STRUCTURE AND STRATIGRAPHY OF THE MAY LAKE INTERPLUTON SCREEN, YOSEMITE NATIONAL PARK, CALIFORNIA

Plutons in the Sierra Nevada batholith commonly are separated by thin screens of metamorphic rock. These interpluton screens (IPSs) preserve structural indicators of emplacement history and information about pre-batholithic stratigraphy and structure. The May Lake IPS, located in central Yosemite National Park, California, separates the granodiorite of Mt. Hoffman (102 Ma, preliminary U/Pb age), on the west of and above the screen, from the Half Dome granodiorite (92 Ma) of the Tuolumne intrusive series (TIS), on the east and below.

The May Lake IPS mainly comprises clean white quartzite and impure quartzite, with minor coarse-grained marble and calc-silicates. The rocks are unlike nearby strata in the White and Inyo Mountains and may correlate with the Stirling Quartzite and Johnnie Formation of southeastern California. The screen also contains abundant mafic dikes that resemble those of the Independence dike swarm. The rock assemblage thus resembles the nearby Snow Lake pendant, which has been inferred to have been transported from the Mojave Desert area by early Cretaceous dextral slip (Lahren and Schweickert, 1989, Geology).

Compositional banding and foliation in the metamorphic screen, and magmatic foliation at pluton margins, are generally parallel to pluton contacts, although compositional banding is locally folded and discontinuous. Foliation within the screen strikes ~N20E and dips ~65 to the west, and lineation trends N63W plunging 54.

The Half Dome granodiorite contains xenoliths of the screen, which may indicate stoping of the pluton’s roof or side, but no evidence for stoping is present along the granodiorite of Mt. Hoffman contact. Most mesoscopic shear-sense indicators (e.g., asymmetric boudins) indicate east side down displacement of TIS rocks relative to older rocks. Hutton and Miller (1994 GSA abstract) also found Tuolumne down displacement along the eastern margin of the TIS, indicating downward displacement of the TIS relative to all of its wall rocks. Pluton-down shearing has been widely observed and inferred variously to record diapiric return flow, postmagmatic sinking, or downward (lopolithic) inflation.