科研之友
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摘要
The ages of orthogneisses exposed in massifs of the Variscan chain can determine whether they are part of a pre-Neoproterozoic basement, a Neoproterozoic, Panafrican arc, or are, in fact, lower Paleozoic, and their isotopic compositions can be used to probe the nature of their source rocks, adding to the understanding of the types, distribution, and tectonic evolution of peri-Gondwanan crystalline basement. Using SHRIMP U-Pb zircon geochronology and Nd isotopic analysis, pre-Variscan metaigneous rocks from the Nuria massif in the Eastern Pyrenean axial zone and the Guilleries massif, 70 km to the south, have been dated and their Nd signatures characterized.. All dated orthogneisses from the Nuria massif have the same age within error, similar to 457 Ma, including the Ribes granophyre, interpreted as a subvolcanic unit within Caradocian sediments contemporaneous with granitic magmas intruded into Cambro-Ordovician sediments at deeper levels. Orthogneisses in the Guilleries massif record essentially continuous magmatic activity during the Ordovician, beginning at the Cambro-Ordovician boundary (488 +/- 3 Ma) and reaching a peak in the volume of magma in the early Late Ordovician (similar to 460 Ma). Metavolcanic rocks in the Guilleries massif were extruded at 452 +/- 4 Ma and appear to have their intrusive equivalent in thin, deformed veins of granitic gneiss (451 +/- 7 Ma) within metasedimentary rocks. In orthogneisses from both massifs, the cores of some zircons yield Neoproterozoic ages between similar to 520 and 900 Ma. The age of deposition of a pre-Late Ordovician metapelite in the Guilleries massif is bracketed by the weighted average age of the youngest detrital zircon population, 582 11 Ma, and the age of cross-cutting granitic veins, 451 +/- 7 Ma. Older detrital zircons populations in this metapelite include Neoproterozoic (749-610 Ma; n = 10), Neo- to Mesoproterozoic (1.04-0.86 Ga; n = 7), Paleoproterozoic (2.02-1.59 Ga; n = 5), and Neoarchean (2.74-2.58 Ga; n = 3). Nd isotopic analyses of the Nuria and Guilleries orthogneisses yielded negative epsilon(Nd) values ranging between -2.1 and -5.2 at 450 Ma, the same as Ediacaran sediments from northwestern Iberia. We interpret these slightly negative epsilon(Nd) values as a signature of Panafrican arc rocks, mixed with an older crustal component as indicated by the inherited and detrital zircon ages we analyzed. The crustal level in which Ordovician melting occurred has not been recognized and the absence of pre-Neoproterozoic basement is a striking feature of the southern part of the Variscan Chain.
- 出版日期2011-11
