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Petrological modeling and geochronology (Lu–Hf garnet, U–Pb monazite, zircon, and titanite) for seven mid-amphibolite to granulite-facies metamorphic rocks in the Thelon tectonic zone (Ttz) provide new insights into the evolution of this orogen. A Grt-Sp-Sil diatexite records 2.01–2.00 Ga garnet and monazite growth during >830 °C contact metamorphism associated with voluminous convergent margin plutonism. The most widespread, 1.96–1.90 Ga metamorphism is associated with clockwise pressure ( P)–temperature ( T) paths, indicating it was driven by crustal thickening. Earlier (1.96–1.92 Ga) and lower temperature (630 °C–730 °C; 6–8 kbar) metamorphism in the eastern Slave craton contrast with ∼860 °C (7.5 kbar), 1.91–1.90 Ga diatexites in the central, Paleoproterozoic plutonic rock-dominated, core of the Ttz. These differences are interpreted to reflect lower vs. upper plate settings, respectively. A contribution of mantle heat is suggested by the counter-clockwise P–T path and 885 °C (5.7 kbar) conditions recorded by diatexite associated with c. 1.9 Ga peraluminous leucogranite that dominates a corridor along the Ttz–Rae craton boundary. A tectonic model is proposed wherein the Ttz evolved at an accretionary margin after rifting of Ttz microcontinent (mTtz) off the Rae craton at c. 2.2–2.1 Ga. Voluminous plutonism at 2.01–1.98 Ga formed via east-dipping subduction under mTtz. Following 1.97 Ga collision of the Slave craton and mTtz, west-dipping subduction generated 1.96–1.95 Ga plutonism in the composite mTtz-Slave upper plate. Collision of Rae craton at 1.95–1.94 Ga led to crustal thickening, widespread 1.91–1.90 Ga high-grade metamorphism and extensive crustal melting in the central Ttz.

The Beaverlodge domain is a polydeformed Precambrian terrane in the southwestern Rae craton whose rocks variably record the effects of Archean tectonism and four major Paleoproterozoic tectonic events: the 2.5–2.3 Ga Arrowsmith orogeny, the 1.99–1.93 Ga Taltson orogeny, the ca. 1.90 Ga Snowbird orogeny and the ca. 1.87–1.80 Ga Hudsonian orogeny. Structural–metamorphic analysis and in situ U–Pb SHRIMP zircon and monazite geochronology reported herein demonstrate that this domain consists of two main structural levels variably exposed due to complex interference folding: a lower level comprising mainly high-grade Archean (ca. 3.0 and 2.7–2.6 Ga) metaplutonic and metasedimentary gneisses and a higher level comprising mainly ca. 2.33 to

The results of mapping, geochronology, petrology, and geochemistry of rocks of the Snowbird Lake map sheet (NTS 65D), western Churchill Province, Canada provide new insight into the tectonometamorphic history of the southern Rae Province and significance of the Snowbird tectonic zone, western Churchill Province, Canada. U/Pb TIMS and SHRIMP data document a complex igneous and metamorphic history, with major contrasts between the Rae Province, Chipman domain, and Hearne Province. The Chipman batholith represents the oldest plutonic rock; crystallized at ∼3.4 Ga, metamorphosed at 2.85 Ga, and intruded at 11 kbar by ca. 1.9 Ga mafic dykes. Subsequent Neoarchean activity in the Rae Province involved plutonism at ca. 2.735, 2.67–2.66, and 2.55 Ga, all events that are documented in other portions of the Rae Province. In contrast, the Hearne Province shows only 2.72–2.66 igneous activity. Calcareous sediments are interpreted to have formed on the Rae Province passive margin, having been deposited on exhumed ca. 2.55 Ga basement between 2.07 Ga (youngest detrital zircon) and 1.92 Ga (metamorphic zircon). Burial to ∼7.6 kbar and heating to granulite facies at ca. 1.92 Ga document tectonic thickening in a compressional setting at ca. 1.92 Ga. This compressional setting may reflect terminal collision in the Taltson orogen or accretion of the Hearne Province during the earliest stage of the Hudsonian orogeny. Collectively, these data suggest that the Snowbird tectonic zone was not in an intracratonic setting prior to ca. 1.9 Ga orogenesis.

New U–Pb geochronological and Sm–Nd isotopic data establish the timing of Neoarchean and Paleoproterozoic magmatic and metamorphic events in northeastern Saskatchewan, and give insight into the late Archean assembly of the central and southern Hearne domains of the western Churchill craton. Tonalite orthogneiss within a migmatite complex is 2714 ± 6 Ma in age. Foliated, tonalite gneiss, typical of tonalitic rock that intruded the migmatite complex and mafic volcanic rocks of the Ennadai Group, is 2698 ± 6 Ma in age. These and a third tonalitic gneiss are correlative with ca. 2.72–2.70 Ga volcanic and plutonic rocks previously documented in the Central Hearne supracrustal belt (CHSB). They have juvenile geochemical signatures (ɛNd > 1.43). A rhyolite dated at 2681 ± 4 Ma is consistent with the age and juvenile nature of a younger volcanic cycle in the CHSB. In contrast, a 2681 ± 3 Ma foliated granite dyke that intrudes the tonalite gneiss contains inherited zircons up to 2.99 Ga and has an ɛNd of −0.48, consistent with a melted older Mesoarchean crustal component similar to that of the southern Hearne domain. Three Paleoproterozoic granites also have Nd isotopic signatures consistent with Mesoarchean precursors. We propose that the abrupt change in geochemical signature of the ca. 2.68 Ga magmatism is due to the accretion of the CHSB onto the southern Hearne domain and a change from oceanic to continental arc magmatism. A 2571 ± 5 Ma in situ monazite age determined for porphyroclastic gneiss in the Striding–Athabasca mylonite zone is linked with latest Archean reactivation along the southeastern margin of the Rae province. The Jones Lake granite, dated at 1.86 ± 0.02 Ga, yielded metamorphic monazite ages from 1821 Ma to 1812 Ma. An amphibolitic gneiss yielded a multi-grain, TIMS, zircon age of 1823 ± 4 Ma. The latter ages agree well with the time of metamorphism determined by in situ monazite SHRIMP ages from three samples of metasedimentary gneiss (1826 ± 8 Ma, 1817 ± 42 Ma and 1809 ± 38 Ma), and coincide with the peak of Hudson granite emplacement in the Western Churchill craton. A diatexitic granodiorite crystallized at 1812 ± 4 Ma. U–Pb titanite analyses from three separate rocks yielded ages of 1778 ± 2 Ma, 1761 ± 2 Ma and 1757 ± 5 Ma, while U–Pb from rutile yielded 1751 ± 12 Ma. The 1830–1800 Ma events appear to be related to terminal collision within the Trans-Hudson Orogen, while the younger titanite and rutile ages are cooling ages coeval with emplacement of the Nueltin igneous suite.

The Uvauk complex is an ultramylonite-bounded, granulite-facies suite of anorthosite–gabbro that forms part of the Chesterfield Inlet segment of the Snowbird tectonic zone. Following initial anorthosite–gabbro magmatism at ca. 2.71 Ga and a cryptic 2.62–2.60 Ga event marked by zircon and monazite growth, the Uvauk complex experienced two high-grade tectonometamorphic events at 2.56–2.50 and 1.91–1.90 Ga. Similar to the 2.56–2.50 Ga development of other shear zones in the region, the upper-amphibolite-facies to granulite-facies, moderately high-pressure (8.4–11.0 kbar and 705–760 °C) (1 kbar = 100 MPA) M1 event is interpreted to have involved the structural emplacement of ca. 2.71 Ga Uvauk complex rocks on ca. 2.68 Ga tonalitic rocks to the south. Granulite-facies, high-pressure (11.2–14.7 kbar and 695–865 °C) M2 metamorphism, gabbroic magmatism, and mylonite development within the complex at ca. 1.9 Ga culminated with ~3.5 kbar decompression at high temperature. Clockwise pressure–temperature–time (P–T–t) paths reflect crustal thickening, thought to be related to the early accretionary history of the Trans-Hudson Orogen. A thermal overprint at ca. 1.85–1.75 Ga resulted in retrograde metamorphism (5.8–6.0 kbar and 625–695 °C) associated with post-tectonic granitoid plutonism.

In the western Churchill Province, Canadian Shield, Neoarchean supracrustal and plutonic rocks, intruded by Paleoproterozoic mafic dykes and granitic rocks, comprise the MacQuoid supracrustal belt and the structurally overlying Cross Bay plutonic complex. They form part of the northwestern Hearne subdomain that occupies an intermediate position between the continental Rae domain to the north and west, and the oceanic central Hearne subdomain to the south and east. New geological mapping and supporting geoscience are compatible with the presence of 2550–2500 Ma, southeast-directed, mid-crustal, thick-skinned thrusting that juxtaposed the plutonic complex over the supracrustal belt. The structural contact between the MacQuoid supracrustal belt and the Cross Bay plutonic complex potentially represents a fundamental boundary between isotopically distinct crustal blocks. The ∼2190 Ma MacQuoid mafic dyke swarm cuts across Neoarchean deformation fabrics, but records ∼1.9 Ga, deep-crustal, regional metamorphism that affected both the supracrustal belt and the plutonic complex. Other Paleoproterozoic deformation events that occurred at ∼1850–1810 Ma are of local extent and appear to be relatively minor manifestations of more important events elsewhere, related to the Trans-Hudson orogen.

De nouvelles contraintes sont maintenant disponibles pour elucider l'evolution paleoproterozoique du domaine Rae, dans le secteur ouest de la Province de Churchill, au Nunavut; on se sert d'etudes integrees des aspects structuraux, metamorphiques et geochronologiques, ces derniers etablis in situ, du secteur sud-ouest de la ceinture archeenne de Committee Bay. Au sein des zones protegees de la deformation D 2 voisines d'un pluton synvolcanique d'environ 2.72 Ga, nous avons etabli que la foliation Si est orientee vers le nord, a pendage vers l'ouest, en association avec des plis D 1 et possiblement des chevauchements diriges vers l'ouest. Les structures D 1 sont partiellement obliterees par une foliation S 2 orientee vers le nord-est, a pendage vers le sud-est, en association avec des plis F 2 a plongement axial legerement vers le nord-est et indiquant une contraction vers le nord-ouest. Quoique certaines observations texturales font penser a un evenement thermique cryptique pre-D 1 qui pourrait resulter d'un plutonisme granitique repandu a environ 2.61-2.58 Ga, les relations entre porphyroblastes et leurs matrices dans les roches metapelitiques demontrent bien que deux evenements principaux ont eu lieu, d'abord avec et suivant D 1 (M 1 ), et ensuite avec et suivant D 2 (M 2 ). Les donnees thermobarometriques et l'utilisation de diagrammes de phase quantitatifs indiquent des traces d'evolution metamorphiques P-T-t a faible pression, dans le sens de l'horloge, pour atteindre une croissance post-tectonique de l'andalousite pendant M 1 et pendant M 2 . Les inclusions de monazite dans le grenat et la staurolite tardifs ou posterieurs par rapport a D 1 menent a une population d'âges de 2344 ′ 6 Ma; a la lumiere des microtextures, nous attribuons cet intervalle d'âge a l'evenement M 1 , developpe a un stade tardif de la deformation D 1 . La croissance de la monazite a eu lieu entre 520 et 560°C. L'âge de l'evenement M 2 est fixe par l'âge de la monazite dans la matrice, qui donne 1838 ′ 5 Ma. D'apres l'absence de roches magmatiques d'âge approprie pouvant fournir la chaleur necessaire, les traces P-T-t dans le sens de l'horloge, et la croissance tardive de porphyroblastes associes aux deux evenements de deformation contractionnelle, D 1 et D 2 , le metamorphisme etait une reponse a la contraction et l'epaississement de la croute. Les forces de compression qui ont cause ces evenements d'epaississement de part et d'autre de la ceinture de Committee Bay a environ 2.35 et 1.85 Ga, grosso modo, seraient des effets a distance dans la plaque superieure de deux evenements orogeniques. Le premier (l'orogenese "Arrowsmith") pourrait temoigner d'une collision a environ 2.35 Ga posterieure a une periode de magmatisme dans un arc continental le long de la bordure ouest de la ceinture de Rae. Le second serait lie a une accretion precoce au cours de l'orogenese trans-hudsonnienne impliquant une collision a environ 1.88-1.86 Ga de microcontinents dans la region de la baie d'Hudson.

In situ U–Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of monazite from upper amphibolite-facies paragneiss of the Committee Bay supracrustal belt, central Rae domain, Canada, reveal three age populations: ca. 2350, 1850, and 1780 Ma. The ca. 1850 Ma age also corresponds to growth of low Th/U zircon as indicated by U–Pb SHRIMP analyses of zircon separates from melanosome and leucosome. The contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution. The region is dominated by a northeast-striking S2 (±S1) fabric, axial planar to tight, northwest-vergent F2 folds. Early garnet is enveloped by this biotite–sillimanite ± cordierite S2 fabric. GarnetI hosts ca. 1850 Ma monazite inclusions (with ca. 2350 Ma cores), placing a maximum age on garnetI growth and S2 development. D2 metamorphic conditions progressed through ~3.5 kbar (1 kbar = 100 MPa) and 600 °C to near-peak conditions of ~5 kbar and 675 °C. A minimum age for S2 is provided by unstrained ca. 1820 Ma monzogranite that locally, and regionally, truncates S2. Dominantly ca. 1780 Ma matrix monazite is interpreted to date post-S2 garnetII and cordierite, which record ~5 kbar and 675 °C. These data indicate that the Committee Bay region experienced penetrative D2 tectonometamorphism at ca. 1850–1820 Ma, with a subsequent static overprint. The absence of a ca. 1.85 Ga plutonic suite in the region suggests that low-pressure metamorphism was a response to thick-skinned crustal thickening initiated at ca. 1870 Ma. The new data highlight the importance of Paleoproterozoic reworking of the central Rae domain in the hinterland of the Trans-Hudson orogen.

Penetrative deformation occurred ca. 70 Ma ago throughout the northern Valhalla complex in Valhalla and Passmore domes and in the Gwillim Creek shear zone, exposed at the deepest structural levels in both domes. Intense strain ( S T ) in the Gwillim Creek shear zone (domain II) was synchronous with and outlasted deformation ( D 2 ) throughout the northern complex (domain I). Upper-amphibolite facies peak mineral assemblages define the predominant foliation. Temperature and pressure results, determined from microdomains with established relationships to reaction textures and microstructures, provide constraints on conditions under which deformation occurred. Deformation was synchronous with and outlasted peak metamorphic conditions at all structural levels. Peak conditions of 825°C and 730 MPa and 850°C and 840 MPa were determined for domains I and II, respectively. This was followed by cooling and retrograde garnet breakdown at conditions of 715°C and 490 MPa and 765°C and 730 MPa in domains I and II, respectively. The faster cooling rate per kilometer of exhumation for domain II relative to domain I is consistent with a model of conductive cooling via thrusting of domain II on to a cold footwall. Metamorphism is interpreted to have resulted from crustal thickening and burial to depths of ca. 25 km based on an inferred clockwise P – T path and the paucity of Late Cretaceous intrusions. Lack of retrograde metamorphism throughout the complex and the high degree of annealing of microstructures indicates that the rocks remained above greenschist-facies conditions until they were exhumed in the Early Tertiary on the Valkyr–Slocan Lake extensional shear zone system. Previous workers have determined that the peak of metamorphism occurred at 72–67 Ma in a restricted locality in the core of Passmore dome, near Vallican. Our study links this dated metamorphism with the structural evolution and metamorphic history throughout the area, and shows that supracrustal rocks at all structural levels in Valhalla and Passmore domes underwent the same metamorphic and deformation event as those near Vallican. Therefore, we assign a ca. 70 Ma age to the penetrative, high-temperature deformation in northern Valhalla complex and the Gwillim Creek shear zone. This coincides with a major period of shortening in the Rocky Mountains of the Foreland belt. Strain in northern Valhalla complex may represent a local transient shear zone that accommodated crustal thickening in the hinterland during orogen-scale compression, or it may be an exhumed part of the basal detachment of the Rocky Mountains.

Monazite grains from nine different Precambrian rock samples and having concordant isotope dilution U–Pb and Th–Pb ages were used to develop and evaluate techniques for U–Pb, Th–Pb, and Pb–Pb isotopic dating using an ion microprobe operating at high mass resolution (R=5500, 1%). An unidentified isobar at ∼203.960 amu complicated the determination of 204Pb in the monazites with higher Th, but it appeared to have been eliminated when using a slight energy offset. The 207Pb*/206Pb* ratios of individual spots were determined in 10 min analyses to ±0.3–0.5% (1σ) of the true values using a 20-μm-wide O2− primary beam, whereas the uncertainty in the mean of several spots on a single generation of monazite was typically ±0.1–0.4% (2σ). Instrumental bias in the measured Pb+/UO+ and Pb+/ThO+ values of individual analyses was monitored using linear relationships between 206Pb+/UO+ vs. UO2+/UO+ and 208Pb+/ThO+ vs. UO2+/UO+. The bias appears to vary somewhat with monazite Th content, requiring the use of compositionally matched standards. Individual spot measurements of the 206Pb/238U and 208Pb/232Th ratios in unknown monazites of low to moderate Th can be determined to ±2% (1σ). The dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite-grade metapelite of Archean origin. Both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral (e.g., garnet, staurolite, plagioclase). Euhedral monazite grains armoured in low-CaO garnet and matrix plagioclase yielded an age of 2548±17 Ma, interpreted as dating an early episode of low-pressure metamorphism. Partially resorbed monazite within a high-CaO garnet rim yielded ages of ca. 2500 Ma, but are interpreted as inherited from the first event rather than syn-genetic with respect to the growth of this garnet. An age of 1755±30 Ma was obtained for anhedral monazite associated with late, chlorite-filled fractures, apparently reflecting post-tectonic retrogression.

The northern part of Baffin Island, in Nunavut, lies within the ca. 3.0–2.5 Ga Committee belt, characterized by episodic granitic plutonism and greenschist- to upper-amphibolite-facies belts of supracrustal rocks. Whereas the entire belt on Baffin Island was likely affected by ca. 2.7 Ga plutonism and associated metamorphism, there is also evidence of a high-grade event at ca. 2.55–2.5 Ga. Evolution of the Baffin orogen in south-central Baffin Island led to generation of the ca. 1.86 Ga Cumberland Batholith, ca. 1.85–1.83 Ga northwest-directed thrusting of the Foxe fold-and-thrust belt over the Dexterity granulite belt (DGB), and variable structural and metamorphic reworking of the Committee belt on northern Baffin Island. Thermobarometric data for granulite-facies rocks in the western DGB indicate ~8.7 kbar paleopressure, interpreted to have been achieved in continental crust thickened by thrust imbrication. Paleopressure decreases gradually northward from the western DGB to

The present volume represents the second part of a collection of papers designed to complement the release of a new tectonometamorphic map of the Canadian Shield. Part I, published in 1997 ( The Canadian Mineralogist , volume 35, October issue), consisted for the most part of contributions

The equilibria: have been reversed experimentally at Pfluid 5 P 5 0.5 kbar, 1.0 kbar, and 5.0 kbar and HO 2 at Pfluid 5 P 1 P 5 5 kbar, respectively. Starting materials consisted of natural trem- HO CO 22 olite (St. Gotthard, Switzerland) and quartz (Brazil), and synthetic calcite, forsterite, di- opside, and enstatite mixed in stoichiometric proportions. Reaction direction was deter- mined by comparing XRD patterns of reactant and product assemblages and by examining surface features of experimental products with an SEM. Our new experimental data for Equilibrium 1 are consistent with the natural-tremolite results of Skippen and McKinstry (1985), who used St. Gotthard tremolite, whereas the new bracket for Equilibrium 4 is 25 8C lower than that of Slaughter et al. (1975) who also used St. Gotthard tremolite. Comparison of our results with other studies indicates that use of the St. Gotthard tremolite in place of synthetic tremolite in the starting material displaces these equilibria toward higher temperatures by about 25 and 5 8C, respectively. Tremolite stability differences reflected in these data, as well as in phase equilibrium data for nine additional equilibria involving synthetic and natural tremolite can be accounted for with a simple ideal on-site mixing model to describe tremolite compositional differ- ences. Our analysis leads us to conclude, however, that tremolite growth in some experi- ments near the equilibrium boundary occurs with respect to metastable end-member py- roxenes used in starting materials, whereas pyroxene-stable half-brackets involve growth of stable pyroxene compositions. Thermodynamic properties for end-member tremolite, retrieved by mathematical programming analysis of the experimental phase equilibrium data with these assumptions, provide the most sound basis for prediction of calcic amphi- bole stability relationships in natural assemblages, as well as improved calibration of quan- titative amphibole geothermobarometers. Our success in extracting consistent thermody- namic properties for end-member tremolite from experimental data obtained with both synthetic and natural tremolite, assuming the former to contain 10 mol% magnesiocum- mingtonite component (Jenkins 1987), can be taken either as support for the validity of this assumption or as an indication that chain multiplicity faults (Maresch et al. 1994) produce a similar degree of stabilization as this solid solution.

Amphibole mixing properties and standard state properties for the end members tremolite, tschermakite, pargasite, ferrotremolite, ferrotschermakite, and ferropargasite have been refined through analysis of relevant phase equilibrium experiments, coexisting cummingtonite–hornblende pairs, and natural mineral parageneses. When merged with the mineral properties given by R.G. Berman, these amphibole properties allow the calculation of numerous equilibria suitable for thermobarometry in a wide range of bulk compositions at amphibolite to granulite metamorphic grades. The method was applied to texturally equilibrated garnet–pyroxene–hornblende gneisses and compared both to results based on anhydrous assemblages and to those for garnet–biotite gneisses from the Kapuskasing structural zone. Thermometer resetting is apparent only in garnet–biotite gneisses, whereas touching garnet–pyroxene pairs in mafic gneiss show little evidence of retrograde Fe–Mg exchange. Computed metamorphic temperatures (garnet–clinopyroxene) for the 50 km wide garnet–clinopyroxene zone, in the range 750–790 °C, show no systematic variation with distance from the Ivanhoe Lake fault zone. Paleopressures computed from hydrous and anhydrous equilibria consistently indicate a north-trending high of 10–11 kbar (1 kbar = 100 MPa), decreasing to 3.3 kbar 60 km to the west in the Wawa gneiss domain. The pressure results define west-dipping Archean isobars with gentle (

The ∼2800-km-long Snowbird tectonic zone is one of the most controversial tectonic features of the Canadian shield. Metamorphic and in situ geochronologic data reported here reveal that a 1.9 Ga medium- to high-pressure belt extends along most of this tectonic zone. In contrast to recent interpretations, a collisional origin is indicated by the length of this metamorphic belt, tectonic thickening documented in parts of it, geological contrasts across it, and subduction-type microdiamonds within it. This collisional event marks a pre–1.865 Ga phase of the Hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of Laurentia.

PT-SYSTEM, TX-SYSTEM, and PX-SYSTEM are three FORTRAN 77 programs that calculate complete pressure-temperature ( P-T ), temperature-composition ( T - X H 2 O - CO 2 ), and pressure-composition ( P - X H 2 O - CO 2 ) diagrams. The output of these programs can be plotted for visual representation of phase relations, or inspected from tables. The programs initially generate all possible reactions in the user-specified compositional and PTX space. At the user's option, each point of every reaction is tested for stability with respect to all other phases in the system, and metastable extensions are erased. If a curve is metastable completely, the list of remaining reactions is pruned of those which are metastable entirely within the specified PTX limits of the diagram. Finally, all curves are labeled with stable assemblages. The software is distributed with a recommended thermodynamic database, but a variety of equations of state are provided so that the programs can be used with different data sets. Functions are provided for temperature-dependent disordering, polymorphic transitions, and solid solutions. Run-time options include setting the P, T, X limits of the diagram, the selection of different routines for calculating gas properties, and the specification of fixed phase activities. The user also can specify that only curves stable in the presence of a given phase or assemblage be computed, so that the stability fields of particular phases or assemblages can be identified readily. Copies of the programs (written in ANSI standard FORTRAN 77), the database, and accompanying documentation are available from the senior author.

The Coquihalla Volcanic Complex is part of the Miocene, calc-alkaline Pemberton Volcanic Belt, whose formation in southwestern British Columbia is associated with subduction of the Juan de Fuca plate. The complex consists of voluminous rhyolitic ash-flow tuffs cross-cut by hypabyssal andesitic to dacitic intrusive bodies and a diorite stock. Whole-rock compositions of members of the Coquihalla Volcanic Complex show a range in silica from 54 to 76 wt.% (volatile-free). Uniform initial strontium isotopic ratios (0.7037 ± 0.00008) argue against derivation of magmas by crustal melting or mixing of basaltic magmas with crustal material. Trace element concentrations of whole-rocks cannot be modelled by varying degrees of partial melting of mantle material. Compositional variations among whole-rocks and constituent phenocrysts form the basis for major element mass-balance calculations which indicate that: (1) hornblende dacites can be derived from basaltic andesites by 50% crystallization of a mixture of plagioclase, hornblende, clinopyroxene, titanomagnetite, and apatite, and (2) rhyolites can be derived from dacites by roughly 45% crystallization of a mixture of plagioclase, hornblende, biotite, titanomagnetite, and apatite. Mineral/liquid distribution coefficients for eight trace elements and the observed mineral assemblages (plagioclase, clinopyroxene, hornblende, titanomagnetite, biotite, apatite, and zircon) have been compiled in an appendix. Large uncertainties in these values have led to a reformulation of the Rayleigh fractionation model so that an internally consistent set of distribution coefficients has been calculated for the proposed differentiation sequence.

The Salal Creek area, at the north end of the main group of vents for the Quaternary Garibaldi (Cascade) Volcanic Belt, southwestern British Columbia, was the site of several small eruptions of mafic lava during the past 1 Ma. In contrast to the calc-alkaline character of all other parts of the Garibaldi Belt and the geographically nearly coincident Miocene and older Pemberton Volcanic Belt, the Salal Creek area Quaternary lavas are predominantly alkaline basalt and hawaiite with typical alkaline volcanic petrography, chemistry, and fractionation trends. Trace elements Ti-Zr-Y show within-plate character for the suite. As for other Garibaldi Belt volcanic rocks, Rb is low, Rb/Sr very low, and 87 Sr/ 86 Sr ratio is low, averaging 0.7032. The oxygen isotopic composition average, 18 O = 5.9, is normal for mantle-derived volcanic rocks. This distinct change in magma type at the end of a volcanic are may be the consequence of a smaller degree of melting, melting at a slightly greater depth than calc-alkaline magma production, or a descending-plate edge effect. Ponded flows and pillow-palagonite accumulations indicate that several Salal Creek area eruptions occurred in proximity to ice which filled major valleys during pre-Wisconsin glacial periods.

A thermodynamic model is proposed for calculation of liquidus relations in multicomponent systems of geologic interest. In this formulation of mineral-melt equilibria, reactions are written in terms of the liquid oxide components, and balanced on the stoichiometry of liquidus phases. In order to account for non-ideality in the liquid, a ‘Margules solution’ is derived in a generalized form which can be extended to systems of any number of components and for polynomials of any degree. Equations are presented for calculation of both the excess Gibbs free energy of a solution and the component activity coefficients. Application to the system CaO-Al2O3-SiO2 at one atmosphere pressure is achieved using linear programming. Thermodynamic properties of liquidus minerals and the melt are determined which are consistent with adopted error brackets for available calorimetric and phase equilibrium data. Constraints are derived from liquidus relations, the CaO-SiO2 binary liquid immiscibility gap, solid-solid P-T reactions, and measured standard state entropies, enthalpies, and volumes of minerals in this system. Binary and ternary liquidus diagrams are recalculated by computer programs which trace cotectic boundaries and isothermal sections while checking each point on a curve for metastability. The maximum differences between calculated and experimentally determined invariant points involving stoichiometric minerals are 17°C and 1.5 oxide weight per cent. Because no solid solution models have been incorporated, deviations are larger for invariant points which involve non-stoichiometric minerals. Calculated heats of fusion, silica activities in the melt, and heats of mixing of liquids compare favorably with experimental data, and suggest that this model can be used to supplement the limited amount of available data on melt properties.

Ge0-Calc is an interactive IBM PC and compatible software package for computing and plotting complete stable phase diagrams for any number of phases in systems of any number of components. The general program flow consists of calculation of reaction coefficients among phases in the selected chemical systems, computation of the equilibrium position for each reaction curve whereas each point on the curve is tested for metastability against all other phases, elimination of metastable extensions and of reactions that involve metastable assemblages, and plotting of all resulting curves with labels identifying the stable assemblages. Other programs in the Ge0-Calc package display the phase diagram on the screen (CGA, EGA, or HERCULES graphics adapters), print it on an Epson or compatible dot-matrix printer at the resolution of the printer and improve diagram legibility by moving the labels or replacing them automatically with numbers so reaction identifiers do not overlap. Ge0-Calc is distributed with a recommended thermodynamic database, but a variety of equations of state are provided so that the programs can be used with different data sets. Run time options include selecting the type of diagram P - T , T - X H 2 O  CO 2 , P - X H 2 O  CO 2 ), selecting the chemical system, setting the P - T - X limits of the diagram, selecting ideal or nonideal H 2 OCO 2 mixing and specifying whether phases are to be projected from, included in all reactions, or have a fixed activity other than unity. The minimum hardware requirements for running the Ge0-Calc system are an IBM or compatible personal computer with 512 K memory, math coprocessor, graphics adapter (CGA, EGA, or HERCULES), single floppy disk drive and an Epson or compatible dot-matrix printer.

The Coquihalla Volcanic Complex consists of calc-alkaline acid to intermediate extrusive and intrusive rocks that have an areal extent of roughly 30 km2 near Hope, British Columbia. The oldest and most voluminous members of the complex are rhyolitic pyroclastic rocks that have an overall thickness of approximately 1600 m. Later igneous activity produced numerous andesite to dacite domes, dykes, and sills. A late stage diorite to quartz diorite stock forms the core of Coquihalla Mountain.Pyroclastic rocks rest unconformably on the Jurassic to Cretaceous Eagle pluton and Lower Cretaceous Pasayten Group rocks. Monolithologic avalanche breccias were deposited against a fault scarp of uplifted Pasayten Group rocks in the southwestern portion of the map area. In the southeastern part of the area, monolithologic granitic avalanche breccias formed in response to lilting and uplift of the underlying Eagle pluton as the basin subsided.Three K–Ar dates average 21.4 ± 0.7 Ma, and are concordant with a Rb–Sr isochron (22.3 ± 4 Ma with initial 87Sr/86Sr = 0.70370 ± 0.00008) based on seven whole-rock samples which span the entire compositional range of the suite. These results indicate that the Coquihalla Volcanic Complex is coeval with calc-alkaline centres in the Pemberton Volcanic Belt (PVB).The north-northwest trend of the PVB is parallel to, but roughly 75 km east, of the Pleistocene to Recent Garibaldi Volcanic Belt (GVB). Both volcanic belts appear to have formed as a result of subduction of the Juan de Fuca plate. Genetic models must explain the easterly displacement of the PVB, the development of larger volumes of rhyolitic compositions in this belt as compared to the GVB, and the decrease in age of igneous rocks from south to north within the PVB.

A revised equation is proposed to represent and extrapolate the heat capacity of minerals as a function of temperature: C P=k0+k1 T −0.5+k2 T −2+k3 T −3 (where k1, k2≤0). This equation reproduces calorimetric data within the estimated precision of the measurements, and results in residuals for most minerals that are randomly distributed as a function of temperature. Regression residuals are generally slightly greater than those calculated with the five parameter equation proposed by Haas and Fisher (1976), but are significantly lower than those calculated with the three parameter equation of Maier and Kelley (1932). The revised equation ensures that heat capacity approaches the high temperature limit predicted by lattice vibrational theory (C P=3R+α2VT/β). For 16 minerals for which α and β have been measured, the average C Pat 3,000 K calculated with the theoretically derived equation ranges from 26.8±0.8 to 29.3±1.9 J/(afu·K) (afu = atoms per formula unit), depending on the assumed temperature dependence of α. For 91 minerals for which calorimetric data above 400 K are available, the average C Pat 3,000 K calculated with our equation is 28.3±2.0 J/(afu·K). This agreement suggests that heat capacity extrapolations should be reliable to considerably higher temperatures than those at which calorimetric data are available, so that thermodynamic calculations can be applied with confidence to a variety of high temperature petrologic problems. Available calorimetric data above 250 K are fit with the revised equation, and derived coefficients are presented for 99 minerals of geologic interest. The heat capacity of other minerals can be estimated (generally within 2%) by summation of tabulated ‘oxide component’ C Pcoefficients which were obtained by least squares regression of this data base.

Journal of Petrology, 27 (6), ISSN:0022-3530, ISSN:1460-2415

In this introduction, we briefly describe the project to produce a new tectonometamorphic map of the Canadian Shield. The design of the legend differs from that of traditional metamorphic maps by visually emphasizing differences in metamorphic ages, grade, as well as pressure regime. A web site has been established to facilitate the digital distribution of a 1:3,000,000 scale map of the Shield, in addition to 1:1,000,000 and 1:2,000,000 scale regional maps. Associated P–T, geochronological, and metamorphic assemblage databases for different parts of the Shield also are available. The legend design is illustrated with a simplified version of the 1:2,000,000 scale tectonometamorphic map of the Grenville Province in Quebec and Labrador, for which a brief summary is provided.