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The Early Cretaceous geology of Northwestern (NW) Peru was characterized by emplacement of voluminous felsic volcanic rocks (e.g. the Oyotún Fm.), which have important implications for the understanding of Early Cretaceous crust-mantle interactions in the Central Andes. We present U-Pb-Hf isotopic compositions and whole-rock and trace elemental compositions of a series of Early Cretaceous volcanic rocks from the Western Cordillera and the Eastern Cordillera of the Central Andes. The volcanic rocks outcrop sporadically with limited thickness and uniform volcanic lithofacies in the western region, while the eastern region has distributions of larger scale volcanic rocks with more complex volcanic lithofacies. LA-ICP-MS zircon U-Pb dating defines the age of the Oyotún Fm. range from 138.8 Ma to 131.0 Ma in the Western Cordillera, and from 139.7 Ma to 132.3 Ma in the Eastern Cordillera. In situ Hf isotope analysis on dated zircon yield an εHf(t) range of +3.37 to +8.42 and a TDM2(Hf) range from 983 Ma to 650 Ma with a peak at ca. 840 Ma. The Early Cretaceous felsic magmas are predominantly dacites and rhyolites with minor components of basalt and andesite, and show relatively homogeneous Hf isotopic compositions of zircon in single samples. Both the εHf(t) of zircons and the magma temperature increase from the bottom to the top of the Early Cretaceous volcanic rocks in each studied area. All these facts imply an increasing contribution of contemporaneous underplated hot and dry mantle-derived magmas with time, both as a heat source inducing crustal melting and a source of material (melt) that variably mixed with the ancient continental crustal materials. [ABSTRACT FROM AUTHOR]

The Paleocene–Eocene southern Peru metallogenic belt contains numerous very large to supergiant porphyry Cu–Mo deposits. The deposits were dated previously as coeval with the Incaic I orogeny at ca. 60 Ma. However, tectono-magmatic processes during the formation of the deposits are poorly constrained. Here, we integrate published geochronological, geochemical, and mineralogical data from barren and ore-related granitoids in the region. The syn-orogenic unit (60–53 Ma) has significantly higher average ratios of Sr/Y (104.4), La/Yb (29.6), Dy/Yb (1.9), and Eu N /Eu* (1.07) than the pre-orogenic unit (69–60 Ma; Sr/Y = 18.6, La/Yb = 11.5, Dy/Yb = 1.7, and Eu N /Eu* = 0.66). The syn-orogenic unit exhibits high Dy/Yb ratios in relatively primitive rocks and has a lower average Dy/Yb (zircon) ratio (0.23) and higher Eu N /Eu* (zircon) ratio (0.43) than the pre-orogenic unit (Dy/Yb (zircon) = 0.29 and Eu N /Eu* (zircon) = 0.21). These integrated data suggest that the syn-orogenic magmas underwent high-pressure, garnet-dominated fractionation prior to amphibole-dominated fractionation, distinct from the pre-orogenic magmas which underwent low-pressure, plagioclase-dominated fractionation. Comprehensive investigation of the geological background suggests that the geochemical variations between pre- and syn-orogenic units were caused by the long-term evolution of mantle-derived magmas at the crust–mantle boundary during arc compression, instead of crustal thickening. Fractionation of ferric iron-depleted minerals (e.g., garnet and amphibole) in the mantle-derived basaltic magmas probably has positive effects on magmatic fertility. The dataset presented highlights the critical role of high-pressure differentiation, thus providing an insight into the relationship between the arc compressional regime and giant porphyry Cu deposit formation in the Andes. Zircon trace elements can be useful indicators for porphyry deposit exploration in the Toquepala arc in southern Peru. However, the regional tectonic settings of the Andean arcs should be studied thoroughly before applying these indicators. [ABSTRACT FROM AUTHOR]

[Display omitted] • Arc compression control magmatic maturity (high oxidation state and S content). • High oxidation state and volatile content promote porphyry mineralization. • External fertile sources supplements are not necessary in the formation of high oxidation state and S-rich magma. The Don Javier porphyry Cu–Mo deposit formed contemporaneously with the Incaic I orogeny (∼60 Ma) in southern Peru. The causative dacite porphyry is hosted by the Yarabamba Superunit, which is the youngest batholithic unit in the Toquepala arc. In this study, we conducted elemental and isotopic analyses on samples from the dacite porphyry and Yarabamba Superunit in an effort to clarify the origins and formation mechanism of the deposit. Zircon U–Pb dating shows that the young part of the Yarabamba Superunit was emplaced at 65.4 ± 0.7 to 63.5 ± 0.8 Ma, and the causative dacite porphyry was emplaced at Don Javier between 59.9 ± 0.4 and 59.2 ± 1.1 Ma. The Yarabamba Superunit and dacite porphyry have similar ε Hf (t) and δ18O isotope values, ranging from –4.6 to +1.9 and 5.1‰ to 6.4‰, respectively. The Yarabamba Superunit has initial Sr and Nd isotope values of 0.70533–0.70579 and 0.51242–0.51244, respectively. Isotopic data suggest that the Yarabamba Superunit and dacite porphyry evolved from an isotopically homogeneous magma reservoir with minor crustal assimilation. However, the dacite porphyry has whole-rock and zircon Eu/Eu* values of 0.9 and 0.34, respectively, higher than those of the Yarabamba Superunit (0.6 and 0.13). Additionally, the dacite porphyry has significantly higher apatite S content (0.07 wt%) and X Cl values (0.21) than those of the Yarabamba Superunit (apatite S: 0.01 wt%; X Cl values: 0.15). The calculated magmatic oxidation state shows that the dacite porphyry has a significantly higher magmatic oxygen fugacity (ΔFMQ +1.1) than the Yarabamba Superunit (ΔFMQ –0.5). Together, the data suggest that the magmas formed the Yarabamba Superunit and causative dacite porphyry are characteristics of distinct tectonic regimes. The magma of the Yarabamba Superunit was generated during normal subduction and underwent low-pressure, H 2 O-poor, plagioclase-dominated fractionation while the magma of the dacite porphyry was generated during arc compression and underwent high-pressure, H 2 O-rich, amphibole-(garnet) dominated fractionation, which led to a high oxidation state and volatile-rich magma that promoted porphyry mineralization at Don Javier. This rapid elevation of the magmatic oxidation state and volatile content occurred synchronously with the Incaic I orogeny (∼60 Ma), which indicates that this orogeny had a significant impact on the formation of the very large–super-giant porphyry ore systems in southern Peru. [ABSTRACT FROM AUTHOR]