华北克拉通古老岩石圈地幔的多次地质事件:来自金伯利岩中橄榄岩捕虏体的启示

Multiple-geological events of ancient lithospheric mantle beneath North China craton:As inferred from peridotite xenoliths in kimberlite

本文提供的两件蒙阴岩区金伯利岩中的蛇纹石化石榴石橄榄岩捕虏体,整体发育剪切-变形结构,其中的辉石有三种类型,代表了三次地质事件,他们是:(1)石榴石中的自形单斜辉石包裹体Py;(2)粗粒不规则形状的斜方辉石Py1;(3)具反应边及定向排列的斜方辉石Py2。Py具有高Na2O和Al2O3,及低Mg#和CaO的特征,暗示所赋存的橄榄岩未遭受过明显的熔融作用。推测Py为早期阶段地幔"岩浆海"结晶时被石榴石包裹的矿物。在手标本及薄片中普遍见到Py2切过Py1,表明Py1形成早于Py2。Py1的Cr(669×10-6～9503×10-6),Ni(1941×10-6～4750×10-6)含量和Mg#(0.91～0.94)比值较高,而Py2中的Cr(725×10-6～1926×10-6),Ni(902×10-6～2989×10-6)和Mg#(0.88～0.90)值较低,说明Py1是早期经部分熔融的橄榄岩耐熔残余中的顽火辉石残留。相反,Py2可能是软流圈来源的熔体与耐熔橄榄岩反应的结果。剪切/变形以及交代事件则发生于上述反应之后或者与之同时。依据主元素特征,较早的Py1的耐熔程度反而高于Py2,本文称之为地幔组成的"逆向演化"。看来,这种逆向成分演化不仅发生在中新生代,而且也发生于古老地幔,甚至是贯穿于整个地幔演化的历史时期。与已发表的有关地幔形成年龄的资料对比,Py、Py1和Py2的年龄估计分别是>3.8Ga,2.5Ga/1.4～1.3Ga和0.9～0.7Ga。另外1件碳酸盐化橄榄岩捕虏体,采自复县金伯利岩,具有明显的剪切-变形结构,最终形成时间可能与Py2接近。

Two piece of serpentinized garnet peridotite mantle xenoliths with sheared-deformation texture entrained in kimberlite from Mengyin have been studied in this paper. Three kinds of pyroxenes represented three geological events of lithospheric mantle are recognized. From early to late they are:(1) euhedral diopsite (Py) inclusions in garnet; (2) irregular coarse grain enstatites (Py1); (3) orientated tabular enstatites with reaction rim (Py2). Py have relative high Na2O,Al2O3,and low Mg#(0.91),CaO,implying that Py and the host mantle peridotite have not been undergone the stronger melting events during that time. It is possible that the Py was the crystallized products from 'magma sea' in initial stage of mantle evolution. Py2 cutting across Py1 occur in hand specimen and thin section indicating that Py1 formed early then Py2. Py1s have higher Cr (669×10-6~9503×10-6),Ni (1941×10-6~4750×10-6) contents and Mg# ratio (0.91~0.94),however the Cr (725×10-6~1926×10-6),Ni (902×10-6~2989×10-6) and Mg# ratio (0.88~0.90) in Py2s are lower then that in Py1s showing the Py1 with an origin as a refractory residue of partial melting. In contrast,Py2s are the reaction products of refractory mantle peridotite-melt which derived from asthenosphere. The sheared/deformation and metasomatized events were happened after or as same as the reaction event. In terms of major element,the refractory degrees of Py1s are higher than that of Py2s. In this paper 'reverse evolution' trend of mantle component is called. It may be possible that the peridotite-melt reaction not only occur in Mesozoic-Cenozoic but also in ancient lithospheric mantle even in whole mantle evolution history. Compared with published data for ages relative to mantle,the ages of Py,Py1s and Py2s may have estimably>3.8Ga,2.5Ga/1.4~1.3Ga,and 0.9~0.7Ga respectively. The age of another carbonated peridotite xenolith with sheared/deformation texture sampled from Fuxian kimberlite may be as same as Py2.