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411.
The Tenda crystalline massif (northern Corsica) is a fragment of the western Corsica basement involved in the Alpine orogeny. Rhyolite dykes crosscutting the gabbroic complex of Bocca di Tenda (southern sector of the Tenda crystalline massif) show an unusual metamorphic mineral assemblage, defined by jadeite‐bearing (up to 46 mol percentage) aegirine, riebeckite, celadonite‐rich phengite (Si=3.50–3.65 apfu), quartz, albite and K‐feldspar. Jadeite‐bearing aegirine and riebeckite mostly occur as coronas around jadeite‐free aegirine and arfvedsonite, respectively, which both are relics of igneous origin. This metamorphic assemblage reflects the peralkaline compositions, which are characterised by anomalously high contents of SiO2 and Na2O, and negligible CaO and MgO. The evolved rocks of the gabbroic sequence (quartz‐diorites to tonalites) and the surrounding granitoids are characterised by the development of riebeckite/ferroglaucophane, epidote, celadonite‐rich phengite and albite, thus pointing to a metamorphic crystallization in the epidote‐blueschist facies. In all the studied rocks, metamorphic reactions were controlled by fluid‐assisted mass‐transfer through grain boundaries and microfractures. The different mineral assemblages allow the peak P–T metamorphic conditions to be constrained to between 0.8 GPa/300 °C and 1.1 GPa/500 °C. These estimates attest to a geothermal gradient (dT/dP) of 10–13 °C km?1 and indicate that the Tenda crystalline massif was buried to a minimum depth of 27 km during the Alpine orogeny. The blueschist facies recrystallization in the Tenda crystalline massif has been related to the cessation of an eastward‐dipping subduction event. 相似文献
412.
The Argentine Precordillera, a rifted fragment of Laurentian crust and sedimentary cover, collided with Gondwana in Middle Ordovician time; the time of collision (Ocloyic orogeny) is similar to that of the Taconic orogeny of eastern Laurentia. Three hypotheses have been proposed to explain Ordovician docking of the Precordillera with western Gondwana: (A) the Precordillera microcontinent was rifted from Laurentia in Cambrian time and, following solitary drift, collided with Gondwana, independent of the Laurentian Taconic orogeny; (B) a continent–continent collision of Laurentia with Gondwana, producing a continuous Taconic–Ocloyic orogenic belt, was followed by rifting that left the Precordillera attached to Gondwana; and (C) the Precordillera at the tip of a distal plateau on greatly stretched Laurentian crust collided with Gondwana and subsequently separated from Laurentia.Contrasts in several aspects of Taconic and Ocloyic orogenic history provide for discrimination between the microcontinent and continent–continent-collision hypotheses. Stratigraphic gradients and lithologic assemblages within the synorogenic clastic wedges are incompatible with a single continuous orogenic belt, which, in palinspastic location, places the thin, fine-grained southern fringe of the Taconic clastic wedge adjacent to the thickest and coarsest part of the Ocloyic clastic wedge. Separate temporal and spatial distribution patterns of volcanic ash (bentonite) beds in Laurentia and the Precordillera indicate originally separate dispersal systems. Late Ordovician Hirnantian Gondwanan glacial deposits in the Precordillera indicate substantial latitudinal separation from Laurentia. Post-collision faults with large vertical separation in the Precordillera have no coeval counterparts on the Laurentian foreland. These contrasts indicate originally separate (not initially continuous, and subsequently dismembered) orogenic belts, favoring the microcontinent hypothesis and eliminating the continent–continent-collision hypothesis.Initial Taconic tectonic loading near the southern corner of the Alabama promontory of Laurentia and the lack of post-Taconic extension there are inconsistent with the tectonic history required by the plateau hypothesis, but are consistent with the tectonic history required by the microcontinent hypothesis. Paleobiogeography, distribution of bentonite beds, and the Hirnantian glacial deposits, all indicate wide separation (Iapetus Ocean) between the Precordillera and southern Laurentia at the time of the Ocloyic and Taconic orogenies, further favoring the microcontinent hypothesis. 相似文献
413.
The southern margin of the East European Craton: new results from seismic sounding and potential fields between the North Sea and Poland 总被引:1,自引:0,他引:1
U. Bayer M. Grad T. C. Pharaoh H. Thybo A. Guterch D. Banka J. Lamarche A. Lassen B. Lewerenz M. Scheck A. -M. Marotta 《Tectonophysics》2002,360(1-4):301-314
The extension of eastern Avalonia from Britain through the NE German Basin into Poland is, in some sense, a virtual structure. It is covered almost everywhere by late Paleozoic and younger sediments. Evidence for this terrane is only gathered from geophysical data and age information derived from magmatic rocks. During the last two decades, much geophysical and geological information has been gathered since the European Geotraverse (EGT), which was followed by the BABEL, LT-7, MONA LISA, DEKORP-Basin'96, and POLONAISE'97 deep seismic experiments. Based on seismic lines, a remarkable feature has been observed between the North Sea and Poland: north of the Elbe Line (EL), the lower crust is characterised by high velocities (6.8–7.0 km/s), a feature which seems to be characteristic for at least a major part of eastern Avalonia (far eastern Avalonia). In addition, the seismic lines indicate that a wedge of the East European Craton (EEC) (or Baltica) continues to the south below the southern Permian Basin (SPB)—a structure which resembles a passive continental margin. The observed pattern may either indicate an extension of the Baltic crust much farther south than earlier expected or oceanic crust of the Tornquist Sea trapped during the Caledonian collision. In either case, the data require a reinterpretation of the docking mechanism of eastern Avalonia, and the Elbe–Odra Line (EOL), as well as the Elbe Fault system, together with the Intra-Sudedic Faults, appear to be related to major changes in the deeper crustal structures separating the East European crust from the Paleozoic agglomeration of Middle European terranes. 相似文献
414.
Orogeny, migmatites and leucogranites: A review 总被引:13,自引:0,他引:13
Michael Brown 《Journal of Earth System Science》2001,110(4):313-336
The type ofP-T-t path and availability of fluid (H2O-rich metamorphic volatile phase or melt) are important variables in metamorphism. Collisional orogens are characterized
by clockwiseP-T evolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be
present throughout a significant portion of the evolution. Field observations of eroded orogens show that lower crust is migmatitic,
and geophysical observations have been interpreted to suggest the presence of melt in active orogens. A consequence of these
results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decouples weak crust
from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Migmatites provide a record
of melt segregation in partially molten crustal materials and syn-anatectic deformation under natural conditions. Grain boundary
flow and intra-and inter-grain fracture flow are the principal grain scale melt flow mechanisms. Field observations of migmatites
in ancient orogens show that leucosomes occur oriented in the metamorphic fabrics or are located in dilational sites. These
observations are interpreted to suggest that melt segregation and extraction are syntectonic processes, and that melt migration
pathways commonly relate to rock fabrics and structures. Thus, leucosomes in depleted migmatites record the remnant permeability
network, but evolution of permeability networks and amplification of anomalies are poorly understood. Deformation of partially
molten rocks is accommodated by melt-enhanced granular flow, and volumetric strain is accommodated by melt loss. Melt segregation
and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure
buildup. During clockwiseP-T evolution, H2O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H2O may be evolved above the solidus at lowP by crossing supra-solidus decompression-dehydration reactions if micas are still present in the depleted protolith. H2O dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled H2O may promote wet melting at supra-solidus conditions and retrogression at subsolidus conditions. The common growth of ‘late’
muscovite over sillimanite in migmatite may be the result of this process, and influx of exogenous H2O may not be necessary. However, in general, metasomatism in the evolution of the crust remains a contentious issue. Processes
in the lower-most crust may be inferred from studies of xenolith suites brought to the surface in lavas. Based on geochemical
data, we can use statistical methods and modeling to evaluate whether migmatites are sources or feeder zones for granites,
or simply segregated melt that was stagnant in residue, and to compare xenoliths of inferred lower crust with exposed deep
crust. Upper-crustal granites are a necessary complement to melt-depleted granulites common in the lower crust, but the role
of mafic magma in crustal melting remains uncertain. Plutons occur at various depths above and below the brittle-to-viscous
transition in the crust and have a variety of 3-D shapes that may vary systematically with depth. The switch from ascent to
emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength
or state of stress) the ascent column, or by the ascending magma intersecting some discontinuity in the crust that enables
horizontal magma emplacement followed by thickening during pluton inflation. Feedback relations between rates of pluton filling,
magma ascent and melt extraction maintain compatibility among these processes. 相似文献
415.
416.
Granitoids and Their Magnetic Susceptibility in South Korea 总被引:1,自引:0,他引:1
Abstract: Magnetic susceptibility (MS) measurements were carried out for 1,120 samples in the Middle Proterozoic to Early Tertiary granitoids so far recognized in South Korea, and the lateral and spatial variation of their magnetic susceptibility, i.e., content of magnetite, is studied. The Middle Proterozoic two mica granitoids related to cassiterite (Sn) deposits in northeastern part of the Sobaegsan Massif show very low MS (less than 0.3 A‐ 10‐3 SI unit), and the Permo‐Triassic tonalitic to granodioritic and monzonitic rocks which are barren in mineralization, distributed in the middle part of South Korea also show low MS (less than 1 A‐ 10‐3 SI unit). On the contrary the Late Triassic to Jurassic granitoids (= Daebo granitoids) which were evolved from tonalite through granodiorite to granite, and are most widely distributed in South Korea, show a wide variation on MS. Particularly in the Andong, Igsan, Gimcheon and Geochang areas, the granitoids which are barren in mineralization, are characterized by high MS (more than 10 A‐ 10‐3 SI unit). The Chuncheon, Jecheon, Namyang and Geumsan plutons related to molybdenite (Mo) and/or wolframite or scheelite (W), and fluorite (F) mineralizations show a little high MS (more than 3 A‐ 10‐3 SI unit). However, more than 60% of the Daebo granitoids show low MS (less than 3 A‐ 10‐3 SI unit) and the rest show a little high MS (more than 3 A‐ 10‐3 SI unit). Heterogeneous distribution of magnetite content in the Daebo granitoids is considered to reflect heterogeneity of redox state of the source materials for these granitoids. The Cretaceous to Early Tertiary granitoids (= Bulgugsa granitoids) in the Gyeongsang Basin had been generally evolved in the order of tonalite, diorite, granodiorite, granite and alkali‐feldspar granites, which are closely related to base metal ore deposits, and mostly show higher MS (more than 3 A‐ 10‐3 SI unit) than other granitoids mentioned above, although some exceptions are recognized in highly evolved alkali‐feldspar granites (SiO2 > 76%). In contrast, as most of the highly oxidized or evolved Cretaceous granitoids distributed in areas other than the Gyeongsang Basin show lower MS than those of the Gyeongsang Basin, and appear to be magnetite free, ilmenite‐series granites, but they might be hematite bearing magnetite‐series granitoids. Highly oxidized nature of the Bulgugsa granitoids may be due to high Fe2O3/FeO ratio of the source materials and also high level intrusion style of the granitic magma activities. Most of the granitic rocks of the Middle Proterozoic, Permo‐Triassic and more than 60% of the Late Triassic to Jurassic (Daebo granitoids) belong to ilmenite–series, however less than 40% of the Daebo granitoids and most Cretaceous ones are magnetite–series. Thus, the granitic magma intruded in Korean Peninsula became oxidized while the intrusive ages become younger. 相似文献
417.
YANG Xing-ke CHENG Jian-jun CHENG Hong-bing JI Jin-sheng CHEN Qiang LUO Gui-chang 《大地构造与成矿学(英文版)》2001,25(1)
According to tectono-stratigraphical division principle,different units in KangguertageHuangshan collision zone,Eastern Tianshan,are divided into order and disorder two types of stratum-rock associations,which belong to two kinds of different tectono-volcanic activity belts.The collision ororeny and ductile shear zone have a strong space-time coupling.Based on the island arcbearing volcanic rock on both sides of the collision zone,time of ore-forming and rock-forming,characteristics of collision granit,geochemical province,special multistage collision orogeny and intracontinental orogeny basin-forming developed features have been found.Gold and copper deposits,with the relation to the collision orogeny,are divided into seven genetic types.The ten metallogenic zones are classified into two kinds of ore-forming system of paleo-continental margin.Metallogenesis of gold deposits can be classified into five stages.Gold and copper deposits are distributed in belts with the relation to the development of the collision orogeny.The distribution of main large scale copper deposits in the north part of the collision zone and most large-middle scale gold deposits in the south part of the collision zone can direct the prospecting for gold and copper deposits.The actual targets are put forward. 相似文献
418.
1987年以来,中国科学院青藏高原综合科学考察队开展了对喀喇昆仑山和昆仑山的多学科综合性科学考察.本文根据这次考察的大量资料,论证了青藏高原第五缝合带的存在.考察资料表明,在西昆仑山存在一个被称作原特提斯的海洋,这个洋是在劳亚大陆南部基底上破裂、扩张并形成于震旦纪至奥陶纪时期(大约800-450Ma).原特提斯海洋闭合于加里东造山运动,并形成现在所见的库地-苏巴什缝合带的残迹,即青藏高原第五缝合带,从而完善了青藏高原的地质历史和构造演化. 相似文献
419.
ABSTRACT The Bunger Hills, East Antarctica, experienced a low-pressure granulite facies orogenic event during the Proterozoic. The stable coexistence of the S1 foliation-parallel M1 assemblages, garnet-cordierite-spinel-ilmenite and garnet-sillimanite-spinel-ilmenite-rutile, in quartz-bearing pelitic gneisses is evidence for metamorphic peak pressures of around 4 kbar during M1, at temperatures of about 800°C. The growth of massive reaction coronas of garnet and cordierite around hercynitic spinel and iron-titanium oxides during M2 is evidence for the destabilization of the M1 assemblages during compression. Thermodynamic calculations on the M2 assemblages indicate formation pressures of 6–7 kbar at temperatures of about 750°C. Thus, the gneisses from the Bunger Hills indicate about 2 kbar or more of compression during minimal cooling. Such a P-T path is different from that of many other Proterozoic terranes which are characterized by isobaric cooling or decompression. A large charnockite body, which is undeformed, was intruded at ~950°C, towards the end of compression. The low pressures during M1 can be best explained by metamorphism at mid-crustal levels in thin continental crust in thin lithosphere above a thermal perturbation in the underlying asthenosphere. We suggest that the compression during cooling was a result of gravitational backflow in which the action of body forces between adjacent normal thickness crust and the thin crust of the Bunger Hills is 'switched on’by the thermal perturbation. Within such a model, the timing of intrusion of the charnockite exposed in the Bunger Hills is consistent with its generation by partial melting during the metamorphic maximum of the lowermost crust. 相似文献
420.
江南皖浙赣区段混杂岩带及其区域构造意义 总被引:5,自引:1,他引:5
主要介绍了展布在皖浙赣区段的混杂岩带研究的新进展,其中包括:混杂岩带空间展布的重新划定,混杂岩带内浅变质岩中的化石及浅变质岩的时代归属,混杂岩带的形成时代及混杂岩带的构造变形特征等;进而讨论了其区域构造意义。 相似文献