LITHOSPHERIC MANTLE AND CENOZOIC MAGMATISM IN THE SOUTHERN ROCKY MOUNTAINS

Late Cretaceous and younger magmatic activity in the southern Rocky Mountain region of the western U.S. occurs more than 1,000 km inboard of a contemporaneous convergent plate boundary, in a region of thick (>150km) continental lithosphere. Exactly why magmatism was initiated in this inboard position remains enigmatic. A key issue is whether magmatism was ultimately related to the intrusion of basaltic magmas into the continental crust and if so, whether these magmas were produced in geochemically-defined lithospheric or sublithospheric mantle. In the Late Cretaceous to early Cenozoic, magmatism was largely confined to the Colorado Mineral Belt (COMB) and few volcanic rocks from this episode have been preserved. However, ~66Ma trachybasalt volcanic clasts are preserved in Paleocene sedimentary rocks in northern Colorado and attest to the production of basaltic magmas during this magmatic episode. Lamprophyres and trachybasaltic volcanic rocks are also associated with the mid-Cenozoic ignimbrite flareup, both at large and small volume igneous centers, such as the Southern Colorado and Never Summer volcanic fields, respectively. Initial Nd and Sr isotopic compositions for the most mafic volcanic rocks in each igneous episode are similar (e_Nd (T)~0 to -5, ⁸⁷Sr/⁸⁶Sr (T)~0.705). Furthermore, these isotopic compositions overlap those of clinopyroxenes in lithosphere-derived spinel and spinel-garnet peridotite xenoliths entrained in Devonian kimberlites and in ~8 Ma trachybasaltic andesites in north central Colorado. These data suggest that continental mantle lithosphere (CLM) with isotopic characteristics distinct from the sublithospheric mantle was present throughout the Cenozoic in the southern Rocky Mountain region and that the isotopic characteristics of basaltic magmas generated throughout this time interval were likely inherited from such material. The exact role of lithospheric mantle in basaltic magma generation through time is unclear. The CLM could have served solely as a “melt reactor” for mafic magmas percolating upwards from greater depths during the Late Cretaceous, while during the mid-Cenozoic the CLM could have been a source of primary melts as a result of its refrigeration and hydration during “flat slab” subduction and subsequent flash melting during slab roll back.