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1.
The Cenozoic intracontinental Teletsk basin in the Central Asian Altai Mountains is composed of a complexly structured northern and a more simple southern sub-basin. These sub-basins formed in two distinct kinematic stages when first the NNW-striking Teletsk- and then the NE-striking West-Sayan shear zones became reactivated in the Cenozoic under dominant NS-oriented horizontal compression. Although the entire Teletsk basin strikes roughly NS, the southern sub-basin is parallel to the NNW-trending, amphibolite facies Teletsk ductile shear zone, while the northern sub-basin is NS-striking and flanked by differently structured, greenschist facies basement. Basement reactivation closely controlled the southern sub-basin formation, but this is less clear for the northern sub-basin. Contrasts between northern and southern basement and the exclusive occurrence of pseudotachylytes along the margins of the southern basin are explored for their contribution to the formation of the Teletsk basin with two distinct sub-basins.In the ductile shear fabric of the basement flanking the southern sub-basin, concordantly interleaved pseudotachylytes and isolated breccia lenses reflect local brittle deformation along the ductile fabric. The genetic link between breccia lenses and pseudotachylyte occurrences was defined by microstructural investigation. It allows to explore their possible development in a dextral strike–slip zone. These rocks occur in a large fault-bounded segment of the basement. The geometry of the structures in the segment is comparable with a dextral strike–slip sidewall-ripout structure along the Teletsk shear zone. Seismic slip related to pseudotachylytes is attributed to the sudden stress release on the NNW-striking Teletsk shear zone, when the latter became unconstrained by reactivation of the NE-trending West-Sayan fault zone at its northern boundary. The boundary of the sidewall-ripout structure was reactivated as a large listric fault in a later stage. The northern sub-basins roughly develop along an NS strike and are assumed to reflect reactivation of the ductile shear zone underneath the variably structured greenschist facies basement outcropping along the flanks of the sub-basin. 相似文献
2.
The Port Mouton Shear Zone: intersection of a regional fault with a crystallizing granitoid pluton 总被引：2，自引：0，他引：2
D. Barrie Clarke Krista L. McCuish Ron H. Vernon Victor Maksaev Brent V. Miller 《Lithos》2002,61(3-4):141-159
The peraluminous tonalite–monzogranite Port Mouton Pluton is a petrological, geochemical, structural, and geochronological anomaly among the many Late Devonian granitoid intrusions of the Meguma Lithotectonic Zone of southern Nova Scotia. The most remarkable structural feature of this pluton is a 4-km-wide zone of strongly foliated (040/subvertical) monzogranites culminating in a narrow (10–30 m), straight, zone of compositionally banded rocks that extends for at least 3 km along strike. The banded monzogranites consist of alternating melanocratic and leucocratic compositions that are complementary to the overall composition of that part of the pluton, suggesting an origin by mineral–melt and mineral–mineral sorting. Biotite and feldspar are strongly foliated in the plane of the compositional bands. These compositional variations and foliations originated by a process of segregation flow during shearing of the main magma with a crystallinity of 55–75%. Subsequent minor brittle fracturing of feldspars, twinning of microcline, development of blocky sub-grains in quartz, and kinking of micas demonstrate overprinting by a high-temperature deformation straddling the monzogranite solidus. Small folds and late sigmoidal dykes indicate dextral movement on the shear zone. This Port Mouton Shear Zone (PMSZ) is approximately co-linear with the only outcrops of Late Devonian mafic intrusions in the area, two of which are syn-plutonic with well-developed mingling textures in the marginal tonalite of the Port Mouton Pluton. Also closely co-linear with the mafic intrusions are a granitoid dyke that extends well beyond the outer contact of the Port Mouton Pluton, a swarm of large aligned angular xenolithic slabs, a zone of thin wispy schlieren banding, a large Be-bearing pegmatite, and a breccia pipe with abundant garnetiferous metapelitic xenoliths. In various ways, the shear zone may control all of these features. The Port Mouton Shear Zone is parallel to many other NE-trending faults and shear zones in the northern Appalachians, probably related to the docking of the Meguma Zone along the Cobequid–Chedabucto Fault system. 相似文献
3.
Throughgoing fractures play a major role in subsurface fluid flow yet the kinematics of their formation, which directly impact rock flow properties, are often difficult to establish. We investigate throughgoing fractures in the Monterey Formation of California that developed by the coalescence of pre-existing joints. At Lompoc Landing, throughgoing fractures fall into three main groups: linked, linked with aperture, and breccia zones. The segmented nature of their walls provides numerous piercing points to firmly establish the sense of displacement. Analysis of displacement vectors derived from piercing points demonstrates that the NW–SE trending throughgoing fractures, often interpreted as strike–slip faults, are in fact extensional structures in origin. We suggest that this method may be applied to throughgoing fractures that form by the same mechanism in other geologic settings. Establishing kinematics of throughgoing fractures will lead to a better understanding of their contribution to subsurface fluid flow. 相似文献
4.
A particular variety of volcanogenic country rock breccia is described; a contact breccia that has been identified at Venetia, River Ranch and Wimbledon kimberlite pipes. The contact breccia is clast supported with no juvenile kimberlite component, has tightly packed, angular fragments (with occasional rounding of smaller particles), and has a shear-fabric dipping towards the center of each kimberlite pipe or volcanic event. Clasts have preferred orientations parallel to the fabric. Photographs of the breccia in the open pit face and measured data from drill core are analyzed specifically to quantify the clast size distributions and clast shapes by means of fractal analysis. The fractal dimension is one means of characterizing the breccia because the dimension can be specific to a fragmentation mechanism. Clast size distribution fractal dimensions in the coarser particles (greater than circa 3 cm) range from greater than 3 for nonsheared breccia, down to circa 2.3 for the sheared breccia. Breccia characterization based on this fractal analysis suggests that fragmentation occurred initially from confined high-energy explosions, followed by collapse and abrasion by subsequently gravity-induced rockmass subsidence. All studied contact breccias produced a distinctive fractal signature in the finer particles (<3 cm) of circa 1.6 that can be explained by a comminution fragmentation process in that particular particle size range. It is suggested that these subsidence breccias require a substantial volume deficit at depth within the volcanic pipe in order to explain their origin and size. The methodology used in this study could be used to characterize any other volcanic breccia and further model their origins. 相似文献
5.
Tereza Cristina Junqueira-Brod Jos Carlos Gaspar Jos Affonso Brod Camilla Vasconcelos Kafino 《Journal of South American Earth Sciences》2005,18(3-4):337-353
Kamafugitic rocks intruded the Precambrian basement and Phanerozoic sediments at the northeast border of the Paraná basin as part of the Late Cretaceous Goiás alkaline province (GAP). Plutonic complexes dominate the north of the province, whereas lavas and pyroclastic rocks prevail in the south. The central GAP is characterized by kamafugitic diatremes, which may crop out continuously for up to 850 m and consist of a central breccia body, surrounded and overlain by lava flows and crosscut by dykes. The breccias contain some special spheroidal juvenile fragments—namely, accretionary and armored lapilli, frozen droplets, spinning droplets, and wrapped fragments—whose textural and mineralogical aspects are described in detail. Irregularly shaped tuff pockets that occur within the breccias contain textures and structures similar to those of subaerial surge deposits and formed in confined, high gas to solid+liquid ratio domains in the conduit. Diatreme emplacement affected the country rock through thermal metamorphism, development of columnar jointing, and formation of peperite-like mixtures. There is no evidence of phreatomagmatic activity in the diatremes, and CO2, rather than H2O, seems to have been the major volatile component of the kamafugitic magmas. This finding implies that features such as accretionary lapilli and peperites are not exclusively associated with H2O-dominated processes. 相似文献
6.
隐爆角砾岩筒型金(铜)矿床作为重要的金矿床类型,常伴生关键金属碲的矿化,然而对于此类矿床中碲的分布、分配特征及沉淀机制研究仍较为薄弱。黄屯矿床是长江中下游成矿带近年来发现的最为典型的隐爆角砾岩筒型金铜矿床,伴生有大量的碲化物产出。本文在详细的野外地质工作和岩相学观察基础上,发现碲在矿床中发生了显著富集。通过对黄屯矿床不同蚀变类型矿石及主要富碲矿物开展全岩及微区地球化学分析,明确了碲的分布、分配特征以及初步讨论了碲的沉淀机制。黄屯矿床成矿阶段从早到晚可划分为钠钙硅酸盐、钾硅酸盐、绿泥石-碳酸盐和伊利石-蒙脱石阶段,在不同深度形成了相应的蚀变带,并发育有不同强度的金铜矿化。黄屯矿床伴生的碲储量约有118.71t,平均品位为5.3g/t,达到中型规模,具备重要的综合利用价值。钾硅酸盐蚀变带碲的分布比例最高,占总储量约92.37%,平均品位约为9.6g/t。钾硅酸盐蚀变带内约有89%~99%的碲呈独立矿物,主要以微米级、纳米级的碲铋矿包体的形式分布在黄铁矿中,剩余部分则以类质同象的形式赋存在黄铁矿和黄铜矿内。减压沸腾引起流体温度骤降导致硫化物沉淀,和沸腾过程中释放大量气相H_(2)S,共同导致流体的f(Te_(2))/f(S_(2))比值升高,可能是黄屯矿床中碲沉淀富集的主要机制。 相似文献
7.
Abundant and regionally unique dolostone lithoclast breccias occur throughout the shallow-marine, Lower to Middle Ordovician Pogonip Group in the Nopah Range and adjacent ranges in eastern California and southern Nevada. Breccia bodies display sharply cross-cutting relationships with host dolostone bedrock stratigraphy. They also show stratigraphic variability in size, shape and dolostone clast composition, but similarity in breccia matrix composition and framework texture and fabric. These characteristics are consistent with a palaeokarst origin. Upsection changes in breccia clast lithology as well as multiple occurrences of associated quartz sand-filled grikes (solution-widened fissures) indicate multiple episodes of carbonate platform exposure and karstification. Repeated karstification is also indicated by stratiform bodies of quartz sand and thin terra rossa palaeosols that locally truncate breccias and grike systems, thus bracketing karstified exposure surfaces. Facies successions and stacking patterns between recognized exposure surfaces are developed as transgressive–regressive cycles and thus show depositional sequence architecture. Hence, these breccias and other associated palaeokarst features are related to a succession of disconformities that provide a sequence-stratigraphic framework for assessing Ordovician relative sea-level history of the south-western Cordilleran margin of Laurentia. 相似文献
8.
The Baula-Nuasahi Complex, on the southern flank of the Singhbhum Archaean nucleus in north-eastern India, exposes a series of Mesoarchaean igneous suites. These are (1) a gabbro–anorthosite unit, which is petrographically homogeneous, although mineral-chemistry data hint at a subtle eastward differentiation; (2) a peridotite unit (with three chromitite layers) together with (3) a pyroxenite unit which display cumulate textures, modal layering, and (for the peridotite unit) differentiation trends in both mineralogy and mineral chemistry; and (4) the Bangur gabbro (~3.1 Ga), which defines an oblong intrusion, crosscutting the older igneous suites in the southern part of the complex, with a curvilinear NW-trending apophysis, 2 km long and up to 40 m wide. Magmatic breccia comprising ultramafic and chromitite wall-rock clasts in a gabbro matrix is exposed at the contact of the main Bangur gabbro body and also forms the entire Bangur gabbro apophysis. Concentrations of platinum-group minerals (PGMs) are found where the breccia contains abundant chromitite clasts, and two types of platinum-group-element (PGE) mineralisation are recognised. Type 1 (Pt 1.1–14.2, Pd 0.1–2.1 ppm, with an average Pt/Pd=8–9) is a contact-type mineralisation which occurs in the breccia at the contact between the Bangur intrusion and its ultramafic host. The PGMs—Pt alloys (isoferroplatinum) and sulphides (braggite, malanite)—are enclosed by pyroxene and plagioclase, reflecting a magmatic origin. Significant wall-rock assimilation by the magma (giving rise to the Bangur gabbro) is indicated by changes in pyroxene composition and by the presence of relicts of chromite (from the host) now altered to secondary ferritchromite in the contact zone. Type 2 PGE mineralisation (Pt 0.3–1.6, Pd 1.8–6.0 ppm, with Pt/Pd~0.5–3.0) is restricted to the breccia apophysis of the Bangur gabbro where it occurs in the breccia matrix, associated with an intense hydrothermal alteration which does not exist in the contact zone. PGMs (PGE arsenides, tellurides, bismuthides and antimonides) and, where present, base-metal sulphides (BMSs) form intergrowths with hydrous silicates, reflecting a hydrothermal origin. Oxygen isotope geothermometry documents the main stages of hydrothermal alteration within a decreasing temperature range between 700–1,000 and 500–600 °C, and oxygen, hydrogen and sulphur isotopes show that the hydrothermal fluids were derived from the magma rather than an external source. Pervasive hydrothermal alteration in the breccia apophysis likely represents upward channelling of late-magmatic fluids along a narrow, near-vertical, subplanar conduit which led away from the main magma chamber. We suggest that Type 2 mineralisation was produced by late-magmatic hydrothermal remobilisation and reconcentration of Type 1 PGE mineralisation, and that the composition of the hydrothermal fluids controlled whether BMSs were enriched along with the PGMs.Editorial handling: P. Lightfoot 相似文献
9.
Geochemical signature of orogenic hydrothermal activity in an active tectonic intersection zone,Alpine Fault,New Zealand 总被引：1，自引：0，他引：1
J.?R.?Campbell D.?CrawEmail author R.?Frew T.?Horton C.?P.?Chamberlain 《Mineralium Deposita》2004,39(4):437-451
A highly faulted and fractured rock mass has developed at the intersection of the Alpine and Hope faults, two major active faults in the South Island, New Zealand. The Alpine Fault is an oblique dextral reverse fault at the late Cenozoic-Recent Pacific-Australian plate boundary. The Hope Fault is a strike-slip fault parallel to the plate convergence vector. Hydrothermal fluids driven by the active tectonic processes have passed through the fractured rock mass, causing localised rock alteration and vein formation. Mylonites in the Alpine Fault zone are crosscut by cm-scale veins of quartz and/or ankerite with minor sulphides, with cemented breccias in dilational jogs. Breccia clasts and immediate (cm-scale) host rocks have been variably impregnated with carbonates and quartz. This generation of veins, breccias and altered rocks is post-dated by cataclasite and fault gouge zones which have been cemented by calcite, illite, smectite and chamosite. Ankerite and calcite have 18O between +10 and +30, and 13C between 0 and –8. These minerals are inferred to have formed from water with variable components of both meteoric and crustally exchanged fluid. Rock alteration associated with ankerite–quartz veins has added arsenic (up to 200 ppm As), strontium, and some Y to the rocks. Host-rock mylonites (<2 ppm As) have been depleted in arsenic compared to their precursors (5–15 ppm As). This depletion of arsenic in the middle crust provides the source for arsenic in shallower-level vein systems.Editorial handling: N. White 相似文献
10.
Tereza Cristina Junqueira-Brod Jos Affonso Brod Jos Carlos Gaspar Hardy Jost 《Lithos》2004,76(1-4):261-282
This paper describes the internal organisation of two diatremes (Águas Emendadas and Neuzinha) and one small breccia-filled conduit (Tigre) in the central portion of the Late Cretaceous Goiás Alkaline Province (GAP), central Brazil, and explores the criteria for facies recognition. The GAP kamafugitic diatremes are emplaced into Carboniferous sandstones of the Aquidauana Formation, at the northern margin of the Paraná Basin. They are usually elliptical structures, not longer than 900 m, filled with breccia and partially covered by thin kamafugitic to basanitic lava flows. The breccias are dominated by juvenile pyroclasts, with subordinate amounts of cognate fragments and xenoliths. In addition to variations in ash and lapilli proportions, juvenile fragment types may be used to discriminate genetic processes and the corresponding pyroclastic deposits.
An extensive field, textural and compositional dataset was analysed by multivariate statistical techniques. Combined with field observations, this allowed us to define a set of facies for kamafugitic diatremes, and, more importantly, to understand the internal structure of the studied bodies and to cross-correlate them. Seven distinct facies were recognised. The Fluidised Conduit Facies (FCF) represents high-energy, strongly fluidised but only moderately fragmented systems. It occurs in a confined environment, and is typical of deeper parts of the conduit, before the actual diatreme level is reached by the ascending fluidised magma. Large amounts of spinning droplets are formed within this region. The Fluidised Conduit–Diatreme Facies (FCDF) is characteristic of intermediate depths in the conduit, where highly fluidised and highly fragmented systems produce large amounts of ash. Spinning droplets decrease in abundance, ordinary juvenile fragments become very common, and xenoliths from the country rock in the immediate vicinity of the diatreme are present. The Fluidised Fragmented Facies (FFF) and the Magmatic Fluidised Facies (MFF) produce very heterogeneous deposits that dominate the shallower part of the system, making up most of the diatreme-filling materials. The Fluidised Fragmented Facies can be distinguished by much higher degrees of fluidisation, fragmentation and system energy. It occupies the internal part of the diatreme and is characterised by the common presence of tuff pockets, tuff fragments, and accretionary and armoured lapilli. The Magmatic Fluidised Facies typically occupies the outer portion of the diatreme and can be distinguished from the Fluidised Fragmented Facies by the dominance of lapilli over ash and by the presence of abundant wrapped fragments. The Magmatic Facies (MF) and the Coherent Magmatic Facies (CMF) are volumetrically subordinate and represent late stages, when less fluidised and less fragmented material, or even coherent magma erupts relatively passively, in the aftermath of the main explosive stage that generated the diatreme. The Border Facies is defined by the increased abundance of material from the immediate country rock. At Águas Emendadas and Neuzinha this facies is marked by the presence of fragments of peperite-like rock, formed by the interaction of the fluidised magma with friable sandstone. 相似文献