Formation of the Hengshanli granitic gneiss dome in the Paleoproterozoic Jiao-Liao-Ji Belt, North China Craton

Abstract

A group of granitic gneiss domes that may provide constraints on the tectonic evolution of early Precambrian orogenic belts are well preserved in the LiaoJi segment (or LJOB-Liaoji orogenic belt) of the Jiao-Liao-Ji Orogenic Belt (JLJOB), eastern North China Craton. To deeply understand the genesis of the domes and how the crustal mass flowed in collisional orogenic belt, we carried out structural analysis of the Hengshanli dome in the western part of the LJOB. Detailed analyses of deformation structures, microstructures and fabrics, and geochronology of syntectonic granitic dykes demonstrate that three episodes of structural deformation contributed to the formation of the dome structure. During D1, a pure shear-dominated deformation, penetrative foliations (S1), symmetrical lenses or boudinages, intrafolial folds (F1) and mineral lineations (L1) developed in the interlayered rocks of viscosity contrasts. The D2, the WNW-ESE orientated sub-horizontal simple shear-dominated deformation, is characterized by asymmetrical A-type folds (F2), stretching lineations (L2) , mylonitic foliations (S2), localized ductile shear zones, axial planar foliations, rotated porphyroblasts, L>S tectonites and S-C fabrics that indicate high-strain simple shear deformation. A D3 brittle deformation is evidenced by E-W directed open folds (F3) by flexural folding mechanism. LA-ICP-MS U-Pb dating of zircons from the syntectonic granitic dykes reveals that the main deformation event occurred from ca. 1946 to 1844 Ma. The dating also reveals that the Liaoji granites at the core were emplaced at ca. 2147 Ma and experienced anatexis at ca. 1864 Ma and 1846 Ma. It is suggested that the mechanism of doming is a combination of middle and lower crustal sub-horizontal flow, diapirism and folding superposition. Convergent flow of remobilized magma at depth and sub-horizontal simple shearing of metasedimentary rocks triggered middle and lower crustal flow in association with diapirism during the doming. The granitic gneiss domes are the products of dominant lateral and subsidiary vertical crustal mass flow under E-W directed compressional regime in the LJOB. We suggest that the orogenic belt experienced peak deformation at ca. 1946-1844 Ma, accompanied with syntectonic metamorphism and magmatism at nearly the same time.