Geology, geochemistry, and isotopic character of the Colville Igneous Complex, northeastern Washington

Abstract

The Colville Igneous Complex in northeastern Washington comprises Paleocene orthogneisses and leucocratic granitic rocks and Eocene volcanic‐plutonic suites which were emplaced during early Tertiary extension. The Colville Igneous Complex is divided into three components: (1) 65 ± 4 Ma orthogneisses of the Okanogan dome or metamorphic complex and similar gneisses in the Kettle and Lincoln complexes; (2) 61–51 Ma leucocratic granitic rocks; and (3) 53–47 Ma intermediate to felsic volcanic‐plutonic rocks. The leucocratic granites and younger volcanic‐plutonic rocks are divided on the basis of age, geochemical and isotopic characteristics, and emplacement mechanism. The older leucocratic granites are silica rich, corundum‐normative, and Sr and light rare earth element (REE)‐enriched, and they form several large, discrete, compositionally zoned plutons and intrusive suites. The younger, Eocene volcanic‐plutonic rocks can be divided into two suites: an older compositionally restricted (68–72wt % SiO2), Sr‐enriched, medium‐K suite and a younger intermediate to felsic, high‐K, calcalkalic suite. Sr isotopic measurements of volcanic and plutonic rock in the Colville Igneous Complex yield variable initial 87Sr/86Sr ratios (Sri), ranging from about 0.704 to values greater than 0.710. In general, the volcanic rocks are especially variable with little or no correlation between Sri and geographic location. The leucocratic granites, however, show a systematic west to east increase across the area which probably reflects an increased involvement with Precambrian crust. The variable nature of Sri in the Colville Igneous Complex may also be controlled by crustal thickness and/or depth of melting. Within individual volcanic and plutonic suites, there is a crude correlation of mean Sri with age. There is a systematic decrease in Sri with increasing age which may reflect either a migration of the site(s) of melting or a decreased upper crustal residence time, possibly related to the onset of extension. The leucocratic granites may have formed by small degrees of partial melting in the lower crust, perhaps facilitated by thermal blanketing of an initially overthickened crust. The ascent of large bodies of leucocratic magmas may have increased the geothermal gradient which allowed for upper, more radiogenic, crust to be melted.