Geochronological Constraints on Late Precambrian Intrusion ...

Geochronological Constraints on Late Precambrian Intrusion, Metamorphism, and Tectonism in the Anti-Atlas Mountains J. D. Inglis, R. S. D’Lemos,1 S. D. Samson, and H. Admou2 Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, U.S.A. (e-mail: [email protected])

ABSTRACT The Bou Azzer inlier, Anti-Atlas Morocco, is a critical element for understanding Gondwanan geology because it exposes rocks formed during the paleosuturing of the Gondwanan margin with peri-Gondwanan arc terranes. Numerous intrusions within the inlier allow specific tectonic events associated with the tectonic evolution of the Gondwanan margin to be bracketed. Detailed examination of plutons within the inlier reveal down temperature magmatic to solid-state fabrics and contact relationships indicative of emplacement during oblique collision between the margin and one or more arc terranes. U-Pb geochronological data for the Aı¨t Abdulla diorite (653.8 Ⳳ 1.6 Ma) and Bou Offroh granodiorite (653.0 Ⳳ 1.3 Ma) provide a limit to the onset of collision and regional metamorphism. A precise age of 640.8 Ⳳ 1.4 Ma for the Ousdrat quartz diorite provides an indication of the lower limit of the collision in the region. Tectonothermal activity in the Anti-Atlas at ca. 650 Ma correlates with similar periods of tectonism in the peri-Gondwanan terranes of Avalonia and Cadomia and points to the emergence of continent-wide tectonism in western Gondwana as early as 650 Ma.

Introduction 1998; Murphy et al. 1999, 2000). The tectonothermal evolution of the circum-Atlantic terranes is thought to have been geodynamically linked to global-scale events associated with the evolution of the west Gondwanan margin. Precisely correlating major geological events along this extensive margin is crucial to understanding the configuration and evolution of Gondwana during the Precambrian. The Anti-Atlas region of Morocco (fig. 1) is important since it exposes the northern margin of the West African Craton (WAC), its passive margin sequence, and dismembered parts of Neoproterozoic oceanic crustal fragments and island arcs (Leblanc 1981; Saquaque et al. 1989; Hefferan et al. 2000). The Bou Azzer inlier (fig. 2) is a critical element of the Anti-Atlas region because (i) it contains outcrops of the northern margin of the WAC and a succession of arc and oceanic crustal components that record a progressive history of deformation and metamorphism in the region; (ii) it is unconformably overlain by only slightly deformed Phanerozoic sedimentary sequences, implying that many of the original Neoproterozoic relationships survive; (iii) it is intruded by numerous calc-alkaline intrusions that can be used to bracket the tim-

A key requirement in the reconstruction of events along the margin of Gondwana during the Neoproterozoic lies in providing firm constraints on the timing of oceanic crust formation, the initiation of subduction, collision, and regional deformation. Geochronology has constrained the evolution of Neoproterozoic circum-Atlantic peri-Gondwanan terranes in many regions with unprecedented resolution (e.g., the Cadomian terrane: Samson and D’Lemos 1998, 1999; Miller et al. 1999, 2001; Chantraine et al. 2001; Gibbons and Moreno 2002; Samson et al. 2003; the Avalonian terrane: Bevier and Barr 1990; Keppie et al. 1998; the Carolina terrane: Dennis and Wright 1997; Wortman et al. 2000). It is now generally accepted that these terranes are composed of a collage of units recording repeated cycles of tectonic events associated with the formation, collision, accretion, and dismemberment of island arcs and oceanic crust of different ages between ca. 750 and 540 Ma (Keppie et al. Manuscript received August 3, 2004; accepted February 8, 2005. 1 Deers Cottage, Upper Heyford, Oxon, United Kingdom. 2 Faculte´ des Sciences, Universite´ Cadi Ayyad, Marrakech, Morocco.

[The Journal of Geology, 2005, volume 113, p. 439–450] 䉷 2005 by The University of Chicago. All rights reserved. 0022-1376/2005/11304-0004$15.00

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Figure 1. Regional geology of the Anti-Atlas mountain belt of southern Morocco, including generally accepted ages for the major tectonic elements of the region (e.g., Thomas et al. 2002, 2004). Box shows location of figure 2. Inset, simplified geological map of the West African craton (WAC) and surrounding Neoproterozoic orogenic belts; RR p Reguibat Rise; MS p Man Shield.

ing of events within the inlier. This article outlines the structural and metamorphic evolution of the Bou Azzer inlier and reports new U-Pb dates from key intrusions that resolve the timing of regional greenschist facies deformation along an important section of the Gondwanan margin. Geology of the Bou Azzer Inlier and Surrounding Area In the southern sectors of the Bou Azzer inlier, quartzite- and stromatolite-bearing carbonate units are widespread (fig. 2). These sediments are believed to be the passive margin cover sequence to the Eburnian basement (Saquaque et al. 1989; Leblanc and Moussine-Pouchkine 1994; Hefferan et al. 2002; Bouougri and Saquaque 2004). The passive margin units occur in close proximity to a variety of igneous, metaigneous, and metasedimentary rocks (Leblanc 1981). These include augen granite and muscovite pegmatite and leucogranite, which, on the basis of lithological similarity and deformation, have been correlated with rocks of the nearby Zenaga Massif (Leblanc 1981; Saquaque et

al. 1989) and have been considered to be Eburnian basement of 2 Ga or older age by all previous workers. Northern parts of the Bou Azzer inlier expose volcano-sedimentary sequences (e.g., Tichibinine Formation) considered to be part of an arc/forearcrelated sequence that is Neoproterozoic in age. Mafic rocks, ultramafic rocks, and rare blueschists occur in a discontinuous central zone between the northern and southern parts of the Bou Azzer inlier and have been interpreted to be a dismembered ophiolite fragment (Le Blanc 1975; Saquaque et al. 1989; Hefferan et al. 2002). A folded but only weakly metamorphosed late-orogenic sedimentary sequence, locally referred to as the Tiddeline Formation, occurs in fault-bounded basins (Hefferan et al. 1992). The majority of the above units are unconformably overlain by a thick succession of subhorizontal ignimbrites and conglomerates termed the Ouarzazate Supergroup. A suite of undeformed intrusions occurs in the south, near the village of Bleida (Leblanc 1981; Inglis et al. 2004a). This suite includes the Bleida granodiorite, which has recently been redated at 579.4 Ⳳ 1.2 Ma (Inglis et al. 2004a), superseding the previous date of 615 Ma

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Figure 2. 3.

L AT E P R E C A M B R I A N I N T R U S I O N A N D T E C T O N I S M

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Simplified geological map of the Bou Azzer–El Graara inlier. Boxes define location of the maps in figure

(Ducrot et al. 1979). Inglis et al. (2004a) argue that an age of 579.4 Ⳳ 1.2 Ma for emplacement of the Bleida granodiorite constrains the timing of the end of Pan-African deformation in the eastern AntiAtlas. The Ouarzazate Supergroup is overlain by the shallow marine carbonate Adoudounian Formation, which contains Cambrian members. Block faulting and weak folding during the formation of the Atlas Mountains in Mesozoic time resulted in the uplift and exhumation of the Bou Azzer basement inlier. The earliest identifiable structure within the inlier is confined to the gneissic basement. It consists of a NW-striking, NE-dipping upper greenschist facies ductile fabric (S1) and a mineral stretching lineation (L1) that dips shallowly to the NW. A lowergrade, greenschist facies fabric (S2) overprints S1 and is the dominant structure within the inlier. Much of the inlier S2 is represented by a composite foliation and is associated with the development of folds and several generations of subparallel foliations trending WNW. In the south, D2 structures

are dominated by WNW-striking composite foliations that dip steeply to the north with a common eastward-plunging mineral stretching lineation (fig. 2). In the north, the D2 structures also trend WNW and contain an eastward-plunging mineral stretching lineation but dip to the south. Analysis of S-C fabrics within the composite S2 foliation indicates that D2 deformation involved a significant component of sinistral shear associated with oblique compression. Greenschist facies structures are repeatedly overprinted at successively lower temperatures by increasingly brittle fault zones, duplexes, and cataclastic shear zones with a general WNW orientation (fig. 2). This is consistent with faults and shear zones that crosscut both the Tiddiline sedimentary succession and late orogenic intrusions such as the Bleı¨da granodiorite. A new precise U-Pb date of 579.4 Ⳳ 1.2 Ma for the Bleı¨da granodiorite (Inglis et al. 2004a) provides a firm constraint on the latest stages of brittle transcurrent movement in the Bou Azzer inlier.

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Calc-Alkaline Intrusions Gabbroic, dioritic, and quartz dioritic intrusions that display broadly similar mineralogical, textural, and chemical characteristics occur widely across the Bou Azzer inlier (figs. 2, 3) and include the Bou Zben diorite; Aı¨t Abdulla, Bou Azzer, and Ousdrat quartz diorites; Bou Offroh granodiorite; and smaller unnamed subordinate bodies. Each of the intrusions have boundaries oriented roughly WNW, subparallel to the structural grain of the inlier, and in general take the form of elongate bodies whose long axes are oriented WNW. All the intrusions have been altered to some degree by late brittle faults, generally masking the original contact relationships. The greatest extent of brittle alteration occurs in those intrusions within the ultramafic complex, while the original contact relationships of plutons to the northeast have been largely preserved. Locally, contacts crosscut greenschist facies structures, indicating emplacement after at least some greenschist facies metamorphism and deformation, although the intrusions themselves contain weak to moderately developed foliations, implying emplacement before some elements of regional deformation. Field Relationships. The Aı¨t Abdulla quartz diorite (fig. 3A) is exposed within the center of the inlier and is hosted by ultramafic rocks of the ultramafic complex, including harzburgite and other mafic cumulates. The northern and southern margins of the intrusion are tectonic; in the ESE, an intrusive body of the pluton is present and crosscuts the regional fabric in the surrounding host rocks. At both its western and eastern margins, sheets of quartz diorite invade the host rock and clearly crosscut D2 structures. However, the sheets themselves contain a weak to moderately developed igneous fabric oriented subparallel to S2. The Bou Offroh granodiorite (fig. 3B) is exposed in the NW of the inlier, close to the Bou Azzer Mine, and forms part of the larger Bou Azzer composite body. A faulted contact lies between the southern margin of the granodiorite and the lower units of the Ouarzazate Supergroup. The western margin is marked by an unconformity with subhorizontal members of the Ouarzazate Supergroup. The northeastern margin of the intrusion exposes a contact with serpentinites believed to be deformed and retrogressed parts of the ultramafic complex of the central terrane. At this boundary, crosscutting relationships between the host-rock fabric and the intrusion are preserved. The Ousdrat quartz diorite (fig. 3C) is exposed in the northern part of the inlier hosted by metagreywackes, keratophyres, and metamor-

phosed tuffs of the northern forearc metasediments. A faulted contact with members of the Tiddiline Formation marks the intrusion’s southern boundary. To the north, members of the Ouarzazate Supergroup unconformably overlie the intrusion. Original intrusive relationships between the quartz diorite and the host-metasediments are exposed along the western and eastern boundaries. At the eastern margin of the intrusion, a semibrittle triple junction occurs in which the host-rock greenschist facies foliation swings to parallel the pluton margin (fig. 3C). Within 300 m of the intrusions eastern boundary, a contact aureole is developed (fig. 3C). Locally, the quartz diorite crosscuts greenschist facies fabrics in the host rocks, and the contact metamorphic mineral assemblages overprint the same regional fabrics. Within the inner aureole, palecolored veinlets, interpreted as contact-generated anatectic melt, penetrate and wrap unmelted hostrock blocks that possess greenschist facies foliations. The anatectic veins also contain fabrics that are oriented NW, subparallel to the surrounding foliations in the host rocks. Each intrusion therefore has boundaries that are oriented WNW, subparallel to the structural grain of the inlier but which locally crosscut all folds and ductile deformation features in their wall rocks that appear to be associated with the regional greenschist facies metamorphism. This alone does not distinguish the intrusions as syntectonic, but it does demonstrate that the intrusions were not intruded before regional fabric development. Microstructures. Each of the intrusions carry weak to moderately strong foliations. These fabrics trend WNW and dip steeply to the NE throughout the Aı¨t Abdulla and Bou Offroh bodies, lying at low angles or subparallel to the intrusions margins (fig. 3). Mapping the foliation across the Aı¨t Abdulla intrusion provides a subtle sigmoidal curvature with sinistral kinematics (fig. 3A). Within the Ousdrat quartz diorite, the fabric is subvertical and also trends WNW. Within the marginal facies of the Ousdrat quartz diorite, the foliation is more pronounced and continues into the contact metamorphic migmatites in the surrounding host rocks. Microstructural analysis has helped characterize the timing of fabric development in each intrusion. Foliations developed within the intrusions display common characteristics that suggest that they record broadly similar structural histories. In general, the foliation in each intrusion is defined by the moderate alignment of subhedral to euhedral plagioclase laths, amphibole and quartz ribbons, and rare mafic enclaves. Plagioclase forms an interconnected framework of crystals aligned subparallel to

Figure 3. Simplified geological maps of three foliated calc-alkaline intrusions within the Bou Azzer inlier. Aı¨t Abdulla quartz diorite (A), Bou Offroh granodiorite (B), and Ousdrat quartz diorite (C). The Bou Offroh body forms part of the Bou Azzer pluton. Solid stars illustrate geochronological sample locations.

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the host-rock fabric. Grain-to-grain contacts of the plagioclase phase are plastically deformed, indicating that the phase developed a load-bearing framework during deformation (e.g., Rosenberg 2001). However, many prismatic crystal faces are preserved, implying an original magmatic-preferred orientation. Undeformed ovoid quartz infilling the spaces provided by the plagioclase framework demonstrates melt was still present during the development of the load-bearing framework of the plagioclase crystals (e.g., John and Stu¨nitz 1997). These features are analogous to partially molten fabric development past the rheologically critical melt percentage and therefore indicate that the fabrics initially developed in a partially magmatic state (cf. Azri 1978; Paterson et al. 1989; Renner et al. 2000; Rosenberg 2001). We thus infer that each intrusion suffered at least some deformation during the later stages of their crystallization. The localized development of coparallel penetrative fabrics within each intrusion indicates that fabric development continued in the solid state. As seen in a thin section, solid-state deformation is indicated by the brittle fracturing and disaggregation of feldspar and amphibole and ductile deformation of quartz. Intraand transgranular fractures in plagioclase feldspar are associated with fine cataclastic seams containing albite-rich feldspar that imply temperatures of deformation !450⬚C (FitzGerald and Stu¨nitz 1993). Grain boundary migration within quartz aggregates infers strain accommodation through dislocation glide at temperatures 1350⬚C (e.g., Stipp et al. 2002). Thus, the frictional-viscous nature of the feldspar and quartz microstructures constrains the overprinting solid-state fabric development to greenschist facies temperatures (ca. 350⬚–450⬚C). These estimated temperatures of solid-state fabric development in the intrusion are analogous to the estimated ambient thermal conditions of the surrounding host rocks during S2 fabric development. When viewed in detail, fabric development varies between individual intrusions. The Aı¨t Abdulla quartz diorite possesses localized regions where a strong penetrative, solid-state foliation is developed. The foliation consists of cataclastic seams that have enlarged and linked together to form an interconnected shear band network. The shear bands display sinistral kinematics, similar to the kinematics in the intrusions wall rocks. At the margin of the Aı¨t Abdulla quartz diorite, a downtemperature mylonitic fabric is developed, characterized by ductile quartz ribbons and extensive breakdown of feldspar and amphibole that has formed an ultracataclasite matrix of chlorite and muscovite. We infer that these features suggest that

deformation was localized within the margin of the intrusion following consolidation and cooling to lower greenschist facies temperatures. Within the Bou Offroh quartz diorite, solid-state fabrics are less pronounced; such structures being restricted to NW-oriented shear zones that pass from the intrusion into the surrounding country rocks. The shear zones therefore postdate emplacement and consolidation of the intrusion and thus point to postmagmatic deformation of the intrusion. Within the Ousdrat quartz diorite, magmatic fabrics dominate, defined by the alignment of undeformed euhedral grains of plagioclase, hornblende, and ovoid pools of quartz. The solid-state overprint on this magmatic fabric is limited to minor deformation of the plagioclase framework. This suggests that the intrusion suffered little solid-state deformation following its consolidation and cooling. This indicates either that regional deformation was near completion at the time of emplacement and deformation of the intrusion or that deformation was partitioned away from the intrusion. Each intrusion is posttectonic with respect to at least some deformation but pretectonic with respect to other D2 events in the inlier. Taking all the field and microstructural relationships together, we infer that the intrusions were emplaced and contemporaneously deformed toward the end of regional greenschist facies metamorphism and deformation but before lower-temperature cataclastic fabric development and brittle faulting. U-Pb Geochronology Analytical Techniques. Zircon separates were prepared from samples from each of the three intrusions for U-Pb zircon geochronology. Prismatic and needlelike zircon grains were handpicked using a binocular microscope. Only clear crack-free zircons with few inclusions were chosen for each UPb isotope dilution analysis. All measurements were made using the VG Sector 54 mass spectrometer at Syracuse University. See Samson and D’Lemos (1998, 1999) for details of dissolution, chemistry, and mass spectrometric procedures. Total common Pb amounts (laboratory blank plus initial zircon common Pb) in most samples were !3 pg, and U blanks were !1 pg. Initial common Pb compositions were estimated using the two-stage Pb evolution model of Stacey and Kramers (1975), and the data were reduced and regressed following the routines of Ludwig (1989, 1990). All analytical uncertainties are given at the 2j confidence level; further details are given in table 1. Results. A sample of the Bou Offroh granodiorite

Table 1. U-Pb Isotopic Data for Foliated Calc-Alkaline Intrusions within the Bou Azzer Inlier, Anti-Atlas, Morocco

Analysisa Bou Offroh granodiorite: A (3) B (4) C (4) D (1) E (5) Aı¨t Abdulla quartz diorite: F (1) G (2) H (5) I (2) J (5) Ousdrat quartz diorite: K (3) L (4) M (1) N (3) O (3) P (1)

Total Total Total U Pb common (ng) (pg) Pb (pg)

Ages (Ma)

Atomic ratios 206

Pb/204Pbb

206

Pb/208Pbc

% Pb/238Uc error

206

% Pb/235Uc error

207

% Pb/206Pbc error

206

206

Pb/238U

207

Pb/235U

207

Pb/206Pb

rd

.53 .46 .34 .12 .33

61.1 51.9 37.6 15.6 37.2

3.15 1.1 .72 3.28 1.18

1111 2587 2725 276.9 1799

5.34 5.52 6.06 3.68 5.09

.1063 .1064 .1055 .1067 .1064

.28 .32 .28 1.10 .44

.8996 .9004 .8924 .9039 .9007

.31 .34 .30 1.22 .46

.06141 .06140 .06137 .06143 .06139

.134 .114 .120 .325 .127

651.0 651.5 646.3 653.5 651.9

651.5 651.9 647.6 653.7 652.1

653.5 653.4 652.2 654.8 652.9

.90 .95 .92 .96 .96

.24 .41 .23 .17 .47

29.2 49.8 29.8 21.5 56.5

.95 2.65 3.13 1.79 1.65

1583 1023 514.9 676.4 1835

3.79 3.94 3.35 3.82 4.02

.1058 .1061 .1069 .1068 .1060

.38 .39 .62 .83 .33

.8964 .8979 .9061 .9046 .8973

.40 .44 .67 .86 .35

.06144 .06137 .06145 .06146 .06139

.142 .198 .249 .213 .122

648.4 650.2 654.9 653.9 649.5

649.8 650.6 654.9 654.2 650.3

654.7 652.2 655.0 655.1 652.9

.94 .90 .93 .96 .94

.24 .39 .26 .39 .80 .20

27.8 46.4 31.0 45.0 89.0 23.2

.63 2.77 2.42 1.53 2.48 1.67

2276 930.2 720.3 1603 2035 793.1

4.82 4.30 4.18 4.37 5.60 4.36

.1043 .1040 .1043 .1041 .1039 .1034

.51 .42 .94 .41 .23 1.00

.8779 .8754 .8771 .8764 .8746 .8706

.53 .47 .97 .44 .26 1.04

.06106 .06104 .06102 .06106 .06103 .06107

.132 .195 .251 .152 .113 .250

639.5 637.9 639.3 638.4 637.5 634.3

639.9 638.5 639.4 639.1 638.1 635.9

641.3 640.5 639.9 641.4 640.3 641.6

.97 .91 .97 .94 .90 .97

Note. All zircons were extensively abraded before analysis. a Number of zircon crystals analyzed given in parentheses. b Measured ratio (uncorrected for fractionation). c Corrected for fractionation plus Daly bias (0.18% Ⳳ 0.09 % amu⫺1), spike, blank, and initial Pb. Errors are 2j. Total laboratory Pb blank ranged from 0.5–2.5 pg (Ⳳ50%) during the course of the study; U blank is ≤0.5 pg (Ⳳ50%). Initial common Pb composition is estimated using the crustal growth model of Stacey and Kramers (1975). d 207 Pb/235U-206Pb/238U error-correlation coefficient (following Ludwig 1989).

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was taken at the eastern margin of the intrusion (lat 30⬚32.870⬘ N, long 006⬚59.242⬘ W), for high precision U-Pb zircon analysis. All five analyses (table 1, A–E; fig. 4A), including one single zircon grain and four analyses of small groups of five crystals or fewer, proved slightly discordant (0.2%–1.0%). Together, the five-zircon grains provide an upper intercept age of 653.0 Ⳳ 1.3 Ma (MSWD p 0.2) that is considered the best estimate for the timing of crystallization of the intrusion. A sample of the Aı¨t Abdulla quartz diorite was collected from the eastern part of the intrusion, close to the village of Aı¨t Abdulla (lat 30⬚27.572⬘ N, long 006⬚28.622⬘ W). Five analyses (table 1, F–K; fig. 4B), including one single zircon grain, proved slightly discordant (0.2%– 1.0%). The five-zircon analyses (fig. 4B) provide an upper intercept at 653.8 Ⳳ 1.6 Ma (MSWD p 0.46). The relatively large error ellipses on two of the analyses (table 1, H, I) reflect a large ratio of common Pb, given the very small amount (!30 pg) of total Pb analyzed. The upper intercept date is considered the best estimate for the timing of crystallization of the intrusion. A final sample of the Ousdrat quartz diorite was taken from the central portion of the intrusion (lat 30⬚29.171⬘ N, long 006⬚34.889⬘ W). Of six analyses, including two single grains and small groups of zircons (five crystals or fewer), one analysis proved concordant (fig. 4C) and the other five only slightly discordant (0.3%– 1.2%). The single concordant zircon grain yields a 206 Pb∗/ 238 U date of 639.9 Ma. Together, the six analyses yield an upper intercept of 640.8 Ⳳ 1.4 Ma (MSWD p 0.13), which is regarded as the best estimate for the timing of crystallization of the intrusion. Discussion The emplacement age of the Aı¨t Abdulla quartz diorite (653.8 Ⳳ 1.6 Ma) is indistinguishable from the emplacement age of the Bou Offroh granodiorite (653 Ⳳ 1.3 Ma). This firmly establishes both units as Neoproterozoic in age and implies that the two intrusions were emplaced geologically synchronously. The 640.8 Ⳳ 1.4 Ma crystallization age from the Ousdrat quartz diorite is significantly younger than the other two lithologically similar intrusions. The mineralogical and structural features of these three intrusions are generally similar to a number of other foliated calc-alkaline gabbroic, dioritic, and granodioritic intrusions within the inlier. We thus consider the three new U-Pb dates to be generally representative of the timing of emplacement of the magmatic suite as a whole. The data therefore point to an important period of calc-

Figure 4. Concordia diagrams for the calc-alkaline intrusions of the Bou Azzer inlier. Bou Offroh granodiorite (A), Aı¨t Abdulla quartz diorite (B), and Ousdrat quartz diorite (C). Error ellipses 2j.

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alkaline plutonism in the Bou Azzer region spanning at least 14 Ma between 654–640 Ma. This period of magmatism has hitherto been unrecognized in the Anti-Atlas; it thus represents a crucial step in characterizing the evolution of this section of the Gondwanan margin. Field and structural relationships demonstrate that these intrusions were emplaced and contemporaneously deformed during regional D2 greenschist facies metamorphism and sinistral transpression. Hence, the new dates for the Bou Offroh granodiorite, Aı¨t Abdulla quartz diorite, and Ousdrat quartz diorite can provide constraints on the timing of this regionally significant event. The Aı¨t Abdulla diorite and Bou Offroh quartz diorite are the oldest of the syntectonic intrusions sampled and show that widespread greenschist facies metamorphism in the inlier had commenced by 654 Ma. The 640.8 Ⳳ 1.4 Ma age for the Ousdrat quartz diorite is markedly younger than the Bou Offroh and Aı¨t Abdulla bodies. Significantly, it is also the body that is least affected by a solid-state overprint, while this might be due to a spatial separation from any localized zones of deformation. Given the pervasive nature of D2 structures across the inlier, we consider it more likely that this body was emplaced during only the latest stages of D2 regional deformation. This suggests that pervasive greenschist facies deformation was still active until at least 640 Ma but was in its waning stages (cf. Miller et al. 1999). Because the intrusions of the Bou Azzer inlier typically contain fabrics that are subparallel to the main structural trend of the surrounding rock, it has been suggested previously that all of the intrusions within the inlier, including the 579 Ma Bleı¨da granodiorite (Inglis et al. 2004a), were emplaced syntectonically with regional deformation and greenschist facies metamorphism associated with collision (Saquaque et al. 1989). However, we consider it unlikely that a single period of greenschist facies metamorphism had the ca. 75-million-year duration necessary to encompass the emplacement of the intrusions that range from 654 to 579 Ma. Within suture zones, deformation can involve the development of similarly oriented yet tectonically and thermally distinct structures over a prolonged period of time. Thus, essentially similar, subparallel structures can form during different deformation events that are widely separated in time. Therefore, the use of the term “syntectonic” is most useful if restricted to intrusions that can be proved to have been emplaced and deformed contemporaneously with individual fabrics and/or thermal structures. The Bou Offroh granodiorite,

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Aı¨t Abdulla quartz diorite, and Ousdrat quartz diorite are demonstrably syntectonic with greenschist facies deformation and therefore constrain the age of regional metamorphism in the Bou Azzer-El Graara inlier to 654–640 Ma. The Bleı¨da granodiorite (579 Ma) crosscuts greenschist facies structures and contains a weak, discontinuous, brittle fabric. It is therefore posttectonic with regard to regional penetrative greenschist facies deformation, although it is likely to have been syntectonic with regard to a much later brittle sinistral shearing (Inglis et al. 2004a). Therefore, the date of 579 Ma places a constraint on the reactivation of structures developed during the earlier greenschist facies metamorphism and illustrates that brittle deformation associated with transpression was still occurring late in the evolution of the Anti-Atlas orogen. Timing of Collision in the Anti-Atlas. The occurrence of similar D2 structures across Bou Azzer implies that each component of the inlier must have been amalgamated either before or during the 653– 640-Ma period of metamorphism identified in this study. This period of tectonism has commonly been regarded to be the result of oblique collision of the distal part of a northerly situated arc (e.g., Saquaque et al. 1989; Hefferan et al. 2002). The Bou Azzer inlier may, therefore, represent part of a wider collision zone suturing members of the WAC, units already amalgamated to it, and younger arc components. Ages from the Bou Offroh granodiorite and Aı¨t Abdulla quartz diorite intrusions may infer that collision occurred in the Bou Azzer region as early as 654 Ma. The age of the Ousdrat quartz diorite indicates that the forearc derived northern metasedimentary succession was sutured with the other components of the inlier by at least 640 Ma. The timing of collision in Bou Azzer is similar to the tectonothermal events identified in the Sirwa inlier to the NW, where it is believed a SHRIMP date of 663 Ⳳ 14 Ma from metamorphic rims on the zircons of the Irri migmatites provides the best estimate of arc-continent collision (Thomas et al. 2002). Regional Correlations. Given the presence of (i) ca. 2-Ga basement with Eburnian affinities in northern Cadomia (Samson and D’Lemos 1998; Inglis et al. 2004b) and (ii) residual bauxitic and lateritic sediments in Iberia that correlate with those found on the NW African craton (Quesada 1997), Cadomia remains the most suitable peri-Gondwanan terrane to correlate with the WAC. Recent detrital zircon U-Pb studies have provided insights into the age of the source rocks to Neoproterozoic sedimentary successions in northern Cadomia

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(Miller et al. 2001; Fernandez et al. 2002; Samson et al. 2003), Iberia (Fernandez et al. 2002; Gutie´rrezAlonso et al. 2003), and Saxo-Thuringian (Linemann et al. 2000, 2004). Significantly, the detrital record from these regions features an absence of Mesoproterozoic zircon, a characteristic of proximity to Amazonia. Instead, the dominant presence of ca. 2-Ga “Eburnian” zircon populations in many of the sedimentary units studied suggests a close proximity to the WAC. Younger zircon from the detrital studies in northern Cadomia, NW Iberia, and Saxo-Thuringian include populations with an age range between 680–630 Ma that provide evidence for magmatism in Cadomia at ca. 650 Ma. This ca. 650-Ma event is particularly evident in the sedimentary record of Brioverian Cesson conglomerates exposed in the Baie de St. Brieuc region of Brittany, France (Samson et al. 2003). Two trondhjemite boulders from the Cesson conglomerate yielded identical ages of 665.2 Ⳳ 0.5 Ma and 665.5 Ⳳ 0.7 Ma, while a third cobble yielded a 207 Pb/206Pb date of 637 Ⳳ 2 Ma. This may suggest a proximal link between northern Cadomia and the WAC at the time of this sedimentation. Remnants of a 650-Ma accretionary event are thought to be recorded in the high-grade metamorphism preserved in Eastern Avalonia (Strachan et al. 1996; Nance et al. 2002). Within the Malverns Complex, hornblende 40Ar/39Ar dates of ca. 649 Ma and ca. 652 Ma from two separate metadiorites were inferred by Strachan et al. (1996) to record the timing of cooling following upper greenschist to low amphibolite metamorphism. Similarly, within western Avalonia, a pre-630-Ma age has been postulated for fabrics associated with accretion onto Gondwana (Nance et al. 2002 and references therein). In particular, recent U-Pb studies within

the central Avalon peninsula of eastern Newfoundland (O’Brien et al. 2001) have revealed evidence for calc-alkaline magmatism and volcanism predating an important suite of monzonite plutons locally dated at 640 Ⳳ 2 Ma (O’Brien et al. 2001). Although it is clear that similar tectonothermal events may have occurred in Avalonia and the AntiAtlas at ca. 650 Ma, difficulties exist in directly correlating the two regions because the Avalonian terrane exhibits affinities for the Amazonia craton and may therefore have a disparate history to the margin of the WAC. Alternatively, the similarity between events in the Anti-Atlas and the periGondwanan terranes of Avalonia and Cadomia may point to the emergence of continent-wide tectonism along both the western Gondwanan continental margin and associated arc terranes as early as ca. 650 Ma. Caution must be taken in making broad correlations, and further detailed structural and geochronological studies are clearly needed to correlate Gondwanan events with events in the peri-Gondwanan terranes. The new data from the Bou Azzer inlier provides a first step in this direction.

ACKNOWLEDGMENTS

We thank K. Hefferan for constructive discussions on the geology of Bou Azzer. We would like to thank the geologists at Bou Azzer mine for logistical support during our field studies. We also thank F. Corfu and P. Valverde-Vaquero for their careful reviews that improved the manuscript considerably. This project was supported by a National Science Foundation grant (EAR-0106853) to S. D. Samson.

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