|Title:||Jurassic peraluminous gneissic granites in the axial zone of the Peninsular Ranges, southern California|
|Authors:||S.E. Shaw, V.R. Todd, and M. Grove|
|Publication:||Geol. Soc. Amer. Spec. Pap., v. 374, p. 157‐183.|
The Peninsular Ranges batholith of southern California and Baja California, México, is well recognized as a prime example of an I‐type Cretaceous batholith. Often overlooked, however, is a volumetrically significant amount of pre‐Cretaceous gneissic granite in the axial zone of the batholith. New U‐Pb zircon age data confirm that the metaluminous and peraluminous plutonic bodies were emplaced during the middle Jurassic. Also reported in this paper is a Jurassic U‐Pb age for a metaluminous (I‐type) tonalite‐quartz diorite pluton that is spatially related to the peraluminous suites. This result suggests that other unrecognized Jurassic I‐type plutons may also be present in the batholith. Within San Diego County, Todd and Shaw (1985) recognized and mapped two suites of strongly deformed gneissic granites and migmatites. One is peraluminous (Harper Creek suite) while the other is transitional between metaluminous and peraluminous (Cuyamaca Reservoir suite). These rocks bear a striking resemblance to deformed and, in places, migmatitic, peraluminous (S‐type) examples from the Lachlan fold belt and New England batholith of eastern Australia. The gneissic granite suites are known to extend north along the axial zone of the Peninsular Ranges batholith and cover an area at least 45 km wide by 150 km long. To the south, rocks of similar type are known to extend into Baja California, México, for at least 300 km. Chemical and isotopic studies of these Jurassic suites confirm that they meet the criteria necessary to define them as S‐type and transitional I‐ to S‐type, respectively. However, unlike the majority of Lachlan fold belt S‐type granites that are high level and often associated with their volcanic equivalents, the Peninsular Ranges batholith suites were emplaced at much deeper levels, possibly as much as 11‐16 km. The Harper Creek suite, of S‐type gneissic granodiorite‐tonalite plutons and associated Stephenson Peak migmatitic schist and gneiss facies, is strongly peraluminous and contains biotite, cordierite, sillimanite, abundant graphite, and ilmenite. It has elevated δ18O up to +20 per mil, initial 87Sr/86Sr ratios (Sri) to 0.713, a high aluminum saturation index, and Na2O/K2O ratios that overlap those of the Lachlan fold belt S‐type granites. The Cuyamaca Reservoir suite contains gneissic granodioritetonalite plutons, transitional between metaluminous and moderately peraluminous (I‐ to S‐type), containing reduced biotite, subaluminous amphibole, orthopyroxene, titanite, and ilmenite. It has values of δ18O and Sri greater than the Cretaceous I‐type granites but less than the Harper Creek suite. Leucosome melt phase accumulation from the Julian Schist diatexites to produce a restite‐rich magma is considered the most likely origin for the Harper Creek suite. For the Cuyamaca Reservoir suite, possible source components include young mantle‐derived magma, metaigneous and metasedimentary rocks formed in an arc environment, and evolved basinal fill metasedimentary rocks. The evolved metasedimentary component may be of Julian type. The Harper Creek and Cuyamaca Reservoir suites comprise deformed, steepwalled, north‐northwest‐trending bodies up to 20 km long and with length‐to‐width ratios of 4:1. Textures range from strongly foliated to gneissic or mylonitic. Internal foliation that strikes parallel to the long dimension of the bodies and dips steeply to the east is defined by alignment of relict magmatic feldspar and quartz grains and recrystallized aggregates of quartz and biotite. The concordance of magmatic and subsolidus foliations in the Jurassic plutons and the continuity of these structures with regionally developed metamorphic fabrics in their wallrocks indicate that magmatic foliation was overprinted by high‐temperature, post‐magmatic solid‐state foliation. Foliation in the Harper Creek and Cuyamaca Reservoir suites is concordant with the axial planes of outcrop‐scale isoclinal folds, and map patterns suggest that the plutons underwent regional‐scale isoclinal folding. Their fabric probably formed during multiple episodes of synintrusive deformation that began at least by the Late Jurassic and culminated by the middle Cretaceous.