EARLY ONSET OF RIO GRANDE RIFT MAGMATISM IN WEST TEXAS

Magmatism in Trans-Pecos Texas began as early as 46 Ma, peaked in the period 38-35 Ma, and declined afterward, ending about 16 Ma. Associated igneous rocks include silicic alkalic rocks, such as comendite and pantellerite, as well as nepheline trachyte and phonolite. Mafic rocks are highly variable, including basanite, alkali basalt, and various types of trachybasalt and trachyandesite. We interpret the widespread eruption of extensive silicic lava at ~38 Ma as the onset of major rift magmatism and a 35 Ma, 400 km long, NW-SE trending belt of nepheline trachyte and phonolite (Potter, 1996) as marking the axis of such rifting. In our interpretation, there is neither a magmatic nor a tectonic transition after 38 Ma, merely a decline in silicic acitivity and a shift towards alkalic basaltic eruption as extension increased and faulting became more pronounced.

Globally, silicic peralkaline igneous rocks like pantellerite and silica-undersaturated peralkaline rocks like nepheline syenite are associated with continental rifts and some oceanic islands. Also, subduction-related igneous rocks typically possess distinctive trace element patterns involving relative enrichment of LIL elements like Ba, and depletion of Nb. West Texas rocks show variable enrichment/depletion of these critical elements: mafic rocks (of all ages) show trends on spider variation diagrams typical of OIB; young, Basin and Range basalts show positive Ba anomalies; some show negative Nb and/or Th anomalies. Older mafic units show variable development of negative Nb and/or Th anomalies; a few show positive Ba anomalies. Some completely lack a subduction signature. Evolved rocks show depletion of elements (Ba, Sr, P, Eu, Ti) expected to be removed during extensive crystal fractionation of observed phenocrysts.

This reinterpretation of the Trans-Pecos magmatic province is in accord with recent interpretations of Rio Grande rift magmatism in New Mexico and Colorado beginning as early as 36 Ma.