Ductile Shear Deformation Research Articles

Geology, age, and fluid inclusions of the Tanjianshan gold deposit, western China: Two orogenies and two gold mineralizing events

The Tanjianshan gold deposit, located at the tectonic boundary between the Oulongbuluk block (OB) and North Qaidam orogenic belt (NQO) on the northern margin of the Qaidam basin (NMQB), western China, underwent Early Palaeozoic and Late Palaeozoic to Early Mesozoic multiple accretions and collisional orogenies. It is a typical example of a deposit resulting from two gold mineralizing events related to two collisional orogenies. Two phases of ore-controlling NW-trending shear zones have been identified in the deposit: dextral brittle–ductile shear deformation during the late Early Palaeozoic collisional orogeny and sinistral strike-slip shear deformation during the Late Palaeozoic–Early Mesozoic orogeny. N–S-trending folds developed in the NW-trending shear zone and resulting from sinistral strike-slip motion of the shear zone are nose-shaped in plan view and control most gold orebodies in the deposit. The major gold orebodies trend NNE and are concentrated in the hinge zones of these N–S-trending folds, occurring along the fault zones parallel to both limbs of these folds. These gold orebodies, composed of altered mylonite schist type ore and altered dyke type gold ore, occur as saddle-shaped bodies, beds and lenses mainly within altered carbonaceous quartz–sericite mylonite schist of a highly strained zone and locally in altered igneous dykes of the highly strained zone. Alteration related to gold mineralization includes silicification, pyritization and sericitization. A progressive increase of SiO 2, S, Au, Ag, As and Sb from unaltered or weakly altered rocks through altered rocks to gold ore is consistent with progressively enhanced silicification and pyritization, suggesting the flow of mineralizing fluids along the NW-trending shear zone and N–S-trending folds. Native gold and electrum are present as very fine grains, mainly enclosed in disseminated pyrite or occurring in fractures within the pyrite. Three generations of pyrite and associated hydrothermal quartz are observed. The fabrics of gold ore, generations of auriferous pyrite, field relationship between multiple ore-controlling structures and gold mineralization, and the bimodal distribution of ore elements in the gold district suggest the occurrence of two main hydrothermal and mineralizing events. The first event, marked by a mineral assemblage of first-generation quartz (qz 1) and first-generation pyrite (py 1), was restricted to the carbonaceous mylonite schist within the NW-trending shear zone. Sericite from the ore-controlling NW-trending shear zone gave a 40Ar– 39Ar age of 409.4 Ma, which is consistent with the age of the regional late Early Palaeozoic collisional orogeny in the region. The second event, marked by a mineral assemblage of second-generation quartz (qz 2) and second-generation pyrite (py 2), occurred within the N–S-trending folds and was related to late Palaeozoic orogeny in the region. We have obtained a K–Ar age of 268.9 ± 4.3 Ma and an Rb–Sr isochron age of 288 ± 9.7 Ma for hydrothermal minerals from gold ores. The latter age is closer to the age of this event of gold mineralization. Mineralization of this event was superimposed onto gold mineralization of the early phase and formed most of the gold orebodies in the deposit. The distribution and microthermometric data of fluid inclusions indicate that late Early Palaeozoic ore fluids belong to the H 2O–CO 2–CH 4–NaCl fluid system formed at 186 to 250 °C, whereas Late Palaeozoic fluids comprise two kinds of immiscible fluid: a H 2O–NaCl fluid with salinity of 1.6 to 10.8 wt.% NaCl equiv., which probably formed at temperatures of 381 to 449 °C and, and H 2O–CO 2–NaCl fluids with salinities of 1.8 to 7.9 wt.% NaCl equiv., which are genetically the product of evolution of former immiscible magmatic fluids, and which formed at temperatures of 274 to 410 °C.

Development of a Novel Oil Cavitation Jet Peening System and Cavitation Jet Erosion in Aluminum Alloy, AA 6063-T6

Compressive residual stresses that improve fatigue strength of material are obtained by peening the surface. Unlike traditional processes, a novel process of oil cavitation jet peening was developed. The process is based on implosion generated by the oil cavitation jet that plastically deforms the surface, imparting compressive residual stresses. The process developed involves injection of a high-speed oil jet (∼230 m/s) through a suitably designed nozzle, into an oil-filled chamber containing the specimen to be peened. The region of cavitation generation, growth, and collapse, at the various cavitation numbers, was recorded using high-speed photography. To optimize the process parameters, a simple erosion test was performed in aluminum alloy, AA 6063-T6, specimens. The impact pressure generated during the implosion of cavitation bubbles causes plastic deformation and erosion of the surface. The surface deformation and cavitation jet erosion in the exposed specimens were characterized using optical and scanning electron microscopies. The standoff distance, which measures jet impact zone of the specimen from nozzle, was optimized at 15 mm in a cavitation number (which is a measure of pressure ratio across the nozzle) of 0.0017. The surface deformation produced by collapse of the oil bubble was similar to impact of oil droplet on the surface. The material removal mechanism during implosion of the bubble is predominately by ductile shear deformation.

NE–SW-trending Hepu–Hetai dextral shear zone in southern China: Penetration of the Yunkai Promontory of South China into Indochina

The NE–SW-trending Hepu–Hetai shear zone extends for about 480 km along the boundary between Guangdong and Guangxi provinces in the southern part of South China. Field mapping and kinematic analyses indicate that the zone is characterized by dextral ductile strike-slip deformation, with an estimated displacement along the fault zone in excess of 500 km. Studies of synkinematic phengite and gas–liquid inclusions in quartz within mylonite suggest that the ductile shear deformation occurred under medium temperature/pressure conditions of the greenschist facies, and 40Ar/ 39Ar analyses of muscovite yield ages of 213–195 Ma. Based on these data and Late Paleozoic–Mesozoic paleogeographic data for southern China, we propose that the Hepu–Hetai shear zone originated via the penetration of the Yunkai Promontory of South China into Indochina during the Triassic.

The Pengguan tectonic dome of Longmen Mountains, Sichuan Province: Mesozoic denudation of a Neoproterozoic magmatic arc-basin system

Neoproterozoic igneous and metamorphic complexes occur as tectonic domes in the Longmen Mountains of the western margin of the Yangtze Block, and are important in reconstructing the Rodinian supercontinent and constraining the timing and mechanism of tectonic denudational processes. The Pengguan dome consists of granitic intrusions and metamorphic rocks of the Huangshuihe Group and is tectonically overlain by ductilly deformed Sinian to Paleozoic strata. The plutonic intrusions consist of granites with abundant amphibolite enclaves. New LA-ICP-MS zircon U-Pb dating yielded an emplacement age of 809±3 Ma and a protolith age of 844±6 Ma for the granite. The granitic rocks have geochemical signatures typical of A-type granites, indicating their formation under an extensional environment, by melting of newly formed tonalite-trondhjemite-granodiorite (TTG) rocks. A detachment fault, characterized by variable ductile shear deformation of S-C fabric and ESE-ward kinematics, separates the Pengguan dome from the Sinian-Paleozoic cover. 40Ar/39Ar dating of muscovite from the mylonite in the detachment fault of the dome demonstrates that ductile deformation occurred at ∼160 Ma. This study indicates the existence of a Neoproterozoic magmatic arc-basin system, which was denudated by a Jurassic middle crustal ductile channel flow along the Longmenshan thrust belt.

Multi-level decollement zones and detachment deformation of Longmenshan thrust belt, Sichuan Basin, southwest China

As is well known that many decollement layers were developed in the Longmenshan thrust belt, Sichuan Basin, China. Through field investigation, explanation of seismic profiles and analysis of the balanced sections, we can divide the decollement zones into 3 categories: (1) the deep level decollement zones, including the crust-mantle decollement layer, intracrustal decollement layer, and presinian basal decollement layer. The main structural styles of their deformation are the crust-mantle decoupling detachment deformation, the basal ductile shear deformation, etc.; (2) the middle level decollement zones, including the Cambrian-Ordovician decollement layer, the Silurian decollement layer, etc., the main structural styles of their deformation are the isopachous fold, the angular fold, the saddle structure, and the combination styles of them; and (3) the shallow level decollement zones, including the Xujiahe Formation decollement layer of Upper Triassic and the Jurassic decollement layers, the main structural styles of their deformation are the thrust-nappe tectonic, the pop-up, the triangle zone, the duplex, etc. Multi-level decollement zones not only made the Longmenshan thrust belt develop many different deformation styles from deep place to shallow place, but also made some local areas have the superimposition of the tectonic deformation apparently. This study indicates that the multi-level decollement zones have a very important effect on the shaping and evolution of the Longmenshan thrust belt.

Evolution of quartz microstructure and c-axis crystallographic preferred orientation within ductilely deformed granitoids (Arolla unit, Western Alps)

We have studied quartz microstructures and the c-axis crystallographic preferred orientations (CPOs) in four granitoid samples representative of increasing ductile shear deformation, from a weakly deformed granitoid (stage 1) to a mylonitic granitoid (stage 4). The quartz c-axis CPO measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area. All the samples belong to the Austroalpine Arolla unit (Western Alps) and were deformed at greenschist facies conditions. The quartz c-axis CPO was analyzed using a U-stage and the optical orientation imaging technique. The magmatic plagioclase, forming more than 50% of the volume of the granitoid, is extensively replaced by a mica-rich aggregate even in weakly deformed samples of stage 1. These aggregates flow to form an interconnected weak matrix with increasing deformation, wrapping relatively less strained quartz grains that undergo dominantly coaxial strain. Recrystallization of quartz ranges from less than 1% in the weakly deformed granitoid to up to 85% in the mylonitic granitoid, with average grain strain of 41% and 64%, respectively. With increasing strain and recrystallization, quartz grains in the granitoids show a sequence of transient microstructures and CPOs. Crystal plastic deformation is initially accomplished by dislocation glide with limited recovery, and at 50% grain strain it results in a CPO consistent with dominantly basal 〈 a〉 slip. At 60% grain strain, recrystallization is preferentially localized along shear bands, which appear to develop along former intragranular cracks, and the recrystallized grains develop a strong c-axis CPO with maxima orthogonal to the shear band boundaries and independent of the host grain orientation. Within the granitoid mylonite, at an average quartz grain strain of 64%, recrystallization is extensive and the c-axis CPO of new grains displays maxima overlapping the host c-axis orientation and, therefore, unrelated to the bulk sense of shear. The host-controlled CPO is inferred to reflect pervasive recrystallization by progressive subgrain rotation. The switch from ‘shear band-control’ to ‘host-control’ on c-axis CPO occurred between 40% and 70% of recrystallization. In the quartz mylonite, the quartz c-axis CPO develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions. This study indicates that the CPO evolution of quartz may significantly differ in cases of polymineralic vs. monomineralic rocks under the same deformation conditions, if quartz in the polymineralic rock behaves as a ‘strong’ phase.

Rheological and kinematical responses to flow of two-phase rocks

Numerical simulations have been performed to investigate the strain-dependent behaviour of rheological and kinematical responses to flow of two-phase rocks using the commercial finite-difference program FLAC2D. It was assumed that the two phases have Maxwell rheology. Plane strain and velocity boundary condition, which produces a simple shear deformation, were also assumed. Two types of geometries were considered: strong phase supported (SPS) and weak phase supported (WPS). We calculated strain-dependent variations of effective viscosity and partitioning of strain rate, vorticity and kinematic vorticity number during deformation in both SPS and WPS structure models. The results show that the strain-dependent behaviour is largely influenced by the geometry of the composite. SPS models show both strain hardening and strain softening during the simulations, with strain hardening preceding strain softening. A critical shear strain is necessary to begin the strain softening behaviour. Strain hardening and strain softening are accompanied by a reduction and an increase of the partition of strain rate into the weak phase, respectively. On the other hand, WPS models show only weak strain hardening and strain softening, being the strain-dependent behaviour close to a steady state flow. In addition, the following results are obtained on vorticity and kinematic vorticity number; (1) in both SPS and WPS models the partition of vorticity into weak phase increases with progressive shear strain, i.e. the strong phase becomes less rotational, (2) in SPS models weak inclusions changes from sub-simple shear to super-simple shear with progressive strain, whereas the strong matrix changes from super-simple shear to sub-simple shear, (3) in WPS models the strong inclusions with high viscosity contrasts are less rotational but can be in super-simple shear condition to high strains. The observed strain-dependent behaviours have been compared with previous proposed analytical models. The degree of agreement is variable. Balshin and Ryshkewitch–Duckworth models are only applicable to SPS models. Ji-generalized mixture rule model is applicable to both models. The results suggest that polyphase rocks with SPS structure during ductile shear deformation respond as strain softening materials, after an initial strain hardening stage that may drive to the strain localization into the material.

Nucleation and growth of myrmekite during ductile shear deformation in metagranites

AbstractMyrmekite is extensively developed along strain gradients of continuous, lower amphibolite facies shear zones in metagranites of the Gran Paradiso unit (Western Alps). To evaluate the role of stress, strain energy and fluid phase in the formation of myrmekite, we studied a sample suite consisting of weakly deformed porphyric granites (WDGs), foliated granites (FGs) representative of intermediate strains, and mylonitic granites (MGs). In the protolith, most K‐feldspar is microcline with different sets of perthite lamellae and fractures. In the WDGs, abundant quartz‐oligoclase myrmekite developed inside K‐feldspar only along preexisting perthite lamellae and fractures oriented at a high angle to the incremental shortening direction. In the WDGs, stress played a direct role in the nucleation of myrmekites along interfaces already characterized by high stored elastic strain because of lattice mismatch between K‐feldspar and albite. In the FGs and MGs, K‐feldspar was progressively dismembered along the growing network of microshear zones exploiting the fine‐grained recrystallized myrmekite and perthite aggregates. This was accompanied by a more pervasive fluid influx into the reaction surfaces, and myrmekite occurs more or less pervasively along all the differently oriented internal perthites and fractures independently of the kinematic framework of the shear zone. In the MGs, myrmekite forms complete rims along the outer boundary of the small K‐feldspar porphyroclasts, which are almost completely free of internal reaction interfaces. Therefore, we infer that the role of fluid in the nucleation of myrmekite became increasingly important as deformation progressed and outweighed that of stress. Mass balance calculations indicate that, in Al–Si‐conservative conditions, myrmekite growth was associated with a volume loss of 8.5%. This resulted in microporosity within myrmekite that enhanced the diffusion of chemical components to the reaction sites and hence the further development of myrmekite.

Geometry and kinematics of early Alpine nappes in a Briançonnais basement (Ambin Massif, Western Alps)

Abstract Petrological and structural observations from the Ambin pre-alpine basement dome and from its Brianconnais and Piedmont covers show an early D1 nappe-forming event overprinted by a major D2 (+ D3) ductile shearing deformation. The D1 event is characterised by garnet-blueschist facies metamorphic assemblages retrogressed to greenschist facies conditions during D2 then D3 stages near the top of the dome. North-verging D1 structures preserved in the core of the dome are consistent with alpine evolutionary models, in which exhumation of HP–LT metamorphic alpine rocks occurs initially in a north–south direction. To cite this article: J. Ganne et al., C. R. Geoscience 336 (2004).

Eoalpine (Cretaceous) evolution of the Oman Tethyan continental margin: insights from a structural field study in Jabal Akhdar (Oman Mountains)

ABSTRACT A structural study in Jabal Akhdar (an autochthonous window in the Oman Mountains) shows that the thrusts and imbrications, mapped in the Permian-Cretaceous formations of the Arabian Platform, have a vergence top to the north-northeast and are associated with a regional cleavage resulting from a ductile shear deformation of the same vergence. This deformation affects the entire autochthonous unit, from the southwestern edge of Jabal Akhdar to the northeastern edge of Saih Hatat, with an increasing strain intensity towards the northeast. The same domain is also affected by a HP/LT metamorphism with a northeastward increase from chlorite facies to blueschist and eclogite facies, i.e. characteristic of a northeast-dipping subduction. However, the retrograde character of the metamorphic parageneses associated with the ductile shear, as well as its north-northeast vergence, indicate that this deformation is linked to the exhumation of the autochthon during the Campanian. These observations have been synthesised in a new lithospheric-scale interpretation of the geodynamic development of the North Oman continental margins during the middle to late Cretaceous. The sequential evolution along a transect passing from southwest of Jabal Akhdar to northeast of Muscat can be summarised as follows: An intra-continental subduction zone affected the autochthon of the Arabian Platform with a basal rupture lying in the proximal part of the continental margin, to the south of the northern edge of the carbonate platform. A North Muscat microplate was created between the intra-continental subduction zone and the intra-oceanic subduction that gave rise to the Samail Ophiolite; this microplate includes the outer part of the Arabian Platform, the continental slope and the entire Hawasina Basin. From Early Turonian to Late Santonian the obduction and the intra-continental subduction were coeval and parallel. The northeast edge of the North Muscat microplate plunged below the Samail Nappe whilst the emergent southwest part overthrust the innermost parts of the Arabian Platform. The leading edge of the Samail and Hawasina Nappes advanced across the southwestern border of the North Muscat microplate just before obduction and intra-continental subduction ceased at the Santonian–Campanian boundary. Towards the end of the intra-continental subduction, the lower part of the crust of the subducted autochthon delaminated the upper part, marking the first stage of the metamorphic rocks exhumation. From Early Campanian to Early Maastrichtian, the North Muscat microplate moved to the northeast, its northeastern edge sinking by gravity into the asthenosphere. The subducted autochthon rose up, and came into contact with the base of the obducted units. The resulting uplift of the ophiolite nappes produced its emergence and partial erosion. Local crustal thickening, related to the lithospheric delamination, caused doming at Saih Hatat and subsequent erosion that locally extended to the pre-Permian sedimentary basement during the Early Maastrichtian. The present day domal shape of Jabal Akhdar is however related to Tertiary tectonic events.

Microcosmic analysis of ductile shearing zones of coal seams of brittle deformation domain in superficial lithosphere

The ductile shearing zones of coal seams in a brittle deformation domain in superficial lithosphere are put forward based on the study on bedding shearing and ductile rheology of coal seams. The macrocosmic and microcosmic characteristics include wrinkle fold, mymonitized zones and ductile planar structure of coal seams, etc., while the microcosmic characteristics may also include different optic-axis fabrics and the anisotropy of vitrinite reflectance as well as the change of chemical structure and organic geochemistry components. The forming mechanism is analyzed and the strain environment of ductile shearing zones of coal seams discussed. The result indicates that, in the superficial brittle deformation domain, the coal seams are easy to be deformed, resulting in not only brittle deformation but also ductile shearing deformation under the action of force. Because of simple shearing stress, the interlayer gliding or ductile rheology may take place between coal seams and wall rocks. Therefore, many ductile shearing zones come into being in superficial lithosphere (<5 km). The research on ductile shearing zone of brittle deformation domain in superficial lithosphere is significant not only theoretically for the study of ductile shearing and ductile rheology of the lithosphere but also practically for the structural movement of coal seams, the formation and accumulation of coal-bed methane, and the prevention and harness of gas burst in coal mine.

Anisotropy of magnetic susceptibility in the Montes de Toledo area (Hercynian Iberian Belt, Spain) and its petrostructural significance

Abstract Anisotropy of magnetic susceptibility data are presented for samples from the anatectic complex of Toledo and the Mora batholith. The units located at the axial part of the Central Hercynian belt are separated by an important listric fault. Anatectic granites (Layos granite), epizonal granites (Mora granite) and, to a lesser extent, high-grade metamorphic rocks and metasediments are considered in this work in order to characterize their magnetic fabric and determine their structural significance in the framework of a large-scale extensional deformation of Late Variscan age. Samples from the Layos granites (footwall) show a magnetic lineation compatible with the stretching related to ductile shear deformation of the Toledo shear zone, which was responsible for the exhumation of the anatectic complex. During the exhumation, the folding of a previous metamorphic foliation in the anatectic complex took place, which is also recognized from the magnetic pattern of the minimum susceptibility axes of Layos granite. On the contrary, the magnetic fabric of the epizonal Mora granites (hanging wall), which intruded at the beginning of the extensional Late Variscan tectonics, mainly reflects magmatic processes.

Major and minor structural features of a bedding shear zone along a coal seam and related gas outburst, Pingdingshan coalfield, northern China

Abstract A fractured coal band, traditionally called a soft-coal band, or structurally referred to as a bedding shear zone, within the Wu coal seam in the Pingdingshan coalfield, northern China, was structurally studied. Based on field examination and observations by reflectance microscopy and scanning electron microscopy (SEM), the bedding shear zone is not consistent with a simple bedding fault defined by pioneering geologists, but rather a complexly deformed band locally overprinted by ductile shearing fabrics. Dominant small-scale structures within the bedding shear zone are thrusts, folds, cleavage duplexes and related structures, but local, refolded folds, kink bands, and S–C band structures also indicate ductile shear deformation, typically developed in areas of steep dip such as within the limb of a subordinate fold. Bed-parallel shearing within the coal seam has been considered a serious influence on gas outburst in underground mining production in addition to thickening the seam and reducing the quality of coal. Locally, abnormal thickening of a shear zone and extensive compressive structures where ductile fabrics frequently occur are closely related with the site of an outburst. Thickening of a shear zone increases the capacity of methane storage, and compressive structures within a shear zone may act as a tectonic screen, blocking methane migration, which can result in a pocket of high methane pressure. A potential risk of outburst is indicated as a working face approaches an area of high methane pressure. Three abnormal phenomena encountered during a mining operation presage methane outburst for seams with a bedding shear zone.

Subduction-Accretion History of the Central Asian Orogenic Belt: Constraints from Mongolia

Despite abundant research efforts, no consensus exists about the Carboniferous tectonic evolution of the Eastern Tianshan. The tectonic environment of the Yamansu arc belt and the geodynamic mechanisms of the Kangguer shear zone are not fully understood. To refine the proposed tectonic history, we studied three granitoids with a variety of shear deformation characteristics and emplacement relationships in the Yamansu–Kangguer area. Bulk geochemistry indicates that the mylonitic granite porphyry is I-type, while the undeformed two-mica granite and quartz porphyry are S-type granites. Zircon UPb dating results give concordant ages of the granite porphyry, two-mica granite, and quartz porphyry at 336.4 ± 2.3 Ma (MSWD = 1.9, n = 16), 331.5 ± 6.1 Ma (MSWD = 2.6, n = 6), and 332.8 ± 1.7 Ma (MSWD = 1.5, n = 25), respectively, suggesting an emplacement age in the middle Carboniferous. The whole-rock SrNd and zircon Hf isotopic analyses of the three granitoids indicate that the granitic magmas were derived from a Mesoproterozoic to Neoproterozoic crustal basement. The whole rock geochemical and isotopic data of the studied granitoids indicate partial melting with insignificant crustal contamination (except the two-mica granites) and fractional crystallization during magma origin and evolution. These granitoids show enrichment of LILEs (e.g., Rb, Ba, K) and Pb, depletion of Nb, Ta, P, Ti, and P, with low Sr/Y ratios (3–73) and Sr contents (33–160 ppm), displaying affinities to arc magma, and they were classified as continental arc granites. By combining the structural analysis and geological evidence, we propose that southward subduction of the Kangguer Ocean in the middle–late Carboniferous resulted in the ductile shear deformation of strata in the Kangguer shear zone and voluminous magmatism in the Yamansu–Kangguer continental arc–fore-arc basin system.

Major and minor structural features of a bedding shear zone along a coal seam and related gas outburst, Pingdingshan coalfield, northern China

A fractured coal band, traditionally called a soft-coal band, or structurally referred to as a bedding shear zone, within the Wu coal seam in the Pingdingshan coalfield, northern China, was structurally studied. Based on field examination and observations by reflectance microscopy and scanning electron microscopy (SEM), the bedding shear zone is not consistent with a simple bedding fault defined by pioneering geologists, but rather a complexly deformed band locally overprinted by ductile shearing fabrics. Dominant small-scale structures within the bedding shear zone are thrusts, folds, cleavage duplexes and related structures, but local, refolded folds, kink bands, and S–C band structures also indicate ductile shear deformation, typically developed in areas of steep dip such as within the limb of a subordinate fold. Bed-parallel shearing within the coal seam has been considered a serious influence on gas outburst in underground mining production in addition to thickening the seam and reducing the quality of coal. Locally, abnormal thickening of a shear zone and extensive compressive structures where ductile fabrics frequently occur are closely related with the site of an outburst. Thickening of a shear zone increases the capacity of methane storage, and compressive structures within a shear zone may act as a tectonic screen, blocking methane migration, which can result in a pocket of high methane pressure. A potential risk of outburst is indicated as a working face approaches an area of high methane pressure. Three abnormal phenomena encountered during a mining operation presage methane outburst for seams with a bedding shear zone.

Kilometre-scale folding in the Teslin zone, northern Canadian Cordillera, and its tectonic implications for the accretion of the Yukon-Tanana terrane to North America: Discussion

Despite the often divergent opinions regarding the evolution of the Teslin suture zone (TSZ), previous tectonic models broadly agree that the TSZ comprises a ductile shear zone that constitutes a fundamental crustal boundary (e.g., Tempelman-Kluit 1979; Erdmer 1985; Hansen 1989; Stevens and Erdmer 1996; Hansen and Dusel-Bacon 1998). In contrast, de Keijzer et al. (1999) offer a radical model in which the TSZ is a rootless polydeformed nappe obducted onto North America. According to their interpretation the TSZ does not represent a shear zone. Although the influx of new ideas in science is always commendable, the methodology by which de Keijzer et al. (1999) arrive at their interpretation is scientifically flawed; they ignore or arbitrarily dismiss published TSZ data, and they fail to present evidence in support of key elements of their model. Although their data may be valid, we question: (1) their interpretation of linear fabric elements, (2) the existence of kilometre-scale folding, and (3) evidence for North American rocks in the western TSZ. de Keijzer et al. (1999) discard the interpretation of the TSZ as a complex ductile shear zone and assert that, apart from the western TSZ, most lineations represent crenulations and bedding–cleavage intersections. They state that they did not observe stretching lineations or fabric asymmetries. In contrast, previous workers (Erdmer 1985; Hansen 1989, Hansen et al. 1989, 1991; Stevens and Erdmer 1996; Oliver 1996; Hansen and Dusel-Bacon 1998) describe numerous structural domains within the TSZ and correlative L–S tectonites in east-central Alaska, based on elongation lineation orientation and microstructural asymmetry, which they attribute to widespread ductile shear across the entire TSZ. Thus, knowledge of whether or not the TSZ represents a zone of ductile shear deformation is critical to models of TSZ evolution. The presence and nature of quartz …

Analysis of Ore‐controlling Structure in the Qifengcha‐Detiangou Gold Deposit, Huairou County, Beijing

AbstractThe Qifengcha‐Detiangou gold deposit is a medium‐sized deposit recently found in Huairou County, Beijing. It belongs to the altered mylonite type with superimposed quartz vein type and is related to the early Yanshanian magmatic activity. Characterized by multiperiodic activity, the NE‐trending Qifengcha fault is a regional ore‐controlling structure in the area, and gold mineralization develops only in its southeastern part. Meanwhile, gold mineralization is controlled by the Yunmengshan metamorphic core complex. The nearly N‐S‐ and E‐W‐trending low‐angle detachment faults, reformed by the Qifengcha fault in the northwestern part of the core complex, are the main ore‐bearing faults. All discovered gold deposits are located within an area 1.5–4.0 km away from the boundary of the upwelling centre. The N‐S‐ (NNE‐) and E‐W‐trending ore‐bearing faults are ductile‐brittle structural zones developing in shallow positions and subjected mainly to compressive deformation. The structural ore‐controlling effects are as follows. (1) The attitude, shape, and distribution of gold orebodies are controlled by faults. (2) There is a negative correlation between the gold abundance and the magnetic anisotropy (P) of the altered mylonite samples from the deposit, which shows that the gold mineralization is later than the structural deformation. (3) Quartz vein type mineralization is superimposed on altered mylonite type mineralization. (4) In mineralized mylonite, the stronger the ductile shear deformation, the easier the late‐stage gold mineralization to occur and the higher the gold abundance. The richest gold mineralization occurs only around the centre of the fault subjected to the strongest deformation.

Petrology of gedrite‐bearing rocks in mid‐crustal ductile shear zones from the Eastern Ghats Belt, India

A suite of metapelitic, basic and quartzofeldspathic rocks intruded by enderbitic gneiss from the southernmost tip of the Eastern Ghats Belt, India, and metamorphosed at c. 750–800 °C, 6 kbar, were subjected to repeated ductile shear deformation, hydration, cooling and accompanying alkali metasomatism along narrow shear zones. Gedrite‐bearing assemblages developed in the shear zones traversing metapelitic rocks. Interpretation of the reaction textures in an appropriate P–T grid in the system FMASH, an isothermal–isobaric μH2O–μNa2O grid in the system NFMASH, and geothermobarometric data suggest a complex evolutionary history for the gedrite‐bearing parageneses. Initially, gedrite‐bearing assemblages were produced due to increase in μNa2O at nearly constant but high μH2O accompanying cooling. Gedrite was partially destabilized to orthopyroxene+albite due to progressively increasing μNa2O. During further cooling and at increased μH2O a second generation of gedrite appeared in the rocks.

Cl-scapolite, Cl-amphibole, and plagioclase equilibria in ductile shear zones at Nusfjord, Lofoten, Norway: implications for fluid compositional evolution during fluid-mineral interaction in the deep crust

Cl-rich scapolite and amphibole formed during ductile shear deformation associated with the infiltration of an externally derived Cl-bearing fluid in a gabbroanorthosite of the Flakstadøy Basic Complex, Lofoten, Norway. Amphibole and scapolite formed along the contacts between incompletely altered igneous mafic minerals (orthopyroxene, clinopyroxene, biotite, and ilmenite) and plagioclase and internally in the grains of primary igneous plagioclase. The secondary minerals show large compositional variations on thin-section scale. The Cl content of scapolite varies between 0.3 and 0.95 apfu (atoms per formula unit), whereas amphibole shows Cl concentrations from 0 to 1.5 apfu. The primary igneous plagioclase (An 50-An 60) underwent extensive recrystallization and alteration during the fluid–rock interactions. Secondary plagioclase shows compositions in the range from An 20 to An 55. In general, plagioclase did not equilibrate with the fluid phase during alteration, due to the sluggishness of the cation exchange reactions between plagioclase and fluid. Occasionally, however, equilibrium among plagioclase, scapolite, and the fluid phase was attained. The compositional variations of amphibole, scapolite, and plagioclase that equilibrated with the fluid are principally related to variations in the fluid activity ratios a Cl − /a (CO 3) 2− and a Cl − /a OH − . The large compositional variations of the minerals on thin-section scale thus indicate steep gradients in the fluid activity ratios a Cl − /a (CO 3) 2− and a Cl − /a OH − . The activity gradients of the fluid phase developed as a result of the preferential extraction of water from the grain-boundary fluid during the formation of hydrous silicates. Scapolite and the most Cl-enriched amphibole formed in equilibrium with an evolved fluid phase, enriched in Cl and CO 2.

U-Pb geochronological constraints on Precambrian stratified units in the Avalon Composite Terrane of Nova Scotia, Canada: tectonic implications

Concordant U-Pb ages of detrital zircons in several Proterozoic units in the Avalon Composite Terrane of Nova Scotia, along with previously published data, indicate that deposition of low- and high-grade stratified units in the Cobequid Highlands (Gamble Brook Formation) and central Cape Breton Island (variously assigned to the George River Group or George River Metamorphic Suite and the Bras d'Or Gneiss) started after ~977 Ma, extended through a period of volcanic activity between ~694 and 637 Ma, and ended before the ~629-612-600 Ma volcanic activity associated with deposition of the Georgeville and Jeffers groups in the Antigonish and Cobequid highlands, and the East Bay Hills and Coxheath groups in southern Cape Breton Island. These age data, along with U-Pb analyses of metamorphic and igneous zircon and monazite, also constrain three Late Proterozoic tectonothermal events: (i) oblique sinistral ductile shear deformation and greenschist-amphibolite-facies metamorphism probably occurred between ~694 and 629 Ma; (ii) oblique dextral deformation and greenschist-facies metamorphism is bracketed between ~612 and 607 ± 2 Ma; and (iii) low-angle, normal shear deformation associated with low-pressure, high-temperature metamorphism took place at ~553-550 Ma. These three events may record (i) opening and (ii) closing of the Antigonish-Cobequid back-arc basin, followed by (iii) gravitational collapse of the central Cape Breton magmatic arc following cessation of subduction, respectively. The combination of ~700-550 Ma arc magmatism and ~1.3-1.0 Ga concordant detrital zircon ages in the Avalon Composite Terrane suggests provenance in the Amazon craton or Oaxaquia in Mexico.