Continental Growth Research Articles

Tracking the birth and growth of Cimmeria: Geochronology and origins of intrusive rocks from NW Iran

New geochronological and geochemical data for Late Neoproterozoic to Mesozoic intrusive rocks from NW Iran define major regional magmatic episodes and track the birth and growth of one of the Cimmerian microcontinents: the Persian block.After the final accretion of the Gondwanan terranes, the subduction of the Prototethyan Ocean beneath NW Gondwana during the Late Neoproterozoic was the trigger for high magmatic fluxes and the emplacement of isotopically diverse arc-related intrusions in NW Gondwana. The Late Neoproterozoic rocks of NW Iran belong to this magmatic event which includes intrusions with highly variable εHf(t) values. This magmatism continued until a magmatic lull during the Ordovician, which led to the erosion of the Neoproterozoic arc, and then was followed by a rifting event which controlled the opening of Paleotethys. In addition, it is supposed that a prolonged pulse of rift magmatism in Persia lasted from Devonian-Carboniferous to Early Permian time. These magmatic events are geographically restricted and are mostly recorded from NW Iran, although there is some evidence for these magmatic events in other segments of Iran. The Jurassic rocks of NW Iran are interpreted to be the along-strike equivalents of a Mesozoic magmatic belt (the Sanandaj-Sirjan Zone; SaSZ) toward the NW. Magmatic rocks from the SaSZ show pulsed magmatism, with high-flux events at both ~176–160 Ma and ~130 Ma. The SaSZ magmatic rocks are suggested to be formed along a continental arc but a rift setting is also considered for the formation of the SaSZ rocks based on the plume-related geochemical signatures. The arc signatures are represented by Nb-Ta depletion in the highly contaminated (by upper continental crust) plutonic rocks whereas the plume-related signature of less-contaminated melts is manifested by enrichment in Nb-Ta and high εHf(t) values, with peaks at +0.6 and +11.2. All these magmatic pulses led to pre-Cimmerian continental growth and reworking during the Late Neoproterozoic, rifting and detachment of the Cimmerian blocks from Gondwana in Mid-Late Paleozoic time and further crustal growth and reworking of Cimmeria during the Mesozoic.

Upper mantle structure of the northeastern Arabian Platform from teleseismic body-wave tomography

A thick Infra-Cambrian to Phanerozoic sedimentary sequence in eastern Arabia conceals a poorly known Neoproterozoic basement that once separated the currently exposed Arabian Shield (western Arabia) from NW India in Gondwana. In this study, the Arabian Platform (eastern Arabia) is targeted by relatively high-resolution passive-source seismic tomography for the first time, which aims to illuminate crust and upper mantle structure in order to make inferences on their nature and origin. This is facilitated by a new broadband seismic network of 55 stations in the United Arab Emirates (UAE), which has recorded sufficient teleseismic body wave data to allow for the determination of three-dimensional seismic structure. Using a grid-based eikonal solver and a subspace inversion technique implemented in FMTOMO, relative arrival-time residuals from teleseismic earthquakes are mapped as 3-D P-wave velocity perturbations in the upper mantle beneath the seismic network. The resultant tomographic images reveal a marked transition from higher velocities in the Gulf of Oman to lower velocities in the west that we interpret to indicate the lithospheric boundary between Cretaceous Tethyan oceanic lithosphere and the Arabian passive continental margin. The northern extent of Tethyan oceanic lithosphere appears to terminate against a possible offshore continuation of the Dibba fault, until now only identified onshore. Further inland, the tomographic model exhibits a number of low- and high- velocity anomalies with a predominant NW orientation. Significantly, one well-defined high velocity anomaly is situated along strike of the Neoproterozoic exposure of volcanic-arc granodiorites found in northern Oman. We relate this anomaly to an intra-oceanic arc created during the Tonian subduction of the Mozambique Ocean, which strongly support the idea of continental growth in the Arabian Platform via magmatic arc accretion.

Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle: Insights from igneous rocks of the Pilbara Craton, Western Australia

The continental crust grows via juvenile additions from the mantle. However, the timing of initial continent stabilisation and the rate of subsequent continental growth during the first billion years of Earth history is widely debated, in part due to uncertainty over the composition of the mantle source of new crust. Well-preserved Archean granite-greenstone terranes, as present within the Pilbara Craton (Western Australia), provide insights into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological past at the regional scale. Here, we present zircon U-Pb, O and Hf isotope data from ancient gneissic and granitic rocks of the Pilbara Craton, to decipher magma sources and the timing and processes of craton growth. There is no evidence for depleted mantle compositions, and the simplest interpretation is that the crust of the Pilbara Craton was generated from mantle with a chondritic Hf isotope composition. Our results indicate crustal addition at ~3.59 Ga, represented by emplacement of gabbroic to anorthositic rocks. We suggest that the formation of these igneous rocks, and the foundering of the complementary residues, triggered extensive melting of hot, upwelling mantle, leading to the subsequent accumulation of the >12 km thick greenstone belt eruptive sequences from 3.53 Ga, with emplacement of coeval felsic magmas at depth. This process shaped the initial crustal configuration of the proto-craton, which subsequently underwent gravitationally driven overturn and reworking to generate stable, cratonic continental crust with the distinctive dome and keel architecture. The zircon Hf and O isotope signatures of the Pilbara igneous rocks from ~3.59–3.4 Ga do not support remelting of an ancient (> 3.8 Ga) basement, and reinforce the overwhelmingly chondritic to near-chondritic zircon Hf isotope composition of Eoarchean meta-igneous rocks from a number of different Archean cratons. A corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the Lu-Hf system from the formation of the Earth into the Paleoarchean (up to 3.6–3.5 Ga), as would be the case if stabilised volumes of felsic continental crust prior to 3.5 Ga were relatively small. One implication is that the common assumption of a linear evolution of depleted mantle from 4.5 Ga to the present day is inappropriate for determining the timing and volume of continental crust extraction in the Archean. The nearly identical early evolution of the Pilbara and Kaapvaal cratons suggests a common process to generate Archean granite-greenstone terranes that does not require extensive reworking of ancient crust, but rather involves juvenile crustal addition above persistent zones of upwelling, chondritic mantle.

Arc accretion and crustal reworking from late Archean to Neoproterozoic in Northeast Brazil

New systematic Nd isotope and U-Pb geochronology data were applied to Precambrian rocks of northeastern Brazil to produce a crustal-age distribution map for a small basement inlier (1,500 km2). The results support episodic crustal growth with five short periods of crustal formation at ca. 2.9 Ga, 2.65 Ga, 2.25 Ga, 2.0 Ga, and 0.6 Ga. Based on the frequency histogram of U-Pb zircon ages and Nd isotopic data, we suggest that about 60% of the continental crust was formed during the Archean between 2.9 Ga and 2.65 Ga. The remaining 40% of crust was generated during the Rhyacian to Neoproterozoic (~2.0–0.6 Ga). This overall continental growth is manifested by accretionary processes that involved successive accretions surrounding an older core, becoming younger toward the margin. Strikingly, this repetitive history of terrane accretion show a change from lithospheric peeling dominated accretionary setting during the late Archean to a more, modern-day akin style of arc-accretion during the Proterozoic. Similar tectonic processes are observed only in large continental areas (>1,000,000 km2) as in the North American continent basement and in the Amazonian Craton.

Continuous continental growth as constrained by the sedimentary record

Analysis of a database of ∼600,000 detrital zircon ages constrained by the depositional ages of the respective sedimentary units reveals a dynamic evolution of zircon age peaks through time. This analysis demonstrates that zircon age peaks from ancient sedimentary successions are often out of phase with the detrital zircon record obtained from modern sediments. The growth and diminishment of the zircon age peaks through time implies the presence of continental crust whose age is not proportionately represented in the modern record, and therefore that the current crustal archive is biased. However, when the detrital zircon record is viewed in terms of its evolution through time, that is taken as a time-lapse view of continental growth, it appears there never was a time in Earth history without an associated zircon age peak. The analysis of detrital zircon age peaks presented herein also reveals an evolution that can be broadly divided into three temporal groupings that broadly correspond with phases of Earth9s tectonic evolution, namely pre-supercontinent continental growth (pre-2.1 Ga), Earth9s middle age (2.1–0.8 Ga), and post-onset of modern-day plate tectonics (post-0.8 Ga). These three groupings each display increasing degrees of zircon age diversification with time, and are a likely result of a net increase of preserved continental crust through time. The presence of these three tectonic states in multiple geologic proxies (detrital zircon ages, changing styles of metamorphism, paleogeography/supercontinents) suggests that while the growth of the continental crust is continuous, the tectonic processes that shape the long-term preservation of the crust have evolved over geologic time.

Coupled U-Pb and Rb-Sr laser ablation geochronology trace Archean to Proterozoic crustal evolution in the Dharwar Craton, India

The Western Dharwar Craton (WDC) and the Karwar Block (KB) in southern Peninsular India provide important windows to Earth’s early crustal evolution and continental growth as these domains likely represent fragments of the first landmass or supercontinent on our planet. Here we employ, for the first time, a coupled laser ablation zircon U-Pb dating and in-situ Rb-Sr dating of micas (biotite, muscovite) and feldspars to evaluate the Meso- to Neoarchean crustal evolution and Proterozoic magmatism/metamorphism in the Karwar Block. This novel approach based on integrated geochronological results show that the WDC and KB experienced (i) a major Mesoarchean continental growth event (c. 3.2 to 2.9 Ga), (ii) younger Neoarchean and Paleoproterozoic magmatism/migmatization (syn- and post-tectonic granite formation events at c. 2.8–2.6 Ga and 2.2 Ga, respectively), and (iii) Mesoproterozoic metamorphism/thermal imprint (as young as 1.2 Ga). The zircon εHf(t) values exhibit a range from +0.4 to +9.8 in amphibolite, hornblende gneiss and trondhjemite (3.28–3.08 Ga), as well as the +6.1 to +9.7 range for 2.96 Ga old zircons in porphyritic granite, which suggests juvenile source for the magmatism that constructed this crustal block.Overall, this study illustrates the benefits of using coupled zircon U-Pb and in-situ Rb-Sr mica and feldspar dating approaches, as the former (U/Pb) provides robust constraints or a ‘baseline age’ for initial crystallization of crustal sequences whereas the latter (Rb/Sr) is sensitive to younger magmatic and metamorphic events. Thus, the integration of these two laser ablation ICP-MS based geochronometers applied to crustal evolution studies, has the potential to provide more comprehensive insights into continental growth in the early Earth, and later magmatic and metamorphic history of the region.

Generation of Jurassic high-Mg diorite and plagiogranite intrusions of the Asa area, Tibet: Products of intra-oceanic subduction of the Meso-Tethys Ocean

Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve understanding of plate tectonic processes and the history of continental growth. Several studies of the intra-oceanic subduction of the Meso-Tethys Ocean have been published in recent years, particularly relating to the Bangong–Nujiang suture zone (BNSZ) and the Shiquanhe–Nam Tso mélange zone (SNMZ) in central Tibet. Here, we report newly discovered occurrences of Jurassic diorite (161.2–160.6 Ma) and plagiogranite (161.0 Ma) from the western part of the central SNMZ. The diorite samples have moderate SiO2 and high MgO contents and Mg# values; high Ba/Th and Ba/La ratios; low K2O, TiO2, and REE contents; low (La/Yb)N, (La/Sm)N, and Th/Yb ratios; and positive εHf(t) values, similar to high-Mg diorite formed by partial melting of a subduction-modified mantle wedge. The plagiogranite intrudes gabbro of the ophiolite suite and has high SiO2 and Na2O contents; low Al2O3, K2O, P2O5, TiO2, and total REE contents; and clear negative Eu anomalies, similar to shear-related oceanic plagiogranite, which can be formed by partial melting process of gabbro in the high temperature shear zone near the expansion center. The formation processes of Asa high-Mg diorite and shear-related plagiogranite are consistent with initial subduction within the Meso-Tethys ocean. Based on these data and those for sedimentary and Jurassic magmatic rocks of the region, we propose that, during subduction, the Northern Lhasa terrane was formed from a collage of oceanic island arcs, oceanic plateaus, and accretionary complexes.

The geological history and evolution of West Antarctica

West Antarctica has formed the tectonically active margin between East Antarctica and the Pacific Ocean for almost half a billion years, where it has recorded a dynamic history of magmatism, continental growth and fragmentation. Despite the scale and importance of West Antarctica, there has not been an integrated view of the geology and tectonic evolution of the region as a whole. In this Review, we identify three broad physiographic provinces and present their overlapping and interconnected tectonic, magmatic and sedimentary history. The Weddell Sea region, which lays furthest from the subducting margin, was most impacted by the Jurassic initiation of Gondwana break-up. Marie Byrd Land and the West Antarctic rift system developed as a broad Cretaceous to Cenozoic continental rift system, reworking a former convergent margin. Finally, the Antarctic Peninsula and Thurston Island preserve an almost complete magmatic arc system. We conclude by briefly summarizing the geologic history of the West Antarctic system as a whole, how it provides insight into continental margin evolution and what key topics must be addressed by future research. Understanding the complex geologic history of West Antarctica provides insight into the formation of continental margins across Earth. In this Review, we detail the magmatism, continental growth and fragmentation of West Antarctica over the past 500 million years.

Paleoarchean and Neoarchean Tonalite–Trondhjemite–Granodiorite (TTG) and granite magmatism in the Western Dharwar Craton, southern India: Implications for Archean continental growth and geodynamics

The Western Dharwar Craton in southern India is underlain by Paleoarchean to Neoarchean granitoids. Here, we use major-trace element chemistry and zircon U-Pb-Hf isotopic composition to identify major components of the crust, constrain the timing of juvenile crust extraction, and discuss the implications for Archean tectonic processes. The granitoids are metaluminous to weakly peraluminous, magnesian, and calcic. They were derived from basaltic protoliths with minor components sourced from pre-existing felsic crust. Low La/Yb and Sr/Y indicate shallow garnet-free plagioclase-bearing amphibolitic sources. The granitoids display large variations in concentration of trace elements, attributed to plagioclase accumulation or fluid-assisted mobilization of REEs during metamorphism. Zircon ages help to constrain four major episodes of granitoid crust formation at 3.43–3.41 Ga, 3.36–3.34 Ga, 3.29–3.25 Ga, and 2.66–2.65 Ga. The 3.43–3.41 Ga, 3.36–3.34 Ga, and 3.29–3.25 Ga granitoid suites have positive εHfi (2.7–4.5) and plot on a common εHfi vs. time trend consistent with repeated granitoid extraction at 3.43–3.41 Ga, 3.36–3.34 Ga, and 3.29–3.25 Ga from mafic sources that separated from model depleted mantle between 3.55 Ga and 3.35 Ga. The εHfi (0.4–0.69) of the 2.66–2.65 Ga Neoarchean granitoids can be explained by melting of similar 3.35 Ga mafic crust or by mixing between juvenile magmas and preexisting granitoids. Uranium-Pb ages from metamorphic zircons indicate polyphase metamorphism of the granitoids at 3353–3329 Ma, 3264–3256 Ma, 3187–3141 Ma, 3083–3062 Ma, and 2574–2526 Ma. Hf-isotopic data from zircons in granitoids from several cratons indicate that prior to c. 3.5 Ga most granitoids have chondritic or crust-like εHfi explained by repeated granitoid extraction from long-lived mafic crusts with limited interaction with juvenile magmas. Juvenile εHfi and short protolith residence times of the Western Dharwar Craton Paleoarchean granitoids is suggestive of a tectonic setting with rapid recycling of basalts as in subduction zones. In contrast, greater protolith residence times and crust-like signature of granitoids older than 3.5 Ga in the crustal record indicate a tectonic setting where basalts persisted for prolonged periods of times such as in an oceanic plateau.

Accretion of oceanic plateaus at continental margins: Numerical modeling

The accretion of oceanic plateaus has played a significant role in continental growth during Earth's history, which is evidenced by the presence of oceanic island basalts (OIB) and plume-type ophiolites in many modern orogens. However, oceanic plateaus can also be subducted into the deeper mantle, as revealed by seismic tomography. The controlling factors of accretion versus subduction of oceanic plateaus remain unclear. Here, we investigate the dynamics of oceanic plateau accretion at active continental margins using a thermo-mechanical numerical model. Three major factors for the accretion of oceanic plateaus are studied: (1) a thinned continental margin of the overriding plate, (2) “weak” layers in the oceanic lithosphere, and (3) a young oceanic plateau. For a large oceanic plateau, the modes of oceanic plateau accretion can be classified into one-sided and two-sided subduction–collisional regimes, which mainly depend on the geometry of the continental margin (normal or thinned). For smaller-sized seamounts, accretion occurs only if all three factors are satisfied, of which a thinned continental margin is the most critical. Possible geological analogues for the two-sided subduction–collisional mode include the Taiwan orogenic belt and subduction of the Ontong Java Plateau. The accretion model for small oceanic plateaus applies to the Nadanhada Terrane in Northeast China.

Accretionary processes and metallogenesis of the Central Asian Orogenic Belt: Advances and perspectives

As one of the largest Phanerozoic orogens in the world, the Central Asian Orogenic Belt (CAOB) is a natural laboratory for studies of continental dynamics and metallogenesis. This paper summarizes the research progresses of the accretionary processes and metallogenesis of the CAOB since the People’s Republic of China was founded, and puts forward the prospect for future research. During the early period (1950s–1970s), several geological theories were applied to explain the geological evolution of Central Asia. In the early period of China’s reform and opening-up, the plate tectonics theory was applied to explain the evolution of the northern Xinjiang and Xingmeng regions, and the opinion of subduction-collision between Siberian, Kazakhstan, and China-North Korea-Tarim plates was proposed. The idea of the Solonker-Yanbian suture zone was established. In the 1990s, the study of the CAOB entered a period of rapid development. One school of scholars including geologists from the former Soviet Union proposed a multi-block collision model for the assemblage of the CAOB. In contrast, another school of scholars, led by a Turkish geologist, Celal Şengor, proposed that the Altaids was formed through the growth and strike-slip duplicates of a single island arc, and pointed out that the Altaids is a special type of collisional orogen. During this period, Chinese geologists carried out a lot of pioneering researches on ophiolites and high-pressure metamorphic rocks in northern China, and confirmed the main suture zones accordingly. In 1999, the concept of “Central Asian metallogenic domain” was proposed, and it became one of the three major metallogenic domains in the world. Since the 21st century, given the importance for understanding continental accretion and metallogenic mechanism, the CAOB has become the international academic forefront. China has laid out a series of scientific research projects in Central Asia. A large number of important scientific research achievements have been spawned, including the tectonic attribution of micro-continents, timing and tectonic settings of ophiolites, magmatic arcs, identification and anatomy of accretionary wedges, regional metamorphism-deformation, (ultra)high-pressure metamorphism, ridge subduction, plume-plate interaction, archipelagic paleogeography and spatio-temporal framework of multiple accretionary orogeny, continental growth, accretionary metallogenesis, structural superposition and transformation, etc. These achievements have made important international influences. There still exist the following aspects that need further study: (1) Early evolution history and subduction initiation of the Paleo-Asian Ocean; (2) The accretionary mechanism of the extroversion Paleo-Asian Ocean; (3) The properties of the mantle of the Paleo-Asian Ocean and their spatio-temporal distribution; (4) The interaction between the Paleo-Asian Ocean and the Tethys Ocean; (5) Phanerozoic continental growth mechanism and its global comparison; (6) Accretionary metallogenic mechanism of the Central Asian metallogenic domain; and (7) Continental transformation mechanism.

The criticality of growth, urbanization, electricity and fossil fuel consumption to environment sustainability in Africa

While most African economies are primarily sandwiched with the seemingly unsurmountable task of attaining consistent economic growth and unhindered energy supply, the enormous threat posed by environmental degradation has further complicated the economic and environmental sustainability drive. In this context, the present study examines the effect of economic growth, urbanization, electricity consumption, fossil fuel energy consumption, and total natural resources rent on pollutant emissions in Africa over the period 1980–2014. By employing selected African countries, the current study relies on the Kao and Pedroni cointegration tests to cointegration analysis, the Pesaran's Panel Pooled Mean Group-Autoregressive distributive lag methodology (ARDL-PMG) for long run regression while Dumitrescu and Hurlin (2012) is employed for the detection of causality direction among the outlined variables. The study traces long run equilibrium relationships between examined indicators. The ARDL-PMG results suggest a statistical positive relationship between pollutant emissions and urbanization, electricity consumption and non-renewable energy consumption. Dumitrescu and Hurlin (2012) Granger causality test lends support to the long-run regression results. A bi-directional causality is observed between pollutant emissions, electricity consumption, economic growth and pollutant emissions while a unidirectional causality is apparent between total natural resources rent and pollutant emission. Based on these results, several policy implications for the African continent were suggested. (a) The need for a paradigm shift from fossil fuel sources to renewables is encouraged in the region (b) The need to embrace carbon storage and capturing techniques to decouple pollutant emissions from economic growth on the continent's growth trajectory. Further policy insights are elucidated.

Evaluation of downward movements of Japanese eel Anguilla japonica inhabiting brackish water areas.

This study evaluated the size and age distributions and otolith microchemistry of the Japanese eel Anguilla japonica in freshwater and brackish water areas in the Aki and Tsuchikawa rivers for 1 year, and in brackish water areas in the Asahi River for 3 years to understand the movements of Japanese eels between continental habitats of different salinity after recruitment (n = 759). For all three rivers, the total length (LT ) and age distributions were consistent; yellow eels captured in the upper brackish water (Aki River: 353.5 ± 77.4 mm and 3.0 ± 0.8 years; Tsuchikawa River: 287.7 ± 87.3 mm and 3.7 ± 1.3 years; Asahi River: 418.2 ± 112.1 mm and 4.2 ± 1.7 years) were smaller and younger than not only those in the fresh water of the two rivers but also those in the lowest brackish water sampling areas (Aki River: 436.0 ± 71.6 mm and 3.8 ± 1.1 years; Tsuchikawa River: 370.9 ± 121.7 mm and 4.9 ± 2.3 years; Asahi River: 558.5 ± 85.9 mm and 5.7 ± 1.7 years). In the Asahi River, these tendencies were found throughout the 3 years. Otolith analysis indicated that the majority of the eels captured in the lowest brackish water areas had moved down from upstream. These results suggest that Japanese eels inhabiting saline water generally move from the upper estuary as they grow. The upper estuary can be an important area for the management of this species because these eels spend their early continental growth life there.

Geologic evolution of the Peña flexure, Southwestern Pyrenees mountain front, Spain

The sedimentary and tectonic evolution of Oligocene continental growth strata of the Peña flexure records the development of a thrust-related anticlinorium and foreland-dipping molasse along the Southwestern Pyrenees mountain front. We exploit the coupling of uplift, the production of accommodation space, and resultant synorogenic sedimentation to explore the interplay of processes accompanying thrust faulting. A new magnetic polarity strata through ~3400 m of continuous pre- and syn-growth strata near the town of Sos del Rey Católico, Spain shows coordinated deposition and deformation. The ChRM data reside mostly in magnetite. Correlation to the Geomagnetic Polarity Time Scale suggests our measured stratigraphic section spans from 33.2 Ma (Chron C12r) in the Middle Campodarbe Formation to 23.4 Ma (chron C6C) at the top of the Bernués Formation. Shallowing in dip of the growth strata up-section is related to coeval slip on a mountain front backthrust. The onset of growth strata deposition and fault slip at this position along the Pyrenees mountain front occurred at about 28.6 ± 0.1 Ma. Folding rates calculated from the growth strata at the resolution of the polarity chrons (0.15–1.12 Myr), varies from 03°/Myr to 35°/Myr.

Bimodal magmatism in the Eastern Dharwar Craton, southern India: Implications for Neoarchean crustal evolution

Archean cratons provide windows to the crustal evolution history in the early Earth. The Dharwar Craton is one of the major cratonic blocks in Peninsular India and is composed of several ancient microcontinents where multiple continental growth and recycling occurred from 3.8 to 2.5 Ga. Here we investigate a suite of magmatic rocks (including granitoids, mafic magmatic enclaves, ultramafic inclusions and synplutonic mafic dyke) from the western margin of the Eastern Dharwar Craton which provide insights on bimodal magmatism in a subduction-related arc setting and crustal growth during Neoarchean. Field evidence and geochemical features suggest mixing and mingling of mafic and felsic magmas. Geochemical features including incompatible and high field strength element features of the enclaves indicate heterogeneous sources involving dominantly enriched mantle reservoirs with minor depleted mantle whereas the host granitoids display depleted to enriched source. We present zircon UPb age data from the various rock types and the combined results display unimodal distribution with a single peak of 207Pb/206Pb age at 2531 Ma (n = 159) suggesting a major Neoarchean bimodal magmatic pulse. The zircon UPb ages of the mafic enclaves indicate either similar ages or slightly younger ages as compared to their host granitoids implying that enclaves represent late stage injections into crystallizing host granitoid magmas. The trace and rare earth element data on zircon from these rocks are consistent with magmatic crystallization. Zircons from most of the rocks in our study display uniformly positive εHf(t) (0.3–8.6), suggesting depleted mantle (juvenile) source for the Neoarchean arc building, and formation of new crust. Some of the mafic magmatic and ultramafic enclaves, as well as the mafic dyke and charnockite display mixed positive and negative εHf(t) values (−7.0 to 3.8), implying mixed depleted mantle and reworked crustal components of Neoarchean to Mesoarchean age. Most of the Hf depleted mantle model ages (TDM; 2520–3115 Ma) and crustal residence ages (TDMC; 2507 Ma to 3452 Ma) fall within the region bounding CHUR and depleted mantle lines, suggesting that the dominant magma source involved juvenile (depleted mantle) components. Thus, latest Neoarchean marks a major phase of continent building in the Eastern Dharwar Craton, and the results presented in our study suggest that bimodal magmatism in a subduction-related setting identical to those in modern convergent margins contributed to crustal growth.

Modern-style tectonic cycle in earliest Proterozoic time: Petrogenesis of dioritic-granitic rocks from the Daqingshan–Wulashan Terrane, southern Yinshan Block, North China Craton

Archean granitoids provide us with an insight into the accretion and differentiation of early continental crust. We report the study of a dioritic to granitic rock association occurring in the Hongzhao region of the Daqingshan–Wulashan Terrane at the southern periphery of the Yinshan Block in western North China Craton. Zircon UPb dating reveals that the Hongzhao plutonic suite was emplaced within a short period (2.48–2.46 Ga) at the Archaean–Proterozoic transition. Four types of rocks have been identified based on their petrogeochemistry, which are K–rich diorite (KDG), Na–rich diorite (NDG), metaluminous monzogranite (MMG), and peraluminous syenogranite (PSG). First, the K–rich diorites are characterized by enrichment in both incompatible elements and transition elements with SiO2 contents of 63.4 to 57.2 wt%. Their dual geochemical character resembles the high–Ti sanukitoids, with strongly subchondritic isotopic compositions [εHf (t) = −0.8 to −5.6; εNd (t) =0.4 to −3.1] and high K2O/Na2O ratios (1.19–0.76), implying the contribution of older crustal materials in their origin. Combined with their high Y (46.9–17.9 ppm) and HREE (Yb = 4.35–1.62 ppm) abundances, the K–rich diorites were most likely derived from a mantle source previously enriched by melts from ancient terrigenous sediments at relatively lower pressures. Second, the Na-rich diorites show a crustal–derived, adakite affinity with moderate SiO2 (66.1–55.3 wt%) contents, high Na2O/K2O (8.51–3.17), Sr/Y (147–37), and (La/Yb)N (51–15) ratios, and low MgO (2.96–1.67 wt%), Cr (64–13 ppm), and Ni (38–6 ppm) contents. Na-rich diorites show evolved and homogeneous zircon εHf (t) (4.0–2.8) and whole–rock εNd (t) (3.6–1.2) values, indicating that they were most probably produced by partial melting of pre-existing mafic crust. Third, the metaluminous monzogranites exhibit high SiO2 contents and moderate REE fractionation with variably negative Eu anomalies. Evolved and homogeneous isotopic compositions [εHf (t) = 4.2 to 2.1 and εNd (t) = 1.7 to 1.6] suggest that the metaluminous monzogranites originated from partial melting of pre–existing continental rocks at shallow depth. Fourth, the peraluminous syenogranites show the highest SiO2 (77.3–68.6 wt%) contents and lowest CaO, ∑REE, and Y abundances, extremely high Sr/Y (up to 241), (La/Yb)N (up to 128) ratios, conspicuous positive Eu anomalies (Eu/Eu* = 6.49–0.91), and variable but negative–dominated εHf (t) (1.2 to −5.9) and εNd (t) (0.8 to −0.8) values, suggesting that they might be produced by the reworking of ancient heterogeneous metasedimentary rocks at relatively high-pressure conditions. These 2.48–2.46 Ga dioritic–granitic rock associations suggest a nearly concurrent melting of both the mantle and the crust from deep to shallow levels, implying post-tectonic extension. They are significantly different from widespread and voluminous >2.5 Ga Na–rich TTG suites in the NCC that represented the growth of the continent. This shifting series of magmas documents the tectonic changes from subduction–collision to post–collisional extension at the Archean–Proterozoic transition, corresponding to the transition from continental growth to cratonization of the North China Craton. Featuring diverse and evolving magmatism, including the enrichment of mantle-derived melts with sediments in the mantle wedge, the Hongzhao plutonic suite suggests a modern-style tectonic cycle in earliest Proterozoic time.

U‐Pb Ages and Hf Isotope of Zircons from a Carbonatite Dyke in the Bayan Obo Fe‐REE Deposit in Inner Mongolia: its Geological Significance

Detailed studies on U‐Pb ages and Hf isotope have been carried out in zircons from a carbonatite dyke associated with the Bayan Obo giant REE‐Nb‐Fe deposit, northern margin of the North China Craton (NCC), which provide insights into the plate tectonic in Paleoproterozoic. Analyses of small amounts of zircons extracted from a large sample of the Wu carbonatite dyke have yielded two ages of late Archaean and late Paleoproterozoic (with mean 207Pb/206Pb ages of 2521±25 Ma and 1921±14 Ma, respectively). Mineral inclusions in the zircon identified by Raman spectroscopy are all silicate minerals, and none of the zircon grains has the extremely high Th/U characteristic of carbonatite, which are consistent with crystallization of the zircon from silicate, and the zircon is suggested to be derived from trapped basement complex. Hf isotopes in the zircon from the studied carbonatite are different from grain to grain, suggesting the zircons were not all formed in one single process. Majority of ∊Hf(t) values are compatible with ancient crustal sources with limited juvenile component. The Hf data and their TDM2 values also suggest a juvenile continental growth in Paleoproterozoic during the period of 1940–1957 Ma. Our data demonstrate the major crustal growth during the Paleoproterozoic in the northern margin of the NCC, coeval with the assembly of the supercontinent Columbia, and provide insights into the plate tectonic of the NCC in Paleoproterozoic.

Continental crustal evolution and synchronous metallogeny through time in the North China Craton

The generation and evolution of continent crust have long been matters of debates. There is evidence to support the view that the first formation of TTGs of continent was possibly before 4.4 Ga, followed by the sizable continent crust associated with BIF-bearing supra-crustal rocks at ca. 3.8 Ga. Multiple stages of continental growth include continental nuclei at ~3.1 Ga, enormous crustal growth at 2.9–2.7 Ga and stabilization and cratonization at ~2.5 Ga. Subsequently a stable super-continent was formed and the Earth was evolved into the Proterozoic eon. In the Proterozoic eon, major geological events include the great oxygen event (GOE), global rift and mobile belt event, and the Earth’s middle age event with a 1.0 billion years long period of Earth’s adjustment or silence. After that, the Rodinian super-continental break up event (the Nanhua rift) and subsequent Gondwana amalgamation have been suggested to be involved with mantle-plume and the modern style-like plate tectonics, both of which are still under debates. Recent studies suggest that the continental metallogeny has secular changes with the evolving Earth, showing synchronous evolution with the continental crust through time. The North China Craton (NCC) is one of the oldest cratons in the world, and has undergone six major tectono-metallogenic cycles: (1) the Neoarchean crustal growth and Algoman-type BIFs-greenstone belt and gold metallogenic system, (2) Paleoproterozoic GOE associated with global (Huronian) rifting- Glaciation and Superior-type BIFs-Mg-B metallogenic system, (3) Paleoproterozoic rifting-subduction-accretion-collision event (mobile belt) and Cu-Pb-Zn metallogenic system; (4) Late Paleoproterozoic – Neoproterozoic multi-stage rifting and REE-Fe-Pb-Zn metallogenic system; (5) Paleozoic orogenesis at the margins of the craton and porphyry Cu-Mo metallogenic system; and (6) Mesozoic extensional tectonics associated with lithosphere thinning and intracontinental Au metallogenic systems. Ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities. Changes of metallogenic styles with time, from the dominant BIFs in Archean to polymetallic deposits in Proterozoic and Phanerozoic, demonstrate that the nature and style of metallogeny through time change with different stages of the Earth’s evolution and are the expression of variable materials at the different stages. The changes of metallogeny through time are controlled by the irreversible continental tectonic evolution. The connotation and signification of synchronous evolution of continental crust and mineral deposit systems through time are beyond the study of mineral resources but would provide new insights into the geodynamics of the generation and evolution of the continent. The NCC provides an excellent venue to address the linkage between metallogenic processes and secular changes in the Earth System, including mantle dynamics and crust-mantle interaction on a global perspective.

Appreciation of A.M. Celâl Şengör

Celâl Şengör is one of the leading and most influential geologists of our time. He has made many significant contributions to the understanding of the evolution of our planet in many areas, including continental growth, Phanerozoic orogenic belts, continental rifts, transform faults, and mantle plumes.

Detrital zircons from Late Paleozoic to Triassic sedimentary rocks of the Gongshan-Baoshan Block, SE Tibet: Implications for episodic crustal growth of Eastern Gondwana

The Gongshan-Baoshan Block is a segment of the northern Sibumasu Terrane, which is a continental strip derived from Eastern Gondwana. However, the formation history of the Gongshan-Baoshan Block and its relationship with other East Gondwana-derived blocks remain unclear. Here, we present five hundred and eighty new analyses of U-Pb ages of detrital zircons from Late Paleozoic to Triassic sedimentary rocks in the Gongshan-Baoshan Block, western Yunnan. The detrital zircon age spectra show distinctive age clusters at 2500–2400 Ma, 1750–1500 Ma, 1200–900 Ma, 600–500 Ma, with a few Archean ages of 3500–2600 Ma, suggesting secular, episodic, continental growth. The age spectra are comparable to those from the Sibumasu Terrane, Australia, Lhasa Block, Qiangtang Block, Tethyan High Himalaya and Cathaysia Block. We confirm that the Gongshan-Baoshan Block and Sibumasu Terrane could have occupied a position outboard of the Lhasa Block and close to northern India, along the northwestern margin of Australia in East Gondwana. The youngest ages from the Mesozioc sediments limit the timing of subduction of the Paleo-Tethys Ocean, which initiated by ~273 Ma and finally stopped at ~223 Ma.