Deciphering the shoshonitic monzonites with I-type characteristic, the Sisdaği pluton, NE Turkey: Magmatic response to continental lithospheric thinning

Abstract

Large-scale late Mesozoic to early Cenozoic plutons in the Eastern Pontide orogenic belt mostly show calc-alkaline and I-type geochemical features. However, we identify the Sisdagi pluton that has shoshonitic affinity and I-type character in the region. The pluton was emplaced at shallow depths (<5 km), revealed by Al-in hornblende barometry, in the northern margin of the orogenic belt, with SHRIMP zircon U–Pb age of 41.55 ± 0.31 Ma, interpreted as dating magma crystallization. It is composed of monzonite, monzodiorite and monzogabbro. The samples show low zircon saturation temperature (TZr) ranging from 603 to 769 °C compared to A-type rocks. They exhibit SiO2 contents of 47.6–61.4 wt.%, and high K2O + Na2O (5.0–8.8 wt.%) and K2O/Na2O (0.8–1.8). All the samples are characterized by low Mg# (<47) and relatively high but variable Al2O3 content (16.9–22.2 wt.%). They are also enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), with a weak negative Eu anomaly (Eu/Eu* = 0.58–1.26) in mantle-normalized trace element patterns. The samples possess homogeneous initial Sr and Nd isotopic compositions, marked with low ISr = 0.70376–0.70408 and eNd (42 Ma) = +1.3 to +2.4. TDM ages of the samples range from 0.70 to 0.85 Ga. The Pb isotopic ratios are (206Pb/204Pb) = 18.64–18.72, (207Pb/204Pb) = 15.51–15.58 and (208Pb/204Pb) = 38.31–38.65. These geochemical features imply that the parental magma resulted from melting of chemically enriched lithospheric mantle source. In such a case, a hot upwelling asthenosphere is necessary to partially melt the lithospheric mantle in order to form the parental magma. The pluton is considered to be a post-orogenic intrusion that was emplaced in an environment of lithospheric extension, triggering asthenospheric upwelling. The thermal anomaly induced by asthenospheric upwelling resulted in partial fusion of chemically enriched subcontinental lithospheric mantle beneath the region. Then, the shoshonitic melt, which subsequently underwent fractional crystallization with minor crustal contamination, ascent to shallower crustal levels to generate a monzonitic rock series ranging from monzogabbro to monzonite. All these data combined with the regional geology suggest that the crustal thickening as a consequence of regional compression during the Paleocene changed into a lithospheric extension and thinning throughout the early Cenozoic (at ∼42 Ma) in the Eastern Pontides. Hence, the middle Eocene shoshonitic I-type magmatism is a unique pluton, signifying initiation of lithospheric thinning and thus of hot asthenospheric upwelling in the region. These interpretations also argue against the presence of an early Cenozoic subduction of oceanic slab in the Eastern Pontides.