Pyroxenite Research Articles

PETROLOGICAL CONSTRAINTS ON THE ORIGIN OF PYROXENITE DYKES IN THE LITHOSPHERIC MANTLE OF THE CHESHMEH-BID OPHIOLITIC MASSIF, SOUTHERN IRAN

The Cheshmeh-Bid ophiolitic massif in the Khajeh-Jamali district (Southern Iran) is dominated by harzburgite-dunite tectonites locally intruded by orthopyroxenite dikes. These latter are composed of dominant coarse orthopyroxene with minor olivine, Cr-spinel, clinopyroxene and amphibole. Estimated equilibrium temperatures for Mg-hornblende and edenitic amphibole reveal a late stage magmatic origin. The Cheshmeh-Bid orthopyroxenites are characterized by very low Al2O3, CaO, Na2O and TiO2 abundances coupled to relatively high MgO and SiO2 contents. They display U-shaped REE patterns, selective LILE enrichment and positive Pb and Sr anomalies. The host harzburgites are highly refractory mantle residues resulting from fluid-assisted melting. Field observations and mineral assemblages suggest that the pyroxenites formed by melt injection along fractures within rather cold ambient harzburgites and chromitites at moderate pressure (P > 1 GPa). Based on bulk-rock compositions and mineral chemistry, we infer that the Cheshmeh-Bid orthopyroxenites originated from the intrusion and crystallization of hydrous Si-rich, low-Ca melts with a boninite signature in a suprasubduction environment. Fine-grained neoblastic domains developed in the pyroxenites in response to subsolidus ductile deformation and recrystallization, which were most likely related to the exhumation of the Cheshmeh-Bid ophiolite massif.

Rare and new compounds in the Ni-Cu-Sb-As system: first occurrence in the Gomati ophiolite, Greece

The Gomati ophiolite (Northern Greece) is located in the Serbo-Macedonian Massif, in the Hellenides orogenic belt. It consists of altered peridotites hosting scattered chromitite bodies. The ultramafics are enclosed in Silurian gneisses and schists, and are partially in contact with late Cenozoic granites. The present work focuses on accessory minerals in the Ni-Cu-Sb-As system, found in a chloritized clinopyroxenite in contact with chromitite. The electron microprobe analyses revealed the presence of known minerals such as orcelite (Ni 5 As 2 ) and breithauptite (NiSb) and other new phases that cluster around the following stoichiometries: (Ni,Cu) 7 (Sb,As) 3 , (Ni,Cu) 2 (Sb,As), (Ni,Cu) 11 (Sb,As) 8 , Ni 3 As, Ni 5 (As,Sb) 2 and Ni 7 (As,Sb) 3 . As orcelite is a non-stoichiometric mineral, (Ni,Cu) 7 (Sb,As) 3 , Ni 5 (As,Sb) 2 and Ni 7 (As,Sb) 3 may correspond to Cu and/or Sb-rich terms of this phase. A mineral phase corresponding to (Ni,Cu) 2 (Sb,As) was first described in the Tulameen complex of Canada. A phase with stoichiometry Ni 3 As was found in the Bon Accord oxide body of South Africa. The (Ni,Cu) 11 (Sb,As) 8 probably represents a Cu-rich Sb analogue of the mineral maucherite (Ni 11 As 8 ). Such mineral assemblage in the Gomati ophiolite is puzzling. While ultramafic rocks contain Ni and As of magmatic origin, the presence of Sb, Ag, Au and Cu minerals could be indicative of a metasomatic enrichment, probably linked to the presence of fluids emanating from the granite body in contact with the Gomati ophiolite.

Plagioclase-facies thermobarometric evolution of the External Liguride pyroxenite-bearing mantle (Suvero, Italy)

Plagioclase peridotites are an important marker of the shallow geodynamic evolution of the lithospheric mantle at extensional settings. Based on lowpressure experiments, a recent study by Fumagalli et al. (2017) defined and calibrated a geobarometer for peridotitic bulk compositions, based on the Forsterite-Anorthite-Ca-Tschermak-Enstatite (FACE) pressure-sensitive equilibrium. The Suvero plagioclase-bearing peridotites, on which the FACE geobarometer was calibrated, are primarily associated to plagioclase pyroxenites. Assuming that the pyroxenites record the same Pressure-Temperature evolution than the plagioclase peridotites, they represent ideal candidates to test the applicability of the FACE geobarometer on pyroxenitic compositions. As documented in the plagioclase peridotites, the pyroxenites are characterized by the development of fine-grained neoblastic assemblages, indicative of partial recrystallization under plagioclase-facies conditions. Chemical zonations in these neoblastic mineral aggregates suggest equilibration stages at variable pressure and temperature and allowed to document two re-equilibration stages corresponding to the onset of plagioclase-facies recrystallization (830-850°C, 6.9-8.1±0.5 kbar) and a shallower colder re-equilibration (770-790°C, 5.8-5.9±0.5 kbar), respectively. The decompressional evolution reported for pyroxenitic bulk compositions is consistent with the exhumation history documented in the associated Suvero peridotite, although at slightly higher equilibrium pressures (~ 1 kbar). Remarkably, the much lower XCr in pyroxenites reflects in lower Cr incorporation in pyroxenes and, consequently, in significantly higher Ca-Tschermak activity in clinopyroxene that might introduce the systematic pressure overestimation by FACE geobarometer.

Deformation history of ultra high-pressure ophiolitic serpentinites in the Zermatt-Saas Zone, Créton, Upper Valtournanche (Aosta Valley, Western Alps)

Detailed multiscale structural analyses and mapping (1:20 scale) integrated with petrological investigation were used to study a portion of the Zermatt-Saas serpentinites that crop out in upper Valtournanche (north-western Italy). Results are synthesized in a foliation trajectory map that displays the transposed original lithostratigraphy of a serpentinite body exposed at Creton. The serpentinite body comprises magnetite sheets and rare, decimetre-thick, diopsidite layers and lenses. Moreover, veins and aggregates of Ti-chondrodite and Ti-clinohumite, olivine-rich layers and lenses, veinlets of olivine, and layers of dark pyroxenite are embedded in the serpentinites. Serpentinites and associated rocks record three relative age groups of ductile structures: D1 consists of rare folds and S1 foliation; D2 is a group of isoclinal folds and a very pervasive foliation (S2), which is the dominant structure; D3 includes a crenulation and shear zones affecting S2. The detailed meso-structural and microstructural analyses allowed individuating the metamorphic environment of successive deformation stages and correlating the resulting tectono-metamorphic investigation with those already inferred in surrounding areas. In addition, metre- to submillimetre-sized pre-D2 structural, mineralogical, and textural relics have been clearly identified in spite of the strong transposition imposed during the development of S2 high pressure - ultra-high pressure foliation.

The Dabie-Sulu orogenic peridotites: Progress and key issues

大别-苏鲁造山带橄榄岩原岩属性可分为壳源和幔源橄榄岩, 它们在矿物结构、变质演化以及全岩和单矿物化学成分上存在一系列差别. 幔源橄榄岩来自于俯冲板块之上的古老华北克拉通岩石圈地幔, 记录了俯冲板块与地幔楔之间相互作用的信息. 石榴橄榄岩峰期条件下都显示石榴石和尖晶石共存, 石榴石和尖晶石的稳定性和成分都受全岩富化程度控制, 尖晶石成分不能用来判别部分熔融程度. 难熔幔源纯橄岩中普遍发育低Mg高Ca橄榄石取代早期斜方辉石结构, 指示硅不饱和熔体-橄榄岩反应过程, 它可能是导致纯橄岩Re-Os同位素年龄偏年轻的原因之一. 幔源橄榄岩/辉石岩中的金红石、钛斜粒硅镁石和锆石是俯冲地壳熔/流体迁移高场强元素进入地幔楔的直接矿物学证据, 只有通过精细的原位元素和同位素工作, 才能准确限定交代介质来源、时间和交代过程. 大陆俯冲带地幔楔氧逸度变化范围宽泛(FMQ=5.50~1.75), 并随深度增加而减小, 但其氧逸度计算结果受矿物组合、温压条件以及折返过程中的脱氢-氧化作用等因素影响. 本文对橄榄岩-辉石岩复杂体系相关系、大陆俯冲之前的地幔过程、地壳交代对地幔楔元素-同位素-氧逸度以及物理性质的改造等重要研究方向提出了建议.

CHROMITITES OF THE URALS (PART 1): OVERVIEWOF CHROMITE MINERAL CHEMISTRY AND GEO-TECTONIC SETTING

Published and unpublished compositions of chromite in 333 chromitite samples from 14 ultramafic complexes of the Urals are overviewed. The chromitites occur in the mantle unit and/or the supra-Moho cumulate sequence of ophiolite complexes, as well as in Alaskan-type intrusions. They vary in size from giant ore deposits associated with ophiolites (e.g., Kempirsai, Ray-Iz, Voykar-Syninsky) to sub-economic mineralization in the Alaskan-type complexes (e.g., Svetly Bor, Kachkanar). Mantle-hosted chromitites occur either as discordant, podiform, high-Cr ore bodies and sub-concordant elongated lenses of high-Al chromite. In the supra-Moho sequences of ophiolites, chromitite is mainly of the high-Al variety, and occurs as concordant layers alternated with peridotite and pyroxenite cumulates. In the Alaskan-type intrusions of the Urals, chromitite occurs as centimeter to meter-size pods and lenses having syngenetic or epigenetic relationship with the host dunite. Calculated melt compositions in equilibrium with chromite and comparison of chromite composition with those from various volcanic suites, and chromitites from different plutonic complexes, allow division of the Urals chromitites into four different compositional groups, corresponding to different geodynamic environments of formation: 1) The high-Al, low-Ti suite (Al 2 O 3 > 20 wt%, Cr# 20 wt%, Cr# 0.70, Al 2 O 3 0.70, Al 2 O 3 < 20 wt%, TiO 2 = 0.38-1.30 wt%, Fe 3+ # = 0.20-1.29, δ logf(O 2 ) = +0.9 ÷ +5.9) is represented by chromitites from the Urals Alaskan-type intrusions and the East-Khabarny complex. They have crystallized from Fe-rich magma (av. FeO/MgO = 1.35) under oxygen- fugacity conditions well above the FMQ buffer. The melt is characterized by high-Ti, high-K, calc-alkaline composition, having many geochemical characteristics in common with ankaramites. It was generated by partial melting of a fluid-metasomatized mantle source, in a subduction-influenced arc setting. However, the close similarity with the zoned complexes emplaced in the Russian-Far-East craton suggests that formation of Alaskan-type melts may be not restricted to SSZ, island arc settings.

The nature and age of the Suroyam clinopyroxenite massif (Middle Urals)

The Suroyam alkaline ultrabasitesyenite (mainly pyroxenite) massif, including the Suroyam apatitetita� nomagnetite deposit, is a part of the Nyazepetrovsk structural-formational zone located on the western slope of the Middle Urals, within the Ufimskii promon� tory of the East European Platform. The Nyazepetrovsk complex tectonically overlaps the sedimentary shelf and bathyal deposits of the mar� gin of the Paleozoic continent (Fig. 1) and is the uppermost tectonic sheet developed here. The com� plex is made of volcanogenic-sedimentary and intru� sive rocks of Ordovician-Early Devonian age, approximately correlated with the Early and Late islandarc oceanic formations of the Tagil zone (1). According to the chemical composition and metallo� genic features, the Suroyam massif located in the area of these formations can be compared with massifs of a palladiumbearing "branch" of the Platinum belt (Ural PGEbearing belt) in the Urals. The Rb-Sr iso� chrone age of the massif is 415.2 ± 1.1 Ma. Recent data obtained provide confirmation of the allochthonous origin of the Suroyam massif and its positional con� nection with the Tagil zone. The stratigraphic section of the Nyazepetrovsk zone is represented by volcanic and volcanogenic- sedimentary formations with subordinate siliceous rocks, siltstone, and sandstones. The section is distin� guished into three formations (from ancient to young): basalt, basalt-andesitebasalt, and trachybasalttra� chyte. Based on rare findings of radiolarians, con� odonts, and macrofauna, the age of the formations is regarded as Ordovician-Early Devonian. The Suroyam massif is located in the southern area of the axial part of the Nyazepetrovsk zone, and it is mainly composed of apatitetitanomagnetite clinopy� roxenites, subjected to strong superimposed potassium and sodium metasomatism, connected with manifes� tations of alkaline magmatism (2). The nature of the massif is under discussion. There were ideas expressed in (3, 4) about its platform origin, but more convincing evidence is needed. We have identified, however, sim� ilarities of the Suroyam massif with some specific mas� sifs of the Platinum belt with titanomagnetite mineral� ization. In particular, among the most widespread types in the Suroyam massif (Fig. 2) are highferrugi� nous clinopyroxenites, green and dark green rocks, characterized by fine to coarse grained structure, as well as pegmatoid structure. The main rockforming minerals are clinopyroxene, titanomagnetite, apatite, and phlogopite. The quantitative ratios of these miner� als vary greatly. The central part of the Suroyam massif is composed of coarsegrained titanomagnetite cli� nopyroxenites (like kosvites) with a titanomagnetite content of 6-25%; the marginal part is characterized mainly by apatitetitanomagnetite types of rocks, con� taining 2.9-19.5% of apatite. The undeformed rocks have sideronitic and granoblastic texture. Clinopyroxene forms short and long prismatic grains. According to the chemical composition, cli� nopyroxene is represented by diopsidehedenbergite, containing 25-30% of the hedenbergite molecule. The iron content in pyroxene ranges from 14 to 26% (20 at. % on average). There was noted a high content of alumina (2.49-5.05%) with variations in the augite content from 6 to 11%. The titanomagnetite is lowtitanium, and it is rep� resented by xenomorphic grains with sideronitic tex� ture. In addition it forms finegrained, platy, den� dritic, and dropshaped inclusions in clinopyroxene. In general, titanomagnetite is distributed uniformly in the rock. However, there are sections with a banded distribution of ore minerals. In titanomagnetite there are inclusions of xenomorphic clinopyroxene and prismatic apatite. Sometimes, the ore mineral forms

Geology and geochronology of Monte de Trigo island alkaline suite, southeastern Brazil

The undersaturated syenite-gabbroid alkaline suite of Monte de Trigo Island lies in the northern sector of the Serra do Mar Alkaline Province in southeastern Brazil, and intrudes Precambrian gneissic and migmatitic rocks of the Ribeira Belt. It is made up by a cumulate mafic-ultramafic body, dominantly show- ing nepheline-bearing olivine melagabbros, melatheralites and clinopyroxenites, in association with synplu- tonic microtheralite and microessexite dykes, and by a miaskitic nepheline syenite stock genetically related to nepheline microsyenite dykes of miaskitic and agpaitic affinity. A magmatic breccia pipe having fragments derived from the mafic-ultramafic body and from the Precambrian country-rocks crops out in the eastern side of the island. The last magmatic alkaline event is represented by a series of lamprophyre-to-phonolite dykes. Geological evidences suggest a low emplacement level for the Monte de Trigo Island intrusion. Ar-Ar geo- chronological data for the different rock types yielded an average value of 86.6 Ma (Late Cretaceous), which is comparable to the age of other alkaline intrusions occurring in the northern sector of the Serra do Mar Prov- ince. The alkaline magmatism probably took place in a short time interval, lower than 0.5 Ma. Moreover, age incompatibility makes difficult to support the Trindade mantle plume model proposed for its origin. Thus, other factors such as pressure release and the influence of volatile phases in the source region might have contributed to the generation of the alkaline magmas.

PETROLOGY AND ORIGIN OF PLAGIOGRANITES FROM THE DAĞKÜPLÜ (ESKİŞEHİR) OPHIOLITE ALONG THE İZMİR-ANKARA-ERZİNCAN SUTURE ZONE, TURKEY

The Izmir-Ankara-Erzincan Suture Zone (IAESZ) formed through the closure of the northern branch of the Neotethyan Ocean that once separated the Sakarya Continent and the Anatolide-Tauride Platform and the subsequent collision of these two continental blocks during Late Cretaceous-Paleocene time. Mafic-ultramafic rocks representing oceanic lithosphere, metamorphic sole rocks, and ophiolitic melange comprising oceanic and continental fragments and continental margin metamorphic and sedimentary rocks are preserved along the IAESZ. A northward dipping subduction zone developed within the Izmir-Ankara-Erzincan Ocean, a northern branch of Neotethyan Ocean, in the Early Cretaceous. The suprasubduction zone (SSZ) ophiolites formed in this intra-oceanic setting. The Dagkuplu Ophiolite was, like other ophiolite nappes in the region, thrust over the southern Anatolide-Tauride Platform. The Dagkuplu Ophiolite, exhibiting an incomplete and inverted ophiolite suite, consists mostly of refractory peridotites (harzburgite and dunite) representing mantle unit and lesser dunite and wehrlite cumulates, pyroxenites, and massive-layered gabbros. The plagiogranite volumetrically constitutes about 10% of the entire ophiolitic rocks. Petrographic and geochemical data suggest that plagiogranites were generated from TiO2-poor cumulate gabbros by fractional crystallization at a suprasubduction zone spreading center.

PETROLOGY OF MANTLE XENOLITHS FROM MEGADO AND DILO; KENYA RIFT; SOUTHERN ETHIOPIA

A study has been made of ultramafic xenoliths embedded in the Quaternary basanitic lavas of Dilo and Megado areas; southern Ethiopia. This is the first time that xenoliths have been found at Dilo. In both areas; the xenoliths are spinel lherzolites and pyroxenites; garnet- and/or spinel-bearing pyroxenites occasionally containing amphibole. Temperature/pressure estimates indicate that the lherzolites and most of the pyroxenites last equilibrated in the upper mantle in similar P-T conditions (T ~ 950-1050°C at P ~ 1.3-1.7 GPa); whereas some spinel pyroxenites equilibrated at higher temperatures (~ 1150-1250°C). Based on mineral assemblage; four groups of pyroxenites were distinguished; two of which exhibit spectacular garnet coronas around spinels. Whole-rock and mineral composition (major and trace elements) indicates that garnet-free pyroxenites segregated from OIB-type melt. Successively; melt evolution caused garnet crystallisation (so that garnet bearing pyroxenites formed) and; eventually; pargasitic amphibole caused the generation of garnet- and amphibole- bearing pyroxenites with the consequent LILE-enrichment of residual melts. Melt evolution resembles that hypothesised by Bedini et al. (1997) to explain metasomatism of lherzolites based on reactive porous flow simulations. Thus; pyroxenites may represent the “counter-part” of the melts responsible of the metasomatism in this area. Furthermore; the OIB nature of parental permeating melts suggests that metasomatism (and pyroxenite genesis) was related to Quaternary magmatic activity responsible for the xenoliths ascent to the surface.

GEOCHEMISTRY OF GARNET PYROXENITE LAYERS FROM THE EXTERNAL LIGURIDE MANTLE PERIDOTITES (NORTHERN APENNINE, ITALY): EVIDENCE FOR RECYCLING OF CRUSTAL MATERIAL IN A OPHIOLITIC MANTLE SEQUENCE

The mantle peridotite bodies from the External Liguride ophiolites (Northern Apennine) preserve a subcontinental lithospheric origin and are locally characterised by the occurrence of garnet-bearing clinopyroxenite to websterite layers. Some of these layers preserve relics of a pristine garnet- bearing mineral assemblage, which yielded a thermobarometric equilibration at 1150-1200 °C and 2.6-3.0 GPa (Montanini et al., 2005). The garnet-bearing mantle section subsequently underwent a subsolidus decompression evolution to spinel- and plagioclase-facies conditions (see also Beccaluva et al., 1984). The garnet clinopyroxenite layers reach up to a couple of metres in thickness and may be subdivided into two groups. Group 1 clinopyroxenites have lower mg#, Cr and Ni contents than group 2 clinopyroxenites, and locally preserve accessory amounts of graphite. Group 1 clinopyroxenites display low SiO2/Al2O3 values and a REE pattern characterised by marked LREE depletion and weak positive Eu anomaly. Relict clinopyroxenes from these clinopyroxenites show a bell-shaped REE pattern, with a slight positive Eu anomaly. A weak positive Eu anomaly is also observed for associated garnets, which are characterised by a marked enrichment of MREE and HREE over LREE. Group 2 clinopyroxenites, as well as included clinopyroxenes, show variable LREE fractionation and positive Eu anomaly. The websterite layers are up to 8 cm in thickness. This rock type has been also found along the contacts between Group 2 clinopyroxenites and peridotites. Relative to the clinopyroxenites, websterites differ in the higher mg# (similar to those of the peridotites), Cr and Ni, and in the lower Al2O3, CaO and Na2O. The REE pattern of websterites is characterised by LREE depletion and lack of significant Eu anomaly. The peridotites enclosing the pyroxenite layers are plagioclase- bearing mylonites with relics of a spinel-facies, Tirich amphibole-bearing tectonitic assemblage. These mantle rocks have a fertile geochemical signature, as shown by the analyses of relict spinel-facies minerals. For instance, spinel cores and porphyroclastic clinopyroxenes have low cr# values and rather high Al2O3, Na2O and LREE contents, respectively. Other peridotite bodies from the External Liguride ophiolites locally retain similar compositional features (Rampone et al., 1995; Piccardo et al., 2004). Major and trace element compositions indicate that the garnet clinopyroxenites were derived from plagioclase-rich mafic cumulates. We also propose that at least Group 1 clinopyroxenites underwent an event of partial melting, in the garnet stability field. The websterites likely record interaction processes with the enclosing peridotites which partially obscures their origin. Two major hypotheses may be formulated for their petrogenesis, i.e. recycling of crustal protoliths variably modified by partial melting and/or diffusive mixing with the peridotite, or infiltration of partial melts derived from the nearby clinopyroxenites. Garnet and clinopyroxene separates from the pyroxenite layers have provided oxygen isotope values close to typical mantle. On the other hand, Sm-Nd isotope data argue against an origin from typical MORB-type cumulates, thus implying that the garnet pyroxenite layers from the External Liguride ophiolites do not simply represent oceanic crust recycled through an ancient subduction.

THE EXHUMATION OF THE ERRO-TOBBIO MANTLE PERIDOTITES (LIGURIAN ALPS, ITALY) AS RECORDED BY MULTIPLE MELT INTRUSION EVENTS

THE EXHUMATION OF THE ERRO-TOBBIO MANTLE PERIDOTITES (LIGURIAN ALPS, ITALY) AS RECORDED BY MULTIPLE MELT INTRUSION EVENTS

SIGNIFICANCE OF PYROXENITES MANTLE XENOLITHS FROM THE PERSANI MOUNTAINS (EASTERN TRANSYLVANIAN BASIN) BASED ON PETROLOGIC AND GEOCHEMICAL STUDY

The Persani Mts. in the Eastern Transylvanian Basin is the easternmost Plio-Pleistocene alkali-basaltic volcanic field in the Carpathian–Pannonian Region. The products of the volcanic activity are lava flows and pyroclastic rocks, in which peridotites as xenoliths from the upper mantle can often be found. Besides peridotite xenoliths, pyroxenites are also quite common. The mineralogical composition of these pyroxenite xenoliths is mostly clinopyroxene, olivine, garnet, spinel ± orthopyroxene and amphibole. According to their modal composition, they are olivine clinopyroxenites, clinopyroxenites and websterites. We classified the pyroxenites into spinel-bearing pyroxenites (representing type I/I and type II/I series) and garnet-bearing pyroxenites. In the type I/I series clinopyroxene is diopsidic whereas in type II/I series its composition is augitic. In spinel pyroxenites, besides clinopyroxene, olivine is the second common mineral in some cases accompanied by orthopyroxene and amphibole. In garnet-bearing pyroxenites olivine and amphibole are absent, whereas orthopyroxene can often be observed. Textural observations suggest deformation events and mineral reactions, the results of changes in stress and p-T conditions during the evolution of the mantle rocks. Both clinopyroxene and orthopyroxene occur in each other as exsolution lamellae, which in some cases are curved. Garnet often contains exsolved needles of rutile and is always surrounded by symplectitic intergrowth of secondary orthopyroxene, clinopyroxene, spinel and plagioclase. Thermobarometry was based upon EMP analyses of the “primary” rock forming minerals. Equilibrium pressure values estimated from single clinopyroxene compositions are between 1,1 and 1,4 GPa, whereas garnet orthopyroxene barometry yielded 1,3-1,6 GPa. Orthopyroxene-clinopyroxene equilibrium temperatures are in the range 1070-1140°C, temperatures obtained from garnet-clinopyroxene pairs are between 1030-1170°C (at 1,1-1,3 GPa). Thermobarometric calculations were also made on the symplectite-forming minerals. The clinopyroxene barometer yielded 1,0-1,3 GPa for the symplectite-formation. Temperature values calculated with the compositional data of symplectite-forming clinopyroxene-orthopyroxene are 1070-1120°C. The variation in lithology points to the heterogeneity of the upper mantle. Based on the textural relationships and the thermobarometric results, the evolution of the xenoliths can be outlined: the deformation and pressure decrease (upwelling) in the lithospheric mantle before alkaline basaltic volcanism. The inferred p-T-path of the Persani Mts. pyroxenites fits well previously studied garnet peridotites. We used geochemical data to determine the equilibrium conditions of the xenolith formation, and the p-T path along with the degree of the mantle upwelling in the Eastern- Carpathian region is estimated by thermobarometric calculations, too.

PYROXENITES AND MEGACRYSTS IN ALKALINE BASALTIC MAGMAS FROM NORTHERN VICTORIA LAND (ANTARCTICA): CONSTRAINT ON THE THERMAL EVOLUTION OF SUB-CONTINENTAL LITHOSPHERE

Cenozoic alkaline basaltic magmas of northern Victoria Land, Antarctica yield a wide variety of ultramafic xenoliths including clinopyroxene-rich xenoliths and augite and kaersutite megacrysts. Pyroxenites and megacrysts were collected in Mt Melbourne Volcanic Province, respectively at Browning Pass, Baker Rocks (both sites are near the coast of the Ross Sea) and at the base of Mt. Overlord, about 80 km inland. Sixteen samples have been petrographycally examined and chemically analyzed, including trace element analyses of minerals by ion microprobe. The dominant assemblage of pyroxenites consists of clinopyroxene + olivine ± spinel ± amphibole with strongly variable modal proportions of minerals. Rare interstitial plagioclase is found in Browning Pass samples. All pyroxenites belong to Al-augite Group (Frey and Prinz, 1978); the collection includes wehrlites, clinopyroxenites and rare dunites and olivinewebsterite. Moreover, Baker Rocks xenoliths are sometimes composite showing wehrlite and depleted lherzolite lithologies in sharp contact. Their typical texture is cumulitic and in only few cases deformation fabrics and polygonal texture may be found. Mt Overlord samples show widespread metasomatic effects, evidenced by clinopyroxene-amphibole replacement and by variably re-crystallized melt pools. In the Browning Pass pyroxenites the metasomatic effects are incipient as evidenced by rare amphibole lamellae in substitution of the clinopyroxene. In few samples from Baker Rocks and Mt. Overlord clinopyroxenes show orthopyroxene exsolutions. Major and trace element compositions of bulk rock and minerals indicate that pyroxenites and megacrysts are related to Cenozoic alkaline magmas and that formed through polybaric fractionation processes affecting McMurdo alkaline magmas. We evaluated the P and T of segregation of clinopyroxenes from parent basaltic melts. The obtained data depict a P-T gradient of 15°C/kbar for the whole area in contrast with widespread volcanic activity. If we hypothesize that pyroxenite veins represent the walls of the paths followed by adiabatically rising basaltic melts, we can assess the temperature of the ambient mantle, if we know the mantle potential temperature (Tp) at some depth. From the onset of crystallization in primitive melts we get Tp =1300-1350°C at P= 2- 2.5GPa. In this way we obtain, for the present day geothermal gradient of the area, a more realistic value of 30°C/kbar. This value is considerably higher than the 8°C/kbar obtained from the equilibration coarse crystals of lherzolite nodules in the same area and allows to clearly identify the heating of the mantle due to Cenozoic magmatism. The different thermal profile obtained from the pyroxenites with respect to those obtained by spinel-peridotite xenoliths, thus seems to be linked to the geodynamic evolution in this area. In particular the computed geothermal gradient fits with the heat-flow values of this area that range between 66 e 114 mW/m2 (Blackman et al., 1987; Della Vedova et al., 1992). The gradient that we propose seems to be compatible with a static rift geotherm of 90mW/m2 . See figure.

SLAB-RELA TED MANTLE-METASOMATISM: A SIMS INVESTIGATION OF LLE COMPOSITION OF FINERO PHLOGOPITE-PERIDOTITE (W ALPS)

The Finero mafic-ultramafic complex (Ivrea-Verbano Zone, Western Italian Alps) offers a unique opportunity to study km-scale hydrous melt migration on a outcropping mantle body. The ultramafic mantle section is mainly formed by spinel harzburgites, which contain hydrous mineral assemblages (amphibole + phlogopite ± apatite) and show LILE (Cs, Rb, Ba, K, Sr, LREE) enrichments. Moreover, these peridotites are characterised by large radiogenic Sr and Pb and low radiogenic Nd (Voshage et al., 1987; Cummings et al., 1987; Hartmann and Wedepohl, 1993). Such petrochemical features have been supposed to be related to pervasive migration of slab-derived melts through a supra-subduction mantle wedge (e.g. Zanetti et al. 1999; Prouteau et al., 2001; Morishita et al., 2003). Alternative hypotheses involve the upward migration of different kinds of alkaline melts (from K-rich differentiates to carbonatites), in the frame of variable geodynamic environments (Grieco et al., 2001, 2004; Garuti et al., 2001; Zaccarini et al, 2004). A new SIMS investigation has been accomplished on clinopyroxenes (Cpx) from the mantle sequence of the Finero peridotite, in order to place further constraints on the nature of the ascending melt(s) and on the genetic and temporal relationships occurring among the different ultramafic units so far recognised. Micro-analytical characterisation of selected LILE, HFSE and REE was combined with the insitu determination of Li and B concentration (Light Lithophile Elements: LLE), because the fractionation of these elements represents a valuable geochemical marker about the presence of slab-derived components in uprising magmas (Ottolini et al., 2004). Five spinel-facies samples, namely one phlogopite pyroxenite, two apatite-free phlogopite harzburgites, one apatite- bearing phlogopite wehrlite and one chromitite, representative of the main Finero ultramafics were selected for this study. Pyroxenite is a pegmatitic websterite, in which clinopyroxene (the dominant phase), amphibole and orthopyroxene are up to 16, 13 and 6 mm long, respectively. Phlogopite is also present with crystals up to 3 cm long. Minerals do not show deformation. Amphibole and phlogopite normally occur as interstitial phases. Unlike pyroxenite, the peridotites show amphibole modal concentration exceeding that of clinopyroxene. The apatitefree phlogopite harzburgites (the dominant mantle lithology) have porphyroclastic textures. Olivines, and sometimes orthopyroxenes, show evidence of ductile deformation, which is absent in most of clinopyroxenes and in the hydrous minerals. In the apatite-bearing wehrlites, orthopyroxene is lacking or occurs only in trace amounts. They have a mosaic equigranular texture, consisting of crystals of equant dimension often with 120° triple junctions. Modal abundance of amphibole and phlogopite increases up to 25 and 4% by volume, respectively. Apatite occurs as interstitial grains, sometimes associated with dolomite, and within fractures cross-cutting large olivine. Dolomite is also present as inclusion in spinel. Dunite-chromitite bands have secondary coarse granular texture, modally dominated by very large olivine and spinel grains. Clinopyroxene and amphibole are randomly present as strongly anhedral interstitial phases. Locally, phlogopiterich layers occur. The B content is 3.9-9.7 ppm and 1.3-1.6 ppm in Cpx from phlogopite harzburgites and pegmatitic websterites, respectively. Such concentrations are rather large for mantle minerals and lie in the range of the lower continental crust (Leeman and Sisson, 1996). Conversely, Cpx from apatite-bearing wehrlite and dunite-chromitite bands have low B concentrations (~0.8 and ~0.6 ppm, respectively), which are consistent with those of the upper mantle (<1 ppm) (Chaussidon and Jambon, 1994). Li contents show a narrow range. However, Cpx from phlogopite harzburgites have lower Li concentrations (0.5- 1.1 ppm) than pegmatitic websterite (1.5-1.9 ppm) and apatite- bearing peridotite (1.6-1.7 ppm). The highest Li contents occur in Cpx from dunite-chromitite bands (2.0-2.4 ppm). Clinopyroxenes from websterite and phlogopite harzburgites are similar, being enriched in LILE and depleted in moderately incompatible trace elements (e.g. HREE; LaN/YbN = 15-25, YbN = 0.97-1.82 xC1). Differently, Cpx from apatite-bearing wehrlite are characterised by: i) relatively low LREE/HREE fractionation (LaN/YbN = 2, with YbN = 25 xC1); ii) marked negative HFSE anomalies. The combination of the LLE data on Cpx with the other trace elements relevant for geochemical modelling indicates the presence in the Finero mantle sequence of contrasting metasomatic overprints. In particular, the large B/Li ratios and B concentration shown by Cpx from phlogopite harzburgites and websterite confirm that they formed as the result of the migration of a melt having a crust-derived component. By contrast, the occurrence of crustal contributions is not apparent on the basis of the LLE compositions of the Cpx from apatite-bearing wehrlite and chromitite, which are more consistent with a derivation from basaltic melts of upper mantle origin. Further geochemical investigations are in progress to assure whether such different trace element compositions are actually due to the migration of several melts or can be the product of complex differentiation processes affecting a unique melt with crustal component.

STRATIFORM CHROMITITES IN THE FARYAB OPHIOLITIC MASSIF, SOUTHERN IRAN

The Faryab ophiolitic massif, located in the extreme southernmost Iran, contains stratiform chromites in the ultramafic portion of its cumulate sequence. The stratiform chromites alternate with thick dunitic layers. Their Cr2O3 content varies between 42 and 44% and shows lower values compared to the disseminated chromites of the dunitic layers. Pyroxenites, websterites and wherlites cumulate layers alternate with dunites. Mg# of orthopyroxenes in websterites is higher than those in pyroxenites. Chromites in pyroxenites are relatively rich in Al2O3. The forsterite content of associated olivines varies between 88.5 and 90.5. The temperatures calculated from the chromite-olivine thermometer are very low, and vary from 654 to 710°C and corresponding probably to subsolidus reactions.

THE REGIONAL VARIATION IN THE MANTLE ROCKS OF THE NORTHERN UAE-OMAN OPHIOLITE

THE REGIONAL VARIATION IN THE MANTLE ROCKS OF THE NORTHERN UAE-OMAN OPHIOLITE

INSIGHTS INTO THE ORIGIN OF MANTLE GRAPHITE FROM A NEW OCCURENCE IN GARNET PYROXENITES FROM THE EXTERNAL LIGURIDE PERIDOTITES (NORTHERN APENNINE, ITALY)

The occurrence of elemental carbon in the mantle, either as diamond or, more rarely as graphite, is almost uniquely restricted to peridotite and eclogite xenoliths derived from cratonic lithospheric mantle. In non-cratonic areas, the occurrence of graphite-bearing garnet pyroxenites has been only reported for mantle bodies exhumed in orogenic settings, namely the Ronda (Davies et al., 1993) and Beni Bousera (Pearson et al., 1993) massifs, where the graphite may occur as diamond pseudomorphs and was interpreted as originated from crustally-derived, biogenic carbon recycled into mantle through subduction on the basis of 13C-depleted isotopic composition. Mantle peridotites of the External Liguride units represent slices of subcontinental lithospheric mantle exhumed during the rifting phases leading to the Jurassic Ligurian Tethys formation. These mantle rocks locally enclose up to meter-sized layers of graphite-bearing garnet pyroxenites. The high-pressure protoliths were characterised by coarsegrained isotropic texture and anhydrous bimineralic assemblage, consisting of pyrope-rich garnet (Prp53-45Alm29- 38Grs11-20) + sodic Al-augite (Na2O ~2.5 wt%, Al2O3 ~ 12.5 wt%), with accessory amounts of Fe-Ni sulphides and rutile. Geothermobarometric estimates for the high-pressure equilibration have yielded T = 1150-1200 °C and P = 2.6-3.0 GPa. Graphite (up to 3 vol%) occurs as flakes and stacks of flakes with grain size up to 2-3 mm. No pseudomorphs with octahedral or cubic simmetry suggesting the former ocurrence of diamond have been observed, in agreement with pressure estimates. The enclosing peridotites do not contain any vein or disseminated graphite. Sulphides (mainly pyrrhotine and pentlandite derived from high-T monosulphide solid solutions) occur as tiny rounded blebs into garnet and clinopyroxene or as interstitial crystals; their abundance may locally reach up to ~ 5 vol % . Graphite crystals were analysed by microRaman spectrometry on fresh fracture surfaces. Well defined first (O-) and second order (S-) peaks, and the absence of first order D-peaks indicate a highly ordered structure similar to that of graphite crystallised at high temperature in peridotite and eclogite xenoliths (Pearson et al., 1994). C isotope composition (d13C= - 4.5) reflects a typical mantle signature. Whole-rock and mineral trace element evidence indicate that the graphite-bearing garnet pyroxenites have suffered some partial melting (Tribuzio et al., this meeting). The observed sulphides may have originated from trapped (immiscible) sulphide melt. We propose that graphite precipitation was possibly related to carbon saturation reached in these sulphide liquids or to carbon nucleation on a sulphide medium, in agreement with experimental evidence and common diamond-sulphide association in higher pressure peridotite and eclogite xenoliths (Bulanova, 2002; Bulanova et al., 1998; Haggerty, 1986; Jacob et al., 2004; Litvin et al., 2002).

EXHUMATION HISTORY OF SUBCONTINENTAL GARNET PYROXENITE-BEARING MANTLE FROM THE EXTERNAL LIGURIDE OPHIOLITES (NORTHERN APENNINE, ITALY): IMPLICATIONS FOR RIFTING AND MANTLE EXHUMATION PROCESSES AT OCEAN-CONTINENT TRANSITION

The External Liguride Jurassic ophiolites derive from an ocean-continent transition similar to the West Iberian margin. The mantle sequence consists of spinel and spinel-plagioclase lherzolites recording a lithospheric thinning history culminated into mantle exhumation at the ocean-floor. They represent unroofed subcontinental litosphere of the former Adria-Europe system largely affected by thermochemical erosion, heating and refertilization by asthenospheric melts during the Ligurian Tethys formation (Rampone et al., 1995; Piccardo et al., 2004). Peridotite bodies which escaped melt contamination and intrusion preserve relics of a deep-seated litospheric origin, i.e. occurrence of graphitebearing garnet clinopyroxenite and websterite layers. The garnet pyroxenite-bearing mantle from the External Liguride ophiolites therefore represents a unique tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. Garnet clinopyroxenites are mafic rocks with a primary assemblage of pyrope-rich garnet + sodic Al-augite (Na2O ~2.5 wt%, Al2O3 ~12.5 wt%), with minor amounts of orthopyroxene, graphite, Fe-Ni sulphides and rutile. Decompression yielded Na-rich plagioclase (An50-45) exsolutions in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene + plagioclase (An85-72) + Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine+ plagioclase-bearing assemblage, locally under synkinematic conditions, at the expenses of two-pyroxene + Alspinel. Mg-rich garnet has been also found in websterite layers, which are commonly characterised by the occurrence of symplectites made of orthopyroxene + Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu-Hf and Sm-Nd mineral isochrons (220 ± 13 Ma and 186 ± 1.8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T = 1150-1200 °C and P = 2.6-3.0 GPa. The early decompression was associated with significant cooling, corresponding to T = 900-1050°C, and development of the spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase-olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution was characterized by a widespread polyphase brittle deformation under decreasing temperature conditions, coupled with hydration. In particular, the brittle evolution includes an early amphibolite-facies veining stage, followed by low-temperature hydrothermal alteration and serpentinisation associated with polyphase cataclasis. The exhumation was likely accomplished as a two-step process starting during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, possibly including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence was involved into an extensional detachment structure up to sea-floor exhumation, which was associated with development of serpentinisation-related hydrothermal activity. Integrated values of cooling and uplift rates for the Jurassic rifting evolution are ~35°C/Ma and 0.6-1.4 mm yr-1.

UHP GARNET ORTHOPYROXENITES (DABIE SHAN, CHINA) AS MONITORS OF PERCOLATION OF HYDROUS SI-RICH MELTS IN DEEP SUBDUCTION ENVIRONMENTS

UHP GARNET ORTHOPYROXENITES (DABIE SHAN, CHINA) AS MONITORS OF PERCOLATION OF HYDROUS SI-RICH MELTS IN DEEP SUBDUCTION ENVIRONMENTS