Petrology and Upward Zonation of the Wooley Creek Batholith, Klamath Mountains, California

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Abstract
The Wooley Creek batholith was intruded into metamorphic rocks of the western Paleozoic and Triassic belt of the Klamath Mountains 162 ± 2 m.y. ago. The batholith crosscut a thrust fault between the lowest subunit of the western Paleozoic and Triassic belt, the Rattlesnake Creek terrane and overlying Hayfork terrane metasediments. The batholith and its host rocks were subsequently thrust over low-density rocks of the Galice Formation and then tilted toward the southwest, presumably by regional doming. Metamorphic mineral assemblages of the contact aureole suggest that the pluton was tilted at least 20° to 30°. The Wooley Creek batholith is gradationally zoned from two-pyroxene gabbro in the deepest exposed portion to hornblende–biotite granite at shallowest levels. The plutonic rocks display two distinct chemical trends that correspond to rocks that contain pyroxene and rocks with only hornblende and biotite as mafic minerals. Pyroxene-bearing rocks are structurally lower and are enriched in Mg, Ca, Cr, Ni, Co, and Sc relative to pyroxene-free rocks that occupy the structurally higher levels and are enriched in Al, Na, K, Sr, Zr, and Rb. The two chemical trends can be explained by (1) two coexisting magmas in a single chamber, or (2) crystallization under a vertical gradient. The second hypothesis is examined in detail in this report. Major-element modelling suggests that the trends can be explained by crystal fractionation of a single parent that became successively enriched in H2O in the highest levels of the pluton. Upward H2O-enrichment led to plagioclase stability at lower temperature and consequent enrichment of the magma in Al and alkalies. Trace element abundances show that minor accumulation of clinopyroxene, olivine, and chromian spinel affected the lower part of the pluton and that minor influxes of basaltic magma affected the upper, H2O-enriched part of the pluton. These basaltic magmas were trapped in the interface between the lower, dense, H2O-poor magma and the upper, H2O-enriched magma. As these basaltic liquids cooled and crystallized, they mingled with the H2O-rich magma of the pluton.

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