Hercynian blueschist metamorphism in North Portugal: tectonothermal implications |
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| Authors: | J I GIL IBARGUCHI R D DALLMEYER |
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| Institution: | Departamento de Mineralogía-Petrología, Universidad del País Vasco-E.H.U., Aptdo. 644, 48080 Bilbao, Spain;Department of Geology, University of Georgia, Athens, GA 30602, USA |
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| Abstract: | Abstract Aegirine–jadeite clinopyroxene (>60 mol% jadeite) locally occurs within blueschists of the 'Lower Allochthon'exposed in the Trás-os-Montes region of northern Portugal. Peak conditions attained during blueschist facies metamorphism are estimated to have been c. 420° C and >11 kbar. Porphyroblastic white mica (paragonite/phengite) within the blueschist assemblage records a 36Ar/40Ar versus 39Ar/40Ar isotope correlation age of 329.4 ± 1.6 Ma. In view of the relatively low- T nature of the metamorphism, the c. 330-Ma age is interpreted to date closely the high- P recrystallization. This tectonothermal activity is interpreted to have resulted from structural emplacement of a previously assembled crystalline nappe complex ('Upper Allochthon/Ophiolite Nappe') onto Iberian protoliths of the Lower Allochthon during terminal stages of the Hercynian orogeny. |
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| Keywords: | 40Ar/39Ar geochronology blueschist metamorphism Hercynian Portugal P–T conditions tectonothermal history Träs-os-Montes |
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