Sedimentology and regional significance of the ‘argillite unit’, a probable Cryogenian map unit in southeast Yukon, Canada

An unnamed succession of volcaniclastic argillite, sandstone, and conglomerate (the ‘argillite unit’) is exposed in an inlier in southeastern Yukon (NTS 95C/5). These strata were hornfelsed during emplacement of the Pool Creek syenite (ca. 640–650 Ma) and are correlated with the late Cryogenian Hay Creek Group of the Mackenzie Mountains based on lithostratigraphic evidence and published constraints from detrital zircons. The unit preserves three argillite lithofacies, two sandstone lithofacies, and two conglomerate lithofacies. The argillite and sandstone can be grouped into three facies associations, reflecting differing volumes of sandstone and the presence or absence of chaotic bedding. Deposition was mainly from sediment‐gravity flows, particularly turbidity currents and debris flows. Post‐depositional slumping was common, and the succession is interpreted to have been deposited on a slope that trended NNW and dipped to the ENE. This orientation was subparallel to but facing towards the contemporaneous slope of the Hay Creek Group in the Mackenzie Mountains. Conglomeratic facies are dominated by angular to poorly rounded clasts of subalkali basalt that probably were entrained in debris flows during or soon after eruption and otherwise saw little transport or weathering. Geochemistry of the clasts is permissive of a rift‐related setting. Following deposition, but prior to deposition of overlying Cambro‐Ordovician strata, the argillite unit underwent compression that produced broad, open folds, consistent with recent proposals for late Neoproterozoic transtension–transpression on the present‐day northwest margin of Laurentia. The argillite unit provides a snapshot of the geological evolution of southeast Yukon during the late Cryogenian, providing a new data point for reconstructing the protracted and complex rifting history of Rodinia in western Canada. © 2016 Her Majesty the Queen in Right of Canada Geological Journal © 2016 John Wiley & Sons, Ltd.

The source of Fengjia and Langxi barite–fluorite deposits in southeastern Sichuan, China: evidence from rare earth elements and S, Sr, and Sm–Nd isotopic data

The barite–fluorite metallogenic belt in southeastern Sichuan is one of the most important barite–fluorite ore‐concentrated areas in China. In order to find the ore‐forming source and the mineralization age of the Fengjia and Langxi barite–fluorite deposit, we systematically evaluated the rare earth element (REE) contents and isotopic characteristics of S, Sr, and Sm–Nd. The results show that the δ34S values of barites and pyrites are high and similar to S isotopic values of gypsum in the Cambrian Qingxudong Formation (∈1q), suggesting that the sulphur source was from Cambrian evaporite strata. The initial 87Sr/86Sr ratios of fluorites and barites range from 0.708800 to 0.712999, and these values are similar to Cambrian carbonate rocks and Lower Ordovician limestone. Except for black shales of the Lower Cambrian Niutitang Formation, the Ba content of other strata was generally low. The REE characteristics of barite and fluorite have close relationships, in regard to the source of ore‐forming materials, to the Lower Cambrian Niutitang Formation black shales. Hence, we have concluded that the Cambrian carbonate rocks and Lower Ordovician limestone provide the source of calcium for mineralization and that the source of barium comes from the Lower Cambrian Niutitang Formation black shales. The fluorine contents of the Upper Sinian Doushantuo Formation and the Lower Cambrian Mingxinshi Formation are much higher than other strata, and these formations could provide the source for fluorine. The Sm–Nd isochron age of fluorites is 104 ± 11 Ma, which demonstrates that the mineralization mainly occurred during the late Yanshanian. Copyright © 2016 John Wiley & Sons, Ltd.

Genesis and tectonic implications of cumulate pyroxenites and tectonite peridotites from the Nagaland–Manipur ophiolites, Northeast India: constraints from mineralogical and geochemical characteristics

The Nagaland–Manipur ophiolites (NMO) of Northeast India forms a part of the Tethyan ophiolites and comprises a suite of tectonite peridotites and cumulate mafic–ultramafic sequence with mafic extrusive–intrusive rocks, felsic intrusives and oceanic pelagic sediments along with minor podiform chromitites. However, sheeted dykes, which are considered as a significant component of ophiolites, are absent in the NMO. The tectonite peridotites are distinguished from the cumulate pyroxenites by the presence of pyroxene lineation, deformed bands and strained extinction in olivine, kink twin lamellae in pyroxene. Both the tectonite peridotites and cumulate pyroxenites contain aluminous spinel with Cr number [Cr# = Cr/(Cr + Al)] in the range of 0.14 to 0.29 and 0.27 to 0.48, respectively. Mg number [Mg# = Mg/(Mg + Fe2+)] in Cr‐spinel is higher in tectonite peridotites (0.71–0.76) than cumulate pyroxenites (0.44–0.53). Chondrite‐normalized rare earth elements (REE) patterns of cumulate pyroxenites exhibit depleted at light REEE (LREE) (LaN/SmN = 0.380–0.759) but flat middle REE (MREE) to heavy REE (HREE) (SmN/YbN = 0.622–0.756). However, the tectonite peridotites show gradual decrease in concentrations from HREE to MREE (SmN/YbN = 0.285–0.460) and slight increase in LREE (LaN/SmN = 0.721–2.201). The cumulate pyroxenites show strong enriched PPGE patterns and higher PGE concentrations (∑PGE = 85.8–163.5 ppb) compared with the tectonite peridotites (∑PGE = 34.8–113.0 ppb). The estimated equilibration temperature ranges from 890 to 931 °C for cumulate pyroxenites and 971 to 1156 °C for tectonite peridotites. The olivine–spinel equilibrium along with Cr‐spinel chemistry and PGE data suggests that the tectonite peridotites represent the residual mantle left after limited extraction of basaltic melts by low‐degree partial melting (<15%). Conversely, the presence of highly magnesian orthopyroxene and clinopyroxene in the cumulate pyroxenites in conjunction with their geothermometry suggests that they were formed at high pressure and temperature by magmatic fractionation from the basaltic melt. The geochemical data together with field and petrographical evidences indicate that both the tectonite peridotites and cumulate pyroxenites are essentially spinel‐bearing and devoid of plagioclase, suggesting their derivation in the mantle beyond the stability limit of plagioclase in a mid‐oceanic ridge tectonic setting. We conclude that the ultramafic sequence of the NMO was initially generated at a mid‐oceanic ridge tectonic setting close to the eastern boundary of the Indian passive margin and then thrust over the continental margin of the Indian Plate towards the west during its collisional and subduction process with/beneath the Myanmar Plate. Copyright © 2016 John Wiley & Sons, Ltd.

Revision of echinoids from the Tournaisian (Mississippian) of Belgium and the importance of disarticulated material in assessing palaeodiversity

Articulated echinoids are rare in Palaeozoic strata. In order to gain a better understanding of palaeodiversity and community composition, it is more than useful to incorporate disarticulated specimens into estimates of such metrics. It has been demonstrated that disarticulated ossicles of echinoids from the post‐Palaeozoic can be diagnostic at the species level and can be used to bolster analyses of diversity. Although usually not identifiable to the species level, many families, and some genera, of Palaeozoic echinoids have diagnostic properties that can be recognized from disarticulated plates. Herein, it is demonstrated that such diagnostic properties exist for plates of the Palaechinidae, Archaeocidaridae and Proterocidaridae, and that the utility of using disarticulated echinoid ossicles to aid in palaeodiversity studies extends back into the Palaeozoic. Portions of the echinoid fauna from Tournai, Belgium, are revised and disarticulated plates of echinoids from two upper Tournaisian localities in Belgium, Pair and Petit‐Modave, are examined and described. The fauna of Pair yielded an indeterminate palaechinid and Hyattechinus sp., while the fauna from Petit‐Modave yielded only indeterminate palaechinid plates. Although no articulated specimens are known from these localities, when disarticulated plates are taken into account, a more accurate estimate of palaeodiversity becomes clear. Copyright © 2016 John Wiley & Sons, Ltd.

Early Ordovician island‐arc‐type Manite granodiorite pluton from the Buqingshan Tectonic Mélange Belt in the southern margin of the East Kunlun Orogen: constraints on subduction of the Proto‐Tethyan Ocean

The Buqingshan Tectonic Mélange Belt in the south margin of the East Kunlun Orogen, located in west section of Buqingshan–A’nyemaqen Suture Zone, is one of the key areas to understand continental tectonics and continental dynamics of China. This paper reports zircon U–Pb dating results and geochemistry of the Manite granodiorite (rock mass) in the Buqingshan Tectonic Mélange Belt. Zircons from the granodiorite show oscillatory zoning structures and relatively high Th/U ratios, indicating that they are magmatic zircons. Zircon LA–ICP–MS U–Pb dating for the Manite granodiorite yields ages of 487 ± 11 Ma (MSWD = 2.3) and 479 ± 2 Ma (MSWD = 0.56), implying that the Manite granodiorite were formed in the Late Cambrian to Early Ordovician. Geochemical analyses show that the rocks have high contents of SiO2 (66.06 wt.%–68.60 wt.%) and Al2O3 (14.84 wt.%–16.54 wt.%), and low alkaline (6.17 wt.%–7.43 wt.%), belonging to the middle‐K calc‐alkaline series. The A/CNK (Al2O3/(CaO + Na2O + K2O)) ratios are 1.02–1.15, indicating that the granodiorite is weakly peraluminous. The contents of rare earth elements (REEs) are lower (89.64–130.41 ppm), with weakly negative to weakly positive Eu anomalies (δEu = 0.83–1.12). The primitive mantle‐normalized trace elements are characterized by evidently negative anomalies of Nb, Ta, P, Hf, Ti, etc., and positive anomalies of Th, La, Nd, Zr, Eu, etc. Moreover, the rocks show features of typical I‐type granite. This leads us to conclude that the Manite granodiorites is derived from the partial melting of crust and formed in an island‐arc environment. Combined with previous studies, we believe that the subduction of the Proto‐Tethyan Ocean in the Buqingshan area was ongoing during 487 to 479 Ma and formed island‐arc‐type granites represented by the Late Cambrian to Early Ordovician Manite granodiorite. Copyright © 2016 John Wiley & Sons, Ltd.