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

Abstract

The interplay of irreversible reactions and deformation during regional metamorphism was analyzed in the reaction zones between rodingite and serpentinite. Rodingites are leucocratic rocks found commonly in serpentinites and are considered metasomatic products of Ca-rich fluid. Rodingites occur ubiquitously in serpentinite from the Nomo metamorphic rocks, western Kyushu, a Cretaceous accretionary complex of greenschist to epidote-amphibolite facies condition. We used the singular value decomposition (SVD) method to analyze rodingitization reactions based on mineral compositions of a rodingite sample and a possible protolith (clinopyroxene gabbro). The resultant reaction implied that the rodingite was formed due to the addition of considerable amounts of CaO and H 2 O to the protolith, whereas other components such as SiO 2 , AF (Al 2 O 3 + Fe 2 O 3 ), and FM (FeO + MgO) were conserved. The Nomo rodingites are associated with reaction zones between serpentinite. A careful study on the compositional variations of diopside and chlorite in the rodingite, serpentinite, and the reaction zones between them showed a disequilibrium crystallization of diopside in the reaction zone. Diopside shows a serrated variation in composition across the reaction zone together with disequilibrium signature within grains (composite grain consisting of Fe-rich and Fe-poor parts). These features clearly indicate that diopside crystallized in a relatively short period compared to the metamorphism by irreversible reactions. Another conspicuous feature is that perovskite occurs in some reaction zones, whereas titanite is common in rodingites; this indicates that the reaction zone is poorer in SiO 2 . The tremolite veins are typically derived from the reaction zone into serpentinite; showing this is a good example of hydrofracturing associated with the formation of the reaction zone. All the reactions obtained by the SVD method that contribute to the formation of the reaction zones consumed CaO and evolved H 2 O, strongly suggesting that the hydrofracturing is caused by a rapid increment of fluid pressure during the progression of the reactions. The reactions consumed considerable amounts of SiO 2 in the reaction zone, which is consistent with the occurrence of perovskite, and the conservation of AF and FM components.

The internal structure and composition of a plate-boundary-scale serpentinite shear zone: the Livingstone Fault, New Zealand

“…Metasomatic reaction zones are found at the contact between schist or rodingite pods and the surrounding serpentinite. Such reactions between serpentinite and siliciccalcic lithologies that form tremolite, talc, diopside and chlorite are extensively documented in the literature (Cronshaw, 1923;Read, 1934;Nishiyama, 1990;Boschi et al, 2006;Nishiyama et al, 2017). In the Livingstone Fault, metasomatic reactions resulted in the transformation of foliated serpentinite into a highly indurated metasomatic fault rock largely consisting of networks of tremolite veins (Fig.…”

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.…”

“…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). The Nomozaki Formation and the older metagabbros have undergone pum- pellyite-actinolite facies metamorphism (Miyazaki and Nishiyama, 1989).…”

“…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.…”

Formation of triple–layer coronas between corundum and hornblende from the Lützow–Holm Complex at Akarui Point, East Antarctica

“…The linear combination of the two solutions provides a general solution (cf. Nishiyama et al, 2017), which is described as R6, 9, 12, 15, and 18 in Table 3. We obtained the ranges of the variable so that the solutions have proper signs for the stoichiometric coefficients consistent with the microstructure, that is, 0.30 < p < 0.64 (R6), 0.39 < q (R9), 0.31 < r < 0.67 (R12), and 0.27 < t < 0.53 (R18).…”