云南马关新生代地幔包体中橄榄石的显微构造及其反映的上地幔物理状态特征

MICROSTRUCTURES OF OLIVINE FROM CENOZOIC MANTLE-DERIVED SPINEL LHERZOLITE XENOLITHS BENEATH MAGUAN, YUNNAN PROVINCE AND ITS REFLECTION OF PHYSICAL STATE OF UPPER MANTLE

利用氧化缀饰法研究了云南马关新生代地幔尖晶石二辉橄榄岩包体中橄榄石的位错构造。其位错构造种类多样、形态各异,主要包括(100)和(001)位错壁、亚颗粒构造、位错网格、{100}滑动条带构造等,表明该区上地幔岩具高温高应变和低温高应变特征。包体形成的温度T为1016—1098℃,压力P为18.3×108—20.6×108Pa。马关上地幔温压状态具有大洋地幔地温特征,热流值达85mW·m-2。上地幔的流动应力σ为27.9—59.7MPa,流动速率ε为1.37×10-18—2.17×10-16s-1,等效粘滞度η为9.18×1022—6.92×1024Pa·s。该区上地幔的高流动应力、高热状态和裂谷带型的流动速率、等效粘滞度特征,可能与16MaB.P.以来区域性东西向伸展作用有关。

Dislocation configurations in olivine from Cenozoic mantle-derived spinel lherzolite xenoliths beneath Maguan,Yunnan Province were analyzed by oxidation decorating technique. The observed dislocation substructures are various and different, and include (100) and (001) dislocation walls, subgrain, disclocation net, {100} slip band superimposed on all other features, which reflect that olivine had mainly experienced high-temperature and high-stress as well as low-temperature and high-stress deformations. The temperature of spinel lherzolite xenoliths ranges from 1 016 to 1 098 ℃, and pressure from 18.3×10^(8) to 20.6×10^(8 )Pa. The geotherm of the upper mantle in Maguan is similar to the oceanic geotherm and the value of heat flow is 85mW·m^(-2). The value of flow stress of the upper mantle ranges from 27.9 to 59.7MPa, flow rate from 1.37×10^(-18) to 2.17×10^(-16)s^(-1), and equivalent viscosity from 9.18×10^(22) to 6.92×10^(24)Pa·s. The state of high flow stress and high heat, and the flow rate and equivalent viscosity with characteristics of the type of continental rift of the upper mantal beneath Maguan, Yunnan Province, may be contributed to the east-west extension of eastern Asia during the period of 16—0 Ma B.P.