哀牢山-红河剪切带左行走滑作用起始时间约束

位于哀牢山-红河剪切带NW延伸方向上的点苍山变质杂岩体遭受强烈的左行走滑剪切变形、变质作用改造,岩石中保存了典型的高温矿物组合以及由它们构成的宏观和微观高温变形构造特征,其中糜棱岩中具有极其发育的长石矿物拉伸线理而形成典型的L与LS型构造岩是其一个明显的特征。本文对点苍山地区高温糜棱岩主要矿物开展了显微构造与矿物变形、变形机制及组构分析,并对于遭受高温糜棱岩化改造的一个花岗质岩体开展了SHRIMP锆石U-Pb定年分析。结果表明岩石中长石、角闪石、石英等主要矿物具有典型的达角闪岩相条件下的高温晶质塑性变形和动态生长特征,它们也为走滑剪切变形活动提供了充分的微观构造证据。对于点苍山高温糜棱岩化改造的眼球状或似斑状二长花岗岩的显微构造分析结果表明,这套花岗质岩石从走滑剪切前期岩浆的侵位之后经历了早期强烈的岩浆期后交代作用—亚岩浆流动—高温固态塑性剪切变形的递进演化过程。由此可见,岩浆的上升与就位受左行走滑剪切作用的制约,岩体又遭受了强烈剪切变形改造。同时对这套构造前期就位花岗质岩石中的锆石进行定年分析,获得33.88±0.32Ma的岩浆结晶年龄,为此,我们有充分的理由认为,在点苍山地区哀牢山-红河剪切带左行走滑剪切作用的起始时间至少应该为早渐新世30.88±0.32Ma。

Timing of Initiation of Left-Lateral Slip along the Ailao Shan-Red River Shear Zone: Microstructural, Texture and Thermochronological Evidence from High Temperature Mylonites in Diancang Shan, SW China

Diancang Shan complex, lying to the northwest extension of the Ailao Shan-Red River shear zone, experienced very intense left-lateral stricke-slip shearing and metamorphism. High-temperature mineral assemblages are preserved in sheared rocks and show typical high-temperature macro- and microstructure characteristics. A remarkable feature of the mylonites is their extremely strong stretching lineation fabrics forming L and L>>S tectonites. Such fabrics occur in both sheared metamorphic rocks and sheared granitic plutons. Microstructures, mineral deformation features and textures of deformed minerals from high-temperature mylonites collected from Diancangshan are studied. U-Pb zircons from a granitic high temperature mylonite are dated by SHRIMP. The results reveal features of intensive ductile deformation and oriented growth of feldspar, quartz and amphibole, in responding to shearing and metamorphism at amphibolite facies. Microstructural features also consistent with left-lateral shearing. Structural and microstructural studies of the mylonitized porphyritic monozogranite reveal a progressive evolution from emplacement of magma, subsolidus flow, late magmatic replacement to solid state shearing. It is suggested that the upwelling and emplacement of the magma are controlled by left lateral shearing and progressive shearing strongly affected the emplaced magmatic rocks. On the other hand, macrostructural analysis reveals that the high temperature granitic mylonites are mainly confined to the shear zone, has experienced shear deformation and extends along the shear zone. All above evidences suggest synkinematic feature of the magmatic intrusion. Meanwhile, thermochronological dating of zircons from the granitic mylonite give U-Pb ages of 30.88±0.32 Ma, which provides an upper limit timing of Diancangshan left-lateral shearing. We would strongly argue that the left lateral shearing along the Ailao Shan-Red River shear zone in Diancangshan area initiated no later than 30.88±0.32 Ma in early Oligocene.