Stable isotope compositions of tourmalines from granites and related hydrothermal rocks of the Karagwe-Ankolean belt,northwest Tanzania

Tourmaline is a ubiquitous mineral in the Mid-Proterozoic, peraluminous, syn- to post-tectonic granites and aplites and the related hydrothermal rocks of the Karagwe-Ankolean belt in northwest Tanzania. Electron microprobe analysis indicates that tourmalines from all of the intrusive and hydrothermal lithologies: (1) belong to the schorl-dravite solid-solution series; and (2) plot within the field occupied by tourmaline from Li-poor granitoids on the Fe-Al-Mg classification diagram. Oxygen isotope compositions range from +12.2 to +11.6‰ (SMOW) for magmatic tourmalines and from +10.8 to +9.8‰ for those of hydrothermal origin. Hydrogen isotope compositions vary from −79 to − 65‰ (SMOW) for magmatic tourmalines and from −99 to −84‰ for hydrothermal tourmalines. Water contents measured by manometry are constant at 3.0–3.2 wt.%. Within the broad grouping there arc systematic variations in both chemical [particularly Fe_tot/(Fe_tot + Mg ratio)] and isotopic composition that relate to evolving magmatic and hydrothermal conditions. Igneous differentiation [increasing Fe_tot/(Fe_tot + Mg) in magmatic tourmaline] has produced trends with higher δ¹⁸O in quartz, lower δ¹⁸O in tourmaline, and larger Δ_QTZ.−TOUR.-values, that reflect a combination of a reduction of crystallization temperature and an increase of Fe_tot/ (Fe_tot + Mg) in the residual melt. Subsequent cooling and interaction of an exsolved, B-rich magmatic fluid with the pelitic country rocks, resulted in the deposition of hydrothermal tourmaline with increasing Fe_tot/(Fe_tot + Mg) ratios, and progressively lower δ¹⁸O and δD -values.     

Boron isotopic variations in tourmaline from metacarbonates and associated calc-silicate rocks from the Bohemian Massif: Constraints on boron recycling in the Variscan orogen

Various metacarbonate and associated calc-silicate rocks form minor but genetically significant components of the lithological units in the Bohemian Massif of the Variscan orogen in Central Europe.These rocks vary in terms of their lithostratigraphy,chemical composition and mineral assemblage(dolomite/calcite ratio,silicate abundance).Tourmaline is present in five paragenetic settings within the metacarbonate and calc-silicate units.TypeⅠcomprises individual,euhedral,prismatic grains and grain aggregates in a carbonate-dominant(calcite±dolomite)matrix poor in silicates.TypeⅡis characterized by euhedral to subhedral grains and coarse-to fine-grained aggregates in silicate-rich layers/nests within metacarbonate bodies whereas typeⅢoccurs as prismatic grains and aggregates at the contact zones between carbonate and associated silicate host rocks.TypeⅣis in veins crosscutting metacarbonate bodies,and typeⅣtourmaline occurs at the exocontacts of elbaite-subtype granitic pegmatite.Tourmaline from the different settings shows distinctive compositional features.Typical for typeⅠare Mg-rich compositions,with fluor-uvite>dravite>>magnesio-lucchesiite.Tourmalines from typeⅡsilicate-rich layers/nests are highly variable,corresponding to oxy-schorl,magnesio-foitite,Al-rich dravite and fluor-uvite.Typical for typeⅢtourmalines are Ca,Ti-bearing oxy-dravite compositions.The typeⅣveins feature dravite and fluor-uvite tourmaline compositions whereas typeⅤtourmaline is Li,F-rich dravite.Tourmaline is the only Bbearing phase in paragenetic typesⅠ-Ⅳ,where it is characterised by two principal ranges of B-isotope composition(δ^11B=-13‰to-9‰and-18‰to-14‰).These ranges correspond to regionally different units of the Moldanubian Zone.Thus,the Svratka Unit(Moldanubian Zone s.l.)contains only isotopically lighter tourmaline(δ^11B=-18‰to-14‰),whereas metacarbonates in the Poli?ka unit(Teplá-Barrandian Zone)and Olesnice unit(Moravicum of the Moravo-Silesian Zone)has exclusively isotopically heavier tourmaline(δ^11B=-9‰to-13‰).Tourmalines from metacarbonates in the Variegated Unit cover both ranges of isotope composition.The isotopically light end of the B isotope range may indicate the presence of continental evaporites within individual investigated areas.On the other hand,variations in the range of~8δ-units is consistent with the reported shift in B isotopic composition of metasedimentary rocks of the Bohemian Massif due to the prograde metamorphism from very-low grade to eclogite facies.In contrast to the metacarbonate-hosted settings,tourmaline of paragenetic type V from the exocontact of granitic pegmatites displays a significantly heavier range ofδ^11B(as low as-7.7‰to-0.6‰),which is attributed to partitioning of 10 B to cogenetic axinite and/or different B-signature of the source pegmatite containing tourmaline with heavyδ^11B signature.     

Boron isotopes in tourmaline as a tracer of metasomatic processes in the Bamble sector of Southern Norway

The Bamble sector of southern Norway comprises metagabbros and metasediments that were metasomatically altered to various extents during a late stage of the Sveconorwegian orogeny (~1.06 Ga). The infiltration of highly saline brines along veins led to penetrative scapolitization and albitization on a regional scale and the local deposition of Fe–Ti oxides. Typical secondary mineral assemblages include either scapolite + apatite + amphibole + phlogopite + tourmaline, or albite + epidote + calcite + chlorite + white mica, indicating that the fluids introduced large amounts of Na, Cl, Mg, Ca, K, P, and B to the system. Metasomatic tourmalines associated with different alteration stages as identified by variations in major-element composition and initial ⁸⁷Sr/⁸⁶Sr were analyzed for B isotopic compositions to constrain possible sources and the evolution of the hydrothermal fluid(s). Measured δ¹¹B values range from ?5 to +27 ‰ relative to SRM-951, suggesting marine evaporites interlayered with various amounts of continental detritus and pelagic clay as a possible B source reservoir. The influence of a seawater-derived component is clearly indicated by the heavy B isotope signature of tourmaline related to Al–Mg-rich metapelites. In contrast, negative δ¹¹B values can be explained by the influence of pneumatolytic fluids associated with granitic pegmatites. On a regional scale (i.e., several km), δ¹¹B values in tourmaline vary widely, whereas variations within a single outcrop (tens of m) are typically small and can be ascribed to different generations of tourmaline related to several fluid pulses.     

辽宁古元古代地体中富电气石岩石的成因：蒸发岩硼源的证据

辽宁东部古元古界底部地层(南辽河群)中赋存着大型的硼酸盐矿床，含矿层位中广泛分布含电气石的变粒岩和电英岩。空间上这些含电气石的岩石与硼酸盐有着密切的联系，电气石可以作为区域硼矿找矿的标志。已有研究结果表明，该地区的硼酸盐矿床是变质蒸发岩成因。本研究对该区不同产状的电气石和硼酸盐的地质特征，全岩和矿物成分、硼同位素组成进行了分析。本区的电气石包括层状和脉状两大类，而电气石的富集与硼酸盐关系密切，电英岩往往分布在硼酸盐矿体的上盘。而矿体的下盘一般不产出富电气石的岩石。当长英质脉体穿过硼酸盐矿体时，脉体中往往会富集电气石。含电气石岩石的全岩地球化学分析表明，它们的REE及其他微量元素特征以及相关性关系与周围不含电气石的同类岩石十分相似，反映出一种成因上的联系。本区电气石主要属于镁电气石一铁电气石系列，靠近硼矿体的电气石比远离硼矿体的电气石更加富镁，有着更高的Mg／Fe比值。电气石和硼酸盐的硼同位素成分分析显示出二者在同位素组成上的相似性，前者比后者的δ^11B稍低，这可能是由于热液活动过程中同位素分馏的结果。电气石的硼同位素组成在空间上显示出变化规律：远离硼酸盐矿体的电气石的δ^11B值(-5．2‰- 3．6‰)比矿体附近的电气石低(平均 10．5‰)。以上空间和成分上的关系表明硼酸盐可能是形成电气石主要的硼来源，电气石是在热液过程中通过淋滤下伏含硼蒸发岩中的硼形成含硼热液，在与上覆沉积物交代过程中形成含电气石岩石。电气石的条带是热液顺层选择交代的结果。本区电气石与硼酸盐的关系表明，层状电气石可以通过含硼热液交代的方式形成。变质地体中的层状电气石岩石的出现可能与变质蒸发岩有关。这一认识对区域硼矿勘查工作和变质地体的沉积环境分析有借鉴意义。     

Chemical and boron isotopic compositions of tourmaline from the Paleoproterozoic Houxianyu borate deposit,NE China: Implications for the origin of borate deposit

The Houxianyu borate deposit in northeastern China is one of the largest boron sources of China, hosted mainly in the Paleoproterozoic meta-volcanic and sedimentary rocks (known as the Liaohe Group) that are characterized by high boron concentrations. The borate ore-body has intimate spatial relationship with the Mg-rich carbonates/silicates of the Group, with fine-grained gneisses (meta-felsic volcanic rocks) as main country rocks. The presence of abundant tourmalinites and tourmaline-rich quartz veins in the borate orebody provides an opportunity to study the origin of boron, the nature of ore-forming fluids, and possible mineralization mechanism. We report the chemical and boron isotopic compositions of tourmalines from the tourmaline-rich rocks in the borate deposit and from the tourmaline-bearing fine-grained gneisses.Tourmalines from the fine-grained gneisses are chemically homogeneous, showing relatively high Fe and Na and low Mg, with δ¹¹B values in a narrow range from +1.22‰ to +2.63‰. Tourmalines from the tourmaline-rich rocks, however, commonly show compositional zoning, with an irregular detrital core and a euhedral overgrowth, and have significantly higher Mg, REE (and more pronounced positive Eu anomalies), V (229–1852 ppm) and Sr (208–1191 ppm) than those from the fine-grained gneisses. They show varied B isotope values ranging from +4.51‰ to +12.43‰, which plot intermediate between those of the terrigenous sediments and arc rocks with low boron isotope values (as represented by the δ¹¹B = +1.22‰ to +2.63‰ of the fine-grained gneisses of this study) and those of marine carbonates and evaporates with high boron isotope values. In addition, the rim of the zoned tourmaline shows notably higher Mg, Ti, V, Sn, and Pb, and REE (particularly LREEs), but lower Fe, Co, Cr, Ni, Zn, Mn, and lower δ¹¹B values than the core. These data suggest that (1) the sources of boron of the borate ore-body are mainly the Paleoproterozoic meta-volcanic and sedimentary rocks, and (2) the ore-forming fluids should be the high temperature metamorphic fluids related to the amphibolite-facies metamorphism of the Paleoproterozoic foldbelt, which leach boron from the boron-rich meta-volcanic and sedimentary rocks of the Liaohe Group, and the boron-rich metamorphic fluids subsequently interacted with the marine Mg-rich carbonates and evaporates, forming borate deposit, the tourmaline overgrowth in the rim and the tourmaline-rich rocks.     

Archaean fluid-assisted crustal cannibalism recorded by low δ<Superscript>18</Superscript>O and negative ε<Subscript>Hf(T)</Subscript> isotopic signatures of West Greenland granite zircon

The role of fluids during Archaean intra-crustal magmatism has been investigated via integrated SHRIMP U–Pb, δ¹⁸O and LA-MC-ICPMS ¹⁷⁶Hf isotopic zircon analysis. Six rock samples studied are all from the Nuuk region (southern West Greenland) including two ~3.69 Ga granitic and trondhjemitic gneisses, a 3.64 Ga granitic augen gneiss, a 2.82 Ga granodioritic Ikkattoq gneiss, a migmatite with late Neoarchaean neosome and a homogeneous granite of the 2.56 Ga Qôrqut Granite Complex (QGC). All zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains. Within the zircon analysed, there is no evidence for metamictization. Initial ε_Hf zircon values (n = 63) are largely sub-chondritic, indicating the granitic host magmas were generated by the remelting of older, un-radiogenic crustal components. Zircon from some granite samples displays more than one ²⁰⁷Pb/²⁰⁶Pb age, and correlated with ¹⁷⁶Hf/¹⁷⁷Hf compositions can trace multiple phases of remelting or recrystallization during the Archaean. Model ages calculated using Lu/Hf arrays for each sample indicate that the crustal parental rocks to the granites, granodiorites and trondhjemites segregated from a chondrite-like reservoir at an earlier time during the Archaean, corresponding to known formation periods of more primitive tonalite–trondhjemite–granodiorite (TTG) gneisses. Zircon from the ~3.69 Ga granite, the migmatite and QGC granite contains Eoarchaean cores with chondritic ¹⁷⁶Hf/¹⁷⁷Hf and mantle-like δ¹⁸O compositions. The age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses, further suggesting genesis of these granites by remelting of broadly tonalitic protoliths. Zircon oxygen isotopic compositions (n = 62) over nine age populations (six igneous and three inherited) have weighted mean or mean δ¹⁸O values ranging from 5.8 ± 0.6 to 3.7 ± 0.5‰. The 3.64 Ga granitic augen gneiss sample displays the highest δ¹⁸O with a mildly supra-mantle composition of 5.8 ± 0.6‰. Inherited Eoarchaean TTG-derived zircon shows mantle-like values. Igneous zircon from all other samples, spanning more than a billion years of Archaean time, record low δ¹⁸O sub-mantle compositions. These are the first low δ¹⁸O signatures reported from Archaean zircon and represent low δ¹⁸O magmas formed by the remelting and metamorphism of older crustal rocks following high-temperature hydrothermal alteration by meteoric water. Meteoric fluid ingress coupled with crustal extension, associated high heat flow and intra-crustal melting are a viable mechanism for the production of the low δ¹⁸O granites, granodiorites and trondhjemites reported here. Both high and low δ¹⁸O magmas may have been generated in extensional environments and are distinct in composition from Phanerozoic I-type granitic plutonic systems, which are typified by increasing δ¹⁸O during intra-crustal reworking. This suggests that Archaean magmatic processes studied here were subtly different from those operating on the modern Earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement.     

A two-stage fluid history for the Orocopia Schist and associated rocks related to flat subduction and exhumation,southeastern California

ABSTRACT

Stable isotopes combined with pre-existing ⁴⁰Ar/³⁹Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ¹⁸O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ¹⁸O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ¹⁸O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills.     

Development and Re‐Evaluation of Tourmaline Reference Materials for In Situ Measurement of Boron δ Values by Secondary Ion Mass Spectrometry

Six tourmaline samples were investigated as potential reference materials (RMs) for boron isotope measurement by secondary ion mass spectrometry (SIMS). The tourmaline samples are chemically homogeneous and cover a compositional range of tourmaline supergroup minerals (primarily Fe, Mg and Li end‐members). Additionally, they have homogeneous boron delta values with intermediate precision values during SIMS analyses of less than 0.6‰ (2s). These samples were compared with four established tourmaline RMs, that is, schorl IAEA‐B‐4 and three Harvard tourmalines (schorl HS#112566, dravite HS#108796 and elbaite HS#98144). They were re‐evaluated for their major element and boron delta values using the same measurement procedure as the new tourmaline samples investigated. A discrepancy of about 1.5‰ in δ¹¹B was found between the previously published reference values for established RMs and the values determined in this study. Significant instrumental mass fractionation (IMF) of up to 8‰ in δ¹¹B was observed for schorl–dravite–elbaite solid solutions during SIMS analysis. Using the new reference values determined in this study, the IMF of the ten tourmaline samples can be modelled by a linear combination of the chemical parameters FeO + MnO, SiO₂ and F. The new tourmaline RMs, together with the four established RMs, extend the boron isotope analysis of tourmaline towards the Mg‐ and Al‐rich compositional range. Consequently, the in situ boron isotope ratio of many natural tourmalines can now be determined with an uncertainty of less than 0.8‰ (2s).     

Tourmalinites from the Eastern Sierras Pampeanas, Argentina

In the eastern Sierras Pampeanas, Central Argentina, tourmalinites and coticules are found in close association with stratabound scheelite deposits in metamorphic terranes. In Sierra Grande (Agua de Ramón and Ambul districts) and Sierra de Altautina, tourmalinites are associated with stratabound scheelite deposits related to orthoamphibolites. In the Pampa del Tamboreo area, tourmalinites are located in biotite schists stratigraphically related to acid to intermediate metavolcanic rocks and scheelite-bearing quartzites.The mineral chemistry and boron isotopic compositions of tourmalinite-hosted and vein-hosted tourmalines are investigated. Overall, the tourmalines belong to the dravite-schorl series and are generally aluminous; Fe/(Fe+Mg) ranges from 0.33 to 0.85, Al/(Al+Fe+Mg) from 0.66 to 0.76 and the amount of X-site vacancy (0.12–0.48) indicates significant foitite components. Their boron isotopic compositions (δ¹¹B) are from −24.0‰ to−15.0‰.Similar mineral chemistries and boron isotopic values for tourmaline in tourmalinites related to stratabound scheelite mineralisation and in tungsten-bearing quartz veins suggest a common source for the boron and probably the tungsten. The field, chemical and isotopic relationships are consistent with tungsten and boron in quartz-vein deposits being remobilised from stratabound scheelite and tourmalinite, dominantly by liquid-state transfer associated with regional shear zones. Tungsten and boron in the original sedimentary sequence (now meta-exhalites) are ascribed to volcanogenic exhalations.     

Tourmalinization related to Late Proterozoic-Early Palaeozoic lode gold mineralization in the Bin Yauri area, Nigeria

Three types of tourmaline occurrence have been identified in the area of Bin Yauri, Nigeria, mesothermal lode-gold mineralization. These are: (1) stratabound tourmalinites in pelitic metasediments, (2) tourmaline in a hydrothermal alteration assemblage within hornfelsed wall rocks, (3) tourmaline in auriferous quartz and quartz-carbonate veins. Although the tourmaline occurrences are all within or close to the contact aureole of a granodiorite intrusion, geochemical characteristics of the tourmalines are broadly similar and reflect a common metasedimentary source. Two stages of tourmalinization are envisaged. The earlier (ca. 1100 Ma) involved syngenetic-diagenetic formation of tourmalinites, while the later (ca. 500 Ma) involved epigenetic (hydrothermal) tourmaline-gold mineralization, possibly derived by dehydration and devolatilization of metasedimentary sequences containing tourmaline-rich rocks or tourmalinites. Electron microprobe analyses indicate that the tourmalines are intermediate members of the schorl-dravite solid solution series. Plots involving FeO, MgO, and/or Al₂O₃ from these analyses are used to constrain the sources and processes of tourmalinization. Two metallogenic implications are derived from this study. One is that, although the tourmalinites are barren of gold and base metal mineralization, their occurrences nevertheless encourages exploration for syngenetic-exhalative massive sulphide deposits in the region. The other implication applies to the potential use of tourmaline in deciphering the physico-chemical conditions of gold-mineralizing fluids in the Bin Yauri area.     

Tourmaline in Proterozoic Massive Sulfide Deposits from Rajasthan, India

We have analyzed the chemical composition and boron isotope composition of tourmaline from tourmalinites, granite and a quartz-tourmaline vein from the Deri ore zone and from a pegmatitic band in the Rampura-Agucha ore body. These two Proterozoic massive sulfide deposits occur in the Aravalli-Delhi orogenic belt, Rajasthan, northwest India. Tourmaline from stratiform tourmalinites closely associated with the massive sulfides in the Deri deposit have preserved their original chemical compositions despite regional and thermal metamorphism in the area. These tourmalines have low Fe/(Fe + Mg) ratios (0.19–0.30; mean 0.26) that suggest formation close to the sediment-sea water interface. The δ¹¹B values (−15.5 and −16.4‰) are compatible with boron derived from leaching of argillaceous sediments and/or felsic volcanics underlying the original massive sulfide deposit during its formation. Boron isotope compositions measured in tourmaline from a post-ore granite and quartz-tourmaline vein in the Deri deposit indicate that boron in these tourmalines was derived from the tourmalinites produced during ore formation. The boron isotope systematics of a coarse brown tourmaline crystal from a pegmatitic band on the hanging wall contact of the Rampura-Agucha deposit indicate that 45 ± 25% of the boron within the original tourmaline was lost during upper amphibolite facies regional metamorphism. Received: 3 April 1996 / Accepted: 11 April 1996     

A Nd- and O-isotope study of the REE-rich peralkaline Strange Lake granite: implications for Mesoproterozoic A-type magmatism in the Core Zone (NE-Canada)

It is well established that A-type granites enriched in high field strength elements, such as Zr, Nb and the REE, form in anorogenic tectonic settings. The sources of these elements and the processes controlling their unusual enrichment, however, are still debated. They are addressed here using neodymium and oxygen isotope analyses of samples from the 1.24 Ga Strange Lake pluton in the Paleoproterozoic Core Zone of Québec-Labrador, an A-type granitic body characterized by hyper-enrichment in the REE, Zr, and Nb. Age-corrected εNd values for bulk rock samples and sodic amphiboles (mainly arfvedsonite) from the pluton range from ?0.6 to ?5.7, and ?0.3 to ?5.3, respectively. The εNd values for the Napeu Kainiut quartz monzonite, which hosts the pluton, range from ?4.8 to ?8.1. The ¹⁴⁷Sm/¹⁴⁴Nd ratios of the suite and the host quartz monzonite range from 0.0967 to 0.1659, large variations that can be explained by in situ fractionation of early LREE-minerals (Strange Lake), and late hydrothermal HREE remobilization. Oxygen isotope analyses of quartz of both Strange Lake and the host yielded δ¹⁸O values between +8.2 and +9.1, which are considerably higher than the mantle value of 5.7 ± 0.2‰. Bulk rock oxygen isotope analyses of biotite-gneisses in the vicinity of the Strange Lake pluton yielded δ¹⁸O values of 6.3, 8.6 and 9.6‰. The negative εNd values and positive δ¹⁸O values of the Strange Lake and Napeu Kainiut samples indicate that both magmas experienced considerable crustal contamination. The extent of this contamination was estimated, assuming that the contaminants were sedimentary-derived rocks from the underlying Archean Mistinibi (para-) gneiss complex, which is characterized by low εNd and high δ¹⁸O values. Mixing of 5–15% of a gneiss, having an εNd value of ?15 and a δ¹⁸O value of +11, with a moderately enriched mantle source (εNd = +0.9, δ¹⁸O = +6.3) would produce values similar to those obtained for the Strange Lake granites. Based on analogies between the Nain Plutonic Suite and the Gardar alkaline igneous province (SW-Greenland), we conclude that the Strange Lake pluton and associated REE-mineralized anorogenic bodies formed from a combination of subduction-induced fertilization of the sublithospheric mantle, crustal extension and in situ magma evolution.     

东秦岭花岗伟晶岩中电气石地球化学特征及成矿指示意义

东秦岭地区是我国重要的花岗伟晶岩区及稀有金属成矿区.电气石在东秦岭各类花岗伟晶岩中广泛发育,通常在无矿化伟晶岩、铍矿化及锂矿化伟晶岩中呈黑色-深蓝色.本文旨在通过各类伟晶岩中电气石的对比研究揭示电气石地球化学特征对东秦岭伟晶岩矿化类型的指示作用.本文所研究电气石为作为东秦岭各类伟晶岩贯通矿物的黑电气石系列.在双峰村、碾...     

Acigöl rhyolite field,central Anatolia (part II): geochemical and isotopic (Sr–Nd–Pb, δ<Superscript>18</Superscript>O) constraints on volcanism involving two high-silica rhyolite suites

The Acigöl rhyolite field erupted the most recent high-silica rhyolites within the Cappadocian Volcanic Province of central Anatolia, Turkey. It comprises two sequences of domes and pyroclastic rocks with eruption ages of ~150–200 ka (eastern group) and ~20–25 ka (western group). Compositionally, the eastern rhyolite group lavas are less evolved (SiO₂ = 74–76 wt%), whereas the western group has higher silica abundance (SiO₂ = ~77 wt%) with extremely depleted feldspar-compatible trace elements. Within each group, compositional variability is small and ¹⁴³Nd/¹⁴⁴Nd (0.51257–0.51265) and Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.87–18.88, ²⁰⁷Pb/²⁰⁴Pb = 15.65–15.67 and ²⁰⁸Pb/²⁰⁴Pb = 38.94–38.98) are homogeneous. The western group rhyolites have δ¹⁸O(zircon) overlapping mantle values (5.7 ± 0.2‰), whereas eastern group rhyolites are enriched in δ¹⁸O by ~0.5‰, consistent with a tendency to lower εNd values. By contrast, western group rhyolites have markedly more radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7065–0.7091) compared to those of the eastern group (0.7059–0.7065). The presence of angular granitic xenoliths and a correlation between hydration (based on loss on ignition data) and ⁸⁷Sr/⁸⁶Sr in the western lavas, however, indicates that Sr was added during the eruption or post-eruption alteration. Isotope constraints preclude the possibility that the rhyolite magmas formed by partial melting of any known regional crystalline basement rocks. Basalts and andesites erupted in the periphery of the Acigöl field are characterised by ⁸⁷Sr/⁸⁶Sr ratios between 0.7040 and 0.7053, ¹⁴³Nd/¹⁴⁴Nd = 0.51259–0.51300, ²⁰⁶Pb/²⁰⁴Pb = 18.85–18.87, ²⁰⁷Pb/²⁰⁴Pb = 15.646–15.655, ²⁰⁸Pb/²⁰⁴Pb = 38.90–38.97. The isotopic and trace element data favour an origin of the rhyolites by mixing of basaltic/andesitic magmas with minor amounts of crustal melts and followed by extensive fractional crystallization.     

Mineral chemistry and chemical zoning in tourmalines, Pampa del Tamboreo, San Luis, Argentina

Tourmalinites that are distally associated with tungsten deposits of the Pampa del Tamboreo area, San Luis, Argentina, contain tourmalines retaining evidence for its origin and evolution. Tourmaline grains uncommonly contain small grains of detrital tourmaline. Analysis of a single detrital tourmaline grain reveals that it is a Ca-rich “oxy-dravite”. Proximal to the detrital cores there are inner domains of asymmetric tourmaline overgrowths that developed during low grade metamorphism. Volumetrically dominant tourmaline overgrowths in the outer domain are concentrically zoned aluminous dravite and “oxy-dravite” with Al/(Al + Fe + Mg) = 0.71–0.74 and Mg/(Mg + Fe) = 0.64–0.71. Variability of Al is primarily controlled by the deprotonation substitution R + OH⁻ = Al + O²⁻ (where R = Fe + Mg), and is a function of the activity of H₂O. A likely evolutionary scenario is one in which volcanogenic material is altered by hydrothermal fluids in the sea floor resulting in an aluminous and magnesian residuum. With further hydrothermal circulation and incipient metamorphism, boron-rich fluids are expelled from metasedimentary and metavolcanic basement rocks and develop Mg-rich tourmalinites in the aluminous, magnesian host rocks. The tourmalinization process occurs over a range of metamorphic conditions and with fluids of variable activity of H₂O.     

加拿大Sullivan超大型Pb-Zn-Ag矿床中几个指示成矿环境的特征矿物研究

加拿大Sullivan矿床是世界上最典型的SEDEX型Pb-Zn-Ag矿床。对该矿床中产出的几个特征矿物,如硼矿物电气石,富氯的方柱石和黑云母,富钡的钾长石和白云母,及富锰的石榴子石、绿泥石、碳酸盐和钛铁矿进行了研究。指出这些矿物的存在反映了该矿床产出的物理化学环境。提出成矿流体可能淋滤了深部存在的非海相蒸发岩层,层状硫化物矿石可能形成于海底高盐度热卤水池中。     

Genesis of the Au–Bi–Cu–As, Cu–Mo ± W, and base–metal Au–Ag mineralization at the Mountain Freegold (Yukon, Canada): constraints from Ar–Ar and Re–Os geochronology and Pb and stable isotope compositions

The genesis of mineralized systems across the Mountain Freegold area, in the Dawson Range Cu–Au?±?Mo Belt of the Tintina Au province was constrained using Pb and stable isotope compositions and Ar–Ar and Re–Os geochronology. Pb isotope compositions of sulfides span a wide compositional range (²⁰⁶Pb/²⁰⁴Pb, 18.669–19.861; ²⁰⁸Pb/²⁰⁴Pb, 38.400–39.238) that overlaps the compositions of the spatially associated igneous rocks, thus indicating a magmatic origin for Pb and probably the other metals. Sulfur isotopic compositions of sulfide minerals are broadly similar and their δ³⁴S (Vienna-Canyon Diablo Troilite (V-CDT)) values range from ?1.4 to 3.6 ‰ consistent with the magmatic range, with the exception of stibnite from a Au–Sb–quartz vein, which has δ³⁴S values between ?8.1 and ?3.1 ‰. The δ³⁴S values of sulfates coexisting with sulfide are between 11.2 and 14.2 ‰; whereas, those from the weathering zone range from 3.7 to 4.3 ‰, indicating supergene sulfates derived from oxidation of hypogene sulfides. The δ¹³C (Vienna Peedee Belemnite (VPDB)) values of carbonate range from ?4.9 to 1.1 ‰ and are higher than magmatic values. The δ¹⁸O (V-SMOW) values of magmatic quartz phenocrysts and magmatic least-altered rocks vary between 6.2 and 10.1 ‰ and between 5.0 and 10.1 ‰, respectively, whereas altered magmatic rocks and hydrothermal minerals (quartz and magnetite) are relatively ¹⁸O-depleted (4.2 to 7.9 ‰ and ?6.3 to 1.5 ‰, respectively). Hydrogen isotope compositions of both least-altered and altered igneous rock samples are D-depleted (from ?133 to ?161 ‰ Vienna-Standard Mean Ocean Water (V-SMOW)), consistent with differential magma degassing and/or post-crystallization exchange between the rocks and meteoric ground water. Zircon from a chlorite-altered dike has a U–Pb crystallization age of 108.7?±?0.4 Ma; whereas, the same sample yielded a whole-rock Ar–Ar plateau age of 76.25?±?0.53 Ma. Likewise, molybdenite Re–Os model ages range from 75.8 to 78.2 Ma, indicating the mineralizing events are genetically related to Late Cretaceous volcano-plutonic intrusions in the area. The molybdenite Re–Os ages difference between the nearby Nucleus (75.9?±?0.3 to 76.2?±?0.3 Ma) and Revenue (77.9?±?0.3 to 78.2?±?0.3 Ma) mineral occurrences suggests an episodic mineralized system with two pulses of hydrothermal fluids separated by at least 2 Ma. This, in combination with geological features suggest the Nucleus deposit represents the apical and younger portion of the Revenue–Nucleus magmatic-hydrothermal system and may suggest an evolution from the porphyry to the epithermal environments.     

Oxygen isotopic study of Late Mesozoic cooling of the Mount Barcroft area,central White Mountains,eastern California

The Middle Jurassic Barcroft mafic granodiorite and Late Cretaceous, ternary-minimum McAfee Creek Granite are important components of the igneous arc sited along the SW North American margin. Bulk-rock analyses of 11 samples of the metaluminous, I-type Barcroft comagmatic suite have an average δ¹⁸O value of 7.4±0.6‰ (all values±1σ). Four Barcroft specimens average ε_Nd=?3.6±1.8, ⁸⁷Sr/⁸⁶Sr=0.707±0.001. The pluton consists of petrochemically gradational, Ca-amphibole-rich gabbro/diorite, granodiorite, metadiorite, and rare alaskite–aplite; for most of the pluton, oxygen isotope exchange of quartz, feldspar(s), biotite, and Ca-amphibole accompanied local deuteric alteration. Eight specimens of slightly peraluminous granitic rocks of the muscovite-bearing McAfee Creek series have an average δ¹⁸O of 8.6±0.5‰. Four McAfee-type samples average ε_Nd=?7.8±1.7, ⁸⁷Sr/⁸⁶Sr=0.711±0.004. For both plutons, bulk-rock evidence of exchange with near-surface water is lacking, suggesting ~5–10 km cooling depths. Barcroft minerals exhibit regular oxygen isotopic partitioning from high to low δ¹⁸O in the sequence quartz>plagioclase>K-feldspar>>amphibole≥biotite. Along the SE margin of the pluton, quartz and biotite in Lower Cambrian quartzites are higher in δ¹⁸O, and show slightly larger fractionations than igneous analogues. Exchange with fluids derived from these heated, contact-metamorphosed country rocks increased bulk ¹⁸O/¹⁶O ratios of Barcroft border rocks (and constituent plagioclase+subsolidus tremolite–actinolite), especially of granitic dikes transecting the wall rocks. Oxygen isotope thermometry for seven Barcroft pluton quartz–amphibole and six quartz–biotite pairs indicate apparent subsolidus temperatures averaging 519±49 °C. Quartz–plagioclase pairs from two Barcroft granodiorites yield values of 519 and 515 °C. A quartz–biotite pair from a quartzite adjacent to the Barcroft pluton yields an apparent temperature of 511 °C, in agreement with estimates based on contact metamorphic parageneses. Except for its SE margin, Barcroft pluton silicates evidently exchanged oxygen isotopes under local deuteric conditions. Compatible with Ca-amphibole thermobarometric analyses, areal distributions for quartz–plagioclase, quartz–amphibole, and quartz–biotite pairs reveal that putative annealing temperatures are lowest in NE-trending axial portions of the Barcroft body, so it simply cooled inwards. Intrusion ~70 million years later by the McAfee Creek Granite had no discernable effect on δ¹⁸O values of Barcroft minerals and bulk rocks.     

Local distribution of oxygen isotopes and fluid exchange during genesis of the corundum-bearing rocks of Khitostrov Island

New data are presented on the distribution of oxygen isotopes and conditions of the local isotope equilibrium in high-Al rocks rocks of Khitostrov Island showing abnormally low δ¹⁸O values (below–25‰). The temperatures of isotope equilibrium are within 400–475°C. The minimum δ¹⁸O values have been registered in the in plagioclase, whereas the same phases in kyanite-bearing rocks lacking corundum demonstrate δ¹⁸O values usually 3–5‰ higher. The fluid δ¹⁸O value varies from–22 to–16‰ at 475 ± 15°C, from–18 to–23‰ at 425 ± 25°C, and from–17 to–22‰ at 380 ± 15°C. The results obtained do not require abnormal depletion of δ¹⁸O values owing to the infiltration of an external fluid under the Svecofennian transformations. The association of corundum-bearing rocks with the basic intrusions, the presence of zircon cores of older ages compared to these rocks, and the peculiarities of rock chemistry may be ascribed to the fact that lower crustal layers of ancient rocks depleted in δ¹⁸O before metamorphism were captured by basite melts.     

Age and nature of 560–520 Ma calc-alkaline granitoids of Biarjmand,northeast Iran: insights into Cadomian arc magmatism in northern Gondwana

ABSTRACT

The Biarjmand granitoids and granitic gneisses in northeast Iran are part of the Torud–Biarjmand metamorphic complex, where previous zircon U–Pb geochronology show ages of ca. 554–530 Ma for orthogneissic rocks. Our new U–Pb zircon ages confirm a Cadomian age and show that the granitic gneiss is ~30 million years older (561.3 ± 4.7 Ma) than intruding granitoids (522.3 ± 4.2 Ma; 537.7 ± 4.7 Ma). Cadomian magmatism in Iran was part of an approximately 100-million-year-long episode of subduction-related arc and back-arc magmatism, which dominated the whole northern Gondwana margin, from Iberia to Turkey and Iran. Major REE and trace element data show that these granitoids have calc-alkaline signatures. Their zircon O (δ¹⁸O = 6.2–8.9‰) and Hf (–7.9 to +5.5; one point with εHf ~ –17.4) as well as bulk rock Nd isotopes (εNd(t) = –3 to –6.2) show that these magmas were generated via mixing of juvenile magmas with an older crust and/or melting of middle continental crust. Whole-rock Nd and zircon Hf model ages (1.3–1.6 Ga) suggest that this older continental crust was likely to have been Mesoproterozoic or even older. Our results, including variable zircon εHf(t) values, inheritance of old zircons and lack of evidence for juvenile Cadomian igneous rocks anywhere in Iran, suggest that the geotectonic setting during late Ediacaran and early Cambrian time was a continental magmatic arc rather than back-arc for the evolution of northeast Iran Cadomian igneous rocks.