BULK COMPOSITION OF CONTINENTAL CRUST IN THE RIO GRANDE RIFT BASED ON RIFT FLANK UPLIFTS AND CRUSTAL XENOLITHS

The composition of the crust controls the petrologic and geophysical structure of the continent. Simply put, mafic crust is denser, stronger and less radiogenic than felsic crust. Further, the bulk composition of the crust has implications for how crust is created and destroyed. The southern Rio Grande rift is an excellent laboratory for the study of crustal composition because mountain ranges flanking it expose rocks from different crustal levels (~upper 25 km of the crust) and a suite of xenoliths from the Potrillo volcanic field provide access to the deep crust in a key section of the rift.

Key observations from the mountains flanking the rift are: 1) The upper crust is dominated by sedimentary and lesser volcanic rocks, with a small percentage of intrusive rocks, exclusive of major calderas. Limestone and dolomite are locally the dominant sedimentary rock type in the upper crust; 2) Middle crustal exposures are mixtures of metavolcanic and metasedimentary rocks intruded by intermediate to felsic plutons. Plutons are dominant in some regions, but are most abundant in the 10-20 km depth interval; 3) Some of the deepest crust contains up to 50% supracrustal rocks, requiring efficient deep burial of upper crustal materials.

Diverse crustal xenoliths occur at Kilbourne Hole and Potrillo Maar, and provide insight into the composition of the lower crust within the rift. While a number of distinct rock types occur as xenoliths, the most common are garnet-sillimanite paragneiss and 2-pyroxene granulite. Seismic velocity calculations based on modal mineralogy and corrected for pressure and temperature show that lower crustal seismic velocity can be matched by a 50-50 mixture of paragneiss and 2-pyx granulite.

These observations indicate that the crust within the Rio Grande rift contains a substantial fraction of sedimentary rocks at all crustal levels. Using estimates for the relative volumes of different rock types, we obtain an overall andesitic bulk composition for the continental crust, similar to published estimates. However, this crustal composition reflects mixing of sedimentary material into the middle and deep continental crust, rather than differentiation of a mafic parental magma. Our estimate for crustal composition also matches heat flow, density and seismic velocity structure for the crust in the Rio Grande rift.