Consequently, such reset isochrons may date large-scale events such as regional thrusting and metamorphism. 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. Phengite is the K-bearing product of the breakdown of biotite and K-feldspar. 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. The rocks range in composition from rhyodacite tuffs to andesite lavas. The data demonstrate that whole-rock RbSr ages of volcanic rocks, even lava flows with SiO2 content as low as 57 wt.%, are susceptible to complete resetting. The same rocks have also been dated by the UThPb method on zircon, giving a crystallization age of 166 ? 11 Ma, over twice as old as the RbSr age.

We report a nine-point RbSr whole-rock isochron age of 70?3 Ma (MSWD 3.97) for Mid-Jurassic volcanic rocks.

Resetting of RbSr ages of volcanic rocks by low-grade burial metamorphismĪsmeroma, Y.

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