Zuni-Bandera volcanism, Rio Grande, USA – melt formation in garnet- and spinel- facies mantle straddling the asthenosphere-lithosphere boundary.
*Timothy J. Peters, Martin Menzies, Matthew Thirlwall, and Philip R. Kyle
The Zuni-Bandera Volcanic Field:
The Zuni-Bandera Volcanic Field:
-Located in west central New Mexico (Fig. 1), the Zuni-Bandera Volcanic Field (ZBVF) is a sequence of mixed tholeiite and alkali basalt lava flows, spatter ramparts, scoria cones, small shields, maars, and collapse pits erupted through Proterozoic continental crust over the last 1.5 Ma (Anders et al., 1981; Maxwell, 1986; Baldridge et al., 1991; Menzies et al., 1991).
-The ZBVF is located in the transitional zone between the actively extending terrains of the Rio Grande rift and the Basin and Range province, and the relatively stable southern edge of the Colorado Plateau (Fig. 1). Across this region the asthenosphere-lithosphere boundary varies significantly in depth from 45-55 km beneath the Rio Grande rift, to 120-150 km beneath the Colorado Plateau (West et al., 2004).
-The ZBVF is located in the transitional zone between the actively extending terrains of the Rio Grande rift and the Basin and Range province, and the relatively stable southern edge of the Colorado Plateau (Fig. 1). Across this region the asthenosphere-lithosphere boundary varies significantly in depth from 45-55 km beneath the Rio Grande rift, to 120-150 km beneath the Colorado Plateau (West et al., 2004).
-Over the last 4.5 Ma, volcanic activity associated with the Rio Grande rift has shifted, to some degree, from the rift to the Jemez Lineament (Fig. 1), an 800 km NE-trending crustal discontinuity that cuts across the rift. Seismic tomography has identified regions interpreted as evidence for the presence of melt, and may suggest that the lineament acts as a route towards the surface for magma transport beneath these volcanic fields (Spence and Gross, 1990; Menzies et al., 1991). More recent teleseismic studies by West et al. (2004) across the Rio Grande rift revealed the absence of a deep-mantle low-velocity anomaly beneath the region of extension. This supports the interpretation of lithospheric extension and magmatism as results of stresses localised within the lithosphere and small-scale mantle convection, rather than deep-mantle upwelling due to the presence of an active plume/thermal anomaly beneath the rift (West et al., 2004).
Figure 1. General tectonic and volcanic map of Northern New Mexico, USA. Tectonic features include the extensional Rio Grande rift and Basin and Range, and stable lithospheric regions of the Great Plains and Colorado Plateau. Volcanic fields (VF) can be observed to lie along with the Jemez lineament. The Zuni Bandera Volcanic Field resides on the western flank of the Rio Grand rift in a transitional region with the Colorado Plateau. After Goff and Grigsby (1982).
ŠElsevier (2007)
ŠElsevier (2007)
LANDSAT image of the Zuni-Bandera Volcanic Field. Zuni-Bandera Volcanic Field (click link to field guide)
Mantle source for the ZBVF basalts:
Menzies and Kyle (1990) and Menzies et al. (1991) proposed a model where by the geochemistry of the ZBVF magmas was explained by the interaction of three mantle sources: asthenosphere, lower lithosphere, and a mantle plume. The consequence of this model was that alkali basalts were believed to have originated within the relatively isotopically homogeneous convecting asthenospheric mantle (87Sr/86Sr ratio <0.703), whereas the tholeiitic basalts were extracted from the isotopically heterogeneous lithospheric mantle or a plume-contaminated asthenospheric mantle (87Sr/86Sr ratio >0.706). With no evidence to support a deep mantle plume-like mantle source, heterogeneity is likely to be sourced in the lithospheric mantle.