EARLY CORDILLERAN ARC MAGMATISM AND CRUSTAL REORGANIZATION IN WESTERN LAURENTIA AS RECORDED BY PERMIAN TO TRIASSIC IGNEOUS ROCKS AND SEDIMENTARY STRATA

Initiation of subduction and proposed translation of crustal blocks characterize major changes that occurred along the Late Paleozoic southwestern margin of Laurentia. Resultant distribution of similar-age igneous and sedimentary strata provide clues to the timing of translation and dispersal of early-arc detritus.

The oldest subduction-related plutonic rocks are exposed in three regions: the El Paso terrane (EPM) in the northern Mojave Desert, CA, the Caborca block of Sonora, Mexico (SON), and the central and eastern Mojave Desert (EMD). Intrusive rocks of the EPM range in age from 275 - 250 Ma and have primitive zircon Hf isotope values (0 to +12). Plutons in SON are similar in age, but have very evolved Hf values (-15 to -3). Plutons of the EMD are slightly younger (ca. 250 - 230 Ma), but are geochemically similar to those of SON. These relations bring into question the timing of translation of EPM and the potential for co-genesis of the early arc between these distinct crustal blocks. Igneous and detrital zircon geochronology and trace element and Hf-isotope geochemistry provide evidence regarding timing of translation and genetic relation between blocks.

Intra/forearc strata (Holland Camp strata, EPM; Monos Formation, SON) contain zircon derived from local intrusions, but also have grains whose age and εHf suggest that the original arc was more extensive than what is preserved. In the backarc, zircon grains from the Union Wash Fm, east-central CA, and the arc-distal Moenkopi Fm, AZ, match age and εHf of EPM. Grains from the age-equivalent Buckskin Fm (CA, AZ), in contrast, lie outside plutonic-suite fields, to some extent overlapping the SON suite, but derived largely from an unexposed source. EMD values are not matched by grains in any of the Early Triassic sedimentary strata.

The earliest arc was likely discontinuous, although igneous suites may be hidden or were obliterated by later intrusions or tectonism. Our data do not support correlation of the SON and EPM or EMD, but do support the previous idea that movement on the California-Coahuila fault had ended by middle Permian time. The EPM was in its current location by Middle Triassic time and fed detritus into backarc sedimentary basins. EMD magmatism either was only plutonic and not exhumed by Early Permain time, or was topographically restricted from backarc dispersal.