WHAT TRIGGERED CENOZOIC, MANTLE-DERIVED MAGMATISM IN THE ROCKY MOUNTAIN REGION?

Existing field, geochronologic, and geochemical studies demonstrate that mafic, mantle-derived volcanism occurred in many portions of the Rocky Mountain area during the Cenozoic. However, the processes that triggered mantle melting in a region underlain by Precambrian continental lithosphere, located ~1,000 km inboard of the western margin of southwestern North America (SWNA), and that had previously experienced only minor igneous activity since at least the Mesoproterozoic, remain unclear. The origin of Cenozoic mafic magmatism in the southern Rocky Mountains in northcentral Colorado is uniquely accessible for study because mantle-derived xenoliths entrained in local, Devonian kimberlites reveal the characteristics of the continental lithosphere in this region prior to Cenozoic magmatism. Mafic volcanic rocks in this area occur in two age groups . Paleocene basalts to trachybasalts were emplaced during compressional tectonism related to the Laramide Orogeny and consist of volcaniclastic cobbles and lavas with low ε_Nd (T) (~-6) and trace element abundances similar to island arc basalts (IAB), including low Ta/Th values (<0.2). Younger basaltic rocks were syn-extensional and also have ε_Nd (T) (~-6) but most have intermediate Ta/Th values (0.2 to 0.6) and lack obvious IAB trace element patterns. Fertile peridotite xenoliths from deeper portions (up to 250 km) of the Devonian continental lithospheric mantle (CLM) have ε_Nd (T) that overlap values for Cenozoic basalts and demonstrate that Nd in the latter was CLM-derived. The progression from low ε_Nd (T) <0), low Ta/Th volcanism, to low ε_Nd (T), intermediate Ta/Th volcanism in northcentral Colorado also occurs elsewhere in SWNA and suggests a common origin for mafic volcanism throughout the region. As in other portions of SWNA, older volcanism in Colorado likely involved low-angle subduction and dehydration of oceanic lithosphere of the Farallon plate during the Laramide Orogeny, which triggered flux-induced melting of the asthenosphere even below thick Precambrian lithosphere. Younger magmatism represents decompression melting of upwelling asthenosphere induced by degradation and thinning of the CLM upon slab removal. In both time periods, mafic melts acquired low ε_Nd (T) values via interaction with Precambrian CLM during ascent to the surface.