Zircon U-Pb geochronology and petrogenesis of Middle Eocene monzonitic plutons in NE Turkey: constraints for generation of post-collisional I-type shoshonitic magmas

We present U – Pb zircon geochronology and whole-rock Sr-Nd-Pb isotopic compositions of monzonitic plutons from NE Turkey to elucidate their genesis and provide significant insights into the generation of post-collisional I-type shoshonitic magmas. Zircon U – Pb dating of the monzonitic plutons yielded an emplacement of the Middle Eocene (44–39 Ma). Geochemically, the monzonitic plutons are I-type, metaluminous, and shoshonitic in composition, and have high contents of total alkalis and LILE, high ratios of LREE/HREE and LREE/HFSE, and show negative Nb-Ta-Ti and Eu (Eu_N/Eu* = 0.69–0.84) anomalies. The monzonitic plutons have homogeneous and moderately low initial ⁸⁷Sr/⁸⁶Sr (0.70464–0.70495), positive εNd_(t) values (0.33 to 1.28), and narrow variation of the initial Pb isotope contents with ²⁰⁶Pb/²⁰⁴Pb (18.74–18.82), ²⁰⁷Pb/²⁰⁴Pb (15.64–15.65), and ²⁰⁷Pb/²⁰⁴Pb (38.81–38.90). The combined use of whole-rock trace elements and isotopic compositions with regional geology suggests that parental magmas of the monzonitic plutons were generated from mainly melt-metasomatized sub-continental lithospheric mantle source and lesser crustal components in an extensional orogenic setting, and then monzonitic magmas have evolved by dominantly fractional crystallization in crustal levels.

本研究报道了土耳其东北部二长岩岩体的铀-铅（U-Pb）锆石年代学与全岩Sr-Nd-Pb同位素组成，旨在阐明其成因，并为碰撞后I型钾玄质岩浆的形成机制提供重要见解。对该二长岩岩体的锆石U-Pb测年结果显示，其侵位时代为始新世中期（44~39 Ma）。地球化学特征表明，该二长岩岩体属于I型、准铝质且钾玄质岩石，具备较高的总碱含量与大离子亲石元素（Large Ion Lithophile Elements, LILE）丰度，轻稀土元素/重稀土元素（LREE/HREE）与轻稀土元素/高场强元素（LREE/HFSE）比值较高，且呈现显著的Nb-Ta-Ti负异常与铕（Eu_N/Eu* = 0.69~0.84）负异常。该二长岩岩体具有均一且中等偏低的初始87Sr/86Sr比值（0.70464~0.70495），正的εNd(t)值（0.33~1.28），且初始Pb同位素组成变化范围较窄，其中206Pb/204Pb为18.74~18.82，207Pb/204Pb为15.64~15.65，207Pb/204Pb为38.81~38.90。结合全岩微量元素、同位素组成与区域地质特征综合分析可知，该二长岩岩体的母岩浆主要来源于经熔体交代改造的大陆下岩石圈地幔源区，辅以少量地壳组分，形成于伸展造山环境，随后二长质岩浆主要通过地壳层面的分离结晶作用完成演化。