Mantle and Crustal Xenoliths of the Puerco Necks, New Mexico: Constraints on Lithospheric Evolution at the Transition Between the Colorado Plateau and the Rio Grande Rift

新墨西哥州西北部的里约普尔科火山颈部含有丰富多样的捕虏体套装，此前几乎没有人注意到它。颈部位于科罗拉多高原和里奥格兰德裂谷之间的Jemez线内，因此捕虏体有可能提供关于构造重要区域的岩石圈结构、化学和蚀变的大量信息。这项研究涉及Jemez线的性质和意义、岩石圈在伸展过程中转化为软流圈的物理和化学过程，以及导致不同包体保留的岩浆上升特征的变化。该项目的重点是三个具体的假设：(1)科罗拉多高原和里奥格兰德裂谷交界处的早期伸展在化学和物理上改变了地幔。大陆裂谷玄武岩的许多地球化学和同位素研究表明，随着伸展的进行，岩石圈熔体的丰度源区向软流圈的贫化源区过渡。地幔包体应为这一转变提供直接证据。通过对普鲁科包体与邻近地区包体的化学、同位素和显微结构特征的比较，确定了在伸展早期阶段发生的地幔改造程度。(2)Jemez线是一种岩石圈尺度的构造，标志着地下元古界的主要界线。Jemez线条的定义是基于新生代火山中心向东北方向的排列；这些火山被认为利用了在该地区元古代组装期间建立的管道系统。然而，沿线断裂的地表表现很少，关于深部存在主要省边界的地球物理证据也不明确。如果Jemez线确实代表了一个主要的地壳边界，那么地壳捕虏体应该从边界的不同一侧显示出明显的变形组构和不同的压力-温度历史。微结构和变质研究正在进行中，以检验这一假说。(3)不同火山口带到地表的包体的丰度、大小和类型随寄主玄武岩的化学成分而有系统的不同。在里约普尔科山谷内大约有50个火山口，其中一些带有丰富的地幔包体，另一些只含有地壳包体，有些既有地幔包体，也有地壳包体，有些完全没有包体。捕虏体种群和寄主玄武岩化学之间的相关性限制了岩浆上升速度、分馏程度和到达地表过程中围岩的相互作用、岩浆挥发分含量、岩浆源区的变化以及火山场内随时间的热演化。

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.