秦岭丹凤群蛇绿岩古地磁学再研究

A PRELIMINARY PALEOMAGNETIC RESULT OF THE DANFENG OPHIOLITE FORMATION OF THE SHANGDAN ZONE IN THE QINLING BELT (CHINA)

本文是对秦岭丹凤县桃花铺郭家沟剖面和商县三十里铺剖面丹凤群蛇绿岩标本的古地磁学研究结果.逐步热退磁和剩磁方向主分量分析表明,60％以上的标本含有可明显区分的3个剩磁组分.另外的约30％—40％的标本,尽管它们具有很高的天然剩磁强度,在150—200℃时,其剩磁强度即衰减50％以上,且显示出不稳定的方向.两种不同的磁性表现与不同的岩石组成和结构有关,特别是与岩石的结晶程度有关.经对3个剩磁组分成因的分析及与变质变形关系的研究,表明高温剩磁组分是原生.由高温剩磁组分得到丹凤群古地磁极位置为43.0°N,274.5°E,古纬度值为12.0°S.比较丹凤群的古地磁极位置和华北扬子块体古地磁极移曲线,发现它们之间存在显著差异,这表明丹凤群在古生代不从属于华北或扬子块体,而是一个独立的地体,很可能是秦岭古洋壳的残片,从而给丹凤群蛇绿岩赋予新的大地构造含义.

Detailed paleomagnetic study was performed on the Danfeng Ophiolite Formation (Ordo-vician age) of the Shangdan Zone in the Qinling Belt, China. Twenty-six specimens (8 hand-samples) from the Guojiagou section (33.7°N, 110.3°E) at Danfeng County and 21 specimens (7 handsamples) from the Sanshilipu section (33.8°N, 109.8°E) at Shangxian County were measured in Beijing and Paris respectively. Stepwise thermal demagnetization and principal component analysis revealed that there were three distinct remanent components in about 60% of the specimens. The other 40% specimens which were well crystallized with coarse crystal grains, despite their higher intensity of NRM, decreased in intensity by 50% at about 150-200℃ and gave unstable directions. The magnetic beavior of the specimens showed that the ferromagnetic minerals in coarse-grains rocks are goethkes and in fine-grains rocks are magnetites which have three distinct components. The distribution of the high temperature component for all the specimens (n= 30) of the two section were calculated using standard Fisher statistics. The mean direction after tectonic correction is D = 11.5°,I=-23.1°,α95 = 6.0°, which gives a positive fold test at 99% level. The paleopole positions (43.0°N, 274.5°E) and paleolatitude (12.0°S) of the Danfeng Formation are considered to be from the primary remanence on the basis of analysis of origin and formation age of the remanence which was formed before the principal orogenic period, as well as the impact of metamorphism and deformation of rocks on the characteristic component. Comparison of the pole position and paleolatitude with those from the North and South China Blocks of early-Paleozoic age shows wide differences between them, and thus the Danfeng Formation with the Shangdan Zone represented probably a independent terrane which was a relic of a Qinling-ocean-plate.