Search

Based on a brief introduction of the spatial distribution, rock types and formation ages of the late Mesoarchean–early Neoarchean (2.6~3.0 Ga) rocks in some key areas of the North China Craton, this paper summarizes the ages and geochemical and Nd-Hf-O isotopic compositions of the granitoids all over the craton. The late Mesoarchean–early Neoarchean basement shows the following features: (1) The late Mesoarchean–early Neoarchean magmatism is almost continuous, with a peak period of 2.70~2.75 Ga; (2) The late Mesoarchean–early Neoarchean rocks widely occur in the North China Craton, mainly in the Eastern Ancient Terrane, the Central Ancient Terrane and the Southern Ancient Terrane; (3) The intrusive rocks are mainly tonalite in composition, with trondhjemite, granodiorite, K-rich granite and gabbro-diorite; (4) The supracrustal rocks are commonly small in scale and scatter in granitoids. The rock types are mainly meta-basaltic rocks. In some areas, there are meta-komatiites, meta-andesitic-dacitic rocks and meta-clastic sedimentary rocks; (5) 2.6 Ga can be regarded as the boundary between the early and late Neoarchean in the North China Craton; (6) TTG rocks show large Sr/Y and La/Yb variations, plotting in the high-, medium- and low-pressure TTG areas in the Sr/Y–Y and La/Yb–Yb diagrams. Except for a few K-rich granites, the late Mesoarchean–early Neoarchean rocks are commonly depleted in Nd-Hf isotope compositions, with the magmatic zircon being similar in O isotope composition to that of the Archean magmatic zircon worldwide; (7) Many regions have similar geological characteristics, but some regions show great uniqueness. The research futher supports the understanding that, similar to many other typical cratons worldwide, the late Mesoarchean–early Neoarchean is the most important period of continental accretion in the North China Craton, and the main difference is that the North China Craton underwent a strong and widespread magmato-tectonothermal event at the end of the Neoarchean.

The recycling of supracrustal materials, and in particular hydrated rocks, has a profound impact on mantle composition and thus on the formation of continental crust, because water modifies the physical properties of lithological systems and the mechanisms of partial melting and fractional fractionation. On the modern Earth, plate tectonics offers an efficient mechanism for mass transport from the Earth's surface to its interior, but how far this mechanism dates back in the Earth's history is still uncertain. Here, we use zircon oxygen (O) isotopes to track recycling of supracrustal materials into the magma sources of early Archean igneous suites from the Kaapvaal Craton, southern Africa. The mean δ 18O values of zircon from TTG (tonalite–trondhjemite–granodiorite) rocks abruptly increase at the Paleo-Mesoarchean boundary (ca. 3230 million years ago; Ma), from mantle zircon values of 5‰–6‰ to approaching 7.1‰, and this increase occurs in ≤3230 Ma rocks with elevated Dy/Yb ratios. The 18O enrichment is a unique signature of low-temperature water–rock interaction on the Earth's surface. Because the later phase was emplaced into the same crustal level as the older one and TTG magmas would derive from melting processes in the garnet stability field (>40 km depth), we suggest that this evident shift in TTG zircon O isotopic compositions records the onset of recycling of the mafic oceanic crust that underwent seawater hydrothermal alteration at low temperature. The onset of the enhanced recycling of supracrustal materials may also have developed elsewhere in other Archean cratons and reflects a significant change in the tectonic realm during craton formation and stabilization, which may be important processes for the operation of plate tectonics on early Earth. [ABSTRACT FROM AUTHOR]

Abundant late Neoarchean granitoids occur in southwestern Liaoning Province, part of the Eastern Ancient Terrane of the North China Craton. These rocks include intermediate gneiss, TTG gneisses and potassic granitoids, and we report on the geochemistry and zircon SHRIMP ages as well as Hf-in-zircon isotopes of these granitoids in order to determine their petrogenesis. Field relationships suggest that most of these granitoids experienced widespread metamorphism and deformation, associated with anatexis at some localities. The intermediate gneisses, TTG gneisses and potassic granitoids were all emplaced at the end of the Neoarchean (2.50–2.53 Ga), and CL images document widespread recrystallization in the zircons. The intermediate and TTG gneisses yielded similar Hf isotopic systematics (ε Hf(t) = −3.73 to +6.42) as the associated potassic granitoids (ε Hf(t) = −2.44 to +7.80), and both rock types yielded mean Hf crustal model ages of 2.8–2.9 Ga. Combined with the geochemistry, we propose that the formation of the intermediate and TTG gneisses was related to partial melting of mafic rocks at different depth, whereas the potassic granitoids have variable petrogenesis. The nearly coeval TTG gneisses and potassic granitoids and their widespread metamorphism, deformation and zircon recrystallization suggest that a large-scale heat source must have been present at or near the base of the crust in southwestern Liaoning Province at the end of the Neoarchean. We propose that collision and post-collisional extension is the most likely tectonic environment for generation of the above granitoids, and the formation of widespread potassic granitoids played an important role in the maturation of continental crust in the North China Craton. Unlabelled Image • TTG gneisses and potassic granitoids show indistinguishable formation ages and Hf isotopes. • Widespread zircon recrystallization occurred immediately after formation of these granitoids. • A long-lived heat source is necessary for the formation and metamorphism of these granitoids. • Post-collision extension is the most likely environment for these granitoids. [ABSTRACT FROM AUTHOR]

Eastern Shandong is one of the important early Precambrian areas in the North China Craton, where extensive 2.5, 2.7 and 2.9 Ga magmatic activities have been identified, followed by very strong late Neoarchean and Paleoproterozoic tectono-thermal events. We carried out SHRIMP zircon dating, Hf-in-zircon isotope analysis and a whole-rock geochemical study on ca. 2.9 Ga Mesoarchean magmatic rocks. These include gneissic diorite, gneissic quartz diorite, gneissic tonalite and gneissic high-Si trondhjemite and were identified in five new areas. All these rocks contain ca. 2.5 Ga metamorphic zircons. The dioritic and tonalitic rocks are enriched in large ion lithosphile (LIL) elements (K, Rb, Ba) and depleted in Nb and P with moderate to highly fractionated REE patterns, similar in chemical compositions to Archean TTG rocks worldwide. The high-Si trondhjemites are characterized by very high SiO 2 and very low FeOt + MgO. The magmatic zircon grains from all these rocks are similar in their Hf isotopic compositions and have ɛ Hf ( t ) values ranging from + 1.3 to +9.9. Combined with previous work, our main conclusions are: (1) eastern Shandong underwent a major magmatic event at ca. 2.9 Ga; (2) diorites and tonalites formed by melting of thickened lower crust or the root of an oceanic plateau, whereas the high-Si trondhjemites were derived from melting of intermediate to felsic rocks; (3) the Mesoarchean (ca. 2.9 Ga) was a major period of juvenile crustal addition from the depleted mantle, earlier than the major crustal growth period at 2.7–2.8 Ga in the North China Craton; (4) the 2.7–2.9 Ga rocks underwent strong metamorphism at ca. 2.5 Ga, probably as a result of magmatic underplating. [ABSTRACT FROM AUTHOR]

The North China Craton (NCC) is the largest craton on the Chinese Mainland, with 3.8–3.0 Ga rocks occurring mainly in three areas, namely: Anshan-Benxi (Anben), eastern Hebei, and Xinyang. In addition, 3.0 Ga and 3.1 Ga TTG (tonalite-trondhjemite-granodiorite) rocks have also been identified in north Liaoning and the Bohai Bay Basin, respectively. Detrital and xenocrystic zircons dated at 4.1–3.1 Ga occur more widely in the craton. In Anben, >3.3 Ga rocks are mainly distributed in six complexes, whereas in eastern Hebei, 3.8 Ga TTG rocks and 4.0–3.9 Ga detrital zircons have recently been discovered. Anben is similar to eastern Hebei in terms of the Archean zircon age records, but >3.3 Ga TTG rocks are different in both rock-type and composition. The recent identification of 3.8 Ga granitic xenoliths in young volcanic rock at Xinyang indicates the existence of Eoarchean rocks in the deep crust of the craton. Taking the NCC as a whole, the most significant zircon-forming magmatic events occurred at 3.8 Ga, 3.7–3.65 Ga, 3.45 Ga, 3.3 Ga and 3.1 Ga. The >3.0 Ga TTG rocks are mainly trondhjemite, with their low Sr/Y and La/Yb ratios indicating formation under low- to medium-pressure conditions. These are different from 3.0 to 2.5 Ga TTG rocks in the NCC and 3.8–3.0 Ga TTG rocks in southern West Greenland, which are dominated by tonalite, with some having high Sr/Y and La/Yb ratios. The >3.0 Ga rocks have large variations in whole-rock ε Nd (t) and in-situ zircon ε Hf (t), with many showing negative values. Zircons have δ18O values mainly between 5.0 and 7.0‰, consistent with Hadean and Archean magmatic zircon data worldwide. The diversity of Eoarchean granitoid magmatism across the craton indicates that continental evolution on Earth at that time already displayed complexity and maturity. It is further considered that the NCC may contain several different ancient continental nuclei with long-term formation and evolutionary histories back to the Hadean. [ABSTRACT FROM AUTHOR]

The accretion and recycling of Archean-Paleoproterozoic continental crust is the key to understanding the formation and evolution of early continental crust. Early Precambrian metamorphic basement is well developed in the Aktashtagh area, southeastern margin of the Tarim Craton, northwestern China. We report zircon SHRIMP U Pb dating, LA-MC-ICP-MS Lu Hf isotopic analysis and whole-rock geochemical analysis on 16 samples (mainly TTG, tonalite-trondhjemite-granodiorite) from Aktashtagh. Multiple episodes of early Precambrain magmatism, including 3.72 Ga, 3.63 Ga, 3.10 Ga, 2.83–2.69 Ga, 2.55–2.50 Ga, 2.05–2.02 Ga and 1.93–1.82 Ga events, have been identified in the Aktashtagh region. The most intense magmatism occurred in the late Mesoarchean to early Neoarchean. Significantly, 3.63 Ga and 3.10 Ga TTG rocks are identified, together with more extensive 2.8–2.7 Ga and ∼ 2.5 Ga TTG rocks. The TTG suites are dominated by tonalitic, with some trondhjemitic and grandioritic rocks. They show large variations in REE contents, are mainly distributed in the high- and medium-pressure fields in the Sr/Y-Y and La/Yb-Yb diagrams, and the 2.7 Ga TTG rocks commonly show higher and more variable Sr/Y and La/Yb ratios. The 3.63–3.10 Ga TTG rocks have magmatic zircon ε Hf (t) values of −3.9 to +3.1 and t DM1 (Hf) model ages of 3.9–3.6 Ga, whereas the ∼2.7–2.51 Ga TTG rocks have magmatic zircon ε Hf (t) values of −13.4 to +2.1 and t DM1 (Hf) model ages of 3.3–3.0 Ga. The ∼2.7 to ∼2.0 Ga meta-gabbroic-dioritic rocks have magmatic zircon ε Hf (t) values of −19.7 to +0.6 and t DM1 (Hf) model ages of 3.2–2.4 Ga. Metamorphic zircon ages of 3.56 Ga, 2.75–2.73 Ga, 2.53–2.51 Ga, 2.03–1.96 Ga and ∼ 1.85 Ga are also recorded, whereas ∼2.83 Ga, 2.53–2.40 Ga, 2.05–2.02 Ga and 1.86–1.83 Ga crustally-derived K-rich granites are identified, with the most important occurring in the late Neoarchean and late Paleoproterozoic. In this regard, the Tarim Craton is similar to the North China Craton in its early Precambrian geological evolution. The main difference is that the Paleoproterozoic tectonothermal event in the Tarim Craton started as early as 2.00 Ga, whereas in the North China Craton it occurred between 1.95 and 1.80 Ga. • Aktashtagh of the southeastern Tarim Craton has multiple episodes of magmatism from 3.72 to 1.82 Ga. • TTG show large Sr/Y-Y and La/Yb-Yb variations and magmatic zircons have ε Hf (t) of −15.2 to +3.1. • There were multiple Paleoarchaean to Paleoproterozoic events leading to crustal recycling. • The early Precambrian geological records are similar to those in the North China Craton. • The Tarim and North China cratons may be different portions of a larger craton. [ABSTRACT FROM AUTHOR]

Highlights: [•] ∼2.7 Ga rocks have been identified in eight areas of the North China Craton. [•] They are mainly tonalitic in composition and frequently underwent late alteration. [•] Their formation was a result of juvenile additions to the continental crust. [•] The North China Craton is similar in main period of crustal growth to other cratons. [ABSTRACT FROM AUTHOR]

• Cold avalanche vs classical subduction. • Mantle overturn and rise up of plumes. • Flat subduction of an oceanic plateau. • Hydrous partial melting of the oceanic crust under amphibolite facies conditions and the formation of a tonalitic melt. • Formation of quartz-diorite under granulite facies conditions by metasomatism and anatexis of the gabbro. Tonalite, trondhjemite, granodiorite (TTG) rocks in Viti Levu, Fiji Islands formed through hydrous partial melting of gabbroic oceanic crust at low-pressure amphibolite-facies conditions caused by flat subduction of an oceanic plateau from Yavuna creek during late-Eocene time. This was followed by the formation of a quartz diorite unit by anatexis of the gabbro in the mid-Miocene. The TTGs occur as migmatitic discordant stocks and dykes within the older gabbroic unit of the Yavuna Pluton. Zircon ages show the parental gabbro to be ∼ 47.5 Ma in age, whereas the TTGs, consisting of a tonalite and a quartz diorite suite, are ∼ 37.1 Ma and ∼ 16.5 Ma old, respectively. The average δ18O value of 4.8‰ in zircons from the gabbro and the tonalite suggest a homogenous mantle source. However, about 50% of the analyzed zircons from the gabbroic and tonalitic rock samples show lower δ18O values than this average, and are interpreted as reflecting interaction of hydrothermally altered seafloor with the deep mantle source. The zircon εHf of 13 in the analyzed TTGs is interpreted as reflecting typical juvenile continental crust. PerpleX whole-rock calculations supports that the tonalite formed by hydrous partial melting of the gabbro through decompression under water-saturated amphibolite-facies conditions at a temperature of ∼ 770 °C at ∼ 3.8 kbar, whereas the quartz diorite formed by anatexis under granulite-facies conditions at ∼ 800 °C at ∼ 1.7 kbar at very shallow depth. Our investigation provides new evidence that the tonalite formed by hydrous partial melting and the quartz diorite by metasomatism and anatexis of an extant gabbroic rock/crust at < 0.1 Ga in the South Pacific region. [ABSTRACT FROM AUTHOR]