The origin and significance of metamorphosed tectonic blocks in mélanges: evidence from Sulawesi, Indonesia

Abstract

ABSTRACTMetamorphosed tectonic blocks (or ‘knockers’) are widespread but volumetrically minor constituents of many circum‐Pacific mélange belts, Due to the common lack of an exposed in situ provenance and to the seemingly chaotic field disposition of most block‐bearing mélanges, their origin and uplift history are problematic and controversial. On the Indonesian island of Sulawesi a block‐bearing mélange is overlain by an ophiolite nappe, the base of which is characterized by a metamorphic sole sequence. Petrological, geochemical and geochronological data indicate a direct genetic relationship between high‐grade tectonic blocks in the mélange and amphibolites in the metamorphic sole. Amphibolite precursors to lower temperature blueschist assemblages are virtually ubiquitous in the tectonic blocks and subdivisions based on the nature of the overprinting relationships can be systematically correlated with block distribution patterns orientated subparallel to the strike of the mélange belt. It is suggested here that the high‐grade tectonic blocks originated in a thin, thermally zoned metamorphic sheet welded to the oceanic hangingwall plate at the inception of subduction. Break‐up of this sequence at depth, by tectonic erosion, led to dispersal of fragments into a newly developed serpentinite mélange wedge. Blocks experienced abrupt changes in P‐T‐X conditions due to a combination of hydration in the new fluid‐rich environment, gradual cooling of the hangingwall over time and continuing underflow dragging sheared blocks deeper into the subduction zone, prior to upflow. Blocks plucked from the hangingwall at different depths and at different times evidently experienced uplift in different flow channels, resulting in block concentrations, with P‐T‐t paths characteristic of their source and flow trajectory, at systematically greater distances from the subduction zone hangingwall. The elucidation of the origin and significance of tectonic blocks in Sulawesi has important implications not only for the tectonometamorphic evolution of similar inclusions in other mélange belts, but also for models of the inception and early stages of subduction.