大巴山构造演化的石英ESR年代学研究

Structural evolution of Dabashan Mountain:Evidence from ESR dating

运用α石英热活化ESR定年法对大巴山构造带58件样品进行了研究。结果显示北大巴山经历了三个主要构造演化阶段,分别为243.7～201.5Ma(晚印支期)、165.6～91.2Ma(中晚燕山期)、68.6Ma至今(喜山期)。南大巴构造带则经历了170～160Ma(早燕山期)、147.3～94.1Ma(中晚燕山期)、66.7Ma至今(燕山末期—喜山期)三个构造演化阶段。南北大巴在变形时间上相关,显示南北大巴在构造变形上的耦合性。代表南大巴强烈变形的轴面劈理和破劈理主要发育于147.2～94.1Ma,显示主变形期在中晚燕山期,并具有明显的前展式变形特征。破劈理中石英脉ESR年龄还显示出南大巴构造带两端变形早(早燕山期即已卷入强烈变形),中间变形稍晚(中燕山期开始强烈变形)。喜山期(66.7Ma至今)南大巴以构造隆升和右行走滑拉张为特点,构造活动渐趋稳定。ESR测年结果与区域构造响应及同位素年代学和沉积年代学结果一致。

Fifty eight samples from tectogenetic α quartz veins in Dabashan Mountain have been studied by using ESR dating method. The results showed that the evolution history of the North Dabashan Mountain （N. DBS） since the Mesozoic can be divided into three stages： 243. 7-201.5 Ma （the Late Triassic）, 165.6-91.2 Ma （the Late Jurassic to the Early Cretaceous） and 68. 6 Ma-present （the Cenozoic）, while the South Dabashan Mountain （S. DBS） can be divided into another correspondent three stages： 170-160 Ma （the Bajocian to the Callovian）, 147. 3-94. 1 Ma （the end of the Late Jurassic to the Early Cretaceous） and 66. 7 Ma-present （the Cenozoic）. Thus, the S. DBS is closely related to the N. DBS in structural evolution. Axial plane cleavages and fracture cleavages that represent the major strong deformation period of the S. DBS were chiefly formed in about 147. 2-94.1Ma （the end of the Late Jurassic to the Early Cretaceous） with the deformational characteristics of a piggyback type. The ESR dating of quartz veins in fracture cleavages also showed that the two terminal strata （deformed before 160 Ma） of S. DBS were involved into deformation earlier than the middle sections （deformed after 140 Ma）. Since the Cenozoic, the tectogenesis of S. DBS transformed to the uplift and extensional slipping movements and became stable gradually. The ESR dating results are consistent with other geochronology evidences very well.