New Mexico Geological Society Annual Spring Meeting — Abstracts

This poster 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 and two samples from ferroan basalt dikes within the Franklin Mountains of El Paso, Texas. Zircon U–Pb geochronology yielded Concordia ages of 1121.3 ± 2.9 Ma and an 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. Our new zircon U-Pb ages coupled with the previous geochronological efforts revealed the subtle differences in the timing of emplacement for the different stages. In fact, U-Pb ages are overlapping within the error. This suggests that the earlier less-evolved magmas and subsequent more differentiated magmatic bodies emplaced within a short span of time not more than 3 Ma. We consider the concordant dates those are 10 to 20 Ma older than the weighted mean/Concordia age as the antecrystic zircons. The range for antecrystic zircons is from 1130 to 1138 Ma. Xenocrystic inheritance from 1141 to 1260 Ma also occurred. The initial Hf isotope composition of the plutons studied remained mostly positive. 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 scattered εHf (t) values with weighted mean εHf (t) of +6.7 ± 2.6 and +5.2 ± 3.3. Geochemistry shows characteristics of A-type granites, indicate strong plagioclase fractionation of basaltic magma and are an indication against metasomatism or crustal contamination. Given the absence of older inheritance such as 1.65-1.60 Ga Mazatzal, geochemistry, and radiogenic Hf isotopic compositions of the Red Bluff granites, a minimal contribution from older Mesoproterozoic crust is suggested. Based on the timing and geochemical composition, we suggest that the magmatism at the Franklin Mountains and Llano Uplift recorded the partial melting of the Subcontinental Lithospheric Mantle at around 1.1 Ga, Possibly, during a rifting event within the overall convergence zone of southwestern Laurentian Front. Alternatively, partial melting of a recently-placed basaltic underplate is suggested. Temporal, tectonic and isotopic evidence presented here supports distinct ages and sources of SW Laurentia during the Grenville Orogeny for southwest U.S. and northern Mexico. Temporal and Hf isotopes suggest three episodes of magma generation at 1) 1125-1100, 2) 1097-1082, and 3) 1081-1068 Ma.