John D. Greenough

Cooling history and differentiation of a thick North Mountain Basalt flow (Nova Scotia, Canada)

A thick (<175 m) North Mountain Basalt flow at McKay Head, Nova Scotia (Canada) shows ∼25-cm-thick differentiated layers separated by ∼130 cm of basalt in its upper 34m. Upper layers (∼5 m below the lava top) are highly vesicular whereas lower ones are pegmatitic and contain a thin (∼2 cm) rhyolite band. The layering of the flow closely resemble that of some Hawaiian lava lakes. The eesicular basalts and mafic pegmatites are inferred to be liquid-rich segregations which drained into horizontal cracks that formed within a crystalline mush. The cracks resulted from a thermal contraction associated with cooling and shrinkage of the mush. Rhyolites were formed by in situ differentiation. Gas overpressures fractured the pegmatites and gas effervescence filter pressing forced silicarich residual liquid from pegmatite interstices into the fractures creating bands. Chemical differences between the pegmatitic layers and early formed, highly differentiated upper vesicular layers may reflect a role for volatiles in the differentiation process along with crystal fractionation.

Platinum-group element geochemistry of continental tholeiites: Analysis of the Long Range dyke swarm, Newfoundland, Canada

Abstract The rift-related, Late Proterozoic Long Range continental dykes of western Newfoundland have platinum-group element (PGE; Pd, Pt, Rh, Ru and Ir determined) concentrations that correlate with indicators of magma evolution [e.g., Mg ′ = Mg ( Mg + 0.9 Fe tot ) ] and magma alkalinity (La/Sm), suggesting that metamorphism had a minimal effect on the PGE. The low-melting-point (LMP) PGE (Pd, Pt and Rh) and Cu behaved incompatibly as magma evolved, indicating that sulphide fractionation did not occur and that the dykes hold little economic promise. Hypotheses that augite (a major fractionating phase) has an affinity for the PGE are not supported. Primitive rocks tend to have lower Ir than evolved rocks and all samples display a negative high-melting-point (HMP) PGE (Ru and Ir) anomaly on mantle-normalized diagrams. Olivine and early chromite fractionation may have decreased the concentrations of these elements. The Long Range dykes confirm that Pd/Ir ratios and absolute LMP PGE concentrations tend to decrease with increasing hot-spot magma alkalinity. The lower PGE concentrations in most MORB compared with hot-spot basalts imply that there are at least two mantle reservoirs for the PGE. Lithosphere overlying mantle sources (continental or oceanic) has little effect on PGE concentrations. The apparently similar source-region PGE concentrations for hot-spot basalts suggest that differences in PGE concentrations between MORB and hot-spot basalts reflect incomplete segregation of the PGE into the core during its formation and rise of the homogeneously dispersed elements to upper-mantle depths in deep mantle plumes.

Mildly alkaline basalts from Pavagadh Hill, India: Deccan flood basalts with an asthenospheric origin

SummaryOf twelve flows at Pavagadh Hill, the two three-phenocryst-basalt flows with Mg#∼0.70 and Ni/MgO∼33 are the most primitive and perhaps as primitive as any basalts in the Deccan province. Scatter on variation diagrams and the occurrence of primitive flows at two different levels in the volcanic sequence implies that most rocks are probably not, strictly speaking, comagmatic. Nevertheless, mass balance calculations indicate a generalized differentiation scheme from primitive basalt to hawaiite that involved removal of olivine, augite, plagioclase and Fe-Ti oxides in the proportions 40:33:22:5 with ∼ 50% of the magma remaining. Crustal assimilation had a minimal effect on evolution of the basalts but rhyolites at the top of the volcanic sequence may have been produced by crustal melting following prolonged heat release from alkali basalt pooled along fault zones in the continental crust. Major element based calculations indicate that the most primitive basalts were generated by 7 to 10% melting of mantle peridotite. These low percentages of melting, typical of alkali basalts, are consistent with the steep slopes on chondrite-normalized REE diagrams. Low heavy REE concentrations point to residual garnet in the source region. Incompatible element concentrations (e.g. Rb, Ba, Zr, La) in Pavagadh basalts exceed those in Deccan tholeiitic basalts but are substantially lower than those reported for some other Deccan alkali basalts. Obviously Pavagadh basalts do not reflect the lowest percentages of melting and greatest amount of source region metasomatic enrichment attained in the Deccan province. Deccan tholeiitic and alkali basalts are largely characterized by low La/Nb ratios and high La/Ba ratios similar to those in oceanic island basalts. This indicates minimal involvement of the subcontinental lithospheric mantle in their petrogenesis. Comparison with continental mafic magma provinces where a subcontinental lithospheric mantle imprint is common indicates long periods of extension and/or melting of mantle lithosphere still hot from pre-extension subduction are more likely to produce magmas bearing the lithospheric imprint.ZusammenfassungIm Gebiet von Pavagadh Hill, Indien, treten 12 Spät-Deccan und rhyolithische alkalibasaltische Ergüsse und Intrusiva auf. Variationsdiagramme zeigen, daß die Abfolge nicht komagmatisch ist. Zusammen mit Berechnungen der Massenbilanz unterstützen sie vielmehr ein Zwei-Stadienmodell für die Entstehung von Hawaiiten aus sehr primitiven (i.e. Mg#=Mg/(Mg+.(0.9*Fetotat)) at.%∼0.70) Basalten. Olivin und Augit dominierten die frühe Fraktionierung während Augit vorherrschte als der Magmaanteil von 65% auf 50% sank. Die Entfernung von Plagioklas spielte bei der Differentiation nur eine geringe Rolle. Niedrige Th/Nb (∼0,2), Rb/Sr(<0,12) und K/NbVerhältnisse geben keine Hinweise auf signifikante Assimilation von Krustenmaterial. Die Seltene-Erd-Verteilungsmuster (SEE), niedrige Gehalte an schweren SEE sowie die Hauptelementspektren der Alkalibasalte weisen auf eine granatführende Ursprungsregion und auf einen Aufschmelzungsgrad von nur 7% bis 10% hin. Es gibt jedoch auch stärker alkalische (höhere Rb, Zr etc.) Deccanbasalte (i.e. Rajpipla). Die Assoziation von Deccanalkalibasalten, Rhyolithen und Störungszonen zeigt, daß letztere die Extraktion von Magma aus dem Mantel erleichterten und dazu führten, daß Magma aus Magmenkammern Krustenschmelzen (Rhyolithe) produzierte. Deccanbasalte tendieren zu hohen La/Ba und niedrigen La/Nb-Verhältnissen; dies weist auf eine asthenosphärische Herkunft hin, selbst wenn die Gesteine verhältnismäßig spät gebildet wurden (i.e. Pavagadh). Längere Perioden von Krustenextension oder von Subduktion, die der Extension vorhergeht, führt offensichtlich zur Entstehung von Magmen mit einer lithosphärischen Komponente.

Provenance of temper in a New Kingdom Egyptian pottery sherd: Evidence from the petrology and mineralogy of basalt fragments

A New Kingdom spinning bowl from Karnak (Luxor) Egypt is similar in form to spinning bowls commonly found at other Egyptian sites and has a bulk chemical composition in the range for other Egyptian marl vessels. These data support a domestic origin. The matrix of the bowl contains unaltered, sand-sized, mafic rock fragments with volcanic, subophitic textures. Over 20% of the sand-sized grains consist of angular, unweathered rock fragments, and of these ∼20% are volcanic. Apparently they were added as temper. Electron microprobe analyses show that augite, plagioclase, and, where present, pigeonite, in nine of these have compositions typical of mafic igneous rocks. Geothermometry confirms crystallization at ∼1100°C. Pyroxene discrimination diagrams indicate geological sources ranging from within-plate alkali basalts to within-plate, continental tholeiites. Suitable sources for the temper are rare in Egypt. Both alkaline and tholeiitic, postorogenic (unaltered) late Cenozoic basalts occur in the Cairo area, making this the most likely but not the only possible source for the temper. The pottery may have also originated in Cairo because raw rock materials were moved upstream less commonly than down the Nile. A Cairo provenance for the Karnak artifact is consistent with the everyday movement of people and goods along the Nile between the ancient twin capitals of Memphis and Thebes. These results and the common occurrence of volcanic rocks as temper indicate that microbeam analytical techniques may help narrow the provenance of ancient pottery.

The Sr isotopic composition of early Jurassic mafic rocks of Atlantic Canada: Implications for assimilation and injection mechanisms affecting mafic dykes

Abstract Early Mesozoic rift-related mafic rocks of Atlantic Canada (Caraquet, Minister Island, Shelburne and Avalon dykes and North Mountain Basalt) show high initial 87Sr/86Sr ratios (0.705–0.709) suggesting derivation from a LILE-enriched subcontinental mantle or assimilation of continental crust during ascent. Despite wide geographic distribution, Rb, Sr and Sr isotopic data for each body tend to overlap on variation diagrams, implying a comagmatic origin and/or similar petrogenetic processes. Modelling of these data supports assimilation of average continental crust by a fractionally crystallizing normal MORB magma. Rb/Sr ratios show a slightly positive or negative correlation with 87Sr/86Sr (depending on igneous body). Apparently assimilation involved early absorption of a Rb-rich partial melt with progressive assimilation of residual material enriched in radiogenic Sr. This Rb/Sr 87Sr/86Sr relationship is difficult to explain with a LILE-enriched subcontinental mantle model. Chilled contact samples generally display lower 87Sr/86Sr ratios than dyke interiors, implying that early pulses of magma through dyke systems were less evolved than later pulses. Assimilation occurred prior to injection or at deep crustal levels. Once in the dyke fracture system magma was quenched against cool upper-crustal rocks preventing further assimilation.

Incompatible element ratios in French Polynesia basalts: describing mantle component fingerprints

The chemical composition and origin of the isotopically identified end-member mantle components have been difficult to describe because their melted samples—oceanic basalts—are affected by many processes such as variable degrees of partial melting. Exploratory data analysis [multidimensional scaling (MDS)] applied to ∼200 basalts from French Polynesia reveals over 100 ratios of similarly incompatible elements (SIER) that are minimally affected by these processes. Ratios from elements with dissimilar incompatibility are identified as affected by melting percentages. When basalt samples are compared simultaneously using ∼100 SIER and MDS, they organise in the same way that they would with isotopes, according to mantle component type. Applying discriminant analysis to the most extreme French Polynesia samples representing each mantle component yields preliminary discrimination diagrams that improve the description of chemical variation in the mantle. As a test of their utility and reliability, they are used ...

Magma injection directions inferred from a fabric study of the Popes Harbour dike, eastern shore, Nova Scotia, Canada

A 15-m-wide dike at Popes Harbour on the eastern shore of Nova Scotia shows an increase in both the size (up to 1 m) and density (up to ∼50%) of predominantly pelitic xenoliths toward the dike center. These features reflect flowage differentiation processes during dike emplacement. Flow direction is indicated by sillimanite xenocrysts that represent the unassimilated products of xenolith disaggregation. The xenocrysts are highly aligned parallel to the dike margins (northwest-southeast), with a mean plunge of 20°SE, suggesting that the magma last moved, and may have been injected, in a subhorizontal, northwestward direction. The available information indicates that lateral injection of magma in mafic dikes is more common than predominantly vertical movement. This study shows that an analysis of fabrics in mafic dikes should be useful in further evaluating the role of lateral magma injection in dike formation.

Petrology of sapphirine granulite and associated sodic gneisses from the Indian Head Range, Newfoundland

Migmatitic granulites from the Indian Head Range (IHR) are dominated by granoblastic, Opx-bearing (quartz) dioritic gneiss with subordinate garnet+orthopyroxene+biotite+albite (±quartz±microcline±cordierite±sillimanite) gneiss and comparatively biotite-rich, sapphirine+cordierite+orthopyroxene+albite (±microcline±sillimanite±corundum) gneiss. The latter contains at least two generations of sapphirine and cordierite, one apparently predating migmatization, the other associated with or following this event. Mineral thermobarometers yield temperatures up to ∼800 °C at 8 kbar. The IHR granulites have very high δ18O values (+10.8 to +14.5) which most likely indicate a sedimentary precursor. Their REE and HFSE contents resemble the trace element signature of post-Archean Australian shale (PAAS). Lithological analogues from other Grenvillian inliers in western Newfoundland have similar oxygen isotopic and immobile-element signatures. They too are interpreted as metasedimentary rocks. The IHR rocks, however, have unusually sodic compositions (e.g., Na2O/CaO and Na2O/K2O=2.1–22.5 and 0.93–13.8, respectively) and contain albite or sodic oligoclase despite their high metamorphic grade. The geochemical data indicate that these rocks were albitized prior to high-grade metamorphism. No counterpart of this event is recorded in granulites from the other inliers, indicating that the IHR may be highly allochthonous with respect to these other Grenvillian terranes.

Petrology of the Cormacks Lake Complex, Newfoundland: decompressional reaction relations in cordierite+orthoamphibole-bearing gneisses and associated rocks

Abstract Cordierite+orthoamphibole (Crd+Oam)-bearing gneisses in the Cormacks Lake complex are regionally associated with metapelites containing prismatic sillimanite and K-feldspar, metabasites that locally contain metamorphic orthopyroxene, and other high-grade rocks in the Central Gneiss (Dashwoods) subzone, in the southwestern Newfoundland Appalachians. Retrograde features formed at the expense of the granulite-facies assemblages are ubiquitous. For example, in some migmatitic rocks, garnet is resorbed by Crd+Oam, and in metapelites, cordierite separates corroded garnet and sillimanite. Mineral thermobarometry suggests that, following granulite-facies metamorphism (T<785°C, P<7.5 kbar), retrogression occurred as the Cormacks Lake gneisses cooled through Mg-Fe diffusional blocking temperatures as they decompressed to a pressure of ~3−4 kbar. Given the absence of Barrovian (or higher pressure) mineral assemblages in the metapelites, regional tectonic reconstructions involving the thrusting of a neighbouring terrane (Notre Dame subzone) over the Central Gneiss subzone appear to be supported only by the moderate pressure determined for the granulite facies event. Although scarcely discernible given re-equilibration effects and the imprecision of thermobarometers, subsequent decompression nonetheless had a marked impact on the mineralogy of the gneisses.

Petrogenesis of the Potato Hill pluton, Newfoundland: transpression during the Grenvillian orogenic cycle?

The Potato Hill pluton is a nested, composite, Grenvillian intrusion (c. 1020 Ma) consisting of pyroxene-bearing quartz monzodiorite, equigranular biotite + hornblende + garnet granite, and K-feld-spar megacrystic hornblende + biotite quartz monzodiorite. All members are potassic with high Fe/Mg ratios, and have elevated concentrations of trace elements such as Ba, Zr, Nb and Y. Mass balance calculations successfully relate the concentration of major elements between the most primitive (megacrystic quartz monzodiorite) and evolved (equigranular granite) samples to the fractional crystallization of orthopyroxene, clinopyroxene, hornblende, biotite, garnet and plagioclase, and of trace elements (e.g. P, La and Zr) to the removal of accessory phases such as apatite, allanite and zircon. Lithologic zonation of the pluton is therefore attributed to in situ differentiation processes rather than varying degrees of crustal assimilation or the emplacement of discrete magma batches. This is supported by (1) contrasting δ18O values for the Potato Hill pluton (δ18O = +6.8 to +8.5º/∞) and the gneissic country rock (δ18 O = +10.5 to + 12.1º/∞), and (2) uniformly low ɛNd values (= -1.6 to + 1.2) for the granitoid rocks. Tectonomagmatic discrimination diagrams indicate a within-plate, late-orogenic setting for the Potato Hill pluton. Furthermore, trace element signatures closely resemble shoshonitic lamprophyres. Compositional criteria therefore suggest that the Potato Hill pluton is not directly subduction-related, but was formed in a transpressional regime following ocean closure. Younger (post-1 Ga) Grenvillian plutons in the area may record crustal thickening or the subsequent extensional collapse of the eastern Grenville Orogen.