EVIDENCE FOR SYNOROGENIC MANTLE-DRIVEN MAGMATISM AT ~1.7 GA, NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO

Metaluminous to peraluminous dioritic to granitic intrusive rocks were emplaced in the Needle Mountains complex during a period of deformation and amphibolite-grade metamorphism from 1.73-1.7 Ga. On the basis of new and existing isotopic and geochemical data we propose that calc-alkaline magmas were generated when mantle magmas invaded and melted pre-1.75 Ga accreted-arc crust. Differentiation of mantle extracts emplaced at ~1.7 Ga may also have contributed to some of the intermediate magmas produced in this period of plutonic activity.

Field evidence has established that the ~1.7 Ga plutonic rocks formed in distinct pulses and crystallized over a continuum of deformation, and that magma emplacement was controlled in part by existing structures and fabrics in older basement rocks. Ductile-deformation fabrics in early intrusive phases record ~N-S contraction and ~E-W extension that produced two generations of isoclinal to tight folds with related NE-SW to E-W trending penetrative foliation, shallow stretching lineation with mostly NE trends, and transposed and attenuated dikes. Intrusive rocks that crystallized in the late stages of ~1.7 Ga magmatism are undeformed or weakly deformed and cut the earlier generation of intrusive rocks and all deformational fabrics preserved in them.

On the basis of field relationships and geochemical signatures of ~1.7 Ga plutonic rocks, and structural trends developed at ~1.7 Ga in southwestern Colorado, we propose a working model in which regional transpressive strain at ~1.7 Ga (i.e., Yavapai orogeny) acted on pre-existing crustal anisotropies in older basement rocks causing local crustal extension and relaxation. This allowed mantle magmas to underplate "juvenile" crust, generating intermediate to felsic plutonic rocks that contributed to intracrustal thickening and growth.