Magma Mixing and Cretaceous Crustal Growth: Geology and Geochemistry of Granites in the Central Sierra Nevada Batholith, California

Abstract

Major-and trace-element data suggest that some granites exposed in the Sierra Nevada batholith represent appropriate end members in a mixing process that generated voluminous granodiorites. Geochemical trends in a range of rock types indicate that the highest-silica granites have been variably modified by partial melting and/or fractional crystallization. Trends in initial Sr, Nd, and common Pb isotope ratios reveal little variation within a wide range of rock types in the same suite, but significant spatial and isotopic variation among individual suites of rocks. These observations are consistent with the interpretation that granites in the Sierra are generated by partially melting juvenile, mantle-derived diorites and mixed with penecontemporaneous mantle melts. With the exception of one spatially limited suite in this study, there is little evidence that ancient continental crust was involved in the generation of the central Sierra Nevada batholith. Instead, isotopic trends from east to west in this study indicate that the majority of granites in the central Sierra Nevada are likely juvenile crust. These observations may dramatically increase estimates of crustal growth rates in the western United States during the Mesozoic.

The persistence of lead from past gasoline emissions and mining drainage in a large riparian system: Evidence from lead isotopes in the Sacramento River, California

“…These lead isotope ratios are interpreted to represent the natural composition of lead transported to the mouth of the Sacramento River: lead derived from rock weathering in the drainage basin prior to the onset of industrial-age human activity in the system (Ritson et al, 1999;Dunlap et al, 2000). These interpreted pre-anthropogenic lead isotope ratios fall within the range of measured lead isotope ratios in the rocks of the Sierra Nevada mountains (e.g., Chen and Tilton, 1991;Wenner and Coleman, 2004).…”

Geochronology, geochemistry and Nd, Sr and Pb isotopes of syn-orogenic granodiorites and granites (Damara orogen, Namibia) — Arc-related plutonism or melting of mafic crustal sources?

“…As a general feature in these arc-related belts, mafic magmas may come directly from the mantle and may be interpreted to serve as heat sources to melt crustal material , Holden et al 1987). On the other hand, mafic magmas may undergo pure crystal fractionation to yield more evolved magma, negating the need for crustal contributions to produce felsic magma (Sisson et al 1996 ;Coleman and Glazner 1997;Ratajeski et al 2001;Wenner and Coleman 2004). In the former case, no new continental crust is formed; in the latter, Although the AFC model presented above seems to be a plausible scenario to explain the elemental and isotopic composition of the Gawib granodiorites, some points require…”

Intrusive history and petrogenesis of the Ash Mountain Complex, Sierra Nevada batholith, California (USA)

“…The fi rst pulse of volumetrically signifi cant magmatism lasted from ca. 180 to 165 Ma in the Jurassic, with a signifi cantly more voluminous phase between 102 and 85 Ma (e.g., Coleman and Glazner, 1997;Ducea, 2001;Wenner and Coleman, 2004;Coleman et al, 2004;Memeti et al, 2010a;Davis et al, 2012).…”

“…Although mixing of high-and low-SiO 2 magmas may contribute to the majority of granodiorite crust, the origin of the felsic end member involved with mixing is not entirely understood and may involve more than one source (Wenner and Coleman, 2004). For example, partial melting of intermediate country rock may generate high-SiO 2 melts (e.g., Skjerlie and Johnston, 1993;Patiño-Douce, 1997), but mafi c underplate and hydrous cumulates may also partially melt to generate high-SiO 2 magmas (e.g., Rushmer , 1991;Ratajeski et al, 2005;Sisson et al, 2005).…”

“…Other workers proposed that mixing and mingling of mafi c and silicic end-member magmas were important in generating this diversity (e.g., Reid et al, 1983;Frost and Mahood, 1987;Dorais et al, 1990;Sisson et al, 1996;Ratajeski et al, 2001;Wenner and Coleman, 2004;Pignotta et al, 2010).…”