New Mexico Geological Society Annual Spring Meeting — Abstracts

A reconciliation of conflicting models suggests that Proterozoic rocks in New Mexico record a polyphase tectonic history involving orogenic activity at 1.75-1.65, 1.4 and 1.3-1.0 Ga. Early deformation and metamorphism took place during several pulses of Early Proterozoic convergence. These events accompanied 1.75-1.65 Ga plutonism and assembly ofjuvenile arcs (Indonesian model) in Arizona and Colorado; similar events probably affected New Mexico. A regional NE-striking, steeply dipping (S₂) shortening fabric predates 1.7 Ga (Arizona, Needle Mts), 1.65 Ga (Magdalena Mts) and 1.45 Ga plutons (Picuris Mts, Manzano Mts) and thus in part records Early Proterozoic continental assembly. Early Proterozoic metamorphism was perhaps dominantly greenschist facies but reached amphibolite facies where plutons were closely spaced in time and space (e.g. NW Arizona). In central New Mexico, peak metamorphic conditions vary from greenschist to upper amphibolite facies and metamorphic field gradients pass close to the Al₂SiO₅ triple point. The age of this triple point metamorphism remains controversial. Isograds crosscut contractional structures, suggesting that peak metamorphism post-dated regional contraction. Elevated geotherms imply either pluton-enhanced metamorphism (at 1.65 and/or 1.45 Ga) or conductive decay resulting from crustal thickening. Metamorphic P-T paths suggest substantial amounts of decompression on the retrograde path, but the timing and character of the prograde path remains poorly constrained.

In New Mexico, Early Proterozoic deformation and metamorphism were variably overprinted by Middle Proterozoic tectonic events. One model suggests that regional deformation and metamorphism were the product of a complex interplay of contraction and extension (Himalayan model) broadly synchronous with emplacement of 1.4 Ga plutons (Grambling and others, 1989). This model lumps all the higher grade metamorphism and deformation into a single Middle Proterozoic event. It may overestimate the magnitude of 1.4 Ga contraction, but it highlights the importance of 1.4 Ga tectonism and the uncertainties in dating deformational and metamorphic fabrics. Some areas show the direct juxtaposition of triple point rocks tightly infolded with but generally above higher grade gneisses (750°C, 8 kb; Cimarron, Taos, and Rincon Mts). This geometry is interpreted to represent a mid-crustal extensional shear zone. ⁴⁰Ar/³⁹Ar hornblende and U-Pb monazite ages of 1.4 Ga from the higher grade rocks indicate regional heating in areas where no 1.4 Ga plutons have been identified. Extensional deformation and accompanying metamorphism may have taken place at 1.4 Ga. A second model suggests that 1.4 Ga plutons produced locally enhanced deformation and metamorphism in their aureoles. Either model is compatible with broad contact aureoles and syn-pluton ductile deformation around 1.4 Ga granites, both extensional (Sandia) and contractional (priest), and by decompressional P-T paths for metamorphism in several areas. It is clear that 1.4 Ga was not a time of "anorogeny", but of regional heating, uplift, and deformation. The extent of 1.4 Ga metamorphism (regional heating versus pluton-enhanced metamorphism) and the kinematic regime for 1.4 Ga ductile deformation (regional contraction and extension versus pluton-enhanced local contraction and extension accompanied by reactivation of earlier fabrics) remain to be established.

New evidence indicates that post-1.4 Ga NW-verging contractional deformation affected several areas (priest, Sandia, Picuris, EI Oro, Cimarron, Taos Mts.). Microfabrics associated with this deformation indicate low temperature deformation (< 350°C). Regional resetting of ⁴⁰Ar/³⁹Ar muscovite ages at 1.35-1.0 Ga suggests that this deformation was at least in part an inboard expression of Grenville convergence.