Showing papers on "Archean published in 1982"

Andesites: Orogenic Andesites and Related Rocks

Abstract: The orogenic andesite association is the basalt-andesite-dacite-rhyolite association characteristic of island ares, active continental margins, and continental collision zones. This book is intended as a reference text for under graduate, postgraduate, and research workers who wish to gain an insight into the orogenic andesite association. It contains invited contributions covering the geology, volcanology, petrology, and geochemistry of orogenic andesites and related rocks, the role of such rocks through geological time, and the wider implications of andesite volcanism such as continental growth and the formation of economic mineral deposits. There were several reasons for producing such a volume. These include the abundance and importance of orogenic andesite volcanism in continental areas, and the important advances in our understanding of such volcanism achieved since the development of pi ate tectonics in the late 1960s. Noting the existence of several volumes devoted to the rarer alkaline rocks, it was felt that it would be valuable to produce a volume devoted entirely to the orogenic andesite association. The volume is divided into 10 sections, chosen to cover all aspects of the association. Section I is a brief introduction to the setting of orogenic andesite volcanism and how the problems posed by these volcanic rocks have evolved during the last few decades. Section II is a general account of the terminology and classification of orogenic volcanic rocks, with a review of the petrology and mineralogy. In Section III, the characteristics of each of the major orogenic volcanic provinces are reviewed and Section IV outlines the evolution of volcanism through time in relation to plate tectonic activity in contrasted provinces. The later sections of the book outline the characteristics of andesite volcanoes and their products (Section V), the relationship of andesitic volcanism to intrusive activity (Section VI), and experimental, chemical, and isotopic characteristics of andesites and related orogenic volcanic rocks (Sections VII and VIII). Some characteristics of andesite volcanism throughout geological time are explained by reference to Archaean volcanic rocks and to Upper Proterozoic volcanism in north-eastern Africa and Arabia and Lower Palaeozoic volcanism in the Caledonian orogenic belt of Britain (Section IX). In conclusion, Section X emphasizes the importance of andesitic volcanism by examining the role of such volcanism in continental growth, in large-scale geochemical cycles, in the eruption of volcanic gases, and in the formation of mineral deposits.

Archean Plate Tectonics: Constraints and Inferences

Abstract: The earth has been cooling since Archean time. The higher temperatures beneath Archean ridges resulted in more partial melting which extended down to greater depths than at present. The Archean oceanic crust was much thicker (>20 km) than modern crust (~5 km). This inference is compatible with previous ideas of suspected oceanic crust. The thicker oceanic crust in the Archean tended to resist subduction similar to modern aseismic ridges, but could not prevent it. Short-lived episodes of intra-arc spreading followed by Cordilleran-type compression may have produced Archean greenstone belts. Archean meta-tonalites represent a significant part of the continental crust and were probably produced by melting of the subducted slab which was favored by the higher mantle temperatures.

Geochemical Constraints on the Growth of the Continental Crust

Abstract: Several lines of evidence indicate that the Archean upper crust was considerably more mafic than the present-day upper crust. There has been no significant change in REE and Th abundances in post-Archean clastic sedimentary rocks, suggesting that there has been no change in the composition of the upper crust during the post-Archean. This indicates that if there have been any additions to the post-Archean upper crust, they must have had similar composition to the upper crust itself. Geochemical modelling of REE and Th abundances in sedimentary rocks suggests that the minimum ratio of post-Archean to Archean upper crustal composition required to eliminate the Archean upper crustal trace element signature, within analytical uncertainty, is about 4:1. Such a model also is supported by isotopic data. Using plausible assumptions regarding the volume of Archean crust, isostatic relations, and extreme models of the earth's degassing history, it is proposed that approximately 65-75% of the continental crust formed...

Evolution of the south-central part of the Archean Abitibi Belt, Quebec. Part I: Stratigraphy and paleogeographic model

Abstract: The region comprises parts of the southern zone of the Abitibi greenstone belt and of the Bellecombe gneiss belt, and the Belleterre–Angliers greenstone belt. The stratigraphic sequence of the Abit...

Archean metavolcanics from the Rouyn - Noranda district, Abitibi greenstone belt, Quebec. 2. Mobility of trace elements and petrogenetic constraints.

Abstract: Qualitative evaluation of altered mafic pillows from the Rouyn–Noranda district of the Abitibi metavolcanic belt reveals that, for low-grade metamorphism, the high field strength elements (HFS: Zr,...

Plate-tectonics model for Proterozoic continental accretion in the southwestern United States

Abstract: Proterozoic crustal provinces in the southwestern United States decrease in age south from the Archean Wyoming province. Supracrustal rocks define three major age provinces from north to south: 1.72 to 1.80, 1.65 to 1.72, and 1.1 to 1.2 b.y. Successions within each province are similar and define two assemblages: a bimodal volcanic assemblage commonly overlain by a quartzite-shale assemblage. Precambrian granitic plutons intrude all three provinces. Geochemical and Sr-isotope data indicate a variably depleted upper mantle source for basalts and a short-lived ( These observations can be explained by a plate-tectonics model involving successive marginal basin closures and Andean-type orogenies associated with southward-migrating arcs. The net result is to accrete ≥ 1,300 km of continental crust to the southwestern margin of North America between 1.8 and 1.1 b.y. ago.

Archean abiogenic and probable biogenic structures associated with mineralized hydrothermal vent systems and regional metasomatism, with implications for greenstone belt studies

Abstract: Abiogenic structures, formed by fluid emission in subaerial mud pools and nearby stromatolites, from stratigraphically related carbonate sediments, occur in the southern part of the approximately 3.3 to 3.5-b.y.-old Barberton greenstone belt. The mud pool structures are associated with ferruginous shales, banded iron-formations, and in places barite deposits; they relate stratigraphically to transgressive pods of ironstone. The pods probably represent buried, mineralized hydrothermal channels and chimneys. Soft sediment deformation structures adjacent to these pods suggest fossil fluid circulation patterns in the sediments with horizontal flow over distances greater than 300 m and vertical motions an order of magnitude less. Associated metasomatism has significantly altered the original chemistry of the host rocks on a regional scale, prior to 13.8 b.y. This is described with specific reference to silicification. Silicification patterns in the igneous rocks underlying the sediments also indicate fluid circulation patterns an order of magnitude greater than those in the sediments. Thin but regionally extensive and originally horizontal alteration planes were probably zones of high fluid pressure along which large-scale, gravity-induced mass transportations occurred. Such transportations greatly influenced the subsurface mineral precipitation and the surface geologic processes within the extensive area of hydrothermal activity, in which the inferred high geothermal gradient was probably related to shallow-level igneous activity. At least minor mineralization, including local gold concentration several orders of magnitude greater than that found in average crustal rocks (e.g., ppb to ppm), occurred simultaneously with these processes of metasomatism during sedimentation, igneous activity, and tectonism within the belt, and prior to the intrusion of the greenstone belt by surrounding granitoids.

Geochemistry of the Archean low- to high-grade transition zone, Southern India

Abstract: The transition zone between Archean low- and high-grade rocks in southern India represents eroded crustal levels representative of 15–20 km. It is comprised chiefly of tonalitic gneisses with some varieties showing incipient charnockitization and of minor amounts of granitic gneiss and charnockite, both of which appear to have developed from the tonalitic gneisses.

Mantle buffering of the early oceans

Abstract: Archean calcites and dolomites, if compared to their Phanerozoic counterparts, are enriched in Sr2+, Ba2+, Mn2+, Fe2+, depleted in 18O, (Na+), and contain mantle-like 87Sr/86Sr and, in associated S phases, mantle-like 34S/32S. This may be a consequence of massive seawater pumping through, and equilibration with, the coeval basaltic oceanic crust. The exponential decline of oceanic geothermal gradient in the course of terrestrial evolution led to a waning of this “mantle” flux and to the enhancement of the continental river discharge as the controlling factor of seawater composition; the major transition occurring probably during the late Archean — early Proterozoic time interval. Such evolution is consistent with the observed tectonic, sedimentological, geochemical and metallogenic secular patterns and may also provide an alternative, or complementary, inorganic explanation for the development of the post-Archean oxygenic atmosphere.

Geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica: evidence for mantle metasomatism

Abstract: Two episodes of tholeiite dyke emplacement have been identified in Archaean high-grade metamorphics of the Napier Complex in Enderby Land Middle Proterozoic Amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low-pressure crystal fractionation Group I Amundsen tholeiites were derived from a relatively homogeneous source region 1,190±200 my ago, whereas that of the group II Amundsen tholeiites was chemically and isotopically heterogeneous Group II dykes have various degrees of enrichment in incompatible elements, and commonly show normalised trace element abundance patterns with negative Nb anomalies These features imply variable metasomatism of the source region by a volatile-rich fluid phase (rather than a melt of any observed igneous composition) enriched in K, Rb, Ba, Th, and possibly La and Ce Early Proterozoic (2,350±48 my) tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite-facies metamorphism and include a high-Mg suite of possible komatiitic affinity, ranging in composition from hypersthene-rich tholeiite (norite) to quartz-rich tholeiite They tend to have higher ratios of highly to moderately incompatible elements (eg, K/Zr, K/Ce), and larger Nb anomalies (ie, higher K/Nb) compared with middle Proterozoic tholeiites, suggesting derivation from more enriched source regions Isotopic data are not compatible with significant crustal contamination, but constrain source metasomatism to a time immediately before emplacement Metasomatism of the source region of the much younger group I tholeiites may have been contemporaneous with that of the high-Mg suite

The geologic evolution of south america during the archaean and early proterozoic

Abstract: ln the South American continent, Archaean and Lower Proterozoic terranes can be found in all tectonic domains, but especially within the Amazonian and Sao Francisco cratons. They can also be found within the Sao Luis, Luis Alves and Rio de La Plata cratonic fragments, as well as within all mobile belts of the Brasiliano orogenic cycle, as reworked basement. The Arequipa massif is lhe only identified Lower Proterozoic unit in the Andean belt. Within the Amazonian craton, the Central Amazonian province is made up of Archaean rocks, exhibiting cratonic conditions since at least the Lower Proterozoic. It is bounded to the north by the Maroni-Itacaiunas mobile belt, in which the major tectonomagmatic episode occurred in the early Proterozoic, the Transamazonico orogenic cycle. It comprises large portions of supracrustal rocks, identified as greenstone belts, associated to gneisses and migmatites, as well as fragments of high-grade polymetamorphic terranes in which Archaean ages were obtained. Within the Sao Francisco craton, three main types of ancient geologic terranes occur: 1) Archaean granite-greenstone terranes, such as the Brumado-Anajo area in Bahia, and the Quadrilatero Ferrifero area in Minas Gerais; 2) Lower Protcrozoic supracrustal belts, such as the Jacobina, Serrinha and Contendas-Mirante sequences in Bahia, and the Minas Group in Minas Gerais; and 3) Medium to high-grade metamorphic terranes, subjected to extensive granitization during the Transamazonic orogeny (Salvador-Juazeiro mobile belt), and including Archaean "cratonic fragments", of grunulitic composition, such as the Jequie-Mututpe complex in Bahia. Ancient terranes were also found within the mobile belts of the Brasiliano cycle, where they make up their ensialic basement. Lower Proterozoic type radiometric ages are predominant, especially within the Borborema province and the Ribeira belt. Taking into account the general distribution of the ancient terranes, it seems that a major part of the South American continental crust was already existent as such just alter the Transamazonico cycle. Moreover, since the known Transamazonico belts are ensialic, it is clear that large portions of continental material were formed earlier, in Archaean times.

Late Proterozoic rifting in NW Scotland: the genesis of the ‘Torridonian’

Abstract: The two major “Torridonian” successions, viz . the Stoer Group (~968 Ma) and the Sleat–Torridon Groups (~777 Ma) occupy NNE-trending rifts cutting Archaean and early Proterozoic crust. Palaeocurrents, pebble petrography and age, and the initial 87 Sr/ 86 Sr ratios of clay-rich sediments indicate derivation from flanking areas of this old crust, rather than from late Proterozoic Grenville or Morarian metamorphic rocks. The earlier rift seems to be roughly contemporaneous with Grenville metamorphism but genetically unrelated to it. The rifts perhaps mark initiation of an Iapetus-related ocean in the same way as very similar fault-bounded Triassic and Lower Jurassic sediments mark the start of continental break-up and initiation of the present Atlantic. The western margin of both rifts probably coincides with the Outer Hebrides Thrust and the eastern margin with the Moine Thrust Zone. Reactivation of the old rift-margining faults by Caledonian compression may have generated the lower thrusts of the Moine Thrust Zone, while the Moine nappe itself, by analogy with Lower Devonian events in southern Appalachia, represents a slice of the Grenville microcontinent emplaced after closure of the intervening ocean.

COCORP profiling across the Rocky Mountain Front in southern Wyoming, Part 2: Precambrian basement structure and its influence on Laramide deformation

Abstract: The influence of Precambrian basement structures on subsequent deformation is of considerable relevance to studies of continental evolution COCORP deep seismic-reflection profiles were recently recorded in southeastern Wyoming, where several major, but temporally separated, tectonic elements of the western United States are superimposed Of these, a fundamental boundary between Archean and Proterozoic basements and the eastern front of Laramide deformation were the principal targets of the reflection survey The former may represent an ancient Proterozoic plate boundary; the latter is a prominent physiographic feature that signifies crustal deformation far within the North American craton, more than 1,500 km from the nearest coeval plate margin The major crustal feature controlling a lateral, north-south variation in Laramide tectonic style appears to be the Archean-Proterozoic crustal boundary, known in the nearby Medicine Bow Mountains as the Mullen Creek-Nash Fork shear zone COCORP data in the Laramie Mountains and the Laramie Basin suggest that this shear zone dips ∼ 55° to the southeast Northwest of the shear zone, the seismic basement is also characterized by southeast-dipping events, suggesting that the early Proterozoic tectonics that produced the shear zone were distributed over a wide region Complex reflections down to 15-km depth or more under the Laramie Basin may correspond to structures or erosional truncations in metasediments overlying the Archean basement complex Deep crustal events (between 35 and 40 km) north of the shear zone are short and discontinuous, in contrast with flat, laterally continuous reflections south of the shear zone at about 48-km depth which are interpreted as the crust-mantle transition Thus, COCORP data and published results of regional refraction and gravity surveys suggest that the crust is significantly thinner in the Archean basement terrane northwest of the shear zone than it is in the Proterozoic province to the southeast Differences in crustal thickness may be partly responsible for the difference between Laramide structures in Wyoming and Colorado, and a thin crust may also have facilitated Laramide deformation farther east in the Black Hills, located north of the shear zone

Tectonic evolution of lower Proterozoic rocks, Uinta Mountains, Utah and Colorado

Abstract: Rocks of the early Proterozoic orogenic system that fringes the Archean-aged Wyoming province are exposed along the Uinta fault in the northeastern Uinta Mountains of Utah and Colorado Exposed here are the Red Creek Quartzite, an early Proterozoic-type miogeoclinal metasedimentary sequence more than 4 km thick, and an underlying, newly recognized, Archean gneiss complex more than 27 by old During the Hudsonian orogenic period, the miogeoclinal sequence was emplaced northward over the Archean complex by tectonic translation along a thick mylonite zone in the waning phases of upper amphibolite metamorphism The orogen was disrupted by east-trending block faults with several kilometres displacement during initiation of the Uinta aulacogen and was buried by more than 7 km of middle Proterozoic sediments of the Uinta Mountain Group The middle Proterozoic block faults were reactivated with reversed sense of displacement during the Laramide uplift of the Uinta Mountain block

Rb–Sr and U–Pb ages of volcanism and granite emplacement in the Michipicoten belt—Wawa, Ontario

Abstract: The Michipicoten greenstone belt at Wawa, Ontario is typical of Archean volcanic belts in the Superior Province. The supracrustal rocks are divisible into lower, middle, and upper metavolcanic sequences, which are separated by iron formation and clastic metasedimentary rocks. These are intruded by granitic stocks and embayed by granitic batholiths.This study reports whole rock Rb–Sr and zircon U–Pb ages for the lower and upper metavolcanics, for the granitic rocks that are physically within the greenstone belt (internal granites), and for the granitic rocks that embay the greenstone belt (external granites). The apparent Rb–Sr ages for the lower metavolcanics are 2530 ± 90, 2285 ± 70, and 2680 ± 490 Ma. The U–Pb ages are 2749 ± 2 and 2744 ± 10 Ma. The internal granites give an Rb–Sr age of 2560 ± 270 Ma and a U–Pb age of 2737 ± 6 Ma. The external granite at Hawk Lake indicates an Rb–Sr age of 2550 ± 175 Ma and a U–Pb age of 2747 ± 7 Ma. It is possible that this unit contains elements older than 2812 Ma as...

Evolution and subsidence of early Precambrian sedimentary basins

Abstract: Many of the models for modern sedimentary basins postulate two-stage subsidence; a rapid initial subsidence due to thinning or loading of the crust, followed by a more protracted thermal stage as the lithosphere, which is thinned during the initial stage, relaxes to equilibrium thickness The geology of a number of Archaean greenstone belts and early Proterozoic cratonic basins in South Africa may be explained by such a model Rapidly erupted shallow marine or subaerial volcanic rocks predominate in the lower parts of sedimentary-volcanic sequences These are thought to relate to initial subsidence as (1) accommodation of relatively thick volcanic sequences requires substantial and rapid subsidence, and (2) marginal uplift following these early volcanic intervals is consistent with viscous relaxation following the initial elastic response of the lithosphere to localized loading Sedimentary sequences overlying initial volcanic dominated intervals may have been deposited during the ensuing phase of more widespread subsidence related to thermal relaxation of the thinned lithosphere If so, sediment-filled subsidence of ca 55 km in greenstone terrains at 35 and 26 Ga and of 7-10 km in cratonic shelf basins between 27 and 21 Ga require increases of lithosphere thickness between ca 60 and 90 km These minimum estimates of early Precambrian lithosphere thickness, although crude, are similar to estimates of present lithosphere thickness In some early Precambrian basins, the cause of subsidence may have been crustal extension with development of faulted grabens that evolved into continental margins, but in cratonic shelf basins faulting did not occur during or after the initial subsidence, and some less obvious causal mechanism must be sought

Archean mafic metavolcanics from the Rouyn–Noranda district, Abitibi Greenstone Belt, Quebec. 1. Mobility of the major elements: Reply

Abstract: In a suite of Archean mafic pillows from the Rouyn–Noranda region of Quebec's Abitibi Greenstone Belt, including both tholeiitic and calc-alkaline varieties spanning the prehnite–pumpellyite to upper greenschist metamorphic facies, three types of alteration can be defined: (I) chlorite–epidote–actinolite; (II) chlorite–epidote; and (III) chlorite ± sericite; the number of mineral phases decreases as a result of progressive hydration from type I to type III alteration. Albitization, resulting from substitution of , in calcic plagioclase, is highly variable in type I alteration, but in types II and III the plagioclase is totally albitized and in some cases silicified. Chloritization is closely linked to increasing hydration and Ca leaching with MgO and FeO substituting for CaO in ferromagnesian minerals.Calcium was mobilized and carried by solutions, as evidenced by the variable concentration of epidote at the margins of pillows. This calcium leaching generated an excess of Al2O3 with respect to the combine...

Gold distribution in supracrustal rocks from Archean greenstone belts of southern Africa and from Paleozoic ultramafic complexes of the European Alps; metallogenic and geochemical implications

Abstract: Many Archean gold veins are thought to have formed as a result of metamorphic secretion processes, with the source of gold being gold-enriched supracrustal rocks. However, in many gold provinces no elevated gold concentrations can be detected in the proposed source beds.In this study, 98 samples of mafic and ultramafic rocks, and 32 samples of ferruginous chemical sediments from South African greenstone belts and from the Belingwe greenstone belt in Zimbabwe were analyzed for their Mg, Al, Cr, Fe, Co, Ni, and Au contents. The analyses were carried out by means of XRF, AAS, and neutron activation analysis, and the data evaluated by various statistical methods. The gold values of the volcanics range from 0.1 to 372 ppb, with the mean content at 10.8 ppb, and the gold values of the sediments vary from 0.5 to 667 ppb, with the mean content being 129.9 ppb. As a comparative population, 56 samples from Paleozoic European ultramafic complexes were investigated for the same suite of elements. The gold values range from 0.1 to 25 ppb and possess a mean content of 2.0 ppb.For the samples from southern Africa, statistical analyses indicate a complete lack of relationship between gold and rock-forming minerals. Significant differences in gold abundance were observed in samples originating from geologically and geochemically similar greenstone terranes. These differences may be a result of heterogeneous gold abundance in the upper mantle. Differentiation trends of gold in the ultramafic to mafic range were not detected in this study. Ferruginous chemical sediments of the Algoma type were found to contain more gold than younger ones of the Superior type.For volcanic rocks, the analytical data suggest that the principal gold carriers are accessory sulfides and, to a lesser extent, intergranular gold in particulate form. Rock-forming minerals appear to contribute less than 0.5 ppb to the gold in these rocks. Most supracrustal rocks contain sulfides and, thus, contain gold readily accessible to leaching metamorphic hydrothermal solutions which may deposit gold in nearby dilatant zones. It is proposed that in greenstone terranes metamorphic overprint constitutes the key factor determining the formation of gold veins. The conspicuous abundance of such gold mineralizations in Archean greenstone terranes is considered to be a result of the unique evolution of these terranes.

Proterozoic tectonic evolution and late Svecokarelian plate deformation of the Central Baltic Shield

Abstract: The continental crust of the Central Baltic Shield evolved by accretion towards the west during the Svecokarelian orogeny 1700–2200 Ma ago. The following features are consistent with a plate tectonic mechanism involving subduction of oceanic crust below an Archean craton in the east: flysch-sediments with serpentinite masses and pillow lavas, linear high-grade metamorphic zones, island-arc type volcanic belts and late tectonic batholiths with porphyry type Cu-Mo deposits. Semi-consolidated new crust was affected by late Svecokarelian deformation (Dn) after 1850 Ma; NNE-trending folds with crenulation cleavage were overprinted on older structures together with associated NW trending ductile transcurrent shear zones that curve the Fn folds into gentle S and Z shapes. The late tectonic batholiths intruded partly at the same time as and partly after the Dn deformation.

Rare earth elements in volcanic rocks associated with Cu–Zn massive sulphide mineralization: a preliminary report

Abstract: Massive sulphide deposits are closely associated with felsic volcanism. This association is believed to be genetic and it forms the cornerstone for most exploration programs, but unfortunately not all felsic volcanic rocks contain ore. It seems likely that ore-bearing felsic volcanic rocks have a different genetic history from those that are barren and, if this is so, these differences should be reflected in their REE geochemistry.A preliminary study of REE in Archean felsic volcanic rocks has shown that those associated with ore have flat REE patterns with well-developed Eu anomalies whereas those from barren volcanic rocks have steep REE patterns with weak or absent Eu anomalies. The felsic volcanic rocks associated with ore can be subdivided into two types: tholeiitic and calc-alkaline. Kam-Kotia, Matagami, and South Bay are tholeiitic whereas Sturgeon Lake, Golden Grove, and Kuroko are calc-alkaline.The well-developed Eu anomalies in the ore-related felsic volcanic rocks indicate that the melt has und...

Some petrologic and oxygen isotopic relationships in the Amulet Mine, Noranda, Quebec, and their bearing on the origin of Archean massive sulfide deposits

Abstract: A concentrically zoned alteration pipe is present at the Amulet "A" Cu-Zn mine in the Archean Abitibi greenstone belt. The pipe consists of a central core zone of so-called "dalmatianite" (=spotted cordierite-anthophyllite rock) surrounded by a zone of biotite-bearing grid-fracture alteration. Study of the dalmatianite indicates that these rocks experienced two episodes of recrystallization. First, the original andesite was completely reconstituted during a hydrothermal event, producing a quartz-chlorite assemblage. This transformation was associated with the formation of the massive sulfide lens and presumably took place in a submarine environment. Later contact metamorphism associated with the intrusion of the nearby Lake Dufault granodiorite resulted in partial recrystallization of the chlorite and quartz to cordierite and anthophyllite.Whole-rock delta 18 O values decrease from about 6 to 10 in most of the Abitibi belt to 5 to 7 in the country rocks surrounding the ore deposit, to values as low as 3.6 in the core of the dalmatianite zone. This oxygen isotopic zoning must have been produced by the hydrothermal activity, because the dehydration reactions associated with contact metamorphism cannot have affected delta 18 O by more than 0.5 per mil. Assuming an alteration temperature of 300 degrees + or - 50 degrees C and a water/rock ratio greater than two, the hydrothermal fluid must have had delta 18 O = 0.5 + or - 1.0. This indicates that the Amulet ore deposit formed from a hydrothermal fluid that had a delta 18 O value similar both to modern seawater and to the fluids which formed the Phanerozoic massive sulfide deposits of the Kuroko and Cyprus types. The apparent constancy of delta 18 O of seawater during the Precambrian and the continued importance of seawater hydrothermal processes are important constraints which must be considered in developing models of the history of the earth.

Geochemistry and Petrology of the Jequie Granulitic Complex (Brazil): An Archean Basement Complex

Abstract: The Jequie granulitic complex is part of the extensive high-grade metamorphic terrain located within the Sao Francisco craton of northeastern Brazil. Some Jequie rocks appear to have been formed in the middle Archean (∼ 3.1 Ga) from preexisting sialic crust. The dominant mineral composition of these rocks is quartz-microcline-plagioclase-hyperstene and occurs over an extensive area (∼ 2,000 km2).

Reworking of sialic crust as represented in late Archean–age gneisses, northern Labrador

Abstract: Regionally reworked sialic crust has been recognized in the form of ∼ 2,800-m.y.-old nebulitic rocks (Kiyuktok gneisses) with high initial 87Sr/86Sr ratios (∼ 0.7081) and Pb isotope ratios that suggest a history of the source back to at least 3,500 m.y. B.P. The field, geochemical, and isotopic data are consistent with these rocks having formed by in situ mobilization of granitic components in the ∼ 3,600-m.y.-old Uivak gneisses by the introduction of aqueous fluids some 2,800 m.y. ago. Contemporary but subordinate tonalitic-granodioritic orthogneisses derived from discrete intrusive bodies have contrasting Sr isotope compositions. Some display low initial 87Sr/86Sr ratios (∼ 0.7030) that suggest derivation from a source with mantlelike Sr-isotope composition. However, most of the late Archean–age intrusive rocks display higher initial 87Sr/86Sr ratios (∼ 0.7056, 0.7059). Recognition of these contrasting isotopic patterns suggests that processes of late Archean crust formation and stabilization in the North Atlantic craton were more complex than previously realized. Crustal reworking due to interaction of old crust with aqueous fluids must have been accompanied by the introduction of granitic melts derived by melting of ancient deep crustal rocks or contamination of juvenile granitic melts by ancient deep crustal material.