Extreme fractionation in a granite–pegmatite system documented by quartz chemistry: The case study of Tres Arroyos (Central Iberian Zone, Spain)

Abstract

The Tres Arroyos granite–pegmatite system is located in the SW margin of the Nisa-Alburquerque Variscan batholith. Two granitic facies (monzogranite and marginal leucogranite) and three types of aplite–pegmatite dykes (barren, intermediate and highly evolved Li-rich), have been distinguished in the area, with a zoned distribution from the granite southwards. Trace elements in quartz from the five facies have been analysed by LA-ICP-MS in order to obtain information about the petrogenetic links among the different lithologies of this system, as well as to better understand the regional and individual fractionation processes that led to the distinct rocks. Aluminium, Ti, Li and Ge show continuous trends from the monzogranite, through the marginal granitic facies, the barren and intermediate aplite–pegmatites, up to the most evolved Li-rich dykes. Titanium and Ge contents, respectively, decrease and increase gradually with fractionation. In contrast, Al and Li show a more complex trend, with an initial descending trend to the marginal granitic facies, and then showing the highest Al and Li contents in the quartz from the most fractionated Li-rich aplite–pegmatites. This suggests the influence of different competing factors controlling the incorporation of these trace elements in quartz, such as the chemical composition of the melt, the P and T conditions and the rate of crystallization. Based on the good correlation between Al and Li, the substitution Si4+↔Al3++Li+ seems to be the dominant mechanism of Li incorporation into quartz. The negligible amount of other trace elements suggests that the remaining Al was mainly compensated with H+ ions, via the Si4+↔Al3+ + H+ substitution.A continuous fractionation trend from the monzogranite up to the most fractionated aplite–pegmatites is inferred from geochemical modelling by applying the Rayleigh equation for fractional crystallization. Fractionation rates over 50% are needed to obtain the marginal granite and the barren aplite–pegmatites compositions, and over 99% for the most evolved dykes. No pattern in the chemical variation of the trace elements in quartz from different layers in the layered aplite–pegmatites has been found, suggesting the lack of internal fractionation processes, most probably due to the rapid crystallization of the pegmatitic melt, intruded into a colder country rock.