D1 Structures Research Articles

Inferred thrust imbrication, deformation gradients and the Lachlan Transverse Zone in the Eastern Belt of the Lachlan Orogen, New South Wales

Re‐examination of the Ordovician geology between Mandurama and Bigga in the Lachlan Orogen of central western New South Wales has produced new interpretations of the stratigraphy and structural geology. The Abercrombie beds have been previously inferred to comprise an Ordovician turbidite package with interbedded black shale bands. Although hampered by a paucity of fossil ages, new data suggest that the Ordovician geology of this region instead represents an imbricate stack of Lower Ordovician turbidites (Adaminaby Group) and Upper Ordovician black shales (Warbisco Shale). Structural data from the north of this region suggest that duplication occurred in a D1 event (with formation of broadly east‐west to west‐northwest‐trending thrust slices or fold limbs) and was accompanied by formation of cleavage and isoclinal folds. Thrusting of the Adaminaby Group and Warbisco Shale over or under the Lower Ordovician Coombing Formation (southern part of the Molong volcanic belt) also occurred at this time. East‐vergent imbrication and thrusting and formation of a regional near‐meridional steeply west‐dipping cleavage occurred in the D2 event, when D1 thrusts or folds were folded around overturned (east‐vergent) D2 folds. These new data also suggest that there is a north‐to‐south gradient in the intensity of the D2 deformation, with D2 effects decreasing from south to north approaching the Lachlan Transverse Zone. Such a gradient mirrors similar but more subtle local changes from the north. Together, they imply that the Lachlan Transverse Zone was a major zone of weakness during north‐south shortening that resulted in the formation of D1 structures but was relatively rigid in local areas during the regional D2 deformation that resulted from east‐west shortening when it formed a major tear/accommodation zone. This D2 rigidity may be caused by strength imparted by the earlier emplacement of large (variably mineralised) intrusive/volcanic complexes along the transverse zone.

Evidence for the Pathway of Metal Triangle Rotation in Solid M3(CO)12: Variable-Temperature X-ray Structures of Fe2Ru(CO)12 and FeRu2(CO)12

Fe2Ru(CO)12 has the Fe3(CO)12 structure, while FeRu2(CO)12 has an all-terminal D3 structure: the metal disorder in FeRu2(CO)12 at high temperatures provides direct evidence for the pathway of metal triangle rotation in the solid state.

Triterpene saponins from Abrus cantoniensis (Leguminosae). II. Characterization of six new saponins having a branched-chain sugar.

The chemical structures of abrisaponins So1 (1), So2 (2), D2 (3), D3 (4), F (5) and SB (6), six of twenty-three saponins from Abri Herba, the whole plants of Abrus cantoniensis (Leguminosae), were investigated. They were elucidated to be 3-O-alpha-L-rhamnopyranosyl-(1 --> 2)-[O-beta-D-glucopyranosyl-(1-->3)]-beta-D-galactopyranosyl-(1 --> 2)-beta-D-glucuronopyranosyl [sequence: see text] (designated as beta-abritetraosyl) sophoradiol (1), 3-O-beta-abritetraosyl sophoradiol 22-O-beta-D-xylopyranoside (2), 3-O-beta-abritetraosyl abrisapogenol D (3), O-beta-abritetraosyl abrisapogenol D 22-O-beta-D-glucopyranoside (4), 3-O-beta-abritetraosyl abrisapogenol F (5) and 3-O-beta-abritetraosyl soyasapogenol B (6), respectively.

Structural features of granulites from Rayagada, Eastern Ghats, India: some preliminary observations.

A suite of granulates including metasedimentary rocks (khondalite, quartzite and calc-silicate granulite), mafic granulite, orthopyroxene granulite, and garnetiferous granite gneisses (leptynite, sillimanite granite and porphyritic granite) exposes in the Rayagada area, north-central part of Eastern Ghats Granulite Belt of Peninsular India. Among these, important marker horizons like calc-silicate granulate and quartzite have been used to establish the sequence and geometry of folds on a regional scale. Signatures of three phases of deformations (D1, D2, D3 from the earliest to the latest) are present in metasedimentary rocks and mafic granulite, whereas most of the massive orthopyroxene granulite, leptynite and sillimanite granite exhibit foliation corresponding to the second deformation event. These three deformations correspond to three folding episodes: F1 (intrafolial fold), F2 (tight to isoclinal fold) and F3 (open fold) respectively. Axial surfaces of F1 and F2 are mostly parallel, trending NNE-SSW and F3 axial plane is at right angle to that of F1 and F2 folds. Corresponding axial plane foliations are gneissic banding in khondalite and mafic granulate (S1), a pervasive foliation in granite gneiss (S2) and a sporadic fracture cleavage in all the rock-types (S3). In most cases, structures of first deformation (D1) were superimposed by the second one (D2), resulting D2 structures (both folding and foliation) as the dominantly preserved deformation structure. D1 deformation and D2 deformation have been inferred to be related to the E-W or ESE-WNW directed compression and progressive simple shear and D3 deformation might be related to the strike-slip displacement and NNE-SSW transtension.

Late Archean and Early Proterozoic gold-tungsten mineralization in the Zimbabwe Archean craton: Rb-Sr and Sm-Nd isotope constraints

The Sm-Nd technique has been successfully used to date scheelites from lode gold and tungsten deposits in the Archean Midlands greenstone belt, Zimbabwe. Two phases of mineralization have been established which correlate with Late Archean and Early Proterozoic magmatic events in the region. The gold deposits are mainly structurally controlled and are spatially associated with high strain zones and internal synvolcanic to late-tectonic tonalite plutons. Most gold-bearing structures can be categorized into one of two preferred orientations: parallel to acutely oblique with the main S1 fabric, or crosscutting S1 at a high angle and striking parallel to locally developed D2 structures or fabrics. Sm-Nd dating of scheelite demonstrates that these two structural populations are characterized by discrete phases of gold mineralization. The main S1-parallel gold deposition, dated at 2.66 ± 0.05 Ga, correlates with the Late Archean trondhjemite-tonalite-granodiorite event in Zimbabwe. The S1 discordant phase has yielded an age of 2.41 ± 0.07 Ga, broadly corresponding to the timing of emplacement of the Great Dyke and reactivation of the Late Archean high-strain zones. Sr isotope data provide important constraints on the evolution of the hydrothermal fluids.

The Structure of Hexamethyltungsten, W(CH3)6: Distorted Trigonal Prismatic with C3 Symmetry

Ab initio and density functional calculations show that the equilibrium structure of hexamethyltungsten is a distorted trigonal prism of C3 symmetry (with local C3v symmetry for the WC6 skeleton). A regular prismatic D3 structure (with D3h skeleton) is found to be ca. 20 kJ mol-1 higher in energy at correlated levels of theory. It is a transition state connecting two C3 minima. These results extend a recent gas-phase electron diffraction study which favored a regular prismatic structure but could not rule out a distortion to C3v. The failure of a previous theoretical study to locate the distorted minimum is due to the neglect of electron correlation and to some other restrictions during the structure optimizations. Correlation is important, e.g. for the description of hyperconjugative “agostic” C−H → W interactions which are found to be pronounced in W(CH3)6. Structures optimized with gradient-corrected or hybrid density functionals, or at the MP2 level, describe these interactions well. The observed single-line 13C and 1H NMR spectra are explained by dynamic motions due to the low D3 inversion and methyl rotation barriers. 13C chemical shifts calculated using density functional theory differ by ca. 18 ppm between the two nonequivalent sets of methyl groups in the distorted trigonal prismatic structure. Low-temperature NMR experiments could be useful to confirm this value and thus the distortion. Harmonic vibrational frequency analyses are consistent with experimental results and have been used to characterize stationary points on the potential energy surface. Differences between the structural preferences of W(CH3)6 and WH6 are investigated via detailed bonding analyses.

Crustal evolution of the East Antarctic mobile belt in Prydz Bay: continental collision at 500 Ma?

The East Antarctic mobile belt as exposed in Prydz Bay presents an excellent example of a poly-metamorphic terrain where complicated high-grade structures can be grouped on the basis of lineation direction, sense of shear and metamorphic grade. In this way local intricacies resulting from truncating and interfering foliations and folds can be simplified by ordering structures with respect to their kinematic connotation. The resulting deformation scheme is simple and facilitates regional correlations, and simultaneously places metamorphic textures in a kinematic—structural background. The mobile belt in Prydz Bay is exposed along 200 km of coastline, and consists of granulite-facies gneiss in which low- to mid-crustal, compressional D1–2 structures, and mid- to upper-crustal, extensional D3–6 structures and relateddecompression-cooling textures are best explained in an exhumation model involving extensional collapse of upper crust in an overall compressional tectonic setting. This implies that compression and extension structures are genetically related. However, D3–6 structures in the Larsemann Hills appear to have formed between 550 and 500 Ma, whilst D2 structures in the Rauer Group have been dated at 1100–1000 Ma. Reinterpretation of the latter dates is possible, implying that most high-grade deformation in Prydz Bay is early Palaeozoic.

Two phase syntectonic gold mineralization and barite remobilization within the main ore body of the Golden Giant mine, Hemlo, Ontario, Canada

Structural analysis of the central part of the Hemlo gold deposit in the Hemlo-Heron Bay greenstone belt has revealed four generations of deformation structures (D1–D4). The second (D2) and fourth (D4) deformations significantly influenced the orebody and controlled the localization of gold mineralization. Development of D3 dextral shearing during retrogressive conditions resulted in strain partitioning around the feldspathic orebody, such that this deposit appears to be controlled by the Hemlo Shear Zone. However, the orebody dominantly preserves vertical D2 parasitic folds rather than D3 structures. The earliest occurrence of gold is with molybdenite as S1 inclusion trails in vanadium-muscovite grains that grew during D2. This suggests that the gold deposit was emplaced by at least early D2. Syngenetic banded barite was partially dissolved, transported and redeposited during each deformation event, and in particular in association with brittle deformation that occurred during D4. Microstructural relationships indicate two distinct generations of gold: the first generation is associated with molybdenite and potassic alteration and was deposited before or early in D2 during progressive amphibolite-facies metamorphism. The second generation formed by redistribution and concentration of gold, together with stibnite in ductile-brittle D4 structures after peak metamorphic conditions.

Orogenesis at the southern tip of the Americas: the structural evolution of the Cordillera Darwin metamorphic complex, southernmost Chile

New, detailed lithologic and structural data are presented from three separately mapped areas along the southern boundary of the Cordillera Darwin metamorphic complex of southernmost Chile. Cordillera Darwin is a unique uplift because it exposes the highest grade rocks in the Andes south of Peru and averages 1 km higher in elevation than adjacent areas. The structural data indicate that Cordillera Darwin experienced mid-Late Cretaceous trans-pressional deformation with a partitioned strike-slip component localized along the Beagle Channel that forms the southern boundary to the range. Foliation, lineation and fold axis trends indicate NE-SW-directed contraction and NW-SE strike-slip shearing (present directions) during progressive D1 D2 Andean deformation. D2 deformation is marked by outcrop-to 10 km-scale south-southwest-vergent folds. Late Cretaceous-Tertiary brittle-ductile and brittle left-lateral strike-slip faults and shear zones crosscut all D1 D2 structures. Although limited structural evidence for extensional tectonics was documented in this study, apparent normal offsets across both arms of the Beagle Channel and previously documented field evidence for extension from other areas in Cordillera Darwin suggest that transtensional displacements also may have occurred in southern Cordillera Darwin during the Late Cretaceous-Early Tertiary. Cordillera Darwin's position within the evolving Patagonian Orocline adjacent to an evolving Mesozoic-Cenozoic left-lateral transform boundary between the South American and Antarctic plates, and later the South American and Scotia plates, necessitates consideration of the possible effects of regional counterclockwise rotation on development of structures. Regional counterclockwise rotation of Cordillera Darwin may have controlled the temporal and spatial transition of deformational regimes within Cordillera Darwin. Exhumation of the metamorphic core of Cordillera Darwin during the Late Cretaceous-Early Tertiary is interpreted to have been due to rapid uplift and normal rates of erosion. Rapid uplift may have been due to migration of contraction to northern Cordillera Darwin and crustal relaxation in southern Cordillera Darwin as contractional structures rotated into parallelism with σ 1 and either ceased to be active or were reactivated as strike-slip or oblique-slip faults. Rapid isostatic rebound of Cordillera Darwin may have also been a response to aborted continental subduction of southern Cordillera Darwin and detachment of a subducted mafic root during mid-Cretaceous closure of the Rocas Verdes marginal basin.

Compressional and extensional tectonics in low-medium pressure granulites from the Larsemann Hills, East Antarctica

AbstractMeta-sediments in the Larsemann Hills that preserve a coherent stratigraphy, form a cover sequence deposited upon basement of mafic–felsic granulite. Their outcrop pattern defines a 10 kilometre wide east–west trending synclinal trough structure in which basement–cover contacts differ in the north and the south, suggesting tectonic interleaving during a prograde, D1 thickening event. Subsequent conditions reached low-medium pressure granulite grade, and structures can be divided into two groups, D2 and D3, each defined by a unique lineation direction and shear sense. D2 structures which are associated with the dominant gneissic foliation in much of the Larsemann Hills, contain a moderately east-plunging lineation indicative of west-directed thrusting. D2 comprises a colinear fold sequence that evolved from early intrafolial folds to late upright folds. D3 structures are associated with a high-strain zone, to the south of the Larsemann Hills, where S3 is the dominant gneissic layering and folds sequences resemble D2 folding. Outside the D3 high-strain zone occurs a low-strain D3 window, preserving low-strain D3 structures (minor shear bands and upright folds) that partly re-orient D2 structures. All structures are truncated by a series of planar pegmatites and parallel D4 mylonite zones, recording extensional dextral displacements.D2 assemblages include coexisting garnet–orthopyroxene pairs recording peak conditions of ∼ 7 kbar and ∼ 780°C. Subsequent retrograde decompression textures partly evolved during both D2 and D3 when conditions of ∼ 4–5 kbar and ∼ 750°C were attained. This is followed by D4 shear zones which formed around 3 kbar and ∼ 550°C.It is tempting to combine D2–4 structures in one tectonic cycle involving prograde thrusting and thickening followed by retrograde extension and uplift. The available geochronological data, however, present a number of interpretations. For example, D2 was possibly associated with a clockwise P–T path at medium pressures around ∼ 1000 Ma, by correlation with similar structures developed in the Rauer Group, whilst D3 and D4 events occurred in response to extension and heating at low pressures at ∼ 550 Ma, associated with the emplacement of numerous granitoid bodies. Thus, decompression textures typical for the Larsemann Hills granulites maybe the combined effect of two separate events.

U–Pb geochronology of the Opatica tonalite-gneiss belt and its relationship to the Abitibi greenstone belt, Superior Province, Quebec

U–Pb ages for zircon, monazite, and titanite from samples collected from two transects across the Opatica tonalite-gneiss belt in the Superior Province of Quebec indicate that the belt contains rocks that are significantly older than those in the adjacent low-grade northern Abitibi belt. Tonalite and tonalite gneiss, which make up most of the belt, formed over an interval of 100 Ma, from pre-2800 to 2702 Ma. Five samples have ages of ca. 2807 ± 13, 2773 ± 23, [Formula: see text], [Formula: see text], and 2702 ± 3 Ma. Zircon growth at 2718–2721 Ma in the two oldest samples may record an early period of high-grade metamorphism in the belt. Hornblende diorite, monzodiorite, and tonalite plutons were intruded at 2693–2696 Ma, particularly along the southern boundary with the Abitibi belt. These include the Canet pluton at [Formula: see text][Formula: see text] Ma, the Lac Ouescapis pluton at 2693 ± 2 Ma, and the Barlow pluton at 2696 ± 3 Ma. Pink granite plutons and dykes are the youngest intrusive rocks in the belt; three samples have yielded zircon ages of 2690 ± 2 and 2686 ± 4 Ma and a monazite age of 2678 ± 2 Ma. The timing of D1 deformation is bracketed by the age of the youngest gneiss sample containing the D1 structures, at 2702 ± 3 Ma, and the 2690 ± 2 Ma age of a granite dyke that cuts D1 structures. The south-vergent D2 event is recorded in the 2693–2696 Ma plutons and must have occurred synchronously with or after this plutonism. D3 dextral strike-slip movement on the Nottoway River shear zone occurred after 2686 ± 4 Ma, and may be associated with post-regional metamorphic titanite growth at 2672 and 2657 Ma. Titanite ages cluster at 2678–2681 Ma along the Matagami transect and at 2665 Ma north of Chibougamau, and record the time of cooling of the belt below the titanite closure temperature.

Early Cretaceous intra‐arc ductile strain in Triassic‐Jurassic and Cretaceous continental margin arc rocks, Peninsular Ranges, California

The Cuyamaca‐Laguna Mountain shear zone (CLMSZ) lies along the axis of the Peninsular Ranges batholith, separating it into an eastern and western plutonic zones. The shear zone involves Triassic‐Jurassic and Early Cretaceous plutonic units which intruded the Triassic Julian Schist and transects the eastern edge of a cryptic lithospheric boundary, separating oceanic crust on the west from continental crust on the east. The Julian Schist crops out on either side of the cryptic lithospheric boundary and is interpreted to represent an overlap sequence. This structural/stratigraphic relationship indicates that the contrasting lithospheric types must have been juxtaposed prior to approximately the Triassic time, and as a result, the CLMSZ probably developed in an intra‐arc setting. At least two periods of deformation produced the polygenetic CLMSZ. Structures that formed during D1 include S1 and L1. In Triassic‐Jurassic and Early Cretaceous orthogneisses, S1, a pervasive NW striking and NE dipping mylonitic gneissosity, obliterates nearly all traces of an older magmatic fabric. L1 plunges steeply to the NE, lies within the plane of S1, and is locally a well‐developed stretching lineation. D1 structures can be traced from the ∼115 Ma Oriflamme Canyon protomylonite into the adjacent Julian Schist and are represented by a well developed S‐C mylonitic structures indicative of NE–SW contraction. D1 structures in the Oriflamme Canyon protomylonite and in the ∼118 Ma Pine Valley granodiorite developed while these plutons were incompletely solidified. Hence D1 probably occurred between ∼118 and ∼115 Ma and had culminated in the 105 My emplacement of the Las Bancas tonalite. Normal convergence, ∼125 to 115 Ma, between the North American and Farallon plates is coincident with D1 and the syntectonic emplacement of the Pine Valley granodiorite and the Oriflamme Canyon protomylonite. This relationship suggests that the mechanically weak, thermally and melt‐softened cryptic lithospheric interface between oceanic and continental lithosphere may have yielded during the normal convergence event, resulting in the concentration of strain into the CLMSZ during arc magmatism. Such a conclusion underscores the possibility of using intra‐arc structures to deduce convergence patterns, as elegantly argued in several recent papers. A >12‐km long normal sense shear zone transects D1 structures and formed during D2. Mesoscopic structure associated with the normal sense shear zone includes S2, L2, and C2. D2 structures are the record of NE–SW extension between ∼105‐ and ∼94 Ma. They may be related to the vertical loading of the CLMSZ by the hanging wall block of the westward verging Santa Rosa and Borrego Springs mylonite belts or they may represent an early, local response to magmatically and structurally overthickened, gravitationally unstable crust. In the latter interpretation, D2 structures are the harbingers of Tertiary‐aged, gravity‐driven collapse of the SW Cordilleran margin.

Brunswick subduction complex in the Canadian Appalachians: Record of the Late Ordovician to Late Silurian collision between Laurentia and the Gander margin of Avalon

The Brunswick subduction complex in the New Brunswick part of the Canadian Appalachians records the Late Ordovician to Late Silurian collision between Laurentia and the Gander margin of Avalon. The Brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the Quebec reentrant of the Laurentian margin. This part of the margin experienced less underthrusting and exhumation and overprinting by orogen‐parallel faulting than the adjacent promontories where collision started earlier. The early, southeast to east vergent thrust‐related structures represent a progressive D1 deformation that formed in response to northwestward subduction of the previously extended Gander margin and subsequent tectonic unroofing of the subduction complex. The original, shallow northwestward dipping envelope to S1 was deformed in the Late Silurian by D2 upright folds and associated shear zones into a steep belt during terminal collision. The D2 structures probably formed in response to sinistral transpression. Together, D1 and D2 indicate that convergence was oblique and sinistral. Most rocks incorporated in the subduction complex formed in the Tetagouche back arc basin that evolved from rifting of an Arenig magmatic arc built on the Gander margin into a wide marginal basin. Subduction was initiated in the Late Ordovician (≈455 Ma) in the back arc basin following collision of the Middle Ordovician Popelogan arc with Laurentia in the Caradoc, shortly after its opening in the late Arenig (≈473 Ma). The closure history of the Iapetus Ocean involved more than one subduction zone and arc‐continent collision and rivals the southwestern Pacific Ocean in its complexities.

Structural geology across a proposed Archaean terrane boundary in the eastern Yilgarn craton, Western Australia

The Eastern Goldfields Province of the Archaean Yilgarn craton, Western Australia, can be divided into several NW—SE- trending terranes with different stratigraphic signature. At least two main phases of penetrative ductile deformation affect the terranes. D1 is a phase of nappe-type transport, mainly to the north, along gently dipping ductile shear zones. It is interpreted as a phase of regional extension contemporaneous with intrusion of massive granitoid plutons and possibly contemporaneous with deposition of felsic volcanics and metasediments in the greenstone belts. Terrane boundaries were probably formed during D1. D2 gave rise to steep NW—SE- to N—S-trending foliations and upright folds. D2 is strongly partitioned over the area for strain and vorticity. The pattern of partitioning seems to be dependent on the position of granitoid plutons which give rise to development of D2 strain shadows. D1 structures have been preserved mainly in these strain shadows. Terrane boundaries are transected by boundaries of D2 structural domains. Minor intrusion of granitoids continued during and after D2. D1 is possibly associated with arrival of a mantle plume below the Yilgarn craton, and D2 may be a phase of regional shortening in a thinned lithosphere weakened by the thermal event.

Synchronous syntectonic granite emplacement in the south palmer river region, Hodgkinson Province, Australia

Deformation partitioning in pluton wall‐rocks during granite intrusion that is synchronous with regional tectonism potentially creates structures suggesting different timing of emplacement. This is due to variations in style and intensity of fabric development, particularly porphyroblast‐matrix microstructures. In the South Palmer River region, detailed mapping plus microstructural examination of matrix and porphyroblast‐matrix relationships assist correlation of deformation elements across variations in deformation style and intensity. The results indicate that the emplacement of each granite body occurred during the compressional Permian D4 event. The fabrics that developed regionally and in the pluton/wall‐rock systems during D4 show differing degrees of intensity and style, which are spatially related to the intensity of D4 fabric development in the adjacent country rock. Granite isotopic ages support non‐diachronous formation of D4 structures across the region.

Crustal structure and kinematic framework of the northwestern Pontiac Subprovince, Quebec: an integrated structural and geophysical study

Structural mapping, gravity and magnetic modelling, and interpretation of a deep-seismic profile in the northwestern Pontiac Subprovince outline the crustal structure and early structural development of the region. Penetrative D1 fabrics in the Pontiac Group and in the underlying Opasatica Gneiss may record south-vergent thrusting of a high-grade nappe. D2 and D3 structures record southeast-vergent folding and thrusting within the Pontiac Group. Steeply dipping northeast-trending ductile shear zones may represent oblique ramps initiated during D1. Gravity and magnetic model profiles are consistent with north-dipping structures in the shallow crust, and indicate that the Pontiac Group is a wedge underlain by north-dipping slabs of different densities and magnetic susceptibilities. Interpretation of a seismic reflection profile shows mid-crustal duplex structures overlying a deeper thrust between 16 and 24 km. From the surface to the deep crust, the structure of the northwestern Pontiac Subprovince records south- to southeast-directed thrusting and important crustal thickening during a collisional event. In light of field observations, available isotopic ages suggest that D1 deformation began no earlier than 2694 Ma, and that deformation continued until at least 2668 Ma.

Structural relationships and SrNd isotope systematics of polymetamorphic granitic gneisses and granitic rocks from central Rajasthan, India: implications for the evolution of the Aravalli craton

Abstract This paper reviews the state of structural and geochronological knowledge of Precambrian cratonic rocks in part of central Rajasthan, India, and contributes new observations and isotopic data on the structural features and age of granitic rocks in the area. The granitic rocks comprise plutons of two suites (BGC and Sendra suites), which intrude ortho- and paragneisses the Archean Banded Gneiss Complex (BGC) and its inliers, as well as the Proterozoic Delhi Supergroup. Nearly all the plutons show a foliation and a lineation, which are mostly the result of solid state deformation, although a magmatic component of strain is apparent in some of them. The intensity of these structural fabrics varies from weak to extremely strong. Samples of granitic rocks from the BGC suite yield ages from SmNd isotopic data which scatter around a 2.83 Ga reference isochron [ϵNd(t) of ∼+4.1 ], with depleted mantle model ages ranging from 2.6 to 3.0 Ga. Scatter of the data suggests that the BGC is a composite group of gneisses, and confirms the antiquity of the BGC within this part of the Aravalli-Delhi orogenic belt. RbSr isotopic data of the BGC suite lie on a 1.7 Ga reference isochron, indicating a thermal event at that time. Data from plutons of the Sendra suite, on the other hand, show some scatter but define a model II isochron with slope equivalent to an age of 0.966±250 Ga (initial 87 Sr 86 Sr of 0.70390±0.01889, MSWD-311). The scatter of data could reflect open system behavior during a subsequent thermal event (Malani volcanics? ), although independent evidence for such is lacking in the study area. Ages calculated from our RbSr data are within the error of published results from previous workers. Structural observations on granitic rocks within the BGC inlier at Ajmer suggest that the inlier could represent a remobilized diapiric ridge emplaced into the Delhi Supergroup during the 1.7 Ga thermotectonic event contemporaneously with formation of D1 structures in the wall rock, although the radiometric age of the latter structures is not well-constrained. Microtexture, mineralogy, and field relations of granitic rocks of the Sendra suite point to those plutons being emplaced into the Delhi Supergroup during the 0.966 Ga thermotectonic event late syntectonic relative to D2 indicating that D2 is ∼ 0.96–1.0 Ga. D3 represents retrograde deformation synchronous with cooling of the regional thermal framework and local pluton cooling, probably between ∼ 0.96 and 0.90 Ga. Further geochronological, geochemical and structural work on the plutons and wall rock of both suites is necessary to establish a rigorous thermotectonic chronology by which to accurately model the crustal evolution of this part of central Rajasthan.

The Sandamap Noord Gold Prospect, Central Namibia: Discovery of a New Style of Turbidite-Hosted Gold Mineralization

Early Paleozoic metaturbidite-hosted Au-As-W-B mineralization, believed to be of a style previously unknown in southern Africa, has been discovered in the ensialic inland branch of the Pan-African Damara Orogen. A 2500-m-long zone of gold mineralization is hosted by altered Kuiseb Formation schist, quartz veins, and mylonite rocks in the aureole of a leucogranite and rare element pegmatite dome on the farm Sandamap Noord, west of Usakos, central Namibia. Au/As fluids either mobilized from the metasediments by and/or expelled from the stanniferous pegmatites and leucogranite were concentrated in NNE-trending Damaran D4 structures. Rb-Sr whole-rock dates and Pb-Pb model ages indicate that the gold mineralizing event occurred at 469-464 Ma. The three main types of alteration/mineralization are: a kaolinite-alunite rock (possibly advanced argillic alteration) that contains < 1 g/t Au, a jarosite schist that contains up to 46 g/t Au, and a massive grunerite rock that contains up to 0.5 g/t Au. Associated alter...

Lithology and structure of the Wandilla terrane, Gladstone‐Yeppoon district, central Queensland, and an overview of the Palaeozoic subduction complex of the New England Fold Belt

The Wandilla terrane consists of, from west to east, the Doonside Formation and the Wandilla Formation. The Doonside Formation is dominated by chert and mudstone. Conodonts from a single locality (Devils Bend) indicate an age in the interval Late Silurian to Middle Devonian. The Wandilla Formation consists of mudstone with greywacke, tuff, chert and greenstone. The greywacke is dominated by volcanic detritus with sparse but persistent ooliths that suggest contemporaneity with Early Carboniferous strata of the Yarrol shelf to the west. The Wandilla terrane is structurally complex with two main deformation events. The first deformation (D1) formed widespread lenticular melange during offscraping at the toe of a subduction complex. The second deformation (D2) formed a moderately to shallowly east‐dipping cleavage that typically dips less steeply than the north‐northwest‐trending D1 structures, and contains a down‐dip elongation lineation. D2 structures were produced by the Late Permian to Triassic Hunter‐Bow...

The deformational history of the Larsemann Hills, Prydz Bay: the importance of the Pan-African (500 Ma) in East Antarctica

The Larsemann Hills represent a low-pressure granulite terrain with a complex structural-metamorphic history that comprises two parts: 1) granulite facies D1 structures transposed within an early form surface that probably formed at 1000 Ma, and 2) a sequence of progressive, upper amphibolite to lower granulite facies D2–D6 structures that formed during the Pan-African at 500 Ma and were associated with the emplacement of granites and pegmatites with high-grade alteration zones. D2–D6 events comprise an early form surface that has been tightly folded and sheared twice after which it was warped and transected by discrete mylonites. D2–D6 assemblages are associated with decompression textures on D1 peak-assemblages, such as cordierite coronas on garnet + sillimanite in metapelite and plagioclase coronas on garnet in metabasite. This suggests that D2–D6 formed at slightly lower pressures than D1 structures. However, the spatial correlation between the coronas and alteration zones around pegmatitic intrusives indicates that the apparent decompression textures may have partly resulted from transient fluxes in water pressure following melt crystallization. Throughout East Antarctica tectonic provinces have been recognized in which the 1000 Ma tectonothermal events are identified as the main stage in the evolution, and Pan-African events are dismissed as a minor thermal overprint. Although the Larsemann Hills are small in area, they are representative of a great many granulite terrains in East Antarctica, and suggest that great care is needed in the structural-metamorphic analysis of such terrains to ensure the separation of tectonic stages before an interpretation of the tectonic path is attempted.