Presence Of Continental Crust Research Articles

U–Pb and Hf isotopes in granitoids from the Eastern Bolivian basement: Insights into the Paleoproterozoic evolution of the western part of South America

Granitoids from the Eastern Bolivian basement at Santo Corazón, Correreca, and Santa Terezita, and the Santana Gneiss from the Corumbá region (Mato Grosso de Sul state, Brazil) were studied by geochemistry and Sm–Nd and U–Pb isotope analysis. The granites have a medium to high calcium-alkaline potassic character and are typically peraluminous S-type granites. For The Santo Corazón quartz monzonite and the Correreca granite we obtained Orosirian upper intercept U–Pb zircon ages of 1874 ± 15 Ma and 1862 ± 7 Ma. For Santa Terezita granite two upper intercept U–Pb zircon ages of 1849 ± 11 and 1852 ± 6 Ma were obtained. The geochemical and Nd and U–Pb isotope data suggest that these complexes were part of a crust created between 2.23 and 1.96 Ga, which was reworked mainly in Orosirian times in a magmatic arc setting. This is further supported by Hf isotope data for zircon from the Correreca granite. The garnet-hornblende-biotite Santana Gneiss gave an upper intercept zircon U–Pb age of 1764 ± 23 Ma, which is interpreted as the crystallization age of the igneous protolith, a quartz-monzonite to granodiorite. The petrological, geochemical, and Nd as well as U–Pb isotope data show similarity between the Eastern Bolivian basement granites (EBB) and the Santana Gneiss with the Alumiador Intrusive Suite and the Caracol Gneiss, respectively. This suggests that this crustal fragment could have been an extension to the west of the Rio Apa Block, which today extends for 300 km from the southwest of Mato Grosso do Sul and northeast of Paraguay to Eastern Bolivia. The presence of continental crust that dates back to 1.7–1.8 Ga is inferred from isotope data for the cores of zircon from the Santana Gneiss and the Eastern Bolivian granites (EBB). In contrast, U–Pb ages obtained in rims of zircon crystals may indicate that two collisional events took place: a first one at 1.7 Ga, which we can relate to the collision of the Eastern and Western terranes of the Rio Apa Block with the Paraguá Block located to the west; and a second one at 1.3 Ga, related to the collision between the Paraguá Block, Eastern Bolivian basement granite terrane and the Rio Apa Block (EBB + RAB) with the Amazonian Craton during the San Ignacio Orogeny. This assemblage remained stable during the amalgamation into the Rodinia supercontinent.

The pre-breakup stratigraphy and petroleum system of the Southern Jan Mayen Ridge revealed by seafloor sampling

The Jan Mayen Microplate Complex (JMMC) in the NE Atlantic is interpreted to mostly consist of continental fragments with possible interstitial embryonic oceanic crust. A complex Cenozoic rifting history accompanied by extensive extrusive and intrusive volcanism have made the geological characterization of the JMMC challenging especially due to poor seismic imaging beneath the breakup basalt succession. The presence of continental crust in the JMMC is inferred by seismic and magnetic data, but ground truthing evidence have yet to be provided. Here, we present the results from a seafloor sampling campaign undertaken in 2011 on the Southern Jan Mayen Ridge complex. Seabed samples were recovered using a gravity corer and a dredge along a 1000 m high escarpment with a 19° slope. Sampling locations were selected based on the interpretation of seismic profiles that suggest the presence of possible pre-breakup successions outcropping along this steep escarpment. Results include a sequence of samples with age diagnostic palynomorph assemblages ranging from Permian/Triassic to Eocene, and including igneous samples related to the Early Eocene breakup volcanism. Importantly, the samples were retrieved from hard substrate in an erosional gully lacking overburden sediments and have ages arranged in younging upward sequential order, supporting their near in-situ position. The sampling results were integrated into a lithostratigraphic pseudo-well that can be used to constrain the evolution and breakup of the JMMC. Additionally, evidence for active migration of Jurassic sourced hydrocarbons comprise the first indication of a working hydrocarbon system, with important implications for the petroleum prospectivity of the Dreki area. Finally, these results confirm that the Southern Jan Mayen Ridge is indeed a sliver of continental crust.

THE ALUMINOUS GNEISSES OF KOLA SERIES, BALTIC SHIELD (GEOCHEMISTRY, NATURE AND AGE OF PROTOLITH)

The geochemical and isotopic data were obtained for gneisses of the Kola Series, Baltic Shield. They are metamorphosed in granulite facies and interpreted according to the igneous nature of zircons and gneisses. The biotitic and garnet-biotitic gneisses are paralleled to the rocks of TTG associations in terms of the structure of correlation between the main elements. Sil ± Crd-Grt-Bt gneisses are formed on Grt-Bt gneisses as a result of blastomylonitization and partial melting. U-Pb zircon ages (SHRIMP-II) were determined for both gneiss species. All analytical points of magmatic zircons in the concordia diagram for Grt-Bt gneisses form a continuous trend extended in concordia in the range of 2909 ± 8 – 2732 ± 13 Ma. It is interpreted as the result of lead loss from magmatic zircons during the slow cooling of tonalite in the lower crust. Minimal age of magmatism and early granulitic metamorphism estimated by most ancient zircon is 2.9 Ga. The time of manifestation of younger granulite metamorphism determined by newly-formed zircon shells from Crd-Sil-Grt-Bt gneiss corresponds to the interval 2707 ± 9 – 2656 ± 8 Ma. Part of the zircon from Crd-Sil-Grt-Bt gneiss is profoundly altered crystals with a discordant age of 2740 ± 6 – 2639 ± 9 Ma, which is similar to that of metamorphic shells, suggesting an event happened at that time which disrupted the crystalline grid and caused the loss of radiogenic lead. Three ancient zircons with a maximum age of 3461 ± 5 MA are interpreted as captured zircons, which indicate the presence of continental crust at this point in the northern part of the Baltic Shield.

Geochemical and petrological evidence for subduction–accretion processes in the Archean Eastern Indian Craton

The composition of sandstones often provides key evidence about the tectonic, weathering and transport processes operating on the surface at the time of deposition. Petrologic and geochemical analyses of little-metamorphosed middle–late Archean sandstones from the Eastern Indian Craton show that the sedimentary rocks were derived from dominantly 3.3-Ga-old amphibolites of the Older Metamorphic Group (OMG) and tonalites of the Older Metamorphic Tonalite Gneisses (OMTG), the two oldest lithologic units of this craton. Chondrite-normalized rare earth element (REE) patterns of the sandstones show a light REE-enriched signature with (La/Sm)N varying from 5.2 to 6.7 with no Eu anomaly, while the heavy REEs display flat patterns with (Gd/Lu)N values of 0.9–1.6. Primitive mantle-normalized incompatible and compatible trace element plots of these sandstones demonstrate an overall similarity with global Proterozoic–Archean sandstones, including strong Nb–Ta negative anomalies. In an fSm/Nd vs. ϵNd(0) diagram, the sandstones plot precisely between the regional amphibolites and tonalites. We infer from the REE abundances and the fSm/Nd vs. ϵNd(0) plot that the sandstones represent a bimodal mechanical mixture of OMG and OMTG. The low Ce/Pb ratios of these rocks of 1–4 indicate a variably Pb-enriched Archean crust and that the Ce/Pb ratio acquired the continental crustal signature, distinctly different from those of the bulk silicate earth and mantle values at least as early as mid-Archean. Strong Nb–Ta depletion relative to the primitive mantle suggests the sandstones were derived from subduction-related magmatic arc sources. This latter suggestion is strongly supported by the low Nb/Ta and high Zr/Sm ratios of these sandstones, identical to Archean tonalite–trondhjemites that require, based on recent trace-element partitioning results, their protoliths to have formed by subduction melting of low-magnesium amphibolites or metamorphosed hydrous basalts [1,2]. The average Nd model ages of the sandstones are greater than the Sm–Nd crystallization ages of the OMG and OMTG at ∼3.3 Ga. The geochemical data presented here can be collectively interpreted to suggest the presence of subduction–accretion processes operational in the mid-Archean Eastern Indian Craton. Because the OMG and OMTG, the source rocks of the sandstones, formed in a subduction-related arc setting, the basement rocks upon which this arc was constructed must have been older. This observation and the depleted mantle Nd model ages (TDM) of the sandstones, ranging from 3.6 to 4.0 Ga, strongly indicate the presence of continental crust in this Eastern Indian craton older than 3.3 Ga and possibly as old as 4.0 Ga.

On the partitioning of mantle heat loss below oceans and continents over time and its relationship to the Archaean paradox

Summary The Archaean paradox stems from the fact that while certain lines of evidence suggest that Archaean continental geotherms were similar to those at present, other lines of evidence suggest that the average mantle heat flux was considerably greater. The simplest qualitative solution, which holds that a greater proportion of heat loss was carried by the creation and subduction of oceanic lithosphere in the Earth’s past, has lacked a clear physical basis. At a fundamental level, it requires that the ratio of mantle heat loss below oceans to that below continents be an increasing function of overall convective vigour and, to date, no self-consistent model has been used to suggest a means by which this could be so. A simple means is suggested by models that allow accumulations of chemically light near-surface material, analogues to continents, to form within the upper thermal boundary layer of a thermally convecting and chemically dense fluid, an analogue to the mantle. The physical properties of continental crust cause the thermal coupling condition between a model continent and the convectively unstable mantle below to be different from that which exists at the interface between mantle and oceans. For the latter, a spatially constant temperature condition holds due to (1) the participation of oceanic crust in convective mantle overturn and (2) the large effective thermal conductivity of oceans relative to the mantle. For the former, a spatially near-constant equilibrium heat flux condition holds due to (1) the long-term stability of continental crust against remixing into the mantle and (2) the comparable thermal conductivities of distinct rock types. This leads to a laterally variable thermal condition at the surface of the convecting mantle, which, in turn, leads to different local mantle heat flux behaviour in continental versus oceanic regions. As a result, heat flux below continents can increase at a slower rate, with increasing convective vigour, than it does elsewhere. Model heat-flux scalings are used to assess the degree to which continental geotherms can be time-stabilized by this means. Results suggest that the thermal surface heterogeneity imposed on the mantle by the presence of continental crust can prevent wide-scale crustal melting in the Archaean by forcing oceans to carry a greater proportion of the Earth’s heat-loss load in the past.

40Ar–39Ar geochronology of silicic and basic volcanic rocks on the margins of the North Atlantic

At the submerged margins of the North Atlantic, andesitic to dacitic and basaltic volcanic rocks occur together. The silicic rocks were derived by processes requiring the presence of continental crust (crustal anatexis and/or contamination of mafic magmas) while the majority of the basaltic lavas had little or no contact with continental crust. We report 40Ar–39Ar incremental heating ages for several dacitic and basaltic rocks recovered from three offshore localities of the North Atlantic Igneous Province. Dacitic lavas and tuffs at the southeast Greenland margin and trachytic lavas in the Scottish Hebrides erupted contemporaneously with basaltic lavas at 62–61 Ma. In contrast, the silicic lavas from the northern Rockall Trough (offshore western Scotland) and the Vøring Plateau (offshore Norway) erupted at ∼55 Ma followed shortly by basaltic volcanism. At this time, silicic magmatism at the southeast Greenland margin had ceased and only oceanic basalts were erupted. Similarly, ∼55 Ma lavas on the southwest Rockall Plateau are wholly basaltic. The compositions of all of the dated silicic volcanic rocks are consistent with derivation from partial melting of either continental crust or sediments. The heat necessary for partial melting appears to have been provided by basaltic magmas. Therefore, the existence of the silicic rocks indicates the presence of continental crust as well as a stable tectonic environment that allowed the stagnation and pooling of basaltic melts within the crust. With this in mind, it is apparent that at 62–60 Ma, both western and eastern sides of the present North Atlantic margins were characterized by extensional environments within continental crust that were restrictive to the passage of mafic magmas. By 55 Ma, at the time of continental breakup, the proximal margins at southeast Greenland and the Rockall Plateau were devoid of continental crust. But the presence of 55 Ma silicic magmatism on the eastern North Atlantic margin can be attributed to a broader zone of magmatism and sediment-filled Mesozoic rift basins.

The Andean subduction zone between 22 and 25°S (northern Chile): precise geometry and state of stress

One year of seismicity recorded by a local network is used to obtain more precision about the geometry and the stress regime of the Andean subduction between 22 and 25°S in the northern Chile seismic gap. A sharp image of the Wadati-Benioff Zone (WBZ) is obtained down to 270 km in depth. A seismically quasi-quiescent zone is observed in the WBZ below the volcanic arc, between 150 and 210 km in depth. Hypocentres of distant intermediate depth earthquakes located with the local network are compared with worldwide seismic network hypocentres in order to evaluate the accuracy of the WBZ image at depth greater than 100–150 km in depth. No shallow microearthquakes have been observed in the continental crust but some seismic activity is likely to occur locally at the deep root of the Atacama Fault. The stress field and the characteristics of faulting along the subducted slab are investigated. Underthrusting and localized reverse faulting earthquakes define the seismically coupled plate interface from 20 to 50 km in depth (Locked zone). Downdip, intra-slab normal faulting prevails (Tensile zone), but some strike-slip faulting is observed. A transition between normal faulting with variable fault azimuth and normal faulting with nearly homogeneous NNW- to NW-oriented fault plane is found at about 80 km in depth. It is found that the stress axes σ 1 and σ 3 in the Locked zone are oriented in the convergence direction (75–80°E). Downdip, in the tensile zone, σ 3 has a mean azimuth 60–65°E. There, the slab is hence submitted to a tensional force (slab pull) oblique relatively to the convergence. The transition between seismic underthrusting and intraplate normal faulting downdip occurs at the depth where the continental Moho encounters the Wadati-Benioff Zone, suggesting that a relationship exists between seismic coupling and the presence of continental crust at the plate interface. The pre-seismic state of this segment of the Andean subduction zone is confirmed by the occurrence of strong earthquakes located by the global network around the presumed rupture area and by the stress regime found along the Wadati-Benioff Zone.

U-Pb Geochronology of Archean magmatism and Proterozoic metamorphism in the Quadrilátero Ferrífero, southern São Francisco craton, Brazil

Research Article| September 01, 1992 U-Pb Geochronology of Archean magmatism and Proterozoic metamorphism in the Quadrilátero Ferrífero, southern São Francisco craton, Brazil N. MACHADO; N. MACHADO 1Department of Geology, Royal Ontario Museum, Toronto, Ontario M5S 2C6, Canada Search for other works by this author on: GSW Google Scholar C. M. NOCE; C. M. NOCE 2Departamento de Geologia, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil Search for other works by this author on: GSW Google Scholar E. A. LADEIRA; E. A. LADEIRA 2Departamento de Geologia, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil Search for other works by this author on: GSW Google Scholar O. BELO DE OLIVEIRA O. BELO DE OLIVEIRA 3Rio Doce Geologia e Mineração (DOCEGEO), Belo Horizonte, Minas Gerais, Brazil Search for other works by this author on: GSW Google Scholar Author and Article Information N. MACHADO 1Department of Geology, Royal Ontario Museum, Toronto, Ontario M5S 2C6, Canada C. M. NOCE 2Departamento de Geologia, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil E. A. LADEIRA 2Departamento de Geologia, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil O. BELO DE OLIVEIRA 3Rio Doce Geologia e Mineração (DOCEGEO), Belo Horizonte, Minas Gerais, Brazil Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (9): 1221–1227. https://doi.org/10.1130/0016-7606(1992)104<1221:UPGOAM>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation N. MACHADO, C. M. NOCE, E. A. LADEIRA, O. BELO DE OLIVEIRA; U-Pb Geochronology of Archean magmatism and Proterozoic metamorphism in the Quadrilátero Ferrífero, southern São Francisco craton, Brazil. GSA Bulletin 1992;; 104 (9): 1221–1227. doi: https://doi.org/10.1130/0016-7606(1992)104<1221:UPGOAM>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Quadrilátero Ferrífero is a metallogenic district (Au, Fe, Mn) located in the southernmost São Francisco craton in eastern Brazil. It is composed of Archean granitoids, gneisses, and a greenstone belt sequence (Rio das Velhas Supergroup), overlain by a Proterozoic clastic-chemical sedimentary sequence hosting a thick Lake Superior-type banded iron formation (Minas Supergroup). A U-Pb study of the main units leads to the following conclusions: granitoid emplacement was dated at 2776 +7/-6 Ma and 2721 ± 3 Ma; two felsic volcanic rocks from the Rio das Velhas Supergroup yielded ages of 2776 +23/-10 Ma and 2772 ± 6 Ma. These data indicate that greenstone-belt volcanism was coeval with granitoid emplacement, a common feature of greenstone-belt terrains, but so far undocumented in the southern São Francisco shield. The younger granitoid intrusion marks the stabilization of the shield in Late Archean time. Zircon cores older than 2776 Ma, together with other data, indicate the presence of continental crust 2.88-3.1 Ga old in the Quadrilátero Ferrífero. A detrital zircon from the uppermost formation in the Minas Supergroup (Sabará Formation) is concordant at 2125 ± 4 Ma and provides the first age constraint on the deposition of the Supergroup. Monazite from two pegmatites and titanite from an amphibolite enclave in gneisses from the Bação Complex, in central Quadrilátero Ferrífero, yielded ages between 2060 Ma and 2030 Ma. These data may constrain the minimum age for the deposition of the Minas Supergroup. In addition, they indicate that the Early Proterozoic Transamazomian orogeny is represented by amphibolite-facies metamorphism and partial melting of Archean crust. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.