研究“钾质和钠质两个地幔富碱岩浆体系”的刍议

A Humble Opinion on the "Potassic and Sodic Two Mantle Alkali-Rich Magma Systems"

钾质富碱岩浆岩组成的岩带研究表明,这类岩石普遍富碱和CaO,高钾,富Rb,Sr,Ba和放射性元素Th,U等,Rb/Sr相对较低;稀土元素总量较高,强烈富轻稀土,δEu较小;I_(Sr)多数介于0.705～0.706之间;I_(Nd)较低,铅同位素较均一。Sr-Nd-Pb同位素特征表明,该岩类岩石的岩浆来源于EMⅡ型地幔源区,岩浆很可能直接来源于钾质富碱地幔。钠质富碱岩浆岩带岩石具富碱、高钠(Na_2O>K_2O)的特点,同时富钛及大离子亲石元素和相对富集Nb,Ta,Zr以及Rb/Sr偏高等;I_(Sr)相对低,而I_(Nd)稍高。这类岩石起源于钠质的富碱地幔源区。这些异常地幔源的形成与地幔流体交代作用有关。而超深大断裂作为岩浆上升的通道,是钾质和钠质地幔富碱岩系形成的重要条件。

'The potassic and sodic mantle alkali-rich magma systems' are the integral of all ig- neous rocks that are formed in a short geological period, and include the ultrabasic, basic, inter- mediate and acidic alkali-rich rocks along deep faults. The integral of potassic rocks is named potassic mantle alkali-rich magma system, and the integral of sodic rocks is named sodic mantle alkali-rch magma system. The study for Ailaoshan-Jingshajiang potassic alkali-rich igneous rocks belt show that in intrusion depth, these rocks can be hypogene, hypabyssal and extrusive; in lithology, they can be ultrabasic, basic, intermediate and acidic; and in alkalinity most rocks are alkaline and weakly alkaline. Their striking features are K>Na, high potassium and alkali-rich. In petrochemistry, these rocks are generally highly alkaline ( Na_2O + K_2O range from 4.55% to 12.5%), potassium-rich (K_2O/Na_2O ratio is 1.15-3.20), with high CaO (2.20% - 14.9%), high Rb, Sr, Ba, Th, U, and low Rb/Sr. The total contents of rare earth elements are higher, the light rare earth elements are strongly enriched, and the δEu values are low. Most Isr values range from 0. 705 to 0. 706. I_(Nd) range from 0. 512305 to 0. 512630. The Sr-Nd-Pb isotopic data imply that the magma of this rock belt was derived from EM Ⅱ type mantle source, and probably directly came from a kind of potassie mantle. The Anning river sodic alkali-rich igneous rock belt are also hypogene, hypabyssal and extrusive in intrusion depth, ultrabasic, basic, intermediate and acidic in lithology and mostly are alkaline and weakly alkaline in alkalinity. Most of sodic al- kali-rich rocks are high in Na_2O + K_2O (5.67% - 14.44% ), sodium-rich (the K_2O/Na_2O ratio is 0.22-0.90), with high contents of TiO_2 (0.15% -6.44% ) and large ion lithophile elements. Nb, Ta, Zr and Rb/Sr are relatively higher. The contents of light rare earth elements are lower than those of potassic rock belt, but δEu values are higher, and sometimes even positively abnor- mal. I_(sr_ values range from 0. 705 to 0. 706. I_(Nd) value is about 0. 512630. These rocks are similar to the igneous rocks in the rift area derived from mantle. They are mainly derived from mantle multi-period sodie alkali-rich magma and multi-stage evolved. The study shows that there are not only potassium-rich mantle source areas, but also sodium-rich mantle source areas. These mantle magmas were formed in the deep mantle under the high pressure and high temperature conditions. As the passageway of magma uprising, the ultra-deep faults are important for formation of the potassic and sodic mantle alkali-rich magma systems.