Lower Paleozoic detrital zircon U-Pb geochronological characteristics of the Baoshan Block in western Yunnan and its constraints on the Proto-Tethys tectonic evolutionSCIEI北大核心CSCD

滇西保山地块是东特提斯构造域的主要微陆块之一,但对其物源和古地理位置仍存在较大争议。本文通过对保山地块西缘早古生代地层进行碎屑锆石U-Pb定年来约束其物源及古地理位置,并进一步探讨原特提斯洋早古生代构造演化模式。保山地块西缘早古生代地层具有相似的年龄分布模式,主年龄峰期为-0.95Ga、次级年龄峰期为-1.2Ga和-2.5Ga。寒武系公养河群最小锆石年龄为526Ma,结合其上部年龄为499.2Ma的火山岩夹层,约束其沉积时代为早寒武世早期。对比保山地块不同区域早古生代地层的碎屑锆石年龄数据,它们都具有相似的锆石年龄分布模式和年龄峰值。-0.95Ga主年龄峰期和-2.5Ga的次级年龄峰期指示保山地块早古生代的沉积物主要来自于印度大陆,而-1.2Ga的次级年龄峰期表明有部分沉积物来自于西澳大利亚,其早古生代古地理位置位于印度和西澳大利亚之间。结合沉积学证据及滇西地区广泛发育的早古生代岩浆作用,本文认为早古生代冈瓦纳大陆北缘为活动大陆边缘。     

U–Pb ages and Hf isotopic record of zircons from the late Neoproterozoic and Silurian–Devonian sedimentary rocks of the western Yangtze Block: Implications for its tectonic evolution and continental affinity

The relationship of the Yangtze Block with other continental blocks of the Rodinia and Gondwana supercontinents is hotly debated. Here we report U–Pb and Lu–Hf isotopic data for zircons from the latest Neoproterozoic Yanjing Group and the overlying Silurian–Devonian rocks on the western margin of Yangtze Block, which provide critical constraints on the provenance of these sediments and further shed light on the crustal evolution and tectonic affinity of the western Yangtze Block in the context of Rodinia and the subsequent Gondwanaland. Mica schist from the middle part of the Yanjing Group contains dominant Neoproterozoic detrital zircons (0.72–0.80 Ga) with a pronounced age peak at 0.75 Ga. Based on the euhedral to subhedral shapes, high Th/U ratios and exclusively positive εHf(t) values (+ 6 to + 14) for the zircon crystals, and the lack of ancient zircons, we consider the sediments as products of proximal deposition near a Neoproterozoic subduction system in western Yangtze. Combined with the age of rhyolite from the lower part of the Yanjing Group, these strata were estimated to have been deposited in a period between 0.72 and 0.63 Ga. In contrast, the Silurian–Devonian sediments exhibit dominant Grenvillian ages (0.9–1.0 Ga), with middle Neoproterozoic (0.73–0.85 Ga), Pan-African (0.49–0.67 Ga) and Neoarchean (~ 2.5 Ga) age populations, suggesting a significant change of sedimentary provenance and thus a different tectonic setting. Although the shift occurred in the Silurian, the age spectra turn to be consistent along the western margin of the Yangtze Block until the Devonian, indicating persistence of the same sedimentary environment. However, the related provenance of these Paleozoic sediments cannot be found in South China. The presence of abundant Grenvillian, Pan-African and Neoarchean ages, along with their moderately to highly rounded shapes, indicates the possibility of exotic continental terrane(s) as a possible sedimentary provenance. Considering the potential source areas around the Yangtze Block when it was part of the Rodinia or Gondwana, we suggest that the source of these Paleozoic sediments had typical Gondwana affinities such as the Himalaya region, north India, which is also supported by their stratigraphic similarity, newly published paleomagnetic data and the tectono-thermal events of northwestern fragments of Gondwana. This implies that after a prolonged subduction in the Neoproterozoic, the western margin of the Yangtze Block began to incorporate into the assembly of the Gondwana supercontinent and was able to accept sediments from northwestern margin of Gondwanaland as a result of early Paleozoic orogeny.     

Provenance and paleogeography of the Early Paleozoic basin in the northern Yangtze Block: Constraints from detrital zircon U-Pb-Hf data

The Proto-Tethys was a significant post-Rodinia breakup ocean that eventually vanished during the Paleozoic. The closure timing and amalgamation history of numerous microblocks within this ocean remain uncertain, while the Early Paleozoic strata on the northern margin of the Yangtze Block archive valuable information about the evolution of the Shangdan Ocean, the branch of the Proto-Tethys. By comparing the detrital zircon U-Pb-Hf isotopic data from Cambrian, Ordovician, and Silurian sedimentary rocks in the northern Yangtze Block with adjacent blocks, it was found that detrital zircons in Cambrian strata exhibit a prominent age peak at ∼ 900–700 Ma, which indicates that the primary source of clastic material in the basin was the uplifted inner and margin regions of the Yangtze Block. In the Silurian, abundant detrital material from the North Qinling Block was transported to the basin due to the continuous subduction and eventual closure of the Shangdan Ocean. This process led to two distinct age peaks at ∼500–400 Ma and ∼900–700 Ma, indicating a bidirectional provenance contribution from both the North Qinling Block and the Yangtze Block. This shift demonstrates that the initial collision between these two blocks occurred no later than the Silurian. The northern Yangtze Basin transitioned from a passive continental margin basin in the Cambrian to a peripheral foreland basin in the Silurian. Major blocks in East Asia, including South Tarim, North Qilian, North Qinling, and North Yangtze, underwent peripheral subduction and magmatic activity to varying degrees during the late Early Paleozoic, signifying the convergence and rapid contraction of microplates within northern Gondwana and the Proto-Tethys Ocean. These findings provide new insights on the tectonic evolution of the Proto-Tethys Ocean.     

Tectonic evolution of the eastern Central Asian Orogenic Belt: Evidence from zircon U–Pb–Hf isotopes and geochemistry of early Paleozoic rocks in Yanbian region,NE China

We present the first evidence of an early Paleozoic terrane in the southern Yanbian region, NE China. We used LA-ICP-MS zircon U–Pb and Hf isotope techniques to analyze one plagioclase gneiss and two garnet-bearing two-mica quartz schists from the early Paleozoic Jiangyu Group, as well as two tonalites that intruded the Jiangyu Group. The tonalites yield weighted mean ²⁰⁶Pb/²³⁸U zircon crystallization ages of 423 and 422 Ma. Zircons from the Jiangyu Group gneiss and two schist samples yield maximum depositional ages of 439 ± 4, 443 ± 2, and 443 ± 5 Ma, respectively. These constraints, together with the age of the tonalite intrusion, indicate that the Jiangyu Group was deposited between 443 and 423 Ma (i.e., Silurian). In addition, detrital zircon age spectra of the three Jiangyu Group samples exhibit prominent age peaks at 442, 473, 513, 565, 600, 635, 671, 740, 1000, and 1162 Ma, as well as secondary peaks between 1344 and 3329 Ma. The occurrence of the prominent Meso- and Neoproterozoic detrital zircon age populations for the Jiangyu Group, combined with the corresponding zircon Hf isotopic data, reveals that the Jiangyu Terrane has a tectonic affinity with northeastern Gondwana. The early Paleozoic magmatism, as suggested by the medium-K calc-alkaline I-type tonalite intrusion and Jiangyu Group detrital zircon age spectra, corresponds to coeval subduction–accretion events along the southern margin of the eastern Central Asian Orogenic Belt (CAOB). Accordingly, we propose that the Jiangyu Group is part of an exotic terrane that rifted from northeastern Gondwana, drifted northward, and ultimately became involved in the early Paleozoic tectonic evolution of the southern margin of the eastern CAOB after the Early Cambrian.     

Provenance of lateritic bauxite deposits in the Wuchuan–Zheng’an–Daozhen area,Northern Guizhou Province,China: LA-ICP-MS and SIMS U–Pb dating of detrital zircons

The provenance of the large and super-large scale bauxite deposits developed in the Wuchuan–Zheng’an–Daozhen (WZD) alumina metallogenic province in the Yangtze Block of South China is poorly understood. LA-ICP-MS and SIMS U–Pb dating of detrital zircons from bauxite ores and the underlying Hanjiadian Group in the WZD area provide new constrains on the provenance of the WZD bauxite and provide new insight on the bauxite ore-forming process. The ages of the detrital zircons in the bauxites and the zircons in the Hanjiadian Group are similar suggesting that the bauxites are genetically related to the Hanjiadian sediments. The detrital zircon populations of the four samples studied show four primary age peaks: 2600–2400 Ma, 1900–1700 Ma, 1300–700 Ma and 700–400 Ma. The age distribution of detrital zircons indicates that they are probably derived from various sources including Neoproterozoic, Mesoproterozoic, Paleoproterozoic, Archean and some minor Paleozoic sources. The most abundant age population contains a continuous range of ages from 1300 to 700 Ma, ages consistent with subduction-related magmatic activities (1000–740 Ma) along the western margin of the Yangtze Block and the worldwide Grenville orogenic events (1300–1000 Ma). Thus, it is suggested that the main provenances of the WZD bauxite and the Hanjiadian Group are the Neoproterozoic igneous rocks in the western Yangtze Block and the Grenville-age igneous rocks in the southern Cathaysia Block. In addition, this work verifies that the global Grenville orogenic events and subduction-related magmatic activities associated with the Yangtze Block had a significant influence on the formation of the WZD bauxite deposits.     

U–Pb ages and Lu–Hf isotopes of detrital zircons from the southern Qinling Orogen: Implications for Precambrian to Phanerozoic tectonics in central China

The Qinling Orogen, central China, was constructed during the Mesozoic collision between the North China and Yangtze continental plates. The orogen includes four tectonic units, from north to south, the Huaxiong Block (reactivated southern margin of the North China Craton), North Qinling Accretion Belt, South Qinling Fold Belt (or block) and Songpan Fold Belt, evolved from the northernmost Paleo-Tethys Ocean separating the Gondwana and Laurentia supercontinents. Here we employ detrital zircons from the Early Cretaceous alluvial sediments within the Qinling Orogen to trace the tectonic evolution of the orogen. The U–Pb ages of the detrital zircon grains from the Early Cretaceous Donghe Group sediments in the South Qinling Fold Belt cluster around 2600–2300 Ma, 2050–1800 Ma, 1200–700 Ma, 650–400 Ma and 350–200 Ma, corresponding to the global Kenorland, Columbia, Rodinia, Gondwana and Pangaea supercontinent events, respectively. The distributions of ages and εHf(t) values of zircon grains show that the Donghe Group sediments have a complex source comprising components mainly recycled from the North Qinling Accretion Belt and the North China Craton, suggesting that the South Qinling Fold Belt was a part of the united Qinling–North China continental plate, rather than an isolated microcontinent, during the Devonian–Triassic. The youngest age peak of 350–200 Ma reflects the magmatic event related to subduction and termination of the Mian-Lue oceanic plate, followed by the collision between the Yangtze Craton and the united Qinling–North China continent that came into existence at the Triassic–Jurassic transition. The interval of 208–145 Ma between the sedimentation of the Early Cretaceous Donghe Group and the youngest age of detrital zircons was coeval with the post-subduction collision between the Yangtze and the North China continental plates in Jurassic.     

New insights into peri-Gondwana paleogeography and the Gondwana super-fan system from detrital zircon U–Pb ages

We present a synopsis of detrital zircon U–Pb ages of sandstones from North Africa and neighboring Israel and Jordan, which allows us to identify zones with characteristic sediment provenance along the northern Gondwana margin (in present-day coordinates) in Cambrian–Ordovician times, and helps us to unravel the peri-Gondwana jigsaw puzzle. A special feature of the early Paleozoic cover sequence of North Africa is the eastward increase of 1.1–0.95 Ga detrital zircons, which become ubiquitous in the early Paleozoic sandstones of the Saharan Metacraton. Detrital zircons aged about 2.7–2.5, 2.15–1.75 and 0.75–0.53 Ga are also present. Early Paleozoic sandstones with similar provenance are known from peri-Gondwana terranes in the Eastern and Western Mediterranean and from NW Iberia. These terranes need not be transported from western Gondwana (Amazonia) as suggested previously. They were likely located to the north of the Saharan Metacraton during the early Paleozoic before they rifted off from Gondwana. Furthermore, we recognize an increase, as stratigraphic ages get younger, of ca. 1.0 Ga detrital zircons at some point between the Late Cambrian and late Middle Ordovician. We speculate that this might be linked to far-field tectonics and regional uplift in central Gondwana related to plate-tectonic reorganization along the Gondwana margin, leading to erosion of ca. 1.0 Ga basement and country rocks of the Transgondwanan supermountain and fluvial dispersal of detritus toward the Gondwana margin.     

Identifying Laurentian and SW Gondwana sources in the Neoproterozoic to Early Paleozoic metasedimentary rocks of the Sierras Pampeanas: Paleogeographic and tectonic implications

The provenance of Neoproterozoic to Early Paleozoic sedimentary rocks in the Sierras Pampeanas has been established using U–Pb SHRIMP age determination of detrital zircons in twelve metasedimentary samples, with supplementary Hf and O isotope analyses of selected samples. The detrital zircon age patterns show that the western and eastern sectors of the Sierras Pampeanas are derived from different sources, and were juxtaposed during the Early Cambrian ‘Pampean’ collision orogeny, thus defining initiation of the supercontinent stage of southwestern Gondwana. The Western Sierras Pampeanas (WSP), which extend northwards to the southern Puna (Antofalla) and the Arequipa Massif (Peru), constitute a single large continental basement of Paleoproterozoic age — the MARA block — that was reworked during the Grenvillian orogeny. The MARA block probably extends eastwards to include the Río Apa block (southern Brazil), but in this case without a Mesoproterozoic overprint. Detrital zircons from the WSP and Antofalla yield age peaks between 1330 and 1030 Ma, remarkably similar to the range of ages in the Grenville province of eastern Laurentia. The WSP Neoproterozoic sedimentary cover to this basement shows the same 1330–1030 component, but also includes important 1430–1380 Ma zircons whose juvenile Hf and O isotopic signatures strongly suggest derivation from the Grenville and the Southern Granite–Rhyolite provinces of eastern Laurentia. In contrast the Eastern Sierras Pampeanas metasedimentary rocks have a typically bimodal detrital zircon pattern with peaks at ca. 1000 and 600 Ma, which respectively indicate sources in the Natal–Namaqua belt and the East African orogen and/or the Dom Feliciano belt of SE Brazil and Uruguay. Sedimentary rocks in the Eastern Sierras Pampeanas and Patagonia deposited during the Late Early Cambrian–Early Ordovician interval, after the Pampean orogeny, have detrital patterns common to many sectors along the Terra Australis orogen, reflecting increasingly dominant input to the Paleozoic basins from the Neoproterozoic to Early Cambrian orogenic belts of the Gondwana margin.     

Provenance of Late Carboniferous bauxite deposits in the North China Craton: New constraints on marginal arc construction and accretion processes

The North China Craton (NCC) is bounded by two Paleozoic accretionary arc terranes: the North Qinling terrane to the south and the Bainaimiao terrane to the north. The timing of arc accretion to the NCC and the architecture of the Bainaimiao arc remain unclear. During the building and accretion of the arcs along its margins, the NCC experienced a long sedimentary hiatus since the Ordovician, which ended with the deposition of bauxite-bearing sediments in the Late Carboniferous. In this paper we report the U–Pb and Hf isotopes of detrital zircons from the Late Carboniferous bauxite layer and use these data to constrain the tectonic evolution of the margin of the NCC. The detrital zircons yield a minimum U–Pb age of ca. 310 Ma and a prominent age peak at ca. 450 Ma. Zircon crystals with ages of ca. 330 Ma and ca. 1900 Ma are more common in the bauxite samples from the northern part of the NCC than in those from the central part. The εHf(t) values of the ca. 450 Ma detrital zircon crystals of the bauxite samples from the NCC are similar to those of the contemporaneous detrital zircon crystals from the North Qinling arc terrane to the south, but different from those of the contemporaneous detrital zircon crystals from the Bainaimiao arc terrane to the north. The ca. 450 Ma detrital zircon crystals in the ca. 310 Ma bauxite deposits are therefore interpreted to have been derived from the North Qinling arc terrane. The source of the ca. 330 Ma detrital zircon crystals of the bauxite deposits is interpreted to be the northern margin of the NCC, where intermediate-felsic plutons formed at ca. 330 Ma are common. The results from this study support the interpretation that the Paleozoic continental arc terranes and their concomitant back-arc basins were developed along the margins of the NCC before ca. 450 Ma, and the arc complexes were subsequently accreted to the craton in the Late Carboniferous. This was then followed by the formation of a walled continental basin within the NCC.     

Early Paleozoic tectonic evolution of the Xing-Meng Orogenic Belt: Constraints from detrital zircon geochronology of western Erguna–Xing’an Block,North China

To better constrain the Early Paleozoic tectonic evolution of the western part of the Erguna–Xing’an Block, detrital zircon U–Pb dating was applied on the Ordovician to Devonian sedimentary strata along the southeast part of the China–Mongolia border. Most of the zircons from five sedimentary samples display fine-scale oscillatory growth zoning and Th/U ratios higher than 0.1, indicating a magmatic origin. All five Ordovician–Devonian samples display the similar age distribution patterns with age groups at ∼440 Ma, ∼510 Ma, ∼800 Ma, ∼950 Ma, and few Meso- to Paleo-Proterozoic and Neoarchean grains. This age distribution pattern is similar to those from adjacent blocks in the southeastern Central Asian Orogenic Belt. Considering previous tectonic studies, we propose bidirectional provenances from the Erguna–Xing’an Block and Baolidao Arc.Consequently, a new model was proposed to highlight the Early Paleozoic tectonic evolution of the western Erguna–Xing’an Block, which constrains two main Early Paleozoic tectonic events of the Xing-Meng Orogenic Belt: (a) pre-Late Cambrian collision between Erguna–Kerulen Block and Arigin Sum-Xilinhot-Xing’an Block; (b) the Early Paleozoic subduction of Paleo-Asian Ocean and pre-Late Devonian collision between Erguna–Xing’an Block and Songliao-Hunshandake Block.     

Provenance of Permian–Triassic Gondwana Sequence units accreted to the Banda Arc in the Timor region: Constraints from zircon U–Pb and Hf isotopes

Analysis of zircons from Australian affinity Permian–Triassic units of the Timor region yield age distributions with large age peaks at 230–400 Ma and 1750–1900 Ma, which are similar to zircon age spectra found in rocks from NE Australia and crustal fragments now found in Tibet and SE Asia. It is likely that these terranes, which are now widely separated, were once part of the northern edge of Gondwana near what is now the northern margin of Australia. The Cimmerian Block rifted from Gondwana in the Early Permian during the initial formation of the Neo-Tethys Ocean. The zircon age spectra of the Gondwana Sequence of NE Australia and in the Timor region are most similar to the terranes of northern Tibet and Malaysia, further substantiating a similar tectonic affinity. A large 1750–1900 Ma zircon peak is also very common in other terranes in SE Asia.Hf analysis of zircon from the Aileu Complex in Timor and Kisar Islands shows a bimodal distribution (both radiogenically enriched and depleted) in the Gondwana Sequence at ~ 300 Ma. The magmatic event from which these zircons were derived was likely bimodal (i.e. mafic and felsic). This is substantiated by the presence of Permian mafic and felsic rocks interlayered with the sandstone used in this study. Similar rock types and isotopic signatures are also found in Permian–Triassic igneous units throughout the Cimmerian continental block.The Permian–Triassic rocks of the Timor region fill syn-rift intra-cratonic basins that successfully rifted in the Jurassic to form the NW margin of Australia. This passive continental margin first entered the Sunda Trench in the Timor region at around 7–8 Ma causing the Permo-Triassic rocks to accrete to the edge of the Asian Plate and emerge as a series of mountainous islands in the young Banda collision zone. Eventually, the Australian continental margin will collide with the southern edge of the Asian plate and these Gondwanan terranes will rejoin.     

Detrital zircon provenance of upper Cambrian-Permian strata and tectonic evolution of the Ellsworth Mountains,West Antarctica

Detrital zircons from the upper Cambrian-Devonian sandstones (Crashsite Group; n = 485) and Carboniferous tillite (Whiteout Conglomerate; n = 81) of the Ellsworth Mountains, Antarctica record a steady supply of Neoproterozoic (“Pan-African”) orogeny (~ 550–600 Ma), Grenville (~ 1000 Ma) and Neoarchean (~ 3000–3500 Ma) zircons into the northern marginal basin of Gondwana. The overlying Permian Glossopteris-bearing Polarstar Formation shales (n = 85) have the same zircon provenance as underlying units but also include a dominance of depositional-age (263 Ma) euhedral zircons which are interpreted to be of local, volcanic arc origin. Modeling of detrital zircon provenance suggests that source areas were present in Pan-African and Laurentian crust throughout the Paleozoic. We also report calcite twinning strain results (12 strain analyses; n = 398 twins) for the Cambrian Minaret Fm. in the Heritage range which is predominantly a layer-parallel shortening strain in the direction (WSW-ENE) of Permian Gondwanide orogen thrust transport. There is a secondary, sub-vertical twinning strain overprint. The initiation of localized lower-middle Cambrian rifting (Heritage Group deposition) in Grenville-aged crust as Gondwana amalgamated and the subsequent Jurassic counterclockwise rotation of the Ellsworth-Whitmore terrane out of the Permian Gondwanide belt into central Antarctica each remain tectonic curiosities.     

U–Pb detrital zircon data of the Rio Fuerte Formation (NW Mexico): Its peri-Gondwanan provenance and exotic nature in relation to southwestern North America

U–Pb detrital zircon studies in the Rio Fuerte Group, NW Mexico, establish its depositional tectonic setting and its exotic nature in relation to the North American craton. Two metasedimentary samples of the Rio Fuerte Formation yield major age clusters at 453–508 Ma, 547–579 Ma, 726–606 Ma, and sparse quantities of older zircons. The cumulative age plots are quite different from those arising from lower Paleozoic miogeoclinal rocks of southwestern North America and of Cordilleran Paleozoic exotic terranes such as Golconda and Robert Mountains. The relative age-probability plots are similar to some reported from the Mixteco terrane in southern Mexico and from some lower Paleozoic Gondwanan sequences, but they differ from those in the Gondwanan-affinity Oaxaca terrane. Major zircon age clusters indicate deposition in an intraoceanic basin located between a Late Ordovician magmatic arc and either a peri-Gondwanan terrane or northern Gondwanaland. The U–Pb magmatic ages of 151 ± 3 Ma from a granitic pluton and 155 ± 4 Ma from a granitic sill permit a revision of the stratigraphic and tectonic evolution of the Rio Fuerte Group. A regional metamorphism event predating the Late Jurassic magmatism is preliminarily ascribed to the Late Permian amalgamation of Laurentia and Gondwana. The Late Jurassic magmatism, deformation, and regional metamorphism are related to the Nevadan Orogeny.     

Detrital zircons from Neoproterozoic sedimentary rocks in the Yili Block: Constraints on the affinity of microcontinents in the southern Central Asian Orogenic Belt

The Yili Block is one of the Precambrian microcontinents dispersed in the Central Asian Orogenic Belt (CAOB). Detrital zircon U–Pb ages and Hf isotopic data of Neoproterozoic meta-sedimentary rocks (the Wenquan Group) are presented to constrain the tectonic affinity and early history of the Yili Block. The dating of detrital zircons indicates that both the lower and upper Wenquan Groups have two major populations with ages at 950–880 Ma and 1600–1370 Ma. Moreover, the upper Wenquan Group has two minor populations at ~ 1100 Ma and 1850–1720 Ma. According to the youngest age peaks of meta-sedimentary rocks and the ages of related granitoids, the lower Wenquan Group is considered to have been deposited during the early Neoproterozoic (900–845 Ma), whereas the upper Wenquan Group was deposited at 880–857 Ma. The zircon ε_Hf (t) values suggest that the 1.85–1.72 Ga source rocks for the upper Wenquan Group were dominated by juvenile crustal material, whereas those for the lower Wenquan Group involved more ancient crustal material. For the 1.60–1.37 Ga source rocks, however, juvenile material was a significant input into both the upper and lower Wenquan Groups. Therefore, two synchronous crustal growth and reworking events were identified in the northern Yili Block at ca. 1.8–1.7 Ga and 1.6–1.3 Ga, respectively. After the last growth and reworking event, continuous crustal reworking took place in the northern Yili Block until the early Neoproterozoic. Comparing the age patterns and Hf isotopic compositions of detrital zircons from the Yili Block and the surrounding tectonic units indicates that the Yili Block has a close tectonic affinity to the Chinese Central Tianshan Block in the Precambrian. The Precambrian crustal evolution of the Yili Block is distinct from that of the Siberian, North China and Tarim Cratons. Such difference therefore suggests that the Yili Block and the Chinese Central Tianshan Block may have been united in an isolated Precambrian microcontinent within the CAOB rather than representing two different blocks rifted from old cratons on both sides of the Paleo-Asian Ocean.     

Geochemistry and U–Pb detrital zircon dating of Paleozoic graywackes in East Junggar,NW China: Insights into subduction–accretion processes in the southern Central Asian Orogenic Belt

The southern Central Asian Orogenic Belt (CAOB) is characterized by multiple and linear accretionary orogenic collages, including Paleozoic arcs, ophiolites, and accretionay wedges. A complex history of subduction–accretion processes makes it difficult to distinguish the origin of these various terranes and reconstruct the tectonic evolution of the southern CAOB. In order to provide constraints on the accretionary history, we analyzed major and trace element compositions of Paleozoic graywackes from the Huangcaopo Group (HG) and Kubusu Group (KG) in East Junggar. The HG graywackes have relatively low Chemical Index of Alteration (CIA) values (50 to 66), suggesting a source that underwent relatively weak chemical weathering. The identical average Index of Compositional Variability (ICV) values (~ 1.1) for both the KG and HG samples point to an immature source for the Paleozoic graywackes in East Junggar, which is consistent with an andesitic–felsic igneous source characterized by low La/Th ratios and relatively high Hf contents. These graywackes are geochemically similar to continental island arc sediments and therefore were probably deposited at an active continental margin. U–Pb dating of detrital zircons from the lower subgroup of the HG yielded a young age peak at ~ 440 Ma, indicating a post-Early Silurian depositional age. However, the youngest populations of detrital zircons from the KG graywackes and the upper subgroup of the HG yielded ²⁰⁶Pb/²³⁸U ages of ~ 346 Ma and ~ 355 Ma, respectively, which suggest a post-Early Carboniferous depositional age. Because of similarities of rock assemblages, these two units should be incorporated into the Early Carboniferous Nanmingshui Formation. The detrital zircon age spectrum of the Early Paleozoic HG graywackes resembles that of the Habahe sediments in the Chinese Altai, which suggests that the ocean between East Junggar and the Chinese Altai was closed before the deposition of the sediments and that the Armantai ophiolite was emplaced prior to the Early Devonian. The differences in age spectra for detrital zircons from the post-Early Carboniferous graywackes in East Junggar and the Harlik arc indicate that the emplacement of the Kalamaili ophiolite postdates the Early Carboniferous. Therefore, a long-lasting northward subduction–accretion process is suggested for the formation of East Junggar and the reconstruction of the Early Paleozoic evolution of the southern CAOB.     

Provenance variations in the Late Paleozoic accretionary complex of central Chile as indicated by detrital zircons

We present detrital zircon UPb SHRIMP age patterns for the central segment (34–42°S) of an extensive accretionary complex along coastal Chile together with ages for some relevant igneous rocks. The complex consists of a basally accreted high pressure/low temperature Western Series outboard of a frontally accreted Eastern Series that was overprinted by high temperature/low pressure metamorphism. Eleven new SHRIMP detrital zircon age patterns have been obtained for meta-turbidites from the central (34–42°S) segment of the accretionary complex, four from previously undated metamorphic complexes and associated intrusive rocks from the main Andean cordillera, and three from igneous rocks in Argentina that were considered as possible sediment source areas. There are no Mesozoic detrital zircons in the accretionary rocks. Early Paleozoic zircons are an essential component of the provenance, and Grenville-age zircons and isolated grains as old as 3 Ga occur in most rocks, although much less commonly in the Western Series of the southern sector. In the northernmost sector (34–38°30′S) Proterozoic zircon grains constitute more than 50% of the detrital spectra, in contrast with less than 10% in the southern sector (39–42°S). The youngest igneous detrital zircons in both the northern Western (307 Ma) and Eastern Series (345 Ma) are considered to closely date sedimentation of the protoliths. Both oxygen and LuHf isotopic analyses of a selection of Permian to Neoproterozoic detrital zircon grains indicate that the respective igneous source rocks had significant crustal contributions. The results suggest that Early Paleozoic orogenic belts (Pampean and Famatinian) containing material recycled from cratonic areas of South America supplied detritus to this part of the paleo-Pacific coast. In contrast, in the southern exposures of the Western Series studied here, Permian detrital zircons (253–295 Ma) dominate, indicating much younger deposition. The northern sector has scarce Early to Middle Devonian detrital zircons, prominent south of 39°S. The sedimentary protolith of the northern sector was probably deposited in a passive margin setting starved of Devonian (Achalian) detritus by a topographic barrier formed by the Precordillera, and possibly Chilenia, terranes. Devonian subduction-related metamorphic and plutonic rocks developed south of 39°S, beyond the possible southern limit of Chilenia, where sedimentation of accretionary rocks continued until Permian times.     

From Gondwana to Europe: The journey of Elba Island (Italy) as recorded by U–Pb detrital zircon ages of Paleozoic metasedimentary rocks

Elba Island, located midway between Corsica and mainland Italy, is a small but important fragment of the Adria Plate. It has a rich sedimentary record preserved in a stack of tectonic nappes of both continental margin and oceanic origin. Especially the detrital zircons in early Paleozoic to early Mesozoic metasedimentary rocks provide an archive of many important geological events in the island's history. Elba Island and Adria originated along the northern margin of Gondwana, but drifted north in Silurian times to become part of Europe. A large new dataset of LA-ICP-MS and SIMS U–Pb zircon ages allows us to trace this history. Three main stratigraphic units have been investigated. The oldest Porto Azzurro Unit was deposited in the early Cambrian and has zircon age distributions indicating a typical northern African provenance, most likely sourced from the Saharan Metacraton. The Ortano Unit has a simple, mostly unimodal Ordovician age distribution that is entirely dominated by metavolcanic rocks and their erosional products; a sample of the metavolcanic Ortano Porphyroids provided a SIMS U–Pb zircon age of 460 ± 3 Ma. This phase of intense volcanism is related to the subduction of the Rheic Ocean beneath Gondwana, terminating with initial rifting and subsequent opening of the Paleotethys. This also marks the onset of the separation of a range of European terranes, including Adria and future Elba Island, from Gondwana. The Permo-Triassic Monticiano–Roccastrada Unit is the first to show a European provenance with the appearance of large amounts of Variscan and late to post-Variscan detritus. The presence of Variscan detrital zircons in the Permo-Triassic sediments is unexpected, since a Variscan age signature is so far not well recorded in the Adria Plate. This dataset is the most comprehensive detrital zircon dataset so far available for the Adria Plate and documents Adria's close affinity to Africa in the Lower Paleozoic, as well as its initial rifting within an active continental margin setting during the Ordovician and its final separation and independent evolution since late Palaeozoic times.     

Tracing the position of the South China block in Gondwana: U–Pb ages and Hf isotopes of Devonian detrital zircons

U–Pb detrital zircon geochronology from Lower Devonian quartz arenites of the northwestern margin of the Yangtze block yields dominant early Neoproterozoic (0.85–1.0 Ga), Pan-African (0.5–0.65 Ga) and middle Neoproterozoic (0.68–0.8 Ga) age populations and minor Mesoproterozoic to middle Mesoarchean (1.0–3.0 Ga) ages. Middle Mesoarchean to Mesoproterozoic rocks, however, are widespread in the South China block. Although Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all the age groupings, the crust growth, essentially mantle-derived, occurred mainly around 3.1 Ga, 1.9 Ga and 1.0 Ga, respectively. Zircon typology and youngest grain ages indicate that this suite of quartz arenites was the product of multiphase reworking. Abundant magmatic zircon detritus with concordant U–Pb Grenvillian and Pan-African ages, together with accompanying various ε_Hf(t) values, indicate an exotic provenance for the quartz arenite external to the South China block. Qualitative comparisons of age spectra for the late Neoproterozoic sediments of the Cathaysian Block, early Paleozoic sediments of pre-rift Tethyan Himalaya sequence in North India and lower Paleozoic sandstone from the Perth Basin in West Australia, show that they all have two the largest age clusters representing Grenvillian and Pan-African orogenic episodes. The resemblance of these age spectra and zircon typology suggests that the most likely source for the Lower Devonian quartz arenites of the South China block was the East African Orogen and Kuunga Orogen for their early Grenvillian and Pan-African populations, whereas the Hannan–Panxi arc, Jiangnan orogen, and the Yangtze block basements might have contributed to the detrital zircon grains of the Neoproterozoic and Pre-Grenvillian ages. Hf isotopic data indicate that the crustal evolution of the drainage area matches well with the episodic crust generation of Gondwana. These results imply that the previously suggested position of the SCB in Gondwana should be re-evaluated, and the South China block should be linked with North India and West Australia as a part of East Gondwana during the assembly of Gondwana, rather than a discrete continent block in the paleo-Pacific.     

U–Pb ages of detrital zircons within the Inthanon Zone of the Paleo-Tethyan subduction zone,northern Thailand: New constraints on accretionary age and arc activity

U–Pb dating of detrital zircons was performed on mélange-hosted lithic and basaltic sandstones from the Inthanon Zone in northern Thailand to determine the timing of accretion and arc activity associated with Paleo-Tethys subduction. The detrital zircons have peak ages at 3400–3200, 2600–2400, 1000–700, 600–400, and 300–250 Ma, similar to the peaks ages of detrital zircons associated with other circum-Paleo-Tethys subduction zones. We identified two types of sandstone in the study area based on the youngest detrital zircon ages: Type 1 sandstones have Late Carboniferous youngest zircon U–Pb ages of 308 ± 14 and 300 ± 16 Ma, older than associated radiolarian chert blocks within the same outcrop. In contrast, Type 2 sandstones have youngest zircon U–Pb ages of 238 ± 10 and 236 ± 15 Ma, suggesting a Middle Triassic maximum depositional age. The youngest detrital zircons in Type 1 sandstones were derived from a Late Carboniferous–Early Permian ‘missing’ arc, suggesting that the Sukhothai Arc was active during sedimentation. The data presented within this study provide information on the development of the Sukhothai Arc, and further suggest that subduction of the Paleo-Tethyan oceanic plate beneath the Indochina Block had already commenced by the Late Carboniferous. Significant Middle Triassic arc magmatism, following the Late Carboniferous–Early Permian arc activity, is inferred from the presence of conspicuous detrital zircon U–Pb age peaks in Type 2 sandstones and the igneous rock record of the Sukhothai Arc. In contrast, only minimal arc activity occurred during the Middle Permian–earliest Triassic. Type 1 sandstones were deposited between the Late Permian and the earliest Triassic, after the deposition of associated Middle–Late Permian cherts that occur in the same mélanges and during a hiatus in Sukhothai Arc magmatism. In contrast, Type 2 sandstones were deposited during the Middle Triassic, coincident with the timing of maximum magmatism in the Sukhothai Arc, as evidenced by the presence of abundant Middle Triassic detrital zircons. These two types of sandstone were probably derived from discrete accretionary units in an original accretionary prism that was located along the western margin of the Sukhothai Arc.     

Sedimentary response to the paleogeographic and tectonic evolution of the southern North China Craton during the late Paleozoic and Mesozoic

With the aim of constraining the influence of the surrounding plates on the Late Paleozoic–Mesozoic paleogeographic and tectonic evolution of the southern North China Craton (NCC), we undertook new U–Pb and Hf isotope data for detrital zircons obtained from ten samples of upper Paleozoic to Mesozoic sediments in the Luoyang Basin and Dengfeng area. Samples of upper Paleozoic to Mesozoic strata were obtained from the Taiyuan, Xiashihezi, Shangshihezi, Shiqianfeng, Ermaying, Shangyoufangzhuang, Upper Jurassic unnamed, and Lower Cretaceous unnamed formations (from oldest to youngest). On the basis of the youngest zircon ages, combined with the age-diagnostic fossils, and volcanic interlayer, we propose that the Taiyuan Formation (youngest zircon age of 439 Ma) formed during the Late Carboniferous and Early Permian, the Xiashihezi Formation (276 Ma) during the Early Permian, the Shangshihezi (376 Ma) and Shiqianfeng (279 Ma) formations during the Middle–Late Permian, the Ermaying Group (232 Ma) and Shangyoufangzhuang Formation (230 and 210 Ma) during the Late Triassic, the Jurassic unnamed formation (154 Ma) during the Late Jurassic, and the Cretaceous unnamed formation (158 Ma) during the Early Cretaceous. These results, together with previously published data, indicate that: (1) Upper Carboniferous–Lower Permian sandstones were sourced from the Northern Qinling Orogen (NQO); (2) Lower Permian sandstones were formed mainly from material derived from the Yinshan–Yanshan Orogenic Belt (YYOB) on the northern margin of the NCC with only minor material from the NQO; (3) Middle–Upper Permian sandstones were derived primarily from the NQO, with only a small contribution from the YYOB; (4) Upper Triassic sandstones were sourced mainly from the YYOB and contain only minor amounts of material from the NQO; (5) Upper Jurassic sandstones were derived from material sourced from the NQO; and (6) Lower Cretaceous conglomerate was formed mainly from recycled earlier detritus.The provenance shift in the Upper Carboniferous–Mesozoic sediments within the study area indicates that the YYOB was strongly uplifted twice, first in relation to subduction of the Paleo-Asian Ocean Plate beneath the northern margin of the NCC during the Early Permian, and subsequently in relation to collision between the southern Mongolian Plate and the northern margin of the NCC during the Late Triassic. The three episodes of tectonic uplift of the NQO were probably related to collision between the North and South Qinling terranes, northward subduction of the Mianlue Ocean Plate, and collision between the Yangtze Craton and the southern margin of the NCC during the Late Carboniferous–Early Permian, Middle–Late Permian, and Late Jurassic, respectively. The southern margin of the central NCC was rapidly uplifted and eroded during the Early Cretaceous.