Earth and Planetary Science Letters 225 (2004) 147 – 161
www.elsevier.com/locate/epsl
Franciscan subduction off to a slow start: evidence from
high-precision Lu–Hf garnet ages on high grade-blocks
Robert Anczkiewicz a,b,*, John P. Platt a, Matthew F. Thirlwall b, John Wakabayashi c
b
a
Research School of Earth Sciences at UCL-Birkbeck, Gower Street, London WC1E 6BT, UK
Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK
c
1329 Sheridan Lane, Hayward, CA 94544, USA
Received 11 November 2003; received in revised form 15 March 2004; accepted 3 June 2004
Available online 21 July 2004
Editor: B. Wood
Abstract
Lu – Hf analyses of garnet from metabasic amphibolite, glaucophane schist and eclogite facies blocks from the Franciscan
complex give highly precise ages that allow us to place new constraints on the early thermal history of the Franciscan
subduction zone. Garnets yield 176Lu/177Hf ratios ranging from 1.5 to 28 with the highest ratios from garnets with high
spessartine/pyrope ratio. Sulphuric acid leaching (SAL) of garnets revealed the presence of inclusions with significantly higher
Lu/Hf ratios than those of garnet itself (most likely apatite). Their removal by SAL brings the 176Lu/177Hf ratios in garnets down
by as much as 40%. This suggests that 176Lu/177Hf ratios of apparently pure garnets can be greatly overestimated due to the
presence of such inclusions. Sm – Nd garnet analyses were dominated by inclusions (mainly sphene), and failed to provide
precise and accurate age information.
The oldest Lu – Hf ages are 168.7 F 0.8 and 162.5 F 0.5 Ma on plagioclase-bearing garnet amphibolite from Panoche Pass
and the Berkeley Hills, respectively, which suggests initiation of the subduction zone at about 169 Ma, coeval with the
formation of the tectonically overlying Coast Range Ophiolite. Relatively high temperature conditions persisted for about 14
Ma as indicated by 153.4 F 0.8 Ma garnet growth recorded in epidote amphibolite and 157.9 F 0.7 in eclogite from Ring
Mountain and Jenner, respectively. A 146.7 F 0.7 Ma age was obtained from garnet glacuophane schist, metamorphosed at
around 400 jC. The sequence of ages from central and northern California shows a younging trend with decreasing
metamorphic grade, which supports previous suggestions that the high-grade metamorphic blocks and slices resulted from
progressive underthrusting and underplating in a cooling subduction system. Combining geothermometry with
geochronological data allow us to estimate cooling rate along the subduction zone interface from amphibolite to blueschist
facies conditions as ca. 15 jC/Ma. The thermal history requires high initial geothermal gradients within both the footwall and
the hangingwall of the subduction zone and a relatively slow subduction rate of the order of 10 km/Ma during the initial stages
of Franciscan subduction. Such conditions are consistent with initiation of the subduction zone at or close to an oceanic
spreading centre. The data also suggest slow exhumation rates and significant residence time at depth of the earliest
Franciscan rocks.
* Corresponding author. Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK. Tel.: +44-1784414045; fax: +44-1784-471780.
E-mail address: rob@gl.rhul.ac.uk (R. Anczkiewicz).
0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2004.06.003
148
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
A much younger age of 114.5 F 0.6 Ma on garnet hornblendite from Santa Catalina Island confirms significantly younger
initiation of the subduction zone in Southern California.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Franciscan complex; subduction; geochronology; garnet; Lu – Hf; Sm – Nd
1. Introduction
The initiation of subduction zones has long been
a topic of speculation [1,2], related to a central
paradox: the primary driving force for subduction is
the negative buoyancy of old, cold oceanic lithosphere, but such lithosphere is inherently strong and
difficult to rupture. The emplacement of young, hot
oceanic lithosphere onto continental margins in
convergent tectonic settings to form ophiolite complexes such as the Semail ophiolite in Oman [3]
and the Bay of Islands ophiolite in Newfoundland
[4] has led to suggestions that subduction may in
fact be initiated at or close to spreading ridges as a
result of a local change in plate kinematics. Ophiolites in these settings commonly have ‘‘soles’’ of
high-temperature metamorphic rocks along their
lower boundaries.
The mid-Mesozoic Coast Range Ophiolite of California formed immediately before the initiation of the
subduction zone that led to the formation of the
Franciscan accretionary complex, suggesting that similar processes may have been involved in this event.
Various more or less complicated scenarios have been
suggested for this episode e.g. [5 –7], and it seems
likely that collision of either an island arc or the Coast
Range Ophiolite itself with the earlier Nevadan active
margin in eastern California resulted in the westward
step-out of active subduction into what are now the
Coast Ranges. The close association in time between
the ophiolite at 164 – 170 Ma [8] and the oldest
Franciscan rocks is generally accepted [9,10]. The
distinctive eclogite and garnet-amphibolite blocks that
litter the Franciscan are believed to be the disrupted
remnants of a thin zone of relatively high-temperature
metamorphism lying immediately beneath the hanging-wall mantle wedge in the newly initiated subduction zone [9,11– 13], at a time when temperatures in
the hangingwall of the subduction zone were sufficiently high to cause significant transient heating in
the immediate footwall [14]. Estimated pressure –
temperature (PT) conditions for these blocks are in
the range 550 – 700 jC, 1.0– 1.4 GPa (equivalent to
depths of 32 – 45 km beneath oceanic crust and
lithosphere). Hangingwall temperatures at these
depths must have been >1000 jC to cause footwall
temperatures to rise to the temperatures inferred for
the highest grade blocks, implying that the ocean
lithosphere was very young at the time. Hence the
age of the highest grade blocks should be a good
indicator of the time of inception of the subduction
zone. There is a close analogy between this proposed
zone of high-T metamorphism beneath the Coast
Range ophiolite and the metamorphic soles found
beneath ophiolites emplaced onto continental margins.
Ar – Ar and K – Ar data from Franciscan tectonic
blocks suggest mainly Jurassic ages of metamorphism
in the range 140– 160 Ma [15], close to the generally
accepted age of the Coast Range ophiolite [8]. Ar
dates on amphibole and white mica are likely to be
cooling ages, however, and their interpretation is
hampered by the fact that metamorphic rocks with
anhydrous protoliths are particularly susceptible to
both inherited and excess Ar (see [16] for review).
In view of this, we have determined the timing of
eclogite – amphibolite- and glaucophane-schist facies
metamorphism in a number of high-grade blocks
using the Lu – Hf isotopic system applied to garnet.
Garnet is a good indicator of deep burial and high P/T
ratio of metamorphism, particularly in mafic rock
compositions. Its ability to strongly fractionate Lu
and Hf results in very high 176Lu/177Hf ratios, which
enables very precise ages to be obtained. High Lu/Hf
ratios together with slow diffusion rates, and the
possibility of determining a direct link between ages
and PT conditions, e.g. [17,18], make this technique
particularly powerful and suitable for dating highgrade metamorphism. High age resolution among
the various blocks in the Franciscan Complex allows
us to place some limits on the thermal structure and
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
the rate of motion during the earliest stages of
Franciscan subduction.
2. Analytical procedures
Sample preparation, sulphuric acid leaching (SAL)
and sample digestion follow [19]. Below we indicate
modifications made to those procedures in order to
adapt the chemistry for combined Sm –Nd and Lu –Hf
analyses on a single mineral separate.
All mineral fractions are dissolved on a hotplate in
TeflonR beakers. The major advantage of using hotplate rather than hydrothermal dissolution is that
zircons, which are one of the main Hf carriers, do
not dissolve well under such conditions and hence
their contribution to Lu – Hf budget in garnet is
limited.
Cleaned mineral separates are spiked and treated
with a 3:1 HF:HNO3 mixture for 1– 2 days at 120–
160 jC. After evaporating to dryness, the residue is
treated three times with 150– 250 Al of concentrated
HNO3 in order to break down residual fluorides.
Subsequently 2 –6 ml (depending on sample size) of
6N HCl:0.1N HF is added and left on a hotplate for at
least 24 h at 120– 160 jC. At this stage samples are
completely dissolved and are assumed to be equilibrated with the spikes. In the next step, samples are
evaporated to dryness and treated twice with ca. 1 ml
of 6N HCl.
Hf, Lu + Yb and light REE fractions are first
separated on a standard cation exchange column
(AG50W-X8 resin, 200 – 400 mesh size) based on
the modified procedure of [20]. Column size was
scaled down by a factor of two and cleaning was
achieved by using alternating 6N HCl and 6N HCl:1N
HF. Hf fractions of large samples were passed once
more through the same column in order to achieve
better purification of matrix elements. Final purification of Hf from other HFSE takes place on a LnspecR column based on [21]. Such Hf purification
completely eliminates Lu and Yb interferences on
176
Hf.
Sm and Nd are separated on a smaller size LnspecR column following a procedure modified from
[22]. The Lu + Yb fractions eluted from the first
column contain some Gd, Dy and Tb whose oxides
and hydroxides cause undesirable interferences on Yb
149
and Lu masses. Although for all samples analyzed in
this study such ‘‘contamination’’ was small, routine
purification of the Lu + Yb fraction from interfering
elements is achieved using the same Ln-specR column as for Sm – Nd separation. The column is cleaned
with 6N HCl and the sample is loaded and eluted in
3N HCl. This method eliminates all interfering elements and also allows reduction of the Yb/Lu ratio to
about 1:1. This leaves sufficient amount of Yb for
precise fractionation correction and reduces interference correction of Yb on 176Lu.
Because only a small amount of Hf was available
for these analyses (usually about 10 ng), all elements
were analyzed in a static, hard extraction mode using
the Royal Holloway IsoProbeR. Mass spectrometry
procedures follow [23]. Total procedure analytical
blanks for Hf and Nd were < 20 pg. External reproducibility and the reference ratios are reported in the
footnote to Table 2. Non-radiogenic ratios for the
studied samples are reported in [23].
3. Sample locations and petrography
We have sampled eclogites and garnet amphibolites from the following five locations along the
length of the Franciscan Complex in California
(Figs. 1 and 2).
PG 5 is a garnet hornblendite from a coherent
slice several hundred meters thick of amphibolite
facies rocks on Santa Catalina Island, the most
southerly exposure of Franciscan rocks in California. This unit is made up of several rock-types,
including mafic orthogneiss, migmatitic paragneiss,
and variably altered ultramafic rocks. It crops out
over an area of about 15 km2, and structurally
overlies a slice of high-pressure greenschist facies
rocks, and then (lowest) jadeite-lawsonite-bearing
blueschists [11]. Estimated PT conditions for the
Catalina Amphibolite Unit are 0.8 – 1.1 GPa, 640 –
750 jC [25]. PG5 comes from a garnet hornblendite interlayer in migmatitic paragneiss, and is
composed of garnet, hornblende, diopsidic clinopyroxene, and sphene, with traces of rutile and ilmenite. Clinopyroxene shows coarse symplectitic
intergrowths of hornblende and minor plagioclase.
Garnet is up to 2 mm diameter, has cores dusted by
very fine-grained inclusions of sphene and relatively
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
Fig. 1. Geological sketch-map of the Franciscan complex. Sample locations.
150
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
clean rims. Outer rims are separated from the rest
of the grain by coarse-grained sphene inclusions
(Fig. 2A).
PG 14 was sampled from a disrupted garnet
amphibolite block exposed in the hillside above El
Cerrito, in the Berkeley hills east of San Francisco
Bay (Fig. 1). This forms part of the Tiburon mélange, the highest of several regionally subhorizontal
tectonic units or nappes in the San Francisco Bay
[26], and overlies coherent lawsonite-bearing blueschist and metagreywacke in the Angel Island nappe.
The sample is made up of hornblende, garnet,
plagioclase and sphene. Garnets are usually small
( < 1 mm size) with inclusions of matrix minerals
(Fig. 2B). Temperature estimates based on garnet –
hornblende geothermometry are in the range 580–
610 jC (Table 1).
151
PG 23 comes from one of a large number of
apparently closely related eclogite and amphibolite
facies blocks exposed on Ring Mountain, on the
Tiburon peninsula north of San Francisco Bay (Fig.
1), which is the type area for the Tiburon mélange
[26]. Most of these blocks have a predominantly
eclogitic assemblage, with a strong lower-temperature
overprint under glaucophane-schist facies conditions,
and the garnets are commonly crowded with inclusions of sphene, rutile, and silicates. PG 23 is somewhat unusual in that it consists mainly of hornblende,
rather clean garnet, and minor epidote, white mica,
rutile and sphene (Fig. 2C). Hornblende shows some
alteration towards sodic amphibole, and rutile is partly
replaced by sphene. Garnet is usually < 2 mm and
contains very few inclusions of amphibole and
sphene. This block, referred to as TIBB, was studied
Fig. 2. Photomicrographs of analyzed samples. A, B, C, F crossed polarized light, D and E-plain polarized light. Abbreviations: grt—garnet,
sph—sphene, plag—plagioclase, amph—amphibole, epi—epidote, apat—apatite. See text for details.
152
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
Table 1
Representative microprobe analyses of mineral pairs used for geothermometry
Sample
SiO2
TiO2
Al2O3
Cr2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
Oxygens
Si
Ti
Al
Cr
Fe3
Fe2
Mn
Mg
Ca
Na
K
Sum
PG 14 g
PG 14
PG23
PG 23
PG 80
PG 80
Garnet
Hornblende
Garnet
Hornblende
Garnet
Clinopyroxene
37.63
0.21
20.71
0.09
1.25
21.00
6.08
2.35
10.81
100.14
12
2.9803
0.0125
1.9339
0.0056
0.0741
1.3905
0.4076
0.2777
0.9179
8.0000
45.13
0.67
12.50
0.08
2.81
13.87
0.29
10.01
11.01
1.59
0.33
98.29
23
6.6132
0.0742
2.1600
0.0089
0.3100
1.7000
0.0359
2.1869
1.7282
0.4513
0.0615
15.3297
38.65
0.00
21.16
0.00
0.02
24.20
2.40
3.51
10.07
100.01
12
3.0304
0.0000
1.9561
0.0000
0.0013
1.5868
0.1593
0.4099
0.8471
7.9988
intensively by [9,10], who reported post-amphibolite overprints in eclogite and blueschist facies from
parts of the block. Their estimated conditions for
the amphibolite facies stage are 660– 680 jC, at a
minimum pressure of 0.8 –0.9 GPa, followed by
decreasing temperature and increasing pressure into
the eclogite facies. Garnet-hornblende geothermometry on PG23, however suggests significantly lower
T equilibration conditions at ca. 513 F 34 jC
(Table 1).
PG 31 was taken from a float block on the beach
immediately north of Jenner at the mouth of the
Russian River, about 100 km north of San Francisco in the northern California Coast Ranges (Fig.
1). Abundant blocks on the beach are derived from
a body several hundred meters in extent that is
poorly exposed in the brush-covered hillside above.
The body overlies weakly metamorphosed greywacke exposed in the cliff face. The sampled block
is composed of omphacite, garnet, plagioclase,
sphene, rutile and glaucophane. Krogh [27] obtained
47.12
0.54
14.62
0.00
2.50
10.97
0.02
10.55
9.14
2.37
0.47
98.28
23
6.73
0.06
2.46
0.00
0.27
1.31
0.00
2.25
1.40
0.66
0.08
15.22
38.60
0.28
21.01
0.06
1.05
21.20
1.10
5.08
11.74
50.45
0.54
5.93
0.13
2.99
6.26
0.06
10.88
21.40
1.65
100.12
12
2.9913
0.0161
1.9195
0.0039
0.0611
1.3740
0.0720
0.5864
0.9760
100.30
6
1.8710
0.0149
0.2594
0.0038
0.0835
0.1943
0.0018
0.6015
0.8507
0.1189
8.0000
4.0000
an anticlockwise PT evolution for these rocks with
peak eclogite facies metamorphism at P = 1.3 GPa
and T = 440 –520 jC. Garnet is up to 1 cm size and
typically very rich in inclusions of all matrix
minerals (mainly omphacite, and rare glaucophane)
(Fig. 2D).
PG 73 is a garnet glaucophane schist from a poorly
exposed but apparently coherent slice of high-pressure
metamorphic rocks in the Willow Springs Canyon
area of the southern Diablo Range in the central
California Coast Ranges (Fig. 1). The rock is composed of glaucophane, lawsonite, garnet, white mica,
epidote and sphene. Garnet is euhedral, up to 0.5 mm
size, with relatively few inclusions of matrix minerals:
mainly glaucophane, some epidote, sphene, and rare
zircon (Fig. 2E). The assemblage suggests P>0.8 GPa
and T in the range 300 – 450 jC.
PG 80 was sampled from a tectonic slice of
amphibolite about 1 km in areal extent near Panoche
Pass, also in the southern Diablo Range, a few km
SE of Willow Springs Canyon (Fig. 1). It is likely
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
that this body lies structurally above the Willow
Springs sequence, but the immediately underlying
rocks are lawsonite-bearing metagreywackes of the
Eylar Mountain unit. The petrology of this body has
been described in some detail by [28], but no
estimates of the metamorphic conditions have been
published. The sample analyzed is composed of
hornblende, clinopyroxene, plagioclase, garnet, and
sphene; secondary glaucophane, lawsonite and white
mica are present in other samples from this body.
Garnet is up to 1.5 cm, rich in sphene and amphibole
inclusions, some large apatite (Fig. 2F). Garnet –
clinopyroxene and garnet –hornblende geothermometry points to 650 –750 jC crystallization temperature (Table 1). 40 Ar/ 39 Ar hornblendes ages of
153
160.6 F 2.2 and 163.0 F 2.8 Ma were reported from
this body by Ross and Sharp [29].
3.1. Garnet compositions
Garnets from most samples consist mainly of
almandine (50 – 60%) and grossular (20 –30%) with
little zonation (Fig. 3). The exception is the garnetglaucophane schist (PG73) which contains 40% spessartine, increasing abruptly to 65% half way from core
to rim, followed by a more gradual decline. PG23 and
PG31 show slight prograde zonation with increasing
pyrope and decreasing spessartine and Fe/(Fe + Mg)
ratio from core to rim; whereas the higher grade
samples (PG14 and PG80) have flatter profiles. PG
Fig. 3. Chemical composition of garnets. Traverses show mol fractions of Fe, Mg, Ca, Mn and #Fe from core to rim.
154
Table 2
Lu – Hf and Sm – Nd isotopic results
Mineral
Sample
wt.
(mg)
Hf
(ppm)
Dawn Valley
0.047 0.497
2.924 0.228
2.463 0.220
3.165 0.163
176
177
Lu
Hf
176
Hf/177Hf
147
Initial
176
Hf/177Hf
eHf
(t)
Age
(Ma)
Sm
(ppm)
2.971
1.197
1.122
1.201
9.020
2.116
2.331
2.218
0.1991
0.3422
0.3168
0.3275
0.078
0.650
0.042
0.038
143
Nd/144Nd
Initial
143
Nd/144Nd
eNd
(t)
Age
(Ma)
0.512926 F 43
0.513060 F 10
0.513046 F 8
0.513135 F 12
0.512773 F 91
5.9
130 F 43
0.0723
0.5129648 F 18
0.512875 F 24
9.3
187 F 15
0.080
0.074
0.3075
0.3075
0.513249 F 23
0.513254 F 22
0.512872 F 12
9.0
178 F 11
0.512735 F 12
5.9
159 F 7
Nd
(ppm)
144
Sm
Nd
0.0135
1.8151
1.5815
2.7464
0.283072 F 15
0.286930 F 18
0.286437 F 20
0.288903 F 25
0.283079 F 15
12.2
114.5 F 0.6
0.0398
26.9008
23.4517
21.7933
0.283163 F 11
0.364824 F 32
0.354265 F 37
0.349205 F 37
0.283041 F 10
13.1
162.5 F 0.5
0.0113
0.283094 F 6
0.283062 F 5
13.6
153.4 F 0.8
7.9014
0.305700 F 34
0.830
0.020
0.020
2.634
0.015
0.011
0.1906
0.8074
1.0809
0.513078 F 10
0.513258 F 134
0.513696 F 108
0.0416
3.4884
5.5221
0.283137 F 15
0.293285 F 32
0.299331 F 48
0.283014 F 15
12.0
157.9 F 0.7
2.682
0.526
0.722
10.248
1.182
1.333
0.1583
0.2672
0.3272
0.513056 F 7
0.513183 F 7
0.513252 F 11
PG 73 Glaucophane schist, Willow
Glau
47.6
0.049
Grt A (SAL)
79.8
16.671
Grt B (SAL)
70.2
14.105
Grt C
29.2
12.768
Grt D
83.7
13.562
Springs Creek
0.363
0.0191
0.095
25.1955
0.073
27.8883
0.082
22.3641
0.093
20.9281
0.282919 F 09
0.351825 F 21
0.358984 F 30
0.344258 F 55
0.340058 F 19
0.282866 F 9
6.5
146.7 F 0.7
0.277
0.068
0.060
0.915
0.190
0.171
0.1833
0.2161
0.2106
0.512953 F 11
0.512959 F 14
0.512971 F 16
0.084
0.238
0.2132
0.512924 F 8
PG 80 Garnet amphibolite, Hermes
Hbl
56.9
0.030
Grt A (SAL)
42.6
6.032
Grt B (SAL)
42.4
7.863
block
0.101
0.180
0.216
0.283151 F 11
0.297940 F 22
0.299347 F 26
0.283017 F 11
2.940
0.606
0.417
11.556
0.961
0.654
0.1539
0.3812
0.3855
0.512898 F 7
0.513146 F 13
0.513139 F 14
PG 14 Garnet amphibolite, Berkely
Plag
Hbl
82.1
0.089
Grt A (SAL)
88.9
15.294
Grt B (SAL)
99.4
15.097
Grt C
82.1
14.967
Hills
0.317
0.082
0.092
0.098
PG 23 Garnet amphibolite, Ring Mountain
Hbl
66.2
0.050 0.630
Grt A (SAL)
39.3
Grt B
43.8
6.096 0.110
PG 31 Eclogite,
Cpx
Grt A (SAL)
Grt B
Jenner
70.0
53.9
35.3
0.034
2.794
3.171
0.115
0.116
0.081
0.0426
4.7542
5.1718
12.4
168.7 F 0.8
All errors are 2SE and relate to the last significant digits. All mineral fractions used for constructing individual isochrons were measured on a single day to minimize correction for secular variation in static
176
Hf/177Hf of JMC475. 176Lu/177Hf errors are 0.5%, JMC475 yielded 0.282186 F 32 (n = 21) over the period of analyses but single day reproducibility was at least 50% more precise. Daily variations in
176
Hf/177Hf ratios were normalized to 176Hf/177Hf = 0.282165. Standards were run at concentrations similar to that in the samples (usually 30 – 50 ppb) and showed no significant difference to standards run at
higher intensity. Mass bias correction to 179Hf/177Hf = 0.7325. Decay constant k176Lu = 1.865 10 11 yr 1 [31,44]. Values used for eHf(t) calculations: 176Hf/177HfCHUR(0) = 0.282772 and
176
Lu/177HfCHUR(0) = 0.0332 [45] 147Sm/144Nd errors are 0.3%. Mass bias correction to 146Nd/144Nd = 0.7219. Reproducibility of Aldrich Nd standard 143Nd/144Nd = 0.511364 F 34 over a period of
analyses. Daily variations in 143Nd/144Nd ratios were normalized to 143Nd/144Nd = 0.511421. Decay constant k147Sm = 6.54 10 12 yr 1. Values used for eNd(t) calculations: 143Nd/144NdCHUR(0) = 0.512647
and 147Sm/144NdCHUR(0) = 0.1966 [24]. See [23] for complete account on mass spectrometric procedures.
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
PG5 Hornblende eclogite,
Omph
60.8
Grt A (SAL)
81.4
Grt B (SAL) 143.3
Grt C
53.9
Lu
(ppm)
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
Fig. 4. Lu – Hf and Sm – Nd isochron diagrams of dated samples. Grt—garnet, SAL—fractions leached with sulphuric acid.
155
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R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
5 from Santa Catalina shows a broadly prograde
profile, but with irregular fluctuations in the rim
region, which may reflect the transition from an
eclogite to an amphibolite-facies assemblage during
the final stages of garnet growth.
4. Results
The isotopic results are summarized in Table 2 and
Fig. 4. Ages were calculated using Isoplot [30]. All
errors are quoted at 2j level.
Lu –Hf dating yielded high quality internal isochron ages for all analyzed samples. Glaucophane
schist from the Diablo range (PG 73) and garnet
amphibolite from the Berkeley Hills (PG 14) gave
176
Lu/177Hf ratios for garnets between 21 and 28,
which are >3 times higher than the highest previously reported (Table 2 and Fig. 4A,B). Although
these two samples formed under very different
metamorphic conditions, and yielded different ages,
it is noteworthy that in both samples spessartine
dominates over pyrope and both have low ( < 5%)
modal proportions of garnet (Fig. 3). Both samples
yielded very highly precise dates of 146.7 F 0.7
and 162.5 F 0.5, respectively. Because of unexpectedly high Lu/Hf ratios, sample PG 73 was strongly
underspiked for Lu and therefore errors on 176Lu/
177
Hf ratios for Grt B, C are 1.2% and 1.6% for
Grt A. Other samples show 176Lu/177Hf ratios
between 1.6 and 8, which is more common for
garnets [31 –33]. PG 80 garnet amphibolite yielded
the oldest age among all studied blocks. Its
168.7 F 0.8 Ma age is established by two garnet
fractions and hornblende (Fig. 4C). Eclogite PG 31
from Jenner and garnet amphibolite PG 23 from
the Ring Mountain gave 157.9 F 0.7 and 153.4 F
0.8 Ma, respectively (Fig. 4D, E). Hornblende
eclogite from Santa Catalina gave a significantly
younger age of 114.5 F 0.6 Ma defined by three
garnet fractions and omphacite (Fig. 3F). All Lu –
Hf isochrons show good regression lines with
MSWD V 1.6, which together with the high
176
Lu/177Hf ratios, gave precisions on the ages
better than 0.5%.
Hf concentrations in all analyzed garnet fractions
fall in a rather narrow range between 70 and 230 ppb,
which is similar to previously reported values for
metamorphic garnets [31 – 33]. High Lu concentrations (2.5 – 16 ppm) reflect strong heavy REE enrichment in garnets (Table 2).
Sm –Nd dating on the other hand led to ambiguous
results. Low 147Sm/144Nd ratios (Fig. 4G – K) either
did not permit obtaining any age information (PG 23,
PG 73) or yielded very imprecise dates (PG 5, PG 14,
PG 31, PG 80). Estimates on the basis of the very
limited spread in isotopic ratios ( < 0.2) made for
samples PG14, PG 31 and PG 80 (Table 2) yielded
187 F 15, 178 F 11 and 130 F 43 Ma ages respectively. Grt C from sample PG 5 yielded anomalously high
143
Nd/144Nd ratio and was excluded from the regression line. Sample PG 23 gave high 147Sm/144Nd ratios
(0.8 and 1.1) for two garnet fractions but high scatter
of the data did not allow the age to be determined
(Fig. 4L). Garnet from this sample has a particularly
low Nd concentration and the analyzed separates
contained less than 1 ng of Nd (Table 2). Because
of very low signal intensities, inaccuracy in baseline
corrections are hugely magnified, and most likely
caused the observed scatter of the garnet analyses.
Comparison of Sm – Nd with Lu –Hf results shows
some discordances among the obtained ages (Fig. 4).
Only in the case of sample PG 5 is the 114.5 F 0.6 Ma
Lu – Hf age concordant with 130 F 43 Ma Sm –Nd
age. This comparison, however, is not very meaningful due to the very poor precision on the latter date. In
the case of PG 80 the 159.4 F 7.4 Ma Sm – Nd age is
slightly younger than the 168.7 F 0.8 Ma Lu –Hf age.
The 187 F 15 Ma Sm – Nd age of PG 14 is at least 9
Ma older than the 162.5 F 0.5 Lu – Hf age. A similar
age difference is shown by sample PG 31, which has a
Sm –Nd age of 178 F 11 and a 157.9 F 0.7 Ma Lu –
Hf age (Table 2 and Fig. 1). Two remaining samples
(PG 73 and PG 23) display highly scattered datapoints, which did not permit any age information to be
obtained.
5. Discussion
5.1. Lu – Hf and Sm –Nd results
Available geochronological data for the Franciscan
complex is scarce. There is a particular shortage of
high-temperature geochronology. This allows only
very limited comparison with previous dating.
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
Sample PG 5 from Santa Catalina Island yielded a
Lu – Hf garnet age of 114.5 F 0.8 Ma, which is
concordant within error with a 113.3 F 1.5 Ma U –
Pb garnet –amphibole – sphene –apatite isochron age
obtained by [34] from two similar amphibolite samples. About 95– 100 Ma Ar – Ar white mica dates
were obtained for all main units on Santa Catalina
[35]. Since the crystallization temperature reported
for the amphibolites lies in the range 640 –750 jC
[25], the younger Ar –Ar ages reflect cooling below
the closure temperature for the K – Ar system in
amphibole.
The 168.7 F 0.8 Lu – Hf age obtained for sample
PG 80 can only be compared with Ar –Ar data. Two
hornblende ages reported for this unit by [29] are
160.6 F 2.2 Ma and 163.0 F 2.8 Ma. As the metamorphic temperature is 650 –750 jC, a younger age
for Ar – Ar system is expected due to its lower isotopic
closure temperature.
For PG 23 (the garnet amphibolite from Tiburon
peninsula) there is very good agreement between the
153.4 F 0.8 Ma Lu – Hf age and a 153 F 4 Ma laser
Ar – Ar white mica age obtained from a sample from
the same area by [36]. An identical Rb –Sr mica age
of 153 F 1 was reported by [37]. Concordant ages
derived by all three systems are consistent with the
equilibration temperature of around 500 jC that we
estimate from garnet – hornblende thermometry, as it is
equal to or below the closure temperature for all three
methods [38]. There is, however, some uncertainty
about the equilibration temperature for this sample, as
[26] estimated metamorphic temperatures for the
TIBB block, from which it comes, at 660 – 680 jC.
The source of this discrepancy is not clear, but may
relate to disequilibrium, given the complex metamorphic evolution of this block documented by [26]. The
radiometric data, however, support the lower equilibration temperature.
The closure temperature for Nd diffusion in garnet
is estimated at about 700 –750 jC [39]. A similar or
higher range for isotopic closure of the Lu – Hf system
was suggested by [32]. Metamorphic temperatures
reported from our samples lie in the range 300 – 770
jC, and hence are unlikely to have exceeded the
closure temperature for the Lu –Hf system. The Lu –
Hf ages are therefore most readily interpreted as
dating, or closely approximating, garnet growth on a
prograde PT path.
157
Sm –Nd geochronology appears to be obscured by
inclusions and is discussed in details in the next
section.
Nearly all samples show q Hf(t) values between 12.0
and 13.6 pointing to the same depleted source. Only the
glaucophane schist PG 73 shows a significantly lower
value of eHf(t) = 6.5, possibly implying some crustal
contamination. In general initial 143Nd/144Nd values
support Hf data. Equivalent eNd(t) shows values of 5.9
for samples PG 5 and PG 80 and about nine for samples
PG 14 and PG 31. Larger variations in eNd(t) values
suggest some isotopic decoupling of the Sm – Nd and
Lu– Hf systems.
5.2. Influence of inclusions on Sm – Nd and Lu– Hf
garnet dating
Although every attempt was made to obtain pure
garnet fractions, not all microscopic and submicroscopic inclusions can be eliminated by standard mineral separation techniques. Certainly, the cause for the
low Sm/Nd ratios in nearly all dated samples is the
presence of Nd-rich inclusions in the mineral separates. The most likely mineral is sphene, which occurs
in all measured samples and has Nd concentration up
to several hundreds of ppm [40]. A small fraction of a
percent of contamination would be sufficient to bring
down Nd isotopic ratios to the observed values.
Additional contribution from accessory inclusions
(epidote, plagioclase, apatite and zircon) occurring
in much smaller amounts certainly had some influence as well [41,42]. Discordance of Sm – Nd ages
relatively to Lu –Hf dates suggests that some of the
inclusions were not in complete isotopic equilibrium
with garnet.
The minerals that lowered the 147Sm/144Nd and
143
Nd/144Nd ratios did not have such a profound
influence on the 176Lu/177Hf ratios. Sulphuric acid
leaching, however, did reveal the presence of inclusions, which significantly influenced Lu – Hf budget.
5.2.1. Sulphuric acid leaching
Sulphuric acid leaching (SAL) aims at dissolving
phosphate inclusions leaving garnets (in practice all
silicates) undisturbed [19]. In order to investigate the
influence of SAL on Lu – Hf analyses in metabasites,
we compared leached and unleached garnet fractions
in selected samples.
158
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
In samples PG 14 and PG 73 SAL eliminates
inclusions with Lu/Hf ratios lower than those of
garnet, resulting in higher isotopic ratios for leached
fractions in comparison to unleached fractions. The
large spread among all analyzed garnets (also among
unleached fractions alone) suggests that SAL made a
rather modest improvement and the higher or lower
ratios are mainly a result of variations in the amount
of silicate inclusions, which are not affected by H2SO4
leaching.
On the other hand, SAL garnets from both PG 5
and PG 31 yielded ratios up to about 40% lower than
unleached fractions. In this case SAL eliminates
inclusion(s) that are more radiogenic than garnet
itself. The most likely mineral with a potentially
higher 176Lu/177Hf ratio than garnet is apatite [43].
Apatite is soluble in H2SO4 and would easily be
removed by leaching, hence lowering 176Lu/177Hf
ratios. However, only direct analyses of apatites from
these samples could verify this theory. This was not
possible due to its small size and amount.
The good fit of isochrons obtained for all samples
for leached and unleached garnet fractions demonstrates that there is no Lu/Hf fractionation induced by
SAL.
6. Geological interpretation: slow start to
Franciscan subduction
Two important points arise from our results. One is
the clear difference in age between the amphibolites
on Santa Catalina Island in southern California and
the eclogite and amphibolite blocks in the central and
northern California Coast Ranges. This was already
recognized on the basis of the Ar – Ar ages from the
two areas, and we can now confirm that garnet growth
ages differ by as much as 55 Ma. This clearly implies
that the subduction zone was initiated later at the
latitude of Catalina. After correction for Tertiary
dextral slip along the faults of the San Andreas
system, Catalina lies about 1000 km SE of the San
Francisco Bay area and the Diablo Range [15].
Possible explanations for the difference in age are a
significantly different history of arc accretion and
consequent step-out of the subduction zone in southern California [35], or the progressive migration of a
triple junction down the paleo-margin of North Amer-
ica, eliminating the Coast Range ophiolite spreading
centre and replacing it with the Franciscan subduction
zone. The second alternative would imply that the
triple junction migrated SE at an average rate of about
18 km/Ma.
The second, and previously unrecognised, result of
this study is that there are significant and systematic
differences in age among the amphibolite, eclogite,
and high-grade blueschist blocks and slices in the
central and northern Coast Ranges (Fig. 5). Our garnet
growth ages range from 153 to 169 Ma in the eclogite
and amphibolite blocks, and we have a still younger
age of 147 Ma from a garnet –glaucophane schist. The
analyzed blocks are distributed over nearly 300 km
distance along strike, but there is no correlation
between age and along-strike position: two blocks
from the San Francisco Bay area differ in age by 9
Ma, for example. After correction for dextral slip
along Tertiary faults west of the San Andreas Fault,
with a total slip of 180 –280 km [15], the blocks are
more closely clustered than they are now, with an
along-strike dispersion of about 100 km, and there is
no correlation between age and position. Hence we
cannot reasonably attribute the scatter in ages to
diachronous inception of the subduction zone within
this region.
It therefore appears that although high-grade metamorphism started early, overlapping the age of the
structurally overlying Coast Range Ophiolite, epidote
amphibolite to eclogite facies metamorphism continued for as much as 15 Ma in the newly formed
subduction zone, and garnet – glaucophane schist fa-
Fig. 5. Geothermometric estimates vs. age diagram for analyzed
samples. Slope of the regression line suggests ca. 15 jC/Ma cooling
rate for the Franciscan subduction.
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
cies metamorphism persisted for a further 6 Ma. This
places severe constraints on the thermal structure and
rate of motion of material in the subduction zone.
Thermal modeling [14] shows that in a newly formed
subduction zone, with a thermal structure in the
hangingwall corresponding to 10 Ma old oceanic
lithosphere and a subduction rate of 100 km/Ma,
temperatures along the interface at 30 km depth drop
below 400 jC in about 0.5 Ma. Under those conditions the high-grade rocks should all give the same
age within the limits of resolution of the Lu – Hf
method. The only conditions under which temperatures corresponding to low-T eclogite or epidote
amphibolite facies conditions (>500 jC) could persist
for 14 Ma would be if both footwall and hangingwall
had initially very high thermal gradients, and the rate
of subduction was very slow (10 km/Ma or less).
Detailed prediction of the thermal structure and evolution of this situation requires numerical modeling,
which has not yet been done, but it is clear that the
observed age distribution requires conditions well
outside the range of values considered by [14].
If subduction is driven mainly by the negative
buoyancy of the subducted slab, it is likely that
subduction of very young oceanic lithosphere will
be slow. Hence our suggestion of high initial thermal
gradients and a slow start to Franciscan subduction is
reasonable. As older lithosphere entered the subduction zone, the rate of subduction would have increased: and it is likely that several tens of thousands
of kilometers of oceanic lithosphere were eventually
subducted along this margin during its ca. 130 Ma
lifespan.
There are two further interesting and important
implications that follow from these results. Firstly,
during slow subduction the zone of elevated temperature beneath the hangingwall of the subduction zone
will be quite broad, and the inverted temperature
gradient slight [14]. Hence the present situation, in
which small blocks and slices of high-grade rock sit
directly on low-T blueschists, does not represent a
fossilized inverted thermal gradient, but results from
the progressive underthrusting and underplating of
rock under conditions of decreasing temperature over
a significant period of time. The present structural
relations are likely to have been significantly modified
by tectonic processes accompanying exhumation. The
evidence from our isotopic ages for progressive
159
underplating is clear: plagioclase-bearing amphibolites from Panoche Pass and the Berkeley Hills yield
the oldest ages, epidote amphibolite and glaucophane
eclogite from Ring Mountain and Jenner are 5– 15 Ma
younger, and garnet-bearing blueschist from the Diablo Range is 8 Ma younger again (Fig. 5). The
evidence from the Diablo Range is particularly compelling: 169 Ma amphibolite at Panoche Pass crops
out a few kilometers from 147 Ma garnet glaucophane
schist, and may have originally overlain it.
The slope of the regression line in Fig. 5 suggests a
cooling rate of about 15 jC/Ma, which implies that
exhumation within the Franciscan Complex was not
particularly rapid. The juxtaposition of amphibolite
and blueschist, both formed at depths of around 30 –
40 km but with ages differing by 22 Ma, implies that
the earliest formed rocks had a significant residence
time at depth.
7. Conclusions
Internal isochrons obtained for the high grade
metamorphic blocks and tectonic slices from the
Franciscan complex yield highly precise Lu –Hf ages,
but low quality Sm – Nd dates. Sm –Nd garnet analyses are dominated by ‘‘non-radiogenic’’ inclusions,
which either led to inaccurate dates, or prevented age
determinations. The same inclusions have very limited
influence on the Lu – Hf budget. The 176Lu/177Hf
ratios obtained for two samples with high spessartine/pyrope ratio range between 21 and 28 and are the
highest yet reported. Taking into account the large
amount of non-radiogenic inclusions with Hf concentrations several times higher than that of garnet, even
higher Lu/Hf ratios for garnets are to be expected.
Sulphuric acid leached out inclusions with relatively high Lu/Hf ratios (apatite?), which lowered
176
Lu/177Hf garnet ratios even by 40% in comparison
with the ratios obtained for the unleached fractions.
This indicates that some 176Lu/177Hf ‘‘garnet’’ ratios
may be significantly overestimated due to the presence of such inclusions.
In the case of two samples with very high
176
Lu/177Hf ratios (PG 14 and PG73) SAL led to an
increase in the 176Lu/177Hf ratios. However, the large
spread among all analyzed garnets suggests that SAL
had a rather small influence on garnets and the differ-
160
R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161
ences in ratios are mainly a result of variations in the
amount of silicate inclusions, which are not affected
by SAL. The good fit of isochrons for leached and
unleached garnet fractions in all analyzed samples
prove that there is no Lu/Hf fractionation induced by
SAL.
High resolution Lu – Hf garnet ages allow us to
place new constraints on the early thermal history of
the Franciscan subduction zone. The oldest ages
obtained, on plagioclase-bearing garnet amphibolites
from Panoche Pass and the Berkeley Hills, suggest
initiation of the subduction zone at or about 169 Ma,
coeval with the formation of the tectonically overlying
Coast Range Ophiolite. Relatively high temperature
conditions persisted for about 14 Ma as indicated by
153 – 158 Ma garnet growth recorded in epidote
amphibolite and eclogite blocks from the Ring Mountain and Jenner. This requires high initial geothermal
gradients within both the footwall and the hangingwall of the subduction zone, and a relatively slow
subduction rate of the order of 10 km/Ma.
The present structural relationships between the
various metamorphic blocks and slices resulted from
progressive underthrusting and underplating in a cooling subduction system, and do not directly reflect an
inverted thermal gradient. In the central to northern
Coast Ranges, the highest temperature amphibolites
yield the oldest ages (169 – 163 Ma), whereas epidote
amphibolite and eclogite are 10 –15 Ma younger, and
garnet-glaucophane schist is 8 Ma younger still.
Hence, cooling along the subduction zone interface
from amphibolite to blueschist facies conditions took
place over about 22 Ma at the rate of about 15 jC/Ma,
which suggest slow exhumation rates and significant
residence time at depth of the earliest Franciscan
rocks.
Acknowledgements
This work was supported by grant NERC/A/S/
1999/00083 from the Natural Environmental Research Council of Great Britain. We are indebted to
Nuria Jareño for her skilled and painstaking work on
the mineral separation required for this study. We
thank D. Vance, J. Schumacher and an anonymous
reviewer for critical comments, which improved the
manuscript.
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