The timing for the onset of plate tectonics, along with the secular changes in the tectonic settings of continental crust formation, continue to be debated. Recent interpretations based on the increase in zircon 176Hf/177Hf ratios at the time of crystallisation (expressed as εHf(t) with respect to chondritic evolution) have been used to ascertain changes in geodynamic settings in the early Earth, specifically in the 3.8–3.6 Ga interval. This increase is widely interpreted as a change in magma generation, from source(s) dominated by ancient crust to source(s) dominated by juvenile inputs from the mantle. At issue, Hf isotope variations remain limited in the early Earth due to the long decay of the LuHf system. This feature, along with the scarcity of rocks and minerals of Eo/Mesoarchaean and Hadean ages, generate large uncertainties over the nature and the timing of the interactions between mantle and crustal reservoirs. The distinction between mantle and crustal sources becomes much clearer in the Palaeoproterozoic, and the study of ancient terranes with several billion years of protracted crustal evolution may hold the key to unlock complex mantle–crust interactions. Here we investigate high-grade metamorphic rocks from the Anabar shield, which contain zircons that crystallised between the Eoarchaean (oldest core at 3814 ± 16 Ma) and the Palaeoproterozoic (youngest core at 2251 ± 15 Ma and youngest rim at 1910 ± 21 Ma). The combination of in situ UPb and Hf isotope analyses in zircon indicates the formation of the continental crust in the Siberian Craton in the Eoarchaean, and a conspicuous metamorphic event at 2.0–1.9 Ga. We demonstrate that 2.0–1.9 Ga zircon ages reflect recrystallisation processes under subsolidus conditions, involving the breakdown of high-Lu/Hf minerals (i.e. garnet and pyroxene). The εHf(t) shift at 2.0–1.9 Ga towards more radiogenic values may not be related to a change in magmatic style and sources, but rather to resetting of the LuHf system in response to heating and metamorphic reactions on a mineral scale. Our findings challenge the widely-evoked mechanism of changes in tectonic style and magma sources to account for vertical arrays in the εHf(t) versus crystallisation age space. This calls for considering alternative options, including those based on petrographic data, when interpreting Hf isotope variations in the Hadean/Archaean detrital zircon record.
Research Article| August 01, 2007 Origin of the island arc Moho transition zone via melt-rock reaction and its implications for intracrustal differentiation of island arcs: Evidence from the Jijal complex (Kohistan complex, northern Pakistan) Carlos J. Garrido; Carlos J. Garrido 1Departamento de Mineralogía y Petrología & Instituto Andaluz de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Granada & CSIC, 18002 Granada, Spain Search for other works by this author on: GSW Google Scholar Jean-Louis Bodinier; Jean-Louis Bodinier 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Bruno Dhuime; Bruno Dhuime 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Delphine Bosch; Delphine Bosch 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Ingrid Chanefo; Ingrid Chanefo 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Olivier Bruguier; Olivier Bruguier 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Search for other works by this author on: GSW Google Scholar Shahid S. Hussain; Shahid S. Hussain 3Pakistan Museum of Natural History, Garden Avenue, Shakarparian, 44000 Islamabad, Pakistan Search for other works by this author on: GSW Google Scholar Hamid Dawood; Hamid Dawood 3Pakistan Museum of Natural History, Garden Avenue, Shakarparian, 44000 Islamabad, Pakistan Search for other works by this author on: GSW Google Scholar Jean-Pierre Burg Jean-Pierre Burg 4Structural Geology and Tectonics, ETH Zürich & Universität Zürich, Geologisches Institut, Leonhardstrasse, 19/LEB, CH-8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Author and Article Information Carlos J. Garrido 1Departamento de Mineralogía y Petrología & Instituto Andaluz de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Granada & CSIC, 18002 Granada, Spain Jean-Louis Bodinier 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Bruno Dhuime 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Delphine Bosch 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Ingrid Chanefo 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Olivier Bruguier 2Laboratoire Géosciences Montpellier, Equipe Manteau-Noyau, UMR 5243, CNRS & Université de Montpellier 2, cc 49, 34095 Montpellier cedex 05, France Shahid S. Hussain 3Pakistan Museum of Natural History, Garden Avenue, Shakarparian, 44000 Islamabad, Pakistan Hamid Dawood 3Pakistan Museum of Natural History, Garden Avenue, Shakarparian, 44000 Islamabad, Pakistan Jean-Pierre Burg 4Structural Geology and Tectonics, ETH Zürich & Universität Zürich, Geologisches Institut, Leonhardstrasse, 19/LEB, CH-8092 Zürich, Switzerland Publisher: Geological Society of America Received: 19 Jan 2007 Revision Received: 12 Mar 2007 Accepted: 18 Mar 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2007) 35 (8): 683–686. https://doi.org/10.1130/G23675A.1 Article history Received: 19 Jan 2007 Revision Received: 12 Mar 2007 Accepted: 18 Mar 2007 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Carlos J. Garrido, Jean-Louis Bodinier, Bruno Dhuime, Delphine Bosch, Ingrid Chanefo, Olivier Bruguier, Shahid S. Hussain, Hamid Dawood, Jean-Pierre Burg; Origin of the island arc Moho transition zone via melt-rock reaction and its implications for intracrustal differentiation of island arcs: Evidence from the Jijal complex (Kohistan complex, northern Pakistan). Geology 2007;; 35 (8): 683–686. doi: https://doi.org/10.1130/G23675A.1 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 SocietyGeology Search Advanced Search Abstract If the net flux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafic keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo–island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafic crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display flat or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafic crust that have higher REE concentrations and are derived from light rare earth element (LREE)–enriched melts similar to island arc basalt. This observation, along with the lower 208Pb/204Pb and 206Pb/204Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafic rocks derive from a common melt via crystal fractionation. In the 208Pb/204Pb versus 206Pb/204Pb diagram, ultramafic rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafic rocks. These data are consistent with a scenario whereby the Jijal ultramafic section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafic cumulates complementary to the evolved mafic plutonic rocks implies either (1) that a substantial volume of such ultramafic cumulates was delaminated or torn out by subcrustal mantle flow from the base of the arc crust in extraordinarily short time scales (0.10–0.35 cm/yr), or (2) that the net flux to the Kohistan arc crust was more evolved than primitive arc basalt. 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The continental crust is the archive of Earth's history.Its rock units record events that are heterogeneous in time with distinctive peaks and troughs of ages for igneous crystallization, metamorphism, continental margins, and mineralization.This temporal distribution is argued largely to reflect the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly.Isotopic and elemental data from zircons and whole rock crustal compositions suggest that the overall growth of continental crust (crustal addition from the mantle minus recycling of material to the mantle) has been continuous throughout Earth's history.A decrease in the rate of crustal growth ca.3.0 Ga is related to increased recycling associated with the onset of plate tectonics.We recognize five stages of Earth's evolution: (1) initial accretion and differentiation of the core/mantle system within the first few tens of millions of years; (2) generation of crust in a pre-plate tectonic regime in the period prior to 3.0 Ga; (3) early plate tectonics involving hot subduction with shallow slab breakoff over the period from 3.0 to 1.7 Ga; (4) Earth's middle age from 1.7 to 0.75 Ga, characterized by environmental, evolutionary, and lithospheric stability; (5) modern cold subduction, which has existed for the past 0.75 b.y.Cycles of supercontinent formation and breakup have operated during the last three stages.This evolving tectonic character has likely been controlled by secular changes in mantle temperature and how that impacts on lithospheric behavior.Crustal volumes, reflecting the interplay of crust generation and recycling, increased until Earth's middle age, and they may have decreased in the past ~1 b.y.
Subduction zones, such as the Andean convergent margin of South America, are sites of active continental growth and crustal recycling. The composition of arc magmas, and therefore new continental crust, reflects variable contributions from mantle, crustal and subducted reservoirs. Temporal (Ma) and spatial (km) variations in these contributions to southern Central Andean arc magmas are investigated in relation to the changing plate geometry and geodynamic setting of the southern Central Andes (28–32° S) during the Cenozoic. The in-situ analysis of O and Hf isotopes in zircon, from both intrusive (granitoids) and extrusive (basaltic andesites to rhyolites) Late Cretaceous – Late Miocene arc magmatic rocks, combined with high resolution U–Pb dating, demonstrates distinct across-arc variations. Mantle-like δ18O(zircon) values (+5.4‰ to +5.7‰ (±0.4 (2σ))) and juvenile initial εHf(zircon) values (+8.3 (±0.8 (2σ)) to +10.0 (±0.9 (2σ))), combined with a lack of zircon inheritance suggests that the Late Cretaceous (∼73 Ma) to Eocene (∼39 Ma) granitoids emplaced in the Principal Cordillera of Chile formed from mantle-derived melts with very limited interaction with continental crustal material, therefore representing a sustained period of upper crustal growth. Late Eocene (∼36 Ma) to Early Miocene (∼17 Ma) volcanic arc rocks present in the Frontal Cordillera have 'mantle-like' δ18O(zircon) values (+4.8‰ (±0.2 (2σ) to +5.8‰ (±0.5 (2σ))), but less radiogenic initial εHf(zircon) values (+1.0 (±1.1 (2σ)) to +4.0 (±0.6 (2σ))) providing evidence for mixing of mantle-derived melts with the Late Paleozoic – Early Mesozoic basement (up to ∼20%). The assimilation of both Late Paleozoic – Early Mesozoic Andean crust and a Grenville-aged basement is required to produce the higher than 'mantle-like' δ18O(zircon) values (+5.5‰ (±0.6 (2σ) to +7.2‰ (±0.4 (2σ))) and unradiogenic, initial εHf(zircon) values (−3.9 (±1.0 (2σ)) to +1.6 (±4.4 (2σ))), obtained for the Late Oligocene (∼23 Ma) to Late Miocene (∼9 Ma) magmatic rocks located in the Argentinean Precordillera, and the Late Miocene (∼6 Ma) volcanic rocks present in the Frontal Cordillera. The observed isotopic variability demonstrates that the assimilation of pre-existing continental crust, which varies in both age and composition over the Andean Cordillera, plays a dominant role in modifying the isotopic composition of Late Eocene to Late Miocene mantle-derived magmas, implying significant crustal recycling. The interaction of arc magmas with distinct basement terranes is controlled by the migration of the magmatic arc due to the changing geodynamic setting, as well as by the tectonic shortening and thickening of the Central Andean crust over the latter part of the Cenozoic.
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