Geochemical characteristics and formation conditions of bimodal series volcanics of Dzabkhan microcontinent

Urmantseva L.N.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

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One of the largest continental blocks within Central Asia is Dzabkhan microcontinent (Central Mongolia). Precambrian Baydaric block of Dzabkhan microcontinent is represented by early Precambrian basement (Bumbuger and Baidaragin Complexes) and Riphean cover (Ulzitgol Formation). The Baidaragin Complex rocks were formed in Late Archean. The oldest age values of about 2.8 Ga were determined for tonalite gneiss and two-pyroxene crystalline schist of Baidaragin Complex and correspond to crystallization time of their protoliths [3]. Probable formation interval of the Bumbuger Complex rocks is 2.44-2.37 Ga [3]. Riphean carbonates and terrigenous rocks of the Ulzitgol Formation unconformably overlie the basement in the eastern part of the microcontinent [4]. Rocks of Dzabkhan Formation take part in the structure of microcontinent in its south-western part. Dzabkhan Formation is mainly represented by felsic volcanites and to a lesser degree by andesitic-basalts and andesites with little amounts of terrigenous and carbonate sediments [4]. Concordant age values of zircons from Dzabkhan rhyolites limit their formation between 805 and 770 Ma [4]. Definition of geodynamic conditions of Dzabkhan sedimentary-volcanogenic rocks formation will give contribution to the question solution concerning belonging of Dzabkhan microcontinent to Gondwanan or Siberian group of terranes.

Dzabkhan mafic volcanites, studied around Bayan-Ula somon are characterized by TiO₂ contents of 1.02-2.73 wt.% over a range of SiO₂ contents (50.0-54.6 wt.%). They span 12.47-16.26 wt.% Al₂O₃, have increased contents of P₂O₅ (0.34-1.23 wt.%) and low values of Mg# between 29 and 47. High FeO_tot/MgO ratio defines tholeitic series of rocks. They have weakly fractioned REE spectra ((La/Yb)_n=3.7-9.2, (La/Sm)_n=2.0-3.3) with higher REE concentrations in titanous varieties (fig. a). Volcanites are characterized by enrichment in HFS-elements on spider diagrams, but strongly Nb (Nb/Nb*=0,3-0,5) and Ti anomalies. Such HFSE and LREE enrichment of mafic volcanites is typical of intraplate rocks (fig. 1, b). The presence of negative Nb anomalies, as well as Th and La enrichment indicate crustal material influence on mafic volcanites composition.

Dzabkhan felsic volcanites and volcanoclastic rocks are highly siliceous (SiO₂=74.5-79.3 wt.%). Associated tuffs have lower SiO₂ concentrations (65.3-73.2 wt.%). Volcanic rocks and their tuffs are characterized by moderate alkalis contents (Na₂O+K₂O=5.5-6.3 wt.%). Rhyolites differ from tuffs by lower TiO₂ contents (0.07-0.34 and 0.29-0.51 wt.%, accordingly), Al₂O₃ (10.74-13.18 and 12.02-18.05 wt.%) and higher CaO (0.35-0.89 and 0.23-0.45 wt.%). Characteristic feature of volcanic and volcanoclastic rocks is their high Fe* (FeO/(FeO+MgO)=0.78-0.90).

Fig. Chondrite-normalized REE (a) and primitive mantle-normalized (b) diagrams for Dzabkhan mafic (samples U-1-11 and U-5-11) and felsic (samples U-8-11 and U-10-11) volcanites. Patterns of OIB, N-MORB, E-MORB are from [6]. The normalization values of chondrite after [1], primitive mantle – after [6].

REE patterns of rhyolites and accompanying tuffs show both LREE and HREE enrichment ((La/Yb)_n=4.8-7.5, (La/Sm)_n=3.4-4.2) and strong Eu minimum (Eu/Eu*=0.4-0.6) (fig. 1, a). Their trace element diagrams display high HFSE, REE concentrations and have remarkable troughs in Nb, Sr, Ti (fig. 1, b). Such features like high Fe*, HFSE enrichement, low (La/Yb)_n ratios and strong Eu minimum in rhyolites and tuffs are compared with those of A-granites or intraplate felsic volcanites. Based on Y-Nb-Ce values felsic volcanites are correspond to A-granites Formation of this A-type granites is explained by melting of sialic lower crustal source, which could be affected by previous melting and/or dehydration, that in turn is the reason of high Fe-number of new acid melts [2]. The studied rocks show high Y/Nb (1.9-4.7) and Yb/Ta (3.0-9.9) ratios which are characteristic of melts produced from sialic sources [2]. Experiments on the dehydration melting of tonalite gneisses demonstrate that melts with A-granite characteristics can be derived at p=6-10 kbar and T=900-1075 ºC [5]. Dzabkhan felsic volcanites show high Ba concentrations (>800 ppm). The main host of Ba in restite during melting of TTG source is biotite [5]. Thus, high Ba contents could be related to the high formation temperatures of volcanites.

Finally, petrogeochemical characteristics of Dzabkhan felsic volcanites (high Fe*, high HFSE, REE concentrations, distinct Eu negative anomaly) are indicative of their intraplate origin. They are similar to A-type granitoids formed during melting of lower crustal source. The possible lower crustal source could be TTG rocks of Dzabkhan microcontinent basement (Baidaragin Complex).

References:

1. Boynton, W.V. Cosmochemistry of the rare earth elements: meteorite studies // Rare earth element geochemistry / Ed. Henderson, P. Amsterdam: Elsevier, 1984. P.63-114.

2. Eby, G.N. The A-Type Granitoids: A Review of Their Occurrence and Chemical Characteristics and Speculations on Their Petrogenesis // Lithos. 1990. V. 26. P. 115–134.

3. Kozakov, I.K., Sal’nikova, E.B., Wang, T., Didenko, A.N., Plotkina, Yu.V., Podkovyrov, V.N. Early Precambrian Crystalline Complexes of the Central Asian Microcontinent: Age, Sources, Tectonic Position // Stratigraphy and Geological Correlation. 2007. V. 15. N. 2. P. 121–140.

4. Levashova, N.M., Kalugin, V.M., Gibsher, A.S., Yff, J., Ryabinin, A.B., Meert, J.G., Malone, S.J. The origin of the Baydaric microcontinent, Mongolia: Constraints from paleomagnetism and geochronology // Tectonophysics. 2010. V. 485. P. 306-320.

5. Skjerlie, K.P. and Johnston, A.D. Fluid-Absent Melting Behavior of an F-Rich Tonalitic Gneiss at Mid-Crustal Pressures: Implications for the Generation of Anorogenic Granites // J. Petrol. 1993. V. 34. P. 785–815.

6. Sun, S.S., McDonough, W.F. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes // Magmatism in the Oceanic Basins / Eds. A.D. Saunders, M.J. Norry. Geol. Soc. Spec. Publ., 1989. V. 42. P. 313–345.

Mineralogy and Geochemistry of the Bitu-Dzhida multiphase intruded massif of Li-F rare-metal granites (Northern Mongolia)

Zolboo Ts.¹, Perepelov A.B.¹, Tatarnikov S.A.¹, Antipin V.S.¹, Kanakin S.V.²

¹A.P. Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia; ²Institute of Geology SB RAS, Ulan-Ude, Russia

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Rare metal mineralization associated with magmatic and post-magmatic processes of multiphase granite massifs evolution, is characterized by large reserves of mineral raw materials, but as a rule, scattered nature of the assignment of useful components. One of the most challenging for practical purposes is the geochemical type of Li-F granites with intrusive complexes of which the development of mineralization of Li, Rb, Sn, W, Be, Ta and Nb is associated. During the studying of these complexes important issues are to identify genetic interdependence between intrusive phases, to determine the mineralogical and geochemical features of rocks and factors of their potential ore content, processes of scattering and concentrating of useful components through the evolution of ore-magmatic systems.

The Bitu-Dzhida multiphase intrusion of Li-F granites located in the southern spurs of Khamar-Daban Ridge. It is localized in the Cambrian metamorphic sequence (the crystalline schist of the Bitu-Dzhida suite). The massif was discovered in 1933 [5] and was studied through geologic survey and prospecting for Li, Rb, Ta, Nb (1954-1960) as well as through thematic scientific research [2, 3]. New geological and geochemical surveys of the massif were carried out in 2004 in Russia and in 2008 in Mongolia. Previously, by means of K-Ar - dating the massif age was determined as the Permian - Triassic – 262-218 Ma [2]. According to the new data [6] the time of intruding of the 1^st initial phase of granites the Bitu-Dzhida intrusion is regarded as the Late Carbonaceous (C₂) that is 311 ± 10 Ma.

By the results of recent investigations we can distinguish three main phases of intruding of granite magma. The 1^st phase includes small outcrops of Pl-Kfs-Qtz-Bt middle-grained and porphyry granites; the 2^nd phase consists of Qtz-Kfs-Pl-Bt leucocratic ones. And, finally, amazonite-albite-zinnwaldite (Amz-Ab) rare-metal granites refer to the 3^rd phase.

The rocks of all the three intrusive phases belong to the group of plumasite granites of Li-F geochemical type. Although, their substantial features essentially differ. A common feature for the granites of all intrusive phases of the massif is their high level of concentration of Li and F. The composition of the rocks of the early phase differs from the granites of the 2^nd and 3^rd phases in lower silica content and the lowest total alkalinity. The granites of the 1^st phase have the most differentiated REE spectrum allocation (La_N/Yb_N 10.6-16.3), whereas the 2^nd phase rocks show a decrease in REE fractionation (La_N/Yb_N 3.6-6.6) resulting from an essential enrichment by heavy elements of the spectrum and possess more considerable Eu_N deficiency (Eu* 0.18-0.29 vs 0.49-0.65 as compared to the rocks of the 1^st phase). The Amz-Ab leucogranites of the final 3^rd phase are characterized by a sharp LREE depletion (La/Yb<1) and a deep Eu-minimum (Eu*≤0.05).

Recent advances in our investigation have continued by microprobe analyses of the Bitu-Dzhida massif rocks, which helped to identify the composition of the rock-forming minerals and the mineral-concentrators: Nb, Ta, LREE, HREE, Th, U, Pb, Sn, Li – allanite, columbite, tantalite, sinhizit, monazite, cassiterite, xenotime, furdite, changbait, zinnwaldite, protolitionite, Li-muscovite.

The rock-forming minerals of the granites are represented by feldspar, quartz and mica. Evolution of their composition indicates a genetic alliance between the rocks of all three intrusive phases. The feldspar of the 1^st and 2^nd intrusive phases granites mainly consist of albite, microcline and rare oligoclase (An 11-17). There are solely albite and microcline in the 3^rd intrusive phase. Another group of the rock-forming minerals of the Bity-Dzhida massif granites is mica. The types of mica and evolution of their compositions in the rocks of the intrusive phases essentially differ too. In the granites of the 1^st and 2^nd phases mica is represented by ferriferous biotite and muscovite. There was no significant level of F and Li detected in biotites and muscovite. On the contrary, in the granites of the 3^rd phase mica is solely composed of the Li-F varieties. The content of F in Li-mica varies from 2 to 8 wt.%, the content of Li in such micas could not been estimated by chemical analyses but was determined due to empirical formulae [7] depending on the contents of F and Si. Among the Li-micas protolitionite and zinnwaldite were identified. In the amazonite-albite granites of the 3^rd intrusive phase Li-F micas contain a considerable concentrations of Na₂O and ZnO, that can be adduced as an important feature of special conditions of their crystallization.

The occurrence of a wide variation of accessory minerals in the rocks of the massif, such as carbonates, phosphates, silicates and oxides of series of elements, such as Sn, TR, Pb, Ta, Nb, Th and U shows the significant variations in crystallization conditions of Li-F magma and the potential ore content of the latest intrusive phases.

Recent advances show that isotope features for granites of the 1^st and 2^nd intrusive phases, as ⁸⁷Sr/⁸⁶Sr_(t) (0.705312-0.706187), ¹⁴³Nd/¹⁴⁴Nd_(t) (0.512088-0.512290), ²⁰⁶Pb/²⁰⁴Pb_(t) (17.761-17.961), ²⁰⁷Pb/²⁰⁴Pb_(t) (15.454-15.491), ²⁰⁸Pb/²⁰⁴Pb_(t) (37.426-37.587), are similar. At the same time, the isotope features of the granitiods of the 3^rd intrusive phase are notable for increasing radiogenic isotope system of component ⁸⁷Sr/⁸⁶Sr_(t) (1.100426-1.135334) and for reducing isotope system of ²⁰⁶Pb/²⁰⁴Pb_(t) (17.208-17.480). So, we can conclude that the source of Li-F granitoid melt was enriched in Rb, Th and depleted in U. These data correspond with the model of forming the initial Li-F granitoid melts at the lower levels of the ancient Precambrian continental crust with an average model age T_DM2 = 1260 Ma and the maximal = 1600 Ma.

References

1. Gao S., Luo T.C., Zhang B.R., Zhang H.F., Han Y.W., Hu Y.K., Zhao Z.D. Chemical composition of the continental crust as revealed by studies in East China // Geochim. Cosmochim. Acta. 1998. V. 62. P. 1959-1975.

2. Kosals Ya.A. Geochemistry of amazonite granites. Publishers of the Institute of geology and geophysics, SB USSR Academy of Sciences. Issue 219. Novosibirsk: Nauka. 1976. 190p.

3. Koval P.V. Petrology and geochemistry of albited granites. Novosibirsk: Nauka. 1975. 258p.

4. Kovalenko V.I., Kostitsyn Yu.A., Yarmoluk V.V., Budnikov S.V., Kovach V.P., Kotov A.B., Salnikova E.B., Antipin V.S. Magma sources and the isotopic (Sr and Nd) evolution of Li-F rare-metal granites. Petrology. 1999. V.7. №4. P.383-409.

5. Naletov P.I., Shalaev K.A., Deulya T.T. geology of Dzhida ore area. Publishers of VSGU. Issue. 27. Irkutsk. 1941. 282p.

6. Perepelov A.B., Tatarnikov S.A., Dril S.I., Antipin V.S., Vladimirova T.A., Sandimirova G.P. Geochemical features, magma sources and age of the Bity-Dzhida multiphase intrusive of Li-F granites (Khamar-Daban). Abstracts of 1^st International Geological Congress “Granites and evolution of the Earth: geodynamic position, petrogenesis and ore content of granitoids”. Ulan-Ude. Publisher of Buryat Scientific Center SB RAS. 2008. P.291-293.

7. Tischendorf G. On Li-bearing micas: estimating Li from electron microprobe analyses and an improved diagram for graphical representation // Mineralogical Magazine. 1997. V. 61. P. 809-834.

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