滇西丽江地区新生代富碱斑岩年代学、地球化学特征及其地质意义

滇西丽江地区新生代富碱斑岩位于滇西“三江”褶皱造山带与扬子板块西南缘结合部位，岩石类型多样，可分为碱性岩和碱性花岗岩，前者主要有正长斑岩、正长岩及粗面斑岩；后者主要包括花岗斑岩、二长花岗斑岩及石英二长斑岩。锆石LA ICP MS U Pb定年研究确定区内斑岩结晶年龄为346～371 Ma，侵位于始新世晚期。碱性岩石的SiO2含量为5454%～6770%，K2O＞Na2O，具有钾玄岩的特征；碱性花岗岩的SiO2含量为6716%～7066%，K2O＜Na2O，具有高钾钙碱性的特征。两类岩石都具有富碱(Na2O+K2O=703%～1170%)和准铝质—弱过铝质(A/CNK=064～115)的特征。富集轻稀土元素和大离子亲石元素(Sr、Ba、K、Pb等)，相对亏损重稀土元素和高场强元素(Nb、Ta、Ti等)，且具有高Sr、Sr/Y和La/Yb比值及低Y、Yb和镁值(Mg#＜05)，表明区内斑岩兼具钾玄质岩石和埃达克质岩石的特征。综合研究认为，区内斑岩形成于印度－亚洲大陆碰撞造山的后碰撞阶段，其形成与减压熔融及幔源岩浆底侵促使增厚下地壳底部岩石发生部分熔融有关，是青藏高原东南缘构造转换带对主碰撞带造山作用过程的岩浆事件响应，进而分析确认本区具有斑岩-热液型金多金属矿床的找矿潜力，为深入认识丽江地区构造-岩浆-成矿作用提供了重要约束。

Characteristics of Chronology and Geochemistry for the Cenozoic Alkali-rich Porphyry in Lijiang Area,Western Yunnan and Their Significance

The Cenozoic alkali rich porphyry in Lijiang region of Western Yunnan is located in the connection part between Sanjiang fold orogenic belt and southwest of Yangtze plate, including alkali rock and alkali granite. Alkali rock includes orthophyre, syenite and trachyte prophyry, while alkali granite includes granite prophyry, monzonitic granite prophyry and masanophyre. The zircon U Pb dating indicates that the ages of those porphyry are between 346 and 371 Ma, which is consistent with the ages of Cenozoic alkali rich porphyry in Western Yunnan. Geochemical results showed that alkali rock belongs shoshonite series, with SiO2 content of 5454%-6770%, K2O＞Na2O. Alkali granite belongs high K calc alkalic series, with SiO2 content of 6716%-7066%, K2O＜Na2O. They are both enriched in alkali (Na2O+K2O=703%-1170%), showing metaluminous and weak peraluminous (A/CNK=064-115) characteristics. The porphyry shows alkali and adakitic geochemical characteristics, with enrichment of LREE and LILE(Sr, Ba, K, Pb, and so on), deficit of HREE and HFSE(Nb, Ta, Ti, and so on), with high Sr content and ratio of Sr/Y, La/Yb, low Y and Yb contents, and low values of Mg# (＜05). The comprehensive study shows that the porphyry is associated with partial melting of the base of thickened lower crust caused by decompression melting and underplating of mantle derived magmas, formed from the structure transition phase of late collisional between Indian and Asian continents, and was the magmatic event response to syn collisional processes in the structural transform zone of southeast of the Tibetan orogen. It is considered that there is great gold polymetallic prospective reserves of porphyry hydrothermal deposits. This paper provides important constraints for understanding deeply the tectonic magmatic metallogenic processes of the Lijiang porphyry metallogenetic belt.