Lead and strontium isotopic evidence for crustal interaction and compositional zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of the Coso volcanic field, California

Abstract

These rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values super 87 Sr/ super 86 Sr = 0.7036, super 206 Pb/ super 204 Pb = 19.05, super 207 Pb/ super 204 Pb = 15.62, super 208 Pb/ super 204 Pb = 38.63. The initial isotopic composition of typical rhyolite super 87 Sr/ super 86 Sr = 0.7053, super 206 Pb/ super 204 Pb = 19.29, super 207 Pb/ super 204 Pb = 15.68, super 208 Pb/ super 204 Pb = 39.00 is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic endmember compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.--Modified journal abstract.

18 samples from this reference