Homogeneous fractionation, involving crystal nucleation, growth, and separation from a melt-dominant magma, and boundary-layer fractionation, involving the separation of a fractionated interstitial melt from a crystal-dominant boundary layer and its mixing with the main magma body, have been advocated as two major differentiation mechanisms in a crustal magma chamber. In this study, we focus on one of the dolerite sills in the Aosawa area, Yamagata Prefecture, Japan, to elucidate the roles of the two mechanisms in the differentiation of a sheet-like magma body. The intrusion is concordantly intruded into black mudstone and is ∼ 100 m thick and more than 5 km in lateral extension. The chilled margins contain olivine (5.3 vol%) and plagioclase (1.9 vol%) as phenocrysts. There is an absence of clinopyroxene in the chilled margins, and there are systematic sill-scale variations in the whole-rock major- and trace-element contents that require the addition or removal of clinopyroxene. These findings suggest sill-scale differentiation, involving the transportation of crystals that nucleated and grew in the sill, and/or the transfer of the residual melt. The downward increases in mode and size of clinopyroxene and in the size of the most dominant plagioclase, all with maxima near the bottom, and the occurrence of pigeonite rimming augite only near the bottom, suggest a slower cooling rate in the lower boundary layer compared to the upper one. Clinopyroxene crystals with Cr-rich cores that show textures suggesting rapid growth, such as remarkable sector zoning, melt inclusions, and small euhedral plagioclase, are more abundant at 5-15 m above the lower contact. These observations lead us to believe that clinopyroxene nucleated and grew near the upper boundary layer, where large supersaturation occurred, and then settled to concentrate in the lower boundary layer, where slower cooling provided suitable conditions for pigeonite crystallization. The upward increase in the incompatible elements and the decrease in compatible elements from the zone containing a high concentration of Cr-rich clinopyroxene suggest that a fractionated melt, which was formed through enhanced overgrowth on settled clinopyroxene, was transported upwards. The sill-scale magmatic differentiation in the Aosawa dolerite intrusion took place through crystal settling from the roof boundary layer, followed by the upward transportation of the fractionated melt from the bottom boundary layer. The lower-boundary-layer fractionation was probably very effective because the settling of the crystals thickened the bottom boundary layer to facilitate an effective melt-crystal separation.
The presence of itacolumites, or flexible quartzites, and the precise geological descriptions of the same have hitherto been reported only in India, Brazil, and the Appalachian region of the USA. In this paper, we report the discovery of itacolumites from three localities in the central highlands of Madagascar. The localities are underlain by the Mesoproterozoic Itremo Group. The occurrence and petrography of these itacolumites and the results of SEM observations are summarized in this paper. The itacolumites from Madagascar contain very coarse grained one beside the fine-to-medium-grained varieties that are common to India, Brazil, and the Appalachians. Furthermore, the itacolumites of Madagascar occur over a wide area of exposure, supporting the opinion that their formation was a result of chemical weathering.
Biotite- and amphibole-bearing monzogranite, quartz monzonite, and tonalite intrude into the biotite granitoids and biotite-hornblende gneisses of the Pan-African belt in the Nkambe area, northwestern Cameroon. The common mineral assemblage in these rocks is amphibole + biotite + plagioclase + K-feldspar + quartz + zircon + apatite; however, their modal composition varies in each rock-type. Most amphibole grains are relatively homogeneous and locally have slightly Al-rich rims. They are mostly ferro-edenite or ferropargasite, and in a tonalite sample, there is magnesiohornblende with less aluminous and more magnesian composition. The amphibole + biotite + plagioclase + K-feldspar + quartz assemblage was affected by several pressure-sensitive and H2O-free reactions. The ferro-edenite- and/or ferropargasite-bearing samples show similar pressure-temperature (P-T) ranges of 0.46-0.50 GPa/655-690 °C and 0.43-0.69 GPa/675-705 °C. In contrast, the magnesiohornblende-bearing tonalite records lower temperature conditions of 0.77 GPa/610 °C, which is attributed to subsolidus reequilibration during cooling. The P-T estimates for the biotite-amphibole granitoids in this and previous studies suggest that relatively high-pressure granitoids are widespread in the Nkambe area.
We report the occurrence of preiswerkite and högbomite as inclusion phases within the garnets of eclogite from the Aktyuz area of Northern Tien Shan, Kyrgyzstan. Preiswerkite and högbomite occur both as a constituent of multiphase solid inclusions (MSI) and as single discrete grains in the mantle and rim of the garnets. However, they do not occur in the core of the garnet and in the matrix of the eclogite. Preiswerkite is associated with the minerals paragonite ± staurolite ± Mg-taramite ± Na-biotite ± hematite ± högbomite ± chlorite ± titanite ± phengite ± magnetite, and högbomite is associated with paragonite ± preiswerkite ± staurolite ± hematite ± chlorite ± Na-biotite ± magnetite in MSI. The average compositions of preiswerkite and högbomite are (Na0.96K0.02Ca0.01)0.99(Mg1.52Fe2+0.54VIAl0.93)2.99(IVAl1.93Si2.07)4.00O10(OH)2 and (Mg1.47Fe2+3.02Zn0.04Fe3+1.45)5.98(Fe3+0.31Al15.13Ti0.56)16O30(OH)2, respectively. Na-biotite, with an average composition of (Na0.89K0.07Ca0.01)0.97(Mg1.66Fe2+0.69VIAl0.63)2.98(IVAl1.57Si2.43)4.00O10(OH)2, corresponding to the intermediate composition between preiswerkite and aspidolite (i.e., Na-phlogopite), is also observed. The compositions of the newly found preiswerkite and Na-biotite with similar XMg values (0.66-0.78) are arrayed along preiswerkite-aspidolite solid solution series. The mode of occurrence of inclusion phases in garnets may suggest that the activity of Na-Al-rich and Si-undersaturated aqueous fluids played a major role in the formation of preiswerkite during the prograde stage of high-pressure eclogitic metamorphism.
The Heilongjiang Complex is mainly composed of high-P/T type metamorphic rocks of blueschists and pelitic schists, and is distributed along the western margin of the Jiamusi Massif in NE China. LA-ICP-MS U-Pb ages of detrital zircons in pelitic schist (LG1-3) from the Mudanjiang district give a weighted mean age of 250.6 ± 2.3 Ma (MSWD = 2.2). Pelitic schist from the Yilan district (09YL10-1) contains 193-348 Ma and 392-561 Ma detrital zircons, with minor amounts of 783-987 Ma zircons. The weighted mean ages of the youngest detrital zircon age group are 230.6 ± 3.5 Ma (MSWD = 1.2; LG1-3) and 199.1 ± 3.1 Ma (MSWD = 1.0; 09YL10-1), and these constrain the maximum depositional age of the protoliths of the Heilongjiang pelitic schists. Phengites from pelitic schists in the Yilan (422YQ-3) and Luobei districts (408HB-1) yielded 40Ar/39Ar plateau ages of 179.9 ± 0.8 Ma and 164.7 ± 0.2 Ma, respectively. A 40Ar/39Ar phengite age of 189.8 ± 0.8 Ma as total gas age (apparent age distribution from 183 Ma to 196 Ma) was also obtained for Yilan garnet-barroisite schist (423YJ-1). Reliable geochronological data suggest that a paleo-ocean located between the Jiamusi Massif and the Songnen Massif to the west was still present at least up to 199-231 Ma, and subduction-related high-P/T type metamorphism occurred during the Jurassic at 145-184 Ma.
Pelitic schists adjacent to the Sebadani metagabbro mass in the Sambagawa metamorphic belt of central Shikoku, Japan consist mainly of garnet, phengite, albite and quartz, and small or scarce amounts of Na-Ca and Ca-amphiboles, epidote, omphacite, kyanite, rutile and carbonaceous matter. Garnets are zoned from pale reddish brown cores to pale yellowish-green or colorless rims. The cores of the garnets are Mg- and Ca-rich (MgO 3.50-11.30 wt%; CaO 7.27-12.98 wt%), and are overgrown by Fe-rich inner and outer rims. Kyanite occurs as inclusions in the high-Mg cores of the garnets. Inclusion assemblages of the high-Mg core of the garnets indicate relatively high-T conditions such as the kyanite eclogite facies (821 ± 32 °C and 19.4 ± 1.6 kbar). The cores of the garnets are anhedral with irregular embayments, and the outlines of the inner rims also show similar texture. The cores are also intensely fractured and filled by Fe-rich garnet with the same composition as the inner rims. The texture and chemical compositions of the high-Mg cores in the garnets are similar to those of garnets in the eclogites in the Sebadani metagabbro mass. The possible origin of the high-Mg cores are (i) in situ high-T regional metamorphism, (ii) detrital origin, and (iii) mechanical mixing of eclogites in the Sebadani metagabbro and surrounding pelitic schists. It is probable that high-Mg garnets were stripped from the Sebadani metagabbro mass and mechanically introduced into the pelitic schists when the Sebadani metagabbro mass was incorporated into the pelitic schists.