Geochemistry and zircon U-Pb geochronology of Miocene plutons in the Urumieh-Dokhtar magmatic arc, east Tafresh, Central Iran

The Tafresh plutons that include Ahmadabab diorite, Vasfonjerd monzonite, Mehrezamin diorite and Chahak diorite, located to the east of Tafresh city, north-central Iran, are part of Urumieh-Dokhtar magmatic arc. U-Pb dating of zircon grains provides emplacement ages of 22.3 ± 1 Ma for the Ahmadabad diorite, and tightly clustered ages of 22.2 ± 0.2 Ma, 21.3 ± 0.2 Ma, and 21.7 ± 0.4 Ma for Vasfonjerd monzodiorite, Mehrezamin diorite-monzonite, and Chahak diorite-monzonite plutons, respectively. These rocks are metaluminous to weakly peraluminous, calc-alkaline, and characterized by enrichment in light rare earth elements, Nb-Ta negative anomalies, and high LILE/HFSE ratios. Tafresh plutonic rocks originated from a parental magma source and experienced different degrees of partial melting. Geochemical signatures of Tafresh plutonic rocks, such as a wide range of Y/Nb (2.7–8.4) and low Zr/Nb (19.5–35.) ratios, Nb/Ta (11.46–18.15), argue for mantle–crust interaction during generation of Tafresh magmas. Relatively low Nb/La ratios further indicate that the lithospheric mantle played a significant role in melt generation. HREE signatures (i.e. decrease Dy/Yb with increasing SiO₂) preclude substantial involvement of garnet either in the residue, both during partial melting and fractionation of the magma. The plutons are a product of final stages of subduction-related magmatism prior to the collision between the Arabian and Eurasian tectonic plates.

位于伊朗中北部塔夫雷什市东侧的塔夫雷什岩体群，包含艾哈迈达巴德闪长岩（Ahmadabab diorite）、瓦斯丰杰尔德二长岩（Vasfonjerd monzonite）、迈赫拉扎明闪长岩（Mehrezamin diorite）与查哈克闪长岩（Chahak diorite），属于乌鲁米耶-多赫塔尔岩浆弧（Urumieh-Dokhtar magmatic arc）的组成部分。对锆石颗粒（zircon grains）开展铀-铅定年（U-Pb dating）结果显示，艾哈迈达巴德闪长岩的侵位年龄为22.3 ± 1 Ma；瓦斯丰杰尔德二长闪长岩、迈赫拉扎明闪长岩-二长岩以及查哈克闪长岩-二长岩岩体的侵位年龄则分别集中为22.2 ± 0.2 Ma、21.3 ± 0.2 Ma与21.7 ± 0.4 Ma。该套岩石为准铝质-弱过铝质钙碱性岩系，以轻稀土元素（light rare earth elements）富集、铌-钽（Nb-Ta）负异常以及高大离子亲石元素/高场强元素（LILE/HFSE）比值为典型特征。塔夫雷什侵入岩源自一套初始岩浆源区，并经历了不同程度的部分熔融作用。塔夫雷什侵入岩的地球化学特征，如宽泛的Y/Nb比值范围（2.7–8.4）、较低的Zr/Nb比值（19.5–35.）以及Nb/Ta比值（11.46–18.15），均表明塔夫雷什岩浆形成过程中发生了壳幔相互作用（mantle–crust interaction）。相对较低的Nb/La比值进一步证实，岩石圈地幔（lithospheric mantle）在熔体形成过程中发挥了重要作用。重稀土元素（HREE）的地球化学特征（即镝/镱（Dy/Yb）比值随二氧化硅（SiO₂）含量升高而降低）表明，在岩浆部分熔融及分异过程中，残留相中几乎不含石榴子石（garnet）。该岩体群是阿拉伯板块与欧亚板块（Arabian and Eurasian tectonic plates）碰撞前，俯冲相关岩浆作用（subduction-related magmatism）晚期阶段的产物。