Paleofluids circulation associated with the Gerês late-orogenic granitic massif, northern Portugal

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• 作者：L. Jaques^a ; ^{luis.jaques@gmail.com} ; F. Noronha^b ; N. Liewig^c ; I. Bobos^b
• 关键词：Episyenitization ; Albitization ; Hydrothermal alteration ; Fluid inclusions ; Oxygen isotopes ; K-Ar dating ; Gerê ; s granites ; Portugal
• 刊名：Chemie der Erde - Geochemistry
• 出版年：2016
• 出版时间：December 2016
• 年：2016
• 卷：76
• 期：4
• 页码：659-676
• 全文大小：6179 K
• 卷排序：76

文摘

The Hercynian late-orogenic granites from the Gerês massif, northern Portugal, underwent intense hydrothermal activity along tectonic structures striking N-S and NE-SW. The first hydrothermal stage is characterized by the albitization of feldspars (primary K-feldspar and plagioclase) followed by the dissolution of magmatic quartz, the chloritization of biotite, and the muscovitization of magmatic biotite and feldspars and occasionally of authigenic albite. Whole-rock geochemistry shows a decrease of SiO2, K2O and Rb and an increase of Na2O, Al2O3 and Sr amounts during the albitization. The second hydrothermal stage is characterized by a mineral assemblage consisting of secondary quartz, albite, chlorite, hematite, apatite, muscovite, epidote, sphene and carbonates, which infilled the cavities produced by the early quartz leaching.Mass balance calculations demonstrate a noticeable element mobilization during the granite alteration. The average ∑REE of the Gerês granite is nearly constant at 154 ppm, but ranges from 91 to 163 ppm in the altered rocks. A relative LREE depletion and a slight HREE enrichment associated with a negative Eu anomaly characterize the feldspathization process of Gerês granite. The average ∑REE of the Carris granite is at 159 ppm but in the altered epidote + chlorite + hematite assemblage it reaches about 201 ppm with a slight HREE increase.Two different fluids were involved in the hydrothermal alteration. A first aqueous fluid (Lw1), with a low to intermediate salinity (<10 wt.% eq. NaCl) circulated along the main structures (N-S, NE-SW and NW-SE), characterized by entrapment temperature lower than 350 °C and maximum pressure of 115 MPa followed by a later colder and more saline fluid (Lw2) under a temperature of 220 °C and a pressure of about 27 MPa.Oxygen isotope data obtained on magmatic quartz indicate δ18Oquartz of + 9.3 and + 11.0‰, pointing to an interaction of magmatic fluids with others of meteoric origin. For the secondary quartz, δ18Owater of +0.5 and +0.7‰ were calculated for a temperature of 250 °C, suggesting a meteoric fluid with a possible seawater signature.The K-Ar data of K-feldspar from the altered rocks confirm an age of 273.6 ± 11.7 Ma, attributed to the first alteration process. Younger K-Ar ages between 155.8 ± 6.7 Ma and 124 ± 5.3 Ma were also obtained in the feldspathized rocks, confirming the late hydrothermal activity.Albitization and quartz dissolution of granitic rocks from the Gerês massif occurred at depths shallower than 5 km, induced by the circulation of fluids along brittle structures, during the orogenic uplift and extensional tectonics which affected the Iberian Massif in the Early Permian. A second major event is attributed to late hydrothermal circulations of aqueous brines until a depth of about 3 km, presumably derived from interaction with sub-surface evaporites throughout Late Jurassic to Early Cretaceous. These late hydrothermal events probably reflect the rifting episodes and the rising of geothermal gradient, associated with the opening of the Atlantic Ocean and Gulf of Biscay, respectively.