Implications of Franciscan Complex graywacke geochemistry for sediment transport, provenance determination, burial‐exposure duration, and fluid exchange with cosubducted metabasites

Abstract

[1] Interpretation of graywacke provenance has long been used to evaluate the record of tectonic process in orogenic belts. Our geochemical data from graywackes of the Franciscan subduction complex, California, show that the connection between sedimentary record and geologic processes may be more complex than previously believed. Trace elements and Nd-Sr-Pb isotopes of Franciscan graywackes indicate two sources types. One group lacking negative Eu anomaly (Eu/Eu * > 0.9), shows slightly concave-up heavy rare earth elements, arc-like trace element patterns, and western Pacific island arc-like Pb isotopes, reflecting derivation from older accreted oceanic-arc terranes in the Sierra Nevada-Klamath Mountains. The other group displays small negative Eu anomalies, with trace element patterns resembling post-Archean Australian shale and Pb isotopes similar to Jurassic-Cretaceous Sierran batholith. There is no systematic separation of these two groups by depositional ages. Thus, geochemistry of the graywackes may partly reflect variation in location of sediment delivery systems, rather than solely reflecting evolution of the neighboring arc. Variation of Nd-Sr isotopes with stratigraphic-age for the graywackes mimics the trends of the coeval Great Valley Group clastic-rocks, suggesting that (1) they share the same sediment sources, (2) there are no "exotic" sediment sources that fed the Franciscan trench, and (3) burial-exposure cycles for Franciscan clastic rocks were comparatively brief. Comparison of Franciscan graywacke and metabasite geochemistry corroborates earlier conclusions that metabasites had little or no chemical exchange with fluids from cosubducted graywacke. Detrital zircon age populations, major element chemistry, and detrital framework modes, when compared to our data suggest that the former three parameters underrepresent the mafic component of clastic sediment provenance.Citation: Ghatak, A., A. R. Basu, and J. Wakabayashi (2013), Implications of Franciscan Complex graywacke geochemistry for sediment transport, provenance determination, burial-exposure duration, and fluid exchange with cosubducted

Sr–Pb isotope compositions of lawsonites in a Pacheco Pass metagraywacke, Franciscan Complex, California

“…5a and 5b). These initial Pb isotopic values are similar to the whole rock data of Cascade sediment (Church, 1976;Prytulak et al, 2006), the Franciscan metagraywacke, Catalina mélange rock, and granodiorites from the Sierra Nevada batholith (Chen and Tilton, 1991;King et al, 2007;Ghatak et al, 2013) (Figs. 5c and 5d).…”

“…and the analyzed Sr isotope ratios can be considered as initial ratios. Granodiorites from the Sierra Nevada batholith present more similar whole-rock trace element and Sr-Pb isotope compositions to Franciscan metagraywackes in other localities than the Pacheco Pass area (Ghatak et al, 2013). However, the Sr isotope compositions of the Pacheco Pass lawsonites show a different trend compared to those of the whole-rock granodiorites from Sierra Nevada volcanic-plutonic arc (Fig.…”

Detrital zircon U–Pb reconnaissance of the Franciscan subduction complex in northwestern California

“…Dickinson et al 1982;Seiders 1983;Ghatak et al 2013). However, others have speculated that some Franciscan clastics, particularly sandstones with anomalous compositions (e.g.…”

“…Each mega-allochthon appears to have been assembled as an imbricate stack of indistinctly defined thrust sheets (Ernst and McLaughlin 2012) that are interpreted as roof thrusts in a series of eastwardly underthrust structural wedges (McLaughlin et al 2000, sheet 6). Petrofacies analyses and bulk-rock geochemistry indicate that Franciscan sedimentary rocks were derived chiefly from landward volcanic-plutonic arc(s) (Dickinson et al 1982;Seiders 1983;Underwood and Bachman 1986;Ghatak et al 2013), similar to the sources of the coeval strata of the directly inboard Great Valley forearc basin (Ingersoll 1979(Ingersoll , 1983Linn et al 1992;Surpless 2015). The simple interpretation that Franciscan sedimentary rocks were derived from nearby Sierra Nevada-Klamath arc rocks was questioned and re-interpreted to reflect fartravelled, northward translation for some Franciscan units (e.g.…”

Geochemistry of the Great Valley Group: an integrated provenance record

“…The integrated provenance signature for the GVG throughout the Cretaceous may help constrain proposed links between the main GVG outcrop belt and faulted GVG exposures west of the San Andreas fault system, and strengthen existing links between the forearc and the accreted Franciscan Complex (e.g. Ghatak et al 2013).…”