Skeletal Plagioclase Research Articles

Petrography and geochemical framework of guébaké dolerites dyke swarms (north Cameroon, central Africa)

Dolerites dykes of 4.5 to 25 m wide extend from 200m to 1km, crosscut the Guébaké granitoids basement in northern Cameroon along EW to N100E directions. Petrographic studies reveal the microlitic porphyritic texture to classical doleritic texture of ophitic to sub-ophitic types. Guébaké dolerites are mainly composed in various proportions of skeletal plagioclase and feldspar, clinopyxene, amphibole and oxides crystals. ICP-MS and ICP-AES geochemical analyses have distiguished a lava series composed of trachybasalts, basaltic trachyandesites, trachyandesites, trachytes and rhyolites of continentale tholeiites affinity. Lavas have been differenciated trough fractional crystallization process coupled with crustal contamination and metasomatism. Guébaké dolerites are products of relatively high partial melting rate of E-MORB mantle component. They stand as fingerprints of the post pan African crustal consolidation and precursor of the development of central African rift system at Cretaceous times. Â

New U-Pb zircon age and petrogenesis of the plagiogranite, Troodos ophiolite, Cyprus

The Troodos ophiolite is a classic and is one of the most complete and best-studied ophiolites on Earth. However, there are few studies on the geochronology of the plagiogranite in the Troodos ophiolite. With the advancement of zircon U-Pb in situ dating method developed in recent decades, it is necessary to revisit the age of the plagiogranite. In this study, we carried out detailed petrography, zircon U-Pb dating and mineral compositional analysis for the plagiogranite. Our new zircon ages of ~90–91 Ma confirm the previous age data. Our findings also offer new perspectives on the petrogenesis of the Troodos plagiogranite. It is common that some of the Troodos plagiogranite outcrops show intense “epidotization”. The Troodos plagiogranite can thus be divided into epidote-free plagiogranite and epidote plagiogranite. Zircons in the epidote plagiogranite have mottled texture with high contents of most elements (especially for P, Y, REEs, Nb and Ta), which differs distinctly from those in the epidote-free plagiogranite with a clean appearance and oscillatory zoning. The two types of zircons all having high Th/U ratio (0.25–1.46 with an average of 0.69) are consistent with being of magmatic origin. The texture and composition of zircons from the epidote plagiogranite suggest that they are crystallized from a highly dissolving agent that transforms existing minerals such as plagioclase and amphiboles while concentrating rare earth elements (REEs) and high field strength elements (HFSEs, including Nb, Ta, Ti, Zr, Hf, Th and U4+) to precipitate accessory minerals such as zircon and titanite as well as the more abundant epidote. The intergrowth of quartz and skeletal plagioclase together with the precipitation of zircon, titanite and epidote indicates that the highly dissolving agent is best understood as late-stage magmatic fluid or more likely supercritical fluid/melt whose nature and origin deserve further investigation.

Nature and composition of interbedded marine basaltic pumice in the ∼52–50 Ma Vastan lignite sequence, western India: Implication for Early Eocene MORB volcanism offshore Arabian Sea

The recognition of pyroclasts preserved in sedimentary environments far from its source is uncommon. We here describe occurrences of several centimetres-thick discontinuous basaltic pumice lenses occurring within the Early Eocene Vastan lignite mine sedimentary sequence, western India at two different levels – one at ∼5 m and the other at 10 m above a biostratigraphically constrained 52 Ma old marker level postdating the Deccan Volcanism. These sections have received global attention as they record mammalian and plant radiations. We infer the repetitive occurrence of pumice have been sourced from a ∼52–50 Ma MORB related to sea-floor spreading in the western Arabian Sea, most plausibly along the Carlsberg Ridge. Pyroclasts have skeletal plagioclase with horsetail morphologies ± pyroxene ± Fe–Ti oxide euhedral crystals, and typically comprise of circular polymodal (radii ≤10 to ≥30 μm), non-coalescing microvesicles (>40–60%). The pumice have undergone considerable syngenetic alteration during oceanic transport and post-burial digenesis, and are a composite mixture of Fe–Mn-rich clay and hydrated altered basaltic glass (palagonite). The Fe–Mn-rich clay is extremely low in SiO 2, Al 2 O 3, TiO 2, MgO, alkalies and REE, but very high in Fe 2 O 3, MnO, P, Ba, Sr contents, and palagonitization involved significant loss of SiO 2, Al 2 O 3, MgO and variable gain in Fe 2 O 3, TiO 2, Ni, V, Zr, Zn and REE. Bubble initiation to growth in the ascending basaltic magma (liquidus ∼1200–1250 ∘C) may have occured in ∼3 hr. Short-distance transport, non-connected vesicles, deposition in inner shelf to more confined lagoonal condition in the Early Eocene and quick burial helped preservation of the pumice in Vastan. Early Eocene Arabian Sea volcanism thus might have been an additional source to marginal sediments along the passive margin of western India.

Melting behaviour of feldspar clasts in high-fired Sue ware

This contribution examines the melting behaviour of feldspar clasts in 9 th century CE Sue ware from the Nakadake Sanroku kiln site, Kagoshima prefecture (Kyūshū), southern Japan. Sue ware is stoneware of typical grey to brownish-grey appearance fired at high temperature; its surfaces may be untreated or covered by natural ash glaze. Firing was performed in sloping single-chamber tunnel kilns at reducing conditions and temperatures exceeding 1000 °C. Set in a vitrified matrix (glass, mullite, minor spinel and cristobalite), quartz and feldspar constitute ubiquitous clast components. The varying degree of vitrification (15–60 wt.% glass phase) and reversely correlated modal proportions of quartz and feldspar clasts likely reflect superposed effects of varying exposure time to peak-firing temperature and temperature gradients in the kilns. Sodic plagioclase clasts (An 18-25 ) melted congruently, producing sieve-textured domains of vitreous phase and residual calcic plagioclase, while calcic plagioclase clasts (An 35-63 ) were dissolved incongruently by reaction with matrix melt, generating narrow vitreous rims embedding residual calcic plagioclase laths at the immediate contact with the host plagioclase and acicular mullite towards the ceramic matrix. Elevated Fe, Mg, and K contents of the melt phase with development of concentration gradients indicate bi-directional diffusive element transfer between reaction domains and the ceramic matrix. Melting phenomena of alkali feldspar clasts, depending on the intensity of vitrification, range from few micrometre-thin vitreous margins to completely melted blistered clasts. With advanced melting, pronounced chemical zonations are developed with continuous gradation from potassic core compositions (Or 90-80 Ab 10-20 An ) to sodic compositions (Or 68-51 Ab 30-46 An 2-3 ) in domains adjoining the vitreous domain. The chemically homogeneous vitreous phase retains the 1:3 Al:Si-stoichiometry of the feldspar framework, whereas K and Na are depleted and Ca, Fe and Mg enriched relative to the precursor feldspar. These findings demonstrate that (1) alkali feldspar clasts did not melt through eutectic reaction with silica/quartz nor via the incongruent reaction K-feldspar → leucite + liquid; (2) melting was initiated by diffusive K-Na exchange with the matrix, thereby shifting the pristine clast compositions towards the thermal minimum (Or 40 Ab 60 ) in the Ab-Or-An ternary system; (3) while sodic marginal domains (Or 68-51 Ab 30-46 An 2-3 ) melted congruently, residual core domains (Or 75-66 Ab 24-33 An 1 ) melted incongruently producing micrometre-scale intergrowths of skeletal plagioclase and melt; (4) melting involved diffusive cation exchange through a quasi-stationary aluminosilicate framework, viz . influx of Ca + Fe + Mg to the feldspar melt domains and concomitant release of the fluxing components K + Na to the ceramic matrix. The evaluation of feldspar melting phenomena suggests firing temperatures did not exceed ca . 1150 °C.

Investigation of the H7 ordinary chondrite, Watson 012: Implications for recognition and classification of Type 7 meteorites

Despite the fact that the number of officially classified meteorites is now over 45,000, we lack a clearly defined sequence of samples from a single parent body that records the entire range in metamorphic temperatures from pristine primitive meteorites up to the temperatures required for extensive silicate partial melting. Here, we conduct a detailed analysis of Watson 012, an H7 ordinary chondrite, to generate some clarity on the textural and chemical changes associated with equilibrium-based silicate partial melting in chondritic meteorites. To do this we compare the textures in the meteorite with those preserved in metamorphic contact aureoles on Earth. The most distinctive texture generated by the partial melting that affected Watson 012 is an extensively interconnected plagioclase network, which is clearly observable with a petrographic microscope. Enlarged metal–troilite grains are encapsulated at widenings in this plagioclase network, and this is clearly visible in reflected light. Together with these features, we define a series of other characteristics that can be used to more clearly classify chondritic meteorites as being of petrologic Type 7. To provide comprehensive evidence of silicate partial melting and strengthen the case for using simple petrographic observations to classify similar meteorites, we use high-resolution X-ray computed tomography to demonstrate that the plagioclase network has a high degree of interconnectedness and crystallised as large (cm-scale) skeletal crystals within an olivine–orthopyroxene–clinopyroxene framework, essentially pseudomorphing a melt network. Back-scattered electron imaging and element mapping are used to show that some of the clino- and orthopyroxene in Watson 012 also crystallised from silicate melt, and the order of crystallisation was orthopyroxene→clinopyroxene→plagioclase. X-ray diffraction data, supported by bulk geochemistry, are used to show that plagioclase and ortho- and clinopyroxene were added to the Watson 012 sample by through-flowing basaltic melt. Along with the absence of glass and granophyre, this interconnected network of coarse-grained skeletal plagioclase indicates that the sample cooled slowly at depth within the parent body. The evidence of melt flux indicates that Watson 012 formed in the presence of a gravitational gradient, and thus at significant distance from the centre of the H chondrite parent body (the gravitational gradient at the centre would be zero). Our interpretation is that incipient silicate partial melting in Watson 012 occurred when a region of radiogenically heated H6 material located at considerable depth (possibly at ∼15–20km from surface) was heated by an additional ca. 200–300°C in association with a large shock event. Due to insulation at depth within an already hot parent body, the post-shock temperature equilibrated and remained above the solidus long enough for widespread equilibrium-based silicate partial melting, and for melt to migrate. Although the observed melting may have been facilitated by additional heating from an impact event, this is not an example of instantaneous shock melting, which produces thermal disequilibrium at short length scales and distinctly different textures. A small number of H, L and LL chondrites have been previously classified as being of petrologic Type 7; with our new criteria to support that classification, these represent our best opportunity to explore the transition from high temperature sub-solidus metamorphism through the onset of silicate partial melting in three different parent bodies.

THE LATE-STAGE CRYSTALLIZATION HISTORY OF THE JURASSIC NORTH MOUNTAIN BASALT, NOVA SCOTIA, CANADA. II. NATURE AND ORIGIN OF SEGREGATION PIPES

The Jurassic (202 Ma) North Mountain Basalt (NMB) is a sequence of quartz-normative continental tholeiites emplaced within the Fundy Basin of southern Nova Scotia, Canada. The sequence is subdivided into three units referred to (from base to top) as the East Ferry (EFM), Margaretsville (MM) and Brier Island (BIM) members. Whereas the lower and upper members are thick (≤150 m) and composed of massive, medium-grained, holocrystalline to hypocrystalline, dark green basalt flows, the middle member is composed of multiple, thin (≤10–20 m), fine-grained and vesicle-rich basalt emplaced as inflated sheeted pahoehoe flows. Locally, at the base of the BIM, abundant, pipe-shaped features (2–60 cm wide, ≤1.5 m vertical dimension) of composite nature are present, consisting, in decreasing proportion, of: (1) a medium to dark green to black igneous phase of variable texture (aphanitic to coarse grained); (2) clear, crystalline to cloudy white, rarely red, green or blue, silica material; (3) aphanitic, red-brown, highly indurated igneous material; (4) amygdules of silica and zeolite, and (5) irregularly shaped blebs (≤1–2 cm) of altered igneous material inferred to have been micromafic enclaves. In plan section, the pipes are dominated by either the dark material or silica, but the silica phase does core pipes, and rarely, the two are mixed. The red-brown phase occurs at pipe margins and blebs occur randomly in pipes of medium- to coarse-grained texture. Rare pipe profiles indicate that pipe compositions can vary vertically, as can their shape and width. Pipe compositions range from basaltic to rhyolitic, the continuum possibly reflecting mixing of the two identified end-members. Microscopically, the pipes are holocrystalline to holohyaline, may contain fresh felsic glass in addition to quenched material ( i.e., skeletal plagioclase and pyroxene) and granophyre. Mineralogically, the pipes are dominated by calcic plagioclase (An 40–70 ) and clinopyroxene, with both having textural and chemical features indicating growth in a Fe-rich melt with late-stage resorption. Importantly, the basalt hosting the pipes is locally enriched in intergranular felsic glass. A model for pipe formation involves initial collection of silicic melt via filter pressing with entrainment of earlier-formed plagioclase and clinopyroxene within the melt and some mixing and mingling with residual basaltic melt. The density contrast between the felsic and basic melts and basalt created a buoyancy instability resulting in the rise of felsic diapirs, which are seen today uniformly dispersed on outcrop surfaces and which reflect paleohorizontal surfaces of the basalt flows.

Evidence for refilling of previously emptied basaltic pillows in the Hooggenoeg Complex, Barberton Greenstone Belt

AbstractSome large metabasaltic pillows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex in the Barberton Greenstone Belt, South Africa, exhibit flow-banded margins and homogeneous cores that have a different texture and compositional variation. The margins consist of millimetre to several centimetre thick alternating bands of pale green spherulitic and darker, conspicuously variolitic varieties of non-vesicular and aphyric metabasalt, previously inferred to be due to mingling of two different types of lava. The dark cores have sharp, aphanitic contacts with the flow-banded carapaces. They lack flow banding, have coarse-grained interiors and exhibit well-preserved primary textures with pseudomorphs after prismatic pyroxene set in a groundmass containing skeletal plagioclase. The compositional range of samples from these cores is unlike that of the flow-banded metabasalt but is similar to a 19 m thick lobate metabasalt flow ~150 m stratigraphically further up, at the local top of the Hooggenoeg volcanic sequence. The pillow cores are inferred to result from the later refilling of drained hollow pillows.

Komatiites and nickel sulfide ores of the Black Swan area, Yilgarn Craton, Western Australia. 2: Geology and genesis of the orebodies

The Black Swan Ultramafic Succession hosts a number of magmatic Fe–Ni–Cu–PGE sulfide ore shoots, ranging from high grade massive ore to low grade disseminated sulfides. Of these, the most economically significant is the Silver Swan massive sulfide orebody, associated with the basal contact of the succession. The deposit varies in thickness between 5 and 20 m, reaches a N–S strike length of 75 m, extends for at least 1.2 km of vertical plunge and is open at depth. Overlying matrix (net-textured) ore is rare. Inclusions of dacite are abundant within the lower 5 m of the massive sulfide. They range from angular fragments through smooth sinuous and plumose morphologies to fine lace-like intergrowths with the sulfide matrix, and comprise variable proportions of cores of porphyritic dacite and carapaces with skeletal plagioclase phenocrysts. Dynamic crystallisation and kinetic melting textures in the carapaces indicate that the inclusions have been heated to various temperatures, some well above their liquidus temperature. The composition of the inclusions ranges from a perfect match with the immediate footwall dacites to mixtures of dacite with up to 30% komatiite. The consistent thickness of the inclusion-bearing basal layer within the massive sulphide is interpreted as the extent of 3-D physical connectivity between the inclusions and a partially molten underlying hybrid layer. Primary contacts between the Silver Swan massive sulfide orebody and overlying ultramafic rocks are marked by thin rinds containing coarse-grained chevron-textured chromites with skeletal textures. Compositions of these chromites match those from Kambalda, Perseverance and other localities, and are inconsistent with a metamorphic origin. They are interpreted as markers of primary magmatic contacts. The combination of this feature with the general paucity of matrix ore implies that the massive ore accumulated and solidified before the accumulation of the overlying thick sequence of olivine cumulates. Taken together with observations on the internal fractionation of platinum group elements within the massive ores, these observations are consistent with a model where the massive ore were emplaced at the floor of a small partially drained lava tube. The floor of the tube had been previously heated by passage of large volumes of lava, such that it had reached its melting range. The felsic inclusions within the ore are the result of buoyant ascent of partially molten substrate into the ore magma. This constitutes strong evidence for the operation of thermo-mechanical erosion during ore emplacement. The disseminated Cygnet and Black Swan orebodies show a number of distinctive features. Cygnet contains a assemblage of clasts and inclusions which are interpreted as the result of rip-up, transport and redeposition of sulfides from a pre-existing massive sulfide orebody, of which Black Duck may be a remnant. The Black Swan orebody, by contrast, does not show xenolithic features, but is characterised by an association of sulfide blebs with segregation vesicles, and by unusually coarse-grained olivine. The Black Swan orebody is interpreted as the result of transport of sulfide droplets within a lava charged with a suspended load of coarse olivine crystals.

The Petrology of Mullite-bearing Peraluminous Xenoliths: Implications for Contamination Processes in Basaltic Magmas

A suite of high-level inclined sheets ranging in composition from basalt through to rhyolite is intruded into the Palaeogene lava field and underlying Moine Supergroup basement rocks around Loch Scridain, Isle of Mull, Scotland. Many of the sheets are highly xenolithic, containing a wide variety of crustal xenolith types derived from the Moine metasedimentary rocks, along with various gabbroic cumulate xenoliths. The most common xenolith types are almost pure quartzites and a variety of mullite-bearing aluminous buchites, many of the latter having thick crystalline selvages of plagioclase, corundum and aluminous spinel. The plagioclase is highly calcic (up to An87), and adjacent to the host basalt is commonly oscillatory zoned, implying crystallization from a melt. Trapped between plagioclase crystals are pockets of quenched, contaminated basic melt, which contain skeletal plagioclase and clinopyroxene, and preserve evidence of local mixing between the host basalt and the aluminous crustal melts. Sr and Nd isotope values of the buchite cores [e.g. (87Sr/86Sr)55 = 0.7074–0.7115], plagioclase selvages [e.g. (87Sr/86Sr)55 = 0.7137–0.7148], and associated trapped melts [e.g. (87Sr/86Sr)55 = 0.7126–0.7128], imply a complex series of magma–xenolith interactions. The textural characteristics, mineral chemistry and isotope geochemistry of these rims suggest that they have crystallized from a hybrid liquid formed by the complex interaction of the aluminous liquids with basic magmas. Such interaction proceeded via liquid–liquid diffusion, physical mixing of melts and a variety of reactions between the crystallization products of the buchites and the basaltic liquids. These crustal xenoliths preserve a detailed record of mineral–melt reactions within a suite of basaltic sheets, dominated by both the production of granitic melts and the ‘bulk’ melting of Al-rich micaceous lithologies.

Preservation of Shallow Plagioclase Dissolution Porosity During Burial: Implications for Porosity Prediction and Aluminum Mass Balance

The abundance and distribution patterns of secondary porosity formed by dissolution of plagioclase were examined in 216 samples of middle Eocene-Oligocene shallow marine to nonmarine arkosic sandstones from the southern San Joaquin basin, California. The proportion of plagioclase intragranular porosity and plagioclase porosity is depth dependent. For purposes of this study, pores containing less than approximately 40% of remnant plagioclase were counted as moldic plagioclase porosity. Pores within skeletal plagioclase grains or mostly enclosed by skeletal grains were counted as intragranular plagioclase porosity. Moldic porosity is most significant at shallow (< 5000 ft or 1500 m) depths, whereas the relative importance of intragranular porosity increases belo approximately 5000 ft (1500 m). These trends are most likely caused by gradual, preferential collapse, with increasing burial, of porosity generated predominantly at shallow depths. Intragranular porosity in more deeply buried sandstones is partly inherited from shallow depths. Feldspar dissolution porosity formed in shallow, open-system settings typically is not accompanied by aluminosilicate (e.g., kaolinite) by-products. This porosity, either or intragranular, can be preserved during burial, particularly in sandstones with high percentages of nonductile detrital grains. Removal of much of the aluminum released by feldspar dissolution in shallow open-system settings coupled with incorrect petrographic interpretations, placing extensive feldspar dissolution at deep (late) burial conditions, may account for the controversial aluminum source-sink imbalance. The presence of plagioclase dissolution porosity (moldic and intragranular) is important, but it does not have a significant impact on the accuracy of empirical porosity predictions. Such predictions are based on an approach that uses a calibration data set and multiple regression analysis to determine the relationship between known, geologically significant variables and reservoir quality. High accuracy of porosity predictions (within 2% porosity of the mean measured porosity) in sandstones containing dissolved plagioclase is possible because the presence of secondary porosity is implicitly accounted for by independent variables (e.g., composition, grain size, sorting, burial history) in the calibration data set that provides the basis for empirical predictions.

Investigation of Aluminum Mass-Transfer in Sandstones, Southern San Joaquin Basin, California

Dissolved plagioclase is the main aluminum source and authigenic caolinite is the principal aluminum sink in quartzofeldspathic San Joaquin basin sandstones. Samples include Oligocene to late Miocene age marine sandstones, which presently contain modified-marine and meteoric pore fluids. Plagioclase porosity and kaolinite are found from 600-1,500 m (30-50C) in the meteoric zone and from 2,100-3,300 m (80-105C) in the modified-marine zone. Leached plagioclase is absent in some meteoric-zone reservoirs but appears in up to 60% of uncemented sandstones in other reservoirs. In contrast, skeletal plagioclase is pervasive in Stevens sandstones, appearing in greater than 80% of all uncemented samples. Point-count data imply that aluminum is conserved in Stevens sandstones with modified-marine pore waters, despite enrichment in organic acids and anions capable of mobilizing aluminum in soluble complex ions. Some Stevens sandstones have excess kaolinite but no samples have a pronounced aluminum deficiency. The average Stevens sandstone has gained 0.06 {plus minus} 0.1 g of aluminum per 100-cm{sup 3} sample. Aluminum is conserved in Stevens reservoirs with high-temperature, slow-moving pore fluids. In contrast, aluminum may be exported from sandstones subject to meteoric flushing, although the aluminum deficiency is not extreme and complementary zones of aluminum import are not found. Average aluminummore » loss from Vedder sandstones in meteoric reservoirs is 0.3 {plus minus} 0.1 g per 100-cm{sup 3} sample. Aluminum apparently is mobilized in low-temperature, fast-moving meteoric waters, which may be focused along shallow faults and fractures, resulting in local zones of intense diagenesis.« less

Petrologic and Experimental Evidence for the Etching of Garnets by Organic Acids in the Upper Jurassic Morrison Formation, Northwestern New Mexico

ABSTRACT The upper Jurassic Morrison Formation in the southern part of the San Juan Basin contains uniquely etched detrital garnets characterized by smooth, crystallographically controlled faceted surfaces. The etched garnets occur in relatively homogeneous Morrison sandstones in discrete stratigraphic zones that are bounded above and below by sandstones bearing unetched garnets. Diagenetic alterations associated with etched garnets include etched staurolite, skeletal plagioclase, regularly interstratified illite-smectite, iron-rich chlorite, ankerite, albite, diagenetically altered (organic-poor) uranium ore, and extensive secondary porosity. Etching of garnets and partial to complete dissolution of other aluminosilicate minerals were caused by high concentrations of organic acids generated during the maturation of epigenetic organic matter (predominantly type-III kerogen) in the Morrison Formation. The presence of authigenic phases that form near 100°C indicates that temperatures were high enough during diagenesis to cause the thermal degradation of kerogen. The subsequent release of reactive organic acids promoted a variety of water-rock reactions. This hypothesis was corroborated by experiments in which dicarboxylic acid solutions created facets on garnets identical to those developed on naturally etched Morrison garnets. The excellent complexing ability of organic acid anions accounts for the characteristic lack of alteration products on etched surfaces.

Mantled feldspars from the Golden Horn batholith, Washington

Mantled feldspars that formed by resorption, development of skeletal plagioclase crystals, and filling with alkali feldspar are common in the Golden Horn batholith, Washington. Subhedral plagioclase mantles have weak normal zoning from An 17 to An 10. Plagioclase zoning and twinning are crosscut by resorption channels. Resorption cavities and channels are coated with albite (An 10). Anhedral, perthitic orthoclase within the plagioclase is optically continuous with orthoclase in channels and on the mantle exterior. This texture resulted from resorption of calcic cores of plagioclase as pressure decreased when water-undersaturated granite magma intruded to a shallow crustal level. At shallow level, only alkali feldspar and quartz crystallized and were available to fill the skeletal plagioclase.