Some Notes on the Lugiin Gol , Mushgai Khudag and Bayan Khoshuu Alkaline Complexes , Southern Mongolia

Volcanic-plutonic alkaline complexes from Lugiin Gol, Mushgai Khudag and Bayan Khoshuu, southern Mongolia (244, 139 and 131 Ma, respectively) occur within grabens in E-W lineaments. They are represented by syenitic rock-types (silica undersaturated to slightly silica oversaturated) potassic rocks and are associated to stockworks of carbonatitic veins, dykes and so on. Geochemical characteristics and isotope systematics point to a veined mantle source particularly enriched in LILE and LREE. The carbonatitic veins show high contents of Ba, Sr, Th and REE and are suitable as potential ore deposits.


Introduction
Alkaline province in Southern Mongolia is well known for associated alkaline-carbonatite complexes (e.g.Lugiin Gol, Mushgai Khudag and Bayan Khoshuu) (Figure 1).These complexes and a number of smaller plutons with carbonatite dykes and veins are controlled by E-W faults and formed in continental rift environment.Alkaline and carbonatitic rocks [1][2][3][4][5][6][7] outcrop along the northern and south Gobi rift zones (Late Paleozoic to Early-Late Mesozoic aged) and consist mainly of volcanic-plutonic complexes.
We present here a review of the petrological and geochemical features of these alkaline-carbonatite complexes.

Geological Background
The alkaline complexes in South Mongolia are controlled by large extensional structures (Figure 1) and are related to occurrences of Late Paleozoic-Early Mesozoic (Lugiin Gol) and Late Mesozoic (Mushgai Khudag and Bayan Khoshuu) ages, respectively ( [8], and therein references).

Lugiin Gol Complex (LGC)
LGC outcrops at the Southeastern side of the Gobi-Tien Shan fold belt, near the boundary with the Sulinkheer suture zone (Figure 1).Gobi-Tien Shan belt is composed mainly of Late Pre-Cambrian limestones, amphibolites and gneisses with Late Paleozoic granitoids and intermediate volcanics.The Sulinkheer zone is characterized by a Paleozoic ophiolitic complex and by Late Paleozoic sediments, in the southern and in the northern areas, respectively.The eastern Lugiin Gol district (108˚20'E) represents a magmatic arc, indicating that remnant oceanic basin (Sulinkheer Sea) existed between North China and a South Gobi microcontinent until Late Permian [9,10].
Notably, the northern area is mainly underlain by black shales, siltstones and sandstones (i.e.Lugiin Gol Formation), intruded by the LGC complex.A silica undersaturated syenitic stock outcrops in an area of about 12 sq.km,showing subcircular outlines with a diameter of 3.5 km.The stock is composed of nepheline-bearing syenite, nepheline syenite and ijolites (Figure 2), surrounded by hornfels [4,11], fenites and skarns.An alkali-granite dyke, 20 m thick, crosscuts the Lugiin Gol complex.Moreover, a stockwork with two main types of dykes and veins, i.e. carbonatites and phonolites (and phonolites with tinguaitic texture) intrude the complex (cf.

Mushgai Khudag Complex (MKC)
MKC, along the main Mongolian lineament (Figure 1) consists mainly of stocks and necks of trachytic and syenitic varieties (Figure 3; [13] cut by carbonatitic veins and plugs (F-S-Sr-Ba-REE rich: s. later).Host rocks are represented by Paleozoic sedimentary-volcanic sequences and by Carboniferous granitoids [3].The K-Ar ages for MKC are between Early Cretaceous (Valanginian) and Late Cretaceous (Cenomanian) times [5].Genesis of these rocks was interpreted as a result of a mantle plume activity during subduction (Enkhtuvshin, 1995), or, more probably, as rifting related [3].

Silicate Rocks
The petrography is based here above all on the [16] and [17] classifications.
The granitic dyke of the Lugiin Gol complex is mainly course, but locally also very coarse, with large phenocrysts of microcline, oligoclase, biotite and quartz set in a groundmass made of the same phases.Accessory minerals are mainly amphibole, titanite, apatite, magnetite and ilmenite.
Notably, carbonatites usually form veins within magmatic rocks, or they are mainly located in brecciated rocks and eruptive breccias.There are rare crater facies at the contact of syenite plutons [3].On the whole the carbonatitic rocks from the three complexes are similar, but Mushgai Khudag complex shows relatively high concentrations of apatite, and barite-celestine as well fluorocarbonates may be important in the veins from Bayan Khoshuu complex.

Classification and Nomenclature
The classification and nomenclature is after the TAS [17,23,24] cf. Figure 5(a)), and on the basis of QAPF diagram [16].The silicate rocks of the analyzed complexes are rich in alkalis, with total alkali content (Na 2 O + K 2 O) ranging from 10.1 to 16.7 wt% (cf.Tables 1 and  2): on the basis of the Na 2 O/K 2 O ratio they are "potassic"; only one phonolitic (tinguaitic) dyke from Lugiin Gol complex plots inside the "highly potassic" field (Figure 5(b)).Agpaitic index ranges from 0.76 to 1.18 pointing to a tendency towards peralkaline rock-types.Silica and MgO contents range from 52.0 to 58.7 and from 3.48 to 0.11 wt%, respectively, indicating that the rocks are evolved (from intermediate to acid, following) [23].Analogously to similar occurrences, the rocks may derive, via fractional crystallization, by original basanitic magmas [22,25,26].
Following the Q-A-P-F classification, where the distribution of the Ab molecule between alkali feldspar and plagioclase was obtained according to Le Maitre (1976) on the CIPW norms (Figure 5(c)), the samples are foidsyenites/phonolites and foid bearing syenites/trachytes (Lugiin Gol) and syenites/trachytes (Mushgai Khudag and Bayan Khoshuu).Representative analyses of Ca-carbonatites are reported in the Tables 1 and 2, along the analyses of F-REE rich variants from the Bayan Khoshuu complex.

Geochemistry
Notably, all the Lugiin Gol samples, granitic dyke excepted, are Ne normative (Ne 2.66 to 28.68 wt%; cf.Table 1), whereas the alkaline rocks from the Mushgai Khudag and Bayan Khoshuu are oversaturated or saturated in silica (Q = 0.0 to 7.6 wt%; cf.Table 2).All the samples display a variable Al 2 O 3 content (14.74 -17.96 wt%).On the whole, the rocks appear enriched in LILE and depleted in HFS elements such as Nb, Ta, P and Ti.They show a strongly enriched pattern in Pb (Tables 3, 4 and Figure 6).On the other hand, the rocks are enriched in LREE and depleted in HREE (showing flat patterns), with a weak negative Eu anomaly (Tables 3, 4 and Figure 7).
In particular, the carbonatite dykes and veins from the complexes (Figure 7 and Tables 3, 4) show a very high concentration of U, Th, (up to 3843 and 114, ppm, respectively) and REE (REE = 0.6 -3.3, up to 23.5 wt%; LREE = 6928 up to 224,291 ppm) and a very strong fractionation LREE/HREE (Nd/Lu = 854 -24,100).It can be stressed that the highest values are typical of the later veins from Bayan Khoshuu complex and "high grade" carbonatites from Lugiin Gol complex, linked to the presence of fluorocarbonate minerals as bastnaesite, synchysite and parisite: this suggests an economic importance for these occurrences, almost for LREE (cf.also [30] and therein references).As matter of fact, from mass balance (Tables 2 and 4) the MK vein contains approximately barite 13 wt%, celestine + strontianite 17 wt% and bastnaesite + synchysite+ parisite (up to 28 wt%).Moreover, an exceptional content of apatite is present in the MK-01 carbonatite (about 13.5 wt%).Considering major and trace elements, REE contents of intrusive rocks from the Mushgai Khudag and Bayan Khoshuu complexes (shown in Tables 2 and 4), it is apparent that these rocks are strongly enriched in Sr, Pb and LILE and depleted in HFSE (Ta, Nb, Hf, Ti, and Y) (Figure 6).They show enriched LREE pattern (Figure 7).Geochemically these rocks are characterized by relatively low HFSE with enrichment in LILE and LREE, depletion in Ta, Nb, Ti and Zr with HFSE troughs on the primordial mantle normalized spiderdiagrams [31], which are commonly observed in island arc basalts and in the Paraná volcanic, the latter linked to extensional (rifting) processes [32].It suggests that the original (basanitic?) magmas were derived from continental lithospheric, mantle, metasomatized by fluids, variably enriched in CO 2 and H 2 O (and F, LILE and so on: cf.veined mantle of [33,34].On the whole, all the data, suggest that the Mongolian alkaline complexes from southern Mongolia are related to the lithospheric mantle, where the contribution of asthenospheric components are not appreciable in terms of geochemical signatures (cf.[32], and therein   references).

Age of the Complexes
An errorchron calculated following the Rb/Sr systematics on the syenitic rocks from Lugiin Gol complex gives an age of 244.9 ± 22.4 Ma (initial Sr isotope ratio, Sr i , = 0.70800; Figure 8(a)), roughly corresponding to "Sensitive High Resolution Ion Microprobe" (SHRIMP) ages (245 -240 Ma) on the zircons from the same complex [19].On the other hand, the whole rock-minerals isochron ages where determined for the three complexes by Rb-Sr method: 87 Rb/ 86 Sr and 87 Sr/ 86 Sr data are reported in Table 5.A Lugiin Gol undersaturated syenite sample shows an age of 222.Notably the Rb-Sr age from the isochron relative to the foid syenite from Lugiin Gol is too low (222 vs. 245 Ma) with respect to the whole complex: the younger age is probably due to the alteration processes that affected some host rocks after the intrusions of the dyke systems.
Moreover, an errorchron (not shown) relative to the whole rocks from Lugiin Gol foid syenites to foid-bearing syenites + associated phonolitic dykes and carbonatitic dyke, give the same age of the intrusive rocks, i.e. 244.9 ± 22.4 Ma.However this age, in our opinion, cannot be accepted because: 1) it is unlikely that the intruding dykes have the same age of the complex; 2) the Sm-Nd systematic does not show any correlation.An age younger than 222 Ma (probable age of hydrothermal alteration on the foid syenite 82306), or similar to that of the granitic dyke, i.e. 210 Ma, may be believed most probable also for phonolitic and carbonatitic dykes.
At any way, a Mesozoic age is confirmed, i.e.Triassic for Lugiin Gol complex, and Early Cretaceous for Mushgai Khudag and Bayan Khoshuu complexes.

Sr-Nd Systematics
All the available isotopic data, initial 87 Sr/ 86 Sr (Sr i ) -143 Nd/ 144 Nd (Nd i ) and εSr -εNd notation are reported in Table 6 along with the model age (T DM ) relative to the depleted mantle [38].Lugiin Gol complex (calculated using the previous age results; cf. Figure 8) are in the ranges 0.70772 -0.70827, 0.51216 -0.51226 and 49.81 -57.07, −4.24 -−1.25, respectively (syenitic variants: age 245 Ma); the phonolitic dykes (222 Ma) have Sr i 0.70745 -0.70888 and Nd i 0.51220 -0.51228, with εSr and εNd varying between 45.54 and 66.36, and between −3.02 and −1.73, respectively.The latter are similar to the carbonatitic dyke (εSr = 64.88 and εNd = −1.53).Notably, all these samples plot in the IV quadrant in the Sr i vs. Nd i diagram (Figure 9): according to [39] and therein references) these volcanics were enriched in Rb (f Rb > 0) and depleted in Sm (f Sm < 0), indicating that a hypothetical basalt source or may has formed by contamination by igneous and metamorphic rocks of the continental crust or, more probably remained isolated for a sufficient time to acquire distinctive isotopic composition of Sr and Nd.
The granitic dyke shows a slight enrichment in Nd (Nd + 0.48 to + 1.27), but with Sr (47.4 -48.9) inside the same compositional variation of the phonolitic dykes, indicating a coupled enrichment in both Rb and Sm.The Mushgai Khudag syenites-trachytes have, as average, Sr i = 0.70605 ± 0.00001 and Nd i = 0.51244, corresponding to Sr = 24.3 and Nd = −0.33,respectively (cf.

Table 6).
Bayan Khoshuu syenites-monzonites show Sr i = 0.70594 (Sr = 22.6), similar to the Mushgai Khudag volcanic-plutonic complex, but the Nd i = 0.51224 (Nd = −4.4)indicate a major depletion in Sm in the source (Figure 9).works are present in the two complexes.On the other hand, the Lugiin Gol (LGC) complex outcrops at the central-eastern Gobi-Tien Shan fold belt and it is Triassic in age (244 -210 Ma).MKC and BAK consist mainly of slightly oversaturated syenites (monzonites) and trachytes, while in the LGC the rock-types are mainly foid-syenites and foid-bearing syenites and the complex is cut by a stockwork made of phonolitic and carbonatitic veins and dykes.
All the alkaline rocks are potassic, rarely highly potassic, and mainly intermediate silica-types, alkali granitic dyke excepted, and then they are believed derivative from a basic magma, probably a basanitic liquid, analogously to other world-wide occurrences and on the basis also of the trace element behavior ( [50] and therein references); [32][33][34]; clinopyroxene and calcic plagioclase are evolving by fractional crystallization of olivine during ascent to the present position.
Stockworks of LREE rich carbonatites, carbonatefluorite, fluorite, and carbonate-fluorite-celestine-barite REE ore bearing rocks are associated with the complexes and point to an economic interest of the region.
On the whole, geochemical characteristics of alkaline complexes in South Mongolia were interpreted as formed within-plate tectonic environment and associated with extensional rift tectonics [3].
All the samples plot in the IV quadrant in the Sri vs. Ndi diagram showed enrichment in Rb (f Rb > 0) and depletion in Sm (f Sm < 0), indicating that a hypothetical basalt probably remained isolated for a sufficient time to gain distinctive isotopic composition of Sr and Nd.
Finally, on the basis of almost constant behaviour of the Sm/Nd ratio in the analyzed samples ( 147 Sm/ 144 Nd = 0.0100 ± 0.017), the Nd model ages can represent the main metasomatic events affecting the lithospheric sources beneath the Mongolian regions (veined lithospheric mantle).T DM (Table 6) for the whole population (33 samples) fit 992 ± 119 Ma, indicating a Neoproterozoic event ended with the Mushgai Khudag complex (797 ± 110 Ma).The latter dates are similar to the youngest Os model ages (1.1 and 0.7 -0.5 Ga) [42], and may support significant melt extraction from the subcontinental mantle or metasomatic events, suggesting also involvement of the Central Asian Orogenic belt and development of significant juvenile crustal growth.