Late Middle Cambrian Research Articles

Petroleum accumulations in the tarim basin during various tectonic stages as revealed by U–Pb dating of multi-phase calcite veins in deeply buried carbonate reservoirs

Significant discoveries in deeply buried marine carbonate petroleum exploration have been made recently in the Tarim Basin, NW China. Constraining the evolutionary history of fluid activity and the process of oil charging in the deeply buried carbonate reservoirs is crucial for further hydrocarbon exploration and development. In this study, we conducted an integrated study combining petrography, geochemistry, fluid inclusion analysis, and in-situ U–Pb dating of multi-phase calcite veins collected from the Ordovician carbonate reservoirs, Akeyasu area, Tarim Basin. Four distinct phases of calcite veins were found within the Ordovician carbonate reservoirs. The U–Pb ages of these four phases of calcite veins are as follows: 459.8 ± 4.7 Ma, 418.8 ± 3.7 Ma, 329.7 ± 1.6 Ma to 310.0 ± 3.6 Ma, and 291.7 ± 6.1 Ma to 220.5 ± 7.3 Ma. These ages closely correspond to key tectonic events that influenced the study area, specifically, the Middle Caledonian, Late Caledonian, Middle Hercynian, and Late Hercynian orogenies, respectively. The first phase (C1) calcite veins are believed to have precipitated from meteoric water, closely associated with the first episode of oil charging. Phase Ⅱ (C2) calcite veins primarily formed from Early-Middle Ordovician formation water. Phase Ⅲ and phase Ⅳ (C3 and C4) calcite veins originated from Middle-Late Cambrian formation water and hydrothermal fluids, respectively. Significantly, the second and third episodes of oil charging occurred concurrently with the precipitation of C3 and C4 calcites. Seismic data analysis of Paleozoic strata in the Akeyasu area revealed four stages of fault activity, corresponding to the Early Caledonian, Middle Caledonian, Late Caledonian-Early Hercynian, and Middle-Late Hercynian orogenies. Our findings strongly suggest that the evolution of fluid activity and oil charging was primarily governed by regional orogeny. These large-scale tectonic shifts led to intensified fault activity, ultimately controlling fluid activity and oil charging dynamics. Furthermore, this intensified faulting eased the connection between deep fluid systems and the Ordovician carbonate reservoirs, serving as the primary conduits for fluid migration and oil charging within the deeply buried Ordovician carbonate reservoirs.

Distribution, evolution and structural properties of Wushenqi paleo-uplift in Ordos Basin, NW China

This paper depicts the distribution of the Wushenqi paleo-uplift in the Ordos Basin by using the latest drilling and seismic data, and analyzes the tectonic evolution of the paleo-uplift with the support of Bischke curve and balanced section. The compressional Wushenqi paleo-uplift which developed in the Early Caledonian orogeny (Huaiyuan orogeny) is approximately a ellipse extending in S-N direction. Its long axis is about 194 km and short axis is about 55–94 km in nearly W-E direction. The denudation thickness and area of the Cambrian in the core are 170–196 m and 11 298 kmbrespectively. It was mainly formed during the Huaiyuan orogeny according to the chronostratigraphic framework. It was in the embryonic stage in the Middle–Late Cambrian, denuded after developed obviously at the end of Late Cambrian. The paleo-uplift of the 3rd member of the Ordovician Majiagou Formation was reactivated and reduced in area. In the sedimentary period of the Ma4 Member–pre-Carboniferous, the paleo-uplift experienced non-uniform uplift and denudation. It entered the stable period of burial and preservation in the Carboniferous and later period. The Wushenqi paleo-uplift was formed on the weak area of the basement and tectonic belts, into an compressional structure with irregular morphology, under the control of the non-coaxial compression in the south and north and the stress transmitted by the uplift in the basin. The Wushenqi paleo-uplift has a controlling effect on the sedimentary reservoirs and hydrocarbon accumulation.

Environmental and trilobite diversity changes during the middle-late Cambrian SPICE event

The Steptoean Positive Carbon Isotope Excursion (SPICE) event at ca. 497−494 Ma was a major carbon-cycle perturbation of the late Cambrian that coincided with rapid diversity changes among trilobites. Several scenarios (e.g., climatic/oceanic cooling and seawater anoxia) have been proposed to account for an extinction of trilobites at the onset of SPICE, but the exact mechanism remains unclear. Here, we present a chemostratigraphic study of carbonate carbon and carbonate-associated sulfate sulfur isotopes (δ13Ccarb and δ34SCAS) and elemental redox proxies (UEF, MoEF, and Corg/P), augmented by secular trilobite diversity data, from both upper slope (Wangcun) and lower slope (Duibian) successions from the Jiangnan Slope, South China, spanning the Drumian to lower Jiangshanian. Redox data indicate locally/regionally well-oxygenated conditions throughout the SPICE event in both study sections except for low-oxygen (hypoxic) conditions within the rising limb of the SPICE (early-middle Paibian) at Duibian. As in coeval sections globally, the reported δ13Ccarb and δ34SCAS profiles exhibit first-order coupling throughout the SPICE event, reflecting co-burial of organic matter and pyrite controlled by globally integrated marine productivity, organic preservation rates, and shelf hypoxia. Increasing δ34SCAS in the “Early SPICE” interval (late Guzhangian) suggests that significant environmental change (e.g., global-oceanic hypoxia) was under way before the global carbon cycle was markedly affected. Assessment of trilobite range data within a high-resolution biostratigraphic framework for the middle-late Cambrian facilitated re-evaluation of the relationship of the SPICE to contemporaneous biodiversity changes. Trilobite diversity in South China declined during the Early SPICE (corresponding to the End-Marjuman Biomere Extinction, or EMBE, of Laurentia) and at the termination of the SPICE (corresponding to the End-Steptoean Biomere Extinction, or ESBE, of Laurentia), consistent with biotic patterns from other cratons. We infer that oxygen minimum zone and/or shelf hypoxia expanded as a result of locally enhanced productivity due to intensified upwelling following climatic cooling, and that expanded hypoxia played a major role in the EMBE at the onset of SPICE. During the SPICE event, global-ocean ventilation promoted marine biotic recovery, but termination of SPICE-related cooling in the late Paibian may have reduced global-ocean circulation, triggering further redox changes that precipitated the ESBE. Major changes in both marine environmental conditions and trilobite diversity during the late Guzhangian demonstrate that the SPICE event began earlier than the Guzhangian-Paibian boundary, as previously proposed.

Facies analysis and depositional environment of a late Cambrian mixed carbonate-siliciclastic ramp from the Zard Kuh Mountain (Zagros Basin, south-western Iran)

Mixed carbonate-siliciclastic rocks were particularly widespread during the Furongian (late Cambrian), a critical time interval marked by biotic turnovers, extinction events and carbon cycle perturbations. In this context, the Zagros Basin (SW Iran) is a key area for the study of this time interval due to the well-exposed mixed carbonate-siliciclastic successions. Nevertheless, these are still poorly constrained from a sedimentological point of view. The current study aims to contribute to the palaeoenvironmental reconstruction of the Zagros Basin during the Furongian through high-resolution facies and microfacies analysis of the Tang-e-Ilbeyk section. The latter includes, in ascending order, the upper Mila C Member and Ilbeyk Formation. The facies and microfacies analysis recognized six facies associations, representing different settings of a mixed carbonate-siliciclastic ramp at the north-eastern Gondwanan margin. The vertical stacking pattern of these facies associations revealed five medium-scale Transgressive-Regressive cycles, superimposed by larger-scale sequences. In particular, during the latest Miaolingian (formerly late middle Cambrian) to early Furongian, a climate change from arid to humid conditions caused a major regressive event with the progradation of clastic shorelines and the following pollution and drowning of the Mila C Member's carbonate factory. The latter was replaced by a storm-dominated siliciclastic shelf (Ilbeyk Formation) that, throughout middle/late Furongian, experienced a large-scale transgressive event, culminating with a maximum flooding interval. A significant forced regression occurred in the latest Furongian when shallow water conditions were re-established (uppermost Ilbeyk Formation). The palaeoenvironmental reconstruction of this study allowed correlation with other coeval depositional events at regional (Arabian plate) and global (Laurentia and Baltica) scales, contributing to new constraints for the lower Palaeozoic evolution of north-eastern Gondwana.

No Furongian Biodiversity Gap: Evidence from South China

The middle–late Cambrian is a crucial transition interval linking the Cambrian Explosion (CE) and the Great Ordovician Biodiversification Event (GOBE). Previous studies have indicated the Furongian Biodiversity Gap with significantly lower richness and indistinct changes during this time interval. However, in addition to newly discovered deposits and Konservat-Lagerstätten (i.e., Guole Konservat-Lagerstätte in South China), and well-preserved continuous stratigraphic successions, the detailed richness pattern of this interval has emerged, filling the gap between the CE and the GOBE. The high-resolution richness curve of marine invertebrates in South China based on Constrained Optimization analysis revealed several biodiversity events, including three extinctions, two radiations, and one fluctuation interval. The first biotic crisis, the Guzhangian–Paibian extinction, accounted for a 45.0% richness loss and coincided with the global Marjuman extinction event previously identified in the Laurentian trilobite and brachiopod records. The second extinction, the Jiangshanian extinction, resulting in a 55.2% richness loss, can be correlated to the Steptoean–Sunwaptan biomere extinction in Laurentia. Finally, a third extinction mainly affecting cephalopods and conodonts occurred near the Cambrian/Ordovician boundary in South China, immediately followed by the GOBE, a rapid, extraordinary biodiversity increase in the Early and Middle Ordovician. Two radiation intervals have been recognised in the middle–late Drumian and the middle Paibian, respectively, before the first two extinctions. A long-term richness fluctuation started from the latest Jiangshanian until the Cambrian/Ordovician boundary. Some of the events in South China can be identified in other continents, suggesting a global pattern and mechanism. We further discussed the possible relationships between these biotic and environmental changes. The combined Fe speciation change results showed that the species richness seemed to follow the oceanic redox changes. Thus, there is no significant biodiversity gap but marked fluctuations in marine life associated with environmental volatility between the CE and the GOBE in South China.

A Guzhangian (late Middle Cambrian) fauna from the Gidgealpa 1 drillhole, Warburton Basin, South Australia

A Cambrian Series 3 (Guzhangian, Boomerangian) fauna is described from Core 12 of the Gidgealpa 1 drillhole from the Warburton Basin, northeastern South Australia. There are four agnostoid taxa: Ammagnostus laiwuensis, Lejopyge calva, L. armata and an unassigned pygidium, and six polymerid trilobite taxa: Fuchouia sp., Dorypyge sp., Solenoparia changi sp. nov., Pianaspis sp., an unassigned cranidium and an unassigned pygidium. This fauna suggests a correlation with the Goniagnostus nathorsti Zone or the lower part of the Lejopyge laevigata Zone (Boomerangian on the northern Australian biostratigraphic scheme), with the latter preferred. These are the first published descriptions of fossils of this age from the Warburton Basin. Xiaowen Sun [Xiaowen_s@yahoo.com.au], Sun Petroleum Geoservices, 4 Jolie Grove, Box Hill, Victoria 3128, Australia; Christopher J. Bentley [ bigfossil@bigpond.com], 30 Albert Street, Clare, South Australia, 5453, Australia; James B. Jago [jim.jago@unisa.edu.au], University of South Australia–STEM, Mawson Lakes, South Australia 5095, Australia.

Quantifying the middle–late Cambrian trilobite diversity pattern in South China

The middle–late Cambrian is a key interval between the Cambrian Explosion and the Great Ordovician Biodiversification Event (GOBE) that marks a major biologic evolutionary transition in the geologic record. Diversity (i.e. richness) patterns in this interval show that macro-biota experienced evolutionary setbacks for the first time. However, relatively little attention has been paid to this interval in previous macroevolutionary studies and very different conclusions about the diversity patterns in coarse time-resolution have been documented. As a dominate group of marine benthic organisms in the Cambrian, trilobites provide insightful information to understand the history of biodiversity in the Cambrian marine ecosystem from the Cambrian Explosion to the GOBE. South China is a region with relatively good preservation of Cambrian fossil materials and sections, and 4 out of 6 Cambrian GSSPs were established in South China. In this paper, we use constrained optimization (CONOP) to correlate 19 sections from South China to reduce the Signor-Lipps effect and integrate the results with high-resolution chronostratigraphic data to evaluate the middle–late Cambrian trilobite diversity pattern in South China. This study, for the first time, constructs a high time-resolution (about 60 kyr) trilobite species-level richness curve with estimated time from Age 4 to the Paibian. A double-peaked species diversification is separated by a minor extinction in Stage 4; this is followed by a gradual decline of species diversity over the entire Wuliuan Age. A radiation of three times increase of species diversity in the Drumian and Guzhangian is exhibited and the diversity remains high for most of the Guzhangian Stage; this is followed by a great extinction at the end of the Guzhangian with around 50% species lost. Detailed comparison with previous composite studies and fossil material from single sections revealed that the Signor-Lipps effect could produce large biases in diversity pattern. In addition, a precise analysis of species evolution features compared with environmental indices provides a perspective about trilobite evolutionary mechanisms associated with environmental changes related to the carbon cycle and biotic competition.

A New Species of Modocia (Trilobita: Ptychoparioidea) in the Late Middle Cambrian (Guzhangian: Miaolingian) Devoncourt Limestone, Northwestern Queensland

A New Species of Modocia (Trilobita: Ptychoparioidea) in the Late Middle Cambrian (Guzhangian: Miaolingian) Devoncourt Limestone, Northwestern Queensland

Addendum: A new species of Modocia (Trilobita: Ptychoparioidea) in the late middle Cambrian (Guzhangian: Miaolingian) Devoncourt Limestone, Northwestern Queensland

Addendum: A new species of Modocia (Trilobita: Ptychoparioidea) in the late middle Cambrian (Guzhangian: Miaolingian) Devoncourt Limestone, Northwestern Queensland

Different order cyclicity of the Paleozoic reef formation

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Geology of the Nelson Limestone, Postel Nunatak, Patuxent Range, Antarctica

AbstractPostel Nunatak in the Patuxent Range has been previously mapped as Nelson Limestone but there was no biostratigraphic support for that interpretation until now. We confirm that limestone exposures at Postel Nunatak are at least partly correlated with the Nelson Limestone of the Neptune Range, 160 km north-east, and are not correlative with the lower Cambrian Schneider Hills Limestone of the Argentina Range. Upper beds have yielded the trilobitesSuludella? davniiPalmer & Gatehouse, 1972 andSolenopleura pruinaPalmer & Gatehouse, 1972, which provide a basis for assignment to Cambrian Series 3 (late middle Cambrian), within the Drumian or lower Guzhangian stages. Limestone beds were deposited in a shallow marine setting, ranging from supratidal to lagoonal facies with rare subtidal intervals. These settings contrast with deeper water facies of the Neptune Range. Despite limitations in sampling density, isotopic analysis indicates that a greater than +2.5‰ shift in δ13C is consistent with δ13C trends documented for the Drumian Stage. Because the upper and lower contacts at Postel Nunatak are covered by snow and ice, the relationship with rocks mapped as the Patuxent Formation in the Patuxent Range remains uncertain, but part of it may belong to the Precambrian Hannah Ridge Formation.

Fingerprinting Gondwana versus Baltica provenance: Nd and Sr isotopes in Lower Paleozoic clastic rocks of the Małopolska and Łysogóry terranes, southern Poland

Neodymium and strontium isotope compositions of clastic material in Lower Paleozoic rocks, in addition to detrital zircon and mica geochronology and faunal data are used to constrain the geotectonic provenance of the Małopolska and Łysogóry terranes in southern Poland. Being characterized by markedly different geological histories during Early Paleozoic times, Małopolska and Łysogóry were most often interpreted as exotic terranes, but their identity, provenance and accretionary history are still being discussed. This study presents the first systematic Nd and Sr isotope survey of the Lower Paleozoic siliciclastic rocks. Cambrian clastic rocks of Małopolska differ in their Nd and Sr isotope compositions from those of Łysogóry. The former are characterized by a wide range of the εNd(505Ma) values, from −11.7 to −5.7, Sr isotope ratios from 0.712 to 0.767, and model ages (T2DM) predominantly younger than 1.9Ga, whereas the εNd(t) values of the latter vary within a fairly narrow range of the εNd(505Ma), from −11.0 to −8.3, Sr isotope ratios from 0.722 to 0.754, and model ages are older than 1.9Ga. New Nd data combined with detrital zircon ages provide evidence that Małopolska must have been supplied with clastic material from Amazonian sources during Cambrian times, being also progressively sourced from the Baltic Svecofennian crust in the Middle Cambrian. Detritus recognized in Łysogóry has mostly been derived from the Svecofennian basement of Fennoscandia, but it has also a small addition from Cadomian sources. All provenance and biogeographic data suggest that Małopolska was situated close to the Amazonian Craton near the western end of the Cadomian Belt during the Early Cambrian, and it was not until the late Middle Cambrian that Małopolska collided with the Baltica margin. During the Late Cambrian Łysogóry occupied a position between Baltica and the Cadomian Belt but it was probably not yet accreted to the Baltic crust at that time.

The sedimentary facies characteristics and lithofacies palaeogeography during Middle-Late Cambrian, Sichuan Basin and adjacent area

Abstract Combined with the regional strata filling characteristics of Middle-Upper Cambrian, the present paper conducts a systematic research on sedimentary facies in the basin and its peripheral area by utilizing 164 field outcrops and drilling and coring data. Further, the method of “multi-factor comprehensive synthesis based on single-factor analysis” was employed to investigate the sedimentary facies and palaeogeography of the study area and establish the sedimentary facies model. Stratigraphic reveals that the study area represents the pattern of thin-northwest and thick-southeast by stretching northeast-southwest. Within the present basin, the pattern of “one thin and two thick” predominates, while outside the basin “four thin and three thick” filling feature was found. Sedimentary facies shows that the study area was featured by rimmed carbonate platform. Specifically, carbonate platform, slope and northeastern corner Qinling paleooceanic Basin and southeastern corner Jiangnan Bain was identified from the west to the east. The carbonate platform contains restricted platform, evaporation-restricted platform, semi-restricted platform and the platform margin. Single factor analysis and lithofacies palaeogeographic characteristics manifests that during Middle-Late Cambrian, the western Old land evolved into peneplain stage, and that the eastern and southwestern sub-sags remained connected to the open-sea to some extent. At the time, the shllow seawater circulation was relatively restricted, while the ancient seabed tended to be flat and evaporation characteristics significantly diminished. Secondary sea-level fluctuation intensively influenced the development of scaled grain beach. It is suggested that tide marginal beach, intraplatform shoal subfacies zone, along with Shiqian-SangZhi in southeast and Zhenba-Xinshan in northeast platform-margin beach subfacies zone to be preferable targets for the favorable reservoir facies zone and potential oil and gas reservoir area.

Late Palaeozoic tectonic setting of the southern Alxa Block, NW China: constrained by age and composition of diabase

ABSTRACTThe ages of diabase outcropping on the southern Alxa Block and in related tectonic setting are not well constrained, but are important for exploring the tectonic evolution of the southern Alxa Block. The ages of the zircons from the diabase intruding the Middle–Late Cambrian Xiangshan Group were measured using sensitive high-resolution ion microprobe II (SHRIMPII) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and the weighted mean ages of 276.8 ± 7.9 Ma and 276.2 ± 9.7 Ma were obtained, respectively. The major elements in the diabase indicate that it may belong to the tholeiitic series. With specific features of the rare earth elements (REEs) and trace elements, potentially affected by the continental crust based on the observed strong positive Pb anomaly, the diabase was produced in an intra-plate extension environment. Similar diabase/basalts were also found along the southern margin of the Alxa Block. Combined with sedimentary, palaeo-current, mafic, and felsic rocks in the southern Alxa Block, a back-arc extension environment potentially occurred in the southern Alxa Block during the Early–Middle Permian that resulted from the convergence between the North China Block (NCB) and the Yangtze Block.

Evolution of the Paleozoic granitoid magmatism in the Kuznetsk Alatau: new geochemical and U-Pb (SHRIMP-II) isotope data

A U-Pb isotope analysis has revealed zircons, dated at ~510-490 and ~430-400 Ma, in granitoid intrusions on the eastern slope of the Kuznetsk Alatau Range (Batenev Ridge). This suggests two stages of the regional granitoid magmatism: Middle-Late Cambrian and Silurian-Early Devonian. The ages of two zircon grains from the rocks of the Tigertysh pluton are ~1.9 and ~2.6 Ga; they suggest the involvement of fragments of the Paleoproterozoic continental crust in magma generation. The granitoids, independently of their age, are mostly products of rocks of the calc-alkalic (K2O/Na2O ≈ 0.5-1.7) series. In the proportions of alumina, calcium, and alkalies most of them correspond to I-type granites (A/CNK≤1.1) with reduced Fe/(Fe + Mg) values and contents of HFSE, including REE (ΣREE ≈ 100–300 ppm). The trace-element patterns of the rocks point to the possible participation of both IAB and OIB in magma genesis. Presumably, the influence of the within-plate component increased as a result of the superposition of plume on accretion-collisional structures, and their melting might have favored the preservation of geochemical features of subduction magmatism in the granitoids. The isotopic inhomogeneity (εNd(T) = 2.3-4.8, (87Sr/86Sr)T ≈ 0.7036-0.7051) of the granitoid complexes suggests the heterogeneous composition of the material sources, which consisted of the matter of moderately depleted (PREMA) and enriched (EM) lithospheric mantle and crustal substrate, mixed in different proportions. Some products of the regional alkali-basic magmatism are coeval with the Cambrian granitoids of the Kuznetsk Alatau and are similar to them in isotope composition. This magmatism confirms the interaction of plume with the active continental margin at the Caledonian stage of evolution of the Central Asian Fold Belt.

A new family of Cambrian rhynchonelliformean brachiopods (Order Naukatida) with an aberrant coral-like morphology

Tomteluva perturbata gen. et sp. nov. and Nasakia thulensis gen. et sp. nov., two new rhynchonelliformean brachiopod taxa, are described from carbonate beds from the lower middle Cambrian (Series 3, Stage 5) basinal Stephen Formation, Canada, and the upper lower Cambrian (Series 2, Stage 4) Henson Gletscher Formation, North Greenland, respectively. The two taxa are characterized by an unusual coral-like morphology typified by a high conical ventral valve with an anteriorly curved umbo and a tube-like structure inside the ventral valve, interpreted as pedicle tube. Both resemble the problematic late middle Cambrian (Drumian) species Anomalocalyx cawoodi Brock from Australia, whose systematic affiliation is controversial. Together, the three genera are interpreted as representatives of a new family of rhynchonelliformean brachiopods, the Tomteluvidae fam. nov., which is interpreted as an aberrant or derived taxon within the Order Naukatida. Convergence between the Tomteluvidae and the coralla of small solitary Cambrian coralimorphs, as well as the late Palaeozoic reef-building richthofenioid brachiopods, might indicate adaptation to a similar life habits and environments. However, their small size (length 4 mm), well-developed pedicle and perfect morphological symmetry make it more likely that tomteluvids lived attached to frondose algae or sponges, above the seafloor, in a similar fashion to the acrotretoid brachiopods with which they show a high degree of morphological convergence. Morphological features of the pedicle tube of N. thulensis suggest that the tomteluvid pedicle is homologous to that in modern rhynchonelliformean brachiopods. This is the first evidence of the pedicle type within the Naukatida and represents the oldest confirmation of a rhynchonellate pedicle.

Biostratigraphy of the Peri-Gondwana Cambrian trilobite fauna (northern Kerman, Iran) and correlation with other countries

Polymeroid trilobite fauna from two stratigraphic sections (Godbondar and Kuhbanan) of the Kuhbanan Formation in northern Kerman (central Iran) were studied and subjected to biostratigraphic analysis. Eleven genera and species are recognised from the latest Early Cambrian and Middle Cambrian Peri-Gondwanian successions of the study sections. The recognised fauna includes Afghanocare lategenatum, Blountia blountia, Iranoleesia sp., Iranoleesia pisiformis, Kermanella kuhbananensis, Kermanella lata lata, Kermanella lata minuta, Kermanella sp., Redlichia chinensis, Redlichia noetlingi and Redlichia sp. Based on trilobite distribution, three trilobite biozones were recognised in the study sections, namely Redlichia noetlingi biozone, Kermanella kuhbananensis biozone and Iranoleesia pisiformis biozone. The age of the study sections is late Early Cambrian to late Middle Cambrian based on the recognised trilobite biozones. The recognised late Early Cambrian trilobite assemblages (especially Redlichia and Kermanella) from northern Kerman are similar to those found from some other parts of Gondwana or Peri-Gondwana terrains (north India, Pakistan, northwestern Kashmir, Tajikistan, South Australia, South China and Afghanistan) and show affinities with fauna found in some other parts of Iran (Alborz, northern Iran; Tabas, eastern Iran; southeast Karman).

Linking the Alxa Terrane to the eastern Gondwana during the Early Paleozoic: Constraints from detrital zircon U–Pb ages and Cambrian sedimentary records

The detrital zircon ages of the Early Paleozoic Xiangshan Group in the southeastern Alxa Terrane show that the lower age limit for the Xiangshan group is 512Ma. The Xiangshan Group is older than the Middle Ordovician strata in the study region and is the Middle–Late Cambrian in age, which is the same age as the fossils found in it. The detrital zircon ages record the Grenville and Pan-African Orogenic Events, and the major age peaks are 963Ma, 1179Ma, 551Ma and 2517Ma. The North China Terrane, the Alxa Terrane and the Qilian Orogenic Belt were not the provenances of the Xiangshan Group. The age spectrum of the Xiangshan Group is similar to terranes associated with eastern Gondwana. The sandstone composition modes of the Xiangshan Group indicate that the source areas were the recycled orogenic belts. The zircons in the Xiangshan Group with an age peak at ca. 963Ma may come from the Eastern Ghats–Rayner orogenic belt or other terranes located to the north of eastern Gondwana. The zircons with a peak at ca. 1179Ma may come from the Wilkes–Albany–Fraser orogenic belt. Those with a peak at ca. 551Ma may come from the Prydz–Darling orogenic belt, and the zircons with a peak at ca. 2517Ma may be from northern India and the Yilgarn Craton in Western Australia. Similar paleontology, paleomagnetic data and detrital zircon age spectra indicate that the Alxa Terrane was located to the east of the South China Terrane and to the north of the eastern Gondwanan continent (present coordinates).

Nainlin Formation, a new Middle Cambrian map unit from the Mackenzie Mountains, Northwest Territories

Abstract Nainlin Formation (new) is proposed to unify two Middle Cambrian successions of “red beds” that crop out in the eastern Mackenzie Mountains, Northwest Territories. Strata east of Mackenzie Arch previously were treated as a proximal facies of the Saline River Formation whereas equivalent strata further west were included as a basal member within the Franklin Mountain Formation. Both successions are characterized by red-weathering siliciclastic facies that preserve evidence for deposition in shallow to marginal marine settings, commonly under restricted conditions. Whereas typical Saline River Formation is dominated by mudrocks and evaporites, Nainlin Formation is rich in quartzose sandstone, lacks evaporites, and locally preserves conglomerate, notably in sections studied along the east flank of Mackenzie Arch. Although Nainlin Formation locally contains dolostone, its siliciclastic-dominated character distinguishes it from the dolostone-dominated Franklin Mountain Formation. Nainlin Formation lies with angular unconformity upon Neoproterozoic and earlier Cambrian formations and is overlain conformably, generally gradationally, by Franklin Mountain Formation. It is in part age-equivalent to Hess River and Rockslide Formations, which lie to the west and south, but has not been mapped in continuity with them. The western limit of Nainlin Formation is a zero edge in the footwall of the Plateau Fault in NTS 106A; the formation extends from thence into the easternmost Mackenzie Mountains, where it passes laterally into Saline River Formation. Published age controls from underlying and overlying units constrain Nainlin Formation as no older than Delameran (early Middle Cambrian) and not younger than late Marjuman (late Middle Cambrian). Nainlin Formation generally is less than 100 m thick, although locally it can exceed 300 m in thickness. It is preserved in three outcrop belts separated by elongate paleohighs that apparently were “corrugations” on the broad crest of Mackenzie Arch. The eastern high is here dubbed “Sheep Lick high”. Further west, paleohighs can be mapped in the footwall and hanging-wall of Plateau Fault; these are dubbed “Deadend high” and “Sayunei high”, respectively. The paleohighs and intervening outcrop belts are oriented NW-SE, as is the facies transition from Nainlin Formation into Saline River Formation.

Pan-African and early Paleozoic tectonothermal events in the Nyainrong microcontinent: Constraints from geochronology and geochemistry

To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the constraints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in northern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The 206Pb/238U concordant age of zircon is 505–517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have characteristics of high silicon, alkali-rich, alkalic rate AR =1.73–3.7, the differentiation index DI = 70.78–90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 × Ga/Al ranges from 2.12 to 2.41, and P2O5 and Al2O3 content decreased with SiO2 increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with regional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The preliminary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean-type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.