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1.
Some recent calc-alkaline andesites and dacites from southern and central Martinique contain basic xenoliths belonging to two main petrographic types: The most frequent one has a hyalodoleritic texture (« H type ») with hornblende + plagioclase + Fe-Ti oxides, set in an abundant glassy and vacuolar groundmass. The other one exhibits a typical porphyritic basaltic texture (« B type ») and mineralogy (olivine + plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides and scarce, or absent hornblende). Gradual textural and mineralogical transitions occur between these two types (« I type ») with the progressive development of hornblende at the expense of olivine and pyroxenes. Mineralogical and chemical studies show no primary compositional correlations between the basaltic xenoliths and their host lavas, thus demonstrating that the former are not cognate inclusions; they are remnants of basaltic liquids intruded into andesitic to dacitic magma chambers. This interpretation is strengthened by the typical calc-alkaline basaltic composition of the xenoliths, whatever their petrographic type (« H », « I » or « B »). The intrusion of partly liquid, hot basaltic magma into colder water-saturated andesitic to dacitic bodies leads to drastic changes in physical conditions. The two components; the basaltic xenoliths are quenched and homogeneized with their host lavas with respect to To;fO2 andpH2O conditions. « H type » xenoliths represent original mostly liquid basalts in which such physical changes lead to the formation of hornblende and the development of a vacuolar and hyalodoleritic texture. The temperature increase of the acid magma depends on the amount of the intruding basalt and on the thermal contrast between the two components. The textural diversity which characterizes the xenoliths reflects the cooling rate of the basaltic fragments and/or their position relative to the basaltic bodies (chilled margins or inner, more crystallized, portions). In addition to physical equilibration (T, fO2) between the magmas, mixing involves: mechanical transfer of phenocrysts from one component to another, in both directions; volatile transfer to the basaltic xenoliths, with chemical exchanges. It is here demonstrated that a short period of time (some ten hours to a few days) separates the mixing event from the eruption, outlining the importance of magma mixing in the triggering of eruption. The common occurrence of basaltic xenoliths (generally of « H » type) in calc-alkaline lavas is emphasized, showing that this mechanism is of first importance in calc-alkaline magma petrogenesis. 相似文献
2.
The Nevado de Toluca, in the middle of the Mexican volcanic belt, has been built by two very dissimilar phases. The first one that lasted more than one million years is mainly andesitic. Numerous massive and autobrecciated lava flows of this phase pass outwards into thick conglomeratic formations. The volume of this primitive volcano represents the essential part of the Nevado. After an intense periode of erosion, the second phase is of very short duration (about 100.000 years) and is dacitic in nature. Three main episode can be distinguished:
- Eruption of important ash and pumice pyroclastic flows related to caldera collapse above a shallow magmatic reservoir.
- Extrusions of several dacitic domes within and outside the caldera with numerous associated «nuées ardentes» surrounding the volcano.
- Plinian eruption leading to widespread pumiceous air-fall and to the opening of the present crater inside the caldera. Extrusion of a new small dacitic dome and late phreatic explosions.
3.
J. Keller 《Bulletin of Volcanology》1969,33(3):942-959
Rhyolitic pumice deposits on some East Aegean Islands are the remnants of a tuff sheet which covered formerly more than 2500 km2. The rhyolites originated by melting of granitic crustal rocks. The petrological properties leading to this conclusion are as follows: -mineralogical composition of the pumice being 60 wt % of glass and 40 % of relictic granitic minerals (quartz, plagioclase, potash feldspar, biotite) in «phenocrysts» up to 1 cm Ø. -phenomena of strong corrosion, resorption and melting of all light colored «phenocrysts» in the rhyolitic pumice. -mineralogical and chemical identity between pumice and granitic xenolites found in great quantity in the pumice tuff. -structurally transitional types between pumice and the granites in different state of melting. -eutectic composition of the pumice glass for a pressure of 2 kb. 相似文献
4.
G. M. Di Paola 《Bulletin of Volcanology》1974,38(3):944-987
Nisyros is a totally volcanic island located at the eastern limit of the quaternary calc-alkaline island arc system of the South Aegean Sea. Its age is rather young since K/Ar dating has given an age of 0.2 m.y. B.P. for one of the oldest outcropping products of this volcano. The volcanological evolution of Nisyros has tentatively been reconstructed as follows:
- after a period (from 3.0? to 0.2 m.y. B.P.) of submarine activity, evidenced by the presence of pillow-lavas and hyaloclastites, the volcano grew above sea level;
- effusive and explosive subaerial activity from different vents built up a complex stratovolcano, probably around 0.2 m.y. B. P.;
- at the end of an intense explosive activity (between 0.2 m.y. B.P. and Present) the top of the volcano collapsed forming a caldera which is still perfectly preserved. A post-caldera activity with eruptions of huge and viscous domes and lava flows of uniform composition, both inside and outside the caldera, concluded this stage of the volcano evolution;
- in historical times, spectacular phreatic explosion craters formed on the caldera floor;
- presently, a large area of the caldera floor is affected by a considerable hydrothermal activity. The hypothesis is formed that Nisyros volcano is not yet extinct.
5.
The rocks outcropping along the Kombolcha-Eloa traverse (western Afar, Ethiopia) can be attributed chiefly to the so-called Oligocenic « Alaji Rhyolitic Series » (32–28 m.y. B.P.) and to the overlying « Termaber Basalts » (28–18 m.y. B.P.), these latter being emitted from a central volcano — Mt. Garfa — rising on the Afar margin and tilted during Miocene. The age and the genesis of the dikes crossing the investigated volcanic succession are discussed in the light of the radiometric ages quoted inMegrue et al. (1972), distinguishing the pre-, the syn- and the post-tilt dikes. The dikes connected with a central volcanism are fed by shallow basins, whereas the others, connected with a fissural volcanism, are to be related to strong tensional movements. 相似文献
6.
J. M. Bardintzeff 《Bulletin of Volcanology》1984,47(3):433-446
A particular nuée ardente type (Merapi-type avalanche nuée) has been defined at the Merapi volcano because of its prominent role in the recent activity of the volcano: gravity plays a significant role during the eruption. However, some other eruption styles occur too producing surges and ashfalls. Three types of tephra, deposited in a very short time-span (15 years) are compared: chemistry and mineralogy are similar, but grain-size analyses are different. There is no vesicular glass, and it is concluded that there is an absence of new magma. This example shows clearly the variety of volcanic styles, with similar chemistry in a very short period. Avalanche nuées from collapsed domes or flows are separated into two types:
- Merapi-typesensu stricto, without any fresh glass, derived from a wholly solidified dome.
- Arenal-type, containing pumiceous glass, derived from a dome, the interior of which is still liquid.
7.
Some trace element data for volcanic rocks found at different levels, from Tertiary to Holocene, in south-eastern Sicily (Iblean Plateau and Mt. Etna) are presented and discussed in the present paper in order to better the information about the origin and relationships of the various rock types. Four groups of volcanic rocks have been recognized on the basis of their major element chemistry: 1) low-K tholeiites, 2) associated alkali basalts to nephelinites of the Iblean Plateau (Upper Pliocene to Lower Pleistocene), 3) the basal subalkaline lavas of Mt. Etna, and 4) the alkalic suite rocks that make up the bulk of the volcano. The distribution of Rb, Sr, Ni, Cr, Co, Cu, REE, Th and Sc suggests:
- an origin of the Iblean magmas by a different degree of partial melting of a Rb-poor and possibly slightly hetereogeneous mantle;
- quite distinct source compositions for the Etnean magmas, relative to those of the Iblean area, on the basis of their Rb and Sr contents;
- an origin of the alkalic rocks of Mt. Etna from independently generated magma(s) rather than by crystal fractionation of the Etnean subalkaline magmas or of a magma having the geochemical features of the Iblean alkali basalts; evidence for this is given by the distribution features of the incompatible elements showing an origin for these rocks from compositionally different parent magmas and/or an evolution under widely variable environmental conditions;
- the primary character for the chemical differences observed in some of the Etnean subalkaline rocks that can be accounted for by different physico-chemical conditions at their source rather than by crystal fractionation processes.
8.
The geological model about volcanism of the Green Tuff geosyncline deduced from the field observations consists of the following processes:
- Dome-shaped uplift with a mean diameter of 30 km.
- Collapse of the central part of the domes forming basins with a mean diameter of 10 km.
- Volcanic activity inside the collapse basins. It is considered that these consecutive processes resulted from the magmatic uplift from a deep part of the crust.
9.
G. Clerc J-P. Décriaud G. Doyen M. Halbwachs M. Henrotte J. Rémy X-C. Zhang 《Surveys in Geophysics》1984,6(3-4):291-304
A campaign of audio-magnetotelluric soundings in the crater of Momotombo has shown that the structure of this volcano is suitable to a surveillance by this method; it has an only central activity, and the resistivity versus the depth decreases suddenly at least 100 times at about 270 m. So, this technical paper describes the equipment designed and built in order to measure every 2 hr the apparent resistivity in a given direction, at seven frequencies regularly spaced from 5 to 312 Hz. The data are preprocessed to fill only 32 bytes: they are - on the one hand, printed locally for the close surveillance; - on the other hand, transmitted by an ARGOS platform and received in Garchy via satellite for searching studies 相似文献
10.
Liparitic volcanism is a typical feature of the orogenic phase giving rise to the Kazakhstan, Middle Asia and Caucasus folded systems. The main characteristics of the liparitic volcanism common to these three regions are the following:
- Geo-structural zonation of the volcanic structures.
- Dismembered Moho surface within the volcanic structures.
- Synchronous, yet independent evolution of liparitic and andesitic volcanisms.
- Ignimbritic character of the liparitic volcanism.
- Lateral petrochemical zonation with some features common to the liparitic and andesitic rock series.
11.
A. B. Kampunzu R. T. Lubala R. Brousse J. P. -H. Caron D. Cluzel L. Lenoble P. J. Vellutini 《Bulletin of Volcanology》1984,47(1):79-103
The December 1981 — January 1982 eruption which started in the Christmas night on the SE side of Nyamulagira, gave the longest historical flow (26 km) representing the highest production rate of this volcano in this century (280×106m3 of erupted magmas in 19 days). This eruption built Rugarambiro, a composite spatter cinder-cone. The ejected lava is a K-hawaiite (kivite) whose basicity decreased during the eruption (first emission: D.I. = 40; last products: D.I. = 35). This chemical evolution is reflected by: --the modal composition of lavas. The first emissions are poor in ferromagnesian phenocrysts (olivine + clinopyroxene: 3%) and rich in plagioclase (12%); the contrary is observed in the last ejected lavas (livine + Cpx: 16%; plagioclase: 1%); --the nature of the crystallizing minerals in the groundmass. In fact, only the first ejections include alkaline feldspars, nepheline and Tiphlogopite; --the glassy phase composition which is more differentiated in the first lavas (D.I. 68–84) than in the last ones (D.I. 42–61). A stratification of the Nyamulagira magmatic chamber is proposed where magmatic differentiation has probably occurred for fractional crystallization. Mineralogical thermobarometers lead to locate this magmatic reservoir at the depth of 6–7 km that we had already hypothesized. The presence of phenocrysts of bytownite, basic chrysolite, diopside and salite indicates a basaltic paragenesis which marks a hawaiitic magma chamber feeding. 相似文献
12.
Volcan Pico de Orizaba, which marks the eastern end of the Trans-Mexican Volcanic Belt, is one of the largest andesitic composite volcanoes in America. It is located above a series of crustal distensive faults making the boundary of the Coast Plains of the Gulf of Mexico from theAltiplano. For this reason, the volcano shows an asymmetry: from the west, its elevation is about 3,000 m whereas on the eastern side it reaches 4,000 to 4,500 m from its base. The Pico de Orizaba is composed of a primitive stratovolcano raised by a recent summit cone. It has been built by three very distinct volcanic and magmatic phases.
- The first one, probably discontinuous effusive activity, lasted more than one million years. It is mainly composed of two pyroxenes-andesites with scarce associated basaltic and dacitic lava-flows. Amphibole is an accessory mineral in most differentiated lavas. On the eastern flank, numerous massive and autobrecciated lava-flows pass outward into thick conglomeratic formations. This effusive phase has built a primitive central volcano and a parasitic cone: the Sierra Negra.
- The second phase is of short duration — about 100,000 years or less — in comparison with the first period. It seems that this period began with the formation of a caldera followed by the extrusion of amphibole dacite domes and the overflow of viscous silica-rich (andesite to dacite) lava flows on the northern flank. An intense explosive activity develops:pelean nuées ardentes are associated with extrusion of the domes; numerous plinian eruptions leading to widespread dacitic pumiceous air-falls are produced by both the central and the adventive volcanoes. This sequence of events is interpreted as the progressive emptying of a superficial chamber containing differenciated magma. A rhyolite flow erupted during this phase.
- The age of the recent phase is better defined. It started 13,000 years B.P. with the eruption of a dacitic ash-flow containing pumice and scoria-bombs. This was such an intense event that products were found 30 km S.E. of the summit, erasing the top of the former volcano and creating a large crater (4–5 km wide). The present cone, of 1,400–1,500 m elevation, grew in this crater. During a period of 7,000 to 8,000 years, the new stratovolcano experienced various important pyroclastic eruptions with a cycle of the order of 1,000 to 1,500 years. The pyroclastic flows (ash, pumice, and bombs) associated with air-fall deposits are of Saint-Vincent type. They present an heterogeneous dacitic and andesitic magma. The dacitic component is similar to previous differenciated materials. On the other hand, the andesitic magma appears somewhat similar to lava-flows from morphologically young cones erupted outside the central vent system. This eruptive cycle can be interpreted as the result of reoccurring injections of deep basic magma within the crustal chamber. For the last 5,000 years the activity of the modern Pico de Orizaba has again been essentially effusive (andesites) with periodic plinian eruptions.
13.
Federico Von Giovanoli André Lambert 《Aquatic Sciences - Research Across Boundaries》1985,47(2):159-178
Results obtained by means of an ultrasonic current meter in the plume of the Rhone river are summarized as follows:
- Currents of Rhone river water entering the lake were clearly discernible up to a distance of about 1 km from the river mouth. The interflow was observed at depth of 10 to 30 m.
- Interflow velocities decreased with increasing distance from the river mouth: from 40cm/s at a distance of 350 m to about 15 cm/s at 1 km.
- Short-term variations of current velocities and directions documented the highly turbulent nature of the interflow.
- The entering river water (inflow direction to the NNW) interfered with a persisting northeastward current of the lake water. At a distance of 1–2 km from the mouth the interflow gradually assumed the same direction, possibly due to deflection by Coriolis forces.
- Current velocities showed considerable variations within a time scale in the order of hours at the same measuring position. The reasons for these fluctuations remain unclear. Possible causes may be lateral oscillations of the entering river water or its deviation by river mouth bars during periods of reduced river discharge. Variations of the discharge alone cannot explain these current fluctuations.
14.
Steam samples from six wells (Colombaia, Pineta, Larderello 57, Larderello 155, Gabbro 6, and Gabbro 1) in a south to north section across the Larderello geothermal field have been analyzed for inorganic and hydrocarbon gases and for oxygen-18 and deuterium of steam. The wells generally decrease in depth and increase in age toward the south. The steam samples are generally characterized by
- Total gas contents increasing south to north from 0.003 to 0.05 mole fraction;
- Constant CO2 (95±2 percent); near constant H2S (1.6±0.8), N2 (1.2±0.8), H2 (2±1), CH4 (1.2±1), and no O2 in the dry gas;
- Presence of numerous, straight chain and branched C2 to C6 hydrocarbons plus benzene in amounts independent of CH4 contents with highest concentrations in the deeper wells;
- Oxygen-18 contents of steam increasing south to north from ?5.0‰ to ?0.4‰ with little change in deuterium (?42±2‰).
- Decreasing gas contents with amount of production because the proportion of steam boiled from liquid water increases with production;
- Synthesis of CH4 from H2 and CO2 with CO2 and H2 produced by thermal metamorphism and rock-water reactions;
- Extraction of C2 to C6 hydrocarbons from rock organic matter;
- Either oxygen isotope exchange followed by distillation of steam from the north toward the south (2 plates at ～220°C) or mixture of deeper more-exchange waters from the north with shallow, less-exchanged recharging waters from the south.
15.
J. L. Cheminee 《Bulletin of Volcanology》1970,34(1):168-195
Concentrations in uranium, thorium and potassium were determined by gamma spectrometry in more than 100 samples of volcanic rocks from the material obtained during the French-Italian expedition in Dankalia (Afar-Ethiopia) (December 1967 – February 1968). These samples were taken from two geographically distinct areas: in the Erta Ale range, for basalts, K: 0,5%, U: 1,3 ppm, Th: 3,3 ppm, Th/U: 2,6. for alkaline rhyolites, U: 4,1 ppm, K: 2,3%, Th: 15,2 ppm, Th/U: 3,7. in the Pierre Pruvost massif, for acid rocks, K: 3,9%, U: 3,6 ppm, Th: 13,5 ppm, Th/U: 3,8. in the Borawlu centre, K: 2,8%, U: 4,6 ppm, Th: 14,7, Th/U: 3,4 for the rhyolites, K: 0,6%, U: 0,9 ppm, Th: 3,1 ppm for the basalts. on the Ethiopian high lands, for the Adigrate basaltic trap rocks, K: 0,6%, U: 0,6 ppm, Th: 1,1 ppm, Th/U: 1,7 and for the Axum zone of phonolitic pinnacles, K: 3,9%, U: 2,7 ppm, Th: 14,9 ppm, Th/U: 4,9. The Th/K and U/K ratios are particularly high compared with the usual data, except for the Pierre Pruvost massif which seems to be different from the others. The difference of Th/U ratios for basic rocks and acid rocks is meaningful. For the magma which gave the volcanism of the studied area, one can think of an evolution more especially connected with transfers of volatile elements. The strong correlations existing between potassium, uranium and thorium probably indicate the presence of groups which are distinct from a geochemical point of view in the Dankalia lavas. 相似文献
- the danakile depression (Erta Ale range, Pierre Pruvost massif, Alayta, Afdera, Borawlu).
- Ethiopian high lands (Adigrate — Axum).
Dash
16.
B. B. Turner 《Bulletin of Volcanology》1970,34(4):777-791
One of the problems associated with massif-type anorthosite in high grade metamorphic terranes is the absence of anorthositic extrusives. However, if anorthosite formed by crystal segregation, volcanic equivalents of the final fluid differentiate might occur. Massive, stratiform hornblende-garnet granitic gneisses occur between two metasedimentary sequences in asynclinorium south of the main Adirondack anorthosite massif and north of the Thirteenth Lake anorthosite dome. The granitic gneisses are divided into three stratigraphic units on the basis of mineralogy. The entire stratigraphic configuration has been traced 70 kilometers laterally, and the interpretation of the literature indicates that it may extend for over 100 kilometers. Earlier workers considered the granitic gneisses to be sill-like intrusives. The following evidence suggests a volcanic origin for these gneisses:
- A thin (3 to 5 meters) but continuous zone of cale-silicates and quartzite occurs within the sequence of granitic gneisses at the same horizon for some 25 kilometers laterally and does not appear to be truncated nor intruded by the gneisses. This observation virtually precludes an intrusive origin.
- 2V, of plagioclase (An24,25) in granulated, augen-shaped zones in the granitic gneisses variesfrom 109° to 120°, whereas 2V, for groundmass plagioclase (also An20, 25) is 84° to 88°. According to the data of Slemmons, the 2V of the granulated augen is characteristic of volcanic (high temperature) plagioclase and that of the groundmass is characteristic of plutonic (low temperature) plagioclase.
- The great lateral extent of the granitic gneisses suggests that the materials were deposited as volcanic ash or pyroclastics.
- The stratigraphic coherency of metasedimentary sequences and granitic gneisses indicates that the gneisses are an integral part of the stratigraphy.
17.
The simultaneous eruption of Mt. Pelée, Martinique and Soufrière, St. Vincent are regarded as the first recognized examples of Pelean-type and St. Vincent-type pyroclastic eruptions. Both produced nuées ardentes, the former usually laterally directed because of the presence of a dome and the latter vertically directed from an open crater. Both volcanoes have subsequently erupted for a second time this century. The 1902–05 and 1929–32 eruptions of Mt. Pelée produced andesite lava of almost identical composition and mineralogy. Both contain two generations of plagioclase, orthopyroxene, Fe-Ti oxide, corroded brown amphibole and olivine rimmed by pyroxene. In contrast, the Soufrière material is more basic in composition varying from basaltic andesite to basalt in 1902–03 and basaltic andesite in 1971–72. The Soufrière material contains two generations of plagioclase (with those of 1971–72 having additional zones of labradorite), clinopyroxene, orthopyroxene, olivine and Fe-Ti oxide. The pyroclastic deposits are strikingly different, those from the Pelean-type eruption are termed «block and ash deposits» being characterised by poorly vesicular lava blocks up to 7 m in diameter, while the St. Vincent-type eruption produced «scoria and ash deposits» containing vesicular ropey blocks or bombs no larger than 1 m in diameter. The differences in styles of eruption are attributed to differences in viscosity and mechanism of eruption of the magmas. Stratigraphic studies of Mt. Pelée reveal that the volcano has produced basaltic andesite scoria and ash deposits from St. Vincent-type eruptions. It is concluded that the recent eruptions of Pelée tapped a deep level magma during both eruptions releasing magma of similar composition, while the 1971 Soufrière magma is thought to be a remnant of the 1903 basaltic magma which remained at a high level within the volcano where it underwent enrichment in plagioclase and loss of olivine and oxide. 相似文献
18.
László Szarka 《Surveys in Geophysics》1987,9(3-4):287-318
The paper gives a summary of geophysical aspects of man-made electromagnetic noise in the Earth as follows: - EM distortion effects of man-made constructions below and over the Earth's surface defined as ‘passive-noise’, - field observation of EM disturbances due to ‘active’ man-made sources, - EM source mechanism of some important active sources from a geophysical point of view, - efforts in order to improve the signal-to noise ratio by instrumental, methodological and dataprocessing ways, - application of man-made EM noise for geophysical prospecting. The paper is based on world-wide EM noise survey studies published mainly in geophysical journals. 相似文献
19.
An extensive set of samples representative of the evolution of Etna since the beginning of its activity in the Quaternary has been analyzed for trace element contents by neutron activation on whole rock samples. Major element data were taken fromTanguy (1980). Several petrogenetic processes have played a role in the observed chemical variations: binary trace element plots form linear arrays extrapolating to the origin for most second row and higher transition elements and the rare alkali metals thus indicating their hygromagmaphile (HYG) character; 3d transition elements show two types of behavior when plotted against an HYG element. Concentrations remain relatively constant for the less evolved rocks in the range from tholeiite through transitional basalts to alkali basalts. On the contrary, concentrations drop rapidly in the series evolving from alkali basalts to trachytes; in addition to the latter two general trends, geochemical regularities specific to Etnean lavas are born out. Rare alkali metals, as well as potassium, for instance, show a trend of abnormal enrichment in the most recent historical eruptions. The recurrence in time of rather primary magmas indicates that the petrogenetic evolution of the lavas cannot be explained by the differentiation of a single batch of mantle derived magma. However, the chemical composition of the mantle source has remained relatively homogeneous with a nature very near the chondritic one. Mineralogically, the source must differ from that for oceanic basalts to account for Th/Ta differences. Early tholeiitic primary magma evolving into later alkali basalts is adequately modeled by variations of the degree of partial melting of a closed system source. Highly evolved products have episodically been emitted as a result of extensive crystal fractionation, during which no basalts are erupted. These episodes are abruptly interrupted by the eruption of basalts presumably indicating recharge of the magma plumbing system by pristine mantle derived material. Selective contamination in alkali elements from the sedimentary basement is also indicated by the data. The presence of partially digested sedimentary xenoliths is an additional argument for this process. 相似文献
20.
Bevan M. French 《Bulletin of Volcanology》1970,34(2):466-517
Recent investigations indicate the importance of meteorite impact as a process which has operated throughout geologic time to produce numerous originally circular structures as much as 50 km in diameter. One such structure, at Sudbury, Ontario, is associated with large volumes of internally derived igneous rock. Geological and experimental studies have demonstrated that rocks subjected to intense shock waves produced by hypervelocity meteorite impacts and by nuclear or chemical explosions develop distinctive and uniqueshock-metamorphic features, including: (1) high-pressure minerals such as coesite and stishovite; (2) crystal lattice deformation features such as isotropic feldspar (maskelynite) and « planar features » (shock lamellae) in quartz; (3) ultra-high-temperature reactions not produced by normal geological processes, such as decomposition of zircon to baddeleyite and melting of quartz to lechatelierite. These petrographic features, currently regarded as unequivocal evidence for meteorite impact, can be preserved and recognized even in very old and deeply eroded structures. Such features have now been observed in more than 50 « crypto-explosion » structures ranging in size from 2 km to more than 60 km in diameter. The recent discovery of shock-metamorphic features in rocks of the Sudbury structure, Ontario, indicates that this old and complex structure was also produced by a large meteorite impact. Petrographic shock effects are widespread in inclusions of « basement » rock in the Onaping « tuff », a unit now regarded as afallback breccia deposited in the original crater immediately after impact. Similar shock effects also occur in the footwall rocks around the basin, associated with shatter cones and unusual Sudbury-type breccias. Study of Sudbury specimens has establishedgrades of progressive shock metamorphism comparable to those recognized at younger impact structures (Brent, Ontario; Ries basin, Germany). Igneous activity associated with known meteorite impact structures takes two forms:
- direct production of impact melt. At many structures (e.g., Brent, Ontario; Lake Mien, Sweden; Clearwater Lakes and Manicouagan, Quebec), breccias containing shock-metamorphic features occur with «sills» and «dikes» of fine- to medium- grained crystalline igneous rock. Such units, previously regarded as internal volcanic products, now appear to have been formed by complete fusion, injection, and rapid crystallization of large volumes of target rock during the impact event.
- emplacement of internally derived magma. The presence of the clearly internally-derived Nickel Irruptive within the Sudbury basin indicates that large meteorite impacts may also control the emplacement of internally-generated magmas through « unroofing » or by the production of deeply-extending zones of weakness below the crater.