REE partition among zircon, orthopyroxene, amphibole and garnet in a high-grade metabasic system

Abstract

A mafic amphibole-bearing granulite with porphyroblastic garnet was investigated to evaluate: (1) the rare earth element (REE) partition among garnet, zircon, orthopyroxene and amphibole during the metamorphic evolution; (2) the significance of the REE distribution along lobes and bights of reabsorbed garnet rim; and (3) REE distribution coefficient values (DREE) suggestive of chemical equilibrium, assuming garnet as a reference. The results have been compared with those deriving from an intermediate granulite containing porphyroblastic garnet, without amphibole. Porphyroblastic garnet from both samples is rimmed by a continuous corona formed during post-peak decompression characterized by REE-enriched lobes and REE-poor bights. The amphiboles from corona have various REE abundances, reflecting a different dissolution rate of original garnet rim. The initial slow rate of garnet dissolution caused high REE concentration in the new garnet rim due to intra-crystalline diffusion, leading to the formation of REE-poorer amphiboles in corona. Subsequently, under an increasing geothermal gradient and fluid-present conditions, the faster dissolution of garnet determined the formation of bights and the transfer of REEs towards the corona. The timing of garnet growth and its dissolution were checked by U–Pb zircon ages. The zircons dated from 339 Ma to 303 Ma in two rock types combined with the garnet domains (core, outer core, rim) show similar distribution of patterns relative to heavy rare earth elements for zircon and garnet (DHREEzrn/grt), suggesting chemical equilibrium. Zircons dated at c. 300 Ma do not appear in equilibrium with REE-rich garnet lobes, and younger zircons (278 Ma) show a new equilibrium with REE-poor garnet bights. On this basis, the DHREEamph/grt values obtained in specific textural sites might be interpreted as suggestive of equilibrium under granulite conditions.

Geochemical Characterization of Zircon in Fyfe Hills of the Napier Complex, East Antarctica

“…The C1-chondrite-normalized REE patterns obtained from Group II are characterized by a weak fractionation between MREE and HREE, namely lower HREE/MREE (Figures 7 and 8). The REE patterns with flat MREE-HREE are found in the zircon that grows or is modified in equilibrium with garnets [13,[15][16][17]. The quartzo-feldspathic gneiss (YH05021606A) contains porphyroblastic garnet (Figure 2), and the REE patterns of Group II reflect the zircon overgrowth or recrystallization under high temperature and in equilibrium with garnet.…”

“…Minerals 2020, 10, x FOR PEER REVIEW 2 of 16 metamorphic events [15][16][17]. Oxygen isotopes in zircon also provide useful information about magmatic evolution, genesis of magmas, isotope exchange between zircon and surrounding minerals during metamorphism, hydrothermal alteration, etc.…”

“…Zircon, which is one of the most common minerals used for U-Th-Pb isotope dating, also provides an opportunity for integration between age and geological processes. The partitioning of heavy rare earth elements (HREE) between zircon and garnet has been considered as the most robust method for evaluating the relationship between zircon and garnet during metamorphic events [15][16][17]. Oxygen isotopes in zircon also provide useful information about magmatic evolution, genesis of magmas, isotope exchange between zircon and surrounding minerals during metamorphism, hydrothermal alteration, etc.…”

Recognising Mineral Deposits from Cover; A Case Study Using Zircon Chemistry in the Gawler Craton, South Australia

“…As feldspars are a major mineral phase, their presence or absence may potentially be visually assessed if there is uncertainty in Eu/Eu* ratio results and followed up with feldspar mineral chemistry if needed. Heavy REEs can be enriched in several metamorphic mineral phases including orthopyroxene, amphibole, and garnet (e.g., [62,63]), and also in the common accessory phase xenotime (YPO 4 [64]). The REE pattern in zircon can commonly be influenced by the cocrystallisation of these mineral phases (e.g., orthopyroxene, amphibole, garnet, and xenotime).…”

Building an Orogen: Review of U-Pb Zircon Ages from the Calabria–Peloritani Terrane to Constrain the Timing of the Southern Variscan Belt

“…Indeed, zircons with flat HREE pattern and Th/U ratio < 0.1 could be indicative of metamorphic growth synchronous with HREE-rich garnet competitor. However, a careful assessment of mineral parageneses (such as garnet and amphibole) coexisting with zircon at P-T conditions of amphibolite-granulite and eclogite facies [9,10], may help to constrain the P-T-t trajectories and to delineate the evolution of poly-metamorphic terranes [6,8,[11][12][13][14][15][16]. Apart the growth from melt, also the interaction with a fluid phase under low grade metamorphic conditions might produce a resetting of U-Pb system of zircons [7].…”