Morphological and trace element study on zircons from a metasomatic xenolith (Hyblean Plateau, SE Sicily)

Rare chlorite-alkali feldspar xenoliths of metasomatic origin occur among the crustal xenolith suite of the Hyblean Plateau (SE Sicily). These rocks consist of fractured alkali feldspar porphyroclasts sit in a pseudo-fluidal matrix of alkali-feldspar neoblasts, chlorites and other clay minerals. The alkali feldspar, exhibiting chessboard extinction and a low-temperature structural state, consists of albite crosscut by a complex set of K-feldspar veinlets. Abundant zircon grains occur in the phyllosilicate matrix. These rocks have SiO2 = 55-58 wt%, low MgO and CaO contents (1.5-2.7 and ~1.3 wt% respectively), P2O5 below detection limits, high alkali contents (Na2O ~ 4.6 wt%; K2O = 3.4-4.7 wt%). Loss on Ignition (LOI) is relatively high (7-10 wt%) and reflect the high modal abundance of hydrated phases. Zr (511-639 ppm) contents reflect the modal abundance of zircon. Ni content is 12-61 ppm, Cr is below detection limits. The spoon-shaped REE patterns and the oxygen isotope ratios ( 18OSMOW = 14-15 ‰) are compatible with the presence of secondary hydrated phases. Backscattered electron-cathodoluminescence (BSE/CL) images of a hundred twenty-one zircon grains from sample FBZ have been hand-picked under binocular microscope to look for its morphology and internal structure. U and Th data are available for sixty-six grains, while the LAM-ICPMS trace element analyses were performed on eight grains. The external morphology, internal texture and geochemical features allow the recognition of three zircon types. Type 1 zircons are reddish and show near euhedral shapes and weak zoning with REE distribution (LaN < CeN, HREE > LREE; Ce/Ce* > 1 with Ce* = CeN/(LaN+PrN)/2) compatible with the igneous nature. Type 2 zircons are opaque and whitish, elongated and ovoid-shaped. These zircons are internally structureless. REE patterns (spoon-shaped from Ce to Gd, HREE > LREE) suggest that some of them may have suffered at least partial hydrothermal alteration. Th/U ratio in type 1 and 2 zircons ranges from 0.2 to 1.66. Type 3 zircons are turbid opaque sugar-like anhedral fragments with different colors (brownish, pinkish and purplish). These grains are fractured and pitted, with variable degrees of porosity due to abundant tiny milky inclusions (spongy texture). The REE distribution (similar to type 2 zircons), the spongy texture and anomalously high Th/U ratio (up to ~ 34) of type 3 zircons may indicate a hydrothermal origin. Type 1 zircons are considered to represent igneous relics, while type 2 zircons probably are partially altered igneous zircons. Geochemical features of type 3 zircons (namely REE patterns, Th/U ratio) suggest their derivation through hydrothermal processes with the active participation of a F-rich aqueous fluid, which can carry Th and U (and maybe Zr). The hydrothermal nature of type 3 zircons is in agreement with the geochemical and petrographic features of the whole rock.