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

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• 作者：Orhan Karsli^a ; ^{okarsli@gmail.com} ; Abdurrahman Dokuz^a ; 陌brahim Uysal^b ; Murat Ketenci^c ; Bin Chen^d ; Raif Kandemir^a
• 关键词：Mantle melting ; Lithospheric thinning ; Asthenospheric upwelling ; Shoshonitic I-type Sisda臒i pluton ; Eastern Pontides
• 刊名：Journal of Asian Earth Sciences
• 出版年：2012
• 期刊代码：40_13679120
• 类别：ear
• 出版时间：2 June, 2012
• 卷：51
• 期：Complete
• 页码：45-62
• 文件大小：2774 K

摘要

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 Sisda臒i 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 卤 0.31 Ma, interpreted as dating magma crystallization. It is composed of monzonite, monzodiorite and monzogabbro. The samples show low zircon saturation temperature (T_Zr) ranging from 603 to 769 掳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 I_Sr = 0.70376-0.70408 and 蔚_Nd (42 Ma) = +1.3 to +2.4. T_DM 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 鈭?2 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.