Lower Permian basaltic agglomerate from the Tsengel River valley, Mongolian Altai

A new occurrence of Permian volcanic and volcaniclastic rocks in the Mongolian Altai south of the Main Mongolian Lineament was described between soums of Tugrug and Tseel in Gobi-Altai aimag. Studied vitrophyric pyroxene basalt lies in a layer of agglomerate and amygdaloidal lavas, which is a part of NE–SW trending subvertical sequence of varicolored siltstones and volcaniclastic rocks in the Tsengel River valley. This high-Mg basalt is enriched in large ion lithophile elements, Pb and Sr and depleted in Nb and Ta. LA-ICP-MS dating on 44 spots reveals several concordia clusters. The whole rock geochemistry of sample fits volcanic arc characteristic in the geotectonic discrimination diagrams. Dominant zircon data yield Upper Carboniferous and Permian magmatic ages 304.4 ± 2.3 and 288.6 ± 1.9 Ma. Two smaller clusters of Upper Devonian (376 ± 4.7 Ma) to Lower Carboniferous ages (351.9 ± 3.5 Ma) indicate probably contamination of ascending magmatic material. Youngest Triassic age found in three morphologically differing grains reflects probably lead loss. Described high-Mg basalt lava represents sub-aerial volcanism in volcanic arc environment developed over the N dipping subduction zone in the southwestern Mongolia in the time span from Uppermost Carboniferous to Permian during terminal stage of its activity.


INTRODUCTION
The presence and character of volcanic rocks can provide number of valuable geological data enabling to decipher environment and processes of evolution of the particular blocks of the Earth's crust. This scheme can be also applicable for evolution history of the Central Asian Orogenic Belt (CAOB, Mossakovsky et al., 1993), developed between the Siberian Craton in the north and the Tarim and Sino-Korean blocks in the south, in the time span between 1000 and 250 Ma (Windley et al., 2007). The continental accretion in the area of the Gobi and Mongolian Altai was accompanied by massive magmatism of volcanic arc character (e.g. Hanžl et al., 2008;Cai et al. 2015;Soejono et al., 2016;Janoušek et al., 2018). Calc-alkaline

Original article
Mongolian Geoscientist subduction-associated magmatism ceased in the region of southwestern Mongolia during the Upper Carboniferous (Hanžl et al., 2020). In this section of CAOB, A-type granitoids and a bimodal alkaline volcanic series developed in the environment analogous to continental rifts during the Permian (Kozlovsky et al., 2015). This contribution describes a new occurrence of Permian volcanic rocks south of the Main Mongolian Lineament in the area of the Tsengel River northwest of the Tseel soum (Gobi-Altai aimag) in the Mongolian Altai. The presented petrological and geochemical data, including radiometric age, complement and specify the lithostratigraphic classification of units in the area which is generally based on the geological map 1: 200,000, sheet L-46-XXIV (Togtokh et al., 1995).

GEOLOGICAL SETTING
The studied locality lies south of a severalkilometers-wide tectonic zone (the Main Mongolian Lineament) separating different geological blocks of Mongolia. The northern block consists of the Precambrian microcontinents and the Cambrian magmatic arc with the Lower Paleozoic oceanic and volcanic arc complexes. Variegated Lower to Upper Paleozoic sedimentary and volcanosedimentary units are exposed in the south. The adjoining units (the Lake Zone in the north and the Gobi-Altai Zone in the south) of these superterranes are imbricated in a complicated tectonic melange just along Main Mongolian Lineament. The valley of the Tsengel River ( Fig. 1) intersects E-W oriented belts of Lower Paleozoic sequences belonging to the Gobi-Altai Terrane in the sense of Badarch et al. (2002). These rocks are covered by Upper Permian continental sediments with coal seams. Newly described Lower Permian rocks were detached from to sequence depicted on the geological map 1: 200,000 as the Cambrian Durvuljinuul Formation (Togtokh et al.,995). They lie south of the E-W oriented significant fault, along which the layers of Silurian limestones are exposed. In the northern part of the Permian sequence, basic amygdaloid lavas predominate over volcaniclastic rocks and grey siltstones and sandstones. Varicolored siltstones with layers of coarse-grained sandstones, basalts and andesites dominate in the southern part. The bedding orientation is variable in the north and subvertical with predominant SW-NE strike in the south. The studied sample PH026 (WGS84 95.504677 E; 45.779629 N, Fig. 2A) was sampled from a massive medium-grained block of dark green lava (Fig. 2B) in agglomerate at the boundary of both subunits.

METHODOLOGY
Approximately 10 kg of rock was taken in the field at the reference point PH026 for geochemistry and heavy mineral separation. The contents of the main and trace elements in the sample were determined in the ACTLabs laboratories by the ICP-MS method, the dissolution of the sample for the ICP-MS determination was performed in LiBO 2 / Li 2 B 4 O 7 (REE and high melting point metals) and aqua regia (precious metals). Whole rock analyses were processed using the GCDkit program (Janoušek et al., 2006). The zircons were separated using conventional techniques (crushing, grinding, sieving, Wilfley gravity separation table, magnetic separation and separation in heavy liquids) at the Central Geological Laboratory in Ulaanbaatar. Subsequently, the individual zircons were manually selected from the zircon concentrate using a microscope, embedded in epoxy resin and polished. The internal structure and zoning of the selected zircon grains were studied by the cathodoluminescence (CL) method using a scanning electron microscope in laboratories of the Czech Geological Survey in Prague. Analyses of mineral chemistry were performed at a joint laboratory of the Faculty of Science of Masaryk University and the Czech Geological Survey in Brno on a Cameca SX-100 instrument using an accelerating voltage of 15 kV. Zircons were dated by U-Pb using an Analyte Excite 193 nm excimer laser ablation system (LA, Proton Machines) equipped with a twovolume HelEx ablation cell in tandem with an Agilent 7900x ICPMS MSCP-MS (Agilent Technologies Inc., Santa Clara, USA) in laboratories of the Czech Geological Survey in Prague. The ablation took place in a He atmosphere (0.8 l. min -1 ), the laser frequency was 5 Hz, the laser fluence was 7.59 J. cm -2 and the laser beam diameter was 25 μm. Each measurement consisted of 20 s integration of the entrainment background signal and another 40 s integration of the sample ablation signal for masses 202,204,206,207,208,232 and 238 using a SEM detector with one point per mass peak and mass integration time 10, 10, 15, 30, 20, 10 and 15 ms (total cycle time 0.134 s). To monitor the stability of the instrument and ensure the reliability of the measured results, zirconium standards (91500, Plešovice and GJ-1) were measured after every twenty samples. Instrumental drift throughout the measurement was monitored by repeated analyses of reference zircon 91500 (Wiedenbeck et al., 1995). The processing of the measured data was performed using Iolite software as is described in Paton et al. (2010), including background correction, followed by laser-induced elemental fractionation (LIEF) correction based on comparison with behaviour of reference zircon 91500 (1065 Ma, Wiedenbeck et al., 1995) whose concordant age 1062.9 ± 2.8 Ma measured during this study corresponds to the reference value. No correction was made for normal Pb. During this study, reference samples of Plešovice zircon (337 Ma, Sláma et al., 2008) and GJ-1 (~ 609 Ma, Jackson et al., 2004) were periodically analysed, which gave a concordant age of 337.8 ± 1.7 Ma and 609.3 ± 2.3. Ma (2σ), what corresponds to the reference values within the analytical error. Concordance diagrams and calculation of relevant U-Pb ages were processed by ISOPLOT / Ex version 3.0 (Ludwig, 2003).

Petrographical and geochemical
characteristics Sample PH026 is a medium-grained basaltic rock with an amygdaloidal structure and observable fluidal textures (Fig. 2C). The rock consists of altered glassy groundmass and pyroxene phenocrysts in, locally rimmed by amphibole and epidote grains. Amygdales are filled by chlorite and rarely by calcite. Chlorite, tiny epidote grains, Fe-oxides and titanite can be identified in the mass of devitrified glass (Fig. 2  C, D). Pyroxene is mostly hypautomorphic up to 6 mm in size, automorphic crystals are less common. The individual zones are relatively "massive", having a diameter of 0.1 to 0.7 mm, the number of zones in individual grains ranges from 5 to 8. The pyroxene can be defined as diopside with a proportion of esseneite and a negligible admixture of alkaline pyroxenes. Well observable optical zoning reflects chemical composition with alternation of zones corresponding to 93.5-94.2 wt. % of diopside and zones in which the diopside content increases to 96.5-97 wt. %. The pyroxenes are sometimes replaced along the margins by a thin (10-15 m) magnesiohornblende rim (Fig. 2D). In individual zones, it is possible to correlate the increase of Si, Mg and Cr with the decrease of Fe, Ti and Al contents and vice versa. Nevertheless, it should be noted that the chemical changes are negligible and pyroxene grains are quite uniform. The chlorite dispersed in the devitrified glass forms irregular flakes up to 100 m in size of clinochlore composition with X Fe = 0.48. The last observable mineral is the epidote, represented by cracked irregular grains about 100-150 m in size, characterized by high relief and varicolored interference, chemically relatively inhomogeneous, with pistacite component contents of 32-55 mol%. The chemical composition of the sample corresponds to basalt according the TAS classification (Le Bas 1986), as well as in the classification based on the contents of trace elements (Pearce 1996). Studied rock is thus a basalt with SiO 2 content 49.57 wt.%, with relatively high contents of FeOt (9.16 wt.%) and CaO (11.76 wt.%). The high mg # (71.6) corresponds to the position in the komatiitic basalt field in the Jensen (1976) classification. The normalized REE trend (Boynton 1984) is weakly fractionated (La N /Yb N = 7.02) with a negligible negative Eu anomaly (Eu/Eu* = 0.85). Compared to NMORB (Sun and McDonough 1989), the sample is enriched in LILE, Pb and Sr, shows significant negative anomaly in Nb and Ta, and less significant negative anomaly in Zr and Ti contents (Fig. 3). The contents of trace elements in geotectonic discrimination diagrams (e.g. Wood 1980;Agrawal et al. 2008;Pearce et al. 1984) correspond to a volcanic arc.

U-Pb LA ICP-MS zircon dating
Zircon grains are generally automorphic, transparent with a size from 50 × 120 to 80 × 300 m. The crystals are shortly prismatic with an aspect ratio of 1:2 or 1:3 (with the exception of the long prismatic grain with an aspect ratio of 1:5, Fig. 4A) with rounded fuzzy edges and short pyramids, pastel luster and light honey-like color. However, fragments of broken originally automorphic to hypautomorphic grains prevail. The magmatic oscillatory zoning of most of the zircon grains is clear in the CL, in some cases it is possible to observe the replacement of the oscillatory zoning by sector zoning. Sometimes also different degrees of recrystallization, or even resorption of the grain is present (Fig. 5) Fig. 4A) with a clear oscillatory zoning and in the marginal zone of a well crystallized automorphic short prismatic grain (No. 19), with sector zoning replacing original oscillatory zoning with the small number of relatively massive (approx. 10 mm) individual zones. Marginal zones in this grain show signs of later probably magmatic dissolution (Fig. 4B).

2)
The second group yielding the Lower Carboniferous age of 351.9 ± 3.5 Ma (Fig. 7) was determined in the marginal zone of the automorphic long prismatic long grain mentioned above (No. 45; spot 2; Fig. 4A). Furthermore, these ages were found in three spots across short prismatic 360 mm long oscillatory zoned zircon (No. 15; spots 1, 2 and 3; Fig. 4C) with oval quartz inclusions and in the central part of a fragment of an automorphic short prismatic grain (No. 35; Fig. 4D) with detailed very fine oscillatory zoning. All of these three grains (No. 45,15,35; Fig. 4A, C, D) morphologically differ from each other studied zircons. No. 45 grain differs in a long prismatic shape, the other in sizes, detailed zoning, darker luminescence and in grain No. 15 in a large number of quartz inclusions. Therefore, it can be assumed that these represent xenocrysts (long prismatic grain) and/or clastic zircons, that have contaminated ascending magma. The dominant ages -34 of the total 44 measurements -form two groups: 304.4 ± 2.3 Ma and 288.6 ± 1.9 Ma (Fig. 7, Table 1).

3) Eleven measurements correspond to Upper
Carboniferous age of 304.4 ± 2.3 Ma. These ages were measured in: (a) the centres of fragments of originally automorphic short prismatic grains with well-developed oscillatory zoning (No. 7, 8; Fig. 5A Table 1. usually with rounded edges of pyramids. These are usually oscillatory zoned; sector zoning is less frequent but not exceptional (grains No. 37,17,60,9;Fig. 5E,F,G,H). 5) The youngest Upper Triassic age 218 ± 3.4 Ma (Fig. 7, Table 1) was detected in three of studied grains (Fig. 6). First grain (No. 31) is a hypautomorphic, very slightly luminescent short prismatic 125 m long grain with indistinct simple zoning formed by the core and two darker incremental zones about 8 m thick, separated by a lighter thin zone (Fig.  6A). The second grain (No. 30) is nonluminescent short prismatic automorphic 150 m long grain with a noticeable indistinct original oscillatory zoning replaced by a more distinct sector zoning (Fig. 6B). The last one (No. 28) is a nearly non-luminescent short prismatic grain 90 m long with an indistinct simple zoning, formed by a dark core and a slightly lighter 12 m thick marginal zone (Fig. 6C). The indistinct luminescence of zircons suggests the lead loss resulting in geologically improbable Triassic age.

SUMMARY AND DISCUSSION
The new occurrence of Permian volcanic and volcaniclastic rocks was found in the valley of the Tsengel River (western Gobi-Altai Terrane of Badarch et al., 2002) inside the area formerly considered to be Lower Paleozoic (Togtokh et al., 1995). The studied sample forms a massive part of fluidal agglomerate lava and it is classified as pyroxene basalt composed of glassy matrix with pyroxene phenocrysts. The chemical composition of this high-Mg basalt does not correspond to the alkaline character of Permian intraplate volcanism described further to the east in the Khar Argalantyn area by Buriánek et al. (2012) or on the NE slopes of the Khantaishir ridge by Kozakov et al. (2015). Dominating concordia cluster with two age peaks corresponding to 288 Ma resp. 304 Ma is associated with two consecutive magmatic events in relatively near time succession of uppermost Carboniferous -Lower Permian age. Inherited zircons of two groups with an age of ~ 376 resp. 352 Ma probably reflect Devonian to Carboniferous magmatic events in wider surroundings. Ages of c. 380-370 Ma are known from granitic rocks of the metamorphic unit Tseel lying south of the studied area (Burenjargal et al. 2014(Burenjargal et al. , 2016Hanžl et al. 2016;Cai et al. 2015). Ages of c. 350 Ma are known from the granite massifs of the Mongolian Altai further west (Cai et al. 2015) or from the Chandman Massif in the east Lehmann et al., 2017). The basalt agglomerate represents the final stage of arc magmatism, which was reported over the subduction zone retreating towards the south in the Trans-Altai Gobi further to the south (Nguyen et al. 2018) or the extension phase immediately following the subduction plate break-off, as described by Buriánek et al. (2016)    on the example of the Permian to Carboniferous Sagsai pluton situated about 75 km SE from the studied locality.