On the timing, fluid sources, and behavior of skarn formation : lessons from oxygen isotopes in skarn garnets of the North American Mesozoic Cordilleran arc

Abstract

Skarns, the calc-silicate products of fluid-facilitated decarbonation reactions occurring at the interface of carbonate-bearing rocks and igneous bodies, are best studied for the base metals and ore minerals they often host (e.g., Meinert, 2005). Skarn garnets preserve the compositions of fluids present during the initial stages of skarn formation because they form early in the skarn metasomatic paragenetic sequence. Although skarns constitute a volumetrically insignificant proportion of the crust, they represent the seldom preserved crustal-level interactions between plutons and their host rocks and as such preserve information regarding the local and regional conditions prevailing during arc magmatism. Skarn garnet crystalizes early, if not first, during metasomatic skarn-forming reactions and records the oxygen isotope composition of the initial hydrothermal fluids (e.g., Einaudi and Burt, 1982; Brown et al., 1985; Meinert et al., 2005). Therefore, the skarn garnet oxygen isotope record is a powerful proxy for hydrothermal fluid sources and a useful monitor of the relative depth of skarn formation, which may be critical to skarn metal classification and, as we show here, paleogeography and tectonic rifting. This study focuses on three aspects of skarn formation. First, the timing of skarn formation in relation to assumed causative plutons, which also serves as a method for dating the presence of meteoric fluid within the subsurface. Second, an investigation of the various fluid sources that contribute to the fluid budgets in skarn systems spanning the spatial and temporal extent of a long-lived continental margin arc. And third, a detailed investigation of the relative timing of fluid source contributions and fluid flow mechanisms that result in large degrees of isotopic heterogeneity. Through the combined use of U-Pb geochronology and stable isotope geochemistry it is possible to provide radiometric ages to constrain the spatial and temporal extent of regional hydrothermal systems. The presence of meteoric fluids captured by skarn garnet correspond to skarns emplaced during tectono-magmatic regimes with extensional components or with plutons experiencing shallow emplacement. Moreover, use of regional paleogeographic reconstructions, U-Pb geochronology, and the presence of meteoric fluid at depths of skarn formation provides a new proxy for constraint on the location and proximity of the Panthalassan paleo-shoreline in the Jurassic, and provides evidence for the Jurassic Andean-style emergence of the arc edifice many millions of years earlier than previously hypothesized. Data presented here suggests that skarn garnet, in comparison with other mineral records of oxygen isotopes, provides the highest-fidelity proxy for the compositions and sources of metasomatic fluids regardless of age or pressure-temperature conditions subsequent to garnet crystallization.