Proterozoic history and crustal evolution in southwestern Colorado: Insight from U/Pb and Sm/Nd data

Abstract

New U/Pb age constraints and Sm/Nd isotope data further establish the timing of magmatic and tectonic events from 1.8 to 1.4 Ga in southwestern Colorado, and give insight into the interaction of crust and mantle systems during the assembly and evolution of orogenic belts along the southern margin of Laurentia. Proterozoic orogenic events in the Needle Mountains led to crustal assembly, thickening and tectonic reworking that was punctuated by periods of relative crustal stability. Pre-1.75 Ga arc successions in southwestern Colorado were produced and assembled into juvenile crust in an orogenic period that culminated in regional deformation and metamorphism that accompanied intrusion of synorogenic plutonic rocks from 1.73 to 1.71 Ga. Emplacement of post-orogenic plutonic rocks at ∼1.70 Ga was followed by uplift and erosion, and the deposition and deformation of siliciclastic sedimentary sequences. Paleoproterozoic crust underwent local melting and thermal metamorphism during invasion of felsic to mafic magmas at ∼1.43 Ga that coincided with a pulse of intracratonic deformation. Invasion and melting of first-cycle 1.8–1.7 Ga crust at ∼1.71 and ∼1.43 Ga involved heat and volatiles from mantle melts that were a catalyst for crustal melting and magmatic recycling. Sm/Nd isotopic signatures of 1.8–1.4 Ga volcanic and plutonic rocks in southwestern Colorado lend evidence to the emplacement of melts from depleted mantle and juvenile crust in the production continental crust, with minor, if any, contamination by crust older than 1.8 Ga. The Proterozoic magmatic record in southwestern Colorado is consistent with continental growth by the addition of juvenile crust extracted from the mantle, followed by periodic intracrustal growth during orogenic pulses in which mantle magmas underplate and melt the crust with consequent thickening.