Crustal Growth Processes Research Articles

Growth of the Central Orogenic Belt, North China Craton through accretion of different Neoarchean arc terranes: Perspective from the Linshan complex

The onset of plate tectonics and crustal growth processes in the early Earth have been controversial scientific issues in the geoscience. The North China Craton (NCC) preserves widespread 3.8–2.5 Ga rocks, providing an ideal place to understand early continent formation and evolution. The Linshan complex located in the southern segment of the Central Orogenic Belt (COB) of the NCC, is mainly composed of TTG (tonalite-trondhjemite-granodiorite)-diorite gneisses and metamorphic volcanic-sedimentary units dominated by gabbro, basalt, basaltic andesite and biotite-plagioclase gneiss. Detailed mapping on the scale of 1:100 of a structural transect shows that the Linshan complex has mainly experienced two major deformation events including top-to-the-SE thrust faults and late NE-trending high-angle normal faults. Detailed zircon U-Pb dating shows that gabbro, basaltic andesite, and TTG gneiss mainly formed at ca. 2.52–2.50 Ga. Gabbros and basalts display enrichment of LREE and negative Nb and Ta anomalies, and basaltic andesites display mixed MORB-IAT geochemical affinities. Basalts and basaltic andesites are members of the Nb-enriched basalt series with high absolute Nb contents (>6 ppm). TTG gneisses are geochemically divided into high-pressure and low-pressure TTGs. High-pressure TTGs are characterized by high ratios of La/Ybcn (26.29–45.73) and fall into the adakitic region in the La/Ybcn-Ybcn diagram. Considering the close contact with Nb-enriched basaltic series, it is proposed that high-pressure TTGs may have formed by partial melting of a subducting oceanic slab with garnet and amphibole and/or rutile as residues. Low-pressure TTGs are characterized by low ratios of La/Ybcn and Sr/Y with marked negative Eu anomalies, indicating partial melting at shallow crustal levels. Regional tectonic relations have defined the Neoarchean Dengfeng island arc-forearc accretionary complex to the east of the Linshan complex. Thus, we propose that the gabbros-basalts-basaltic andesites in the Linshan complex mostly formed in a Neoarchean suprasubduction back-arc basin by rifting of a TTG-dominated island arc terrane. The final closure of the back-arc basin resulted in their tectonic juxtaposition forming thrust-imbricated structures. There may have been several Neoarchean “forearc-island-arc-backarc” systems in the NCC that are similar to modern accretionary tectonic orogens, indicating that plate tectonics has been in operation since at least 2.55–2.50 Ga.

Petrogenesis of Early Cretaceous andesites and mafic dikes in central Tibet: Implications for the growth of continental crust in collision zones

Post-collisional magmatic rocks in central Tibet provide an opportunity to study the processes of continental crustal growth in a collision zone. Here, we report data for samples of andesites and coeval mafic dikes obtained from the Jiaqiong area of the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating shows that the andesites and mafic dikes were emplaced concurrently during the late Early Cretaceous (ca. 115 Ma). The andesites have high Mg# values (55.2–61.6) and initial 87Sr/86Sr ratios (0.7086–0.7088), negative whole-rock εNd(t) (−6.5 to − 6.1) and zircon εHf(t) (−5.1 to − 1.7) values, and elevated zircon δ18O (6.8‰–7.5‰) values, indicating that the andesites were probably derived by the partial melting of an ancient subduction-modified lithospheric mantle. The coeval mafic dikes have moderately high Mg# values (49.3–50.0) and initial 87Sr/86Sr ratios (0.7076–0.7082), and negative εNd(t) values (−5.4 to − 5.3), implying that they were derived from an enriched lithospheric mantle source. However, the zircon εHf(t) (−5.8 to + 1.6) and δ18O (5.2‰–6.3‰) values of these mafic dikes indicate the influence of depleted mantle components. Furthermore, results of Sr–Nd–Hf–O isotopic mixing modeling suggest that these mafic dikes could have been produced by interaction between asthenospheric and lithospheric mantle. Therefore, we suggest that the parental magmas of the mafic dikes were probably generated by interaction between asthenospheric and overlying metasomatized lithospheric mantle. Combined with data on nearby Cretaceous magmatic rocks and sedimentary strata, we propose that the studied magmatic rocks were formed in a post-collisional slab breakoff setting in central Tibet and have made a substantial contribution to the vertical growth of continental crust in this continental collision zone.

Continental Crustal Growth Processes Recorded in the Gangdese Batholith, Southern Tibet

The continental crust in the overriding plate of the India-Asia collision zone in southern Tibet is characterized by an overthickened layer of felsic composition with an underlying granulite-eclogite layer. A large data set indicates that this crust experienced magmatism from 245 to 10 Ma, as recorded by the Gangdese Batholith. Magmatism was punctuated by flare-ups at 185−170, 90−75, and 55−45 Ma caused by a combination of external and internal factors. The growth of this crust starts with a period dominated by fractional crystallization and the formation of voluminous (ultra)mafic arc cumulates in the lower crust during subduction, followed by their melting during late-subduction and collision, due to changes in convergence rate. This combined accumulation-melting process resulted in the vertical stratification and density sorting of the Gangdese crust. Comparisons with other similarly thickened collision zones suggests that this is a general process that leads to the stabilization of continental crust. ▪ The Gangdese Batholith records the time-integrated development of the world's thickest crust, reaching greater than 50 km at 55–45 Ma and greater than 70 km after 32 Ma. ▪ The Gangdese Batholith records three magmatic flare-ups in response to distinct drivers; the last one at 55−45 Ma marks the arrival of India. ▪ Magmatism was first dominated by fractional crystallization (accumulation) followed by crustal melting: the accumulation-melting process. ▪ Accumulation-melting in other collision zones provides a general process for vertical stratification and stabilization of continental crust.

Relics of 2.6–2.5 Ga oceanic crust from the ultramafic-mafic complex of Goa, western India: Magmatic response to a progressive subduction system

Subduction initiation in the Early Earth and the onset of plate tectonics are topics of wide interest in understanding the formation and evolution of continents and supercontinents on our planet. Here we report results from integrated petrological, geochemical and geochronological (Zircon U Pb ages) studies on an ultramafic-mafic complex from the Bondla-Usgao-Durigini (BUD) sectors of Goa to address its petro-tectonic evolution and correlation with Neoarchean crustal growth processes and cratonization events. The ultramafic-mafic cumulate sequence from BUD sectors of Goa comprising dunite, peridotite, troctolite, olivine gabbro and gabbro represents a dismembered crustal section of oceanic lithosphere formed either in a MOR or SSZ fore-arc extensional regime. The geochemical attributes marked by boninitic-IAT trend, depleted to enriched mantle compositions, selective enrichment in fluid-mobile LILE and LREE over HFSE enunciate a multistage petrogenetic evolution associated with a SSZ fore-arc tectonic setting. The ocean-continent transition featured by the geochemical signatures can be translated in terms of oceanic crust formation in a juvenile extensional fore-arc in a SSZ setting, its maturation and accretion at active continental margin during Neoarchean ocean basic closure and arc-continent collision. The studied ultramafic-mafic suite fingerprints the transition of depleted mantle wedge to a LILE-LREE enriched metasomatized one through influx of subduction components and hydrous arc magmatism typical of SSZ oceanic lithosphere formed above a nascent subduction zone. The magmatic precursors of the studied lithologies were derived from a transitional depleted to enriched mantle regime marked by hydration and metasomatism of a depleted MOR-type mantle by the influx of subduction-derived fluids. Zircon grains in the peridotite sample yield positive εHf(t) of +3.2 to +7.6, together with T DM C of 2610–2877 Ma suggesting depleted mantle source. The gabbro sample shows positive εHf(t) values of +3.7 to +6.3 and T DM C age of 2731–2868 Ma also attesting to depleted mantle source. Zircon U Pb ages of 2602.42 ± 0.82 Ma for peridotite and 2479–2684 Ma for gabbro represent the magma emplacement time and Neoarchean-early Paleoproterozoic melt activity. The highly variable ages from zircon grains in the studied samples might suggest multiple stages of metasomatism and melt-rock interaction in a long-lived sub-continental lithospheric mantle. The tectonic evolution can be equated with Neoarchean global subduction-accretion-collision and arc-continent amalgamation events. This study deciphers a 2.6–2.5 Ga Neoarchean oceanic crust formation and magmatic response to subduction initiation, arc maturation and accretion processes correlatable with global crustal growth processes and amalgamation of western Dharwar Craton (WDC) and Antongil-Masora blocks into the Greater Dharwar Craton (GDC). • Ultramafic-mafic cumulate sequence from Goa represents dismembered SSZ oceanic crust. • Depleted to enriched melt composition reflects mantle heterogeneity. • Paleo-ocean closure and arc accretion culminated to GDC formation. • 2.6–2.5 Ga age coincides with Neoarchean global subduction-accretion event.

An Ordovician ophiolitic complex in West Junggar, NW China: Implications for subduction initiation and oceanic arc evolution of the Paleo-Asian Ocean

As a key step of the Wilson cycle, subduction initiation is regarded as a start hallmark of the ocean-continent transformation. However, the crust-mantle interaction and crustal growth processes related to subduction initiation remain enigmatic. Here we report a newly recognized complete magmatic sequence containing three rock assemblages in the Hongguleleng ophiolitic complex (HOC) in West Junggar, NW China. Group I contains typical Mid-Oceanic Ridge (MOR)-derived rocks including abyssal peridotite, gabbro, plagiogranite, and basalt. Group II consists of forearc basalt (FAB) and boninitic gabbro and has a high continentality index (CI > 130), similar to the rock assemblages in the Mariana proto-arc. Group III composed of calc-alkaline andesite and arc tholeiite basaltic andesite exhibits middle CI values (50 < CI < 100), suggesting that they form in a transitional oceanic arc. Zircon U-Pb ages show that the N-MORB gabbro and plagiogranite samples are emplaced at 483–481 Ma, and the boninitic rocks, calc-alkaline andesites, and arc tholeiite basaltic andesites have the crystalline ages of 472–467 Ma, 456 Ma, and 452 Ma, respectively. An increasing trend of Ba/Th and V/Ti ratios from Group I to Group II suggests the addition of aqueous fluid and elevated oxidation in the mantle source during subduction initiation. However, it exhibits an uptrend in Th/Nb and (La/Sm)cn ratios but a downtrend in Ba/Th ratios from Group II to Group III, indicating a significant contribution of slab-derived sediments to the mantle during oceanic arc evolution. Combined with regional data, we consider that there developed an E-W–trending sub-ocean basin of the Paleo-Asian Ocean spanning northern West Junggar and East Kazakhstan during the Middle Cambrian–Ordovician. The MOR-type and forearc ophiolites in northern West Junggar as well as our results suggest that the ocean happened mid-ocean ridge spreading at ca. 512–478 Ma and subduction initiation at ca. 476–467 Ma, and developed a transitional oceanic arc at ca. 456–452 Ma.

U-Pb geochronology of the Silurian-Devonian Bega Batholith, south-eastern Australia: Insights into the origin and development of I-type granites

Earth’s continental crust is primarily composed of buoyant, felsic igneous (granitic) material that has been extracted from the mantle along convergent plate margins. Establishing the timescales of subduction-related granite batholith assembly is key to understanding the processes of magma generation, segregation, and transport that define the dynamics and rates of crustal growth. The ‘Cordilleran’ Bega Batholith is the largest I-type granite batholith in the Lachlan Fold Belt (LFB) of south-eastern Australia. The product of accretionary orogenesis along the Terra Australis Orogen, it has been extensively characterised by isotopic and geochemical studies yet lacks a robust geochronological framework. We present new ID-TIMS and SHRIMP U-Pb zircon ages for twenty-two plutonic rocks of the Bega Batholith. This geochronological survey indicates a prolonged duration for I-type magmatism, with crystallisation ages ranging from ca. 420 Ma in the west, to 385 Ma in the eastern Bega Batholith. These I-type granites collectively occupy ∼90,000 km3 and were emplaced over a period of ∼20–35 Ma, corresponding to a magma production rate of ∼8–15 km3/Ma km−1, considerably lower than that of S-type granites of the region and other subduction-related granite terranes globally. Fundamentally, the temporal overlap between the Bega I-type granites and S- and A-type granites also argues against a hiatus between each phase of granite magmatism in the LFB, suggesting a continuum between the granite types and derivation from common source components. This new geochronological data provides the first comprehensive compilation of U-Pb ages from across the Bega Batholith, establishing initial constraints on the development and evolution of I-type magmatism in south-eastern Australia. Our results demonstrate that I-type granite petrogenesis is linked with crustal growth processes, requiring the modification of global crustal growth curves during the Phanerozoic to account for new continental crust generated within I-type granite terranes of the Paleo-Pacific Gondwanan margin.

Petrogenesis of Neoproterozoic magmatic rocks in the Songnen Massif (northeastern China): Implications for basement composition and crustal growth

This paper reports new zircon U–Pb geochronological, whole-rock geochemical, and zircon Hf isotopic data for Neoproterozoic intrusive rocks in the Songnen Massif in the eastern Central Asian Orogenic Belt (CAOB), with the aim of revealing the petrogenesis of the Neoproterozoic intrusive rocks, and constraining the basement composition and crustal growth process of the Songnen Massif. Dating results identify three episodes of Neoproterozoic magmatism at ca. 950, ca. 856, and ca. 790 Ma in the eastern Songnen Massif. The ca. 950 Ma magmatic rocks consist of syenogranites, the ca. 856 Ma magmatic rocks comprise mainly gneissic monzogranites, and the ca. 790 Ma magmatic rocks consist of gneissic monzonites and monzogranites. The Neoproterozoic granitoids have high contents of SiO2 and K2O, low contents of total Fe2O3 and MgO, and show characteristics of enrichment in large-ion lithophile elements (e.g., K and Rb), and depletion in high-field-strength elements, implying that their primary magmas could have been derived from partial melting of the lower crust. However, compared with the ca. 790 Ma monzogranites [εHf(t) = +7.3 to +9.5] in the south of the Songnen Massif, the ca. 950 and ca. 856 Ma granitoids in the north have much lower zircon εHf(t) values (−2.0 to −0.2 and −10.6 to −8.9, respectively), suggesting that the ca. 950 and ca. 856 Ma magmas were generated by the reworking of ancient mafic lower crust, whereas the ca. 790 Ma granitic magma was generated by the reworking of juvenile mafic lower crust. In contrast, the ca. 790 Ma gneissic monzonites were formed by mixing of the mantle-derived and crust-derived melts. Considering the Precambrian magmatism, detrital and captured zircons, and zircon Hf model ages together, the Precambrian basement of the Songnen Massif includes not only the Neoproterozoic crustal materials, but also the Neoarchean–Mesoproterozoic components. Zircon εHf(t) values of the Neoproterozoic granitoids in the Songnen Massif appear to progressively decrease from south to north, reflecting a lateral heterogeneity of basement composition of the eastern Songnen Massif, and indicating that crustal growth occurred mainly during the late Mesoproterozoic and Neoproterozoic in the south, while during the late Paleoproterozoic in the north.

A new geological map of the Lau Basin (southwestern Pacific Ocean) reveals crustal growth processes in arc-backarc systems

A 1:1,000,000-scale lithostratigraphic assemblage map of the Lau Basin (southwestern Pacific Ocean) has been created using remote predictive mapping (RPM) techniques developed by geological surveys on land. Formation-level geological units were identified in training sets at scales of 1:100,000–1:200,000 in different parts of the basin and then extrapolated to the areas where geological data are sparse. The final compilation is presented together with a quantitative analysis of assemblage-level crustal growth based on area-age relationships of the assigned units. The data sets used to develop mapping criteria and an internally consistent legend for the compilation included high-resolution ship-based multibeam, satellite- and ship-based gravity, magnetics, seafloor imaging, and sampling data. The correlation of units was informed by published geochronological information and kinematic models of basin opening. The map covers &amp;gt;1,000,000 km2 of the Lau-Tonga arc-backarc system, subdivided into nine assemblage types: forearc crust (9% by area), crust of the active volcanic arc (7%), backarc rifts and spreading centers (20%), transitional arc-backarc crust (13%), relict arc crust (38%), relict backarc crust (8%), and undivided arc-backarc assemblages (&amp;lt;5%), plus oceanic assemblages, intraplate volcanoes, and carbonate platforms. Major differences in the proportions of assemblage types compared to other intraoceanic subduction systems (e.g., Mariana backarc, North Fiji Basin) underscore the complex geological makeup of the Lau Basin. Backarc crust formed and is forming simultaneously at 12 different locations in the basin in response to widely distributed extension, and this is considered to be a dominant pattern of crustal accretion in large arc-backarc systems. Accelerated basin opening and a microplate breakout north of the Peggy Ridge has been accommodated by seven different spreading centers. The result is an intricate mosaic of small intact assemblages in the north of the basin, compared to fewer and larger assemblages in the south. Although the oldest rocks are Eocene (~40 m.y. old basement of the Lau and Tonga Ridges), half of the backarc crust in the map area formed within the last 3 m.y. and therefore represents some of the fastest growing crust on Earth, associated with prolific magmatic and hydro-thermal activity. These observations provide important clues to the geological evolution and makeup of ancient backarc basins and to processes of crustal growth that ultimately lead to the emergence of continents.

Ca. 815 Ma intra-plate granitoids and mafic dykes from Emeishan pluton in the western Yangtze Block, SW China: A record of rifting during the breakup of Rodinia

The western Yangtze Block, South China is characterized by voluminous Neoproterozoic extension-related volcano-sedimentary sequences and intrusions (e.g. Emeishan granitoid intrusion) and spatially associated mafic dykes. The origin and tectonic setting of Emeishan granitoid intrusion and mafic dykes in the Yangtze Block is poorly study. Understanding the origin and geodynamic setting of dykes and granitoid intrusion are significant in deciphering thermal structure and composition of mantle, tectonic evolution as well as crustal growth processes in the Yangtze Block. SIMS Zircon U–Pb dating of Emeishan granitoid intrusion and mafic dykes suggests that the granitoids (ca. 818 Ma) and mafic dykes (ca. 814 Ma) are contemporaneous. Compositionally, the Emeishan granitoids are restricted and characterized by high SiO2 (69.11–70.50 wt%), Na2O + K2O (3.52–4.13%) and Al2O3 (14.16–14.47%), but low CaO (1.78–2.03%), MgO (0.72–0.91%) contents. They feature strong LREE-enrichment (La/YbN = 7.2–12.3) in conjunction with negative Eu anomalies. The mafic dykes have low SiO2 (48.09–48.65 wt%), K2O (1.94–2.12 wt%) and Na2O (1.64–1.75 wt%) contents, and high Mg# (61.67–62.26). On primitive mantle-normalized trace element plot, they are characterized by enrichment of LILEs and depletion of HFSEs. Both the mafic dykes and granitoids record similar zircon εHf(t) values (+2.2 to +12.1, +1.4 to +6.8), and zircon δ18O values (4.47‰ and 10.41‰, 4.49‰ and 12.05‰, respectively). The rifting-related geochemical signature of the Emeishan pluton and contemporaneous margin and intra-plate plume-related magmatism suggest that they were derived from mantle melting in extensional tectonic setting during mid-Neoproterozoic.

Early Jurassic high εNd(t)-εHf(t) granites in the Southeastern South China Block: Early Jurassic crustal growth or crustal reworking?

Granites that have high εNd(t) and zircon εHf(t) (here phrased as ‘HNHGs’) can offer significant insights into the processes of crustal growth and reworking. This study investigates Early Jurassic HNHGs in the Southeastern (SE) South China Block (SCB) to determine whether they reflect crustal growth or reworking processes, which has implication for understanding the tectonic setting of the SE SCB. We presented new zircon geochronological and Hf-O isotopic, and geochemical analyses on these HNHGs to assess the fundamental crustal processes. Zircon U-Pb geochronological analyses yield the crystallization ages of 192.9 ± 0.8 Ma, 186.7 ± 2.5 Ma and 188.6 ± 1.3 Ma for the Wengong, Gaokeng and Chabei granites, respectively. These HNHGs display high SiO2 (69.75–77.50 wt%), low MgO (0.07–0.83 wt%) and Rb/Sr (0.74–14.3), belonging to unfractionated granites. They have low (87Sr/86Sr)i and zircon δ18O, high εNd(t) and zircon εHf(t), originating from partial melting of a juvenile crust. The HNHGs show a spatial distribution that overlaps with Early Paleozoic lithospheric mantle-derived mafic igneous rocks in the SE SCB. Isotope modeling results imply that the juvenile crust was likely a consequence of mixing among depleted mantle, subordinate enriched lithospheric mantle and SCB basement rocks during Early Paleozoic. These HNHGs record crustal reworking, rather than crustal growth during Early Jurassic, and formed in a continental extension setting.

High- and Low-Mg Adakitic Rocks in Southern Tibet: Implication for the Crustal Growth and Geodynamic Process in the Late Cretaceous

High- and Low-Mg Adakitic Rocks in Southern Tibet: Implication for the Crustal Growth and Geodynamic Process in the Late Cretaceous

Applications of Pb isotopes in granite K-feldspar and Pb evolution in the Yilgarn Craton

The isotopic composition of Pb in a mineral or rock at the moment it formed – often referred to as common Pb – provides an important tool to track geological processes through time and space. There is a wide range of applications of common Pb isotopes including understanding magma sources, melt production, fractionation, contamination, and crystallization in the crust. Pb but not U is incorporated into the structure of K-feldspar during crystal growth, which, together with its widespread occurrence as a framework mineral, makes it an excellent common Pb tracer. Consequently, common Pb isotopes in granite K-feldspar crystals provide a potential signature of source composition and a link to crustal growth processes in the mid to lower crust. Hence, combining common Pb isotopes with Sm-Nd (or Lu-Hf) isotopic signatures from the same dated rocks allows assessment of the degree of isotopic communication from deep fractionation systems to those higher in the crustal column. In this contribution, we analyze common Pb isotopic signatures in K-feldspar from a granite sample transect through the Archean Yilgarn Craton in Western Australia. This transect crosses the major crustal-scale Ida Fault that is apparent on Nd and Hf isotopic maps and interpreted as a fundamental lithospheric boundary across which magma sources change. Our results yield a difference in median values of the Pb isotope derivative parameters µ (238U/204Pb) and ω (232Th/204Pb) across the Ida Fault, with higher µ and ω associated with more evolved Nd and Hf isotopic signatures on the western side of the fault. Pb evolution in the Yilgarn Craton is distinct from the widely applied Stacey & Kramers (1975) model. New Yilgarn-specific Pb evolution models are developed with implication for common Pb correction. A correlation in the spatial trends of granite K-feldspar common Pb signatures with those of upper crustal Pb ores and also the Sm-Nd and Lu-Hf systems reveals geochemical communication all the way through the crustal column, implying a common source for the entire lithospheric section on each side of the Ida Fault. Pb isotopes in granite K-feldspar are not an independent geochronometer but may yield important source context on major phase silicate growth that helps refine U-Pb geochronology interpretations (e.g., distinguishing magmatic versus metamorphic zircon growth).

Late Cretaceous adakitic and A-type granitoids in Chanang, southern Tibet: Implications for Neo-Tethyan slab rollback

The co-existence of Cretaceous adakitic and A-type granitoids in southern Tibet can offer fundamental constraints on subduction of the Neo-Tethyan oceanic plate and associated crustal growth processes, if their petrogenetic relationships are understood. Here, we report whole-rock geochemistry, zircon U-Pb geochronology, in situ zircon Hf isotopes and zircon trace elements from the Chanang intrusive suite, including the Zeyu and Cuojielin plutons. The Cuojielin pluton is composed of A-type quartz syenites with high K2O + Na2O, FeOT, and Zr contents and distinct negative Eu anomalies, consistent with fractional crystallization of mafic magma in a reducing, low-pressure, high-temperature environment. The adakitic Zeyu pluton, by contrast, has a strikingly high density of mafic enclaves. The elemental and isotopic signatures of the Zeyu mafic enclaves indicate a hybrid origin by mixing of magmas derived from the mantle wedge and from juvenile lower crust. The adakitic Zeyu host rocks, with low MgO, Cr, and Ni contents, show typical characteristics which originate from residual or fractionated amphibole of juvenile lower crust. The co-existence of aluminous A2-type granites and adakitic rocks indicates an anomalous high-temperature and relatively H2O-rich environment. The synchronous regional occurrence of calc-alkaline magmatism, charnockites, granulite-facies metamorphism, back-arc extension, and generation of the two studied types of Chanang intrusive rocks can best be explained in the context of an episode of rollback of the Neo-Tethyan slab in the late Cretaceous.

Tectonic juxtaposition of plume and subduction derived magmatic sequences in the Bababudan greenstone terrane, western Dharwar Craton, India: Constraining crustal accretion processes in a Neoarchean subduction-collision orogeny

Archean greenstone belts in the Western Dharwar Craton preserve important clues on Early Earth geodynamics. Here we investigate the ultramafic-mafic metavolcanic rock sequences from the Santaveri Formation of Bababudan greenstone terrane to address the tectonic evolution, mantle dynamics and crustal growth processes of western Dharwar Craton. The ultramafic volcanic rocks are geochemically classified as komatiite-picrite-basanites, whereas the mafic volcanic rocks correspond to magnesian andesite -Niobium-enriched basaltic andesite association. The pristine mineralogy for these ultramafic volcanic rocks is overprinted by tremolite-actinolite-biotite assemblage marking greenschist facies of metamorphism. The komatiites include Al-undepleted, Al-depleted and Ti-enriched types and share overlapping geochemical characteristics with the picrites. The basanites are distinguished as alkaline ultramafic rocks with total alkali content >3 wt% and MgO > 18 wt%. The coherent association of komatiite-picrite-basanite suggests pervasive plume-lithosphere interaction and complies with polybaric melting of a chemically heterogeneous mantle plume through a subduction-modified sub-continental lithospheric mantle at a rifted cratonic margin. Our results substantiate the role of an Archean mantle refertilized by subduction-driven recycling of Mesoarchean supracrustals concurrent with the onset of subduction event at ≥3.2 Ga. The mafic rocks were derived through melting of a hot oceanic slab and mantle hybridization. The xenocrystic zircon grains from komatiite suggest lower crustal inheritance and reworking of basement gneisses by ascending ultramafic melts. The 3177 Ma magmatic age obtained from these zircon grains indicate a Mesoarchean magmatic event in the genesis of the basement gneisses. The emplacement of komatiite-picrite-basanite association and magnesian andesite -Niobium-enriched basaltic andesite lithologies envisages a post 3.1 Ga Neoarchean crustal growth over Mesoarchean protocontinent of WDC. This crust generation episode can be translated in terms of plume-driven bottom-up and subduction-driven top-down tectono-magmatic processes at continental rift and ocean-continent convergence set-up respectively. Geochemical characteristics and petrogenetic aspects suggest that the studied volcanic supracrustals of Bababudan greenstone terrane encapsulate a Neoarchean subduction-collision orogenic system comprising plume-derived komatiite-picrite-basanite and subduction-derived magnesian andesite -niobium-enriched basaltic andesite magmatic sequences tectonically juxtaposed during Neoarchaen (~2.7 Ga) subduction-accretion-collision event.

Evaluating zircon initial Hf isotopic composition using a combined SIMS–MC-LASS-ICP-MS approach: A case study from the Coompana Province in South Australia

The accessory mineral zircon is the most widely used geological timekeeper and tracer of crustal growth processes. Specifically, U–Pb isotopes in zircon offer a means to accurately determine the timing of magmatic events and their Hf isotopic composition provides a means to constrain magma source composition and potentially approximate source age. The high spatial resolution provided by in situ techniques such as secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have the advantage of being able to target different growth zones within individual zircon crystals, and unravel complex magmatic histories. However, utilising this power effectively requires calculation of accurate initial Hf compositions, which are founded on the assumption that the information obtained via the U–Pb and Lu–Hf systems are correctly integrated. A typical Hf isotope LA-ICP-MS analysis ablates a sample volume that is two orders of magnitude greater than a typical SIMS analysis. Thus, when age determination has been carried out by SIMS it is necessary to demonstrate that each subsequent Hf isotope analysis has sampled a similar isotopically homogeneous volume. Here, we use a combined SIMS and laser ablation split stream (LASS)-ICP-MS approach, whereby U–Pb isotopic measurement concurrently on the same sample volume as the Hf isotope measurement is compared to prior lower volume SIMS measurements. Using a suite of new drill core magmatic rock samples from the comparatively unexplored Coompana Province in South Australia, we demonstrate how such an approach can be used to filter Hf isotope datasets by identifying LASS-ICP-MS analyses that sampled mixtures of different zircon growth domains. The robust initial 176Hf/177Hf compositions obtained from the filtered Coompana data set indicate that the province represents part of a juvenile Paleoproterozoic-Mesoproterozoic arc system formed through hyper-extension of the margin of the Archean Gawler Craton. This arc system can be correlated to the Musgrave Province and Madura Province in central and Western Australia respectively.

Continental Crustal Growth Processes Revealed by Detrital Zircon Petrochronology: Insights From Zealandia

AbstractEstablishing spatial, temporal, and geochemical relationships between magmatism and the broader tectonics of accretionary orogens is important for understanding continental crustal growth processes. Here we reconstruct the Paleozoic and Mesozoic evolution of the active continental margin of Zealandia (eastern Gondwana), using a combination of detrital zircon geochronology, trace element geochemistry and Hf isotope data. We find that zircon grains dated 360–160 Ma from New Zealand are characterized by εHfi (+15 to +2) and trace element compositions typical of predominantly juvenile magmatic sources. In contrast, the εHfi (+15 to −5) and trace element compositions of detrital zircon grains dated 245–140 Ma from New Caledonia reflect a mix of juvenile and evolved crustal sources. Secular trends in trace element and Hf isotope compositions of zircon grains suggest that magmatism and continental crustal growth in Zealandia during the Devonian‐Cretaceous were controlled by switches from trench advance to trench retreat. Orogenesis and crustal growth were controlled by a long‐lived westward dipping subduction system, which during the Permian‐Triassic, was intermittently affected by distinct phases of arc accretion (e.g., of the Brook Street intraoceanic arc) and orogenesis (e.g., driven by trench advance). These phases of orogenesis coincided with the Gondwanide Orogen (265–230 Ma), which might have been controlled by a plate‐scale reorganization event following the final assembly of Pangea supercontinent.

Proterozoic tectonic evolution of the Tarim Craton: New insights from detrital zircon U-Pb and Lu-Hf isotopes of metasediments in the Kuruktag area

The Proterozoic is the longest Eon in geological history and witnessed the assemblies and breakups of the Columbia and Rodinia supercontinents. The Tarim Craton has been proposed to be a building block of these supercontinents. However, its tectonic evolution during late Paleoproterozoic to earliest Neoproterozoic (ca. 1.8–0.8 Ga) has been poorly understood, due to the paucity of geological record. In this paper, we present U-Pb and Lu-Hf isotopic data for detrital zircons from the metasediments of the early Neoproterozoic Paergangtag Group and the Paleoproterozoic Xinditag Group in the Kuruktag area, northern Tarim Craton.Although formed approximately 1 billion years later, the Paergangtag Group contains detrital zircons almost identical to those inthe Xinditag Group. LA-ICP-MS U-Pb dating yields a ca. 2.0 Ga age peak, as well as a group of older zircons of ca. 2.5 Ga in six samples from these two groups. Lu-Hf data show that the ca. 2.0 Ga zircons have variableεHf(t) values (−28.1 to 0.1), indicating the detritus were mostly derived from reworked continental crusts. The older zircons around 2.5 Ga have both negative and positiveεHf(t) values (−28.5 to 9.5), indicating a prolonged and complicated crustal growth and reworking process in the source area. It is inferred that the zircons in these samples were likely derived from southeastern Tarim Craton, where a ca. 2.0 Ga orogenic event has been documented.The fact that the Neoproterozoic Paergangtag Group contains only Paleoproterozoic and older zircons suggests that the Tarim Craton was not involved in the Grenville Orogeny, which developed between the colliding continents in the interior of the Rodinia Supercontinent. In contrast, the Tarim Craton was probably on the periphery of the Rodinia Supercontinent and was involved in the peripheral subduction zone of the supercontinent shortly after the deposition of the Paergangtag Group. Our new data imply that the Tarim Craton might have been a tectonically stable continent for nearly 1 billion years from ca. 1.8 to 0.9 Ga.

Petrogenesis of highly differentiated I‐type volcanic rocks: Reinjection of high‐temperature magma—An example from Suolun silicic volcanic rocks, central Great Xing'an Range, China

Silicic volcanic rocks are the main components of the upper crust. Understanding their genesis can help us better understand the processes of crustal growth, reconstruction, and crust–mantle interactions. In this study, we examine silicic volcanic rocks from Suolun in the Great Xing'an Range, China. These silicic volcanic rocks are of Cretaceous age (140–123 Ma), with 68–76 wt% SiO2, and the majority are of the high‐K (calc‐alkaline) series. εHf(t) ranges between +1.7 and +9.9, mostly around 7. The Early Cretaceous silicic volcanic rocks from Suolun are cold‐wet‐oxidised, highly differentiated I‐type volcanic rocks, and plagioclase and K‐feldspar are the main fractionation phases. TTiZ shows that silicic magma undergoes a reheating process in the magma chamber, which causes the early rheological lock‐up magma to reactivate and thus drives it to further differentiation. This process is probably driven by the injection of high temperature magma at depth. The wet upwelling of the Mongol‐Okhotsk Oceanic plate in the deep resulted in large‐scale magmatism in the area. These results provide a possible explanation for the formation of highly differentiated magmatic rocks and further confirm the evolutionary link between volcanic rocks and plutonic systems.

Thermal structure and evolution of an Archean large hot orogen: Insights from the Tasiusarsuaq terrane, SW Greenland

The Tasiusarsuaq terrane represents an Archean continental upper plate that was assembled and thickened by a series of igneous and tectonic events between c. 2970 and 2700 Ma. A review of the available data combined with thermodynamic modelling and U-Pb titanite and zircon dating allows detailed insight into the processes of crustal growth during the Meso- and Neoarchean. Following the intrusion of the c. 2970–2950 Ma Fiskenæsset complex in an arc-type setting, the southern part of the Tasiusarsuaq terrane was intruded by an early TTG generation between c. 2920 and 2880 Ma to create the Bjørnesund block. Subsequently, from c. 2880 to 2830 Ma, TTG magmatism migrated to the Sermilik block in the central and northern part of the Tasiusarsuaq terrane, whereas monzogranite magmatism occurred in the south. We interpret this pattern of igneous activity to be a result of accretion of the Sermilik block to the Bjørnesund block and the formation of a new plate interface behind the accreted terrane. Fabrics related to this early accretionary stage are preserved in the prograde amphibolite facies (580–630 °C; 4–6 kbar) lithologies from the Bjørnesund block that have U-Pb titanite ages of c. 2820 Ma. Afterwards, ongoing southwards-directed subduction led to substantial thickening, manifested by high-pressure granulite facies metamorphism (870 °C, 9.0–9.2 kbar) and the intrusion of the c. 2805–2785 Ma Ilivertalik intrusive complex. Convergence and underthrusting by Eoarchean continental crust (Færingehavn terrane) at c. 2760 to 2720 Ma led to the extrusion of hot, ductile granulite nappes into the mid crust. At this time, large sections of the Tasiusarsuaq terrane underwent extensive amphibolite facies reworking, at conditions ranging from ~725 °C to 5.0–5.5 kbar in the central parts to 6.5–7 kbar and 700 °C in the north. These conditions lasted until the final collision of the terranes at 2720–2700 Ma. Collectively, our model describes the evolution of an Archean large hot orogen that is characterized by continuous convergence rather than intermittent subduction. In many respects, it resembles more recent large hot orogens such as the Grenville, including the accretion of terranes prior to collisional orogeny.

Granulite-facies gneisses and meta-igneous xenoliths from the Campo de Calatrava volcanic field (Spain): Implications for the tectonics of the Variscan lower crust

The microstructural, petrofabric and geochronological study of deep-seated xenoliths from the La Encomienda volcano (Neogene Campo de Calatrava Volcanic Field) has helped to characterize the nature and dynamics of processes operative in the lower crust and subcontinental mantle domains of the Central Iberian Zone in the Iberian Massif (Spain). We present the finding of three new xenolith types: noritic andesine meta-anorthosites, aluminous granulites and felsic granulites. Plagioclase (andesine), hypersthene, ilmenite, apatite and zircon make up the mineral assemblage in equilibrium in the anorthosite xenoliths, whereas K-feldspar (sanidine), antiperthitic plagioclase (oligoclase-andesine), quartz, sillimanite, garnet, rutile and biotite conform the mineral assemblage in the aluminous granulites. Felsic granulites differ from the aluminous type in the smaller amount of garnet, the occurrence of perthitic microcline and the lack of sillimanite and biotite. The presence of significant fabrics in the constituent minerals indicates that they underwent high-temperature deformations representative of low crustal domains.U-Th-Pb analyses on anorthositic zircons provide a Variscan Late Carboniferous Concordia age of 309 ± 3 Ma, interpreted as the age of the igneous protolith of the granulitic anorthosite. Estimations for zircons in the felsic granulites yield three age groups at 288 ± 2 Ma, 398 ± 2 Ma and 490 ± 2 Ma. The first one would represent a late- to post-Variscan (Cimmerian) regional metamorphic event under granulite-facies conditions. The last two might be related to the age of an igneous component of the protolith at ca. 490 Ma and possibly that of a previous tectonothermal event at 400 Ma. 206Pb/238U results on the aluminous and felsic granulitic rutiles give ages of 5 ± 1 Ma that may correspond to resetting during Neogene volcanic events.A tentative scenario would contemplate the existence of Upper Carboniferous (late-Variscan) lithospheric shear zones along which mantle-derived components would have raised until the mantle-crust interface. The associated thermal budget would have acted as an important heat source responsible for heat conduction into the lower crust, along with eventual crustal melting. Removal of the mafic portions would have led to formation of low-density plagioclase-rich accumulations propagating upwards until its eventual incorporation in low crustal domains along with the formation of deep-seated granulites. The occurrence of these lithologies and the large volumes of hybrid monzogranites to granodiorites in the Central Iberian Zone might indicate, congruently, that, more than expected, mantle-derived magmas would have also played an important role during crustal growth processes related to the final stages of the Variscan orogeny in this part of the Iberian Massif.