Mafic Crust Research Articles

A petrochronology window into near-surface fluid/rock interaction within Archaean ultramafic-mafic crust: Insights from the 3.25 Ga Stolzburg Complex, Barberton Greenstone Belt

The Stolzburg Complex is one of several early Archaean ultramafic-mafic layered complexes situated along the northern edge of the Barberton Greenstone Belt on the eastern Kaapvaal Craton. In order to constrain timing and mode of emplacement, as well as post-emplacement processes affecting this igneous complex, we present whole rock trace element compositions as well as isotopic (U-Pb, O, Hf) and REE-Ti concentration data for zircon and titanite from three representative meta-gabbroic samples from the central and western portions of the complex. Zircon (SHRIMP) and titanite (LA-MC-ICPMS) analyses yield precise and identical U-Pb ages of 3247 ± 3 Ma, 3252 ± 16 Ma and 3259 ± 6 Ma (2SE), respectively. Cathodo-luminescence imaging of zircon reveals remnant, faint oscillatory and convolute zoning that is cross-cut by luminescent, irregular fracture fillings. Zircon crystals from one meta-gabbro sample yield oxygen isotopic compositions (δ18O) of <1 to +6‰, which are uncorrelated with the initial Hf isotopic compositions (εHf) of 0 to +3 epsilon units. Zircon REE systematics are highly variable across crystal domains and characterized by enriched light and medium REE. Micro-textural evidence of post-emplacement modification, along with zircon δ18O and trace element systematics, are indicative of pervasive but heterogeneous/irregular overprinting involving high-T hydrothermal fluids (>250 °C) soon after magma emplacement. Some recent metamictization may have also contributed to the low δ18O component identified in the magmatic zircon, but is difficult to disentangle from hydrothermal effects. Based on zircon trace element systematics, the gabbros appear to have formed in an oceanic setting, consistent with the lack of whole rock geochemical evidence for assimilation of more evolved felsic crustal material. Considering analytical uncertainty, zircon εHf values are relatively uniform and point to derivation of the mafic magma from a Depleted Mantle source. Titanium-in-zircon temperatures (assuming αSiO2 = 1.0, αTiO2 = 1.0) range from 840 to 1020 °C. Although such apparent crystallization temperatures do neither constrain magma petrogenesis nor tectonic setting uniquely, they match temperatures recorded by modern mid-oceanic gabbros that experienced extensive hydrothermal alteration. Thus, the meta-gabbros of the ca. 3.25 Ga Stolzburg Complex provide a rare record of fluid-rock interaction at high temperatures within mafic crust on an ancient seafloor.

Rock probes and seismic methods to constrain the structure, composition and evolution of the deep crust beneath North China Block

<p indent="0mm">Deep continental crust archives critical information on the bulk crustal composition as well as its formation. It is commonly the inherent zone for complex crust-mantle interaction. Unlike the shallow crust, the deep crust is generally inaccessible for direct geological sampling. Instead, knowledge of the deep crust can be obtained from a combination study of geophysical techniques and “lithoprobe” (deep crustal fragments entrained in magmatic outcrops). Wide-angle reflection/refraction seismic profiles have the highest resolution on crustal velocity structure and provide information on the averaged present-day nature of large deep crust domains; “lithoprobe”, instead, give detailed information of locality-specific deep crustal sections through determining the physical and chemical properties, dating the formation and alteration ages and constraining the melting and equilibration depths of the xenoliths. Here, the two methodologies are integrated to constrain the crustal structure on the North China Block. The seismic crustal structure model of the block is compiled from all available seismic wide-angle reflection/refraction profiles and receiver function results. The model parameters include Moho depth, <italic>V</italic>p velocity and thicknesses for four crustal layers (sedimentary cover, and the upper, middle and lower crust). The crust as a whole is thin (&lt; <sc>36 km)</sc> in the eastern part relative to the western part <sc>(38−44 km).</sc> In addition, the crust is shown to have a thick upper and middle crust and a relatively thin (typically <sc>&lt;5 km)</sc> lower mafic crust; no spatial variations in average crustal <italic>V</italic>p velocity and densities are observed. The average crustal <italic>V</italic>p and density for the whole North China Block are <sc>6.33−6.37 km/s</sc> and <sc>2.72−2.78 g/cm³,</sc> respectively. The overall absence of high P-wave velocity layer and the relatively low average <italic>V</italic>p and density indicate that the block has an intermediate to felsic bulk crustal composition. In contrast, regional crustal xenoliths include mafic to felsic granulites and amphibolites and rare eclogite xenoliths (e.g., Xinyang and Xuhuai), indicating the presence of thick lower crust (e.g., <sc>&gt;45 km)</sc> at that time. Meanwhile, mafic xenoliths in Hannuoba and Nüshan show strong mafic underplating at the lower crust in the Meso-Cenozoic while no high-velocity layer is shown in the seismic crustal model. These inconsistences can be explained by the fact that the two methodologies tap the crustal information at different timeslices and dimensions and be complementary to understand the chemical and physical structure of the deep crust. Therefore, the combination study of petrological and geochemical of the crustal xenoliths and geophysical observations can provide essential information on making and destroying the deep continental crust through time.

Metamorphic history and Neoarchean–Paleoproterozoic crustal growth of the central Trans-North China Orogen: Evidence from granulite- to amphibolite-facies rocks of the Hengshan complex

The Trans-North China Orogen (TNCO) is a major Paleoproterozoic collisional orogen exposed within the North China Craton (NCC). The orogen has undergone a complex tectonic evolution, but its role in the growth of continental crust is contentious. High-grade metamorphic rocks may carry key information required to clarify these issues. Here, using petrography, mineral chemistry, and zircon U–Pb–Hf isotopes, we investigate a suite of Neoarchean–Paleoproterozoic high-grade metamorphic rocks in the Hengshan Complex within the central TNCO. Garnet-bearing mafic granulites and amphibolites occur as lenses or dikes within tonalite–trondhjemite–granodiorite (TTG) gneisses. Three stages of metamorphic mineral assemblages are identified in a mafic granulite from the northern Hengshan Complex: (1) the prograde assemblage is preserved as small plagioclase and apatite inclusions within garnet porphyroblasts; (2) the peak mineral assemblage is recorded by a high-Ca garnet core coexisting with clinopyroxene, plagioclase, hornblende, biotite, and quartz; and (3) the retrograde assemblage is represented mainly by vermicular symplectites of hornblende + plagioclase and plagioclase coronae surrounding low-Ca garnet rims. Phase equilibria modeling and conventional geothermobarometry suggest that the mafic granulite records a clockwise P–T path, including decompression and cooling from the granulite-facies peak (12.5–14 kbar and >750 °C) to amphibolite-facies conditions during retrogression (7.3–8.3 kbar and 615–721 °C). In contrast, the amphibolite in the southern Hengshan Complex preserves only amphibolite-facies assemblages (hornblende + plagioclase + quartz), consistent with the metamorphic pressures of <7.1 kbar and temperatures below 720 °C.Zircon U--Pb dating of magmatic zircons shows that the protoliths of the TTG gneisses and amphibolite were emplaced at ca. 2.5 and 2.1 Ga, respectively. The metamorphic zircons reveal ages of 1.92–1.80 Ga for the mafic granulite (weighted mean of ca. 1.87 Ga) and 1.92–1.83 Ga for the amphibolites (weighted mean of ca. 1.88 Ga). These 1.88–1.87 Ga metamorphic ages are interpreted to represent the time when the rocks cooled to the solidus. By comparing Paleoproterozoic metamorphic ages across the TNCO, we infer that the subduction–collision–amalgamation between the Western and Eastern Blocks of the NCC lasted from 1.95 to 1.80 Ga at least, marking the final cratonization of the NCC. The ca. 2.5 Ga magmatic zircons within the TTG gneisses show positive εHf(t) values (+0.49 to +7.12) with model ages of 2.8–2.7 Ga, suggesting that the protoliths were derived from the partial melting of Neoarchean juvenile mafic crust. Zircon Hf model ages (TDM = 2.1–2.0 Ga) of the mafic granulite may represent the approximate time at which the mafic magma was generated in the mantle. Thus, the Hengshan Complex within the central TNCO records two episodes of crustal growth at 2.8–2.7 and 2.1–2.0 Ga, respectively. To further understand the processes involved in crustal evolution, we reconstructed a layered crust model for the Hengshan Complex, and the layered crust was strongly influenced by Paleoproterozoic thermal events.

Molybdenum isotope tracing petrogenesis of adakitic rocks and associated ore-forming process

Molybdenum (Mo) isotopes can be fractionated during redox-related hydrothermal and Earth’s surface processes. Distinct Mo isotope features of different reservoirs, thus, make Mo isotopes as a potential tracer for the recycling of crustal materials, particularly subduction sediments. Here, we report the Mo isotope compositions of 61 early Cretaceous high-K calc-alkaline rocks (defined as adakitic rocks) with distinct source affinities from central-eastern China, including low-Mg adakitic rocks from the Dabie orogen, ore-barren high-Mg adakitic rocks from the South Tan-Lu Fault and ore-bearing high-Mg adakitic rocks from the Lower Yangtze River Belt. Low-Mg adakitic rocks have the low δ98MoNIST3134 of −0.48‰ to −0.03‰, and both ore-barren and ore-bearing high-Mg adakitic rocks display heavier isotope compositions and larger variations: for example, −0.38‰ to 0.41‰ and −0.58‰ to 1.39‰, respectively. Magma evolutions (e.g., hornblende crystallizations) appear to dominate the Mo isotope variation in low-Mg adakitic rocks but do not account for those in high-Mg adakitic rocks. Some ore-bearing high-Mg adakitic rocks characterized by high S contents and hydrothermal alteration minerals, display positive correlations between S and Mo, Cu contents, δ98Mo values, reflecting the effects of isotopically heavy hydrothermal fluid.Having eliminated the effects of magmatic evolution and hydrothermal alteration, both ore-barren and ore-bearing high-Mg adakitic rocks display multiple-end-member mixing trends according to the correlations of Ce/Mo, Th/Yb, and Ba/Th ratios to δ98Mo. First, a common end-member with extremely low Ce/Mo, low Th/Yb, high Ba/Th and mantle-like δ98Mo values was identified, representing an enriched mantle origin metasomatized by fluids from subduction oxidized sediments. The other end-member of ore-barren high-Mg adakitic rocks displays high Ce/Mo, Ba/Th ratios and low δ98Mo values similar to low-Mg adakitic rocks, thus, it represents the lower continental crust derived melt component. By contrast, the other end-member of ore-bearing high-Mg adakitic rocks displays extremely high δ98Mo (>1.5‰), implying that initial adakitic melts involve reduced sediments in addition to the oceanic mafic crust. Notably, the common enriched mantle end-member for both ore-barren and ore-bearing adakites is characterized by high Mo and Cu concentrations, implying that the incorporation of oxidized sediments into the mantle likely plays a key role in the initial enrichment of the chalcophile metals in the mantle. This work demonstrates that Mo isotopes can be useful for understanding adakitic source affinities and associated Cu-Au-Mo ore-forming processes.

Thermal evolution of mafic granulites of Princess Elizabeth Land, East Antarctica

Outcrops of East Antarctic Shield exposed along the coast of Princess Elizabeth Land (PEL) provide a peek into its Neoproterozoic evolution. The terrain comprises of interleaved pelitic, felsic and mafic granulites. Examination of mafic granulites from Sostrene Islands - Larsemann Hills - Brattstrand Bluffs terrain indicate multiple sources for their protolith. The ensemble of mafic rocks intimately associated with sedimentary protoliths are sourced from older mafic crust as well as syn-sedimentary intrusives, prior to multiple stages of deformation and high-grade metamorphism, often including felsic melt generation. High-grade metamorphism (peak conditions of ~900 °C and 11 kbar) followed by two stages of decompression, which are indicated by the presence of symplectitic growth of orthopyroxene over garnet and in cracks in garnetiferous mafic granulite. Pseudosection modelling confirms that the mafic granulites underwent a near isothermal decompression of ~2–3 kbar, indicating a clockwise PT path. Chemical dating of monazites from associated paragneisses show three growth zones viz. a Grenvillian aged core (884–940 Ma) followed by a thin Upper Tonian- Cryogenian aged middle rim (689–754 Ma) and a typical Pan African aged outer rim (523 Ma). We suggest that the thermo-tectonic evolution of PEL outcrops should be reassessed taking into account the newly emerging evidence of Tonian metamorphism.

Petrogenesis and tectonic implications of the ca. 2.5 Ga granitoid assemblage in the eastern Zhongtiao region, southern North China Craton

The key to unraveling the crustal evolution and extensive magmatism of the North China Craton (NCC) at the Archean–Proterozoic boundary (ca. 2.5 Ga) is to investigate the petrogenesis of the volumetrically dominant granitoids. In this contribution, we report new petrological, mineralogical, and geochemical data for the Huping and Luojiahe trondhjemites, which are ca. 2.5 Ga granitoid assemblages from the eastern Zhongtiao region of the southern NCC. The Huping trondhjemites have high SiO2 and Na2O contents and high (La/Yb)N and Sr/Y values, which are consistent with most Tonalite–trondhjemite–granodiorite (TTG) rocks worldwide. Evolved whole-rock Nd and Hf isotopic compositions indicate that the Huping trondhjemites were derived from partial melting of preexisting ca. 2.7 Ga mafic crust, and high MgO, Cr, and Ni contents suggest the possible involvement of depleted mantle in their source region. Integrated petrological, mineralogical, and geochemical data indicate that the Huping trondhjemites were likely formed by partial melting of delaminated lower crust. In contrast, the Luojiahe trondhjemites are Na- and Si-rich with negative Eu and Sr anomalies. Their geochemical characteristics suggest that they were likely formed by partial melting of low-K mafic crust that was shallower than the source of the Huping trondhjemites. The higher LREE content of allanite from the Luojiahe trondhjemites relative to that from the Huping trondhjemites indicates the Luojiahe trondhjemites formed at a higher melting temperature. The occurrence of the two types of coeval granitoids indicates that ca. 2.5 Ga crustal anatexis occurred at different depths. Furthermore, a strong heat flux at the base of the southern NCC at ca. 2.5 Ga is evident, which might have been introduced by mafic magma underplating. Based on our new data and the records of coeval magmatism in the region, this study suggests that lithospheric delamination and large-scale mantle-derived magma underplating beneath the thickened crust could have played important roles in the extensive magmatism of the southern NCC at the Archean–Proterozoic boundary.

Geochemical and isotopic studies of potassic granite from the western Dharwar Craton, southern India: Implications for crustal reworking in the Neoarchean

The Neoarchean potassic granite from the Hosadurga region adjacent to the Chitradurga greenstone belt (CGB), part of the western Dharwar Craton (WDC), is studied for its petrogenesis. The combined field, petrographic, whole‐rock elemental, and Nd–Sr isotope data of granite and associated Tonalite–Trondhjemite–Granodiorite (TTG) gneiss from this region are presented here. The granites are alkali‐calcic to calc‐alkalic in terms of modified alkali lime index (MALI) and are weakly peraluminous (molar Al2O3/(CaO + Na2O + K2O) = 1.0–1.1). The geochemical data indicate that these are formed by the melting of TTG gneiss at depths above the garnet stability field. Isotopic data (εNd = −2.9 to −7.6 at T = 2,600 Ma, ISr = 0.715–0.729, TDM &gt; 3.0 Ga) also show the involvement of Archean crustal sources. Trace element modelling was carried out using non‐modal equilibrium melting equation, which constraints partial melting of TTG source to a low degree (11%) for the formation of this granite. This model could also produce a major element composition consistent with this granite. The upper bounds on the magmatic temperatures (724–785°C) assessed from zircon saturation thermometry suggest that fluid influx is required. TTG samples show variable and very high positive epsilon Nd values (1.5–8.9), higher than the contemporaneous mantle, suggesting re‐melting of mafic crust. The observed high Nd isotope ratios in TTG indicate involvement of a mafic source that formed as early as ~3.8 Ga, probably as a result of global differentiation of the early Earth.

Early Permian Granitic Magmatism in Middle Part of the Northern Margin of the North China Craton: Petrogenesis, Source, and Tectonic Setting

The late Palaeozoic was an important period of tectonic evolution for the northern margin of the North China Craton (NCC). The source(s) and tectonic setting of early Permian granitoid rocks emplaced along the northern margin of the NCC are still unclear. These granitoids formed between ~295.4–276.1 Ma (uncertainties ranging from ±1.5 to ±7.8 Ma) according to zircon laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb data. The Dadongou (DDG) pluton is an A1-type granite and the Dananfangzi (DNFZ) pluton is an A2-type granite. The Erdaowa (EDW), Lisicun (LSC), Wuhai (WH) and Gehuasitai (GHST) plutons are I-type granites. The Yuanbaoshan (YBS) dykes are diorite and syenodiorite. All the granitoids are enriched in large ion lithophile elements and light rare earth elements, depleted in high field strength elements and have negative εNd(t) and εHf(t) values. The A1-type granite was formed by the melting of the mafic crust. The A2-type granite was derived from partial melting of tonalite gneiss from the NCC crust and mantle materials. The EDW, LSC, WH and GHST granites mainly originated from partially melted granulite, with some mantle input. The YBS dykes are formed by the magma mixing of hot mantle melt and the relatively cold crustal magma. The northern margin of the NCC experienced anorogenic and collision tectonic stages, and the structural setting started to transform to post-collision at the later period of early Permian.

Preliminary result for crustal properties derivation related to tectonics for hazard mitigation in Eastern Indonesia using Teleseismic P Coda

Eastern Indonesia is tectonically complex, formed by different plates and microplates interactions from different origins. This complexity gives geoscientists a challenge to solve the ’jigsaw’ of the complex interactions. The understanding of tectonic processes can lead to a breakthrough in both resource exploration and disaster risk reduction. We utilize teleseismic P wave coda for random coda from scattering and deterministic coda originated from the crust-mantle boundary (Moho) to derive the crustal properties, including thickness, Vp/Vs, and qualitative scattering characteristics. For the scattering properties, we apply Iterative Cross-Correlation and Stacking (ICCS) to align the waveform. At the same time, for the crust characteristic, we employ the Receiver Functions (RF) method alongside H-k stacking. The crustal thickness recovered from the RF and H-k stacking has a good correlation with the crustal origin, where the thickness in older and stable crust originated from Sundaland and Gondwana is thicker than a younger plate of the crust arc and subduction origin. The Vp/Vs is high in a region that is interpreted to be dominated by mafic lower crust originated from oceanic-oceanic subduction during Eocene, anisotropy, or by a magmatic anomaly. The P coda also correlated well with the subsurface magmatic anomaly by providing a unique pattern.

Petrogenesis of Pleistocene Basalts from the Western Snake River Plain, Idaho

Abstract We present new geochemical, Sr, Nd, and Pb isotope, and 40Ar/39Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New 40Ar/39Ar dates of AlB and FeB2 basalts range from 0·920 ± 0·049 to 0·287 ± 0·014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15 % crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12 % equilibrium melting of a garnet–spinel-enriched mantle source, slightly different from that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronological data for the FeB2 basalts suggests that they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2·5 kbar) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr–Nd–Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are probably products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone–SRP hotspot track, and to the west owing to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across-strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume-head impingement in east–central Oregon with the subsequent Yellowstone–SRP hotspot plume-tail track.

Early Paleozoic accretionary orogenesis in the northeastern Indochina and implications for the paleogeography of East Gondwana: constraints from igneous and sedimentary rocks

The Indochina block is one of a number of continental fragments that lay along or outboard of the East Gondwana northern margin. Its early Paleozoic tectonic nature and the relationship with East Gondwana remain poorly-known. This paper shows the general geology of northeastern Indochina and presents a set of new geochronological and geochemical data with respect to its early Paleozoic tectonics in East Laos. These results reveal early Paleozoic (~485-410 Ma) igneous activity in the northeastern Indochina, which are divisible into four geochemical groups. The ~450-434 Ma Group 1 and ~485-434 Ma Group 2 mafic-intermediate igneous rocks were derived from MORB-like and supra-subduction zone wedge source modified by the recycled-sedimentary components, respectively. The Group 3 rhyolites at ~424 Ma represent the melting products of juvenile mafic crust. The Group 4 granitoids and equivalent volcanics formed at ~485-407 Ma and were derived from crustal materials with variable input of juvenile crust. The detrital-zircon age-spectra are marked by peaks at ~560 Ma, ~960 Ma, ~1600 Ma, ~1850 Ma and ~2450 Ma for the pre-Silurian paragneiss in the NE Indochina, reflective its geographical proximity to the India-Antarctica segment of East Gondwana until the earliest Paleozoic. The age-peaks of ~447 Ma and ~780 Ma is additionally given for the detrital-zircon from the Silurian sandstones in the Truong Son zone. An accretionary assemblage model is proposed for the early Paleozoic tectonic pattern of the SW Indochina with Yangtze fragments. Such an orogenesis formed the part of the North Indo-Australie Orogen related to subduction of the Prototethyan Ocean and the subsequent accretion of Asian micro-continental fragments along the Peripheral East Gondwana until earliest Devonian.

Multiple Early Paleozoic granitoids from the southeastern Qilian orogen, NW China: Magma responses to slab roll-back and break-off

The slab dynamic processes, such as roll-back during oceanic subduction and break-off during collision orogenesis, would affect the movement of mantle material and the interaction between crust and mantle, accompanied by significant amounts of magmatism. Providing accurate depictions for these processes are extremely difficult, especially in the case of ancient orogens. Here, we present an integrated study of petrology, geochronology, geochemistry and Sr–Nd–Hf isotopes for Early Paleozoic granitoids from the Duojielaka, Ledu and Binglingsi plutons of the southeastern Qilian orogen. Our LA-ICP-MS UPb zircon data show that these granitoids were emplaced at 476–432 Ma. The petrological and geochemical data suggest that the ~476 Ma Duojielaka porphyritic syenogranites formed by feldspar-dominated fractionation from relatively K-rich basaltic rock-derived adakitic parental magma. The ~454 Ma Duojielaka biotite monzogranites are highly fractionated I-type granites and were derived from basement orthogneisses mixed with minor mafic rocks. The Ledu fine- and medium-grained quartz diorites (~446 and ~452 Ma) are high-K calc-alkaline to shoshonitic and were produced by partial melting of relatively old K-rich mafic crust followed by minor mixing with mantle-derived magmas. The ~432 Ma Binglingsi biotite monzogranites with adakitic characteristics most likely originated by partial melting of thickened mafic lower crust. Based on the new data, in combination with previously published data, we suggest that the roll-back of the South Qilian oceanic slab occurred during the Middle Cambrian to early Middle Ordovician. The Duojielaka porphyritic syenogranites represented crustal anatexis in an arc front associated with slab roll-back. The Late Ordovician–early Middle Silurian intrusive rocks probably resulted from diachronous slab break-off in the post-collisional stage. The rocks record the opening of the break-off window from east to west and the gradual heat propagation from the central part to the southern and northern margins of the eastern Central Qilian due to the widening of break-off window and asthenospheric upwelling. We propose that complex geodynamic processes and their thermal effects can be traced by a comprehensive consideration of the petrogenetic, temporal and spatial evolution of related magmatism.

Onset of plate tectonics by the Eoarchean

One of the most contentious areas of Earth Science today is when, or whether or not modern-style plate tectonics was in operation in the Archean Eon. In this review we present evidence that the onset of plate tectonics was not at 3.2 Ga, as popularly conceived, but was in operation during the Eoarchean by at least ca. 4.0 Ga. Following a review of the main Eoarchean supracrustal belts of the world, constrained by relevant geochemical/isotopic data, we present evidence that suggests that from at least ca. 4.0 Ga Earth produced considerable juvenile mafic crust and consequent island arcs by Accretionary Cycle Plate Tectonics. From ~3.2 Ga there was a gradual transition in geodynamics to more abundant active continental margin magmatism in the form of voluminous TTGs and sanukitoids. From 3.2 Ga to 2.5 Ga juvenile oceanic crust and arcs continued to form, accompanied by more active continental margin magmatism until ~2.7–2.5 Ga, by which time there were sufficient crustal rocks to amalgamate into incipient large continents, the fragmentation of which started the first complete classical Wilson Cycle Plate Tectonics of breaking apart and re-assembling large continental masses. In other words, there were two types of plate tectonics in operation in the early Earth, Accretionary Cycle Plate Tectonics and Wilson Cycle Plate Tectonics, but Wilson Cycle type plate interactions only became more common after contiguous continental landmass became voluminous and extensive enough around 2.7–2.5 Ga. Failure to realize this dual mechanism of continental growth may lead to erroneous ideas such as “plate tectonics started at 3.2 Ga”, or “mantle plumes generated early Archean magmatic rocks.” We present new geochemical data that together with lithological and structural relationships, negate the various plume-type speculations including stagnant lids, heat pipes, and mushy-lid tectonics. It is interesting to consider that the way Earth’s crust developed in the first Gigayear of the geological record continued later, albeit in more advanced forms, into the Phanerozoic, where we can still recognize Accretionary Cycle Plate Tectonics and orogens still with short boundaries in examples including the Altaids of Central Asia, the Arabian-Nubian Shield, the Japanese Islands, and in incipient form in Indonesia, as well as Wilson Cycle Plate Tectonics that leads inexorably to continental collisions as in the Alpine-Himalayan orogen with its long plate boundaries. We recommend this holistic view of crustal growth and the evolution of continents that leads to a robust, viable, and testable model of Earth evolution.

Neoproterozoic tectonic switch on the southwestern Yangtze Block: evidence from zircon U-Pb-Hf isotopes and geochemistry of the A- and I-type granites

ABSTRACT The petrogenesis and tectonic affinity of late Mesoproterozoic to early Neoproterozoic magmatic records in the Yangtze Block is important to understand its tectonic evolution within the context of the Rodinia supercontinent’s reconstruction. In this paper, our new SHRIMP zircon U-Pb dating results demonstrate that the Luonie granodiorites and Gonghe granites were emplaced at 1019.2 ± 5.3 Ma and 925.0 ± 4.9 Ma, respectively. The Luonie granodiorites show high HFSE and HREE contents and Ga/Al ratios, resembling A-type granites. They exhibit negative εHf(t) values (−1.6 to −3.9) with TDM2 ages of 1980–2125 Ma, suggesting that they were derived from partial melting of pre-existing continental crust. The Gonghe granites are calc-alkaline, slightly peraluminous and have low P2O5 contents, which are typical features of I-type granites. Zircons from Gonghe granites possess positive εHf(t) values (+10.8 to +13.1) with TDM2 ages of 969–1113 Ma, indicating juvenile mafic lower crust source. In conclusion, Luonie granodiorites were formed in an intra-plate rifting environment, whereas Gonghe arc-related intrusion was emplaced into an arc-setting. On the basis of these results, we propose that the rifting-related igneous rocks and subduction-related magmatism in the southwestern Yangtze Block record a tectonic transition from a continental-rifting basin in the passive plate margin to a collisional setting in an active continental margin in the early Neoproterozoic.

The Mesoarchaean Akia terrane, West Greenland, revisited: New insights based on spatial integration of geophysics, field observation, geochemistry and geochronology

The northern part of the North Atlantic Craton (NAC) in southern West Greenland comprises a large tract of exposed Meso-Neoarchaean continental crust, divided into the ca 3300–2900 Ma Akia and ca 2900–2500 Ma Tuno terranes. We combine aeromagnetic, stream sediment geochemical, new litho-chemical and zircon geochronological data with previously published data to re-evaluate the crustal architecture and evolution of the Akia terrane and its boundary towards the Tuno terrane.The previously recognised, but overlooked, Alanngua complex, situated between the Akia and Tuno terranes is bounded by aeromagnetic lineaments interpreted as Neoarchaean shear zones and has a distinct spectrum of Neoarchaean magmatic and metamorphic zircon ages that are rare in the Akia terrane. The Alanngua complex comprises components derived from both the Akia and Tuno terranes and is interpreted as a tectonic melange created during the Neoarchaean assembly of the NAC.Within the Akia terrane, the chemistry of orthogneiss samples indicate that a large percentage is too mafic to classify as TTG s.s., implying that not only partial melting of mafic crust, but also some yet unaddressed mantle involvement is necessary in their formation. Previous models for the generation of the ca. 3015–2990 Ma quartz-dioritic Finnefjeld and Taserssuaq complexes conflict with their geochemical variation. We argue that the complexes may be genetically linked to buried gabbro-dioritic rocks indicated by their strong aeromagnetic response. Formed at same time are carbonatite, high-Mg gabbro and tonalite-trondhjemite, and we propose that this wide spectrum of rocks was formed by lithospheric and crustal melting in response to asthenospheric upwelling possibly in an extensional setting.Periods of extensive magmatism in the Akia terrane were previously recognised at ca. 3220–3180 Ma and 3070–2970 Ma. We now subdivide the latter period into three episodes: juvenile basaltic-andesitic volcanism at 3070–3050 Ma; tonalitic and dioritic plutonism at 3050–3020 Ma, and gabbroic-dioritic plus tonalitic-trondhjemitic plutonism at 3020–2985 Ma. This last episode was immediately followed by crustal reworking during collision at 2980–2950 Ma.

Post-collisional ca. 800 Ma A-type felsic volcanic rocks in the eastern Jiangnan orogen, South China

Abstract Neoproterozoic igneous rocks in the Jiangnan orogen can provide important information to unravel the tectonic evolution of the South China Block and are thus important for the reconstruction of Rodinia. Dacites and rhyolites in the Shangshu region, the eastern Jiangnan orogen, contain zircon grains with U-Pb ages of ca. 800 Ma. These felsic rocks have high Ga/Al ratios (2.3–3.7) and HFSE and HREE contents relative to the primitive mantle, thus resembling A-type granitoids. Dacites have negative zircon eHf(t) values (–2.8 to –18.7), indicative of derivation from primary magmas generated by partial melting of ancient continental crust. In contrast, rhyolites show positive eHf(t) values (+1.9 to +8.8) and are explained as derived from partial melting of juvenile lower mafic crust. Felsic volcanic rocks in Shangshu were formed in a post-orogenic extension setting shortly after the final amalgamation of the Yangtze and Cathaysia blocks at around 830 Ma. Combining our results with the previously published ones, we demonstrate that the 800 Ma extension-related magmatic activity was likely the earliest phase of the Neoproterozoic rifting in the South China Block. The tectonic transition from continental collision to a post-orogenic setting should have occurred after ca. 800 Ma.

Petrogenesis of the Neoproterozoic low-δ18O granitoids at the western margin of the Yangtze Block in South China

The western margin of the Yangtze Block in South China preserves voluminous arc-like intrusive and extrusive igneous rocks which were formed in an active continental margin during the Neoproterozoic time. Rocks from the ca. 800 Ma Moutuo pluton are calc-alkaline I-type granitoids which have high SiO2 (67.13–77.96 wt%), K2O (2.08–4.32 wt%) and low MgO (0.16–1.81 wt%). They show concave chondrite-normalized rare earth element patterns, and are rich in large ion lithophile elements (e.g. Rb, Ba, Th, U and Pb), but depleted in high field strength elements (e.g. Nb, Ta and Ti). The magmatic zircons have low δ18O (3.29–4.80‰) and high εHf(t) values (+6.10 to +13.16) with two-stage Hf model ages (TDM2) of 0.87 to 1.33 Ga. These lines of evidence suggest that the juvenile mafic crust, protolith of the low-δ18O granitoids, was derived from the sub-arc mantle that was modified by slab-derived materials (~90% altered oceanic crust and ~10% oceanic sediments). Partial melting of the newly formed mafic crust followed by fractional crystallization played an important role in the petrogenesis of the Moutuo granitoids. Based on the available oxygen and hafnium isotopic data for the Neoproterozoic magmatic zircons in this region, the low-δ18O character of the I-type granitoids in active continental margins was probably inherited from the sub-arc mantle that was mixed with the altered oceanic crust and oceanic sedimentary materials.

Origin of Carboniferous intra-oceanic arc granitoids from the eastern Pamir and implications for the Paleo-Tethyan ocean

The expanding and diminishing of the Paleo-Tethyan ocean recorded in the Pamir, and relationships between different segments of the Paleo-Tethyan suture from Pamir to Tibet are poorly constrained. Here we present new zircon U–Pb ages and geochemical data of the Waqia granitoid pluton from the eastern Pamir. Both the host granitoids and gabbroic to dioritic enclaves have similar zircon U–Pb ages of 323–318 Ma, being contemporaneous in origin. These rocks are metaluminous, calc-alkaline, tholeiitic series. They exhibit low K2O, low Sr/Y ratios, low REE contents with depleted to flat LREE patterns, slight enrichment in Rb, Ba, Zr, Hf, and negative Nb, Ta, P, and Ti anomalies. They have similar initial 87Sr/86Sr ratios of 0.7040 to 0.7045, positive εNd(t) (+4.63 to +5.02) and εHf(t) (+9.9 to +14.7) values. Detailed elemental and isotopic data indicate that these coeval rocks in the Waqia pluton were cognate and formed by hydrous partial melting of a juvenile mafic crust within an intra-oceanic arc setting. This study is the first to report reliable isotopic ages of the early Paleo-Tethyan subduction in the eastern Pamir. Further, the Tanymas-Mazha Paleo-Tethyan suture in the eastern Pamir should be the western extension of the Jinshanjiang Paleo-Tethyan suture in the central to eastern Tibetan plateau. An integration of published results leads us to conclude that closure of the Jinshanjiang Paleo-Tethyan ocean was diachronous along its 3000–4000 km length, with closure times of ca 240 Ma in the Pamir and ca 220 Ma in the Sanjiang region.

Lu-Hf analyses of zircon from the Makoppa Dome and Amalia-Kraaipan area: implications for evolution of the Kimberley and Pietersburg blocks of the Kaapvaal Craton

Abstract Previously dated zircon crystals from the Amalia-Kraaipan granite-greenstone belts and Makoppa Dome were analysed for their Lu-Hf isotopic characteristics to refine the geological evolution of these areas. Samples from the Makoppa Dome, belonging to the Pietersburg Block, largely fall within the epsilon Hf-age range for granitoids from the eastern part of the block. However, the oldest 3.01 to 3.03 Ga trondhjemitic gneisses show that reworking of juvenile mafic crust started earlier in the western than the eastern part of the block, suggesting a diachronous tectonic evolution. The three granitoids from the Amalia-Kraaipan area fall within the field for Pietersburg and Kimberley block granitoids. Contribution from older crustal material is seen in a 3.08 Ga schist, likely derived from a volcanic protolith, from the Madibe Belt, in the far east of the Kimberley Block, with a mantle extraction age of 3.25 to 3.45 Ga. The data suggest that the Kimberley Block, like the Pietersburg Block, also contains (minor) ancient crustal components, derived from a depleted mantle source prior to 3.1 Ga. The new data suggest that the Kimberley and Pietersburg blocks underwent a very similar Paleo- to Mesoarchean crustal evolution, with a major crust formation event at 3.1 to 3.0 Ga followed by successive crust reworking until 2.77 Ga. Lavas of the Ventersdorp Supergroup, for which zircon grains from a ca. 2.75 lapilli tuff give εHfi of +2, are the first evidence of a juvenile source, after 300 Myr of crustal reworking.

Plume — Lid interactions during the Archean and implications for the generation of early continental terranes

Many Archean terranes are interpreted to have a tectonic and metamorphic evolution that indicates intra-crustal reorganization driven by lithospheric-scale gravitational instabilities. These processes are associated with the production of a significant amount of felsic and mafic crust, and are widely regarded to be a consequence of plume-lithosphere interactions. The juvenile Archean felsic crust is made predominantly of rocks of the tonalite–trondhjemite–granodiorite (TTG) suite, which are the result of partial melting of hydrous metabasalts. The geodynamic processes that have assisted the production of juvenile felsic crust, are still not well understood. Here, we perform 2D and 3D numerical simulations coupled with the state-of-the-art of petrological thermodynamical modelling to study the tectonic evolution of a primitive Archean oceanic plateau with particular regard on the condition of extraction of felsic melts. In our numerical simulations, the continuous emplacement of new, dry mafic intrusions and the extraction of the felsic melts, generate an unstable lower crust which drips into the mantle soon after the plume arrival. The subsequent tectonic evolution depends on the asthenosphere TP. If the TP is high enough (≥ 1500 ∘C) the entire oceanic crust is recycled within 2 Myrs. By contrast at low TP, the thin oceanic plateau slowly propagates generating plate-boundary like features.