湘南地区深部岩石剖面的建立——来自深源岩石包体的证据

The deep lithological section in southern Hunan province──evidence from some xenoliths in alkaline basalt

研究了湘南道县玄武岩中深源岩石包体尤其是下地壳岩石包体的特征 ,对代表下地壳化学组成的典型岩石———麻粒岩包体和辉长岩包体进行了详细研究 ,利用多种地质温压计计算了包体的平衡压力及深度 ,并就此建立了该区的深部岩石剖面 :①上地幔顶部主要为尖晶石二辉橄榄岩及辉石岩 ,深度在 85～ 3 8km ;②下地壳主要由基性麻粒岩组成 ,平衡深度为 3 8～ 2 5km ;③中地壳下部由尖晶石辉长岩层状侵入体及花岗质片麻岩组成 ,以波速为 6 0 5～ 6 40km/s为特征 ,深度为 2 5～ 1 9km ;中地壳上部主要由元古界浅变质岩组成 ,深度为 1 9～ 4km ;④上地壳表层主要由沉积岩组成 ,深度为 4～ 0km。从包体的岩石类型来看 ,下地壳的物质组成以基性玄武质岩石为主 ,麻粒岩和辉长岩则分别代表了不同时期幔源岩浆对下地壳的添加。

The paper studies some features about the deep-seated rock xenoliths entrained by volcanos,especially the typical samples graunlite and gabbro xenoliths are described in detail.Many geo-thermobar meters are used to calculate the balanced temperature and pressure.According to xenolith types and the minerals composition and balanced temperatures and pressures,the deep section is shown:1)upper mantle(85～38 km) spinellherzolite;2)lower crust(38～25 km): two-pyroxene granulite and cumulate spielgabbro,measured elastic velocity about 6.05～6.40 km/s;3)middle crust(19～4 km) gneiss and schist at deep part,low-grade metamorphic rocks at upper part;4)upper crust(0～4 km) sedimentary rocks.The composition of xenoliths reveals that the lower crust is basaltic while granulite and gabbro suggest that there are mantle magmas underplating to lower crust in different ages respectively.