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GEODYNAMICS & TECTONOPHYSICS
PUBLISHED BY THE INSTITUTE OF THE EARTH'S CRUST SIBERIAN BRANCH OF RUSSIAN ACADEMY OF SCIENCES
2017 VOLUME 8 ISSUE 3 PAGES 411-415 ISSN 2078-502X
https://doi.org/10.5800/GT-2017-8-3-0250
Proceedings of the Second Russia-China International Meeting on the Central Asian Orogenic Belt (September 6-12, 2017, Irkutsk, Russia)
Geochemistry and origin of the Eastern Sayan ophiolites, Tuva-Mongolian microcontinent (Southern Siberia)
V. A. Belyaev1, 2, K.-L. Wang1, 3, M. A. Gornova2, S. I. Dril'2, A. A. Karimov2, A. Ya. Medvedev2, Yu. V. Noskova2
1 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
2 A.P. Vinogradov Institute of Geochemistry, Siberian Branch of RAS, Irkutsk, Russia
3 Department of Geosciences, National Taiwan University, Taipei, Taiwan
For citation: Belyaev V.A., Wang K.-L., Gornova M.A., Dril'S.I., KarimovA.A., MedvedevA.Ya., Noskova Yu.V., 2017. Geochemistry and origin of the Eastern Sayan ophiolites, Tuva-Mongolian microcontinent (Southern Siberia). Geodyna-mics & Tectonophysics 8 (3), 411-415. doi:10.5800/GT-2017-8-3-0250.
The Eastern Sayan ophiolites (1020 Ma) of the Tuva-Mongolian microcontinent are believed to be the most ancient ophiolite of the Central Asian Orogenic Belt [Khain et al., 2002]. The ophiolites obducted as a single nappe onto the Gargan block composed of Early Precambrian gneisses and Meso-Neoproterozoic sediments. The Eastern Sayan ophiolites form two belts surrounding Gargan block, with Dunzhugur massif in northern belt, and Ilchir massif in southern belt, which are the subject of this work. Previous studies reported boninites and boninite-series andesites, peridotite-pyroxenite-gabbroic rocks and mantle harzburgites and argued for the supra-subduction zone setting and formation of the ophiolites in an incipient Duznhugur island arc [Dobretsov et al., 1985; Khain et al., 2002; Kuzmichev, 2004; Sklyarov et al., 2016]. In this study,
elemental and Nd isotope geochemistry of lavas and dikes in the Dunzhugur and Ilchir massifs are investigated to constrain the origin of the Eastern Sayan ophi-olites.
Geochemistry and geodynamic implications. The
whole-rock geochemistry is summarized on Fig. 1. The studied lavas and dikes from the Dunzhugur massif are low-TiO2 (<0.5 %) varying from boninites (MgO > 8 %) to boninite-series andesites (MgO < 8 %) and subordinate dacites. One sample from dike complex is tholeiite (0.6 % TiO2). Metamorphosed lavas from the Ilchir massif are boninites to andesites resembling Dunzhugur rocks, as well as tholeiite and basaltic andesites with elevated TiO2 (0.85-1.30 %). The Dunzhugur and Ilchir boninite-series rocks are variously LREE-and Th-enriched with Nb and Ti depletions typical of
MgO, wt.%
A
20
15
10
100 -,
10 -
10
0.1
0.01
Boninites
o ^
o Ao A0
fcSTg o
00
-r~
50
-r~
55
-r~
60
Rock / Chondrite
-1—i—i—i—i—i—i—i—i—i—i—i—i—i
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Th/Yb
Continental arcs
Troodos lavas
OIB
I I INI
0.1
I I INI
1
TT1—
10
TT
TiO2, wt.%
B
-1-1-1
65 70
SiO2, wt.%
100 -,
10 -
1 -
0.1 -
Rock / N-MORB
£
r
15
20
MgO, wt.%
D
Nb Ce Pr Nd Zr Eu Gd Y Yb
-1—I—I—I—
-1—I—I—I—I—I—I—I—I—I—I—I
Th La Pb Sr Sm Hf Ti Dy Er Lu
500
400
Papua New 300 E-MORB Guinea ■ boninites
, D • 200 Izu-Bonin-
forearc
boninites
iN-MORB and related rocks 100
T1
100
Nb/Yb
1
15
Ti / 1000, ppm
Fig. 1. Geochemical characteristics of volcanics and dikes from the Eastern Sayan ophiolites.
A, B - variations of SiÜ2, MgO, TiÜ2 contents. Field of boninite composition is after [Le Bas, 2000], border line between boninite and basalt-andesite-dacite-rhyolite (BADR) series is after [Pearce, Robinson, 2010]. C, D - chondrite normalized REE and N-MORB-normalized trace element patterns. Normalization values are from [McDonough, Sun, 1995]. E - Nb/Yb - Th/Yb diagram [Pearce, 2008, 2014]. Composition of Troodos lavas [Pearce, 2014], Izu-Bonin forearc boninites and related rocks [Pearce et al., 1992], and Papua New Guinea boninites [König et al., 2010] is shown for comparison. F - Ti - V diagram, [Shervais, 1982] with field subdivision of [Pearce, 2014]. Previously reported data for the Dunzhugur lavas and dikes [Dobretsov et al., 1985; Khain et al., 2002; Kuzmichev, 2004; Sklyarov et al., 2016] are shown in A, B, E.
5
0
0
6 -, 4 -2 -0 --2 _ -4 _
£Nd
aa *
O Q O
° X ¿o
o
O
6 £Nd
0
~i-1-1-1-1-1-1-1
12 3 4
& *
o *
O
6 £Nd
~i—I—I—I—I—I—I—I—I—I
0.5 1 1.5 2 2.5
4 -2 -0 --2 _ -4
O Dunzhugur lavas % Dunzhugur dikes A Ilchir lavas n Dunzhugur dikes [Sklyarov et al., 2016]
O
m X O
O
0
~i-1-1-1-1-1-1-1
2 4 6 8
Zr/Ti (PM)
Th/Yb
La/Yb (PM)
Fig. 2. Correlation of sNd (calculated at 1020 Ma) with selected trace element ratios. Zr/Ti and La/Yb ratios are normalized to primitive mantle [McDonough, Sun, 1995]. Compositions of the Dunzhugur dikes reported by [Sklyarov et al., 2016] are shown.
0
subduction-related magmas. Tholeiites from Dunzhugur and Ilchir have higher REE abundances without LREE enrichment, show negative Nb anomaly and lack of Ti depletion. The HREE contents of all studied samples are lower than in N-MORB, indicating derivation from the source more depleted than MORB mantle. Relative enrichment in LREE-Th can be explained as the mantle source metasomatised by fluids and/or melts from the subducting slab. On Nb/Yb vs. Th/Yb diagram [Pearce, 2008, 2014], tholeiites plot within oceanic arc, whereas boninites and andesites locate within oceanic to continental arc fields. On Ti-V plot [Shervais, 1982], tholeiites are classified as island arc tholeiites, and boninites to andesites plot within the boninite field. Nd isotope analysis revealed variations in sNd(1020) from -2.8 to +2.7 in Dunzhugur boninites and andesites, -0.2 to +0.4 in Ilchir boninites and andesites, and +2.3 to +4.5 in tholeiites from Ilchir and Dunzhugur massifs. The range of sNd(1020) from -1.0 to + 1.5 was previously reported for the Dunzhugur andesite and boninite lavas, dikes, and gabbro [Sklyarov et al., 2016]. Overall, samples with lower sNd tend to be more enriched in terms of trace element ratios such as Zr/Ti, Th/Yb, La/Yb (Fig. 2).
The boninites represent second-stage melts derived from previously depleted mantle in subduction zone environment [König et al., 2010; Shchipansky, 2016]. They are classified into high-Ca type, generally originated from less depleted source under fluid flux, and low-Ca type, derived from more depleted source, generally fluxed by melts from subducting slab [Crawford et al., 1989; König et al., 2010]. The studied Eastern Sayan boninite-series rocks are similar in major-element composition to low-Ca and intermediate-Ca boninites and related rocks of Izu-Bonin-Mariana forearc [Pearce et al., 1992], but in terms of trace elements vary between Izu-Bonin and Papua New Guinea low-Ca boninites [König et al., 2010]. Low-Ca type of boninites is more typical of subduction initiation settings, while
high-Ca boninites occur in environments varying from subduction initiation to back-arc and subduction zone influenced by a mantle plume [Reagan et al., 2017; Shchipansky, 2016; Sklyarov et al., 2016]. Boninite-series rocks in association with tholeiites are common for intra-oceanic subduction initiation settings recorded in modern forearcs [Reagan et al., 2010, 2017] and ophiolites [König et al., 2010; Pearce, Robinson, 2010]. Similarly, the Eastern Sayan ophiolites are likely to be originated in subduction initiation environment, but their enriched trace element and Nd isotope composition needs further consideration.
Phanerozoic boninite-series mostly have isotope characteristics of depleted mantle variously affected by an enriched component: sNd(T) +6.2 to +8.3 (Izu-Bo-nin-Mariana [Pearce et al., 1992; Reagan et al., 2010]), +3.9 to +5.1 (Papua New Guinea [König et al., 2010]), +5 to +6.8 (Troodos ophiolite [König et al., 2008]), +0.6 to +3.0 (Pindos ophiolite [Pe-Piper et al., 2004]). Boninites of these localities formed from depleted mantle variously affected by fluids and melts from subducting slab (subduction component), and additional OIB-type source is inferred for the Pindos ophiolite. Lack of typical intraplate alkaline basalts among the studied Eastern Sayan ophiolite rather rules out an OIB-source hypothesis. Slab melts and fluids can potentially represent an enriched component. Subducting basaltic crust is supposed to have MORB-like isotope variations and cannot explain alone such enriched isotope compositions observed in the Eastern Sayan ophiolites. Following [Sklyarov et al., 2016], subduction of sediments derived from a continental block, probably Gargan block with Neoarchean gneiss basement (sNd(1020) from -20.7 to -28.0), can cause enrichment in Nd isotopes. If this is the case, the melt derived from the sediment will be more effective in transporting trace elements than the fluid. Such process is not unusual for ophiolites. Felsic intrusions with negative sNd(T) interpreted as melts of subducted sediments were re-
ported in Oman ophiolite [Haase et al., 2015] and some Tibetian ophiolites [Zeng et al., 2016]. The studied Eastern Sayan ophiolites might record similar case of sediment melt participation in ophiolite petrogenesis. Another explanation of enriched Nd isotope and trace element characteristics of the boninite-series is involvement of ancient lithosphere. The subcontinental li-thospheric mantle is characterized by wide variations in Nd isotopes [e.g., Carlson, Irving, 1994]. Ancient li-thospheric blocks are preserved in the modern ocean basins, e.g. in the Izu-Bonin-Mariana forearc [Parkinson et al., 1998]. If such ancient lithosphere existed beneath the Eastern Sayan (or Dunzhugur) forearc and if it was enriched in terms of Nd isotopes, its second-stage melting above the newly formed subduction zone might yield boninite-series magmas and explain the ophiolite geochemistry. This is supported by the Re-Os isotope study of the mantle peridotites from Ulan-Sardag and Hara-Nur massifs of the Eastern Sayan ophiolites, re-
vealed low 187Re/188Os ratios and TMA model ages of 1.22 to 2.38 Ga, much older than ophiolite formation age, and reflecting ancient depletion event [Wang et al., 2016]. Further studies will help to clarify the ophiolite formation model.
Conclusion. Lavas and dikes from the Eastern Sayan ophiolites vary from boninites and boninite-series andesites to tholeiites. Similar chemistry of the Dunzhugur and Ilchir rocks support their occurrence as remnants of a single ophiolite nappe covered the Gargan block. Geochemical data point to the formation of the Eastern Sayan ophiolites in forearc setting during subduction initiation. Coupled trace-element and Nd isotope enrichment in boninite series relative to tholeiite basalts indicate progressive increase in enriched component in boninite-series petrogenesis, which might represent melts of subducted sediments or ancient lithospheric mantle.
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