Geochemistry And Geochronology Characteristics Of Nanlao Gneisses In Southeast Yunnan And Its Tectonic Signigicance

The Laojun mountain area in the Southeast Yunnan, locating on the contact placebetween Yangzte Blocks and Cathaysia Blocks, is an important study place for thetectonic evolution of the South China Block. The petrology, major elementsl, traceelements, rare elements, LA-ICP-MS zircon U-Pb geochronology、Hf isotopic studieswere carried out for granites gneisses in the Laojun mountain area of southeast ofYunnan. The experiment results show that biotite monzogneisses are characterized byhigh SiO₂（SiO₂=71.14～71.44%）， high aluminium（Al₂O₃=14.79～15.18%），potassic（K₂O/Na₂O=0.92～1.90），Alumina saturation index greater than1，highALK（K₂O+Na₂O=6.26%～7.38%）, Rittman index=1.33～2.00, A/CNK=1.03～1.21,FeO^T=2.98～3.05, FeO^T/（FeO^T+MgO）=0.67～0.72. Nanlao gneisses are potassicaluminium-peraluminous in-K calc-alkaline series S-type granite. REE distributionpatterns show LREE enrichment, tilting right slightly. LREE fractionation are obvious,with negative Eu anomalies, similar to crustal model. Nanlao gneisses show negativeanomalies in HFSEs （eg., Nb, Ta, Ti, Sr, Ba）, positive anomalies in large ion element（eg., Rb,Nd,Pb）. U-Pb dating of LA-ICP-MS zircon secleted from upper and lowerparts of Nanlao gneisses are inconsistent with its corresponding²⁰⁶P/²³⁸U weightedaverage ages with406.61±1.3（MSWD=0.86） and431.8±4.1（MSWD=022）, combinedwith the CL images that metamorphic age,indicating that caledonianmagmatic-hydrothermal event last for25Ma in the Laojunshan area of SouthernYunnan. Zircon Lu-Hf isotope results show that the value of ε_Hf（t） was-11.8～0.8，andmodern ages t_DM^C of two stages of Hf isotope mainly in the range of1395～1544Ma，indicating that material source of biotite monzogneisses are mainly partial meltingform from lower crust matter. Based on the regional geological background，Nanlaogneisses formed because of collision of Cathaysia Block and Yangtze Block，mantlesource inducing partial melting from metamorphic sandstone of lower crust, thengenerated by highly fractional crystallization of parent magma, finally magmaupwelling.