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

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 57.7–70.2wt.% SiO2, 13.1–17.5wt.% Al2O3, 3.06–3.64wt.% Na2O, 3.66–5.92wt.% K2O, 0.33–3.3wt.% 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 Fe2O3T, 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.