Early Proterozoic Supracrustal Rocks Research Articles

The Crust of the Colorado Plateau: New Views of an Old Arc

Abstract Combined field, seismic, xenolith, and heat flow data from the Four Corners region of the Colorado Plateau permit construction of a four‐layer crustal model in which a thin sediment layer is underlain sequentially by Early Proterozoic supracrustal rocks and granitoids, intermediate plutonic rocks, and finally by mafic granulites and amphibolites. Both upper and lower crust show Nb‐Ta depletion, Pb enrichment, and negative Eu anomalies, suggesting an arc origin in which plagioclase was an important fractionating phase throughout the crust. The relative abundance of felsic rocks in the section suggests that the arc formed on continental lithosphere. The bulk crust has approximately andesitic composition, and mass balance calculations indicate that significant volumes of mafic cumulate and/or restite must have been recycled into the mantle during crustal formation in the Proterozoic. Neither the xenolith nor the seismic data support mafic underplating of the Colorado Plateau crust since Proterozoic ...

Basement–cover relationships in the East Greenland Caledonides: evidence from the Eleonore Bay Supergroup at Ardencaple Fjord

AbstractThe Eleonore Bay Supergroup (EBG) is a 16 km-thick shallow-water sequence of Neoproterozoic age that is preserved within the East Greenland Caledonides in several tracts, surrounded by crystalline gneisses and schistose supracrustal rocks. The apparent downward transition from non-metamorphic EBG into gneiss gave rise to the classic ‘stockwerke’ hypothesis, in which all the metamorphism was regarded as Caledonian, and differences in grade were ascribed to the ascent of a migmatite front to different levels within the orogen. Field and isotopic studies in the 1970s however revealed that the underlying gneisses and schists had undergone orogenic reworking in mid-Proterozoic time; the EBG–basement contact was then interpreted as an approximately bedding-parallel décollement with apparent lag geometry, that is with EBG cover rocks in its hangingwall.Recent work in the northernmost EBG tract, at Ardencaple Fjord, has shed light on the problems posed by the basal relationships of the EBG, and together with regional structural and stratigraphic data leads to the following interpretation. There are two regionally important basement-cover interfaces within the East Greenland Caledonides. The earlier one is between Archaean/early Proterozoic gneisses and early Proterozoic supracrustal rocks, which were pervasively deformed in mid-Proterozoic time and form the basement to the Neoproterozoic Eleonore Bay cover sequence. This was deposited on a vast, continually subsiding shelf that is now preserved in East and NE Greenland and Svalbard, and contains Grenville detritus. EBG deposition was terminated by major extensional faulting of Vendian age; the succeeding Tillite Group is interpreted as a syn-rift sequence, presumably associated with the opening of Iapetus.The EBG–basement contacts that are not late faults are inferred to be extensional shear zones of Vendian age. These were reactivated in compression during the Caledonian orogeny in the Silurian, with metamorphic and fabric convergence, which accounts for the apparent downward transition from sedimentary rocks through schists into gneisses. Caledonian shortening was not large; inversion of the Vendian grabens was incomplete, so that the marginal shear zones retained their lag geometry and large tracts of low grade Eleonore Bay sediments are preserved at the present erosion level, surrounded by Proterozoic basement rocks, within the Caledonian belt of East Greenland.

Origin of Late Archean and Early Proterozoic rocks and associated mineral deposits from the Zhongtiao Mountains, east-central China

Abstract Late Archean and Early Proterozoic supracrustal rocks in the Zhongtiao Mountains of east-central China include five successions separated by unconformities or intrusive contacts. The oldest successions, the Shushui Complex (⩾ 2500 Ma), and the Jiangxian (2400-2200 Ma) and Zhongtiao Groups (∼ 2100 Ma) are composed chiefly of metapelites, quartzites and marbles with small amounts of bimodal felsic-mafic volcanics in which felsic ash flow tuffs dominate. The overlying Danshanshi Group includes chiefly quartzites and conglomerates and the Xiyanghe Group (∼ 1830 Ma) comprises bimodal volcanics and red beds. Granitoids range from Late Archean to Early Proterozoic in age and vary from tonalite to granite in composition. Precambrian igneous rocks show a subduction zone geochemical component (depletion in Nb-Ta relative to LILE), and Early Proterozoic granitoids range in composition from I to S types. Geochemical and isotopic constraints favor an origin for Early Proterozoic Zhongtiao felsic magmas either by melting of sediments or tonalites in the crust or by melting of amphibolite/eclogite in a descending slab. Zhongtiao pelites are similar in composition to Phanerozoic shales and exhibit negative Eu anomalies on chondrite-normalized graphs. Stratiform copper deposits in the Zhongtiao Proterozoic rocks are of three types: volcanic-hosted deposits of hydrothermal or exhalative origin; primary sedimentary deposits; and remobilized sedimentary deposits. The Jiangxian and Zhongtiao lithologic assemblages of quartzite-pelite-carbonate together with primary textures and structures in these rocks suggest deposition in shallow-marine, tectonically stable basins, whereas coarse terrigenous sediments in the Danshanshi and Xiyanghe Groups appear to reflect near-shore marine to fluvial deposition in tectonically active basins. Most data are consistent with deposition of the Early Proterozoic supracrustal rocks in a continental-margin back-arc basin.

The geology and geochemistry of an Early Proterozoic volcanic-arc association at Cartwright Lake: Lynn Lake greenstone belt, northwestern Manitoba

The geology of the Cartwright Lake area consists of a >2 km thick conformable sequence of Early Proterozoic supracrustal rocks intruded by calc-alkaline granitoid plutons. The supracrustal succession comprises a basal series of tholeiitic basalts, an overlying bimodal sequence, and an uppermost series of calc-alkaline andesites. The bimodal sequence incorporates abundant tholeiitic basalts and associated mafic tuffs, lesser felsic hyaloclastites and pyroclastics, and minor interflow greywacke–mudstone turbidites.Petrogenetic models involving trace-element concentrations indicate that most of the extrusive and intrusive rocks were derived from similar parent magmas that formed by extensive partial melting of a garnet lherzolite upper-mantle source. The parent liquids fractionated along an early tholeiitic trend and a later calc-alkaline trend, producing the observed geochemical variations in the mafic and intermediate volcanic assemblages. Ponding of mafic magma at the base of the crust may have promoted crustal fusion, thereby generating felsic liquids, which erupted and formed the dacite–rhyolite suite.The geology and geochemistry of the volcanic assemblages are consistent with a subduction-related origin in a volcanic-arc setting. The majority of geochemical evidence favours the interpretation that the Cartwright Lake segment of the arc developed on relatively thin sialic crust.

Geochemistry and petrogenesis of early Proterozoic amphibolites, west-central Colorado, U.S.A.

Early Proterozoic supracrustal rocks exposed in west-central Colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments. Three successions are recognized. The Dubois succession (1770-1760 Ma) is comprised chiefly of amphibolites and felsic volcanics. The amphibolites exhibit low concentrations of light REE and HFSE and are broadly similar in composition to tholeiites from immature island arcs and, in some respects, to NMORB. The Cochetopa succession (1745-1730 Ma) includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible-element enrichment similar to basalts from evolved island-arc systems and associated back-arc basins. The Black Canyon succession is coeval with the Cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite. These amphibolites exhibit significant incompatible-element enrichment similar to basalts from continental-margin arc systems. Geochemical modeling indicates that the amphibolites evolved by closed-system fractional crystallization of depleted (for the Dubois succession) to variably enriched parental basalts (for the Cochetopa and Black Canyon successions), chiefly at shallow depths. Incompatible-element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted (1770-1760 Ma) to variable but slightly enriched (1745-1730 Ma). A provisional model calls upon the Dubois succession forming in an island-arc system followed by collision with the Archean Wyoming Province at ∼ 1750 Ma. The Cochetopa succession was deposited in an intra-arc basin developed within a new continental-margin arc, and the Black Canyon succession formed in a back-arc basin associated with this arc.

Geochemistry and Tectonic Setting of Early Proterozoic Supracrustal Rocks in the Southwestern United States

A revision of previous models of Proterozoic crustal accretion in the southwestern U.S. is presented, based on the more extensive data now available. Five Early Proterozoic predominantly volcanic supracrustal terranes are recognized, with ages of 1760-1800 Ma, 1730-1740 Ma, 1720 Ma, 1680-1700 Ma, and 1650 Ma. In some areas they are overlain by a sixth, comprised chiefly of quartzite-pelite. In the oldest three terranes mafic volcanic rocks of largely submarine origin are more abundant than felsic. In the 1680-1700 Ma and 1650 Ma terranes felsic rocks exceed mafics, volcanics are mixed submarine and subaerial, and non-volcanogenic sediments are important. Least mobile incompatible element distributions in basalts from the 1760-1800 Ma and 1720 Ma terranes are similar to those in basalts from evolved oceanic arcs and associated back-arc basins. Basalts from the other terranes resemble those from continental-margin arcs and associated back-arc basins at early stages of opening. Incompatible element ratios in...

Early Proterozoic bimodal volcanic rocks in central Colorado, U.S.A., Part II: Geochemistry, petrogenesis and tectonic setting

The early Proterozoic supracrustal rocks of the Salida area of central Colorado consist of strongly bimodal sequences of volcanogenic rocks. The mafic rocks — basalts, basaltic volcaniclastics, and related gabbro sheets — are distinctly tholeiitic, display a strong iron-enrichment trend, and typically contain less than 50% SiO 2. The felsites are rhyolites to dacites and contain more than 70% SiO 2. Major and trace element modeling show that the mafic rocks underwent two stages of crystal fractionation, the first involving olivine and plagioclase, the second involving plagioclase and clinopyroxene. Fractionation occurred within individual injections as they rose toward the surface rather than in a single magma chamber at depth. Field relations and major element data support the derivation of the felsic rocks from a magma generated by anatexis of sialic crust. However, the low Sr and high heavy REE concentrations in these rocks are not compatible with a partial melting model and suggest that the felsic volcanic rocks could have been derived by extensive fractional crystallization of the mafic magma. Normalized trace element abundances and trace element ratios of the mafic rocks are most like continental flood basalts such as the Columbia River basalts. They also display some similarity to immature back-arc basin tholeiites developed on continental crust, such as those of the Sarmiento complex. The felsic rocks have strong chemical affinities to within-plate rhyolites rather than calc-alkaline rhyolites from orogenic areas. The chemical data, as well as the petrographic, stratigraphic, and regional field data all indicate that the early Proterozoic supracrustal rocks of the Salida area developed along a continental margin, probably within an immature back-arc basin underlain by sialic crust. Remnants of the arc system of similar age may lie to the south in northern New Mexico and southwestern Colorado.

Stratigraphy, petrology, and platinum-group element mineralization of the early Proterozoic Penikat layered intrusion, northern Finland

The Penikat layered intrusion is part of an early Proterozoic belt of layered intrusions, 2,440 m.y. in age, which extends across northern Finland. The intrusion is located between late Archean granitoids and early Proterozoic supracrustal rocks. The Marginal series, with slightly contaminated chilled margin, separates the Penikat intrusion from the underlying basement complex. The Layered series is divided into five megacyclic units, I to V from bottom to top, each unit comprising an ultramafic lower part and a gabbroic upper part.Three major mineralized zones, enriched in platinum-group elements (PGE), the Sompujaervi (SJ), Ala-Penikka (AP), and Paasivaara (PV) mineralizations, have been delineated along almost 23 km of strike and occur in megacyclic unit IV. The Sompujaervi mineralization, about 1 m thick, is mostly associated with pyroxenitic rocks at the lower contact of the unit, whereas the Ala-Penikka zone (about 0.35 m thick) is associated with gabbronorite and poikilitic-textured anorthosite in the middle part and the Paasivaara zone (about 1 m thick) with leucogabbro and anorthosite very close to the upper contact.The ratios of platinum-group elements are relatively constant for each mineralization. The Paasivaara mineralization represents the most platinum-rich variety and the Ala-Penikka the most palladium-rich one, the ratio of Pd to Pt being typically about 2 for the sulfide-free type, and 1.5, 3.5, and 0.7 for the sulfide-bearing type for the Sompujaervi, Ala-Penikka, and Paasivaara mineralizations, respectively.The sulfur-free platinum-group minerals (PGM), sperrylite and isomertieite are the dominant species encountered in the Sompujaervi mineralization, whereas sulfur-bearing platinum-group minerals, together with sperrylite, dominate in the Ala-Penikka and Paasivaara mineralizations. These are represented by braggite and vysotskite in the Ala-Penikka one and by braggite and cooperite in the more platinum-rich Paasivaara mineralization.According to the new classification of platinum-group element deposits proposed by Cabri and Naldrett (1984), the typically sulfide-free Sompujaervi mineralization may be included in the oxide-silicate association as a possible new type, whereas the Ala-Penikka and Paasivaara mineralizations may be classified as Merensky reef-type of the sulfide associations.

Geochemistry and origin of early Proterozoic supracrustal rocks, Dos Cabezas Mountains, southeastern Arizona

Early Proterozoic supracrustal rocks in the Dos Cabezas Mountains in southeastern Arizona include felsic volcanic and hypabyssal rocks, amphibolite, feldspathic quartzite, arkose and conglomerate, quartzite, and phyllite. Felsic volcanics are chiefly water-laid tuff and associated volcaniclastic sediments. Feldspathic quartzites, arkoses, and conglomerates are derived from granitic and felsic volcanic sources. Felsic and basaltic igneous rocks define a bimodal suite, and felsic rocks can be divided into three populations: rhyodacites, quartz latites, and rhyolites. Volcanics share major- and trace-element distributions in common with modern volcanics from continental rift systems and few in common with arc or oceanic volcanics. Dos Cabezas basalts can be produced by ∼20% melting of an incompatible-element-enriched Iherzolite source in the mantle followed by as much as 40% fractional crystallization of olivine. Rhyodacites can be produced by partial melting of an undepleted granulite source in the middle crust, and quartz latites and rhyolites by fractional crystallization of rhyodacite magma. The Proterozoic succession in the Dos Cabezas Mountains appears to have formed in a continental rift system similar to or part of early Proterozoic rift systems in central and southern New Mexico and, perhaps, in central Arizona.

A Proterozoic Volcano-Plutonic Terrane, Gunnison and Salida Areas, Colorado

Early Proterozoic supracrustal rocks near Gunnison and Salida, Colorado include tholeiitic basalt, dacite to rhyolite, and intercalated sedimentary rocks. These were intruded by essentially synchronous gabbroic sheets, folded, and intruded by major plutons ranging from quartz diorite to granite. Preservation of primary volcanic and sedimentary features is good to excellent; sedimentary rocks in the Gunnison area are mostly turbidite wackes whereas near Salida they are mostly near-source volcaniclastic rocks. U-Pb age measurements on zircons indicate that the earliest bimodal volcanic rocks erupted between 1770 and 1760 m.y. ago and were intruded by granitic plutons between 1760 and 1750 m.y. ago. Renewed bimodal volcanism occurred between 1740 and 1730 m.y. ago and was followed by a period of deformation producing two fold sets before 1725 m.y. ago when the first of a series of younger plutons was emplaced. The compositions and features of the sedimentary rocks and their association with volcanics are con...

The origin of the early Proterozoic supracrustal rocks in the Garpenberg district, south central Sweden

Abstract The early Proterozoic supracrustal sequence in the Garpenberg district, south central Sweden, consists mainly of quartz-feldspar gneissses (‘leptites’) and subordinate amphibolites, quartz gneisses, mica schists, skarns and marbles. Stratabound iron and sulfide deposits are intercalated within this sequence. Most of the quartz-feldspar gneisses and amphibolites are interpreted as acid and basic metavolcanic rocks. The quartz gneisses and mica schists represent the metamorphic equivalents of originally altered felsic volcanic and sedimentary rocks. The geologic development of the Garpenberg district was dominated by extensive acid volcanism of calc-alkaline affinity, which was interrupted by periods of intense hydrothermal activity, ore deposition and sedimentation. Most of the volcanic activity occurred was submarine in a shallow-water environment. Subsequently, the supra crustal sequence was affected by the Svecokarelian orogeny (1800–1750 Ma) and the rocks were metamorphosed in the amphibolite ...

Basement–cover relationships during polyphase deformation in the Svecokarelides of the Kaavi district, eastern Finland

ABSTRACTFor much of its length in eastern Finland, the contact between the late Archaean granitoid terrane and early Proterozoic supracrustal rocks is that of a depositional unconformity tectonically modified during the Svecokarelian orogeny. In the Kaavi district, this modification has been extensive, particularly because of thrusting which resulted in tectonic slices of basement being present as inliers within the cover rocks and cover rocks as outliers within the main basement outcrop. Both basement and cover have undergone repeated deformation and metamorphic recrystallisation reflected by the presence of multiple tectonic fabrics and sequential mineral growths. However, it is only locally in major zones of movement that the basement rocks of the Presvecokarelides show major modification as the result of the deformation which had such a profound effect on the cover rocks during the development of the Svecokarelides. Over large areas, the products of four phases of deformation (DI–IV) remain in the basement gneisses together with the products of basic and acidic intrusion whose positions in the deformational sequence can be established‥In the cover rocks, early nappe emplacement (Outokumpu nappe) was followed by the development of isoclinal folds associated with prograde metamorphism (D1). Regionally expressed asymmetrical folds, overturned to the E, with attenuation of overturned limbs and thrusting, involving basement, followed (D2). Subsequent deformation related to wrench faulting (D2c) was followed by the development of more open folds associated with declining geotherms (D3, D4); F3–F4 interference structures are expressed as domes, some of which were a locus of granitoid emplacement (Maarianvaara granitoid suite). D5 and D6 are expressed as cleavages and fractures.Correlation of the structural features in the Kaavi district with those in the Outokumpu and adjacent districts forms the basis both for establishing the regional disposition of basement and cover units and for determining the structural evolution of this part of the Svecokarelides.

The evolution of the Precambrian Shield of Greenland

The Archaean gneiss block of Greenland is made up of gneisses, amphibolites, anorthositic rocks and minor supracrustals. It contains the oldest crustal rocks yet recorded on earth. The Archaean gneiss block is bordered to the north and to the south by Proterozoic mobile belts. The Nagssugtoqidian and Rinkian mobile belts to the north, differentiated on the basis of differences in the tectonic development, consist mainly of reworked Archaean rocks. Early Proterozoic supracrustal rocks are prominent in the Rinkian mobile belt, where they overlie the Archaean basement. The Ketilidian mobile belt to the south consists mainly of Proterozoic supracrustal rocks and granites. After renewed denudation late Proterozoic supracrustal rocks were deposited in North and South Greenland where they are associated with large amounts of late Proterozoic intrusive rocks.

Structural analysis of sheath folds with horizontal X-axes, northeast Canada

Early Proterozoic supracrustal rocks occur below a thick nappe of Archaean basement gneiss in the Melville Peninsula where sheath folds are exposed in a wide zone of middle Proterozoic dynamothermal metamorphism. Outcrop patterns of truncated isoclinal sheath folds resemble cylindrical folds except in relatively small areas around the paraboloidal caps. Bulk extension axes are parallel to strike in the belt as shown by isoclinal sheath folds with horizontal central axes ( X-axes), as well as similarly aligned mullion structure and rotated scapolite prisms. Extension axes converge from northeast to southwest in the apparent flow direction.