GEOCHEMISTRY AND GEOCHRONOLOGY OF THE PALEOZOIC SEDIMENTARY ROCKS IN THE SHAR KHUTUL AREA , CENTRAL MONGOLIA

The study area is located in the central part of Tsetserleg terrane in the southwestern margin of the Khangai-Khentey orogenic system. The paper presents new data on geochemistry and geochronology of sedimentary rocks from the Shar Khutul area, where the Tsetserleg terrane consists of Silurian-Devonian oceanic plate stratigraphic unit and Carboniferous shallow water sediment. The Upper Silurian to Middle Devonian Erdenetsogt Formation (S3-D2er), which is an oceanic plate stratigraphic unit, is mainly composed of siliceous siltstone, volcanites, tuffs, quartzite, and cherts. The shallow water sediments are divided into Upper Devonian to Lower Carboniferous Tsetserleg Formation (D3-C1cc) and Lower–Middle Carboniferous Dzargalant Formation (C1-2dz). The Tsetserleg Formation (D3-C1cc) consists of only sedimentary rocks such as bluish-grey sandstones and siltstones, and Lower–Middle Carboniferous Dzargalant Formation (C1-2dz) is principally composed of mediumto coarse-grained, brown-greenish grey sandstones with thin-layers of dark siltstones and gravelites. The SiO2 content of the Shar Khutul area sandstones ranges from 63.85 to 67.95 wt.% and the average content of TiO2 is 0.72 wt.% and Al2O3 content is 14.38 wt.%. The Chemical Index of Alteration (CIA) value ranges from 48.71 to 56.94 and the range of Index of compositional variations (ICV) is from 0.98 to 1.24. Moreover, the samples studied show that most of the sandstones are generally immature and were derived from weakly weathered source rocks. The ratios of Eu/Eu* (0.83), La/Sc (3.81), La/Co (5.30), and Cr/Th (13.81) indicate that the derivation of the Shar Khutul area sandstones from felsic rock sources and confirm the signatures of a felsic igneous provenance and suggest an active continental margin tectonic setting of the source area. The clastic zircons from the medium grained sandstone (Erdenetsogt formation) yield ages between 2.5 Ga and 236 Ma and the detrital zircons exhibit four peak ages at 1.7-2.5 Ga (n = 13), 455-499 Ma (n = 6), 337382 Ma (n = 13) and 236–250 Ma (n = 5).


INTRODUCTION
The Devonian-Carboniferous Tsetserleg terrane takes place in the Khangai mountain ranges.The terrane is divided into the Lower-Middle Devonian chert-basalt-terrigenous (Erdenetsogt Formation), Middle-Upper Devonian flyshoid (Tsetserleg Formation) and Lower Carboniferous (Dzargalant Formation) [38,12,15,39].A type-section of the Lower-Middle Devonian Erdenetsogt Formation (D 1-2 er) was made by Tomorchudur et al. [38] at about 15 km northeast from the village of Erdenetsogt soum and divided into two members.Lower member consists of polymictic, oligomictic conglomerate, sandstone and siltstone.Upper member is mainly composed of reddish-grey coloured chert-quartzite, metabasalt, conglo-breccia, and bluish, green-grey coloured sandstone, conglomerate and siltstone.The Middle-Upper Devonian Tsetserleg Formation (D 2-3 cc) consists of blue, bluish, grey, fine-medium grained sandstones with thin layers of dark, dark-grey siltstones.The Lower Carboniferous Dzargalant Formation(C 1 dz) is represented by brownish green, brownish grey, medium-course grained polymictic, oligomictic sandstones, gravelites and siltstones.Numerous studies, which aimed at determining the provenance, source weathering, and tectonic setting, were made by some researchers [24,29,30,2] in the Khangai region.This paper presents provenance, depositional period, weathering condition and tectonic setting of sedimentary units according to their geochemical analysis and geochronological data.

Geological setting
Previous studies inferred that the age of the Tsetserleg Formation belongs to the Middle-Upper Devonian, the Erdenetsogt Formation is Lower-Middle Devonian, and Dzargalant Formation is Lower Carboniferous in age.We have modified the age of those Formations based on the field observation, geochemical and geochronological data.According to our study, the Erdenetsogt Formation is Upper Silurian-Middle Devonian, the Tsetserleg Formation is Upper Devonian-Lower Carboniferous, and Dzargalant Formation is Lower-Middle Carboniferous, respectively [10].The study area is dominated by Upper Devonian-Lower Carboniferous Tsetserleg Formation (D 3 -C 1 cc) which consists of bluish grey siliceous siltstone, sandstone, gravelite and rarely radiolarian siltstone (Figure 1).
The Tsetserleg Formation borders on the Upper Silurian-Middle Devonian Erdenetsogt Formation (S 3 -D 2 er), which is recognized as an oceanic plate stratigraphic (OPS) succession and exposed by tectonic wedge (Figure 2).The Erdenetsogt Formation is lithologically divided into three different sequences.The first sequence is characterized by whitishgrey coloured quartzite, greenschist facies metamorphosed basalt, which is classified into Upper Silurian-Lower Devonian.The second sequence is composed of greengreenish coloured basalt, red-reddish coloured chert, quartzite-jasperoid and tuff, which is classified into Lower Devonian.The third sequence consists of green coloured basalt, bluish-grey siltstone and tuff, which is classified into Lower-Middle Devonian.The Tsetserleg Formation is composed of greybluish coloured sandstone, siltstone and some layers of conglomerate and gravelite, which is conformably overlaid by Lower-Upper Carboniferous Dzargalant Formation (C 1-2 dz).Based on geochronological data of the detrital zircon and radiolarian fossils (palaeoscenidium sp., tetratormentum sp., albaillella sp.), the Tsetserleg Formation age is inferred to Upper Devonian-Lower Carboniferous period.The Dzargalant Formation consists of sandstone, siltstone and conglomerate, which is probably Lower-Upper Carboniferous.Its analogical unit is named by Dulaankhairkhan Formation (C 1-2 dh) in the southeastern part of the Tsetserleg terrane, which is dated as 348 Ma on the detrital zircon [27].
Figure 1.The geological map of the Shar Khutul area modified after Tomorchudur et al. [38] and location of the study area [39].

MATERIALS AND METHODS
Nine thin sections of the sedimentary rocks from different units were studied using petrographic microscope and geochemical (major, trace and rare earth elements) analysis and one sample from the Tsetserleg Formation in the Shar Khutul area, Central Mongolia was dated by U-Pb detrital zircon dating.

Geochemical analysis
The samples were analysed for major, trace and rare earth elements (REE) at SGS Mongolia LLC, Ulaanbaatar, Mongolia.The major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K and P) were analysed by X-ray fluorescence spectrometer (XRF), and trace and rare earth elements (54 elements) by inductively coupled plasma mass spectrometry (ICP-MS).

U-Pb zircon dating
The samples were first crushed to 80-100 mesh, and the low-density materials were then removed using conventional heavy-liquid techniques.High-purity zircon samples were then separated from remaining heavy minerals using magnetic separation techniques at the Laboratory of Petrography, Institute of Paleontology and Geology, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia.High-quality non-fractured zircon grains were inlaid in epoxy, polished down to half their original thickness and washed in an acid bath before analysis.The cathodoluminescence (CL) images were collected using field emission scanning electron microscope FEG-SEM Tescan Mira3 GMU at the Center of Lithospheric Research of the Czech Geological Survey.Based on analysis of the CL images, distinct location within the zircons were selected as the best point for laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS).An Agilent 7900x Quad ICP-MS equipped Analyte Excite 193nm excimer laser was used to measure the U-Pb isotope ages of the zircons.

Petrography
The petrographic analysis (Figure 3) shows that sandstones of Shar Khutul area are mainly characterized by medium to coarse grained particle.The sandstones are mostly poorly sorted with angular to subrounded grains.The main mineralogical constituents are quartz, feldspars, and lithic fragments (55-60%).Quartz is the most predominant constituent mineral and is present as monocrystalline (Qm) and polycrystalline (Qp).Two types of feldspars have been identified, which include albite (plagioclase), orthoclase and microcline (k-feldspar).K-feldspars is more abundant than plagioclase.Lithic fragments are predominantly composed of granite, mudstones, schist and andesite.Cement has a percentage range of 40 to 45, and include very fine grained quartz, feldspars, mudstones, biotite, chlorite, carbonate, and iron oxide.The accessory minerals are composed of mainly non-opaque minerals (zircon, apatite, rutile, and minor tourmaline).In addition, the K 2 O and Na 2 O contents of sandstone samples and their ratios (Na 2 O/ K 2 O>1) are consistent with the petrographic observations, in which plagioclase dominates over K-feldspar, and the Na 2 O/K 2 O ratio is more than 1, therefore, the sandstones Na 2 O/ K 2 O ratio represent immature sediments near to the source (Table 1) [16].Thus, the sediments K 2 O/Al 2 O 3 ratio can be used as an indicator of ancient sediments original composition.The K 2 O/Al 2 O 3 ratios for clay minerals and feldspars are different (0.0 to 0.3, 0.3 to 0.9, respectively) according to Cox [6].In this present study, the average K 2 O/Al 2 O 3 ratio of the sandstone is 0.21, which is their original composition may have been enriched by clay minerals.

Geochemical composition
The sandstones of the Shar Khutul area with their chemical composition are greywacke affinity (Figure 5).
The composition of major element in sandstones has also been used to determine sedimentary provenance by the application of discriminant function analysis [34].In this discrimination diagram, sandstones of both the formations are plotted in the felsic igneous provenance (Figure 6).The upper continental crust-normalized trace element diagram shows high content in large ion lithophile elements (LILE) and high field strength elements (HFSE), lower content in transition trace elements (TTE), therefore, we believe that their origin is derived from felsic source rocks (Figure 7).

Rare earth elements
The REE patterns were used to infer sources of sedimentary rocks.The Eu/Eu* (0.83), La/Sc (3.81), La/Co (5.30) ratios indicate derivation of the Shar Khutul area sandstones from felsic rock sources.
The Tsetserleg Formation sandstones have (La/Yb)N ratios ranging from 7.81 to 11.18 (av.=9.48) and the Dzargalant Formation sandstones range from 9.32 to 10.36 (av.=9.96).The rare earth element contents of the sandstones from the Shar Khutul area are presented in Table 2.The basic rocks generally have low LREE/ HREE ratios and no Eu anomalies, whereas, felsic rocks usually contain high LREE/HREE ratios and negative Eu anomalies [7,8].In this study, the sandstones are characterized by high LREE/HREE ratios (11.55-14.19,av.=12.74) and negative Eu anomalies (Eu/Eu*=0.71-0.95,av.=0.83), therefore, we consider that their origin derived from felsic source rocks, which is similar to UCC (Figure 8).
In total, 52 single detrital zircon grains were analyzed from this sample, and 37 grains are selected for the final interpretation (Table 3).The sample yielded ages between 2.5 Ga and 236 Ma, and the detrital zircons exhibit four peak ages at 1.7-2.5 Ga (n=13), 455-499 Ma (n=6), 337-382 Ma (n=13) and 236-250 Ma (n=5).(Figure 9b,c).We regard that the third main peak can indicate depositional period, because the fourth peak would be related to reaction of metamorphism, derived from the Khangai intrusion complex (P 2 -T 1 ) or weathering.
The ancient peak is 2.5-1.7Ga and corresponds to the Paleoproterozoic basement in the Baidrag block [13,5], the second peak (499-455Ma) corresponds to the Dzag series sediments [14], and the fourth peak (250-236Ma) includes 5 zircon grains, which correspond to Middle Permian-Early Triassic Khangai intrusive complex [42,25,26].The zircon grains and morphology of the fourth peak represent re-crystallization event.It is probably related to Middle Permian-Early Triassic granitoid pluton.
In the previous literatures, our newly classified Tsetserleg formation belonged to Lower-Middle Devonian Erdenetsogt formation.Based on geochronology, the Tsetserleg formation is assigned Upper Devonian-Lower Carboniferous periods.Otherwise, the Tsetserleg formation should be distinguished for lithology from the Erdenetsogt formation.In addition, according to radiolarian study, we classified that the Erdenetsogt formation is Upper Silurian -Middle Devonian period, which is composed of basalt, quartzite, chert, minor siliceous siltstone, mudstone, probably none of sandstone.

Sorting and weathering effects
The sedimentary sorting and recycling can be monitored by a plot of Th/Sc versus Zr/Sc [19].A simple positive correlation between Th/Sc and Zr/Sc ratio is exhibited by first cycle sediments, whereas there is a substantial increase in Zr/Sc with far less increase in Th/ Sc in recycled sediments [1,31].On the Th/ Sc versus Zr/Sc diagram, the sandstones of the Shar Khutul are shown simple positive correlation, which is consistent with that of the first cycle sediments (Figure 10).
The Th/U ratio is also a useful parameter in determining the source characteristics of clastic sedimentary rocks [32].In sedimentary rocks, Th/U values higher than 4.0 may indicate intense weathering in source areas or sediment recycling [20].In this study, all sandstones Th/U ratios range between 3.60 and 3.89, (av.=3.75).This suggests that the sandstones were derived from unweathered rocks, which indicate they were similar to UCC (Th/U=3.80).The Th/U versus Th plot for the sandstones (Figure 11)shows typical distribution similar to the average values of the upper continental crust and follows the normal weathering trend [19].The higher chemical index of alteration (CIA), ranging between 76 and 100, indicates the intensive chemical weathering in the source areas [22,11], whereas values <50 indicate near absence of chemical alteration and reflect cool and arid conditions or unweathered source areas [11].Furthermore, sandstones with index of compositional variation (ICV)>1are compositionally immature with the first cycle of sediments deposited in tectonically active settings and ICV<1 are compositionally mature and were deposited in the quiescent or cratonic environment where sediment recycling was active [6].In the present study, the CIA values range between 48.71 and 56.94, with an average of 52.21, while ICV values range between 0.98 and 1.24 with an average of 1.12 (Table 1).The binary plot of CIA against ICV for studied samples shows that most of the sandstones are geochemically immature and were derived from weak weathered source rocks (Figure 13).
The CIA values of the studied sandstones are also plotted in the Al 2 O 3 -(CaO*+Na 2 O)-K 2 O (A-CN-K) diagram, which may express much of the chemical variation resulting from weathering.Unweathered rocks cluster along the left-hand side of the plagioclase-K-feldspar join line in the A-CN-K system [23].All samples of the Shar Khutul area are plotted nearly in the plagioclase-K-feldspars join line, which is perhaps because these samples were derived from felsic-intermediate igneous and unweathered source rocks (Figure 13).Provenance and tectonic settings The SiO 2 content and K 2 O/Na 2 O ratios in sandstones appear to be particularly sensitive indicators of geotectonic setting of the source area [33].The quartz-poor greywackes (quartz<15%, average SiO 2 =58%, K 2 O/ Na 2 O<1) are indicative of magmatic island arcs and quartz intermediate greywackes (quartz=15-65%, SiO 2 =68-74%, K 2 O/ Na 2 O<1) are indicative of Andean-type continental margins and have approximately the same composition as the upper continental crust.Quartz-rich greywackes (quartz>65%, SiO 2 =89%, K 2 O/Na 2 O>1) are characteristic of Atlantic-type continental margins and are similar in composition to the sand fraction of the continental platform cover [35].The sandstones of study area are quartz intermediate greywackes (SiO 2 =67-70%, K 2 O/ Na 2 O<1) and it is similar in composition with upper continental crust.
Therefore, K 2 O/Na 2 O ratio and SiO 2 content discriminate between passive-margin and active-margin sandstones [17].They suggest that passive-margin sandstones have K 2 O/Na 2 O ratios greater than 1 and activemargin sandstones have ratios less than 1.Also passive-margin sandstones are enriched in SiO 2 compared to active margin sandstones.K 2 O/Na 2 O versus SiO 2 diagram shows the studied sandstones may be classified as active continental margin provenance (Figure 14), furthermore the sandstones plotted in the field of active continental margin in La/Sc-Ti/Zr diagram [3] which describes the geotectonic environments (Figure 15).

Figure 2 .
Figure 2. New schematic section of the Shar Khutul area

Figure 4 .
Figure 4. Harker variation diagrams for the Shar Khutul area sandstones

Figure 5 .
Figure 5. Geochemical classification of the sandstones[28] Figure6.Discriminant function diagram using major elements for the provenance signature of the sedimentary rocks[34] Trace elementsTrace elements, including Cr, Ni, V,Co, Y, Sc, Zr, Hf and Th, reside in minerals that are resistant to secondary processes and are commonly used to constrain the composition of the source[37,21,40].The relative abundance of Co and Cr (indicative of mafic source), La and Th (indicative of felsic source) and ratio such as Co/Th and La/Sc are generally used to examine the geochemical nature of the source rocks [18, 9, 40, 41].The sandstones from the Shar Khutul area have Co/Th and La/ Sc ratios generally similar to UCC, indicating dominantly felsic sources.Furthermore, depletion of compatible elements (Cr, V, Ni and Sc) and slight enrichment of HFSE (Zr and Hf) relative to the UCC, are consistent with such felsic source.The upper continental crust-normalized trace element diagram shows high content in large ion lithophile elements (LILE) and high field strength elements (HFSE), lower content in transition trace elements (TTE), therefore, we believe that their origin is derived from felsic source rocks (Figure7).

Figure 9a )
Figure 9a) Representative CL images of the detrital zircon, b) Condordia plot of detrital zircon, and, c) Detrital zircons age histogram

Table 1 .
Major and trace element analyses of the sandstones from the Shar Khutul area Lithology Tsetserleg Formation (D 3 -C 1 cc) Dzargalant Formation (C 1-2 dz)

Table 1 )
. The average values of Al 2 O 3 range between 14.11 and 14.60.Variations in the major geochemistry elements of the Shar Khutul area sandstones are shown on Harker diagrams (Figure 4).Al 2 O 3 against major oxides were plotted on variation diagrams, K 2 O, P 2 O 5 , TiO 2 , Fe 2 O 3 , MgO and MnO show positive correlation with linear trend.CaO, Na 2 O and SiO 2 negatively correlated with Al 2 O 3 .

Table 2 .
Rare earth element concentrations of sandstones from the Shar Khutul area