Terence M. Gordon

Mylonite to megabreccia: Tracking fault events within a transcurrent terrane boundary in Nova Scotia, Canada

Transcurrent terrane boundaries commonly evolve into long-lived faults that preserve little evidence for early docking events. A remarkable exception is exposed at Clarke Head along the Appalachian Meguma terrane boundary in Nova Scotia, Canada, where a Late Carboniferous fault megabreccia contains Devonian (369 Ma: U-Pb zircon) granulite-grade mylonite fractured by veins filled with Visean amphibole (ca. 335 Ma: Ar-Ar). This fractured mylonite was later mixed with Early Carboniferous sedimentary rocks during megabrecciation (ca. 315–310 Ma). These three fault events are reflected in the tectonostratigraphic record. Devonian (ca. 370–360 Ma) transpressional terrane docking ramped Meguma up against Avalonia and shed clastic detritus across the fault system. The Visean brittle deformation recorded by the amphibole veins was coeval with marine regression at surface. The late Namurian megabrecciation event similarly produced unconformity followed by renewed nonmarine clastic sedimentation. The Clarke Head megabreccia therefore preserves an episodic late Paleozoic fault history spanning some 55 m.y. during convergence between Laurentia and Gondwana and the assembly of the Pangean supercontinent.

Generalized thermobarometry: Solution of the inverse chemical equilibrium problem using data for individual species

Abstract The inverse chemical equilibrium problem is the determination of unknown equilibrium pressure, temperature, and chemical potentials of s species, given measurements of their thermochemical constants and the compositions of phases in which they occur. Of the s species for which free energy approximations can be made, c will be compositionally independent, i.e., form a basis of composition space and hence of chemical potential space. This means that if the equilibrium model is correct, it should be possible to express the chemical potentials of all s species as linear combinations of the chemical potentials of any c basis species. The inverse chemical equilibrium problem can then be stated: g i (p,T) − μ i = 0 i= 1…c g j (p,T)− ∑ i=1 c v ij μ i =0 j=c+1…s where g i ( p , T ) is a measured approximation to the apparent free energy of formation of species i , determined from thermochemical constants and observed compositions; μ i is the unknown equilibrium chemical potential of species i ; and v ij are stoichiometric coefficients relating the compositions of the s − c compositionally dependent species to the c basis species. The problem has s equations and c + 2 unknowns, hence is underdetermined, exact, or overdetermined, depending on the relative magnitudes of s and c . Because of errors in measurement, or failure to preserve equilibrium compositions, overdetermined systems will usually be inconsistent. In such cases, the ordinary least-squares solution to the problem may be found by finding p , T , and μ i that minimize ƒ T ƒ where ƒ is the s component vector obtained by evaluating (1). If desired, an error covariance matrix V can be incorporated to obtain a generalized least-squares solution at the minimum of ƒ T V −1 ƒ . Confidence regions can be approximated by contouring the sum of squares and using Monte Carlo techniques. The formulation is readily extended to include data from directly calibrated equilibria as equations of form: ΔG j ( p , T ) − ∑ ν ij μ i = 0. Solutions to underdetermined problems can be constrained by replacing some of the equations with inequalities such as: g i 0 ( p , T ) − μ i ≥ 0.

Application of INVEQ to the geothermobarometry of metamorphic rocks near a kyanite-sillimanite isograd, Mica Creek, British Columbia

Abstract A kyanite-sillimanite isograd is well-exposed in the Mica Creek area, British Columbia, and both pelitic and basic rocks occur within a few hundred meters of the isograd. This provides us with an opportunity to test geothermobarometry in pelitic and basic bulk compositions against an independent metamorphic equilibrium, kyanite-sillimanite. The kyanite-sillimanite isograd is modeled by a univariant P-T curve that passes through the following points (T °C, P kbar): 564/5; 610/6; 656/7; and 796/10. The quoted experimental uncertainty is 25 °C or about 350 J in ΔG. Samples that bracket the kyanite-sillimanite isograd in the Mica Creek area have the following mineral assemblages: quartzmuscovite- biotite-garnet-plagioclase-kyanite or sillimanite in pelitic rocks; and hornblende-plagioclase- clinopyroxene-garnet-quartz in basic rocks. Using the thermodynamic database of Berman (1988) and the INVEQ program of Gordon (1992), we have calculated metamorphic pressures and temperatures for 17 samples that bracket the kyanite-sillimanite isograd. For the basic rocks, the P-T estimate fell entirely within the kyanite stability field, even for samples containing sillimanite, and the 68.3% confidence region overlapped the ±25 °C uncertainty in the kyanite-sillimanite P-T curve. For the pelitic rocks, the P-T estimate and the 68.3% confidence region usually fell within the sillimanite stability field and the confidence region overlapped the ±25 °C uncertainty in the kyanite-sillimanite P-T curve.

WEBINVEQ thermobarometry: an experiment in providing interactive scientific software on the World Wide Web

Abstract Specialized scientific software is volatile and often has a small number of users with a wide variety of computer platforms. The problems of distributing, updating and maintaining such packages can be ameliorated by using the World Wide Web to distribute graphical interface processing to individual users’ Web browsers and performing the numerical computations on a single server. This application uses perl as the CGI scripting language and MATLAB® as the computational language. The use of ascii files for communication between the perl scripts and MATLAB code and for storing error messages greatly simplifies debugging. The system has worked successfully for 12 months and the techniques have been adapted to other applications.