黔东—湘西早古生代岩石圈热状态与高产热率HHPRM型地幔源区

岩石圈热结构和热状态的研究对于大陆裂解、变质作用及活化构造形成机制等大陆动力学问题的研究十分重要。通过地球化学、构造地质学和综合分析,以贵州省镇远地区金伯利岩-钾镁煌斑岩和黔东—湘西一带的晚元古代地层为例,研究了扬子地块东南缘早古生代岩石圈热结构与热状态。提出高放射性元素产热率的HHPRM型地幔源区,进一步讨论HHPRM型地幔源区形成与演化的大陆动力学机制。认为古富集地幔(HHPRM型地幔源区)是镇远一带的早古生代钾镁煌斑岩-金伯利岩的岩浆源区,镇远地区金伯利岩-钾镁煌斑岩岩浆源区深度(H)在208～244km,形成温度在1547～1403℃间,该温度和深度范围属软流层,具有形成金刚石矿床的地质条件。建议在贵州—湖南—广西三省交界区的金伯利岩和钾镁煌斑岩岩群中应进一步开展金刚石矿床研究与勘查工作。

EARLY PROTEROZOIC HEAT STATE OF LITHOSPHERE AND HHPRM-TYPE MANTLE-SOURCE AREA IN NORTHEASTERN GUIZHOU AND WESTERN HUNAN, CHINA

Research on heat state and structure of lithosphere is important to better understand continent breakup, metamorphism and reactive tectonics mechanism. Based on geochemistry, structural geology and comprehensive analysis on kimberlite and lamprophyre in Zhenyuan area and the Late Proterozoic strata in northeastern Guizhou and western Hunan Provinces, heat state of lithosphere in southeastern margin of the Yangtze massif in the Upper Palaeozoic has been discussed in this paper. The kimberlite and lamprophyre in Zhenyuan area are still of high heat production of radioactive elements, respectively 2.83 mWm-3 and 8.09 mWm-3, as imply that there is a high heat production of radioactive elements derived from the earth mantle. It is estimated that kimberlite and lamprophyre are probable derived from the depth from 208 to 244 km at temperature 1 547 1 403℃, which might be attached to asthenosphere, i.e., radioactive elements of mantle source area (HHPRM type mantle source, Fig.1) have a high heat production. On the other hand, heat production of radioactive elements in the Late Proterozoic strata in northeastern Guizhou and western Hunan Provinces is 1.54 mWm-3. The relative cold paleo mantle plume that might be formed by after image of continental subduction during Grenville orogeny from 1200 to 950 Ma. HHPRM type mantle source may be closed related to melting of the relative cold paleo mantle plume by the CO H2O rich of mantle fluid metasomatism that may extract K、U and Th from the lower crust and mantle. At least, three main evolutional processes can be recognized as followings: Firstly, the magma source of kimberlite and lamprophyre in southeastern margin of the Yangtze massif may be derived from the Archaeozoic Proterozoic strata. It is suggested 2 410 Ma by U Pb ages of apatite in the Zhenyuan area and from 2 610 to 1 900 Ma by Pb Pb isochronous ages of zircon in the Dahongshan area, Hubei Province. Zircon and apatite belong to mantle capture type, and their ages may indicate the magma source of kimberlite and lamprophyre. Secondly, that K Ar ages of Ti phlogopite in the Zhenyuan kimberlite and lamprophyre are from 654 to 536 Ma, which may be time limits of mantle fluid metasomatism. The CO H2O rich mantle fluid metasomatism may extract K、U and Th from the lower crust and mantle, resulting in HHPRM type mantle source in asthenosphere in the earth mantle. Finally, when the studied area was transformed into a type of syncollsion rift basin in the Lower Palaeozoic, the CO H2O rich mantle fluid with high heat production of radioactive elements was intruded in the Zhenyuan area from 503 to 497 Ma and this event of tectonic magma ended in the Zhenyuan area from 442.67 to 435.54 Ma.