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ratios of both lamproite groups and other Cenozoic igneous rocks from the
Wyoming Province indicate an ancient metasomatic enrichment (>1.0 Ga) of the
mantle source associated with the subduction of carbonate-bearing sediments.
Other chemical characteristics of the Leucite Hills lamproites, especially their
high K
2
O and volatile contents, are attributed to more recent metasomatism
(<100 Ma) involving in
ux from upwelling mantle during back-arc extension or
plume activity.
4.5.9 The Potassic Lava Suite from Central Sierra Nevada,
California, U.S.A
Small volumes of absarokite and minette lavas (43
57 wt% SiO
2
) erupted along
the western slope of Sierra Nevada, California, during the late Pliocene, within
the Kings River drainage system (Dodge and Moore 1981; Feldstein and Lange
1999, Table
4.13
). The absarokites contain phenocrysts of olivine
-
±
augite,
±
±
whereas the minettes are constituted of K-feldspar and phlogopite
oliv-
ine; Absarokits are distinguished by lack of K-feldspar phenocrysts. Feldestein
and Lange estimated that pre-eruptive magmatic temperatures and pressures for a
felsic and a ma
augite
c minette of that region to be 1138 and 1,144 (
±
50)
°
C, and 1.2
and 1.6 (
0.4) GPa, respectively. These magmas were characterized by extreme
enrichments in the large ion lithophile elements (e.g. 1.9
±
8.1 wt% K
2
O,
-
1,380
3,719 ppm Ba), depletion in high
field strength elements (Ba/Na
pm
of
-
7
33), and high oxygen fugacities (1
3 log units above the Ni
NiO buffer). Trace
-
-
-
element ratios (e.g. Ba/Rb 20
100) of the rocks are distinct from those observed
for the mid-oceanic ridge basalts and ocean island basalts. According to them
variation in K and Ba with respect to other incompatible elements would require
that phlogopite with or without potassic amphibole was an important accessory
phase during the magma generation. According to Feldstein and Lange (1999) the
buoyant ascent of the Kings River magmas through
-
40 km thick sialic crust
should require pre-eruptive volatile concentrations (H
2
O and F) >2 wt%. Vol-
canism probably was triggered as a part of regional response to Basin and Range
extension, which resulted in asthenospheric upwelling and therefore higher heat
*
flow to the subduction-modi
ed lithosphere.
Van Kooten (1980) described ultrapotassic lava
flows from southeastern part of
Yoshemite National Park (Fig.
4.15
). According to him, the eruption took place
through a number of isolated vents. In this region, they occur as plugs and
ows,
which are sometimes 5 km across. The rocks described by him as potassium-rich
olivine basalt are characterized by K
2
O content ranging between 3 and 4.36 wt%.
This, observation together with the fact they are associated with leucite basanite
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