Retrograde Shear Zones Research Articles

Sources and Effects of Fluids in Continental Retrograde Shear Zones: Insights from the Kuckaus Mylonite Zone, Namibia

Midcrustal rocks in retrograde metamorphic settings are typically H2O-undersaturated and fluid-absent and have low permeability. Exhumed continental retrograde faults, nonetheless, show evidence for the operation of fluid-mediated weakening mechanisms during deformation at midcrustal conditions. To explore the origin and effects of fluids in retrograde faults, we study the Kuckaus Mylonite Zone (KMZ), an exhumed crustal-scale, strike-slip shear zone in the southern Namibian Namaqua Metamorphic Complex. The KMZ deformed quartzofeldspathic migmatised gneisses at midcrustal retrograde conditions (450-480°C, 270-420 MPa) in the Mesoproterozoic, 40 Ma after granulite facies peak metamorphism at 825°C and 550 MPa. The mylonites contain fully hydrated retrograde mineral assemblages, predominantly adjacent to anastomosing high-strain zones, providing evidence of local H2O saturation and fluid presence during deformation. Whole rock and quartz veinδ18O values suggest that at least some of the fluids were meteoric in origin. The rocks across the shear zone retain the effect of different protoliths, implying little effect of fluid-rock interaction on whole rock major element chemistry. Together with a general scarcity of quartz veins, this suggests that fluid/rock ratios remained low in the KMZ. However, even small amounts of H2O allowed reaction weakening and diffusion-precipitation, followed by growth and alignment of phyllosilicates. In the ultramylonites, a fine grain size in the presence of fluids allowed for grain size sensitive creep. We conclude that the influx of even small volumes of fluids into retrograde shear zones can induce drastic weakening by facilitating grain size sensitive creep and retrograde reactions. In retrograde settings, these reactions consume fluids, and therefore elevated fluid pressures will only be possible after considerable weakening has already occurred. Our findings imply that the range of seismic styles recently documented at active retrograde transform faults may not require high fluid pressures but could also arise from other local weakening mechanisms.

Tectonic and chemical implications of cathodoluminescent microstructures in quartz, Parry Sound domain, Ontario, Canada

Cathodoluminescence (CL) imaging of quartz in orthogneiss of the Parry Sound domain, Grenville Province, Ontario, Canada, reveals four dark microstructures—medium-dark grains, mantles, sinuous lines, and straight lines. The Parry Sound domain experienced granulite-facies deformation and metamorphism followed by variable degrees of amphibolite-facies retrogression associated with the development of retrograde shear zones. We integrate our observations of CL-visible quartz microstructures with structures observable with optical and scanning transmission electron microscopy, analyses of water species concentration, and trace element distribution in quartz, as well as with microstructures in plagioclase, to determine their characteristics and elucidate their timing and mechanisms of formation relative to the deformational history. Medium-dark grains are primarily located in and near kilometer-scale shear zones at the margins of the Parry Sound domain, have notably darker cores than most samples in the interior, and may have higher water contents than other samples in the interior. These grains formed during late shearing, at a lower temperature than CL-bright grains; they thus provide evidence that the large shear system narrowed with time. Dark mantles occur at grain boundaries and correlate with higher concentrations of Fe. They formed after the first shearing event, but while still at amphibolite facies, and they provide evidence for post-kinematic alteration at grain boundaries. Dark sinuous lines correspond to subgrain boundaries that formed after retrograde shearing, but while still at amphibolite facies. Dark straight lines correspond to fluid inclusion trails and most likely formed at low temperatures and pressures during exhumation.

Monazite Dating of Prograde and Retrograde P–T–d paths in the Barrovian terrane of the Thaya window, Bohemian Massif

Monazite laser ablation–split-stream inductively coupled plasma–mass spectrometry (LASS) was used to date monazite in situ in Barrovian-type micaschists of the Moravian zone in the Thaya window, Bohemian Massif. Petrography and garnet zoning combined with pseudosection modelling show that rocks from staurolite–chlorite, staurolite, kyanite and kyanite–sillimanite zones record burial in the S1 fabric under a moderate geothermal gradient from 4–4·5 kbar and ∼530–540°C to 5 kbar and 570°C, 6–7 kbar and 600–640°C, 7·5–8 kbar and 630–650°C, and 8 kbar and 650°C, respectively. In the kyanite and kyanite–sillimanite zones, garnet rim chemistry and local syntectonic replacement of garnet by sillimanite–biotite aggregates point to re-equilibration at 5·5–6 kbar and 630–650°C in the S2 fabric. Heterogeneously developed retrograde shear zones (S3) are marked by widespread chloritization, but minor chlorite is present in the studied samples. Monazite abundance and size increase with metamorphic grade from 5 µm in the staurolite–chlorite zone to >100 µm in the kyanite and kyanite–sillimanite zones. Irrespective of the monazite-forming reaction, this is interpreted as the onset of limited prograde monazite growth at staurolite grade, and continued prograde monazite growth after the kyanite-in reaction, compatible with conditions of about 5·5 kbar and 570°C and 7·5 kbar and 630°C from pseudosection modelling. Monazite is zoned, showing embayments and sharp boundaries between zones, with low Y in the staurolite zone, high-Y cores and low-Y rims in the kyanite zone, and high-Y cores, a low-Y mantle and a high-Y rim in the sillimanite zone. The 207Pb-corrected 238U/206Pb ages from three samples range from 344 ± 7 to 330 ± 7 Ma, irrespective of metamorphic grade. The dates from monazite inclusions are interpreted as the ages of the staurolite- and kyanite-in reactions along the prograde path at 340 and 337 ± 7 Ma, respectively. The monazite in the matrix (and some inclusions) is interpreted as dating the prograde crystallization at (340–337) ± 7 Ma within the S1 fabric, and then being affected by recrystallization at or down to 332 ± 7 Ma in the S2 and S3 fabrics. The two groups of data, for 340–337 and 332 Ma, are significantly different when only their in-run uncertainties (±1–3 Myr) are compared and indicate a 9 ± 3 Myr period of monazite (re)crystallization. A systematic increase in heavy rare earth element (HREE) content with decreasing monazite age from 344 to 335 Ma is correlated with growth on the prograde P–T path; the drop in HREE of monazite at 335–328 Ma is assigned to recrystallization. The presence of chlorite even in the least retrogressed samples witnesses limited external fluid availability on the retrograde P–T path. Migration of this fluid was probably responsible for heterogeneous fluid-assisted recrystallization and resetting of original prograde monazite, even where included in garnet, staurolite or kyanite. It is suggested that the rocks passed the chlorite-in reaction on the retrograde path at 332 ± 7 Ma. The timing of burial in the Thaya window, a deformed part of the underthrust Brunia microcontinent, was coeval with exhumation of granulites and migmatites of the Moldanubian orogenic root at c. 340 Ma.

Rates of Retrograde Metamorphism and their Implications for the Rheology of the Crust: an Experimental Study

We have carried out experimental studies of the rate at which water is consumed by hydration reactions under mid-crustal conditions. Both pelitic and mafic assemblages are susceptible to extensive hydration in the laboratory on a time scale of weeks to months. Quantitative hydration rate determinations were made using enstatite–oligoclase ± diopside powder mixtures and a natural hypersthene hornfels. Under all conditions, the main hydration product was saponite clay with variable amounts of talc according to the initial proportion of enstatite to plagioclase. The experiments yield consistent rates for water consumption of around 10−8 g H2O per m2 of mineral surface per second at 400°C and 300 MPa (3 kbar). Additional experiments were run at 300°C and 500°C and at lower pressures (40 MPa), as well as with NaCl; rates appear to be faster at higher temperatures and in the presence of salt, but slower at low pressure. Comparison of powder and core experiments on the natural hornfels indicates that it is primarily the outer surface of the rock core that is available for hydration, with only minor infiltration along grain boundaries. The hydration rates reported here appear to be typical for the types of lithology that demonstrate moderate to high degrees of retrogression along joints and deformation zones in crystalline rocks of the upper crust. Assuming that the surface roughness and damage effects in a natural fault zone are comparable with those of the materials used here in the experiments, the measured hydration rates imply that a natural fracture in crystalline rocks of the middle crust that becomes filled with a water film 0·2 mm in thickness will dry out through incorporation of the water into hydrous phases on a time scale of the order of 10–100 years. This clearly implies that free water has only a short residence time in crystalline rocks of the middle crust or deeper, provided they have cooled below their original temperature of formation and therefore have the potential to undergo retrograde hydration. We infer that the strength of retrograde shear zones in the middle to lower crust will fluctuate through time, with episodes of water infiltration resulting in short periods of water weakening before the water is fully consumed and the rocks become stronger once more.

Three‐stage evolution of lower crustal gneiss domes at Breaksea Entrance, Fiordland, New Zealand

AbstractNew structural analyses reveal the presence of several kilometer‐scale gneiss domes and many subdomes within an exhumed section of Cretaceous lower crust in SW Fiordland. The domes are defined by complex, nearly concentric trajectories of foliations and steep high‐strain zones in migmatitic, granulite, and eclogite facies orthogneiss. Structural patterns combined with evidence of deformation that was synchronous with partial melting and high‐grade metamorphism, steep mineral lineation orientations, sense of shear indicators, and near‐isothermal decompression suggest that buoyancy‐driven diapiric flow of hot, partially molten crust aided initial dome formation. Mafic‐intermediate magmatism appears to have enhanced this process. Superimposed on all high‐grade structures are upper amphibolite facies shear zones that record a change in the style and conditions of lower crustal flow. Lineation orientations, sense of shear, evidence of cooling with decompression, and other patterns suggest that the retrograde shear zones record bulk horizontal flow and crustal thinning driven by regional tectonic forces. A preferred sequence begins with the initial emplacement of mafic‐intermediate magma into the lower crust at depths up to 70 km (D1). Diapirism and initial dome formation occurred during a period of waning arc magmatism and episodic partial melting and high‐grade metamorphism (D2). Following melt crystallization, dominantly horizontal flow of a cooler, strain‐hardened lower crust occurred during Late Cretaceous continental extension (D3). This example highlights the interplay between mechanisms of horizontal and vertical flow at the root of a continental arc and illustrates their roles in the formation of gneiss domes.

Understanding the 1720–1640 Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province, Southeastern Australia: Implications for tectonics, basin evolution and ore genesis

The ∼1720–1640 Ma Willyama Supergroup was deposited in an epi-continental rift basin accompanied by felsic and mafic magmatism, and hosts the largest known Pb–Zn–Ag accumulation at Broken Hill. The Supergroup occupies the southern portion of the Curnamona Province, a largely buried ∼50,000 km 2 crustal block, possibly the southernmost part of the Eastern Australian Pb–Zn belt (which includes Mt Isa). Additionally, the Province is a component of the early Mesoproterozoic IOCG ‘super-province’ that includes the Gawler Craton (Olympic Dam), Eastern Succession of the Mt Isa region (Ernest Henry) and perhaps parts of NW North America (Wernecke Mountains). The Curnamona Province, but a small part of the original basin, is divided into domains on depositional, tectonic and magmatic criteria with variably distributed stratigraphic units reflecting syn-depositional faulting and onlapping relationships resulting from rifting during the earlier extensional stage of basin evolution. Rifting controlled the variable distribution and thickness of the succession. In the Olary Domain to the west, the palaeo-flank exposes the ∼1720–1715 Ma Curnamona Group, but overlying sediment packages are but sporadically developed. To the east and closer to the rift axis, sedimentation in the Broken Hill Domain was more complete, comprising ∼1710–1705 Ma migmatitic-metasediments, and the overlying ∼1705–1695 Ma Thackaringa, ∼1695–1685 Ma Broken Hill and ∼1685–1670 Ma Sundown Groups. The ∼1670–1640 Ma Paragon and Strathearn Groups represent sag-phase deposition with a change in basin configuration and provenance. Earliest syn-depositional magmatism is recorded in the Olary Domain by ∼1720–1710 Ma A-type felsic volcanics and intrusives and restricted basaltic lavas. The locus of magmatism then moved to the Broken Hill Domain with partial melting of either the lower parts of the Willyama Supergroup, or underlying rocks of similar composition, producing S-type granitic sills of the ∼1705–1685 Ma Silver City Suite. Magmatism climaxed in the upper Broken Hill Group with emplacement of Fe-rich tholeiite sills and the ∼1685 Ma volcaniclastic felsic Hores Gneiss hosting the Broken Hill Pb–Zn–Ag orebody. Polyphase deformation and high T, low P metamorphism, ranging from greenschist to lower granulite grade, resulted from the ∼1620–1590 Ma Olarian Orogeny. Early layer-parallel schistosity was deformed progressively, first by large ductile recumbent and sheath-style folds, and later by upright folding. The mainly S-type granites of the ∼1595–1580 Ma Ninnerie Supersuite were intruded late in the deformation history. An extensive network of post-1580 Ma retrograde shear zones dismembered the Willyama Supergroup and was partially reactivated during the ∼500 Ma Delamerian Orogeny. Of the wide range of mineralisation styles in the Curnamona Province, Broken Hill-type Pb–Zn–Ag deposits and epigenetic Cu–Au and U are currently of greatest interest. Broken Hill-type mineralisation probably formed in a shallow depositional environment at or below the sea-floor, with maximum development approximating cessation of magmatic activity. Cu–Au (±Co, Mo) deposits (e.g. Kalkaroo, Portia) are located in chemical and structural traps near a regionally extensive redox boundary. By sharing geological aspects of the eastern Gawler Craton, the Curnamona Province has excellent prospectivity for Olympic Dam-style IOCG mineralisation. Uranium deposits, some economic, are located both in basement rocks and as roll-front concentrations in overlying Tertiary channel sands.

Coupled Deformation and Reaction Softening Processes: Retrograde Shear Zones in the Rosslare Complex, South-East Ireland

Coupled Deformation and Reaction Softening Processes: Retrograde Shear Zones in the Rosslare Complex, South-East Ireland

EMP chemical age dating of monazites from a complex terrain: The Paleo-Proterozoic of broken hill, Australia

A regional program of EMP age-dating has been undertaken on samples, chiefly from meta-sediments, covering all groups in the stratigraphy [1], main metamorphic assemblages and HT and LT shears (ca. 80 specimens, ca. 3000 spot ages). A cumulative plot of the spot (and isochron) ages reveals two concentrations: 1) a broad peak ca. 1720 to 1600 Ma and 2) twin peaks at ca. 520 & 480 Ma. All are independent of the position of the specimen in the stratigraphic sequence. The high grade metamorphic rocks and mylonites in 1) are characterized by any or all of the And, fibrolite (Fib) and Sil minerals. Texturally, on a regional basis, And is overprinted by Fib which is overprinted by at least two generations of prismatic Sil. The age spread 1) is thought to reflect the above textural complexity. Fib is often included in monazites within this age-peak, including those with ages of circa 1720 Ma. Monazite cores with ages in the older part of 1) are often rimmed by overgrowths in the younger part which concentrate in mylonites. A protracted thermo-tectonic event is indicated. Samples with monazites from the 520 & 480 Ma peaks have St + Ky ± Cld assemblages and are found mainly, but not exclusively, in retrograde shear zones. These monazites may be new grains or rim the older ones. All monazites corrode/alter when positioned in Bt-Chl-Ms-Ab assemblages. Monazites included in Qtz, Fsp or Grt may retain older ages, some older than 1720 Ma and which may be due to older metamorphic rocks. The spread of ages from 1720-1600 Ma is basically similar to that seen in the zircon-geochronology from the area [2]. This spread supports a thermo-tectonic origin [3] rather than one also involving deposition [4].

The geology, SHRIMP U–Pb geochronology and metallogenic significance of the Ankisatra-Besakay District, Andriamena belt, northern Madagascar

The Ankisatra-Besakay District (A-BD), located about 200 km north of Antananarivo and 75 km east of Maevatanana in central-northern Madagascar, hosts two historical mines, the Ankisatra Pb–Zn ± Au and Besakay Pb–Ag mines. These shear-hosted en echelon quartz veins at Besakay and deformed tensional quartz veins at Ankisatra produced a total of 4446 t of lead and 156 t of zinc in the early 1940s. In addition, there is Pb–Zn–Cu mineralisation in both quartz-feldspar leucosome veins/bands and metasomatised granulite-facies mafic orthogneiss, Cu–Zn and associated Fe–Mn mineralisation in magnetite–pyrite enderbite breccia, and Cu–Zn mineralisation in retrograde shear zones in granulite-facies paragneiss in the A-BD. The country rocks in the A-BD consist of amphibolite to granulite-facies mafic and granitoid orthogneisses and paragneisses with horizons of silicate-facies BIF. The paragneissic rocks in the district tectonically overlie the biotite–granitoid hornfels and sub-volcanic mangeritic rocks, which are separated from amphibolite-facies alkali-feldspar granitoid and enderbitic rocks by a major structure. The A-BD is structurally characterised by: (1) E–W-trending tensional fractures, quartz veins and dolerite dykes; (2) buckling-related axial-planar fractures; and (3) N–S, NE–SW and NNE–SSW trending dextral strike-slip shear zones, dolerite sills and quartz veins in transpressional extensional zones. Uranium–Pb SHRIMP II geochronology of zircon constrains the peak of magmatic, metamorphic, deformational and metasomatic events in the A-BD. An important constraint is whether hosting terranes contain signatures of the ca. 1690–1590 Ma critical age window for world-class BHT Pb–Zn–Ag deposits elsewhere in the world. At least two magmatic events are recorded from the A-BD. An early magmatic event is recorded by a 2725 ± 12 Ma single xenocrystic magmatic zircon in the >2676 ± 6 Ma precursor to the granulite-facies mafic orthogneiss. A ca. 2503–2460 Ma event is recorded by a 2465 ± 6 Ma minimum age of magmatism for the precursor to metasomatised granulite-facies mafic orthogneiss and 2483 ± 20 Ma for the precursor to biotite–granitoid hornfels. Zircons extracted from both the metasomatised and unaltered granulite-facies mafic orthogneisses record peak metamorphic ages of 2465 ± 12 and 2390 ± 10 Ma, probably representing compressional deformation, partial melting, and associated local magmatic events within the ca. 2475–2380 Ma period. Inherited zircons from the quartzo-feldspathic granulite-facies paragneisses return ages of protolithic supracrustal rocks ranging from ca. 2870 to 1700 Ma. A widespread period of rifting, anatectic magmatism, basic-ultrabasic and mangerite magmatism, and related granulite-facies metamorphism occurred between ca. 820 and 780 Ma. The possible exhalative units (silicate-facies BIF and metasomatic, garnet–quartz–plagioclase rock) are of late-Archaean to early-Palaeoproterozoic, rather than Mesoproterozoic age. The terrane lacks the critical evolution age window of ca. 1770–1550 Ma, characteristic of well-documented BHT Provinces in the Broken Hill Block and Mt. Isa Block, Australia and ca. 1959–1135 Ma from the Bushmanland Ore District, South Africa. This suggests that either such an event did not occur in the crust now forming the A-BD or that the equivalent supracrustal rocks containing these age signatures were eroded during Proterozoic times. It is less likely that the intense 780–820 Ma event destroyed evidence for their prior existence. The galenas from the Ankisatra and Besakay deposits have signatures characteristic of small-scale mineralised systems which derived most of their lead from local crustal rocks older than ca. >2.7 Ga. They are thus atypical of BHT deposits and associated vein-style mineralisation from well-endowed terranes. It is concluded that there are neither direct signs nor indirect temporal signals of giant stratiform/stratabound BHT Pb–Zn–Ag mineralisation, nor clear evidence for the presence of characteristic transitional sequences and alteration styles associated with BHT mineralisation in the A-BD, thus downgrading its prospectivity.

Ordovician metamorphism and plutonism in the Sierra de Quilmes metamorphic complex: Implications for the tectonic setting of the northern Sierras Pampeanas (NW Argentina)

Towards unravelling the geodynamic setting of the northern Sierras Pampeanas (NW Argentina) we describe the tectono-metamorphic and geochronologic evolution of sub-greenschist to granulite facies metamorphic sediments, granitoid plutons, and pegmatites in the Ordovician Sierra de Quilmes metamorphic complex. The protoliths of the metasediments are represented by a sequence of turbidites and minor calcsilicate rocks of the Neoproterozoic to Cambrian Puncoviscana Formation. The metamorphic complex consists of four zones including the (1) chlorite, (2) biotite–muscovite, (3) garnet–cordierite–sillimanite, and (4) orthopyroxene zones. Zones (3) and (4) show an increasing degree of anatexis, reaching large-scale diatexis in the orthopyroxene zone at P– T conditions exceeding ∼800 °C and 600 MPa. At, or shortly after the metamorphic peak, the granitic to tonalitic Cafayate pluton intruded approximately along the boundary between anatectic and non-anatectic rocks. Retrograde near-isobaric cooling of the middle crust was accompanied by non-penetrative ductile shearing at granulite to amphibolite facies P– T conditions. Evidence for significant prograde deformation is absent in the Sierra de Quilmes metamorphic complex. Monazite and titanite U–Pb isotopic data constrain the metamorphic peak in migmatites and calcsilicate rocks to be at or slightly prior to ∼470 Ma. Retrograde amphibolite facies mineral reactions led to continuous formation of monazite and titanite during slow cooling between ∼470 Ma and 455 Ma (U–Pb data). The composite Cafayate pluton intruded over a time interval of several million years between ∼477 Ma (Sm–Nd isochron) and ∼460 Ma (monazite and titanite U–Pb isochron), followed by pegmatites. A younger group of pegmatites was emplaced in the country rocks at the end of the Ordovician (∼440 Ma, Rb–Sr mineral isochrons), postdating most of the retrograde shear zones. Resetting of the muscovite K–Ar and 40Ar– 39Ar system in weakly deformed pegmatites and crystallisation of new muscovite in low-grade metamorphic sediments at ∼400–416 Ma is attributed to minor late greenschist and sub-greenschist facies metamorphism and deformation. The massive Early Ordovician heat transfer, the absence of major prograde deformation, and subsequent, prolonged phases of non-penetrative deformation in the high-grade metamorphic zones during slow near-isobaric cooling contradict crustal thickening and continent collision models, as have been proposed for the southern Sierras Pampeanas. We suggest continuous or stepwise extensional tectonics in a back-arc or a mobile belt tectonic environment for the Ordovician Sierra de Quilmes, and the northern Sierras Pampeanas in general. An extensional setting of the northern Sierras Pampeanas in the Ordovician is in agreement with the coeval formation of marine extensional sediment basins in vicinity of the Sierras Pampeanas in northwestern Argentina and southern Bolivia.

Timing of subduction and exhumation along the Cambrian East Gondwana margin, and the formation of Paleozoic backarc basins

Research Article| January 01, 2005 Timing of subduction and exhumation along the Cambrian East Gondwana margin, and the formation of Paleozoic backarc basins David A. Foster; David A. Foster 1Department of Geological Sciences, PO Box 112120, University of Florida, Gainesville, Florida 32611, USA Search for other works by this author on: GSW Google Scholar David R. Gray; David R. Gray 2School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia Search for other works by this author on: GSW Google Scholar Catherine Spaggiari Catherine Spaggiari 3Department of Applied Geology, Curtin University of Technology, Perth, Western Australia 6845, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2005) 117 (1-2): 105–116. https://doi.org/10.1130/B25481.1 Article history received: 08 Sep 2003 rev-recd: 10 Jun 2004 accepted: 15 Jun 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David A. Foster, David R. Gray, Catherine Spaggiari; Timing of subduction and exhumation along the Cambrian East Gondwana margin, and the formation of Paleozoic backarc basins. GSA Bulletin 2005;; 117 (1-2): 105–116. doi: https://doi.org/10.1130/B25481.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The inversion of the Neoproterozoic-Cambrian passive margin of East Gondwana occurred during the early Paleozoic Delamerian-Ross orogeny. We present 40Ar/39Ar and structural data from deformed and metamorphosed Neoproterozoic clastic rocks beneath the Tasmanian ophiolite and the footwall of a high-pressure metamorphic complex in northern Tasmania. These data reveal the timing of accretionary deformation and the initiation of backarc extension along the Australian margin of Gondwana. 40Ar/39Ar analyses of muscovite from lower greenschist facies fault slices bounding the Forth metamorphic complex give plateau ages of 521.4 ± 2.5 and 520.7 ± 1.6 Ma. These data suggest that deformation within an accretionary prism off the margin of Tasmania, and possibly ocean arc collision, were under way by ca. 521 Ma. Muscovite from upper amphibolite and upper greenschist facies rocks in five locations of the Forth metamorphic complex, including retrograde shear zones, give 40Ar/39Ar cooling ages that average 508.1 ± 2.6 Ma. Identical muscovite cooling ages from rocks originally at very different metamorphic P-T conditions suggest rapid cooling of the Forth complex at ca. 508 Ma, due to the juxtaposition of higher-grade against lower-grade rocks. Rapid cooling is also indicated by concordant 40Ar/39Ar ages of hornblende and muscovite in the high-grade core. Cooling was probably due to rapid exhumation along extensional shear zones within a regional extensional setting that also produced the Mount Read–Mount Stavely volcanic complexes (505–495 Ma) along with rift basins in Tasmania and southeast Australia. This continental rift magmatism and extension were caused by west-dipping subduction under the Australian margin of Gondwana after the collisional phase of the Delamerian-Ross orogeny. Rollback of subduction in the Australian sector of the margin between ca. 508 and 460 Ma produced a backarc basin >1000 km wide that became the basement for the Lachlan orogen turbidites. Similar amounts of subduction rollback seem not to have occurred in Antarctica at this time (unless the record is lost), suggesting significant along-strike differences in the early Paleozoic geodynamics of the Delamerian-Ross orogenic system. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrology and lithogeochemistry of the mineralized tanzanite-grossular bearings rocks in the Merelani-Lelatema area, northeastern Tanzania

The Merelani-Lelatema area, which is occupied by late Proterozoic metasedimentary rocks, lies along the Lelatema fault zone. The rocks are mainly pelitic and semi-pelitic gneisses, psammitic gneisses, hydrothermally altered rocks, crystalline dolostones, pegmatites and quartz veins. The occurrence of gem-quality tanzanite and grossular has been found to occur mainly in boudinaged pegmatitic veins and hydrothermal fracture fillings. These fracture fillings occur in brecciated and hydrothermally altered graphite-bearing gneiss in a mineral association containing glass-clear quartz, diopside, zoisite, graphite and calcite. The psammitic gneisses are more siliceous and enriched in feldspars. They are grouped into graphite-kyanite gneiss with thin interlayers of quartz-feldspar fels. The pelitic and semi-pelitic gneisses are aluminium-rich and thus have been grouped into garnet-biotite-sillimanite and/or kyanite graphite gneisses. The hydrothermally altered rocks are strongly deformed and contain varying amounts of clay minerals, which characteristically show distinct banded layers due to alterations of minerals. The rocks have been folded and the fold structures have undergone three deformation episodes (D1, D2 and D3) D1 and D2 were accompanied by high-grade metamorphism whereas D3 was associated with retrograde metamorphism and the formation of retrograde shear zones. Evidence from metamorphosed graded bedding confirms the existence of the antiform and also shows that the macroscopic F2 folds are superimposed on a previously inverted stratigraphic succession. This inversion is the consequence of D1 deformation. The polyphase nature of these rocks with which pegmatites and quartz veins occur is shown by rotation of boudins, presence of overturned tight isoclinal folds resembling F1, presence of F2 folds and refolded folds shown by quartz veins. Features like boudins shown by quartz veins and pegmatites at the limbs and along the axial planes of F1 folds with S1 development in the host rocks and disruption of limbs of tight folds resembling F1, shown by these veins also point to significant amount of flattening during F1 folding phase and longitudinal stretching of the late stage of their development. Slippage in the later stages of deformation phase (D1) is also distinct in displaced veins. Lithogeochemical studies reveal that the gneissses originated from miogeosynclinal sediments composed of sand; organic matter rich in V, U and fine Al-rich materials. These underwent advanced weathering, deposited in a lake and during metamorphism and after the Pan-African tectonothermal event, the silicates were altered, trace elements released and migrated with hydrothermal solutions rich in Ca, Mg, CO2 and SO3. Finally these elements together with V, U, Sr, Zn and heavy REE elements were deposited in the hydrothermal zone. Emplacement of gemstones (tanzanite and green grossular) in the area is structurally controlled along the Lelatema fault system. Tanz. J. Sci. Vol.29(2) 2003: 55-70

Mapping Mineralogical and structural relationships with satellite-borne ASTER and airborne geophysics at Broken Hill

The recent availability of 14 band multi-spectral and 15 metre DEM data from the satellite borne ASTER (Advanced Spaceborne Thermal Emission Reflection Radiometer) instrument has introduced new possibilities for geological mapping and integration with traditional geophysical data sets. Calibrated radiance at the sensor data and atmospheric corrected ASTER products, such as surface reflectance and emissivity imagery, have been made available by NASA/USGS and the Japanese agency, ERSDAC. ASTER radiance data measures five bands at 30 metre resolution, within the short-wave infrared 2.1 to 2.5 mm wavelength region, and also five bands at 90 metre resolution within the 8 to 12 mm thermal infrared region. This compares with only one band as measured by Landsat TM for each of these regions. An investigation into the capabilities of ASTER and strategies for its integration with geophysical data (using ER Mapper) was undertaken at Broken Hill, given its high spatial geological control and extensive airborne geophysical data sets. Mineral (group) maps derived from multi-scene ASTER data, collected over the Broken Hill Block, proved useful for discriminating stratigraphic units, regolith and areas of alteration. In particular ASTER imagery highlighted several sericite-rich as well as quartz-rich colluvial regolith. Variations in ASTER?s derived AlOH mineral spectral signatures correlated with higher potassium radiometric responses and indicated a change in the muscovite chemistry, possibly due to retrograde metamorphic alteration. However structural features associated with retrograde shear zones were identified most clearly using aeromagnetics. Overall ASTER data products provided complementary mineralogical information to the structural interpretation afforded by geophysical data sets.

Gneiss-cored interference dome associated with two phases of late Pan-African thrusting in the Central Eastern Desert, Egypt

Structural analysis of the Wadi Um Had area in the Central Eastern Desert (CED) of Egypt reveals the presence of another NW-trending elliptical gneissic and migmatitic domal structure west of Gabal Meatiq, outlined by a thick schistose and mylonitized carapace. The sheared carapace formed during a NW/NNW-ward thrusting event which emplaced thrust slices of ophiolitic mélange, Dokhan Volcanics and Hammamat Group sediments over the higher grade gneisses. The gneisses are subdivided by retrograde shear zones of this event. The sheared carapace is interpreted as the roof thrust of an imbricate stacked antiformal duplex. A second thrusting and folding event associated with NE-SW shortening deformed the earlier structures producing the dome and re-thrusting low-grade units against it as SW- and NE-dipping thrusts. The features of this gneiss dome are inconsistent with it being a metamorphic core complex. Caution must be exercised in the recognition of transcurrent shears in the Eastern Desert since folded low-angle thrust-related mylonite zones may be steepened by folding, producing steep shear zones with low-pitching slip lineations. Extension, bi-directional thrusting and Najd faulting in the CED may be explained in an escape tectonic deformation regime involving extrusion of material away from the pivotal collision zone in the Mozambique belt, to the south.

A moderate exhumation rate for the high‐pressure Maksyutov Complex, southern Urals, Russia

AbstractThe high‐pressure Maksyutov Complex in the southern Urals contains a regionally coherent crustal unit that experienced peak metamorphic conditions of ∼2 GPa (∼60 km) and ∼550°C at 378 ± 6 Ma. Exhumation of the high‐pressure unit occurred under retrograde blueschist and middle/lower greenschist facies conditions, indicating a pronounced cooling during decompression. The rise of the high‐pressure unit to a mid‐crustal depth (∼20 km) was accommodated along retrograde ductile shear zones that formed in quartz‐rich lithologies. Rb‐Sr white mica ages from these quartzitic mylonites cluster at ∼360 Ma, while Ar‐Ar ages are more variable, between 370 ± 7 and 344 ± 7 Ma. We interpret the Rb‐Sr ages to date the mylonitic deformation in the retrograde shear zones and infer that by 360 ± 8 Ma the high‐pressure Maksyutov Complex has reached mid‐crustal depth. The pressure–temperature–time data suggest an average exhumation rate of 2 (+2/−0.5) mm a−1 for the first exhumation stage from 60 to 20 km. Using our Rb‐Sr data and published fission‐track data, a slower exhumation rate of 0.3 ( ± 0.1) mm a−1 is inferred for the rise of the high‐pressure rocks from the middle to the upper crust. The moderate exhumation rate of 2 mm a−1 is in accordance with published thermal models, which demonstrate that a relatively slow exhumation is required for high‐pressure rocks to experience syn‐decompressional cooling. Such syn‐decompressional cooling, as inferred for the Maksyutov Complex, contrasts with isothermal decompression observed in many other regional high‐pressure terrains that experienced a relatively fast exhumation. The two main factors that controlled the unusual thermal evolution of the southern Urals may be the low radiogenic heat production of the crust and the stable, ∼200 km thick lithospheric mantle beneath the orogen. Copyright © 2000 John Wiley & Sons, Ltd.

The role of fluids in retrograde shearing: Broken Hill, Australia

Shear zones at Broken Hill, Australia, are highly deformed structures that have provided a focus for migrating aqueous fluids, resulting in the widespread retrogression of rocks within and adjacent to their boundaries. Shearing probably occurred soon after regional metamorphism at greenschist to lower amphibolite facies. Lowering of δ18O values in the range of 10–12‰ in metasedimentary host rocks to 7–8‰ in sheared metasediments suggests that isotopic exchange occurred due to pervasive fluid flow. Time integrated fluid fluxes of 1–7×106m3/m2 and calculated metasomatism suggest that retrograde shear zones were enriched in silica and were subjected to a high degree of fluid flow. This is consistent with petrographic observations showing the crystallisation of abundant quartz and muscovite.

Regional structure and distribution of magnetite: implications for the interpretation of aeromagnetic data in the Broken Hill region, New South Wales

Magnetic anomalies in the Palaeoproterozoic Willyama Supergroup in the Broken Hill region have a number of different sources, including stratiform magnetite, structurally controlled magnetite and magnetite in igneous rocks. Magnetic sedimentary units are relatively uncommon; most metasedimentary rocks are chemically reduced and host few stratiform anomalies. Exceptions include units within the Paragon Group and volumetrically minor banded iron formation. Structurally controlled anomalies are common across the region and are caused by magnetite in hightemperature shear zones, tectonic fabrics, or in high-strain zones developed along the contact between rocks of contrasting competency. Magnetite formation occurred more than once during the deformational history, during both high-grade and lower-grade metamorphism. Many of the observed linear anomalies coincide with the regional S3 fabric, which formed during upper greenschist to amphibolite-facies metamorphism. Retrograde shear zones are generally magnetite-destructive and form linear zones of low magnetic intensity. Amphibolite has a generally low magnetic susceptibility, although certain units have a high susceptibility, possibly due to alteration as a result of fluid flow. The abundance of structurally controlled anomalies and the relative paucity of stratigraphic anomalies hamper the detailed extrapolation of lithological units beneath cover and means that a ‘magnetic stratigraphy’ is difficult to construct for the Willyama Supergroup. Rather, aeromagnetic data are more useful in the delineation of high-strain zones that have acted as channels for fluid flow, resulting in the formation or destruction of magnetite. Tracing these zones of fluid flow has potential benefits for the reconstruction of mineralising systems and magnetite formed in these zones may have acted as a chemical trap for metals such as gold.

Multistage Extension and Age Dating of the Xiaoqinling Metamorphic Core Complex, Central China

Abstract There are two extensional systems in the Xiaoqinling metamorphic core complex (XMCC). One is the detachment fault system developed along the peripheries of the XMCC, which extended in an ESE‐WNW direction and whose upper plate moved towards the WNW. The other extensional system includes the retrograde shear zones and normal faults developed within the XMCC, which represent the collapse of the XMCC. Ar‐Ar and K‐Ar dating shows that the extension of the detachment fault system continued from 135 to 123 Ma, i.e. in the late stage of its evolution at about 127 Ma. The collapse represented by the extensional system within the XMCC was operative during 120–106 Ma, and its main activity occurred about 116 Ma ago. These suggest that the XMCC experienced two extensional stages in its evolution, i.e., the syn‐orogenic regional extension and post‐orogenic collapse extension.

Chemical changes associated with deformation of granites under greenschist facies conditions: the example of the Zawidów Granodiorite (SE Lusatian Granodiorite Complex, Poland)

Small-scale, subvertical, retrograde shear zones are developed in the Zawidów Granodiorite, located in the eastern part of the Lusatian Massif, southwest Poland. Heterogeneous deformation, under conditions of greenschist-facies metamorphism, has produced mylonite zones that show a broad range of microstructural development. Three fabric types in the granodiorite deformation have been distinguished: (A) weakly deformed protolith; (B) moderately deformed in lens-shaped domains; (C) strongly foliated. In fabric types B and C whole-rock analyses show a decrease in CaO, MnO, Cr, Sr, Pb, whereas Na 2O and SiO 2 increase slightly overall. Al 2O 3, TiO 2 and Zr remain essentially constant. This pattern of element variation is consistent with the observed feldspar breakdown and increase of sericite and also with the transformation of biotite into chlorite. Mass-balance calculations, based on major- and trace-element geochemistry, indicate constant volume or less than 10% volume loss. From this study it is apparent that a low-grade metamorphic deformation event does not have to have a profound influence on whole-rock geochemical composition.

Eclogites from the Czech part of the Erzgebirge: multi-stage metamorphic and structural evolution

A structural and petrological study of Saxothuringian eclogites in the central Erzgebirge, Czech Republic, revealed their polyphase thermal and tectonic evolution. The first stage of deformation is related to eclogite-facies metamorphism of mafic rocks at pressures of 26 kbar and temperatures of 650–700°C. The clinopyroxene crystallographic preferred orientation is consistent with high-temperature flow. The second stage of deformation is characterized by transport of eclogitic rocks at the base of a crustal nappe and their large-scale folding. During this stage, HP–HT metamorphic assemblages developed in the surrounding metapelites (14.5 kbar/600°C) while eclogite assemblages were preserved in passively transported eclogitic boudins. Heat advection and similar P–T conditions in the surrounding crustal material are responsible for the stability of mineral assemblages and preclude closure of isotopic systems in the eclogites. Therefore the published isotopic ages of the eclogites do not reflect the subduction stage but the age of cooling under crustal conditions. Subsequent large-scale (D3) folding is responsible for reorientation of previously flat-lying earlier fabrics (D1–D2). The final stage of deformation is marked by the development of a heterogeneous network of ductile retrograde shear zones. The thermometry and geochemical study of amphibolitized eclogites in the cores of shear zones indicates thermal conditions not exceeding those in surrounding metapelites and fluid influx. The fluid influx is interpreted to be the result of water-producing prograde reactions in the footwall metasediments and fluid chanelling into the base of the nappe during brittle deformation of the eclogites.