GEOCHEMICAL CHARACTERISTICS OF THE EARLY JURASSIC LIGHTS CREEK AND MCKENZIE MEADOWS STOCKS, TAYLORSVILLE, CALIFORNIA; EVIDENCE OF SUBDUCTION-RELATED, CORDILLERAN ARC ACTIVITY IN NORTHEASTERN CALIFORNIA

The Lights Creek (LCS) and McKenzie Meadows (MMS) stocks and their related wall rock represent Early Jurassic arc activity in NE California. The fine-grained, quartz-monzonite stocks display variable degrees of hydrothermal alteration and are characterized by abundant K-feldspar, hydrothermal biotite, and < 2.0mm rosettes of tourmaline. Two locations in the LCS have recently yielded U/Pb zircon ages of 184.0 ± 15.0 and 178.1 ± 3.9 Ma via SHRIMP. New geochemical analyses show the stocks display a limited range of ASI values (0.84 – 1.13) and have generally metaluminous compositions. SiO₂ contents range from 62.6 to 69.0 wt%. The scatter present in K₂O (2.5 – 5.6 wt %), Na₂O (2.8 – 5.3 wt %) and CaO (1.2 – 4.8 wt %) when plotted against the immobile elements Zr and Nb is consistent with the observed hydrothermal alteration and associated copper-sulfide ore deposits (LCS). On spider diagrams normalized to primordial mantle, stock samples display strong enrichment in Rb, Th, Pb and U and depletion in Nb, Ta, and Ti (attributes of convergent margin arc magmas). Enrichment in U may reflect both subduction-type mobile fluid components, fractionation of LIL-enriched parental magmas, and/or crustal contamination. Chondrite normalized REE patterns display a moderate negative Eu anomaly suggestive of plagioclase fractionation. Stock samples have uniformly low Sr/Y ratios (2-9) indicating LCS and MMS parental magmas had relatively low water contents and differentiated in thin crust with fractionating assemblages including plagioclase but not garnet.

The new data provide information on the tectonic setting of the stocks in the Early Jurassic. Both stocks plot within volcanic arc fields on granite discriminant diagrams (Nb vs Y; Ta vs. Yb; Rb vs. Y+Nb; Rb vs. Yb+Ta); although some MMS samples lie near or just inside within-plate or orogenic granite fields. The stocks intrude a similar-aged, lower-greenschist basement of shallow marine, plagioclase-phyric, proximal-facies, basalts, andesites, and dacites which generally plot within volcanic arc fields on Zr vs Ti; Ta/Yb vs Th/Yb; and Th-Hf/3-Ta discriminant diagrams. The LCS, MMS, and their volcanic wall rock display geochemical characteristics consistent with Early Jurassic, subduction-related island-arc magmatism developed in thin continental crust.