Plagioclase-rich Rocks Research Articles

Brittle initiation of dissolution–precipitation creep in plagioclase-rich rocks: insights from the Bergen arcs, Norway

The initiation of ductile shear zones commonly occurs spatially associated with fluid-rock reactions along brittle precursors. In many cases the relative timing of fracturing, fluid infiltration, reaction, and recrystallisation is unclear, making it difficult to disentangle mechanisms of shear zone initiation from subsequent deformation and recrystallisation. Here we present the study of the transition from a dry plagioclase-diopside-garnet-scapolite host granulite-facies lithology to (1) a low strain amphibolite-facies rock, and (2) a transition from low strain to high strain amphibolite-facies lithologies. Hydration of the granulite-facies precursor at amphibolite-facies conditions produces an assemblage comprised dominantly of plagioclase-amphibole-zoisite-clinozoisite-kyanite-scapolite-quartz. Detailed study of plagioclase chemistry and microstructures across these two transitions using Electron Backscatter Diffraction (EBSD) and Wavelength Dispersive Spectrometry (WDS) allows us to assess the degree of coupling between deformation and fluid-rock reaction across the outcrop. Plagioclase behaves dominantly in a brittle manner at the hydration interface and so the initial weakening of the rock is attributed to grain size reduction caused by fracture damage and fluid infiltration at amphibolite-facies conditions. Extensive fracturing-induced grain size reduction locally increases permeability and allows for continuing plagioclase and secondary mineral growth during shear. Based on plagioclase microstructures, such as, an inherited but dispersed crystallographic preferred orientation (CPO), truncation of chemical zoning, and the dominance of fine (5–150 µm), slightly elongate, polygonal grains we conclude that deformation is dominantly facilitated by dissolution–precipitation creep assisted by grain boundary sliding in the shear zone.

U-Pb isotope systematics and impact ages recorded by a chemically diverse population of glasses from an Apollo 14 lunar soil

Glass beads formed by ejection of impact-melted lunar rocks and soils are an important component of lunar soils. These glasses range from 10′s of microns to up to a few cm in diameter and contain variable, but usually relatively low (several hundred ppb to a few ppm), quantities of U. Because Pb is a volatile element, it tends to be lost from the melts, so individual impact glasses can be dated by the U-Th-Pb isotopic systems. The presence of two additional Pb components in lunar glasses, likely linked to addition of lunar Pb to the beads during their residence on the lunar surface and from terrestrial laboratory contamination, require corrections to the data before accurate formation ages of the glasses can be determined. Here we report a U-Th-Pb isotopic and geochemical study of impact glasses from the Apollo 14 soil 14163, which documents multiple impacts into chemically diverse targets that can be linked to the main groups of rocks found on the Moon, i.e., mare basalts, highlands plagioclase-rich rocks, and KREEP (from high contents of K, REE and P) enriched rocks. The impact ages show a bimodal distribution with peaks at ∼3500–3700 Ma and <1000 Ma, similar to that obtained previously by 40Ar-39Ar dating of other suites of lunar regolith glasses. Our data suggest two predominant age peaks at ∼100 Ma and ∼500 Ma, with other statistically definable clusters of ages also possible. As Pb is relatively resistant to subsolidus diffusive loss in these glasses, the age clusters probably represent primary formation ages during impact events, although processes such as preferential preservation of young glasses and impact conditions necessary for production of regolith glasses need further quantification.

A plagioclase-rich rock measured by Yutu-2 Rover in Von Kármán crater on the far side of the Moon

The Chang'E-4 spacecraft successfully landed in Von Kármán crater inside the South Pole-Aitken basin on the lunar far side on January 3, 2019 and the Yutu-2 Rover has performed explorations on the lunar surface for nine lunar days as of September 2019. Our earlier analysis of the visible and near-infrared spectrometer measurements made by the Yutu-2 rover during the first two lunar days shows that the regolith of the landing site may have come from the nearby Finsen crater and is dominated by plagioclase with lesser amount of mafic minerals. During its third lunar day explorations, the Yutu-2 photographed a small piece of lunar rock and measured its reflectance spectra. Compared with the spectra of its surrounding regolith, this rock's spectrum has deeper absorption features, indicating its fresher nature. To obtain the mineralogy of the rock, we compared the rock's spectrum with the spectral library data of NASA's reflectance experiment laboratory of returned lunar rocks and lunar meteorites. We found that this rock is also plagioclase-rich with a possible plagioclase abundance of 60–80 vol%. A source region analysis using the Moon Mineralogy Mapper's remote sensing observations indicates this rock was ejected from the Zhinyu crater, about 30 km west of the landing site, rather than directly from the Finsen crater. Numerical simulations of the Zhinyu crater on the impact cratering process and ejecta thickness distribution confirmed our findings and imply that the surficial materials at the CE-4 landing site experienced a complicated evolution rather than simply retaining the pristine or primordial ejecta directly from the Finsen crater.

Evaluating the roles of melt-rock interaction and partial degassing on the CO2/Ba ratios of MORB: Implications for the CO2 budget in the Earth’s depleted upper mantle

Abstract Carbon content in the Earth's depleted upper mantle has been estimated in previous studies using CO2/Ba ratios of CO2 undersaturated depleted mid-ocean ridge basalt (D-MORB) glasses and melt inclusions. However, CO2/Ba ratios in CO2 undersaturated MORB may not necessarily record those of the mantle source, as they may be affected by (1) assimilation of Ba-rich plagioclase-bearing rocks in the oceanic crust and (2) CO2 degassing through partial degassing and mixing. In this study, we evaluate these effects on the CO2/Ba ratios as well as other volatile to refractory trace element ratios (H2O/Ce, F/Nd, Cl/K, and S/Dy) in D-MORBs using the compositions of olivine-hosted melt inclusions and glasses from the Siqueiros and Garrett transform faults. The Siqueiros and Garrett melt inclusions are CO2 undersaturated and highly depleted in incompatible trace elements, and their average CO2/Ba ratios show relatively large ranges of 90 ± 34 and 144 ± 53 respectively. A subset of melt inclusions in lavas from both transform faults show potential signatures of contamination by plagioclase-rich rocks, such as correlations between major elements contents (e.g., FeO, Al2O3, and MgO), and trace element ratios (e.g., Sr/Nd). We find that (1) assimilation fractional crystallization (AFC) of gabbro into D-MORB and (2) mixing between partial melts of gabbro and D-MORB can reproduce the observed range in Sr/Nd ratios as well as the general trends between major elements. However, we find that these processes had limited effects on the CO2/Ba ratio of the melt inclusions and it is unlikely that they can account for the observed range in the CO2/Ba ratio. On the other hand, while a partial degassing and mixing model can generate melts with large range of CO2/Ba ratios (as proposed by Matthews et al. (2017)), it cannot reproduce the Pearson correlation coefficients between CO2/trace element and 1/trace element ratios observed in the Siqueiros and Garrett melt inclusions. Instead, when analytical uncertainties on the elemental concentrations are considered, a model without partial degassing can adequately reproduce the majority of the observed range in CO2/Ba ratio and Pearson correlation coefficients. Hence, we postulate that the Siqueiros and Garrett melt inclusions are undegassed and use their average CO2/Ba ratios to estimate the Siqueiros and Garrett mantle source CO2 contents (21 ± 2 ppm and 33 ± 6 ppm respectively). We also evaluate the effects of shallow level crustal processes on H2O/Ce, F/Nd, Cl/K, and S/Dy ratios, and after which we filter those effects, we estimate the H2O, F, Cl and S contents in the mantle sources of the Siqueiros (40 ± 8 ppm, 8 ± 1 ppm, 0.22 ± 0.04 ppm, and 113 ± 3 ppm) and Garrett (51 ± 9 ppm, 6 ± 1 ppm, 0.27 ± 0.07 ppm, and 128 ± 7 ppm) melt inclusions.

The Upper Critical Zone of the Rustenburg Layered Suite in the Swartklip Sector, north-western Bushveld Complex, on the farm Wilgerspruit 2JQ: I. Stratigraphy and PGE mineralization patterns

AbstractThe Upper Critical Zone of the Rustenburg Layered Suite (RLS) in the Swartklip Sector, north-western Bushveld Complex, is considerably attenuated relative to other parts of the Complex. The interval between the UG2 chromitite and the Merensky Reef amounts to as little as 25 m in places. Within this interval, the aggregate thickness of orthopyroxenite-dominated ultramafic layers hosting the UG1 and UG2 chromitites and the Merensky and Bastard reefs does not differ significantly from the area around Rustenburg, to the south. The total thickness of ultramafic lithologies is, in fact, increased by the presence of the 3 to 5 m thick olivine-rich Pseudo Reef Unit, which is developed between the UG2 and Merensky Reef units in the Swartklip Sector, but does not occur in any significant form elsewhere in the Bushveld intrusion. The substantial thinning of the succession is due almost entirely to the fact that plagioclase-rich rocks (norite and anorthosite) between the ultramafic layers are radically thinned in the Swartklip Sector relative to virtually all other parts of the Bushveld Complex. The ultramafic layers, although dominated by orthopyroxenite, are characterized by higher proportions of olivine than in other parts of the Bushveld Complex. In our logging of the substantial number of exploration drill cores that form the basis of this study, we have found it expedient to define stratigraphic units that are either exclusively plagioclase-rich (norite and anorthosite) or plagioclase-poor (consisting of varying proportions of orthopyroxenite, harzburgite and chromitite). This effectively binary system of lithological classification has no overt genetic connotations.Our nomenclature has, in fact, enabled us to rigorously document the nature of contacts between ultramafic and plagioclase-rich units, and thus to identify unconformities between the ultramafic units (orthopyroxenite and harzburgite) and intervening noritic and anorthositic units, which have in the past been ascribed to localized thermo-chemical erosion of pre-existing plagioclase-rich cumulates. Apart from the well-documented evidence of erosional unconformities at the basal contacts of ultramafic units, we also provide evidence for unconformities at the tops of these units.

The Upper Critical Zone in the Swartklip Sector, north-western Bushveld Complex, on the farm Wilgerspruit 2JQ: II. Origin by intrusion of ultramafic sills with concomitant partial melting of host norite-anorthosite cumulates

Abstract In the Bushveld Complex, the ultramafic (orthopyroxenite/harzburgite with chromitite) layers that host most of the PGE and chromite mineralization in the Upper Critical Zone display well-documented discordant basal contacts with their anorthositic and noritic host rocks. Whilst not so well documented, there is evidence that the upper contacts of these units are also discordant. We review the nature of the contacts between the ultramafic units and adjacent plagioclase-rich lithologies. These include contact phenomena like pegmatoidal lithologies and thin magmatic reaction chromite stringers. We conclude that most, if not all, ultramafic layers were intruded as sills into pre-existing norite/anorthosite cumulates. The sequence of norites and anorthosites that hosts the ultramafic layers was built up by a prior series of multiple tholeiitic (A-type) magma intrusions. The spectrum of lithologies from melanorite through to (mottled) anorthosite represents differing degrees of partial melting in response to these successive magma influxes. Density and competence contrasts between layers of plagioclase-rich rocks in turn provided pathways for sill propagation of subsequent ultramafic (U-type) magmas. The ultramafic magmas further modified the host norites and anorthosites by processes of partial melting and metasomatism. The ultramafic units themselves accumulated as composite sills in response to multiple magma injections. In the western Bushveld Complex, particularly including the Swartklip Sector in the north-western part of the complex, the Merensky Reef is represented by various facies that occur at different levels in the host stratigraphy. This phenomenon has been referred to by the term “regional potholing”, and has been attributed to the erosion of footwall cumulates by new influxes of magma. We suggest that a series of step-and-stair-type transitions of intruding sills to successive stratigraphic levels might more appropriately explain the various facies of the Merensky Reef.

Mapping the wavelength position of mineral features in hyperspectral thermal infrared data

The Wavelength Mapper is an algorithm that searches for the deepest absorption feature in each pixel of a hyperspectral image. On a per pixel basis, it extracts the wavelength position, which serves as a proxy of the mineralogy and the feature depth as a proxy for the relative abundance. This algorithm has been used with near and shortwave infrared data, but has not yet been tested on hyperspectral thermal infrared images. It is unclear what results are expected when the Wavelength Mapper algorithm is applied to hyperspectral thermal infrared data since reststrahlen features characteristically overlap in emissivity spectra. In this paper, the Wavelength Mapper is tested on a multi-flightline airborne hyperspectral TIR dataset acquired over the Yerington Batholith, Nevada. Observations were made in the 8.05–11.65 μm wavelength range to include thermal spectral features of major rock-forming minerals, and a new color ramp is created to separate quartz-rich rocks from plagioclase-rich rocks. Our results indicate that the Wavelength Mapper creates coherent spatial patterns across flightlines. The results displayed represent different types of igneous and sedimentary rocks, as well as the products of hydrothermal alteration via different colors, mainly based on the relative abundance of quartz, feldspar and garnet, as well as mica and epidote. Comparison with published maps indicate that the Wavelength Mapper represents for each pixel a parameter value that can be linked to the spectrally dominate rock-forming mineral of that area, as mapped with traditional fieldwork methods. In conclusion, the Wavelength Mapper can be applied to airborne hyperspectral TIR data to achieve a simple, repeatable, per-pixel overview map of the dominating rock-forming mineral occurrences.

Geochemistry of Ahoko claystone-shale sequence, south eastern Bida Basin, Nigeria: implications for tectonic setting, provenance and source area weathering

Results of geochemical investigation of shale and claystone samples from two different sections of the exposed Ahoko claystone-shale sequence along the Koton-Karifi/Abaji highway are presented and discussed. Chemical variation of SiO2/Al2O3 and Na2O/K2O is ascribed to varying degrees of weathering of plagioclase rich rock either at the source and/or during transportation to the depositional basin. The high Chemical Index of Alteration (CIA) and Chemical Index of Weathering (CIW) values indicate intense alteration/weathering and removal of mobile cations. The CIW/CIA ratios indicate material that has undergone short distance transportation before final deposition. Besides, the overall Index of Compositional Variability (ICV) values and lower contents of Rb and Cs than upper continental crust (UCC) and post Archean Australian shale (PAAS) of both profiles (i.e. AH-1 and AH-2) indicate intense weathering. The average Th/U ratio is less than the average values for UCC and PAAS, which indicates that the source area is rich in uranium. Furthermore, the average Zr/Sc values are considerably higher than the average value for both UCC and PAAS, suggesting a high recycled sedimentary source. The geochemistry of our samples is consistent with felsic rocks, mixed with some old sediment components and should be derived from UCC. On the log (Fe2O3/K2O) versus log (SiO2/Al2O3) scattergram, the Ahoko claystone-shale samples mainly plot in the shale and Fe-shale fields. The variability in the geochemical classification of our samples is attributed to variation in the relative proportion of matrix, feldspar and lithic components. The major, trace and rare-earth elements assemblage of the studied Ahoko claystone-shale samples plot on various discrimination diagrams, revealing that the tectonic setting belongs to a passive continental margin and a continental island arc.

Pulses of Plagioclase-laden Magmas and Stratigraphic Evolution in the Upper Zone of the Bushveld Complex, South Africa

Cumulate rocks of the Upper Main Zone and Upper Zone (UUMZ) of the Bushveld Complex, South Africa, contain the world's major resources of Fe-Ti-V ±, hosted in Ti-magnetite and apatite, and are commonly considered as having crystallized from the last major injection of magma into the magma chamber. In this study, we present the petrography, modal proportions, whole-rock major element chemistry (260 samples), electron microprobe data (∼10 000 analyses for plagioclase, olivine, and pyroxene), and compiled analyses of Cr in magnetite (239 samples) for the UUMZ sampled over 2.1 km of the Bierkraal drill cores in the western limb of the Complex. The UUMZ section exhibits a broad normal fractionation trend upwards, but a series of reversals to more primitive anorthite contents in plagioclase, Mg# in pyroxenes and olivine, Cr in whole-rocks and Cr in magnetite separates are observed, accompanied by the appearance or disappearance of various minerals. Anorthosite or leucogabbro layers are closely linked to these reversals; the reversals in An % of plagioclase are used as boundaries to divide the UUMZ into 18 cycles. These cycles are interpreted as indications of magma chamber replenishment by plagioclase-laden magmas (up to 20 vol. % plagioclase) and are also marked by spikes in Cr content. In addition, abundant Fe-Ti oxide-bearing plagioclase-rich rocks are identified in the lower half of the UUMZ. These have crystallized from a hybrid melt produced by the mixing of a new plagioclase-bearing magma batch and the resident magma. Further crystallization of this hybrid liquid may lead to the formation of magnetite layers in the lower part of the UUMZ. The Bushveld UUMZ therefore grew by multiple emplacements of crystal-laden magmas coming from deep-seated chambers. Slow cooling in a shallow chamber explains the systematic bottom-up compositional evolution in the cumulate pile within individual cycles. The residual melt reached silicate liquid immiscibility soon after the saturation of apatite. Thereafter, segregation of conjugate Fe-rich and Si-rich melts and crystallization of the paired melts produces cumulates with a smooth upward decrease in Fe-Ti oxides, whereas plagioclase mode increases in each apatite-bearing cycle. A comparison of systematic geochemical analyses and a detailed lithological stratigraphy between the Bushveld limbs demonstrates the possible connectivity between the western and eastern Upper Zone but indicates notable differences from the Bellevue section of the northern limb.

Wishstone to Watchtower: Amorphous alteration of plagioclase-rich rocks in Gusev crater, Mars

Previous observations by the Spirit rover in Gusev crater revealed a suite of rocks dubbed Wishstone and Watchtower Class in which the parent lithology and daughter products of a distinctive style of aqueous alteration are evident. Results from Spirit’s Miniature Thermal Emission Spectrometer (Mini-TES; ~2000–340 cm −1 ) were compromised by dust contamination of one of the instrument’s mirrors, for which a correction has since been developed. Now we have documented nearly 200 examples of rocks encompassing the span of alteration from Wishstone Class, which spectrally resemble minimally altered plagioclase-phyric basalt, to the most altered Watchtower Class. Among them is a rock dubbed Bruce that may be a previously unrecognized alteration spectral end-member. We employed factor analysis/target transformation and linear least-squares modeling to investigate the spectral characteristics and mineralogy of these rocks. Our results amplify those of a prior preliminary analysis showing that alteration produced a material resembling basaltic glass that masks the spectral features of plagioclase. The association of this amorphous silicate component with a ferric iron nanophase oxide phase identified via Spirit’s Mossbauer spectrometer is now clearly shown by our data, further characterizing the distinctive mineralogic expression of the alteration. These components and the absence of any recognizable secondary silicates or opaline silica may be an expression of alteration in the extreme aridity and cold of the martian environment. Similar mineralogic characteristics of soil measured with the CheMin X-ray diffraction instrument on the Curiosity rover in Gale crater may be an indication that this alteration process is widespread on Mars.

Epithermal and arc-related layered mafic platinum-group element mineralisation in the mafic–ultramafic rocks of eastern Papua

ABSTRACTThis paper describes contrasting platinum-group element (PGE) mineralisation in the mafic–ultramafic rocks of eastern Papua, discovered nearly 100 years after the first record of detrital ‘osmiridium’ in alluvial gold production from the region. The two mineralisation types include: (1) the Doriri Pd–Pt–Ni lode, a new type of epithermal shear zone-hosted PGE mineralisation from the Ada'u Valley; and (2) stratiform Pt–Pd ± Au mineralisation hosted in an arc-related layered mafic intrusion at the Goroa prospect in the Waria Valley. Doriri is the first description of the transport and deposition of PGE in epithermal conditions, confirming the case long made by geochemists for the solubility, transport and deposition of significant concentrations of Pd and Pt in hydrothermal fluids up to 300°C. Doriri's temperature of formation (100–220°C) is based on chlorite thermometry, clearly placing the deposit in the epithermal field. Other epithermal-hydrothermal features of the deposit include formation at a shallow depth, episodic mineral deposition and the development of rhythmically layered chlorite and magnetite, hydrothermal brecciation, cross-cutting vein textures, the presence of low-temperature marcasite in the lode's pyrite–pentlandite–pyrrhotite sulfide mineralisation and a Pd:Pt ratio >7. The stratiform Goroa Pt–Pd ± Au mineralisation contains concentrations up to 311 ppb Pt + Pd over a 3 m interval in a plagioclase-rich gabbro. Grey–black micrograbbo commonly forms alternating thin layers within plagioclase-rich gabbro and many of these mafic layers contain visible sulfides. Sulfide content of the plagioclase-rich gabbro is ≪1 vol%. A Pt:Pd ratio of ∼1 is typical of the cumulate layered mineralisation. The association of Goroa mineralisation with (1) plagioclase-rich gabbroic rocks, rather than ultramafic rocks, (2) trace sulfides, rather than chromite, and (3) the occurrence with Pt–Pd detrital alloys, suggests a genetic relationship with arc-related layered mafic intrusions. The Goroa diorite–gabbro–ultramafic sequence may be genetically linked to late Paleocene–middle Eocene tonalite, quartz diorite and porphyry arc-related stocks present in the lower Waria River district.

Effect of small-scale heterogeneities on interpretation of crustal compositions exemplified by a layered anorthosite

The composition of the lower crust has a significant effect on geodynamic processes because it influences physical rock properties such as densities and seismic velocities. Compositional differences in lower crustal rocks are potentially large and exist on the scales of centimeters up to kilometers resulting in non-unique seismic and gravity data. While larger heterogeneities can be detected as reflections on seismic profiles, irregular small-scale compositional variations are not likely to be discovered, but will influence the averaged seismic velocities and densities of an area. The extent and effects of such small-scale heterogeneities are explored on an exposed high-grade layered anorthositic body by providing a detailed field map, petrological descriptions, pycnometry measurements as well as whole rock and mineral analyses combined with thermodynamic phase equilibria calculations. To evaluate the results of our thermodynamic calculations, densities and mineral modes obtained from the modeled phase equilibria are compared to measured densities and estimated mineral modes from rock samples. The proportion of mafic to ultramafic (plagioclase-poor) rocks in the mapped field area amounts to 10–15% but higher proportions of these rock types in the lower crust are feasible. To further study the effects of compositional variations, we have generated mixtures of mafic to ultramafic and anorthositic/intermediate rocks until the average properties of these mixtures are comparable to those of mafic granulites (3000–3100 kg/m3; 7.1–7.3 km/s). Mixtures of anorthosite with 40–45% and of tonalite with 50–60% high-grade mafic to ultramafic rocks yield average densities and seismic velocities similar to mafic granulites although they still contain 50–60 vol.% plagioclase. Hence small-scale mixing of certain rock types may result in the overestimation of the proportion of mafic (garnet) granulites in the lithologic interpretation of crustal compositions from seismic data. Since the transition to eclogite-facies in plagioclase-rich rocks is shifted to higher pressures and anorthositic/intermediate eclogites yield lower densities, a lower crust with higher modal amounts of plagioclase may not always provide the significant densification needed for certain geodynamic settings (e.g. delamination or subsidence).

A database of plagioclase crystal preferred orientations (CPO) and microstructures – implications for CPO origin, strength, symmetry and seismic anisotropy in gabbroic rocks

Abstract. This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO characteristics as a function of the deformation regime (magmatic or crystal-plastic) are outlined and discussed. The studied samples are dominantly from slow- and fast-spread present-day ocean crust, as well as from the Oman ophiolite. Plagioclase is the dominant mineral phase in the studied samples. Plagioclase CPOs are grouped into three main categories: Axial-B, a strong point alignment of (010) with a girdle distribution of [100]; Axial-A, a strong point maximum concentration of [100] with parallel girdle distributions of (010) and (001); and P-type, point maxima of [100], (010), and (001). A majority of CPO patterns are Axial-B and P-type, in samples showing either magmatic or crystal-plastic deformation textures. Axial-A CPOs are less common; they represent 21% of the samples deformed by crystal-plastic flow. Although fabric strength (ODF J index) does not show any consistent variation as a function of the CPO patterns, there is a significant difference in the relationship between the ODF and pole figures J indices; the magmatic type microstructures have high (010) pole figures J indices, which increase linearly with ODF J index, whereas the high [100] pole figures J indices of plastically deformed samples vary in a more scattered manner with ODF J index. The multistage nature of plastic deformation superposed on a magmatic structure compared with magmatic flow, and the large number of possible slip-systems in plagioclase probably account for these differences. Calculated seismic properties (P wave and S wave velocities and anisotropies) of plagioclase aggregates show that anisotropy (up to 12% for P wave and 14% for S wave) tends to increase as a function of ODF J index. In comparison with the olivine 1998 CPO database, the magnitude of P wave anisotropy for a given J index is much less than olivine, whereas it is similar for S wave anisotropy. Despite a large variation of fabric patterns and geodynamic setting, seismic properties of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a small difference in the aggregate elastic symmetry, with magmatic microstructures having higher orthorhombic and hexagonal components, whereas plastic deformation microstructures have a slightly higher monoclinic component, possibly correlated with predominant monoclinic simple shear flow in plastically deformed samples. Overall, plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, although our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry (triclinic) structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs.

New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China

The Hongge magmatic Fe-Ti-V oxide deposit in the Panxi region, SW China, is hosted in a layered mafic–ultramafic intrusion. This 2.7-km-thick, lopolith-like intrusion consists of the lower, middle, and upper zones, which are composed of olivine clinopyroxenite, clinopyroxenite, and gabbro, respectively. Abundant Fe-Ti oxide layers mainly occur in the middle zone and the lower part of the upper zone. Fe-Ti oxides include Cr-rich and Cr-poor titanomagnetite and granular ilmenite. Cr-rich titanomagnetite is commonly disseminated in the olivine clinopyroxenite of the lower parts of the lower and middle zones and contains 1.89 to 14.9 wt% Cr2O3 and 3.20 to 16.2 wt% TiO2, whereas Cr-poor titanomagnetite typically occurs as net-textured and massive ores in the upper middle and upper zones and contains much lower Cr2O3 (<0.4 wt%) but more variable TiO2 (0.11 to 18.2 wt%). Disseminated Cr-rich titanomagnetite in the ultramafic rocks is commonly enclosed in either olivine or clinopyroxene, whereas Cr-poor titanomangetite of the net-textured and massive ores is mainly interstitial to clinopyroxene and plagioclase. The lithology of the Hongge intrusion is consistent with multiple injections of magmas, the lower zone being derived from a single pulse of less differentiated ferrobasaltic magma and the middle and upper zones from multiple pulses of more differentiated magmas. Cr-rich titanomagnetite in the disseminated ores of the lower and middle zones is interpreted to represent an early crystallization phase whereas clusters of Cr-poor titanomagnetite, granular ilmenite, and apatite in the net-textured ores of the middle and upper zones are thought to have formed from an Fe-Ti-(P)-rich melt segregated from a differentiated ferrobasaltic magma as a result of liquid immiscibility. The dense Fe-Ti-(P)-rich melt percolated downward through the underlying silicate crystal mush to form net-textured and massive Fe-Ti oxide ores, whereas the coexisting Si-rich melt formed the overlying plagioclase-rich rocks in the intrusion.

The initiation of strain localisation in plagioclase-rich rocks: Insights from detailed microstructural analyses

In order to shed light on the cause for onset of strain localisation in plagioclase-rich rocks we have performed detailed microstructural analyses on a sheared anorthosite–leucogabbro using optical microscopy, electron backscatter diffraction (EBSD) and chemical analyses. The analysed sample is from an Archaean unit, SW Greenland, deformed at lower to mid crustal conditions ( T = 675–700 °C and moderate pressure). The initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation. Recrystallised plagioclase grains (average size 80 μm) are dominantly found in (i) clusters, (ii) lenses and (iii) continuous bands subparallel to shear zone boundaries. Recrystallised grains in clusters and lenses display inherited crystallographic orientations. Their bulk crystallographic preferred orientation (CPO) is random; however, crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems. Recrystallised grains in continuous bands show a CPO with a single dominant active slip system, (001)<110>, aligned with the structural (XYZ) framework. For these parent and daughter grains, misorientation axes are random and the dominant slip system is different. Grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast. We infer that the cause for strain localisation is recrystallisation and development of a CPO in continuous recrystallised bands. Microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation-accommodated grain boundary sliding (DisGBS) in continuous bands. This inferred shift in dominant deformation mechanism would lower the strength of the shear zone.

Trace element compositions of rocks and minerals from the Chilas Igneous Complex, Kohistan, northern Pakistan

...... The Kohistan terrane in the western Himalayan region is considered as a Cretaceous island-arc sequence sandwiched between the Asian and Indian continental crusts. It is a 300-kmlong plutonic body that extends parallel to the general trend of the Kohistan terrane. Rocks of this complex are petrographically and compositionally similar to plutonic xenoliths found in island arcs (Khan et al. 1989). We report geochemical data of rocks and minerals in the Chilas Complex and discuss the melt compositions and magmatism in middle to lower arc crust. The Chilas Complex is well exposed along the Indus and Swat Rivers. Most of the complex consists of generally homogeneous gabbronorite, pyroxene diorite and pyroxene quartz diorite, and these are called as the Main facies rocks. Some of the rocks were re-equilibrated under granulite facies conditions at 750-850 °C and 6-8 kbar (Swat valley, Jan and Howie 1980). Rocks with layered structure are also found in the Chilas Complex, which often occur in km-scale masses. Some of the masses are abundant in peridotites associated with layered gabbroic rocks (ultramafic-mafic association), which are deduced to be crystal accumulates. One of the bodies of the ultramaficmafic association, Thak body just to the east of Chilas town, shows excellent outcrops. This body is included in the Main facies rocks, and consists of cyclic units of layered cumulate rocks, which is classified into olivine-dominant cumulate (dunite-wehrlite), plagioclase-dominant cumulate (troctolite-gabbronorite) and pyroxene-dominant cumulate (websterite-clinopyroxenite). Hornblende and spinel are often included in the rocks. Mg values of the pyroxenes and An content of plagioclase are high in these cumulative rocks, but systematic change of major chemical composition of the minerals are not recognized through the Thak body. In addition, one gabbronorite mass along the Indus River (Basehri body) is characterized by well-developed rhythmic layering, and most of the layers were overturned. The Basehri body is intruded by the Main facies rocks, which is considered as crystal cumulate from the magma of the Main facies. Main facies rocks of the Chilas Complex have the characteristics of subduction-related calc-alkaline magmas with depletion of Nb relative to other incompatible elements (Khan et al. 1989). The concentrations of K2O, Y, Zr, Th and rare earth elements (REE) in the Main facies rocks are positively correlative with SiO 2 content. In the chondrite-normalized diagram, the light REE are enriched relative to heavy REE in the rocks, and the REE patterns are slightly concave upward. The chemical variation of the most of the Main facies rocks can be explained by a weak segregation of melt and early-formed crystals composed of plagioclase, clinopyroxene and orthopyroxene. The major element compositions of the rocks from the Thak body show the wide variation reflecting accumulation of earlystage crystals. Rocks are generally poor in REE and other incompatible elements, implying the separation of the melt and crystals are effective. Even the gabbroic rocks without olivine are still poor in REE and other incompatible elements relative to the Main facies rocks. REE concentrations of the layered rocks of the Basehri body are generally lower than those of the Main facies rocks. For characterization of magmas of these layered rocks, trace element compositions of clinopyroxene and plagioclase are determined by ICP-MS after mineral separation. Clinopyroxene fractions from wehrlite and websterite, and whole-rock sample of clinopyroxenite in the Thak body have REE, Ba, Nb, Sr and Zr concentrations similar to each other. The clinopyroxenite and the clinopyroxene from websterite have weak negative Eu anomaly and REE concentrations slightly higher than the clinopyroxene from wehrlite. The clinopyroxene from a plagioclase-rich part of the layered gabbronorite of the Basehri body shows the REE concentrations 3-5 times higher than clinopyroxenes from the Thak body, with clear negative Eu anomaly. Plagioclase fractions from a troctolite in the Thak body and from the plagioclase-rich rock of the Basehri body have REE concentrations with positive Eu anomalies, and they have similar Sr concentrations although the plagioclase from the troctolite are poor in most of the incompatible elements. The melts calculated from the clinopyroxenes of the pyroxene- and olivine-dominant cumulates in the Thak body are enriched in light REE relative to heavy REE, showing chemical characteristics closer to calc-alkaline or high-alumina basalt magmas in island arcs rather than depleted island-arc tholeiites. The calculated melts have REE, Sr and Zr concentrations similar to the Main facies rocks. The melts calculated from the plagioclase fractions have Sr, Ba and Rb concentrations similar to the Main facies rocks. These data suggest a possibility that the original magmas of the cumulates of the Thak body and the magma of the Main facies were derived from common or similar source materials, in spite of large petrographic variations. Trace element abundances in clinopyroxene and plagioclase from the Basehri body may have been affected by subsolidus equilibration, and also affected by a small amount of trapped melt.

Genesis of crustal xenoliths from andesite in the Southeast Mátra Mts (Hungary) based on petrologic and geochemical studies

The newly discovered crustal xenoliths from Miocene andesites in the Matra Mts (Northeast Hungary) were classified into three petrographic types. Type 1 is a garnetiferous, plagioclase-rich cumulate rock, presumed to originate from the felsic portion of the nearby Szarvasko-Darno mafic igneous complex. The almandine-rich garnet has been inferred to be of granulite facies origin. Garnet - orthopyroxene geothermobarometry on the orthopyroxene - plagioclase corona structure around it indicates a later high temperature (800-950 °C) low pressure (3-4 kbar) retrogression event. Type 2 is also plagioclase-rich cumulate rock with contact metamorphic assemblage containing spinel, corundum and andalusite, which crystallized in the course of dehydration reactions of biotite and/or smectite in hornblende hornfels to sanidinite facies conditions. Type 3 is microsyenite, which contains abundant anorthoclase and minor amount of Na- and K-rich kaersutitic-barkevikitic amphibole.

Prolonged KREEP magmatism on the Moon indicated by the youngest dated lunar igneous rock

Primordial solidification of the Moon (or its uppermost layer) resulted in the formation of a variety of rock types that subsequently melted and mixed to produce the compositional diversity observed in the lunar sample suite. The initial rocks to crystallize from this Moon-wide molten layer (the magma ocean) contained olivine and pyroxene and were compositionally less evolved than the plagioclase-rich rocks that followed. The last stage of crystallization, representing the last few per cent of the magma ocean, produced materials that are strongly enriched in incompatible elements including potassium (K), the rare earth elements (REE) and phosphorus (P)--termed KREEP. The decay of radioactive elements in KREEP, such as uranium and thorium, is generally thought to provide the thermal energy necessary for more recent lunar magmatism. The ages of KREEP-rich samples are, however, confined to the earliest periods of lunar magmatism between 3.8 and 4.6 billion years (Gyr) ago, providing no physical evidence that KREEP is directly involved in more recent lunar magmatism. But here we present evidence that KREEP magmatism extended for an additional 1 Gyr, based on analyses of the youngest dated lunar sample.

The source and origin of terrigenous sedimentary rocks in the Mesoproterozoic Ui group, southeastern Russia

Arenites, wackes, and associated shales and siltstones of the Ui group of Riphcan age in southeastern Siberia have been analyzed for petrographic, major element and selected trace element compositions (including the REE). Petrographic results suggest that the arenites may have been derived from granites and that the wackes may have been derived from more plagioclase-rich rocks like tonalites or granodiorites. The high quartz relative to other materials in many sandstones suggest that they may have been derived from intense chemical weathering or diagenesis so the mineralogic ratios may have been changed during these processes. The shales and siltstones are composed mostly of illite and quartz. In addition, A–CN–K diagrams indicate that the wackes and associated shales and siltstones underwent K-metasomatisim. Extrapolation of the shales and siltstones associated with the wackes back to the plagioclase–alkali feldspar line in the A–CN–K diagram suggests a high average plagioclase to alkali feldspar ratio in the provenance (tonalite to granodiorite). Extrapolation of shales and siltstones associated with the arenites suggest a low plagioclase to alkali feldspar ratio in the provenance (granite). In addition, the index of compositional variability (ICV) and chemical index of alteration (CIA) parameters of the shales and siltstones suggest the weathering of the first cycle material was intense. Trace element ratios suggest all the sedimentary rocks were derived from mostly granitoids. Shales and siltstones were plotted as elemental ratios (e.g. Th/Sc vs. Eu/Eu*). These plots suggest a mix of a granodiorite–tonalite source (for the source of the shales and siltstones associated with the wackes) with a granite source (for the shales and siltstones associated with the arenites). The arenites also contain lower Eu/Eu* and higher Th/Co and Th/Sc ratios than the wackes. This, as is consistent with the petrography, suggests that the arenites were derived from a source with more granite and less granodiorite–tonalite than the wackes.

Seismic evidence for a mantle source for mid-Proterozoic anorthosites and implications for models of crustal growth

Abstract Voluminous anorthosite intrusions are common in mid-Proterozoic crust. Historically, two end-member models have been proposed for the origin of these anorthosites. In the first model anorthosites derive from fractionation of a mantle source leaving a residue of metagabbro in the lower crust; in the second model anorthosites are the product of partial melting of the lower crust with residual pyroxene and high-grade minerals (i.e. a pyroxenitic and/or metapelitic lower crust). Although a general consensus has developed that the first model provides the best fit to petrological and geochemical constraints, the sparse evidence for mafic and ultramafic counterparts to the anorthosites leaves the issue still unresolved. We use the absolute P-wave velocity and the ratio between P- and S-wave velocities ( V P / V S ) to infer the composition of the lower crust beneath the Marcy Anorthosite (New York State, USA). Seismic refraction data reveal a lower crust 20 km thick, where V P and V P /V S range from top to bottom between 7.0 km s −1 and 7.2 ± 0.1 and 1.84 km s −1 and 1.81 ± 0.02, respectively. Laboratory measurements on rock samples indicate that these seismic properties are typical of plagioclase-rich rocks. Magmatic underplating of basaltic melts is a mechanism to form plagioclase-rich bulk composition for the Grenville crust. At the bottom of the lower crust, increase of P-wave velocity, slight decrease of V P / V S ratios and the presence of a low-reflective seismic Moho are additional observations supporting crustmantle interactions related to magmatic underplating. High P-wave velocity (8.6 km s −1 ) in the upper mantle may indicate that the ultramafic portion (e.g. pyroxenites) of the underplated magma has become eclogite. High average P-wave velocity (6.7 km s −1 ) and V P / V S (1.81), and the exceptional abundance of anorthosites-norites-troctolites among the rocks exposed at the surface, indicate that the Grenville Proterozoic crust may have a unique plagioclase-rich bulk composition. We suggest magmatic underplating, occurring either over a wide time span or with separate syn- and post-collisional magmatic pulses, as being a major crust-forming mechanism operating in mid-Proterozoic time.