Dynamic evolution in a Cretaceous high-P/T subduction channel evidenced by the juxtaposition of amphibolite blocks with different P-T paths: an example from the Kamuikotan belt, northern Japan
Abstract
A subduction channel developed at the boundary between a subducting oceanic plate and an overlying plate could be geologically defined as the place, where accretionary sediments were dragged down to great depth with an oceanic plate to suffer a high-P/T type metamorphism, and transformed to high-P/T metamorphic rocks (e.g. blueschist and eclogite). In the study area, while typical high-P/T metamorphic rocks (blueschist), which originated from Cretaceous accretionary sediments, amphibolites and metacherts also occur as tectonic blocks in mélange surrounded by either serpentinite or pelitic matrix, which originally suffered intermediate-P/T type metamorphism, but later the same high-P/T type metamorphism as the sediments did. In this research, we have analyzed mineral assemblages in these amphibolites and metacherts, and conducted micro-chemical analyses of compositional zoning in amphibole and garnet from these rocks with an EPMA. As a result, compositional zoning in some constituent amphibole can be divided into 3 types. Type I is a dominant type, where actinolite is overgrown by glaucophane, indicating pressure increase. Type II, which has been found in only one sample, is defined as the compositional zoning in amphibole consisting of magnesiohornblende, actinolite and glaucophane from core to rim. The compositional zoning shows a change of the temperature gradient from low-P/T (or intermediate-P/T) type to high-P/T type, which could reflect a cooling of the subduction channel with time from the onset of subduction to a steady state. Type III is characterized by the compositional zoning in amphibole from tschermakite to glaucophane-magnesioriebeckite. This also shows a cooling of the subduction channel with time. In this sample, garnet also shows a compositional zoning from a Mn-rich and Ca-poor inner core to a Mn-poor and Ca-rich outer core, which is surrounded by a Mn-rich rim, showing a compositional discontinuity across the core-rim boundary. The compositional zoning in garnet indicates an increase in both temperature and pressure during the initial growth, followed by temperature decrease during the later growth, comparable with the P-T paths inferred from the amphibole zoning. These different types of compositional zonings in amphibole and garnet show different P-T paths. Hence, these amphibolite tectonic blocks with different temperature-pressure-time paths could have been juxtaposed perhaps by large-scale ductile flow in the ancient subduction channel, which occurred during the initial stage of subduction from the onset to a steady state. This kind of dynamic evolution in subduction channel at the initial stage has now been reported from other well-known high-pressure terrains (e.g. Sambagawa, Japan; Samaná, Dominican Repúblic).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T43E2700O
- Keywords:
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- 8170 TECTONOPHYSICS Subduction zone processes;
- 8104 TECTONOPHYSICS Continental margins: convergent;
- 3652 MINERALOGY AND PETROLOGY Pressure-temperature-time paths;
- 3613 MINERALOGY AND PETROLOGY Subduction zone processes