Eocene I-type magmatism in the Eastern Pontides, NE Turkey: insights into magma genesis and magma-tectonic evolution from whole-rock geochemistry, geochronology and isotope systematics

The Eastern Pontides orogenic belt in NE Turkey hosts numerous I-type plutons of Eocene epoch. Here, we report new U–Pb SHRIMP zircon ages and in situ zircon Lu-Hf isotopes along with bulk-rock geochemical and Sr-Nd-Pb-O isotope data from the Kemerlikdağı, Aydıntepe and Pelitli plutons and mafic microgranular enclaves (MMEs) to constrain their parental melt source(s) and evolutionary processes. U-Pb SHRIMP zircon dating yielded crystallization ages between 45 and 44 Ma for the studied plutons and their MMEs. The plutons range from gabbro to granite and have I-type, medium to high-K calc-alkaline, and metaluminous to slightly peraluminous characteristics. On the primitive mantle-normalized multi-trace-element variations, the plutons and their MMEs are characterized by significant enrichment in LILE/HFSE. Chondrite-normalized REE patterns of the plutons and their MMEs are close to each other and show moderate enrichment with variable negative Eu anomalies. The studied plutons have fairly homogeneous isotope composition (⁸⁷Sr/⁸⁶Sr_(i) = 0.70502 to 0.70560; εNd_(i) = +0.9 to – 1.4; δ¹⁸O = +5.0 to +8.7‰, εHf_(i) = – 2.2 to +13.5). The MMEs show medium to high-K calc-alkaline and metaluminous character. Although the isotope signatures of the MMEs (⁸⁷Sr/⁸⁶Sr_(i) = 0.70508 to 0.70542; εNd_(i) = +0.9 to −1.1; δ¹⁸O = +5.8 to +8.0, εHf_(i) = +4.3 to +10.4) are very similar to those of the host rocks. Fractionation of plagioclase, amphibole, pyroxene and Fe-Ti oxides played an important role in the evolution of the plutons. The isotopic composition of the studied plutons and MMEs are similar to I-type plutons derived from mantle sources. The MMEs show incomplete magma mixing/mingling, representing small bodies of mafic parental magma. The parental magma(s) of the studied plutons were generated from the enriched lithospheric mantle and then modified by fractional crystallisation, and lesser assimilation and mixing/mingling in the crustal magma chambers.

土耳其东北部的东蓬蒂造山带（Eastern Pontides orogenic belt）产出大量始新世（Eocene epoch）的I型岩体（I-type plutons）。本文报道了采自Kemerlikdağı、Aydıntepe与Pelitli岩体以及镁铁质微粒包体（mafic microgranular enclaves, MMEs）的最新U-Pb SHRIMP锆石定年结果、原位锆石Lu-Hf同位素数据，连同全岩地球化学（bulk-rock geochemical）与Sr-Nd-Pb-O同位素数据，以约束其母岩浆源区与演化过程。U-Pb SHRIMP锆石定年结果显示，研究岩体及其包体的结晶年龄介于45~44 Ma之间。研究岩体的岩性范围从辉长岩至花岗岩，兼具I型、中-高钾钙碱性、准铝质（metaluminous）至弱过铝质（peraluminous）的地球化学特征。在原始地幔标准化（primitive mantle-normalized）的多微量元素配分模式中，岩体及其包体均表现出大离子亲石元素（large ion lithophile elements, LILE）与高场强元素（high field strength elements, HFSE）的显著富集。岩体及其包体的球粒陨石标准化（chondrite-normalized）稀土元素（rare earth elements, REE）配分模式彼此接近，均呈现中等程度的稀土元素富集，并伴随可变的负铕异常。本次研究的岩体同位素组成均一性较好：(⁸⁷Sr/⁸⁶Sr)_(i) = 0.70502~0.70560；εNd_(i) = +0.9~–1.4；δ¹⁸O = +5.0‰~+8.7‰，εHf_(i) = –2.2~+13.5。镁铁质微粒包体同样具有中-高钾钙碱性与准铝质的特征，尽管其同位素组成（(⁸⁷Sr/⁸⁶Sr)_(i) = 0.70508~0.70542；εNd_(i) = +0.9~−1.1；δ¹⁸O = +5.8‰~+8.0‰，εHf_(i) = +4.3~+10.4）与寄主岩体极为相似。斜长石、角闪石、辉石与铁-钛氧化物的分异作用在岩体演化过程中发挥了关键作用。研究岩体与包体的同位素组成与源自地幔源区的I型岩体相似。镁铁质微粒包体表现出不完全的岩浆混合/混杂作用，代表小型的镁铁质母岩浆体。本次研究的岩体母岩浆均源自富集型岩石圈地幔（lithospheric mantle），随后在壳内岩浆房内经历了结晶分异作用，并伴随程度较弱的地壳同化混染与岩浆混合/混杂过程。