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1.
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   

2.
The Midcontinent Rift (MCR) of North America comprises a series of basaltic sheets, flows and intrusive rocks emplaced in the Lake Superior region during the Mesoproterozoic. The mafic rocks preserved on the northern flank of Lake Superior represent the older portions of the rift sequence and offer insights into the early development of the rift. New geochronological, geochemical and paleomagnetic data are presented for the dikes and sills located in and south of Thunder Bay, Ontario. Three sill suites are recognized within the study area; an earlier, spatially restricted ultramafic unit termed the Riverdale sill, the predominant Logan sills and Nipigon sills in the north of the study area. In addition three dike sets are recognized, the north-east trending Pigeon River swarm, the north-west trending Cloud River dikes and the Mt. Mollie dike. The geochemical data demonstrate that the majority of sills south of Thunder Bay are of Logan affinity and distinct from those of broadly similar age in the Nipigon Embayment to the north. The Pigeon River dikes that intrude the sills are geochemically coherent but distinct from the Logan sills and could not be feeders to the sills. The new age of 1109.2 ± 4.2 Ma for the Cloud River dike and its R polarity are consistent with published magnetostratigraphy. The Mt. Mollie dike age (1109.3 ± 6.3 Ma) indicates that it is not coeval with the spatially associated Crystal Lake gabbro as previously thought. The complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south. The geochemistry and geochronology of the intrusions suggest a long-lived and complex magmatic history for the Midcontinent Rift.  相似文献   

3.
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   

4.
《China Geology》2021,4(1):56-66
The timing of the initial Indo-Asian collision is a subject of debate for a long time. Besides, the magmatic trace of the collisional process is also unclear. In the present study, the authors report Early Eocene leucocratic sill/dike swarms in the northern edge of the Nymo intrusive complex of the Gangdese belt, southern Tibet. The Nymo intrusive complex was emplaced at ca. 50 –47 Ma and surrounded by the metamorphosed Jurassic-aged Bima Formation volcano-sedimentary sequence along its northern side. At outcrops, the leucocratic sills/dikes intruded along or truncated the deformed foliations of the host Bima Formation, which has been subject to high-temperature amphibolite-facies metamorphism at ca. 50 –47 Ma. Detailed cathodoluminescence image analyses reveal that the zircon grains of the leucocratic sills/dikes have core-mantle textures. The cores yield the Jurassic ages comparable to the protolith ages of the Bima Formation. In contrast, the mantles of zircon grains yield weighted mean ages of ca. 49–47 Ma, representing the crystallization timing of these leucocratic sills/dikes. The coeval ages for the Nymo intrusive complex, the high-temperature metamorphism, and the leucocratic sills/dikes indicate that a close relationship exists among them. The authors tentatively suggest that these leucocratic sills/dikes were generated from partial melting of the Jurassic-aged Bima Formation volcanic rocks, triggered by the high heat from the magma chamber of the Nymo intrusive complex. This Early Eocene tectono-thermal event of coeval magmatism, metamorphism and partial melting was most likely formed during the Indo-Asian collisional setting.©2021 China Geology Editorial Office.  相似文献   

5.
In the Yangliu Mine in Huaibei Coalfield, violent magma intrusion produced two layers of the igneous cover sills and the annular dike and caused the ejection of gas and water in the bed splitting from the surface well that destroyed the gas extraction system on the surface. The source, flow characteristics and hazard of the gas in bed splitting under igneous sills were researched via laboratory experiments, theoretical analyses and numerical simulations. The experimental results indicate that the shorter the distance from the igneous sill and the larger the thickness of the igneous sill, the greater will be the amount of the thermal evolution, the pyrolysis stoma, the coal adsorption and desorption ability and the risk of outburst, six coal samples collected at different distances from the igneous sill. The first breaking length of the igneous sill, the volume of the bed splitting, the gas pressure and flow rate in the bed splitting and the accumulative volume of the gas ejected from the surface well #2 were theoretically calculated. The laws of the bed splitting development under the igneous sills were simulated, consistent with the theoretical calculation results. The regional control and prevention technologies for coal seam gas under igneous sills were proposed and applied. Then the safety of the first-mining seam production and the gas extraction in the adjacent seam and bed splitting via the surface well was ensured, and no accidental gas and water ejection occurred.  相似文献   

6.
由于蕴含着潜在的油气勘探和资源储藏信息,岩床复合体在全球范围内被广泛研究。本文基于南海北部三维地震资料以及南极罗斯海地区二维多道地震资料,对岩床复合体的形态特征及分布进行分析,对两个地区岩床复合体的形成时间进行判断,阐述岩床复合体的侵位机制。研究发现,岩床复合体代表性产状除了菱形、碟形及复合型外,还存在沿着层位平行侵入的板状,推测它可能是夭折了的岩床侵入体,以及由几个小型岩床堆叠而成的透镜状岩床复合体。分布上,南海北部岩床主要发育于珠江口盆地白云凹陷三维工区西部,东部只零星可见;南极西罗斯海岩床主要发育于西北角的阿黛尔盆地内。根据岩床上覆的强制褶皱结合区域构造背景,推测南海北部岩床复合体的形成时间为早中新世(15.5 Ma),海底扩张结束之时。残余岩浆将盆地内裂谷时期形成的断层作为岩浆运移的通道,并最终在断层终点处即下中新统近海相砂泥岩地层中实现侵位。南极西罗斯海岩床复合体的形成时间则非常晚,不早于5.5 Ma。该区这期岩浆活动可能与构造作用无关,而是受到冰川消融的影响。研究发现岩床复合体内部的岩床个体互相供给,即深部的大岩床为上覆浅层的小岩床提供岩浆供给,并且互相组合,形成大的岩床复合体。然而,每个岩床复合体则可能由不同的岩浆供给发育而成。  相似文献   

7.
The question of whether continental flood basalts are contaminatedwith crustal material during ascent through the lithosphereis addressed through a study of the feeder dike to the firstof the Mesozoic flood basalts in the Hartford basin of Connecticut.Faulting associated with basin formation exposed the dike, notonly where it connects with the flood basalt, but also at levelsthat may have been as deep as 10 km at the time of crystallization.Wallrocks at all levels of exposure and along the entire 250-kmlength of this 50–60 m wide dike show evidence of partialmelting and contamination of the diabase magma. Melting of the wallrocks took place on grain boundaries betweenquartz and feldspar, especially where these boundaries werefluxed with water released from the breakdown of biotite. Themelts were simple mixtures of quartz and the particular feldsparinvolved rather than equilibrium minimum compositions, probablybecause heating rates were faster than melting rates. The meltscrystallized to form granophyre. Feldspars in direct contactwith the diabase partly melted, with plagioclase developingthe fingerprint texture and orthoclase being converted to anexceptionally fine-grained mixture of plagioclase and quartz. Melts generated in the wallrocks were forced into the marginsof the dike by the volume expansion caused by melting. The earliestmelts to enter the dike are preserved as felsic wisps in thechilled margins, but at distances of more than a few centimetersfrom the edge of the dike they appear to have been assimilatedby the diabase magma. Melts that entered the dike after themargins had solidified were intruded as crosscutting granophyreveins. Chemical profiles across the dike reveal that the contaminantswere not restricted to narrow marginal zones but entered themain body of the dike. Thus, by the time this magma had risento the surface and erupted as the Talcott basalt it had assimilateda significant amount of crustal material, which accounts forthe change from olivine normative compositions in the deepestlevel exposures of the dike to quartz normative ones in thebasalt. Mass balance calculations indicate that the Talcottbasalt may have had {small tilde}6% granophyre added to it duringthe magma's ascent through the last 10 km of the crust. Theamount assimilated in traversing the entire continental crustwould therefore be greater than this.  相似文献   

8.
Recent seismic reflection studies of large-volume, anorogenic basaltic provinces at passive continental margins have challenged the traditional viewpoint that erupted magmas predominantly ascend through the lithosphere via dykes that exploit high-angle faults. However, such seismic-based methods are yet to be applied to identify the magmatic plumbing systems of low-volume basaltic provinces, such as the Cretaceous–Cenozoic Newer Volcanic Province (NVP) in southeastern Australia (total volume <20 000 km3). The South Australian Quaternary Basalt (SAQB) represents the most recent phase of activity both within the NVP and on the Australian continent. This province is located within the Otway Basin, and a large amount of seismic data from gas exploration is available for this region. Consequently, the SAQB represents a superb natural laboratory in which geochemical and seismic data can be combined in order to study the magmatic plumbing system of a low-volume continental basalt province and discriminate between the competing hypotheses for magma transport through the Earth's crust. Geochemical analyses and thermodynamic modelling suggests that the magma that fed the SAQB was generated by adiabatic decompressional melting of a secondary mantle plume. These models imply that melt segregation took place at successively lower pressures from about ～4000 to 3000 MPa during the ca 1 Ma SAQB eruptive history that culminated in the Northern Group, Mt Schank and Mt Gambier eruptions. During ascent, the magma underwent 34–41% fractional crystallisation and cooled ～200 °C, while residing in the crust for a time period on the order of days to weeks. Interpretation of a 3D seismic survey that overlaps with the northeastern part of the SAQB reveals a saucer-shaped sill with an unusual morphology, exhibiting a series of vertical concentric steps towards its outer rim. This sill appears to be fed by magma that intruded along steep normal faults from a feeder dyke that is hosted by the regional, NW–SE-trending Hungerford-Kalangadoo Fault. Our results suggest that the melt that fed the SAQB rose through the crust via dykes and high-angle normal faults, with less evidence for significant horizontal transport of magma than observed in large-volume basaltic provinces in sedimentary basins at rifted continental margins, possibly highlighting a correlation between the extent, volume and magma supply rate within a basaltic province and the nature of magma ascent.  相似文献   

9.
The shapes of sills and laccolithic intrusions and associated host rock deformation were studied at several locations on the flanks of the Henry Mountains. Diorite sills range from 0.5 to 10 m in thickness, are less than 1 km2 in areal extent, and have blunt terminations. The laccolithic intrusions range from 10 to 200 m in thickness, and from 1 to 3 km2 in areal extent. The host rock, principally sandstone and shale, is deformed along closely spaced cataclastic shear planes. This deformation is concentrated at contacts, especially near sill terminations and over laccolith peripheries. The diorite contains plagioclase phenocrysts which are usually sheared in a thin zone adjacent to each contact. Field observations suggest that sills are the forerunners of laccolithic intrusions which form only after magma has spread far enough laterally (greater than about 1 km2 in the Henry Mountains) to gain leverage to bend the overburden upward. Further injection of magma results in laccolithic peripheries or terminations with one of three distinct cross-sectional forms: (1) blunt termination of the diorite accompanied by bending and minor faulting of the host rock; (2) termination at a peripheral diorite dike cutting upward across the host rock; or (3) abrupt termination of the diorite against a nearly vertical fault zone.In order to study some of the processes of sill and laccolith intrusion, mechanical models for the driving pressure, physical properties, and flow behavior of the diorite magma are derived and discussed. A static driving pressure (equal to the difference between total magma pressure and lithostatic pressure) of up to 700 bar is estimated. The rheological behavior of the magma in the Henry Mountains is unknown. However, flow behavior is calculated assuming three of the more common models for fluids: Newtonian viscous, pseudoplastic, and Bingham. Suspended crystals probably contributed to the finite strength of the magma (estimated to be at least 103 dyn/cm2 for the Henry Mountains magma) which enables it to support dense zenoliths and also fixes maximum limits on the lengths of sills or dikes. Pressure in magma flowing along tabular intrusions of uniform thickness drops linearly in the flow direction for all three rheological materials. Thickening of tabular intrusions tends to make the pressure drop less rapidly, but pressure drops more rapidly in the tapered region near a termination. Pressure distributions under these and other conditions are derived in order to use them in the models of host rock deformation presented in Part II.  相似文献   

10.
The stratigraphy of western Australia   总被引：2，自引：2，他引：0  
Abstract

Measurements of intensity of magnetization and susceptibility have been made at intervals of 5 ft. on some bore cores from Tasmanian tholeiites of Jurassic age. In all cases the direction of magnetization was nearly vertical. One sill appears to have its upper 700 ft. magnetized normally and its lower part reversed.

The magnetic properties vary very rapidly and show a structure in which narrow regions with high values are superposed on a general increase to a maximum at about 300 ft. from the top of the sill. This coincides with a minimum of the density and a maximum of normative magnetite and total iron and also in the size of the areas of mesostasis.

It is suggested that this behaviour is due to differentiation and that magnetic measurements form an extremely sensitive tool for the study of this process. An attempt is made to show that differentiation by the settling of individual crystals through the sill is impossible in thick sills in which convection does not occur. and that the mechanism must be one of roof stoping in which large blocks of mixed crystals and residual magma fall from the roof to the floor of the sill. The regions with exceptionally high magnetic values which have been observed are regarded as composed of magma trapped between these blocks.  相似文献   

11.
武当地块西部顺层侵位了大量的基性岩席。本文对其进行的岩石学、地球化学和Sr、Nd同位素等特征研究表明，该基性岩席群的岩浆属于大陆拉斑玄武岩系列，来源于混合异常大陆岩石圈地幔型源区，为武当地块裂解和中古生代时南秦岭地区曾发生大规模的地幔岩浆底侵及壳幔相互作用重要标志之一。此外，该基性岩席群的岩石地球化学特征与勉略地区玄武岩及变辉绿岩总体的特点具有较大的一致性，结合该基性侵入岩席群的形成时代(401—407Ma)、武当地块伸展构造(425—260Ma)与勉略带火山岩(230Ma)同位素年代学的差异，武当地块西部基性岩席群很可能是由于勉略洋拉开前地幔柱活动使大陆岩石圈拉伸减薄发生拆离、深部岩浆上涌的结果。因此，它们应该是勉略洋打开的早期阶段在武当地块区的响应。  相似文献   

12.
Deformation of host rocks during growth of a laccolithic intrusion is analyzed using the theory of bending a stack of thin elastic plates. The theoretical model suggests that magma spreading laterally in the form of a sill will eventually gain sufficient leverage on the overlying strata to deflect them upward and form a laccolith. The amount of bending increases as the fourth power of the distance the magma spreads, whereas the overburden resists bending as the third power of its effective thickness. Effective thickness is the thickness of a single layer which has the same resistance to bending as a multilayer of similar length and elastic modulus. The effective thickness of overburden in the Henry Mountains is estimated as between and of the actual thickness. The form of bending is similar for Newtonian, pseudoplastic, and Bingham magmas. The magnitude of the bending depends upon the total upward force and its distribution and is not simply related to magma viscosity as has been suggested by several previous investigators.After elastic bending strata should fail over the periphery of an intrusion, the site of maximum bending strain and differential stress predicted by the theory. Field observations described in Part I correlate well with these predictions. Because bending strains are proportional to layer thickness, strata of comparable strength but different thicknesses fail at different stages of laccolith development. This leads to the different cross-sectional forms of laccoliths observed in the field.The effect of host rocks on sill form and growth is analyzed using the elastic solution for an elliptical hole under uniform pressure. The theory suggests that sill thickness increases in proportion to length. The concentration of high stresses near the sill termination should induce permanent deformation and account for the blunt terminations described in Part I. This blunting is most likely to occur in relatively ductile rocks whereas sills simply split brittle rocks and maintain sharp terminations. The driving pressure in sills can be calculated from measurements of length and termination radius of curvature, if the yield strength of the host rocks can be estimated. This driving pressure must be greater than the overburden pressure, but sills apparently do not form or propagate by lifting their overburdens. Instead they propagate by locally deforming the host rock. After spreading over a distance about three times the effective overburden thickness, the overlying layers begin to bend upward significantly. This stage marks the transition from a sill to a laccolithic intrusion.  相似文献   

13.
新疆塔里木盆地东北缘库鲁克塔格地区的中寒武统莫合尔山组沉积地层中发现三条形成于二叠纪末到中三叠世的基性岩床。三条岩床沿沉积岩层面顺层侵入,按照从下到上的顺序,分别为辉绿岩床(厚3m)、辉长岩床(厚20m)和粗玄岩床(厚2.5m)。三条岩床的地球化学特征相似,稀土含量高(∑REE=210×10-6~297×10-6),轻稀土元素富集((La/Yb)N =13.54~17.96),Eu负异常不明显(δEu=0.97~1.21),Ce稍具正异常(δCe=1.11~1.25)。大离子亲石元素(Rb、Ba、Th、Sr)富集,Sr具明显正异常; Ta、Nb和Hf具负异常。低氧化度(0.13~0.40)。三条岩床为同源岩浆活动的产物,岩浆源于上地幔或下地壳,但有下陆壳成分混染。三个样品的MgO含量在4%~6%之间,Mg#值在37.6~55.6之间,固结指数SI值位于18.7~29.3之间,反映原始的岩浆经历了比较明显的分异作用。粗玄岩床K-Ar年龄为255.2±7.3Ma,辉绿岩床K-Ar年龄为242.8±5.7Ma,辉长岩床Ar-Ar年龄为229.7±3.7Ma。本次岩浆侵入事件发生在南天山洋闭合之后,构造背景为造山后碰撞伸展环境。  相似文献   

14.
It has been advocated that convective and crystal settling processes play significant, and perhaps crucial, roles in magmatic differentiation. The fluid dynamics of magma chambers have been extensively studied in recent years, both theoretically and experimentally, but there is disagreement over the nature and scale of the convection, over its bearing on fractionation and possibly over whether it occurs at all. The differential distribution of modal olivine with height in differentiated alkaline basic sills provides critical evidence to resolve this controversy, at least for small to medium-large magma chambers. Our own and others' published data for such sills show that, irrespective of overall olivine content, modal olivine contents tend to increase in a roughly symmetrical manner inwards from the upper and lower margins of the sill, i.e. the distribution patterns are more often approximately D-shaped rather than the classic S-shape generally ascribed to gravity settling. We concur with the majority of other authors that this is an original feature of the filling process which has survived more or less unchanged since emplacement. We therefore conclude that the magmas have not undergone turbulent convection and that gravity settling has usually played only a minor modifying role since the intrusion of these sills. We offer a possible explanation for the apparent contradiction between fluid dynamical theory and the petrological evidence by suggesting that such sills rarely fill by the rapid injection of a single pulse of magma. Rather, they form from a series of pulses or a continuous pulsed influx over a protracted interval during which marginal cooling severely limits the potential for thermal convection.  相似文献   

15.
《Journal of Structural Geology》1999,21(8-9):1125-1130
Integrated studies that correlate gravity data with magmatic structures or geochemical analysis reveal that the organization of granitic plutons (shape and feeder) varies according to the tectonic regime (brittle or ductile) and type (extension, strike-slip or compression) under which magma was generated. Deformation clearly controls granite emplacement. It also has influence during the ascent, segregation of melt and indirectly during the phase of initial melting. Because of the contrast of viscosity between melt and its plastic matrix, strain partitioning develops during magma ascent that facilitates melt flow, which bears consequences for the chemical evolution of the magma. Fast rate of melt extraction out of the source may lead to chemical disequilibrium. At a larger scale, petrochemical zoning of plutons is described as a dynamic process that results from the competition between the rate of magma emplacement (the rate of the room provided by deformation) and the rate of magma delivery.  相似文献   

16.
The western part of Cuddapah basin has been considered as a foreland to the fold-thrust belt represented by eastern Nallamalais and Nellore schist belt. The igneous activities, in the form of lava flows and sills, in western Cuddapahs have been a subject of controversy since tectonogenetic models ranging from a mantle plume to lithospheric extension have been suggested. The present work covers a detailed study of the mafic sills and lava flows of Vempalle and Tadpatri formations in western Cuddapahs, which earlier workers have also noted. A detailed field and laboratory work in this study brings out, for the first time, five distinct petrographic and geochemical varieties of sills. Geochemical studies also suggest that the different sill types have originated from varying degrees of partial melting of a metasomatized subcrustal lithospheric mantle, the metasomatism of source being attributed to older Archaean convergent settings/greenstone belts that the basin unconformably overlies. The possible contribution of any small plume is only heating and doming of overlying mantle that induced incipient crustal rifting. Our geochemical studies on the lava flows of Vempalle Formation also suggest that the magma was derived from small degree of partial melting of spinel-garnet peridotite mantle in a rift setting.  相似文献   

17.
A. Audtat  T. Pettke  D. Dolej 《Lithos》2004,72(3-4):147-161
A quartz-monzodioritic dike associated with the porphyry-Cu mineralized stock at Santa Rita, NM, has been studied to constrain physico-chemical factors (P, T, fO2, and volatile content) responsible for mineralization. The dike contains a low-variance mineral assemblage of amphibole, plagioclase (An30–50), quartz, biotite, sphene, magnetite, and apatite, plus anhydrite and calcite preserved as primary inclusions within the major phenocryst phases. Petrographic relationships demonstrate that anhydrite originally was abundant in the form of phenocrysts (1–2 vol.%), but later was replaced by either quartz or calcite. Hornblende–plagioclase thermobarometry suggests that several magmas were involved in the formation of the quartz-monzodiorite, with one magma having ascended directly from ≥14 km depth. Rapid magma ascent is supported by the presence of intact calcite inclusions within quartz phenocrysts.

The assemblage quartz+sphene+magnetite+Mg-rich amphibole in the quartz-monzodiorite constrains magmatic oxygen fugacity at logfO2>NNO+1, in agreement with the presence of magmatic anhydrite and a lack of magmatic sulfides. The same reasoning generally applies for rocks hosting porphyry-Cu deposits, seemingly speaking against a major role of magmatic sulfides in the formation of such mineralizations. There is increasing evidence, however, that magmatic sulfides play an important role in earlier stages of porphyry-Cu evolution, the record of which is often obliterated by later processes.  相似文献   


18.
The stratigraphy and geological position of the eastern compartment of the Bushveld Complex are described. A mechanical model for the initiation and growth of the eastern compartment of the Bushveld intrusion has been developed using thin elastic plate theory, assuming linked conical magma chambers. It is shown that the contribution to the pressure at the base of a cell by the restitutional force exerted by the roof of Rooiberg felsites is 104 times as great as that of the layers of host in the cone. Both are minimal compared to the lithostatic pressure exerted by the magma pile. Roof deformation is therefore seen to be a more important process than sagging of the floor during intrusion—a feature which probably occurred during cooling, solidification and isostatic readjustment of the area.A stratigraphie model is proposed in which the intrusion of basic rocks into the Transvaal sequence is discussed in the light of continuous basin subsidence. Early submarine sedimentation in an irregularly-floored basin some 620 km in diameter situated on the Archaean craton gave rise to a 7.7 km thick sedimentary pile, to which was added some 7 km of subaerial basalts and felsites. Depression of the floor of the basin into the regime of maximum horizontal compression induced favourable conditions for the intrusion of a total of 2.5 km of diabase sills which further assisted the subsidence. The 9 km thick Bushveld Complex was intruded into the basal sections at points along a 010° trend in a regime characterised by shear failure. Early magma influxes gave rise to a laminated marginal zone forming a shallow cone, with associated sill activity, whilst continued later influxes filled the conical cell, transgressed the floor and uparched the roof. Partial melting in the regions beneath the Complex, exacerbated by continued crustal depression, gave rise to the late Bushveld granites.  相似文献   

19.
The production of Earth’s granitoids is generally attributed to magma intrusion, fractional crystallization and assimilation but the details of how granitoid plutons form remains widely debated. In light of recent experimental results which indicate that partially molten wet andesite in a temperature gradient evolves into a granitic bulk composition at the cooler end of the gradient (in a process called thermal migration), I present a model for at least some of Earth’s granitoids forming by a top-down thermal migration zone refining process. According to this model, convergent margin igneous activity builds a thick volcanic pile which becomes a barrier to further ascent of magma, leading to magma underplating by injection of sills at the base of the pile. When magmas arrive at the location of underplating, they react and release heat and water to the overlying materials (previously intruded sills), resulting in a downward moving zone having a near-steady-state temperature gradient. This leads to compositional differentiation by wet thermal migration taking place over million year time scales; this in situ differentiation process occurs in the middle of the underplated region but not on the more rapidly cooled edges of the sills. Modeling using the IRIDIUM program shows this process can produce sequences of granitoid that are kilometer or greater in thickness; regardless of granitoid thickness, the bottom of the system maintains a near constant thickness of hornblende gabbros. The model provides a logical connection between andesitic stratovolcanoes and underlying, more silicic intrusive series plutons—both reflect ascent of andesitic composition magmas, with the implication that convergent margin magmatic systems evolve temporally from stratovolcanoes to plutons once magma ascent is inhibited and underplating begins.The model provides an alternative to the standard view that granitoids result from cooling of large bodies of magma and could help to resolve long-standing questions concerning: geophysical observations of magma chambers; the compositions of minerals in granitoids; and the development of preferred mineral orientations in granitoids. It provides a consistent model in that it explains the systematic normal compositional zoning of plutons within the context of an incremental growth process dictated by geochronology. Most importantly, the model is predictive, emphasizing the importance of examining granitoids in the vertical dimension. The hypothesis that thermal migration plays a role in granitoid formation can be tested by analysis of non-traditional stable isotope systems such as Fe, Mg and Si that should show a signature of thermal diffusion. The model predicts that the tops of overlying granitoids will have relatively heavy isotopic compositions whereas underlying hornblende gabbros will have relatively light isotopic compositions. Examination of existing iron isotope data and new silicon isotope data are consistent with the hypothesis and point to the need for more thorough testing.  相似文献   

20.
The role of regional extension on the rise and emplacement of granites in the crust is still debated. Pluton ascent and emplacement widely occurred in Tuscany (Italy) since late Miocene during the post-orogenic collapse of the inner Apennines, and are presently occurring in the geothermal areas of Amiata and Larderello. Tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels. Ductile extension enhanced the segregation and ascent of granitic melts in the lower crust, controlling pluton emplacement in correspondence with the brittle–ductile transition. In the brittle crust, magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction; pluton emplacement mainly occurred by means of roof lifting. The case of Tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust, but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle-extending crust. The estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates. This suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes (roof lifting) accommodated the space required for pluton emplacement.  相似文献   

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