MAGMA DIFFERENTIATION AS RECORDED BY CHANGES IN MINERAL STRATIGRAPHY WITHIN LAYERED MAFIC CUMULATES (WESTERN SIERRA NEVADA BATHOLITH, CALIFORNIA)

Variations in the chemical composition of minerals with stratigraphic position within layered mafic cumulates record magmatic differentiation processes that were operative within the Early Cretaceous Sierra Nevada batholith. Such stratigraphic variations have been used to characterize evolution and event chronology within other layered intrusions, such as the vast Bushveld Intrusion of South Africa (Ashwal et al., 2005). The Stokes Mountain cumulate section, located in the westernmost foothills of the Sierra Nevada, comprises as much as 220 m of layered troctolite stratigraphic section in discontinuous outcrops. The uppermost 100 m is this studies' focus and has been further divided into a 14-m-thick massive section with diffusely layered packages sandwiched between 37- and 49-m-thick well layered sections showing cm- to m-scale layering. Intercumulus hornblende is plentiful both in the massive layer and in the abundant coarse-grained pods that intrude and disrupt the layered section. Major element analysis of olivine, plagioclase, orthopyroxene and clinopyroxene from 6 samples suggest a history of overall increasing fractionation with rising stratigraphic position. Minerals from the massive section record a significant break in this overall trend that is hypothesized to have resulted from a venting or recharge event. Transects across plagioclase grains reveal reverse and oscillatory zoning in both the upper and lower layered sections with variations up to 10 %An. Zonations in the massive section are generally small (< 5% An) but a few crystals show oscillations and/or reverse zoning with differences up to 9% An. High resolution mineral stratigraphy of nickel concentrations in olivine from a larger sample set (n=24) will test the hypothesized origin of the massive section, as well as support a more detailed analysis of the specific processes active within the Stokes Mountain magma chamber.

Ashwal, L.D., J., W.S., and Knoper, M.W., 2005, Magmatic stratigraphy in the Bushveld Northern Lobe: continuous geophysical and mineralogical data from the 2950 m Bellevue drillcore: Geological Society of South Africa, p. 35.