1975/01/01 by J-C. C. MERCIER, J-C.C. Mercier, A. NICOLAS +1 · 6 citations
Earth and Planetary Sciences · #Geological and Geochemical Analysis #Geology and Paleoclimatology Research #Paleontology and Stratigraphy of Fossils
paper · doi:10.1093/petrology/16.2.454
A structural study of lherzolite xenoliths mainly from Western Europe and Hawaii leads to a classification of three representative groups with every transitional stage. Their origin in the upper mantle and their kinematic meaning can be best understood by referring to the Alpine-type peridotite massifs in which the large scale structures can also be observed. The first structural group, called protogranular, has the relatively oldest texture recorded in xenoliths and is present in about 20 per cent of them. It may have developed first in a harzburgitic paragenesis with high Al-Ca enstatite or alternatively in a high-Al pigeonitebearing peridotite. It is mainly due to a recrystallization process which is regarded as occurring during partial melting. By plastic flow, it grades into the second textural group, called porphyroclastic (55 percent of the xenoliths studied here). This new group has the same textures that characterize the Alpine-type peridotite massifs where it has also been observed superimposed on the protogranular type. Compared with massifs, the porphyroclastic texture in xenoliths shows less evidence of intracrystalline gliding and more syn- and post-tectonic recrystallization possibly due to a higher-temperature deformation. The plastic flow mainly operates in a shear regime. The third, or equigranular texture (20 percent of the investigated xenoliths develops by complete recrystallization of a porphyroclastic texture, probably after an intensive flowage. It compares with the textures of crustal granulites, except for a small grain size. This third group has been divided into two subgroups, one with an equant mosaic texture and a poor fabric, the other with a tabular texture and a good fabric. The specific conditions of recrystallization leading to each of these subtypes are not completely understood and several distinct processes may even have produced the same texture. The remaining 5 percent of the lherzolite xenoliths mostly belongs to a poikiloblastic textural group not studied here. The spinel-silicates relation is shown to be a geological marker which allows the textural types to be defined with respect to a given deformation-recrystallization cycle.