Investigation into the Episodic Behavior and Potential Causes of Jurassic Magmatism in the Northern Sierra Nevada, California

Jurassic plutons that occupy three metamorphic belts in the northern Sierra Nevada are important for understanding the early development of batholithic rocks in the Mesozoic. These plutons provide a suitable location to study orogenic influences and the spatial and temporal roles of episodic magmatism on early convergent margin batholiths of western North America. New U-Pb zircon geochronology and whole rock major and trace element geochemistry from 7 plutons are presented. New and existing ages, combined with pluton areas estimated from existing mapping, reveal two episodes of increased magma production. The first episode occurs in the Middle Jurassic at ca. 165 – 160 Ma, and the second occurs in the Early Cretaceous at ca. 145 – 140 Ma. Between these two episodes of enhanced magmatism, there is an apparent magmatic lull at ca. 155 – 150 Ma. Pre-Nevadan plutons intruded during the Middle Jurassic episode are calcic to calc-alkalic, metaluminous to weakly peraluminous, and exhibit SiO2 at 53 – 70%. Trace element data reveals that these plutons are variable in Sr/Y with values that range from 10 – 75. By comparison, post-Nevadan plutons that intruded during the Late Jurassic-Early Cretaceous episode are also calcic to calc-alkaline, but are more peraluminous and more silicic (60 – 80% SiO2), with Sr/Y values that range from 2 – 48, with a single pluton as high as 95. Our geochemistry indicates that plutons from the northern Sierra are products of fractional crystallization of mantle-derived magmas from a convergent arc, consistent with previous works. Sr/Y trace element data reveals that there are very little direct correlations between high and low Sr/Y and timing of pluton emplacement, however, it does present an indication that there are significant variations of Sr/Y concentration depending on which metamorphic belt the pluton was emplaced in. While the Western belt primarily contains plutons of low Sr/Y, plutons within the Central and Eastern belts are both significantly higher in Sr/Y concentrations. We propose that these differences suggest a thickening period to the crust during the accretion process of an island arc terrane 155 – 150 Ma. The timing of the lull in magmatism and the change from low to high Sr/Y from west to east offers new constraints on non-steady state magmatism during the Mesozoic. Based on this study, it appears that the timing and volumetric significance of early plutons in the northern Sierra are more complex than previously recognized, and it is likely that two episodes of magmatism are likely due to a crustal thickening period during the Nevadan orogeny, refining the single episode model depicted in previous studies.