The conditions of formation of sapphire and zircon in the areas of alkali basaltoid volcanism in central vietnam

Russian Geology and Geophysics 51 (2010) 719–733 www.elsevier.com/locate/rgg The conditions of formation of sapphire and zircon in the areas of alkali-basaltoid volcanism in Central Vietnam A.E Izokh a,*, S.Z Smirnov a, V.V Egorova a, Tran Tuan Anh b, S.V Kovyazin a, Ngo Thi Phuong b, V.V Kalinina a a V.S Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, prosp Akad Koptyuga 3, Novosibirsk, 630090 Russia b Geological Institute of the Vietnamese Academy of Sciences and Technologies, Hanoi, Vietnam Received February 2009; received in revised form 27 July 2009; accepted 14 August 2009 Abstract Study of the chemical composition of clinopyroxene and garnet megacrysts from the Dak Nong sapphire deposit and model calculations have shown that megacrysts originated from the crystallization of alkali basaltoid magma in a deep-seated intermediate chamber at 14–15 kbar, which is close to the Moho depth (50 km) in this part of southeastern Asia The chamber was a source of heat and CO2 fluids for the generation of crustal syenitic melts producing sapphires and zircons The formation conditions of sapphires and zircons are significantly different The presence of jadeite inclusions in placer zircons points to high pressures during their crystallization, which is confirmed by the ubiquitous decrepitation of CO2-rich melt inclusions Sapphires crystallized from iron-rich syenitic melt in the shallower Earth’s crust horizons with the participation of CO2 and carbonate–H2O–CO2 fluids The subsequent eruptions of alkali basalts favored the transportation of garnet and pyroxene megacrysts as well as sapphire and zircon xenocrysts to the surface It is shown that sapphire deposits can be produced only during multistage basaltic volcanism with deep-seated intermediate chambers in the regions with thick continental crust The widespread megacryst mineral assemblage (clinopyroxene, garnet, sanidine, ilmenite) and the presence of placer zircon megacrysts can be used as indicators for sapphire prospecting © 2010, V.S Sobolev IGM, Siberian Branch of the RAS Published by Elsevier B.V All rights reserved Keywords: basaltic volcanism; continental Earth’s crust; sapphire; zircon; Vietnam Introduction Placers of gem sapphire and zircon are widespread in Southeastern Asia, Australia, and Russian Far East (Primor’e), where they are related to multistage Cenozoic basaltic magmatism These placers form the West Pacific belt (Sutherland et al., 2004; Vysotskii and Barkar, 2006) Zircon and corundum megacrysts were also revealed in Cenozoic basalts in Central and Southeastern Mongolia (Agafonov et al., 2006) In Central Vietnam, sapphire placers are explored in the Gia Lai (Pleiku), Dak Lac (Dak Nong), Bin Phuoc, Bin Tuan (Hong Liem), Lam Dong, and other provinces (Fig 1) Zircon and garnet placers are exploited in the Dong Nai (Gia Kiem) province There are also noncommercial sapphire occurrences in this area In the placers confined to volcanic areas zircon dominates over sapphire, whereas in the distant placers corundum is predominant Alkali basalts in these regions favor * Corresponding author E-mail address: izokh@uiggm.nsc.ru (A.E Izokh) transport of clinopyroxene, sanidine, garnet, and titanomagnetite megacrysts to the surface, which evidences the existence of deep-seated magma chambers In the Gia Kiem Village region (Dong Nai province), clinopyroxene, garnet, sanidine, and titanomagnetite megacrysts and large (up to cm) zircon crystals were found in the cinder cones of alkali basalts We also discovered zircon placers and occasional sapphire grains in the Pleiku region In the placer confined to the cinder cone of Nui Boong Volcano, sapphire and zircon are associated with clinopyroxene, sanidine, and ilmenite megacrysts The occurrence of sapphires and zircons exclusively in alkali basalts and their assemblage with minerals of the deep-seated parageneses permit them to be considered indicators of petrogenesis in deep-level zones of the Earth’s crust or the upper mantle Thus, complex mineralogical and thermobarometrical studies of sapphires, zircons, and minerals of the megacryst assemblage confined to the same volcanic area will help to reconstruct the petrogenetic conditions In this work we present results of the study of sapphires, zircons, and clinopyroxene and garnet megacrysts from the 1068-7971/$ - see front matter D 2010, V S Sabolev IGM, Siberian Branch of the RAS Published by Elsevier B.V All rights reserved doi:10.1016/j.rgg.2010.06.001 720 A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 The state of the art of the problem and object of study Fig Distribution of Neogene (1) and Quaternary (2) basalts and sapphire placers (3) in Central Vietnam Sapphire and zircon placers: 1, Dak Nong; 2, Gia Kiem; 3, Hong Liem Dak Nong placer (Dak Lac province) The placer is localized in eluvial laterites developed after the alkali-basalt flow of the Dak Nong Volcano (Garnier et al., 2005) The heavy concentrates from this placer contain not only dark blue, green, and yellow sapphires but also corroded zircon grains up to 0.5 cm in size Among the typical megacryst minerals associated with alkali basaltoids, the authors found clinopyroxene, garnet, sanidine, and ilmenite Other abundant minerals are garnet, olivine, clino- and orthopyroxenes, and spinel, which originated from lherzolite xenoliths Also, occasional grains of tourmalines of the schörl-dravite and uvite-schörl-dravite series and unusual Y-containing garnet were discovered The main goal of our study was to elucidate the formation conditions of garnet, clinopyroxene, sapphire, and zircon megacrysts and to construct a model relating mantle basaltoid magmatism to the formation of zircon and sapphire parageneses The Neogene–Quaternary intraplate basaltoid magmatism covered vast areas in Eastern and Southeastern Asia—from Northern Mongolia and Tuva to Southern Vietnam,—where basalts compose large fields with volcanic centers of different preservation degrees In Central and Southern Vietnam, basaltic plateaus are often more than 100 km across and up to several hundred meters in thickness, with the total area of basaltic flows being more than 23,000 km2 (Hoang and Flower, 1998) (Fig 1) Tholeiitic basalts are predominant in the basaltic sequences, and alkaline rocks are subordinate Two stages of volcanic activity were recognized The first stage included eruptions of quartz-normative and olivine tholeiites with subordinate alkali basalts, and the second, eruptions of olivine tholeiites, alkali basalts, basanites, and, more seldom, nephelinites (Hoang and Flower, 1998) It is at the second stage that megacrysts of pyroxenes, garnet, amphiboles, K-Na-feldspars, phlogopite, garnet and spinel lherzolites, websterites, and pyroxenites were let out to the surface There are also sapphire and zircon placers in the reported areas Most of the existing models relate the formation of sapphire and zircon to the crystallization of alkali-basaltic magma in deep-seated intermediate chambers There are several groups of models for sapphire and zircon genesis (Vysotskii and Barkar, 2006) Some researchers relate this genesis to fractional crystallization of alkali basalts in deep-seated chambers (Hong-sen et al., 2002; Irving, 1986; Kievlenko et al., 1982) They suggest that the highly differentiated melt from which sapphire crystallized corresponded in composition to phonolite (nepheline syenite) or trachytes (syenite) In this case, corundums should be considered phenocrysts despite the signs of their corrosion or the formation of spinel and titanomagnetite rims (Chen et al., 2006) Other researchers suggest that the sapphire and zircon are xenogenic and that alkali basalts served as transporters of xenocrysts of these minerals, which formed from different crustal or mantle sources Study of mineral inclusions in corundums and zircons showed that the minerals crystallized from alkaline-salic or salic melts in the lower or middle Earth’s crust (Aspen et al., 1990; Guo et al., 1996a; Smirnov et al., 2006; Vysotskii et al., 2002) In some cases, the metamorphic or metasomatic origin of sapphires is substantiated (Graham et al., 2004; Sutherland and Schwarz, 2001; Sutherland et al., 2002) Some researchers admit that corundums result from the contamination of basaltic magma with aluminous (including boxites) or carbonate rocks (Levinson and Cook, 1994) Several xenoliths of corundum-bearing rocks in alkali basalts were discovered For example, corundum-anorthoclase rock was found in Australia (Stephenson, 1976), and xenolith of alkali-feldspathic rock bearing corundum, zircon, magnetite, ilmenorutile, Y-titanoniobates, biotite, and apatite was revealed in alkali basalts in Scotland (Aspen et al., 1990; Upton et al., 1999) Similar xenoliths are known in China In all these cases, corundum is in paragenesis typical of syenites 721 A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 Table Contents of the major (wt.%) and trace (ppm) elements in clinopyroxene and garnet megacrysts from alkali basalts from the Dak Nong volcanic plateau, Central Vietnam Mineral Clinopyroxene Garnet Sample 288/3 SiO2 49.80 50.10 49.90 50.64 40.34 40.75 41.17 40.96 39.82 TiO2 0.99 0.80 0.80 0.70 0.52 0.44 0.42 0.46 0.60 288/4 288/5 288/9 286/3 286/4 287/1 287/3 287/5 Al2O3 8.24 8.12 8.13 7.27 22.67 22.87 23.12 22.97 22.21 Cr2O3 0.04 0.04 0.05 0.17 11.77 10.88 9.87 10.70 14.15 FeO 7.27 6.96 6.77 6.58 0.04 0.06 0.14 0.08 0.02 MnO 0.18 0.18 0.15 0.16 0.41 0.34 0.30 0.33 0.42 MgO 15.50 16.12 16.44 17.13 18.08 18.99 19.77 19.01 16.21 CaO 16.45 15.88 15.66 15.58 5.48 5.22 5.14 5.15 5.60 Na2O 1.46 1.38 1.39 1.18 0.03 0.03 0.03 0.03 0.03 Total 100.0 99.6 99.3 99.4 99.3 99.6 100.0 99.7 99.1 0.06 Ba 0.12 0.14 0.12 0.12 0.07 0.21 0.09 0.54 Th 0.031 0.022 0.026 0.025 b.d.l 0.006 0.010 0.008 0.060 Nb 0.21 0.16 0.13 0.14 0.04 0.09 0.11 0.09 0.07 La 1.03 0.85 0.85 0.81 0.03 0.06 0.08 0.06 0.02 Ce 3.96 3.67 3.17 3.02 0.26 0.34 0.43 0.56 0.22 Pr 0.68 0.58 0.54 0.53 0.08 0.10 0.20 0.11 0.06 Sr 42 45 39 43 0.64 0.64 0.37 0.39 0.40 Nd 3.69 3.24 2.89 2.89 0.96 1.14 2.50 1.44 0.85 Zr 13 8.9 6.7 9.6 46 55 125 61 37 Hf 0.59 0.48 0.39 0.53 1.11 1.36 2.67 1.63 0.90 Sm 1.22 1.09 1.12 1.03 1.06 1.17 3.03 1.24 0.88 Eu 0.49 0.47 0.38 0.43 0.62 0.77 1.30 0.87 0.54 Gd 1.34 1.24 1.14 1.26 3.35 3.73 6.15 4.21 2.51 Tb 0.21 0.19 0.17 0.2 0.79 0.92 1.63 1.05 0.69 Dy 1.38 1.2 1.04 1.3 7.25 7.88 13.70 10.76 6.22 1.69 Ho 0.26 0.24 0.2 0.26 1.86 2.01 3.31 2.47 Y 6.3 5.4 5.1 5.9 50 53 87 72 48 Er 0.62 0.56 0.55 0.69 6.1 6.2 11.7 9.6 5.9 Yb 0.62 0.52 0.45 0.53 6.9 6.1 12.4 13.6 8.0 Lu 0.072 0.054 0.064 0.07 1.04 0.86 1.70 1.79 1.32 Prp – – – – 63.2 65.8 68.2 66.2 57.5 Alm – – – – 23.1 21.2 19.1 20.9 28.2 Grs – – – – 13.8 13.0 12.7 12.9 14.3 En 49.4 51.3 52.2 53.5 – – – – – Fs 13.0 12.4 12.1 11.5 – – – – – Wo 37.7 36.3 35.7 35.0 – – – – – Mg# 79.2 80.5 81.2 82.3 73.2 75.7 78.1 76.0 67.1 P, kbar 13.9 14.5 14.7 14.2 – – – – – Note Major elements were determined by electron microprobe analysis, and trace elements, by ICP MS Mg# = Mg ⋅ 100/(Mg + Fe) at.%; b.d.l., below detection limit; dash, not determined Thus, the above review shows that the genesis of sapphire and zircon is related to alkali-basaltoid magmatism, but there were only few complex studies of minerals throwing light on the evolution of basaltic-magma chambers and sapphire and zircon parageneses in a particular volcanic center, and the results of these studies are contradictory The Dak Nong placer in the Dak Lac province (Central Vietnam) seems to be the most appropriate for such studies Its eluvial nature and the coexistence of minerals of megacryst assemblage (garnet, clinopyroxene, K-Na-feldspar), sapphire, and zircon permit the reconstruction of the history and depth of their formation along with the degree of the influence of basaltic melts on the formation of crustal parageneses containing sapphire and zircon Methods of investigation The compositions of mineral phases and glasses of melt inclusions were studied on a Camebax Micro electron micro- 722 A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 Fig Compositional variations of clinopyroxene megacrysts from alkali basalts from the Dak Nong volcanic plateau, Vietnam 1, clinopyroxene megacrysts; 2, clinopyroxenes from lherzolite xenoliths Fig REE (a) and trace-element (b) patterns of clinopyroxenes of the megacryst assemblage from alkali basalts from the Dak Nong volcanic plateau, Vietnam probe at the Analytical Center of the Sobolev Institute of Geology and Mineralogy (SIGM) at accelerating voltage 20 kV Analysis of mineral phases and inclusions was performed at the probe current of 50 and 20–30 nA, respectively Calibration was made using natural minerals and synthetic compounds whose compositions were earlier determined by different methods The trace-element composition of clinopyroxene and sapphire megacrysts and zircon inclusions in sapphire was determined by LA ICP MS at the Analytical Center of the SIGM Analysis of each grain was made in parallel with analysis of standard glass samples (NIST 612 and NIST 614) The analytical error (standard deviation) was no more than 25% for element contents of 1 ppm The trace-element composition of placer zircons was deter- mined by secondary ion mass-spectroscopy (SIMS) (ion microprobe) Analyses were made on a CAMECA IMS-4f ion microprobe at the Institute of Microelectronics and Informatics RAS, Yaroslavl’ To obtain secondary ions, a primary O2− beam with an energy of ~14.5 keV was used Secondary ions were collected from the field of view 25 µm across To suppress the interfering molecular and cluster ions and reduce the matrix effect, the method of energy filtration was applied The absolute concentration of each element was calculated from the measured intensities of positive single-atomic secondary ions of elements normalized to the intensity of secondary 30 + Si ions The concentrations of SiO2 at all points were determined independently by electron microprobe analysis To suppress the contribution of interfering molecular ions to the intensities of analytical signals of 153Eu+ and 174Yb, we used A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 723 the subtraction procedure The accuracy of determination of concentrations is 5–10 rel.% for >1 ppm and 15–20 rel.% for 1–0.1 ppm To identify mineral inclusions and daughter phases of fluid and melt inclusions, we also applied Raman spectroscopy The Raman spectra were recorded on a X-Y Dilor OMARS multichannel spectrometer with a Peltier cooled CCD detector Gas Ar laser with line 514 nm was used for exitation Microthermometric studies of melt inclusions were carried out on a Sobolev–Slutsky heating stage at 500 ºC to 1300 ºC Results Clinopyroxenes of the megacryst assemblage from the Dak Nong placer compositionally correspond to high-alumina augites (En42-54, Fs12-20, Wo34-44), whose Mg# varies from 74 to 81 at.% They contain 7.7–8.6 wt.% Al2O3, 0.7– 1.54 wt.% TiO2, and 1.42–1.76 wt.% Na2O; the contents of these oxides increase as Mg# decreases (Table 1, Fig 2) The content of Cr2O3 in all clinopyroxenes is below its detection limit Compared to clinopyroxenes from lherzolite and websterite xenoliths (Mg# = 90–92) transported with the same basalts, the studied ones have lower Mg# and Cr2O3 contents and higher TiO2 content Their REE contents are 2–6 chondrite units (Fig 3) The chondrite-normalized REE patterns of the minerals show their enrichment in MREE ((Ce/Sm)n = 0.68–0.81), have a negative slope in the HREE field ((Sm/Yb)n = 2–2.6), and lack a Eu anomaly Garnets of the megacryst assemblage correspond in composition to pyrope-almandine (Prp59-66, Alm20-27, Grs13-15) and show Mg# = 69–76 at.% (Table 1, Fig 4) In contrast to garnets of lherzolite xenoliths, which contain up to 0.25 wt.% Cr2O3, the studied megacrysts lack chromium but have high TiO2 contents (0.48–0.69 wt.%) Similarly to the clinopyroxenes, the compositions of pyrope and the host basalts are intimately related The Vietnamese garnets are more magnesian (Mg# = 69–76 at.%) than the Mongolian ones (Mg# = 61–63 at.%) The total content of LREE in the garnets is 0.1–40 chondrite units The REE patterns are strongly fractionated and show a LREE depletion and strong HREE enrichment Fig Compositional variations of garnet megacrysts from alkali basalts from the Dak Nong volcanic plateau, Vietnam 1, garnet megacrysts; 2, garnets from lherzolite xenoliths ((Nd/Yb)n = 0.04–0.07)) (Fig 5) The multielemental patterns show Sr and Ba anomalies The HREE content reaches 40 chondrite units Zircon- and sapphire-bearing assemblages Sapphires are moderately or poorly shaped crystals and crystal clastics Most of them are dark blue and blue-green Sometimes, pink sapphires also occur in the placer The mineral color is unevenly distributed in growth zones and sectors (Fig 6) Most crystals have a weakly colored or colorless core and an intensely colored outer zone Sapphyres from the Dak Nong placer are poorer in Ga than those from other basalt placers and lack Cr (Table 2) Zircon is the only mineral from the Dak Nong placer that was found both as placer crystals and as inclusions in sapphire, which suggests their genetic relationship Cathodoluminescent studies showed that all placer zircons have an oscillatory polygonal zoning (Fig 7, a) In some of them, the zoning is superposed by a curved pattern called a convolute zoning (Fig 7, b) Most of the placer zircons contain 0.6–1.3 wt.% HfO2 and minor U and Th The Th/U ratio is 0.3–1.6 (Table 3), which is close to those in magmatic zircons (Hoskin and Schaltegger, 2003) Fig REE (a) and trace-element (b) patterns of garnets of the megacryst assemblage from alkali basalts from the Dak Nong volcanic plateau, Vietnam 724 A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 Table Compositions of sapphires from the Dak Nong placer, wt% Component Dark blue X (13) SD Light blue X (9) SD Greenish blue X (6) SD TiO2 0.094 0.11 0.073 0.09 0.070 0.04 Al2O3 98.69 0.59 99.51 0.42 99.44 0.80 Cr2O3 0.004 0.00 0.008 0.00 0.003 0.00 FeO 1.23 0.32 1.08 0.40 1.66 0.50 Total 100.03 100.67 101.17 – Note X, average content of element (parenthesized is the number of grains); SD, standard deviation; FeO, all iron as Fe2+ The Ga content in these sapphire varieties measured by LA ICP MS is 33–43 ppm These zircons are characterized by a significant domination of HREE over LREE The REE patterns show a positive Ce and a negative Eu anomalies (Table 3, Fig 8, a) The total REE content varies from 957 to 5509 chondrite units The placer zircons are characterized by great variations in the depth of Eu anomaly (Eu/Eu* = 0.05–0.66) In most of the placer zircons Eu/Eu* = 0.5–0.6 The REE contents and patterns of the core and peripheral zones of crystals are almost identical, but the cores have somewhat higher REE contents than the peripheral ones (Fig 8, a) Inclusions of mineral phases and mineral-forming media in sapphire and zircon Sapphires and zircons contain inclusions of syngenetic minerals (Fig 9) and mineral-forming media—melts and fluids (Fig 10) Sapphires also contain crystalline inclusions resulted from corundum solid solution breakdown (topotactic inclusions) Mineral inclusions Topotactic inclusions in sapphire are tabular crystals of hematite, pseudobrookite, and högbomitelike mineral (Fig 9, a, Table 4) regularly oriented relative to corundum A specific feature of hematite inclusions is high contents of Al2O3 and TiO2 In the inclusion-rich zones, the content of Al2O3 in hematite reaches 17–18 mol.%, whereas the content of Fe2O3 in corundum is ~3 mol.% (Tables and 4) Inclusions of syngenetic minerals in sapphire are zircon (Fig 9, b, c), plagioclase, and ferrocolumbite crystals Plagioclase trapped by sapphire during its growth is oligoclase (An12 mol.%) Ferrocolumbite has a low MnO (~3.6 wt.%) and an increased ZrO2 (~0.8 wt.%) contents (Table 4) Zircon occurs in sapphire as prismatic crystals (Fig 9, c, d) Compared with placer zircons, the zircon inclusions have higher concentrations of HfO2 (~2.5 wt.%), Th, and U (Table 3, runs 13–15) The Th/U ratio in them is 0.8–2.3, which is higher than that of placer zircons but falls into the range of magmatic-zircon values (Hoskin and Schaltegger, 2003) The total REE content in the inclusions is 1230–3487 chondrite units, which corresponds to the range of REE concentrations in placer zircons The REE patterns of the inclusions are similar to those of placer zircons (Fig 8, a) but show a narrower range of Eu/Eu* variations (0.31–0.46) The REE contents in the zircon inclusions from the Dak Nong placer sapphire are close to the lowest REE contents in the zircon inclusions from other deposits in Southeastern Asia (Fig 8, b) Syngenetic minerals in the placer zircon are rare inclusions of baddeleyite and tourmaline, most likely, of foititic composition Electron microprobe analysis revealed a jadeite ((Na0.91K0.04)0.95(Al1Fe0.05)1.05[(Si1.91Al0.1)2.01O6]) inclusion Fig The internal structure of sapphires from the Dak Nong deposit: a, irregular distribution of color in dark blue sapphire (core is colorless); b, sector distribution of color and inclusions (the latter are concentrated in the crystal core and form a six-ray star pattern) A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 725 Fig The growth (a) and convolute (b) zonings of placer zircon, revealed by cathodoluminescence Fig Chondrite C1-normalized REE pattern of zircons from inclusions in sapphire (solid lines) and from placer zircons a, Comparison of the REE compositions of zircon inclusions in sapphire and placer zircons from the Dak Nong placer: 1, REE of zircon inclusions; 2, composition range of placer zircons; 3, composition range of the cores of placer zircon crystals; 4, composition range of the margins of placer zircon crystals; b, comparison of the REE compositions of zircon inclusions in the Dak Nong placer sapphires (1) and zircon inclusions in sapphires from Thailand, Laos, and China (2), after Guo et al (1996b) and Sutherland et al (1998); c, comparison of the REE compositions of zircon inclusions in sapphires (1) and the Dak Nong placer zircons (2) with the composition of zircons from corundum-containing syenites (3), after Hinton and Upton (1991) Chondrite composition is given after Sun and McDonough (1989) 726 A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 Table Compositions of placer zircons and zircon inclusions in sapphires from the Dak Nong placer Compo- DLS195-6-12 nent DLS195-6-13 DLS195-6-25 DLS195-6-26 DLS195-6-31 DLS195-6-32 DLS192 DN01 DN01 10 11 12 13 14 15 32.10 32.21 32.30 32.20 32.09 32.19 32.00 32.05 32.23 31.93 32.27 32.34 Electron microprobe analysis (wt.%) SiO2 32.17 32.12 32.00 HfO2 0.93 1.85 0.83 0.78 0.99 0.90 0.85 0.83 1.35 0.89 0.84 0.80 2.60 2.58 2.44 ZrO2 67.33 66.82 67.41 67.23 67.46 67.50 67.28 67.30 66.83 67.29 66.78 67.24 65.63 64.39 63.96 ThO2 b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l 0.25 0.69 0.76 UO2 b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l 0.06 0.30 0.93 Total 100.43 100.79 100.24 100.11 100.66 100.70 100.33 100.22 100.37 100.18 99.67 100.27 100.23 100.24 100.43 Ion microprobe analysis (ppm) LA ICP MS analysis (ppm) Y 28.28 1109.00 289.24 200.51 670.40 181.61 90.02 355.83 74.89 249.47 278.51 490.06 880.81 181.12 182.93 Ta – – – – – – – – – – – – 107.03 36.57 – Nb 24.00 80.71 15.53 11.68 14.79 8.92 9.94 10.60 8.28 6.58 5.17 6.54 337.57 37.31 – Ba 1.21 1.29 1.36 1.03 1.24 1.29 0.78 0.40 0.90 0.12 0.20 0.25 – – – La 0.02 0.09 0.02 b.d.l 0.00 0.01 0.04 b.d.l 0.03 0.00 b.d.l b.d.l 0.30 0.05 16.84 Ce 0.45 7.39 0.96 0.79 1.65 0.66 0.50 1.17 0.43 0.87 0.65 1.54 4.10 0.74 20.69 Pr 0.01 0.07 0.01 0.01 0.03 0.01 0.01 0.01 0.01 0.02 0.00 0.01 0.04 2.22 Nd 0.02 0.73 0.12 0.09 0.40 0.07 0.06 0.17 0.03 0.18 0.07 0.15 0.45 0.03 7.32 Sm 0.06 2.27 0.46 0.35 1.18 0.29 0.18 0.57 0.10 0.49 0.38 0.68 1.20 0.24 1.65 Eu 0.01 1.23 0.27 0.18 0.84 0.14 0.07 0.37 0.05 0.35 0.24 0.48 0.46 0.14 0.42 Gd 0.21 14.35 3.09 2.18 8.39 1.84 0.95 4.00 0.57 3.48 2.75 5.29 8.75 2.32 4.30 Tb – – – – – – – – – – – – – 1.16 1.45 Dy 1.93 93.12 22.05 14.30 53.53 13.71 6.77 28.03 4.98 20.69 20.28 35.41 71.98 18.02 18.24 Ho – – – – – – – – – – – – – 6.84 6.07 Er 5.24 179.73 52.19 35.37 113.69 32.26 16.06 60.02 14.08 45.38 52.78 91.43 161.22 36.11 27.70 Tm – – – – – – – – – – – – – 7.82 4.85 Yb 12.44 272.23 99.06 74.54 200.60 65.51 30.00 101.07 33.58 85.46 107.32 174.05 367.71 69.40 39.33 Lu 1.89 32.85 14.22 11.84 27.80 9.74 4.41 14.55 4.89 12.48 17.52 26.75 – 11.41 6.00 Th 3.12 837.92 44.47 9.73 128.49 17.89 6.43 45.56 5.73 60.06 15.82 61.21 1092.71 – – U 11.41 508.22 88.15 26.93 196.16 44.74 18.73 83.76 17.02 70.01 30.33 82.50 1252.73 – – Th/U 0.27 1.65 0.50 0.36 0.66 0.40 0.34 0.54 0.34 0.86 0.52 0.74 0.87 0.82 2.33 Note 1–12, placer zircons (odd, crystal cores; even, crystal margins); 13–15, zircon inclusions in sapphires b.d.l., Below detection limit; dash, not determined in zircon This differs the placer zircon from the sapphire, which contains inclusions of acid plagioclase, and points to the higher pressures of zircon crystallization Inclusions of mineral-forming media in sapphire and zircon Fluid and melt inclusions in sapphire and zircon crystals from the Dak Nong placer are shown in Fig 10 The sapphire crystals contain scarce primary melt inclusions (MIs) (Fig 10, a) Most of the MIs are secondary and are localized along the healed cracks (Fig 10, b) Their silicate phase is either glass or crystallized aggregate Sometimes, iron oxide crystals (probably, magnetite) are present Both the primary and secondary inclusions in the sapphires show wide variations in the volume fractions of fluid segregation (Fig 10, a, b) This suggests the heterogeneous trapping of coexisting fluid and silicate phases The fluid inclusions (FIs) accompanying the secondary MIs in the sapphire (Fig 10, b) are mainly of carbon dioxide composition One of the sapphire crystals contains primary FIs containing an aqueous solution and a bubble consisting of liquid and gas CO2 (Fig 10, c, d) One of these FIs contains an aggregate of crystalline phases Raman spectroscopic analyses showed the presence of carbonates among these phases (weak but distinct line at 1086 cm–1) The studied sapphires lack such phases as individual crystalline inclusions, which permits them to be considered daughter phases Most of primary MIs in the zircon contain glass and a fluid segregation (Fig 10, e–h) There are also subordinate partly crystallized inclusions containing crystals of silicate phases compositionally corresponding to amphibole, albite, and Kfeldspar Many inclusions contain rather large crystals of A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 727 Fig Mineral inclusions in the Dak Nong placer sapphire a, Topotactic inclusions of Al-hematite and other Fe-Ti oxides; b, inclusion of zircon-columbite intergrowth; c, inclusion of intergrown prismatic zircon crystals complex Fe–Cu oxide (cuprospinel?) The fluid segregations in the vitreous inclusions are either heterogeneous (gas + liquid) or homogeneous (liquid) bubbles The Raman spectra of these segregations show that they consist of carbon dioxide with a nitrogen impurity The main difference between the mineral-forming media in the zircon and in the sapphire is the absence of FIs cogenetic with the MIs But most of the MIs are surrounded by a swarm of finer inclusions localized in radial fissures (Fig 10, g, h) This arrangement suggests that the inclusions were decrepitated, probably, during the transportation of zircon crystals to the surface Thermometry of melt inclusions Crystallized aggregate of MIs in sapphires starts melting at 685 ºC, and the last crystal dissolves in the melt at 740–780 ºC The glass of vitreous inclusions softens at 725–860 ºC When it becomes a liquid, the bubble becomes mobile and starts moving within the vacuoles This argues for the extremely low viscosity of the melt Since the substance of MIs might be heterogeneous at the moment of trapping, their homogenization temperature might not reflect the trapping temperature For this reason, we determined the trapping temperatures, using homogenization temperatures of inclusions with the finest bubbles, in which a gas bubble is mainly the fluid phase of MIs Thus, we succeeded in estimating the temperature of MI trapping, which is ~930–1100 ºC The primary and secondary MIs in sapphire are characterized by similar features of behavior, close temperatures of phase transitions, and close minimum homogenization temperatures This suggests that the secondary and primary MIs were trapped at similar temperatures and might be portions of the same sapphire-producing melt, which were trapped in different time Since the MIs in zircons are unsealed, we could not obtain reliable data on their trapping temperatures The thermometric experiments showed that the glass of MIs in zircon passes into a liquid (without crystallization) at 800–900 ºC (the bubble becomes mobile), but the homogenization temperature was not reached even after the 3–4 h exposure at 1300 ºC 728 A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 Fig 10 Inclusions of mineral-forming media in the Dak Nong placer sapphires and zircons a, b, Melt inclusions in sapphire: a, primary vitreous inclusion with abnormal gas phase, b, series of secondary vitreous melt and fluid (low-density CO2) inclusions; c, d, primary fluid inclusions in sapphire, with heterogeneous CO2 segregation (liquid (lCO2) and gas (gCO2)) and daughter carbonate crystals; e–h, vitreous melt inclusions in placer zircon (CO2, high-density CO2 bubble; gl, glass; cr, unsealing cracks); oc, opaque crystal 729 A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 Table Microprobe analyses of mineral inclusions in sapphires from the Dak Nong placer (except for zircon) Component SiO2 b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l – 64.2 ZrO2 b.d.l b.d.l b.d.l b.d.l 0.11 b.d.l 0.79 b.d.l TiO2 0.53 1.47 1.55 1.57 24.9 23.4 b.d.l b.d.l Nb2O5 b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l 77.1 – Ta2O5 b.d.l b.d.l b.d.l b.d.l b.d.l b.d.l 3.61 – Al2O3 14.3 12.8 11.9 11.9 2.90 1.72 – 22.1 Cr2O3 b.d.l 0.03 0.02 0.02 b.d.l b.d.l – b.d.l Fe2O3 83.2 84.5 85.6 85.6 69.3 73.1 – – FeO – – – – – – 15.9 0.06 MnO 1.68 1.12 1.05 1.02 0.23 0.26 2.93 0.02 MgO 0.44 0.29 0.32 0.35 0.15 0.15 – b.d.l CaO – – – – – – – 3.13 Na2O – – – – – – – 9.71 K2O – – – – – – – 0.82 Total 100.2 100.3 100.5 100.5 97.5 98.7 100.4 100.1 Note Content of oxides, wt.% 1–4, Al-Ti-hematite lamellae (topotactic inclusions); 5, 6, pseudobrookite lameallae (topotactic inclusions); 7, Fe–columbite inclusion; 8, oligoclase inclusion In runs 1–6, total iron is converted to Fe3+; in runs and 8, total iron is converted to Fe2+ b.d.l., Below detection limit; dash, not determined Compositions of melt inclusions The glasses of unheated inclusions in sapphire are similar in composition to syenitic melts (SiO2 = 56–60 wt.%, total alkalies ~6 wt.%) (Table 5) The total content of alkalies in vitreous inclusions is also ~6 wt.% The contents of FeO are low (0.1–0.3 wt.%), and those of CaO and MgO are close to their electron-microprobe detection limits We also established an extremely high content of Al2O3 (23–30 wt.%) and a high content of TiO2 (~0.2 wt.%) The sizes of the analyzed inclusions exclude the possibility of contamination with the host mineral during an electron microprobe analysis Table Microprobe analyses of unheated vitreous melt inclusions in sapphires and zircons from the Dak Nong placer Component Vitreous inclusions in sapphire in zircon SiO2 56.95 60.19 59.78 56.71 TiO2 0.15 0.13 0.03 0.05 Al2O3 30.16 24.65 18.36 21.35 FeO 0.29 0.11 2.72 1.75 MgO 0.01 0.01 1.64 0.12 MnO 0.05 0.01 b.d.l b.d.l CaO 0.06 0.04 0.57 0.31 K2O 1.00 1.17 3.32 3.20 Na2O 5.47 4.17 3.41 2.57 Total 94.17 90.47 89.89 86.09 Deficit 5.83 9.53 10.11 13.91 Note Contents of oxides, wt.% Deficit, total components not determined by microprobe analysis The glasses of MIs in zircons are similar in SiO2 contents (57–60 wt.%) and the total contents of oxides of alkaline elements (~6–7 wt.%) to the inclusions in sapphires But they have lower contents of Al2O3 (18–21 wt.%) and TiO2 and higher contents of FeO (2–3 wt.%), MgO (0.1–1.6 wt.%), and CaO (0.3–0.6 wt.%) (Table 5) Discussion The conditions of formation of basaltoid-magma chambers Pyroxene and garnet megacrysts resulted from the crystallization of basaltoid melt in magma chambers at different depths Our data on the compositions of clinopyroxene from alkali basalts and sapphire- and zircon-containing placers permit us to reconstruct the melt composition in the corresponding basaltic-magma chambers and to determine the depth of their formation For this purpose we estimated the compositions of melts equilibrated with the clinopyroxenes of the megacryst assemblage of the Dak Nong placer The estimation was made from the REE contents, using the mineral–melt partition coefficients for alkali-basaltoid systems (Fujimaki et al., 1984): CL = CCpx/K, where CL is the concentration of element in the melt, CCpx is the concentration of element in clinopyroxene, and K is the coefficient of element partition among the melt and the mineral The calculated REE contents are close to those in the alkali basalts from the Dak Nong volcanic plateau (Garnier et al., 2005) (Fig 11) Along with the positive correlations between the compositions of clinopyroxenes and the host basalt, this confirms the genetic relationship between the megacrysts and the Dak Nong alkali basalts The pressure of formation of clinopyroxenes of the Dak Nong megacryst assemblage was determined using the cli- 730 A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 Fig 11 Estimated compositions of melts equilibrated with clinopyroxenes (a) and garnets (b) of the megacryst assemblage from alkali basalts from the Dak Nong volcanic plateau, Vietnam 1, calculated compositions of melts, 2, composition of real alkali basalt, after Garnier et al (2005) The element contents are normalized to chondrite (Boynton, 1984) nopyroxene barometer (Nimis, 1999) based on the relationship between pressure and the volumes of the unit cell and polyhedron M1 in the mineral structure These pressures are 14–17 kbar The experiments (Green and Hibberson, 1970; Thompson, 1975) showed that clinopyroxenes of similar composition crystallize from natural alkaline olivine basalt melt at 14–16 kbar and 1200 ºC, which agrees with our estimates The above data evidence that the clinopyroxenes of the megacryst assemblage of the Dak Nong volcanic center and placer crystallized from alkali-basaltic magma in the deepseated intermediate chamber at 14–17 kbar, which corresponds to a depth of 56–60 km In Eastern Asia, these depths are close to the crust–lithospheric-mantle boundary At depths of >50 km, these magma chambers were, most likely, long-living sources of heat and fluids that influenced the rocks of the Earth’s crust lower horizons This might have led to the partial melting of crustal rocks in the immediate vicinity of alkalibasaltic magma chamber and the production of melts of the system quartz-feldspar Earlier it was established that at high pressures, the line of the cotectic compositions of this system shifts to the composition of feldspars; therefore, the melts must have a near-syenitic composition The conditions of formation of zircon- and sapphirebearing assemblages As mentioned above, the modern concepts of zircon and sapphire genesis are rather contradictory All researchers agree that sapphire is xenogenic relative to basalt, whereas the viewpoints of zircon genesis are different Since zircon is a typical accessory mineral of igneous rocks, whose composition varies from gabbroids to granitic pegmatites, let us consider the genesis of the Dak Nong placer zircon first Zircon that occurs as inclusions in the sapphire crystals is undoubtedly xenogenic relative to the basalt Therefore, let us first consider the origin of placer zircons The presence of primary MIs in them and the oscillatory polygonal zoning of the zircons unambiguously argue for magmatic genesis Additional evidence is high Th/U ratios Based on the compositions of MI glasses, we can state that the zircon formation was related to the crystallization of syenitic magmas A similar conclusion is drawn from the similar REE compositions of placer zircons and zircons from corundum-bearing syenites (Fig 8, c) Wark and Miller (1993) showed that the Hf contents in the zircon can roughly reflect the degree of differentiation of the parental melt Thus, the wide scatter of Hf contents in the placer zircons suggests that they formed at different stages of syenitic-magma evolution At the same time, the similar contents of REE and their distributions in the cores and margins of placer zircon crystals indicate that the latter are, most likely, of a single generation The alkaline-salic melt from which the zircon crystallized was strongly enriched but not saturated in CO2 This might be related to the high pressure of zircon crystallization The finding of jadeite inclusion evidences the high-pressure formation of some zircon crystals The crystal ascent led to overpressure in the inclusion vacuoles, cracking of the host mineral surrounding the inclusions, and, probably, fluid bubble formation But since the pressure remained high, the released fluid consisted of liquid CO2 In contrast to zircons, most of the studied sapphire crystals contain secondary MIs Inclusions characterized as primary are extremely rare Below, we will hold the viewpoint that the revealed inclusions reflect the late stages of corundum crystallization These inclusions, like those in the zircons, have a syenitic composition and evidence that the sapphire, similarly to the zircon, crystallized from syenitic magma The presence of inclusions of acid plagioclase, ferrocolumbite, and zircon in the sapphire additionally proves that it was produced as a result of acid- or intermediate-magma crystallization but not metamorphic processes The ferrocolumbite inclusions in the sapphire from the Dak Nong placer are similar in composition (Table 4) to the inclusions found in sapphires from other placers and to columbites from granitic pegmatites (Guo et al., 1996b; Sutherland et al., 1998) The presence of acid-plagioclase inclusions and the estimated temperatures of MI trapping evidence that the crystallization of sapphires took place in the crust at 900–1100 ºC A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 Most probably, dark blue corundums crystallized at high Fe3+ contents Experimental studies showed that at ~3 kbar, crystallization of high-Fe corundums similar to the Dak Nong sapphires is possible at 1100–1200 ºC and higher (Feenstra et al., 2005) Since the temperatures were estimated for the late stages of corundum crystallization, we can consider them to be minimum The crystallization of the Dak Nong placer sapphires seems to have occurred in a wider temperature range than that determined from the MI homogenization As mentioned above, the presence of zircon inclusions in the sapphires suggests the genetic relationship between the Dak Nong sapphires and zircons But the results obtained in this work, namely: (1) the drastic difference in Hf, Th, and U contents between the placer zircons and those from inclusions in sapphires, (2) the absence from the placer zircons of inclusions of plagioclase, columbite, and other minerals reflecting the high degree of differentiation of parental melt, (3) the findings of jadeite in the placer zircon, (4) the lower contents of Fe, Mg, and Ca in the MIs in the sapphire as compared with those in the zircon, show that, in contrast to the placer zircons, the sapphires and trapped zircons were produced from alkaline-salic melts with a higher degree of differentiation, which crystallized at lower pressures The MI assemblages in the Dak Nong placer sapphires and zircons point to the principal differences in the fluid regime of the mineral formation The placer zircon crystallized, most likely, from a homogeneous melt rich but not saturated in CO2 At the late stages marked by MIs, the sapphires and syngenetic zircons crystallized from volatile-rich magmas But the presence of secondary (CO2-rich) and primary (CO2-H2O) FIs suggests that the syenitic magma was saturated with water and CO2 and contained a free fluid phase, whose composition might have varied with time This confirms our hypothesis that the sapphires and syngenetic zircons were produced at lower pressures than the placer zircons An important feature is the presence of carbonate phases in the CO2-H2O inclusions, which indicate that carbon dioxide existed in the fluid mainly as carbonate The Fe- and Ti-enrichment of the sapphire-producing syenitic melts determined the predominant dark blue, yellow, and green colors of the corundum Model for the formation of the Dak Nong placer sapphires and zircons A review of the literature on the formation of sapphire- and zircon-bearing mineral assemblages in basalts showed that a noncontradictory model explaining the available factual data, including those for the studied object, was proposed by Guo et al (1996a) According to this model, corundums and zircons are produced from salic melts that form in the crust under the influence of the heat released from intermediate chambers of tholeiitic and alkaline basaltic magmatism The authors suppose that the alumina oversaturation of the melts results from the interaction of crustal melts with hypothetic carbonatite melts Sapphire and zircon crystallize in the crust from a hybrid salic melt This model is consistent with the presence of specific mineral inclusions in sapphires: Ta-Nb oxides, alkali feldspars, zircon, apatite, and 731 monazite, i.e., minerals typical of alkaline-salic magmas rather than basalts (Guo et al., 1996a; Sutherland et al., 1998) Geochronological studies of zircon inclusions in sapphires from Australia, Laos, and Vietnam showed that their formation was almost synchronous with the eruptions of alkali basalts (Garnier et al., 2005; Sutherland et al., 2002), which also agrees with the above model But the presence of carbonatites is a weak link of this model, because real carbonatite melts in association with basalts are known only in Kenya (e.g., Oldoinyo Lengai Volcano), where commercial sapphire placers also exist in places All other regions of sapphire-bearing alkali basalts, including Central and Southern Vietnam, lack carbonatites and evidence for their presence Thus, the analysis of the published data and results of our studies showed a number of specific features of the sapphire formation, which must be taken into account on the elaboration of models for this process, namely: – conjugation of sapphire placers with the multistage occurrences of Cenozoic basaltic volcanism developed in the areas with thick continental crust; – wide occurrence of pyroxene, garnet, sanidine, and ilmenite megacrysts and titanomagnetite in basalts, which evidences the existence of deep-seated intermediate chambers; – presence of zircon crystals in sapphire-bearing placers, whose crystallization was synchronous with the basalt eruptions (Garnier et al., 2005; Sutherland et al., 2002); – similar geochemical features of sapphires with high Fe3+ and Ti and low Cr contents; – similar parageneses of mineral inclusions in sapphires and syenitic composition of melt inclusions; – carbonate-water-CO2 composition of fluid inclusions Corundum crystallization is possible from low-Si melts with alumina strongly dominating over alkalies, e.g., alkali syenites of the miaskite series and normal rocks of monzonite composition Crystallization of corundum directly from alkalibasaltoid magma requires neutralization of the alkali effect, otherwise alumina will be bound with alkalies to form alkali feldspars or nepheline The saturation of salic melt with CO2 and the presence of fluid phase containing CO2 and carbonate seem to have played a key role in the formation of corundum mineralization related to alkaline-salic magmas Free carbon dioxide and carbonate favor the binding of alkalies into mobile alkali-carbonate complexes and cause an excess of alumina, especially in melts of miaskite composition Thus, we can propose a model for the formation of sapphires, which is similar to the model by Guo et al (1996a) but excludes the participation of carbonatite melts Our data on the minerals of the megacryst assemblage evidence the existence of a system of deep-seated magma chambers beneath the Dak Nong volcanic plateau, in which the crystallization of alkali-basaltic and/or tholeiitic melt took place These chambers are localized at depths close to the Moho beneath Eastern Asia In the region of the Dak Nong placer, these depths are ~50–60 km The heat and fluid separated from these intermediate chambers caused the melting of lower-crustal rocks with the formation of syenitic melts According to our data, the crystallization of zircons occurred at high pressures 732 A.E Izokh et al / Russian Geology and Geophysics 51 (2010) 719–733 and at the high content of CO2 in the parental melt The formation of sapphires proceeded, most likely, in shallower horizons of the Earth’s crust with the participation of highly differentiated syenitic melts and a fluid phase of CO2 and CO2–H2O composition These melts are not fractionates of basalts, as evidenced from the oxygen isotopy of corundums (Garnier et al., 2005) and the extremely low contents of Ca and Mg in the glasses of MIs in the sapphires (Table 5) The main volatile of melts of intraplate basalts is CO2 (Bakumenko et al., 1999) The fluid conserved in mantle rock xenoliths let out with these basalts is also of essentially CO2 composition (Andersen et al., 1984; Bergman and Dubessy, 1984; Golovin and Sharygin, 2007; Murck et al., 1978; Roedder, 1984) The concentration of CO2 in basic alkaline melts is so high that at the early stages of their crystallization the magma can segregate into silicate and carbonatite melts (Solovova et al., 1996, 2006) Thus, not only the basaltic magma but also its mantle source can be the source of CO2 fluid phase To sum it up, the syenitic melts generated under the influence of intermediate basaltic-magma chambers and, probably, the mantle source of fluid must have high content of CO2 Differentiation or fractionation of syenitic magmas led to the accumulation of trace elements, formation of alkali-carbonate complexes, oversaturation of rocks with alumina, and appearance of corundum in the most highly differentiated chambers The subsequent intrusions of alkali basalts broke through magmatic bodies of crustal origin during their ascent, trapped sapphire- and zircon-containing fragments of these bodies, and transported these minerals to the surface During the following lateritization, the minerals were concentrated, thus forming commercial placers Conclusions Study of the chemical composition of clinopyroxene and garnet megacrysts from the Dak Nong sapphire deposit and model calculations have shown that they originated from the crystallization of alkali basaltoids in a deep-seated intermediate magma chamber at 14–17 kbar, which is close to the Moho depth (~50 km) in this part of Southeastern Asia The chamber was a source of heat and CO2 fluids for the generation of lower-crustal syenitic melts producing zircons Sapphires crystallized from a more fractionated iron-rich syenitic melt with the participation of CO2 and CO2–H2O fluids in shallower crustal horizons The subsequent eruptions of alkali basalts favored the transport of garnet and pyroxene megacrysts as well as sapphire and zircon xenocrysts to the surface Thus, sapphires can be produced only during multistage basaltic volcanism with deep-seated intermediate chambers in the regions with thick continental crust The widespread megacryst mineral assemblage (clinopyroxene, garnet, sanidine, ilmenite) and the presence of zircon can be used as a criterion for sapphire prospecting The placer in the Gia Kiem Village region abounds not only in zircon crystals but also in minerals of the basalt megacryst assemblage Regard- ing the presence of these minerals, it is also promising for sapphire placers Acknowledgments We thank RAS Corresponding Member E.V Sklyarov (Institute of the Earth’s Crust, Irkutsk) and Prof A.A Tomilenko (Sobolev Institute of Geology and Mineralogy, Novosibirsk) for useful comments and advice as well as Dr V.V Sharygin (Sobolev Institute of Geology and Mineralogy, Novosibirsk) for detailed discussion of the manuscript We are also grateful to Dr L.N Pospelova, Dr S.V Palesskii, and Dr I.V Nikolaeva (Analytical Center of the Sobolev Institute of Geology and Mineralogy, Novosibirsk) for analyses The SIMS analysis of zircons was carried out by S.G Simakin (Institute of Microelectronics and Informatics, Yaroslavl’) This work was supported by grant 07-05-90005 from the Russian Foundation for Basic Research and Basic Research and Program 70.79.06 from the Vietnamese Academy of Sciences and Technologies References Agafonov, L.V., Antipin, V.S., Batzhargal, Sh., Berzina, A.P., Vladykin, N.V., Genshaft, Yu.S., Dorfman, M.D., Ivanova, G.F., Kepezhinskas, V.V., Lesnov, F.P., Kovalenko, V.I., Kostikov, A.T., Lkhamsuren, Zh., Maksimyuk, I.E., Nenasheva, S.N., Pinus, G.V., Saltykovskii, A.Ya., Sotnikov, V.I., Chernikov, A.A., Shcherbakov, Yu.G., 2006 Minerals of Mongolia [in Russian] Nauka, Moscow Andersen, T., O’Reily, S., Griffin, W.L., 1984 The trapped fluid phase in upper mantle xenoliths from Victoria—implications for mantle metasomatism Contrib Mineral Petrol 88, 72–85 Aspen, P., Upton, B.G.J., Dicken, A.P., 1990 Anorthoclase, sanidine and associated megacrysts in Scottish alkali basalts: high pressure syenitic debris from upper mantle sources? 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the Sweetwater Wash pluton, southeastern California, U.S.A Chem Geol 110, 49–67 Editorial responsibility: A.S Borisenko ... with the degree of the influence of basaltic melts on the formation of crustal parageneses containing sapphire and zircon Methods of investigation The compositions of mineral phases and glasses of. .. impurity The main difference between the mineral-forming media in the zircon and in the sapphire is the absence of FIs cogenetic with the MIs But most of the MIs are surrounded by a swarm of finer inclusions... also sapphire and zircon placers in the reported areas Most of the existing models relate the formation of sapphire and zircon to the crystallization of alkali-basaltic magma in deep-seated intermediate