J. V. Owen

Prograde metamorphism and decompression of the Gföhl gneiss, Czech Republic

Quartzofeldspathic rocks of the Gfohl gneiss from the Moldanubian of the Czech Republic span amphibolite-to granulite-facies, and are associated with eclogite. Protomylonitic fabrics related to terminal tectonic emplacement and reworking of the gneiss are common. Some non-mylonitic rocks, however, preserve early, prograde features (e.g., Opx-rimmed Hbl in metabasites), whereas others have characteristics generally associated with near-isothermal decompression (e.g., Pl-Opx moats separating Grt and Qtz in metabasites; Crd ± Spl coronas on Grt and aluminosilicates in metapelites); the unequivocal distinction between prograde and decompressional features in these rocks, however, may not be possible or even justified. For example, some metapelites contain growth-zoned (i.e., rimward increase in XMg) garnets that also record evidence (i.e., rimward decrease in XCa, compensated by the presence of reversely-zoned plagioclase in the same rock) of decompression. In rare instances, eclogitic rocks (P > 11 kbar) interpreted as tectonic enclaves within the gneiss also record mineralogic evidence of decompression (e.g., Crd-Opx-Spr coronas on pyrope). In metapelites, plagioclase-cored coronal garnets with high PrpGrs ratios (~ 2.5) record near-isobaric cooling from near the thermal maximum at a relatively shallow but undetermined crustal level. n nUnlike Gfohl gneisses elsewhere (e.g., in Austria), the rocks described here do not preserve evidence of extreme metamorphic conditions. Texturally stable Grt-Bt pairs in non-mylonitic samples give Tmax < 750 °C. Pmax is not known, but prograde metamorphism apparently progressed from the kyanite to sillimanite fields, implying P ~ 8 kbar at the maximum Grt-Bt temperature. At these conditions, dehydration of mafic gneiss occurred in the presence of a CO2-rich (XCO2 ~ 0.85) pore fluid

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.

'Decompressional' reaction textures formed by isobaric heating: an example from the thermal aureole of the Taylor Brook Gabbro Complex, western Newfoundland

Abstract Reaction textures including corona structures in granulites from the Proterozoic Long Range Inlier of western Newfoundland are spatially associated with a Silurian (0.34 Ga) mafic intrusion, the Taylor Brook Gabbro Complex. They comprise, in metabasites and tonalitic gneiss, coronal orthopyroxene and plagioclase on garnet and, in metapelites, cordierite and spinel formed at the expense of sillimanite, garnet and quartz. Although generally interpreted to indicate near-isothermal decompression (ITD) following regional metamorphism, which in the inlier occurred at ~1.10-1.03 Ga, these features appear to be absent elsewhere. Therefore they are interpreted to be products of contact metamorphism (near-isobaric heating - IBH) within the thermal aureole of the gabbro. Thus, there is a ~0.7 Ga difference (i.e. mid-Proterozoic vs. mid-Silurian) between the age of the regional metamorphic mineral assemblages and the contact aureole assemblages. The observation that classic ITD features occur in this aureole environment underscores the fact that P-sensitive reactions can progress during IBH as well as by pressure release.

Influence of the mode and distribution of garnet and biotite on Grt-Bt thermometry: evidence from a single-sample case study

Abstract Detailed microprobe and modal data for a sample of layered, garnetiferous, quartzose paragneiss reveal significant differences in garnet-biotite Mg-Fe distribution coefficients (Kd) - and hence paleotemperatures - determined for leucocratic (modal Grt+Bt<25 vol.%) and mesocratic (Grt+Bt> 25 vol.%) layers. In leucocratic layers, lnKd determined from both the core and rim compositions of minerals shows a range of values that varies sympathetically with the absolute amount of garnet and biotite, and, as demonstrated in other studies, inversely with the distance separating both minerals. Due to the small size (<2 mm) of garnets, which facilitated diffusional re-equilibration during cooling from peak metamorphic temperature, lnKd does not correlate to modal Bt/Grt ratios. The largest garnets, which occur in mesocratic layers, nonetheless tend to preserve the most pronounced (retrograde) zoning patterns (i.e. rimward increase in Fe/Mg), consequently mineral core composition lnKd values correlate with grain diameter except where garnets contain abundant biotite inclusions. The highest Grt-Bt temperatures (~ 700°C) are recorded by: (1) the composition of relatively widely-separated (>0.3 mm) grains in highly leucocratic layers; and (2) the core compositions of relatively large (>1 mm), inclusion-free grains in mesocratic layers. More closely spaced garnets and biotites in leucocratic layers, and small grains in mesocratic layers, give a range of temperatures intermediate between Tmax and diffusional blocking temperatures ( ~ 560°C) recorded by the rim compositions of contiguous grains.

Recrystallization of quartzofeldspathic rocks in a paleozoic thrust belt, Grand Lake, Newfoundland: Implications for garnet-biotite thermometry

Migmatitic granitic gneiss and associated garnetiferous granite at Grand Lake are tectonically interleaved with high-pressure ( ∼ 9 kbar) metapelites that contain Barrovian (Ky-St-Grt-Bt-Ms-Rt) mineral assemblages. The migmatites contain metabasites that are compositionally similar to the latest Proterozoic Long Range dykes, suggesting that the gneisses correlate to Grenvillian inliers in western Newfoundland. n nIn the metapelites, the Barrovian porphyroblastic assemblage overprints microfolds and is itself overprinted by greenschist-facies assemblages in thrust-related high-strain zones. Evidence that the quartzofeldspathic rocks also experienced Barrovian metamorphism is provided by low Ti>Al amphiboles in metabasites in the gneiss, and, despite the migmatitic character of these rocks, the presence of growth-zoned garnets with bell-shaped compositional profiles (e.g., rimward increase in XPrp and decrease in XSps) similar to those exhibited by garnet in the metapelite. The range of garnet-biotite paleotemperatures for the quartzofeldspathic rocks is only marginally higher (Tmax=530–660°C) than that determined for the metapelites (Tmax=500–615°C). There are no systematic differences in Grt-Bt temperatures between (proto) mylonitic rocks and nearby precursors. n nThese results indicate that the quartzofeldspathic rocks and metapelites share a common, post-migmatization (of the gneisses) metamorphic history; they may well have originally been linked by a basement/cover relationship. Mineralogical evidence for the earlier, higher grade metamorphic history of the migmatites has virtually been eradicated even outside the high-strain zones. In this area, there is no clear relation between qualitatively-estimated superimposed strain and the degree of resetting of the Grt-Bt geothermometer.

Igneous layering in a dacite; on the origin and significance of Layer Cake Mountain, Kelowna, B.C., Canada

Abstract A Tertiary, dacitic volcanic land-form in Kelowna, British Columbia, shows layering that has not been recognized elsewhere. Layering is expressed as thin (0.5 m) layers separated by thick (4.5 m) layers exposed along a weathered fault scarp. The major elements show that both thick and thin layers are dacitic and geochemically very similar. Trace element modelling indicates that thin layers formed from thick layers via crystal fractionation involving removal of plagioclase, biotite and magnetite in the proportions 75:20:5, and with only 12% fractionation. The thin layers represent segregation veins generated during crystallization of the dacite. They formed when the crystal mush at the bottom of the upper crust successively, thermally contracted, fractured and foundered, siphoning evolved interstitial liquid from the mush into the horizontal crack. Cooling of the segregation veins led to further fracturing. Later, fluids following these fractures altered the thin layers and precipitated secondary carbonate minerals. The altered thin layers weather preferentially, thus visually accentuating the small primary chemical differences between thick and thin layers. The scale of layering, mode of formation and differentiation mechanisms appear different from those in felsic magma chambers and it is unclear how common this phenomenon is. However, similar layering is more easily identified and commonly developed in mafic lava flows.

Mineralogical controls on the distribution of trace elements in metasomatized peridotite enclaves from Planany, Czech Republic

Abstract Small (m-scale) peridotite enclaves at Planany (central Czech Republic) are separated from their gneissic host rocks by a narrow (cm-scale) reaction rim comprising an inner, tremolite + phlogopite zone and an outer, essentially monomineralic phlogopite zone. Both retain an Mg# very similar to that of the peridotite (Mg# = 81), but relative to this reference frame, show large increases in LILE (K, Rb, Ba) and radionuclides (U, Th). On a smaller scale, however, there has been a mineralogically-controlled decoupling of various components, particularly among the HFSE and REE, the former favouring the phlogopite-rich outer layer of the reaction rim, the latter the amphibole-rich inner zone. Taken together, however, the reaction zones preserve key compositional features of their inferred protolith.

Migmatites from Grenville, Quebec: metamorphic P-T-X conditions in transitional amphibolite/granulite-facies rocks

Abstract Migmatitic rocks near Grenville, Quebec, preserve features indicative of reactions at the onset of granulite facies metamorphism. In this area, metapelites and metacarbonates of the classic Grenville Series are spatially associated with granitic gneiss and metabasite, and flank a Paleozoic, Fe-rich syenite stock. Near this intrusion, the metapelite is diatexitic and nearly devoid of biotite, indicating the involvement of biotite during melting in the contact aureole of this intrusion. Outside of the contact aureole, metapelites and associated rocks contain biotite and are metatexitic. These features suggest two episodes of migmatization, the earlier predating the syenite, the later, synchronous with this intrusion. Hornblende-rich metabasites near the syenite contain a two-part neosome consisting of coarse-grained leucosome veins and patches that are enclosed by fine-grained, pyroxene-rich envelopes. Migmatization is attributed to dehydration melting in the presence of CO2-rich fluids possibly derived from nearby carbonate rocks prior to and/or during emplacement of the syenite. The occurrence of isolated mafic clots in the mesosome and rarity of melanosome seams on leucosomes suggest that some melts were mobile on an outcrop scale. These observations suggest that the leucosomes formed by the segregation of melts, which, coupled with CO2 flux, dehydrated the wallrock along narrow margins, forming the pyroxene-rich neosomes. Back-reaction with residual fluids led to the local scapolitization of plagioclase and the concomitant formation of coronal garnet on pyroxene in neosomes. Thermobarometry of corona structures within the contact aureole generates diffusional Mg-Fe blocking temperatures (∼ 550 °C at 5.5 kbar). Extrapolated up-temperature, P-sensitive equilibria for the coronas yield similar pressures (8–9 kbar) as texturally-equilibrated assemblages for which high temperatures (∼ 750 ± 50 °C; XCO2 = 0.90−0.95) were determined for rocks sampled inside and outside of the contact aureole. This suggests that the Grenville migmatites had not been substantially decompressed by the time that the syenite was emplaced.