Reference 522

Abstract

This study investigates an extensive system of mid-Tertiary igneous intrusions, including plutons, stocks, dikes, sills, plugs, and laccoliths, as well as late-Tertiary and Quaternary basaltic and andesitic lava flows in the Raton basin and adjacent northern Rio Grande rift. The igneous rocks are derived from melts associated with the various stages of northern Rio Grande rifting and formed due to partial melting of the upper mantle and possibly the associated melting of granitic and amphibolitic source areas in the upper and lower crust. Geochronology, petrochemistry, and isotopic analyses were conducted for a select group of igneous rocks from the Raton basin and adjacent northern Rio Grande rift (San Luis Valley and southern Culebra Range) in order to constrain the petrogenesis of these igneous rocks and put together a magmatic history for the region. Of particular importance are intrusions related to East and West Spanish Peaks in the northern part of the Raton basin and their genetic relationship to northern Rio Grande rifting. The results of (super 40) Ar/ (super 39) Ar dating place new age constraints on the timing of magmatism within the Raton basin. Lamprophyre magmas were generated prior to (33 to 25.3 Ma) and during (25.3 to 19.7 Ma) the initial stages of rifting. The age results also show chronological changes in the location of magmatic fields within the basin, with the youngest (10 to 0.056 Ma) igneous rocks, mostly basalts, occurring in the eastern part of the basin, and older (33 to 19.7 Ma) lamprophyres, andesites, and rhyolites occurring in the northern and western margins of the basin. The volcanic rocks preserved in the southern Culebra Range and San Luis Valley span an apparent age range from approximately 35 to 1 Ma, the youngest basaltic and andesitic lavas being from within the San Luis Valley. New (super 40) Ar/ (super 39) Ar dating has helped place the timing of the initiation of rifting close to 25 Ma, contemporaneous with the eruption of the Amalia Tuff from the Questa caldera in northern New Mexico. Also, uplift rates were determined for two geomorphic datums; one from the Culebra Range that yielded a local uplift rate of 58 m/m.y., and the other from offset San Pedro Mesa lavas in the San Luis Valley that yielded a local uplift rate of 87 m/m.y. Rb-Sr and Sm-Nd isotopic compositions for the dated rocks indicate that magmas were being generated from several isotopically distinct source areas in the crust and upper mantle. The Sm-Nd isotopic values indicate that mafic magmas are less contaminated by crustal components through time.

74 samples from this reference