Addition Of Juvenile Crust Research Articles

Provenance of the Paleoproterozoic Hutuo Group basal conglomerates and Neoarchean crustal growth in the Wutai Mountains, North China Craton: Evidence from granite and quartzite pebble zircon U-Pb ages and Hf isotopes

Zircon U-Pb ages (SHRIMP and LA-ICPMS) and Lu-Hf isotope data (LA-ICPMS) are presented for two granite and two quartzite pebbles from the basal conglomerates of the Sijizhuang Formation in the Hutuo Group from the Wutai Mountains area in the North China Craton. These two granite pebbles give zircon 207Pb/206Pb ages of 2513±8 Ma and 2527±8 Ma respectively, which are consistent with the emplacement ages of the Wangjiahui grey granite and Guangmingsi or Shifo granite in the Wutai Mountains. Detrital zircons from those two quartzite pebbles are mostly 2550–2490 Ma old with lesser number of 2800–2550 Ma grains, which is similar to the ages of detrital zircons from quartzites in the Gaofan Subgroup of the Neoarchean Wutai Group. Thus, the pebbles in the Hutuo Group basal conglomerates were derived locally from Wutai Mountains Neoarchean sources. Zircons from the Sijizhuang Formation conglomerate granite and quartzite pebbles mostly have positive ɛ Hf(t) values, a minority with ɛ Hf(t) values like model depleted mantle (DM) of the same age, but with most showing DM model ages 200-100 Ma. This indicates that most of the source materials were derived from the mantle within the previous 200 million years, whereas some are derived from 2550-2510 Ma juvenile crustal additions. This additional evidence suggests that in the North China Craton there was important initial polycyclic crustal formation and cratonization in the late Neoarchaean, prior to superimposed Palaeoproterozoic orogenic cycles.

Petrogenesis of the Xihuashan Granite in Southern Jiangxi Province, South China: Constraints from Zircon U‐Pb Geochronology, Geochemistry and Nd Isotopes

Abstract:Mesozoic granitic intrusions are widely distributed in the Nanling region, South China, Yanshanian granites are closely connected with the formation of tungsten deposits. The Xihuashan granite is a typical representative of tungsten‐bearing granite. The Xihuashan granite consists mainly of medium‐grained porphyritic biotite granite, medium‐grained biotite granite and fine‐grained two‐mica granite, which correspond to LA‐ICP‐MS zircon U‐Pb ages of 155.5±0.4 Ma, 153.0±0.6 Ma and 152.8±0.9 Ma, respectively. Rocks from the Xihuashan mining area displays high SiO2 (73.85% to 76.49%) and Na2O+K2O contents (8.09% to 9.43%), belonging to high‐K calc‐alkaline series. They are metaluminous to weakly peraluminous with A/CNK values ranging from 0.96 to 1.06. All granites in this study area are rich in Rb, Th, U and Pb, and depleted in Ba, Sr, P, Ti, Nb and Eu, especially depleted in medium‐grained biotite granite and fine‐grained two‐mica granite. The medium‐grained porphyritic biotite granites usually have high LREE concentrations, whereas medium‐grained biotite granite and fine‐grained two‐mica granite displays high HREE contents. Our geochemical data reveal that the studied rocks are highly fractionated I‐type granite. The magma underwent strong magma differentiation with decreasing temperature and increasing oxygen fugacity, which may explain the formation of three types of distinct granites. Variations of Rb, Sr and Ba concentrations in different type granites were controlled by fractional crystallization of biotite and feldspar. Fractional crystallization of monazite, allanite and apatite resulted in LREE changes in granite, and formation of garnet mainly caused HREE changes. Granites from the Xihuashan mining area have relatively high εNd(t) values (–9.77 to –11.46), indicating that they were probably generated by partial melting of underlying Proterozoic metasedimentary rocks with minor addition of juvenile crust or mantle‐derived magmas.

Major geological cycles substantiated by U–Pb ages and εHfi of detrital zircon grains from the Lower Rhine Basin

Detrital zircon grains of the Cenozoic Lower Rhine Basin were dated by the U–Pb method and simultaneously analyzed for initial Hf signatures to (1) identify distinct geodynamic events recorded in the W-European basement and cover rocks and (2) estimate the addition of juvenile crust in relation to these. Approximately 350 zircon grains extracted from 4 Miocene to Quaternary sand samples were investigated by back-scattered electron (BSE) and cathodoluminescence (CL) imaging. One third of these zircons were found to be sufficiently homogeneous for laser ablation U–Pb dating, which was performed by two individual and spatially well separated laser-spot analyses on the same grain, substantiating zircons have identical ages for the two intra-grain analyses. Approximately 40 zircons showed different ages. Our U–Pb dating results document distinct periods of primary magmatic zircon growth between 2.7 and 0.03Ga. These ages correspond to worldwide known subduction–collision cycles. The oldest terrains exposed today along the Rhine River and the Alps belong to the Variscan cycle. Therefore, all zircon grains older than ca. 0.4Ga must have been extracted from Paleozoic or Mesozoic–Cenozoic cover rocks containing detrital grains inherited from pre-Variscan basement; only the Cenozoic zircons require direct transport to the basin.From the dated homogeneous zircons 34 grains were selected for Hf isotope analysis, showing large variations in initial Hf isotope ratios (εHfi) between −10.9 and +15.2. Twelve of the 13 Precambrian grains yield positive or zero εHfi values, with a single 1025-Ma old grain having a negative value of −10.9. For the 21 zircon grains younger than 0.6Ga, a series of 9 grains yield positive or zero εHfi, whereas the remaining 12 zircons have negative values as low as −5.8. Approximately 60% of the zircon crystals reflect addition of juvenile crust having positive or zero εHfi being extracted from the mantle around the time indicated by their U–Pb ages. For some cases, juvenile crust addition may be somewhat older than indicated by the U–Pb ages because zircon growth may occur during later differentiation of rocks originally devoid of zircon. For such cases, εHfi values are necessarily less positive than that of the juvenile magma, depending on the Lu/Hf and crustal residence times of the zircon-free rock. A few εHfi values between +11.4 and +15.2 corroborate the existence of highly fractionated, Lu-enriched and Hf-depleted asthenospheric mantle in Mid-Proterozoic times. In agreement with earlier detrital zircon studies, our new data demonstrate that crustal recycling during continent collision events significantly increased in Phanerozoic times, being relatively rare during Precambrian times where intra-plate plume activity and oceanic arc attachment were dominant.

Continental growth and secular evolution: Constraints from U-Pb ages and Hf isotope of detrital zircons in Proterozoic Jixian sedimentary section (1.8–0.8 Ga), North China Craton

In order to understand the Precambrian crustal growth and reworking history of the North China Craton, in situ U-Pb and Lu-Hf isotope analyses were carried out on the detrital zircons from a well-preserved, nearly undeformed Proterozoic sedimentary section with time interval of 1.8–0.8 Ga. Totally, 806 zircons separated from 7 samples collected from each representative stratigraphic unit were measured for U-Pb and Lu-Hf isotopic compositions, of which 430 grains yielded concordant U-Pb ages. The concordant zircons show three prominent age groups of 1.6–1.8 Ga, 1.9–2.2 Ga and 2.3–2.6 Ga. Both positive and negative ɛHf( t) values were exhibited by zircons from all groups, suggesting both juvenile crust addition and crustal reworking for all age groups. The Hf crustal model ages of zircons reveal that the juvenile materials were continuously added to the crust between 4.0 and 2.0 Ga with a prominent peak at 2.7–3.0 Ga and a less significant peak at 3.1–3.4 Ga. The juvenile contributions to the newly formed crust decrease stepwise through time, and crustal reworking has dominated over juvenile inputs to the continental crust since 2.6 Ga. Compared with the detrital zircons from modern rivers running within the North China Craton, the sedimentary rocks contain more older zircons, implying the relatively poor preservation of Archean crustal rocks in modern river sand, and accordingly the weights of Archean crystallizations ages and crustal growth inferred from detrital zircons in modern river are significantly underestimated. The integrated studies of detrital zircons from both modern river sands and ancient sedimentary rocks are essential to decipher the early crustal growth history more faithfully.

Coupled U–Pb–Hf of detrital zircons of Cambrian sandstones from Morocco and Sardinia: Implications for provenance and Precambrian crustal evolution of North Africa

U–Pb–Hf of detrital zircons from diverse Cambrian units in Morocco and Sardinia were investigated in order to clarify the sandstone provenance and how it evolved with time, to assess whether the detrital spectra mirror basement crustal composition and whether they are a reliable pointer on the ancestry of peri-Gondwanan terranes. Coupled with Hf isotopes, the detrital age spectra allow a unique perspective on crustal growth and recycling in North Africa, much of which is concealed below Phanerozoic sediments.In Morocco, the detrital signal of Lower Cambrian arkose records local crustal evolution dominated by Ediacaran (0.54–0.63Ga) and Late-Paleoproterozoic (1.9–2.2Ga; Eburnian) igneous activity. A preponderance of the Neoproterozoic detrital zircons possess positive εHf(t) values and their respective Hf model ages (TDM) concentrate at 1.15Ga. In contrast, rather than by Ediacaran, the Neoproterozoic detrital signal from the Moroccan Middle Cambrian quartz-rich sandstone is dominated by Cryogenian-aged detrital zircons peaking at 0.65Ga alongside a noteworthy early Tonian (0.95Ga) peak; a few Stenian-age (1.0–1.1Ga) detrital zircons are also distinguished. The majority of the Neoproterozoic zircons displays negative εHf(t), indicating the provenance migrated onto distal Pan-African terranes dominated by crustal reworking. Terranes such as the Tuareg Shield were a likely provenance. The detrital signal of quartz–arenites from the Lower and Middle Cambrian of SW Sardinia resembles the Moroccan Middle Cambrian, but 1.0–1.1Ga as well as ~2.5Ga detrital zircons are more common. Therefore, Cambrian Sardinia may have been fed from different sources possibly located farther to the east along the north Gondwana margin. 1.0–1.1Ga detrital zircons abundant in Sardinia generally display negative εHf(t) values while 0.99–0.95Ga detrital zircons (abundant in Morocco) possess positive εHf(t), attesting for two petrologically-different Grenvillian sources. A paucity of detrital zircons younger than 0.6Ga is a remarkable feature of the detrital spectra of the Moroccan and Sardinian quartz-rich sandstones. It indicates that late Cadomian orogens fringing the northern margin of North Africa were low-lying by the time the Cambrian platform was deposited. About a quarter of the Neoproterozoic-aged detrital zircons in the quartz-rich sandstones of Morocco (and a double proportion in Sardinia) display positive εHf(t) values indicating considerable juvenile crust addition in North Africa, likely via island arc magmatism. A substantial fraction of the remaining Neoproterozoic zircons which possess negative εHf(t) values bears evidence for mixing of old crust with juvenile magmas, implying crustal growth in an Andean-type setting was also significant in this region.

Mafic granulite xenoliths from the East Indian Shield: Evidence for recycled continental crust in the Archean mantle

Massif-type charnockites of the Eastern Ghats granulite terrain, India, abound in mafic enclaves, which are generally hornblende mafic granulites with relatively minor occurrences of pyroxenite enclaves in the marginal segments only. The mafic granulite enclaves may be interpreted as earlier mafic melts within plutonic charnockite, where prograde heating in the hornblende in these mafic granulite enclaves was probably due to the host charnockite crystallization. Pyroxenite enclaves, on the other hand, are likely to be cumulates from an episode of mafic magmatism. The trace-element characteristics of hornblende–mafic granulite xenoliths are akin to arc-derived basalt, indicating a tectonic setting of subduction and slab melting. Further, low values of primitive mantle–normalized Nb/U ratios and enriched radiogenic isotopic compositions in the mafic xenoliths clearly indicate recycled continental crust in the mantle source region. While most of the internal segments of the Eastern Ghats mobile belt are Paleoproterozoic domains, the marginal (cratonic) segments in the north and west are Archean domains. The average Nd model age of ca. 2.5 Ga for the protoliths of hornblende–mafic granulite xenoliths for the Paleoproterozoic domains may be interpreted as the age of arc magmatism. For the marginal segments, the average Nd model age of ca. 3.3 Ga probably represents earlier Archean arc magmatism. Initial 87 Sr/ 86 Sr ratios calculated at these periods of mafic magmatism are high, which, together with negative e Nd values calculated for 2.5 and 3.3 Ga mafic magmatism, indicate recycled continental crust in their mantle source region. However, juvenile crustal addition seems to have been significant at 2.5 Ga, as is evident from the positive e Nd values for the majority of samples representing 2.5 Ga magmatism.

Age and growth of the Archean Kongling terrain, South China, with emphasis on 3.3 ga granitoid gneisses

The North China craton and the Yangtze craton (South China) both contain Archean rocks in eastern China. Unlike the North China craton, where Archean rocks are widespread, in the Yangtze craton the exposed Archean rocks are only known in the Kongling terrain (360 km²). Zircon U-Pb ages and Lu-Hf isotopic compositions of three granodioritic-trondhjemitic gneisses and three metasedimentary rocks from the Kongling terrain were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Igneous zircons in one trondhjemitic gneiss in the north of the Kongling terrain have an age of 3302±7 (1σ) Ma. Evidence from cathodoluminescence imaging, variations in Th/U and degree of U-Pb age discordance suggest that apparently younger zircons in the same population are variably disturbed 3302 Ma grains. Thus, this trondhjemitic gneiss is the oldest known rock in South China and predates the earlier reported ∼2900 Ma granitoid magmatism by 400 Ma. Zircon cores from one granodioritic gneiss in the north of the Kongling terrain also give a concordant age group at 3200 to 3300 Ma. Regardless as inherited or not, these cores crystallized from a magma indistinguishable in age with the trondhjemite. Concordant U-Pb ages for igneous zircons in one granodioritic gneiss in the south of the Kongling terrain yielded a weighted average ²⁰⁶Pb/²⁰⁷Pb age of 2981±13 Ma (2σ, MSWD=9.7, n=21). The zircon age and initial Hf isotopic compositions are similar to those of widespread granitoid gneisses from the north of the Kongling terrain (2903-2947 Ma), and indicate that the south and north of the Kongling terrain are correlative. The results also reinforce that magmatism of the whole Kongling terrain mainly occurred at 2900 Ma. Available Hf isotopic data from the Kongling terrain show that juvenile crustal additions occurred mainly between 3150 and 3800 Ma with a significant peak at 3300 to 3500 Ma. The ∼3300 Ma zircons from the trondhjemitic gneiss have Hf crust formation ages of 3450 to 3730 Ma, some of which have nearly chondritic ε_Hf (t). The whole-rock depleted mantle Nd model age of this rock is 3400 Ma, close to its age of magmatism and consistent with the Hf model age. Its ε_Nd value at 3300 Ma is nearly chondritic (1.26). These lines of evidence suggest that the 3300 Ma trondhjemite represent juvenile crust additions to the pre-existing continental crust.

∼2.7 Ga juvenile crust formation in the North China Craton (Taishan-Xintai area, western Shandong Province): Further evidence of an understated event from U–Pb dating and Hf isotopic composition of zircon

The evolution of the North China Craton (NCC) is well known for a marked 2.55–2.50Ga tectonothermal event. However, supracrustal and intrusive rocks of 2.75–2.70Ga are in fact widely distributed in the western Shandong Province, the most important area of Archaean basement in the eastern part of the NCC. This paper reports SHRIMP U–Pb dating and LA-ICPMS Hf isotopic composition of zircons from 2.75–2.70Ga supracrustal and trondhjemite–tonalite–granodiorite (TTG) rocks in that area. Three fine-grained (hornblende) biotite gneiss samples (known locally as leptynite, with meta-volcanic or volcanosedimentary rock protoliths) and five TTG samples have SHRIMP zircon U–Pb ages varying from 2.75 to 2.70Ga and 2.74 to 2.71Ga, respectively. Zircons from most of the samples have high positive ɛHf(t) values (+4.7 to +10.0) and tDM(Hf) ages (2.85–2.60Ga) similar to their zircon U–Pb ages. This indicates that the rocks represent largely juvenile crustal additions derived from depleted mantle only a short time before. However, some granitoids show ɛHf(t) zircon values of −13.6 to +5.1 and tDM(Hf) of 3.51–2.80Ga. Therefore, the strong 2.75–2.70Ga tectonothermal event in the western Shandong Province involved not only juvenile addition to the continental crust but also intracrustal recycling of older components. Combined with craton-wide data, it is shown that the NCC is similar to many other cratons around the world where tectonothermal events of ∼2.7Ga are well developed. However, the main difference is that in the NCC, superimposed ∼2.5Ga tectonothermal events were much stronger than in most other cratons.

Permian-Triassic (260-220 Ma) crustal growth of Eastern Central Asian orogenic belt as revealed by detrital zircon studies

Present global compilations of ages and isotopic data suggest insignificant crustal growth after 450 Ma. Previous Nd isotopic studies of whole rocks from the Central Asian Orogenic Belt suggest large volumes of juvenile crustal additions in the Phanerozoic. To test this, we studied detrital zircons from the Xinglonggou Formation of western Liaoning at the northern margin of the North China craton, which was deposited after the collision between the Siberian Plate and the North China craton to form this part of the Central Asian Orogenic Belt. Zircons from the Xinglonggou sandstones are characterized by two major groups of U-Pb ages (2.6-2.4 Ga and 260-220 Ma) except for three grains (1.9-1.6 Ga). The 2.6 to 2.4 Ga zircons have positive eHf (t) values up to coeval depleted mantle value, which suggest juvenile crustal addition. The results are consistent with existing data for the North China craton (NCC). eHf (t) values of 260 to 220 Ma zircons range widely from −15.4 to 13.3. While zircons with the negative eHf (t) values are similar to igneous zircons from intrusive rocks of the North China craton and indicate recycling of ancient continental crustal materials, those with the positive values and young model ages point to a significant period of crustal generation with the source provenance from the eastern Central Asian Orogenic Belt (CAOB) to the north. Mixing of detritus from both the North China craton and the eastern Central Asian Orogenic Belt suggests the ca. 250 Ma closure of the Paleo-Asian ocean and collision between the North China craton and the Siberian Plate along the eastern Solonker zone. The youngest zircons constrain uplift of the eastern Central Asian Orogenic Belt to be no older than 208 Ma. Thus, the 260 to 220 Ma crustal growth is related to the subduction of the Paleo-Asian ocean Plate along the Solonker suture, which records the termination of the Central Asian Orogenic Belt. The Solonker suture occupies an area of 700 km long and 60 km wide and extends from Solonker via Sonid Yuoqi to Linxi in Inner Mongolia and further west and northeast. This implies extensive juvenile crustal additions during this period. It may change our present views of Phanerozoic crustal growth.

Hannuoba intermediate-mafic granulite xenoliths revisited: Assessment of a Mesozoic underplating model

The intermediate-mafic granulite xenoliths from Tertiary Hannuoba alkali basalts at the northern margin of the North China Craton have been commonly considered to be formed by Mesozoic basaltic underplating due to their abundant Mesozoic zircons and compositionally higher Mg# (molecular Mg/(Mg+Fe)) than rocks of the adjourning Archean granulite terrain. However, such a Mesozoic underplating model cannot account for the presence of Precambrian zircons in the xenoliths and their evolved Sr–Nd isotopic compositions which are dissimilar to expected basaltic melts from both the lithospheric and asthenospheric mantle. Instead, their Sr–Nd isotopic compositions resemble those of the granulite terrain and of the Mesozoic low Mg# granitoids. Here newly acquired data and published data are combined to assess the Mesozoic basaltic underplating model and to discuss the origin of the Hannuoba intermediate-mafic granulite xenoliths in terms of their relationships with the Mesozoic magmatism and the granulite terrain. Zircons from a mafic granulite xenolith (JN0724), which has geochemical and Sr–Nd isotopic compositions indistinguishable from other mafic granulite xenoliths, yield ∼2.5Ga U–Pb ages, providing unambiguous evidence that the mafic xenoliths represent portions of ancient lower crust. Combined data reveals that the pyroxene-rich granulite xenoliths can be best explained as restites left after partial melting of the ancient lower crust to produce the Mesozoic low Mg# granitoids. Hf isotope analysis on Mesozoic zircons from the Hannuoba feldspar-rich granulite xenoliths demonstrates that the zircons crystallized from Mesozoic granitic magmas that were derived from ancient lower crust rather than from Mesozoic underplated magmas. The proposed scenarios successfully explain the age and compositional differences between granulite xenoliths and the granulite terrain and also provide new insights into derivation and evolution of granitic magmas. It is concluded that most of the Hannuoba intermediate-mafic granulite xenoliths and the Mesozoic granitoids reflect reworking of ancient lower crust rather than juvenile crust addition. It provides strong evidence that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust. Our data imply that whether younger zircon ages in granulite xenoliths from many other Archean cratons register post-Archean basaltic underplating could be tested by integrated analysis of U–Pb age and Hf isotopes.

Timing of crust formation and recycling in accretionary orogens: Insights learned from the western margin of South America

Abstract Accretionary orogens are considered major sites of formation of juvenile continental crust. In the central and southern Andes this is contradicted by two observations: siliciclastic fills of Paleozoic basins in the central Andean segment of the accretionary Terra Australis Orogen consist almost exclusively of shales and mature sandstones; and magmatic rocks connected to the Famatinian (Ordovician) and Late Paleozoic magmatic arcs are predominantly felsic and characterized by significant crustal contamination and strongly unradiogenic Nd isotope compositions. Evidence of juvenile crustal additions is scarce. We present laser ablation (LA)-ICPMS U–Pb ages and LA-MC-ICPMS Hf isotope data of detrital zircons from seven Devonian to Permian turbidite sandstones incorporated into a Late Paleozoic accretionary wedge at the western margin of Gondwana in northern Chile. The combination with Nd whole-rock isotope data permits us to trace the evolution of the South American continental crust through several Proterozoic and Paleozoic orogenic cycles. The analyzed detrital zircon spectra reflect all Proterozoic orogenic cycles representing the step-wise evolution of the accretionary SW Amazonia Orogenic System between 2.0 and 0.9 Ga, followed by the Terra Australis Orogen between 0.9 and 0.25 Ga. The zircon populations are characterized by two prominent maxima reflecting input from Sunsas (Grenville) age magmatic rocks (1.2–0.9 Ga) and from the Ordovician to Silurian Famatinian magmatic arc (0.52–0.42 Ga). Grains of Devonian age are scarce or absent from the analyzed zircon populations. The Hf isotopic compositions of selected dated zircons at the time of their crystallization (eHf(T); T = 3.3–0.25 Ga) vary between − 18 and + 11. All sandstones have a significant juvenile component; between 20 and 50% of the zircons from each sedimentary rock have positive eHf(T) and can be considered juvenile. The majority of the juvenile grains have Hf-depleted mantle model ages (Hf TDM) between 1.55 and 0.8 Ga, the time of the Rondonia–San Ignacio and Sunsas orogenic events on the Amazonia craton. The corresponding whole-rock eNd(T) values fot these same rocks are between − 8 and − 3 indicating a mixture of older evolved and juvenile sources. Nd-depleted mantle model ages (Nd TDM⁎) are between 1.5 and 1.2 Ga and coincide broadly with the zircon Hf model ages. Our data indicate that the Paleo- and Mesoproterozoic SW Amazonia Orogenic System, and the subsequent Neoproterozoic and Paleozoic Terra Australis Orogen in the region of the central and southern Andes, developed following two markedly different patterns of accretionary orogenic crustal evolution. The SW Amazonia Orogenic System developed by southwestward growth over approximately 1.1 Ga through a combination of accretion of juvenile material and crustal recycling typical of the extensional or retreating mode of accretionary orogens. In contrast, the central Andean segment of the Terra Australis Orogen evolved from 0.9 to 0.25 Ga in the compressional or advancing mode in a relatively fixed position without the accretion of oceanic crustal units or large scale input of juvenile material to the orogenic crust. Here, recycling mainly of Mesoproterozoic continental crust has been the dominant process of crustal evolution.

The Musgrave Province: Stitching north, west and south Australia

The Musgrave Province in central Australia is an east-west trending ∼120,000 km 2 region composed of lithologic units that range in age from earliest Mesoproterozoic to Neoproterozoic. It is one of the few terrains in Australia that records significant Grenvillian-aged tectonism, and has a distinctly younger geological history than the surrounding fragments of Proterozoic Australia. Compared to other Proterozoic terranes of Australia the evolution of the Musgrave Province is poorly understood. The region contains evidence for the development of earliest Mesoproterozoic arc-related magmatism, with major juvenile crustal additions comprising intercalated felsic and mafic gneisses with protolith ages ranging from ca. 1.60 to 1.54 Ga. This period of arc magmatism was followed by closure of the ocean separating the North Australian Craton (NAC) from the South Australian Craton (SAC) at ca. 1.54 Ga. Arc magmatism and collision was followed by a period of extension during which protoliths to the metasedimentary rocks of the eastern Musgrave Province were deposited at ca. 1.4 Ga. The combined juvenile Nd isotopic signature and detrital zircon ages from these metasediments suggests provenance from non-Australian sources, with possible sources originating from Laurentia. Granitic magmatic events occurred at ca. 1.3 and 1.20–1.14 Ga, the latter comprising the voluminous A-type granites of the Pitjantjatjara Supersuite. The intrusion of these magmatic units provide key temporal constraints on deformational events, and are coeval with the voluminous A-type suites of Laurentia which would invite correlation of Australia with Laurentia at ca. 1.30–1.14 Ga. These magmatic episodes are accompanied by deformation at granulite facies at ca. 1.30 Ga (unnamed event), and 1.23–1.15 Ga (Musgrave Orogeny). These deformation/metamorphic events have completely recrystallised any primary layering within the pre 1.30 Ga rocks, forcing interpretations on their genesis to be inferred from geochemical and isotopic data. The tectonic significance of these events is not clear, however they may form part of a larger Grenville-aged deformational belt extending into the western Australian Albany-Fraser Province and Antarctica. Emplacement of the voluminous mafic–ultramafic Giles Complex and associated granulite facies metamorphism of the Giles Event occurred at ca. 1.078 Ga. The layered intrusions of the Giles Complex are generally fault bound, and are considered to represent discrete bodies emplaced in an extensional regime associated with generation of the Warakurna Large Igneous Province. The ca. 1.08 Ga Alcurra Dolerite, unnamed ca. 1.0 Ga olivine dolerites, and ca. 0.8 Ga Amata Dolerite dissect much of the Musgrave Province. The Alcurra Dolerite and unnamed olivine dolerite dykes have been proposed as originating from a modified sub-continental lithospheric mantle, whereas the Amata Dolerite is proposed to be plume related, coinciding with the initiation of rifting creating the Centralian Superbasin. An earliest Cambrian intraplate transpressional event is recorded in the ca. 550 Ma Petermann Orogeny. This resulted in deep crustal exhumation of much of the northern Musgrave Province from beneath the Centralian Superbasin, with mylonitic deformation recorded at pressures of ∼12 kbar and temperatures of 700–750 °C.

Precambrian crustal growth of Yangtze Craton as revealed by detrital zircon studies

Clastic sedimentary rocks are natural samples of the exposed continental crust over large areas. The age and evolution of the Yangtze craton is not well known, because much of the craton is covered by thick Nanhua (850 –635Ma), Sinian (635 –543Ma) and Phanerozoic sedimentary rocks. The Nanhua clastic sedimentary rocks including tillites provide ideal samples for studies of age and evolution of the craton. 1130 zircons in 8 sandstone and tillite samples of three Nanhua formations (Liantuo, Gucheng and Nantuo) from the Yangtze Gorges area were dated by LA-ICP-MS. 251 of the 711 concordant zircons were analyzed for Hf isotopic compositions by LA-MC-ICP-MS. The results reveal four major age groups of 720 to 910 Ma, 1.90 to 2.05 Ga, 2.40 to 2.55Ga and 2.60 to 2.70 Ga with few grains >3.2Ga. Although Hf isotopic compositions show both juvenile crustal growth and reworking of old crust for all the age groups, the Paleoproterozoic is a period of prominent crustal reworking with negative eHf(t) values. The Neoproterozoic is a period of significant juvenile crustal additions, accounting for 68 percent of zircons with positive eHf(t) values similar to those of the depleted mantle. Crustal additions at 3.2 to 3.8Ga are also significant, as indicated by the zircon Hf continental model ages. Our results highlight the importance of analysis of a sufficient number of zircons for provenance studies. In addition, our results illustrate that possible temporal and spatial provenance variations have to be taken into account for better characterizing formation and evolution of the related continental crust. Our obtained youngest age for each formation shows a strong negative correlation with stratigraphic height. Youngest ages provide good constraints on the maximum ages of the three Nanhua formations: ≤770Ma, ≤733Ma and ≤704Ma for the base, middle unit and top of the Liantuo Formation, respectively; ≤703Ma for the Gucheng Formation and possibly ≤600 to 606Ma for the Nantuo Formation. Although the North China and Yangtze cratons show some apparently similar zircon ages, the two cratons have distinct histories of formation and evolution. While the North China craton is dominated by Archean crustal growth, the Yangtze craton is characterized by prominent crustal additions in the Neoproterozoic, which is almost lacking in the North China craton.

Mesozoic decratonization of the North China block

Research Article| June 01, 2008 Mesozoic decratonization of the North China block Jin-Hui Yang; Jin-Hui Yang * 1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China *E-mail: jinhui@mail.igcas.ac.cn. Search for other works by this author on: GSW Google Scholar Fu-Yuan Wu; Fu-Yuan Wu 1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China Search for other works by this author on: GSW Google Scholar Simon A. Wilde; Simon A. Wilde 2Department of Applied Geology, Curtin University of Technology, P.O. Box U1987, Perth, Western Australia 6845, Australia Search for other works by this author on: GSW Google Scholar Elena Belousova; Elena Belousova 3Australian Research Council's National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia Search for other works by this author on: GSW Google Scholar William L. Griffin William L. Griffin 3Australian Research Council's National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia Search for other works by this author on: GSW Google Scholar Geology (2008) 36 (6): 467–470. https://doi.org/10.1130/G24518A.1 Article history received: 17 Oct 2007 rev-recd: 13 Feb 2008 accepted: 15 Feb 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Jin-Hui Yang, Fu-Yuan Wu, Simon A. Wilde, Elena Belousova, William L. Griffin; Mesozoic decratonization of the North China block. Geology 2008;; 36 (6): 467–470. doi: https://doi.org/10.1130/G24518A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Cratons are old, stable parts of the continental crust that have survived at least since Proterozoic time and have not undergone strong magmatism or tectonism since their stabilization. Traditionally, the growth of continental crust in the North China craton is considered to have been complete by Paleoproterozoic time, Phanerozoic events being largely restricted to surrounding orogenic belts. However, the eastern part of the North China craton contains large volumes of Mesozoic igneous rocks, with widespread metamorphic core complexes and pull-apart basins. Hf isotope compositions of magmatic zircon grains from igneous rocks in the Liaodong Peninsula indicate that widespread late Mesozoic granitoids formed by partial melting of ancient crust, but with significant input of a mantle component via magma mixing and crustal assimilation. This magmatism has resulted from removal and modification of lithospheric mantle, accompanied by asthenospheric upwelling. The Hf isotopic signatures thus record the addition of juvenile crust beneath the eastern part of the North China craton, which appears related to major extension and possibly slab rollback of the Pacific plate. Whatever the mechanism, it is apparent that since ca. 200 Ma, ancient lithosphere beneath the eastern North China craton has been progressively reactivated and replaced, resulting in “decratonization.” Tertiary to Holocene volcanism in the area and major subsidence around the Gulf of Bohai suggest that the effects of the process are continuing. Similar processes probably operated in the geological past, leading to significant modification of continental crust and requiring reconsideration of mechanisms for continental breakup and dispersal and of continental growth rates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Proterozoic history and crustal evolution in southwestern Colorado: Insight from U/Pb and Sm/Nd data

New U/Pb age constraints and Sm/Nd isotope data further establish the timing of magmatic and tectonic events from 1.8 to 1.4 Ga in southwestern Colorado, and give insight into the interaction of crust and mantle systems during the assembly and evolution of orogenic belts along the southern margin of Laurentia. Proterozoic orogenic events in the Needle Mountains led to crustal assembly, thickening and tectonic reworking that was punctuated by periods of relative crustal stability. Pre-1.75 Ga arc successions in southwestern Colorado were produced and assembled into juvenile crust in an orogenic period that culminated in regional deformation and metamorphism that accompanied intrusion of synorogenic plutonic rocks from 1.73 to 1.71 Ga. Emplacement of post-orogenic plutonic rocks at ∼1.70 Ga was followed by uplift and erosion, and the deposition and deformation of siliciclastic sedimentary sequences. Paleoproterozoic crust underwent local melting and thermal metamorphism during invasion of felsic to mafic magmas at ∼1.43 Ga that coincided with a pulse of intracratonic deformation. Invasion and melting of first-cycle 1.8–1.7 Ga crust at ∼1.71 and ∼1.43 Ga involved heat and volatiles from mantle melts that were a catalyst for crustal melting and magmatic recycling. Sm/Nd isotopic signatures of 1.8–1.4 Ga volcanic and plutonic rocks in southwestern Colorado lend evidence to the emplacement of melts from depleted mantle and juvenile crust in the production continental crust, with minor, if any, contamination by crust older than 1.8 Ga. The Proterozoic magmatic record in southwestern Colorado is consistent with continental growth by the addition of juvenile crust extracted from the mantle, followed by periodic intracrustal growth during orogenic pulses in which mantle magmas underplate and melt the crust with consequent thickening.

Complex 3670–3500 Ma Orogenic Episodes Superimposed on Juvenile Crust Accreted between 3850 and 3690 Ma, Itsaq Gneiss Complex, Southern West Greenland

The Itsaq Gneiss Complex of the Nuuk region, southern West Greenland, is dominated by 3850–3690 Ma tonalites intruded into and intercalated with lesser amounts of different ≥3850–3700 Ma supracrustal units. Published whole‐rock Sr and Nd isotopic studies demonstrate that the tonalites are juvenile crustal additions from a depleted mantle source. From our field studies and SHRIMP U/Pb zircon dating, we argue that this juvenile crustal accretion was spread over ca. 170 m.yr. (≥3850–3690 Ma). Following 3850–3690 Ma juvenile crustal accretion events, the evolution of the Itsaq Gneiss Complex continued with numerous crustal‐reworking events between 3670 and 3500 Ma. Examples of these reworking events are as follows: (1) there is intrusion of several generations of geochemically diverse granites (sensu stricto) with subordinate gabbros and diorites; (2) there are superimposed, multiple episodes of amphibolite to granulite facies metamorphism (illustrated with Akilia Island samples by detailed dating of metamorphic zircon overgrowths in several rocks); (3) part of the complex around Amiitsoq and the hills to the north contains the youngest tonalitic gneisses at ca. 3660 Ma, indistinguishable in age from the oldest recognized granites (sensu stricto) elsewhere; and (4) the hills north of Amiitsoq also contain tectonized remnants of volcanosedimentary basins that were still being filled between 3650 and 3600 Ma (from dating youngest detrital zircons) and that had been inverted and metamorphosed by ca. 3570 Ma (dating of oldest in situ metamorphic overgrowths). These 3670–3500 Ma events are interpreted as reflecting a complex orogen (tentatively collisional and/or strike slip) superimposed on the products of the earlier (3850–3690 Ma) juvenile crustal accretion regimes. Implications for early crustal evolution studies are discussed.

Geochemistry and petrogenesis of 270 Ma Ni–Cu–(PGE) sulfide-bearing mafic intrusions in the Huangshan district, Eastern Xinjiang, Northwest China: implications for the tectonic evolution of the Central Asian orogenic belt

The Central Asian orogenic belt (CAOB) is composed of remnants of microcontinents, island arcs and ophiolites. The presence of voluminous Phanerozoic A-type granites suggest that continental growth occurred through addition of juvenile crust, and the presence of mafic–ultramafic intrusions provides direct evidence for a mantle contribution. The origin of the mafic–ultramafic intrusions in the Huangshan area, Eastern Xinjiang, has been debated because of the lack of precise age data or trace element and isotopic analyses. We have studied the geology, petrology, geochemistry and geochronology of two of the intrusions: the Huangshanxi and Huangshandong intrusions. Both are typical layered mafic–ultramafic intrusions composed of diorite, troctolite,norite, gabbronorite, gabbro wehrlite, websterite and lherzolite with systematic geochemical variations that are consistent with the following crystallization sequence: Ol–(Chr), Ol–Pyx–(Chr), Cpx–Pl and Hbl–(Bt). Igneous zircons separated from a diorite in the Huangshanxi intrusion yield a 269±2 Ma SHRIMP U–Pb age. The rocks are enriched in Rb–Ba–U–K–Sr relative to Th and LREE, depleted in Nb–Ta relative to Th and exhibit positive εNd values (+6.7 to +9.3). Ni–Cu–(PGE) sulfide mineralization occurs in the lower parts of the intrusions, primarily strata-bound within wehrlite or lherzolite and locally stratiform along the margins of the intrusions. The parental magmas appear to have been derived from a mantle source that was previously contaminated by subduction of oceanic crust and to have been variably contaminated by upper continental crustal components. The intrusions appear to have formed by plume-related activity within a continental setting, likely related to post-orogenic extension following Carboniferous amalgamation of arc-related terranes in the CAOB. This plume activity was most likely also responsible for providing heat sources to generate large amounts of A-type granites across the CAOB.

Petrogenesis and significance of the Mesozoic North Taihang complex: Major and trace element evidence

The Mesozoic North Taihang complex consists mainly of intermediate rocks and leucogranites as well as minor, synplutonic enriched-mantle derived mafic rocks. The intermediate rocks show high-K calc-alkaline affinity, and high Sr (and Ba) and La/Yb ratios with negligible Eu anomalies, but are low in Y; these signatures are shared by the mafic rocks. The intermediate rocks were probably produced by magma mixing between the evolved enriched-mantle derived mafic magma and a crustal granitic magma, followed by fractional crystallization. The leucogranites show high SiO2 and low ferromagnesian contents; they probably represent residual melts produced from fractional crystallization of the intermediate rocks. Therefore, the North Taihang complex represents significant addition of juvenile crust in the Mesozoic. Mobilized by an upwelling asthenosphere, enriched portions of subcontinental lithospheric mantle experienced partial melting and were consumed significantly, resulting in the intensive magmatism within the North China Craton (NCC). This process is possibly an important mechanism responsible for the lithosphere thinning beneath the NCC. Although the Taihang (and the whole eastern China) granites display rare earth patterns similar to adakites, they appear to have no genetic link with the latter. The model of “eastern plateau’ proposed by some authors is not supported by our new data for the Taihang rocks.

Proterozoic evolution of the northeastern Canadian Shield: Lithoprobe Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT), introduction and summary

The Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT) has focused on studies of the processes by which the northeastern Canadian (Laurentian) Shield evolved, firstly by the 1.9–1.8 Ga coalescence of Archean cratons and then by the addition of juvenile crust from the south in the period 1.9–0.9 Ga. The transect area is a microcosm of continental accretion and also offers a unique opportunity to link the geological evolution of the Canadian Shield with once-adjacent Greenland.

Geochemistry and tectonic setting of Paleoproterozoic metavolcanic rocks of the southern Front Range, lower Arkansas River Canyon and northern Wet Mountains, central Colorado

Across a 5000-km 2 area of central Colorado, previously unstudied Paleoproterozoic metabasalts (amphibolites) and metarhyolites (felsic gneisses) comprise a bimodal metavolcanic association within a dominantly metasedimentary terrain. Extending southward from about 39° N latitude in the Southern Front Range to about 38° 15′ N latitude in the Wet Mountains, and from the mountain front westward to the Wet Mountain Valley and Pleasant Valley fault system, this area includes the exceptional exposures within the lower Arkansas River Canyon from Howard downstream to Canon City. Regional metamorphism from garnet to sillimanite grade, pervasive deformation, and intrusion by three generations of Proterozoic plutons have largely obscured original stratigraphic relationships and primary structures within these metamorphic basement rocks, although a few pyroclastic features persist locally within the felsic members. These metavolcanic rocks are compositionally similar to the much better preserved bimodal section in the Salida area, dated at 1728 ± 6 Ma by Bickford (1986), which emerges from beneath Paleozoic cover rocks about 15 km beyond the western edge of the area of this report. Geochemical studies of 45 samples (30 amphibolites and 15 felsic gneisses) delineate two groups of metavolcanic rocks ranging in silica content from 45–55% in one group and from 65 to almost 80% in the other. Along a 100-km transect from north to south, metavolcanic rocks of the lower-silica group (amphibolites) show an increase in total alkalies (from 2% t o 5–6%) and large ion lithophile trace elements as well as an increase in degree of enrichment in light rare earth elements (from La N /Lu N N /Lu N ∼5). Rocks with higher silica content (felsic metavolcanic rocks) occur mostly in the Arkansas Canyon area and contain 6–8% total alkalies; they show strong fractionation between light and heavy rare earth elements with moderate to pronounced negative europium anomalies. Tectonic discriminant diagrams using relatively immobile high-field-strength elements indicate volcanic arc settings for both mafic and felsic populations. Metavolcanic rocks from the northern Wet Mountains and Arkansas Canyon suggest a mature arc environment, possibly on an expanding continental margin. The isolated metabasalts to the north in the southern Front Range, where no felsic metavolcanic rocks have been identified, are more primitive island-arc tholeiites; they may represent pyroclastic rocks with a source beyond the study area. These new data from a wide area of central Colorado reinforce results from the well-studied Paleoproterozoic bimodal are assemblages to the west near Salida and Gunnison. They also allow the extension across a wider geographic area of previous tectonic models for the Paleoproterozoic evolution of the Colorado province (as defined by Bickford et al., 1986). These models (Condie, 1986; Reed et al., 1987; Karlstrom et al., 1987) portray the rapid addition of juvenile crust to the southern margin of the Wyoming province by accretion of individual volcanic arcs or larger, previously amalgamated are terranes, resulting in the southward expansion of the craton by about 1300 km from 1800–1650 Ma.