ABSTRACT

In the Mariana forearc region, blueschist-facies metabasite clasts have been recovered from a serpentinite seamount during Ocean Drilling Program Leg 125. To clarify the origin of these metamorphic clasts, major and rare earth elements (REE), Ba, and Sr were measured in seven blueschist-facies metabasite clasts. Two out of seven clasts have zig-zag REE patterns reflecting the lanthanide tetrad effect, suggesting intense interaction with sea water. The other five clasts, however, show REE patterns with light REE depletion similar to N-type mid-ocean ridge basalts. Barium and Sr enrichments, relative to light REE abundances, are not conspicuous and most clasts show Sr and/or Ba depletions, indicating that they have chemical affinity with mid-ocean ridge basalt. This result is supported by the similarity of the TiO₂-Al₂O₃ systematics of the clasts to those of mid-ocean ridge basalts. These MORB-like clasts metamorphosed under blueschist-facies conditions must have been taken into uprising serpentinite diapir at a depth where materials from oceanic crust extensively accreted. Two possible sources for the MORB-like protolith of blueschist-facies clasts are considered: (1) the oceanic crust of the subducted Pacific plate, (2) oceanic crust trapped within the area between trench axis and volcanic front when the subduction of the Pacific plate started. The second possibility is likely because the subducting Pacific plate crust is overlain by 600-800 m of oceanic sediment which is not represented in the clast population. It is proposed that the clasts provide evidence of tectonic erosion of oceanic crust which was trapped in the Mariana forearc at the initiation of subduction.