A Tale of Two Plutons: Geochemical Evidence Bearing on the Origin and Differentiation of the Red Lake and Eagle Peak Plutons, Central Sierra Nevada, California

Abstract

The Eagle Peak pluton is zoned with an equigranular granodiorite margin and porphyritic granitic core; in contrast, the nearby, and coeval Red Lake granodiorite pluton is relatively unzoned. Major element abundance variations in both plutons can be explained by segregation of the early phases, plagioclase and hornblende. However, the preferential decrease of heavy rare-earth elements (HREE) from margin to core in the Eagle Peak pluton cannot be explained by fractional crystallization of a melt composition similar to the marginal rocks. The HREE decrease is best explained by segregation of hornblende from a mixture of melt and minerals equivalent in composition to the marginal rocks. Both plutons contain aplites that crystallized from highly evolved melts formed by segregation of mafic minerals, feldspars, and an accessory phase rich in light REE. Mafic inclusions are related to their host granitoids, and they may be remnants of restite or early accumulates. The small negative Eu anomalies in both plutons are consistent with significant amounts of residual hornblende and plagioclase. Geochemical differences between these plutons mimic those between the sodic and main series plutons in the Boulder batholith; e.g., the Eagle Peak pluton and the adjacent older Mount Givens pluton have lower Na/K, higher Rb, Ta, Th, and U contents but lower Sr and Ba contents than the Red Lake pluton. These differences are not easily explained by models relating these magmas to a common source. However, depletions in K, Rb, Cs, Th and U relative to Sr and Ba are common in high-grade metamorphic rocks. Therefore, these inter-pluton compositional differences may reflect differences in source composition developed in the lower crust during earlier magmatism and metamorphism. The compositional heterogeneity inferred for the source in this localized region is a significant fraction of the source heterogeneity inferred from transects across the Sierra Nevada batholith.