锆石U-Pb年代学和地球化学对浙皖马鞍山高分异岩体成因的约束

为了解由早期(伟晶、巨晶)斑状二长花岗岩、中期(细粒)花岗岩和晚期花岗(斑)岩脉组成的马鞍山杂岩体的成因, 采用SHRIMP和LA-ICP-MS锆石U-Pb法厘定其侵入时代, 年龄显示伟晶斑状二长花岗岩为132.2±1.6 Ma, 巨晶斑状二长花岗岩为127. 7±1.2 Ma, 细粒花岗岩为128.3±1.1 Ma, 花岗斑岩脉为127.4±1.8 Ma.岩石地球化学研究结果表明岩体从早到晚具有从钾玄岩系列向高钾钙碱系列演变特征, 分异演化程度逐渐变高; 斑状二长花岗岩具有高REE含量, 轻重稀土分异较为明显, 具较强负铕异常和弱右倾的配分曲线特征, 富集K、Th、U、Rb等元素, 弱亏损Ba、Sr、P、Nb、Ta、Ti等元素; 细粒花岗岩及花岗(斑)岩脉具有较低REE含量, 轻重稀土分异不明显, 具强负铕异常和"V"型配分曲线特征, 富集K、Th、U、Rb等元素, 强亏损Ba、Sr、P、Nb、Ti等元素.马鞍山岩体为浙西北-皖南地区早白垩世俯冲造山后陆内拉张作用环境下下地壳部分熔融的同源岩浆侵位结晶分异作用形成的产物, 具有高分异I型花岗岩的特征.      

Metasomatic crystallization of muscovite in granite and tourmaline in schist related to pegmatite emplacement near Yellowknife,Canada

Numerous pegmatite dikes occur in the Sparrow pluton (muscovite-biotite granite) and in the adjacent cordierite-zone schist-hornfels of the Yellowknife Supergroup. Where pegmatite dikes cut granite, the adjacent granite is enriched in muscovite and apatite, and depleted in K-feldspar. Mass transfer calculations, based on rock, mineral, and modal analyses, indicate that H, P, and locally B, Ti, Fe, and Ca were added, and K, Sr, Ba, and locally Na were removed (hydrogen metasomatism). In one alteration zone (8 cm wide) the calculated change (in terms of mols/gram of unaltered granite) is, 600 K-feldspar+24 biotite+190 plagioclase +[770 H+36 P+3 Ti+13 Fe+13 Ca] 400 muscovite+1100 quartz +11 apatite+[240 Na+260 K]. Where pegmatite dikes cut schist-hornfels (biotite-plagioclase-quartz), the adjacent rock is, in places, enriched in tourmaline, apatite, and quartz, and depleted in biotite and plagioclase. These alteration zones are variable in width; most are less than 20 cm wide. Mass transfer calculations, based on rock, mineral, and modal analyses, indicate that B, P, Zn, and locally Ca, Fe, and Al were added, and that Na, K, Fe, Rb, Sr, Ba, and locally Mg and Si were removed (boron metasomatism). In one zone, 2 cm wide, the calculated reaction (in units of mols/gram of unaltered schist) is, 730 biotite+1530 plagioclase +[1080 B+600 H+430 P+360 Ca] 480 tourmaline+480 quartz+115 apatite +[3630 Si+870 Na+590 K+110 Fe]. Changes in the volume fraction of muscovite, K-feldspar, tourmaline, and biotite, relative to distance from pegmatite, are progressive, and in most alteration zones may be expressed by use of an error-function equation. Some tourmaline zones are more complex. Zone formation is considered in terms of a steady-state reaction model in which grainboundary diffusion is the transport mechanism.     

Zur Geochemie einiger Pegmatite der Ostalpen

Zusammenfassung 139 Proben von Gesteinen und Mineralen des Villacher Granites und von Pegmatitvorkommen im Kärntner Altkristallin (Kärnten/Österreich) sowie vergleichsweise 13 Proben von anderen Pegmatiten der Ostalpen und solchen der Böhmischen Masse wurden mit Hilfe flammenphotometrischer, emissionsspektrographischer, röntgenfluoreszenzspektrometrischer und teilweise auch gammaspektrometrischer Methoden auf die Elemente Li, Na, K, Rb, Be, Ca, Sr, Ba, B, Sc, Y, Yb, (Th, U), Ti, Zr, Ni und Ga analysiert.Es konnte gezeigt werden, daß zwischen dem voralpidischen Villacher Granit und dem pegmatitischen und aplitischen Ganggefolge im Kärntner Altkristallin zwischen Ossiacher See und Drautal Blutsverwandtschaft bestehen sollte.Mit zunehmender Größe des Pegmatitvorkommens und offenbar auch mit Annäherung an den Granit nimmt der Gehalt an Li, Rb, Be, B, Ga und Pb zu, der Gehalt an Ca, Sr, Ba, Ti, Sc u. a. hingegen ab. Das K/Rb-Verhältnis ist ebenso ein verläßlicher Indikator. In kleineren Pegmatitkörpern nähert sich der Gehalt der pegmatophilen Spurenelemente dem des Nebengesteins.Es werden Abhängigkeiten zwischen Mindestkonzentrationen und Mineralisationen gefunden: Granite der Ostalpen mit Berylliumgehalten über 5 ppm Be lassen Beryllmineralisationen in ihren Pegmatiten erwarten. In Pegmatit-Proben mit über 50 ppm Be sind unter dem Mikroskop Beryllnädelchen nachweisbar.Zinnstein tritt auf, wenn das Pegmatitgestein mehr als 20 ppm Sn enthält, Spodumen bei>0,1% Li und Xenotim bei>20 ppm Y.

---

Contribution to the geochemistry of some pegmatites in the Eastern Alps

Summary 139 samples of rocks and minerals from the granite of Villach and from pegmatites of the Carinthian Old Cristalline Complex (Austria) and additional 13 samples for comparison purposes from other pegmatites in the Eastern Alps and the Bohemian Mass were analyzed for the elements Li, Na, K, Rb, Be, Ca, Sr, Ba, B, Sc, Y, Yb, (Th, U), Ti, Zr, Mn, Co, Ni, and Ga using flamephotometric, emission-spectrographic, x-ray-fluorescence spectrometric and partially gamma-spectrometric methods.It could be shown that the prealpidic granite of Villach and the pegmatites of the Carinthian Old Cristalline Complex lying between the Lake of Ossiach and the Drau Valley are probably consanquineous. The contents of Li, Rb, Be, B, Ga, and Pb increase with the size of the pegmatite and with increasing proximity to the granite. Other elements, like Ca, Sr, Ba, Ti, Sc, show the reverse relation.Also the K/Rb-ratio is a good indicator. The trace-element content of small pegmatite bodies approaches that of the wall rocks.Relationships between the concentration of some elements and the mineralization have been found: One may expect beryllium minerals in pegmatites of granites of the Eastern Alps if the granite contains more than 5 ppm Be. Pegmatites samples containing more than 50 ppm Be show needles of beryll under the microscope. Cassiterite appeares, if the content of the pegmatitic rock reaches 20 ppm Sn, spodumen, if it exceeds 0,1% Li, and Xenotim, if it exceeds 20 ppm Y.

---

---

Mit 7 Abbildungen     

Trace element chemistry of lithium-rich micas from rare-element granitic pegmatites

Summary Granitic pegmatites characterized by advanced accumulation and fractionation of incompatible rare lithophile elements (Li, Rb, Cs, Be, Ta Nb, B, P and F), often contain mineral assemblages which host lithium-rich micas. Lepidolite and lithian muscovite occur in high-pressure spodumene, low-pressure petalite, phosphorus-enriched amblygonite and fluorine-rich lepidolite subtypes of orogenic affiliated complex type granitic pegmatites and rarely in anorogenic affiliated amazonite-bearingTrace element data determined by X-ray fluorescence for lepidolite of various pegmatite subtypes, morphology (book, scaly, fine-grained), position within the pegmatite (primary zones, replacement units, pockets), mineral assemblages and tectonic affinity (orogenic vs anorogenic) show extreme fractionation of Rb and Cs; modest levels of T1, Ga, Nb, Ta, Sn and Zn; and typically low abundances of Ba, Sr, Ni, Pb, Y, V, W and Zr. Extreme fractionation is indicated by low values of K/Rb, K/Cs and Nb/Ta which are lowest in lepidolite from petalite subtype pegmatites.No systematic differences in trace element content is evident among the different lepidolite morphologies or paragenetic position. Lepidolite from spodumene subtype pegmatites are generally slightly less fractionated than those from petalite or lepidolite subtype pegmatites.

---

Spurenelement-Chemie von Lithium-reichen Glimmern aus granitischen Pegmatiten

Zusammenfassung Granitische Pegmatite, die durch fortgeschrittene Anreicherung und Fraktionierung von inkompatiblen, seltenen, lithophilen Elementen (Li, Rb, Cs, Be, Ta Nb, B, P und F) charakterisiert sind, enthalten häufig Mineralparagenesen mit Lithium-reichen Glimmern. Lepidolith und Li-Muskowit treten in Hochdruck-Spodumen, in Niedrigdruck-Petalit, in mit Phosphor angereichertem Amblygonit und in Fluor-reichen Lepidolith-Unterarten aus komplexen orogenen granitischen Pegmatiten und selten auch aus anorogenen, Amazonit-führenden Pegmatiten, auf.Spurenelement-Daten aus der Röntgenfluoreszenzanalyse von Lepidolith aus verschiedenen Pegmatit-Untertypen, die Morphologie (tafelig, schuppig, feinkörnig), die Position innerhalb des Pegmatits (primäre Zonen, verdrängte Einheiten, Taschen), Mineralbestände und tektonische Affinität (orogen gegen anorogen) zeigen eine extreme Fraktionierung von Rb und Cs, bescheidene Gehalte an TI, Ga, Nb, Ta, Sn und Zn; und typischerweise geringe Häufigkeiten von Ba, Sr, Ni, Pb, Y, V, W und Zr. Die extreme Fraktionierung wird durch niedrige Werte von K/Rb, K/Cs und Nb/Ta angezeigt, die in Lepidolith von Pegmatiten des Petalit-Subtyps am niedrigsten sind.Aus den verschiedenen Morphologien oder paragenetischen Positionen von Lepidolith sind keine systematischen Unterschiede im Spurenelementgehalt ersichtlich. Lepidolith aus Pegmatiten des Spodumen-Subtyps sind generell etwas weniger fraktioniert als jene von Pegmatiten des Petalit- oder Lepidolith-Subtyps.

---

---

With 4 Figures     

Characteristics of geochemical evolution of trace elements and REE in Gejiu granites,Yunnan Province

The evolution characteristics of Gejiu granites, Yunnan Province are described in terms of their petrology, especially their trace elements and REE geochemistry. The three major types of Gejiu granites: porphyritic biotite monzonitic granite (stage I), medium-coarse-grained biotite-K-feldspar granite (stage II) and two-mica alkali-feldspar granite (stage III) are thought to have been formed successively from the same granite magma source through fractional crystallization (Rayliegh fractionation), because linear correlations are found between log(Rb/Sr)-log Sn, log(Rb/Ba)-log Sn, log(Rb/Ba)-log(Rb/Sr), log La-log Sr, log Ce-log Sr, log Eu-log Sr, etc. In addition, the characteristics of REE distribution patterns in these three major types of granites also reflect the magmatic differentiation features of Gejiu granites. Of the three major types, the two-mica alkali-feldspar granite of stage III underwent the strongest differentiation, and thus has the closest genetic relationship with the Gejiu tin-polymetallic ore deposit. Such tin-polymetal mineralized granites are characterized by high Rb/Sr and Rb/Ba ratios, low K/Rb and ΣCe/ΣY ratios and remarkable Eu depletion.     

The baryte-bearing beryl-phosphate pegmatite Plössberg—A missing link between pegmatitic and vein-type barium mineralization in NE Bavaria, Germany

Pegmatites and aplites enriched in P, Be, Nb, Ta and Li occur in the high-temperature metamorphic lithological units of the NE Bavarian Basement, SE Germany. They are accompanied by Ba mineralization, in vein-type deposits in the basement as well as in its foreland. Locally, Ba minerals are encountered in the late Variscan pegmatites and aplites too. The shallow discordant stock-like pegmatites (Hagendorf-type) are barren as to Ba, but in the tabular, concordant aplites and pegmatites Ba was concentrated (Plössberg-type). These concordant pegmatites and aplites are supposed to be the root zone of the intrusive pegmatites. In the rare case of low sulfur fugacity, Ba forms Ba–Zr–K–Sc phosphates/silicates in the pegmatites (transition of magmatic into the hydrothermal stages I/II). Under high sulfur fugacity, Ba is accommodated within the same stages in the structure of baryte. Barium is not accommodated in the lattice of phosphates during or in the immediate aftermaths of the emplacement of these Be–P–Nb–Ta pegmatites (stage III). This element shows up again in APS minerals during supergene alteration under acidic conditions (stage IV). Considering the host rocks of baryte mineralization, the Sr contents of baryte increased from the early Paleozoic to the Late Triassic. The Sr contents of baryte are a function of the depth below ground in the vein-type deposits and in the shear-zones bounding the tabular concordant pegmatites. Beryl is not only a marker mineral for the shear-zone-hosted pegmatites but can also be used as a tool for the geodynamic positioning of these pegmatites using its oxygen isotopes. A subdivision of the pegmatites into intrusive and shear-zone hosted may be achieved by its REE and minor elements.     

Geochemical diversity of minerals of the pyrochlore group

A systematic and rational nomenclature of minerals of the pyrochlore group are developed based on the results obtained by processing 671 chemical analyses of pyrochlore-group minerals from carbonatite complexes, alkali rocks, and their pegmatites, granite pegmatites, and alkali and albitized granites. The proportions of Nb, Ta, and Ti are typomorphic of pyrochlore from these four types of geological environments. The paper lists pervasive characteristics of the distribution of Na and Ca, REE, Th and U, Sr and Ba, K and Cs, Pb, Sn, Sb, and Bi in the minerals. Based on the occurrence of compositions with elevated concentrations of typomorphic elements at site B in the structure of the minerals, pyrochlore subspecies are recognized: pyrochlore, Ta-pyrochlore, Ta,Ti-pyrochlore, Zr-pyrochlore, Nb-betafite, Ta-betafite, Ti-betafite, Ti-microlite, Nb,Ti-microlite, Nb-microlite, and microlite, as well as 60 geochemically significant varieties with the predominance of certain cations at site A (REE-pyrochlore, U-pyrochlore, etc.). Aspects of a rational systematics of minerals of complicated isomorphic series are discussed.     

Petrology and Li-Be-B geochemistry of muscovite-biotite granite and associated pegmatite near Yellowknife,Canada

Prosperous granite (Rb-Sr 2520±25 Ma) occurs as several plutons (1–380 km² outcrop area) in a thick succession of metamorphosed greywacke-mudstone of the Yellowknife Supergroup. The average mineral content of the Sparrow pluton (in vol.%) is quartz (32), plagioclase (31), K-feldspar (24), muscovite (9), biotite (3), and apatite (<1). Average trace-element concentrations (in ppm) are Li (140), Be (4), B (28), Zn (47), Rb (250), Sr (76), Zr (75) and Ba (360). The central portion of the pluton is slightly richer in K, Sr, and Ba than the margin. Li is concentrated in mica (Li in biotite/Li in muscovite=4.7), and Be and B in muscovite and plagioclase. Countless pegmatite dikes occur in the Sparrow pluton and in schist-hornfels to the east; the outer limit is marked by the cordierite isograd, 9 km from the granite contact. Dikes vary greatly in size (1 km to a few cm in length), in mineral content (quartz, albite, K-feldspar, muscovite, tourmaline, beryl, spodumene), in major element composition (especially the NaK ratio), and in trace-element content (Li 18–5000 ppm, Be 5–260 ppm, B 20–150 ppm). Compared with Prosperous granite, the pegmatite bodies are richer in P and Rb, and poorer in Ti, Fe, Mg, Zr, and Ba. Dikes rich in tourmaline, beryl, and spodumene occur in overlapping zones situated progressively farther from the centre of the Sparrow pluton. The composition of tourmaline is related to host rock; the highest concentrations of Fe and Zn occur in crystals from pegmetite and the highest concentrations of Mg and V occur in crystals from tourmalinized schist, while those from granite and quartz veins occupy on intermediate position. Complex compositional zoning is present in some tourmaline crystals in pegmatite. Estimates of temperature (500°–600° C) and pressure (2–4 kb) of granite emplacement, based on the distribution of andalusite and sillimanite in the contact rocks, suggest that the final stage of granite emplacement occurred at sub-solidus conditions. A vaportransport model is proposed to explain the widespread distribution of the pegmatite dikes and their extreme compositional variability. Some of the pegmatite constituents, including Li, Be, and B, were possibly derived from Yellowknife graywacke and mudstone.     

Geochemistry and evolution of the Wolf River Batholith,a late Precambrian Rapakivi Massif in North Wisconsin,U.S.A.

The Wolf River Batholith is an anorogenic rapakivi massif in central and northeastern Wisconsin with an age of 1.5 Ga. The Batholith has alkaline affinities and consists of biotite granite and biotite-hornblende adamellite with minor occurrences of quartz syenite and older monzonite and anorthosite. The batholith is part of a major Late Precambrian (1.4–1.5 Ga) magmatic event of continental proportions, represented by separate intrusions extending from Labrador to southern California (Silver et al., 1977).The major and trace element composition (Li, Rb, Sr, Ba, and REE) of 40 samples from the anorthosite, monzonite, and rapakivi granite and adamellite plutons precludes a comagmatic (although not cogenetic) model between all three rock units. However, the monzonite may be related to the anorthosite alone by fractional crystallization of plagioclase, orthopyroxene, clinopyroxene, and apatite. Alternatively, the monzonite may be a separate parent melt or a hybrid associated with the granite and adamellite plutons. The high REE content of the monzonite precludes it from being related to the rapakivi granite and adamellite plutons as a source material, a residuum, or a cumulate.A major portion of the Batholith is an undifferentiated intrusive sequence ranging from older rapakivi granite to younger adamellite. The compositions of these plutons suggest a crustal fusion origin at intermediate to lower levels of the crust (25–36 km). The trace element data are consistent with partial fusion of tonalitic to granodioritic source material.During crystallization and emplacement into the upper crust (less than 4 km), 55–70% fractionation of two feldspars, biotite and hornblende from one of the granite plutons produced a small volume of differentiated granitic melt high in Si, Fe/Mg, Rb, Li, and REE (except Eu), and low in Ca, Mg, Al, Ca/Na, Sr, Ba, and K/Rb and with a large negative Eu anomaly. Presumed associated cumulate material ranges from silica-poor quartz monzonite and quartz syenite.The chemical and mineralogical similarity between the Wolf River Batholith and younger magmatic analogs associated in continental break-up (Nigerian younger granites, White Mountain magma series, and the peralkaline volcanics of the Red Sea Region) are suggestive but not conclusive of an extensional tectonic setting. A preliminary tectonic model suggests that the 1.4–1.5 Ga event is in response to thermal doming in an extensional regime leading to continental separation in the western Cordillera (pre-Belt) and extensive crustal fusion with no rifting or separation across the North American Craton.     

Petrological and geochemical evolution of the Kymi stock, a topaz granite cupola within the Wiborg rapakivi batholith, Finland

The 6×3 km Kymi monzogranite stock represents the apical part of an epizonal late-stage pluton that was emplaced within the 1.65 to 1.63 Ga Wiborg rapakivi batholith. The stock has a well-developed zonal structure, from the rim to the center: stockscheider pegmatite, equigranular topaz granite, porphyritic topaz granite. The contact between the two granites is usually gradational within a few centimeters, but local inclusions of the porphyritic granite in the equigranular granite indicate that the latter solidified later. Hydrothermal greisen and quartz veins, some of which contain genthelvite, beryl, wolframite, cassiterite, and sulfides, cut the granites of the stock and the surrounding country rocks. The equigranular granite contains 1 to 4 vol.% topaz, and its biotite is lithian siderophyllite; the porphyritic granite has 0 to 3 vol.% topaz, and the mica is siderophyllite. The equigranular granite is geochemically highly evolved with elevated Li, Rb, Ga, Ta, and F, and very low Ba, Sr, Ti, and Zr. The REE patterns show deep negative Eu anomalies and tetrad effects indicating extreme magmatic fractionation and aqueous fluid–rock interaction. The zonal structure of the stock is interpreted as a result of differentiation within the magma chamber. Internal convection in the crystallizing magma chamber and upward flow of residual melt as a boundary layer along sloping contacts resulted in accumulation of a layer of highly evolved, volatile-rich magma in the apical part of the chamber. Crystallization of this apical magma produced the stockscheider pegmatite and the equigranular granite; the underlying crystal mush solidified as the porphyritic granite. Much of the crystallization took place from volatile-saturated melt, and episodic voluminous degassing expelled fluids into opened fractures where they or their derivatives reacted with country rocks and caused alteration and mineralization.     

浙西北萧山—诸暨地区晚中生代侵入岩地球化学特征及构造意义

浙西北萧山—诸暨地区晚侏罗世末期花岗(石英)闪长岩与早白垩世末期(辉石)闪长岩具有以下地球化学特征:低硅、贫碱、富钙镁铁,弱负铕异常,稀土配分曲线右倾,富集K,Rb,Ba,Th,U等,亏损Nb,Ta等,Sr,Ti亏损不明显。晚侏罗世末期花岗岩与早白垩世末期花岗(斑)岩均具有高硅、富碱、贫钙镁,强负铕异常的特征;晚侏罗世侵入岩具"V"型稀土配分曲线,强富集K,Rb,Th,U等,相对富集Nb,Ta,强亏损Ba,Sr,Ti等特征;早白垩世侵入岩具右倾稀土配分曲线,富集K,Rb,Th,U等,亏损Nb,Ta,Sr,Ti,Ba不亏损等特征。岩石构造地球化学图解分析表明,晚侏罗世末期I-A型花岗岩类成岩构造环境具有从俯冲挤压碰撞作用向后碰撞或后造山作用演化的特征;早白垩世末期(高分异)I型花岗岩类岩石主要与火山-次火山喷发-侵入作用关系密切,成岩构造环境主要与区域伸展作用有关。     

冈底斯带谷露区中新世花岗岩地球化学特征及构造环境

谷露花岗岩是冈底斯构造带上念青唐古拉花岗岩的一部分,分布在当雄-拉萨大型北北东向伸展断裂两侧。岩石类型主要有斑状花岗闪长岩、巨斑黑云母花岗岩、黑云二长花岗岩、含石榴石花岗岩等。花岗岩同位素K-Ar测年结果在11 Ma左右,形成于中新世。花岗岩具有富硅、铝和碱,贫铁、镁、钙的特点,为准铝质高钾钙碱性花岗岩。花岗岩轻稀土富集,重稀土相对亏损,具有较明显的负铕异常。岩石相对富集不相容元素（LILE）,贫化高场强元素（HFSE）,在原始地幔标准化蛛网图上显示Rb、Th强烈富集,Nb与Ta亏损近似,Ba、Sr 和Ti强烈亏损的特点。谷露中新世花岗岩形成于后碰撞构造环境,与该区地壳东西向拉伸阶段的构造环境有关,是碰撞造山后地壳伸展、快速隆升背景下减压深熔的结果。     

The geochemistry and evolution of early precambrian mantle

Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and ⁸⁷Sr/⁸⁶Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:

1. starting with primordial Sr, a short period of relatively rapid ⁸⁷Sr/⁸⁶Sr growth, followed by Rb depletion;
2. a period between ≧ 3.5 b.y. and ～ 1.7 b.y. when closed-system Sr isotope evolution occurred (with Rb/Sr ～ 0.023);
3. development of large-scale Rb/Sr heterogeneities in the mantle at ～ 1.7 b.y., leading to a present-day mantle with ⁸⁷Sr/⁸⁶Sr ranging from 0.7023 to 0.7065 and Rb/Sr ranging from ～ 0 to 0.065.

     

Petrogenetic and metallogenetic characteristics of the Xikeng granitic pegmatites,Fujian Province

The Xikeng pegmatite field lies on the eastern margin of the south China fold system in Fujian Province, and it is located at the junction of three major tectonic units. The distribution of pegmatites is obviously controlled by the fold system. There exists apparent injection relationship between the pegmatites and the surrounding Sinian schist and granulitite. The granitoids extensively distributed in the field belong either to the Variscan or to the Yenshanian cycle, and it is evident that the pegmatites are genetically related to Variscan migmatitic granites. The pegmatites can be grouped into four types: muscovite-orthoclase-albite pegmatite (I), muscovite albite-orthoclase pegmatite (II), muscovite-orthoclase-albite pegmatite (III), and muscovite-albite-spodumene pegmatite (IV). Owing to strong metasomatism and multi-stage emplacement of pegmatitic meltsolution, the sequence of interior assemblage zones in the pegmatites does not always represent the sequence of original crystallization. The mineral composition of the pegmatites is extremely complicated. 81 kinds of minerals have so far been found. From type I to type IV, the mineral assemblage tend to get increasingly complex, together with the synchronous intensification of rare-metal and Sn mineralizations. Most of the type-IV pegmatites are of economic value. The features of fluid inclusions in the minerals are significantly different not only in different types of pegmatite, but also in different parts of a single pegmatite vein. Theδ ¹⁸O values of migmatitic granite and pegmatites are comparatively low (9.3–10.4‰), and those of rock-forming fluids are higher than 9.5‰. Isotopic ages of the pegmatites range from 235 to 328 Ma with initial⁸⁷Sr/⁸⁶Sr ratios being 0.715–0.746. According to the temporal and spatial relationships between the pegmatites and the migmatitic granite, combined with the features of the pegmatites themselves, it can be concluded that the Xikeng pegmatites are the product of differentiation closely related to the migmatitic granite.     

鹰峰环斑花岗岩地球化学特征及其构造意义

柴达木北缘鹰峰环斑花岗岩出露于柴达木地块与南祁连地体之间的柴北缘造山带,是我国发现的又一元古宙环斑花岗岩体.初步研究表明,鹰峰环斑花岗岩是具环斑结构和A-型花岗岩特征的典型的元古宙环斑花岗岩体,且属于A1亚型,岩浆组合具双峰式特征.环斑结构主要由几个钾长石斑晶颗粒形成聚斑,中心有一斜长石内核,斑晶表面具不均匀高岭土化,条纹构造明显且有规律,基质由细粒-微粒的石英组成,有明显重结晶及定向构造; 岩石化学组成以高钾为特征,A/ NKC < 1,A/NK > 1,属准铝质; 微量元素组成上富集Ba、U、Th、Ce、Hf、Sm,亏损Sr、Ta、Nb、Zr、Y,Rb/Sr (0.17~0.6)和Rb/Ba (0.03~0.24) 很低,岩石分异演化程度不高; 稀土元素: REE、Ce、Zr含量高,Ga含量高达25×10-6以上,远远高出其他类型花岗岩,但Eu (0.75~4.3) ×10-6轻度亏损,属轻稀土富集型.通过对微量元素和稀土元素的地球化学行为分析,鹰峰环斑花岗岩是发生在板内的一种岩浆作用,是下地壳的麻粒岩受底侵或拆沉作用地幔上涌影响,发生部分熔融,然后经过分异演化形成了碱性的“干”岩浆,并在后碰撞的区域拉伸构造环境下侵位.同时伴随温度的降低,钠质的斜长石从钾长石中出溶,并迁移到钾长石的边沿,形成了具环斑结构的A1型花岗岩.      

Fluid inclusion,geochemical, Rb–Sr and Sm–Nd isotope studies on tungsten mineralized Degana and Balda granites of the Aravalli craton,NW India

Granitic plutons occurring within and to the west of the Delhi Fold Belt in the Aravalli craton, northwestern India are the result of widespread felsic magmatism during Neoproterozoic, some of which are associated with greisen and skarn tungsten deposits. In this paper, we present the result of our study on fluid inclusions, geochemistry and geochronology of two such tungsten mineralized granite plutons at Degana and Balda, and interpret the nature of ore fluid, and petrogenesis and age of these mineralized granites. Fluid inclusion study reveals coexistence of moderate and hyper-saline aqueous fluid inclusions along with aqueous-carbonic inclusions, suggesting their origin due to liquid immiscibility during fluid–rock interaction. Geochemically, the granites are peraluminous, Rb enriched, Sr and Ba depleted and highly differentiated. The Rb–Sr isotopic systematics yielded \(795\pm 11\) Ma for Balda granite and \(827\pm 8\) Ma for Degana granite. We show that major phase of widespread granitoid magmatism and mineralization during the Neoproterozoic (840–790 Ma) in NW India is coeval with breakup of the Rodinia supercontinent and infer a causal relationship between them.     

Trace and minor element geochemistry of the rhyolitic volcanic rocks,Central North Island,New Zealand

Analytical data are presented for the following elements: Cs, Rb, Ba, K, Sr, Ca, Na, Fe, Mg, Cu, Co, Ni, Li, Sc, V, Cr, Ga, Al, Si, La, Y, and Zr. Eight samples were analysed by the spark source method for rare earths, Tl, Pb, Hf, Sn, Nb, Mo, Bi, and In. In addition to data on rhyolitic volcanics, a small number of intermediate volcanics and eugeosynclinal sediments were analysed for comparative purposes. The following features are shown by the trace element data:

1. The rhyolitic rocks have consistently lower concentrations of most trace and minor elements when compared with recent estimates of average concentrations in granites. None of the criteria for strong fractionation (e.g. low K/Rb, Ba/Rb and K/Cs ratios) are present.
2. The data do not indicate any systematic differences between the rhyolitic lavas and ignimbrites, although the very young rhyolitic pumices are consistently more “basic” in their element concentrations compared to the other rhyolitic analyses.
3. The residual glasses (and devitrified matrices) are depleted, relative to the total rock compositions, in Fe, Mg, Ca, Sr, V, Sc, and Al, and enriched in Cs, Rb, K, Ba, and Si. Zr is depleted in the residual glasses separated from rhyolites, but not in the andesitic residual matrices.
4. The rare earth fractionation patterns of the rhyolitic and andesitic extrusives are very similar, being intermediate between chondritic and sedimentary patterns i.e., there is no evidence of strong fractionation. The rhyolitic patterns also indicate a slight Eu depletion.
5. Comparable trace and minor element behaviour (with the possible exception of Zr) seems to exist through the rhyolite-andesite compositional range. This is supported by the whole rock-residual liquid trends for the various elements studied, which broadly coincide with the observed whole rock trends, both through the rhyolitic-andesitic compositonal range, and within the rhyolitic compositional range.

The data are finally discussed in the light of the possible origin of the rhyolitic magmas. It is believed that the analytical data presented are qualitatively consistent with the recently proposed idea that the magmas are derived by partial fusion of the associated Triassic-Jurassic eugeosynclinal greywacke-argillite sedimentary sequence.     

The Ackley City Batholith,southeastern Newfoundland: Evidence for crystal versus liquid-state fractionation

The 345 ± 10 Ma old composite Ackley City Batholith of southeastern Newfoundland, consists largely of very felsic K-feldspar megacrystic granite and alaskite. Spatially related to the southeast contact of the alaskite are younger aplites and pegmatite, intrusive phases which are interpreted to be pan of a tilted, high level roof zone complex to the batholith. The compositions of the alaskite and roof zone complex define major and trace element gradients similar to those in voluminous high-silica eruptive suites; i.e., the alaskite is more chemically evolved (higher in Rb, lower in Ca, Fe, Mn, Ti, P, Sr, Ba and LREE) toward the roof. Apparently these chemical gradients in the batholith are restricted to the top 2 to 3 kms of the former magma chamber. Fractional crystallization is a plausible process for generating the chemical dispersion in the granites, although very high feldspar partition coefficients for Ba, Sr and Eu are required to generate the observed chemical gradients by a reasonable degree of fractional crystallization. Restriction of crystal fractionation to near the roof of the batholith may reflect a decreased viscosity which would facilitate crystal-liquid separation by processes such as filter pressing, flow differentiation or convective fractionation.The chemical gradients in these granites closely resemble those attributed in high-silica volcanics to the process of thermogravitational diffusion (TGD). Compositional gradients in the upper portion of a magma chamber are consistent with the TGD model. This model, although still poorly understood, is, like fractional crystallization, a plausible mechanism to generate the chemical features of the Ackley City granites.     

东昆仑祁漫塔格阿确墩地区侵入岩U-Pb年代学、地球化学及其地质意义

祁漫塔格地区岩浆岩的成岩时代和形成环境的确定能对东昆仑造山带加里东期构造演化时限加以约束.对祁漫塔格西北部阿确墩地区石英闪长岩和二长花岗岩进行了年代学和岩石地球化学研究,结果显示,石英闪长岩属准铝质-弱过铝质钙碱性系列岩石;轻重稀土分馏明显,具中等-轻微铕负异常（δEu=0.79~0.90）;相对富集Rb、K、Hf、Zr、Tb、Nd等元素,不同程度地亏损Ba、P、Ti、Nd、Ta、Y;具有I型花岗岩类特征.二长花岗岩属弱过铝质钙碱性系列岩石;轻重稀土分异程度极大,具明显铕负异常（δEu=0.42~0.45）;富集大离子亲石元素（如Rb、K、La、Ce、Nd、Tb等）,亏损高场强元素（P、Ti、Nd、Ta）和Ba、Sr、U等元素;为高分异I型花岗岩.Nd/Th、Nb/Ta、Mg#值等指标显示石英闪长岩为壳源特征且受到幔源岩浆的影响,推测是幔源岩浆底侵地壳物质发生部分熔融形成的;二长花岗岩则是壳源的,可能与幔源岩浆底侵诱发的上地壳物质部分熔融有关,且经历了强烈的结晶分离作用.石英闪长岩和二长花岗岩的LA-ICP-MS锆石U-Pb年龄分别为448.8±3.9 Ma和405.2±3.6 Ma,代表其形成时代.石英闪长岩总体显示出与俯冲消减作用有关的岛弧岩浆岩地球化学特征;二长花岗岩在构造环境图解中显示为碰撞背景,但微量元素与同碰撞花岗岩典型特征不符,综合分析认为形成于后碰撞构造背景下.结合区域构造演化,推测东昆仑祁漫塔格地区在晚奥陶世持续处于俯冲消减环境中,早泥盆世之前进入后碰撞造山阶段.      

湘中印支期关帝庙岩体地球化学特征及成因

关帝庙岩体位于湘中盆地,据年龄资料可分为中三叠世末和晚三叠世中期两个形成时期。中三叠世花岗岩中发育闪长质暗色微粒包体,自早至晚依次为细中粒角闪石黑云母花岗闪长岩、细中粒斑状角闪石黑云母二长花岗岩、细-细中粒(斑状)黑云母二长花岗岩。晚三叠世花岗岩自早至晚依次为细中粒-粗中粒斑状二云母二长花岗岩、细粒二云母二长花岗岩。岩石高硅、富铝、高钾,(Na2O+K2O)含量为6.80%~8.87%,平均7.74%;K2O/Na2O比值在1.35~2.66之间,平均为1.58;ASI值为0.99~1.40。总体属镁质、高钾钙碱性系列过铝质花岗岩类。中三叠世花岗岩Ba、Nb、Sr、P、Ti表现为明显亏损,Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等则相对富集。中、晚三叠世花岗岩ΣREE含量为121.60~197.56μg/g,平均为158.70μg/g;δEu值0.28~0.68,平均为0.53;(La/Yb)N值为5.94~17.53,平均13.80。中三叠世花岗岩ISr值为0.71302~0.71758,εSr(t)值为121~186,εNd(t)值为-9.95~-8.74,t2DM为1.72~1.82Ga。C/MF-A/MF图解显示源岩主要为碎屑岩、少量基性岩和泥质岩。地质地球化学特征表明,印支期关帝庙花岗岩属S型花岗岩,形成于碰撞-后碰撞构造环境。