ISSN:0072-1050    

DOI:10.1002/gj.3350250302

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350250303

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractA concise review of concepts dealing with the genesis of granite in the late 18th, 19th, and early 20th centuries is given. Arguments are provided for the repeated recurrence of genetic concepts in an altered form which were separated by periods in which a rival concept was dominant. This type of development may be likened to Eldredge and Gould's (1972) punctuations of biological equilibrium. On the other hand phases of development may be recognized in which rival concepts tend to converge towards a unified content, thus conforming to the ‘tree of knowledge’ hypothesis of Popper (1972). In this context the two metaphors do not appear to be mutually exclus

ISSN:0072-1050    

DOI:10.1002/gj.3350250304

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractAccording to some early definitions, ‘magma’ had to be completely molten so that granitic rocks containing residual material from their source (restite) were not considered to be magmatic. Granites and volcanics with abundant recognizable restite were called migmatites or migmatitic rocks, and this terminology appears to be still in use by some. However, many felsic granites and volcanics commonly contain at least some restite, and for this reason we suggest that magma be defined either as melt or as rock material with enough melt for it to become mobile and intrusive or extrusive. The rocks crystallizing from these magmas are igneous. They are not migmatites. On the other hand, we suggest that migmatites be defined as rocks that did not have sufficient melt to become mobile; they retain a structural coherency even though the structures may be complex. Differences between the two rock types result from the amount of melt phase. When a critical melt fraction is reached during partial melting, there is a rapid change from migma to magma (migmatite to granite).The geochemistry and mineralogy of migmatites tell us little about reactions that produce more melt and hence about granite formation because of back reactions and other processes that occur during the cooling of migmatite. Back reactions mostly involve H2O that is not lost from the migmatitic system. Because H2O is lost by processes of second boiling and pressure quenching during the crystallization of granite magmas at or near the surface, it is these rocks and their inclusions, not migmatites, that provide the most information about sources and processes of granite format

ISSN:0072-1050    

DOI:10.1002/gj.3350250305

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractComparison of the twenty most frequently used petrogenetic classifications of granitoids is summarized using petrography, nature and abundance of enclaves, mineralogy, major and trace element, and isotope geochemistry. This comparison indicates considerable agreement between most authors concerning the main divisions. Differences result either from different genetic models or from the weighting of the criteria used, such as nature of the source, relative proportions of crustal and mantle contributions, magmatic processes, and types of emplacement. In the classification proposed here, three main groups of granitoids correspond respectively to a crustal, a mantle, or a mixed (crustal and mantle) origin of the magmas. Each of these main groups is subdivided into two or three types. Distinct petrographic, mineralogical, and chemical characteristics result from differences in the origin and tectonic setting of the granitoid magmas. Consequently, granitoids with well‐defined petrographic, mineralogical, chemical features and ages may provide information on the changes of tectonic settings with space, time, or both, i.e. they may be used as geotectonic tracer

ISSN:0072-1050    

DOI:10.1002/gj.3350250306

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractThe Kerguelen Islands (6,500 km2) are the third largest oceanic archipelago after Iceland and Hawaii. One of their most remarkable geological features is the existence of several igneous complexes thought to represent the deep levels of volcano‐plutonic structures of which the upper part has been eroded. These plutonic bodies have ring shapes with diameters ranging from 1 to 15 km. The oldest (39–23 Ma) have transitional petrological features while the youngest (26 Ma–Recent) are typically alkaline. This sequence is also observed in the surrounding flood‐basalts. Both the basalts and the plutonic rocks have a mantle origin. The alkaline plutonic suites evolve towards silica‐over or undersaturated rock types depending on whether they are located to the west or to the east of a NNW–SSE boundary. The divergence in alkaline trends may be related to magmatic differentiation or linked to the evolution of two different initial magmas, one being mid‐alkaline and the other high‐alkaline. In both hypotheses fluids, particularly H2O, pl

ISSN:0072-1050    

DOI:10.1002/gj.3350250307

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractGeological data are seldom reliable enough to quantify the shape of granitic bodies at depth. Remote sensing measurements and geophysical methods may allow hidden features inside plutons to be recognized. Seismic methods can be employed to display the interface between the granite and its sedimentary cover. Unfortunately, impedance contrasts are very low inside the intrusive body resulting in a transparency to seismic waves. In addition, the high cost of seismic lines with sufficient resolution excludes full 3D shaping. The use of magnetic methods is limited by the low content of magnetic minerals in acidic rocks and the observed magnetic field is often flat over plutons. Some high amplitude magnetic anomalies are restricted to the contact metamorphic aureoles surrounding granites. Acidic rocks generally have a lower density than the surrounding country rocks. A negative gravity signature is therefore associated with the intrusion. To assist with geological interpretation, a detailed gravity survey should be made over the pluton with a coverage density of about 1 measurement per km2. This is necessary to register the effects of facies variations. Interpretation should be restricted to 3D methods rather than 2D modelling on profiles because granitic bodies are not uniform along any one direction. However, 3D inversion of data must take into account the regional gravity field and variations of density in and outside the pluton. With these assumptions, models are presented which can be correlated with the structures acquired during the magmatic stage of pluton emplacement. The zones where magmatic foliations are vertical correlate with deep zones identified from gravity data. They are interpreted as magma feeding zones. The information provided by these methods represents a valuable contribution to determining the mode of emplacement of granitic intrusions.

ISSN:0072-1050    

DOI:10.1002/gj.3350250308

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractMany orogenic belts display associations of calc‐alkaline and alkaline igneous centres which are closely related in space and time. The sequence: calc‐alkaline batholith→uplift and unroofing→alkaline plutonic–volcanic complexes, lasts less than 100 Ma and documents a very fast switch from orogenic to anorogenic geodynamic conditions. The magmatic suites have a mantle origin with decreasing crustal contribution, but the sources and the types of differentiation processes differ.Alkaline granites can be subdivided in two groups:1Post‐orogenicBa and Sr‐rich red granites having Mg‐rich mafic minerals and high content of Mn whether they are peralkaline or peraluminous. Crustal contribution is indicated by Sr isotopic signatures. Water‐rich fluids are responsible for the subsolvus crystallization of alkali feldspars and for the high oxygen fugacity. Magmatic centres were emplaced less than 10 Ma after the late‐orogenic formations.2Early anorogenicBa and Sr‐poor greenish to whitish hypersolvus granites having Fe‐rich mafic minerals, low contents of Mn, and virtually no Mg F‐rich aqueous fluids are probably less abundant and promote subsolidus hydrothermal alteration with only subordinate late‐stage oxidation. Crustal contamination can be ruled out for the non‐mineralized complexes, but can be important in the hydrothermal mineralized areas. The magmatic centres are considerably younger than the first group.During the first 100 Ma following the end of a major orogeny, a new mantle source replaces the old complex system of mixed oceanic–continental crust‐mantle sources and produces alkaline melts which are subsequently less and less contaminated by crustal host rocks. Relations between hydrothermal events, mineralizations, and crustal isotope signatures suggest that crustal contribution relates to percolating fluids and not to anat

ISSN:0072-1050    

DOI:10.1002/gj.3350250309

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractThe Flamanville pluton was emplaced during the upper Carboniferous in Palaeozoic sedimentary cover forming one limb of the Siouville syncline, a Hercynian kilometre‐scale fold in Normandy, Northwest France. We review the geological environment of the pluton with special emphasis on the Siouville syncline. The features described include gravity data, metamorphic environment, fold patterns in the contact metamorphic aureole, trajectories of principal strains in and around the pluton, finite strain and anisotropy of magnetic susceptibility within the pluton, and physical and kinematic aspects of pluton fabrics. The results emphasize that (1) the Flamanville pluton is syntectonic, (2) its emplacement involves lateral expansion of magma rather than spherical ballooning, and (3) granite fabrics do not reflect magmatic flow but are strain‐controlled irrespective of grain‐scale deformation processes and rheological state.The example of the Flamanville granite further suggests (1) that grain‐scale deformation features are not critical in distinguishing tectonic from magmatic origin of granite fabrics and (2) that pluton formation within the soft sediments of the shallow upper crust most likely results from lateral expansion of magma injected through the brittle crust rather than from ballooning of a diapir

ISSN:0072-1050    

DOI:10.1002/gj.3350250310

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY

摘要:

AbstractThe southernmost part of the early Proterozoic Ketilidian mobile belt (1·9–1·7 Ga) of southern Greenland comprises a thick series of largely flat‐lying migmatitic metasediments and metavolcanics of upper amphibolite to lower granulite facies. Regional tectonics are extensional and there is little structural evidence for earlier crustal thickening. Several episodes of granite magmatism are recorded, culminating in the late‐ to post‐orogenic rapakivi granites, which are associated with with a coeval suite of norites. These were emplaced atca. 1740 Ma, roughly synchronously with the peak of regional metamorphism (2–3·5 kbar, 650–800°C). The rapakivi granites have been emplaced into major crustal‐scale extensional shear zones and have ramp‐flat geometries. ‘Black’ and ‘white’ rapakivi granites are found. The mineralogy of the black granites is dominated by orthopyroxene–amphibole assemblages with rare fayalite. In the white granites, biotite and amphibole are the only Fe–Mg silicates. All Fe–Mg silicates have high Fe/(Fe+Mg) ratios. In terms of bulk chemistry, the suite is bimodal. Both norites and granites are strongly fractionated and have Fe/(Fe+Mg) ratios of 0·50–0·65 (norites) and 0·65–0·90 (granites). REE modelling shows that the granites can be derived from 40 per cent partial melting of a metasedimentary protolith. The sequence of magmatic events in this region reflects the progressive metamorphism and dehydration melting of juvenile sediments in an extending basin. The heat source for melting may have come from the underplating of olivine tholeiite melt

ISSN:0072-1050    

DOI:10.1002/gj.3350250311

出版商:John Wiley&Sons Ltd    

年代:1990

数据来源: WILEY