Magmatic Diapir Research Articles

Geological Interpretation and Hydrocarbon Exploration Potential of Three Types of Mound‐shaped Reflectors in the Meishan Formation, Southern Qiongdongnan Basin

AbstractMany mound‐shaped reflectors with different features and shapes are evident in the Upper Member of the Meishan Formation from seismic profiles taken in the deep‐water area of the Southern Qiongdongnan Basin. Based on the drilling, 2D and 3D seismic data from the study area, descriptions of the seismic reflection characteristics as well as the geometric shape, wave impedance inversion, analogy and comparative analyses are carried out. Taken in conjunction with research on the paleostructure and paleosedimentary background, we consider that the mound‐shaped seismic reflectors are distributed in the Southern Slop belt and the Southern High‐rise of the Qiongdongnan Basin, which can be subdivided into three types: reefs, contourite mounds and magmatic diapirs. The first type, reefs, includes patch reefs, platform marginal reefs and pinnacle reefs. Patch reefs present mound‐shaped seismic facies with medium frequency and a moderately strong amplitude, being distributed at the uplift of the fault control platform on the Southern Slop belt. The platform marginal reefs have flat mound‐shaped seismic facies with strong amplitude and medium frequency, developing at the margin of the carbonate platform in the Southern High‐rise. The pinnacle reefs have mound‐shaped seismic facies with strong amplitude and medium frequency and are developed on an isolated volcanic cone. The boundaries between individual reefs are clear on the seismic section, with reef ridge and reef ditch developed, the phenomenon of ‘front product’ being visible within, two‐way superelevation between wings is developed and they exist visibly as mounds in any viewed direction of the cross‐section. They are slightly asymmetrical in the direction perpendicular to the paleodepth, the reef body being steep near the deep‐water side, while being gentle near the shallow water side. The wave impedance of a patch reef is about 7 kg/m3×m/s, while the wave impedance of a platform marginal reef is about 7.5 kg/m3×m/s. The second type ‐ contourite mounds ‐ are mainly developed under the slope break of the southern slope fault control platform's edge. They are subdivided into two types: conical and flat. The former has mound‐shaped seismic facies with medium‐strong amplitude and low frequency, the latter having mound‐shaped seismic facies with medium amplitude and low frequency. The internal texture of the mounds is not clear on the seismic section, with the boundaries between contourite mounds being blurred. They are mound‐shaped only in cross‐section, being banded in the extending direction. The upper surface of a single contourite mound is relatively gentle near the deep‐water side, while being steep near the shallow water area. The wave impedance of contourite mounds is about 5.8 kg/m3×m/s, which is speculated to represent a marly to calcareous clastic deposit. The third type is the magmatic diapir, which has ‘roots’. They have a dome‐shaped upper boundary, are bottomless, with a chaotic interior. They penetrated multiple formations, opening towards the base. There are two major accumulation assemblages of reefs, one is the platform margin reef accumulation assemblage with distant source rocks and long‐distance migration through an unconformity surface in the Southern High‐rise, the other is the patch reef accumulation assemblage with twin sources and short distance migration through faults in the Southern Slop belt near the Central Depression zone. The latter is the main exploration targets at present. There are contourite mound accumulation assemblages with nearby source rocks and short distance vertical migration through faults, which are potentially important targets. The magmatic diapirs pierce the overlying strata and form good hydrocarbon traps and migration pathways, thus representing potential prospecting targets.

Huizhou Movement and its significance in Pearl River Mouth Basin, China

The Huizhou Movement refers to the Middle Eocene tectonic transition from the early to the late Wenchang Rifting stage (about 43 Ma ago) in the Pearl River Mouth Basin. Based on seismic reflection, drilling, logging and geological data, fault characteristic analysis, denudation thickness recovery, magmatism statistics, regional tectonic dynamics comparison and other methods are used to reveal the characteristics, properties and dynamic mechanism of the Huizhou Movement. The Huizhou Movement mainly shows the North-South transition of rifting and the migration along the faults, basement uplift, magmatic diapir and stratigraphic denudation. It is believed that the Huizhou Movement is a comprehensive reflection of plate interaction and lithospheric thinning process in the Pearl River Mouth Basin, which is closely related to the transition of lithosphere from initial rifting to rapid thinning, the India-Eurasia hard collision and the change of subduction direction of the Pacific plate. The Huizhou Movement has significant influence and control on the Paleogene hydrocarbon-generating sags and the development of hydrocarbon source rocks, sedimentary system and deep high-quality reservoir, hydrocarbon migration and accumulation in the Pearl River Mouth Basin.

Generation of lithium-bearing pegmatite deposits within the Songpan-Ganze orogenic belt, East Tibet

As the largest lithium ore deposits in China, the Jiajika and Markam lithium pegmatites formed in the Songpan-Ganze orogenic hinterlands, East Tibet. The lithium pegmatites, hosted in the Upper Triassic turbiditic metasedimentary rocks, were related to granitic intrusions currently expressed as gneiss domes. Combined previous studies with new field observations, drilling data and geochemical and geochronological results from Jiajika and Markam gneiss domes, we determined that both gneiss domes, related to lithium ore deposits, are characterized by cored granites and mantling high-medium temperature/middle-low pressure metamorphic rocks intruded by numerous lithium-bearing pegmatites. We confirm that the granitic plutons within the gneiss domes were mainly intruded at ∼227-215 Ma during crustal shortening associated with Upper Triassic orogeny. The gneiss domes are metamorphic core complex generated by magmatic diapirism or a combination of top-to-the-south ductile detachment and diapirism, respectively, during the Late Triassic to Early Jurassic (∼216-170 Ma).Our synthesis indicates that the lithium-bearing pegmatites occur mainly in the staurolite-andalusite metamorphic zones of both core complexes, and that they were produced by strong differentiation of parental S-type, calc-alkaline and peraluminous granites.We suggest that the Markam gneiss dome formed between ∼212 and 170 Ma, as indicated by the age of the Markam detachment, the crystallization ages of the lithium-bearing pegmatites and the associated metamorphic events. In contrast, formation of the Jiajika gneiss dome was initiated at least by ∼216-214 Ma, as indicated by ages of the lithium-bearing pegmatite veins and other mineralization ages. Therefore, we suggest that the lithium-bearing pegmatites were emplaced during formation of the gneiss domes, and that the deformation, metamorphism, magmatism and mineralization in the eastern Songpan-Ganze orogenic belt are spatially and temporally related to the exhumation stage of the Indosinian orogeny.

Paleogene geological framework and tectonic evolution of the central anticlinal zone in Lufeng 13 sag, Pearl River Mouth Basin

The Paleogene geological framework and evolution process in the central anticline zone in the Lufeng 13 sag in Pearl River Mouth Basin is well analyzed through seismic data and drilling data, and control of tectonic evolution on hydrocarbon accumulation is also discussed. The results show that the central anticline zone in the Lufeng 13 sag develops the upper deformation layer and lower deformation layer. The “arched graben system” is developed in the upper deformation layer, and the magmatic diapir structure and flowing deformation of plastic strata is developed in the lower deformation layer. The evolution process of the central anticline zone can be divided into four stages, i.e. fault block uplifting stage, prototype stage, strengthening stage and finalization stage. The geological framework and tectonic evolution of the central anticline zone control Paleogene hydrocarbon accumulation. The Paleogene two-layer geological framework is favorable for development of structural traps and composite traps; the paleostructure highs are the direction of hydrocarbon migration, and the gravitational gliding faults are the main carrier bed for vertical hydrocarbon migration; the tectonic uplift is a key factor for reservoir diagenesis improvement and preservation of primary pores, and also controls distribution of high-quality reservoirs.

The nature of the heat source of mafic magmatism during the formation of the Vilyui rift based on the ages of dike swarms and results of numerical modeling

Possible mechanisms of rifting and the thermal regime of the lithosphere beneath the rift zone of the Vilyui sedimentary basin are considered based on the available isotopic ages of dike swarms, rates of sedimentation, and results of numerical modeling. Temporal correlations between the intrusion of mafic magma and a sharp increase in the rate of subsidence and sedimentation in the rift basin prove the contribution of both plate-tectonic and magmatic factors to the formation of the Vilyui rift. The results show a relationship between the rapid extension of the lithosphere and the formation of mafic dike swarms in the Yakutsk-Vilyui Large Igneous Province of the Siberian Platform at the Frasnian-Famennian boundary, with a peak at ~374.1 Ma, and at the end of the Late Devonian, with a peak at ~363.4 Ma. There were two pulses of dike formation during rapid subsidence of the basin basement in the period 380-360 Ma, with a sedimentation rate of 100-130 m/Myr, at a background rate of 10-20 m/Myr. Analysis of numerical thermomechanical models revealed that the best-fit model is that combining the mechanisms of intraplate extension (passive rifting) and the ascent of a mantle magmatic diapir (active rifting). A conclusion about the nature of the heat source of trap magmatism has been drawn: The plume-driven regime of the lithosphere can better explain the dynamics of extension during rifting than the decompression melting mechanism.

Seismic interpretation of the diapiric structures and gas chimneys in the Liwan gas field, Pearl River Mouth Basin, northern South China Sea

Voluminous diapiric intrusions are present in 3D seismic data from the Liwan gas field area in the Baiyun sag of the Pearl River Mouth Basin, South China Sea. These diapirs are subdivided into magmatic diapirs and mud diapirs according to their seismic signatures and formation mechanisms. Saucer- and mound-shaped reflectors with anomalously high amplitudes distributed throughout the Early Miocene strata are interpreted as sills and laccoliths and are typified as magmatic diapirs. Mud diapirs in the study area can be divided into two types: wavy reflectors occurring at the basement of the sag caused by the gravitational instability of plastically deformed mudstones and columnar upheaval anomalies rooted in Paleogene rift sequences. Gas chimneys are characterized by vertical weak/blank zones accompanied by pull-down effects. Vertical gas chimneys and mud diapirs pierce the Middle Miocene strata. The vast majority of them are triggered by normal faulting; however, no deep-seated faults can be detected around the peripheries of igneous intrusions. Migration forces and pathways are represented by vertical mud diapirs and gas chimneys and thus control the distribution of shallow gases with negative-amplitude anomalies as well as gas hydrates.

Temporal correlation between dyke swarms and crustal extension in the middle Palaeozoic Vilyui rift basin, Siberian platform

This paper presents results from new 40Ar/39Ar isotope dating of nine mafic dykes from three large dyke swarms (Vilyui–Markha, Kontai–Dzherba, and Chara–Sinsk) of the Yakutsk–Vilyui large igneous province (LIP), in addition to a reconstruction of the subsidence history of the middle Palaeozoic Vilyui paleorift basin (eastern Siberian platform). All previously published 40Ar/39Ar and U–Pb dates are summarized. Statistical analysis of the dyke ages reveals repeated magmatic events in the study area. Two major pulses of mafic magmatism are identified: one at the Frasnian–Famennian boundary, with a main peak at ca. 374.1Ma, and another in the latest Devonian with a peak at ca. 363.4Ma. The time of maximum intensity of dyke intrusion coincides (within uncertainty) with rapid subsidence in the Vilyui basin. The minimum total volume of middle Palaeozoic magmatism produced in the Yakutsk–Vilyui LIP is 100–215Kkm3, which is much less than earlier estimates. Most of the mafic material within the Yakutsk–Vilyui LIP is related to the Vilyui basin and associated dyke swarms. Backstripping analysis of sedimentation in depressions of the Vilyui basin was carried out. Estimates were obtained for the spatial distribution of the stretching factor of the crust and mantle lithosphere, averaging 1.17 and 1.44, respectively. The amount of extension due to dyke intrusion is estimated to be ~6%. Backstripping analysis of sedimentation in the Vilyui basin was used to assess the effect of both intraplate far-field forces and upwelling magma flows initiated by a mantle plume. A numerical thermomechanical model was developed to investigate the relations between two possible mechanisms by which the Vilyui rift was initiated: intraplate extension (passive rifting) and the ascent of a mantle magmatic diapir (active rifting). A model considering both of these mechanisms shows the contribution of the far-field extension forces and the effect of convective flows around the mantle plume, assuming a temperature of 1500°C for a heat source operating for ≥10Myr. The best-fit model predicts the formation of the Vilyui basin under extensional conditions with a depth-integrated horizontal driving force of 2.0×1013N/m, which corresponds to a lithostatic pressure of 0.73.

Structural analysis and deformation characteristics of the Yingba metamorphic core complex, northwestern margin of the North China craton, NE Asia

The presence of the Yingba (Yinggete–Bagemaode) metamorphic core complex (MCC) is confirmed near the Sino-Mongolian border in China. We report its structural evolution and the rheological features of ductile shear zones within this complex. Three deformations (Ds, Dm, and Db) since the Late Jurassic are identified. Ds is characterized by ductile structures that resulted from early NW-oriented, low-angle, extensional ductile shearing. Dm is associated with partial melting and magmatic diapirism, which accelerated the formation of the dome-like geometry of the Yingba MCC. Synchronously with or slightly subsequently to Ds and Dm, the Yingba MCC was subjected to brittle, extensional faulting (Db), which was accompanied by the exhumation of the lower crust and the formation of supracrustal basins. The ductile shearing (Ds) developed under greenschist-to amphibolite-facies metamorphic conditions (400–650 °C), as indicated by microstructures in quartz and feldspar, quartz [c] axis fabrics, and two-feldspar geothermometry. The mean kinematic vorticity estimates of 48–62% show a pure shear-preferred flow during Ds. The Yingba MCC provides an excellent sample that recorded an intermediate to high temperature shearing, which also implies the widely extensional regime in northeastern Asia at that time.

New evidence for the structure of conical seamounts and hills at the foot of the Sea of Okhotsk margin of the Kuril island arc: Continuous seismic-profiling data

This paper is concerned with results from the interpretation of continuous seismic profiling (CSP) data that were acquired during the 21st cruise of the R/V Pegas in 1980. The data are of relevance for the structure of conical seamounts and sea-hills at the foot of the Sea of Okhotsk margin of the Kuril island arc. The seamounts are extrusive domes (volcanoes) or magmatic diapirs with thick sedimentary caps of contrasting (at the top) and transparent (at the bottom) Cenozoic deposits. They were mostly formed in the Cenozoic, largely during Pliocene to Quaternary time; they resulted from viscous magma being emplaced into the sediments. There are also several small, buried domes with flattened bottoms that were formed by liquid magma being emplaced into the sediments (laccoliths or subvolcanoes). We also touch on questions relating to the terminology, geography, and history of submarine volcanism in the region and its relationship to the Benioff zone.

Numerical modeling of mantle diapirism as a cause of intracontinental rifting

The numerical model of mantle diapirism and active rifting is developed. The model describes the possibility of extension and thinning of the Earth’s crust under the action of a local 100-km long heat source in the sublithospheric mantle, which causes melting and rising of the magmatic diapir through the cratonic lithosphere. The model combines the mechanisms of the uplifting of the anomalously hot material due to its gravitational instability, underplating of magma beneath the continental crust, and its extension by the forces of the convective flows at the base of the plate. The obtained results shed light on some geological features of the joint formation of the large Vilyui igneous province and Vilyui sedimentary basin.

A quasi-asymptotic model for the ascent of magmatic diapirs

Recent asymptotic analysis for the slow, steady ascent of a zero-traction sphere in a power-law fluid with temperature-dependent viscosity is applied to the derivation of a model for magmatic diapirism in the Earth’s mantle and continental crust. As an example, the model is applied to the case of a mantle diapir rising through country rock that is treated as a strongly shear-thinning fluid. It is found that, within the geologically relevant timescale of years, the diapir would only encounter two of the four possible asymptotic regimes that the earlier analysis uncovered, before stalling at a considerably greater depth below the Earth’s surface than was predicted by an earlier model. The implications of this result for the modelling of magmatic diapirism are discussed.

Seeing Is Believing

Satellite data and computational modeling provide evidence for a spheroidal magma body rising within the crust below the Altiplano Plateau in the central Andes.

Shallow laccolithic emplacement of the Land’s End and Tregonning granites, Cornwall, UK: Evidence from aureole field relations and P-T modeling of cordierite-anthophyllite hornfels

The Land’s End and Tregonning-Godolphin granites of the >250 km-long Permian Cornubian Batholith are heterogeneous medium- to coarse-grained peraluminous biotite-, tourmaline-, and lithium-mica granites traditionally thought to be emplaced as massive magmatic diapirs. Although S-type characteristics are dominant (quartz + biotite + muscovite + tourmaline ± topaz ± lithium-micas in the melt, numerous greisen and pegmatite veins, Sn-W mineralization), some characteristics of evolved I-type granites are also exhibited (hornblende-bearing enclaves, elevated ɛNd, Cu mineralization, batholithic dimensions). Here, we present an investigation focusing on the contact metamorphism and deformation of the aureole rocks adjacent to the Land’s End and Tregonning granites as an approach to better determine the method of granite emplacement and the depth at which it occurred. New 1:5000-scale geological maps are presented for ∼15 km of coastal exposure of the granites and their aureoles. We propose that the granites were emplaced non-diapirically by intrusion of sills that amalgamated to form a sheeted laccolith-type body. Granite contacts cleanly truncate all faults, folds, and cleavages generated during both Variscan convergence and subsequent latest Carboniferous–Early Permian (end-Variscan) extension, and it is likely that granite was emplaced during continuation of this extensional episode. There is evidence for stoping of the country rocks by an outward-migrated sill and dyke network, and uplift and doming of the host rocks can be partially attributed to laccolith inflation. Host meta-siltstones of the Devonian Mylor Slate Formation formed a contact aureole of cordierite + biotite + chlorite ± andalusite “spotted slates.” Several interspersed pillow basalts and dolerites, previously affected by hydrothermal alteration, underwent isochemical contact metamorphism to form cordierite- and orthoamphibole-bearing hornfels, including cordierite-anthophyllite rocks that are present in Kenidjack cliff, NW Land’s End aureole. THERMOCALC P-T modeling and pseudosection construction for these rocks in the large Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCKFMASHTO) chemical system indicates contact metamorphism occurred at 1.5 ± 1.0 kbar and 615 ± 50 °C. This ultra-low pressure metamorphism equates to a likely emplacement depth of 5–6 km for the adjacent granite sheets. The Cornubian Batholith is highly composite and likely comprises an amalgamation of discrete shallow-seated sheeted laccoliths that are dyke-fed from a common lower-crustal/upper-mantle melt region to result in the batholith’s mixed S-type/I-type character.

Origin of the Mt Ashmore structural dome, west Bonaparte Basin, Timor Sea

New insights into the 3D structure, composition and origin of the Mt Ashmore dome, west Bonaparte Basin, Timor Sea, are enabled by reprocessed seismic-reflection data and by optical microscopic, X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive spectrometry (EDS) and transmission electron microscopy (TEM) analyses of drill cuttings. The structural dome, located below a major pre-Oligocene post-Late Eocene unconformity and above a ∼6 km-deep-seated basement high indicated by marked gravity and magnetic anomalies, displays chaotic deformation at its core and a centripetal kinematic deformation pattern. A study of drill cuttings of Lower Oligocene to Lower Jurassic sedimentary rocks intersected by the Mt Ashmore 1B petroleum-exploration well reveals microbrecciation and extreme comminution and flow-textured fluidisation of altered sedimentary material. The microbreccia is dominated by aggregates of poorly diffracting micrometre to tens of micrometres-scale to sub-millimetre particles, including relic subplanar fractured quartz grains, carbonate, barite, apatite and K-feldspar. A similar assemblage occurs in fragments in basal Oligocene sediments, probably derived from the eroded top section of the dome, which protrudes above the unconformity. SEM coupled with EDS show the micrometre to tens of micrometres-scale particles are characterised by very low totals and non-stoichiometric compositions, including particles dominated by Si, Al–Si, Si–Ca–Al, Si–Al–Ca, Si–Mg, Fe–Mg–Ca, Fe–Mg and carbonate. XRD analysis identifies a high proportion of amorphous poorly diffracting material. TEM indicates internally heterogeneous, fragmented and recrystallised structure of the amorphous grains, which accounts for the low totals in terms of the high-volatile and porous nature of the particles. Another factor for the low totals is the uneven thin-section surfaces which affect the totals. No volcanic material or evaporites were encountered in the drillcore, militating against interpretations of the structure in terms of magmatic intrusion or salt diapirism. Such models are also inconsistent with the strong gravity and magnetic anomalies, which signify a basement high below the dome. An interpretation of the dome in terms of a central rebound uplift of an impact structure can not be proven due to the lack of shock metamorphic effects such as planar deformation features, impact melt or coesite. However, an impact model is consistent with the chaotic structure of the domal core, centripetal sense of deformation, microbrecciation, comminution and fluidisation of the Triassic to Eocene rocks. In this respect, an analogy can be drawn between the Mt Ashmore structural dome and likely but unproven impact structures formed in volatile (H2O, CO2)-rich sediments where shock is attenuated by high volatile pressure, such as Upheaval Dome, Utah. In terms of an impact hypothesis the Mt Ashmore dome is contemporaneous with a Late Eocene impact cluster (Popigai: D = 100 km, 35.7 ± 0.2 Ma; Chesapeake Bay: D = 85 km, 35.3 ± 0.1 Ma).

Magmatic diapirism of the Fangshan pluton, southwest of Beijing, China

Diapirism as a crustal magma ascent and emplacement mechanism has been increasingly questioned due to lack of unambiguous natural examples. In this paper we report the structure of the Cretaceous Fangshan pluton in the hinterland of the North China Craton, ca. 40 km southwest of Beijing, China. This pluton is a compositionally and texturally concentrically zoned granodiorite with an approximately circular shape. Our detailed structural and petrological investigations reveal a high-temperature shear aureole with pluton-side-up kinematic indicators and a rim syncline around the pluton. In addition, an intensely deformed Archean complex occurs at the northern and southern margins of the pluton requiring at least 4 km upward movement through the 4–5 km thick overlying cover sequence during magma ascent. Based on comparison of stratigraphic units inside and outside the aureole, the bulk wall-rock shortening by ductile flow is estimated to be about 4 km, which is roughly the same as the radius of the pluton. These features are consistent with results of numerous modeling studies and diagnostic criteria of magmatic diapirism, indicating that the Fangshan pluton is a magmatic diapir intruded into a ductile upper crust.

Structures, kinematics, thermochronology and tectonic evolution of the Ramba gneiss dome in the northern Himalaya

The Ramba gneiss dome, one of the north Himalayan gneiss domes, is composed of three tectono-lithologic units separated by an upper and a lower detachment fault. Low-grade metamorphic Tethyan Himalayan sedimentary sequence formed the upper unit above the brittle upper detachment fault. Mylonitic gneiss and a leucogranite pluton made up the lower unit beneath the ductile lower detachment fault. Mylonitic middle-grade garnet-, staurolite- and andalusite-schist constituted the middle unit between the two faults, which may be that the basal part of the upper unit experienced detachment shear. The Ramba dome underwent three episodes of deformation in its tectonic evolution. The first episode was a top-down-to-north-northwest sliding possibly related to the activity of the south Tibetan detachment system (STDS). The second episode was the dominant deformation related to a east–west extension, which resulted in a unique top-down-to-east kinematics and the major tectonic features of the dome. The third episode was a collapse sliding toward the outsides of the dome. The Ramba gneiss dome is possibly a result of the east–west extension and magmatic diapir. The lower detachment fault is probably the main detachment fault separating the sedimentary sequence from the crystalline basement during the east–west extension in the dominant deformation episode. The diapir of the leucogranite pluton formed the doming shape of the Ramba gneiss dome. This pluton intruded in the core of the dome in a late stage of the dominant deformation, and its Ar–Ar cooling ages are about 6 Myr. This indicates that the dominant deformation of the dome happened at the same time of the east–west extension represented by the north–south trending rifts throughout the northern Himalaya and southern Tibet. Therefore, the formation of the Ramba gneiss dome should be related to this east–west extension.

Characteristics of the Bumo Gold Field, Hainan Island, China and Gold Exploration Through Soil Geochemical Surveys

AbstractThe Bumo gold deposit is a low‐sulfide quartz vein type deposit, situated in the west of Hainan Island, approximately 25 km southeast of Dongfang City (termed Basuo in alias). Tectonically, the SW‐NE‐oriented Gezhen Ductile‐Shearing Zone was largely attenuated in Bumo. It is hosted by gneisses and schists of Proterozoic age (locally called the Baoban Group), but the radiometric age of ores mainly is Jurassic–Cretaceous, being coeval to the regional metallogenesis along the Eurasian margin. The affinity with the Phanerozoic orogenic gold deposits is indicated by low salinity (2.4 wt% NaCl–10.3 wt% NaCl), intermediate temperature (150–300°C) and appropriate δ18O value (approximately 7–11‰). The gold metal tonnage, proved previously, within an area smaller than 1.0 km2 in the gold belt II amounted to 11.59 t, with the average gold grades for the major Au quartz veins (7 and 8) in this belt of 52.7 and 71.1 g/t, respectively. This shows significant enrichment of gold and an unusual “nugget effect”. A ring structure, mainly represented by ridges, valleys, dikes, big bull quartz veins and lithological borders or pertinent faults, marks the gold field, and its origin was linked to a possible magmatic diapir. It was viewed as macroscopic evidence that the dip angles of strata tend to decline gradually from deeper ones in the center area to nearly horizontal in the outskirts, and certain intrusion‐affiliated elements such as Bi, As and W are considerably abundant in soil samples. Two areas, namely the area A (3.06 km2) in the outskirts of the ring structure or in northeastern Bumo and the area B (1.32 km2) in the field center or to the west of the gold belts I and II, were selected for the geochemical survey in a 50 × 50‐m sampling grid. The analytical results of the samples from horizon C and in a −80 mesh size showed that the average contents of Bi, As and W amounted to 0.55, 19.3 and 3.67 ppm in the area A (1225 samples) and to 0.79, 68.3 and 4.14 ppm in the area B (529 samples), respectively. This geochemical exploration led to the discovery of several Au quartz veins in both areas A and B, which also manifested an Au nugget effect (the average Au grades between 50 and 100 g/t) in spite of their limited scales (from a couple of centimeters to 50 cm wide and from several meters to 15 m long). A mylonite zone in area A was also found by this study. An intimate spatial correlation between a lithological border (gneiss and schist or pegmatite) or the mylonite zone and Au anomalies indicates an important prospect for further gold exploration.

Rediscovery of the Mattawa Anorthosite Massif, Grenville Province, Quebec

We present new field observations and laboratory data confirming the presence of the Mattawa Anorthosite Massif (MAT), whose existence in south-central Quebec was hinted at more than 35 years ago. MAT thus represents a newly recognized member of the late- to post-tectonic ~1060–1010 Ma andesine anorthosite belt that includes the Château-Richer, St. Urbain, and Labrieville massifs. The dominant rock type at MAT is foliated andesine anorthosite or leuconorite, and orientations of foliations indicate that the pluton has the shape of a dome. MAT contains separate core and border zones, which are distinguished on the basis of plagioclase composition and concentrations of Ga, Rb, Sr, and Ba. Xenoliths of labradorite anorthosite having Ga, Sr, and Ba concentrations different from those of the host andesine anorthosites occur sporadically throughout the pluton as well. Lastly, rocks enriched in Fe, Ti, and P (jotunite, oxide–apatite gabbronorite, nelsonite, ilmenitite) also occur at MAT, primarily near the core–border transition or at the pluton margins. Compared with other anorthosites in the andesine belt, MAT is compositionally most similar to Labrieville. By analogy with Labrieville, we interpret the concentric zoning of MAT (more sodic core, more calcic border) to reflect polybaric crystallization accompanying upward intrusion as a magmatic diapir, which also produced the overall domal structure. The labradorite xenoliths bear little physical or compositional resemblance to typical Lac St. Jean rocks. Therefore, if the xenoliths were derived from the Lac St. Jean Anorthosite, their present character must reflect considerable modification by the Mattawa magma.

Evolution of an Archean basement complex and its autochthonous cover, southern Slave Province, Canada

The Sleepy Dragon Complex (SDC) is one of several antiformal exposures of a 4.0–2.85 Ga basement block underlying the west-central region of the Archean Slave Province, northwestern Canada. This basement block, the Central Slave Basement Complex (CSBC), is overlain by an autochthonous to locally parautochthonous, dominantly quartz-rich clastic sequence, the 2.85–2.80 Ga Central Slave Cover Group. Together these units comprised a fundamental building block during Neoarchean growth and assembly of the Slave craton. The well-exposed Patterson Lake–Morose Lake area represents a type locality for study of the CSBC, Central Slave Cover Group, and overlying rocks of the Yellowknife Supergroup. Field mapping and U–Pb geochronology, employing both thermal ionisation mass spectrometry (TIMS) and sensitive high resolution ion microprobe (SHRIMP), document a prolonged crustal history. Oldest intact basement units consist of foliated to gneissic tonalite dated in two places at 2955 ± 12 Ma and 2944 ± 9 Ma. Indirect evidence for a ca. 3150 Ma basement component is obtained both from xenocrystic zircon in tonalite and detrital zircons from the overlying Central Slave Cover Group, which here consists of the Patterson Lake Formation. A granite boulder from this formation is dated at 2934 ± 3 Ma and may be locally derived. A sheared mafic volcanic unit along the basement-cover contact contains 2942 ± 3 Ma metamorphic titanite which provides a minimum deposition age. This volcanic unit is therefore also a part of the basement complex. Younger events in the SDC mainly reflect tonalite-trondhjemite-granodiorite plutonism during construction of the overlying Cameron River and Beaulieu River greenstone belts of the Yellowknife Supergroup. A unit of K-feldspar megacrystic granodiorite is dated at 2726 ± 3 Ma, and tonalite and granodiorite bodies located toward the centre of the SDC have primary crystallization ages of 2683 + 3/−2 Ma, 2677 ± 2 Ma, and 2672 + 7/−6 Ma. The core of the southern SDC is occupied by a late-tectonic to post-tectonic granite pluton previously dated at 2586 ± 2 Ma. An intravolcanic unconformity along the southern margin of the SDC (developed in part on 2.7 Ga granodiorite) is constrained by existing U–Pb data to have formed sometime between ca. 2685–2660 Ma. An abundance of synvolcanic plutons of similar age and the presence of a >3-km thick mafic volcanic sequence overlying a less dense, magmatically active felsic crust suggest that gravitational instability and magmatic diapirism may have played important roles in doming and uplift of the SDC prior to regional D 1 deformation. The younger-toward-centre zonation of plutonic rocks in the SDC is documented in other domal granite–gneiss complexes of Archean age (e.g., Pilbara craton, western Australia) where it is attributed to upward and outward displacement of older crust during repeated magma channeling into a growing mid-crustal dome. In the west-central Slave craton, such a process might account for the regional presence of local unconformities of apparently similar age. The domal basement–plutonic complexes would have been further amplified after 2660 Ma during the development of regional F 1- and F 2-fold belts.

Diapiric emplacement in the upper crust of a granitic body: the La Bazana granite (SW Spain)

The ascent and emplacement of granites in the upper crust is a major geological phenomenon accomplished by a number of different processes. The active processes determine the final geometry of the bodies and, in some favourable cases, the inverse problem of deducing mechanisms can be undertaken by relying on the geometry of plutons. This is the case of the La Bazana granitic pluton, a small Variscan igneous body that intruded Cambrian rocks of the Ossa-Morena Zone (SW Iberian Massif) in the core of a large late upright antiform. The granite shows no appreciable solid-state deformation, but has a late magmatic foliation whose orientation, derived from field observations, defines a gentle dome. The regional attitude of the main foliation in the country rock (parallel to the axial plane of recumbent folds) is NW–SE, but just around the granite, it accommodates to the dome shape of the pluton. Flattening in the host rock on top of the granite is indicated by boudinaged and folded veins, and appears to be caused by an upward pushing of the magma during its emplacement. The dome-shaped foliation of the granite, geometrically and kinematically congruent with the flattening in the host rock, can be related in the same way to the upward pushing of the magma. The level of final emplacement was deduced from the mineral associations in the thermal aureole to be of 7–10 km in depth. Models of the gravity anomaly related to the granite body show that the granite has a teardrop–pipe shape enlarged at its top. Diapiric ascent of the magma through the lower middle crust is inferred until reaching a high viscous level, where final emplacement accompanied by lateral expansion and vertical flattening took place. This natural example suggests that diapirism may be a viable mechanism for migration and emplacement of magmas, at least up to 7–10 km in depth, and it provides natural evidence for theoretical discussion on the ability of magmatic diapirs to pierce the crust.