Primary Assemblage Research Articles

New evidence for polyphase metamorphism of glaucophane schist and eclogite exotic blocks in the Franciscan Complex, California and Oregon

ABSTRACT The early metamorphic history of high‐grade exotic blocks in the Franciscan Complex may be more complicated than previously supposed. The different assemblages of high‐grade glaucophane schist, eclogite, amphibolite and hornblende schist are commonly considered to have formed at the same time from essentially unmetamorphosed oceanic crust. However, new textural and mineralogical data presented here suggest that high‐grade glaucophane schist and eclogite have replaced an earlier epidote‐amphibolite facies assemblage that is identical to the primary assemblages in many of the hornblende‐rich blocks. At least some of the hornblende‐rich blocks may therefore be well‐preserved remnants of the earlier metamorphism. Comparison of the mineral assemblages and element partitioning in the mixed‐assemblage blocks suggests that the glaucophane schist and eclogite metamorphism took place at slightly lower temperatures but at the same or higher pressures than the earlier, hornblende‐forming stage.

Petrology of a gneiss-amphibolite lower crustal unit from Zabargad Island, Red Sea

A metamorphic unit, consisting of amphibolites interlayered with felsic gneisses, is exposed on Zabargad Island, in tectonic contact with mantle-derived peridotites. The amphibolites contain relicts of igneous pyroxenites and gabbros which have recrystallized into mafic granulites. The chemistry of the gabbros-pyroxenites' primary assemblage (Al-augite ± Al-orthopyroxene + kaersutitic pargasite ± plagioclase) suggests that the gabbro-pyroxenites are remnants of a basic layered complex which crystallized at pressures not lower than 9–10 kbar and probably not higher than 12–13 kbar. The felsic gneisses were originally made of quartz + plagioclase + orthopyroxene ± garnet ± clinopyroxene ± magnesiohastingsite, an assemblage characteristic of high pressures (− 5–12 kbar) garnet granulite facies. The high- P tonalitic-trondhjemitic composition of some of the gneisses suggests they were originally part of the lower continental crust, an interpretation supported by the presence of characteristic zircon crystals with ovoidal morphology. The association of granulitic gneisses with high- P gabbroic rocks suggests that the lower continental crust was intruded by basaltic magmas. The pyroxenite-gabbro complex underwent deformation when still hot, followed by recrystallization at P of about 5–6 kbar. During this event, breakdown of the Al-pyroxenes under hydrous conditions resulted in granulitic Al-poor pyroxenes + pargasitic hornblende + plagioclase assemblages. Both the mafic and sialic granulites subsequently underwent further decompression under increasing fluid partial pressure and oxygen fugacity, which resulted in widespread amphibolization of the rocks in the presence of relatively high- T, chlorine-rich, metasomatic fluids. The resulting assemblage Mg-hornblende + plagioclase + ilmeno-hematite ± sphene ± quartz suggests T of 600–700°C, P tot < 2 kbar and high ƒO 2 . Finally, rising fluid partial pressure leads to breakdown of Mg-hornblende and crystallization of metasomatic phases such as alkali-richer, alumina-poorer Mg-hornblende, actinolite. Cl-biotite, Cl-scapolite and Cl-apatite. The Zabargad metamorphic unit probably results from underplating of the lower continental crust by basic magma intrusions before and during the early stages of rifting in the central Red Sea. Subsequently, the lower crustal gabbro-gneiss complex was subjected to a number of retrometamorphic events under decreasing P- T conditions, probably related to progressive uplift during rifting and crustal thinning. The last, low- P metasomatic event is probably related to the final stages of the emplacement of Zabargad Island.

Petrology and tectonic setting of blueschist facies metabasites from the Emo Metamorphics of Papua New Guinea

Blueschist facies metabasites from the Emo Metamorphics of Papua New Guinea contain the assemblage quartz-albite-phengite-stilpnomelane-ferroglaucophane-chlorite-almandine-epidote-sphene-apatite + relict igneous pyroxene with omphacitic rims. The assemblage is similar to the lawsonite-epidote transition zone on New Caledonia. Textural variation from unfoliated to foliated blueschist is marked by an increase in retrograde modal actinolite and chlorite, suggesting that strain has accelerated the actinolite- and chlorite-forming reaction. Application of standard geobarometers and geothermometers to the primary assemblage suggests pressure and temperature conditions of metamorphism of approximately 7.0 kbar and 320°C. The textural variation suggests that the Emo Metamorphics may be a sliver of oceanic crust caught up in the thrusting that accompanied obduction of the Papuan ophiolite. The resulting depressurization led to retrograde growth of actinolite and chlorite.

Petrology and tectonic setting of blueschist facies metabasites from the Emo Metamorphics of Papua New Guinea

Blueschist facies metabasites from the Emo Metamorphics of Papua New Guinea contain the assemblage quartz-albite-phengite-stilpnomelane-ferroglaucophane-chlorite-almandine-epidote-sphene-apatite + relict igneous pyroxene with omphacitic rims. The assemblage is similar to the lawsonite-epidote transition zone on New Caledonia. Textural variation from unfoliated to foliated blueschist is marked by an increase in retrograde modal actinolite and chlorite, suggesting that strain has accelerated the actinolite- and chlorite-forming reaction. Application of standard geobarometers and geothermometers to the primary assemblage suggests pressure and temperature conditions of metamorphism of approximately 7.0 kbar and 320°C. The textural variation suggests that the Emo Metamorphics may be a sliver of oceanic crust caught up in the thrusting that accompanied obduction of the Papuan ophiolite. The resulting depressurization led to retrograde growth of actinolite and chlorite.

An analysis of the textures within the primary assemblage peridotite, the Lizard, Cornwall

Rock samples from the primary assemblage peridotite of the Lizard, Cornwall, contain a range of textures. These textures, principally between olivine and pyroxene, range from those with a linear or planar fabric associated with evident penetrative deformation to those of considerable complexity in which only an initial deformation is apparent. The diverse textures are described in terms of a set of texture elements and by this method a semi-quantitative assessment is made of the varieties of texture elements present in a range of samples. New electron probe microanalyses of co-existing pyroxenes are presented from various sites across the primary assemblage peridotite. From these analyses, temperatures of re-equilibration are obtained. The new data, both textural and thermometric, illustrate the consequences upon an original texture set as the primary assemblage peridotite underwent penetrative deformation and re-equilibrated with declining temperatures and pressures within the spinel-lherzolite facies of the upper mantle. It is concluded that at 1200°C ± 70°C and 15 Kb ± 4 Kb a large part of the primary assemblage peridotite contained extensive sets of the texture elements described, in an undeformed state, and that these represent primary textures from an original equilibration within the upper mantle.

Mantle-derived sapphirine

AbstractA xenolith from the Delegate breccia pipe (New South Wales, Australia) contains sapphirine in equilibrium with aluminous clinopyroxene, garnet, and plagioclase (An48). This unusual assemblage probably developed from a clinopyroxene (±spinel ± plagioclase) cumulate during cooling from &gt; 1400°C to c. 1000°C at pressures near 15 kbar. The sapphirine is close to the 7:9:3 composition, suggesting that bulk composition is more important than P-T conditions in determining the stoichiometry of natural sapphirines. A similar occurrence of sapphirine has also been recorded in mantlederived xenoliths from the Stockdale kimberlite in Kansas. Re-examination of sapphirine granulites from Finero suggests that their primary assemblages and origin may have been similar to those of the Delegate xenolith. Sapphirine is clearly stable under upper-mantle conditions in Ca-Al-Mg-rich bulk compositions.

Toward the definition of criteria for the recognition of artificial bone alterations

Cutting, fracturing, flaking, and polishing of bones and other osseous materials may in some instances be interpreted as evidence of former human activity. Such interpretations must avoid confusion with the wide variety of natural processes that alter bones. Reliable criteria are prerequisite to sound inferences based on bone, regardless of whether they have been redeposited or recovered from primary assemblages. Criteria must be defined by means of actualistic studies (neotaphonomy) and experiments that demonstrate causal relationships between patterns of alteration and the processes that produce them. The criteria can be empoyed in the interpretation of fossil bones (paleotaphonomy) on the basis of uniformitarian principles. At the present time, such relationships are relatively well understood in the case of cut marks on bones, and modest progress has been made in investigating fractured and flaked bones. Polished bones are little understood and difficult to interpret.

Meta-igneous granulite and ultramafic xenoliths from basalts of the Midland Valley of Scotland: petrology and mineralogy of the lower crust and upper mantle

ABSTRACTUltramafic xenolith lithologies representative of the mantle beneath the Midland Valley of Scotland comprise magnesian peridotite (predominantly spinel lherzolite) and cumulate wehrlites and clinopyroxenites. The lherzolites are typical of the worldwide type I (Cr-diopside) xenolith suite; their textures and mineral chemistry record a complex thermal and deformational history. The petrographical and mineralogical features of the wehrlite-clinopyroxenite suite can be interpreted within the context of a sequence of cpx + ol ± sp cumulates that have undergone a protracted period of subsoli dus re-equilibration. Mineral compositions are similar to those of type II (Al-augite) ultramafic xenolith suites.Although xenolith populations imply widespread lower crustal heterogeneity, meta-igneous basic granulites form a major component of this region beneath the Midland Valley. They are principally composed of pi + cpx + mt ± opx ± ap; a modal continuum exists from clinopyroxenite to anorthosite, skewed towards plagioclase-rich lithologies. Garnet is rare. Locally, evidence indicates re-equilibration from garnet granulite precursors. Rock densities range from c. 2·8 to c. 3·2 gm . cm−3, implying P-wave velocities in the range 6·5–7·5 km . s−1 consistent with the known seismic properties of the lower crustal layer beneath the Midland Valley. Phase relations are also consistent with equilibration of primary assemblages at depths of 20–35 km. Retrograde reactions indicate that portions of the crust may have had a complex pressure-temperature-time evolution. Major element compositions of the granulites are broadly basaltic, ranging from ne to hy normative with the mean corresponding to alkali olivine basalt. They are distinct, chemically, from basic granulites of the Lewisian complex of NW Scotland.

Geochemistry of early mesozoic basalts from Tunisia

The Late Triassic-Liassic evaporites of the Tunisian Central Atlas commonly contain fault blocks of layered or brecciated metabasalts and tuffs. The metabasalts show the effects of two alteration episodes: (1) a greenschist-facies, spilitic assemblage composed of albite, chlorite, actinolite, epidote, calcite, quartz and leucoxene; and (2) a later, potassic alteration assemblage composed of potassium-feldspar, sericite, calcite, chlorite, and hematite. The primary assemblage of plagioclase + titanomagnetite + ophitic pyroxene ± olivine can be identified from crystal pseudomorphs and rare, relict crystals. Based on the abundances and interrelations of immobile elements (Ti, P, Y, Zr, Nb, Ni and Cr), the metabasalts are shown to be tholeiitic basalts with slight alkaline tendencies. Based upon these features, their field relations, and the extensive occurrence of Triassic-Liassic tholeiites across the northwest African margin, the Tunisian metabasalts are considered to be remnants of a widespread basalt province associated with the breakup of Pangaea in the Early Jurassic.

The primary ore mineralogy of the Alderley Edge deposit, Cheshire

AbstractThe Alderley Edge deposit, Cheshire, England has been mined principally for Cu and Pb. Here, veins and disseminations occur in coarse-grained clastic sediments of Triassic age and are classified as belonging to the red-bed type of mineralization.Examination of material in polished and thin section has shown the mineralization to be multiphase. The earliest opaque phases are associated with the diagenesis of the sandstones and include authigenic anatase, plus bravoite, pyrite, and chalcopyrite within authigenic quartz overgrowths. This stage of mineralization was followed by the formation of intergrown bravoite, pyrite, chalcopyrite, sphalerite, and galena accompanied by minor amounts of Ni-Co-Fe sulpharsenides, marcasite, and tetrahedrite cementing the clastic grains. This primary assemblage has undergone extensive alteration resulting in the formation of djurleite, covelline, and blaubleibender covelline accompanied by Pb and Zn carbonates and sulphates. Later, extensive shattering of the cemented clastic grains has been infilled by supergene Cu, Pb, and Zn carbonates and sulphates, limonite, and other secondary minerals.Electron probe microanalysis of the sulphides shows the pyrite to contain significant Ni and Cu but lesser amounts of Co; the sphalerite to be an Fe-poor variety but to contain Cd, and the chalcopyrite to contain Sb. Ag is present in appreciable amounts in tetrahedrite and occurs in trace amounts in both sphalerite and chalcopyrite but not in galena. The concentration of Ag increases with the alteration of the primary sulphides to secondary sulphides. This is most clearly demonstrated by the alteration of chalcopyrite to an idaite-like phase and finally to djurleite when the Ag content increases from 0.10 to 2.35 wt. %.

Electron microprobe studies of Iceland Research Drilling Project high‐temperature hydrothermal mineral geochemistry

The geochemistry of epidote‐ and chlorite‐dominated hydrothermal assemblages in the Iceland Research Drilling Project drill core has been studied by electron microprobe. The association of epidote, andradite, and Cu‐Fe sulphides suggests maximum basalt alteration temperatures of 350°–300°C. Near‐pure andradite implies a fluid XCO2 &lt;0.05. The sequence of successively lower temperature minerals (epidote, prehnite, laumontite) in veins and vesicles suggests precipitation during rapid cooling from &gt;300°C to &lt;200°C and together with zonation in garnet and epidote implies fluid depletion in Fe3+ with time. The eradication of probable early zeolite facies assemblages and this cooling sequence suggest that temperatures rose to &gt;300°C during burial of the lower part of the IRDP sequence before vigorous fluid circulation, producing rapid cooling and intense alteration, was established. Transition metals (Fe, Mn, Cu) were released from primary assemblages but were incorporated in epidote, chlorite, and sulphide. Epidote contains average SrO = 0.23%, but intersample variations in trace element contents of secondary minerals are complex, and there are no correlations with depth. It is concluded that the effects on whole rock chemistry (including Sr) and secondary mineral chemistry were largely controlled by kinetic factors such as (1) differing responses to alteration by primary assemblages and (2) permeability variations.

The primary crescumulates of the Lizard peridotite, Cornwall

SummaryPreserved in the deformed peridotites of the Lizard, Cornwall, are relics of intricate primary precipitates of olivine. The mineral compositions of those rocks with an abundance of primary relics are typical of the primary assemblage Lizard peridotite. In detail the olivine primary structures resemble, but are not identical to, the crescumulates described from various layered ultrabasic intrusions. The available evidence shows distinctive specific features in the Lizard crescumulates in the scale and morphology of the structures and in the conditions of growth.

奈良県五代松鉱床におけるスカルンの産状

The Goyomatsu magnetite skarn deposit consists of large and small irregular massive bodies interlayered in upper Chichibu formations (limestone, sandstone, slate etc.), which have been metamorphosed by Miocene Omine acidic rocks (granodiorite).On the basis of mineralogic observations and thermodynamic data, the principal conditions of formation of the skarns have been determined. Results are as follows:1) Field evidence reveals that in the deposit hedenbergite is predominant in and near the granodiorite, and andradite is predominant in and near the limestone. This contrasting occurrence appears to show that the μO2 in the limestone gradually increased away from the granodiorite-limestone contact, assuming these skarns were formed at nearly the same stage.2) Generally, andradite rarely coexists with actinolite in skarns. This field evidence iswell in agreement with thermodynamic data, which shows that the andradite+actinolite phase cannot be a stable assemblage at constant temperature and pressure asseen in Fig. 3.3) Fig. 3 suggests that in general, under constant μCO2 conditions, the following mineral zonation formed with increasing μO2: zones of hedenbergite+actinolite, hedenbergite+andradite, and andradite, respectively. However primary assemblages of hedenbergite+actinolite havenot been observed. On this point, we need more detailed examinations.4) It is also found that in the skarns, hedenbergite has sometimes altered into aggregates of actinolite, calcite and quartz. Assuming constant temperature and pressure it can be interpreted that such alteration was caused by an increase in μCO2 at low μO2 environments.

Plate-Tectonic Controls on Diagenesis

Plate tectonics can be used as a framework for relating diagenesis to geological and geophysical parameters: regional tectonics, including rates of subsidence, uplift and deformation; volcanism and plutonism; heat flow; and transport of formation waters. These parameters affect diagenetic variables: the primary detrital-chemical assemblage; composition of early interstitial waters; rates and depth of burial; length of time buried; geothermal and pressure gradients; and oscillatory vertical movements. The consequences of changes in the values of these variables are illustrated for the diagenesis of the silica polymorphs, Ca- and Mg-carbonates, zeolites, and clay minerals. The unstable nature of biogenic opaline silica components of the primary assemblage leads to successive polymorphic inversions, ultimately to microcrystalline quartz chert. Pressure and temperature control the solubility and rate of inversion of all silica polymorphs. Carbonate systems also are controlled both by the presence of unstable phases (Mg-calcite, aragonite) in the primary assemblage and the effects of pressure and temperature on solubility. In later diagenesis oscillatory movements lead to reversals or replacements of carbonates. Zeolites are derived from the marine and nonmarine alteration of volcaniclastics. Burial, accompanied by pressure and temperature increases, produces the diagenetic zeolite facies. Clay minerals of both terrigenous and marine volcanic derivation are responsive to ion-exchange and silicate reconstitution reactions in early interstitial waters. Pressure and temperature increases lead to the stable chlorite-illite assemblage under deep-burial conditions. Plate-tectonic environments are for simplicity classified as mid-ocean ridge, trailing edge (passive), subduction, continent-continent collision, rift valley, and intraplate (cratonic). Mid-ocean ridges are sites of thin pelagic sediment accumulation affected by high heat flow, hydrothermal activity, and volcanic activity, followed by decreasing heat flow and slow burial as the sediments spread from the ridge. Geothermal gradients are higher in these marine sediments than they are on the continents. Muddy and sandy sediments remain largely unlithified except for layers of chert and limestone. Trailing edges are characterized by lowered heat-flow rates and reduced geothermal gradients, as well as rapid subsidence and burial of terrigenous detrital and chemical sediment after the initial opening phase in which evaporites and heavy metal deposits are formed. The resulting rocks are moderately cemented and lithified by carbonate and quartz and, in deeper zones, have the stable chlorite-illite mineral pair. Volcaniclastics are rare, as are zeolites. Subduction zones, a diverse group of subenvironments, are regions where low heat flow, rapid subsidence, and burial, accompanied by crumpling, operate on turbidites and pelagics, together with large amounts of volcaniclastic debris from nearby island arcs. Zeolites, smectites, and low-temperature-high-pressure mélanges, including ophiolites, are produced. Continent-continent collisions produce enormous quantities of terrigenous detritus deposited in both terrestrial and marine environments, from intermontane basins with subaerial exposure and meteoric water infiltration to submarine fans and plains. Geothermal gradients are low, sedimentation and burial are rapid, and diagenesis is mild, except in sections that have remained deeply buried for 100 m.y. or more. Diagenesis of rift-valley sediment is related to the abundance of volcaniclastics, mixed with alluvial and lacustrine deposits, all buried rapidly in regimes of high heat flow and hydrothermal activity. Intraplate diagenesis is the result of low subsidence and sedimentation rates in low continental heat-flow regimes. But oscillatory movements are frequent, and complex parageneses of authigenic mineral replacements indicate multiple changes in formation-water composition. A uniformitarian approach to this subject is valid only for the Phanerozoic, for present-ay patterns of plate tectonics may not have existed during earlier times when heat flow from the interior was differently distributed.

Unusual sulphide replacement textures in altered olivine-rich rocks of the Bulong Complex near Kalgoorlie, Western Australia

SummaryUnusual replacement-type sulphide mineralization occurs in the northern part of the Bulong Complex, about 30 km east of Kalgoorlie, Western Australia. The mineralization is non-economic (up to 0·5% Ni, 0·4% Cu, 0·1% Co, and 8·6% S) and occurs in altered unlayered olivine-rich rocks immediately above a thin sheet-like inclusion of country rock. The supergene-modified primary opaque assemblage pyrrhotine-magnetite-pyrite-chalcopyrite-(pentlandite) is texturally unusual in that the opaques are largely pseudomorphic after primary olivine grains, mainly within irregular fracture networks in a manner similar to ‘early’ serpentine. Textural relations between opaques and silicates indicate that the mineralization was introduced during the early stages of serpentinization prior to the onset of deformation and regional metamorphism. Monoclinic pyrrhotine is the main opaque phase, with some grains containing relict cores of the hexagonal variety. Magnetite associated with the mineralization is Ni-poor (&lt; 0·1% Ni) compared with ‘serpentinization magnetite’ from elsewhere in the Bulong Complex, which contains 0·5–0·8% Ni. As the mineralization was intersected at relatively shallow depth, supergene alteration effects are evident; pyrrhotine is locally altered to pyrite and marcasite (texturally and chemically distinct from the primary pyrite), and pentlandite is largely replaced by cupriferous violarite.Textural features and consideration of phase relations in the system Cu-Fe-S-O suggest that the mineralization is of low-temperature (350± °C) hydrothermal origin. By contrast, the more commonly developed Fe-Ni-Cu sulphide mineralization of the Kalgoorlie region is generally considered to be of high-temperature (1200± °C) magmatic origin.

Structural relationships in the Antalya ophiolite complex, Turkey: Possible model for an oceanic ridge

Research Article| December 01, 1977 Structural relationships in the Antalya ophiolite complex, Turkey: Possible model for an oceanic ridge T. JUTEAU; T. JUTEAU 1Laboratoire de Minéralogie–Pétrographie, Institut de Géologie, 1 Rue Blessig, 67084 Strasbourg Cedex, France Search for other works by this author on: GSW Google Scholar A. NICOLAS; A. NICOLAS 2Laboratoire de Géologie Structurale–Tectonophysique, B.P. 1044, 44037 Nantes Cedex, France Search for other works by this author on: GSW Google Scholar J. DUBESSY; J. DUBESSY 1Laboratoire de Minéralogie–Pétrographie, Institut de Géologie, 1 Rue Blessig, 67084 Strasbourg Cedex, France Search for other works by this author on: GSW Google Scholar J. C. FRUCHARD; J. C. FRUCHARD 3Ecole Nationale Supérieure de Géologie Appliquée, B.P, 452, 54001 Nancy Cedex, France Search for other works by this author on: GSW Google Scholar J. L. BOUCHEZ J. L. BOUCHEZ 2Laboratoire de Géologie Structurale–Tectonophysique, B.P. 1044, 44037 Nantes Cedex, France Search for other works by this author on: GSW Google Scholar Author and Article Information T. JUTEAU 1Laboratoire de Minéralogie–Pétrographie, Institut de Géologie, 1 Rue Blessig, 67084 Strasbourg Cedex, France A. NICOLAS 2Laboratoire de Géologie Structurale–Tectonophysique, B.P. 1044, 44037 Nantes Cedex, France J. DUBESSY 1Laboratoire de Minéralogie–Pétrographie, Institut de Géologie, 1 Rue Blessig, 67084 Strasbourg Cedex, France J. C. FRUCHARD 3Ecole Nationale Supérieure de Géologie Appliquée, B.P, 452, 54001 Nancy Cedex, France J. L. BOUCHEZ 2Laboratoire de Géologie Structurale–Tectonophysique, B.P. 1044, 44037 Nantes Cedex, France Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (12): 1740–1748. https://doi.org/10.1130/0016-7606(1977)88<1740:SRITAO>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 T. JUTEAU, A. NICOLAS, J. DUBESSY, J. C. FRUCHARD, J. L. BOUCHEZ; Structural relationships in the Antalya ophiolite complex, Turkey: Possible model for an oceanic ridge. GSA Bulletin 1977;; 88 (12): 1740–1748. doi: https://doi.org/10.1130/0016-7606(1977)88<1740:SRITAO>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 From an extensive field study of the structural orientations within the Antalya ophiolite complex, a model for an oceanic ridge is proposed. The magmatic cumulate plane in ultramafic and mafic cumulates is restored to a horizontal orientation, and the strike of the ridge is chosen as the normal in this horizontal plane to the mineral lineation found in the foliation of the ultramafic tectonites. The lineation in these tectonites is the trace of the ductile-flow direction. It is parallel to the axis of folds developed in pyroxenite layers during ductile flow. Along the ridge strike, as defined above, the foliation has a 32° mean dip and is almost horizontal in the direction normal to the ridge strike. This suggests a mantle diapiric intrusion beneath the ridge. In the ultramafic tectonites, which are deformed in a dominantly rotational regime, the shear-flow plane has a somewhat more pronounced dip toward the ridge than does the foliation. The gabbro and pyroxenite veinlets, generated during the flow by partial melting of the peridotite, are intruded along planes containing the mineral lineation in tectonites. The isolated diabase dikes that cut all the members of the assemblage are principally subvertical with various trends and have nothing to do with the primary assemblage. 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.

Petrology of Volcanic Sedimentary Rocks, Vieja Group, Southern Rim Rock Country, Trans-Pecos Texas

ABSTRACT Primary mineral facies, depositional facies, and diagenetic assemblage zones can be mapped in the Vieja Group (Eocene-Oligocene). Variations of primary mineralogy can be ascribed to both source and environmental effects. By recognizing those attributable to source effects, and by consideration of the depositional environments present, the sequence of eruption of volcanos and the nature of their activity can he deduced. When they were deposited, volcanic-sedimentary rocks of the Vieja Group were composed mostly of three types of volcano-derived detrital constituents: glass shards and pumice fragments, volcanic rock fragments (VRF's), and monocrystalline mineral grains. Crystal fragments were most abundant in medium sandstones, glass fragments predominated in finer grained rocks and VRF's predominated in coarse sandstones and conglomerates. The actual abundance of any type of constituent was also a function of the abundance in which it was provided by the various volcanic sources and of the environment of deposition of the sediment. The volcano active during deposition of the Colmena formation provided abundant VRF's and plagioclase. One erupting during deposition of the Chambers formation produced de ris rich in sanidine and quartz. During deposition of the Capote Mountain formation, the Chinati Mountains-Pinto Canyon center produced abundant glass. The rocks were deposited on a bajada-like apron by streams or mudflows that flowed radially away from volcanic eruptive centers, or in valleys where streams flowed between eruptive centers to the north and south. The streams entered the Southern Rim Rock Country from the west and continued eastward across the Trans-Pecos volcanic field. Stream channel and overbank deposits, with common paleosoil features, make up the valley facies. Medium sandstones in the valley facies are enriched in crystals relative to those deposited on aprons, either because of sorting by depositional processes or selective abrasion of glass fragments during transportation. Non-volcanic constituents are present in minor amounts in most valley facies rocks. They are the predominant grain type only in the valley f cies part of the Colmena Formation. Detrital clay is present in valley facies rocks and in the mudflow deposits of the Colmena Formation. Some rocks are the result of deposition by aeolian processes.

Irreversible Reactions in High-Grade Metapelitic Rocks

At least two discontinuous reactions and additional divariant (continuous) compositional shifts were overstepped in pelitic rocks of the inner aureole around the Ronda peridotite massif, southern Spain, probably as a result of upward drag and rapid decrease of Pt during high-grade metamorphism. The resulting reactions were garnet or biotite+A1-silicate↑cor-dierite+hercynite±excess component phases present throughout reaction (quartz, K-feld-spar, ilmenite, plagioclase). Both reactant and product phases are preserved. The extent of reaction, estimated from the amount of cordierite and hercynite produced by reaction, can vary markedly within a single thin section. Coexisting phases were analysed in twelve domains representing varying degrees of reaction in one thin section. A domain, the unit of comparison, was taken as the smallest volume that contained the major reacting phases. Compositions of products in a thin section vary little from domain to domain and probably can be considered a function only of the intensive variables (P, T, μH2o); product phases appear to approach equilibrium as determined by partitioning of major elements. However, reactant compositions are a function of extent of reaction and are not generally in equilibrium with the products. Thus, the reactions are irreversible. The original assemblage before reaction (quartz-K-feldspar-sillimanite and kyanite-garnet-biotite) is preserved within unzoned cores of garnet grains. As the extent of reaction increases, Mg/Fe of biotite in the matrix migrates from the initial value toward a more Fe-rich composition. The analytical data are used to reject the local equilibrium or domain equilibrium model of reaction on the scale of a domain. Product compositions do not vary as expected in this model as a function of extent of reaction, variable reactant composition, or distance from non-equilibrium reactants. The reactant grains must be chemically isolated from the equilibrium matrix products and fluid in partial equilibrium; this model is distinguished as the 'dissolution model'. Diffusion gradients still can be present in this model and are responsible for the distribution of product phases; however, the reaction rate is not diffusion-controlled. Reaction rate can be limited by interface reactions or by diffusion within crystals. Two models of reaction are examined assuming that reaction rate is controlled by reactant dissolution. The first model of irreversible reaction examined, assuming the reacting composition to be that for which the decrease of free energy of reaction is greatest, does not satisfactorily explain the data. A second, descriptive model is proposed in which the overall irreversible reaction is divided into component reactions and the rate of these reactions is assumed to be a monotonic function of free energy change or growth rate of product phases. The model can be shown qualitatively to explain observed garnet zoning and changing biotite composition during reaction. Because reactant phases and compositions of the primary assemblage are preserved along with the equilibrium product phases in these samples, more information is available from these ‘disequilibrium’ rocks than from samples that have completely equilibrated. The variability of extent of reaction in these samples emphasizes that contact of phases in a sample is not evidence of mutual compatibility.

Mineral chemistry of peridotite xenoliths from the Lashaine volcano, Tanzania

Eight garnet peridotite and ten garnet-free peridotite xenoliths from the Lashaine volcano in northern Tanzania, and one garnet peridotite from the Matsoku pipe in Lesotho, were studied petrographically and by electron-microprobe techniques. The primary assemblages are ol + opx + cpx + ga, ol + opx + cpx + chr, ol + opx + ga + chr, ol + opx + ga, ol + opx + chr, and ol + cpx + chr. With the exception of the ol + cpx + chr assemblage, which is more iron-rich, the composition of each of the primary mineral species is essentially the same for all the xenoliths and is independent of the abundances of the minerals. The primary minerals are Ni-rich very low Ca olivine (Fo 92), low Ca, Al, Cr, Ti, Mn,enstatite (Wo 1En 93Fs 7), low Al, Ti, Mn chrome diopside (WO 44En 52Fs 4), chrome pyrope and Mg, Al-rich chromite. Comparisons with phase equilibria studies suggest that the primary assemblages are stable at approximately 1050°C and 50 kbar, equivalent to a depth of approximately 150 km. The primary xenolith assemblages have been locally altered by three types of secondary processes. (1) Garnet has reacted with the adjacent olivine to produce reaction rims of aluminous orthopyroxene plus aluminous clinopyroxene plus spinel. (2) Some chrome-diopsides show marginal or total alteration that may result from secondary melting. (3) Extremely localized partial melting has occurred at some grain boundaries and the interstitial melt quenched to various combinations of olivine, clinopyroxene, orthopyroxene, spinel, phlogopite and glass. Garnet-bearing and garnet-free assemblages differ in bulk composition but have formed under similar physical conditions. The xenoliths derive from a region of the upper mantle that is homogeneous in terms of mineral compositions but heterogeneous, at least on the scale of the Lashaine xenoliths, in mineral proportions and bulk composition. Phlogopite in one garnet peridotite appears to be primary and several peridotites contain regions that have formed by localized melting of K-rich areas. Either the peridotites are not refractory residues from partial melting in the mantle or K-rich material has been added to them after partial melting but within the mantle, before incorporation into the ankaramite host.

The Igwisi Hills extrusive “kimberlites”

The Igwisi Hills are a group of volcanic hills in Tanzania where a unique series of eruptions has produced volcanic rocks that have many of the features of kimberlites. The rocks are “igneous conglomerates” that are characterized by the presence of abundant ellipsoids of olivine in a fine grained matrix. The matrix contains major amounts of carbonate, serpentine, and complexly zoned MgAlCr spinels, minor perovskite, apatite and limonite. Large magnesian olivines are highly rounded and some have partially recrystallized to more Fe-rich and Ni-poor subhedral to euhedral grains. Several olivine ellipsoids are partly or wholly rimmed by a black coating consisting of perovskite, and MgAl spinel. Mineral inclusions within the olivines include chrome pyrope, similar in composition to garnets in kimberlites, low Al enstatite, low Al magnesian chrome diopside, MgAl chromite, and a highly magnesian phlogopite ( Mg Mg + Fe ∼.94 ). All of these are apparently primary phases in equilibrium with olivine. The total assemblage is similar in many respects to that found in garnet peridotite xenoliths such as those from the Lashaine volcano in northern Tanzania. The Igwisi irruptives apparently contain material derived from phlogopite-bearing, garnet peridotites with a primary mineral assemblage (assuming that all these phases coexist at depth) indicative of equilibrium at upper mantle temperatures and pressures. This primary assemblage was disrupted and brought rapidly to the surface in a gas-charged, carbonate-rich fluid. Rapid upward transport, extrusion, and rapid cooling have preserved inclusions of upper mantle ultramafics in a carbonate-rich matrix and have tended to prevent reaction between inclusions and matrix that might otherwise have yielded a more typical kimberlite.