Internal Zoning Research Articles

A geological, geochemical and textural study of an LCT pegmatite: implications for the magmatic versus metasomatic origin of Nb-Ta mineralization in the Moose II pegmatite, Northwest Territories, Canada

*Corr esponding author The internal zonation and chemical evolution of Nb–Ta oxides and muscovite have been characterized in the Moose II pegmatite, Northwest Territories, Canada, to distinguish primary magmatic mineralization from that formed during late-stage metasomatic processes. In addition, muscovite associated with Nb–Ta oxides was examined in order to assess the evolution of the pegmatite melt and the nature of the late-stage fluids. Detailed mapping shows that the studied body (430 × 61 m) is a highly fractionated, irregularly zoned, spodumene-subtype rare-element pegmatite dominated by coarse-grained to megacrystic albite, K-feldspar, and spodumene, with intergranular assemblages of quartz, K-feldspar, albite, spodumene, muscovite, and minor amblygonite–montebrasite. Monomineralic core zones (quartz and amblygonite–montebrasite), aplitic albite ‘pods,’ and units characterized by phyllic alteration occur in the central portions of the pegmatite. Columbite–tantalite minerals occur throughout the pegmatite, excluding the quartz cores, and patterns of internal zoning include: (1) normally zoned ferrocolumbite with early Ta–Ti-rich, and later Nb–W-rich overgrowths; (2) oscillatory zoned Ti-rich ferrotantalite with Nb-rich patchy replacement; (3) reversely zoned ferrocolumbite, with Ta-rich cores and (4) com plexly zoned Ti-rich ferrotantalite with abundant late Nb- and Ta-rich patches and sieve textures. The textures and chemical patterns demonstrate an evolution from columbite to tantalite, whereby the Ta/(Ta + Nb) ratio increased with progressive fractionation (0.13–0.59) but the Mn/(Mn + Fe) ratio remained nearly constant (0.15–0.31). The chemical evolution of the Nb- Ta oxides from columbite to tantalite is consistent with crystallization from a magmatic to late-stage magmatic Na-rich melt, with a sequence of crystallization progressing from those of the wall zone, to the first intermediate zone, and finally the late aplitic albite zones. Minor remobilization of Nb, and to a lesser extent Ta, was responsible for some of the replacement features found in the Nb–Ta oxides. Textural observations and trace-element analyses of fine-grained, secondary muscovite found throughout the pegmatite indicate hydrothermal metasomatism by a late F- and Nb-rich vapor-like “supercritical” fluid.

Timescales of growth response of microbial mats to environmental change in an ice-covered antarctic lake.

Lake Vanda is a perennially ice-covered, closed-basin lake in the McMurdo Dry Valleys, Antarctica. Laminated photosynthetic microbial mats cover the floor of the lake from below the ice cover to >40 m depth. In recent decades, the water level of Lake Vanda has been rising, creating a “natural experiment” on development of mat communities on newly flooded substrates and the response of deeper mats to declining irradiance. Mats in recently flooded depths accumulate one lamina (~0.3 mm) per year and accrue ~0.18 µg chlorophyll-a cm−2 y−1. As they increase in thickness, vertical zonation becomes evident, with the upper 2-4 laminae forming an orange-brown zone, rich in myxoxanthophyll and dominated by intertwined Leptolyngbya trichomes. Below this, up to six phycobilin-rich green/pink-pigmented laminae form a subsurface zone, inhabited by Leptolyngbya, Oscillatoria and Phormidium morphotypes. Laminae continued to increase in thickness for several years after burial, and PAM fluorometry indicated photosynthetic potential in all pigmented laminae. At depths that have been submerged for >40 years, mats showed similar internal zonation and formed complex pinnacle structures that were only beginning to appear in shallower mats. Chlorophyll-a did not change over time and these mats appear to represent resource-limited “climax” communities. Acclimation of microbial mats to changing environmental conditions is a slow process, and our data show how legacy effects of past change persist into the modern community structure.

Variations in mercury concentration within and across lichen Xanthoparmelia spp. individuals: implications for evaluating histories of contaminant loading and sampling design

Environmental context Lichens have been widely used as biomonitors of atmospheric pollution in the absence of high-density ambient monitoring networks. This study examines the potential for the lichen Xanthoparmelia spp. as a recorder of temporal histories of mercury deposition to the landscape. Abstract Effects of thallus size and internal zonation on the Hg concentration in the foliose lichen Xanthoparmelia spp. were investigated. Size and zonation effects, if present, provide the potential for temporal records of atmospheric deposition to be recorded in lichens. Our results (n=49; 0.4–13.8cm in diameter) indicated no significant relationship between Hg and size, although thalli less than 2cm in diameter tended towards lower Hg concentrations; and no zonation of Hg within thalli. Distinct zonation of Hg in thalli has been reported in some studies, but not in others, indicating regulatory mechanisms result by which Hg is released or relocated within the thallus under certain conditions. A secondary objective was to evaluate the variability of Hg in lichen individuals to drive future sampling designs. Within a size range of 2–8cm in diameter, we observed Hg=154±30ppb (mean±s.d., n=38). Bootstrap analysis of this dataset indicated that for a sample size of n=3 thalli, we can expect a 94% probability that the variability in our sample set will be at least as low as that observed in other studies of Hg in lichen (s.d. ≈50ppb Hg).

Detrital zircon geochronology and polycyclic sediment sources, Upper Jurassic – Lower Cretaceous of the Scotian Basin, southeastern Canada 1This article is one of a series of papers published in this CJES Special Issue on the theme of Mesozoic–Cenozoic geology of the Scotian Basin.

Sources of Tithonian–Albian sediment in the Scotian Basin are interpreted from detrital zircon geochronology to test previous hypotheses about the sources and pathways of sediment to thick deltaic successions that are important hydrocarbon reservoirs. Sediment provenance influences reservoir quality, but also provides information on tectonism during rifting of the North Atlantic Ocean. More than 760 zircons were dated by laser ablation U–Pb methods from nine offshore wells and one borehole on land and were characterized by external morphology, internal zoning, and Th/U ratio. A Meguma terrane source to the LaHave Platform was confirmed by peaks in detrital zircon abundance at 550–650 Ma, 1.0–1.2 Ga, and ∼2.1 Ga. Samples from the Sable Subbasin show a large peak in detrital zircon abundance at ∼1050 Ma, with lower peaks from 400–650, ∼1480, ∼1650, ∼1860 Ma and 2.7 Ga, characteristic of inboard Appalachian terranes of Laurentide affinity. Many late Paleozoic to Neoproterozoic zircons are euhedral or subhedral, and apparently first cycle, as are a few older zircons that indicate transport from the rising rift shoulder in southern Labrador as far north as the Makkovik Province (∼1860 Ma). About half the zircons are rounded and polycyclic. Samples from the Abenaki Subbasin are similar, but late Paleozoic to Neoproterozoic zircons are rare and ∼40% of the Mesoproterozoic zircons are subhedral, implying a different Laurentide source through the Humber valley. Euhedral–subhedral unzoned zircons yielded two groups of Cretaceous dates: ∼105 Ma from the Cree Member, and ∼120 Ma from the Missisauga Formation.

Beryl composition and evolution trends: an example from granitic pegmatites of the beryl-columbite subtype, Western Carpathians, Slovakia

*Corresponding author Beryl is an uncommon but characteristic mineral of Hercynian granitic pegmatites associated with S-type granites– granodiorites of the Bratislava and Bojna massifs, Tatric Unit, Central Western Carpathians (SW Slovakia). Beryl represents the only essential rare-element phase in majority of the pegmatites, whereas accessory Nb–Ta–(Sn) oxide minerals occur in the most evolved ones (Moravany nad Vahom, Jezuitske Lesy). Beryl forms columnar pale green crystals on the boundary between blocky K-feldspar zone and muscovite plus quartz core (beryl I) or locally in saccharoidal and cleavelandite albite unit (beryl II). The EMPA, LA-ICP-MS and XRD data show mostly the presence of common Li, Cs-poor beryl with variable amounts of Na and occasionally small Fe, Mg-enriched domains (up to 3.5 wt. % FeO and 1.8 wt. % MgO), with dominant channel–octahedral substitution represented by the Na(Fe 2+ ,Mg) -1 Al -1 exchange. Beryl of the most evolved pegmatites contains moderately elevated Li (up to 5600 ppm; Moravany nad Vahom) or Cs contents (up to 9800 ppm, 1 to 2 wt. % Cs 2 O in some zones; Jezuitske Lesy), which indicates also a possibility of channel–tetrahedral substitutions. A common patchy internal zoning of primary magmatic beryl I crystals represents a late-magmatic to subsolidus, partial dissolution-reprecipitation phenomenon. On the contrary, crystals of beryl II show relatively homogeneous internal texture and lower content of Cs than associated beryl I. Consequently, compositional variations and internal texture of the beryl demonstrate magmatic to subsolidus evolution of less fractionated, beryl– columbite subtype populations of the granitic pegmatites. The primary evolution trend of beryl is characterized by increasing Cs and Cs/Na with decreasing Mg and Mg/Fe from less evolved to more fractionated pegmatites. However, a secondary evolution trend, probably connected with post-magmatic partial dissolution-reprecipitation, shows decreasing Cs and increasing Mg/Fe in the beryl.

Relation of organic and inorganic substances in formation of urolyths

Macro- and microscopic structural features of human urolyths have been studied to reveal the relation between organic and inorganic components and the role of the latter in the process of urolyth formation and growth. The combined study of the shape, color, surface, mineral and organic composition, and internal structure and zoning of 788 urolyths has been performed. Crystallomorphological, polarization-optical, X-ray-diffraction, electron-microscopic, chemical, neutron-activation, and computer technologies of study of the urolyth structure have been used. It has been found that macroscopic characteristics of uroluths (color, shape, surface, and size) did not reflect the chemical composition of urolyths. In the structure of abundance of urolyths in Tomskand the Tomsk Region, oxalates (58%) and urolyths of complex mineral composition (29%) prevail. It has been found that an urolyth is an organomineral aggregate having the internal structure formed by a rhythm of alternation of organic and inorganic layers and the character of arrangement of individual mineral crystals, and the internal structure is independent of the chemical composition of the urolyth and uniform in urolyths of different chemical composition. The most part of urolyths has a mixed (44%) or druzy (35%) type of crystal arrangement and the combined (45%) or grainy (31%) rhythm of alternation of organic and inorganic components.

Small-scale polygenetic mélanges in the Ligurian accretionary complex, Northern Apennines, Italy, and the role of shale diapirism in superposed mélange evolution in orogenic belts

The Argille varicolori (Varicolored scaly clays) of the External Ligurian Units of the Northern Apennines have been widely described as a typical unmetamorphosed broken formation (i.e., a chaotic unit without exotic blocks), produced by offscraping and tectonic imbrication during the evolution of the Ligurian accretionary wedge. Geological mapping and integrated structural and stratigraphic observations show that the Argille varicolori consist of diverse types of small-scale mélanges (non-mappable at a 1:25,000 scale) forming a composite chaotic unit, in which the superposition of tectonic, sedimentary and diapiric processes resulted in the occurrence of polygenetic chaotic bodies at different scales. These mélange units record the evolution of the Ligurian accretionary wedge from subduction to collision and intracontinental deformation. Tectonically Disrupted Body 1 (TDB1) comprises boudinage and pinch-and-swell structures formed by layer-parallel extension/contraction at the wedge front of the Ligurian accretionary complex during the late Cretaceous–middle Eocene. It is interleaved with non-mappable Gravity-driven Chaotic Bodies (GCB) developed during alternating episodes of accretion and removal of material at the wedge front. The late Oligocene–early Miocene out-of-sequence thrusting related to the collisional episodes in the Apennines overprinted the previously formed chaotic bodies and formed a polygenetic tectonic mélange (Tectonically Disrupted Body 2, TDB2). This unit is characterized by a structurally ordered block-in-matrix fabric and by the gradual decrease of stratal disruption away from the regional thrust. Overpressurized fluids concentrated along the shear surfaces, and the scaly cleavage planes facilitated the diapiric upward movement of unconsolidated sediments in the early Miocene. This process produced non-mappable shale dike injections (DDB1) and mappable Diapirically Disrupted Bodies (DDB2), which show an internal zonation of deformation. This deformation reworked the previously formed chaotic bodies. Although some of these polygenetic mélanges cannot be mapped at a 1:25,000 scale, their careful documentation provides a better understanding of time-progressive, scale-independent mélange-forming processes.

Controls on emplacement of the Caledonian Ross of Mull Granite, NW Scotland: Anisotropy of magnetic susceptibility and magmatic and regional structures

Research Article| May 01, 2012 Controls on emplacement of the Caledonian Ross of Mull Granite, NW Scotland: Anisotropy of magnetic susceptibility and magmatic and regional structures M.S. Petronis; M.S. Petronis 1Environmental Geology, Natural Resource Management Department, New Mexico Highlands University, P.O. Box 9000, LasVegas, New Mexico 87701, USA Search for other works by this author on: GSW Google Scholar B. O'Driscoll; B. O'Driscoll 2School of Physical and Geographical Sciences, Keele University, Keele ST5 5BG, UK Search for other works by this author on: GSW Google Scholar C.T.E. Stevenson; C.T.E. Stevenson 3School of Geography, Earth, and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK Search for other works by this author on: GSW Google Scholar R.J. Reavy R.J. Reavy 4Geology, School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland Search for other works by this author on: GSW Google Scholar Author and Article Information M.S. Petronis 1Environmental Geology, Natural Resource Management Department, New Mexico Highlands University, P.O. Box 9000, LasVegas, New Mexico 87701, USA B. O'Driscoll 2School of Physical and Geographical Sciences, Keele University, Keele ST5 5BG, UK C.T.E. Stevenson 3School of Geography, Earth, and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, UK R.J. Reavy 4Geology, School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland †E-mail: mspetro@nmhu.edu Publisher: Geological Society of America Received: 26 Jun 2010 Revision Received: 28 Jul 2011 Accepted: 09 Aug 2011 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2012 Geological Society of America GSA Bulletin (2012) 124 (5-6): 906–927. https://doi.org/10.1130/B30362.1 Article history Received: 26 Jun 2010 Revision Received: 28 Jul 2011 Accepted: 09 Aug 2011 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M.S. Petronis, B. O'Driscoll, C.T.E. Stevenson, R.J. Reavy; Controls on emplacement of the Caledonian Ross of Mull Granite, NW Scotland: Anisotropy of magnetic susceptibility and magmatic and regional structures. GSA Bulletin 2012;; 124 (5-6): 906–927. doi: https://doi.org/10.1130/B30362.1 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 Anisotropy of magnetic susceptibility (AMS) was measured from 139 sites across the exposed portion of the Caledonian Ross of Mull Granite, Argyllshire, NW Scotland, to investigate the internal architecture of the pluton. Field and petrographic observations support the results of the AMS study, and a complementary suite of rock magnetic experiments was used to quantify the magnetic mineralogy of the granite. AMS data reveal a fabric that records a partial tectonic overprint of an emplacement-related magma inflow fabric. The partially preserved inflow pattern indicates a south to north emplacement of subhorizontal sheets that coalesced to form a tabular pluton. The AMS data are remarkably consistent across the intrusion, with clear north-south strikes and trends dominating the magnetic foliation and lineation patterns, respectively. These AMS fabrics are discordant to internal zonation in the granite. In the southernmost part of the granite, AMS data are parallel to bedding-cleavage orientations preserved in many large (>100 m) Proterozoic Moine blocks. The scarcity of Moine blocks in the northern part of the intrusion and the prevalence of randomly oriented stoped blocks suggest that this part of the intrusion is nearer to the ceiling of the pluton. The partial tectonic overprint, consistent with east-west compression, occurred during the latter stages of emplacement and mainly affected the younger granite facies (RM2), while the oldest granite facies (RM1) and certain magmatic structures (diorite enclaves) preserve the original emplacement fabric. An alternative explanation interprets the magnetic lineations as an intersection fabric between a shallowly dipping emplacement fabric and a steep tectonic fabric. We argue that emplacement was not associated with eastward orogenic collapse of the Scandian Moine nappes, as proposed by previous workers, but occurred either before orogenic collapse or during a period of compressional reactivation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

An Early Ordovician tonalitic–granodioritic belt along the Schistose-Greywacke Domain of the Central Iberian Zone (Iberian Massif, Variscan Belt)

AbstractThe Zarza la Mayor and Zarza de Montánchez tonalites and Arroyo de la Luz granodiorite are part of a tonalitic–granodioritic belt located along the Schistose-Greywacke Domain of the Central Iberian Zone. These intrusions are also part of the Central Extremadura Batholith, a set of plutons ranging from tonalite to leucogranite that have been considered a prime example of Variscan syn-kinematic plutonism. New LA-ICP-MS and CA-ID-TIMS U–Pb dating reveals that the Zarza la Mayor tonalite–granodiorite is an Early Ordovician intrusion. The LA-ICP-MS data show that there is an absence of inherited cores, despite some complex internal zoning with obvious resorption features in some of the zircon crystals. Dating of monazite and zircon by CA-ID-TIMS provides a concordant age of 478.1 ± 0.8 Ma. This age coincides with electron microprobe analysis (EMPA) monazite chemical ages for the Zarza de Montánchez (482 ± 10 Ma) and Arroyo de la Luz (470 ± 15 Ma) intrusions. These new data indicate the presence of an Early Ordovician belt of calc-alkaline tonalite–granodiorite in the Schistose-Greywacke Domain – the Beira Baixa–Central Extremadura tonalite–granodiorite belt – which resembles a continental magmatic arc. This belt is contemporaneous with the Ollo de Sapo magmatic event further north in the Central Iberian Zone.

Trail Accessibility as a Tool for Sustainable Management of Protected Areas: Case Study Ceahlău National Park, Romania

Abstract Accessibility is an indicator that involves two components: the place - with its morphological and morphometric features -, and man - who wants to reach that place. The relief accessibility map is a useful tool both for tourists and national park managers. Sustainable management in a park and planning involves the opening of new hiking trails which must take into account the relief accessibility, internal zoning, land use and the presence of tourist interest objectives. In this study we produced a map of relief accessibility for Ceahlau National Park (CNP) in Romania using two indicators: the slope and the land use. The model integrates both primary data and secondary data resulted from the reclassification operations. From the intersection of these secondary data in a transition matrix we have established five types of accessibility levels that are represented for the entire park and trails in the CNP. Of the total area of the park - which is located in Romania's eastern Carpathians, within a mountain unit with less than 1900 m altitude -, over 90% encompasses high and medium accessibility levels (types 2 and 3) which shows a high potential for planning. Data resulted from this model can be used for an efficient management of trails in a national park and their evaluation according to the degree of accessibility.

Geochemical characterization of zoned zircon from Wadi Abu Rusheid psammitic gneiss, south Eastern Desert, Egypt

A unique zircon was studied in the gneiss samples collected from the Wadi Abu Rusheid psammitic gneiss using electron scanning microscope and electron probe microanalyses. This zircon can be categorized into two types according to the texture and trace element content: (l) magmatic zircon slightly enriched in HfO2 with ordinary zone. (2) Overgrowths of zircon occur as two species, the first species being highly enriched in HfO2 with irregular zoning. The second species is highly enriched in HfO2 forming a rim around the second species with a very sharp thinner boundary. The first type shows a distinct oscillatory internal zoning pattern without change in shape of this zone and has conspicuous inclusion-free zircon overgrowths with distinct poor concentrations in Y, Hf, Th, U, Nb, and Ta in both rim and core. The second type shows two species, the first one displays distinct irregular interval zoning and irregular overgrowth with abrupt change in composition of these zones with distinct enrichment in Y, Hf, Th, U, Nb, and Ta in the rim relative to the core. The second species is forming a rim around the first species also with distinct enrichment in Y, Hf, Th, U, Nb, and Ta content. These indicate that two events (crystallization environment) have played an important role in the formation of this zircon and largely reflect differences in whole-rock trace element contents between the successive generations of this zircon. The first event is believed to be of magmatic origin giving rise to normal composition of magmatic zircon. The second event shows an intense successive process of metasomatic activity during the formation of the Abu Rusheid radioactive gneiss. Electron microprobe analysis indicates that oscillatory zoned zircon shows poor content of Y, Hf, Th, U, Nb, Ta, and rare earth elements (REE) in the rim and core, while overgrowths of zircon are slightly enriched by these elements. Also, these analyses indicate that the Abu Rusheid psammitic gneiss has been significantly enriched by the thorite mineral (Th content up to 54.72% ThO2) and columbite-bearing minerals (Nb content up to 64.74% Nb2O5, Ta content up to 9.32% Ta2O5). The poor content of REE in overgrowths of zircon indicates mobilization of REE during the metamorphism processes of gneiss.

The case for a cognate, polybaric origin for kimberlitic olivines

Kimberlitic olivines typically show a continuous range in size and texture rather than two discrete populations. The cores of small euhedral olivines commonly provide the template for the final crystal shape, which in turn closely matches morphologies produced by crystallization from a moderately under-cooled magma. Cores and edges of the majority of all olivines define a continuous compositional field, which can be interpreted in terms of Raleigh crystallization. Marked chemical gradients at the olivine margins are linked to rapid physico-chemical changes to the magma associated with loss of volatiles during the late stages of emplacement. Thus, rapid crystallization of groundmass olivines would deplete the magma in Ni, but increase Ca activity. The latter would be enhanced by decreasing pressure coupled with loss of CO 2 from the carbonate-bearing kimberlite magma. For mantle olivines and the most refractory olivines in kimberlites (~ Fo 94) to be in equilibrium with bulk rock compositions matching those of Mg-rich macrocrystic and aphanitic kimberlites (Mg# ~ 88) requires a mineral-melt Mg–Fe distribution coefficient of 0.47. This is well within the experimentally determined range for this distribution coefficient in carbonate-bearing systems. In southern African post-Gondwana alkaline pipe clusters, the average bulk rock Mg# and composition of the associated most Mg-rich olivine both decrease sympathetically from the interior to the continental margin, which is also consistent with a cognate origin for the olivines. A kimberlite magma following a plausible P-T trajectory relative to the CO 2/H 2O peridotite solidus would initially experience superheating, resulting in partial resorption of early-formed olivines that crystallized on the cool conduit walls. It would become supersaturated as it crossed the carbonated peridotite “ledge”, resulting in tabular and hopper growth forms typical of euhedral olivine cores. With further ascent, the magma would once again become superheated, resulting in partial resorption of these cores. Thus, apparently complex textures and internal zonation patterns of kimberlitic olivines are predicted by a plausible magma P-T trajectory.

Mechanism of metamorphic zircon growth in a granulite-facies quartzite, Adirondack Highlands, Grenville Province, New York

Granulite-facies quartzites from the Adirondack Highlands (Grenville Province, New York) contain accessory zircon with ubiquitous metamorphic overgrowths. Detrital zircon cores are resorbed, preserve various internal zoning styles and inclusions, and have ages of 1.3 to 2.7 Ga. These ages constrain the timing of deposition of the protolith sandstone and suggest that the Adirondack Highlands were proximal or part of Laurentia during deposition. Metamorphic overgrowths formed in the quartzites during the Shawinigan orogeny (1.20-1.14 Ga). The average volume of overgrowths in eight samples ranges from 62-87%, with a positive correlation between zircon growth and feldspar content (melt productivity during metamorphism). Melt volumes and Zr solubility were too small to allow the overgrowths to have formed in one dissolution-precipitation event. Crystal-size distributions suggest zircon coarsening by the dissolution of small crystals and Zr transfer via a partial melt, and thus zircon overgrowths date anatexis. These results have implications for provenance studies, as dissolution of small zircon grains and growth of new zircon could bias age spectra of metasedimentary rocks.

Phanerozoic reactivation of the Archean North China Craton through episodic magmatism: Evidence from zircon U–Pb geochronology and Hf isotopes from the Liaodong Peninsula

The Archean lithospheric root of the North China Craton (NCC) has been considerably eroded and modified by Phanerozoic magmatic processes. Here we investigate the decratonization of the NCC through U–Pb and Hf isotopic analyses of zircons from Cenozoic basalts in the Liaodong Peninsula using ion-probe and MC-ICPMS techniques. The U–Pb zircon geochronology identifies three zircon populations: Precambrian, Paleozoic and Mesozoic. The Precambrian zircons yield 207Pb/ 206Pb ages of 2275–2567 Ma with a peak at around 2.5 Ga. They define a U–Pb discordia with upper intercept ages of 2447 ± 50 Ma to 2556 ± 50 Ma and a wide range of Hf T DM ages with a mode at 2.7–2.8 Ga. Our results clearly demonstrate the presence of an Archean lower crust in the Liaodong region. The Paleozoic zircons from the Liaodong region lack the clear internal zoning and are subhedral to rounded in shape, and yield a narrow 206Pb/ 238U concordant ages of 419–487 Ma with a weighted mean age of 462 ± 16 Ma. The Mesozoic zircons predominantly show crystallization in the early Cretaceous and yield a relatively large range in 206Pb/ 238U ages from 100 to 138 Ma ( n = 53) with a peak around 120 Ma. Three samples give indistinguishable weighted mean 206Pb/ 238U ages of 120 ± 5 Ma, 120 ± 4 Ma and 121 ± 2 Ma. These early Cretaceous zircons have enriched Hf isotope compositions with ε Hf( t) values from −26 to −16. Our results provide important constraints on episodic magmatism during the Phanerozoic in the Liaodong region, which led to substantial reactivation of the Archean basement of the North China Craton.

FTIR spectroscopy with a focal plane array detector: A novel tool to monitor the spatial OH-defect distribution in single crystals applied to synthetic enstatite

Single crystals of pure enstatite (Mg2Si2O6) were synthesized under water-saturated conditions at 6 GPa and 1250 °C and variable silica activity. Run products were investigated using a novel technology: a FTIR spectrometer equipped with a focal plane array detector enabling IR-imaging with a spatial pixel resolution of 2.7 μm. IR spectra within the OH-absorption region show strong pleochroic behavior: absorption bands at 3687 and 3592 cm−1 show strongest absorptions for E||nβ, whereas absorption bands at 3362 and 3067 cm−1 show strongest absorptions for E||nγ. Furthermore, absorption bands are sensitive to the silica activity—the ratio of the integral absorbance of the absorption bands at 3687 and 3592 cm−1 to the absorption bands at 3362 and 3067 cm−1 increases with increasing Mg/Si-ratio of the bulk charge. The most probable interpretation is an assignment of the high-energy absorption bands to tetrahedral (T-site) defects caused by a lower availability of Si and the low-energy absorption bands to octahedral (M-site) defects caused by a lower availability of Mg. All crystals show an internal zonation pattern with an increasing T-site to M-site defect ratio from core to rim, which is interpreted to be caused by changing silica-activity and T during the experiments. The defect ratio and the zonation pattern are applied as a monitor of crystal growth conditions.

Dissolution–reprecipitation of igneous zircon in mid-ocean ridge gabbro, Atlantis Bank, Southwest Indian Ridge

Zircons recovered from oceanic gabbro exposed on Atlantis Bank, Southwest Indian Ridge, typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas. In one rock (of twenty investigated), weak-oscillatory-zonation patterns are overprinted by secondary textural features characterized by mottled, convoluted and wavy internal zonation patterns that are frequently associated with secondary micron- to submicron-scale micro-porosity. These zircons are hosted in a felsic vein that intruded an oxide gabbro, both of which are cross-cut by monomineralic amphibole- and quartz-rich veinlets. Zircons with weak-oscillatory-zonation patterns record a weighted-average 206Pb/ 238U age of 12.76 ± 0.20 Ma (mswd = 1.5), and have high trace element concentrations [e.g., ΣREEs (∼ 0.4–2.2 wt.%), Y (∼ 0.6–2.8 wt.%), P (∼ 0.4–0.9 wt.%)], and Th/U (0.1–0.5). These zircons are anomalously old (≥1 Myr) relative to the magnetic age for this portion of oceanic crust (11.75 Ma). In contrast, zircons with non-igneous, secondary textures have a younger weighted-average 206Pb/ 238U age of 12.00 ± 0.16 Ma (mswd = 1.7), and have lower trace element concentrations [e.g., ΣREEs (∼ 0.2–0.8 wt.%), Y (∼ 0.3–1.0 wt.%), P (∼ 0.1–0.3 wt.%)], and slightly lower Th/U (0.1–0.3). The weighted-average age of these zircons is similar to the magnetic anomaly age, and other 206Pb/ 238U ages of nearby rocks. We do not observe a correlation between crystallographic misorientation, internal texture, or trace element chemistry. We suggest that the decrease in trace element concentrations associated with the development of non-igneous alteration textures is attributed to the purging of non-essential structural constituent cations from the zircon crystal lattice at amphibolite-facies conditions. The mechanism of alteration/re-equilibration was likely an interface-coupled dissolution–reprecipitation processes that affected pre-existing, anomalously old zircons during shallow-level magmatic construction of Atlantis Bank at ∼ 12.0 Ma.

Rapid, high-temperature formation of large-scale rheomorphic structures in the 2.06 Ma Huckleberry Ridge Tuff, Idaho, USA

In the Teton River Canyon, eastern Idaho, the ca. 2.06 Ma, 130-m-thick Huckleberry Ridge Tuff exhibits large-scale rheomorphic fold geometries defined by eutaxitic fabrics parallel to both the primary internal zonation and the basal contact with older strata. Paleomagnetic data from a large-amplitude (>150 m), northwest-trending, overturned fold near the failed Teton Dam indicate folding above maximum magnetization unblocking temperatures (>580 °C). The in situ characteristic remanent magnetization (ChRM) direction is indistinguishable from previous studies of undeformed Huckleberry Ridge Tuff, and a fold test is negative ( k minimized at 100% unfolding). Anisotropy of magnetic susceptibility data reveal magnetic foliation planes that dip northeast, roughly parallel to the axial surface of the fold. Because deformed and undeformed Huckleberry Ridge Tuff exposures preserve the same anomalous ChRM direction, large-scale rheomorphic structures in the tuff must have formed rapidly at high temperatures shortly after development of compaction fabrics. Post-welding, high-temperature deformation is consistent with field evidence indicating rapid, plastic secondary deformation of much of the tuff prior to devitrification.

Niobium-tantalum minerals in the Skoddefjellet NYF granitic pegmatite, Svalbard Archipelago, Norway: Primary versus secondary assemblage

The first documented occurrence of Nb-Ta mineralization in a granitic pegmatite from the Svalbard Archipelago, Arctic Caledonides and the relationships between primary and secondary Nb,Ta-bearing phases are discussed. The Skoddefjellet NYF pegmatite located in southwest Spitsbergen (Wedel Jarlsberg Land) is characterized by enrichment in Nb, U, Zr, Y, REE and Pb, low Zr/Hf (∼8-10), and (∼8-12) Nb/Ta ratios. Columbite-(Fe) and columbite-(Mn) (locally enriched in Ti, Y, Sc and Ca) and Nb,Ta, Y-bearing titanite (≤3.8 wt. % Nb2O5, ≤1.9 wt. % Ta2O5, ≤2.9 wt. % Y2O3) represent the primary magmatic Nb-Ta minerals. The secondary assemblage comprises Nb,Ta-bearing rutile (≤23 wt. % Nb2O5, ≤1.7 wt. % Ta2O5), ilmenite, fersmite and pyrochlore group minerals: including yttropyrochlore-(Y), plumbopyrochlore, yttrobetafite-(Y) and plumbobetafite. Internal zoning of columbite group minerals is expressed by distinct rimward increase of the Mn/(Mn+Fe) ratio (0.07 to 0.83) and low Ta/(Ta+Nb) ratio (0.03 to 0.10), suggesting alkaline environment and increasing fluorine activity during the pegmatite evolution. The formation of secondary phases is related to the hydrothermal stage of the pegmatite. Notably, the formation of Nb,Ta-bearing rutile and ilmenite seems to be a product of Nb,Ta-bearing titanite alteration.

A MULTI-STAGE ORTHOMAGMATIC AND PARTIAL MELTING HYPOTHESIS FOR THE DRIEKOP PLATINIFEROUS DUNITE PIPE, EASTERN LIMB OF THE BUSHVELD COMPLEX, SOUTH AFRICA

The occurrence of platiniferous dunite pipes is restricted to four localities in the eastern limb of the Bushveld Complex. They comprise wholly discordant bodies aligned perpendicular to the regional layering, and reveal a complex internal zonation. They remain poorly understood, despite a number of contributions dealing with the ore mineralogy, due mainly to the fact that field relationships and the silicate and oxide mineralogy have not been adequately addressed. The principal geological features of one such pipe, at Driekop, are revisited on the basis of detailed mapping and sampling of mine workings that are not generally accessible. The pipe forms a subcircular body (diameter 300 m) that is persistent to considerable depth. The principal unit is a stock-like body composed entirely of fine-to-medium grained magnesian dunite (Fo 83.4 ). Chromite is a prominent accessory phase of the magnesian dunite. The chromite is locally altered to highly unusual, polyphase grains in which discrete Cr-rich, Al-rich, and Fe-rich phases are identified. The magnesian dunite is rimmed by a zoned outer envelope that consists of an inner ring of iron-rich wehrlite (Fo 65 ) and an outer ring of iron-rich clinopyroxenite pegmatite. Mineralisation is mostly contained in a near-cylindrical core-zone (diameter 18 to 24 m) characterised by segregations of iron-rich dunite (Fo 73 ) and subordinate iron-rich wehrlite within the magnesian dunite. Mineralised veins of iron-rich dunite and wehrlite, as well as barren veins of iron-rich clinopyroxenite pegmatite, crosscut the magnesian dunite outside of the core-zone. All of the iron-rich ultramafics at Driekop are distinguished by the absence of a spinel phase. The pipe crosscuts a thick sequence of leucocratic cumulates (Merensky Footwall unit), that in turn are underlain by layers of chromitite and feldspathic orthopyroxenite (UG3 and UG2 units). The wall rocks are severely disrupted and form a km-wide downwarped feature. Layering proximal to the pipe on the southern margin dips at 75° (north-east) in comparison to the regional dip of 12° (west). The leucocratic wall rocks are mostly unaltered, apart from a thin zone of sausseritization, but include discrete satellite bodies of iron-rich clinopyroxenite pegmatite (Fo 63 ). The underlying, more mafic UG3 and UG2 units, however, are anomalous. They are considerably thinner in comparison to the regional stratigraphy and some layers are entirely absent. The feldspathic orthopyroxenite in these units has been pervasively replaced by magnesian peridotite (Fo 80-78 ). The downwarped UG3 and UG2 chromitite layers are partly dismembered and include spots of pipe-related magnesian olivine. Mineralogical and chemical similarities with dunite layers from the Lower Zone (Fo 85.8 ) and Lower Critical Zone (Fo 85.1 ) suggest that the magnesian dunite in the Driekop pipe is orthomagmatic. This hypothesis is supported by mapping of a meter-wide sill of magnesian dunite (Fo 85 ), in the wall rocks at Driekop. Field relationships demonstrate this sill to have been intruded between layers of leuconorite and anorthosite. The magnesian dunite in the pipe formed as a consequence of “flowage differentiation” of an ultramafic magma through a vertical conduit. Magnesian olivine and Cr-spinel were the only liquidus phases during this phase of pipe formation. Heat associated with the intrusion resulted in formation of magnesian olivine spots within orthopyroxenitic wall rocks (albeit located some distance from the pipe); they are ascribed to the incongruent melting relationship between orthopyroxene and olivine. Within the leuconorite-anorthosite wall rocks, however, the increased temperature associated with the intrusion resulted in selective partial melting of ferromagnesian components (plagioclase was refractory). This produced a moderately iron-rich ferromagnesian melt that was relatively dense and drained downward into fractures propagated by the primary conduit. The downward draining of melt, together with the partial melting of more mafic layers, caused a catastrophic downwarping of the wall rocks. The ferromagnesian melt reacted with the leucocratic wall rocks to produce an iron-rich clinopyroxenite pegmatite (outermost part of the outer envelope and discrete satellites). Reaction with earlier-formed magnesian dunite resulted in either an iron-rich dunite (e.g. core-zone) or an iron-rich wehrlite (e.g. innermost part of the outer envelope). The occurrence of polyphase Cr-spinel in selective samples of magnesian dunite, as well as the entire absence of a spinel phase from the iron-rich ultramafics, suggests the pipe chromite was in disequilibrium with the downward-draining ferromagnesian melt. The origin of the mineralisation is not addressed here but the new hypothesis is consistent with the suggestion of Wagner (1929) that PGE were transported into the platiniferous dunite pipes by iron-rich melts derived by melting of earlier-formed layered reefs.

On the development of internal chemical zonation in small mafic dykes

AbstractThree dolerite dykes (Small, 3.2 cm; Middle, 13.5 cm; Thick, 50 cm) from Torsholma Island, SW Finland, reveal distinctly different internal zonation that becomes more complicated with increasing thickness of the dykes. The Small Dyke shows a systematic inward increase in normative Pl (An+Ab+Or) and a decrease in normative An (100*An/(An+Ab)), whole-rock MgO, Mg number (100*Mg/(Mg+Fetotal)), TiO2, K2O and Zr. The Middle Dyke exhibits the same compositional pattern at the margins, while the centre is distinguished by an abrupt increase in normative Pl and An, whole-rock Sr and Mg number. From the margins inwards, the Thick Dyke displays first a compositional pattern identical to that observed in the Small Dyke and the margins of the Middle Dyke. This is followed by a region where whole-rock MgO, Mg number and normative An start increasing inwards, while the transition to the centre of the dyke is characterized by a compositional pattern similar to that in the centre of Middle Dyke. The origin of chemical zonation in these dykes is attributed to the operation of three independent physico-chemical processes, namely: the Small Dyke formed exclusively by progressive changes in the composition of inflowing magma; the Middle Dyke by changes in composition of inflowing magma (margins) and concentration of plagioclase and olivine phenocrysts by flow differentiation (centre); the Thick Dyke by changes in composition of inflowing magma (margins),in situcumulate growth against dyke sidewalls (middle) and flow differentiation (centre). Systematic changes in these processes and, as a result, in internal chemical zonation, likely take place in response to crystallization of magma under less supercooled conditions with increasing dyke thickness. A comprehensive geochemical study of the internal zonation of small mafic dykes worldwide is required to develop a complete understanding of the processes operating in mafic dykes.