Rapid cooling of the Yanshan Belt, northern China: Constraints from 40Ar/39Ar thermochronology and implications for cratonic lithospheric thinning

The Yanshan Orogenic Belt is located in the northern part of the North China Craton (NCC), which lost similar to 120 km of lithospheric mantle during Phanerozoic tectonic reactivation. Mesozoic magmatism in the Yanshan fold-and-thrust belt began at 195-185 Ma (Early Jurassic), with most of the granitic plutons being Cretaceous in age (138-113 Ma). Along with this magmatism, multi-phase deformational structures, including multiple generations of folds, thrust and reverse faults, extensional faults, and strike-slip faults are present in this belt. Previous investigations have mostly focused on geochemical and isotopic studies of these magmatic rocks, but not on the thermal history of the Mesozoic plutons. We have applied 40Ar /39Ar thermochronology to biotites and K-feldspars from several Lower Cretaceous granitic plutons to decipher the cooling and uplift history of the Yanshan region. The biotite 40Ar/39Ar ages of these plutons range from 107 to 123 Ma, indicating that they cooled through about 350 degrees C at that time. All the K-feld-spar step-heating results modeled using multiple diffusion domain theory yield similarly rapid cooling trends, although beginning at different times. Two rapid cooling phases have been identified at ca. 120-105 and 100-90 Ma. The first phase of rapid cooling occurred synchronously with widespread extensional deformation characterized by the formation of metamorphic core complexes, A-type magmatism, large-scale normal faults, and the development of half-graben basins. This suggests rapid exhumation took place in an extensional regime and was a shallow-crustal-level response to lithospheric thinning of the NCC. The second phase of rapid cooling was probably related to the regional uplift and unroofing of the Yanshan Belt, which is consistent with the lack of Upper Cretaceous sediments in most of the Yanshan region. (C) 2014 Elsevier Ltd. All rights reserved.