滇西保山地块东缘晚三叠世何珠次英安斑岩的发现及其地质意义

摘要

Geochronology,petrogenetic type and tectonic significance of the Hezhu subvolcanic dacite porphyry in the eastern margin of the Baoshan terrane were studied in this paper.In situ U-Th-Pb geochronology indicates that the porphyry has an age of 216.8Ma±2.2 Ma,suggesting Late Triassic emplacement instead of Early Cenozoic emplacement as proposed by previous researchers.This means that the abundance and distribution of magmatism of the Cenozoic intrusive province in the south of Baoshan terrane should be re-evaluated.This is also the first discovery of Late Triassic subvolcanic dacite porphyry in the west of the Changning-Menglian suture zone.Petrographic observation and petrochemical data suggest that the Hezhu porphyry has both S-and I-type characteristics,and was formed by mixing of mantle-derived mafic and crustal felsic magmas.The participation of mantle components in Hezhu porphyry further implies that mantle upwelling prevailed in Tengchong and Baoshan terrane in the Late Triassic period.%在野外地质调查和岩相学观察的基础上,对保山地块东缘何珠次英安斑岩进行了LA-ICP-MS锆石原位U-Th-Pb同位素测定,得到16个测定点的206Pb/238U年龄加权平均值为216.8Ma±2.2Ma.该年龄值反映了岩体的侵位时间,表明其属于晚三叠世,而非前人认为的新生代.结合岩相学和前人地球化学研究资料分析认为,岩体具有Ⅰ型和S型花岗岩的特征,反映了其在矿物和地球化学组成上的不平衡,应为幔源高温基性岩浆与壳源岩浆混合形成的,且前者占主导地位.这是昌宁-孟连结合带西侧首次报道幔源物质对晚三叠世岩浆活动的参与,表明保山地块在碰撞后地壳熔融过程中伴随地幔物质的上涌.研究表明,以何珠岩体为代表的次英安斑岩并非前人认为的新生代,保山地块南端前人划分的新生代岩体群的分布范围和规模可能需要重估.