Neoproterozoic metamorphism and deformation at the southeastern margin of the East European Craton,Uralides, Russia

The eastern margin of the East European Craton (EEC) has a long lasting geological record of Precambrian age. Archaean and Proterozoic strata are exposed in the western fold-and-thrust belt of the Uralides and are known from drill cores and geophysical data below the Palaeozoic cover in the Uralides and its western foredeep. In the southern Uralides, sedimentary, metamorphic and magmatic rocks of Riphean and Vendian age occur in the Bashkirian Mega-anticlinorium (BMA) and the Beloretzk Terrane. In the eastern part of the BMA (Yamantau anticlinorium) and the Beloretzk Terrane, K-Ar ages of the <2-µm-size fraction of phyllites (potassic white mica) and slates (illite) give evidence for a complex pre-Uralian metamorphic and deformational history of the Precambrian basement at the southeastern margin of the EEC. Interpretation of the K-Ar ages considered the variation of secondary foliation and the diagenetic to metamorphic grade. In the Yamantau anticlinorium, the greenschist-facies metamorphism of the Mesoproterozoic siliciclastic rocks is of Early Neoproterozoic origin (about 970 Ma) and the S1 cleavage formation of Late Neoproterozoic (about 550 Ma). The second wide-spaced cleavage is of Uralian origin. In the central and western part of the BMA, the diagenetic to incipient metamorphic grade developed in Late Neoproterozoic time. In post-Uralian time, Proterozoic siliciclastic rocks with a cleavage of Uralian age have not been exhumed to the surface of the BMA. Late Neoproterozoic thrusts and faults within the eastern margin of the EEC are reactivated during the Uralian deformation.     

Perspectives of phosphorite mineralization in Lower Paleozoic rocks of the Zilair synclinorium,Southern Urals

Data on the phosphorite mineralization, lithology, and variability of Lower Paleozoic rocks in the Zilair synclinorium and Yuryuzan basin are given. Comparison of Lower Paleozoic rocks with type sections of phosphate-bearing rocks in the world suggested their compliance with the phosphorite-enclosing rocks of the fine-grained phosphorite association. Analysis of the facies variability of rocks indicate the possibility of discovery of large phosphorite deposits in the Yuryuzan and Syuren subzones of the Western structural-facies zone. Phosphorite deposits are unknown in the synclinorium because of their instability during hypergenesis and burial under allochthonous loose sediments of the Cenozoic.     

New data on tectonics of the western slope of the Southern Urals (Based on reflection profile No. 4)

This paper presents an alternative interpretation for the fragment of regional profile no. 4 made by Public Corporation Bashneftegeofizika in 2004?C2005. This profile traverses the southeast part of the East European Craton within the limits of Bashkirian plain and the frontal folds of the Urals. Two thrusts having west vergence are of special importance and are distinct for a big horizontal amplitude that allows us to classify them as overthrusts. These are the Muradymovo thrust, which confines the Zilair Synclinorium from the west, and the Pokrovka thrust in the center of the synclinorium. These thrusts and the Shikhan-Volostnovka thrust are smoothed towards the east and join at the depth of 17?C18 km (7?C8 s) with a common detachment.     

Proterozoic magmatic and tectonometamorphic evolution of the Taratash complex, Central Urals, Russia

The Taratash Complex (TC) in the northernmost Bashkirian Anticlinorium (Middle Urals) is unique among the pre-Uralian polymetamorphic complexes along the eastern margin of the East European Craton because it experienced granulite facies peak metamorphic conditions (850–900°C/10 kbar). Herein, we constrain the post-granulite facies polystage evolution of the complex, which records various increments of the geodynamic history of the East European continental margin. Formation of granite and migmatite associated with amphibolite facies events are dated at 2,344±29 and 2,044±8 Ma (U–Pb, zircon) in different structural units. At 1,810±41 Ma, the TC was affected by a greenschist facies retrogressive metamorphism which was probably related to a stage of granite formation in the eastern part of the East European Craton. This is confirmed by a U–Pb–zircon age of 1,848±8 Ma obtained from a sheared granite in the adjacent Alexandrovskiy Complex (AC). Greenschist facies shear zones which separate different structural units of the TC formed before 1,350 Ma. Partial re-equilibration of Rb–Sr- and K–Ar-isotope systems between 1,350 Ma and 1,200 Ma is attributed to fluid flow probably induced by anorogenic magmatism in the Bashkirian Anticlinorium. Meso- to Neoproterozoic basaltic dykes indicate that the TC had been exhumed to upper crustal levels at that time. Evidence for a Grenvillian event or for the Timanian orogeny which affected other pre-Uralian complexes in the Urals is lacking. Uralian orogenic shortening and thrusting on Devonian limestones is recorded by shear zones in the AC to the east of the TC and has been dated at 300 Ma (Rb–Sr, ⁴⁰Ar/³⁹Ar).     

Evolution of the Murphy synclinorium,southern Appalachian Blue Ridge,USA

The western Blue Ridge allochthon of the southern Appalachians is dominated by the >180 km-long Murphy synclinorium, paired with anticlinoria to the northwest. These are first generation, northwest overturned, doubly plunging, large amplitude and wavelength (>10 km) isoclinal folds contemporaneous with peak Neo-Acadian orogeny (Visian, ∼335–345 Ma) regional metamorphism. The synclinorium folds a regional unconformity separating Neoproterozoic rift and lower Paleozoic drift sequences from a younger successor-basin sequence. Strain analysis of metaconglomerates from lithologic groups above and below the unconformity indicates coaxial, low to moderate, oblate to nearly plane strain in both groups. The synclinorium evolved via NNW-SSE-crustal shortening (∼32%), combined with orthogonal NNE-SSW-sub-horizontal flow (stretching) (∼35–45%) sub-parallel to the developing fold axes. Differences in metamorphic grade and paleodepth (∼10–17 km) of the exposed synclinorium had essentially no effect on strain magnitudes. Retrodeformation of the embedded regional unconformity reveals a very broad synclinal warping of the rift and drift-facies units predating superposition of the Murphy synclinorium, suggesting tectonic inheritance in the latter structure's origin. The earlier mild deformation is post-Early Cambrian and may represent the only vestige of the dynamic effects of the Middle Ordovician Taconic orogeny to be found in this region.     

Thermotectonic evolution of the western fold-and-thrust belt, southern Uralides, Russia, as revealed by apatite fission track data

The Uralides, a linear N–S trending Palaeozoic fold belt, reveals an intact, well-preserved orogen with a deep crustal root within a stable continental interior. In the western fold-and-thrust belt of the southern Uralides, Devonian to Carboniferous siliciclastic and carbonate rocks overlay Mesoproterozoic to Neoproterozoic sedimentary rocks. Deformation in the Devonian, Carboniferous and Permian caused thick-skinned tectonic features in the western and central parts of the western fold-and-thrust belt. A stack of several nappes characterizes the deformation in the eastern part. Along the E–W transect AC-TS'96 that crosses the western fold-and-thrust belt, apatite fission track data record various stages of the geodynamic evolution of the Uralide orogeny such as basin evolution during the Palaeozoic, synorogenic movements along major thrusts, synorogenic to postorogenic exhumation and a change in the regional stress field during the Upper Jurassic and Lower Cretaceous. The Palaeozoic sedimentary cover and the Neoproterozoic basement of the Ala-Tau anticlinorium never exceed the upper limit of the PAZ since the Devonian. A temperature gradient similar to the recent one (20 °C/km) would account for the FT data. Reactivation of the Neoproterozoic Zilmerdak thrust was time equivalent to the onset of the Devonian and Carboniferous collision-related deformation in the east. West-directed movement along the Tashli thrust occurred in the Lower Permian. The Devonian and Carboniferous exhumation path of the Neoproterozoic siliciclastic units of the Tirlyan synclinorium mirrors the onset of the Uralian orogeny, the emplacement of the Tirlyan nappe and the continuous west-directed compression. The five main tectonic segments Inzer Synclinorium, Beloretzk Terrane, Ala-Tau anticlinorium, Yamantau anticlinorium and Zilair synclinorium were exhumed one after another to a stable position in the crust between 290 and 230 Ma. Each segment has its own t–T path but the exhumation rate was nearly the same. Final denudation of the western fold-and-thrust belt and exhumation to the present surface probably began in Late Tertiary. In Jurassic and Cretaceous, south-directed movements along W–E trending normal faults indicate a change in the tectonic regime in the southern Uralides.     

Anchimetamorphism of the Neoproterozoic and the Lower Paleozoic along the Profile of Yuanguping in Western Hunan Province, China

The Neoproterozoic and Lower Paleozoic along the profile of Yuanguping in western Hunan Province, China underwent anchimetamorphism. The illite crystallinity (IC) of the <2 μm fractions ranges from 0.23-0.34°△2θfor the Neoproterozoic to 0.23-0.35°△A2θ for the Lower Paleozoic (calibrated with the Kisch IC set, Kisch, 1991). This indicates that the metamorphic grade of the Neoproterozoic and Lower Paleozoic is the anchizone. The peak metamorphic temperature is estimated to be 290-210℃. This result does not agree with the greenschist or subgreenschist facies of the Banxi Group, nor with the lower-greenschist facies or sedimentary cover of the Sinian to Lower Paleozoic, as most previous researchers thought. The illite (K-mica) b0 values range from 0.9074 to 0.8963 (nm) for the Neoproterozoic and the Lower Paleozoic. Based on cumulative frequency curves of the illite (K-mica) b0, the peak metamorphic pressure of the Banxi Group was derived to be of a type that is slightly higher than that of the N. Ne     

Problems of Siderite Formation and Iron Ore Epochs: Communication 1. Types of Siderite-Bearing Iron Ore Deposits

It is shown that siderite is unstable during sedimentation, diagenesis, and metamorphism of sedimentary and volcanosedimentary rocks. Regularities in the distribution of siderite in Precambrian jaspilites (iron formations), metasomatic ores of the Bakal type, continental–marine coaliferous formations, and oolitic iron ores are discussed. The genesis of the Precambrian iron formations and Riphean–Lower Paleozoic elisional–hydrothermal deposits is considered. The genetic relation of nodular siderites from coaliferous formations and oolitic iron ores with lowmoor coal-forming peat deposits is noted.     

The Almadén mercury mining district, Spain

The Almadén district is the largest mercury concentration in the world, with a total content of about 250 000 t of mercury, nearly one third of the known total mercury resources of the Earth. Mercury has been exploited since the Celtic and Roman times, with peak production during the Renaissance and between 1939–1945. The district is hosted by a Paleozoic synclinorium overlying Precambrian rocks. The Paleozoic sequence comprises epicontinental quartz arenite rocks, including black shales and quartzites. Diatremes, alkaline lavas of different composition, and late tholeiitic diabases account for the Ordovician to Devonian magmatism. The tectonic setting of this complex suite corresponds to the intraplate type. The mercury deposits of Almadén can be classified into two main types: type 1, early stratiform type ores characterized by cinnabar deposition on the lower Silurian quartzites (Criadero quartzite; e.g. the Almadén and El Entredicho deposits), and type 2, late discordant orebodies (e.g. Las Cuevas), largely hosted or related to diatremes (the `frailesca rocks') of alkaline basaltic composition. In type 1 cinnabar was deposited during diagenesis, in relation to hydrothermal circulation driven by magmatic activity. Type 2 include a variety of deposits having in common the discordant character of the orebodies (e.g. veins, stockworks, massive replacements), and their wide dispersion along the stratigraphic column, i.e. from Lower Silurian (e.g. Nueva Concepción) to Upper Devonian (e.g. Corchuelo). Received: 23 October 1998 / Accepted: 4 January 1999     

Partitioning of strontium between calcite,dolomite and liquids: An experimental study under higher temperature diagenetic conditions,and a model for the prediction of mineral pairs for geothermometry

The partitioning of Sr between calcite, dolomite and liquids is essentially independent of temperature between 150° and 350° C. The partition coefficients corrected for number of cation sites are b _calc=0.096 and b _dol= 0.048 for 1 mol cations/6 mol H₂O liquid. Upon dilution the partition coefficients increase, but their ratio stays constant at about 2∶1. This ratio is due to the fact that calcite has twice as many Ca-sites for Sr-substitution as dolomite. The 2∶1 relationship is also observed in natural calcite and dolomite which have undergone diagenesis. The temperature independence of partitioning is caused by the relatively small thermal expansion of calcite and dolomite. Thermal expansion between 25° and 400° C was found to follow the equations V _calc=7.0·10⁻⁴ T(°C)+36.95 and V _dol=6.9·10⁻⁴ T(°C)+32.24, V: cm³/mol. Therefore calcite and dolomite cannot serve as a temperature indicator. To have an ideal geothermometer a mineral pair with high and low thermal expansion is required. Literature date demonstrate that wurtzite, sphalerite, and galena are such minerals.     

Analysis and simulated diagenesis of kerogen in a Recent bottom mud from Mono Lake,California: a comparison with selected ancient kerogens

The insoluble organic matter, or kerogen, in a Recent bottom mud (<1000yr old) from Mono Lake, California, has been analyzed by vacuum pyrolysis-GC-MS and compared with the kerogen from several Precambrian rocks, including the Belingwe and Transvaal stromatolites. The Mono Lake kerogen consists mainly of cyclic and acyclic aliphatic components with lesser amounts of aromatics present. It is less aromatic and more susceptible to thermal degradation than the Precambrian kerogens, and its products show a much greater diversity especially among the unsaturated aliphatics and the heteroatomic components. The presence of these compounds, most notably 2,5-dimethylfuran, in the Mono Lake kerogen indicates a relatively rapid formation and incorporation into a young kerogen and suggests that their presence in the Precambrian kerogens, such as the Belingwe stromatolite, may be consistent with an ancient biological origin.In simulated diagenesis experiments the Recent mud was heated at 150°C for 3 months or 225°C for 8 months. The former was insufficient to affect the kerogen pyrolysis products. The latter, however, caused a large decrease in the heteroatomic components and a slight increase in the abundance of n-alkanes relative to that of the cyclic and branched alkanes. This suggests that the presence of some of these components in Precambrian rocks should be consistent with a high degree of preservation of these rocks, as appears to be the case for the Belingwe and Transvaal stromatolites.     

Präorogene Versenkungstiefe und Orogenese als Diagenesefaktoren für altpaläozoische Kontinentalrandbildungen der Nördlichen Appalachen in Quebec,Kanada

This paper presents data and preliminary interpretations on the diagenesis of Early Paleozoic continental margin deposits along a traverse of the Quebec Appalachians near Quebec City, Canada. Regional variations in diagenesis were studied using the thermal maturation of organic matter in shales (reflectance measured on asphaltic bitumen, 105 samples) and illite crystallinity (330 samples). These revealed a regional southeastward increase in grade from the late middle and late stage of diagenesis to epimetamorphism, which is reflected in the distinction of four zones: Zone I representing the late middle diagenetic stage has a mean reflectance in oil (R₀) between 1.0 and 1.5% and illite crystallinity between 5.5 and 8.0 mm. Zone II (late diagenetic stage) is characterized by R₀=1.5–2.6% and illite crystallinity between 3.5 and 5.5 mm. Anomalously poor illite crystallinities in Zone II (i. e. 5.5 to 8.0 mm) were obtained for black shales, in which improvement of crystallinity lags behind red and green shales. Zone II is subdivided into subzones IIA and IIB. In the former, reflectance and illite crystallinity increase, within individual nappes, as a function of age or depth of burial. In the latter no such dependence is observed, instead diagenetic grade increases regionally in a southeastward direction as it does in zones III and IV. Zone III represents the anchizone in which observed reflectance values R₀ range from 2.6 to 4.0% and illite crystallinities from 2.0 to 3.1 mm. In Zone IV (epizone) illite crystallinity is less than 2.0 mm (In terms of reflectance the anchi-zone/epizone boundary was not defined). Zones I and IIA are anomalous in that lower tectonic units are diagenetically less altered than higher tectonic units: R₀ varies from 1.71 to 2.30% for the highest tectonic unit (Cambrian Chaudière Nappe), 1.53 to 1.90% (Cambro-Ordovician Bacchus Nappe) and 1.08 to 1.46% (Lower Ordovician Pointe-de-Lévy Nappe) for the middle tectonic units, and 1.01 to 1.15% for the lowest tectonic unit (Middle Ordovician Quebec Promotory Nappe). Thermal maturation and mineral diagenesis in zone IIA are probably due solely to sedimentary burial at the original site of deposition (by an estimated 6 to 7 km of younger sediments) because in this zone the highest diagenetic grade occurs in the highest tectonic unit. Diagenesis in the nappes of zone I probably required additional tectonic burial by the higher nappes because original sedimentary thicknesses that once overlay these Lower and Middle Ordovician rocks appear insufficient to have caused the observed degree of diagenesis. Diagenesis in zone IIA, therefore, was most likely formed entirely before orogenesis; in zone I it is probably partly pre-orogenic in origin and has been transported during nappe-movement. In contrast, diagenesis and metamorphism in zones IIB to IV are interpreted as related to regional synorogenic heating in conjunction with the Taconic orogeny. Thermal maturation levels in zone I indicate that the rocks have not yet passed the “oil window” which is of interest for petroleum exploration in Quebec. An extended English version of this paper is in preparation for the Bulletin of Canadian Petroleum Geology (Ogunyomi et al., ms.).     

Conodont colour alteration indices (CAI) of Upper Ordovician limestones from the Iberian Peninsula

Conodont colour alteration index (CAI) data in Upper Ordovician rocks from several areas of the Variscan domain in the Iberian Peninsula indicate conditions ranging from diagenesis to low-grade metamorphism. In most of the areas, where studies using other indicators, such as illite crystallinity (IC) or where vitrinite reflectance are lacking, the CAI method has permitted a preliminary estimation of the metamorphic grade. In the Almadén syncline (Central-Iberian Zone), where IC studies are available, the thermal conditions inferred from CAI data agree with those obtained by the IC method. In the Puertollano–Almuradiel syncline, the thermal interval obtained primarily from fluid inclusions (270–370°C) overlaps considerably with that obtained from CAI data (180–340°C). In general, cleavage in rocks is present in anchizonal or epizonal conditions, whereas in diagenetic conditions with CAI 2.5, cleavage is scarce. The conodont texture changes with increasing metamorphism, and apatite recrystallisation appears in general with CAI 5. Variation of CAI values within a single sample and/or within short stratigraphic distances observed at several localities is due to hydrothermal activity.     

Phase transition of potassium sulfate,K2SO4 (III); thermodynamical and phenomenological study

The orthorhombic-hexagonal phase transition of K₂SO₄ has been investigated by measurements of the temperature dependencies of the specific heat, expansion, and X-ray intensity of superstructure reflections, correlated with the structural point of view. The values of the net enthalpy and entropy changes are ΔH=4.28 KJ/mol and ΔS=4.98 J/mol·K at the phase transition temperature (587°C), respectively. The thermal expansion along the c axis shows strong anisotropic character above about 300°C and exhibits a very large discontinuous increase at 587°C, whereas those along the a and b axes increase linearly and exhibit small discontinuous decreases at 587°C. The X-ray intensity of superstructure reflections in the low-temperature form gradually decrease with increasing temperature, and come to extinction at 587°C, exhibiting a discontinuity. The observed entropy change and pressure dependence of the phase transition temperature were explained successfully by the use of results of the structural analysis and measured physical properties. The temperature dependencies of the spontaneous strain, X-ray intensity of superstructure reflection, and birefringence were consistently described by introducing a transition parameter on the basis of an instability at the M point in the Brillouin zone of the hexagonal phase.     

大洋洲地区大地构造格架与优势矿产资源

大洋洲横跨印澳板块、太平洋板块和欧亚板块三大板块,区内从太古宙至今经历了漫长的构造演化历史。根据区域地质特征,该区可划分为3个一级构造单元,即澳大利亚中西部前寒武纪克拉通、澳大利亚东部古生代造山带和环太平洋中新生代岛弧区,12个二级构造单元和40个三级构造单元。3个一级构造单元由于经历了不同的构造演化过程,各自发育了不同的岩石组合和矿化作用。在研究区域构造演化与成矿作用耦合关系的基础上,总结出3个构造单元的成矿特征：澳大利亚中西部前寒武纪克拉通成矿过程与克拉通的生长有关,成矿时代主要为太古宙和元古宙,优势矿种为金、铜、镍、锰、铁、铀、稀土、铅锌、铝土等;东部古生代造山带成矿多和古太平洋与古冈瓦纳板块的相互作用有关,成矿时代为古生代,优势矿种为铜、金、铅锌和钨锡;环太平洋中新生代岛弧区的成矿作用多与印澳板块、欧亚板块与太平洋板块的相互作用有关,成矿时代多集中在中一更新世,优势矿种为铜、金、镍、钴等。对大洋洲地区矿产资源潜力进行了评估,认为大洋洲地区的铁、锰、铜、铝、镍、金、铀、稀土等矿产资源均具有较好的潜力。     

甘新蒙北山地区构造格局及演化

甘新蒙北山地区地处天山东段、北山及阿拉善地区 ,为古生代哈萨克斯坦板块、塔里木板块和华北板块交汇地带。可分为 7个二级构造单元 (构造区 )、2 6个三级构造单元 (地块、褶皱带 )。该区构造演化史漫长而复杂。太古宙时期 ,是古陆核形成萌芽阶段 ,古元古代初陆壳开始生成 ,出现薄壳结构的宽广的裂陷活动带 ,到了中、新元古代后期显露出板块构造运动 ,新元古代晚期经历重大热事件———晋宁运动 ,发生陆块间汇聚—碰撞并形成了榴辉岩—花岗岩岩浆带。震旦纪在晋宁期的拼合古陆上再一次裂解 ,经历了板块构造演化史。晚古生代 ,本区主体转入板内构造时期 ,以开合构造为主 ,花岗岩浆活动广泛发育。中、新生代进入板内造山和现今的盆山构造格局时期     

Metamorphic zonal sequences of pelitic schists and gneisses from the area around Kandra (Jharkhand): Constraints from field and textural relationship

The pelitic schists of the area around Kandra, Singhbhum district, Jharkhand belong to the Chaibasa Formation of the Singhbhum Group, which constitute a part of the youngest Precambrian orogenic cycle of the Singhbhum region. Structurally, the area represents the Singhbhum anticlinorium and is overlain by Dalma traps which form the synclinorium towards the north of the area around Kandra. This area mainly consists of medium to high grade rocks belonging to greenschist and amphibolite facies. These rocks are folded in the E-W trending doubly plunging folds (F₁) overturned towards the south with low plunges and superposed by cross-folds (F₂). The spatial distribution of the index minerals in the pelitic schists of the area shows Barrovian type of metamorphism. Four isograds, viz. biotite, garnet, staurolite and sillimanite have been delineated by the first appearance of the index minerals and also by isograd reactions. The textural relation suggests that sillimanite is formed from staurolite consumption reaction instead of kyanite consumption.     

Polyphase metamorphism in the eastern Carnic Alps (N Italy–S Austria): clay minerals and conodont Colour Alteration Index evidence

Thermal evolution of the Palaeozoic–Triassic sequences of the Carnic Alps has been characterized by b cell dimension and Kübler Index (illite “crystallinity”) of K-white micas (KI), árkai Index (AI) of chlorites, clay mineral assemblages and conodont Colour Alteration Index (CAI). Data indicate at least two metamorphic events, Variscan and Alpine. In the older event high anchizonal conditions predominated although epizonal conditions were reached over wide areas. It was characterized by low-intermediate pressure facies. The thermal peak was mainly due to an extensional regime during the Bashkirian. A younger thermal overprint generated by Alpine orogeny was of lower grade, reaching high diagenetic–anchizonal conditions characterized by high-pressure facies. Inverted metamorphic patterns are associated with middle to late Miocene thrusting. Hydrothermal alteration in the northern part of the region can be linked with emplacement of Oligocene plutons and high heat flow along the Periadratic lineament. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Geometry and pressure-temperature significance of the kyanite-sillimanite isograd in the Mica Creek area,British Columbia

Late Precambrian and Lower Paleozoic metapelitic rocks near Mica Creek, British Columbia, range in metamorphic grade from biotite zone to sillimanite zone. The kyanite-sillimanite isograd was established after phase 2 folding but was deformed by phase 3 folding. Topographic relief of about 2 km, combined with phase 3 folding, permits reasonably precise determination of the geometry of the isogradic surfaces. The effects of phase 3 folding have been accounted for and the isobaric surfaces at the time of metamorphism are inferred to have been dipping gently. Using the kyanite-sillimanite experimental phase diagram, intersection of the isobars and the isograd permits estimates of differences in temperature and pressure along a cross section. These estimates are about 400 bars and 20° C. Mineral geothermometry (garnet-biotite) and geobarometry (garnetplagioclase-Al ₂SiO₅-quartz) does not provide a fine enough resolution to detect these predicted differences.     

青藏高原拉萨地体北部的前寒武纪变质作用及构造意义

青藏高原南部拉萨地体中分布的角闪岩相至麻粒岩相变质岩一直被认为是前寒武纪变质基底,但并没有获得可靠的年代学证据。本文运用原位锆石U-Pb定年方法,在拉萨地体北部那果地区的变质岩中获得了约720Ma的变质年龄,从而证明拉萨地体北部在新古元代经历了角闪岩相变质作用和近同期的岩浆作用。基于这一成果和在拉萨地体中、南部高级变质岩中陆续获得的中、新生代变质年龄,对拉萨地体变质作用的时、空变化及其成因进行了初步探讨。