MINGLED METAMORPHISM: COMPUTATIONAL MODELLING OF AN OXIDIZED FRANCISCAN BLUESCHIST-ECLOGITE AND IMPLICATIONS FOR METAMORPHIC EVOLUTIONARY MODELS

Although intergrown eclogite and blueschist assemblages have been well studied, their origin remains uncertain. Two theories predominate: one suggests these rocks are the result of multi-step development, and another claims that differing bulk compositions are responsible for the distinct mineralogies formed at the same P-T. In this study, computational thermodynamic modelling is employed to evaluate the applicability of these two models in the Franciscan.

Two distinct cm-scale domains of one tectonic block from the Ward Creek locality near Cazadero, California were examined in thin section and analyzed by XRF. The eclogitic assemblage is composed of omphacite (70%), epidote (12%), garnet (8%), sphene (8%), chlorite (1%), and white mica (1%). The blueschist domain consists of zoned sodic-calcic amphibole (40%), omphacite (20%), garnet (20%), chlorite (10%), sphene (5%), epidote (2%), and white mica (1%). In comparison to the eclogite domain, the blueschist domain is depleted in Si, Ca, and Na and enriched in Fe, Mg, and K.

The bulk compositions of each domain were used along with independent oxygen fugacity measurements based on garnet-epidote equilibria to create pseudosections. Modelling of the eclogitic portion predicts the observed assemblage with the exception of rutile + quartz rather than sphene at 560-600°C and 13-19 kbar. Relict rutile inclusions within sphene yield [Zr] temperatures consistent with those modelled and with retrograde formation of sphene. Despite differences in bulk composition, the blueschist domain is predicted to have an eclogitic assemblage at the same P-T, but between 325-350°C and 6.3-7.3 kbar, the observed blueschist assemblage is predicted.

Results of thermodynamic modelling support the model of multi-step evolution for eclogite and blueschist intergrowth. The domains’ differing bulk compositions do not adequately explain the differing mineralogies; since models predict the blueschist and eclogite assemblages at different P-T conditions, the two must record two stages of metamorphism. While the pseudosection approach is highly sensitive to the redox environment and to the choice equilibrium volume, it is invaluable in the evaluation of rocks with heterogeneous composition to discriminate the effects of bulk composition on a rock’s metamorphic evolution.