An overview of the geology of the Transvaal Sequence and Bushveld Complex,South Africa

The Late Archaean-Early Proterozoic Transvaal Sequence is preserved within the Transvaal, Kanye and Griqualand West basins, with the 2050 Ma Bushveld Complex intrusive into the upper portion of the succession within the Transvaal basin. Both Transvaal and Bushveld rocks are extensively mineralized, the former containing large deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin ores, and the latter some of the World's major occurrences of PGE, chromium and vanadium ores. Transvaal sedimentation began with thin, predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) which grade up into a thick package of carbonate rocks and BIF (Chuniespoort-Ghaap-Taupone Groups). These lithologies reflect a carbonate-BIF platform sequence which covered much of the Kaapvaal craton, in reaction to thermal subsidence above Ventersdorp-aged rift-related fault systems. An erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups within the three basins, under largely closed-basin conditions. An uppermost predominantly volcanic succession (Rooiberg Group-Loskop Formation) is restricted to the Transvaal basin. A common continental rift setting is thought to have controlled Pretoria Group sedimentation, Rooiberg volcanism and the intrusion of the mafic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The dipping sheets of the Rustenburg magmas cut across the upper Pretoria Group stratigraphy and lifted up the Rooiberg lithologies to form the roof to the complex. Subsequent granitic rocks of the Lebowa and Rashoop Suites of the Bushveld Complex intruded both upper Rustenburg rocks and the Rooiberg felsites.     

New Rb-Sr age determinations on the Archaean basement of Eastern Sierra Leone

The Archean basement of Sierra Leone is a typical example of granite-greenstone terrains found in ancient continental nucleii. Reconnaissance field mapping showed that the area can be subdivided into old gneiss, which predates the greenstone belts, and young granite which is later than the greenstone belts.New Rb-Sr whole-rock age determinations on two suites of old tonalitic gneiss yield ages of 2786 ± 49 Ma and 2770 ± 137 Ma, which either reflect the time of formation of the original tonalites or their metamorphism. Three new Rb-Sr whole-rock age determination on young granites yield ages of 2786 ± 143 Ma, 2780 ± 79 Ma and 2770 ± 50 Ma, which are interpreted as the time of emplacement. The widespread occurrence of similar young granites, throughout the Archaean of West Africa, suggests that these results date a major event in the evolution of this segment of the crust.A published Pb-Pb age of the old gneiss and the new ages of the young granite bracket the age of the greenstone belts to 3000-2770 Ma. However, if the Rb-Sr ages of the old gneiss reported in this paper reflect the time of their formation, the age of the greenstone belts is tightly bracketed to ca. 2770 Ma. There is no isotopic evidence for rocks substantially older than 3000 Ma in the West African Archaean.     

Transvaal Supergroup inliers: geology, tectonic development and relationship with the Bushveld complex, South Africa

Several deformed Transvaal Supergroup inliers occur in the Bushveld complex. The most prominant are the Crocodile River dome and the Rooiberg fragment in the western Transvaal basin and the Dennilton-Marble Hall dome and Stavoren fragment in the eastern Transvaal basin. Several other smaller Transvaal Supergroup inliers are situated in the Bushveld complex to the east and west of the central inliers. The geology and tectonic relationship of these inliers with the Bushveld complex imposed important constraints on the tectonic evolution of the Transvaal basin and the subsequent distribution of the Bushveld complex.The central inliers are subdivided into two groups. The Crocodile River, Marble Hall and Dennilton domes consist of highly deformed, lower Transvaal strata that were subjected to low-grade metamorphism. The domes were formed by interference folding that was accentuated by the intrusion of the Bushveld complex. They acted as physical barriers to the emplacement of the mafic rocks of the Bushveld complex in the centre of the Transvaal basin.The Rooiberg and Stavoren fragments are synforms of upper Transvaal strata. The strara that comprise them are less deformed than those in the domes. These fragments were subjected to low-grade metamorphism because of the intrusion of Bushveld granite beneath them. They acted as roof pendants to the emplacement of the Bushveld complex.Other smaller Transvaal Supergroup inliers in the Transvaal basin are shown to be either attached or detached structures, depending on their tectonic setting and relation to the Bushveld complex.     

Rb–Sr and Sm–Nd isotopic compositions of the Rooiberg Group, South Africa: early Bushveld-related volcanism

The Rooiberg Group is a 6-km-thick sequence of mostly volcanic rocks, which represent the first phase of magmatic activity associated with the Bushveld Complex. These strata include, in ascending stratigraphic order, the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof Formations. Units of the lower Dullstroom Formation range from basalts to andesites and comprise two compositional suites: high Ti and low Ti. Compositional data indicate that melts represented by the overlying, more siliceous volcanic rocks, which include dacites and rhyolites, were derived from low Ti melts by fractional crystallization and assimilation of crustal material (AFC processes).

Rb–Sr isotopic data (28 samples) for units of the Dullstroom and Damwal Formations loosely constrain a crystallization age of 2071+94/−65 Ma (these errors and those below: 95% confidence limits), which agrees with previously reported age data. These data suggest an initial value for ⁸⁷Sr/⁸⁶Sr of 0.70655+0.00087/−0.00051 for the Rooiberg Group. In contrast, Rb–Sr isotopic compositions of six samples of the Kwaggasnek Formation indicate post-crystallization alteration, which was probably associated with the Lebowa Granite Suite of the Bushveld Complex. Sm–Nd isotopic data (29 samples) for volcanic units of the Rooiberg Group provide a poorly constrained age of 1837+360/−320 Ma with an initial value for ¹⁴³Nd/¹⁴⁴Nd of 0.50976+0.00026/−0.00035. These Rb–Sr and Sm–Nd isotopic compositions are similar to those indicated for melts that crystallized to form the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Extruded and intruded melts probably were derived from the same or similar sources and may have resided in the same magma chambers before emplacement.     

Economic potential of the Rooiberg Group: volcanic rocks in the floor and roof of the Bushveld Complex

Volcanic rocks of the Rooiberg Group are preserved in the floor and roof of the mafic Rustenburg Layered Suite of the Bushveld Complex. Field and geochemical characteristics of these volcanic rocks imply that they are genetically related to the Rustenburg Layered Suite. Four major ore-forming events are identified in the Rooiberg Group. The first phase was accompanied by volcanic hosted, fault controlled, hydrothermal copper mineralisation, which is found in the lowermost portion of the Rooiberg Group, underlying the Rustenburg Layered Suite. This type of mineralisation is tentatively linked to initial Rustenburg Layered Suite intrusions. Stratabound arsenic mineralisation that possibly formed in response to contact metamorphism, characterises the second phase, and occurred after extrusion of the Damwal Formation, possibly due to shallow granophyric intrusion. The third mineralising event occurred in response to contact metamorphism during the final stages of the Rustenburg Layered Suite, where especially Pb and Zn were introduced into the felsite roof rocks. This type of mineralisation affected the majority of the Rooiberg Group, but is most pronounced towards the contact with the Rustenburg Layered Suite. The fourth phase is restricted to the Rooiberg Group in the Nylstroom area and is linked to the granite intrusions of the Lebowa Granite Suite, from which Sn and F were introduced into the uppermost felsite succession. Mineralisation in the Rooiberg Group appears to be controlled by the character and intrusion level of the associated Bushveld magmas. Different styles of mineralisation in Rooiberg Group volcanic rocks are encountered at various stratigraphic levels. Major primary volcanogenic ore deposits appear to be absent.     

羌塘基底质疑

部分学者根据西藏1：100万改则幅区调查资料和戈木日，果干加年山浅变质岩中获得的14个锆石Pb-Pb年龄(3204～509Ma），建立了太古宙一元古宙岩石地层系统，提出了“结晶基底”与“软基底”的双层结构模式。资料证实样品取自沉积层，锆石为碎屑锆石，无法确切反映成岩或变质年龄。作者等在原划为下元古界都古尔片麻岩中获得了384Ma的锆石年龄，日湾茶卡东山下石炭统之下火山岩（原划AnD）获得346Ma的Rb-Sr年龄，玛依岗日组玄武岩夹层中获得318Ma的Rb-Sr年龄，野外工作中将“基底岩系”与上覆石英二叠系角度不整合关系厘定为整合关系，基于野外地质立状和同位素年龄资料分析，对羌塘地区基底和双层结构模式提出质疑，认为羌塘中部的浅变质岩系主体时代为晚石炭世。太古宙一元古宙岩石地层，变质期次，双层基底结构及其演化的认识显然是不正确使用同位素年代数据得出的结论。     

Mantle sources and magma evolution of the Rooiberg lavas,Bushveld Large Igneous Province,South Africa

We report a new whole-rock dataset of major and trace element abundances and ⁸⁷Sr/⁸⁶Sr–¹⁴³Nd/¹⁴⁴Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)_N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNd_i (??5.2 to ??9.4) and radiogenic εSr_i (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.     

“Calc-alkaline” magmatism of the Omgon Range: Evidence for Early Paleogene extension in the western Kamchatka segment of the Eurasian continental margin

The hypabyssal rocks of the Omgon Range, western Kamchatka, that intrude Upper Albian-Lower Campanian deposits of the Eurasian continental margin belong to three coeval (62.5–63.0 Ma) associations: (1) ilmenite gabbro-dolerites, (2) titanomagnetite gabbro-dolerites and quartz microdiorites, and (3) porphyritic biotite granites and granite-aplites. The Early Paleocene age of the ilmenite gabbro-dolerites and biotite granites was confirmed by zircon and apatite fission-track dating. The ilmenite and titanomagnetite gabbro-dolerites were produced by the multilevel fractional crystallization of basaltic melts with, respectively, moderate and high Fe-Ti contents and the contamination of these melts with rhyolitic melts of different compositions. The moderate-and high-Fe-Ti basaltic melts were derived from mantle spinel peridotite variably depleted and metasomatized by slab-derived fluid prior to melting. The melts were generated at variable depths and different degrees of melting. The biotite granites and granite aplites were produced by the combined fractional crystallization of a crustal rhyolitic melt and its contamination with terrigenous rocks of the Omgon Group. The rhyolitic melts were likely derived from metabasaltic rocks of suprasubduction nature. The Early Paleocene hypabyssal rocks of the Omgon Range were demonstrated to have been formed in an extensional environment, which dominated in the margin of the Eurasian continent from the Late Cretaceous throughout the Early Paleocene. Extension in the Western Kamchatka segment preceded the origin of the Western Koryakian-Kamchatka (Kinkil’) continental-margin volcanic belt in Eocene time. This research was conducted based on original geological, mineralogical, geochemical, and isotopic (Rb-Sr) data obtained by the authors for the rocks.     

赣南燕山早期双峰式火山—侵入杂岩的Rb—Sr同位素定年及意义

通常认为中国东南大陆中生代时地壳裂解作用发生在燕山晚期。对赣南地区广泛分布的双峰式火山岩和Ａ型花岗岩进行了Ｒｂ－Ｓｒ同位素定年研究,确定基性端元玄武岩的Ｒｂ－Ｓｒ等时线年龄为１７０－１８０Ｍａ,酸性端员流纹岩和Ａ型花岗岩的Ｒｂ－Ｓｒ等时线年龄为１６０－１８０Ｍａ,表明它们形成于中侏罗世早期。由此提出东南太陆中生代时最早的裂解作用应该始于燕山早期。     

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.     

赣东北-浙西北登山群和松木坞群中火山岩的地球化学

赣东北－浙西北新元古代登山群和松木坞群主要由板岩、砂岩、流纹岩、流纹质英安岩、玄武岩和玄武质安山岩组成，火山岩的Ｒｂ－Ｓｒ和Ｓｍ－Ｎｄ等时线年龄以及流纹岩中单颗粒锆石Ｕ－Ｐｂ年龄显示其成岩时代为８００－９００Ｍａ。登山群和松木坞群火山岩呈双峰式产出，其中镁铁质火山岩在化学成分上呈现出演化程度较高的特点（ｅｖｏｌｖｅｄｎａｔｕｒｅ），高强场元素（Ｔｉ、Ｎｂ、Ｔａ等）相对于碱土元素未发生明显亏损，反映其形成于大陆裂谷环境。镁铁质火山岩具低的Ｎｄ值，表明其来源于弱亏损的地幔源，也可能遭受一定程度的地壳混染作用改造。酸性火山岩在化学成分上表现出Ｓ型火山岩特点，但其变化于－１．９～＋２８，指示其为先存的长英质基底分熔作用的产物，但存在强烈的地慢物质添加作用。     

Evidence for synchronous extrusive and intrusive Bushveld magmatism

The intracratonic, 2.06 Ga volcanic rocks of the Rooiberg Group of southern Africa consist of nine magma types, varying in composition from basalt to rhyolite. Basalts and andesites, intercalated with dacites and rhyolites, are found towards the base; rhyolite is the chief magma composition in the upper succession. The absence of compositions intermediate to the magma types and variations in major and trace element concentrations suggest that fractional crystallization was not prominent in controlling magma compositions. REE patterns are comparable for all magma types and concentrations increase for successively younger magmas; LREE show enriched patterns and HREE are flat. Elevated Sr_i-ratios and high concentrations of elements characteristically enriched in the crust suggest that the Rooiberg magmas were crustally contaminated or derived from crustal material. Some Rooiberg features are related to the intrusive events of the Bushveld complex.Petrogenesis of both the Rooiberg Group and the mafic intrusives of the Bushveld complex is linked to a mantle plume, melting at progressively higher crustal levels. The basal Rooiberg magmas have undergone a complex history of partial melting, magma mixing and crustal contamination. Crustal melts extruded as siliceous volcanic flows to form the Upper Rooiberg Group, simultaneously intruding at shallow levels as granophyres. Crustally contaminated plume magma synchronously intruded beneath the Rooiberg Group to produce the mafic rocks of the Rustenburg Layered Suite. Granite intrusions terminated the Bushveld event. The Bushveld plume was short-lived, which conforms, together with other features, with younger, voluminous plume environments.     

Geochronometry of the Middle Paleozoic volcanics of the Omolon Massif in Northeast Asia: A comparison of the K-Ar, Rb-Sr, and U-Pb data and their geological interpretation

The results of the K-Ar, Rb-Sr, and U-Pb (SHRIMP zircon method) dating of the Middle Paleozoic volcanogenic rocks of the Omolon Massif are summarized. It was concluded that they are principally consistent with each other, as well as with the geological data. The formation of the Kedon Group, which makes up the main volume of the Middle Paleozoic volcanics, began at the Early-Middle Devonian boundary about 400 Ma ago (U-Pb dates of 400.5 ± 4.4 and 387 ± 6.4; Rb-Sr isochron age of 402 ± 6 Ma). The isotopic age of the upper boundary of the Kedon Group remains unclear due to disagreements concerning its stratigraphic assignment. The histogram based on the 111 K-Ar dates of the volcanic rocks from the Kedon Group gives a polymodal distribution, which indicates that the K-Ar isotopic system was disturbed by thermal events, which occurred 310–290 (terminal Carboniferous—beginning of the Permian) and 240–220 (Middle-beginning of the Late Triassic) Ma ago. Both thermal events were associated with mantle (ultrabasic-basic) magmatism, which spanned a significantly wider territory than the distribution area of the Kedon Group     

The sedimentary and tectonic setting of the Transvaal Supergroup floor rocks to the Bushveld complex

The Palaeoproterozoic Transvaal Supergroup floor to the Bushveld complex comprises protobasinal successions overlain by the Black Reef Formation, Chuniespoort Group and the uppermost Pretoria Group. The protobasinal successions comprise predominantly mafic lavas and pyroclastic rocks, immature alluvial-fluvial braidplain deposits and finer-grained basinal rocks. These thick, laterally restricted protobasinal sequences reflect either strike-slip or small extensional basins formed during the impactogenal rifting and southeasterly-directed tectonic escape, which accompanied collision of the Zimbabwe and Kaapvaal cratons during Ventersdorp times. The erosively-based sheet sandstones of the succeeding Black Reef Formation reflect northwand-directed compression in the south of the basin. Thermal subsidence along the Ventersdorp Supergroup and Transvaal protobasinal fault systems led to shallow epeiric marine deposition of the sheet-like Chuniespoort Group carbonate-BIF platform succession. After an estimated 80 Ma hiatus, characterized by uplift and karstic weathering of the Chuniespoort dolomites, slower thermal subsidence is thought to have formed the Pretoria Group basin. Widespread, closed basin alluvial fan, fluvial braidplain and lacustrine sedimentation, as well as laterally extensive, subaerial andesitic volcanism (Rooihoogte to Strubenkop Formations), gave way to a marine transgression, which laid down the tuffaceous mudrocks, relatively mature sandstones and subordinate subaqueous volcanic rocks of the succeeding Daspoort, Silverton and Magaliesberg Formations. Poorly preserved post-Magaliesberg formations in the Upper Pretoria Group point to possible compressive deformation and concomitant rapid deposition of largely feldspathic detritus within smaller closed basins.     

On the relationships between the Bushveld Complex and its felsic roof rocks, part 1: petrogenesis of Rooiberg and related felsites

A major question concerning the Bushveld Complex is the relationship between the layered mafic rocks and the overlying Rooiberg Group felsites and related granophyres. Here, we assemble bulk-rock analyses to gain insight into this question and investigate the petrogenesis of the felsic rocks. The data indicate that the Rooiberg Group consists of distinctive magnesian and ferroan lavas. The former dominates the basalts to rhyolites of the basal Dullstroom Formation, while nearly all the dacites to rhyolites of the overlying Damwal, Kwaggasnek, and Schrikkloof Formations are ferroan. The ferroan rocks also include the Stavoren Granophyre, which exists regionally as a several-hundred-meter-thick concordant sheet between the Bushveld Complex and Rooiberg lavas. The compositions of the magnesian lavas are similar to calc-alkaline granitoids found in convergent margins, suggesting that the lavas could have originated in a mantle affected by previous Archean subduction events that are recorded by xenoliths and inclusions in diamonds from most Kaapvaal kimberlites. In contrast, the compositions of the ferroan lavas indicate formation by fractional crystallization of basaltic liquids and are essentially identical to ferroan rhyolites associated with mafic rocks from other settings. The hypothesis that these rocks are fractional crystallization products of Bushveld mafic liquids is consistent with published radiogenic and stable isotope data and known age relations. Based on compositional characteristics and geologic relations, the Stavoren Granophyre is the most likely candidate for the residual liquid that escaped from the top of the Bushveld Complex. Whether the bulk of the Bushveld Province ferroan rhyolites formed in the chamber of the extant layered mafic sequence or in a deeper, hidden crustal magma reservoir remains unclear.     

The Bushveld granites around Rooiberg,Transvaal, South Africa

The detailed investigation of the Bushveld granites, around the Rooiberg area, has revealed the existence, in the field, of at least three main types:

1. Granophyric rocks, which form a belt (transition-zone) between the country rocks (sediments and felsite) and the Main granite.
2. Main granite, which is the most common type of granite.
3. Younger granites, which are responsible for the tin mineralization.

All evidence points to a metasomatic origin for the granophyric rocks. For the Main granite, a probable anatectic origin is proposed, which is borne out by the presence of ghost-stratigraphy patterns, detected by trend surface analysis. The younger granites are undoubtely magmatic. On account of parallelism between the structural lines of the granitic rocks and the country rocks, an hypothesis based on deformation contemporaneous with the emplacement of the granites is advanced to explain the structure of the Rooiberg Area. In view of this hypothesis, the “Rooiberg roof-pendant” is considered to be probably a portion of the roof in its normale position.     

电气石——成岩成矿作用的灵敏示踪剂

本文详细论述了电气石的化学组成（主化学元素、微量元素和稀土元素）和同位素组成Ｈ、Ｏ、Ｂ、Ｓｉ、Ｒｂ／Ｓｒ、Ｓｍ／Ｎｄ、ｐｂ／ｐｂ和Ｋ／Ａｒ）特征,提出电气石可以作为研究成岩成矿作用的一个灵敏示踪剂。利用电气石的主化学元素和微量、稀土元素组成,可以有效指示其形成的物理化学环境,还可以作为一种有用的找矿标志,指标矿化的存在,并可能用于区分矿化类型和矿体规模。电气石的稳定同位素和放射性同位素研究,已经用     

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.     

U-Th-Pb systematics in hydrothermally altered granites from the Granite Mountains,Wyoming

U-Th-Pb systematics were investigated in 15 samples representing two types of deuterically altered Archean granite (albitized and silicified-epidotized granite) from the Granite Mountains, Wyoming. The loss of K-feldspar during both types of deuteric alteration was accompanied by an extreme reduction of Pb content from roughly 40 ppm to less than 12 ppm in the most altered samples. Nine of the 15 samples yield anomalously young whole-rock Pb-Pb and Th-Pb ages compared to concordia ages for zircons and to whole-rock Pb-Pb and Th-Pb ages for samples of unaltered granite. The young ages are interpreted to be the result of radiogenic Pb loss during a middle Proterozoic metamorphism that disturbed several isotopic systems in the unaltered granite. The loss of radiogenic Pb from the whole-rock systems of the deuterically altered granites is most likely due to the absence of microcline. In many granitic rocks, potassium feldspar tends to act as a receptor for Pb that has been mobilized, and this effect may account for the closed-system behavior of Pb in whole-rock samples of the unaltered granite. It may also account for the apparent gain of radiogenic Pb during the Proterozoic by one sample which was collected at the edge of an alteration zone. The deuterically altered granites may have also lost U during the Proterozoic, but, except for two samples, the dominant U loss occurred relatively recently, probably during Tertiary uplift and erosion.The low common-Pb contents and rather high U and Th contents of the hydrothermally altered granites might seem to indicate that these rocks are well suited for geochronologic studies in the U-Th-Pb system because such rocks should have a high percentage of radiogenic Pb. Unfortunately, open-system behavior in response to post-crystallization metamorphism shows that apparent ages for these types of rocks must be interpreted with caution.     

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。