Magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of California: Effects of assimilation, crustal thickness, and depth of crystallization

In order to better understand geologic fac-tors controlling pronounced regional variations in whole-rock chemistry, mineralogy, and mineral chemistry in the batholiths of California, we calculate the magmatic intensive variables f_HF/f_H2O, f_HF/f_HCl, and f_O2.Regional-scale west-to-east increases in F/OH in mafic silicates, corresponding to the systematic I-WC to I-MC and I-SC progression, reflect orders-of-magnitude increase in f_HF/f_H2O attending pluton crystallization. Low f_HF/ f_H2Oof formation of western I-WC types is consistent with their derivation from low-fluorine source rocks in subducted oceanic slabs or the upper mantle. In contrast,higher f_HF/f_H2Oof crystallization of I-MC and I-SC types to the east implies the involvement of (1) progressively greater amounts of continental crustal source material such as biotite-bearing metamorphic rocks, their unweathered sedimentary derivatives retaining F-rich mafic minerals, or their fusion products and/or (2) source materials which become more F-rich toward the continental interior. The regional distribution of I-MC and I-SC types suggests that the Precambrian craton of western North America, or derivative sediments, may extend farther north in California and be morphologically more complex than previously thought. From seemingly out-of- place occurrences of I-WC plutons on the eastern slopes of the Sierra Nevada batholith, we infer the existence of regions where Precambrian basement was thin or absent in Mesozoic time, which prevented extensive cratonal contamination of subducted slab or upper mantle-derived magmas.

New methods for estimating T-f_O2 relations in the magmas demonstrate that I-SCR granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of I-WC, I-MC, and I-SC types under conditions at or below the maximum stability limit of graphite in equilibrium with a C-O-H-S gas phase. The local-scale formation of I-SCR granites in plutonic belts within specific wall-rock terranes containing highly reducing sediments or metasediments may occur by contamination of I-types with graphitic pelite or, in some cases, by the direct fusion of this reducing pelitic wall rock. The spatial distribution of I-SCR granite provinces therefore is controlled simply by wall-rock lithology.

Amphibole geobarometry demonstrates a general west-to-east decrease in crystallization pressure across the Sierra Nevada batholith. In contrast, the Peninsular Ranges batholith displays a west-to-east crystallization pressure increase. The bulk of the California batholiths crystallized at pressures less than 4-5 kb and depths less than about 15-19 km. In the southern Sierra Nevada batholith, the San Gabriels, and the eastern Peninsular Ranges, however, plutons crystallized at pressures exceeding 6 kb at deep crustal levels (>23 km). Reconstruction of the pre-erosion top of the batholith shows that in an east-west cross section, the central Sierra Nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one.