A STUDY OF THE ORIGIN OF RHYOLITE AT MID-OCEAN RIDGES: GEOCHRONOLOGY AND PETROLOGY OF TRACHYDACITE AND RHYOLITE FROM SALTON SEA, CALIFORNIA, AND TORFAJÖKULL, ICELAND

This study aims to examine the genetic relationship between felsic and voluminous basaltic lavas in mid-ocean ridge settings. Salton Sea rhyolites erupted through sediments blanketing the intersection of the East Pacific Rise and the San Andreas fault zone. The Torfajökull volcanic complex is located near the Mid-Atlantic Ridge, at the intersection of the active East Rift Zone and the Southeastern Zone. Chemical analyses reveal that the samples include trachydacite and rhyolite. Torfajökull samples have 63-71% wt. silica, high amounts of alumina (13-15% wt.) and low phosphorous (<0.1% wt.) and Sr (80-120 ppm). Salton Sea rhyolite is comparatively silica rich (74%), has lower alumina and Sr, and higher K/Na. Loss on ignition to 700C indicates that all trachydacites and rhyolites contain <0.5% wt. H2O.

The Salton Sea rhyolite is a series of five small domes. Zircon Th-U ages are ~17 ka, identical to a ~16 ka K-Ar whole rock eruption age (Muffler and White, 1969). Inherited zircons of Jurassic age suggest this rhyolite represents small volume partial melts of Jurassic granitic basement, or deltaic sediments derived from Jurassic basement rocks exposed on the northeastern shoulder of the Salton trough.

Torfajökull is host to several subglacial and postglacial (<10 ka) rhyolite eruptions; we focused on three postglacial flows from the Domadalshraun vent, which erupted trachydacite and rhyolite between 8 and 2 ka (McGarvie, 1985). Th-U ages of zircon from two samples are 22-60 ka, ~20-50 ka older than eruption ages; the earliest flow has relatively older zircons. The earliest flow is more homogeneous, felsic, and has An4-7 feldspar phenocrysts and matrix grains, whereas later flows are more heterogeneous, mafic, have An7-20 plagioclase phenocrysts, and include more abundant xenocrysts (An30-70) and mafic to intermediate inclusions. These observations suggest that the flows erupted from a thermally and compositionally zoned Domadalshraun magma chamber formed at least 60 ka.