地幔岩包体中斜方辉石的GOD化现象

Godization of Orthopyroxene in Mantle Xenoliths

对我国东部四个地点碱性玄武岩中地幔岩样品的研究分析表明，地幔岩包体被带上来的过程中，斜方辉石比单斜辉石更易发生部分熔融，而形成熔体玻璃（Ｇｌａｓｓ）、橄榄石（Ｏｌｉｖｉｎｅ）和铬透辉石（ＣｈｒｏｍｅＤｉｐｓｉｄｅ）集合体，此作用可称为ＧＯＤ化。ＧＯＤ化沿包体边部最发育，向内逐渐减弱，以至消失。当玻璃极富Ｋ２Ｏ时，出现强烈ＧＯＤ化。在强烈ＧＯＤ化外面的寄主岩石中，还形成销沸石和／或方沸石聚集体。斜方辉石的ＧＯＤ化不仅是由于升温降压而导致的不一致熔融所致，而且还伴有物质的交换（带入ＣａＯ＋Ｎａ２Ｏ＋Ｋ２Ｏ，带出ＳｉＯ２＋ＭｇＯ）。

Microscopic and microprobe investigation of mantle xenolith samples from 4 localities in eastern China indicates that during transporting to the surface, orthopyroxene in xenolith can partly be melted easier than clinopyroxene, forming aggregate of glass (G ) (20 %～60% ), olivine (O ) (10%～70% ) and chrome-diopside (D) (5 %～25% ). The numerous microcrystals of chrome-diopside are characterized by nearly identical crystallographic orientation with that of relic orthopyroxene. This process can be named as godization. The godization decreases from the margin of xenolith towards the inner part. Intense godization occurs when glass is extremely rich in K2O. Outside the intense godization in the host alkalic basalt there is lensoid accumulation of natrolite and/or analcite. Godization of orthopyroxene is not only caused by the incongruent melting with increasing temperature and decreasing pressure under volatile-bearing circumstance,but also accompanied by compositional exchange (introduction of CaO+Na2O+K2O and depletion of SiO2+ MgO).