青藏高原凯蒙蛇绿混杂岩中碱性火山岩的发现及意义

班公湖-怒江洋的关闭时间直接制约青藏高原早期构造演化的认识。最近,在班公湖-怒江缝合带南侧凯蒙蛇绿混杂岩中发现一碱性火山岩,岩性主要是橄榄粗安岩,具粗面结构,斑晶主要是更长石和少量普通辉石,基质主要由更长石、普通辉石和少量填隙的碱性长石组成,有的具辉绿结构。岩石化学成分较一致,Si O2含量介于51.34%~53.91%之间,Ti O2含量为1.02%~1.55%,具有高Al2O3(17.06%~18.46%)和Na2O(4.90%~6.36%)、低K2O(0.05%~0.88%)含量特点,大多数Mg#大于60,最高68.62,里特曼指数(σ)介于3.65~4.47之间,为碱性系列火山岩;富集Sr、Rb、Ba等大离子亲石元素,亏损Nb、Y、Yb等高场强元素,相对富集Zr、Ti,Nb/U、Zr/Nb、La/Yb等比值稳定,分别为7.45~8.51、15.92~17.26和7.26~8.06;(87Sr/86Sr)i值变化范围较小,介于0.706~0.707之间,(143Nd/144Nd)t值在0.512 368~0.512 548之间,说明源区较为一致,结合Ce/Pb-Si O2图解判断结果,认为凯蒙碱性火山岩具有原始地幔、陆壳和深海沉积物源区混合特征。锆石U-Pb同位素定年结果表明该火山岩年龄为101.8±1.1 Ma,可能形成于洋壳俯冲阶段末期,由大陆边缘陆壳与俯冲洋壳板片断离导致软流圈地幔上涌诱发部分熔融所致,推测班公湖-怒江洋大约在早白垩世晚期关闭。     

The crystal-chemical basis for Ar retention in micas: inferences from interlayer partitioning and implications for geochronology

 Cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length/strength in these minerals. These data therefore provide a useful crystal-chemical perspective on relative mass-transfer kinetics of radiogenic isotopes, and account for the observation that biotite is generally less retentive of ⁴⁰Ar and ⁸⁷Sr than coexisting muscovite. Partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite. First, the preferences of large (Rb, Cs)⁺ in biotite and of small La³⁺ and Na⁺ in muscovite indicate a relatively spacious interlayer volume in biotite (suggesting a longer mean K−O bond). Second, the preference of interlayer vacancies in biotite (with some/all possibly H₂O/H₃O⁺-filled) suggests that its adjacent 2:1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite. Third, the relative exclusion of large Ba²⁺ from biotite despite its large interlayer sites is attributed to O−H bonds pointing into the interlayer cavity sub-normal to (001); (K⁺, Ba²⁺)-H⁺ repulsion thereby induced by the bare proton both destabilizes Ba²⁺ and weakens K−O bonds. In contrast, muscovite offers a more favorable electrostatic environment for Ba²⁺ substitution since its O−H bonds are directed into the vacant M ₁ octahedral site sub-parallel to (001). This hypothesis is supported by the observation that progressive F(OH)₋₁ exchange enhances Ba²⁺ partitioning into biotite/phlogopite relative to coexisting muscovite. These crystal-chemical differences between biotite and muscovite are mirrored in calculated values of “ionic porosity”, Z _i , defined here as the percentage of their interlayer unit-cell volume not occupied by ions. A monitor of ionic packing density and geometry, Z _i is inversely correlated with K−O bond strength, which appears to be the rate-determining “kinetic common denominator” for a variety of processes affecting micas – including those responsible for loss of radiogenic isotopes in biotite and muscovite. Accordingly, the relatively longer/weaker K−O bonds of biotite are envisioned as being more easily stretched (during volume diffusion) or broken (during recrystallization or retrograde alteration). This in turn accounts for common observations of enhanced radiogenic Ar/Sr loss and younger ⁴⁰Ar/³⁹Ar and Rb/Sr ages in natural biotite (high Z _i ) relative to coexisting muscovite (lower Z _i ). Significantly, this pattern may arise irrespective of isotopic loss mechanism (diffusion or recrystallization, etc.), and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field, petrographic, and petrologic constraints. Extension of this partitioning/porosity-based synthesis leads to prediction of corollary age-retentivity-composition effects among chemically diverse trioctahedral and dioctahedral micas, which are best field tested in terranes that cooled slowly under dry, static conditions. Pressure effects on argon retention are also inferred from the porosity model. Received: 9 February 1995 / Accepted: 8 September 1995     

Transport of Pb and Sr in leaky aquifers of the Bufa del Diente contact metamorphic aureole, north-east Mexico

The Sr and Pb isotopes from the 31.6 ± 0.3 Ma (2σ) old Diente del Bufa alkali syenite, northeastern Mexico, and marbles of its contact aureole were used to trace the sources and the mobility of these metals during hydrothermal activity. Chert layers form aquifers within the marbles. The marbles represent aquitards. During fluid-wallrock reaction, the chert layers developed wollastonite rims. Early wollastonite rims have Sr and Pb isotopic compositions similar to those of their immediate host marbles, which indicates that the isotopic composition of Sr and Pb is initially buffered by the marble. Later wollastonite and other replacement minerals rimming the aquifer have Sr and Pb isotopic compositions that carry with time increasingly larger contributions from the high-salinity magmatic brine. The Sr and Pb contributions from the alkali syenite can be traced isotopically for more than 90 m away from the contact of the intrusion. In contrast, Sr and Pb originating from the alkali syenite are traceable within the marbles only for 3 to 5 cm from the aquifer-marble boundary. This distance is comparable to the spatial distribution of isotopic alterations of C and O implying that Sr and Pb were transported into the marbles through a fluid phase. The isotopic variation of Sr, Pb, C, and O across the aquifer-marble profiles reflects infiltration as a transport mechanism rather than diffusion. Because Sr and Pb are minor components in both the infiltrating fluid and the rock and because their concentrations are strongly affected by the distribution coefficients among the solid phases present, there is little correlation between the isotopic compositions of the trace elements Sr and Pb and those of C and O, which are major components in fluid and rock. Very thin meta-argillite rinds at the outer margin of the aquifer represent residual material after the dissolution of calcite. They are distinctly enriched in Rb, Sr, and U. The Rb and Sr are to some extent residual from the original limestone mineralogy, whereas U is dominantly derived from the magmatic fluid and leaked from the aquifer with the escaping immiscible CO₂-rich H₂O-CO₂ fluid that was produced by decarbonation. The ²³⁸U/²⁰⁴Pb values ranging from 100 to 250 and distinctly lowered Th/U in the meta-argillite rims (1) demonstrate that U was transported with the magmatic fluid along the aquifer and (2) imply that during unmixing of the highly saline magmatic fluid U fractionated into the CO₂-rich H₂O-CO₂ fluid from which it precipitated selectively in the meta-argillite band across the aquifer. Radioautographs demonstrate that the upper meta-argillite rim has 20 to 40 times more U than the lower rim, which implies that 20 to 40 times more CO₂-rich H₂O-CO₂ fluid has left through the upper aquifer contact. Received: 30 September 1997 / Accepted: 15 December 1997     

Extreme variations in strontium initial ratios in ore-related fluids

Magmato-hydrothermal cassiterite-topaz ore at the Carboniferous East Kemptville (EK) greisenhosted tin deposit (Nova Scotia, Canada) is cross-cut by veins containing apatite, triplite, vivianite and fluorite. Initial ⁸⁷Sr/⁸⁶Sr ratios of these minerals have an extreme range (0.7135 to 0.8284). The initial ratios of the host rocks, EK quartz-topaz rock and Davis Lake biotite monzogranite (0.729±0.001, 0.727±0.004), are also high. The adjacent Meguma Group metasedimentary rocks are more typical of crustal material (0.712–0.719 at 330 Ma). Rb and Sr contents of EK fluorite (max.: 13.0, 1420 ppm) and apatite (max.: 88.1, 6660 ppm) are unusually high and variable. Unexpectedly, high Sr contents correlate positively with the high initial ⁸⁷Sr/⁸⁶Sr ratios. Fluorite and phosphate minerals from the first set of post-greisen veins at East Kemptville reflect the unusual chemistry of a high ⁸⁷Sr/⁸⁶Sr fluid present in the deposit after ore formation. The most extreme composition of this fluid was characterized by ⁸⁷Sr/⁸⁶Sr>0.8284, high Rb/Sr, high P, Rb, Cu, Zn and Fe contents, but low abundances of Ca, Pb and Sn with respect to the Davis Lake monzogranite. Such a fluid could have been derived from the greisen fluid and modified by reaction with the overlying Meguma metawacke. A second alternative, which cannot be well constrained at present, is that an extremely radiogenic fluid entered the deposit after ore formation and mixed with the postore fluid. In either case, the modified fluid subsequently mixed with meteoric water and precipitated the minerals with much lower ⁸⁷Sr/⁸⁶Sr ratios and Rb and Sr contents.     

Strontium and hydrogen isotope geochemistry of fresh and metabasalt dredged from the Mid-Atlantic Ridge

Fresh basalt and metabasalt dredged from the Mid-Atlantic Ridge were studied for Na, K, Rb, Sr, and H₂O(+) contents, and strontium and hydrogen isotope ratios. Na, K, Rb, and Sr contents of these samples are within the range of those of oceanic tholeiite. H₂O(+) content, strontium, and hydrogen isotope ratios vary widely. The variation in water content of metabasalt is apparently related to the chlorite content. The metamorphic temperature was about 550 °C based on the estimated δD value of chlorite. There is positive linear relationship between water content and strontium isotope ratio. Based on this relationship, the variation of strontium isotope ratio of the metabasalt was interpreted as follows: complete exchange occurred between strontium in the chlorite portion of the metabasalt and strontium in sea water (⁸⁷Sr/⁸⁶Sr ratio=0.7090), while the original strontium (⁸⁷Sr/⁸⁶Sr∼0.7023) was retained in the non-altered portion of the basalts.     

相山铀矿田西部地区深部多金属矿化成矿年代与成矿流体演化:Rb-Sr同位素体系的制约

江西相山铀矿田科学深钻3号孔在深部-700 m发现大量铅锌多金属矿化脉,垂向上呈"上铀下多金属"的分布特征。本文选取深部多金属矿脉主成矿阶段(S3)自形闪锌矿样品6件和不同阶段的毒砂、黄铁矿、方铅矿、方解石等样品12件,以及围岩全岩样品17件,进行了Rb、Sr同位素组成研究。结果表明:(1)由闪锌矿Rb-Sr等时线法确定的相山铀矿田深部多金属矿化形成于121. 0±3. 5Ma,与围岩火山岩存在较大时差,可能与晚于围岩的深部次火山有关。根据穿插关系,多金属矿化略晚于碱性交代铀矿化,但明显早于酸性交代铀矿化;(2)多金属矿化脉体中金属矿物的Rb和Sr含量分别介于0. 041×10~(-6)～1. 38×10-6和2. 35×10-6～23. 11×10-6之间,Sr同位素初始比值(87Sr/86Sr)i变化较大,介于0. 706114～0. 718814之间,平均值为0. 713579,暗示相山铀矿田深部多金属矿化的成矿物质主要来源于地壳。初始流体Sr同位素值(0. 718665)明显高于成矿时赋矿围岩(流纹英安岩为0. 714581,碎斑流纹岩为0. 714417)的Sr同位素组成,表明多金属成矿流体和物质并非来自围岩火山岩;(3)由早到晚阶段的(87Sr/86Sr)i呈明显降低的演化趋势,表明成矿流体演化过程中受到大气降水的不断稀释作用。相山矿田的铀矿和深部多金属矿化同形成于华南中生代板内伸展构造背景。     

Halogen-bearing minerals in syenites and high-grade marbles of Dronning Maud Land,Antarctica: monitors of fluid compositional changes during late-magmatic fluid-rock interaction processes

Meta-sedimentary rocks including marbles and calcsilicates in Central Dronning Maud Land (CDML) in East Antarctica experienced a Pan-African granulite facies metamorphism with peak metamorphic conditions around 830 ± 20 °C at 6.8 ± 0.5 kbar which was accompanied by the post-kinematic intrusion of huge amounts of syenitic (charnockitic) magmas at 4.5 ± 0.7 kbar. The marbles and calcsilicates may represent meta-evaporites as indicated by the occurrence of metamorphic gypsum/anhydrite and Cl-rich scapolite that formed in the presence of saline fluids with X _NaCl in the range 0.15–0.27. The marbles and calcsilicates bear biotite, tremolite and/or hornblende and humite group minerals (clinohumite, chondrodite and humite) which are inferred to have crystallized at about 650 °C and 4.5 kbar. The syenitic intrusives contain late-magmatic biotite and amphibole (formed between 750 and 800 °C) as well as relictic magmatic fayalite, orthopyroxene and clinopyroxene. Two syenite and two calcsilicate samples contain fluorite. Corona textures in the marbles and calcsilicates suggest very low fluid-rock ratios during the formation of the retrograde (650 °C) assemblages. Biotite in all but two syenite samples crystallized at log(f _{H 2 O}/f _HF) ratios of 2.9 ± 0.4, while in the calcsilicates, both biotite and humite group minerals indicate generally higher log(f _{H 2 O}/f _HF) values of up to 5.2. A few samples, though, overlap with the syenite values. Log(f _{H 2 O}/f _HCl) derived from biotite covers the range 0.5–2.6 in all rock types. Within a single sample, the calculated values for both parameters vary typically by 0.1 to 0.8 log units. Water and halogen acid fugacities calculated from biotite-olivine/orthopyroxene-feldspar-quartz equilibria and the above fugacity ratios are 1510–2790 bars for H₂O, 1.3–5.3 bars for HF and 7–600 bars for HCl. The results are interpreted to reflect the reaction of relatively homogeneous magmatic fluids [in terms of log(f _{H 2 O} /f _HF)] derived from the late-magmatic stages of the syenites with both earlier crystallized, still hotter parts of the syenites and with adjacent country rocks during down-temperature fluid flow. Fluorine is successively removed from the fluid and incorporated into F-bearing minerals (close to the syenite into metamorphic fluorite). In the course of this process log(f _{H 2 O} /f _HF) increases significantly. Chlorine preferably partitions into the fluid and hence log(f _{H 2 O} /f _HCl) does not change markedly during fluid-rock interaction. Received: 28 November 1997 / Accepted: 27 April 1998     

Relationships between O isotope equilibrium,mineral alteration and Rb–Sr chronometric validity in granitoids: implications for determination of cooling rate

A combined study of mineral O and Rb–Sr isotopes was carried out for a number of Mesozoic granitoids in China in order to compare the degree of O isotope equilibrium between coexisting minerals, with the validity of mineral Rb–Sr isochrons for granitoids. A scrutiny of both O isotope geothermometry and Rb–Sr internal isochron dating for corresponding minerals indicates that equilibrium O isotope fractionation between Rb–Sr isochron minerals corresponds to geologically meaningful isochron ages if the variation in ⁸⁷Rb/⁸⁶Sr ratio is big enough to provide reasonably small uncertainties in age. Significant deviation of the Rb–Sr isochron age from the actual age appears to depend on the difference in Sr isotopic composition between an external fluid and the igneous minerals. As a result, O isotope disequilibrium is often caused by interaction between the rock and the external fluid that results in mineral alteration. Post-magmatic alteration can cause isotope exchange between the minerals and an internally buffered fluid that is isotopically identical to the host rock. The O isotope composition of coexisting minerals in studied samples changed principally due to a decrease in temperature. Both Rb and Sr concentrations and the Sr isotope ratios of isochron minerals also changed due to the mixing of different Sr reservoirs. Nevertheless, the isochron age can remain unchanged if the mixing took place along the isochron chord between the internal fluid and the minerals from that newly altered minerals formed. This provides an insight into the effect of internal and external fluids on the validity of mineral Rb–Sr chronometry. In addition, an alternative approach is proposed to construct the cooling curve by a combined use of O isotope temperature and mineral isotope age for the granitoids of interest. Comparing with the traditional method using the empirical closure temperature for Rb–Sr chronometry, the proposed approach utilizes fewer variables with smaller uncertainties than the traditional way.     

The geochronology of Iqna Granite (wadi Kid pluton), Southern Sinai

The wadi Kid pluton of Iqna Granite, Southern Sinai, which was intruded during the last Precambrian magmatic phase, yields a Rb-Sr total rock isocrhon age of 580±23 m.y., and an initial ⁸⁷Sr/ ⁸⁶Sr ratio of 0.7028±0.0028. The magma of the Iqna Granite was derived from a low Rb/Sr source shortly before its crystallization. Partial resetting of biotite ages is detected by both Rb-Sr and K-Ar methods. Mineral isochrons yield higher initial values (0.7045–0.7065) as a result of Sr isotopic redistribution within a closed total rock system. The Rb-Sr resetting of the biotites is expressed by radiogenic Sr loss accompanied by a proportional enrichment of common Sr. The Rb content was unaffected by this process. Oxidation of the iron within the biotite indicates the opening of the biotite interlayer space, thus making the Sr exchange possible. These effects are attributed to a thermal event some 510–540 m.y. ago.     

Coupling of anatectic reactions and dissolution of accessory phases and the Sr and Nd isotope systematics of anatectic melts from a metasedimentary source

Advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes. Incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the Rb-Sr and Sm-Nd isotope systems in anatectic melts from a muscovite-bearing metasedimentary source. Modeling results show that (1) the Sm/Nd ratios and Nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt, and (2) the relative proportion of micas (muscovite and biotite) and feldspars (plagioclase and K-feldspar) that enter the melt is a key parameter determining the Rb/Sr and ⁸⁷Sr/⁸⁶Sr ratios of the melt. Furthermore, these two factors are not, in practice, independent. In general, nonmodal partial melting of a pelitic source results in melts following one of two paths in ε_Nd-⁸⁷Sr/⁸⁶Sr ratio space. A higher temperature, fluid-absent path (Path 1) represents those partial melting reactions in which muscovite/biotite dehydration and apatite but not monazite dissolution play a significant role; the melt will have elevated Rb/Sr, ⁸⁷Sr/⁸⁶Sr, Sm/Nd, and ε_Nd values. In contrast, a lower temperature, fluid-fluxed path (Path 2) represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue; the melt will have lower Rb/Sr, ⁸⁷Sr/⁸⁶Sr, Sm/Nd, and ε_Nd values than its source. The master variables controlling both accessory phase dissolution (and hence the Sm-Nd system), and melting reaction (and hence the Rb-Sr systematics) are temperature and water content. The complexity in Sr-Nd isotope systematics in metasediment-derived melts, as suggested in this study, will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component.     

Weak compositional zonation in a silicic magmatic system: Incesu ignimbrite,Central Anatolian Volcanic Province (Kayseri – Turkey)

The Central Anatolian Volcanic Province (CAVP), one of four major volcanic provinces in Turkey, plays a significant role in the interpretation of the tectonic evolution of Central Anatolia. The CAVP developed within a complex collisional system involving the African, Arabian and Eurasian plates during the Miocene. The volcanism exhibits complicated variations in mineralogical, petrological and geochemical compositions resulting from post-collisional lithospheric dynamics. The Incesu ignimbrite has 5–20 m thick and covers an area of ～7800 km². It is composed of three stratigraphic levels. The lower level (LL) shows blackish brown and glassy welded structure. The middle level (ML) is a well-welded, reddish pink in color and has large amounts of fiamme. The upper level (UL) is grayish pink, weakly welded and has rock fragments of different compositions. The Incesu ignimbrite is composed of plagioclase (oligoclase, andesine) + pyroxene (augite, clinoenstatite) + opaque minerals and low amount of amphibole, biotite and quartz. Eutaxitic texture is dominant in ML and LL samples; these levels are more strongly and contain more flattened pumice fragments and volcanic glass shards than in the UL. A sharp color contrast defines the contact between LL and ML.Major, trace and rare earth element of the Incesu ignimbrite, characterized by their rhyolite, rhyodacite–dacite composition, medium–high K, calcalkaline and peraluminous nature, show fractional crystallization primarily controlled by plagioclase, clinopyroxene, magnetite, ilmenite and titanomagnetite. Sr and Nd isotopic ratios of Incesu ignimbrite display isotopic variations between the ignimbrite levels; they exhibit a limited range in ⁸⁷Sr/⁸⁶Sr (0.7043–0.7049) and ¹⁴³Nd/¹⁴⁴Nd (0.512716–0.512760). The Sr–Nd isotopic ratio of Incesu ignimbrite reveals an age of 3 Ma. The ignimbrite evolved through fractional crystallization and crystal contamination of the parent magma derived from Ocean Island Basalt (OIB) like magma. This suggestion is supported by the AFC modeling based on the trace elements and Sr isotope data.Variation of several major oxide concentrations (Fe₂O₃, TiO₂, CaO and K₂O), trace element concetrations (V, Sr, Cs and Rb) and trace element ratios (Ba/Rb, Sr/, K/Sr, K/Nb, Rb/Sr, Rb/Y and Rb/Nb) versus SiO₂ concentration show the magma chamber that generated the Incesu ignimbrite was compositionally zoned. All geochemical and Sr–Nd isotpic datas can be interepreted to be the result of a subduction related source.     

Fluid and gas migration in the North German Basin: fluid inclusion and stable isotope constraints

Fluid inclusions have been studied in minerals infilling fissures (quartz, calcite, fluorite, anhydrite) hosted by Carboniferous and Permian strata from wells in the central and eastern part of the North German Basin in order to decipher the fluid and gas migration related to basin tectonics. The microthermometric data and the results of laser Raman spectroscopy reveal compelling evidence for multiple events of fluid migration. The fluid systems evolved from a H₂O–NaCl±KCl type during early stage of basin subsidence to a H₂O–NaCl–CaCl₂ type during further burial. Locally, fluid inclusions are enriched in K, Cs, Li, B, Rb and other cations indicating intensive fluid–rock interaction of the saline brines with Lower Permian volcanic rocks or sediments. Fluid migration through Carboniferous sediments was often accompanied by the migration of gases. Aqueous fluid inclusions in quartz from fissures in Carboniferous sedimentary rocks are commonly associated with co-genetically trapped CH₄–CO₂ inclusions. P–T conditions estimated, via isochore construction, yield pressure conditions between 620 and 1,650 bar and temperatures between 170 and 300°C during fluid entrapment. The migration of CH₄-rich gases within the Carboniferous rocks can be related to the main stage of basin subsidence and stages of basin uplift. A different situation is recorded in fluid inclusions in fissure minerals hosted by Permian sandstones and carbonates: aqueous fluid inclusions in calcite, quartz, fluorite and anhydrite are always H₂O–NaCl–CaCl₂-rich and show homogenization temperatures between 120 and 180°C. Co-genetically trapped gas inclusions are generally less frequent. When present, they show variable N₂–CH₄ compositions but contain no CO₂. P–T reconstructions indicate low-pressure conditions during fluid entrapment, always below 500 bar. The entrapment of N₂–CH₄ inclusions seems to be related to phases of tectonic uplift during the Upper Cretaceous. A potential source for nitrogen in the inclusions and reservoirs is C_org-rich Carboniferous shales with high nitrogen content. Intensive interaction of brines with Carboniferous or even older shales is proposed from fluid inclusion data (enrichment in Li, Ba, Pb, Zn, Mg) and sulfur isotopic compositions of abundant anhydrite from fissures. The mainly light δ³⁴S values of the fissure anhydrites suggest that sulfate is either derived through oxidation and re-deposition of biogenic sulfur or through mixing of SO₄²⁻-rich formation waters with variable amounts of dissolved biogenic sulfide. An igneous source for nitrogen seems to be unlikely since these rocks have low total nitrogen content and, furthermore, even extremely altered volcanic rocks from the study area do not show a decrease in total nitrogen content.     

Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada,California

We provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick Jurassic pelite unit at mid-crustal levels that produced a migmatite complex in conjunction with the intrusion of part of the southern Sierra Nevada batholith at ca. 100 Ma. Field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products. Hence, this area yields insights into potential mid-crustal level contributions of crustal components into Cordilleran-type batholiths. Major and trace-element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host. Compared with the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. The initial ⁸⁷Sr/⁸⁶Sr ratios of leucosomes range from 0.7124 to 0.7247, similar to those of the metapelite protoliths (0.7125–0.7221). However, the leucosomes have a much wider range of initial ε_Nd values, which range from −6.0 to −11.0, as compared to −8.7 to −11.3 for the metapelites. Sr and Nd isotopic compositions of the leucosomes, migmatites, and metapelites suggest disequilibrium partial melting of the metapelite protolith. Based on their Sr, Nd, and other trace-element characteristics, two groups of leucosomes have been identified. Group A leucosomes have relatively high Rb, Pb, Ba, and K₂O contents, Rb/Sr ratios (0.15<Rb/Sr<1.0), and initial ε_Nd values. Group B leucosomes have relatively low Rb, Pb, Ba, and K₂O contents, Rb/Sr ratios (<0.15), and initial ε_Nd values. The low Rb concentrations and Rb/Sr ratios of the group B leucosomes together suggest that partial melting was dominated by water-saturated or H₂O-fluxed melting of quartz + feldspar assemblage with minor involvement of muscovite. Breakdown of quartz and plagioclase with minor contributions from muscovite resulted in low Rb/Sr ratios characterizing both group A and group B leucosomes. In contrast, group A leucosomes have greater contributions from K-feldspar, which is suggested by: (1) their relatively high K concentrations, (2) positive or slightly negative Eu anomalies, and (3) correlation of their Pb and Ba concentrations with K₂O contents. It is also shown that accessory minerals have played a critical role in regulating the partitioning of key trace elements such as Sm, Nd, Nb, and V between melt products and residues during migmatization. The various degrees of parent/daughter fractionations in the Rb–Sr and Sm–Nd isotopic systems as a consequence of nonmodal crustal anatexis would render melt products with distinct isotopic signatures, which could profoundly influence the products of subsequent mixing events. This is not only important for geochemical patterns of intracrustal differentiation, but also a potentially important process in generating crustal-scale as well as individual pluton-scale isotopic heterogeneities.     

Occurrence of calcite in Sanbagawa pelitic schists: implications for the formation of garnet, rutile, oligoclase, biotite and hornblende

The frequency of occurrence of minerals in 1876 samples of Sanbagawa pelitic schist in central Shikoku is summarized on the basis of microscopic observation accompanied, in part, by use of an electron microprobe. All samples contain quartz, plagioclase, phengite, chlorite and graphite. More than 90% of samples contain clinozoisite, titanite and apatite. Garnet is present in 95% of samples from the garnet zone, and biotite is present in 64% of samples from the albite‐biotite zone. Calcite is found in about 40% of samples of the pelitic schist collected from outcrop, but occurs in 95% of the pelitic schist from drill cores. Calcite was apparently ubiquitous in the pelitic schist during the Sanbagawa metamorphism, but must have been dissolved recently by the action of surface or ground water. The mineral assemblages of the Sanbagawa pelitic schist have to be analyzed in the system with excess calcite, quartz, albite (or oligoclase), clinozoisite, graphite and fluid that is composed mainly of H₂O, CO₂ and CH₄. In the presence of calcite, reactions that produce garnet, rutile, oligoclase, biotite and hornblende, some of which define isograds of the metamorphic belt, should be written as mixed volatile equilibria that tend to take place at lower temperature than the dehydration reactions that have been proposed. The presence of calcite in pelitic schist suggests that fluid composition is a variable as important in determining mineral assemblages as pressure and temperature. Thus Ca‐bearing phases must be taken into account to analyze the phase relations of calcite‐bearing pelitic schist, even if CaO content of Sanbagawa pelitic schist is low. As calcite is a common phase, the mineral assemblages of the biotite zone pelitic schist may contravene the mineralogical phase rule and warrant further study.     

Evolution of feldspars at the amphibolite-granulite-facies transition in augen gneisses (SW Norway): geochemistry and Sr isotopes

The Sveconorwegian Augen Orthogneisses of Rogaland — Vest-Agder (SW Norway) were emplaced as amphibole- and biotite-bearing granodiorites at 1040 Ma (concordant Rb/Sr and zircon U/Pb ages). They underwent prograde metamorphism which increased from lower amphibolite-facies in the eastern zone to granulite-facies in the western zone, close to the Rogaland anorthosite complex. K-feldspar megacrysts initially crystallised as phenocrysts and were chemically equilibrated during metamorphism, as shown by the flat Ba concentration profiles and the increase of the anorthite content from An_1.1 in the amphibolitefacies to An_2.6 in the granulite-facies. This increase of the An content suggests an increase in metamorphic temperature. The REE content of the megacrysts is related to the associated accessory minerals which depend upon the metamorphic grade: sphene + allanite + apatite + zircon and rarely thorite in amphibolite-facies, and apatite + zircon + monazite ± thorite in lower amphibolite-and granulite-facies. Amphibole and biotite inclusions in megacrysts were also equilibrated during metamorphism. Groundmass K-feldspar and plagioclase experienced late-metamorphic changes during uplift. An internal Rb/Sr mineral isochron (plagioclase, apatite, K-feldspar) defines an age of 870 Ma, which represents the closure of the Rb/Sr isotopic system in minerals of the augen gneisses. This age also represents a K-feldspar cooling age in regionally distributed augen gneiss samples. The K-feldspar cooling age appears to be similar to or slightly older than the biotite cooling age.     

Late Neoproterozoic strongly peraluminous leucogranites, South Eastern Desert, Egypt – petrogenesis and geodynamic significance

Summary Late Neoproterozoic garnet-bearing leucogranites are developed locally along thrust faults in the South Eastern Desert, Egypt. This work presents field observations, whole rock major and trace element abundances, Rb–Sr isotope data and mineral chemistry for three occurrences in the Sikait-Nugrus area. Field observations show that the leucogranites cut the faults and their contact with the country rocks is sharp with no indication of contact metamorphism. They were intruded into a low-grade metamorphosed ophiolitic melange and a high-grade metamorphosed metasedimentary succession of biotite schist composition. Numerous biotite schist enclaves, having irregular and diffuse contacts, are recorded within the leucogranites. Whole rock Rb–Sr ages of the leucogranites from two different localities are 610±20 and 594±12Ma respectively; they are interpreted as emplacement ages. The leucogranites contain more than 70% SiO₂, and they are strongly peraluminous (A/CNK>1.1) with low TiO₂, Fe₂O₃*, MgO, CaO, Ba, Sr, LREE, Eu/Eu* and Sr/Ba and high Rb, Rb/Zr, Rb/Sr and Rb/Ba. These geochemical parameters and the low initial ⁸⁷Sr/⁸⁶Sr ratios (0.703) indicate crustal derivation by dehydration partial melting from a juvenile protolith similar to the exposed biotite-rich metasediments. Models for the tectonic setting of these leucogranites suggest their emplacement during an extensional tectonic stage that follows continental collision. It is proposed that crustal heating, caused by decompression along shear zones, is responsible for the production of these granitic melts. The results support previous hypotheses and further document a regional late Neoproterozoic extensional tectonic event, which is probably related to the initial break-up of Gondwana.     

Hydrothermal alteration of Variscan granites,southern Schwarzwald,Federal Republic of Germany

Hercynian S-type granites from the southeastern Schwarzwald granite series represent cogenetic biotite-and two-mica granites. Oxygen- and hydrogen-isotope data show that hydrothermal alteration invoking isotopically light surface waters resulted in a drastic reduction in ¹⁸O and D and pronounced disequilibrium between the minerals. Effective water-rock ratios are calculated to be high, about 0.8 vol units. A shift in the¹⁸O/¹⁶O and the chemical composition of the fluid due to water-rock interaction is continuously traced from pure H₂O with meteoric isotopic character in the deep-seated biotite granites to slightly saline water with rock-equilibrated isotopic composition in the two-mica granites at a shallower level. Substantial retrograde hydrometamorphism in the temperature range 500° to 200° C resulted mineralogically in high-temperature chloritization of biotite, and low-temperature muscovitization as well as feldspar alteration, respectively. Another result of the re-equilibration of cations is strong disturbance of the Rb–Sr system which affects measured ages and initial⁸⁷Sr/⁸⁶Sr values. Hydrothermal differentiation and alteration probably overlap to a very large extent magmatic differentiation processes.     

The timescale and mechanism of granulite formation at Kurunegala,Sri Lanka

Incipient charnockite formation at Kurunegala in Sri Lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss. Mineral equilibria in the charnockite assemblage record pressure-temperature (P-T) conditions of 738±60° C and 6.9±1.2 kbar at-17.0±1.2 log fO₂ and aH₂O=0.18±0.16. Wholerock trace-element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of Sm>Rb>Pb>U>Sr>Nd, with a fractionation of Rb/Sr, Sm/Nd and Th/U ratios, and crystallization of the charnockite assemblage at 535±5 Ma. Major element (Fe–Mg–Ca) and Sm–Nd equilibration between minerals occurred at 524±9 Ma, whereas, Pb and Rb–Sr underwent continued exchange to 501±5 Ma and 486±1 Ma, respectively. Trace-element data for both amphibolite and charnockite minerals show that depletion on a whole-rock scale can be accounted for either by changes in mineral modes or trace-element abundances, within the immediate area of dehydration. The fractionation of Sm/Nd on a whole-rock scale is controlled by the breakdown of amphibole, without the growth of a major new host-phase for Sm in the charnockite. Rubidium and Sr are dependent on the relative behaviour of biotite, plagioclase and alkali-feldspar. Modelling of dehydration-melting involving the breakdown of amphibole, biotite, and alkali-feldspar reproduces the observed Sm/Nd and Rb/Sr fractionation, and indicates the loss of small melt fractions, on a cm scale, from the charnockite. These observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation.     

Petrogenesis of the Puerto Edén Igneous and Metamorphic Complex,Magallanes, Chile: Late Jurassic syn-deformational anatexis of metapelites and granitoid magma genesis

A suite of schists, gneisses, migmatites, and biotite granitoids from the Puerto Edén Igneous and Metamorphic Complex (PEIMC) and biotite–hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied. For that purpose, the chemistry of minerals and the bulk rock composition of major and trace elements including Rb–Sr and Sm–Nd isotopes were determined. Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca. 600–700 °C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U–Pb zircon ages). Structures in schists, gneisses, migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions. Andalusite schists and sillimanite gneisses yield initial ⁸⁷Sr/⁸⁶Sr ratios of up to 0.7134 and εNd₁₅₀ values as low as − 7.6. Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial ⁸⁷Sr/⁸⁶Sr ratios between 0.7073 and 0.7089, and εNd₁₅₀ values in the range − 7.6 to − 4.4. This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source. Thus, a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids. The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times. A biotite–hornblende granodiorite and a muscovite–garnet leucogranite show initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7048 and 0.7061, and εNd₁₀₀ values of − 2.6 and − 1.8, respectively, and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks.     

Geochemistry of Ore Fluids and Rb-Sr Isotopic Dating for the Wulong Gold Deposit in Liaoning, China

On the basis of detailed geological studies of the Wulong gold deposit, three metallogenic stages can be identified. With quartz fluid inclusions as an object of study, the authors investigated phase characteristics, compositional variations, temperature and pressure changes, fluid evolution, Pb isotope tracing and Rb-Sr isotopic dating of fluid inclusions entrapped in the above three metallogenic stages. The results show that Na+ is decreased obviously with metallogenic evolution, while K+ and other cations and gas compositions (H2, CO, CH4 and CO2) are increased slightly, and that the temperature and salinity vary in a pulsating manner along with the metallogenic evolution. Inverse calculation of hydrogen and oxygen isotopes indicate that at the first metallogenic stage the fluids were magmatic water, at the second stage they were dominated by magmatic water with a minor amount of meteoric water involved, and at the third stage, i.e., the final stage of metallogenesis, the fluids were composed complete