PETROLOGIC EVOLUTION OF MG-AL-RICH GNEISSES DURING MIGMATITE DOME FORMATION, OKANOGAN DOME, WASHINGTON

The Okanogan dome (WA, USA) is one of three migmatite domes in the northern Cordillera, in addition to the Thor-Odin and Valhalla domes (B.C., Canada), that contain orthoamphibole-cordierite gneisses as layers and lenses surrounded by anatectic migmatites. Mg-Al-rich rocks in these domes record metamorphic conditions and path information not recorded in the host migmatites, and therefore provide insights into gneiss dome tectonics.

In the Okanogan dome, Mg-Al-rich bulk compositional layers are part of the Tunk Creek unit, which occurs at the periphery of a structurally underlying migmatite domain. Bulk compositional layers in this unit consist of gedrite-dominated, hornblende-dominated, and biotite-bearing layers that contain variable amounts of gedrite ± hornblende, anorthite, cordierite, spinel, sapphirine, corundum, kyanite, and/or staurolite, and therefore provide information about different parts of the petrogenetic history. Gedrite-dominated layers containing relict kyanite preserve evidence of the highest-P conditions, and symplectitic and coronal reaction textures around relict kyanite suggest decompression. Gedrite-dominated layers lacking these reaction textures contain intergrown layers of spinel + sapphirine in apparent textural equilibrium and record a later high-T – low-P part of the path. Pseudosection analysis for bulk compositional domains lacking obvious disequilibrium textures indicate T ~ 720-750 °C over a range of P (> 8 to < 4 kbar) following decompression. Elevated crustal temperatures in the Tunk Creek unit and the underlying migmatite domain, and concordant structural fabrics in both units, suggest that the calculated P-T conditions recorded in Tunk Creek rocks were coeval with anatexis, extension, and dome formation in Paleocene-Eocene time.

In contrast to gedrite-cordierite gneisses in the other Cordilleran domes, the Tunk Creek unit occurs in a discontinuous km-scale unit rather than as smaller pods, is more calcic, and lacks garnet. Additionally, kyanite did not transform to sillimanite, and spinel commonly occurs as a blocky matrix phase. The differences in textures within the Mg-Al-rich gneisses of the Cordilleran domes are likely related to differences in bulk composition, but also to differences in their tectonic evolution (and therefore P-T path).