Transition from High- to Moderate-Mg Magmatism Beneath Vitim Plateau, Siberia

Geoscience Frontiers xxx (2016) 1e22

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China University of Geosciences (Beijing) Geoscience Frontiers

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Focus paper Mid-Miocene thermal impact on the lithosphere by sub-lithospheric convective mantle material: Transition from high- to moderate-Mg magmatism beneath Vitim Plateau, Siberia

Irina Chuvashova a,b,*, Sergei Rasskazov a,b, Tatyana Yasnygina a a Institute of the Earth’s Crust, Siberian Branch of RAS, Irkutsk, Russia b Irkutsk State University, Irkutsk, Russia article info abstract

Article history: High-Mg lavas are characteristic of the mid-Miocene volcanism in Inner Asia. In the Vitim Plateau, small Received 7 July 2015 volume high-Mg volcanics erupted at 16e14 Ma, and were followed with voluminous moderate-Mg lavas Received in revised form at 14e13 Ma. In the former unit, we have recorded a sequence of (1) initial basaltic melts, contaminated 10 May 2016 by crustal material, (2) uncontaminated high-Mg basanites and basalts of transitional (KeNaeK) com- Accepted 21 May 2016 positions, and (3) picrobasalts and basalts of K series; in the latter unit a sequence of (1) initial basalts Available online xxx and basaltic andesites of transitional (NaeKeNa) compositions and (2) basalts and trachybasalts of KeNa series. From pressure estimation, we infer that the high-Mg melts were derived from the sub- Keywords: Volcanism lithospheric mantle as deep as 150 km, unlike the moderate-Mg melts that were produced at the e Geodynamics shallow mantle. The 14 13 Ma rock sequence shows that initial melts equilibrated in a garnet-free Cenozoic mantle source with subsequently reduced degree of melting garnet-bearing material. No melting of Asia relatively depleted lithospheric material, evidenced by mantle xenoliths, was involved in melting, Asthenosphere however. We suggest that the studied transition from high- to moderate-Mg magmatism was due to the Lithosphere mid-Miocene thermal impact on the lithosphere by hot sub-lithospheric mantle material from the Transbaikalian low-velocity (melting) domain that had a potential temperature as high as 1510 S. This thermal impact triggered rifting in the lithosphere of the Baikal Rift Zone. Ó 2016, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

1. Introduction 2011; Wang and Gaetani, 2008; Zhao, 2009; Hanano et al., 2010; Herzberg, 2011). The origin of basalts from continents and other The explanation for the origin of within-plate volcanism from ocean islands, where no high-Mg liquids erupted, is attributed to deep mantle sources is currently based on global seismic tomography extension of the lithosphere and precursor metasomatism without images in combination with results of ultrahigh pressure experi- high temperature processes (Grifﬁn et al., 1988; Barry et al., 2003). ments (Anderson, 2007; Ghosh et al., 2007, 2009; Maruyama et al., The Emeishan and Siberian large igneous provinces (LIPs) that 2007; Karato, 2012). The emphasis in the ongoing debate about hot formed in Asia in the late Permian and at the PermianeTriassic mantle plumes rising from the coreemantle boundary is placed on boundary were dominated by moderate-Mg (moderate-tempera- the crucial value of seismic velocity structure of the mantle ture) basaltic melts with minor high-Mg (high-temperature) mei- (Hofmann, 1997; Foulger, 2010). The structure of the mantle and mechites and, probably, komatiites (Vasiliev and Zolotukhin, 1995; components of volcanic rocks associated with the Hawaiian plume Fedorenko et al., 1996; Fedorenko and Czamanske, 1997; Kogarko are adequately interpreted by high-temperature processes respon- and Ryabchikov, 2000; Hanski et al., 2004; Carlson et al., 2006; sible for generating high-Mg magmatic liquids (Sobolev et al., 2005, Zhang et al., 2006; Sobolev et al., 2009; Rasskazov et al., 2010; Shellnutt, 2014). The Siberian LIP was simulated by a thermo- chemical plume in the upper mantle and transition zone with a * Corresponding author. Institute of the Earth’s Crust, Siberian Branch of RAS, diameter of nearly 1000 km (Sobolev et al., 2011). However, no low- Irkutsk, Russia. velocity region that might correspond to the Siberian LIP is recor- E-mail address: [email protected] (I. Chuvashova). ded (Koulakov and Bushenkova, 2010). Peer-review under responsibility of China University of Geosciences (Beijing). http://dx.doi.org/10.1016/j.gsf.2016.05.011 1674-9871/Ó 2016, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC- ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Cenozoic volcanic fields of Asia are often underlain by low-velocity highlighted by both a drastic lateral change of S-wave velocities at anomalies residing in the upper mantle (Rasskazov et al., 2003a; Lei the depth of 250 km and high- to moderate-Mg transition of lava and Zhao, 2005; Zhao et al., 2009; Chuvashova et al., 2012; Wei compositions (Fig. 1). et al., 2012). Decreasing velocities were firstly detected therein at a Most of the high-Mg low-volume lavas with 12e32 wt.% MgO shallow level of the upper mantle through a temporal delay of seismic (Le Bas, 2000; Kerr and Arndt, 2001) erupted in the outline of the waves from nuclear explosions in Nevada, USA (Rogozhina and Transbaikalian domain in the short time interval of 16e14 Ma. Kozhevnikov, 1979). At the beginning of the 1990s, some volcanic Voluminous moderate-Mg lavas flooded large areas afterward areas in Central and East Asia were referred to deep mantle plumes and lasted for millions of years. In the Kamar volcanic field, a (Nakamura et al., 1990; Yarmolyuk et al.,1990,1994; Rasskazov, 1991, single high-Mg lava flow was found within the 16e15 Ma unit 1994). There was also an attempt to locate mantle plumes through that was preceded and followed by moderate-Mg basalts dated at geophysical data (Zorin et al., 2003, 2006). It was suggested that the 18.1e17.6 and 13e12 Ma, respectively (Rasskazov et al., 2003b, dispersed within-plate Cenozoic volcanism of Asiawas derived due to 2013b). In the Udokan volcanic field, the initial low-volume a broad asthenospheric upwelling from a deeper mantle level (“hot (few km3) eruptions of high-Mg olivine melaleucitites occurred region”)(Tatsumi et al., 1990) or convective flow upraised from the at ca. 14 Ma, whereas the subsequent high-volume (450 km3) coreemantle boundary (“hot mantle field”)(Zonenshain et al., 1991). alkali olivine basaltetrachyte and basanitesetephriphonolite The extensive asthenospheric upwelling was assumed to be marked volcanic series flooded the area of 3000 km2 in the last 8 Ma by the DUPAL isotopic anomaly (Flower et al., 1998). However, from (Rasskazov et al., 1997, 2000). Small outcrops of high-Mg rocks the measured Pb-isotope ratios in basalts, no anomaly was detected in and extensive moderate-Mg volcanic strata were identified also Cenozoic basalts erupted within the Caledonian terranes of Inner Asia, in the Vitim and Dariganga volcanic fields (Ashchepkov, 1991; although the anomaly was traced in lavas erupted through the Ashchepkov et al., 2003; Chuvashova et al., 2012, 2016). In this TuvaeMongolian and other Riphean micro-continents that separated paper, we present results of a comparative study of mid-Miocene from East Gondwana and drifted across Paleo-Asian Ocean from the rocks that erupted in the Vitim volcanic field at 16e14 and 14e13 Southern to Northern Hemisphere in the late Precambrian. It was Ma and exhibited high- and moderate-Mg liquids, respectively. inferred that the anomaly was brought by these tectonic units and could not belong to the deep “hot mantle region” (Rasskazov et al., 2. Geologic background and sampling 2002). The S-wave model of Asia by Yanovskaya and Kozhevnikov The Vitim Plateau is situated within the BaikaleVitim Fold (2003) was used for interpreting spatial-temporal distribution of System, the geological structure of which has long been the object Cenozoic basalts in terms of large low-velocity (melting) domains of lively debate. Comprehensive geological mapping of the area of two mantle levels: the Transbaikalian of 200e410 km and the showed sedimentary and volcano-sedimentary units of late Pre- Sayan-Mongolian, Sea of Okhotsk, and Philippine Sea of cambrian and early Paleozoic designated as Baikalides and early 50e200km(Rasskazov et al., 2003a, 2004). The identities of the Caledonides, respectively (Belichenko, 1977). Later on, the whole SayaneMongolian and Transbaikalian melting domains were area of Western Transbaikal was referred to a single Barguzin

Figure 1. Spatial distribution of high- and moderate-Mg mid-Miocene volcanic rocks in Inner Asia relative to the S-wave velocity image at a depth of 250 km after Yanovskaya and Kozhevnikov (2003). Volcanic ﬁelds with mid-Miocene rocks: high-Mg e Vitim (VT), Udokan (UD), Kamar (KM), Dariganga (DR); moderate-Mg e Tunka (TN), Dzhida (DZ), Eastern Sayans (ES), Khubsugul (KH), Dzabkhan (DB), Valley of Lakes (VL), Eastern Hangay (EH), Ugii-Nur (UN), Hannuoba (HN). Next to each of mid-Miocene locations of high- and moderate-Mg rocks, a maximum content of MgO (wt.%) is shown in parentheses.

Figure 2. Mid-Miocene river valleys buried beneath lavas of the Vitim volcanic ﬁeld (a) and tectono-stratigraphic terranes of Western Transbaikal; (b) In the scheme (a), the Northern (N), Central (C) and Southern (S) paleovalleys and volcanic centers: Bereya (Br), Sharbahtay (Sh), Ingur (In), Salbulin (Sl), Antase (An), Kolichikan (Kl), Db (Dybryn), Nm (Namaru), Sirinikta (Sr), Muhal (Mh), Yaole (Ya), Khoigot (Kh), and Malyi Amalat (Ma) are shown after Rasskazov et al. (2000). In the scheme (b), the terrane boundaries of the island arc Eravna (Er), back-arc turbidite Ikat (Ik), Riphean BaikaleMuya (BM), and composite West Stanovoy (WS) are designated after Belichenko et al. (2006).

Figure 3. Location of the quarry and lines of drill holes that exposed, respectively, high- and moderate-Mg rocks in the Bereya volcanic center.

Figure 4. Relations between the 14e13 Ma and younger (12.6e0.6 Ma) volcano-sedimentary units in the Bereya volcanic center. The drill-hole lines of the sections BA, BC, and DE are shown in Fig. 3. NBS SRM-987 (USA) were used. The obtained ratio 4. Major oxides 143Nd/144Nd ¼ 0.512104 4(2s) was close to the standard value 0.512103 (JNd-1), the ratio 87Sr/86Sr ¼ 0.710248 0.000013 (2s)was The volcanic rocks from the mid-Miocene units of the Bereya comparable to the standard value 0.71025 (NBS SRM-987) (Faure, volcanic center are well grouped on the classiﬁcation diagrams K2O 2001). Major oxides were measured by classic “wet chemistry”. vs. K2O/Na2O, K2O vs. SiO2, (Na2O þ K2O) vs. SiO2, and MgO vs. SiO2 Data on volcanic rocks were included into the database (Rasskazov (Fig. 6). Representative compositions of the groups are placed in et al., 2013b). Additional data on volcanic rocks from the studied Table 1. units were used after Ashchepkov (1991), Harris (1998), Litasov and The KeNa ratio is usually used as the main criterion for Taniguchi (2002), Johnson et al. (2005). deﬁnition of potassic (K2O > Na2O), sodic (K2O < 0.25 Na2O),

and potassicesodic (Na2O > K2O > 0.25 Na2O) series (in wt.%) (Foley et al., 1987; Rollinson, 1993). In the diagram of K2O vs. K2O/ Na2O, the mid-Miocene rocks from the Bereya volcanic center overlap a field of KeNa series and partly occupy potassic and sodic fields. Among the 16e14 Ma rocks, the high-Mg groups of picrobasaltsebasalts and contaminated basaltsetrachybasalts belong to K and KeNa series, respectively. The group of high-Mg basanitesebasalts shows intermediate position, i.e. transition from K to KeNa series. Unlike the UgiieNur volcanic field of Central Mongolia, where potassic affinity was provided by increasing K concentrations in moderate-Mg lavas, the potassic high-Mg series of the Bereya volcanic center was due to relatively decreasing Na content. Among the 14e13 Ma rocks, the group of basaltsetrachybasalts exhibits KeNa series, the group of basaltsebasaltic andesites demonstrates a transition from Na to KeNa series. Similarly, rocks of 16e14 and 14e13 Ma are well separated on Figure 5. Schematic cross-section that demonstrates relations between the 16e14 and the total alkalis vs. silica (TAS) diagram. High-Mg basanitesebasalts 14e13 Ma volcano-sedimentary units in the Bereya volcanic center. Location of the and picrobasaltsebasalts show low SiO2 contents (42.8e45.0 and quarry that exposed sediments with fragments of high-Mg volcanic rocks is shown in 44.2e45.6 wt.%), whereas high-Mg basaltsetrachybasalts demon- Fig. 3. strate the elevated SiO2 concentrations (46.2e50.2 wt.%). The most

Figure 6. Coviriation diagrams of K2O vs. K2O/Na2O(a), K2O vs. SiO2 (b), total alkalis vs. silica (TAS) (c), and MgO vs. SiO2 (d) diagrams for mid-Miocene rocks from the Bereya volcanic center. Among volcanic rocks from the 16e14 Ma layers (Fig. 5), we define three groups: (1) high-Mg basalts and trachybasalts of KeNa series, (2) basanites and basalts of transitional (KeNaeK) compositions, and (3) picrobasalts and basalts of K series. Among volcanic rocks of the 14e13 Ma unit (Fig. 4), we recognize two groups: (1) basalts and basaltic andesites of transitional (NaeKeNa) compositions and (2) basalts and trachybasalts of KeNa series. In the diagram (a), a field of K series of moderate-Mg lavas from the Ugii-Nur volcanic field, Central Mongolia is shown after Rasskazov et al. (2012). In the diagram (b), the dividing lines of the series in terms of the potassium contents are used after Rollinson (1993). Fields in the diagram (c): P e picrite, PB e picrobasalt, BSN, TPH e basanite, tephrite, PHT e phonotephrite, B e basalt, TB e trachybasalt, BA e basaltic andesite, BTA e basaltic trachyandesite, TA e trachyandesite, F e foidite. The dividing lines in (c) are after Le Bas and Streckeisen (1991). Contents of major oxides are recalculated to 100% without loss on ignition.

Table 1 Major oxides (wt.%), trace elements (ppm), and Sr, Nd isotope ratios in representative compositions of the rock groups from the Bereya volcanic center.

Age (Ma) 16e14 14e13

Group Contaminated rocks, High-Mg basanitesebasalts Picrobasaltsebasalts, Basaltsebasaltic Basaltsetrachybasalts, KeNa series of transitional (KeNaeK) K series andesites of transitional KeNa series composition (NaeKeNa) composition

Sample VT-13-9 VT-13-9 gm VT-13-20 VT-13-19 V9 V18

SiO2 56.20 47.90 42.55 42.51 52.89 50.71 TiO2 1.13 1.89 1.85 2.09 1.85 2.28 Al2O3 15.30 13.03 9.79 10.27 14.36 16.13 Fe2O3 3.95 5.84 4.20 5.46 11.18* 10.75* FeO 2.96 5.29 7.54 6.61 N.d. N.d. MnO 0.11 0.16 0.16 0.15 0.18 0.17 MgO 5.52 10.12 17.23 13.87 7.23 5.61 CaO 5.22 7.11 8.06 8.99 8.87 8.69

Na2O 3.80 2.90 2.22 1.28 3.08 3.88 K2O 3.31 2.11 1.60 1.61 0.63 1.70 P2O5 0.36 0.62 0.64 0.73 0.23 0.48 LOI 2.04 2.98 4.07 6.61 0.00 0.00 Total 99.90 99.95 99.91 100.18 100.50 100.40 Sc 15.9 22.2 12.3 11.5 22.7 19.0 V 105 163 147 156 169 183 Co 30 47 63 57 44 34 Cu 5 35 40 35 55 40 Ni 214 326 744 458 131 54 Cr 238 550 1301 651 193 85 Rb 73 45 26 22 9 24 Sr 632 769 791 4042 389 713 Y 22.4 26.1 18.9 20.9 23.0 24.0 Zr 88 156 201 227 123 198 Nb 21.9 38.2 49.1 57.0 14.5 32.7 Cs 1.26 0.62 0.19 0.25 N.d. N.d. Ba 1103 845 651 1048 194 470 La 48.3 56.7 44.4 54.3 10.8 25.8 Ce 95.6 114.5 89.6 107.0 23.4 52.1 Pr 10.9 13.1 10.9 12.7 3.2 6.6 Nd 42.2 50.9 41.3 49.4 14.2 26.5 Sm 7.23 9.87 8.64 9.71 4.08 6.16 Eu 2.37 2.92 2.85 3.34 1.52 2.09 Gd 6.12 9.68 6.59 7.71 3.89 5.83 Tb 0.77 1.24 0.98 1.13 0.73 0.87 Dy 4.69 5.73 4.54 5.08 4.15 4.59 Ho 0.91 0.97 0.72 0.79 0.78 0.84 Er 2.26 2.35 1.56 1.68 2.00 2.08 Yb 1.72 1.80 0.86 0.97 1.57 1.63 Lu 0.21 0.26 0.09 0.11 0.22 0.22 Hf 1.96 4.33 4.70 5.47 3.12 4.47 Ta 1.36 2.33 3.18 3.62 0.88 2.03 Pb 12.4 9.7 4.5 5.1 N.d. N.d. Th 5.39 5.77 5.51 6.51 1.18 2.88 U 0.81 1.11 1.17 2.44 0.32 0.81 87Sr/86Sr 0.706556 N.d. 0.704069 0.704243 0.70374 0.70431 2s 0.000036 0.000050 0.000075 0.00001 0.00001 144Nd/143Nd 0.512444 N.d. 0.512852 0.512809 0.512934 0.512788 2s 0.000049 0.000024 0.000044 0.000009 0.000009

The group of high-Mg contaminated rocks is exhibited with bulk and groundmass compositions VT-13-9 and VT-13-9 gm, respectively. *Deﬁned only total iron content in the form of Fe2O3. N.d. e no data. Data on samples from the 16e14 Ma unit were obtained by authors, on those from the 14e13 Ma unit by Harris (1998). silica-rich sample VT-13-09 (SiO2 57.6 wt.%) falls into the tra- Mg basaltsetrachybasalts contain 10.3e12.6 wt.% MgO. The chyandesite ﬁeld. We assign this rock to a trachyandesite-like 14e13 Ma lavas display a dramatic decrease of MgO contents to composition. While compared to bulk compositions, ground- 6e10 wt.%. masses separated from the high-Mg basaltsetrachybasalts reveal The rock groups of 16e14 and 14e13 Ma are clearly seen systematically lower SiO2 abundances in the range of in diagrams of normative minerals (Fig. 7). High- 45.9e49.6 wt.%. Lavas of 14e13 Ma have the whole range of SiO2 Mg basaltsetrachybasalts differ from high-Mg basanitesebasalts from 47.3 to 53.5 wt.%. KeNa basaltsetrachybasalts differ from the and picrobasaltsebasalts by lower contents of normative anorthite transitional (NaeKeNa) basaltsebasaltic andesites by lower SiO2 100 An/(An þ Ab). High-Mg basanitesebasalts show 3e9% Ne, contents. whereas picrobasaltsebasalts 0e8% Hy. In terms of these petro- MgO contents in high-Mg basanitesebasalts and pic- chemical characteristics, picrobasaltsebasalts are similar to absar- robasaltsebasalts range from 12.5 to 20.4 wt.%. With increasing okites from Southwest Japan (Tatsumi and Koyaguchi, 1989). The SiO2 contents in the bulk compositions of the high-Mg basaltetrachybasalt, Na basalt, and basaltebasaltic andesite groups basaltsetrachybasalts, MgO contents decrease slightly relative yield sub-parallel trends of slightly increasing normative anorthite to high-Mg basanitesebasalts and picrobasaltsebasalts, ranging consistent with increasing 100 An/(An þ Ab) and a general from 10.7 to 16.8 wt.%. The groundmass separates from the high- transition from Ae-normative to AtzeHy-normative compositions.

Figure 7. Diagrams of CIPW normative Nee(Hy þ Qtz) vs. 100 An/(An þ Ab) (a) and Nee(Hy þ Qtz) vs. MgO (b). Symbols are as in Fig. 6.

Simultaneous reduction of MgO concentrations and normative incompatible elements from Ba to Sr, although peaks and troughs of anorthite from 16e14 to 14e13 Ma rocks demonstrates decreasing these two samples are consistent with those in moderate-Mg lavas. crystallization temperatures of magmatic liquids. Basalts of Na series reveal speciﬁc minimum of K relative to U and Nb. The trachyandesite-like rock VT-13-9 indicates an effect of crustal contamination by elevated Cs, Rb, K, Pb and reduced Nb, Ta, 5. Trace elements Sr, P, Zr, Hf, and Ti abundances.

The diagrams of rare earth elements (REE) show distinct separa- 6. Discussion tion of rocks from the 16e14 and 14e13 Ma units (Fig. 8). Light REE (LREE) decrease from the high-Mg basanite VT-13-14 and picrobasalt For understanding magmatic processes that led to successive VT-13-19 to moderate-Mg rocks, whereas heavy REE (HREE) increase eruptions of high- and moderate-Mg liquids, we associate the accordingly. As a result, REE patterns for high- and moderate-Mg mid-Miocene appearance of high-Mg magmas with high temper- rocks intersect each other. Chondrite-normalized La/Yb ratios range atures in the Transbaikalian melting domain, define geochemical from 22 to 38 in the groups of high-Mg basanitesebasalts and pic- signatures of melts erupted from deep and shallow mantle sour- robasaltsebasalts and from 5 to 18 in the groups of moderate-Mg ces and, finally, present a model of a thermal impact of a hot sub- basalts. The trachyandesite-like rock VT-13-9 is comparable to the lithospheric material on the lithosphere beneath the Vitim high-Mg basanite VT-13-14 in terms of LREE and to moderate-Mg Plateau. lavas in terms of HREE. Four samples of the high- Mg basaltsetrachybasalts demonstrate elevated concentrations of REE in groundmass relative to bulk compositions (not shown in 6.1. Crustal contamination diagrams). The rock grouping is supported by a multi-element diagram. High-Mg volcanic rocks of the initial volcanic episode 16e14 Ma Compared to moderate-Mg lavas, the high-Mg basanite VT-13-14 are sufficiently contaminated by crustal material. In diagrams, data and picrobasalt VT-13-19 show elevated concentrations of points of high-Mg contaminated basaltsetrachybasalts and the

Figure 8. Pyrolite-normalized concentrations of incompatible elements (a) and chondrite-normalized contents of REE (b). The pyrolite and chondrite compositions are after McDonough and Sun (1995). trachyandesite-like rock VT-13-9 are offset from the trends of high- moderate-Mg basaltsetrachybasalts to the basaltic trachyandesite Mg rocks. The contamination is expressed in increasing SiO2 and V-9. The shift is resulted from the generally increasing degree of decreasing 100 An/(An þ Ab) to values typical of moderate-Mg partial melting in the mantle sources. Meanwhile, Sr isotope ratios rocks (Fig. 6). may be overstated due to rock alteration. Although this effect can In the diagram Th/Yb vs. Ta/Yb, high-Mg contaminated be reduced by acid treatment of a sample (Song et al., 1990; basaltsetrachybasalts and trachyandesite-like rock VT-13-9 are Rasskazov et al., 2015), no criterion for a strontium isotope ratio shifted above the mantle array. This shift is a characteristic of in an unaltered rock is substantiated and no true value of this data points not only for bulk compositions of rocks, but also for parameter can be estimated. groundmasses (Fig. 9a). Thus, crustal material was partially melted and introduced into crystallized liquids. Bulk composition 6.2. Trends of high- and moderate-Mg rocks of the trachyandesite-like rock VT-13-9 reveals lower Ta/Yb, while its groundmass is comparable with those of other On plots of compatible (Ni) and incompatible (Th, La, K2O, Rb, contaminated species. This sample is the most contaminated by Sr, Ba) trace elements versus MgO, high- and moderate-Mg rocks crustal material. yield individual groups (Fig. 9aeg). On the Ni vs. MgO plot, data Crustal contamination of high-Mg basaltsetrachybasalts and points of bulk compositions for contaminated rocks show the trachyandesite-like rock VT-13-9 is reﬂected in high initial elevated Ni and MgO contents, as compared with groundmass 87 86 87 86 Sr/ Sr (Fig. 9b). In the diagram ( Sr/ Sr)i vs.1000/Sr, these rocks compositions, except strongly contaminated trachyandesites-like demonstrate a typical effect of crustal contamination (i.e. rock VT-13-9. Basaltsetrachybasalts from the lower 16e14 Ma 87 86 increasing both ( Sr/ Sr)i and 1000/Sr) relative to uncontami- unit resulted from partial assimilation of crust material accom- nated 16e14 Ma rocks. Various groups of uncontaminated rocks are panied with fractional crystallization of olivine. From shifted along the ordinate with increasing 1000/Sr from pic- an assimilationefractionalecrystallization (AFC) model (DePaolo, robasaltsebasalts through high-Mg basanitesebasalts and 1981) on the Ni vs. MgO plot (Fig. 10a), groundmasses of these

Figure 9. Binary diagrams of Th/Yb vs. Ta/Yb (a) and initial 87Sr/86Sr vs. 1000/Sr (b). Due to young ages of rocks and low Rb/Sr, the initial Sr isotopic ratios do not differ from the measured values. E-MORB and OIB are from McDonough and Sun (1995). Symbols are as in Fig. 6.

rocks underwent fractional crystallization F ¼ 9e13%. For pic- region, whereas the trend of moderate-Mg melts results from robasalts this value decreases to a range of 3e10% and for melting in a shallow mantle source. Increasing MgO consistent with basanites to a range of 0e4%. decreasing concentrations of incompatible trace elements in the The AFC model curve assumes a possible origin of the mid- groups of high-Mg basanitesebasalts and picrobasaltsebasalts in- Miocene rocks from a common primary high-Mg melt at the pro- dicates increasing temperature and degree of partial melting. In the portion of initialecontaminated compositions 1:4. This proportion group of high-Mg contaminated basaltsetrachybasalts, the highest is adopted from concentrations of MgO (16.98 wt.%), Ni (756 ppm) concentrations of incompatible elements are defined in a ground- in the sample VT-13-8 and from concentrations of MgO (1.97 wt.%), mass of the basalt VT-13-21 (Th ¼ 7.3 ppm, La ¼ 68 ppm, Ni (78.5 ppm) in the sample VT-13-9, respectively. From trace- Sr ¼ 4912 ppm, Ba ¼ 1508 ppm). Conodes of these elements in bulk element modeling, a magma source is calculated to contain Ol compositions and groundmasses have steeper slopes than the (57%), Opx (25%), Cpx (5%), Grt (6%), and Amph (5%) (see details in trend of other high-Mg rocks. The conodes of uncontaminated Section 6.3). Concentrations of MgO and Ni in minerals and min- high-Mg basaltic samples lay sub-parallel to those of the basalt VT- eral/melt distribution coefficients are presented in Tables 2 and 3, 13-21 on the Th vs. MgO and La vs. MgO plots and more gentle on respectively. Bulk distribution coefficients Dmineral/melt(Ni) ¼ 7.39, the Sr vs. MgO and Ba vs. MgO ones. Minimal concentrations of La, Dmineral/melt(Mg) ¼ 4.48 are calculated from the mineral proportions Sr, and Ba in bulk compositions and groundmasses of high-Mg in the source with Ni abundance of 1750 ppm. The melting line of basalts overlap each other. As MgO decreases in the peridotite with the Ni concentration of 1900 ppm (Sobolev et al., trachyandesite-like rock VT-13-9 from the bulk composition to the 2005) demonstrates compositions with higher MgO contents. De- groundmass, La and Th increase and Ba decreases with minor viations of MgO from 20.4 to 12.5 wt.% in high-Mg rocks appear to variations of Sr (Fig. 10b,c,f and g). Unlike Th, La, Sr, and Ba, con- reflect relative variations mineral proportions in a source region centrations of K and Rb depend substantially on crustal contami- (see Fig. 13a in Section 6.3). nation. The group of high-Mg contaminated basaltsetrachybasalts Assumption on the origin of both moderate and high-Mg rocks and trachyandesite-like rock VT-13-9 show enrichment by both from a common primary high-Mg melt contradicts the distribution elements relative to other high-Mg groups (Fig. 10d and e). of incompatible elements. We suggest that the trend of high-Mg Grouping of volcanic rocks is expressed also in trace-element rocks reflects temperature variations in a sub-lithospheric source ratios. Zr/Nb is lower in high-Mg rocks than in moderate-Mg ones

Figure 10. Ni, Th, La, K2O, Rb, Sr, and Ba vs. MgO (aeg) and Zr/Y vs. Zr/Nb (h). Symbols are as in Fig. 6.

(2.3e4.3 and 5.0e12.2, respectively). Zr/Y is higher in pic- Table 3 robasaltsebasalts than in high-Mg basanitesebasalts. In the dia- The mineral/basaltic melt distribution coefficients used for the partial melting and AFC modeling. gram Zr/Y vs. Zr/Nb, the high-Mg groups of basalts-trachybasalts, basanitesebasalts, and the moderate-Mg basalt group of KeNa Element Ol Opx Cpx Grt Phl Amph Ap series show correlated trends of these ratios. Those of moderate- Rb 0.00002 0.0001 0.0004 0.00002 5.18 0.2 0.01 Mg transitional (NaeKeNa) basaltsebasaltic andesites demon- Ba 0.000005 0.00006 0.0007 0.00007 3.48 0.16 1.555 strate sufficient variations of Zr/Nb and limited ranges of Zr/Y Th 0.000007 0.0002 0.013 0.0021 0.0014 0.0039 2.131 U 0.000009 0.00004 0.006 0.011 0.0011 0.0041 2.131* (Fig. 10h). K 0.00002 0.0001 0.07 0.013 3.67 0.58 N.d. Nb 0.00005 0.0013 0.0027 0.02 0.0853 0.159 0.001 Ta 0.00005 0.0025 0.012 0.05 0.2255 0.16 0.001* 6.3. Components of mantle sources La 0.0002 0.003 0.054 0.0007 0.0004 0.055 7.39 Ce 0.00007 0.0021 0.086 0.0026 0.0006 0.096 5.965 In terms of Nd and Sr isotope ratios, sources of the 16e14 Ma Pr 0.0003 0.0022 0.154 0.01 0.0009 0.17 4.64* Sr 0.00004 0.0015 0.096 0.0007 0.183 0.298 3.055 high-Mg magmas have a common depleted isotopic component Zr 0.001 0.012 0.123 0.2 0.0232 0.18 0.012 (depleted mantle, DM) and other less depleted compositions. This Hf 0.0029 0.019 0.256 0.4 0.048 0.29 0.01 is indicative for convective isotopic homogenization of a sub- Sm 0.0009 0.0037 0.291 0.1 0.0008 0.33 4.603 lithospheric mantle material and its fractional differentiation, Ti 0.015 0.12 0.384 0.32 1.768 1.29 N.d. accompanied by incompatible element enrichment of resulting Y 0.0082 0.02 0.467 2.5 0.007 0.52 3.43 Yb 0.024 0.032 0.43 6.4 0.023 0.47 0.93 melts (Figs. 11 and 12). The separated melts, therefore, are derived Ni 7 3.1 1.4 0.19 5 3.4 N.d. due to high-temperature sub-lithospheric convective processes in Mg 5 4.7 5.4 N.d. N.d. 3.8 N.d. the mantle and decreasing melting temperatures connected with Data sources: olivine (Ol): Ni, Mg (Wang and Gaetani, 2008), other elements increasing incompatible element abundances in a source. This type (Halliday et al., 1995); orthopyroxene (Opx): Ta (Green et al., 2000), Ti (Girnis et al., of magma generation differs from the generation of komatiite 2006), Yb (Kennedy et al., 1993), Ni (O’Reilly et al., 1991), Mg (Henderson, 1982), magmas in a plume that shows depletion in both isotopic and other elements (Halliday et al., 1995); clinopyroxene (Cpx): Rb (Halliday et al., elemental signatures (Arndt et al., 2008). 1995), Ba, K, REE, Zr, Hf, Ti, Y (Hart and Dunn, 1993), Th and U (Hauri et al., 1994), Nb and Sr (Foley et al., 1996), Ta (Green et al., 1989), Ni (O’Reilly et al., 1991), Mg High-Mg liquids from the Vitim Plateau are enriched by LREE, (Villemant et al., 1981); garnet (Grt): Rb, Ca, Th, U, K, REE, Sr, Y (Halliday et al., 1995), Th, Zr, and other incompatible elements that are introduced into Nb, Ta, Zr, Hf, Ti (Girnis et al., 2006), Ni (O’Reilly, Griffin, 1995); phlogopite (Phl): Rb, melts due to prevailing partitioning from various mineral phases. Ba, Nb, Sr, Zr, Y (Foley et al., 1996), Th, U, K, Ti (LaTourrette et al., 1995), Ta, La ’ Peridotite xenoliths, extracted from the spinelegarnet transition (Gregoire et al., 2000), Ce, Pr, Hf, Sm, Yb (Ionov et al., 1997), Ni (O Reilly et al., 1991); amph (Amph): Ta (Zack et al., 1997), Zr, Hf, Sm (Ionov et al., 1997), Ni (O’Reilly et al., zone by ascending liquids of this area, show evidence on melt 1991), Mg (Henderson, 1982), other elements (LaTourrette et al., 1995); apatite infiltration in the upper mantle (Glaser et al.,1999). Various types of (Ap): Rb, Nb, Ta, Hf (Ionov et al., 1997), other elements (Chazot et al., 1996). *Values mantle xenoliths have mutual genetic relationships. It was sug- after interpolation. N.d. e no data. gested that the high-temperature garnet-bearing rocks result from repeated affection of basaltic melts on the ambient upper mantle material. From the AFC calculations, it was also inferred that metasomatized lherzolites and pyroxenites are examined as megacrysts and shallow xenoliths crystallized during the rise of possible sources for the mid-Miocene liquids. basaltic liquids to the surface (Ashchepkov and Andre, 2002; The group of high-Mg basanitesebasalts has initial Ashchepkov et al., 2011). From these observations, 143 144 87 86 ( Nd/ Nd)t ¼ 0.512867e0.512892 and ( Sr/ Sr)i ¼ 0.703831 e0.704183 in the range of the depleted mantle. The group of picrobasaltsebasalts reveals less depleted isotopic signatures: initial 143 144 87 86 Table 2 ( Nd/ Nd)t ¼ 0.512730e0.512842, ( Sr/ Sr)i ¼ 0.703778 Trace element concentrations (ppm) in the pyrolite and mean concentration in e0.704406 with a relative shift of data points along the mantle array minerals from xenoliths used for calculations of the model magmatic sources towards the primitive mantle composition. Crustal contamination of beneath Vitim Plateau. high-Mg basaltsetrachybasalts and the trachyandesite-like rock VT- Element Pyrolite Cpx Grt Phl Amph Ap 87 86 143 144 13-9 is reflected both in high ( Sr/ Sr)i and low ( Nd/ Nd)t.The Rb 0.6 0.45 N.d. 263 6 15 group of moderate-Mg basaltsetrachybasalts overlaps both the high- Ba 6.6 11 0.03 3364 178 187 Mg basaniteebasalt and picrobasaltebasalt compositions. The Th 0.0795 0.07 N.d. 0.005 1.50 97 U 0.0203 0.01 N.d. N.d. 0.50 23 basaltic trachyandesite V-9 shows a more depleted signature: 143 144 87 86 K 240 83 N.d. 73390 15994 N.d. ( Nd/ Nd)t ¼ 0.512934, ( Sr/ Sr)i ¼ 0.703740 (Fig. 11a). Nb 0.658 0.79 0.14 19.1 42.8 0.66 The differences between the eruption age of 16e14 Ma and Ta 0.037 0.028 N.d. 0.38 3.23 o. e. SmeNd ages of 42e24 Ma obtained for garneteclinopyroxene pairs La 0.648 2.33 0.01 0.69 6.01 2300* of xenoliths from tuffaceous-sedimentary strata of the Bereya vol- Ce 1.675 9.42 0.07 0.9 21.3 4042 Pr 0.254 2* 0.02 0.27 4* 377 canic center was explained by incomplete isotopic equilibrium in Sr 19.9 104 0.18 212 467 13295 the xenoliths at the time of eruption due to insufficiently rapid Zr 10.5 95.5 24.3 18 119 24 diffusional exchange in coarse-grained rocks. SmeNd and RbeSr Hf 0.283 2.76 0.24 0.31 4.94 0.71* isotope data on xenoliths have been interpreted in terms of a Sm 0.406 3.25 0.48 0.14 5.5 170* w Ti 1260 7008 1112 47098 25120 N.d. 2 Gyr depletion event relative to the primitive mantle values after Y 4.3 8.29 27.7 0.78 11.4 173 Zindler and Hart (1986). It was suggested that the mantle perido- Yb 0.441 0.40 2.92 0.06 0.52 7.67 tites have been attached to the continental lithosphere since that Ni 1960 468 47 N.d. 868 N.d. time and survived major tectono-thermal events (Ionov et al., MgO (wt.%) 37.80 14.68 20.72 19.40 16.76 N.d. 2005). In the area of study, no geological units of w2 Gyr have Data sources: Ionov et al., 1997; Glaser et al., 1999; Litasov et al., 2000a,b; O’Reilly been recorded, though. fi and Grif n, 2000; Ashchepkov and Andre, 2002; Ashchepkov et al., 2003, 2011. From Pb isotope data on Cenozoic volcanic rocks, it was inferred Th and U in Amph is after Zheng et al. (2006). The pyrolite composition is after e McDonough and Sun (1995). *The values obtained after interpolation from that the mantle of the Baikal Mongolian region was isotopically normalized spectra. N.d. e no data. homogenized by vigorous convection at about 700e600 Ma in

Figure 11. Initial 143Nd/144Nd vs. initial 87Sr/86Sr (a) and 143Nd/144Nd vs. 147Sm/144Nd (c). Insets (b) and (d) show data on xenoliths from the 16e14 Ma high-Mg rocks (Ionov et al., 2005). Symbols are as in Fig. 6. connection with disintegration of the supercontinent Rodinia Plateau. On Fig. 11c and d, data points of high-Mg basanites and (Chuvashova, 2013). This event seems to be displayed also in Nd mantle xenoliths with extremely high 143Nd/144Nd ratio are isotopic composition of the lithospheric mantle beneath the Vitim distributed along a line with a slope of 640 Ma. One can assume that the material of lithospheric mantle xenoliths is a comple- mentary product of mantle depletion of convective material responsible for disintegration of the supercontinent Rodinia, while the high-Mg basanites show time-integrated 143Nd/144Nd ratio of sub-lithospheric convective mantle. Offset of individual data points of xenoliths with increasing 147Sm/144Nd ratio might reflect minor transformation of the lithospheric mantle, associated with the formation of the island-arc Eravna terrane at about 440 Ma. Evolution of the convective mantle material at shallow depths is reflected in shifting data points parallel to abscissa-axis with increasing 147Sm/144Nd. Under generally depleted isotopic signatures of Nd in garnet and spinel peridotites from the 16e14 Ma tuffaceous-sedimentary stratum, the only composite xenolith of garnet-bearing xenolith 313-4 falls into a field of volcanic rocks. In terms of isotopic compositions of Nd, volcanic rocks are comparable also to minerals from veins of composite xenoliths. Consequently, the shallow-mantle volcanic rocks cannot be derived from a source of the depleted lithospheric mantle and might be related to the processes, exhibited by the formation of veins in mantle rocks. Figure 12. Initial 143Nd/144Nd vs. 100/Nd. Symbols are as in Fig. 6. The reported data on veins (Ionov et al., 2005) exhibit heterogeneities of the lithospheric mantle on the scale of individual

Figure 13. (a) (La/Yb)N vs. (Yb)N and (b) La/Yb vs. MgO diagrams. For simplification the diagram (a), data points of bulk and groundmass compositions for only two samples of the contaminated basalts that yield the highest (La/Yb)N are plotted. Abbreviations: Grt e garnet, Cpx e clinopyroxene, Amph e amphibole, Phl e phlogopite, Ap e apatite. Ratios of minerals in sources are based on trace-element modeling (see the text). Mineral/melt distribution coefficients for La and Yb are shown in Table 3. The primitive mantle composition is adopted after McDonough and Sun (1995). In diagram (b), data obtained are compared with data fields of Miocene lavas from the Hannuoba Province of China: the groups of evolved alkali olivine basalts (EVeAOB), primitive alkali olivine basalts (PReAOB), transitional basalts (TRB), and tholeiitic basalts (TLB) after Zhi et al. (1990).

Please cite this article in press as: Chuvashova, I., et al., Mid-Miocene thermal impact on the lithosphere by sub-lithospheric convective mantle material: Transition from high- to moderate-Mg magmatism beneath Vitim Plateau, Siberia, Geoscience Frontiers (2016), http://dx.doi.org/ 10.1016/j.gsf.2016.05.011 I. Chuvashova et al. / Geoscience Frontiers xxx (2016) 1e22 15 samples. On the one hand, the isotopic differences that might exist mantle source, which expresses the mantle temperature, projected among mineral grains of chemical layering on centimeter or meter along the adiabatic line to the atmospheric pressure at the Earth’s presumably, will not be seen except in xenolithic studies. On the surface. This parameter can vary from 1250 to 1400 C in mid-ocean other hand, references to lithospheric mantle components imply ridges and rise up to 1550 C in hot plumes such as Hawaii. Change volumes of rock large enough to represent the source of individual to the conductive transfer of geothermal heat flow in the litho- batches of magma, perhaps of the dimension of several cubic ki- sphere causes a large temperature gradient, in which the temper- lometers. For instance, volume of 15 km3 moderately-Mg magmas, ature increases with depth for 500 K GPa 1 (16 C km 1)or erupted in the Bereya volcanic center at 14e13 Ma, assumes a 20% more. As a result, melting of the mantle is dominated by decom- partial melting and segregation of magmas from a mantle batch as pression mechanisms that cause eruptions of basaltic magma large as 75 km3. through the extended lithosphere (Asimow, 2005; Herzberg et al., Mantle components of volcanic rocks are identified by cooper- 2007). 143 144 # ative use of the diagrams ( Nd/ Nd)t vs. 100/Nd and (La/Yb)N vs. Rocks enriched in Ni and Cr and showing Mg ¼ Mg/ 2þ 3þ YbN (Figs. 12 and 13). The high-Mg basanite 93VBS-4 with the (Mg þ Fe )>70 with corrections Fe ¼ 0.15 Fetotal are usually 1 143 144 lowest 100/Nd (1.7 ppm ) and relatively high ( Nd/ Nd)t related to the primary mantle liquids. Besides, the composition of (0.512892, εNd ¼þ5) is the starting point for trends of high-Mg primary melt should be in equilibrium with the most magnesian basanitesebasalts (BSN e high Grt), picrobasaltsebasalts (PB), olivine (Herzberg et al., 2007). High-Mg volcanic rocks from the and high-Mg basanites with increasing degree of partial melting Bereya volcanic center are characterized by Ni and Cr as high as 788 (BSN e high F). This composition is designated as the common and 1503 ppm, respectively, and by interval of Mg# ¼ 66.3e77.5 component of the depleted mantle (DM). The trend of high-Mg (predominated values 75e76). Some Mg# values decrease to 64 143 144 contaminated rocks yields low ( Nd/ Nd)t and 100/Nd because of crustal contamination and increase to 78 due to variable (elevated Nd concentrations). The data points of the 14e13 Ma mineral proportions in the mantle source (see Figs. 10a and 13a). moderate-Mg lavas offset from compositions of the 16e14 Ma high- Unlike high-Mg rocks with typical signatures of primary mantle Mg rocks with elevated 100/Nd (reduced Nd concentrations). The liquids, moderate-Mg rocks show relatively low Cr and Ni con- common trend of moderately-Mg lavas and minerals from veins centrations and interval of Mg# ¼ 53.9e63.2 that demonstrate (Fig. 12) is indicative for origin of the lavas through melting of the generation of such magmas from a modified mantle source. vein-bearing shallow mantle source. A maximum liquidus temperature of olivine crystallization from In the Bereya volcanic center, high-Mg liquids were followed an erupted melt or primary eruption temperature (Tpe) is estimated with moderate-Mg ones within a time interval of 1e2 million years. from an assumption on anhydrous composition of magma by There was drastic transition from high-Mg (high temperature) to various equations. Three of these by Kutolin (1966), Herzberg et al. moderate-Mg (moderate temperature) melts. In Miocene lavas (2007), and Arndt et al. (2008): Tpe ( C) ¼ 1056.6 þ 17.34 MgO, Tpe 2 from the Hannuoba province (North China), a similar contrast ( C) ¼ 935 þ 33 MgO e 0.37 MgO , Tpe ( C) ¼ 20 þ 1000 MgO existed between the groups of evolved alkali olivine basalts with yield approximately comparable results. For the high-Mg basanite high La/Yb and three other groups of basalts (tholeiitic, transitional VT-13-2 (MgO ¼ 17.1 wt.%), that belong to the predominated group and primitive alkaline) with low La/Yb. In the La/Yb vs. MgO dia- with Mg# ¼ 75e76, the maximum liquidus crystallization tem- gram, the three groups of Hannuoba basalts are compared with the perature obtained is in the range of 1340e1390 C. Calculation of a moderate-Mg lavas from the Bereya volcanic center (Zhi et al.,1990) potential temperature using equation Tp ( C) ¼ 1463 þ 12.7 MgO (Fig. 13b). e 2924/MgO (Herzberg et al., 2007) provides more satisfactory The group of evolved alkali olivine basalts from the Hannuoba approximation to the temperature conditions in the mantle. The province is shifted relative to the high-Mg rocks from the Bereya high-Mg basanite VT-13-2 yields an estimate Tp ¼ 1510 C. center with decreasing MgO content. The Hannuoba and Bereya The assessment of potential temperature for the mantle beneath high La/Yb trends are parallel to each other. They both can be the Vitim Plateau was at its maximum in the outline of the Trans- interpreted in connection with increasing degree of partial melting baikalian melting domain. The Tp for generation of high-Mg olivine at elevated temperatures. From model calculations by Chuvashova melaleucetites from the Udokan volcanic field does not exceed et al. (2012), it was inferred that the Hannuoba evolved alkali 1430 C. This estimate is comparable with the potential mantle olivine basalts were derived by 2e5% partial melting of a source temperature for rocks from Ontong Java Plateau and Iceland (early that had 4.5e6.0% of garnet. These parameters are close to those of Tertiary). The modern rocks from Iceland yield the lower Tp esti- a source for high-Mg rocks from the Bereya volcanic center. mates. Velocity models are indicative for association of Ontong Java Interestingly, that MgO contents strongly vary in the high-Mg Plateau and Iceland with upper mantle melting anomalies, not Bereya rocks only under elevated La/Yb and decrease with extending below the transition layer (Foulger, 2010). It is noteworthy decreasing La/Yb (i.e. at the elevated temperature and increasing that the calculated Tp ¼ 1550 C for Hawaii (Herzberg et al., 2007) degree of partial melting). The Hannuoba evolved alkali olivine exceeds the upper mantle Tp interval and is comparable with the basalts, affected with olivine fractionation (Zhi et al., 1990), had higher Tp interval for high-Mg rocks from Gorgona Island (Fig. 14). primary liquids compositionally similar to high-Mg rocks from the These high temperatures might reflect raising hot plumes from the Bereya volcanic center and other localities of the Transbaikalian deeper mantle according to the model suggested by Morgan (1971). melting domain shown on Fig. 1. The unfractionated high-Mg liq- The maximum temperatures in the mantle can be constrained uids from the Bereya volcanic center might be overheated above a also from temperature calculations of mineral equilibrium in liquidus. mantle xenoliths. The highest temperatures were obtained for xenoliths of pegmatoid orthopyroxenites and websterites with la- 6.4. Temperature estimates mellas in pyroxenes. These xenoliths were found in basanites of the Udokan and Kamar volcanic fields. The temperature interval of The mantle melts at relatively high temperature and low pres- 1350e1450 C was calculated for initial Ca solubility in orthopyr- sure. Convective heat transfer in the asthenosphere provides ver- oxene solid solutions (Rasskazov, 1985; Rasskazov and Chuvashova, tical temperature gradient reflected in a material flux without 2013). In the Udokan volcanic field, the high-temperature xenoliths additional or lost heat. Asthenospheric adiabatic gradient of about occur in moderate-Mg basanite lavas erupted along the axial part of 1 10 K GPa corresponds to the potential temperature (Tp)ofa Udokan Range in the last 4 Ma. These basanites (as well as their

Figure 14. Primary eruption and mantle potential temperatures as a function of the trations of Cr shift a reaction of garnet introduction to the higher 2þ MgO contents in primary magmas. Primary eruption temperatures are anhydrous pressures. The presence of Fe has the opposite effect (Klemme, olivine liquidus temperatures at 1 bar calculated for the primary magmas using the 2004; Klemme and O’Neill, 2000). Similar estimates for the gar- ﬁ ¼ þ e 2 simpli ed equation Tpe ( C) 935 33 MgO 0.37 MgO . The bracket shows the netespinel transition have been obtained through experiments on uncertainty at the 2s level. Potential temperatures are calculated from the equation enriched and depleted peridotites. Under lower pressure condi- Tr ( C) ¼ 1463 þ 12.7 MgO e 2924/MgO (Herzberg et al., 2007). Arrows display the effects of H2O content on liquidus temperature. Partial crystallization of primary tions, melting is assumed to begin in garnet pyroxenite veins magmas will yield actual eruption temperatures that are lower than primary eruption (Robinson and Wood, 1998). temperatures. Abbreviations: MORB e mid-ocean ridge basalts, OJP e Ontong Java Plateau, I (ET) e Island (early Tertiary), I (PD) e Island (present day). Highlighted in 6.6. Partial melting modeling three temperature ranges are: (1) Tr ¼ 1550 C and above e plumes rising from the deeper parts of the mantle (Gorgona, Hawaii), (2) Tr ¼ 1430e1510 C e upper-mantle melting anomalies extending from the mantle transition zone (Ontong Java Plateau, For modeling partial melting of enriched sources, we use mean Iceland, Transbaikalian domain), and (3) Tr ¼ 1280e1400 C e melting anomalies in compositions of minerals from Grt- and GrtePhl lherzolites and the shallow part of the upper mantle (mid-ocean ridges, SayaneMongolian domain). also pyroxenites with phlogopite, ilmenite, and kaersutite, as well

Please cite this article in press as: Chuvashova, I., et al., Mid-Miocene thermal impact on the lithosphere by sub-lithospheric convective mantle material: Transition from high- to moderate-Mg magmatism beneath Vitim Plateau, Siberia, Geoscience Frontiers (2016), http://dx.doi.org/ 10.1016/j.gsf.2016.05.011 I. Chuvashova et al. / Geoscience Frontiers xxx (2016) 1e22 17 as other deep-seated xenoliths that satisfy to the conditions: (1) a content of 6% towards melt fractions F up to 0.075. The maximal mineral assemblage Ol þ Cpx þ Grt þ Amph Phl Ap represents depth estimates up to 150 km from these data points are consistent a possible model source, (2) PeT parameters of xenoliths corre- with increasing MgO content due to high-temperature magma spond to the upper mantle conditions (i.e. T>1000 C), and (3) generation. A trend for picrobasaltsebasalts (PB) crosses the middle analytical data for trace-element concentrations in minerals are of part of the BSN e high Grt one. Unlike the latter source, garnet high quality. The trace element concentrations were defined in content of the PB trend remains constant under varying degree of xenoliths from picrobasalts and basanites of the Vitim Plateau melting (F ¼ 0.039e0.050). Contaminated basaltsetrachybasalts (Ionov et al., 1997; Glaser et al., 1999; Litasov et al., 2000a,b; with elevated (La/Yb)N fall at the line of sources with 12% of garnet. Ashchepkov and Andre, 2002; Ashchepkov et al., 2003, 2011). In the (La/Yb)N vs. YbN diagram, the data points of lavas erupted Additional trace-element data on Ta, Th, U concentrations in about 14e13 Ma are plotted between model lines that show garnet amphibole were taken for xenoliths from basalts of the SW Tian content 3.5% and no garnet in a source region. Firstly, melted ma- Shan (Zheng et al., 2006). The data on the type-A apatite were used terial from the shallow garnet-free source (or containing a small after O’Reilly and Griffin (2000) (Table 2). amount of garnet) provided voluminous eruptions (2/3 of the Equilibrium partial melting was modeled using the equation whole 14e13 Ma volume). The degree of partial melting was at a ð Þ ¼ Ci 0 e maximum (F ¼ 0.15e0.20). The mantle source for coeval small (Shaw, 1970): Ci D þFð1P Þ, where Ci concentration of element i i i portion of Na-basaltic lavas, derived from the garnet-bearing in a melt; C (0) e concentration of element i in the source; D e bulk i i source (1e2% of Grt), experienced relatively weak fusion distribution coefficient for the source material, F e melt fraction; P (F ¼ 0.05e0.07). The final lava portion, which amounted to 1/4 of P ¼ XjKj e bulk distribution coefficient for material involved in i i the volume of all the 14e13 Ma eruptions, was also derived from melting; Xj e weight fraction of phase j; Kj e mineral/melt distri- the garnet-bearing source (up to 3.5% Grt) with an elevated degree bution coefficient for phase j. Mineral/melt distribution coefficients of partial melting (F ¼ 0.05e0.10). are given in Table 3. The clinopyroxene fraction in the source material was increased about two times and the olivine fraction was decreased relative to their contents in the source according to the 6.7. Change of magmatism at 16e13 Ma experimental study of garnet-bearing rocks (Walter, 1998). From results of trace-element modeling, we infer that the 16e14 We suggest that the mid-Miocene temporal change from high- Ma high-Mg basanitesebasalts and picrobasaltsebasalts were to moderate-Mg liquids in the Bereya volcanic center was due to originated from a source that has: Ol (48e57%), Opx (25%), Grt a thermal impact on the lithosphere from the convective mantle of (6e12%), Cpx (5%), Amph (5e7%), Phl (2e3%), and Ap (<0.07%). the Transbaikalian melting domain (Fig. 16). Some discrepancies between rocks and model melts were found in In the time interval of 16e14 Ma, initial high-Mg magmas were terms of Rb, Th, and K. Besides, the maximum of Sr in the trace contaminated by disintegrated crustal material. The composition of element patterns are not explained by this model. Melt fractions (F) primary magmas that rise from the mantle source was distorted vary from 0.04 to 0.06 (Fig. 15a and b). due to the partial assimilation of this material. Perhaps, the primary In Fig. 13, some data points of high-Mg basanitesebasalts fall at mantle melts were similar in composition to the high-Mg basani- a line of the melt fraction F ¼ w0.045. Neodymium isotope ratios of teebasaltic melts erupted in the Bereya volcanic center after high- this sub-group decrease as the garnet content in the source in- Mg contaminated basaltsetrachybasalts. creases from 6 to 12%. Another sub-group of high-Mg basani- The high-Mg basaniteebasaltic and picrobasaltsebasaltic tesebasalts yields melt fractions increasing up to 0.075 in the magmas of the second and third episodes exhibited uncontami- source with 6e8% of garnet. The neodymium isotope ratios of this nated primary mantle liquids. In adiabatically ascending mantle sub-group range in narrow interval as Nd concentrations decrease material, melting depth depends on a heat content of a magmatic due to increasing melt fractions. The source of picrobasaltsebasalts system. When heat supply is high, melting is initiated at a deeper shows about 10% Grt and melt fraction F ¼ 0.04e0.05. level; when heat supply is low, melting is only provided in rela- The 14e13 Ma lavas were derived from sources with relatively tively shallow conditions (Fukuyama, 1985). low garnet (or garnet-free) and elevated clinopyroxene contents: The mid-Miocene eruptions of high-Mg magmatic liquids in the basalts and trachybasalts e Ol (49.5%), Opx (25%), Grt (<3.5%), Bereya volcanic center from the 115e150 km mantle source were Cpx (12%), Amph (8%), Phl (2%), Ap (<0.07%), F ¼ 0.05e0.1; due to adiabatic introduction of heat enough to melting at such a basaltsebasaltic andesites e Ol (54.5%), Opx (25%), Cpx (15%), deep level. Created buoyancy of partially molten material exceeded Amph (5%), Phl (<0.5%), Ap (<0.05%), F ¼ 0.15e0.20. that of crystalline phases in the ambient asthenospheric mantle. As In basaltsetrachybasalts of KeNa series, LREE are variable in a result, the melts, released from the matrix, moved upwards and transitional (NaeKeNa) rocks, while HREE remain constant erupted on the surface. (Fig. 8b). These REE patterns are typical for equilibrium partial The deepest high-Mg basanitesebasalts were produced through melting in a mantle source. Increasing La/Yb reflects decreasing relatively homogeneous melting by excessive heat that caused degree of melting (Rollinson, 1993). moderate melting in the magmatic column F ¼ 6%. An initial melt From the results of trace element modeling in the (La/Yb) vs. N region had the upper limit at the depth 115 km governed by release Yb coordinates, high-Mg basanitesebasalts are examined as de- N of buoyant high-Mg liquids from a fractionated matrix with the rivatives from a source with the garnet content from 6 to 12%. A garnet content varying from 6 to 12%. Shortly afterward melting high-garnet trend (BSN e high Grt) in Fig. 12 corresponds to 6e12% processes localized within a region that had more constant garnet of Grt at relatively low degree of melting (F ¼ 0.042e0.046) in content and yielded picrobasaltebasaltic liquids. Fig.13a. A similar source was identified for basanites from the Kamar At the initial stage of 16e14 Ma, igneous channels provided heat volcanic field located near the South Baikal basin (Rasskazov et al., supply to the shallow part of the lithospheric mantle. High-degree 2013a,b). In the source region, garnet concentrations might in- decompression melting of a garnet-free (or low-garnet) source crease due to depth-dependent increasing rock density and due to caused thermal impact on the lithosphere, uplift and erosional garnet fractionation relative to less dense pyroxene and olivine. dissection of the area and resulted in voluminous eruptions of Some data points of the high-Mg basaniteebasalt group from the moderate-Mg basalts and basaltic andesites about 14e13 Ma. This Bereya volcanic center are shifted, however, along a line with garnet impulse of the shallow melting, however, was short-lived and

Figure 15. Comparisons of trace element (a, s) and REE (b, d) patterns of rocks (lines with markers) and calculated model liquids (black lines without markers). Symbols are as in Fig. 6. (a, b) e high-Mg basanitesebasalts and picrobasaltsebasalts of 16e14 Ma, (c, d) e moderate-Mg basalts, basaltic andesites, and trachybasalts of 14e13 Ma. Normalization values are after McDonough and Sun (1995). The calculated degrees of melting (melt fractions) and garnet contents in sources are indicated (see discussion in the text).

changed to less voluminous melting in the deeper garnet-bearing reflect adiabatic heat transfer, whereas the less advanced melting at part of the lithospheric mantle. shallower depths (up to 115 km) was caused by relative decreasing temperature beneath the lithosphere. Variations of major oxides, trace elements, and isotopes in the 14e13 Ma rock sequence 7. Conclusion showed initial melts equilibrated at the shallow garnet-free mantle source with subsequently reduced degree of melting of garnet- A comparative study of high- and moderate-Mg volcanic rocks bearing material. The shallow melts revealed no melting of rela- erupted in the Bereya volcanic center of the Vitim Plateau, at 16e14 tively depleted lithospheric material, evidenced by mantle xeno- and 14e13 Ma, respectively, has revealed specific temporal varia- liths from the lithosphere. tions of rock compositions in these units. The former (low-volume) The main purpose of this paper is to connect the mid-Miocene unit shows the sequence of high-Mg rocks: (1) contaminated volcanic activity, initiated in deep and further developed in basaltsetrachybasalts of KeNa series, (2) basanitesebasalts of shallow mantle sources, with destabilization and rifting of the transitional (KeNaeK) compositions, and (3) picrobasaltsebasalts lithosphere in the Baikal Rift Zone. It is suggested that voluminous of K series. The latter (voluminous) unit demonstrates the sequence lava eruptions from the shallow lithospheric source at about of moderate-Mg rocks: (1) basaltsebasaltic andesites of transitional 14e13 Ma reflected a thermal impact on the lithosphere beneath (NaeKeNa) compositions and (2) basaltsetrachybasalts of KeNa the Vitim volcanic field by hot convective material of the Trans- series. baikalian melting domain that had a potential temperature T as It has been found that the initial portion of high-Mg liquids in p high as 1510 C, comparable with that parameter in the upper the 16e14 Ma unit was strongly affected by crustal contamination. mantle of the Ontong Java Plateau and Iceland hot spots. This Later on, the uncontaminated melt portions of this unit revealed thermal impact caused extension of the lithosphere, accompanied various temperature effects in a mantle anomaly; the elevated by subsidence of rift basins on the surface. In other volcanic fields of degree of melting in its source as deep as 150 km appeared to

Figure 16. Change of sources for rocks of the Bereya volcanic center in terms of the mid-Miocene thermal impact on the lithosphere by a sub-lithospheric hot convective material from the Transbaikalian melting domain. Tp e potential temperature, F e degree of partial melting.

e the Transbaikalian melting domain (Udokan, Dariganga, Kamar), Tp petrogenetic model of magmatism at the asthenosphere lithosphere bound- e estimates decreased to 1430 C, whereas in the shallower ary. Geodynamics & Tectonophysics 4, 385 407. http://dx.doi.org/10.5800/GT- 2012-3-4-0081. SayaneMongolian melting domain, Tp did not exceed 1300 C. Chuvashova, I.S., Rasskazov, S.V., Yasnygina, T.A., Rudneva, N.A., 2016. Activation and termination of late Cenozoic extension in the lithosphere of the marginal part of the Baikal Rift Zone: changing sources of volcanism in the Vitim Plateau. Acknowledgments Volcanology and Seismology (in press). DePaolo, D.J., 1981. Trace element and isotopic effects of combined wallrock This work was supported by the Russian Science Foundation for assimilation and fractional crystallization. Earth and Planetary Science Letters 53, 189e202. Basic Research (project 14-05-31328). Sample preparation for Faure, G., 2001. Origin of Igneous Rocks: the Isotopic Evidence. Springer, p. 496. trace-element and isotope analytical work was performed by M.E. Fedorenko, V.A., Czamanske, G.K., 1997. Results of new field and geochemical Markova and E.V. Saranina, isotope mass-spectrometric de- studies of the volcanic and intrusive rocks of the Maymecha-Kotuy area, Si- berian flood-basalt province, Russia. International Geology Review 39, terminations by N.N. Fefelov, and major-oxide determinations by 479e531. G.V. Bondareva and M.M. Samoilenko. Agilent 7500ce and Finnigan Fedorenko, V.A., Lightfoot, C., Naldrett, A.J., Czamanske, G., Hawkesworth, C., MAT 262 mass spectrometers are affiliated with the Common Use Wooden, J., Ebel, D., 1996. Petrogenesis of the Siberian flood-basalt sequence at ’ e Centers “Ultramicroanalysis” and “Geodynamics and Geochro- Noril sk, north central Siberia. International Geology Review 38, 99 135. Flower, M.F.J., Chung, Sun-Lin, Lo, Ching-Hua, Lee, Tung-Yi (Eds.), 1998. 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Crust SB RAS, Irkutsk, Russia (since 1996), Head of Dy- Walter, M.J., 1998. Melting of garnet peridotite and the origin of komatiite and namic Geology Chair at the Irkutsk State University (since depleted lithosphere. Journal of Petrology 39 (1), 29e60. 2012), M.S. (1976) from the Irkutsk State University, Ph.D. Wang, Z., Gaetani, G.A., 2008. Partitioning of Ni between olivine and siliceous (1982), Dr. (1992), and Professor (2005) of Petrology and eclogite partial melt: experimental constraints on the mantle source of Ha- Volcanology from the Institute of the Earth’s Crust SB RAS. waiian basalts. Contributions to Mineralogy and Petrology 156 (5), 661e678. He has studied Cenozoic rifts in Central and East Asia, Wei, W., Xu, J., Zhao, D., Shi, Y., 2012. East Asia mantle tomography: new insight into plate North America, and East Africa. His main interest is origin subduction and intraplate volcanism. Journal of Asian Earth Sciences 60, 88e103. of volcanic rocks, erupted at the latest geodynamic stage of Yanovskaya, T.B., Kozhevnikov, V.M., 2003. 3D S-wave velocity pattern in the upper the evolving Earth. He has published more than 500 works mantle beneath the continent of Asia from Rayleigh wave data. Physics of the that include 17 monographs and 132 peer-reviewed Earth and Planetary Interiors 138, 263e278. papers. Yarmolyuk, V.V., Kovalenko, V.I., Bogatikov, O.A., 1990. South Baikal “hot spot” of the mantle and its role in the formation of the Baikal Rift. Doklady of the USSR Academy of Sciences 312 (1), 187e191. Yarmolyuk, V.V., Ivanov, V.G., Kovalenko, V.I., Samoilov, V.S., 1994. Dynamics of formation and magmatism in the Late Mesozoic-Cenozoic South Khangai Yasnygina Tatyana: Senior Research Scientist at the mantle hotspot (Mongolia). Geotektoniks 5, 28e45. Institute of the Earth’s Crust SB RAS, Irkutsk, Russia (since Yasnygina, T.A., Markova, M.E., Rasskazov, S.V., Pakhomova, N.N., 2015. Determi- 2004), M.S. (1993) from the Irkutsk State University, Ph.D. nation of rare earth elements, Y, Zr, Nb, Hf, Ta, and Th in geological reference (2004) from the Institute of the Earth’s Crust SB RAS. Her materials of the DV series by ICP-MS. Zavodskaya Laboratoriya. Diagnostika scientific interests are geochemistry of volcanic rocks and materialov 81 (2), 10e20. crude oils, trace-element melting modeling. The results of Zack, T., Foley, S.F., Jenner, G.A., 1997. A consistent partitioning coefficient set for her research have been presented in more than 110 publi- clinopyroxene, amphibole and garnet from laser ablation microprobe analyses cations that include 2 monographs and 29 peer-reviewed of garnet pyroxenite from Kakanui, New Zealand. Neues Jahrbuch fur Miner- papers. alogie. Abhandlungen 172 (1), 23e41. Zhang, Z., Mahoney, J.J., Mao, J., Wang, F., 2006. Geochemistry of picritic and associated basalt flows of the Western Emeishan Flood Basalt Province, China.