Origin of 1.8 Ga zircons in Post Eocene mafic dikes in the Roshtkhar area, NE Iran

The Roshtkhar area is located in the Khaf-Kashmar-Bardaskan volcano-plutonic belt to the northeastern Iran along the regional E–W trending Dorouneh Fault, northeastern of the Lut Block. There are several outcrops of subvolcanic rocks occurring mainly as dikes in the area, which intruded into Cenozoic intrusive rocks. We present U–Pb dating of zircons from a diabase dike and syenite rock using LA-ICP-MS that yielded an age of 1778 ± 10 Ma for the dike, indicating this Cenozoic dike has zircon xenocrysts inherited from deeper sources; and 38.0 ± 0.5 Ma, indicating an Late Eocene crystallization age for the syenite. Geochemically, the dikes typical of high-K calc-alkaline to shoshonitic magmas. Petrographic observations and major and trace element variations suggest that diabase melts underwent variable fractionation of clinopyroxene, olivine, and Fe-Ti oxides and minor crustal contamination during the differentiation process. Primitive mantle-normalized multi-element diagrams display enrichment in LILE, such as Rb, Ba, Th, U, and Sr compared to HFSE, as well as negative anomalies of Nb, Ta, P, and Ti, suggesting derivation from subduction-modified mantle. Chondrite-normalized REE plots show moderately LREE enriched patterns (N/YbN versus La/Yb and La/Sm ratios suggests a low-degree of batch melting (~1–3%) of a phlogopite-spinel peridotite source to generate the mafic dikes. The geochemical signatures suggest that the Roshtkhar mafic dikes cannot be related directly to subduction and likely resulted from melting of upper mantle in an extensional setting where the heat flow was provided from deeper levels. These dikes presumably derived the zircon xenocrysts from the assimilation of upper crust of Gondwanian basement. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in NE Iran was triggered by heating due to asthenospheric upwelling in an extensional setting.Roshtkhar地区地处伊朗东北部Lut地块东北部的Khaf-Kashmar-Bardaskan火山-深成岩带内，沿区域东西向展布的Dorouneh断裂带分布。区内发育多块次火山岩露头，该类岩石多以岩脉形式产出，侵入于新生代侵入岩体中。 本文采用LA-ICP-MS（激光剥蚀电感耦合等离子体质谱法）对一件辉绿岩脉与一件正长岩样品中的锆石开展U-Pb定年分析：辉绿岩脉的定年结果为1778 ± 10 Ma，指示该新生代岩脉含有源自深部的继承锆石捕虏晶；正长岩的定年结果为38.0 ± 0.5 Ma，表明其结晶年龄为始新世晚期。 地球化学特征显示，该类岩脉的岩浆属于高钾钙碱性系列至钾玄岩质岩浆系列。岩相学观察以及主量、微量元素变化特征表明，辉绿岩熔体在分异演化过程中经历了单斜辉石、橄榄石以及铁钛氧化物的不同程度分异结晶，并伴随少量地壳混染作用。 原始地幔标准化多元素图解显示，相较于高场强元素（HFSE，High Field Strength Element），该岩脉富集大离子亲石元素（LILE，Large Ion Lithophile Element），如Rb、Ba、Th、U及Sr，同时表现出Nb、Ta、P与Ti的负异常，指示其源区为受俯冲作用改造的地幔。 球粒陨石标准化稀土元素（REE，Rare Earth Element）配分图显示，其轻稀土元素（LREE，Light Rare Earth Element）呈现中等富集的配分模式；通过N/Yb_N与La/Yb_N、La/Sm_N比值计算可知，该基性岩脉源于金云母-尖晶石二辉橄榄岩源区约1%~3%的批式部分熔融作用。 地球化学特征表明，Roshtkhar基性岩脉与俯冲作用无直接成因联系，其更可能形成于伸展背景下由深部热流供给的上地幔熔融过程。该类岩脉中的继承锆石捕虏晶，推测源自冈瓦纳基底上部地壳的同化混染作用。 伊朗东北部交代岩石圈地幔的部分熔融以及碰撞后岩浆作用的成因过程，均由伸展背景下软流圈上涌引发的热效应所触发。