Early Crustal Evolution Research Articles

Neoarchean and Paleoproterozoic magmatism in the Longjiang area, NE China: Implications for the recycling history of supracrustal components in the eastern Central Asian Orogenic Belt

Substantial Phanerozoic crustal growth and reworking in the eastern Central Asian Orogenic Belt has partly obscured the Precambrian geological record. Accurate identification of such Precambrian magmatism and its petrogenesis is paramount to unveiling the early crustal evolution of the eastern Central Asian Orogenic Belt. Two outcrops of Neoarchean and Paleoproterozoic magmatism in the Longjiang area of NE China, eastern Central Asian Orogenic Belt, were recently discovered. The Kaoquantun outcrop consists of monzogranites that yield a zircon 207Pb/206Pb age of 2568 ± 8 Ma and are similar to peraluminous A1-type granites. They were derived from the partial melting of a hybrid crustal source comprising newly accreted Mesoarchean crust and abundant supracrustal metapelites in a post-collisional extensional setting. Such metapelites were initially part of the upper crust but were buried in the middle−lower crust due to crustal thickening prior to ca. 2.6 Ga. In the Zhanbeitun outcrop, basaltic andesites overlie monzogranites, and both have been intruded by syenogranites. The monzogranites and syenogranites yield zircon 207Pb/206Pb ages of 1881 ± 10 Ma and 1843 ± 3 Ma, respectively, whereas the basaltic andesites are inferred to have formed at 1.88−1.84 Ga. The 1.88 Ga monzogranites have relatively enriched zircon Hf-O isotopic compositions and were derived from a hybrid melt of mafic rocks in the lower crust and ancient metabasalts that had undergone low-temperature alteration at supracrustal depths. The 1.88−1.84 Ga basaltic andesites are Nb-rich and were generated by the partial melting of a mantle wedge that was metasomatized by subduction-derived fluids, recycled sediments, and slab-derived melts. The 1.84 Ga syenogranites are peraluminous and K-rich, and record a profound mixing and assimilation process involving melts derived from supracrustal sediments and the lower crust. Three stages of Paleoproterozoic magmatism emerged from subduction-related crustal thickening, slab breakoff, and collision, respectively. The zircon Hf isotopic data further indicate that the eastern Central Asian Orogenic Belt underwent at least one stage of proto-crustal growth at ca. 3.0 Ga, followed by multiple phases of crustal reworking during the Neoarchean and Paleoproterozoic, with dominant involvement of supracrustal components.

Petrogenesis of Neoarchean granitic gneisses from the Daqishan area of the Dabie orogen and implications for the early crustal evolution of the Northern Yangtze Craton

ABSTRACT The Yangtze Craton is one of the crucial cratons constitued the eastern Asia continent. It is characterized by Archaean to Paleoproterozoic basement rocks, making it an excellent area to study the Precambrian geologic and tectonic regime. Recently, abundant Archaean rocks have been reported from the Dabie Orogen, which providing important information to understanding the early evolution of the Northern Yangtze Craton. In this contribution, we present petrological, whole-rock geochemical, and zircon U – Pb – Hf isotopic analyses on newly discovered Neoarchean granite in the Dabie Orogen. We identified 2744 ± 10 Ma and 2715 ± 11 Ma (~2.74 Ga) Na-rich granitic gneisses, 2691 ± 6 Ma and 2696 ± 8 Ma (~2.69 Ga) A2-type granitic gneisses, and 2664 ± 8 Ma, 2664 ± 6 Ma, 2672 ± 7 Ma, 2642 ± 10 Ma (~2.66 Ga) A1-type granitic/syenite gneisses from the Daqishan area of the Dabie Orogen. Geochemical and zircon Hf isotope data indicate that the ~ 2.74 Ga Na-rich granitic gneisses were partial melting products of thickened mafic crust with limited depleted mantle input, while the 2.69 Ga and 2.66 Ga A-type granitic gneisses represent felsic magmatism in a post-collisional to an extensional tectonic setting. Based on our research, we categorized the Northern Yangtze Craton into the Kongling domain and Dabie domain by comparing with the distribution of the zircon U-Pb magmatic ages and metamorphic events. We suggest that the initial unified basement of the northern proto-Yangtze Craton formed during the Nuna supercontinent cycle at ~ 2.0 Ga.

From 3.4 Ga TTG generation to 2.9 Ga crustal anatexis: The Archean crustal evolution of Porteirinha Complex (SE, Brazil)

The Porteirinha Block (PB) provides an important record of the origin and evolution of South American crust. The PB records the generation of Paleoarchean TTG, Mesoarchean anatexis, and Paleoproterozoic crustal reworking. It is exposed as a basement inlier in the Araçuaí Orogen as a result of a tectonic involvement of the continental margins of São Francisco Congo Paleocontinent (PSFC) during the collisional processes of Gondwana amalgamation at the Neoproterozoic. The PB basement unit is the Porteirinha Complex (PC), which consists of TTG orthogneisses, migmatites, amphibolites, metaultramafics rocks and Neoarchean granitoids. In this paper, we discuss the evolution of the TTG orthogneisses and migmatites from PC based on field, geochronological and whole-rock geochemistry data. Zircon U–Pb ages (LA-ICP-MS) show a complex and polycyclic evolution of these rocks. Concordant ages were obtained for crystallization of TTG magma at ca. 3.4 Ga and the geochemical signature supports the idea of TTG generation by partial melting of hydrated mafic crust under garnet-amphibolite facies conditions. Zircon U–Pb ages also suggest crustal reworking events at 3.1 Ga. Metatexite and diatexite domains related to partial melting of TTG orthogneisses are individualized. The migmatites record processes of crustal anatexis and melt generation that occurred between 2.8 and 2.9 Ga (LA-ICP-MS, U–Pb in zircon) and are linked to crustal reworking and heat generation events recognized regionally. The new data presented in this work allow us to establish correlations between Archean and Paleoproterozoic units of the PB and those that occur regionally, particularly those related to Gavião block, where a series of stages and events of early crustal evolution are shared.

Paleoproterozoic crustal evolution of the Tarim Craton, NW China: Constraints from geochronology and geochemistry of orthogneisses and granitic veins in the Xingdi region of the Quruqtagh Block

The Archean–Paleoproterozoic basement rocks, exposed in the Xingdi region of the Quruqtagh Block, provide a great opportunity to characterize the early crustal evolution of the Tarim Craton and reconstruct the Columbia supercontinent. Here we report zircon U–Pb ages, Lu–Hf isotope compositions, and bulk-rock geochemical data of orthogneisses and granitic veins from the Xingdi region of the Quruqtagh Block in the northeastern margin of the Tarim Craton. The orthogneisses formed in the late Neoarchean to early Paleoproterozoic (2504 ± 33 Ma, 2419 ± 24 Ma and 2450 ± 27 Ma), and were metamorphosed in the late Paleoproterozoic (1789 ± 43 Ma and 1748 ± 35 Ma). The orthogneisses have relatively high Sr (312–710 ppm) contents, and low Y (5.3–13.8 ppm) and Yb (0.46–1.10 ppm) contents, resulting in high Sr/Y (32–118) and La/Yb (17–64) ratios that are characteristics of adakites. The relatively high Mg# (49–55) and depleted zircon Hf isotope characteristics (positive εHf(t), with TDM2 of ca. 2.5–2.8 Ga) indicate that the orthogneisses crystallized from partial melting of garnet-bearing amphibolite in a subduction environment. The granitic veins that intrude the orthogneisses formed during the late Paleoproterozoic (1760 ± 25 Ma), which is roughly contemporaneous with the metamorphic ages of the orthogneisses; the granitic veins are characterized by extremely high SiO2 (>80 %), enrichments in Rb, Ba and U, and depletions in Nb and Ta. It is inferred that the granitic veins formed by in situ anatexis of the orthogneisses during the late Paleoproterozoic. Based on the zircon ages with Hf isotope compositions, it is evident that the continental crust in the Xingdi region of the Tarim Craton largely formed between 2.5 and 2.8 Ga and was reworked at ca. 1.8 Ga; this evolutionary history is similar to that of the Dunhuang Block. Comparison of the Paleoproterozoic tectono-thermal events in the different terranes of the Tarim Craton, it is suggested that the Tarim Craton is composed of discrete terranes that detached from different ancient cratonic nuclei prior to the late Paleoproterozoic, and assembled within the Columbia supercontinent respectively.

Zircon Hf isotopic compositions of the basement gneiss in Bayan Obo: Constraints on crustal evolution of the Yinshan Block in the North China Craton

Bayan Obo, located in the north margin of the Yinshan Block, North China Craton (NCC), contains the Archean to Paleoproterozoic basement rocks which may shed light on the early crustal evolution of the NCC. In this paper, to reveal the formation and modification of the basements, we report Hf isotope of zircons from the biotite plagiogneisses in Bayan Obo. Combined with the published U–Pb ages and Hf isotope compositions of the zircons from the Bayan Obo basement rocks, a comprehensive review was employed to provide new insights into the formation and modification of the basement in this area. The oldest formation age of the basement rocks was found to be approximately 2.6 Ga, with positive εHf(t) values indicating the contribution from the depleted mantle to the rocks. The age of ∼2.0 Ga marks the major formation period of the basement rocks. The varied εHf(t) values (−17.2 to +8.9) of the magmatic zircons suggest a mixture origin between the depleted mantle and crustal rocks for the gneiss. The basement went through extensive metamorphism during 1.95–1.9 Ga. In addition, zircon U–Pb–Hf isotope data of the Precambrian units in Guyang and Wuchuan, which are adjacent to Bayan Obo, were compiled for comparison. The comprehensive review on the data of the concordant magmatic zircons indicates the existence of 3.5 Ga crustal fragments in the Yinshan Block. The melting of the depleted mantle and the addition of juvenile materials into the crust at 2.6–2.9 Ga and 2.45–2.55 Ga were confirmed by the high positive εHf(t) values of the magmatic and inherited zircons. An increase in remelting of the crust at 2.0 Ga is deduced from the extremely unradiogenic Hf isotope of the magmatic zircons.

Tracing the Early Crustal Evolution of the North China Craton: New Constraints from the Geochronology and Hf Isotopes of Fuchsite Quartzite in the Lulong Area, Eastern Hebei Province

Understanding the composition, formation and evolution of the oldest continental crust is crucial for comprehending the mechanism and timing of crustal growth and differentiation on early Earth. However, the preservation of the ancient continental crust is limited due to extensive reworking by later tectonothermal events. In the Lulong area of eastern Hebei, abundant ca. 3.8–3.4 Ga detrital zircons of the fuchsite quartzite have been previously identified. Nonetheless, the provenance and Hf isotopic compositions of the fuchsite quartzite remain unclear. In this study, we present new detrital zircon ages and Hf isotopic for the fuchsite quartzite in the Lulong area to establish the timing of deposition, the provenance and the regional stratigraphic relationship. Zircon U-Pb dating indicates that the fuchsite quartzite was deposited between 3.3–3.1 Ga and most grains were sourced from the 3.8 Ga TTG gneisses and Paleoarchean magmas. Field investigations and regional correlations reveal that the fuchsite quartzite from the Lulong area is equivalent to that of the Caozhuang area. Zircon Hf isotopic data from eastern Hebei Province (Lulong and Caozhuang areas) and Anshan and Xinyang areas indicate that the oldest crustal growth event of North China Craton occurred in the Hadean.

Petrogenesis and Geodynamic Evolution in the Northern Westfjords, Iceland, Elucidated by Iceland's Oldest Silicic Rocks

AbstractIceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present new zircon U‐Pb ages bolstered with zircon trace element and isotopic compositions, and whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers—Hrafnsfjörður, Árnes, and Kaldalón—to understand the petrogenesis of large silicic volcanic centers in the northern Westfjords, Iceland. Our data confirm Hrafnsfjörður as the oldest known central volcano in Iceland (∼14 Ma) and establish an older age for Árnes (∼13 Ma) than previously estimated. We also report the first U‐Pb zircon dates from Kaldalón (∼13.5 Ma). Zircon oxygen isotope compositions range from δ18O∼+2 to +4‰ and indicate involvement of a low‐18O component in their source magmas. Hrafnsfjörður zircon Hf (mean sample εHf ∼ +15.3–16.0) and whole rock Hf and Nd (εHf = +14.5 to +15; εNd = +7.9 to +8.1) isotopic compositions are more radiogenic than those from Árnes (zircon sample εHf ∼ +11.8–13; whole rock εHf = +12.8 to +15.1; εNd = +7.3 to +7.7), but Hrafnsfjörður whole rock Pb isotope compositions (208/204Pb = 37.95–37.96; 206/204Pb = 18.33–18.35) are less radiogenic than those from Árnes (208/204Pb = 38.34–38.48; 206/204Pb = 18.64–18.78). Kaldalón has zircon Hf isotope compositions of εHf∼+14.8 and 15.5 (sample means). These age and isotopic differences suggest that interaction of rift and plume, and thus the geodynamic evolution of the Westfjords, is complex. Isotopic compositions of Hrafnsfjörður and Árnes support involvement of an enriched mantle (EM)‐like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle.

Isotope evidence for Archean accordion-tectonics in the Superior Province

Archean cratons preserve the oldest continental crust, but their tectonic mode of formation and assembly remain key unknowns. The Superior Province is Earth’s largest Archean craton and comprises Eo-Neoarchean plutonic-gneiss terranes separated by Meso-Neoarchean granite-greenstone terranes and metasedimentary belts. Resolving whether the plutonic-gneiss terranes represent dismembered fragments of a once contiguous proto-craton or a series of unrelated accreted crustal fragments is critical to understanding the early evolution of the Superior Province and Archean tectonics. We present zircon U-Pb-Hf isotopic data from the Tannis and Cedar Lake TTG gneisses, which record the early crustal evolution of the Winnipeg River plutonic-gneiss terrane and are some of the oldest rocks in the western Superior Province. The gneisses yield a large range of zircon εHf(t) signatures (−6 to +4) at igneous formation (ca. 3.3–3.25 Ga) indicating coeval crustal growth and reworking of isotopically depleted and evolved sources. Crustal reworking of Eo-Paleoarchean sources is supported by sub-chondritic ca. 3.5–3.4 Ga zircon xenocrysts in the Cedar Lake gneiss. The early crustal evolution of the central Winnipeg River terrane is similar to the Hudson Bay and Minnesota River Valley terranes, on the northern and southern margins of the Superior Province, respectively. Correlation of the early history amongst the three plutonic-gneiss terranes supports a tectonic model in which a once coherent Eo-Paleoarchean proto-craton disaggregated into three fragments during formation of intervening granite-greenstone terranes in the Mesoarchean. These fragments reaggregated in the Neoarchean. The complex history of the Superior Province highlights that Archean cratons are not simple entities that formed in a single event, but may have experienced times of cratonic breakup and reassembly.

Structural restoration of an Eo-Mesoarchean (3.8–2.9 Ga) terrane, Eastern China, dissected by the Tanlu fault zone

Ancient gneissic terranes are critical rare materials that can reveal the Earth's early crustal evolution. This paper presents reconstruction of the oldest Eo-Mesoarchean gneissic terrane (3.8–2.9 Ga) in the eastern North China Craton that was partitioned by the ENE-WSW striking Tanghe-Hanling strike-slip fault (THF). The THF is a splay off the Tanlu fault zone and has been traced for 130 km and is 1–1.5 km wide. The THF has a 28 km sinistral offset, which has been established by detailed field observation. Restoring the 28 km offset, means that: (1) A sizeable potential area is recognized for searching for more 3.8–2.9 Ga Eo-Mesoarchean rocks; (2) A large NW-striking late Archean BIF belt is correlated with another at Anshan. Our data integrate with regional structural information of the Tanlu fault zone; we suggested that the Tanlu fault zone has an eastwards-fan splay pattern geometric feature in its northern part. Accordingly, we further restore the ancient tectonic configuration of eastern Asia before the Tanlu fault zone initiation. Eo-Mesoarchean rocks and 2.7–2.9 Ga terranes cropping out in Eastern Block of the North China Craton are arranged in line with NNW-SSE strike, which is consistent with their oriented gneissic foliations.

U–Pb age and Hf isotopic characteristics of detrital zircon from the Nanhua System in Longsheng, northern Guangxi Province, and their geological significance

U–Pb dating and Hf isotopes of detrital zircons from the Nanhua System in Longsheng, northern Guangxi, were investigated to determine the depositional age and provenance of the Nanhua System and the early crustal evolution stages of the South China. U–Pb ages of detrital zircons from the Chang'an Formation of the Nanhua System in Longsheng, northern Guangxi, are mainly 0.66 ~ 0.72 Ga and 0.72 ~ 0.90 Ga, with εHf(t) ranging from −17.05 to 12.79. The dominant provenance is composed of Neoproterozoic magmatic rock from the southeastern margin of the Yangtze Block and the Nanhua period detrital material from Cathaysia Block. U–Pb ages of detrital zircons from the Fulu Formation can be divided into four major groups, that is, 0.64 ~ 0.72 Ga, 0.72 ~ 0.90 Ga, 1.80 ~ 2.20 Ga, and 2.30 ~ 2.80 Ga, with εHf(t) ranging from −27.66 to 11.68. The main provenance is Neoproterozoic magmatic rock from the southeastern margin of Yangtze Block, followed by the Nanhua period detrital material from the Cathaysia Block and geological body equivalent to the assembly of the Columbia Supercontinent. U–Pb age of detrital zircons from Lijiapo Formation is concentrated in 0.72 ~ 0.90 Ga, with εHf(t) ranging from −16.02 to 11.87, and the main provenance comes from Neoproterozoic magmatic rock in the south‐east margin of Yangtze Block. The second‐stage Hf model age (TDM2) of the Nanhua System is 0.84 ~ 4.51 Ga, with two obvious peak intervals of 0.90 ~ 1.30 Ga and 1.40 ~ 2.10 Ga, and two secondary peak intervals of 2.40 ~ 3.00 Ga and 3.10 ~ 3.60 Ga, which can be interpreted that the research area has experienced four stages of crustal growth: 3.10 ~ 3.60 Ga, 2.40 ~ 3.00 Ga, 1.40 ~ 2.10 Ga, and 0.90 ~ 1.30 Ga. The detrital zircon age spectra indicate that the Chang'an Formation and Fulu Formation of the Nanhua System contain more detrital zircons which is contemporaneous with strata due to the Xuefeng orogeny, while the Lijiapo Formation contains a very small amount of detrital zircon which is contemporaneous with strata, suggesting that the tectonic setting has shifted to a passive continental margin or pull‐apart basin during the period of the Lijiapo Formation.

Petrogenesis of Archaean granites in the Barberton region of South Africa as a guide to early crustal evolution

ABSTRACT The Barberton region of South Africa is characterized by a broad variety of granite types that range in age from ca. 3.5 Ga to 2.7 Ga and reflect the processes involved in the formation of Archaean continental crust on the Kaapvaal Craton. These granites are subdivided into three groups, as follows: A tonalite-trondhjemite-granodiorite (TTG) suite diapirically emplaced at 3 450 Ma and 3 250 Ma into pre-existing metamorphosed greenstone belt material. TTG melts were derived from melting amphibolite in the lower crust, with individual plutons being emplaced at various crustal levels. The dome-and-keel geometry that characterizes the TTG-greenstone dominated crust at this time is inconsistent with a plate tectonic domain and reworking was likely controlled by gravity inversion or ‘sagduction’; Regionally extensive potassic batholiths (the GMS suite) were emplaced at 3 110 Ma during a period of crustal thickening and melting of a TTG-dominated lower crust. Subsequent to emplacement of the voluminous GMS granites, the thickened continental crust had stabilized sufficiently for large sedimentary basins to form; Late granite plutons were emplaced along two distinct linear and sub-parallel arrays close to what might have been the edge of a Kaapvaal continent at 2 800 to 2 700 Ma. They are subdivided into high-Ca and low-Ca granites that resemble the I- and S-type granites of younger orogenic episodes. The high-Ca granites are consistent with derivation from older granitoids in the lower crust, whereas the low-Ca granites may have been derived by melting metasedimentary precursors in the lower-mid crust. Granites with similar characteristics are associated with a subduction zone in younger terranes, although the recognition of such a feature at Barberton remains unclear. The petrogenesis of granites in the Barberton region between 3.5 Ga and 2.7 Ga provides a record of the processes of Archaean crustal evolution and contributes to discussions related to the onset of plate tectonics.

Discovery of rhyolitic tuffaceous slate in the southwestern margin of Yangtze Craton: Zircon U-Pb ages (2491 Ma) and tectonic-thermal events

The Mesoproterozoic Dongchuan Group that is widely exposed in Yimen area, central Yunnan Province is a series of sedimentary sort of low-grade metamorphic rocks interbedded with volcanic rocks, which are closely related to the early tectonic evolution of the Earth. However, its formation era, sedimentary filling sequence, and geotectonic characteristics have always been in dispute. In this study, several rhyolitic tuffaceous slate interlayers with a centimeter-level thickness were found in the previously determined Heishan Formation of the Dongchuan Group located to the western part of Yimen-Luoci fault zone. This paper focuses on the study of the rhyolitic tuffaceous slate in Qifulangqing Village, Tongchang Township, Yimen County. LA-ICP-MS zircon dating was conducted, achieving the crystallization age of magma of 2491 ± 15 Ma and the metamorphic ages of about 2.3 Ga, 2.0 Ga, and 1.8 Ga for the first time. Meanwhile, according to in-situ Hf isotope analysis, the zircon εHf(t) values were determined to range from −3.0 to 7.6, with an average of 2.7. Furthermore, the first-stage Hf model age (TDM1) was determined to be 2513−2916 Ma, indicating that the provenance of the rhyolitic tuffaceous slate is the depleted mantle or juvenile crust between the Middle Mesoarchean and the Late Neoarchean. Therefore, it is believed that the strata of the slate were deposited in the Late Neoarchean, instead of the Mesoproterozoic as determined by previous researchers. Accordingly, it is not appropriate to group the strata into the Mesoproterozoic Dongchuan Group. Instead, they should be classified as the Maolu Formation of the Neoarchean Puduhe Group given the lithologic association and regional information. Furthermore, the magma ages of 2491 ± 15 Ma are highly consistent with the eras of the large-scale Late Neoarchean orogenic magmatic activities on the northern margin of the Yangtze Craton, and thus reflect the orogenic process consisting of subduction and collision from Late Neoarchean to Early Paleoproterozoic. The magmatic activities during this period were possibly caused by the convergence of the supercontinent Kenorland. Meanwhile, the metamorphic ages of 2.3 Ga, 2.0 Ga, and 1.8 Ga are highly consistent with three metamorphic ages of 2.36 Ga, 1.95 Ga, and 1.85 Ga of the northern margin of the Yangtze Craton, indicating that the strata experienced Paleoproterozoic tectonic-thermal events. The study area is located on the eastern margin of Qinghai-Tibet Plateau, and thus was possibly re-transformed by magmatism subjected to the subduction of the Meso-Tethys Ocean during the Early Cretaceous. The discoveries made in this study will provide strong petrological and chronological evidence for analyzing the early crustal evolution of the Yangtze block.©2021 China Geology Editorial Office.

Three episodes of Precambrian mafic magmatism in the southern Central Tianshan Block (NW China): Insight into an evolving geodynamic model

Precambrian microcontinents are an essential component in the tectonic evolution and architecture of the Central Asian Orogenic Belt (CAOB). However, the early crustal evolution and origin of Precambrian microcontinents remain poorly constrained due to the scarcity of early Precambrian rocks. The Chinese Central Tianshan Block (CTB) constitutes a microcontinent in the western CAOB with Precambrian basement rocks along its southern margin. The basement rocks, comprising amphibolite-facies supracrustal assemblages of possible volcano-sedimentary origin, are exposed in the Wuwamen Complex, which occurs as tectonic slivers in the late Paleozoic Wuwamen ophiolitic mélange. Here we present in situ zircon U–Pb and Lu–Hf isotopic data and the whole-rock geochemistry of three suites of (meta-)mafic rocks from the Wuwamen Complex and an associated Neoproterozoic gabbro body. The zircon dating revealed that the Wuwamen Complex contains two suites of meta-mafic rocks with protolith formation at ca. 2.55 Ga and 2.21 Ga followed by an amphibolite-facies metamorphic overprint at ca. 1.83 Ga. In addition, an individual gabbro body adjacent to the Wuwamen Complex yielded an emplacement age of ca. 0.69 Ga, reflecting a mid-Neoproterozoic mafic magmatic event in the CTB. Furthermore, zircon U–Pb dating of a crosscutting diorite dyke and a gneissic diorite sourced from a composite batholith in the CTB yielded weighted mean intrusive ages of ca. 320 Ma and 454 Ma, respectively. In conjunction with the field observation, the emplacement ages suggest that the Wuwamen Complex represents a part of the continental arc basement of the CTB. The three suites of (meta-)mafic rocks show geochemical characteristics similar to typical arc-related basalts. Along with the evolved zircon Hf isotopic compositions, it is suggested that these rocks originated from partial melting of a metasomatized sub-continental lithospheric mantle during different events. In conjunction with previous studies, the here presented new data further suggest that the CTB and other microcontinents in the western CAOB record similar Precambrian magmatic and metamorphic episodes, which are comparable to those of the northern Tarim during Proterozoic times. The here outlined new model indicates that the microcontinents of the western CAOB and the Tarim Craton may have shared a common geodynamic evolution.

Early crustal evolution of the Superior craton – A U–Pb, Hf and O isotope study of zircon from the Assean lake complex and a comparison to early crust in other cratons

We report U–Pb/Hf/O isotopic measurements of zircon extracted from meta-igneous rocks exposed in the Assean Lake crustal complex, northwestern Superior craton (Canada). With crystallization ages of ca. 3.22 Ga, zircon from the Assean Lake crustal complex are among the oldest reported in the Superior craton. The Hf and O isotope compositions of zircon indicate that these meta-igneous rocks derive from a mantle component with a relatively primitive Hf isotope composition, similar to chondrites, that was uncontaminated by elevated δ18O material (εHf,initial ~ 0 and δ18OVSMOW ~ 5.5‰). These features are shared by the large majority of the zircon extracted from the oldest (meta)-igneous rocks exposed in Archean terranes. Positive εHf deviations in Archean zircons are typically observed in magmas that post-date the oldest magmatic activity in a given terrain. This temporal variability in magma source regions may reflect a sequence of events comprising: (i) felsic magmatism derived from undepleted mantle components followed by, (ii) re-melting of the same mantle component producing the depleted Hf signatures in zircon and then, (iii) accretion of the proto-craton and reworking of the whole-system enabling stabilization.

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

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

Hidden Eoarchean crust in the southwestern Central Asian Orogenic Belt

U-Pb and Hf-isotope analyses of zircon xenocrysts from drilling-sampled Carboniferous volcanic rocks suggest the presence of unexposed Eoarchean crust beneath the Junggar Basin (NW China), southwestern Central Asian Orogenic Belt (CAOB). The oldest zircon population with U-Pb ages of 3.8 Ga shows chondritic εHf(t) of −0.7 to +0.7, and Hf model ages of 3900–3952 Ma (TDM) and 3973–4062 Ma (Tcrust). Three slightly younger populations at 3.7, 3.62 and 3.45 Ga have εHf(t) of −5.7 to +2.4, TDM of 3.6–3.9 Ga and Tcrust of 3.7–4.1 Ga. The youngest xenocrysts yield U-Pb ages of 2.55 Ga with εHf(t) values of +3.7 to +8.0. Crustal mineral inclusions and the trace-element compositions of all these zircons show that they probably crystallized from granitic magmas. The observation of deeply hidden Archean crust significantly older than the exposed Phanerozoic upper crust suggests that the juvenile origin in CAOB may be overestimated, and the study of crustal growth rates through time for young orogens will require revision. By comparing the Hf isotopic signatures of ancient zircons (>3.4 Ga) worldwide, we infer that there may have been a great change in early crustal evolution at the Hadean–Archean transition (~3.9 Ga), possibly linked to the Late Heavy Bombardment (LHB).

Late Neoarchean reworking of the Mesoarchean crustal remnant in northern Liaoning, North China Craton: A U–Pb–Hf–O–Nd perspective

How the earth's crust formed and evolved during the Precambrian times is one of the key questions to decipher the evolution of the early Earth. As one of the few cratons containing well-preserved Eoarchean to Neoarchean basement on Earth, the North China Craton is an ideal natural laboratory to unravel the early crustal evolution. It is controversial whether the Archean tectonothermal events in this area represents reworking or growth of the continental crust. To solve this issue, we have compelled field-based mapping, zircon U–Pb dating by SHRIMP RG and LA–ICP–MS U–Pb, zircon SHRIMP SI oxygen and LA–MC–ICP–MS Hf isotope, and whole-rock Nd–O isotope analyses from the Archean granitoids in northern Liaoning, North China Craton. On the basis of zircon U–Pb isotopic dating and measured geological section investigation, two distinct magmatic suites as enclaves in the Jurassic granites are recognized, viz. a newly discovered 3.0 Ga crustal remnant and a 2.5 Ga granitoid. The Mesoarchean zircons from the 3.0 Ga granodioritic gneisses exhibit heterogeneous Hf isotopic compositions, with the most radiogenic analysis (εHf(t) = +3.8) following the depleted mantle evolution array and the most unradiogenic εHf(t) extending down to −3.4. This implies that both ancient continental crust at least as old as 3.4 Ga and depleted mantle contributed to the magma source of the protoliths of the Mesoarchean gneisses. The εHf(t) values of the Neoarchean zircons from these gneisses overlap the 3.4–3.0 Ga zircon evolution trend, indicating that the ancient crustal materials have been reworked during the late Neoarchean. The Neoarchean zircons from the 2.5 Ga granitoids have a relatively small variation in the Hf isotope and are mainly plotted in the 3.0–2.8 Ga zircon evolution field. However, taking all the εHf(t) values of the Neoarchean zircons into the consideration, we find that the Hf model age of the Neoarchean zircon does not represent the time of crustal growth or reworking but are artifacts of magma mixing. The interaction between the magmas derived from the ancient crustal materials and the depleted mantle is also supported by zircon O isotopic data and Hf–O isotopic modeling of the Neoarchean granitoids. Both Mesoarchean and late Neoarchean tectonothermal events involved synchronous crustal growth and reworking, which may be applicable to other parts of the world.

No evidence for high-pressure melting of Earth\u2019s crust in the Archean

Much of the present-day volume of Earth’s continental crust had formed by the end of the Archean Eon, 2.5 billion years ago, through the conversion of basaltic (mafic) crust into sodic granite of tonalite, trondhjemite and granodiorite (TTG) composition. Distinctive chemical signatures in a small proportion of these rocks, the so-called high-pressure TTG, are interpreted to indicate partial melting of hydrated crust at pressures above 1.5 GPa (>50 km depth), pressures typically not reached in post-Archean continental crust. These interpretations significantly influence views on early crustal evolution and the onset of plate tectonics. Here we show that high-pressure TTG did not form through melting of crust, but through fractionation of melts derived from metasomatically enriched lithospheric mantle. Although the remaining, and dominant, group of Archean TTG did form through melting of hydrated mafic crust, there is no evidence that this occurred at depths significantly greater than the ~40 km average thickness of modern continental crust.

Initial 87Sr/86Sr as a sensitive tracer of Archaean crust-mantle evolution: Constraints from igneous and sedimentary rocks in the western Dharwar Craton, India

Among the rocky planets of the solar system only the Earth has “granitic” continental crust. The timing and processes involved in the formation of Earth’s first extensive crust is still enigmatic. The chemical and isotope compositions of ancient crustal rocks preserve a record of their genesis. The Rb-Sr system proves to be an efficient proxy for the reconstruction of crust-mantle evolution since it can bring together information from seawater as preserved in chemical sedimentary rocks and information from magmatic rocks that can trace the time and extent of crust formation and concomitant mantle depletion during the Archaean eon. The Dharwar Craton in India preserves a suite of metamorphosed igneous and sedimentary rocks that record its early crustal evolution. To overcome the susceptibility for resetting and the difficulty in determining initial 87Sr/86Sr, the minerals barite and apatite are used to obtain precise and accurate 87Sr/86Sr, because these minerals preferentially incorporate Sr and exclude Rb and preserve the initial Sr isotope compositions at the time of their formation. Initial 87Sr/86Sr of apatite were obtained in situ using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry. The robustness and only minor dispersion of the isotope ratios demonstrate the capability of matrix apatites in preserving initial Sr ratios. The least radiogenic value is used as the best estimate for the initial 87Sr/86Sr. The 87Sr/86Sr ratios of apatite from igneous rocks that formed from 3.5 Ga to 2.6 Ga constrain the Rb/Sr of the source over the whole time-span. A comparison of the Sr isotopes between seawater-derived barite and initial Sr isotope ratios in apatite from igneous rocks reveals that significant mafic to intermediate crust had formed by 3.2 Ga. Studying the entire Archaean time window, a dominantly mafic crust was the main source for the granitoid rocks in the Dharwar Craton from 3.5 to 3.1 Ga, whereas the rocks from 2.9 to 2.7 Ga were extracted dominantly from the depleted mantle.

Neoarchean and Paleoproterozoic K-rich granites in the Phan Si Pan Complex, north Vietnam: Constraints on the early crustal evolution of the Yangtze Block

Precambrian igneous and metamorphic rocks of the Phan Si Pan Complex, North Vietnam, constitute the southern extension of the Yangtze Block, and provide a valuable record of the early evolution of the continental crust. We present results of U-Pb zircon geochronology and geochemistry for Precambrian granites in this complex to constrain their emplacement age and genesis. Granites from three plutonic bodies yielded ages of 2848 ± 15 Ma, 2768 ± 19 Ma and 1869 ± 30 Ma, which represent newly-recognized late Archean to Paleoproterozoic potassic granite plutonism in the southern Yangtze Block. The average εHf(t) values range from −6.2 to 0.1 for the 2.85 to 2.77 Ga granitic rocks and −13.1 to −9.2 for the ca. 1.86 Ga granitic rocks, with two-stage model ages of 3.64 to 3.20 Ga and 3.31 to 3.07 Ga, respectively, suggesting derivation from partial melting of Paleoarchean and Mesoarchean crust. The late Archean potassic granites exhibit high K2O, and high Sr/Y and (La/Yb)N ratios with negligible Eu anomalies, indicating derivation from melting of the thickened lower crust, which is inferred to have occurred in an active margin setting. The late Paleoproterozoic alkali feldspar granites are characterized by high FeOT/(FeOT + MgO)(0.96–0.99) and 10000 * Ga/Al (2.75–2.94) ratios, showing an affinity of A-type granite. These A-type granites exhibit flat chondrite-normalized HREE patterns and strong negative Eu anomalies, and low Sr/Y and (La/Yb)N ratios, corresponding to melting at a shallow depth, probably in a post-collisional extension setting.Comparison of the rock units and events recorded by the Phan Si Pan Complex with other Archean to Paleoproterozoic complexes (Houhe, Dongchuan, Yudongzi, Douling, Zhongxiang and Kongling complexes) in the Yangtze Block indicate spatially distinct histories of crustal growth, and thus may reflect independent terranes. The ca. 1.86 Ga post-collisional magmatism, which succeeds a 2.0–1.9 Ga metamorphic event, is distributed throughout the Yangtze Block, including the Phan Si Pan Complex, suggesting assembly of the disparate terranes and final cratonization of the Yangtze Block overlaps with, and may be related to, assembly of the Nuna supercontinent.