S3 Crenulation Cleavage Research Articles

Structures and deformation phases affecting the Diagorou-Darbani greenstone belt south of Téra (Liptako, West Niger, and West African Craton)

The present study focuses on the geological formations of the Diagorou-Darbani greenstone belt of southern Téra in the Niger Liptako (NE portion of the Man Shield of the West African Craton). The main objective of this study is to characterize the deformation phases that affected the Diagorou-Darbani greenstone belt to the south of Téra. The methodology implemented is based on a structural analysis using field data. These formations are affected by two major deformation phases, D1 and D2. Phase D1 comprises three episodes: D1a, D1b and D1c. The first episode (D1a) developed the S1 cleavage (NE-SW). This episode is linked to a NW-SE shortening (N130°). The second episode (D1b) is marked by S2 crenulation cleavage, micro-folding and the dextral reactivation of N45° shear zones. This episode is linked to an E-W shortening. The third episode (D1c) is highlighted by the NW-SE S3 fracture cleavage and the sinistral reactivation of the N45° shear zones. A N-S to NNE-SSW shortening was obtained for episode D1c. Phase D2 is essentially brittle. It favoured the development of submeridian fractures and conjugate WNW-ESE (sinistral) and ENE-WSW (dextral) shear fractures. This deformation is compatible with a generally E-W direction of shortening.

Lower Continental Crusts Involved in the Alpine Orogeny: New Insight from the Santa Lucia Nappe Metasediments (Corsica, France)

AbstractIn the Alpine Corsica (France), the Santa Lucia Nappe represents a peculiar unit preserving the unique relicts of Paleozoic lower to medium continental crust. It consists of composite basement affected by Permian granulite facies metamorphic conditions unconformably covered by a Late Cretaceous clastic sequence (Tomboni Conglomerate and Tralonca Flysch) affected by polyphase deformation and low‐grade‐metamorphism. In this work, we present a new reconstruction of the deformation events registered by the Tralonca Flysch during the Alpine orogeny. The D1 phase was testified by rare isoclinal folds. The D2 phase produced a continuous foliation and a map‐scale sheath‐fold with a top‐to‐W sense of shear. The D3 phase produced E‐verging non‐cylindrical folds and S3 crenulation cleavage that is not associated to metamorphic blastesis. We present the first temperature‐pressure‐deformation (P‐T‐d) path for the Tralonca Flysch, demonstrating that the Santa Lucia Nappe underwent accretion and then first stage exhumation in the Alpine wedge during the D1 phase with pressure and temperature peaks both occurred under blueschist metamorphic conditions. The D2 phase occurred at lower pressure‐temperature conditions during a second stage exhumation. This pressure‐temperature‐deformation path is comparable with those of the Lower Units (i.e., the subducted continental units of Alpine Corsica) suggesting a common geodynamic history.

Metamorphic and deformational characteristics recorded by Langzishan Barrovian series rocks within the Jiao-Liao-Ji Orogenic Belt, North China Craton

Orogenesis always involves multiple tectonic events accompanied by metamorphic cycles. The Barrovian metamorphic series often records complicated prograde metamorphism and deformational features, which are crucial for understanding the related orogenic evolution. While Barrovian metamorphism have been widely documented in the Phanerozoic orogenic belts, limited stuides have been conducted on Paleoproterozoic Barrovian metamorphic series. In this contribution, Barrovian metamorphic series extending from biotite to kyanite zones in the middle of Jiao-Liao-Ji Orogenic Belt (JLJOB) from eastern part of North China Craton (NCC) was investigated. The current studies identified the occurrence of polyphase deformation and metamorphism within the Langzishan Barrovian metamorphic series. D1 fabrics are characterized by the development of S1 foliation and L1 lineation in association with greenschist facies metamorphism (M1) that occurrs at ca. 490 ℃ and ca. 5.7 kbar. D2 fabrics that include different folds (F2), S2 crenulation cleavage, S2 foliation and L2 lineation, accompanied by the low amphibolite metamorphism (M2) that occurrs at ca. 670 ℃ and 8.2–8.6 kbar. D3 fabrics involving kink band postdate sillimanite growth that occurrs at ca. 660 ℃ and ca. 6.5 kbar, and predate chlorite occurrence. Monazite U-Pb dating suggests the peak and retrograde metamorphic ages of ca. 1.93 Ga and 1.86 Ga, respectively. Thus, a clockwise pressure–temperature-time-deformation (P-T-t-D) path is established, possibly related to the crustal thickening process. These features are very similar to those documented in Phanerozoic Barrovian metamorphic series, suggesting that a comparable orogenesis mechanism could be shared in the Paleoproterozoic and Phanerozoic.

Effects of tectonically induced fabrics on geomechanical properties of rocks, NW Pakistan

The Chakdara Granitic Gneisses (CGG) of the Indian plate and Kamila Amphibolite of the Kohistan Island Arc (KIA) along the Main Mantle Thrust (MMT) in Shigo Kas, Talash Dir Lower, indicate that tectonically induced foliations and lineations strongly affected the geomechanical properties of these rocks. The earlier S1 crenulated cleavages are well preserved in the microlithon of a well-developed ENE-WSW trending S2 crenulation cleavage. The pervasive S2 foliations, D2 fold axes, and L22 lineations are induced by NNW-SSE horizontal bulk shortening. The core samples obtained parallel and perpendicular to the main ENE-WSW trending S2 and L22 have higher and lower uniaxial compressive strength (UCS) values, respectively. The UCS and uniaxial tensile strength (UTS) average values of four core samples obtained parallel and perpendicular to the main S2 are 51.8 MPa and 12.21 MPa versus 45.65 MPa and 12.45 MPa, respectively. Core samples from the weakly foliated S-2 specimen shows little variation in the UCS and UTS values. The variation in the UCS values in the core samples cut perpendicular and parallel to the main tectonic fabric has been controlled by micro-shear zones at the contact zones of crenulated and crenulation cleavages and sigmoidal mica fish. The UCS values are higher in the core samples parallel to the pervasive S2 and L22 because the parallel shear on the sigmoidal crenulated cleavages in microlithon of the S2 and S2 mica fish counterbalance the parallel external applied load. However, the UCS values decrease in the core samples that were cut perpendicular to the pervasive S2 and L22 because the perpendicular shear on the sigmoidal crenulated cleavages in microlithon of the S2 and S2 mica fish enhances the external applied load, which lead to the failure of core samples.

Initiation and development of the Pennine Basal Thrust (Swiss Alps): a structural and geochronological study of an exhumed megathrust

The Pennine Basal Thrust (PBT) is an exhumed megathrust developed during continental collision from late Eocene to Miocene. To trace its evolution, five samples, with indications for up to three microstructurally diachronous white-mica generations, were investigated by laser in-situ and step-heating 40Ar–39Ar dating. Three deformation-related crystallization ages can be distinguished: (1) D1, characterized in the PBT hanging wall by an S1 foliation defined by white mica + chloritoid, began at or before ∼38.0 Ma; (2) D2 formed a pervasive S2 cleavage and synchronous white-mica rich veins dated at ∼27 Ma; (3) D3 produced an S3 crenulation cleavage and chlorite + white-mica veins dated at ∼23 Ma. Older ages of ∼96 Ma (footwall) and ∼115 Ma (hanging wall) are interpreted as minimum ages for the detrital component. Finally, discrete faulting produced fault gouge, with an illite K–Ar age of ∼19 Ma. A simplified back-restored reconstruction provides a tectonic context for the dated structures. In this framework, D1 occurred during middle to late Eocene tectonic accretion. After late Eocene initiation of continental collision, D2 reflects Oligocene top-to-NW shearing, with both in- and out-of-sequence thrusting. D3 then developed from 23 to 19 Ma during the progressive deactivation of the PBT.

Transpressional imbricate thrust zones controlling gold mineralization in the Central Eastern Desert of Egypt

Strongly deformed volcaniclastic metasediments and ophiolitic slices hosting the Sukari gold mineralization display evidence of a complex structural evolution involving three main ductile deformational events (D1–D3). D1 produced ENE-trending folds associated with NNW-propagating thrust slices and intrusion of the Sukari granite (689±3Ma). D2 formed a moderately to steeply dipping, NNW-trending S2 foliation curved to NE and developed arcuate structure constituting the Kurdeman shear zone (≤595Ma) and East Sukari imbricate thrust belt. Major NE-trending F2 folds, NW-dipping high-angle thrusts, shallow and steeply plunging mineral lineation and shear indicators recorded both subhorizontal and subvertical transport direction during D2. D3 (560–540Ma) formed NNE-trending S3 crenulation cleavage, tight F3 folds, Sukari Thrust and West Sukari imbricate thrust. The system of NW-trending sinistral Kurdeman shear zone (lateral ramps and tear faults) and imbricate thrusts (frontal ramps) forming the actuate structure developed during SE-directed thrusting, whereas the prevailing pattern of NNE-trending dextral Sukari shear zone and imbricate thrusts forming Sukari thrust duplex developed during NE-directed tectonic shearing. Sukari granite intruded in different pluses between 689 and 540Ma and associated with at least four phases of quartz veins with different geometry and orientation. Structural analysis of the shear fabrics indicates that the geometry of the mineralized quartz veins and alteration patterns are controlled by the regional NNW- and NE-trending conjugate zones of transpression. Gold-bearing quartz veins are located within NNW-oriented sinistral shear zones in Kurdeman gold mine area, within steeply dipping NW- and SE dipping thrusts and NE- and NS-oriented dextral and sinistral shear zones around Sukari mine area, and along E-dipping backthrusts and NW-SE and N-S fractures in Sukari granite. The high grade of gold mineralization in Sukari is mainly controlled by SE-dipping back-thrusts branched from the major NW-dipping Sukari Thrust. The gold mineralization in Sukari gold mine and neighboring areas in the Central Eastern Desert of Egypt is mainly controlled by the conjugate shear zones of the Najd Fault System and related to E-W directed shortening associated with oblique convergence between East and West Gondwana.

Tectonic Imprints of the Hazara Kashmir Syntaxis on the Mesozoic Rocks Exposed in Munda, Mohmand Agency, Northwest Pakistan

AbstractTwo well‐developed mesoscopic folds, D2 and D3, which postdate the middle amphibolite metamorphism, were recognized in the western hinterland zone of Pakistan. NW–SE trending D2 folds developed during NE–SW horizontal bulk shortening followed by NE–SW trending D3 folds, which developed during SE–NW shortening. Micro‐ to mesoscopically the NW–SE trending S2 crenulation cleavage, boudins and mineral stretching lineations are overprinted by D3. The newly established NW–SE trending micro‐ to mesoscopic structures in Munda termed D2, which postdated F1/F2, is synchronously developed with F3 structures in the western hinterland zone of Pakistan. We interpret that D2 and D3 folds are counterclockwise rotated in the tectonic event that has evolved the Hazara Kashmir Syntaxis after the main phase Indian plate and Kohistan Island Arc collision. Chlorite replacement by biotite in the main matrix crenulation cleavages indicates prograde metamorphism related with D2. The inclusion of muscovite and biotite in garnet porphyroblasts and the presence of staurolite in these rocks indicate that the Barrovian metamorphic conditions predate D2 and D3. We interpret that garnet, staurolite and calcite porphyroblasts grew before D2 because the well developed S2 crenulation cleavage wraps around these porphyroblasts.

The Archean BIF-hosted Lamego gold deposit, Rio das Velhas greenstone belt, Quadrilátero Ferrífero: Evidence for Cambrian structural modification of an Archean orogenic gold deposit

ABSTRACT The Lamego orogenic gold deposit (1.3 Mt measured resources at 7.26 g/t Au) is located at the south-western end of the 5 km-long Cuiaba–Lamego trend, Quadrilatero Ferrifero region, Brazil. Both Archean orogenic gold deposits are situated in the Rio das Velhas greenstone belt with the lithological succession at Lamego consisting of metamorphosed (greenschist facies) mafic volcanic rock, chert and banded iron formation (BIF), and carbonaceous and micaceous pelites. The Lamego fold, which controls the Lamego deposit, is the most visible structure related to the D1–D2 event, with a perimeter of about 4.8 km and a maximum width of 450 m. The fold is defined by the layering of the mafic unit with minor BIF and large exposures of carbonaceous and micaceous pelites. It is a rootless, reclined, isoclinal, cylindrical fold with an axial trace striking northwest–southeast dipping 20° to 30°. The hinge zone is thickened and the limbs are thinned, with the limbs dipping 20° to 30° to the SE. Orebodies consist of the Lamego BIF, where gold-mineralized zones are related to iron-rich bands, and associated silicification zones. Replacement-style mineralization is associated with sulfide bands; mainly pyrite, As-rich pyrite, and arsenopyrite. Two structural generations, G1 and G2, are recognized and encompass a set of structural elements. The G1 structural generation developed in a progressive deformation event and resulted in structures oriented from NE to SW and dipping to the SE. Structures pertaining to the G2 structural generation are oriented N–S dipping to the W. The four major high-grade gold orebodies are Carruagem, Queimada, Arco da Velha, and Cabeca de Pedra. Their gold grade shows a spheroidal pattern and a distribution that varies along the S1–2 foliation. These lenses represent the hinge zone of F2 reclined folds with the plunge of the orebodies controlled by the F2 fold axes. The lower-grade gold lenses are controlled by pinch and swell, and locally expressed quartz boudins developed during D1–D2. They have two orthogonal directions, one to the NW–SE and the other to the NE–SW, thereby defining chocolate-tablet style boudinage. Hydrothermal monazite grains in a mineralized mafic volcanic rock indicate that mineralization formed at 2730 ± 42 Ma (U–Pb SHRIMP). Younger monazite dated at 2387 ± 46 Ma, and xenotime dated at 518.5 ± 9 Ma suggest Siderian and Cambrian imprint in the Lamego deposit area. The Cambrian age reflects the late stages of the Brasiliano orogenic cycle, which is expressed in the Lamego deposit by the S3 crenulation cleavage (trend NS and dip steeply to the E). Importantly, the Cambrian structural modification is responsible for the present geometry of the Lamego orebodies, but is not associated with hydrothermal alteration; it has not introduced any new gold and also has not caused remobilization of the orebodies or ore minerals.

신보광산 주변지역에서 변성광물의 성장과 변형작용 사이의 상대적인 시간관계: 우라늄 광화대의 상대적인 광화시기

옥천변성대의 남서부에 위치하는 전라북도 진안군 신보광산과 그 동부지역에서 고품위 우라늄 지화학 이상대가 보고된 바가 있다. 본 논문은 선캠브리아기 변성퇴적암류(규암, 변성이질암, 변성사질암)와 이를 관입하는 시대미상의 페그마타이트와 백악기 반암류 등이 분포하는 신보광산 동부지역에서 이들 구성암류가 경험한 변형작용과 변성광물의 성장 사이의 상대적인 시간관계에 대한 미구조의 연구결과를 보고하고, 기존 연구결과를 종합하여 광화작용의 상대적인 시기를 고찰하였다. D1 변형작용은 홍주석 반상변정 내에서 주로 신장된 석영, 흑운모, 불투명광물 등으로 정의되는 직선 형태의 내부엽리 Si 를 형성시켰다. Si 엽리를 미습곡시키고 S2 파랑엽리를 형성시키는 D2 변형작용은 전기 파랑습곡작용과 후기 파랑습곡작용으로 구분된다. 전자는 홍주석의 맨틀부에서 곡선 형태의 Si 를 형성시켰다. 후자는 홍주석을 포획하며 그 외부에서 S1-2 복합엽리를 형성시켰다. 홍주석 반상변정은 직선 Si 엽리에 해당하는 S1 엽리가 형성된 이후의 비변형작용 조건하에서 출현하여 후기 파랑습곡작용 이전까지 성장하였다. 시대미상의 페그마타이트는 D2 변형 이후에 관입하여 S1-2 복합엽리를 무질서하게 덮는 섬유상 규선석을 성장시켰다. D3 변형작용은 S1-2 복합 엽리와 D2 파랑습곡 그리고 섬유상 규선석을 미습곡시키는 F3 습곡을 형성시켰다. 이는 섬유상 규선석의 성장과 관련된 페그마타이트의 관입은 D2 변형과 D3 변형 사이의 비변형조건하에서 발생하였음을 지시한다. 흑운모의 녹니석화작용과 홍주석 반상변정 내부에서 S1-2 복합엽리와 F3 습곡축면에 평행한 두 방향의 벽개 라멜라에 의해 인지되는 후퇴변성작용은 각각 D2 후기변형과 D3 변형과 함께 발생하였다. 따라서 우라늄의 초생적 근원암을 페그마타이트로 간주하는 기존 연구결과를 고려해 볼 때, 시대미상의 페그마타이트는 지각이 상승하는 후퇴변성작용 동안에 해당하는 D2 변형과 D3 변형 사이의 비변형조건하에서 관입하여 신보광산 주변지역에 우라늄 광화대를 형성시킨 것으로 고찰된다. The geochemical high-grade uranium anormal zone has been reported in the Shinbo mine and its eastern areas, Jinan-gun, Jeollabuk-do located in the southwestern part of Ogcheon metamorphic zone, Korea. In this paper is reported the time-relationship between deformation and growth of metamorphic minerals in the eastern area of Shinbo mine, which consists of the Precambrian metasedimentary rocks (quartzite, metapelite, metapsammite) and the age-unknown pegmatite and Cretaceous porphyry which intrude them, and is considered the relative mineralization time on the basis of the previous research's result. The D1 deformation formed the straight-type Si internal foliation which is defined mainly as the arrangement of elongate quartz, biotite, opaque mineral in andalusite porphyroblast. The D2 deformation, which is defined by the microfolding of Si foliation, formed S2 crenulation cleavage. It can be divided into two sub-phases, early crenulation and late crenulation. The former occurs as the curvetype Si foliation in the mantle part of andalusite. The latter occurs as S1-2 composite foliation which warps around the andalusite. The andalusite porphyroblast began to grow under non-deformation condition after the formation of S1 foliation which corresponds to the straight-type Si foliation. It continued to grow before the late crenulation phase. The age-unknown pegmatite intruded after the D2 deformation and grew the fibrous sillimanite which random masks the S1-2 composite foliation. The D3 deformation formed F3 fold which folded the S1-2 composite foliation, D2 crenulation, fibrous sillimanite. It means that the intrusion of pegmatite related to the growth of the fibrous sillimanite took place during the inter-tectonic phase of D2 and D3 deformations. The retrograde metamorphism is recognized by the chloritization of biotite and two-way cleavage lamellae which is parallel to the S1-2 composite foliation and the F3 fold axial surface in the andalusite porphyroblast. It occurred during the D2 late crenulation phase and D3 deformation. In considering of the previous research's result inferring the most likely candidate for the uranium source rock as pegamatite, it indicates that the age-unknown pegmatite intruded during the inter-tectonic phase of D2 and D3 deformations, i.e. during the retrograde metamorphism related to the uplifting of crust, and formed the uranium ore zone around the Shinbo mine.

Structural Controls on Gold Mineralization at the Burnt Timber Mine, Lynn Lake Greenstone Belt, Trans-Hudson Orogen, Manitoba

The Burnt Timber gold deposit is located in the Paleoproterozoic Lynn Lake greenstone belt of the Trans-Hudson Orogen, Manitoba. The deposit occurs along the Johnson shear zone, and is hosted by enriched and depleted tholeiitic volcanic arc basalts cut by feldspar porphyry dikes. The Johnson shear zone is a regional, belt-parallel structure characterized by the intensification of the regional S2 foliation. At the Burnt Timber deposit, S2 is folded by centimeter- to meter-scale, Z-shaped, F3 chevron folds that have an axial planar S3 crenulation cleavage. Within the 20 to 30 m-wide core zone of the shear zone, S2 and F3 are transposed parallel to S3 and the volcanic rocks are strongly sheared parallel to S3. The presence of a steeply plunging stretching lineation, together with dextral shear sense indicators along both S2 and S3 on horizontal surfaces, suggest that the Johnson shear zone is a dextral transpression zone. Gold was introduced in pyritic, biotitic, and carbonatized mafic volcanic rocks along the core of the shear zone. Mylonitic mafic volcanic rocks contain localized high-grade zones (≥10 g/t Au) that differ from wider carbonatized, low-grade zones (<3 g/t Au) by the presence of deformed gold-bearing quartz- pyrite veins. High-grade zones are also associated with swarms of quartz-pyrite veins that cut across sericitized and carbonatized feldspar porphyry dikes, which acted as brittle, competent bodies during shearing. Gold deposition in the high-grade zones was controlled by the channeling of hydrothermal fluid along the core of the shear zone during shearing of the volcanic rocks, and brittle fracturing of the porphyry dikes. © 2006 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved.

Structure of the Early Palaeozoic Cape River Metamorphics, Tasmanides of north Queensland: evaluation of the roles of convergent and extensional tectonics

The Early Palaeozoic Cape River Metamorphics consist mainly of psammitic gneiss and schist and occur as an extensive linear belt at the western margin of the Charters Towers Province 200 km southwest of Townsville in the northern Tasmanides. A prominent foliation (S2) is the main structure in the belt and is associated with tight to isoclinal folds, subparallel mineral and intersection lineations, and boudinaged pods of vein quartz and pegmatite. In the southwest, the main foliation is a crenulation cleavage (S2) related to D2 deformation. It overprints steeply dipping foliation (S1) formed in a D1 deformation but no associated folds have been found. Gently plunging, upright, open folds (D3 deformation) with axial planar S3 crenulation cleavage have affected the main foliation (S2). These deformations were associated with upper greenschist to lower amphibolite facies metamorphism. Amphibolite-grade orthogneiss containing S2 and S3, deformed granite and migmatite of the Fat Hen Creek Complex occurs in the northeast. In the southwest, the main foliation (S2) is folded around a map-scale, gently plunging synclinorium indicating that S2 formed with a subhorizontal orientation. In metamorphic rocks, the origin of widespread, intense subhorizontal foliation, usually associated with recumbent folds, has been considered problematic and in many cases is attributed to crustal extension. We relate the origin of D2 structures to subvertical shortening (i.e. extension) resulting in orientations that are strikingly divergent to those of upright D1 and D3 structures that were induced by compression. The proposed extensional event is poorly constrained in timing but it affected much of the Fat Hen Creek Complex, the oldest known phase of which is 493 Ma, and occurred prior to 40Ar/39Ar cooling ages at 423 – 409 Ma that also post-dated the D3 deformation.

Relative timing of deformation and two-stage gold mineralization at the Hutti Mine, Dharwar Craton, India

Gold mineralization at Hutti is confined to a series of nine parallel, N–S to NNW–SSE trending, steeply dipping shear zones. The host rocks are amphibolites and meta-rhyolites metamorphosed at peak conditions of 660±40°C and 4±1 kbar. They are weakly foliated (S1) and contain barren quartz extension veins. The auriferous shear zones (reefs) are typically characterized by four alteration assemblages and laminated quartz veins, which, in places, occupy the entire reef width of 2–10 m, and contain the bulk of gold mineralization. A <1.5 m wide distal chlorite-sericite (+biotite, calcite, plagioclase) alteration zone can be distinguished from a 3–5 m wide proximal biotite-plagioclase (+quartz, muscovite, calcite) alteration zone. Gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization. An inner chlorite-K-feldspar (+quartz, calcite, scheelite, tourmaline, sphene, epidote, sericite) alteration halo, which rims the laminated quartz veins, is characterized by a pyrrhotite, chalcopyrite, sphalerite, ilmenite, rutile, and gold paragenesis. The distal chlorite-sericite and proximal biotite-plagioclase alteration assemblages are developed in microlithons of the S2–S3 crenulation cleavage and are replaced along S3 by the inner chlorite-K-feldspar alteration, indicating a two-stage evolution for gold mineralization. Ductile D2 shearing, alteration, and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions (560±60°C, 2±1 kbar). The reefs were reactivated in the D3 dextral strike-slip to oblique-slip environment by fault-valve behavior at lower greenschist facies conditions (ca. 300–350°C), which formed the auriferous laminated quartz veins. Later D4 crosscutting veins and D5 faults overprint the gold mineralization. The alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization, which took place either during isobaric cooling or at different levels of the Archean crust. From overlaps in the tectono-metamorphic history, it is concluded that gold mineralization occurred during two tectonic events, affecting the eastern Dharwar craton in south India between ca. 2550 – 2530 Ma: (1) The assemblage of various terranes of the eastern block, and (2) a tectono-magmatic event, which caused late- to posttectonic plutonism and a thermal perturbation. It differs, however, from the pre-peak metamorphic gold mineralization at Kolar and the single-stage mineralization at Ramagiri. Notably, greenschist facies gold mineralization occurred at Hutti 35–90 million years later than in the western Dharwar craton.

Microstructural features of albite porphyroblasts as indicators of sequential Barrovian metamorphic mineral growth in the Caledonides of the SW Scottish Highlands

AbstractInclusion – porphyroblast and porphyroblast – porphyroblast relationships show that abundant albite in mica schists in the Caledonides of the SW Scottish Highlands are part of the Barrovian metamorphic assemblage. Growth early in the D2 deformational phase of porphyroblast cores followed the growth of Mn‐rich garnet but preceded the growth of porphyroblasts of the index mineral almandine. Two sets of inclusion trails in the albite correspond to the regionally expressed S1 and S2. Straight trails of muscovite, chlorite, quartz, epidote and the earliest growth of biotite make up S1. Crenulated trails express deformation of S1 early in D2 with muscovite, chlorite, biotite, quartz, epidote and the Mn‐rich garnet associated with the development of S2 crenulation cleavage. The geometries of these trails uniquely record early stages of D2 deformational history. An 0−3 growth is related to the temporal coincidence of the formation of S1–S2 crenulation cleavage hinges as favourable sites for nucleation and the release of large amounts of water from prograde reactions during tectonothermal reconstitution of first cycle immature sediments with a volcanic component. The main characteristics of the regionally expressed D2 schistosity were developed during the major grain coarsening that followed both albite and almandine porphyroblast growth. Essentially inclusion‐free An 4−19 rims grew on the inclusion‐containing cores in the almandine zone in the later stages of schistosity growth and unoriented porphyroblasts of muscovite, biotite and chlorite indicate that mineral growth extended from the later stages of D2 to post‐D2. Previous interpretations of the albite porphyroblast growth having been during D4 to post‐D4 contemporaneous with retrogression are inconsistent with the microstructural evidence.

Late-orogenic backfolding and extension in the Brooks Range collisional orogen, northern Alaska

New structural observations across a crustal section spanning relatively unmetamorphosed fold–thrust belt rocks and epidote–amphibolite grade rocks document the late-orogenic kinematic history of the Brooks Range, Alaska, and provide insight into the character, relative timing, and geodynamic setting of late-orogenic backfolding and extension in collisional orogens. Greenschist-grade rocks on the north flank of a metamorphic culmination display mesoscopic, open-to-close, S-vergent folds (and associated cleavages) and small-displacement normal faults, suggesting a sequence of D2 backfolding and D3 extension following the protracted phase of N-directed D1. At deeper levels, epidote–amphibolite facies rocks display a pervasive gently N-dipping S2 crenulation cleavage that is axial-planar to kilometer- and smaller-scale tight to isoclinal recumbent F2 folds. Porphyroblast–matrix relationships suggest that folds at the deeper levels initiated during synmetamorphic D2 backfolding. Much of the strain recorded by S2 and the tightening of folds, however, probably accumulated during post-metamorphic D3 extension.40Ar/39Ar data suggest an early- to mid-Albian age for D2 backfolding, which may be related to formation of a regional-scale backfold. Backfolding, together with apparently synchronous thrusting near the range front may represent the final stages of Cretaceous contractional deformation in the orogen. Contraction was immediately followed by extension, which, aided by erosion, produced a short-lived period of cooling and exhumation beginning at ∼103Ma. The available data suggest that extension is post-contractional, and thus is not viewed as a driving force of thrusting in the foreland. Extension was not limited to range-bounding normal faults, but rather was distributed over much of the southern half of the Brooks Range, with character of extensional fabrics being dependent on the geometry of D2 fabrics.Inasmuch as backfolding and extension are common features within collisional orogenic belts, the conclusions drawn here have implications regarding fundamental processes associated with the late-stages of collisional orogenesis. In particular, the study provides an example of the roles that backfolding and extension have played in the geodynamic development of a collisional orogen.

Deformation history of the blueschist-facies sequences from the Villa de Cura unit (Northern Venezuela)

The Serrania del Interior terrane is located in Northern Venezuela, that represents the southern margin of the Caribbean plate. This terrane includes the Villa de Cura unit, which mainly consists of multiple sub-units of metamorphosed and deformed Cretaceous volcano-sedimentary sequences. These sequences are commonly interpreted as generated in a supra-subduction zone setting. The structural history of the Villa de Cura blueschists-facies units includes four main deformational phases, from D1 to D4. The first D1 phase is mainly represented by a relict S1 schistosity developed under HP/LT conditions. The relic S1 schistosity is deformed by isoclinal to subisoclinal F2 folds showing similar geometry. The F2 folds are characterized by a continuous S2 crenulation cleavage developed under greenschist-facies metamorphism. The parallelism between the A2 axes and the related L2 mineral lineations suggests an interpretation of the F2 folds as sheath folds developed during non-coaxial deformation. The kinematic indicators suggest a top-to-W sense of shear for the D2 phase. The D3 phase is distinguished by asymmetric, parallel F3 folds with a S vergence. Finally, the D4 phase consists of F4 open, gentle folds with high-angle to sub-vertical axial planes. The collected data suggest a complex deformation history, characterized by coupling of strike-slip tectonics and shortening during the retrograde evolution of the blueschist-facies sequences.

Cretaceous structural evolution of the Pelagonian crystalline in western Voras Mt (Macedonia, Northern Greece)

The kinematic of the Cretaceous deformation and the relationship between deformation and metamorphism of the Pelagonian crystalline was studied in Voras Mt (northern Greece). The Pelagonian crystalline in this area has been subdivided into a lower, core part, consisting mainly of gneissic rocks and schists and an upper, cover part, consistine of schists and quartzites with marble intercalations. Intensely deformed granitic rocks of Upper Paleozoic age intrude the Pelagonian crystalline basement. An S j foliation is the oldest fabric recognized in the Pelagonian crystalline. Sj is mainly defined by syn-Sjgarnet(Gr1), biotite(Btj), white mica (Wnij), chloritoid, kyanite and plagioclase in the metapelitic rocks and green amphibole, epidote, plagioclase, and biotiteiBtj) in the amphibolite. Garnet grows also in some cases post-kinematically. Ilmenite and tourmaline are often found in the pelitic rocks as well. S is overprinted by an S2 foliation that developed as a crenulation cleavage. In most places, however, S2 has destroyed all earlier fabrics and a single S2 fabric is present related to, isoclinal or sheath folds intrafolial in places. S2 in the metapelitic rocks is characterized by the syn-S2 development of chlorite, white mica(Wm2) and plagioclase. In the amphibolite S2 is mainly defined by the syn-kinematic development of actinolite, plagioclase, biotite(Bt2), white mica(Wm2) and chlorite. During D2 garnet(Gr1) and biotiteiBtj) are partially replaced by chlorite, while green amphibole is replaced by actinolite and chlorite. Chloritoid remains generally stable along the S2-planes but in some places transforms to chlorite and sericite. Furthermore, D2 was locally followed by a static post-kinematic annealing indicated by polygonal quartz microfabrics with equilibrated grain boundaries and triple points. The overall orientation of S2 is dome shaped with a gentle SW-ward and NE-ward dip in the southwestern and northeastern flanks of the dome respectively. Syn-S2 minerals defined a very well exposed NW-SE trending stretching lineation. Kinematic indicators show a main top to the SE sense of movement. An S3 crenulation cleavage associated with asymmetric NW-SE trending folds is also present in most parts of the core and cover rocks, possibly, related to a constrictional type of deformation. A well developed, S4 shear band cleavage is mainly present in the upper parts of the metamorphic dome and formed under cooler conditions. S4 shear bands are associated with a NW-SE developed stretching lineation defined by elongated and dynamically recrystallized quartz grains and a preferred orientation of white mica and chlorite. Along the S4 shear bands a transformation of garnet, biotite, chloritoid and amphibole into chlorite is always observed. S4 shear bands indicate a main top to the SE sense of movement. The P-T metamorphic conditions were derived from textural equilibria and mineral assemblages, as well as from the spatial distribution of the metamorphic minerals. Syn-Dj metamorphism reached the conditions of the boundaries between greenschist and amphibolite facies. Syn-D2 retrogression took place under greenschist facie conditions. K/Ar radiometric datings on coarse-grained syn-St and younger fine-grained syn-S2 micas define an Early Cretaceous cooling age ('135Ma) for the older event and a Mid- to Late Cretaceous age ('90-80Ma) for the second event. A white mica age of ca. 65Ma correlates with S4 shear band clevage. Furthermore, the intrusion age of a granitic body into the Pelagonian crystalline is dated using the Pb/Pb single zircon evaporation method. The estimated intrusion age of 300±3Ma suggests that the Pelagonian crystalline was affected by a pre-kinematic magmatic activity relative to its Cretaceous deformation.

Sub-greenschist facies assemblages of metabasites from south-eastern Peloritani range (NE-Sicily)

Pre-Hercynian magmatic rocks are widespread in the Palaeozoic basement of the Peloritani range. The metabasites of the Mongiuffi-Melia and Gallodoro areas represent the largest and the most important lower-Ordovician magmatic products of this range. These rocks were metamorphosed during the Hercynian event and preserve relict igneous minerals and textures. A detailed study of the metamorphic assemblages has allowed the identification of two stages of metamorphic crystallisation; they are mainly distinguished by the differentXCO2 of the fluid phase. The first metamorphic event produced three calcite-free sub-greenschist facies assemblages that contain ubiquitous quartz+albite+titanite+chlorite+epidote along with pumpellyite or prehnite or actinolite. The second metamorphic episode produced a “Calcite+Chlorite” assemblage, non-diagnostic to evaluate P-T conditions of metamorphism. The first stage assemblages are only preserved in small domains of the rock. As even very low amounts of CO2 in the fluid phase drastically inhibit the formation of diagnostic sub-greenschist facies calc-silicate assemblages, it appears that a more CO2-rich fluid must have been introduced during the second event. We suggest that this introduction of more CO2-rich fluid occurred during the development of S2 crenulation cleavage.

Mesostructures and deformational history of the Central Crystallines: an example from Garhwal Himalaya, India

Structural analysis reveals that the Central Crystallines in the Garhwal region were subjected to four phases of deformations (D1 to D4). The D1 deformational phase is highly obliterated and usually found as F1 intrafolial (rootless) tight isoclinal folds in migmatites and gneisses. The D2 deformational phase produced strong pervasive S2 schistosity and asymmetric and open fold (F2) plunging 20-30° towards ENE-WSW. The L2 lineation plunge 5-10° towards east-west is well developed in medium grade metamorphic rocks. The D1 deformations were responsible for F3 folds reflected in large scale anticlinal and synclinal, overturned and recumbent folds, which have 10-40° plunges towards NW. The late D3 deformational stresses were responsible for shearing along the middle limbs of F1 folds and they ultimately initiated thrusting. The NNE­ SSW plunging mineral or stretching lineation (L3), S3 crenulation cleavage and S-C fabrics were developed during the dominant ductile shearing related to the late D3 deformation. The D4 phase characterised by brittle-ductile deformation (minor kinks, puckers, transverse/transcurrent faults, and S-C' fabrics) and extensive cataclasis along thrust- and fault-zones reflects the last episode of deformation. The structural and geochronological data indicate that D1 and D2 deformation episodes may be related to the Precambrian time while D3 and D4 are exclusively of the Tertiary age.

On the deformation sequence in the New Harbour Group of Holy Island, Anglesey, North Wales

Four phases of deformation are recorded by minor structures in the New Harbour Group (NHG) of southern Holy Island. The regional schistosity in these rocks is a differentiated crenulation cleavage of D2 age. An earlier preferred orientation (S1) is commonly preserved as crenulations within the Q-domain microlithons of the S2 schistosity and is demonstrably non-parallel to bedding. F3 folds are widely developed in S2 and, to a lesser extent, in bedding. S3 crenulation cleavage is sporadically developed but can be intense locally. A major antiformal fold exists in the NHG near Rhoscolyn. This fold is of D3 age since it clearly deforms S2 schistosity and is consistent with the vergence of F3 minor structures. All planar structures are deformed by folds of D4 age. © 1997 John Wiley & Sons, Ltd.

Microstructure, metamorphism, thermochronology and P‐T‐t‐deformation history of the Port aux Basques gneisses, south‐west Newfoundland, Canada*

Abstract The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent folds (F2), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW‐directed reverse shearing. The NE‐trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright‐to‐steeply inclined, periclinal folds (F3) and a variably penetrative schistosity (S3).Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast‐matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K‐feldspar, and the development of an increasing volume of anatectic migmatite in south‐eastern sectors of the area record syn‐ to late‐D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P‐T space for units of the Port aux Basques gneisses. Peak metamorphic conditions are estimated to have been 620–650° C at ≤8kbar in the west and 700–750° C at ≤8 kbar in the east.Titanite from an upper amphibolite facies calc‐silicate gneiss yields U‐Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in these gneisses. Variable D3 strain was associated with some recrystallization of hornblende and micas. 40Ar/39Ar hornblende plateau isotope correlation ages range from c. 419 to c. 393 Ma, from east to west across the area, and are interpreted to record cooling through c. 500° C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential uplift from south‐east to north‐west associated with displacement on major shear zones. 40Ar/39Ar muscovite plateau ages cluster at c. 390 Ma, and date cooling through c. 375° C during regional exhumation. Cooling rates are moderate to fast and may indicate a component of tectonic exhumation.The Port aux Basques gneisses are a product of Silurian collisional tectonics. The higher grade of metamorphism in comparison with adjacent areas of the Canadian Appalachians is interpreted to reflect greater thickening due to juxtaposition of the St Lawrence promontory (Laurentian margin) with the Cabot promontory (Avalonian margin) during closure of the Iapetus Ocean.