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Geologic Trips, Sierra Nevada
by Ted Konigsmark
ISBN 0-9661316-5-7
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40 - Geology of the Sierra Nevada
SIERRA NEVADA BATHOLITH
The granitic rocks that form the backbone of the Sierra Nevada are part
of the large Sierra Nevada batholith. This batholith is not just one
homogenous body of granite. Instead, it is made up of over one hundred
individual plutons. Each pluton covers an area of from one to several
hundred square miles and represents a separate intrusion of magma.
Although most plutons are distinguished by their composition and age,
plutons may have many other distinct characteristics. Some plutons are
fine grained, some are coarse grained, and some have large phenocrysts.
Some plutons have layering or segregation of the minerals. And some
plutons have dark blobs, called xenoliths, formed when parts of the
intruded roof or wall rock were assimilated in the magma. The plutons
may also be cut by dikes of lighter or darker rocks. These dikes are
usually formed during the final stages of crystallization when the last
magma is injected into cracks in the previously crystallized parts of the
pluton. Thus, there are many things to look for when you see granitic
rocks, and many ways to distinguish one pluton from another.
Most of the granitic plutons of the Sierra Nevada were intruded from 80
to 150 million years ago, during the period of greatest subduction within
the Franciscan subduction zone. At that time, the Franciscan subduction
zone was located along the present-day Coast Ranges and the Farallon
plate was being carried eastward into the subduction zone. Much of the
Farallon plate consisted of oceanic crust and serpentine. As the rocks
were carried deep into the subduction zone, high temperatures drove the
water out of the serpentine. The water, which was a supercritical fluid at
these high temperatures and pressures, rose into the overlying rocks and
melted the rocks to form magma. The magma was lighter than the
surrounding rocks, and rose within the crust. As the magma rose, it
mixed with the silica-rich rocks of the crust and formed the silica-rich
magma that resulted in the Sierra plutons. Most plutons were emplaced
at depths of around six miles in the earth’s crust, but there is evidence
that some plutons were emplaced at depths of as much as twenty miles.
The granite formed at these great depths is now exposed at the surface,
indicating that the Sierra Nevada has been uplifted several miles, and
that great thicknesses of overlying rock have been removed by erosion.
40
Batholith
- 41
ORIGIN OF THE SIERRA BATHOLITH
Future
Coast
Ranges
Franciscan
subduction
zone
Future Sierra Nevada
Old Nevadan
subduction
zone
North American plate
W
te
es
e
of
Fa
n
ic
rph
llo
ra
o
tam
Me
rn
pl
lt
Be
ne
rra
g
Te
in
st
lle
ru
tvi
ar
rth
de
Sm
un
e
at
n
Zo
Mantle
Plutons
Partial
melting
85 MYA
The large rectangular grains in this photo are phenocrysts of potassium feldspar in
the Cathedral Peak Granodiorite on Pothole Dome. The granodiorite was polished
by glaciers during the last glacial episode. The hard phenoncrysts have retained
the polish whereas most of the other minerals in the granodiorite have
decomposed and lost their polish.