Temperature and timing of ductile deformation of the Longquanguan shear zone, Trans-North China Orogen

Abstract

Determination of temperature conditions and timing of ductile shearing is important in understanding the deformation mechanisms and tectonic evolution. The Longquanguan shear zone locates in the middle section of the Trans-North China Orogen, extending ~ 100 km long in NNE-SSW direction and exposing abundant granitic and granodioritic mylonites, characterized by well-developed NW-dipping mylonitic foliation and penetrative NW-plunging mineral stretching lineation. The mylonite exhibits regionally heterogeneous mechanisms of dynamic recrystallization, dominated by subgrain rotation. Ti-in-quartz (TitaniQ) geothermometer was applied to assess the deformational temperature along five profiles across this shear zone. In the northern part of this shear zone, quartz ribbons of the three narrower profiles display relatively homogeneous Ti content and record similar temperatures of 474–505 °C, 500–539 °C, and 492–521 °C, respectively. In the southern part, quartz ribbons of the two wider profiles show eastward increasing Ti content and record temperature increasing from 440 to 557 °C, and from 461 to 553 °C, respectively. A temperature gradient is estimated to be 20–25 °C/km, which possibly relates to the geothermal gradient between the hanging wall and the footwall. The shear zone, along with the temperature gradient narrows out to the north, possibly resulted from differential erosion/denudation. The hornblende and mica 40Ar/39Ar age data indicate that the ductile deformation occurred at ~ 1.85–1.80 Ga, followed by quick cooling occurred at 1.83–1.78 Ga. Therefore, the Longquanguan shear zone records ductile shear deformation, which possibly occurred during the collision between the Western and Eastern Blocks of the NCC in the Late Paleoproterozoic.