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 H2O to the protolith, whereas other components such as SiO2, AF (Al2O3 + Fe2O3), 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 SiO2. 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 H2O, 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 SiO2 in the reaction zone, which is consistent with the occurrence of perovskite, and the conservation of AF and FM components.
Highlights
Metamorphism is a significant crustal process that results in the reconstruction of crustal rocks through various chemical reactions and deformation
The results are consistent with the field observation that the tremolite veins are derived from the reaction zone into the serpentinite (Fig. 4a)
The singular value decomposition (SVD) method is a powerful tool by which reaction relations among minerals with complex compositions can be systematically derived and the amount of mobile components evaluated
Summary
Metamorphism is a significant crustal process that results in the reconstruction of crustal rocks through various chemical reactions and deformation. Nishiyama et al Progress in Earth and Planetary Science (2017) 4:1 under the constant solid volume conditions For this purpose, the reaction zones between rodingite and serpentinite in the Nomo metamorphic rocks, western Kyushu, Japan, were used as a case study. In an example of mass transfer analysis associated with rodingitization, Barriga and Fyfe (1983) compared the bulk compositions of rodingites and protolith gabbros from East Liguria, Italy and reported significant enrichment in CaO and H2O in rodingites with remarkable depletion in SiO2, Na2O, K2O, and TiO2. They suggested that these results were caused by interactions of the rocks with highly alkaline Ca–Mg(OH)2-rich solutions derived from serpentinites. The SVD method can show which components are inert components because linear dependence among minerals for a set of components implies the conservation of those components through the reaction produced by the linear dependence. Shigeno et al (2012) applied both the isocon and SVD methods to jadeitite reaction zones in the Nagasaki Metamorphic Rocks and determined that both the methods produce consistent results
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