Petrology and geochemistry of adakitic intrusions and dykes at Sarcheshmeh porphyry Cu‐Mo ± Au deposit, Iran: Insights into their source

Abstract

The Sarcheshmeh porphyry copper deposit (PCD) is located in the southeast part of the arc‐related Urumieh‐Dokhtar volcano‐plutonic belt. This PCD was formed by intrusion of a Middle Miocene granodiorite to tonalite stock into the volcano‐sedimentary rocks that are mainly andesite exhibiting Early Tertiary thrusting and faulting. On the basis of geochemistry of intrusive rocks, they are calc‐alkalic and alkali‐calcic suites that are consistent with oxidized I‐type magmas. The Fe2O3/FeO ratio in the Sarcheshmeh PCD is 1 to 3, consistent with a high oxygen fugacity. The igneous rocks of Sarcheshmeh PCD belong to a mature arc, but related to a post‐collision arc setting. Considerable evidence supports adakitic affinities of the Sarcheshmeh rocks, including geochemical values of Sr > 300 ppm (ave., 594 ppm), high ratios of Sr/Y > 20 (ave., 75), La/Yb > 20 (ave., 30), and enrichment of LREE and LILE relative to HREE and HFSE. The adakites of Sarcheshmeh belong to high‐silica adakites (HSA) that can be formed by melting of subducting seawater‐altered oceanic plate, which interact with the mantle wedge then followed by slab break‐off with local transtension in the middle part of the Dehaj‐Sarduieh belt. In rocks of the region, the ratios of Th/Ce > 0.12 and Nb/Zr > 0.05 suggest that the subducting crust has a role in forming different groups of these rocks. Geotectonic discrimination diagrams are used to distinguish between normal arc and slab failure magmatic systems, including Nb versus Y, La/Sm versus Sm/Yb, La/Yb versus Gd/Yb, and Rb versus Nb + Y diagrams. The plutonic rocks of the Sarcheshmeh PCD have characteristics consistent with adakites derived via slab failure, which are obviously metallogenically favorable magmatic systems. Adakitic melts generated rapidly during sinking of a broken slab (into higher temperature mantle), buoyantly migrate through the subcontinental lithospheric mantle (SCLM), which is related to structural evolution of that SCLM with local extension or transpression.