Pre‐Middle Jurassic accretionary metamorphism in the southern Klamath Mountains of northern California, USA

Abstract High‐P/T metamorphic parageneses are preserved within two late Palaeozoic to early Mesozoic assemblages of the southern Klamath Mountains that show contrasting structural styles and mineral parageneses reflecting formation in different parts of a subduction‐zone regime. Blueschist facies tectonites of the Stuart Fork terrane represent a coherent subduction complex formed at relatively deep crustal levels, whereas the chaotic metasedimentary mélange of the eastern Hayfork terrane contains a diverse range of metamorphic parageneses reflecting complex structural mixing of metamorphic components at shallower levels. The convergent‐margin‐type accretionary metamorphism evident in both terranes pre‐dates Middle Jurassic low‐P/T metamorphism resulting from regional tectonic contraction and magmatism. The epidote‐ to lawsonite‐zone Stuart Fork blueschists (and eclogites locally) formed at pressures of about 6‐11 kbar and temperatures of 250‐400° C. Deformed matrix material of the eastern Hayfork mélange formed at similar temperatures but lower pressures, on the order of 3‐6 kbar. The mélange contains a diverse assemblage of tectonic blocks that formed under a range of P‐T conditions, including those of the blueschist, pumpellyite‐actinolite, greenschist and upper greenschist to amphibolite facies. The succession of mineral parageneses and inferred P‐T conditions of the eastern Hayfork blocks reflect those of igneous protolith formation, structural mixing, subduction‐zone metamorphism, olistolith transport, and tectonic and erosional denudation. Although temporal relations are not well constrained, the evolution of these terranes is consistent with formation within a single convergent‐margin system.