U-Pb zircon,monazite and Rb-Sr whole rock systematics of granitic gneiss and psammitic to semi-pelitic host gneiss from Glenfinnan,Northwestern Scotland

New U-Pb zircon data from a segregation pegmatite in the granitic gneiss at Glenfinnan yield discordant points which appear to be aligned along a chord on a concordia diagram with upper and lower intersection ages of 1,517±30 Ma and 556±8 Ma, respectively. The results are similar to published U-Pb zircon data from the granitic gneiss but the lower intersection age does not correspond to concordant ages of 455±3 Ma obtained for monazites from the segregation pegmatite and from paragneiss which hosts the granitic gneiss. The apparent U-Pb zircon chord also gives no indication of a 1,030±50 Ma (large sample) Rb-Sr whole rock isochron age for the granitic gneiss (Brook et al. 1976). A traverse of adjacent 5–8 cm thick slabs in the paragneiss yields a Rb-Sr errochron of 455±60 Ma which also does not agree with the U-Pb zircon lower intersection age. The scale of this Sr whole rock diffusion (ca. 10 cm) is not at variance with existing thermal, temporal and experimental constraints.A two episodic loss model has been applied to the zircon data from the segregation pegmatite, to the previously published zircon data on the granitic gneiss and to new U-Pb zircon data on the host paragneiss. The first lead loss event, if assumed to be in Grenville time, was computed to be strongest in the granitic gneiss and segregation pegmatite. For the three suites of zircon considered, primary ages converge in the 1,700–1,800 Ma range with a final disturbance event at ca. 490 Ma, i.e., close to a plausible prograde stage of Caledonian metamorphism.The zircons in both the granitic gneiss and the paragneiss are believed to have been derived from the ubiquitous early Proterozoic shields bordering the North Atlantic. Furthermore the above model is consistent with the hypothesis that the zircons in the granitic gneiss were largely derived from the paragneiss. However, the U-Pb zircon data are not inconsistent with new Sr-isotopic evidence which suggests an additional, possibly deeper source with lower ⁸⁷Sr/ ⁸⁶Sr ratios.     

Metamorphic Evolution of Garnet-Epidote-Biotite Gneiss from the Moine Supergroup, Scotland, and Geotectonic Implications

Metapelitic gneisses from the Glenfinnan Group of the MoineSupergroup, Scotland, contain sparse large and numerous smallgarnets, associated with complex zoned epidote and plagioclasein a biotite matrix. The large garnets show four zones (AI–AIV),whereas the small garnets show three or fewer zones, indicatingsuccessive garnet nucleation with increasing nucleation densities.Garnet zones AI and AIV grew under static conditions, whereasthe formation of AII and AIII was accompanied by deformation.Garnet zones AI and AII were formed in the assemblage (all +biotite + epidote + plagioclase + quartz + fluid + apatite)garnet + chlorite + muscovite ± ilmenite ± sphene± magnetite; zone AIII in the assemblage garnet + muscovite+ sphene ± magnetite; and zone AIV in the assemblagegarnet + sphene ± ilmenite. The chemical zonation andmicrostructures of garnet A indicate two important discontinuities;one at the transition between garnet zones AI and AII, and asecond between zones AII and AIII, which correlate with complexzonation shown by epidote and plagioclase. These discontinuitiesmay result from polymetamorphic garnet growth during differentorogenic cycles affecting the Moine Supergroup. Geothermobarometriccalculations and Gibbs method modelling provide evidence thatgarnet zone AI grew rapidly during heating from about 550 to560°C at pressures of about 4–6 kbar. In contrast,the formation of zone AII was accompanied by nearly isothermalcompression from 6 to 8·5 kbar (560 575°C), indicatingcrustal stacking. After a certain period of cooling, garnetzone AIII grew during renewed heating at P–T conditionsof about 640°C and pressures between 5 and 9 kbar. Growthof garnet AIV was accompanied by further temperature rise, reachingmaximum conditions of about 670°C at 5 kbar. KEY WORDS: epidote; garnet; Gibbs method; Moine Supergroup; P–T path     

内蒙古北山地区发现新元古代片麻状花岗岩:锆石U-Pb定年证据

<正>1研究目的(Objective)北山造山带位于中亚造山带最南缘,经历了复杂的地质演化历史。北山造山带存在两个古老地块为敦煌地块和明水—旱山地块。在敦煌地块之上的柳园地区,梅华林等曾发现有榴辉岩和新元古代花岗质片麻岩。而对于明水—旱山地块,前人鲜有报道存在有新元古代的花岗岩。本次对于明水—旱山地块之上片麻状花岗岩年代学的研究,将为北山造山带北带是否存在新元古代片麻状花岗岩、同时确立该区构造演化历史提供新的基础资料。     

Three metamorphic events recorded in a single garnet: Integrated phase modelling,in situ LA‐ICPMS and SIMS geochronology from the Moine Supergroup,NW Scotland

In situ LA‐ICP‐MS monazite geochronology from a garnet‐bearing diatexite within the Moine Supergroup (Glenfinnan Group) NW Scotland records three temporally distinct metamorphic events within a single garnet porphyroblast. The initial growth of garnet occurred in the interval c. 825–780 Ma, as recorded by monazite inclusions located in the garnet core. Modelled P–T conditions based on the preserved garnet core composition indicate an initially comparatively high geothermal gradient regime and peak conditions of ～650 °C and 7 kbar. Monazite within a compositionally distinct second shell of garnet has an age of 724 ± 6 Ma. This is indistinguishable from a SIMS age of 725 ± 4 Ma obtained from metamorphic zircon in the sample, which is interpreted to record the timing of migmatization. This second stage of garnet growth occurred on a P–T path from ～6 kbar and 650 °C rising to ～9 kbar and 700 °C, with the peak conditions associated with partial melting. A third garnet zone which forms the rim contains monazite with an age of 464 ± 3 Ma. Monazite in the surrounding matrix has an age of 462 ± 2 Ma. This corresponds well with a U–Pb SIMS zircon age of 463 ± 4 Ma obtained from a deformed pegmatite that was emplaced during widespread folding and reworking of the migmatite fabric. The P–T conditions associated with the final phase of garnet growth were ～7 kbar and 650 °C. The monazite ages coupled with the phase relations modelled from this multistage garnet indicate that it records two Neoproterozoic tectonothermal events as well as the widespread Ordovician Grampian event associated with Caledonian orogenesis. Thus, this single garnet records much of the Neoproterozoic to Ordovician thermal history in NW Scotland, and highlights the long history of porphyroblast growth that can be revealed by in situ isotopic dating and associated P–T modelling. This approach has the potential to reveal much of the thermal architecture of Neoproterozoic events within the Moine Supergroup, despite intense Caledonian reworking, if suitable textural and mineralogical relationships can be indentified elsewhere.     

U-Pb ages and morphologies of zircon in microgranitoid enclaves and peraluminous host granite: evidence for magma mingling

 Granites of the S-type Wilson's Promontory Batholith (Lachlan Fold Belt, Australia) contain zircons which are euhedral and relatively large; their age is 395 Ma, which can be considered as the best available estimate of the crystallysation age of the granites. Contrary to their dominance in other S-type granites of the Lachlan Fold Belt, very few zircon cores give inherited ages, varying between 500 and 1700 Ma. Microgranitoid enclaves contained within the granites contain a zircon population that is dominated by relatively small, anhedral or elongated crystals. These give ages that are indistinguishable from the crystallisation age of the granite. Some enclaves, which are characterised by the presence of megacrysts, contain a proportion of larger, euhedral zircons. These zircons give inherited ages similar to the zircons from the granitic host rocks. The data are in agreement with a magma mingling origin for the microgranitoid enclaves. The large euhedral zircons are interpreted to have been introduced into the “enclave magma” during a hybridisation event which also introduced quartz and plagioclase megacrysts into the magma. The relatively high proportion of inherited cores within the “large” zircon population of the enclaves is related to the timing of mixing between “enclave” and host magma. This mixing event took place before the majority of the magmatic zircons crystallised in the granitic magma. The small, anhedral zircons within the enclaves crystallised during quenching of the globules of enclave magma against the cooler granitic magma. Received: 21 August 1995 / Accepted: 9 October 1995     

聂荣微陆块花岗片麻岩锆石LA-ICP-MS U-Pb定年——新元古代基底岩石的发现及其意义

聂荣微陆块呈透镜体状夹持在班公湖-怒江板块缝合带内,其上出露有黑云母花岗片麻岩以及二长花岗片麻岩等不同类型的古老片麻岩,本文报道产出于聂荣微陆块上花岗片麻岩的锆石LA-ICP-MS U-Pb定年结果,样品锆石的岩浆震荡环带十分发育,多数锆石具有典型的岩浆结晶锆石的特征。所有测点中有16个测点的Th/U值较高,介于0.58~1.24之间,平均值为0.79,获得的²⁰⁶Pb/²³⁸U年龄基本一致,加权平均值为819.6±5.2Ma,笔者等认为该年龄代表了花岗片麻岩的原岩结晶时代为新元古代。我们在羌塘南部地区的碎屑岩中获得了同样的年龄峰值(809Ma)。此外,我们认为年龄中具有507Ma左右的信息反映了泛非事件之后另外一次强烈构造热事件的影响在该区的存在。     

广西桂林地区东部栗木花岗岩体SHRIMP锆石U-Pb年龄

栗木矿区位于南岭钨锡多金属成矿带的中段,成矿作用与花岗岩密切相关。通过对栗木金竹源与三个黄牛地区云英岩化花岗岩的年代学研究,获得2个SHRIMP锆石U-Pb年龄分别为218.3Ma±2.4Ma和214.0Ma±5.0Ma,代表了栗木花岗岩体的形成年龄。结合前人的研究数据,栗木矿区成岩、成矿作用年龄近乎一致,应为晚三叠世同一期次岩浆作用的产物。     

广西桂林地区东部栗木花岗岩体SHRIMP锆石U-Pb年龄

栗木矿区位于南岭钨锡多金属成矿带的中段,成矿作用与花岗岩密切相关。通过对栗木金竹源与三个黄牛地区云英岩化花岗岩的年代学研究,获得2个SHRIMP锆石U-Pb年龄分别为218.3Ma±2.4Ma和214.0Ma±5.0Ma,代表了栗木花岗岩体的形成年龄。结合前人的研究数据,栗木矿区成岩、成矿作用年龄近乎一致,应为晚三叠世同一期次岩浆作用的产物。     

柴达木山花岗岩的岩石学、地球化学及锆石U-Pb年代学研究

正柴北缘高压—超高压变质带内发育的榴辉岩、蛇绿岩、麻粒岩及片麻岩等前人进行了大量的研究。大柴旦地区是柴北缘古生代超高压带的重要组成部分,与超高压岩石相伴的花岗岩十分发育,查明该超高压带上花岗质岩浆活动期次及其与超高压变质作用、造山带构造演化之间的关系,对探讨中国西部大地构造演化具有十分重要的意义(吴才来等,2007)。     

鞍山地区营城子古太古代片麻岩杂岩的识别与锆石U-Pb年代学研究

鞍山地区分布有始太古代-新太古代花岗岩杂岩,是研究早期地壳物质组成及其演化的经典地区之一.通过大比例尺岩性调查工作,新近在营城子识别出一套片麻岩杂岩和奥长花岗岩组合,为本区古老岩系的对比、构造格局恢复、太古代地壳形成与演化研究提供了新的依据和线索.片麻岩杂岩主要由条带状黑云斜长片麻岩、(脉状)奥长花岗质片麻岩以及黑云母片岩组成,呈不同规模的包体产出于细粒、均匀块状的奥长花岗岩之中.片麻岩杂岩的岩石组合以及复杂多变、明显不均一的岩石组构特征表明具有深熔混合岩的成因特点.SIMS锆石U-Pb定年表明,片麻岩杂岩中条带状黑云斜长片麻岩和黑云母片岩形成年龄分别为3312±14Ma、3304±7Ma和3324±7Ma～3235±14Ma,明确反映古太古代热事件,此外个别样品中存在3.68～3.60Ga和～3.47Ga继承锆石.细粒黑云奥长花岗岩的年龄为3.14～ 3.13Ga,与全区中太古代岩浆热事件一致.区域对比分析表明:营城子片麻岩杂岩的岩石组合、产状关系和年代学特征与东山杂岩和深沟寺杂岩十分相似,为古大古代(3.33 ～3.24Ga)深熔作用的产物.营城子片麻岩杂岩是鞍山地区另一个保留有古老锆石信息和太古宙早期地壳岩石的地质体.     

塔里木盆地库鲁克塔格地区新元古代冰碛岩锆石SHRIMP U-Pb年龄新证据

塔里木盆地库鲁克塔格地区新元古代冰碛岩剖面是目前世界各大陆中唯一发育有4套新元古代冰碛岩和多期火成岩事件的剖面.近年来,"雪球事件"的提出为全球新元古代冰碛岩的研究注入了新的活力,各国地质学家根据新元古代冰碛岩在全球各大洲发育的特点、同位素年龄和C/O、Sr同位素在新元古界划分出4个冰期.因此,新疆库鲁克塔格地区新元古界发育的4套冰碛岩的沉积特征、沉积环境和年代地层学研究的进展始终为各国地质学家所注目.本次获得的库鲁克塔格地区特瑞爱肯组的锆石SHRIMP U-Pb新年龄,为完善库鲁克塔格地区新元古代冰碛岩的年代地层学提供了新的证据.     

Neoproterozoic subductions and differential exhumation of western Hoggar (southwest Algeria): new structural, petrological and geochronological evidence

The western terranes exposed east of the Pan-African suture in western Hoggar (southwest Algeria), are reexamined in the light of new structural, petrologic and by the ⁴⁰Ar/³⁹Ar laser probe data on metamorphic micas and amphiboles. To the north, the Tassendjanet nappe includes the Paleoproterozoic basement, its Mesoproterozoic cover and mafic rocks representing the roots of a ca. 680 Ma arc overlain by Late Neoproterozoic andesites and volcanic greywackes. The nappe preserved at rather shallow crustal level in the east was emplaced southward (D_1a) to southeastward (D₂). In the south, two metamorphic suites are distinguished. The Tideridjaouine–Tileouine high-pressure metamorphic belt (T=550–600 °C, P=1.4–1.8 GPa) represents a slab of subducted continental material exposed along the western edge of the In Ouzzal granulite unit interpreted as a microcontinent. Differential exhumation of tectonic slices from the high-pressure belt occurred around 615–600 Ma through a system of west-directed recumbent folds (D_1b). The Egatalis high grade belt in the west was intruded by syn-metamorphic gabbro–norite bodies. It includes unretrogressed low-pressure granulite facies rocks (T around 750–800 °C, P0.45 GPa) cooled at a rate of 15°/m.y. between 600 and 580 Ma, and followed by the emplacement of several late-kinematic granitic plutons. Final exhumation of the low-pressure, high-temperature metamorphic rocks, that are not found as pebbles in the molasse, took place in the Late Cambrian. The early and relatively fast cooling of the high-pressure and high-temperature metamorphic rocks of the southern part of the Tassendjanet terrane is at variance with the slow cooling of central Hoggar where repeated magmatic activity as young as Late Cambrian occurred [Lithos 45 (1998) 245].     

Crystal Size Distribution (CSD) and Textural Evolution of Accessory Apatite, Titanite and Allanite during Four Stages of Metamorphism: an Example from the Moine Supergroup, Scotland

Crystal size distributions (CSD) and shapes of accessory apatite,titanite and allanite are investigated in three texturally distinctgarnet zones (Z1–Z3) and in the matrix (Z4) of a garnet–epidote–biotitegneiss from the Moine Supergroup. Additionally, textural relationshipsand interactions between the accessory minerals and surroundingrock-forming minerals are considered, results of numerical CSDmodelling are presented, and geochronological and geologicalconsequences of the inferred CSD evolutions are discussed. Texturesand CSDs indicate that the accessory minerals were in, or near,a stage of nucleation and initial growth immediately prior togarnet Z1 overgrowth, and formed within less than 20 000 years,either by a size-independent or size-dependent growth mechanism.Subsequently, the CSDs were modified by different growth mechanisms,as supported by several parameters including CSDs, grain numbers,grain sizes, specific volumes and others. The apatite CSD evolutionfrom Z1 to Z4 is consistent with open-system LPE (Law of ProportionateEffects) growth accompanied and followed by supply controlledrandom ripening, whereas transformation of the original titaniteCSD is more consistent with Ostwald ripening, temporarily accompaniedby positive or negative McCabe growth. The allanite CSDs alsopoint to Ostwald ripening between Z3 and Z4. The textural observationsindicate that the growth evolution of the accessory phases wasinfluenced by mineral reactions with surrounding rock-formingminerals, as well as by deformation and matrix coarsening, ina manner similar to that found in more simple ceramic systems.The observed textures require a successive temperature risethroughout the tectono-metamorphic evolution of the investigatedrock, in agreement with existing P–T data. Fast nucleationand initial growth of the accessory minerals during Z1 was perhapsinitiated by contact metamorphism, whereas subsequent growthand annealing (Z2–Z4) result from regional metamorphicevents. KEY WORDS: apatite; titanite; allanite; CSD; Moine Supergroup; textures; kinetics     

SHRIMP U-Pb zircon geochronology and Nd-Sr isotopic study of the Mamianshan Group: implications for the Neoproterozoic tectonic development of southeast China

Northwestern Fujian contains abundant well-studied Precambrian basement, and was a composite terrane in Cathaysia during the Neoproterozoic; however, its magmatic activity, petrogenesis, and tectonic evolution remain controversial. This article focuses on the geochronology and geochemistry of the Neoproterozoic Group in order to resolve the above problems. We provide new SHRIMP U-Pb zircon dating for the Mamianshan Group: 851.9 ± 9.2 to 825.5 ± 9.8 Ma for the Longbeixi Formation, 796.5 ± 9.3 Ma for the Dongyan Formation, and 756.2 ± 7.2 Ma for the Daling Formation. These ages document the existence of Neoproterozoic magmatism in the northwestern Cathaysia Block. Dongyan Nd-Sr isotopic data show that mafic amphibolite schists, mafic greenschists, and quartzofeldspathic schists were derived from a more depleted mantle (initial ε_Nd ? +5.5 and ⁸⁷Sr/⁸⁶Sr ratio 0.703409643), a mixture of depleted mantle and crustal components (initial ε_Nd ? ?1 and ⁸⁷Sr/⁸⁶Sr ratio 0.702045282–0.704147714), and late Palaeoproterozoic continental crustal materials (initial ε_Nd < ?1 and ⁸⁷Sr/⁸⁶Sr ratio 0.71083603), respectively. These new data, together with previous studies, suggest a bi-subduction-collision orogenic model for the Neoproterozoic evolution of the Yangtze and Cathaysia blocks. Our plate tectonic scenario involves earlier NW-dipping subduction during 1.0 Ga–860 Ma along the southeastern margin of the Yangtze Block and later NW-dipping subduction near the northwestern margin of the Cathaysia Block starting at ca. 850 Ma. The 796.5 ± 9.3 Ma age of the volcanic Dongyan Formation suggests that the final assembly of the Yangtze and Cathaysia blocks probably occurred after ca. 800 Ma. The 756.2 ± 7.2 Ma age of the Daling Formation indicates that post-orogenic extensional magmatism took place after 800 Ma along the northwestern margin of Cathaysia.     

广西桂林地区东部栗木花岗岩体SHRIMP锆石U-Pb年龄

栗木矿区位于南岭钨锡多金属成矿带的中段,成矿作用与花岗岩密切相关。通过对栗木金竹源与三个黄牛地区云英岩化花岗岩的年代学研究,获得2个SHRIMP锆石U-Pb年龄分别为218.3Ma±2.4Ma和214.0Ma±5.0Ma,代表了栗木花岗岩体的形成年龄。结合前人的研究数据,栗木矿区成岩、成矿作用年龄近乎一致,应为晚三叠世同一期次岩浆作用的产物。     

塔里木盆地库鲁克塔格地区新元古代冰碛岩锆石SHRIMP U-Pb年龄新证据

塔里木盆地库鲁克塔格地区新元古代冰碛岩剖面是目前世界各大陆中唯一发育有4套新元古代冰碛岩和多期火成岩事件的剖面。近年来,“雪球事件”的提出为全球新元古代冰碛岩的研究注入了新的活力,各国地质学家根据新元古代冰碛岩在全球各大洲发育的特点、同位素年龄和C/O、Sr同位素在新元古界划分出4个冰期。因此,新疆库鲁克塔格地区新元古界发育的4套冰碛岩的沉积特征、沉积环境和年代地层学研究的进展始终为各国地质学家所注目。本次获得的库鲁克塔格地区特瑞爱肯组的锆石SHRIMP U-Pb新年龄,为完善库鲁克塔格地区新元古代冰碛岩的年代地层学提供了新的证据。     

塔里木盆地库鲁克塔格地区新元古代冰碛岩锆石SHRIMP U-Pb年龄新证据

塔里木盆地库鲁克塔格地区新元古代冰碛岩剖面是目前世界各大陆中唯一发育有4套新元古代冰碛岩和多期火成岩事件的剖面。近年来,“雪球事件”的提出为全球新元古代冰碛岩的研究注入了新的活力,各国地质学家根据新元古代冰碛岩在全球各大洲发育的特点、同位素年龄和C/O、Sr同位素在新元古界划分出4个冰期。因此,新疆库鲁克塔格地区新元古界发育的4套冰碛岩的沉积特征、沉积环境和年代地层学研究的进展始终为各国地质学家所注目。本次获得的库鲁克塔格地区特瑞爱肯组的锆石SHRIMP U-Pb新年龄,为完善库鲁克塔格地区新元古代冰碛岩的年代地层学提供了新的证据。     

塔里木盆地库鲁克塔格地区新元古代冰碛岩锆石SHRIMP U—Pb年龄新证据

塔里木盆地库鲁克塔格地区新元古代冰碛岩剖面是目前世界各大陆中唯一发育有4套新元古代冰碛岩和多期火成岩事件的剖面。近年来。“雪球事件”的提出为全球新元古代冰碛岩的研究注入了新的活力,各国地质学家根据新元古代冰碛岩在全球各大洲发育的特点、同位素年龄扣C／O、Sr同位素在新元古界划分出4个冰期。因此,新疆库鲁克塔格地区新元古界发育的4套冰碛岩的沉积特征、沉积环境和年代地层学研究的进展始终为各国地质学家所注目。本次获得的库鲁克塔格地区特瑞爱肯组的锆石SHKIMPU-Pb新年龄．为完善库鲁克塔格地区新元古代冰碛岩的年代地层学提供了新的证据。     

阿拉善地区新元古代岩浆事件及其地质意义

在阿拉善西部地区分布有一些眼球状片麻岩和条带状片麻岩,以往将它们作为阿拉善群的地层对待。本文研究表明它们是受到变形改造的正片麻岩。通过锆石LA-ICPMSU-Pb分析,4个样品分别获得了913±7Ma、921±7Ma、926±15Ma和904±7Ma的年龄结果。新元古代早期变形花岗岩的发现表明,阿拉善地区经历了新元古代早期(格林威尔期)造山作用的强烈改造,它们可能与祁连地块等相似,应属于不同于华北克拉通的独立的变质地块。