A COMPARISON OF THE PETROCHEMICAL SIGNATURES OF ~1.7 GA GRANITIC ROCKS IN THE NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO: IMPLICATIONS FOR MAGMA GENESIS AND CRUSTAL EVOLUTION

Granitic plutonic rocks were emplaced into the pre-1750 Ma metamorphic complex in the western and northern Needle Mountains from 1735 to 1690 Ma during and after deformation related to the Yavapai-orogenic event. Previous age constraints and isotopic studies on these ~1.7 Ga plutonic rocks provided insight into the timing of magmatism and potential magma sources. A detailed and comprehensive comparison of the petrochemical signatures of three different magmatic complexes in the western and northern Needle Mountains was done to gain further insight into the evolution and interaction of the crust and mantle through time, and the production and growth of continental lithosphere.

Deformed ~1.7 Ga plutonic complexes in the Needle Mountains range from diorite to granite in composition, and are predominantly calc-alkaline to alkaline with metaluminous to peraluminous signatures. The Tenmile Granite plutonic complex is the largest exposure of syn-orogenic ~1.7 Ga intrusive rocks. These rocks are strongly foliated with complex internal intrusive relationships, and abundant dioritic to gabbroic enclaves and xenoliths of metamorphosed country rock. Geochemical and isotopic data indicate mixed mantle and crustal magma sources for all phases of these rocks.

The undeformed ~1.7 Ga Bakers Bridge Granite in the western Needle Mountains is dominated by calc-alkaline and metaluminous biotite-hornblende granite with minor biotite-muscovite phases. In the Coal Bank Pass area ~1.7 Ga intrusive rocks are mostly peraluminous S-type garnet-muscovite granite generated from a more evolved crustal source. We will also present new results of geochemical modeling for the undeformed S-type granites exposed at Coal Bank Pass to test the hypothesis that these rocks were generated by partial melting of older crystalline basement gneisses in the region.

We propose that the petrologic and geochemical signatures of ~1.7 Ga plutonic complexes in the western Needle Mountains reflect a long-lived pulse of magmatism in which mantle magmas invaded newly-formed continental crust. Fractionation of mantle derivatives and partial melting of crust led to the spectrum of rock types and compositions, with more evolved granites with crustal signatures dominating the waning phases of magmatism after deformation.