Felsic Metavolcanics Research Articles

The origin of Archaean gneisses in the Fyfe Hills Region, Enderby Land; field occurrence, petrography and geochemistry

Sandiford, M. and Wilson, C.J.L., 1986. The origin of Archaean gneisses in the Fyfe Hills region, Enderby Land: field occurrence, petrography and geochemistry. Precambrian Res., 31: 37--68. Granulite and amphibolite facies gnelsses at Fyfe Hills in Enderby Land, East Antarctica, form part of an early Archaean (c. 3.8 Ga) terrain termed the Napier Complex. The gneissic sequence at Fyfe Hills is composed of both supracrustal and intrusive rocks. The oldest lithological association, the Layered Gnelssic Series, is defined by interlayered felsic and basic metavolcanics and metasediments derived from psammitic, pelitic, ferruginous and calcareous precursors. Felsic gneisses in the Layered Gneissic Series have low Y/Ce, low Ga/A1 and are low in incompatible elements and, therefore, correspond to Sheraton and Black's depleted granulites. The Layered Gneissic Series has been intruded by ultrabasic rocks and by subconcordant sheets of charnockite, enderbite and pyroxene granulite which form the Massive Gneissic Series. The 'undepleted' felsic gneiases of the Massive Gneissic Series probably derive from crustal anatexis associated with granulite facies metamorphism of the Napier Complex. Dehydration during this unusually high grade metamorphism was achieved by the removal of partial melts, rather than by 'CO2-flushing', as metamorphism occurred either during vapour-absent conditions or, if vapour was present, then the composition of the vapour phase was internally buffered. Anatectic pegmatites generated during granulite facies metamorphism differ little in chemistry from the felsic granulites from which they derive. Chemical modification accompanied rehydration during waning granulite facies metamorphism and during the development of retrograde shear zones, the latter are associated with a magmatic episode in which numerous pegmatites were intruded into the Napier Complex.

Sm-Nd and Rb-Sr dating of an Archean massive sulfide deposit: Kidd Creek, Ontario

Highly altered felsic metavolcanics associated with the Kidd Creek, Ontario, Cu-Zn massive sulfide deposit show a large range of Sm/Nd ratios and yield a Sm-Nd isochron of 2674 ± 40 Ma (initial ratio ϵ Nd = 1.55 ± 0.30), which represents the time of rare-earth-element redistribution during intense hydrothennal alteration. That the Sm-Nd age is consistent with age constraints on ore deposition provided by precise U-Pb zircon data indicates contemporaneity of ore deposition, hydrothennal alteration, and rare-earth mobility. The age is therefore interpreted as a minimum age of ore deposition. In contrast, the Rb-Sr age of the altered rocks, as well as the metavolcanic rocks outside the alteration zone, has been reset at 2576 ±26 Ma, most likely as a result of widespread low-temperature metasomatism unrelated to ore deposition. Our results suggest that Sm-Nd dating could be a useful tool in the study of ore deposits and, potentially, in the study of a wide range of mineralizations. Initial ϵ Nd (T) values for massive ore, altered felsic volcanics, and their weakly altered precursors are identical, indicating derivation and redistribution of light rare-earth elements within the altered footwall volcanics. These data suggest that the footwall volcanics have also supplied part of the base metals to the stratiform ore.

U–Pb zircon ages for rocks from the Island Lake greenstone belt, Manitoba

The Archean Hayes River Group of the Island Lake greenstone belt (Superior Province, Sachigo Subprovince) comprises mafic to felsic metavolcanics, subvolcanics, and associated metasedimentary rocks. The Hayes River Group is intruded by granitoid rocks belonging to the early intrusive complex. One such pluton, the Bella Lake tonalite, is intrusive into the metabasalt of the Hayes River Group and has a U–Pb zircon age of 2886 ± 15 Ma. Similar intrusives of this complex, either internal or marginal to the greenstone belt, yield zircon ages of 2801 ± 8 Ma (Pipe Point tonalit) and 2768 ± 22 Ma (Linklater Island prophyry). This suggests that the early intrusive complex was emplaced over an ~ 120 Ma long interval by at least three separate intrusive episodes.Subsequent to the emplacement of the early intrusive complex, the isoclinally folded Hayes River Group and the early intrusive complex were uplifted, eroded, and followed by the unconformable deposition of the Island Lake Group, comprising fluvial to marine metasedimentary rocks. The stratigraphically lower part of the Island Lake Group is bracketed by the 2768 ± 22 Ma age of the Linklater Island porphyry and the 2729 ± 3 Ma age obtained for the late tectonic suite—the Pipe Point quartz diorite and feldspar porphyry. A feldspar quartz porphyry belonging to the post-tectonic intrusive rocks intrudes higher stratigraphic levels and has been dated at 2699 ± 4 Ma (Horseshoe Island quartz feldspar porphyry).

The geology and genesis of the Proterozoic massive sulfide deposit at Garpenberg, central Sweden

The Garpenberg deposit of central Sweden is a metamorphosed, strata-bound Zn-Pb-Cu-Ag sulfide deposit intercalated in an early Proterozoic supracrustal sequence of felsic metavolcanics and subordinate metasedimentary rocks, which have been folded, metamorphosed, and intruded by synorogenic granitoids. The magmatism is calc-alkaline and most of the volcanic activity occurred as explosive submarine volcanism.The Garpenberg deposit is emplaced at the contact between metasedimentary and metavolcanic rocks and consists of 32 lens-shaped orebodies. Stratiform Zn-Pb-Cu mineralization is underlain by Cu-bearing stockwork ore with an extensive alteration zone of quartz-phlogopite rocks with minor chlorite, cordierite, garnet, staurolite, and andalusite. Chemically, the altered zone is enriched in magnesium, iron, ore metals, manganese, and volatiles, and is depleted in sodium and calcium.The proposed volcanogenic origin, related to calc-alkaline magmatism, is supported by the morphology, wall-rock alteration, and ore mineralogy. The similarity of the Garpenberg deposit to the Archean massive sulfide deposits of Canada and the Kuroko deposits of Japan supports this theory but also reveals possible differences between the compositions of Precambrian and Phanerozoic seawater. Dissimilarities in the metal ratios (Cu:Pb:Zn) of the different deposits and their associated lithologies suggest differences in their tectonic setting. The Garpenberg deposit is thought to have formed in an active continental margin setting rather than in an island arc.

Geological and genetic aspects of the Hällefors sulfide ores, Bergslagen, Sweden

Abstract The Hallefors sulfide ores are located in the westernmost part of Bergslagen Ore Province, south central Sweden. The ore-bearing rock sequence consists of K-rich felsic metavolcanics. In the eastern part of the field, stratiform ore occurs in the upper part of a 30–40 m thick limestone. This ore consists of carbonate-bearing manganiferous skarn rock with conformable sulfide- and/or magnetite-rich beds. In the ore there are interlaycred beds of limestone, dolomite and tuffite. The sulfide skarn ironoxide ore is interpreted as a product of regional metamorphism of pre-existing strata. It comprises four chemical components, which are suggested to represent tuff materials (Si, Al, K), and chemically precipitated oxides (Fe, Mn), carbonates (Ca, Mg) and sulfides (S, Zn, Pb, As, Ag). The precipitates are considered to be products of submarine hydrothermal activity. The absence of wallrock alteration and channelways for ore-bearing solutions indicates that the ore was deposited on the sea floor distally...

Rb-Sr geochronology of the Bitlis Massif, Avnik (Bingöl) area, S.E. Turkey

Summary In the Avnik area, felsic metavolcanics are interbedded with banded and massive apatite-rich iron ores, and are intruded by the Avnik and Yayla granitoids. These rocks are unconformably overlain with depositional contact by micaschists and (?) Permian marbles, which were folded and metamorphosed during the Alpine orogeny. The metavolcanics and the granitoids are extensively feldspathized and silicified. The Yala granite gives a poorly defined Rb-Sr age of 347 ± 52 Ma (IR = 0.7217 ± 80) but the albitized Avnik body gives only a scatter of data suggesting an age of 250–425 Ma. Feldspathized metavolcanics from a 100-metre section define an age of 91 ± 9 Ma. One sample of the Avnik granitoid gives an amphibole-rock-feldspar age of 71 ± 28 Ma and another gives a biotite-rock age of 41 ± 1 Ma; a micaschist gives a chlorite-muscovite age of 38 ± 2 Ma. An isochron age of 454 ± 13 Ma (IR = 0.7105) for metavolcanics in the Cacas area can be extracted from the data of Yilmaz et al. (1981). We interpret this as the age of eruption, and suggest a similar age for the Avnik metavolcanics. Our 90 Ma (Eoalpine) data on these metavolcanics requires metamorphic resetting of whole-rock Rb-Sr systems over tens of metres, and probably records the time of pervasive feldspathization and silicification. The data for the Yala granite are interpreted as the age of intrusion; the high IR indicates an origin by anatexis of older crust. The late-Alpine mineral ages probably reflect recrystallization during folding and thrusting.

Metamorphic feldspathization of metavolcanics and granitoids, Avnik area, Turkey

In the Avnik area of the Bingol massif the Lower Unit consists of basic to felsic metavolcanics (ca. 450 Ma), intruded by granitoids (ca. 350 Ma). These are unconformably overlain by an Upper Unit of micaschists and Permian marbles; both units have been deformed and metamorphosed in Alpine time. The metavolcanics and granitoids are extensively feldspathized and silicified. The granitoids, and basic-intermediate volcanics, are albitized, while felsic volcanics are K-feldspathized. Metasomatism has severely modified K/Rb and Rb/Sr ratios, but not REE patterns, and is inferred to have occurred at relatively low T. Nametasomatism of the Upper Unit micaschists has produced albite porphyroblasts.

Optimum second vertical derivatives in geologic mapping and mineral exploration

The second vertical derivative is often used in gravity interpretation to enhance localized near‐surface features. Since high‐frequency noise is amplified considerably in a true second derivative map, some smoothing is always necessary. One of the methods available to the interpreter is the optimum second derivative technique which utilizes Wiener theory to design filters from an analysis of the power spectrum to suit the degree of noise in the data. The power spectrum of the Bouguer gravity field obtained from over 5100 gravity stations covering an area of approximately [Formula: see text] in Archean greenstone belts of northern Ontario was computed. The radially averaged spectrum decreased monotonically with frequency and flattened at the high‐frequency end. The spectrum, separated into its signal and noise components, was used to design an optimum second vertical derivative filter which has a peak at 0.35 cycles/grid interval with low‐ and high‐frequency cut‐off at 0.12 and 0.44 cycles/grid intervals, respectively. The resultant second derivative map corresponds remarkably well with the known surface geology. In most of the mapped regions the zero contours coincide with the lithological boundaries; positive and negative anomalies match surface exposures of the mafic and felsic rock units, respectively. In the central part of the area which covers the Birch‐Uchi greenstone belt, where the geology has been well mapped, the second derivative map is quite successful in delineating the successive groups of mafic to felsic metavolcanic rocks that represent the product of cyclic volcanism. The distribution of the mineral deposits in the study area can be related to the positive or negative second derivative anomalies. For example, most of the known gold and silver mineralization occurring in mafic metavolcanics is associated with positive second derivative anomalies. Similarly, the polymetallic Zn‐Cu‐Ag deposits are mainly located in the negative second derivative anomalies caused by felsic metavolcanics. A properly designed second derivative map can thus be an important supplement to geologic mapping in the identification of lithological units, in the study of structure, and as an indirect tool in regional mineral exploration.

Recambrian evolution of the Beartooth Mountains, Montana-Wyoming, U.S.A.

The Beartooth Mountains ofMontana and Wyoming contam more than 5,000 km2 of exposed Archean rocks. The southeastern portion of the range contains the only well documented segments of early Archean crust. The oldest rocks identified are a supracrustal assemblage contaning basaltic, ultramafic and felsic metavolcanics, metagraywackes, quartzites, meta-ironstones, and metapelites. These rocks yield a Rb-Sr whole-rock age of 3,400 Ma which is interpreted as the time of a granulite fades metamorphic event (800°C and 6 kb). This granulite facies event was followed at about 3,000 Ma age by the development of extensive volcanic and sedimentary packages with some minor silicic plutonism. The last major episode of crustal development began with an amphibolite fades rnetamorphic deformational event (about 2,850 Ma) and culminated with the intrusion of large volumes of granitoids (about 2,800 Ma). These granitoids were subsequently intruded by mafic dikes at least four separate times between 2,800 and 700 Ma age. Many of these events, including mafic dike emplacement, are correlative with geologic events reported from other ranges in the Wyoming Province; however, the Beartooth region appears to contain the longest and most complete record.

Epiclastic sedimentation and stratigraphy in the North Spirit Lake and Rainy Lake areas: A comparison

In the North Spirit Lake greenstone belt within the Sachigo geological subprovince epiclastic metasediments interlayered with mafic and intermediate to felsic metavolcanics unconformably to disconformably overlie a sequence of ultramafic to felsic metavolcanics and quartz-magnetite iron-formation. Polymictic conglomerate of mixed volcanic and plutonic provenance overlies the unconformity. In the northern part of the belt it is followed by cross-bedded sandstone, mudstone and marble: alluvial fan, braided fluvial, and lacustrine deposits. In the southern part of the belt the basal conglomerate is overlain by channelled submarine fan deposits: inverse to normally graded and normally graded conglomerate; sandstone showing dish structure, Bouma cycles, and flame structures; mudstone and quartz-magnetite iron-formation. The submarine fan deposits are separated from the alluvial fan deposits by an area of non-deposition. In the Wabigoon geological subprovince near Rainy Lake polymictic conglomerate of volcanic provenance unconformably overlies volcanic rocks, a gabbro-anorthosite body, and a granitic subvolcanic intrusion. This alluvial fan deposit is followed upwards by braided river deposits: parallel laminated or low-angle cross-stratified sandstone and trough cross-bedded sandstone. Siltstone and magnetite iron-formation associated with the sandstone may be lacustrine deposits. To the south in the adjacent Quetico subprovince are medium-grade submarine fan sandstones. They are separated from the alluvial fan deposits to the north by a fault. Within the context of present tectonic concepts the North Spirit Lake area is comparable to a marginal basin whereas the Rainy Lake area shows characteristics more compatible with an arc-continent interface.

Chemical discontinuities in Archean metavolcanic terrains and the development of Archean crust

Most large Archean greenstone belts ( ⋍ 2.7 Ga ), comprise thick (12–15 km) mafic to felsic metavolcanics sequences which exhibit consistent but discontinuous geochemical patterns resulting from mantle-crust processes. In a typical Archean metavolcanic sequence, thick (5–8 km) uniform tholeiitic basalt is followed by geochemically evolved rock units (4–7 km thick) containing intermediate and felsic calc-alkaline rocks. This major geochemical discontinuity is marked by a change from LIL-element depleted basalts which show unfractionated REE abundance patterns, to overlying andesites with higher LIL-element contents, fractionated REE patterns and relatively depleted HREE. A less well marked discontinuity separates andesitic rocks from still later more felsic dacite-rhyolite extrusive assemblages and their intrusive equivalents, and is identified by a further increase in LIL element content and REE fractionation. The major geochemical discontinuity apparently separates rocks derived by partial melting of mantle (either directly or through shallow fractionation processes) from those which originated either by partial melting of mantle material modified by crustal interactions or by partial melting of crustal material. We suggest that accumulation of a great thickness of mantle derived volcanic rocks can lead to sagging and interaction of the lower parts of the volcanic piles with upper mantle material. The resulting modified mantle acts as a source for some of the geochemically evolved rocks observed in volcanic successions. Subsequent direct melting of the volcanic pile produces the felsic magmas observed in the upper parts of Archean volcanic successions. This process, termed sag-subduction, is the inferred tectonic process operating in the comparatively thin, hot Archean crustal regime. By this process, large masses of ultimately mantle-derived material were added to the crust.

Geochronology and geochemistry of felsic rocks in an Archean volcanic–plutonic suite in the Wabigoon belt, northwestern Ontario

Selected rock units of an Archean volcanic–plutonic assemblage from the Kakagi Lake area, northwestern Ontario, were sampled for geochronological and geochemical studies. The units sampled were felsic metavolcanic rocks from the upper third of the sequence (Middle Felsic Group) and a felsic pluton (the Stephen Lake stock) intruded into these metavolcanics. A Rb/Sr whole rock isochron from the Stephen Lake stock yielded an age of 2534 ± 96 Ma with an initial 87Sr/86Sr ratio of 0.7015. This date is interpreted as a minimum age for the end of volcanism in the Kakagi Lake area, and the low initial ratio indicates that older sialic material was not a significant contributor to the Stephen Lake magma.Major and trace element (Rb, Sr, Ba, Zr, Y, Ce) compositions show that the Kakagi Lake felsic metavolcanics are similar to high-alkali andesite and depleted siliceous volcanics as recognized by Condie. Geochemical indicators such as K/Rb, the potash index, and the Rb/Sr thickness grid index suggest that the Stephen Lake pluton is not simply the intrusive equivalent of the felsic metavolcanics, but that different magma sources are probably required.

Formula omitted] of Archean clastic metasedimentary rocks: a petrogenetic indicator for Archean gneisses?

The oxygen isotopic compositions of over 100 Archean clastic metasedimentary, felsic metavolcanic and gneissic rocks from selected areas within the Superior Province have been determined. δ 18 O values of low grade, immature clastic metasediments range from 8.0 to 13.3%. and hence are somewhat depleted in 18O relative to other clastic metasediments. The lower 18O content is attributed to the large proportion of unaltered rock fragments in the Archean metasediments. Felsic metavolcanics have a similar range of δ 18 O values (7.3 to 13.0%.. δ 18 O values for the middle amphibolite facies Pakwash paragneiss (8.8–11.7%.) are higher than those determined for the amphibolite to granulite facies Twilight paragneiss (7.3–9.2%.). Archean orthogneisses such as the Clay Lake gneiss (7.0–9.0%.), the Cedar Lake gneiss (7.4–8.7%.) and the Footprint gneiss (5.9-8.8%.) have δ 18 O values similar to those of the Twilight paragneiss. Therefore, at least at metamorphic grades higher than middle amphibolite facies, Archean metasedimentary gneisses may have δ 18 O values that are indistinguishable from those of Archean orthogneisses, and hence are no longer isotopically recognizable as metasedimentary rocks.