Early Cretaceous trachybasalt-trachyte-trachyrhyolitic volcanism in the Nyalga basin (Central Mongolia): sources and evolution of continental rift magmas

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Early Cretaceous trachybasalt–trachyte–trachyrhyolitic volcanism in the Nyalga basin (Central Mongolia): sources and evolution of continental rift magmas I.S. Peretyazhko *, E.A. Savina, S.I. Dril’ A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia Received 20 March 2018; accepted 27 July 2018

Abstract As shown by geological, mineralogical, and isotope geochemical data, trachybasaltic–trachytic–trachyrhyolitic (TTT) rocks from the Nyalga basin in Central Mongolia result from several eruptions of fractionated magmas within a short time span at about 120 Ma. Their parental basaltic melts formed by partial melting of mantle peridotite which was metasomatized and hydrated during previous subduction events. Basaltic trachyandesites have high TiO2 and K2O, relatively high P2O5, and low MgO contents, medium 87Sr/86Sr(0) ratios (0.70526–0.70567), and almost zero or slightly negative εNd(T) values. The isotope geochemical signatures of TTT rocks are typical of Late Mesozoic basaltic rocks from rift zones of Mongolia and Transbaikalia. The sources of basaltic magma at volcanic centers of Northern and Central Asia apparently moved from a shallower and more hydrous region to deeper and less hydrated lithospheric mantle (from spinel to garnet-bearing peridotite) between the Late Paleozoic and the latest Mesozoic. The geochemistry and mineralogy of TTT rocks fit the best models implying fractional crystallization of basaltic trachyandesitic, trachytic, and trachyrhyodacitic magmas. Mass balance calculations indicate that trachytic and trachydacitic magmas formed after crystallization of labradorite-andesine, Ti-augite, Sr-apatite, Ti-magnetite, and ilmenite from basaltic trachyandesitic melts. The melts evolved from trachytic to trachyrhyodacitic and trachyrhyolitic compositions as a result of prevalent crystallization of K–Na feldspar, with zircon, chevkinite-Ce, and LREE-enriched apatite involved in fractionation. Trachytic, trachyrhyodacitic, and trachyrhyolitic residual melts were produced by the evolution of compositionally different parental melts (basaltic trachyandesitic, trachytic, and trachyrhyodacitic, respectively), which moved to shallower continental crust and accumulated in isolated chambers. Judging by their isotopic signatures, the melts assimilated some crustal material, according to the assimilation and fractional crystallization (AFC) model. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: trachybasalt; basaltic trachyandesite; trachyte; trachydacite; trachyrhyodacite; trachyrhyolite; Early Cretaceous volcanism; rift; Nyalga basin; Central Mongolia

Introduction The Mongolia–Okhotsk belt in Mongolia and Transbaikalia formed by ocean closure in the Early–Middle Jurassic (Kuzmin et al., 2010; Parfenov et al., 2003). The Late Mesozoic continental evolution of the orogen was accompanied by magmatism that produced high-alkali plutonic and volcanoplutonic complexes in rifts of Central and East Mongolia and in Transbaikalia (Kuzmin and Yarmolyuk, 2014; Yarmolyuk et al., 2000). The territory comprises three major units: East Mongolian volcanic belt (EMVB) and two rift zones of South Khingan and West Transbaikalia (Fig. 1). The Late Mesozoic West Transbaikalian rifting was concurrent with Late Jurassic

* Corresponding author. E-mail address: [email protected] (I.S. Peretyazhko)

shoshonite-latite and Late Jurassic–Early Cretaceous bimodal trachybasalt–trachyrhyolite volcanism in East Transbaikalia (Kazimirovskii, 2001; Pervov et al., 1987; Sasim et al., 2016; Tauson et al., 1984). The East Mongolian volcanic belt consists of several parallel and quasi-parallel basins bounded by uplifted blocks of metamorphic clastic and Paleozoic granitic rocks (Batulzii et al., 2011). The basins extend in an SW–NE 300 km wide strip along the Kerulen River in Mongolia and on into China and East Transbaikalia. In the northeastern direction, the volcanic belt broadens and the basins become larger; the largest ones are the Nyalga basin (also referred to as Choir-Nyalga or Nilga) in Central Mongolia and the North Choibalsan basin in Northeastern Mongolia. Late Mesozoic lavas originally made a 1500 m thick cover of ~500,000 km2 in total within Mongolia and East Transbaikalia (Frikh-Khar and Luchitskaya, 1978), but they have been

1068-7971/$ - see front matter D 201 8, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 8.12.011

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Fig. 1. Location map of Late Mesozoic volcanic areas in Central Asia, complemented after (Parfenov et al., 2003; Vorontsov et al., 2016; Yarmolyuk et al., 1995). 1, grabens in rift zones; 2, Late Mesozoic rift zones; 3, volcanic fields of Bolshoi Khingan belt; 4, Late Cenozoic basins in the Baikal rift system; 5, cratonic areas; 6, Central Asian Orogenic Belt. Abbreviations stand for WTR, West Transbaikalian rift; SHR, South Hangayn rift; EMVB, East Mongolia volcanic belt; BHVB, Bolshoi Khingan volcanic belt. Boxes are areas of Late Mesozoic volcanic rocks, with their names abbreviated as NB, Nyalga basin; Ud, Uda segment; TK, Tungnui–Khilok segment. White dots show Ingoda (In) and Usugli (Us) basins in East Transbaikalia.

eroded to a subvolcanic depth of 200–300 m and remain exposed over a smaller area. The lavas, mainly of trachybasaltic and basaltic trachyandesite compositions, make up large fields of successive lava flows. Mafic lavas in rifts are locally overlain by trachyrhyolite and trachyte to thrachyrhyodacite rocks of the shoshonite–latite series. Other igneous rocks in the basin occur as small fissure bodies, extrusions, tuffs, ignimbrites, trachyrhyolitic and dacitic lava breccias, as well as subvolcanic syenite sills and dikes that crosscut trachybasalts and trachyrhyolites. Late Mesozoic lavas in the region belong to the bimodal trachybasalt–trachyrhyolite, shoshonite– latite, and alkali-basalt (basanite) series revealed by geological and geochronological data (Batulzii et al., 2011, 2015). Most of previously obtained K/Ar ages of bimodal basaltic volcanism fall within 115–122 Ma. Shoshonite–latite rocks form extrusive–intrusive domes or calderas, with lava flows often dipping toward the dome center, as well as eruptive vents composed of clastic lavas. The shoshonite–latite domes are

arranged in long chains across the basin strike. Shoshonite from the Tsagandeler district in the southwestern Nyalga basin and basanite from the EMVB alkali-basaltic series, which is known from a single area in northeastern Mongolia (Batulzii et al., 2015), have 40Ar/39Ar ages of 114 ± 0.7 Ma and 104 ± 0.7 Ma, respectively. Geochemical and isotopic signatures in samples of the series have been studied at few sites of the large EMVB territory (Bars et al., 2018; Batulzii et al., 2015), but they require more thorough investigation in many lava sections along the belt. This study aims at gaining new mineralogical, chemical, and geochronological evidence of volcanism in the Nyalga basin (Figs. 1, 2), with implications for magma sources. We discuss models that simulate partial melting of mantle protoliths and results of mass balance calculations for fractional crystallization of magma differentiated into basaltic trachyandesitic, trachytic, trachyrhyodacitic, and trachyrhyolitic melts.

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Fig. 2. Local geology of Nyalga basin. 1, undifferentiated Mesozoic–Cenozoic sediments; 2, undifferentiated Cretaceous sediments; 3, Saishand Formation (K1) clastic sediments; 4, Dzunbain Formation. (K1) clastic sediments; 5, trachybasalt–basaltic trachyandesite (K1); 6, trachyte–trachyrhyodacite (K1); 7, trachyrhyolite (K1); 8, Upper Jurassic granite; 9, Upper Paleozoic (mainly Permian) metamorphic rocks; 10, faults; 11, main sampling sites. Map is based on 1:200,000 and 1:500,000 geological surveys (1952–1953), Google satellite images, and field observations by I.S. Peretyazhko and E.A. Savina.

Geology of the Nyalga Basin Volcanic rocks from the Nyalga basin belong to the Early Cretaceous Dzunbain Formation (Fig. 2). The formation section begins with a 100 m thick sedimentary member, which is overlain by alternated trachybasaltic and basaltic trachyandesitic lavas with few layers of limy sandstone, marl, and shale. According to drilling evidence, the volcanic rocks with scarce sedimentary, tuff, and sedimentary-pyroclastic intercalations are from 200–250 to 600 m thick. Mafic volcanics, especially abundant in the area (Fig. 2), build lava fields and flows that reach 10–15 km long and hundreds of meters thick. Basaltic magma epurted upon an almost flat eroded surface of sediments (Early Cretaceous Saishand and Dzunbain Formations) and upon Upper Paleozoic (mostly Permian) metamorphic rocks. Trachytes and compositionally similar trachytic rocks of the shoshonite–latite series (trachydacites and trachyrhyodacites) occur as lava fields and flows up to 50–70 m thick, which lie over Cretaceous sediments in the southern part of the basin and over trachybasalts in the northeast. Trachyrhyolites are found at several sites in the southeastern and northeastern basin flanks, within isolated centers of felsic volcanism. Trachyrhyolitic magma erupted mainly upon Cretaceous sediments, trachybasalts, and

trachyte–trachydacites and produced from 1 m to tens of meters thick fields, the largest one reaching a surface area of ~20 × 7 km2 and a thickness of 50–150 m (Peretyazhko and Savina, 2014; Peretyazhko et al., 2014). The relative position of volcanics in local sections within the Nyalga basin are as follows (bottom to top): trachybasalt–trachyte (trachydacite), trachybasalt–trachyrhyolite, trachyte (trachydacite)–trachyrhyolite, trachybasalt–trachyte (trachydacite)–trachyrhyolite. Basaltic trachyandesites overlie trachyrhyolites in the Shibe locality (northeastern part of the basin), while alternated 1–2 m layers of mafic and felsic lavas are known from vicinities of Lake Deret Nuur (Fig. 2). Trachytic (trachydacite and trachyrhyodacite) rocks sometimes enclose trachyrhyolite xenoliths. Geological observations reveal eruptions of mafic, intermediate, and felsic magmas which followed one shortly after another and produced trachybasalt–trachyte–trachyrhyolite (TTT) rocks found in the Nyalga basin.

Analytical procedures Bulk chemistry of rock samples was analyzed by the XRF method on a CPM-25 X-ray fluorescence spectrometer, at the Analytical Center for Isotopic and Geochemical Studies,

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Vinogradov Institute of Geochemistry (Irkutsk). Trace elements were determined by mass spectrometry with inductively coupled plasma (ICP-MS) on a Perkin Elmer NexION 300D mass spectrometer, applying open-vessel acid digestion (analyst L.S. Tauson). Mineral phases and glasses in rock samples were investigated in polished thin sections by scanning electron microscopy with energy-dispersive spectrometry (SEM EDS) at the Analytical Center for Multielement and Isotope Studies, Sobolev Institute of Geology and Mineralogy (Novosibirsk), using a MIRA-3 LMU scanning electron microscope equipped with an Oxford Instruments INCA Energy 450 microanalysis system (analyst N.S. Karmanov). The analyses were performed at 20 kV accelerating voltage, 1.5 nA beam current, and 20 s counting time, to a precision of 0.2–0.3 wt.% (Lavrent’ev et al., 2015). Silicate glasses and minerals were analyzed by scanning over >10 µm2 rectangles to minimize the losses of alkalis. Samples for isotope analyses were preconditioned at the Analytical Center for Isotope and Geochemical Studies, Vinogradov Institute of Geochemistry (Irkutsk). Pure Sr fractions were extracted using ion exchange resin BioRad AG 50W-X8, 200–400 mesh and BioRad AG 50-X12, 200–400 mesh. The Sr isotope composition and Rb and Sr concentrations in samples were determined by isotope dilution using the mix tracer 85Rb + 84Sr. Pure Nd fractions were obtained using ion exchange resin BioRad AG-50WX12 200–400 mesh and Ln-Eicrome. The Nd isotope composition and the concentrations of Nd and Sm were determined by isotope dilution using the mix tracer 149Sm + 150Nd (analysts T.A. Vladimirova and V.V. Yarovaya). The Sr and Nd isotope compositions were analyzed in a static mode on a Finnigan MAT-262 mass spectrometer with 7 collectors (analyst N.S. Gerasimov), at the Analytical Center for Geodynamics and Geochronology, Institute of the Earth’s Crust (Irkutsk). Sr isotope composition was determined in 50 ng specimens placed on Ta or W cathode with a Ta2O5 activator, using a monoenergetic ion beam, at a 88Sr beam current of 2–3 × 10–11 A. The results were checked against the respective values for the NBS-987 and BCR-2 standards, which were 87 86 Sr/ Sr = 0.710254 ± 7 (2SD, n = 45) and 87Sr/86Sr = 0.705011 ± 14 (2SD, n = 7), respectively, in the period of observations. The Nd isotope composition was analyzed in 100-200 ng specimens placed on Rh cathodes, using a dual-energy ion beam, at a 146Nd beam current of 0.5–1.0 × 10–11 A. The presence of Sm traces in the Nd spectrum was monitored according to the 147Sm/144Nd ratio which was always below 0.00005. The results were checked against the JNdi-1 and BCR-2 standards, with 143Nd/144Nd = 0.512107 ± 4 (2SD, n = 35) and 143Nd/144Nd = 0.512629 ± 8 (2SD, n = 18), respectively, in the period of observations. Oxygen isotope composition was analyzed in bulk rocks samples on a FINNIGAN MAT-253 stable isotope ratio mass spectrometer (analyst V.F. Posokhov) at the Geological Institute (Ulan-Ude). The values of δ18O were calculated relative to the international standard of quartz NBS-28 and the domestic standards of GI-1 (Geological Institute, Moscow) and

Polaris (Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Moscow), to an error of ± 0.2‰ δ18O. Crystal chemical formulas of minerals and mass balance models of fractional crystallization were computed using the CRYSTAL software (Peretyazhko, 1996).

Major-oxide and trace-element compositions Major oxide contents in the Nyalga TTT samples (see Table 1 for representative analyses) were recalculated to 100% without 1 to 3.5 wt.% loss on ignition (LOI), for correct comparison. The normalized values were used for classification of rocks, mass balance calculations, and data interpretation. Mafic lavas mostly contain 50 to 56 wt.% SiO2, 5.6– 6.6 wt.% Na2O + K2O (Fig. 3) with 2–3.2 wt.% K2O, which corresponds to high-K basaltic trachyandesite. Only few out of thirty points in the TAS diagram fall within the field of trachybasalt (two), phonotephrite and trachyandesite (one in each field). The rocks contain quite high concentrations of TiO2 (2–3.3 wt.%), Fe2O3 (8.6–12.6 wt.%), P2O5 (1.1– 1.7 wt.%), and LOI (2–3 wt.%), while MgO does not exceed 4 wt.% and Al2O3 is below 18 wt.% (Table 1, analyses 1–7). SiO2 is in negative correlation with TiO2 and Fe2O3 (Fig. 4). Mafic samples in our collection are mostly basaltic trachyandesites, which is likely proportional to their true percentage in the sampled area of the Nyalga basin (Fig. 2). In the TAS diagram, the compositions of volcanics belong to the fields of trachyte, trachydacite, trachyrhyodacite, and alkali rhyodacite and trachyrhyodacite within the ranges 66–74 wt.% SiO2 and 8–11 wt.% Na2O + K2O. In the SiO2– K2O and Na2O–K2O diagrams, trachytic rocks are the highest-silica varieties of the shoshonite–latite series (Fig. 3). The SiO2 difference between basaltic trachyandesite and trachyte rocks may reach 10–15 wt.%, i.e., the volcanism is bimodal (Figs. 3, 4). Trachyrhyolites contain up to 75–80 wt.% SiO2, 7.8–8.9 wt.% Na2O + K2O, 4.8–5.3 wt.% K2O, 2.7–3.6 wt.% Na2O, and 0.4–0.6 wt.% H2O+. The A/CNK ratio is haplogranitic (0.96–1.08) to plumasitic (1.10–1.18). Intermediate and felsic rocks are mainly trachyte–trachydacites and trachyrhyolites, according to geological surveys and our field observations (Fig. 2). Trachytic rocks vary strongly in composition (Table 1, analyses 8–13) and often show gradual transitions within one or several lava flows. The contents of many major oxides (Al2O3, TiO2, Fe2O3, MgO, CaO, Na2O, K2O, and P2O5) decrease systematically from trachyte to trachyrhyolite with increasing SiO2 (Fig. 4). Trace element patterns normalized to primitive mantle (PM) are shown in spider diagrams (Figs. 5 and 6). Incompatible elements change only slightly in mafic rocks (except for Cs). The spider diagram of Fig. 5a shows prominent lows in Nb–Ta ((La/Nb)PM = 1.8–3.4), Sr, Zr, Hf, Ti, and Y, and highs in Ba, La, Pb, P, Nd, Gd, and Ho. The concentrations of Ti in basaltic trachyandesites are similar to those in ocean-island basalts (OIB), while Ba, K, REE, Pb, Sr, P, Zr,

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I.S. Peretyazhko et al. / Russian Geology and Geophysics 59 (2018) 1679–1701 Table 1. Representative analyses of TTT rocks from Nyalga basin Component

SiO2, wt.% TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 F LOI Total Li, ppm Rb Cs Ba Sr Zr Hf Ta Nb Be Sc V Cr Co Ni Cu Zn Ga Ge Mo Sn Sb W Pb Th U Tl Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

542

550

566

1

2

3

4

46°49.17′ N

46°48.82′

46°48.37′

46°47.63′

107°58.14′ E 107°55.83′

107°55.74′

107°55.73′

49.75 2.95 15.76 9.04 1.62 0.10 3.51 7.32 3.56 2.36 1.38 0.16 2.50 99.94 49 43 0.3 1106 989 347 7.8 1.6 23 2.2 17 197 62 28 41 38 179 22 0.9 1.2 1.6 0.3 0.6 14 4.0 1.2 – 32 76 167 19 81 15 3.8 12 1.4 7.5 1.4 3.2 0.4 2.2 0.4

49.20 3.24 15.33 7.92 3.23 0.07 2.48 7.83 3.50 1.99 1.83 0.17 3.30 100.01 26 52 0.6 1309 1216 434 9 1.8 31 3.3 19 227 57 27 34 38 202 25 1.0 1.9 2.0 0.0 0.6 17 4.2 1.2 – 40 98 224 27 109 19 4.7 15 1.7 9.5 1.6 4.0 0.5 2.7 0.4

49.80 3.05 16.50 9.94 2.07 0.06 1.88 6.58 3.70 2.32 1.50 0.21 2.51 100.02 55 28 0.1 1221 1124 387 8.6 1.6 25 2.5 17 178 68 28 37 32 205 24 0.9 1.2 1.7 0.1 1.0 15 4.0 1.6 – 32 83 181 22 90 16 4.3 12 1.4 7.7 1.4 3.3 0.4 2.4 0.3

48.80 2.89 17.19 9.24 2.15 0.06 2.72 6.68 3.88 2.94 1.21 0.20 2.13 99.99 44 35 0.2 1131 1112 355 8 1.9 30 2.9 17 207 41 26 29 42 174 23 1.0 0.7 2.0 0.2 1.1 15 4.3 1.4 – 28 84 179 21 83 14 3.7 11 1.2 6.6 1.2 2.8 0.4 2.0 0.3

569

576

1107

1139

1117

1068

1147

5

6

7

8

9

10

46° 47.64′

47°1.23′

47°0.64′

47°1.99′

47°0.02′

47°0.49′

107°55.68′

108°2.66′

108°2.59′

107°59.43′

108°4.91′

108°4.48′

51.80 2.52 15.11 3.53 6.65 0.13 3.77 6.64 3.48 2.32 1.28 0.20 2.51 99.85 20 75 0.7 1094 877 478 12 1.7 29 3.2 18 205 48 26 30 37 179 23 1.0 2.5 2.2 0.1 0.7 18 6.0 1.5 – 42 94 198 24 96 18 4.2 14 1.7 9.6 1.8 4.4 0.6 3.3 0.5

53.37 2.34 14.91 5.14 4.13 0.12 3.43 6.37 3.12 2.80 1.15 0.18 2.86 99.84 13 74 0.7 1074 917 602 12 1.6 33 3.0 15 169 51 23 31 48 179 23 1.5 4.4 2.3 0.1 1.3 16 6.0 1.5 0.5 34 79 167 20 83 15 3.1 13 1.3 7.5 1.3 3.4 0.4 2.8 0.4

53.85 2.26 14.67 4.06 5.21 0.12 3.35 6.33 2.88 2.66 1.10 0.16 2.57 99.15 12 80 1.0 1079 966 641 12 1.5 32 2.9 14 164 54 22 31 51 189 23 1.6 3.5 2.3 0.1 1.2 16 6 1.4 0.5 33 79 168 20 82 14 3.0 13 1.3 7.2 1.3 3.2 0.4 2.7 0.4

65.44 0.94 16.53 2.68 0.93 0.07 0.43 1.60 3.91 5.54 0.21 0.06 1.37 99.68 19 122 3.1 2105 400 944 16 1.8 36 3.0 7.0 30 1.8 3.4 4.7 12 83 22 1.7 2.4 2.5 0.4 8.4 23 9 1.8 0.5 32 82 165 20 78 13 3.0 13 1.2 6.8 1.2 3.2 0.4 2.8 0.4

67.87 0.62 16.02 2.31 0.43 0.04 0.36 0.96 4.57 5.98 0.11 0.01 0.60 99.88 23 133 1.8 897 140 80 3.0 1.8 34 3.5 4.4 14 2.1 1.4 2.7 4.0 78 22 1.0 1.9 2.8 0.2 1.6 27 11 1.1 0.6 29 87 194 22 81 14 2.3 10 1.2 7.1 1.3 3.1 0.4 2.4 0.3

69.01 0.73 14.90 2.66 1.15 0.05 0.44 1.25 3.17 5.28 0.15 0.08 0.84 99.67 18 104 2.3 1617 259 556 11 1.5 30 2.7 4.9 18 10 2.3 5.2 13 79 21 1.5 1.8 2.6 0.4 3.0 26 8 1.5 0.4 27 77 155 19 75 12 2.4 11 1.4 6.0 1.1 2.7 0.4 2.3 0.3

(continued on next page)

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Table 1 (continued) Component

SiO2, wt. % TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 F LOI Total Li, ppm Rb Cs Ba Sr Zr Hf Ta Nb Be Sc V Cr Co Ni Cu Zn Ga Ge Mo Sn Sb W Pb Th U Tl Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

1141 11 47°0.76′ N 108°2.02′ E 67.55 0.57 14.98 1.38 1.15 0.06 0.57 0.97 3.19 5.76 0.07 0.14 3.53 99.91 16 156 2.6 345 64 399 9.9 2.3 44 4.1 4.9 10 1.5 1.2 2.2 7.8 91 24 2.1 4.4 3.6 0.4 2.1 33 15 2.6 1.4 53 160 349 38 147 24 1.8 23 2.1 12 2.0 5.1 0.7 4.1 0.6

1103 12 47°2.72′ 108°1.64′ 71.17 0.50 14.65 1.92 0.65 0.05 0.32 0.51 3.40 5.94 0.06 0.06 0.72 99.88 27 158 1.7 208 63 291 7.9 2.2 41 3.9 4.2 12 7.0 1.3 3.4 7.0 64 23 1.7 2.1 3.3 0.3 1.8 26 15 2.3 0.7 46 136 278 34 131 23 1.5 21 2.5 11 1.9 4.7 0.6 3.6 0.5

1110 13 47°0.77′ 108°3.62′ 73.27 0.48 13.42 1.93 0.36 0.04 0.25 0.68 3.17 5.16 0.08 0.04 1.06 99.92 16 133 1.6 405 66 239 6.6 1.9 34 3.1 3.4 9.5 4.9 1.0 2.9 7.0 61 21 1.5 1.9 2.2 0.3 2.6 23 13 2.0 0.3 34 128 277 33 126 20 1.5 18 2.1 8.8 1.4 3.5 0.4 2.6 0.4

540 14 46°48.10′ 107°55.93′ 76.81 0.07 11.24 0.42 1.51 0.10 0.20 3.25 5.06 0.02 0.02 1.26 100.20 – 253 3.2 41 26 114 4.7 2.2 35 8.0 1.9 5.0 4.7 1.5 6.9 11.4 32 18 1.3 2.5 3.1 0.4 1.6 30 26 2.8 0.8 25 70 141 16 52 8.8 0.3 7.6 0.9 5.2 1.0 2.5 0.4 2.3 0.3

555 15 46°48.75′ 107°55.42′ 75.70 0.14 11.92 1.82 0.48 0.03 0.11 0.32 3.40 5.15 0.03 0.07 0.99 100.13 – 238 2.1 38 18 119 4.8 2.2 34 7.2 1.9 16 1.5 1.3 3.8 7.1 57 20 1.4 2.9 4.0 0.6 1.5 40 26 3.3 1.2 28 74 147 17 59 10 0.4 8.9 1.1 5.9 1.2 2.8 0.4 2.6 0.4

560 16 46°50.19′ 107°55.03′ 76.74 0.21 12.10 0.42 0.18 0.03 0.05 0.44 3.40 5.15 0.02 0.04 0.46 99.22 – 254 2.5 49 24 89 3.5 2.2 29 3.9 1.0 2.0 0.7 0.3 1.6 4.9 8 21 1.3 0.4 18 0.2 1.0 28 26 2.5 0.4 19 79 182 17 56 8.8 0.4 7.8 0.8 4.1 0.8 1.8 0.3 1.9 0.3

1072 17 46°49.99′ 107°59.04′ 76.31 0.17 12.11 2.16* – 0.03 0.23 0.41 2.88 5.09 0.05 – 0.46 99.90 38 216 2.8 551 50 125 4.9 2.5 32 6.2 1.6 9.1 4.0 1.4 3.6 5.2 46 19 1.4 2.4 2.6 0.4 1.1 26 27 3.1 0.6 38 76 147 17 55 9.7 0.6 8.6 1.1 6.4 1.3 3.9 0.6 4.4 0.6

1218 18 46°55.06′ 108°1.92′ 78.71 0.14 11.18 0.31 0.65 0.01 0.10 0.50 2.68 4.56 0.03 0.03 1.56 100.42 15 202 5.0 121 25 138 5.4 2.4 27 5.5 2.0 5.1 5.2 0.8 2.6 5.1 24 19 1.6 4.6 3.2 0.5 3.3 32 35 7.1 0.7 20 61 116 12 38 6.3 0.4 6.2 0.8 4.0 0.8 2.3 0.4 2.6 0.4

1120 19 47°1.54′ 107°59.17′ 71.12 0.19 10.99 0.67 0.80 0.06 0.03 5.00 3.46 4.75 0.03 2.45 1.63 100.14 30 212 2.6 39 178 172 5.9 2.4 45 7.2 1.8 4.2 21 0.8 4.7 6.5 50 18 1.9 4.2 2.7 1.1 2.3 29 18 3.0 0.3 26 45 81 8.0 26 4.1 0.3 4.8 0.5 3.4 0.7 2.2 0.4 2.5 0.4

Note. All sample numbers have the prefix MN-. Locations of sampling sites are defined as latitude (N) and longitude (E) degrees and minutes with a decimal divider. Major oxides are in wt.% and trace elements are in ppm. 1–7, basaltic trachyandesites; 8–10, trachyte–trachydacites; 11–13, trachyrhyodacites; 14–18, trachyrhyolites; 19, fluorite-bearing trachyrhyolite. LOI is loss on ignition. Trace element contents are determined by ICP-MS. Sample 19 is analyzed by wet chemistry; other analyses are by XRF method. Fluorine is determined by injection AES. Totals are corrected for F contents. Dash means no data. Rocks are identified according to TAS classification according to The Petrographic Code of Russia (2009) as compositions normalized to 100% without LOI (Fig. 3). *All iron as Fe 2O3.

Fig. 3. TTT rocks, matrix, and melt inclusion glasses in TAS diagram. 1–4, rocks in Nyalga basin: basaltic trachyandesite (1), trachytic rocks (trachyte, trachydacite, and alkali rhyodacite) (2), trachyrhyodacite (3), trachyrhyolite (4); 5, basaltic rocks of Tugnui–Khilok segment of West Transbaikalian rift zone (Vorontsov and Yarmolyuk, 2007); 6, 7, basaltic trachyandesites of Ingoda and Usugli rift zones in East Transbaikalia, respectively (Kazimirovskii et al., 2001; unpublished data by Peretyazhko et al.); 8-10, matrix glasses in samples MN-1139 (8), MN-576 and MN-1107 (9), MN-1141 (10); 11, glasses of melt inclusions in quartz from trachyrhyolites (Peretyazhko et al., 2014). Thin dotted lines with arrows lead from rock compositions to fields of matrix glasses. Composition fields are according to The Petrographic Code of Russia (2009).

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Fig. 4. Major oxide variations, TTT rocks. Arrows show evolution trends from basaltic trachyandesites to trachytes. Legend same as in Fig. 3.

and Hf are higher. Most of REE are light lanthanides ((La/Yb)PM = 12–31). Compared with OIB, the chondrite-normalized REE spectra of basaltic trachyandesites have a worse pronounced negative Eu anomaly, higher concentrations of LREE and slightly higher HREE ((La/Yb)N = 20–31), the average ΣREE being 399 ppm (Fig. 7a). The PM-normalized REE patterns of trachyte–trachydacites in the spider diagram of Fig. 5b differ from those for basaltic trachyandesites (Fig. 5a) in large negative anomalies of Sr, P, and Ti and in greater K and Pb peaks, and in large Ba, Zr, and Hf variance. As SiO2 increases, trachyte–trachydacites decrease in Ba, Zr, and Hf, this trend being still more prominent in trachyrhyodacites (Fig. 5b, 6a). The spider diagrams of trachyrhyodacites (Fig. 6a) show distinct lows in Ba, Sr, P, Zr, Hf, and Ti, a well pronounced negative Eu anomaly, and average ΣREE reaching 636 ppm (Fig. 7c). In spite of a large difference in major oxides, trachyte–trachydacites (Table 1, analyses 8–10) are very similar to basaltic trachyandesites (Table 1, analyses 1–7) in chondrite-normal-

ized REE spectra (Fig. 7b) and average ΣREE (412 ppm), but have a more prominent negative Eu anomaly. Trachyrhyolites have the lowest concentrations of Ba, Sr, P, REE, and Ti possible for the Nyalga TTT rocks and, correspondingly, negative anomalies of these elements in normalized patterns (Fig. 6b). Trachyrhyolites have the highest concentrations of Cs, Rb, Th, U, Ta, and Pb (Table 1, analyses 14–19). Figure 8 shows variations in relationships of Sr with Ba, Zr, Ta, Nb, La, Rb, Th, and Pb. Ba, Zr, La (and other REE) are much greater in trachyte–trachydacites than in basaltic trachyandesites and then systematically decrease to trachyrhyodacites and trachyrhyolites. Other trace elements change gradually from mafic to felsic rocks. All samples have similar (La/Yb)N and Sm/Nd ratios (Fig. 7). Mineralogy Mafic (trachybasalt and basaltic trachyandesite) rocks are dark to almost black or locally reddish-brownish (altered

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Fig. 5. Multielement patterns in basaltic trachyandesites and trachyte–trachydacites normalized to primitive mantle (McDonough and Sun, 1995). Gray shade shows fields of basaltic trachyandesites (a) and trachyte–trachydacites (b) in Nyalga basin; solid lines show analyzed samples and OIB compositions. Arrows in panel (b) show Ba, Sr, P, Zr, Hf, and Ti in trachyte rocks decreasing with increasing SiO2.

Fig. 6. Multielement patterns in trachyrhyodacites and trachyrhyolites normalized to primitive mantle (McDonough and Sun, 1995). Gray shade shows fields of trachyrhyodacites (a) and trachyrhyolites (b) in Nyalga basin; solid lines show analyzed samples and OIB compositions.

varieties). They have massive structures and aphyric or less often fine porphyritic textures with a fine to medium-grained hypocrystalline or locally porous matrix. They contain 10– 15 vol.% of plagioclase, clinopyroxene, titanomagnetite, and rare ilmenite phenocrysts; some samples contain olivine. Phenocrysts of most calcic plagioclase are up to 1–3 mm, have corroded grain boundaries and sometimes enclose antiperthite ingrowths of feldspar, acicular clinopyroxene, and fine grains of Fe and Ti oxides. Clinopyroxenes and olivines are 1–2 mm grains. Titanomagnetite segregations often enclose exsolution lamelli of ilmenite. Plagioclase, clinopyroxene, titanomagnetite, and fluorapatite occur also as microliths. Plagioclase microliths in crystalline varieties of mafic rocks have rims of K–Na feldspar, the latest phase in the matrix. The interstitial space between microliths is sometimes occupied by dark residual glass with phenocrysts and submicron dendritic segregations of Fe-rich phases (possibly, titanomagnetite). Altered rocks contain pseudomorphs of chlorite and epidote

after clinopyroxene, serpentine and chlorite after olivine, and hematite after titanomagnetite. The chemistry of minerals and glasses was analyzed by SEM-EDS in five basaltic trachyandesite samples: three (MN-576, MN-1107, MN-1139) olivine-bearing and two (MN-542 and MN-569) olivine-free ones. See Tables 2–5 for average compositions of mineral phases. Olivines are mostly Mg-rich varieties intermediate between forsterite and fayalite (Fo56–67Fa44–33) (Table 2, analyses 1–3). The feldspar compositions are especially variable (Fig. 9). Plagioclase (labradorite-andesine) is more calcic (An45Ab51Or4) in olivinebearing basaltic trachyandesites than that (An38Ab57Or5) in the olivine-free samples (Table 3, analyses 10–19). The matrix of all samples contains fine segregations of Ca–Na–K feldspar (An9–27Ab59–67Or12–23) and sanidine (An3–4Ab42–47Or42–48), with 0.2–1.3 wt.% BaO, which crystallized after plagioclase; plagioclase in rims of phenocrysts likewise contains 0.2– 0.3 wt.% BaO. Clinopyroxene has a relatively uniform com-

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Fig. 7. REE patterns normalized to chondrite CI (McDonough and Sun, 1995). Gray shade shows fields of basaltic trachyandesites (a), trachyte–trachydacites (b), trachyrhyodacites (c), and trachyrhyolites (d) in Nyalga basin; solid lines show analyzed samples and OIB compositions. Panels (a) to (d) show average values of (La/Yb)N, Sm/Nd and ΣREE.

position (En29–37Fs29–37Wo42–43) with 1.5 to 5.7 wt.% TiO2, which corresponds to Ti-augite (Table 2, analyses 4–8). Fe and Ti oxides occur as titanomagnetite with 13–20 wt.% TiO2 and ferrian ilmenite with a hematite component, up to 10% (Table 4, analyses 24–32). Titanomagnetite in sample MN-542 is exsolved into Ti-bearing magnetite and ilmenite. All samples bear fluorapatite phenocrysts and microliths with up to 1.4 wt.% SrO (Table 5, analyses 37–40). Trachyte–trachydacite glasses occur in interstitials between phenocrysts and microliths in sample MN-1139 (Table 5, analysis 49), along with micrometer acicular amphibole, possibly, kaersutite (Table 5, analyses 43). Interstitials between matrix minerals in

samples MN-576 and MN-1107 are occupied by glass with large composition variations from alkali rhyodacite to trachyrhyodacite and trachyrhyolite (Table 5, analyses 47, 48). The compositions of matrix glasses in mafic rocks are presented in the TAS diagram of Fig. 3. Shoshonite–latite trachytic lavas (trachytes, alkali rhyodacites, trachydacites, and trachyrhyodacites) share mineralogical similarity and look similar. They are pinkish or sometimes reddish rocks with coarse porphyritic textures. Sanidine is obviously predominant in 30–40 vol.% of phenocrysts, where it forms up to 5–7 mm crystalline aggregates. Many phenocrysts are cut by multiple dissolution channels.

Table 2. Mineral chemistry of olivine and clinopyroxene Component

576

1107

1139

542

569

576

1107

1139

1141

(12)

(11)

(20)

(8)

(6)

(8)

(11)

(4)

(19)

1

2

3

4

5

6

7

8

9

SiO2

35.66

36.33

37.16

49.92

50.49

48.94

48.92

44.67

52.79

TiO2

–

–

–

1.93

1.61

2.22

2.87

5.80

0.39

Al2O3

–

–

–

2.67

2.39

4.00

3.53

5.34

1.06

FeO

36.74

33.17

28.91

12.09

13.23

14.96

13.00

16.25

10.58

MnO

0.69

0.60

0.43

0.30

0.39

0.33

0.34

0.29

0.88

MgO

26.57

29.57

33.14

12.59

11.95

9.97

11.40

8.91

13.72

CaO

0.37

0.33

0.35

20.07

19.39

19.01

19.42

17.91

19.97

Na2O

–

–

–

0.43

0.56

0.58

0.52

0.83

0.62

Note. Here and in Tables 3–5, 7: All sample numbers have the prefix MN-. Numerals in braces after sample number show number of SEM-EDS analyses, for which average compositions were calculated. No. is sample number: 1–3, olivine; 4–9, clinopyroxene. Compositions are normalized to 100%. All iron as FeO. Dash means concentrations below SEM-EDS detection.

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Fig. 8. Concentrations of Sr and trace elements in TTT rocks. Arrows show fractional crystallization trends from basaltic trachyandesites to trachyrhyolites (see text for explanation). UC, Average composition of upper continental crust (Taylor and McLennan, 1995). Legend same as in Fig. 3.

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Fig. 9. Chemistry of feldspars. 1, plagioclase in basaltic trachyandesites; 2–6, Ca–Na–K feldspar in basaltic trachyandesites: 2, MN-569, 3, MN-576, 4, MN-1107, 5, MN-1139, 6, MN-542; 7, field of Ca–Na–K feldspar in basaltic trachyandesites and trachyte–trachyrhyodacites; 8, 9, Ca–Na–K feldspar in trachyte–trachyrhyodacites: 8, MN-1068, 9, MN-1141; 10, field of sanidine in trachyte–trachyrhyodacites and trachyrhyolites. Crystal chemical formulas of feldspars are based on 8 oxygens. Mineral names are abbreviated as: An, anorthite; Or, orthoclase; Ab, albite; By, bytownite; Lab, labradorite; And, andesine; Olg, oligoclase; Ant, anorthoclase; Sa, sanidine.

The matrix locally contains phenocrysts of titanomagnetite, ilmenite, fluorapatite, and biotite. Trachyrhyodacites enclose sporadic grains of quartz and segregations of quartz-sanidine symplectite and sometimes bear clasts of quartz grains and aphyric trachyrhyolite matrix. The percentages of melanocratic

minerals decrease from trachytes to trachyrhyodacites. The trachytic matrix consists mainly of aggregated acicular sanidine microliths, which are often aligned with flow texture, and encloses fine phenocrysts of Fe and Ti oxides. The accessories are zircon and chevkinite-Ce.

Table 3. Mineral chemistry of feldspars Component

SiO2 TiO2 Al2O3 FeO CaO Na2O K2O BaO An, % Ab Or

542

569

576

1107

1139

1068

1141

(12)

(15)

(20)

(12)

(34)

(3)

(11)

(11)

(39)

(13)

(14)

(40)

(9)

(35)

10

11

12

13

14

15

16

17

18

19

20

21

22

23

57.84 0.07 25.78 0.80 7.84 6.57 0.84 0.25 38 57 5

65.84 0.32 18.58 0.95 0.56 5.22 8.17 0.35 3 47 50

57.52 0.08 26.10 0.82 8.09 6.54 0.64 0.19 39 57 4

66.14 0.11 18.44 0.70 0.43 4.74 8.98 0.45 2 43 55

56.35 0.09 26.84 0.92 9.30 5.74 0.76 – 45 51 4

65.38 0.49 18.68 0.92 1.02 5.40 6.78 1.33 5 50 45

56.36 0.21 26.77 0.88 9.23 5.78 0.77 – 45 51 4

66.27 0.70 17.93 1.04 0.62 4.92 7.62 0.90 3 47 50

56.49 0.22 26.41 1.14 9.18 5.68 0.82 0.06 45 50 5

60.89 0.24 23.23 0.94 5.72 6.73 2.06 0.20 28 60 12

61.53 – 23.36 0.38 5.22 7.62 1.38 0.50 25 66 9

66.09 0.19 18.79 0.36 0.56 5.09 8.75 0.17 3 45 52

62.65 – 22.50 0.35 3.98 7.64 2.57 0.31 15 65 20

65.89 0.11 19.40 0.27 0.93 6.03 7.22 0.15 3 49 48

Note. Compositions are normalized to 100%. All iron as FeO. Dash means concentrations below SEM-EDS detection. An, Ab, and Or are anorthite, albite, and orthoclase components, respectively.

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542

542

569

569

576

576

1107

1107

1139

1068

1068

1141

1141

(7)

(14)

(12)

(13)

(19)

(6)

(12)

(4)

(13)

(9)

(5)

(11)

(10)

24

25

26

27

28

29

30

31

32

33

34

35

36

SiO2

0.28

0.20

0.27

0.17

0.55

0.39

0.66

0.32

0.63

1.07

–

0.68

0.23

TiO2

2.73

52.04

13.87

49.53

20.38

48.74

19.45

49.16

15.81

11.25

50.24

12.75

44.87

Al2O3

0.70

0.14

0.87

0.73

2.32

0.29

2.24

0.27

2.99

1.35

0.48

1.60

0.34

FeO

32.63

41.26

40.25

38.52

47.55

39.64

46.94

39.11

41.91

40.84

42.54

43.41

34.83

Fe2O3

61.35

3.65

42.95

6.63

25.53

8.10

27.43

7.69

32.20

43.29

5.15

41.03

15.90

MnO

0.30

0.69

0.24

0.16

0.61

0.63

0.59

0.61

0.44

1.82

1.60

0.06

1.39

MgO

0.72

1.67

0.86

3.35

1.81

2.07

1.77

2.58

2.90

0.09

–

0.36

2.44

CaO

0.11

0.15

0.04

0.04

0.14

0.13

0.11

0.27

0.20

0.16

–

–

–

Cr2O3

0.44

0.04

0.40

0.65

0.37

–

0.39

–

2.49

0.07

–

–

–

V2O3

0.75

0.17

0.25

0.21

0.73

–

0.44

–

0.42

0.06

–

0.11

–

Note. 24, 26, 28, 30, 32, 33, 35, titanomagnetite; 25, 27, 29, 31, 34, 36, ilmenite. Compositions are normalized to 100%. FeO and Fe2O3 are calculated according to stoichiometry. Dash means concentrations below SEM-EDS detection.

Table 5. Mineral chemistry of fluorapatite, amphibole, fluor-biotite, and matrix glasses Component

542

569

576

1107

1068

1141

1139

1141

1068

1141

576

1107

1139

1141

(9)

(11)

(3)

(2)

(12)

(23)

(5)

(4)

(17)

(14)

(16)

(12)

(13)

(33)

37

38

39

40

41

42

43

44

45

46

47

48

49

50

SiO2

0.40

0.74

1.55

1.15

1.64

1.86

48.46

48.99

39.84

40.02

72.00

70.90

66.66

73.90

TiO2

–

–

–

–

–

–

4.14

1.41

4.05

5.38

1.25

1.90

2.35

0.42

Al2O3

–

–

–

–

–

–

7.35

6.40

11.37

12.48

14.56

14.15

14.59

14.56

FeO

0.69

0.71

1.37

0.99

0.70

0.68

14.07

11.70

13.49

12.35

1.71

2.16

3.87

1.13

MnO

–

–

–

–

–

–

0.23

0.69

0.40

0.22

–

–

–

–

MgO

0.32

0.32

0.68

0.39

–

–

7.30

16.52

17.20

17.83

0.19

0.09

0.42

0.03

CaO

52.53

52.33

51.44

51.70

50.89

50.59

14.92

10.82

–

–

0.55

0.50

1.79

0.35

Na2O

–

–

–

–

–

–

1.22

2.44

0.63

0.87

3.84

3.43

3.59

3.84

K2O

–

–

–

–

–

–

1.48

1.04

9.25

9.00

5.75

6.71

5.90

5.72

P2O5

42.19

42.02

41.15

41.46

39.89

40.22

0.83

–

–

–

0.06

–

0.66

–

SrO

1.08

1.23

0.94

1.39

–

–

–

–

–

–

–

–

–

–

La2O3

–

–

–

–

0.56

0.63

–

–

–

–

–

–

–

–

Ce2O3

–

–

–

–

2.10

2.12

–

–

–

–

–

–

–

–

Pr2O3

–

–

–

–

0.10

0.12

–

–

–

–

–

–

–

–

Nd2O3

–

–

–

–

1.16

1.16

–

–

–

–

–

–

–

–

Cl

0.16

0.13

0.09

0.16

0.13

0.17

–

–

0.11

–

0.12

0.17

0.13

0.05

F

4.62

4.40

4.85

4.85

4.95

4.29

–

–

6.37

3.08

–

0.04

0.05

–

Note. 37–42, fluorapatite; 43, 44, amphibole; 45, 46, fluor-biotite; 47–50, matrix glass. Compositions are normalized to 100%, with regard to corrections for F and Cl. All iron as FeO. Dash means concentrations below SEM-EDS detection.

According to SEM-EDS, sanidine phenocrysts in the trachyte (MN-1068) and trachyrhyodacite (MN-1141) samples retain zones of relict Ca–Na–K feldspar (An10–30Ab60–80 Or5–25), often in the core (Table 3, analyses 20, 22). Sanidine in microliths and phenocrysts typically have large ranges of Na and K: An1–8Ab30–75Or35–70 (Fig. 9; Table 3, analyses 21,

23). The rocks often contain fluorapatite microliths with REE enrichment (up to 4 wt.% of summed oxides, Table 5, analyses 41, 42) and coarse laths of fluor-biotite (Table 5, analyses 45, 46). The matrix of MN-1068 trachyte is composed of quartzsanidine symplectite. The trachyrhyodacite sample MN-1141 comprises up to 50 vol.% of trachyrhyolite glass (Table 5,

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analysis 50; Fig. 3) with phenocrysts and microliths of sanidine, augite (Table 2, analyses 9), titanomagnetite, and ilmenite (Table 4, analyses 35, 36), sporadic accessory fluorapatite, chevkinite-Ce, and zircon, as well as few plagioclase, amphibole, and orthopyroxene (En60Fs37Wo3) xenocrysts with augite overgrowths. Trachyrhyolites are light gray, pale lilac, pinkish-grayish, white, or pale yellow porphyritic rocks (sometimes with banded or flow textures), with microspherolithic, felsitic, or micropoikilitic matrix textures, which coexist in some thin sections. Quartz and sanidine phenocrysts (An1–5Ab40–65 Or35–60), 1–4 mm in size, occupy up to 30–40 vol.%. Quartz grains have isometrical or dipyramid habits, often with dissolution signatures. Sanidine crystals are most often euhedral; some varieties underwent dissolution. The rock matrix consists of coarse to fine quartz-sanidine symplectites with fine titanomagnetite and ferrian ilmenite grains; some titanomagnetites are fully or partly replaced by hematite. The accessories are zircon, fluorite, fluorapatite, monazite-Ce, xenotyme, and chevkinite-Ce. The mineralogy of trachyrhyolites was described in detail by Peretyazhko et al. (2014). Fluorite-bearing trachyrhyolites found within a 0.5 km2 site in the northeastern Nyalga basin (Table 1, analysis 19) retain of relicts compositionally and rheologically unusual O-bearing F–Ca melts (Peretyazhko et al., 2018).

Ages and Sr, Nd, and O isotope compositions The isotope systematics of the Nyalga TTT rocks is summarized in Table 6. The rocks have been dated by the Rb–Sr method, using data processed in Isoplot/Ex v.4.15 and assuming errors within 1.0 rel.% for 87Rb/86Sr and 0.01 rel.% for 87Sr/86Sr. An isochron age of 120.1 ± 3.0 Ma was obtained

for five trachyrhyolite (MN-540, MN-560, MN-555, MN1072, and MN-1120) and three trachyrhyodacite (MN-1103, MN-1110, MN-1141) samples, assuming the initial isotope ratio of 87Sr/86Sr(0) = 0.70858 ± 0.00072 and MSWD = 16 (Fig. 10). The respective values with two additional trachyte samples (MN-1147, MN-1117) are: 122.8 ± 4.1 Ma, 87 86 Sr/ Sr(0) = 0.70775 ± 0.00091 and MSWD = 161. The dating based on fourteen basaltic trachyandesite, trachyte, trachydacite, trachyrhyodacite, and trachyrhyolite samples corresponds to 126.3 ± 4.6 Ma, at 87Sr/86Sr(0) = 0.70668 ± 0.00088 and MSWD = 303 (Fig. 10). The large uncertainty in the Rb–Sr age and high MSWD result from variance in 87 86 Sr/ Sr(0) in different types of TTT rocks. However, the quite large range of Rb/Sr ratios reliably brackets the time of volcanism in the Nyalga basin between 130 and 117 Ma, or in the second half of the Early Cretaceous (accurate to measurement error for all isotope data). Proceeding from geological data, rock mineralogy, and mass balance calculations (see below), we infer that mafic, intermediate, and felsic magmas erupted successively within a short time span in the Nyalga basin area we studied. Therefore, the initial Sr isotope ratio for all TTT rocks we studied was estimated assuming a 120 Ma Rb–Sr age of trachyrhyolite-trachyrhyodacites (Table 6). The 87Sr/86Sr(0) ratio increases gradually as the rocks contain more SiO2: 0.70526–0.70567 in basaltic trachyandesites, 0.70665–0.70678 in trachyte–trachydacites, and 0.70803–0.70934 in trachyrhyodacite-trachyrhyolites. The Nd isotope composition in the series from basaltic trachyandesite to trachyrhyolite is uniform, and εNd(T) varies in a small range from –0.02 to –1.54, as recalculated to the 120 Ma age. Oxygen isotope composition was determined in several TTT samples (Table 6). The δ18O ratio is the lowest in basaltic trachyandesites (7.5 and 8.5‰) and is slightly greater in

Table 6. Isotope systematics No.

Sample

Rb

Sr

87

Rb/86Sr

87

Sr/86Sr

±2σ

(87Sr/86Sr)0 Sm

147

143

1

MN-542

36

888

0.1184

0.70573

1

0.70553

14.87 82.40

0.10993

0.512553

10

–0.3

7.5

2

MN-566

42

1038

0.1194

0.70587

3

3

MN-550

31

1029

0.0873

0.70541

5

0.70567

18.94 107.10

0.10772

0.512531

19

–0.7

–

0.70526

13.98 80.76

0.10541

0.512538

10

–0.5

–

4

MN-1139

73

849

0.2529

0.70602

1

0.70559

14.19 82.30

0.10490

5

MN-1117

100

324

0.9087

0.70820

2

0.70665

84.9

14.08

0.10097

0.512518

13

–0.9

8.5

0.512523

3

–0.8

9.4

6

MN-1147

95

220

1.2583

0.70893

1

0.70678

13.11 80.70

0.09880

7

MN-1110

116

60

5.6390

0.71788

1

0.70826

22.08 138.10

0.09731

0.512516

9

–0.9

–

0.512559

14

0.0

–

8

MN-1103

149

49

8.9820

0.72408

1

0.70876

25.99 152.60

9

MN-1141

150

59

7.4920

0.72122

1

0.70844

26.45 160.10

0.10371

0.512486

8

–1.5

9.6

0.10059

0.512527

8

–0.7

10.9

10

MN-540

210

27

22.7258

0.74679

2

0.70803

8.47

11

MN-560

208

23

26.8940

0.75413

1

0.70826

8.50

50.30

0.10238

0.512535

17

–0.6

–

54.70

0.09449

0.512528

13

–0.6

–

12

MN-555

209

19

32.6450

0.76482

2

0.70915

10.05 58.10

13

MN-1072

195

48

11.8790

0.72960

2

0.70934

10.05 60.32

0.10528

0.512526

13

–0.8

–

0.10148

0.512524

3

–0.8

9.4

14

MN-1120

196

176

3.2490

0.71414

3

0.70860

8.47

0.10238

0.512512

10

–0.9

15

Nd

50.30

Sm/144Nd

Nd/144Nd ±2σ

εNd(T) δ18OSMOW, %

Note. Rb, Sr, Sm, and Nd are determined by isotope dilution. 1–4, basaltic trachyandesites; 5, 6, trachytes and trachydacites; 7–9, trachyrhyodacites; 10–14, trachyrhyolites. 87Sr/86Sr(0) and εNd(T) values calculated for an assumed isotope age of 120 Ma age for trachyrhyolites.

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trachytes, trachyrhyodacites, and trachyrhyolites (9.4 to 10.9‰) where it is independent of SiO2 contents. The δ18O range is the largest (up to 15‰) in fluorite-bearing trachyrhyolites described in detail by Peretyazhko et al. (2018).

Discussion The basaltic rocks with different mineralogy, major- and trace-element compositions, and isotope signatures originated from heterogeneous mantle protoliths that underwent partial melting of different degrees; their parent melts assimilated some crustal material and showed local features of fractionation patterns in intermediate magma chambers. Intermediate to felsic melts which produced trachytic rocks and trachyrhyolites in rift zones may result from fractional crystallization of mafic melts that separated from the source basaltic magma, as well as from partial melting of upper continental crust under the effect of heat and fluid fluxes. Major- and trace-element compositions of basaltic trachyandesites and sources of mafic melts

Fig. 10. Isotope evolution: 87Rb/86Sr–87Sr/86Sr diagram. Solid line is isochron for seven samples: five trachyrhyolites (MN-540, MN-560, MN-555, MN-1072, MN-1120) and three trachyrhyodacites (MN-1103, MN-1110, MN-1141). Dashed line is regression line with high MSWD for fourteen samples: five trachyrhyolites, three trachyrhyolites, two trachytes (MN-1147, MN-1117) and four basaltic trachyandesites (MN-542, MN-566, MN-550, MN-1139).

Mafic volcanic rocks from the Nyalga basin, which are mainly basaltic trachyandesites, may result from fractional crystallization of trachybasaltic melts. Mafic phenocrysts in these melts apparently precipitated by gravity, judging by the presence of abundant olivine-free (clinopyroxene–andesine) aphyric or fine porphyritic rocks and by their major-element compositions. Basaltic trachyandesites contain <4 wt.% MgO (Fig. 4) and very unlikely could form by direct mantle melting (Palme and O’Neill, 2003). As SiO2 in the samples increases from 50 to 56 wt.%, the contents of Al, Ti and Fe decrease systematically (Fig. 4). Basaltic trachyandesites, though having high SiO2, have Nb/Ta ratios from 15 to 24, which is common to mantle compositions where it is ~17.5 on average (Green, 1995). They also have relatively high TiO2 (2–3 wt.%) and average element ratios of K/Ce = 121, Th/Y = 0.14, Ba/Nb = 14, and Ba/Ce = 7 close to those in ocean-island basalts (TiO2 = 2.9 wt.%, K/Ce = 150, Th/Y = 0.14, Ba/Nb = 9, Ba/Ce = 4). On the other hand, basaltic trachyandesites differ markedly from OIB in trace-element signatures common to suprasubduction basaltic rocks: depletion in HFSE (Nb, Ta) and enrichment in LILE (Cs, Rb, K, Ba, Pb) and LREE (Fig. 5a). Mafic melts with such compositions commonly form in subduction zones during melting of metasomatized mantle wedge (Hawkesworth et al., 1993; McCulloch and Gamble, 1991). The Nyalga basaltic rocks cannot have trace-element features associated with subduction because they formed in a setting of continental rifting about 120 Ma ago, i.e., long after the North Asian active margin of the paleocontinent had disappeared in the Jurassic (Kuzmin et al., 2010; Parfenov et al., 2003). The PM-normalized trace-element compositions of basaltic rocks in rift zones of Mongolia and Transbaikalia, with depletion in Nb and Ta and enrichment in Pb and K, are most often explained by the effect of a mantle plume on lithospheric mantle which was metasomatized during Paleo-

zoic and Early Mesozoic subduction (Kozlovsky et al., 2006; Kuzmin et al., 2010; Vorontsov and Yarmolyuk, 2007; Vorontsov et al., 2002; Yarmolyuk et al., 2013; etc.). In the same way, the high-Ti mafic parent melts of basaltic magmas that erupted in the Nyalga basin may likewise have formed during continental rifting by partial melting of the mantle more or less strongly metasomatized upon interaction with the plume. Late Jurassic–Cretaceous magmatism in Central and North Asia acted in several large independent areas of multistage within-plate continental volcanism (Fig. 1). Magmatism of that time span was extensively studied in the rift zones of South Hangayn (Yarmolyuk et al., 1994, 1995; etc.) and West Transbaikalia (Kazimirovskii et al., 2001; Pervov et al., 1987; Sasim and Dril, 2013; Sasim et al., 2016; Tauson et al., 1984; Vorontsov and Yarmolyuk, 2007; Vorontsov et al., 2002, 2016; Yarmolyuk et al., 1994, 1998; etc.). However, the mineralogy, isotope systematics, and geochronology of volcanics in the East Mongolia volcanic belt remain poorly investigated and were discussed in few publications only (Bars et al., 2018; Batulzii, 2011; Batulzii et al., 2015; Frikh-Khar and Luchitskaya, 1978, 1983). The Nyalga Early Cretaceous basaltic trachyandesites and Late Mesozoic basaltic rocks have particular trace-element signatures. The TAS diagram in Fig. 3 shows, for comparison, the compositions of Cretaceous basaltic rocks from the Tugnui-Khilok area (Vorontsov and Yarmolyuk, 2007), as well as from the Ingoda and Usugli rift zones in the West Transbaikalian rift zone. Like the case of the Nyalga basin, most of Late Mesozoic mafic rocks from rift zones in Transbaikalia are basaltic trachyandesites with 2–3 wt.% TiO2, <4–5 wt.% MgO, 2–3 wt.% K2O, and 1–2 wt.% P2O5. Their distinct trace-element signatures easily discriminate them from

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Fig. 11. Multielement patterns in basaltic rocks from Late Paleozoic–Late Mesozoic rifts in Mongolia and Transbaikalia normalized to primitive mantle (McDonough and Sun, 1995). Gray shade shows field of Late Paleozoic–Early Mesozoic basaltic rocks of rift zones in North Asia (Gobi–Tien Shan, Gobi–Altai, North Mongolia, Mongolia–Transbaikalia), after (Yarmolyuk et al., 2013). Solid lines are average compositions of Late Mesozoic basaltic rocks: 1, Nyalga basin; 2, West Transbaikalia rift zone (Vorontsov and Yarmolyuk, 2007); 3, 4, Ingoda and Usugli basins of East Transbaikalia, after unpublished data by Peretyazhko et al.; 5, OIB composition. Arrows show trends from Late Paleozoic to Late Mesozoic basaltic rocks.

their Late Paleozoic–Early Mesozoic counterparts in North Asian rift zones (Yarmolyuk et al., 2013). Compared with Late Paleozoic–Early Mesozoic mafic volcanics from the North Asian rift zones, the Late Mesozoic (J3–K) basaltic rocks from Mongolia and Transbaikalia have greater enrichments in Ti (Fig. 11) and in LILE (Cs, Rb, K, Ba, Pb) and HFSE (Nb, Ta, Zr, Hf, Th, U, Y), as well as in REE (La, Ce, Pr, Sm, Eu, Gd), which produce PM-normalized patterns convex from K to Y relative to OIB (Fig. 5a). Quite prominent minimums in Nb–Ta and Ti in PM-normalized patterns for mafic melts are commonly attributed to involvement of fluids into partial melting of the mantle protolith, whereby Ti and Fe oxides (ilmenite and rutile) that act as concentrators of Ti, Nb and Ta crystallized and accumulated in the residue (Hawkesworth et al., 1993; Kelemen et al., 2003; Ryerson and Watson, 1987). The basaltic rocks with moderate Nb–Ta depletion and relatively high Ti enrichment (Fig. 11) were derived from mafic melts produced by partial melting of the mantle protolith which partially lost its fluid phase. Therefore, the sublithospheric mantle involved into melting and generation of mafic melts became progressively drier from Late Paleozoic to latest Mesozoic time. The reason is that basaltic magmatism was associated with subduction of oceanic crust beneath North Asia (Siberia) in the Paleozoic and Early Mesozoic and with melting of the partially dehydrated metasomatized mantle wedge in the Late Mesozoic (J3–K1), after complete closure of the paleoocean (Donskaya et al., 2013). REE contents in mafic melts depend on the melt fraction and mineralogy of the mantle sources. Melts with high LREE/HREE ratios can form at low melt fractions, within the

garnet stability field (Wang et al., 2002). The chondrite-normalized Tb/Yb ratio (Furman et al., 2004) is among most sensitive indicators of partial melting, which is almost independent of the fractional crystallization of basaltic melts: (Tb/Yb)n > 1.8 and (Tb/Yb)n < 1.8 in the melts formed by partial melting of garnet and spinel peridotites, respectively. According to this criterion, the parent melts of the Nyalga basin and Transbaikalian Late Mesozoic basaltic rocks formed by partial melting of garnet-bearing peridotite. The mantle mineralogy and melt fraction were simulated using the La, Dy, Yb and Nb contents for basaltic rocks, because the La/Yb and Dy/Yb ratios are the most sensitive indicators of the garnet presence and melting degree, while the Nb/Yb ratio is diagnostic of the subduction component. The model parameters, assuming non-modal batch melting, are listed in the caption to Fig. 12. Garnet, with HREE exceeding LREE, preserves a large portion of HREE from the melt residue left after partial melting of garnet peridotite, while the mantle basaltic melts become depleted in these elements and have higher La/Yb and Dy/Yb ratios. On the contrary, partial melting of spinel peridotite fails to cause significant changes to the Dy/Yb and La/Yb ratios in mafic melts. Modeling for the Nyalga basaltic trachyandesite predicts partial melting of both garnet and spinel peridotites, at ratios changing from 30/70 to 80/20 (Fig. 12). Estimates of melt fraction at mantle sources largely depend on the coefficients KD assumed for La, Dy, Yb, and Nb partitioning between mantle minerals (olivine, orthopyroxene, clinopyroxene, garnet, and spinel) and mafic melt. The calculated KD for La, Dy, and Yb (McKenzie and Onions, 1991) lead to very low melt fractions (F from < 0.1 to 0.3%), with some points on the right of the F = 0 line, which is unrealistic. Meanwhile, the empirical KD (Adam and Green, 2006; Foley et al., 1994) provide a more realistic F value: 0.1–1 wt.% for the system La/Yb–Dy/Yb (Fig. 12a) and 1–5% for Nb/Yb–Dy/Yb (Fig. 12b). The modeling indicates prevalent garnet peridotite (60–80 wt.%) as a source of parent melts for basaltic rocks from the Ingoda and Usugli basins in East Transbaikalia. Abnormal La/Yb ratios in the Usugli basaltic trachyandesites (Fig. 11) result from 0.2–0.6 wt.% partial melting of mantle sources composed mainly (65–80 wt.%) of garnet peridotite (Fig. 12a). In the εNd(T)–87Sr/86Sr(0) diagram (Fig. 13), the compositions of the Nyalga basaltic trachyandesites plot a compact group in the fields of Late Paleozoic–Mesozoic within-plate basaltic rocks of the Central Asian Orogenic Belt (Yarmolyuk et al., 2003) and trachybasalts of rift zones from East and West Transbaikalia (Sasim and Dril, 2013; Vorontsov and Yarmolyuk, 2007). They have weakly negative εNd(T) of –0.1 to –1.54 and moderately radiogenic initial Sr isotope ratios 87 86 Sr/ Sr(0) = 0.70526–0.70567. Therefore, the parent mafic melts of the Nyalga Early Cretaceous basaltic trachyandesites, as well as other Late Paleozoic–Mesozoic rift-related basaltic rocks in the Central Asian orogen (Kuzmin et al., 2010; Yarmolyuk and Kovalenko, 2000; Yarmolyuk et al., 2000), formed by partial melting of mantle protolith from moderately depleted (PREMA) and enriched (EM-II) sources. The basal-

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Fig. 12. Model of nonmodal batch melting at garnet- and spinel-peridotite mantle sources. For source compositions after (McKenzie and O’Nions, 1991), garnet peridotite: 59.8 wt.% olivine, 21.1 wt.% orthopyroxene, 7.6 wt.% clinopyroxene, and 11.5 wt.% garnet; spinel peridotite: 57.8 wt.% olivine, 27 wt.% orthopyroxene, 11.9 wt.% clinopyroxene, and 3.3 wt.% spinel. For source compositions after (Thirlwall et al., 1994), garnet peridotite: 5 wt.% olivine, 20 wt.% orthopyroxene, 30 wt.% clinopyroxene, and 45 wt.% garnet; spinel peridotite: 10 wt.% olivine, 27 wt.% orthopyroxene, 50 wt.% clinopyroxene, and 13 wt.% spinel. Calculations are made using La, Dy, Yb, and Nb partition coefficients for olivine, orthopyroxene, clinopyroxene and garnet after (Adam and Green, 2006) and for spinel after (Foley et al., 1994). Initial concentrations at mantle source: La, Dy, and Yb after (Peters et al., 2007), Nb in primitive mantle after (Sun and McDonough, 1995). F, Melting fraction of mantle protolith; Sp, spinel peridotite; Gr, garnet peridotite. Other symbols same as in Fig. 3.

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Fig. 13. εNd(T)–87Sr/86Sr(0) diagram. 1, basaltic trachyandesite; 2, trachyte; 3, trachyrhyodacite; 4, trachyrhyolite. Roman numerals are composition fields: I, Cenozoic within-plate basaltic rocks from Central Asian Orogenic Belt (Yarmolyuk et al., 2003); II, Late Paleozoic–Mesozoic within-plate basaltic rocks from Central Asian Orogenic Belt (Yarmolyuk et al., 2003); III, trachybasalts of Cretaceous rifts in East Transbaikalia (Sasim and Dril, 2013); IV, trachybasalts of Late Mesozoic rifts in West Transbaikalia (Vorontsov and Yarmolyuk, 2007). Mixing line is between average model compositions of North Asian continental basalt (Yarmolyuk et al., 2003) and granite of Unda complex (Kozlov et al., 2003).

tic melts may also have assimilated some amount of continental crust. Trachytes, trachydacites, trachyrhyodacites, and trachyrhyolites plot a flat trend preserving weakly negative εNd(T) as in basaltic trachyandesites (Fig. 13, Table 3), while Sr contents change significantly and initial 87 86 Sr/ Sr(0) ratios reach 0.70665–0.70678 in trachyte–trachydacites and 0.70803–0.70934 in trachyrhyodacite–trachyrhyolites (Fig. 14). Crustal contamination alone cannot account for these variations of the Nd and Sr isotope compositions in the Nyalga TTT rocks; therefore, other petrogenetic models are required (see below). The effect of crustal contamination on the composition of basaltic and fractionated melts can be estimated approximately using the 87Sr/86Sr(0)–1/Sr × 1000 diagram (Fig. 14), which shows the compositions of the Nyalga rocks, trachybasalts from rifts in East Transbaikalia (Sasim and Dril, 2013; Sasim et al., 2016), and Cretaceous (135–90 Ma) basaltic rocks from the Tugnui-Khilok area in the West Transbaikalian rift zone (Vorontsov and Yarmolyuk, 2007). The same figure shows mixing lines between the average compositions of North Asian continental basalts (Yarmolyuk et al., 2003), upper continental crust (Taylor, 1977; Taylor and McLennan, 1995), and the Uda palingenic collisional granite from East Transbaikalia (Kozlov et al., 2003). Late Mesozoic basaltic rocks of rifts

Fig. 14. 87Sr/86Sr(0)–1/Sr × 1000 diagram. 1, basaltic trachyandesite; 2, trachyte; 3, trachyrhyodacite; 4, trachyrhyolite; 5, average composition of North Asian continental basalt (Yarmolyuk et al., 2003); 6, mixing line between model basaltic melt and bulk upper continental crust (Taylor, 1977; Taylor and McLennan, 1995) compositions; 7, mixing line between model basalt and granite of Unda complex (Kozlov et al., 2003). I, Trachybasalts of Cretaceous rifts in West and East Transbaikalia (Sasim and Dril, 2013; Sasim et al., 2016; Vorontsov and Yarmolyuk, 2007). Abbreviations at trend arrows stand for fractional crystallization (FC), assimilation and fractional crystallization (AFC), mixing of sources during assimilation (C), source change (SC) (De Paolo, 1988).

plot an elongate quasi-horizontal trend at the account of Sr variations, possibly, because of local features in the fractionation of mafic melts that erupted over a large territory of Mongolia and Transbaikalia. The 87Sr/86Sr(0) variations are within 0.7049–0.7058, which is consistent with the 10– 15 wt.% amount of crustal material assimilated by basaltic melts. The small contribution of crustal material to the parent melts of basaltic trachyandesites follows also from Th/Yb ratios near the metasomatized mantle trend and OIB-type basalt (Fig. 15). δ18O in basaltic trachyandesites are higher than in mafic mantle melts: 7.5–8.5‰ (Table 6) against 4.5–6.1‰ according to Pokrovskiy (2000). Origin and composition evolution of TTT rocks Increase in SiO2 in the Nyalga TTT rocks is attendant with systematic changes in chemistry of their minerals, especially, feldspars (Fig. 9). The most calcic plagioclase (andesine-labradorite) occurs in olivine-bearing basaltic trachyandesites. Most of feldspar in olivine-free mafic samples occurs as andesine. The matrix microlith assemblage that crystallized after plagioclase contains triple (Ca–Na–K) feldspar and sanidine. Triple feldspar in trachytic rocks occurs only as a residual phase in the cores of coarse sanidine phenocrysts. Sanidine with large K and Na variations exists as phenocrysts and microliths in the trachyrhyodacite and trachyrhyolite matrix. Plagioclase and K–Na feldspar bear Ba only in basaltic trachyandesites, trachytes, and trachydacites (Table 3), while sanidine in trachyrhyolites is free from Ba. Quartz appears in trachyrhyo-

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dacites and becomes a main phase in trachyrhyolites. Note also large variations in fluorapatite chemistry: it contains up to 1.4 wt.% SrO in mafic rocks and 4 wt.% of total LREE in trachytic rocks (Table 5, analyses 37–42). The structure of volcanic sections (intercalated mafic and felsic lavas), petrography of rocks, compositions of residual glasses in basaltic trachyandesites, as well as Rb–Sr isotope systematics indicate that the eruptions of mafic, intermediate, and felsic lavas within the study area followed one shortly after another. The presence of mineral xenocrysts in the trachyrhyodacite sample MN-1141 and trachyrhyolite xenoliths in trachyrhyodacites provide evidence of possible magma mixing. Residual trachyte–trachyrhyodacite glasses in basaltic trachyandesites (MN-1139, MN-576, MN-1107) and trachyrhyolite glasses in trachyrhyodacite (MN-1141) formed after the crystallization of the phenocryst assemblage in the primary basaltic trachyandesitic and trachytic melts. Note also that the compositions of glasses in melt inclusions we studied earlier in quartz from trachyrhyolites (Peretyazhko et al., 2014, 2018) overlap considerably with those of residual glasses from basaltic trachyandesites and trachyrhyodacites (Fig. 3). Proceeding from systematic changes in the mineralogy and majorand trace-element compositions of TTT rocks, as well as glasses and melt inclusions, we applied mass balance calculations (Tables 7–10) using data on chemistry of minerals and glasses (Tables 2–5) to simulate fractional crystallization of differentiated melts that produced the TTT rock series from basaltic trachyandesites to trachytes and trachyrhyolites. The contents of SiO2 are the lowest (51 wt.%) in olivinefree mafic rocks (MN-542 and MN-569) and are slightly higher (53–55 wt.%) in olivine-bearing basaltic trachyandesites (MN-1139, MN-576 and MN-1107). Correspondingly, mass balance calculations cannot lead to olivine-free mafic residual melts (MN-542 and MN-569) derived from higherSiO2 olivine-bearing basaltic trachyandesite compositions. Modeling in a reverse sequence is possible but it contradicts the observed mineral chemistry trends (Table 8, models 2–4, 6–8). Residual melts that would crystallize olivine and plagioclase (labradorite-andesine) in these models require fractionation of andesine. The parent melts for olivine-bearing and olivine-free basaltic trachyandesites appear to have no direct relationship. They may result from local variations in the conditions for crystallization of the primary mafic (trachybasaltic) melts which followed different trends in intermediate magma chambers. Mafic melts that correspond to the compositions of the MN-569, MN-1139, MN-576 and MN-1107 basaltic trachyandesites may produce, with minor misfit in oxides (∆X2 < 1), from 31 to 53 wt.% residual trachyte–trachyrhyodacitic melts with compositions as in the MN-1117, MN-1068 and MN1141 trachyte–trachyrhyodacite samples, by crystallization of labradorite–andesine (20–37 wt.%), Ti-augite (10–15 wt.%), titanomagnetite (3–9 wt.%), ilmenite (0.5–3 wt.%), and apatite (2–3 wt.%) (Table 9, models 10–12, 14–16, 18–20, and 22–24). Mass balance calculations show that fractionation of basaltic trachyandesites does not involve amphibole (which crystallizes at the final stage of matrix formation). The

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Fig. 15. Ta/Yb–Th/Yb diagram. Gray shows trend of mantle metasomatism. Abbreviations stand for model basalt compositions: N-MORB, E-MORB and OIB; PM is primitive mantle (Sun and McDonough, 1995); UC is average upper continental crust (Taylor and McLennan, 1995). Basaltic trachyandesites have slightly higher Th/Yb ratios than in metasomatic mantle, possibly as a result of fractional crystallization (FC), assimilation and fractional crystallization (AFC) of parent mafic melts. Legend same as in Fig. 3.

evolution of trachytic melts with compositions as in MN-1068 and MN-1141 can lead to a trachyrhyolitic residual melt in the case of prevalent crystallization of K-Na feldspar (Table 10, models 26, 27, 29, 30). Biotite did not participate in fractionation of trachytic melts. The contents of P2O5 decrease progressively from 0.1–0.2 wt.% in trachytes to 0.02– 0.05 wt.% in trachyrhyolites, which may result from crystallization of abundant fluorapatite from the primary trachyte–trachyrhyodacitic melts. Quantitative trace-element modeling of fractional crystallization (FC) is impossible without rigorous constraints on element partitioning between minerals and residual melts. The available data on the compositions of rocks and minerals can provide only a qualitative idea of the behavior of some trace elements during the evolution of magmas that produced the TTT rocks. Namely, increase in Ba along with decrease in Sr and P on transition from basaltic trachyandesites to trachytes with the greatest Ba enrichment (sample MN-1117) is consistent with the models that imply removal of labradorite-andesine and Sr-rich apatite during FC of basaltic trachyandesitic melts. REE remained almost invariable in the series from basaltic trachyandesites to trachyte–trachydacites (Fig. 7), possibly because the combined coefficients of REE partitioning between main crystallizing phases (plagioclase, clinopyroxene, and apatite) and residual melts approach unity. Gradual decrease in Ba from trachyte (MN-1117) to trachyrhyolites (Fig. 8) via trachydacite (MN-1068) and trachyrhyodacite (MN-1141) is consistent with FC models where Baand Sr-bearing K–Na feldspar is the main fractionating phase.

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Table 7. Compositions of TTT rocks used in mass balance calculations Component

Basaltic trachyandesite

Trachyte–trachyrhyodacite

Trachyrhyolite

542

569

1139

576

1107

1117

1068

1141

1072

1218

SiO2

51.01

51.01

55.48

52.87

54.83

66.52

68.33

70.10

76.74

79.56

TiO2

3.02

3.13

2.33

2.57

2.40

0.95

0.62

0.59

0.17

0.14

Al2O3

16.16

16.90

15.11

15.42

15.32

16.80

16.13

15.54

12.18

11.30

FeO

10.00

11.27

9.13

10.03

9.00

3.39

2.53

2.48

1.95

0.94

MnO

0.10

0.06

0.13

0.13

0.12

0.07

0.04

0.06

0.03

0.01

MgO

3.60

1.92

3.45

3.85

3.52

0.44

0.36

0.59

0.23

0.10

CaO

7.51

6.74

6.53

6.78

6.55

1.63

0.97

1.01

0.41

0.51

Na2O

3.65

3.79

2.97

3.55

3.21

3.97

4.60

3.31

2.90

2.70

K2O

2.42

2.38

2.74

2.37

2.87

5.63

6.02

5.98

5.12

4.61

P2O5

1.42

1.54

1.13

1.31

1.18

0.21

0.11

0.07

0.05

0.03

Note. Compositions are normalized to 100%.

Table 8. Fractional crystallization of basaltic trachyandesitic melts: mass balance models Index

Model No. 1

2

3

4

MN-542

5

6

7

8

MN-569

Residual basaltic trachyandesitic melts, wt.% MN-1139

–

55.36

–

–

–

56.81

–

–

MN-576

–

–

74.99

–

–

–

57.70

–

MN-1107

–

–

–

66.62

–

–

–

59.29

6.49

2.85

4.48

15.00

–

0.32

–

Crystallizing phases, wt.% CPX

19.91

PL

38.36

25.31

15.21

21.37

36.15

23.48

20.12

22.48

KFS

28.67

3.87

1.87

–

33.85

8.93

13.04

8.27

MGT

5.66

2.99

1.30

2.51

9.65

6.52

5.90

6.49

ILM

4.58

2.91

1.91

2.43

2.59

1.54

1.21

1.30

AP

1.76

1.46

1.20

1.29

2.39

2.18

2.06

2.02

Total

98.94

98.37

99.32

98.70

99.64

99.47

100.36

99.85

ΣX2

2.185

1.020

0.316

0.650

1.550

0.087

0.390

0.158

Note. Models 1 and 5 present modal rock compositions. Mass balance calculations are made assuming that both primary (MN-542, MN-569) and residual (MN-1139, MN-576, MN-1107) melts have basaltic trachyandesitic compositions. Mineral names are abbreviated as CPX, clinopyroxene; PL, plagioclase; KSF, K–Na feldspar; TMG, titanomagnetite; ILM, ilmenite; AP, apatite. See Tables 2–5 for compositions of minerals and glasses and Table 7 for rock compositions. Major oxides used in mass balance calculations: SiO2, TiO2, Al2O3, FeO, MnO, MgO, CaO, Na2O, K2O, and P2O5. ΣX2 is sum of rms errors in oxide contents (difference between original and calculated values).

Trachyrhyodacites contain high concentrations of REE, Zr and Hf, possibly because triple feldspar and sanidine, which are free from these elements, crystallize from trachytic melts. High contents of these elements are common to trachyte from differentiated series, especially alkaline volcanics (Peretyazhko et al., 2015). The contents of REE (mainly, LREE), as well as Zr, Hf and P in trachyrhyolites are low due to crystallization of zircon, chevkinite-Ce and fluorapatite with 4 wt.% LREE (in total). Residual melts in FC models should accumulate incompatible elements that lack from crystallizing minerals. This behavior of elements in the TTT rocks from basaltic trachyandesites to trachyrhyolites is observed for Rb,

Ta, Nb, Th, U and Pb (Fig. 8). Note also that it is impossible to obtain the composition features of trachyte–trachyrhyodacites with 66–74 wt.% SiO2 in models implying mixing of melts corresponding to the most mafic and felsic TTT rocks (basaltic trachyandesites and trachyrhyolites). The Nyalga trachyrhyodacites–trachyrhyolites contain much more REE, Rb, Ta, Nb, Th, and Pb than the upper continental crust (Taylor and McLennan, 1995) (Fig. 8) and cannot have formed by its complete melting. In the Th/Yb– Ta/Yb diagram, trachytic and trachyrhyolitic rocks plot within the trend of fractional crystallization from primary basaltic trachyandesitic melts, while the TTT field is off the average

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I.S. Peretyazhko et al. / Russian Geology and Geophysics 59 (2018) 1679–1701 Table 9. Fractional crystallization of basaltic trachyandesitic melts that produced trachyte–trachyrhyodacites: mass balance models Index

9

10

11

12

13

MN-569

14

15

16

17

MN-1139

18

19

20

MN-576

21

22

23

24

MN-1107

Residual trachyte–trachyrhyodacitic melts, wt.% MN-1117

–

40.09

–

–

–

52.94

–

–

–

41.63

–

–

–

50.43

–

–

MN-1068

–

–

35.71

–

–

–

48.16

–

–

–

38.06

–

–

–

46.00

–

MN-1141

–

–

–

30.63

–

–

–

46.13

–

–

–

36.31

–

–

–

44.02

GL

–

–

–

–

47.68

–

–

–

33.61

–

–

–

39.95

–

–

–

Crystallizing phases, wt.% OL

–

–

–

–

6.55

5.37

5.50

5.74

11.11

9.95

10.10

10.36

8.16

6.46

6.62

6.98

CPX

15.00

10.44

10.90

10.11

11.97

15.45

15.46

12.80

6.91

9.56

9.50

7.78

8.38

11.34

11.32

9.19

PL

36.15

33.07

35.28

37.32

27.99

19.94

23.91

26.91

37.77

29.94

32.87

35.22

34.39

23.65

27.29

29.97

KFS

36.85

1.16

4.41

–

–

–

–

–

–

–

–

–

–

–

–

MGT

9.65

8.79

9.07

9.41

4.31

3.92

4.48

4.89

4.09

2.69

3.14

3.42

5.20

3.75

4.30

4.52

ILM

2.59

2.90

3.22

3.10

–

0.54

0.75

0.98

2.32

2.85

2.98

2.99

0.64

1.66

1.82

1.90

AP

2.39

3.07

3.16

3.19

1.91

2.24

2.33

2.59

3.32

2.94

3.04

3.19

2.97

2.62

2.73

2.95

Total

99.64

98.37

98.51

98.17

100.42 100.40 100.59 100.05 99.13

99.56

99.70

99.26

99.69

99.90

100.07 99.53

ΣX2

1.550

0.219

0.315

0.245

0.356

0.067

0.057

0.091

0.040

0.056

0.307

0.350

0.712

0.103

0.028

0.015

Note. Models 9, 13, 17, and 21 present modal rock compositions. Mass balance calculations made assuming that basaltic trachyandesites (MN-569, MN-1139, MN-576, MN-1107) are initial compositions and trachyte–trachyrhyodacites (MN-1117, MN-1068, MN-1141) are residual melts. OL, olivine; GL, matrix glass. Other abbreviations are as in Note to Table 8.

Table 10. Fractional crystallization of trachyte–trachyrhyodacitic melts that produced trachyrhyolites: mass balance models Index

25

26

27

28

MN-1068

29

30

MN-1141

Residual trachyrhyolite melts, wt.% MN-1072

–

30.88

–

–

49.90

–

MN-1218

–

–

27.37

–

–

42.85

GL

–

–

–

70.48

–

–

Crystallizing phases, wt.% Q

14.80

3.96

3.39

–

–

–

CPX

–

–

–

2.73

2.05

1.98

PL

14.61

13.48

13.16

–

–

–

KFS

67.36

49.14

52.94

25.18

46.67

53.08

MGT

2.56

1.82

2.28

1.58

1.02

1.83

ILM

0.41

0.54

0.45

0.14

0.71

0.54

Total

99.73

99.83

99.59

100.10

100.34

100.27

0.180

0.161

0.167

1.041

1.041

1.355

2

ΣX

Note. Models 25 and 28 present modal rock compositions. Mass balance calculations made assuming that trachyte–trachyrhyodacites (MN-1068, MN-1141) are initial compositions and trachyrhyolites (MN-1072, MN-1218) are residual melts. Q, Quartz. Other abbreviations are as in Note to Table 8.

upper continental crust composition and the AFC trend (Fig. 15). The mineralogy, chemistry, and δ18O (varying from 9.4 to 10.9‰ irrespective of SiO2, Table 6) of trachytes and trachyrhyolites are poorly consistent with significant contribution of assimilated crustal material to trachyte–trachyrhyolitic melts. The Nd isotope composition (Fig. 13) remained almost invariable during the evolution of the Nyalga TTT rocks, which is confirmed by similarity in Sm and Nd contents and stability of the Sm/Nd ratio (Fig. 7). On the other hand, their

87

Sr/86Sr(0) ratios depend on Sr contents and, correspondingly, on the fractionation degree of the parent melts. As follows from the 87Sr/86Sr(0)–1/Sr × 1000 diagram (Fig. 14), the 87 86 Sr/ Sr(0) ratio increases significantly with SiO2 in the series from basaltic trachyandesites (0.7053–0.7057); trachyte– trachydacites (0.7067–0.7068); to trachyrhyodacite–trachyrhyolites (0.7080–0.7093). This systematic change in the initial Sr isotope ratio as a function of 1/Sr × 1000 may be due to fractional crystallization of the parent melts which assimilated

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crustal material (AFC vector in Fig. 14). Note that the magmas evolved towards progressively more felsic melt compositions with Rb/Sr increasing at the account of higher Rb and lower Sr (Fig. 8). The trachyte–trachyrhyodacitic and trachyrhyolitic differentiated melts with large ranges of Rb/Sr and 87Sr/86Sr(o) variations could accumulate in isolated magma chambers. The AFC model simulating the evolution of Rb, Sr, and Nd isotope systematics has to be supported further by numerical modeling of the process in which the melts assimilate crustal material and, possibly, also crustal fluids enriched in 87Sr to different degrees.

Conclusions The reported geological, mineralogical, and isotope data on the Nyalga TTT rocks show that they formed by eruptions of mafic, intermediate, and felsic magmas following one shortly after another within a short time span in the Early Cretaceous, about 120 Ma ago. Basaltic trachyandesites are differentiated rocks with relatively high contents of TiO2, K2O, and P2O5, low MgO, moderately enriched Sr isotope composition (87Sr/86Sr(0) = 0.70526–0.70567), and nearly zero or weakly negative εNd(T) values. Judging by these compositions, the parent mafic melts result from partial melting of mantle protolith coming from moderately depleted PREMA and enriched EM-II sources and may contain a 10–15% component of assimilated upper continental crust. The PM-normalized REE patterns of the Nyalga TTT rocks, which look convex relative to the OIB pattern from K to Y and show moderate depletion in Nb and Ta and enrichment in Pb, as well as their Rb–Sr and Sm–Nd isotope signatures, are typical of Late Mesozoic rift-related basaltic rocks widespread in Mongolia and Transbaikalia. This allows discriminating reliably the Late Mesozoic basaltic rocks from both Late Paleozoic–Early Mesozoic and Cenozoic mafic volcanics in rift zones of the Central Asian orogen, according to their geochemistry. The high-Ti Nyalga basaltic trachyandesites are compositionally similar to OIB and suprasubduction basalts as they bear the geochemical features of mafic melts produced by partial melting of mantle protolith which was metasomatized and hydrated during previous subduction events. Magma generation in the volcanic centers of Northern and Central Asia likely moved from a shallower and more hydrous to a deeper and less hydrous mantle source (from spinel- to garnet-bearing peridotite) between the Late Paleozoic and the latest Mesozoic. The mafic melts that produced basaltic rocks of the respective ages formed by 0.1–5% partial melting of the protolith, in which the garnet-to-spinel peridotite ratio changed successively from 30/70 to 80/20. Garnet-bearing peridotite presumably occurred in lithospheric mantle at depths below 55–60 km. The mass balance relationships, major- and trace-element compositions, and mineralogy of the analyzed TTT samples fit the best the models implying fractional crystallization of basaltic trachyandesitic, trachytic, and trachyrhyodacitic melts.

According to mass balance calculations, trachytic and trachydacitic melts formed after the phases of labradorite-andesine, Ti-augite, Sr-apatite, titanomagnetite, and ilmenite had crystallized from basaltic trachyandesitic melts. The evolution from trachytic to trachyrhyodacitic and trachyrhyolitic melts was possible due to prevalent crystallization of K–Na feldspar, as well as to the involvement of zircon, chevkinite-Ce, and LREE-rich apatite in fractionation. Intermediate and felsic (trachytic, trachyrhyodacitic, and trachyrhyolitic) residual melts that evolved from compositionally different primary magmas (basaltic trachyandesitic, trachytic, and trachyrhyodacitic, respectively) could migrate to shallow continental crust where they accumulated in isolated magma chambers. As indicated by isotope data, these melts assimilated some amount of crustal material, in correspondence with the AFC model. The study was supported by grants 16-05-00518 and 17-05-00928 from the Russian Foundation for Basic Research and was carried out as part of program 0350-2016-0029 of the Research Foundation.

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