Regional geochemical and isotopic characteristics of high-grade metamorphics of the Prydz bay area: The extent of proterozoic reworking of Qrchaean continental crust in East Antarctica

Preliminary isotopic data for Late Proterozoic (～ 1100 Ma) granulite-facies metamorphics of the Prydz Bay coast indicate only very minor reworking (i.e., remetamorphism) of Archaean continental crustal rocks. Only two orthopyroxene—quartz—feldspar gneisses from the Rauer Group of islands, immediately adjacent to the Archaean Vestfold Block, show evidence for an Early Archaean origin (～ 3700—3800 Ma), whereas the vast majority of samples have Middle Proterozoic crustal formation ages (～ 1600–1800 Ma). The Prydz Bay rocks consist largely of garnet-bearing felsic gneisses and interlayered aluminous metasediments, although orthopyroxene-bearing gneisses are common in the Rauer Group; in contrast, Vestfold Block gneisses are predominantly orthopyroxene-bearing orthogneisses. The extensive Prydz Bay metasediments may have been derived by erosion of Middle Proterozoic rocks, such as the predominantly orthogneiss terrain of the Rauer Group, and deposited not long before the Late Proterozoic metamorphism. Data from nearby parts of the East Antarctic shield also suggest only limited Proterozoic reworking of the margins of the Archaean cratons.As in the Prydz Bay area, high-grade metamorphies in nearby parts of the East Antarctic shield show a secular increase in the sedimentary component. Archaean terrains like the Vestfold Block consist mainly of granitic orthogneisses derived by partial melting of igneous protoliths (I-type), whereas Late Proterozoic terrains (such as the Prydz Bay coast) include a much higher proportion of rocks derived either directly or by partial melting (S-type granitic orthogneisses) from sedimentary protoliths. Related chemical trends include increases in K₂O₂, Rb, Pb, and Th, and decreases in CaO, Na₂O₂ and Sr with decreasing age, essentially reflecting changes in the proportions of plagioclase and K-feldspar.     

Crustal contamination as an indicator of the extent of early Archaean continental crust: Pb isotopic evidence from the late Archaean gneisses of West Greenland

Ph isotopic analyses are reported for 119 samples of late Archaean (ca. 3000-2800 Myr) calc alkaline orthogneisses and associated anorthosites from southern West Greenland. Over most of the area. $\text{Pb}\text{Pb}$ whole rock isotope systematics indicate derivation of the magmatic precursors of the gneisses and anorthosites from a source region with a typically mantle-type $\text{U}\text{Pb}$ ratio (μ₁ value of 7.5) at. or shortly before, ca. 3000-2800 Myr ago. In contrast, in the Godthaabsfjord region, late Archaean Nûk gneisses and associated anorthosites were emplaced into or through early Archaean (ca. 3700 Myr) Amîtsoq gneisses, and crystallised with variable proportions of two isotopically distinct types of Pb which commenced their respective crustal developments at ca. 3000-2800 Myr and at ca. 3700 Myr ago. Isotopic and other geochemical constraints demonstrate that Nûk gneisses and their temporal equivalents were not produced by reworking or melting of Amîtsoq gneisses. Mixing of early and late Archaean Pb results from contamination of the magmatic precursors of Nûk gneisses and anorthosites (characterised by mantle-type Pb at time of emplacement) with ancient, unradiogenic Pb derived from ca. 3700 Myr-old Amîtsoq-type continental crust invaded by the Nûk magmas. The contaminant is considered to be a trace-element enriched fluid phase released from dehydrating older continental crust during progressive burial and heating by emplacement of calc alkaline magmas in the late Archaean ‘accretion differentiation superevent’. This was followed by mixing of the released fluids with younger Nûk magmas.Pb isotopic compositions of late Archaean gneisses and anorthosites outside the Godthaabsfjord region provide no evidence for the presence of early Archaean Amîtsoq-type continental crust in southern West Greenland in areas more than a few tens of km outside the known outcrop of Amîtsoq gneisses. We suggest that early Archaean crust does not exist at depth in late Archaean areas with undisturbed Pb-isotope systematics, either in Greenland or elsewhere in the North Atlantic craton.Pb-isotope evidence for crust magma interaction, involving selective extraction of certain trace elements by a fluid phase from wall rock and subsequent mixing between magma and contaminant fluid, provides a powerful tool for detection, sub-surface ‘mapping’, and geochronological and geochemical characterisation of deep, ancient continental crust.     

Discordant RbSr and PbPb whole rock isochron ages for the Archaean basement of Sierra Leone

Field mapping and structural studies in northern Sierra Leone by an I.G.S. team have established a stratigraphic sequence in this part of the Archaean of the West African Craton. An older “Leonian” granite-greenstone terrain is identified which experienced a tectonic-metamorphic event before the formation of the granite-greenstone terrain which ended with the Liberian tectonic-metamorphic event. Granite gneisses in the Fadugu district with Leonian structures yield statistically acceptable but discordant Pb-Pb and Rb-Sr whole-rock isochron ages of 2959±50 Ma and 2753±61 Ma, respectively (2 σ errors). These ages may be correlated with radiometric ages for the Leonian and Liberian structures elsewhere in Sierra Leone, and it is concluded that the Fadugu Rb-Sr whole-rock isochron has been reset by the Liberian event. The Pb-Pb whole-rock isochron for the Fadugu gneisses and a previously determined (but recomputed and partially checked) Rb-Sr whole-rock isochron age of 2980+80 Ma for granite gneisses from southeastern Sierra Leone provide a definitive age for the Leonian tectonic-metamorphic event at about 2970 Ma. Both the initial ⁸⁷Sr/⁸⁶Sr ratios and present-day first-stage model ²³⁸U/²⁰⁴Pb value for the Leonian granitoids are indistinguishable from mantle values, but do not preclude the possibility that these granitoids were derived from parental material with a short history in the crust or lower crust. The Rb-Sr whole-rock isochron age of 2753+61 Ma for the Fadugu granite gneiss provides a definitive age for the Liberian event in northern Sierra Leone. A succession of rocks older than the Leonian (i.e., older than 2970 Ma) has been identified in the field but not yet dated.     

Lead ages,reset rubidium-strontium ages and implications for the Archaean crustal evolution of the Diemals area,Central Yilgarn Block,Western Australia

The Pb-Pb whole-rock geochronology of Archaean granitic and gneissic rocks from the Diemals area in the Central Yilgarn granite-greenstone terrain provides important constraints on crustal evolution. The regionally extensive banded gneisses, previously considered as candidates for basement to the greenstones give a Pb-Pb whole-rock age of 2700 ± 97 Ma (2σ errors). This is within error of previously published Rb-Sr and Sm-Nd gneiss ages and also within error of the Sm-Nd ages on the greenstones in the Eastern Goldfields Province. Two synkinematic plutons give Pb-Pb whole-rock ages (2737 ± 62 Ma and 2700 ± 100 Ma) and Pb isotopic compositions consistent with the hypothesis, based on field and geochemical relations, that these plutons were derived by partial melting of the precursors to the banded gneisses. Assuming this, the combined data date the melting event at 2723 ± 25 Ma with a model source μ value of 8.18 ± 0.02. This source μ value is close to the range postulated for mantle values and restricts the crustal history of the precursors to less than ~200 Ma. A post-kinematic pluton with a whole-rock Pb-Pb age of 2685 ± 26 Ma and μ value of 8.26 ± 0.02 puts a younger limit on this relatively short lived crustal accretion-differentiation event.Comparison of Pb-Pb and Rb-Sr whole-rock dates for the plutons suggests that the latter became closed systems up to 200 Ma after the Pb-Pb ages, and that the plutons gained or lost Rb or Sr at this time.     

冀东三屯营地区片麻岩的早太古宙全岩年龄

冀东三屯营地区麻粒岩相片麻岩由三组中酸性岩石组成。三组片麻岩Pb-Pb全岩等时线年龄为。三组片麻岩也各给自出了早太古宙年龄。岩石的这一年龄,是麻粒岩相变质过程中U和Th亏损之后发生均匀化的结果,因而表明麻粒岩相的变质作用结束于3563Ma之前。岩石在3563～2500Ma期间处于角闪岩相的环境。文中还给出了全岩Rb-Sr同位素的年龄成果,为2500～2800Ma。     

RbSr isotope systematics at amphibolite facies conditions,Uppsala Region,Eastern Sweden

On the basis of U-Pb, Rb-Sr and K-Ar isotope analyses of Proterozoic rocks and minerals, a chronology has been established for the tectonic, intrusive and metamorphic evolution of the Svecokarelian orogeny 1750–1950 Ma ago in the Uppsala Region, Eastern Sweden. It is suggested that when synkinematic granitoids intruded the orogenic belt, at a stage of general subsidence and at medium metamorphic conditions (600°C and 3.5–4 kbar), the U-Pb isotope system in zircons closed earlier than the Rb-Sr whole-rock system. The zircon age (1886 Ma) reflects the intrusion and crystallization of the rock melt and the Rb-Sr whole-rock age (1830 Ma) the time when the temperature had decreased below the threshold for ⁸⁷Sr migration. The Rb-Sr whole rock age (1898 Ma) determined for metaandesites and metadacites reflects a recrystallization related to the intrusion of the granitoids. On the contrary, the more silicic metarhyodacitic volcanic rocks have a Rb-Sr whole rock age (1830 Ma) reflecting the cessation of the synkinematic metamorphism. The difference in the way the Rb-Sr isotope system responds in subsilicic or silicic metavolcanics is probably dependent on the amount of radiogenic ⁸⁷Sr and on the fixation of ⁸⁷Sr in Ca-rich minerals. Subsequent, late-kinematic, low amphibolite facies metamorphism has not altered the Rb-Sr ages of the granitoids and the recrystallized metavolcanics.     

Geochronology of an archaean tonalitic gneiss dome in Northern Finland and its relation with an unusual overlying volcanic conglomerate and komatiitic greenstone

Archaean gneiss-greenstone relationships are still unresolved in many ancient cratonic terrains although there is growing evidence that most of the late Archaean greenstone assemblages were deposited on older tonalitic crust.We report here well defined basement-cover relationships from a late Archaean greenstone belt in Lapland, north of the Polar Circle. The basal greenstone sequence contains quartzite, schist, komatiitic volcanics and an unusual volcanic conglomerate with well preserved granite pebbles of an older basement. These rocks surround a gneiss dome composed of foliated tonalite which shows a polyphase deformation pattern not seen in the neighbouring greenstones.Zircon fractions of the gneisses plot on two discordia lines and give upper intercept ages with concordia at 3,069±16 Ma and 3,110±17 Ma respectively. One fraction contains metamict zircons with components at least 3,135 Ma old. These are the oldest reliable ages yet reported from the Archaean of the Baltic Shield. Rb-Sr whole-rock dating of the tonalitic gneiss yielded an isochron age of 2,729±122 Ma and an I_Sr of 0.703±0.001. This is interpreted to reflect a resetting event during which the gneisses may have acquired their present tectonic fabric.Rb-Sr model age calculations yield mantle values for I_Sr at about 2,950±115 Ma and suggest that the tonalite was intruded into the crust as juvenile material at about 3.1 Ga ago as reflected by the zircon ages. It was subsequently deformed and isotopically reset at about 2.7 Ga ago, prior to greenstone deposition.Comparison with tonalitic gneisses of eastern Karelia displays significant differences and suggests that the Archaean of Finland may contain several generations of pre-greenstone granitoid rocks.     

Abyssal peridotites >3,800 Ma from southern West Greenland: field relationships, petrography, geochronology, whole-rock and mineral chemistry of dunite and harzburgite inclusions in the Itsaq Gneiss Complex

Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to ~500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite-harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of ca. 3,650 Ma, but with an age of ca. 3,800 Ma most likely. The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at ca. 3,800 Ma. This is interpreted to have occurred during the development of a 3,810-3,790-Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. This northern part of the Itsaq Gneiss Complex is the most favourable for the geochemical study of early Archaean protoliths because it experienced peak metamorphism only within the amphibolite facies with little or no in-situ melt segregation, and contains some areas that have undergone little deformation since ca. 3,800 Ma. Most of the ultramafic enclaves are thoroughly altered, and largely comprise secondary, hydrous phases. However, the centres of some enclaves have escaped alteration and comprise dunite and harzburgite with >95% olivine (Fo_89-91) + orthopyroxene (En₈₉) + Al-spinel (Cr_8-20) assemblages. The dunites and harzburgites are massive or irregularly layered and are olivine-veined on 5-10-m to 10-cm scales. Their whole rock major and rare earth element, and olivine and spinel compositions differ significantly from xenoliths representing the Archaean cratonic lithospheric mantle, but are typical of some modern abyssal peridotites. The harzburgites and dunites show both LREE depleted and enriched patterns; however, none show the massive REE depletion associated with the modelled removal of a komatiite. They are interpreted as being the products of small degrees of melt extraction, with some showing evidence of refertilisation. These Greenland dunites and harzburgites described here are currently the best characterised 'sample' of the early Archaean upper mantle.     

Geochemistry of the Shushui Complex,South Shanxi Province,China

The Shushui Complex can be divided into three rock units based on field investigation, petrography and geochemistry:(1) felsic gneisses, (2) supracrustal rocks consisting of amphibolite, marble and quartzite, and (3) late granites. Of the complex, felsic gneisses are dominant and formed in the Late Archaean, which were intruded by a basic dyke with a whole-rock Rb-Sr isochron age of 2264±219 Ma. The data on rare-earth elements as well as on major and trace elements presented for most of the rock types making up the complex suggest that (1) basic gneisses were produced by partial melting of mantle peridotite, followed by fractional crystallization, and (2) felsic gneisses produced by varying degree of melting of a mafic source. The most suitable tectonic setting to account for the generation of both types would be similar to the underplate setting.     

Early Archaean Amîtsoq tonalites and granites of the Isukasia area,southern West Greenland: development of the oldest-known sial

Amphibolite facies early Archaean Amîtsoq gneisses envelop and intrude the c. 3,800 Ma Isua supracrustal belt, Isukasia area, southern West Greenland. Most of these gneisses are strongly deformed, but in a c. 75 km² augen of lower deformation, the Amîtsoq gneisses are seen to comprise predominantly 3,750–3,700 Ma tonalitic grey gneisses that were intruded first by thin bodies of mafic to dioritic composition, known collectively as the Inaluk dykes, and then by c. 3,600 Ma white gneisses and finally by sporadic c. 3,400 Ma pegmatitic gneiss sheets. The grey gneisses could have formed by partial melting of crust consisting predominantly of basic rocks. The Inaluk dykes are interpreted as strongly fractionated basic melts of mantle origin, contaminated by crustal material. The white gneisses consist mostly of medium grained granite and occur as lenses and anastomosing sheets throughout their host of grey gneisses with subordinate inclusions of supracrustal rocks. The white gneisses have chemistry compatible with formation by partial melting at depth of a source dominated by grey gneisses. The igneous chemistry, including REE abundances, of the grey gneisses and white gneisses has been modified to varying degrees by metasomatism and assimilation reactions during the crystallisation of the white gneisses and also during subsequent tectonometamorphic events. The white gneisses are evidence for considerable reworking by anatexis of sialic crust in the early Archaean, 150 to 100 Ma after its formation. The white gneisses and the pegmatitic gneisses show that granitic rocks s.s. were important in the earliest Archaean, and are further evidence of the diversity of the oldest-known sial.Previously at and the Geological Survey of Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark     

鞍山地区太古代岩石同位素地质年代学研究

鞍山本溪地区太古代变质岩可分为三套,即含铁的表壳岩建造、侵入于铁建造中的花岗质片麻岩和铁架山奥长花岗质-花岗质片麻岩,后者为表壳岩的基底。原划为上鞍山群樱桃园组(齐大山矿带)和山城子组(歪头山-北台矿带)的斜长角闪岩分别获得2729Ma和2724Ma的Sm-Nd等时线年龄。这就为有争议的鞍本地区铁建造属于同一时代提供了依据,并讨论了表壳岩中的变质沉积岩以及铁架山基底片麻岩的同位素年代。     

Discrepancies between neodymium,lead and strontium model ages from the Precambrian of southern East Greenland: Evidence for a Proterozoic granulite-facies ev

Sm-Nd (t_DM) model ages for gneisses from the coastal region between 66° and 68°N in southern East Greenland range from 3.02 to 2.79 Ga and indicate that these basement rocks were formed in a major late Archaean episode of sialic crust formation between 3.0 and 2.8 Ga ago. Very low concentrations of U have resulted in unradiogenic Pb-isotopic compositions, so that most samples do not yield chronologically useful Pb/Pb isochrons. The data have been used to calculate Pb/Pb model ages, and in the northern and southern parts of the study area these are in good agreement with the Sm-Nd model ages. In the central part of the area, however, Pb/Pb model ages are much lower than Sm-Nd ages, locally as low a.80 Ga. This discrepancy is attributed to loss of U long after the crust formation event, and probably related to a phase of Proterozoic granulite-facies metamorphism that has not previously been recognised. Rb-Sr isotope data support this interpretation.     

黔西北福来厂铅锌矿床Pb同位素研究及地质意义

单阶段演化正常铅用H-H法可获得高准确性的模式年龄,在黔西北福来厂铅锌矿床中采集16件矿脉和围岩样品,测定它们的Pb同位素组成,206 pb/204 Pb为18.5346 ～ 18.7294(均值18.5935),207pb/204 Pb为15.7408～15.7603(均值15.7519),208 pb/204 Pb...     

The nature of the basement in the Archaean Dharwar craton of southern India and the age of the Peninsular Gneiss

The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrustal rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhF₁, DhF_1a and DhF₂) that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation (DhF_*) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inference that the supracrustal rocks were deposited on a gneissic basement. Whole rock Rb-Sr ages of gneisses showing only the DhF₁ structures fall in the range of 3100–3200 Ma. Where the later deformation (DhF₂) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonalitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ～2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300–3100 Ma, and 2800–3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma.     

陕西太白南部西坝复式岩体的成因及其与双王金矿床的关系

本文认为西坝复式岩体由第一期的石英二长闪长岩（Rb-Sr同位素年龄213.5Ma）和第二期的二长花岗岩（K-Ar同位索年龄198.3～202Ma）组成。两期岩浆岩的岩石化学、稀土元素组成和同位素特征等表明,它们同属一个系列,由来源于上部地幔的岩浆经过演化而形成。从岩体的Au含量、岩体和矿床的Pb同位素组成推测,岩体和双王金矿床之间无物质成分方面的联系。借助于传热学方法分析岩体的热影响,亦未发现有控制金矿床形成的迹象。总之,岩体与双王金矿床无关。     

Geochemical evolution of Archaean granulite-facies gneisses in the Vestfold Block and comparisons with other Archaean gneiss complexes in the East Antarctic Shield

The Vestfold Block, like other Archaean cratons in East Antarctica and elsewhere, consists predominantly of felsic orthogneiss (Mossel and Crooked Lake gneisses), with subordinate mafic granulite (Tryne metavolcanics) and paragneiss (Chelnok supracrustals). Two major periods of continental crust formation are represented. The Mossel gneiss (metamorphosed about 3,000 Ma ago) is mainly of tonalitic composition, and is similar to much of the roughly coeval Napier Complex in Enderby Land. The Crooked Lake gneiss was emplaced under high-grade conditions about 2,450 Ma ago and comprises a high proportion of more potassic rocks (monzodioritic and monzonitic suites), as well as tonalite and minor gabbro and diorite. Both Mossel and Crooked Lake gneisses are depleted in Y and have moderate to high Sr, Ce/Y, and Ti/Y, consistent with melting of a mafic source (?subducted hydrated oceanic crust) leaving major residual hornblende (± garnet). Most Crooked Lake gneisses are more enriched in incompatible elements (P, Sr, La, Ce, and particularly Rb, Ba, and K) than Mossel gneisses, suggesting derivation from a more enriched mafic source. The Vestfold Block contains few orthogneisses derived by melting of older felsic crustal rocks, in marked contrast to the Archaean Napier Complex and, in particular, southern Prince Charles Mountains. Both Mossel and Crooked Lake tonalites are strongly depleted in Rb, K, Th, and U, and have very low Rb/Sr and high K/Rb; more potassic orthogneisses are depleted in Th, U, and, to lesser extents, Rb. Tryne metavolcanics are depleted in Th and Rb, but appear to have been enriched in K (and probably Na), possibly during early low-grade alteration.     

Stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt,southern west greenland

The highly deformed c. 3800 Ma Isua supracrustal belt is a fragment of a more extensive Early Archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic Amftsoq gneisses in the period 3800-3600 Ma. The supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 Ma, in the Late Archaean and locally at c. 1800 Ma. Layered sequences of rock of sedimentary and probable volcanic origin form over 50% of the belt. Bodies of high MgAl basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of Amitsoq gneisses. The layered rocks which are < 1 km thick are divided into two sequences, that are in faulted contact with each other. The way-up of these sequences has been determined from facing-directions of locally-preserved graded layering in felsic metasediments at several localities. The overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments. Clastic sedimentation took place against a background of chemical sedimentation, shown by interlayers of banded iron formation, metachert and calc-silicate rocks throughout the sequences. The felsic rocks locally preserve graded bedding and possible conglomerate structures, indicating deposition from turbidite flows and possibly as debris flows. Nodules in the felsic rocks contain structures interpreted as fiammé. There is an irregular enrichment in K₂O/Na₂O in many of the felsic rocks at constant SiO₂ and Al₂O₃ content, interpreted as owing to alteration of original andesitic to dacitic volcanic rocks. Banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking, possibly under shallow water conditions. Lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source.     

柴北缘夹榴辉岩的片麻岩（片岩）地球化学、Sm-Nd和U-Pb同位素研究——深俯冲的前寒武纪变质基底？

在柴北缘的鱼卡-锡铁山-沙柳河一带,出露夹有榴辉岩透镜体的花岗质片麻岩(正片麻岩)和副片麻岩(片岩)。地球化学和Sm-Nd同位素数据显示副片麻岩(片岩)与正片麻岩具有类似的地球化学成分和一致的Nd模式年龄(1.88～2.18Ga),结合副片麻岩(片岩)局部包在正片麻岩中的野外关系,正片麻岩可能为副片麻岩(片岩)原地熔融作用的产物。U-Pb锆石测定表明熔融作用产生的正片麻岩的岩浆形成时代为952Ma。另外,这些夹榴辉岩的片麻岩(片岩)也与柴北缘北侧不夹榴辉岩的深变质基底片麻岩和中南祁连地块的变质基底片麻岩有相似Sm-Nd同位素特征和近一致Nd模式年龄(1.87～2.26Ga)。表明它们具有明显的亲缘关系,可能来源于具有古元古代晚期地壳形成年龄的同一变质基底。然而,与柴北缘北 侧和祁连地块的深变质基底岩石不同的是,这套含榴辉岩的片麻岩(片岩)明显遭受了早古生代变质作用的影响,正片麻岩锆石U-Pb测定获得的下交点年龄为478±44Ma,与柴北缘地区榴辉岩的变质锆石的年龄在误差范围内一致;而已在都兰地区副片麻岩锆石中柯石英包体的发现也证明了含榴辉岩的片麻岩(片岩)与榴辉岩一样同样经历了UHP变质作用。因此,我们认为柴北缘含榴辉岩的片麻岩虽然具有与相邻变质基底相似的早期演化历史,但在早古生代又与所夹的榴辉岩     

华北地台北缘鞍山白家坟奥长花岗岩地球化学特征、锆石U-Pb年龄及意义

太古宙TTG岩石的成因及年代学研究是理解前板块构造和板块构造开始及其演化过程的关键,是陆壳增生的重要标志,为太古宙地质体研究的热点和难点之一。对鞍山地区的白家坟岩体进行了地球化学研究和锆石U-Pb测年。数据结果显示,该岩体具有正片麻岩特征,Si、Na含量高,TFeO、MgO、CaO含量低,轻、重稀土元素分异程度低,且具有一定变化。稀土元素总量偏低(12.6×10~(-6)～119.34×10~(-6)),LREE/HREE值为8.98～13.11。微量元素富集K、Rb、Ba,亏损Nb、Ce、Zr、Sm、Ti、Y、Yb;K/Rb平均值为229.37,Rb/Sr平均值为0.58,Ba/Sr平均值为3.28,Y/Nb平均值为3.36。表明该岩体为岩浆作用的产物,也有部分为壳内再循环的产物或受到陆壳物质影响。测得~(207)Pb/~(206)Pb年龄加权平均值为3816±14Ma,锆石具有清晰的振荡环带结构,所有核部锆石分析点的Th/U值在0.05～0.54之间,具有岩浆锆石的特点,代表了白家坟奥长花岗质岩石的结晶时代。3613～3651Ma和3331Ma两组年龄数据代表早太古代的一次岩浆事件及重要的陆壳增生时期的热事件。     

Tectonostratigraphic and geochronologic reappraisal constraining the growth and evolution of Singhbhum Archaean craton, eastern India

Reappraisal of field relationships between the different lithological ensembles supported by available geochronological data, and taking due note of the tectono-metamorphic, magmatic and sedimentation history helped to build up a coherent crustal evolutionary history of the Singhbhum Archaean craton, eastern India. The evolution of the earliest sialic crust, as the isotope ages suggest, was around 3700 Ma or even earlier. Deposition of the oldest, dominantly metasedimentary supracrustals, the Older Metamorphic Group (OMG), was initiated at around 3380 Ma, i.e. after a gap of about 320 million years. The closing of OMG basins synchronously with the emplacement of a granitoid phase was at ca.3285 Ma. No other fabric-forming ductile deformation and metamorphism associated with the development of foliation and mineral lineation is known in the rocks of the Singhbhum Archaean craton subsequent to this event. Formation of the succeeding geological ensembles including the deposition of BIF-bearing Iron Ore Group (IOG) and the emplacement of the post-IOG granitoids at ca.3100 Ma can be described as ??lsnon-orogenic?? event taking place during the phase of tectonic quiescence. Supracrustals like the Dhanjori and Simlipal mafic volcanics with intercalated beds of arenite evolved later during the phase of Plume outburst at around 2800 Ma. The end-Archaean intrusion of Newer Dolerite dykes in conjugate sets and the deposition of Kolhan Group in an N-S oriented basin during an E-W stress system mark the culmination of the Archaean crust-building activity in the Singhbhum Archaean craton.