A combined K-Ar and 40Ar/39Ar geochronological study of the Ruby Mountains-East Humboldt Range, Nevada, a Cordilleran metamorphic core complex, has revealed a complex tectonothermal history. The three structural subdivisions of the core complex (migmatitic core, mylonitic zone, and cover) share magmatic and deformational histories in part, but also display important contrasts in structural style, metamorphic paragenesis, and thermal history. During the Late Jurassic (ca 160 Ma), miogeoclinal rocks were polyphase-deformed, metamorphosed under amphibolite facies conditions, and pervasively intruded by peraluminous granitic magmas, thereby forming an igneous and metamorphic complex. Late Jurassic muscovite granite porphyry plugs also intruded unmetamorphosed cover rocks. Late Cretaceous cooling of the igneous and metamorphic complex is locally suggested by poorly resolved Cretaceous incremental-release 40Ar/39Ar hornblende spectra and by U-Pb monazite ages from pelitic schist exposed in the northern Ruby Mountains. Between ca 45 Ma and 20 Ma, the igneous and metamorphic rocks experienced an episode of complex extensional tectonism which involved the development of low-angle, normal-sense, simple shear zones. As a result, rocks metamorphosed at depth in the Mesozoic were translated to higher crustal levels which resulted in cooling through temperatures required for the intracrystalline retention of argon within minerals. Near concordance of hornblende and biotite plateau ages from the migmatitic core of the complex suggest relatively rapid exhumation. However, it is not certain if this cooling followed a prolonged maintenance at elevated temperatures after the ca 160 Ma metamorphism or if distinct, locally superposed late Mesozoic and/or middle Tertiary thermal overprints had affected the cooling history. A major extensional simple shear zone is marked by an approximately 1-kilometer-thick mylonite zone that experienced late kinematic, superposed brittle deformation. A geometric consequence of the development of low-angle extensional fault zones is the eventual translation of colder rocks over hotter rocks. Such a history is supported by the 40Ar/39Ar data. For example, mylonitic mafic orthogneisses from a major allochthon record 40Ar/39Ar hornblende plateau ages between 44 and 48 Ma and biotite plateau ages between 32 and 33 Ma, and are structurally positioned above mylonitic rocks of the northern Ruby Mountains which record biotite cooling ages of ca 22–24 Ma. Such relations suggest either multiple episodes of Tertiary mylonitization or a protracted, multiphase history of mylonitization. However, after ca 20 Ma, all of the presently exposed mylonitic rocks were maintained below 100°C and have only experienced upper crustal brittle deformation.
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