GEOCHEMICAL AND GEODYNAMIC MODELING EVIDENCE OF MELTS GENERATED BY FLUID INFILTRATION OF THE CONTINENTAL CRUST DURING LARAMIDE FLAT-SLAB SUBDUCTION

The Laramide orogeny is widely accepted as a period of flat-slab subduction in the NA Cordillera (~80–55 Ma) that created basement cored uplifts and shut-off magmatism due to truncation of the mantle wedge and suprasubduction refrigeration. However, in the SE California and Arizona portion of the system, voluminous crustal melting occurred in the overriding plate, suggesting heating rather than refrigeration. Here we explore how melting may have occurred in light of published geochemical data from crustal melts and geodynamic modeling of the Laramide system. In SE California we examined data from the Cadiz Valley batholith (CVB; 83–74 Ma) and the Sweetwater Wash pluton (SWP) in the Old Woman Mountains (72.6 ± 1 Ma), and in south-central Arizona the Pan Tak granite (PTG) in the Coyote Mountains (58 Ma) and the Sierra Pozo Verde granite (SPP) in the southern Baboquivari Mountains. All are peraluminous, two-mica granites. εNd values were determined for two of these: SWP: -15.8 and -16 to -16.9, and CVB: -10.4 to -16.5, and are consistent with dominantly crustal derivation. Crustal melting via fluid-fluxed melting is predicted from time-dependent geodynamic models of Laramide flat-slab subduction produced using THERMODSUBDUCT. Here we test this hypothesis using published criteria for evaluating the melting mechanism (fluid-fluxed verses dehydration melting) of peraluminous leucogranites. One test of melting mechanism is melt temperature, with temperatures <800 °C being indicative of fluid-fluxed melting. We calculated melt temperatures using zircon-saturation thermometry: CVB: 724°C–734°C, SWP: 757°C, PTG: 714°C, and SPP: 721°C. We also applied trace element criteria established in a recent study of Himalayan leucogranites, in particular, Sr, Rb, Ba, Rb/Sr, Sr/Y, and Zr/Hf. After filtering the data to select largely undifferentiated samples (Fe₂O₃* > 0.75 wt %, SiO₂ < 75 wt %), all but 3 of the selected 59 samples (CVB n=37, SWP n=5, PTG n=6, SPP n=11) fall within the criteria for fluid-fluxed melting. Our finding corroborates earlier studies of the SWP. Thus, a possible resolution to the apparent contradiction between predicted crustal refrigeration and voluminous crustal melting is slab-derived fluids infiltrating the continental crust shortly after the transition to low-angle subduction.