Trace element geochemistry of the Mt Vulture carbonatites,southern Italy

The Mt Vulture carbonatites are the only carbonatite occurrence in the southern Apennines. We present new trace element data for these rocks in order to evaluate the factors influencing rare earth element (REE) and other trace element fractionations and their REE grade. This study focuses on massive hyalo-alvikites from two lava flows and one dike, which have different relative abundances of silicate and carbonate (i.e. Si/Ca). These differences are also evident from CaO/(CaO + MgO + FeO(T) + MnO) and Sr/Ba ratios. The REE grade of the Mt Vulture carbonatites is very similar to that of the global average for calcio-carbonatites. R-mode factor analysis shows that most of the trace element variance reflects the relative roles of carbonate and silicate minerals in influencing trace element distributions. Silicates largely control heavy rare earth element (HREE), transition metal, Zr, and Th abundances, whereas carbonate minerals control light rare earth element (LREE), Ba, and Pb abundances. In addition, apatite influences LREE concentrations. Increasing silica contents are accompanied by decreases in (La/Yb)N and (La/Sm)N ratios and less marked LREE enrichment. In contrast, higher carbonate contents are associated with increases in (La/Yb)N and (La/Sm)N. The Si/Ca ratio has little influence on Eu anomalies and middle rare earth element (MREE) to HREE fractionations. Apatite has a negligible effect on inter-REE fractionations amongst the carbonatites.     

Rare earth and trace element distribution in zircons from miaskite lamproites of the Panozero complex,Central Karelia

Ion microprobe study of rare-earth and trace element composition of zircons from miaskite lamproites of the Panozero sanukitoid complex in Central Karelia made it possible to divide them into weakly altered and metasomatically altered zircons. The latter differ in elevated contents of LREE, Ca, Al, Fe, Ti, Sr, Ba, and Th, a flat REE distribution pattern, and a reduced Ce anomaly. Zircons show moderate to strong degrees of alteration, which are recorded in their geochemical features. The metasomatic alteration was caused by fluid related to the mantle alkaline magmatism. At the final stage, this fluid caused the crystallization of zircon domain with extremely high contents of REE (55000 ppm), Ti, Ba, Sr, and Th. The use of a Ti thermometer for unaltered zircons from the miaskite lamproites yields strongly underestimated temperatures, which require additional explanation or revision of the thermometer. For strongly altered zircons with sharply elevated Ti content, the use of this thermometer seems to be impossible.     

The phenomenon of deficient electron microprobe totals in radiation-damaged and altered zircon

The phenomenon of deficient electron microprobe analyses, with sums of analyzed constituents often below 95 wt%, is assigned to the analysis of altered, porous minerals. With the example of three zircon populations we show that low totals are related to textural features (i.e., numerous pores of tens to hundreds of nanometers size) as well as to the chemical composition (i.e., water content well within the wt% range, which may affect partial sample degradation under the electron beam). The formation of the spongy texture is explained by the alteration of a previously radiation-damaged and, thus, volume-expanded material in a fluid-driven replacement reaction. The smaller volume of the reaction product (crystalline, non volume-expanded zircon) accounts for the formation of numerous voids and pores, which are perfect candidates for the incorporation of water. The alteration has also resulted in uptake of non-formula elements such as Al, P, Ca, Fe, Y, and REEs whereas Si and Zr are depleted. In one case, strong uptake of non-radiogenic Pb in altered zircon was observed. Because porous, low-total zircon has formed in secondary alteration process, its occurrence can be considered as an indicator of a secondary alteration history of the host rock. Low-total zircon is easily recognized by very low electron back-scatter intensities, which are closely related to the two main causes of the analytical shortfall (i.e., water content and porosity) and often lowered furthermore by the presence of light non-formula elements (especially P and Fe) up to the wt% range.     

Geology and Geochemistry of Highly Metamorphosed Footwall Alteration Zones in the Hongtoushan Volcanogenic Massive Sulfide Deposit,Liaoning Province,China

The Hongtoushan Archean Cu–Zn volcanogenic massive sulfide (VMS) deposit, which was metamorphosed (3.0–2.8 Ga) to upper amphibolite facies at temperatures between 600 and 650°C, occurs in the Hunbei granite–greenstone terrane, Liaoning Province of NE China. Stratiform cordierite–anthophyllite gneiss (CAG) that occurs hundreds of meters below the ore horizon in the Hongtoushan district corresponds to the metamorphosed semi‐conformable alteration zone of the VMS hydrothermal system, whereas the CAG that contains abundant deformed sulfide‐bearing quartz veins immediately below the main ore layer represents the metamorphosed discordant alteration zone. Whole‐rock geochemistry indicates that stratiform CAG was derived ultimately from five lithologies (basalt, basaltic andesite, andesite, dacite, and rhyolite), while discordant CAG derived from a single lithology (rhyolite). Amphibolite and biotite‐rich gneiss are identified as a metamorphosed least‐altered precursor for these CAGs. Mass change calculation indicates that, compared to the least‐altered rocks, stratiform CAG is enriched in Fe and Mg, and depleted in Na, K, Ca, Cu, Pb and Zn, while discordant CAG is enriched in Fe, Mg, Si, Na, Pb, Cu and Zn, and depleted in K. HREE and HFSE (Zr, Ti, Nb and Ta) behaved inertly during submarine alteration, whereas Rb, Sr, Ba and LREE (especially Eu) were leached off. Both stratiform and discordant CAGs are depleted in ¹⁸O, with values up to 7‰ lower than their corresponding least‐altered precursors. Addition of Fe, Mg, and depletion of Ca, K, Sr, and ¹⁸O, indicate that hydrothermal alteration for both types of CAGs was characterized by chloritization prior to metamorphism. Stratiform CAG could be used to evaluate the mineralization potential of VMS in metamorphic terranes, while discordant CAG containing sulfide‐bearing quartz veins could be a good indication for overlying stratiform massive sulfide ores as well as an exploration target itself.     

Metavolcanic host rocks,mineralization, and gossans of the Shaib al Tair and Rabathan volcanogenic massive sulphide deposits of the Wadi Bidah Mineral District,Saudi Arabia

The Wadi Bidah Mineral District of Saudi Arabia contains more than 16 small outcropping stratabound volcanogenic Cu–Zn–(Pb) ± Au-bearing massive sulphide deposits and associated zones of hydrothermal alteration. Here, we use major and trace element analyses of massive sulphides, gossans, and hydrothermally altered and least altered metamorphosed host rock (schist) from two of the deposits (Shaib al Tair and Rabathan) to interpret the geochemical and petrological evolution of the host rocks and gossanization of the mineralization. Tectonic interpretations utilize high-field-strength elements, including the rare earth elements (REE), because they are relatively immobile during hydrothermal alteration, low-grade metamorphism, and supergene weathering and therefore are useful in constraining the source, composition, and physicochemical parameters of the primary igneous rocks, the mineralizing hydrothermal fluid and subsequent supergene weathering processes. Positive Eu anomalies in some of the massive sulphide samples are consistent with a high temperature (>250°C) hydrothermal origin, consistent with the Cu contents (up to 2 wt.%) of the massive sulphides. The REE profiles of the gossans are topologically similar to nearby hydrothermally altered felsic schists (light REE (LREE)-enriched to concave-up REE profiles, with or without positive Eu anomalies) suggesting that the REE experienced little fractionation during metamorphism or supergene weathering. Hydrothermally altered rocks (now schists) close to the massive sulphide deposits have high base metals and Ba contents and have concave-up REE patterns, in contrast to the least altered host rocks, consistent with greater mobility of the middle REE compared to the light and heavy REE during hydrothermal alteration. The gossans are interpreted to represent relict massive sulphides that have undergone supergene weathering; ‘chert’ beds within these massive sulphide deposits may be leached wall-rock gossans that experienced silicification and Pb–Ba–Fe enrichment from acidic groundwaters generated during gossan formation.     

High and low temperature alteration of uranium and thorium minerals, Um Ara granites, south Eastern Desert, Egypt

The Um Ara area, in the south Eastern Desert of Egypt contains a number of uranium occurrences related to granitic rocks. U-rich thorite, thorite and zircon are the main primary uranium- and thorium-bearing minerals found in mineralized zones of the Um Ara alkali-feldspar granites; uranophane is the most common secondary uranium mineral. U-rich thorite contains blebs of galena, has rims of uranophane and contains inclusions of Zr-rich thorite. Electron probe microanalysis (EPMA) provides an indication of a range of solid solution between thorite and zircon, in which intermediate phases, such as Th-rich zircon and Zr-rich thorite, were formed. These phases have higher sum of all cations per formula (2.05 to 2.06 apfu, for 4 oxygen atoms) than that of ideal thorite and zircon. This is attributed to the presence of substantial amount of interstitial cations such as Ca, U and Al in these phases. Some zircon grains are stoichiometric in composition, other altered grains display lower SiO₂ and ZrO₂ contents. Enrichment of Th and U in altered zircon preferentially involves coupled substitution (Ca²⁺ + (Th,U)⁴⁺ ↔ 2Zr⁴⁺ + 2Si⁴⁺), implying that significant U and Th may enter the Zr and Si position in zircon. Negative correlation of Zr vs. Hf and Al may indicate that Hf and Al have been introduced to the zircon during later fluid alteration rather than during the primary magmatic event. A two-stage metallogenetic model is proposed for the alteration processes and origin of U- and Th-bearing minerals in the Um Ara alkali-feldspar granite: 1) the first stage was dominated by hydrothermal alteration and accompanied by albitization, k-feldspathization, desilicification, chloritization, hematitization, silicification, argillization, fluoritization and corrosion of primary U-bearing minerals. Solid-solution between thorite and zircon occurred during this stage. The second stage occurred at the near-surface profile where circulating meteoric water played an important role in mobilizing the early formed primary U-bearing minerals. Uranium was likely transported as a calcium uranyl carbonate complexes. When these complexes lost their stabilities by precipitation of calcite, they decomposed in the presence of silica to form uranophane.     

Extreme enrichment of Sb, Tl and other trace elements in altered MORB

We have analyzed 25 trace elements (e.g., Sb, Tl, Sn, rare earth elements (REE), Th, U, Nb, Pb, Zr, Hf, and Y) in altered mid-ocean ridge basalts (MORB) from locations near the mouth of the Gulf of California. Our results imply that the heavy REE and Y are not seriously affected by seawater alteration, in agreement with previous studies. The elements Zr, Hf, Nb, light REE and Sr are enriched up to a factor of 2 in some extremely altered samples. However, element ratios between Zr, Hf, and Nb (e.g., Zr/Hf, and Zr/Nb) are not greatly affected, presumably due to the chemical similarity of these elements during any exchange process. The enrichment of Th and Sn is even higher. Antimony, Tl, Cs, Rb, Rb, and Ba are most easily altered by water-rock interaction and are therefore the best indicators for seawater alteration. The enrichment factor of the most mobile element Sb is up to 2000.

There is a weak correlation between the concentration in seawater and the enrichment factors. On the other hand, the worldwide pelagic clay pattern matches the enrichment pattern much more closely, and the limited data available for local oceanic sediments give an even better correlation. A plausible model to explain the enrichment pattern may be an elemental exchange between basalt and seawater that had interacted earlier with overlying sediments.     

Geochemistry of High-silica Peralkaline Rhyolites, Naivasha, Kenya Rift Valley

The Recent (<15000 y) volcanic complex of southwest Naivasha,Kenya, consists of mildly peralkaline (comenditic) rhyolitedomes, lava flows, air fall pumices, and lake sediments, withminor, peripheral, basalts and hawaiites. The comendites areeither aphyric or sparsely porphyritic, few samples containing>5 per cent phenocrysts. Phenocryst minerals are quartz-sanidine-ferrohedenbergite-fayalite-titanomagnetite-ilmenite-riebeckite-arfvedsonite-aenigmatite-biotite-zircon.Ferrohedenbergite and zircon are restricted to less peralkaline,and amphibole, aenigmatite, and biotite to more peralkaline,rocks. The comendites show unusually strong enrichment in Cs, F, Hf,Nb, Rb, REE, Ta, Th, U, Y, Zn, and Zr, and extreme depletionin Mg, Ca, Ba, Co, and Sr. REE patterns are moderately LREE-enriched,with large, negative Eu anomalies. Values of LIL/HFS elementratios, such as Th/Ta and Rb/Zr, are unusually high for peralkalinerhyolites, and are consistent with a substantial crustal componentin the comendites. Parameters such as LREE/HREE and Zr/Nb ratiosindicate that the Naivasha rhyolites represent several pulsesof closely related, but subtly different, magmas. Sanidine/glasspartition coefficients for Ba, Pb, Rb, Sr, U, and the REE arepresented for one specimen. Major and trace element modelling, and feldspar-rock relationships,show that closed system crystal fractionation cannot alone accountfor the overall compositional variations in the comendites.A model involving partial melting of variable crustal sourcerocks and migration of dissolved volatile-metal complexes maybe appropriate at Naivasha.     

Effects of alteration on element distributions in archean tholeiites from the Barberton greenstone belt,South Africa

Major and trace element and modal analyses are presented for unaltered, epidotized, and carbonated tholeiite flows from the Barberton greenstone belt. Au, As, Sb, Sr, Fe⁺³, Ca, Br, Ga, and U are enriched and H₂O, Na, Mg, Fe⁺², K, Rb, Ba, Si, Ti, P, Ni, Cs, Zn, Nb, Cu, Zr, and Co are depleted during epidotization. CO₂, H₂O, Fe⁺², Ti, Zn, Y, Nb, Ga, Ta, and light REE are enriched and Na, Sr, Cr, Ba, Fe⁺³, Ca, Cs, Sb, Au, Mn, and U are depleted during carbonization-chloritization. The elements least affected by epidotization are Hf, Ta, Sc, Cr, Th, and REE; those least affected by carbonization-chloritization are Hf, Ni, Co, Zr, Th, and heavy REE. Both alteration processes can significantly change major element concentrations (and ratios) and hence caution should be used in distinguishing tholeiites from komatiites based on major elements alone. The amount of variation of many of the least mobile trace elements in the altered flows is approximately the same as allowed by magma model calculations. Hence, up to about 10% carbonization and 60% epidotization of tholeiite do not appreciably affect the interpretation of trace-element models for magma generation.     

阿尔金南缘构造混杂岩带中角闪辉长岩地球化学特征及同位素测年

阿尔金茫崖地区发育的角闪辉长岩,属钙碱性岩石系列.岩体相对富集LREE、大离子亲石元素(Rb、Ba、Sr)以及U和Pb,亏损高场强元素(Nb、Ta、Zr、Hf),类似于岛弧环境产物.推测其源区为岩石圈地幔,原生岩浆通过幔源物质部分熔融产生,在上升就位过程中遭受地壳混染.LA-ICP-MS锆石U-Pb测年获得谐和年龄44...     

榴辉岩韧性剪切变形中的流体作用及地球化学效应

韧性剪切带是在较高温条件下机械与化学作用高度耦合的系统。韧性剪切带普遍含有一种或多种流体,流体是韧性剪切变形高度局域化和剪切变形组构形成的主要影响因素。在强应变岩石中,可利用变形矿物的元素地球化学的变化特征来反演参与变形的流体的地球化学性质。苏鲁-大别超高压变质带中,多数榴辉岩经历了强烈的韧性剪切应变和流体作用,形成高度局域化、尺度不等的韧性剪切变形带,为研究在高压-超高压条件下,参与流体的性质,应变和流体相互作用提供了重要机会。样品DH-2G是一强烈剪切榴辉岩,由石榴石、绿辉石、金红石,及少量的多硅白云母、黝帘石、角闪石、斜长石、石英、锆石和磷灰石组成。在不同应变域,矿物的粒度变化明显。在类变斑晶域,石榴石粒度比剪切条带中的小2~4倍。在以下讨论中,与细粒石榴石(石榴石-I)共生的绿辉石称为绿辉石-I,与粗粒石榴石(石榴石-II)共生的绿辉石称为绿辉石-II。电子探针和原位LA-ICP-MS测试结果表明:(1)除Ni外,两类石榴石没有显著区别;(2)两类绿辉石在主量元素上也没有明显的差异,但微量元素特征,尤其是稀土元素上,存在明显的变化。与绿辉石-I相比,绿辉石-II具有以下特征:(1)轻稀土明显降低,达25~40倍左右;(2)重稀土明显富集,升高5~10倍;(3)Ni、Zn、Co、Sc、Ba、Zr、Y、Rb、Hf和HREE明显升高,而 V、Sr、U、Th、Pb和LREE明显降低;和(4)微弱的正Eu异常, Eu/Eu^*=1.04~1.45,绿辉石-I基本上没有Eu异常。并且,剪切带域金红石的Nb/Ta比值从核部(<19.0)到边部(>20.0)显著增加。岩相学、元素和温压估计研究表明:在剪切过程中不存在外来流体,流体是由无水矿物(绿辉石、石榴石、金红石等)在超高压榴辉岩折返减压过程中通过压力降低出溶结构羟基或分子水,释放出流体,流体富含V、Sr、U、Th、Pb、Zr、Nb和LREE等微量元素。这些流体显著影响深俯冲地壳的流变学性质,是迁移活动元素(LILE和LREE)及保守元素(HFSE等)的重要载体,对超高压岩片的快速折返发挥关键作用。     

内蒙古乌拉特中旗索伦山地区中元古代石英闪长岩的发现及其构造意义

在内蒙古乌拉特中旗索伦山地区首次发现了中元古代的石英闪长岩岩体。岩石富镁、碱(尤其富钠)和钙,贫硅、钛,属碱性系列岩石。轻稀土富集,重稀土相对亏损,Eu异常不明显。微量元素富集Rb、Ba、Sr等大离子亲石元素,亏损Nb、Ta、Zr、Hf、Ti等高场强元素,显示出与板块俯冲作用有关的弧岩浆特点。岩体形成于与俯冲消减作用有关的火山弧构造环境,其岩浆来源于受俯冲的板片流体/沉积物交代的岩石圈地幔部分熔融,源岩可能为变质玄武岩或变质英云闪长岩。采用LA- ICP- MS锆石U- Pb法获得的207Pb/206Pb加权平均年龄为1410±11Ma,代表了该石英闪长岩体的结晶年龄。结合区域地质背景,认为至少在1410Ma时华北北缘发生了板块俯冲汇聚作用,石英闪长岩可能是板块俯冲汇聚作用的直接产物。该成果为华北板块北缘俯冲汇聚作用开始的时限提供了新证据,同时为华北板块北缘在中元古代构造演化提供了重要依据。     

Crystallogenetic models for metasomatic replacement in zircons: implications for U–Pb geochronology of Precambrian rocks

Recrystallization of zircons under the influence of fluids was studied using examples from Precambrian rocks (microcline granites, metasedimentary, and mafic rocks) of the Kola Peninsula. All zircon crystals showed complex internal textures visible by cathodoluminescence and backscattered electron (BSE) imaging. Detailed mineralogical and geochemical studies with subsequent secondary ion mass spectrometer U–Pb dating of different zircon domains show that secondary texture formation can be interpreted in terms of metasomatic replacement of zircon crystals on the base of crystallogenetic experimental models. Mechanisms of zircon replacement and interpretation of U–Pb ages for secondary zircon domains are dependent on the degree of damage of the zircon structure and the fluid composition. The recrystallization of metamict zircon without additional supply of new zircon substance (Zr, SiO₂) goes with the dissolution of amorphous domains and precipitation of new polycrystalline zircon, which preserves the U–Pb initial age, but loses radiogenic lead, and the lower intercept of Discordia lines with the Concordia curve determines the time of fluid influence. The recrystallization of metamict zircon or crystalline zircon with high contents of impurities with additional supply of Si and Zr forms monocrystalline replacements. Dissolution of primary zircon is accompanied by growth of new zircon domains differing in the composition of isomorphic impurities and zones of transitional composition, whose ages have no geological sense. The study is of particular importance for zircons from Precambrian rocks with long and complex histories.     

Hydrothermal alteration of felsic volcanic rocks associated with massive sulphide deposition in the northern Iberian Pyrite Belt (SW Spain)

Massive sulphide deposits of the northern Iberian Pyrite Belt (IPB) are mainly hosted by felsic volcanic rocks of rhyolitic to dacitic composition. Beneath most of the massive ores of this area (e.g., Concepción, San Miguel, Aguas Teñidas Este or San Telmo deposits) there is usually a wide hydrothermal alteration halo associated with stockwork-type mineralization. Within these alteration envelopes there are two principal rock types: (1) chlorite-rich rocks, linked to the inner and more intensely altered zones and dominantly comprising chlorite+pyrite+quartz+sericite (+carbonate+rutile+zircon+chalcopyrite), and (2) sericite-rich rocks, more common in the peripheral zones and showing a dominant paragenesis of sericite+quartz+pyrite+chlorite (+carbonate+rutile+zircon+sphalerite). Mass-balance calculations comparing altered and least-altered felsic volcanic rocks suggest that sericitization was accompanied by moderate enrichment in Mg, Fe and H₂O, with depletion in Si, Na and K, and a slight net mass loss of about 3%. Chloritization shows an overall pattern which is similar to that of the sericitic alteration, but with large gains in Fe, Mg and H₂O (and minor enrichment in Si, S and Mn), and a significant loss of Na and K and a minor loss of Ca and Rb. However, chloritization has involved a much larger net mass change (mass gain of about 28%). Only a few elements such as Nb, Y, Zr, Ti, P and LREE appear to have remained inert during hydrothermal alteration, whilst Ti and Al have undergone very minor mobilization. The results point to the severity of the physico-chemical conditions that prevailed during the waxing stage of the ore-forming hydrothermal systems. Further, mineralogical and geochemical studies of the altered footwall rocks in the studied deposits indicate that hydrothermal ore-bearing fluids reacted with host rocks in a multi-stage process which produced a succession of mineralogical and chemical changes as the temperature increased.     

福建东山变质岩LA-ICP-MS同位素年龄及其地质意义

LA-ICPMS锆石U-Pb法测得东山地区变质岩的年龄为176.8 Ma、203 Ma、230 Ma、1320 Ma、1810 Ma等,其中176.8 Ma为火山-沉积岩中的岩浆锆石,代表着地层形成的年龄;203 Ma、230 Ma为继承岩浆碎屑锆石,代表了晚三叠世源区火成岩的物源年龄;1810 Ma、1320 Ma为变质岩继承锆石,代表了基底物源年龄。地球化学分析结果表明:岩石具贫硅、高铝、铁,富钾,Co、Ni、V含量低,Ba、Rb、Zr、Hf、U、Th及La含量高,具壳源特征;稀土元素总量较高,轻稀土略富集,轻、重稀土元素分异不明显,无明显的负铕异常,表明变质岩的部分物质与地幔有关。福建东山地区亲营山组可能属早侏罗世晚期华夏陆块边缘进入裂陷期海陆交互相火山-沉积的产物。     

大兴安岭鄂伦春地区满克头鄂博组火山岩形成年代及地球化学

对大兴安岭鄂伦春地区满克头鄂博组火山岩年代学与岩石地球化学特征进行分析,锆石U-Pb测年结果表明火山岩形成时代为晚侏罗世(152.9±1.7 Ma).地球化学特征表明该组火山岩具有富硅、碱,贫钙、镁特征,且具较低Mg^#值及极低的Cr、Co、Ni值的特点.微量元素相对富集大离子亲石元素(LILE) K、Rb、Ba和LREE,亏损高场强元素Nb、Ta、Ti.火山岩具有地壳源区特征,Rb/Sr值平均为0.66,远高于N-MORB (0.03)、OIB (0.047)和E-MORB (0.033),Ti/Zr值平均为13.94,与壳源岩浆的范围(<20)一致,表明该类岩石的原始岩浆起源于地壳岩石的部分熔融.根据岩石地球化学特征,结合区域已有研究资料,本区满克头鄂博组火山岩的形成应与蒙古-鄂霍次克洋向南东方向俯冲作用有关.     

山东省日照市岚山头片麻状碱性花岗岩的岩石地球化学、年代学和成因研究

岚山头片麻状碱性花岗岩是由古生代的沉积岩变质而成,其主量元素具高硅(SiO₂=75% ～77%)、富碱(Na₂O+K₂O=8.38% ～8.60%)、低铝(Al₂O₃=11.72% ～12.05%)、贫钙(CaO=0.09% ～0.53%)等特征,其微量元素特征显示为,富集Rb、Ba、Th、U等大离子亲石元素和Pb,贫Nb、Ta、Zr、Hf、Ti 等高场强元素,以及富集轻稀土(LREE/HREE=6.32～9.86)和铕负异常明显到中等(δ_Eu=-0.03～0.51)等,在研究锆石"CL"图像基础上,对具核-幔-壳3层结构的变质复合锆石的幔(第一次变质作用的产物),获得年龄为245 Ma和 235 Ma,反映第一次超高压变质时间,也就是它的成岩时间,相当于早-中三叠世,变质复合锆石的新壳,年龄为222～198 Ma,第二次变质时间,也就是退变质作用的时间,相当于晚三叠世,对变质复合锆石老核"继承锆石"定年显示:既含有大量的属于元古代不同年龄段的岩浆碎屑锆石(864 Ma～781 Ma～701 Ma～688 Ma～631 Ma～598 Ma～592 Ma),又含有比较多的属于古生代不同年龄段的岩浆碎屑锆石等(544 Ma～500 Ma～447 Ma～339 Ma～306 Ma),表明其寄主岩的时代不可能为元古代,只能是古生代。     

CHIME dating of monazite, xenotime, zircon and polycrase: Protocol, pitfalls and chemical criterion of possibly discordant age data

This paper outlines the CHIME (chemical Th–U-total Pb isochron method) dating method, which is based on precise electron microprobe analyses of Th, U and Pb in Th- and U-bearing accessory minerals such as monazite, xenotime, zircon and polycrase. The age-mapping technique that is applicable to young monazite and zircon is also described. CHIME dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation, and then these data are used to construct a “pseudo-isochron” from which an age can be obtained via regression. This method, when coupled with discrimination of possibly concordant age data by chemical criteria such as the (Ca + Si)/(Th + U + Pb + S) ratio for monazite and Ca and S contents for zircon, has the potential advantage of significant precision, and the ability to work with minerals that have a significant initial common Pb component. This technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis. Many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data. Critical factors include the roles of collimator slit, detector gas, background estimation, accelerating voltage, probe current, X-ray interferences and count rate in affecting the accuracy, and a way to apply the Th and U interference correction without pure Th- and U-oxides or synthesized pure ThSiO₄. The age-mapping procedure for young monazite and zircon includes acquiring PbMα (or PbMβ) intensity of individual pixels with multiple spectrometers, correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement, calibrating of intensity with standards and calculating of ages from the Th, U and Pb concentrations. This technique provides age maps that show differences in age domains on the order of 20 Ma with in monazite as young as 100 Ma. The effect of sample damage by irradiation of intense and prolonged probe measurement is also described.     

Retention of uranium in complexly altered zircon: An example from Bancroft,Ontario

Mesoproterozoic (~ 1050 Ma; Stenian) zircon crystals from the Saranac Prospect, Bancroft, Ontario, contain up to ~ 1 wt.% U and ~ 0.15 wt.% Th and, correspondingly, they are for the most part extensively radiation-damaged (calculated total α-doses 2.3?35.3 × 10¹⁸/g). The crystals show textures of complex, intense chemical alteration that is attributed to multiple, low-T replacement events along fluid-controlled reaction fronts. Centers of crystals appear totally replaced; the primary zoning is virtually erased and the material has high porosity and numerous inclusions. Interior regions surrounding the central reworked areas still exhibit primary igneous-type zoning; in those regions the alteration emanates from fractures and then follows the more radiation-damaged growth zones. Altered areas are typically recognized by their high porosity, low BSE intensity, and deficient analytical totals. Those regions often have lost a significant fraction of their radiogenic Pb. They are in general somewhat depleted in Zr, Si, and U, and are notably enriched in Ca and Fe. Element maps reveal elevated concentrations of Al and Y within filled fractures. Our observations indicate that the fluid-driven ion exchange is mainly controlled by the accessibility of micro-areas with elevated levels of radiation damage to transporting fluids via “fast pathways”. Most importantly, there is apparent Zr?Si?U equilibrium between initially existing and newly formed zircon. The retention of U after the chemical replacement (94 ± 14% relative to the original U content in the respective zones) does not significantly fall below the retention of two major cations Zr (95 ± 4%) and Si (95 ± 2%). In spite of the partially extreme hydrothermal alteration overprinting, the original U zoning in the crystals is well preserved. These observations suggest that preferential chemical leaching of U from zircon is clearly not a general feature of this mineral. This in turn seems to question the general validity of hydrothermal experiments to low-T, fluid-driven alteration of zircon in geological environments. The observed apparent immobility of U may affect the interpretation of U?Pb discordance in zircon, and the performance assessment of this mineral as potential waste form for actinides.     

华北克拉通东南缘蚌埠隆起带荆山—涂山岩体地质地球化学特征再认识

对位于华北克拉通东南缘蚌埠隆起带的荆山—涂山岩体进行了野外观察、年代学和地球化学研究,测得弱片麻状二长花岗岩体中U Pb年龄为(1604±13)~(1628±18) Ma,为晚侏罗世岩浆活动的产物;岩石地球化学测试结果显示,岩体总体富硅、富碱,具有高钾钙碱、准铝—过铝质特征,LREE相对富集,具明显的正Eu异常,Ce异常不明显或无异常,K、P和Ti有较明显的亏损,Rb、Ba、U、La、Sr、Zr和Sm元素相对富集。成因类型判别图显示其具备I型花岗岩特征;锆石Lu Hf同位素及全岩Sm Nd同位素组成暗示其物质起源主要为新太古代陆壳,且岩浆源区主要为华北克拉通下地壳物质;构造环境判别图中,投影点落于火山弧花岗岩区域,暗示岩体形成于扬子克拉通沿郯庐断裂带俯冲于华北克拉通之下的构造背景,与华北克拉通晚侏罗世期间的动力学背景相一致。