Amphibolites of the Stalemate Ridge and Shirshov Rise, Northwest Pacific: Indicators of the Geodynamic Regime of Metamorphism at the Convergent Plate Boundary

Abstract

The paper presents data on amphibolites dredged during Cruise 249 of the German R/V Sonne at three sites: one at the northwestern termination of the Stalemate Ridge (ocean-side slope of the Aleutian Trench) and two at the western slope of the north Shirshov Rise (in the western sector of the Bering Sea). Data on the petrography and geochemistry of the amphibolites and their estimated P–T metamorphic parameters provide an insight into the probable geodynamic environments in which their protoliths were formed and into the geodynamic regime of their metamorphism. The magmatic protolith of amphibolites in the Shirshov Rise was produced by the melting of a strongly depleted mantle source and, perhaps, was a product of suprasubductional magmatism. The isotope composition and distribution of HFSE in amphibolites from the Stalemate Ridge suggest that the protolith of these rocks was a derivative of tholeiitic melt parental for N-MORB. The typical mineral assemblage and P–T metamorphic parameters of glaucophane-bearing amphibolites from the Shirshov Rise indicate that the protolith of these rocks was metamorphosed within a subduction zone related to the Cretaceous–Paleogene Olyutorsky Arc. Interpretations of the age and geodynamic nature of metamorphism of amphibolites in the Stalemate Ridge based on currently available data cannot be regarded as absolutely realistic, and the most likely scenario for the origin of the amphibolites is that they were produced within a subduction zone that was no younger than 55–50 Ma and preceded the modern Aleutian Arc. The extinction of this ancient subduction zone should have been associated with the development of the Aleutian Arc north of it. According to this geodynamic scenario, an ophiolite sequence was produced where the ancient subduction zone had occurred. The ophiolite consisted of rocks of an mafic–ultramafic association and amphibolites. This scenario may, however, be a matter of further discussion.