藏北布若错地区新生代火山岩及其成因探讨

布若错地区新生代火山岩发育两种组合,一是由粗面安山岩、粗面岩、响岩等组成的钾质-超钾质火山岩组合,二是由玄武粗面安山岩、流纹岩组成的双峰式火山岩组合,K-Ar同位素年龄分别为31.5、39.3 Ma.它们都是北羌塘新生火山岩带的组成部分.火山岩具有高碱（K20＋ Na20=6.66％～14.34％）、富钾（3.78％～8.22％,K20/Na2O=1.15 ～5.37）、高∑REE（中基性岩平均值1 161.22×10-6,酸性岩154.19×10-6）,强烈富集LREE（∑LREE/Σ HREE=20.52～40.29）和大离子亲石元素（LILE） Sr、Ba、K、Th、Rb,高场强元素（HFSE） Ta、Nb、P、Ti相对亏损等特征,兼有板内和岛弧火山岩的双重地球化学特性.结合区域地质构造背景研究表明,该火山岩形成于晚侏罗世末一早白垩世早期北羌塘陆内碰撞造山后的拉张环境,为富集地幔或壳-幔混合层发生部分熔融产物,但岩浆向上隆升过程中发生了一定的分离结晶和下地壳部分熔融作用.

Petrogenesis of Cenozoic Volcanic Rocks of Buruocuo in North Tibet

The Buruocuo Cenozoic volcanic rocks include two series of rock association. One is potassic and ul- tra-potassic volcanics including trachyandensites, trachytes and phonolites. The other is bimodal volcanics of basaltic-trachyandesites and rhyolites. The K - Ar isotopic age of both volcanics are 31.5 Ma and 39.3 Ma, re- spectively. They belong to the part of Cenozoic volcanic rocks of Northern Qiangtang. The geochemical charac- teristics of volcanics display high alkaline （ K20 ＋ Na20 = 6. 66% - 14. 34% ）, rich potassium （ K20 = 3.78% -8.22% , K20/Na20 = 1.15 -5.37）, high rare earth elements （basic to intermediate rocks： ∑ REE = 1 161.22 x 10-6, silicic rocks： ∑ REE = 154. 19 x 10-6）, extremely enriched light rare earth elements （ LREE/∑ HREE = 20. 52 -40. 29） and large ion lithophile elements （Sr, K, Th and Rb）, but relative deple- ted high field strength elements （Ta, Nb, P and Ti）. The whole geochemical signatures of volcanic rocks dis- play characteristics from the intra-continental to island arc transitional. By regional geological tectonic back- ground researche, it is suggested that the volcanic rocks were formed in intra-continental post-collisional exten- sion setting from late Jurassic to early Cretacious of North Qiangtang rich mantle or crust-mantle mixed layer. However, to some extent, tion and partial melting of lower crust during magma upwelling. block, the production of partial melting of it developed with the fractional crystalliza-tion and partial melting of lower crust during magma upwelling.