The Colville batholith: Tertiary plutonism in northeast Washington associated with graben and core-complex (gneiss dome) formation

The Colville batholith in the central Okanogan Highlands of Washington is composed of numerous individual granitoid intrusions (plutons), most of which may be grouped on the basis of field evidence, modal analysis, and major-element chemical composition into three successive intrusive suites: the Keller Butte suite (KBS), the Devils Elbow suite (DES), and the Herron Creek suite (HCS). The KBS is distinguished from the younger intrusive suites by (1) modes restricted to the fields of granite and granodiorite; (2) lack of horn-blende; (3) a relatively Fe-rich AFM variation trend; (4) an alkali-lime index classification of calcic, as opposed to calc-alkalic and alkali-calcic for the younger suites; (5) compositions >64% SiO₂ and (6) low MgO relative to SiO₂ The DES is distinguished from the HCS by (1) its lower modal alkali feldspar and correspondingly low K₂O relative to SiO₂ and (2) a slightly more Fe-rich AFM variation trend.

Biotite and hornblende K-Ar ages, the most believable and geologically significant of which are concordant ages from plutons of the DES, indicate the batholith is early Tertiary in age. Norms of highly evolved residual aplitic granite suggest that the magmas completed crystallization at 2 to 3 km depth. The structural relations of the plutonic rocks suggest that associated core-complex and graben structures were active both before and after emplacement of KBS and DES plutons, and that the graben structures continued to develop until after emplacement of the HCS. The structural relations and radiometric ages, taken together, lead to the conclusion that the batholith, core complexes, and graben structures are all broadly contemporaneous and chiefly of early Tertiary age.