Extensional magmatism caused by strain partitioning: insights from the mafic dikes hosted in Biesituobie batholith in West Junggar, CAOB

The connection between magmatism and the tectonic environment is essential yet highly debatable. The West Junggar, CAOB, outcropped a swarm of norite-gabbro dikes hosted in the Biesituobie batholith. The dikes are a family of mafic magma consisting of nortite, norite-gabbro, and gabbro with equigranular, porphyritic, and cumulate textures. The mafic dikes represent various chemical compositions due to the magma evolution suggested by the different lithological textures, especially for MgO, CaO, Sr, and Eu, possibly caused by the crystal fractionation. But commonly, they show depleted Nb, Ta, Zr, Hf, Ti, and enriched Ba, U. Geochronology study by LA-ICP-MS gives zircon U–Pb age of 265 ± 4 Ma, 269 ± 1 Ma, 270 ± 2 Ma for the mafic dikes, respectively. The mafic dikes show geochemical similarities with E-MORB type lava with minor subduction affinity. The low fraction on the LREE/HREE indicates that they were generated from a relatively shallow mantle with low pressure. The norite-gabbro dikes and the host batholith suggest a regional extension with crust thining upwelling of the mantle caused by strain partitioning. Regionally, the Permian extensional magmatism is distributed mainly in the “Yumin–Miaoergou axis, ” perpendicular to the Darburt fault. The activation of the Darburt fault might cause the partition of regional transpression strain into simple shear along the strike-slip Darburt fault and pure shear perpendicular to the fault. The T-fracture parallel with the maximum principal stress of the pure shear causes crustal thinning and mantle upwelling, generating the extensional magmatism in the “Yumin–Miaoergou axis.” This tectonic scenario might be the essential geodynamic background for extensional magmatism, which suggest that local extension might be coordinated with regional transpression, and a unified regional extension is not needed in such a model.