EFFICIENT FRACTIONAL CRYSTALLIZATION AND EARLY ZIRCON SATURATION IN AN UPPER CRUSTAL ARC MAFIC INTRUSION: HIDDEN LAKES MAFIC COMPLEX, SIERRA NEVADA BATHOLITH

Fractional crystallization has long been invoked as a process driving magma evolution to produce evolved magmas from initially basaltic melts. This process requires separation of crystals and residual melt. Simple Stoke’s settling and hindered settling calculations suggest that the process should occur rapidly in mafic melts. However, direct measures of the efficiency of this process in geological examples have been limited because most layered mafic intrusions are composed of rocks that did not crystallize the minerals that are useful for high-precision geochronology. We present a case study of the Hidden Lakes mafic complex (HLMC), which is interpreted to preserve a cogenetic magmatic sequence consisting of norite, gabbro, monzodiorite, and monzonite within the upper crust (~12 km paleodepth) of the central Sierra Nevada batholith (Lewis et al., 2021). Zircon saturation in HLMC occurred early, and zircon is present in all observed rock types. This allows us to use CA-ID-TIMS U-Pb zircon geochronology to test if the complex was formed from a single large intrusion that underwent in situ fractional crystallization, or whether the complex represents a series of intrusions. Zircon ages from all rock types overlap to within the highest achievable analytical uncertainty at ~95.6 Ma, with minimal intra- and inter- sample dispersion. These results suggest that much of the HLMC is derived from a single intrusion that differentiated in place within the upper crust. The evolution of magmas from basaltic (49 wt.% SiO₂) to evolved (61 wt.% SiO₂) within a relatively small intrusive body (~3 km²) suggests efficient crystal-liquid separation in the upper crustal magmatic system.

Although zircon is found in norite and gabbro cumulates within the HLMC, mineral saturation models do not predict zircon crystallization in basaltic compositions. Zircon saturation temperatures for modeled melt compositions that produced the mafic cumulates in HLMC are significantly lower than temperatures recorded by plagioclase-amphibole thermometry. Thus, either small amounts of evolved, zircon saturated melt persisted to low temperatures within the mafic cumulates, or zircon saturated pockets of melt existed within the magma reservoir to allow for localized early zircon crystallization in these melts.