The long-lived partial melting of the Greater Himalayas in southern Tibet, constraints from the Miocene Gyirong anatectic pegmatite and its prospecting potential for rare element minerals

The Cenozoic Himalayan leucogranite-pegmatite belt has been a hotspot for rare metal exploration in recent years.To determine the genesis of the pegmatite in the Himalayan region and its relationship with the Greater Himalayan Crystalline Complex(GHC),the Gyirong pegmatite in southern Tibet was chosen for geochronological and geochemical studies.The dating analyses indicate that the U-Th-Pb ages of zircon,monazite,and xenotime exhibit large variations(38.6‒16.1 Ma),with the weighted average value of the four youngest points is 16.5±0.3 Ma,which indicates that the final stage of crystallization of the melt occurred in the Miocene.The age of the muscovite Ar-Ar inverse isochron is 15.2±0.4 Ma,which is slightly later than the intrusion age,showing that a cooling process associated with rapid denudation occurred at 16‒15 Ma.TheεHf(t)values of the Cenozoic anatectic zircons cluster between−12 and−9 with an average of−11.4.The Gyirong pegmatite shows high contents of Si,Al,and K,a high Al saturation index,and low contents of Na,Ca,Fe,Mn,P,Mg,and Ti.Overall,the Gyirong pegmatite is enriched in Rb,Cs,U,K,Th and Pb and depleted in Nb,Ta,Zr,Ti,Eu,Sr,and Ba.The samples show a high 87Sr/86Sr(16 Ma)ratio of ca.0.762 and a lowεNd(16 Ma)value of−16.0.The calculated average initial values of 208Pb/204Pb(16 Ma),207Pb/204Pb(16 Ma)and 206Pb/204Pb(16 Ma)of the whole rock are 39.72,15.79 and 19.56,respectively.The Sr-Nd-Pb-Hf isotopic characteristics of the Gyirong pegmatite are consistent with those of the GHC.This study concludes that the Gyirong pegmatite represents a typical crustal‒derived anatectic pegmatite with low metallogenic potential for rare metals.The Gyirong pegmatite records the long‒term metamorphism and partial melting process of the GHC,and reflects the crustal thickening caused by thrust compression at 39‒29 Ma and the crustal thinning induced by extensional decompression during 28‒15 Ma.

Petrogenesis of the Late Eocene to Early Oligocene Yao'an Shoshonitic Complex,Southeastern Tibet:Partial Melting of an Ancient Continental Lithospheric Mantle beneath the Yangtze Block

Cenozoic potassic-ultrapotassic igneous rocks are widespread in the southeastern Tibetan Plateau.Their petrogenesis and magmatic processes remain subject to debate in spite of numerous publications.Almost all of the Cenozoic extrusive and intrusive rocks in the Yao’an area,western Yunnan Province,SW China,are geochemically shoshonitic,collectively termed here the Yao’an Shoshonitic Complex(YSC).The YSC is located in the(south)easternmost part of the ENE-WSW-trending,~550 km-long and~250 km-wide Cenozoic magmatic zone;the latter separates the orthogonal and oblique collision belts of the India-Eurasia collision orogen.Previously published geochronological and thermochronological data revealed that the rocks of the YSC were emplaced over a short timespan of 34-32 Ma.This and our new data suggest that the primary magma of the YSC likely was formed by partial melting of ancient continental lithospheric mantle beneath the Yangtze Block.This part of the continental lithospheric mantle had likely not been modified by any oceanic subduction.Fractionation crystallization of an Mg-and Ca-bearing mineral and TiFe oxides during the magmatic evolution probably account for the variable lithologies of the YSC.

Partial Melting and Its Implications for Understanding the Seismic Velocity Structure within the Southern Tibetan Crust

In order to constrain the crustal wave velocity structure in the southernTibetan crust and provide insight into the contribution of crustal composition, geothermal gradientand partial melting to the velocity structure, which is characterized by low average crustalvelocities and widespread presence of low-velocity zone(s), the authors model the crustal velocityand density as functions of depth corresponding to various heat flow values in light of velocitymeasurements at high temperature and high pressure. The modeled velocity and density are regarded ascomparison standards. The comparison of the standards with seismic observations in southern Tibetimplies that the predominantly felsic composition at high heat flow cannot explain the observedvelocity structure there. Hence, the authors are in favor of attributing low average crustalvelocities and low-velocity zone(s) observed in southern Tibet mainly to partial melting. Modelingbased on the experimental results suggests that a melting percentage of 7-12 could account for thelow-velocity zone(s).

Partial Melting Processes During Exhumation of Ultrahigh-Pressure Metamorphic Rocks in Dabieshan, China

Study practice has proved that the ultrahigh pressure metamorphic rocks iu Dabieshan must have exPerienced botk the retrograde metumorphism and partial melting under decompression and amphibolite-facies conditions during their exhumation from mantel depth to lower-middle crust.The retrometamorphism and partial melting of the ultrahigh pressure rocks in association with thermal state changing in the middle-lower crust, under amphibolite-facies conditions, are important physical and chemical processes. It would result in a great detrease in the integrated yield strength, and the enhancement of the de formabilitY or the rocks, promoting the transition from contractional (collision) to extensional defoemational regime. The statement of tbe retrometamorphism and partial melting of the ultrahigh pressure rocks has proved the in-site model for the ultrahigh pressure rocks in Dabieshan. It not only clarifies the evolutiou from the UHP eclogite to the surrounding gneissic rock (so called UHP gueiss) and to the garnet-beariug roliated granites (non-UHP country rocks), but also provides scientific arguments for the establiskment of the dynamic model of the exhumation of UHP metamorphic rocks in Dabiesban. In general, Purely conductive heat transfer from the crust itself is probably insurficient to produce temperature conditions for partial melting, and additional heat sources must have been present during partial melting. We infer that the partial melting and extensional flow are probably driveu by delamination and magmatic underplating of thickeued lithospkeric mantle following the continental oblique collision.

Partial melting of ultrahigh-pressure metamorphic rocks at convergent continental margins: Evidences, melt compositions and physical effects

Ultrahigh-pressure（UHP） metamorphic rocks are distinctive products of crustal deep subduction,and are mainly exposed in continental subduction-collision terranes. UHP slices of continental crust are usually involved in multistage exhumation and partial melting, which has obvious influence on the rheological features of the rocks, and thus significantly affect the dynamic behavior of subducted slices. Moreover,partial melting of UHP rocks have significant influence on element mobility and related isotope behavior within continental subduction zones, which is in turn crucial to chemical differentiation of the continental crust and to crust-mantle interaction.Partial melting can occur before, during or after the peak metamorphism of UHP rocks. Post-peak decompression melting has been better constrained by remelting experiments; however, because of multiple stages of decompression, retrogression and deformation, evidence of former melts in UHP rocks is often erased. Field evidence is among the most reliable criteria to infer partial melting. Glass and nanogranitoid inclusions are generally considered conclusive petrographic evidence. The residual assemblages after melt extraction are also significant to indicate partial melting in some cases. Besides field and petrographic evidence, bulk-rock and zircon trace-element geochemical features are also effective tools for recognizing partial melting of UHP rocks. Phase equilibrium modeling is an important petrological tool that is becoming more and more popular in P-T estimation of the evolution of metamorphic rocks; by taking into account the activity model of silicate melt, it can predict when partial melting occurred if the P-T path of a given rock is provided.UHP silicate melt is commonly leucogranitic and peraluminous in composition with high SiO_2,low MgO, FeO, MnO, TiO_2 and CaO, and variable K_2 O and Na_2 O contents. Mineralogy of nanogranites found in UHP rocks mainly consists of plagioclase ＋ K-feldspar ＋ quartz, plagioclase being commonly albite-rich.Trace element pattern of the melt is characterized by significant enrichment of large ion lithophile elements（LILE）, depletion of heavy rare earth elements（HREE） and high field strength elements（HFSE）,indicating garnet and rutile stability in the residual assemblage. In eclogites, significant Mg-isotope fractionation occurs between garnet and phengite; therefore, Mg isotopes may become an effective indicator for partial melting of eclogites.

Adakitic Rocks Resulting from Partial Melting of Metabasite at High-Pressure Granulite-Facies Condition during Continental Collision

Objective Previous studies on adakitic rocks with high Sr/Y and La/Yb ratios have established that such rocks may form in a variety of tectonic settings through different petrogenetic processes including： （1） partial melting of subducted young （〈25 Ma）, hot and hydrated oceanic slab; （2） partial melting of thickened lower crust; （3） assimilation and fractional crystallization processes involving basaltic magma; （4） partial melting of delaminated lower crust; and （5） partial melting of hydrous garnet peridotite. The various origins for adakites provide important constraints on crustal growth and evolution throughout the Earth＇s history.

On the Relationship between Mantle Metasomatism and Partial Melting—Evidence from Mantlederived Lherzolite Xenoliths from Nshan,China

Studies of the mantle-derived iherzolites from Nushan show that in addition to CO2,there were present H2O and small amounts of CO, CH4, SO2,Cl and F in the initial mantlc fluids derived fron the asthenospheric mantlc plumc .The imitial fluids accumulated in some regions of the mantle, resulting in lowering of the mantle solidus (and liquidus) and partial melting of the upper mantle. Melts formed from low-degree of fluid-involved partial melting of the upper mantle would be highly enriched in incompatible elements.Fluies and melts are allthe metasomatic agents for mantle metasomatism, and the interaction between them and the depleted mantle could result in the substan-tial local enrichment of LREE and incompatible elements in the latter.In case that the concentration of H2O in the fluids (and melts) is lower ,only cryptic metasomatism would occur, in case that the concentration of H2O is higher,the degree of partial melting would be higher and hydrous metasomatic phases(e.g. amphiboles )would nucleate. Under such circumstances, there would occur model metasomatism.

Characterization of partial melting events in garnet-cordierite gneiss from the Kerala Khondalite Belt,India

Phase equilibria modelling coupled with U–Pb zircon and monazite ages of garnet–cordierite gneiss from Vallikodu Kottayam in the Kerala Khondalite Belt,southern India are presented here.The results suggest that the area attained peak P–T conditions of^900
C at 7.5–8 kbar,followed by decompression to 3.5–5 kbar and cooling to 450–480
C,preserving signatures of the partial melting event in the field of high to ultra-high temperature metamorphism.Melt reintegration models suggest that up to 35%granitic melt could have been produced during metamorphism at^950
C.The U–Pb age data from zircons(~1.0–~0.7 Ga)and chemical ages from monazites(~540 Ma and^941 Ma)reflect a complex tectonometamorphic evolution of the terrain.The^941 Ma age reported from these monazites indicate a Tonian ultra-high temperature event,linked to juvenile magmatism/deformation episodes reported from the Southern Granulite Terrane and associated fragments in Rodinia,which were subsequently overprinted by the Cambrian(~540 Ma)tectonothermal episode.

Experimental Study of Partial Melting of Mantle Peridotite -A Discussion about the Genesis of Sih'ca-rich Fluids (Melts)

Experiments on partial melting of mantle lherzolite have been realized at 0.6 and 1.0 GPa and the chemical compositional variations of melts during different melting stages have been first discussed. The results show that the trends of variations in SiO2, CaO, Al2O3, Na2O and TiO2 are different at different melting stages. The melts produced at lower pressure are richer in SiO2 than those at higher pressure. The mantle-derived silica-rich fluids (silicate melts) are polygenetic, but the basic and intermediate-acid silicate melts in mantle peridotite xenoliths from the same host rocks, which have equivalent contents of volatile and alkali components and different contents of other components, should result from in-situ (low-degree) partial melting of mantle peridotite under different conditions (e.g. at different depths, with introduction of C-O-H fluids or in the presence of metasomatic minerals). The intermediate-acid melts may be the result of partial melting (at lower pressure) Opx + Sp + K-Na-rich fluid ±(Am-phi) ± (Phlog) = OI + melt. But the intermediate-acid magmas cannot be produced from the partial melting of normal mantle peridotite unless the crustal materials are introduced to some extent.

The Evolution of Alpine Ultramafic Rocks and Partial Melting of the Upper Mantle

Ultramafic rocks of Tibet and Xinjiang are the products of partial melting of the upper mantle. The evolution of their mineral composition is marked by two parallel evolutionary series: one is the progressive increase of the 100 Mg/(Mg+Fe^(2+) ratio of silicate minerals in order of lherzolite→harzburgite→dunite, i.e. the increase in magnesium; the other is the increase of the 100 Cr/(Cr+Al) ratio of accessory chrome spinel in the same order, i. e. the increase in Chromium. The above-mentioned evolutionary trends are contrary to that of magmatic differentiation. The evolution of fabrics of ultramafic rocks is characterized by progressive variation in order of protogranular texture→melted residual texture, symplectic texture and clastophyritic texture→equigranular mosaic texture and tabular mosaic texture. Experiments of partial melting of lherzolite have convincingly shown that the evolution of Alpine ultramafic rocks resulted from the partial melting of pyrolite. Various subtypes of them represent different degrees of partial melting. The vertical zoning marked by more basic rocks in the upper part and more acid rocks in the lower actually belongs to the fusion zoning of pyrolite.

Mantle Partial Melting and Melt-peridotite Interaction: A Case Study of Peridotite Xenoliths in Cenozoic Basalt of Nanjing, SE China

The peridotite xenoliths are widely distributed in the Cenozoic basalts, Eastern China. However, their petrogenesis is hotly controversial. The peridotite xenoliths of Nanjing are found embedded in Cenozoic alkali basalt. Most of the xenoliths are rounded and small to moderate in size(typically 5~10 cm in diameter), though larger ones have been found. Nearly all small xenoliths are harzburgite and dunite. However, the big ones have zoned structure: lherzolite core and harzburgite or dunite rim with new growth clinopyroxene(Cpx) as eyeliner along their margins. Petrology, mineralogy, and Major and trace element compositions of the Nanjing peridotite xenoliths in the Cenozoic basalts are measured to provide an insight into the nature of their mantle sources and processes. Our works suggest that they were suffered from a partial melting process and subsequently underwent a process of melt-peridotite interaction. The evidences of partial melting are as follows. Firstly, the lherzolite core is mostly composed of olivine(Ol) + orthopyroxene(Opx) + Cpx with minor spinel(Spl), however, the harzburgite or dunite rim is mostly complosed of Ol + Opx with minor Cpx. Secondly, from the lherzolite core to the harzburgite or dunite rim, Ol and Opx contents are gradually increased, inversely, Cpx contents are decreased, and the Spl disappears. Thirdly, Mg# values of Ol are increased from the core(~89.5) to the rim(>92) of the peridotite xenolith, but FeO contents(from 11.0 to 8.1) in Ol are opposite.Forthly, Mg#(from 90 to 93) and Cr#(from 4 to 17) values of the Opx are increased, but its Al2O3 contents(from 5.0 to 2.0) are decreased from the core to the rim. The evidences of meltperidotite interaction are mostly from clinopyroxenes. The clinopyroxenes can roughly divided into two groups: original clinopyroxenes of the peridotite and new growth clinopyroxenes generated by melt-peridotite interaction. The original clinopyroxenes are generally in the inner of peridotite xenoliths such as lherzolite. They are mostly coarse-grained, euhedral and bottle-green. However, the new growth clinopyroxenes generally occur as eyeliner along the margin of the peridotite xenoliths. They are generally fine-grained, irregular and light green. Compared with the original clinopyroxenes, the new growth ones have low Na2O and Al2O3 and high CaO contents. The Nanjing peridotite xenoliths in the Cenozoic alkali basalts suggest that the SCLM beneath eastern China might be a fertile mantle which has had a complex history, and is now a mixture of refractory and fertile mantle domains modified by a number of events.

PARTIAL MELTING AND RECRYSTALLIZATION OF ISOTACTIC POLYPROPYLENE

A relatively high predetermined crystallization temperature （135℃） was chosen to grow well developed iPP spherulites, then the partial melting was carried out at a temperature of 165℃, where the preformed spherulites were seen to only decrease their size but not completely melted. The crystallization behavior of partially melted isotactic polypropylene （iPP） has been carefully examined by different scanning calorimetry （DSC） and polarized light microscopy （PLM）. The experimental results show that at a special annealing temperature （165℃） the melting behavior of iPP includes two parts with different mechanism, one part is the melting of iPP spherulite outside, another is the partial lamellae perfection during longer annealing time in the unmelted spherulite. The conformational orders of the iPP melt decrease with the increase of the annealing temperature.

Experimental Investigation on Low-Degree Dehydration Partial Melting of Biotite Gneiss and Phengite-Bearing Eclogite at 2 GPa

The ultrahigh-pressure（UHP） eclogite and gneiss from the Dabie（大别）-Sulu（苏鲁） oro-gen experienced variable degrees of partial melting during exhumation.We report here dehydration partial melting experiments of biotite gneiss and phengite-bearing eclogite at 2 GPa and 800-950 ℃.Our results show that the partial melting of gneiss is associated with the breakdown of biotite into almandine-rich garnet starting at 900 ℃.About 10% granitic melt can be produced at 950 ℃.In con-trast,the partial melting of phengite-bearing eclogite exists at slightly lower temperatures（800-850 ℃）.The melt fraction is in general more in biotite gneiss than in phengite-bearing eclogite under similar pressure and temperature conditions.Both melts are rich in silica and alkali,but poor in FeO,MgO and CaO.These results suggest that low-degree partial melting of gneiss and eclogite is often associated with dehydration of hydrous mineral,such as micas.The dehydration temperature and melt composi-tion can place important constraints on the partial melting phenomena（granitic leucosome and multi-phase mineral inclusions） recorded in UHP rocks.

Temperature of Prograde Metamorphism, Decompressional Partial Melting and Subsequent Melt Fractional Crystallization in the Weihai Migmatitic Gneisses,Sulu UHP Terrane:Constraints from Ti-in-Zircon Thermometer

In order to constrain temperature during subduction and subsequent exhumation of fel- sic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai region in the Sulu ultra-high pres- sure （UHP） metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of in- tercalated compositional layers of melanosome and plagioclase （Pl）-rich lencosome and K-feldspar （Kfs）-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompres- sional partial melting during early stage exhumation and subsequent fractional crystallization of pri- mary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the pro- tolith of the migmatitic gneiss is Mid- Neoproterozoic （-780 Ma） magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as over- growth mantles on the protolith magmatic zir- con cores. The estimated growth temperatures （625-717 ＂C） of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eciogite-facies condition during subduction. Zircon recrystal- lized rims （228-2 Ma） in the PI-rich ieucosome layers provide the lower limit of the decompress-sional partial melting time during exhumation. The ages from 228^-2 to 219~2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 , and the temperatures of new growth zircon grains （219±2 Ma） in Kfs-rich vein are between 769 and 529 . The estimated temperatures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indi- cating that a process of fractional crystallization of the partial melt during exhumation.

Time Scale of Partial Melting of KLB-1 Peridotite: Constrained from Experimental Observation and Thermodynamic Models

Partial melting experiments were carried on KLB-1 peridotite, a xenolith sample from the Earth＇s upper mantle, at 1.5 GPa and temperatures from 1 300 to 1 600 ℃, with heating time varies from 1 to 30 min. We quantify the axial temperature gradient in the deformation-DIA appa- ratus （D-DIA） and constrain the time scale of partial melting by comparing experimental observa- tions with calculated result from pMELTS program. The compositions of the liquid phase and the coexisting solid phases （clinopyroxene, orthopyroxene, and olivine） agree well with those calculated from pMELTS program, suggesting that local chemical equilibrium achieves during partial melting, although longer heating time is required to homogenize the bulk sample. The Mg# （=Mg/（Mg＋Fe） moi.%） of olivines from the 1-minute heating experiment changed continuously along the axial of the graphite capsule. A thermal gradient of 50 ℃/mm was calculated by comparing the Mg# of oli- vine grains with the output of pMELTS program. Olivine grains at the hot end of the graphite cap- sule from the three experiments heated at 1 400 ℃ but with different annealing time show consis- tence on Mg#, indicating that partitioning of Fe2＋ between the olivine grains and the silicate melt happened fast, and partial melting occurs in seconds.

An Experimental Study of Partial Melting of Metafelsic Rocks:Constraints on the Feature of Anatectic Melts and the Origin of Garnets in Collisional Orogens

Crustal anatexis in continental subduction zones has great bearing on chemical differentiation of the continental crust at convergent plate boundaries.This was experimentally investigated for ultrahigh-pressure(UHP)metafelsic rocks at 0.5-3.0 GPa and 650-900℃.The results show that partial melting begins at about 750℃ when pressure drops from 3.0 to 2.0 GPa,corresponding to decompressional exhumation of the deeply subducted continental crust.As the pressure further decreases to 1.0 GPa,the partial melting degree reaches the maximum of~25%at 900℃.Partial melts produced in these experiments are rich in silica and alkali,and poor in iron,manganese and magnesium.As the degree of partial melting increases,the composition of partial melts gradually converges toward homogeneous one.In the absence of free water,the partial melting of metafelsic rocks were triggered by the breakdown of hydrous minerals.At low temperatures of~750℃at 1.0-2.0 GPa,phengite dehydration melting occurs at first,giving rise to small amounts of felsic melts and peritectic K-feldspar.As the temperature rises up to 850-900℃,biotite begins to break down and gives rise to large amounts of felsic melts and peritectic minerals such as garnet,K-feldspar and orthopyroxene.It is noted that peritectic garnet is much different from anatectic garnet crystallized from anatectic melts and metamorphic garnet formed through metamorphic dehydration reaction under subsolidus conditions.The peritectic garnet is characterized not only by anhedral shapes with many multiphase crystal inclusions but also by compositions poor in spessartine and grossular but rich in almandine and pyrope.On the other hand,the anatectic garnets are characterized not only by euhedral shapes with few inclusions but also by compositions rich in grossular and spessartine but poor in almandine and pyrope.These observations provide experimental constraints on the origin of garnets in UHP metamorphic rocks,which have great bearing on understanding of anatectic metamorphism in collisional orogens.

Formation of Melt Pocket in Mantle Peridotite Xenolith from Western Qinling, Central China: Partial Melting and Metasomatism

Two types of melt pockets, closed melt pocket （CMP） and open melt pocket （OMP）, are recognized from the peridotite xenoliths entrained in the Cenozoic kamafugites in western Qinling （秦岭）, Central China. The Haoti （好梯） CMPs have a mineral assemblage of olivine＋ clinopyroxene＋amphibole＋K- feldspar, whereas the Baiguan （白关） CMPs are composed of olivine＋clinopyroxene＋Umenite＋carbonate. The components of the OMPs are more complicated. In the Haoti OMPs, there are olivine, ciinopyroxene, glass, low modal abundances of amphibole, K-feldspar （Kfs）, Umenite, sulfide, chlorite, perovskite, chromite and phlogopite. The Baiguan OMPs contain olivine, clinopyroxene, glass, chlorite and chromite. Compositionally, ofivines in the CMPs and OMPs are both apparently depleted in Ni, and those in the OMPs are also depleted in Fe and Mg, and enriched in Ca compared to the primary ones. Ciinopyroxenes display large and systematical compositional variations between the CMPs and OMPs, particularly in Al, Cr, Na, Ca and Ti. Glasses are generally depleted in Si compared to the worldwide glasses in melt pockets, although they still have large variations. Amphiboles and K-feldspars have relatively restricted compositional variations. The petrographical observations and mineral chemistry suggest that the Haoti and Baiguan CMPs were generated by the in-sitn decompression melting of orthopyroxenes, olivines and clinopyroxenes, and by the addition of minor external K-rich and Ca-rich melt/fluids. The OMPs formed during the latest metasomatic event in the lithospheric mantle beneath the western Qinling.

Petrographic and Geochemical Characteristics of the Pouni Palaeoproterozoic Formations North of the Léo Square Degree (Burkina Faso, West Africa)

The Pouni area is made up of basalts belonging to the Boromo belt, lamprophyres and granitoids. These geological formations are similar to geological formations of the same type in other regions of the Palaeoproterozoic domain of the Man/Leo shield. This study, which focused on the petrographic and geochemical characteristics of these geological formations, led to the following main conclusions: The lamprophyres are basic plutonic rocks that cut through other geological formations. The basalt belongs to the northern part of the Borormo belt and is thought to be a relic of overthickened oceanic plateaus. There are two groups of granitoid rocks. The granodiorite has a geochemical signature close to that of Archean TTGs and is metaluminous in character. It has a low potassium content. The minor element and rare earth element spectra indicate that it could be derived from partial melting of basic magmatic rocks. Biotite granites are peraluminous and highly potassic. Minor element contents and rare earth spectra indicate that they could be derived from partial melting of felsic materials. Geotectonic diagrams show that the granitoids identified in the Pouni zone were emplaced in an active tectonic context, similar to that of present-day subduction zones.

Petrographic and Geochemical Characteristics of the Granitoids in the South of Godé (North of the Square Degree of Léo-Burkina Faso, West Africa)

South of Godé, in the central-western region of Burkina Faso, granitoids of Paleoproterozoic age are similar to those of the Man/Leo shield. This study focused on the petrographic and geochemical characteristics of these granitoids, with the following results: 1) The tonalite that outcrops in the south-west of the study area belongs to the TTG group or first generation granitoids. They are most often ribboned at outcrop and have a geochemical signature close to that of Archean TTGs. Tonalite has a metaluminous character and the REE spectrum indicates that it may be derived from partial melting of basic magmatic rocks. 2) Biotite granites have no outcrop structure. They are weakly metaluminous to peraluminous and potassic to highly potassic. Their rare earth spectra indicate that they may be derived from the partial melting of TTG granitoids. 3) Geotectonic diagrams show that the granitoids studied to the south of Godé were emplaced in an active tectonic context similar to that of present-day subduction zones.

Mantle melting beneath the Southwest Indian Ridge: signals from clinopyroxene in abyssal peridotites

The mineral chemistry and texture of clinopyroxenes in peridotite from the Kingkong tectonic zone of the Southwest Indian Ridge segment in an effort to constrain mantle melting beneath this slow-spreading ridge are reported. There are three types of clinopyroxenes in the abyssal peridotites： coarse-grained, intergranu- lar and exsolved. The compositional variations among these three types suggest that the coarse-grained clinopyroxene is a mantle-derived source. The A1, Na and Ti contents and the Na/Ti ratio of the coarse- grained clinopyroxene may be used to monitor the degree of partial melting, combined with the contradis- tinction with Spinel Cr#, which is calculated to be between 7.9% and 14.9%, and may represent low degrees of melting in the global ocean ridge system. The along-axis compositional variations in the coarse-grained clinopyroxene suggest that the degree of partial melting is primarily controlled by the transform faults on both sides of the ridge. Nonetheless, the northwestern side of the ridge may be affected by a hypothesised detachment fault as documented by the calculated P-T conditions. Simultaneously high Na and low Ti con- tents in the coarse-grained clinoovroxene points to mantle heterogeneities along the ridge axis.