Geochemical and isotopic constraints on the petrogenesis of Early Ordovician granodiorite and Variscan two-mica granites from the Gouveia area, central Portugal

摘要

A biotite granodiorite and seven Sn-bearing two-mica granites crop out in the Gouveia area, central Portugal. A SHRIMP U-Th-Pb zircon age from the granodiorite, and monazite ages from four of the two-mica granites, show that they are of Early Ordovician (～480 Ma) and Permo-Carboniferous, i.e. Variscan (～305 and 290 Ma) age respectively. The Variscan two-mica granites are late- and post-D3. Major and trace element variation in the granitic rocks and their biotite and muscovite indicate mainly individual fractionation trends. The granitic rocks are mostly depleted in HREE relative to LREE. The biotite granodiorite is probably derived from igneous lower crust, as evidenced by low initial ~(87)Sr/~(86)Sr (0.7036), high εNd_T ( + 2.5) and moderate δ~(18)O (8.8‰). The two-mica granites are probably derived by partial melting of heterogeneous mid-crustal metasediments, mainly metapelite and some metagraywacke, as evidenced by their high initial ~(87)Sr/~(86)Sr (0.7076-0.7174), δ~(18)O (10.7-13.4‰) and major element compositions. However, variation diagrams for major and trace elements from two of the muscovite > biotite granites and their micas define fractionation trends. Rb-Sr whole-rock analyses from the two granites are perfectly fitted to a single isochron and the rocks have sub-parallel REE patterns; the younger granite is derived from the older by fractional crystallization of quartz, plagioclase, biotite and ilmenite (tested by modelling major and trace elements). Most of the Sn-bearing granites are derived from distinct magma batches. They result from partial melting of a heterogeneous mid-crustal metasediment. They do not represent a crustal anomaly in tin. Fractional crystallization is responsible for the increase in the Sn contents of the granites and their micas. Muscovite has a higher Sn content than coexisting biotite and is the principal host mineral for Sn in these rocks.