PALEOPROTEROZOIC GNEISSES FROM THE QUITOVAC AREA IN NW MEXICO: ASSESSING MAGMA SOURCES ALONG THE SW MARGIN OF LAURENTIA USING IN-SITU HF ZIRCON MICROANALYSIS

The Quitovac area in NW Mexico has been the focus of multiple studies on Paleoproterozoic (~1.66–1.78 Ga) gneissic and granitic rocks that define two distinct basement blocks consisting of probable island arc-related rocks. A block to the west has similarities to typical Yavapai-province crust; one to the east has Mazatzal-type characteristics (more juvenile), as originally defined in the SW United States.

The overall juvenile character of these arc-related rocks, defined mostly by Sm-Nd whole-rock determinations, is also observed in our preliminary in-situ zircon laser ablation MC-ICPMS Hf data. Mazatzal rocks from Quitovac have epsilon Hf (initial) values for single zircons ranging between +1.7 and +9.2 (weighted mean +6.5, n = 47, 4 rock samples) with corresponding Hf model ages between 1.75–2.04 Ga (largest age peak at ~1.82 Ga). We suspect these rocks were coming directly from mantle sources (~10 epsilon Hf units for mantle) without significant crustal contribution, indicating their juvenile nature. This contrasts with Hf data collected in zircons from the Yavapai rocks in this study that yielded slightly less positive epsilon Hf (initial) between -0.9 to +8.7 (weighted mean +4.5, n = 105, 9 rock samples) and older Hf model ages in the 1.79–2.17 Ga range (peak at ~2.05 Ga), and suggest more crustal involvement in the precursor magmas that formed these Yavapai rocks. Results from these two basement blocks in Quitovac contrast sharply with our Hf isotopic determinations in Mojave-like crust on gneissic rocks from the Bámuri area, south of Quitovac, where epsilon Hf (initial) values range between -3.8 and +1.9 (weighted mean -1.0, n = 20, 2 rock samples) with corresponding Hf model ages between 2.14–2.57 Ga (peak at ~2.20 Ga). At Bámuri Neoarchean crustal components were likely involved in magma genesis during the Paleoproterozoic (~1.74 Ga).

The spatial delimitation of Proterozoic basement provinces in northern Mexico (Sonora and Chihuahua), by means of geological studies that include mapping, geochronology, geochemistry and isotopes (including whole-rock Sm-Nd and in-situ Hf zircon microanalysis), should help reconstructing the history of basement amalgamation—and later breakup—of the Mesoproterozoic supercontinent Rodinia for this part of SW Laurentia.