江南古陆南缘四堡群同位素地质年代学研究

THE GEOCHRONICAL STUDIES OF SIBAO GROUP IN THE SOUTHERN MARGIN OF JIANGNAN MASSIF

四堡群浅变质火山－沉积岩系主要出露于桂北九万大山地区，是华南最古老的地层单元之一。对宝坛地区罗布山枕状玄武岩中细粒原生锆石进行单颗粒锆石蒸发－沉积法年龄测定，两组锆石样品获得￣（２０７）Ｐｂ／￣（２０６）Ｐｂ年龄分别为１７３４±２０（２σ）Ｍａ和１８６３±７（２σ）Ｍａ。对宝坛地区基性和超基性火山岩进行了Ｓｍ－Ｎｄ同位素研究，获等时线年龄为１７８２±８２（２σ）Ｍａ，与锆石单颗粒￣（２０７）Ｐｂ／￣（２０６）Ｐｂ年龄一致，充分说明四堡群形成于中元古代早期。火山岩的ε＿（Ｎｄ）（ｔ）为０．６，表明基性－超基性岩浆源自于亏损地幔。这套岩石的Ｒｂ－Ｓｒ同位素体系已明显遭受到扰动，￣（８７）Ｓｒ／￣（８６）Ｓｒ和￣（８７）Ｒｂ／￣（８６）Ｓｒ之间不存在相关关系，构不成有意义的等时线年龄。在江南古陆，存在一系列可与四堡群相对比的浅变质火山－沉积岩系，故四堡群成岩时代的厘定必将引起人们对这些岩系形成时代的重新认识，同时也排除了四堡群作为中生代残余蛇绿岩套的可能性。结合本区地质特征，由于四堡群中科马提岩套形成时代较晚，故其金、镍矿化前景不好。

The ￣（207）Pb/￣（206）Pb-ages on single zircons by using kober’s method,and Sm-Nd and Rb-Sr isotopicsystems have been studied for the mafic-ultramafic volcanic rocks from Sibao Group in Baotan districtalong the southem margin of Jiangnan massif.The￣（207）Pb-￣（206）Pb determinations on single zirconse separat-ed from pillowed basalt lava in the upper part of Wentong Formation show that two groupe of fine-grained primary zircons give the ￣（207）Pb/￣（206）Pb ages of 1734±20 Ma and 1863±7 Ma,respectively.Eight samples of mafic and ultramafic volcanic rocks collected from Wentong,Yangjia,Shaping,andJiepei gives a Sm-Nd isochron age of 1782±82 Ma,with the initial Nd value of 0.51035,which isvery consistent with the ￣（207）Pb/￣（206）Pb ages of fine-grained primary zircons,which indicate the formationof Sibao Group from the late Ealy Proterozoic to early Middle Proterozoic time.The εNd（t） of 0.6 forthe analyzed samples implies the derivation of mafic-ultramafic magma from a depleted mantle source.The Rb-Sr isotopic systein has been strongly disturbed probably by the late tectonic movement,mag-matic intrusions, and metamorphism,and reveals the lack of correlation between ￣（87）Sr/￣（86）Sr and￣（87）Rb/￣（86）Sr,which can not give any meaning of rock-forming age.The delineation of the forming agefor Sibao Group causes our reconsidering the forming age of the corresponding metavolcano-sedimen-tary sequences in Jiangnan massif,and excludes the possibility of the relict ophiolite suites for thosemetavolcano-Sedimentary sequences.