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New structural and geochronological data are presented for two orogenic events, the Blezardian and Yavapai orogenies, which affected the Paleoproterozoic Southern Province near Sudbury, Ontario, Canada. The Southern Province comprises ca. 2452 Ma metavolcanic rocks and metasedimentary rocks of the Huronian Supergroup, which were deposited along the southern margin of the Archean Superior craton during its evolution from a rifted to passive continental margin. Emplacement of the ca. 2415 Ma Creighton pluton during rifting was followed by its deformation and the development of a penetrative gneissic fabric during the ca. 2415--ca. 2219 Ma Blezardian Orogeny. New laser ablation--inductively coupled plasma--mass spectrometry (LA-ICP-MS) U-Pb zircon ages of 2343 ± 17 and 2344 [+ or -] 47 Ma on two granitic dikes that cut this fabric provide a new minimum age of ca. 2.34 Ga for the Blezardian Orogeny. The Sudbury area was then impacted by a large extraterrestrial bolide at ca. 1.85 Ga and deformed during the Penokean Orogeny. The southern part of the Southern Province was later reworked by regional folding and north-directed thrusting during the younger 1.7 Ga Yavapai Orogeny. The 1744 ± 29 Ma Eden Lake Complex was emplaced and deformed during this event, which produced a strong foliation overprinting the complex. The foliation formed at pressures of 2.8-4 kbar (1 kbar = 100 MPa) and temperatures of 540-565°C and was intruded by a weakly deformed 1704 ± 13 Ma old granitic dike, bracketing the Yavapai event between 1744 ± 29 and 1704 ± 13 Ma in the Sudbury segment of the Southern Province. Crustal thickening associated with the Yavapai event resulted, locally, in minor pressure increases before or during regional metamorphism as revealed by phase equilibria modeling in the Raft Lake area; this evolution may be recorded elsewhere in the Ontario segment of the Southern Province. De nouvelles donnees structurales et geochronologiques sont presentees pour deux evenements orogeniques : blezardien et de Yavapai, qui ont marque la Province Sud (Paleoproterozoique) a proximite de Sudbury, Canada. La Province Sud comprend des roches metavolcaniques et metasedimentaires (~2452 Ma) du Supergroupe de l'Huronien qui ont ete deposees le long de la bordure sud du craton du Superieur (Archeen) lors de son evolution d'une marge continentale de divergence a passive. La mise en place du pluton de Creighton (~2415 Ma) durant la divergence a ete suivie par sa deformation et le developpement d'une fabrique gneissique penetrative au cours de l'orogene blezardien (~2415--~2219 Ma). De nouveaux ages de 2343 ± 17 Ma et de 2344 ± 47 Ma, determines par LA-ICP-MS-U-Pb sur deux dykes granitiques qui recoupent cette fabrique, fournissent un nouvel age minimum de ~2,34 Ga pour l'orogene blezardien. La region de Sudbury a ensuite subi l'impact d'un bolide extraterrestre vers 1,85 Ga et elle a ete deformee durant l'orogene penokeen. Un plissement regional et un chevauchement en direction nord au cours de l'orogene plus recent de Yavapai (1,7 Ga) ont par la suite retravaille la partie sud de la Province Sud. Le complexe d'Eden Lake (1744 ± 29 Ma) a ete mis en place et deforme durant cet evenement, lequel a produit une forte foliation de surimpression sur le complexe. La foliation a ete formee a des pressions de 2,8 a 4 kbar (1 kbar = 100 MPa) et a des temperatures de 540 a 565°C et elle a ete penetree par un dyke de granite (1704 ± 13 Ma) legerement deforme, cadrant ainsi l'evenement de Yavapai entre 1744 ± 29 Ma et 1704 ± 13 Ma dans le segment Sudbury de la Province Sud. Tel que revele par la mobilisation de l'equilibre des phases dans le secteur du lac Raft, lepaississement de la croute associee a l'evenement Yavapai a conduit, localement, a de petites augmentations de pression avant ou durant le metamorphisme regional; cette evolution pourrait etre enregistree ailleurs dans le segment Ontario de la Province Sud. [Traduit par la Redaction], Introduction The Paleoproterozoic Southern Province of the Canadian Shield defines a fold belt that stretches from Minnesota to the Cobalt-Noranda area in eastern Ontario and western Quebec (Card et al. [...]

P-T pseudosections, constructed in MnNCKFMASH and adjusted for chemical compositional changes resulting from zoned garnet growth (chemical fractionation) in a pelitic rock, show negligible changes in the position of the peak metamorphic mineral assemblage field (garnet + biotite + plagioclase + sillimanite + quartz) compared to the position of this field calculated with the bulk-rock composition. Pelitic rock samples with less than 5% modal garnet were modeled using bulk-rock chemical compositions (unfractionated), and compositions adjusted for 1, 2, and 5% garnet growth, in order to model the effects of changes in effective composition on pseudosection topology. Differences in the location of mineral mode zero lines along the garnet growth P-T path and in the peak mineral assemblage field are generally less than 10 [degrees]C and/or less than 0.3 kbar. However, at some P-T conditions, significant changes in topology are observed. For example, at pressures above 9 kbar, large temperature shifts in the zoisite mode zero line change the pseudosection topology so that biotite+zoisite stability in the pseudosection with 5% garnet fractionation has a larger temperature range (>120 [degrees]C) than in the unfractionated pseudosection (

Software for processing quantitative X-ray maps collected on an electron probe microanalyzer is described. The techniques used in collection of maps and capabilities of the software to produce data useful for petrologic analysis are illustrated using three examples from pelitic schists from Mica Creek, British Columbia. The program utilizes a Bence-Albee algorithm to convert raw X-ray counts at each map pixel to wt% oxide, and allows for spectrometer deadtime corrections and background subtraction. Additional features include phase identification, modal analysis, calculation of cations, calculation of integrated rock composition over specified regions of the map, display of maps and compositional graphs, output of compositional statistics, and the ability to remove pixels and screen for bad analyses. The first example utilizes a sample lacking phases with significant chemical zonation, and outlines the basic functionality of the program. The second example compares an integrated map composition to an X-ray fluorescence analysis on the same rock chip. The results indicate it is possible to obtain integrated map compositions approaching that obtained from an X-ray fluorescence analysis. The third example compares mean X-ray map compositions for individual phases to means obtained using routine point analysis procedures, and the results indicate mean phase compositions obtained from the X-ray maps are of generally high quality. The sample in this example contains compositionally zoned garnet, and is used to show the ability to remove selective portions of garnet to model the approximate change in effective bulk composition during garnet growth.

Low‐pressure and high‐temperature (LP–HT) metamorphism of basaltic rocks, which occurs globally and throughout geological time, is rarely constrained by forward phase equilibrium modelling, yet such calculations provide valuable supplementary thermometric information and constraints on anatexis that are not possible to obtain from conventional thermometry. Metabasalts along the southern margin of the Sudbury Igneous Complex (SIC) record evidence of high‐grade contact metamorphism involving partial melting and melt segregation. Peak metamorphic temperatures reached at least ~925°C at ~1–3 kbar near the SIC contact. Preservation of the peak mineral assemblage indicates that most of the generated melt escaped from these rocks leaving a residuum characterized by a plagioclase–orthopyroxene–clinopyroxene–ilmenite‐magnetite±melt assemblage. Peak temperatures reached ~875°C up to 500 m from the SIC lower contact, which marks the transition to metabasalts that only experienced incipient partial melting without melt loss. Metabasalts ~500 to 750 m from the SIC contact are characterized by a similar two‐pyroxene mineral assemblage, but typically contain abundant hornblende that overgrew clino‐ and orthopyroxene along an isobaric cooling path. Metabasalts ~750 to 1,000 m from the SIC contact are characterized by a hornblende–plagioclase–quartz–ilmenite assemblage indicating temperatures up to ~680°C. Mass balance and phase equilibria calculations indicate that anatexis resulted in 10–20% melt generation in the inner ~500 m of the aureole, with even higher degrees of melting towards the contact. Comparison of multiple models, experiments, and natural samples indicates that modelling in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCFMASHTO) system results in the most reliable predictions for the temperature of the solidus. Incorporation of K2O in the most recent amphibole solution model now successfully predicts dehydration melting by the coexistence of high‐Ca amphibole and silicate melt at relatively low pressures (~1.5 kbar). However, inclusion of K2O as a system component results in prediction of the solidus at too low a temperature. Although there are discrepancies between modelling predictions and experimental results, this study demonstrates that the pseudosection approach to mafic rocks is an invaluable tool to constrain metamorphic processes at LP–HT conditions. [ABSTRACT FROM AUTHOR]

Whole-rock Zr-Hf systematics combined with zircon petrography, geochronology, and geochemistry in partially melted mafic rocks offer new insights into Zr-Hf decoupling processes during metamorphism. Zirconium and Hf are frequently used to interpret the petrogenesis of mafic igneous rocks, but their behavior during dehydration and partial melting is still controversial. The contact aureole of the 1850 Ma Sudbury Igneous Complex (SIC, Canada) includes pyroxene hornfels facies metabasalts with systematically strong negative whole-rock Zr/Zr* (down to 0.25) and Zr/Hf (down to 29.7) anomalies. This signature only occurs in the highest grade portions of the aureole where partial melting occurred proximal to the SIC, and indicates that Zr-Hf decoupling is linked to partial melting and melt segregation. In this zone, the metabasalts contain intergranular melt with rare interstitial and poikilitic zircon grains yielding an 1850 ± 24 Ma U-Pb age, connecting zircon growth to partial melting processes. In addition, zircon texturally overgrows the peak contact metamorphic mineral assemblage and locally shows straight crystal faces in microleucosomes. This is consistent with zircon crystallizing from trapped melt and in agreement with models suggesting that metamorphic zircon does not necessarily grow during peak pressure-temperature conditions. The apparent requirement for melt to facilitate zircon growth and Zr-Hf mobility illustrates the importance of melting for understanding zircon and Zr-Hf behavior in mafic rocks. An increased understanding of whole-rock Zr-Hf decoupling combined with zircon analysis provides the opportunity of better constraining high-temperature crustal processes involving silicate melts. [ABSTRACT FROM AUTHOR]

A series of MnO-[Na.sub.2]O-CaO-[K.sub.2]O-FeO-MgO-[Al.sub.2] [O.sub.3]-Si[O/sub.2]-[H.sub.2]O (MnNCKFMASH) metapelite pseudosections highlights the dependence of predicted mineral assemblages on bulk rock [Al.sub.2][O.sub.3] and Mg# [MgO/(MgO + FeO)]. T-[X.sub.1], pseudosections portray the dependence of staurolite, biotite, and aluminum silicate on [Al.sub.2][O.sub.3] content, allowing the distinction between high-Al and low-Al pelite, as commonly portrayed with KFMASH modeling. The MnNCKFMASH system also shows the effect of [Al.sub.2][O.sub.3] on plagioclase and zoisite stability, which cannot be done in the KFMASH system. Comparison of MnNCKFMASH to KFMASH pseudosections highlights the consequence of ignoring the important rock constituents MnO, [Na.sub.2]O, and CaO when constructing pseudosections. KFMASH cannot model important phases such as plagioclase and zoisite, and there are significant differences in predicted garnet, biotite, and chloritoid stability in the two different systems. In particular, KFMASH does not model garnet stability appropriately at low pressures and temperatures because it cannot account for the stabilizing effect of Mn. The comparisons also show that the method of calculating a KFMASH bulk rock composition equivalent to a real rock composition is problematic and has significant implications for the predicted pseudosection assemblage stability. Comparison of the MnNCKFMASH pseudosections to natural assemblages observed in the Waterville Fm., Maine, indicates that the MnNCKFMASH system comes very close to modeling naturally developed mineral assemblages successfully. The only major discrepancy between predicted and observed assemblages is the inability to predict the paragenesis staurolite + andalusite using an average or natural Waterville Fm. composition. Garnet thermobarometric results from the Waterville Fm. are in poor agreement with pseudosection topology for an average Waterville Fm. composition. This suggests that if quantitative P-T path information is to be derived through a combination of pseudosections and thermobarometry, samples will have to be investigated on an individual basis in more detail than was done in this study.