RESETTING OF RB-SR AGES OF VOLCANIC-ROCKS BY LOW-GRADE BURIAL METAMORPHISM

We report a nine-point Rb-Sr whole-rock isochron age of 70 +/- 3 Ma (MSWD 3.97) for Mid-Jurassic volcanic rocks. The same rocks have also been dated by the U-Th-Pb method on zircon, giving a crystallization age of 166 +/- 11 Ma, over twice as old as the Rb-Sr age. The data demonstrate that whole-rock Rb-Sr ages of volcanic rocks, even lava flows with SiO2 content as low as 57 wt.%, are susceptible to complete resetting. The rocks range in composition from rhyodacite tuffs to andesite lavas. The complete breakdown of all major minerals that contain Rb and Sr resulted in an alteration mineral assemblage consisting of phengite, albite, secondary quartz, and minor amounts of chlorite and epidote. Phengite is the K-bearing product of the breakdown of biotite and K-feldspar. Pressure during low-grade metamorphism of the volcanic rocks, estimated from phengite composition to have been in the range of 4 to 6 kbar, points to thrust-related burial as the main cause of resetting. Consequently, such reset isochrons may date large-scale events such as regional thrusting and metamorphism. The coherent resetting of the Rb-Sr isochron suggests large-scale pervasive fluid movement during thrust-related burial metamorphism.