Jurassic peraluminous gneissic granites in axial zone of Peninsular Ranges, southern California

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 signiÞ cant amount of pre-Cretaceous gneissic granite in the axial zone of the batholith. New U-Pb zircon age data conÞ rm 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 conÞ rm that they meet the criteria necessary to deÞ ne them as S-type and transitional Ito 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 δO up to +20 per mil, initial Sr/Sr ratios (Sr i ) to 0.713, a high aluminum saturation index, and Na 2 O/K 2 O 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 (Ito S-type), containing reduced biotite, subaluminous amphibole, orthopyroxene, titanite Shaw, S.E., Todd, V.R., and Grove, M., 2003, Jurassic peraluminous gneissic granites in the axial zone of the Peninsular Ranges, southern California, in Johnson, S.E., Paterson, S.R., Fletcher, J.M., Girty, G.H., Kimbrough, D.L., and Martin-Barajas, A., eds., Tectonic evolution of northwestern México and the southwestern USA: Boulder, Colorado, Geological Society of America Special Paper 374, p. 000–000. For permission to copy, contact [email protected]. © 2003 Geological Society of America. 2 S.E. Shaw, V.R. Todd, and M. Grove spe374-07 page 2 of 27