Deformation of amphibolites from the Paleoproterozoic Liaohe Group, Liaodong Peninsula, China: Implications to the crustal structure of the Jiao‐Liao‐Ji mobile belt in the eastern block, North China Craton

Structural evolution of the Paleoproterozoic Jiao‐Liao‐Ji orogenic belt in eastern North China Craton has been a subject of extensive study in the past decades. Studies of the characteristics of the crustal structure during orogenesis are essential to the understanding of the formation and evolution of the orogenic belt. In this paper, we present detailed studies of deformed amphibolites in the southern part of the orogenic belt to illustrate the changes of deformation conditions and variation of deformation mechanisms during the orogenic processes. Based on differences in structural characteristics of foliations, amphibolites from the Paleoproterozoic South Liaohe Group at Caohekou–Jiguanshan area are subdivided into four types (massive, weak‐gneissic, moderate‐gneissic, and strong‐gneissic). Our study shows that the different characteristics in microstructures, rock fabrics, and deformation mechanisms of the amphibolites were resulted from varying temperatures and pressures in relation to the tectonic deformation. The widely distributed massive amphibolites (P = 0.39 GPa, T = 639°C) display no or weak deformation, which is inferred to represent steady‐state crystallization. Weak‐gneissic amphibolites (P = 0.52 GPa, T = 753°C) were resulted from stress‐controlled grain growth. Amphiboles in such rocks show obvious (100) LPOs but minor evidence for crystal plastic deformation. Evidences for crystal plastic deformation of amphibole grains are common in moderate‐gneissic amphibolites (P = 0.61 GPa, T = 771°C). The amphibole grains have strong (100) and (001) LPOs. Dislocation slip is considered as the main deformation mechanism of the amphibole grains in the rocks. Both subgrains and mechanical twins of amphiboles from strong‐gneissic amphibolites (P = 0.77 GPa, T = 686°C) are long‐columnar shaped and show the same orientation as their host crystals. They are resulted from reactivation of (100)(001) slip system controlled by dynamic recrystallization, twinning, and twinning nucleation recrystallization. The evident pressure difference from 0.39 to 0.77 GPa during the formation of the various types of amphibolites may be related to the tectonic thickening during contractional orogenesis.