| Alkali-rich porphyries of Xiaolongtan intrusions, lying in the Jinsha River-AilaoMount suture zone, belong to the Jinsha River-Ailao Mount alkali-rich intrusive rock belt. The geochmeisty and geochronology characteristics of alkali-rick porphyries are researched in this paper to reveal the lithogenisis.(1)The alkali-rick porphyries in Xiao Longtan area are characterized by enrichment in alkaline(K20+Na20=9.05%～12.45%) and high value of potassium(K20/Na20=0.85-8.39) and aluminum (A/CNK=0.93～1.16), low value of titanium(TiO2=0.27%-0.37%). It is suggested that the alkali-rick porphyries in Xiaolong Tan area belong to shoshonitic rocks.(2) Geochemically, the porphyries are enriched in large ion lithophile elements (Ba, K, Rb, LREE, etc.), depleted in high field strength elements (such as Ti, Nb, Ta, HREE).(3) The high ΣLREE/ΣHREE and (La/Yb)N rations indicate that they are enriched in LREE and lacking in HREE. The alkali-rick porphyries have middle degree Eu anomalies while lack of obvious Ce anomalies.(4) It is predicted that the intrusions of Xiaolong Tan has the characteristics of EMⅡ type mantle by datas of Rb-Sr-Pb isoplot from BeiYa and Ma Changqin area.(5) The zircon U-Pb MA-ICP-MS age,34.7±0.28Ma, of alkali-rick porphyries from Xiao Longtang intrusions are published for the first time, which suggests that the intrusions are formed in Cenozoic.(6) On the basis of the geochemical characteristics and regional tectonic history, the alkali-rich magma were derived from the partial melting of metasomatized litho spheric mantle, which is resulted from the subdiction of Yangtze Plate. The partial melting owes to the upward intrusion of the asthenosphere. The detachment of metasomatized lithospheric mantle from the lower crust induced the upward intrusion. It is suggested that the the magma intrusion was induced by strike-slip shearing in the post-cllosion stage. |
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