Paper No. 6
Presentation Time: 17:00-18:30
INTRUSIVE RELATIONSHIPS AND DEFORMATION OF MAFIC AND TONALITIC ROCKS IN PART OF 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 magmatic systems at different crustal depths. The 6-8 kb, ~91 Ma Seven Fingered Jack pluton (SFJ) is a markedly elongate body (~30 by 5 km), which has been previously mapped as a highly heterogeneous sheeted pluton in the NW, but as largely tonalite along strike to the SE. Detailed mapping of a ~2 km2 area in the Klone Peak area shows that the eastern margin of the southern portion of the pluton is also very heterogeneous and sheeted, and intrudes the Triassic Dumbell orthogneiss. In the study area, the SFJ pluton is composed of four units: (1) a 50-100-m-thick mafic complex, which contains xenoliths, irregularly shaped patches, and enclaves of hornblendite ranging from 2-30 cm in length, enclosed in gabbro and diorite, all of which are cut by felsic dikes; (2) a heterogeneous diorite, mapped in three 75-100-m thick sheets, with less abundant gabbro and hornblendite enclaves and dikes (3 cm-3 m wide); (3) a 100-300-m-thick diorite; and (4) a much larger internal mass of hornblende-biotite tonalite. Each unit contains internal heterogeneities, which are less common in more felsic magmas. Mingling occurs between rocks of the heterogeneous diorite and the mafic complex. Contacts of all units trend NW and are gradational. Foliations, both magmatic and solid-state, strike NNW, concordant to sheet/unit contacts, and normal to the inferred regional shortening direction. Dips range from steep to nearly vertical, and are to the SW and NE. Foliation ranges in intensity from moderate to strong in all units. Lineations in the area mainly plunge gently to moderately to the NW or SE. In summary, the eastern margin of part of the SFJ pluton is a heterogeneous, sheeted, mid-crustal pluton constructed by multiple pulses of intermediate to mafic magma that dynamically interacted.