伊朗大理矿区中新世成矿及无矿斑岩地球化学对比及其对成矿的启示

伊朗乌尔米耶-达克塔尔弧岩浆带(Urumieh-Dokhtar magmatic arc,UDMA)是特提斯域最重要的斑岩铜矿省,发育大量中新世大型超-大型斑岩铜矿床;同时,该带也发育大量同时代无矿岩体,但控制岩体成矿潜力的关键因素尚不清楚.为此,选择该带中段、成矿及无矿岩体同时发育的大理矿区,针对成矿及无矿岩体开展了系统的锆石岩相学、年代学、微量元素地球化学及Hf同位素地球化学对比.结果显示,无矿闪长岩(锆石U-Pb年龄:17.4±0.3 Ma)比成矿石英闪长斑岩(锆石U-Pb年龄:15.6±0.1 Ma)形成略早,但近乎同期;闪长岩εHf(t)值变化介于+2^+4,石英闪长斑岩εHf(t)值变化介于+2^+5,两者具有类似的Hf同位素组成;闪长岩中锆石常含老的继承核(172~920 Ma),石英闪长斑岩则不发育继承锆石;闪长岩及石英闪长斑岩中的中新世锆石具有类似的稀土配分模式,且Eu负异常不明显,而闪长岩中的锆石继承核则显示出明显的Eu负异常,配分模式与中新世锆石不同.基于上述结果,我们提出大理矿区的两套中新世岩体具有相同的岩浆源区,但经历了不同的地壳演化过程,成矿的石英闪长斑岩浆形成后,与古老地壳没有明显交互,而无矿的闪长岩浆,在上升过程中与地壳物质、特别是古老还原性物质发生了交互,交互过程中岩浆氧逸度降低,是该套岩浆不成矿的主要原因.进而我们提出UDMA带中段斑岩成矿与否不仅与前人所认为的受岩浆源区控制,也与岩浆演化过程密不可分.

Geochemical Constraints on Fertile and Infertile Miocene Magmatic Suite in Dalli Area,Iran and Its Insights for Metallogeny

The Urumieh-Dokhtar magmatic arc (UDMA), hosting three giant and six large Miocene porphyry copper deposits, is the most important post collisional porphyry belt in the Tethyan domain. In the UDMA belt, there are also abundant coeval barren porphyry intrusions, but the factors controlling fertility of Miocene porphyry intrusions in the belt remains enigmatic. The Dalli deposit, located in the central segment of UDMA, contains both fertile and barren Miocene intrusions. In this study, we report zircon petrography, geochronology, trace element geochemistry and Hf isotopic compositions for the barren diorite and mineralization-related quartz diorite porphyry at Dalli. The results show that the diorite has inherited zircon cores overgrown by new zircons, whereas quartz diorite only has one type of zircon. The diorite yields a zircon U -Pb age of 17.4±0.3 Ma, which is slightly older than the quartz diorite porphyry (zircon U-Pb:15.6±0.1 Ma), and the inherited zircon cores yielded as zircon U-Pb of 172-920 Ma. Both the diorite and quartz diorite porphyry show slightly positive ε_Hf(t) values, which are respectively +2-+4 and +2-+5.Miocene zircons from both diorite and quartz diorite porphyry are characterized by obviously positive Ce anomalies and slightly depleted Eu anomalies. However, the inherited zircon cores exhibit more depleted Eu anomalies, suggesting low oxygen fugacity. These results suggest that the barren diorite possibly interacted with ancient crust, particularly with reduced material, which subsequently decreased magma oxygen fugacity whereas the fertile quartz diorite porphyry emplaced without such material exchange. On the basis of above observations, we propose magma process may be the first order of key factors controlling the fertility of intrusions in the central segment of the UDMA.