Morphology and geochemistry of zircon: a case study on zircon from the microgranitoid enclaves

There are three types of zircon (i.e. Zircon A, Zircon B and Zircon C) in the microgranitoid enclaves from the Qingtian granite. Zircon A is of the smallest I_pr, I_py and I_el values with the largest range of variations; Zircon C is of the largest I_pr, I_py and I_el values with the smallest range of variations; and Zircon B is intermediate among the three types. The microprobe analysis of zircon demonstrates that the contents of trace elements (Hf, U, Y, Th) increase progressively with larger and larger variation from Zircon A through Zircon B to Zircon C. These characters snggest that the three types of zircon in the enclaves may have formed successively during the cooling mess of enclave magma, corresponding to different sites along with the intrusion of enclave magma. Because of positive correlations of the UO₂/HfO₂ratio with I_pr, the Th0₂/Y₂0₃ ratio with I_py, and the UO₂/(ThO₂+Y₂0₃) ratio with I_el, it is suggested that the variation in zircon typology is caused by selective substitution of trace elements on different surfaces of zircon. On the basis of enclave petrology and zircon typomorphism, it is believed that the enclave magma may be generated from partial melting of residual materials in deep regions where the granitic magma has been extracted. Project supported by the National Natural Science Foundation of China (Grant No. 49632080).     

南海晚第四纪表层古生产力与东亚季风变迁

根据南海南北陆坡两个站位的柱状样中底栖有孔虫分布,结合有机碳、稳定同位素分析,获得南海近4万年来高分辨率的表层古生产力记录,进而探讨晚第四纪东亚古季风的变迁。研究表明,由于冬季风在本次冰期、夏季风在全新世早期约10000aB．P．急剧增强,分别在南海南北陆坡产生海岸上升流,加上陆源营养物输入增多,使得表层古生产力明显增高、有机碳通量增大至3．5gC／(m²·10³a)以上,从而底栖有孔虫群中Buliminaaculeata和Uvigerinaperegrina占绝对优势。     

Multistage progressive evolution of rare osumilite‐bearing assemblages preserved in ultrahigh‐temperature granulites from In Ouzzal (Hoggar,Algeria)

Osumilite is reported in Palaeoproterozoic Al–Mg‐rich granulites from the Khanfous area (Tekhamalt, In Ouzzal, Hoggar, Algeria). The main peak assemblages are osumilite + sapphirine + biotite + orthopyroxene + sillimanite and osumilite + orthopyroxene + sillimanite + quartz ± biotite (±K‐feldspar) in silica‐deficient and silica‐saturated granulites respectively. Osumilite coexists with F‐rich biotite (X_F ≈ 0.6). The observed microstructures, the mass balance of metamorphic reactions and P–T pseudosections modelled for bulk‐rock and reaction‐microdomain compositions indicate a clockwise P–T metamorphic evolution at ultrahigh temperatures, without substantial post‐peak deformation. The peak P–T conditions recorded by the osumilite‐bearing assemblages are 8.5–9.0 kbar and 930–980 °C. During retrogression, osumilite was partially or totally replaced by fine‐grained pseudomorphs of cordierite + orthopyroxene + K‐feldspar + quartz at ~7 kbar and ~850 °C. This study confirms that osumilite can occur only in Mg‐rich metamorphic rocks that experienced ultrahigh‐temperature metamorphism under anhydrous conditions. In the presence of a hydrous fluid, it is replaced, even at high temperatures, by cordierite‐bearing assemblages. This important feature explains the rarity of osumilite in granulite facies rocks and its common replacement by cordierite + orthopyroxene + K‐feldspar + quartz pseudomorphs. The peak conditions suggest that a delamination of the lithospheric mantle underneath the In Ouzzal crust brought the asthenosphere close to the Mohorovi?i? discontinuity.     

Structure and metamorphism of the Karakorum gneisses in the Braldu-Baltoro Valley (North Pakistan)

Abstract

The Karakorum gneisses outcrop north of the complex suture separating the Indian-Pakistan plate from the Europe-Asia block; they grade to deformed earlier members of the Karakorum batholith ranging in age from Cretaceous to Miocene and are cross-cut by its later members. The main interest of the region lies in the fact that very young high-grade gneisses (Miocene), outline the southern edge of the Europe-Asia Plate. The tectonic and metamorpic evolution of the Braldu-Baltoro region is interpreted here as resulting from a poly phased history. The following structural sequence has been defined : - (1) A Dl isoclinal folding was accompanied by subparallel healed shear zones and by intense boudinage, and cross-cut by a dense net of post-Dl hetero-geneous leucogranitic veins and stocks; - (2) a major phase of EW trending recumbent folds (D2), is followed by (3) large open D3 folds generating EW trending domai structures (Dassu and Panmah domes); and (4) a late set of brittle to locally more ductile structures such as the southern thrust contact of the Karakorum gneisses with the Shyok suture zone. The sequence proposed here differs from other interpretations (Rex et al. 1988). We consider that the Dl event only may be attributed to the main India-Asia collision and that the D2-D3 events, interpreted as having occurred in a continuum, correspond to a late reactivation of the major thrusts and sutures related to continuing continental subduction.

A Dl-related intermediate pressure assemblage is preserved (Grt-St-Ky) in the upper levels of the tectonic pile; the estimated PT conditions determined are 10-4 Kb and 700°--525°C. In the core of the large D3 domes, late granoblastic recrystallization is widespread together with almost complete S1-S2 transposition, incipient melting and development of a low-pressure sillimanite-bearing assemblage where relicts of higher pressure minerals are locally preserved. Corresponding PT conditions are 650°-550°C and a lower pressure (5.5 to 2.5 Kb). As most of the observed structures at the lower levels (mineral lineations, boudinage) are clearly associated with (or reworked by) D2 and accentuated by D3 which was accompagnied by partial melting, D2 and D3 are interpreted as representing a continuum developed in the same PT field. It can be assumed also that the Baltoro granite was emplaced by the end of this combined D2-D3 event. From the Miocene ages published for the Baltoro granite (20 Ma to 8 Ma), the low-pressure evolution of the Karakorum gneisses may represent a very young high-grade assemblage. The age of Dl is less defined but at least older than 36 Ma old leucogranites.

The sharp contact along the Shyok Suture zone, interpreted as a large thrust (Main Karakorum Thrust - MKT) of this young high-grade metamorphic terrene against the older (older than 30-45 Ma from late undeformed intrusives) Kohistan-Ladakh island-arc domain, is interpreted, following Mattauer (1985), as resulting from the interaction between the still-ongoing northward movement of the Indo-Pakistan plate and an opposite southward continental subduction, seismically active, operating in Pamir.     

The distribution of late-Quaternary woody taxa in northern Eurasia: evidence from a new macrofossil database

We present a database of late-Quaternary plant macrofossil records for northern Eurasia (from 23° to 180°E and 46° to 76°N) comprising 281 localities, over 2300 samples and over 13,000 individual records. Samples are individually radiocarbon dated or are assigned ages via age models fitted to sequences of calibrated radiocarbon dates within a section. Tree species characteristic of modern northern forests (e.g. Picea, Larix, tree-Betula) are recorded at least intermittently from prior to the last glacial maximum (LGM), through the LGM and Lateglacial, to the Holocene, and some records locate trees close to the limits of the Scandinavian ice sheet, supporting the hypothesis that some taxa persisted in northern refugia during the last glacial cycle. Northern trees show differing spatio-temporal patterns across Siberia: deciduous trees were widespread in the Lateglacial, with individuals occurring across much of their contemporary ranges, while evergreen conifers expanded northwards to their range limits in the Holocene.     

Les « Porphyroı̈des » de Bretagne méridionale : une unité de HP–BT dans la chaı̂ne hercynienneEvidence for HP–LT Hercynian metamorphism within the ‘Porphyroı̈des’ of South Brittany (France)

The so-called ‘Porphyro??des’ Unit of South Brittany is located below units marked by an early HP–LT event (blueschists from the Île de Groix) and above units marked by Upper Carboniferous HT metamorphism. P–T estimates in the Porphyro??des (at Belle-Île-en-Mer), using the phengite–chlorite thermobarometer, indicate pressures around 8 kbar and temperatures of 350–400 °C. These new estimates suggest that the early tectonic history of the ‘Porphyro??des’ should be linked to that of the overlying Blueschists, and imply a much larger areal extension of the HP–LT domain in this part of the Hercynian Belt. To cite this article: F. Le Hébel et al., C. R. Geoscience 334 (2002) 205–211.     

Textures and phase relationships in ferrous granulites from Tidjenouine (Hoggar, Algeria): fayalite–ferrossilite–quartz secondary assemblage

Ferrous granulites in the area of Tidjénouine (Central Hoggar) exhibit a remarkable mineralogical composition characterized by the association orthoferrossilite–fayalite–quartz. These granulites are metamorphosed mafic igneous rocks showing the juxtaposition of different metamorphic parageneses. Peak paragenesis with garnet–clinopyroxene–amphibole–plagioclase–quartz reach to assemblage with orthopyroxene–plagioclase₂. Secondary orthopyroxene reacted with garnet to produce symplectites with fayalite + plagioclase + quartz. The latest stage corresponds to an orthopyroxene–fayalite–quartz–plagioclase assemblage. The metamorphic history of the ferrous granulites is inferred by combining the study of phase relations with the construction of a petrogenetic grid and pseudosection in the CFMASH and CFAS systems using the Thermocalc program of [J. Metamorph. Geol. 6 (1988) 173]. The evolution of paragenetic minerals indicates a metamorphic P–T path through the following conditions: 7.1 ± 1 kbar at 880 °C, 4.9 ± 1.6 kbar at 750 °C and 3–4 kbar at 700 °C, which is consistent with a clockwise P–T path recorded throughout the area.     

Periglacial landscape evolution and environmental changes of Arctic lowland areas for the last 60 000 years (western Laptev Sea coast, Cape Mamontov Klyk)

Non-glaciated Arctic lowlands in north-east Siberia were subjected to extensive landscape and environmental changes during the Late Quaternary. Coastal cliffs along the Arctic shelf seas expose terrestrial archives containing numerous palaeoenvironmental indicators (e.g., pollen, plant macro-fossils and mammal fossils) preserved in the permafrost. The presented sedimentological (grain size, magnetic susceptibility and biogeochemical parameters), cryolithological, geochronological (radiocarbon, accelerator mass spectrometry and infrared-stimulated luminescence), heavy mineral and palaeoecological records from Cape Mamontov Klyk record the environmental dynamics of an Arctic shelf lowland east of the Taymyr Peninsula, and thus, near the eastern edge of the Eurasian ice sheet, over the last 60 Ky. This region is also considered to be the westernmost part of Beringia, the non-glaciated landmass that lay between the Eurasian and the Laurentian ice caps during the Late Pleistocene. Several units and subunits of sand deposits, peat–sand alternations, ice-rich palaeocryosol sequences (Ice Complex) and peaty fillings of thermokarst depressions and valleys were presented. The recorded proxy data sets reflect cold stadial climate conditions between 60 and 50 Kya, moderate inderstadial conditions between 50 and 25 Kya and cold stadial conditions from 25 to 15 Kya. The Late Pleistocene to Holocene transition, including the Allerød warm period, the early to middle Holocene thermal optimum and the late Holocene cooling, are also recorded. Three phases of landscape dynamic (fluvial/alluvial, irregular slope run-off and thermokarst) were presented in a schematic model, and were subsequently correlated with the supraregional environmental history between the Early Weichselian and the Holocene.