Rb-Sr ages of Precambrian dolerite and alkaline dikes,southeast Mysore state,India

Rb-Sr isochron ages have been determined for two suites of Precambrian dikes in the Bidadi-Harohalli area of southeast Mysore State. Whole-rock samples of unmetamorphosed dolerites yield an age of 2420±246 (2σ) m.y., which is a minimum value for the intruded Peninsular Gneiss and Closepet Granite. The dolerite magma originated in the mantle, as indicated by the initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7012±0.0010 (2σ). A suite of alkaline dikes, also referred to as felsite and feldspar porphyry dikes, has an age of 832±40 (2σ) m.y., which correlates with the intrusion of the Chamundi Hill Granite and the feldspar porphyry dikes near Srirangapatnam. One of the alkaline dikes has a K-Ar age of 810±25 m.y., indicating an absence of subsequent thermal events in the area.     

Rb/Sr geochronology of granites and gneisses from the Mount Everest region,Nepal Himalaya

Nineteen samples of granites, orthogneisses and paragneisses from the High Himalaya basement nappe(s) of the Mount Everest region have been dated by the Rb/Sr method. The post-metamorphic tourmaline leucogranites of the upper Imja Drangka (Nuptse, Lhotse Glacier) have high initial Sr⁸⁷/Sr⁸⁶ characteristic of an anatectic origin from crustal material. A whole-rock isochron age of 52 m. y. (Early Eocene) has been obtained for the samples from the granite body of Lhotse Glacier; apparently Sr isotopic homogenization was not reached throughout the much larger Nuptse granite. The granite precursor of the migmatitic orthogneisses from the upper Dudh Kosi valley has an age of 550 ± 16 m. y. (whole rock isochron) and a high initial Sr⁸⁷/Sr⁸⁶ ratio indicating its origin from an older basement complex. The Rb/Sr data on paragneisses from the south face of Lhotse do not define an isochron, possibly reflecting isotopic hetero-geneity in the sedimentary protoliths and incomplete homogenization during a late Precambrian metamorphism. All the mineral ages fall in the time span from 15 to 17 m. y. They represent cooling ages reflecting a regional phase of major uplift in the Middle Miocene and post-dating the peak of the Himalayan metamorphism which the data from the Mt. Everest region place in pre-Eocene times.     

Pb isotopic dating of some Precambrian rocks from North China—An ensuing discussion on Precambrian Geochronological Scale of China

Pb isotopic ages of some Precambrian rocks from North China were determined. New data from the Chuanlinggou Formation of the Great Wall Group in the Yanshan region, together with published data, show that their whole-rock Pb-Pb isochron age is 1,922±92 (δ) m.y. Thus, 2,000±100 m.y. as the lowest age of the Sinian Geochronological Scale of China is important. In view of the fact that the whole-rock Pb-Pb isochron age from the Liaohe Group in Liaoning Province is estimated to be 1,977±49 (δ) m.y., which is in agreement with the possible maximum deposition age calculated from Rb?Sr data, we prefer to consider the Liaohe Group to be Sinian rather than pre-Sinian in age. Zircon, apatite and other minerals from the Tiejiashan granitic gneiss at Anshan, Liaoning Province, lie on two U-Pb discordant lines with ca. 3,300 and 2,800 m.y. respectively. Zircon, on the discordia with 3,300 m.y., is non-magnetic and free from dark inclusions. Apatite with common Pb isotopic composition, lying on the line of older age, is close to the concordia, indicating that it has not undergone any geological thermal events since it was formed. Zircon and apatite with inclusions and magnetism lie on the discordia of younger age. A U-Pb age of ca. 3,300 m.y. for the Tiejiashan granitic gneiss seems to be possible. However, it is necessary to examine the ages of these rocks with Rb?Sr and other methods in the future. A precursory Precambrian Geochronological Scale of China is propsed based on the Sinian Geochronological Scale of China published in 1977^[1] as well as on new data from this study.     

Geology and geochronology of the Arunta complex north of Ormiston Gorge,Central Australia

A major west‐trending lineament marked by a wide belt of highly deformed rocks (the Redbank Zone), lies in the Arunta Complex, north of the Amadeus Basin. Along its southern margin the Zone has been progressively affected by, and is hence older than, migmatization and granite intrusion. The migmatization event yields a Rb‐Sr isochron age of 1076 ± 50 m.y. Within the migmatite complex, relicts of a pre‐migmatite metasedimentary sequence around the Chewings Range yield a Rb‐Sr isochron age of 1620 ± 70 m.y. The migmatites are unconformably overlain by the basal unit of the Amadeus Basin sequence, the Heavitree Quartzite. The 1076 ± 50 m.y. date thus provides a maximum age for the start of sedimentation along the northern margin of the Basin. The existence of a major zone of weakness in the basement probably exerted a strong control on basement and cover deformation during the Palaeozoic Alice Springs Orogeny.     

Rb−Sr age of Gaik granite,Ladakh batholith,northwest Himalaya

The Gaik Granite is a part of the Ladakh batholith outcropping between Gaik and Kiari in NW Himalaya. This is a pink porphyritic granite rich in biotite and poor in hornblende. Rb-Sr analyses have been made on six whole-rock samples of the Gaik Granite. Though the samples are poorly enriched in radiogenic Sr, they define a reliable isochron corresponding to an age of 235±13 (2σ) m.y. and initial⁸⁷Sr/⁸⁶Sr ratio of 0·7081±0·0004 (2σ). Biotite, plagioclase and potash feldspar fractions separated from two of the samples have yielded a much younger mineral isochron at 30±1·5 m.y. indicating a nearly complete redistribution of Sr isotopes between mineral phases at a time much later than the primary emplacement of the granite. The present results show that at least some components of the Ladakh batholith are of Permo-Triassic age. These rocks were isotopically re-equilibrated on a mineral scale during Upper Oligocene in response to the Himalayan orogeny.     

Rubidium‐strontium geochronology of the encounter bay granite and adjacent Metasedimentary rocks,South Australia

Rubidium‐strontium and strontium isotope data for eight whole‐rock samples of granite varieties from the Encounter Bay area, South Australia, yield an isochron age of 487 ± 37 m.y. Two specimens of albitised granite, formed as a result of late‐stage metasomatic alteration of original megacrystic granite, conform to this isochron. These data support a genetic relation between granites and late‐stage metasomatic alteration as suspected from field, petrographical and geochemical studies. Eight samples from contiguous Kanmantoo Group metasedimentary rocks have an isochron age of 487 ± 60 m.y. Thus this metamorphic event is coincident with emplacement of the Encounter Bay Granite.

The initial Sr⁸⁷Sr⁸⁶ ratio for the Encounter Bay Granite (0.719) is significantly higher than initial ratios for the Palmer (0.709) and Anabama (0.705) Granites from the same region and can be attributed to either remobilisation or incorporation of strontium from older crustal material in the intrusion. The apparent initial Sr⁸⁷/Sr⁸⁶ ratio for the Kanmantoo Group metasedimentary rocks (0.722) can not be distinguished from that for the Encounter Bay Granite within the analytical uncertainties. Compatability of ages and high initial Sr⁸⁷Sr⁸⁶ ratios suggest that the granites formed by remobilisation of associated crustal rock.     

Rb-Sr age of Godhra and related granites,Gujarat, India

Rubidium and strontium determinations are reported for Godhra and geographically related granites from central Gujarat. The whole rock data define a Rb-Sr isochron corresponding to a common age of 955±20 m.y. and initial Sr ratio of 0·7130±0·001. This age is distinctly older than the age of 735 m.y. reported for the Erinpura suite of rocks from Mount Abu in western Rajasthan and from Idar in northern Gujarat. There are at least two generations of post-Delhi intrusive rocks in the Gujarat precambrian. Biotites associated with these granites have the same age as the whole-rocks within experimental error indicating the absence of significant metamorphic heating since the time of emplacement. It is significant that rocks of similar age occur in the Rajasthan Precambrian mainly in the axial zone of the Aravalli Mountains.     

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.     

Sm-Nd, Pb-Pb and Rb-Sr Isotopic Dating and Its Dynamic Implications for the Proterozoic Augen Granite in the Yunkai Area, Western Guangdong Province

The samples of ductile-rheologic deformational augen granite from the Yunkai uplift area, western Guangdong province, were determined by the whole-rock Sm-Nd, Pb-Pb and Rb-Sr isotopic dating to have an Sm-Nd isochron age of 1414±68 Ma, a Pb-Pb isochron age of 1388±90 Ma and a Rb-Sr isochron age of 490±36 Ma. The first two ages are interpreted as the formation age of this suite of granite and the last age represents the timing of the tec-tono-thermal event of Caledonian ductile-rheologic shear partial melting. It is indicated that in the study area not only an orogeny took place in the Caledonian, but also a more important tectono-magmatic activity occurred in the Meso-proterozoic there , which may be related to the subduction-collision between the Yangtze block and Cathaysia block.     

Rb/1bSr isochrons for the granitic plutons around Farsund,southern Norway

The granitic and charnockitic intrusions in the Precambrian migmatites around Farsund in southern Norway have previously been shown to be successively intruded as well as being geochemically different. Rb- and Sr-isotope data support the conclusion that they are intruded with a separate time interval and that the Farsund charnockite is younger (852±41 m.y.) than the Lyngdal hornblende granite (932±38 m.y.). Furthermore, the different initial ratios (0.7128 and 0.7054, respectively) disprove a comagmatic origin unless large-scale contamination has occurred.     

Isotopic evidence of middle proterozoic magmatism from bombay high field: Implications to crustal evolution of western offshore of India

Precambrian granitic basement rocks obtained from well BH-36 of Bombay High Field, western offshore of India has been studied both by Rb-Sr and K-Ar dating methods. Seven basement samples chosen from two cores have yielded whole rock Rb-Sr isochron age of 1446 ± 67 Ma with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7062 ± 0.0012. This age has been interpreted as the formation/emplacement time of the granite. Two biotite fractions of different grain size separated from a sample CC6B2T have yielded Rb-Sr mineral isochron age of 1385 ± 21 Ma. However, these fractions when studied by K-Ar dating method have yielded slightly higher but mutually consistent ages of 1458 ± 43 Ma and 1465 ± 43 Ma, respectively. Further, two biotites separated from additional samples CC5B9T and CC6B3B have yielded K-Ar ages of 1452 ± 42 Ma and 1425 ± 40 Ma with an overall mean age of 1438 ± 19 Ma. This mean K-Ar age is indistinguishable from whole rock Rb-Sr isochron as well as mineral isochron age within experimental error. The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement/formation around 1450 Ma ago. The present study provides the evidence for the existence of an important Middle Proterozoic magmatic event around 1400-1450 Ma on the western offshore of India which, hitherto, was thought to be mainly confined to the eastern Ghats, Satpura and Delhi fold belt of India. This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of western Indian shield.     

Geological Characteristics of the Furong Tin Orefield, Hunan,~(40)Ar-~(39)Ar Dating of Tin Ores and Related Granite and Its Geodynamic Significance for Rock and Ore Formation

Furong, Hunan, is a large tin orefield discovered in China in recent years, which is mainly of the skarn-greisen-chlorite type. On the basis of the geological characteristics of the orefield, 40Ar-39Ar dating was performed on muscovite from greisen-type tin ore and biotite from related amphibole-biotite granite, which yielded three sets of age data, i.e., a plateau age of 157.5±0.3 Ma and an isochron age of 156.9±3 Ma for amphibole-biotite granite; a plateau age of 156.1±0.4 Ma and an isochron age of 155.7±1.7 Ma for the Sanmen greisen-type tin ore; and a plateau age of 160.1±0.9 Ma and an isochron age of 157.5±1.5 Ma for the Taoxiwo greisen-type tin ore. The three sets of age data coincide well with each other. They not only accurately reflect the timing of rock and ore formation but also indicate close relations between granite and tin deposits. In addition, the plateau ages of all three sets suggest that no subsequent thermal perturbation event occurred after the formation of granite and tin dep     

The rubidium‐strontium ages of some Tasmanian igneous rocks

Rb‐Sr isotopic age measurements relate emplacement of the Pieman and Meredith Granites (356 ± 9 and 353 ± 7 m.y., respectively), and the Bischoff and Renison Bell Porphyries (349 ±4 and 355 ± 4 m.y., respectively) to the Tabberabberan Orogeny. The genetic relationship of the Bischoff Porphyry to mineralization and the agreement between the age of this porphyry and the age of the adjacent Meredith Granite, strongly suggests that the Bischoff mineralization resulted from granite intrusion.

The Pieman Granite is closely similar to the white Heemskirk Granite in displaying a high initial ⁸⁷Sr/⁸⁶Sr ratio (0.7354 ± 0.0018), feldspar discordance patterns and open system total‐rock behaviour. The high initial ⁸⁷Sr/⁸⁶Sr ratio is attributed to contamination during intrusion by Precambrian metasediments containing appreciable radiogenic strontium.

A basic intrusion (McIvors Hill Gabbro) gave a pre‐Tabberabberan age (518 ±133 m.y.) and a high initial ⁸⁷Sr/⁸⁶Sr ratio (0.7132 ± 0.0031).     

湖南金船塘锡铋矿床辉钼矿Re-Os同位素测年及其地质意义

湖南东坡矿田位于南岭成矿带的西段,构造位置上处于扬子板块与华夏板块的对接地带,矿田内以千里山岩体为中心,发育一系列与燕山期花岗质岩浆作用有关的超大型、大型和中型钨锡钼铋多金属矿床。金船塘锡铋矿床是东坡矿田内一个以锡铋为主的大型矽卡岩型多金属矿床。本文对该矿床的矽卡岩型矿石中的辉钼矿进行了Re-Os同位素测年。结果显示,辉钼矿Re-Os同位素模式年龄范围为157.2±2.8Ma至162.4±2.4Ma,加权平均值为159.8±2.9Ma,对应的Re-Os等时线年龄为158.8±6.6Ma;这些年龄数据与柿竹园矿床辉钼矿的Re-Os等时线年龄(151.0±3.5Ma)在误差范围内基本一致,亦与区内千里山岩体锆石U-Pb年龄(152±2Ma)接近,指示金船塘Sn-Bi矿床与区内花岗岩具有密切的时间和成因关系。结合区域上已有的研究成果,包括金船塘矿床在内的东坡矿田的成岩成矿作用主要集中在149～161Ma,与南岭地区大规模的钨锡多金属成矿作用时限(150～160Ma)一致;另外,区域上的研究表明,幔源物质广泛参与了湘南钨锡矿集区晚中生代的成岩成矿作用,指示该区中-晚侏罗世爆发式的成岩成矿作用可能是区域地壳拉张-岩石圈伸展减薄背景下,强烈的壳幔相互作用的结果。     

西秦岭略阳地区鱼洞子杂岩变形花岗岩锆石LA-ICP-MS U-Pb测年及地质意义

鱼洞子杂岩是秦岭地区最古老的复杂的地质体。前人对鱼洞子杂岩不同的地质体采用多种方法进行了年龄测定,但结果差异较大,且总体测试精度不高。依据锆石CL影像的特征,结合锆石成因分析和锆石微区的U-Pb同位素,进行了LA-ICP-MS测定,在鱼洞子杂岩糜棱岩化细粒黑云母花岗岩和强片理化黑云母花岗岩中分别获得2661Ma±17Ma和2703Ma±26Ma的岩浆结晶年龄,同时在糜棱岩化细粒黑云母花岗岩中获得2647Ma±65Ma的变质锆石年龄。其中2703Ma±26Ma是秦岭造山带目前发现的最古老的侵入岩的形成年龄,为研究秦岭造山带早前寒武纪构造岩浆事件和地球早期的形成演化提供了新的资料。     

论河北丰宁牛圈银(金)矿床的成矿时限问题

对河北牛圈银(金)矿区细粒钾长花岗岩株和含矿隐爆角砾岩墙钾长石样品进行了详细的40Ar/39Ar同位素年龄测定。细粒钾长花岗岩株钾长石样品主要加热阶段所获得数据构成一条相关性很好的直线,其对应的同位素等时线年龄值为(216.6±3.2)Ma(2σ),40Ar/39Ar初始比值为297±28。相比之下,含矿隐爆角砾岩墙钾长石样品特定加热阶段(1200～1300℃和1360～1450℃)所获数据同样也构成两条直线,其对应的同位素等到时线年龄值分别为(227.9±4.8)Ma(2σ)和(216.7±6.1)Ma(2σ),40Ar/39Ar初始比值分别为307±41和269±39。考虑到所测样品均采自未受到明显后期构造-岩浆活动或其他热事件影响的岩(矿)体,因此,细粒钾长花岗岩株和含矿隐爆角砾岩墙的形成时代为216Ma,与印支期富钾岩浆作用具有密切成因联系。根据含矿隐爆角砾岩墙的野外地质证据,结合矿物共生组合关系和钾长石同位素年龄数值,可以推测牛圈银(金)矿区早期成矿作用发生在(227.9±4.8)Ma,而主要成矿期为(216.7±6.1)Ma,是印支期多阶段富碱质成矿流体活动的产物。牛圈银(金)矿区成矿作用时间的厘定对于提高该区成矿理论研究水平和寻找隐伏金属矿床均具重要意义。     

Early Paleozoic Magmatism and Gold Mineralization in the Northern Aitun, NW China

Abstract This paper discusses the relationships between granitic magmatism and gold mineralization and the exhumation history of the Dapinggou gold deposit in northern Altun, NW China based on the geochronological data, including zircon U‐Pb ages, Rb‐Sr isochron age and ⁴⁰Ar‐³⁹Ar dating and MDD modeling data. The main granitic magmatism age in this area is attained from the ID TIMS U‐Pb geochronology of zircons from the Kuoshibulak granite, the biggest granite in the northern Altun area, which gives a concordant age of 443±5 Ma in the Late Ordovician. Zircon ID TIMS U‐Pb geochronology of the West Dapinggou biotite granite west of the Dapinggou gold deposit gives concordant ages around 485±10 Ma, representing the early stage of Ordovician magmatism. The Rb‐Sr isochron age (487±21 Ma) of 6 quartz inclusion samples from quartz veins in this gold deposit is very close to that of the West Dapinggou granite. MDD modeling of step heating ⁴⁰Ar‐³⁹Ar data of K‐feldspar from the same West Dapinggou biotite granite gives a rapid cooling history from 300°C to 150°C during 200–185 Ma. According to the age data and the geological setting of this area, we conclude that the Dapinggou gold deposit was formed at the early stage of the Early Paleozoic granitic magmatism in northern Altun, and exhumed in the Early Jurassic due to the normal faulting of the Lapeiquan detachment. The Early Paleozoic magmatism may provide heat source and produce geological fluids, which are very important for gold mineralization. Exhumation in the Mesozoic caused the uplift of the deposit towards the ground surface.     

内蒙古沙麦钨矿白云母40Ar/39Ar年龄及其地质意义

内蒙古沙麦钨矿床位于中亚造山带东段（或称兴蒙造山带）,矿体主要赋存在黑云母二长花岗（斑）岩内,主要矿化类型为伟晶岩型和云英岩型。本文利用40Ar/39Ar同位素测年方法对沙麦钨矿成矿阶段形成的白云母进行了年龄测定,获得白云母Ar-Ar坪年龄为138.4 ± 0.84 Ma,对应的正、反等时线年龄分别为137.32 ± 0.73 Ma和137.35 ± 0.73 Ma。所测坪年龄与正反等时线年龄具有很好的一致性,可以代表矿床钨矿体的形成年龄,表明该矿床的形成与沙麦地区燕山晚期的岩浆活动有关,这与区域上的成矿事件相吻合。结合区域地球动力学背景的研究成果,认为沙麦钨矿床形成于陆-陆碰撞造山后的陆内伸展环境。     

Zircon U-Pb ages of the Franzfontein granitic suite,northern South West Africa

U-Pb isotopic data are presented for composite and size fractions of zircons from 15 samples of the Franzfontein granitic suite. These data reveal two distinctly different discordance trends that yield concordia intercept ages of 1730±30 m.y. and 1870±30 m.y. that are believed to encompass the age of emplacement of the suite. The previously published Rb-Sr isochron age of 1580±20 m.y. is now interpreted as recording a time of Rb and/or Sr migration through the system. The stratigraphic implications of the new zircon data are discussed.     

内蒙古乌拉特后旗查干花钼矿床地质特征及Re-Os测年

查干花钼矿是近年来在内蒙古西部中蒙边境地区新发现的一个具大型规模隐伏矿床,区内辉钼矿化呈浸染状、团块状、细脉状分布于中细粒二长花岗岩与古元古界宝音图群地层北东接触带部位,具斑岩型矿床特点.本文在对矿区矿化和蚀变特征初步研究的基础上,开展了辉钼矿Re-Os测年,获得模式年龄为(239.3±3.3)～(242.6±3.6)...