Tectonic Stacking of HP/LT Metamorphic Rocks in Accretionary Wedges and the Role of Shallowing Slab‐Mantle Decoupling

Abstract

AbstractHigh‐pressure/low‐temperature (HP/LT) chloritoid‐bearing micaschists crop out widely in the central part of northern Turkey and represent deep‐seated subduction‐accretionary complexes. Three peak metamorphic assemblages are identified in the area studied: (1) garnet‐chloritoid‐glaucophane with pseudomorphs after lawsonite; (2) chloritoid with pseudomorphs after glaucophane; and (3) chloritoid with pseudomorphs after jadeite in addition to phengite, paragonite, quartz, chlorite, rutile, and apatite. The latter is interpreted as transformation of a chloritoid + glaucophane assemblage to chloritoid + jadeite with increasing pressure; PT modeling indicates ~17 and 22–25 kbars for the two peak parageneses. The diversity of peak metamorphic assemblages and the PT estimates suggest that basal accretion occurred at different depths within the wedge. The depth of the basal accretion is possibly controlled by the slab‐mantle decoupling depth. Stretching and thinning of the lithospheric fore arc induced by the slab rollback possibly caused shallowing of the slab‐mantle decoupling depth which limited depth of the basal accretion from 70–80 km to ~55 km within the subduction channel. A slab‐mantle coupling depth‐controlled basal accretion may also explain the scarcity of eclogite and high‐grade blueschist facies metamorphic rocks in active intraoceanic subduction zones. Because the overriding plate is young and hot in intraoceanic subductions, the slab and mantle are coupled at a relatively shallow depth before eclogitization of the oceanic crust. This prevents accretion and exhumation of eclogite along the subduction channel.