Prograde Regional Metamorphism Research Articles

Release of CO2 from carbonate rocks during regional metamorphism of lithologically heterogeneous crust

Prograde regional metamorphism drives CO 2 from carbonate rock to crustal fluids that ascend and ultimately interact with the atmosphere and oceans. The observed loss of CO 2 from metamorphic belts remains problematic, however, because the cooling that accompanies fluid ascent favors reactions that add CO 2 to metacarbonate rock by removing CO 2 from fluids. A new two-dimensional model of coupled mass transfer, chemical reaction, and heat transport was developed to assess how rock devolatilization proceeds along the upward escape paths of crustal fluids during prograde metamorphism. The model is based on upper greenschist to lower amphibolite facies growth of amphibole in metacarbonate layers and garnet and biotite in intercalated metapelite layers of the Wepawaug Schist, Connecticut (Acadian orogeny). The modeling indicates that during heating, CO 2 concentrations were larger in metacarbonate layers than in adjacent metapelite layers because amphibole growth in metacarbonates produced CO 2 , whereas garnet and biotite growth in metapelites produced H 2 O. The resulting cross-layer concentration gradients drove H 2 O into the metacarbonate layers and CO 2 out by diffusion and the transverse component of mechanical dispersion. Such cross-layer mass transfer can continually force rock decarbonation while fluids ascend, dominating the effects of cooling, unless fluid fluxes are large and prograde heating rates are small. Consequently, prograde metamorphism of carbonate-bearing sedimentary sequences containing significant amounts of pelitic rock will release CO 2 to regionally migrating fluids in a wide range of orogenic settings, regardless of whether flow is in a direction of increasing or decreasing temperature. Regional CO 2 release can be driven by outcrop-scale processes of volatile exchange between contrasting lithologies.

Self-organisation and fracture connectivity in rapidly heated continental crust

Volume expansion (∼1–5% volume strain with Δ V melting positive) and fluid-absent partial melting, in which Δ V melting is positive, of continental crust by intruding basaltic magma is a strongly irreversible process involving the dissipation of both thermal energy and matter (partial melt). Using a simple random graph model we show by analogy how isolated fractures that form during rapid thermal perturbation in the source region can combine to form a single, interconnected structure with high permeability. Once connected, the fracture network may be thought of as a single structure or pattern that will remain stable so long as a strong temperature gradient is maintained in the source region. Estimates of fracture permeability that take into account changes in connectivity and fracture spacing range from approximately 10 −10 to 10 −5 m 2, many orders of magnitude greater than values considered typical during large-scale crustal deformation and prograde regional metamorphism. The ability of the isotropic fracture network to develop a top–bottom directionality is crucial for buoyancy-driven melt transport. A physical model based on non-linear evolution rules during thermal expansion is given that predicts the emergence of directionality (vertical fracture alignment) on a time scale of the order of 10 5 y. The necessary ingredients are a deviatoric strain path, a heterogeneous medium and a stiffness that evolves as a function of the local strain.

Devolatilization‐generated fluid pressure and deformation‐propagated fluid flow during prograde regional metamorphism

The obstruction to fluid flow formed by the rocks overlying a metamorphic devolatilization front causes the fluid pressure gradient in the reacting rocks to diverge from lithostatic. This drives deformation in tandem with the fluid pressure anomaly generated by the volume change of the reaction. Numerical simulations show that once the vertical extent of the reacted rocks is comparable to the compaction length, compaction processes caused by the difference between confining and fluid pressure gradients generate a positive fluid pressure anomaly (effective pressure <0) above the reaction front, irrespective of the reaction volume change. Consequent dilational deformation propagates the anomaly upward, leading to underpressuring and densification at the reaction front and detachment of a wave of anomalous fluid pressure and porosity. Creep is a viable mechanism for such wave propagation for crustal viscosities <1015 MPa s. Continuous upward strengthening of the crust increases the wavelength and amplitude of the fluid pressure waves and thereby the likelihood of hydrofracture. Order of magnitude strength contrasts are adequate to arrest wave propagation, forming water sills that become increasingly stable in the absence of deviatoric stress. Although the fluid pressure gradient within a wave may be near hydrostatic, Rayleigh convection is unlikely. Thus, in the absence of lateral perturbations, fluid flow is upward and episodic, despite continuity of devolatilization. Porosity waves provide a mechanism for temporal focusing of metamorphic fluid fluxes with the potential to increase the efficacy of heat and mass transport.

TEM and XRD determination of crystallite size and lattice strain as a function of illite crystallinity in pelitic rocks

Average crystallite size and mean‐square strain of illite in rock specimens and clay separates were measured independently in TEM images and by single‐line Fourier (Voigt method) profile analysis of the c. 1 nm peak of XRD patterns for a prograde sequence of pelitic rocks (illite crystallinity indices=0.17–0.58°Δ2θ) from the Gaspé Peninsula, Quebec. The TEM‐determined crystallite sizes in clay separates approximate those determined by Fourier profile analyses and those calculated from illite crystallinity indices by the Scherrer equation, with the exception of the diagenetic sample. The crystallite sizes and mean‐square strains of illite in rock samples exhibit a trend similar to that determined by profile analyses, but the average crystallite sizes are up to five times larger than those measured for clay separates.TEM images show that all rock samples have a wide range of crystallite sizes, and the proportions of larger crystallites increase with metamorphic grade. The diagenetic illite is defect‐rich, fine‐grained (mean thickness by volume=c. 70 nm), 1Md material. Anchizonal illite tends to occur as separate aggregates of small 1Md and larger 2M1 crystals (c. 200 nm), comprising arrays of subparallel coalescing packets. The epizone sample has thick (c. 400 nm), defect‐free crystals of muscovite occurring in stacks of parallel layers, or subhedral crystals intergrown with large‐angle boundaries. Cleaved crystals that are free of intracrystalline layer terminations are dominant in clay separates of all samples, having ranges of smaller sizes with volume‐average thicknesses of c. 43, 43, and 81 nm (c. 14, 28, 67 nm by the Voigt method), respectively, for the three zones.The results suggest that illite crystallinity indices do not provide a direct measure of a single microstructural state of illite in rocks, although they yield consistent limits for average crystallite sizes for the anchizone (23 & 48 nm here). Therefore, they serve as a general parameter of the degree of recrystallization on a relative basis, in part because the contributions of all peak‐broadening variables (mixed layering, size and strain) decrease regularly with prograde regional metamorphism of pelites. The microstructural changes caused by rock disaggregation are probably a function of those variables as well. The data collectively demonstrate a trend from metastable, defect‐rich, small crystals towards a stable assemblage of larger, defect‐free crystals, through dissolution of strained crystals and neocrystallization, consistent with the Ostwald step rule.

Structural control and metamorphic setting of the shear zone-related Au vein mineralization of the Adola Belt (southern Ethiopia) and its tectono-genetic development

Structural study of the Adola Belt shows that most of its known and potential Au deposits occur in quartz veins which are localised within shear contacts between lithological units, and along major shear zones that divide the Adola Belt into different lithostructural domains. Analysis of the shear zone-related ore bodies and their host volcano-sedimentary succession and gneisses indicates that Au mineralization in the Adola Belt is pre-dated by two stages of deformation and a regional prograde metamorphism. The first deformation event (D 1) is a fold-and-thrust event which is characterised by low-angle thrusts, associated recumbent folds and axial planar S 1 foliation, and is related to nappe-style deformation. The second event (D 2) has folded and/or reactivated the thrust-related structures and formed upright folds and high-angle reverse shear zones and is related to the collision event. Gold mineralization occurred over a prolonged deformation history but is closely related to alteration, retrograde greenschistfacies assemblages and brittle-ductile deformation of late D 2 and D 3 transpressional shear zones that accommodate regional shortening both by crustal thickening and lateral displacement. The mineralization occurs in associated dilational jogs or bends that might have formed during the lateral or vertical expulsion. The Au-hosting shear zones are characterised by extensive development of heterogeneous mylonitic fault rocks which reveals that the accompanied deformation is characterised by processes that can increase the porosity and permeability of the rocks within the shear zones. This gave rise to further extensive dilatancy within the major dilational jogs and produced a suitable structural regime for vein-hosted Au mineralization. This implies that the Au mineralization is epigenetic in origin and that it resulted from precipitation from metamorphic hydrothermal fluids circulating through major shear zones and associated structures late during the deformation and metamorphic history of the shear zones, through a hydrodynamic process of vein formation. The association of Au with low temperature ore minerals like galena gives an independent evidence for the deposition of Au in low temperature retrograde conditions. The increase in Au concentration along the Megado greenschist-facies metavolcano-sedimentary terrane as compared to the gneissic rocks is related to its development in a rapid extensional zone and rapid subsidence environment in an inter-arc/back-arc setting, where a thicker amount of volcanic rocks containing trace amounts of massive sulphide-related Au can best be developed and rapidly buried. The occurrence of tellurides in association with the Au mineralization in the Megado Terrane confirms that the mineralization is genetically related to the subduction event.

Fluid—Rock Interaction during Low-Pressure Polymetamorphism of the Reynolds Range Group, Central Australia

Marbles and metapelites from the Reynolds Range Group (central Australia) were regionally metamorphosed at low pressure during M^ at ~/-6~ Go. M^ ranged in grade from gnenschist to granulite fades along the length of the Reynolds Range, and overprinted ~l-78 Ga granites and their contact aureoles in the Reynolds Range Group metasediments. At all M.2 grades the marbles and metapelites have highly variable oxygen isotope ratios [marbles: 5I8O(carb) ~14~20%o; metapelites: b180 ~ 6-14%o]. Similarly, ~l-78 Ga granites have highly variable oxygen isotope ratios (8I8O ~ 5-13%o), with the lowest values occurring at the granite margins. In all rock types, the lowest oxygen isotope values are consistent with the infiltration of channelled magmatic and/or meteoric fluids. The variable lowering of oxygen isotope values resulted from pre-Mi2 contact metamorphism and fluid—rock interaction around the ~l-78 Ga granites. In contrast, mineral assemblages in the marbles define a trend of increasing Xco, with increasing grade from <0-05 (gnenschist fades) to ~0-71 -0 (granulite fades). This, together with the lack of regionally systematic resetting of oxygen isotope ratios, implies that there was little fluid—rock interaction during prograde regional metamorphism. KEY WORD& law pressure; polymetamorphism; fluids; stable isotopes; petrology

Controls on the nucleation and growth of porphyroblasts: Kinetics from natural textures and numerical models

AbstractQuantitative textural analysis of garnetiferous rocks from diverse metamorphic environments has demonstrated that diffusionally influenced nucleation and thermally accelerated diffusion‐controlled growth are commonly among the dominant controls on porphyroblast crystallization. A new numerical model of diffusion‐controlled nucleation and growth that incorporates time as an explicit variable is capable of replicating the essential features of many of these natural porphyroblastic textures using geologically reasonable values for the kinetic parameters governing crystallization. This model has been used to interpret quantitative textural data from four rocks in which garnet grew during prograde regional metamorphism, by assuming constant heating rates and by accepting a previously published estimate for the activation energy for intergranular diffusion. From fits of the model to natural textures, estimates have been extiacted for three previously undetermined kinetic quanities that control the nucleation and growth of garnet porphyroblasts, namely the activation energy for nucleation and the pre‐exponential rate constants for nucleation and intergranular diffusion. The uncertainty in derived values for the pre‐exponential rate constant for intergranular diffusion is probably not much larger than a factor of two because it is proportional to the uncertainty in prograde heating rates, given a defined temperature interval over which crystallization occurs. The uncertainty in estimates of the activation energy and pre‐exponential rate constant for nucleation is larger, perhaps a factor of ten, because values for these quantities are derived from fits of the model to natural crystal size distributions; the precision of these estimates appears to be limited by the fact that the crystal size distributions are affected by factors not encompassed within the model (probably variable heating rates and inhomogeneity in the distribution of nutrients in the rock's precursor).

Strain‐related differences in the crystal growth of white mica and chlorite: a TEM and XRD study of the development of metapelitic microfabrics in the Southern Uplands thrust terrane, Scotland

Abstract TEM and XRD techniques were used to study crystal growth characteristics of the fabric‐forming phyllosilicates which developed in response to low‐grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding‐parallel. TEM‐measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD‐measured crystallinity indices. Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric‐forming phyllosilicates. White micas are paragonite‐poor phengites with a mean b lattice parameter of 9.037 Å, and indicate an intermediate pressure series of metamorphism with a field gradient of &lt;25° C km‐1. Chlorite compositions evolved from diabantite (with intergrown corrensite) to ripidolite over an estimated temperature range of 150–320° C. Field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, and resulted in similar size distributions which resemble typical Ostwald ripening curves. Under anchizonal and epizonal conditions, white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. In contrast, chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain‐induced crystal growth indicate that white mica (illite) and chlorite crystallinity indices are likely to show significant differences where low‐grade metamorphism is closely associated with tectonic fabric development.

Development of inverted metamorphic gradient in the internal domain of the Taconian belt, Gaspé Peninsula

The Mont Logan Nappe is part of the Taconian internal domain of the Quebec Appalachians, and is entirely made up of synrift to passive margin elastics and volcanics of the Shickshock Group. Rocks of the Mont Logan Nappe were affected by both Taconian and Acadian deformations but regional prograde metamorphism is Taconian and limited to the D1 deformational event. Thermobarometry and mineral assemblages indicate that the metasedimentary and metavolcanic rocks of the Mont Logan Nappe have recorded peak temperatures in the range 610–700 °C under pressures of approximately 600–700 MPa, and that prograde metamorphism was accompanied by the development of an inverted metamorphic gradient of −40 to −75 °C/km. The preferred interpretation of the cause of the inverted gradient is dissipative heating accompanying deformation along an intracontinental synmetamorphic thrust fault located at the top of the inverted metamorphic sequence.

The mineral chemistry of hydrothermally altered and metamorphosed wall-rocks at the Stollberg Fe-Pb-Zn-Mn(-Ag) deposit, Bergslagen, Sweden

The c. 1.9 Ga old Stollberg sulphide and Mnrich skarn iron ores and sulphide ores in Bergslagen, south-central Sweden are hosted by hydrothermally altered and metamorphosed felsic volcanic and volcaniclastic rocks. The ores are underlain by comformable alteration zones characterized by albite-gedrite-quartz and biotite-muscovite-plagioclase-K-feldspar-quartz +/− garnet assemblages. The present mineralogies are interpreted as medium-grade metamorphic equivalents to the original alteration mineral assemblages. PT-conditions during prograde regional metamorphism are semiquantatively determined to be 510 to 560 °C at approximately 3 kbar. With increasing modal content of gedrite and biotite in the alteration zones, the Mg/Fe ratios and XMg's in octahedral positions of these minerals also increase. In the gedrite-bearing strata, whole-rock Mg/Fe ratios remain constant, whereas in the biotite-rich unit the wholerock Mg/Fe trend is parallel to that of the biotites.

Paleoatmospheric consequences of CO 2 released during early Cenozoic regional metamorphism in the Tethyan orogen

The Eocene was the warmest epoch of the Cenozoic, with published estimates of Eocene atmospheric CO 2 content ranging from two to six times the current value. Calculations of CO 2 consumption by silicate weathering show that CO 2 fluxes to the atmosphere of ca. 10 18/Myr could account for inferred Paleocene/Eocene atmospheric CO 2 contents, and the resulting greenhouse effect would have contributed to Eocene warmth. Extensive portions of the Tethyan orogen underwent regional metamorphism culminating in the Eocene. Prograde metamorphism may have been contemporaneous with the Late Paleocene global warming. We calculated the amount of metamorphic CO 2 produced at depth in the Himalayan orogen with data on the timing of the India/Asia collison, duration of prograde regional metamorphism, and proportions and bulk compositions of metamorphic CO 2 source rocks. If CO 2 was generated at a constant rate over a 10 Myr period of prograde metamorphism in the Himalayan orogen, we estimate that ca. 10 18–10 19 moles/Myr of metamorphic CO 2 were produced at depth. Significant expulsion of metamorphic CO 2 to the atmosphere may have occurred by focused fluid flow along shear zones such as the extensive Main Central Thrust in the Himalayan orogen. An additional ca. 10 18/Myr could have been contributed by Eocene regional metamorphism in the Mediterranean Tethys (i.e., from the Alps to Turkey). The extensive metamorphism associated with the India/Asia collision, and the closing of Tethys, may have contributed to CO 2-greenhouse warming in the early to mid Cenozoic.

Porphyroblast–fabric relationships: an example from the Appin Group in the Glen Roy area

Synopsis The Neoproterozoic Grampian and Appin groups exposed in the Glen Roy area were deformed during an initial progressive tectono–thermal event of the Grampian orogeny. Prograde regional metamorphism to amphibolite facies accompanied deformation with the thermal peak coinciding with the second phase of folding (F2) and the imposition of an S2 crenulation to transposition fabric. In the pelitic to semipelitic schists of the Lochaber Subgroup (Appin Group), metamorphism resulted in the development of the assemblage: quartz + muscovite + biotite ± garnet, plagioclase (An 13 to 22%), staurolite and opaques. The complexity of porphyroblast–fabric relationships recorded from the schists varies systematically across the area and can be directly related to the intensity of S2. Six progressive stages of S2 crenulation cleavage development are recognized, preserved within multitextured, syn-kinematic garnet porphyroblasts. These stages show a progression from a planar S1 fabric to differentiated S2 crenulation cleavage, through to the development of a differentiated S2 transposition fabric. Porphyroblast–fabric relationships are further complicated by the localized reactivation of S1 in response to movement along the Grampian–Appin lithostratigraphic boundary during F2 folding. Biotite porphyroblasts developed early during the imposition of S2 and were subsequently deformed during D2 to form lenticular mica-fish. Staurolite (rare) nucleation and growth, representing peak metamorphism recorded by these schists, post–dated S2 development.

Ca-rich scapolite in quartz amphibolites from the Sarti area, Northern Greece

Ca-rich scapolite occurs in quartz amphibolites from the Sarti area, Chalkidiki peninsula, northern Greece. The mineral composition of these rocks is amphibole + quarz + clinopyroxene + epidote + zoisite + scapolite + calcite + sphene + plagioclase + opaques (pyrite, graphite). The mineralogy and mineral chemistry indicate prograde regional metamorphism in the upper amphibolite facies in a high geothermal gradient at low pressure conditions, in the presence of a volatile-rich fluid phase. The association amphibole + clinopyroxene + zoisite represents the peak conditions reached during prograde metamorphism. The availability of CO 2 and probably SO 3− in the fluid phase as well as the bulk rock composition appear to have controlled the scapolite composition. A retrograde stage under low temperature and low fO 2 is also recorded

Geologic implications of the oxygen isotope profile of the Toa Baja Drill Hole, Puerto Rico

The whole‐rock O‐isotopic compositions of volcanic and volcaniclastic samples from the Toa Baja drill hole demonstrate that low‐temperature (&lt;200°C) processes have strongly enriched the island arc materials in 18O. Subsequent to eruption, processes such as subaerial weathering, alteration during transport and deposition in volcaniclastic aprons, submarine weathering, burial diagenesis, and prograde regional metamorphism through the beginning of the prehnite‐pumpellyite facies have raised average whole‐rock δ18O values by ∼4‰ for basalt and andesite lava flows, and by ∼8‰ for volcaniclastic sandstones. These O‐isotopic disturbances were probably caused by oxygen exchange with regionally circulating seawater under rather high water/rock conditions. The processes associated with “ageing” of volcanic and volcaniclastic materials in the oceanic environment are probably more important to the global budgets of the oxygen isotopes than has been assumed in the past. Integration of these results into global models for the oxygen isotopes awaits analysis of more varied oceanic terranes, to determine the generality of the O‐isotopic conclusions preferred here, and to more carefully evaluate the relative sizes of volcanic, volcaniclastic, and oceanic oxygen reservoirs and their variabilities in time.

Low‐pressure regional metamorphism in the Omeo Metamorphic Complex, Victoria, Australia

The Omeo Metamorphic Complex forms the southern end of the Wagga Metamorphic Belt, which is the main locus of Palaeozoic low‐pressure metamorphism in the Lachlan Fold Belt, south‐eastern Australia. It comprises metamorphosed Ordovician quartz‐rich turbidites originally derived from Precambrian cratonic rocks. Prograde regional metamorphism occurred in the early Silurian, very soon after sedimentation had ceased. The sequence of metamorphic zones, with increasing grade, is: chlorite, biotite, cordierite, andalusite–K‐feldspar and sillimanite–K‐feldspar. Migmatites occur in the sillimanite–K‐feldspar zone, but large bodies of S‐type granite were derived from rocks underlying the exposed Ordovician sequence. P and T estimates for the highest grade rocks are T= 700°C and P= 3.5 kbar, indicating a very high P–T gradient of 65°C/km.The high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre‐metamorphic back‐arc basin environment, and intrusion of hot, mantle‐derived magmas into the lower and middle crust.Regional retrograde metamorphism coincided with a general reheating of the crust in the Siluro‐Devonian, accompanied by intrusion of many I‐type plutons and resetting of the K–Ar dates of some earlier plutons. The Omeo Metamorphic Complex was exposed to erosion at this time.

Crustal paleorheology of the southeastern Canadian Cordillera and its influence on the kinematics of Jurassic convergence

The kinematic evolution of the southeastern Canadian Cordillera (52°N–54°N) during the Jurassic is characterized by an eastward migration of the active boundary of convergence between North America and allochthonous terranes and by two reversals in regional structural vergence. To investigate possible mechanical controls on the kinematics of convergence, the thermal and geological structure of the Jurassic continental margin at three stages in its history of shortening were palinspastically reconstructed and analyzed with the use of strength versus depth profiles. The resulting models reproduce to a first order the observed distribution of brittle and ductile deformation and provide a mechanical basis for the localization and orientation of displacement zones during shortening of the continental margin. The initial eastward migration of the active boundary of convergence between North America and allochthonous terranes from outboard of the continental margin sediment prism into the thick central part of the continental margin prism is attributed to thermal weakening of the continental margin lithosphere during obduction of the oceanic terrane. The subsequent eastward migration of the convergence boundary into the inner part of the continental margin lithosphere is attributed to thermal weakening of the lithosphere during prograde regional metamorphism. Reversals in regional structural vergence accompanying each migration apparently occur because the push necessary to continue displacement on one displacement trajectory exceeds the failure strength of another displacement trajectory that dips in the opposite direction.

Temperature-time relationships across metamorphic zones: evidence from porphyroblast—matrix relationships in progressively deformed metapelites

In a rock pile undergoing prograde regional metamorphism, the isotherms corresponding to certain metamorphic reactions shift continuously, while the same rock volume develops progressive deformation fabrics as a result of increasing strain. Porphyroblasts in pelitic schists can provide crucial information on both these aspects of metamorphism. Not only do they reflect the pressure-temperature evolution of that terrain, but their inclusion trails are clues to the state of deformation at the time of porphyroblast growth. Microstructural relationships of inclusion trail patterns versus matrix foliations, combined with petrological data on prograde reaction sequences, can be used to reconstruct temperature-time relationships in different zones within metamorphic terrains. In two Proterozoic areas in northern Queensland, the western Mary Kathleen Fold Belt and the Robertson River Subgroup, a systematic variation of inclusion trail patterns has been found across metamorphic zones. Each porphyroblast species contains more advanced stages of S 2 foliation development towards its isograd, compared with higher-grade zones in which these porphyroblasts are present. The combined microstructural-petrographic observations allow to place constraints on the T−t evolution of the various zones. The regional distribution of inclusion trail patterns in the two study areas indicates that deformation affected high- and low-grade zones simultaneously.

Chemical mineralogy and geothermometry of the Middle Shire granulites, Malawi

The Middle Shire area of Malawi lies roughly midway between the southern tip of Lake Malawi and the Zambesi river. It includes the western side of the Shire Highlands and the central section of the Shire rift valley. It is occupied by gneisses and granulites of the Mozambique Belt, a polymetamorphic fold belt, which extends down the eastern side of Africa from Ethiopia to southern Mozambique. In the Middle Shire section the eastern part of the area is occupied by two-pyroxene granulites together with subordinate skarns and biotide-garnet granulites. The western part consists of biotite-gneisses and hornblende-biotite gneisses together with subordinate pelitic and calcareous metasediments. A well defined transition zone lies between them separating prograde amphibolite facies gneisses from the granulite facies to the east. Later retrograde effects are related to a subsequent episode of folding and produced secondary amphibole and biotite. Mineral compositions reflect a temperature range of 680–1000°C, during the prograde regional metamorphism and pressures of 6–8 kbars in the granulites. Transition zone temperatures were 550–680°C at 4–7 kbars. Amphibolite facies temperatures were 450–600°C at pressures of 4–5 kbars.

The stratigraphy, metamorphism and tectonics of the Early Proterozoic Litchfield Province and western Pine Creek Geosyncline, Northern Territory

The Litchfield Province is a 9000 km 2 area of Early Proterozoic, low- to high-grade metamorphics and granitoids, which occurs immediately to the west of the Pine Creek Geosyncline and at the northern extremity of a major fault zone extending from the Halls Creek Mobile Zone. The oldest unit, the Hermit Creek Metamorphics in the southern half of the province, is essentially metamorphosed semi-pelitic sediments with some intercalated basic igneous rocks. The first two of at least three phases of deformation are related to regional prograde metamorphism, which formed low- to medium-grade phyllite and mica schists, and high-grade migmatitic gneisses and metabasites. The northern part of the province contains the Welltree Metamorphics, consisting predominantly of low- to medium-grade metamorphosed pelitic and arenaceous sediments, with carbonaceous, iron-rich, and calcareous and dolomitic sequences. These are considered equivalent to sediments of the Pine Creek Geosyncline. The western margin of the Pine Creek Geosyncline is formed by low-grade, metamorphosed pelitic to rudaceous, terrigenous sediments of the Finniss River Group, which to the south contain basal volcanics. The sediments were derived from a proximal, westerly source, and exhibit a vertical transition from deep- to shallow-water depositional environments. An angular unconformity between the Finniss River Group and the Hermit Creek Metamorphics is exposed in the southern part of the province. Elsewhere, the relationship is obscured or faulted. Prograde metamorphism of the Hermit Creek Metamorphics was followed by regional, low-grade retrogression. The retrogression is believed to have been caused by the main metamorphic event of the Top End Orogeny which began at ∼ 1870 Ma and accompanied regional deformation of both the sediments of the Pine Creek Geosyncline and the Welltree Metamorphics. Extensive syn- and post-orogenic granite intrusion (at ∼ 1850 Ma) was followed by a regional, low-grade, essentially hydrothermal event (1780 Ma). The Litchfield Province and Pine Creek Geosyncline are overlain with marked unconformity by post-1800 Ma plateau-forming sandstone and by mainly sandstone, limestone and basalt of Palaeozoic age. Eroded remnants of a once extensive sheet of Mesozoic sandstone overlies older rocks in many places.

Structural control of greenstone-hosted Gold mineralization by transcurrent shearing: A new interpretation of the Kalgoorlie mining district, Western Australia

The Kalgoorlie mining district encloses the Golden Mile and Mt. Charlotte mining areas, and has produced 1200 metric tons of gold. It is classed as one of the largest Archaean gold deposits in the world. The district is located in the Norseman-Wiluna greenstone belt of the Yilgarn craton, Western Australia. This greenstone belt is characterized by NNW-trending transcurrent shear zones, some of which are traceable over more than 500 km strike length. Mineralization in the district is controlled by second-, third- and fourth-order structures located between principal shear zones of regional extent. An early deformation phase of regional folding (D1), associated with prograde regional metamorphism of low grade, preceded two phases of wrenching. The geometry of mineralized shear zones in both mining areas, as well as field and microstructural studies, allow us to distinguish an early phase of transpressional sinistral shearing (D2) from a later phase of transcurrent dextral shearing (D3). The earlier sinistral shear system in the Golden Mile area was in part reactivated, but its gross original geometry was preserved. These observations are consistent with clay model experiments carried out by the authors. Hydrothermal carbonate-sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event. Mineralization within individual shear zones forms tabular orebodies, whereas pipe-shaped ore shoots occur at the intersection of shear zones. The ore shoots contain high-grade telluride mineralization locally. Relative timing relationships show that mineralization postdates the peak of regional metamorphism. Hydrothermal activity in the district was preceded by the intrusion of calc-alkaline porphyry dykes of granodioritic to tonalitic composition, and accompanied by the intrusion of lamprophyre dykes.