Fingerprints of forearc element mobility in blueschist-facies metaconglomerates,Catalina Schist,California

ABSTRACT

Metaconglomerates in the lawsonite–blueschist facies unit of the Catalina Schist (California) contain gabbroic and dioritic clasts exhibiting evidence for extensive metasomatism during high-P/T metamorphism. We performed whole-rock and in situ analyses of these metaconglomerate clasts to better constrain the composition of infiltrating fluids and to elucidate the history of chemical alteration. Petrographic evidence for this alteration includes replacements of plagioclase by phengite and sodic amphibole rims developed on igneous hornblende. These observations regarding mineral replacement are reinforced by corresponding shifts in chemical compositions. Relative to compositions of presumed protoliths, whole-rock compositions of the metaconglomerate clasts show enrichments in elements that are relatively mobile in aqueous fluids (LILE: K, Rb, Cs, and Ba; Li, B, N), and elevated δ¹⁵N, and show depletions in Ca and Sr. Electron and ion microprobe data, and analyses of mineral separates, show that phengite and sodic amphibole are enriched in LILE and Li and B, respectively, relative to the igneous phases they have replaced. Oxygen and C isotope compositions of finely disseminated calcite in the clasts, and of calcite in veins cross-cutting or mantling the clasts, are consistent with crystallization from fluids previously equilibrated with metasedimentary rocks within the same unit. The same fluids are implicated as the source for the Li, B, N, and LILE enrichments. These metaconglomerate clasts provide unique records of forearc metasomatism due to the presumed extremely low and well-constrained concentrations of fluid-mobile elements in their protoliths and the previously published, larger-scale fluid–rock context into which the observed metasomatic changes can be placed.     

A Clovis well at the type site 11,500 B.C.: The oldest prehistoric well in America

An enigmatic circular pit uncovered during archaeological excavations at the Clovis type site, Blackwater Draw, New Mexico, in 1964 has been reexposed and posited as a water well excavated by Clovis people around 11,500 B.C. The prehistoric well, the oldest in the New World, was probably a dry hole. Other Clovis wells may exist in the area. The excavation of wells near where there had been surface water shortly before adds to the evidence for drought during the Pleistocene-Holocene transition. © 1999 John Wiley & Sons, Inc.