New Zircon U-Pb Geochronology of the Post-kinematic Granitic Plutons in the Diancang Shan Metamorphic Massif along the Ailao Shan-Red River Shear Zone and Its Geological Implications

The Ailao Shan-Red River fault zone is the boundary between the Yangtze block to the northeast and the Indochina block to the southwest.It is an important tectonic zone due to its role in the southeastward extrusion of the Indochina block during and subsequent to the Indian-Eurasian collision.Diancang Shan（DCS） high-grade metamorphic complex,located at the northwest extension along the Ailao Shan-Red River（ASRR） shear zone,is a representative metamorphic complex of the ASRR tectonic belt.Structural and microstructural analysis of sheared rocks in the high-grade metamorphic rocks reveals that they are coherent with solid-state high-temperature ductile deformation,which is attributed to left-lateral shearing along the ASRR shear zone.New LA-ICP-MS zircon U-Pb geochronological and microstructural studies of the post-kinematic granitic plutons provide a straightforward time constraint on the termination ductile left-lateral shearing and exhumation of the metamorphic massif in the ASRR shear zone.It is suggested that the left-lateral shearing along the ASRR shear zone ended at ca.21 Ma at relative lower-temperature or decreasing temperature conditions.During or after the emplacement of the young dikes at ca.21 Ma,rapid brittle deformation event occurred,which makes the DCS massif start fast uplift/exhumation and cooling to a shallow crustal level.

Crust and upper mantle structure of the Ailao Shan-Red River fault zone and adjacent regions

Using arrival data of the body waves recorded by seismic stations, we reconstructed the velocity structure of the crust and upper mantle beneath the southeastern edge of the Tibetan Plateau and the northwestern continental margin of the South China Sea through a travel time tomography technique. The result revealed the apparent tectonic variation along the Ailao Shan-Red River fault zone and its adjacent regions. High velocities are observed in the upper and middle crust beneath the Ailao Shan-Red River fault zone and they reflect the character of the fast uplifting and cooling of the metamorphic belt after the ductile shearing of the fault zone, while low velocities in the lower crust and near the Moho imply a relatively active crust-mantle boundary beneath the fault zone. On the west of the fault zone, the large-scale low velocities in the uppermost mantle beneath western Yunnan prove the influence of the mantle heat flow on volcano, hot spring and magma activities, however, the upper mantle on the east of the fault zone shows a relatively stable structure similar to the Yangtze block. The low velocities of the deep mantle beneath the southeastern extending segment of the fault zone are probably related to the mantle convection produced by the pull-apart of the South China Sea.

New evidence of tectonic uplift and transform of movement style along Ailao Shan-Red River shear zone

Using cooling curves of K-feldspars obtained by specific40Ar/39Ar step heating procedure and multiple diffusion domain modeling (MDD model), together with results of dating hornblende, biotite, muscovite, and apatite, this research relates the cooling history of 3 areas, from Ejia in NW to Jinping in SE of Ailao Shan-Red River shear zone, and near Hekou (distance about 390 km). The results indicate that the slip distance of Ailao Shan-Red River shear zone is more than 350 km (at least based on the study done within China). And between the end of the left-lateral slip movement and the beginning of the right-lateral slip movement, the uplift of shear zone from SE to NW was homogeneously extended.

滇西瑶山杂岩变形特征与新生代剥露隆升的磷灰石裂变径迹证据

瑶山杂岩是位于哀牢山-红河剪切带(ASRR)上最南端的一个杂岩体,即瑶山-大象山杂岩的中国境内部分。为了揭示杂岩体的低温热演化与浅部剥露历史,并正确理解ASRR变质杂岩的剥露与构造演化,本文开展了杂岩体的宏观构造以及深入的显微构造分析,并在平行杂岩延伸方向上和垂直杂岩延伸方向上进行了磷灰石裂变径迹分析。结果显示,瑶山杂岩作为一个宏观线性穹窿,自渐新世以来经历了多阶段热演化过程。磷灰石的热历史反演结果表明,在约30Ma的时候,瑶山杂岩就已经剥露到距地表4.3km处,继而存在三次抬升过程:30~25Ma,是一个快速的抬升过程,冷却速率为8~8.9℃/Myr;25~12.5Ma,是一个相对缓慢的抬升过程,冷却速率为1.5~1.9℃/Myr;从中新世约13Ma至今,抬升速度又开始加快,但小于第一阶段的速度,冷却速率为3.4~4.1℃/Myr。根据磷灰石裂变径迹年龄的分布,在平行杂岩体的延伸方向上,剥露过程不是整体抬升的,而是一个不均匀的剥露过程;而在垂直杂岩体延伸方向上,是一个整体的均匀的剥露过程。区域尺度上,沿着瑶山-大象山杂岩体延伸方向上,从南东到北西,磷灰石年龄有逐渐变新的趋势,说明剥露具有穿时性;而在瑶山杂岩局部,剥露具有不均匀性,类似于波瓦状剥露特点。

Chronological and Geochemical Study of the Cenozoic Potassic Felsic Igneous Rocks in Western Yunnan,SE Tibet:Implications for their Tectonic Mechanisms

Whether there is a relationship between widespread Cenozoic igneous rocks and the activated Ailao Shan-Red River shear zone(ASRR)in western Yunnan,SE Tibet,remains highly controversial.New geochronological,and geochemical data derived from Cenozoic igneous rocks both within and far from the ASRR offers answers.Although these coeval felsic rocks(36.9-33.5 Ma)are enriched in large-ion lithophile elements relative to high field strength elements,they also display some special geochemical features,such as high Y and Yb values of syenitic porphyries,which contrast with the high Sr and low Y contents of monzoporphyries and granitic porphyries.The syenitic porphyries exhibit geochemical characteristics similar to those of coeval mafic magmas,e.g.,high K 2 O,Y,and Yb contents,indicating that the former were probably generated by fractional crystallization of the latter.However,the high Sr/Y(56.1-109)monzoporphyries and granitic porphyries,combined with their low MgO,and Cr(or Ni)contents,suggest that they were mainly derived from partial melting of a thickened mafic lower crust.Given that these Cenozoic magmatic events exhibiting planar distributions occurred outside of the activation of the ASRR(31-20 Ma),then they were not controlled by the latter.Instead,they were tied to convective removal of a thickened lower lithospheric mantle in western Yunnan during the Paleogene.