PREDICTING PARTIAL MELT TEMPERATURES FOR PELITIC ROCKS IN THE BANDED GNEISS, CASCADES CRYSTALLINE CORE, WA

The Banded Gneiss comprises interlayered tonalite gneiss, pegmatite, mica schist, hornblende schist, and amphibolite. The unit has been interpreted as the upper amphibolite facies 'core of a major antiform' comprised of rocks 'derived from schist by metasomatic and igneous processes'. Published estimates for peak P-T estimates for metamorphism range from 548 to 645°C @ 5.9 to 8.9 kbar. Our P-T estimates for a single sample are 7.2±1.0 kbar @ 593±72°C (2-sigma) (Grt rim using 4 reactions, including GASP, GABI, and GRAIL). Field relations are compatible with either partial melting of schist or intrusion of tonalite dikes and sills. Thermodynamic modeling is presented in order to determine viability of partial melting for the origin. Pseudosections (MnNaCaKFMASH) were constructed for a pelitic schist (sample 99NC37) using THERMOCALC and thermodynamic data of Holland and Powell (2001). Activity models were those of Holland and Powell (1998) and those supplied with THERMOCALC, with some solutions extended to include Mn.

Banded Gneiss sample 99NC37 contains a peak metamorphic assemblage of Bt+Grt+Ky+Pl+Ms+Qtz and retrograde Chl. A T-H₂0 pseudosection constructed for this sample at 7.5 kbars indicates: i) that H₂0 undersaturated melting would occur between 730 and 825°C ii) that H₂0 saturated melting would occur at ca. 680°C, iii) that Ky would only be stable at low H₂0 contents without melt in equilibrium, and iv) that Ksp should be present. A P-T pseudosection constructed for the lowest H₂0 content providing a fluid phase at initial melting indicates that the peak mineral assemblage would only be stable ca. 650±10°C and 7±0.1 kbars. Modeling of 99NC37 is compatible with peak metamorphism at H₂0 contents and P-T conditions ca. 25°C below the solidus. Although this appears to preclude partial melting, numerous other samples from the Banded Gneiss and Chiwaukum Schist contain Sil and/or Ser/Ms after Ky. These samples may have partially melted due to higher temperatures and/or H₂0 contents.