UPb zircon and rutile chronology of Archean greenstone formation and gold mineralization in the Val d'Or region, Quebec

UPb geochronology provides an absolute time framework for the evolution of the Sigma gold deposit and surrounding rocks at Val d'Or, southern Abitibi subprovince. The Bourlamaque batholith, the largest pluton in the area, gives a 2699.8 ± 1.0 Ma UPb zircon age. This pluton cuts the Val d'Or Formation which hosts the mineralization. A UPb zircon age of 2704.9 ± 1.1 Ma on a felsic volcanic rock, the Colombière “rhyolite”, 13 km east of the mine dates that formation. The gold-bearing quartz vein system at Sigma is hosted by andesites and two generations of porphyry intrusions, all metamorphosed to the greenschist facies. The oldest porphyry (“porphyritic diorite”) shows the same deformation as the volcanic rocks, and has a 2703.7 ± 2.5 Ma UPb zircon age. The porphyritic diorite and volcanic rocks are cut by feldspar-porphyry dykes which post-date regional folding and have a 2694.0 ± 2.2 Ma UPb zircon age.Regional greenschist metamorphism has been dated directly, with a UPb date of 2684 ± 7 Ma on rutile in the Colombière “rhyolite”. The mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals. Hydrothermal rutile, from an alteration halo around the veins in andesite, has a 2599 ± 9 Ma UPb age. Textural evidence clearly indicates that the wall-rock alteration and vein filling are contemporaneous, and hence the vein system and gold mineralization appear to have developed at least 80 m.y. after the formation and metamorphism of host greenstones.     

Transpressional tectonics of the Mineoka Ophiolite Belt in a trench–trench–trench-type triple junction, Boso Peninsula, Japan

Abstract   Structures developed in metamorphic and plutonic blocks that occur as knockers in the Mineoka Ophiolite Belt in the Boso Peninsula, central Japan, were analyzed. The aim was to understand the incorporation processes of blocks of metamorphic and plutonic rocks with an arc signature into the serpentinite mélange of the Mineoka Ophiolite Belt in relation to changes in metamorphic conditions during emplacement. Several stages of deformation during retrogressive metamorphism were identified: the first faulting stage had two substage shearing events (mylonitization) under ductile conditions inside the crystalline blocks in relatively deeper levels; and the second stage had brittle faulting and brecciation along the boundaries between the host serpentinite bodies in relatively shallower levels (zeolite facies). The first deformation occurred during uplift before emplacement. The blocks were intensively sheared by the first deformation event, and developed numerous shear planes with spacing of a few centimeters. The displacement and width of each shear plane were a few centimeters and a few millimeters, respectively, at most. In contrast, the fault zone of the second shearing stage reached a few meters in width and developed during emplacement of the Mineoka Ophiolite. Both stages occurred under a right-lateral transpressional regime, in which thrust-faulting was associated with strike-slip faulting. Such displacement on an outcrop scale is consistent with the present tectonics of the Mineoka Belt. This implies that the same tectonic stress has been operating in the Boso trench–trench–trench-type triple junction area in the northwest corner of the Pacific since the emplacement of the Mineoka Ophiolite. The Mineoka Ophiolite Belt must have worked as a forearc sliver fault during the formation of a Neogene accretionary prism further south.     

Re-Os age for molybdenite from the Gangdese porphyry copper belt on Tibetan plateau: Implication for geody- namic setting and duration of the Cu mineralization

It is recognized that there are at least two sorts of significant environments for porphyry copper deposits, i.e. magmatic arcs and collisional orogens[14]. The deposits in the former environments are exampled by the circle-Pacific porphyry copper belt, such as An-dean-type deposits, which mainly formed in the period of the Andean tectonic cycle characterized by trans- pressional and transtensional movements along the arc-parallel strike-slip fault zone in the Late Eo-cene-Early Oligocene[5…     

Rhyolite volcanism in the Krafla central volcano,north-east Iceland

At the Krafla central volcano in north-east Iceland, two main phases of rhyolite volcanism are identified. The earlier phase (last interglacial) is related to the formation of a caldera, whereas the second phase (last glacial) is related to the emplacement of a ring dike. Subsequently, only minor amounts of rhyolite have been erupted. The volcanic products of Krafla are volumetrically bimodal. Geochemically, there is a series of basaltic to basalto-andesitic rocks and a cluster of rhyolitic rocks. Rocks of intermediate to silicic composition (icelandites and dacites) show clear signs of mixing. The rhyolites are Fe-rich (tholeiitic), and aphyric to slightly porphyritic (plagioclase, augite, pigeonite, fayalitic olivine and magnetite). They are minimum melts on the quartz-plagioclase cotectic plane in the granite system (Qz-Or-Ab-An). The rhyolites at Krafla were produced by near-solidus, rather than nearliquidus fractionation. They are interpreted as silicic minimum melts of hydrothermally altered crust, mainly of basaltic composition. They were primarily generated on the peripheries of an active basaltic magma chamber or intrusive domain, where sufficient volumes of crust were subjected to temperatures favorable for rhyolite genesis (850–950° C). The silicic melts were extracted crystal-free from their source in response to crustal deformation.     

Intrusion of ultramafic magmatic bodies into the continental crust: Numerical simulation

Intrusions of ultramafic bodies into the lower density continental crust are documented for a large variety of tectonic settings spanning continental shields, rift systems, collision orogens and magmatic arcs. The intriguing point is that these intrusive bodies have a density higher by 300-500 kg m⁻³ than host rocks. Resolving this paradox requires an understanding of the emplacement mechanism. We have employed finite differences and marker-in-cell techniques to carry out a 2D modeling study of intrusion of partly crystallized ultramafic magma from sublithospheric depth to the crust through a pre-existing magmatic channel. By systematically varying the model parameters we document variations in intrusion dynamics and geometry that range from funnel- and finger-shaped bodies (pipes, dikes) to deep seated balloon-shaped intrusions and flattened shallow magmatic sills. Emplacement of ultramafic bodies in the crust lasts from a few kyr to several hundreds kyr depending mainly on the viscosity of the intruding, partly crystallized magma. The positive buoyancy of the sublithospheric magma compared to the overriding, colder mantle lithosphere drives intrusion while the crustal rheology controls the final location and the shape of the ultramafic body. Relatively cold elasto-plastic crust (T_Moho = 400 °C) promotes a strong upward propagation of magma due to the significant decrease of plastic strength of the crust with decreasing confining pressure. Emplacement in this case is controlled by crustal faulting and subsequent block displacements. Warmer crust (T_Moho = 600 °C) triggers lateral spreading of magma above the Moho, with emplacement being accommodated by coeval viscous deformation of the lower crust and fault tectonics in the upper crust. Strong effects of magma emplacement on surface topography are also documented. Emplacement of high-density, ultramafic magma into low-density rocks is a stable mechanism for a wide range of model parameters that match geological settings in which partially molten mafic-ultramafic rocks are generated below the lithosphere. We expect this process to be particularly active beneath subduction-related magmatic arcs where huge volumes of partially molten rocks produced from hydrous cold plume activity accumulate below the overriding lithosphere.     

The thermal regime of the San Juan basin since late cretaceous times and its relationship to San Juan mountains thermal sources

Heat-flow and coal-maturation data suggest that the thermal history of the San Juan Basin has been influenced by magmatic and volcanic activity in the San Juan Mountains-San Juan volcanic field located to the north of the Basin. Time-dependent isothermal step models indicate that the observed heat flow may be modelled by a (near) steadystate isothermal step extending from 30–98 km depth whose edge underlies the northern San Juan Basin. The observed maturation levels of the Fruitland formation coals in the northern and central Basin, however, require more heat than can be associated with conduction from a deep thermal source (steady-state step) and from the shallow crustal batholith which underlies the San Juan volcanic field. Magmatic activity within the Basin does not appear to be a reasonable source of additional heat. Increased burial depths of the coals may explain some of the elevated maturation levels observed in the central and northern Basin, but it seems likely that an additional source of heat is still required. Heat advection by groundwater movement may have influenced the coal maturation levels in the Basin as well. Both magmatic activity associated with the emplacement of the San Juan batholith and elevated geothermal gradients associated with the steady-state thermal source at depth may have contributed to heating of the groundwater. An appreciation of heat advection by groundwater flow may therefore be most important to understanding regional patterns of heat flow and hydrocarbon maturation.     

The use of seismic reflection profiles to probe the crystalline basement in Britain

In an attempt to assess the feasibility of a future programme of seismic reflection profiling to investigate major structural features deep within the crust of Britain, existing commercial offshore data across shallow basement have been investigated together with lines on land, fired conventionally with explosives, in which record lengths have been extended to at least 12 s two-way time. Profiles crossing Lewisian basement, a granite batholith, and Caledonian and Variscan structures have been examined. The seismic information has been combined with gravity and magnetic surveys, bore-hole logs and surface geology to give self-consistent interpretational models. Features such as intrusive bodies and thrust faults can be discerned within the basement. These are clearly visible on the seismic record section obtained along the MOIST line, fired along the north coast of Scotland especially for deep reflections, and, for the first time, normal-incidence reflections from the Moho are clearly observed.     

Petrological influence on differential weathering and inselberg development in the Kora area of central Kenya

In the Kora area of central Kenya domed inselbergs are well developed on outcrops of granitoid migmatite, while positive relief features are rare on the surrounding gneiss. Block-strewn, vegetated hills occur on restricted areas of granoblastite, gabbro, and metagabbro. Schmidt Hammer measurements have shown that the apparent differences in resistance to weathering and erosion are not due to variations in rock hardness, since all the rock types have similar ‘R’ values. The results of geochemical analyses have shown that the migmatites are significantly more potassic than the surrounding gneiss. Samples of migmatite from the inselbergs were also found to be slightly richer in potassium than migmatite samples from the inter-inselberg areas. The variations in potassium content probably reflect differences in protolith composition, chemical fractionation during partial melting, and the effects of metasomatism. These findings support earlier suggestions that, other things being equal, potassium-rich granitoid rocks weather more slowly than less potassic rocks.     

Rock mass strength and the form of some inselbergs in the central namib desert

A rock mass strength classification, involving eight parameters, has been applied to selected inselbergs in the Namib Desert. The inselbergs are formed of schists, gneiss, granite and marble. Their slope inclinations are in a strength equilibrium with their rocks. The study suggests that the strength classification is consistent and has a general validity, and that rock slopes, undergoing uniform weathering, retreat to form slopes which are adjusted to their rock mass strength: this is a more general statement than the common hypothesis of parallel retreat. The favoured hypothesis of slope evolution in the Namib is one of retreat rather than downwearing.     

Aeromagnetic and Landsat TM structural interpretation for identifying regional groundwater exploration targets, south-central Zimbabwe Craton

Aeromagnetic (AM) and Landsat Thematic Mapper (TM) data from the south-central Zimbabwe Craton have been processed for the purpose of regional structural mapping and thereby to develop strategic models for groundwater exploration in hard-rock areas. The lineament density is greater on TM than on AM images, partly due to the resolution of the different datasets, and also because not all TM lineaments have a magnetic signature. The derived maps reveal several previously undetected lineaments corresponding to dykes, faults, shear zones and/or tectonically-related joints, striking predominantly NNE, NNW and WNW. We suggest the possible hydrogeological significance of some of these patterns as follows: the aeromagnetic data can be used to map faults and fractures of considerable depth which are likely to be open groundwater conduits at depth (typically under tension), while TM lineaments, although not necessarily open (mostly under compression), represent recharge areas.The interpreted persistent lineation and well developed fracture patterns are correlated with existing boreholes and indicate a spatial relationship between regional structures and high borehole yields (> 3 m³/h). This relationship is combined with other lithological and hydrogeological information to identify potential regional groundwater sites for detailed ground investigations. These are defined as dyke margins, faults, fractures/joints or intersections of any combination of these structures. Priority should be given to coincident AM/TM lineaments (e.g., NNW and NNE fractures) and continuous structures with large catchment areas (e.g., NNE and WNW faults). The late Archaean (2.6 Ga) granites are considered the most favourable unit because of their associated long and deep brittle fractures between numerous bornhardts (inselbergs) and kopjes. Several small-scale TM lineaments also form important local sources of groundwater for hand-dug wells. Based on measured rock susceptibilities from the area, we present a model of the typical magnetic responses from the possible groundwater exploration targets. The developed magnetic model could be applicable to similar terrains in other Archaean Cratons.     

The Christmas Mountains caldera complex,Trans-Pecos Texas

The Christmas Mountains caldera complex developed approximately 42 Ma ago over an elliptical (8×5 km) laccolithic dome that formed during emplacement of the caldera magma body. Rocks of the caldera complex consist of tuffs, lavas, and volcaniclastic deposits, divided into five sequences. Three of the sequences contain major ash-flow tuffs whose eruption led to collapse of four calderas, all 1–1.5 km in diameter, over the dome. The oldest caldera-related rocks are sparsely porphyritic, rhyolitic, air-fall and ash-flow tuffs that record formation and collapse of a Plinian-type eruption column. Eruption of these tuffs induced collapse of a wedge along the western margin of the dome. A second, more abundantly porphyritic tuff led to collapse of a second caldera that partly overlapped the first. The last major eruptions were abundantly porphyritic, peralkaline quartz-trachyte ash-flow tuffs that ponded within two calderas over the crest of the dome. The tuffs are interbedded with coarse breccias that resulted from failure of the caldera walls. The Christmas Mountains caldera complex and two similar structures in Trans-Pecos Texas constitute a newly recognized caldera type, here termed a laccocaldera. They differ from more conventional calderas by having developed over thin laccolithic magma chambers rather than more deep-seated bodies, by their extreme precaldera doming and by their small size. However, they are similar to other calderas in having initial Plinian-type air-fall eruption followed by column collapse and ash-flow generation, multiple cycles of eruption, contemporaneous eruption and collapse, apparent pistonlike subsidence of the calderas, and compositional zoning within the magma chamber. Laccocalderas could occur else-where, particularly in alkalic magma belts in areas of undeformed sedimentary rocks.     

The influence of complex intra- and extra-vent processes on facies characteristics of the Koala Kimberlite,NWT, Canada: volcanology,sedimentology and intrusive processes

The Koala kimberlite, Northwest Territories, Canada, is a small pipe-like body that was emplaced into the Archean Koala granodiorite batholith and the overlying Cretaceous to Tertiary sediments at ~53 Ma. Koala is predominantly in-filled by a series of six distinct clastic deposits, the lowermost of which has been intruded by a late stage coherent kimberlite body. The clastic facies are easily distinguished from each other by variations in texture, and in the abundance and distribution of the dominant components. From facies analysis, we infer that the pipe was initially partially filled by a massive, poorly sorted, matrix-supported, olivine-rich lapilli tuff formed from a collapsing eruption column during the waning stage of the pipe-forming eruption. This unit is overlain by a granodiorite cobble-boulder breccia and a massive, poorly sorted, mud-rich pebbly-sandstone. These deposits represent post-eruptive gravitational collapse of the unstable pipe walls and mass wasting of tephra forming the crater rim. The crater then filled with water within which ~20 m of non-kimberlitic, wood-rich, silty sand accumulated, representing up to 47,000 years of quiescence. The upper two units in the Koala pipe are both olivine rich and show distinct grain-size grading. These units are interpreted to have been deposited sub-aqueously, from pyroclastic flows sourced from one or more other kimberlite volcanoes. The uppermost units in the Koala pipe highlight the likelihood that some kimberlite pipes may be only partially filled by their own eruptive products at the cessation of volcanic activity, enabling them to act as depocentres for pyroclastic and sedimentary deposits from the surrounding volcanic landscape. Recognition of these exotic kimberlite deposits has implications for kimberlite eruption and emplacement processes.     

Regional tectonic control on large size explosive eruptions: Insights into the Green Tuff ignimbrite unit of Pantelleria

Original geological and structural data, which derive from the analysis of the rheomorphic Green Tuff ignimbrite unit of Pantelleria, have offered the opportunity to define its modes of emplacement and the location of the eruptive sources in terms of distribution and geometry. The Green Tuff displays a wide range of rheomorphic structures consisting of preserved penetrative foliations, lineations and folds which, developed at distinct times, have been assigned to three major (D₁–D₃) deformation events accompanying and following the ignimbrite unit emplacement. The first D₁ event produced distinct sets of structures developed along ductile shear zones generated during the emplacement of pyroclastic density currents along current-deposit boundaries. Palaeoflow directions of this event are completely independent from topography and are directly related to high-energy currents generated from the eruption. D₂ event is characterized by folding due to down-slope post-emplacement flows related to gravity sliding processes whereas the D₃ event was dominated by semi-brittle to brittle structures developed after the complete emplacement of the flow units and their subsequent cooling and compaction. The statistical analysis of these structural data has led to the hypothesis that the Green Tuff eruption developed from fissural sources that are largely superimposed on the NNE-trending dip-slip normal fault zones of the island (the Zinedi and the Montagna Grande faults). Our model also implies that the Green Tuff ignimbrite deposit can be the result of several events within a single eruptive cycle. The orientation of the fissural eruptive systems is evidence that the feeding structures for this large-size explosive event were strongly controlled by the E-W to ESE-WNW directed extension structures that affect the island of Pantelleria and, as a whole, the entire region of the Sicily Channel.     

Facies architecture of a silicic intrusion-dominated volcanic centre at Highway–Reward, Queensland, Australia

The Highway–Reward massive sulphide deposit is hosted by a silicic volcanic succession in the Cambro-Ordovician Seventy Mile Range Group, northeastern Australia. Three principal lithofacies associations have been identified in the host succession: the volcanogenic sedimentary facies association, the primary volcanic facies association and the resedimented syn-eruptive facies association. The volcanogenic sedimentary facies association comprises volcanic and non-volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base. Lithofacies of the primary volcanic facies association include coherent rhyolite, rhyodacite and dacite, and associated non-stratified breccia facies (autoclastic breccia and peperite). The resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes, but then redeposited by mass-flow processes. Resedimentation was more or less syn-eruptive so that the deposits are essentially monomictic and clast shapes are unmodified. This facies association includes monomictic rhyolitic to dacitic breccia (resedimented autoclastic facies), siltstone-matrix rhyolitic to dacitic breccia (resedimented intrusive hyaloclastite or resedimented peperite) and graded lithic-crystal-pumice breccia and sandstone (pumiceous and crystal-rich turbidites). The graded lithic-crystal-pumice breccia and sandstone facies is the submarine record of a volcanic centre(s) that is not preserved or is located outside the study area. Pumice, shards, and crystals are pyroclasts that reflect the importance of explosive magmatic and/or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings.The lithofacies associations at Highway–Reward collectively define a submarine, shallow-intrusion-dominated volcanic centre. Contact relationships and phenocryst populations indicate the presence of more than 13 distinct porphyritic units with a collective volume of 0.5 km³. Single porphyritic units vary from <10 to 350 m in thickness and some are less than 200 m in diameter. Ten of the porphyritic units studied in the immediate host sequence to the Highway–Reward deposit are entirely intrusive. Two of the units lack features diagnostic of their emplacement mechanism and could be either lavas and intrusions. Direct evidence for eruption at the seafloor is limited to a single partly extrusive cryptodome. However, distinctive units of resedimented autoclastic breccia indicate the presence nearby of additional lavas and domes.The size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption/intrusion, the style of emplacement, and the subaqueous emplacement environment. Due to rapid quenching and mixing with unconsolidated clastic facies, the sills and cryptodomes did not spread far from their conduits. The shape and distribution of the lavas and intrusions were further influenced by the positions of previously or concurrently emplaced units. Magma preferentially invaded the sediment, avoiding the older units or conforming to their margins. Large intrusions and their dewatered envelope may have formed a barrier to the lateral progression and ascent of subsequent batches of magma.     

Flow and emplacement direction determined for selected basaltic bodies using magnetic susceptibility anisotropy measurements

Low-field anisotropy of magnetic susceptibility (AMS) has been determined for a total of 248 basaltic specimens taken from cross sections between the cooling interfaces of 6 subaerial lavas, 6 deep-sea lavas, and 6 intrusives (5 dikes and 1 sill). Statistically significant AMS clusters are exhibited by all the dikes examined and, based upon these clusters, derivation of emplacement direction becomes possible. Two lavas are observed to have statistically significant AMS clusters which can be used for flow direction determinations. The methods of emplacement and flow direction analysis are discussed as well as the statistics used. It is concluded that most of the dikes examined have low angle emplacement directions. A classification scheme for AMS data distributions is presented.The AMS analysis shows that intrusives and deep-sea lavas can be distinguished from subaerial lavas approximately 80% of the time by the random AMS ellipsoid orientations exhibited in subaerial lavas. Contrasts in the fluid properties, degassing, wall effects with subsequent distortion of the fluid, and grain interaction during the extrusion of subaerial lavas can be expected to distort magnetic grain alignment. Further effects such as convection and secondary processes contribute to yield the random distributions observed for most of these bodies.     

蒙古及周边地区重力异常和地壳不均匀体分布

基于全球EGM2008自由空气重力异常模型,本文计算了蒙古及周边地区的布格重力异常和AiryHeiskanen均衡重力异常.在此基础上,本文采用Crust 1.0地壳模型为参考,通过重力正演方法,对蒙古及周边地区不同深度地壳密度结构模型的重力异常进行了计算,并对得到的正演布格重力异常与实际重力异常进行了对比和分析.研究结果表明:蒙古西部杭爱山地区与阿尔泰山地区的构造变形差异性明显,现今均衡重力异常中杭爱山周边没有明显的均衡异常高值区,而阿尔泰山地区西南方向存在均衡重力异常高值分布,分析与新构造运动密切相关;Crust 1.0模型给出的壳幔横向密度不均匀体分布对于计算Moho面起伏引起的重力异常作用明显;Crust 1.0给出的地壳内界面变形可以反映深大活动断裂的深部构造变形.研究结果对于认识蒙古东西部构造特征差异,以及现今西部活动断裂的地球物理场特征具有参考意义,也可以为进一步应用Crust 1.0模型为参考开展三维密度结构反演提供一定帮助.     

Isotopic evidence for meteoric-hydrothermal alteration of plutonic igneous rocks in the Yakutat Bay and Skagway areas,Alaska

Oxygen and hydrogen isotope ratios were measured on coexisting minerals from quartz diorites and quartz monzonites from a section across the Coast Range batholith in the Skagway area, Alaska, including a variety of outlying plutons west of the batholith in the Yakutat Bay-Mt. St. Elias region (latitudes 59–60°N). The extremely low and variable δ¹⁸Oand δD results indicate widespread meteoric-hydrothermal alternaiton of the Coast Range batholith, and to a lesser extent, of the Yakutat Bay plutons as well. In the Yakutat Bay area, the plutons with K—Ar ages younger than 50 m.y. have widely varying δD values of ?72 to ?148, compared to δD = ?69to?90 for all but one sample in the 50–225 m.y. age grouping (one biotite has δD = ?109). This suggests that the major meteoric-hydrothermal episodes in this area occurred during the Eocene and Miocene. This involved relatively small meteoric water/rock ratios(<0.1), as none of the δ¹⁸O values show any clear-cut evidence of alteration (δ¹⁸O_quartz= 7.4 ?11.8; δ¹⁸O_feldspar= 5.7?10.0). However, in the section across the Coast Range batholith, 85% of the plutonic rocks have very low δD values of ?100 to ?167, and the δ¹⁸O values are extremely variable δ¹⁸O_feldspar= + 10.3to?4.0 and Δ¹⁸O_{quartz-feldspar}= 0.4?10.5. These data indicate that a major portion of the batholith, particularly the quartz monzonite-rich eastern part, but also including many of the quartz diorite plutons as well, interacted with meteoric-hydrothermal convective systems that involved water/rock ratios of about 0.3–1.4. The quart diorite plutons are most depleted in¹⁸O near their northeast contacts against the younger quartz monzonite intrusions. The primary igneous δ¹⁸O values of the quartz diorites were apparently higher than those of the quartz monzonites; they are also unusually high in¹⁸O compared to most other analyzed quartz diorites, suggesting derivation from, exchange with, or assimilation of high-¹⁸O metasediments or altered volcanic rocks. These data and conclusions are very similar to those reached previously on a similar isotopic study of the Coast Range batholith in British Columbia, 700 km to the southeast at latitudes 54–55°N, except that in the Skagway area an even greater proportion of the batholith was apparently depleted in deuterium. This implies that deep (?5km?) circulation of meteoric groundwaters is probably a characteristic of the later stages of emplacement of the Cordilleran batholiths of western North America, suggesting that the eastern sections of these batholiths in particular were emplaced at relatively shallow depths.     

Miocene emplacement and deformation of the Konga Shan granite (Xianshui He fault zone, west Sichuan, China): Geodynamic implications

The presently active sinistral Xianshui He strike-slip fault (XSH) is a lithospheric scale strike-slip fault in the eastern Himalaya. In the study area this fault affects the eastern edge of the Konga Shan granitic massif, where it has caused both brittle and ductile deformation. A RbSr isochron and Nd and Pb isotope study of three samples, and a UPb zircon study of a single sample, were completed on the granite.

UPb data indicate a granite emplacement age of 12.8 ± 1.4 Ma. The RbSr isochrons show that the granite emplacement and the deformational event were synchronous, at around 12-10 Ma (minimum age for deformation). The Nd and Pb isotope compositions of whole rocks and K-feldspars indicate the involvement of Proterozoic continental crust, which is confirmed by UPb systematics indicating inherited zircons.

Sinistral faulting along the XSH began at the latest at 12 Ma and marks the extrusion toward the east of the West Sichuan and South China blocks, following their extrusion along the Red River fault zone between 50 and 21 Ma.     

Magma batches in the Timber Mountain magmatic system, Southwestern Nevada Volcanic Field, Nevada, USA

The common occurrence of compositionally and mineralogically zoned ash flow sheets, such as those of the Timber Mountain Group, provides evidence that the source magma bodies were chemically and thermally zoned. The Rainier Mesa and Ammonia Tanks tuffs of the Timber Mountain Group are both large volume (1200 and 900 km³, respectively) chemically zoned (57–78 wt.% SiO₂) ash flow sheets. Evidence of distinct magma batches in the Timber Mountain system are based on: (1) major- and trace-element variations of whole pumice fragments; (2) major-element variations in phenocrysts; (3) major-element variations in glass matrix; and (4) emplacement temperatures calculated from Fe-Ti oxides and feldspars. There are three distinct groups of pumice fragments in the Rainier Mesa Tuff: a low-silica group and two high-silica groups (a low-Th and a high-Th group). These groups cannot be related by crystal fractionation. The low-silica portion of the Rainier Mesa Tuff is distinct from the low-silica portion of the overlying Ammonia Tanks Tuff, even though the age difference is less than 200,000 years. Three distinct groups occur in the Ammonia Tanks Tuff: a low-silica, intermediate-silica and a high-silica group. Part of the high-silica group may be due to mixing of the two high-silica Rainier Mesa groups. The intermediate-silica group may be due to mixing of the low- and high-silica Ammonia Tanks groups. Three distinct emplacement temperatures occur in the Rainier Mesa Tuff (869, 804, 723 °C) that correspond to the low-silica, high-Th and low-Th magma batches, respectively. These temperature differences could not have been maintained for any length of time in the magma chamber (cf. Turner, J.S., Campbell, I.H., 1986. Convection and mixing in magma chambers. Earth-Sci. Rev. 23, 255–352; Martin, D., Griffiths, R.W., Campbell, I.H., 1987. Compositional and thermal convection in magma chambers. Contrib. Mineral. Petrol. 96, 465–475) and therefore eruption must have occurred soon after emplacement of the magma batches into the chamber. Emplacement temperatures of the pumice fragments from the Ammonia Tanks Tuff show a continuous gradient of temperatures with composition. This continuous temperature gradient is consistent with the model of storage of magma batches in the Ammonia Tanks group that have undergone both thermal and chemical diffusion.     

Morphology and emplacement of flows from the Deccan Volcanic Province,India

The present study is probably the first of its kind in the Deccan Volcanic Province (DVP) that deals in detail with the morphology and emplacement of the Deccan Trap flows, and employs modern terminology and concepts of flow emplacement. We describe in detail the two major types of flows that occur in this province. Compound pahoehoe flows, similar to those in Hawaii and the Columbia River Basalts (CRB) constitute the older stratigraphic Formations. These are thick flows, displaying the entire range of pahoehoe morphology including inflated sheets, hummocky flows, and tumuli. In general, they show the same three-part structure associated with pahoehoe flows from other provinces. However, in contrast to the CRB, pahoehoe lobes in the DVP are smaller, and hummocky flows are quite common. 'Simple' flows occur in the younger Formations and form extensive sheets capped by highly vesicular, weathered crusts, or flow-top breccias. These flows have few analogues in other provinces. Although considered to be a'a flows by previous workers, the present study clearly reveals that the simple flows differ considerably from typical a'a flows, especially those of the proximal variety. This is very significant in the context of models of flood basalt emplacement. At the same time, they do not display direct evidence of endogenous growth. Understanding the emplacement of these flows will go a long way in determining whether all extensive flows are indeed inflated flows, as has recently been postulated.Most of the studies relating to the emplacement of Continental Flood Basalt (CFB) lavas have relied on observations of flows from the CRB. Much of the current controversy surrounding the emplacement of CFB flows centers around the comparison of Hawaiian lava flows to those from the CRB. We demonstrate that the DVP displays a variety of lava features that are similar to those from the CRB as well as those from Hawaii. This suggests that there may have been more than one mechanism or style for the emplacement of CFB flows. These need to be taken into account before arriving at any general model for flood basalt emplacement.Editorial responsibility: T. Druitt