Ultramafic Rock Types Research Articles

Mid-ocean ridge and supra-subduction affinities in the Pindos ophiolites (Greece): implications for magma genesis in a forearc setting

The Pindos ophiolitic massif is considered an important key area within the Albanide–Hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units: (1) a lower unit, including an intrusive and a volcanic section; and (2) an Upper Ophiolitic Unit, mainly including mantle harzburgites. Both units share similar metamorphic soles and tectono-sedimentary mélanges at their bases. The intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock-types, including dunites, lherzolites, olivine-websterites, olivine-gabbros, anorthositic gabbros, gabbros and rare gabbronorites. The volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks: (1) basalts and basaltic andesites of the lower pillow section showing a clear high-Ti affinity; (2) basaltic andesites of the upper pillow section with high-Ti affinity, but showing many geochemical differences with respect to the first group; (3) very low-Ti (boninitic) basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections, and dykes widespread throughout the Pindos ophiolites. These different magmatic groups originated from fractional crystallization from different primary magmas, which were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extractions. Group 1 volcanics may have derived from partial melting (ca. 20%) of an undepleted lherzolitic source, while group 2 basaltic rocks may have derived from partial melting (ca. 10%) of a mantle that had previously experienced mid-ocean ridge basalt (MORB) extraction. Finally, the Group 3 boninites may have derived from partial melting (ca. 12–17%) of a mantle peridotite previously depleted by primary melt extraction of Groups 1 and 2 primary melts. In order to explain the coexistence of these geochemically different magma groups, two petrogenetic models formerly proposed for the Albanian ophiolites are discussed.

Midinfrared spectra of Mercury

Observations of Mercury in the mid infrared (8–12.5 μm region) obtained over a variety of longitudes using the Fourier Transform Spectrometer (FTS) on the McMath‐Pierce 1.5 m solar telescope at Kitt Peak reveal low‐contrast spectra, except in cases where atmospheric noise or difficulties in guiding the telescope caused spurious signals. Although there are features that are suggestive of emissivity maxima (Christiansen features), their proximity to the telluric absorption band, and the noise in the spectra, preclude their unambiguous interpretation. We see little evidence of reststrahlen bands in our data; however, there are strong indications of transparency features. A shallow emissivity minimum occurs at 12 μm in the spectra centered at 80°, 256°, and 266° Hermean longitudes. A minimum occurs at 12.5 μm in the spectra centered at 15°, and a doublet minimum, with one trough at 12.2 μm and a second trough at 12.4–12.6 μm, is seen in the spectra centered at 229°. These features indicate the presence of a fine powder on the Mercurian surface, and their low spectral contrast suggests a significant percentage of agglutinitic material. On the basis of the position of the transparency features, we conclude that our spectra are indicative of intermediate, mafic, and ultramafic rock types. Further specificity is not warranted by the data.

The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks

Compositional fields for spinels from a wide variety of mafic– a wide range of conditions from mafic and ultramafic ultramafic igneous rock types and tectonic environments have been magmas and, in the case of chromites, are often among determined from a global database of over 26 000 analyses. the first phases to crystallize. They also exhibit a wide These fields are defined using contoured data density plots based range of solid solution, the thermodynamics of which has on the spinel prism, and plots of TiO2 vs ferric iron, for mantle been studied extensively (O’Neill & Wall, 1987; Mattioli xenoliths, ophiolitic rocks, continental layered intrusions, alkalic & Wood, 1988; Wood, 1990; Sack & Ghiorso, 1991; and lamprophyric rocks, tholeiitic basalts, Alaskan ultramafic Poustovetov, 2000). They are relatively refractory and complexes and komatiites. Several trends appear regularly in the resistant to alteration, particularly compared with other various environments: a trend of widely variable Cr/(Cr + Al) high-temperature igneous minerals such as olivine. They at low Fe/(Mg + Fe) (the Cr–Al trend); increasing Fe, occur in a high proportion of terrestrial mafic and ultraFe/(Mg + Fe) and TiO2 at constant Cr/(Cr + Al) mafic rocks, and a very large volume of microprobe data (Fe–Ti trend); a trend found primarily in kimberlites, similar is available on their compositions. to Fe–Ti but at constant Fe/(Mg + Fe); and an unusual Publications on spinels (particularly chromites) have trend of increasing Al found only in layered intrusions. The routinely used compositional fields based on the spinel Cr–Al and Fe–Ti trends are both found to varying degrees in prism to compare populations of analyses. Some of tholeiitic basalts. The Cr–Al trend is prevalent in rocks that these fields have remarkable longevity, particularly those have equilibrated over a range of pressures, whereas the Fe–Ti defined for layered intrusions and Alpine peridotites by trend is dominantly due to low-pressure fractionation. The most Irvine (1967, 1977). Many new data have become availCr-rich chromites found in nature occur in boninites, diamondable in the last 20 years, however, and it has become bearing kimberlites, some komatiites and ophiolitic chromitites. apparent that a new compilation of data is necessary to Exceptionally reduced chromites are found in some komatiites reflect this. The prime purpose of this paper is to analyse and in ophiolitic chromitites. Detrital chromites from the a global database of about 26 000 spinel analyses from Witwatersrand conglomerates are of komatiitic provenance. terrestrial igneous and metamorphosed igneous rocks, and to extract from this database the characteristic compositional fields of spinels from the wide variety of magma types and tectonic environments in which they occur.

The metamorphosed mafic-ultramafic complex of Mokuro, Ilesha Schist Belt, southwestern Nigeria

In the Proterozoic Schist Belt of Nigeria, lenticular bodies of metabasites and meta-ultramafics are frequently intercalated within staurolite bearing metapelitic schists. Such a metamorphosed mafic-ultramafic complex is particularly well exposed in the Mokuro riverbed between the towns of Ife and Ilesha. These outcrops display contact relationships with the surrounding metasediments, as well as between the individual mafic and ultramafic rock types. The most common mafic rocks are indistinctly layered amphibolites, accompanied by apatite rich amphibolites and massive amphibolites, in part rich in ilmenite and pyrrhotite. Among the generally massive ultramafic rocks, nearly monomineralic amphibole rocks predominate, while chlorite-amphibole, talc-chlorite-amphibole and talc bearing olivine-chlorite-amphibole rocks occur in subordinate amounts. Field, textural and geochemical evidence suggest that the mafic-ultramafic complex derived from a thick, structurally differentiated basaltic sill that contained doleritic portions in its interior. Slow cooling rates in these inner parts enabled crystal settling with the formation of ultramafic cumulates. Due to the enrichment of volatiles during the crystallisation process, higher amounts of apatite and sulphides, as well as late magmatic amphibole, were formed in parts of the mafic-ultramafic body. Mineral assemblages in the mafic-ultramafic complex testify to a metamorphic overprint under amphibolite-facies conditions. Thermodynamic modelling in the system CMFASH leads to an estimated P–T range of 1.5–3 kbar and 550–620°C for the metamorphic peak assemblage talc-olivine-chlorite-Ca amphibole-orthoamphibole.

Parental Magma Characterization of Salitre Cumulate Rocks (Alto Paranaíba Alkaline Province, Brazil) as Inferred from Mineralogical, Petrographic, and Geochemical Data

The Alto Paranaiba alkaline province (∼82 Ma), tectonically associated with a NW-trending linear structure bordering the São Francisco craton, consists of a wide variety of igneous forms and magma types. It includes several alkaline-carbonatite complexes, some of which are quite important economically. The Salitre complex is represented by two interconnected oval-shaped bodies, N-S-aligned and variable in size, emplaced into Proterozoic metasedimentary rocks. It consists mainly of syenites showing clear evidence of fenitization, ultramafic rocks (clinopyroxenites, mica-bearing clinopyroxenites, locally referred to as bebedourites, dunites, glimmerites, and per-ovskitites), radial dikes of trachytes/tinguaites, and carbonatites as small veins and also forming an elongated plug, approximately 500 m2 in area, associated with the Salitre I intrusion. In a few cases the carbonatitic rocks grade into phoscorites because of an increase in magnetite content. Texturally, the ultramafites are described as adcumulates, but meso- and orthocumulate rock types also are found. Clinopyroxene, olivine, phlogopite, perovskite, and apatite represent the main cumulus phases, sphene being less common. Intercumulus material consists of the same minerals in addition to melanite. Fresh rocks are rarely present at the surface; thus the entire set of data derives almost entirely from borehole samples. Incompatible-element distributions normalized to primordial mantle for the different ultramafic rock types exhibit many similarities, showing patterns clearly dominated by the abundance of perovskite and phlogopite. Syenitic rocks and carbonatites generally are less enriched than the cumulates. Distribution patterns of REE display LREE enrichment and strong LREE/HREE fractionation typical of alkaline suites; higher REE concentrations are related to the perovskitic rocks. Smooth, almost linear patterns are indicated for all the rock types excluding the syenites (which show a characteristic U-shaped distribution curve, probably the result of the removal of sphene and amphibole). It should also be noted that the dunitic rocks in general are more fractionated than the clinopyroxenitic ones. On the basis of texture and mineralogy, the Salitre ultramafites are interpreted as being formed by accumulative processes. No distinctive layering is evident in the rocks at the borehole scale. The crystallization sequence of the main rock-forming minerals of the ultramafites shows perovskite and opaques (Cr-spinel) as early phases, followed by olivine, clinopyroxene, and finally phlogopite; the last mineral is clearly a two-generation phase, the latest being related genetically to the fenitization processes. Geological, petrographic, and mineralogical evidence for the Salitre rocks indicates a highly complex origin involving multistage crystallization processes from an initial ultrapotassic magma source. This may explain the absence of ijolitic rocks over the entire Alto Paranaíba province, which is contrary to the situation in other alkaline complexes of southern Brazil (e.g., Jacupiranga, Juquiá). Carbonatitic rocks are interpreted as resulting from an unmixing process that developed during the evolution of the parental magma.

Nd-Sr isotopic and trace element study of rocks and fluids from the continental deep drilling Project (KTB), Germany

Geochemical analyses were interpreted on the dominant lithological units and on a deep crustal fluid from the Continental Deep Drilling Project (KTB) Pilot Hole, situated at the western margin of the Variscan Bohemian Massif. The biotite gneiss (from 384 m depth) shows a rare earth element pattern very similar to the European shale composite with Nd model ages of 940 Ma (CHUR) and 1.4 Ga (DM). The lamprophyre dike in the upper profile (1549 m), a nepheline and olivine normative basalt, is geochemically and isotopically similar to rocks from the Tertiary Central European Volcanic Province. The lower metabasite sequence (3575–4000 m), with an intrusion age of approximately 500 Ma, is made up primarily of highly metamorphosed subalkalic olivine basalts. The geochemical characteristics of the metabasites are a (La/Yb)N of 5–10, an La concentration of 20–50 times chondrite as well as (87Sr/86Sr)i of 0.7035–0.7038 and ɛeNd(T) of 4–6. These values suggest a depleted mantle source for the igneous precursors, evolving by assimilation-fractional crystallization processes with up to 25% of upper crust into the ultramafic, basaltic, and intermediate rock types of the metabasite sequence. The strong geochemical and chronological similarities between the KTB metabasites and rocks from the Munchberg Massif suggest that these units belong to the same lithological complex. The high salinity as well as the radiogenic 87Sr/86Sr ratio of 0.709413 in the KTB fluid from 4000 m depth might be the result of migrating fluids reacting with the regional Permo-Mesozoic evaporite deposits, followed by extensive Sr isotopic exchange with the upper crust.

Thermal state of the lithosphere beneath Central Mongolia: evidence from deep-seated xenoliths from the Shavaryn-Saram volcanic centre in the Tariat depression, Hangai, Mongolia

A suite of garnet-two pyroxene granulites, garnet pyroxenites and garnet peridotites from the pyroclastic facies of the Shavaryn-Saram volcanic centre in the Tariat depression in the northern part of the Hangai dome, Central Mongolia, yields pressure and temperature information for the lower crust and upper mantle in that region. Although a real geotherm cannot be constructed because of the common zoning of the minerals in some of the xenoliths, it can be inferred that the P-T locus from about 900 °C at 45 km to 1050 °C at 60 km defines a likely approximate geothermal gradient for the region around the time of entrainment of the xenoliths (about 1 Ma ago). This geothermal gradient is high relative to cratonic geotherms but is 50–100 °C lower than that for typical alkali basaltic provinces worldwide. The crust-mantle boundary inferred from the incoming of ultramafic rock types in this region is located at about 45 km and granulite rock types extend well into the mantle. This interpretation is consistent with the most recent seismic sections for the area.Analytical data for major and trace elements (by electron- and proton-microprobe respectively) in clinopyroxenes indicate that the Cr-diopside series xenoliths are enriched in basaltic components (including Al2O3, Na2O, TiO2, Sr, Y and Zr).The combination of elevated temperature and fertile composition of the uppermost mantle as revealed by the xenoliths could explain the observed anomalous seismic signatures seen beneath this region.

The Evolution of Serpentine Endemics: A Chloroplast DNA Phylogeny of the Streptanthus glandulosus Complex (Cruciferae)

The Streptanthus glandulosus complex constitutes an ideal group for the study of evo- lution and edaphic endemism. The complex comprises both narrowly and widely distributed taxa, most of which are restricted to serpentine soils in the central Coast Ranges of California. Populations are typically highly disjunct, occurring on isolated outcrops of serpentine substrate. To investigate evolutionary patterns and infer processes of diversification in these edaphic endemics, we assessed chloroplast DNA (cpDNA) diversity among 55 populations within the complex and 11 additional species of Streptanthus and the closely related Caulanthus. Variation at 948 restriction sites yielded 12 most parsimonious trees. The strict consensus of cpDNA relationships suggests that the complex is monophyletic and consists of four major clades, each of which corresponds to a discrete geographic region. The levels and geographic patterns of cpDNA variation are consistent with the hypothesis that S. glandulosus subsp. glandulosus is a paleoendemic taxon that may be the progenitor of the entire complex. Some clades are not well resolved and include taxa that comprise only two or three populations; these taxa may have arisen relatively recently and could be considered neoendemics. These results complement and corroborate those from allozyme analyses and interpopulational crossing experiments. The evolutionary processes revealed in the S. glandulosus complex are likely to be common to many other endemics of the California Floristic Province, given the similar biogeographic histories of many species of the region. Despite the cosmopolitan occurrence of ser- pentine floras (Brooks 1987), surprisingly few studies have addressed hypotheses regarding the evolution of species endemic to serpentine soils. Stebbins (1942) classified endemics in general as either insular or depleted species, now considered neoendemics or paleoendem- ics, respectively (following Stebbins and Major 1965). A neoendemic is a taxon that has arisen relatively recently and has undergone minimal range expansion, whereas a paleoendemic is a taxon that has suffered range contraction, often leaving it with a disjunct distribution. We will show that the concepts of neoendemism and paleoendemism apply well to the evolution of serpentine plants. We will also demonstrate how the integration of phylogenetic and geographic patterns can be used to distinguish between neoendemics and paleoendemics and to reveal much about the routes to species rarity in both serpentine and non-serpentine environments. The unique physical and floristic character- istics of serpentine habitat have been apparent to naturalists for hundreds of years (Brooks 1987). Serpentine soils, which are derived from an array of ultramafic rock types, bear physical and chemical properties that render them un-

Petrology and geochemistry of peridotites and associated vein rocks of Zabargad Island, Red Sea, Egypt

Zabargad (St. John's) Island in the Red Sea contains three ultramafic bodies, one of which bas produced the famous gem olivine (peridot). The ultramafic rock types consist of two major groups—the peridotites and the vein rocks within them. The peridotites are divided into three groups: primitive, depleted and metasomatized. The primitive peridotites are the most abundant and are represented by mainly pristine spinellherzolites which have chemical compositions representative of the subcontinental upper mantle. The depleted peridotites are mainly harzburgites and nome dunites and both are similar to worldwide occurrences. The most depleted peridotites also appear to have the greatest metasomatic additions of incompatible elements, as has been noted at other localities. Metasomatic additions were clearly accompanied by tectonic shearing. Metasomatism included infiltration of incompatible elements and the formation of porphyroblasts of clinopyroxene, amphibole, Al-spinel and plagioclase; il took place under a variety of p-T conditions and with fluids of differing compositions.

Criteria for the recognition of metamorphosed or altered lamprophyres: A case study from the Archaean of Kambalda, Western Australia

Abstract The ability to distinguish ancient lamprophyres from other mafic and ultramafic intrusive rock types has become increasingly important. Lamprophyres have been used as marker horizons in the mapping of several Precambrian terranes, they have been recognized for decades as possible indicators of gold mineralization, and their presence has implications for the tectonic interpretation of ancient greenstone belts. Whilst fresh, undeformed lamprophyres are readily recognized, metamorphosed, altered and/or deformed examples are more problematical. The intrusive ages of Kambalda lamprophyres are constrained by available field and geochronological evidence, including direct dating of one kersantite, to the period 2684-2660 Ma. These rocks therefore belong to the world's oldest group of alkaline rocks and lamprophyres so far recognized, which also includes contemporaneous examples from Canada, Zimbabwe and, probably, Tanzania. Despite their extreme age and low- to medium-grade regional metamorphic history, these rocks still preserve characteristic lamprophyric textures, chemistry and mineralogy. Only where metamorphism has been overprinted by gold-related alteration and deformation do the lamprophyres cease to be recognizable petrographically. However, their unique chemistry persists and, despite losses of the most mobile elements from presumed magmatic compositions, altered and deformed specimens still retain far higher contents of Sr, K, Ba, Th, Nb, P, Zr and Ce than basalts of the local greenstone succession. Even lamprophyres which have been converted to biotite-plagioclase-carbonate-pyrite schists retain this distinctive chemical signature. During both alteration and metamorphism, ‘mobile’ elements (K, Rb, Sr, Ba) display far larger changes than archetypal ‘immobile’ elements (Ce, Nb, Zr, P, Ti), clearly sanctioning traditional preference for the latter in assessing altered rocks. However, the surprising loss of Nb during biotitization of hornblende indicates that negative Nb anomalies, conventionally regarded as indicators of subduction-related magmatism, can also be induced purely by alteration processes.

Pyroclastic rocks: another manifestation of ultramafic volcanism on Gorgona Island, Colombia

Tertiary ultramafic volcanism on Gorgona Island, Colombia, is manifested not only by komatiite flows, but also by a more voluminous sequence of tuff breccias, which is cut by comagmatic picrite dikes. The ultramafic pyroclastic rocks are chaotic to stratified mixtures of angular to subrounded glassy picritic blocks and a fine grained volcaniclastic matrix that consists primarily of plastically-deformed, glassy globules. The entire deposit is interpreted to have formed by an explosive submarine eruption of phenocryst-laden picritic magma. MgO contents of tuff breccias and picrite dikes range from 21 to 27 wt%. Relative to nearby komatiite flows, these rocks are MgO-rich, and FeO-, TiO2- and Ni-poor. HREE concentrations are very low (<seven times chondrite), and similar to those of the komatiites, whereas the LREE are extremely fractionated ((Ce/Sm)N<0.2), indicating derivation from a more highly depleted mantle source region. Nd isotopic data suggest a cogenetic relationship between the picritic magma that formed the tuff breccia and associated dikes and that which produced the komatiite flows. Nevertheless Pb isotopic data as well as whole rock geochemistry preclude such a connection, either due to olivine fractionation/accumulation or to different degrees of partial melting. These ultramafic rock types crystallized from magmas which most likely were extracted from distinct mantle source regions.

Statistical analysis of thematic mapper simulator data for the geobotanical discrimination of rock types in southwest oregon

An evaluation of Thematic Mapper Simulator (TMS) data for the geobotanical discrimination of rock types based on vegetative cover characteristics is addressed in this research. A methodology for accomplishing this evaluation utilizing univariate and multivariate techniques is presented. TMS data acquired with a Daedalus DEI-1260 multispectral scanner were integrated with vegetation and geologic information for subsequent statistical analyses, which included a chi-square test, an analysis of variance, stepwise discriminant analysis, and Duncan's multiple range test. Results indicate that ultramafic rock types are spectrally separable from nonultramafics based on vegetative cover through the use of statistical analyses.

The Patuki intrusive suite: closed-system fractionation beneath a slow-spreading ridge

A wide range of mafic and ultramafic rock types, together with cogenetic silicic plagiogranites, form a structurally coherent intrusive sequence within the Patuki Volcanics at south D'Urville Island, New Zealand. In addition, gabbroic rocks comprise abundant tectonic inclusions in highly-sheared, concordant serpentinite bands which intrude the Patuki suite. Chemical evidence suggests many of the gabbros, including those in which recrystallization has obliterated original textures, represent magmatic cumulates and indicates extensive closed-system fractionation analogous to that known to occur beneath slow-spreading mid-oceanic ridges. Dyke intrusion occurred throughout the generation of the suite. An early stage of spreading is suggested by the anomalously low thickness of the sequence, the non-sheeted nature of the dyke suite and chemical characteristics of the lavas which comprise the extrusive component of the ophiolite.

Petrology and mineralization of the Sakhakot-Qila ophiolite, Pakistan

Summary The Sakhakot-Qila complex is an incomplete, dominantly ultramafic ophiolite. The geology, rock and mineral compositions, and occurrences of chromite ore deposits are described. The predominant ultramafic rock types are harzburgite, wehrlite and dunite, and dykes of pyroxenite. The mafic components of the complex are a metamorphosed gabbro and dykes of metadolerite. Postmagmatic alteration is common, and several kinds of rodingite are abundant. They formed by addition of Ca, Al, Cl and Cu and removal of Si, Mg, Fe and Ni from pre-existing rocks, and contain corundum, clintonite and Al-depleted ferritchromite in addition to the more usual minerals such as grossular and vesuvianite. The rodingites appear to have developed independently of serpentinization, probably with the participation of seawater. There are numerous small chromite deposits, varying widely in texture and composition. There is a strong reciprocal Cr-Al variation and lesser Mg-Fe 2+ variation, partly related to the stratigraphic position of the chromite. Nickeliferous opaque mineral assemblages are well developed.

THE NOMENCLATURE OF POLYMICT BASALTIC ACHONDRITES

The subgroups within the basaltic achondrite suite are defined using the structural criterion of Wahl (1952). The ‘monomict’ meteorites are samples of a single lithology while the polymict meteorites are those containing two or more lithologies. The ‘monomict’ subgroups eucrites, cumulate eucrites and diogenites are subdivided into both brecciated and unbrecciated meteorites. The polymict achondrites sample a petrological‐compositional continuum that contains both mafic and ultramafic rock types and may be subdivided into several groups. Two groups of polymict basaltic achondrites, the polymict eucrites and howardites are separated using an arbitrarily defined criterion. The recommended criterion is based on the amount of magnesian ortho‐pyroxenite (diogenite) component in the meteorite. Howardites contain more than 10% and polymict eucrites contain less than 10%.The criteria proposed (perhaps with minor variations), appear to reconcile the ambiguities caused by the polymict eucrites. These meteorites, using earlier structural criteria, are howardites, but using mineralogical‐chemical criteria are eucrites. As a subgroup of the polymict achondrites, their relationship with the howardites is clear, and the preservation of the term ‘eucrite’ in their name highlights their modal affinity to the monomict eucrites.

Structural petrology of the Olympus ultramafic complex in the Troodos ophiolite, Cyprus

Research Article| June 01, 1978 Structural petrology of the Olympus ultramafic complex in the Troodos ophiolite, Cyprus RICHARD P. GEORGE, JR. RICHARD P. GEORGE, JR. 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 940252Present address: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California 90024 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1978) 89 (6): 845–865. https://doi.org/10.1130/0016-7606(1978)89<845:SPOTOU>2.0.CO;2 Article history first online: 01 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 RICHARD P. GEORGE; Structural petrology of the Olympus ultramafic complex in the Troodos ophiolite, Cyprus. GSA Bulletin 1978;; 89 (6): 845–865. doi: https://doi.org/10.1130/0016-7606(1978)89<845:SPOTOU>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 SocietyGSA Bulletin Search Advanced Search Abstract The Olympus ultramafic complex is one of three ultramafic complexes that crop out at the lowermost stratigraphic levels of the Troodos ophiolite, Cyprus. The Olympus ultramafic complex comprises two types of ultramafic and related rocks: (1) harzburgite tectonite, a residuum of partial fusion, and (2) cumulus chromitite, dunite, wehrlite, pyroxenite, and gabbro, the products of fractional crystallization and magmatic sedimentation of basaltic magma. The harzburgite tectonite is the basement or “country rock” intruded by the magma from which the cumulates crystallized. Pyrometamorphic textures in the harzburgite (magmatic pyroxenes in metamorphic peridotite) near its contact with the cumulates perhaps record “contact metamorphism” of the harzburgite by the magma.The stratigraphically lowest cumulates were penetratively deformed (and hence are termed “ultramafic metacumulates”) during the same event that produced the dominant pyroxene foliation (S1) in the underlying harzburgite, whereas the stratigraphically highest cumulates were not penetratively deformed. The structural transition between metacumulates and cumulates is gradual and occurs in olivine cumulates that contain intercumulus clinopyroxene (clinopyroxene-bearing dunites and wehrlites).Two mechanisms can explain the structural transition from metacumulates to cumulates: (1) Deformation of the harzburgite basement occurred during accumulation of the cumulates (syntectonic magmatic sedimentation); the lowermost (oldest) cumulates consequently deformed more than the uppermost (youngest) cumulates. (2) The uppermost cumulates, perhaps rich in intercumulus liquid (now crudely represented by postcumulus clinopyroxene and plagioclase) at the time of the deformation, may have accommodated the strain by grain boundary sliding (“crystal-mush flow”) and consequently left little evidence of solid-state, penetrative deformation.If syntectonic and posttectonic magmatic sedimentation, crystal-mush flow, partial fusion of metamorphic peridotite, and multiple intrusion of magma act simultaneously during the formation of ophiolites, then the resulting field relations, structures, textures, and fabrics will be exceedingly complex, particularly if subsequent transport and emplacement impose strong metamorphic and tectonic overprints. One should expect that differences in the relative chronology and intensity of the processes of the formation of ophiolites make every ophiolite unique. 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.

The nature of chrysotile asbestos occurrences in southern Africa; a review

The principal chrysotile asbestos occurrences in Rhodesia, South Africa, and Swaziland are described, emphasis being placed on the regional geological settings and host rock stratigraphy of the mineralized areas. All the more important asbestos ore deposits in southern Africa are Archean in age ( approximately 2.5-3.5 b.y. old), being associated with ultramafic complexes occurring either as sill-like bodies in greenstone belts or as later cross-cutting intrusions.The ultramafic bodies can be grouped into three varieties. These, in order of decreasing age, are: (1) the layered complexes associated with basaltic and peridotitic komatiite extrusives and forming part of the Lower Ultramafic Unit of southern African greenstone belts, (2) layered ultramafic bodies associated with the intermediate to acid volcanic rocks that constitute part of the Mafic-to-Felsic Unit of greenstone belts, and (3) ultramafic intrusive bodies that postdate the greenstone belts but which are still affected by Archean tectonic disturbances that arise from the emplacement of granites.All the principal asbestos-bearing complexes show magmatic segregation into layered, often cyclically repetitive, differentiation sequences. These single or multicyclic sequences may consist of two or more of the following rock types: dunite, peridotite, harzburgite, Iherzolite, wehrlite, bronzitite-enstatolite, websterite, gabbro, norite, and gabbroic anorthosite. Where fractional crystallization of the ultramafic magma has been most efficient, many layers at, or near, the base of the complexes comprise monomineralic cumulate phases. These commonly consist of dunites and Mg-rich orthopyroxenites. With increasing distance from the base, progressive Mg depletion and Fe enrichment of the successive layers take place. Although chrysotile asbestos may commonly be encountered in all serpentinized ultramafic rock types, optimum development of economically exploitable fiber generally occurs in dunites, peridotites, or harzburgites.The Archean layered complexes were derived from magma of ultramafic composition in contrast to magmas of tholeiitic parentage that gave rise to the great stratiform intrusions like the Bushveld, Stillwater, and many others, including the Great Dyke in Rhodesia. The Great Dyke, unlike the others, however, acts as host to small chrysotile deposits developed in serpentinized dunites or harzburgites.In addition to the asbestos mineralization found in the layered complexes, subordinate deposits, occurring in serpentinized dolomitic rocks associated with the approximately 2.0-b.y.-old Transvaal Supergroup, are briefly described.Whereas faulting and fracturing is generally acknowledged as being largely responsible for the local development of asbestos fiber, examples from the southern African green-stone belts demonstrate that folding is often a dominant regional controlling factor in the localization of asbestos mineralization in ultramafic rocks.

D/H and O 18/O 16 studies of serpentinization of ultramaflc rocks

Oxygen and hydrogen isotope analyses have been obtained on serpentine and related metasomatic talc and ‘blackwall’ minerals from the following ultramaflc-rock types: 1. (1) alpine associations in unmetamorphosed or low-grade metamorphic terranes; 2. (2) alpine associations from medium- to high-grade regionally metamorphosed areas; 3. (3) magmatic ultramafic intrusions such as the layered stratiform complexes, the concentrically-zoned Alaskan-type bodies, the high-temperature Mt. Albert alpine intrusion, and the kimberlite body at Moses Rock, Utah; 4. (4) dikes and sills of Precambrian shield areas; and 5. (5) deweylites, which are serpentine-like mineraloids of probable near-surface weathering origin. The δD values of the lizardite-chrysotile serpentines from all of the ultramafic rock types exhibit a total range from −59 to −205 per mil and, except for the Precambrian samples, most show an exceptionally good correlation with geographic position and latitude of the sample locality. The lizardite-chrysotile samples show a progressive change in δD that parallels the present-day variation in δD of meteoritic waters across the North American continent: Caribbean (−59 to −71); Central America (−78 to -97); California (−85 to −108); Oregon (−107 to −149); Washington (−127 to −142); S.E. Alaska ( − 127 to −140); southern British Columbia ( − 150 to −163); northern British Columbia ( −162 to −205). The δO 18 values of lizardite-chrysotile range from +8.7 to −5-0 per mil and are more correlative with the δO 18 of the country rocks than with latitude. These relationships, combined with model estimates of the isotopic compositions of waters involved in such serpentinization, suggest that much of this lizardite-chrysotile serpentinization probably formed by waters of meteoric-hydrothermal origin at relatively shallow levels in the Earth's crust. Inasmuch as sedimentary formation waters typically contain a major meteoric-water component, they are also possible agents of this type of serpentinization. In contrast to lizardite-chrysotile, the pure antigorites have very restricted ranges of δD (−39 to δ-66) and δO 18 ( +4.7 to +8.7) similar to values in metamorphic chlorites. This indicates that the antigorites generally have formed during regional metamorphism in the presence of non-meteoric waters; similar relationships are also observed for all metasomatic talc and ‘blackwall’ minerals. The deweylites are consistently higher in δO 18 than all true serpentines, compatible with a very low temperature of formation (~15–30°C) and serpentine-H 2O fractionation factors of 1.0185 ( O 18 O 16 ) and 0.946, (D/H). The observed isotopic values of all serpentines, combined with our experiments on isotopic exchange rates in the serpentine-H 2O system, suggest that most samples examined in this work probably have largely preserved the isotopic compositions they acquired at the time of their formation.

Heavy oxygen isotope of the olivine from the ultramafic rocks as indicator of their genesis

The Isotropic composition of olivine in several types of ultramafic rocks shows variations that can be explained by the different depths of origin in the upper mantle. The δO18 values of olivine increase in the sequence alpine ultramafics, gabbro pyroxenite-dunite associations, and alkaline ultramafic associations. The alpine ultramafics form in the shallowest upper mantle regions, whereas the alkaline ultramafics form in deeper regions underlying stable continental crust.

Bedrock geology of the nunataks and semi-nunataks in the Frederikshåbs Isblink area of southern West Greenland

The Precambrian geology of an area of 250 km2 within the old gneiss block ofWest Greenland is described. The area is composed of a gneiss-granite-amphibolite complex which is cut by later basic dykes. The largest part of the complex is made up of folded and migmatised biotite-, hornblende- and epldotebearing gneisses in which granodiorites and adamellites can be mapped as separate granite units. Different types of amphibolites and ultramafic rocks occur in the complex. The oldest amphibolites occur as migmatised and folded inclusions and tracts within the gneisses and these are cut by different generations of metabasite dykes which are themselves migmatised and deformed. Metavolcanic rocks containing pillow lavas and metagabbro layers exist on some nunataks and these are considered to form cover rocks to an earlier basement. Aspects of the deformational and metamorphic history are discussed. At least six main phases of deformation ean be recognised in rocks which are polymetamorphic. The gneisses and granites of the complex display amphibolite facies mineralogy but greenschist mineral assemblages characterise some of the amphibolite and ultramafic rocks. Relics of an early high-grade metamorphism, possibly reaching granulite facies conditions, are preserved in some basic rocks. The basic dykes which cut this polymetamorphic complex vary from dolerite to lamprophyre. The late tectonism and metamorphism is described. An attempt is made to place the area into the regional chronology of this part of West Greenland.