白垩纪时期赣杭构造带的伸展作用

前人研究认为赣杭构造带在晚中生代有两个阶段拉张活动:第一阶段发生于晚侏罗世,造成中酸性-基性火山岩喷发;第二阶段发生于早白垩世晚期,形成断陷红盆带,伴随基性玄武岩喷溢。以白垩纪基性火山岩地球化学和同位素数据为依据,认为两个阶段玄武岩浆喷发均发生于白垩纪时期,但对早期构造-岩浆活动的认识与之不同,且赣江断裂带对这期活动有限制和转换作用。早期((143±1.1)~(139±0.7)Ma)在赣杭带西端没有酸性岩出露,赣江断裂带附近的玄武岩浆来自亏损地幔,没有地壳物质混染,属于碱性玄武岩系列,属于板内拉张作用的产物。晚期(105~98 Ma)的玄武岩浆来自于富集地幔,有俯冲陆壳成分的不均匀混染,岩石属于拉斑玄武岩和碱性橄榄玄武岩系列,玄武岩浆喷发伴随着强烈的区域伸展作用,形成一系列红层盆地。

The Cretaceous tectonism of the Gan-Hang Tectonic Belt, southeastern China

This paper reports new geochemical and isotopic data from the Cretaceous volcanic rocks of the Gan-Hang Tectonic Belt,southeast China.Two periods of basaltic eruptions((143±1.1)~(139±0.7) Ma and 105~98 Ma),each representing results of separate tectonic events,are recognized in the Gan-Hang Belt.Rock types associated with these two volcanic events exhibit significant differences in mineral assemblage and chemical compositions.For the(143±1.1)~(139±0.7) Ma basalts,~(87)Sr/~(86)Sr ratio(0.704 1~0.704 8) and Sm-Nd isotopic compositions (ε(Nd)=+3.78~+4.38) suggest that the magma originated from the depleted mantle and did not mix with crustal materials.During the second episode(105~98 Ma),volcanism was accompanied by intense regional extension and by development of a series of red basins in the belt.The geochemical characteristics of these basalts exhibit both tholeiite and alkaline olivine-type,indicating inhomogeneous mixing of magma with crustal materials.The tholeiites have medium-high ~(87)Sr/~(86)Sr ratio(0.705 4~0.707 0) and low ε(Nd) value(1.69~0.70),indicating that the magma originated from an undepleted mantle.The alkaline olivine basalts,on the other hand,have relatively higher ~(87)Sr/~(86)Sr ratio(0.707 6~0.708 0) and relatively lower value of ε(Nd)(-6.43~-5.18),suggesting that these magmas originated from enriched mantle by mixing with subducted crustal material.Based on our comprehensive geologic analyses,we conclude that an extensional environment existed in the region from the early period of the Early Cretaceous,and the Gan-Hang Belt represents the main rift zone in SE China during these time intervals.