摘要
采用目前认为可靠的几种地质温压计计算了广东省雷州半岛 1件石榴石二辉橄榄岩、1 8件石榴石辉石岩 ,以及 2 0件尖晶石二辉橄榄岩包体样品的平衡温度、压力。在所获数据的基础上建立了该区上地幔的地温线以及层序剖面并计算了其速度结构。壳幔边界大致在2 8%D 30km之间 ,在深度为 50km左右发现有尖晶石相向石榴石相的过渡 ,推测该地区岩石圈的底界在 85km左右。采用显微构造应力计和橄榄岩的高温流变律计算了上地幔差异应力、应变速率和等效粘滞度 ,建立了雷州半岛地区上地幔的流变学剖面。差异应力在 2 5—50MPa之间 ,随深度的变化没有明显的规律。应变速率在 1 0 — 1 8— 1 0 — 1 3s— 1 之间 ,有明显的随深度而增高的趋势 ;等效粘滞度的数值大致在 1 0 2 1 — 1 0 2 5Pa·s之间 ,有明显的随深度而变小的趋势。在 65km的深度 ,应变速率和等效粘滞度已达到软流圈的数值。这是由于在该深度有较高的应力所致 ,可能与软流层的底辟上涌有关。
Cenozoic basalts widely spread in Leizhou Peninsula, Guangdong Province. They contain relatively abundant mantle xenoliths mainly composed of lherzolite and pyroxenite, which can provide valuable information ofthe upper mantle beneath this region. Several tens of mantle xenoliths have been collected and studied in detail. The equilibrium temperature and pressure of spinel-garnet lherzolite and garnet pyroxenite xenoliths are estimated by using geothermo-barometers that are commonly considered to be reliable. An upper mantle geotherm for Leizhou Peninsula is constructed on the basis of the estimation results. This geotherm is much higher than the oceanic geotherm, but lower than that ofsoutheast Australia proposed by OReilly et al. According to this geotherm and the temperature distribution of lherzolite xenoliths, a lithospheric section of the studied area is constructed. From this section, it can beseen that the crust-mantle boundary is about 28―30km deep. The transition from spinel lherzolite to garnet lherzolite seems to initiate at thedepth about 55km, and the lower boundary of the lithosphere is at thedepthof 80km. The velocity structure of the upper mantle inthis area is calculated by using the density-velocity correlation equation of mantle rocks proposed by Christensen & Mooney (1995). The rheological parameters of the upper mantle are estimated by using microstructural piezometers and the flow law of peridotitic rocks, and rheological sections of the upper mantle for this area are constructed on the basis of estimation results. The results show that the differential stress is within the range of 25―50MPa, showing no obvious tendency to increase with increasing depth. The strain rate is within the range of 10 -18―10 -14s -1, showing an obvious tendency to increase with increasing depth. The equivalent viscosity is within the range of 10 21―10 24Pa·s, showing an obvious tendency to decrease with increasing depth. The strain rate and equivalent viscosity reach the asthenospheric values at the depth greater than 65km,which may be attributed to the high differential stress at this depth and might be closely related to upper mantle diapir that occurred in this region in late Tertiary.
出处
《热带海洋学报》
CAS
CSCD
北大核心
2003年第2期49-62,共14页
Journal of Tropical Oceanography
基金
中国科学院知识创新工程项目 (KZCX2 - 2 0 9)
国家自然科学基金 (4 992 530 8)资助项目
