SYNTECTONIC EMPLACEMENT OF THE WHITE CHIEF PLUTON, MINERAL KING PENDANT, SIERRA NEVADA,CALIFORNIA: IMPLICATIONS FOR EARLY CRETACEOUS DEFORMATION

The Mineral King pendant is a 10-km-long screen of NW-striking, steeply dipping Mz metasedimentary and metavolcanic rocks that separates granitic plutons emplaced at ~98 Ma. Rocks are highly deformed with NW-striking foliation and subvertical stretching lineation. Deformation history is not well understood but likely involves several phases of contractional deformation. It has been suggested that the dominant deformation was vertical stretching after 104 Ma, possibly as a result of return flow associated with emplacement of the adjacent plutons.

The White Chief pluton is a granitic body that intruded the SW edge of the pendant at ~135 Ma and thereafter behaved as part of the pendant. The pluton shows a moderate, NW-striking foliation with a similar component of vertical deformation to the rest of the pendant. The foliation is cross-cut by plastically deformed zones that range from millimeter thick zones evenly distributed throughout the rock to decimeter thick bands spaced tens of meters apart. Zones of plastic deformation trend NW and are parallel to foliation in the pluton. These zones both crosscut and are crosscut by aplite dikes in the pluton. In thin section, undeformed grains tend to be elongate in a NW direction, only grain boundaries show evidence of plastic deformation, and Albite and Carlsbad-style twinning in feldspar grains shows a persistent, foliation-parallel extinction. Stringers of quartz in plastically deformed zones have been thermally reset into distinct sub-grains. The inconsistent cross-cutting relationship with late-stage dikes and lack of solid-state deformation suggest that plastic deformation occurred while it was still relatively hot. This is supported by recrystallization of quartz stringers, which also suggests a relatively high temperature persisting after deformation. The aligned axes in feldspar grains suggest that the grains were under stress while cooling and that their elongation and alignment comes as a result of that stress rather than later deformation. The data suggest a syntectonic emplacement of the White Chief Pluton and thus a significant episode of contractional deformation at ~135 Ma. Thus the Mineral King pendant experienced at least two, separate, and parallel phases of deformation that came ~ 31 Ma apart rather than a single phase of deformation at ~104 Ma.