Metasomatic origin of garnet xenocrysts from the V.Grib kimberlite pipe,Arkhangelsk region,NW Russia  

作　　者：E.V.Shchukina A.M.Agashev N.P.Pokhilenko 

机构地区：[1]Sobolev Institute of Geology and Mineralogy,Siberian Branch Russian Academy of Sciences

出　　处：《Geoscience Frontiers》2017年第4期641-651,共11页地学前缘（英文版）

基　　金：supported by the FBRF(Grants No.15-05-07758);by state assignment project No.Ⅷ.72.1.1

摘　　要：This paper presents new major and trace element data from 150 garnet xenocrysts from the V.Grib kimberlite pipe located in the central part of the Arkhangelsk diamondiferous province（ADP）.Based on the concentrations of Cr_2O_3,CaO,TiO_2 and rare earth elements（REE） the garnets were divided into seven groups：（1） lherzolitic ＂depleted＂ garnets（ ＂Lz 1＂）,（2） lherzolitic garnets with normal REE patterns（ ＂Lz2＂）,（3） lherzolitic garnets with weakly sinusoidal REE patterns（ ＂Lz 3＂）,（4） lherzolitic garnets with strongly sinusoidal REE patterns（ ＂Lz 4＂）,（5） harzburgitic garnets with sinusoidal REE patterns（ ＂Hz＂）,（6）wehrlitic garnets with weakly sinusoidal REE patterns（ ＂W＂）,（7） garnets of megacryst paragenesis with normal REE patterns（ ＂Meg＂）.Detailed mineralogical and geochemical garnet studies and modeling results suggest several stages of mantle metasomatism influenced by carbonatite and silicate melts.Carbonatitic metasomatism at the first stage resulted in refertilization of the lithospheric mantle,which is evidenced by a nearly vertical CaO-Cr_2O_3 trend from harzburgitic（ ＂Hz＂） to lherzolitic（ ＂Lz 4＂） garnet composition.Harzburgitic garnets（ ＂Hz＂） have probably been formed by interactions between carbonatite melts and exsolved garnets in high-degree melt extraction residues.At the second stage of metasomatism,garnets with weakly sinusoidal REE patterns（ ＂Lz 3＂, ＂W＂） were affected by a silicate melt possessing a REE composition similar to that of ADP alkaline mica-poor picrites.At the last stage,the garnets interacted with basaltic melts,which resulted in the decrease CaO-Cr_2O_3 trend of ＂Lz 2＂ garnet composition.Cr-poor garnets of megacryst paragenesis（ ＂Meg＂） could crystallize directly from the silicate melt which has a REE composition close to that of ADP alkaline mica-poor picrites.P-T estimates of the garnet xenocrysts indicate that the interval of-60-110 km of the lThis paper presents new major and trace element data from 150 garnet xenocrysts from the V.Grib kimberlite pipe located in the central part of the Arkhangelsk diamondiferous province（ADP）.Based on the concentrations of Cr_2O_3,CaO,TiO_2 and rare earth elements（REE） the garnets were divided into seven groups：（1） lherzolitic ＂depleted＂ garnets（ ＂Lz 1＂）,（2） lherzolitic garnets with normal REE patterns（ ＂Lz2＂）,（3） lherzolitic garnets with weakly sinusoidal REE patterns（ ＂Lz 3＂）,（4） lherzolitic garnets with strongly sinusoidal REE patterns（ ＂Lz 4＂）,（5） harzburgitic garnets with sinusoidal REE patterns（ ＂Hz＂）,（6）wehrlitic garnets with weakly sinusoidal REE patterns（ ＂W＂）,（7） garnets of megacryst paragenesis with normal REE patterns（ ＂Meg＂）.Detailed mineralogical and geochemical garnet studies and modeling results suggest several stages of mantle metasomatism influenced by carbonatite and silicate melts.Carbonatitic metasomatism at the first stage resulted in refertilization of the lithospheric mantle,which is evidenced by a nearly vertical CaO-Cr_2O_3 trend from harzburgitic（ ＂Hz＂） to lherzolitic（ ＂Lz 4＂） garnet composition.Harzburgitic garnets（ ＂Hz＂） have probably been formed by interactions between carbonatite melts and exsolved garnets in high-degree melt extraction residues.At the second stage of metasomatism,garnets with weakly sinusoidal REE patterns（ ＂Lz 3＂, ＂W＂） were affected by a silicate melt possessing a REE composition similar to that of ADP alkaline mica-poor picrites.At the last stage,the garnets interacted with basaltic melts,which resulted in the decrease CaO-Cr_2O_3 trend of ＂Lz 2＂ garnet composition.Cr-poor garnets of megacryst paragenesis（ ＂Meg＂） could crystallize directly from the silicate melt which has a REE composition close to that of ADP alkaline mica-poor picrites.P-T estimates of the garnet xenocrysts indicate that the interval of-60-110 km of the l

关 键 词：Lithospheric mantle Mantle xenolith Lherzolite-harzburgite-wehrlite-megacryst paragenesis Trace element Major element Kimberlite 

分 类 号：P542.5[天文地球—构造地质学] P587[天文地球—地质学]