Mantle Exhumation in an Early Paleozoic Passive Margin, Northern Cordillera, Yukon

Orogenic peridotite occurs as a megaboudin structurally juxtaposed with smaller boudined masses of corona troctolite, skarn, and garnet amphibolite in metasedimentary rocks of the Yukon‐Tanana Terrane, Yukon. The peridotite shows well‐developed plagioclase coronae on spinel and records cooling from ∼1000° to 600°C and decompression across the spinel‐plagioclase peridotite facies boundary at ∼0.7 GPa. The troctolite boudins record cooling to 850°C through the same facies reaction at ∼0.8 GPa. In an aureole surrounding the peridotite body, the mainly quartzofeldspathic country rock contains leucosome with garnet, orthopyroxene, and sillimanite formed from breakdown of biotite, which records temperatures of ∼900°C at pressures of at least 0.7 GPa. Exhumation of the peridotite body from the mantle during rifting of continental lithosphere at least 25 km thick subjected continental margin metasediments to conditions above the fluid‐absent metapelite solidus. The contrasting strength of upper mantle and crustal lithologies during extension may explain the structural juxtaposition and boudinage of upper mantle peridotite and troctolite on various scales. The Os isotopic compositions of the peridotite body show that it represents either relatively young mantle lithosphere with an age only slightly older than the Devonian metasedimentary rocks in which it is hosted or mantle that is metasomatized in the same events related to rifting. The rocks in the study area bear striking similarities to those in and surrounding the Zabargad (Red Sea) and Ronda (Spain) peridotite massifs and are interpreted to have formed in an Early Paleozoic preoceanic rift. Other enigmatic mantle tectonite occurrences in continental margin metasediments in Yukon and Alaska may have a similar origin.

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