PETROGRAPHIC AND GEOCHEMICAL TRENDS WITHIN VOLCANIC ROCKS OF THE SIERRA MADRE OCCIDENTAL SILICIC LARGE IGNEOUS PROVINCE, CHIHUAHUA-DURANGO BORDER, NORTHERN MEXICO: INSIGHTS INTO TEMPORAL CHANGES IN PETROGENESIS

The c. 45 – 23 Ma Sierra Madre Occidental (SMO) silicic large igneous province in northern and western Mexico encompasses over half the volume (~3.9 x 10⁵ km³) of the mid-Cenozoic ignimbrite flare-up of western North America. We present petrographic, whole-rock major and trace element geochemistry, and mineral chemistry (SEM-EDS) data for a suite of 54 samples (7 mafic lavas, 6 silicic lavas, and 41 silicic ignimbrites) collected along a recently established geological transect along Highway 24 across the north-central portion of the SMO, approximately along the border between Chihuahua and Durango states, between the towns of Hidalgo de Parral and Guadalupe y Calvo. This is the first new transect to be established across the SMO in over 20 years, and was chosen specifically because it straddles the boundary between extended and non-extended parts of the SMO. Silicic lavas and ignimbrites are rhyolites, rhyodacites, and two crystal-rich dacites. The phenocryst assemblage is plagioclase-quartz-K-feldspar-biotite with amphibole or pyroxene. Phenocryst abundances range from 1-50%. Two compositional-petrological suites are identified within the silicic rocks: (i) a high-temperature (>825°C), plagioclase-rich, high-Ba (>900 ppm), and phenocryst-poor (<10%) suite spatially associated with the non-extended core of the SMO; and (ii) a lower temperature (750-800°C), phenocryst-rich (plagioclase = quartz = K-feldspar), low-Ba suite associated with the extended margins. Preliminary dating results suggest that the high-T suite occurred between c. 35 – 31 Ma, whereas the low-T suite occurred in two phases: c. 40 – 37 Ma and c. 31 Ma. Our preliminary interpretation is that the increase in Ba and decrease in phenocryst abundance can be explained by re-heating and rejuvenation of older partly-crystallized silicic magma, perhaps c. 40 – 37 Ma suite magma.

Future work will attempt to model the parental magma elemental composition(s) to constrain relative mantle and crustal mass contributions, and to compare to new isotopic data. The SMO has the potential to be one of the largest net increases in continental crust in the Phanerozoic; however, previous studies have produced little consensus regarding the relative contributions of mantle and crustal components to the parental magmas.