POST-GRENVILLIAN TECTONOMAGMATIC EVOLUTION OF THE SOUTHWESTERN LAURENTIAN FRONT

Precise timing and characterization of magma sources are critical to better understand the tectonomagmatic and geodynamic evolution of southwestern margin of Laurentia. This work provides LA–MC–ICP–MS U–Pb geochronology and LA–MC–ICP–MS time-constrained zircon Hf isotope composition on five samples from two stages of the Red Bluff Granitic Suite (RBGS) and two samples from ferroan basalt dikes within the Franklin Mountains of El Paso, Texas. Zircon U–Pb dating yielded concordia ages of 1121.3 ± 2.9 Ma and 1118.4 ± 5.4 Ma for the stage-1 syenite and main body of stage-2 granite, respectively. The basaltic dikes yielded a similar weighted mean age of 1124 ± 14.1 Ma. The weighted mean εHf (t) for the stage 1 and 2 varies from +5.3 to +7.2. Two basaltic dike samples revealed the similar but more heterogeneous εHf (t) values with weighted mean εHf (t) values of +5.2 to +6.7. Whole-rock geochemical data show all the RBGS, granitic dike, and mafic dikes plot as A2-type post-orogenic granites. Trace element geochemistry discriminated the magmatic rocks as within-plate, fractionated granites with REE pattern similar to OIB. Anomaly in Eu is typical of a feldspar fractionated trend, and flatter HREE patterns might suggest magma was generated at the garnet stability field. Lower Ta and Nb abundances are typical of island arc basalts. Given the absence of older inheritance such as 1.65-1.60 Ga Mazatzal and older Mesoproterozoic basement of 1.4-1.3 Ga and radiogenic Hf isotopic compositions of the Red Bluff granites, a minimal contribution from older crust is suggested in the post-Grenvillian magmatism. We also integrated zircon U-Pb and zircon hafnium isotopic data with the spatially-related granite plutons emplaced in Llano. Based on the timing and geochemical composition, we suggest that the magmatism at the Franklin Mountains and Llano represent a two stage process after the development of a convergence zone along southern Laurentia. During the first stage, post-orogenic collapse caused asthenospheric upwelling that heated sub-continental lithospheric mantle in conjunction with basaltic underplating from a pre-existing arc. During the second stage, the alkaline basaltic underplate (now mafic amphibolite) remelted a pre-existing arc and through extensive fractional crystallization produced southwestern A-type magmas.