PLUTON ROOFS IN ARCS AND EMPLACEMENT MECHANISMS: INSIGHTS FROM THE BLACK PEAK INTRUSIVE COMPLEX, NORTH CASCADES

Pluton roofs and continuous roof-wall transitions provide important insights into magma emplacement, but are not widely described in Cordilleran arcs. We studied the well-exposed roof of the Black Peak intrusive complex (BPIC), a voluminous mid- to upper-crustal pluton in the Cretaceous North Cascades arc. The BPIC is exposed over ~1.3 km of topographic relief and preserves the abrupt (10s of m) transition from its steep eastern wall to the gently dipping roof. The roof consists of medium-grade, schists, quartzites, and amphibolites, and is intruded by the oldest and most mafic rocks of the complex, the 91.8 to 91.2 Ma Crescent Mountain phase, and the wall is marked by this unit and the 90.3-89.7 Ma eastern Stiletto Mtn. phase. These phases intrude a steep host rock contact between the metamorphic tectonites and weakly metamorphosed clastic rocks.

The BPIC is more heterogeneous near the extensively altered and mineralized roof than elsewhere. Texturally variable diorite, local tonalite, and minor hornblendite are cut by porphyritic diorite and tonalite bodies, at least some of which are coeval with the Stiletto Mtn. phase. Sheets and irregularly shaped porphyritic bodies extensively intrude the roof rocks for distances of up to 500 m upward from the roof contact. In some 100s of m² areas, intrusive rocks are more abundant than host rocks, which form xenoliths and screens. Xenoliths are generally rare within the BPIC, but are present within 500 m of the roof, ranging from <1 cm to 10s of m wide, and in a much narrower zone next to the wall. Foliation in the roof host rocks is mostly discordant to the pluton contact and in the wall foliation is generally concordant to the contact, and where the clastic rocks are in contact with the pluton, bedding is deflected in a narrow (<100-m wide) aureole.

The absence of syn-emplacement faults in the roof and along the wall, and the discordance of foliation and lack of extension of the roof rocks argue against significant lateral dilation during intrusion. Stoping of the roof was important, and some magma wedging likely occurred, whereas limited ductile flow took place next to the wall. These observations support vertical transport of host rocks, as inferred from a number of other roofs. Many other roof contacts, however, are much sharper and less extensively injected by magmatic sheets.