Present Erosion Surface Research Articles

Precambrian structure and Late Pleistocene strike-slip tectonics around mega (southern Ethiopia)

Regional structure of Precambrian rocks suggests a tectonic style of large scale upright open synforms and antiforms, with axial-trace trending and fold axes plunging to the northwest. We propose that Precambrian structural highs, emerging through the present erosional surface, are northwest-southeast trending tectonic wedges formed by strike-slip faults. During the late Jurassic uplift of the Somali plateau, these faults initiated and were reactivated in the Late Pleistocene period. A right-lateral component is deduced from regional geometry of fault-pattern, spatter cones and maars alignment. In spite of the spectracular morphology shown by this fault system, geological considerations suggest that faults have reactivated ancient structures.

7131. Two-stage vacuum pumps for high working pressures and high water vapor tolerances: W Kaiser,J Vac Sci Technol, A8, 1990, 2761–2763

Impact melt and country rocks from the 23 km diameter Lake St. Martin structure in Manitoba were analysed for major and trace element abundances and for Rb-Sr isotopic compositions. Our aim was to better understand the formation of the melt rocks in this probable impact structure, to search for traces of the meteoritic projectile, to date the melt sheet in order to improve the chronological record of impact events on the Earth's surface, and to constrain the target, from which the melt rocks could have formed.Major element analyses are consistent with the observation by Simonds and McGee (1979) that many of the melt rocks from Lake St. Martin are chemically homogenized, to a degree similar to that of melt bodies from other impact structures. However, some samples are of slightly different composition. This can be explained by post-impact hydrothermal alteration (particularly near the present erosion surface), chemical exchange between melt-bearing breccia and basement rock near the bottom of the melt sheet, or limited mixing of different target lithologies. So-called “pseudotachylites” from the central uplift of the structure are chemically diverse and do not always represent the composition of their host rock. We conclude that such samples are not pseudotachylite but probably represent injected impact breccia.REEs and other lithophile elements are well homogenized in the melt in comparison with the country rock variation. There is a slight enrichment of the siderophile elements Co and Au and of Cr in the melt rocks, but further analyses are required to verify whether this is caused by a small contribution from the meteoritic projectile. The melt rocks are also homogenized isotopically, when compared with the spread of isotopic ratios determined for the country rocks. The melt rock isotopic composition, however, can only be achieved by mixing of various granitoid country rock types with another component of high 87Rb86Sr ratio. Furthermore, the melt rocks are isotopically reequilibrated: whole rock Rb-Sr isotope data yield an age of 212.4 ± 43 Ma (ISr = .7132 ∓ 3) at 2σ level. Whole rock and mineral separate data yield an age of T = 219 ± 32 Ma (ISr = .7131 ∓ 3). This age, consistent with previously reported K-Ar ages of 200 ± 25 and 250 ± 25 Ma, is thought to be the best approximation to the age of the melt rocks and thus of the impact event. However, the 2σ error of 32 Ma is too high to allow this age to become a significant addition to the data base of well-constrained impact ages for analysis of periodicity in the cratering record. An “age” of T = 2785.3 ± 157 Ma (2σ) for ISr = .6997 ∓ 11 was obtained for the basement rocks of the Lake St. Martin structure, but the true meaning of this “age” is still unclear. While the age obtained is comparable to other Superior Province data, it appears that the Lake St. Martin country rock data define a mixing line.The new chemical and isotopic results are consistent with an impact origin for the Lake St. Martin structure. Harmonic least-squares mixing calculations suggest that the melt rocks were formed from ca. 80% monzonitic and 15% granitic basement plus a small addition of ca. 6% carbonate rock. This is in good agreement with the results by Simonds and McGee (1979) and favors their suggestion that carbonaterich target rocks, even when they form the upper portion of the target stratigraphy, do not significantly take part in impact melt formation but are largely removed in a CO2-rich vapor phase.

Evolution of aqueous-carbonic fluids during contact metamorphism, wall-rock alteration, and molybdenite deposition at Trout Lake, British Columbia

The Trout Lake deposit is associated with a granodiorite-tonalite stock that intruded greenschist facies calcareous and pelitic metasedimentary rocks. It is typical of the fluorine-deficient or granodiorite class of molybdenum deposits. The stock is surrounded by a contact metamorphic aureole in which the mineral assemblages record evidence of fluid infiltration. At the present erosional surface, temperatures were below 400 degrees C and X (sub CO 2 ) in the calcareous rocks was <0.01. The confining pressure is estimated to have been 2 kbars. Contact metamorphism was followed successively by skarn and potassic alteration. The latter developed as biotite halos around quartz-albite veins in the metasedimentary rocks and as K feldspar halos around quartz veins in the intrusive rocks. Fluid inclusions in veins formed during potassic alteration are dominantly aqueous and have low to moderate salinities. The ensuing quartz-feldspar-muscovite and muscovite-ankerite alteration events were relatively pervasive. Aqueous-carbonic fluid inclusions are commonly observed in quartz-feldspar-muscovite-altered samples and are the dominant fluid inclusion type present in veins associated with muscovite-ankerite alteration. The aqueous fluid inclusions from all stages of alteration homogenize at temperatures between 170 degrees and 310 degrees C, with a peak at 250 degrees C. The aqueous-carbonic fluid inclusions homogenize to the H 2 O phase between 240 degrees and 340 degrees C. Oxygen isotope compositions for muscovite-quartz pairs yield temperatures of 370 degrees to 400 degrees C for quartz-feldspar-muscovite and muscovite-ankerite alteration. This indicates that the fluids were trapped at approximately 1,400 to 1,700 bars, i.e., above the two-phase region in the H 2 O-CO 2 -NaCl system. Calculated delta 18 O values for the fluid range from 8.0 to 8.4 per mil and, taken in conjunction with pressure and temperature estimates, suggest a magmatic and/or modified ground-water source. Phase relations of alteration minerals suggest that pH decreased during alteration. Mineralization is spatially and temporally associated with quartz-feldspar-muscovite alteration; molybdenite is typically intergrown with muscovite that replaced alkali feldspar. This textural relationship is interpreted to indicate that molybdenite deposited in response to decreases of temperature and/or pH. The common occurrence of these intergrowths in other fluorine-deficient and -enriched deposits suggests that the controls of mineralization for these deposits were similar to those at Trout Lake.

Geochemistry of the melt and country rocks of the Lake St. Martin impact structure, Manitoba, Canada

Impact melt and country rocks from the 23 km diameter Lake St. Martin structure in Manitoba were analysed for major and trace element abundances and for Rb-Sr isotopic compositions. Our aim was to better understand the formation of the melt rocks in this probable impact structure, to search for traces of the meteoritic projectile, to date the melt sheet in order to improve the chronological record of impact events on the Earth's surface, and to constrain the target, from which the melt rocks could have formed. Major element analyses are consistent with the observation by Simonds and McGee (1979) that many of the melt rocks from Lake St. Martin are chemically homogenized, to a degree similar to that of melt bodies from other impact structures. However, some samples are of slightly different composition. This can be explained by post-impact hydrothermal alteration (particularly near the present erosion surface), chemical exchange between melt-bearing breccia and basement rock near the bottom of the melt sheet, or limited mixing of different target lithologies. So-called “pseudotachylites” from the central uplift of the structure are chemically diverse and do not always represent the composition of their host rock. We conclude that such samples are not pseudotachylite but probably represent injected impact breccia. REEs and other lithophile elements are well homogenized in the melt in comparison with the country rock variation. There is a slight enrichment of the siderophile elements Co and Au and of Cr in the melt rocks, but further analyses are required to verify whether this is caused by a small contribution from the meteoritic projectile. The melt rocks are also homogenized isotopically, when compared with the spread of isotopic ratios determined for the country rocks. The melt rock isotopic composition, however, can only be achieved by mixing of various granitoid country rock types with another component of high 87Rb 86Sr ratio. Furthermore, the melt rocks are isotopically reequilibrated: whole rock Rb-Sr isotope data yield an age of 212.4 ± 43 Ma ( I Sr = .7132 ∓ 3) at 2σ level. Whole rock and mineral separate data yield an age of T = 219 ± 32 Ma ( I Sr = .7131 ∓ 3). This age, consistent with previously reported K-Ar ages of 200 ± 25 and 250 ± 25 Ma, is thought to be the best approximation to the age of the melt rocks and thus of the impact event. However, the 2σ error of 32 Ma is too high to allow this age to become a significant addition to the data base of well-constrained impact ages for analysis of periodicity in the cratering record. An “age” of T = 2785.3 ± 157 Ma (2σ) for I Sr = .6997 ∓ 11 was obtained for the basement rocks of the Lake St. Martin structure, but the true meaning of this “age” is still unclear. While the age obtained is comparable to other Superior Province data, it appears that the Lake St. Martin country rock data define a mixing line. The new chemical and isotopic results are consistent with an impact origin for the Lake St. Martin structure. Harmonic least-squares mixing calculations suggest that the melt rocks were formed from ca. 80% monzonitic and 15% granitic basement plus a small addition of ca. 6% carbonate rock. This is in good agreement with the results by Simonds and McGee (1979) and favors their suggestion that carbonaterich target rocks, even when they form the upper portion of the target stratigraphy, do not significantly take part in impact melt formation but are largely removed in a CO 2-rich vapor phase.

The Acadian thermal history of western Maine

ABSTRACT Following the Middle Devonian Acadian deformation an extensive belt of high grade metamorphism was formed in New England. In south‐western Maine, at the northern end of this belt, there occurs a transition along the strike from regional low‐pressure/high‐temperature metamorphism to contact metamorphism in low‐grade rocks. Petrological studies indicate that this transition occurs along a surface plunging to the north‐east at about 3.5°, with respect to the Middle‐to‐Late Devonian erosion surface. In addition, detailed petrological mapping has defined a history of temporally separate, localized metamorphic events associated with plutonism and occurring at increasingly deeper levels to the south‐west. Geochronological studies constrain ambient temperatures in the transition zone at the time of metamorphism to be less than 300° C in the north‐east and between 350° C and 500° C in the south‐west. They also establish a pattern of diachronous cooling due to differential uplift and erosion, with cooling occurring later and most rapidly to the south‐west. Geophysical evidence suggests that along with this spatial variation in metamorphic style the shapes of the plutons in Maine undergo a transition from laterally extensive sheet‐like bodies in the high grade terrane to more equant‐shaped bodies in the low‐grade terrane. Using the results of these petrological, geochronological and geophysical studies, as well as those of stratigraphical and structural studies we construct a thermal model for the transition zone. The model suggests that the Acadian metamorphism in south‐western Maine is a result of deep‐level contact metamorphism near laterally extensive granitic sills dipping to the north‐east with respect to the present erosion surface. The plutons themselves are interpreted to be a result of lower crustal melting in response to crustal thickening in the presence of normal or slightly augmented mantle heat flux.

Age, petrology and mineralisation associated with two Neogene intrusive types in the Eastern Highlands of Papua New Guinea

Numerous Neogene porphyritic basic to intermediate intrusive rocks crop out as batho­ liths, stocks and dyke swarms in the eastern highlands of Papua New Guinea between 144 °E and 146°E. Their radiometric ages range from 18 Ma to 7 Ma, and are consistent with known more definitive stratigraphic relationships. Our work has shown that within this time range, two distinct phases of plutonism occurred and that the second phase from 9 Ma to 7 Ma was often associated with magmatic­ hydrothermal Cu-Au-Ag mineralisation. We have named the earlier phase Akuna-type, and the later phase Elandora-type. Akuna-type intrusives tend to form large complexes (eg. Akuna Intrusive Complex, Bismarck Intrusive Complex) up to 800 km2 in outcrop area, displaying a wide variety of fractionated compositions from pyroxenite, gabbro, diorite to granodiorite. Other smaller intrusions (Oipo and Kimil Diorites) are more homogeneous. In contrast to Akuna-type, Elandora-type intrusives (Elandora Porphyry, Yandera porphyries) generally form microdioritic, often tabular stocks less than 10 km2 in area, dykes and dyke swarms, some of which intrude Akuna-type masses. Many bodies and parts of individual bodies display propylitic alteration assemblages, with argillic, phyllic and silicic alteration being locally dominant. Elandora-type intrusives are apparently associated with either areas of outcropping basement or areas underlain at shallow depth by basement. Exposure of many Elandora-type intrusions at the present erosion surface is commonly a function of two fault sets trending 120° and 040°, which intersect to form elevated fault-bound basement blocks and areas of cover underlain at shallow depth by basement. Some Elandora-type intrusions occur in erosional basement highs. The two intrusive types have calc-alkaline affinities. Analysed Elandora-type intrusives have a narrow compositional range between 60 and 70% Si02 compared with the wide range of Si02 values shown by analysed Akuna-type intrusives. Elandora-type analyses plot on or adjacent to Akuna-type major element variation diagram trend lines at 60% Si02• However, at 60% Si02, Elandora-type intrusives have higher Sr (700 ppm) and lower K,O (< 2%) than Akuna-type intrusives. These empirical geochemical features, coupled with field criteria, serve to distinguish the two intrusive types.

Post-Aphebian uplift deduced from remanent magnetization, Yellowknife area of Slave Province

Remanent magnetization in five Indin, three Dogrib, two Archean, and one Mackenzie dyke contact zone(s) from the Yellowknife area of the Canadian Precambrian Shield has been studied in order to establish the ambient temperature of the host rock and the depth of burial of the present erosion surface at the time of intrusion. A positive baked-contact test for an Aphebian Indin dyke demonstrates the primary nature of the dyke magnetization. From this contact, eight individual specimen determinations of the ambient host-rock temperature at the time of intrusion yield an average of 211 °C, with a standard deviation of 18 °C and probable error of ± 36 °C. Assuming a paleogeothermal gradient of 44 °C/km (± 30%) the estimated depth of burial of the present erosion surface is [Formula: see text]. None of the remaining contacts yield estimates of the depth of burial. However, the results from the three Dogrib contacts illustrate complications such as chemical overprinting that must be considered when analysis is made of magnetization in igneous contact zones. Thus, only one spot reading of the depth of burial of the area has been obtained. Correlation of the Indin result with a result from the Matheson area of Ontario and with stratigraphic indicators is uncertain because of large intervening distances and the occurrence of younger structural zones.

Porphyry copper and tourmaline breccias at Los Bronces-Rio Blanco, Chile

The Los Bronces-Rio Blanco deposit is located on the west side of the Andes in central Chile about 69 km from Santiago. Los Bronces is a breccia complex superimposed on the west side of an earlier major porphyry copper system. The Rio Blanco mine currently exploits the north-central part of this porphyry deposit and has started operating a large copper-bearing tourmaline breccia, Sur-Sur, about 2 km south of the present mine.The Los Bronces-Rio Blanco deposit was formed on the east side of the San Francisco batholith. This intrusion is strongly peraluminous and has a calc-alkaline composition with an alkali-calcic affinity. The batholith took a minimum of 11.5 m.y. to form from the early Miocene (20.1 m.y.) to the late Miocene (8.6 m.y.). The porphyry copper mineralization, alteration, and copper tourmaline breccias were formed over a period of at least 2.5 m.y. between 7.4and 4.9 m.y. ago.A postmineral volcanic neck or diatreme at La Copa erupted within, and removed a large segment of, the northern part of the porphyry copper system, thus marking the last evidence of magmatic activity in the area. K-Ar age determinations of biotites indicate the diatreme erupted in the early Pliocene between 4.9 and 3.9 m.y. ago.The prebreccia porphyry system exhibits propylitic, sericitic, silicic, and potassic alteration. A unique alteration feature of this system is the replacement of mafic minerals by specularitc and/or tourmaline within the propylitic zone. The porphyry system contains disseminated and stockwork copper-iron-molybdenum sulfide mineralization within an area of about 12 km 2 .Los Bronces is composed of at least seven different copper-bearing tourmaline breccias that form one large contiguous kidney-shaped body about 2 km long and 0.7 km wide, at the present erosion surface. The breccia body crops out at elevations between 4,150 and 3,450 m. The various breccias are characterized by their locations, matrices, clasts, shapes, types, and degrees of mineralization and alteration. The breccias are usually monolithic but in some cases are bilithic or heterolithic with most clasts consisting of quartz monzonite or andesitc with locally minor amounts of quartz latite porphyry, monzodiorite, and vein quartz. The breccia matrices consist of variable amounts of quartz, tourmaline, specularitc, anhydrite, pyrite, chalcopyrite, bornitc, molybdenite, sericite, chlorite, and rock flour.The seven different breccias types are identified from oldest to youngest as Ghost, Central, Western, Infiernillo, Anhydrite, Fine Gray, and Donoso. The breccia complex has sharp contacts with the surrounding intrusive rocks and andesites. Internally, the breccia contacts are locally well defined, but elsewhere they coalesce, interfinger, or display gradational contacts. Breccias at Los Bronces are interpreted as being emplaced explosively, followed by collapse after pressure release of hydrothermal fluids.The primary mineral distribution is best known in the Donoso breccia which has been the center of mining activity since its discovery in 1864. In spite of the coarse and irregular nature of the sulfides in the matrix, chalcopyrite, pyrite, and specularitc at the 3,670-m, open-pit operating level show a tendency to be distributed in irregular shells in which one of the three minerals predominates in any one shell. The transitions between shells are rapid. Several semiellipsoidal shells of alternating high and low copper grades are also apparent from the copper distribution of underground level 3640 and from various cross sections. The shells are approximately vertical, subparallel to the Donoso breccia contacts, which dip inward.Secondary enrichment enhanced the primary grade in the southern two-thirds of the Los Bronces breccia complex and in much of the surrounding porphyry copper system. The degree and depth of enrichment are a function of breccia and fracture permeability and extend to a depth of more than 500 m in certain favorable sectors. The shape and depth of the enrichment blanket and overlying leached capping suggest-that the enrichment process is related to the present ground-water regime and is still active.

The Acadian Thermal History of the Merrimack Synclinorium in New Hampshire

The Acadian structural history of the Merrimack Synclinorium involves a phase of nappe emplacement and thrusting at 400 m.y.B.P. followed by a second thickening event before 370 m.y.B.P.; at the end of this phase the present erosion surface was buried at pressures of 6 to 8 kbar and subsequently experienced peak metamorphic temperatures in the range of 500°C to 650°C. The thermal history of the Acadian metamor-phism of the Merrimack Synclinorium is examined in a well-studied area of central New Hampshire to determine the conditions required for the high grade metamorphism of the region. Constraints are provided by metamorphic pressure estimates, by information on the erosional history, and by present day heat flow determinations in the Synclinorium. One-dimensional heat transfer calculations show that the high grade metamorphism could readily have been produced during thermal relaxation of the thickened continental crust under conditions of normal continental heat flow. These conditions also permit the melting of the lower continental crust between 50 and 100 m.y. after the end of crustal thickening, which could account for the generation of some of the post-tectonic Concord Series plutons (370-275 m.y.B.P.). If portions of the Acadian orogen experienced rapid erosion (faster than about 1 mm/yr) at the end of crustal thickening, then they would achieve peak metamorphic temperatures 100°C to 150°C lower than calculated here; this perhaps was the case for the Bronson Hill Anticlinorium to the west. Equally, if it can be shown that the Merrimack Synclinorium was eroded rapidly at the end of the Acadian, then an augmented heat supply during the metamorphism is required. More data on the rates of Paleozoic uplift and on the Pressure-Temperature-Time paths followed by these rocks are needed in order to place tighter constraints on the thermal history of the Acadian metamorphism.

The ages of fractures in the Eye-Dashwa pluton, Atikokan, Canada

Ages of fractures in the Eye-Dashwa granite pluton north of Atikokan, Ontario, are estimated from analyses of their filling materials (pegmatite, aplite, hornblendeporphyry, epidote, chlorite, gypsum, diabase, iron oxides, carbonate and clays). The analyses include radiometric age determinations of filling materials, comparison of crystallization temperatures of filling materials with the expected temperature of the pluton through time, and paragenetic relations derived from textures. The major fractures (primarily epidote and chlorite-filled) were produced by penetrative deformation during cooling immediately after intrusion of the pluton. From 2.6 Ga to 2.4 Ga they transected the pluton and largely defined the present overall fracture network. Less severe penetrative deformation occurred about 1.1 Ga, when a few diabase dikes and small gypsum fractures developed. A second source of fracturing appears to have existed through most of geological time, and it reopened segments of old fractures within about 400 m of the present erosional surface. It is unknown whether the near-surface, or supergene, fractures formed more or less continuously through time or in distinct episodes, such as during glacial periods. Open fractures, which are most common in the near-surface layer, tend to transmit groundwater, but fracture sealing due to the accumulation of filling materials seems to occur with time. Underground excavations such as nuclear fuel waste disposal vaults, would be less susceptible to the incursion of groundwater, if they were located below the near-surface layer.

Uplift deduced from remanent magnetization: Sudbury area since 1250 Ma ago

Two contacts between Sudbury norite and northwest-trending diabase dikes and two contacts between the overlying micropegmatite and northwest dikes were investigated in order to estimate the depth of burial of the present erosion surface at the time of dike emplacement. A zone of hybrid paleomagnetic direction representing the vectorial sum of an older host component and an intrusion component of decreasing highest blocking temperature and intensity with distance from the intrusion was sought. Subtracting the calculated thermal effect of the intrusion from this highest blocking temperature yields the temperature of the host at the time of magma emplacement. Dividing this host temperature by an estimated paleogeothermal gradient yields the burial depth of the present erosion (or sampling) surface at the time of magma emplacement. Remanence direction in one of the dikes and norite contact zones is not typical for the Sudbury dike swarm of 1250 Ma age, and this contact is not further considered. An earlier published result for a norite-dike contact was reconsidered because of complicated dike geometry and included in this study. In one of the four usable contacts the hybrid zone is represented by three samples, in another by one sample, and in the remaining two only the contact zone width could be used. The final host temperature results are based on 4 individual calculations and show fair consistency with mean values of 287°C (s.d. 13°) and 267°C (s.d. 11°) calculated without and with a correction for viscosity of the host remanence respectively. Using a gradient of 26°C/km for 1250 Ma ago indicates a burial depth of 9.5 ± 2km at that time. The fair consistency encourages the use of the method to deduce quantitatively the history of vertical motions of Precambrian terranes, the detail obtained being dependent on the presence of hybrid zones and of intrusions of various ages.

Uplift estimated from remanent magnetization: Munro area of Superior Province since 2150 Ma ago

A method for determining depth of burial from remanent magnetization was applied to three igneous contacts of two different ages in Munro Township of the Superior Province of the Canadian Shield. Sampling profiles perpendicular to the contact of a 2150 Ma diabase dike, one collected in an older (2690 Ma) diabase dike and a second in gabbro country rock, yielded ambient temperatures for the present erosion surface at the time of dike emplacement of 219 ± 23 and 181 ± 7 °C, respectively. A third sampling profile perpendicular to a 2690 Ma dike failed to provide a usable magnetic contact zone.A secondary magnetic component with shallow inclination and easterly declination was detected in individual samples collected from both dikes as well as from the surrounding country rock. Converging remagnetization circles indicate that this component, acquired some time after emplacement of the younger (2150 Ma) dike, has the direction D = 70°, I = −39° and corresponding paleopole 39°E, 4°S.

The magmatic evolution of the Vermilion greenstone belt, NE Minnesota

Abstract The Vermilion greenstone belt, a nearly linear belt of steeply dipping, low grade, metavolcanic and metasedimentary rocks and adjacent areas in northeastern Minnesota compose a typical Archean (∼2.75 Ga) greenstone—granite association. In the western part of the belt, four formally designated units have been defined, which together form a complex volcanic—sedimentary pile that apparently formed through the coalescence of petrologically distinct volcanic centers. The oldest supracrustal unit, the lower Ely Greenstone, consists of calc-alkaline basalts, andesites, heterolithic agglomerates and tuffs deposited in a relatively shallow subaqueous environment. The lower part of this unit apparently was destroyed by emplacement of a granitic batholith. Following a period of volcanic quiescence, during which the Soudan Iron Formation and associated rocks were deposited, basaltic (tholeiitic) volcanism commenced and produced a subaqueous shield volcano (upper Ely Greenstone); the basalt interfingered laterally with calc-alkaline, dominantly dacitic material. The termination of basaltic volcanism was followed by a period of eruption of largely dacitic volcanic material accompanied by rapid erosion and turbidite deposition represented by the Lake Vermilion Formation and Knife Lake Group. This period was followed by renewed volcanism that changed laterally from dominantly calc-alkaline lava, tuff and breccia (felsic member of Newton Lake Formation) to dominantly basaltic lava and mafic—ultramafic intrusions (mafic member of Newton Lake Formation). Within the mafic sequence five petrologically distinct basalt types were erupted. Assuming that the present erosion surface in the Vermilion greenstone belt is a representative cross-section of the original supracrustal sequence, the volume of calc-alkaline, dacitic volcanic material appears to exceed that of basaltic volcanic rocks. If current petrologic models proposed for these rocks are valid, a dynamic tectonic environment capable of generating and rapidly recycling large volumes of volcanic and derived sedimentary material is required to account for the development of the Vermilion greenstone belt.

Positive test of the paleomagnetic method for estimating burial depth using a dike contact

Detailed magnetic measurements were made on specimens from profiles across the contact zones of two unmetamorphosed Miocene diabase dikes. The dikes cut Cretaceous plutonic rocks of the Coast Crystalline Complex near Bella Bella, British Columbia. The dikes feed flat-lying basalts that outcrop nearby. This indicates post-Miocene uplift is less than 1 km in the area. Alternating field demagnetization, magnetic susceptibility, thermal demagnetization, and magnetic force balance measurements were made on the specimens. From them, the distance from the contact and the maximum reheated temperature of the magnetic hybrid zone were determined. Results are reported for the Thorburne Island contact only because the other contact had unsuitable remanence characteristics. Using standard conduction heat flow equations the ambient temperature of the host rock at the time of intrusion was found to be 18 ± 5 °C giving a determined depth for the present erosion surface at the time of intrusion of 0.3 ± 0.1 km. This is the first positive test of the magnetic paleogeobarometric method.

The Margnac and Fanay uranium deposits of the La Crouzille District (western Massif Central, France); geologic and fluid inclusion studies

The uranium deposits of the La Crouzille district which are the subject of this study (Margnac and Fanay mines) are located in the Saint-Sylvestre granitic plutonic complex, approximately 20 km north of Limoges (Haute-Vienne). The Saint-Sylvestre pluton in its entirety and the Saint-Sylvestre granite in particular were first reinterpreted on the basis of new observations made in the mines along the contact zone between the Brame and Saint-Sylvestre granites (Peny area). A similar origin for members of the Saint-Sylvestre plutonic complex, recognized earlier by Chenevoy (1958), was confirmed. Structural information (synthesized by Autran and Guillot, 1974) and isotopic data (Duthou, 1977) are reinterpreted in a new tectonic framework. The genesis of the leucogranitic Saint Sylvestre pluton is believed to be comparable to that of Himalayan leucogranites.The existence of two granitic facies (Brame and Saint-Sylvestre) is due to magmatic and deuteric processes (muscovitization, albitization) operating within the same plutonic complex. These phenomena obliterated the primary features and caused a reequilibration of the initial catazonal paragenesis (sillimanite-orthoclase) to mesozonal conditions (muscovite-biotite).The emplacement of the two granitic facies took place around 350 to 360 m.y. The age determination of 315 m.y. is now interpreted to be the result of partial reequilibration of Rb-Sr isotopes.The intrusion of lamprophyres at 285 m.y. corresponds with brittle fracture tectonic activity, in contrast to the earlier plastic deformation. A similar sequence of magmatic and tectonic styles is found elsewhere in the Hercynian chain. Two fracture systems are present (EW-NS and NW-SE). Lamprophyric magmatic activity produced localized hot spots. This heat flux caused the reheating of the cold waters which impregnated the plutonic complex and produced a convective fluid circulation as in modern geothermal systems. The fluids were channeled into faults and fractures of the east-west-north-south fracture system causing the micaceous episyenitization of the granite (dissolution of quartz, total muscovitization of plagioclase and biotite, and partial replacement of orthoclase). An earlier type of episyenite (feldspathic episyenite produced by quartz dissolution, chloritization of biotite, and feldspathization of muscovite) is considered to have been produced during the plastic P 4 deformation which folded the Saint-Sylvestre plutonic complex into an anticlinorium (NNE to NE fold axes).Pitchblende and pyrite mineralization immediately followed the emplacement of the lamprophyres and the micaceous episyenitization of the granite and a minimum age of 275 m.y., rather than 240 to 250 m.y., is accepted. The mineralization was precipitated from a CO 2 -rich fluid by the unmixing of complex CO 2 -H 2 O mixtures following a drop in pressure. The temperature of the solutions in the 132 episyenite column of the Margnac mine was approximately 345 degrees C.After pitchblende deposition, the fluid became progressively more water-rich during the precipitation of microcrystalline quartz and hematization of pyrite. The deposition of quartz plus marcasite and the transformation of pitchblende to coffinite took place during falling temperatures (330 degrees to 140 degrees C) related to the progressive decay of the geothermal system. The formation of the deposit ended with the deposition of fluorite, barite, and calcite, beginning at 135 degrees C and continuing to a lower temperature.Supergene modification caused the formation of secondary minerals (black products, gummite, etc.) and the adjustment of the deposits and prospects to the present erosion surface. These supergene modifications are recent. Based on a reinterpretation of U-Pb isotopic measurements, they date from about 30 m.y. and are still continuing to some extent.

Manganese deposits and the associated rocks of Northland and Auckland, New Zealand

Abstract Small manganese deposits described here are associated with mafic lava, chert, and volcanic argillite and are contained in a thick sequence of indurated Permian to Jurassic eugeosynclinal sediments. Chert, volcanic argillites, and primary manganese mineralisation all result from the diagenesis and lithification of manganese and iron-enriched clay and silica sediments. These sediments precipitated out of sea water enriched with Si, Al, K, Mn, and Fe from submarine volcanic exhalations. Primary and redeposited mineralisation bodies have been distinguished on the basis of their gross structure, microfabric relations, and relation to the present erosion surface. Microscopy of polished sections indicates that primary mineralisation contains minerals formed during sedimentation, diagenesis, deep burial, and supergene in situ oxidation. Redeposited mineralisation contains only supergene minerals precipitated from ground- and surface water. The manganiferous chert, volcanic argillite, lava assemblage cou...

An unusual hydrothermal copper deposit at Messina, South Africa

Hydrothermal copper mineralization emplaced within Archean granulite facies metamorphic rocks are aligned along a northeast-trending line passing through the Artonvilla mine. The orebodies in the district include breccia pipes, fissure deposits, and disseminated replacement bodies, the latter being best developed in the Artonvilla mine, where four separate lodes are developed.The Emery Lode is a flattened ovoid (600 X 400 X 50 meters) dipping at 45 degrees with the stratigraphy. Elongated apophyses extend downdip, and the most westerly is known to extend to 700 m below surface.Mineralization, which is accompanied by intense hydrothermal alteration, is confined to a cordierite-biotite-garnet-gneiss and a pyroxene granulite. This alteration and mineralization, which does not extend to the present erosion surface (in the cases of the Emery and Reggie Lodes), is confined to restricted structural traps as described by Jacobsen (1974). The disseminated orebodies reveal a distinct concentric zonation of hydrothermal minerals from an outer zone of sericitization followed inward by zones of albitization, chloritization, and epidotization; concomitant with these changes the sulfides show a zonation from an outer pyrite, through chalcopyrite and bornite, to chalcocite and occasional native copper zones at the center.Chemically the most important changes are related to the increase in the K 2 O content of the sericite-bearing outer aureole and the sharp decline of this component as well as SiO 2 with increasing alteration. The abundance of Al 2 O 3 and Na 2 O increases with advancing states of alteration; and downward nepheline and acmite normative compositions are encountered in rocks of the conduit below the 500m level. The extent of oxidation of iron increases markedly with increasing alteration and accompanies the economically important copper sulfide mineralization.Indications are that the initial hydrothermal fluid entered the structural traps at elevated temperatures, probably in excess of 650 degrees C, and passed through the rocks down a temperature gradient, finally cooling to about 400 degrees C at the outer extremity. The remaining trapped solutions cooled inward in much the same fashion as pegmatites do, thereby creating the well-developed mineral zonation. The central parts of the orebody indicate mineral associations compatible with temperatures in the range of 140 degrees C to 280 degrees C.The extensive aluminum and sodium metasomatism suggest that the fluid which caused mineralization was rich in these components and was capable of albitizing quartz and dissolving silica in the deeper levels of the conduits, thereby producing nepheline normative compositions. The fluids were therefore not of granitic parentage.We suggest that these features are most easily reconciled with a concentrated magmatic fluid which did not reach surface but reacted to completion by processes akin to pegmatite-type differentiation.A single injection of ore fluid is therefore envisaged, which would evolve by reaction with the enclosing host rocks and cause the observed mineral zonation. Successive minor zones then represent increasingly lower equilibration temperatures.

Discussion of Torridonian tourmaline-quartz pebbles and the Precambrian crust northwest of Britain

M r A. D. G ibbs writes: Allen et al. suggest that tourmaline-quartz pebbles in the Torridonian of Scotland may be used to reconstruct contemporary erosion levels in the Precambrian rocks to the northwest. The authors state that the pebbles are indicators of granite intrusions in the pre-Ketilidian block at a level much higher than the present erosion surface. In the course of my mapping in the Buksefjorden region to the south of Godthab, West Greenland, (Ghadwick &amp; Coe 1972) radiating tourmaline aggregates up to 0.7 m in diameter in quartz segregations have been found along the margins of pegmatites in Malene supracrustal sillimanite gneisses and amphibolites. The pegmatites are associated with the roof zone of the Qprqut granite, which is a high level sheeted complex. The Qprqut granite intrudes amphibolite facies gneisses of Amitsoq and Nuk types (McGregor 1973) with ages around 3.7 b.y. and 3.2 b.y. respectively (Black et al. 1971). The granite complex gives an age around 2.6 b.y. (unpublished age quoted in Black et al. 1973), i.e. within the range quoted by Allen et al. for the Torridonian pebbles. The erosion of the roof zones of granites such as the Qorqut at or near the present level of erosion could easily provide a source of quartz-tourmaline pebbles such as those described by the authors. I acknowledge the assistance of K. Ellitsgaard-Rasmussen of the Greenland Geological Survey. P rofessor J. S utton replied on behalf of the authors: Mr Gibbs suggestion is most welcome. It will be interesting to see if

Recent dedolomitization and the origin of the rusty crusts of Northumberland; a reply

ABSTRACT Two forms of dedolomitization. surface and near-surface, are found in the dolomitic Carboniferous rocks of the Middle Limestone Group of the northeast coast of Northumberland, England. This work is primarily concerned with the surface form of dedolomitization which is taking place in the Recent and sub-Recent, rusty-looking weathering crusts of the ferroan (iron-rich) dolostones. The rusty colour is caused by the presence of iron hydroxides in the weathering crusts. These iron hydroxides and the associated dedolomites are genetically related. The susceptibility of ferroan dolomites to dedolomitization is apparently caused by their metastability in the surface environment. Circulating sea or fresh water along permeable zones in the carbonates exposed at or near the present erosional surface is responsible for the oxidation and hydration of the ferrous iron content of the metastable ferroan dolomites as well as the dedolomitization of these dolomites. The dedolomitization mechanism has Been found to be a process of great potential in converting highly impermeable and impervious dolomitic rocks into a porous and permeable state. The detection of such a phenomenon in carbonate rocks at depth may, therefore, not only indicate an unconformity, but may help in the prediction of potential oil or gas reservoirs.

Discussion

Dr M. K. W ells congratulated the author on the interesting and unique nature of his data, and Dr Borley on the way she had presented it. The explanation offered for the chemical variation of the dykes appeared to imply that they were intruded more or less simultaneously. If, in fact, they showed any appreciable age difference it would be reasonable to suppose that some of the differentiation was achieved by processes operating through time, rather than being due entirely to the form of the postulated magma chamber. Was there any evidence concerning the relative ages of the dykes, and had the author considered the time factor in relation to possible differentiation mechanisms? Dr D. G. H elm commented that one of Dr Baker’s illustrations showed individual dykes crossed by the total-alkali percentage contours, part of a dyke occurring in one alkali percentage zone and part in an adjacent zone. Did such dykes show variation in alkali value along their length ? If so it might be that the magma chamber was tapped by certain fissures at continuously varying depth along their length as well as by a number of independent fissures at different depths. Dr R. R. S kelhorn asked whether the Author had been able to use the hade of the dykes in coming to a conclusion as to the depth of the top of the magma chamber below the present erosion surface. It seemed that before one could account for the compositional variation found in the dyke swarm one needed