PROTEROZOIC BASEMENT IN THE PINACATE REGION OF NW SONORA, MEXICO: POSSIBLE CONNECTIONS TO NORTHEASTERN AUSTRALIA?

Geologic mapping and zircon U-Pb analyses from the Pinacate region of Sonora, Mexico provide new constraints on the age of Proterozoic basement strategically located near the truncated southwestern margin of Laurentia. We recognize two geographically distinct blocks characterized by contrasting ages of pluton emplacement and style/timing of metamorphic overprint. The southwestern block contains four sheets of granite-granodiorite augen gneiss (1725 to 1696 Ma) intruded into finely banded metavolcanic gneisses. The metaplutonic rocks form "stratigraphic" markers repeated in northwest-trending folds, and display top-to-the SW S-C fabrics. Mesoscopic folds in augen gneiss and host strata record consistent SW-vergent asymmetries. Crosscutting intrusions include 1.1-Ga(?) diabase and Late Cretaceous quartz diorite. Directly northeast across a possible suture, the basement is composed of a zoned alkali-syenogranite batholith (six U-Pb SHRIMP ages between 1650 and 1640 Ma) intruded into arkosic and quartzose sandstone host rocks with a detrital zircon component predominantly 1665 to 1650 Ma (plus 2 grains ~1690 Ma). All Paleoproterozoic rocks of the northeastern block are variably foliated and lineated and so pervasively recrystallized that only patches of NW-vergent S-C fabric remain in the granites (now augen gneisses).

Cathodoluminescence images of zircon and consistent patterns of discordance suggest that many of the zircons were isotopically disturbed during a profound episode of solid-state recrystallization, tentatively constrained at 1590±8 Ma, but potentially as young as 1550 Ma. This metamorphism is distinct from low Th/U zircon overgrowths caused by emplacement of megacrystic porphyritic granite at 1432±6 Ma. We are intrigued by the cryptic 1590-1550 Ma thermal event that appears to lack associated plutonism in western Laurentia. Interestingly, basement in northeastern Australia (Queensland region) preserves a major 1550-1590 Ma plutonic event associated with regional metamorphism. Affected host rocks include igneous units with ages between 1725 and 1640 Ma, many of which display discordant zircon arrays remarkably similar to our data set from Pinacate.