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.     

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.     

Rb-Sr and Sm-Nd ages and isotopic geochemistry of Archaean granodioritic gneisses from Eastern Finland

Twenty granodioritic rocks and one amphibolitic enclave of the “basement” of the Suomussalmi-Kuhmo Archaean (2.65 Ga) greenstone belts (central-eastern Finland), have been chosen together with one greenstone sample for Rb-Sr and Sm-Nd geochronological and isotopic studies.The granitoïd rocks are subdivided into three groups: two generations of grey gneisses and a post-belt augen gneiss. The Rb-Sr ages of the first and second generation of grey gneisses are 2.86 ± 0.09 and 2.62 ± 0.07 Ga, respectively. These results are corroborated by Sm-Nd data. The post-belt augen gneiss gives an age of 2.51 ± 0.11 Ga. The results show that the two generations of grey gneisses, the greenstone belts and the post-greenstone augen gneiss, were developed over a period > 350 Ma. The two generations of grey gneisses show identical I_Sr values (0.7023 ± 8 and 0.7024 ± 6) which contrast with that of the augen gneiss (0.7049 ± 8). The low I_Sr and the near-chondritic ?_T^CHUR values indicate that the grey gneisses cannot derived from much older continental materials. Trace element studies suggest that these grey gneisses have had a multi-stage development. The augen gneiss with a moderately high I_Sr is likely to be derived from a granodiorite originated by partial melting of older sialic crust. The more probable parent rock seems to be the first generation grey gneisses. The I_Sr and average Rb/Sr values preclude the greenstone belt and the second generation of grey gneisses as the protolith.     

Middle to late Archaean geology of the eastern Baltic shield,with a note on its similarity and contrast with the Archaean of southern India

The middle to late Archaean rocks of Kola and Karelia in the eastern Baltic shield consist of the Infracomplex overlain by the Saamian complex, and the Lopian greenstone belts. The Infracomplex which forms the basement is a polymigmatite, parts of which are at least 3100 Ma old. The Saamian in the central Belomorian region comprises granite gneiss, amphibolite, garnet-kyanite gneiss and high alumina gneisses which belong to the Keret, Hetolombina and Chupa suites. The Lopian greenstone belts ranging in age from 3000 to 2700 Ma are composed of peridotitic, pyroxenitic and basaltic komatiites, tholeiitic basalts, andesites, dacites and rhyolites, together with tuffs, graywackes and iron formations. Whereas there is a dominance of volcanic over sedimentary rocks in the greenstone belts of the Baltic shield, a significant proportion of detrital and chemogenic sedimentary rocks characterizes the Dharwar succession of approximately the same time span in the southern Indian shield. Association of mature and immature detrital sedimentary rocks with bimodal volcanic assemblages points to a back-arc setting for the Dharwar belts. This contrasts with the association of immature sediments with calc-alkaline volcanic rocks in the greenstone belts of the eastern Baltic shield, suggesting an island arc environment there.     

Mineral isotope evidence for the contemporaneous process of Mesozoic granite emplacement and gneiss metamorphism in the Dabie orogen

Kinetics of isotopic equilibrium in the mineral radiometric systems of igneous and metamorphic rocks is an important issue in geochronology. It turns out that temperature is the most important factor in dictating isotopic equilibrium or disequilibrium with respect to diffusion mechanism. Contemporaneous occurrence of Mesozoic granites and gneisses in the Dabie orogen of China allows us to evaluate the thermal effect of magma emplacement and associated metamorphism on mineral radiometric systems. Zircon U-Pb, mineral Rb-Sr and O isotope analyses were carried out for a Cretaceous granite and its host gneiss (foliated granite) from North Dabie. Zircon U-Pb dating gave consistently concordant ages of 127 ± 3 Ma and 128 ± 2 Ma for the granite and the gneiss, respectively. A direct correspondence in equilibrium state is observed between the O and Rb-Sr isotope systems of both granitic and gneissic minerals. Mineral O isotope temperatures correlate with O diffusion closure temperatures under conditions of slow cooling, indicating attainment and preservation of O isotope equilibrium in these minerals. The mineral Rb-Sr isochron of granite, constructed by biotite, feldspar, apatite and whole-rock with the O isotope equilibrium, yields a meaningful age of 118 ± 3 Ma, which is in accordance with the mineral Rb-Sr isochron age of 122 ± 1 Ma for the host gneiss. The consistency in both U-Pb and Rb-Sr ages between the granite and the gneiss suggests a contemporaneous process of crystallizing the zircons and resetting the Rb-Sr radiometric systems during magma emplacement and granite foliation. Whereas the zircon U-Pb ages for both granite and gneiss are interpreted as the timing of magma crystallization, the young Rb-Sr isochron ages record the timing of Sr diffusion closure during the slow cooling. Protolith of the gneiss crystallized shortly before intrusion of the granite, so that it was able to be foliated by voluminous emplacement of coeval mafic to felsic magmas derived by anatexis of orogenic lithospheric keel. Therefore, extensional collapse of collision-thickened crust at Early Cretaceous is suggested to trigger the post-collisional magmatism, which in turn serves as an essential driving force for the contemporaneous high-T deformation/metamorphism.     

Constraints on Archaean crustal evolution of the Zimbabwe craton: a U-Pb zircon,Sm-Nd and Pb-Pb whole-rock isotope study

 The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic, continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide event and heralding cratonization. The similarity between U-Pb zircon ages and T_DM model ages (2.65–2.62 Ga) and the positive ɛNd_T values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and minor contamination with older crust. The rather high model μ₁ values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older T_DM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction between depleted mantle-derived melts and older crustal material. Received: 15 August 1995 / Accepted: 12 January 1996     

The radiometric age of the Mount Keith Granodiorite,a maximum age estimate for an Archaen greenstone sequence in the Yilgarn block,Western Australia

The ‘granite’ pluton previously shown by Durney (1972) to be unconformably overlain in the Jones Creek area of Western Australia by a boulder conglomerate and a younger greenstone sequence has a Rb-Sr total rock age of $\text{2689 ± 17}$ Ma. Two boulders analysed from the conglomerate are not distinguished isotopically from the underlying ‘granite’. Biotite ages from the ‘granite’ reflect the final effects of a regional upper greenschist facies metamorphism at approximately $\text{2622 ± 15}$ Ma. Potassium feldspars in the ‘granite’ indicate ages less than 2600 Ma and are presumably related to late stage tectonic adjustments.Thus the isotopic dating shows that the ‘granite’, now formally named the Mount Keith Granodiorite, belongs to the early part of the 2750–2600 Ma Yilgarn age group (Compston and Arriens, 1968) and places an older limit on the age of the upper greenstone sequence. This greenstone was laid down within a period of 70 Ma or less and was followed by metamorphism which had ended by 2622 Ma, the time of closure of the biotite Rb-Sr system.Details of the petrography, particularly the metamorphic textures and paragenesis, indicate that despite their externally massive appearance, all the samples except the pegmatite have been recrystallized to varying degrees. Major-element analyses are given also, to aid in the chemical classification of the Granodiorite and in understanding its petrogenesis.     

花岗岩锆石U-Pb年龄与全岩Rb-Sr等时线年龄对比研究及其地球化学意义

对国内外32个花岗岩体的锆石U-Pb年龄与全岩Rb-Sr等时线年龄之间差值（Δt）进行的频数统计分析表明：Δt呈对称正态分布（偏度系数CSK=0.36;峰度系数CKU=2.99）;年龄差（Δt）既呈正值又有负值,其均值为2.08Ma;相对年龄差（Rt）小于5%。采用最小二乘法计算,花岗岩体锆石U-Pb年龄（tZr）对全岩Rb-Sr等时线年龄（tRb）拟合出相关系数很高（r=0.998）,回归系数接近l（α=1.003）的线性回归方程（tRb =1.003tZr +1.258）。这些统计特征表明,从总体来看,花岗岩体的Rb-Sr等时线定年测定结果与锆石U-Pb定年测定结果是一致的,花岗岩全岩Rb-Sr等时线定年方法是成熟、可信的,同时也为花岗岩锆石U-Pb年龄代表结晶年龄而不代表花岗岩侵位年龄提供了依据。     

SOVETSKAYA GEOLOGIYA (SOVIET GEOLOGY) IN COVER-TO-COVER TRANSLATION, 1960, NOS. 11 AND 12

The Río Negro-Juruena Province (RNJP) occupies a large portion of the western part of the Amazonian Craton and is a zone of complex granitization and migmatization. Regional metamorphism, in general, occurred in the upper amphibolite facies. The granites and gneisses of the RNJP yield Rb-Sr and Pb-Pb whole-rock isochron dates ranging from 1.8 Ga to 1.55 Ga, with initial ⁸⁷Sr/⁸⁶Sr ratios of ～ 0.703 and a single-stage model μ₁ value of ～ 8.1. In order to improve the geochronological control, SHRIMP U-Pb zircon ages, conventional U-Pb zircon ages, and additional Pb-Pb whole-rock isochron ages were determined for samples of granitoids and gneisses from the Papuri-Uaupés and Guaviare-Orinoco rivers areas (northern part of the province) and Jamari-Machado rivers and Pontes de Lacerda areas (southern part). The granitoids from the northern part of the province yield conventional U-Pb zircon ages of 1709 ± 17 Ma and 1521 ± 31 Ma, and SHRIMP U-Pb concordant zircon results of 1800 ± 18 Ma. Samples of gneissic rocks from the southern part of the RNJP yielded SHRIMP U-Pb concordant ages of 1750 ± 24 Ma and 1570 ± 17 Ma and a Pb-Pb whole-rock isochron age of 1717 ± 120 Ma. These new U-Pb and Pb-Pb results confirm the previous Rb-Sr and Pb-Pb geochronological evidence that the main magmatic episodes within the RNJP occurred between 1.8 and 1.55 Ga, and suggest that this crustal province constitutes a segment of continental crust newly added to the Amazonian Craton at the end of the Early Proterozoic. In the area of the RNJP, there are several anorogenic rapakivi-type granite plutons. Because of the absence of recognized Archean material within the basement rocks, it is reasonable to consider the Early to Middle Proterozoic continental crust as the magmatic source for the rapakivi granite intrusions.     

A new fragment of the early earth crust: the Aasivik terrane of West Greenland

The Aasivik terrane is a ∼1500 km² complex of gneisses dominated by ∼3600 Ma components, which has been discovered in the Archaean craton of West Greenland, ∼20–50 km south of the Paleoproterozoic Nagssugtoqidian orogen. The Aasivik terrain comprises granulite facies tonalitic to granitic gneisses with bands of mafic granulite, which include disrupted mafic dykes. Four gneiss samples of the Aasivik terrain have given imprecise SHRIMP U–Pb zircon ages of 3550–3780 Ma with strong loss of radiogenic lead and new growth of zircon probably associated with a granulite facies metamorphic event(s) at ∼2800–2700 Ma. To the Southeast, the Aasivik terrane is in tectonic contact with a late Archaean complex of granitic and metapelitic gneisses with apparently randomly distributed mafic and ultramafic units, here named the Ukaleq gneiss complex. Two granitic samples from the Ukaleq gneiss complex have U–Pb zircon ages of 2817 ± 10 and 2820 ± 12 Ma and t_zircon ε_Nd values of 2.3–5.4. Given their composition and positive ε_Nd values, they probably represent melts of only slightly older juvenile crust. A reconnaissance SHRIMP U–Pb study of a sample of metasedimentary rock from the Ukaleq gneiss complex found ∼2750–2900 Ma zircons of probable detrital origin and that two or more generations of 2700–2500 Ma metamorphic zircons are present. This gneiss complex is provisionally interpreted as a late Archaean accretionary wedge. A sample of banded granulite facies gneiss from a complex of banded gneisses south of the Aasivik terrain here named the Tasersiaq gneiss complex has yielded two zircon populations of 3212 ± 11 and 3127 ± 12 Ma. Contacts between the three gneiss complexes are mylonites which are locally cut by late-post-kinematic granite veins with SHRIMP U–Pb zircon ages of ∼2700 Ma. The isotopic character and the relationships between the lithologies from the different gneiss complexes suggest the assembly of unrelated rocks along shear zones between 2800 and 2700 Ma. The collage of Archaean gneiss complexes were intruded by A-type granites, here named the Umiatsiaasat granites, at ∼2700 Ma, later than the tectonic intercalation of the gneiss complexes.     

Rb-Sr and Sm-Nd Geochronology of the Eppawala Metamorphic Rocks and Carbonatite,Wanni Complex,Sri Lanka

Petrological studies on the surrounding metamorphic rocks of the Eppawala carbonatite body, Wanni complex, Sri Lanka, revealed that these rocks had been metamorphosed under amphibolite to granulite facies conditions. Garnet-sillimanite-biotite gneiss shows lower range of metamorphic temperature (730–770°C) than the migmatite gneiss (750–780°C) and the pressure varies from 6.6–7.8 kbar to 5.6–6.4 kbar respectively. The metamorphic age of the garnet-sillimanite-biotite gneiss and migmatite gneiss dated 607±23 Ma and 626±16 Ma, respectively for mineral — whole rock isochron in Sm-Nd system. These ages are compatible with the ages of regional high-grade metamorphism occurred 610–550 Ma in the three crustal units in Sri Lanka.Rb-Sr system for biotite, apatite and whole-rock fractions suggests 493±5 Ma for the Eppawala carbonatite body. This age indicates the cooling age of the biotite. The presence of non-crystalline carbonatite matrix and large hexagonal apatite crystals suggests a slow cooling history. Further, low closure temperature of biotite in Rb-Sr system suggests that the intrusion age of carbonatite body should be more than 493 Ma, but non-metamorphosed nature provides evidence that the intrusion age of the carbonatite body should be less than the period of regional metamorphism 610–550 Ma. Therefore, Eppawala carbonatite body has a strong possibility to be a late to post magmatic intrusion. The other late to post magmatic intrusions in the Wanni complex and Highland complex are dated between 580–550 Ma. Therefore, the most probable intrusion age of the Eppawala carbonatite body is suggested to be around 550 Ma.     

Rb-Sr geochronology and petrogenesis of granitoids from the Chhotanagpur granite gneiss complex of Raikera-Kunkuri region,Central India

The Precambrian Chhotanagpur granite gneiss complex (CGGC) terrain covers more than 80,000 sq km area, and is dominated by granitoid gneisses and migmatites. Recent geochronological data indicate that the CGGC terrain has witnessed five tectonomagmatic thermal events at: (i) 2.5–2.4 Ga, (ii) 2.2–2.0 Ga, (iii) 1.6–1.4 Ga, (iv) 1.2–1.0 Ga, and (v) 0.9–0.8 Ga. Of these, the third and the fourth events are widespread. The whole-rock Rb-Sr isotopic analysis of twenty granite samples from the CGGC of Raikera-Kunkuri region, Jashpur district, Chhattisgarh, Central India, yields two distinct isochrons. The eleven samples of grey granites define an isochron age of 1005±51 Ma with moderate initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7047±0.0065, which corresponds to the fourth tectonomagmatic event. On the other hand, the nine samples of pink granites indicate younger isochron age of 815±47 Ma with a higher initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7539±0.0066 that matches with the fifth phase of the thermal event. The data suggest emplacement of large bodies of grey granite at ∼1005 Ma that evolved possibly from precursors of tonalitic-granodioritic composition. Furthermore, the younger age (∼815 Ma) suggests the age of metasomatism, involving isotopic resetting, that resulted in genesis of pink granite bodies of limited areal extent. By analogy, the age of metasomatism (∼815 Ma) may also be taken to represent the age of Y-mineralisation in the Raikera-Kunkuri region of the CGGC terrain.     

Direct Isotope Dating of W(−Y) Mineralization at Kyzyltau (Mongolian Altai): Preliminary Results

Absolute ages of granite magmatism, as well as of tungsten mineralization, are poorly constrained in the Mongolian Altai and adjacent areas. There are no reports focusing on special isotopic investigations of the tungsten deposits. For the deposits in the Achit nuur and the Zagaan-Shibetin tectonic zones, two concepts that are discussed in the literature assume Paleozoic or Mesozoic ages for mineralization and related granite magmatism. We report the first results of a combined Sm-Nd and Rb-Sr isotope investigation of rocks and vein minerals of the Ulaan uul tungsten deposit at Kyzyltau; the results suggest Paleozoic ages for vein mineralization and for albitization of the host granite. The Sm-Nd isotope system, and the structure of vein minerals used for isotopic dating, were only slightly affected by late alteration processes. Sm-Nd mineral isochrons for wolframite and fluorite from the veins define an age of 303 ± 17 Ma (MSWD = 1.8, εNd = +0.9 ± 0.2). The Rb-Sr isotope system of the vein-hosting granite was strongly influenced by alteration processes. The Rb-Sr whole-rock isochron (282.2 ± 2.4 Ma, Sr_i = 0.70667 ± 0.00032, MSWD = 0.53) is interpreted as a mixing line and the age so defined has no direct geological meaning. Nevertheless, using Rb-Sr model ages and data on the degree of alteration of the samples, an age somewhat below 316 Ma can be estimated for albitization of the vein-hosting granite. This age estimation is in good agreement with the Sm-Nd isochron age for the vein mineralization. High εNd values obtained for wolframite and fluorite from the ore veins indicate an important role for material derived from the upper mantle in the ore formation processes.     

大别山高级变质岩云母Rb—Sr等时线年龄测定及其地质意义

应用云母Rb Sr等时线法对大别山高级变质岩的后期改造历史进行了初步的探讨。采用重液变温技术将云母按比重的区别 ,分成若干组分 ,然后用常规的Rb Sr方法测定。应用这种技术对大别山黄土岭太古代麻粒岩相岩石中的黑云母和产于超高压变质带内的石马含石榴石片麻岩的白云母进行了Rb Sr等时线年龄测定。黄土岭太古代麻粒岩相片麻岩中的黑云母 全岩Rb Sr等时线年龄为 12 7± 9Ma ,与该区片麻岩中的角闪石的K Ar和Ar Ar年龄 ,及燕山期花岗岩的Rb Sr年龄一致 ,说明这一太古代下地壳岩石受到了燕山期深熔 岩浆事件的影响 ;大别山东南部石马含石榴石片麻岩 (榴辉岩相 )中的白云母Rb Sr等时线年龄为 191± 10Ma ,反映了印支 早燕山期区域性低级变质事件的时代。研究表明 ,大别山区的不同构造单元中产出的不同类型的高级变质岩自中生代以来有不同的后期演化历史。     

A 3500 Ma plutonic and volcanic calc-alkaline province in the Archaean East Pilbara Block

Variably foliated, predominantly granodioritic plutonic rocks from the northern part of the Shaw Batholith in the east Pilbara Archaean craton are dated at 3,499±22 Ma (2σ errors) by a whole-rock Pb-Pb isochron. These rocks intrude the surrounding greenstone sequence, and their age is indistinguishable from that sequence. High strain grey gneisses which occupy much of the western and southern Shaw Batholith are chemically and isotopically similar to the North Shaw suite and are inferred to have been derived from this suite by tectonic processes. Felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc-alkaline volcanic and plutonic province at ～3,500 Ma. This volcanic and plutonic suite is similar to modern calc-alkaline suites on the basis of major element, rare earh element and most other trace element contents. The Archaean suite contrasts with modern equivalents only in having lower concentrations of HREE and higher concentrations of Ni and Cr. The average composition of the North Shaw suite is similar to that of Archaean gneiss belts for most elements and is consistent with the previously formulated hypothesis that the Shaw Batholith is transitional to the upper crustal level of a high-grade gneiss belt. Enrichment of the gneissic crust in the Shaw Batholith in alkali and heat-producing elements is inferred to have taken place by both igneous and hydrothermal processes over a protracted time interval. Late- and post-tectonic adamellite and granite melts intrude the gneissic rocks and there is isotopic evidence consistent with the gneisses being substantially enriched in Rb by pegmatite injection at ～3,000 Ma.     

Single zircon ages for felsic to intermediate rocks from the Pietersburg and Giyani greenstone belts and bordering granitoid orthogneisses, northern Kaapvaal Craton, South Africa

Previous models for the temporal evolution of greenstone belts and surrounding granitoid gneisses in the northern Kaapvaal Craton can be revised on the basis of new single zircon ages, obtained by conventional U---Pb dating and Pb---Pb evaporation. In the Pietersburg greenstone belt, zircons from a metaquartz porphyry of the Ysterberg Formation yielded an age of 2949.7±0.2 Ma, while a granite intruding the greenstones, and deformed together with them, has an age of 2853 + 19/−18 Ma. These data show felsic volcanism in this belt to have been coeval with felsic volcanism in the Murchison belt farther east, and the date of 2853 Ma provides an older age limit for deformation in the region. In contrast, a meta-andesite of the Giyani greenstone belt has a zircon age of 3203.3±0.2 Ma, while a younger and cross-cutting feldspar porphyry has an emplacement age of 2874.1±0.2 Ma. The meta-andesite is intercalated with various mafic and ultramafic rocks and, therefore, the age of 3.2 Ga appears plausible for the bulk of the Giyani greenstones.Granitoid gneisses surrounding the Pietersburg and Giyani belts vary in composition from tonalite to granite and texturally from well-layered to homogeneous but strongly foliated. These rocks yielded zircon ages between 2811 and 3283 Ma. The pre-3.2 Ga gneisses are polydeformed and may have constituted a basement to the Giyani greenstone sequence, while the younger gneisses are intrusive into the older gneiss assemblage and/or into the greenstones. The Giyani and Pietersburg belts probably define two separate crustal entities that were originally close together but were later displaced by strike-slip movement.     

满洲里地区印支期花岗岩Rb—Sr等时线年代学证据

满洲里－西旗地区为一重要的燕山期斑岩－次火山岩脉型浅成低温矿化区,较早期的花岗岩往往作为矿化围岩。关于本区是否存在印支期花岗岩,一直存在疑问。本文就区内四大矿区的早期花岗岩体（原推断为海西晚期或燕山早期）进行了系统的岩矿和Ｒｂ－Ｓｒ同位素年代学研究,得到两条线性关系甚好的等时线,年龄分别为２１１±２１Ｍａ和２２５．４±７．９Ｍａ,证明该区存在印支期花岗岩。原划分的海西晚期、燕山早期花岗岩相当一部分要解体划为印支期花岗岩。     

Orthogneisses in the Spessart Crystalline Complex,north-west Bavaria: Silurian granitoid magmatism at an active continental margin

The Spessart Crystalline Complex, north-west Bavaria contains two orthogneiss units of granitic to granodioritic composition, known as the Rotgneiss and Haibach gneiss, respectively, which are structurally conformable with associated metasediments. The igneous origin of the Rotgneiss is apparent from field and textural evidence, whereas strong deformation and recrystallization in the Haibach gneiss has obscured most primary textures. New geochemical data as well as zircon morphology prove the Haibach gneiss to be derived from a granitoid precursor, which was chemically similar to the Rotgneiss protolith, thus suggesting a genetic link between those two rock units. Both gneiss types have chemical compositions typical of anatectic two-mica leucogranites. They show characteristics of both I- and S-type granites. Rb-Sr whole rock data on the Haibach gneiss provide an isochron age of 407±14 Ma (IR = 0.7077±0.0007; MSWD 2.2), which is slightly younger than the published date for the Rotgneiss (439±15 Ma; IR=0.7048±0.0026; MSWD 4.9). Single zircon dating of six idiomorphic grains, using the evaporation method, yielded a mean ²⁰⁷Pb/²⁰⁶Pb age of 410±18 Ma for the Haibach gneiss and 418±18 Ma for the Rotgneiss. Both zircon ages are within analytical error of the Rb-Sr isochron dates and are interpreted to reflect the time of protolith emplacement in Silurian times. Three xenocrystic zircon grains from the Rotgneiss yielded ²⁰⁷Pb/²⁰⁶Pb ages of 2278±12, 2490±13 and 2734±10 Ma, respectively, suggesting that late Archaean to early Proterozoic crust was involved in the generation of the granite from which the Rotgneiss is derived. Although it is assumed that the granitic protoliths of the two gneisses were formed through anatexis of older continental crust, the relatively low ⁸⁷Sr/⁸⁶Sr initial ratios of both gneisses may also indicate the admixture of a mantle component. The Rotgneiss and the Haibach gneiss thus document granitic magmatism at an active continental margin during late Silurian times.     

内蒙古红山子地区新太古代变质侵入岩年代学及地球化学特征

华北克拉通西部陆块阴山地块花岗-绿岩带中出露的花岗岩类型比较复杂,研究其类型、成因及其与绿岩带之间的联系,对于探讨花岗-绿岩带的形成和演化有重要意义。本文对内蒙古三合明铁矿西侧红山子地区的花岗-绿岩带进行了研究。其不仅包含有大量的变质科马提岩、科马提质玄武岩和拉斑玄武岩等绿岩带的表壳岩,而且发育着多种变质侵入岩。研究表明,区内与绿岩带相关的侵入岩主要有3种,即变质英云闪长岩、变质花岗闪长岩、变质花岗岩,本文对其中的变质英云闪长岩和变质花岗岩两种岩石进行了地球化学和锆石测年的研究。变质英云闪长岩是一套富铝、低钾、贫铁镁和稀土元素强烈分异的太古宙高铝型TTG岩,SHRIMP锆石U-Pb测年为(2 534±7)Ma(MSWD=1.3),与研究区南部的赞岐岩和TTG的年龄相同;而变质花岗岩具有高硅、富碱,低铁、镁、钛、锰的特点,是一套钙碱性的过铝质岩石,SHRIMP锆石U-Pb测年为(2 509±7)Ma(MSWD=1.5),野外地质现象表明变质花岗岩明显地侵入到区内的绿岩地层和变质英云闪长岩之中。研究发现变质英云闪长岩形成于板块俯冲时TTG岩系岩浆弧构造环境,而变质花岗岩成因则可能与加厚地壳中变沉积岩的熔融有关。