Batholithic rocks of Southern California: A model for the petrochemical nature of their source materials

Abstract

Major-element analyses of 497 composite samples of batholithic rocks (quartz diorites, granodiorites, and quartz monzonites) from the northern Peninsular Ranges and Transverse Ranges Provinces, southern California, form the basis for a mixing model that accounts for most of the compositional variation in the rocks. The compositional structure in the batholithic rocks as a group was found to be similar to that in the Sierra Nevada batholith, and indicates that four end members are sufficient to account for 85-97 percent of the variability in each of the eight major oxides. According to the model, the batholithic rocks formed from the mixing of basaltic and quartzofeldspathic end-member magmas, and the removal of variable proportions of plagioclase and mafic minerals, principally hornblende. Gabbroic rocks, common only in the western part of the region, could have formed from nearly uncontaminated magmas of the basaltic end member. Variations in the mixtures of basaltic and quartzofeldspathic magmas are presumed to reflect variations in their source materials at depth. According to the model, the compositions of the source materials do not vary smoothly over the region, but display a discontinuity along a line approximately coincident with the present San Jacinto fault zone. The discontinuity is roughly coincident with previously noted petrologic and isotopic discontinuities in the northern Peninsular Ranges and is interpreted as the western limit of significant contribution of continental materials to the batholithic rocks. The model gives no evidence of a discontinuity in the vicinity of the San Andreas fault zone. 'Average of eight analyses from Larsen and Draisin (1950, p. 69). 2Analysis 7 from Larsen and Draisin (1950, p. 71). ' See figure 3 for sampling localities and Appendix 1 for the analyses.

Geophysical evidence for wedging in the San Gorgonio Pass structural knot, southern San Andreas fault zone, southern California

“…Peninsular Ranges-type basement can be divided into a relatively mafi c western and a felsic eastern part characterized by lithologic differences in both prebatholithic and batholithic rocks (Silver et al, 1979;Baird and Miesch, 1984;Silver and Chappell, 1987;Todd et al, 1988). San Gabriel Mountainstype basement is separated into two units by the region-wide Vincent thrust, which places various upper-plate Mesozoic plutons and prebatholithic rocks on top of lower-plate Pelona Schist.…”

Geology and geochronology of granitic batholith complex, Sinaloa, México: Implications for Cordilleran magmatism and tectonics

“…1) may be its most prominent feature and the most permanent record of plate convergence and related magmatism. Batholiths of the Sierra Nevada and Peninsular Ranges of California and northern Baja California have been extensively studied (e.g., Evernden and Kistler, 1970;Gastil, 1975;Gastil et al, 1975;Krummenacher et al, 1975;Silver et al, 1979;Walawender and Smith, 1980;Baird and Miesch, 1984;Silver and Chappell, 1988;Todd et al, 1988;Walawender et al, 1990Walawender et al, , 1991Bateman, 1992;Ortega-Rivera et al, 1997;Johnson et al, 1999aJohnson et al, , 1999bTate et al, 1999;Kimbrough et al, 2001; Ortega-Rivera, this volume, Chapter 11), but little has been done on the batholiths of mainland México (Anderson and Silver, 1974;Damon et al, 1983a, b;McDowell et al, 2001). Indeed, as with many aspects of Mexican geology that are critical to a full understanding of the geology and evolution of North America, there seems to be little recognition even of their existence.…”

Controls on orogenesis along an ocean-continent margin transition in the Jura-Cretaceous Peninsular Ranges batholith

“…Across-strike variation in pluton chemistry, including major element (Baird et al, 1974;Baird and Miesch, 1984), and Figure 4. Map of Peninsular Ranges batholith showing tectonostratigraphic assemblages, "gabbro" and "tonalite" belts west and east of gabbro/tonalite line respectively ("granodiorite-granite" belt lies to east in Sonora), the U-Pb age step between western and eastern zones, the La Posta pluton (largest recognized pluton in the batholith), and K-Ar and 40 Ar/ 39 Ar biotite apparent age isopleths in the Peninsular Ranges Batholith.…”