Geodynamic Nature of Magmatic Sources in the Northwest Pacific: An Interpretation of Data on the Sr and Nd Isotope Composition of Rocks Dredged at the Stalemate Ridge, Ingenstrem Depression, and Shirshov Rise

Abstract

The paper presents the first data on the Sr and Nd isotope composition of rocks from a unique collection of magmatic rocks of all petrographic types (basalts, gabbro, and peridotites) belonging to the lithosphere at the convergent plate boundary in the Aleutian Arc area. The rock samples were collected over large areas in the Northwest Pacific and Bering Sea. The results of this study provide reliable information on the geodynamic nature of the central segment of the Shirshov Rise, which is made up of magmatic rocks whose isotopic–geochemical features indicate that they were produced by magmatic melts that evolved within the crust and were derived by the partial melting of MORB parental source, which may have belonged to a mantle wedge. The isotopic–geochemical characteristics of the rocks at the Shirshov Rise suggest that the mafic–ultramafic rocks association may have been produced at a backarc spreading center. Petrological and isotopic–geochemical traits of rocks dredged at the northwestern flank of the Stalemate Ridge indicate that the sources of the parental melts were heterogeneous. Data on the rocks suggest that magmatism in this part of the Northwest Pacific may have involved a source responsible for the origin of the oldest seamounts in the Hawaiian–Emperor Volcanic Chain. Ultramafic rocks in the northwestern segment of the Stalemate Ridge show similarities with plutonic rocks found as xenoliths in volcanics of the Aleutian Island Arc. With regard for the scarcity of information on the structure of the lithosphere in this part of the Pacific, it can be cautiously suggested that the oceanic slope of the Aleutian Trench and adjacent segment of the Stalemate Ridge involve fragments of the basement of the Aleutian Island Arc.