青藏高原南部地幔包体的发现及其意义

Mantle xenoliths in southern Tibet:geochemistry and constraints for the nature of the mantle.

尽管青藏高原碰撞后超钾质岩石代表了上地幔低度部分熔融的产物，增加了对地幔的了解，但是对青藏高原陆下岩石圈地幔的性质依然缺乏清楚认识，其中一个最主要的问题是高原腹地的超钾质岩石中一直缺少幔源包体和巨晶。本文报导了青藏高原南部赛利普粗面安山岩（具有典型的超钾质岩石特征，年龄约为17Ma）中产出的地幔包体的矿物主量元素成分。包体大小为0．5cm—1．5cm，主要为两类，一类是辉石岩（Opx＋Cpx），另一类为二辉橄榄岩（01＋Opx＋Cpx±Phl±Sp）。包体中橄榄石（Mg^#=89—90，CaO=0．05％-0．12％，TiO2〈0．03％，NiO=0．29％-0．80％），单斜辉石（Mg^#=88—91，Al2O3=5．5％-7％），斜方辉石（TiO2=0．05％-0．15％，Al2O3=2％-5％）和尖晶石（Mg^#=58—76，Cr^#=10—44，Cr2O3=9％-35％，MnO=0．09％-0．24％，FeO=11％-18％，Al2O3=29％-57％，MgO=16％-21％）的成分与中国东部新生代玄武岩中的地幔包体特征一致。包体的温度为990—1140℃、压力为16—20kb，显示的地温曲线与中国东部、东非肯尼亚等世界典型裂谷区的上地幔特征一致，表明青藏高原南部在中新世尽管处于印度与亚洲大陆的挤压汇聚状态，但是仍具有区域性伸展作用存在，这与藏南广泛发育的南北向裂谷和地表高热流是吻合的。包体中含水金云母与石英的出现以及尖晶石成分不均一性等揭示了包体中多期交代作用过程。与金云母共生的尖晶石后期改造作用表明导致藏南上地幔改造的交代流体应是与寄主火山岩成分接近的富K，Si和H2O的流体。藏南地幔包体的深入研究将对揭示青藏高原地幔的成分、状态与深部作用过程以及为更好解释超钾质岩石的成因提供更多的证据。

The nature of the subcontinental mantle of Tibetan Plateau is not well-known, although lots of postcollisional volcanic rocks in Tibet have been inferred to be generated by low degrees of partial melting from the upper mantle. The key problem is that mantle-derived xenoliths and megacrystals hosted in potassic and ultrapotassic volcanic rocks are rarely found in the hinterland of Tibetan plateau. Here we report our major element data of minerals of the mantle xenoliths hosted in the Sailipu volcanic rocks （trachyandesite, with an age of - 17Ma, have typical ultrapotassic features that similar to that found in southern Tibet）. The xenoliths, ranging in size from 0. 5cm to 1.5cm in diameter, can be divided into two groups. The first group is pyroxenites （Opx ＋ Cpx）, and the second is lherzolite （ O1 ＋ Opx ＋ Cpx ＋ Phi ＋ Sp）. The compositions of olivine （ Mg^# = 89 - 90, CaO = 0.05% - 0. 12%, TiO2 〈0.03%, NiO =0.29% -0.80%）, Cpx （Mg^# =88-91, Al2O3 =5.5% -7%）, Opx （TiO2 =0.05% -0. 15%, Al2O3 =2% -5% ） and spinel （Mg^# =58 -76, Cr^# = 10-44, Cr2O3 =9% -35% , MnO =0. 09% -0. 24%, FeO = 11% - 18% , Al2O3 = 29% - 57%, MgO = 16% - 21% ） show similar features to that in the Cenozoic mantle xenoliths in eastern China. The calculated temperatures of the xenoliths are 990 - 1140℃ at the given pressures of 16 - 20kb. The geotherm suggested by this P-T conditions is similar to that in eastern China, Kenya and other rift-related upper mantle regimes, implying that the regional extension beneath southern Tibet in Miocene, although India were colliding with Asia during that time. Multi-stage metasomatic processes could be found in the samples, including water-bearing phlogopite, quartz in pyroxenites, and the rim-core composition variation in spinel. The secondary spinels along with neighboring phlogopite suggest that a potassic, host-rock like metasomatic agent that enriched in K, Si and H2O, should have played an important role in modifying the upper mantle beneath southern Tibet. Further study on the Sailipu mantle xenoliths will be helpful in revealing the composition, regime, and processes of the upper mantle beneath southern Tibet, and in discussing the origin of the orogenic uhrapotassic rocks.