Syntectonic emplacement of the Triassic biotite-syenogranite intrusions in the Taili area, western Liaoning, NE China: Insights from petrogenesis, rheology and geochronology

Abstract

The North China Craton (NCC) is one of the oldest cratons in the world, and it recently becomes a hot study area because of large volumes of Mesozoic intrusions associated with lithospheric thinning contributing to cratonic destruction in late Mesozoic times. However, the timing of initial thinning and destruction is still controversial. The Taili area, western Liaoning Province, in the northeastern part of the NCC well exposes the Archean basement rocks and the Mesozoic magmatic rocks with variable plastic deformation. This study focuses on the syntectonic emplacement of the Triassic biotite-syenogranite intrusions, in order to understand their petrogenesis, timing as well as the geological significance. Zircon LA-ICP-MS U-Pb ages reveal that the biotite-syenogranites formed between 246 and 191Ma, and contain many ancient (2564–2317Ma) zircon xenocrysts. Geochemical data suggests that the biotite-syenogranites display an adakitic affinity with high Sr/Y=135–167 and (La/Yb)N=48–69, as well as negligible Eu anomalies (δEu=0.87–0.94), high negative zircon εHf(t) values (−15.5 to −21.5) and ancient TDM2 ages (2246–2598Ma). This data suggests that the parent magmas were generated from partial melting of thickened Archean lower crustal rocks probably due to the bidirectional amalgamation of the NCC with the NE China micro-blocks and the Yangtze Craton in its north and south, respectively. In the middle part of the Taili area, magmatic fabrics are well preserved in the biotite-syenogranite intrusion characterized by the strong preferred orientation of biotite and hornblende crystals, which parallel to the intrusion margin and are slightly oblique to the gneissosity of the sheared host Neoarchean granitic gneisses. The quartz grain size piezometer suggests that the paleo-differential stresses weaken toward to the central part of the intrusion, ranging from 21.40–22.22MPa to 16.74–19.34MPa, during quartz crystallization in the emplacement stage. This allow deduce much higher strain rates in the center (1.26×10−11–2.24×10−9s−1) than at the margin (9.07×10−12–1.31×10−9s−1) of the pluton. These observations are interpreted by the rheological behavior of magma during the magmatic “pipe” flow. The adakitic source melts ascended through the conduits along weak NE-trending sinistral shear zones, and emplaced at the shallower depth of ∼16km before Early Jurassic (∼190Ma). The biotite-syenogranites were still in a semisolid state, when garnet-bearing granitic aplites injected at ∼220Ma. This stage records elongate (constrictional) strain under the sinistral shear stresses, particularly in quartz grains occurring in the margin of intrusions. In combination with previous studies, an exhumation rate of the NCC’s Archean basement (from ∼25km to ∼11km in depth) is calculated as initial low exhumation rate of ∼4.0mm/kyr from Neoarchean to Late Triassic, and subsequent a rapid exhumation process of ∼63mm/kyr between Late Triassic to Early Cretaceous. All the results presented here allow us to consider the geodynamic evolution of the eastern NCC and constrain the onset of lithospheric thinning and cratonic destruction of the NCC as early as Middle Triassic (∼240Ma) triggered by the amalgamation of adjacent blocks. It developed prosperously since Late Triassic, due to the oblique subduction of the Paleo-Pacific Plate.