and Eastern Pontides, the latter is also called Eastern Pontide Magmatic Arc (Yılmaz-Şahin et al., 2004) or as in this paper the Eastern Pontide Orogenic Belt (EPOB) (Ketin, 1966, Özdamar, 2016; Fig. 1). The EPOB is a well-preserved continental magmatic arc (Dokuz et al., 2010, Eyüboğlu et al., 2011, Eyüboğlu et al., 2016, Özdamar et al., 2013, Özdamar, 2016) and is mainly composed of volcanic rocks including their pyroclastics, and plutonic bodies, which were products of Paleozoic to early Cenozoic magmatic activities as well as cover lithologies (Boztuğ et al., 2004, Boztuğ et al., 2006, Delibaş et al., 2016, Karslı et al., 2007, Karslı et al., 2010, Karslı et al., 2011, Karslı et al., 2012, Kaygusuz et al., 2010, Kaygusuz and Öztürk, 2015, Özdamar et al., 2014, Özdamar, 2016, Temizel et al., 2012, Topuz et al., 2005, Topuz et al., 2011, Yılmaz and Boztuğ, 1996). Three models were proposed for Eocene magmatism within the EPOB: (1) post-collisional slab breakoff (Boztuğ et al., 2007, Dilek et al., 2010, Keskin et al., 2008); (2) post-collisional crustal thickening (Karslı et al., 2012, Topuz et al., 2005, Topuz et al., 2011) and delamination of the thickened crust (Arslan et al., 2013, Aslan et al., 2014, Karslı et al., 2010, Kaygusuz and Öztürk, 2015, Temizel et al., 2012); and (3) slab window-related processes (Eyüboğlu et al., 2011).

Despite the previous studies outlined above, the geodynamic evolution of the region is still a subject of debate because of the lack of systematic geochemical, isotopic, and geochronological data on a regional scale. In this paper, we report new geochemical, geochronological and isotopic data on the Eocene Çambaşı pluton: an Eocene I-type granitoid with shoshonitic affinity. The new data, including biotite ⁴⁰Ar/³⁹Ar dating, whole-rock major and trace element geochemistry and Sr–Nd isotope compositions, allow us to discuss the petrogenesis of this predominantly monzonitic pluton and the generation of shoshonitic plutons in an extensional setting as well as the implications for evolution of the Neo-Tethys Ocean in the northern part of the Gondwanaland.