U-Pb isotopic relationships in zircon suites from a para- and ortho-gneiss from the Ceneri Zone,Southern Switzerland

U-Pb isotopic analyses have been determined on four size splits from an orthogneiss zircon population and on seven size and magnetic splits from a paragneiss zircon population from the Ceneri Zone, Southern Switzerland. The orthogneiss zircon results suggest that a severe isotopic disturbance occurred in this region between 400–450 m. y. ago which, besides initiating new zircon growth, almost completely reconstituted older zircon. The U-Pb systems of the orthogneiss zircons are nearly concordant, except the coarsest size split where some older lead is present.The paragneiss zircon U-Pb systems have Precambrian apparent ages and appear to be an old detrital zircon suite which has been severely isotopically disturbed by the same Caledonian event which affected the orthogneiss zircons. Zircon U-Pb data-points of paragneiss size-and-magnetic fractions are linearly correlated on a concordia plot. The age significance of this is not fully understood, however these results indicate that Precambrian crustal rocks, at least 1,500 m. y. old, contributed detritus to the original sediments of the Alpine gneisses.The gneissic rocks in Southern Switzerland have been postulated as a possible source or root zone for petrologically and chemically similar gneisses in the Silvretta Nappe to the north. The close similarity of zircon U-Pb systems of orth- and para-gneisses between the two regions supports this hypothesis.     

羌塘盆地变质岩锆石SHRIMP U-Pb年龄及其地质意义

笔者在位于北羌塘的玛尔果茶卡发现了具有区域变质特点的副变质岩组合,岩性为含蓝晶石矽线石长英质片麻岩、含蓝晶石夕线石绢云母石英片岩。1：25万区域地质调查将这套岩石命名为前奥陶纪齐陇乌如岩组,呈断续出露的小型穹隆分布在中央隆起带北缘附近。笔者对片麻岩中的锆石进行了SHRIMP年龄分析,获得锆石年龄347～2827Ma。本文对不同年龄段所反映的构造热事件及其地质意义进行讨论。     

Age and origin of detrital zircons from the pre-Permian basements of the Bohemian Massif and the Alps

U-Pb isotopic analyses were made on detrital zircon populations from sandstones and quartzites of the pre-Permian basement in an attempt to shed light on the presedimentary history of the zircons and the age of their primary source rocks. Eight rock samples were collected from the Saxothuringian and Moldanubian parts of the Bohemian Massif, the western part of the Upper Austroalpine Nappes, and the Southern Alps. The heterogeneous populations were separated into fractions of different size, magnetic susceptibility, color, and shape. Because of their typically pitted surface all zircon grains from the sandstones and quartzites appear to be detrital. Only in three samples from the Alps—one from a contact metamorphic aureole—the zircons show surface recrystallization and minor new growth. With the exception of some euhedral crystals in the Saxothuringian quartzites all zircon fractions have highly discordant U-Pb ages. On a concordia diagram their data points scatter slightly around best-fit lines with upper intersections between 2000 and 2300 m.y. From this pattern the following conclusions are reached:

1. A large proportion of the material of the metasedimentary basement rocks in the Bohemian Massif as well as in the Alps derives from one or more sources, about 2000 to 2300 m.y. old.
2. The estimated proportion of detrital zircons with primary ages of 700 to 1500 m.y. is less than 10%.
3. The existence of a regional high-grade metamorphism in the Bohemian Massif as well as in the Alps during 700 to 1500 m.y. can be excluded. From Rb-Sr isotopic data, a metamorphism for the time prior to 1500 m.y. is very unlikely.

The lower intersections of the best-fit lines with the concordia curve cannot be clearly correlated with an episodic disturbance of the U-Pb systems during weathering and sedimentation and/or during regional metamorphism. For the zircons of the Bohemian Massif a disturbing event, about 550 to 600 m.y. ago, is likely. Clear, euhedral, but nevertheless detrital zircons found among the zircon populations of two Saxothuringian quartzites (“Plattenquarzit” of the pre-Ordovician “Arzberger Serie” and Lower Ordovician “FrauenbachQuarzit”) crystallized most probably during the Upper Proterozoic and/or the Assyntian petrogenesis. The highly discordant age pattern of the detrital zircons from the Alps is likely to be the result of the Caledonian and/or Hercynian (=Variscan) metamorphism. Differences in concentration levels of common lead in detrital zircons and the problem of red zircons as indicators of Precambrian origin are discussed.     

U/Pb Systematics of zircons during dynamic metamorphism

Eight zircon fractions from the Henderson Gneiss were analyzed from the Brevard mylonites and adjacent Inner Piedmont rocks near Rosman, North Carolina, to determine variations of U/Pb systematics of zircons in a pressure-dominated metamorphism. Substantial uranium gain occurred during zircon recrystallization and size reduction.At Rosman, the Henderson Gneiss zircons probably formed about 600 m.y. ago and now show evidence of being binary mixtures. They experienced mylonitization at about 450 m.y. ago during Taconic deformation and metamorphism in the almandine amphibolite facies. The lower intensity, upper greenschist or greenschist-amphibolite Acadian (360–390 m.y.) metamorphism and mylonitization (recrystallization) had little effect on the zircons but disturbed Rb/Sr systems.The ages of dynamic metamorphism and thrusting (450, 360–390) in the Piedmont and Blue Ridge near and along the Brevard zone at Rosman, North Carolina, agree well with the tectonic interpretations of the Valley and Ridge Taconic (ca. 470-400 m.y.) and Acadian (ca. 375-330 m.y.) clastic wedges. Intermittent thrusting, folding, and uplift rather than continuous secular uplift of the crystalline terrane seems indicated.     

Paleozoic migmatites affected by high-grade tertiary metamorphism in the central Alps (Valle Bodengo,Italy)

The Lepontine Gneiss Complex of southern Switzerland and northern Italy is characterized by high-grade metamorphism and intensive deformation of Alpine age with migmatites prevalent in the area with the highest metamorphic grade. Petrological and structural observations are generally inconclusive but indicate in some places an Alpine age for the migmatite formation. To determine the time of migmatite formation a geochronologic study was undertaken in one of the best exposed areas, the Valle Bodengo, Italy. Rb-Sr whole-rock errorchrons of intrusive migmatite phases and of two rather homogeneous granitoid gneiss bodies yield apparent ages between 280 and 350 m.y. They suggest a Hercynian or older igneous history for these rocks. The U-Pb ages of the euhedral zircons are highly discordant, but they do point to the presence of zircon components more than 450 m.y. old. The concordia-intercept ages are incompatible with the Rb-Sr data and the low initial ⁸⁷Sr/⁸⁶Sr ratios of about 0.706. These low initial ratios suggest that either the bulk of the granitoid material is not much older than Hercynian, or older crustal material was isotopically homogenized on a regional scale with rocks that had low Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios (e.g. the lower crust or upper mantle) during a Hercynian metamorphism. Rb-Sr small-scale whole-rock isochrons and tie lines of adjacent, lithologically different rock phases give Alpine ages, the best isochron yielding 22 m.y. This coincides with concordant U-Pb ages of monazites of 23 to 24 m.y. Rb-Sr mineral isoohrons (muscovite, biotite, feldspars, apatite) give ages of 18–21 m.y. Our interpretation is that this age pattern resulted due to rapid cooling after the climax of the last phase of the Alpine metamorphism and we conclude that high-grade metamorphic conditions existed during the upper Oligocene or early Miocene. Other investigators have suggested that the Alpine metamorphism had a climax 35–40 m.y. ago and that the younger mineral ages are a result of simple continuous cooling due to uplift. Based on this study and other recent geochronological studies in the Lepotine Gneiss Complex we suggest that there had to be a thermal maximum at about 20–25 m.y. The example of Valle Bodengo demonstrates that the areal coincidence of the zone of highest-grade metamorphism with the occurrence of migmatites does not necessarily mean that metamorphism and migmatite formation were coeval and related to each other.     

238U/206Pb-235U/207Pb-232Th/208Pb zircon geochronology in alpine and non-alpine environment

Based on regular arrays in a U-Pb diagram of data points of zircons from polymetamorphic rocks of the Alps quantitative models are developed that include several episodic lead losses, continuous lead losses during discrete periods and combinations of both.The data from the Alps can be explained by assuming an initial age of the zircons of 2000 and/or 2500–2600 m.y. and three episodic lead losses 520–580 m.y., 300 m.y. and 30 m.y. ago with regionally varying intensities of the events as can be seen from the efficiency parameters which define the shape of the discordia. On the other hand, zircon data from North American basement rocks satisfy a model assuming a discrete interval of continuous lead loss that starts with the time of uplift of the rocks.The Th-U-Pb diagrams are valuable for recognising whether the history of the zircons and their host rock is a complex one or not which is not always noticeable in a U-Pb diagram.     

Evidence of multiple intrusion,possible resetting of U-Pb ages,and new crystallization of zircons in the post-tectonic intrusions (‘Rapakivi granites’) and gneisses from South Greenland

U-Pb data for zircons from post-tectonic monzonite, syenite and norite, and country rock gneiss and migmatite from the Kap Farvel-Prins Christian Sund area of South Greenland indicate two distinct intrusive episodes at ～1740 m.y. and ～1755 m.y. The norites have the same age as the older granitic rocks. Similar intrusions further north along the southeast coast have the same age (～1755 m.y.) and geochemical character as those of the Kap Farvel-Prins Christian Sund area. Our U-Pb ages are about 100 m.y. older than previously determined K-Ar ages.The youngest peak of regional Ketilidian metamorphism in this area is placed at about 1800 m.y. based on a linear array of seven magnetic and size fractions from a syntectonic granitic part of a migmatite. This age is distinctly different from the post-tectonic intrusions and, along with other data, precludes the possibility of in situ formation of the intrusions by remelting of the country rocks.In one hypersthene migmatite sample collected near a post-tectonic intrusion, clear overgrowths and separate clear grains with low uranium concentrations were identified. The clear grains of zircon have the same age as the intrusion, indicative of new zircon growth during the granulite facies recrystallization of the gneiss. In contrast, rounded red zircons from an early subconcordant granitic sheet have clear uranium-rich overgrowths which probably formed during regional metamorphism. If the deep-red zircons observed in one of the post-tectonic intrusions were derived from the surrounding metamorphic rocks, they have had their U-Pb systems reset to the time of intrusion.In accord with observations (Krogh, 1971) on non-magnetic zircon fractions from volcanics, the Greenland zircons contain very low amounts of common lead ranging from <0.01 ppm in the least magnetic fractions in the post-tectonic intrusions, to 6.5 ppm in the most magnetic fractions of the gneisses. These two rock groups can be differentiated on the basis of their common lead content.     

Age,origin, and thermal histories of some plutonic rocks from the Salinian block of California

U-Pb isotopic measurements on zircons from some granitic rocks of the Salinian block indicate emplacement and crystallization of these rocks about 104 m.y. ago (mid-Cretaceous). The relatively radiogenic nature of initial Sr and common Pb in these rocks, and the presence of an inherited component of zircon in some of them strongly suggest the involvement of continental crust in the genesis of the magmas. Possibly the magmas were generated in a zone of melting that overlapped the boundary between the mantle and the continental crust. U-Pb measurements on sphene, apatite, and feldspar from the plutons, along with previously published K-Ar and fission-track ages shed light on the post-emplacement thermal histories of the plutons. Most of the samples from the northern part of the Salinian block (Bodega Head, Point Reyes) show relatively simple thermal histories. Evidently these plutons were emplaced at moderate levels in the crust, crystallized, and cooled to moderate temperatures over an interval of about 10–15 m.y. Plutons from the central Salinian block (Santa Lucia Range) show more complex and prolonged thermal histories. These plutons evidently were emplaced at greater depths in the crust than were the plutons from the northern Salinian block. They remained at elevated temperatures for ca. 25 m.y., then cooled fairly rapidly, probably as a result of rapid uplift and erosion. One sample from the northern Salinian block shows an even longer span of time between emplacement and cooling. The thermal evolution of the Salinian plutonic rocks predates major offset along the San Andreas fault zone and thus reflects the thermal evolution of the undisrupted source terrane of the Salinian block. Isotopic measurements of the type reported here might therefore prove valuable in correlations across the San Andreas fault zone. Moreover, detailed study of thermal evolution within the Salinian block could shed light on major offsets within the block.     

西大别浒湾变质带洋壳和陆壳耦合俯冲的锆石U-Pb年龄记录及其动力学意义

西大别浒湾高压变质带是研究秦岭-大别-苏鲁造山带演化的关键区域.本文对该变质带熊店和学河两地的两个榴辉岩样品进行了LA-(MC)-ICPMS锆石U-Pb定年、微量元素分析及Hf同位素测定.熊店榴辉岩岩浆锆石得到的年龄为406±14Ma,具有高的εHf(t)值(εHf(t)=11.3±1.3),年轻的亏损地幔模式年龄(tDM=578±52Ma),其来源可能为亏损地幔,进而说明它们的原岩可能为古特提斯洋壳物质;学河榴辉岩岩浆锆石的年龄为703±8Ma,具有略低的εHf(t)值(εHf(t)=4.11±0.94),较老的亏损地幔模式年龄(tDM=1105±37Ma),其原岩可能为扬子克拉通新元古代裂谷岩浆作用产生的新生陆壳物质.这些结果表明浒湾地区存在原岩形成于新元古代和志留纪两个时期的榴辉岩.熊店榴辉岩中变质锆石的微量元素特征与岩浆锆石类似,可能为完全重结晶成因锆石,其206Ph/238U加权平均年龄为316±1Ma,代表了洋壳榴辉岩榴辉岩相峰期变质的最早时间.学河榴辉岩变质锆石以低Th/U、Nb/Ta比值为特征,其REE组成模式为不明显的Eu负异常,HREE呈平坦型.这些特征反映了这些锆石形成时出现了较大数量的石榴子石与金红石,而缺乏长石.根据锆石Ti温度计计算学河榴辉岩变质锆石形成的温度范围是704～741℃,与榴辉岩相变质温度一致.它们对应的206Pb/238U加权平均年龄为312±3Ma,可作为浒湾变质带榴辉岩相峰期变质年龄,这一结果表明浒湾变质带陆壳成因榴辉岩存在石炭纪榴辉岩相变质事件.空间上相近的古生代洋壳和新元古代陆壳具有相同的榴辉岩相变质年龄表明,浒湾变质带的洋壳和陆壳榴辉岩可能存在石炭纪的耦合俯冲作用.     

Dating of granulites involved in the hercynian fold-belt of Europe: An example taken from the granulite-facies orthogneisses at La Picherais,Southern Armorican Massif,France

Situated within the crystalline metamorphic complex of Champtoceaux NE of Nantes, the orthogneiss of La Picherais (near St Mars-du-Désert, Loire Atlantique, France) show relicts of a granulite facies paragenesis. Comparison with other granulitic rocks in the Hercynian fold-belt suggest possible ages ranging from Lower Proterozoic to Phanerozoic. The Rb-Sr whole rock method yields an errorchron of 570±110 m.y. for the Picherais orthogneiss, whereas the U-Pb zircon method indicates an upper intersection on Concordia at 1,880±120 m.y. and a lower intersection at 423±10 m.y. Several interpretations are possible for these data: the granite emplacement age was (1) 1,900 m.y. ago. (2) more likely Upper Proterozoic — Lower Palaeozoic. The zircons concordant at 1,900 m.y. were either present in the granitic magma at its time of origin or were introduced into the magma during emplacement. These zircons could be derived from sedimentary horizons such as found in the Lower Ordovician sandstones of the Armorican massif whose zircon age data are presented here.     

Comparative geochronology in the reversely zoned plutons of the Bottle Lake Complex,Maine: U-Pb on zircons and Rb-Sr on whole rocks

The Bottle Lake Complex is a composite granitic batholith emplaced into Cambrian to Lower Devonian metasedimentary rocks. Both plutons (Whitney Cove and Passadumkeag River) are very coarse grained hornblende and biotite-bearing granites showing petrographic and geochemical reverse zonation. Two linear whole rock Rb/Sr isochrons on xenolith-free Whitney Cove and Passadumkeag River samples indicate ages of 379±5 m.y. and 381±4 m.y., respectively, in close agreement with published K-Ar ages for biotite from Whitney Cove of 377 m.y. and 379 m.y., and for hornblende ⁴⁰Ar/³⁹Ar determinations from Passadumkeag River which indicate an age of 378±4 m.y. The initial Sr isotopic ratio for Whitney Cove is 0.70553 and for Passadumkeag River is 0.70414. A whole-rock isochron on a suite of xenoliths from the Passadumkeag River granite indicates a whole rock Rb-Sr age of 496±14 m.y., with an initial Sr isotopic ratio of 0.70262.Two types of zircon exhibiting wide petrographic diversity are evident in variable proportions throughout the batholith. One of these types is preferentially found in a mafic xenolith and it is widely dispersed in the host granites forming discrete grains and probably as inclusions in the other type of zircon. U-Pb analyses of zircons give concordia intercept ages of 399±8 m.y. for Whitney Cove, 388±6 m.y. for Passadumkeag River, 415 m.y. for a mafic xenolith in Passadumkeag River, and 396±32 for combined Whitney Cove and Passadumkeag River granite. The zircons show a spread of up to 20 m.y. in the ²⁰⁷Pb/²⁰⁶Pb ages. Omitting the finest zircon fraction in the Passadumkeag River results in a concordia intercept age of 381±3 m.y., in better agreement with the whole-rock Rb-Sr and mineral K-Ar ages. For the Whitney Cove pluton, exclusion of the finest fraction does not bring the zircon age into agreement with the Rb-Sr data.Age estimates by the whole rock Rb-Sr, mineral K-Ar and Ar-Ar methods suggest that the crystallization age of the plutons is about 380 m.y., slightly younger than the U-Pb zircon intercept ages. A possible reason for this discrepancy is that the zircons contain inherited lead. Thus, zircon U-Pb ages might represent a mixture of newly developed zircon and older inherited zircon, whereas the Rb-Sr whole rock age (380 m.y.) reflects the time of crystallization, and the argon ages result from rapid cooling after emplacement.     

U-Pb zircon and Rb-Sr whole-rock dating of low-grade metasediments example: Montagne Noire (Southern France)

Three detrital, Proterozoic zircon suites extracted from siltstones progressively metamorphosed between chlorite- and staurolite-grade independently date the major Caledonian metamorphism within the gneiss dome of the Montagne Noire (Southern France). From this, the following conclusions concerning U-Pb systematics of zircons in low-, medium- and highgrade metamorphic rocks can be drawn:

1. Temperatures of at most 350–400 °C are sufficient to open U-Pb systems of metamict zircons or domains within zircons.
2. The observed open U-Pb system behaviour during metamorphism of the host rocks was found to be due to a low-temperature recrystallisation of highly radiation damaged zircon lattices, probably enhanced by high concentrations of fluid phases in the dehydrating rock volumes.
3. Recrystallisation of metamict zircons under low temperatures causes maximum U-Pb ages for the thermal climax of metamorphism of medium-and high-grade metamorphic rocks, as annealing and accompaning closing of U-Pb systems took place before the maximum temperatures of metamorphism were reached.
4. Low-temperature recrystallisation of old — generally Proterozoic—zircons can readily help to explain the fact that the numerous zircon suites from ancient shield areas yield “lower intercept ages” which are not correlated to any known geological event. Thus, either a weak thermal pulse, not necessarily registered by the mineral assemblage of the host rock, and/or elevated temperatures during burial in the crust might supply enough energy for a structural reordering and simultaneous lead loss of at least the most disordered lattice domains.

In contrast to the U-Pb zircon method, no unambiguous dating of the Caledonian main metamorphism was possible using the Rb-Sr whole-rock technique for phyllites and mica schists sampled in the same metamorphic profile from which the zircon samples were taken. The scatter of data points can best be explained by their rotation around a probable Caledonian isochron. This rotation very probably took place during the later Hercynian orogeny, not significantly affecting the slope of the least square regression line through the scattered data points.     

北大别片麻岩中锆石U—Pb年龄离子探针初步测定

对北大别鹿吐石和道士冲二个片麻岩中锆石进行了离子探针Ｕ－Ｐｂ年龄测定。二个样品１３个颗粒的２０个有效测定点的年龄表明,鹿吐石片麻岩主要分布在２３３￣３９４Ｍ道士冲片麻岩在３０７￣７１８Ｍａ之间。道十冲片麻岩年龄的一致性程度要高于鹿叶石,两个样品均无低于２２０Ｍａ的年龄信息。由于存在年龄的不一致性,这些年龄可能代表了由古老岩浆锆石和三叠纪的超高压变质锆石或者早白垩纪岩浆锆石所代表的混合、增生锆石。Ｈ     

北秦岭西段红花铺俯冲型侵入体LA-ICPMS定年及其地质意义

北秦岭造山带西段红花铺侵入体的岩石类型为中细粒英云闪长岩,具俯冲型花岗岩类的地球化学特征。通过对其中所获锆石的形态及CL图像特征的研究,分辨出3种不同成因的锆石,通过高精度的LA-ICPMS(激光剥蚀等离子体质谱)单颗粒锆石微区U-Pb同位素年龄测定,获得(1 765.1±8.4）Ma、(450.5±1.8）Ma、(413.5±1.3）Ma的3组年龄值,分别为继承锆石年龄、成岩锆石年龄和变质锆石年龄,代表了3期构造 热事件的时限,可能是吕梁运动和加里东运动在北秦岭西段的地质记录。红花铺侵入体英云闪长岩(450.5±1.8）Ma的成岩年龄可能代表了北秦岭西段早古生代俯冲带岛弧的形成时代。红花铺俯冲型侵入体的形成时代及构造环境的确定,对研究北秦岭造山带元古宙-早古生代大地构造格局、构造演化及中国大陆动力学机制具有重要意义。     

中国东北麻山杂岩晚泛非期变质的锆石SHRIMP年龄证据及全球大陆再造意义

中国东北地区佳木斯地块南部麻山杂岩正、副片麻岩 7个样品的锆石 SHRIMP年龄数据首次明确地表明 ,东北地区存在 500 Ma的晚泛非期高级变质作用事件。峰期麻粒岩相变质导致柳毛地区 (502± 10)Ma (2σ )深熔花岗岩的形成。正、副片麻岩变质年龄的一致性表明它们已在变质前发生了构造叠置。西麻山副片麻岩中含有在后期麻粒岩相变质过程中未重结晶的碎屑锆石，由此形成从协和一致的 550 Ma到弱不一致 1 900 Ma的较大 207Pb/206Pb年龄变化范围，表明其原岩具有从新元古代到中元古代-古元古代的年龄。柳毛地区变质的片麻状闪长岩中所含的古老锆石的 207Pb/206Pb年龄为 546～ 1 460 Ma表明，该闪长岩大约在 1 400 Ma就位，并受到 500 Ma变质事件的影响，从而说明柳毛地区存在中元古代基底。然而，与以前的认识相反，麻山杂岩不存在具有太古宙基底的同位素证据。晚泛非期变质事件年龄的确定对重塑晚前寒武纪-显生宙早期麻山杂岩和佳木斯地块的古地理位置具有重要意义。根据目前获得的有关证据，认为佳木斯地块可能曾经位于冈瓦纳大陆北缘的华北克拉通附近。     

柴北缘大陆深俯冲板片折返过程中的深熔作用研究

柴北缘锡铁山地区长英质（花岗）片麻岩普遍经历了不同程度的部分熔融作用,常见新生的花岗质浅色体呈层状、脉状或网络状分布于长英质片麻岩中,并显示出混合岩化的特征。岩相学观察结果显示长英质片麻岩保留了关键的深熔作用显微结构证据:（1）石榴石内部发育有钾长石、石英和斜长石组成的矿物包裹体;（2）长石颗粒边界出现由石英+钾长石±斜长石±白云母组成的楔形矿物集合体;（3）云母颗粒边界发育尖锐的、不规则的微斜长石,而且云母边界溶蚀明显,形成锯齿状不规则的边界;（4）石英、斜长石或钾长石颗粒边界发育圆珠状（stringofbeads）结构,而且颗粒边界或三联点中尖锐状微斜长石与周围矿物的形成较小的二面角。阴极发光图像和锆石U-Pb定年结果表明花岗质浅色体中的锆石具有明显的核、幔、边三层结构,而且具有明显不同的年龄结果。发光较强的继承性锆石岩浆核部的²⁰⁶Pb/²³⁸U年龄约为~910Ma,而且具有高的Th/U比值;弱发光的变质锆石幔的²⁰⁶Pb/²³⁸U年龄结果约为~450Ma。新生的锆石增生边中等程度发光,并发育震荡环带和较低的Th/U比值,与世界典型地区混合岩中深熔锆石的特征十分相似,其²⁰⁶Pb/²³⁸U年龄结果为432±3Ma。野外关系、显微结构特征和年代学的研究结果显示柴北缘锡铁山地区花岗质浅色体可能是其寄主岩石长英质片麻岩在折返到高压麻粒岩相条件下深熔作用的产物,而且白云母的脱水熔融是引发岩石发生深熔作用的主要机制。柴北缘地区已有的资料综合研究表明,大陆深俯冲板片在俯冲/碰撞和折返过程中可能经历了多重深熔作用。     

秦岭佛坪变质结晶岩系年龄和物质组成特征——SHRIMP锆英石U-Pb年代学和全岩Sm-Nd年代学数据

秦岭是长期多阶段发展的复杂大陆碰撞造山带。本文报道了位于造山带中部佛坪变质结晶杂岩系的SHRIMP锆英石U-Pb年龄和全岩Sm-Nd年龄同位素分析结果。变质岩SHRIMP锆英石U-Pb年龄十分复杂，除有少量太古宇形成的锆英石外，还存在大量元古宇至显生宇的锆英石。并在2000Ma，1200Ma，800Ma，400Ma，200Ma左右形成峰期。片麻岩Nd模式年龄tDM 1372～2081Ma。佛坪变质结晶岩系主体可能形成于古元古代，年龄2000Ma左右。在后期强烈地质作用中有少量地幔物质加入。岩系形成于新太古代的意见笔者没有得到证实。秦岭造山带中部可能存在或存在过太古宙结晶基底岩石。     

鄂尔多斯盆地周缘地壳形成与演化历史:来自锆石U-Pb年龄与Hf同位素组成的证据

锆石U-Pb定年及Hf同位素测定结果表明,鄂尔多斯盆地周缘的华北板块北缘、兴蒙造山系及扬子板块-秦岭-大别-苏鲁造山带等构造单元系统具有明显不同的形成与演化历史。华北板块北缘锆石年龄平均值为1 837 Ma,最强烈的岩浆活动出现于2 200~1 800 Ma,该期锆石约占全部的40%;次为强烈的岩浆活动在2 800~2 200 Ma,其众数在全部锆石中约占30%;1 500~1 200 Ma、500~100 Ma这两个阶段形成的锆石在全区所占比例各约为15%。华北板块北缘最突出的特征是基本不含1 000~700 Ma期间形成的锆石,>3 000 Ma的锆石在全区分布极为有限。锆石Hf同位素亏损地幔模式年龄表明华北板块北缘平均值为2.55 Ga,较U-Pb平均年龄老,说明2 200~1 800 Ma期间形成的锆石含有较多的古老地壳再循环组分。Hf亏损地幔模式年龄最强峰值约为2.8 Ga,与全岩Nd亏损地幔模式年龄的峰值相一致,Hf模式年龄为3.0~2.25 Ga的颗粒占全部的近95%,证明华北板块北缘的地壳增生主要在太古宙至古元古代期间。Hf同位素亏损地幔模式年龄>3.0 Ga的锆石颗粒所占比例不到0.1%,另外近5%锆石的模式年龄分布于中元古代。晚古生代-中生代所形成的锆石均是先存地壳组分,尤其是中元古代增生地壳的熔融作用形成。兴蒙造山系中锆石U-Pb年龄平均值为497 Ma,最强峰分布于石炭纪(约320 Ma),石炭纪-二叠纪末(350~250 Ma)形成的锆石所占比例达30%以上。新元古代至早古生代(600~440 Ma)形成的锆石占全部锆石的55%以上,而中元古代末-新元古代期间(1 200~600 Ma)形成的锆石在全区仅占4%。中元古代以前形成的锆石非常有限,说明该区最早形成的地壳组分在兴蒙造山系的形成过程中较充分地参与到后期的岩浆作用过程中。兴蒙造山系中锆石相应的Hf同位素亏损地幔模式年龄平均为1.13 Ga,明显较相应的U-Pb年龄老,最强峰值出现于约0.6 Ga。Hf亏损地幔模式年龄为0.7~0.28 Ga的颗粒在兴蒙造山系所占比例达57%,证明该区最强烈的地壳增生发生于新元古代至古生代期间。Hf同位素亏损地幔模式年龄分布于1.5~0.7 Ga的锆石在全区约占38%,说明此期间也是该区地壳较强烈的增生期。Hf亏损地幔模式年龄大于1.5 Ga的锆石所占比例不到5%,古生代以后兴蒙造山系也基本没有明显的地壳增生。扬子与秦岭-大别-苏鲁造山带构造单元中的锆石U-Pb年龄平均为799 Ma,年龄为1 300~750 Ma的锆石在全部锆石中约占70%。晚古生代-燕山期形成的锆石约占20%。年龄在3 500~2 650 Ma、2 118~1 680 Ma的锆石在该区各约占5%。结合扬子与秦岭-大别-苏鲁造山带平均为1.56 Ga的Nd亏损地幔模式年龄特征,说明1 300~750 Ma期间该区较强烈的岩浆作用事件中有较多的古老地壳组分加入其中。锆石U-Pb年龄及Hf同位素组成均说明鄂尔多斯盆地周缘各构造单元具有不同的形成演化历史。地壳是幕式增长的,但各构造单元每幕发生的时间、强度存在明显差别。因此,由盆地中不同时代地层中碎屑锆石U-Pb年龄及Hf同位素组成及全岩Nd同位素特征的系统研究可反演盆地物源供给与周围构造单元之间的关系。     

U-Pb data on zircons for the thermal peak of metamorphism in the Variscan Odenwald,Germany

U-Pb data are presented for zircons from metamorphic rocks of the crystalline Odenwald that comprises part of the Mid-German Crystalline Rise. The Odenwald can be divided into four principal domains with different magmatic and metamorphic histories on the basis of published structural and petrological data and the presented ages. In the eastern (Böll-stein) Odenwald the metasedimentary rocks contain zircons derived from basement rocks that exceed 2.2 Ga in age. The intrusion of granitic magmas into these metasedimentary rocks took place before or close to the end of the thermal peak of metamorphism at 375 ± 5 Ma ago. Rapid uplift of the Böllstein Odenwald occurred at about 320 Ma bp, later than in the Bergsträsser Odenwald. The western (Bergsträsser) Odenwald can be subdivided, by geochronological, petrological and especially structural data, into a northern, a central and a southern part. In the northern part basic magmatism has an age of 362 ± 7 Ma. The peak of thermal metamorphism in the southernmost Odenwald was accompanied by partial melting of metasedimentary rocks and the formation of new zircons with ages between 332 and 342 Ma. Probable peak metamorphic conditions, revealed by two samples from the heterogeneous central part, occurred at about 336 Ma, i.e. in the same time span as in the southernmost part, but later than in the Böllstein area and the northern Odenwald. Rapid uplift started earlier in the central and southern part and 10–20 Ma earlier than in the Böllstein Odenwald. Cooling from 500 to 300°C occurred in the northern part in the time interval between the elevation of the other parts of the Bergstraesser Odenwald and the Böllstein Odenwald. There is no indication for any thermal equilibrium between 2.25 Ga and 375 Ma recorded in the zircons of both areas.     

内蒙古黄土窑孔兹岩系的锆石与金红石年龄研究

内蒙古黄土窑及其毗邻地区孔兹岩系已有一些１８００￣１９００Ｍａ的锆石Ｕ－Ｐｂ年龄数据,但因孔兹岩是高级变质的沉积岩,其锆石Ｕ－Ｐｂ年龄往往代表峰期变质年龄而不是原岩沉积年龄,因此孔兹岩系属太古宙还是古元古代长期未决。内蒙古黄土窑是晋蒙孔兹岩系的经典产地之一,对其锆石和金红石研究和年龄测试,得到孔兹岩系的碎屑锆石年龄为２３１０Ｍａ,变质成因锆石年龄为１８７３Ｍａ,金红石代表的冷却年龄为１７９３Ｍａ。