Evaluation of the Lithospheric Contribution to Southern Rio Grande Rift Mafic Melts

As continental rifting proceeds, the accommodation of lithospheric thinning by mechanical extension and magmatic intrusion represents an important but poorly constrained tectonic process. Insight into role of the magmatic component may come from the composition of volcanic products, which can record magma-lithosphere interactions. The volcanic activity in continental rift environments is frequently characterized by bimodal associations of mafic and silicic volcanism with heterogenous lithospheric contributions. We present a new integrated data set from several mafic volcanic fields in the Rio Grande Rift, consisting of major and trace element compositions, as well as isotopes. This data set provides insight into asthenospheric melting processes and interactions with the overlying lithosphere. The melting processes and the related extensional volcanism is the result of foundering of the Farallon slab. Large volume silicic eruptions such as those in the Sierra Madre Occidental originate from a large contribution of lithospheric melting, with a subordinate asthenospheric contribution. In contrast, Late Tertiary and Quaternary basaltic volcanic fields in the Rio Grande Rift were likely sourced in the asthenosphere and did not reside in the lithosphere for substantial periods. As a result the region is the ideal natural laboratory to investigate the interaction of asthenospheric melts with the lithosphere. In particular the wide array of volcanic fields contain multiple xenolith localities, such as Kilbourne Hole, providing direct samples of lithosphere and crust. Although previous studies have focused on correlations between amount of extension related to Farallon slab foundering, volcanic compositions, and their mantle sources, we present data that suggest that some compositional signatures may pre-date current tectonic processes. Radiogenic isotope data from several volcanic fields in New Mexico show a converging pattern in Pb isotope compositions, focusing on the unradiogenic Pb isotope composition of lower crustal xenoliths from Kilbourne Hole. The opposite ends of the converging trends are more radiogenic for some volcanic fields than the (lithospheric) mantle xenoliths of the Potrillo, San Carlos and Geranimo volcanic fields. Combined Pb-Sr isotope compositions for these fields are consistent with a trend from lower crustal xenoliths to mantle xenoliths, but show more variability. This variability may be explained by a small upper crustal contribution, in agreement with the Pb isotope systematics. Therefore, a common unradiogenic lower crustal composition likely contributed to the asthenospheric melts, followed by upper crustal contamination. The unradiogenic character of the lower crust implies an ancient event created the required low U/Pb ratios that generated the present-day Pb isotope compositions.