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

urmantseva@gmail.com

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.