franciscan subduction off to a slow start: evidence...

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Earth and Planetary Science Letters 225 (2004) 147–161

Franciscan subduction off to a slow start: evidence from

high-precision Lu–Hf garnet ages on high grade-blocks

Robert Anczkiewicza,b,*, John P. Platta, Matthew F. Thirlwallb, John Wakabayashic

aResearch School of Earth Sciences at UCL-Birkbeck, Gower Street, London WC1E 6BT, UKbDepartment of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK

c1329 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 168.7F 0.8 and 162.5F 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 153.4F 0.8 Ma garnet growth recorded in epidote amphibolite and 157.9F 0.7 in eclogite from Ring

Mountain and Jenner, respectively. A 146.7F 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.

0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2004.06.003

* Corresponding author. Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK. Tel.: +44-1784-

414045; fax: +44-1784-471780.

E-mail address: [email protected] (R. Anczkiewicz).

R. Anczkiewicz et al. / Earth and Planetary Science Letters 225 (2004) 147–161148

A much younger age of 114.5F 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 immediate footwall [14]. Estimated pressure–

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 litho-

sphere, 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 com-

plexes 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. Ophio-

lites in these settings commonly have ‘‘soles’’ of

high-temperature metamorphic rocks along their

lower boundaries.

The mid-Mesozoic Coast Range Ophiolite of Cal-

ifornia formed immediately before the initiation of the

subduction zone that led to the formation of the

Franciscan accretionary complex, suggesting that sim-

ilar 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 hang-

ing-wall mantle wedge in the newly initiated subduc-

tion zone [9,11–13], at a time when temperatures in

the hangingwall of the subduction zone were suffi-

ciently high to cause significant transient heating in

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 high-

grade 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) 147–161 149

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 hot-

plate 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 concentratedHNO3 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 equili-

brated 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 purifica-

tion of Hf from other HFSE takes place on a Ln-

specR column based on [21]. Such Hf purification

completely eliminates Lu and Yb interferences on176Hf.

Sm and Nd are separated on a smaller size Ln-

specR 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

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 col-

umn 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 ele-

ments 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 interfer-

ence 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 repro-

ducibility 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 amphibo-

lites 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 Califor-

nia. 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 hornblen-

dite interlayer in migmatitic paragneiss, and is

composed of garnet, hornblende, diopsidic clinopyr-

oxene, and sphene, with traces of rutile and ilmen-

ite. 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

Fig.1.Geological

sketch-m

apoftheFranciscan

complex.Sam

ple

locations.



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 me-

lange, the highest of several regionally subhorizontal

tectonic units or nappes in the San Francisco Bay

[26], and overlies coherent lawsonite-bearing blues-

chist 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).

Fig. 2. Photomicrographs of analyzed samples. A, B, C, F crossed polariz

sph—sphene, plag—plagioclase, amph—amphibole, epi—epidote, apat—

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 melange

[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 inclu-

sions of sphene, rutile, and silicates. PG 23 is some-

what 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

ed light, D and E-plain polarized light. Abbreviations: grt—garnet,

apatite. See text for details.

Table 1

Representative microprobe analyses of mineral pairs used for geothermometry

Sample PG 14 g PG 14 PG23 PG 23 PG 80 PG 80

Garnet Hornblende Garnet Hornblende Garnet Clinopyroxene

SiO2 37.63 45.13 38.65 47.12 38.60 50.45

TiO2 0.21 0.67 0.00 0.54 0.28 0.54

Al2O3 20.71 12.50 21.16 14.62 21.01 5.93

Cr2O3 0.09 0.08 0.00 0.00 0.06 0.13

Fe2O3 1.25 2.81 0.02 2.50 1.05 2.99

FeO 21.00 13.87 24.20 10.97 21.20 6.26

MnO 6.08 0.29 2.40 0.02 1.10 0.06

MgO 2.35 10.01 3.51 10.55 5.08 10.88

CaO 10.81 11.01 10.07 9.14 11.74 21.40

Na2O 1.59 2.37 1.65

K2O 0.33 0.47

Total 100.14 98.29 100.01 98.28 100.12 100.30

Oxygens 12 23 12 23 12 6

Si 2.9803 6.6132 3.0304 6.73 2.9913 1.8710

Ti 0.0125 0.0742 0.0000 0.06 0.0161 0.0149

Al 1.9339 2.1600 1.9561 2.46 1.9195 0.2594

Cr 0.0056 0.0089 0.0000 0.00 0.0039 0.0038

Fe3 0.0741 0.3100 0.0013 0.27 0.0611 0.0835

Fe2 1.3905 1.7000 1.5868 1.31 1.3740 0.1943

Mn 0.4076 0.0359 0.1593 0.00 0.0720 0.0018

Mg 0.2777 2.1869 0.4099 2.25 0.5864 0.6015

Ca 0.9179 1.7282 0.8471 1.40 0.9760 0.8507

Na 0.4513 0.66 0.1189

K 0.0615 0.08

Sum 8.0000 15.3297 7.9988 15.22 8.0000 4.0000


intensively by [9,10], who reported post-amphibo-

lite 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 geothermom-

etry on PG23, however suggests significantly lower

T equilibration conditions at ca. 513F 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 Fran-

cisco 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 grey-

wacke exposed in the cliff face. The sampled block

is composed of omphacite, garnet, plagioclase,

sphene, rutile and glaucophane. Krogh [27] obtained

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 com-

posed 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


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 geothermom-

etry points to 650–750 jC crystallization tempera-

ture (Table 1). 40Ar/39Ar hornblendes ages of

Fig. 3. Chemical composition of garnets. Traverses show mol

160.6F 2.2 and 163.0F 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 garnet-

glaucophane schist (PG73) which contains 40% spes-

sartine, 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

fractions of Fe, Mg, Ca, Mn and #Fe from core to rim.

Table 2

Lu–Hf and Sm–Nd isotopic results

Mineral Sample

wt.

(mg)

Lu

(ppm)

Hf

(ppm)

176Lu177Hf

176Hf/177Hf Initial176Hf/177Hf

eHf(t)

Age

(Ma)

Sm

(ppm)

Nd

(ppm)

147Sm144

Nd

143Nd/144Nd Initial143Nd/144Nd

eNd(t)

Age

(Ma)

PG5 Hornblende eclogite, Dawn Valley

Omph 60.8 0.047 0.497 0.0135 0.283072F 15 0.283079F 15 12.2 114.5F 0.6 2.971 9.020 0.1991 0.512926F 43 0.512773F 91 5.9 130F 43

Grt A (SAL) 81.4 2.924 0.228 1.8151 0.286930F 18 1.197 2.116 0.3422 0.513060F 10

Grt B (SAL) 143.3 2.463 0.220 1.5815 0.286437F 20 1.122 2.331 0.3168 0.513046F 8

Grt C 53.9 3.165 0.163 2.7464 0.288903F 25 1.201 2.218 0.3275 0.513135F 12

PG 14 Garnet amphibolite, Berkely Hills

Plag 0.078 0.650 0.0723 0.5129648F 18 0.512875F 24 9.3 187F 15

Hbl 82.1 0.089 0.317 0.0398 0.283163F 11 0.283041F10 13.1 162.5F 0.5

Grt A (SAL) 88.9 15.294 0.082 26.9008 0.364824F 32

Grt B (SAL) 99.4 15.097 0.092 23.4517 0.354265F 37 0.042 0.080 0.3075 0.513249F 23

Grt C 82.1 14.967 0.098 21.7933 0.349205F 37 0.038 0.074 0.3075 0.513254F 22

PG 23 Garnet amphibolite, Ring Mountain

Hbl 66.2 0.050 0.630 0.0113 0.283094F 6 0.283062F 5 13.6 153.4F 0.8 0.830 2.634 0.1906 0.513078F 10

Grt A (SAL) 39.3 0.020 0.015 0.8074 0.513258F 134

Grt B 43.8 6.096 0.110 7.9014 0.305700F 34 0.020 0.011 1.0809 0.513696F 108

PG 31 Eclogite, Jenner

Cpx 70.0 0.034 0.115 0.0416 0.283137F 15 0.283014F 15 12.0 157.9F 0.7 2.682 10.248 0.1583 0.513056F 7 0.512872F 12 9.0 178F 11

Grt A (SAL) 53.9 2.794 0.116 3.4884 0.293285F 32 0.526 1.182 0.2672 0.513183F 7

Grt B 35.3 3.171 0.081 5.5221 0.299331F 48 0.722 1.333 0.3272 0.513252F 11

PG 73 Glaucophane schist, Willow Springs Creek

Glau 47.6 0.049 0.363 0.0191 0.282919F 09 0.282866F 9 6.5 146.7F 0.7 0.277 0.915 0.1833 0.512953F 11

Grt A (SAL) 79.8 16.671 0.095 25.1955 0.351825F 21 0.068 0.190 0.2161 0.512959F 14

Grt B (SAL) 70.2 14.105 0.073 27.8883 0.358984F 30 0.060 0.171 0.2106 0.512971F16

Grt C 29.2 12.768 0.082 22.3641 0.344258F 55

Grt D 83.7 13.562 0.093 20.9281 0.340058F 19 0.084 0.238 0.2132 0.512924F 8

PG 80 Garnet amphibolite, Hermes block

Hbl 56.9 0.030 0.101 0.0426 0.283151F11 0.283017F 11 12.4 168.7F 0.8 2.940 11.556 0.1539 0.512898F 7 0.512735F 12 5.9 159F 7

Grt A (SAL) 42.6 6.032 0.180 4.7542 0.297940F 22 0.606 0.961 0.3812 0.513146F 13

Grt B (SAL) 42.4 7.863 0.216 5.1718 0.299347F 26 0.417 0.654 0.3855 0.513139F 14

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 static176Hf/177Hf of JMC475. 176Lu/177Hf errors are 0.5%, JMC475 yielded 0.282186F 32 (n= 21) over the period of analyses but single day reproducibility was at least 50% more precise. Daily variations in176Hf/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 and176Lu/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 = 0.511364F 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) = 0.512647and 147Sm/144NdCHUR(0) = 0.1966 [24]. See [23] for complete account on mass spectrometric procedures.

R.Anczkiew

iczet

al./Earth

andPlaneta

ryScien

ceLetters

225(2004)147–161

154

Fig. 4. Lu–Hf and Sm–Nd isochron diagrams of dated samples. Grt—garnet, SAL—fractions leached with sulphuric acid.



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 iso-

chron ages for all analyzed samples. Glaucophane

schist from the Diablo range (PG 73) and garnet

amphibolite from the Berkeley Hills (PG 14) gave176Lu/177Hf ratios for garnets between 21 and 28,

which are >3 times higher than the highest previ-

ously 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 146.7F 0.7

and 162.5F 0.5, respectively. Because of unexpect-

edly high Lu/Hf ratios, sample PG 73 was strongly

underspiked for Lu and therefore errors on 176Lu/177Hf 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

168.7F 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 157.9F 0.7 and 153.4F0.8 Ma, respectively (Fig. 4D, E). Hornblende

eclogite from Santa Catalina gave a significantly

younger age of 114.5F 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 high176Lu/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 concentra-

tions (2.5–16 ppm) reflect strong heavy REE enrich-

ment 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

187F 15, 178F 11 and 130F 43 Ma ages respective-

ly. Grt C from sample PG 5 yielded anomalously high143Nd/144Nd ratio and was excluded from the regres-

sion 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 114.5F 0.6 Ma

Lu–Hf age concordant with 130F 43 Ma Sm–Nd

age. This comparison, however, is not very meaning-

ful due to the very poor precision on the latter date. In

the case of PG 80 the 159.4F 7.4 Ma Sm–Nd age is

slightly younger than the 168.7F 0.8 Ma Lu–Hf age.

The 187F 15 Ma Sm–Nd age of PG 14 is at least 9

Ma older than the 162.5F 0.5 Lu–Hf age. A similar

age difference is shown by sample PG 31, which has a

Sm–Nd age of 178F 11 and a 157.9F 0.7 Ma Lu–

Hf age (Table 2 and Fig. 1). Two remaining samples

(PG 73 and PG 23) display highly scattered data-

points, 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.


Sample PG 5 from Santa Catalina Island yielded a

Lu–Hf garnet age of 114.5F 0.8 Ma, which is

concordant within error with a 113.3F 1.5 Ma U–

Pb garnet–amphibole–sphene–apatite isochron age

obtained by [34] from two similar amphibolite sam-

ples. 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 168.7F 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

160.6F 2.2 Ma and 163.0F 2.8 Ma. As the meta-

morphic 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

153.4F 0.8 Ma Lu–Hf age and a 153F 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 153F 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 metamor-

phic evolution of this block documented by [26]. The

radiometric data, however, support the lower equili-

bration 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.

Sm–Nd geochronology appears to be obscured by

inclusions and is discussed in details in the next

section.

Nearly all samples show qHf(t) values between 12.0and 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 5.9for samples PG 5 and PG 80 and about nine for samples

PG 14 and PG 31. Larger variations in eNd(t) valuessuggest 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 submicro-

scopic inclusions can be eliminated by standard min-

eral 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 sepa-

rates. 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 influ-

ence 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 and143Nd/144Nd ratios did not have such a profound

influence on the 176Lu/177Hf ratios. Sulphuric acid

leaching, however, did reveal the presence of inclu-

sions, 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.


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 demon-

strates that there is no Lu/Hf fractionation induced by

SAL.

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.

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 south-

ern 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 meta-

morphism started early, overlapping the age of the

structurally overlying Coast Range Ophiolite, epidote

amphibolite to eclogite facies metamorphism contin-

ued for as much as 15 Ma in the newly formed

subduction zone, and garnet–glaucophane schist fa-


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 con-

ditions 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 temper-

atures 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 evo-

lution 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 subduc-

tion zone, the rate of subduction would have in-

creased: 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 temper-

ature 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

underplating is clear: plagioclase-bearing amphibo-

lites 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 Dia-

blo Range is 8 Ma younger again (Fig. 5). The

evidence from the Diablo Range is particularly com-

pelling: 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 analy-

ses 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 spessar-

tine/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 concen-

trations several times higher than that of garnet, even

higher Lu/Hf ratios for garnets are to be expected.

Sulphuric acid leached out inclusions with rela-

tively high Lu/Hf ratios (apatite?), which lowered176Lu/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 pres-

ence of such inclusions.

In the case of two samples with very high176Lu/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-


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 Moun-

tain and Jenner. This requires high initial geothermal

gradients within both the footwall and the hanging-

wall 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 cool-

ing 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 Re-

search Council of Great Britain. We are indebted to

Nuria Jareno 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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franciscan subduction off to a slow start: evidence...

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