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northern Gondwana cratons. Most researchers haveso far favoured a Gondwana origin, but Siberia hasalso been considered. The age pattern for theMongolian microcontinents favours a Palaeo- toMesoproterozoic origin although, in the BaydraghBlock, a small Archaean terrain has beenrecognized.

The Mongolian xenocrystic and detrital zircon agepattern is characterized by a predominance of agesin the range 500-2100 Ma. The younger ages, upto about 700 Ma, may be derived from cannibalisticreworking of crustal components formed during theearly stages of arc formation in the CAOB. Markedage groupings between 800 and 1100 and 1300-2100 Ma are distrinctly different from the agepatterns observed in Siberia and northern China,although there is a marked peak in Siberia at 1700-2100 Ma. Northern China shows one single peakat about 1700 Ma, and many ages are in the bracket2100-2700 Ma with a few going up to 3800 Ma.The dissimilarity in the age patterns of Mongoliaand northern China makes it unlikely that the latter

was a major source for crustal material in the centralCAOB.

Scattered ages in the range 2400-2700 Ma inMongolia show a reasonable match with a groupingfrom 2300 to 3300 Ma in Siberia and this, togetherwith peaks in both regions at 1700-2100 Ma makesit likely that Siberia contributed considerable detritusand perhaps crustal fragments to the CAOB.

It is difficult to assess any potential input fromGondwana, since the late Palaeoproterozoic andArchaean zircon ages found in our samples alsoreflect well established age patterns in thissupercontinent. However, the ages around 800-1100and 1300-1700 Ma are distinct from the patterns inSiberia and northern China, and the noticeablecomponent of Grenvillian-age zircons in Mongoliasuggests a source other than these two cratons.Since the Grenvillian event is not widely distributedin northern Gondwana, other source terrains shouldalso be considered such as the Baltic Shield.

GRANITOIDS OF MONGOLIA AND RELATED METALLOGENY:EXAMPLE ON SOUTH MONGOLIA

O. Gerel, S. Myagmarsuren, S. Oyungerel, B. Munkhtsengel, B. Batkhyshig, S. Amar-Amgalan

Mongolian University of Science & Technology, P.O. 46, Box 520, Ulaanbaatar 210646, Mongolia

Introduction

The integrated GIS, geology, petrography andpetrochemical databases for the territory ofMongolia provide a revised geodynamic frameworkand tectonic interpretation of key magmatic arcs inMongolia. The use of these data with the availablemineral deposit database connects the evolution ofmagmatic arcs with metallogeny. The GIS packagehas been used to illustrate the evolution of magmaticarcs during the Late Paleozoic to Late Mesozoic.

A GIS product includes terrane maps, consisting ofa series of time slices describing the spatialdistribution of different terrane types with ageoreferenced set of spreadsheets. Thespreadsheets contain whole rock geochemical data

of granitoids and some volcanic rocks for the majorarc successions, information on petrography, post-magmatic alteration, metamorphism, structuralfeatures, geochronology, occurrence ofmineralization, and many others. The system ishighly flexible, compiled in ARC and MapInfoformats (Fig.1) with databases compiled in the formof Excel spreadsheets. It can be used not only tooutline prospective belts for exploration but can alsobe used as a tool for petrological, tectonic andmetallogenic research in the region.

Granitoid interpretation includes chemicalclassification, tectonic discrimination, magmasource, origin of granitoids and evolution of graniticmagmatism trough the time. Also location of majormineral deposits and mineralization associated with

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accreted belts and interpretation of this relation isdone. Examples of map and interpretation aredemonstrated for the southeast Mongolia theGurvansaikhan island arc terrane. Examples of the

maps and interpretation will be demonstrated forthe Gurvansaikhan terrane, southern Mongolia (Fig.2.).

Fig. 1. Granitoid distribution map of Mongolia. Granitoids are shown by red color and mafic intrusive rocksby light green. Volcanics, namely from the Dariganga province are shown by dark green.

Fig. 2. Gurvansaikhan terrane map shows distribution of Devonian to Mesozoicgranitoids.

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South Mongolia

The Gurvansaikhan terrane is composed ofdismembered ophiolite mélanges, Ordovician-Silurian greenschist facies metamorphosedsandstone, siltstone, chert, volcaniclastic rocks,Upper Silurian-Lower Devonian radiolarian cherts,tholeiitic pillow basalt, andesite, tuff, MiddleDevonian-Mississippian volcaniclastic rocks, chertscontaining Frasnian conodonts, and minorolistostrome with coral limestone casts (Badarchet al., 2002). The structure of the terrane is complexand dominated by imbricate trust sheets,dismembered blocks, melanges, and high strainzones. There are several melange zones containingblocks of pillow lavas, fossiliferous limestone,sandstone, gabbro and dolerite dikes, andamphibolites.

Granitoid magmatism is represented by Devonian,Carboniferous, Permian and early Mesozoic ageplutons and volcanic-plutonic complexes. Within theterrane there are widely distributed Carboniferouspost-accretion syenite plutons which have K-Arages of 307±4 Ma at Oyu Tolgoi, and a monzonitedike at Tsagaan Suvarga has an Ar-Ar mice age of313±2.9 Ma (Lamb and Cox, 1998; Perello et al.,2001). The terrane is overlain by Carboniferous,Permian, Jurassic and Cretaceous volcanic andsedimentary rocks (Badarch et al., 2002).

Devonian granitoids form small bodies ofmonzodiorites, with which the Oyu Tolgoi porphyryCu-Au mineralization is associated. This monzoniticseries is K-rich calc-alkaline metaluminous. TheOyu Tolgoi has a K-Ar biotite age of 411±3 Ma forK silicate alteration (Perello et al., 2001).

Fig.3. Trace element distribution in monzodiorite (a) showing depletion in Nb, light REE, P and Zr, and(b) Y+Nb versus Rb plot after Pearce et al. (1984). Auqmd-Au quartz monzodiorite, Eqmd-Early quartzmonzodiorite, Lqmd-Late quartz monzodiorite, Qmd- quartz monzodiorite, OT-Qmd-Oyu Tolgoi quartzmonzodiorite, xQmd-xenolite. Syn COLG-syn-collisional granite, VAG-volcanic arc granite, WPG-within-plategranite, ORG-ocean- ridge granite.

Fig.4. N-MORB normalized trace element (a) and REE (b) patterns for the Shuteen complex. Triangles -granitoids, solid circles - andesites, solid squares - pyroclastic rocks (Batkhyshig, 2005).

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Monzodiorites show very typical depletion in Nb,light REE, P, Zr, and Ti (Fig. 4 a). Trace elementdata indicate a volcanic arc setting (Fig. 4b).

The Shuteen volcano-plutonic Complex consists ofco-magmatic andesites and biotite-hornblendegranite and granodiorite with relatively rare quartzdiorites, monzonites, monzodiorite and syentesoriginated of one magmatic source (Batkhyshig,2005). The pluton has a Rb-Sr whole rock age of321±9 Ma (Fig. 7) with initial Sr isotope ratio of0.70387±0.0006 (Batkhyshig et al.,2003). Granitoidsare related to I-type granites, metaluminous, oxidizedmagnetite series. The inferred magmatic processesresponsible for development of the ShuteenComplex are; subduction, dehydratation, partialmelting, magma uplifting and subsequaent intrusions(Batkhyshig, 2005). Granitoids show typical I-typegranite features, depleted in Nb, P, and Ti, andenriched in K, Rb and Ba (Fig. 5a). REE plot isfractionated without Eu anomaly (Fig. 5b). Trace

Fig.5. Y+Nb versus Rb plot after Pearce et al.(1984) for Shuteen Complex (Batkhyshig, 2005)

Fig. 6. Rb-Sr whole rock isochron age of theShuteen volcano- plutonic Complex shows 321±9Ma.

Fig. 8. Rb-Sr isochron diagram of KhanbogdComplex

element data indicate a typical volcanic arc setting(Fig.6).

The explanation for these rocks represent asubduction related continental arc existed inCarboniferous. Porphyry Cu-Au mineralization isassociated with the Shuteen Complex.

Khanbogd alkali granites are hypersolvus graniteswith high alkali content, low MgO and CaO, highFeO*/MgO and Ga/Al ratio, high HFSE, enrichmentin Rb, Th, U, K and Pb and depletion in Ba, Sr, Nb,Ta and Ti (Fig. 8a). REE pattern with strong Eunegative anomaly (Fig. 8b). All these featuresindicate A-type garnites, A2 post-orogenic. Granitesfrom the Khanbogd complex have almost the sameinitial Sr and Nd isotope ratios, suggesting that themagmas were derived from the same andisotopically depleted source (Fig. 9) (Amar-Amgalan, 2004, Amar-Amgalan et al., 2005).Carboniferous host for the Khanbogd Complex

Fig. 7. Rb-Sr whole rock isochron diagram ofandesite from Tsokhiot Complex

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Fig. 10 b). Ga/Al vs. Zr discrimination diagram of the Khanbogd Complex (Whalen et al. ,1987). Comparisonof patterns for Khanbogd granitoid with Orogenic A-type granitoids.

Fig. 9. N-MORB normalized (Sun&McDonough, 1989) trace element variation diagrams for the KhanbogdAlkaline Granite.

andesites have 334±19 Ma Rb-Sr whole rock age(Amar-Amgalan et al., 2005) and are similar to theShuteen Complex anadesites.

Conclusion

Phanerozoic magmatism in South Mongolia exhibitsa variable composition from calc-alkaline to potassicand ultrapotassic rocks. In Paleozoic SouthernMongolia underwent the accretion of a number ofmagmatic arcs and continental blocks. These

include island arc, Andean-type magmatic arcs,rifted basins accretion wedges and continentalmargins (Badarch et al., 2002).

Subduction related calc-alkaline magmatism in earlyand late Paleozoic occurs as extensional west-easttrending belts, where plutonic rocks are associatedwith volcanics, forming the volcanic-plutoniccomplexes or as large batholithic composites bodies(Table 1.).

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References

Amar-Amgalan, S., Sh. Iizumi, O. Gerel and D. Garamjav.(2005). Khanbogd Complex in South Mongolia: anexample of A-type granite. Geology. No. 12. pp. 115-123.

Badarch, G., Cunningham W.D., and Windly B.F. (2002)A new subdivision for Mongolia: implications forthe Phanerozoic crustal growth of Central Asia.Journal of Asian EarthSciences. 21. pp. 87-110.

Batkhishig B., B. Bignall, G. Kimura and N. Tsuchiya.(2003). Geochemical relationsheeps betweenandesite and granodiorite in the Shuteen area, SouthGobi, Mongolia. Mongolian Geoscientist. No.21.pp.15-21.

Batkhyshig B. (2005). Magmatic-hydrothermal systemof the Shuteen mineralized complex, South Gobi,Mongolia. PhD thesis. Tohoku University, Japan, pp.143 p.

Perello, J., Cox, D., Garamjav D, Sanjdorj S, Diakov S.,Schissel D., Munkhbat T and Oyun G. 2001. OyuTolgoi, Mongolia: Siluro-Devonian porphyry Cu-Au(Mo) and high- sulfidation Cu mineralization with aCretaceous chalcocite blanket. Economic Geology96: pp. 1407-1428.

Watanabe Y and H. J. Stein. 2000. Re-Os ages for theErdenet and Tsagaan Suvarga porphyry C u - M odeposits, Mongolia and tectonic implications.Economic Geology, Vol. 95, pp 1537-1542.

Table 1. Evolution of Phanerozoic mgmatism in the Gurvansaikhan terrane.

GEOCHEMICAL CONSTRAIN ON THE ORIGIN OF BAYANHONGOR OPHIOLITECOMPLEX (CENTRAL MONGOLIA)

D.Tomurhuu, E.Munkh-Erdene

Institute of Geology and Mineral Resources, Mongolian Academy of Sciences

Abstract

The Bayanhongor Ophiolite in Central Mongolia isa one of largest Ophiolite complex in Central AsianOrogenic belt (CAOB). The igneous rocks of thiscomplex consist both mantle and crustal suites andinclude metamorphic peridotite, cumulate seriesrocks, sheeted dike complex and pillow basalts. Theassociated sedimentary rocks are represented byboth siliceous and carbonate rocks filling the

interpillow space or sometimes occur as lenses orthin broken layer in volcanic sequences.

The detailed investigation of the geochemistry ofthe volcanic suites of the Bayanhongor Ophioliteresulted in the following features have beenunderlined.

In mantle normalized trace element spidegrams, themajority of the rocks reveal positive Nb and negative