中国西南地区CO2释放点的He同位素分布不均一性及大地构造成因

在印度板块向欧亚大陆俯冲碰撞过程中,产生一系列深大断裂带,形成大量CO2释放脱气点。这些点除了释放大量的CO2、N2、H2S气体以外,还会附带释放CH4、He等各种各样的气体。释放出来的氦为惰性元素,是判识幔源气体最灵敏的地球化学示踪指标,He同位素随形成的构造部位不同而不均匀地分布,且与不同的大地构造成因关系密切。藏中及藏北区块在大地构造位置上属于班公湖-怒江断裂带发生了明显的地幔脱气作用,显示其与地幔相连通而且深度达到上地幔,为一条岩石圈深断裂带;但样品中幔源氦约占总氦的1.4%~1.7%,反映该断裂带深部的开放性程度较低,而闭合性程度相对较高,由此反映了该区处于强烈挤压的构造环境和地壳增厚的地质背景。在滇西南地区(重点在腾冲热海地区)地热流体逸出气体中含有大量幔源岩浆挥发组份,表明该区地壳浅部存在幔源岩浆侵入活动。该区怒江断裂带发生着明显的地幔脱气作用,显示该断裂带与地幔相连通且深度达到上地幔,为一条岩石圈深断裂带;同时所有样品中的幔源氦平均约占总氦的26.2%,最高可达48.8%以上。反映该裂谷带代表伸展性构造环境,且是地幔脱气作用最强烈的构造区之一,是俯冲碰撞的中心地带。而在滇中地区—小江断裂带却是另一番景象,该区样品中的幔源氦平均约占总氦的2.27%,最高也才8.9%。反映此区为印度板块向欧亚大陆以NNE方向碰撞影响到的最东缘。小江断裂带局限在地壳范围内,故而氦同位素显示出在此断裂带以西则明显存在幔源来源,而往东则幔源氦极微。氦同位素在自西至东由藏西到藏南(藏南西与藏南东)至滇西到滇中,R/Ra值由较低到较高再到较低到最高再降低的平面分布规律。川西—鲜水河断裂带则因特殊的构造部位,是中国“地质百慕大”;样品中幔源氦平均约占总氦的8.1%,平均值高于西藏各分区以及云南除滇西南以外的各区,但又远远小于滇西南—腾冲地区。说明鲜水河断裂带,部分地贯穿整个岩石圈,并有切割上地幔之趋势。

Distributing Inhomogeneity of Helium Isotope of CO2 Degasification Point and Its Geotectogenesis in Southwest of China

There are come into being a series of deep seated fault zone and a great deal CO2 degasification point through the collision between Indian and Asian plates. Meanwhile the degasification point will release various gases including to N2, CO2, H2S, CH4, He, and so on. He gases is an inertia chemical element that cannot react chemical action but physical process. So Helium and its isotope is a sensitive geochemistry tracer reagent that can distinguish the mantle-derived gases. There are different mother earth constitution will formative uneven distributing Helium isotopes. The mother earth constitution of the Central and the Northern of Tibet attribute Bangong Co-Nujiang suture zone. It takes place obvious mantle degasification that shows the suture zone' depth reaches the upper mantle and it's a lithosphere deep seated fault zone. The mantle-derived Helium of all samples averagely occupies the whole Helium about 1.4%~1.7% which reflects the bottom of the fault zone is lower opening degree and tight close. As a result it lies on the intensity extrusion constitution environment and incrassated crust. The terrestrial heat current release large number of the mantle-derived helium from the southwest of Yunnan province. It show the section exist mantle-derived magma intrude activity on shallow crust. Nujiang suture zone takes place obvious mantle degasification that shows the suture zone' depth reaches the upper mantle and it's a lithosphere deep seated fault zone. The mantle-derived Helium of all samples averagely occupies the whole Helium about 26.2%, the tiptop about 48.8% which reflect the fault zone is repressing extent structure environment. Meanwhile the area is the strongest mantle-derived helium assoil. On the other way round the Central of Yunnan province (Xiaojiang fault) entirety is differ from the southwest of Yunnan province. The mantle-derived Helium of all samples averagely occupies the whole Helium about only 2.27%, the maximum about 8.9%. We can suppose that the Xiaojiang fault zone is the furthest east edge of the collision between Indian and Asian plates. It is a crust fault but is the boundary of the Yangzi ancient land plate to the collision zone. So the helium isotope shows mantle-derived on the west to this crust fault and less mantle-derived helium on the east. The Xianshuihe fault zone on the west of Sichuan province is an especial mother earth constitution place that is know as "the Bermuda Triangle of Geology". The mantle-derived Helium of all samples averagely occupies the whole Helium about only 8.1%. The mean value larger than the subarea of Tibet' value and!the subarea of Yunnan' value except the southwest of Yunnan. So the great strike-slip fault-the Xianshuihe fault zone run through the whole lithosphere in part and have a tend to incision the upper mantle.