PHASE RELATIONS IN MIGMATITES OF THE SEQUOIA PENDANT, WEST-CENTRAL SIERRA NEVADA

Metamorphic rocks of the Kings Sequence are widely distributed in the west-central Sierra Nevada and record P-T conditions during batholith emplacement. In the Sequoia Pendant, Kings Sequence biotite schists are locally migmatized at the contact of the pendant with the Fry's Point Granite. Field and petrographic observations from the migmatite are consistent with high-T metamorphism and partial melting in this moderate depth (10-13 km; Ague and Brimhall 1988, GSAB) area of the Sierra.

Migmatite morphology is typically stromatic, with leucosome concentrations approaching 30 vol. % at the contact. Leucosomes are granitic with approximately equal amounts of quartz and K-feldspar, plus minor plagioclase and cordierite. Textural evidence suggesting crystallization of melt in leucosomes includes “pooling” of K-feldspar between quartz grains, as well as domains of pure K-feldspar that show euhedral crystal form and equiangular triple junctions between grains.

Melanosomes contain cordierite, andalusite, biotite, ilmenite, zincian-hercynite, tourmaline, sillimanite, and granitic phases. Andalusite (AND) exhibits several key phase relations including the following: (1) hercynite is only found as inclusions within AND; (2) numerous ilmenite inclusions are found in AND, but are less abundant in the matrix; (3) early cordierite is included in AND and forms coronas around AND in the highest grade rocks; (4) biotite fringes AND and also forms symplectites with it; (5) sillimanite after AND is observed in the highest grade rocks. These textures suggest early simultaneous production of AND + cordierite, such as by the reaction muscovite + chlorite + quartz = cordierite + AND + biotite + H₂O. Such a reaction would produce biotite fringes around AND and chlorite breakdown could liberate Fe, Ti, and Zn to form oxides within AND. Subsequent higher temperature reactions involving biotite + AND, (e.g., biotite + AND + plagioclase + quartz = cordierite + ilmenite + K-feldspar + melt), would explain biotite + AND symplectites, growth of cordierite coronas on AND, and would generate partial melts. Further analysis of mineral composition will be used to quantify peak P-T conditions in the migmatites and evaluate the conditions of partial melting in the pendant.