U-PB ZIRCON GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY OF THE MOUNTAIN HOME METAMORPHIC COMPLEX, BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON

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. In the Mountain Home Metamorphic Complex (MHMC), an enigmatic sequence of medium-grade metavolcanic and metasedimentary rocks is intruded by two groups of variably deformed plutons. Metamorphic rocks include chlorite+biotite+garnet±sillimanite±staurolite schist, and keratophyre and metatuff which grade into chlorite schist near fault contacts. This degree of metamorphism is uncommon in the BMP, and contrasts with adjacent rocks in the Baker and Wallowa terranes.

Two distinct plutonic suites intrude metamorphic host rocks: 1) deformed hornblende tonalite and trondhjemite (Group 1), and 2) undeformed biotite-hornblende tonalite, biotite-hornblende quartz diorite, and hornblende gabbronorite (Group 2). Quartz microstructures in deformed plutons display evidence for grain-boundary migration and subgrain-rotation recrystallization, features which are absent in Group 2 plutons. ²⁰⁶Pb/²³⁸U zircon ages for Group 1 plutons range from 161.3 to 149.5 Ma; whereas Group 2 plutons yield ages ranging from 147.8 Ma to 145.5 Ma. These ages bracket deformation to between 149.5 to 147.9 Ma, which contrasts with the timing of previously documented tectonic events in the BMP. Initial εHf values range from +16.3 to +10.2 in Group 1 plutons and +13.8 to +4.2 in Group 2 plutons. These values overlap and extend to lower values than average modern island arc crust. Based on the new deformational period and degree of metamorphism in the MHMC we suggest that it may be a new terrane in the BMP.

Unit				206Pb/238U age			error	εHfi
					(Ma)	(2σ)			(range)
Group 1: Deformed
Metatrondhjemite of Pearson Creek					161.3	1.2			n.d.
Metatonalite of California Gulch					152.7	1.2			16.3-10.2
Trondhjemite of Johnson creek					149.5	1.7			13.9-10.5
Group 2: Undeformed
Quartz diorite of Alexander Creek					147.9	2.4			13.8-8.3
Gabbronorite of Carney Butte					147.8	1.6			12.8-5.8
Tonalite of Granite Meadows					146.5	1.2			11.5-7.2
Gabbronorite of Ridenor Canyon					145.6	0.5			10.7-4.2