Geochemical diversity of metabasite pods from the Neoarchean Baidaragin grey gneiss complex, Central Mongolia

Belyaev V.A.

A.P. Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia

belyaev_vasya@mail.ru

The Central Asian orogenic belt contains several microcontinents, with the most ancient Baydrag block located in Central Mongolia south of the Bayankhongor ophiolite melange. The Baydrag block of the Dzabkhan microcontinent (also termed the Baydrag microcontinent) has Neoarchean to Paleoproterozoic crystalline basement overlain by Mid-Neoproterozoic shales. Two metamorphic complexes have been recognized: Neoarchean Baidaragin and Paleoproterozoic Bumbuger. This study focuses on the Baidaragin complex outcropped in the valley of Baydrag River, the southern slope of Khangai Ridge.

The Baigaragin complex consists of abundant tonalite-trondjhemite-granodiorite (TTG) grey gneisses with the pods of mafic granulites and amphibolites from 20-30 cm to several meters long. Mafic pods are concordant with foliation of grey gneisses, that is, mafic granulites and amphibolites are considered to be older than grey gneisses. The Baidaragin mafic rocks are currently investigated to constrain evolution of the Baydrag block in the Archean time.

Geochronology of the Baidaragin complex is mainly summarized by I.K. Kozakov et al. [1]. As the paper suggests, the SHRIMP II dating of zircons from the Baidaragin grey gneisses revealed formation of their protolith at ~ 2.65 Ga and subsequent metamorphism at ~ 2.65-2.5 Ga. Besides, zircons from a mafic pod in grey gneisses yielded age ~ 2.76 Ga, considered as the protolith emplacement age, and 2.65-2.5 Ga age of high temperature metamorphism. The veins of late undeformed potassium granites (1825 Ma) cut the Baidaragin metamorphic rocks.

V.A. Belyaev et al. [2] investigated petrography and geochemistry of the Baidaragin mafic granulites and amphibolites in detail. The mineral assemblages of Opx + Cpx + Pl and Opx + Cpx + Hrb + Pl result from granulite metamorphism. Considering the texture observations it can be assumed that granulite assemblages were transformed into amphibolite-facies ones as a result of retrogressive cooling. The most common assemblage is Hbl + Cpx + Pl ± Bt. There are also single- or two-mineral rocks composed of hornblende and/or tremolite-actinolite. In this abstract, all the Baidaragin mafic rocks are further referred to as metabasites.

Having examined major element whole-rock chemistry, V.A. Belyaev et al. [2] distinguished three groups within the Baidaragin metabasites. On classification Mg – Fe+Ti – Al triangular plot [3] and binary plots of oxides vs. MgO, the analyzed rocks are similar with (1) tholeiitic basalts of oceanic plateau, e.g. Ontong Java Plateau [4], (2) Al-undepleted basaltic komatiites, e.g. Vetreny greenstone belt, Baltic Shield [5], and (3) Al-depleted basaltic komatiites, e.g. Mendon formation, Barberton greenstone belt, South Africa [6]. Some of the Baidaragin metabasites have “primary” trace-element patterns resembling those of oceanic plateau basalts and Al-undepleted komatiites. Moreover, several samples show evidence of crustal contamination on primitive mantle-normalized plots, such as Nb and Ti negative anomalies, LREE enrichment, slight positive peaks of Zr-Hf, and Th/La_PM > 1. These are the features of basalts and komatiites contaminated with the TTG-crust [5, 7]. Calculation of assimilation-fractional crystallization process (AFC) and investigation of (La/Sm)_PM vs. (Nb/Th)_PM plot [5] were used to evaluate 2-10% contamination of the Baidaragin metabasites (groups 1 and 2) with average TTG composition [8]. Contaminated and non-contaminated basalts and komatiites are common in the Archean greenstone belts [5, 6, 7]. Thus, contaminated and “primary” Baidaragin metabasites could be the greenstone remnants, now available as pods in grey gneisses.

More interestingly, it was found that the other Baidaragin metabasites, with non-primary and non-contaminated features, all having similar trace element patterns. That is, rocks belonging to different groups (oceanic plateau-basalts, Al-undepleted and Al-depleted basaltic komatiites) are characterized by nearly identical LREE enrichment, deep HFSE troughs and Th/La_PM < 1 on primitive mantle-normalized plots. This trace element systematics does not feature the magmatic precursors. So it can be concluded that unusual chemical characteristics of these LREE-enriched and HFSE-depleted rocks arise after their emplacement into the earth’s crust and juxtaposition in the Baidaragin complex. One can suggest that these rocks were affected by metasomatic process during emplacement of TTG in greenstone sequence of basalts and basaltic komatiites, or, alternatively, during high-grade metamorphism. This hypothesis is supported by geochemical zoning found in two metabasite pods: concentrations of LREE, Th and SiO₂ increase from the cores to rims. More likely, because this change of composition occurred in a solid state it was a “metasomatic” process. It is hard to define strictly the time and mode of “metasomatism”. Surprisingly, studied “metasomatic” metabasites include both granulite- and amphibolite-facies rocks.

To conclude, the pods of metabasites of the Baidaragin grey gneiss complex belong to different geochemical groups, namely (1) tholeiitic plateau-like basalts, (2) Al-undepleted basaltic komatiites and (3) Al-depleted basaltic komatiites. Some of the rocks of the first and second groups experienced crustal contamination with TTG-like composition. The other metabasites of every group have geochemical fingerprints of “metasomatic” process.

References:

1. Kozakov, I.K., Sal’nikova, E.B., Wang, T., Didenko, A.N., Plotkina, Yu.V. (2007) Early Precambrian crystalline complexes of the Central Asian Microcontinent: age, sources, tectonic position // Stratigraphy and Geological Correlation, v. 15, p. 121-140.

2. Belyaev, V.A., Gornova, M.A., Medvedev, A.Ya., Pakhomova, N.N. (2012) Geochemical features of metabasalt enclaves in grey gneisses of the Baidarick Block (Central Mongolia) // Russian Geology and Geophysics, v. 53, in press.

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4. Tejada, M.L.G., Mahoney, J.J., Dunkan, R.A., Hawkins, M.P. (1996) Age and geochemistry of basement and alkalic rocks of Malaita and Santa Isabel, Solomon Islands, southern margin of Ontong Java Plateau // Journal of Petrology, v. 37, p. 361-394.

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8. Martin, H. (1994) The Archaean grey gneisses and the genesis of continental crust / In: Condie, K.C. (Editor) Archean Crustal Evolution. Amsterdam, Elsevier, p. 205-259.