Crustal control on the redox state of granitoid magmas: tectonic implications from the granitoid and metallogenic provinces in the circum-Japan Sea Region

Abstract

Felsic magmatism has occurred over a large region of East Asia since Jurassic times and has provided important mineral resources such as tin, tungsten, base metals and gold. The circum-Japan Sea region preserves various geological records of active continental margins, including Jurassic to Early Tertiary magmatic arcs and subduction zones and pre-Jurassic continental basements, which were separated by the opening of the Japan Sea during the Miocene. The felsic magmatism in this region shows a wide variation in terms of redox state and related mineralisation, encompassing east-west contrasts around the Pacific Ocean. A review of granitoids and associated ore deposits in this region indicates that the character of the crust, sedimentary versus igneous, is an essential factor to control the redox state, and a tectonic setting may be an additional factor in some cases.The reduced-type granitoids, characterised by tin mineralisation, were generated in carbonbearing sedimentary crust which was composed mainly of accretionary complex material and not influenced by previous magmatism. Involvement of sedimentary materials is corroborated by oxygen, sulphur and strontium isotope data. The oxidised-type granitoids, characterised by gold or molybdenum mineralisation, were generated in igneous crust which was depleted in reducing agents as a result of previous magmatism. Granitoid magmatism in a given area tends to become more oxidised with time.Jurassic accretionary complexes in East Asia are thought to have been largely displaced from the original place of accretion and stacked up against the northeastern margin in the Khingan and Sikhote-Alin Mountains. This region, dominated by sedimentary crust, was subsequently subjected to Cretaceous felsic magmatism and converted to a large province of reduced-type granitoids and tin-tungsten mineralisation. Diverse geodynamic processes, including the change of the arc-trench system, the creation and collapse of the back-arc basin and the collision of continents, may have prepared many favourable sites for the generation of reduced-type granitoids in northeast Asia. These processes may have resulted in a remarkable contrast with the Pacific margin of North America, where repeated arc magmatism during the Mesozoic formed granitoid batholiths of the oxidised-type.The granitoid types may also be controlled by the tectonic setting and mode of magma emplacement. In the northern Kitakami area of Northeast Japan, Early Cretaceous episodic magmatism occurred in a Jurassic accretionary complex, and formed the oxidised-type granitoids accompanied by submarine bimodal volcanism associated with kuroko mineralisation. Granitoids of fissure-filling type emplaced under extensional environments may be oxidised, irrespective of basement geology, because of insignificant crustal input.

Why No Porphyry Copper Deposits in Japan and South Korea?

“…The reduced character of the ilmenite-series intrusions is generally considered to reflect a reduced crustal source, principally the carbonaceous sedimentary component of the late Paleozoic, Jurassic, and Cretaceous-Paleogene accretionary complexes and their high-P/T metamorphic equivalents that underlie much of Japan (Ishihara, 1977(Ishihara, , 1981(Ishihara, , 2004Sato et al, 2004;Sato, 2012;Fig. 2).…”

“…Porphyry Cu deposits worldwide are hosted by magnetite-series intrusions of upper mantle and/or lower-crustal provenance, which are thought to attain their oxidized states by contributions of volatiles, SO 4 2− , and Fe 3+ from subducted oceanic slabs (Ishihara, 1977(Ishihara, , 1981Mungall, 2002;Kelley & Cottrell, 2009). The original oxidized states of the magmas are variably retained, depending on rate of ascent through the crust, whether the latter was originally oxidized or underwent some degree of oxidation during earlier magnetite-series magmatism (Sato et al, 2004;Sato, 2012), and potentially other factors (e.g., magnetite fractionation; Jenner et al, 2010). It is important to stress that oxidized melts and fluids are a fundamental requirement for effective Cu transport through the mantle and crust, which also maximises their S (as SO 4 2− )-carrying capacity (e.g., Richards, 2015).…”

“…6), although none is assigned high Cu potential. This conclusion stems from the fact that the lithocapsincluding the main examples in the Nansatsu district (Izawa & Urashima, 1983)are likely to be underlain by chemically reduced crust unless there has been wholesale substitution during earlier magnetite-series intrusive activity (Sato et al, 2004;Sato, 2012). Although this is unlikely in the Nansatsu district, with a basement dominated by the Shimanto accretionary complex (Figs 2, 6), such substitution may have been possible during the Cretaceous-Paleogene in the NE Japan arc, especially during caldera development (e.g., Fig.…”

Sedimentary Crust and Metallogeny of Granitoid Affinity: Implications from the Geotectonic Histories of the Circum‐Japan Sea Region, Central Andes and Southeastern Australia

“…In order to emphasize this aspect, the ilmenite/magnetite-series can also be named the reduced/oxidized types (e.g. Sato et al, 1992Sato et al, , 2004aSato, 2003). The reduced/oxidized types are used in this paper.…”

“…The genesis of this contrast is considered to be a result of global geodynamic processes. Sato et al (2004aSato et al ( , 2006b and Sato (2006) suggested that repeated magmatism along a convergent margin of the North and South American continents for a long period could have produced oxidized-type granitoids due to the depletion of sedimentary carbon, an essential reducing agent, while sedimentary crusts could have been involved in magmatism in the Asian side in various stages during more complicated tectonic history than the monotonous convergent margin of the American side. However, this insight is to be reexamined through careful analyses of geotectonic information for individual regions.…”