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.

In the Peninsular Ranges batholith of southern California, a central belt of Jurassic metagranites was intruded by a Cretaceous magmatic arc that migrated from west to east across the belt. The Cretaceous batholith has been divided into western and eastern zones, zones that correspond to age, lithologic, geochemical, and geophysical zonations. In this study, density and magnetic susceptibility measurements performed on ~960 hand samples show that, in the eastern zone of the Peninsular Ranges batholith, values of magnetic susceptibility are uniformly low (<0.5 × 10 −3 cgs [centimeter-gram-second] units), while density values are in general lower and have less scatter than those in the western zone. A relatively sharp break between western and eastern zones indicates the existence of two crustal types separated by a tectonic suture: on the west, oceanic crust (mainly Mesozoic and older mantle and mantle-derived rocks) and on the east, continental crust (Neoproterozoic, Paleozoic, and early Mesozoic rocks). Previous studies in the San Diego County segment of the Peninsular Ranges batholith revealed petrologic distinctions between two Jurassic metagranite suites (S-type and transitional I-S type) and nine Cretaceous granite suites (exclusively I-type). The results of electron microprobe (EM) analyses of mafic minerals from Jurassic and Cretaceous plutonic rocks in general confirm plutonic suite subdivision. On biotite and hornblende variation diagrams, Early Cretaceous plutons tend to plot in distinct fields/trends that are characteristic of their various plutonic suites. Hornblende from three Early Cretaceous tonalite suites is Mg enriched, as expected from melts of mafic-intermediate composition that were H 2 O rich and contained hornblende as an early-crystallizing phase. Hornblende from gabbro plutons (Cuyamaca Gabbro) shows the greatest Mg enrichment for a given whole-rock SiO 2 value, reflecting cumulate processes in the evolution of gabbroic magmas. Biotite and hornblende from highly evolved leucomonzogranite-leucogranodiorite plutons assigned to three leucogranite suites have the most Fe- and Mn-rich compositions. Hornblende compositions of two Late Cretaceous tonalite suites overlap those of the Early Cretaceous tonalite suites, but, in general, Late Cretaceous hornblende does not show the extreme fractionation shown by hornblende of Early Cretaceous suites with similar SiO 2 contents. Biotite of two Jurassic plutonic suites has the most aluminous compositions of all Peninsular Ranges batholith suites, with biotite of the S-type Harper Creek suite markedly more Al rich than that of the transitional I-S–type Cuyamaca Reservoir suite. Complete overlap of Harper Creek biotite compositions with those of metasedimentary rocks of the Triassic–Jurassic Julian Schist indicates that partial melting of the latter was an appropriate source for Harper Creek magma. The existence of two Cuyamaca Reservoir biotite trends suggests that its parental magma originated by fractionation and contamination of an I-type magma by aluminous metasedimentary material, thus producing transitional I-S characteristics. All but one sample of hornblende from the Cuyamaca Reservoir suite falls in the subaluminous compositional range.

Forty kilometers south of Tecate, Baja California Norte, is an extensive area of pre–medial Cretaceous, greenschist-facies flysch, poorly exposed, and of unknown thickness. These rocks are lithocorrelative with the Triassic(?) French Valley and Julian Schist Formations and the Upper Triassic-Middle Jurassic Bedford Canyon Formation, north of the international border. The flysch type strata of the area consist of tabular, thin- to thick-bedded subarkosic metasandstone interbedded with metapelite. Isotopic studies on detrital zircon suggest a mixed population of discordant Middle to Late Proterozoic zircon and late Paleozoic and Triassic zircon. The flysch strata, herein named the Rancho Vallecitos Formation, is divided into a predominantly sandstone lithofacies and a predominantly shale lithofacies. Vertical sequence patterns and associations of sedimentary structures (graded bedding, outsized clasts, meniscate and diffuse laminae, etc.) indicate that the sandstones of both lithofacies were deposited by high-density turbidity currents. Rare ripple-laminated contourites of the shale lithofacies indicate deposition by northerly and southerly oriented paleocurrents. Vertical sequence patterns of sandstone beds in the sandy lithofacies suggest deposition in the outer part of a submarine fan-like system. Features of the predominantly shale lithofacies suggest a basin-plain depositional environment. Subordinate pebbly mudstone and local olistostrome/mélange deposits of the shaly lithofacies indicate proximity to slope areas. The overall fine-grained, poorly sorted, and matrix-rich character of all the sandstones suggests deposition in a large submarine fan or cone system of unrestricted open ocean basins, characteristically fed by large river/delta systems. Modal analyses of the sandstone framework indicates sediment sources of recycled orogens and possibly craton interior.