Special Issue "Continental Accretion and Evolution"

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A special issue of Geosciences (ISSN 2076-3263).

Deadline for manuscript submissions: closed (30 April 2013)

Special Issue Editor

Guest Editor
Prof. Dr. David A. Foster
Department of Geological Sciences, University of Florida, 241 WIlliamson Hall, P.O. Box 112120, Gainesville, Florida 32611, USA
Website: http://www.clas.ufl.edu/users/dafoster/
E-Mail: dafoster@ufl.edu
Phone: +1 3523922241
Interests: tectonics; geochronology; thermochronology; structural geology; continental evolution; isotope geochemistry

Special Issue Information

Dear Colleagues,

Understanding the evolution of the continental crust continues to be a challenge because of the diversity of environments where continental crust and subcontinental lithosphere is formed, recycled, and stabilized. This is further complicated by long-term changes in continental formation and growth processes over geological time, and subsequent modifications to the continents. Advances over recent years have come from large-scale geophysical experiments, improvements analytical methods for in-situ isotopic and elemental analysis of accessory phases, experimental petrology (igneous and metamorphic), and geodynamic modeling. This special issue will focus on the accretion and evolution of continents in the broadest sense including: (1) continental growth from juvenile materials extracted from the mantle; (2) recycling of continental lithosphere through subduction, sediment subduction/accretion, and delamination; (3) continental evolution in convergent margins through arc magmatism and accretion; and (4) role of large igneous provinces and mantle plumes in the evolution and growth continents.

Prof. Dr. David A. Foster
Guest Editor

Submission

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Keywords

  • continental lithosphere
  • arc magmatism
  • accretionary orogens
  • continental crust
  • large igneous provinces
  • tectonics
  • isotope geochemistry
  • granitic magmatism
  • metamorphism
  • continental growth

Published Papers

No papers have been published in this special issue yet, see below for planned papers.

Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Title: Continental accretion at the western margin of South America: case study of the Peruvian Andes
Authors: O. Adrian Pfiffner & Laura Gonzalez
Abstract: Based on the structural style and physiographic criteria, the Central Andes of Peru can be divided into five segments running parallel to the Pacific coast. The westernmost segment, the Coastal Belt, consists of a Late Jurassic – Cretaceous volcanic arc sequence that was accreted to the South American craton in the Cretaceous. Its structural style is characterized by relatively open folds that were intruded by the Coastal Batholith in Late Cretaceous times. The Mesozoic strata of the adjacent Western Cordillera represent an ENE-verging fold-and-thrust belt. In its western part, tight upright folds developed above a detachment horizon in the Early Cretaceous shales of the Oyón Formation. In contrast, more open folds are observed in the eastern part of the Western Cordillera and the neighboring Central Highlands. Here the Mesozoic strata are harmonically folded and a detachment occurred possibly along the Devonian phyllites. The folds in the Western Cordillera are connected at depth to NE-verging thrust faults that level off into the detachment horizons. A completely different style with steeply dipping reverse faults and open folds is observed in the Neoproterozoic crystalline basement and the Paleozoic sediments of the Eastern Cordillera. The reverse faults are in part of transpressive nature and uplifted large blocks of basement rocks next to Paleozoic strata. In the Subandean Zone, Paleozoic and Cenozoic strata are affected by mainly NE-verging imbricate thrusting. A quantitative estimate of the shortening of the orogen was obtained from the construction of two transects that run from the Pacific coast to the undeformed Amazonas foreland. Total shortening of the two transects is 120 – 150 km (24 –27%), roughly 80 km of which is taken up by the Western and Eastern Cordilleras and the Central Highlands. This orogenic shortening occurred in the framework of plate convergence whereby more than 3000 km of oceanic lithosphere of the Nazca plate was subducted beneath the South American Plate.

Three major deformation phases can be recognized in the Andes of Peru. The earliest, the Mochica Phase, corresponds to the open folding of the Coastal Belt and is sealed by the Coastal Batholith. The tight folding and thrusting in the Western Cordillera and the neighboring Central Highlands and Eastern Cordillera is attributed to the following Inca Phase. This Early Paleogene phase is sealed by the unconformity at the base of the Eocene-Miocene volcanics of the Calipuy Group. The last phase, called Quechua Phase, can be subdivided into several episodes and unlike the earlier phases it affected the entire orogeny. In in the Subandean Zone it accounts for the imbricate thrusting involving Neogene and even Pliocene sediments. The Central Highlands were uplifted as a block in the process. A major system of Quechua Phase thrust faults, the Raura-Viuda-Quera fault system (RVQ), straddles the eastern boundary of the Western Cordillera and uplifted this mountain range relative to the Central Highlands in post-Eocene times. The most recent episode is held responsible for folds within unconsolidated Pleistocene sediments in the Central Highlands and a steep reverse fault bordering the Cordillera Blanca. The Quecha Phase coincides in time with the Neogene westward drift of the South American Plate and was responsible for the high elevation of the Central Andes.

Title: Testing the existence of the South Gobi Microcontinent: Protolith studies of metamorphic tectonites in southeastern Mongolia
Authors: Joshua P. Taylor1, Laura E. Webb2, Cari L. Johnson3, Matthew J. Heumann3
Affiliations: 1 Department of Earth Sciences, 204 Heroy Geology Lab, Syracuse University, Syracuse, NY 13244
2 Department of Geology, University of Vermont, Burlington, VT 05405
3 Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112

Abstract: The Central Asian Orogenic Belt is an amalgamation of volcanic arcs and microcontinent blocks that records a complex Late Precambrian–Mesozoic accretionary history. Microcontinents cored by Precambrian basement have been proposed to play an integral role in the accretion process, however a lack of isotopic data precludes accurate volume estimates of newly produced arc-derived versus old cratonic crust in southeastern Mongolia. This study investigates metamorphic tectonites within this region that have been mapped as Precambrian in age largely on the basis of their high metamorphic grade and high strain. Microstructural analyses and U-Pb zircon geochronology on samples from Tavan Har and the Yagan-Onch Hayrhan metamorphic core complex provide no compelling evidence for Precambrian basement in southeastern Mongolia. Rather, the protoliths to all tectonites examined are Paleozoic–Mesozoic age rocks, formed during Devonian–Carboniferous arc magmatism and subsequent Permian–Triassic orogenesis during collision of the South Mongolia arc with the North China craton. These results yield important insights into the Paleozoic accretionary history of southern Mongolia, including the volume of Precambrian cratonic crust, as well as implications for subsequent intracontinental reactivation.

Title: Modification of the Continental Crust by Subduction-Zone Magmatism and Vice-Versa: Across-Strike Geochemical Variations of Silicic Lavas At Individual Eruptive Centers In The Andean Central Volcanic Zone
Authors: Todd Feeley
Affiliations: tfeeley@montana.edu
Abstract: In an effort to better understand the origin of across-strike K2O enrichments in volcanic rocks from the Andean Central Volcanic Zone, we compare geochemical and isotopic compositions of Quaternary (<1.0 Ma) silicic lava flows (60–68 wt.% SiO2) erupted from three well-characterized composite volcanoes situated along a northwest striking transect between 21o and 22oS. From northwest to southeast these are Volcáns Aucanqilcha, Ollagüe, and Uturuncu. Aucanqilcha is located on the arc front entirely within Chile; Ollagüe is located ~25 km to the east of the arc front within the transition zone between the arc front and the Bolivian Altiplano; Uturuncu is located ~75 km east of the arc front on the SW Bolivian Altiplano. Trends observed include the following. At a given SiO2 content lavas erupted with increasing distance from the arc front display systematically higher K2O, P2O5, TiO2, Rb, Th, and REE and HFSE contents, Rb/Sr elemental ratios, and Sr isotopic ratios (0.7055 - 0.7165), in addition to more negative Eu anomalies. In contrast, the lavas display systematically decreasing Al2O3, Na2O, Sr, and Ba contents, Ba/Nb, Ba/Zr, K/Rb, and Sr/Y elemental ratios, and Nd isotopic ratios (0.51239 – 0.51214) with distance from the arc front. It is unlikely that the across-arc geochemical variations solely reflect differences in mantle source compositions or degrees of melting for parental magmas given the highly modified isotopic ratios of the lavas relative to primitive intra-oceanic arc magmas, implying extensive contamination by or derivation within the continental crust. Instead, these data favor a model in which lower crustal source rocks for the silicic lavas become progressively older and more feldspar-rich with increasing distance from the arc front. In this regard, our preliminary interpretation is that silicic magmas erupted along the arc front reflect melting of young, mafic composition amphibiotic source rocks with a garnet- (but not feldspar) bearing residual mineralogy and that the lower crust becomes increasingly older with a more felsic bulk composition in which residual mineralogies are progressively more feldspar-rich, but garnet-poor. One implication of this interpretation is that large-scale regional trends in magma compositions at volcanic arcs may reflect a process wherein the continental crust becomes progressively hybridized beneath frontal arc localities due to protracted intrusion of subduction-derived basaltic magmas, with a diminishing effect behind the arc front because of smaller degrees of mantle partial melting and primary melt generation.

Title: Nd isotope mapping of crustal terranes in the Parent-Clova area, Quebec: implications for the evolution of the Laurentian margin in the central Grenville Province
Authors: Mark Zelek and Alan Dickin
Affiliation: School of Geography & Earth Sciences, McMaster University, Hamilton, Ontario; dickin@univmail.cis.mcmaster.ca

Abstract: Over 100 new Nd isotope analyses for the central Grenville Province in the Parent-Clova region of Quebec help fill a major gap in understanding the crustal accretion history of the province. Nd model ages show that the Parent-Clova region consists of three crustal blocks: Archean parautochthon in the north; a central block with mixed ages interpreted as an ensialic arc; and a southerly block forming an extension of the Mesoproterozoic Quebecia arc terrane. The Allochthon Boundary Thrust is believed to define the edge of the Archean parautochthon, which is bordered for a distance of 300 km by the ensialic arc block, within which model ages decrease consistently away from the craton. A similar negative correlation between Nd model age and distance from the craton is seen in published data for the Algonquin terrane in Ontario, but with a lower range of model ages. These comparisons show that in the Parent-Clove region a Mesoproterozoic ensialic arc was established directly on the Archean margin, but further west the Mesoproterozoic arc was built on a younger margin consisting of accreted Paleoproterozoic arc crust. The use of large Nd data sets allows these distinct regional growth patterns to become clear, and hence allows an understanding of Mesoproterozoic crustal evolution in the province as a whole.

Title: Evaluating magma mixing in evolving continental crust via polytopic vector analysis (PVA): Papagayo tuff, northern Costa Rica
Authors: David Szymanski
Affiliations: DSZYMANSKI@bentley.edu
Abstract: Magma mixing is a common process in volcanic arcs. Over the last forty years, research has revealed the importance of magma mixing as a trigger for volcanic eruptions, as well as its role in creating the diversity of magma compositions found in most evolved volcanic arcs. Although sensitive isotopic and microchemical techniques reveal subtle evidence of magma mixing in igneous rocks, more robust statistical techniques for bulk chemical data can help evaluate mixing relationships within and among volcanic units. Relatively new to igneous petrology, polytopic vector analysis (PVA) is a multivariate statistical technique that can be used to evaluate suites of samples that are produced by mixing of two or more magma batches.

The previously undescribed Papagayo Tuff in northern Costa Rica presents a unique opportunity to demonstrate the utility of PVA for evaluating magma mixing in an arc located on oceanic crust with many of the chemical signatures of continental arc magmatism. The ignimbrite contains banded pumice fragments that are petrographically and chemically consistent with the mingling of rhyolitic and andesitic magma batches to produce a single volcanic deposit. However, PVA yields a three-end member solution for the suite of samples. One end member is andesitic (57 wt.% SiO2); the other two are rhyolitic (71 wt.% SiO2) with differing trace element compositions. We demonstrate that this solution is consistent with observations from bulk chemistry, microchemistry, and mineralogic and petrographic data from the rocks.

Title: Slab Extrusion, Crustal Diapirism and Post-Orogenic Melting: a Spectrum of Mechanisms for the Exhumation of Subducted Continental Crust.
Authors: Hannes K. Brueckner
Affiliations: Lamont-Doherty Earth Observatory of Columbia University

Abstract: The presence of high pressure (HP) and ultrahigh pressure (UHP) terranes within most mountain systems and the presence of mantle fragments (orogenic peridotites) within many of these terranes provide evidence that continental crust is subducted into the upper mantle to depths sometimes exceeding 150 km during the continent-continent and arc-continent collisions that occur as ocean basins narrow and close. Continental subduction is usually caused by the pull exerted by the subduction of previously subducted oceanic lithosphere. However, the exhumation of these HP/UHP terranes appears to occur through several mechanisms, all the result of the bouyancy of the silica-rich continental crust relative to the surrounding, denser mantle. Some terranes return as relatively rigid coherent slabs that return towards the surface along the same route they were subducted, bounded below by thrust faults and above by low angle normal faults. These terranes show limited high temperature overprinting and melting. Other terranes return as spaced diapirs (i.e. “massifs”) characterized by an overall domal structure. Their more ductile return towards the surface is consistent with evidence for a high temperature overprint during exhumation accompanied by extensive melting. Still other terranes may not return as coherent solids, but instead melt and return as magmas that rise through the overlying mantle wedge and intrude the overlying continetal crust as post-orogenic and even anorogenic granitoids. Exhumation of some terranes may be a hybrid process combining slab return, diapirism and melting. Some factors that determine which of these mechanisms predominates include, but are not restricted to: 1) the chemistry and mineralogy of the subducted slab (particularly the presence or absence of radioactive elements and of hydrous minerals); 2) the initial temperatures of the subducted slab and the enclosing mantle, 3), the length of time the crustal slab lingers in the mantle (i.e. “hang time”); 4) the angle of subduction and 5) the depth of penetration into the mantle. Mountain chains that do not contain HP/UHP terranes, particularly Archean and Proterozoic systems, may nevertheless have evolved through the subduction of continental crust into the mantle where they melted rather than returning as coherent HP/UHP bodies.

Last update: 27 April 2013

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