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1.
Sub-volcanic intrusive networks, of which cone sheets are recognised as a major constituent, partially control volcano growth and eruption style. The accepted cone sheet model is that these confocally dipping intrusions originate from an unexposed central magma chamber through dip-parallel magma flow. However, the emplacement mechanism of cone sheets remains poorly understood. The ∼58 Ma cone sheet swarm of Ardnamurchan, NW Scotland, offers an excellent opportunity to further resolve the emplacement dynamics of cone sheets, through studying magma flow, and their importance in volcanic edifice construction. Structural measurements and anisotropy of magnetic susceptibility (AMS) analyses have constrained a lateral magma flow pattern, consistently oriented NW–SE, in the majority of the Ardnamurchan cone sheets. Field observations also highlight the importance of host rock structure and interference between competing local and regional stress fields in controlling intrusion geometry. Our observations suggest cone sheet formation may be linked to laterally propagating NW–SE trending regional dykes, sourced from laterally adjacent magmatic systems (likely the Palaeogene Mull Central Complex), which are deflected around the central complex by stress field interference. Implicitly, edifice construction and potential eruption precursors observed at a volcano may instigate, or result from, magmatic activity in laterally adjacent volcanic systems. 相似文献
2.
Occurrence and Dispersal of Magmas in the Jurassic Ferrar Large Igneous Province, Antarctica 总被引：1，自引：0，他引：1
Tholeiitic rocks of the Ferrar Large Igneous Province (FLIP) occur in a linear belt from the Theron Mountains to Horn Bluff in the Transantarctic Mountains and extend into southeastern Australasia. The FLIP was emplaced during the initial stages of Gondwana break-up from a source suggested to be in the proto-Weddell Sea region. Magma transport from its source (Weddell triple junction) was controlled by an Early Jurassic zone of extension. The FLIP comprises the Dufek intrusion, Ferrar Dolerite sills and dykes (sheet intrusions), and extrusive rocks consisting of pyroclastic strata overlain by Kirkpatrick Basalt lavas. The Dufek intrusion occurs in deformed supracrustal rocks of the foldbelt along the paleo-Pacific Gondwana margin. A few sills were emplaced in basement rocks, but the majority of the sheet intrusions occur in flat-lying Devonian to Triassic Beacon strata. Only in the central Transantarctic Mountains (CTM) and south and north Victoria Land (SVL, NVL) are extrusive rocks preserved overlying Beacon strata. The greatest cumulative thicknesses of magmatic rocks (ca. 2 km) occur in areas where lavas are preserved (CTM and SVL). Sheet intrusions have complex relationships. Dyke swarms (sensu stricto) are unknown and dykes cutting basement rocks are uncommon. Nevertheless, these dykes, including a 30-m-wide dyke in SVL, suggest that some magmas locally migrated up through basement rocks. In CTM and NVL the outcrop belt has a width of about 160 km. Sills originally extended farther toward the plate margin but have been cut out by erosion and Cenozoic faulting, most clearly in CTM; geophysical data suggest extension under the East Antarctic ice sheet for at least 100 km. Although Early Jurassic extension is documented in CTM, major rift-bounding faults have not been observed. Models for magma emplacement include transport along the axis of the Transantarctic Mountains and off-axis transport from major rift-bounding faults. Contrasts in geochemistry between lavas of NVL (MgO=67%) and CTM (MgO=24%) and the presence of massive dolerite bodies (CTM, SVL) suggest discrete episodes and locations of magma emplacement, and that there was no long range interconnection along the mountain range in supracrustal rocks. 相似文献
3.
Numerous batches of initially heterogeneous magma aggregated to form the I-type Lysterfield Granodiorite, with four geochemically distinct series of granodioritic rocks, emplaced in the shallow crust. The compositional heterogeneities originated through variations in the stoichiometries of melting reactions in the protolith terrane and variable degrees of peritectic assemblage entrainment. The high-K series contains igneous, microgranular enclaves that most probably formed through deep-level hybridisation of enriched mantle magmas with crustal melts. In the model presented here, this heterogeneous collection of magmas ascended to form a thin, sheet-like intrusion, quenched against cold wall rocks. Later, laccolithic inflation, through ingress of voluminous more felsic magmas, arched the pluton roof and fragmented the initial sheet, pieces of which fell back into the Granodiorite to become enclaves, some of which were further hybridised by plastic deformation and mechanical incorporation of host-derived crystals. This may be a common mechanism for the formation of such enclave suites. 相似文献
4.
David D. Pollard 《Tectonophysics》1973,19(3):233-269
A two-dimensional mechanical model for sheet intrusions based on the elastic deformation around a pressurized elliptical hole is derived. After evaluating the effects of irregularities in contact shape, possible magma pressure distributions, regional boundary conditions, host rock properties, and interference from adjacent intrusions the model can be applied with confidence to understand many features associated with dike or sill emplacement. Three mechanisms for sheet intrusion propagation are extension fracturing, brittle faulting, and ductile faulting. This deformation is concentrated at the intrusion termination, the site of large principal stress differences and large stress gradients. In both brittle and ductile host rock the magma pressure needs only barely to exceed the regional stress acting normal to the intrusion's length for propagation to occur. Propagation direction is controlled by the regional stress orientation, interference from adjacent intrusions, planar discontinuities, and changes in host rock properties. The classic method of distinguishing dilational from non-dilational intrusions may not work because contacts are not planar or parallel and displacements are not everywhere perpendicular to the intrusion's length. Equations are derived for calculating the magma pressure for intrusions in relatively simple tectonic settings. 相似文献
5.
6.
F. A. Temporim Ricardo Ivan Ferreirae da Trindade Marcos Egydio-Silva Tiago Valim Angelo Eric Tohver Caroline Cibele Soares Lucas Pequeno Gouvêa Julio Cesar Mendes Silvia Regina Medeiros Antônio Carlos Pedrosa-Soares Gabriel Gomes Silva 《地学学报》2023,35(1):23-31
Featuring 3 000-km-long large and hot orogen, the Mantiqueira Province provides a rare opportunity to study the process of gravitational collapse at mid to deep crustal levels. Distinct but contemporary (~500 Ma) post-collisional intrusions show structures and anisotropy of magnetic susceptibility (AMS) fabrics related to their emplacements, recording different flow patterns. In southern deep-seated intrusions, ellipsoidal-shaped roots with gabbroic-to-hybrid cores surrounded by granitic rocks show concentric patterns of AMS fabrics that cut across the NE-trending regional foliation. In contrast, northern intrusions, exposed as the upper sections of batholith-size bodies of coarse-grained granite emplaced at the shallow to mid-crust, show general NS-trending magnetic fabrics roughly parallel to strike of the orogen and the regional foliation of host rocks. These contrasting magnetic patterns from shallow to deeper crust suggest vertical magma migration from the overthickened orogenic core to be emplaced across its thinner stretched flanks during the gravitational collapse of the orogenic edifice. 相似文献
7.
The Jinchuan Ni-Cu sulfide deposits, NW China, are hosted in small ultramafic intrusions that were emplaced into Paleoproterozoic metamorphic rocks. The ultramafic intrusions were previously thought to be the segments of a single elongate intrusion that was dismembered by late faults into eastern and western portions, each of which have distinct stratigraphic sequences. 相似文献
8.
GUO Rui ZHANG Gongcheng ZHANG Jinwei ZHAO Xingbin LIU Junbang YUAN Dawei SONG Shuang 《《地质学报》英文版》2013,87(5):1306-1318
An extensive suite of igneous sills was intruded into the Tertiary sedimentary section of the Jiaojiang sag,East China Sea.This suite has been well imaged offshore through 2D and 3D seismic surveys,showing a close relationship with CO2 content in nearby gas discoveries.A new observational model,which incorporates simple upward propagation,simple horizontal propagation,and transgressive propagation,was proposed to interpret these sill intrusions.In this model,the source magma of the saucer-shaped sills was injected from their lowest points near the center.The transgressive propagation can be interpreted as a combination of the vertical and horizontal propagation.Most shallow sub-volcanic intrusions in the Jiaojiang sag exhibit fingered characteristics,both vertically and horizontally.The vertical fingered propagation produced saucer-shaped sills arranged in the flower style.Along the brims of lower sills could usually be found the upper sills,which are interpreted to have formed simultaneously with or later than the lower feeder sills.In the second type,the chilled paths of the older sills were reutilized by subsequent intrusions.The horizontal fingered propagation formed sheet intrusions that exhibit groove,lobate,tubular,and crevasse splaylike geometry in plan view.In 3D view,the sheet intrusions have still preserved the lower center and higher rim that similar with the sub-rounded saucer-shaped sills.Although fracture propagation may be important,flow inflation was the key mechanism of magma intrusion.Further consideration of the mechanisms underlying sill formation may help explain the fingered characteristics of sill propagation. 相似文献
9.
Chromite is a ubiquitous accessory mineral in the olivine-pyroxene cumulate bodies that host massive and disseminated nickel sulphide mineralization in intrusions of the Kabanga-Musongati-Kapalagulu Alignment in East Africa. Its composition is related to the conditions of emplacement and petrologic evolution of its host magma in a spectrum of intrusions ranging from classical lopolithic layered intrusions to groups of smaller, discrete sill-like chonoliths.The Kapalagulu lopolithic intrusion, emplaced into polymetamorphosed Archæan-Palæoproterozoic crust, contains abundant chromite with relatively oxidized compositions, whereas chromites from the highly-mineralized Kabanga chonolith intrusions, emplaced into graphitic and sulphidic schists, are strongly reduced in terms of their Fe3+/Fetotal ratio. Ni in chromite correlates with Ni in olivine: Ni in both is depleted in the more strongly sulphide-mineralized intrusions. The Musongati intrusion, also emplaced through graphitic schists, but much larger and less-well mineralized in sulphides than Kabanga, has chromites intermediate in character. The compositions of the chromites can be used to determine the petrologic history of the intrusions, and may prove to be a useful exploration tool in such mineralized belts. 相似文献
10.
Yang Li Wentian Liang Guowei Zhang Yazhou Ran Qi Shen Jingli Wang Chunsheng Jin 《地学前缘(英文版)》2018,9(1):191-205
The Huamenlou pluton,is an elongated granite intrusion with high aspect ratio,emplaced within the southern margin of the North Qinling(central China).Here we investigate this pluton through multiple techniques including the fabric study,microstructural observation and zircon geochronology.Our zircon U-Pb data confirm that the granite crystallized at ca.462 Ma which is consistent with the ages of other linear plutons in North Qinling.Microstructural observations of the Huamenlou granites illustrate that the pluton has undergone superimposed deformation during its emplacement,from magmatic to hightemperature solid state conditions.The internal fabric obtained by anisotropy of magnetic susceptibility(AMS)and shape preferred orientation(SPO)show similar results.The fabrics are relatively concordant and generally vary from NE-SW to NEE-SWW which are roughly oblique to the trend of the pluton elongation and the regional structures.Meanwhile,scalar parameters reflect two completely different strain regimes for the pluton and its host rocks,i.e.,the fabrics within host rocks are mainly oblate while the central part of the intrusion displays mainly prolate fabrics.It is inferred that the structural pattern recorded in this pluton was caused by local dextral transtension in consequence of oblique convergence between the South and North China Blocks.We propose that the local transtension in convergence setting probably evolved from vertical extrusion tectonics that provided room for the magma emplacement and imparted prolate fabrics in the Huamenlou pluton. 相似文献
11.
Deposits of Ni–Cu–Co–(PGE) sulfide often occur in association with small differentiated intrusions that reside within local transtensional spaces in strike-slip fault zones. These faults often develop in response to incipient rifting of the crust and the development of large igneous provinces due to far-field stresses generated by plume-induced continental drift. We review the geology of a number of large and small nickel sulfide deposits and the associated intrusions, and show that the geometry of the host intrusion and localization of the mineral zones can be classified into three main groups. Further, we show that the morphology of each is controlled by space created in response to deformation on structures.One group of intrusions has the plan shape of an asymmetric rhomboid with the long axis sub-parallel to a fault zone, and contacts which have often been structurally modified during and/or after emplacement of the magma. The typical cross section is a downward-closing cone shape with curved walls and often a dyke-like keel at the base. This morphology is found in the Ovoid and Discovery Hill Zones of the Voisey's Bay Deposit (Canada), the Jinchuan, Huangshan, Huangshandong, Hongqiling, Limahe, Qingquanshan, and Jingbulake (Qingbulake) Intrusions in China, and the Eagle and Eagle's Nest deposits in the USA and Canada, respectively.A second group of deposits is associated with conduits within dyke and sheet-like intrusions; these deposits are often associated with discontinuities in the dyke which were created in response to structural controls during emplacement. Examples include the Discovery Hill Deposit and the Reid Brook Zone of the Voisey's Bay Intrusion, where there are plunging domains of thicker dyke which control the mineralization inside the dyke, and thin discontinuous segments of the dyke which are associated with structurally controlled mineralization in the surrounding country rock gneisses. The Oktyabrysk, Taimyrsk, Komsomolsk, and Gluboky Deposits in the Noril'sk Region of Russia are localized at the base of thicker parts of the Kharaelakh Intrusion which appear to be a conduit that follows synformal features in the country rocks located west of the Noril'sk–Kharaelakh Fault. Other examples of dyke-like bodies with both variation in width and the development of discontinuities are the Copper Cliff and Worthington Offset Dykes which radiate away from the Sudbury Igneous Complex (Canada). The distribution of ore bodies in these Sudbury Offset Dykes is principally controlled by variations in the thickness of the dyke, interpreted to reflect the presence of conduits within the dyke.A third group of mineralized intrusions located within structural corridors have the geometry of oblate tubes; examples include Kalatongke in China, Northeastern Talnakh and Noril'sk 1 in Russia, Babel–Nebo in Australia, and Nkomati in South Africa. Sometimes these oblate tube-like intrusions form in bridging structures between larger intrusions hosted in the more significant structures. Examples include the Tamarack Intrusion in Minnesota, USA, and the Current Lake Complex in Ontario, Canada, both of which contain magmatic Ni–Cu sulfide mineralization.In all of these deposits, the intrusions appear to be open system magma pathways, and so the term “chonolith” can be applied to describe them as a group. All of these intrusions are characterized by a high ratio of sulfide/silicate; there are 1–3 orders of magnitude more sulfide in the intrusion than the magma contained in the intrusion is capable of dissolving. The formation of these deposits is considered to have taken place in open system magma conduits. It is possible that the metal tenor of the sulfides were upgraded by equilibration of successive batches of silicate magma passing through the conduit, and equilibrating with a stationary pool of magmatic sulfide. At Voisey's Bay there appears little doubt that the sulfides were injected through a conduit dyke into higher level magma chambers. A similar model has been proposed for the formation of the deposits at Jinchuan and Noril'sk–Kharaelakh. Economically significant nickel sulfide deposits that tend to be high in Ni tenor, are often related to the late injection of magma that form distinct parts of the intrusion, and the localization of mineralization tends to be related to changes in the geometry of the magma chamber. Strongly deformed and metamorphosed komatiite-associated deposits (e.g. Pechenga, Thompson, and the Yilgarn komatiite associations) appear to be the remains of open system magma conduits which are now represented by segmented and boudinaged ultramafic bodies as a result of more than 4 phases of post-emplacement deformation.LIP activity at craton margins has long been recognized as a key control on the genesis of magmatic sulfide deposits; we show that the principal regional controls of strike-slip tectonics underpin the local geometry of the intrusions, and we provide an explanation for why so many of the global nickel sulfide ore deposits are associated with intrusions that share common morphologies and characteristics. This model provides a framework for more detailed structural investigations of nickel sulfide deposits, and it is a predictive framework for mineral exploration. 相似文献
12.
Most magmatic Ni–Cu–PGE sulphide deposits occur within long-lived magma pathways fed by high degree partial melts of the mantle. Holistic mineral-system analysis for such deposits has some parallels with dominantly hydrothermal systems, but also some important differences.Major provinces are associated with large volumes of magma erupted at margins of ancient Archaean cratons, and are associated with small intrusions through which large volumes of magmas have passed. There is no demonstrable association with any particular magma type, although in most provinces the ores are found associated with the most primitive available magmas, whatever these may be. Ore-bearing intrusions tend to form early in the evolution of the host province, although exceptions exist to this rule, and these intrusions typically account for very small proportions of the volumes of the province as a whole.Ore deposition is favoured by prolonged high-volume flow over a horizontal floor. This floor may take the form of the base of a channelized sill, tube or blade-shaped dyke, which account for most of the known host igneous bodies to significant ore deposits. Deposition mechanisms may be chemical or physical, but large high-grade deposits require a major component of transported sulphide liquid, initially carried as droplets. Late stage migration of sulphide liquid as gravity currents within intrusion networks, coupled with infiltration and melting of floor rocks, accounts for the common observation in mafic intrusion hosted deposits of cross cutting relationships between massive sulphides, host intrusions and country rocks.The following set of criteria is proposed in targeting and evaluating Ni–Cu–PGE sulphide systems: 1) nature of magmatism and relationship to pre-existing cratonic architecture; 2) magmatic and structural controls on the development of protracted-flow magma conduits; 3) access to crustal S sources at some point along the pathway; 4) favourable intrusion geometry and emplacement style for deposition, reworking and upgrading of sulphide magmas, and 5) favourable structural history and erosional level for preservation and detectability.These components can be translated into mappable geological criteria. At the predictive targeting scale, the key features are proximity to ancient cratonic boundaries and long-lived, trans-crustal structures, and relationship to voluminous mafic or ultramafic magmatism typically with high Mg and low Ti contents, but otherwise lacking distinctive characteristics. At the detection scale, there are two distinct approaches: recognition of high volume magma pathways with prolonged flow-through operating at length scales of km based on morphological, petrological, geophysical and structural observations; and identification of the petrographic and geochemical signals of accumulation or extraction of sulphide liquid. 相似文献
13.
The Lilloise Intrusion, East Greenland: Fractionation of a Hydrous Alkali Picritic Magma 总被引：2，自引：1，他引：1
The Lilloise is an 8 km4 km layered mafic intrusion which cutsthe plateau basalts of the East Greenland Tertiary province.Lilloise was intruded at 50 Ma, 4–5 Ma after cessationof the voluminous tholeiitic magmatism which accompanied riftingof the East Greenland continental margin. Lilloise is unusualamong layered intrusions in the province because it had a hydrousalkali picrite parent magma and generated a late-stage effluxof magmatic water from the intrusion into the aureole rocks.The three major subdivisions of the layered rocks are: olivine-clinopyroxene,olivine-clinopyroxene-plagioclase and plagioclase-amphibolecumulates. Massive subsidence of the intrusion before completesolidification resulted in deformation of the internal layeringand downturn of the bedding in the surrounding basalts. A strikingfeature of the intrusion is the injection of the layered rocksby a plexus of magmatic sheets which formed at the time of subsidence.The composition of these sheets is representative of the fractionationtrend of the intrusion and ranges from hawaiite to mildly saturatedquartz trachyte. The fractionation trend is successfully explainedby extraction of cumulus minerals of the layered rocks froma parent magma represented by alkali picrite dykes of a contemporaneousregional dyke swarm. Saturated to mildly over-saturated syenitesare a major component of the East Greenland province and theLilloise intrusion is illustrative of an important magmatictrend towards such compositions at this stage in the openingof the North Atlantic. KEY WORDS: Lilloise intrusion; East Greenland; alkali picrite magma; layered intrusion; magmatic differentiation
*Corraponding author 相似文献
14.
The Novate intrusion is a Late Alpine leucogranite that intruded the structures related to dextral back‐thrusting along the Periadriatic Fault System in the Eastern Central Alps. The Novate granite was heterogeneously deformed from amphibolite to greenschist facies conditions during cooling of the intrusion. The deformation inside the granite is characterized by strongly localized and anastomosed ductile shear zones surrounding lenses of weakly deformed granite and by late faults formed at the brittle–ductile transition. The fault kinematic analysis of conjugated shear zones suggests that the Novate leucogranite was emplaced at 25 Ma in an extensional regime along the southern tip of the Forcola Fault. A model of extensional jog opening by vertical shearing along the Forcola Fault provided the space for magma accommodation. The Novate granite is the first evidence for orogen‐parallel syn‐extensional leucogranite emplacement during the Oligocene collision in the Alps. 相似文献
15.
《Journal of Structural Geology》2001,23(6-7):1141-1150
Emplacement of the Proterozoic Gunnison annular complex, Colorado, involved brittle failure and both subhorizontal sheeting and steeply-dipping dyking. The annular complex consists of a central diorite body (1730 Ma), surrounded by a ring of metamorphosed supracrustal rocks, in turn ringed by tonalite and granodiorite (1721 Ma). The older central diorite was emplaced as sills parallel to bedding, prior to regional deformation of the Gunnison volcanic arc, ∼1730–1710 m.y. ago. This central body was deformed during regional shortening, into an upright bowl with inward-dipping walls. The surrounding country rocks were folded and locally transposed against the central body, forming an arcuate foliation, conforming roughly to the shape of the body. This foliation and bedding acted as mechanical planes of weakness, which localized the syn-deformational emplacement of the outer ring intrusions, as multiple magma sheets parallel to steeply dipping bedding and foliation. Portions of the outer rings were also injected along concentric, inward-dipping, shear fractures generated under high magma pressure, a process similar to cone sheet emplacement in volcanic ring-dyke complexes. Multiple mechanisms were therefore involved in the emplacement of this sheeted annular plutonic complex, which otherwise superficially resembles a subvolcanic ring-dyke complex. 相似文献
16.
A series of linked extensional detachments, transfer faults, and sediment- and volcanic-filled half-grabens that pre-date regional folding are described in the Late Archaean Margaret anticline, Eastern Goldfields Province, Yilgarn Craton, Western Australia. Coeval structures and rock units include layer-parallel extensional detachments, transfer faults (high-angle rotational faults rooted in the detachments and linking layer-parallel shear zones with varying amounts of extension); felsic intrusions, either as granitoids emplaced in or below the detachments, or as fine-grained intrusive bodies emplaced above the detachments and controlled by the high-angle faults; and half-grabens controlled by the high-angle faults and filled with clastic sedimentary and volcanic rocks. At least 1500 m of section is excised across the detachments. The detachments and high-angle faults are folded by the east-northeast regional compression that formed the Margaret anticline. Extensional deformation in the Margaret anticline is correlated with the regionally recognised felsic magmatism and associated volcanic and volcaniclastic basin fill dated at approximately 2685–2670 Ma across the Eastern Goldfields Province. This suggests the extensional event was province-wide and post-dated initial greenstone deposition (at around 2705 Ma) but pre-dated regional compressive deformation. We suggest the extension is the result of a thermal anomaly in the crust, generated by the insulating effect of a thick pile (of the order of 10 km or greater) of mafic and ultramafic volcanic rocks on precursor Archaean felsic crust. The thermal anomaly has generated renewed production of felsic and mafic volcanic rocks, coeval with uplift and extension in the upper crust. 相似文献
17.
A Lower Ordovician age for the Oughterard Granite is established by Rb–Sr dating of magmatic white micas from pegmatites cutting two of the satellite bodies found 12–15 km west of the main granitic intrusions. These micas give a minimum age of c. 473 Ma for the emplacement of the satellite bodies, and for the completion of the D3 deformation in the Dalradian host rocks. The main granite intrusions are post-D4 in age, but some of the small outlying sheet-like intrusions are either deformed by D4 folds, or were present during this deformation and were responsible for the development of locally disharmonic D4 minor folds. The correlation of the smaller granite bodies with the main bodies of Oughterard Granite is supported by their similar field appearance, petrography and trace element geochemistry. In common with other late orogenic granites, such as the 470 Ma Aberdeen Granite in NE Scotland, the Oughterard Granite is thought to have been part of a discontinuous magmatic arc, which formed along the southern margin of the Dalradian outcrop in Connemara and in NE Scotland in the early Ordovician. Members of the Oughterard Granite suite were emplaced during the final stages of the Grampian orogeny, from post-D3 to post-D4, while the country rocks were still at an elevated temperature (>500°C) following the peak of the regional metamorphism. The new minimum age of intrusion of c. 473 Ma, together with existing radiometric data, confirms that emplacement of the metagabbro and gneiss complex in southern Connemara, followed by the D3 structural and metamorphic events, all took place during a period of less than 20 Ma. © 1997 John Wiley & Sons, Ltd. 相似文献
18.
The Agnew supracrustal belt consists of a greenstone sequence (interlayered metabasalt, differentiated gabbroic sills, ultramafic bodies, and black volcanogenic sediment) unconformably overlain by granitoid-clast conglomerate and meta-arkose. The base of the preserved sequence is intruded by grey tonalite with a crudely concordant upper contact, and by small discordant bodies of leucogranite.An early deformation (D1) produced isoclinal folds and a regional penetrative foliation. These structures were probably gently dipping when formed. D2 produced large-scale NNW-trending upright folds, a regional foliation, and a vertical N-trending ductile fault on the west side of the belt. D2 structures indicate a combination of ENE-WSW shortening, and right-lateral shear along the ductile fault. Both D1 and D2 were accompanied by metamorphism under upper greenschist to lower amphibolite facies conditions.The interpreted sequence of tectonic events is (1) deposition of the greenstone sequence on an unknown basement; (2) intrusion of large volumes of tonalite, separating the supracrustal rocks from their basement; (3) erosion of mafic rocks and tonalite to produce the clastic sedimentary sequence; (4) the first deformation; (5) intrusion of small volumes of leucogranite; (6) the second deformation.The bulk of the granitoid rocks were emplaced before the first recognisable deformation. Thus the granitoid magma cannot have been produced by partial melting of previously downbuckled ‘greenstone belt’ rocks, nor can the large-scale upright folds (D2) be a result of forceful emplacement of the magma — two common postulates for Archaean terrains. The D2 folds are closely related to the ductile fault bounding the zone: these structures, which give the present N-trending tectonic belt its form, are the youngest features in the terrain. 相似文献
19.
PETROLOGY AND AGE OF THE KINNAR KAILAS GRANITE:EVIDENCES FOR AN ORDOVICIAN POST-OROGENIC EXTENSION IN THE HIGHER HIMALAYAN CRYSTALLINE, SUTLEJ, INDIA 相似文献
20.
Nadine Friese Axel Vollbrecht David C. Tanner Wiebke Fahlbusch Miriam Weidemann 《International Journal of Earth Sciences》2012,101(5):1149-1167
The emplacement of the Mesoproterozoic G?temar Pluton into Paleoproterozoic granitoid host rocks of the Transscandinavian Igneous Belt is re-examined by microfabric analysis, including cathodoluminescence microscopy. Field data on the pluton-host rock system are used to strengthen the model. The G?temar Pluton, situated on the Baltic Shield of SE Sweden, is a horizontally zoned tabular structure that was constructed by the intrusion of successive pulses of magma with different crystal/melt ratios, at an estimated crustal depth of 4–8?km. Initial pluton formation involved magma ascent along a vertical dike, which was arrested at a mechanical discontinuity within the granitoid host rocks; this led to the formation of an initial sill. Subsequent sill stacking and their constant inflation resulted in deformation and reheating of existing magma bodies, which also raised the pluton roof. This multi-stage emplacement scenario is indicated by complex dike relationships and the occurrence of several generations of quartz (Si-metasomatism). The sills were charged by different domains of a heterogeneous magma chamber with varying crystal/melt ratios. Ascent or emplacement of magma with a high crystal/melt ratio is indicated by syn-magmatic deformation of phenocrysts. Complex crystallization fabrics (e.g. oscillatory growth zoning caused by high crystal defect density, overgrowth and replacement features, resorbed and corroded crystal cores, rapakivi structure) are mostly related to processes within the main chamber, that is repeated magma mixing or water influx. 相似文献