Peridotite Body Research Articles

The Ronda high temperature peridotite: Geochemistry and petrogenesis

The Ronda peridotite in southern Spain is a large (~300 km 2) exposure of upper mantle which provides direct information about mantle processes on a scale much larger than that provided by mantle xenoliths in basalt. Ronda peridotites range from harzburgite to lherzolite, and vary considerably in major element content, e.g., Al 2O 3 from 0.9 to 4.8%, and trace element abundances, e.g., Sr, Zr and La abundances vary by factors of 20 to 40. These compositional variations are systematic and correlate with (pyroxene + garnet)/olivine ratios and olivine compositions. The data are consistent with formation of residual peridotites by variable degrees of melting (~0 to 30%) of a compositionally homogeneous peridotite. None of the peridotites have geochemical characteristics of residues formed by extensive (⪢5%) fractional melting and the data can be explained by equilibrium (batch) melting, possibly with incomplete melt segregation in some samples. Based on compositional differences between Ronda peridotites, the segregated melts were picritic (12–22% MgO) with relative rare earth element abundances similar to mid-ocean ridge basalt (MORB). Prior to the melting event the Ronda peridotite body was a suitable source for MORB. The compositional characteristics of Ronda peridotites are consistent with diapiric rise of a fertile mantle peridotite with relatively small degrees of melting near the diapir-wall rock interface yielding residues of garnet iherzolite, and larger degrees of melting in the diapir interior yielding residues of garnet-free peridotite. Subsequently these residual rocks were recrystallized at sub-solidus conditions ( Obata, 1980), and emplaced in the crust by thrusting (Lundeen, 1978).

Eclogite-plagiopyrigarnite relations in the catazonal complexes of northwest Spain

Eclogites and plagiopyrigarnites in the catazonal complexes have identical major- and trace-element bulk compositions and very similar lattice fabrics of their constituent minerals. On the other hand, the chemical composition of and the element partitioning between the respective clinopyroxenes and garnets are significantly different. The dimensional fabric of the respective clinopyroxenes is also different, indicating dynamic crystallization for the eclogites followed by static recrystallization during their transformation to plagiopyrigarnites. The eclogites are of B-type facies and the plagiopyrigarnites are high-pressure granulites of the clinopyroxene + almandine ± hornblende subfacies. Oriented omphacite of the eclogites is gradually transformed into mosaic-equigranular sodic augite and plagioclase via symplectitic intergrowths in transitional rock types, indicating an increase in temperature possibly accompanied by a slight drop in pressure. The inferred P— T—time path is supported by the temperature and pressure of coeval recrystallization established in closely associated peridotite bodies.

Petrology of ultramafic and mafic rocks from the Lanzo peridotite body (Western Alps)

The Lanzo lherzolites (west of Torino, Italy) exhibit a history of decompression at relatively high temperature (1150-950°C) from the spinel-lherzolite field to the plagioclase-lherzolite field. These ultramafics have clinopyroxenes with Na contents intermediate between those of the sub-oceanic peridotites and those of the sub-continental peridotites; moreover they do not exhibit a marked depletion in easily fusible elements (e.g. Na, Ti and Al) with respect to primitive mantle abundances, and are less depleted than other ophiolitic ultramafics of the Western Alps. The associated dykes of gabbro and porphyritic basalt derive from tholeiitic melts which show many affinities with N-MORB and with the other Piedmont ophiolites; they were produced by partial melting and low-pressure fractionation of an already depleted asthenospheric source. Thermobarometric data and mantle source/extracted melts relationships suggest that the Lanzo lherzolites cannot represent residua after extraction of N-MORB-type melts. Consequently the Lanzo ultramafics might be a section of sub-continental lithosphere which suffered a polyphasic history of partial melting and a decompression during attenuation and rifting of the European-“African” continental crust. Later, during the Jurassic, they were intruded by N-MORB-type melts produced at depth and were involved in more “mature” oceanic-type stages which led to the opening of the small Piedmont-Ligurian basin.

K-Ar Ages of Allochthonous Mafic and Ultramafic Complexes and Their Metamorphic Aureoles, Western Brooks Range, Alaska: ABSTRACT

End_Page 656------------------------------New K-Ar ages from allochthonous mafic and ultramafic complexes of the western Brooks Range (Brooks Range ophiolite) show that igneous rocks yield ages nearly identical to those of underlying metamorphic aureole rocks. Dated rocks of the Misheguk igneous sequence from Tumit Creek consist of (1) hornblende gabbro with minor greenschist and lower grade alteration, hornblende age 147.2 ± 4.4 Ma; and (2) hornblende-bearing diorite, also slightly altered, age 155.8 ± 4.7 Ma. Both samples come from presumed higher levels of the Misheguk sequence. Dated samples of metamorphic aureole rocks come from outcrops near Kismilot Creek and lie structurally beneath the Iyikrok Mountain peridotite body. The rocks consist of amphibolite and garnet-bearing biotite-hornblende gneiss considere to be metamorphosed Copter igneous sequence and related sedimentary rocks. Hornblende ages are 154.2 ± 4.6 Ma and 153.2 ± 4.6 Ma. Metamorphism is clearly related to the structurally overlying peridotite, as the degree of alteration decreases downward. We suggest that the K-Ar ages of these rocks represent the effects of thermal metamorphism post-dating igneous crystallization and are related to tectonic emplacement of the complex. Earlier K-Ar data on igneous rocks give similar ages and have been interpreted as reflecting tectonothermal events. The age of igneous crystallization of the mafic and ultramafic rocks of the Misheguk igneous sequence remains uncertain. End_of_Article - Last_Page 657------------

Zabargad (St. John’s) Island: an uplifted fragment of sub-Red Sea lithosphere

Zabargad is a small island located in the Red Sea about 50 km W of its axis, between 23º and 24ºN. The island is not volcanic but probably represents an uplifted block of upper mantle and crustal rocks, and yields information on the nature of the underlying lithosphere. Peridotites are the main rock type, consisting of exceptionally fresh spinel lherzolites, which equilibrated last at a depth >30 km in the mantle. Other ultramafic facies, some representing rocks which mixed with a basaltic melt fraction, and some which underwent metasomatic exchange at depth, are also present. The peridotite bodies are in tectonic contact with a metamorphic formation consisting of gneissic and amphibolitic rocks similar to those outcropping in the Pan African metamorphic basement of eastern Egypt, Peridotites and metamorphic rocks are both crossed by several generations of basaltic dykes. The sedimentary Zabargad Formation, of probable Cretaceous or Palaeocene age, consists of alternations of silicified limestones, sandstones, black shales and phosphoritic lenses, and is overlain by evaporites, probably of Miocene age, and by reef limestone units, one probably of early Pleistocene age, the other probably deposited during the last interglacial. Various formations are affected by localized metasomatic mineralizations (recrystallized large, gem-quality olivine, cancrinite, scapolite. garnierite). The uplift of upper mantle ultramafic bodies with fragments of crust, which created the island, might have occurred in connection with the early stages of development of the rift which preceded the formation of the Red Sea. An alternative explanation, that the Zabargad peridotites are remnants of a Pan-African Palaeozoic ophiolitic complex, is possible but less likely.

Ultramafic intrusions in the Lewis Hills Massif, Bay of Islands Ophiolite Complex, Newfoundland: Implications for igneous processes at oceanic fracture zones

The Coastal Complex of western Newfoundland has been interpreted as oceanic crust and upper mantle which formed in an oceanic fracture zone. Within the Coastal Complex, a suite of peridotite bodies and associated basaltic dikes occur. The least-deformed peridotites in these bodies have cumulate textures and have apparently been separated from any gabbroic cumulates which formed in association with them. Intrusive relationships with surrounding metamorphic and cumulate rocks are well preserved and suggest that the peridotite bodies are crystal mush intrusions derived from mobilized ultramafic cumulates which were originally deposited within relatively small isolated magma chambers. These processes are believed to be characteristic of ridge-transform intersections or “leaky” transform regions. Two petrologically distinct types of peridotite have been identified. One type has lithologies and mineral chemistry similar to those of many oceanic plutonic rocks, and these lithologies may be interpreted as differentiates produced in the evolution of typical Mid-Ocean Ridge Basalts (MORBs). The second type of peridotite, although lithologically similar to the first, has relatively high CaO/Al 2 O 3 ratios and relatively low TiO 2 and Na 2 O. This second type of peridotite cannot be simply related to the genesis of typical MORBs. Chilled margins and fine-grained dikes along the margins of some of these peridotite bodies are high-magnesium basalts.

Possible high-temperature origin of pyroxenite lenses within garnet peridotite, northern Italy

A garnet websterite lens carrying pyroxene megacrysts with unmixing texture and a garnet clinopyroxenite lens are described from within the garnet peridotite body at Seefeld Aim, Ultimo valley, northern Italy. Texture, bulk rock and mineral chemical data suggest that the garnet pyroxenites resulted from exsolution and recrystallization from originally monomineralic pyroxenite that consisted of aluminous and subcalcic clinopyroxenes and small amount of probable silicate melt. From extremely high temperatures of crystallization, that is, > 1 450 °C as inferred from the pre-exsolution composition of the pyroxene megacrysts, it was suggested that the primary pyroxenes were igneous precipitates from a magma that had injected into a solid peridotite probably in the upper mande.

Petrology of garnet-clinopyroxene rocks in a granulite facies environment, Bohemian Massif of Lower Austria

Whole rock and mineral analyses are presented for garnet-clinopyroxene rocks found intimately associated with possible mantle derived garnetiferous peridotite bodies in a high pressure granulite facies terrain. Although garnets are pyropic (48-76 % pyrope) these rocks are not true eclogites since their constituent clinopyroxenes are not omphacites but rather Al-augites or Cr-diopsides with AlIV/AlVI ratios, mostly > 1.0. Element partitioning considerations between garnet-clinopyroxene and clinopyroxene-orthopyroxene pairs are taken to indicate arrested equilibration under pressure-temperature conditions compatible with those previously deduced for the surrounding granulite facies rocks. However, mineral zoning profiles and exsolution textures clearly indicate that the current metamorphic mineral assemblages have formed in response to cooling from an earlier temperature (and pressure ?) maximum. It is considered that these garnet-clinopyroxene rocks may represent the partially recrystallised crystalline products of trapped upper mantle partial melts of olivine tholeiite to picrite composition in which highly aluminous pyroxenes were the dominant original phases formed on the liquidus. The present metamorphic character and lithological associations of such rocks may accordingly bear witness to deep-seated tectonic interactions between the upper mantle and lower continental crust during a profound compressional orogenic event.

Formation of zoning of olivine with progressive metamorphism of serpentinite. An example from the Ryumon peridotite body of the Sanbagawa metamorphic belt, Kii peninsula.

Olivine formed by the reaction antigorite+brucite=2 olivine+3 water from the Ryumon peridotite body, a regionally metamorphosed serpentinite of the Sanbagawa metamorphic belt, exhibits zoning with Fe, Mn and Ni-rich core and Mg-rich rim. From the core to the rim, XMn decreases at much higher rate than XNi does. The chemical relations between olivine and coexisting antigorite and brucite suggest that the zoning was formed by fractional crystallization of olivine from antigorite and brucite at the time of progressive metamorphism. The higher decreasing rate of XMn than XNi can be explained by the smaller values of KDanti-ol Mn-Mg and KD bru-ol Mn-Mg than KDanti-ol Ni-Mg and KDbru-ol Ni-Mg, respectively. This type of zoning may be characteristic of the zoning of olivine crystallized from serpentine. The zoned olivine suggests that local equilibrium was maintained even at low-grade in metamorphosed serpentinite.

Structural study of the Almklovdalen peridotite massif (southern Norway)

The Almklovdalen ultramafic body included in the basal gneisses of southern Norway is composed mainly of hydrated harzburgite and dunite with cores of layered garnet lherzolite. From field and gravity evidence, the body can be interpreted as a deformed cone surrounding a gneiss central mass. Based on structural, microstructural and mineral preferred orientation (MPO) studies a continuous kinematic and metamorphic history is inferred. The major events of this history starting 1400–1800 million years ago are 1. (1) Intrusion of a garnet-bearing peridotite slice from mantle into crust and folding of garnet-rich S 0 layers inducing an S 1 foliation and an L 1 lineation marked by flattening and elongation of garnet and olivine crystals. Traces of high-temperature slip in olivine porphyroclasts are found. Strong annealing had a scattering effect on MPO. 2. (2) Pervasive deformation in the peridotites and in the surrounding gneisses coinciding with introduction of water in the peridotites. It is responsible for isoclinal folding with S 2 foliation and L 2 lineation marked by chlorite and amphibole flattening and elongation in the peridotites. 3. (3) Gravity sinking of the whole slice in the acidic gneisses of the crust creating a cuplike shape. This event is responsible for an S 3 foliation marked by chlorite flattening parallel to the peridotite—gneiss contact and for an L 3 lineation, marked by chlorite elongation and spinel strings, parallel to the generatrix of the imperfect cone. MPO's of the peridotite indicate slip in olivine following different systems. Enlargement of olivine porphyroblasts occurs due to annealing. The gravity sinking is contemporaneous with the almandine amphibolite metamorphism in the gneiss where it develops a foliation and lineation parallel to those in the peridotites. 4. (4) Deformation possibly due to a north-south compression which induces an S 4 foliation and L 4 , lineation in the gneiss, parallel to the axial planes and the axes of the two folds affecting the peridotite body.

Two contrasting types of zoned chromite of the Mt. Higashi-Akaishi peridotite body of the Sanbagawa metamorphic belt, central Shikoku.

Two contrasting types of zoned chromite that were formed at different stages of the thermal history of the Mt. Higashi-Akaishi peridotite body were examined by an electron microprbe analyzer. The zoned chromite of retrogressive-type occurs associated with antigorite and chlorite, and consists of chromite core and ferritchromite rim. In the rim, compositional gradients in regard to Al-Fe2+ and Mg-Fe2+ substitutions are conspicuous. The Fe2+-Mg partitioning between the cores and neighbouring olivines indicates the equilibration temperature of about 600°C (the Sanbagawa metamorphism). At the retrogressive stage of the Sanbagawa metamorphism, the peridotit body was hydrated and the introductin of H2O and O2 induced the reaction between chromite and olivine to form ferritchromite rim and aluminous antigorite (or chlorite). The other type of zoned chromite, igneous-type, occurs in the olivine matrix of dunite with or without garnet from Gongen valley where the upper-most part of the body is exposed. The zoned chromite exhibits chemical zoning characterized by chromite core and Fe2+-rich aluminous chromite rim in garnet-free dunite, and by Fe8+-rich aluminous chromite core and ferrian chromite rim in garnet-bearing dunite. Cation substitution is represented by Cr-(Al, Fe3+) in the former and by (Cr, Al)-Fe3+ in the latter. Fe2+-Mg zoning probably existed at the igneous stage, but would have been homogenized by the Fe2+-Mg substitution at the time of the Sanbagawa metamorphis. As subsolidus cation redistribution can not explain the zoning in regard to Cr, Al and Fe3+, it is considered to be a relic of the igneous stage mineralogy.

Upper mantle beneath a young oceanic rift: Peridotites from the island of Zabargad (Red Sea)

Research Article| October 01, 1981 Upper mantle beneath a young oceanic rift: Peridotites from the island of Zabargad (Red Sea) Enrico Bonatti; Enrico Bonatti 1Lamont-Doherty Geological Observatory, Palisades, New York 10964 and Laboratorio di Geologia Marina del CNR, Via Zamboni, 65, Bologna, Italy Search for other works by this author on: GSW Google Scholar Paul Hamlyn; Paul Hamlyn 2Lamont-Doherty Geological Observatory, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Giulio Ottonello Giulio Ottonello 3Istituto di Petrografia, University of Pisa, Via S. Maria, 53, Pisa, Italy Search for other works by this author on: GSW Google Scholar Geology (1981) 9 (10): 474–479. https://doi.org/10.1130/0091-7613(1981)9<474:UMBAYO>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Enrico Bonatti, Paul Hamlyn, Giulio Ottonello; Upper mantle beneath a young oceanic rift: Peridotites from the island of Zabargad (Red Sea). Geology 1981;; 9 (10): 474–479. doi: https://doi.org/10.1130/0091-7613(1981)9<474:UMBAYO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Information on the nature of the upper mantle in a young rift zone with lithosphere transitional from continental to oceanic was obtained from the island of Zabargad, probably an uplifted fragment of Red Sea lithosphere. The island exposes exceptionally fresh, mantle-derived peridotite (spinel Iherzolite) bodies. The mineralogy and major-element, rare-earth element, and mineral chemistry of the spinel Iherzolites suggest that they equilibrated last at depth ≥ 30 km in the mantle and that they have oceanic rather than continental affinity. It is unlikely that the Zabargad ultramafics are part of a late Precambrian–early Paleozoic ophiolite, as found elsewhere in circum–Red Sea regions; rather, they were uplifted probably in connection with the development of the Red Sea Rift. The Zabargad peridotites are in tectonic contact with continental metamorphic basement rocks; it appears, therefore, that regions of an embryonic rift such as the Red Sea can be covered with continental-type crust but be underlain by an upper mantle having already oceanic affinities. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

An analytical electron microscopic study of a pyroxene-amphibole intergrowth

Samples of a garnet granulite from the mafic border units of the Lake Chatuge, Georgia alpine peridotite body were found to contain lamellar intergrowths of a pargastic amphibole in augite having the typical appearance of an exsolution feature. Single crystal X-ray diffraction, optical, electron microprobe and conventional and analytical electron microscopic studies have provided data limiting the compositions and structures of the coexisting phases. Individual lamellae of both materials are from 0.5 to 2.0 μm in width with the lamellar interface parallel to {0 1 0}. The formulae of the minerals, as determined by a combination of electron microprobe and analytical electron microscopy, are (Na0.1Ca1.0Mg0.6Fe3+ 0.3)(Si1.8Al0.2)O6 for the pyroxene and Na0.7Ca1.9(Mg2.1Fe2+ 1.4Fe3+ 0.5Ti0.1Cr0.1Al0.8)(Si5.9Al2.1) O22(OH)2 for the amphibole. Several other studies have described intergrowths similar to those observed in this work, in general favoring exsolution as the formation mechanism for the intergrowths. In the Lake Chatuge samples however, replacement of pyroxene by amphibole is in part indicated by continuous gradation of amphibole lamellae into amphiboles rimming the clinopyroxenes.

Nikubuchi peridotite body in the sanbagawa metamorphic belt; thermal history of the ?Al-pyroxene-rich suite? peridotite body in high pressure metamorphic terrain

The Nikubuchi peridotite body occurs as a tectonic body in the Sanbagawa metamorphic belt. The peridotites consist of dunite, spinel wehrlite, spinel websterite and spinel-bearing metagabbro, and are characterized by the presence of aluminous pyroxenes and green spinel.

The Lizard complex as an ophiolite

The Lizard complex of south Cornwall consists of an assemblage of peridotites, gabbros, hornblende schists, metasediments, dolerites and metadolerites and acid-basic banded gneisses (the Kennack gneisses). It is bordered to the north by a zone of chaotic sediments with phacoidal inclusions of pillow lavas, greywackes and quartzites (the so-called Meneage crush zone). This zone can be traced eastwards into the Roseland district of south Cornwall1. The age of the complex is uncertain. Dates (largely K–Ar) obtained from the Kennack Gneiss and the majority of hornblende schists are mainly between 370 and 390 Myr, although Green2,3considered the peridotite to be older than the oldest date obtained from the hornblende schists, at 492 Myr. Metadolerite dykes have recently4 been given ages of approximately 400 Myr. All the above are thought to represent minimum ages due to the possibility of argon loss during subsequent uplift, cooling and mild metamorphism. Since the work of Green3, the Lizard complex has been regarded by many as the type example of a hot diapiric mantle intrusion into continental crust. Intrusion is thought to have occurred during a period of regional metamorphism and to have resulted in the imposition of a contact dynamothermal aureole on the hornblende schists at the margin of the peridotite body. The gabbro is regarded as a later and unrelated intrusion, and the Kennack gneisses were attributed to intrusion of acid magma along sheared basic dykes in peridotite. Thayer5 suggested that the complex was ophiolitic, basing his views on the field relationships of gabbros and peridotites which indicated that the two lithologies might be temporally and genetically related. General comparisons have since been made between the Lizard complex and the well-documented ophiolote sequences6–10, although little detail has generally been given. I provide here relevant details which suggest that the Lizard complex is ophiolitic in origin.

Trace elements and Sr-isotopes in some mantle-derived hydrous minerals and their significance

New analyses of K, Rb, Sr and Ba contents and the 87Sr 86Sr ratios of eight amphiboles, one phlogopite, two diopsides and one host alkalic basalt for an amphibole are reported: The samples are mostly inclusions in alkalic basalts and occur in association with peridotite inclusions. Two of the samples are from alpine-type peridotite bodies — one from the Etang de Lhers massif in the French Pyrenees and the other from the Finero massif in the Ivrea zone in northern Italy. The kaersutites come from the following localities: Hoover Dam, Arizona; Deadman Lake, California; Massif Central, France; Queensland; Spring Mountain, New South Wales. The data indicate that kaersutitic amphiboles are genetically unrelated to their host basalts. The isotopic and trace element data of these amphiboles further strengthens the suggestion of BASU and MURTHY (1977) that kaersutites play a significant role in ocean ridge basalt genesis. In addition, pargasitic amphibole with higher 87Sr 86Sr ratios, if present, may be important in the source regions of alkalic basalts. The bulk amphibole lherzolite from Lherz has the K Rb ratio and 87Sr 86Sr ratio appropriate for source material of ridge tholeiites. If the diopside and the amphibole in this rock had isotopically equilibrated under upper mantle conditions, the data show the time of last equilibration to be approximately 735 m.y., in contrast to the young emplacement age of the ultramafic massif. The coexisting phlogopite and diopside in the spinel lherzolite inclusion from Kilbourne Hole, New Mexico, show, surprisingly, isotopic equilibration under upper mantle conditions despite their drastically different Rb Sr ratios. The data show that the phlogopite must have formed very recently in the upper mantle. This phlogopite also has a high K Rb ratio (1133), contrary to the commonly held view that mantle phlogopites have low K Rb ratios. The coexisting diopside shows high K content (778 ppm) and a lower K Rb ratio than the phlogopite. This phlogopite lherzolite has trace elemental and isotopic characteristics that may be adequate for the origin of alkalic basalts upon partial melting.

熊本県肥後変成帯に産する”コマチアイト様”かんらん岩について

Peridotites characterized by the texture of elongated olivine crystals, are found in the Higo metamorphic belt mainly composed of psammitic gneiss. The peridotite mainly consists of olivine, orthopyroxene, tremolite, serpentine, and talc. Spinifex-like olivine crystals, partly altered to serpentine and magnetite, consist of randomly oriented plates parallel to (010). The elongated olivine crystals in same handspecimen are chemically homogeneous, and the compositional zonation of the crystals has not been observed by microprobe analysis. However, the chemical composition of olivine ranges from FO83 to FO92 in different handspecimens. The NiO content of olivine varies from 0.2 to 0.5 weight per cent and the MnO content from 0.1 to 0.2 weight per cent. The NiO and MnO contents are about the same as those of komatiitic olivines. On the other hand, the olivines in the Higo perioditite contain a negligible amount of CaO and remarkabIly differ from those of extrusive peridotite (komatiite). The rocks are free from clinopyroxene and contain large and prismatic orthopyroxene. The extremely Ca-depleted orthopyroxenes (En89-En92) are interlocked with tabular olivines in each other. The Al2O, and Cr2O3 contents of orthopyroxenes are relatively lower than those of orthopyroxenes in alpine peridotites. From the mineral assemblages of peridotites and metamorphic rocks, and from the chemical composition of olivine and orthopyroxene, it is suggested that the peridotite bodies suffered from the regional metamorphism of amphibolite facies (approximately 700°C, 3-4kb). This conclusion is also supported by the data concerning Mg-Fe2+ distribution between olivine and chromian spinel in the Higo peridotite.

A crustal origin for eclogites and a mantle origin for garnet peridotites: Strontium isotopic evidence from clinopyroxenes

Strontium isotopic data suggest that the classic eclogite-facies rocks of western south Norway described by Eskola (1921) formed from several parental materials in a variety of environments. Mineral separates from essentially basic, bi-minerallic (clinopyroxene and garnet) eclogites that occur as lens-shaped masses within high grade gneisses (country rock eclogites) have Sr87/Sr86 values that range from 0.704 for fine-grained varieties to 0.716 for coarse-grained, orthopyroxene-bearing varieties. These high, varied ratios contrast with the very low, restricted ratios (0.701 to 0.704) of similar minerals from ultrabasic, garnet-clinopyroxene-orthopyroxene-olivine assemblages (garnet peridotites) that occur as lenses within large peridotite bodies. The eclogite-facies metamorphism that generated the garnet peridotites may have occurred in the mantle. However, the metamorphism that generated at least the more radiogenic country-rock eclogites must have occurred in the crust. The high Sr87/Sr86 ratios of these eclogites could be generated either by forming them from crustal parental rocks or by contaminating mantle-derived parental rocks with radiogenic strontium from the country rocks. If this contamination occurred after intrusion and before eclogite-facies metamorphism, a rather contrived history must be postulated that involves intrusion, contamination accompanied by hydration, subsequent dehydration, and finally eclogite-facies metamorphism. These processes could have occurred within the long, complicated history of the enclosing country rocks. Alternatively, if the contamination occurred during eclogite-facies metamorphism, the presence of some hydrous fluid appears to be required to transport the radiogenic strontium from the enclosing country rocks. The eclogites with the highest Sr87/Sr86 ratios are also the most coarse-grained and it is possible that the presence of some intergranular fluid enabled these eclogites to recrystallize to a much larger grain size than would have been possible in a totally anhydrous environment. The garnet peridotites and fine-grained country rock eclogites may have formed from mantle material in the crust but escaped contamination by radiogenic strontium as a result of their position in a dry environment in the crust.

Scandium, chromium, iron, cobalt and nickel in some basic and ultrabasic rocks from west Norway

Sixteen eclogites and ultrabasic rock samples from west Norway have been analyzed for Sc, Cr, Fe, Co and Ni by instrumental neutron-activation analysis. In gneiss-eclogites, Sc and Co give values from about 30 to 60 ppm and Cr and Ni from 180 to 700 ppm. An eclogite surrounded by a peridotite body shows nearly the same content of Sc and Co as do the gneiss-eclogites, but up to 20 times more Cr. Ni is only slightly more enriched in this particular eclogite than in the others. The ultrabasites have only a few ppm Sc, Co concentrates around 100 ppm while the Cr and Ni values are mainly found between 2 and 3,000 ppm. Fe shows an average value of nearly 10%.

CHROMIAN SPINEL AS A PETROGENETIC INDICATOR: PART 2. PETROLOGIC APPLICATIONS

The crystallization of chromium-bearing spinel from silicate magmas is first examined, and then an attempt is made to apply the theory developed in Irvine (1965) to the principal occurrences of the mineral for which data are available. It is concluded that the chromium-rich varieties commonly known as chromite have generally formed simultaneously with olivine, and that their crystallization has in many occurrences been terminated by a peritectic (reaction) relation leading to formation of a pyroxene. The origin of the aluminium-rich varieties is more problematical and perhaps more varied; however, they evidently occur only in alpine-type peridotite bodies and peridotite "nodules" in basaltic volcanic rocks, and their formation may generally have involved high pressures. It is found that there is a rough correlation between the Mg/Fe++ ratios of chromian spinels, olivines, and pyroxenes occurring in the same rock bodies, and there is some evidence that the Mg–Fe++ distribution coefficients of spinel–silicate pairs may significantly be sensitive to temperature. The chromites of stratiform intrusions reflect an appreciable range of oxygen fugacities, whereas the spinels of alpine- type peridotite bodies seem generally to have formed at about the same relatively low oxygen fugacity. Chromites from ultra mafic bodies of the type common to southeastern Alaska have exceptionally high Fe+++/Cr + Al + Fe+++ ratios, probably because the bodies crystallized from magma that was extremely poor in silica.