STRUCTURE AND CONSTRUCTION OF THE HIGHLY ELONGATE, CRETACEOUS SEVEN-FINGERED JACK PLUTON IN THE DEVIL'S SMOKESTACK AREA, NORTH CASCADES, WASHINGTON

Within the Chelan block of the North Cascades crystalline core, a 20-25-km-wide region of 92-71 Ma, markedly elongate, and commonly internally sheeted plutons intrude amphibolite-facies host rocks. This study focuses on the structure of one of these elongate (50 km by 7 km) plutons, the ~92-90 Ma, mid-crustal (20-25 km) Seven-Fingered Jack (SFJ) pluton. Previous work suggests that large parts of the pluton are relatively homogeneous. Detailed mapping of three ~ 0.5-1.0-km² domains at a scale of 1:10,000 concentrated on internal heterogeneities, internal contacts, and magmatic fabric patterns with the goal of evaluating whether the apparently homogeneous rocks were constructed by multiple bodies with subtle contacts.

The pluton is dominantly tonalite, which is relatively homogeneous, particularly in the central and southern domains. Host rock xenoliths or rafts are rare. Schlieren and other layering are only locally developed. Schlieren are typically several meters long, thin (cm-scale), wispy, and biotite-rich. A ~15-cm wide zone of alternating mafic and felsic layers, which extends for ~10-15 m, may represent partially digested xenoliths or mingling between separate injections of melt. Textural and modal variations, although subtle, are gradational over several centimeters across internal contacts (traceable ≤ 10 m) and gradational at larger scales (10s to 100s of m). These contacts, schlieren, and the layered zone strike WNW and dip steeply to the ENE. Enclaves are abundant and were divided in the field into five types based on texture and color index. A ~5x5-m grid was used at each station to estimate enclave abundance, which ranged from 0 to 25 enclaves. Enclaves are elongate parallel to well-developed magmatic foliation, defined by hornblende and biotite; foliation strikes NW and has moderate to steep dips. Foliation orientations are less uniform in the northern domain, reflecting steeply plunging magmatic folds with 10s of m wavelengths.

These field results imply that the SFJ pluton was constructed by thin (< 1 m) and thicker (up to 500 m?) steeply dipping sheets, which strike parallel to the margins of the pluton and amalgamated into km-scale bodies with subtle internal contacts.