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The Naozhi deposit of northeast China comprises intermediate-sulfidation epithermal-style veins and subeconomic, porphyry-style Cu mineralization hosted by dioritic to granitic plutons, which formed at ca. 130 Ma. These intrusive phases were fed by magmas that ponded at mid- to upper-crustal levels and were finally emplaced at < 0.6 kbar based on amphibole geobarometry. The magmas were moderately oxidized (∆NNO − 0.35 to 2.21; with NNO being the nickel–nickel–oxide buffer) and water-rich (mostly > 4 wt. %), and likely experienced a relatively prolonged evolution in the shallow crust rather than at depth. Magma devolatilization and fluid exsolution at shallow-crustal levels lead to decreases in both oxygen fugacity (log ƒO2 − 10.96 to − 14.13) and water content (6.53 to 2.26 wt. %) at < 2.5 kbar pressures. Thermochronological data of zircon and apatite, thermal history modeling, and geological preservation record complex time–temperature histories of the mineralized intrusive rocks, from the cooling of the parent magma itself through burial by younger volcanic rocks to protracted erosion with time. It is estimated that < 2 km of pre-mineralization material was removed from the top of the Naozhi magmatic–hydrothermal system from the Early Cretaceous to present. A 108 ± 2 Ma volcanic event possibly tilted the hydrothermal system, but buried the ores, protecting them from extensive erosion. [ABSTRACT FROM AUTHOR]

Oligocene to early Miocene volcanic rocks are preserved in the Central Cordillera range and Cagayan Valley of northern Luzon, Philippines. Basaltic and andesitic rocks of the Pugo Formation in the Baguio district of the Central Cordillera were intruded by the -27 to 20 Ma calc-alkaline Central Cordillera Diorite Complex. In the southern Cagayan Valley the subalkaline to alkaline late Oligocene Mamparang Formation overlies the Cretaceous Caraballo Formation and has been intruded by the Didipio Igneous Complex, the Cordon Syenite Complex, and the Palali batholith. The Didipio complex comprises an early suite of diorites, which were intruded by the strongly mineralized stocks of the Dinltidi Cu-Au porphyry deposit. Whole-rock geochemical data for intrusive and extrusive rocks of the Baguio district range from low K calc-alkaline to shoshonitic basalts to dacites with rare earth element (REE) and high field strength element (HFSE) characteristics of suprasubuction zone magmas and are all interpreted to have been sourced from the same parent melt. Samples from Didipio display higher alkali contents but similar trace element characteristics. New age dates for the Didipio area range from 25.7 to 24.8 Ma. The potassic magmas of the Cagayan Valley are interpreted to have formed in a back-arc coeval to the main-arc sequence that is preserved in the Baguio Miocene rocks. This contradicts earlier models, which invoke an early Miocene arc reversal in the northern Luzon archipelago with the switch from early westward subduction to later eastward subduction attributed to a variety of causes. The lack of a single compelling trigger for arc reversal combined with the coeval emplacement of arc magmas in the west and back-arc magmas in the east in northern Luzon is best interpreted as the result of eastward subduction since the late Oligocene. The presence of -20 Ma adakitic magmas in the Baguio district may indicate that flattening of the downgoing slab resulted in a hiatus in magmatism and termination of back-arc rifting. [ABSTRACT FROM AUTHOR]

Protracted magma storage in the deep crust is a key stage in the formation of evolved, hydrous arc magmas that can result in explosive volcanism and the formation of economically valuable magmatic-hydrothermal ore deposits. High magmatic water content in the deep crust results in extensive amphibole ± garnet fractionation and the suppression of plagioclase crystallization as recorded by elevated Sr/Y ratios and high Eu (high Eu/Eu*) in the melt. Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of central Chile. These rocks formed in a magmatic cycle that culminated in high-Sr/Y magmatism and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu*, and Mg in apatite to track discrete stages of arc magma evolution. We apply fractional crystallization modeling to show that early-crystallizing apatite can inherit a high-Sr/Y and high-Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modeling shows that crystallization of the in situ host-rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk-magma chemistry. Understanding this decoupling behavior is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies. [ABSTRACT FROM AUTHOR]