Leveraging detrital zircon geochemistry to study deep arc processes: REE-rich magmas mobilized by Jurassic rifting of the Sierra Nevada arc

Diane Clemens-Knott,Kathleen Degraaff Surpless,Andrew P Barth,Joseph L Wooden

doi:10.1016/j.ringeo.2021.100010

Abstract

Anomalous trace element compositions of Middle to Late Jurassic detrital zircon separated from Sierra Nevada forearc and intra-arc strata reveal processes of differentiation occurring within the deep arc lithosphere. REE-Sc-Nb-Ti-Hf-U-Th covariations define three populations of atypically REE-rich grains that we interpret as crystallizing from (1) differentiates produced by olivine+clinopyroxene+plagioclase+garnet±ilmenite fractionation; (2) mixing between mafic arc magmas and partial melts of Proterozoic Mojave province crust; and (3) compositionally transient, low Gd/Yb magmas generated by hornblende resorption during decompression. We interpret a fourth population of Middle Jurassic to Early Cretaceous zircons having REE contents similar to “typical” arc zircon but with atypically high Gd/Yb ratios as having crystallized from partial melts of recycled arc crust and from deep-arc differentiates that evolved down-temperature through hornblende saturation. We hypothesize that latest Jurassic extension ripped open the arc, facilitating upward migration and eruption of geochemically anomalous zircon-bearing magmas. The anomalous compositions relative to “typical” arc zircon imply that these zircons and their host magmas rarely reach the upper arc crust, where eruption and/or erosion would release their zircon cargo to the clastic system. Focusing on the trace element compositions of zircons of syn-extensional age represents a productive new strategy for learning about deep magmatic reservoirs and early differentiation pathways within the thick lithosphere of continental margin arcs.

Highlights

The use of trace element contents of magmatic zircon to discriminate between tectonomagmatic environments is well established (Grimes et al, 2007, 2015), and applications to complex magmatic environments are rapidly diversifying (e.g., Carley et al, 2014; Padilla et al, 2016). Surpless, Clemens-Knott, Barth and Gevedon (2019) evaluated the extent to which the trace element geochemistry of detrital zircon from Great Valley forearc strata records the temporal and geochemical variability of magmas in the Mesozoic Sierra Nevada arc
A few ocean island (OI)-like zircons have elevated Ce concentrations (Fig. 3a), the Rare earth elements (REE) anomaly is dominated by the MHREE (i.e., Gd, Yb; Fig. 3bc)
We evaluate the hypothesis that the predominantly Late Jurassic, anomalously REE-rich detrital zircon preserved in the Sierra Nevada forearc and intra-arc basins are geochemically unlike “typical” arc zircon that are sourced in the felsic volcanoplutonic arc mass because they crystallized from mafic arc magmas that seldom escape the deep arc crust (e.g., Sisson et al, 1996)

Summary

Introduction

The use of trace element contents of magmatic zircon to discriminate between tectonomagmatic environments is well established (Grimes et al, 2007, 2015), and applications to complex magmatic environments are rapidly diversifying (e.g., Carley et al, 2014; Padilla et al, 2016). Surpless, Clemens-Knott, Barth and Gevedon (2019) evaluated the extent to which the trace element geochemistry of detrital zircon from Great Valley forearc strata records the temporal and geochemical variability of magmas in the Mesozoic Sierra Nevada arc. Alkalic OI-type basalts have been identified within other arcs, including the Cascades arc to the north and the Trans-Mexico volcanic belt to the south (Bacon et al, 1997; Ferrari, Petrone & Francalanci, 2001; Mullen, Weis, March & Martindale, 2017; Richter & Carmichael, 1992), so these OI-like detrital zircon may record a distinct mantle source. These detrital zircons may instead reveal magmatic processes operative in the deep arc crust.

Results

Discussion

Conclusion