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1.
The age of pseudotachylite formation in the crustal-scale Cauvery Shear Zone system of the Precambrian Southern Granulite Terrain (South India) has been analyzed by laser-probe 40Ar–39Ar dating. Laser spot analyses from a pseudotachylite from the Salem–Attur shear zone have yielded ages ranging from 1214 to 904 Ma. Some evidence for the presence of excess Ar is indicated by the scatter of ages from this locality. The host gneiss preserves Palaeoproterozoic Rb–Sr whole rock–biotite ages (2350 ± 11 to 2241 ± 11 Ma). A mylonite in the Koorg shear, ca. 200 km to the north, yielded an age of 895 ± 17 Ma the consistency of the age distribution from spot analyses precludes the presence of significant excess Ar. Despite published evidence for the growth of high-grade minerals within some components of the Cauvery Shear Zone during the Pan-African event (700–550 Ma), the pseudotachylites in this study provide no evidence for Pan-African formation. Instead they document the first evidence for Mesoproterozoic tectonism in the Cauvery Shear Zone system, thus prompting a review of the correlation between the Cauvery Shear Zone system and the large-scale shear zones located elsewhere in eastern Gondwana.  相似文献   

2.
We present a detailed, new time scale for an orogenic cycle (oceanic accretion–subduction–collision) that provides significant insights into Paleozoic continental growth processes in the southeastern segment of the long-lived Central Asian Orogenic Belt (CAOB). The most prominent tectonic feature in Inner Mongolia is the association of paired orogens. A southern orogen forms a typical arc-trench complex, in which a supra-subduction zone ophiolite records successive phases during its life cycle: birth (ca. 497–477 Ma), when the ocean floor of the ophiolite was formed; (2) youth (ca. 473–470 Ma), characterized by mantle wedge magmatism; (3) shortly after maturity (ca. 461–450 Ma), high-Mg adakite and adakite were produced by slab melting and subsequent interaction of the melt with the mantle wedge; (4) death, caused by subduction of a ridge crest (ca. 451–434 Ma) and by ridge collision with the ophiolite (ca. 428–423 Ma). The evolution of the magmatic arc exhibits three major coherent phases: arc volcanism (ca. 488–444 Ma); adakite plutonism (ca. 448–438 Ma) and collision (ca. 419–415 Ma) of the arc with a passive continental margin. The northern orogen, a product of ridge-trench interaction, evolved progressively from coeval generation of near-trench plutons (ca. 498–461 Ma) and juvenile arc crust (ca. 484–469 Ma), to ridge subduction (ca. 440–434 Ma), microcontinent accretion (ca. 430–420 Ma), and finally to forearc formation. The paired orogens followed a consistent progression from ocean floor subduction/arc formation (ca. 500–438 Ma), ridge subduction (ca. 451–434 Ma) to microcontinent accretion/collision (ca. 430–415 Ma); ridge subduction records the turning point that transformed oceanic lithosphere into continental crust. The recognition of this orogenic cycle followed by Permian–early Triassic terminal collision of the CAOB provides compelling evidence for episodic continental growth.  相似文献   

3.
The Precambrian geology of west-central Madagascar is reviewed and re-interpreted in light of new field observations, Landsat Thematic Mapper image analysis, and U–Pb geochronology. The bedrock of the area consists of: (1) late Archean (to Paleoproterozoic) migmatite gneiss and schist; (2) Mesoproterozoic stratified rocks (Itremo, Amborompotsy, and Malakialina Groups) perhaps deposited unconformably on the older metamorphic rocks (1, above); (3) Proterozoic ( 1000 Ma–720 Ma) plutonic rocks emplaced into both units above (1 and 2), and; (4) latest Neoproterozoic to middle Cambrian ( 570–520 Ma) granitoids emplaced as regionally discordant and weakly foliated plutons throughout the regions.

The effects of Neoproterozoic orogenic processes are widespread throughout the region and our observations and isotopic measurements provide important constraints on the tectonic history of the region: (i) Archean gneisses and Mesoproterozoic stratified rocks are the crystalline basement and platformal sedimentary cover, respectively, of a continental fragment of undetermined tectonic affinity (East or West Gondwanan, or neither). (ii) This continental fragment (both basement and cover) was extensively invaded by subduction-related plutons in the period from  1000 Ma to  720 Ma that were emplaced prior to the onset of regional metamorphism and deformation. (iii) Continental collision related to Gondwana's amalgamation began after  720 Ma and before  570 Ma. Collision related deformation and metamorphism continued throughout the rest of the Neoproterozoic with thermal effects that lasted until  520 Ma. The oldest structures produced during continental collision were km-scale fold- and thrust-nappes with east or southeast-directed vergence (present-day direction). They resulted in the inversion and repetition of Archean and Proterozoic rocks throughout the region. During this early phase of convergence warm rocks were thrust over cool rocks thereby producing the present distribution of regional metamorphic isograds. The vergence of the nappes and the distribution of metamorphic rocks are consistent with their formation within a zone of west or northwest-dipping continental convergence (present-day direction). (iv) Later upright folding of the nappes (and related folds and thrusts) produced km-scale interference fold patterns. The geometry and orientation of these younger upright folds is consistent with E–W horizontal shortening (present-day direction) within a sinistral transpressive regime. We relate this final phase of deformation to motion along the Ranotsara and related shear zones of south Madagascar, and to the initial phases of lower crustal exhumation and extensional tectonics within greater Gondwana.  相似文献   


4.
207Pb/206Pb ages from 420 zircons from 9 upper amphibolite and granulite facies quartzites of the post-1.8 Ga metasedimentary cover from the Occidental and Oriental terranes from the Neoproterozoic central Ribeira belt, in eastern Brazil, are discussed. Ages from the Occidental terrane show that Proterozoic ages predominate with a modal class at 2.1–2.2 Ga and that Archean ages are less common and lie within the 2.5–3.6 Ga range. A wider dispersion on Proterozoic ages down to 1.1 Ga and youngest Neoproterozoic ages (550–700 Ma) were also observed and could be related to the Brasiliano/PanAfrican metamorphic events. The data show that detrital sediments of the Occidental terrane were derived predominantly from erosion of a 1.8–2.2 Ga Paleoproterozoic terrane with a small proportion of a 2.6–3.6 Ga Archean crust. Ages from the Oriental terrane are greatly dispersed, but show a significant proportion of ages between 1.7 and 2.1 Ga, although the great majority are between 0.5 and 1.6 Ga. Only 3 out of 144 grains yield Archean ages, between 2.5 and 2.9 Ga. Based on the greatest errors observed on the data from the Oriental Terrane (100–300 Ma), and a continuum of obtained ages between 2072 Ma (Paleoproterozoic) and 517 Ma (Cambrian), the more plausible interpretation is that the dispersion of ages results from partial resetting of the U–Pb system by Brasiliano metamorphism in Proterozoic–Archean zircons, although a Mesoproterozoic source could not be discarded.  相似文献   

5.
A vast supracrustal belt of khondalites (granulite facies metapelites) occur along the northern margin of the North China Craton. We report here for the first time spinel + quartz equilibrium assemblage from these rocks in two textural settings: (1) high ZnO (up to 14.47 wt.%) spinel with quartz as inclusions within garnet porphyroblasts defining pressure above 12 kbar and temperature of 900 °C; and (2) low ZnO (down to 1.2 wt.%) spinel in association with quartz in the matrix assemblage formed during peak ultrahigh-temperature conditions (ca. 975 °C and 9 kbar). We present a unique case of decompression where the metamorphic conditions of the rocks traversed mostly through the spinel + quartz (extended) stability field. Monazite grains in textural association with both types of spinel + quartz textures were analysed for age determination, and the data define two age peaks at 1927 ± 11 Ma and 1819 ± 11 Ma. Since the peak thermal regime of the khondalites was close to or exceeded the theoretical closure temperature of Pb in monazite, we infer the 1819 ± 11 Ma age as the timing of ultrahigh-temperature event in this craton. Our data lend support to the idea of ca. 1.9–1.8 Ga E–W collisional orogen at the northern margin of the North China Craton. We correlate the extreme crustal metamorphism with tectonics associated with the assembly of the North China Craton within the Columbia supercontinent.  相似文献   

6.
Zircons from an eclogite and a diamond-bearing metapelite near the Kimi village (north-eastern Rhodope Metamorphic Complex, Greece) have been investigated by Micro Raman Spectroscopy, SEM, SHRIMP and LA-ICPMS to define their inclusion mineralogy, ages and trace element contents. In addition, the host rocks metamorphic evolution was reconstructed and linked to the zircon growth domains.

The eclogite contains relicts of a high pressure stage (ca. 700 °C and > 17.5 kbar) characterised by matrix omphacite with Jd40–35. This assemblage was overprinted by a lower pressure, higher temperature metamorphic event (ca. 820 °C and 15.5–17.5 kbar), as indicated by the presence of clinopyroxene (Jd35–20) and plagioclase. Biotite and pargasitic amphibole represent a later stage, probably related to an influx of fluids. Zircons separated from the eclogite contain magmatic relicts indicating Permian crystallization of a quartz-bearing gabbroic protolith. Inclusions diagnostic of the high temperature, post-eclogitic overprint are found in metamorphic zircon domain Z2 which ages spread over a long period (160 – 95 Ma). Based on zircon textures, zoning and chemistry, we suggest that the high-temperature peak occurred at or before ca. 160 Ma and the zircons were disturbed by a later event possibly at around 115 Ma. Small metamorphic zircon overgrowths with a different composition yield an age of 79 ± 3 Ma, which is related to a distinct amphibolite-facies metamorphic event.

The metapelitic host rock consists of a mesosome with garnet, mica and kyanite, and a quartz- and plagioclase-bearing leucosome, which formed at granulite-facies conditions. Based on previously reported micro-diamond inclusions in garnet, the mesosome is assumed to have experienced UHP conditions. Nevertheless, (U)HP mineral inclusions were not found in the zircons separated from the diamond-bearing metapelite. Inclusions of melt, kyanite and high-Ti biotite in a first metamorphic zircon domain suggest that zircon formation occurred during pervasive granulite-facies metamorphism. An age of 171 ± 1 Ma measured on this zircon domain constrains the high-temperature metamorphic event. A second, inclusion-free metamorphic domain yielded an age of 160 ± 1 Ma that is related to decompression and melt crystallization.

The similar age data obtained from the samples indicate that both rock types recorded a high-T metamorphic overprint at granulite-facies conditions at ca. 170 – 160 Ma. This age implies that any high pressure or even ultra-high pressure metamorphism in the Kimi Complex occurred before that time. Our findings define new constraints for the geodynamic evolution for the Alpine orogenic cycle within the northernmost Greek part of the Rhodope Metamorphic Complex. It is proposed that the rocks of the Kimi Complex belong to a suture zone squeezed between two continental blocks and result from a Paleo-ocean basin, which should be located further north of the Jurassic Vardar Ocean.  相似文献   


7.
Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.  相似文献   

8.
The Variscan Hauzenberg pluton consists of granite and granodiorite that intruded late- to postkinematically into HT-metamorphic rocks of the Moldanubian unit at the southwestern margin of the Bohemian Massif (Passauer Wald). U–Pb dating of zircon single-grains and monazite fractions, separated from medium- to coarse-grained biotite-muscovite granite (Hauzenberg granite II), yielded concordant ages of 320 ± 3 and 329 ± 7 Ma, interpreted as emplacement age. Zircons extracted from the younger Hauzenberg granodiorite yielded a 207Pb–206Pb mean age of 318.6 ± 4.1 Ma. The Hauzenberg granite I has not been dated. The pressure during solidification of the Hauzenberg granite II was estimated at 4.6 ± 0.6 kbar using phengite barometry on magmatic muscovite, corresponding to an emplacement depth of 16-18 km. The new data are compatible with pre-existing cooling ages of biotite and muscovite which indicate the Hauzenberg pluton to have cooled below T = 250–400 °C in Upper Carboniferous times. A compilation of age data from magmatic and metamorphic rocks of the western margin of the Bohemian Massif suggests a west- to northwestward shift of magmatism and HT/LP metamorphism with time. Both processes started at > 325 Ma within the South Bohemian Pluton and magmatism ceased at ca. 310 Ma in the Bavarian Oberpfalz. The slight different timing of HT metamorphism in northern Austria and the Bavarian Forest is interpreted as being the result of partial delamination of mantle lithosphere or removal of the thermal boundary layer.  相似文献   

9.
T. Andersen  W.L. Griffin  A.G. Sylvester   《Lithos》2007,93(3-4):273-287
Laser ablation ICPMS U–Pb and Lu–Hf isotope data on granitic-granodioritic gneisses of the Precambrian Vråvatn complex in central Telemark, southern Norway, indicate that the magmatic protoliths crystallized at 1201 ± 9 Ma to 1219 ± 8 Ma, from magmas with juvenile or near-juvenile Hf isotopic composition (176Hf/177Hf = 0.2823 ± 11, epsilon-Hf > + 6). These data provide supporting evidence for the depleted mantle Hf-isotope evolution curve in a time period where juvenile igneous rocks are scarce on a global scale. They also identify a hitherto unknown event of mafic underplating in the region, and provide new and important limits on the crustal evolution of the SW part of the Fennoscandian Shield. This juvenile geochemical component in the deep crust may have contributed to the 1.0–0.92 Ga anorogenic magmatism in the region, which includes both A-type granite and a large anorthosite–mangerite–charnockite–granite intrusive complex. The gneisses of the Vråvatn complex were intruded by a granitic pluton with mafic enclaves and hybrid facies (the Vrådal granite) in that period. LAM-ICPMS U–Pb data from zircons from granitic and hybrid facies of the pluton indicates an intrusive age of 966 ± 4 Ma, and give a hint of ca. 1.46 Ga inheritance. The initial Hf isotopic composition of this granite (176Hf/177Hf = 0.28219 ± 13, epsilon-Hf = − 5 to + 6) overlaps with mixtures of pre-1.7 Ga crustal rocks and juvenile Sveconorwegian crust, lithospheric mantle and/or global depleted mantle. Contributions from ca. 1.2 Ga crustal underplate must be considered when modelling the petrogenesis of late Sveconorwegian anorogenic magmatism in the region.  相似文献   

10.
兴地Ⅱ号铜镍矿床为岩浆多期次分异、侵位形成的基性-超基性杂岩体;本文通过对兴地辉长岩进行SIMS锆石U-Pb精确测年,获得206Pb/238U加权平均年龄为737±2 Ma,该数据表明兴地Ⅱ号杂岩体形成于南华纪,并非形成于中元古代。塔里木北缘广泛发育的碳酸岩、基性-超基性岩和双峰式火山岩,为Rodinia 超大陆长期的和持续的裂解产物。  相似文献   

11.
Ar/Ar analyses of phengites and paragonites from the ultrahigh-pressure metamorphic rocks (zoisite–clinozoisite schist, garnet–phengite schist and piemontite schist) in the Lago di Cignana area, Western Alps were carried out with a laser probe step-heating method using single crystals and a spot dating method on thin sections. Eight phengite and two paragonite crystals give the plateau ages of 37–42 Ma with 96–100% of 39Ar released. Each rock type also contains mica crystals showing discordant age spectra with age fractions (20–35 Ma) significantly younger than the plateau ages. Phengite inclusions in garnet give ages of 43.2 ± 1.1 Ma and 44.4 ± 1.5 Ma, which are significantly older than the spot age (36.4 ± 1.4 Ma) from the matrix phengites, and the plateau ages from the step-heating analyses. Inclusion ages (43 and 44 Ma) are consistent with a zircon SHRIMP age (44 ± 1 Ma) in this area. These results suggest that the oceanic materials that underwent a simple subduction related UHPM, form excess 40Ar-free phengite and that the peak metamorphism is ca. 44 Ma or little older. We suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic 40Ar from the crystals. This has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age.  相似文献   

12.
Crustal architecture of the Himalayan metamorphic front in eastern Nepal   总被引：4，自引：0，他引：4  
The Himalayan Metamorphic Front consists of two basinal sequences deposited on the Indian passive margin, the Mesoproterozoic Lesser Himalayan Sequence and the Neoproterozoic–Cambrian Greater Himalayan Sequence. The current paradigm is that the unconformity between these two basinal sequences coincides with a crustal-scale thrust that has been called the Main Central Thrust, and that this acted as the fundamental structure that controlled the architecture of the Himalayan Metamorphic Front. Geological mapping of eastern Nepal and eight detailed stratigraphic, kinematic, strain and metamorphic profiles through the Himalayan Metamorphic Front define the crustal architecture. In eastern Nepal the unconformity does not coincide with a discrete structural or metamorphic discontinuity and is not a discrete high strain zone. In recognition of this, we introduce the term Himalayan Unconformity to distinguish it from high strain zones in the Himalayan Metamorphic Front. The fundamental structure that controls orogen architecture in eastern Nepal occurs at higher structural levels within the Greater Himalayan Sequence and we suggest the name; High Himal Thrust. This 100–400 m thick mylonite zone marks a sharp deformation discontinuity associated with a steep metamorphic transition, and separates the Upper-Plate from the Lower-Plate in the Himalayan Metamorphic Front. The high-T/moderate-P metamorphism at  20–24 Ma in the Upper-Plate reflects extrusion of material between the High Himal Thrust and the South Tibet Detachment System at the top of the section. The Lower-Plate is a broad schistose zone of inverted, diachronous moderate-T/high-P metamorphic rocks formed between  18 and 6 Ma. The High Himal Thrust is laterally continuous into Sikkim and Bhutan where it also occurs at higher structural levels than the Himalayan Unconformity and Main Central Thrust (as originally defined). To the west in central Nepal, the Upper-Plate/Lower-Plate boundary has been placed at lower structural levels, coinciding with the Himalayan Unconformity and has been named the Main Central Thrust, above the originally defined Main Central Thrust (or Ramgarh Thrust).  相似文献   

13.
Deposits of coral-bearing, marine shell conglomerate exposed at elevations higher than 20 m above present-day mean sea level (MSL) in Bermuda and the Bahamas have previously been interpreted as relict intertidal deposits formed during marine isotope stage (MIS) 11, ca. 360–420 ka before present. On the strength of this evidence, a sea level highstand more than 20 m higher than present-day MSL was inferred for the MIS 11 interglacial, despite a lack of clear supporting evidence in the oxygen-isotope records of deep-sea sediment cores. We have critically re-examined the elevated marine deposits in Bermuda, and find their geological setting, sedimentary relations, and microfaunal assemblages to be inconsistent with intertidal deposition over an extended period. Rather, these deposits, which comprise a poorly sorted mixture of reef, lagoon and shoreline sediments, appear to have been carried tens of meters inside karst caves, presumably by large waves, at some time earlier than ca. 310–360 ka before present (MIS 9–11). We hypothesize that these deposits are the result of a large tsunami during the mid-Pleistocene, in which Bermuda was impacted by a wave set that carried sediments from the surrounding reef platform and nearshore waters over the eolianite atoll. Likely causes for such a megatsunami are the flank collapse of an Atlantic island volcano, such as the roughly synchronous Julan or Orotava submarine landslides in the Canary Islands, or a giant submarine landslide on the Atlantic continental margin.  相似文献   

14.
The eastern Amery Ice Shelf (EAIS) and southwestern Prydz Bay are situated near the junction between the Late Neoproterozoic/Cambrian high-grade complex of the Prydz Belt and the Early Neoproterozoic Rayner Complex. The area contains an important geological section for understanding the tectonic evolution of East Antarctica. SHRIMP U–Pb analyses on zircons of felsic orthogneisses and mafic granulites from the area indicate that their protoliths were emplaced during four episodes of ca. 1380 Ma, ca. 1210–1170 Ma, ca. 1130–1120 Ma and ca. 1060–1020 Ma. Subsequently, these rocks experienced two episodes of high-grade metamorphism at > 970 Ma and ca. 930–900 Ma, and furthermore, most of them (except for some from the Munro Kerr Mountains and Reinbolt Hills) were subjected to high-grade metamorphic recrystallization at ca. 535 Ma. Two suites of charnockite, i.e. the Reinbolt and Jennings charnockites, intrude the Late Mesoproterozoic/Early Neoproterozoic and Late Neoproterozoic/Cambrian high-grade complexes at > 955 Ma and 500 Ma, respectively. These, together with associated granites of similar ages, reflect late- to post-orogenic magmatism occurring during the two major orogenic events. The similarity in age patterns suggests that the EAIS–Prydz Bay region may have suffered from the same high-grade tectonothermal evolution with the Rayner Complex and the Eastern Ghats of India. Three segments might constitute a previously unified Late Mesoproterozoic/Early Neoproterozoic orogen that resulted from the long-term magmatic accretion from ca. 1380 to 1020 Ma and eventual collision before ca. 900 Ma between India and the western portion of East Antarctica. The Prydz Belt may have developed on the eastern margin of the Indo-Antarctica continental block, and the Late Neoproterozoic/Cambrian suture assembling Indo-Antarctica and Australo-Antarctica continental blocks should be located southeastwards of the EAIS–Prydz Bay region.  相似文献   

15.
The island of Madagascar experienced widespread magmatism at ca. 90 Ma due to its interaction with the Marion hotspot. Previous paleomagnetic data from igneous rocks in the southwestern and northwestern regions of the island indicated that the Marion hotspot has remained fixed for the past 90 Ma. We report paleomagnetic data from northeastern Madagascar (d'Analava Complex). Samples were collected from basalts, rhyolites, gabbros and a dolerite dyke. Sixty samples from 5 sites yield a paleomagnetic pole at 66.7°S, 43.5°E (A95 = 10.7°) and a grand mean pole (GMP) calculated from 10 different studies covering the entire island of Madagascar falls at 68.9°S, 49.0°E (A95 = 4.4°). This pole translates to a paleolatitude for the Volcan de l'Androy (focal point of the hotspot) at 45.2° + 6°/− 5°S compared to the current location of the Marion hotspot at  46°S. Our results confirm, and expand upon, previous studies that argue for the fixity of the Marion hotspot for the past 90 Ma.  相似文献   

16.
A particular non-stromatolitic carbonate succession making up the third member of the Mesoproterozoic Gaoyuzhuang (高于庄) Formation might demonstrate that a stromatolite decline of the Mesoproterozoic occurring at ca. 1 450 Ma besides other three events of the Proterozoic,respectively,occurred at ca. 2 000 Ma,ca. 1 000 Ma,and ca. 675 Ma. The forming duration of this non-stromatolitic carbonate succession can be generally correlative to that of a similar depositional succession in North America,i.e. a non-stromatolitic carbonate succession made up by the Helena Formation of the Belt Supergroup,which suggests that the stromatolite decline occurring at ca. 1 450 Ma may be a global event. This information endows the non-stromatolitic carbonate succession making up the third member of the Gaoyuzhuang Formation in the Yanshan (燕山) area with important significance for the further understanding of Precambrian sedimentology. The Mesoproterozoic Gaoyuzhuang Formation in Yanshan area is a set of more than 1 000 m thick carbonate strata that can be divided into four members (or subformations). The first member (or the Guandi (官地) subformation) is marked by a set of stromatolitic dolomites overlying a set of transgressive sandstones; the second member (or the Sangshu'an (桑树鞍) subformation) is a set of manganese dolomites with a few stromatolites; the third member (or the Zhangjiayu (张家峪) subformation) is chiefly made up of leiolite and laminite limestones and is characterized by the development of molar-tooth structures in leiolite limestone; the fourth member (or the Huanxiusi (环秀寺) subformation) is composed of a set of dolomites of stromatolitic reefs or lithoherms. Sequence-stratigraphic divisions at two sections,i.e. the Jixian (蓟县) Section in Tianjin (天津) and the Qiangou (千沟) Section of Yanqing (延庆) County in Beijing (北京),demonstrate that a particularly non-stromatolitic succession making up the third member of the Mesoproterozoic Gaoyuzhuang Formation is developed in the Yanshan area of North China,in which lots of grotesque matground structures (wrinkle structures and palimpsest ripples) are developed in beds of leiolite limestone at the Qiangou Section and lots of molar-tooth structures are developed in beds of leiolite limestone at the Jixian Section. The time scale of the Gaoyuzhuang Formation is deduced as 200 Ma (from 1 600 Ma to 1 400 Ma). The duration of an obvious hiatus between the Gaoyuzhuang Formation and the underlying Dahongyu (大红峪) Formation is deduced as 50 Ma to 100 Ma,thus the forming duration of the GaoyuzhuangFormation is thought as 100 Ma (1 500 Ma to 1 400 Ma). Furthermore,the age of the subface of the third member of the Gaoyuzhuang Formation that is just in the mid position of the Gaoyuzhuang Formation can be deduced as about 1 450 Ma,which is the basis to infer a stromatolite decline of the Mesoproterozoic occurring at ca. 1 450 Ma. Importantly,several features of both the molar-tooth structure and the stromatolite,such as the particular forming environment,the important facies-indicative meaning,and the episodic distribution in the earth history,might express the evolutionary periodicity of the surface environment of the earth and can provide meaningful clues for the understanding of the Precambrian world,although their origin and forming mechanism is highly contentious. Therefore,like other three stromatolitic declines,respectively,occurring at ca. 675 Ma,ca. 1 000 Ma,and ca. 2 000 Ma,the identification of the stromatolite decline occurring at ca. 1 450 Ma during the Golden Age of stromatolites (2 800 Ma to 1 000 Ma) has important meaning for the further understanding of the evolving carbonate world of the Precambrian.  相似文献   

17.
This paper presents a regional scale observation of metamorphic geology and mineral assemblage variations of Kontum Massif, central Vietnam, supplemented by pressure–temperature estimates and reconnaissance geochronological results. The mineral assemblage variations and thermobarometric results classify the massif into a low- to medium-temperature and relatively high-pressure northern part characterised by kyanite-bearing rocks (570–700 °C at 0.79–0.86 GPa) and a more complex southern part. The southern part can be subdivided into western and eastern regions. The western region shows very high-temperature (> 900 °C) and -pressure conditions characterised by the presence of garnet and orthopyroxene in both mafic and pelitic granulites (900–980 °C at 1.0–1.5 GPa). The eastern region contains widespread medium- to high-temperature and low-pressure rocks, with metamorphic grade increasing from north to south; epidote- or muscovite-bearing gneisses in the north (< 700–740 °C at < 0.50 GPa) to garnet-free mafic and orthopyroxene-free pelitic granulites in the south (790–920 °C at 0.63–0.84 GPa). The Permo-Triassic Sm–Nd ages (247–240 Ma) from high-temperature and -pressure granulites and recent geochronological studies suggest that the south-eastern part of Kontum Massif is composed of a Siluro-Ordovician continental fragment probably showing a low-pressure/temperature continental geothermal gradient derived from the Gondwana era with subsequent Permo-Triassic collision-related high-pressure reactivation zones.  相似文献   

18.
The boundary between the Archean cratons and the Eastern Ghats Belt in peninsular India represents a rifted Mesoproterozoic continental margin which was overprinted by a Pan-African collisional event associated with the westward thrusting of the Eastern Ghats granulites over the cratonic foreland. The contact zone contains a number of deformed and metamorphosed nepheline syenite complexes of rift-related geochemical affinities. In addition to the nepheline-bearing rocks, metamorphosed quartz-bearing monzosyenitic bodies can also be identified along the suture in the region between the Godavari-Pranhita graben and the Prakasam Igneous Province. One such occurrence at Jojuru near Kondapalle is geochemically comparable to the nepheline syenites and furnishes a weighted mean concordant U–Th–Pb SHRIMP zircon age of 1263 ± 23 Ma (2σ), which provides a lower age bracket for the rift-related magmatic activity. The original igneous mineral assemblage in the monzosyenite was partially replaced by the formation of coronitic garnet during the Pan-African metamorphism of the rocks. P–T estimates of garnet corona formation at the interface between clinopyroxene–orthopyroxene–ilmenite clusters and plagioclase indicate mid to upper amphibolite facies condition (5.5–7.0 kbar and 600–700 °C) during the thrust induced deformation and metamorphism associated with the Pan-African collisional tectonics.  相似文献   

19.
This study is a comprehensive, stable isotope survey of the marine carbonate-dominated, upper Paleo- to lower Neoproterozoic stratigraphy of Jixian County, China. Carbonate-associated sulfate (CAS) was extracted and measured for δ34SCAS using the same samples analyzed for δ13Ccarbonate. This integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of Proterozoic seawater. We specifically sampled marine carbonate intervals from the lower section of the Chuanlinggou Formation, Changcheng Group (ca. 1700 Ma) to the top of the Jingeryu Formation, Qingbaikou Group (ca. 800 Ma). δ13Ccarbonate values are mostly negative in the upper Paleoproterozoic Changcheng Group, with an ascending trend from −3‰ to 0‰. We observed variation of approximately 0 ± 1‰ in the Mesoproterozoic Jixian Group, and positive values of +2 ± 2‰ characterize the lower Neoproterozoic Qingbaikou Group. Stratigraphic variations in δ34SCAS are more remarkable in their ranges and magnitudes, including conspicuously high values exceeding +30‰ in the three intervals at ca. 1700 Ma, 1300-1100 Ma, and 1000-900 Ma. In the Changcheng Group, δ34SCAS values are typically higher than +25‰, with only a few values of less than +15‰. In contrast, most of the data spanning from the Mesoproterozoic Tieling Formation of the Jixian Group to the lower Neoproterozoic Jingeryu Formation of the Qingbaikou Group are highly variable between +10‰ and +25‰, with some values exceeding +25‰.In the late Paleoproterozoic (1700-1600 Ma), a >10‰ decrease in δ34SCAS and ∼3‰ increase in δ13Ccarbonate are coincident with, and likely related to, the breakup of Columbia, a supercontinent that predated Rodinia. Carbon and sulfur isotope data from the Mesoproterozoic, when global tectonic activity was comparatively weaker, fall mostly in the ranges of +15 ± 10‰ and 0 ± 1‰, respectively, but fluctuations of >20‰ for δ34SCAS and >3‰ for the δ13Ccarbonate at ca. 1450-1400 Ma may reflect subduction and large-scale magmatic activity in island arcs marking the end of Columbia breakup. From the late Mesoproterozoic (ca. 1300-1100 Ma) to the early Neoproterozoic (ca. 800 Ma), the δ13C and δ34S of seawater increased gradually with increasing variability. Most impressive areδ34SCAS values that exceed +30‰ in two intervals at ca. 1300-1100 Ma and ca. 1000-900 Ma, which may reflect the assembly and early breakup of Rodinia. Although gaps in the record remain, and studies of even higher resolution are warranted, our results suggest that changes in paleoceanographic conditions linked to global tectonics strongly influenced the biogeochemical cycles of C and S. Furthermore, periods of the Proterozoic previously noted for their isotopic invariability show clear isotopic expressions of this tectonic activity.  相似文献   

20.
敦煌复合造山带前寒武纪地质体的组成和演化   总被引：3，自引：3，他引：0  
赵燕  孙勇 《岩石学报》2018,34(4):963-980
敦煌复合造山带位于塔里木克拉通东端,是连接塔里木克拉通和华北克拉通的重要纽带。近年来,敦煌基础地质研究取得了重大进展。本文简要回顾了敦煌基础地质研究历史和现状,系统归纳了区内前寒武纪地质单元时空分布特征及前寒武纪构造-热事件序列,初步讨论了前寒武纪大陆地壳形成和演化规律、前寒武纪结晶基底亲缘性及构造演化过程,提出:(1)敦煌造山带前寒武纪结晶基底形成于ca.3.1~1.6Ga,构造-热事件主要划分为新太古代(ca.2.7~2.6Ga和2.6~2.5Ga)、古元古代晚期(ca.2.0~1.8Ga)和中元古代早期(1.8~1.6Ga)三个阶段;(2)新太古代早期(ca.2.7~2.6Ga)和新太古代晚期(2.6~2.5Ga)是敦煌造山带大陆地壳形成的主要阶段;古元古代晚期(ca.2.0~1.8Ga)和中元古代早期(1.8~1.6Ga)主要是古老大陆地壳物质再循环阶段,也有少量新生陆壳物质的形成;(3)敦煌造山带前寒武纪结晶基底最初拼合事件可能发生在新太古代末期(~2.5Ga),之后经历了古元古代晚期(ca.2.0~1.8Ga)汇聚、碰撞造山过程,直到中元古代早期(1.8~1.6Ga),造山活动结束,前寒武纪结晶基底最终固结,进入稳定发展阶段;(4)前寒武纪结晶基底最终稳定固结之后,即～1.6Ga之后,敦煌前寒武纪结晶基底可能进入长达12亿年的静寂期,一直处于稳定状态,目前没有发现相关的岩浆-变质-沉积记录(类似于地盾状态),直至古生代志留纪开始活化(～440Ma),卷入古亚洲洋南缘俯冲、碰撞造山过程并被强烈改造。  相似文献   

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