鹤庆盆地早更新世以来湖相沉积物中早期还原成岩过程的岩石磁学证据

ROCK MAGNETIC EVIDENCE FOR EARLY DIAGENESIS IN THE PLEISTOCENE LACUSTRINE SEDIMENTS FROM HEQING BASIN

鹤庆盆地位于我国西南地区,受印度季风影响。在盆地内获取的665.83m的湖相岩芯具有沉积连续,时间跨度长的特点,是研究印度季风在第四纪以来发展演化特征的理想材料。在岩芯的下部(1.8Ma以前),磁化率大幅度降低,而烧失量则较高。通过对这部分沉积物详细的岩石磁学测试和扫描电镜分析可知,岩芯下部(2.5～1.8Ma)磁性矿物含量较低,低磁化率值与沉积后的还原溶解作用密切相关。在岩芯下部有自形程度较高的单畴(SD)胶黄铁矿存在,可能为早期还原成岩过程的产物。

Heqing Basin（26°27＇ -26°46＇N, 100°08＇ -100°17＇E） is a Late Cenozoic north-south oriented pull-apart basin near the northwestern margin of the Yunnan-Guizhou Plateau. The Basin is located at the southeastern margin of the Tibetan Plateau and belongs to the terminal of Hengduan Mountains. The local climate is dominated by Indian monsoon. Therefore, the continuous and thick sediments in the basin make it a perfect continental record for evolution of Indian summer monsoon. The Heqing drill core was obtained in the centre of the Heqing Basin. Using the internal lining method, recovery of the HQ drill core （26°33＇43. I＂N, 100°10＇14.2＂E;2200m） which has an accumulative depth of 665.83m reaches up to 96.7%. The lake sediments mainly consist of horizontally laminated greyish-green calcareous clay and silty clay with thin-bedded silt and fine sand layers,except two intervals of sand layers with fine gravels. With the equal interval of 50em,we obtained 1368 samples from the drill core and chose 108 from different depositional units for detailed rock magnetic measurements. Based on detailed rock magnetic measurements like magnetic susceptibility versus high/low temperature curves, thermal demagnetization curve, hysteresis loops and FORC diagrams, we have investigated the magnetic properties of Heqing core lacustrine sediments. Magnetic measurements showed that sediments from the lower section （before 1.8Ma） of the core contain both magnetite and greigite while sediments from the upper section （ after 1.8Ma）contain magnetite only. The abrupt decrease at about 580℃ in %-T curves revealed that magnetite could be the dominant magnetic minerals in the upper section. FORC diagram displayed a characteristic PSD distribution. Hysteresis loop of low % units in upper sections showed a wasp waist shape which is due to admixture of different eoercivity and different domain state. This could be attributed to magnetic dissolution which decrease ferromagnetic component like magnetite and maghemite （low coercivity components） and increase the relative content of antiferromagnetic component like hematite and goethite（high coercivity components）. By contrast,the behaviors of samples from lower section （before 1.8Ma）in thermal demagnetization hunt the presence of greigite. SIRM imparted before and after thermal demagnetization decreased sharply at about 225 250℃ which could be attributed to the thermal decomposition of greitgite. Hysteresis loop displayed a characteristic SD distribution. Because of no Verwey transition （magnetite） in low temperature measurement, SD distribution and peak value of about 60mT in FORC diagram must be caused by autogenetie greigite. The SEM image showed octahedral iron sulfide proved the presence of greigite in the lower section. Rock magnetic measurements revealed that magnetite was the dominant magnetic mineral in the upper section while SD greigite and PSD magnetite dominated the magnetic property in the lower section. At upper section,high % and low loss in ignition indicate that post-depositional dissolution could be weak. In contrast,magnetic susceptibility （Z） decrease and loss in ignition increase both in a large degree in the lower section. Detrital components in this section had undergone a strong post-depositional dissolution and caused low ~. Greigite which presented in lower section （before 1.8Ma）had an octahedrite crystal and displayed SD magnetic behavior. The maximal diameter of this large grained autogenetic greigite could achieve about 21xm. The strong post-depositional diagenesis indicated that the paleolake before 1.8Ma in Heqing Basin would have deep water,and climate here would be suitable.