Temporal and genetic link between incremental pluton assembly and pulsed porphyry Cu-Mo formation in accretionary orogens Hervé Rezeau1,*, Robert Moritz1, Jörn-Frederik Wotzlaw2, Rodrik Tayan3,†, Rafael Melkonyan3, Alexey Ulianov4, David Selby5, François-Xavier d’Abzac1, and Richard A. Stern6 Department of Earth Sciences, University of Geneva, 1205 Geneva, Switzerland Institute of Geochemistry and Petrology, ETH Zurich, 8092 Zurich, Switzerland 3 Institute of Geological Sciences, National Academy of Sciences, 0019 Yerevan, Armenia 4 Institute of Earth Sciences, University of Lausanne, 1015 Lausanne, Switzerland 5 Department of Earth Sciences, University of Durham, DH1 3LE Durham, UK 6 Canadian Centre for Isotopic Microanalysis, University of Alberta, Edmonton, Alberta T6G 2E3, Canada 1 2
ABSTRACT Economically important porphyry Cu-Mo deposits (PCDs) are generally hosted by uppercrustal plutons of variable chemical compositions related to distinct geodynamic settings. The absolute timing and duration of pluton assembly and PCD formation are critical to understanding the genetic relationship between these interrelated processes. Here, we present new comprehensive zircon U-Pb and molybdenite Re-Os ages that tightly constrain the timing and duration of pluton assembly and the age of mineralization in one of the largest ore-bearing plutons of the central Tethyan metallogenic belt, the Meghri-Ordubad pluton, southern Armenia and Nakhitchevan, Lesser Caucasus. This composite pluton was incrementally assembled during three compositionally distinct magmatic episodes over ~30 m.y., comprising Middle Eocene (48.9–43.1 Ma) calc-alkaline subduction-related magmatism lasting 5.8 ± 0.8 m.y., followed by postsubduction Late Eocene–Middle Oligocene (37.8–28.1 Ma) shoshonitic magmatism over 9.7 ± 0.9 m.y., and Late Oligocene–Early Miocene (26.6–21.2 Ma) adakitic magmatism consisting of shoshonitic dikes and high-K calc-alkaline granodioritic magmas emplaced over 5.4 ± 0.4 m.y. Despite the distinct geodynamic settings and magma compositions, each intrusive suite culminated in the formation of variably sized PCDs, including the giant Oligocene Kadjaran porphyry Cu-Mo deposit associated with high-Sr/Y shoshonitic magmas. Complementary in situ zircon hafnium (eHf = +8 to +11.3) and oxygen (d18Ozircon zircon = +4.6‰ to +6.0‰) isotope data support a mantle-dominated magma source with limited crustal contribution and/or cannibalization of young and juvenile lower-crustal cumulates. We conclude that, independent of geodynamic setting and magma composition, long-lived (5–10 m.y.) incremental mantle-derived magmatism is a prerequisite to form fertile magmatichydrothermal systems, and especially giant PCDs. INTRODUCTION The majority of porphyry Cu-Mo deposits (PCDs) are associated with subduction-related calc-alkaline upper-crustal plutons (Sillitoe, 2010). However, porphyry Cu-Mo systems have recently also been recognized in postsubduction settings, particularly along the Tethyan metallogenic belt (e.g., Richards, 2015; Hou et al., 2015a, 2015b; Moritz et al., 2016). This raises a number of questions concerning the primary control on PCD formation, as most models require active oceanic subduction to generate large volumes of hydrous, oxidized, S-rich and Cu-rich magmas derived from a subcontinental lithospheric mantle metasomatized by slab fluids, and repeated mafic magma injections to provide sulfur, metals, and volatiles into *E-mail:
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upper-crustal reservoirs (e.g., Hattori and Keith, 2001; Scaillet, 2010; Audétat and Simon, 2012; Tapster et al., 2016). Ore-forming processes in PCDs occur over short time scales of
