The early Mesozoic Julian Schist, Julian, California

The formation name Julian Schist is applied to prebatholithic metasedimentary wall rocks of the Peninsular Ranges batholith of central and eastern San Diego County. This study focuses on the type area of the Julian Schist located approximately 64 km east-northeast of San Diego, near Julian, California. Field mapping and petrographic analysis distinguish several types of andalusite-and sillimanite-bearing pelitic schist and gneiss and a sequence of calc-silicate-bearing feldspathic metasandstones. These rocks have a strong foliation that generally strikes northwest. The axes of tightly appressed isoclinal folds, boudins, and elongate minerals plunge steeply. Relict bedding and other sedimentary structures, such as graded laminations, channel scours, and rip-up clasts, preserved in a section of interbedded quartz-mica schist and feldspathic metasandstone, indicate that the protolith was a sequence of turbidites. Several turbidite facies are recognized and suggest deposition in the middle- to outer-fan lobes and fringe of a submarine fan. In the early Mesozoic these rocks were part of a large clastic wedge shed from the craton in the east into a marginal basin. The wedge contains shallow deltaic, turbidite-fan, and basin-plain deposits. These rocks were deformed and metamorphosed during mid- to late Mesozoic subduction, island-arc collision, and emplacement of the Peninsular Ranges batholith.