STRUCTURE AND CONSTRUCTION OF THE EL CAPITAN GRANITE AND YOSEMITE CREEK GRANODIORITE IN YOSEMITE NATIONAL PARK

Recent research in the central Sierra Nevada batholith (CSNB) has led to different interpretations about pluton structure, size of magma chambers and number of pulses constructing plutons. To evaluate these issues, research in an 80 km² area in NW Yosemite National Park focused on two Cretaceous plutons, the ~102 Ma El Capitan Granite (Kec) and the 96-97 Ma Yosemite Creek Granodiorite (Ky), two plutons for which little is known about the structure and emplacement. The Kec is a major unit of the CSNB, covering ~500 km², and was probably much larger before removal by younger magmatism. In the study area, two major facies of Kec are observed; medium-grained granite intruded coarse-grained, K-feldspar phyric granite. Contacts between the facies range from sharp to gradational with minor mingling. The medium-grained rocks form two smaller, more texturally heterogeneous bodies than the coarse-grained Kec. Diorites have also locally mingled with the coarse-grained granite. Ky sharply intrudes the Kec, forming elongate (~4 km x 1 km) bodies of granodiorite with abundant enclaves and thin (~30 m) NE-trending sheets of hornblende diorite extending up to 4 km. The units appear homogenous, locally displaying mingling along contacts. Ky internal contacts range from sharp to gradational. The field relations (internal contacts, overall homogeneity) indicate construction of the plutons by multiple pulses, but perhaps not a large number (<10?), which were intruded close in time.

Magmatic foliation is dominantly contact-parallel, NE striking and steep. A second enigmatic, steep foliation formed locally in the Kec and strikes roughly E-W. Lineations plunge relatively steeply (>50º). Steeply E-dipping, reverse-slip ductile shear zones occur in a 1-km-wide, NE-striking corridor that extends >7 km in the Kec. Recrystallized feldspars in these zones indicate deformation at >450° C. These structures likely formed during or shortly after emplacement given the high temperatures and their absence in adjacent younger units and may represent a previously unrecognized regional zone of strong NW-SE shortening.