Magmatism, metamorphism, and deformation in the Mountain Home Metamorphic Complex, Blue Mountains Province, Oregon, and its role in late Jurassic deformation in the western North American Cordillera
Orogenesis in the Blue Mountains Province (BMP) of NE Oregon involved the accretion of various tectonostratigraphic terranes in concert with syn-tectonic magmatism. Timescales of deformation, metamorphism and magmatism in this region of the Cordillera are poorly constrained, hindering detailed understanding of the timing of tectonic events and the magmatic and metamorphic processes accompanying them. The Mountain Home Metamorphic Complex (MHMC) is a sequence of medium-grade metavolcanic and metasedimentary rocks that borders the Wallowa and Baker terranes of the BMP. The MHMC is intruded by two groups of plutons distinguished based on their respective presence and absence of crystal-plastic deformational textures. This study utilizes U-Pb zircon dating on both crystal-plastically deformed and undeformed pluton suites to bracket the timing of metamorphism and deformation in the MHMC. The whole rock geochemistry of the MHMC was compared to the adjacent Baker and Wallowa terranes to assists in determining the relationship between these terranes and the MHMC. In addition, the petrogenesis of plutonic and metamorphic rocks in the MHMC is determined by Lu-Hf isotopic ratios of zircons. Two distinct plutonic suites are present in the MHMC: 1) deformed hornblende (±clinopyroxene) gabbro, hornblende tonalite and trondhjemite (Group 1), and 2) undeformed biotite-hornblende tonalite, biotite-hornblende quartz diorite, and hornblende gabbronorite (Group 2). 206Pb/238U zircon ages for Group 1 plutons range from 159.46 ± 0.67 to 149.5 ± 1.6 Ma; whereas Group 2 plutons yield ages ranging from 149.4 ± 2.1 Ma to 145.66 ± 0.50 Ma. These ages bracket deformation to between 149.5 to 149.4 Ma, which contrasts with the timing of previously documented tectonic events in the BMP. Epsilon Hf values in the MHMC range from +16.3 to +10.2 in Group 1 plutons and +13.8 to +4.2 in Group 2 plutons. These values exclude correlation between the MHMC and the Baker terrane, but not the Wallowa terrane. On a large scale, the MHMC is metamorphosed to amphibolite facies, a significantly higher grade than any other regional scale metamorphism in the BMP. The timing of deformation constrained in this study does not correspond with any of the previously proposed deformational models for the BMP. The geographically closest known deformational event that corresponds with the timing of deformation in the MHMC is the 150 ±1 Ma Nevadan Orogeny of the Klamath Mountains. The unique timescale of deformation and higher grade metamorphism imply that the MHMC does not belong to the Baker or Wallowa terranes, but may be a new terrane in the BMP.