Pb Geochronological Studies Research Articles

Early Ediacaran and Middle Ediacaran Granitoids in the Provenances of Early Cambrian Coarse-Grained Rocks of the Bayan-Kol Formation of the Systyg-Khem Depression (Tuva)

Abstract —The Tuva segment of the Central Asian Orogenic Belt is characterized by the ubiquitous presence of conglomerates few tens of meters to a kilometer in thickness in early Paleozoic volcanosedimentary sequences. We present the first results of geochemical, isotopegeochemical (Sm–Nd and Rb–Sr), and U–Pb geochronological studies of granitoid boulders and pebbles from the conglomerate sequence of the early Cambrian Bayan-Kol Formation of the Systyg-Khem depression. These studies made it possible to establish several sources of clastic material as a result of the destruction of granitoids of different ages and isotope-geochemical compositions. At least two complexes of granitoids were denuded in the pre-Ediacaran tectonic block in the early Cambrian: (1) middle Ediacaran (~590 Ma) and (2) early Ediacaran (~630 Ma); the latter resulted from the melting of pre-Ediacaran island arc crust formed from a depleted mantle source (εNd(T) = +8.0 to +8.6). At present, no granitoids of this age and with such isotope-geochemical characteristics have been found within the Tuva segment. Probably, the granitoid complexes reconstructed from the results of study of clastic conglomerates are eroded or buried beneath younger deposits and do not expose. Thus, the study of clastic conglomerates from the Bayan-Kol Formation provided the first information about the Precambrian history of the tectonic block whose destruction led to the accumulation of this terrigenous sequence.

First Results of U–Th–Pb (SIMS) Geochronological Study of Zircon from Serpentinized Ultramafic Rocks of the Tekturmas Ophiolite Zone (Central Kazakhstan)

First Results of U–Th–Pb (SIMS) Geochronological Study of Zircon from Serpentinized Ultramafic Rocks of the Tekturmas Ophiolite Zone (Central Kazakhstan)

U–Pb geochronology of calcite carbonatites and jacupirangite from the Guli alkaline complex, Polar Siberia, Russia

AbstractThis work is the first in situ U–Pb geochronological study of perovskite and calcic garnet (andradite) from the Guli complex in the Maimecha–Kotuy alkaline province (Polar Siberia, Russia). The U–Pb isotopic compositions of perovskite from contact zones of the two carbonatite stocks (Southern and Northern) and from jacupirangite separating the stocks were determined by laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICPMS) and isotope-dilution thermal-ionisation mass-spectrometry (ID-TIMS). The LA-ICPMS and ID-TIMS data for perovskite from the Northern carbonatite stock are in good agreement, yielding 206Pb/238U ages of 250.4 ± 1.1 Ma and 249 ± 2 Ma, respectively. These ages are also within the analytical uncertainty from the ID-TIMS results for perovskite from jacupirangites (250 ± 1 Ma). The LA-ICPMS results for perovskite from the Southern carbonatite stock indicate its somewhat older age (255.3 ± 2.4 Ma), implying the possibility of small-volume mantle magmatism predating the eruption of the Siberian flood basalts at ca. 252–251 Ma. This interpretation is supported by reports of pre-flood magmatism elsewhere in eastern Siberia. Andradite crystals from the contact between the Southern stock and metasomatised melilitolite were analysed by ID-TIMS. These measurements are inconclusive (247 ± 6 Ma) and could not be used to constrain further the timing of carbonatitic magmatism in the southern part of the complex. The present contribution also presents a refined methodology for LA-ICPMS geochronological studies of perovskite with elevated levels of common lead, and addresses some of the problems with previously proposed calibration standards.

North China block underwent simultaneous true polar wander and tectonic convergence in late Jurassic: New paleomagnetic constraints

Jurassic witnessed two synchronous events on the Earth, named true polar wander (TPW) and closure of the Mongol–Okhotsk ocean (MOO) between Siberia and eastern Asian blocks. The paleogeographic change of the north China block (NCB) should thus include both TPW and plate tectonic components in Jurassic. We performed a new combined paleomagnetic and U–Pb geochronological study for Middle and Late Jurassic volcanic rocks from the northern NCB and obtained three new well-dated paleopoles: 82.7°N, 282.6°E, A95=3.2° (ca. 160 Ma); 84.1°N, 228.9°E, A95=6.6° (ca. 165 Ma); and 89.3°N, 216.8°E, dp/dm = 5.4°/7.1° (ca. 170 Ma). A positive untilting test indicates a pre-folding origin interpretation for the ca. 160 Ma pole. Quality of these poles is enforced by three positive conglomerate tests, the presence of magnetic reversals and secular variation analyses. Comparison of the apparent polar wander paths of Eurasia and the NCB suggests that the NCB accreted to Eurasia around 140 Ma. The NCB and its then connected blocks did not undergo any significant southward paleogeographic shift during Jurassic. In context of TPW, we estimate a net northward plate moving velocity at 0.50–0.75°/myr for the NCB in Jurassic. The long-term accumulation of MOO subducted slabs at the high paleolatitudes may have critically contributed to, or perhaps led to, the Jurassic TPW.

Geochronology and trace elements of zircon in the Southern Chinese Altay: Implications for tectonic setting

Zircon (ZrSiO4) trace elements are commonly used to distinguish the tectonic settings of their host rocks. The Chinese Altay is a key area for deciphering the accretionary history of the Central Asian Orogenic Belt (CAOB), and its tectonic setting during the Devonian is debated. In this study, we describe the systematic investigations of the magmatic and metamorphic rocks from the Qiemuerqieke and Qinghe areas situated to the west and east of Southern Altay, respectively. In doing so, we present the first attempt to elucidate the Devonian tectonic setting of the Southern Altay based on zircon trace elements. Zircon U–Pb geochronological studies on the magmatic and metamorphic rocks in Southern Altay yielded ages of 395–360 Ma, which indicate that these metamorphic rocks formed in the Devonian rather than the Precambrian. Furthermore, most of the zircons from these rocks were concentrated in the arc‐related orogenic setting of the discrimination diagrams of Th/U versus Nb/Hf and Th/Nb versus Hf/Th, and in the continental field of the discrimination diagrams of Yb versus U, Hf versus U/Yb, Y versus U/Yb, and Nb/Yb versus U/Yb. These results are consistent with the geochronology and whole‐rock geochemistry of Devonian granitoids in Southern Altay. Therefore, we propose that the continental arc along the Southern Chinese Altay was triggered by the active continental margin of the Altay‐Mongolian Terrane during the Devonian.

Early Paleozoic Protoliths of Gneisses and Granitic Gneisses in the Eastern South Urals: Results of U–Th–Pb (SIMS) Geochronological Studies

The Early Paleozoic age of the protolith for gneisses in the East Uralian megazone (South Urals) is proved by zircon dating. Two metamorphic complexes have been identified in the eastern framing of the Middle–Late Paleozoic Suunduk granitoid pluton. The Zelenodolsk complex is represented by granitic gneisses, while the Kusakan complex (Kusakan Formation) is represented by metavolcanic gneisses, crystalline schists, and quartzites. U–Th–Pb analyses (SHRIMP II) of accessory magmatic zircons from the Zelenodolsk granitic gneiss yielded dates of 478 ± 5 Ma. The protolith age of the Kusakan gneiss (andesitic metatuff) is 529 ± 6 Ma. The origin of volcanic, volcanosedimentary rocks, and granitoids in the East Uralian megazone is associated with the evolution of the volcanoplutonic belt in the active continental margin.

Age of Detrital Zircons and Sources of Terrigenous Deposits of the Olokit Zone (Northern Baikal Region)

This work presents the results of U–Th–Pb (LA–ICP–MS) geochronological study of detrital zircons from terrigenous deposits of the Olokit zone of the Baikal–Patom fold–thrust belt of the Siberian Craton. It was established that the age of deposits of the Olokit Formation is in the range of 0.86–0.72 Ga. Archean and Early Proterozoic rocks of the southern part of the Siberian Craton and the Neoproterozoic complexes of the Baikal–Muya belt were the sources of detrital zircons of the terrigenous rocks of the Olokit zone. There is no evidence of rocks that could have been the sources of Mesoproterozoic (1.08–1.46 Ga) and Late Paleoproterozoic (1.65 Ga) detrital zircons for the deposits of the Ondok paleo-uplift in the Olokit zone, in the Baikal mountain region, and in the Siberian Craton. The data obtained and the results of paleomagnetic reconstructions indicate that Siberia and Laurentia in the Meso- and Early Neoproterozoic were in a fixed position relative to each other; an unknown continental block composed of Mesoproterozoic rocks could have occupied the space between them.

Neoproterozoic–paleozoic detrital sources in the Variscan foreland of northern Iberia: primary v. recycled sediments

Abstract The Cantabrian Zone, in the Variscan belt of Western Europe, has one of the most continuous Ediacaran–Paleozoic stratigraphic successions in the world. This succession has been extensively studied, including several detrital zircon U–Pb geochronological studies on 30 samples spanning the aforementioned time slice. In this work, we present data from three new samples covering previously unsampled time-slices and perform multidimensional analysis on the 33 samples in order to quantify the similarity/dissimilarity among all of them with the aim of detecting potential changes in source areas through time and the role of recycling. The results indicate a continuous source of sediments from Ediacaran to late Devonian times punctuated by a sudden ephemeral change in the Early Cambrian that can be attributed to local causes during the inception of the Paleozoic passive margin.

The First Evidence of Cambrian Granitoid Magmatism during the Formation History of the Bureya Continental Massif of the Central Asian Fold Belt

The results of U–Pb geochronological studies of Early Paleozoic granitoids from the northern part of the Bureya continental massif are presented. It has been established that porphyroblastic hornblende–biotite granites of the Kivili massif referring to the complex of the same name are of 502+6 Ma in age, while weakly gneissic biotite granites of the Kivity massif, Sularin complex, are of 492+6 Ma in age. Thus, granitoids of both complexes are of Cambrian rather than Ordovician age, as was previously assumed. In combination with materials already obtained, these data indicate that intensive granitoid magmatism manifested itself in the Middle Cambrian–Early Ordovician within the Bureya continental massif. It is noteworthy that, in the structure of the Jiamusi–Khanka continental massif, which is adjacent to the Bureya one, Early Paleozoic magmatism manifested itself in a broader range, which is from the Early Cambrian to the Silurian. This may indicate both differences in the formation history of these continental massifs and insufficient knowledge of the Bureya massif.

Late Paleozoic Age and Nature of Protolith of Metamorphic Rocks of the Djagdagle Formation, Bureya Terrane, Central Asian Fold Belt

The results of geological, geochemical, Sm–Nd isotopic geochemical studies of metamorphic rocks of the Djagdagle Formation and U–Pb geochronological studies of its zircons are presented. In correlation schemes of geological complexes of the Bureya Terrane, this formation is the oldest (conditionally, Late Archean) amid other formations of the terrane; thus, it is considered its basement. The biotite and two-mica garnet-bearing gneisses of the formation formed owing to transformation of primary sedimentary rocks. The maxima on the relative probability curve of zircon age correspond to 487, 541, 690, 778, and 896 Ma. Thus, the Djagdagle Formation is Paleozoic rather than Early Precambrian, as was considered before. The lower age boundary of the accumulation of the protolith is identified by the age of the youngest zircon population of ~487 Ma. Similar ages of some cores and rims of zircons indicate that the rocks of the protolith underwent structural metamorphic transformations under PT conditions of amphibolite facies. The results of Sm–Nd and Lu–Hf isotopic studies indicate that the geological complexes which originated by means of reworking of the Mesoproterozoic crust could have been the source of primary sedimentary rocks. Taking into account the lack of Mesoproterozoic (and older) rocks in the structure of the Bureya Terrane, it can be suggested that the sources included some amount of ancient recycled material. Similar geological evolution of the Bureya and Jiamusi terranes is suggested on the basis of similar age ranges of metasedimentary rocks of the Djagdagle Formation and Turan Group of the Bureya Terrane and the Kimkan Formation of the Jiamusi Terrane. This is also evident from their synchronous Neoproterozoic magmatism.

Source and deposition age of the Dialé-Daléma metasedimentary series (Kédougou-Kéniéba Inlier, Senegal) constrained by U–Pb geochronology on detrital zircon grains

The Dialé-Daléma metasedimentary series is exposed in the Kédougou-Kéniéba Inlier that corresponds to the northwestern branch of the Eburnean orogenic belt in the southern West African Craton. Here we conducted a U–Pb geochronological study on metapelites, metagraywackes and metavolcanic breccia of the Dialé-Daléma metasedimentary series in order to identify the sedimentary sources and establish the lithostratigraphic sequence of the Kédougou-Kénieba Inlier. This new U–Pb geochronological dataset from five samples representing different stratigraphic levels yields a dominant population of Paleoproterozoic detrital zircon grains with ages ranging from c. 2200 to 2100 Ma. The youngest weighted mean ages at c. 2120 Ma are identical for all five samples within error and provide a maximum deposition age for the sediments of the Dialé-Daléma series. The dominant ages are similar to those obtained on metamorphosed plutonic and volcanic rocks of the Mako belt and thus suggest a dominant proximal source for the clastic sediments of the Dialé-Daléma series and a distal or geographically isolated position relative to the Archean Leo-Man or Réguibat craton nuclei. Deposition ages are only a few Myr older than available ages for metamorphism and intrusion of plutons forming the Saraya batholith. This is consistent with deposition along an active convergence zone marked by the succession, within a few tens of Myr, of (1) magmatic accretion of the Mako plutonic and volcanic rocks in the context of volcanic arc, associated with local uplift, exhumation and erosion at c. 2200-2160 Ma, (2) deposition of clastic sediments forming the Dialé-Daléma series at c. 2120-2110 Ma followed by (3) their burial and exhumation at c. 2090–2060 partially contemporaneous with (4) the intrusion of the Saraya batholith at c. 2080–2070 Ma.

SHRIMP U–Pb ages of zircons from the Lengshuikeng ore field, North Wuyi Mountains, South China: Implications for dating volcaniclastic rocks

AbstractThe Lengshuikeng Ag‐Pb‐Zn ore field is located in the North Wuyi Mesozoic volcanic belt south of the Qinzhou–Hangzhou suture zone between the Yangtze and Cathaysia paleo‐plates. Previous zircon U–Pb geochronological studies on ignimbrites and tuffs from this area have yielded conflicting ages of 157–161 Ma (Early Upper Jurassic) and 137–144 Ma (Early Lower Cretaceous). Volcanic rocks in the ore field have even been proposed to include both ages. Our SHRIMP zircon U–Pb dating of the ignimbrite and tuff samples from the ore field, along with field observations and results from geochronological work on other volcanic and sub‐volcanic rocks in the region, shows that two populations of magmatic zircons, one autocrystic and the other xenocrystic, are present in the pyroclastic rocks. The autocrystic zircons have ages suggesting formation/eruption at approximately 140 Ma, whereas the xenocrystic zircons give ages of 155–159 Ma, indicating intrusion of granitic porphyries in the Early Upper Jurassic. Therefore, the pyroclastic rocks in the Lengshuikeng Ag–Pb–Zn ore field formed in the Early Lower Cretaceous. The youngest zircon U–Pb ages from pyroclastic rocks may not represent the formation/eruption ages of the host rock, depending most likely on the existence and/or abundance of juvenile or vitric pyroclasts in the rocks.

Calcic Garnets as a Source of Information on the Age of Ultramafic Alkaline Intrusions in the Kola Magmatic Province

The paper presents data of U–Pb (ID-TIMS) geochronologic studies of schorlomite–morimotoite–andradite garnet from the Vuoriyarvi, Sallanlatva, and Salmagorsky massifs in the Kola alkaline province. The garnet is relatively rich in U (5–9 ppm) and poor in common Pb (Pbc/Pbt = 0.2–0.7), contains U as an isomorphous component, and does not host inclusions of any U-bearing minerals. The U–Pb ages of garnet from the various rocks of the massifs (373 ± 2 to 377 ± 1 Ma) are mutually consistent and correspond to the main pulse of alkaline ultramafic magmatism in the Kola province (360–380 Ma). The geochronologic data indicate that the U–Pb system of schorlomite–morimotoite–andradite garnet is stable and can be utilized as a reliable geochronometer for alkaline ultramafic rocks.

Geochemistry of Metasedimentary Rocks, Sources of Clastic Material, and Tectonic Nature of Mesozoic Basins on the Northern Framing of the Eastern Mongol–Okhotsk Orogenic Belt

Abstract —We present results of geochemical studies of the upper Mesozoic deposits of the Strelka and Malaya Tynda depressions and U–Th–Pb (LA-ICP-MS) geochronological and Lu–Hf isotope-geochemical studies of detrital zircons from these deposits. It is shown that the Strelka and Malaya Tynda depressions, adjacent to the Mongol–Okhotsk Orogenic Belt in the north and extending along the boundary between the southern framing of the North Asian Craton and the orogenic belt, are marginal troughs. These troughs are filled with thick beds of Mesozoic marine (at the bottom) and continental (at the top) metaterrigenous rocks, with an increase in the grain size of clastic material up the section; the rocks should be regarded as molasses. The results of U–Th–Pb geochronological studies of detrital zircons from metaterrigenous rocks of the Strelka and Lesser Tynda depressions, on the one hand, and the eastern part of the Mongol–Okhotsk Orogenic Belt, on the other, show that orogenic processes in the east of the belt were completed at the Early–Middle Jurassic boundary. The depressions began to form after the complete closure of the Mongol–Okhotsk basin and the formation of an orogenic structure at its place. Then they were filled with material supplied both from the Selenga–Stanovoi and Dzhugdzhur–Stanovoi superterranes on the southern framing of the North Asian Craton and from the Mongol–Okhotsk Belt, which was a mountain-folded structure in the Middle Jurassic.

Detrital Zircon U–Pb Geochronology and Provenance of Bayan Obo Group, Northern Margin of North China Craton: New Implications for the Position of NCC in Rodinia

The paleoposition of North China Craton in Rodinia has long been in controversial. This paper mainly focuses on the U–Pb geochronological studies of detrital zircons obtained from Bayan Obo Group exposed in the Shangdu area, Inner Mongolia, aiming to provide more information for interprating this problem. Based on the acquired data, this paper comes to the following conclusions. Firstly, the depositional age of Bayan Obo Group might be from Meso– to Neoproterozoic according to the zircons U–Pb dating results. The lower succession of this group, namely Dulahala and Jianshan formations deposited between 1800 and 1650 Ma. The Halahuogete and Bilute formations deposited between 1500 and 1350 Ma. For Baiyinbaolage and Hujiertu formations, their depositional age was 1250–900 Ma. Secondly, for the provenance of Bayan Obo Group, this paper believes detrital zircons with age of 2.51–2.71 Ga and 2.00–2.48 Ga were from Guyang, Xi Ulanbulang and Zhuozi area; the Khondalite Belt provided detrital zircons with age of 1.95–1.80 Ga; zircons with age of 1.60–1.75 Ga might come from granitic rocks in Miyun Area. The magmatism after 1.60 Ga was rarely recorded in the NCC, therefore those zircons with ages younger than 1.60 Ga might come from outside of NCC. The magmatism with the same age existed in Baltic, Amazonia and Laurentia. Based on previous paleomagnetic researches, this paper proposes that NCC might receive detritus from Baltic during 1560–1350 Ma and had affinity with Laurentia and Amazonia at ∼0.9 Ga in Rodinia. Baltic, Amazonia and Laurentia might be potential provenances for non–NCC detritus in Bayan Obo Group.

Garnet U–Pb Age of Skarns from Dashkesan Deposit (Lesser Caucasus)

The results of the U–Pb geochronological study of garnet from scarns of the Dashkesan deposit are reported. The first “direct” estimate of the age of the skarn formation and ore genesis for this deposit (147 ± 2 Ma) is obtained. The possibility of using the U–Pb garnet system to obtain estimates of the scarns age is shown.

Kumystin granosyenites complex of the late Cryogenian in Bolshoi Karatau (South Kazakhstan), age substantiation

The results of U–Pb geochronological studies of quartz syenites of the Kumystin complex of the Bolshoi Karatau ridge in southern Kazakhstan are presented and their late Neoproterozoic (717 ± 4 Ma) age is substantiated. Kumystin syenites complex together with rhyolites and basalts of the Kainar Formation are the youngest formations taking part in the basement of Karatau-Dzhebagly precambrian massif and formed in the second half of Cryogenian. The data set about the ages of the youngest complexes taking part in the basement of precambrian massifs of the Western part of the Central Asian belt indicates that the ending of magmatic activity within various massifs occurred asynchronously during the Neoproterozoic.

Age and sources of the Lower Mesozoic metasedimentary rocks of the Un'ja-Bom terrane in the Mongol-Okhotsk fold belt: results of the U–Th–Pb geohronological (LA–ICP–MS) AND Sm–Nd isotope studies

The results of the U—Th—Pb geochronological studies indicate that in the Un’ja-Bom terrane the youngest peaks on the relative probability curves of ages are 207 and 212 Ma for zircons from the metasiltstone of the Kurnal formation and from the metasiltstone of the Amkan formation. Similar age estimates for the youngest peaks were obtained for the detrital zircons from the metasedimentary rocks of the Tukuringra terrain of the eastern part of the Mongol-Okhotsk belt, which indicates that the Lower Mesozoic flyschoid complexes in the structure of this belt are developed much wider than is currently believed. The results obtained in our U—Th—Pb geochronological and Sm—Nd isotope geochemical studies suggest’s that the sedimentary rocks material was carried to the sedimentation basin mainly from the continental massifs of the Amur superterrane.

Early Ordovician alkali-ultramafic Zhilandy complex (Central Kazakhstan): structure and age of formation

U–Pb geochronological study of amphibole-bearing quartz monzodiorites of the alkali-ultramafic Zhilandy complex in Central Kazakhstan, whose formation is deduced at the Early Ordovician era (479 ± 3 Ma). The obtained data indicate three stages of intra-plate magmatism in the western part of the Central Asian Orogenic Belt: Late Neoproterozoic stage of alkali syenites of the Karsakpay complex intrusion, Early Cambrian stage of ultramafic-gabbroid plutons of the Ulutau complex formation, and Late Cambrian–Early Ordovician stage of formation of the Zhilandy complex and Krasnomay complex intrusions.

Age and Sources of Detrital Zircons from Jurassic Conglomerates of Strelka Depression (Northern Framing of the Mongol–Okhotsk Fold Belt)

This paper presents the results of U–Th–Pb geochronological study of detrital zircons from cement of the Jurassic conglomerates of the Strelka Depression extending in the sublatitudinal direction along the border between the southern margin of the Selenga-Stanovoy superterrane and the Mongol–Okhotsk fold belt. It has been shown that Paleoproterozoic and Neoarchean zircons overwhelmingly dominate in the conglomerate cement. This indicates that the main input of the material to the sedimentation basin was carried from the Southeast border of the North Asian craton (from the north in modern coordinates). In our opinion, the Strelka Depression formed after the completion of orogenic processes associated with the formation of the Mongol–Okhotsk structure.