DYNAMOTHERMAL METAMORPHIC AUREOLES, ANATEXIS, AND THE FOUNDERING OF ULTRAMAFIC-MAFIC PLUTONS

Amphibolite-facies dynamothermal aureoles are associated with oceanic-arc plutonic suites on Tobago, West Indies, and in the Klamath Mountains, California (i.e., Bear Mountain intrusive complex=BMIC). The plutonic suites and associated host rocks exhibit many similarities despite significant differences in age and tectonic history. These similarities include: 1) compositional ranges from ultramafic to silicic for the plutonic suites, 2) abundant cumulate rocks in the plutonic suites (dunite, wehrlite, olivine +/- hornblende clinopyroxenite, and various gabbroic rocks), 3) a piecemeal emplacement history for the plutonic suites, 4) L-S tectonite fabrics with normal-sense shear criteria (in present orientation) in the host rocks (i.e., pluton side down), and 5) local anatexis in the host rocks (leucosomes in amphibolite-facies rocks, especially mafic rocks). These characteristics suggest: an early history of crystal fractionation of H2O-rich basaltic magma that yielded abundant olivine- and augite-rich cumulates and later calcic plagioclase and hornblende; emplacement of younger gabbroic to dioritic magmas; partial melting of the host rocks and overall mechanical weakening of these rocks; foundering of early ultramafic and mafic cumulates along amphibolite-facies, normal-sense shear zones in the host rocks (and the creation of a dynamothermal aureole); and emplacement of late tonalitic to granitic rocks, perhaps in part derived during melt segregation and mobilization during downward displacement of the plutonic rocks along crystal-plastic shear zones. The ultramafic-mafic suite exposed on Tobago was emplaced during crustal extension (mid-Cretaceous intra-arc spreading ?), whereas the BMIC was emplaced immediately after regional thrusting (i.e., Late Jurassic Nevadan orogeny). Therefore, in both tectonic settings, several key aspects that apparently led to the late foundering of the cumulates were intrusion of younger magmas into the cumulate pile, which led to instability, and the partial melting of the host rocks, which weakened these rocks and facilitated the localization of strain as a marginal, high-temperature, normal-sense shear zone.