Download Magmatic growth and batholithic root development in the northern

00729 1st pages / page 1 of 15
Magmatic growth and batholithic root development
in the northern Sierra Nevada, California
M.R. Cecil1,*, G. Rotberg2, M.N. Ducea2, J.B. Saleeby1, and G.E. Gehrels2
1
California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, California 91125, USA
University of Arizona, Department of Geosciences, Tucson, Arizona 85721 USA
2
ABSTRACT
In contrast to the much-studied central
and southern Sierra Nevada, relatively little
is known about the growth and petrogenesis
of the batholith in its northern reaches, making it difficult to evaluate range-wide, spatiotemporal trends in batholithic development
and the regional extent of eclogite root production and/or loss. New U-Pb ages from
northern Sierra plutons reveal a shift between
the age of Cretaceous magmatism recorded
in the northern Sierra and the timing of an
apparent flare-up in the main batholith,
indicating: (1) that the northern batholith
was more spatially dispersed and emplaced
into regions beyond the modern topographic
range; and (2) that the Cretaceous high-flux
event may have occurred over a longer period
of time than previously suggested. Relative
to the southern Sierra, Nd and Sr isotopic
signatures in northern plutons are more
primitive, mimicking the predominantly
juvenile nature of the terranes into which
the plutons are built. Despite differences
in isotopic character, however, major and
trace element trends are remarkably similar
between northern plutons and the rest of the
batholith, suggesting that emplacement into
juvenile and/or oceanic lithosphere does not
inhibit the generation of evolved, arc-type
magmatic products. Northern plutons have
relatively high La/Yb and Sr/Y and steep
rare-earth element patterns, with small to no
Eu anomalies. Taken together, these trends
are interpreted to indicate deep processing of
magmas in equilibrium with a feldspar-poor,
amphibolite-rich residue, containing modest
amounts of garnet. It is therefore likely that
*Present address: California State University,
Northridge, Department of Geosciences, Northridge,
California 91330-8266, USA; robinson.cecil@csun
.edu.
the northern Sierra Nevada batholith was
emplaced into relatively thick crust and
developed a dense mafic to ultramafic root.
Because it is not seismically imaged today, we
posit that the root was subsequently lost, perhaps in response to encroachment of protoCascade arc volcanism.
INTRODUCTION
The Sierra Nevada, one of the largest batholiths in North America, strikes NNW and can
be traced ~600 km along the eastern spine of
California, varying in width between ~80 and
120 km. The batholith comprises mainly subduction-related intermediate to felsic intrusive rocks
and associated metamorphic screens and pendants (Bateman and Wahrhaftig, 1966; Evernden
and Kistler, 1970; Ducea, 2001; Saleeby et al.,
2003). Although the Sierra Nevada is commonly
classified as a single, exhumed continental
mega-plutonic complex, major lateral (W to E)
changes in the age (Kistler and Peterman, 1973,
1978; Stern et al., 1981; Chen and Moore, 1982),
composition (Moore, 1959; Bateman, 1992), and
isotopic signature (Doe and Delevaux, 1973;
DePaolo, 1980, 1981; Masi et al., 1981; Saleeby
et al., 1987a; Kistler, 1990; Kistler and Ross,
1990; Chen and Tilton, 1991; Kistler and Fleck,
1994; Ducea, 2001; Lackey et al., 2005, 2008) of
the batholith have long been recognized. These
changes are largely attributed to the flaring up
and migration of magmatism across heterogeneous crustal and upper mantle domains.
In addition to significant transverse changes,
the Sierra Nevada also changes along strike in
several important ways: (1) plutonic rocks are
volumetrically more significant in the central
and southern Sierra Nevada (between ~35 °N
and 37.5 °N; Fig. 1), comprising nearly the
entire exposed arc in those portions of the
range; (2) Eocene–Miocene clastic sedimentary
and volcanic rocks are preserved in the northern Sierra Nevada (N of 38 °N), whereas they
are conspicuously absent in other parts of the
range (Fig. 1); (3) the estimated depths of pluton emplacement become shallower toward the
north (Ague and Brimhall, 1988; Pickett and
Saleeby, 1993; Ague, 1997; Nadin and Saleeby,
2008); (4) the basement into which the batholith
is built changes from mainly late Proterozoic
continental and transitional lithosphere in the
south to a tapestry of accreted Phanerozoic belts
in the north (Kistler, 1990; Fig. 1); and (5) the
southern and central part of the batholith had a
documented dense root that has recently delaminated from the base of the crust (e.g., Ducea and
Saleeby, 1998b; Zandt et al., 2004), whereas the
development and/or loss of such a root in the
northern Sierra is unclear (Frassetto et al., 2011).
Although a wealth of information exists about
the main batholithic body of the Sierra Nevada
in its southern and central extents, much less is
known about the age and petrogenesis of granitic plutons in the north. Because of the northsouth changes enumerated above, we examined
the geochronologic, geochemical, and isotopic
character of northern plutons with the aim of
(1) describing their petrology and emplacement history, (2) identifying range-wide spatial
and temporal patterns in batholith genesis, and
(3) identifying fundamental differences, if any,
between magma sources regions in the north
and other parts of the range. This work was
conducted as part of a larger, interdisciplinary
project—the Sierra Nevada Earthscope Project
(SNEP)—aimed at resolving the nature and complexity of the Sierran lithosphere. Of particular
interest to principal investigators were: the presence or absence of a dense residue at the base
of the felsic crust, the distribution of that residue
through both time and space, and its influence
on recent volcanism and surface deformation.
In light of the greater context of this work, the
isotopic and geochemical data presented herein
are interpreted with an eye toward understanding
the evolution of the lower crust and mantle lithosphere in the northern Sierra Nevada.
Geosphere; June 2012; v. 8; no. 3; p. 1–15; doi:10.1130/GES00729.1; 11 figures; 3 tables; 1 supplemental file.
MANUSCRIPT RECEIVED 15 JUNE 2011 ◊ REVISED MANUSCRIPT RECEIVED 17 JANUARY 2012 ◊ MANUSCRIPT ACCEPTED 14 FEBRUARY 2012 ◊ FIRST PUBLISHED ONLINE XX MONTH 2012
For permission to copy, contact [email protected]
© 2012 Geological Society of America
1
00729 1st pages / page 2 of 15
Cecil et al.
122°W
121°W
120°W
119°W
118°W
87
Sr/86Sri = 0.706
isopleth
Tertiary volcanics
Sierra Nevada batholith
G05
39.5°N
G08
Sutter
Buttes
G01
a
Yub
Panthalassan (oceanic)
affinity
N. American (continental)
affinity
Pendant rocks
0
25
G06
G02
2
G03 G12
R.
G10
G11
T
39°N
G22
Sample location
G02 Sample name
Km
50
100
G20
G19
G23
G17
G21
Sacramento
G16
G18
.
Con
st
es R
n
sum
cre
38.5°N
N
Basin and Range
G12
Sierra
Nevada
a
at V
Gre
38°N
Rivers
Mafic/ultramafic rocks
G04 G09
117°W
lley
NV
37°N
CA
NV
CA
o
nt
te
ex ith
rn hol
ste at
we .N. b
S
s
ge
R an
706
>0. 6
70
<0.
ast
Co
37.5°N
f
Fresno
36.5°N
Mt. Whitney
Figure 1. Generalized geologic map of the central and northern Sierra Nevada, modified after Irwin
and Wooden (2001), Saucedo and Wagner (1992), and Wagner et al. (1987). Sample locations (and their
corresponding names) are shown in white. The Sri 0.706 line is modified after Kistler and Peterman
(1978) and Kistler (1990). Belts of metamorphic rocks in the northern Sierra foothills have been grouped
according to the two different interpreted lithosphere types (Panthalassan and North American) of
Kistler (1990). The western extent of the Great Valley is based on the presence of tonalitic and gabbroic
arc-related basement sampled in well cores (Williams and Curtis, 1977; Saleeby, 2007).
GEOLOGIC BACKGROUND
The Sierra Nevada batholith formed as a result
of long-lived subduction of the Pacific plate
beneath western North America (Dickinson,
1981). Magmatism was in general continuous
from ca. 248 to 80 Ma, but most active during episodic events in the Late Jurassic (165–145 Ma)
and Late Cretaceous (100–85 Ma), building
a thick Mesozoic crustal column—at least in
the central and southern Sierra—composed of
~35 km of batholithic rocks (Pickett and Saleeby,
1993; Ducea, 2001). In general, plutons making
up the batholith tend to become younger, more
silicic, and more voluminous toward the east.
Unlike in the central and southern stretches of the
Sierra Nevada, where batholithic rocks comprise
the vast majority of the crust, the northern Sierra
Nevada batholith is a loosely assembled patch-
2
work of quartz diorite, granodiorite, and tonalite
plutons. In the north, plutons are more variable in
their composition and size, collectively comprising less than 50% of the exposed surface geology
north of latitude 38.5 °N.
Prebatholithic Rocks of the Northern
Sierra Nevada
Granitoid rocks of the northern Sierra are
intruded across range-parallel, accretionary
tectonostratigraphic packages of Paleozoic
and Mesozoic metamorphic rocks, which are
juxtaposed along Late Jurassic and older faults
(Saleeby, 1982; Edelman and Sharp, 1989; Snow
and Scherer, 2006). We have grouped these packages into a western, Panthalassan (oceanic) belt
and a central, North American (continental) belt,
after the two different lithosphere types defined
Geosphere, June 2012
by Kistler (1990) (Fig.1). The western belt
of Panthalassan affinity is composed mainly of
Jura–Triassic metavolcanic arc rocks and associated Upper Jurassic accretionary metasediments.
Basement rocks to this belt are primarily midocean ridge basalt (MORB)–affinity Paleozoic
ophiolitic rocks (Saleeby, 1982; Sharp, 1988;
Ernst et al., 2008). Rocks of the North American belt consist primarily of the early Paleozoic
Shoo Fly Complex, which contains mostly deep
marine siliciclastic rocks deposited in proximity
to a continental source (e.g., Girty et al., 1996;
Harding et al., 2000). Although derived from a
continental source, the Shoo Fly and associated
assemblages are floored by oceanic or transitional lithosphere, as inferred from their relatively juvenile isotopic nature.
In the southern Sierra Nevada, the western boundary of the Precambrian continental
00729 1st pages / page 3 of 15
Northern Sierra Nevada magmatism
Sample
Northern transect
G01
G02
G03
G04
G05
G06
G08
G09
G10
G11
G12
Latitude
Longitude
39.2978
39.3307
39.3219
32.3247
39.4433
39.3334
39.3782
39.3436
39.3276
39.3123
39.3061
–121.0882
–120.1947
–120.633
–120.5981
–120.0112
–120.2906
–120.671
–120.3385
–120.3901
–120.4946
–120.6328
TABLE 1. SAMPLE INFORMATION AND U-Pb GEOCHRONOLOGY
Distance E of
range crest
(km)
Rock type
Location or unit
–59.5
–68.6
–18.3
–17.7
25
8.5
–18.1
4.4
0
–8.9
–20.7
Granodiorite
Granodiorite
Granodiorite
Quartz monzonite
Granite
Tonalite
Granite
Granodiorite
Granodiorite
Granodiorite
Diorite
Southern transect
G14
38.3721
–120.5542
–39.6
Granodiorite
G16
38.4356
–120.5509
–39.3
Granodiorite
G17
38.5463
–120.352
–22.2
Granodiorite
G18
38.561
–120.2663
–20.5
Granodiorite
G19
38.6457
–120.1343
–3.8
Tonalite
G20
38.7048
–120.0899
0
Granodiorite
G21
38.6945
–119.9956
8.0
Tonalite
G22
38.776
–119.8968
16.5
Granodiorite
G23
38.7648
–119.8484
20.6
Granodiorite
Note: n.d.—not determined; MSWD—mean standard of weighted deviates.
margin is geochemically outlined by the classic 87Sr/86Sri = 0.706 isopleth, which bisects
the west-central part of the batholith (Kistler
and Peterman, 1978; Kistler, 1990) (Fig. 1).
Eastward transition into older continental lithosphere is evidenced by more evolved isotopic
signatures in the batholith (Chen and Tilton,
1991; Sisson et al., 1996; Coleman and Glazner,
1998) and the presence of ancient, continentderived miogeoclinal rocks in metamorphic
screens and pendants (Saleeby and Busby,
1993). To the north, that boundary becomes
less well defined and the zone that is inferred to
separate the more juvenile, accreted metamorphic belts from the ancient continental margin
is generally thought to lie well east of the Sierra
Nevada batholith (Davis et al., 1978). We present new geochemical and isotopic data from the
northern Sierra, which is used to elucidate the
influence of strikingly different prebatholithic
lithospheric conditions on the magmatic development of the batholith.
SAMPLE SELECTION, PREPARATION,
AND ANALYSIS
Plutons were sampled from 20 locations
along two range-perpendicular transects in the
northern Sierra Nevada: a northerly transect
at the latitude of the Yuba River (~39.25 °N),
and another at the approximate latitude of the
Consumnes River (~38.5 °N) (Fig. 1). Only
samples with no visible weathered surfaces
and/or secondary minerals were crushed and
pulverized to a coarse-sand size. The bulk of
the crushed material was processed for zircon
separation using standard density and magnetic
Pb/ 238U age
(±2σ)
MSWD
Yuba River pluton
Pleasant Valley pluton
Lake Spaulding (S)
Lake Spaulding (E)
N. Flonston
Donner Lake
Bowman Lake
Donner Pass
Soda Springs
Cascade Lake (W)
Emigrant Gap
162.2 (2.2)
162.0 (3.0)
166.5 (2.6)
145.3 (3.0)
n.d.
109.0 (2.5)
372.0 (6.0)
116.4 (2.6)
111.6 (2.3)
120.8 (2.8)
n.d.
2.8
1.7
1.8
3.0
—
5.5
1.9
3.3
4.0
0.5
—
West Point (N)
West Point (S)
Cooks Station
Bear River Reservoir
Silver Lake (W)
Caples Lake (W)
Carson Pass (W)
Freel Peak
Freel Peak
166.3 (2.0)
163.2 (3.2)
148.7 (2.4)
121.5 (1.3)
105.2 (1.5)
101.7 (1.5)
109.8 (2.1)
90.2 (1.7)
106.8 (1.9)
2.1
1.5
2.9
4.6
2.5
1.4
5.3
3.8
2.4
techniques, and small aliquots were set aside
and powdered in an Al2O3-lined shatter box for
whole-rock geochemical and isotopic analysis. Zircons separated from each sample were
picked and mounted, along with Sri Lanka and
R33 standards, in 2.5 cm epoxy discs. U-Pb
zircon geochronology data were collected via
laser ablation–multicollector–inductively coupled
plasma mass spectrometry (LA-MC-ICPMS)
(GV Isoprobe) at the Arizona LaserChron Center
(see Gehrels et al., 2008, for instrumentation
and methodology details). Sample information,
location, and U-Pb age data are summarized
in Table 1. Individual U-Pb measurements and
concordia plots are provided in the Supplemental File1. Some samples (G06 and G10,
for example) show a high scatter in individual
zircon U-Pb ages. These are attributed to analytical error, as opposed to meaningful geologic
heterogeneity, due to the fact that: (1) zircon age
standards analyzed in the same session show
similar scatter, and (2) there is no observed relationship between calculated age and U concentration, U/Th, or any other geologic indicator.
Major and trace element analyses were performed at the GeoAnalytical Laboratory in the
School of Earth and Environmental Sciences
at Washington State University using X-ray
fluorescence (XRF) (Johnson et al., 1999) and
ICPMS. Major and trace element data are summarized in Table 2. Isotopic analyses were per1
Supplemental File. Weighted mean plots and Concordia diagrams for all reported U-Pb analyses. If you
are viewing the PDF of this paper or reading it offline,
please visit http://dx.doi.org/10.1130/GES00729.S1
or the full-text article on www.gsapubs.org to view
the Supplemental File.
Geosphere, June 2012
206
formed at the University of Arizona via isotopedilution–thermal ionization mass spectrometry
(ID-TIMS), using both an automated VG Sector
multicollector instrument with adjustable 1011
Ω Faraday collectors and a Daly photomultiplier (Otamendi et al., 2009) and a VG Sector
54 instrument (Ducea, 2002) (see Appendix for
details of Sm-Nd and Rb-Sr isotopic measurements). All isotope data are summarized in
Table 3.
RESULTS
U-Pb Geochronology
U-Pb zircon ages of plutons from the northern
Sierra Nevada record the presence of a Late Jurassic arc, active between 167 and 145 Ma, and a
mid-Cretaceous arc, with magmatism continuous
between 125 and 90 Ma. The distribution of ages
presented here is similar to range-wide estimates
of magmatic flux, in that both reveal pronounced
magmatic episodes in the Jurassic and in the Cretaceous, but they differ in that: (1) the Jurassic
event is apparently more significant in the north;
and (2) the distribution of ages from the northern
Sierra is offset from the apparent intrusive flux
curve for the greater Sierra Nevada, as defined by
Ducea (2001) (Fig. 2). Although the similarities
and differences between these trends are significant, it is important to note that magmatic rocks
in general comprise a much smaller proportion of
the crust in the northern Sierra Nevada and that
much of which is presumably underlain by granitic rocks in the higher, eastern reaches of the
range, is presently covered by younger volcanic
and clastic deposits (e.g., Saucedo and Wagner,
3
00729 1st pages / page 4 of 15
Cecil et al.
TABLE 2. MAJOR AND TRACE ELEMENT CHEMISTRY*
Northern transect
G01
G02
G03
SiO2
67.82
69.66
65.73
TiO2
0.28
0.53
0.4
Al2O3
16.63
15.06
16.55
FeO†
3.48
3.86
4.24
MnO
0.09
0.09
0.09
MgO
1.26
1.23
2.03
CaO
4.12
4.11
4.73
Na2O
4.17
3.98
3.57
K2O
2.16
1.35
2.53
P2O5
0.09
0.13
0.15
Total
100.1
100
100.1
La
11.47
10.96
15.81
Ce
20.27
20.85
30.01
Pr
2.45
2.58
3.51
Nd
9.35
10.62
13.34
Sm
2.02
2.75
2.71
Eu
0.67
1.04
0.77
Gd
1.83
3.03
2.19
Tb
0.28
0.53
0.32
Dy
1.67
3.35
1.82
Ho
0.34
0.71
0.35
Er
0.96
2.04
0.99
Tm
0.15
0.33
0.14
Yb
0.98
2.18
0.93
Lu
0.17
0.37
0.15
Ba
753.1
472.0
886.1
Th
2.67
2.33
4.44
Nb
4.14
2.67
4.28
Y
9.47
18.87
9.28
Hf
2.38
4.29
2.96
Ta
0.37
0.4
0.3
U
0.99
0.77
1.19
Pb
9.18
2.96
11.2
Rb
47.84
35.41
65.68
Cs
1.75
0.88
2.14
Sr
405.2
218.9
691.9
Sc
6.61
12.08
9.3
Zr
84.52
162.3
108.3
Southern transect
G14
G16
G17
SiO2
63.13
62.38
78.31
TiO2
0.71
0.53
0.12
Al2O3
16.63
17.29
11.53
†
FeO
6.01
5.96
1.75
MnO
0.12
0.11
0.02
MgO
2.0
1.98
0.01
CaO
4.43
4.79
0.07
Na2O
2.51
2.84
3.84
K2O
4.19
3.88
4.33
P2O5
0.28
0.24
0.01
Total
100.1
100
99.98
La
30.2
27.85
55.39
Ce
68.87
56.07
121.2
Pr
8.11
6.54
14.85
Nd
31.78
24.9
51.66
Sm
6.95
5.11
11.17
Eu
1.41
1.27
0.18
Gd
6.0
4.18
9.61
Tb
0.92
0.59
1.88
Dy
5.23
3.19
12.29
Ho
1.02
0.6
2.59
Er
2.71
1.61
7.38
Tm
0.39
0.23
1.11
Yb
2.44
1.43
6.95
Lu
0.38
0.22
1.07
Ba
979.5
988.2
17.08
Th
14.67
14.13
17.34
Nb
20.68
9.48
24.28
Y
26.77
16.13
70.54
Hf
6.96
4.74
15.22
Ta
1.62
0.76
1.75
U
3.65
3.53
3.59
Pb
13.47
13.24
18.99
Rb
165.9
135.7
156.7
Cs
5.95
8.06
0.47
Sr
560.7
699.2
7.54
Sc
13.55
10.73
1.67
Zr
276.5
181.2
595.7
*All values are unnormalized, given in wt%.
†
All Fe is calculated as FeO.
4
G04
60.02
0.93
19.43
4.16
0.09
1.33
2.81
5.28
5.58
0.26
99.89
56.02
6.46
24.2
4.21
2.02
3.44
0.48
2.69
0.54
1.46
0.22
1.44
0.27
0.27
1630
1.98
8.75
13.33
16.28
0.35
0.74
13.75
45.73
0.37
415.8
6.91
867.9
G05
78.95
0.06
12.09
0.5
0.04
0.07
0.43
3.3
4.55
0.01
100
13.65
24.7
2.72
9.14
2
0.14
1.87
0.35
2.21
0.48
1.44
0.24
1.75
0.3
97.46
20.91
6.58
14.44
2.95
1.03
27.8
32.75
244.6
7.88
34.86
1.24
60.48
G06
66.55
0.51
16.28
4.05
0.08
1.72
4.2
3.75
2.73
0.12
99.8
17.47
35.95
4.36
16.57
3.36
0.85
2.95
0.45
2.57
0.51
1.36
0.2
1.3
0.21
825.9
12.41
6.75
13.75
3.18
0.79
2.46
10.83
79.27
3.91
431.3
7.89
106.1
G08
76.09
0.16
13.16
2.2
0.04
0.14
1.97
2.91
3.32
0.03
100.1
44.57
66.31
10.26
36.89
7.25
1.12
6.58
1.08
6.49
1.29
3.61
0.54
3.45
0.53
720.3
13.53
8.03
33.39
4.71
0.72
3.05
14.34
114.2
1.03
187.0
10.37
158.3
G09
67.87
0.44
15.86
3.65
0.08
1.51
3.61
3.57
3.25
0.12
99.9
14.53
29.13
3.59
14.01
3.03
0.75
2.64
0.43
2.65
0.54
1.52
0.24
1.61
0.27
763.8
10.4
7
14.58
4.27
0.81
4.04
13.1
111.8
4.5
362.2
7.91
143.9
G10
63.21
0.68
16.87
5.4
0.1
2.63
5.22
3.43
2.43
0.16
100.2
18.41
37.58
4.58
17.86
3.73
0.92
3.29
0.51
3.08
0.61
1.66
0.25
1.61
0.26
829.8
8.73
6.77
16.17
3.27
0.65
2.78
9.02
86.34
3.4
461.8
12.06
112.6
G18
76.66
0.11
13.03
1.05
0.06
0.12
0.6
3.74
4.51
0.03
99.92
18.65
56.02
6.46
24.2
4.21
2.02
3.44
0.48
2.69
0.54
1.46
0.22
1.44
0.31
270.2
20.33
18.87
18.79
3.84
1.6
5.05
21.88
168.7
1.53
49.55
2.43
105.4
G19
62.35
0.79
16.92
5.61
0.1
2.68
5.57
3.38
2.47
0.15
100.1
19.06
34.45
3.87
14.44
3.17
1.22
2.91
0.44
2.51
0.5
1.29
0.18
1.11
0.18
1187
12.7
12.19
12.19
4.72
0.71
3.25
9.98
78.91
2.38
414.3
15.78
180.0
G20
69.49
0.43
15.33
3.28
0.07
0.12
3.21
3.47
3.38
0.12
100.1
27.6
50.85
5.53
18.69
3.2
0.78
2.54
0.39
2.27
0.45
1.27
0.2
1.35
0.23
784.6
17.7
12.89
12.89
3.72
0.95
4.84
17.23
121.2
4.89
337.1
6.02
121.6
G21
66.97
0.53
15.78
4.27
0.07
1.67
4.02
3.45
2.99
0.14
99.89
25.63
46.77
5.2
18.18
3.36
0.85
2.61
0.38
2.17
0.41
1.08
0.16
1.0
0.16
831.7
13.85
10.98
10.98
3.98
0.78
3.5
11.15
85.94
2.57
453.3
8.28
138.4
G22
70.42
0.37
15.46
2.48
0.05
0.83
2.69
4.05
3.53
0.11
99.9
20.07
35.65
3.9
13.69
2.45
0.67
1.76
0.24
1.3
0.23
0.58
0.08
0.56
0.1
865.8
15.39
6.5
6.5
3.45
0.5
3.83
18.58
110.5
2.84
432.6
3.58
109.3
G23
67.1
0.52
16.69
3.65
0.11
1.08
3.1
5.17
2.41
0.17
100
21.13
42.34
5.29
21.0
4.65
1.06
4.22
0.69
4.16
0.84
2.22
0.32
2.01
0.32
783.1
6.58
21.71
21.71
5.38
0.41
2.24
11.72
80.84
3.63
315.1
7.52
209.6
Geosphere, June 2012
G11
66.13
0.51
16.21
4.23
0.09
2.01
4.54
3.66
2.45
0.14
99.7
16.27
32.44
3.95
15.22
3.19
0.82
2.75
0.42
2.47
0.5
1.36
0.21
1.34
0.22
659.3
7.48
6.87
13.36
3.75
0.7
3.0
12.25
83.32
2.97
444.5
9.45
128.8
G12
54.77
0.68
16.38
8.57
0.16
6
9.03
2.73
1.49
0.18
99.9
10.13
19.85
2.6
10.95
2.56
0.92
2.48
0.39
2.27
0.46
1.26
0.19
1.2
0.19
476.7
1.97
2.74
11.83
1.08
0.18
0.63
5.58
33.41
1.45
601.5
30.17
33.66
00729 1st pages / page 5 of 15
Northern Sierra Nevada magmatism
TABLE 3. Nd AND Sr ISOTOPE DATA
87
147
Sr/ 86Sr
Sr/ 86Sri
Sm/144Nd 143Nd/144Nd
87
Sample
Rb/ 86Sr
Northern transect
G01
0.6852
G02
0.4816
G03
0.2743
G04
0.3137
G05
17.875
G06
0.4667
G08
0.5423
G09
0.7513
G10
0.5082
G11
0.4817
G12
0.1078
Southern transect
G14
0.919
G16
0.488
G17
134.614
G18
8.8334
G19
0.712
G20
0.9927
G21
0.5254
G22
0.6417
G23
0.783
87
143
Nd/144Ndi
εNdi
0.70448
0.70451
0.70548
0.70545
0.73463
0.70562
0.70568
0.70722
0.70629
0.70666
0.70500
0.70293
0.70340
0.70483
0.70480
0.70798
0.70489
0.70283
0.70598
0.70549
0.70584
0.70476
0.1241
0.1489
0.063
0.1086
0.1284
0.1111
0.1165
0.1334
0.1209
0.1197
0.1381
0.51275
0.51293
0.51250
0.51243
0.51266
0.51248
0.51261
0.51245
0.51245
0.51234
0.51255
0.51262
0.51277
0.51243
0.51238
0.51257
0.51239
0.51233
0.51231
0.51236
0.51225
0.51241
–0.31
2.7
–3.9
–5.1
–1.2
–4.6
–6.0
–6.3
–5.3
–7.5
–4.3
0.70752
0.70615
1.04810
0.72381
0.70649
0.70736
0.70644
0.70639
0.70616
0.70538
0.70501
0.70668
0.70807
0.70542
0.70593
0.70562
0.70557
0.70498
0.1229
0.1162
0.1233
0.1325
0.1219
0.1078
0.1133
0.1013
0.1009
0.51248
0.51258
0.51256
0.51234
0.51244
0.51234
0.51237
0.51242
0.51258
0.51235
0.51246
0.51244
0.51223
0.51236
0.51226
0.51228
0.51236
0.51248
–5.5
–3.4
–3.7
–7.9
–5.4
–7.2
–6.8
–5.4
–2.9
8
n = 17
Northern transect
3 × 104
Southern transect
6
Number
S. Sierra intrusive flux
5
2 × 104
4
3
1 × 104
2
1
0
50
Apparent intrusive flux (km2/My)
7
370 Ma
100
206
150
Figure 2. Histogram of Jurassic–Cretaceous U-Pb zircon
ages from the northern Sierra
Nevada. Apparent intrusive
flux in the southern Sierra
Nevada (dotted black line) is
from Ducea (2001). A single
Devonian age (370 Ma) from
the Bowman Lake batholith is
not plotted.
200
238
Pb/ U age (Ma)
1992; Busby and Putirka, 2009) (Fig. 1). Thus,
the ages we present may not accurately reflect the
relative volumes of magmatic pulses in this part
of the range. For example, a comparison of the
age histogram from this study with the estimated
age-area distribution of previously dated plutons
from the northern Sierra Nevada (between 39 °N
and 40 °N; Cecil et al., 2010) suggests that the
large ca. 165 Ma age peak is overrepresented.
In addition to revealing temporal variability
(periods of high and low magmatic flux), U-Pb
zircon ages are also spatially variable. In both
the northern and southern transects, Cretaceous
plutons become systematically younger to the
east (Fig. 3). This is similar to trends previously reported in the southern and central Sierra
Nevada (Chen and Moore, 1982; Nadin and
Saleeby, 2008), the Coast Mountain batholith of
British Columbia (Gehrels et al., 2009), and the
Peninsular Ranges (Silver et al., 1979; Silver and
Chappell, 1988). Jurassic plutons are restricted
to the western parts of both transects, and their
ages have no apparent migratory trends. A
single Late Devonian age (370 Ma) from the
Bowman Lake pluton was also measured, and
is in good agreement with previously published
zircon ages from the same composite batholith
(Saleeby et al., 1987b; Hanson et al., 1988).
Because its petrogenetic history is not related to
that of the other Mesozoic intrusions, it is not
considered in subsequent sections.
Major and Trace Element Geochemistry
In this and subsequent sections and figures,
northern Sierra plutons from this study are compared with a range-wide suite of Jurassic and
Cretaceous intrusive rocks, compiled from the
crest
Age (Ma)
60
Figure 3. Arc migratory trends for the Sierra Nevada batholith.
U-Pb ages for the central and southern Sierra Nevada (small circles)
80
are from Saleeby and Sharp (1980), Stern et al. (1981), Chen and
2
Moore (1982), Saleeby et al. (1987a, 2008), Chen and Tilton (1991),
100
Tobisch et al. (1995), Clemens-Knott and Saleeby (1999), and Cole3
1
man et al. (2004). U-Pb zircon ages from plutons in north-central
120
Nevada (large red circles) are from Van Buer and Miller (2010).
Ar-Ar ages from plutons in north-central Nevada (black crosses) are
U-Pb ages from the southern and
central Sierra Nevada
from Smith et al. (1971). U-Pb ages from the Great Valley basement
140
U-Pb ages from the northern Sierra
are from Saleeby (2007); location of batholithic rocks in the Great
Nevada (this study)
U-Pb ages from north central Nevada
Valley subsurface at the latitude of the northern Sierra Nevada from
160
Williams and Curtis (1977). Migratory trend 1 (wide-dashed line) =
Ar-Ar ages from north central Nevada
2.7 km/My (from Chen and Moore, 1982; at latitude ~37 °N). MigraU-Pb ages from the Great Valley
180
basement
tory trend 2 (dotted line) = 2.0 km/My. This is a fit to Cretaceous
data from the northern Sierra Nevada (38.5 °N–39.5 °N) presented
200
-100
-50
0
50
100
150
200
in this study. Migratory trend 3 (dot-dash line) = 4.7 km/My. This is
Distance east of the range crest (km)
a fit to early to mid-Cretaceous ages of batholithic rocks in the Great
Valley subsurface (Saleeby, 2007), Cretaceous ages (this study), and U-Pb ages from north-central Nevada (Van Buer and Miller, 2010). This
eastward migration rate is a minimum, because it uses maximum estimates of Neogene east-west extension across northwestern Nevada and
the northeastern Sierra Nevada (35%; from Surpless et al., 2002; Colgan et al., 2006a).
Geosphere, June 2012
5
00729 1st pages / page 6 of 15
Cecil et al.
western North American volcanic and intrusive
rock database (NAVDAT; see Fig.4 caption for a
list of references). For discussions of elemental
and isotopic geochemistry, the northern Sierra
Nevada plutons investigated in this study are
divided into a Jurassic and a mid-Cretaceous
group, based on the distribution of ages presented. Both groups are similar compositionally,
but differ in some of their geochemical and/or
isotopic trends. In general, Jurassic intrusions
tend to have greater compositional heterogeneity,
showing greater scatter and less well defined
trends, both geochemically and isotopically.
Plutons of the northern Sierra are calc-alkaline to slightly calcic and form a trend that is
nearly identical to all other Sierra Nevada intrusive rocks when plotted as Na2O + KO-CaO
versus SiO2 (Fig. 4A). With the exception of
few, high SiO2 (>70%) samples, northern Sierra
plutons are metaluminous to mildly peraluminous (Fig. 4B). With the except of K2O, most
major oxides in both age groups form linear
arrays and are negatively correlated with silica
content, which varies between 54 and 78 wt%
SiO2, trending toward higher values in mid to
Late Cretaceous rocks (Fig. 5). In both Jurassic and Cretaceous plutons, FeO is higher than
that of most Sierra Nevada intrusions, across
all values of SiO2. Trace element compositions
are generally characterized by enrichments in
large ion lithophile elements (LILE) and strong
depletions in Nb and Ti (Fig. 6). Jurassic plutons
show greater depletion in high field strength elements (HFSE), which is reflected in higher average Ba/Th ratios (average Jurassic Ba/Th = 235;
average Cretaceous Ba/Th = 65).
Like most major and trace element patterns, chondrite-normalized, rare-earth element
K2O + Na2O - CaO
12
Figure 4. General classification
of northern Sierra Nevada pluA
tons. (A) Plot of K2O + Na2O –
8
CaO versus SiO2. Major element
alic
data from northern plutons
Alk
4
ic
(this study) are compared here,
alc
li-c
a
and in subsequent figures, with
Alk
data from a range-wide suite
lic
0
ka
-al
of Jurassic and Cretaceous
c
l
Ca
lcic
plutons, compiled from North
Ca
–4
American volcanic and intrusive rock database (NAVDAT;
All Sierra Nevada data
Cretaceous plutons
–8
www. navdat.org). Data are
Jurassic plutons
from: Kistler and Peterman
(1978); Bateman and Chappel
–12
45
50
55
60
65
70
75
80
(1979); Bateman et al. (1984,
1988); Saleeby et al. (1987a);
SiO2
Ague and Brimhall (1988);
3
Moore (1991); Clemens-Knott
B
low SiO2
(1992); Coleman et al. (1992);
Peraluminous
Sisson (1992); Sisson et al.
FG
(1996); Macias (1996); Truschel
2
(1996); Ratajeski et al. (2001);
Gray (2003); Hirt (2007); Pater- A/NK
T
SB
son et al. (2008). (B) A/CNK
Metaluminous
High SiO2
1
(Al2O3 /(CaO + Na2O3 + K2O)
mol%) versus A/NK (Al 2 O 3 /
Peralkaline
(Na2O3 + K2O) mol%) diagram
for discriminating between
0
metaluminous, peraluminous,
0.6
0.8
1
1.2
1.4
and peralkaline compositions.
A/CNK
Most plutons from the northern
Sierra are metaluminous to mildly peraluminous. Three samples plot as outliers to the main
trend and correspond with those samples having unusually high or low SiO2 contents. Samples
overlap most closely with data from plutons of the Fine Gold intrusive suite (central Sierra
Nevada foothills) and the Sahwave batholith (north-central Nevada). T (light-green field)—
Tuolumne intrusive suite (Bateman and Chappell, 1979); SB (gray-striped field)—Sahwave
batholith, Nevada (Van Buer and Miller, 2010); FG (blue horizontal–striped field)—Fine Gold
intrusive suite (Truschel, 1996).
6
Geosphere, June 2012
(REE) patterns are also similar to those of the
greater Sierra Nevada batholith (Figs. 7A and
7B). Rare-earth element patterns show enrichment in the light rare-earth elements (LREE)
(up to >100× chondrite) and relative depletion
in heavy rare-earth elements (HREE), which
have flat to slightly concave trends. La/Yb
ratios range from 5 to 35, but are slightly higher
(steeper REE slope) and more homogeneous
in mid-Cretaceous plutons. Most samples have
small to no Eu anomalies, with the exception
of rare high SiO2 intrusions, which have large
negative anomalies (Fig. 7C).
Sr and Nd Isotopes
Strontium and neodymium isotope systematics of northern Sierra Nevada plutons were
investigated in order to identify any spatial and
temporal patterns in magmatic source and to
estimate the degree to which magmas interacted
with older, preexisting crust. Initial 87Sr/86Sr
ratios in most plutons range between 0.7047
and 0.7059, with the notable exceptions of samples G01 and G02, which have more primitive
ratios (0.7029 and 0.7034, respectively). G01
and G02 are from Jurassic plutons emplaced
into Panthalassan lithosphere of the northwestern Sierra Nevada foothills, the significance of
which will be discussed in subsequent discussion sections. Additionally, samples from high
SiO2 intrusions have low Sr, high Rb/Sr ratios,
and high 87Sr/86Sri ratios (0.7070–0.7081). Initial εNd values for most northern Sierra plutons
are negative and vary between –7.5 and –2.9 and
show expected negative correlation with initial
Sr ratios (Fig. 8). Low 87Sr/86Sri samples (G01
and G02) have correspondingly high εNd(i) values (–0.3–2.7). High SiO2 (>75%) rocks with
high 87Sr/86Sri (indicated by asterisks in Fig. 8),
however, have εNd(i) values within the normal
range. These anomalously high 87Sr/86Sri values
likely reflect minor contamination by crustal
components, such as Calaveras Formation wall
rocks (Fig. 8), or, potentially, seawater-altered
metabasalts. Given the exceptionally low Sr
concentrations in these rocks (<49 ppm), only
very small degrees of contamination are necessary to greatly elevate 87Sr/86Sri ratios.
DISCUSSION
Flare-ups and Migration of the
Magmatic Arc
Plutons of the northern Sierra Nevada are
mainly calc-alkaline, subduction-related products that were generated episodically during
intrusive events in the Middle to Late Jurassic and
the mid-Cretaceous. Although there are plutons
00729 1st pages / page 7 of 15
Northern Sierra Nevada magmatism
15
3
All Sierra Nevada data
Cretaceous plutons
Jurassic plutons
2
TiO2
FeO
5
1
0
8
0
7
Rock / Chondrite
10
Cretaceous plutons
Jurassic plutons
100
10
All Sierra
Nevada data
A
1
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb
6
K2O
5
4
4
100
NaO2
3
2
2
1
Rock / Chondrite
6
B
0
15
0
80
10
1
La Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm Yb
2.0
60
10
CaO
5
20
0
40
50
60
80
70
SiO2
40
50
60
70
0
80
1.0
1000
G20 101.7 Ma; Ba/Th = 60; Sr/Y = 41; La/YbN = 18
G01 162.2 Ma; Ba/Th = 282; Sr/Y = 42; La/YbN = 9
Sample / N-MORB
100
10
1
0.1
Rb Ba Th K U Nb Ta La Ce Sr Nd Sm Zr Ti Hf Eu Gd Dy Y Er Yb Lu
of this general age in the main body of the batholith to the south, the distribution of ages reported
here is offset from recognized major, range-wide
apparent high-flux events (Ducea, 2001; Fig. 2).
There is an observed paucity of post–100 Ma
plutons in north compared with great volumes
of batholithic material emplaced into the central
and southern Sierra during the mid- to Late Cre-
taceous flare-up (ca. 100–85 Ma). Likewise, the
observed pulse of northern magmatism between
ca. 130 and 100 Ma (Fig. 2) is not as prevalent
in the southern and central parts of the batholith.
This could be explained by a southward shift in
magmatism through time, although intermediate arc-related basement rocks that are early to
mid-Cretaceous in age have been identified in
Geosphere, June 2012
Average = 0.93 ± 0.25
0.5
C
SiO2
Figure 5. Harker diagrams showing the variation of major oxides and Mg # (MgO/[MgO +
FeO]) with SiO2. All iron is represented as FeO.
Figure 6. Normal–mid-ocean
ridge basalt (N-MORB)–normalized trace element abundance
diagrams for representative
samples from the northern
Jurassic Sierra Nevada (G01)
and the northern Cretaceous
Sierra Nevada (G20). Both
show large-ion–lithophile element (LILE) enrichment, relative high field strength element
(HFSE) depletion, and strong
depletions in Ti and Nb. The
N-MORB values are from Sun
and McDonough (1989).
1.5
Eu/Eu*
Mg # 40
0.0
50
55
60
65
70
75
80
SiO2
Figure 7. (A and B) Rare-earth element
(REE) abundance diagrams normalized
to chondrite values (Sun and McDonough,
1989) for Cretaceous (A) and Jurassic
(B) plutons of the northern Sierra Nevada.
Shaded curve encompasses all REE data
available for batholithic rocks of the central
and southern Sierra Nevada (see Fig. 4 caption for a list of references). (C) Plot of europium anomaly (Eu/Eu*) versus SiO2. Dashed
lines indicate the average and 2σ standard
deviation of Eu/Eu* for all samples except
for the high (>70 wt%) SiO2 rocks outlined.
well cores from the eastern Great Valley at central to southern Sierra latitudes (Saleeby, 2007).
Alternatively, the age distribution presented
here, together with new insights from the Great
Valley subsurface, could be indicating that the
Cretaceous flare-up event was less punctuated
and occurred over a longer period of time than
previously estimated (Fig. 2).
The apparent north-south difference in the
timing of Cretaceous high-volume magmatism could be due in part to the covering of
younger plutons in the northeastern Sierra by
7
00729 1st pages / page 8 of 15
Cecil et al.
15
5
Jurassic plutons
DM
y
rra
ea
ntl
Ma
10
Cretaceous plutons
Average Nd-Sr of Sahwave
batholith, NV
SN wallrocks (Calaveras Fm)
εNd(i) 0
EM
*
–5
T
Y
*
–10
Crustal
contamination
–15
0.700
0.702
0.704
0.706
0.708
0.710
0.712
0.714
87
Sr/86Sr(i)
Figure 8. Initial 87Sr/ 86Sr versus initial εNd for northern Sierra Nevada
plutons, compared with initial Nd-Sr fields from the Tuolumne intrusive suite (T; Bateman and Chappell, 1979), Yosemite (Y; Ratajeski
et al., 2001), and the Sahwave batholith, north-central NV (yellow
diamond; Van Buer and Miller, 2010). Sierra Nevada wallrock data
are from the Late Paleozoic Calaveras Formation (DePaolo, 1981).
DM—depleted mantle reservoir. EM—enriched (slab + pelagic sediments) mantle reservoir.
late Cenozoic volcanic and clastic deposits. It is
more likely, however, to be due to the widening of the batholith to the north. In the southernmost Sierra Nevada (35.5 °N), the width of
the exposed batholith is ~60 km, and it widens
gradually along strike to the north such that at
the latitudes of this study, it is roughly 100 km,
from outcroppings of Jura–Cretaceous plutons
in the western foothills to exposed granites east
of the range crest. In the north, the locus of Late
Cretaceous (post–100 Ma) magmatism coeval
with large intrusive suites of the eastern Sierra
Nevada (e.g., the Sonora Pass, Tuolumne, John
Muir, and Mount Whitney suites) is shifted to
the northeast, up to 200 km east of the modern
range crest (Smith et al., 1971; Barton et al.,
1988; Van Buer and Miller, 2010).
Because of its age, presumed volumetric
significance (covering an area of >1000 km2),
geochemistry, and zonation pattern, the Sahwave batholith of NW Nevada is considered
part of the greater Sierra Nevada batholith, suggesting that it is continuous across the modern
divide and into Nevada (Van Buer and Miller,
2010). Even accounting for maximum amounts
of Cenozoic extension across northern California and northwestern Nevada (~35%; Surpless
et al., 2002; Colgan et al., 2006a, 2006b), the
eastern extent of the magmatic Sierra in the
north is far greater than it is in south, stretching the batholith to ~180 km in width at 40 °N.
Subsurface sampling of the north-central Great
8
Valley basement near Sutter Buttes (Fig. 1)
reveals that it is underlain by mafic and intermediate batholithic products (Williams and
Curtis, 1977). Similar rocks have been observed
in cores from other parts of the eastern Great
Valley and, where dated, yield early Cretaceous
ages ranging from ca. 140 to 130 Ma (Saleeby,
2007). This further increases the overall dispersion of the northern Sierra Mesozoic batholith to
≥200 km. However, the width of the arc at any
given 10 My time period was probably not more
than 60 km, similar to the modern frontal arcs of
the Andes (e.g., Mamani et al., 2010).
The presence of Cretaceous Sierran magmatism both east and west of the modern topographic range helps account for the apparent
reduction in batholithic volume in the northern
Sierra Nevada, while highlighting the difference
between voluminous, spatially-concentrated
magmatism in the south and dispersed magmatism in the north. Because of the northward
widening of the batholith, the apparent rate of
eastward migration of Cretaceous magmatism
in the north is also greater (Fig. 3). In the central Sierra, Chen and Moore (1982) estimate
eastward propagation of the arc at 2.7 km/Ma,
whereas in the northern Sierra, that estimate
increases to 4.8 km/Ma. The widening of the
batholith to the north coincides with the widening and dispersal of the Mesozoic thrust belt
in the retroarc. At the latitudes of the southern
Sierra Nevada (~35 °N–37.5 °N), the eastern
Geosphere, June 2012
Sierra thrust belt is confined to a narrow belt
of folds and mylonitic thrusts along the eastern margin of the batholith (Dunne et al., 1978,
1983; Walker et al., 1990; Dunne and Walker,
1993). This narrow belt transitions to a wider
belt to the north, where the westernmost zone of
Mesozoic deformation, the Luning-Fencemaker
fold and thrust belt, defines a broader zone of
mainly thin-skinned imbricate thrusts (Wyld,
2002; DeCelles, 2004). This pattern suggests
that the delivery of continental lithosphere components from the foreland region into the magma
source region by retroarc thrusting was more
vigorous in the south, potentially driving a more
spatially concentrated pattern of magma genesis
and resulting composite batholith growth.
High-Flux Events in the Northern
Sierra Nevada
The northern Sierra Nevada arc was assembled in a non-steady state, with times of higher
fluxes separated by magmatic lulls (Fig. 2),
similar to the southern Sierra Nevada and most
other Cordilleran arcs studied in some detail to
date (e.g., Gehrels et al., 2009; Coast Mountains batholith). What stands out for the northern Sierra Nevada transects is the high apparent
intrusive flux during the earlier, Jurassic stage
of magmatism. This is similar to the Coast
Mountains batholith, where the initial stages of
magmatism were characterized by high apparent fluxes, intermediate to felsic compositions,
and high δ18O (Wetmore and Ducea, 2011), suggesting in that case, that the fertile framework
represented by continental miogeoclinal rocks
provided an important mass contribution to the
magmatic budget.
We suggest a similar scenario for the northern Sierra Nevada, where the arc was built
from its inception onto the continental margin, albeit one comprising some accreted (and
primitive) terranes. This contrasts with various
peri–American island arcs of Jurassic age that
are primitive and more mafic in composition
(e.g., the Talkeetna arc in Alaska; DeBari and
Sleep, 1991) or the Jurassic arc segments found
along the western margin of South America in
Chile (Oliveros et al., 2007). There are no good
estimates of magmatic fluxes for those arcs, but
there is every indication that they were built as
island arcs away from the continental margin
and were subsequently docked. The northern
Sierra Nevada example supports a previous suggestion that arcs developing along a continental margin have high magmatic fluxes initially
because they are inevitably built on a meltfertile sequence of passive margin (miogeoclinal) assemblages (Ducea et al., 2010). The
later, mid-Cretaceous flare-up in the northern
00729 1st pages / page 9 of 15
Northern Sierra Nevada magmatism
8
Sierra Nevada and the latest Cretaceous one
east of the range, represent more typical mature
arc events that formed in response to the overall
cyclic nature of foreland deformation, crustal
thickening, and magmatism (DeCelles et al.,
2009), which are superimposed on the constant baseline flux of hydrous melting from the
mantle wedge (Grove et al., 2003).
6
Cretaceous plutons
Jurassic plutons
Central Sierra plutons
4
εNd(i)
2
0
~ –0.2, 100 km E
–2
–4
–6
Emplacement of the Northern Sierra
Nevada Batholith into Primitive and
Heterogeneous Crust
–8
–10
0.707
0.705
~ 0.7048, 100 km E
Sri 0.706 line
87
Strontium and neodymium isotopes from
both Jurassic and Cretaceous plutons in the
northern Sierra Nevada are variable but trend
toward more primitive values than those in the
south. The distribution of Sr-Nd signatures
from northern plutons is consistent with a twocomponent mixing of depleted, mantle-derived
magmas with evolved metasedimentary wall
rocks (Fig. 8). The Jurassic Yuba River (G01)
and Pleasant Valley (G02) plutons intruding Panthalassan lithosphere in the western
foothills have distinctively primitive 87Sr/86Sri
(0.7030–0.7035) and εNd(i) (–0.3–+2.7), limiting
the reservoir to isotopically juvenile oceanic arc
rocks and/or altered MORB. Large, Late Cretaceous intrusive suites of the central and southern
Sierra show a greater enrichment in radiogenic
strontium than Cretaceous plutons in the northern Sierra and northwest Nevada, indicative of
a major along-strike change in magma source,
which incorporates older, continental material
in the south.
Initial εNd and 87Sr/86Sri plotted along rangeperpendicular transects reveal a trend toward
more evolved isotopic compositions from the
foothills east to near the range crest, and a trend
toward more primitive values eastward from the
range crest (Fig. 9). This is in contrast to the central and southern Sierra, which was emplaced
across a presumed boundary between accreted
Phanerozoic rocks to the west and Proterozoic
continental lithosphere to the east, such that
isotopic values evolve consistently eastward
toward more continental values (Fig. 9B). New
investigation of pendant rocks in the central and
southern Sierra indicates that that portion of the
batholith was likely emplaced into transitional
lithosphere and the western margin of continental lithosphere coincided with the location of
the eastern Sierra thrust belt (J. Saleeby, 2011,
personal commun.). The close proximity to,
and underthrusting of, continental lithosphere
from the retroarc imparts the eastward progressive and more evolved isotopic signatures in
the south. To the north, the boundary between
Panthalassan and continental lithosphere not
only becomes more diffuse, but is truncated by
Sr/86Sr(i)
0.709
0.703
0.701
–100
–50
0
50
Distance east of the range crest (km)
Figure 9. Initial εNd and 87Sr/ 86Sr plotted as a function of distance
across the range. Nd and Sr both show systematic eastward trend
toward more evolved values, which are most pronounced immediately west of the range crest (gray zone). East of the range crest,
values trend back toward more primitive values, mimicking the
pronounced bend in the Sr 0.706 isopleth in the northern Sierra
Nevada (see Fig. 1). At ~100 km east of the range crest (accounting
for postmagmatic extension), isotopic values of the Sahwave batholith are yet more primitive than those from the northeastern Sierran
(Sri ≅ 0.7045; εNdi ≅ 0). With regard to Sr, this is in contrast to the
central (and southern; not shown) parts of the batholith, shown in
black squares. Central Sierra Sr data are from a range perpendicular transect at ~36.5 °N (compiled by Lackey et al., 2008; data from
Chen and Tilton, 1991; Sisson et al., 1996; Coleman and Glazner,
1998; Wenner and Coleman, 2004). ).
strike-slip faults in the backarc (e.g., Oldow,
1983), thereby apparently bending the Sri 0.706
line and imparting an appendage-like shape to
the isotope data. Ignoring the ca. 162 Ma Yuba
River and Pleasant Valley plutons, there is no
correlation between age and isotopic character
of plutons, suggesting that observed isotopic
variability is controlled by heterogeneity in
relatively primitive crustal and upper mantle
magma source regions.
Despite clear along-strike changes in the
character of magmatic source, as evidenced
from changing isotopic values, major and trace
element data are remarkably similar along all
segments of the Sierran batholith. This can be
seen via comparison of major element trends
(Figs. 4 and 5), REE element trends (Fig. 7),
and selected trace element ratios (Fig. 10). This
Geosphere, June 2012
indicates that the juvenile nature of the lithosphere in the northern Sierra Nevada (its age,
chemistry, mineralogy, thickness, etc.) does
not preclude the generation of significant volumes of fractionated granitoids. This has been
documented in other geologic settings and is
perhaps best exemplified by the generation of
the great Coast Mountains batholith of British
Columbia and southeast Alaska (e.g., Mahoney
et al., 2009; Wetmore and Ducea, 2011). The
non-dependence of source lithosphere type on
granitoid melt production can be seen in plots
showing no relationship between Sr/Y and
La/Yb, proxies for depth of melting, and initial
87
Sr/86Sr, a proxy for type and/or maturity of preexisting lithosphere (Fig. 11). The geochemical
similarity of plutons emplaced into fertile continental lithosphere (southern Sierra), an amalga-
9
00729 1st pages / page 10 of 15
Cecil et al.
40
120
10
100
7% garnet
amphibolite
30
La/Yb 20
80
Sr/Y
Cretaceous plutons
Jurassic plutons
Sahwave Batholith
Yosemite Intrusives
SW Sierra foothills
A
Cretaceous plutons
Jurassic plutons
Average metabasalts
(Smartville Complex, SN)
30% garnet amphibolite
20
30
60
20
10
Hi Sr/Y
“Adakite” field
120
40
30
60
50
20
100
Normal intermediate
arc field
B
80
Sr/Y 60
0
0
10
20
30
40
50
Y
20
Figure 10. Plot of Sr/Y versus Y for northern Sierra Nevada plutons (large
circles) and regional data for the entire Sierran batholith. High Sr/Y (“adakite”) and normal intermediate (dacite-andesite) arc fields are from Drummond and Defant (1990). Partial-melting trends from an amphibolite source
with variable garnet abundances (7%–30%) are from Petford and Atherton (1996). Yellow star is the average of metabasaltic basement rocks of the
Smartville complex (part of the Panthalassan belt; see Fig. 1) (Beard, 1998)
and is similar to source compositions used by Petford and Atherton (1996) in
their modeling.
mation of accreted Phanerozoic belts (northern
Sierra), and basinal terranes (northwest Nevada;
Van Buer and Miller, 2010), attests to alongstrike similarity in style of Sierran batholithic
petrogenesis.
Constraints on Depth of Melting and
Associated Batholithic Residues
Plutons of the northern Sierra Nevada are
interpreted to have been generated through
melting of deep crustal sources, based on REE
fractionation patterns, the absence of Eu anomalies, and elevated La/Ybnorm and Sr/Y ratios, all
of which are discussed in detail below.
Rare-earth element patterns for northern
Sierra plutons are moderately steep, characterized by both enrichment in LILE and relative
depletion in HREE, and show mildly concave upward “dished” middle-REE depletion
(Fig. 7). These trends are consistent with low
to moderate degrees of partial melting in equilibrium with a residue rich in amphibole, which
preferentially incorporates middle-REE, and
modest amounts of garnet. The above described
REE trends are evident in both Jurassic and
Cretaceous plutons, although REE patterns of
10
40
0
0.702
0.704
0.705
0.706
0.707
0.708
0.709
87
Sr/86Sr(i)
Figure 11. Plots revealing a lack of correlation between
La/Yb (A) and Sr/Y (B) and initial 87Sr/ 86Sr. Data are
from Van Buer and Miller (2010; Sahwave batholith);
Ratajeski et al. (2001; Yosemite); and Clemens-Knott
(1992; SW Sierra foothills).
Jurassic rocks are more variable and tend to be
slightly less steep (average Jurassic La/Ybnorm ≅
9; average Cretaceous La/Ybnorm ≅ 11). Given
that average Gd/Yb ratios (a measure of HREE
differentiation) are the same, but La/Sm (a
measure of LREE differentiation) are higher
in Cretaceous plutons, the observed difference in
overall steepness is apparently due to greater
LREE enrichment in Cretaceous plutons. This is
similar to REE trends for the Peninsular Ranges
batholith observed by Gromet and Silver (1987),
who document increases eastward and through
time in La/Ybnorm and LREE enrichment. The
difference in La/Ybnorm between Jurassic and
Cretaceous magmatic episodes could be due to a
decrease in the degree of partial melting, and/or
a change in the mineralogy of the source residua
over time.
All rocks, with the exception of the high SiO2
plutons, have very small Eu anomalies (average
Eu/Eu* = 0.93; Fig. 7C). Eu2+ can substitute
for inter-tetrahedral cations in feldspars and is
strongly partitioned by plagioclase (Weill and
Drake, 1973) and alkali feldspars (e.g., Ren,
2004). It does not, however, have an affinity for
occupying Ca sites in amphibole or clinopyroxene. In fact, partition coefficient patterns of REE
Geosphere, June 2012
0.703
between amphibole and silicic melts show negative Eu anomalies (e.g., Sisson, 1994). Thus, the
lack of a large Eu depletion signal in Sierran
granitoids indicates the generation of granitoid
melts in equilibrium with a relatively feldsparpoor source. Some residual feldspar is certainly
permissible, however, given the competing
effects of feldspar (Eu relatively compatible)
and amphibole (Eu relatively incompatible).
In addition to having small to no Eu anomalies, most northern Sierran intrusives have high
Sr concentrations (average ≅ 450 ppm), high
Sr/Y ratios (up to 75), and low Y concentrations
(average ≅ 14 ppm) (Fig. 10). Relative depletion in the HREE, high Sr/Y, high Sr, and low
Y are all characteristics of magmas produced
at depth from the melting of a subducting
mafic slab (Kay, 1978; Defant and Drummond,
1990). They are also characteristic of magmas
produced via melting of amphibolite in the
deep subarc crust in equilibrium with a garnet
+ amphibole + clinopyroxene residue (Petford
and Atherton, 1996; Stevenson et al., 2005;
Mahoney et al., 2009). A plot of Sr/Y against
Y concentration in northern Sierra Nevada plutons reveals trends consistent with those from
experimental batch partial-melts of source
00729 1st pages / page 11 of 15
Northern Sierra Nevada magmatism
amphibolite with garnet abundance ranging
between 7% and 30% (Petford and Atherton,
1996) (Fig. 10). Batholithic residua likely to be
amphibole rich, relatively feldspar poor, and to
have modest amounts of garnet, are consistent
with high pressure (>10 kbar) melting experiments of a basaltic source (e.g., Rapp et al.,
1991; Wolf and Wyllie, 1993, 1994). The production of northern Sierra Nevada granodiorites
and tonalites is therefore loosely constrained to
depths ≥35 km and is interpreted to result from
the processing of metabasaltic sources in the
lower arc crust.
The Presence and Fate of a Dense
Batholithic Root in the North
Studies of the southern Sierra Nevada suggest that it was composed of a thick (35–40 km;
Pickett and Saleeby, 1993, 1994; Ducea, 2001)
column of felsic and intermediate rocks underlain
by a dense, garnet and clinopyroxene-rich residue (Ducea and Saleeby, 1996, 1998a; Saleeby
et al., 2003). This deep residue, on the basis of
its mineralogy, was termed an “eclogitic” root
(Saleeby et al., 2003), also informally known as
“arclogite.” Fundamental changes between lower
crustal and upper mantle xenoliths entrained in
Miocene basalts and those entrained in Plio–
Quaternary basalts, provide evidence that such
an eclogitic root existed at the base of the felsic
batholith in the Miocene, but was subsequently
lost from beneath the southeastern Sierra by
ca. 3.5 Ma (Ducea and Saleeby, 1996, 1998a,
1998b; Farmer et al., 2002; Saleeby et al., 2003).
Foundering of the eclogite root accounts for the
present-day modest crustal thickness (~35 km)
of the southern Sierra Nevada (Park et al., 1995;
Jones et al., 1994; Zandt et al., 2004), and its
replacement by hot asthenosphere has been
called upon to explain recent uplift and high
topography in the southern Sierra (Jones et al.,
2004) and Cenozoic basaltic volcanism (Manley
et al., 2000; Farmer et al., 2002). The continued
attachment of the root beneath the west-central
Sierra Nevada has likewise been associated with
tectonic subsidence there (Saleeby and Foster,
2004). As such, the development and removal of
an eclogitic root has played a fundamental role in
the evolution of the central and southern Sierra.
Much less understood in this context, however,
is the northern Sierra, which may have had a
much different tectonic history, as inferred from
the different ages and distribution of plutons and
the lower and more subdued topography. Indeed,
a major focus of SNEP was to image the lithospheric structure of the northern Sierra Nevada
and to learn more about the potential development and/or loss of a high-density batholithic
root there.
In terms of the exposure of batholithic rocks
within the modern topographic range, the northern Sierra Nevada appears to have volumetrically less material than the main batholith to the
south. Xenolith studies, as well as experimental
studies of partial melts derived from hydrous
mafic lithologies, indicate that only magmatic arcs producing large volumes and thick
(>25 km) columns of felsic material will generate gravitationally unstable roots (e.g., Ducea,
2002). Similarly, it has been suggested that
sourcing of melts in oceanic lithosphere reduces
the potential for generating garnet-rich lithologies (e.g., Ducea and Saleeby, 1998a). It could
be argued, therefore, that the northern batholith
was less likely to have developed a dense residual root. The lack of lower crustal and/or mantle
xenoliths in the northern Sierra Nevada prevents
direct sampling of the batholith at depth there.
However, diagnostic trace elements evidence
presented here for deep processing of magmas
in the northern Sierra implies: (1) that the crust
was reasonably thick (>>35 km) in the Mesozoic; and (2) that a dense mafic residue was
produced during batholith generation. Thus,
despite the patchy exposure of northern Sierra
plutons and their emplacement into relatively
juvenile lithosphere, it is likely that the bulk
of the crustal column there was composed of a
thick section of intermediate to felsic batholithic
products floored by a dense residual root. This
is in contrast to proposed estimates of thin crust
during Mesozoic time in northwestern Nevada,
based on limited Cenozoic crustal extension
there (Colgan et al., 2006a, 2006b).
Seismic receiver-function analysis of crust
and upper mantle structure in the Sierra Nevada
reveals a bright, “delamination” Moho at ~30 km
depth in the southern and eastern parts of the
range, which is interpreted to be tectonic in origin and generated by a large step in wave speed
between relatively homogeneous continental
crust and asthenosphere below (Zandt et al.,
2004; Frassetto et al., 2011). This delamination
Moho is thought to result from the foundering
of dense, negatively buoyant lithosphere, and its
replacement by asthenosphere at shallow levels.
The Moho dips westward, becoming deep and
significantly weaker, in seismic expression,
beneath the central and western portions of the
Sierra Nevada. Areas where the Moho appears
weak correlate well with a tomographically
imaged, high-speed anomaly at depth (Gilbert
et al., 2008; Reeg et al., 2008; Schmandt and
Humphreys, 2010). Together, these observations
have been interpreted as a dense root, which is
attached to the base of the Sierran crust and
presently in the process of foundering into the
mantle. This explains the high-speed anomaly
at depth, as well as the weak Moho, since the
Geosphere, June 2012
crustal root appears seismically fast and similar
to the mantle with which it is in contact. Seismic imaging (e.g., Zandt et al., 2004; Schmandt
and Humphreys, 2010; Frassetto et al., 2011),
together with heat-flow measurements (Saltus
and Lachenbruch, 1991) and studies of xenoliths (Ducea and Saleeby 1996, 1998a, 1998b)
and young volcanics (Manley et al., 2000;
Farmer et al., 2002), form a coherent picture of
evolving lithosphere through time and space in
the southern and eastern Sierra Nevada region.
Conversely, the crustal structure and lithospheric evolution of the northern Sierra Nevada
has remained ambiguous. Large-scale tomographic studies of the western United States
reveal the absence of dense, over-thickened
crust in the northern Sierra Nevada region
(e.g., Yang et al., 2008). A recent study using
receiver-function analysis (Frassetto et al.,
2011) shows a continuous Moho at ~35 km
depth beneath the northern Sierra Nevada that
dips shallowly westward and appears stepped
(deeper to the east) at the approximate eastern margin of the metamorphic foothills belt.
Although brighter than the weak Moho recognized in the central and western Sierra, the
amplitude of the northern Moho is not as high
as that of the delamination Moho to the south
and east, and has been interpreted as resulting
from extension and volcanism associated with
the westward encroachment of the Basin and
Range (Frassetto et al., 2011).
Given the geochemical evidence supporting
the development of a dense, mafic batholithic
residue beneath the northern Sierra, we propose
that the observed northern Moho is the result
of sharpening due to post-Mesozoic removal of
that residue. We accept, however, that this assertion is problematic in the absence of volcanism
attributable to convective removal of the lithosphere and replacement by hot asthenosphere
and/or geomorphic signals that can be clearly
related to a specific tectonic event or events.
Although marked incision of river channels
in the central and northern Sierra (e.g., Wakabayashi and Sawyer, 2001; Stock et al., 2004)
indicate relative base-level changes in the Pliocene, it is not clear whether or not such baselevel changes were generated by a climatic or a
tectonic perturbance. It is also not clear what the
triggering mechanism for root loss would have
been. The northern Sierra Nevada batholith is
more dispersed and likely less voluminous than
the southern part of the batholith. It is plausible
that instead of a singular, large plug of dense
material at the base of the felsic crust, there
developed multiple, but smaller lenses of maficultramafic cumulates. Given appropriate thermal and rheological conditions not uncommon
in arc settings, it is possible for those cumulates
11
00729 1st pages / page 12 of 15
Cecil et al.
to be convectively removed shortly after they are
formed (Jull and Kelemen, 2001). These downwellings would have occurred in Mesozoic time
and may not require a dramatic, wholesale overturning of the upper mantle, potentially explaining the lack of delamination-driven geomorphic
and volcanic features in the northern Sierra
region. Alternatively, the northern Sierra Nevada
could have developed a dense eclogitic root
similar to that proposed for the southern Sierra,
but it was thermally destabilized by eruptive
events of the proto–Cascade arc at ca. 15 Ma,
which initiated along the eastern margin of the
northern Sierra Nevada batholith (Busby et al.,
2008a, 2008b). It is conceivable that proto–Cascade arc volcanism weakened the lithosphere,
leading to a delamination event, the formation
of the Sierra Nevada microplate (Busby et al.,
2008b; Busby and Putirka, 2009), and eruption
of the Table Mountain latite at ca. 10.4 Ma (e.g.,
Pluhar et al., and references therein). In such
a scenario, modification of the northern Sierra
Nevada landscape and/or volcanism resulting
from foundering of the dense root could be
attributed to Basin and Range extension and the
development of the ancestral Cascades.
garnet residua. On this basis, we propose that
the northern Sierra Nevada developed a dense,
“arclogitic” root similar to the one sampled by
Miocene lower crustal xenoliths in the southern
Sierra, even though the greater spatial distribution of plutons in the north implies a more dispersed and thinner root mass there. Because the
crust is only ~35 km thick today and is underlain by a seismically strong Moho, we suggest
that the northern dense root was convectively
recycled back into the mantle, perhaps in
response to the initiation of proto–Cascade
arc volcanism and the initiation of the Sierra
Nevada microplate.
ACKNOWLEDGMENTS
The authors thank Victor Valencia for help in the
University of Arizona LaserChron laboratory. We are
grateful to Craig Jones and an anonymous reviewer for
their thorough and thoughtful reviews, which greatly
improved the manuscript. This research was supported
by National Science Foundation (NSF) awards EAR0606967 (Continental Dynamics Program) to Ducea,
EAR-0732436 for support of the Arizona LaserChron
Center (Gehrels), and by the George and Betty Moore
Foundation, Caltech Tectonics Observatory Number
171 (Saleeby and Cecil).
APPENDIX
CONCLUSIONS
The data presented here afford new insights
into the petrogenetic and tectonic development
of the northern segment of the Sierra Nevada
batholith. Documentation of the geochemical
and isotopic character of a regional suite of
northern plutons allows for the identification
of spatial and temporal variations in a longlived continental magmatic arc system. New
U-Pb geochronology data argue for the widening and dispersal of the batholith to the north,
which has important tectonic implications for
the spatial concentration of magmatic products
and their related residues. A preponderance of
early to mid-Cretaceous pluton ages further suggests that the documented Cretaceous flare-up
event was less punctuated and longer-lived than
previously estimated. Principal findings, based
on new major and trace element chemistry, and
Nd-Sr isotopes, are that plutons of the northern Sierra Nevada were sourced from—and
emplaced into—Phanerozoic oceanic and juvenile lithospheric terranes, but are nonetheless
geochemically very similar to those emplaced
into less heterogeneous and more evolved lithosphere in the southern Sierra. This suggests that
younger and/or thinner lithosphere does not preclude the development of thick crust or dense
residual phases. Perhaps most interesting is the
recognition that northern plutons have clear geochemical signatures consistent with equilibration
of magmas at depth (>35 km) with amphibole +
12
Concentrations of Rb, Sr, Sm, and Nd were determined by isotope dilution, with isotopic compositions determined on the same spiked runs. An off-line
manipulation program was used for isotope-dilution
calculations. Typical runs consisted of acquisition of
100 isotopic ratios. The mean result of ten analyses
of the Rb standard NRbAAA performed during the
course of this study is: 85Rb/ 87Rb = 2.61199 ± 20. Fifteen analyses of the Sr standard Sr987 yielded mean
ratios of: 87Sr/ 86Sr = 0.710285 ± 7 and 84Sr/ 86Sr =
0.056316 ± 12. The mean results of five analyses of
the Sm standard nSmb performed during the course
of this study are: 148Sm/147Sm = 0.74880 ± 21, and
148
Sm/152Sm = 0.42110 ± 6. Fifteen measurements
of the La Jolla Nd standard were performed during
the course of this study. The standard runs yielded the
following isotopic ratios: 142Nd/144Nd = 1.14184 ± 2,
143
Nd/144Nd = 511853 ± 2, 145Nd/144Nd = 0.348390 ±
2, and 150Nd/144Nd = 0.23638 ± 2. The Sr isotopic
ratios of standards and samples were normalized to
86
Sr/ 88Sr = 0.1194, whereas the Nd isotopic ratios were
normalized to 146Nd/144Nd = 0.7219. The estimated
analytical ±2σ uncertainties for samples analyzed in
this study are: 87Rb/ 86Sr = 0.35%, 87Sr/ 86Sr = 0.0011%,
147
Sm/144Nd = 0.2%, and 143Nd/144Nd = 0.0010%. Procedural blanks averaged from five determinations
were: Rb 6 pg, Sr 110 pg, Sm 2.7 pg, and Nd 5.0 pg.
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