摘要:

ABSTRACTMicrostructural and petrological data from the Jumping Brook metamorphic suite, western Cape Breton Highlands, suggest that a single episode of syntectonic prograde metamorphism, followed by uplift, cooling and associated retrogression, affected these rocks during mid‐Palaeozoic times. Microstructures indicative of progressive crenulation foliation development can be traced from low‐grade (chlorite zone) through high‐grade (kyanite zone) rocks, allowing a clear sequence of porphyroblast growth to be established. Metamorphic reactions andP‐Tcalculations suggest metamorphic conditions of 700‐750°C at 8‐10 kbar were achieved in kyanite zone rocks. Although a completeP‐T‐tpath was not defined, combined petrological and geochronological data can be used to constrain computedP‐T‐tmodels. These models suggest that a component of post‐metamorphic tectonic exhumation is required to explain the observed times of cooling and uplift. The microstructural and petrological data to not support the interpretation that the high‐grade rocks represent pre‐existing crystalline basement. Indeed, the metamorphic history, geochronology and computed tectonic models all point to a single, short‐lived episode of Silurian‐Devonian volcanism, intrusion, convergence, regional metamorphism and uplift, probably resulting from collision tectonics at a

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00606.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTDissolution and solution transfer during deformation/metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components. Progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries.Progressive shortening is accommodated by the cores of most other minerals (including sulphides). These minerals develop strain, and hence dislocation density gradients, on their rims due to progressive shearing along grain boundaries. These gradients are particularly large when the mineral abuts phyllosilicate or graphite. The resulting chemical potential gradients between the core and rim drive dissolution, causing removal of the highly strained grain margins.Removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation. Interlayers and interfaces on boundaries lying at a low angle to the direction of shearing, and oriented relative to the sense of shear such that they can open, gape by small amounts. Water present in these interlayer spaces becomes destructured, considerably enhancing diffusion rates along the foliation.Penetrative volume loss, especially in deforming/metamorphosing pelitic rocks, is large at all metamorphic grades, increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies. Transference of material by fluid flow from deep to high levels in the earth's crust is precluded because thousands to tens of thousands of rock volumes of fluid are required, necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells, unless some extremely large‐scale form of fluid channelling is possible. Reassessment of diffusion mechanisms, and hence rates, during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid, indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required. The amount of fluid can be reduced sufficiently to allow large volume losses by a one‐way flow of fluid to the earth's surface, in deforming/metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure.Deformation partitioning‐controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation/metamorphism because matrix minerals, other than phyllosilicates and graphite, are preferentially removed. The large size of porphyroblasts, if present, tends to preserve them from dissolution. Hence, the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew, in rocks in which bedding is still well pres

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00607.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Bunger Hills, East Antarctica, experienced a low‐pressure granulite facies orogenic event during the Proterozoic. The stable coexistence of the S1 foliation‐parallel M1 assemblages, garnet‐cordierite‐spinel‐ilmenite and garnet‐sillimanite‐spinel‐ilmenite‐rutile, in quartz‐bearing pelitic gneisses is evidence for metamorphic peak pressures of around 4 kbar during M1, at temperatures of about 800°C. The growth of massive reaction coronas of garnet and cordierite around hercynitic spinel and iron‐titanium oxides during M2 is evidence for the destabilization of the M1 assemblages during compression. Thermodynamic calculations on the M2 assemblages indicate formation pressures of 6–7 kbar at temperatures of about 750°C. Thus, the gneisses from the Bunger Hills indicate about 2 kbar or more of compression during minimal cooling. Such aP‐Tpath is different from that of many other Proterozoic terranes which are characterized by isobaric cooling or decompression. A large charnockite body, which is undeformed, was intruded at ∼950°C, towards the end of compression.The low pressures during M1 can be best explained by metamorphism at mid‐crustal levels in thin continental crust in thin lithosphere above a thermal perturbation in the underlying asthenosphere. We suggest that the compression during cooling was a result of gravitational backflow in which the action of body forces between adjacent normal thickness crust and the thin crust of the Bunger Hills is 'switched on’by the thermal perturbation. Within such a model, the timing of intrusion of the charnockite exposed in the Bunger Hills is consistent with its generation by partial melting during the metamorp

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00608.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThermobarometric studies on various granulite facies areas along the Prydz Bay coast, East Antarctica (73°‐79°E, 68°‐70°S), show that, at around 1100 Ma, during a late Proterozoic orogeny, the rocks of the Larsemann Hills suffered a lower pressure metamorphic peak than the surrounding areas. Along the Prydz Bay coast, the rocks affected by this event include parts of the Vestfold Hills block plus all of the Rauer Group, the Larsemann Hills and the Munro Kerr Mountains. The dykes in the south‐west corner of the Vestfold Hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south‐west (650°C, 6.5 kbar) (Collersonet al., 1983), the Rauer Group was metamorphosed at 800°C and 7.5 kbar (Harley, 1987a), the Larsemann Hills at 750°C and 4.5 kbar, and the Munro Kerr Mountains probably at around 850°C and 5 kbar. Retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas, followed by isobaric cooling at this depth.This paper shows that the peak metamorphism in the Larsemann Hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow. Although other areas in Prydz Bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model, the inferred pre‐metamorphic peak histories and the requirement of consistency with the Larsemann Hills, make it unlikely that collision followed by erosion‐driven decompression is an appropriate model. We suggest that the thermal regime of the crust in the Larsemann Hills region was controlled by a perturbation in the asthenosphere, with magma invasion of the crust. We suggest that the 500 Ma event, represented in Prydz Bay by granitic outcrops at Landing Bluff and by several K/Ar ages from the Larsemann Hills area, was responsible for the final excava

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00609.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00610.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY