Late Cretaceous Plutonic and Metamorphic Overprint of Proterozoic Metasediments of Ontario Ridge, Eastern San Gabriel Mountains, Ca

Evaluating the Shinumo-Sespe drainage connection: Arguments against the “old” (70–17 Ma) Grand Canyon models for Colorado Plateau drainage evolution

“…(2) Discordance is a geologically significant feature in potential source terranes of rocks considered here (e.g., Barth et al, 2009); ignoring such grains can obscure important signal. (3) Several rock units considered here have undergone medium-to high-grade metamorphism that led to significant Pb loss in zircon (Barth et al, 2009;Zylstra, 2017); using only statistically concordant zircon from these samples would disqualify an excessive number of analyses. Accordingly, we re-filtered any data sets (with one exception to be described below) that did not already adhere to the following criteria, which follow the widely used protocol of the Arizona LaserChron Center (e.g., Gehrels et al, 2006Gehrels et al, , 2008: (1) Rejection of analyses with concordance of <80% or >105%, where concordance is defined as the ratio of 206 Pb/ 238 U age to 207 Pb/ 206 Pb age.…”

“…The age and significance of the sedimentary protoliths of the Ontario Ridge rocks are not well constrained owing to metamorphism within the upper-amphibolite facies, strong deformation, and intrusion by pre-and postmetamorphic Cretaceous granitoids. Nonetheless, these rocks are considered a potential correlative of the Big Bear Group (Zylstra, 2017). The relatively weak separation of the Grenville, Picuris, and Yavapai-Mazatzal-age peaks in the Ontario Ridge quartzites is likely the result of Pb loss during metamorphism.…”