Geochemical Characterization Of Silurian–Carboniferous Ulaanbaatar Terrane Sedimentary Rocks in The HangayHentey Basin, Central Mongolia Narantuya PUREVJAV1., Molor BOLD1., Daham JAYAWARDANA2 Ukhnaa GENDEN3., Nyambayar PUREVJAV1
1st International Conference
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School of Geology and Petroleum Engineering, Mongolian University of Science and Technology, MONGOLIA 2
Geoscience for Energy, Mineral Resources, and Environment applieds 2014
Sri Jayewardanepura University, SRILANKA
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Traning center of Mineral Resource & Geo-information Corresponding author: Narantuya PUREVJAV Email:
[email protected]
The Mongolia is a key part of the Central Asian Orogenic Belt (CAOB) of East Asia. The Ulaanbaatar terrane forms part of the COAB, and lies within the Hentey sub-basin of the larger Hangay-Hentey basin. The terrane is mainly composed of Silurian–Carboniferous sediments. Whole-rock XRF analyses were made of a suite of turbidite sandstones and mudrocks to determine their provenance signatures. Geochemical classification and Index of Compositional Variability (ICV) values show the sediments are immature, as also indicated by moderate SiO2 contents (all 0.84 are typical of major rock-forming minerals such as feldspars, amphiboles and pyroxenes, whereas values < 0.84 are typical of alteration products such as
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kaolinite, illite and muscovite (Cox et al., 1995). The Ulaanbaatar terrane sediments have ICV values ranging from 0.85 to 1.10, and CIA ratios from 50 to 62 (Fig 3). The ICV values are thus greater than PAAS (Post-Archaean Australian Shale; Taylor & McLennan, 1985; ICV= 0.85), whereas CIA is lower (Fig. 3).
Figure 3. ICV-CIA plot (after Long et al., 2012b) for the Ulaanbaatar sandstone and mudstone. PAAS range from Taylor & McLennan (1985); UCC from Rudnick & Gao (2005)
Immobile elements provenance: Immobile trace element ratios are also commonly used to identify the source rock composition of sediments; as such elements are transported quantitatively from source to the depocenter (McLennan et al., 1993). Numerous models for the provenance of the Hangay–Hentey sediments have been proposed, as outlined above. Derivation from the Precambrian basement of Mongolia, either the Siberian Craton or north China (Badarch et al., 2002; Jahn et al., 2004; Tomurtogoo et al., 2005; Kelty et al., 2008) now seems unlikely, as shown by the general lack of old detrital zircon U/Pb ages (Kelty et a., 2008, Bussien et al., 2011). The low ICV, CIA and Th/U ratios in the Ulaanbaatar sediments and their distributions in A–CN–K space described above also suggest a stable cratonic source is unlikely. The Ulaanbaatar patterns also closely correspond with that for average felsic volcanic rock (Condie, 1993). The Ulaanbaatar sediments plot of Th/Sc vs Zr/Sc close to UCC and along a primary source evolution trend samples concentrated at high ratios between average dacite and rhyolite (Fig. 4a). This confirms the relatively felsic source indicated by the major element data too (not shown). The Altanovoo samples trend across the source evolution trend, with higher Zr/Sc ratios in the sandstones relative to the mudrocks (Fig. 4a). This trend is typical of zircon concentration by hydraulic sorting, as also indicated by Zr–Al2O3 variations (not shown). A companion Ti/Zr–Ce/Sc plot of the Ulaanbaatar sediments, fall between average dacite and rhyolite, confirming and
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intermediate to felsic source (Fig. 4b). Gorkhi sandstones fall near rhyolite, along with most Altanovoo sandstones, but Altanovoo mudrocks are displaced toward average dacite, indicating a minor sorting effect toward more intermediate composition (Fig. 4 a, b).
Figure. 4 (a, b) Immobile trace element provenance plots: (a) Zr/Sc-Th/Sc (McLenann et al., 1993); (b) Ti/Zr-Ce/Sc (Floyd & Leveridge, 1987). Stars: BAS, LSA, AND, DAC and RHY are average basalt, low-silica andesite, dacite, and rhyolite (Roser & Korsch 1999).
Tectonic setting and depositional environment: The depositional setting of the Hangay-Hentey basin in which the Ulaanbaatar terrane occurs has been a matter of debate. Proposed approaches were the basin originated as an oceanic gulf, was a post-orogenic successor basin, an accretionary complex origin, passive margin, and derivation from an island arc system of some sort, or from an Andean continental margin (ACM) arc (Sengör & Natal’in (1996); Ruzhentsev & Mossakovskiy (1996); Zorin (1999), Natal’in (2007); Martin-Gombojav & Winkler (2007); Kelty et al., (2008); Bussien et al., (2011). The Hangay–Hentey basin floored by continental crust, although this model has recently been dismissed (Kurihara et al., 2009). The tectonic setting of the Ulaanbaatar terrane was here evaluated using (K2O/Na2O)– (SiO2/Al2O3) (Roser & Korsch, 1986). The Ulaanbaatar terrane sediments lie within the A2 evolved arc setting on the former, and within the CIA (Continental Island Arc) field on the latter. Other tectonic discriminants (not illustrated) such as the ThSc-Zr plot of Bhatia & Crook (1986) and the Basicity Index-Al2O3/SiO2 scheme of Kumon & Kiminami (1994) yield similar results. On the former the Ulaanbaatar terrane sediments are classified as CIA, whereas on the latter Basicity Index values of ~0.05 to 0.10 yield EIA (evolved island arc) classification.
5. Conclusions Geochemically the Ulaanbaatar sandstones are classed as wackes, and most of the mudstones as shales and weak sorting in texturally and mineralogically immature sediments derived from weakly weathered source a relatively average felsic. Low CIA (
