ISOTOPE AND ELEMENTAL CHEMISTRY OF SEDIMENTARY CLASTS OF THE CRETACEOUS AND
EOCENE GUALALA BASIN, NORTHERN CALIFORNIA: TIES TO SALINIA, MOJAVE, OR
SIERRA NEVADA CORDILLERAN BATHOLITHS?
        JOHNSON, Clark and SCHOTT, Ronald, Dept. Geology, Univ. Wisc.,
Madison, WI 53706;
                O'NEIL, James, Dept. Geology, Univ. Mich., Ann Arbor, MI,
94109
Large granitoid cobbles (up to 1 m diameter) from the Cretaceous and Eocene
Gualala Basin (180 km north of San Francisco), the northernmost on-land
exposure of the Salinian block, have major element and REE compositions
that broadly overlap those of plutonic rocks from the Salinian, Sierra
Nevada, and west-central Mojave Mesozoic arcs.  A distinctive lithology in
the Gualala clasts includes garnet-bearing granitoid clasts, which are
relatively uncommon in the Sierran and Salinian plutonic arcs.  New U/Pb
ages indicate these granitoid clasts are 95-135 Ma (see Schott and Johnson,
this meeting).  eNd values for three granitoid clasts range from +0.1 to
-3.2, values that are not observed in granitoid rocks of this age in the
Sierran or Salinian arcs; the age and isotopic compositions are probably
best matched to a Mojave provenance.  If this correlation is valid, it
requires the Gualala Basin to have been adjacent to the central- western
Mojave Desert during the Eocene, and thus necessitating several hundred km
of pre-San Andreas fault translation in the early Cenozoic.
        An additional lithology that is common in the Gualala Basin is
LREE-depleted gabbro clasts ([Ce/Yb]N=0.3-0.7), which have ages of 145-160
Ma, initial 87Sr/86Sr down to 0.7025 and  eNd of +6.5 to +9.2.  Although
new U/Pb data suggest a range in ages for these gabbros (Schott and
Johnson, this meeting, and references within), Jurassic mafic plutonic
rocks that have such chemical and isotopic compositions are not found in
Salinia, and rarely in the Sierran arc.  The Nd and Sr isotope data support
correlation of the gabbro clasts with the Eagle Rest Peak (ERP) parent body
(southern Sierra tail), as originally proposed by Ross (1970).  dD and d18O
values of the clasts and gabbros at ERP indicate a distinctive prior early
history involving seawater hydrothermal interaction, which may indicate a
near-trench, partially submerged magmatic arc setting, similar to those
found in some western Pacific arcs; the H and O isotope compositions of the
ERP body are unique to the western U.S. Mesozoic ophiolites or Cordilleran
plutonic arcs.