Liangshu Shu

Early Paleozoic depositional environment and intraplate tectono-magmatism in the Cathaysia Block (South China): Evidence from stratigraphic, structural, geochemical and geochronological investigations

The early Paleozoic geological evolution of the South China Craton composed of the Yangtze and Cathaysia Blocks has been the focus of long debate. The Cathaysia block has been central to the controversy regarding convergent margin versus intraplate environment in the early Paleozoic. In order to address the early Paleozoic evolution of Cathaysia, we undertook a systematic study of the stratigraphic sequences, deformational features and geochronology of magmatic event. Our results show that (1) during the early Paleozoic, the Jiangnan domain of the SE Yangtze block was characterized by a carbonate platform and the Cathaysia block by a graptolite-facies clastic rock assemblage, (2) in the Cathaysia block, a littoral-neritic depositional environment prevailed in Cambrian whereas a neritic-bathyal setting dominated during the early-middle Ordovician, and (3) the Late Ordovician depositional sequence in Cathaysia witnessed a period of transition from neritic-bathyal to littoral-land environment, marking the initial uplift process. Paleo-current measurements on the crossbeds revealed northwestward and westward transport directions, suggesting a source area to the east-southeast. All samples collected from the Cambrian-Ordovician strata show similar chemical characteristics; they have negative εNd(t) values (−9.7 to −13.7) and two-stage εNd(t) model ages at ca.2.04 to 2.36 Ga. This suggests that the early Paleozoic rocks were derived from the eroded Paleoproterozoic basement, and little or no mantle component was identified. During the Silurian, the Cathaysia block underwent strong folding, thrusting, weak metamorphism and large-scale anatexis accompanied by granitoid emplacement, building the South China Fold Belt. The maximum shortening is estimated at 67 percent. A kinematic analysis of the ductile sheared rocks revealed a fan-shape thrust pattern, with top-to-the southeast in the southeastern and top-to-the northwest in the northwestern Cathaysia block. Zircon U-Pb dating of four granitic plutons yielded 206Pb/238U ages of 435 ± 4 Ma, 424 ± 5 Ma, 428 ± 3 Ma and 427 ± 2 Ma. All the zircon εHf(t) values are negative (−6 to −9) and show a peak of two-stage Hf model ages around 1.9 Ga, indicating that the Silurian granitic magma was derived from the recycling of Paleoproterozoic basement. Major features of the early Paleozoic South China Fold Belt include the lack of early Paleozoic ophiolites and volcanic rocks, the absence of coeval HP-type blueschists, and the absence of mantle-derived juvenile magmatic rocks. Consequently, a subduction-collision-type orogeny is excluded. The magmatism most probably took place in an intraplate tectonic setting with little or no input of mantle components. We therefore conclude that the South China Fold Belt was an intraplate orogen, and is possibly related to the global early Paleozoic continental assembly.

Neoproterozoic ages of the Kuluketage diabase dyke swarm in Tarim, NW China, and its relationship to the breakup of Rodinia

The widely exposed Kuluketage diabase dyke swarm, Tarim Block, NW China, has been considered to have been emplaced in Permian times. New precise zircon U–Pb SHRIMP ages for two samples from the dyke swarm yield Neoproterozoic ages of 823.8 ± 8.7 Ma and 776.8 ± 8.9 Ma. Correlated with peaks of magmatism in South China and Australia at c. 825 Ma and c. 780 Ma, these two new ages provide significant information for palaeocontinental reconstructions. The prolonged duration of the magmatic events, combined with regional stratigraphic relationships, imply that the Tarim Block may have been affected by a mantle plume during the breakup of Rodinia.

Subducted Precambrian oceanic crust: geochemical and Sr–Nd isotopic evidence from metabasalts of the Aksu blueschist, NW China

Abstract: The Aksu Proterozoic blueschist terrane in northwestern China is regarded as one of the oldest well-substantiated Precambrian blueschist terranes in the world. Previous work has focused on the high-pressure metamorphism: both the age and P–T conditions of the metamorphic event have been well discussed. However, little attention has been paid to protolith identification of the blueschist terrane. In this paper, we present geochemical and Sr–Nd analyses of metabasalt samples from the blueschist terrane. The results show that the protoliths of these mafic schists were enriched mid-ocean ridge basalt (E-MORB). Crystal fractionation has led to differentiation of trace element concentrations. This provides the opportunity to obtain a protolith Sm–Nd isochron age of 890 ± 23 Ma (MSWD = 0.68). We conclude that the Aksu blueschist terrane represents part of an ocean crust at the NW edge of Rodinia around Tarim at c. 890 Ma. It experienced subduction and exhumation followed by accretion to the Tarim craton. If the north margin of present Tarim faced outboard of Rodinia during subduction, it might form one section of the arcuate subduction zone around the supercontinent. Alternatively, the subduction zone may have been located between Tarim and Australia and would later be closed.

Thermotectonic evolution of Precambrian basement rocks of the Kuruktag uplift, NE Tarim craton, China: evidence from apatite fission-track data

The Kuruktag uplift is located directly northeast of the Tarim craton in northwestern China. Neoarchaean-to-Neoproterozoic metamorphic rocks and intrusive rocks crop out widely in the uplift; thus, it is especially suited for a more complete understanding of the thermal evolution of the Tarim craton. Apatite fission-track (AFT) methods were used to study the exhumation history and cooling of these Precambrian crystalline rocks. Nine apatite-bearing samples were collected from both sides of the Xingdi fault transecting the Kuruktag uplift. Pooled ages range from 146.0 ± 13.4 to 67.6 ± 6.7 Ma, with mean track lengths between 11.79 ± 0.14 and 12.48 ± 0.10 μm. These samples can be divided into three groups based on age and structural position. Group A consists of five samples with AFT apparent ages of about 100–110 Ma and is generally associated with undeformed areas. Group B comprises three specimens with AFT apparent ages lower than 80 Ma and is mostly associated with hanging wall environments close to faults. Group C is a single apatite sample with the oldest relative apparent age, 146.0 ± 13.4 Ma. The modelled thermal history indicates four periods of exhumation in the Kuruktag uplift: late-Early Jurassic (180 Ma); Late Jurassic–Early Cretaceous (144–118 Ma); early-Late Cretaceous (94–82 Ma); and late Cenozoic (about 10 Ma). These cooling events, identified by AFT data, are assumed to reflect far-field effects from multi-stage collisions and accretions of terranes along the south Asian continental margin.