Continental Growth Research Articles

The success of the African Continental Free Trade Area: potentials and constraints

Purpose. This study aims to demonstrate the variables on which African nations depended to realise their desire to create an area of free trade. It also intends to investigate these countries’ constraints in establishing the area, which may stymie their efforts.
 Results. After thoroughly examining the Free Trade Area’s opportunities and limitations, the study came to the following findings: (1) the African countries’ long-term economic growth and social progress will be made possible by the African Continental Free Trade Area, whose very ambitious goals are based on Agenda 2063’s objectives; (2) African countries and the African Union aim to create a thriving free trade area based on several vital factors. One key factor is the potential to expand the African market, which could reach up to 1.2 billion people, providing abundant opportunities for intra-trade. Many African countries hope to boost neighbouring economies by reducing barriers and improving growth rates. Additionally, the continent's youthful population and wealth of natural resources can facilitate the development of various industries and the availability of diverse products for the continental market; (3) the countries in the continent have been facing long-standing issues that are not new to them. Firstly, trade constraints are seen as non-tariff barriers and high levels of informal trade, making it challenging to establish transparent, free trade zones. In addition, there are weaknesses in intra-regional trade structures and multiple regional agreements. Secondly, there are structural constraints that impede intra-African trade. A lack of manufacturing capabilities is one of these constraints, as most African countries generate income by exporting raw materials without converting them. Furthermore, the youth’s lack of education and skills makes structural transformation in African countries difficult. Furthermore, due to the size of the debt, the vast majority of revenues are directed towards debt repayment rather than investment. Thirdly, because of the social, political, and security constraints that poverty, food insecurity, and political and security instability represent in many African countries, solving these issues for those nations should take precedence over taking part in the free trade area implementation; (4) to sum up, even though there are several necessary components for creating a thriving trade area, they are not enough to overcome the multitude of obstacles and problems that many African nations face. These barriers will hinder the formation of a prosperous free trade zone that includes all African countries, which is essential for achieving overall continental growth that aligns with the region’s goals and Agenda 2063.
 Scientific novelty. The study explored the potential success of the African Continental Free Trade Area, examining the resources and strengths African countries possess to achieve their goals, as well as the challenges and risks they face. After weighing these factors, the study concluded that achieving the ambitious goals of the free trade zone will be difficult due to the many obstacles it faces.
 Practical value. The findings of this study hold immense value for African nations aspiring to forge lucrative commercial alliances under the Free Trade Area Agreement. Armed with the insights presented, these nations’ businesses are empowered to select politically and securely steadfast countries, economically and commercially varied nations, and those least burdened by social challenges, particularly poverty and food insecurity.

Cross‐Orogen Granite Migration as an Indicator of Slab Rollback Along Eastern Gondwana

AbstractSlab rollback during subduction plays a key role in controlling continental growth at convergent plate boundaries. The dynamics of currently subducting slabs can be precisely constrained using geophysical techniques. In contrast, ancient episodes of slab rollback can be difficult to constrain, yet are critical to unlocking the tectonic evolution of long‐lived orogens such as the Phanerozoic Australian Tasmanides of eastern Gondwana. Recognition of ancient slab rollback relies on the identification of the progressive migration of magmatic arcs. Here, we investigate the timing and isotopic variation of ∼90 km of the trans‐orogen migration of the Carboniferous Bathurst Batholith as a potential indicator of slab rollback. U–Pb–Hf isotopes, combined with a regional zircon Hf isotope data set, suggest that the eastward migration of the batholith over ∼18 Myr, from 340.1 to 322.4 Ma, records a maximum slab steepening rate of ∼0.6–1.2° Myr−1 in a relatively stable trench setting. These results provide a magmatic record of Carboniferous slab rollback and establish a missing link between the long‐lived Lachlan and New England orogens of the eastern Gondwanan Tasmanides.

Thermobarometry of diamond inclusions: Mantle structure and evolution beneath Archean cratons and mobile belts worldwide

Thermobarometric calculations for mineral inclusions in diamonds provide a systematic comparison of P-T-fO2 conditions for different cratons and mobile belts worldwide, using a database of 4440 mineral EPMA analyses. Beneath all cratons, the cold branch of the mantle geotherm (35-32 mWm−2) relates to PT estimates for sub-calcic garnets and rare omphacitic diamond inclusions, referring to major continental growth events in Archean. High-temperature plume-related geotherms are common in Proterozoic kimberlites such as Premier, Mesozoic-Roberts Victor etc. and are common in Slave and Siberian cratons. Eclogitic and pyroxenitic pyroxenes and garnets and Fe- and Ca-enriched pyrope diamond inclusions prevail in mobile belts like Limpopo, Magondi, Ural, Khapchan and in the marginal parts of cratons (Kimberly in Australia) They refer to diamondiferous eclogite-pyroxenite lens in the mididdle parts of lithospheric mantle. Eclogitic garnets and clinopyroxenes show trends of joint decreasing Mg# and pressures due to differentiation of rising melts. Dunitic Ca pyropes giving high-pressure estimates prevail in the central parts of cratons. The PT for the chromites reflect conditions just above the lithosphere-asthenosphere boundary formed due to interaction with the hydrous plume protokimberlite melts. Archean diamond Ca-enrich pyropes inclusions from Wawa province Canada give low-temperature conditions. Inclusions from younger kimberlites in Superior and Slave, Siberian and East European cratons show complex high-temperature geotherms due to plumes influence. Peridotite garnets beneath the Amazonian craton indicate complex layering in the lithosphere base and a pyroxene layer in the middle part of mantle lithosphere. Diamond inclusions from the Kimberley Craton of Australia show the greatest variations in the temperatures and composition. The mobile belts and marginal parts of cratons are favorable for the finding of the abundant colored diamonds. The huge plume influence for hydrated and eclogite-rich mantle like in Premier, Venetia and other kimberlites brings to the formation of giant diamonds. Diamond inclusion reveal the huge variations of PT conditions formed by different agents due to long complex stories of craton deciphered using thermobarometry and trace element and isotopic geochemistry.

Paleo-Asian oceanic crust trapped by irregular continental margin contributes to continental growth: Evidence from airborne gravity and magnetic imaging across the Alxa tectonic belt, Central Asia

Unsubducted, and trapped or accreted oceanic crust contributes greatly to continental growth. The Alxa tectonic belt (ATB) is located at the critical intersection of the Central Asian Orogenic Belt (CAOB) and the North China Craton. However, little attention has been paid to the influence of the evolution of the Beidashan-Langshan arcuate tectonic belt in the ATB on the closure of the Paleo-Asian Ocean, the formation of the CAOB, and the growth of continental crust. Here we report new integrated airborne gravity and magnetic surveys in the ATB that image a high-amplitude magnetic anomaly zone distributed across the Quagan Qulu ophiolite belt. The three-dimensional magnetic structure inversion reveals that the Quagan Qulu magnetic anomaly zone consists of mid-lower crustal high-value magnetic susceptibility. Gravity-magnetic forward modeling reveals that the Quagan Qulu magnetic anomaly zone may have been generated by a crustal mafic pluton characterized by high magnetism and density, approximately same to the magnetic susceptibility and density of the exposed Late Paleozoic gabbros measured in the field, which are consistent with those of the Yueyashan-Xichangjing ophiolite belt in the Beishan orogenic belt adjacent to the ATB. We argue that the mafic pluton beneath the Quagan Qulu zone may be the trapped oceanic crust derived from the Late Paleozoic-Triassic Paleo-Asian Ocean (i.e., the Quagan Qulu Ocean). In conjunction with the integrated petrologic, chronologic and geochemical observations, we argue that the Beidashan-Langshan arcuate tectonic belt dominated the differential subduction of the Paleo-Asian Ocean and finally trapped partial Quagan Qulu oceanic crust. Therefore, we propose that, in addition to oroclines, the irregular continental margins could alternatively provide tectonic space for trapping oceanic crust, contributing to the lateral growth of the continental crust and serving as an important means of continental accretion in the CAOB.

Discovery of a giant 3.3–3.1 Ga terrane in the Rae craton, Canada: Implications for the timing and extent of ancient continental growth

Abstract We report the discovery of one of the largest ancient (>3.0 Ga) crustal terranes on Earth. Granitoids with crystallization ages >3.0 Ga and/or Sm-Nd depleted mantle model ages ≥3.2 Ga define a ~1000 × 100 km belt on the western margin of the Rae craton, Canada, referred to herein as the Perry River terrane (PRT). Zircon U-Pb-Hf-O isotope and whole-rock geochemical data from granitoids show that the PRT is a predominantly juvenile 3.3–3.2 Ga terrane that was partially reworked by more evolved ca. 3.1 Ga magmatism. These findings call for a reassessment of the timing and extent of ancient continental growth on Earth. A global compilation of zircon Hf isotope data from 3.6 to 3.0 Ga igneous rocks reveals clusters of relatively juvenile (initial εHf −2 to +3) rocks at ca. 3.31 and ca. 3.23 Ga, which include samples from the PRT and 13 other terranes worldwide. Other global zircon data sets also document age peaks between 3.3 and 3.2 Ga, and a cluster of broadly chondritic initial εHf values around 3.2 Ga. The 3.3–3.2 Ga period may therefore have been a time of enhanced net continental growth on Earth, and the PRT is one of the largest terranes preserved from that time. Furthermore, zircon Hf isotope data from 3.3–3.1 Ga PRT granitoids and 3.5–3.0 Ga igneous rocks worldwide yield little evidence for parent magmas that interacted with or derived from pre–3.6 Ga continental material. Contrary to some continental-growth models, this latter observation suggests that the volume of continental crust established by 3.6 Ga was relatively small.

Quantitative characterization of orogens through isotopic mapping

The relationship between orogens and crustal growth is a fundamental issue in the Earth sciences. Here we present Nd isotope mapping results of felsic and intermediate igneous rocks from eight representative and well-studied Phanerozoic orogens. The results illustrate the distribution of isotopic domains that reflect the compositional architecture of the orogens. We calculated the areal proportion of juvenile crust and divided the orogens into five types: (i) highly juvenile (with >70% juvenile crust); (ii) moderately juvenile (70–50%; e.g., the Altaids with ~58% and the North American Cordillera with ~54%); (iii) mixed juvenile and reworked (50–30%; e.g., the Newfoundland Appalachians with ~40% and the Lachlan Orogen with ~31%); (iv) reworked (30–10%); (v) highly reworked (<10%; e.g., the Tethyan Tibet (~3%), Caledonides (~1%), Variscides (~1%), and the Qinling-Dabie Orogen (<1%)). This study presents an approach for quantitatively characterizing orogens based on compositional architecture through isotope mapping, and for investigating the relationships between orogenesis and continental growth.

The provenance of Avalonia and its tectonic implications: a critical reappraisal

Abstract The late Neoproterozoic–Cambrian interval is characterized by global-scale orogenesis, rapid continental growth and profound changes in Earth systems. Orogenic activity involved continental collisions spanning more than 100 myr, culminating in Gondwana amalgamation. Avalonia is an example of arc magmatism and accretionary tectonics as subduction zones re-located to Gondwana's periphery in the aftermath of those collisions, and its evolution provides significant constraints for global reconstructions. Comprising late Neoproterozoic ( c. 650–570 Ma) arc-related magmatic and metasedimentary rocks, Avalonia is defined as a composite terrane by its latest Ediacaran–Ordovician overstep sequence: a distinctive, siliciclastic-dominated cover bearing ‘Acado-Baltic’ fauna. This definition implies that Neoproterozoic Avalonia may consist of several terranes, and so precise palaeomagnetic or provenance determination in one locality need not apply to all. On the basis of detrital zircon and Nd isotopic data, Avalonia and other lithotectonically related terranes, such as Cadomia, have long been thought to have resided along the Amazonian–West African margin of Gondwana between c. 650 and 500 Ma – Avalonia connected to Amazonia, and Cadomia to West Africa. These interpretations have constrained Paleozoic reconstructions, many of which imply that the departure of several peri-Gondwanan terranes led to the Early Paleozoic development of the Rheic Ocean whose subsequent demise in the Late Paleozoic led to Pangaea's amalgamation. Since these ideas were proposed, several new lines of evidence have challenged the Amazonian affinity of Avalonia. First, there is evidence that some Avalonian terranes may have been ‘peri-Baltican’ during the Neoproterozoic. Baltica was originally excluded as a potential source for Avalonia because, unlike Amazonia, arc-related Neoproterozoic rocks were not documented. However, subsequent recognition of Ediacaran arc-related sequences in the Timanides of northeastern Baltica invalidates this assumption. Second, detailed palaeontological and lithostratigraphic studies have been interpreted to reflect an insular Avalonia, well removed from either Gondwana or Baltica during the Ediacaran and early Cambrian. Third, recent palaeomagnetic data have raised the possibility of an ocean (Clymene Ocean) between Amazonia and West Africa in the late Neoproterozoic, thereby challenging conventional reconstructions that show the ‘peri-Gondwanan’ terranes as a contiguous belt straddling the suture zone between these cratons. In this contribution, we critically re-evaluate the provenance of the so-called ‘peri-Gondwanan’ terranes, the contiguity of the so-called ‘Avalonian–Cadomian’ belt and the validity of the various plate tectonic models based on the traditional interpretation of these terranes. In addition, we draw attention to critical uncertainties and the challenges that lie ahead.

The combined Hf and Nd isotope evolution of the depleted mantle requires Hadean continental formation

The onset and rates of continental growth are first-order indicators of early Earth dynamics, and whether substantial crust existed in the Hadean or much later has long been debated. Here, we present a theoretical analysis of published Hf and Nd isotopic data representing the depleted mantle and demonstrate that continental growth must have started in the early Hadean. Whereas the traditional interpretation of depleted mantle signatures in crustal rocks assumes unrealistic instantaneous mantle mixing, our modeling incorporates the effect of a finite mixing time over which these signatures are recorded in rocks produced through mantle melting. This effect is shown to delay, by as much as 0.65 to 0.75 billion years, the appearance of the earliest depleted mantle signatures in continental crust. Our results suggest that published observations of εHf, ε143Nd, and μ142Nd require Hadean growth of continental crust, with a minimum of 50% of today's continental volume already existing by the end of Hadean.

Fossil divergent double-subduction zone in the Great Xing’an Range, NE China: Evidence from a deep seismic reflection profile

Divergent double-subduction is an important component of plate tectonics and continental growth at modern active convergent margins. However, such a process is rarely identified by deep seismic reflection (DSR) profiles, which casts doubt on its past significance. We report here a 416 km DSR profile of large dynamite shots across the Great Xing’an Range (NE China), which shows multiple arc-shaped reflections in the upper-middle crust of the Xing’an terrane, and densely layered reflection structures suggesting tectonic nappes in the lower crust. This new evidence, together with recent geological studies, indicates that the Xing’an terrane may not be a micro-continental block as previously thought, but a Paleozoic accretionary complex. In addition, there is a strong west-dipping reflection at the boundary between the Xing’an and Erguna terranes in the western DSR profile extending from the lower crust to the lithospheric mantle, suggesting that this fossil subduction zone is most likely the Toudaoqiao-Xinlin suture, which separates the Xing’an and Erguna terranes. In addition, an east-dipping strong reflection was also found in the eastern profile between the Xing’an and Songliao terranes, corresponding to the Hegenshan-Nenjiang suture at the surface. Given the newly identified crust-mantle structure, the opposite subduction zone, and other geological evidence, we propose a new model that the Xing’an terrane was developed by divergent double-subduction of the Paleo-Xing’an Ocean (part of the Paleo-Asian Ocean in the area) from ca. 500 Ma to ca. 300 Ma. Our study provides the first DSR evidence for a divergent double-subduction in the Paleozoic, which emphasizes its importance in past accretionary orogens.

Land Fraction Diversity on Earth-like Planets and Implications for Their Habitability.

A balanced ratio of ocean to land is believed to be essential for an Earth-like biosphere, and one may conjecture that plate-tectonics planets should be similar in geological properties. After all, the volume of continental crust evolves toward an equilibrium between production and erosion. If the interior thermal states of Earth-sized exoplanets are similar to those of Earth-a straightforward assumption due to the temperature dependence of mantle viscosity-one might expect a similar equilibrium between continental production and erosion to establish, and hence a similar land fraction. We show that this conjecture is not likely to be true. Positive feedback associated with the coupled mantle water-continental crust cycle may rather lead to a manifold of three possible planets, depending on their early history: a land planet, an ocean planet, and a balanced Earth-like planet. In addition, thermal blanketing of the interior by the continents enhances the sensitivity of continental growth to its history and, eventually, to initial conditions. Much of the blanketing effect is, however, compensated by mantle depletion in radioactive elements. A model of the long-term carbonate-silicate cycle shows the land and the ocean planets to differ by about 5 K in average surface temperature. A larger continental surface fraction results both in higher weathering rates and enhanced outgassing, partly compensating each other. Still, the land planet is expected to have a substantially dryer, colder, and harsher climate possibly with extended cold deserts in comparison with the ocean planet and with the present-day Earth. Using a model of balancing water availability and nutrients from continental crust weathering, we find the bioproductivity and the biomass of both the land and ocean planets to be reduced by a third to half of those of Earth. The biosphere on these planets might not be substantial enough to produce a supply of free oxygen.

Accretion of an early Paleozoic Alaska-type arc onto northern North China: Implications for continental growth of the Central Asian orogenic belt

The accretionary orogenesis of the Central Asian orogenic belt is essential for understanding the reconstruction and growth of the Asian continent. The origin and accretion of the Bainaimiao arc along the northern margin of North China remain controversial. Here, a comprehensive study of field geology, geochemistry, and geochronology was performed on plutons and an ophiolite-bearing accretionary complex in the Ganqimaodu region, Inner Mongolia. Geochemical data indicate that ca. 445−431 Ma plutons were generated in an intra-oceanic arc and represent the western extent of the Bainaimiao arc. Field mapping and zircon U-Pb dating revealed that the ophiolitic mélange contains ca. 470−435 Ma tectonic blocks of ultramafic rocks, gabbros, plagiogranites, and metavolcanic and siliceous rocks in a matrix of intensely foliated quartz schist. The pillow basalt−limestone sequence might have originated from a seamount and then been incorporated into the accretionary complex. Northward subduction of the South Bainaimiao Ocean was responsible for the early Paleozoic tectonic evolution of the Bainaimiao arc, based on the spatial-temporal configuration of arc magmatism and accretionary complexes. Integration of new data with previous studies indicates that the Bainaimiao arc was an Alaska-type arc with various components of intra-oceanic and continental arcs. We suggest that a scissor-like arc-continent collision led to the accretion of the Bainaimiao arc onto the northern margin of North China, which played a fundamental role in the continental growth of the southern Central Asian orogenic belt.

Post-collisional batholiths do contribute to continental growth

Post-collisional voluminous silicic magmatism is represented in most orogens across the world in the form of large granodiorite batholiths and minor intermediate and mafic intrusions, postdating 5-30 Ma the age of the collisional paroxysm responsible of the main mountain building events. Post-collisional mafic intrusions are acknowledged as a mechanism that contributes to long-term yet minor continental growth. The silicic magmas forming the large batholiths, however, have been dismissed from the crustal growth discussion due to bias in the conception that they always generate by recycling older lower crustal igneous rocks. Contrary to this, geochemical and isotopic relations together with new experimental data provided in this paper suggest that the post-collisional signature can be reproduced without the implication of a crustal component, supporting a potential common origin for the two suites, intermediate and silicic. That is, both suites can be derived from a metasomatized mantle source, thus representing the injection of largely juvenile material to produce new continental crust. This inference is contextualized within the supercontinent cycle, showing that the timing of post-collisional magmatism accounts for the generation and preservation rates predicted by the existing models, since both reach maximum values in the amalgamation-collisional stage of the supercontinent cycle, rather than in the subduction stage. All together these inferences lead to think that post-Archean, post-collisional magmatism has been significantly underestimated when computing continental crustal growth through time.

Intra-oceanic subduction in the southeastern Paleo-Asian ocean: Evidence from early Permian gabbro of the Diyanmiao ophiolite in central Inner Mongolia, North China

Investigation of intra-oceanic subduction can improve our understanding of plate tectonic processes and the history of continental growth. Evidence for intra-oceanic subduction in the Paleo-Asian Ocean has recently become an important focus of research, including the Diyanmiao ophiolite in central Inner Mongolia, North China. Here, we report a newly discovered occurrence of early Permian gabbro in the Diyanmiao ophiolite zone. The gabbro yields a weighted mean zircon U–Pb age of 294.4 ± 2.2 Ma. The gabbro samples are characterized by moderate SiO2 (47.32–50.51 wt%), low TiO2 (0.26–0.54 wt%) and K2O (0.04–0.75 wt%), and high Na2O (1.84–4.52 wt%) contents, high Na2O/K2O ratios (2.92–58.29), and depleted chondrite-normalized light rare Earth element patterns that are similar to N-MORB. The gabbros show slightly lower contents of high-field-strength elements (e.g., Nb, Ta, and Ti) and slightly higher contents of large-ion lithophile elements (e.g., K, Rb, Ba, and U) relative to N-MORB. In addition, the gabbros show high εNd (t) values (8.0–9.8) that are similar to those of forearc basalt in the Diyanmiao ophiolite and N-MORB. Integrating these new data with available results for ophiolite and arc-magmatic rocks of central Inner Mongolia, we propose that the studied gabbro was formed during the initial stage of intra-oceanic subduction and that the Paleo-Asian Ocean was still in a subduction setting during the early Permian.

A fossil oceanic lithosphere preserved inside a continent

The recycling of oceanic lithosphere into the deep mantle at subduction zones is one of the most fundamental geodynamic processes on Earth. During the closure of an ocean, ancient oceanic slabs are thought to be consumed entirely in subduction zones due to their negative buoyancy. Yet, it is recently suggested that small pieces of oceanic slabs could be trapped along paleo-subduction zones. What remains far more enigmatic is whether significant portions of paleo-oceanic lithosphere could eventually avoid the fate of subduction and be accreted to continental lithosphere, thus contributing to continental growth through time. We present seismic evidence for a preserved paleo-oceanic lithosphere beneath the Junggar region in northwestern China. We show that unsubducted oceanic lithosphere in the West Junggar has been preserved beneath the Junggar Basin, becoming a piece of the Eurasian continent. This scenario is likely to have occurred in other continents throughout Earth’s history, providing an additional and commonly underestimated contribution to the growth of continental lithosphere.

A fossil oceanic lithosphere preserved inside a continent

Abstract The recycling of oceanic lithosphere into the deep mantle at subduction zones is one of the most fundamental geodynamic processes on Earth. During the closure of an ocean, ancient oceanic slabs are thought to be consumed entirely in subduction zones due to their negative buoyancy. Yet, it is recently suggested that small pieces of oceanic slabs could be trapped along paleo-subduction zones. What remains far more enigmatic is whether significant portions of paleo-oceanic lithosphere could eventually avoid the fate of subduction and be accreted to continental lithosphere, thus contributing to continental growth through time. We present seismic evidence for a preserved paleo-oceanic lithosphere beneath the Junggar region in northwestern China. We show that unsubducted oceanic lithosphere in the West Junggar has been preserved beneath the Junggar Basin, becoming a piece of the Eurasian continent. This scenario is likely to have occurred in other continents throughout Earth’s history, providing an additional and commonly underestimated contribution to the growth of continental lithosphere.

The Wulanmoren Accretionary Complex Unravels Early Devonian to Late Triassic Multiple‐Arc Amalgamation in the Tianshan Orogen (NW China)

AbstractReconstructing the tectonic architecture of the southern Altaids is of great importance to piece together the continental growth history of the Central Asian continent. In this paper, we address the subduction‐accretion process of the Paleo‐Tianshan Ocean, one branch of the South Tianshan Ocean, in the southern Altaids, through a detailed geological and geochemical investigation of the Wulanmoren Accretionary Complex (WAC). The WAC is composed of ophiolitic mélanges, ocean plate stratigraphy mélanges, and turbiditic coherent units. In this study, we present key geological, petrological, and geochemical evidence indicating back‐arc and ocean island affinity of the igneous rocks of the WAC. Zircon U‐Pb dates of these rocks ages range between 380.2 ± 1.3 and 302.0 ± 5.3 Ma. The sedimentary matrix units of the mélanges were deposited in the Early Devonian (∼405 Ma) to the Late Triassic (∼222 Ma), rather than Silurian strata as previously thought. However, the ages of some detrital zircons in the sedimentary matrix are Precambrian, pointing to a complex evolution involving multiple ocean island terrains and island arcs of different ages that accreted from the mid‐Paleozoic to the Early Mesozoic.

The factors controlling along-arc and across-arc variations of primitive arc magma compositions: A global perspective

Arc volcanism is a key process influencing Earth’s climate, continental growth, and the formation of mineral deposits. Therefore in this contribution, we have compiled whole-rock geochemistry of frontal arc and rear-/back-arc basalts, crustal thickness, and slab parameters (e.g., slab age, slab dip, and convergence velocity) from global convergent margins to investigate the factors controlling along-arc and across-arc variations. Crustal thickness or lithosphere thickness plays a dominant role in elements immobile in aqueous fluids (e.g., Zr/Yb and Nb/Yb). The effect is imposed through mantle partial melting for both frontal arc and rear-/back-arc rocks. Slab thermal structure also affects these immobile elements and gives rise to along-arc variations. Both slab sediment and altered oceanic crust can melt especially in hot subduction zones which yield across-arc variations (e.g., Nd isotope). Aqueous fluids (represented by element ratios such as Ba/Nb and Sr/Nd) also show across-arc variations as they decrease toward rear-/back-arc. This meanwhile decreases mantle wedge melting as rear-/back-arcs show higher Zr/Yb and Nb/Yb. However, no correlations between aqueous fluids and slab parameters suggest aqueous fluids in arc rocks are controlled by complex processes. We summarize factors such as slab alteration, slab dehydration, and mantle metasomatism might impose an effect on the content of fluid mobile elements in arc rocks.

Voluminous continental growth of the Altaids and its control on metallogeny.

The Altaids is generally considered to be the largest Phanerozoic accretionary orogen on Earth, but it is unclear whether it was associated with extensive continental crustal growth and whether there is a link between the crustal growth and ore mineralization. This paper reviews whole-rock Nd and zircon Hf isotope data for felsic-intermediate-mafic igneous rocks in the Altaids and presents Nd+Hf isotopic contour maps for this region. The maps highlight the 3D lithospheric compositional architecture of the Altaids and make it possible to quantitatively evaluate the crustal growth and its relationship with ore deposits. The Altaids hosts ∼4 107 350km2 and ∼184 830 750km3 (assuming a crustal thickness of 40-50km) juvenile crust (ϵ Nd(t) > 0), accounting for 58% by isotope-mapped area (∼7 010 375km2) of almost all outcrops of the Altaids (∼8 745 000km2) and formed during 1000-150 Ma (mainly 600-150 Ma). The juvenile crustal, slightly juvenile-reworked crustal and slightly reworked crustal provinces controlled the Cu-Au, the Pb-Zn-Ag and the Li-Be, Nb-Ta and W-Sn ore deposits. According to the crustal architecture and background of deep compositions, we propose that the ore deposits can be grouped into three types: juvenile crust-related, mixed-source (or slightly juvenile crust)-related and reworked crust-related. This highlights the close relationship between accretion, continental growth and mineralization, and will facilitate exploration for specific ore-deposit types in the Altaids.

Passive margins in accreting Archaean archipelagos signal continental stability promoting early atmospheric oxygen rise

Significant changes in tectonic style and climate occurred from the late Archaean to early Proterozoic when continental growth and emergence provided opportunities for photosynthetic life to proliferate by the initiation of the Great Oxidation Event (GOE). In this study, we report a Neoarchaean passive-margin-type sequence (2560–2500 million years ago) from the Precambrian basement of China that formed in an accretionary orogen. Tectonostratigraphic and detrital zircon analysis reveal that thermal subsidence on the backside of a recently amalgamated oceanic archipelago created a quiet, shallow water environment, marked by deposition of carbonates, shales, and shallow water sediments, likely hosts to early photosynthetic microbes. Distinct from the traditional understanding of passive margins generated by continental rifting, post-collisional subsidence of archipelago margins represents a novel stable niche, signalling initial continental maturity and foreshadowing great changes at the Archaean-Proterozoic boundary.

A linked evolution for granite-greenstone terranes of the Pilbara Craton from Nd and Hf isotopes, with implications for Archean continental growth

Granite-greenstone terranes, such as in the Pilbara Craton (Western Australia), record the growth and stabilisation of Archean (>2.5 Ga) continental crust. Yet, uncertainty surrounds the formation of the silica-rich portions of these terranes, important for preservation of continental lithosphere, specifically whether this involved reprocessing of older continental nuclei over hundreds of millions of years, or rapid differentiation of juvenile mantle additions, as preserved in the greenstone belt volcanic sequences. In part, the uncertainty reflects difficulties with establishing the isotope evolution of the early Archean mantle, critical for ascertaining the time gap between the extraction of new crust and granitic emplacement in Archean cratons. To investigate the formation of silica-rich Archean crust, we report Hf and Nd isotope data for 46 mafic–ultramafic samples and zircon Hf isotope data for 83 felsic igneous rocks from the Pilbara Craton, selected to be representative of the major magmatic episodes from 3.56 to 2.72 Ga. The strikingly similar secular isotope trends defined by mafic and felsic rocks reveal a linkage between mantle and crustal processes throughout the history of the craton. From 3.56 Ga to 3.12 Ga, craton evolution was dominated by the transformation of juvenile mantle derived inputs into felsic igneous rocks on short timescales, thus recording sustained continental growth over 400 million years. Incorporation of older crust by felsic magmas was minor during this interval. Evolution toward sub-chondritic Nd and Hf after collision of the East and West Pilbara terranes at 3.1 Ga signals greater involvement of pre-existing crust in magma generation, possibly by recycling of crustal material into mantle source regions or by enhanced reworking of older, thickened cratonic lithosphere. Mafic–ultramafic rocks provide no evidence for anomalous or strongly depleted mantle domains in the Paleoarchean. Rather, the flat zircon εHf–time array indicates extraction of continental materials from mildly super-chondritic mantle from ca. 3.6 to 3.12 Ga and attests to a distinctive style of crust production that operated in the Archean. This study highlights the importance of comparing the mantle and crustal isotope records in order to reconstruct the crustal growth rates in ancient terranes.