CONSTRUCTION AND RELATIONSHIPS OF MAFIC AND TONALITIC ROCKS IN THE SEVEN FINGERED JACK PLUTON, NORTH CASCADES, WASHINGTON

The crystalline core of the North Cascades preserves a crustal section through a Cretaceous continental magmatic arc and provides an excellent opportunity for study of a mid-to-deep-magmatic system. One pluton in this section, the 6-8 kb, Seven Fingered Jack pluton is an elongate body (~50 by 7 km), which has been previously mapped as a highly heterogeneous sheeted pluton in the NW, but largely as tonalite along strike to the SE. Previous U/Pb zircon geochronology on tonalite from the NW tip and pluton interior to the SE yielded ages of ~91 Ma, whereas mafic rocks in the NE margin are ~79 Ma. Detailed mapping was carried out on a southern part of the pluton in the Klone Peak area, which extends across strike from the margin SW into the pluton interior. Here, the eastern margin is very heterogeneous and sheeted, and the pluton is composed of: (1) a mafic complex mapped in three 50-600-m-thick sheets, which contain hornblendite xenoliths, patches, and enclaves enclosed in gabbro and diorite; (2) a heterogeneous diorite forming four 75-200-m-thick sheets, with less abundant gabbro and hornblendite enclaves and dikes; (3) a diorite mapped in two 100-700-m-thick sheets; and (4) a quartz diorite in two ~100 m-wide masses. To the SW of the ~1.5 km-wide mafic marginal units is a much larger (> 3 km wide) internal mass of relatively homogeneous hornblende-biotite tonalite. The tonalite contains abundant mafic and local felsic enclaves, minor xenoliths, and meter-wide felsic sheets. Two zones, 50-200 m wide, have abundant heterogeneities, which include concentrations of enclaves, hornblendite xenoliths, felsic sheets (5 cm-1 m wide), and schlieren, and most likely represent internal contacts between different tonalite bodies. The mafic units are on strike with the dated mafic rocks ~20 km to the NW, whereas the relatively homogeneous tonalite is probably part of the > 25 km-long interior belt of ~91 Ma tonalite. The mafic rocks intruded during a short-lived pulse of mafic magmatism in the area that includes the ~78 Ma Riddle Peak gabbro and mafic tip of the Cardinal Peak pluton. In summary, the composite mid-crustal Seven Fingered Jack pluton contains an internal mass of homogeneous tonalite and an eastern margin of younger heterogeneous mafic rocks that were constructed by multiple pulses of magma that dynamically interacted.