The Character Of Geochemistry And Chronology And Its Geological Significance Of Milipo Gneissic Granite In Southeast Yunnan

To further discuss the formation, evolution and prospecting of gneissic granite in the Song Chay dome of the Sino-Vietnamese border, the paper studies the chronology and geochemistry of the Malipo gneissic granite in the north of the dome. The results are shown below.(1) Malipo gneissic granite is composed of three sets of acid rock, which are Laochengpo, Nanlao and Tuantian gneissic granite, respectively. Through comparing their lithology, geochemistry and chronology, we found that these gneissic granites have similar structure, similar geochemical charateristics and closed diagenetic age, the only discrepancy is the tectonic deformation degree. It concludes that the three sets of acid rock probably come from the same magma source area.(2) Their Si O2 value is 69.94% ~ 76.36% with higher total alkalis(K2O+Na2O=6.11 % ~ 8.31%). All of them belong to high potassium calc-alkaline S type granite. LILE elements Rb and K are relatively enriched, and Ba, P and Sr are depleted, significantly. HFSE elements U and Hf are relatively enriched while Ti and Nb are depleted. The distribution of Chondrite curve represents weakly right-inclining ―V‖ type. These gneissic granite have the characteristcs of S-type granite, and some of them have the characteristcs of I-type granite, suggesting that magma source-rocks comes from the production that the most early crustal material blending in intermediate-basic mantle material were partially melted after the late crustal thickening.(3) LA-ICP-MS zircon U-Pb weighted average age of Laochengpo gneissic granite sample ZK40-1b8 is 415.4±5.2 Ma; weighted average age of Laochengpo gneissic granite sample MLP156-Q21 is 432.7±2.6 Ma; weighted average age of Tuantian gneissic granite sample MLP133-Q14 is 419.8±3.2 Ma, which belong to the production of late Silurian magma intrusion.(4) The value εHf(t) of Laochengpo gneissic granite(MLP156-Q21) changes for-4.75 ~-10.52, and two-stage model ages(TDM2) range from 1771 Ma to 2094 Ma; The value εHf(t) of Tuantian gneissic granite(MLP133-Q14) changes for-12.25 ~ 10.69, and two-stage model ages(TDM2) range from 728 Ma to 2271 Ma. It shows that their magma source area ia mainly composed of Proterozoic crustal material, involved in mantle material in the process of diagenesis. They belong to the crust-mantle mixed source granite.(5) Malipo gneissic granite was formed in the syn-collision setting. When the Kwangxi movement occurs, Yunkai terrane experienced interaction with Guidian-North Vietnamese terrane, which resulted in magma invading into the South China fold belt.