Late Paleozoic subduction–accretion along the southern margin of the North Qinling terrane,central China: Evidence from zircon U-Pb dating and geochemistry of the Wuguan Complex

The Qinling Orogen separating the North China plate from the Yangtze plate is a key area for understanding the timing and process of aggregation between the two plates. Two competing and highly contrasting tectonic models currently exist to explain the timing and nature of collision; one advocates a Devonian continental collision while the other favors a Triassic collision. The Wuguan Complex, between the early Paleozoic North Qinling and the Mesozoic South Qinling terranes, can provide important constraints on the late Paleozoic evolutionary processes of the Qinling Orogen. Metamorphosed sedimentary rock of the Wuguan Complex have a detrital zircon age spectrum with two major peaks at 453 Ma and 800 Ma, several minor age populations of 350–430 Ma and 1000–2868 Ma, and a youngest weighted mean age of 358 ± 3 Ma, indicating a mixed source from the North Qinling terrane. The recrystallized zircons yield a weighted mean age of 333 ± 2 Ma, representing the metamorphic age. Geochemical analyses imply that the sedimentary rocks were originally deposited in an active continental margin dominated by an acidic-arc source with a subordinate mafic-ultramafic source. The youngest population of detrital zircons (358 Ma) suggests that the Wuguan Complex developed as forearc basin along the southern accreted margin of the North Qinling terrane during the early Carboniferous, whereas the ca. 520–460 Ma mafic rocks with E-MORB, N-MORB, OIB or island arc basalt signatures probably derived from the Danfeng Group. In combination with regional data, we suggest that the depositional age of the Wuguan Complex is ca. 389–330 Ma, but it was subsequently incorporated into tectonic mélange by the northward subduction of the Paleo-Qinling Ocean. A long-lived southward-facing subduction-accretionary system in front of the North Qinling terrane probably lasted until at least the early Carboniferous.     

Comment on “Structural geology of the Upper Cretaceous Chalk Central Mass,Isle of Wight,UK” by Rory Mortimore. Proceedings of the Geologists’ Association,vol. 122 (2), 2012, pp. 298–331

The Chalk Group of the Central Downs of the Isle of Wight forms a relay ramp between two major inverted extensional faults. Mortimore (2011) presented a structural model of this key area based on a geological map constructed from detailed logging of a limited set of exposed sections. The area has been recently mapped at 1:10,000 scale by the British Geological Survey. Our interpretation of the geological structure differs significantly from that proposed by Mortimore, and suggests that Chalk has relatively uniform dips that progressively steepen towards the hanging wall of the E-W oriented basement faults. However, a suite of mapped extensional faults indicates an element of differential movement or transpression along the main basement structures.     

The Vendian passive continental margin in the southern Siberian Craton: geochemical and isotopic (Sr,Sm–Nd) evidence and U–Pb dating of detrital zircons by the LA-ICP-MS method

Geochemical and isotopic (Sm–Nd and Sr) studies of deposits of the Baikal and Oselok Groups in the southern Siberian Craton and LA-ICP-MS U–Pb dating of detrital zircons show that they accumulated in passive continental-margin settings in the Vendian. The time limits of sedimentation were assessed on the basis of Sr chemostratigraphy of carbonate deposits of the Baikal Group and LA-ICP-MS U–Pb dating of detrital zircons in first-cycle terrigenous deposits of the Oselok Group. The main provenances for rocks of these groups were constant. These were rocks of the cover and basement of the Siberian Craton. Tuffite horizons in upper portions of the groups are the only sign of Late Vendian activation of this block, which is reflected in changes of geochemical indices of terrigenous rocks and their younger Sm–Nd model ages.     

Discussion: Oil seepage or fossil podzol? An Early Oligocene oil seepage at the southern rim of the North Sea Basin,near Leuven (Belgium) by E.D. van Riessen & N. Vandenberghe,Geologie en Mijnbouw 74: 301-312 (1996)

Geologie en Mijnbouw -