Phase analysis in the model K2O‐poor aluminous rock system (FMASH) illustrates the following sequence of reactions during retrograde metamorphism in the Botswanan Limpopo Central Zone: Opx+Sil+Qtz=Crd , Opx+Sil=Spr+Crd , Grt+Qtz=Opx+Crd ; Opx+Crd+W=Ged+Qtz , Grt+Opx+Crd+W=Ged ; and Grt+Qtz+W=Ged+Crd . A quantitative petrogenetic grid with phase relations shows that sapphirine results from nearly isothermal decompression in the quartz‐undersaturated portions of the grid, and that gedrite formation by reactions (4)–(6) records isobaric cooling from high temperature (c. 800° C) after the decompression. Conditions for hydration in the western part of the area were 700–800° C and c. 6 kbar, based on microthermometric data and the available garnet–cordierite geothermometer. On the basis of these conditions and predicted thermodynamic properties of gedrite, phase relations in T–XMg space were constructed to investigate the isobaric cooling event. The results are in good agreement with the hydration P–T  path. Further, the T–XMg topologies show that hydration of orthopyroxene in the central part of the area (reaction 4) occurred at about 800° C and c. 5 kbar. Therefore, we conclude that the Botswanan Limpopo Central Zone has suffered isothermal decompression, similar to the Central Zone in South Africa and Zimbabwe, followed by isobaric cooling. The isobaric cooling event in the western (at c. 6 kbar) and central (at c. 5 kbar) parts of the area commenced at nearly the same temperature (c. 800° C), and appear to be consistent with a tectonic model that involved westward movement (thrusting) of the Central Zone.

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Metapelites, migmatites and granites from the c. 2 Ga Mahalapye Complex have been studied for determining the P-T-fluid influence on mineral assemblages and local equilibrium compositions in the rocks from the extreme southwestern part of the Central Zone of the Limpopo high-grade terrane in Botswana. It was found that fluid infiltration played a leading role in the formation of the rocks. This conclusion is based on both well-developed textures inferred to record metasomatic reactions, such as Bt Þ And + Qtz + (K 2 O) and Bt ± Qtz Þ Sil + Kfs + Ms ± Pl, and zonation of Ms | Bt + Qtz | And + Qtz and Grt | Crd | Pl | Kfs + Qtz reflecting a perfect mobility (Korzhinskii terminology) of some chemical components. The conclusion is also supported by the results of a fluid inclusion study. CO 2 and H 2 O (a fl H2O ¼ 0.6) are the major components of the fluid. The fluid has been trapped synchronously along the retrograde P-T path. The P-T path was derived using mineral thermobarometry and a combination of mineral thermometry and fluid inclusion density data. The Mahalapye Complex experienced low-pressure granulite facies metamorphism with a retrograde evolution from 770°C and 5.5 kbar to 560°C and 2 kbar, presumably at c. 2 Ga.

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Peridotites exposed in the Yugu area in the Gyeonggi Massif, South Korea, near the boundary with the Okcheon Belt, exhibit mylonitic to strongly porphyroclastic textures, and are mostly spinel lherzolites. Subordinate dunites, harzburgites, and websterites are associated with the lherzolites. Amphiboles, often zoned from hornblende in the core to tremolite in the rim, are found only as neoblasts. Porphyroclasts have recorded equilibrium temperatures of about 1000°C, whereas neoblasts denote lower temperatures, about 800°C. Olivines are Fo 90-91 in lherzolites and Fo 91 in a dunite and a harzburgite. The Cr# (= Cr/(Cr + Al) atomic ratio) of spinels varies together with the Fo of olivines, being from 0.1 to 0.3 in lherzolites and around 0.5 in the dunite and harzburgite. The Na 2 O content of clinopyroxene porphyroclasts is relatively low, around 0.3 to 0.5 wt% in the most fertile lherzolite. The Yugu peridotites are similar in porphyroclast mineral chemistry not to continental spinel peridotites but to sub-arc or abyssal peridotites. Textural and mineralogical characteristics indicate the successive cooling with hydration from the upper mantle to crustal conditions for the Yugu peridotites. Almost all clinopyroxenes and amphiboles show the same U-shaped rare earth element (REE) patterns although the level is up to ten times higher for the latter. The hydration was associated with enrichment in light REE, resulting from either a slab-derived fluid or a fluid circulating in the crust. The mantle-wedge or abyssal peridotites were emplaced into the continental crust as the Yugu peridotite body during collision of continents to form a high-pressure metamorphic belt in the Gyeonggi Massif. The peridotites from the Gyeonggi Massif exhibit lower-pressure equilibration than peridotites, with or without garnets, from the Dabie-Sulu Collision Belt, China, which is possibly a westward extension of the Gyeonggi Massif.