The electrical structure of the Slave craton

The Slave craton in northwestern Canada, a relatively small Archean craton (600×400 km), is ideal as a natural laboratory for investigating the formation and evolution of Mesoarchean and Neoarchean sub-continental lithospheric mantle (SCLM). Excellent outcrop and the discovery of economic diamondiferous kimberlite pipes in the centre of the craton during the early 1990s have led to an unparalleled amount of geoscientific information becoming available.

Over the last 5 years deep-probing electromagnetic surveys were conducted on the Slave, using the natural-source magnetotelluric (MT) technique, as part of a variety of programs to study the craton and determine its regional-scale electrical structure. Two of the four types of surveys involved novel MT data acquisition; one through frozen lakes along ice roads during winter, and the second using ocean-bottom MT instrumentation deployed from float planes.

The primary initial objective of the MT surveys was to determine the geometry of the topography of the lithosphere–asthenosphere boundary (LAB) across the Slave craton. However, the MT responses revealed, completely serendipitously, a remarkable anomaly in electrical conductivity in the SCLM of the central Slave craton. This Central Slave Mantle Conductor (CSMC) anomaly is modelled as a localized region of low resistivity (10–15 Ω m) beginning at depths of 80–120 km and striking NE–SW. Where precisely located, it is spatially coincident with the Eocene-aged kimberlite field in the central part of the craton (the so-called “Corridor of Hope”), and also with a geochemically defined ultra-depleted harzburgitic layer interpreted as oceanic or arc-related lithosphere emplaced during early tectonism. The CSMC lies wholly within the NE–SW striking central zone defined by Grütter et al. [Grütter, H.S., Apter, D.B., Kong, J., 1999. Crust–mantle coupling; evidence from mantle-derived xenocrystic garnets. Contributed paper at: The 7th International Kimberlite Conference Proceeding, J.B. Dawson Volume, 1, 307–313] on the basis of garnet geochemistry (G10 vs. G9) populations.

Deep-probing MT data from the lake bottom instruments infer that the conductor has a total depth-integrated conductivity (conductance) of the order of 2000 Siemens, which, given an internal resistivity of 10–15 Ω m, implies a thickness of 20–30 km. Below the CSMC the electrical resistivity of the lithosphere increases by a factor of 3–5 to values of around 50 Ω m. This change occurs at depths consistent with the graphite–diamond transition, which is taken as consistent with a carbon interpretation for the CSMC.

Preliminary three-dimensional MT modelling supports the NE–SW striking geometry for the conductor, and also suggests a NW dip. This geometry is taken as implying that the tectonic processes that emplaced this geophysical–geochemical body are likely related to the subduction of a craton of unknown provenance from the SE (present-day coordinates) during 2630–2620 Ma. It suggests that the lithospheric stacking model of Helmstaedt and Schulze [Helmstaedt, H.H., Schulze, D.J., 1989. Southern African kimberlites and their mantle sample: implications for Archean tectonics and lithosphere evolution. In Ross, J. (Ed.), Kimberlites and Related Rocks, Vol. 1: Their Composition, Occurrence, Origin, and Emplacement. Geological Society of Australia Special Publication, vol. 14, 358–368] is likely correct for the formation of the Slave's current SCLM.     

Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.     

Physical modelling of deformation in the Tasman Orogenic Zone

Structural vergence within the Western Subprovince of the Lachlan Fold Belt is towards the hinterland rather than the foreland, in contrast to many well-known orogenic belts. High angle-reverse faults and upright folds verge eastwards, away from the Australian craton, towards the inferred centre of orogenic and magmatic activity. We designed a series of analogue models to test the anomalous vergence in the western Lachlan Fold Belt, particularly the interaction of a stable Australian craton with Tasman Line geometry, interacting with weaker oceanic or transitional lithospheric material. We found consistently that vergence direction in the models was towards the hinterland, not the foreland, as in the western Lachlan Fold Belt, irrespective of the way the model was deformed. Strength gradients between the oceanic and cratonic lithosphere control the deformation patterns. An important result of the models is that they demonstrate that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere.     

关于内蒙与晋冀交界地区早前寒武纪灰片麻岩和孔兹岩的讨论

晋冀内蒙边界地区的早前寒武纪岩石大致以大同－兴和一线为界分为南东部麻粒岩系和北西部孔兹岩系。麻粒岩系中称作灰片麻岩的浅色麻粒岩与世界各地太古代典型的灰片麻岩不同，其矿物组合含斜方辉石或二辉石，岩石化学成分高铁、高镁又高钙，不是典型的太古代灰片麻岩ＴＴＧ岩套，而应称作紫苏斜长花岗岩。如果原来是ＴＴＧ岩浆，则可能是因受表壳岩的混染所致。研究区内孔兹岩系的特征是堇青石片麻岩发育，沿韧性剪切带含堇青石的Ｓ－型花岗岩，以及作为熔融残留体的含堇青石麻粒岩也时有发现。三者的稀土元素分布型式表现出其间的成因关系。孔兹岩系中的夕线石多具近于平行（００１）的变形扭折带，在土贵乌拉还发现夕线石的（１１０）聚片双晶。这可能是夕线石双晶的首次发现。本文对夕线石作了详细测定。其＋２Ｖ＝２６°±０．５°．晶胞参数α＝β＝γ＝９０°，ａ＝７．５０，ｂ＝７．６５，ｃ＝５．８１，Ｖ＝３３０．０６３。     

Mantle Xenoliths from the Southeastern Slave Craton: Evidence for Chemical Zonation in a Thick, Cold Lithosphere

We present the first data on the petrology of the mantle lithosphereof the Southeastern (SE) Slave craton, Canada. These are basedon petrographic, mineralogical and geochemical studies of mantlexenoliths in Pipe 5034 of the Cambrian Gahcho Kué kimberlitecluster. Major types of mantle xenoliths include altered eclogite,coarse garnet or spinel peridotite, and deformed garnet peridotite.The peridotites belong to the low-temperature suite and formedat T=600–1300°C and P= 25–80 kbar in a thick(at least 220–250 km), cool lithosphere. The SE Slavemantle is cooler than the mantle of other Archaean cratons andthat below other terranes of the Slave craton. The thick lithosphereand the relatively cool thermal regime provide favourable conditionsfor formation and preservation of diamonds beneath the SE Slaveterrane. Similar to average Archaean mantle worldwide, the SESlave peridotite is depleted in magmaphile major elements andcontains olivine with forsterite content of 91–93·5.With respect to olivine composition and mode, all terranes ofthe Slave mantle show broadly similar compositions and are relativelyorthopyroxene-poor compared with those of the Kaapvaal and Siberiancratons. The SE Slave spinel peridotite is poorer in Al, Caand Fe, and richer in Mg than deeper garnet peridotite. Thegreater chemical depletion of the shallow upper mantle is typicalof all terranes of the Slave craton and may be common for thesubcontinental lithospheric peridotitic mantle in general. Peridotiticxenoliths of the SE Slave craton were impregnated by kimberliticfluids that caused late-stage recrystallization of primary clinopyroxene,spinel, olivine and spinel-facies orthopyroxene, and formationof interstitial clinopyroxene. This kimberlite-related recrystallizationdepleted primary pyroxenes and spinel in Al. The kimberliticfluid was oxidizing, Ti-, Fe- and K-rich, and Na-poor, and introducedserpentine, chlorite, phlogopite and spinel into peridotitesat P < 35 kbar. KEY WORDS: kimberlite xenolith; lithosphere; mantle terrane; chemical zoning; thermobarometry; Slave craton     

Deep seismic reflection profiling in the Archaean northeastern Yilgarn Craton, Western Australia: implications for crustal architecture and mineral potential

Deep seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia, have provided information on its crustal architecture and on several of its highly mineralised belts. The seismic reflection data allow interpretation of several prominent crustal scale features, including an eastward thickening of the crust, subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low-angle shear zones within and beneath the granite–greenstone terranes. Where the low-angle shear zones intersect the major crustal-penetrating structures, a wedge shaped geometry is formed. This geometry forms a suitable fluid focusing wedge in which upward to subhorizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones.     

Glacial dispersal studies using indicator minerals and till geochemistry around two eastern Finland kimberlites

Diamondiferous kimberlites occur in eastern Finland, in the areas of Kaavi–Kuopio and Kuhmo. Active diamond exploration has been ongoing in the country for over two decades, but the Karelian craton still remains under explored given its size and potential. In order to develop techniques that can be applied to diamond exploration in glaciated terrains, the Geological Survey of Finland (GTK) carried out a detailed heavy mineral and geochemical survey of Quaternary till in 2001–2003 around two of the known kimberlitic bodies in Finland, Pipe 7 in Kaavi and Dyke 16 in Kuhmo. The mineralogical and geochemical signatures of these two kimberlites were studied in the basal till deposited down-ice from the targets. The kimberlites were selected to represent two different types in terms of shape, size, age and petrology, as well as showing contrasting country rocks and Quaternary deposits. Till samples up to 60 kg in weight were taken by excavator and by drill rig. Kimberlitic indicator mineral grains (0.25–1.0 mm) were concentrated using a GTK modified 3″Knelson Concentrator. Fine fractions (< 0.063 mm) of selected samples were analyzed by XRF and ICP-MS. The indicator grains down-ice from Pipe 7 form a well-defined fan in the basal till that can be followed for at least 2 km with a maximum concentration at 1.2 km distance from the pipe. Another kimberlitic body discovered during the study 300 m down-ice from Pipe 7 demonstrates that there are in fact at least two superimposed indicator fans. The results do not rule out the possibility of even more undiscovered kimberlitic sources in the area. In contrast, the indicator dispersal trail from Dyke 16 is shorter (1 km) and less well-defined than that at Kaavi, mainly due to the lower indicator content in the kimberlite itself and subsequently in till, as well as a large population of background chromites in till. The latter population is likely having been derived from the Archean Näätäniemi serpentinite massif and the associated ultramafic metavolcanics of the Kuhmo greenstone belt, located ca. 30 km up-ice from the sampling area. The indicator maximum at Seitaperä dyke swarm occurs immediately down-ice from the kimberlite, after which the concentration drops rapidly. Results of this study contribute to the overall understanding of the Quaternary history of the Kaavi and Kuhmo areas, and more importantly, provide key information to diamond exploration in these particular regions and also elsewhere in glaciated terrains.     

Isotope geochemistry and Re–Os geochronology of the Yanjiagou Mo deposit in the central North China Craton

The Yanjiagou deposit, located in the central North China Craton (NCC), is a newly found porphyry‐type Mo deposit. The Mo mineralization here is spatially associated with the Mapeng batholith. In this study, we identify four stages of ore formation in this deposit: pyrite phyllic stage (I), quartz–pyrite stage (II), quartz–pyrite–molybdenite stage (III), which is the main mineralization stage, and quartz–carbonate stage (IV). We present sulphur and lead isotope data on pyrite, and rhenium and osmium isotopes of molybdenite from the porphyry deposit and evaluate the timing and origin of ore formation. The δ³⁴S values of the pyrite range from ‐1.1‰ to −0.6‰, with an average of −0.875‰, suggesting origin from a mixture of magmatic/mantle sources and the basement rocks. The Pb isotope compositions of the pyrite show a range of 16.369 to 17.079 for ²⁰⁶Pb/²⁰⁴Pb, 15.201 to 15.355 for ²⁰⁷Pb/²⁰⁴Pb, and 36.696 to 37.380 for ²⁰⁸Pb/²⁰⁴Pb, indicating that the ore‐forming materials were derived from a mixture of lower crust (or basement rocks) and mantle. Rhenium contents in molybdenite samples from the main ore stage are between 74.73 to 254.43 ppm, with an average of 147.9 ppm, indicating a mixed crustal‐mantle source for the metal. Eight molybdenite separates yield model ages ranging from 124.17 to 130.80 Ma and a mean model age of 128.46 Ma. An isochron age of 126.7 ± 1.1 Ma (MSWD = 2.1, initial ¹⁸⁷Os = 0.0032 ± 0.0012 ppb) is computed, which reveals a close link between the Mo mineralization and the magmatism that generated the Mapeng batholith. The age is close to the zircon U–Pb age of ca. 130 Ma from the batholith reported in a recent study. The age is also consistent with the timing of mineralization in the Fuping ore cluster in the central NCC, as well as the peak time of lithosphere thinning and destruction of the NCC. We evaluate the spatio‐temporal distribution of the Mo deposits in the NCC and identify three important molybdenum provinces along the northern and southern margins of the craton formed during three distinct episodes: Middle to Late Triassic (240–220 Ma), Early Jurassic (190–175 Ma), and Late Jurassic to Early Cretaceous (150–125 Ma). The third period is considered to mark the most important metallogenic event, coinciding with the peak of lithosphere thinning and craton destruction in the NCC. Copyright © 2014 John Wiley & Sons, Ltd.     

华南壳幔结构与动力学的宽频地震观测研究

了解华南各岩石圈块体壳幔结构和各向异性方面的差异是揭示华南深部构造演化的基础。本文利用布设于华南的两条宽频地震测线观测数据,采用多种地震学方法对华南的地壳上地幔结构和各向异性进行了研究。接收函数结果表明,华南地区地壳厚度和岩石圈厚度都较薄,地壳厚度自东南沿海向西北内陆增厚,扬子克拉通的泊松比(波速比)低于华夏块体,表明扬子克拉通地壳较华夏块体更偏长英质。约北纬29°以北的扬子克拉通地幔转换带厚度明显增厚,可能是由地幔转换带底部停滞的冷的古太平洋板片或中生代克拉通碰撞残留造成的。层析成像结果显示华南上地幔具有很强的横向差异性,上地幔中的强烈低速异常体可能对应了晚中生代发生广泛岩浆作用时的岩浆房和岩浆通道。台湾下方的上地幔存在南北横向差异明显的高速异常,分别对应台湾南部向东俯冲的欧亚板块及台湾北部向北俯冲的菲律宾海板块。俯冲的欧亚板块在台湾南部是连续的,而在台湾中北部,由于与菲律宾海板块的相互作用,俯冲的欧亚板块被折断。剪切波分裂结果显示,以江绍断裂为界,华夏块体与扬子克拉通的岩石圈地幔各向异性存在明显的横向变化,表明两者的构造演化过程有显著差异。     

Evidence for a shock-metamorphic breccia within a buried impact crater,Lake Raeside,Yilgarn Craton,Western Australia

Mineral exploration drilling 60 km west of Leonora in 2008 intersected >95 m of poorly consolidated granitoid-dominated breccia at the base of a Cenozoic paleochannel beneath Lake Raeside. The breccia, initially interpreted as a kimberlite, is composed of poorly consolidated fragments of granitic gneiss, felsite and metamorphosed mafic rock within a matrix of fine to medium-grained breccia. Microscopic examination revealed quartz grains displaying well-developed planar deformation features (PDFs) dominated by the ω? {1013} planar set, diaplectic silica glass and diaplectic plagioclase glass. These features constitute the diagnostic hallmarks of shock metamorphism owing to high-velocity impact of a large meteorite or asteroid. The PDFs in quartz grains of the breccia are distinctly different from metamorphic deformation lamellae produced tectonically or in diatremes. Airborne total magnetic intensity data suggest an outline of an 11 km-diameter crater, consistent with the significant thickness of the shock-metamorphosed breccia at >95 m, suggestive of the existence of a large impact structure.