Tectonics and crustal evolution

We thank the Natural Environment Research Council (grants NE/J021822/1 and NE/K008862/1) for funding.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.Publisher PDFPeer reviewe

Neoproterozoic subduction along the Ailaoshan zone, South China : geochronological and geochemical evidence from amphibolite

This study was supported by China Natural Science Foundation (41190073 and 41372198), National Basic Research Program of China (2014CB440901) and Natural Environment Research Council (grant NE/J021822/1).Lenses of amphibolites occur along the Ailaoshan suture zone at the southwestern margin of the Yangtze Block, South China. Petrological, geochemical and zircon U-Pb geochronological data indicate that they are divisible into two coeval groups. Group 1, represented by the Jinping amphibolite, has mg-number of 71-76 and (La/Yb)cn ratios of 7.2-7.7, and displays a geochemical affinity to island arc volcanic rocks. Group 2 amphibolites occur at Yuanyang and are characterized by high Nb contents (14.3-18.4 ppm), resembling Nb-enriched basalts. The epsilon(Nd)(t) values for Group 1 range from -3.45 to -2.04 and for Group 2 from +4.08 to +4.39. A representative sample for Group 1 yields a U-Pb zircon age of 803 7 Ma, whereas two samples for Group 2 give U-Pb zircon ages of 813 +/- 11 Ma and 814 +/- 12 Ma. Petrogenetic analysis suggests that Group 1 originated from an orthopyroxene-rich source and Group 2 from a mantle wedge modified by slab-derived melt. In combination with other geological observations, these amphibolites are inferred to constitute part of an early Neoproterozoic (similar to 815-800 Ma) arc-back-arc basin system. The Neoproterozoic amphibolites and related rocks along the Ailaoshan zone may be the southward extension of the Neoproterozoic supra-subduction zone that developed along the western margin of the Yangtze Block. (C) 2014 Elsevier B.V. All rights reserved.PostprintPeer reviewe

Constraining timing and tectonic implications of Neoproterozoic metamorphic event in the Cathaysia Block, South China

We acknowledge the financial support by the Major State Research Development Program of China (Grant No. 2016YFC0600202), National Natural Science Foundation of China (Nos. 41330208 and 41572200) and State Key Laboratory for Mineral Deposits Research (Nanjing University) (ZZKT-201603).The Cathaysia Block of the South China Craton includes a Proterozoic basement that experienced a prolonged Precambrian crustal evolution but to date lacks evidence of Proterozoic metamorphic ages. At Lichuan and Jianning, in the Wuyi Domain of the eastern Cathaysia Block, Proterozoic rock units include migmatized paragneiss of the Wanyuan Group and minor amphibolite of the Tianjingping Formation, which are enveloped by schist of Mayuan Group, and all are intruded by Paleozoic and Mesozoic igneous rocks. Detrital zircon grains from the Wanyuan paragneiss display metamorphic rims that yield concordant weighted average 206Pb/238U ages of 860 ± 6 Ma and 435 ± 5 Ma, along with variably disconcordant ages with lower intercept ages of 442 ± 41 Ma. The zircon core ages range from 3015 Ma to 851 Ma, with three major age populations at 930–865 Ma, 1850–1200 Ma and 2650–2400 Ma. Detrital zircon grains from Mayuan schist samples at Jianning generally lack core-rim structures and yield three main age populations at 860–736 Ma, 1835–1775 Ma and 2720–2500 Ma. Metamorphic ages of ca. 860 Ma and ca. 435 Ma for the Wanyuan paragneiss along with the youngest detrital zircon constrain the depositional age of the protolith to ca. 865–860 Ma, whereas the Mayuan Group is younger and probably deposited after ca. 736 Ma. Characteristics of detrital zircon age populations along with regional geological data suggest accumulation of the Wanyuan Group in a convergent and/or collisional setting. Metamorphism and a possible subduction -collision process within the Cathaysia Block at around 860 Ma suggest it was not a unified block in early Neoproterozoic. The growth of ca. 440 Ma metamorphic rims is likely related to granitic magmatism, such as that exposed in the Lichuan region. The sparse evidence for early Neoproterozoic metamorphism likely reflects widespread overprinting by the Paleozoic tectonothermal event at around 440 Ma.PostprintPeer reviewe

Jiangnan Orogen, South China : a ~970–820 Ma Rodinia margin accretionary belt

Authors thank the Major State Research Development Program of China (Grant No. 2016YFC0600202) for financial support, as well as financial support from National Natural Science Foundation of China (Nos. 41330208, 41572200 and 41190070) and State Key Laboratory for Mineral Deposits Research (Nanjing University) (ZZKT–201603). Peter A. Cawood acknowledges support from Australian Research Council grant FL160100168.The Neoproterozoic Jiangnan Orogen in South China records a succession of arc-trench-basin assemblages culminating in accretion of the bounding Yangtze and Cathaysia blocks to form the stabilized South China Craton. The orogen can be traced over some 1500 km and extends up to 100 km across strike. It is divisible into three domains: the northeast domain (also referred to as the Huaiyu or Shuangxiwu Terrane), the central domain (Jiuling Terrane), and an undifferentiated southwest domain. The northeast domain contains arc type volcanic suites and I-type granitoids dated at ca. 970–850 Ma. It is interpreted as an intra–oceanic terrane based on the juvenile radiogenic isotopic signature of the igneous rocks, the absence of older detritus and inherited xenocrysts, and the presence of ophiolites along its southwestern and western margins. The central and southwest domains contain trench-arc-basin assemblages of clastic sedimentary units, mappable magmatic arc suites and ophiolitic mélanges (Sibao and equivalent groups) that range in age from ca. 880 to 820–815 Ma. The presence of old zircon grains within these two domains, both as detritus within sedimentary units and as inherited zircon in arc basalt, suggest they formed at convergent continental margins. S-type granites dated at 845–815 Ma are a distinctive element of the central and southwest domains. The ages of these granites overlap with convergent plate magmatism in the two domains, arguing against previous models for plume-rift and post-collisional geodynamic settings. Instead, these bodies likely formed in an accretionary orogenic margin setting in which granitic magmatism occurred in an extensional regime triggered by slab rollback. The slab-rollback process triggered mantle-sourced thermal input and partial melting of the older and buried arc-bounding basin sediments. Early Paleozoic S-type granites in the Lachlan and New England belts in eastern Australia and Jurassic ones in the Cordillera belt of the western US provide analogous geodynamic environments. Isotopic data indicate that the central Jiangnan domain experienced significant crustal growth, whereas in the southwest domain there was a greater degree of crustal reworking. The character and distribution of the early Neoproterozoic sedimentary and igneous succession in the orogen suggests it represents a ca. 970–820 Ma accretionary orogen. Upper age limits on the Jiangnan Orogen are provided by a regional angular unconformity in the central and southwest domains at ca. 810–805 Ma, and in the northeast domain at ca. 825 Ma, along with the overlying bimodal volcanic and clastic sedimentary successions mostly dated at ca. 810–730 Ma. Thus, timing of final assembly of South China displays variations across the Jiangnan Orogen, from ca. 825 Ma in the northeast to ca. 820–805 Ma in the central and southwest of the orogen. Post-assembly successions are parts of the Nanhua Basin and are interpreted to have formed during regional lithospheric extension across the eastern and central South China Craton. The age patterns across the South China Craton are indicative of northwest directed accretion of fragments and suggest an external rather than an internal position of the craton within the assembled Rodinia supercontinent. Paleomagnetic data, regional correlations and sedimentary records are consistent with a position along the northern margin of Rodinia, adjacent to India and Australia. The Jiangnan Orogen recorded the accretion of trench-arc assemblages and ultimately the Yangtze Block to the Cathaysia Block that was already located on the margin of Rodinia. The Panxi-Hanan belt, which lies along the western and northwestern margin of the Yangtze Block, formed on the upper plate to a subduction system that both overlaps with, and is younger than, the Jiangnan Orogen. The belt provides a record of ongoing accretion on the Rodinia margin until the mid-Neoproterozoic.PostprintPeer reviewe

Continental growth seen through the sedimentary record

This work was supported by the Natural Environment Research Council [NERC grant NE/K008862/1], the Leverhulme Trust [grant RPG-2015–422], and the Australian Research Council [grant FL160100168].Sedimentary rocks and detrital minerals sample large areas of the continental crust, and they are increasingly seen as a reliable archive for its global evolution. This study presents two approaches to model the growth of the continental crust through the sedimentary archive. The first builds on the variations in U-Pb, Hf and O isotopes in global databases of detrital zircons. We show that uncertainty in the Hf isotope composition of the mantle reservoir from which new crust separated, in the 176Lu/177Hf ratio of that new crust, and in the contribution in the databases of zircons that experienced ancient Pb loss(es), adds some uncertainty to the individual Hf model ages, but not to the overall shape of the calculated continental growth curves. The second approach is based on the variation of Nd isotopes in 645 worldwide fine-grained continental sedimentary rocks with different deposition ages, which requires a correction of the bias induced by preferential erosion of younger rocks through an erosion parameter referred to as K. This dimensionless parameter relates the proportions of younger to older source rocks in the sediment, to the proportions of younger to older source rocks present in the crust from which the sediment was derived. We suggest that a Hadean/Archaean value of K = 1 (i.e., no preferential erosion), and that post-Archaean values of K = 4–6, may be reasonable for the global Earth system. Models built on the detrital zircon and the fine-grained sediment records independently suggest that at least 65% of the present volume of continental crust was established by 3 Ga. The continental crust has been generated continuously, but with a marked decrease in the growth rate at ~ 3 Ga. The period from > 4 Ga to ~ 3 Ga is characterised by relatively high net rates of continental growth (2.9–3.4 km3 yr−1 on average), which are similar to the rates at which new crust is generated (and destroyed) at the present time. Net growth rates are much lower since 3 Ga (0.6–0.9 km3 yr−1 on average), which can be attributed to higher rates of destruction of continental crust. The change in slope in the continental growth curve at ~ 3 Ga is taken to indicate a global change in the way bulk crust was generated and preserved, and this change has been linked to the onset of subduction-driven plate tectonics. At least 100% of the present volume of the continental crust has been destroyed and recycled back into the mantle since ~ 3 Ga, and this time marks a transition in the average composition of new continental crust. Continental crust generated before 3 Ga was on average mafic, dense, relatively thin (< 20 km) and therefore different from the calc-alkaline andesitic crust that dominates the continental record today. Continental crust that formed after 3 Ga gradually became more intermediate in composition, buoyant and thicker. The increase in crustal thickness is accompanied by increasing rates of crustal reworking and increasing input of sediment to the ocean. These changes may have been accommodated by a change in lithospheric strength at around 3 Ga, as it became strong enough to support high-relief crust. This time period therefore indicates when significant volumes of continental crust started to become emergent and were available for erosion and weathering, thus impacting on the composition of the atmosphere and the oceans.PostprintPeer reviewe

An early Neoproterozoic accretionary prism ophiolitic mélange from the western Jiangnan orogenic belt, South China

The authors acknowledge the financial support provided by the National Basic Research Program of China (973 Program, 2012CB416701) and the National Natural Science Foundation of China (41330208 and 41572200).The Neoproterozoic Jiangnan orogenic belt delineates the suture zone between the Cathaysia and Yangtze blocks of the South China Craton. The western part of the belt, in the Longsheng region, consists of a disrupted mafic-ultramafic assemblage of pillow basalt, gabbro, diabase, and peridotite along with siliceous marble, ophicalcite, and jasper mixed with basalt. Significant talc deposits occur on the margins of the ultramafic bodies as well as in the transition zone between marble and basalt. Primary rock relations are largely overprinted by pervasive shearing, resulting in disruption of the assemblage into series of discontinuous blocks within a phyllite matrix. West-dipping thrust faults mark the eastern contact of blocks, and the overall succession has the appearance of a tectonic mélange. U-Pb zircon age data from the gabbros and diabases yield crystallization ages of 867 ± 10, 863 ± 8, and 869 ± 9 Ma, with positive εHf(t) values. The gabbro, basalt, serpentinite, and some talc samples display minor light rare earth element?enriched patterns with obvious depletion of Nb and Ta, indicating a subduction-related setting. The tuffaceous phyllite shows similar geochemical features. A few mafic rocks and the altered ultramafic rocks display mid-ocean ridge basalt (MORB) affinity. Overall lithostratigraphic relationships, age data, and geochemical signatures suggest a forearc setting that was imbricated and disrupted within an accretionary prism environment to form an ophiolitic mélange. The pillow basalt, red jasper, and MORB-type mafic-ultramafic rocks within the mélange occur as exotic blocks derived from the subducting oceanic plate, whereas the arc-type mafic rocks occur as autochthonous blocks, which are all exposed in a matrix of sandy and tuffaceous phyllite.Publisher PDFPeer reviewe

Neoproterozoic crustal growth of the Southern Yangtze Block : Geochemical and zircon U–Pb geochronological and Lu-Hf isotopic evidence of Neoproterozoic diorite from the Ailaoshan zone

This study was supported by National Natural Science Foundation of China (41190073 and 41372198), National Basic Research Program of China (2014CB440901), Basic Operation Expense of Sun Yat-Sen University and Startup Foundation for Doctors of Guilin University of Technology (002401003475).Abstract Neoproterozoic felsic igneous rocks associated with mafic-ultramafic bodies along the margins of the Yangtze Block, South China, can be used to constrain the continental crustal growth and secular evolution of the region. LA-ICPMS zircon U-Pb dating of the Adebo quartz diorite pluton in the Ailaoshan tectonic zone on the southern margin of the Yangtze Block gives the Neoproterozoic age of 800 ± 7 Ma and ɛHf(t) values in the range of -1.03 to +3.75 with two-stage model age of 1.3-1.6 Ga. The pluton is characterized by relatively low SiO2 (60.97-64.41 wt. %) and total alkalis (K2O + Na2O, 7.35-9.14 wt. %) and high Al2O3 content (16.98-18.21 wt. %) with mg-number of 36-39. REE-normalized patterns show enrichment in LREE with (La/Yb)cn of 11.36 to 19.77 and Europium negative anomalies with Eu/Eu* = 0.61- 0.74. The samples are characterized by negative Nb-Ta ((Nb/La)n = 0.18-0.35) and P, Ti, Sr anomalies and high Y concentrations (33.79-41.04 ppm) and low Sr/Y ratios (5.65-10.16). Their isotopic composition are similar to those of the Neoproterozoic mafic igneous rocks in the Ailaoshan zone and the southwestern Yangtze Block, indicating that the quartz diorite was produced by partial melting of mafic lower crust. The diorite also shows the similar geochemical characteristics with adakitic rocks from thickened lower crust or amphibolite and eclogite experimental melts. In combination with their arc-related geochemical signatures and synchronous developed adakitic rocks in the region, the Adebo quartz diorite pluton might be produced in a subduction-related tectonic setting during Neoproterozoic crustal growth along the margins of Yangtze Block.PostprintPeer reviewe

Continental crustal volume, thickness and area, and their geodynamic implications

We appreciate support from Australian Research Council grant FL160100168 and Leverhulme Trust grants RPG-2015-422 and EM-2017-047\4.Models of the volume of continental crust through Earth history vary significantly due to a range of assumptions and data sets; estimates for 3 Ga range from 120% of present day volume. We argue that continental area and thickness varied independently and increased at different rates and over different periods, in response to different tectonic processes, through Earth history. Crustal area increased steadily on a pre-plate tectonic Earth, prior to ca. 3 Ga. By 3 Ga the area of continental crust appears to have reached a dynamic equilibrium of around 40% of the Earth's surface, and this was maintained in the plate tectonic world throughout the last 3 billion years. New continental crust was relatively thin and mafic from ca. 4–3 Ga but started to increase substantially with the inferred onset of plate tectonics at ca. 3 Ga, which also led to the sustained development of Earth's bimodal hypsometry. Integration of thickness and area data suggests continental volume increased from 4.5 Ga to 1.8 Ga, and that it remained relatively constant through Earth's middle age (1.8–0.8 Ga). Since the Neoproterozoic, the estimated crustal thickness, and by implication the volume of the continental crust, appears to have decreased by as much as 15%. This decrease indicates that crust was destroyed more rapidly than it was generated. This is perhaps associated with the commencement of cold subduction, represented by low dT/dP metamorphic assemblages, resulting in higher rates of destruction of the continental crust through increased sediment subduction and subduction erosion.PostprintPeer reviewe

Cyclic formation and stabilization of Archean lithosphere by accretionary orogenesis : constraints from TTG and potassic granitoids, North China Craton

This study is financially supported by the National Natural Science Foundation of China (Grant Nos. 41502179, 41530207, 41472165) and Central University Basic Scientific Research Business Expenses of China University of Geosciences (Beijing) (Grant No. 2652015038). PAC acknowledges support from Australian Research Council grant FL160100168.Accretionary orogens are major sites of modern continental growth, yet their role in the development of Archean continental crust remains enigmatic. Diverse granitoid suites from tonalite-trondhjemite-granodiorite (TTG) to potassic granitoids appeared during late Archean, representing a period of major continental formation and stabilization. In this study, whole-rock geochemical and zircon U-Pb and Lu-Hf isotopic data are reported for Neoarchean granitoid gneisses from the Northern Liaoning Terrane, northeastern North China Craton (NCC). Older granitoid gneisses (~2592-2537 Ma) define three magmatic zones migrating from southeast to northwest, each showing a common magmatic evolution from high-pressure TTGs to medium-/low-pressure TTGs and potassic granitoids. They have depleted zircon ƐHf(t) of +0.5 to +8.7. Younger ~2529-2503 Ma potassic granitoids and TTGs occur throughout the terrane, which are marked by variable zircon ƐHf(t) of -4.7 to +8.1, and are coeval with regional high grade metamorphism. Petrogenetic modeling and changing Sr/Y and (La/Yb)N of the granitoids suggest that the crust experienced episodic thickening and thinning, and became progressively evolved through development of potassic granitoids and sedimentary successions. The metavolcanic basement to the granitoids display tholeiitic to calc-alkaline affinities, together with the top-to-the-northwest thrusting and associated VMS-type Cu-Zn deposits, suggesting cyclic crustal formation of Northern Liaoning within an accretionary orogen with a SE-dipping subduction polarity. Cyclic crustal thickening and thinning is related to tectonic switching from advancing to retreating relations between the downgoing and overriding plate. After ~2530 Ma, this accretionary system accreted to the ancient continental nucleus of NCC (Anshan-Benxi Terrane), signifying final lithosphere stabilization.Publisher PDFPeer reviewe