西藏南部绒布地区基性岩脉岩石地球化学、年代学特征及地质意义

绒布地区位于西藏南部特提斯喜马拉雅构造域中段北缘,夹持于北侧邛多江断裂和南侧的绒布-古堆断裂之间。该地区发育大量的基性岩脉。通过岩石学、年代学及岩石地球化学等方面的研究,认为区内出露的基性岩脉类型为辉绿玢岩、辉长辉绿岩及辉长岩等。辉绿玢岩结晶年龄为137. 3±1. 6Ma,具有E-MORB的特征,未遭受岩石圈地幔或地壳混染,主要形成于大洋板内环境,受洋中脊源区的影响明显。辉长辉绿岩、辉长岩结晶年龄为147. 3±3. 6Ma,与时代接近的拉康组、桑秀组基性火山岩具有相同OIB的地球化学特征,带有岩石圈地幔物质混染的痕迹,形成于强烈拉伸的大陆边缘裂谷环境。晚侏罗世-早白垩世特提斯喜马拉雅被动陆缘处于强烈拉伸、岩石圈减薄的构造背景之下,OIB型辉长辉绿岩及辉长岩与措美大火成岩省诸多OIB型基性岩具有相似岩石成因,是大陆裂谷背景下Kerguelen地幔柱与岩石圈地幔相互作用的产物;而E-MORB型辉绿玢岩则可能是靠近大陆边缘的热点以下地幔柱与软流圈地幔相互作用的产生的岩浆沿区域深大断裂运移至大陆边缘侵位的结果。OIB型辉长辉绿岩、辉长岩的结晶年龄明显早于Kerguelen地幔柱活动的峰期(132Ma),可能是地幔柱早期活动的产物; E-MORB型辉绿玢岩的存在可作为目前对于措美大火成岩省基性岩脉类型的补充,对认识措美大火成岩省具有一定的意义。

Geochemistry, geochronology and geological significances of the basic dykes in Rongbu area, southern Tibet

The Rongbu area, which tectonically belongs to the north edge of East Tethys Himalaya tectonic belt of southern Tibet, is located between Qiongduojiang Fault and Rongbu-Gudui Fault. A large amount of mafic dykes have been found in this area. The petrological, geochronological and geochemical research indicates that the three types of mafic dykes including allgovite, gabbro-diabase and gabbro were recognized in our study area. The allgovite''s crystallization age is 137.3±1.6Ma and it shows the characteristics of E-MORB. The geochemistry characteristics of samples show that allgovite was not contaminated by lithosphere mantle or crust, formed in oceanic within-plate environment and significantly influenced by the source area of middle ocean ridge. The gabbro-diabase and gabbro samples, with a crystallized age of 147.3±3.6Ma, exhibit typical geochemistry characteristics of OIB with the contamination marks of lithosphere mantle materials, and they were formed in a strongly extensional rift zone in continental margin. The Tethys Himalaya passive margin was under the strongly extensional tectonic environment with lithospheric attenuation. The OIB-type gabbro-diabase and gabbro have similar petrogenesis with many OIB-type mafic rocks from Comei Large Igneous Province and should be the interactional product of the Kerguelen mantle plume and lithosphere mantle. The formation of E-MORB-type allgovite dykes was probably caused by the magma, which was the interactional product of the mantle plume and asthenosphere mantle under the hotspot near the continental margin, transferred along the regional large fault and intruded into the continental margin. The OIB-type gabbro-diabase and gabbro dykes'' crystallization ages are significantly earlier than the peak active period of Kerguelen mantle plume and they might be the product of mantle plume''s early activities. The E-MORB-type allgovite dykes could be considered as the supplements of the mafic dyke type of Comei LIP, which brings some new significance for the understanding of the Comei LIP.