Petrogenesis of a late-Variscan rhyodacite at the Ossa Morena-Central Iberian zones boundary, Iberian Massif, Central Portugal: Evidence for the involvement of lithospheric mantle and meta-igneous lower crust

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• 作者：S.B.A. Henriques^a ; ^{sbahgeo@gmail.com" class="auth_mail" title="E-mail the corresponding author} ; ^{susana.henriques@lneg.pt" class="auth_mail" title="E-mail the corresponding author} ; A.M.R. Neiva^b ; ^{neiva@dct.uc.pt" class="auth_mail" title="E-mail the corresponding author} ; G.R. Dunning^c ; ^{gdunning@mun.ca" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Carboniferous ; Igneous monazite ; Inherited zircon ; ID-TIMS U-Pb geochronology ; Sr ; Sm-Nd and δ18O isotopes
• 刊名：Chemie der Erde - Geochemistry
• 出版年：2016
• 出版时间：October 2016
• 年：2016
• 卷：76
• 期：3
• 页码：429-439
• 全文大小：3665 K

文摘

A late-Variscan rhyodacite is exposed at the contact between the Ossa Morena Zone and the Central Iberian Zone of the Iberian Massif, Central Portugal. Dykes of rhyodacite intruded the Série Negra Unit and the Sardoal Complex that are part of the Cadomian basement. The igneous crystallization age of the rhyodacite (308 ± 1 Ma) was obtained on igneous monazite by the ID-TIMS U-Pb method. It is broadly coeval with the emplacement of late-Variscan granitoids during the last deformation phase of the Variscan Orogeny (ca. 304–314 Ma) and with the development of the large late-Variscan strike-slip shear zones (ca. 307 Ma). The rhyodacite samples are calc-alkaline, show identical composition and belong to the same magmatic sequence. The rhyodacite isotopic signatures (Sm-Nd and δ¹⁸O) are consistent with depleted-mantle juvenile sources and the contribution of the meta-igneous lower crust. The input of mantle juvenile sources is related to Variscan reactivation of lithospheric fractures. The inherited Neoproterozoic (ca. 619 Ma) and Mesoproterozoic (ca. 1054 Ma) zircon ages, are similar to those of the Central Iberian Zone. This suggests that lower crust of the Central Iberian Zone was involved in the magma generation of the rhyodacite. Coeval late-Variscan magmatic rocks display a larger contribution from ancient crustal components, which may be attributed to the smaller volume and faster cooling rate of the rhyodacite and consequent lower melting of the crust. Mixing of juvenile mantle-derived melts with melts from the lower continental crust was followed by fractional crystallization of garnet and amphibole that remained in the source. Fractional crystallization of plagioclase, biotite, quartz and zircon occurred in shallower magma chambers. Fractional crystallization of zircon was not significant.