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奥普尔科火山颈含有丰富多样的捕虏体套件，几乎没有受到事先的关注。颈部位于Jemez线性带之间的过渡科罗拉多高原和格兰德河裂谷，捕虏体，因此有可能提供大量的信息岩石圈结构，化学和蚀变在构造上的重要区域。这项研究解决了一些问题的性质和意义的Jemez线性，物理和化学过程，岩石圈在伸展过程中转化为软流圈，并在岩浆上升特性的变化，导致保存不同的捕虏体套件。该项目侧重于三个具体的假设：（1）在科罗拉多高原和格兰德河裂谷之间的边界处的初始伸展改变了地幔的化学和物理性质。许多来自大陆裂谷的玄武岩的地球化学和同位素研究暗示，随着伸展的进行，从富集的岩石圈熔体源向亏损的软流圈源过渡。地幔捕虏体应为这种转变提供直接证据。地幔改造的程度，发生在早期阶段的扩展正在确定通过比较的化学，同位素和微观结构特征的Puerco捕虏体与捕虏体从邻近地区。 (2)Jemez Lineament是一个岩石圈规模的结构，标志着地下主要的元古代省边界。Jemez Lineament是根据新生代火山中心的东北走向排列而定义的;这些火山被认为利用了该地区元古宙组装期间建立的管道系统。然而，几乎没有地表断层沿着线性体的表现，并且在深部存在一个主要省份边界的地球物理证据是模糊的。如果Jemez线状体确实代表了一个主要的地壳边界，地壳捕虏体应该表现出明显的变形组构和不同的压力-温度历史，从不同的边界两侧。显微构造和变质研究正在进行中，以验证这一假设。 (3)由不同火山颈带至地表的捕虏体的丰度、大小和类型随寄主玄武岩的化学成分而系统地变化。在Rio Puerco山谷内大约有50个火山颈，其中一些含有丰富的地幔捕虏体，另一些只含有地壳捕虏体，一些同时含有地幔和地壳捕虏体，还有一些完全没有捕虏体。捕虏体的人口和主机玄武岩化学之间的相关性的限制岩浆上升速度，分馏和围岩相互作用的程度，途中的表面，岩浆挥发分含量，岩浆源区的变化，和热演化内的火山场作为时间的函数。

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.