Martins, F.1,2*, Azevedo, M.R.1, Valle Aguado, B.1, Gomes, M.E.3, Tassinari, C.4, Nogueira Neto,
J.A. 2
1 Universidade de Aveiro, Departamento de Geociências; Unidade de Investigação GeoBioTec (UID/GEO/04035/2019), Campus Santiago, 3810-193 Aveiro, Portugal
2 Departamento de Geologia, Universidade Federal do Ceará, Campus do Pici, Bloco 912, 60455-760, Fortaleza, CE, Brasil 3 Universidade de Trás-os-Montes e Alto Douro, Departamento de Geologia, Quinta de Prados, 5000-801 Vila Real, Portugal, CEMMPRE (UID/EMS/00285/2019)
4 Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, 05508-900 São Paulo, SP, Brasil * fabioamartins@ua.pt
Resumo: O Complexo Migmatítico de Bemposta (CMB), exposto no sector nordeste da Zona Centro- Ibérica é composto por metassedimentos do Ediacariano-Câmbrico e ortognaisses glandulares mostrando um bandado migmatítico estromático bem marcado, intruídos por vários corpos de leucogranitos anatécticos. Os dados estruturais e petrográficos revelam que as rochas do CMB sofreram metamorfismo regional do tipo Barroviano durante a primeira fase de deformação varisca (D1), foram afectadas por descompressão a alta temperatura num episódio extensional subsequente (D2) e terminaram a sua evolução, seguindo uma trajectória de arrefecimento, durante a tectónica transcorrente (D3). No final da D1, início da D2, as rochas metamórficas de Bemposta terão atingido as condições de fusão parcial através da reacção de desidratação da moscovite (topo da fácies anfibolítica) e deram origem a quantidades significativas de “melts” graníticos peraluminosos. Com o arrefecimento (nos estádios finais da D2), ocorreu a cristalização dos leucossomas e dos corpos graníticos sin-D2.
O leucogranito de Faia d’Água Alta ocorre no CMB como uma intrusão sub-horizontal tabular, concordante com o bandado estromático S2 dos metatexitos encaixantes. A idade de cristalização (324 ± 3 Ma), obtida por SHRIMP em zircões deste granito fornece uma boa estimativa para a idade do evento anatéctico sin-D2.
Palavras-chave: Complexo Migmatítico de Bemposta, leucogranito sin-D2 de Faia d’Água Alta, idades U-Pb por SHRIMP
Abstract: The Bemposta Variscan high-grade metamorphic complex (BMC), exposed in the northeastern sector of the Central Iberian Zone, consists of migmatitic metasediments and orthogneisses, intruded by several bodies of anatectic leucogranites. Structural and petrographical evidence reveal that the host metasedimentary and metaigneous rocks experienced an early Barrovian- type metamorphism coeval with contractional deformation (D1), followed by nearly isothermal decompression at high temperature related to an extensional episode (D2) and subsequent cooling during late Variscan transcurrent shearing (D3). By the end of D1 begginning of D2, the BMC metamorphic rocks reached upper-amphibolite facies conditions and anatexis via muscovite dehydration melting, leading to the generation of significant amounts of peraluminous granite melts. Cooling from peak metamorphic conditions to the granite solidus started at the final stages of D2 and resulted in the crystallization of leucosomes and syn-D2 plutons.
The Faia d’Água Alta leucogranite occurs in the BMC as a sub-horizontal tabular body, emplaced concordantly with the S2 stromatic layering of the host metasedimentary-derived metatexites. Therefore, the U-Pb crystallization age obtained by SHRIMP in zircon grains from this leucogranite (324 ± 3 Ma) provides a good estimate for the age of the D2-anatectic event.
Keywords: Bemposta Migmatic Complex, Faia d’Água Alta syn-D2 leucogranite, SHRIMP U-Pb zircon ages
100
1. Geological setting
The Bemposta Migmatite Complex (BMC) is a NE-SW trending Variscan high-grade metamorphic complex bounded by a major detachment, exposed in the northeastern sector of the Central Iberian Zone in NE Portugal (Fig. 1). It consists of stromatic migmatites (probably derived from Ediacaran - Cambrian metapelites and metagreywackes) and subordinate migmatized augen orthogneisses, intruded by several bodies of anatectic leucogranites (Dias da Silva, 2013; Fig. 1a). Structural and petrographical evidence reveal that the BMC rocks experienced an early Barrovian-type metamorphism coeval with contractional deformation (D1),
followed by nearly isothermal decompression at high temperature related to an extensional episode (D2). Subsequent
cooling occurred during D3 late Variscan
transcurrent shearing (e.g. Escuder Viruete et al., 2000; Dias da Silva, 2013).
By the end of D1 begginning of D2, the BMC
metamorphic rocks reached upper- amphibolite facies conditions (locally granulite facies) and anatexis via muscovite dehydration melting, leading to the generation of significant amounts of peraluminous granite melts.
Cooling, indicated by recrossing of the muscovite melting reaction, started during the final stages of the D2 extensional
episode and was accompanied by the crystallization of millimeter- to centimeter- scale leucosomes to kilometer-scale concordant flat-lying syn-D2 leucogranites
and pegmatites (Fig. 1).
In the studied area, D3 deformation is much
less pervasive than D2. The D3 deformation
event was related to narrow, strike-slip, subvertical, dextral and sinistral shear zones, along which the rocks show S-C structures and a steeply dipping mylonite foliation.
At deep crustal levels, anatectic conditions could have persisted during D3, as
suggested by the emplacement of large volumes of allochtonous syn- and late-post- D3 granitoids in the region.
2. Faia d’Água Alta leucogranite
The Faia d’Água Alta syn-D2 leucogranite,
located near the Lamoso village, is a sub- horizontal tabular body, emplaced concordantly with the S2 stromatic layering
of the host metasedimentary-derived metatexites (Fig. 1b).
Sample FD-97 collected at Faia d’Água Alta is a weakly deformed medium-grained muscovite-biotite leucogranite, containing quartz, K-feldspar, plagioclase, muscovite, biotite, apatite, zircon, monazite and opaques.
Geochemically, this leucogranite exhibits high silica contents (SiO2 = 72%), a strongly
(a) (b)
Fig. 1 – (a) Simplified geological map of the Bemposta migmatite Complex, showing the location of the Faia d’Água Alta leucogranite. Modified from Dias da Silva, (2013); (b) Faia d’Água Alta leucogranite.
peraluminous character (A/CNK = 1.24) and relatively high Ba, Rb, Zr and REE abundances (Ba = 291 ppm; Rb = 321 ppm; Zr = 203 ppm; REE = 229 ppm), suggesting a provenance from metasedimentary / metaigneous crustal sources.
3. Analytical techniques
After crushing and separation, zircons extracted from sample FD97 were imaged by cathodoluminescence (CL). U-Pb isotopic ratios were measured by SHRIMP at Centro de Pesquisas Geocronológicas (CPGeo) of the University of São Paulo (Brasil), following the analytical procedures described by Sato et al. (2014). U-Pb ratios were calibrated against the TEMORA reference zircon. Data reduction was performed using appropriate software and plotted via Isoplot 3.0. (Ludwig, 2003). Additional trace element data on zircons were obtained by LA-ICP-MS, at the Laboratory of Chemistry of the Institute of Geosciences of the University of São Paulo (Brazil).
4. SHRIMP zircon age data
Zircon grains from sample FD97 are mostly prismatic and frequently display oscillatory zoning. U and Th contents range from 1070-3184 ppm and 130-1342 ppm, respectively, and Th/U ratios are higher than 0.1 (Th/U = 0.11-1.17) indicating an igneous origin.
From the 16 analysed zircon spots, only two yielded pre-Variscan 206Pb/238U
concordant ages (507 ± 7 Ma; 432 ± 10 Ma). The 507 Ma age may represent Cambrian inherited / xenocrystic components, while the 432 Ma age can correspond to a mixing age.
The other 14 analyses gave highly scattered subconcordant 206Pb/238U ages
(disc. < 10%), varying between 377 ± 3 Ma and 295 ± 6 Ma and two younger ages at 230 ± 11 Ma and 223 ± 9 Ma, which may reflect Pb loss (Fig. 2a).
Five of these analyses show broadly coherent 206Pb/238U ages and define a
Concordia age of 324 ± 3 Ma (2σ) with a MSWD=0,046 (Fig. 2b), representing the crystallization age of the leucogranite. The zircon spots with ages older than 324 Ma (377-365 Ma) probably represent mixed
ages between different zircon populations, whilst the younger ages may reflect Pb loss along Concordia, due to post-crystallization perturbing events.
As the Faia d’Água Alta granite corresponds to a syn-D2 intrusion, its
crystallization age provides a reasonable estimate for the age of the D2 anatectic
event.
The good agreement between the 324 ± 3 Ma crystallization age obtained in this study and the ID-TIMS U-Pb zircon ages reported for migmatites from adjacent areas (325– 320 Ma; Valverde-Vaquero et al. 2007) supports such an assumption.
Fig. 2 – (a) Distribution of 206Pb/238U ages in zircon grains from sample FD97; (b) Concordia diagram for
five zircon spots from sample FD-97
5. Ti-in-zircon thermometry
The Ti-in-zircon geothermometer (Ferry & Watson, 2007) was used for estimating the minimum temperatures of zircon crystallization. Some of the zircons have anomalously high Ti contents (Ti > 25 ppm), possibly due to presence of ilmenite inclusions, and were not considered in the calculations.
102 The Ti-temperatures for the remaining grains range from 761°C to 812°C, suggesting crystallization from melts formed under fluid-absent conditions.
6. Zircon REE chemistry
In addition to ilmenite, accidental sampling of REE-bearing submicroscopic mineral inclusions (including monazite and apatite) and/or metamictization may have been responsible for the high REE abundances, particularly LREE, found in several zircon grains from sample FD-97 (not discussed here).
Excluding these anomalous analyses, the igneous zircon from the Faia d’Água Alta leucogranite show total REE contents ranging from 194 to 1102 ppm and chondrite-normalized REE patterns characterized by LREE depletion, pronounced Eu negative anomalies (Eu/Eu* = 0.09–0.16) and flat MREE - HREE profiles (LuN/DyN = 1.2-1.9) (Fig. 3).
The lack of significant HREE enrichment in the targeted grains strongly suggests that zircon growth occurred in the presence of HREE-sequestering minerals (peritectic garnet), under granulite facies conditions. Because garnet is very scarce in the host migmatites, it is likely that the anatectic leucogranite melt was injected in the BMC from slightly deeper crustal levels.
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 10 100 1000 10000 R o c k /c h o n d ri te
Fig. 3 – Chondrite-normalized REE patterns of zircons from sample FD97. Normalizing values from
McDonough & Sun (1995)
7. Conclusions
The new SHRIMP U-Pb zircon age obtained in this study (324 ± 3 Ma) provides reliable time constraints for granite
crystallization and the age of the D2
anatectic event in the BMC.
The results of Ti-in-zircon thermometry point to leucogranite crystallization from an evolving melt at temperatures of 761- 812°C, while zircon REE chemistry appears to indicate synchronous growth with peritectic garnet, implying that the Faia d’Água Alta leucogranite may represent an injection vein.
Acknowledgements
This work received financial support from GeoBioTec
(UID/GEO/04035/2019) sponsored by FCT
(Portuguese Foundation of Science and Technology).
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