Origin and tectonic significance of the Aldrich Mountains serpentinite matrix melange, northeastern Oregon

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The Permian ‐ Triassic Aldrich Mountains serpentinite melange consists of a matrix of massive and foliated serpentinite (lizardite + chrysotile) that encases a variety of metamorphic, igneous, and sedimentary block types. Serpentinite matrix foliation trends NE‐SW, dips steeply (> 75°), and is parallel to long dimensions of blocks. At block‐matrix contacts, the foliation traces block margins. The volumetrically most abundant block types are greenstone, chert, and metagabbro; subordinate block types are amphibolite tectonites, epiclastic sediments (breccia, conglomerate, sandstone, and argillite), silicic volcanic rocks, silicic plutonic rocks, and ultramafic rocks. Three types of metamorphism are evident from textures and mineral assemblages of blocks and matrix: (1) serpentinization and related hydrous alteration confined to matrix, ultramafic blocks, and margins of mafic blocks; (2) lower to middle greenschist and prehnite‐actinolite facies metamorphism characterized by static, non penetrative fabrics of some blocks, and; (3) dynamothermal epidote amphibolite to upper amphibolite/lower granulite metamorphism exhibited by strongly schistose amphibolite tectonites. Mineral assemblages and mineral chemistry indicate amphibolite tectonites formed at moderate to high temperatures (550°–780°C) and low to moderate pressures (3–5 kbar). Most blocks were affected by variable degrees of post metamorphic brittle deformation prior to immersion in serpentinite matrix. The melange is interpreted to have formed within a Permian to Late Triassic sediment‐starved, tectonically active forearc. Fragments of abyssal oceanic crust and Early Permian ensimatic arc crust were mixed by tectonic and sedimentary processes within the forearc. Highly mobile serpentinite, generated beneath the forearc, was protrusively emplaced into the tectonized forearc and mixed with the blocks, forming the melange. Melange generation probably began in the late Early Permian and ceased by the Late Triassic.