Isotope chemostratigraphy of marbles in northeastern Mozambique: Apparent depositional ages and tectonostratigraphic implications

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Precambrian Research 162 (2008) 540–558

Isotope chemostratigraphy of marbles in northeastern Mozambique: Apparent depositional ages and tectonostratigraphic implications V.A. Melezhik a,∗ , B. Bingen a , A.E. Fallick b , I.M. Gorokhov c , A.B. Kuznetsov c , J.S. Sandstad a , A. Solli a , T. Bjerkg˚ard a , I. Henderson a , R. Boyd a , D. Jamal d , A. Moniz e b

a Geological Survey of Norway, 7491 Trondheim, Norway Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, Scotland, United Kingdom c Institute of Precambrian Geology and Geochronology, nab. Makarova, 2, 199034 St. Petersburg, Russia d Eduardo Mondlane University, P.O. Box 257, Maputo, Mozambique e Provincial Directorate for Geology, Lichinga, Niassa Province, Mozambique

Received 20 February 2007; received in revised form 22 October 2007; accepted 10 November 2007

Abstract Marbles are minor but characteristic components of metasedimentary units within nappes in the Pan-African Mozambique Belt in NE Mozambique. Metasedimentary units remain largely undated, and carbon and strontium isotope stratigraphy of marbles has been used for indirect dating of the depositional history in this part of the Mozambique Belt. Sixty-nine samples from nine occurrences of dolomite, calcite and magnesite marbles in the Montepuez, Xixano, Lalamo, Ocua and Nampula metamorphic complexes were analysed for major and trace elements, and a subset of 39 samples for C, O and Sr isotopes. The least altered ␦13 C values range from −3.5 to +7.1‰ (V-PDB) and 87 Sr/86 Sr ratios from 0.70504 to 0.70671. These values are considered as the best proxy to seawater composition at the time of deposition. The apparent deposition ages, derived from available seawater evolution curves, range from c. 1250 to c. 660 Ma. An age of 1250–910 Ma is obtained from a tripartite marble unit in the Montepuez Complex which is exposed in the Montepuez quarries. Five other age-groups are represented by marble units with apparent depositional ages of 800–750 Ma (Xixano North), 800–660 Ma (Montepuez West), c. 750 Ma (Nampula), c. 740 Ma (Xixano South and Lalamo), and 740–670 Ma (Montepuez East). The data suggest that: (i) Pan-African nappes in NE Mozambique include Neoproterozoic and probable Mesoproterozoic sediments; (ii) Neoproterozoic rocks of the Xixano and Nampula complexes might have different ancestry and were tectonically juxtaposed during the Neoproterozoic Pan-African orogeny. © 2007 Elsevier B.V. All rights reserved. Keywords: Mozambique; Marble; Calcite; Dolomite; Magnesite; Carbon; Oxygen; Strontium; Isotopes

1. Introduction The Mozambique Belt in East Africa was first recognised by Holmes (1951) who demonstrated a structural discontinuity between the Tanzanian Craton and metamorphic complexes of an apparent Mezoproterozoic age to the east. Later studies by Pinna et al. (1993), Muhongo et al. (2001), Kr¨oner et al. (2003) and Johnson et al. (2005) have shown a complex history of the belt, which experienced several tectonometamorphic events that occurred between 1100 and 500 Ma. In northern Mozambique, reconstruction of the geological evolution of the Mozambique

∗

Corresponding author. Tel.: +47 73 90 42 41; fax: +47 73 92 16 20. E-mail address: [email protected] (V.A. Melezhik).

0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.11.002

Belt is based on very few radiometric dates that constrain the ages and provenances of the different tectonostratigraphic complexes that constitute the belt. These dates (reviewed by Johnson et al., 2005), obtained from igneous and metamorphic rocks by the U–Pb, Sm–Nd and Rb–Sr methods, are rather few, generally lack high precision, and thus allow various interpretations of the geological history (Holmes, 1951; Pinna et al., 1993; Shackleton, 1996; Muhongo et al., 2001; Meert, 2003). Moreover, there is general lack of dates constraining depositional ages of various supracrustal rocks that constitute the belt. To date, there has been only one attempt to obtain depositional age from metasedimentary rocks. The low-grade metalimestones of the Geci group which was tectonically juxtaposed with granulitefacies complexes in NW Mozambique, have been constrained to 630–585 Ma by means of isotope chemostratigraphy (Melezhik et al., 2006).

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Marbles associated with metamorphosed complexes are minor but consistent components of allochthonous units in the Mozambique Belt in NE Mozambique, Tanzania and Madagascar (Pinna et al., 1993; Muhongo, 1994; De Wit et al., 2001; Fernandez and Schreurs, 2003). The purpose of this work is to provide further progress in understanding the geological history of the region by constraining the depositional age of high-grade marble formations in NE Mozambique with the tool of Sr and C isotope chemostratigraphy. Extensive isotopic work in the Caledonian orogenic belt (Melezhik et al., 2001a, b, 2002a, b, 2003, 2005; Thomas et al., 2004; Slagstad et al., 2006) has demonstrated that carbonate chemostratigraphy can be in principal applied to high-grade marbles, thus providing the age constraints necessary for improving reconstructions of the depositional history and tectonostratigraphic assembly of areas where it is difficult to use other methods. 2. Geological background Originally the Mozambique Belt, a high-grade north–south trending orogenic belt in East Africa, was defined to have formed as the result of a c. 1300 Ma single orogenic event (Holmes, 1951). As more radiometric dates were generated, it became apparent that the belt is the product of at least two orogenic cycles. Pinna et al. (1993) recognised high-grade gneisses, granulites and migmatites comprising a Precambrian basement which was intruded by orogenic plutonic rocks. These rocks were emplaced and deformed during the 1100–850 Ma Mozambican orogeny, approximately equivalent to the Mezoproterozoic Kibaran orogeny of central Africa. A younger cycle involved the deposition of limestones, phyllites, sandstones and conglomerates of marine origin, and some other rocks with possible glacial affinities. The Geci group metalimestones, belonging to this cycle, have an apparent depositional age of 630–585 Ma (Melezhik et al., 2006). Muhongo et al. (2001) and Kr¨oner et al. (2003) constrained the peak of high-grade metamorphism in northern Mozambique to c. 615 Ma. Johnson et al. (2005) reviewed the newly obtained radiometric dates and suggested that previous tectonostratigraphic subdivisions in northern Mozambique should be considered with caution. New geological mapping at the 1:250,000 scale was accomplished in northern Mozambique between 2003 and 2006 by the Geological Survey of Norway, the British Geological Survey and the National Directorate of Geology of Mozambique. It resulted in acquisition of new petrographic, mineralogical, geochemical, structural and geochronological data (Bingen et al., 2006a, b, 2007). A simplified version of the new geological map over northeasternmost Mozambique is presented in Fig. 1. This map integrates new field, airborne geophysics, structural data and a substantial database of U–Pb zircon and monazite ages constraining magmatic and metamorphic events (Bingen et al., 2006a, b, 2007). The map features a number of lithotectonic units, designated as complexes. New geochronological data (Bingen et al., 2006a, b, 2007) combined with results reported by Kr¨oner et al. (2001, 2003) and Jamal (2005) suggest that the lithological complexes were metamorphosed, deformed and

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imbricated during the Neoproterozoic Pan-African orogeny, and were intruded by several generations of granites; the present-day tectonostratigraphy in NE Mozambique is the result of several phases of Pan-African orogenic processes. Available data provide evidence for a Mesoproterozoic basement, and an overlying Meso- to Neoproterozoic nappe system. The basement is an integral component of the Nampula, Marrupa and Nairoto complexes, and probably the Meluco Complex. The basement is mainly made up of felsic orthogneiss with magmatic crystallisation ages ranging from c. 1150 to 940 Ma (Bingen et al., 2006a, b, 2007. It is locally unconformably overlain by a Neoproterozoic metasedimentary cover (e.g., Mecuburi group, located SW of the area of Fig. 1). The Pan-African nappes include the Xixano and Lalamo complexes. They are mainly Neoproterozoic mafic to intermediate granulites, various metasediments, metavolcanic rocks and felsic orthogneiss. Enderbites, metagranites and metarhyolites yield magmatic crystallisation ages between c. 820 and 740 Ma. The nappes carry evidence for a pre- or early-Pan-African granulite-facies metamorphism dated to 735 ± 5 Ma in the Xixano Complex. The nappe system probably represents exotic or outboard continental fragments accreted to the basement during the Pan-African orogeny. The main Pan-African orogeny took place between 580 and 530 Ma, and included at least two main phases, partially overlapping in time (Bingen et al., 2006a, b, 2007). These are (i) NW verging imbrication of the basement and transport of the nappes over the basement, and (ii) folding of the basement and nappes, especially along a NNE–SSW trend. The Ocua and Montepuez complexes are partially interpreted as a melange containing tightly folded lithologies belonging to both the basement and the nappes. Available data provide only limited constraints on the deposition of sedimentary sequences (e.g., Melezhik et al., 2006). The present study addresses this problem. 3. Marble formations studied Amphibolite-faces marbles occur sporadically in most supracrustal complexes (Fig. 1) where they range in thickness from less than a meter to hundreds of meters with lateral extension over 100 km. Those involved in our study are from the Nampula, Montepuez, Xixano, Lalamo, and Ocua complexes (Fig. 1). Marbles from the Montepuez Complex are the main target of the research. They were studied in commercially exploited quarries near Montepuez (Fig. 1). Excellent man-made exposures provide an opportunity for detailed study and systematic sampling. The level of exposures of marbles from other complexes allowed only limited sampling. The present-day lithology of the Montepuez Complex includes a variety of amphibolite-facies schists, amphibolites, quartzites, felsic volcanites and several marble units. Marbles exposed in four small quarries east of Montepuez comprise three distinctive lenses dipping at 46–60◦ southeast. Their visible thicknesses range between 20 and 50 m. The lenses are isoclinally folded, tectonically dissected, separated by thin slices of mica schists and apparently represent a tectonically imbricated unit. Three disparate marble lenses identified in the quarries are designated as a tripartite marble unit. A structurally lower

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Fig. 1. Schematic geological map of northeastern Mozambique with emphasis on marble formations.

member of the unit is termed as Calcite-dolomite marble, a structurally upper as White dolomite marble, and a middle as Grey dolomite marble. The Calcite-dolomite marble is mainly composed of pale grey, impure, thinly layered and medium to coarsely crystalline calcite marbles (Fig. 2a). One 1.5 m-

thick bed of bluish, giantly crystalline, massive marble (Fig. 2b) occurs in the middle part of the lens. The calcite marbles are structurally overlain by pale grey, medium crystalline, layered, dolomite marble. The Grey dolomite marble is dominated by pale grey, medium-crystalline, dolomite marbles with thin lay-

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Fig. 2. Lithological varieties of some of the studied marbles. Montepuez Complex marbles: (a) Photograph of an inner corner of sawn surfaces in the quarry № 4 (Calcite-dolomite marble member) showing the typical style of tectonic deformation in the grey, banded, calcite marble; (b) photograph of a banding-parallel surface in the quarry № 4 (Calcite-dolomite marble member) showing the bluish marble composed of giant calcite crystals; coin = 9 mm; (c) photograph of a sawn surface in the quarry № 1 (White dolomite marble member) showing banded, pale grey, dolomite marble with numerous inclusions of amphibolite structurally overlain by massive to weakly banded white dolomite marble with banding-parallel inclusions of amphibolite; hammer head = 12 cm. Marbles of other complexes: (d) Pale pink, massive, medium-crystalline, dolomitised, magnesite marble from the Lalamo Complex; (e) back-scattered electron image of the micro-brecciated and retrogressively dolomitised (Dol), calcitised (Cc) and serpentinised (Spt) Lalamo magnesite (Mg); (f) weathered surface of depositional carbonate breccia from the Lalamo Complex; (g) photomicrograph of the Lalamo carbonate breccia showing dark and pale brown fragments cemented by white calcite.

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ers of a dolomitised calcite marble. The White dolomite marble is distinguished by its white colour and high purity. It contains both dolomite and calcite marbles though the latter are subordinate. The marbles include roughly equal volumes of massive and layered varieties and contain sporadic, layer-parallel, strongly foliated amphibolite inclusions, apparently dissected and dismembered thin mafic dykes or sills (Fig. 2c). In the tripartite marble unit at Montepuez, both calcite and dolomite marbles are composed of crystals arranged in hypidiomorphic, allotriomorphic and granoblastic textures. In calcite marbles, accessory minerals are tremolite, quartz and muscovite, whereas dolomite marbles additionally contain sizable amounts of graphite and are commonly devoid of quartz. Calcite and dolomite marble units occurring in the Montepuez Complex outside the quarries (termed in this article as East and West marbles, Fig. 1), as well as most marble units hosted by other complexes, are very similar to the marbles of the tripartite unit. They are tectonically dissected bodies associated with paragneisses. They are commonly rather pure and composed of medium-crystalline calcite with variable amount of dolomite. Impure, dolomitised marbles contain tremolite as the result of quartz + dolomite metamorphic reaction. The marble lens sampled in the Ocua Complex is composed of pure dolomite. One lens from the Lalamo Complex (JS307, Fig. 1) consists of magnesite and calcite marbles. The lens has a thickness exceeding 200 m and it is traceable along strike over a distance of 20 km. It is hosted by amphibolite-facies layered arenites with layers of biotite gneiss and quartzite. The magnesite marble bed, of unknown thickness, is composed of a pale pink, mediumcrystalline, massive lithology (Fig. 2d), which microscopically appears to be brecciated (Fig. 2e). Extensional micro-fractures are cemented by retrogressive dolomite and calcite, which are replaced by later chlorite and serpentinite (Fig. 2e). The same marble lens, though exposed 5 km north of the magnesite bed, comprises brown, brecciated, and white, massive varieties of a calcite marble. The former has a visible thickness of over 2 m and a vuggy appearance on weathered surfaces (Fig. 2f). The breccia consists of 0.5–5 cm large, red to pale brown, calcite marble fragments cemented by coarsely crystalline white calcite (Fig. 2g). There is no field evidence suggesting a postdepositional tectonic brecciation, therefore the breccia is categorised as a collapse breccia originated either from dissolution of evaporate minerals or karstification. 4. Geochemistry 4.1. Analytical techniques Major and trace elements were analysed by X-ray fluorescence spectrometry at the Geological Survey of Norway (NGU), Trondheim, using a Philips PW 1480 X-ray spectrometer. The precision (1σ) is typically around 2% of the major oxide present. Acid-soluble Fe, Ca, Mg and Mn were analysed by ICP-AES at NGU using a Thermo Jarrell Ash ICP 61 instrument. Detection limits for Fe, Mg, Ca, Mn and Sr are 5, 100, 200, 0.2 and 2 ppm, respectively. The total analytical uncertainty includ-

ing element extraction (1σ) is ±10% rel. Standard analytical procedures have been used for measurement of total organic carbon (TOC) at NGU. The TOC content has been measured from acid-washed material via sealed tube combustion using a Leco SC-444 instrument with a total analytical uncertainty of ±15% rel. Oxygen and carbon isotope analyses of whole-rock marble samples were carried out at the Scottish Universities Environmental Research Centre, Glasgow, using the phosphoric acid method of McCrea (1950) as modified by Rosenbaum and Sheppard (1986) for operation at 100 ◦ C. Carbon and oxygen isotope ratios in carbonate constituents of the whole-rock samples were measured on a VG SIRA 10 mass spectrometer. Analyses were calibrated against NBS 19, and precision (1σ) for both isotope ratios is better than ±0.2‰. Oxygen isotope data for dolomites were corrected using the fractionation factor 1.00913, and 1.00933 for magnesite recommended by Rosenbaum and Sheppard (1986). The ␦13 C data are reported in per mil (‰) relative to V-PDB and the ␦18 O data in ‰ relative to V-SMOW. Rb–Sr analyses were carried out at the Institute of Precambrian Geology and Geochronology of the Russian Academy of Sciences (St. Petersburg) as specified in Gorokhov et al. (1995). The Rb and Sr concentrations were determined by isotope dilution. Rb isotopic composition was measured on a MI 1320 mass spectrometer. Strontium isotope ratios were measured on the Thermo-Finnigan Triton thermal ionisation mass spectrometer in static collection mode. All 87 Sr/86 Sr ratios were normalised to a 86 Sr/88 Sr of 0.1194, and measurements of the NIST SRM-987 run with every batch averaged 0.710255 ± 8 (2␴mean , n = 11). During the course of the study, the value obtained for the 87 Sr/86 Sr ratio of the USGS. EN-1 standard was 0.709191 ± 25 (2σ mean , n = 3). 4.2. Major elements The major and trace element geochemistry of the tripartite marble unit sampled in four quarries at Montepuez is based on analyses of 56 samples (Table 1). Calcite (CM), dolomite (DM) and dolomitised calcite (DCM) marbles have been identified in all three members of the unit. Both the calcite and the dolomite marbles are relatively pure rocks. They are almost devoid of Al2 O, Na2 O and K2 O (Table 1). The DM and DCM are relatively enriched in SiO2 (up to 18.7 wt%, Fig. 3a). Several samples of the DM contain sizable amounts of total organic carbon (TOC, up to 0.6 wt%). We tentatively suggest a Mg/Ca ratio of 0.1 as the boundary between the CM and the DCM. A Mg/Ca ratio of 0.5 sets a lower limit for the DM. The CM is characterised by variable Mg/Ca ratios ranging between 0.02 and 0.09 (Fig. 3a), whereas dolomitised varieties show ratios between 0.37 and 0.50 (Table 1). Mg/Ca ratios of the DM range between 0.51 and 0.63 (average, 0.58 ± 0.03, n = 43), which is lower than that for stoichiometric dolomite (0.62), thus indicating incomplete dolomitisation. The DCM does not plot along the limestone–dolostone mixing line (Fig. 3b) because both Mg and Ca concentrations are low due to the high SiO2 content (Fig. 3a). The East and West marbles in Montepuez Complex (Fig. 1) are both pure calcite marbles with Mg/Ca ratios of 0.01 (Table 1).

Table 1 Chemical composition of Montepuez Complex marbles Field #

Lithology

Al2 O3 (wt%)

Na2 O (wt%)

K2 O (wt%)

TOC (wt%)

– 0.40 – – – 0.04 –

– – – – – 0.04 –

– – – – – – –

– 0.04 – – – 0.03 –

– – – – – – –

Calcite marbles, intensively dolomitised BB039 Pale brown, medium-crystalline

6.0

2.98

0.11

0.70

Grey dolomite marbles Mvm046 Medium-crystalline, banded Mvm047 Medium-crystalline, banded BB038 Medium-crystalline, banded

1.3 0.48 –

0.05 0.02 –

– – –

Montepuez quarry 3, UTM 0497650, 8552251 Grey dolomite marble member Calcite marbles, intensively dolomitised Mvm052 Grey, banded, medium-crystalline

2.1

–

Montepuez quarry 3, UTM 0497650, 8552251 Grey dolomite marble member Dolomite marbles Mvm051 Grey, medium-crystalline, banded Mvm048 Grey, medium-crystalline, banded Mvm049 Grey, medium-crystalline, banded Mvm050 Grey, medium-crystalline, banded Mvm053 White, medium-crystalline, banded Mvm054 Grey, medium-crystalline, banded Mvm055 Grey, medium-crystalline, banded Mvm056 Grey, medium-crystalline, banded BB034 White, coarsely crystalline, massive BB035 Pale grey, medium-crystalline, banded

6.9 0.1 3.0 7.5 2.8 7.1 3.0 0.82 0.33 7.7

Montepuez quarry 2, UTM 0497683, 8552084 White dolomite marble member White calcite marbles, intensively dolomitised BB031 Medium-crystalline, massive

Mg (wt%)

Ca (wt%)

Fe (ppm)

Mn (ppm)

Sr (ppm)

Mg/Ca

3.1 1.9 0.69 2.4 1.4 2.3 2.5

33.1 34.7 37.0 34.3 35.7 34.3 33.4

456 312 72 358 343 344 405

28 18 4 14 20 12 18

160 167 116 135 140 145 132

0.09 0.05 0.02 0.07 0.04 0.07 0.07

–

8.7

19.9

4080

150

83

0.44

0.05 0.03 0.03

– – –

12.2 12.4 12.2

20.6 21.0 20.8

1300 1210 1530

48 52 57

58 93 61

0.59 0.59 0.59

–

–

–

10.4

22.7

1690

91

83

0.46

– – – – – – – – – –

– – – – – – – – – –

– – – – – – – – – –

0.37 0.13 0.60 0.33 – 0.30 – 0.32 0.17 0.26

10.8 12.7 12.0 10.9 11.7 10.7 12.0 12.7 12.4 10.2

20.2 20.6 20.4 19.9 21.3 19.3 20.5 21.4 20.8 19.2

632 1540 1090 798 1280 666 2220 1440 782 386

23 45 35 27 43 24 81 43 33 16

99 71 65 104 102 76 58 86 86 99

0.53 0.62 0.59 0.55 0.55 0.55 0.59 0.59 0.60 0.53

–

–

–

–

–

11.3

22.6

298

12

119

0.50

White dolomite marbles BB029 Medium-crystalline, massive BB030 Medium-crystalline, massive BB032 Medium-crystalline, massive

1.8 3.6 0.36

– – –

– – –

– – –

– – –

12.4 12.3 12.6

20.9 19.9 20.4

1530 395 513

73 20 19

83 106 71

0.59 0.62 0.62

Montepuez quarry 1, UTM 0497684, 8552072 White dolomite marble member White calcite marbles, intensively dolomitised Mvm065 Finely crystalline, massive Mvm066 Finely crystalline, massive Mvm074 Finely crystalline, massive Mvm075 Finely crystalline, massive

17.6 8.4 18.7 0.04

– – – –

– – – –

– – – –

– – – –

6.7 9.4 6.4 11.2

18.0 20.6 17.4 22.9

328 493 736 758

13 24 52 61

82 99 70 112

0.37 0.46 0.37 0.49

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SiO2 (wt%)

Montepuez quarry 4, UTM 0497505, 8552297 Calcite-dolomite marble member White to pale grey calcite marbles, partially dolomitised Mvm040 Coarsely crystalline, weakly banded Mvm041 Coarsely crystalline, weakly banded Mvm042 Giant crystalline, massive Mvm043 Coarsely crystalline, banded Mvm044 Giant and medium-crystalline interbedded Mvm045 Coarsely to medium-crystalline, banded BB037 Very coarsely-crystalline, massive

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Table 1 (Continued ) Field #

Lithology

SiO2 (wt%)

Al2 O3 (wt%)

Na2 O (wt%)

K2 O (wt%)

TOC (wt%)

Mg (wt%)

Ca (wt%)

1.5 0.72 11.9 2.2 3.8 – 6.8 4.7 0.73 0.08 0.83 0.22 – 0.57 0.56 –

– – – – – – – – – – – – – – – –

– – – – – – – – – – – – – – – –

– – – 0.02 – – – – – 0.01 – – – – – –

– – – – – – – – – – – – – – – –

12.6 12.4 9.2 12.5 11.0 12.1 10.3 11.0 12.1 12.5 12.3 11.6 12.4 12.1 12.4 13.3

20.9 20.7 18.1 20.8 19.9 21.8 19.4 20.5 21.6 20.7 21.1 21.7 21.3 22.1 20.5 21.1

1550 1130 1540 715 2740 984 2050 806 1700 1530 1090 1150 447 211 1570 2120

73 47 79 36 98 39 79 40 49 58 38 42 26 10 53 97

79 76 72 78 86 108 88 85 102 80 89 108 77 98 94 65

0.60 0.60 0.51 0.60 0.55 0.56 0.53 0.54 0.56 0.60 0.58 0.53 0.58 0.55 0.60 0.63

Montepuez quarry 1, UTM 0497684, 8552072 White dolomite marble member White dolomite marbles Mvm077 Medium-crystalline, massive Mvm078 Medium-crystalline, massive Mvm079 Medium-crystalline, massive Mvm080 Medium-crystalline, massive Mvm082 Medium-crystalline, massive Mvm081 Medium-crystalline, massive Mvm083 Medium-crystalline, massive Mvm084 Medium-crystalline, massive BB040 Medium-crystalline, massive BB041 Medium-crystalline, massive BB042 Medium-crystalline, massive

– – 0.23 – 1.8 0.52 2.1 4.4 4.7 – –

– – – – – – – – – – –

– – – – – – – – – – –

– – – – – – – – – – –

0.10 – – – – – – – – – –

13.0 12.9 12.8 13.1 12.5 12.2 11.3 11.6 11.1 12.5 12.5

21.2 20.9 20.7 21.0 20.6 21.6 21.2 20.0 20.1 20.5 20.9

2360 2290 1100 1180 863 507 2160 522 976 1210 829

69 72 48 54 36 26 70 20 55 84 36

89 86 91 84 83 90 92 61 121 96 81

0.61 0.62 0.62 0.62 0.61 0.56 0.53 0.58 0.55 0.61 0.60

Marbles sampled outside the quarries Calcite marbles BB022 White, coarsely-crystalline TBM-143A Pale grey, medium-crystalline

– 1.8

– 0.14

– –

0.01 0.040

0.17 n.d.

37.2 37.2

155 715

17 41

4120 1070

0.01 0.01

0.34 0.38

(–) Below detection limits: 0.01 wt% for K2 O; 0.1 wt.% for SiO2 , Al2 O3 , Na2 O and Corg . Mg, Ca, Fe, Mn, and Sr are acid soluble constituents determined by ICP–AES.

Fe (ppm)

Mn (ppm)

Sr (ppm)

Mg/Ca

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White dolomite marbles Mvm057 Medium-crystalline, massive; amphibolite inclusions Mvm058 Medium-crystalline, massive; amphibolite inclusions Mvm059 Medium-crystalline, massive; amphibolite inclusions Mvm060 Medium-crystalline, massive; amphibolite inclusions Mvm062 Medium-crystalline, massive; amphibolite inclusions Mvm061 Medium-crystalline, massive; amphibolite inclusions Mvm063 Finely crystalline, massive Mvm064 Finely crystalline, massive Mvm067 Finely crystalline, massive Mvm068 Finely crystalline, massive Mvm069 Finely crystalline, massive Mvm070 Finely crystalline, massive Mvm071 Finely crystalline, massive Mvm072 Finely crystalline, massive Mvm073 Medium-crystalline, massive Mvm076 Weakly banded with amphibolite inclusions

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Fig. 3. Cross-plots illustrating major geochemical and isotopic features of the tripartite marble unit of the Montepuez Complex.

Marbles of the Xixano and Nampula complexes commonly contain a few percent of SiO2 and show variable degrees of dolomitisation (Table 2). Some varieties contain sizable amounts of Al2 O3 and TOC. The marble of the Ocua Complex is devoid of SiO2 and Al2 O3 and is dolomitic in composition. The Lalamo Complex contains numerous lenses of a relatively pure calcite marble containing 0.6 to 6% SiO2 . The magnesite marble (Mg/Ca = 1.1) has a low SiO2 content (1.8%) and is devoid of Al2 O3 .

transitional position between the two end-members (Table 1). The Mn and Fe concentrations in the Montepuez West and East calcite marbles are 14–41 ppm and 155–755 ppm, respectively. The dolomite and calcite marbles of the Nampula and Ocua complexes are enriched in Mn (Table 2) relative to those in the Montepuez Complex. The Xixano and Lalamo calcite marbles are rather similar to the Montepuez Complex marbles. The Lalamo magnesite marble has low Mn (23 ppm) and Fe (250 ppm) concentrations.

4.3. Iron and manganese

4.4. Strontium

In the tripartite marble unit at Montepuez, average Mn (47 ± 23 ppm, n = 43) and Fe (1208 ± 623 ppm) contents of the DM are much lower than those commonly reported from Proterozoic dolostones (Mn = 180 ppm, Fe = 2020 ppm, e.g., Melezhik et al., 2005). The CM contains less Mn (16 ± 7.3 ppm, n = 7) and Fe (327 ± 122 ppm), whereas the DCM occupies a

In the tripartite marble unit at Montepuez, the Sr concentration in the CM is low (142 ± 17 ppm, n = 7) and shows no significant correlation with the Mg/Ca ratio (Fig. 3c). Concentrations of Sr and Mn are both low, and so is the Mn/Sr ratio (0.11 ± 0.04, n = 7). The DCM and DM contain less Sr (88 ± 15 ppm, n = 6; 86 ± 15 ppm, n = 43, respectively), with

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Table 2 Chemical composition of marbles from from Xixano, Lalamo, Ocua and Nampula complexes in NE Mozambique Sample #

Lithology

Lalamo Complex Calcite marble JS200 Marble Calcite, partially dolomitised marble JS303 Impure, tremolite-bearing Magnesite marble JS307 Pink, brecciated, dolomitised Ocua Complex Dolomite marble JS330 Grey, medium-crystalline Nampula Complex Calcite, partially dolomitised marbles TBM097 Grey, medium-crystalline TBM098 Pale grey, medium-crystalline RB823 Impure, tremolite-bearing Calcite, intensively dolomitised marbles TBM-099 Impure, tremolite-bearing TBM-105 Pale grey, medium-crystalline AS04-26 Pale grey, medium-crystalline

Al2 O3 (%)

Na2 O (%)

K2 O (%)

TOC (%)

Mg (%)

Ca (%)

Fe (ppm)

Mn (ppm)

Sr (ppm)

Mg/Ca

1.2

0.29

–

0.04

–

0.40

36.2

798

81

1140

0.01

7.6 14.3

– –

– –

0.038 0.013

n.d. n.d.

2.7 4.0

29.4 22.5

236 225

117 65

58 70

0.09 0.18

2.2

–

–

0.015

n.d.

10.2

22.9

373

18

88

0.45

0.57

0.05

–

0.03

0.41

0.22

36.8

36.4

15

2160

0.01

6.1

–

–

0.037

n.d.

1.1

33.7

63.3

14

n.d.

0.03

1.8

–

–

–

n.d.

10.5

9.9

249

23

235

1.06

–

–

–

0.003

n.d.

12.1

21.3

331

150

139

0.57

– 1.5 5.3

– 0.21 1.04

– – –

– 0.17 0.39

0.36 – n.d.

3.78 1.53 n.d.

31.9 34.0 n.d.

736 1030 n.d.

271 187 n.d.

234 239 n.d.

0.12 0.05 n.d.

7.9 1.3 2.1

1.24 – 0.09

0.19 – –

0.19 0.02 0.01

– 0.34 0.33

6.07 8.65 6.62

22.4 24.3 26.3

1590 792 321

230 186 201

347 189 163

0.27 0.36 0.25

(–) Below detection limits of 0.002 for MnO, 0.003 for K2 O, 0.004 for TiO2 , 0.01 for SiO2 , Al2 O3 , Stot, P2 O5 , 0.1 for Na2 O and TOC.

V.A. Melezhik et al. / Precambrian Research 162 (2008) 540–558

Xixano Complex Calcite marble IH04.027 Pale grey, medium-crystalline Calcite, partially dolomitised marbles JS285 Impure, tremolite-bearing JS299 Impure, tremolite-bearing Calcite, intensively dolomitised marble TBM126 Pale grey, medium-crystalline

SiO2 (%)

V.A. Melezhik et al. / Precambrian Research 162 (2008) 540–558

maximum concentrations of 112 and 121 ppm, thus approaching only the lowest concentration in the CM (Fig. 3c). The average Sr concentration of the DM is higher than that reported for non-metamorphosed Proterozoic dolostones (54 ppm, n = 344; Melezhik et al., 2005). Mn/Sr ratios range between 0.1 and 1.5 averaging 0.58 ± 0.33 (n = 43). In contrast, the Montepuez West and East calcite marbles are rich in Sr (1070 and 4120 ppm, respectively). The dolomitised calcite and dolomite marbles of other complexes are characterised by moderate to low Sr concentrations. However, pure calcite marbles of the Xixano and Lalamo complexes contain 1140 and 2160 ppm Sr, respectively (Table 2). The Lalamo magnesite has a low Sr content (235 ppm). 4.5. Oxygen and carbon isotopes Carbon and oxygen isotope data from the tripartite marble unit at Montepuez comprise 24 analyses (Table 3). The δ18 O values of the CM range between 14.6 and 20.3‰ (17.2‰ on average, n = 7). The δ13 C values are all positive and cluster tightly between +1.6 and +2.1‰ (+1.9 ± 0.2‰ on average) (Table 3). There is a very weak positive correlation between δ18 O and δ13 C (r = +0.53, n = 7, >80%). The DM is enriched in 18 O with respect to the CM (Fig. 3d) with δ18 O values tightly clustering around 22.1 ± 0.4‰ (n = 12). δ13 C fluctuates between −1.7 and +1.8‰ (Fig. 3e), and low values are associated with high TOC content (Fig. 3f); all the samples with negative δ13 C values contain graphite. There is no correlation between δ18 O and δ13 C. The DCM shows both δ13 C (0.4–1.7‰) and δ18 O (19.3–22.6‰) transitional from the CM to the DM. The sample obtained from the Montepuez West calcite marble is characterised by δ13 C (+0.1‰) and δ18 O (16.7‰), which are within the range of the tripartite marble unit. In contrast, the Montepuez East calcite marble shows much higher values for both δ13 C (+4.7‰) and δ18 O (24.7‰). The Xixano calcite marbles have δ13 C values fluctuating between −1.1 and +3.8‰, whereas δ18 O ranges from 15.1 to 22‰ (Table 4). The Lalamo calcite marbles are characterised by the highest δ13 C (+3.5 to +7.1‰) and δ18 O (26.1–28.4‰) values. Similarly, the magnesite marble has a high δ18 O (24.3‰) whereas δ13 C is negative (−1.1‰). The Nampula calcite marbles are uniformly depleted in 13 C (δ13 C = −3.5 to −2.0‰) and have δ18 O in the range of 18.6–20.8‰. 4.6. Strontium isotope ratios In the tripartite marble unit at Montepuez, strontium isotope ratios have been obtained from 23 samples (Table 3). 87 Sr/86 Sr ratios measured from the CM range between 0.70508 and 0.70521 (n = 7), whereas those obtained from the DCM are more radiogenic and fluctuate between 0.70531 and 0.70560 (n = 4). The DM has the most radiogenic 87 Sr/86 Sr ratios ranging from 0.70540 to 0.70565 (n = 12) (Fig. 3h) with one outstanding outlier at 0.70488. The 87 Sr/86 Sr ratios of the CM show a significant negative correlation with the Sr concentration (Fig. 3g) (r = −0.66, n = 7, 90%) and do not exhibit a correlation with the Mg/Ca ratio.

549

The Montepuez West marble has a 87 Sr/86 Sr ratio of 0.70566, which is more radiogenic than the calcite marbles in the tripartite unit. The Montepuez East calcite marble has 87 Sr/86 Sr ratio of 0.70654, which is the most radiogenic of all in the Montepuez Complex. Among marbles of other complexes, the Xixano calcite marbles show the greatest spread in 87 Sr/86 Sr ratios (0.70574 to 0.70666), with no dependence on the Sr concentration. In contrast, 87 Sr/86 Sr ratios of the Lalamo calcite marbles correlate negatively with the Sr concentrations within the range of 0.70627–70699 (Table 4). The magnesite has the most radiogenic value of 0.70699. 5. Geochemical screening for postdepositional resetting of C-, O- and Sr-isotope systems In general, the carbon isotopes are strongly buffered by the high C concentrations in carbonate minerals relative to fluids (Banner and Hanson, 1990; Jacobsen and Kaufman, 1999) and, consequently, infiltration of externally derived fluids is likely to have a relatively greater effect on O and Sr isotopes. Traditional screening procedures for postdepositional alteration of carbonate, originally specified for non-metamorphosed or lowgrade rocks (Brand and Veizer, 1980; Veizer, 1983; Banner and Hanson, 1990; Denison et al., 1998, 1994; Kaufman and Knoll, 1995; Azmy et al., 1998; Jacobsen and Kaufman, 1999), are of very limited use in the case studied here. In high-grade metamorphic rocks, such screening could only detect the latest geochemical alteration. For these reasons, the discrimination technique has been based essentially on geochemical criteria, namely on relative abundances of Mn, Rb and Sr (e.g. Brand and Veizer, 1980). Elemental ratios, such as Mn/Sr, Fe/Sr, Ca/Sr and Rb/Sr, as well as δ18 O values, are widely used as geochemical criteria for detecting the least disturbed C-, O- and Rb-Sr systems. Very variable values and dissimilar combinations of the ratios have been used in the past (Asmerom et al., 1991; Derry et al., 1992; Kaufman et al., 1993; Gorokhov et al., 1995; Semikhatov et al., 2002; Kuznetsov et al., 2003). In all cases, the choice is empirical and to some extent arbitrary. The published data on non-metamorphosed limestones suggest Mn/Sr = 0.065–0.02 and Mg/Ca < 0.02, whereas a significant database obtained from high-grade calcite marbles (Melezhik et al., 2003) suggests Mn/Sr < 0.02, Mg/Ca < 0.02. However, such values have been chosen empirically during intensive studies in the Scandinavian Caledonides. The values may well be different in cases where the rocks experienced a different geological history, and should therefore be used with caution. For instance, Pili et al. (1997a, b) performed isotopic studies on marbles from a large-scale granulite section in Madagascar, from geological environments which are rather similar to those in the study area. They reported that marbles from major shear zones are depleted in 13 C down to −3‰ due to exchange with mantle-derived carbon. Marbles from minor shear zones may show significant isotopic variations at the metre scale reflecting the heterogeneous distribution of fluid flow associated with variable permeabilities. However, even granulite-grade marbles

550

Table 3 Minerals in insoluble residue, Rb and Sr concentrations and isotopic ratios of marbles from the Montepuez Complex Sample #

Minerals in insoluble residue

Montepuez quarry 4, UTM 0497505, 8552297 Calcite-dolomite marble member Calcite marbles, partially dolomitised Mvm040 – Mvm041 Mu, Am (Qu, Tlc) Mvm042 – Mvm043 – Mvm044 – Mvm045 (Mu) BB037 –

Montepuez quarry 2, UTM 0497683, 8552084 White dolomite marble member Calcite marble, intensively dolomitised BB031 – Montepuez quarry 1, UTM 0497684, 8552072 White dolomite marble member Calcite marble, intensively dolomitised Mvm065 Am Mvm066 Am Mvm075 Am Dolomite marbles Mvm061 – Mvm064 Am Mvm067 Am (Tlc) Mvm070 – Mvm071 Qu Mvm072 Am Mvm081 Tlc, Am, Qu Marbles sampled outside the quarries Calcite marbles BB022 – TBM143A Qu, Fsp

Mn/Sr

δ13 C (‰)

δ18 O (‰)

Rb (ppm)

0.09 0.05 0.02 0.07 0.04 0.07 0.07

0.17 0.11 0.04 0.10 0.15 0.08 0.13

+2.0 +1.7 +2.1 +2.0 +1.6 +1.9 +1.8

17.9 18.9 20.3 16.8 14.6 15.1 16.6

0.12 0.14 0.13 0.11 0.12 0.13 0.09

0.46

1.09

+0.4

19.3

0.53 0.55 0.55 0.55 0.59

0.23 0.26 0.42 0.32 0.50

−1.5 −1.7 +1.4 −1.6 −0.4

0.50

0.097

0.37 0.46 0.55

87 Rb/86 Sr

87 Sr/86 Sr

170 169 123 144 150 145 128

0.0021 0.0024 0.0031 0.0022 0.0023 0.0026 0.0021

0.70510 0.70516 0.70521 0.70514 0.70508 0.70518 0.70520

0.70507 0.70512 0.70516 0.70511 0.70504 0.70514 0.70517

0.18

83

0.0063

0.70531

0.70521

22.7 21.7 22.4 22.0 22.7

0.22 0.09 0.12 0.12 0.14

122 110 106 78 85

0.0053 0.0024 0.0033 0.0045 0.0048

0.70565 0.70561 0.70546 0.70553 0.70555

0.70557 0.70557 0.70541 0.70546 0.70547

+1.8

22.0

0.07

124

0.0017

0.70540

0.70537

0.16 0.24 0.55

+1.3 +1.5 +1.7

22.2 22.6 21.4

0.14 0.1 0.08

80 99 116

0.0051 0.0030 0.0020

0.70561 0.70560 0.70551

0.70553 0.70555 0.70548

0.56 0.54 0.56 0.53 0.58 0.55 0.56

0.36 0.47 0.48 0.38 0.34 0.11 0.29

+0.7 +1.6 +1.4 +1.7 +1.8 +1.8 +1.3

19.5 22.6 22.0 22.2 21.8 21.2 21.7

0.11 0.09 0.09 0.09 0.07 0.08 0.06

108 92 104 113 82 102 92

0.0030 0.0029 0.0025 0.0023 0.0025 0.0023 0.0019

0.70488 0.70565 0.70548 0.70556 0.70562 0.70552 0.70556

0.70483 0.70560 0.70544 0.70552 0.70558 0.70548 0.70553

0.01 0.01

0.004 0.04

+0.1 +4.7

16.7 24.7

0.23 1.31

4125 1040

0.0002 0.0037

0.70566 0.70654

0.70566

Sr (ppm)

(measured)

87 Sr/86 Sr

(initial)a

a For marbles sampled in quarries, the initial strontium ratio was calculated under the assumption that the age of these rocks is equal to 1200 Ma; for other samples—equal to 750 Ma. Abbreviations: Am – amphibole; Fsp – feldspar; Mu – muscovite; Qu – quartz; Tlc – talc. Rb and Sr concentrations were determined by standard isotope dilution and solid-source mass spectrometry.

V.A. Melezhik et al. / Precambrian Research 162 (2008) 540–558

Montepuez quarry 3, UTM 0497650, 8552251 Grey dolomite marble member Calcite marble, intensively dolomitised Mvm052 Am, Qu Dolomite marbles Mvm051 Am, Qu Mvm050 Am, Qu Mvm053 Am (Tlc) Mvm054 Am Mvm056 –

Mg/Ca

Table 4 Minerals in insoluble residue, Rb and Sr concentrations and isotopic ratios of marbles from Xixano, Lalamo, Ocua and Nampula complexes in NE Mozambique Sample #

Minerals in insoluble residue

Mn/Sr

δ13 C ‰

δ18 O

Rb

Sr

87 Rb/86 Sr

87 Sr/86 Sr

0.01

0.07

−1.1

15.1

0.25

1190

0.0006

0.70616

0.70615

0.09 0.18

2.0 0.9

+2.4 −0.2

22.0 19.0

0.25 0.29

60.6 73.5

0.0121 0.0116

0.70666 0.70574

0.70653 0.70562

0.45

0.2

+3.8

19.0

0.49

82

0.0175

0.70631

0.70612

(measured)

87 Sr/86 Sr

Lalamo Complex Calcite marble JS200 Grph, Am, Mu, Fsp Calcite, partially dolomitised marble JS303 Qu, Am, Px

0.01

0.007

+7.1

26.1

0.12

2105

0.0002

0.70671

0.70671

0.03

n.d.

+3.5

28.4

0.73

3930

0.0005

0.70627

0.70626

Ocua Complex Dolomite marble JS330

0.57

1.1

+0.4

23.1

0.22

130

0.005

0.70684

0.70679

0.12 0.05

1.16 0.78

−3.5 −2.0

19.6 19.9

0.04 n.d.

229 n.d.

0.0005 n.d.

0.70594 n.d.

0.70593 n.d.

0.27 0.36 0.25

0.66 0.98 1.23

−2.8 −2.9 −2.9

18.6 20.8 20.1

n.d. n.d. n.d.

n.d. n.d. n.d.

n.d. n.d. n.d.

n.d. n.d. n.d.

n.d. n.d. n.d.

–

Nampula Complex Calcite, partially dolomitised marbles TBM-097 – TBM-098 n.d. Calcite, intensively dolomitised marbles TBM-099 n.d. TBM-105 n.d. AS04-26 n.d.

(initial)a

V.A. Melezhik et al. / Precambrian Research 162 (2008) 540–558

Xixano Complex Calcite marble IH04.027 (Mu) Calcite, partially dolomitised marbles JS285 Am, Mu JS299 Px Calcite, intensively dolomitised marble TBM-126 Am (Tlc, Qu, Mu)

Mg/Ca

a The initial strontium ratio was calculated under the assumption that the age of these rocks is equal to 750 Ma. Abbreviations: Am – amphibole; Fsp – feldspar; Grph- graphite; Mu – muscovite; Px – pyroxene; Qu – quartz; Tlc – talc. Rb and Sr concentrations were determined by standard isotope dilution and solid-source mass spectrometry.

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outside shear zones have carbon and oxygen isotopic compositions similar to their protoliths (Pili et al., 1997a, b). 5.1. Montepuez Complex marbles The Montepuez CM from the tripartite unit shows significant fluctuations in δ18 O and a weak positive correlation between δ18 O and δ13 C, which are indicative of postdepositional resetting. Based on the observed alteration trends (Fig. 3e, d and f), we tentatively suggest that +2‰ and 20‰ represent the leastaltered δ13 C and δ18 O values, respectively. In the DM, low δ13 C is associated with high TOC content (Fig. 3f), and such an effect can be attributed to a local isotopic exchange between organic matter/graphite and dolomite. There is no correlation between δ18 O and δ13 C, and the overall δ18 O values show an overlap with those reported from non-metamorphosed, Proterozoic dolostones (e.g., Melezhik et al., 2005). This may suggest a high degree of preservation of original δ18 O values in the DM. The observed alteration trend (Fig. 3d and f) suggests +1.8‰ and 22‰ as the best proxies for initial ␦13 C and ␦18 O values, respectively. In the tripartite unit, the 18 O depletion of the CM relative to the DM can be attributed to a sequence of events. The partial dolomitisation of limestones commonly creates enhanced porosity and permeability, whereas intensively and completely dolomitised rocks become largely impermeable. As the result, the partially dolomitised rocks should be more sensitive towards exchange of their oxygen isotopes with infiltrating postdepositional fluids, whereas the δ13 C would be sufficiently buffered by the dissolving calcite precursor (cf. Banner and Hanson, 1990; Land, 1992). Because diagenetic and metamorphic alterations affect carbonate material in a similar way (Nabelek, 1991), we are not in a position to distinguish between the two. In the tripartite marble unit, the strontium isotope ratios of the CM are less radiogenic than DCM, and both have less radiogenic ratios in comparison with the DM (excluding one outlier, Fig. 3h). Such features can be reconciled if the dolomite exchanged Sr with dolomitising fluids. This inference is in agreement with the relative enrichment of the dolomite in 18 O. However, significant negative correlation between 87 Sr/86 Sr ratios and the Sr concentration in the CM (Fig. 3g), and the absence of 87 Sr/86 Sr correlation with the Mg/Ca ratio, suggest that the CM were affected by an additional episode of postdepositional alteration, which was not associated with the dolomitisation. Based on the reasoning outlined above and considering all observed alteration trends (Fig. 3g and h), the least radiogenic 87 Sr/86 Sr ratio of 0.70508 is tentatively suggested as the least altered value for the CM. However, the relatively low Sr concentration in the CM was probably not sufficient to provide a strong buffer for the Sr isotope system against multiple alterations, although this is currently impossible to quantify.

Sample Mvm061 of the dolomite marble, the outlier, yielded an exceptionally low 87 Sr/86 Sr ratio of 0.70488. This is coupled with depletion in 18 O, and therefore, has no obvious explanation in terms of preservation potential. The sample was obtained from the unit containing abundant dismembered amphibolite bodies. The amphibolite occurring in the quarries is characterised by a low 87 Sr/86 Sr ratio (0.70403). The amphibolite contains 320 ppm Sr (Table 5), which is three times higher than in the dolomite marbles. Thus, the most probable explanation for the low 87 Sr/86 Sr in the outlier is an isotopic exchange between the amphibolite and the dolomite. The low ␦18 O value is consistent with this model. The Montepuez West marble shows similarities to those studied in the quarries, whereas the Montepuez East marble is distinctly different in that its δ13 C (+4.7‰) and δ18 O (24.7‰) values are enriched in heavy isotopes. These enriched values plot outside any alteration trends associated with the tripartite marble unit and therefore cannot be explained as the result of a higher degree of preservation. We tentatively suggest that the calcite was precipitated from seawater with different C- and O-isotopic composition. The Montepuez West and East calcite marbles are distinguished by higher 87 Sr/86 Sr ratios (Table 3). Because these rocks have Sr concentrations (1040–4125 ppm) one order of magnitude higher than that of the CM in the tripartite unit, their more radiogenic strontium isotope ratios are very unlikely to be attributable to postdepositional alterations. Instead, they apparently represent marbles whose carbonate precursor was originally precipitated from seawater with a different isotopic composition. This is particularly true for the Montepuez East 13 C-rich marble. As far as the Montepuez West marble is concerned, it was certainly derived from an aragonite precursor as indicated by the very high Sr abundance. This suggests that the latter may represent not only a different age but also that it may signify a different depositional setting or a different postdepositional geochemical history. 5.2. Marbles of other complexes A limited database on marbles (Table 4) sporadically occurring in several other complexes (Fig. 1) does not allow for robust screening against postdepositional resetting of C, O and Rb/Sr isotope systems. However, several samples are characterised by exceptionally high Sr concentrations, which should provide a sufficient buffer against postdepostional resetting. Such samples can be considered as retaining 87 Sr/86 Sr ratios close to depositional values. In the Xixano Complex, this applies to the South marble lens (IH04.027, Fig. 1) having 1190 ppm Sr and 87 Sr/86 Sr of 0.70616. Interestingly, the Xixano North marble (JS299, Fig. 1), has a significantly lower Sr concentration

Table 5 Chemical composition and strontium isotope ratio of amphibolite boudin in marbles from the Montepuez quarry Sample #

SiO2 (%)

Al2 O3 (%)

Fe2 O3 (%)

TiO2 (%)

MgO (%)

CaO (%)

Na2 O (%)

K2 O (%)

Rb (ppm)

Sr (ppm)

87 Rb/86 Sr

87 Sr/86 Sr

BB036

40.7

15.9

15.6

3.63

6.37

11.2

2.41

0.16

1.18

320

0.0118

0.70403

V.A. Melezhik et al. / Precambrian Research 162 (2008) 540–558

553

(73 ppm) combined with the lower 87 Sr/86 Sr ratio of 0.70574, which cannot be attributed to a postdepositional resetting. Two samples, representing different marble units in the Lalamo Complex, namely the Centre and South marbles (Fig. 1), yielded very high Sr concentrations (3939 and 2102 ppm), but however have different 87 Sr/86 Sr ratios (Table 4). Because of strong buffering abilities against postdepositional alterations, both ratios are probably very close to depositional values; thus calcite in these two marbles was very likely precipitated from isotopically different seawaters. The Nampula marbles have negative δ13 C values, which distinguish them from other marbles in the study area. The δ13 C values show a very limited spread, implying that they may reflect the isotopic composition of seawater. Importantly, despite a low Sr concentration, the 87 Sr/86 Sr ratio (0.70594) is much lower in comparison to the Sr isotope ratios of the Sr-rich Lalamo and Xixano marbles. This again suggests that the Nampula carbonates reflect the isotopic composition of a different seawater. 6. Implications for isotope chemostratigraphy Strontium isotope stratigraphy has the potential to provide indirect dating of Neoproterozoic sedimentary carbonates because of the gradual, though irregular, increase in 87 Sr/86 Sr from c. 850 to c. 550 Ma (Fig. 4); and also because Sr has a long residence time relative to seawater, resulting in a uniform 87 Sr/86 Sr ratio in seawater at any given time (Veizer et al., 1999). However, sampling of carbonate sequences through Neoproterozoic time remains incomplete and precise radiometric ages are not always available. Because of this, there have been several disagreements among investigators as to how to construct the seawater reference curve (see discussion in Melezhik et al., 2001b and Kuznetsov et al., 2003, 2005). The result of the discussion was the reference curve (Fig. 4) which has been used in this study. In the case of high-grade metamorphic rocks, when the Sr concentration in carbonates is low, the Rb/Sr system might have been reset while the carbon isotope system may still retain the original ratio. This is because the sediment/water ratios of diagenetic-metamorphic systems (cf. Banner and Hanson, 1990) are about 101 , 102 –103 and 104 for oxygen, strontium and carbon, respectively (cf. Land, 1992). Therefore, although the strontium isotope ratios of carbonates are a better proxy for seawater isotopic composition, δ13 C may provide a better result for low-Sr marbles. The apparent depositional ages of the studied rocks have been constrained by projection of the ‘least altered’ isotopic values obtained from the marble formations onto the δ13 C and the 87 Sr/86 Sr seawater reference curves. The time when both the δ13 C and the 87 Sr/86 Sr intercepts are in agreement has been considered to represent an apparent depositional age (Fig. 4). Fig. 4. Temporal trends of 87 Sr/86 Sr and ␦13 C in seawater and apparent depositional ages of the studied marbles from NE Mozambique. The 87 Sr/86 Sr reference curve (green) is based on Melezhik et al. (2001 and references therein) and Kuznetsov et al. (2003 and references therein) (red symbols) modified by using data from Kuznetsov et al. (2005, 2006) and the database compiled by Shields

and Veizer (2002 and references therein) (green symbols). The least altered 87 Sr/86 Sr δ13 C ratios of the marble units are shown by horizontal lines. Vertical, arrowed-head lines indicate apparent depositional ages (see discussion in the text).

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6.1. The Montepuez Complex marbles The upper age limit for sedimentation within the Montepuez Complex can be constrained by the penetrative 580–530 Ma high-grade metamorphism (e.g., Bingen et al., 2006a, b, 2007). However, the lower age limit cannot be constrained by any, currently available, published data. Thus, within limitations of ‘isotope stratigraphy’, one should initially consider a wide time interval from the Archaean up to 580 Ma. Within this time span, the least altered initial ratio 87 Sr/86 Sr of 0.70504 projected onto the seawater curve sets the apparent depositional age of the tripartite marble as Mesoproterozoic at around c. 1250 Ma (Fig. 4). The global carbon isotope records offer additional constraints. Kah et al. (1999) reported that δ13 C values of −1.0 to +4.0‰ are characteristic for the interval between approximately 1300 and 800 Ma. This pattern is distinct from that of older Mesoproterozoic successions, which record values near 0‰, and suggests that the 1300–800 Ma moderately positive values may be useful for broad time correlation. In the late Palaeoproterozoic (1800–1500 Ma) the secular δ13 C curve is flat, relatively monotonous and fluctuates close to zero much like that seen in the Mesoproterozoic (Lindsay and Brasier, 2002). Thus, considering all limitations, ages older than 1300 Ma are not credible, as the Montepuez carbonates have slightly elevated δ13 C values fluctuating between +1.6 and +2.0‰, thus distinct from those close to zero. Moreover, Palaeoproterozoic carbonates are characterised by a much lower 87 Sr/86 Sr (Shields and Veizer, 2002). The currently available Sr-isotope database includes a rather limited number of analyses involved in the constructed reference curve. Therefore, there is chance for more fluctuations than it is shown in Fig. 4, however, combined Sr- and C-isotope data suggest that an apparent depositional age cannot be younger than 910 Ma. The relatively low Sr concentrations in the Montepuez marbles might not have provided sufficient buffer against postdepositional resetting of the Sr isotope system. Consequently, the apparent depositional age interval of 1250–910 Ma should be considered as a minimum age. The 87 Sr/86 Sr (0.70566) and δ13 C (+0.1‰) values obtained the Montepuez West marble, when projected onto the seawater curve, give several interceptions; however, both ratios agree at 800, 750 and 660 Ma, which can be considered as apparent depositional ages. The Montepuez East marble has 87 Sr/86 Sr of 0.70654. This, combined with δ13 C of +4.7‰, corresponds to a 740 or 700–660 Ma seawater (Fig. 4). 6.2. Marbles of other complexes In the Xixano Complex only the South marble (IH04.027) could be considered to retain an 87 Sr/86 Sr ratio close to the depositional value due to the high Sr concentration (1190 ppm). The 87 Sr/86 Sr of 0.70616 and δ13 C of −1.1‰ intersects on the seawater reference curve agree at 740, 720 and 670 Ma. Two younger intercepts should not be considered because the Xixano Complex underwent metamorphism at 735 ± 5 Ma (Bingen et al., 2006a, b, 2007). The 87 Sr/86 Sr and ␦13 C of the North Xixano marbles intercepts the reference curve at 800, 750 and

660 Ma. The younger intercept should not be considered for the reason explained above. The least altered 87 Sr/86 Sr of 0.70574 is significantly lower than the ratio in the South marble. However, the low Sr concentration (73 ppm) suggests that the Rb/Sr system might have experienced a postdepositional alteration, and therefore 800, and 750 Ma should be considered as the upper limit for an apparent depositional age. In the Lalamo Complex, both the Centre and South marbles (Fig. 1) are exceptionally rich in Sr and their 87 Sr/86 Sr ratios may therefore be a robust proxy for coeval seawater. The 87 Sr/86 Sr of 0.70627 and δ13 C of +3.5‰ for the Lalamo Centre marble has a joint intercept at 740 or 670 Ma. Although both 87 Sr/86 Sr (0.70671) and δ13 C (+7.1‰) ratios of the Lalamo South marble differ, they provide a similar apparent depositional age at 740 or 670 Ma (Fig. 4). As in the Xixano Complex, the younger intercept at 670 Ma is in conflict with magmatic ages ranging between 753 ± 10 and 696 ± 13 Ma (Bingen et al., 2006a, b, 2007), and thus should not be considered as an apparent depositional age. All marbles sampled in the Nampula Complex have relatively low Sr concentration (up to 350 ppm, Table 3). The limited database (Table 4) does not allow reliable discrimination against postdepositional resetting of the Rb/Sr system. Consequently the 87 Sr/86 Sr ratio of 0.70594 obtained from one of the Nampula Complex marbles may not represent a robust proxy for coeval seawater. However, the ratio can set an upper age limit for deposition. Several intercepts on the 87 Sr/86 Sr reference curve can be reconciled with δ13 C of −3.5‰ at c. 750 Ma (Fig. 4). The age cannot be older than 800 Ma, because δ13 C values significantly below −1.0‰ are not characteristic of the 1300–800 Ma interval (Kah et al., 1999) and the late Palaeoproterozoic (1800–1500 Ma) (Lindsay and Brasier, 2002). 7. Implications for tectonostratigraphy and basin evolution Apparent carbonate depositional ages obtained by isotope chemostratigraphy (Table 6) can provide insight into basin evolution during the Mezo-Neoproterozoic in NE Mozambique, which cannot be done at present by any other means. However, apparent ages should be treated with caution for two reasons: (i) a possible, unresolved, postdepositional resetting; and (ii) a possible bias towards exposed and selectively sampled marble units. Therefore, initial 87 Sr/86 Sr ratios might have been lower, and so apparent depositional ages older, and the age coverage obtained for each complex might be incomplete. Further, we consider only complexes that contain marbles for which apparent depositional ages have been obtained. The apparent carbonate depositional ages cluster in six groups (Table 6), namely (1) 1250–910 Ma (tripartite marble unit of the Montepuez Complex); (2) 800 or 750 Ma (Xixano North marble); (3) 800, 750 or 660 Ma (Montepuez West marble); (4) c. 750 Ma (Nampula marble); (5) c. 740 Ma (Xixano South and Lalamo Centre and South marbles); and (6) 740, or 700–670 Ma (Montepuez East marble). All groups, except group 1, have overlapping apparent ages. The apparent depositional ages of groups 2, 3 and 6 are tentatively considered undistinguishable. Although the isotopic composition of the Nampula marble (group 4) has

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Table 6 Summary of the least altered 87 Sr/86 Sr and δ13 C ratios, and apparent depostional ages of marble units in NE Mozambique Complex

Occurrence

87 Sr/86 Sr

Montepuez Xixano Montepuez Nampula Xixano Lalamo Lalamo Montepuez

Montepuez quarries Xixano North Montepuez West Nampula Xixano South Lalamo Centre Lalamo South Montepuez East

0.70504 0.70574 0.70566 0.70594 0.70616 0.70627 0.70671 0.70654

a unique resolution at c. 750 Ma, this apparent depositional age overlaps with those of the Xixano North (group 2) and Montepuez West (group 3) marbles. Similarly, the c. 740 Ma Xixano South and Lalamo Centre and South marbles (group 5) overlaps with the apparent depositional age of the Montepuez East marble (group 6). Individual marble units within the c. 750 and c. 740 Ma groups are rather inhomogeneous in terms of 87 Sr/86 Sr ratios, highly variable in Sr abundance; the Nampula marble is significantly depleted in 13 C, whereas the Lalamo South marble is considerably enriched in 13 C (Table 6). All these differences suggest that their carbonates originally precipitated from different seawater, and that the depositional ages of individual marble units might be also different. This is, however, impossible to resolve within the limitations of the isotope chemostratigraphy approach (Fig. 4) 7.1. The Montepuez Complex The Montepuez Complex contains marble units showing at least two different depositional ages. The oldest age of 1250–910 Ma was obtained from the tripartite marble unit at Montepuez quarries. The two younger marble units, one with the apparent depositional ages at c. 800, 750 or 660 Ma (West marble), and the other at c. 740 or between 700–670 Ma (East marble), have carbon and Sr isotopic compositions that correspond to significantly different seawaters (Table 6). From the tectonostratigraphic implications, it is important to note that the 800–660 Ma marble is closely linked spatially to the 1250–910 Ma marbles exposed in the Montepuez quarries (Fig. 1). The 1250–910 Ma marbles represent the oldest dated carbonate rocks in the Montepuez Complex. No depositional basement to the marbles is currently known at the present day. The 1250–910 Ma estimate suggests that the Pan-African nappe system in NE Mozambique may carry Mesoproterozoic sedimentary rocks, and not only Neoproterozoic lithologies, as suggested by zircon-monazite geochronology of orthogneisses. The 1250–910 Ma marbles are low-Sr, intensively dolomitised rocks. Compilation of depositional settings of MezoNeoproterozoic dolostones suggests that dolostones and dolomitised limestones are common features of intertidal, supratidal and peritidal settings, whereas limestones tend to accumulate in shelf and deep basinal environments, very often below storm waves (Muir et al., 1980; Delaney, 1981; Tucker, 1983a, b; Lindsay, 1987; Preiss, 1987; Southgate, 1989). In

␦13 C (‰)

Age (Ma)

Age-group

+2.0 −0.2 +0.1 −3.5 −1.1 +3.5 +7.1 +4.7

c. 1200 800 or 750 800, 750 or 660 c. 750. c. 740 c. 740 c. 740 700–670 or 740

1 2 3 4 5 5 5 6

view of the very limited sedimentological evidence, the robust reconstruction of depositional settings is not possible. The 800–660 Ma (Montepuez West) and 740–670 Ma (Montepuez East) marble units are composed of Sr-rich calcite, suggesting an aragonite precursor. The marbles of these two units show considerable isotopic differences (Table 3). The significant enrichment of the East marble in 13 C suggests enhanced burial of organic matter coeval with the carbonate deposition. Overall, the isotopic dissimilarities of the East and West marbles, combined with a considerable spatial separation by thick piles of gneisses and schists, would argue for their accumulation in two unrelated basins. However, care must be taken in comparison of the carbon and strontium isotopic records due to the greatly different residence times of C (∼105 years) and Sr (∼3 × 106 years) in oceanic waters, and consequently their respective response time to perturbations (e.g., Derry et al., 1992; Richter and Turekian, 1993). The greatest temporal change confidently documented in seawater 87 Sr/86 Sr is at a rate of ∼69 × 10−6 Myr−1 that has occurred between 40 and 25 Ma in the Cenozoic (e.g., Elderfield, 1986). If such a rate is applied to the two youngest carbonate units, the required difference in their time of deposition should be not less than 10 Ma. However, this can neither prove nor disprove whether or not the two younger marbles belonged to two separated depositional cycles. We tentatively imply isolated carbonate platform settings for the two younger marble units of the Montepuez Complex because they are composed of high-Sr, pure, calcite marbles. However, the limited nature of the geological information available is acknowledged. 7.2. The Xixano Complex The Xixano Complex includes two marble units whose isotopic composition is respectively consistent with c. 800 and 750 Ma, and c. 740 Ma seawater. Although their apparent depositional ages are rather similar, the marble units are separated in space and have a significant difference in 87 Sr/86 Sr (Table 6). These two sedimentation episodes are preceded by a volcanicarc type felsic volcanism dated to 818 ± 10 Ma (Bingen et al., 2006a, b, 2007). However, detailed geological relationships between the sedimentary and volcanic rocks remain unknown. There is no sedimentological evidence elucidating depositional settings of carbonates in the Xixano Complex. Tentatively, one volcanic and two sedimentary events are suggested as one continuous depositional cycle, until new data would suggest

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otherwise. The 818 Ma volcanic rocks and 800–740 Ma carbonate units and volcanic rocks underwent only amphibolite-facies metamorphism, and thus their deposition-metamorphic history must be considered separately from that of the mafic and enderbitic orthogneisses affected by granulite-facies metamorphism at 735 ± 4 Ma in the same complex (Bingen et al., 2006a, b, 2007). Such heterogeneity is considered to be the result of tectonic juxtaposition of two plates with different ancestries during the Neoproterozoic stacking event.

3.

4.

7.3. The Lalamo Complex Based on the results described above, the geological history of the Lalamo Complex very likely includes development of one carbonate basin with an apparent age of c. 740 Ma. Two synorogenic granitic plutonic events dated to 753 ± 10 Ma and 696 ± 13 Ma in the Lalamo Complex (Jamal, 2005; Bingen et al., 2006a, b, 2007) may imply that the development of this carbonate-depositing basin falls in between the two orogenic events. This also may imply that these complexes could have different ancestries and were tectonically juxtaposed prior to or during the 560–530 Ma Pan-African orogeny. However, since the time constraint for the carbonate deposition remains imprecise, both above inferences require corroboration by more precise age determinations. 7.4. The Nampula Complex The Nampula Complex is regarded as one of the basement complexes in NE Mozambique. It is mainly made up of Mesoproterozoic orthogneiss ranging in age between c. 1150 and 1020 Ma (Kr¨oner et al., 1997; Bingen et al., 2006a, b, 2007). The Sr- and C-isotopic compositions of marble interbedded with graphite schist and paragneiss in the northeastern part of the complex, south of the Ocua Complex (Fig. 1), suggest much younger depositional ages, at around c. 750 Ma. The difference over 300 Ma between the older igneous rocks and the younger marbles, both subsequently involved in the Pan-African deformation and high-grade metamorphism, cannot easily be reconciled within a model of the coherent tectonostratigraphic unit which experienced a single cycle of deposition–deformation–metamorphic history. The data suggest that the marble-bearing sequences represent either an unconformable Neoproterozoic cover, or tectonic slices, belonging to the Neoproterozoic nappe system, folded together with the basement lithologies. 8. Conclusions 1. Calcite, dolomite and, to a lesser extent, magnesite marbles are minor but characteristic components of several metasedimentary units of unknown age within tectonic nappes in the Neoproterozoic Mozambique Belt of NE Mozambique. 2. Carbon and strontium isotope stratigraphy of marble units has been applied for indirect dating of carbonate sedimentation and to provide a complementary and independent insight into the depositional history and tectonic development of NE

5.

6.

7.

Mozambique within a previously established tectonostratigraphic and geotectonic framework. Despite amphibolite-facies Neoproterozoic metamorphism, some marbles retain δ18 O (V-SMOW) as high as 28.4‰, suggesting well-preserved depositional signatures. High Sr-abundances in several marble units (1000–4000 ppm) provided strong buffering of the Sr-isotope system against postdepositional resetting. The overall spread of the least altered δ13 C (−3.5 to +7.1‰) and 87 Sr/86 Sr (0.70504–0.70671) values from the Nampula, Montepuez, Xixano and Lalamo marbles suggests several discrete apparent depositional ages within the 1250–660 Ma time interval. The apparent depositional age of the Ocua marble was not resolved. The 300 Ma time lag between felsic volcanism (1250–1200 Ma) and carbonate deposition (c. 750 Ma) in the Nampula Complex suggests either a non-depositional unconformity between the two cycles or different ancestry and tectonic juxtaposition during the Neoproterozoic Pan-African orogeny. Juxtaposition of amphibolite-grade Xixano Complex marbles (originally deposited between 800–750 and at c. 740 Ma) with 735 Ma granulites implies different ancestry of these rock assemblages. Apparent carbonate depositional ages suggest that the Nampula, Montepuez, Xixano and Lalamo complexes do not represent contiguous tectonostratigraphic units and that they may in fact consist of juxtaposed nappe units.

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