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 g ranite with the latter greatly dominating. There is no apparent relationshi p between chemical or isotopic composition and age, geographic location, or degree of alteration or deformation of the xenoliths. Two of the major age s of Proterozoic plutonism recognized in the Southwest (1.75-1.70, Yavapai, and 1.45-1.41 Ga, 'anorogenic') are represented by the NVF xenoliths, in s ome instances within a single diatreme. We did not find any xenoliths in th e 1.65-1.62 Ma (Mazatzal) range. The xenoliths are dominantly I-types and b oth Nd and Sr isotopes and trace element distributions indicate juvenile, a re sources approximately 1.8 Ga. The granite magmas may have been produced by either vapor absent partial melting or fractional crystallization of a t onalite parent. Juvenile sources must have been available for melting in ea ch 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 o f 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 li mited to the far northern part of the volcanic field consistent with a poss ible crustal boundary in the basement. Compositional and thermobarometric r esults suggest that the biotite schist xenoliths are not restites, but that the paragneisses may be mixtures of melt and restite. Sedimentary protolit hs may have been deposited in a forearc basin, and later tectonically trans ported to the deep crust during one or more are-continent collisions in the late Paleoproterozoic. (C) 1999 Elsevier Science B.V, All rights reserved.