Wenbin Ji

Geochronological and geochemical constraints for a middle Paleozoic continental arc on the northern margin of the Tarim block: Implications for the Paleozoic tectonic evolution of the South Chinese Tianshan

In order to reveal the tectonic evolution of the South Chinese Tianshan orogenic belt, we conducted structural, geochemical, and geochronological studies and identified granitic and volcanic rocks along the northern margin of the Tarim block. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of two samples from granitic plutons yielded crystallization ages from 404.8 ± 2.0 Ma to 388.1 ± 2.2 Ma, and two samples of dacite and rhyolite yielded ages of volcanism at ca. 404 Ma, highlighting a middle Paleozoic igneous event. Geochemical data suggest that these Devonian igneous rocks are metaluminous, high-K calc-alkaline felsic volcanic and plutonic rocks. All the samples display relatively enriched light rare earth element (LREE) contents, weak or no negative Eu anomalies, and relative depletion in Nb, Ta, Ti, and P, but they show enrichment in K, Rb, and Ba. In situ zircon Hf isotopic results show a positive range of e Hf (t). These geochemical and isotopic features suggest that this magmatic event probably occurred in an Andean-type arc. Combined with zircon U-Pb and Hf isotopic studies on the basement rocks, the results indicate multiphase Precambrian (ca. 2600–2300 Ma, 1900–1800 Ma, 1140–830 Ma) continental growth and minor involvement of basement in Devonian arc magmatism. Finally, we integrated the structural, geochemical, and geochronological data into a geodynamic model of the South Chinese Tianshan that emphasizes south-directed subduction of the South Tianshan Ocean along the northern margin of the Tarim block during the early to middle Paleozoic.

Multiple emplacement and exhumation history of the Late Mesozoic Dayunshan-Mufushan batholith in Southeast China and its tectonic significance: 2. Magnetic fabrics and gravity survey

The Late Mesozoic magmatic province is a prominent feature of the South China block (SCB). However, the tectonic regimes associated with the magmatism are still elusive. A combined anisotropy of magnetic susceptibility (AMS) and gravity study has been carried out to determine the fabric patterns and shape at depth of the Dayunshan–Mufushan composite batholith in the north-central SCB. This is a companion paper to Part 1 that presented the structural and geochronological data of this batholith. The magnetic fabrics in the batholith interior predominantly reflect magma flow structures. Two distinct patterns of the magnetic lineations are defined, around NNE–SSW and WNW–ESE trends for the early-stage and late-stage intrusions of the batholith, respectively. The gravity survey reveals that the early-stage intrusion has a main feeder zone located below its northern part while several linear feeder zones trending NNE–SSW are inferred for the late-stage intrusion. Integrating all results, a two-stage construction of the batholith with distinct tectonic regimes has been established. It is concluded that the early-stage intrusion experienced a southward magma transport during its emplacement, partially assisted by far-field compression from the north at ca. 150 Ma. Conversely, the emplacement and exhumation of the late-stage intrusion was accommodated by a NW–SE crustal stretching involving a lateral magma expansion above the multiple feeder zones (likely corresponding to extensional fractures) and ductile shearing during 132–95 Ma localized mainly along the Dayunshan detachment fault. Finally, we discuss the geodynamic linkage between the Paleo-Pacific subduction and the Late Mesozoic tectono-magmatism in the SCB.