Monazite geochronology and geochemistry of meta-sediments in the Narryer Gneiss Complex, Western Australia: constraints on the tectonothermal history and provenance     

Tsuyoshi Iizuka  Malcolm T. McCulloch  Tsuyoshi Komiya  Takazo Shibuya  Kenji Ohta  Haruka Ozawa  Emiko Sugimura  Kenneth D. Collerson 《Contributions to Mineralogy and Petrology》2010,160(6):803-823

Mt. Narryer and Jack Hills meta-sedimentary rocks in the Narryer Gneiss Complex of the Yilgarn Craton, Western Australia are of particular importance because they yield Hadean detrital zircons. To better understand the tectonothermal history and provenance of these ancient sediments, we have integrated backscattered scanning electron images, in situ U–Pb isotopic and geochemical data for monazites from the meta-sediments. The data indicate multiple periods of metamorphic monazite growth in the Mt. Narryer meta-sediments during tectonothermal events, including metamorphism at ~3.3–3.2 and 2.7–2.6 Ga. These results set a new minimum age of 3.2 Ga for deposition of the Mt. Narryer sediments, previously constrained between 3.28 and ~2.7 Ga. Despite the significant metamorphic monazite growth, a relatively high proportion of detrital monazite survives in a Fe- and Mn-rich sample. This is likely because the high Fe and Mn bulk composition resulted in the efficient shielding of early formed monazite by garnet. In the Jack Hills meta-sediments, metamorphic monazite growth was minor, suggesting the absence of high-grade metamorphism in the sequence. The detrital monazites provide evidence for the derivation of Mt. Narryer sediments from ca. 3.6 and 3.3 Ga granites, likely corresponding to Meeberrie and Dugel granitic gneisses in the Narryer Gneiss Complex. No monazites older than 3.65 Ga have been identified, implying either that the source rocks of >3.65 Ga detrital zircons in the sediments contained little monazite, or that >3.65 Ga detrital minerals had experienced significant metamorphic events or prolonged sedimentary recycling, resulting in the complete dissolution or recrystallization of monazite.  相似文献   

The Weathering of Granodiorite Porphyry in the Thiel Mountains, Inland Antarctica   总被引：2，自引：0，他引：2  

Satoshi Ishimaru  & Kenji Yoshikawa 《Geografiska Annaler: Series A, Physical Geography》2000,82(1):45-57

The weathering of granodiorite porphyry is examined in the extremely cold and dry environment of the inland part of Antarctica. Weathering features include granular disintegration, rock varnish, and sheeting. Sheeting has gradually proceeded since the exposing of the nunatak, but the other types of weathering have not actively advanced in recent times.
Granular disintegration primarily comprises the release of individual crystals. The mechanism of this release is that cracks are created along crystal boundaries and cleavages in phenocrysts mainly due to differential thermal expansion. Plagioclase and hypersthene are released more readily than quartz. Disintegration of plagioclase has produced many prism-shaped holes, 0.5 to 2.0 mm in length, while fine-grained quartz crystals, 0.05 to 0.1 mm in diameter, are incidentally released without cracking.
The reddish brown (10R4/4) rock varnishes result from oxidation and consist of limonite, which fills cracks and penetrates into crystals. Ferrous iron in hypersthene, biotite, ilmenite and magnetite is transformed into limonite by oxidation. Manganese is not found in the varnishes. Sulfur, which is important for oxidation and which may have originated from adhered snow, is concentrated on the surface of the rock.
Sheeting has precipitated rock falls, and has gradually formed a gray mosaic on the varnished wall. The sheeting was caused by gravitational body force, which is internal stress of the rock body due to the mass of the overburden.  相似文献   

A Late Holocene geomagnetic secular variation record from Erhai Lake, southwest China   总被引：1，自引：0，他引：1  

Masayuki Hyodo  Arata Yoshihara  Kenji Kashiwaya  Takashi Okimura  Toshiyuki Masuzawa  Ryotaro Nomura  Shingo Tanaka  Tang Bang Xing  Liu Su Qing  & Liu Shi Jian 《Geophysical Journal International》1999,136(3):784-790

A secular variation record of the geomagnetic field direction for the last 6.5  kyr has been obtained from the magnetization of sediment cores from Erhai Lake, southwest China. In order to make a comparison with this record, secular variation in east-central China was investigated by combining available magnetic field data from historical records and archaeomagnetic measurements since about 350 bc . The secular variation in Erhai Lake shows features consistent with the combined record, except for the oldest three observed declination swings in Sian from 720 to 900 ad . Many features of declination and inclination in China also occur in Japan. From 500 to 1000 ad , declination was westerly ranging from about −20° to −5° in Erhai Lake, east-central China, and Japan.  相似文献   

Timescale of magma chamber processes revealed by U–Pb ages,trace element contents and morphology of zircons from the Ishizuchi caldera,Southwest Japan Arc          下载免费PDF全文

Mami Takehara  Kenji Horie  Kenichiro Tani  Takeyoshi Yoshida  Tomokazu Hokada  Shoichi Kiyokawa 《Island Arc》2017,26(2)

U–Pb geochronology and trace element chemistry of zircons in a microscale analysis were applied to the Ishizuchi caldera in the Outer Zone of Southwest Japan in order to estimate the timescale of the magma process, in particular, the magma differentiation. This caldera is composed mainly of ring fault complexes, major pyroclastic flow deposits, and felsic intrusion including central plutons. Using SHRIMP‐IIe, our new U–Pb zircon ages obtained from the major pyroclastic flow deposits (Tengudake pyroclastic flow deposits), granitic rocks from central plutons (Soushikei granodiorite and Teppoishigawa quartz monzonite), and rhyolite from the outer ring dike (Tenchuseki rhyolite) and the inner ring dike (Bansyodani rhyolite) are 14.80 ±0.11 Ma, 14.56 ±0.10 Ma, 14.53 ±0.12 Ma, 14.55 ±0.11 Ma and 14.21 ±0.19 Ma, respectively. Based on the U–Pb ages, the Hf contents and the REE patterns of the zircons, three stages are recognized in the evolutionary history of the magma chamber beneath the Ishizuchi caldera: (i) climactic Tengudake pyroclastic flow eruption; (ii) Tenchuseki rhyolite intrusion into the outer ring dike and central pluton intrusion; and (iii) Bansyodani rhyolite intrusion in the inner ring dike. These results indicate a magma evolution history of the Ishizuchi caldera system which took at least ca 600 kyr from the climatic caldera‐forming eruption to the post‐caldera intrusions. Our new geochronological data suggest that the Ishizuchi caldera formed as part of the voluminous and episodic magmatism that occurred in the wide zone along the Miocene forearc basin of Southwest Japan during the inception of the young Philippine Sea Plate subduction.  相似文献