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.

扎罗斯造山成矿带的花岗岩类侵入体主要沿萨南达季-西尔詹带（Sanandaj-Sirjan Zone, SaSZ）和乌鲁米耶-多赫塔尔岩浆弧（Urumieh-Dokhtar magmatic Arc, UDMA）侵位，其侵位时代分别为侏罗纪-白垩纪和始新世晚期-上新世。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-西尔詹带缺乏斑岩成矿作用（porphyry mineralization），而乌鲁米耶-多赫塔尔岩浆弧广泛发育含斑岩侵入体（porphyry-bearing intrusions）？对萨南达季-西尔詹带内6处典型花岗岩类侵入体（阿尔万德、阿尔莫加拉格、阿利古达尔兹、布鲁杰尔德、德赫诺与科拉-加齐）的地球化学特征表征结果显示，这些侵入体大多属于低-高钾钙碱性过铝质岩浆岩套，该特征与此前报道的乌鲁米耶-多赫塔尔岩浆弧含斑岩铜侵入体的岩浆组合高度相似。但萨南达季-西尔詹带侵入体具有Sr/Y比值极低、显著负铕异常的特征，且重稀土元素（HREE）配分模式分馏程度较弱；在分馏程度较低的样品中可见洋岛玄武岩（OIB）地球化学特征，指示其源区以交代改造的大陆下岩石圈地幔（subcontinental lithospheric mantle, SCLM）熔融为主。本研究提出，新生代平板俯冲的起始，以及随后在陆陆碰撞持续阶段中约29 Ma的交代型大陆下岩石圈地幔发生脱水熔融，对岩浆向更富水、更具氧化性的性质转变起到决定性作用。相较于扎罗斯造山带及相关成矿带内的中生代古老火成活动，这些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。