EVOLUTION OF SOUTHERN LAURENTIAN LITHOSPHERE; SOME NEW OBSERVATIONS FROM CRUSTAL DOMAINS AND MID-PROTEROZOIC ALKALINE MAGMATIC SUITES IN MOJAVIA AND MAZATZAL

Proterozoic North America was assembled around pre-existing cratonic nuclei by progressive addition of juvenile volcanic arcs and oceanic plateaus. This led to an amalgamation of Proterozoic crustal domains that, in the SW US, become progressively younger from W-NW to E-SE. Nd isotope composition of exposed cratonic rocks of the southern Laurentian region delineates specific crustal provinces (Nd provinces 1 – 3) marked by decreasing depleted mantle model ages eastwards (Bennett and DePaolo, 1987). We have examined two Mesoproterozoic ultrapotassic magmatic suites and their cratonic country rocks, one from Nd province 1 (Mojavia) and one from Nd province 3 (Mazatzal). The former is the Mountain Pass rare-earth carbonatite and associated ultrapotassic rocks in SE California and comprises a series of low- to high-SiO₂ silicate rocks (shonkinite to alkali granite) and calcio- and magnesiocarbonatites, emplaced into older cratonic rocks at ~1400 Ma. The latter is a ~1460 Ma assemblage of comingled ultrapotassic minette and potassic granite in the northern Burro Mountains, SW New Mexico. ε_Nd(T) values for the shonkinites, syenites, and carbonatites in Mountain Pass range from -4 to -2, whereas those for the minettes and granites in the Burro Mountains are between +2 and +4. The exposed granitoid and metamorphic rocks of Mojavia and Mazatzal have ε_Nd(1400 Ma) values of -9 to -3 and -1 to +4, respectively. The ε_Nd(T) values of the Mountain Pass carbonatites and ultrapotassic rocks are all negative (but, on average, markedly higher than those of the exposed crust) and probably register an ancient subcontinental mantle domain, formed while Mojavia was assembled. The Mountain Pass ultrapotassic rocks and carbonatites have different initial ⁸⁷Sr/⁸⁶Sr (0.7078-0.7089 and 0.7048-0.7057, respectively) and were probably not derived from identical mantle sources. The Burro Mountain minettes tapped a more juvenile, depleted mantle domain attached to the Mazatzal province. The two crustal provinces, Mojavia and Mazatzal, are grossly different in terms of their mantle separation ages. Both, however, register incorporation of pre-existing crustal material in the form of sedimentary detritus (increasing aluminum saturation with decreasing eNd values) during mantle to crust differentiation.