湘桂地区泥盆纪硅岩Rb-Sr、Sm-Nd同位素地球化学特征及构造沉积背景研究

The Charactoristics of Strontium and Neodynimu Isotope Geochemistry and Tectonic-depositional Setting of the Devonian Chert,Hunan-Guangxi Province

泥盆纪时,由于扭张裂谷作用,华南地区形成复杂的台盆相间格局。层状硅岩广泛分布,一般与凝灰岩互层,产出于狭长的台间盆地(或沟槽)中。为了揭示湘桂地区硅岩形成机制、分布及与断裂活动的关系,进行了系统的RbSr、Sm-Nd同位素研究。研究结果表明研究区硅岩(87Sr/86Sr)0值一般在0.721 000~0.731 000之间,表明硅岩形成时受到陆源和海水的影响。硅岩Nd同位素模式年龄(tDM或t2DM)和εNd(0)值大多分布区间分别为1.5~2.1与-16^-21,表明硅质来源于深部元古代地壳。εNd(0)值(-0.22~14.7)高的一些地区,大多沿狭长海槽分布,表明硅质可能来源于深部地幔,通过延伸到地幔的地块边缘断裂带上升到地表。

During the Devonian, a complicated carbonate platform-basin configuration was created through transten- sional rifting; extensive bedded chert, commonly interbedded with tuffaceous beds, occurred in the narrow, elongate interplatform basins （or troughs） in South China. In order to unravel the origin and distribution of the bedded chert successions, and their relationships to basement faulting activities during the opening of the Devonian South China Sea, geochemistry and Rb-Sr, Sm-Nd isotopic systematics are carried out upon the chert deposits. The initial （87Sr/ 86Sr） 0 ratios of chert generally vary from 0.712 000 to 0.730 000, suggesting influences both from terrigenous influx and seawater. The Nd isotopic model ages （tDM or t2DM） and initial εNd（0） values of chert vary mostly from 1.5 to 2.1 Ga, and from -16 to -21, respectively, implying that the silica sources were derived from the provenances of the Palaeoproterozoic crust relics at depth. The high εNd（0） values of chert （-0.22 to 14.7） in some localities, mostly a- long the elongate troughs, suggest that silica sources may have been derived from deeper-seated mantle, being chan- neled through the interplate boundary fault zones extending downwards to the mantle.