DEEP CRUSTAL ASSIMILATION BY DEPLETED-MANTLE-DERIVED BASALTS MASQUERADING AS MELTING OF ENRICHED MANTLE: HF-O MODELING OF EASTERN SIERRA NEVADA ARC GABBROS

Zircon hafnium and oxygen isotope data from Jurassic plutons of the Kern Plateau (southern Sierra Nevada, CA) may provide insight regarding the composition of the Mesozoic mantle source. Although isotopic data confirm the western Sierra Nevada arc was derived from depleted mantle, questions remain regarding the enriched or depleted identity of the mantle source of the eastern arc. The composition of the mantle source has important implications for estimating the amount of juvenile continental crust generated during the ~180 m.y. span of Sierran arc magmatism. We present zircon data from 5 eastern gabbroic-to-granodioritic plutons, with εHf values ranging from -0.3 to -11.4 and δ¹⁸O values from +4.0 to 11.1‰ VSMOW. We note that gabbroic zircon having δ¹⁸O values of 4.0 to 4.7‰ cannot have crystallized from exclusively mantle-derived melts; given this isotopic data, arguments may be made to support derivation from either enriched or depleted mantle sources.

We hypothesize these data are the product of depleted-mantle-derived magmas modified by assimilation of a hydrothermally altered continental crust. The measured Hf-O isotope compositions of the gabbroic zircon can be generated through simple binary mixing of 11 to 23 percent of a partial melt derived from hydrothermally altered Mojavia crust with depleted-mantle-derived basalt. Paleogeographic reconstructions of the Laurentian craton place the Mojavia crustal province in the region of the modern-day southern Sierra Nevada; after correction for radiogenic ingrowth, Hf data of Mojavia province rocks (Wooden et al., 2012) constrain the end-member assimilant. New Proterozoic U-Pb ages and eHf-values of partially resorbed inherited zircon grains recovered from a Kern Plateau olivine-pyroxene-bearing gabbro document that even the most mafic plutons found in the Sierra Nevada to date may have experienced crustal assimilation.

We propose that deep crustal assimilation mimics the signature of an eastern depleted mantle source. This data-driven model makes reasonable predictions for the quantity of contamination involved in eastern Sierra magma differentiation, is rooted in regional geology, and provides a mechanism for making isotopically modified magmas of mafic to intermediate compositions without invoking an enriched mantle endmember.