本文首次对中国大陆科学钻探工程100～2000m榴辉岩中的绿辉石和片麻岩中的长石进行了普通Pb同位素研究。钻孔上部岩段(0-800m)样品放射成因Pb同位素含量比较高且变化幅度较小(206Pb／204Pb=16．81-17．29,207Pb／204Pb= 15．37-15．45,208Pb／204Pb=37．20-37．66),说明其原岩的U／Pb和Th／Pb或者初始Pb同位素比值比较高。钻孔中部岩段(800～1600m)发育有韧性剪切带,相应的Pb同位素比值开始降低且有起伏(206Pb／204Pb=16．26—16．72,207Pb／204Pb=15．27 -15．38,208Pb／204Pb=36．73-37．40)。钻孔下部岩段(800-1600m)样品放射成因Pb同位素含量低但变化幅度较小(206Pb／204Pb=16．05-16．46,207Pb／204Pb=15．22～15．29,208Pb／204Pb=36．68-37．48),说明其原岩的U／Pb和Th/Pb或者初始Pb同位素比值比较低。实测数据同模拟数据对比表明,钻孔上、下两岩段样品普通Pb同位素不同一方面是由于继承了原岩初始Pb同位素不同,另一方面是由它们的μ值不同所致。结合氧同位素研究,本文认为800m以上和1600m以下分属上地壳和中地壳,前者是从俯冲大陆岩石圈解耦的上地壳,在大陆岩石圈俯冲过程中,上部岩片同俯冲的大陆岩石圈发生挤离,率先沿着这一个挤离面逆冲折返至浅部。下岩片是随后从俯冲岩石圈解耦的中地壳岩片,并折返至浅部下伏在先期折返的上部岩片之下。考虑到中部岩段(800-1600m)既发育有韧性剪切带又具有异常低18O峰值(Xiao et al,2006),挤离面是在地壳内部晚元古代的断裂带及冷水活动通道基础上发育起来的。
Zircon SHRIMP U-Pb dating was per- formed for olivine gabbro at Wangmuguan in the Beihuaiyang zone of the Dabie orogen and its coun- try rock (garnet-bearing epidote-mica-quartz schist). The results show that the gabbro was crystallized at 635 ± 5 Ma, in the late Neoproterozoic rather than in the late Paleozoic as previously suggested; its coun- try rocks formed at 464±7 Ma, younger than the en- closed gabbro. The U-Pb age for the gabbro is in good agreement with ages for tuff interbedded with sediments from the Doushantuo Formation in the South China Block and late-Neoproterozoic basic dyke swarms distributed on a large scale over areas of Suizhou to Zaoyang of Hubei Province in the northern margin of the South China Block. This sug- gests a large-scale magmatic activity occurred at the late Neoproterozoic in the South China Block, so that the gabbro at Wangmuguan in the western segment of the Beihuaiyang zone is geotectonic affinity to the northern margin of the South China Block. Since the olivine gabbro occurs within the schist of Ordovician protolith with tectonic contact between them but forming in different tectonic settings, it is concluded that the late-Neoproterozoic gabbro was detached from the Precambrian basement of the South China Block during the Triassic subduction of the South China Block, and tectonically thrusted over the metamorphosed rocks in the southern margin of the North China Block.
