川西江浪穹窿煌斑岩地球化学特征及锆石U-Pb定年

Geochemical Characteristics and Zircon U-Pb Dating of Lamprophyres in the Jianglang Dome,Western Sichuan

为精确厘定川西江浪穹窿煌斑岩的形成时代并探讨其成因,对其进行了LA-ICP-MS锆石U-Pb定年及岩石地球化学研究。结果表明,煌斑岩中发育大量2784~604Ma的捕获锆石,最年轻一组岩浆锆石206Pb/238U加权平均年龄为29.85±0.35Ma。岩石地球化学分析显示,煌斑岩具有较低的Si O2含量（47.66%~50.93%）及高的K2O含量（4.98%~6.77%）、（Th/Ta）PM值（14.7~17.6）、（La/Nb）PM值（9.69~13.0）,富集轻稀土和大离子亲石元素（如Rb、Sr、Ba和U）,亏损高场强元素（如Nb、Ta和Ti）。综合前人研究成果,认为煌斑岩结晶年龄为~30Ma,形成于印度-亚洲大陆后碰撞伸展背景下的板内岩浆作用,且基性岩浆上升过程中明显遭受地壳物质混染。煌斑岩与雅砻江断裂具有密切的时空联系,暗示区内发育的新生代走滑断裂体系可能为岩浆侵位提供了通道。此外,两粒207Pb/206Pb年龄为2784Ma与2439Ma的锆石表明江浪穹窿很可能存在太古宙-古元古代变质基底。

A set of lamprophyres,which were newly discovered in the Liwu Group of the Jianglang dome,are mainly composed of biotite（~ 40%）,plagioclase（~ 45%）,hornblende（~ 10%） and pyroxene（~ 3%）.In order to accurately determine the formation age of lamprophyres and to discuss their petrogenesis,we have performed LA-ICP-MS zircon U-Pb dating and petrogeochemical studies on the lamprophyres.Dating results show that the lamprophyres have numerous captured zircons with U-Pb ages varying from 2784 Ma to 604 Ma,and magmatic zircon with the youngest206 Pb /238 U weighted mean age of 29.85 ± 0.35 Ma.Geochemical results show that the lamprophyres contain low Si O2（47.66% ~ 50.93%）but high K2 O contents（4.98% ~ 6.77%）,with（Th / Ta）PMratios varying from 14.7 to 17.6 and（La / Nb）PMratios varying from 9.69 to 13.0,enriched light rare earth elements and large ion lithophile elements（e.g.,Rb,Sr,Ba and U）,and depleted high field strength elements（e.g.,Nb,Ta and Ti）.Combining with the results of previous studies,we proposed that the lamprophyres were formed by the intra-plate magmatism in the Indo-Asian post-collision extensional setting at about 30 Ma,with the basic magma obviously contaminated by crustal materials in the ascending process.The lamprophyres were temporally and spatially associated with the Yalongjiang fault,suggesting that the Cenozoic active regional strike-slip faults may have provided channels for the magma emplacement.Moreover,two zircon207 Pb /206 Pb ages of2784 Ma and 2439 Ma may indicate that there could be Palaeoproterozoic or Archean metamorphic basements existed under the Jianglang dome.