New Mexico Geological Society Annual Spring Meeting — Abstracts

Preliminary examination of 85 crustal xenoliths from 6 diatremes in the Navajo Volcanic field provides information on crustal compositions in basement beneath the Colorado Plateau. Each of the diatremes brought up mafic granulites, felsic orthogneisses, and granitoids. In addition, the three northwestern diatremes (Moses Rock, Garnet Ridge, and Red Mesa) brought up abundant paragneisses and garnet amphibolites that are lacking in the xenolith suites from the southeastern diatremes (Shiprock, The Thumb, and Mitten Rock). This observation indicates either a fundamental difference in lower to midcrustal makeup between the two suites or an inherent sampling bias by the different diatremes.

The metasedimentary rocks of the northern diatremes fall into two categories, both of which are represented at all three sites: (1) garnet-sillimanite-perthite-qtz±plag±kyanite± hercynite gneiss, and (2) isoclinally folded biotite-sillimanite-Fe-oxide gneiss. The latter rock type occurs at musc+qtz grade in the Red Mesa suite and at 2nd sillimanite grade in the Moses Rock suite; both metamorphic grades are represented in the Garnet Ridge suite. Approximately half of the Type (2) metasedimentary xenoliths experienced pervasive low-T, moderate-P hydration, as documented previously by Broadhurst (1986) and confirmed in this study; Type (1) xenoliths were largely unaffected by this hydration.

P-T calculations carried out on the Type (1) metasedimentary xenoliths indicate that a I thermal peak of ~700-850°C was attained at pressures between ~6 and 10 kbar (~20-35 km), followed by prolonged cooling to temperatures of $500°C at unknown depths. The extensive diffusional cooling profiles indicate that the "peak" conditions were attained prior to Tertiary entrainment in the host kimberlite and most likely represent part of the Proterozoic metamorphic history. Numerous petrographic observations in single samples place further constraints on the PTt path shape: (a) Inclusions of intergrown sill+kspar in garnet indicate initial garnet growth around mus grains, followed by the reaction muse+qtz= sill+kspar+ H₂O at 2<P<10 kbar. (b) Herc inclusions in the outer portion of garnets and as discrete grains in the presence of quartz in the matrix indicates continued garnet growth up to high T followed by the reaction gar+sill= herc+qtz at P>3 kbar. (c) Partial transformation of sill to kya in the presence of kspar indicates T>750-800°C at P>9-10 kbar. (d) Extensive perthitic exsolution of kspar implies slow cooling at high T. (e) A few samples show retrogression of kspar+Al₂SiO₅ to musc. These observations are consistent with counterclockwise PTt paths that reached granulite grade followed by slow cooling at depth; such paths are typical of pluton-enhanced metamorphism in collisional settings.

Paragneisses with similar bulk composition and metamorphic grade occur in Proterozoic exposures in the Cimarron Range (Grambling & Dallmeyer 1993) and the Grand Canyon (Williams 1991) but have not thus far been reported from rocks further to the south. Model Nd
ages of 1.8-2.0 Ga (Wendlandt 1992) obtained on Type (1) xenoliths are in agreement with model ages determined from the Yavapai Province to the N. Broadhurst (1986) argued that hydration of the type (2) xenoliths might represent Tertiary devolatilization of the Farallon slub, but the lack of scatter in the model ages argues against such a young event, and the lack of similar hydration features in the southern diatremes is also difficult to explain by this model. An alternative hypothesis is that the hydration was related to underthrusting and devolatilization of hydrous material in the Proterozoic. Eclogite xenoliths with Proterozoic Nd model ages (Wendlandt 1992) are also confined to the northern diatremes. This geographic restriction of Proterozoic metasediments, hydration, and eclogites to the northern diatremes supports the existence of a major crustal boundary in the Four Corners region, as postulated from other lines of evidence by Bennett & DePaolo (1987) and Karlstrom & Bowring (1988). The locus of crustal hydration and restricted occurrence of eclogitic mafic rocks further suggest an original northward dip along this boundary.