MESOZOIC TRANSPRESSION, TRANSTENSION, AND SUBDUCTION-INDUCED METALLOGENESIS IN NORTHERN AND CENTRAL CALIFORNIA

Devonian-mid Jurassic terrane amalgams in the Klamaths and Sierran Foothills consist of serpentinized peridotites, disrupted hypabyssal gabbros + seafloor basalts, and capping deep-water chert-argillite sequences. Mafic-ultramafic complexes are oceanic, whereas fine-grained terrigenous strata were derived chiefly from previously docked continental-margin belts. Klamath-Sierran sutured terranes reflect ~230 m. y. of margin-parallel strike slip involving minor subduction, as mirrored by ubiquitous ophiolite-chert-argillite lithologies. Quartzofeldspathic clastic sedimentary units, and high-pressure metamorphic rocks are rare. Little devolatilization occurred at magmagenic depths, hence coeval hydrothermal ore deposits and calcalkaline plutons are uncommon in this terrane assembly. In contrast, the Klamath-Sierra Nevada calcalkaline arc reflects voluminous magmatism attending a Late Jurassic transition to transpressive motion, generating deposition of Galice-Mariposa erosional products of an incipient arc. Nearly head-on subduction of the Farallon plate beneath the margin began in Cretaceous time. Great Valley forearc basin and Franciscan oceanic trench deposits are first-cycle debris shed mainly from the Klamath-Sierran igneous arc, as shown by detrital petrofacies, paleocurrents, and bulk-rock geochemistry. The three contemporaneous belts record ~70 m.y. of rapid sialic growth. Dewatering of the descending oceanic plate at magmagenic depths promoted formation of Cretaceous gold deposits in the calcalkaline arc and surrounding wall rocks.

Spatial association links Klamath-Sierran Foothills metallogenesis to Cretaceous granitoids and coeval fluids that invaded the wall rocks. The ultimate origin of magmatic + hydrothermal activity probably was subduction-zone devolatilization. Au-bearing aqueous fluids exsolved from the plutons and/or expelled from heated wall rocks were mobilized during magmatic arc construction. Precipitation of gold-bearing quartz veins occurred where hot aqueous fluids met major geochemical discontinuities such as wall-rock serpentinites. The Franciscan Complex also is typified by serpentinized peridotites, but ascending subduction-channel fluids were derived from much shallower depths than the magmagenic zone, hence calcalkaline plutons and gold ore deposits are lacking.