PROTEROZOIC ORTHOGNEISSIC XENOLITHS WITHIN THE TERTIARY BASASEACHIC IGNIMBRITE IN WESTERN CHIHUAHUA, MEXICO: NEW EVIDENCE FOR THE DISTRIBUTION OF PALEOPROTEROZOIC BASEMENT PROVINCES IN SOUTHWESTERN LAURENTIA

The large blanket of Tertiary ignimbrites within the Sierra Madre Occidental province of northern Mexico have obscured the view of any previous geological events, including those occurring in Proterozoic and Paleozoic times. Subsurface geological sampling through drilling for oil and gas exploration is very limited in the region. However, Tertiary magmas have provided excellent sources of basement information in the lithologies of xenoliths carried upward from different levels of the crust.

In this study, we present U-Pb age data determined by the LA-ICPMS technique on various suites of gneissic and granitic xenoliths hosted by the Tertiary Basaseachic ignimbrite from western Chihuahua. A suite of three coarse-grained orthogneissic xenoliths yielded U-Pb zircon crystallization ages ranging between 1619–1649 Ma. These Paleoproterozoic zircons exhibit various degrees of Pb loss in addition to some inherited zircons (cores) with slightly older ages (1.65–1.80 Ga). A second suite of fine-grained orthogneissic xenoliths yielded Mesoproterozoic U-Pb zircon ages between 1404–1432 Ma (3 samples). Zircons from this suite have Pb-loss patterns similar to those in the older gneissic rocks, in addition to abundant inherited zircons of Paleoproterozoic age (1.62-1.75 Ga). Lastly, an undeformed granitic xenolith has been dated by U-Pb at 31.04 ± 0.87 Ma, which is interpreted as the time of crystallization of the granite and representing the maximum possible age for the eruption of the Basaseachic ignimbrite dated by K-Ar at ~29 Ma in an earlier study.

Preliminary whole-rock geochemical and Sm-Nd data on some of the Paleoproterozoic samples indicate that these rocks where most likely formed from mantle sources in a juvenile volcanic arc environment without much, if any, interaction with preexisting continental crust, a result similar to that from the basement igneous rocks studied in the Yavapai and/or Mazatzal provinces of northern Sonora and southern Arizona. This new occurrence of Paleoproterozoic rocks in western Chihuahua expands the distribution of basement provinces in northern Mexico. This expanded view would allow further testing of current and future models that propose assembling and dismembering the Rodinia supercontinent in this part of the SW margin of Laurentia.