ANATECTIC VERSUS NON-ANATECTIC ORIGINS FOR MIGMATITE IN THE BANDED GNEISS, CASCADES CORE, WA

The Banded Gneiss of the Cascades Core (CC) is a migmatitic unit comprising pelitic schist and gneiss, amphibolite, tonalite gneiss, and cross cutting tonalite quartz-rich granitoid, and pegmatite. There are several generations of ‘igneous’ lithologies (leucosomes) some of which are concordant; others clearly cross-cut the strongly-deformed host rocks. The host rocks are interpreted to have metasedimentary (schists and some gneisses) and metavolcanic origins (amphibolites). Metamorphic fabric in the Banded Gneiss is characterized by penetrative foliation (mineral alignment), banding, mineral lineation, and non-coaxial deformation features (asymmetric augen, grain offsets, rotated porphyroblasts, etc.). This fabric is faulted and folded by mesoscopic structures. Banding is characterized by quartz-plagioclase lenses and patches, and by large variations in biotite content. The most strongly foliated leucosomes (gneissic tonalites) are concordant with the regional trend of foliation, while weakly foliated leucosomes (tonalites) and pegmatitic veins cross-cut host rock and gneissic tonalites. Thin layers of melanosomes (biotite schist and amphibole schist) are developed around some of the quartz-plagioclase lenses and patches. These and other field and petrographic data suggest that at least some of the leucosome bodies were derived from partial melting.

Metamorphic temperatures reported for Banded Gneiss rocks and the nearby Chiwaukum schist are 550 to 740 °C. Our preliminary thermodynamic modeling indicates that hydrous partial melting would start ca. 680 °C. The clearly intrusive character and the sharp contacts between some leucosome bodies and host rock support an externally-derived origin for these tonalitic melts. However, some of these bodies may have originated from partial melting of the host Chiwaukum schist then traveled a short distance before crystallization or been modified by deformation so as to obscure textural evidence for local derivation. Our preliminary data are compatible with derivation of migmatitic rocks in the Banded Gneiss from two non-exclusive processes: partial melting of the host rock and intrusion of externally-derived tonalite melts.