Early Paleozoic Age Research Articles

Precambrian protoliths and Phanerozoic overprinting on the Wuyishan terrain (South China): New evidence from a combination of LA-ICPMS zircon and EMP monazite geochronology

Wuyishan is one of the most important terrains exposing the Precambrian basement of the Cathaysia Block, South China. Conventionally, Paleoproterozoic and Neoproterozoic sequences have been proposed in this terrain, however, the thermal histories affecting each unit within these sequences remain unclear. We report here spot analysis of 448 LA-ICPMS U-Pb zircon and >1600 EMP Th-U-Pb monazite ages for twenty samples, including schist, gneiss, migmatite and amphibolite, from different units within the basement. U-Pb zircon dating yield some concordant Neoproterozoic and/or Early Paleozoic age clusters with a wide spread of Neoarchean to Mesoproterozoic inherited ages (except amphibolite), indicating the formation of the Mayuan/Chencai core in the Early Neoproterozoic (887–827 Ma) and peripheral units in two stages of the Late Neoproterozoic (746–731 Ma and 681–660 Ma). The in-situ EMP monazite analyses give consistent Early Paleozoic ages (440–426 Ma) from inclusions in various host minerals for all but two samples from same locality that are Triassic in age (239–237 Ma). A large population of Early Paleozoic interstitial monazites shows breakdown texture and ages of secondary monazites range from 376 Ma to 358 Ma, conforming with the U-Pb zircon age of the amphibolite (370 ± 5 Ma). A few samples exhibit relict ages of ∼540 Ma from monazite cores. We conclude that the Wuyishan terrain is dominated by Early and Late Neoproterozoic basement sequences related to collisional and continental rifting orogenies, respectively, while the Badu Group in Zhejiang as a Paleoproterozoic protolith. The Neoproterozoic basement incorporating some Early Cambrian detritus, has thoroughly metamorphosed during the Silurian and further affected by the regional fluid-aided metasomatism coeval with the basic magma, now represented by amphibolites, around ∼370 Ma. The EMP monazite dating results for the Wuyishan terrain are quite different from the Yunkai massif where Triassic ages predominate, although the Wuyi-Yunkai metamorphic belt is often referred to as the same orogen in the Cathaysia Block.

Variations in Zircon Provenance Constrain Age and Geometry of an Early Paleozoic Rift in the Pinjarra Orogen, East Gondwana

AbstractThe Tumblagooda Sandstone in Western Australia documents Early Paleozoic rifting in East Gondwana and provides the earliest evidence for terrestrial activity of multicellular animals on Earth. We constrain the provenance of this sequence using 737 concordant U–Pb ages of detrital zircons from two stratigraphic wells in the northern Perth Basin and Southern Carnarvon Basin and outcrops of the type sections near Kalbarri. Detrital zircon age signatures are linked to infrared spectral data and stratigraphic logs. These ages span 3,312–466 Ma, including major Precambrian age peaks at 1,079–1,544 Ma, 1,695–2,403 Ma, and 2,640–2,879 Ma, consistent with igneous sources in the West Australian Craton. Significant Early Paleozoic age peaks at 499–541 Ma suggest a North Indian Orogen source. The maximum depositional age is constrained by the youngest detrital zircon, which yields an age of 466 ± 8 Ma. Our age constraints imply that terrestrial activity of multicellular animals on Earth may not be older than Middle Ordovician (Darriwilian). Rifting resulted in the exposure of the Yilgarn Craton and the segmentation of the Pinjarra Orogen. The Northampton Complex segment of the Pinjarra Orogen constituted a basement high that separated subbasins during the onset of rifting. Discordant Archean zircons provide a consistent record of radiogenic Pb loss at ~470 Ma, which we interpret as being related to the denudation of the Yilgarn Craton. The Pb loss event suggests that intracratonic rifting in the Pinjarra Orogen was initiated in the Middle Ordovician, after the Kuunga Orogeny completed the final amalgamation of Gondwana.

Phanerozoic polyphase orogenies recorded in the northeastern Okcheon Belt, Korea from SHRIMP U-Pb detrital zircon and K-Ar illite geochronologies

We present the SHRIMP U-Pb detrital zircon and K-Ar illite 1Md/1M and 2M1 ages, suggesting new insight into the Phanerozoic polyphase orogenies preserved in the northeastern Okcheon Belt, Korea since the initial basin formation during Neoproterozoic rifting through several successive contractional orogens. The U-Pb detrital zircon ages from the Early Paleozoic strata of the Taebaeksan Zone suggest a Cambrian maximum deposition age, and are supported by trilobite and conodont biostratigraphy. Although the age spectra from two sedimentary groups, the Yeongwol and Taebaek Groups, show similar continuous distributions from the Late Paleoproterozoic to Early Paleozoic ages, a Grenville-age hiatus (1.3–0.9Ga) in the continuous stratigraphic sequence from the Taebaek Group suggests the existence of different peripheral clastic sources along rifted continental margin(s). In addition, we present the K-Ar illite 1Md/1M ages of the fault gouges, which confirm fault formation/reactivation during the Late Cretaceous to Early Paleogene (ca. 82–62Ma) and the Early Miocene (ca. 20–18Ma). The 2M1 illite ages, at least those younger than the host rock ages, provide episodes of deformation, metamorphism and hydrothermal effects related to the tectonic events during the Devonian (ca.410Ma) and Permo-Triassic (ca. 285–240Ma). These results indicate that the northeastern Okcheon Belt experienced polyphase orogenic events, namely the Okcheon (Middle Paleozoic), Songrim (Late Paleozoic to Early Mesozoic), Daebo (Middle Mesozoic) and Bulguksa (Late Mesozoic to Early Cenozoic) Orogenies, reflecting the Phanerozoic tectonic evolution of the Korean Peninsula along the East Asian continental margin.

Sandstone memory of a Late Paleozoic continental arc in southeast China (Lower Yangtze region)

Provenance analysis was carried out on Paleozoic siliciclastic rocks exposed in the South China Block (Lower Yangtze region, Chaohu and Nanjing areas) by sandstone petrography, U-Pb detrital-zircon geochronology, and zircon Hf isotopes. Sandstones of the Silurian Fentou Formation are feldspatho-quartzose with a few igneous and low-grade metamorphic rock fragments. Sandstones of the Devonian Wutong Formation are quartzose with a few quartz-rich metamorphic rock fragments. The Fentou and Wutong formations share similar detrital-zircon age spectra, with peaks mainly at 465−420 and 838−812 Ma. Zircons of Early Paleozoic age were with all likelihood derived from magmatic rocks associated with the Kwangsian orogeny, documented by a widespread unconformity across the South China Block. Neoproterozoic zircons were probably shed from Proterozoic basement of the Wuyi terrane in the Cathaysian block, interpreted to have been uplifted south of the Lower Yangtze region during the Kwangsian orogeny. The Permian Longtan Formation in Chaohu area and Nanjing area are litho-quartzose and feldspatho-quartzose, respectively, with dominant felsic and intermediate volcanic lithic fragments, indicating that detrital source areas did not change markedly after the Kwangsian orogeny. Zircon-age spectra from the Longtan Formation in the Chaohu area display a diagnostic peak at ~ 1866 Ma, indicating provenance from the Paleoproterozoic basement of the Wuyi terrane. Instead, detrital zircons in the Longtan Formation in the Nanjing area display peaks at ~ 445 and ~ 970 Ma, similar to those in the underlying Paleozoic sandstones. The Wuyi terrane in the Cathaysian block thus remained as a major source area for long, as supported by independent sedimentological and seismic evidence. The Longtan Formation in both Chaohu and Nanjing areas also contain detrital zircons with ages distributed continuously from 341 to 254 Ma and a major peak around 270 Ma, reflecting renewed intermediate and felsic magmatism in the source area. These zircons have e Hf( t ) ranging from −19.8 to +14.9, indicating melting of old crust and mixing with crustal magmas. Such a continuously magmatic source supplied sediments all across the South China Block from east to west. These and other lines of evidence including probability accumulation-curve analysis favor deposition in a convergent plate-tectonic setting. We suggest that source rocks were part of a continental arc developed along the southeast coast of China during the Late Paleozoic, shedding sediments to as far as north as the northern South China Block. According to previous studies, such continental arc would be related to the subduction of the paleo-Pacific plate. Magmatism generated at first by melting of old crust was mixed later on with felsic and intermediate magma newly generated during mantle upwelling and associated uplift. The continental arc remained as the main detrital source to the Lower Yangtze region and other parts of the South China Block until it was eventually denudated or buried by the Mesozoic times.

Shoshonitic- and adakitic magmatism of the Early Paleozoic age in the Western Kunlun orogenic belt, NW China: Implications for the early evolution of the northwestern Tibetan plateau

The Western Kunlun orogenic belt in the northwestern margin of the Tibetan plateau contains two magmatic belts; early Paleozoic belt in the northern part of Western Kunlun Terrane (WKT), and early Mesozoic belt in the southern part of WKT. Both formed from northward subduction of the Paleo-Tethys. The early Paleozoic belt contains large Datong and Qiukesu igneous complexes and many smaller plutons. The Datong complex is mainly composed of dark-colored porphyritic syenite and monzonite with minor light-colored dykes of granite and monzonite. The dark-colored rocks are characterized by moderate SiO2 (58.2–69.3wt.%), and high Al2O3 (15.3–17.1wt.%), total alkali (Na2O+K2O=8.07–10.2wt.%) and ratios of K2O/Na2O (0.77–1.83). They plot in “shoshonite” field, and show high abundances of LILE including LREE ((La/Yb)n=15.4–26.2; mean 20.2) with pronounced negative anomalies of Nb–Ta–P–Ti in normalized trace elemental patterns and weak negative anomalies of Eu (δEu=2Eun/(Smn+Gdn)=0.68–0.80). The light-colored rocks contain slightly higher concentrations of SiO2 (60.3–72.0wt.%), similar Al2O3 (14.7–17.6wt.%), and slightly lower total alkalis (6.57–9.14wt.%) than dark-colored rocks. They show adakitic geochemical signatures with low Y (5.80–17.2ppm) and Yb (0.63–1.59ppm), and high Sr/Y (>40). U–Pb zircon dating indicates that shoshonitic rocks and adakitic dykes formed at ~444Ma to ~443Ma, and a separate small adakitic plug at ~462Ma. The mean εHf(t) values of zircon range from −1.6 to −0.94 (n=14) with TDM2 of ~1.5Ga for shoshonitic rocks and εHf(t) values from −1.8 to +0.72 (n=12) with TDM2 of 1.4 to 1.5Ga for adakitic rocks. Shoshonitic rocks show initial 87Sr/86Sr and εNd(t) of 0.7092–0.7100 and −3.9 to −3.2, respectively, and adakitic rocks yield initial 87Sr/86Sr and εNd(t) of 0.7099–0.7134 and −3.6 to −3.1, respectively. Similar Sr, Nd, and Hf isotope compositions for the shoshonitic and adakitic rocks suggest similar ancient rocks as their sources.Combined with the geological development of the area, we propose that a local extension in the WKT over the subducting Paleo-Tethys has induced the upwelling of asthenospheric mantle and partial melting in the lower crust during mid-Ordovician to early Silurian. The primary shoshonitic melt evolved into the parental magmas for the shoshonitic rocks and adakitic rocks through fractional crystallization of hornblende.

Exceptional preservation of soft-bodied Ediacara Biota promoted by silica-rich oceans: REPLY

The Ediacara Biota, Earth’s earliest fossilized ecosystem of complex, macroscopic, multicellular organisms, occurs in terminal Ediacaran strata worldwide, yet how the fossils are preserved remains controversial. Ediacara assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology and phylogenetic affinity. Many of these fossil assemblages are anactualistically preserved as casts and molds in sandstones (“Ediacara-style” preservation). Here we present evidence from the Ediacara Member of South Australia that Ediacara-style preservation was due to rapid, early-stage precipitation of silica cements, facilitated by the high silica saturation state of the oceans prior to the appearance of prolific silica biomineralizers. An early silicification model provides a coherent, mechanistic and empirically supported explanation for the widespread preservation of soft-bodied organisms of Ediacaran–early Paleozoic age as sandstone casts and molds. The prevalence of early silicification confirms that Ediacara-style fossil assemblages can provide an accurate window into life on the Ediacaran seafloor that can be used to reconstruct critical steps in the development and diversification of early animal ecosystems.

Using zircon U–Pb ages to constrain the provenance and transport of heavy minerals within the northwestern shelf of the South China Sea

Numerous ore-grade heavy mineral placer deposits occur in the northern South China Sea region. Previous studies on these deposits have focused on the heavy-mineral ore resources themselves, but the provenance and transport pathways of these heavy minerals are poorly constrained. This paper presents U–Pb ages of detrital zircons from sediments within the northwestern shelf of the South China Sea, and uses this new dataset to determine the provenance and transport pathway of the sediments. Zircons in sediments from ten areas of the northwestern shelf exhibit distinct age populations, suggesting that they have multiple provenances. Zircons in sediments from the Pearl River Mouth, Shangchuan Island, and Moyang River Mouth areas all have an obvious peak of Mesozoic ages, indicating that they have similar sediment provenances; i.e., mainly from the Pearl River, and to a lesser degree from the Moyang River. Zircons in sediments from the areas around the Jian River Mouth and Leizhou Bay, and off Hainan Island have an early Paleozoic age, suggesting that the sediments predominantly originate from the Yunkai massif. Zircons of the sediments from the remaining four areas, the Leiqiong Strait, Wanquan River Mouth, Qiongdongnan, and the Outer Shelf, have Yanshannian and Indosinian age peaks in addition to an obvious early Paleozoic population, implying mixed provenances, including the Yunkai massif and Hainan Island. The sediment transport may have involved two hydrodynamic conditions in two distinct stages. First, the Guangdong Longshore Current carried the river sediments to where they dispersed in the inner shelf; subsequently, wave-induced strong currents further transported sandy sediments southeastward to the outer shelf. In addition to explaining the provenance and transport pathways of heavy minerals within the northwestern shelf of the South China Sea, these results provide new information relevant to exploration for heavy-mineral placer deposits.

Exceptional preservation of soft-bodied Ediacara Biota promoted by silica-rich oceans

Abstract The Ediacara Biota, Earth’s earliest fossilized ecosystem of complex, macroscopic, multicellular organisms, occurs in terminal Ediacaran strata worldwide, yet how the fossils are preserved remains controversial. Ediacara assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology and phylogenetic affinity. Many of these fossil assemblages are anactualistically preserved as casts and molds in sandstones (“Ediacara-style” preservation). Here we present evidence from the Ediacara Member of South Australia that Ediacara-style preservation was due to rapid, early-stage precipitation of silica cements, facilitated by the high silica saturation state of the oceans prior to the appearance of prolific silica biomineralizers. An early silicification model provides a coherent, mechanistic and empirically supported explanation for the widespread preservation of soft-bodied organisms of Ediacaran–early Paleozoic age as sandstone casts and molds. The prevalence of early silicification confirms that Ediacara-style fossil assemblages can provide an accurate window into life on the Ediacaran seafloor that can be used to reconstruct critical steps in the development and diversification of early animal ecosystems.

The provenance and tectonic setting of the Lower Devonian sandstone of the Danlin Formation in southeast Yangtze Plate, with implications for the Wuyi-Yunkai orogeny in South China Block

The South China Block was subject to widespread tectonic and magmatic events during the middle Ordovician to earliest Devonian which are collectively called the Wuyi-Yunkai orogeny. Two different hypotheses were formulated about the origin of the orogeny: collisional orogenesis and intracontinental orogenesis. Ages of 215 detrital zircons were obtained from quartz sandstones in the Lower Devonian Danlin Formation exposed in Dushan County, south Guizhou Province. The results show that the detrital zircons came from multiple source areas but with little indication of the Wuyi-Yunkai orogeny. The detrital zircons of early Paleozoic age account only for 1.9% of all samples. These zircons range from 476 to 402Ma and originated from the early Paleozoic granites situated east of Dushan County. The mean 206Pb/238U age of the two youngest detrital zircons was 404Ma, which constrains the maximum depositional age of the Danlin Formation. The detrital zircons are mostly within Neoproterozoic age (59.5%) and in the range 997 to 557Ma. The zircon age pattern, morphology, and trace element characteristics combined with paleogeographic reconstruction interpret that these detrital zircons were supplied from igneous rocks in the Neoproterozoic Sibao and Danzhou Groups within the west Jiangnan orogen to the east of the study area. Mesoproterozoic zircons made up 20.9% of the grains and range from 1569 to 1055Ma; the ages of most of these zircons coincide with those of zircons indicating the Grenvillian orogeny (1300–1000Ma) in the Cathaysia Plate. The detrital zircon age data reveal that the study area was located in an intracontinental tectonic setting, both before and after the Wuyi-Yunkai orogeny. Therefore, the Cathaysia Plate was not separated from the Yangtze Plate by an ocean and the study area received sediments primarily from the Cathaysia Plate located to the southeast since the middle Ordovician. By the early Devonian, the sediments were supplied by the west Jiangnan orogen. Although a source change was caused by Wuyi-Yunkai orogeny, our data imply that this orogeny represented an intracontinental event rather than an orogenesis involving subduction-collision and joining of the Yangtze and Cathaysia Plates during the early Paleozoic.

Widespread Paleoproterozoic basement in the eastern Cathaysia Block: Evidence from metasedimentary rocks of the Pingtan–Dongshan metamorphic belt, in southeastern China

The Cathaysia Block in South China Block (SCB) is famous for its polyphase granitic magmatism and world-class mineralization, especially W, Sn and REE. Although isotopic data suggest that widespread igneous rocks were derived from Precambrian basement, outcrops of the basement rocks are rare in the Cathaysia Block. Therefore, the distribution of the Archean-Paleoproterozoic basement beneath Cathaysia Block is still unclear.The northeastern-striking Pingtan–Dongshan metamorphic belt (PDMB) is located along the coastal region of southeastern China, and consists mainly of voluminous granites and volcanic rocks with minor metasedimentary rocks. These metasedimentary rocks were intruded or captured by 160–143Ma granites. U–Pb dating of detrital and metamorphic zircons from seven metasedimentary rocks indicates that their protoliths were deposited between 183Ma and 160Ma, rather than pre-Devonian as previously proposed. Dating of metamorphic zircons suggests that the most important metamorphism took place at ca 97Ma, consistent with emplacement of A-type granites and mafic magmatism in the coastal region, and implying an extensional setting.Age spectra of detrital zircons in samples from the middle and southern PDMB exhibit a bimodal distribution with abundant Late Paleoproterozoic and Early Mesozoic zircons. In contrast, age spectra of detrital zircons from the northern PDMB display pronounced Neoproterozoic and Early Paleozoic age peaks as well as Late Paleoproterozoic and Early Mesozoic ones, suggesting a different provenance. The comparisons of age and Hf-isotope with the exposed rocks of the SCB suggest that the terrigenous material in these sediments was sourced from the Wuyi terrane, in the eastern Cathaysia Block.The age spectra of detrital zircons from Early Paleozoic to Late Mesozoic sedimentary rocks in the SE SCB show that late Paleoproterozoic (1.9–1.8Ga) detritus increases markedly in Middle Permian to Early Cretaceous sedimentary rocks. Comparison with zircon U–Pb–Hf isotopic data from exposed Paleoproterozoic rocks suggests that a Paleoproterozoic basement existed extensively beneath the eastern Cathaysia Block. It has been uplifted and eroded since Middle Permian time, and then covered by Late Cretaceous volcanic rocks. The uplift of the Paleoproterozoic basement was probably caused by the Late Paleozoic orogeny in the eastern Cathaysia Block.

The first paleomagnetic data on dolerites from Jeannette Island (New Siberian Islands, Arctic)

The first paleomagnetic data on dolerite dikes from the volcanogenic–sedimentary section of Jeannette Island (De Long Archipelago, New Siberian Islands) are discussed. The petromagnetic data and results of the baked contact and fold tests are used to substantiate the nature of the characteristic magnetization component, which in combination with the 40Ar/39Ar dates implies its likely Late Precambrian–Early Paleozoic age. The calculated paleomagnetic pole makes it possible to extend the trajectory of the apparent polar movement for the New Siberian Islands block and confirms the assumption that this structural element of the Arctic shelf evolved as a terrane. Two variants of paleotectonic interpretation of the obtained data and their consistency with the available data on the geology and tectonics of the New Siberian Islands are considered.

Evaluate the geological structure, petroleum potential by interpretation the 2D seismic data of Phu Quoc basin

So far, the understanding of Pre-Tertiary formations in the Phu Quoc basin is eliminated due to lack of drilling and the absence of outcrops except on Phu Quoc Island and some other isles. By analogue with stratigraphic information from surrounding areas, seismic horizons may be correlated with known-age strata. Strata of pre-Cambrian and Early Paleozoic ages have not been discovered yet in SW Viet Nam but by analogue may consist of metamorphosed rocks such as chloritized phylite, sericite schist, and quartzite sandwiched with ultra basic rocks. Therefore, PetroVietnam assigned PVEP to perform acquisition of 2D seismic on the area of blocks 41- 44 in order to get additional data for more precise evaluation of the petroleum potential and defining the exploration strategy in the Phu Quoc area.Based on seismic data interpretation in block 42 of Phu Quoc Basin, the Isodepth maps were constructed by approach of conversingtime to depth from isochrone maps. The results of interpretation allowed to identify many seismic closures particularly on the isodepth map of the Brown section, but these closures are located deeply above 5000m, which reliability is fair and economic effect is weak. Hence, the Brown horizon did not considered as structures identified on and concentrate to describe those determined in the Orange and Purple horizons. Result of interpretation indicated 8 closures, which could be identified in the Orange and Purple horizons

Extensional Carboniferous magmatism at the western margin of Gondwana: Las Lozas valley, Catamarca, Argentina

The Las Lozas volcanic sequence, which crops out at northwestern border of the Famatina belt-southeastern Puna, NW Argentina, is constituted mainly by rhyolites and a lesser volume of basalts and trachytes, and volcanoclastic deposits. These rocks, previously considered of Early Paleozoic age, are now assigned to the Lower Pennsylvanian (320 Ma U-Pb age). They represent a bimodal volcanic succession that plot in the subalkaline/tholeiitic (rhyolites), alkaline basalts (basalts) and alkaline (trachytes) fields on the total alkali-silica diagram. The basalts display features comparable to transitional MORB and within-plate tholeiites, with contributions from a mantle source affected by crustal contamination. The acid members also show geochemical affinities to within-plate magmas, and their composition suggest a derivation from continental crustal material with mantle source interaction or a juvenile essentially mantle derived crust. The 320 Ma age from the Las Lozas volcanic succession as well as the 342 and 348 Ma U-Pb ages, from rocks in the nearby Cazadero Grande section, to the south, and the U-Pb ages from Sierra Pampeanas granites (332-357 Ma) highlight the importance achieved by Carboniferous magmatic activity in that region, framed between 320 and 350 Ma. Low strontium initial ratios from the Las Lozas (0.70479-0.70164) indicate a predominant contribution by a juvenile component, while the ratios in the nearby Cazadero Grande (0.71433-0.71233) and Sierras Pampeanas granites (0.717 to 0.7124) point to an input by enriched sources with restricted contribution of a mantle component. Nd isotopes from a basalt from the Las Lozas section (εNd(320) with +3.11, TDM with 774.6 Ma) indicates a possible asthenospheric source, with evidence for some mixture between juvenile and reworked crustal material. In contrast Sr-Nd isotopes in a rhyolite from Cazadero Grande (εNd(t) of -2.91 and -0.3, TDM of 1.09 and 1.1 Ga) and of Sierra Pampeanas granites (εNd(t) of -0.6, TDM of 1.19 and 1.1 Ga) suggest a crustal source with minor juvenile input. Taking in account the age difference in the region between Mississippian and the Lower Pennsylvanian magmatism, this would indicate a change in the magma source consistent with a more pronounced extensional tectonic regime for the Lower Pennsylvanian. This assumption need to be considered together with observations along a transect at 27º30’S, where the Carboniferous Eastern Sierras Pampeanas granitic rocks show, regardless of age, a greater contribution of juvenile material of mantle character to the west. Among these rocks, with a major juvenile component, are those of the Las Lozas succession as well the Cerro Gloria Granite, the eastern manifestations of the Carboniferous Eastern Sierras Pampeanas magmatism. From the foregoing emerges that the contribution of juvenile material could be continuously varied in the region through the Carboniferous, due to varying lithospheric extension. An example of this arise from the presented data of the Lozas succession, which indicate that this rocks resulted from reworking of supra-crustal material with input of juvenile magmas, linked to the change in the lithospheric extension

U-Pb and Hf isotope analysis of detrital zircons from Paleozoic strata of the southern Alexander terrane (southeast Alaska)

The Alexander terrane is an unusual tectonic fragment in the North American Cordillera in that it contains a long and very complete stratigraphic record, including sedimentary or volcanic rocks representing every period and nearly every epoch between Neoproterozoic and Late Triassic time. The terrane is also unusual in that the southern portion of the terrane experienced arc-type magmatism during Neoproterozoic−early Paleozoic time, whereas the northern portion of the terrane consists mainly of Paleozoic shelf-facies strata. This long and diverse history provides opportunities to reconstruct the evolution and displacement history of the terrane, and specifically test the prevailing interpretation that the terrane formed in the paleo-Arctic realm. This study presents U-Pb geochronologic data and Hf isotopic information for detrital zircons from arc-type rocks in the southern portion of the terrane. Information has been generated from seven samples of Ordovician through Devonian age, complementing the information available from previous studies of Ordovician through Triassic strata. Together, these data sets yield a robust record of the magmatic history of the southern Alexander terrane, with dominant age groups of 640−550 Ma, 490−400 Ma, 380−340 Ma, and 310−275 Ma (dominant ages of 579, 441, 361, and 293 Ma). There are few pre−640 Ma grains in any of the samples. Hf isotope compositions of the detrital zircons are exceptionally juvenile, with most epsilon Hf (t) values between +15 and +5. Collectively, the available geologic, U-Pb geochronologic, and Hf isotopic evidence suggests that the southern Alexander terrane formed within a juvenile Neoproterozoic−early Paleozoic arc system, with little continental influence, whereas the northern portion of the terrane formed in proximity to a continental landmass that experienced similar Neoproterozoic−early Paleozoic ages of continental-affinity magmatism. Our data are consistent with previous suggestions that the Alexander terrane resided in the paleo-Arctic realm during early Paleozoic time, with the northern portion of the terrane adjacent to Baltica and the Caledonides, and the southern portion of the terrane forming further offshore as a juvenile north-facing oceanic arc.

The first data on the geology of Jeannette Island (De Long Archipelago, New Siberian Islands)

This paper presents the first actual information on the geology of Jeannette Island, one of the islands of the De Long archipelago located in the East Siberian Sea. We show that Jeannette Island has a volcano-sedimentary section dominated by volcaniclastic turbidites. The sequence identified on the southwestern coast of the island has a submonoclinal plunge complicated by secondary deformation structures, which indicate a general E-W direction of tectonic transport (in present-day coordinates). The sequence is intensely cut by multiple thin (up to a few meters) gabbro-dolerite dikes that are deformed conformably with the host rocks. The general geological framework of the island bears a close resemblance to that of nearby Henrietta Island located some 70 km to the east, which consists of a volcano-sedimentary cover of Early Paleozoic age. No organic remains have been found in the studied section of Jeannette Island. The preliminary results of isotope geochronological and paleomagnetic studies confirm the Late Precambrian-Early Paleozoic age assigned to the entire rock complex of Jeannette Island. The measured paleomagnetic directions are generally consistent with the directions of Lower Paleozoic rocks of Bennett Island (De Long archipelago) and Kotelny Island (Anzhu archipelago), which confirms our earlier conclusion that the New Siberian Islands shared a common tectonic history and that this structural element of the Arctic shelf appears to have been a terrane during the Early Paleozoic. This new information can help elucidate the possible relations between the marginal-continental, shelf, island and deep-seated structures of the Eastern Arctic.

Detrital zircon record of the early Paleozoic meta-sedimentary rocks in Russian Altai: Implications on their provenance and the tectonic nature of the Altai–Mongolian terrane

An integrated U–Pb and Hf-isotope study on detrital zircons from the early Paleozoic meta-sedimentary rocks along the Charysh–Terekta–Ulagan–Sayan suture zone in Russian Altai was conducted in order to trace their provenance and tectonic setting. Most of the zircons possess oscillatory zoning and high Th/U ratios (>0.1), indicating their magmatic origin. The investigated samples yield similar zircon populations, i.e., dominant groups with late Neoproterozoic to early Paleozoic ages, followed by those from Mesoproterozoic to late Neoproterozoic and minor ones from Archean to middle Mesoproterozoic, indicating multiple tectono-thermal events in the source area. Comparison with surrounding tectonic units shows that the Tuva–Mongolian terrane and its adjacent island arcs possibly provided substantial materials to the sedimentary basin. These rocks show detrital zircon age patterns and Hf-isotope compositions similar to their counterparts in the Chinese Altai and Tseel terrane in western Mongolia, but different from those in the Gorny Altai terrane. Therefore, the investigated meta-sedimentary units possibly represented the northernmost segment of the Altai–Mongolian terrane. With combination of previous studies in the Chinese Altai and Tseel terrane, our data suggest that the Altai–Mongolian terrane possibly represents a coherent continental arc-accretionary prism system built upon the active margin of the western Mongolia during the Cambrian to Ordovician and thus does not support the micro-continent model with a passive margin.A compilation of U–Pb and Hf-isotope data of detrital zircons from the whole Altai–Mongolian terrane shows that the source area (i.e., the western Mongolia) underwent two most extensive magmatic activities at ca. 1.02–0.67Ga and 0.67–0.43Ga. These zircons possess both positive and negative εHf(t) values, suggesting significant crustal growth and reworking during the magmatic activities. Our study underlines a crucial role of Precambrian micro-continents (e.g., the Tuva–Mongolian terrane) and island arcs surrounding them in the formation of the Central Asian Orogenic Belt.

Geochemistry and U–Pb zircon dating of the Toudaoqiao blueschists in the Great Xing’an Range, northeast China, and tectonic implications

The Toudaoqiao Complex is a sequence of high-pressure metamorphic rocks located along the suture zone that separates the Xing’an and Erguna blocks in northeast China. The rocks were metamorphosed up to epidote–blueschist facies at P–T conditions of approximately 0.9–1.1GPa and 320–450°C. Lithological associations and major and trace element compositions indicate that the blueschists are metabasalts with ocean island basalt (OIB) and normal mid-ocean ridge basalt (N-MORB) affinities, similar to those of the Philippines intraoceanic accretionary complex formed by subduction of oceanic crust. Magmatic zircons extracted from two samples of the epidote–blueschist facies metabasalts from the North Mountain in the Toudaoqiao Complex exhibit 206Pb/238U ages of 511±5 and 516±17Ma, whereas a greenschist from the South Mountain has a 206Pb/238U age of 511±2Ma; These data show that the protoliths of the Toudaoqiao Complex are of early Paleozoic age. In addition, a granitic dike from the South Mountain exhibits a weighted mean 206Pb/238U age of 492±1Ma. These ages constrain the timing of high-pressure metamorphism in the Toudaoqiao Complex to 490–510Ma. The new data support the view that the suture zone between the Xing’an and Erguna blocks is the Toudaoqiao–Xinlin Fault, rather than the Derbugan Fault as previously thought. This suture, which extends from Xinlin, through Toudaoqiao, and southern Mongolia, named the ‘South Mongolia–Toudaoqiao–Xinlin Suture Zone’, may be an important boundary between the Central Mongolia–Erguna and Xing’an blocks.

Early paleozoic age of the Isagachi Sequence (Chalaya Group, Gonzha Terrane) in the eastern part of the Central Asian fold belt

Early paleozoic age of the Isagachi Sequence (Chalaya Group, Gonzha Terrane) in the eastern part of the Central Asian fold belt

Morphometric characterization of a large scale rockslide, and probable seismogenic origin of landslides on the western flank of Central Precordillera, Argentina

The distribution of late Pleistocene to Holocene rockslides in the western flank of the eastern mountain ranges of Central Precordillera (30° 40′–31° S), has been analyzed to determine the triggering mechanisms. The mode of failure was determined to be translational sliding. This paper presents the results of research on one of the Holocene rockslides located in the western flank of Sierra de La Dehesa, which is composed of stratified limestones of early Paleozoic age. In order to characterize this slide, high resolution satellite imagery interpretation was carried out, along with the recognition of the landslide detachment zones and landslide bodies with the aim of reconstructing the pre-slide topography. The model proposed for this slide is a translational or planar slide, as the mode of failure was along a broadly planar surface accompanied by shear or tensile fractures and joints. The estimated rock volume involved is 1.24 Mm3. We compared rockslide occurrence patterns to slope, topography, lithology, geological structures, and seismicity records. After analysis of the spatial relationships among all the slides and the distribution of seismic epicenters, historical earthquakes and neighboring Quaternary faults as seismogenic sources, we hypothesize that these Quaternary rockslides in the Central Precordillera have been triggered by shallow seismicity associated with active faults.

Ordovician and Cretaceous tectonothermal history of the Southern Gemericum Unit from microprobe monazite geochronology (Western Carpathians, Slovakia)

The Southern Gemericum basement in the Inner Western Carpathians experienced a polyphase regional deformation. Differences in the pre-Alpine and Alpine events have been constantly discussed. To address this, monazites from metapelites and acid metavolcanic rocks were dated using the Th–U–Pb electron microprobe method. Three monazite generations, such as Precambrian, Early Paleozoic, and Alpine, have been recognized in the greenschist facies pelites and acid metavolcanic rocks of the Southern Gemericum basement. Both inherited magmatic monazite grains in metavolcanites and rare relics of detrital monazites within the polyphase monazite grains in metapelites yielded the Precambrian age in the time span of 550–660 Ma. They prove the provenance and derivation from deeper crustal Cadomian fragments. High-Y magmatic monazites of Early Paleozoic age (444 ± 13 and 477 ± 7 Ma) have been recorded in the acid metavolcanites and their metavolcaniclastics. These ages roughly fit within the previously published magmatic zircon age determinations (at 494 ± 1.7 and 464 ± 1.7 Ma) that clearly indicate two-phase volcanic activity in the Early Paleozoic Southern Gemericum basin. The Early Paleozoic magmatic monazites were partly overprinted by the low-Y Alpine monazites (133 ± 5 and 184 ± 16 Ma) at their rims. In Al-rich metapelites, the newly formed low-Y monazites of Alpine age commonly occur, reflecting the polystage compression geodynamic evolution with three distinct peaks at 100 ± 8, 133 ± 5, and 190 ± 16 Ma, respectively. No data as the evidence of the pre-Alpine metamorphic events were observed in metapelites. Only some monazites yield the age indications for the Permian extensional thermal re-heating (260–290 Ma). The monazite age data from the Southern Gemericum basement indicate the strong overprinting due to the polyphase Alpine deformation at least in the greenschist facies conditions.