STRUCTURE OF THE EOCENE GOLDEN HORN BATHOLITH, NORTH CASCADES, WASHINGTON

The 48 Ma Golden Horn batholith is a ~310 km², shallow intrusion in the crystalline core of the North Cascades of Washington. It is the only large granite body in an orogen dominated by 96-45 Ma tonalite, and may have intruded during ridge subduction. Alkaline and calc-alkaline granites dominate, and include a hypersolvus biotite granite, a two-feldspar biotite granite, and a sodic amphibole-bearing alkaline granite containing large miarolitic cavities. Internal contacts are moderately steep and sharp between the alkaline and hypersolvus granite, whereas the latter grades into the two-feldspar granite. Diorite forms a narrow, discontinuous belt along the SW margin, and the NW end of the intrusion is granodiorite and tonalite. The alkaline granites are inferred to be structurally highest and the NW end may represent the structurally deepest rocks, supporting the interpretation that the batholith is tilted NW-side-up and is more mafic with depth. Schlieren and enclaves are rare in the alkaline and hypersolvus granites, but are progressively more common in the two feldspar and relatively mafic rocks. The batholith displays a weak to moderately developed magmatic foliation, and a weaker lineation. It has a dominant NW-striking, typically SW dipping fabric and a weaker NE-striking fabric with both NW and SE dips, which is restricted to the center of the intrusion. Dips are moderate to steep. Foliations near the NW contact are coupled with the adjacent host rock, apparently recording regional strain. The weaker interior fabric probably reflects internal magmatic processes. Golden Horn dikes (n=61) intruding host rock are felsic and less commonly mafic, are typically ~1-m wide, but range up to 20 m. These steep dikes strike NW and E-W in roughly equal amounts. Their orientations may reflect regional transtension. Contacts of the batholith are typically steep. To the NW, the contact is a mixed zone, with Golden Horn dikes extending up to 100 m into the slightly older and deeper(?) Ruby Creek plutonic complex. At the southern contact, the ~90 Ma Black Peak pluton is highly fractured and incorporated into the Golden Horn rock as 10s-of-m-scale stoped blocks. The diking, stoping and lack of a ductile aureole are compatible with the shallow emplacement levels.