Geochemistry of the Upper Neoproterozoic and Lower Cambrian siliciclastic rocks and U-Pb dating on detrital zircons in the Central Iberian Zone,Spain |
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| Authors: | Email author" target="_blank">J?M?UgidosEmail author K?Billstr?m M?I?Valladares P?Barba |
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| Institution: | (1) Departamento de Geología, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, Spain;(2) Swedish Museum of Natural History, Box 50 007, 104 05 Stockholm, Sweden |
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| Abstract: | Sandstones and shales from the Upper Neoproterozoic (UN) succession in the Central Iberian Zone (CIZ) show parallel REE patterns and relatively restricted and similar ranges and average values of some element ratios such as Al2O3/TiO2, Ti/Nb, Eu/Eu*, (La/Yb)n, (Gd/Yb)n and Th/U. This remarkable geochemical homogeneity for related medium- and fine-grained rocks is unusual, and strongly suggests a recycled source area. However, the Lower Cambrian (LC) equivalent rocks are, in general terms, geochemically less mature, more heterogeneous and more fractionated. Their average REE patterns are practically coincident, probably as a consequence of REE redistribution related to the reworking of sediments during a stage of sea level fall in Lower Cambrian times. Ti and Zr abundances, chemical index of alteration (CIA) values and element ratios such as Al2O3/TiO2, K/Rb, Ti/Nb and Rb/Zr can be used for discriminating purposes between the UN and LC siliciclastic rocks. Detrital zircons from a UN and a LC sandstone display morphological differences. However, U-Pb data are discordant and cannot be interpreted in a straightforward manner. Nevertheless, age data are compatible with a model in which the continent of Gondwana would have supplied zircons, showing a bimodal age distribution, in variable proportions to the respective sandstone units. |
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| Keywords: | Geochemistry U-Pb dating Siliciclastics Upper Neoproterozoic– Lower Cambrian Iberian Massif |
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