Geothermometry, Geobarometry and T-X(Fe-Mg) Relations in Metapelites, Snow Peak, Northern Idaho

An inverted metamorphic gradient associated with the northern mylonite zone of the Cheyenne belt, a deeply eroded Precambrian suture in southern Wyoming, has been documented within metasedimentary rocks of the Early Proterozoic Snowy Pass Supergroup. Metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite, garnet, and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone. A minimum temperature increase of approximately 100" C over a km-wide zone is required for this transition. Parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the Snowy Pass Supergroup.Field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite-grade rocks over rocks of the Snowy Pass Supergroup along the northern mylonite zone. Mineral equilibria and garnet-biotite geothermometry on synkinematic mineral assemblages within the Snowy Pass metasedimentary rocks indicate deformation at minimum temperatures of 480" C and pressures of 350-400 MPa (3-5-4.0 kbar). This implies tectonic burial or upper plate thickness of 13-15 km.The narrow character of metamorphic zonation and microtextures within the Snowy Pass Supergroup which indicate late synkinematic growth of garnet and staurolite, preclude rotation of pre-existing isograds by folding as a mechanism for development of the inverted gradient. Conductive transport of heat from the upper into the lower plate across the originally low-angle thrust is insufficient to produce the necessary temperatures in the lower plate. Shear heating is considered insufficient to produce the observed metamorphic transition unless 'Present address: De artment of Geoscience, University of Nevada, Las%egas, Nevada 89154, USA.high shear stresses are postulated. Up-dip advection of metamorphic fluids is a feasible, but unproven, mechanism for heat transport. The possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present.

Section: Conditions Of Deformation and Metamorphismmentioning

confidence: 72%

Evidence for an inverted metamorphic gradient associated with a Precambrian suture, southern Wyoming

Duebendorfer

1988

“…Grambling (personal communication, 1990) the transition from the garnet to staurolite zone in New Mexico involves a monotonic increase in calculated temperature and a similar result is described by Holdaway et al (1988) for the transition in western Maine. Lang & Rice (1985) report very similar temperatures across the staurolite isograd in Idaho (470 and 487" C, respectively), but they also discuss the inconsistency of the calculated temperatures with other phase equilibria constraints. Based on the considerations of this paper, a monotonic increase in temperature should only happen if either (a) garnet is not consumed during staurolite production or (b) diffusion is sufficiently rapid at the staurolite isograd to permit substantial re-equilibration of the garnet rim during consumption.…”

Section: Discussionmentioning

confidence: 84%

On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling

Spear

1991

271

156

Finite difference models of Fe-Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet-biotite Fe-Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (Tcarc) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (vg,,,,[/vb,,,,,,) and the cooling rate. For garnets with radii of 1 mm and Vgarne1/Vbtotltc<< 1, Tcalc is 633, 700 and 777°C for Cooling rates of 1, 10 and lO(l"C/Ma. For VgarnrI/Vblotlle = 1 and 4 and a cooling rate of 10" C/Ma, T,,,, is approximately 660 and 610" C, respectively.Smaller and larger garnets have lower and higher T,,,,, respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below T,,, of the garnet core, provided that VgameI/Vb,ot,re is sufficiently small (cO.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling.Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550-600" C generate a 'well' in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the 'well' with matrix biotite range from 23-43°C to 5-12°C below the peak metamorphic temperature for cooling rates of 1 and 100" C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70" C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet-biotite Fe-Mg exchange thermometry on amphibolite facies rocks (T,,,,, > 550" C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.

“…Compared to the significant predicted changes with grade in Mg/(Mg+Fe) of biotite and staurolite going through Fe–Mg divariant reaction , and in related Fe–Mg divariant reactions at lower grade that produce staurolite from chlorite (Thompson, ; Tinkham et al., ; White, Powell, Holland, et al., ), commonly there is little change in the measured compositions (e.g. Craw, ; Lang & Rice, ).…”

Section: Width Of Zones Of Coexisting Staurolite and Kyanite Or Andalmentioning

confidence: 99%

“…These incongruities contribute to what has become known as “the staurolite problem” (Pigage & Greenwood, ), originally identified based on the mismatch between experimentally constrained staurolite phase equilibria and P–T estimates of natural rocks from classical thermobarometry (e.g. Carmichael, ; Lang & Rice, ; Pigage & Greenwood, ). In the next section, we explore possible explanations for the above incongruities.…”

Section: Summary Of Incongruities Between Thermodynamic Prediction Anmentioning

confidence: 99%

Kinetic control of staurolite–Al₂SiO₅ mineral assemblages: Implications for Barrovian and Buchan metamorphism

Pattison

Spear

2018

The distribution and textural features of staurolite–Al2SiO5 mineral assemblages do not agree with predictions of current equilibrium phase diagrams. In contrast to abundant examples of Barrovian staurolite–kyanite–sillimanite sequences and Buchan‐type staurolite–andalusite–sillimanite sequences, there are few examples of staurolite–sillimanite sequences with neither kyanite nor andalusite anywhere in the sequence, despite the wide (~2.5 kbar) pressure interval in which they are predicted. Textural features of staurolite–kyanite or staurolite–andalusite mineral assemblages commonly imply no reaction relationship between the two minerals, at odds with the predicted first development (in a prograde sense) of kyanite or andalusite at the expense of staurolite in current phase diagrams. In a number of prograde sequences, the incoming of staurolite and either kyanite, in Barrovian sequences, or andalusite, in Buchan‐type sequences, is coincident or nearly so, rather than kyanite or andalusite developing upgrade of a significant staurolite zone as predicted. The width of zones of coexisting staurolite and either kyanite, in Barrovian sequences, or andalusite, in Buchan‐type sequences, is much wider than predicted in equilibrium phase diagrams, and staurolite commonly persists upgrade until its demise in the sillimanite zone. We argue that disequilibrium processes provide the best explanation for these mismatches. We suggest that kyanite (or andalusite) may develop independently and approximately contemporaneously with staurolite by metastable chlorite‐consuming reactions that occur at lower P–T conditions than the thermodynamically predicted staurolite‐to‐kyanite/andalusite reaction, a process that involves only modest overstepping (<15°C) of the stable chlorite‐to‐staurolite reaction and which is favoured, in the case of kyanite, by advantageous nucleation kinetics. If so, the pressure difference between Barrovian kyanite‐bearing sequences and Buchan andalusite‐bearing sequences could be ~1 kbar or less, in better agreement with the natural record. The unusual width of coexistence of staurolite and Al2SiO5 minerals, in particular kyanite and andalusite, can be accounted for by a combination of lack of thermodynamic driving force for conversion of staurolite to kyanite or andalusite, sluggish dissolution of staurolite, and possibly the absence of a fluid phase to catalyse reaction. This study represents an example of how kinetic controls on metamorphic mineral assemblage development have to be considered in regional as well as contact metamorphism.