The formation of rodingite in the Nagasaki metamorphic rocks at Nomo Peninsula, Kyushu, Japan – Zircon U–Pb and Hf isotopes and trace element evidence

Abstract

Rodingites occur in serpentine-matrix mélange of the Nagasaki metamorphic rocks in Japan. Two types of rodingites can be distinguished on the basis of their mode of occurrence and mineralogical composition. One occurs as dikes, which contain a mineral assemblage of grossular, vesuvianite, diopside, apatite, titanite, and zircon. The other occurs as a block, which consists of zoisite, clinozoisite, diopside, chlorite, apatite, titanite, and zircon. The former type of rodingites posses two types of zircons: prismatic and porous. The prismatic zircons contain primary fluid inclusions indicating their crystallization in the presence of fluids. The porous zircons have extensive fractures filled by zircon, which are indicative of a hydrothermal origin. Both zircon types were thought to have formed under the influence of fluids. U-Pb ion probe analyses of prismatic zircons from the rodingites yield a weighted mean age of 108-105 Ma, suggesting the Early Cretaceous as the time of rodingitization in the subduction zone. Hafnium isotopic compositions of prismatic zircons are close to or overlap with the mid ocean ridge basalt (MORB) Hf isotopic ratio. This indicates that the fluid composition may have been reflected by the MORB composition during rodingitization. On the other hand, the low ε Hf values (11.8-18.9) of porous zircons suggest that they incorporate a small amount of Hf from fluid contaminated by subducted sediments. The rodingites are significantly enriched in Sr and depleted in large ion lithophile elements (LILE) (Cs, Rb, Ba). The fluid during rodingitization is able to extracts LILEs from the protolith of rodingites and adds Sr to the protolith of rodingites. The high field strength elements (HFSE) (Zr, Th, U, Nb, Ta) concentrations in the rodingites are similar to those of MORB, thus indicate their relatively immobile nature during rodingitization.

Interplay of irreversible reactions and deformation: a case of hydrofracturing in the rodingite–serpentinite system

“…The rodingites occur ubiquitously in the serpentinite. Fukuyama et al (2014) conducted geochemical and geochronological studies on these rodingites and determined that the U-Pb age of the zircons from the rodingites was 108-105 Ma, indicating that the formation of the rodingites predates the peak metamorphism of the Sanwa Formation. The rodingites mostly occur as metamorphosed dykes, typically 20 to 50 cm thick in the serpentinite, and occasionally as tectonic blocks with chlorite-rich reaction zones in a serpentinite mélange.…”

“…Li et al (2014) found perovskite-bearing rodingites in a mélange type blueschist-bearing accretionary wedge complex. In our study area, Fukuyama et al (2014) carried out geochemical studies on the rodingite. They found two types of zircons, prismatic, and porous types, from dyke-type rodingites.…”

Jadeitites and associated metasomatic rocks from serpentinite mélanges in the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan: a review

“…Blocks of Type B consist of garnet-bearing epidote-blueschist-barroisite rock, meta-hornblendite, albitite, zoisite rock, rodingite, omphacite-bearing metagabbro, omphacitite, and jadeitite. Rodingite (grossular + clinozoisite + diopside) is very rare in the Nishisonogi unit, whereas it is ubiquitous in the Nomo unit (Fukuyama et al, 2014;Nishiyama et al, 2017). These features of the serpentinite mélange in this region indicate that tectonic mixing of various rocks occurred in a narrow serpentinite channel within a subduction zone under the ductile condition together with extensive metasomatism.…”

“…The Nomo metamorphic rocks consist of three geological units of different ages in fault contact with one another: the older metagabbros of 480-460 Ma (hornblende K-Ar ages : Shibata et al, 1977;Igi et al, 1979), the Nomozaki Formation (the Suo unit) of 250-150 Ma (phengite K-Ar ages: Nishimura, 1998), and the Sanwa Formation of 90-70 Ma (phengite K-Ar ages: Ueda and Onuki, 1968; hornblende K-Ar age: Shibata et al, 1977; phengite K-Ar ages: Nishimura, 1998). Fukuyama et al (2014) reported the U-Pb ages of zircons from rodingites in serpentinites associated with the Sanwa Formation as 108-105 Ma, indicating that the formation of rodingites predates the peak metamorphism of the Sanwa Formation. The coherent schists in the Sanwa Formation have undergone greenschist to epiote-amphibolite facies metamorphism; only rocks associated with the serpentinite in the Sanwa Formation represents epidote-blueschist facies metamorphism according to the occurrence of omphacite and magnesio-riebeckite (Nishiyama and Miyazaki, 1987;Miyazaki and Nishiyama, 1989;Nishiyama et al, 2017).…”

“…It consists mostly of antigorite with or without dolomite, magnesite, talc, magnetite, and tremolite. Olivine rarely occurs as a relict mineral, contrasting to serpentinite in the Nomo unit which commonly contains relict clinopyroxene (Fukuyama et al, 2014;Nishiyama et al, 2017). Possible protoliths are dunite or hartzburgite for the Nishisonogi serpentinite, and lherzolite or wehrlite for the Nomo serpentinite.…”