HOW DO PLUTONIC AGES ALIGN WITH AGES OF OROGENY: A CASE STUDY OF ZIRCON AGES FROM A KLAMATH RIVER SAND SAMPLE FROM NORTHERNMOST CALIFORNIA (Invited Presentation)

Zircon Pb/U ages are the most commonly cited for the crystallization age of a plutonic rock. The mineral zircon, because of its physical and chemical robustness, outlasts other phases and accumulates in clastic sediments. The Klamath River’s path runs almost entirely through the Klamath Mountain Province which consists of Mesozoic plutonic rocks intruded into variably metamorphosed Mesozoic and older rocks. At its headwaters, the river flows over the Cascade Province (relatively zircon poor), and near its mouth it cuts through the Coast Ranges Province where the sand was sampled by Cal Barnes in 2002. One hundred newly sampled grains (~200 analyses) are compared to the original 2002 dataset of 100 points on 100 grains. There are some subtle but important differences of interpretation from having more data; accuracy of the data sets is similar. The time required for data acquisition has shrunk dramatically. Main improvements are stacking of sample imaging (CL, plane light, etc.), more sensitivity so more ability to sample trace elements as well as the few required for Pb/U dating, and the constraints from sampling more grains. A new insight is the impact of sector zoning on zircon trace elements content and their interpretation.

The Nevadan Orogeny is a compressional event that divides Klamath geological history. In situ plutonic rocks of different petrologies have been divided into pre- and post-orogenic, and potentially syn-orogenic. The new Klamath River data give concordia ages ranging from 175 to 134.4 Ma with individual point in-run uncertainty of about 3.5 Ma (propagated about twice in-run uncertainty) and one grain gives an age of 96.6+/-2 Ma; it was struck 3 times. Ignoring the young (Coastal Province?) outlier, the rest cluster into 3 groups: 174.4-165.7, 162.9-151 and 145.4 to 132.7 Ma. There are no data in the 152-146 Ma period commonly assigned to the Nevadan Orogeny. This is consistent with an age gap detected in the 2002 dataset. Using the data gap as an age divider, post-Nevadan rocks on average are less continental (more oceanic) in Yb vs U composition and are more oxidized using the hydrobarometer of Loucks et al. (2020) than are pre-Nevadan rocks.

Loucks, R.R., Fiorentini, M.L. and Henríques, G. (2020) New magmatic oxybarometer using trace elements in zircon, Journal of Petrology 61, egaa034.