Why are there no Cu-porphyry deposits in Jurassic Sanandaj-Sirjan zone intrusions of Iran?

Granitoid intrusions of Zagros orogenic and metallogenic belt were mainly emplaced during the Jurassic-Cretaceous and Upper Eocene – Pliocene along the Sanandaj-Sirjan Zone (SaSZ) and Urumieh-Dokhtar magmatic Arc (UDMA), respectively. The present work tried to answer the question of whether or not there are fundamental differences in magmatic systems of Mesozoic versus Cenozoic intrusions of Zagros orogen resulting in the lack of porphyry mineralization in SaSZ compared with the widespread occurrence of porphyry-bearing intrusions in the UDMA. Characterizing the geochemical features of six major granitoid intrusions in the SaSZ (Alvand, Almogholagh, Aligoodarz, Boroujerd, Dehno, and Kolah-Ghazi) show they mostly represent low to high-K calc-alkaline and peraluminous magmatic suite, which is analogous to those reported previously for the UDMA porphyry Cu intrusions. However, the SaSZ intrusions are characterized by extremely low Sr/Y contents, distinct negative Eu anomalies, and less fractioned HREE patterns with OIB signature in the less fractionated samples indicating a source dominated by melting of subcontinental lithosphere mantle. This study proposed that the inception of Cenozoic flat slab subduction followed by dehydration melting of ~ 29 Ma of metasomatized subcontinental lithospheric mantle (SCLM) during the continuous stages of continental-continental collision has a decisive role in magmatic change to a more hydrous and oxidized nature. These processes provided unique conditions for increasing metal fertility of these Miocene intrusions compared to those of older Mesozoic igneous activities in the Zagros orogenic and associated metallogenic belt.

扎罗斯造山带与成矿带的花岗岩类侵入体，分别主要侵位于萨南达季-锡尔詹带（SaSZ）和乌鲁米耶-杜赫塔尔岩浆弧（UDMA）的侏罗纪-白垩纪时段与始新世晚期-上新世时段。本研究旨在解答如下科学问题：扎罗斯造山带中生代与新生代侵入体的岩浆系统是否存在本质差异，进而导致萨南达季-锡尔詹带缺乏斑岩成矿作用，而乌鲁米耶-杜赫塔尔岩浆弧却广泛分布含斑岩侵入体。对萨南达季-锡尔詹带内6处主要花岗岩类侵入体（阿尔万德、阿尔莫加拉格、阿里古达尔兹、博罗耶尔德、德赫诺与科拉-加齐）的地球化学特征进行表征后发现，这些侵入体大多属于低钾至高钾钙碱性过铝质岩浆组合，这与此前报道的乌鲁米耶-杜赫塔尔岩浆弧斑岩型铜侵入体的特征高度相似。然而，萨南达季-锡尔詹带侵入体的显著特征为极低的Sr/Y比值、明显的负铕异常，以及分馏程度较弱的重稀土元素配分模式；分馏程度较低的样品呈现洋岛玄武岩（OIB）地球化学特征，指示其源区以交代型大陆下岩石圈地幔（SCLM）的熔融为主。本研究提出，新生代平板俯冲的起始，伴随陆陆碰撞持续过程中约29 Ma交代型大陆下岩石圈地幔（SCLM）的脱水熔融，对岩浆体系向更富水、更氧化的性质转变起到了决定性作用。相较于扎罗斯造山带及相关成矿带内的中生代古老岩浆活动，上述过程为这些中新世侵入体提升金属成矿潜力提供了独特的有利条件。