The Rio Puerco volcanic necks in northwestern New Mexico contain a rich and varied xenolith suite that has received almost no prior attention. The necks lie within the Jemez Lineament at the transition between the Colorado Plateau and the Rio Grande rift, and the xenoliths thus have the potential to provide a large amount of information on lithospheric structure, chemistry, and alteration in a tectonically important region. This study addresses a number of questions regarding the nature and significance of the Jemez Lineament, the physical and chemical processes whereby lithosphere is transformed into asthenosphere during extension, and the variations in magma ascent characteristics that lead to preservation of different xenolith suites. The project focuses on three specific hypotheses: (1) incipient extension at the boundary between the Colorado Plateau and the Rio Grande rift modified the mantle both chemically and physically. Many geochemical and isotopic studies of basalts from continental rifts imply a transition from enriched lithospheric melt sources to depleted asthenospheric sources as extension progresses. Mantle xenoliths should provide direct evidence for this transition. The degree of mantle modification that has occurred in the early stages of extension is being determined through comparisons of chemical, isotopic, and microstructural features of the Puerco xenoliths with xenoliths from the adjacent regions. (2) The Jemez Lineament is a lithospheric-scale structure that marks a major Proterozoic province boundary in the subsurface. The Jemez Lineament is defined on the basis of a northeast-trending alignment of Cenozoic volcanic centers; these volcanoes are thought to have taken advantage of a plumbing system established during Proterozoic assembly of the region. There is, however, little surface expression of faulting along the Lineament, and the geophysical evidence for the existence of a major province boundary at depth is ambiguous. If the Jemez Lineament does represent a major crustal boundary, crustal xenoliths should show pronounced deformational fabrics and different pressure-temperature histories from different sides of the boundary. Microstructural and metamorphic studies are in progress to test this hypothesis. (3) The abundance, size, and type of xenoliths brought to the surface by different volcanic necks varies systematically with the chemistry of the host basalts. There are approximately 50 volcanic necks within the Rio Puerco valley, some of which carry abundant mantle xenoliths, others that contain only crustal xenoliths, some with both mantle and crustal xenoliths and some of which are entirely devoid of xenoliths. Correlations between xenolith populations and host basalt chemistry place constraints on magma ascent velocities, degree of fractionation and wallrock interaction en route to the surface, magma volatile contents, changes in magma source region, and thermal evolution within the volcanic field as a function of time.
