in volcanic rock series from Archean greenstone belts and, accordingly, for other geodynamic mechanisms responsible for the formation of the Neoarchean con.
ISSN 1028334X, Doklady Earth Sciences, 2009, Vol. 429A, No. 9, pp. 1575–1579. © Pleiades Publishing, Ltd., 2009. Original Russian Text © A.B. Vrevskii, 2009, published in Doklady Akademii Nauk, 2009, Vol. 429, No. 6, pp. 794–798.
GEOCHEMISTRY
Geochemical and Isotopic Signatures of Nonsubduction Mechanisms of Formation of the Neoarchean Continental Lithosphere of the Fennoscandian Shield A. B. Vrevskii Presented by Academician F.P. Mitrofanov April 6, 2009 Received April 13, 2009
Abstract—This study presents new data on the geochemical and Sm–Nd isotope compositions, as well as the U–Pb age and geodynamic nature, of the Neoarchean basalt–andesite–dacite (BAD) association from the Kolmozero–Voron’ya greenstone belt. As it was first demonstrated by the example of the Neoarchean green stone belt, the formation of BAD associations within a single Neoarchean greenstone structure may be explained by the longlasting evolution of separate mantle or crustal sources not related to subduction processes. DOI: 10.1134/S1028334X09090347
The origin and geodynamic evolution of the juve nile continental crust in Earth’s early history represent some of the most fundamental problems of Archean geology. Basalt–andesite–dacite associations of the Archean greenstone belts are generally interpreted as primary and sole indicators of the formation of the continental lithosphere of granite–greenstone areas in the active continental margin and islandarc settings. The role of such volcanic complexes, including the Fennoscandian shield, as an indicator is underpinned by the similarity of their geochemical signatures to Phanerozoic adakitic and boninitic volcanic series [1– 4]. At the same time, the existence of such geody namic settings in the Early Precambrian can only be admitted under certain limitations due to the specific thermal state and the distinctive isotopic and geochemical signatures of the mantle in Earth’s early history [5]. This study presents new geochemical and isotopic data on the basalt–andesite–dacite (BAD) associa tion of the Polmos–Porosozero structure, which pro vide evidence for a nonsubduction origin of the BAD in volcanic rock series from Archean greenstone belts and, accordingly, for other geodynamic mechanisms responsible for the formation of the Neoarchean con tinental lithosphere of the Fennoscandian shield. The Polmos–Porosozero structure located at the presentday junction zone between the Murmansk and Central Kola domains of the Kola region is the largest and best preserved part of the more than 300kmlong
Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, Saint Petersburg, Russia
Uraguba–Kolmozero–Voron’ya greenstone belt. The volcanosedimentary complex of this belt comprises two suites or three formations [5]. The base of the sec tion is a metaterrigenous suite (Lyavozero Forma tion), consisting of garnet–biotite and biotite schists with conglomerate lenses. The base of the overlying volcanic suite (Polmostundra and Voron’ya Tundra Formations) is made up of metamorphosed tholeiitic basalts or komatiitetype lavas with interbeds of vol canosedimentary rocks [5, 6]. Metavolcanic rocks of the komatiite–tholeiitic series grade up, with no strati graphic break or unconformity, into a suite, which is lithologically more variable and represented by the interbedding metamorphosed basalts, andesibasalts, andesites, and dacites, exhibiting a lava or tufflike texture [5]. A rock succession of this BAD association also contains thin (up to several meters) interlayers of greywackes, calcareous dolomites, ferruginous quartz ites, and carbonaceousschists [5]. Metavolcanic rocks of the BAD association are unconformably overlain by the upper terrigenous suite (Chervurt Formation), which consists of aluminous gneisses and schists with a member of polymictic conglomerates at the base of the section. The basalt–andesite–dacite association is typical calcalkaline rocks of normal alkalinity (Table 1). However, the variations in the major, trace, and REE compositions of the studied metavolcanics enabled us to recognize in this association two geochemical groups of anadesites. Group I volcanics (andesites I) show a continuous range of composition in terms of their SiO2 content (53.2–65.3 wt.%) from the lowTi (TiO2 = 0.43 on average) andesibasalts to andesidacites of the sodium series (K2O/Na2O = 0.58 on average). The entire spectrum of normalized REE patterns in these
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Table 1. Average chemical composition of the BAD rock association from the Polmos–Porosozero structure Compo nent
1 (7)
2 (7)
3 (4)
SiO2 TiO2 Al2O3 Fe2O3 FeO FeO* MnO MgO CaO Na2O K2O P2O5 K2O/Na2O Ti/Zr Zr/Y Sr/Y
58.07 0.43 14.46 1.67 6.56 8.06 0.16 5.64 7.63 2.41 1.14 0.15 0.58 53.7 6.5 15.9
53.37 0.69 15.01 2.55 6.95 9.25 0.13 4.61 7.08 3.67 0.27 0.13 0.07 25.0 3.7 6.6
50.58 0.31 14.20 2.00 9.83 11.63 0.22 8.41 8.93 2.04 0.26 0.13 0.15 76.0 2.4 9.1
Compo nent
1 (8)
2 (6)
3 (5)
Ba 26.0 35.1 19.9 Sr 243.7 132.4 103.0 Y 15.3 20.1 11.3 Zr 99.29 74.04 27.1 Nb 7.62 4.69 1.53 Ta 0.47 0.29 0.18 Hf 2.61 2.14 0.66 Sc 13.6 25.7 44.0 V 139.3 214.4 214.8 Cr 247 118 296 Co 56.0 44.9 51.3 Ni 171.8 63.8 133.3 Ti 2481 3973 2061 (Gd/Yb)N 1.81 1.21 1.12 (Ce/Yb)N 2.52 0.35 0.33 (Ce/Sm)N 2.43 0.81 0.86
Compo nent
1 (8)
2 (6)
3 (5)
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu mg* (Nb/La)N
13.51 29.03 3.67 14.21 2.87 0.87 2.80 0.43 2.45 0.49 1.41 0.20 1.26 0.20 0.53 0.90
2.47 7.33 1.16 6.04 2.14 0.82 2.58 0.48 3.06 0.67 1.89 0.28 1.78 0.27 0.50 2.56
1.57 3.84 0.61 3.11 1.05 0.40 1.32 0.26 1.71 0.40 1.11 0.18 1.07 0.18 0.56 1.10
Note: (1) Andesites I; (2) andesites II; (3) basalts. Major oxides (wt %) were determined by XRF; trace and REE abundances (in ppm) were measured by ICPMS at VSEGEI. The number of analyses is shown in parentheses.
Table 2. U and Pb isotopes in zircons from andesites I (nos. 2–7) and andesites II (no. 1) Nos.
Sample no.
1 2 3 4 5 6 7
1.1 2.1 3.1 3.2 4.1 6.1 6.2
206Pb , c
% U, ppm
0.07 7.02 0.03 0.04 17.96 0.30 0.80
646 486 296 347 542 839 212
232Th/238U
0.74 0.56 0.45 0.40 0.54 0.26 0.60
206P , rad
206Pb/238U
ppm
Age, Ma
273 124 146 161 246 385 98.7
2575 ± 52 1554 ± 56 2925 ± 58 2787 ± 55 2209 ± 90 2746 ± 54 2761 ± 61
207Pb*/235U
206Pb*/238U
Correla tion coef ficient
13.19 ± 2.5 7.27 ± 5.5 15.33 ± 2.5 14.41 ± 2.5 11.90 ± 8.7 14.23 ± 2.4 14.22 ± 3.8
0.491 ± 2.4 0.273 ± 4 0.574 ± 2.5 0.541 ± 2.4 0.409 ± 4.8 0.531 ± 2.4 0.535 ± 2.7
0.993 0.730 0.987 0.985 0.553 0.993 0.720
Note: The 4nA primary ion beam was focused to a diameter of 18 μm. The U/Pb isotope ratios were normalized to a value of 0.0668 for the TEMORA reference zircon [13]. Pbc and Pbrad are common and radiogenic lead, respectively. Asterisks denote isotopic ratios corrected for measured 204Pb. Errors on single analysis (ratios and ages) are given at the 1σ level, and those on concordant ages are at the 2σ level.
rocks ranges from a strongly fractionated LREE ((Ce/Sm)N = 2.38–2.82) pattern to weakly fraction ated patterns of heavy lanthanides ((Gd/Yb)N = 1.62– 2.35) (Fig. 1). As can be seen in variation diagrams, group I volcanics have continuous geochemical trends, which could be accounted for by fractionation of the pri mary melt and segregation of the Ol + Cpx + Grt ± Pl mineral assemblage. Group II andesites (andesites II) are lowmagnesium (mg# =0.44–0.56) andesibasalts and andesites (SiO2 54.2–61.6 wt %) of the potassium– sodium series (K2O/Na2O = 0.03–0.09) (Table 1). Unlike group I andesites, these are characterized by
higher Ti, V, and Ba and lower Sr, Cr, Co, Ni, Ti con tents, as well as lower Ti/Zr and Zr/Y ratios (Table 1). The normalized REE patterns of this group volcanics show LREE depletion ((Ce/Sm)N = 0.62–0.88) and slight HREE fractionation ((Gd/Yb)N = 1.01–1.58) (Fig. 1), which are atypical for andesites. As opposed to the Archean and Phanerozoic adakite assemblages [2, 4], the volcanics of both geochemical groups have a lower concentration of indicator elements such as Ba, Sr, and Y and lack a negative Nb anomaly ((Nb/La)N = 0.9–2.6) (Table 1).
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GEOCHEMICAL AND ISOTOPIC SIGNATURES OF NONSUBDUCTION MECHANISMS
U–Pb dating of zircons, constraining the age of andesites, was performed on the SHRIMP II ion microprobe at the VSEGEI Center for Isotopic Research using the procedure presented in [7]. Cathodoluminescence microscopy of accessory zir cons in both types of andesites revealed that most zir cons grains are wellfaceted and exhibit oscillatory zoning. There were two types of crystals identified: (1) longprismatic, small and transparent; and (2) short prismatic, mediumsized, transparent, light brown. Seven zircon crystals of both morphological types were analyzed for U–Pb isotope systematics (Table 2), and the measurements yielded a discordia with an upper intercept age of 2778.4 ± 5.4 Ma (MSWD = 0.75). The Nd isotope compositions of both groups of andesites and associated basalts show a significant variation in the initial 143Nd/144Nd ratios (Table 3). These andesites have an εNd(T2570) value ranging from –1.0 to –6.5, indicative of their origin from an isoto pically enriched mantle source or input from crustal material. For calculation of the Sm–Nd model age on the basis of geochemical signatures of group I andes ites, available experimental data [8] on the andesite melts and model compositions of the sources of calc alkaline rock series we propose a twostage model (TDM2), which assumes that the likely protolith was represented by lower crustal basic granulites with 147 Sm/144Nd =0.15. The 3.59+0.42/–0.25 Ga model ages suggest that such a protolith of andesites II pri mary melts had a prolonged evolutionary history of isotopic composition. The andesites I have εNd(T = 2778) values ranging from +0.8 to +3.7, approximately coincident with those of the metabasalts from the same section (εNd(T = 2778) from +1.5 to +3.5). On the Sm–Nd evolution diagram, the basalts and andesites I define a regression line with a slope of 2911 ± 29 Ma (MSWD = 1.03) with εNd = +1.7 ± 0.3 (Fig. 3). This age is, within error, identical to a Pb–Pb age of 2930 ±
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Rock/PM 100 Andesite I
1 2 3
10
Andesite II
1 10
Basalt
1
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 1. Primitive mantle–normalized [14] REE distribution patterns in rocks of the basalt–andesite–dacite association from the Polmos–Porosozero structure. (1–3) Intermediate compositions: (1) andesites II, (2) andesites I, (3) basalts.
90 Ma for andesites from the Voronoya Tundra Forma tion [9, 10] and to an earlier Sm–Nd age for komati ites from the Uraguba–Kolmozero–Voron’ya green stone belt [6]. The results of Sm–Nd and U–Pb isoto pic study suggest that the evolution of primary melts of the basalt–andesite–dacite association from their extraction from the mantle source until their crystalli zation lasted at least 130 Myr. On the basis of geologi cal features of the Polmos–Porosozero structure, the
Table 3. Sm–Nd data for the BAD association from the Polmos–Porosozero structure Sample no.
Rock
Sm, ppm
Nd, ppm
147Sm/144Nd
143Nd/144Nd
εNd(T = 2880)
63a 155 161 137 521 523 5371 529 257a 240
Basalt “ “ Andesite II “ “ Andesite I “ Dacite I Andesite I
0.81 1.44 0.98 1.39 2.53 3.03 4.65 1.17 2.16 2.91
2.36 4.32 2.87 4.15 6.77 8.41 25.84 3.49 10.97 13.37
0.2121 0.2018 0.2069 0.2024 0.2263 0.2175 0.1118 0.2024 0.1167 0.1314
0.513027 0.512826 0.512907 0.512416 0.513149 0.512958 0.511087 0.512832 0.511200 0.511454
+3.5 +1.8 +1.5 –6.5 –1.0 –1.5 +3.7 +2.9 +0.8 +1.1
Note: Nd and Sm isotope measurements were performed on a Finnigan MAT261 at the Institute of Precambrian Geology and Geochronol ogy, Russian Academy of Sciences, analysts E.S. Bogomolov and V.F. Guseva. DOKLADY EARTH SCIENCES
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Pb/235U Intercepts 0 ± 0 & 2778.4 ± 5.4 Ma MSWD = 0.75
0.65
3000 2800
0.55 2600 1
2400
0.45
0.62
Concordant age 2776.7 ± 9.5 Ma MSWD = 0.81
2200 0.35
3
0.58
2000
2900
5
0.54 4
2700 2
0.25
0.50 12.5
0.15
4
6
7 6
8
10
12
14
13.5
16
14.5
15.5
16.5
18
207Pb/235U
Fig. 2. Concordia plot for zircons from andesites. Numbers 1–7 are the item numbers in Table 2.
continuity of volcanic supracrustal rocks and the pre viously defined nature of the komatiite–tholeiite asso ciation [6], the results of this study can be interpreted in the context of geodynamic evolution of a mantle plume. This mechanism of juvenile crust formation agrees reasonably well with the concept [1] of a long lived Archean mantle plume (>200 Myr) and is there fore sufficient for explaining the andesite I data as being derived from melting of the peridotite mantle in a plume head enriched in the fluid components and 143Nd/144Nd
0.5134 0.5130
T = 2911 ± 29 Ma εNd = +1.7 MSWD = 1.03
the primary basaltic melts as being the melting prod ucts of the hottest axial zone of the plume. As was demonstrated by the example of the Neoarchean Kolmozero–Voron’ya greenstone belt, the formation of BAD associations within a single Neoarchean greenstone structure can be explained by a longlasting evolution of separate mantle or crustal sources not related to subduction processes. The inter mediate volcanics (andesite II) with abnormal isotopic and geochemical signatures found in the BAD associ ation can be explained by melting of older eclogites (>3.9 Ga) in the lower crust as a result of plume–litho sphere interaction. These basic eclogites can be inter preted as products of melts from the underplated mafic part of the lower crust, which are close in their compo sition to the amphibolites from the Voldozero block of the Fennoscandia shield [11] or the 4.28 Ga Nuvvuag ittuq mafic complex of the Canadian shield [12].
0.5126
ACKNOWLEDGMENTS
0.5122
This work was supported by the Russian Founda tion for Basic Research (project no. 070500570) and Program No. 6 of the Earth Science Division, Russian Academy of Sciences.
0.5118 0.5114
1 2
0.5110 0.5106 0.09
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0.13
0.15
0.17
0.19
0.21 0.23 Sm/144Nd
147
Fig. 3. Sm–Nd evolution diagram for basalts (1) and andes ites I (2) from the Polmos–Porosozero structure.
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