EVIDENCE FOR LOWER CRUSTAL SYNTECTONIC FORE-ARC EMPLACEMENT OF THE HICKS BUTTE PLUTON BREAKING THE CORRELATION OF HP-LT ROCKS ACROSS THE CASCADES OF WASHINGTON STATE

Similar subduction related (HP-LT) units exposed in the northwestern and southern Cascades range are correlated across the Straight Creek Fault with ~120 km of dextral offset, are interpreted to represent the same accretion event to North America in the late Jurassic to early Cretaceous. This correlation is problematic because the sequence of exhumed HP-LT rocks above LP-LT rocks shows opposite arc polarity. Establishment of an arc body as a test of polarity in the southern exposure has been difficult. The Hicks Butte pluton (~140-145 Ma) had been discounted because if its younger isotopic age than the country rock of the Shuksan schist (>160 Ma Ar/Ar on amphibole ages). Recent work has shown that near by arc related ~157 Ma Quartz Mountain stock to be arc-magmatism active during formation of blueschist in the Shuksan schist.

We present new metamorphic constrains for the P-T paths of both the Hicks Butte and blueschist component of the Shuksan schist that link these arc magmatism and exhumation processes. Hicks Butte gabbro-diorite contains symplectites of Opx + Hbl +/- Spinel (hercynite) + Mt on igneous Ol + Pl, an assemblage previously shown to be caused by a rapid cooling from 1100˚C to around 700-800˚C at pressures between 6-8 kbars (Claeson, 1998). At the pluton margin, this assemblage is mylonitic suggesting syntectonic emplacement. Shuksan blueschist sampled 150 m down section from the contact between pluton displays fabrics co-planar to the mylonite of Act + Ep + Ab at the expense of Gln + Ep + Pl, interpreted to be a greenschist overprint on a Ep-blueschist. PERPLEX modeling with this bulk composition shows the P-T window for the greenschist assemblage is ~ 550˚C at pressures between 5-7 kbars. This sample also contains minor inclusions of omphacite in albite, which could only have occurred above 8 kbars, but below 400˚C for this bulk composition confirming residence in the subduction channel.

The congruence and pervasiveness of ductile fabrics between these two radically different P-T paths meet at the same conditions. This suggests that the Hicks Butte was emplaced into the Shuksan schist under lower crustal conditions during exhumation of the Shuksan during subduction processing, possibly near the subduction surface. This establishes the contemporaneous arc with an opposite polarity to correlated units to the north.