Neoproterozoic Volcanic Rocks Research Articles

Au-Pd mineralization and ore-forming fluids of the Bleïda Far West deposit (Anti-Atlas, Morocco)

Research subject. Au-Pd mineralization of the Bleïda Far West deposit, represented by an unusual association of palladium gold, minerals of the Pd-Bi-Se system, as well as silvery gold and minerals of the Pd-Bi-Te system. The deposit is localized in the Neoproterozoic volcanic rocks of the central Anti-Atlas (Morocco).Methods. A chemical analysis of minerals was carried out at the Center for Collective Use of Multielement and Isotopic Studies of the Siberian Branch of the Russian Academy of Sciences (Novosibirsk, Russia) using the electron probe microanalysis (EPMA) method. Fluid inclusions were studied using cryometry and homogenization using a THMSG-600 microthermochamber. The composition of the gas phase and the determination of the solid phases of the inclusions were carried out by Raman spectroscopy. The qualitative chemical composition of fluid inclusion salts was determined by the EPMA method.Results and conclusions. It is suggested that palladium gold and minerals of the Pd-Bi-Se system were formed under the conditions close to those during the formation of Au-Pd infiltration deposits in Brazil, while silvery gold and minerals of the Pd-Bi-Te system could be formed under the conditions similar, but not identical, to the formation conditions of Au-Pd low-temperature mineralization in porphyry deposits. According to the study of fluid inclusions in quartz and calcite, Au-Pd mineralization was formed at temperatures from 384 to 75°C with the participation of homogeneous or heterophasic CaCl2-NaCl highly saline hydrothermal solutions at a depth of 2.8–2.7 km. Minerals of the Pd-Bi-Se system, previously unknown in Bleïda Far West ores, were found in association with native gold: osterboshite (Pd, Cu)7Se5, paladsite (Pd17Se15), (Au,Ag)Se, padmaite PdBiSe, native Se, as well as a number of unidentified phases – Pd2BiSe, Pd3BiSe, Pd4BiSe and Pd5BiSe.

Geochemistry of the Neoproterozoic Volcanic Rocks of the Nakora Area of Malani Igneous Suite, Barmer District, Western Rajasthan, India

The Neoproterozoic rocks of the Nakora Ring Complex (NRC) consist of three phases (Extrusive, Intrusive and Dyke) that are based on the detailed geological mapping (contact relationship, mode of occurrence, position of xenolith, flows, dykes and veins) and their stratigraphic position. NRC consists mainly of acid volcanic rocks besides minor amount of basic rocks with intermediate calc-alkaline to tholeiitic affinities and occurs in the form of ring structures. The Nakora basaltic rocks show LREE enriched nature and they have consistent negative Nb, Ta, Sr and Zr anomalies. The HREE pattern is showing parallel arrangement with HREE pattern of other basic rocks. The Nakora acid volcanic rocks exhibit high LREE enrichment than the HREE with negative Ba, Sr, Eu and Ti anomalies in primitive mantle normalized multi-element diagrams. All the samples show negative Ba, Sr, Eu and Ti anomalies. The diminution in amounts of Sr and Eu is apparently related to the fractionation of feldspars or their retention in the refractory minerals resistant to melting in the lower crust. As compared to trachytes, the rhyolites show high SiO2, high Al2O3, low total alkalis, low total iron, low TiO2, high CaO and high MgO. The petro-mineralogical and geo-chemical data specifies that the NRC rocks are generated from a co-magmatic source through a co-genetic process in a rift tectonic context. Petrogenetic modeling indicates that both the basic rocks and acid volcanic rocks of Nakora may have been derived from rocks akin to Bhilwara mafic metavolcanic/mixed Nakora gabbros and Siwana rhyolite/banded gneiss from Kolar Schist Belt by different degrees of partial melting respectively.

Phase equilibria and geochronology of Triassic blueschists in the Bikou terrane and Mesozoic tectonic evolution of the northwestern margin of the Yangtze Block (SW China)

The northwestern margin of the Yangtze Block is an important part of the northeastern-most margin of the Tibetan Plateau and is composed of several terranes. However, how and when many terranes or microplates accreted to the Yangtze Block remain to be resolved. Among these, the Bikou terrane consists of Archean to Paleoproterozoic basement and Neoproterozoic volcanic rocks. Blueschists occur as lenses along a ductile shear zone in the southern margin of the Bikou terrane. The age of blueschist facies metamorphism remains an unsolved issue but is important to understand the tectonic evolution. The mineral assemblage of the blueschists is sodic amphibole, epidote, chlorite, albite and quartz. The calculated pseudosection constrains the peak P-T conditions of the blueschist-facies metamorphism to be 5.2–6.5 kbar at 350–385 °C. The blueschists contain zircon grains with an age of 765.2 ± 11 Ma, representing the age of the protolith similar to mafic volcanic rocks in the Bikou terrane. Deformed granitic veins in the blueschists have been subjected to blueschist facies metamorphism and contain phengites with Si contents of 3.5–3.6 per formula unit (pfu). The deformed veins contain zircon grains with ages of 232.3 ± 6.8 Ma and 230.3 ± 4.3 Ma, indicating that blueschist metamorphism is younger than 230 Ma. The new data, combined with previous studies, suggest that the Bikou terrane was accreted to the Yangtze Block in the Late Triassic during the collision between the North China and South China blocks. The blueschist belt, which marks the Late Triassic accretionary boundary between the Bikou terrane and the Yangtze Block, merged with the Qinling-Dabie Orogen to the northeast.

Geochronology, Geochemistry, Whole Rock Sr‐Nd and Zircon Hf‐O Isotopes of the Early Neoproterozoic Volcanic Rocks in Jiangshan, Eastern Part of the Jiangnan Orogen: Constraints on Petrogenesis and Tectonic Implications

AbstractThis paper presents the results of combined studies of geochronology, geochemistry, whole rock Sr‐Nd and zircon Hf‐O isotopes carried out upon the rhyodacite and ignimbrite of Shangshu village, Shangyu town and Shanghupeng village of Jiangshan City in Zhejiang Province, along the northwestern side of the Jiangshan–Shaoxing suture. SHRIMP zircon U‐Pb dating of samples in the three areas yielded weighted mean 206Pb/238U ages of 842.8 ± 6.9 Ma and 850.0 ± 7.3 Ma, 839 ± 9 Ma and 832.2 ± 8.1 Ma, 828.3 ± 8.5 Ma and 836.9 ± 9.9 Ma, respectively. These ages are older than the volcanic rocks of the Shangshu Formation dated at around 780 Ma distributed in Fuyang City, Hangzhou City, Kaihua County, etc. The volcanic rocks generally have high SiO2 (54.08–76.80 wt%) and Al2O3 (12.40–21.31 wt%), low Fe2O3 (0.68–8.92 wt%), MgO (0.29–2.49 wt%), CaO (0.12–2.86 wt%), TiO2 (0.10–1.59 wt%) and P2O5 (0.01–0.39 wt%), with variable total alkalis (K2O + Na2O =5.42–8.29 wt%). There exists a clear negative correlation between SiO2 and P2O5. The volcanic rocks have A/CNK ratios of 1.03–2.77 and thus are peraluminous. They are characterized by enrichment in LREE, Rb, Ba, Zr, Hf, K, Th, La, U and depletion in Nb, Sr, P, Ti, with distinct LREE and HREE fractionation of (La/Yb)N values of 5.68–11.67, and with a moderate negative Eu anomaly (&Eu=0.58–0.89). Whole‐rock geochemical data shows that the Jiangshan volcanic rocks are possibly I‐type granitic rocks, even though they have some characteristics of A&S‐type granites due to the magma fractional crystallization and water‐rock interaction. Zircon δ18O values are 3.97‰–5.49‰ (average 4.50‰), 2.90‰–5.21‰ (average 4.32‰) for ignimbrite from Shangshu village section, and Shanghupeng village section, respectively. They are slightly lower than the average δ18O values of igneous zircons in equilibrium with mantle magmas (5.3 ± 0.6‰ (2σ)), the lower δ18O value also demonstrating the presence of high temperature water‐rock interactions. The ignimbrite rocks have positive ∊Nd(t) (4.02, 3.37, 3.91, 4.74, 2.85, 4.39, totals from the three areas) and ∊Nd(t) (in‐situ zircon) (4.3–14.6, a weighted mean of 8.4; 6.6–12.7, a weighted mean of 9.0; 8.1–12.0, a weighted mean of 9.5, respectively, from the three areas). In conjunction with the trace element studies, they indicate that the source region of the Jiangshan volcanic rocks was mainly composed of juvenile lower crustal material, mixed with some mantle‐sourced magma. Detailed elemental and isotopic data suggest that the Jiangshan volcanic rocks were formed in a continental arc setting. There is a series of ca. 860–830 Ma volcanic rocks formed in a back‐arc extensional setting in the southern margin of the eastern Jiangnan Orogen, along the northwest side of the Jiangshan–Shaoxing suture. The first stage rift‐related anorogenic magmatism may have occurred as early as ca. 860 Ma in the eastern Jiangnan Orogen.

New data on age of the Neoproterozoic volcanic rocks of Isakovka Terrain from the Sayan-Yenisei accretion belt (U-Pb, zircon)

The Late Neoproterozoic U-Pb age for zircon of island-arc metadacites (691 8.8 million years) and basalts (572 6.5 million years) of the Kiselikhinskaya Formation of the Kutukasskaya Group was established for the first time. The manifestation of basaltic volcanism is associated with rift-related processes. The studies clarify the Late Precambrian stratigraphy of the Yenisei Ridge and the features of the evolution of the Sayan-Yenisei accretionary belt at the Neoproterozoic stage of its history. Folded-thrust structures of the junction zone of the Yenisei Ridge with the West Siberian Plate may be favorable in relation to the search for unconventional oil and gas traps.

New Data on the Age of Neoproterozoic Volcanic Rocks of the Isakovka Terrane, Sayan–Yenisei Accretion Belt (U–Pb, Zircon)

A Late Neoproterozoic U–Pb zircon age is established for the first time for arc metadacites (691 ± 8.8 Ma) and basalts (572 ± 6.5 Ma) from the Kiselikha Formation of the Kutukas Series. The manifestation of basaltic volcanism is associated with rifting processes. Our studies specify the Late Precambrian stratigraphy of the Yenisei Range and the evolutionary features of the Sayan–Yenisei accretion belt at the Neoproterozoic stage of its history. The fold-and-thrust structures of the junction zone between the Yenisei Range and the West Siberian Plate may be favorable in terms of the search for unconventional oil and gas traps.

Neoproterozoic Low‐δ18O Zircons Revisited: Implications for Rodinia Configuration

AbstractNeoproterozoic magmatic rocks and detrital zircons with low‐δ18O values were frequently identified in the Yangtze Block, Madagascar, and India. Here we first report Neoproterozoic low‐δ18O zircons from the central part of the Cathaysia Block in South China. These zircons include detrital grains from the Early Paleozoic metamorphic rocks, inherited domains from the Early Paleozoic granitic rocks, and igneous ones from the Neoproterozoic volcanic rocks. In combination with the available data for Neoproterozoic 18O‐depleted magma and zircons worldwide, we conclude that, globally, 18O‐depleted magmatic activities generally began after 800 Ma and reached the peak at 780–760 Ma, corresponding to the rifting of Rodinia supercontinent. The occurrence of low‐δ18O magmatism after approximately 730 Ma in the northern Yangtze Block possibly indicates extended rifting that was associated with Neoproterozoic glaciations. The similarity in low‐δ18O magmatism between northwest India, Madagascar, and South China suggests close connections in the Rodinia supercontinent during the middle Neoproterozoic.

Generation of ca. 900–870 Ma bimodal rifting volcanism along the southwestern margin of the Tarim Craton and its implications for the Tarim–North China connection in the early Neoproterozoic

Large scale rift related volcanic suites are widely accepted as the evidence of the initial breakup of supercontinents and thus can be used to reconstruct the relative position of the cratons. Numerous early Neoproterozoic volcanic rocks have been identified in the Tiekelik Belt, southwestern part of the Tarim Craton. The Sailajiazitage volcanic rocks in the Tiekelik Belt are composed of a suite of bimodal volcanics and related pyroclastic rocks. The geochemical characteristics resemble those of continental flood basalts (CFBs) formed in the shallower mantle dominated by spinel–lherzolite with EM2 affinities. The rhyolites have been dated at 896±11Ma at the base, 881±14Ma at the middle and 872±8Ma at the top by U–Pb method on zircon. They erupted in an intraplate rift environment and are interpreted to have formed by crustal melting. Hf isotopes of individual dated zircon grains show a wide range of initial εHf values, from strongly negative (down to −28) to highly positive (up to +12.8), indicating a mixture of mantle- and crustal-derived materials. The bimodal volcanic suite is overlain by the Bochatetage Formation sediments, deposited sometime after 797±12Ma. An identical ca. 900–870Ma age for rift-related volcanic rocks indicate Neoproterozoic regional extensional processes in the southern margin of the Tarim–North China, which allow us to speculate on the correlation of the Tarim and North China cratons during the Neoproterozoic. This voluminous bimodal magmatism resembles the rift-related magmatism in the North China, West African, Congo and São Francisco cratons of age ca. 925–870Ma, suggesting that the Tarim Craton was located close to these cratons during early Neoproterozoic, or alternatively, that such early Neoproterozoic rift magmatism was widespread over large parts of Rodinia.

Neoproterozic anorogenic rhyolite-granite volcanoplutonic association of the Aktau-Mointy sialic massif (Central Kazakhstan): Age, source, and paleotectonic position

Rhyolite-granite volcanoplutonic association was identified among the Precambrian basement complexes of the Aktau-Mointy Massif, Central Kazakhstan. This association comprises rhyodacites, rhyolites, subalkaline rhyolites, tuffs, and felsic volcanogenic-sedimentary rocks of the Altyn Syngan and Urkendeu formations, as well as granitoids of the Uzunzhal Complex. U-Pb (ID-TIMS) dating of accessory zircons from the volcanic rocks and granites showed that the association was formed in the Neoproterozoic (Tonian, 925–917 Ma). The Neoproterozoic volcanic rocks and granites are the youngest Precambrian magmatic complexes and mark the final stage in the formation of the Precambrian crust of the Aktau-Mointy Massif. In terms of major and trace element composition, the volcanic rocks and granites resemble A-type granites, thus indicating the within-plate settings of their formation. It was established that their primary magma could be derived by melting of metatonalitic or metagraywacke protolith at T ≥ 940°C and P ∼ 8–10 kbar in response to mantle magma underplating. Sm-Nd isotope data on the volcanic rocks and granites (TNd(DM) = 1.9−1.7 Ma, ɛNd(T) from −1.9 to −3.5) testify the Paleoproterozoic age of their crustal protolith. Available data have revealed strong similarity between the Neoproterozoic tectonomagmatic evolution of the Aktau-Mointy Massif and the Congo-Sao Francisco paleocontinent, which, with other cratons, composed the southern Rodinia supercontinent. This suggests that the formation of the Tonian anorogenic volcanoplutonic association of the Aktau-Mointy sialic massif was related to the global-scale divergent processes in the southern Rodinia supercontinent (Congo-Sao Francisco paleocontinent).

Peri-Gondwanan terrane interactions recorded in the Cambrian–Ordovician detrital zircon geochronology of North Wales

Precambrian to Ordovician sedimentary basins of Wales display contrasting histories across major NE-striking fault systems. Northeast of the Menai Strait Fault System the Monian Supergroup is a deformed succession of mainly metaclastic rocks. Within the fault system the Arfon Basin contains sandstone and slate overlying Neoproterozoic volcanic rocks. South of the fault system, in the Harlech Dome of the Welsh Basin, a distinctive succession of clastic rocks has been correlated with those of the Meguma Terrane in the northern Appalachian orogen, together comprising the domain Megumia. A detrital zircon analysis from Cambrian sandstone in the Harlech Dome is consistent with this correlation. However, the Early Ordovician Dol-cyn-afon Formation higher in the succession and the overlying Late Ordovician Conway Castle Sandstone show a more diverse provenance, consistent with derivation from the Monian Supergroup. Cambrian sandstone from the Llanberis Slates Formation in the Arfon Basin shows a distinct provenance dominated by a Neoproterozoic source. None of the samples analyzed contains Laurentian detritus. These results suggest that the Welsh Basin was juxtaposed with the Monian Supergroup and its Precambrian substrate along the Menai Strait Fault System by the Tremadocian, and indicate that the Iapetus Ocean remained open at least until the Silurian. The Cambrian detrital zircon record from the Arfon Basin does not show clear similarity to the Monian Supergroup, nor to the Welsh Basin and adjacent Midland Platform, indicating that the basin was isolated from these sedimentary sources within the fault system. The juxtaposition of these terranes probably took place during strike-slip to transpressional tectonism close to the northern margin of Gondwana during the Early Ordovician.

Detrital zircon provenance of the Lower Yangtze foreland basin deposits: constraints on the evolution of the early Palaeozoic Wuyi–Yunkai orogenic belt in South China

AbstractThe Lower Yangtze foreland basin is situated to the northwest of the early Palaeozoic Wuyi–Yunkai orogen in South China. To demonstrate its provenance history and the denudation of the orogen, seven sandstone samples were collected from the upper Ordovician to Silurian strata for U–Pb dating. The zircons show a broad range of ages that can be linked with the ages of specific units in the Wuyi–Yunkai orogen. The zircon spectra in the late Ordovician samples are similar to those in the pre-orogenic strata, suggesting a recycled source. The dominant age population of 880–740 Ma in the early Llandovery samples indicates that the middle Neoproterozoic volcanic rocks were the primary source. A significant age population of 460–425 Ma in the late Llandovery to Wenlock samples reflects the fact that the synorogenic magmatic and metamorphic rocks were exposed to provide detritus. The youngest zircons from the uppermost Silurian strata yield an age of 425 Ma, which approximates the inferred depositional age. This age, together with available biostratigraphic data, indicates that the foreland basin was formed 448–425 Ma ago. We surmise a possible link between the Wuyi–Yunkai orogen and the Appalachian–Caledonian orogen based on the geological constraints and palaeomagnetic data.

Neoproterozoic Volcanic Rocks in the Southern Quruqtagh of Northwest China: Geochemistry, Zircon Geochronology and Tectonic Implications

AbstractThe Early Neoproterozoic Beiyixi Formation volcanic rocks of the southern Quruqtagh comprise mainly of a suite of tholeiitic basalts, alkaline andesites, and calc‐alkaline rhyolites. The rhyolites are characterized by variably fractionated enrichment in light rare earth elements (LREE) and flat in heavy rare earth elements (HREE), and strongly negative Eu anomalies. Compared to the rhyolites, the andesites also exhibit enrichment in LREE and flat HREE (chondrite‐normalized values of La/Yb, and La/Sm are 13.30–41.09, 3.18–6.89 respectively). Their rare earth element patterns display minor negative Eu anomalies. Both of them exhibit coherent patterns with strongly to moderately negative anomalies of Nb, Zr, Ti, and Hf on spider diagrams. Two rhyolite and one andesite magmatic zircons with defined oscillatory zoning yielded weighted mean 206Pb/238U ages of 743 ± 7 Ma, 741±2 Ma, and 727±4 Ma. These ages are interpreted to represent the riming of volcanic eruptions. According to geochemistry and rock type, these volcanic rocks formed within a continental island‐arc environment following subduction of the oceanic crust during the Early Neoproterozoic period.

Mid–Late Neoproterozoic rift-related volcanic rocks in China: Geological records of rifting and break-up of Rodinia

Early Cambrian and Mid–Late Neoproterozoic volcanic rocks in China are widespread on several Precambrian continental blocks, which had aggregated to form part of the Rodinia supercontinent by ca. 900 Ma. On the basis of petrogeochemical data, the basic lavas can be classified into two major magma types: HT (Ti/Y > 500) and LT (Ti/Y < 500) that can be further divided into HT1 (Nb/La > 0.85) and HT2 (Nb/La ≤ 0.85), and LT1 (Nb/La > 0.85) and LT2 (Nb/La ≤ 0.85) subtypes, respectively. The geochemical variation of the HT2 and LT2 lavas can be accounted for by lithospheric contamination of asthenosphere- (or plume-) derived magmas, whereas the parental magmas of the HT1 and LT1 lavas did not undergo, during their ascent, pronounced lithospheric contamination. These volcanics exhibit at least three characteristics: (1) most have a compositional bimodality; (2) they were formed in an intracontinental rift setting; and (3) they are genetically linked with mantle plumes or a mantle surperplume. This rift-related volcanism at end of the Mid–Neoproterozoic and Early Cambrian coincided temporally with the separation between Australia–East Antarctica, South China and Laurentia and between Australia and Tarim, respectively. The Mid–Late Neoproterozoic volcanism in China is the geologic record of the rifting and break-up of the supercontinent Rodinia.

Precambrian microcontinents of the Ural-Mongolian Belt: New paleomagnetic and geochronological data

The knowledge on the early stages of evolution of the Ural-Mongolian Belt (UMB) (Late Neoproterozoic-Cambrian) is a key for understanding of its evolution in the Paleozoic. Unfortunately, this stage remains poorly studied. The tectonic reconstructions of the UMB for this time primarily depend on the views on the kinematics and tectonic evolution of numerous sialic massifs with Precambrian basement in the structure of the Tien Shan, Kazakhstan, Altai, and Mongolia. At present, the concept of the origin of these massifs is largely based on the lithostratigraphic similarity of the Neoproterozoic and Lower Paleozoic sections of the Tarim, South China, and Siberian platforms with coeval sections of Precambrian massifs within the UMB. New paleomagnetic and geochronological data can serve as additional sources of information on the origin and paleotectonic position of the microcontinents. In this paper, we present new isotopic datings and a new paleomagnetic determination for the Neoproterozoic volcanic rocks of the Zabhan Formation from the Baydrag microcontinent in central Mongolia. It is established that 805−770 Ma ago (U-Pb LA-MC-ICP-MS age of zircon) the Baydrag microcontinent was situated at a latitude of 47 ± 14° in the Northern or Southern hemisphere. These data provide new insights into the possible origin of the Precambrian micro-continents in the UMB. Analysis of paleomagnetic data and comparison of the age of the basement beneath various plates allow us to state rather confidently that ∼800 Ma ago the micro-continents of the UMB belonged to one of the North Rodinian plates: Indian, Tarim, or South China; their Australian origin is less probable.

内蒙狼山新元古代酸性火山岩的发现及其地质意义

Bedded acid volcanic rocks, which preserve blasto-glomeroporphyritic or porphyritic crystals consisting of quartz and albite with clear evidence of alterations and deformations, from the Mesoproterozoic Zha’ertaishan Group in the southwestern section of Langshan, Inner Mongolia were analyzed. Zircons from two acid volcanic rock samples, A8-0 and A14-5, were separated. The zircons exhibited an igneous morphology, oscillatory zonings and relatively high Th/U ratios, and they were mostly prismatic or short prisms either colorless or brown in color, suggesting a magmatic origin. SHRIMP U-Pb dating was carried out and the 20 measurement points obtained from sample A8-0 yielded a weighted average 206 Pb/ 238 U age of 805±5 Ma. Twenty measurements were also taken on the A14-5 sample, but five points (3.1, 6.1, 7.1, 20.1 and 21.1) were discarded because of their higher 206 Pb/ 238 U ages (867.0 ± 10, 829.1 ± 8.0, 849.7 ± 9.8, 839.2 ± 9.7 and 830.7 ± 9.3 Ma, respectively). The remaining 15 points yielded a weighted average 206 Pb/ 238 U age of 814 ± 4.6 Ma. These ages are interpreted as the timing of an acid volcanic eruption and thus represent a Neoproterozoic (805–814 Ma) submarine volcanic eruption in the southwestern section of Langshan, Inner Mongolia. Geochemical data show that the acid volcanic rock is enriched in light rare earth and high field strength elements, but depleted in heavy rare earth and lithophile elements that have well-developed Eu anomalies similar to those of the Neoproterozoic volcanic rocks. These rocks are comparatively depleted in high field strength elements Th, Nb, Ta, Hf, Zr, and from the western Yangtze block and Ciwu, Zhejiang, revealing that the acidic magma had erupted in. We suggest that this submarine volcanic eruption is probably related to the breakup of Rodinia in Langshan, Inner Mongolia. This discovery, together with the ages of 814–872 Ma reported for the neighboring terranes of Jinningian deformed granites in the Alax area and the age of 827 Ma for the ultramafic rock (ultramafite) in Jinchuan, as well as the recent report of magma activities during the Neoproterozoic in the North China Platform and adjacent areas, is of significance help in understanding the Rodinia evolution in the western section of Langshan, Inner Mongolia.

Geochemical, U–Pb zircon, and Nd isotope investigations of the Neoproterozoic Ghawjah Metavolcanic rocks, Northwestern Saudi Arabia

New geochemical, Nd-isotope and U–Pb zircon data from Neoproterozoic volcanic rocks from Wadi Sawawin in northwestern Saudi Arabia provide important constraints on the evolution of the crust in this part of the Arabian–Nubian Shield (ANS). The Ghawjah volcanic rocks range from tholeiitic to calc-alkaline and are metamorphosed to greenschist facies. U–Pb zircon analyses for Ghawjah andesite yield a weighted mean 206Pb/ 238U age of 763 ± 25 Ma, indicating that these are some of the oldest rocks of the Midyan terrane. Ghawjah volcanic rocks are mostly moderately fractionated, as indicated by Mg-numbers between 28 and 67, Cr between 5 to 537 ppm and Ni from 4 to 175 ppm, REE patterns are slightly fractionated [(La/Yb)N = 1.2 to 4.0], and multi-element diagrams show Ba, Sr, Rb and K enrichments and Nb and Ta depletions, typical of modern convergent-margin igneous rocks. Ghawjah volcanic rocks have positive εNd (+ 5.4 to + 8.2) and a mean model age of 0.71 Ga. Ghawjah volcanic rocks are similar to the “Younger Volcanic” rocks from the Central Eastern Desert (CED) of Egypt, in terms of stratigraphic relations, chemical compositions, Nd-isotopic compositions, and U–Pb zircon ages (~ 750 Ma), indicating that both were generated by partial melting of subduction-modified depleted mantle. The Ghawjah volcanic rocks are interpreted to have formed at ~ 750 Ma in an arc setting during an important episode of crust formation.

Geochemistry, Nd isotopes and U–Pb SHRIMP zircon dating of Neoproterozoic volcanic rocks from the Central Eastern Desert of Egypt: New insights into the ∼750 Ma crust-forming event

Submarine tholeiitic and calc-alkaline basalt and andesite with subordinate dacite, metamorphosed to greenschist facies, are exposed at Wadi Kareim and Wadi El Dabbah, Central Eastern Desert of Egypt. These metavolcanic sequences are representative of early stages of Neoproterozoic crust formation in the northern part of the Arabian-Nubian Shield. The magmas were primitive to moderately fractionated, as indicated by average Mg# (54), Cr (191ppm) and Ni (69ppm). Their REE patterns are slightly fractionated [(La/Yb)N=1.3–3.9], and multi-element diagrams show Ba, Sr, K and Rb enrichments and Nb and Ta depletions, typical of modern convergent margin igneous rocks. Tectonic discrimination diagrams also show affinities with modern convergent margin magmas, suggesting that Wadi El Dabbah lavas formed in a volcanic arc that experienced cryptic crustal contamination, whereas Wadi Kareim lavas formed in a back-arc basin. Positive initial ɛNd (+5.1 to +8.9) and Nd model ages (0.64–0.79Ga) indicate that the crust is juvenile and was extracted from a depleted mantle source. Wadis Kareim and El Dabbah metavolcanic rocks are dated at ∼750Ma (U–Pb SHRIMP) but contain abundant pre-Neoproterozoic zircons which might be derived from underlying sediments or perhaps older continental crust. These results indicate that a major ∼750Ma crust-forming event involving ophiolite generation and eruption of juvenile melts was important for the formation of the crust.

Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U–Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.

U–Pb zircon geochronology and geochemistry of Neoproterozoic volcanic rocks in the Tarim Block of northwest China: implications for the breakup of Rodinia supercontinent and Neoproterozoic glaciations

As the lowest volcanics-bearing unit of the Neoproterozoic succession, the Beiyixi Formation is the key to understanding the early response to the breakup of the Roninia supercontinent in the Tarim Block. The SHRIMP analyses of zircons from the volcanic rocks at the bottom of the Beiyixi Formation yield a weighted mean 206Pb/ 238U age of 755 ± 15 Ma. This is interpreted as the eruption age of the Beiyixi volcanic rocks. The Beiyixi volcanic rocks consist of bimodal basalt and dacite-rhyolite with a SiO 2 gap between 55% and 65%. The mafic rocks display negative ɛ Nd (755 Ma) values (−9.9 to −10.8), moderate enrichment in LILE and variable depletion in Nb, Ta and P, resembling those of the tholeiitic basalts in continental rift. Geochemical and Nd isotopic characteristics suggest that the mafic rocks were derived from partial melting of an enriched lithospheric mantle reservoir. The felsic rocks show negative ɛ Nd (755 Ma) values (−7.9 to −9.2), negative Nb, Ta, P and Ti anomalies, very high La N/Yb N (62–92) ratios and LILE abundances, and may be generated by melting of eclogites or garnet amphibolites in the lower crust, as a result of basalt emplacement into continental crust during continental rifting. The age of 755 ± 15 Ma indicates that the Beiyixi glaciation took place later than 755 Ma and it could be correlated with the Chang’an glaciation in the Yangtze Block and the Sturtian–Rapitan glaciation in other Rodinia Blocks. The geochemical characteristics of the Beiyixi volcanic rocks resemble those of the rift-related magmatism in other Rodinia Blocks, suggesting that the Beiyixi volcanism was a part of global magmatism during the breakup of Rodinia supercontinent. The age and geochemical features of the Beiyixi volcanic rocks also reveal that the mantle plume activity spread to the northwestern margin of the Rodinia supercontinent and probably resulted in the breakup between Australia and Tarim Blocks.

Significance of Late Paleozoic Fission-track Ages in Volcanic Rocks from the Lavras Do Sul Region, Southernmost Brazil

This paper describes the application of apatite fission-track thermochronology to four outcrop samples of the basaltic to andesitic volcanic rocks of the Hilário Formation, located in the Precambrian Sul-rio-grandense Shield, in southernmost Brazil. The investigated unit was generated during the late stages of the Neoproterozoic Brasiliano/Pan-African Cycle (900-500 Ma), in a context of strike-slip motion along NE-trending structures and consequent basin inception. The apparent fission-track (FT) ages fall in the range 270.1 to 298.2 Ma, late Carboniferous to early Permian. The mean confined fission-track lengths range between 10.76 and 12.20 mm and the estimate for initial fission-track recording (below 110°C) is positioned around 359 and 344 Ma. Considering this, the Neoproterozoic volcanic rocks of the Hilário Formation were buried under at least 4 km of volcanic and sedimentary rocks, resulting in thermal fading of all the original fission-tracks, and remained so until the late Paleozoic. Then, the investigated rocks and the whole region acted as the southern margin and source area for the Paleozoic/Mesozoic intraplate Paraná Basin. A long-lived cooling trend, from the early Carboniferous (359-344 Ma) to the Permian/Triassic boundary (ca. 250 Ma) can be inferred from the analysis of the thermal history curves. This tendency can be associated to the conjunction of uplift and denudation of this region. During the Carboniferous, it probably reflects a response to the compressive events in the southwestern Gondwanan margin, in the context of the Eo-Hercynian (Chañic) Orogeny. This period, in the studied region, was also characterized by extensive ice caps related to the Gondwanan glaciation and by lack of sedimentation. With the collapse of the glaciers, alluvial fan and glaciogenic sedimentation took place during the late Carboniferous and early Permian. This cooling phase is probably recorded by the apparent FT ages. A subsequent uplifting event is revealed by the analysis of the thermal history curves, which show a strong inflection between 90 and 70 Ma. These data are here tentatively correlated with significant re-arrangements of the Brazilian southeastern coast, such as the Serra do Mar and Serra da Mantiqueira uplifts.