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1.
Abstract Understanding the evolution and destruction of past oceans not only leads to a better understanding of earth history, but permits comparison with extant ocean basins and tectonic processes. This paper reviews the history of the Early Paleozoic circum-Atlantic oceans by analogy with the Pacific Ocean and Mesozoic Tethys. Rifting and continental separation from 620 to 570 Ma led to the development of passive margins along parts of the northern margin of Gondwana (the western coast of South America); eastern Laurentia (eastern North America, NW Scotland and East Greenland), and western Baltica (western Scandinavia). Meagre paleomagnetic data suggest that western South America and eastern North America could have been joined together to form facing margins after breakup. Although western Baltica is an apparently obvious candidate for the margin facing NW Scotland and East Greenland, the paleomagnetic uncertainties are so large that other fragments could have been positioned there instead. The Iapetus Ocean off northeastern Gondwana was probably a relatively wide Pacific-type ocean with, during the late Precambrian to early Ordovician, the northern margin of Gondwana as a site of continentward-dipping subduction zone(s). The 650-500 Ma arc-related igneous activity here and the associated deformation gave rise to the Cadomian, ‘Grampian’, Penobscotian, and Famantinian igneous and orogenic events. By 490-470 Ma, marginal basins had formed along the eastern Laurentian margin as far as NE Scotland, along parts of the northern margin of Gondwana, and off western Baltica, but none are known from the East Greenland margin. These basins closed and parts were emplaced as ophiolites shortly after their formation by processes that, at least in some cases, closely resemble the emplacement of the late Cretaceous Semail ophiolite of Oman. This orogenic phase seems to have involved collision and attempted subduction of the continental margin of Laurentia, Gondwana and Baltica. In Baltica it gave rise to some eclogite facies metamorphism. Marginal basin development may have been preceded by arc formation as early as ca 510 Ma. A double arc system evolved outboard from the eastern Laurentian and western Baltica margins, analogous to some of the arc systems in the present-day western Pacific. At 480-470 Ma, there was a second phase of breakup of Gondwana, affecting the active Gondwanan margin. Eastern and Western Avalonia, the Carolina Slate Belt, Piedmont, and other North American exotic continental blocks rifted away from Gondwana. Farther east, Armorica, Aquitainia, Iberia and several European exotic continental blocks also rifted away, though it is unlikely that they all rifted at the same time. Between 460-430 Ma, peaking at ca 450 Ma, orogenic events involved continuing arc-continent collision(s). From 435-400 Ma the remaining parts of the Eastern Iapetus Ocean were destroyed and the collision of Baltica with Laurentia caused the 430-400 Ma Scandian orogeny, followed by suturing of these continents during the Siluro-Devonian Acadian orogeny or Late Caledonian orogeny to 380 Ma, leaving a smaller but new ocean south of the fragments that had collided with the Laurentian margin farther south. The Ligerian orogeny 390-370 Ma collision of Gondwana-derived Aquitaine-Cantabrian blocks with Eastern Avalonia-Baltica and removed the part of the Iapetus south of Baltica. Prior to any orogenic events, the Eastern Iapetus Ocean between Baltica and Laurentia may have resembled the present-day central Atlantic Ocean between Africa and North America. The ocean appears to have closed asymmetrically, with arcs forming first outboard of the western margin of Baltica while the East Greenland margin was unaffected. The Western Iapetus Ocean between Laurentia and Gondwana also closed asymmetrically with a dual arc system developing off Laurentia and an arc system forming off the northern margin of Gondwana. Like the Pacific Ocean today, the Eastern Iapetus Ocean had a longer and more complex history than the Western Iapetus Ocean: it was already in existence at 560 Ma, probably developed over at least 400 million years, by mid-Cambrian time was many thousands of kilometres wide at maximum extent, and was associated with a < 30 million year phase of marginal basin formation. In contrast, the Western Iapetus Ocean appears to have been much narrower, shorter lived (probably < 100 million years), and associated with the rifting to form two opposing passive carbonate margins, analogous to the Mesozoic Tethys or the present-day Mediterranean.  相似文献   

2.
Recent models for the post-750 Ma Rodinian supercontinent dispersal (e.g. Hoffman, 1991) envision that cratons margined by Grenvillian belts, were reorganized before ca 540 Ma to form the Gondwanan supercontinent. Laurentia and Baltica distanced themselves from Gondwana by moving out of the Rodinian cratonal cluster. West Gondwana, of which Avalon was a part during the late Proterozoic to Cambrian cratonal assembly, consisted mainly of Africa and South America.The main geological evidence is presented for: (1) a transition from continental platform conditions to those of a subduction-related volcanic arc regime in Late Proterozoic time during the dispersal of the Rodinian supercontinent, and the resulting assembly of the Gondwanan supercontinent; and (2) a second transition that marked a reversal from the volcanic arc regime to marine platformal environments by early Cambrian time.Evidence for progressive instability of the continental shelf margining the Rodinian supercontinent is contained in late Proterozoic olistostromes, mylonite zones, calc-alkaline magmatism, and arc-derived clastic rocks, some being glacigenic, during three phases of the Avalonian orogeny.By early Cambrian time the reversal from a tectonically unstable volcanic arc regime to more stable platformal conditions took place as Avalon, Armorica and related microcontinental blocks rifted from Gondwana. These Gondwanan fragments sequentially come into collision, first with each other and Baltica, and then with Laurentia in Mid to Late Paleozoic time as Pangaea was being assembled.  相似文献   

3.
A regional terrane map of the New Jersey Coastal Plain basement was constructed using seismic, drilling, gravity and magnetic data. The Brompton-Cameron and Central Maine terranes were coalesced as one volcanic island arc terrane before obducting onto Laurentian, Grenville age, continental crust in the Taconian orogeny [Rankin, D.W., 1994. Continental margin of the eastern United States: past and present. In: Speed, R.C., (Ed.), Phanerozoic Evolution of North American Continent-Ocean Transitions. DNAG Continent-Ocean Transect Volume. Geological Society of America, Boulder, Colorado, pp. 129–218]. Volcanic island-arc rocks of the Avalon terrane are in contact with Central Maine terrane rocks in southern Connecticut where the latter are overthrust onto the Brompton-Cameron terrane, which is thrust over Laurentian basement. Similarities of these allochthonous island arc terranes (Brompton-Cameron, Central Maine, Avalon) in lithology, fauna and age suggest that they are faulted segments of the margin of one major late Precambrian to early Paleozoic, high latitude peri-Gondwana island arc designated as “Avalonia”, which collided with Laurentia in the early to middle Paleozoic. The Brompton Cameron, Central Maine, and Avalon terranes are projected as the basement under the eastern New Jersey Coastal Plain based on drill core samples of metamorphic rocks of active margin/magmatic arc origin. A seismic reflection profile across the New York Bight traces the gentle dipping (approximately 20 degrees) Cameron's Line Taconian suture southeast beneath allochthonous Avalon and other terranes to a 4 sec TWTT depth (approximately 9 km) where the Avalonian rocks are over Laurentian crust. Gentle up-plunge (approximately 5 degrees) projections to the southwest bring the Laurentian Grenville age basement and the drift-stage early Paleozoic cover rocks to windows in Burlington Co. at approximately 1 km depth and Cape May Co. at approximately 2 km depths. The antiformal Shellburne Falls and Chester domes and Chain Lakes-Pelham dome-Bronson Hill structural trends, and the synformal Connecticut Valley-Gaspe structural trend can be traced southwest into the New Jersey Coastal Plain basement. A Mesozoic rift basin, the “Sandy Hook basin”, and associated eastern boundary fault is identified, based upon gravity modeling, in the vicinity of Sandy Hook, New Jersey. The thickness of the rift-basin sedimentary rocks contained within the “Sandy Hook basin” is approximately 4.7 km, with the basin extending offshore to the east of the New Jersey coast. Gravity modeling indicates a deep rift basin and the magnetic data indicates a shallow magnetic basement caused by magnetic diabase sills and/or basalt flows contained within the rift-basin sedimentary rocks. The igneous sills and/or flows may be the eastward continuation of the Watchung and Palisades bodies.  相似文献   

4.
A method developed recently for constructing tectonic subsidence curves in early Paleozoic miogeoclines has produced new evidence for the breakup of a late Proterozoic supercontinent. Tectonic subsidence analyses in miogeoclines of eastern and western North America, northwestern Argentina, the Middle East and northwestern Australia limit the timing of the continental breakup to between 625 and 555 Ma. These results refine the implications of a much broader range of radiometric ages of rift-related igneous rocks and biostratigraphic ages of the transition from active extension to passive subsidence in miogeoclines.

The recognition of the timing and extent of rifting has led to testable hypotheses for latest Proterozoic and early Paleozoic continental histories. Breakup and onset of drift along an extensive system of continental fractures within a relatively short period of time would generate a large amount of young ocean floor, thereby reducing the volume of the global ocean basin and causing a sea level rise. Maximum reduction of ocean basin volume would postdate the time of breakup, probably by about 70 m.y., placing the transgressive peak at a time not older then about 510–520 Ma. That age agrees well with the time of maximum flooding on the continents close to the end of the Cambrian. Restriction of the breakup to between 625 and 555 Ma reduces the time gap between an essentially intact late Proterozoic supercontinent and the oldest reliable paleomagnetic reconstruction of the dispersed continents at about 560 Ma. A continental reconstruction produced by rotating Laurentia and Baltica into Gondwana a minimum distance from the 560 Ma position is consistent with limited geologic data. However, that reconstruction places Laurentia and Baltica in low latitudes which is difficult to reconcile with the absence of evaporites in syn-rift complexes in both continents.  相似文献   


5.
Nd and Sr isotopic variations of Early Paleozoic oceans   总被引：4，自引：0，他引：4  
We report143Nd/144Nd and87Sr/86Sr isotopic data for Lower Paleozoic phosphatic brachiopod and conodont fossils. The data appear to represent the isotopic values of Early Paleozoic seawaters. We show that different paleoceanic water masses can be distinguished on the basis of their εNd signatures. Two sides of what is classically considered one circulating Iapetus Ocean have different εNd signatures from at least the Middle Cambrian until the Late Middle Ordovician. We infer two ocean basins between North America and Baltica separated by an island and/or shoal circulation barrier. Thus, it appears necessary to redefine the area of the Iapetus Ocean. The εNd signature of the redefined smaller Iapetus Ocean ranges from −5 to −9 and the εNd signature of the larger, coeval Panthalassa Ocean, including part of what was formerly called the Iapetus Ocean, ranges from −10 to −20. The first time that the εNd values are identical in these two water masses is coincident with the onset of the Taconic Orogeny of North America. The paleogeographic geometry we infer from this work is consistent with paleogeographic reconstructions having the Baltica continent at very high latitudes in the Early/Middle Ordovician. The εNd and faunal distribution support temperature-controlled conodont faunal provinciality. A rough mean age for exposed continental crust in the Early Paleozoic can be obtained from the average εNd value of Early Paleozoic Oceans. The data suggest that the mean age of the crust as a function of time has remained essentially constant or even decreased during the past 500 Ma, and suggest substantial additions of new crust to the continents through the Phanerozoic.  相似文献   

6.
Ion microprobe zircon ages, a Nd model age and RbSr whole-rock dates are reported from the high-grade gneiss terrain at Sabaloka on the River Nile north of Khartoum, formally considered to be part of the Archaean/early Proterozoic Nile craton. The granulites, which are of both sedimentary and igneous derivation, occur as remnants in migmatites. Detrital zircon ages range from ≈ 1000 to ≈ 2650 Ma and prove the existence of Archaean to late Proterozoic continental crust in the sedimentary source region. The Nd model age for one sedimentary granulite is between 1.26 (TCHUR) and 1.70 (TDM) Ga and provides a mean crustal residence age for the sedimentary precursor. Igneous zircons in enderbitic gneiss crystallized at 719 ± 81 Ma ago, an age that also corresponds to severe Pb loss in the detrital zircons and which probably reflects the granulite event at Sabaloka. The RbSr data indicate isotopic homogenization at about 700 Ma ago in the granulites and severe post-granulite disturbance at ≈ 570 Ma in the migmatites. We associate this disturbance with hydration, retrograde metamorphism and anatexis that produced undeformed granites ≈ 540 Ma ago. The ≈ 700 Ma granulite event at Sabaloka suggests that this part of the Sudan belongs to the Pan-African Mozambique belt while the ancient Nile craton lay farther west. The gneisses studied here may represent the infrastructure of the ancient African continental margin onto which the juvenile arc assemblage of the Arabian-Nubian shield was accreted during intense horizontal shortening and crustal interstacking of a major collision event.  相似文献   

7.
New UPb zircon crystallization ages and 40Ar/39Ar cooling ages from the Colombian Andes confirm the existence of rocks metamorphosed during the Orinoquian Orogenic Event (ca. 1.0 Ga) of northern South America. εNd (t = 1.1 Ga) for these rocks range from −3.9 to +0.91, which is interpreted as a mixture of Late Archean-Early Proterozoic crust with juvenile material produced during the 1.1 Ga orogenic event. The Colombian Grenville age rocks are part of a much longer metamorphic pericratonal belt, sporadically exposed along the Andes, in western-central Peru, southern Bolivia and northern Argentina. In addition, Nd model (TDM) ages for the Colombian rocks range from 1.9 to 1.45 Ga, similar to those obtained in the Grenville Province of the eastern U.S. and in the Mexican basement, placing constraints on Late Proterozoic-Early Paleozoic paleocontinental reconstructions.  相似文献   

8.
Subsidence of the Bahama Escarpment, determined from deep-diving submersible and dredge samples, is used to constrain the nature of crust underlying the Bahamas. Horizontal disposition of the Hauterivian/Barremian (125 Ma) age boundary along the Bahama Escarpment is inconsistent with an underlying oceanic crust (either normal or thickened) here, and suggests that thinned continental crust underlies the Bahamas. Subsidence curves are then fit based on a stretched lithosphere model to a stratigraphic section (2000–4000 m) off Cat Island. This analysis indicates crustal thinning by a factor (β) of 2.0–2.5, resulting in present crustal thicknesses of 10–12 km. We propose that rifting beneath the Bahamas occurred from middle (175–180 Ma) to late (160 Ma) Jurassic time. The pre-extension Bahamas fit between North and South America and Africa in Early Jurassic time, eliminating overlap of the present Bahamas onto Africa in reconstructions of the North Atlantic.  相似文献   

9.
Recognition of the eastern (Avalonian) margin of the northern Appalachian orogen as a Late Precambrian microcontinental arc terrane, rather than the opposing passive margin of the Proto-Atlantic (Iapetus) Ocean to that of eastern Laurentia, constituted a fundamental advance in Appalachian geology that profoundly influenced subsequent models for the orogen's plate tectonic evolution. This advance was first clearly articulated by Nick Rast and his students in 1976, who, by correlating rocks of the Avalon Platform with those of the British Midlands, established the Avalonian volcanic belt as a Japan-like microcontinent. Contrary to contemporary views of the Avalon Platform, which favored an extensional, Basin and Range-like setting for its volcanism, Rast argued on the basis of this correlation that the association of Avalonian volcanism with compressional orogeny, widespread calc-alkaline plutonism and, in Angelsey, with blueschists and ophiolitic rocks, indicated a convergent plate margin setting. Rast further proposed that the Avalonian volcanic belt was ensialic, and was bordered to the northwest and southeast by Precambrian oceanic domains. Contemporary reconstructions of the Avalonian and Cadomian belts as fragments of a Cordilleran-like accretionary orogen that developed along an active margin of Neoproterozoic Gondwana owe their origin to these early ideas and, while far removed from the tectonic model that Rast envisaged, are a direct heritage of his recognition of the Avalonian volcanic belt as a microcontinental arc terrane.  相似文献   

10.
Geochemical analyses of dikes, sills, and volcanic rocks of the Mesozoic Appalachian Tholeiite (MAT) Province of the easternmost United States provide evidence that continental tholeiites are derived from continental lithospheric mantle sources that are genetically and geochronologically related to the overlying continental crust. Nineteen olivine tholeiites and sixteen quartz tholeiites from the length of this province, associated in space and time with the last opening of the Atlantic, display significant isotopic heterogeneity: initial εNd = +3.8 to −5.7; initial 87Sr/86Sr= 0.7044−0.7072; 206Pb/204Pb= 17.49−19.14; 207Pb/204Pb= 15.55−15.65; 208Pb/204Pb= 37.24−39.11. In PbPb space, the MAT define a linear array displaced above the field for MORB and thus resemble oceanic basalts with DUPAL Pb isotopic traits. A regression of this array yields a secondary PbPb isochron age of ≈ 1000 Ma (μ1 = 8.26), similar to Sm/Nd isochrons from the southern half of the province and to the radiometric age of the Grenville crust underlying easternmost North America. The MAT exhibit significant trace element ratio heterogeneity (e.g., Sm/Nd= 0.226−0.327) and have trace element traits similar to convergent margin magmas [e.g., depletions of Nb and Ti relative to the rare earth elements on normalized trace element incompatibility diagrams, Ba/Nb ratios (19–75) that are significantly greater than those of MORB, and low TiO2 (0.39–0.69%)].Geochemical and geological considerations very strongly suggest that the MAT were not significantly contaminated during ascent through the continental crust. Further, isotope and trace element variations are not consistent with the involvement of contemporaneous MORB or OIB components. Rather, the materials that control the MAT incompatible element chemistry were derived from subcontinental lithospheric mantle. Thus: (1) the MAT/arc magma trace element similarities; (2) the PbPb and Sm/Nd isochron ages; and (3) the need for a method of introducing an ancient (> 2−3 Ga) Pb component into subcontinental mantle that cannot be much older than 1 Ga leads to a model whereby the MAT were generated by the melting of sediment-contaminated arc mantle that was incorporated into the continental lithosphere during arc activity preceding the Grenville Orogeny (≈ 1000 Ma).  相似文献   

11.
A vast sheet of mature quartz sand blanketed north Africa and Arabia from the Atlantic coast to the Persian Gulf in Cambro–Ordovician times. U–Pb geochronology of a representative section of Cambrian sandstone in southern Israel shows that these sediments are dominated by 550–650 Ma detrital zircons derived from Neoproterozoic Pan-African basement. The short time lag between magmatic consolidation of a Pan-African source and deposition of its erosional products indicates that, despite their significant mineralogical maturity, the voluminous quartz-rich sandstones on the northern margin of Gondwana are essentially first-cycle sediments.

Mass production of these voluminous first-cycle quartz-rich sandstones resulted from widespread chemical weathering of the Pan-African continental basement. We suggest that conditions favoring silicate weathering, particularly a warm and humid climate, low relief and low sedimentation rates prevailed over large tracts of Gondwana in the aftermath of the Pan-African orogeny. An unusually corrosive Cambro–Ordovician atmosphere and humid climate enhanced chemical weathering on the vegetation-free landscape. We infer that late Neoproterozoic–Cambro–Ordovician atmospheric pCO2 rose as a consequence of widespread late Neoproterozoic volcanism, followed by an uptake of CO2 by chemical weathering to produce the Cambro–Ordovician sandstone as a negative feedback.  相似文献   


12.
Geologic discontinuities across the Cheyenne Belt of southeastern Wyoming have led to interpretations that this boundary is a major crustal suture separating the Archaean Wyoming Province to the north from accreted Proterozoic island arc terrains to the south. Gravity profiles across the Cheyenne Belt in three Precambrian-cored Laramide uplifts show a north to south decrease in gravity values of 50–100 mgal. These data indicate that the Proterozoic crust is more felsic (less dense) and/or thicker than Archaean crust. Seismic refraction data show thicker crust (48–54 km) in Colorado than in Wyoming (37–41 km). We model the gravity profiles in two ways: 1) thicker crust to the south and a south-dipping ramp in the Moho beneath and just south of the Cheyenne Belt; 2) thicker crust to the south combined with a mid-crustal density decrease of about 0.05 g/cm3. Differences in crustal thickness may have originated 1700 Ma ago because: 1) the gravity gradient is spatially related to the Cheyenne Belt which has been immobile since about 1650 Ma ago; 2) the N-S gradient is perpendicular to the trend of gravity gradients associated with local Laramide uplifs and sub-perpendicular to regional long-wavelength Laramide gradients and is therefore probably not a Laramide feature. Thus, gravity data support the interpretation that the Cheyenne Belt is a Proterozoic suture zone separating terrains of different crustal structure. The gravity “signature” of the Cheyenne Belt is different from “S”-shaped gravity anomalies associated with Proterozoic sutures of the Canadian Shield which suggests fundamental differences between continent-continent and island arc-continent collisional processes.  相似文献   

13.
Thick terminal Proterozoic–lowest Cambrian successions allow reference of the Saint John, New Brunswick, and MacCodrum Brook, southern Cape Breton Island, areas to the marginal platform of the Avalon microcontinent. Marginal-platform siliciclastic-dominated sequences form a cover on Late Precambrian arc successions from southern New Brunswick to North Wales. Their deposition in fault-bounded basins began with the origin of the Avalon microcontinent and development of a persistent transtensional regime in the latest Precambrian. The terminal Proterozoic–lowest Cambrian on the Avalonian marginal platform consists of three successive lithofacies associations: lower subaerial rift to marginal-marine facies; overlying cool-water, wave-influenced, marine platform sandstones and shales; and higher macrotidal quartz arenites (=Avalonian depositional sequences 1–2). Only the Lower Cambrian macrotidal quartz arenites onlap southeast, where they form the oldest Cambrian unit on the inner platform. These major lithofacies are the Rencontre, Chapel Island, and Random formations, respectively, in Avalonian North America. Southwest thinning of the Rencontre–Chapel Island–Random interval in southern New Brunswick reflects slower subsidence of a fault-bounded area in the city of Saint John. The depositional sequence 1–2 unconformity, which falls in the sub-trilobitic Lower Cambrian Watsonella crosbyi Zone of the Chapel Island Formation, persists for 650 km along the marginal platform from southeastern Newfoundland to southern New Brunswick and, potentially, appears in Cape Breton Island. Latest Precambrian-earliest Cambrian epeirogenic and depositional history was very uniform along the marginal platform, and a unified lithostratigraphic nomenclature is appropriate.  相似文献   

14.
In order to test two different proposals for the poorly defined African Paleozoic apparent polar wander path (APWP), a paleomagnetic study was carried out on Ordovician through Carboniferous clastic sediments from the Cape Fold belt, west of the 22nd meridian. One proposal involves a relatively simple APWP connecting the Ordovician Gondwana poles in North Africa with the Late Paleozoic poles to the east of South Africa in a more or less straight line crossing the present equator in the Devonian. The other proposal adds a loop to this path, connecting Ordovician poles in North Africa with poles to the southwest of South Africa and then returning to central Africa. This loop would occur mainly in Silurian time. New results reported herein yield paleopoles in northern and central Africa for Ordovician to lowermost Silurian and Lower to Middle Devonian formations. The best determined paleopole of our study is for the Early Ordovician Graafwater Formation and falls at 28°N, 14°E (k = 25, α95 = 8.8°, N = 28 samples). The other paleopoles are not based on sufficient numbers of samples, but can help to constrain the apparent polar wander path for Gondwana. Our results give only paleopoles well to the north of South Africa and we observe no directions within the proposed loop. Hence, if the loop is real, it must have been of relatively short duration (60–70 Ma) and be essentially of Silurian/Early Devonian age, implying very high drift velocities for Gondwana (with respect to the pole) during that interval.  相似文献   

15.
West of Boston, Mass., Castle and others (1976) recognized an up to 5km wide, possibly folded, NE-SW trending Burlington Mylonite Zone. We have extended mapping south into Natick and Framington quadrangles, and supplemented it by fixing local directions of tectonic motion, which are more variable than reported by Goldstein (1989). In Natick the mylonite zone is partly migmatized and converted into blastomylonites, forming the lithodemic Rice Gneiss and is intersected by the Dedham Granite dated ca 630 Ma. The granite also invades deformed, folded, and commonly mylonitized Westboro Quartzite. Thus mylonitization, folding, and formation of migmatitic blastomylonites are all earlier than ca 630 Ma, and can collectively be attributed to the main phase of the Avalonian orogeny that in Africa is referred to as the Pan-African I. The sense of movements in the Rice Gneiss is generally sinistral strike-slip with a NE-SW trend of foliation. Other local mylonites have more variable directions of motion.A narrower E-W zone of mylonitization has been recognized by Grimes (M.S. thesis 1993, Boston College) and named the Nobscot Shear Zone. It affects the Milford Granite, also about 630 Ma in age, while similar but narrow shear zones affect other local granites including the Dedham. These zones, dipping steeply north and including the Nobscot, are less intensely mylonitized and are not associated with migmatites. Their age is not known, but since they affect only Precambrian rocks, they are assumed to be late Proterozoic. We attribute these zones to the second stage of the Avalonian or the Pan-African II.The older rocks west of Boston are widely affected by numerous brittle faults. These are all of unknown age, but probably Phanerozoic. The most significant brittle fault in the Burlington area is the mid to late Paleozoic Bloody Bluff Fault. We do not associate large scale mylonitization with that fault, because the mylonites are commonly cut by undeformed or little deformed Siluro-Devonian gabbro-diorites.  相似文献   

16.
Granitoid intrusives such as Saishitenshan, Tuanyushan, Aolaohe and Sanchagou occur widely in the western segment of North Qaidam. All these bodies trend NW, roughly parallel to the regional structure. Zircon SHRIMP dating for these granites show that they range in age from Ordovician to Permian; 465.4±3.5 Ma for Saishitenshan, 469.7±4.6 Ma and 443.5±3.6 Ma for Tuanyushan, 372.1±2.6 Ma for Aolaohe, and 271.2±1.5 Ma and 259.9±1.2 Ma for Sanchagou. Both the Tuanyshan and Aolaohe plutons record two distinct intrusive events. Geochemically, the early Paleozoic granites have an island arc or active continental margin affinity, and their protolith may have been Mesoproterozoic oceanic crust derived from depleted mantle. The protolith of the late Paleozoic granites may have been Mesoproterozoic lower crust from the root of an island arc with the magmas reflecting a mixture of mantle and crustal material.  相似文献   

17.
Summary A short review of the Neoproterozoic and Cambrian sedimentary and volcanogenic successions in Poland is concerned with depositional and tectonic developments. The successions accumulated in shallow shelf and deeper slope environments. They overlie Palaeoproterozoic crystalline basement of the Baltic Craton and Gondwana-derived Neoproterozoic Cadomian basement. This discrepancy in the nature of the basement is viewed as indicating distinct terranes. During the course of Palaeozoic time, the discrete terranes were assembled along the Teisseyre-Tornquist Lineament and the Trans-European Suture Zone adjacent to the Baltica palaeocontinent.  相似文献   

18.
Ion microprobe UThPb ages of zircons from granulite facies lower crustal xenoliths from north Queensland, Australia, correlate well with the ages of major orogenic episodes manifest at the earth's surface. About half of the xenoliths contain Proterozoic zircons which are similar in age to the episodes of high-grade metamorphism of the older surface rocks. All the xenoliths contain late Paleozoic zircons which show a real 100 Ma range in206Pb238/U ages (from 320 to 220 Ma), which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust. The Paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash-flows and intrusion of high-level granites in this region (320-270 Ma). Mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths, and coupled with the age information, can be used to infer the geological processes operating in the lower crust. The zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two-stage history for these rocks, involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt. Some of the zircons from the felsic xenoliths contain CO2-rich fluid inclusions, indicating that those zircons grew during high-grade metamorphism. The isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic, mantle-derived, lithologies. The major orogenic event responsible for the formation of that crust occurred in the late Paleozoic, when Proterozoic supracrustal rocks were emplaced into the lower crust, possibly along thin-skinned thrust slices. This was accompanied by intrusion of high-temperature, mantle-derived melts which caused partial melting of pre-existing crust. The most likely setting for such tectonism is a continental margin subduction zone.  相似文献   

19.
In southeastern New England the Narragansett Pier Granite locally intrudes Carboniferous metasedimentary rocks of the Narragansett basin, and yields a monazite UPb Permian emplacement age of 273 ± 2Ma. Zircon from the Narragansett Pier Granite contains a minor but detectable amount of an older, inherited component, and shows modern loss of lead. Zircon from the late-stage, aplitic Westerly Granite exhibits a more pronounced lead inheritance —permitting the inherited component to be identified as Late Archean. Such old relict zircon has not been previously recognized in Proterozoic to Paleozoic igneous rocks in New England, and may be restricted to late Paleozoic rocks of the Avalon zone. We suggest that the Archean crustal component reflects an African connection, in which old Archean crust was underplated to the Avalon zone microplate in the late Paleozoic during collision of Gondwanaland with Avalonia.  相似文献   

20.
The Helong block, located in southeastern Jilin Province, was thought to be an Archean geological unit in the most northeast part of the North China Craton (NCC)[1,2]. Previous geological survey sug-gested that this block is mainly composed of two parts: Jinchengdong Archean metamorphic supracrustal rocks intruded by Archean TTG complex[3―5], and the Bailiping granite[6,7] distributed in the Shiliping-Bai- liping-Guangping area. Both of them were thought as the Jinchengdong (or Helong…  相似文献   

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