Experimental, geochemical, mineralogical and O-isotope constraints on the late-magmatic history of the Fichtelgebirge granites (Germany)

Abstract

Mineralogical, experimental, geochemical and stable-isotope data give evidence for an aqueous late-stage fluid which develops increasingly from the less to the more highly evolved granites of the Fichtelgebirge (G1-G4). In less-evolved granites (particularly G1) only minor chloritization and sericitization, minor hydration and corrosion of zircons, low leachable fractions of Sr and Zr, the nearly chondritic ratios of Zr/Hf and Y/Ho, and rare-earth-element (REE) patterns lacking the tetrad effect provide evidence of a less-differentiated granitic melt system, poor in complexing agents and water, with insignificant late magmatic fluid-rock interaction. The subsequent low-temperature alteration, which is suggested by oxygen-isotope composition of quartz and biotite, did not affect the whole-rock chemical composition. In the highly evolved granites (particularly G4), albitization, sericitization and fluoritization, ubiquitous hydration and corrosion of zircons, high leachable fractions of Sr and Zr, Zr/Hf and Y/Ho ratios very different from the chondritic ratios, and REE patterns showing a marked tetrad effect point to the presence of a highly specialized fluid system, rich in water and complexing agents, such as fluorine, leading to a marked magmatic-hydrothermal alteration. The oxygen isotopes also indicate alteration by high-temperature fluids which were likely exsolved during the final stage of crystallization.

Zircon U-Pb SHRIMP ages of the Taiping (calc-alkaline)-Huangshan (alkaline) composite intrusion: Constraints on Mesozoic lithospheric thinning of the southeastern Yangtze Craton, China

“…The Huangshan intrusion exhibits a strong "tetrad effect" of REE distribution patterns ( Figure 2(e)), which is infrequent partition model of REE, only seen in highly evolved magmatic differentiates with strong hydrothermal interactions [35][36][37][38] or in non-hydrothermal settings in the deep crust [39] , and can be interpreted as transitional between the end-members of magmatic and high-temperature hydrothermal systems [35,38] or in the setting has undergone strong fractionation of plagioclase [39] . Corresponding to the "tetrad effect" of REE distribution patterns, behaviors of many trace elements of the Huangshan granite also show obvious aberrance, termed by Bau [35] as non-CHARAC behavior: K/Rb ratios are usually low (except one or two samples at about 100, but most are less than 65, while K/Rb ratios of ordinary granitoid are usually greater than 150), high K/Ba ratios (among 265-2384, while K/Ba ratios of ordinary continental rocks are less than 50), low Zr/Hf ratios (only 27.5-13.6, while Zr/Hf ratios of most continental rocks are 38 ± 2).…”

Mineralogical and geochemical characteristics of hydrothermal alteration and episyenitization in the Königshain granites, northern Bohemian Massif, Germany

“…8). Such trace element behaviour as shown also by highly fractionated granites of the Fichtelgebirge was attributed to late-magmatic fractionation processes by deuteric fluids rich in strongly complexing agents such as F (Irber et al 1997). In the equigranular Königshain granite fluorine may be responsible for the observed tetrad effect and Zr/Hf fractionation.…”

Paired uraninite and molybdenite dating of the Königshain granite: implications for the onset of late-Variscan magmatism in the Lausitz Block

“…There are several possible reasons for the discrepancy in our age results compared to previous work. As typical for evolved granites from the STZ (e.g., Irber et al 1997, Förster et al 1999, Breiter et al 2006 Molybdenite mineral separates created at AIRIE from quarry hand sample; maximum silicate dilution is 10%…”