The deformation and metamorphic evolution of Ailaoshan massif, Southwest China

Abstract

Being one of the most significant structures for the Cenozoic tectonic evolution in Asia, the Ailaoshan – Red River Shear Zone has attracted much attention over the last two decades. The Ailaoshan massif was considered to mark the boundary between South China block and Indochina block linking the Jinshajiang suture to the North and the Song Ma suture zone to the South. However, the justification of marking the Ailaoshan as a suture zone is debated as no ophiolite suites have been identified. Although enormous amount of petrological, structural and geochronologicaldata regarding the deformation history for all metamorphic complexes has been reported,none of such was reported for the Northern branch of the Ailaoshan. This studyreports the very first petrological, structural, and zircon ages from paragneiss and mylonite from the Northern Ailaoshan region. The most abundant group of zircons reveals a major period of protolith formation during Triassic. This group shows ages ranging from 252Ma to 240 Ma (Th/U > 0.1) in 8 samples from both Zhenyuan and Xishelu-Longjie cross-section. This indicates an intensive magmatism related to the subduction of Ailaoshan Ocean and Indosinian Orogeny. The earliest deformation event (Dn-1) formed Sn-1subhorizontal foliation, which is defined by myrmekite, muscovite + biotite folia, quartz and feldspar. Muscovite and plagioclase fish indicates top-to-SW, bottom-to-NE sense of shear. Based on U/Pb dating results of metamorphic zircon rims (Th/U ratio<0.1) with age ranging from 36 -24 Ma, it is interpreted as the active period of Dn-1 deformation as the P-T condition of Dn-1 can partially reset the zircon system. The second deformation event (Dn) formed upright folds with NW-SE trending sub-vertical fold axial planes (Sn) within gneiss and S/C fabrics within mylonites showing left-lateral sense of shear. The Sn foliation is characterized by biotite + myrmekite and quartz, which suggests the temperature condition should be higher than 450oC under amphibolite facies. The last ductile deformation event (Dn+1) formed upright folds with NNE-SSW trending sub-vertical fold axial planes (Sn+1) with right-lateral sense of shear. The Sn+1 are defined by chlorite + muscovite folia and retrograde reaction of hornblende into chlorite leading us to interpret the metamorphic condition to be at greenschistfacies. Syn-Dnmylonite andlucogranite samples yielded U-Pb zircon ages of 25 – 29 Ma (Th/U > 0.1) suggests the Dn+1left-lateral shear event occurred after 25 Ma. The detrital/inherited zircon age patterns showed significant differences between samples across the Ailaoshan fault. The age patterns obtained from samples East of the fault showed age peaks of: 772 Ma, which resembles the detrital zircon age pattern from South China. The age patterns obtained from samples West of the fault show age peaks of: 435Ma and 956Ma, which resembles the detrital zircon age pattern from Indochina blocks. Combining the Triassic ages(252Ma – 240Ma) from the magmatic zircons with inherented cores, these magmatic zircons are evidences for the amalgamation of Indochina and South China block, thus a new suture zone within the Ailaoshan massif can be confirmed.

Being one of the most significant structures for the Cenozoic tectonic evolution in Asia, the Ailaoshan – Red River Shear Zone has attracted much attention over the last two decades. The Ailaoshan massif was considered to mark the boundary between South China block and Indochina block linking the Jinshajiang suture to the North and the Song Ma suture zone to the South. However, the justification of marking the Ailaoshan as a suture zone is debated as no ophiolite suites have been identified. Although enormous amount of petrological, structural and geochronologicaldata regarding the deformation history for all metamorphic complexes has been reported,none of such was reported for the Northern branch of the Ailaoshan. This studyreports the very first petrological, structural, and zircon ages from paragneiss and mylonite from the Northern Ailaoshan region. The most abundant group of zircons reveals a major period of protolith formation during Triassic. This group shows ages ranging from 252Ma to 240 Ma (Th/U > 0.1) in 8 samples from both Zhenyuan and Xishelu-Longjie cross-section. This indicates an intensive magmatism related to the subduction of Ailaoshan Ocean and Indosinian Orogeny. The earliest deformation event (Dn-1) formed Sn-1subhorizontal foliation, which is defined by myrmekite, muscovite + biotite folia, quartz and feldspar. Muscovite and plagioclase fish indicates top-to-SW, bottom-to-NE sense of shear. Based on U/Pb dating results of metamorphic zircon rims (Th/U ratio<0.1) with age ranging from 36 -24 Ma, it is interpreted as the active period of Dn-1 deformation as the P-T condition of Dn-1 can partially reset the zircon system. The second deformation event (Dn) formed upright folds with NW-SE trending sub-vertical fold axial planes (Sn) within gneiss and S/C fabrics within mylonites showing left-lateral sense of shear. The Sn foliation is characterized by biotite + myrmekite and quartz, which suggests the temperature condition should be higher than 450oC under amphibolite facies. The last ductile deformation event (Dn+1) formed upright folds with NNE-SSW trending sub-vertical fold axial planes (Sn+1) with right-lateral sense of shear. The Sn+1 are defined by chlorite + muscovite folia and retrograde reaction of hornblende into chlorite leading us to interpret the metamorphic condition to be at greenschistfacies. Syn-Dnmylonite andlucogranite samples yielded U-Pb zircon ages of 25 – 29 Ma (Th/U > 0.1) suggests the Dn+1left-lateral shear event occurred after 25 Ma. The detrital/inherited zircon age patterns showed significant differences between samples across the Ailaoshan fault. The age patterns obtained from samples East of the fault showed age peaks of: 772 Ma, which resembles the detrital zircon age pattern from South China. The age patterns obtained from samples West of the fault show age peaks of: 435Ma and 956Ma, which resembles the detrital zircon age pattern from Indochina blocks. Combining the Triassic ages(252Ma – 240Ma) from the magmatic zircons with inherented cores, these magmatic zircons are evidences for the amalgamation of Indochina and South China block, thus a new suture zone within the Ailaoshan massif can be confirmed.