A garnet population in Yellowknife schist, Canada

Abstract Data are presented on a garnet population in a specimen of garnet-biotite-plagioclase-quartz schist from the cordierite zone of an Archaean thermal dome in the Southern Slave Province of the Canadian Shield. Garnet crystals are bounded by planar dodecahedral faces and by trapezohedral faces which on the 10-μm scale are corrugated. Crystal distribution, as revealed by dissection of a small cubic volume of rock, is random. The size distribution is normal, with a mean diameter of 0.81 mm and a standard deviation of 0.32 mm. In the largest crystal of the population (mean radius 0.83 mm), [Mn] = 100 Mn/(Fe + Mg + Mn + Ca) decreases from 14.5 at the centre to 7.5 and then increases in the outer margin to 8.5; [Fe] increases continuously from 67 at the centre to 77 at the surface; [Mg] increases from 12.5 to 13.5 and then falls sharply to 11; [Ca] remains unchanged at 4.0 and then drops to 3.3. Progressively smaller crystals have progressively lower [Mn] and higher [Fe] concentrations at their centres, while all crystals have the same margin composition. Growth vectors extending from given concentration contours to crystal surfaces are of equal length regardless of the size of the crystal in which the vector is located. A garnet-forming model is presented in which reaction was initiated by a rise in temperature. Nucleation sites were randomly selected. The nucleation rate increased with time and then declined. Crystal faces advanced at a constant linear rate, which implies an increase in volume proportional to surface area. Initially, the composition of garnet deposited on crystal surfaces was determined by van Laar equations of equilibrium, which demanded the withdrawal of Mn and Fe from within chlorite crystals. This transfer reaction was then accompanied by an ion exchange reaction which moved Mn and Fe to garnet surfaces from biotite, in exchange for Mg. The exchange reaction provides an explanation for the high overall concentration of Mn and Fe in garnet and for the observed Mn and Mg reversals in the margins of crystals. The increase of garnet volume in the garnet population is found to be parabolic, i.e. Vαα⁵.     

Geochronology,geochemistry, and mineralization of the granodiorite porphyry hosting the Matou Cu–Mo (±W) deposit,Lower Yangtze River metallogenic belt,eastern China

Porphyry and skarn Cu–Fe–Au–Mo deposits are widespread in the Middle and Lower Yangtze River metallogenic belt (MLYMB), eastern China. The Matou deposit has long been regarded as a typical Cu–Mo porphyry deposit within Lower Yangtze part of the belt. Recently, we identified scheelite and wolframite in quartz veins in the Matou deposit, which is uncommon in other porphyry and skarn deposits in the MLYMB. We carried out detailed zircon U–Pb dating and geochemical and Sr–Nd–Hf isotopic studies of the granodiorite porphyry at Matou to define any differences from other ore-related granitoids. The porphyry shows a SiO₂ content ranging from 61.85 wt.% to 65.74 wt.%, K₂O from 1.99 wt.% to 3.74 wt.%, and MgO from 1.74 wt.% to 2.19 wt.% (Mg^# value ranging from 45 to 55). It is enriched in light rare earth elements and large ion lithophile elements, but relatively depleted in Nb, Ta, Y, Yb and compatible trace elements (such as Cr, Ni, and V), with slight negative Eu anomalies (Eu/Eu^* = 0.88–0.98) and almost no negative Sr anomalies. Results of electron microprobe analysis of rock-forming silicate minerals indicate that the Matou porphyry has been altered by an oxidized fluid that is rich in Mg, Cl, and K. The samples show relatively low ε_Nd(t) values from −7.4 to −7.1, slightly high initial ⁸⁷Sr/⁸⁶Sr values from 0.708223 to 0.709088, and low ε_Hf(t) values of zircon from −9.0 to −6.5, when compared with the other Cu–Mo porphyry deposits in the MLYMB. Zircon U–Pb dating suggests the Matou granodiorite porphyry was emplaced at 139.5 ± 1.5 Ma (MSWD = 1.8, n = 15), which is within the age range of the other porphyries in the MLYMB. Although geochemical characteristics of the Matou and other porphyries in the MLYMB are similar and all adakitic, the detrital zircons in the samples from Matou suggest that Archean lower crust (2543 ± 29 Ma, MSWD = 0.25, n = 5) was involved with the generation of Matou magma, which is different from the other porphyries in the belt. Our study suggests that the Matou granodiorite porphyry originated from partial melting of thickened lower crust that was delaminated into the mantle, similar to the other porphyries in the MLYMB, but it has a higher proportion of lower crustal material, including Archean rocks, which contributed to the formation of the porphyry and related W-rich magmatic-hydrothermal system.     

Geochemistry and Ar–Ar muscovite ages of the Daraban Leucogranite,Mawat Ophiolite,northeastern Iraq: Implications for Arabia–Eurasia continental collision

Daraban Leucogranite dykes intruded discordantly into the basal serpentinized harzburgite of the Mawat Ophiolite, Kurdistan region, NE Iraq. These coarse grained muscovite-tourmaline leucogranites are the first leucogranite dykes identified within the Mawat Ophiolite. They are mainly composed of quartz, K-feldspar, plagioclase, tourmaline, muscovite, and secondary phologopite, while zircon, xenotime, corundum, mangano-ilemnite and cassiterite occur as accessories.The A/CNK value of the granite dyke samples varies from 1.10 to 1.22 indicating a strongly peraluminous composition. CaO/Na₂O ranges from 0.11 to 0.15 and Al₂O₃/TiO₂ from 264 to 463, similar to the strongly peraluminous (SP) granites exposed in ‘high-pressure’ collision zones such as the Himalayas.Ar–Ar muscovite step-heating dating yields 37.57 ± 0.25 and 38.02 ± 0.53 Ma plateau ages for two samples which are thought to reflect either their magmatic emplacement or resetting during collision-related metamorphism. Mineral chemistry shows evidence of both primary and secondary types of muscovite, with cores favouring the magmatic interpretation and slight effects of a late syn-serpentinization fluid seen at the rims.Geochemical features of Daraban Leucogranite dykes favour a syn-collisional tectonic setting. They probably formed in response to the continental collision between Eurasia and Arabia during the initial stage of the opening of the Gulf of Aden at 37 Ma. The muscovite ages and geochemical features of Daraban Leucogranite are strong evidence for the timing of the continental collision between northeastern Arabia and Eurasia in Kurdistan region of Iraq.     

Anatomy of garnets in a Jurassic granite from the south-eastern margin of the North China Craton: Magma sources and tectonic implications

The Late Jurassic Jingshan granite located at the south-eastern margin of the North China Craton contains abundant garnets which can be subdivided into three types based on texture and composition: (i) euhedral garnet in mafic biotite and garnet rich enclave (Grt I), (ii) coarse-grained garnet (Grt II) in the host granite, and (iii) small euhedral garnet in aplite (Grt III). In general, Grt I has higher FeO, CaO and lower MnO contents than Grt II. Grt III has higher Mn, but lower Ca contents than others. Grt I has lower MREE and HREE contents than Grt II. Grt III has prominent and distinctly negative Eu anomaly as well as higher MREE composition compared to the others. Systematic variations in oxygen isotope compositions are observed among the three garnet types, with δ¹⁸O values of <3.8‰ in most of Grt I, 3.8–4.7‰ in most Grt II (for inclusion-free garnets), and typically >4.7‰ in Grt III. Some of the Grt II and Grt III display two distinct zonings with cores having similar major and trace element compositions to Grt I.Cathodoluminescence (CL) images revealed that the zircons from different garnet-bearing samples possess fine-scale oscillatory zoned magmatic rims with inherited cores. In situ zircon U–Pb dating and trace element analyses show that the dark-luminescent magmatic rims all have Jurassic concordia ages (∼160 Ma) and similar trace element patterns. Most of the inherited cores also display similar Triassic ages of 210–236 Ma, which is similar to the ages of ultrahigh pressure (UHP) metamorphic rocks of the Dabie–Sulu orogen (230 Ma). In addition, Jurassic concordia ages were also found in a zircon inclusion in Grt I, implying that the Grt I was formed shortly before the main magmatic event. The age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events.Based on the zircon U–Pb ages from different garnet-bearing samples, the major element, trace element, oxygen isotope, and zoning textures of the three kinds of garnet we suggest that Grt I may be peritectic garnet, whereas Grt II and III are probably the results of magmatic dissolution–precipitation processes and re-equilibration of garnets with changing magmatic conditions during melting, differentiation, crystallization, and cooling within the granite. We conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the Jingshan granites are from Dabie–Sulu orogen representing the South China Craton.     

Late Triassic volcanic activity in South-East Asia: New stratigraphical,geochronological and paleontological evidence from the Luang Prabang Basin (Laos)

In South-East Asia, sedimentary basins displaying continental Permian and Triassic deposits have been poorly studied. Among these, the Luang Prabang Basin (North Laos) represents a potential key target to constrain the stratigraphic and structural evolutions of South-East Asia. A combined approach involving sedimentology, palaeontology, geochronology and structural analysis, was thus implemented to study the basin. It resulted in a new geological map, in defining new formations, and in proposing a complete revision of the Late Permian to Triassic stratigraphic succession as well as of the structural organization of the basin. Radiometric ages are used to discuss the synchronism of volcanic activity and sedimentation.The Luang Prabang Basin consists of an asymmetric NE-SW syncline with NE-SW thrusts, located at the contact between Late Permian and Late Triassic deposits. The potential stratigraphic gap at the Permian–Triassic boundary is therefore masked by deformation in the basin. The Late Triassic volcaniclastic continental deposits are representative of alluvial plain and fluvial environments. The basin was fed by several sources, varying from volcanic, carbonated to silicic (non-volcanic). U–Pb dating of euhedral zircon grains provided maximum sedimentation ages. The stratigraphic vertical succession of these ages, from ca. 225, ca. 220 to ca. 216 Ma, indicates that a long lasting volcanism was active during sedimentation and illustrates significant variations in sediment preservation rates in continental environments (from ∼100 m/Ma to ∼3 m/Ma). Anhedral inherited zircon grains gave older ages. A large number of them, at ca. 1870 Ma, imply the reworking of a Proterozoic basement and/or of sediments containing fragments of such a basement. In addition, the Late Triassic (Carnian to Norian) sediments yielded to a new dicynodont skull, attributed to the Kannemeyeriiform group family, from layers dated in between ∼225 and ∼221 Ma (Carnian).     

Petrology and U–Pb zircon dating of coesite-bearing metapelite from the Kebuerte Valley,western Tianshan,China

This paper deals with the petrology and U–Pb dating of coesite-bearing garnet–phengite schist from the Kebuerte Valley, Chinese western Tianshan. It mainly consists of porphyroblastic garnet, phengite, quartz and chlorite with minor amounts of paragonite, albite, zoisite and chloritoid. The well preserved coesite inclusions (∼100 μm) in garnet are encircled by a narrow rim of quartz. They were identified by optical microscopy and confirmed by Raman spectroscopy. Using the computer program THERMOCALC, the peak metamorphic conditions of 29 kbar and 565 °C were obtained via garnet isopleth geothermobarometry. The predicted UHP peak mineral assemblage comprises garnet + jadeite + lawsonite + carpholite + coesite + phengite. The metapelite records prograde quartz–eclogite-facies metamorphism, UHP coesite–eclogite-facies peak metamorphism, and a late greenschist-facies overprint. Phase equilibrium modeling predicts that garnet mainly grew in the mineral assemblages garnet + jadeite + lawsonite + chloritoid + glaucophane + quartz + phengite and garnet + jadeite + lawsonite + carpholite + glaucophane + quartz + phengite. SHRIMP U–Pb zircon dating of the coesite-bearing metapelite yielded the peak metamorphic age 320.4 ± 3.7 Ma. For the first time, age data of coesite-bearing UHP metapelite from the Chinese western Tianshan are presented in this paper. They are in accord with published ages obtained from eclogite from other localities in the Chinese western Tianshan and the Kyrgyz South Tianshan and therefore prove a widespread occurrence of UHP metamorphism.     

Zircon U–Pb age,Hf isotopic compositions and geochemistry of the Silurian Fengdingshan I-type granite Pluton and Taoyuan mafic–felsic Complex at the southeastern margin of the Yangtze Block

This work presents an integrated study of zircon U–Pb ages and Hf isotope along with whole-rock geochemistry on Silurian Fengdingshan I-type granites and Taoyuan mafic–felsic intrusive Complex located at the southeastern margin of the Yangtze Block, filling in a gap in understanding of Paleozoic I-type granites and mafic-intermediate igneous rocks in the eastern South China Craton (SCC). The Fengdingshan granite and Taoyuan hornblende gabbro are dated at 436 ± 5 Ma and 409 ± 2 Ma, respectively. The Fengdingshan granites display characteristics of calc-alkaline I-type granite with high initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7093–0.7127, low εNd(t) values ranging from −5.6 to −5.4 and corresponding Nd model ages (T_2DM) of 1.6 Ga. Their zircon grains have εHf(t) values ranging from −2.7 to 2.6 and model ages of 951–1164 Ma. The Taoyuan mafic rocks exhibit typical arc-like geochemistry, with enrichment in Rb, Th, U and Pb and depletion in Nb, Ta. They have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7053–0.7058, εNd(t) values of 0.2–1.6 and corresponding T_2DM of 1.0–1.1 Ga. Their zircon grains have εHf(t) values ranging from 3.2 to 6.1 and model ages of 774–911 Ma. Diorite and granodiorite from the Taoyuan Complex have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7065–0.7117, εNd(t) values from −5.7 to −1.9 and Nd model ages of 1.3–1.6 Ga. The petrographic and geochemical characteristics indicate that the Fengdingshan granites probably formed by reworking of Neoproterozoic basalts with very little of juvenile mantle-derived magma. The Taoyuan Complex formed by magma mixing and mingling, in which the mafic member originated from a metasomatized lithospheric mantle. Both the Fengdingshan and Taoyuan Plutons formed in a post-orogenic collapse stage in an intracontinental tectonic regime. Besides the Paleozoic Fengdingshan granites and Taoyuan hornblende gabbro, other Neoproterozoic and Indosinian igneous rocks located along the southeastern and western margin of the Yangtze Block also exhibit decoupled Nd–Hf isotopic systemics, which may be a fingerprint of a previous late Mesoproterozoic to early Neoproterozoic oceanic subduction.     

无机生油假说及其在中国的应用前景

无机生油假说认为,原油和天然气和近地表的生物物质没有根本联系,它们是生成于地幔内的非生物来源的碳氢化合物。因而油气不是一个不可再生资源,而是一个可再生资源。无机生油假说得到地质学、物理学和化学等三个基本学科的支持。在地质观察上,发现全球许多大油田的油气储藏与原始生物物质之间数量上有巨大落差,难于解释它们是由生物生成的。此外,有许多地区在结晶基底或变质基底内,或直接位于其上的沉积岩中发现石油。从生物生油假说来说,也是无法理解的。在化学上,早在二战期间,德国已由人工合成石油(费托合成),并生产了占德国战争中用油的9%的石油。无可争辩地说明,无机可以生成石油。根据化学(物理学)热力学理论分析确认,甲烷是唯一一种在标准温压条件(温度为298.15 K;压力为101325 Pa)下稳定的碳氢化合物,从甲烷形成正常烷属烃只有在压力>3×106kPa、温度>700°C时(相当于地下深度约100 km)才有可能。在地壳内的温压条件下由生物变质形成石油的假说,与化学热力学的基本原则相抵触。从氧化的有机分子,如碳水化合物(C6H12O6)形成较高的碳氢化合物在任何条件下都是不可能的。根据我国长期对深部构造的研究,笔者认为在中国东部及西太平洋蘑菇云岩石圈地幔发育的地区是寻找巨型无机油气田的有利地区,建议在发育蘑菇云岩石圈地幔地区开展无机油气田的勘探,并在无机油气田远景地区布置超深参数钻,以评价含油气远景。另外建议加强物探工作,尤其是研究地震勘探处理基底内三维含油气构造的技术。     

The upper Cenomanian–lower Turonian carbonate platform of the Preafrican Trough,Morocco: Biostratigraphic,paleoecological and paleobiogeographical distribution of ostracods

The upper Cenomanian–lower Turonian paleoenvironments of the Preafrican Trough carbonate platform is characterized by analyzing the structure of the ostracod assemblages and the information provided by other groups, and also by linking together the paleontological and geochemical data (detrital influx-redox-paleoproductivity proxies, δ¹³C curve). Two different domains (eastern and western) can be recognized on the platform during the late Cenomanian, before the onset of the OAE2. The western domain corresponds to a low-energy environment developed on a mid and/or outer ramp with hypoxic waters, low detrital influx and low paleoproductivity. The paleoecological assemblages show limited specific diversity but variable density. The ostracods are opportunistic and unspecialized (r strategists), being associated with Buliminidae, surface and intermediate-water planktonic foraminifera, and fishes. The eastern domain corresponds to an inner ramp and/or peritidal environment with oxic waters, low detrital influx and low paleoproductivity, developed in a higher energy environment with paleoecological assemblages showing high diversity but variable density. The ostracods are more specialized (K strategists), being represented by diverse and constant assemblages associated with diversified benthic foraminifera, calcareous sponges and echinoderms, as well as intermediate- and deep-water planktonic foraminifera. The onset of the OAE2 has no influence on the western ostracod assemblages, but leads to the decline of the ostracod fauna and the disappearance of the deep-water planktonic foraminifera in the eastern domain. During the early Turonian, after the OAE2, the platform becomes an outer ramp with increased paleoproductivity, but is associated with a decrease of taxonomic diversity in hypoxic waters. The ostracods are very sparse and unspecialized, associated with siliceous sponges, Buliminidae, surface-living planktonic foraminifera, fishes and pelagic crinoids. Marine paleobiogeographic communication is relatively easy across the carbonate platforms between the Preafrican Trough and other Moroccan regions, as well as between Morocco and different parts of the South Tethyan and East Atlantic margins belonging to the Cenomanian–Turonian South Tethyan Ostracod Province (STOP). Thirteen new species are described: Cytherella tazzouguertensis n. sp., Bairdiacypris chaabetensis n. sp., Bythocypris amelkisensis n. sp., Pontocypris tadighoustensis n. sp., Procytherura? elongatissima n. sp., Loxoconcha akrabouensis n. sp., Hemiparacytheridea sagittaemucronata n. sp., Rehacythereis errachidiaensis n. sp., Rehacythereis zizensis n. sp., Veenia (Nigeria) tardaensis n. sp., Veenia (Nigeria) mediacostarobusta n. sp., Xestoleberis? preafricanensis n. sp., and Xestoleberis circinatus n. sp.     

赛马-柏林川碱性杂岩常量组分变异的分离结晶制约

赛马－柏林川碱性杂岩体为侵位于古元古宙与新元古宙之间的缓倾斜岩席状岩体,缓倾的张裂隙制约了分异岩浆的侵位和含矿溶液的流通,交代富集的上地幔产生的富碱岩浆与富集的地壳产生的岩浆以不同比例混合,形成了正长质岩浆和霞石正长质岩浆。含地壳组分较高的正长质岩浆,受富钙辉石、角闪石和镁质较高黑云母的结晶分离制约向硅酸过饱和方向演化;霞石正长质岩浆受富钙辉石—霓辉石、白榴石、含Ｆｅ较高黑云母、黑榴石的分离结晶制约发生分异。稀土元素矿床是霞石正长质岩浆分异残余熔体的产物,侵位于岩体最高层位。