REVISED METAMORPHIC HISTORY OF THE SAN JUAN ISLANDS

Blueschist facies metamorphism in rocks of the San Juan Island thrust system is currently regarded as having formed during mid-Cretaceous thrust emplacement of the San Juan Island nappes (Brandon and others, 1988; Maekawa and Brown, 1991). Age constraints on the thrusting event are provided by 100 Ma fossils in rocks involved in thrusting and 84 Ma fossils in post-thrust sedimentary rocks. The metamorphic age has never been directly dated but has been inferred to be coeval with thrusting based largely on relations of the blueschist facies minerals to tectonite fabric. We have obtained Ar-Ar ages of 125 +/- 1 Ma on phengites from two samples of metamorphic tectonite occurring at Richardson on Lopez Island. The phengites (100 X 500 microns) are well aligned in foliation. They have crystallized directly from volcanic glass in pillow breccia, and thus are not detrital. Accompanying metamorphic minerals are chlorite, pumpellyite, and aragonite. The volcanic rocks hosting the phengites are pillow breccias and tuff, overall only weakly recrystallized. The foliation in which the phengites are aligned is pervasive throughout the volcanic rocks in the Richardson area. In apparent conflict with the Ar data, the Richardson outcrop contains Albian (100 Ma) fossils previously inferred to mark the maximum age limit of thrusting and metamorphism. This outcrop is the only known occurrence in the SJI of the 100Ma fossils. The fossil-bearing rock is a brown, fecal pellet-rich, weakly foliated mudstone containing forams. The rock lacks blueschist facies indicator minerals and occurs as a fault-bound sedimentary lens in the metamorphosed volcanic section. From these features we suggest that the Albian fossils do not constrain the age of SJI metamorphism. The blueschist metamorphism at Richardson at 125 Ma lies within the age range of Shuksan blueschist metamorphism (120-164 Ma) in the northwestern Cascades. Mica Ar ages reported by Lamb (2001) for the eastern San Juan Islands are 137 and 154 Ma, also in the Shuksan age range. These findings are interpreted to indicate that the regional incipient blueschist facies metamorphism in the San Juan Islands and the associated pervasive fabric are not related to the mid-Cretaceous thrust emplacement event, but like the Shuksan terrane formed elsewhere (in a subduction zone) at an earlier time.