Feldspar concentrations in lower Cambrian limestones of the Moroccan Atlas: Pyroclastic vs authigenic processes

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Feldspar concentrations in lower Cambrian limestones of theMoroccan Atlas: Pyroclastic vs authigenic processes

J. Javier Alvaro a,b,*, Blanca Bauluz a

a Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009-Zaragoza, Spainb Laboratoire de Paleontologie et Paleogeographie du Paleozoıque, UMR 8014 CNRS, Universite des Sciences et Technologies de Lille,

59655-Villeneuve d’Ascq, France

Received 6 April 2006; received in revised form 9 January 2007; accepted 13 September 2007Available online 24 October 2007

Abstract

Carbonate strata containing abundant euhedral feldspar, usually considered as authigenic, are characteristic of some lower Cambrianexposures in the Atlas Mountains, Morocco. Impure limestones are abundant in the Issendalenian Amouslek Formation of the westernAnti-Atlas, and in the Banian Lemdad Formation of the southern High Atlas. Some of their limestone beds contain up to 40% acidinsoluble residue, of which feldspar comprises as much as 65% in some samples, the remainder consisting of detrital and authigenicquartz, apatite, pyrite, and clay minerals. The host limestones are bioclastic tempestites, ooidal–oncoidal–bioclastic shoal complexes,archaeocyathan-microbial peri-reef settings (mainly flanks but not reef cores), and microbial reefs. A volcanic origin is adopted forthe feldspars based on: (i) fluctuations in feldspar concentration paralleling bedding, but unrelated to host-facies (except for the afore-mentioned archaeocyathan-microbial reef cores that were controlled by turbidity); (ii) the local association with glassy fragments (withfeldspars embedded in glass shards); and (iii) the scattered occurrence of gradational lithofacies from silty tuff to tuffitic limestone.

The mineral and chemical composition of the host-rock and diagenetic fluids was the determining factor for the kind of feldspar pres-ervation. Although the primary type of K–feldspar replacement is albitization, secondary albite subsequently suffered from illitization,chloritization, and/or replacement by calcite. A direct implication of this work is the possible geochronological dating of both volcaniceruptions and archaeocyath- and trilobite-bearing host-rocks based on radiometric analyses within unaltered K–feldspar pyroclastsextracted after etching.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Carbonate rocks; Feldspar; Pyroclast; Volcanism; Lower Cambrian

1. Introduction

Authigenic feldspar is ubiquitous in sandstones, but itsabundance sharply decreases in carbonate strata. Oneexception, studied in detail, occurs in the North AmericanMidcontinent, where up to 30% of some Cambrian andOrdovician carbonates are composed of secondary K–feld-spar (Hearn and Sutter, 1985; Harper et al., 1995). Thecrystallization of authigenic feldspar in carbonates requiresan interstitial environment supplied with K (or Na), Al,

and Si ions (Kastner and Siever, 1979; Holness, 2003). Atleast three hypotheses have been proposed to induce feld-spar authigenesis: (i) regional migration of hot brinesinduced by orogenic activities; (ii) interaction of sedimen-tary rocks with saline waters at relatively low temperatures;and (iii) subaerial, low-temperature chemical back-weath-ering in paleosols (Pitman et al., 1997; Spotl et al., 1999;Liu et al., 2003). However, the euhedral shape of silt- andsand-sized feldspars is not necessarily indicative of authi-genesis, because euhedral phenocrysts are also commonin ash-flow tuffs and lava flows (Fisher and Schmincke,1984; Best and Christiansen, 1997; and references therein).As a result, textural analyses of feldspars embedded inlimestones are not adequate for genetic interpretations if

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doi:10.1016/j.jafrearsci.2007.10.005

* Corresponding author. Address: Departamento Ciencias de la Tierra,Universidad de Zaragoza, 50009-Zaragoza, Spain.

E-mail address: [email protected] (J.J. Alvaro).

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these data are isolated from the geodynamic setting andregional context of the host rocks.

This study presents new petrographic and sedimentarydata to document the provenance of feldspar hosted bylower Cambrian impure limestones from the Anti-Atlasand High Atlas, Morocco. Unravelling the origin of theseaccessory components is an integral part of the reconstruc-tion of the sedimentary and diagenetic history of theMoroccan margin of West Gondwana during Cambriantimes.

2. Geological setting and stratigraphy

The Souss Basin (Geyer and Landing, 1995) is one of thesedimentary troughs that bordered the western Gondwa-nan margin from late Neoproterozoic to Middle Palaeozoictimes. This SW–NE-trending, intra-cratonic basin, approx-imately parallel to the High Atlas (Fig. 1), is closely relatedto the structural architecture of the Proterozoic basement(Destombes et al., 1985). After the Pan-African orogeny,a phase of intra-continental extension took place in theMoroccan margin of Gondwana, ranging from late Neo-proterozoic to earliest to middle Cambrian in time. Thisled to development of a multi-stage rifting, which reflectsthe diachronous opening of a finally aborted rift (Piqueet al., 1995; Soulaimani et al., 2003). The magmatism asso-ciated with this intra-plate extension shows tholeiitic andalkaline affinities, and a distinct diachroneity: it started ear-lier (latest Neoproterozoic) in the Anti-Atlas (Gasquetet al., 2005; Alvaro et al., 2006a), reached the western HighAtlas after the earliest Cambrian (Badra et al., 1992; AitAyad et al., 1998), and propagated even later in the Mesetadomains (Ouali et al., 2000, 2003).

Thin (up to 5 cm thick) beds of volcanic ash (frequentlyK-bentonites) are widely interbedded in lower Cambrianstrata of Morocco, whereas coarser-grained pyroclasticdeposits, and lava and pillowed flows are less abundant(Fig. 2). The episodic character of the Cambrian volcanism

was one of the major factors that controlled developmentand demise of carbonate factories in the Souss Basin. Itis usually accepted that siliciclastic input (both volcanogen-ic and derived from source areas) can smother carbonatefactories, so that carbonates commonly formed duringeither quiescent episodes without volcanism or in distalareas of platforms submitted to volcanic activity. However,although pyroclastic-related turbidity was a major ecolog-ical factor that constrained development of filter/suspen-sion-feeder and phototrophic organisms, it did not affectbenthic non-phototrophic microbial communities (Wilson,2000; Wilson and Lockier, 2002), and some lower Cam-brian carbonate strata of the Souss Basin contain moderatequantities of pyroclastic material. This is the case of someexposures of the Amouslek (western Anti-Atlas) and Lem-dad (southern rim of the High Atlas) formations, on whichthis paper is focused, which bear up to 40% in volume offeldspar (Alvaro et al., 2006b; Alvaro and Clausen,2007). The volcaniclastic material can even become domi-nant in numerous exposures of the Amouslek, Lemdadand the overlying Asrir formations (Boudda et al., 1979;Buggisch and Siegert, 1988).

The Amouslek Formation is a 40–220 m thick succes-sion of variegated shale with interbedded limestone thatis rich in archaeocyathan-microbial reefs (Fig. 2). The for-mation can be divided into shallowing-upward parase-quences (up to 20 m thick), grading upward from shale tolimestone, and commonly topped by ooidal–oncoidalshoals and archaeocyathan-microbial reef systems (Alvaroet al., 2006b). Trilobites and archaeocyaths of the Amou-slek Formation belong to the Issendalenian Stage (�Atda-banian sensu Spizharski et al., 1986; Geyer and Landing,1995).

The Lemdad Fomation is another heterolithic succes-sion composed of marlstone, siltstone, limestone and dolo-stone (some of them microbial and archaeocyathan-microbial reefs; Fig. 2). It is geographically restricted to apart of the central High Atlas, and its thickness is estimatedto be 500 m in the Lemdad-oued stratotype. There, a517 ± 1.5 Ma age is available (Landing et al., 1998) fromvolcanic ashes, which lies in the upper Antatlasia guttaplu-

viae Zone of the Moroccan lower-Cambrian Banian Stage(�Botoman; Geyer and Landing, 1995). The formationrepresents a mixed (carbonate-siliciclastic) platform punc-tuated by episodes of pyroclastic input related to magmaticactivity in neighbouring areas. Their mixed shoaling para-sequences, 1.2–10 m thick, change upward from shales andbioclastic wackestones, to grainstone/siltstone alternations,prograding ooidal–peloidal–pyroclastic grainstone shoalsand, in come cases, microbial boundstones and/or oncoidalgrainstones (Alvaro and Clausen, 2007).

3. Samples and methods

The impure limestone strata of the Amouslek Forma-tion were examined in the Tazemmourt jbel (westernAnti-Atlas), and those of the Lemdad Formation along

Fig. 1. Geological setting of the study areas in the High Atlas and Anti-Atlas, Morocco.

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the Lemdad-oued stratotype (southern rim of the HighAtlas; Figs. 1 and 3). Facies and sequence stratigraphy ofthese sections are described in detail in Alvaro et al.(2006b) and Alvaro and Clausen (2007). Host-rock lime-stones are bioclastic tempestites, ooidal–oncoidal–bioclas-tic shoals (Fig. 4A–D), archaeocyathan-microbial peri-reef settings (mainly flanks but not cores), and microbialreefs (Fig. 4E and F). The only feldspar concentrationdirectly controlled by facies is the archaeocyathan-micro-bial boundstone (reef core), where the insoluble residue iscommonly less than 5% in volume. This can be related tothe ecological constraints of these reef cores that did notdevelop under turbid waters. By contrast, microbial coresand archaeocyathan-microbial flanks can reach up to40% in volume (Alvaro and Clausen, 2007).

The studied feldspars were characterized using a combi-nation of methods, including transmitted light microscopy,scanning electron microscopy operating in back-scatteredelectron image and EDS analysis, and cathodolumines-cence. SEM analysis was made by using a JEOL JSM-6400 fitted with an Oxford Instruments D6679 detector.Back-scattered (BSE) imaging and energy-dispersiveX-ray (EDS) analyses were obtained by SEM with the

following measurement conditions: accelerating voltage20 kV, beam current 1–2 nA, and a counting interval of50 s. Analytical results display an error of ±5–7%.

Mineral separation was accomplished by dissolving thehost limestone in dilute acetic acid and hand picking undera binocular microscope. Feldspar concentration was alsoanalysed in thin section by point-counting techniques usingthe Gazzi–Dickinson technique (Ingersoll et al., 1984), andwere stained following Houghton (1980) method.

4. Stratigraphic distribution of feldspar abundance and

textures

Fig. 3 shows modal data from 45 samples from theTazemmourt section (250 m thick) and 32 from the Lem-dad-1 section (85 m thick). Feldspar grains with silt-sizedmicrolites are classed as microlithic and those containingsand-sized microlites as lathwork. Volcanic lithic grains(Lv), where feldspars are embedded in glass shards, aredivided into vitric volcanic lithic (Lvv) and lathwork volca-nic lithic (Lvl) phenocrysts.

Unaltered K–feldspar is rare due to partial to wholereplacement into albite (Fig. 5A). Albitization is easily

Fig. 2. Lower Cambrian stratigraphic chart of the Atlas Mountains; modified from Alvaro and Clausen (2007).

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distinguishable by using back-scattered electron SEM andcathodoluminescence. Secondary albite within a feldsparhost has a distinctive texture consisting of equant domainsof untwined albite. EDS analyses show that the albitizeddomains are pure albite (Table 1). In addition, K–feldsparshave green, yellow, greenish-blue, and blue luminescencecolours depending on their composition (Mora andRamseyer, 1992), whereas albitized feldspars show non-luminescent colours (Morad et al., 1990). Partial albitiza-tion allows identification of preferential fronts of epigenicreplacement, commonly located at feldspar cleavage dis-continuities (Fig. 5B). Both K–feldspar and secondaryalbite are locally flecked with mica (phengite) and chlorite

(clinochlore; Fig. 5C and D; Table 1). The whole frame-work locally suffered from partial to whole replacementby calcite, leading to the formation of sparry mosaics ofcalcite, where K–feldspar/albite pseudomorphs are onlyrecognisable by their outline (Fig. 5A, E). The aforemen-tioned phyllosilicates and, more rarely, calcite also replaceoriginal glass in Lv grains.

Fluctuations in feldspar concentration are parallel tobedding. Feldspars are not significantly flattened in theplane of bedding due to early-diagenetic lithification ofthe host rock. Some of the limestone beds contain up to40% acid insoluble residue, of which feldspar grains andLv comprise as much as 65% in some samples, the remain-

Fig. 3. Stratigraphic sections of the transition across the Igoudine-Amouslek formations (Tazemmourt jbel) and the Lemdad Formation (Lemdad oued)showing the main host-rock facies, and the distribution of feldspar abundance; logs modified from Alvaro et al. (2006b) and Alvaro and Clausen (2007);K–feldspar and untwined pure albite are measured together due to wide development of albitization (see text for description).

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der consisting of detrital and authigenic quartz, apatite,pyrite, and clay minerals (Fig. 3). The feldspars of the

Amouslek Formation are almost entirely K–feldspar(replaced into untwined albite), with an average K–feld-

Fig. 4. Photomicrographs of pyroclast-bearing carbonate textures from the Amouslek (AF) and Lemdad (LF) formations. (A) Oncoidal (on) grainstonerich in lathwork volcanic lithics, LF, Lemdad oued; scale = 4 mm. (B) Peloidal (p) packstone bearing partly albitized K–feldspar phenocrysts, LF,Lemdad oued; scale = 2 mm. (C) Peloidal grainstone rich in lathwork volcanic lithics, LF, Lemdad oued; scale = 2 mm. (D) Composite agglomerateshowing an aggregate of feldspar phenocrysts, AM, Tazemmourt section; scale = 4 mm. (E) Phenocryst-free Girvanella boundstone (g) encrusted byEpiphyton bushes (e) with silt-sized feldspar phenocrysts embedded in the microbial framework; the Girvanella/Epiphyton contact is arrowed, AF,Tazemmourt section; scale = 500 lm. (F) Another example of Epiphyton bush developed under turbid waters, as indicated by the presence of feldsparphenocrysts, AF, Tazemmourt section; scale = 500 lm. (G) Feldspar pseudomorphs partially (p) and completely (c) replaced by calcite, embedded in apeloidal packstone, AF, Tazemmourt section; scale = 2 mm.

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spar/twined plagioclase ratio of 0.9–1.0. They consist ofhomogeneous, complete and broken K–feldspar pheno-crysts, euhedral to subhedral in shape, and up to 2 mm insize. They are compositionally dominated by sanidineand anorthoclase (Table 1), with rare microcline. By con-trast, the feldspars of the Lemdad Formation show vari-able Lv/feldspar and twined plagioclase/K–feldsparratios, ranging from 0.3 to 0.5 and 0.5 to 1, respectively.Lv grains (dominated by plagioclase embedded in glassshards) are subrounded to subangular in shape, and upto 5 mm in size. EDS analyses indicate a compositionalrange of twined plagioclase of An20–An45. In the LemdadFormation, homogeneous K–feldspar phenocrysts and alb-itized pseudomorhs are also euhedral to subhedral inshape, except those larger than 2 mm, which are roundedanhedra. Where feldspar concentration is beyond 40% involume, the host-rock passes gradually into tuffitic lime-stones and silty tuffs. In this case, the siliciclastic frame-work is rich in unstable pyroclastic fragments, composedof microlithic, microgranular felsitic or glassy textures, fel-

sitic volcanic rock fragments of feldspar and femic miner-als, and angular rhyolitic grains (Buggisch and Siegert,1988; Alvaro and Clausen, 2007).

5. Genetic and environmental implications

As textural descriptions of pyroclasts frequently dealonly with vitroclasts (pumice fragments and glass shards),other arguments are necessary to distinguish between pyro-clastic and authigenic feldspars. In addition, as pointed outby Best and Christiansen (1997), euhedral felsic pheno-crysts are common in ash-flow tuffs and lava flows, so thatfeldspars sharply euhedral in outline are not necessarilyindicative of authigenic processes.

Authigenic feldspar embedded in carbonates is domi-nated by albite because sodium is commonly yielded: (i)by neomorphism of carbonate minerals, where is over-abundant with respect to potassium (Molenaar and deJong, 1987); (ii) supplied by dissolution and alteration oflabile glassy and pyroclasts derived from mafic rocks;

Fig. 5. Back-scattered electron micrographs of several subhedral K–feldspar microlites and laths from the Amouslek Formation. (A) K–feldspar lath,partially replaced by albite and calcite, showing the preferential front of albitization; scale = 100 lm. (B) Detail of previous image showing albitization ofcleavage discontinuities (arrowed); scale = 20 lm. (C,D) Partial replacement into mica of an albitized K–feldspar microlite; scale = 10 lm. (E) Relic of aK–feldspar lath partially replaced into albite, calcite and mica; scale = 50 lm. FK = K–feldspar, Ab = albite, ca = calcite, mi = mica, Q = quartz,Fe = iron oxides, mo = monacite, and ap = apatite.

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and/or (iii) derived from the dissolution of evaporates (par-ticularly halite). However, albite is also common in carbon-ate strata via albitization of primary K–feldspars. Theproblem, in this case, is to discriminate the genesis of theprimary K–feldspars: where these albitized feldspars origi-nally pyroclastic or authigenic?

If the primary feldspars are pyroclastic in origin: (i)their grain size is commonly selected by marine hydrody-namics; (ii) the mineral composition of the non-albitizedfeldspars is directly related to volcanism (e.g. sanidineand anorthoclase are abundant where derived fromnearby volcanic sources); (iii) fluctuations in feldspar con-centration are parallel to bedding; and (iv) they are absentin distinct facies associations developed under clearwaters, such as archaeocyathan-microbial reef cores. Thepresence/absence of carbonate inclusions within both pri-mary feldspar and secondary albite cannot be used asindicative of authigenesis because subsequent replacementby calcite can develop partial replacement within feldsparcrystals, a process that only depends on diagenesis.Although the aforementioned diagenetic types of feldsparreplacement are albitization, illitization, and chloritiza-tion, feldspar pyroclasts embedded in limestones also dis-play later-diagenetic replacement by calcite, which rangesfrom microsparitic replacements decreasing in intensitycentripetally to complete replacement and neomorphismleading to mosaic pseudomorphs (Fig. 4G) only recognis-able by their outline.

Although the origin of isolated homogeneous euhedralK–feldspar microlites and laths in the Amouslek Forma-tion can be difficult to constrain, their common associationwith Lv fragments and the local occurrence of gradationallithofacies from silty tuffs to tuffitic limestones supporttheir interpretation as volcanic ejecta. The shards and pum-

ice grains that dominate the Lv content show morphologiestypically produced by subaerial, magmatic, felsic eruptions(Heiken, 1972; Best and Christiansen, 1997). As delicatevesiculated glassy fragments are rapidly altered and cannotsurvive prolonged traction transport, their abundance sug-gests either direct emplacement or redeposition soon aftereruption (Coussineau, 1994). In the same way, larger feld-spar and Lv fragments are rounded anhedra, so that round-ing may be the result of magmatic resorption rather thanabrasion during transport (see Bull and Cass, 1991).

Although pyroclastic dispersal was probably primarilyrelated to sheetwash or hyperconcentrated volcanogenicflow, the pyroclastic material was subsequently reworkedby marine wave and benthic currents. The only reportedlack of reworking occurs in the feldspar-rich microbialboundstones of the Lemdad Formation. There, domaland digitate stromatolites consist of alternating layers ofdense, light-coloured carbonate-rich laminae, extremelyrich (up to 60% in volume) in silt-sized quartz, mica andfeldspar, and darker, organic-rich laminae with pyroclasticmaterial less than 20% in volume (see Fig. 6a in Alvaro andClausen, 2007). These stromatolites represent accretion ofmicrobial mats and biofilms developed under turbidwaters, and fit with Riding (1991) concept of ‘agglutinatedstromatolites’. The feldspar-rich siltstone component of theLemdad thrombolites (mainly composed of Epiphyton andGirvanella) can reach locally up to 30% in volume (Fig. 4Eand F). The siltstone not only occurs dispersed in the mud-stone and grainstone encasing the microbial boundstone,but is also covered by boundstone textures forming bothsilty patches within the microbial boundstone and bound-stone patches embedded in a pyroclastic matrix. This evi-dences growth of frame-building textures related toturbid waters controlled by pyroclastic input.

The widespread distribution of pyroclastic feldspars inthe lower Cambrian limestones of the Atlas Mountainsoffers new possibilities for high-resolution stratigraphiccorrelation. As the time span between eruption and redepo-sition of the felsic pyroclasts is considered to have been‘geologically insignificant’, radiometric analyses of etchedfeldspar phenocrysts should allow geochronological dateof both volcanic eruptions and deposition of host-rocks.As a result, the present-day bio- and chronostratigraphicscale proposed for the lower Cambrian of the Souss Basin,mainly based on archaeocyaths and trilobites, can be fur-ther refined by geochronological date of their associatedfeldspar pyroclasts.

By contrast, although ash beds and tuffs are key markerbeds for event-stratigraphic correlation, regional correla-tion of the lower Cambrian impure limestones of the AtlasMountains is difficult. This is due to local omission ofpeaks in feldspar concentration, lateral changes in key-levelthickness, mixing with surrounding foreshore-to-shorefacesediments (such as grainstone and bioclastic shoals), andmineralogical similarities after feldspar albitization andreplacement by calcite leading to development of feldsparpseudomorphs composed of sparry calcite mosaics.

Table 1Formula of analyzed phyllosilicates and feldspars; mica, chlorite andfeldspar analyses normalized on the basis of eleven, fourteen and eightoxygens, respectively; Roct = total octahedral cations, Int. Ch. = Interlayercharge, F/FM = Fe/(Fe + Mg)

Micas (n = 10) Chlorites (n = 2)

Mean Min Max Mean Min Max

Na 0.13 0.00 0.42 – – –Mg 0.19 0.05 0.30 2.27 2.06 2.47Al 2.06 1.73 2.33 2.10 2.02 2.18Si 3.59 3.32 3.94 3.74 3.59 3.89K 0.73 0.16 0.97 – – –Fe 0.12 0.00 0.23 1.10 1.07 1.13AlIV 0.41 0.06 0.68 0.26 0.11 0.41AlVI 1.65 1.55 1.72 1.84 1.77 1.91Roct 1.95 1.86 2.08 5.21 5.10 5.32Int. Ch. 0.86 0.49 1.02 – – –F/FM 0.37 0.00 0.80 0.33 0.30 0.35

Albites (n = 9) K–feldspar (n = 6)

Na 0.95 0.91 1.04 0.20 0.00 0.61Al 0.94 0.85 1.03 0.91 0.81 0.98Si 3.06 2.97 3.13 3.08 3.06 3.12K 0.02 0.00 0.09 0.74 0.48 0.94

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6. Conclusions

The lower Cambrian carbonates of the Atlas Mountainsinclude discrete intervals rich in silt- to sand-sized pyroclas-tic ejecta episodically derived from explosive felsicvolcanism. The impact of explosive eruptions on the car-bonate-dominated seafloor resulted in episodic variationsin sediment grain-size and composition, and changes inthe style and rate of deposition. Significant fluctuations infeldspar concentration are parallel to bedding and unre-lated to host-facies, except for the cores of archaeocya-than-microbial reefs that developed under clear waters.Therefore, turbidity directly influenced in the episodicdemise of archaeocyathan-microbial frame-building fabricsin substrates neighbouring active volcanoes. Loose pyro-clastic material recorded in the Amouslek and Lemdad for-mations was frequently reworked and redeposited bymarine processes. It mixed with shoal (grainstone barrier)and back-shoal (protected) settings, microbial reef cores,and archaeocyathan-microbial flanks, whereas archeocya-than-microbial cores developed under low pyroclasticinput.

The feldspars of the Amouslek Formation are almostentirely homogeneous K–feldspar phenocrysts commonlyreplaced into untwined pure albite, whereas those of theLemdad Formation show variable Lv/feldspar and twinedplagioclase/K–feldspar ratios. As the vitric particles (Lv)derived from magma and the feldspar microlites and lathsderived from phenocrysts developed in the magma, thecompositional differences between both formations reflect:(i) the presence/absence of mixtures of crystal fragmentsand laths with glass shard in the ash and pumice ejectaand (ii) the subsequent increasing distance for the ventwhere sorting is better and glassy particles decrease inabundance. In both cases, volcanic ejecta were depositedin marine substrates with little reworking and sorting.

The mineral and chemical composition of the host-rockand diagenetic fluids was the determining factor for thekind of feldspar preservation. Although the primary typeof K–feldspar replacement is albitization, secondary albitesubsequently suffered from illitization, chloritization, and/or replacement by calcite. The latter even led to formationof sparry mosaics of calcite, where feldspar pseudomorphsare only recognisable by their outline.

Acknowledgements

The authors thank the useful revisions by H. Ezzouhairi(El Jadida, Morocco) and M. Morad (Uppsala, Sweden),who have greatly improved the ideas expressed in the pa-per. This work is a contribution to IGCP project 485 ‘Cra-tons, metacratons and mobile belts: keys from the WestAfrican craton boundaries’, and CGL 2006-13533/BTEProject financed by Spanish Ministerio de Educacion yCiencia and FEDER.

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