CONTRIBUTION TO GENESIS AND RELATIONSHIPS OF GRANITIC PLUTONS OF

EASTERN MONGOLIAN ALTAY

Hanzl P.*, Hrdlickova K.*, Burianek D.*, Aichler J.*, Gerdes A.**, Byambasuren D.***

*Czech Geological Survey, Klárov 3, 118 21 Praha, Czech Republic, pavel.hanz@geology.cz

**Institut fűr Geowissenschaften, J. W. Goethe Universität, Altenhöferallee 1, 60438 Frankfurt an Main,

Germany, gerdes@em.uni-frankfurt.de

***Geological Investigation Centre, Songino Khayrkhan District, PO Box 37/307, Ulaanbaatar, Mongolia, dbbyamba@yahoo.com

Three individual granitoid massifs and granitoid complex linked with the crystalline unit were recognized during the field work in the frame of the project „Geological mapping of selected parts of Mongolia at a scale of 1:50,000“. Project was held as an integral part of the Program of the Development Assistance Project of the Czech Republic.

The studied area is situated to the SW Mongolia, approximately 740 km SW of Ulaanbaatar right along the boundary of Bogt fault, which separates here the two different structural zones, the Lake Zone in the north and the Gobi Altay Zone in the south (Rauzer et al., 1987)

Granitoid rocks of Palaeozoic age spatially related to the Lake Zone are represented by undifferentiated bodies in the Zamtyn Nuruu Crystalline Complex, the Burdnii Gol Massif (Cambrian), and the Shar Oroy Massif (Permian). The position of Lower Palaezoic metagranites of the Unegt Uul Crystalline Complex stay still questionable, but its assignment to the Lake Zone seems to be highly probable.

The Chandman Massif (Devonian-Carboniferous) represents intrusive body of the Mongolian Altay Zone.

Biotite granodiorite together with gabbro and diorite bodies crop out in small areas along the northern margin of the Zamtyn Nuruu mountain range. Various gabbro and metaggabro types with textures indicating magma mixing form bodies throughout the unit. The cores of zircons in diorites revealed 542 ± 4 Ma (Lower Cambrian), which is likely to coincide with zircon crystallization. However, the emplacement age of undeformed diorite is to be derived from the formation of overgrowths rimming the zircon grains so that it is younger than 542 Ma. Granites of the Zamtyn Nuruu Crystalline Complex has been dated at 517 ± 5 Ma (Lower Cambrian), what is interpreted as time of granite (and probably also diorite) emplacement.

The rocks plot in the gabbro, gabbronorite, gabbrodiorite, granodiorite and granite fields in the diagram by La Roche (1980). Basic rocks are metaluminous, granodiorite metaluminous to peraluminous and granite peraluminous.

Burdnii Gol Massif is situated north of the Chandman Khayrkhan Uul along the river Burdnii gol. The massif is lithologically homogenous, composed of leucocratic granite. Rauzer et al. (1987) ascribed Permian age to the granite but new radiometric dating revealed the Upper Cambrian age. The CHIME monazite dating provided Cambrian-Ordovician magmatic age and the U/Pb zircon and monazite dating yielded 511 ± 4.7 Ma and 506 ± 5.4 Ma, respectively.

The Burdnii Gol Massif consists of calc-alkaline granites according to chemical classification of rocks in the R1-R2 diagram (La Roche, 1980). Rocks are peraluminous with potassium content corresponding to high-K series. Trace elements signatures according to Pearce et al. (1984) indicate volcanic-arc character of the granitoids.

Shar Oroy Massif is exposed in a few individual bodies in the Khar Argalantyn Nuruu.

The Permian age of this massif is confirmed by radiometric dating. The U/Pb zircon age 285 ± 1.3 Ma points to the Lower Permian age of the intrusion.

Three petrographical suites can be distinguished in these calc-alkaline series according to SiO₂ abundance: the mafic rocks with SiO₂ contents in a range of 49–52 wt. %, syenites (60–65 wt.%) and granites (71–78 wt. %). Syenites and gabbros are alkaline, granite sub-alkaline, with high potassium content for granitoids and gabbros. Most of the rocks are metaluminous. Trace elements signatures according to Pearce et al. (1984) indicate within plate character of the granitoids.

The Chandman Massif, the only one, which does not appertain to the northern Lake Zone but to the southern Gobi Altay Zone, is exposed in a region between the Khokh Khadnii Khondii and Guutin Khondii, SE of the Chandman sum. The Massif intruded migmatites of the Chandman Khayrkhan Crystalline Complex.

Intrusive age of the massif was thought to be Ordovician according to Rauzer et al. (1987), but radiometric Pb/U dating of zircon gave the Lower Carboniferous age (344.9 ± 1.9 Ma).

The Chandman Massif can be subdivided into two separate parts. Porphyritic, biotite granodiorite with anatectic textures forms the southern part, whereas the biotite to biotite - amphibole granodiorite to tonalite varieties are situated in the northern part.

Three petrographical types can be distinguished according to the SiO₂ content: it ranges between 71–77 wt. % for the younger granites, 67–71 wt. % for granodiorite and 55–59 wt. % for diorite and gabbrodiorite. Plutonic rocks are calc-alkaline, with potassium abundance related to the high-K series for granites and medium-K series for the tonalite group. The granites are peraluminous, while other rocks are metaluminous. Trace elements signatures according to Pearce et al. (1984) indicate volcanic-arc character of the granitoids but some granitoids tend to be within plate granites.

The different ages and geotectonic positions of granite evolution of studied massifs quite well reflect and ilustrate the geotectonic evolution of whole area of the contact between Lake Zone and Gobi Altay terranes in the area of Zamtyn Nuruu.

The eldest granitoids – Burdnii Gol massif and granite bodies of Zamtyn Nuruu Crystalline Complex are both of the Cambrian age and coincide with final stage of southward subduction accompanied by the collision of accreted geological domains between microcontinents of Baydrag and Zamtyn Nuruu crystallines.

The Paleozoic Unegt Uul and Lower Carboniferous Chandman massifs are adherent to Lower Paleozoic evolution (caledonian) and late extension stage of variscan orogenesis.

Permian Shar Oroy massif is linked with extensive volcanic activity of continental character in the lower Permian – geological and structural relationships point to co-genetic evolution of volcanic and plutonic rocks.

References

La Roche, d. H. (1980) Granites chemistry through multicationic diagrams. Sciences de la Terre, Série Informatique Géologique 13, P.65-88.

Pearce, J.A., Harris, N.B.W. Tindle, A.G. (1984) Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrology 25, P.956-983.

Rauzer, A.A., Zhanchiv, D.I., Golyakov, V.I., Ykhina, I.F., Ivanov, I.G., Tsukernik, A.B., Afonin, V.V., Smirnov, I.G., Bykhover, V.I., Kravtsev, A.V., Baatarkhuyag, A., Skoryukin, M.I., Khodikov, I.V., Mantsev, N.V., Okaemov, S.V., Mischin, V.A. Enkhsajkhan, T. (1987) Otchot o rezultatach gruppovoj geologičeskoj syomki masštaba 1 : 200 000 provedennoj v yugo-vostočnoy časti Mongolskogo Altaya Mongolskoi Narodnoji Respubliki v 1983-1986 g.g. In: Tekhnoeksport. Moskva. Tekhnoexport, Moskva.