Solid-Fluid Equilibria in the System KAlSi3O8-NaAlSi3O8-Al2SiO5-SiO2-H2O-HCl

Activity diagrams in the system KAlSi₃O₈-NaAlSi₃O₈-Al₂SiO₅-SiO₂-H₂O-HClhave been calculated in terms of a_K+/a_H+ and a_N+/a_H+ from existingexperimental data. They show the effect of temperature, pressure,and a_H2O on the stability fields of the alkali feldspars, micas,and aluminium silicate. These activity diagrams are useful in revealing the bufferingcapacity of mineral assemblages and the chemical potential gradientsestablished by changes in T, P, a_H2O, and mineral assemblage.An analysis of mineral paragenesis in terms of these diagramssuggests that mosaic equilibrium, allowing limited metasomatismand internal buffering of chemical potentials, best describemetamorphic systems. Thus the dehydration reaction: muscovite+quartz=K-feldspar+Al₂SiO₅+H₂O which is most important in closed systems, probably fails todescribe in detail the mechanism of natural muscovite decomposition.Rather the decomposition of muscovite is more likely representedby ionic reactions. The replacement of muscovite by feldspar: muscovite+6 SiO₂+2 K⁺=3 K-feldspar+2 H⁺ muscovite+6 SiO₂+3 Na⁺=3 Albite+K⁺+2 H⁺ is favored at high temperature and low pressure, and may accountfor the crystallization of some feldspars in metamorphic rocks.The reaction involving aluminium silicate replacement of muscovite: 2 muscovite+2 H⁺=3 Al₂SiO₅+3 SiO₂+3 H₂O+2 K⁺ is favored at high temperature and pressure and low a_H2O, andcould contribute to the development of the aluminium silicates.It is concluded that both activity diagrams and AKNa projectionsshould be used together to more completely evaluate mineralparagenesis in terms of mosaic equilibria.     

Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina

The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine‐grained biotite + plagioclase, that lacks a well‐defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1‐related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine‐grained folia of muscovite + chlorite ± biotite truncate earlier biotite + plagioclase textures, and coarser‐grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H⁺ and/or H₂O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system.     

The blueschist enigma

Plate tectonics, the new view of Earth dynamics that took shape in the late 1960s, has steadily gained strength over the past IS years. The vast majority of geologists now picture the Earth's outer shell not as a single, solid sheath, but as a jigsaw of shifting plates, whose movements are called upon to explain everything from mountains to the mineral make-up of individual rocks. Emerging plate-tectonics models revolutionised the very approach to geologic problems. One of the early fruits of this powerful and intellectually fertile theory was a new understanding of'blueschists', rocks named for their finely layered, or schistose, structure and for the characteristic colour and nature of their mineral grains.     

Deformation-induced reconstitution and local resetting of mineral equilibria in polymetamorphic gneisses: tectonic and metamorphic implications

A sequence of at least three Al₂SiO₅-bearing mineral assemblages are preserved in successively overprinted ductile shear zones in the Willimantic window, Connecticut. The ductile deformation, localized at and near the boundary between the Putnam-Nashoba terrane and underlying Avalon terrane is characterized by a network of anastomozing shear zones that outline metre-scale tectonic blocks of migmatitic Kfs + Sil + Gt + Bi + Pg + Qtz + Ilm + Ru gneiss. These assemblages record Acadian or older metamorphic conditions of 6 kbar, 700d? C. Mylonitic gneisses in shear zones that define block margins were formed by reconstitution and recrystallization of the migmatitic gneiss. The reconstituted rocks exhibit relict Ky + St + Grt (+Pl + Bt + Qtz + Rt + Ilm) assemblages and require a minimum pressure for the Ky-Str grade metamorphism of 8.5 kbar. Kyanite in block margins is widely replaced by sillimanite, and locally by andalusite, during a period of post-Alleghanian ductile deformation. The interiors of blocks do not record this sequence of polymorphs. The pattern of reconstitution is accounted for by localization of strain along block margins within a regionally extensive terrane-bounding fault zone. Strain provided the activation energy for recrystallization and retrograde mineral reactions. The P-T conditions of post-Alleghanian ductile deformation evolved from 600d? C and 6 kbar to 550d? C and 3 kbar. The occurrence of Ky + Str-bearing assemblages, overprinting Acadian Kfs + Sil-bearing assemblages and subsequently overprinted by Alleghanian sillimanite- and andalusite-bearing assemblages, along with reset hornblende ⁴⁰Ar/³⁹ Ar mineral ages from Mississippian to Permian, requires a prograde Alleghanian metamorphism of rocks previously metamorphosed during the Acadian. Thus, mineral assemblages from gneisses in the Willimantic fault zone retain evidence of a protracted tectonothermal evolution that included high-grade Acadian orogenesis, tectonic loading resulting from Alleghanian collision of Avalon with North America, and tectonic exhumation in Permo-Triassic time. The c.3-kbar pressure decrease between prograde and retrograde Alleghanian metamorphic conditions corresponds to 10 km of crust that must have been tectonically excised from the base of the Putnam-Nashoba terrane cover sequence following Alleghanian orogenesis in south-eastern New England.     

Scales of equilibrium and disequilibrium during cleavage formation in chlorite and biotite-grade phyllites, SE Vermont

Detailed electron microprobe analyses of phyllosilicates in crenulated phyllites from south‐eastern Vermont show that grain‐scale zoning is common, and sympathetic zoning in adjacent minerals is nearly universal. We interpret this to reflect a pressure‐solution mechanism for cleavage development, where precipitation from a very small fluid reservoir fractionated that fluid. Multiple analyses along single muscovite, biotite and chlorite grains (30–200 μm in length) show zoning patterns indicating Tschermakitic substitutions in muscovite and both Tschermakitic and di/trioctahedral substitutions in biotite and chlorite. Using cross‐cutting relationships and mineral chemistry it is shown that these patterns persist in cleavages produced at metamorphic conditions of chlorite‐grade, chlorite‐grade overprinted by biotite‐grade and biotite‐grade. Zoning patterns are comparable in all three settings, requiring a similar cleavage‐forming mechanism independent of metamorphic grade. Moreover, the use of ⁴⁰Ar/³⁹Ar geochronology demonstrates this is true regardless of age. Furthermore, samples with chlorite‐grade cleavages overprinted by biotite porphyroblasts suggest the closure temperatures for the diffusion of Al, Si, Mg and Fe ions are greater than the temperature of the biotite isograd (>～400 °C). Parallel and smoothly fanning tie lines produced by coexisting muscovite–chlorite, and muscovite–biotite pairs on compositional diagrams demonstrate effectively instantaneous chemical equilibrium and probably indicate simultaneous crystallization. These results do not support theories suggesting cleavages form in fluid‐dominated systems. If crenulation cleavages formed in systems in which the chemical potentials of all major components are fixed by an external reservoir, then the compositions of individual grains defining these cleavages would be uniform. On the contrary, the fine‐scale chemical zoning observed probably reflects a grain‐scale process consistent with a pressure‐solution mechanism in which the aqueous activities of major components are defined by local dissolution and precipitation. Thus the role of fluids was probably limited to one of catalysing pressure‐solution and fluids apparently did not drive cleavage development.     

Evidence for deformation-induced K-feldspar replacement by myrmekite

ABSTRACT Several examples of deformation-induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths, namely a muscovite-poor S-C mylonite and a single foliation, muscovite-poor mylonitic gneiss. Back-scattered SEM and conventional optical microscopy show that in both rock types, syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of K-feldspar grains that faced the local inferred incremental shortening direction for the mylonite. Myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction. The replacement of K-feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains. We suggest that the ambient temperature, pressure and chemical activities were such that the replacement reaction was favoured, but the addition of extra strain energy along the high-pressure sides of the grains localized the reaction at these sites. This energy could arise from elastic strain, or strain associated with tangled dislocations or twin boundaries. The relative roles of stress and strain energy concentrations in driving the replacement reaction are not known, but both were probably important.     

Deformation of amphibolites via dissolution–precipitation creep in the middle and lower crust

Continuous compositional zoning in amphibole grains in strongly deformed and lineated amphibolites from the Eastern Blue Ridge, North Carolina indicates that most of the deformation was accommodated by dissolution–precipitation creep. Amphibole in most samples shows moderate prograde and/or retrograde zoning parallel to the long‐axis with compositions ranging between magnesiohornblende and tschermakite. In one sample, grains are zoned from actinolitic (Si = 7.9 p.f.u.) cores to tschermakitic (Si = 6.2 p.f.u) rims. Amphibole‐plagioclase thermometry suggests prograde growth temperatures as low as 400 °C, but typically range from 650 to 730 °C and retrograde growth temperatures <700 °C. These estimates are corroborated quantitatively with amphibole‐garnet‐plagioclase thermobarometry and qualitatively with a positive correlation between TiO₂ concentration in amphibole and calculated temperature. This growth zoning provides persuasive evidence that amphibole precipitation produced the fabric, but evidence for dissolution is less common. It is present, however in the form of truncations of complicated zoning patterns produced by healed fractures and overgrowths in low‐temperature cores by high‐temperature tschermakitic grains lacking similar internal structures. The preservation of this network of straight cracks filled with optically continuous amphibole also provides evidence against the operation of dislocation creep even to temperatures >700 °C because dislocation‐creep would have deformed the fracture network. Thus, these amphibolites deformed by dissolution–precipitation creep that produced a strong linear fabric under upper amphibolite facies, middle‐to‐lower crustal conditions. The significance of this discovery is that dissolution–precipitation creep is activated at lower stresses than dislocation creep and that the strength of the lower crust, where amphibole is the dominant mineral is probably lower than that derived from experimental studies.     

Identification of the scales of differential element mobility in a ductile fault zone

Two fundamentally different scales of element mobility have been identified in the upper-amphibolite-facies Hunts Brook fault zone of south-central Connecticut, USA. Field relations, mineral chemistry, and 80 bulk rock analyses provide overwhelming evidence that the blastomylonitic schists and gneisses of the fault zone were derived from the enclosing granodioritic Rope Ferry orthogneiss. Two-sample mass balance calculations between the Rope Ferry Gneiss and fault rocks strongly suggest that the Rope Ferry Gneiss was segregated into biotite-rich schists and quartz–plagioclase-rich gneisses through the centimetre-scale transfer of Si, Al, Ca, Na, Ba, and Zr from schists to gneisses. Three- and four-sample mass balance calculations provide convincing evidence that Al, Ca, Na, and Ba were lost and Si and K gained by the entire fault zone. Both scales of metasomatism have been identified at two localities separated by 20 km, implying that the metasomatic processes were pervasive throughout the fault zone.
From the mass balance calculations, an apparent disparity in K mobility arises. K appears to be gained by the entire fault zone, but is immobile during the centimetre-scale segregation. This is because mass balance calculations can only detect differential element mobility. Differential element mobility requires that the mechanism of metasomatism produces chemical potential gradients. Therefore, the identification of differentially mobile elements provides insight into the mechanisms of metasomatism. The immobility of K during segregation of schists and gneisses implies that the segregation was not achieved through the extraction of partial melt. However, the element mobility is reconcilable with deformation-driven metamorphic differentiation. The gains of Si and K and losses of Ca, Na, Al, and Ba for the entire fault zone are also inconsistent with the intrusion or extraction of partial melt, but may reflect the infiltration of a metasomatizing aqueous fluid.