THE SYENITE-CARBONATITE COMPLEX OF LUESHE (N-E ZAIRE):PETROGRAPHY, GEOCHEMISTRY AND Rb-Sr CHRONOLOGY

摘要

The Lueshe alkaline complex (N-Kivu. Zaiere) is inainly composed of cancrinite-syenite, calcilc-carbonatite (soevite) and dolomile-carbonatite (beforsite). Intmsion of the carbonatites is accompanied by intense fenitisation of the quartzitic to pelitic precambrian country rocks. Surficial weathering has led to the formation of a residual deposit mined for pyrochlore (Nb). Fine-grained rounded feldspathic nodules, 10cm to 1m in size, are regularly widespread in the soevite. Petrographically, they are of syenitic composition. Major phases are albite, inicrocline and calcite. Pyrochlore and aegyrine are present in variable amounts. Accessory phases are apatite, biotite and zircon. Field observations allow to propose the following sequence of emplaceinent : cancrinite-syenite→beforsite→ soevite←→syenitic nodules. Nb_2O_5 content increases with this sequence of emplacement. The mean value is 0.07% for the cancrinite-syenite, 0.13% for the beforsite. 0.22% for the soevite and 0.62% for the syenitic nodules. Total REE contents show also an increase except for the syenitic nodules. The mean total REE contents run from 11 lppm for the cancrinite-syenile to 540ppm for the soevite (beforsite : 410ppm). An Rb-Sr whole rock isochron (three caiicrinite-syenites and one pyroxenite) yields an age of 619+42 Ma with an initial ~(87)Sr /~(86)Sr ratio of 0.70323. The initial~(87)Sr /~(86)Sr ratios of the different rocks were calculated assuming an age of 619 Ma. The range of values is narrow 0.7028 to 0.7033 (except for one beforsite at 0.7048), which suggests a mantle source region (OIB-type) for the parental magma. The different petrographic types would then form a single comagmatic series. crystallizing without any crustal influence.