Geochemical Evidence for Incremental Emplacement of Palms Pluton, Southern California

Palms Pluton is a NW-trending, elongate pluton, 2 – 10 km in width, composed of texturally homogeneous, seriate to weakly porphyritic biotite granite with weak internal fabrics. Intrusive contacts are typically sharp and discordant to foliation in Proterozoic host gneisses. Although textures and whole rock major oxide concentrations do not vary considerably, trace elements have a much more extensive range. Sr, for example, averages over a 400 ppm increase toward the western contact. These variations cannot be explained by in situ fractionation or mixing processes. U-Pb ages for five samples suggest that components of the pluton were assembled over 5 ± 1 m.y. between 81 and 76 Ma. Magmatic zircons record Hf enrichment with progressive crystallization, and Ti temperatures of 800 – 660°C, consistent with crystallization between calculated zircon saturation temperatures and the wet granite solidus. Pre-magmatic zircons are abundant in all samples; ages range up to 2041 Ma and average 2% discordance. About 75% of pre-magmatic zircons are 1700 to 1800 Ma, with minor amounts of younger Proterozoic zircons, and about 20% pre-magmatic Mesozoic zircons. The provenance age distribution of these zircons is distinct from both local host rocks and the inferred provenance of local older Mesozoic plutons. Zircon REE pattern variability indicates the melt(s) from which pre-magmatic zircons grew varied in chemical composition, temperature, and/or pressure. These geochemical and geochronologic observations imply Palms Pluton was assembled by partial melting of a moderately hybridized Paleoproterozoic source, and emplacement of a series of discrete yet chemically similar intrusions over several million years in Late Cretaceous time.