THE COMPOSITION OF ZIRCON IN THE PERALUMINOUS HERCYNIAN GRANITES OF THE SPANISH CENTRAL SYSTEM BATHOLITH

Abstract

We have investigated the zircon from granites of the Spanish Central System (SCS) batholith. This batholith is composed of plutons of both I- and S-type granite, with different degrees of fractionation. The association provides a natural laboratory for the study of compositional evolution of zircon. We recognize two types of magmatic zircon. Type-1 zircon consists of euhedral and elongate crystals showing an oscillatory zoning, usually hosted in the early major minerals, being more abundant in granodiorite and monzogranite plutons. Type-2 zircon is characterized by smaller and more equant crystals, unzoned, or irregularly zoned, and usually interstitial to the rock-forming minerals, and forming clusters with other accessory minerals. This second type is more abundant in leucogranites, aplites and pegmatites. Type-2 zircon shows the highest concentrations of Hf (16.58 wt.% HfO 2 ), U (4.83% UO 2 ), Th (4.87% ThO 2 ), Y (8.51% Y 2 O 3 ), and HREE (3.78% HREE 2 O 3 ), and the lowest analytical totals (down to 91.2%). The replacement of Zr by tetravalent cations (Th, U and Hf) is favored in the most evolved granites. Micro-areas of zircon with low analytical totals (91–98 wt.%) have a high level of cation substitution. Moreover, their high Th–U contents induce elevated levels of radiation damage. Disturbances in the original structure, combined with severe metamictization of U–Th-rich micro-areas, may have caused an enhanced susceptibility to secondary alteration. This process, in turn, is believed to have formed the observed strongly damaged and cation-deficient zircon. The range of Zr/Hf values in zircon crystals of the granites studied and those of related metamorphic rocks (as possible crustal sources) are similar. The composition of zircon from SCS granulite xenoliths overlaps with the compositional field of zircon from granite, supporting previous models of crustal recycling for the origin of the SCS batholith.