CORRELATION OF MANTLE XENOLITH DATA WITH GEOPHYSICAL IMAGES OF THE TRANSITION ZONE BETWEEN THE COLORADO PLATEAU AND THE RIO GRANDE RIFT

The Pliocene Rio Puerco volcanic necks lie within the transition zone between the Colorado Plateau (CoP) and the Rio Grande Rift (RGR), and also straddle the width of a possible Proterozoic terrane boundary (Jemez Lineament, JL). Mantle xenoliths (N=300) from 9 necks provide a glimpse of deep-seated processes in this tectonically complex area. Spinel lherzolite, spinel websterite, and lherz/webs composites dominate a heterogeneous sample suite that includes both hi- and low-Cr types. All xeno types range from granoblastic to highly sheared. Garnet peridotites in which gar and oliv are largely replaced by opx, cpx, and spinel occur in 3 necks. Decompression breakdown of spinel is apparent in 10-20% of lherz and webs samples. Evidence for mantle metasomatism is widespread and includes melt and fluid inclusions, stable isotopic anomalies, and changes in rock magnetic properties; carbonates are common, but hydrous phases occur in only 2 samples. P-T studies indicate equilibration of most samples at 875-950°C in the spinel lherzo field and of relic garnet peridotites at 1050-1100°C and 22-28 kbar. These T’s lie between those determined from similar xeno types from the CoP and from the RGR. Localized breakdown of gar and spinel are consistent with heating and decompression during incipient extension.

Tomographic data from the RISTRA seismic experiment indicate a prominent region of slow mantle velocities beneath the Puerco Necks. Our T data show moderate heating relative to uppermost mantle beneath the CoP, and our stable isotope data indicate extensive interaction with fluid that could have been derived from the Farallon slab. The velocity anomaly thus likely reflects combined T and compositional effects, as predicted by Gao et al (JGR 04). Combined xeno and basalt data across the JL are ambiguous with respect to the nature of the JL at depth. All gar-bearing xenos come from necks in the center of the JL, however, which may indicate structural control on transport of deep-seated xenos. Overall, the Puerco xenoliths record a history of heating, decompression, and fluid alteration that both augments and correlates well with geophysical images of the transition zone.