Chemical Th-U-total Pb Isochron Method Research Articles

Unravelling the complex sub-ice geology of the Wilkes Subglacial Basin region of East Antarctica from marine sediment provenance analyses

AbstractDeciphering the sub-ice geology in the Wilkes Subglacial Basin region is important for understanding solid earth-ice sheet evolution and for assessing geological ties between East Antarctica and formerly contiguous Australia. We analyse marine sediment samples derived from drill site U1359 of Integrated Oceanic Drilling Program Expedition 318. Our study reports for the first time that the inland sediment source area comprises a complex mafic igneous terrain and a metamorphosed Precambrian subglacial basement. Pyroxene geochemical analyses confirm the presence of tholeiitic to calc-alkaline basalts. The high-grade part of the subglacial terrain contains upper amphibolite to granulite facies rocks that are comparable to Archaean to Palaeoproterozoic rocks exposed in the Terre Adélie Craton and the formerly adjacent Gawler Craton in Australia. Chemical Th-U-total Pb isochron method (CHIME) ages extracted from a subhedral monazite grain associated with the low-grade biotite-muscovite schist rock fragment provide a unimodal age of 799 ± 13 Ma. Rare occurrences of 800 Ma age in the Terre Adélie Craton and/or George V Coast provide evidence for the presence of at least one late Neoproterozoic magmato-metamorphic event in the interior of Wilkes Land. The affinity of the unexposed geological domains of Wilkes Land, East Antarctica, with their Australian counterparts is discussed in the context of the Rodinia supercontinent.

Возраст монацита из метапелитовых сланцев серий Атомфьелла и Моссель (полуостров Ню-Фрисланд, о. Западный Шпицберген) по данным Th-U-Pb (CHIME)-датирования

The article presents the results of CHIME (chemical Th–U-total Pb isochron method) dating of monazite from metamorphic rocks of Precambrian complexes located in the north part of the West Spitsbergen Island. It is shown that for rocks of Atomfjella Series and Mossel Series, monazite ages are coeval within error (Atomfjella Series: 381 ± 18 Ma, Mossel Series: 377 ± 23 Ma). These age estimates show that metamorphism of the crystalline basement possibly took place during the Late Caledonian orogeny.

U-Pb-Th geochronology of monazite and zircon in albitite metasomatites of the Rožňava-Nadabula ore field (Western Carpathians, Slovakia): implications for the origin of hydrothermal polymetallic siderite veins

Sodic metasomatites (albitites) occur around and within siderite veins in the southern part of the Gemeric tectonic unit of the Western Carpathians. Accessory minerals of the metasomatites represented by monazite, zircon, apatite, rutile, tourmaline and siderite are basically identical with the quartz-tourmaline stage of other siderite and stibnite veins of the tectonic unit. Statistical analysis of chemical Th-U(total)-Pb isochron method (CHIME) of monazite dating yielded Jurassic-Cretaceous ages subdivided into 3–4 modes, spreading over time interval between 78 and 185 Ma. In contrast, LA-ICPMS 206Pb/238U dating carried out on the same monazite grains revealed a narrow crystallization interval, showing ages of Th-poor cores with phengite inclusions identical within the error limit with Th-rich rims with cauliflower-like structure. The determined lower intercept at 139 ± 1 Ma overlapped the Vallanginian-Berriasian boundary, thus corroborating the model of formation of hydrothermal vein structures within an arcuate deformation front built up in the Variscan basement as a response to Early Cretaceous compression, folding and thrusting. In contrast, associated zircons are considerably older than the surrounding Early-Palaeozoic volcano-sedimentary rocks, showing Neoproterozoic ages. The zircon grains in albitite metasomatites are thus interpreted as fragments of Pan-African magmatic detritus incorporated in the vein structures by buoyant hydrothermal fluids.

U-Pb zircon and CHIME monazite dating of granitoids and high-grade metamorphic rocks from the Eastern and Peninsular Thailand — A new report of Early Paleozoic granite

Abstract In order to understand the age and tectonic framework of Eastern to Peninsular Thailand from the viewpoint of basement (metamorphic and plutonic) geology, the LA-ICP-MS U-Pb zircon dating and the chemical Th–U-total Pb isochron method (CHIME) monazite dating were performed in the Khao Chao, Hub-Kapong to Pran Buri, and Khanom areas in Eastern to Peninsular Thailand. The LA-ICP-MS U-Pb zircon dating of the garnet-hornblende gneiss from the Khao Chao area gave 229 ± 3 Ma representing the crystallization age of the gabbro, and that of the garnet-biotite gneisses gave 193 ± 4 Ma representing the timing of an upper amphibolite facies metamorphism. The CHIME monazite dating of pelitic gneiss from the Khao Chao gneiss gave scattered result of 68 ± 22 Ma, due to low PbO content and rejuvenation of older monazite grains during another metamorphism in the Late Cretaceous to Tertiary time. The U-Pb ages of zircon from the Hua Hin gneissic granite in the Hub-Kapong to Pran Buri area scatter from 250 Ma to 170 Ma on the concordia. Granite crystallization was at 219 ± 2 Ma, followed by the sillimanite-grade regional metamorphism at 185 ± 2 Ma. Monazite in the pelitic gneiss from this area also preserves Early to Middle Jurassic metamorphism and rejuvenation by later contact metamorphism by non-foliated granite or by another fluid infiltration event in the Late Cretaceous to Tertiary time. The Khao Dat Fa granite from the Khanom area of Peninsular Thailand gave a U-Pb zircon age of 477 ± 7 Ma. This is the second oldest granite pluton ever reported from Thailand, and is a clear evidence for the Sibumasu block having a crystalline basement that was formed during the Pan-African Orogeny. The Khao Pret granite gives U-Pb zircon concordia age of 67.5 ± 1.3 Ma, which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite-grade contact metamorphism against surrounding metapelites and gneisses. Metamorphic rocks in the Doi Inthanon area also share the similar plutono-metamorphic history with the Khanom and the Hub-Kapong to Pran Buri areas. This suggests that these three areas belong to the Sibumasu block, and the Sibumasu block records similar plutono-metamorphic history from Northern to Peninsular Thailand. Relative abundance of oceanic components in the Khao Chao gneiss, their Late Triassic magmatic ages, and the Early Jurassic metamorphic ages prefer the interpretation that the Khao Chao gneiss belongs to the Sukhothai Arc.

Carboniferous inherited grain and age zoning of monazite and xenotime from leucogranites in far-eastern Nepal: Constraints from electron probe microanalysis

Chemical Th-U-total Pb isochron method (CHIME) of monazite and xenotime from three leucogranites in far-eastern Nepal revealed the presence of Carboniferous inherited grains of monazite and intense intrusion of leucogranites around 18-16 Ma in the High Himalaya. In garnet-bearing sillimanitemuscovite- biotite leucogranite, most monazite grains have inherited cores with the chemical dates of ~504-418 and ~342-272 Ma, which were overgrown by Early Miocene mantles and rims of 17.9 ± 1.5 Ma. Early Ordoviciain and Carboniferous ages are rarely found in the same euhedral-subhedral cores. In addition to the previously recognized Early Paleozoic magmatism, monazite cores with Carboniferous ages in Early Miocene leucogranites provide evidence for two periods of magmatism at the base of the High Himalaya prior to the Cenozoic Himalayan orogeny. In muscovite-biotite leucogranite, no inherited domains were observed in monazite and xenotime grains. They yielded the CHIME monazite and xenotime dates of 16.1 ± 2.0 and 19.8 ± 6.5 Ma, respectively. Monazite grains adjacent to xenotime have significantly lower concentrations of UO2 and Y2O3 compared to those isolated from xenotime. These results imply that xenotime influences Y and U contents in monazite, reflecting local equilibrium system. In aplitic leucogranite, monazite grains yielded the mean of apparent chemical date of 18.0 ± 2.2 Ma. The CHIME monazite ages of ~18-16 Ma in three leucogranites reflect the timing of melt crystallization.

New SHRIMP U–Pb zircon ages and CHIME monazite ages from South Harris granulites, Lewisian Complex, NW Scotland: Implications for two stages of zircon formation during Palaeoproterozoic UHT metamorphism

Sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon age dating was applied to several types of granulites and gneisses from the Lewisian Complex in South Harris, NW Scotland, to clarify the timing of Palaeoproterozoic tectono-thermal events. For comparison, monazite was dated using the chemical Th–U–total Pb isochron method (CHIME). The obtained U–Pb zircon age data were evaluated under the constrains of the modal amount of constituent minerals and whole-rock compositions of analysed samples, the zircon internal texture based on Th/U ratios, and the monazite CHIME ages. Zircons in three samples, characterised by zoned overgrowth rims, sector-zoned grains, and structureless high-U cores, yielded concordant ages of approximately 1900Ma (1909±3Ma, 1899±10, and 1888±15Ma, respectively). The age of 1899±10Ma, obtained from melanocratic granulite (garnet and kyanite/sillimanite: 76–91vol.%), is interpreted to represent the timing of zircon crystallisation prior to the formation of restitic garnet and kyanite/sillimanite. Therefore, the age of ∼1900Ma is proposed to represent the timing of early metamorphism, close to the thermal peak (M1). In the leucocratic gneiss (sample 93921-7), the age population at 1868±35Ma obtained from low-U and low-Th overgrowth rims represents a second stage of zircon growth from crystallising anatectic melt (M3), whereas the high-U cores represent the timing of zircon growth and recrystallisation prior to anatexis. Monazites from two leucocratic gneisses yield a relatively narrow range of CHIME ages: 1858±7Ma and 1840±7Ma. These ages probably represent the timing of zircon and monazite crystallisation from partial melt and resetting of the U–Th–Pb system. These results suggest that the timing of zircon formation was controlled by metamorphic processes, such as restite formation and crystallisation from partial melt.

Regional‐Scale Excess Ar wave in a Barrovian type metamorphic belt, eastern Tibetan Plateau

AbstractLaser step heating40Ar/39Ar analysis of biotite and muscovite single crystals from a Barrovian type metamorphic belt in the eastern Tibetan plateau yielded consistent cooling ages of ca. 40 Ma in the sillimanite zone with peak metamorphic temperatures higher than 600°C and discordant ages from 46 to 197 Ma in the zones with lower peak temperatures. Chemical Th‐U‐Total Pb Isochron Method (CHIME) monazite (65 Ma) and sensitive high mass‐resolution ion microprobe (SHRIMP) apatite (67 Ma) dating give the age of peak metamorphism in the sillimanite zone. Moderate amounts of excess Ar shown by biotite grains with ages of 46 to 94 Ma at metamorphic grades up to the high‐grade part of the kyanite zone probably represent incomplete degassing during metamorphism. In contrast, the high‐grade part of the kyanite zone yields biotite ages of 130 to 197 Ma. The spatial distribution of these older ages in the kyanite zone along the sillimanite zone boundary suggests they reflect trapped excess argon that migrated from higher‐grade regions. The most likely source is muscovite that decomposed to form sillimanite. The zone with extreme amounts of excess argon preserves trapped remnants of an ‘excess argon wave’. We suggest this corresponds to the area where biotite cooled below its closure temperature in the presence of an elevated Ar wave. Extreme excess Ar is not recognized in muscovite suggesting that the entrapment of the argon wave by biotite took place when the rocks had cooled down to temperatures lower than the closure temperature of muscovite. The breakdown of phengite during ultrahigh‐pressure (UHP) metamorphism may be a key factor in accounting for the very old apparent ages seen in many UHP metamorphic regions. This is the first documentation of a regional Ar‐wave spatially associated with regional metamorphism. This study also implies that resetting of the Ar isotopic systems in micas can require temperatures up to 600°C; much higher than generally thought.

Petrology and CHIME geochronology of Pan-African high K and Sr/Y granitoids in the Nkambe area, Cameroon

The Central African Belt in the Nkambe area, northwestern Cameroon represents a collisional zone between the Saharan metacraton and the Congo craton during the Pan-African orogeny, and exposes a variety of granitoids including foliated and massive biotite monzogranites in syn- and post-kinematic settings. Foliated and massive biotite monzogranites have almost identical high-K calc-alkaline compositions, with 73–67 wt.% SiO 2, 17–13 wt.% Al 2O 3, 2.1–0.9 wt.% CaO, 4.4–2.7 wt.% Na 2O and 6.3–4.4 wt.% K 2O. High concentrations of Rb (264–96 ppm), Sr (976–117 ppm), Ba (3680–490 ppm) and Zr (494–99 ppm), with low concentrations of Y (mostly< 20 ppm with a range 54–6) and Nb (up to 24 ppm) suggest that the monzogranites intruded in collisional and post-collisional settings. The Sr/Y ratio ranges from 25 to 89. K, Rb and Ba resided in a single major phase such as K-feldspar in the source. Garnet was present in the source and remained as restite at the site of magma generation. This high K 2O and Sr/Y granitic magma was generated by partial melting of a granitic protolith under high-pressure and H 2O undersaturated conditions where garnet coexists with K-feldspar, albitic plagioclase. CHIME (chemical Th–U-total Pb isochron method) dating of zircon yields ages of 569 ± 12–558 ± 24 Ma for the foliated biotite monzogranite and 533 ± 12–524 ± 28 Ma for the massive biotite monzogranite indicating that the collision forming the Central African Belt continued in to Ediacaran (ca 560 Ma).

CHIME monazite ages of metasediments from the Altai orogen in northwestern China: Devonian and Permian ages of metamorphism and their significance

Abstract The chemical Th‐U‐total Pb isochron method (CHIME) has been applied to determine the ages of monazite grains from metasediments of greenscshist‐amphibolite facies in the Altai orogen, northwest China. The area of distribution of these metamorphic rocks is divided into the Permian (261–268 Ma) central‐western and Devonian (377–382 Ma) eastern units on the basis of their metamorphic ages. The Devonian CHIME ages are consistent with the Pb–Pb ages of granitoid in the eastern unit, and support the idea that emplacement of the granitoids was synchronous with regional metamorphism at deep levels. The Permian metamorphic ages (the present study) and igneous ages previously reported from the central‐western unit can be interpreted in terms of subduction of crustal material and oceanic plate, and rapid exhumation.

Monazite, iron oxide and barite exsolutions in apatite aggregates from CCSD drillhole eclogites and their geological implications

We have identified abundant exsolutions in apatite aggregates from eclogitic drillhole samples of the Chinese Continental Scientific Drilling (CCSD) project. Electron microscope and laser Raman spectroscopy analyses show that the apatite is fluorapatite, whereas exsolutions that can be classified into four types: (A) platy to rhombic monazite exsolutions; (B) needle-like hematite exsolutions; (C) irregular magnetite and hematite intergrowths; and (D) needle-like strontian barite exsolutions. The widths and lengths of type A monazite exsolutions range from about 6–10 μm (mostly 6 μm) and about 50–75 μm, respectively. Type B exsolutions are parallel with the C axis of apatite, with widths ranging from 0.5 to 2 μm, with most around 1.5 μm, and lengths that vary dramatically from 6 to 50 μm. Type C exsolutions are also parallel with the C axis of apatite, with lengths of ∼30–150 μm and widths of ∼10 to 50 μm. Type D strontian barite exsolutions coexist mostly with type B hematite exsolutions, with widths of about 9 μm and lengths of about 60–70 μm. Exsolutions of types B, C and D have never been reported in apatites before. Most of the exsolutions are parallel with the C axis of apatite, implying that they were probably exsolved at roughly the same time. Dating by the chemical Th–U-total Pb isochron method (CHIME) yields an U–Pb isochron age of 202 ± 28 Ma for monazite exsolutions, suggesting that these exsolutions were formed during recrystallization and retrograde metamorphism of the exhumed ultrahigh pressure (UHP) rocks. Quartz veins hosting apatite aggregates were probably formed slightly earlier than 202 Ma. Abundant hematite exsolutions, as well as coexistence of magnetite/hematite and barite/hematite in the apatite, suggest that the oxygen fugacity of apatite aggregates is well above the sulfide-sulfur oxide buffer (SSO). Given that quartz veins host these apatite aggregates, they were probably deposited from SiO 2-rich hydrous fluids formed during retrogression of the subducted slab. Such SiO 2-rich hydrous fluids may act as an oxidizing agent, a feasible explanation for the high oxygen fugacity in convergent margin systems.

Discovery of a c.370 Ma granitic gneiss clast from the Hwanggangri pebble-bearing phyllite in the Okcheon metamorphic belt, Korea

The chemical Th–U total Pb isochron method (CHIME) of dating was carried out on accessory minerals in samples from the Okcheon metamorphic belt in Korea. Dated minerals include xenotime and monazite with overgrown mantles in a granitic gneiss clast from the Hwanggangri Formation, metamorphic allanite in garnet-bearing muscovite–chlorite schist of the Munjuri Formation, and polycrase and monazite in post-tectonic granite from the Hwanggangri area. Overgrowth of mantles took place at 369 ± 10 Ma on c. 1750 Ma cores of xenotime and monazite in the granitic gneiss. Allanite, occurring in textural equilibrium with peak metamorphic minerals, yields a CHIME age of 246 ± 15 Ma that is discriminably older than the polycrase (170 ± 6 Ma) and monazite (170 ± 3 Ma) ages of the post-tectonic granite. These chronological data suggest that some of the metasedimentary rocks in the belt formed through a single stage of metamorphism at c. 250 Ma from post-370 Ma sediments. Late Permian age signatures have also been reported from the Precambrian Gyeonggi and Yeongnam massifs that border the Okcheon metamorphic belt, and indicate that parts of the basement massifs and the metamorphic belt were affected by the same regional metamorphic event.

CHIME zircon age of garnet-granite from the Naguri Fault Zone, Kanto Mountains, central Japan

CHIME (chemical Th-U-total Pb isochron method) dating of garnet-granite, which is exposed in the southernmost part of the Naguri Fault Zone in the Kanto Mountains, was carried out by precise microprobe analysis of zircon grains. The CHIME age is 432±40 Ma. This age is concordant with that of granitoids (378-447 Ma) of the Kurosegawa Terrane in the other areas of Southwest Japan. It is a conclusive evidence indicating that the Naguri Fault Zone represents an eastern extension of the Kurosegawa Terrane, in addition to the other characteristics such as the tectonic situation of the Naguri Fault Zone, mode of occurrence as well as petrography and petrochemistry of rocks occurring in this fault zone.

Monazite and Zircon Dating by the Chemical Th‐U‐Total Pb Isochron Method (CHIME) from Alasheyev Bight to the Sør Rondane Mountains, East Antarctica: A Reconnaissance Study of the Mozambique Suture in Eastern Queen Maud Land

Abstract Ages based on electron microprobe analysis using the chemical Th‐U‐total Pb isochron method (CHIME) were determined for monazite and zircon in 12 granulite‐facies paragneiss samples from four exposures on or near the coast of East Antarctica: Mt. Vechernyaya (Alasheyev Bight; 46° E, Rayner Complex), East Ongul Island (39°35 \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\arcmin$\end{document} E, Lutzow‐Holm Complex), the southernmost Yamato Mountains (36° E, Yamato‐Belgica Complex), and the Sor Rondane Mountains (23°–2...

CHIME zircon ages of granodiorite clasts from the Motodo Formation, Hida-Gaien Belt, central Japan

CHIME (chemical Th-U-total Pb isochron method) dating of granodiorite clasts from the Motodo Formation of the Hida-Gaien Belt was carried out by precise microprobe analyses of zircon grains. The CHIME ages are 201±20 Ma and 202±30 Ma, indicating that the granodiorite intrusion in the provenance areas of the Motodo Formation occurred in the Early Jurassic time. The depositional age of the Motodo Formation was constrained by previous authors between the Late Permian and Early Cretaceous, but our data further restrict it between the Early Jurassic and Early Cretaceous. On the basis of age constraints and lithological similarities, the Motodo Formation correlates with the Kyongsang Supergroup in South Korea and the Kanmon Group in Southwest Japan. Early Jurassic granitoids are distributed in the Hida Belt in Japan and in the Korean Peninsula, which are considered as likely provenance areas for the sediments of the Motodo Formation.

Early Permian peak metamorphism recorded in U–Pb system of black slates from the Ogcheon metamorphic belt, South Korea, and its tectonic implication

We report Pb–Pb whole rock and uraninite CHemical Th–U–total Pb Isochron MEthod (CHIME) ages of carbonaceous black slates from the Ogcheon metamorphic belt, South Korea. The Pb isotopic data of whole rock samples yield 207Pb/206Pb ages of 283±33 and 291±13 Ma for two outcrops from the northeastern part of the belt. The uraninite CHIME age is estimated at 283±26 and 281±27 Ma for the northeastern and the middle part of the belt, respectively. All of the above ages are identical within error ranges, and represent the timing of peak metamorphism after the late Precambrian intraplate rifting. On the other hand, the 207Pb/206Pb whole rock age for the southwestern part of the belt is estimated at 194±27 Ma, probably representing the timing of contact thermal metamorphism associated with the intrusion of Jurassic granitic plutons. Rb–Sr isotopic data of the black slates do not define any meaningful isochron. The early Permian metamorphic age of this study does not support any tectonic scheme in favor of major tectonometamorphism at either the Silurian–Devonian or the Triassic time. Instead, it corroborates the probability that the two zones in the Ogcheon belt, the Ogcheon metamorphic belt and the Taebaegsan zone, were separated from each other before the development of major structural framework in the former. Our data do not support an idea that the Ogcheon belt corresponds to the continuation of the Triassic collision belt in east central China.

Chemical Th–U–total Pb dating by electron microprobe analysis of monazite, xenotime and zircon from the Archean Napier Complex, East Antarctica: evidence for ultra-high-temperature metamorphism at 2400 Ma

The chemical Th–U–total Pb isochron method (CHIME) of dating was carried out on monazite, xenotime, zircon, and polycrase in six samples of ultra-high-temperature (UHT) granulites and one of pegmatite from five localities in the Napier Complex, Enderby Land, East Antarctica. Despite the chronological heterogeneity in many grains, the CHIME ages overall show strong evidence for a major event near 2420 Ma. Analyzed monazite and xenotime grains, except monazite from Mt. Riiser-Larsen, have a narrow range of CHIME ages: 2412±42 Ma from Beaver Island and 2429±36, 2421±18 and 2431±22 Ma from Reference Peak. These ages are indistinguishable from CHIME ages on zircon (2436±17 and 2420±20 Ma) and on central older domains of monazite (2404±54 Ma) from Mt. Riiser-Larsen, and on a zircon rim (2430±15 Ma) from Mt. Cronus. We conclude that the ca. 2420 Ma ages relate to the UHT metamorphism, but we do not exclude the possibility that lead loss has lowered the ages by some 50 Ma. This interpretation is consistent with a preponderance of ages obtained by other methods on the Napier Complex particularly the ca. 2475±25 Ma age for a generation of pegmatites coeval with the UHT event and containing the UHT mineral assemblage beryllian sapphirine–khmaralite+quartz. Events prior to 2500 Ma and after 2420 Ma have also left their imprint on the CHIME ages, for example, a 3646 Ma CHIME age suggests that a zircon core is inherited, whereas ca. 2200 and 2000 Ma CHIME ages in zoned monazite could be due to further lead loss during post-UHT activity. The much younger CHIME age of 1094±67 Ma for monazite from a pegmatite at ‘Zircon Point’, Casey Bay, confirms 1000–1100 Ma lead loss ages reported for this region, and could result from fluid activity in an outlier of Rayner activity near the boundary between the Rayner and Napier Complexes. Approximately 2500 Ma ages have been reported from granulite-facies (but not UHT) rocks in the Madras and Nilgiri blocks in South India; these blocks and the Napier Complex could have constituted a single structural unit by late Archean time.

Provenance of granitic anchor stones recovered from the Takashima submerged site: an approach using the CHIME method for dating of zircons

Wooden anchors equipped with a pair of granitic anchor stones were found and recovered form a number of submerged wrecks located in Imari Bay of Takashima, Nagasaki Prefecture. These remains are from wrecked Mongolian warships that were involved in the second Mongol invasion of 1281. All the granitic anchor stones from different anchors are alkali granite, consisting mainly of microcline-micro-perthite and quartz with trace amounts of biotite and plagioclase. The CHIME (chemical Th-U-total Pb isochron method) age determination was carried out on zircons from the recovered granitic anchor stones to try to identify the original source area for the anchor stones. The CHIME zircon ages for 3 anchor stones from different anchors are 110±1Ma, 108±3Ma and 108±2Ma. The ages together with the Sr-poor nature of the bulk chemistry suggest that the anchor stones were derived from a leucocratic alkali granite body found close to Quanzhou, southeastern China. This result implies that the wrecked warships were constructed in Quanzhou and supports the view that the Moslem traders in Quanzhou contributed significantly to the second Mongol invasion by providing ships and seamen.

Permian CHIME ages of leucocratic tonalite clasts from Middle Permian Usuginu-type conglomerate in the South Kitakami Terrane, northeastern Japan.

The CHIME (chemical Th-U-total Pb isochron method) dating of tonalite clasts in the Middle Permian Usuginu-type conglomerate has been carried out by precise microprobe analyses of monazite and zircon. The CHIME ages are 257±19 Ma, 244±42 Ma and 248±11 Ma (zircon), and 247±24 Ma (monazite) for tonalites. The ages indicate the intrusion of tonalite around 250 Ma together with quartz diorite and diorite, and rapid uplift and denudation after the igneous activity occurred in the provenance area for the Usuginu-type conglomerate.

Denudation history of the high T/P Ryoke metamorphic belt, southwest Japan: constraints from CHIME monazite ages of gneisses and granitoids

CHIME (chemical Th–U‐total Pb isochron method) monazite ages were determined for gneisses and granitoids from the eastern and western parts of the Ryoke belt separated by about 500 km. The monazite ages for the gneisses are concentrated between 102 and 98 Ma, and are interpreted as the time of monazite formation under lower amphibolite facies conditions. The peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c. 95 Ma in both the eastern and western parts. In the eastern part plutonism continued from c. 95 Ma to c. 68 Ma at intervals of 2–10 Ma, whereas in the western part it ceased at c. 85 Ma. The CHIME monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts. These lines of evidence are incompatible with a current view that the plutonometamorphism in the Ryoke belt becomes younger towards the east. The CHIME monazite ages, coupled with available data on the depth at which the Ryoke metamorphism took place and the emplacement of individual plutons, show that the western part was eroded more rapidly (about 1.5 mm year−1) than the eastern part (about 0.8 mm year−1) over the time span from 91 to 85 Ma. The denudation rates agree well with those in active orogenic belts like the Alps and Himalayas.

Provenance of Creataceous sandstones in the southeastern Yongdong Basin, Korea: CHIME geochronology of detrital monazites

CHIME (chemical Th-U-total Pb isochron method) ages were determined on detrital monazites in Cretaceous sandstones, southeastern part of the Yongdong Basin and in granitic gneiss (?foliated granite) of the Yongnam Massif, northern part of the Kimchon area. Most detrital monazites in the sandstone samples are chronologically unzoned, and yield CHIME ages of about 180 and 250 Ma. Some detrital monazites show age signatures of middle Precambrian (ca. 1200 and 1750 Ma) and possible Silurian (420 Ma). Monazites in the granitic gneiss yield an unequivocal age of 251.2±3.0 Ma. The age spectrum of detrital monazites coupled with paleoflows and petrographic data suggests that clastic materials in the southeastern part of the Yongdong Basin were largely derived from the granitic and metamorphic rocks of the Yongnam Massif in the Sangju—Kimchon—Keochang area.