Dan-Ping Yan

Reappraisal of the ages of Neoproterozoic strata in South China: No connection with the Grenvillian orogeny

The Jiangnan fold belt separates the Yangtze and Cathaysia blocks in South China and has long been considered Grenvillian in age in order to place South China in central Rodinia. It consists of deformed Early Neoproterozoic strata that are unconformably overlain by undeformed Late Neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons. Zircons from the Early Neoproterozoic strata yield U-Pb ages as young as 830 Ma, and one granitic pluton has a zircon U-Pb age of ca. 827 Ma. The ≥830 Ma mafic rocks along the southeastern margin of the Yangtze block have arc-affinity geochemical characters, whereas mafic rocks younger than 830 Ma have typical ocean island basalt (OIB)—like compositions. Thus, we suggest that the Early Neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the Yangtze and Cathaysia blocks at ca. 830 Ma. The overlying Late Neoproterozoic strata were deposited in the intracontinental rifted Nanhua Basin at 820–730 Ma and probably reflect backarc spreading above the long-lived (950–735 Ma) oceanic subduction zone along the northern and western margin of the Yangtze block. This model is consistent with the secular tectonic evolution of South China during the Neoproterozoic. The Jiangnan fold belt is therefore not a Grenvillian feature as previously suggested, and there is no evidence to place South China in central Rodinia. Instead, we believe that South China was located in a marginal position relative to this supercontinent.

Mesozoic extensional structures of the Fangshan tectonic dome and their subsequent reworking during collisional accretion of the North China Block

The Fangshan area, SW of Beijing, lies at the junction of the NNE-trending Taihang mountain range and the southeastern portion of the Yanshan intraplate orogenic belt and has undergone at least five stages of deformation. Mid- to late Triassic extensional deformation (D1) is represented by the formation of the Fangshan tectonic dome during SE-directed extensional tectonics. This deformation was later modified by NNW-directed thrusting in the late Triassic (D2) and WNW-directed thrusting in the late Jurassic (D3). These D1–D3 structures were deformed by the arcuate Beiling syncline (D4), which formed during the intrusion of the 133–128 Ma Fangshan pluton. D5 deformation is represented by Cretaceous to Quaternary NNE-striking high-angle normal faults. The ages of these events demonstrate that the principal ENE-trending tectonic framework of the Yanshan intraplate orogenic belt was established mainly in the mid- to late Triassic (the Indosinian event). The Jurassic to Cretaceous Yanshanian deformation with thick-skinned thrusting was associated with intrusion of numerous plutons, and strongly modified the older structures. The tectonic evolution of the Fangshan tectonic dome was linked genetically with the collision of the North China Block and the Siberian Craton, and the later collision of the North and South China blocks.

Constraining the mid-crustal channel flow beneath the Tibetan Plateau: data from the Nielaxiongbo gneiss dome, SE Tibet

Gneiss domes involving the South Tibetan Detachment System provide evidence for crustal extension simultaneous with shortening. The Nielaxiongbo gneiss dome is composed of a metamorphic complex of granitic gneiss, amphibolite, and migmatite; a ductilely deformed middle crustal layer of staurolite- or garnet-bearing schist; and a cover sequence of weakly metamorphosed Triassic and Lower Cretaceous strata. The middle crust ductilely deformed layer is separated from both the basement complex and the cover sequence by lower and upper detachments, respectively, with a smaller detachment fault occurring within the ductilely deformed layer. Leucogranites crosscut the basement complex, the lower detachment, and the middle crustal layer, but do not intrude the upper detachment or the cover sequence. Three deformational fabrics are recognized: a N–S compressional fabric (D1) in the cover sequence, a north- and south-directed extensional fabric (D2) in the upper detachment and lower tectonic units, and a deformation (D3) related to the leucogranite intrusion. SHRIMP zircon U–Pb dating yielded a metamorphic age of ∼514 million years for the amphibolite and a crystallization age of ∼20 million years for the leucogranite. Hornblende from the amphibolite has an 40Ar/39Ar age of 18 ± 0.3 million years, whereas muscovites from the schist and leucogranite yielded 40Ar/39Ar ages between 13.5 ± 0.2 and 13.0 ± 0.2 million years. These results suggest that the basement was consolidated at ∼510 Ma and then exhumed during extension and silicic plutonism at ∼20 Ma. Continuing exhumation led to cooling through the 500°C Ar closure temperature in hornblende at ∼18 Ma to the 350°C Ar closure temperature in muscovite at ∼13 Ma. The middle crustal ductilely deformed layer within gneiss domes of southern Tibet defines a southward-extruding ductile channel, marked by leucogranites emplaced into migmatites and amphibolites. We propose a model involving thinned upper crust for the initial extension of the Tibetan Plateau in the early Miocene.

Discussion on Mesozoic extensional structures of the Fangshan tectonic dome and their subsequent reworking during collisional accretion of the North China BlockJournal, Vol. 163, 2006, 127–142

Yu Wang writes: On the basis of descriptions of the stratigraphic sequences and structural features in the field, and interpretations of published data, Yan et al . (2006 a ) reach the surprising conclusion that early Mesozoic (early–middle Triassic) ESE-oriented extension resulted in formation of the Fangshan metamorphic core complex. If correct, the authors have made an important contribution to understanding of the tectonic evolution of North China. However, the way in which they cite previously published work to support their model, and discrepancies between their findings in the field and those of other workers raise a number of problems that will be discussed below. ### Citing of published work. Yan et al . (2006 a ) cited many references (e.g. Lei et al . 1994; Niu et al . 1994; Yu & Zhang 1996; Chen 1999; Fu 1999; Meng et al . 2003) in support of their conclusions about early Triassic extension and formation of metamorphic core complexes, as well as Late Cretaceous normal faulting. In many cases, however, the cited references do not lend support to their interpretations. For example, the Du Shan granite and surrounding Archaean and Proterozoic metamorphic rocks (Yu & Zhang 1996) contain no evidence for extension or development of metamorphic core complex features, but instead record NE-trending, strike-slip ductile shearing, not top-to-the-SE extension. The Malanyu dome is an old metamorphic sequence (Chen 1999), and no one has previously interpreted it as an extensional dome associated with SE-directed extensional shear. Zhang et al . (1991) did not examine any aspect of the geology in the area of the so-called Yuerya metamorphic core complex. Furthermore, no dome has been identified here by any previous geologists; instead, it is a c . 175–174 Ma (U–Pb sensitive high-resolution ion microprobe ages, Luo et al . 2001) granitic sheet-like …

Geochronology of Early Mesozoic Diabase Units in Southwestern China: Metallogenic and Tectonic Implications

Two phases of diabase-sill-forming magmatism are recorded within the Badu anticline where magmas were emplaced into upper Palaeozoic carbonates and clastic rocks of the Youjiang fold-and-thrust belt in the SW South China Block, China. Zircons from these diabase units yield weighted mean U–Pb ages of 249.2±2.0 Ma and 187.1±3.3 Ma, and magmatic oxygen fugacity values from ‒20 to ‒6 (average of ‒12, equating to FMQ +5) and ‒20 to ‒10 (average of ‒15, equating to FMQ +2), respectively. These data indicate that the sills were emplaced during Early Triassic and Early Jurassic times. The discovery of c . 250 Ma mafic magmatism in this area was probably related to post-flood-basalt extension associated with the Emeishan mantle plume or rollback of the subducting Palaeo-Tethys slab. The c . 190 Ma diabase sills indicate that the southwestern South China Block records Early Jurassic mafic magmatism and lithospheric extension that was likely associated with a transition from post-collisional to within-plate tectonic regimes. The emplacement of diabase intrusions at depth may have driven hydrothermal systems, enabling the mobilization of elements from sedimentary rocks and causing the formation of a giant epigenetic metallogenic domain. The results indicate that high-oxygen-fugacity materials within basement rocks caused crustal contamination of the magmas, contributing to the wide range of oxygen fugacity conditions recorded by the Au-bearing Badu diabase. In addition, data from inherited xenocrystic zircons within the Badu diabase and detrital zircons from basement rocks suggest that the Neoproterozoic Jiangshao suture extends to the south of the Badu anticline.