In-situoxygen isotope records of crustal self-cannibalization selectively captured by zircon crystals from high-δ²⁶Mg granitoids

Abstract

Zircon oxygen-hafnium and biotite magnesium isotopic compositions of Late Cretaceous to Oligocene (88-27 Ma) granitoids from the Gyeongsang continental arc, southeastern Korea, collectively provide a new insight into shallow magma petrogenesis. Zircon crystals extracted from eight calc-alkaline or alkaline plutons commonly contain xenocrystic cores and typically show magmatic cathodoluminescence zonings and sharp Raman spectra. Most zircons have positive e Hf values plotting below the mid-ocean-ridge basalt evolution path, indicating that the arc magmatism was dominated by the recycling of juvenile crustal materials. Zircon d 18 O values shifted upward or downward from the mantle range attest to the O isotopic exchange of crustal protoliths with surface waters at various temperature conditions. Small (<1‰) but discernable core-to-rim decreases of d 18 O values are recognized selectively in magmatic zircon grains from four plutons possessing biotite d 26 Mg values (-0.07‰ to +2.32‰) distinctly higher than the mantle range. Such a concomitant O-Mg isotopic variation provides compelling evidence for a series of self-induced hydrothermal alteration and assimilation processes (i.e ., "crustal self-cannibalization"). This study shows that the supracrustal input to magmas in a young and juvenile orogen can be traced effectively by the combination of stable isotope records from the plutonic root.

Why No Porphyry Copper Deposits in Japan and South Korea?

“…The existence of major, regional‐scale caldera flare‐ups in the Late Cretaceous–Paleogene of Japan and South Korea as well as during the Mio‐Pliocene and Quaternary in NE and SW Japan further reduced the likelihood of porphyry Cu formation by evacuation of potential ore‐forming magmatic fluids during large‐volume, silicic ignimbrite eruptions (Sillitoe, ). This process was exacerbated by the extensive crustal melting and fractionation that took place in upper‐crustal magma reservoirs during the caldera cycle, leading to an abundance of intrusive rocks too silicic in composition for major Cu deposit formation, as well as by the potentially large and at least partly reduced crustal contributions to both the intrusive and extrusive components of caldera volcanism (Yoshida et al , ; Kimura et al , ; Jo et al , ).…”

“…The ilmenite-series character of magmatism in the Sanyo belt of SW Japan and elsewhere completely precluded porphyry Cu formation (Ishihara, 1977(Ishihara, , 1981 and even the magnetite-series activity in the Sanin belt, Kitakami region, and SE Korea, which gave rise to some Cu mineralization, has a distinctive lithophilemetal signature, defined by Mo AE W. The existence of major, regional-scale caldera flareups in the Late Cretaceous-Paleogene of Japan and South Korea as well as during the Mio-Pliocene and Quaternary in NE and SW Japan further reduced the likelihood of porphyry Cu formation by evacuation of potential ore-forming magmatic fluids during large-volume, silicic ignimbrite eruptions (Sillitoe, 1980b). This process was exacerbated by the extensive crustal melting and fractionation that took place in upper-crustal magma reservoirs during the caldera cycle, leading to an abundance of intrusive rocks too silicic in composition for major Cu deposit formation, as well as by the potentially large and at least partly reduced crustal contributions to both the intrusive and extrusive components of caldera volcanism (Yoshida et al, 2014;Kimura et al, 2015;Jo et al, 2016).…”

Magmatic response to the interplay of collisional and accretionary orogenies in the Korean Peninsula: Geochronological, geochemical, and O-Hf isotopic perspectives from Triassic plutons

“…The correlation between ε Hf (t) and Lu/Hf observed in synmagmatic zircons from the Taean and Namyang granites suggests that the mixing of the crustal components was accompanied with magmatic differentiation (i.e., DePaolo, 1981). The oxygen isotopic evolution of the source crust is more difficult to trace, because lowand high-temperature surface processes could modify the original value significantly (Valley, 2003;Bindeman, 2008;Jo et al, 2016).…”

LKZ-1: A New Zircon Working Standard for the In Situ Determination of U–Pb Age, O–Hf Isotopes, and Trace Element Composition

“…Zircon can provide U-Pb age constraints, as well as trace element and O-Hf isotope data for given microdomains. The integration of such multifaceted zircon data has proven to be a powerful tool for addressing the diverse issues related to crustal evolution [11][12][13][14].…”