U–Pb Zircon geochronology of the Cambro-Ordovician metagranites and metavolcanic rocks of central and NW Iberia

New U–Pb zircon data from metagranites and metavolcanic rocks of the Schist-Graywacke Complex Domain and the Schistose Domain of Galicia Tras-os-Montes Zone from central and NW Iberia contribute to constrain the timing of the Cambro-Ordovician magmatism from Central Iberian and Galicia Tras-os-Montes Zones which occurred between 498 and 462 Ma. The crystallization ages of the metagranites and metavolcanic rocks from the northern Schist-Graywacke Complex Domain are as follows: (a) in west Salamanca, 489 ± 5 Ma for Vitigudino, 486 ± 6 Ma for Fermoselle and 471 ± 7 Ma for Ledesma; (b) in northern Gredos, 498 ± 4 Ma for Castellanos, 492 ± 4 Ma for San Pelayo and 488 ± 3 Ma for Bercimuelle; (c) in Guadarrama, 490 ± 5 Ma for La Estacion I, 489 ± 9 Ma for La Canada, 484 ± 6 Ma for Vegas de Matute (leucocratic), 483 ± 6 Ma for El Cardoso, 482 ± 8 Ma for La Morcuera, 481 ± 9 Ma for Buitrago de Lozoya, 478 ± 7 Ma for La Hoya, 476 ± 5 Ma for Vegas de Matute (melanocratic), 475 ± 5 Ma for Riaza, 473 ± 8 Ma for La Estacion II and 462 ± 11 Ma for La Berzosa; and (d) in Toledo, 489 ± 7 Ma for Mohares and 480 ± 8 Ma for Polan. The crystallization ages of the metagranites from the Schistose Domain of Galicia Tras-os-Montes Zone are 497 ± 6 Ma for Laxe, 486 ± 8 Ma for San Mamede, 482 ± 7 Ma for Bangueses, 481 ± 5 Ma for Noia, 480 ± 10 for Rial de Sabucedo, 476 ± 9 Ma for Vilanova, 475 ± 6 Ma for Pontevedra, 470 ± 6 Ma for Cherpa and 462 ± 8 Ma for Bande.This magmatism is characterized by an average isotopic composition of (87Sr/86Sr)485Ma ≈ 0.712, (eNd)485Ma ≈ -4.1 and (TDM) ≈ 1.62 Ga, and a high zircon inheritance, composed of Ediacaran–Early Cambrian (65 %) and, to a lesser extent, Cryogenian, Tonian, Mesoproterozoic, Orosirian and Archean pre-magmatic cores. Combining our geochronological and isotopic data with others of similar rocks from the European Variscan Belt, it may be deduced that Cambro-Ordovician magmas from this belt were mainly generated by partial melting of Ediacaran–Early Cambrian igneous rocks

Variations in organic matter composition in sediments from Lake Huguang Maar (Huguangyan), south China during the last 68 ka: implications for environmental and climatic change

This study presents a continuous and long-term palaeoenvironmental and palaeoclimatic record from a lacustrine sediment core (Lake Huguang Maar) at the northern coast of the South China Sea, extending back to about 68 cal ka BP. We provide a comprehensive ecosystem analysis from the opportunity to combine organic geochemical and palynological data. Several climatically induced changes to the flora and fauna have been recognised. From 68 to 58 and 48 to 40.5 cal ka BP, relatively depleted delta(13)C(TOC) (-31.3 to -25.0%) and delta(13)C values of mid- and long-chain n-alkanes (-34.3 to -30.2%), as well as high percentages of tropical pollen, indicate a vegetation dominated by subtropical/tropical forest (C-3 plants). An expansion of a mixed C-3/C-4 open grassland between 58 and 48 and particularly after 40.5 cal ka BP mirrors drier climatic conditions, in conjunction with a lowering of atmospheric CO2 concentrations; delta(13)C(TOC) (-24.1 to -16.8%) and delta(13)C values of n-alkyl lipids (-28.9 to -24.1%) are heavier, higher percentages of non-arboreal pollen were observed and the wood/grass lignin index (WGLI) from open pyrolysis (Py-GC) indicates a higher input of grass lignin. Abundant oxidised terrigenous particles that resulted from increased fire activity and/or greater erosion rates are a further hint of enhanced dryness. The clear drop in moisture availability suggests that this site is sensitive in fluctuations of the summer and winter monsoon activity in accordance with other proxy records in south-east Asia. The response of the aquatic ecosystem is documented by an extremely large delta(13)C shift of algal-derived botryococcenes from -35.4 to -6.2%, which records a change towards a bicarbonate carbon source at low dissolved CO2 concentrations. (C) 2003 Elsevier Ltd. All rights reserved