geochemical characteristics of cretaceous carbonatites from angola

Pergamon Journal of African Earth Sciences. Vol, 29, No. 4, pp. 735-759, 1999

© 2000 Elsevier Science Ltd P I I : S 0 8 9 9 - 5 3 6 2 ( 9 9 ) 0 0 1 2 7 - X A, rights reserved. Printed in Great Britain

0899-5362/00 $- see front matter

Geochemical characteristics of Cretaceous carbonatites from Angola

A. ALBERTI, 1'* F. CASTORINA, 2 P. CENSI, 3 P. COMIN-CHIARAMONTP and C.B. GOMES 5

~Dipartimento di Scienze della Terra, Trieste University, Via E. Weiss 8, 1-34127 Trieste, Italy 2Dipartimento di Scienze della Terra, "La Sapienza" University, Piazzale A. Moro 5,

1-00185, Rome, Italy 31stituto di Mineralogia, Petrografia e Geochimica, Palermo University, Via Archirafi 36,

1-90123 Palermo, Italy 4Dipartimento di Ingegneria Chimica, dell'Ambiente e delle Materie Prime,

Trieste University, Piazzale Europa 1,1-34127, Trieste, Italy Slnstituto de Geoci6ncias, S~o Paulo University, USP, CP 11348,

05422-970 S~o Paulo, Brazil

ABSTRACT--The Early Cretaceous (138-130 Ma) carbonatites and associated alkaline rocks of Angola belong to the Parand-Angola-Etendeka Province and occur as ring complexes and other central-type intrusions along northeast trending tectonic lineaments, parallel to the trend of coeval Namibian alkaline complexes. Most of the Angolan carbonatite-alkaline bodies are located along the apical part of the Mo~:amedes Arch, a structure representing the African counterpart of the Ponta Grossa Arch in southern Brazil, where several alkaline-carbonatite complexes were also emplaced in the Early Cretaceous. Geochemical and isotopic (C, O, Sr and Nd) characteristics determined for five carbonatitic occurrences indicate that: (1) the overall geochemical composition, including the O-C isotopes, is within the range of the Early and Late Cretaceous Brazilian occurrences from the Paran& Basin; (2) the La versus La/Yb relationships are consistent with the exsolution of C02-rich melts from trachyphonolitic magmas; and (3) the l"3Nd/l~Nd and 87Sr/SBSr initial ratios are similar to the initial isotopic ratios (129 Ma) of alkaline complexes in northwest Namibia. In contrast, the Lupongola carbonatites have a distinctly different 143Nd/~Nd initial ratio, suggesting a different source. The Angolan carbonatites have Sr-Nd isotopic compositions ranging from bulk earth to time- integrated depleted sources. Since those from eastern Paraguay (at the western fringe of the Parand-Angola-Etendeka Province) and Brazil appear to be related to mantle-derived melts with time-integrated enriched or B.E. isotopic characteristics, it is concluded that the carbonatites of the Paran&-Angola-Etendeka Province have compositionally distinct mantle sources. Such mantle heterogeneity is attributed to 'metasomatic processes', which would have occurred at ca 0.6-0.7 Ga (Angola, northwest Namibia and Brazil) and ca 1.8 Ga (eastern Paraguay), as suggested by Nd-model ages. © 2000 Elsevier Science Limited. All rights reserved.

RE~SUMI~--Les carbonatites du Cr~tac6 inf6rieur (138-130 Ma) et les roches alcalines associ~es d'Angola, appartiennent ~ la province de Parand-Angola-Etendeka et se pr~sentent comme complexes annulaires ou autres intrusions circulaires le long de lindaments tectoniques orient,s au NE, parall~les ~ la tendance des complexes alcalins contemporains de Namibie. La plupart des corps carbonatitiques et alcalins d'Angola sont Iocalis~s le long de la partie apicale de I'arc de Mo~amedes, une structure constituant I'~quivalent africain de I'arc de Ponta Grossa au sud du Br~sil, o~ une s~rie de complexes de carbonatites et roches alcalines s'est ~galement mise en

* Corresponding author [email protected] (A. Alberti)

Journal of African Earth Sciences 735

A. ALBERTI et al.

place au Cr~tac~ inf~rieur. Les caract~ristiques g6ochimiques et isotopiques (C, O, Sr et Nd) de cinq corps carbonatitiques indiquent que: (1) la composition g6ochimique globale, y compris les isotopes de O et C, se situe dans le champ des occurrences br~siliennes du Cr6tac~ inf6rieur et sup~rieur du bassin du Paran~; (2) les relations Lavs La/Yb sont en accord avec I'exsolution de fluides riches en CO 2 de magmas trachyphonolitiques et (3) les rapports initiaux ~43Nd/l~Nd et 87Sr/88Sr sont similaires aux rapports isotopiques (~ 129 Ma) des complexes alcalins du NW de la Namibie. Au contraire les carbonatites du Lupongola poss~dent des rapports initiaux 143Nd/~Nd diff~rents, sugg~rant des sources mantelliques differentes. Les carbonatites d'Angola poss~dent des compositions isotopiques du Sr et du Nd s'6talant entre la moyenne terrestre et des sources appauvries. Comme les carbonatites du Paraguay oriental (sur le bord occidental de la province de Paran~-Angola-Etendeka) et du Br6sil ont une origine dans le manteau enrichi, nous concluons que les carbonatites de la province du Paran&-Angola-Etendeka poss~dent des sources mantelliques distinctes. Une telle h6t~rog~n6it~ du manteau est attribu6e aux 'processus m~tasomatiques' qui auraient eu lieu vers 0.6-0.7 Ga (Angola, NW Namibie et Br~sil) et vers ca 1.8 Ga au Paraguay oriental, comme le sugg~rent les ~ges modules Nd. © 2000 Elsevier Science Limited. All rights reserved.

(Received 17/3/98: revised version received 15/2/99: accepted 29/3/99)

INTRODUCTION

The Cretaceous carbonatites from Angola occur as ring complexes and other central-type intrusions and are generally associated with alkaline rocks (Lapido- Loureiro, 1968, 1973; Rodrigues, 1973; Lapido- Loureiro and Valderano, 1980). Similar carbonatitic occurrences are widespread also in Damaraland, northwest Namibia (le Roex and Lanyon, 1998).

The Angolan and Namibian carbonatites and associated alkaline complexes are of Early Cretaceous age (138-130 and 130-124 Ma, respectively; Cahen et aL, 1984; AIIsopp and Hargraves, 1985; Milner et aL, 1994, 1995). They are substantially coeval tO:

i) the f lood tholei i tes of the Paran~-Angola- Etendeka Province (133-129 Ma; Renne et al., 1997);

ii) the potassic magmatism of eastern Paraguay, in the westernmost fringe of the Paran& Basin (147- 126 Ma; Comin-Chiaramonti and Gomes, 1996; Renne et aL, 1997);

iii) the alkaline-carbonatite complexes of the Ponta Grossa Arch, southern Brazil (138-128 Ma; Morbi- delli et al., 1995); and

iv) the Anit~polis alkaline-carbonatitic complex, Santa Catarina State, southern Brazil (129 Ma; Amaral et al., 1967).

The emplacement of the alkaline-carbonatitic complexes, in and around the Paran~-Angola-Etendeka Province, occurred along tectonic lineaments active at least since the Early Mesozoic (Fig. 1 ; cf. Comin- Chiaramonti and Gomes, 1996, and references therein). Most of the Angolan carbonatites occur towards the apical part of the northeast trending Mo~amedes Arch (Fig. 1 ), an uplifted mega-structure corresponding to the Ponta Grossa Arch of southern Brazil at

pre-drift times (e,g. LeBas, 1987; Trompette, 1994). In Angola, the carbonatites occur in three main

areas (Issa Filho et aL, 1991; inset of Fig. 1): (1) the central-western area (Catanda); (2) the central area (Monte Verde-Sulima, Bailundo, Coola, Longonjo, Tchivira-Bonga); and (3) the southwestern area, close to the Namibian border (Virulundo, Lupongola).

The distribution in space and time, as well as the available geochemical-petrological features (e.g. Coltorti et al., 1993), consistently indicate that the Early Cretaceous carbonati t ic magmatism from Angola occurred in the easternmost part of the Paran~-Angola-Etendeka Province (cf. Comin-Chiara- monti et al., 1997a, b). In southern Brazil (Paran& Basin) the alkaline and carbonatit ic magmas are believed to have originated from a metasomatised lithospheric mantle whose melting was promoted by the thermal perturbations of the Tristan da Cunha Plume in the Early Cretaceous (Huang et aL, 1995) and by the Trindade Plume in the Late Cretaceous (Gibson et aL, 1995; Thompson et aL, 1998). According to Comin-Chiaramonti et al. (1997a, b), melting of lithospheric mantle, variably metasomatised in Proterozoic times, could explain the genesis of the Paran~ tholeiites, as well as both the Early and Late Cretaceous alkaline-carbonatitic magmatism. In the latter case, the thermal perturbations would be related exclusively to the Tristan da Cunha Plume.

The aim of this study is to make use of the geochemical, stable and radiogenic isotopic composition of the Angolan carbonatites in order to throw light on their possible relations with the overall alkaline- carbonatitic magmatism of the Paran&-Angola-Eten- deka Province.

736 Journal of African Earth Sciences

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https://www.researchgate.net/publication/223635806_Ultra-alkaline_magmatism_with_or_without_rifting?el=1_x_8&enrichId=rgreq-d047e7f5-fbe2-4bd7-92de-63687a2b4a4d&enrichSource=Y292ZXJQYWdlOzIyMzU0OTc4MjtBUzo5OTEwNTYwNjk5NTk4NkAxNDAwNjQwMDI3MTMx


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Angolan Carbonatitic Complexes

Early Cretaceous

ParanS-Angola- Etendeka tholeiitic

r ~ ] basalts and subordinate rhyolites

K-alkaline magmatism

Late Cretaceous

Na-alkaline magmatism

K-alkaline magmatism

Tertiary

~ _ ~ Na-alkaline magmatism

Figure 1. Distribution of the magmatism in the Paran~-Angola-Etendeka Province (modified after Piccirillo et al., 1990; Comm- Chiaramonti et al., 1997a), and location o f the studied carbonatitic complexes in Angola (modif ied after Lapido Loureiro, 1973; Issa Filho et al., 1991). Quoted ages: Monte Verde: 137 + 9 Ma (whole rock, feldspar-bearing carbonatite: K = 1.56 wt %, rad 4°Ar = 8. 62 x 106 ccSTP g -r, atm. A r = 8 . 0 5 % ) ; Tchivira: 135.4 + 3. 1Ma (phlogopite from carbonatite: K = 7.57 wt%, rad ~°Ar =41 .34 x 106 ccSTP g -~, atm. Ar = 16. 78%). The sketch map represents the South-America-Afr ica links at pre-drift times (~ 130 Ma). RGR: Rio Grande Rise; WR: Walvis Ridge.

GEOLOGICAL OUTLINE

The Angolan carbonatites and alkaline complexes from central areas generally intrude the Neo-Palaeo- proterozoic crystalline basement (Carvalho et aL, 1983) of the southern unit of the central African mega-unit comprising the Congo, Kasai and Angolan Cratons (Goodwin, 1991 ). The basement is made up of granitic rocks, medium- to high-grade metamorphic rocks and migmatites. The carbonatites from Lupongola, southwestern area, are instead emplaced in a gabbro-anorthositic complex (1155 Ma, Sm- Nd isochron; Petrini and Sleiko, pers. comm. 1998).

Carbonatite occurrences The lithology, structure, mineralogy and petrography of carbonatite/alkaline occurrences are described by Lapido-Loureiro (1973) and Issa Filho et al. (1991 ). Although more than 35 alkaline and alkaline-carbonatitic complexes were mentioned, carbonatitic bodies have been recognised only in 10 main centres. Six of these were considered in this paper, i.e. Monte Verde-Sulima, Bailundo, Coola, Longonjo, Tchivira- Bonga and Lupongola. The main structural features are summarised here (cf. Lapido-Loureiro, 1973 for detailed description and mapping).


https://www.researchgate.net/publication/248532286_Aspects_of_the_geology_petrology_and_chemistry_of_some_Angolan_carbonatites?el=1_x_8&enrichId=rgreq-d047e7f5-fbe2-4bd7-92de-63687a2b4a4d&enrichSource=Y292ZXJQYWdlOzIyMzU0OTc4MjtBUzo5OTEwNTYwNjk5NTk4NkAxNDAwNjQwMDI3MTMx

A. ALBERTI et al.

Table 1. Main petrographical characteristics of the carbonatitic rocks from Angola

Sample Main Mineralogy Texture Classification

Sulima-MonteVerde SU.1.32 calcite, alkali feldspar

(26 vol%), quartz, apatite (2 vol%)

SU.2.55 calcite

SU.3.64 calcite, apatite (10 vol%), aegirine

SU.4.113 calcite, dolomite

SU.5.144 calcite, aegirine

SU.6.143 calcite, apatite (4 vol%)

SU.7.104 alkali feldspar (64 vol%), calcite, dolomite, apatite (4 vol%)

cataclastic: heterogranular carbonates cementing lithic fragments (mainly syenitic, subordinately granitoid basement rocks) fine- to medium-grained, subequigranular xenomorphic medium-grained, granoblastic

calcite carbonatite

calcite carbonatite

apatite-calcite carbonatite

fine-grained, subequigranular xenomorphic heterogranular, xenomorphic (aegirine in few large crystals) medium-grained, granoblastic

calcite carbonatite

calcite carbonatite

calcite carbonatite

microbrecciated carbonatitic syenite

Bailundo BA.8.55 calcite, apatite (13 vol%)

BA.9.86 apatite (59 vol%), magnetite, calcite

BA.10.89 apatite (91 vol%), calcite

BA.11.90 calcite

BA. 12.126 calcite, dolomite, apatite (8 vol%), magnetite

Longonjo L0.13.18

LO. 14.30

LO. 15.34

dolomite, calcite, Fe- oxides (from ankerite transformation) dolomite, calcite, Fe- oxides (from ankerite transformation)

alkali felspar (12 vol%), ankeritic dolomite, calcite, kalsilite (7 vol%), Fe-oxides

fine- to medium-grained, equigranular xenomorphic with veins of quartz fine-grained; carbonates in small veinlets and pods heterogranular xenomorphic; few carbonates in interstices medium- to coarse-grained, granoblastic fine-grained, subequigranular with flowage features in irregular bands


phoscorite

apatite cumulate

calcite carbonatite


fine-grained, brecciated with quartz-filled network

dolomite carbonatite

fine-grained, brecciated with alkali feldspar and kalsilite xenocrysts and syenitic xenoliths microbrecciated with subrounded clasts (syenite, alkali feldspar, from basement rocks) in oxidised matrix with kalsilite and alkali feldspar ocelli)

dolomite carbonatite

alkali feldspar-dolomite carbonatite

Coola C0.16.42 dolomite, calcite, barite,

fluorite, fluorocarbonates brecciated barite-dolomite carbonatite



Table 1. continued

Sample Main Mineralogy

Tchivira-Bonga TB. 17.14 dolomite, calcite, apatite

(7 vol%)

TB. 18.32 dolomite TB. 19.66 dolomite TB.20.34 calcite, apatite (18 vol%) TB.21.42 calcite, apatite (9 vol%),

dolomite

Texture Classification

medium-grained, equigranular protoclastic, with quartz-filled veinlets and patches medium-grained, equigranular medium-grained, equigranular coarse-grained, brecciated medium-grained, equigranular

apatite-dolomite carbonatite

dolomite carbonatite dolomite carbonatite apatite-calcite carbonatite apatite-calcite carbonatite

Lupongola LU.22.23

LU.23.25

calcite, alkali feldspar (42 vol%), green mica (8 vol%)

calcite, alkali feldspar (33 vol%)

LU.24.45 calcite

LU.25.60 calcite

LU.26.61 calcite LU.27.63 calcite, ankerite

LU.28.65 alkali felspar, opaques, biotite, calcite

LU.29.67 calcite, fluorite

brecciated with subrounded feldspar clasts in heterogranular matrix of xenomorphic carbonates and fine-grained quartz + feldspar aggregates and opaques protoclastic porphyritic with subrounded feldspar clasts in medium-grained carbonatitic groundmass heterogranular fluidal; few small alkali feldspar-nepheline ocelli fine- to medium-grained, equigranular fluidal texture xenomorphic fluidal or banded fine- to medium-grained, xenomorphic fluidal fine-grained, microbrecciated banded heterogranular, banded

carbonatitic syenite

alkali feldspar-calcite carbonatite

calcite carbonatite

calcite carbonatite

calcite carbonatite calcite carbonatite

carbonatitic trachyphonolite

fluorite-calcite carbonatite

The Sulima-Monte Verde complex consists of two main intrusive centres. Monte Verde shows a ring with fenitised rocks including discontinuous outcrops of carbonatite. A hypabissal nepheline syenite body, with associated eruptive breccias, intrudes the western part of the ring structure and extends also to the south. Sulima, 2 km to the southwest of Monte Verde, is a large annular structure of syenitic intrusions. On its northeastern side it is intruded by eruptive breccias related to the Monte Verde centre.

Bailundo shows a discontinuous ring structure of fenitised syenites and a core of carbonatite with apatite cumulates.

Coola is a polygenetic complex showing two main intrusive centres: one consisting of a discontinuous ring of carbonatites and carbonatitic breccias; and the other, about one km to the south, consisting of an elongated body of potassic-ijolite. Late foiditic,

mainly extrusive breccias, outcrop in the central part of the ijolitic body.

Longonjo is a body with a horse-shoe shape, open to the southeast, intrusive in granitic-migmatitic rocks. Eruptive carbonatitic breccias, some of which have extended barite and fluorite mineralisation, are a hallmark of these outcrops. The rocks locally carry syenitic xenoliths and kalsilite xenocrysts. The outer carbonatitic breccias carry up to 70% of alkali felspar (microcline) and quartz xenocrysts from the disrup- tion of the crystalline basement.

Tchivira-Bonga is a polygenetic alkaline-carbonatite complex (Coltorti et al., 1993) showing two main centres. A north-northeast trending swarm of tephritic to phonolitic dykes is also present. The main Tchivira centre consists of early rings of ijolite and nepheline syenite with an inner carbonatitic ring. Late alkali gabbro to syenitic intrusions partially


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A. ALBERT/et al.

disrupt the earlier structure in the western sides. Bonga is a troncated carbonatite plug, surrounded by feldspathic-carbonatitic breccias.

Lupongola is an annular structure in which brecciated carbonatite pods, dykes and veins are associated with syenites in discontinuous outcrops along the ring. A north to northwest trending dyke system (carbonatites and phonolites) cross-cuts the ring structure and extends for several kilometres in the gabbro-anorthosite country rocks.

DESCRIPTION OF THE SAMPLES

A total of 29 samples, selected by A. Alberti from the collection at the Geology Department of the 'A. Nero' University, Luanda, Angola, were chosen as representative of the six described carbonatite- bearing complexes. The samples illustrate a wide spectrum of carbonatitic lithologies, ranging from rocks with more than 95% carbonates to silica-rich rocks, including two apatite-rich specimens. The range is typical for the Angolan volcanic-plutonic complexes, in which comparatively few medium- to coarse-grained calcite carbonatites are present. Brecciated types ('eruptive breccias' of Lapido Loureiro, 1973) are common. Solid-state deformation and recrystallisation is restricted to some 'pure' carbonatites (i.e. carbonate > 95%). In some cases the brecciated rocks con- tain feldspar xenocrysts and syenite and/or basament clasts set in a fine-grained carbonatic matrix, suggestive of an explosive, diatreme-type origin.

Iron oxides, mica (in general tetraferriphlogopite) and kalsilite-rich nepheline are frequent as accessory minerals. REE-fluorocarbonates are present in all carbonatites as radiating fibres or clusters included in carbonate minerals, along with veins of barite, strontianite and accessory pyrochlore (cf. Lapido Loureiro, 1973; Issa Filho et al., 1991 ).

The main petrographical features are summarised in Table 1.

ANALYTICAL PROCEDURES

Major and minor element abundances were determined by X-ray fluorescence on powder pellets using a Philips PW-1400 automated spectrometer. Accu- racy was estimated to be better than 2% and 10% for major and minor elements, respectively. Trace element concentrations were determined by ICP- MS methods following the procedures of Alaimo and Censi (1992). Accuracy is estimated to be ca 5%. Precision, based on triplicate analyses of standards and samples, ranges from 1% to 5%.

Stable isotopes for carbonates and apatite were analysed by means of a Finnigan Mat Delta E mass

spectrometer. The results are given in terms of the conventional 5%0 units, reference standards being PDB-1 and V-SMOW for C and O isotopic compositions, respectively. Typical precision for analyses of in-house standards (Jacupiranga carbonatite and apatite, and Juquid beforsite) are 0.05%0 for both 813C and 5130.

Strontium isotope compositions were measured after Sr separation by standard ion-exchange chro- matography, using a VG54E single collector mass spectrometer. The measured 87Sr/e6Sr ratios were fractionation-corrected to 86Sr/SSSr = 0.1194. Mean and 2~ error for repeated analyses of the NBS 987 standard gave 87Sr/86Sr =0.71021 9 ___0.00001 6 (N = 10). Neodymium isotope compositions were determined using a Finnigan MAT262 multicollector mass spectrometer. The measured 143Nd/144Nd were corrected for fractionation to ~46Nd/~44Nd = 0.7219, La Jolla Nd standard =0.511850; standard error (2~) of the analysis is 0.000008; uncertainty= 1.5%. Detailed information on the sample pre- paration, analytical procedures, precision and accuracy are in Castorina eta/. (1 997) and Speziale et al. (1 997).

GEOCHEMISTRY

The results of the major and trace elements are given in Table 2, and O-C and Sr-Nd isotopic analyses are reported in Tables 3 and 4, respectively.

Major elements The Angolan carbonatites have a wide compositional range comparable to that of the Brazilian analogues. Figure 2 shows the distribution in terms of a CaO- MgO-(FeO + MnO) diagram (Woolley and Kempe, 1989), and of the DI (molar CaO/[CaO+MgO+ FeO + MnO]) versus (FeO + MnO + MgO) plot. The samples include calcio-, magnesio- and some ferrocarbonatites, with a continuous trend from magnesio- to calciocarbonatites (Fig. 2A). The Tchivira- Bonga samples range from calciocarbonatites to magnesiocarbonatites to ferrocarbonatites (inset of Fig. 2A), a feature found in many carbonatite complexes (Woolley, 1982). The other samples correspond to: Sulima-Monte Verde = calciocarbonatites, Bailundo=calcio- to ferrocarbonatites, Lon- gonjo = magnesiocarbonatites, Coola = magnesiocar- bonatite, and Lupongola = calciocarbonatites, alkali feldspar carbonatite (alkali feldspar up to 44 wt%) and carbonatitic trachyphonolite. Notably, the Damaraland (northwest Namibia) carbonatites are represented by ferrocarbonatites and scarce calciocarbonatites (Fig. 2; cf. le Roex and Lanyon, 1998).


https://www.researchgate.net/publication/238438167_A_Discussion_of_Carbonatite_Evolution_and_Nomenclature_and_the_Generation_of_Sodic_and_Potassic_Fenites?el=1_x_8&enrichId=rgreq-d047e7f5-fbe2-4bd7-92de-63687a2b4a4d&enrichSource=Y292ZXJQYWdlOzIyMzU0OTc4MjtBUzo5OTEwNTYwNjk5NTk4NkAxNDAwNjQwMDI3MTMx


Table 3. Isotopic O-C values for carbonates (calcite and Fe-dolomite) and apatite from Angolan carbonatites

8180o, (%o) 5~3C~ (%o) 5~80~ol (%0) 6~3Cdol (%o) 5~80,p (%0) 5~80dol.cc (°C) ~13Cdol.c~ (°C) 5180,p.c~ (°C)

Sulima-Monte Verde SU.1.32 9.35 4.00 5.40 -4.00 SU.2.55 13.98 -1.67 SU.3.64 8.65 -4.88 SU.4.113 5.89 -5.26 12.21 -2.53 SU.5.144 7.08 -6.13 SU.6.143 9.21 -5.20 SU.7.2 7.20 -5.59 8.21 -5.06 292 310 Bailundo BA.8.55 19.04 -4.08 BA.9.86 22.41 -2.55 BA. 10.89 7.21 -7.04 BA.11,90 9.03 -4.89 BA.12.126 11.18 -4.12 12.69 -3.55

18.1 391 4.65 649

6.8 212 398 384 Longo~o LO.13.18 22.13 -5.15 19.90 -2.23 LO.14.30 23.06 4.12 22.58 -1.62 LO.15.34 23.08 4.16 22.12 -1.13

m

m

m

Coola CO.16.42 10.00 4.80 10.97 4.1 300 310

Tchi~m-Bonga TB.17.14 21.49 4.36 15.53 -3.65 TB.18.32 9.42 -3.09 TB.19.66 16.55 -1.88 TB.20.34 11.89 4.00 12.59 -3.37 TB.21.42 9.69 -5.31 10.32 4.85

304

7.21 366 352 359 5.54 388 515 406

Lupongola LU.22.23 7.52 -8.19 LU.23.25 8.25 -8.04 LU.24.45 10.39 -6.27 LU.25.60 8.73 -8.09 LU.26.61 8.45 -8.32 LU.27.63 14.04 -6.43 LU.28.65 8.39 -6.65 LU.29.67 10.23 -6.30

7.67 -7.79 4.75 631 611 602 8.58 -7.58 4.91 512 515 502

5.70 359

Temperatures of isotopic (re)equilibration are according to Sheppard and Schwartz (1 970) and Jenkin et al. (1 991 ).

The DI ratio in calciocarbonatites and magnesio (ferro)carbonatites is negatively correlated wi th (MgO + FeO + MnO), due to Ca-Mg(Fe) substitution (Fig. 2B). Some other correlations (not shown) are significant: SiO 2 is correlated wi th AI203 (r = + 0.88) and Na20 + K20 (r = + 0.95) in the whole population, whereas in the rocks with SIO2<9 wt%, P205 is negatively correlated with L.O.I. (assumed to represent CO2; r =- 0.98), in good agreement with the modal content of alkali feldspar and apatite, respectively (cf. Table 1 ).

Trace elements Of the elements which are generally regarded as trace elements in igneous rocks, Sr, Ba and Ce are

unusually abundant: SrO, max. 2.83 wt% (Sulima- Monte Verde); BaO, max. 5.76 wt% (Lupongola); and Ce203, max 1.55 w t% (Coola). On the whole, trace element scatter of the different carbonatitic types (Table 2) reflects the variable distribution of accessory phases. It should be noted that the Y/Ho ratio for most Angolan carbonatites (mean 25 + 3, N=21 ) is similar to the chondritic value (28; Sun and McDonough, 1989), except for the Tchivira- Bonga carbonatites (mean 13 + 2, N = 5).

In the primitive mantle normalised patterns (Fig. 3A), the Angolan Ca-carbonatites are characterised by:

i) high Ba, Sr and REE contents, similar to other worldwide carbonatites (Nelson etal,, 1988; Woolley


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Figure 2. (A) CaO-MgO-(FeO t + MnO) classification diagram (Woolley and Kempe, 1989) for the Angolan and northwest Namibia carbonatites. Inset: main trends o f the Angolan carbonatites. (B) DI= CaO/ (CaO + MgO + FeO t + MnO) molar ratios versus FeO t + MnO + MgO wt%. Dotted field." alkali feldspar- carbonatites and carbonatitic silicate rocks. Data sources: this study, Lapido Loureiro (1973), Issa Filho et al. (1991), le Roex and Lanyon (1998).


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Figure 3. (,4) Minor and trace element data for the Angolan carbonatitic samples normalised to primitive mantle (Sun and McDonough, 1989).

e t aL, 1991); ii) variable concentrations of Th, U, Nb and Ta,

which roughly parallel those of the carbonatites from southern Brazil;

iii) variable, but marked depletion in Rb, K, Hf(Zr) and Ti, and enrichment in Ba, Ta, La-Ce-Sr and Tb,

as the Jacupiranga (Brazil) Ca-carbonatites (Huang et aL, 1995; Castorina et aL, 1994); and

iv) ranges of incompatible elements similar to those of the Namibian carbonatites (Fig. 3B; cf. le Roex and Lanyon, 1998).

The Angolan Mg(Fe)-carbonatites show patterns



RbBaTh U NbTaK LaCe Sr P NdSmHl Zr Eu TI Tb Y YbLu

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Figure 3. continued. (B) Fields of Ca, Mg-Fe carbonatites, apatite and silicate rocks, as we# as Angolan, Namibian and Brazilian carbonatites (Castorina et al., 1997; le Roex and Lanyon, 1998) shown for comparison.

similar to those of Ca-carbonatites, but they differ from the Brazilian analogues (e.g. Juqui&; Beccaluva et aL, 1992; Castorina et aL, 1996) mainly because the latter Mg(Fe)-carbonatites have positive Sr, and negative Th and Nb spikes. Alkali feldspar-bearing carbonatites also have patterns similar to those of Ca-carbonatites, except for K. Apatite-rich samples are characterised by negative Rb, K, Sr, Hf-Zr and Ti spikes. The Lupongola carbonatitic trachyphonolite shows negative Ta, Ce-La, Hf and Ti, and positive Sr spikes, as do some plagioleucitites from eastern Paraguay (Comin-Chiaramonti etaL, 1992, 1997b).

Rare earth elements Chondrite-normalised REE concentrations display characteristically high La/Yb (mean [La/Yb] N = 64_

31 ), carbonatitic trachyphonolite excepted ([La/Yb] N = 4.5). In general, total REE concentrations increase with increasing P2Os and F, suggesting that phos- phates and fluorocarbonates are the main REE carriers. The chondrite-normalised patterns (Fig. 4) parallel those of their Brazilian Ca-carbonatites analogues (e.g. Jacupiranga), but at higher contents, and differ from the more flattened patterns of the Juqui& Mg-carbonatite (Beccaluva et al., 1992).

The investigated samples cover the variation field of the worldwide carbonatites in terms of La versus La/Yb relationships (Fig. 5; cf. Andersen, 1987). Castorina et al. (1997) illustrated the La versus La/ Yb relationships for basanite-tephrite and trachyphonolite-phonolite suites and primary carbonates in the Paraguay and Brazilian alkaline-carbonatititic


A. ALBERTI et al.

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F igu re 4. REE, c h o n d r i t e - n o r m a l i s e d ( B o y n t o n , 1 9 8 4 ) p a t t e r n s o f A n g o l a n c a r b o n a t i t i c s a m p l e s ( n u m b e r s as in Table 1). The f ie lds o f the Braz i l ian ca rbona t i t es , e.g. J a c u p i r a n g a a n d Juqu i~ , a re f r o m B e c c a l u v a e t al. ( 1 9 9 2 ) , H u a n g e t al . ( 1 9 9 5 ) a n d Cas to r ina e t al.

( 1 9 9 6 ) . The f ie lds o f the n o r t h w e s t N a m i b i a c a r b o n a t i t e s are f r o m le R o e x a n d L a n y o n ( 1 9 9 8 ) .

complexes. In particular, if the differentiation of the C02-bearing parental magma is relatively restricted (e.g. basanite to phonotephrite), the exsolving carbonatitic melts are relatively small and carbonates crystallise either in the groundmass, or as irregular patches and ocelli. On the contrary, if the differentiation leads to phonolitic liquids, these melts may exsolve large amounts of carbonatitic liquids characterised by high La and La/Yb relative to the fluid-

rich parental basic magma (cf. inset of Fig. 5). Thus, if, for example, a basanitic magma containing 7 wt% CO 2 is assumed as the parental magma, the differentiation (fractional crystallisation) may consist of two main steps:

i) the first step leads to moderately differentiated magmas (e.g. phonotephrite-trachyphonolite) where the concentration of CO 2 may be as high as 14 wt%; and

748 Journal o f Afr ican Earth Sciences

Geochemical characteristics o f Cretaceous carbonatites from Angola

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Figure 5. La versus La/Yb ratios for the carbonatitic samples from Angola. The worldwide carbonatite field is from Andersen (1987). Representative samples of northwest Namibia carbonatites are from le Roex and Lanyon (1998). The dotted area represents the limits of trachytic-phonolitic magmas (e.g. TP: LU. 28, 65 trachyphonofite) exsolving carbonatitic liquids, as inferred by mixing (heavy) curves showing fractions of residual liquid (i.e. 20 wt% of carbonatitic magma). Other end members: I: groundmass carbonates from silicate rocks; IHV: carbonatites associated with alkaline complexes (see Castorina et al., 1997 for detailed discussion). Inset: possible evolution path from basanitic to carbonatitic magma (cf. Castorina et al., 1997).

ii) the second step will promote the exsolution of about 20 wt% of carbonatitic liquid from the differentiated phonolitic magma (cf. path of inset of Fig. 5).

Notably, the Angolan carbonatites (and some carbonatites from Damaraland) fit the evolutionary paths of Fig. 5, suggesting that carbonatite exsolution from a trachyphonolitic melt is a possible event.

Oxygen and carbon isotopes The O and C isotopic results are plotted in Fig. 6. The 6180 and 613C values of the carbonates range between 5.5 and 23%0, and between -8.5 and -1%o, respectively; the 6180 of the apatites is in the range 4.7-18.1%0. The carbonate values cover almost the

whole data set from southern Brazil and Angola (Fig. 6, inset B). They follow two main trends (Castorina et al., 1 997) corresponding to:

i) an 'intrusive trend': starting from the box of the primary carbonatites up to 6180 14%o; and

ii) an extrusive trend (6180 ranging from 14-23%0). Castorina et al. (1 997) and Speziale et al. (1997)

developed a model in terms of C-O isotopic fractionation of carbonates during 'magmatic processes' (i.e. fractional crystallisation and liquid immiscibility), and 'fluid processes' (i.e. interaction between rocks and fluids at different hydrothermal temperatures and CO2/H20 ratios: cf. Fig. 6, inset A). On the basis of the above model, about 30% of the samples plot in the box of 'magmatic processes' (i.e. temperatures


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Figure 6. Plot of 5~80 versus 5'3C for carbonates from Angolan carbonatitic samples and from northwest Namibia carbonatites. Data sources: Table 2, Pineau et al. (1973) and le Roex and Lanyon (1998). PC: box of "primary carbonatites" (Taylor et al., 1967). (A) Evolution of the C-O isotopic composition of a carbonatic component: magmatic conditions (i.e. 1200-400°C; CLM=continental fithospheric component) and hydrothermal environment at various CO/H20 molar ratios (cf. Speziale et al., 199 7). (B) Comparison between Angolan and Brazilian carbonatites (Bizzi et al., 1994 and references therein; Castorina et al., 1996, 1997; Speziale et al., 1997). (C) 0-0 and C-C plots for calcite-(Fe)dolomite and apatite-calcite pairs and DO-C, dolomite-calcite and apatite-calcite versus temperature. The isotopic equilibration temperatures (cf. Table 2) give ranges of 212-631 °C and 304-611 °C (6 'aO ~o~.cc and 6~3C do1_c~ respectively; Sheppard and Schwartz, 19 70), and of 359-604 °C (~80ap.c j Jenkin et al., 1991).

in the range 900-400°C, (~180 5.5-9%0), while the other samples follow trends related to 'fluid processes' (i.e. temperatures in the range 400-100°C) at CO/ H20 between 0.8 and 1.0.

Oxygen isotopes of the calcite-dolomite and calcite- apatite pairs are positively correlated (inset C of Fig. 6). The O isotopes of the dolomite-calcite and apatite- calcite pairs yielded temperatures whose distinct trends pinch out at about 920°C. It should be noted that

such a temperature is consistent with those (i.e. 850- 950 °C) estimated for exsolution of carbonatitic liquids from trachyphonolitic magmas in the carbonatitic occurrences from the Paran6 Basin (e.g. Comin-Chiara- monti etal., 1992, 1995; Castorina etal., 1994, 1996).

Sr-Nd isotopes Initial (1 38 Ma) eTSr/SeSr (Sr i) and ~43Nd/144Nd (Nd i) ratios for most Angolan carbonatites and associated



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Jou rna l o f A f r i can Earth Sciences 751

A. ALBERTI et al.

alkaline rocks from the Mo~amedes Arch, brecciated rocks and Lupongola carbonatites excepted, vary between 0.70321 and 0.70466, and between 0.51273 and 0.51237, respectively, showing on the whole depleted characteristics relative to bulk earth (B.E.) (Fig. 7). In comparison, the Early Creta- ceous primary carbonatites from northwest Namibia have a similar Sr~ range (0.70351-0.70466), but almost constant Nd i , i.e. 0.51250 to 0.51244 (Milner and le Roex, 1996; le Roex and Lanyon, 1998). Mixing models between parental magmas with crustal components imply unrealistic (up to 50%) con- taminant fractions (cf. inset A of Fig. 7). According to Bell and Blenkinsop (1989), Sr~-Nd~ values of the analysed samples can result from either mixing between a depleted and a slightly enriched component, or from sampling of several ancient reservoirs formed at the same time from the same parent material.

The carbonatite specimens from Lupongola have Sr~ in the same range as the Mocamedes carbonatites, but very low Nd i (mean Sr~=0.70371 + 0.00038; mean Nd i = O. 51160 + 0.00018). These characteristics suggest complex processes between mantle-derived melts and amphibolites and/or granulites (cf. Bell and Peterson, 1991 ), or anorthosites and/or vapour fluxing of the mantle (cf. Meen et aL, 1989). The latter rocks might be a potential con- taminant since the plagioclase from the surrounding Cunene anorthositic complex (1155 Ma) has Sr~= 0.7046 and Nd~ = 0.51065 (Petrini and Sleiko, pers. comm. 1998).

Alternatively, the Lupongola carbonatites may represent a Precambrian event (cf. worldwide Pre- cambrian carbonatites: Sr~ = 0.70181-0.70372 and Nd~ = 0.51207-0.50999; Bell and Blenkinsop, 1989). Asssuming an age of 1155 Ma, the Lupongola carbonatites plot, in terms of Sr and Nd (inset B of Fig. 7) towards B.E., in the enriched quadrant. In this case the Lupongola carbonatites would represent an extension of the Precambrian carbonatites, as well as those that define the African array (inset B of Fig. 7; cf. Bell and Blenkinsop, 1989, fig. 12.4). Obviously, the age of the Lupongola carbonatites is crucial.

Comparison with the carbonatitic occurrences of the Paranfi-Angola-Etendeka Province (PAEP) In general, the carbonatites from the Paranfi Basin have Sr~ and Nd~ similar to those of the associated (potassic) alkaline rocks (Fig. 8), even in the samples affected by hydrothermal processes, as shown by O-C and Sr-Nd isotopic systematics (Comin-Chiara- monti eta/., 1997a; Castorina eta/., 1997; Speziale et al., 1997),

In eastern Paraguay, the Early Cretaceous potassic rocks and associated carbonatites yielded Sr~ and Ndi within the ranges 0.70612-0.70754 (mean 0.70720_+0.00094) and 0.51154-0.51184 (mean 0.51172 _+ 0.00041 ), respectively, showing a strong time-integrated enriched character. These isotopic characteristics are not easily explained by crustal contamination for the high percentage of crustal component(s) indicated by the mixing equation (up to 90%). Note that the O isotopic data indicate a primary, not contaminated origin for the rock-forming minerals (cf. Comin-Chiaramonti and Gomes, 1996; Comin-Chiaramonti et al., 1997a). Likewise, AFC processes do not account for the isotope data of the potassic rocks, given the poor correlations between LILE and Sr~ and Nd~ (Comin-Chiaramonti et al., 1997a, b).

The Early Cretaceous K-alkaline-carbonatitic complexes from the Ponta Grossa Arch, Brazil, have mean Sr~= 0.70527 _+ 0.00034 and Nd~=0.51224+ 0.00011 (Comin-Chiaramonti et al., 1997b). In northwest Namibia, the Early Cretaceous carbonatites have variable Sr~ (mean 0.70398 +0.00046) and quite constant Nd i (mean 0.51248 _+ 0.00002), whereas the associated coeval alkaline rocks (both sodic and potassic) are characterised by mean Sr i = 0.70414 + 0.00035 and Nd~ = 0.51253 + 0.00007 (cf. Milner and le Roex, 1996; le Roex and Lanyon, 1998).

Late Cretaceous alkaline-carbonatite complexes have the following mean Sr~ and Nd~, respectively (Fig. 8): Alto Paranafba kamafugites, Sr~ = 0.70527 _+ 0.00036 and Nd~ =0.51224-+0.00006 (Bizzi eta/., 1994; Gibson et al., 1995 and references therein); Taiuvfi-Cabo Frio lineament, Serra do Mar plagioleucitites, 0.70447 -+0.00034 and 0.51252-+ 0.00008 (Thompson etaL, 1998); and Lages kamafugites, 0 . 7 0 4 8 5 + 0 . 0 0 0 5 3 and 0.51218_+ 0.00022 (Traversa et al., 1996).

On the whole, the Early Cretaceous alkaline and alkaline-carbonatite complexes from the PAEP have a Sr-Nd isotopic signature which spans from time- integrated depleted, e.g. DMM (eastern side of the province), and time-integrated enriched, e.g. EMI- type, mantle components (western side of the province). The most time-integrated enriched Late Cretaceous rock-types, instead, belong to the northern (Alto Paranafba) and southern (Lages) areas of the PAEP.

In summary, Sr-Nd isotope data support the view that the Angolan carbonatites are related to a time- integrated depleted mantle, while those from southern Brazil and eastern Paraguay are related to time-integrated enriched mantle sources. This suggests that the alkaline-carbonatitic complexes



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Geochemical characterist ics o f Cretaceous carbonati tes f rom Angola

of the PAEP were derived from a heterogeneous subcontinental mantle variously 'contaminated' by 'metasomatic processes' (see later).

Model ages Nd-model ages can provide an estimate of the time when a 'metasomatic' event may have occurred, assuming that Sm/Nd was not substantially modified by melting and magma differentiation. The Angolan carbonatites yielded Nd-model ages (T DM) of 0.53 + 0.7 Ga (0.41-0.61)for Coola, Longonjo, Sulima and Bailundo, 0.88 + 0.15 Ga for Tchivira-Bonga, and 1.81 +0.10 Ga for Lupongola (Fig. 9).

The youngest model ages are similar to those (mean 0.58 +0.08 Ga) of the Late Cretaceous carbonatites of Mato Preto (Ponta Grossa Arch). The T DM of the Tchivira Bonga carbonatites are instead similar to those of the Early Cretaceous carbonatites of Barra do Itapirapua and Jacupiranga (Ponta Grossa Arch), and northwest Namibia, i.e. 0 .7+0.2 and 0.7 _+ 0.2, respectively. The latter model ages virtu- ally coincide with those (mean 0.68+0.02 Ga) of the Marinkas Quellen Complex (southern Namibia) of Early Cambrian (550 Ma) age (Smithies and Marsh, 1998; inset A of Fig. 9). Finally, the T DM of Lupongola approach those (mean 1.8+0.1 Ga) of the Early Cretaceous carbonatites from eastern Paraguay (inset B of Fig. 9). It should be noted that high and low Ti tholeiites and K-alkaline rocks from the Paran~ Basin yielded mean T DM of 1.1 4-0.1 and 1.5 4-0.2 Ga, respectively. Then the range of model ages in the PAEP (inset B of Fig. 9) implies that its magmatism derived from the subcontinental mantle enriched by Meso-Neoproterozoic 'metasomatic processes' (asthenospheric component?; Comin- Chiaramonti et al., 1997b).

On the whole, the model ages indicate that notional distinct mantle metasomatic events may have occurred during the Meso- and Neoproterozoic as precursors to the alkaline and tholeiitic magmatism in the PAEP. These metasomatic processes were chemically distinct, as indicated by the strong differences in LILE and HFSE concentrations between the alkaline rocks (potassic versus sodic types) and tholeiites (low versus high Ti types) (Castorina et al., 1994, 1996, 1997; Comin-Chiaramonti et al., 1997b).

DISCUSSION

Le Roex and Lanyon (1998) and Thompson et al. (1998) suggest that the Early Cretaceous alkaline- carbonatitic and tholeiitic magmatism from northwest Namibia and the Late Cretaceous alkaline magmatism from Alto Paranafba-Serra do Mar (southern

Brazil) would reflect the variable contribution of the asthenospheric mantle components related to the Tristan da Cunha and Trindade Plumes, respectively. On the contrary, Comin-Chiaramonti et al. (1997a, b) and Castorina et al. (1997) suggested that the alkaline and alkaline-carbonatitic magmatism (and flood tholeiites) in the Paran~ Basin originated from lithospheric mantle sources without appreciable plume-derived materials. In this context, the Angolan carbonatites represent the natural link between the Namibian and Brazilian regions of the PAEP.

In the conventional ~'Sr-~tNd isotope diagram (Fig. 10A), the Early Cretaceous alkaline-carbonatitic magmatism from the PAEP appears to be related to heterogeneous source mantle spanning from time- integrated depleted to enriched types. This magmatism overlaps the coeval flood tholeiites (Fig. 10B), as well as the Cretaceous alkaline magmatism (Fig. 10C). However, it should be noted that, in general, the enriched isotopic signature of the PAEP alkaline- carbonatitic magmatism of Early Cretaceous age decreases from west (Paraguay) to east (Ponta Grossa Arch to Angola and northwest Namibia). On the other hand, it is also apparent that a similar shift towards time-integrated depleted isotopic compositions (i.e. bulk earth) is observed in the same area, decreasing the age of the magmatism, e.g. Paraguay (Early-Late Cretaceous to Tertiary) and Ponta Grossa Arch (Early-Late Cretaceous). These data strongly suggest that the PAEP magmatism is related to large- and small-scale heterogeneous (lithospheric) mantle sources. Note that the time- integrated depleted isotopic signature of the Trindade and Abrolhos Islands (Fig. 10D) is similar to that of Angola and northwest Namibia alkaline-carbonatitic magmatism of Early Cretaceous age, but different from that (EMI to B.E. signature) of the Late Creta- ceous-Tertiary analogue of Alto Paranaiba to Ponta Grossa Arch to Cabo Frio. According to Thompson et al. (1 998) this would be the inland surface expression of the 'dogleg' track left by the Trindade Plume. In terms of Sr-Nd isotopes, the contribution, if any, of the asthenospheric components related to the Trindade Plume is difficult to account for.

The alkaline magmatism of Tristan da Cunha, Gough and Inaccessible Islands, and Walvis Ridge (Fig. 10D) have B.E.-EMI Sr-Nd isotopic characteristics which are distinct or partly overlap those of the Early Cretaceous alkaline-carbonatitic (and tholeiitic) magmatism. Therefore, as in this case, the contribution of the asthenospheric components derived from the Tristan da Cunha Plume are not appreciable. Hawkesworth et al. (1986) interpreted the EMI signature of the Etendeka HTZ basalts as resulting from melting of ancient lithospheric mantle, which,


A. ALBERTI et al.

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Figure 10. Synoptical dSFdNd time-integrated diagrams. (A) Early Cretaceous K-alkaline-carbonatitic magmatism (ECACC) from the PAEP. (B) Early Cretaceous "uncontaminated tholeiites " (ECT, L e. measured aTSr/eSSr < O. 706; T: Tafelberg and Tafelkop; P: Paraguay; B: Brazil; E: Etendeka). (C) Late Cretaceous K-alkaline-carbonatitic rnagmatism (Alto Paranaiba, Taiuv~-Cabo Frio lineament and Lages). (D) Post Mesozoic magmatism (h Inaccessible; T: Tristan da Cunha; G: Gough; WR1 and WR2: Walvis Ridge; MAR: Mid- Atlantic Ridge); distribution of magmatism of the Paran~-Angola-Etendeka Province is also shown. Data sources: Bizzi et al. (1994), Comin-Chiaramonti et al. (1991, 1992, 1995, 1997a, b), Castorina et al. (1994, 1996, 1997), Fontignie and Schilling (1996), Gibson et al. (1995), le Roex and Lanyon (1998), Milner and le Roex (1996), Smithies and Marsh (1998), Thompson et al. (1998) and Traversa et al. (1996). DMM, EMI and EMIl components, as from Zindler and Hart (1986).

in the case of the Walvis Ridge (WR2 basalts), was located as remnant within the shallow oceanic asthenosphere following the opening of the South Atlantic. Alternatively, this signature may result from contamination of the oceanic mantle by ancient continental lithospheric mantle that was eroded from the base of the lithosphere at the time of continental break-up (cf. Milner and le Roex, 1996).

In summary, the 'enriched' isotopic signature of the Early and Late Cretaceous magmatism of the PAEP apparently reflects ancient heterogeneities pre- served in subcontinental lithospheric mantle sources. Therefore, the hypothesis of an asthenospheric plume (or plumes) origin for the PAEP magmatism is not in general compelling other than as a thermal perturbation(s) and a possible source(s) for the

756 Journal o f Afr ican Earth Sciences

Geochemical characteristics o f Cretaceous carbonatites from Angola

Mesozoic plume melts which contaminated the continental lithosphere.

CONCLUSIVE REMARKS

The Early Cretaceous alkaline-carbonatitic magmatism from Angola is mainly distributed along the Mo.camedes Arch, corresponding to the Ponta Grossa Arch in southern Brazil, where an analogue coeval magmatism is more extensive.

The Early Cretaceous alkaline-carbonatitic magmatism of Damaraland (northwest Namibia) trends northwest-southeast and, as in the Ponta Grossa Arch, separates the low Ti (southern) from the high Ti (northern) flood tholeiites. Therefore, such magmatism, as well as that of Angola, belongs to the Parand- Angola-Etendeka Province.

The Angola carbonatites are compositionally similar to those from eastern Paraguay and Brazil which, however, are distinct for their positive and negative spikes of Sr, and Th and Nb, respectively, in primitive mantle normalised multi-elemental plots.

The La versus La/Yb relationships suggest that the Angolan carbonatites can be derived by differentiated COj i ch melts (e.g. trachyphonolites) through fractional crystallisation. The exsolution of the carbonate fraction would have occurred at a temperature of ca 920°C, as suggested by the O and C isotopes.

The Angolan (Lupongola excepted) and northwest Namibia carbonatites have Sr-Nd isotopic compositions which range from bulk earth (B.E.) to time- integrated depleted sources. On the contrary, the carbonatites from eastern Paraguay and Brazil appear to be related to mantle-derived melts with time- integrated enriched or B.E. isotopic characteristics.

The carbonatites of Lupongola (Angola) are characterised by low Nd~ (max. 0.5114) and Sr~ (max. 0.7043) isotope ratios which might be related to interaction with appropriate crustal materials (e.g. amphibolites, mafic granulites). Alternatively, the age of the Lupongola carbonatites could be similar to that (ca 1.1 Ca) of the anorthositic complex where they are intruded.

Geochemistry and Sr-Nd isotopes indicate that the carbonatites of the Paran¢5-Angola-Etendeka Province originated from compositionally distinct mantle sources. Such heterogeneity is attributed to 'metasomatic' processes which, on the basis of Nd- model ages, would have occurred at ca 0.6-0.7 Ga (Angola, northwest Namibia and Brazil) and ca 1.6 Ga (eastern Paraguay).

The areal distribution indicates that the time-integrated isotopic enrichment of the carbonatites and associated alkaline rocks decreases from west

(eastern Paraguay) to east (Angola and northwest Namibia), just as there is a concomitant decreasing age of magmatism (Early-Late Cretaceous). This indicates that the alkaline-carbonatitic magmatism from the Paran~-Angola-Etendeka Province originated from a large- and small-scale, heterogeneous subcontinental lithosheric mantle.

All the data consistently indicate that the contribution of asthenospheric components derived from hypothetical mantle plumes (e.g. Tristan da Cunha, Trindade) is not appreciable in the genesis of the alkaline-carbonatitic magmatism of the Paran&- Angola-Etendeka Province, as was also found for the Paran& flood tholeiites (Comin-Chiaramonti et al., 1997a). The possible role of a mantle plume consistent with the data may have been the trig- gering of melting of the underplated lithospheric mantle.

ACKNOWLEDGEMENTS

Permission for using part of the carbonatite samples from the Director of the Department of Geology of the 'A. Neto' University is gratefully acknowledged. This research was supported by grants from Italian (CNR; P.C.-C.) and Brazilian (FAPESP; C.B.G.) agen- cies. This work has benefited greatly through discussion with E.M. Piccirillo and reviews by A. Cundari, J.A. Gittings and P. Bowden. Editorial handling- R.J. Thomas & P. Bowden

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geochemical characteristics of cretaceous carbonatites from angola

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