Geochemistry, Nd and Sr isotopes, and U/Pb Zircon ages of Granitoid and Metasedimentary Xenoliths from the Navajo Volcanic Field, Four Corners area, Southwestern United States

Proterozoic granitoid xenoliths from the Navajo Volcanic Field (NVF) in the southwestern United States range in composition from tonalite–diorite to granite with the latter greatly dominating. There is no apparent relationship between chemical or isotopic composition and age, geographic location, or degree of alteration or deformation of the xenoliths. Two of the major ages of Proterozoic plutonism recognized in the Southwest (1.75–1.70, Yavapai, and 1.45–1.41 Ga, `anorogenicʹ) are represented by the NVF xenoliths, in some instances within a single diatreme. We did not find any xenoliths in the 1.65–1.62 Ma (Mazatzal) range. The xenoliths are dominantly I-types and both Nd and Sr isotopes and trace element distributions indicate juvenile, arc sources approximately 1.8 Ga. The granite magmas may have been produced by either vapor absent partial melting or fractional crystallization of a tonalite parent. Juvenile sources must have been available for melting in each of the three periods of magma production, and hence some juvenile 1.8-Ga crust in this region must have escaped melting in the first two episodes of magmatism and deformation. Metasedimentary xenoliths are chiefly biotite–sillimanite schists, garnet–biotite schists, and garnet paragneisses. These three groups are strikingly similar in chemical and isotopic composition, suggesting similar graywacke-pelite protoliths. The schists appear to be limited to the far northern part of the volcanic field consistent with a possible crustal boundary in the basement. Compositional and thermobarometric results suggest that the biotite schist xenoliths are not restites, but that the paragneisses may be mixtures of melt and restite. Sedimentary protoliths may have been deposited in a forearc basin, and later tectonically transported to the deep crust during one or more arc-continent collisions in the late Paleoproterozoic.