2004_PrecambRes_Tasriwine

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Precambrian Research 135 (2004) 133147

Geochronological, geochemical, and NdHf isotopic constraintson the origin of Neoproterozoic plagiogranites in the Tasriwine

ophiolite, Anti-Atlas orogen, Morocco

S.D. Samsona,, J.D. Inglisa, R.S. DLemosb, H. Admouc,J. Blichert-Toftd, K. Hefferane

a Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USAb Deers Cottage, Aston View, Somerton, Bicester OX25 6NP, UK

c Faculty of Science, Cadi Ayyad University, Marrakech, Moroccod Ecole Normale Superieure de Lyon, 69364 Lyon Cedex 7, France

e Department of Geography and Geology, University of Wisconsin-Stevens Point, Stevens Point, WI 54481, USA

Received 29 March 2004; accepted 10 August 2004

Abstract

Small leucogranite bodies within the Tasriwine ophiolite, Anti-Atlas Mountains, Morocco have chemical characteristics

consistent with being plagiogranites sensu stricto. Total REE abundances are low, as are K2O (


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134 S.D. Samson et al. / Precambrian Research 135 (2004) 133147

The chemical and isotopic data for the plagiogranites are consistent with their generation by partial melting of a chemically

enriched, but isotopically depleted mantle, followed by extensive fractional crystallization.

2004 Elsevier B.V. All rights reserved.

Keywords: Plagiogranite; Neoproterozoic; Morocco; Anti-Atlas; Hf isotopes

1. Introduction

Ophiolites have played a major role in reconstruct-

ing ancient plate boundaries ever since their initial

recognition as on-land slicesof oceanic lithosphere. Es-

tablishing precise ages of ophiolites is oneof the critical

tasks involved in their full characterization, and is nec-

essary in order to maximize the tectonic information

potentially provided by these lithospheric fragments.

Determining robust radiometric ages of the ultramafic

or mafic rocks within an ophiolite is extremely difficult,

thus much work has concentrated on the geochronol-

ogy of spatially, and presumably temporally, associated

plagiogranites. Plagiogranites, as defined by Coleman

and Peterman (1975), are primarily composed of quartz

and plagioclase and contain less than 10% ferromagne-

sian minerals. The bulk chemical composition of pla-

giogranites differs from average continental granites

in that the K2O content is usually less than one weight

percent (and often1%), whereas Na2OandCaOcon-tents are generally higher(Coleman and Donato, 1979).

Because plagiogranites often contain zircon as a pri-

mary accessory mineral, they have been the target of

many UPb geochronological studies (e.g. Mattinson,

1976; Bluck et al., 1980; Tilton et al., 1981; Dunning

and Krogh, 1985; Mukasa and Ludden, 1987; Borsi et

al., 1996; Clark et al., 1998; Whitehead et al., 2000;

Khain et al., 2002, and many others).

In the Neoproterozoic Anti-Atlas Orogen of cen-

tral Morocco dismembered ophiolites occur in well-

exposed erosional inliers, or boutonnieres (Fig. 1).Although the economically important Bou AzzerEl

Graa ophiolite has been the focus of numerous ge-

ological, petrological, and geochemical studies (e.g.

Church and Young, 1974; Leblanc, 1976, 1981 and ref-

erences therein; Bodinier et al., 1984; Saquaque et al.,

1989; Naidoo et al., 1991, and many others), its age has

not yet been directly established. The most commonly

quoted age is 788 10Ma (Clauer, 1976), which is

based on a RbSr date of hornfelsed pelites collected

close to meter-thick mafic sills. Clauer et al. (1982)

argued that Leblanc (1975) considered the mafic sills

as magmatically connected with the ophiolites, and

thus viewed the RbSr date as a reliable constraint of

the age of the Bou AzzerEl Graara ophiolite. Approx-

imately 60 km northwest of the Bou Azzer inlier is the

Siroua inlier. The Siroua inlier has recently been con-

sidered as a westward extension of the Bou Azzer in-

lier based on extensive mapping and structural studies

(Admou, 2000). Two small highly tectonized ophio-lites, the Nqob and Tasriwine ophiolites (nomencla-

ture ofThomas et al., 2002), crop out within the Siroua

inlier. The relationship of the ophiolites in the Siroua

inlier to the main Bou AzzerEl Graa ophiolite is un-

known. One reason for the difficulty in determining

whether a genetic relationship exists between these

different ophiolite sequences stems from the absence

of precise ages of primary units within the ophiolites.

There is also a lack of radiogenic isotopic data for the

sequences that, by being powerful petrogenetic tracers,

could be used as a means of comparison. Leucogran-ite bodies are known to occur within the Tasriwine

ophiolite, however, and these are the focus of the com-

bined geochemical, NdHf isotopic, and UPb zircon

geochronological study presented here.

2. Geologic background

The Khazama region of the Siroua inlier consists

of distinct tectonic blocks (Admou, 2000). A southern

block contains undated mica schists and gneisses of theTachakoucht Formation. Thrust over this formation, or

metamorphic complex, are banded gneisses of the Iriri

Migmatite, which is thought to have a protolith age of

743 14 Ma based on UPb ion microprobe analyses

of zircon from a tonalitic gneiss (Thomas et al., 2002).

Further north, thrust slices of ultramafic rocks (and

their serpentinized equivalents), gabbroic rocks, and

a dike complex occur (Admou, 2000). Collectively

these units comprise what has been considered a dis-

membered ophiolite, named the Tasriwine ophiolite


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Fig. 2. Generalized geologic map of part of the Siroua region that includes the Tasriwine ophiolite. Locality of plagiogranite samples analyzed

in this study are shown by star. Map adapted from Admou (2000). Nomenclature from Thomas et al. (2002).

unrelated intrusive units, but are part of the overall

ultramaficgabbroic sequence. Two of the leucogran-

ite samples are from small (


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S.D. Samson et al. / Precambrian Research 135 (2004) 133147 137

niques for the SmNd isotope measurements followed

those described by Samson et al. (1995). Details of the

zircon dissolution, Hf chemical separation, and mass

spectrometry techniques are fully outlined in Samsonet al. (2003). The Hf isotope ratio measurement of the

zircons from the leucocratic dike sample were made on

a VG Plasma 54 multi-collector inductively-coupled

plasma mass spectrometer (MC-ICP-MS) at the Ecole

Normale Superieure in Lyon, France following tech-

niques outlined by Blichert-Toft et al. (1997). Hf iso-

tope ratios for zircons from the more massive leuco-

cratic body were measured at theUniversity of Arizona,

using the Micromass Isoprobe MC-ICP-MS, follow-

ing similar techniques to those described by Blichert-

Toft et al. (1997). Slight corrections to the measured176Hf/177Hf ratios were made for the samples mea-

sured in Arizona, due to slight isobaric interference

on 176Hf from 176Yb. The sample analyzed in Lyon

required no Yb correction. UPb geochronological

techniques followed those described by Samson and

DLemos (1999). Data were reduced using the pro-

grams ofLudwig (1989, 1990).

5. Geochemical and NdHf isotopic results

5.1. Major element compositions

The Tourtit orthogneiss and the three small

leucogranites all have high SiO2 contents >71%

(Table 1). However, the leucogranites are particularly

silica-rich ranging from 76% to 79% SiO2. In addi-

tion, the three leucogranites have extremely low K2O

(


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Table 3

SmNd and Hf isotopic data for plagiogranite samples from the Tasriwine ophiolite

Sample Nd (ppm) Sm (ppm) 147Sm/144Nd 143Nda/144Nd Nd(0)b Nd(T)b TDMc 176Hfd/

Leucogranite pluton (99-05)

Whole-rock 4.25 0.98 0.1391 0.512656 5 +0.4 +6.0 801

Repeat 4.28 0.98 0.1383 0.512649 8 +0.2 +5.9 806

Leucogranite pluton (99-06)

Whole-rock 3.17 0.68 0.1298 0.512629 4 0.2 +6.3 763

Zircon E (1) 0.28269

Zircon F (1) 0.28271

Zircon G (3) 0.28270

Zircon H (4) 0.28269

Leucogranite dike (99-07)

Whole-rock 3.29 0.62 0.1146 0.512550 6 1.7 +6.3 767

Zircon C (5) 0.28269

Note: letters for zircon analyses correspond to the UPb analyses in Table 2.a Measured ratio, corrected for spike. Normalized to 146Nd/144Nd = 0.7219. Uncertainties are 2m and refer to least significant digit.

b Nd(0) ={[143Nd/144Nd]

Sample[143Nd/144Nd]

BulkEarth}

[143Nd/144Nd]BulkEarth 10

4; Present-day Bulk Earth values: 143Nd/144Nd = 0.512638; 147Sm/144Nd = 0.1966;

c Depleted mantle model age following model ofDePaolo (1981).d Measured ratio, normalized to 179Hf/177Hf = 0.7325. Uncertainties are 2m and refer to least significant digit.

e Hf(0) ={[176Hf/177Hf]

Sample[176Hf/177Hf]

BulkEarth}

[176Hf/177Hf]BulkEarth 10

4; Present-day Bulk Earth values: 176Hf/177Hf = 0.282772; 176Lu/177Hf = 0.0332; = 1

2001).


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Fig. 3. Chondrite normalized (Evensen et al., 1978) plot of REE for the Tasriwine plagiogranite compared to plagiogranites from the Semail

ophiolite (data from Pallister and Knight, 1981), the Troodos ophiolite (data of Kay and Senechal, 1976) and the northern Appenines (data

from Borsi et al., 1996). The LREE enrichment of the plagiogranites in the Tasriwine and Appenine ophiolites contrasts sharply with the flat to

LREE-depleted patterns from the classic Semail and Troodos ophiolites. A LREE enrichment of the mantle source region must have occurred

prior to the petrogenesis of the Tasriwine plagiogranites.

Fig. 4. Nd vs. age diagram for plagiogranites from the Tasriwine ophiolite (this study) and from and the nearby Nqob ophiolite ( Thomas et al.,

2002). The initial Nd values for the Tasriwine whole-rock samples are identical to the depleted mantle model of DePaolo (1981), consistent

with a depleted mantle source for parental magmas that underwent significant amounts of fractional crystallization. Evolution of 2 Ga Eburnian

crust shown for comparison (data from Samson, unpublished).


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the very high average concentration of Hf in zircon

(10,000 ppm) combined with typically very low Lu

concentrations (


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Fig. 5. Hf vs. age diagram for zircons from the Tasriwine plagiogranites. The height of the rectangles includes both the range of determined

Hf values and the 2errors or those measurements. The plagiogranites are the same age but are shown slightly offset in age to allow for better

visualization. The values for the plagiogranite zircon crystals are extremely radiogenic, consistent with formation from a depleted mantle source.

The evolution band for 2 Ga Eburnian crust, estimated from Nd isotopic data using the relationship Hf= 1.36 Nd + 3 determined by Vervoort

et al. (1999), is shown for reference.

ite (sample 99-06). Two of the five analyses are con-

cordant (Fig. 6B), yielding identical 238U/206Pb* dates

of 763.1 3.8 Ma and 762.9 2.3 Ma. The remaining

three analyses overlap one another within the 2 er-ror, but are slightly discordant (1%). A regression

line through all five analyses (forced through 0 Ma) in-

tersects Concordia at 762 + 1/2 Ma (MSWD = 0.25).

This date is considered the best estimate of the age of

crystallization of this leucogranitic pod, and is analyti-

callyindistinguishable fromthe date of the leucogranite

dike.

6. Discussion

The major element compositions of the leucogran-

ites combined with their very low total REE contents

and Rb concentrations are consistent with them be-

ing classified as plagiogranites. Further supporting this

classification is the very radiogenic Nd and Hf isotopic

compositions of the leucogranites, which are entirely

consistent with the granites having been derived di-

rectly from depleted mantle sources. The leucogranites

are thus considered to be true plagiogranites and there-

fore their chemical characteristics and ages have direct

bearing on the evolution of the Tasriwine ophiolite. De-

tails of these characteristics are discussed below.

6.1. REE compositions

The total REE abundances (REE) of the Tasriwine

plagiogranites are extremely low (


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Fig. 6. Concordia diagrams for zircon from the Tasriwine pla-

giogranites. (A) Zircon from the deformed plagiogranite dike. Upper

intercept date of 761.1 + 1.9/1.6 Ma is considered the best estimate

of the timing of emplacement of this dike. (B) Zircon from a more

massive plagiogranite pod. Upper intercept date of 762 + 1/2 Ma

(forced through 0 Ma) is considered the best estimate of the timingof crystallization of this plagiogranite body and is the best current

age estimate of the Tasriwine ophiolite as a whole.

giogranites in the Troodos or Semail ophiolites. In-

stead, the REE patterns of the Tasriwine plagiogranites

suggest that the source region must have been LREE-

enriched. One possibility is that LREE enrichment of

the mantle occurred as the result of fluids released from

subducting lithosphere prior to the formation of the

Tasriwine plagiogranites and associated mafic and ul-

tramafic rocks (e.g. Elliot et al., 1997; Grove et al.,

2002 and references therein). Following melting of that

LREE-enriched lithosphere, extensive fractional crys-

tallization in the plagioclase-stability field must haveoccurred to explain the very well developed negative

Eu anomalies.

Mesozoic plagiogranites from the northern Apen-

nines also have LREE-enriched patterns, and thus are

similar to the Tasriwine plagiogranite patterns, buthave

much higher (REE) (Borsi et al., 1996). Borsi et

al. (1996) invoked enrichment (metasomatism) of the

mantle prior to formation of the plagiogranites, and

suggested it occurred via injection of mantle melts pro-

duced by very small degrees of partial melting in a

deeper, but adjacent part of the mantle. Whether en-

richment occurred by fluids released during slab de-

hydration or via small degrees of partial melt, both the

Apennine and Tasriwine plagiogranites must have been

generated from a source that became LREE-enriched

within a short interval of time before petrogenesis of

the plagiogranites to explain their very radiogenic ini-

tial Nd isotopic compositions. If LREE enrichment had

occurred hundreds of millions of years prior to the for-

mation of the plagiogranite, then their initial Nd values

would be much lower than the Nd(762 Ma) values of

+6 determined for the Tasriwine samples.

6.2. Nd isotopes

The high initial Nd isotopic composition of the

leucogranites is consistent with the production of the

leucogranitic magmas, either directly or indirectly,

from a depleted mantle source region. In fact, the two

samples chosen for UPb zircon analyses have the ex-

act initial Nd value predicted for average depleted

mantle at 762 Ma, based on the model of DePaolo

(1981). Being derived directly from the depleted man-

tle implies a mafic parental magma that underwent veryextensive fractional crystallization. Alternatively, the

leucogranitic magmas could have been produced by

partial melting of rocks that had themselves been pro-

duced by partial melting of depleted mantle.

The Nd isotopic composition of the Tasriwine pla-

giogranite is similar to, but more radiogenic than, the

composition of a plagiogranite in the nearby Nqob

ophiolitic fragment (Fig. 4) (Thomas et al., 2002). The

radiogenic nature of the Tasriwine plagiogranites is

also similar to that observed for younger plagiogran-


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ites in other ophiolitic regions. For example, Borsi et

al. (1996) reported initial Nd values between +8.8 and

+9.7 for 150 Ma plagiogranites within the western Alps

and Northern Apennines, values about an epsilon unithigher than model 150 Ma depleted mantle. Amri et

al. (1996) reported initial Nd values of +6.6 and +7.4

for two plagiogranite samples within the Maqsad area

of the c.100 Ma Oman ophiolite, which although 12

epsilon units lower than model 100 Ma depleted man-

tle are still well within the range of depleted mantle-

derived Mesozoic oceanic basalts.

6.3. Hf isotopes

The measured hafnium isotopic composition of the

zircons from the plagiogranite samples is considered

to accurately reflect the initial isotopic composition of

the leucogranite magmas. Most igneous rocks show

tight coupling between their Hf and Nd isotopic com-

positions, with a general correlation ofHf= 1.36*Nd+3 for world-wide terrestrial samples (Vervoort et al.,

1999). Based on the initial Nd isotopic composition of

the plagiogranite whole-rock samples, the zircon Hfvalues are slightly higher than would be predicted us-

ing either the full terrestrial NdHf correlation line

or a best-fit correlation line for juvenile rocks only

(Vervoort and Blichert-Toft, 1999). This may be duepartly to the zircon Hf values being maximum val-

ues as no correction was made for the slight amounts

of radiogenic in-growth of 176Hf since formation of

the plagiogranites. However, the correlation between

Nd and Hf isotopes is better viewed as an array, with

considerable scatter around a best-fit regression, rather

than a single line, and the Nd and Hf isotopic compo-

sitions of the leucogranite samples do fall well within

that array. Thus, no decoupling between the SmNd

and LuHf isotope systems need be invoked for the

plagiogranites.The very high Hf values of the plagiogranites

strongly reinforce the depleted mantle-like signature

observed from the Nd isotopic compositions. Such

high Hf values are exactly what would be expected if

the leucogranitic magmas were produced by extensive

fractional crystallization of much more basic magmas,

which were themselves produced by partial melting of

depleted mantle. While consistent with such a model,

however, the isotopic compositions cannot distinguish

between an origin by extensive fractional crystalliza-

tion of a basic magma or by a series of small-degree

partial melting events of a mafic source.

6.4. UPb geochronology

The two UPb zircon dates are indistinguishable

within error, consistent with the inference that the dif-

ferent exposures of theseleucogranitic bodies represent

a series of contemporaneous injections of leucogranitic

magmas. The best estimate of the crystallization age

of the plagiogranite suite as a whole, therefore, is

762+1/2 Ma, the more precise of the two leucogran-

ite dates and the one based on two concordant analy-

ses. As there are no reliable estimates of the ages of

any of the more mafic bodies within the overall se-

quence, an age of 762 + 1/2 Ma is thereby also the

current best available estimate for the age of the en-

tire dismembered Tasriwine ophiolite. The only other

unit of similar age yet identified in the Siroua region is

the 743 14 Ma Iriri Migmatite (Thomas et al., 2002).

That unit has been interpreted as having formed during

the early phases of an island arc built offshore of the

West African craton. It is therefore possible that the

Tasriwine ophiolite represents oceanic crust, possibly

oceanic basement, associated with a nascent island

arc that began to form shortly after production of the

Tasriwine ocean crust. Viewed in this way, the Tasri-wine ophiolite could be thought of as corresponding

to approximately the second stage of the idealized life

cycle of suprasubduction zone ophiolites as described

by Shervais (2001).

7. Correlation of the Anti-Atlas ophiolites

The proximity of theTasriwine ophiolite to thesmall

Nqob ophiolite and the larger Bou Azzer ophiolite to

the east raisesthe possibility that they mayall simplybedismembered pieces of a single, larger ophiolite body.

One major difficulty in trying to establish the possible

relationship between the three areas is that there are no

robust dates for any of the units deriving directly from

either the Nqob or Bou Azzer ophiolites. If the RbSr

date of 788 10Ma(Clauer, 1976) from contact meta-

morphosed sediments near mafic intrusions is taken at

face value as constraining the age of the Bou Azzer

ophiolite, then that ocean crust is 26 million years older

than the Tasriwine ocean crust. Even if the youngest


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limit (778 Ma) of the RbSr date is used, given its 10

million year uncertainty, and the 2 million year uncer-

tainty on the plagiogranite date is applied, an age differ-

ence of 14 million years still remains. This discrepancywould appear to preclude a direct genetic relationship

between the two ophiolites, although they could have

formed in similar geographic regions. It is also possi-

ble, however, that the reported RbSr date is not a ro-

bust estimate of the true age of the Bou Azzer ophiolite.

If isotopic resetting of the sedimentary detritus did not

occur completely during the intrusion of the mafic sills,

then the calculated date would not reflect the timing of

contact metamorphism, but would yield an older, geo-

logically meaningless date. Given that no precise radio-

metric dates exist for any magmatic units unambigu-

ously linked to the Bou Azzer ophiolite, its age should

be considered unknown. Small aplitic bodies have been

found within mafic rocks in the area of At Ahmaine

within the Bou Azzer ophiolite, but it is not yet clear

whether these are younger, cross-cutting leucogranitic

intrusions, possibly related to the quartz diorites in the

general area, or true plagiogranites within the ophiolitic

rocks.

Comparison of Nd and Hf isotopic compositions

of similar lithologies from the ophiolites would pro-

vide an additional method of testing possible genetic

relationships. Thomas et al. (2002) report Nd iso-topic data for one sample of plagiogranite from the

Nqob ophiolite. Calculated at the age of the Tasri-

wine ophiolite their sample has an Nd(762 Ma) value

of +5.4 1.4, indistinguishable to the values for the

Tasriwine plagiogranites within analytical error. The

Nd data is thus permissive of a genetic relationship

between the two ophiolitic fragments. However, cur-

rently there are no isotopic data available from rocks

that are part of the Bou Azzer ophiolite sensu stricto

and thus testing the idea that there may be a ge-

netic relationship between this ophiolite and the oth-ers in the Anti-Atlas will have to await the collection

of further robust geochronological and geochemical

data.

8. Comparison with other North African

Neoproterozoic ophiolites

The 762+ 1/2 Ma age of the Tasriwine ophiolite

provides documentation that this period of Neoprotero-

zoic time was important for the production of ophiolitic

assemblages in northwestern Africa. More numerous

ophiolites and ophiolitic fragments are known to oc-

cur in northeastern Africa, particularly throughout theArabian-Nubian Shield. Many of these ophiolites have

been dated allowing for a comparison with the Tasri-

wine ophiolite. Claesson et al. (1984) reported whole-

rock and mineral SmNd isochrons of 743 24Ma

and 782 38 Ma for gabbro bodies from the Jabal al

Wask and Jabal Ess ophiolites, respectively, of NW

Saudi Arabia. Pallister et al. (1988) obtained indistin-

guishable dates based on UPb zircon dating. Initial

Nd isotopic ratios of the two ophiolites range from

+6.6 to +7.6. Thus both the ages and the isotopic

compositions of these two Saudi Arabian ophiolites

are broadly similar to those of the Tasriwine plagio-

granites.

Pallister et al. (1988) investigated several ophio-

lites from the central, southern and eastern regions of

the Arabian-Nubian shield using UPb zircon tech-

niques. However, few of the analyses are either con-

cordant or lie on well-defined chords, making inter-

pretation of the geochronological data difficult. A date

of 838 10 Ma for a diorite in the Bir Umq ophi-

olite is the most reliable, as it is based on three

collinear analyses, one of which is essentially concor-

dant. The Bir Tuluhah ophiolite may also be 840 Maas two discordant analyses of zircon from a pla-

giogranite within the ophiolite yielded 207Pb/206Pb

dates of 823 11 Ma and 847 14 Ma. Pallister et al.

(1988) reported an older date of 870 11 Ma for the

Thurwah ophiolite in the central part of the shield.

However, this was based on one discordant analysis

and since two additional 207Pb/206Pb dates of 1228

and 1259 Ma were also obtained it is possible that

xenocrystic components could have affected all three

dates. In the eastern part of the Arabian-Nubian shield

a younger date of 694 11 Ma was determined fortwo zircon analyses from a gabbro within the Urd

ophiolite.

Kroner et al. (1992) provided 207Pb/206Pb zircon

dates, using the zircon evaporation technique, from

widely separated ophiolites within Egypt and Su-

dan. Reported dates from ophiolites within Egypt in-

clude 770 9 Ma (Wadi Allaqi), 746 19 Ma (Wadi

Ghadir), and 741 21 Ma (Jabal Gerf). The Onib

ophiolite, Sudan, yielded a mean 207Pb/206Pb date of

808 14 Ma.


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Taken as a whole, the Arabian-Nubian shield

contains ophiolites ranging from between about

840700 Ma, and possibly as old as 870 Ma. Most of

these studied ophiolites are intimately associated withrocks of island arc affinity. Thus it has been proposed

that the ophiolites in northeastern Africa were gen-

erated during intra-arc spreading, forming at various

places at different times, rather than being linked to-

gether in large-scale correlations (Kroner et al., 1992).

The Tasriwine ophiolite appears to have formed in a

very similar tectonic setting to the ophiolites exposed

in the Arabian-Nubian Shield, i.e. oceanic lithosphere

generated in a supra-subduction environment. In addi-

tion, the Tasriwine ophiolite formed during the same

time as the majority of northeast African ophiolites

(780700 Ma).

Ophiolite fragments are increasingly being recog-

nized as components of island arc complexes that

formed in distinct tectonic basins over brief periods

of time (e.g. Hawkins, 2003; Pearce, 2003 and refer-

ences therein). Many Phanerozoic ophiolites appear to

have been generated only a short time prior to their

obduction, thus it appears that most are minimally

traveled lithospheric fragments. If this is also true of

Neoproterozoic ophiolites, then both the northeastern

and northwestern margins of the West African Cra-

ton were regions of the simultaneous formation ofbuoyant, oceanic lithosphere, associated with newly

formed arc systems. The obduction of the ophiolitic

fragments onto these two widely separated regions of

the West African Craton presumably occurred during

the accretion of the ophiolite-island arc complexes. De-

termining precisely the timing of these accretionary

events remains as important a challenge as the pre-

cise determination of the formation of the ophiolites

themselves.

Acknowledgements

A. Essaifi is thanked for his discussions about Mo-

roccan geology and his help with translations while

in the field. P. J. Patchett is thanked for providing ac-

cess to the Arizona Isoprobe and clean laboratory. R.

Thomas and F. Corfu are thanked for their helpful re-

views of the manuscript. This work was supported by

NSF grant EAR-0106853.

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