A PRIMARY LITHOSPHERIC SECTION FOR THE SIERRA NEVADA BATHOLITH
(SNB) BASED ON AN OBLIQUE CRUSTAL SECTION AND MIOCENE
VOLCANIC HOSTED LOWER CRUST AND UPPER MANTLE XENOLITHS
Saleeby, J, Division of Geological and Planetary Sciences, Caltech, Pasadena CA 91125,
and Ducea, M., Department of Geosciences, University of Arizona, Tucson AZ 85721
A primary lithospheric section may be reconstructed for the Cretaceous SNB based on
the convergence of rock types that occur at the deepest levels (~35 km) of an oblique
crustal section, exposed at its southern terminous, and batholith-related lower crust-upper
mantle xenoliths that were entrained in Miocene lavas erupted through relatively shallow
levels of the batholith in the central Sierra. The section is best defined for crust and
mantle rocks that formed/equilibrated at ca.100 Ma. The shallowest-level exposures of
the SNB in a number of localities preserve coeval silicic ignimbrites while the deepest
mantle xenoliths are from ~125 km deep; thus an entire lithospheric thickness is
represented. The base of the section consists of garnet peridotite which from ~100 to ~75
km depths includes lenses of garnet clinopyroxentite ("eclogite") and garnet websterite.
From ~75 to ~45 km "eclogite" dominates the section with subordinate lenses of garnet
and shallower spinel peridotite. Between ~45 to ~35 km depths lies a complex zone
containing subordinate "eclogite", abundant garnet granulite and a variety of gabbroids
and granitoids of the batholith. The upper ~35 km of the section is predominately
tonalitic to granodioritic batholithic rock with subordinate wallrock screens and minor
mafic intrusives. The roof of the SNB is predominately its own silicic volcanic cover,
and there is no evidence for batholithic rocks having spread out over a mid-or deep crust
regional metamorphic substrate. Virtually the entire crust was reconstituted by batholith
production with steep primary structures pervading the section to shallower than ~10 km.
A complex mixture of steep multiple dikeform and flatter sillform intrusions charaterizes
the upper batholith. The granulitic and "eclogitic" rocks represent low and high pressure
residues, respectively, left from felsic magma generation from a polygenetic source.
This source consited of crustal basement and cover strata components, and mantle wedge
derived wet mafic intrusives. Much of the peridotie section represents variably
metasomatized and depleted asthenosphere hypothesized to have been emplaced into the
section by a combination of cornerflow and wet rheologically controlled diapirism. The
entire lithosphere appears to have been reconstituted during batholith production.