CRUSTAL AND MANTLE SOURCES OF MAGMAS FROM THE SOLITARIO LACCOLITH/CALDERA COMPLEX, SOUTHWEST TEXAS

The Solitario is a 16 km diameter laccolith/caldera complex. Ar-Ar dating defines magmatic pulses at 36.1, 35.5, and 35.1 Ma separated by repose periods of 200-300 ka. 36.1 Ma magmatism was most prolonged and compositionally diverse, with sills and lavas from 57-75% SiO₂ produced over 420 ka. Three distinct groups are present; syenite and trachyte sills and stocks and porphyritic and aphyric rhyolite sills and lavas. Syenites and trachytes have Pb and Nd isotopic compositions intermediate between mantle and crustal values, and low Sr isotope ratios of 0.704-0.707 indicate the crustal component was granulite facies Grenville lower crust. Rhyolites have Grenville basement Pb isotope ratios, lower Nd and higher Sr isotope ratios than alkalic rocks, indicating a larger crustal component. 35.5 Ma magmatism produced the main laccolith and caldera, with peripheral peralkaline rhyolites. 35.5 Ma laccolith/caldera rocks are intracaldera tuff and radial dike rhyolites. The tuff ranges from ~60-72% SiO₂. Dikes have higher SiO₂ of 74% to 77%, similar to peralkaline rhyolites, and may be cogenetic and feeders to them. Two distinct components are present in the tuff; rhyolite and trachyandesite. The rhyolite has the most mantle-like isotopic composition of analyzed Solitario rocks, with high Pb and Nd and low Sr ratios similar to regional basalts, whereas the mafic end member is represented by more crustal isotopic compositions of quartz trachyte samples which comprise the bulk of the tuff. The caldera eruption may have been triggered by mixing of the two magmas. Radial dikes and peralkaline rhyolite lavas are the most chemically evolved igneous rocks in the Solitario and have Pb isotopic compositions indicating a dominantly crustal origin. 35.1 Ma magmatism produced dikes and sills which cross-cut earlier rocks. This final interval of magmatism was the most brief (~90 ka duration), compositionally restricted and least silicic. Pb isotopic ratios indicate significant crustal and mantle components. The diversity in petrographic, geochemical, and isotopic compositions suggests the presence of multiple, discrete magma batches. Despite the presence of multiple magma systems Solitario magmas over an ~1 Ma interval share similar source regions in the crust and mantle.