燕山地区喀喇沁变质核杂岩的构造特征与发育机制

Structural characteristics and formation mechanism of the Kalaqin metamorphic core complex in the Yanshan area, China

通过详细的野外观察与岩浆岩锆石定年,重新分析了燕山构造带北侧喀喇沁变质核杂岩的变形特征与演化历史,认为其经历了晚侏罗世与早白垩世两期伸展变形,而不是过去认为的仅发育于早白垩世.该变质核杂岩起源于晚侏罗世（156-150 Ma）NE-SW向拉伸中,最初发育了向北东缓倾、上盘向北东运动的拆离韧性剪切带,其下盘也卷入了同期韧性变形,并有岩浆活动与上叠盆地伴生.在随后的149-145 Ma期间仅发生了有限的均衡隆升.早白垩世时（141-100 Ma）,区内拉伸方向转变为NW-SE向,沿着变质核杂岩两侧发育了2条NE-SW走向、倾向相背的大型脆性正断层.这2条正断层外侧的上盘控制发育了早白垩世半地堑式盆地,而其间共同的下盘发生了地垒式隆升而成为伸展穹窿,使得晚侏罗世韧性伸展构造剥露到地壳浅部.在早白垩世伸展隆升中,核部杂岩由早期至晚期先后叠加了局部的韧性变形带与脆性正断层,显示为应变局限化的特征.对该变质核杂岩演化历史与发育机制的重新厘定,一方面为燕山构造带发生过晚侏罗世伸展活动与克拉通破坏提供了构造证据,另一方面也表明该带早白垩世强烈伸展活动普遍存在,代表了克拉通的峰期破坏.

Based on new detailed field observations and magmatic zircon dating, we reanalyzed deformation characteristics and tectonic evolution of the Kalaqin metamorphic core complex in the northern Yanshan tectonic belt. We propose that two stages of extension occurred in formation of the core complex in Late Jurassic and Early Cretaceous, rather than only one in Early Cretaceous as previously suggested. During Late Jurassic（-156–150 Ma）, the core complex originated from a shallow detachment ductile shear zone with a top-to-the-NE shear sense under NE-SW extensional stress. The footwall was ductilely deformed, and shear zone development was associated with magmatic activity and formation of rift basins. Ductile shearing was followed by limited isostatic uplift -149–145 Ma. During Early Cretaceous（-141–100 Ma）, the extensional direction switched to NW-SE, leading to development of two NEstriking major normal faults with opposite dip directions along the eastern and western edges of the core complex. Early Cretaceous half-graben basins developed on the hanging walls of both major faults, as their shared footwall rose in an extensional dome. This doming resulted in exhumation of the Late Jurassic ductile extensional structures to shallow levels. The Kalaqin metamorphic core complex was overprinted by earlier ductile deformation belts and later brittle normal faults during the extensional doming of Early Cretaceous, the deformation shows characteristics of strain localization. Our new structural evidence allows determination of the evolution and formation mechanism of the Kalaqin metamorphic core complex, and confirms extensional deformation in both Late Jurassic and Early Cretaceous in the Yanshan tectonic belt. The Early Cretaceous extension is intense and widespread, and represents the peak destruction of the eastern North China Craton.