Origin of blueschist-facies clasts in the Mariana forearc, Western Pacific.

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 Ti02-A1203 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.

Exhumation of the ultrahigh‐pressure Tso Morari unit in eastern Ladakh (NW Himalaya): A case study

“…In this model, the exhumation within a low‐viscosity channel is effectively controlled by a hydrodynamic return flow along the subducting plate [ Cloos , 1982, 1986]. The Tso Morari dome could be exhumed as a rigid UHP unit embedded in light and soft serpentinite seamounts as observed today in the Mariana zone [ Yamamoto , 1995]. Moreover, Guillot et al [2001] show that the low normal deviatoric stresses (few tens of MPa) developed at the interface between a rigid subducting plate and a low‐viscosity serpentinized wedge can produce a hydrodynamic return flow necessary to move an eclogitic unit back up.…”

Blueschist from the Mariana forearc records long-lived residence of material in the subduction channel

“…Additionally, there has been no in-depth and detailed metamorphic work done on the clasts to constrain the metamorphic conditions of formation and therefore the depth they sample within the subduction system. A high-pressure origin for blueschist clasts from the Mariana serpentinite-mud volcanoes has been suggested before, but never quantified (Maekawa et al, 1993;Yamamoto et al, 1995).…”