The Ti content of biotite can serve as a geothermometer for graphitic, peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4–6 kbar. The relationship between Ti-content, temperature, and Mg/(Mg + Fe) value was calibrated empirically using an extensive natural biotite data set (529 samples) from western Maine and south-central Massachusetts in combination with the petrogenetic grid of Spear et al. (1999). The calculated Ti-saturation surface is curved such that for a given Mg/(Mg + Fe) value, Ti concentration increases as a function of temperature in a nonlinear fashion, and for a given temperature Ti concentrations decrease with an increase in Mg/(Mg + Fe). The fit to the Ti-saturation surface can be reformulated as the geothermometric expression: T = {[ln(Ti) − a − c ( X Mg)3]/ b }0.333, in which T is temperature in degrees Celsius, Ti is the number of atoms per formula unit (apfu) normalized on the basis of 22 O atoms, X Mg is Mg/(Mg + Fe), a = −2.3594, b = 4.6482 × 10−9 and c = −1.7283. The calibration range for this expression is X Mg = 0.275–1.000, Ti = 0.04–0.60 apfu, and T = 480–800 °C. Precision of the Ti-in-biotite geothermometer is estimated to be ±24 °C at the lower temperature range and improves to ±12 °C at higher temperatures. Application of the Ti-in-biotite geothermometer to ilmenite- or rutile-bearing, graphitic, peraluminous metapelites equilibrated at 3–6 kbar is generally consistent with independent temperature determinations, but with some deviations that represent local reequilibration. Consequently, the Ti systematics in biotite can also serve as the basis of a very sensitive indicator of chemical equilibrium, or lack thereof. Application of the geothermometer to metapelites not containing the requisite mineral assemblages can lead to minor-to-significant errors in estimated temperatures.

Biotite Ti-substitution mechanisms are controlled by several factors. Based on the biotite calibration data set, magnesian biotites ( X Mg > 0.65) incorporate Ti in accordance with the exchange vector TiAl2R−1Si−2, where R is the sum of the divalent cations Mg + Fe + Mn. This substitution mechanism is primarily a response to misfit of the octahedral and tetrahedral layers in magnesian biotites. Intermediate biotites ( X Mg <0.65), particularly at higher temperatures, exhibit enhanced Ti concentrations, most consistent with the Ti-deprotonation TiO2R−1(OH) −2 exchange vector. Dominance of Ti-deprotonation substitution is largely a function of reduction of H2O activity at higher metamorphic grades. Supplementary biotite data from metaluminous amphibolites and mafic granulites, metamorphosed isothermally with variable H2O activities, reveal that low-Al biotite incorporates significantly higher concentrations of Ti relative to peraluminous biotite as a result of a combination of the exchange vectors TiO2R−1(OH) −2 and RSiAl−2 substituting in roughly an 8:1 ratio.

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The Ti-saturation surface for low-to-medium pressure metapelitic biotites: Implications for geothermometry and Ti-substitution mechanisms

This study assesses temperatures of formation of common granulites by combining experimental constraints on the P±T stability of granulite-facies mineral associations with a garnet± orthopyroxene (Grt±Opx) thermobarometry scheme based on Al-solubility in Opx, corrected for late Fe±Mg exchange. We applied this scheme to 414 granulites of mafic, intermediate and aluminous bulk compositions. Our findings suggest that granulites are much hotter than traditionally assumed and that the P±T conditions of the amphibolite±granulite transition portrayed in current petrology textbooks are significant underestimates by over 100 C. For aluminous and intermediate granulites, mean corrected temperatures based on our method are 890 17 and 841 11 C, respectively (uncertainties reported as 95% confidence limits on the mean), consistent with minimum temperatures for orthopyroxene production by fluid-absent partial melting in these bulk compositions. In contrast, mean temperatures based on Grt±Opx Fe±Mg exchange equilibria, using the same thermodynamic data, are 732 22 and 723 11 C, respectively, well below the minimum temperatures for Opx stability. For mafic granulites, the mean corrected temperature using our method is 816 12 C, similar to the mean temperature of 793 13 C from Fe±Mg exchange. Reasons for the differences between the mafic granulites and aluminous±intermediate granulites are unclear but may be due to the lower Al concentrations in Opx in the mafic rocks and possible deficiencies in the thermodynamic modelling of these low concentrations. We discuss a number of well-known granulite terrains in the context of our findings, including the Adirondacks, the Acadian granulites of New England, the incipient charnockites of southern India and Sri Lanka, and the Kerala Khondalite Belt. Our findings carry implications for thermotectonic models of granulite formation. A computer program to perform our thermobarometry calculations, RCLC, is available from the Journal of Petrology website at http://www.petrology.oupjournals.org or from the authors at http://www.geo.ucalgary.ca/ pattison/drm_pattison-rclc.htm.

Temperatures of Granulite-facies Metamorphism: Constraints from Experimental Phase Equilibria and Thermobarometry Corrected for Retrograde Exchange

Pseudomorphs of melt-filled pores, recognized by their generally cuspate shape, are used as diagnostic for the former presence of partial melt. They are commonly observed in migmatites from the mid- to deep crust although they occur in the smaller pores in migmatites from shallower levels (1^2 kbar). The pseudomorphing of meltfilled pores is controlled by the kinetics of nucleation and is a consequence of the greater supersaturation required for nucleation in a small pore compared with a larger one. We examine three migmatites in detail: a contact metamorphosed cherty band from an iron formation; an Archaean regional granulite from an accretionary prism; and an amphibolite-facies sample from the roots of an Archaean mountain chain. The greater undercooling required for nucleation in progressively smaller pores is recorded by the composition of plagioclase pseudomorphs. A study of dihedral angles at the corners of pseudomorphed pores demonstrates that melt^solid textural equilibrium was probably attained only in the contact aureole.The regional granulite preserves an almost unmodified reaction-controlled melt distribution, with little evidence for either melt^solid textural equilibration or solid^solid re-equilibration, whereas the reactioncontrolled melt distribution in the regional amphibolite-facies example has been modified by a partial approach to solid^solid textural equilibrium. It is not clear whether the differences in dihedral angle population are due to differences in uplift and exhumation rates or due to the presence of H2O on grain boundaries.

On the Pseudomorphing of Melt-filled Pores During the Crystallization of Migmatites

The Mt Stafford area in central Australia preserves a lowpressure greenschistto granulite-facies regional aureole. The metasedimentary sequence has been divided into five zones from greenschist (Zone 1) to granulite facies (Zone 4) and a zone of hybrid diatexite formed from the introduction of granitic magma into the high-grade migmatites (Zone 5). Melt production was dominated by a series of multivariant biotite breakdown reactions, not the univariant reactions suggested by previous studies. Although the three main metasedimentary rock types produced similar amounts of melt at the highest grades, their melt production histories differed markedly as a function of temperature. Aluminous metapelites produced more melt at lower temperatures (Zones 2 and 3), whereas metapsammite and cordierite granofels experienced an additional major melt-producing step at higher temperatures (upper Zone 3 and Zone 4). This melting step involved the breakdown of biotite to produce garnet, K-feldspar and melt, and in some rocks the production of orthopyroxene. Melt production in Zone 4 exceeded 25 mol %, resulting in the formation of in situ diatexites. Complex relationships involving aluminosilicate porphyroblasts resulted in the breakdown of biotite and aluminosilicate being drawn out over a wide temperature range, from subsolidus conditions to temperatures close to 750 C. Initially, much of the melting developed around the aluminosilicate porphyroblasts during the breakdown of coexisting biotite, aluminosilicate and quartz. However, much of the rock was chemically isolated from the porphyroblasts and could not react to produce melt. As temperatures rose, the presence of the large isolated aluminosilicate porphyroblasts controlled the spatial development of quartzabsent, spinel-present compositional domains, the formation of spinel being governed by the silica-undersaturated breakdown of coexisting biotite and aluminosilicate.

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Prograde Metamorphic Assemblage Evolution during Partial Melting of Metasedimentary Rocks at Low Pressures: Migmatites from Mt Stafford, Central Australia

Re–Os geochronology of the Neoproterozoic–Cambrian Dalradian Supergroup of Scotland and Ireland : implications for Neoproterozoic stratigraphy, glaciations and Re–Os systematics.

I Introduction.- 1 The Setting of the Ballachulish Intrusive Igneous Complex in the Scottish Highlands.- 2 Regional Geology of the Ballachulish Area.- II The Intrusive Complex.- 3 Structure, Petrography and Emplacement of Plutonic Rocks.- 4 Thermal Conditions and Crystallization Sequence, as Deduced from Whole-Rock and Mineral Chemistry.- 5 Nucleation and Growth of Pyroxene in the Hypersthene Diorites.- 6 Microstructures and Thermal Behaviour of Igneous Pyroxenes.- 7 The Shape of the Intrusion Based on Geophysical Data.- III The Contact Aureole and Its Rocks.- 8 Petrography and Mineral Chemistry of Pelites.- 9 Field Relations and Petrography of Partially Melted Pelitic and Semi-Pelitic Rocks.- 10 Decarbonation Reactions in Siliceous Dolomites and Impure Limestones.- 11 Microtextures and Reaction Mechanisms of Carbonate Rocks: A Comparison Between the Thermoaureoles of Ballachulish and Monzoni (N. Italy).- 12 Quartz Grain Coarsening by Collective Crystallization in Contact Quartzites.- 13 Disordering, Re-Ordering and Unmixing in Alkali Feldspars from Contact-Metamorphosed Quartzites.- 14 A Search for Variations in the Structural States of Cordierite in Contact-Metamorphosed Pelites.- 15 Detrital Quartz and K-Feldspar in Quartzites as Indicators of Oxygen Isotope Exchange Kinetics.- IV Interactions Between the Intrusion and the Contact Aureole.- 16 P-T-a(H2O) Conditions in the Thermal Aureole.- 17 Stable Isotope Geochemistry on the Intrusive Complex and Its Metamorphic Aureole.- 18 Thermal Models of Cooling.- V Concluding Discussion.- 19 Evidence of Fluid Phase Behaviour and Controls in the Intrusive Complex and Its Aureole.- 20 Intracrystalline Processes.- 21 Summary and Outlook.- References.

G. Voll

Papers

Equilibrium and kinetics in contact metamorphism : the Ballachulish Igneous Complex and its aureole