Andesine megacrysts in alkali basalts from Japan

Andesine megacrysts up to 3 cm in size occur sporadically in certain alkali basalts and allied mafic rocks in southwestern Japan. They are sometimes accompanied by megacrysts of mafic minerals and ultramafic and mafic inclusions. Nine andesines have been chemically analysed.From the petrography and chemistry and the results of high pressure experimental work, it is suggested that andesine megacrysts crystallized from alkali basalt magma under dry conditions at a depth of about 30 to 60 km.     

‘Thailand was a desert' during the mid‐Cretaceous: Equatorward shift of the subtropical high‐pressure belt indicated by eolian deposits (Phu Thok Formation) in the Khorat Basin,northeastern Thailand

The Tibetan Plateau is a key factor in controlling the present‐day climate and atmospheric circulation pattern in Asia. The pattern of atmospheric circulation after the uplift of the plateau is well known, whereas direct evidence is lacking regarding the nature of the circulation pattern prior to the uplift. The distribution of desert directly reflects the position of the subtropical high‐pressure belt, and the prevailing surface‐wind pattern recorded in desert deposits reveals the position of its divergence axis. Cretaceous eolian sandstone of the Phu Thok Formation is extensively exposed in the northern Khorat Basin, northeastern Thailand. We conducted a sedimentological study on this formation to reconstruct temporal changes in the latitude of the subtropical high‐pressure belt in low‐latitude Asia during the Cretaceous. Spatio‐temporal changes in the paleo‐wind directions recorded in the Phu Thok Formation reveal that the Khorat Basin mainly belonged to the northeast trade wind belt and subtropical high‐pressure belt was situated to the north of the Khorat Basin during the initial stages of deposition, shifted southward to immediately above the basin during the main phase of deposition, and then shifted northward again to the north of the basin during the final stages of deposition. The paleomagnetic polarity sequence obtained for the Phu Thok Formation comprises three zones of normal polarity and two of reversed polarity, correlating to chrons M1n to C34n of the geomagnetic polarity time scale. This result suggests that the Phu Thok Formation is mid‐Cretaceous in age (from c. 126 Ma to c. 99–93 Ma), similar to the age of eolian sandstone in the Sichuan Basin, southern China (the Jiaguan Formation). These results, in combination with paleo‐wind direction data, suggest the development of low‐latitude desert and an equatorward shift of the subtropical high‐pressure belt (relative to the present‐day) in Asia during the mid‐Cretaceous.     

Opening and closing process of the Paleo-Tethys in Northern Thailand

The Paleo-Tethys formed a large ocean basin that existed between Laurasia and Gondwana during Late Paleozoic to Early Mesozoic times. It opened in the Early Devonian by the rifting of Gondwanaland and closed at around latest Triassic time by the collision of the Cimmerian continent to Laurasia (Metcalfe, 1999). We reconstructed opening and closing process of the Paleo-Tethys in Northern Thailand.     

泰国北部Inthanon带外来硅质岩块的中、晚二叠世放射虫：对与古特提斯消减有关的混杂岩形成时代的约束

The Inthanon Zone of Northern Thailand, origi-nally proposed by Barr and Macdonald (1991), is characterized by the occurrence of Paleo-Tethyan pe-lagic sediments including Carboniferous–Permian seamount-type carbonate associated with oceanic ba-saltic rocks and Middle Devonian–Middle Triassic radiolarian chert (Ueno, 1999; Ueno and Hisada, 2001; Ueno and Charoentitirat, 2011). These pelagic rocks have been mainly studied from the viewpoint of bio-stratigraphy to clarify the duration of their deposition. These studies concluded the Paleo-Tethys to be a vast ocean basin once existed between the Indochina and Sibumasu continental blocks during Devo-nian–Triassic times.     

Petrology of the Yugu peridotites in the Gyeonggi Massif,South Korea: Implications for its origin and hydration process

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 Fo₉₁ 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 Na₂O 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.     

Early Cretaceous paleogeography of Korea and Southwest Japan inferred from occurrence of detrital chromian spinels

The Sindong Group was deposited in the north–south trending half‐graben Nakdong Trough, southern Korean peninsula. The occurrence of detrital chromian spinels from the Jinju Formation of the Sindong Group in the Gyeongsang Basin means that the mafic to ultramafic rocks were exposed in its provenance. The chromian spinels from the Jinju Formation are characterized by extremely low TiO₂ and Fe³⁺. Moreover, their range of Cr# is from 0.45 to 0.80 and makes a single trend with Mg#. The chemistry of chromian spinels implies that the source rocks for chromian spinels were peridotites or serpentinites, which originated in the mantle wedge. To more narrowly constrain their source rocks, the Ulsan and Andong serpentinites exposed in the Gyeongsang Basin were examined petrographically. Chromian spinels in the Andong serpentinite differ from those of the Jinju Formation and those in the Ulsan serpentinite partly resemble them. Furthermore, the Jinju chromian spinel suite is similar to the detrital chromian spinels from the Mesozoic sediments in the Circum‐Hida Tectonic zone, which includes the Nagato Tectonic zone in Southwest Japan and the Joetsu Belt in Northeast Japan. This suggests that the basement rocks, which were located along the main fault bounding the eastern edge of the Nakdong Trough, had exposures of peridotite or serpentinite. It is possible that the Nakdong Trough was directly adjacent to the Circum‐Hida Tectonic zone before the opening of the Sea of Japan (East Sea).     

Mineralogy,textures, and whole-rock geochemistry of advanced argillic alteration: Hugo Dummett porphyry Cu–Au deposit,Oyu Tolgoi mineral district,Mongolia

Advanced argillic (AA) alteration is developed over a vertical interval of 500 m, above (and enclosing) Late Devonian quartz monzodiorite intrusions that accompany porphyry-style Cu–Au mineralization at the Hugo Dummett deposit. The AA alteration is mainly in basaltic rocks and locally extends into the overlying dacitic ash-flow tuff for about 100 m. The AA zone overprints porphyry-style quartz veins associated with quartz monzodiorite intrusions, but at least partly precedes high-grade porphyry-style bornite mineralization. Mineralogically, it consists of andalusite, corundum, residual quartz, titanium oxides, diaspore, alunite, aluminum phosphate-sulfate (APS) minerals, zunyite, pyrophyllite, topaz, kaolinite, and dickite, as well as anhydrite and gypsum, but is dominated by residual quartz and pyrophyllite. Alteration zonation is not apparent, except for an alunite-bearing zone that occurs approximately at the limit of strong quartz veining. Whole-rock geochemistry shows that the AA alteration removes most major elements except Si, Al, Ti, and P, and removes the trace elements Sc, Cs, and Rb. V, Zr, Hf, Nb, Ta, U, and Th are relatively immobile, whilst light REEs (La to Nd), Sr, Ba, and Ga can be enriched. Middle REEs (Sm to Gd) are moderately depleted; Y and heavy REEs (Tb to Lu) are strongly depleted except in two unusual samples where middle to heavy REEs are enriched.     

Early Permian ammonoids from the Kaeng Krachan Group of the Phatthalung-Hat Yai area, southern peninsular Thailand

An Early Permian small ammonoid fauna consisting of Neocrimites sp., Agathiceras suessi Gemmellaro, A. girtyi Böse, Agathiceras? sp., and Miklukhoceras sp. was found in nodules of a fine sandstone bed exposed in the Phatthalung-Hat Yai area of southern peninsular Thailand. The ammonoid-bearing bed belongs stratigraphically to the uppermost part of the Kaeng Krachan Group, which is essentially a clastic-dominant, Late Carboniferous (?) to Early Permian stratigraphic unit, widely distributed in western and peninsular Thailand. This ammonoid fauna is considered to be of Bolorian (Kungurian) age and includes Agathiceras girtyi Böse, which is described for the first time from Thailand. The present discovery of Bolorian ammonoids suggests that the uppermost part of the Kaeng Krachan Group is slightly younger than previously considered and around the latest Early Permian. This further implies that the continental margin environment of the Sibumasu Block drastically changed at around Bolorian time from a cool, clastic-dominant shelf condition to a temperate to subtropical, carbonate platform due to rapid northward drift after middle Artinskian rifting.