Edenitic Hornblende Research Articles

Late Neoproterozoic metamorphic assemblages along the Pan-African Hamisana Shear Zone, southeastern Egypt: Metamorphism, geochemistry and petrogenesis

A variety of Late Neoproterozoic gneisses and amphibolites are distributed along the N–S trending Hamisana Shear Zone (HSZ), in southeastern Egypt. The HSZ originated after the accretion of the Arabian–Nubian Shield (ANS) and covers an area of about 1500km2 in southeastern Egypt and northeastern Sudan. The architecture of the northern part of the HSZ is best explained as a tectono-stratigraphic column, in which allochthonous ophiolitic mélange was thrusted onto metamorphosed island-arc assemblages (gneisses and amphibolites). The latter rock units were generally subjected to two successive phases of amphibolite facies metamorphism, followed by a thermal phase and retrograde overprint. The early penetrative, low- to medium-pressure metamorphism (M1) was synchronous with D1-gneissosity and N–S trending lineation, demarcating the high strain HSZ. The mineral assemblages formed during the M1 phase include quartz+andesine+hornblende (I)+biotite (I) in hornblende–biotite gneiss, quartz+andesine+pargasitic hornblende (I)+ferroan pargasitic hornblende (I)+edenitic hornblende (I) in hornblende-schist, quartz+plagioclase+biotite+muscovite in psammopelitic gneiss, and diopside+tremolite+calcite+sphene±garnet in calc-silicates, being characteristic for amphibolite facies with metamorphic conditions of 600±50°C and 5–6.5kbar. The second metamorphic phase (M2) is related to the crystallization of biotite and/or hornblende in S2 foliation demarcating the NE–SW trending dextral shear deformation (D2). The calculated temperature for this M2 phase is about 592°C. Subsequent thermal events are documented by growth of spinel and scapolite in calc-silicate rocks and of cordierite in psammopelitic gneiss in response to uplift, decomposition and heat provided by the nearby late-formed igneous intrusions. Finally, the rocks reached a temperature of about 530°C during the cooling retrogressive stage. Based on geological, petrological and geochemical investigations, the island arc assemblages are grouped into: (a) meta-igneous rocks (hornblende–biotite gneiss, biotite gneiss and amphibolites) and (b) metasedimentary rocks (psammopelitic gneiss, hornblende-schist and calc-silicates). Geochemical inspection revealed the non-consanguineous nature of these rock units. They represent subduction-related, theoleiitic and calc-alkaline magmatic rocks and their concomitant sedimentary derivations as well as minor continental shelf calcareous sediments. In terms of maturity, the geochemical signatures of these subduction-related rocks point to an immature volcanic arc origin.

Li, B and Be Contents of Harzburgites from the Dramala Complex (Pindos Ophiolite, Greece): Evidence for a MOR-type Mantle in a Supra-subduction Zone Environment

The Pindos ophiolite represents oceanic lithosphere obducted during theJurassic.The Dramala mantle section mainly consists of highly depleted spinel harzburgite and minor plagioclase-bearing harzburgite. Textural observations and major element compositions of minerals indicate that the harzburgites experienced impregnation by a mafic, depleted melt and subsequent high-temperature (high-T) hydration and cooling (47508C) forming pargasite and edenitic hornblende. During further cooling (from 350^4008C to 51008C), talcþ tremolite serpentine olivine, serpentineþ magnetite, and finally plagioclase alteration phases formed.To test the hypothesis of a supra-subduction zone origin for the Dramala mantle, we measured Li, B and Be contents of minerals by secondary ion mass spectrometry. Whole-rock contents were measured using inductively coupled plasma^mass spectrometry and prompt gamma neutron activation analysis. We observe low Li and B contents of primary minerals (olivine, orthopyroxene, clinopyroxene) consistent with values for unmetasomatized mantle minerals; only Li contents of clinopyroxene (up to 3 7 g/g) are slightly elevated. The bulk Li contents (0 5^1 1 g/g) are in the upper range of values for unmetasomatized mantle, whereas B contents (50 04^1 1 g/g) are variable and slightly elevated compared with the unmetasomatized mantle as a result of serpentinization. Beryllium abundances in all minerals are very low (50 005 g/g), except for pargasite, where a maximum Be content of 0 012 g/g was measured. The selective addition of Li to clinopyroxene can be related to the interaction with a depleted melt, and/or to partitioning of Li into clinopyroxene upon cooling. During high-T hydration and cooling, the fluid calculated to be in equilibrium with the pargasite or edenitic hornblende (based on Li, Be and B) could have been reactionmodified seawater. Low-T hydration may have led to a very minor increase in bulk B content of most samples and to the formation of serpentine with highly variable B contents (0 1^28 g/g). Low-T hydration decreased the Li content of orthopyroxene, and Li was probably leached from some samples.The lack of correlation between degree of serpentinization and bulk B contents as well as the presence of highand low-B serpentine can be explained by low fluid^rock ratios, decreasingTduring serpentinization and lack of equilibrium as a result of fast obduction^exhumation. The low light-element contents of primary minerals and whole-rock samples clearly argue against a supra-subduction zone (SSZ) origin of the Dramala mantle section, and against the previous hypothesis of hydrous melting of the Pindos mantle above a subduction zone.We therefore conclude that the Dramala harzburgites represent a mid-ocean ridge (MOR)-type mantle, and not an SSZ-type mantle, juxtaposed with MOR-type and SSZ-type oceanic crust, either in a back-arc or in an intra-oceanic subduction zone setting.

Petrology of forearc peridotite from the Hahajima Seamount, the Izu-Bonin arc, with special reference to chemical characteristics of chromian spinel

AbstractForearc peridotite is generally characterized by low Mg# (= Mg/[Mg + Fe2+] atomic ratio) at a given Cr# (= Cr/[Cr + Al] atomic ratio) of chromian spinel compared to common abyssal peridotite. This may be due to (1) smaller modal abundance of spinel and/or (2) lower equilibrium temperature, for the forearc peridotite. Forearc peridotite has the same amount of spinel as abyssal peridotite, eliminating the first possibility. Spinel in harzburgite and dunite from the Hahajima Seamount at the Izu-Bonin forearc, has a large Cr#, >0.57, and the Mg# is slightly variable towards low values at a given Cr#. The Mg# of spinel cores decreases strongly with a decrease in size. This is due to cooling along with hydration, which gave rise to a compositional variation of Ca-amphibole, from edenitic hornblende (high-temperature) to tremolite (low-temperature) in the Hahajima peridotite. The average two- pyroxene temperature of the Hahajima peridotite, 921°C, is less than that of the abyssal peridotite (1138°C), which is not consistent with a size-dependent Mg# of spinel for the latter. Forearc peridotite has been cooled effectively by H2O released from the subducted slab, causing a small Mg# of their spinels.

Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex, South Africa

About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions. Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex. Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different. The Mg/(Mg+Fe2+) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO (26 wt%), Na2O (2.4 wt%) and H2O (2.6 wt%), and low CaO (1.1 wt%) and FeO (4.4 wt%). The δ18O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ∼7‰. This value is consistent with the previous estimates for the Bushveld magma and with the δ18O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.

TEXTURAL AND MINERALOGICAL CONSTRAINTS ON CHROMITITE GENESIS: EVIDENCES FROM THE CUMULATE SEQUENCE OF NURALI MASSIF (SOUTHERN URALS, RUSSIA)

TEXTURAL AND MINERALOGICAL CONSTRAINTS ON CHROMITITE GENESIS: EVIDENCES FROM THE CUMULATE SEQUENCE OF NURALI MASSIF (SOUTHERN URALS, RUSSIA)

Petrology and geochemistry of shoshonitic plutons from the western Kunlun orogenic belt, Xinjiang, northwestern China: implications for granitoid geneses

A series of granitoids from Proterozoic to Cenozoic age occurred in the western Kunlun orogenic belt, Xinjiang, northwestern China. Several intrusions such as the West Datong (Middle Caledonian age), North Kuda (Late Caledonian age) and Kuzigan, Karibasheng, Zankan (Himalayan age) plutons have shoshonitic affinity. Their rock assemblages include (quartz) monzodiorite–(quartz) monzonite–quartz syenite (Middle Caledonian) or monzonitic granite–granite (Late Caledonian) or biotite (monzonitic) granite–diopside granite–diopside syenite (Himalayan). Generally, biotite is iron–phlogopite, with some eastonite and high Mg/(Mg+FeT) and Fe3+/Fe2+ ratio. Amphibole is mainly edenitic hornblende and magnesian hastingsitic hornblende, with some edenite and higher Mg/(Mg+FeT) and Fe3+/Fe2+ ratio. The rocks show SiO2 contents of 52.77–71.85% and high K2O+Na2O (mostly >8%, average 9.14%), K2O/Na2O (mostly >1, average 1.50) and Fe2O3/FeO (0.85–1.51, average 1.01) and low TiO2 contents (0.15–1.12%, average 0.57%). Al2O3 contents (13.01–19.20%) are high but variable. The granitoids are prominently enriched in LILE, LREE and volatiles such as F. However, the studied shoshonitic granitoids among the three intrusive periods also show differences in isotopic compositions and trace element concentrations, suggesting their different geneses: the origin of the West Datong pluton is probably related to the involvement of subducted oceanic crust sediments into the mantle source; the North Kuda and Himalayan plutons could have been generated by partial melting of subducted oceanic crust sediments or metasediments of thickened continental lower crust in the process of late-orogenic slab break-off or lithospheric thinning.

PHLOGOPITE AND HORNBLENDE IN SPINEL HARZBURGITES FROM THE MID-ATLANTIC RIDGE: MINERAL ASSEMBLAGES AND ORIGIN

Phlogopite in mid-oceanic ridge peridotites was detected mainly as inclusions in spinel and rarely as discrete grains from ultramafic cumulates (Arai et al., 1997). Phlogopite and biotite were also found in oceanic gabbro (Cannat et al., 1992; Cannat and Casey, 1995; Silantyev, 1998). In most cases, hornblende is associated with phlogopite. Origin of these minerals in oceanic peridotites and gabbro is now under discussion. In our investigations, phlogopites were detected in spinel harzburgites from a number of localities within the Mid-Atlantic Ridge - at 15oN and 35oN areas. The compositional variety of phlogopites is maximal in harzburgites containing thin gabbroic veins from dredge 68 (16-th cruise of r/v “Akademic Boris Petrov”) located in the west slope of the rift valley at 14o49’N. Since associated minerals are often preserved in these rocks, it is possible to address the problem of the origin of this association. In gabbro veins and of their contact with peridotite, phlogopite forms tables with clear pleochroism. Phlogopite is characterized by high TiO2 (1.5-3.0 wt%), low Mg# (65-85) and low Cr2O3 (0.0-0.65 wt%). Rare small grains of high-Ti spinel (1.1 wt% TiO2) are present at its rims. In peridotites, phlogopite occurs as small (< 0.1 mm) plates in narrow shear zones near the contact with gabbros. Phlogopite is usually colorless or with hardly recognizable pleochroism; it is Ti-rich (2.0-2.3 wt% TiO2), relatively Crpoor (0.1-0.7 wt% Cr2O3), and Mg-rich (Mg# 93.2-93.7). Orthopyroxene and hornblende are associated with phlogopite, both having elevated TiO2 contents (1.3-2.1 and 0.14- 0.23 wt%, respectively). Ti-poor phlogopite (0.3-0.6 wt% TiO2) has been found in a similar textural position. Ti-poor phlogopite has the same ranges of Cr2O3 contents and Mg#, and it is associated with Ti-poor hornblende and orthopyroxene (TiO2 = 0.4-0.7 and 0.02-0.05 wt%, respectively), and sometimes with plagioclase. Ti-rich spinel (TiO2 up to 0.37 wt%) seems to take part in these associations. Phlogopite has also been found in harzburgites that do not show relations with gabbros. Phlogopite occurs either as discrete grains (0.1-0.4 mm) along shear zones, or single grains in olivine, or in contact with spinel, or as plates replacing orthopyroxene along cleavage. It shows low TiO2 (0.04-0.4 wt%), high Mg# (90.1-94.7) and high Cr2O3 (0.65-1.6 wt%) content. No silicates associated with phlogopite are present, and associated spinels are enriched in iron and sometimes in titanium (up to 0.27 wt% TiO2). The orthopyroxenes associated with phlogopite and hornblende are commonly characterized by low Al2O3, Cr2O3 and CaO (1.0-1.8 wt%, 0.18-0.70 wt% and 0.35-0.80 wt%, respectively). Hornblende has high K2O (0.22-0.50 wt%) and is represented by edenitic and pargasitic hornblendes according to Leake’s nomenclature. In harzburgites, clinopyroxene usually is not in contact with phlogopite and rarely with hornblende. According to Wells’s geothermometer, the crystallization of the phlogopite-hornblende mineral association took place at 870-1000oC. Distribution of Ti between coexisting phases is consistent with evaluated coefficients (Ionov et al., 1997) and indicates TiO2 liquid contents in the range of 0.56-1.40 wt% for the Ti-rich mineral association. Therefore, its crystallization possibly took place from fractionated melts. Origin of the low-Ti association is disputable. On the basis of mineral compositions, the low-Ti association is inferred to have crystallized at the same temperature range. The two minerals associations can be detected at one millimeter scale. The preferred hypothesis is crystallization of the low-Ti mineral association from a fluid phase, which was exsolved from the liquid. Alternatively, crystallization derived from extremely low-Ti melts (below 0.24 wt% TiO2). However, some phlogopites show TiO2 contents as low as 0.10 wt%, which lead to unrealistically low calculated TiO2 contents in the liquid (0.04 wt%). Crystallization of the phlogopite-hornblende mineral association in spinel harzburgites from Capo Verde FZ area took place after solidification of peridotites, mainly along shear zones, which was a place for segregation of residual differentiated melts and related fluids. In other places of Mid-Ocean Ridges, a shearing event under the same temperature conditions is associated with the development of plagioclase-bearing mineral associations, where only in rare cases edenitic-pargasitic hornblende occurs (Cannat and Seyler, 1995). This difference is related to the compositions of fractionated melts that interacted with the mantle peridotites, so that generation of phlogopite is restricted to areas of geochemical anomalies along Mid-Atlantic Ridge. Some gap between two areas of water silicate generation in mantle peridotites under MOR seems to exist. In a first one, outlined above, the source of water should be of juvenile origin. The second area is connected with deep penetration of seawater into the lithosphere. In the latter case, the temperature hardly exceeds 800-820oC, phlogopite does not crystallize, hornblende is represented mostly by tremolite, rarely by edenite or edenitic hornblende, and never by pargasitic hornblende. Recrystallized orthopyroxene in this process is characterized by extra-low calcium contents (CaO = 0.15-0.20 wt%). This work was supported by the Russian Foundation for Basic Research grant 97-05-64842.

The metamorphism and migmatization of the Xanthe-Echinos metamorphic complex, Central Rhodope, Greece

The Xanthe-Echinos metamorphic complex in the central Hellenic Rhodope records early Tertiary convergent tectonics on the Eurasian continental margin of Neotethys. It consists principally of layered gneiss and migmatite with minor amphibolite and calc-silicate rocks. Three metamorphic events are recognised: a high-pressure eclogite event (?Cretaceous); medium-pressure, Barrovian-type metamorphism (Eocene); and later low-pressure, greenschist facies metamorphism. Petrographic composition and electron microprobe analysis of minerals in forty four samples of gneiss and migmatite shows that the Eocene Barrovian event is characterized by replacement of clinopyroxene by pargasitic hornblende in clinopyroxene-garnet and clinopyroxene-edenitic hornblende assemblages. The compositional variation and textures of the amphiboles suggest initial growth of edenitic hornblende zoned to hornblende and development of syn- to late-tectonic pargasitic hornblende under higher temperature conditions associated with migmatization. The textures, zoning and paragenetic relationships of sodic-calcic amphiboles are consistent with uplift accompanied by synmetamorphic deformation. Partial melting in the migmatites involved plagioclase+K-feldspar+quartz. The solidification products of the produced melt are present in the form of quartz monzonitic leucosome layers and is also represented in syntectonic plutons. Metamorphic pressures of 7.5–8.5 kbar (at T = 600° C) were estimated from garnet-clinopyroxene pairs in gneisses adjacent to the migmatites. Amphibole-plagioclase thermometry of migmatite samples yielded temperatures of 728°–746°C (at P = 8.5 kbar). The activity of H 2O was probably less than 1.0 in the partially melted rocks. The geothermobarometric results indicate an increase of the geothermal gradient during uplift, probably related to intrusion of basic magma into the crust and/or changes in geothermal gradient associated with crustal thickening.

Field occurrences and petrology of eclogites from the Dabie Mountains, Anhui, central China

Four distinct types of eclogites are recognized according to their field occurrences and mineral parageneses in a gneiss terrane of the Dabie Mountains, a collision zone between the Sino-Korean and Yangtze cratons in central China. Some eclogites contain coesite and its quartz pseudomorphs enclosed in garnet and omphacite. Type I eclogites occur as layers in serpentinites and contain garnet, clinopyroxene, orthopyroxene, phengite, rutile, and coesite pseudomorph. Type II eclogites occur as lenticular bodies inside serpentinites and contain garnet, clinopyroxene, quartz, rutile, and edenitic hornblende. Type III eclogites occur as blocks of 2 cm to 20 m in size in a matrix of hornblende gneiss and biotite gneiss, and Type IV eclogites occur as thin layers interbedded with amphibolites. P- T estimates for these different eclogites indicate that they were formed under different physical conditions. All the eclogites were affected by later regional metamorphism for which the P- T conditions are estimated. This paper provides an introduction to the abundant eclogites from central China which have not been reported previously in Western literature. Specifically, the mode of field occurrence, petrography, mineral chemistry and formation conditions of the four types of eclogites are described. The paper is thus designed to establish a petrological framework for future detailed studies of the eclogites and their country rocks in an ancient zone of collision.

Magma mixing in late Archean tonalitic and mafic rocks of the Lac des Iles area, western Superior province

Abstract Plutonic rocks in the Lac des Iles area of the western Superior Province provide a record of contemporaneous late Archean mafic and tonalitic magmatism. This suite of rocks consists of mafic to ultramafic intrusions, mafic to intermediate dikes, hornblendite, and hornblende tonalite. The tonalite contains net-veined and disaggregated mafic to intermediate dikes, mafic schlieren and zones of hornblendite to mela-tonalite with skeletal hornblende. These textures are interpreted to be the result of magma mixing. The hornblende tonalite is geochemically characterized by high Al 2 O 3 , high Na 2 O/K 2 O, low Rb, moderate Sr and fractionated REE. The hornblendite, mafic to intermediate dikes and tonalite show whole-rock trends of decreasing total REE, increasing depletion of HREE and increasing Eu/Eu∗ with increasing SiO 2 . Amphiboles range in composition from edenitic and pargasitic hornblende to hornblende in dike rocks, mela-tonalite and hornblendite, and from hornblende to actinolite in tonalite. The development of skeletal amphiboles with strong normal zoning and resorbed hornblende with weak reverse zoning are attributed to conditions of supercooling and superheating, respectively, in zones of magma mixing. The least fractionated dike rocks geochemically resemble lamprophyres derived from alkali-basaltic magma. Variation of REE among the mafic to intermediate dikes suggests that they fractionated hornblende. Hornblendites and mela-tonalites can be modeled as hornblende cumulates from the dike rocks. The amphiboles probably crystallized rapidly during mixing, leaving a feldspathic residue. Variation in hornblende tonalites is consistent with mixing of SiO 2 -rich tonalite melt with mafic to intermediate dike phases and associated hornblende cumulates and feldspathic residue. Open-system magmatic processes at Lac des Iles result in a continuum of disequilibrium textures reflecting emplacement of mafic magma into a crystallizing felsic host. The plutonic rocks probably formed from a zoned magma chamber with a mafic base and tonalitic top. Breccia zones in the tonalite suggest that explosive degassing of the chamber occurred. Structures similar to those reported here are common in other granitoids in the Superior Province and suggest that mafic magmatism and magma mixing played an important role in the development of late Archean granitoid plutons.

Mineral chemistry of some gabbroic rocks of the Central Eastern desert, Egypt

The mafic minerals of the two essential gabbroic rocks from the central Eastern Desert of Egypt (Sid Metagabbros, MG, and Younger Gabbro, YG) have been subjected to EPMA. There are distinct geochemical differences between the amphiboles of these two types. The amphiboles of the YG show relatively high TiO 2, A1 2O 3, Na 2O, K 2O and low SiO 2 and Fe 2O 3 than the MG. The YG have also (Na+K) > 0.5 and A1 IV > 1.3. The amphiboles of the MG are mainly magnesio-hornblende with subordinate actinolitic hornblende, whereas, the YG amphiboles is edenitic hornblende in composition. The A1 VI and Ti contents of the amphibole of the MG indicate that these rocks are subjected to low pressure metamorphism (below 5 k bars) within the greenschist-amphibolite facies. The pyroxenes of YG show diopside and augite compositions developed in subalkaline tectonic environments. The olivine of the YG is mainly iron rich (fa = 77.61 − 76.61).

Ultramafic rocks of a fracture-zone ophiolite, North Cascades, Washington

The Ingalls Complex was deformed in a Late Jurassic oceanic fracture zone. An unusually diverse group of ultramafic tectonites comprise three units in this ophiolite. Unit 1 consists mostly of poorly to moderately foliated harzburgite and dunite characterized by porphyroclastic textures. Irregular-shaped and tabular dunite bodies probably represent intrusive bodies or residues of partial melting. Voluminous Unit 2 consists mostly of poorly to strongly foliated Iherzolite and clinopyroxene-bearing harzburgite, plagioclase peridotite is present locally. Olivine and enstatite generally define equigranular mosaics or weakly porphyroclastic textures. Clinopyroxene, however, in some samples displays only weak deformation, compositional zoning, simple (growth?) twins and interstitial, commonly poikilitic texture. Clinopyroxene and plagioclase in these samples probably formed from a melt after recrystallization of olivine and enstatite, indicating that these Iherzolites are impregnated peridotites. Other Iherzolites and clinopyroxene-bearing harzburgites may represent weakly depleted mantle. Pods of metagabbroic gneiss within Unit 2 probably are small intrusions that were deformed as they cooled. Unit 3 represents a major high-temperature ( 700 °≥ 900 ° C ) shear zone that separates Units 1 and 2, and consists of strongly foliated, commonly mylonitic Iherzolites and hornblende peridotites. The latter are the most strongly foliated ultramafites, and olivine in them records stresses as high as 275 MPa. The abundance of hornblende implies a genetic relationship between mylonitization and the hydration and metasomatism necessary to form such rocks from Iherzolites. Mineral chemistry and geothermometry are typical of mantle tectonites in many ophiolites and oceanic fracture zones. There is a particularly strong similarity between the spinels in the Ingalls Complex and the spinels from the Owen and Vema fracture zones. Hornblende in Unit 3 ranges from edenite to edenitic hornblende. Calculated temperatures from CPX-OPX pairs in Unit 2 range from 950 ° to 1000 ° C. These data are best explained by horizontal and vertical displacements in oceanic fracture zones which may juxtapose lithosphère formed at different structural levels and with different petrologic histories. Unit 1 represents a block of upper mantle which probably experienced convective upwelling and partial melting beneath a spreading ridge near its intersection with a fracture zone. During convective upwelling at least parts of the upper mantle represented by Unit 2 were impregnated by a melt. The impregnated Iherzolite may be accounted for by the edge effect at a fracture zone or reduced melt production at a fracture zone due to ribbon spreading. The mylonite zone of Unit 3 reworks Iherzolites and harzburgites which were originally part of the block represented by Unit 2 and probably records deformation in the active segment of the fracture zone. These observations document the complexities that may occur in ultramafic rocks within oceanic fracture zones. They also suggest that impregnation, metasomatism, and high stresses are important in these zones.

Mineralogy and petrology of alkaline granophyric xenoliths from the Thorsmörk ignimbrite, southern Iceland

The Thorsmörk ignimbrite, southern Iceland, contains a suite of granophyre xenoliths displaying magmatic or high-temperature sub-solidus mineral assemblages. These granophyres are consanguineous with the erupting comenditic magma. Four types of mineral assemblages are distinguished: 1. (A) oligoclase, edenitic hornblende, salitic pyroxene, magnesian biotite, magnetite and sphene; 2. (B) oligoclase, manganoan to sodic ferro-augite, fayalite, richterite, ilmenite and magnetite; 3. (C) anorthoclase, ferrohedenbergite to aegirine hedenbergite, ilmenite, magnetite and (riebeckite); 4. (D) cryptoperthite, aegirine hedenbergite to (aegirine), aenigmatite, arfvedsonite, ilmenite and magnetite. Geothermometry shows that the xenoliths have crystallized between 900°C and 500°C at moderate oxygen fugacities, just above the FMQ buffer. It is further demonstrated that a hot vapour phase heavily charged with sodium and halogens, played a major role in the late sub-solidus crystallization of the different types.

Pétrologie et signification géotectonique des plutonites de Kambusi (sud Kivu, Zaïre)

The massif of Kambusi, located about 30 km northwest of Bukavu, is made up of two intrusive plates: microgranite to the south and microsyenite to the north. These plates were emplaced during the Cambrian.The microsyenite is characterized essentially by the association of quartz, perthites, albite + K-feldspar, olivine, hedenbergite, edenitic hornblende, iron-rich biotite, ilmenite, and fluorine. The microgranite is distinguishable from the microsyenite by an absence of olivine, pyroxene, and fluorine, a relative richness of quartz and amphibole, and a scarcity of feldspar in the modal composition.These magmas are typically alkaline (some rocks show a hyperalkaline tendency) and were emplaced at a shallow depth under low values of [Formula: see text] and [Formula: see text] and high T (&gt; 890 °C). They are considered to mark the relaxation stage typifying the change of tectonic regime that took place at the end of the Panafrican.

Reactions involving hydration of orthopyroxene in anorthosite-gabbro

Hydration of orthopyroxene (En 60) in contact with plagioclase (An 36) leads to oronas of the type orthopyroxene-cummingtonite-edenite + quartz-edenitic hornblende. The compositions of the minerals involved indicate that kyanite needles in the adjacent plagioclases form as part of the reaction: orthopyroxene (En 60) + plagioclase (An 36) + H 2O→ cummingtonite + edenite/edenitic hornblende + plagioclase (< An 36) + kyanite + quartz. The reaction proceedings consumption first of the orthopyroxene and later of the cummingtonite to produce a corona structure consisting of a core of intergrown edenite and quartz surrounded by edenitic hornblende. Corona evolution of this type is controlled by diffusion.

Olivine-dolerite intrusionsin the Fastnet Basin

Sills of olivine-microgabbro and olivine-dolerite 0.5–180 m thick intrude Lower Liassic (Hettangian–Sinemurian) sediments in the Fastnet Basin, a SW extension of the N Celtic Sea Basin, about Lat. 50°N, Long. 10°W. The evidence of 3 wells and 2300 km of seismic reflection profiles indicates 6 major sills and sill-complexes, and 3 possible igneous centres or plugs. A high-level sill may intrude Lower Cretaceous sediments, and one plug apparently deforms Cretaceous reflectors. K-Ar ages on separated biotites from a thick sill intruding Liassic sediments gave 170 ± 4 Ma (Bajocian). This sill contains 30% olivine (FO 70-73 ), 20% clinopyroxene (magnesian salite), 45% plagioclase (An 60-78 ), &amp;, with accessory magnetite, ilmenite, edenitic hornblende, Ti-phlogopite, apatite, zircon, spinel (in olivine) and sulphides. Alteration to serpentine minerals, chlorite, sericitic mica and leucoxene is sporadic. The magmatism in the Fastnet province thus appears to have been active from the mid-Jurassic to the early Tertiary, related initially to the time of initial rifting between the European and American plates, and subsequently to their final separation between Greenland and Rockall and associated Thulean magmatism.