Eastern Basement Research Articles

Neoarchean reworking of Mesoarchean and Paleoarchean crust (3.4 ∼ 3.0 Ga) within the North China Craton: Constraints from zircon U-Pb geochronology and Lu-Hf isotopes from the basement of the Bohai Bay Basin

The Bohai Bay Basin basement is mainly composed of Archean granitoid gneisses with minor supracrustal rocks and is the largest basin in the Eastern Block of the North China Craton. Due to a cover of Mesozoic and Cenozoic strata, little is known about the age and crustal evolution of this basement. In this study we report new zircon SHRIMP and LA-ICP-MS U-Pb and Lu-Hf isotope data for drill core samples, including TTGs (granodiorite, tonalite gneiss, trondhjemite gneiss), granites (monzogranite, syenogranite) and a leptite, with the aim of revealing the Archean crustal evolutionary history of the Bohai Bay Basin basement. The U-Pb age of magmatic zircons from these granitoids reveals that basement rocks were mainly generated by two-stage events at ∼3.1 Ga and ∼2.5 Ga. The εHf(t) values of ∼3.1 Ga magmatic zircons vary from +0.56 to +8.27, and their corresponding single-stage model ages range from 3.3 Ga to 3.0 Ga. The εHf(t) values of ∼2.5 Ga magmatic zircons range from −12.87 to −0.07, their corresponding two-stage model ages range from 3.8 Ga to 2.9 Ga with most ages from 3.4 Ga to 3.0 Ga. The Hf isotopic characteristics show that the crustal growth of basement beneath Bohai Bay Basin occurred mainly between 3.4 Ga and 3.0 Ga, different from crustal accretion ages of 2.9–2.7 Ga on the periphery of the Bohai Bay Basin. However, both areas were reworked by the ∼2.5 Ga tectono-thermal event. Integration of this new data from the basin basement with previous data, indicates that the Eastern Block of the North China Craton may be controlled by a mantle plume during the ∼2.5 Ga period. The results from this study are significant in assessing the tectonic environment of the eastern basement in the North China Craton.

U‐Pb zircon dating of metaigneous rocks from the Nordbreen Nappe of Svalbard’s Ny‐Friesland suggests their affinity to Northeast Greenland

AbstractSIMS U‐Pb zircon dating of metaigneous rocks of the Nordbreen Nappe from the West Ny‐Friesland terrane (Eastern Basement Province of Svalbard) yields crystallization ages of 1,761 ± 4 Ma for a felsic metatuff and 1,373 ± 4 Ma for a metagabbro dyke. The Palaeoproterozoic age of the metatuff is similar to previously obtained ages for various felsic rocks from the study area, whereas the Mesoproterozoic age of the metagabbro has not been thus far documented on Svalbard. However, a similar age pattern has been reported from Northeast Greenland. Therefore, we conclude that the West Ny‐Friesland terrane can be correlated with age equivalent units of Northeast Greenland. We also explore similarities and dissimilarities between the study area and other terranes of Svalbard and speculate on regional‐scale correlations. Together, the presented new ages provide critical piercing points in palaeogeographic reconstructions of the relatively poorly understood circum‐Arctic region.

Mesozoic – Cenozoic evolution of the Southern Patagonian Andes fold and thrust belt (47°–48°S): Influence of the Rocas Verdes basin inversion and onset of Patagonian glaciations

The Southern Patagonian Andes (SPA) between 47°–48°S are characterized by relatively high elevations (1500–4000 m) and a 200-km-wide, largely exhumed, basement domain along the backbone of the Cordillera, bounded by a narrow 30-km-wide fold and thrust-belt to the east. The tectonic evolution of the SPA includes widespread Jurassic extension during the opening of the Austral-Magallanes basin and the Rocas Verdes back-arc oceanic basin (RVB), followed by Cretaceous to Cenozoic shortening. Although the main phases of deformation that affected this Andean segment have been the focus of previous studies, its finite geometry and step by step structural and kinematic evolution have not been quantitatively determined. In this work, we present kinematically restored cross sections, showing the variable geometry, trough time, of the eastern basement domain and the fold and thrust-belt. A regional westward-dipping listric decollement, interpreted as formed during the Jurassic extensional event, was reactivated during the Andean deformation constituting the main detachment surface of basement structures and the fold and thrust belt. West verging faults systems are considered a consequence of inversion of north-south trending extensional features as passive roof high-angle west-vergent faults. Using regional data such as field relations, unconformities, and existing ages to differentiate deformational stages, three main phases of contractional deformation are proposed for the area, starting with an Aptian - Cenomanian? phase related to inversion of the extensional depocenter in inmediate response to the closure of northernmost RVB. A second, Miocene phase responsible for most of the shortening occurred between 18 Ma and 10 Ma. Finally, the onset of glaciations and glacial erosion might have triggered out-of-sequence thrusting in the interior of the orogen after 7 Ma.

Records of Mesoproterozoic taphrogenic events in the eastern basement of the Araçuaí Orogen, southeast Brazil

ABSTRACT: The history of palaeocontinents alternates long fragmentation to drift periods with relatively short agglutination intervals. One of the products of a Rhyacian-Orosirian orogeny was a palaeocontinent that brought together the basement of the Araçuaí-West Congo orogen (AWCO) with regions now located in the São Francisco and Congo cratons. From ca. 2 Ga to ca. 0.7 Ga, this large region of the São Francisco-Congo palaeocontinent was spared of orogenic events, but underwent at least five taphrogenic events recorded by anorogenic magmatism and/or sedimentation. The taphrogenic events are well documented in the AWCO proximal portions and neighboring cratonic regions, but lack evidence in the AWCO high-grade core. Our studies on amphibolites intercalated in the Rhyacian Pocrane complex, basement of the Rio Doce magmatic arc, allowed to the recognition of two Mesoproterozoic taphrogenic episodes. The oldest one, a Calymmian episode, is recorded by amphibolites with a zircon magmatic crystallization age at 1529 ± 37 Ma (U-Pb SHRIMP), and lithochemical signature of basaltic magmatism related to continental intraplate settings. Another set of amphibolite bodies records the youngest taphrogenic episode, a Stenian event, with a zircon magmatic crystallization age at 1096 ± 20 Ma (U-Pb SHRIMP), and lithochemical signature similar to mature magmatism of continental rift setting. The Calymmian episode (ca. 1.5 Ga) correlates to the Espinhaço II basin stage and mafic dikes of the northern Espinhaço, Chapada Diamantina and Curaçá domains, while the Stenian episode (ca. 1.1 Ga) correlates to the Espinhaço III basin stage. We also present U-Pb data for 87 detrital zircon grains from a quartzite lens intercalated in the Pocrane complex, the Córrego Ubá quartzite. Its age spectrum shows main peaks at 1176 ± 21 Ma (35%), 1371 ± 30 Ma (18%), 1536 ± 22 Ma (19%), 1803 ± 36 Ma (17%) and 1977 ± 38 Ma (12%), suggesting a Stenian (ca. 1176 Ma) maximum depositional age (although only one zircon with low discordance shows an age of 955 ± 66 Ma). Comparing with data from the western sector of the Araçuaí orogen and São Francisco craton, it is noteworthy that no igneous zircon from the three samples yielded an age older than early Orosirian (~2.05 Ga), showing age spectra essentially limited in the range of ca. 1-2 Ga; i.e., younger than the Late Rhyacian orogeny that amalgamated the basement, and older than the main anorogenic event (930-870 Ma) associated with the Early Tonian precursor basin of AWCO. All together, these continental taphrogenic events testify the several unsuccessful fragmentation attempts that affected the long-lived São Francisco-Congo palaeocontinent, which remained preserved from a complete break-up associated with ocean spreading from the Early Orosirian to the Atlantic opening in Cretaceous time.

Andean stratigraphic record of the transition from backarc extension to orogenic shortening: A case study from the northern Neuquén Basin, Argentina

The temporal transition from backarc extension to retroarc shortening is a fundamental process in the evolution of many Andean-type convergent margins. This switch in tectonic regime is preserved in the 5–7 km thick Mesozoic-Cenozoic stratigraphic record of west-central Argentina at 34–36°S, where the northern Neuquén Basin and succeeding Cenozoic foreland succession chronicle a long history of fluctuating depositional systems and diverse sediment source regions during Andean orogenesis. New findings from sediment provenance and facies analyses are integrated with detrital zircon U-Pb geochronological results from 16 samples of Jurassic through Miocene clastic deposits to delineate the progressive exhumation of the evolving Andean magmatic arc, retroarc fold-thrust belt, and foreland province. Abrupt changes in provenance and depositional conditions can be reconciled with a complex Mesozoic-Cenozoic history of extension, postextensional thermal subsidence, punctuated tectonic inversion, thick- and thin-skinned shortening, overlapping igneous activity, and alternating phases of basin accumulation, sediment bypass, and erosion.U-Pb age distributions constrain the depositional ages of Cenozoic units and reveal a prolonged late middle Eocene to earliest Miocene (roughly 40–20 Ma) hiatus in the retroarc foreland basin. This stratigraphic gap is expressed as a regional disconformity that marks a pronounced shift in depositional conditions and sediment sources, from (i) slow Paleocene–middle Eocene accumulation of distal fluviolacustrine sediments (Pircala and Coihueco Formations) contributed from far western magmatic arc sources (Cretaceous–Paleogene volcanic rocks) and subordinate eastern basement rocks (Permian-Triassic Choiyoi igneous complex) to (ii) rapid Miocene–Quaternary accumulation of proximal fluvial to megafan sediments (Agua de la Piedra, Loma Fiera, and Tristeza Formations) recycled from emerging western thrust-belt sources of Mesozoic basin fill originally derived from basement and magmatic arc sources. The mid-Cenozoic stratigraphic gap signified ∼20 Myr of nondeposition, potentially during passage of a flexural forebulge or during neutral to extensional conditions driven by mechanical decoupling and a possible retreating-slab configuration along the Nazca-South America plate boundary. Neogene eastward propagation of the Malargüe fold-thrust belt involved basement inversion with geometrically and kinematically linked thin-skinned shortening at shallow foreland levels, including late Miocene deposition of accurately dated 10.5–7.5 Ma growth strata and ensuing displacement along the frontal emergent and blind thrust structures. Subsequent partitioning and exhumation of Cenozoic clastic fill of the Malargüe foreland basin has been driven by inboard advance of arc magmatism and Pliocene-Quaternary uplift of the San Rafael basement block farther east.

Geochemical zonation across a Neoproterozoic orogenic belt: Isotopic evidence from granitoids and metasedimentary rocks of the Jiangnan orogen, China

In order to understand how Late Mesoproterozoic to Early Neoproterozoic orogenic belts evolved during the assembly and rifting of the supercontinent Rodinia, we have carried out detailed studies on the geochemical compositions of Early Neoproterozoic crust across the Jiangnan orogen (JO) which connects the Yangtze and Cathaysia blocks on the northwestern margin of Rodinia. A gradual geochemical variation is recognized from east to west, based on comparisons of whole-rock Nd isotopes, U–Pb age spectra of detrital zircons, and Hf isotopes in magmatic zircons and detrital zircons from the Neoproterozoic granitoids and metasedimentary basement sequences in the JO. LA-ICP-MS U–Pb dating of detrital zircons from the sediments and magmatic zircons from interlayered volcanic rocks suggests that the folded basement sequences formed within the span 860–825Ma, implying the final amalgamation of the Yangtze and Cathaysia blocks occurred no older than ca 825Ma. The 950–820Ma detrital zircons strongly dominate in the eastern basement sequences, and most of them show moderately to highly positive ɛHf(t). In contrast, many of the Early Neoproterozoic detrital zircons in the western JO have moderately negative ɛHf(t) and more older (>1.0Ga) detrital zircons were found in this area. We suggest that the metasedimentary basement sequences in the JO were deposited in retro-arc foreland basins which originally evolved from back-arc basins, and the change of provenance controlled the variation in crustal geochemistry of crust across the JO. Sediments in the eastern basement sequences have been sourced mainly from the juvenile subduction-related igneous rocks to the east, with a few from the central Yangtze Block, whereas those in the western JO may have been located far from arc terranes to the east and thus received more older recycled detritus from the southern part of the South China Block. Moreover, the Hf isotopes of the detrital zircons imply episodic crustal growth in the provenance of the JO metasedimentary basement sequences, with age peaks at 1.0–0.8Ga, 1.75–1.50Ga and 2.60–2.45Ga. The early crust in South China may have been formed mainly at around 3.8Ga ago, and contains some Hadean components (ca 4.1Ga).

Geophysical Investigation of Shallow Subsurface Fracture Distribution in Parts of the Eastern Basement Complex of Nigeria

Very low frequency electromagnetic method was carried out at twenty three (23) locations within Parts of the Eastern Basement Complex of Nigeria. The field data were collected using ABEM Wadi VLF (very low frequency) electromagnetic instrument with other accessories. A transmitter located at Cutler, Maine USA, designated as NAA with 24.0 KHz frequency and 1000Kw transmission power was used throughout the data collection exercise. The profiles were of lengths between 120m and 650m. The station interval along each profile was 5m with profile lines oriented in N–S, and E–W directions. Data were analyzed using KHFFILT software. The results of the study showed that majority of the fractures were oriented in the NE – SW direction, followed by fractures oriented in the NW – SE direction indicating that the study area is well fractured and has good potentials for ground water. Key word: ABEM Wadi, Basement, fracture, groundwater, Very low frequency.

Mountain building across a lithospheric boundary during arc construction: The Cretaceous Peninsular Ranges batholith in the Sierra San Pedro Martir of Baja California, Mexico

The Jura-Cretaceous Peninsular Ranges batholith (PRB) of Southern and Baja California contains a remarkable example of variation in crustal composition and structure across a batholith-parallel lithospheric-scale discontinuity. This lithospheric boundary between western oceanic-floored and eastern continental-floored crust influenced contractional deformation, arc magmatism, and differential exhumation of western and eastern zones in the batholith during its evolution. In the Sierra San Pedro Martir of Baja California, Mexico, a ca. 20 km wide, doubly vergent fan structure occurs across the PRB basement transition that consists of inward-dipping mylonite thrust sheets on the sides of the fan that gradually transition to a steeply-dipping tectonized zone in the center. A dramatic inverted metamorphic gradient occurs on the western side of this structure where mid-crustal amphibolite metamorphic grade rocks with peak pressures of 5–6 kbar in the center of the fan were thrust over upper-crustal sub-greenschist grade rocks (peak pressures < 2 kbar) in the western zone footwall. An inverted but smaller gradient occurs on the eastern side of the structure where rocks of the fan interior have been thrust eastwards over amphibolite to upper greenschist grade rocks (peak pressures 4–5 kbar). Gradients in cooling ages determined by 40Ar/ 39Ar biotite and K-feldspar and apatite fission track methods coupled with U–Pb zircon ages and Al in hornblende thermobarometry studies on plutons across this zone indicate that structures focused along the transition zone between contrasting lithosphere in the PRB accommodated nearly 15 km of the differential exhumation of western and eastern basement in the orogen. The western zone of the batholith was a major forearc depo-center for thick clastic sequences derived from the uplifting eastern PRB and remained at low average elevation during the Late Cretaceous and Paleogene. In contrast the eastern zone experienced dramatic uplift subsequent to achieving a crustal thickness in excess of 55 km by mid-Cretaceous time. This region had the isostatic potential for 4–5 km surface elevations, and likely formed a topographically high orogenic plateau. Exhumation of the fan structure initiated after 100 Ma and was largely complete by 85 Ma. Eastward-migrating unroofing of the rest of the eastern PRB continued into the Paleogene. A variety of factors were responsible for exhumation in this region. Structural thickening of the eastern zone of the orogen resulted from more than 30 million years of episodic contractional deformation in the fan structure, much of which followed island arc accretion of the western zone along this segment of the batholith. An episode of voluminous magmatism involving the intrusion of the 99–92 Ma La Posta-type magmatic suite across the eastern zone of the PRB triggered exhumation in the fan structure. Denudation in this region appears to have been solely by erosion; no evidence has been found for extensional tectonics during this time. This arc orogen demonstrates the important influence of inherited tectonic boundaries in controlling the spatial distribution of structural thickening and magmatism. It also displays the complex interrelationships among structural thickening, exhumation, and the role of magmatism in triggering exhumation episodes within orogens.

Cenozoic provenance history of synorogenic conglomerates in western Argentina (Famatina belt): Implications for Central Andean foreland development

In Famatina, at the modern northern flat-slab segment of the Central Andean belt, synorogenicconglomeratefaciesprovideaunique opportunity to reconstruct paleogeography and tectonic setting. Stratigraphically constrained provenance analyses record a multi-event history of the Andean foreland and a complex pattern of exhumed uplands, which, in turn, controlled the depocenter history. Western and eastern sources can be differentiated. The western domain was composed of equigranular granitoids and lower and upper Paleozoic volcano-sedimentary and sedimentary units, whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower Paleozoic clastic rocks. The surrounding regions expose medium- to high-grade metamorphic rocks, which are not recorded in any Cenozoic conglomerate of Famatina. Unroofing sequences can be deduced from the compositional trends. Conglomerate composition and paleocurrent data show input from intraba-sinal sources and important participation of basement since the beginning of the Neogene. We infer an early Miocene broken foreland during deposition of the Del Creston Formation.A significant source inversion, with clasts supplied from the eastern basement domain, is recorded ∼2000 m from the base, within the middle Miocene Del Abra Formation; reactivation of western granitic thrust sheets occurred by the late middle Miocene and late Miocene, as indicated by provenance studies of the Santo Domingo and El Durazno Formations. In contrast to simple asymmetric foreland models, provenance analysis documents early participation of crystalline rocks, suggesting early development of intrafore-land basement highs within the southern Central Andes. At a regional scale, basement thrusting and associated intermontane basin development were diachronous. This interpretation is supported by broad propagation of basement thrusting from the northern Sierras Pampeanas (and Famatina) toward the central and eastern Sierras Pampeanas between early-middle Miocene and late Miocene–Pliocene time. Early Miocene reconstructions of the Juan Fernandez Ridge position on the Central Andean margin suggest that during this interval, shallow subduction was likely unrelated to oceanic plateau subduction under the Sierras Pampeanas.

Characteristics of deep groundwater flow in a basin marginal setting at Sellafield, Northwest England: 36Cl and halide evidence

A number of chemical and physical processes inside and outside a sedimentary basin (e.g. evaporite dissolution and topographic drive, respectively) affect groundwater flow near the basin’s margin. Contrasting formations at the margin, typically basinal sedimentary rocks and basement, are host to the interplay between these processes so that groundwater flows and compositions change within a relatively small volume. To interpret how groundwater flow and geochemistry have evolved, interactions between these processes must be understood. Such interactions were investigated near the margin of the East Irish Sea Basin in NW England, by sampling deep groundwaters (to 1500 m below sea level) from Ordovician volcanic basement rocks and Carboniferous to Triassic sedimentary cover rocks. Variable Br/Cl ratios and Cl concentrations in deep saline waters and brines indicate mixing patterns. Variations in 36Cl/Cl constrain the timing of mixing. Relatively low Br/Cl ratios (ca. 1 × 10 −3 by mass) characterise brine from the western sedimentary cover and reflect halite dissolution further west. Saline water with relatively high Br/Cl ratios (ca. 2 × 10 −3 by mass) of uncertain origin occupies the eastern basement. These two waters mix across the area. However, mixing alone cannot explain variable 36Cl/Cl ratios, which partly reflect differing in situ 36Cl production rates in different rock formations. Most 36Cl/Cl ratios in groundwater sampled from the eastern metavolcanic basement (mean = 25 × 10 −15) and western sedimentary cover (mean = 10 × 10 −15) are at or close to equilibrium with in situ 36Cl production. These variations in 36Cl/Cl across the site possibly took >1.5 Ma to be attained, implying that deep groundwater flow responded only slowly to the Quaternary glaciation of the site. Interplay between varied processes in basin marginal settings does not necessarily imply flow instability.

Structural evolution of the Sierra de San Luis (Eastern Sierras Pampeanas, Argentina): implications for the Proto-Andean Margin of Gondwana

Abstract The Western Basement Complex of the Sierra de San Luis underwent a complex deformational and metamorphic history of presumed Proterozoic age. The Phyllite Group and equivalents consist of a thick clastic sequence, whose time of deposition is compared with that of the Late Precambrian-Early Cambrian Puncoviscana Formation. This is intruded by early Famatinian magmatic arc tonalites to granodiorites, whereas an eastern basement unit was affected by early Famatinian mafic/ultramafic intrusions, metamorphism, and deformation. This Early Ordovician plutonism is related to east-directed subduction beneath the western margin of the Pampean terrane. Subsequent compression, plutonism, and variable-grade metamorphism are considered to be Mid- to Late Ordovician in age. Post-Famatinian c. WNW-ESE contraction under greenschist facies conditions led to folding of the cover sediments and uplift of different metamorphic units along mylonite zones which record an oblique sinistral component in the western part of the sierra. Based on a few radiometric data, this stage is interpreted as Devonian in age. It is possible that the compressive event was the result of the collision with the Precordillera (Cuyania) terrane in the west: oblique kinematic displacements, together with some evidence from outside the Sierra de San Luis, can be interpreted as the effect of the indenting terrane.

Wideband Magnetotelluric Measurements across Izu-Oshima Volcano.

We carried out wideband magnetotelluric (MT) profile measurements in 1989 and 1990 to detect basement of Izu-Oshima volcano, which erupted in 1986. We newly found the difference in basement resistivity, beneath the previously known wide-spread conductor. The eastern basement is resistive. It may be due to older volcanoes, which formed earlier than the present central cone. The older volcanoes actually outcrop at the east coast. On the other hand, the western basement is conductive, suggesting the Miocene formation, the Yugashima Group (altered andesite, basalt and dacite). We also detected another deep conductive layer at several km depth beneath the western part of the profile.

Disko Bugt Project 1991, West Greenland

One of GGU's major field activities in 1991 took place in July and August in the Disko Bugt region of central West Greenland. This was the third year of field investigations under the 'Disko Bugt Project', planned for 1999–1992, with one summer's break in 1990. The project spreads over various activities and over two very different geological provinces: the Archaean-Proterozoic basement east and north-east of Disko Bugt, and the onshore part of the Cretaceous-Tertiary basin in the west, on Disko and the western part of Nugssuaq (Fig. 1). In 1989, the second year of the project, work was concentrated in the eastern basement part of the study area, and in 1991 this work was brought to conclusion. In 1992 field work (including a shallow core drilling campaign) will be concentrated in the sediments and volcanics on Nugssuaq and Svartenhuk Halvø to the north. The main aim of the Disko Bugt Project is to provide a background for the evaluation of the economic potential of the regional mineral resources, mainly in the Precambrian basement, and the hydrocarbon potential of the neighbouring basin offshore (see Kalsbeek, 1989. 1990).

U–Pb geochronology of reactivated Archean basement and of Hudsonian metamorphism in the northern Labrador Trough

The Labrador Trough is a Proterozoic orogenic belt bordering the eastern margin of the Archean Superior Province. The volcano-sedimentary sequences of the trough were deformed and metamorphosed during the Hudsonian Orogeny (ca. 1800 Ma). The eastern basement, present as domal inliers, was remobilized during this orogeny, whereas the western basement remained unaffected.In this study we present U–Pb ages of the western and eastern basement of the Proterozoic Labrador Trough orogen between Leaf River and Koksoak River and of the northernmost extension of the DePas batholithic complex farther east. From the western basement, the Leaf Bay granodiorite has an age of 2721 ± 3 Ma (zircon). From the gneiss domes of the eastern basement the ages are as follows: Lac Moyer—2883 ± 6 Ma (zircon), 1793 ± 5 Ma (monazite), and 1746 Ma (minimum age, rutile); Lac Boulder—an upper-intercept age of 2868 ± 8 Ma and a lower-intercept age of 1783 ± 11 Ma (zircon–titanite regression) and an age of 1740 ± 5 Ma (rutile); Lac Olmstead Lake—2721 ± 4 Ma (zircon) and 1774 ± 5 Ma (titanite); Leaf Strait—2719 ± 7 Ma (zircon) and 1783 ± 2 Ma (monazite). The DePas Batholith contains four generations of zircon: the minimum ages of the three oldest are 2688, 2779, and 2922 Ma; we also obtained a monazite age of 1808 ± 2 Ma.These results lead us to the following conclusions: (i) The age of the Leaf Bay granodiorite, 2721 ± 3 Ma, is a minimum age of the gneisses of the Minto Subprovince, (ii) The westernmost gneiss domes of the eastern basement (Lac Moyer and Lac Boulder), bounded by two major faults, contain zircon formed during metamorphic events dated at 2868 ± 8 and 2883 ± 6 Ma and may be part of an allochthon. (iii) The Lac Olmstead and Leaf Strait domes contain metamorphic zircon dated at 2719–2721 Ma. (iv) The minimum age of migmatization of the DePas complex is 2688 Ma. (v) The monazite ages date the latest phase of metamorphism in the area, (vi) The titanite ages probably represent post-deformational cooling, whereas the rutile ages could represent late-metamorphic hydrothermal activity, (vi) The available data suggest the presence of continuous Archean basement—remobilized during the Hudsonian Orogeny—between the Labrador Trough and the DePas Batholith, which could represent a collision zone. The final stages of this collision could be slightly older (1808 Ma) than the equivalent activity in the Labrador Trough. These speculations raise the possibility that the evolution of the Labrador Trough is closely related in time and space to events occurring in its "hinterland".

Three decades of geochronologic studies in the New England Appalachians

Research Article| August 01, 1988 Three decades of geochronologic studies in the New England Appalachians ROBERT E. ZARTMAN ROBERT E. ZARTMAN 1U.S. Geological Survey, M.S. 963, Denver Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT E. ZARTMAN 1U.S. Geological Survey, M.S. 963, Denver Federal Center, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1988) 100 (8): 1168–1180. https://doi.org/10.1130/0016-7606(1988)100<1168:TDOGSI>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 ROBERT E. ZARTMAN; Three decades of geochronologic studies in the New England Appalachians. GSA Bulletin 1988;; 100 (8): 1168–1180. doi: https://doi.org/10.1130/0016-7606(1988)100<1168:TDOGSI>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 Over the past 30 years, both isotope geochronology and plate tectonics grew from infancy into authoritative disciplines in the geological sciences. Previously, mountain systems like the Appalachians had been viewed almost entirely in the context of the classical geosyncline, implying a gradualism in stratigraphic and structural change throughout the orogen. Age control, determined largely from distant fossiliferous strata, was unabashedly carried to high-grade metamorphic rocks based only on lithological correlations. With the new concepts in tectonics came the realization that abrupt breaks in stratigraphy and structure occur in many cases at the boundaries of lithotectonic zones. Fortunately, the new techniques of isotope geochronology could be brought to bear directly on the rocks of the immediate study area. This paper chronicles some of the major contributions to the geology of the New England Appalachians that resulted from these efforts during the past three decades.In tracing the history of geochronologic research, one encounters an increasingly sophisticated approach to the analytical and interpretive aspects of the discipline. Today, the geochronologist can, under optimum conditions, constrain the age of stratigraphic units, igneous activity, deformation, and metamorphism with accuracy that is capable of resolving fine structure within individual orogenic pulses. He participates in full partnership with other colleagues of the science in unravelling the mysteries of mountain building. Several of the topical problems of New England geology in which geochronology played a key role include (1) the recognition and delineation of Avalonia as a Late Proterozoic eastern basement distinct from more western terranes, (2) the dating of the White Mountain Plutonic-Volcanic Suite, a Mesozoic igneous event spanning 100 m.y., and (3) the temporal and spatial separation of structural and metamorphic features imprinted by the Taconic and Acadian orogenies.The existing geochronology is summarized into a map and table emphasizing the temporal construction of the New England Appalachians. By using lithotectonic zones as the building blocks of the orogen, seven such zones are defined in terms of pre-, syn-, and post-assembly geologic history. From west to east, these lithotectonic zones are (1) Berkshire-Green Mountain, (2) Rowe-Hawley, (3) Connecticut Valley, (4) Bronson Hill, (5) Kearsarge-Central Maine, (6) Tatnic Hill-Nashoba, and (7) Avalonia. Avalonia is further divided into three subzones, Hope Valley, Esmond-Dedham, and Penobscot Bay, which themselves may have had distinct origins and assembly histories. The boundaries between these zones are faults in most cases, some of which may have had recurring movement to further complicate any plate-tectonic scenario.A delineation of underlying Grenvillian, Chain Lakes, and Avalonian basement is also attempted, which now can make use of isotopes in igneous rocks as petrogenetic indicators to supplement the rare occurrences of basement outcrop within mobile zones of the orogen. The belt of Permian thermal disturbance within the Kearsarge-Central Maine zone is hypothesized to reflect rapid rebound following compressional thickening of underlying Avalonian basement during the Alleghanian orogeny. 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.

Paleocurrent, petrography and provenance analyses of the Ajali Sandstone (Upper Cretaceous), southeastern Benue Trough, Nigeria

Paleocurrent, petrographic and heavy-mineral analyses of the Ajali Sandstone (Late Cretaceous) in southeastern Nigeria show that the sedimentary rocks on the Santonian Okigwe-Abakaliki anticlinorium provided the major detritus, whereas minor contributions of the eastern Precambrian basement block (Oban massif) are confined along a narrow belt southeast of Alayi in the Afikpo Basin. The sandstones derived from the major sedimentary source are quartz arenites characterized by a zircon-tourmaline heavy-mineral assemblage and a radial paleocurrent pattern which parallels the paleoslope of the provenance. On the other hand, those sands generated from the eastern basement block are arkose and sub-arkose dominated by a garnet-apatite-rutile heavy-mineral assemblage and southwesterly directed paleocurrent modes. The results of this study support the previous hypothesis that most ancient quartz arenites are multicycle in origin.

Origin of the rhyolitic rocks of the taupo volcanic zone, New Zealand

Abstract Major element, Rb, Sr, Ba, Cr and V analyses as well as 13 new rare earth element (REE) analyses are presented for the greywacke basement surrounding the Taupo Volcanic Zone (TVZ). On this basis the basement rocks are divided into a Western Basement of approximately andesitic composition (∼ 62% SiO2) and an Eastern Basement of approximately granodiorite composition (∼ 75% SiO2). These analyses, 5 new REE analyses for the rhyolites, and published data for the volcanic rocks of TVZ are used to investigate the petrogenesis of rhyolitic rocks in the area. Least-squares mixing calculations for major elements show that 88% fractional crystallisation of high-alumina basalt produces a liquid of rhyolitic bulk composition, but Rayleigh fractionation models show that the trace element concentrations of the rhyolites are inconsistent with basalt fractionation. 57% fractionation of the assemblage plagioclase (35.6%), orthopyroxene (9.7%), clinopyroxene (7.8%), ilmenite (0.6%) and magnetite (3.4%) from a plagioclase-pyroxene andesite can produce liquids of rhyolitic bulk composition. REE concentrations produced by this model are consistent with those observed in the rhyolites but predicted Ba and Rb values are lower and V concentrations are higher than those in the rhyolites. Andesite fractionation also produces an unrealistic fractionation of the Cr/V ratio. A non-modal melting model involving ∼35% melting of a granulitic assemblage (plagioclase + quartz + clinopyroxene + orthopyroxene + biotite + magnetite + cordierite) with a bulk composition equivalent to the Western Basement can reproduce the REE pattern of the rhyolites as well as the concentrations of Rb and Ba. Sr values remain anomalously high, but the Cr/V ratio does not indicate fractionation. Absolute values of Cr and V are within the uncertainties of published crystal—liquid partition coefficients. The rhyolites have relatively flat REE patterns (La/Yb ∼ 7.5), as do the greywackes (La/Yb ∼ 8.2), so it is therefore unlikely that the rhyolites equilibrated with a garnet or amphibole-bearing assemblage.

Tectonic Trends, Timing, and Mechanics of the Egyptian Edge of the Red Sea and Gulf of Suez: ABSTRACT

Two major areas of onshore preservation of both pre-and syn-Red Sea sediments occur at Gebel Zeit, near the mouth of the Gulf of Suez, and at Quseir, 200 km (125 mi) to the south. At Quseir, a belt of 600 m.y. greenstone basement has at least three stages of coaxial N30°W deformation curving northwestward into N60°W grain of the regional Hamrawein Synclinorium. In Late Eocene (?) to Early Miocene time, this northwest-trending structure was further downwarped by block faulting with associated local gravity fold tectonics. The Cenozoic synclinorium dies out southeastward into a synchronous or slightly older system of north-south trending fault blocks, many of which show early stage right-lateral strike-slip slickenslides. In the Mid-Miocene, the Red Sea coast suffered major downwarping along N25°W trends, whereas the interior synclinorium was further broken into generally northwest-trending, northeast-tilted irregular fault blocks reactivating many older fault trends. By contrast, the Gebel Zeit block appears rigidly parallel with the N25°W Red Sea-Gulf of Suez trend and shows systematic long-term tilting away from the Gulf of Suez with at least five stages of Eocene through Miocene uplift, erosion of its eastern basement edge, and concomitant sinking and deposition on its western edge. The evidence points to early stage fault patterns inherited from the local structural grain of the Precambrian basement that pre-dated the principal Red Sea-Gulf of Suez evolution into its dominant N25°W tectonic trend during the Miocene. End_of_Article - Last_Page 571------------

Newport-Inglewood Zone and Mesozoic Subduction, California

The Newport-Inglewood zone is a 45 mi long narrow belt of en echelon folds and faults which trends southeast through Miocene-Pleistocene sediments of the Los Angeles basin. Below is an inferred right slip fault, presumed to follow a contact separating the oceanic Catalina Schist facies from an eastern continental basement facies of granitic and associated metamorphic rocks. The Newport-Inglewood zone of deformation disappears abruptly on the northwest at the faulted south front of the Santa Monica Mountains, and it apparently ends gradually to the southeast near Newport, California. Displacement on the inferred basement fault, which is responsible for deformation in the overlying strata, mainly reflects Pliocene-Holocene north-south shortening (Pasadenan orogeny). The contact between oceanic and continental basement rocks is inferred to continue offshore, southeast of Newport, under Cenozoic and probably Late Cretaceous strata. These strata apparently are not deformed in a belt of folds and faults, as in the Los Angeles basin. The basement rock contact is inferred to continue to the southeast, passing along the west side of Baja California, for a total distance of more than 800 mi. It is assumed to have developed by a sea-floor contraction mechanism of east-west shortening in Cretaceous time (perhaps a late and western element of the Nevadan orogeny). It is named here the Southern California subduction zone, and it should not be confused with the Newport-Inglewood zone of deformation which developed over its northern segment at a later time and within a different strain system. This basement zone, along with what are called here the Coast Ranges and Great Valley subduction zones, are considered to mark the position of a Mesozoic oceanic trench which originally extended for more than 2,000 mi along the margin of the northeast Pacific Basin.