Petrology of the Early Mesozoic ultramafic-mafic Luchina massif (southeastern periphery of the Siberian craton)

Russian Geology and Geophysics 49 (2008) 570–581 www.elsevier.com/locate/rgg

Petrology of the Early Mesozoic ultramafic-mafic Luchina massif (southeastern periphery of the Siberian craton) I.V. Buchko a, A.A. Sorokin a, *, A.E. Izokh b, A.M. Larin c, A.B. Kotov c, E.B. Sal’nikova c, S.D. Velikoslavinskii c, A.P. Sorokin a, S.Z. Yakovleva c, Yu.V. Plotkina c a

Institute of Geology and Nature Use, Far East Branch of the RAS, 2 ul. Khmel’nitskogo, Blagoveshchensk, 675000, Russia b Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Koptyuga, Novosibirsk, 630090, Russia c Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, 2 nab. Makarova, St.Petersburg, 199034, Russia Received 18 April 2007; in revised form 15 October 2007; accepted 26 December 2007

Abstract Three zones of layered series — lower, middle, and upper — composed of dunites and plagiodunites, troctolites and olivine gabbros, gabbros and gabbronorites, respectively, have been recognized in the Luchina massif. The melt that produced the massif rocks was of picrite-basaltic composition (15–16% MgO), and its crystallization took place at 1300–1000 °C and ∼7 kbar. The Early Mesozoic age (248±1 Ma), geochemistry, and location of the massif on the northern periphery of the eastern segment of the Mongolo-Okhotsk fold belt suggest that it formed either in the hinterland of subduction zone dipping beneath the southern margin of the Dzhugdzhur-Stanovoy superterrane or on the periphery of the Siberian superplume during the mantle heating. © 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Layered massifs; primary melt; geochronology; U-Pb method; subduction; plume; Dzhugdzhur-Stanovoy superterrane

Introduction

Object of study

The western part of the Dzhugdzhur-Stanovoy superterrane localized on the southeastern periphery of the Siberian craton (Fig. 1) is composed mainly of ultramafic-mafic complexes, whose age, structural position, and origin are still debatable. This area abounds in ultramafic-mafic massifs of two formational types — dunite-troctolite-gabbro (Luchina) and wehrlite-gabbro (Il’deus, Utanak, Utugai, and Troitsk). Recent data have shown the Paleozoic age of many geologic complexes on the southern periphery of the North Asian craton that were earlier dated to the Precambrian (Buchko et al., 2005, 2007a,b,c; Larin et al., 2001, 2002, 2003, 2006; Sal’nikova et al., 2006). In this paper we discuss the results of geochronological (U-Pb zircon dating) and geochemical studies of the Luchina dunite-troctolite-gabbro massif.

The Luchina massif is localized in the Bryanta block (middle reaches of the Bryanta River) of the DzhugdzhurStanovoy superterrane (Fig. 1). It occurs within the Lower Archean rocks of the Utugei Formation — amphibole and biotite-amphibole schists and amphibole–two-pyroxene and biotite-hornblende gneisses. The Luchina massif is a typical dunite-troctolite-gabbro formation. It is oval and extended northwestward (21×12 km) (Fig. 2). Its layered series includes three zones: (1) lower (dunites, peridotites, and plagiodunites), (2) middle (troctolites alternating with olivine gabbros, gabbros, and pyroxenites), and (3) upper (olivine gabbros with rare troctolite and gabbronorite horizons) (Shcheka, 1963). The lower and middle zones were separated according to the appearance of cumulose plagioclase in the rocks. The upper zone differs from the middle one in higher Fe and Ti contents of rocks and a more ferruginous composition of rock-forming minerals. The vein complex of the massif consists of coarse-grained troctolites, pyroxenites, and gabbros. Peridotites in gabbros, gabbronorites, and troctolites form lenticular bodies of NE strike, which are either grade into these rocks (i.e., are part of the

* Corresponding author. E-mail address: [email protected] (A.A. Sorokin)

1068-7971/$ - see front matter D 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2007.12.008

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Fig. 1. Schematic occurrence of layered ultramafic-mafic massifs on the southeastern periphery of the Siberian craton. Compiled after Karsakov (1984), Martynov et al. (1990), and data from (Geological Map..., 1999). 1–4 — structures on the southern and southeastern peripheries of the Siberian craton: 1 — Selenga-Stanovoy superterrane; 2, 3 — Dzhugdzhur-Stanovoy superterrane: 2 — Dambuki, Ilikan, and Larba blocks, 3 — Bryanta block; 4 — Dzheltulak suture zone; 5 — Kerulen-Argun-Mamyn superterrane; 6 — Mongolo-Okhotsk fold belt; 7 — secondary dislocations; 8 — ultramafic-mafic massifs. Massifs: 1 — Lukinda, 2 — Kengurak, 3 — Mongoli, 4 — Veselyi, 5 — Nyukzha, 6 — Luchina, 7 — Il’deus. In the inset, the position of the Luchina massif is asterisked. Hatched area is the Mongolo-Okhotsk fold belt (MOFB).

layered series) or have sharp contacts (without interaction) with the enclosing rocks (i.e., belong to the vein complex).

Analytical methods The chemical composition of rocks was studied by the X-ray fluorescence method (main components, Sr, Zr, Nb) at the Institute of Geochemistry, Irkutsk, and by ICP MS (Ga, Ge, Rb, Cs, Sr, Ba, Pb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Th, U, Zr, Hf, Nb, Ta, Sc) at the Institute of Mineralogy and Geochemistry of Rare Elements, Moscow. Powdered sample for X-ray fluorescent analysis was homogenized through its fusion with LiBO2 in an incinerator at 1050–1100 °C. The measurements were carried out on a CPM-25 X-ray spectrometer. The intensities of analytical lines were corrected for the background, absorption, and secondary fluorescence. For ICP MS analysis, the samples were subjected to acid dilution in a MULTI-WAVE furnace. The measurements were performed on a Elan 6100 DRC mass spectrometer in the standard regime. The spectrometer sensitivity was calibrated throughout the mass scale against standard solutions containing all elements that were to be determined in the samples. The analysis accuracy was 3–10 rel.%. The U-Pb isotope dating of rocks was made at the Institute of the Precambrian Geology and Geochronology, St. Peters-

burg. Accessory zircon was extracted by the standard heavyliquid technique. Zircon decomposition and chemical extraction of Pb and U were performed following modified Krogh’s (1973) technique. The blank sample contained no more than 50 pg Pb. The isotopic compositions of Pb and U were determined on a Finnigan MAT 261 mass spectrometer in the static regime or with the use of an electron multiplier (discrimination coefficient for Pb was 0.32±0.11 amu). Experimental data were processed using the PbDAT and ISOPLOT computer programs (Ludwig, 1991a,b). The rock ages were calculated using conventional uranium decay constants (Steiger and Jager, 1976). Corrections for terrestrial lead were made in accordance with the model values (Stacey and Kramers, 1975). The technique for Sm-Nd isotope measurements was described by Neymark et al. (1993). The blank sample contained 0.03–0.2 ng Sm and 0.1–0.5 ng Nd. The measured 143 Nd/144Nd values were normalized to 146Nd/144Nd = 0.7219 and reduced to 143Nd/144Nd = 0.511860 of the La Jolla Nd standard sample. The accuracy of determination of Sm and Nd concentrations was 0.5% (2σ), and the accuracy of measurement of 147Sm/144Nd and 143Nd/144Nd was 0.5 and 0.005%, respectively. The average weighted 143Nd/144Nd value of the La Jolla Nd standard sample is 0.511894±0.000008 (2σ) (11 measurements). The εNd(0) values were calculated using the modern CHUR values obtained

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by Jacobsen and Wasserburg (1984) (143Nd/144Nd = 0.512638, 147 Sm/144Nd = 0.1967) and the DM values given by Goldstein and Jacobsen (1988) (143Nd/144Nd = 0.513151, 147Sm/144Nd = 0.2136). The composition of rock-forming minerals was determined on a Camebax-Micro probe at the Institute of Geology and Mineralogy, Novosibirsk.

Petrography of rocks The layered series of the Luchina massif includes a great number of rock varieties owing to the different proportions of rock-forming minerals — olivine, ortho- and clinopyroxenes, and plagioclase. The rock classification was made in accordance with the Petrographic Codex, though part of them is of cumulose texture. Plagiodunites are predominant rocks in the lower zone of the layered series, which grade into melanotroctolites. They are most widespread in the southern part of the massif (Fig. 2). These are dark-gray medium- and coarse-grained massive rocks with hypidiomorphic and corona microtextures. They are composed of olivine (70–90%), plagioclase (5–10%), orthopyroxene (5–10%), hornblende (up to 10%), and ore minerals (1%). Most rocks are of distinct cumulose textures. The cumulose paragenesis consists of olivine, and the intercumulose paragenesis, of augite with plagioclase admixture. The main minerals of plagiodunites are olivine (chrysolite, f = 18%) and orthopyroxene (Fig. 3, A, B). Olivine occurs as euhedral, often prismatic grains up to 2 mm in size. Orthopyroxene is present as fine prismatic crystals measuring 0.4–0.6 mm. The interstices between olivine grains are filled with 1–2 mm long intercumulose plagioclase segregations. At the contact of these minerals, there is a clinopyroxene rim about 0.2 mm thick. Secondary plagiodunite alterations gave rise to pargasite or green hornblende developed after orthopyroxene as well as serpentine–chrysolite assemblage and secondary magnetite in olivine cracks. Olivine gabbros and troctolites compose the middle and upper zones of the layered series. These are medium- and coarse-grained rocks with corona, gabbro-ophitic, and poikilitic textures consisting of plagioclase (50–85%), olivine (10– 35%), ortho- and clinopyroxene (5–20%), and minor amphibole and ore minerals. The femic minerals in these rocks are olivine (f = 29–32%), bronzite (f = 25–26%), and diopside (f = 13–27%), sometimes with poikilitic olivine inclusions. Orthopyroxene and clinopyroxene sometimes form thin serpentine symplectites (Fig. 3, C). Plagioclase is of two generations. The older labradorite-bytownite forms large, almost unaltered polysynthetically twinned crystals up to 3 mm in size. The younger plagioclase occurs as fine segregations about 0.2 mm in size. It resulted from secondary lower-temperature rock alterations accompanied by the development of magnesiohastingsite after clinopyroxene. The rocks permanently contain minor initially magmatic pargasite. At the olivine–plagioclase contact, there is often a reactionary enstatite or diopside rim up to 0.6 mm

thick. Ore minerals fill the interstices between rock-forming silicates; these are xenomorphous segregations of pleonaste, magnetite, pyrrhotite, and chalcopyrite. Gabbronorites are distinguished in the central part of the massif and are most widespread in the upper zone of the layered series. They are associated with veinlet and disseminated pyrrhotite-chalcopyrite mineralization. These are greenish-gray medium-grained massive or trachytoid rocks of gabbro, gabbro-ophitic, and poikilitic textures. They are composed of labradorite-andesine (40–60%), ortho- and clinopyroxene (20–50%), amphibole (up to 10%), and, sometimes, olivine (∼3%) (Fig. 3, D). Accessory minerals are pleonaste, apatite, magnetite, pyrrhotite, and chalcopyrite. The main rock-forming minerals of gabbronorites are orthopyroxene, olivine, and plagioclase. Orthopyroxene occurs as euhedral segregations 1–2 mm in size. Olivine is scarce and is present as euhedral crystals 0.5 to 1 mm in size. Labradorite fills the interstices between grains of femic minerals. It is polysynthetically twinned and almost unaltered. Ferrosilite rims (f = 17.8%) are developed between labradorite and orthopyroxene. The young minerals of gabbronorites are pargasitic hornblende developed after enstatite and serpentinechrysolite developed after olivine. Orthopyroxenites, websterites, and hornblende gabbros are present as thin (up to 20 m) dike-like bodies in the southwest of the massif (right bank of the Bryanta River). They are the latest phase of the Luchina massif rocks. Pyroxenites (Fig. 3, E, F) are composed of orthopyroxene (30–70%), clinopyroxene (25–65%), and hornblende (up to

Fig. 2. Schematic geological map of the Luchina massif. 1 — tentatively Late Archean schists and gneisses; 2–4 — rocks of the Luchina massif: 2 — lower layered series — dunites and plagiodunites; 3 — middle layered series — troctolites, olivine gabbros, and gabbros; 4 — upper layered series — troctolites, gabbros, and gabbronorites; 5 — faults; 6 — locality of sampling for geochronological studies and its number.

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Fig. 3. Photomicrographs of the main rock varieties of the Luchina massif; parallel nicols. A, B — Lower layered series — plagiodunites. Olivine is replaced by serpentine–secondary-magnetite assemblage, with clinopyroxene developed in its marginal zones (A); C, D — upper layered series: C — gabbros (orthopyroxene rim forms at the olivine–plagioclase contact. Orthopyroxene and clinopyroxene from symplectites), D — gabbronorite; E, F — vein complex: E — plagiopyroxenite, F — gabbro. The replacement of clinopyroxene and kaersutite by brown hornblende is clearly seen. Ol — olivine, Opx — orthopyroxene, Cpx — clinopyroxene, Pl — plagioclase, Amf — amphibole, Mt — magnetite, Spl — spinel, Serp — serpentine.

10%); also, minor olivine, biotite, apatite, plagioclase, and ore minerals are present. The main minerals of orthopyroxenites and websterites are olivine (hyalosiderite (f = 31–32)), bronzite-hypersthene (f = 29–30), and fassaite (f = 18%). Hornblende gabbros consist of labradorite-andesine (40–60%), clinopyroxene (30–40%), kaersutite (10–30%), and ore minerals (magnetite, pyrrhotite, and chalcopyrite) filling the interstices between silicates.

Based on the general change of cumulose parageneses in the intrusion section, we suggested the following sequence of parental-magma crystallization: Ol + Shp ⇒ Ol + Pl ⇒ Ol + Cpx + Pl ⇒ Cpx + Pl + Opx + Amf. The appearance of cumulose orthopyroxene in some rocks might be due to the presence of intratelluric phenocrysts in magma during its supply to a chamber. The same is evidenced by the veinedrock series with orthopyroxene as the main liquidus phase.

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Table 1 Chemical compositions of olivines from the Luchina massif rocks (wt.%) Component

1(3)

2(4)

3(2)

4(3)

5(2)

6(4)

SiO2 TiO2 FeO MnO MgO CaO Na2O K2O NiO Cr2O3

39.3 — 16.83 0.22 42.75 0.02 0.02 0.01 — —

40.00 0.01 17.43 0.23 42.45 0.01 0.02 — 0.04 —

37.60 0.01 27.84 0.40 33.27 0.01 0.05 0.01 0.03 —

37.39 0.01 27.32 0.41 33.55 0.02 0.01 0.02 0.05 —

38.00 — 26.60 0.38 35.43 0.01 0.04 0.01 0.07 —

37.58 0.01 27.78 0.45 34.15 0.02 0.02 0.01 0.01 —

Note. 1 — plagiodunite; 2 — pyroxenites; 3–5 — gabbro; 6 — olivine gabbro; parenthesized is the number of runs; dash means no data. Table 2 Chemical compositions of orthopyroxenes from the Luchina massif rocks (wt.%) Component

1(2)

2

3(3)

4(2)

5(4)

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O NiO Cr2O3

53.27 0.13 3.42 15.12 0.41 24.22 3.18 0.20 0.01 — 0.07

54.86 0.11 2.56 11.86 0.26 30.03 0.44 0.03 0.01 — 0.04

54.15 0.10 2.72 15.97 0.36 26.32 0.60 0.05 0.01 0.01 —

53.60 0.01 3.24 15.85 0.28 26.39 0.23 0.01 — — 0.01

51.98 0.03 4.02 16.88 0.45 25.96 0.20 0.02 0.01 — 0.14

Note. 1 — pyroxenites; 2–4 — gabbro; 5 — gabbronorite. Table 3 Chemical compositions of clinopyroxenes from the Luchina massif rocks (wt.%) Component

1(2)

2(2)

3(2)

4(6)

5

6

7(2)

8(2)

9(3)

10(6)

11(2)

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O Cr2O3

52.14 0.58 4.73 3.90 0.06 14.67 21.32 1.21 0.02 0.42

51.49 0.92 4.78 2.96 0.08 15.08 21.51 1.22 0.01 0.67

51.38 0.77 3.86 6.15 0.18 14.31 21.94 0.81 0.01 0.14

51.85 0.70 4.08 6.00 0.17 14.23 21.35 1.03 0.02 0.22

52.43 0.74 4.28 3.98 0.13 14.59 22.11 0.88 0.01 —

52.59 0.49 3.61 5.86 0.19 14.28 22.24 0.79 — —

51.65 0.83 4.12 6.13 0.19 13.87 21.90 0.82 0.01 —

50.86 0.56 3.87 7.59 0.28 14.14 21.16 0.85 — 0.037

51.08 0.88 3.92 8.78 0.40 13.26 19.51 1.07 — 0.01

52.20 0.34 3.66 5.70 0.16 14.72 21.32 0.70 0.01 0.26

51.47 0.41 3.20 8.69 0.26 14.15 19.35 0.77 0.01 0.01

Note. 1, 2 — plagiodunites; 3 — pyroxenites; 4–9 — gabbro; 10 — olivine gabbro; 11 — gabbronorite.

Composition of the main rock-forming minerals Tables 1–3 present the compositions of rock-forming minerals revealed in the Luchina massif rocks. Olivine is a persistent mineral in all zones of the layered series of the Luchina massif. In dunites and plagiodunites of the lower zone it is the first to appear on the liquidus and is represented by chrysolite (f = 18%). In contrast to ultrabasites,

olivine gabbros and troctolites of the middle and upper zones are similar in olivine composition to hyalosiderite (f = 30–32%). Taking into account that the f value of the liquidus olivine increases under the interaction of the mineral with intercumulose melt, we assume that the early olivine was more magnesian. Anyway, this f value of olivine suggests that the primary melt was of picrite-basaltic composition.

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Orthopyroxene is the second most abundant dark-colored mineral of the massif. It is most widespread in pyroxenites and ultrabasites. In dunites and plagiodunites of the lower zone it compositionally corresponds to hypersthene. Olivine gabbros and troctolites of the middle zone contain bronzite (f = 18%), and gabbroids of the upper zone, hypersthene (f = 25–26%). The higher f values of orthopyroxene in the lower part of the section are due to the fact that it is an intercumulose phase here, which crystallized from residual melt, whereas in gabbronorites it forms a cumulose paragenesis. Orthopyroxene has low contents of Ti and Cr but high contents of Al2O3 (up to 2.5–4%) (Table 2), which suggests a high pressure during the crystallization in magma chamber. Some samples are enriched in CaO (up to 3.2%). It is not ruled out that the rock bears low-Ca pigeonite. Clinopyroxene, like olivine, is permanently present in the layered-series rocks. In chemical composition clinopyroxenes from dunites and plagiodunites correspond to diopsides (f = 10–13%). They have high contents of Cr (up to 0.7%) and Na (1.2%) (Table 3) and the maximum contents of Al2O3 (up to 4.8%). This composition is accounted for by the clinopyroxene crystallization from residual melt. Clinopyroxene of gabbroids of the medium zone has higher f values (f = 13–23%) (Table 3). Pyroxene still richer in iron is observed in the upper zone (f = 26%), but it is poor in Cr, Ti, and Na.

Petrochemistry and geochemistry Major elements. Petrochemical data evidence that the Luchina massif is of well-differentiated peridotite-troctolite-

575

gabbro type. On the TAS composition diagram, dunites and plagiodunites fall in the field of normal ultrabasic rocks. Pyroxenites and gabbroids of the middle zone of the massif correspond to normal basic rocks, whereas most of gabbroids of the upper zone fall in the field of subalkalic gabbroids. The content of K in some gabbroids reaches 1%. This is atypical of the CAFB island-arc peridotite-troctolite-gabbro intrusions (Izokh et al., 1998). On the contrary, high contents of K are specific for basites of different formation types related to Permo-Triassic superplume (Borisenko et al., 2006). Gabbroids of the massif are poor in TiO2 (0.5%), though the upper part of the massif contains their high-Ti varieties. The latter are highly ferruginous (f = 50–55%), which is typical of late differentiates. The variation diagrams show distinct composition trends associated with olivine and plagioclase differentiation during the crystallization of primary picrite-basaltic melt (Fig. 4). The most magnesian rocks contain up to 33% MgO and up to 3.5% CaO. Peridotites of the lower zone and troctolites of the middle zone show a distinct olivine-plagioclase differentiation trend ended with gabbroids. There is a drastic gap between the compositions of peridotites and melanotroctolites and the compositions of gabbroids. This might be due to the incomplete sampling or flotation of late plagioclase. Peridotites and early gabbroids show no correlations between the contents of TiO2 and alkalies as the f value (degree of fractionation) increases. Ferruginous gabbroids (f > 35%) are, on the contrary, characterized by a positive correlation between the f value and TiO2 and K2O contents. In general, this pattern corresponds to Fenner’s differentiation trend, which does not

Fig. 4. Petrochemistry of the Luchina massif rocks. 1–3 — compositions of rocks: 1 — dunites and plagiodunites of the lower layered series, 2 — pyroxenites, 3 — troctolites, olivine gabbros, gabbros, and gabbronorites of the middle and upper layered series; 4 — composition of primary melt calculated by Ariskin’s technique (Ariskin and Barmin, 2000); 5 — evolution trend of primary melt at 4 kbar, QFM buffer.

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Fig. 6. Trace-element patterns of the Luchina massif rocks. Designations follow Fig. 4. Primitive-mantle-normalized after Sun and McDonough (1989). Fig. 5. REE patterns of the Luchina massif rocks. 1 — plagiodunites of the lower layered series, 2 — troctolites, olivine gabbros, and gabbronorites of the middle and upper layered series; 3 — gabbronorite; 4 — gabbros of the vein series; 5 — pyroxenites. Chondrite composition taken after McDonough and Sun (1995).

lead to an increase in the SiO2 content of rocks and the accumulation of Fe in residual melt. Rare-earth and trace elements. Plagiodunites of the lower zone of the layered series are characterized by the minimum contents of REE and their weakly differentiated patterns ((La/Yb)n = 1.65–3.8). The presence of a positive Eu anomaly (Eu/Eu* = 1.91–1.97) is due to the presence of intercumulose plagioclase (Fig. 5). Gabbroids of the middle and upper zones of the layered series have higher contents of REE, with their more differentiated patterns ((La/Yb)n = 2.8–9.07). Gabbroids of all layered series show a distinct Eu maximum (Eu/Eu* = 1.71–2.87). Pyroxenites of the dike complex are characterized by extremely high contents of REE, their weakly differentiated patterns ((La/Yb)n = 3.1), and negative Eu anomaly (Eu/Eu* = 0.34) (Fig. 4). Gabbros of the same series are enriched in LREE relative to HREE ((La/Yb)n = 9.1) and show no Eu anomaly (Fig. 5). The REE contents of the Luchina massif gabbroids are significantly lower than those of within-plate basites and are close to the REE contents of island-arc basalts. The only exclusion is rocks of the dike complex, whose REE contents approach those in OIB, including basalts of continental rift zones. In contrast to the latter, they are enriched in HREE (Fig. 5), obviously due to the presence of clinopyroxene or garnet in the source. The spidergrams of the Luchina massif rocks have Ba, Sr, and Eu maxima and Ta, Nb, Zr, and Hf minima (Fig. 6). These geochemical features are specific for magmas generated from the suprasubductional mantle. At the same time, the low contents of Rb, U, and Th are atypical of suprasubductional ultrabasite-basite associations. The conform trace-element patterns and regular increase in the contents of most elements as their basicity is reduced (Figs. 5 and 6) suggest that the ultrabasites and basites of the lower, middle, and upper series of the Luchina massif are differentiates of the same magmatic melt. In contents of trace elements the Luchina rocks are most similar to subductional basites (Izokh et al., 1998), differing

from them in having much higher contents of REE, particularly, HREE (Table 4). The rocks under study are characterized by ancient model ages TNd(DM) = 2.0 Ga (TNd(DM-2st) = 1.8 Ga), εNd(T) = −9.6, and ISr = 0.70453 (Table 5). These isotope data also agree with the generation of primary melt from the depleted subductional mantle.

Model computations of primary-melt crystallization The compositions of primary melts of gabbroid massifs are usually estimated either from its chilled facies or by calculation of their weighted average composition (Ariskin and Barmina, 2000; Sharkov, 1983), or by geochemical thermometry by Ariskin’s technique (Ariskin and Barmina, 2000; Ariskin et al., 1993). We did not detect chilled facies in the Luchina intrusion; therefore, the composition of primary melt was determined from the weighted average composition with correction for the f values of the most magnesian olivine. Geochemical thermometry permits estimation of the composition of rocks crystallized from the same cotectics, which is difficult in our case. To estimate the composition of parental magma, we used the weighted average composition of the layered series of the massif (wt.%) corresponding to picrite basalt (SiO2 = 46.05; TiO2 = 0.81; Al2O3 = 13.25; FeO = 10.64; MgO = 15.65; CaO = 8.61; Na2O = 1.74; K2O = 0.30; P2O5 = 0.14) (Buchko, 2005). The temperature of formation of different rock groups of the Luchina massif and oxygen fugacity were computed by the Petrolog program (Danyushevsky, 1998), using data on the contents of major oxides. The formation temperature of plagiodunites is estimated at 1365–1398 °C, fO2 = 6.3−6.6, and the formation temperature of gabbroids and troctolites is 1369–1380 °C, fO2 = 6.5−6.6. Slightly overestimated values were obtained for pyroxenites: 1400–1500 °C, fO2 = 5.0−6.3. The oxygen fugacity values are similar for all types of rocks and correspond to the QFM buffer (quartz–fayalite–magnetite). The crystallization pressure (depth of formation) of different rock groups was estimated by the Nimis (2000) geobarometer based on a change in Al2O3 (AlVI) contents of

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I.V. Buchko et al. / Russian Geology and Geophysics 49 (2008) 570–581 Table 4 Chemical composition of representative rock samples from the Luchina massif Component

1

2

3

Lower series SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Cs Rb Sr Ba Ga La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th U Zr Hf Nb Ta Zn Cu Co Ni Sc V Cr

40.16 0.41 6.57 11.80 0.21 28.62 3.64 0.93 0.08 0.04 6.54 99.0 0.02 1 159 69 5 0.95 2.45 0.37 1.93 0.53 0.36 0.62 0.10 0.66 0.14 0.37 0.06 0.41 0.07 3.54 0.02 <0.01 10.0 0.3 0.07 0.01 48 23 86 358 10 — 998

4

5

Middle series 42.87 0.49 9.38 10.99 0.19 24.47 5.89 0.30 0.05 0.03 4.61 99.27 0.20 1 35 139 7 1.60 2.66 0.32 1.38 0.40 0.29 0.50 0.08 0.49 0.10 0.35 0.04 0.30 0.03 2.82 0.02 0.01 5.2 0.2 0.11 0.02 94 166 91 633 12 — 1371

44.59 1.40 9.47 15.81 0.23 14.99 8.85 1.66 0.60 0.38 1.29 99.32 — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —

6

7

8

9

Upper series 46.19 3.33 1.71 18.54 0.28 17.37 10.20 0.30 0.03 0.04 0.78 98.77 — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —

46.32 0.39 14.88 10.35 0.13 10.16 12.13 2.31 0.16 0.03 2.66 99.52 0.05 1 587 158 12 2.34 5.88 0.90 4.54 1.20 0.67 1.39 0.25 1.30 0.25 0.74 0.09 0.59 0.09 6.39 0.03 <0.01 11.4 0.6 0.11 0.01 23 780 77 339 31 139 347

10

Vein series 46.34 0.48 17.13 10.76 0.17 13.67 8.40 2.04 0.19 0.03 0.32 99.53 0.05 2 912 299 13 3.49 7.05 0.88 3.73 0.76 0.71 0.73 0.12 0.64 0.13 0.33 0.04 0.33 0.04 3.07 0.07 0.02 9.9 0.3 0.43 0.05 43 70 55 220 10 115 110

47.62 0.25 19.52 8.37 0.12 10.06 10.90 1.63 0.25 0.04 0.91 99.67 0.02 1 791 197 12 3.22 6.76 0.99 4.86 1.13 0.66 1.09 0.20 1.21 0.24 0.50 0.07 0.42 0.06 5.14 0.06 0.02 12.2 0.4 0.36 0.04 37 74 43 82 23 97 289

47.18 0.33 20.30 8.01 0.10 8.11 11.86 2.35 0.32 0.11 1.05 99.72 0.16 3 1060 221 15 4.63 10.36 1.37 6.33 1.51 0.77 1.13 0.16 0.96 0.21 0.54 0.07 0.51 0.07 5.05 0.18 0.05 45.0 0.5 0.70 0.05 49 31 45 74 11 24 279

47.41 0.60 5.77 13.57 0.28 22.48 6.23 0.95 0.49 0.16 0.78 98.72 0.53 14 121 162 6 22.15 63.47 9.91 43.71 10.24 1.12 9.41 1.66 9.13 1.96 5.56 0.75 5.10 0.73 46.9 0.68 0.17 33.4 1.2 3.71 0.21 160 70 91 378 29 94 80

46.98 0.84 17.55 9.98 0.15 7.99 11.40 3.00 0.50 0.19 1.05 99.63 2.93 17 570 377 13 17.64 40.08 5.42 21.42 4.19 1.33 3.69 0.55 3.18 0.56 1.63 0.21 1.39 2.57 15.2 1.01 0.23 43.8 1.1 3.7 0.24 73 1078 81 161 27 160 129

Note. 1, 2 — plagiodunites; 3–7 — olivine gabbros, troctolites, and gabbros; 8 — gabbronorite; 9 — pyroxenites; 10 — gabbros. The contents of oxides are given in wt.%, and those of elements, in ppm. Dash means no data. Table 5 Isotopic compositions of Sr and Nd in gabbronorite from the Luchina massif (sample I-352) Age, Ma Sm, ppm

Nd, ppm

147

143

250

6.16

0.1292

0.512036 ± 6

1.32

Sm/144Nd

Nd /144Nd* εNd(T)

Note. Model age is given in Ma. * The 2σ error corresponds to the last significant figure.

-9,6

TNd(DM) TNd(DM-2st) Rb, ppm

Sr, ppm

87

87

ISr

1997

1224

0.0058

0.70456

0.70453

1826

2.45

Rb/86Sr

Sr/86Sr

578

I.V. Buchko et al. / Russian Geology and Geophysics 49 (2008) 570–581

Fig. 7. Correlation between the pressure (P) calculated by the Nimis (2000) barometer and AlVI contents in clinopyroxenes from the Luchina massif. Designations follow Fig. 4.

diopsides (Fig. 7). We established the following crystallization pressures: dunites and plagiodunites — 7.6–7.9 kbar, gabbroids, troctolites, and olivine gabbros — 3.8–4.0 kbar, pyroxenites — 2.7–3 kbar, and gabbronorites — 1.8–2.0 kbar. High crystallization pressures are also evidenced from the high alumina contents of orthopyroxene (up to 4.5%), even in the rims on olivine, which crystallized in situ. Modeling by the Pluton program (Lavrenchuk, 2004) was performed at 1 to 9 kbar and the oxygen activity corresponding to the QFM buffer, with a water component being ignored. According to the computation results, the crystallization sequence in this pressure range is close to that in the massif; the compositions of cumulates and rock-forming minerals change insignificantly. The computation shows that at low pressures, it is pigeonite that must crystallize, whereas orthopyroxene is either absent or appears only at the end of crystallization. At pressures above 7 kbar, pigeonite and orthopyroxene cocrystallize, with the latter being predominant. Thus, the optimal agreement between the actual and experimental compositions of rocks of the lower, middle, and upper zones of the layered series is observed at 6–7 kbar and oxygen fugacity corresponding to the QFM buffer.

Results of U-Pb geochronological studies We carried out U-Pb geochronological studies of gabbronorite of the upper layered series (sample I-352) taken in the central part of the massif (Fig. 2). Accessory zircon extracted from the sample is transparent subeuhedral prismatic and oval crystals and their fragments (Fig. 8). It has a roughly

zonal and sector internal structure. The grains are 50 to 300 μm in size; elongation coefficient Kelong = 1.0–2.5. For U-Pb isotope studies, we used three zircon samples of the >150 and >100 μm fractions. Two samples (2 and 3 in Table 6) were pretreated with acid (Mattinson, 1994). As seen from Fig. 8, the untreated zircon and the zircon residue after the 3 h acid treatment have a concordia age of 248±1 Ma (MSWD = 0.2, probability = 0.7). The zircon that was subjected to a longer acid treatment is characterized by an older 207Pb/206Pb age, which suggests the presence of a minor inherited ancient component. The lower intersection of the discordia calculated for the three zircon fractions corresponds to an age of 248±1 Ma (the upper intersection corresponds to 3081±3300 Ma; MSWD = 1.7) (Fig. 9). The morphology of zircon points to its magmatic origin. Hence, the obtained age of 248±1 Ma can be accepted as the most precisely estimated age of the crystallization of primary melt of the Luchina massif.

Discussion Three zones are recognized in the layered series of the Luchina massif on the basis of geological, mineralogical, and geochemical data: lower (dunites and peridotites), middle (mainly troctolites), and upper (mainly olivine gabbros with troctolite and gabbronorite horizons). These rocks are broken through by a vein complex of coarse-grained troctolites, gabbronorites, and pyroxenites. The regular change in the chemical compositions of rocks of layered series during melt crystallization is reflected on the composition and proportions of the hosted minerals: (1) the f values of femic minerals increase during this process (from 12 to 34% for olivine, from 25.2 to 30.6% for orthopyroxene, and from 28.5 to 31.2% for clinopyroxene), and (2) the basicity of cumulose plagioclase decreases (from An71 to An48). Similar regularities of changes in major-oxide contents have been established for the massif rocks. The composition points of dunites, plagiodunites, troctolites, olivine gabbros, gabbros, and gabbronorites form common trends on all binary petrochemical diagrams (MgO–SiO2, TiO2, Al2O3, FeO, and CaO) (Fig. 4). The only exclusion is pyroxenites, which

Fig. 8. Photomicrographs of zircon from the Luchina massif gabbronorite (sample I-352) taken on: A — ABT 55 scanning electron microscope (secondary-ion regime), B — CamScan scanning electron microscope (cathodoluminescence regime).

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I.V. Buchko et al. / Russian Geology and Geophysics 49 (2008) 570–581 Table 6 Results of U-Pb isotope studies of zircons from the Luchina massif gabbronorite (sample I-352) No.

Fraction Charge, Content, (μm) and its mg ppm characteristics Pb U

Isotope ratios 206

Pb/204Pb

Rho

207

Pb/206Pb

208

Pb/206Pb

207

Pb/235U

206

Pb/238U

Age, Ma 207

Pb/235U

206

Pb/238U

207

Pb/206Pb

1

>150, >100

4.02

0.82

0.0512 ± 1

0.3124 ± 1

0.2772 ± 11 0.0393 ± 1

0.74

248 ± 1

248 ± 1

250 ± 6

2

>150, 3 h acid treatm.

—

U/Pb*=21.7 2151

0.0512 ± 1

0.2980 ± 1

0.2756 ± 8

0.0391 ± 1

0.52

247 ± 1

247 ± 1

248 ± 6

3

>100, 3.5 h acid treatm.

—

U/Pb*=21.0 974

0.0517 ± 1

0.2896 ± 1

0.2794 ± 6

0.0392 ± 1

0.81

250 ± 1

248 ± 1

271 ± 2

1.70

896

Note. a — Isotope ratios corrected for the blank sample and terrestrial lead; acid treatm. — zircon residue after acid treatment. All errors are at the 2σ level. Rho — correlation coefficient. * Zircon charge was not determined.

formed, most likely, from residual melts at the final stage of the intrusion formation. They have minimum contents of SiO2 (47.41%) and FeO (13.57%). The composition points of the massif rocks lie near the olivine-plagioclase trend (MgO– Al2O3 diagram) (Fig. 4), which is due to the low Mg/(Mg + Fe) and high Al/(Al + Ca) values of the crystallized magmatic melt. The REE patterns of rocks from different layered series, namely, the obvious predominance of LREE ((La/Yb)n = 2.8–74.3) and a distinct Eu maximum (Fig. 5), suggest that ultrabasites and basites of the lower, middle, and upper zones of the Luchina massif are differentiates of the same picritebasaltic melt. The strong LREE enrichment of the rocks is apparently related to the melting of suprasubductional lithospheric mantle. In this case, the enrichment is the result of mantle-wedge metasomatism by hydrous high-K fluids rich in LILE and poor in HFSE, which were separated during the dehydration of subducting oceanic lithosphere (Balashov, 1976; Cox et al., 1979). It is not ruled out that the HREE

Fig. 9. Diagram with concordia for zircons from the Luchina massif gabbronorite (sample I-352). Point numbers follow Table 6.

depletion was due to the presence of garnet in the mantle source during the generation of primary melt (Cox et al., 1979). The depletion of the mantle source (suprasubductional lithospheric mantle) is established from Nd isotope data (εNd(T) = −9.6) (Table 5). This εNd value cannot be due to the crustal contamination of astenospheric- or plume-mantle melts, otherwise the changes in the chemical compositions of rocks would be much greater. Based on the estimated age of the Luchina massif (248±1 Ma), we have recognized a Permo-Triassic stage of ultramafic-mafic magmatism in the geologic evolution of the Dzhugdzhur-Stanovoy superterrane on the southeastern periphery of the Siberian craton. This stage was synchronous with the stage of superplume-related basic magmatism of the Siberian craton (Borisenko et al., 2006; Dobretsov, 1997; Yarmolyuk et al., 2000). Therefore, it seems reasonable that the Luchina ultramafic-mafic massif formed in a Siberian superplume-related within-plate setting, where such plutons are typical. Permo-Triassic peridotite-troctolite-gabbro intrusions were revealed within the Selenge volcanoplutonic belt in Mongolia (Borisenko et al., 2006). Among them, there is the Nomgon troctolite-anorthosite-gabbro intrusion (256±21 Ma) bearing low-sulfide Cu-Pd-Pt mineralization. A belt of layered massifs of this type is traceable in the eastern direction along the Mongolo-Okhotsk suture up to the Zharcha massif in Transbaikalia (Izokh et al., 1998) and, possibly, father eastward. On geological schemes and maps, these massifs are dated either at the Early Paleozoic or at the Mesozoic. It is not ruled out that the above belt includes massifs of the same type but of different ages, which reflect the history of subduction processes of the Mongolo-Okhotsk paleo-ocean. The relationship of these massifs with within-plate formations seems to be inconsistent with the results of geochemical studies. Nevertheless, layered massifs with suprasubductional geochemical characteristics related to the Tarim plume were earlier revealed in the Central Asian Fold Belt (Cambrian-Ordovician massifs (Izokh et al., 2005) and Early Permian picrite-dolerite massifs (Chunming Han et al., 2007)). These massifs resulted from the melting of ancient suprasubductional lithospheric mantle under the effect of the plume heat. Note that layered ultramafic-mafic massifs are specific not only for within-plate areas. There are known basic-ultrabasic

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massifs, including layered ones, that formed in subduction setting, e.g., dunite-clinopyroxenite-gabbro massifs of the Olyutor zone (Koryak upland), which are of the same type as zonal concentric basic-ultrabasic massifs of the Ural-Alaska type (Ledneva, 2000; Ledneva et al., 2000). Though the tectonic setting of formation of these massifs is still debatable (from oceanic arcs at their early stage of evolution to back-arc basin), most researchers relate their formation to subduction processes. The Seinav layered dunite-clinopyroxenite-gabbro massif (Koryakia) is a typical and well-studied massif of this type (Ledneva, 2000; Ledneva et al., 2000). The trace-element spidergrams of rocks of the Luchina and Seinav massifs (Fig. 6) show that they are very close, though not identical, in geochemical composition. This is an additional argument for the formation of the Luchina massif in subduction setting. Note that the Dzhugdzhur-Stanovoy superterrane abounds in metadiorites of the Tok-Algoma complex, which are nearly coeval with the Luchina massif (238±2 Ma) and formed in subduction setting (Sal’nikova et al., 2006). Thus, it is not ruled out that these igneous rocks resulted from the same subduction process at different stages of its evolution. Moreover, one of the stages of formation of the Mongolo-Okhotsk Fold Belt, on the northern periphery of which the Luchina massif is localized, proceeded in the Mesozoic (Parfenov et al., 1999, 2003). This suggests that the Luchina massif formed at the rear of the subduction zone dipping beneath the southern margin of the Dzhugdzhur-Stanovoy superterrane and related to one of the stages of the closure of the Mongolo-Okhotsk paleo-ocean. The results of geochronological studies show that subduction took place on the background of the Siberian superplume activity. We should not rule out the possibility of formation of the Luchina massif in within-plate setting related to the Siberian superplume provided that the lithospheric mantle was the source of “subduction component”.

Conclusions Three zones of layered series — lower, middle, and upper — composed of dunites and plagiodunites, troctolites and olivine gabbros, gabbros and gabbronorites, respectively, have been recognized in the Luchina massif. Pyroxenites cut rocks of layered series; they were produced from melt at the final stages of the intrusion formation. During the crystallization of magmatic melt, the rock-forming minerals and rocks show regular changes in the chemical composition: Their SiO2, TiO2, Al2O3, CaO, and FeO contents increase as the MgO content decreases. Genetic relationship between different zones of the layered series and a high degree of primary-melt fractionation have been established. In composition the primary melt of the Luchina massif was close to picrite basalt, which crystallized at 1350–1000 °C and 6–7 kbar. The K-enriched suprasubductional lithospheric mantle was the main source of the initial massif magmas.

The Early Mesozoic age (248±1 Ma) and localization of the massif on the northern periphery of the eastern segment of the Mongolo-Okhotsk fold belt suggest that it formed either in the hinterland of subduction zone (originated at some stage of the closure of the Mongolo-Okhotsk paleo-ocean) dipping beneath the southern margin of the Dzhugdzhur-Stanovoy superterrane or in within-plate settings related to the Siberian superplume. We thank D.Z. Zhuravlev (Institute of Mineralogy and Geochemistry of Rare Elements, Moscow), V.V. Egorova and L.N. Pospelova (Institute of Geology and Mineralogy, Novosibirsk), and A.L. Finkel’shtein (Institute of Geochemistry, Irkutsk) for help with analyses. This work was supported by grants 04-05-64810, 05-0565347, 05-05-65128, 06-05-64989, and 07-05-00825 from the Russian Foundation for Basic Research, the “Central Asian Mobile Belt: geodynamics and stages of formation of the Earth’s crust” and “Isotope systems and isotope fractionation in natural processes” basic-research programs of the Geoscience Department of the RAS, and grants 06-I-ONZ-115 and 06-II-SO-08-034 from the Presidium of the Far East Branch of the RAS.

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