Peridotite xenoliths in alkali basalt of about 11.0 Ma in age from Boun, which erupted in the Ogcheon belt, a tectonic belt between the Sino-Korean and Yangtze cratons, are examined to understand the petrological nature of the upper mantle beneath the Korean Peninsula. The xenolithic suite is almost composed of lherzolite containing spinel of low Cr# [=Cr/(Cr+Al) atomic ratio] around 0.1, and clinopyroxene with relatively high Na2O content (about 1 to 2 wt%). Predominance of the fertile lherzolite within the mantle may indicate that appreciable amount of melts did not pass through it and not leave dunite and/or pyroxenites with aureole of restite with higher degree of melting. The lherzolite from Boun has mineralogical characteristics of continental rift-zone mantle peridotite and is distinctly different from sub-arc and abyssal mantle peridotites. It is noteworthy that the peridotite xenoliths with arc or abyssal mantle signatures apparently have not been found from the eastern margin of the Asian continent, including the Korean Peninsula, despite possible arc settings experienced through geologic time. It is highly speculative but is possible that the arc-type mantle material had not been accreted upon arc crust accretion. Alternatively the arc type mantle which had once been present beneath the continental margin was replaced later by the continental rift-zone type mantle.
In the Su-Lu UHP terrane, eastern China, coesite-bearing eclogites have been reported in five areas (Weihai, Yangkou, Rizhao, Donghai and Rongcheng). The presence of a coesite inclusion (70 μm) in garnet in a strongly retrograded eclogite at Zekou (about 40 km southwest of Rongcheng) shows for the first time that this region also underwent coesite-eclogite facies metamorphism. In addition, the garnet of the Zekou eclogite preserves a distinct compositional zonation. The eclogite consists of garnet (20%), symplectite (70%: amphibole+plagioclase) and other accessory minerals (amphibole, rutile, quartz and ilmenite). Garnet porphyroclasts (0.5-1 mm in diameter) show compositional zonation that can be divided into three parts: core (Alm58-59 Sps0.7-0.9 Prp15-17 Adr0.1-0.2 Grs23-24), mantle (Alm55-58 Sps0.5-0.8 Prp14-15 Adr0.1-0.2 Grs25-28), and rim (Alm53-56 Sps0.4-0.6 Prp17-18 Adr0.1-0.2 Grs25-27). Each part has a characteristic inclusion mineralogy: the core part contains rutile, quartz and no coesite, the mantle part contains coesite, quartz-pseudomorphs after coesite, rutile, omphacite and zircon, and the rim part contains rutile. The coesite inclusion (70 μm) is located in the outer mantle part and is associated with radial fractures in the surrounding garnet. The different inclusion assemblages and the chemical zonation from core to rim in garnet porphyroclasts suggest that the Zekou eclogite preserves the information of the prograde stage during UHP metamorphism.
Localized deposit of Fe-rich pyrophyllite was encountered in the kaolinite-dominated alteration halo in Solo, Mabini, Philippines. The environment of formation of this dioctahedral mineral is related to low-temperature advanced argillic alteration found in shallow volcanic region. The pyrophyllite crystallizes as a two-layer monoclinic (2M) structure with the following unit cells, a=5.16Å, b=8.95Å, c=18.70Å, β=99.87° while its dehydroxylate form yields a slightly different unit cell parameters with a=5.17Å, b=9.02Å, c=18.76Å, β=99.80°. The expansion of b on dehydroxylation arises largely from the rearrangement of the Al ions in the octahedral layer and the insertions of oxygen ions in the Al planes. The obtained tetrahedral rotation (twisting of SiO4 groups) of 11°52' in pyrophyllite is closely comparable to the 9°40' angle of twist in the dehydroxylate. This strongly suggests that the tetrahedral networks of the original pyrophyllite are almost similarly twisted in the dehydroxylate phase. X-ray powder data demonstrate that the dehydroxylation process is accompanied by a significant enlargement of basal spacing with an observed expansion of about 1.99%. Thermal decomposition of pyrophyllite shows a single dehydration event, which takes place over a temperature range of 500-700°C with its peak at 604°C. The occurrence of this low-temperature dehydroxylation can be regarded as a consequence of significant replacement of Al by Fe in the octahedral layer and the presence of some impurities. Chemical data of pyrophyllite obtained from EDX analysis give an average structural formula of Ca0.01K0.10(Al1.81Fe3+0.16Ti0.01)(Si3.88Al0.12)O10(OH)2 with an average Si/Al3++Fe3+ ratio of 1.86. Analyses of representative samples indicate that the octahedral site is filled with 1.76 to 1.84 Al cations, which is relatively lower than the 1.90 to 1.95 Al cations exhibited by ordinary pyrophyllite. This significant deficiency of Al is believed to be compensated by considerably high Fe3+ occupancy in the octahedral site. The source of Fe can be correlated to the previous decomposition of minor amount of pyrite associated with argillic alteration. SEM observations show common disoriented stacks of pyrophyllite plates, but the most visible component of the samples are the loosely compacted stacks in which thin elongated blade-like particles are randomly dispersed in the form of relatively porous aggregates. The coexistence of pyrophyllite and kaolinite in this environment is a consequence of sluggish crystallization of quartz caused by local variation of silica activity, which explains the crystallization of pyrophyllite instead of stable kaolinite at low temperature (<300°C).
Natural-silica-rich glasses (impactites, tektites and obsidians) have been investigated with infrared (IR), Raman and optical spectroscopy. Comparison with artificial glasses with silica-rich and alkali-rich compositions is made. The vibrational data of these compounds are discussed in relationship with their structures, particularly with respect to (i) Si-O-Si bonding differences, (ii) SiO4-ring arrangements, (iii) lattice disorder. IR spectra are strongly dependent on silica content: frequencies of the ν3 and νD bands increase with the silica content. A general finding in the Raman spectra of tektites, is the relationship between silica, alumina, sodium contents and the presence of vibrational bands peaked at very specific energies. Raman spectra of Lybian desert glass (LDG), Darwin glass (DG) and vitreous silica are almost identical with the typical doublet at 440-490 cm−1 whereas in tektites the band at 440 cm−1 has relatively less pronounced doublet structure. A common character of Raman spectra of tektites and obsidians is the appearance of broad bands centered around 1000 and 1600 cm−1 due to substitutions of silicon by metals. Tektites have a strong absorption band at 1100 nm which originates from Fe2+ ions. In the other glasses, this absorption is slightly shifted towards 1110-1130 nm. Additional sharp features of impactites and obsidians at 1380, 2210-2250 nm are completely absent in the absorption spectra of tektites. These bands are the signature of molecular water trapped inside the structure.