Carbonatite Occurrence in Ambaji-Sendra Belt of NW Indian Shield: Evidence of Carbonatitic Magmatism in the Subduction Setting

The paper represents a new discovery of a late Mesoproterozoic lenticular and discontinuous, carbonatitic body exposed at Basantgarh, Sendra and near the Abu-road area of the Ambaji-Sendra belt of the South Delhi Fold Belt. It is medium to coarse-grained and light to dark coloured compact rock. The common associates of the carbonatitic rock are felsic rocks, rich in alkalies. Carbonatite contains more than 50% carbonate minerals, the majority of which are calcite, dolomite, ankerite, augite-aegirine augite and plagioclase. It is classified as calcite carbonatite of the sövite variety due to its coarse-grained character, chemically as calico-carbonatite and magnesio-carbonatite and even as silico-carbonatites for having more than 20% SiO2. The ∑REE contents of calico-carbonatite samples are nearly 100 times greater than magnesio-carbonatite. Chondrite normalised REE profiles of calcio-carbonatites are LREE enriched with nearly flat HREEs whereas the magnesio-carbonatite is characterised by flat REE patterns. The mantle-normalized incompatible trace element spidergram of Ambaji-Sendra belt carbonatites shows distinct negative anomalies of Ba, Nb, Ta, P, Sm, Eu, Ti and Y and positive at U and Pb by calcio-carbonatite whereas the magnesio-carbonatite displays negative kinks at K, Zr, Nb, Ta and Ti and positive at Th, Pb and Sr. The variable and/or contrasting enrichment/depletion in various elements in the two types of Ambaji-Sendra belt carbonatite is attributed either to significant differences in the type and modal proportion of different accessory mineral species or selective incorporation of metasomatic fluids during the subduction process. The chemical attributes of Ambaji-Sendra belt carbonatite suggest its emplacement in subduction settings.

Isotopic Ages of the Carbonatitic Volcanic Rocks in the Kunyang Rift Zone in Central Yunnan,China

The Mesoproterozoic Kunyang rift, which is located on the western margin of the Yangtze platform and the southern section of the Kangdian axis, is a rare massive Precambrian iron-copper polymetallic mineralization zone in China. The Mesoproterozoic Wulu (Wuding-Lufeng) basin in the middle of the rift is an elliptic basin controlled by a ring fracture system. Moreover, volcanic activities in the basin display zonation of an outer ring, a middle ring and an inner ring with carbonatitic volcanic rocks and sub-volcanic dykes discovered in the outer and middle rings. The Sm-Nd isochron ages have been determined for the outer-ring carbonatitic lavas (1685 Ma) and basaltic porphyrite of the radiating dyke swarm (1645 Ma) and the Rb-Sr isochron ages for the out-ring carbonatitic lavas (893 Ma) and the middle-ring dykes (1048 Ma). In combination of the U-Pb concordant ages of zircon (1743 Ma) in trachy-andesite of the corresponding period and stratum (1569 Ma) of the Etouchang Formation, as well as the Rb-Sr isochron age (1024 Ma) and K-Ar age (1186 Ma) of the dykes in the middle ring, the age of carbonarites in the basin is preliminarily determined. It is ensured that all of these carbonatites were formed in the Mesoprotero/oic period, whereby two stages could be identified as follows: in the first stage, carbonatitic volcanic groups, such as lavas, pyroclastic rocks and volcaniclastic sedimentary rocks, were formed in the outer ring; in the second stage, carbonatitic breccias and dykes appeared in the middle ring. The metamorphic age of the carbonatitic lavas in the outer ring was determined to be concurrent with the end of the first stage of the Neoproterozoic period, corresponding to the Jinning movement in central Yunnan.

Mafic, Ultramafic and Carbonatitic Dykes in the Southern Siberian Craton with Age of ca 1 Ga: Remnants of a New Large Igneous Province?

Virtual absence of igneous complexes with ages between1.8 Ga and 0.8 Ga in southern part of the Siberian Craton allowed to Galdkochub et al.(2010)to formulate a hypothesis of long magmatic quiescence.Most reliable

Geology and mineralization of the Bayan Obo supergiant carbonatite-type REE-Nb-Fe deposit in Inner Mongolia, China: A review

The Bayan Obo supergiant carbonatite-related rare-earth-element-niobium-iron(REE-Nb-Fe) endogenetic deposit(thereafter as the Bayan Obo deposit), located at 150 km north of Baotou City in the Inner Mongolia Autonomous Region, is the largest rare-earth element(REE) resource in the world. Tectonically,this deposit is situated on the northern margin of the North China Craton and adjacent to the Xing’anMongolian orogenic belt to the south. The main strata within the mining area include the Neoarchean Se’ertengshan Group and the Mesoproterozoic Bayan Obo Group. Generally, the rare earth, niobium, and iron mineralization within the deposit are intrinsically related to the dolomite carbonatites and the extensive alteration of the country rocks caused by the carbonatite magma intrusion. The alteration of country rocks can be categorized into three types: contact metasomatism(anti-skarn and skarn alteration), fenitization,and hornfelsic alternation. As indicated by previous studies and summarized in this review, the multielement mineralization at Bayan Obo is closely associated with the metasomatic replacement of siliceous country rocks by carbonatite magmatic-hydrothermal fluids. The metasomatic process is comparable to the conventional skarnification that formed due to the intrusion of intermediate-acid magmatic rocks into limestone strata. However, the migration pattern of Si O2, Ca O, and Mg O in this novel metasomatic process is opposite to the skarn alteration. Accordingly, this review delineates, for the first time, an antiskarn metallogenic model for the Bayan Obo deposit, revealing the enigmatic relationship between the carbonatite magmatic-hydrothermal processes and the related iron and rare earth mineralization.Moreover, this study also contributes to a better understanding of the REE-Nd-Fe metallogenetic processes and the related fluorite mineralization at the Bayan Obo deposit.

The giant Bayan Obo REE-Nb-Fe deposit,China:Controversy and ore genesis

Bayan Obo ore deposit is the largest rare-earth element（REE） resource,and the second largest niobium（Nb） resource in the world.Due to the complicated element/mineral compositions and involving several geological events,the REE enrichment mechanism and genesis of this giant deposit still remains intense debated.The deposit is hosted in the massive dolomite,and nearly one hundred carbonatite dykes occur in the vicinity of the deposit.The carbonatite dykes can be divided into three types from early to late：dolomite,co-existing dolomite-calcite and calcite type,corresponding to different evolutionary stages of carbonatite magmatism based on the REE and trace element data.The latter always has higher REE content.The origin of the ore-hosting dolomite at Bayan Obo has been addressed in various models,ranging from a normal sedimentary carbonate rocks to volcano-sedimentary sequence,and a large carbonatitic intrusion.More geochemical evidences show that the coarse-grained dolomite represents a Mesoproterozoic carbonatite pluton and the fine-grained dolomite resulted from the extensive REE mineralization and modification of the coarse-grained variety.The ore bodies,distributed along an E-W striking belt,occur as large lenses and underwent more intense fluoritization and fenitization.The first episode mineralization is characterized by disseminated mineralization in the dolomite.The second or main-episode is banded and/or massive mineralization,cut by the third episode consisting of aegirinerich veins.Various dating methods gave different mineralization ages at Bayan Obo,resulting in long and hot debates.Compilation of available data suggests that the mineralization is rather variable with two peaks at～1400 and 440 Ma.The early mineralization peak closes in time to the intrusion of the carbonatite dykes.A significant thermal event at ca.440 Ma resulted in the formation of late-stage veins with coarse crystals of REE minerals.Fluids involving in the REE-Nb-Fe mineralization at Bayan Obo might be REE-F-C02-NaCI-H20 system.The presence of REE-carbonates as an abundant solid in the ores shows that the original ore-forming fluids are very rich in REE,and therefore,have the potential to produce economic REE ores at Bayan Obo.the Bayan Obo deposit is a product of mantle-derived carbonatitic magmatism at ca.1400 Ma,which was likely related to the breakup of Columbia.Some remobilization of REE occurred due to subduction of the Palaeo-Asian oceanic plate during the Silurian,forming weak vein-like mineralization.

Carbonatites in China:A review for genesis and mineralization

Carbonatites are commonly related to the accumulation of economically valuable substances such as REE, Cu, and P. The debate over the origin of carbonatites and their relationship to associated silicate rocks has been ongoing for about 45 years, Worldwide, the rocks characteristically display more geochemical enrichments in Ba, Sr and REE than sedimentary carbonate rocks. However, carbonatite＇s geochemical features are disputed because of secondary mineral effects. Rock-forming carbonates from carbonatites at Qinling, Panxi region, and Bayan Obo in China show REE distribution patterns ranging from LREE enrichment to flat patterns. They are characterized by a Sr content more than 10 times higher than that of secondary carbonates. The coarse- and fine-grained dolomites from Bayan Obo H8 dolomite marbles also show similar high Sr abundance, indicating that they are of igneous origin. Some carbonates in Chinese carbonatites show REE （especially HREE） contents and distribution patterns similar to those of the whole rocks. These intrusive carbonatites display lower platinum group elements and stronger fractionation between Pt and lr relative to high-Si extrusive carbonatite. This indicates that most intrusive carbonatites may be carbonate cumulates. Maoniuping and Daluxiang in Panxi region are large REE deposits. Hydrothermal fluorite ore veins occur outside of the carbonatite bodies and are emplaced in wallrock syenite. The fluorite in Maoniuping has Sr and Nd isotopes similar to carbonatite. The Daluxiang fluorite shows Sr and REE compositions different from those in Maoniuping. The difference is reflected by both the carbonatites and rock-forming carbonates, indicating that REE mineralization is related to carbonatites. The cumulate processes of carbonate minerals make fractionated fluids rich in volatiles and LREE as a result of low partition coefficients for REE between carbonate and carbonatite melt and an increase from LREE to HREE. The carbonatite-derived fluid has interacted with wallrock to form REE ore veins. The amount of carbonatite dykes occurring near the Bayan Obo orebodies may support the same mineralization model, i.e. that fluids evolved from the carbonatite dykes reacted with H8 dolomite marble, and thus the different REE and isotope compositions of coarse- and fine-grained dolomite may be related to reaction processes.

Geochronology and mineralogy of the Weishan carbonatite in Shandong province, eastern China

The Weishan REE deposit is located at the eastern part of North China Craton(NCC), western Shandong Province. The REE-bearing carbonatite occur as veins associated with aegirine syenite. LA-ICP-MS bastnaesite Th-Pb ages(129 Ma) of the Weishan carbonatite show that the carbonatite formed contemporary with the aegirine syenite. Based on the petrographic and geochemical characteristics of calcite, the REEbearing carbonatite mainly consists of Generation-1 igneous calcite(G-1 calcite) with a small amount of Generation-2 hydrothermal calcite(G-2 calcite). Furthermore, the Weishan apatite is characterized by high Sr, LREE and low Y contents, and the carbonatite is rich in Sr, Ba and LREE contents. The δ^(13)Cv-PDB(-6.5‰ to -7.9‰) and δ^(13)OV-SMOW(8.48‰-9.67‰) values are similar to those of primary, mantlederived carbonatites. The above research supports that the carbonatite of the Weishan REE deposit is igneous carbonatite. Besides, the high Sr/Y, Th/U, Sr and Ba of the apatite indicate that the magma source of the Weishan REE deposit was enriched lithospheric mantle, which have suffered the fluid metasomatism. Taken together with the Mesozoic tectono-magmatic activities, the NW and NWW subduction of Izanagi plate along with lithosphere delamination and thinning of the North China plate support the formation of the Weishan REE deposit. Accordingly, the mineralization model of the Weishan REE deposit was concluded: The spatial-temporal relationships coupled with rare and trace element characteristics for both carbonatite and syenite suggest that the carbonatite melt was separated from the CO_2-rich silicate melt by liquid immiscibility. The G-1 calcites were crystallized from the carbonatite melt, which made the residual melt rich in rare earth elements. Due to the common origin of G-1 and G-2 calcites, the REE-rich magmatic hydrothermal was subsequently separated from the melt. After that, large numbers of rare earth minerals were produced from the magmatic hydrothermal stage.

From mantle to critical zone:A review of large and giant sized deposits of the rare earth elements

The rare earth elements are unusual when defining giant-sized ore deposits,as resources are often quoted as total rare earth oxide,but the importance of a deposit may be related to the grade for individual,or a limited group of the elements.Taking the total REE resource,only one currently known deposit（Bayan Obo） would class as giant（〉1.7×10^7 tonnes contained metal）,but a range of others classify as large（〉1.7×10^6 tonnes）.With the exception of unclassified resource estimates from the Olympic Dam 10 CG deposit,all of these deposits are related to alkaline igneous activity- either carbonatites or agpaitic nepheline syenites.The total resource in these deposits must relate to the scale of the primary igneous source,but the grade is a complex function of igneous source,magmatic crystallisation,hydrothermal modification and supergene enrichment during weathering.Isotopic data suggest that the sources conducive to the formation of large REE deposits are developed in subcontinental lithospheric mantle,enriched in trace elements either by plume activity,or by previous subduction.The reactivation of such enriched mantle domains in relatively restricted geographical areas may have played a role in the formation of some of the largest deposits（e.g.Bayan Obo）.Hydrothermal activity involving fluids from magmatic to meteoric sources may result in the redistribution of the REE and increases in grade,depending on primary mineralogy and the availability of ligands.Weathering and supergene enrichment of carbonatite has played a role in the formation of the highest grade deposits at Mount Weld（Australia） and Tomtor（Russia）.For the individual REE with the current highest economic value（Nd and the HREE）,the boundaries for the large and giant size classes are two orders of magnitude lower,and deposits enriched in these metals（agpaitic systems,ion absorption deposits） may have significant economic impact in the near future.

Bayan Obo Carbonatites:Texture Evidence from Polyphase Intrusive and Extrusive Carbonatites

Most of the so-called Bayan Obo fine-grained dolomite marbles collected from the main and east orebodies show a microporphyritic texture,namely the microphenocrysts are set in a very finegrained matrix,although nearly all of them have undergone recrystallization caused either by deformation or alteration.The texture seems likely to have maintained the original features.It is known that one of the most characteristic textures of volcanic rocks is the porphyritic texture,and the microporphyritic texture is a variety in which both the phenocrysts and the matrix are only distinguishable with the microscope.Therefore,the dolomite marbles in the main and east orebodies may be related to the extrusive carbonatites.In addition,there also occur some carbonatite sills and dykes with different textures at Bayan Obo.Thus,the Bayan Obo carbonatites are polyphase intrusive and extrusive carbonatites.

A Geochemical Study of an REE-rich Carbonatite Dyke at Bayan Obo,Inner Mongolia,Northern China

An REE-rich carbonatite dyke was found in Dulahala, close to the Bayan Obo superlarge REE-Nb-Fe mineral deposit in Inner Mongolia, northern China. The REE content in the dyke varies greatly, from 1% up to 20% (wt), which might constitute rich REE ores. Light REEs in the carbonatite are enriched and highly fractionated relative to heavy REEs and there is no Eu anomaly. The REE and trace element distribution patterns of the carbonatite are identical to those of fine-grained dolomite marble which is the host rock of the Bayan Obo REE-Nb-Fe superlarge mineral deposit. This indicates a petrogenetic linkage between the REE-rich carbonatite and the mineralizations in this region.

Re-interpretation of zircon date in a carbonatite dyke at the Bayan Obo giant REE-Fe-Nb deposit,China

Recent ce-valuation of the dating of the carbonatite dykes associated with the REE-Re-Nb giant deposit at Bayan Obo

The role of carbonate-fluoride melt immiscibility in shallow REE deposit evolution

The Lugiin Gol nepheline syenite intrusion, Mongolia, hosts a range of carbonatite dikes mineralized in rare-earth elements(REE). Both carbonatites and nepheline syenite-fluorite-calcite veinlets are host to a previously unreported macroscale texture involving pseudo-graphic intergrowths of fluorite and calcite. The inclusions within calcite occur as either pure fluorite, with associated REE minerals within the surrounding calcite, or as mixed calcite-fluorite inclusions, with associated zirconosilicate minerals. Consideration of the nature of the texture, and the proportions of fluorite and calcite present(～29 and 71 mol%,respectively), indicates that these textures most likely formed either through the immiscible separation of carbonate and fluoride melts, or from cotectic crystallization of a carbonatefluoride melt. Laser ablation ICP-MS analyses show the pure fluorite inclusions to be depleted in REE relative to the calcite. A model is proposed, in which a carbonate-fluoride melt phase enriched in Zr and the REE, separated from a phonolitic melt, and then either unmixed or underwent cotectic crystallization to generate an REE-rich carbonate melt and an REE-poor fluoride phase. The separation of the fluoride phase(either solid or melt) may have contributed to the enrichment of the carbonate melt in REE, and ultimately its saturation with REE minerals. Previous data have suggested that carbonate melts separated from silicate melts are relatively depleted in the REE, and thus melt immiscibility cannot result in the formation of REE-enriched carbonatites. The observations presented here provide a mechanism by which this could occur, as under either model the textures imply initial separation of a mixed carbonate-fluoride melt from a silicate magma. The separation of an REEenriched carbonate-fluoride melt from phonolitic magma is a hitherto unrecognized mechanism for REE-enrichment in carbonatites, and may play an important role in the formation of shallow magmatic REE deposits.

Fenitized Wall Rock Geochemistry of the First Carbonatite Dyke at Bayan Obo, Inner Mongolia, China

The first carbonatite dyke at Bayan Obo is well exposed on the surface for a length and width of approximately 60 m and 1.1-1.5 m, respectively. Along its strike, the fenitized H1 （Qs） and H2 （Cs） quartzite is replaced by Na-amphiboles, aegirines, and alkali-feldspars, intermittently stretching as far away as 800 m in length. Based on petrographical characteristics, the dyke＇s fenitized wall rocks are divisible into different zones： （1） outer, （2） middle, and （3） inner. The outer zone is 5-17 m from the NW margin of the dyke. The middle zone is located at 3.5-5 m from the NW margin of the dyke. The inner contact zone is located between direct contact with the dyke and 3.5 m from the dyke. In the outer zone, upon visual examination, no evidence of outcrop fenitization was found and the major elemental rock composition is nearly identical to the unaltered H1 and H2 lithologies. In the thin sections, however, small amounts of Na-amphibole and phlogopite are present. Despite relatively poor development throughout the 5 m of fenitization, the wall rocks have retained at least a small geochemical signature comparable to the original sedimentary protolith. The fenites occurring in the inner zone exhibit distinct variations, not only for the sharp contact at the outcrop scale, but also for variations in major, rare earth elements （REE）, and trace elements and Sm-Nd isotope composition. The wall rocks within 3.5 m have undergone strong fenitization, inheriting the geochemical signature derived from the carbonatite dyke. Fenitization in the middle zone was not as strong, at least compared to the inner zone, but was stronger than the outer zone. Compared to some trace elements and REEs, the major elements are relatively immobile during fenitization. The Sm-Nd isotope data for the carbonatite dyke and the adjacent fenitized wall rocks, where the Sm and Nd originate solely from the dyke, plots as a six-point isochron with an age of 1308~56 Ma. This age is identical to that of ore-bearing dolomite carbonatite and the related ore-forming events, indicating that there may be a petrogenetic link between the two. Based on Sr and Nd isotope compositional data, the first carbonatite dyke may be derived from an enriched mantle.

Geochemical Characteristics of a Carbonatite Dyke Rich in Rare Earths from Bayan Obo, China

The whole-rock geochemistry of a rare earths rich carbonatite dykes that locates at Dulahala and lies 3 km north-east to the East Ore body of the giant Bayan Obo RE-Nb-Fe deposit was analysed. The dyke cuts cross H1 coarse quartz sandstone and H2 fine quartzite of the Proterozoic Bayan Obo group. RE content in the dyke varies greatly up to 20% (mass fraction), which comprises rich RE ores. Light RE in carbonatites are extremely enriched and strongly fractionated relative to heavy RE, but no Eu anomaly. The carbonatite may be produced by mechanisms as follows: the carbonatite mana is directly formed by very low degree (F <1%) partial melting of enriched lithospheric mantle, leaving residual minerals characterized by abundant garnet; then the magma arises into a chamber within the crust where they will undergo fractional crystallization, which makes RE further concentrated in carbonatite. The RE patterns and spider diagrams of the carbonatite are identical to those fine-grained dolomite marble that is the ore-host rock for the Bayan Obo deposit. However, the carbonatite is calcic, which is different from the fine-grained dolomite marble in major element geochemistry. The difference is suggested to be resulted from that the carbonatite dyke is not affected by a large scale dolomitization, while the fine-grained dolomite marble might be the product of dolomitized carbonatite intrusive body that might set up a hydrothermal system in the region, which transported Mg from the Bayan Obo sediments, especially form the shales to the carbonatite intrusion.

Investigation on the Thermo-dynamics of Alkali-activated Carbonatite

The thermo-dynamics of reactions between carbonatite and sodium silicate solution at ordinary temperature （25℃） were investigated. The calculated results indicate that at ordinary temperature, the reactions between dolomite, calcite, Ca2＋ and Mg2＋ in carbonatite and H4SiO4, tl3SiO4- and H2SiO42- in sodium silicate solution to form the cementitious products of hydrated calcium silicate or hydrated magnesium silicate all possibly happen; among these reactions, the reactions to form gyrolite （2CaO.3SiO2.2.5H2O） and serpentine （3MgO.2SiO2-2H20） are the most possible to occur. Further, the dissociation degree of dolomite and calcite and the activity of H3SiO4 , H2SiO42- and H4SiO4 ions are the key factors to influence the reactions.

Setting and Strength Characteristics of Alkali-activated Carbonatite Cementitious Materials with Ground Slag Replacement

The effect of the ground granulated blast-furnace slag （ GGBFS ） addition, the modulus n （ mole rutio of SiO2 to Na2O ） and the concentrution of sodium silicate solution on the compressive strength of the material, i e alkuli-activated carbonatite cemeutitious material （ AACCM for short ） was investiguted. In addition, it is found that barium chloride has a sutisfiwtory retarding effect on the setting of AACCM in which more than 20% （ by mass ） ground carbonatite was replaced by GGBFS. As a result, a cementitious material, in which ground carbonatite rock served as dominative starting material, with 3-day and 28-day compressive strength greuter them 30 MPa and 60 MPa and with continuous strength gain beyond 90 days was obtained.

Geochemical constraints on CO2-rich mantle source for the Kocebu Seamount,Magellan Seamount chain in the western Pacific

The alkaline oceanic island basalts(OIBs)with under-saturated SiO2 and high contents of CaO and alkaline are usually attributed to mantle sources different from typical tholeiitic OIBs.Based on the results of high pressure and temperature experiment study,the genesis of silica under-saturated alkaline basaltic melts could be explained by the role of CO2,thus,the genetic relationship of alkaline basalts with CO2 has become a topic of relevance because it is closely related to the deep carbon cycle.The Magellan Seamount chain in the West Pacific Seamount Province has wide distribution of alkali basalts.For the first time,we collected alkaline basalt samples from the Kocebu Seamount of the Magellan Seamount chain and found that magmatic apatites widely occur in the less evolved volcanic rock samples,and the high contents of phosphorus should be a feature of the alkaline OIBs of the Magellan Seamounts.Compared with typical OIBs,these alkaline volcanic rocks have higher CaO and P2O5,lower SiO2 content,negative anomaly of high field strength elements(HFSEs),more distinctly negative anomaly of potassium(K)and the ubiquity of titanaugite,indicating a CO2-rich mantle source.Based on the relatively high K2O and TiO2 contents and La/Yb ratio and low MgO content of these alkaline rocks,we suggest that the volcanic rocks of the Magellan Seamounts are originated from carbonated eclogites derived possibly from ancient subducted altered oceanic crust.

Mineralogical and Petrological Studies on Carbonatite Dykes in Bayan Ebo，Nei Mongol，China and Their Implication to Rare Earth Mineralization

Two carbonatite dykes separetely located on Dulahala Mountain and within Erdaowa Group strata in Bayan Ebo mineral deposit have been studied.For Dulahala carbonatite the content of RE2O3 is 23.9 wt%,higher than that in the rare earth ores of both Main Ore-body and Eastern Ore-body,the distribution patterns of rare earth elements and rare earth mineral assemblage are consistent with those in both Ore-bodies.The evidence indicates a material source for RE in Bayan Ebo deposit.Fenite occurring at the edge of the carbonatite in Erdaowa Group yields an isochron age of 343.26±7.33 Ma by Rb-Sr method.It implies the geological time of mineralization.

The Forming Mechanisms of the Neoproterozoic Molartooth Carbonatites in North Anhui and Jiangsu Provinces

As a characteristic sedimentary type,molartooth carbonatites veins(MCV) can be found in almost all the Neoproterozoic carbonatite strata in the North Anhui and Jiangsu Provinces.But their forming mechanism is still an enigma,and more than four incompatible forming hypotheses have been put forward according to the structures,mineral components and elements of the MCV.Though all the MCV with the similar shape

CARBONATITES FROM THE EASTERN HIMALAYAN SYNTAXIS: PETROLOGY AND TECTONIC IMPLICATIONS

Most carbonatites occur in relatively stable, intra\|plate areas but some are found to occur in near to plate margins and may be linked with plate separation (Woolley, 1989). Although many carbonatites have been discovered to occur in the orogenic belts in recent years, most of these rocks are related to post\|orogenic magmatism, that is, the rocks occur in the specially extensional setting. Therefore it is unusual that such magmatic rocks occur in the typical convergent environment. Here we report carbonatites and associated ultramafic and mafic rocks in the core of the eastern Himalayan syntaxis. The eastern Himalayan syntaxis consists of three tectonic units: the Gangdise, the Yarlung Zangbo, and the Himalayan units, each of which is bounded by faults (Liu & Zhong, 1997). The Himalayan unit, the northernmost exposed part of the Indian plate, is divided into two complexes, the amphibolite facies complex in the south and the granulite facies complex in the north. The granulite facies complex in the Himalayan unit have been argued to experience high\|pressure metamorphism and represent materials buried to upper\|mantle depths (Liu & Zhong, 1997). The carbonatites and associated ultramafic and mafic rocks only occur in the granulite facies rocks and are divided into two belts: northern and southern belts.The northern belt extends at least 30km, and is about 20km in width. The southern belt extends several kilometers, and is 3km or so in width. Each belt consists mainly of differently compositional dykes, extending parallel to gneissosity of granulite facies gneiss. Carbonatitic agglomerates are observed in the northern belt. From the center of carbonatite dykes to country rocks, five types of rock are observed: the center parts of carbonatites, the rim parts of carbonatites, ultramafic and mafic rocks, altered rocks and country rocks. The gneissosity of country rock was deformed by intrusion of dykes.