Geochemical evolution of a composite pluton: insight from major and trace element chemistry of titanite

Katarzyna Gros,Jiří Sláma,Łukasz Birski,Gabriela Kozub-Budzyń,Ewa Słaby

doi:10.1007/s00710-020-00715-x

Abstract

Titanite from various rocks of the Karkonosze granitoid pluton (South-Eastern Poland) was studied, in order to evaluate its precision in recording magma evolution processes. The rocks are of lamprophyric, dioritic, granodioritic and granitic composition, including hybrid structures such as microgranular magmatic enclaves and composite dykes. Based on textures, chemistry and Zr-in-titanite geothermometry, titanites can be divided into magmatic and post-magmatic populations. Late- to post-magmatic titanite is present in almost all rock types, especially in the most evolved ones (where magmatic titanite is absent) and can be characterized by low trace element and high Al and F contents. Magmatic titanite crystallized in temperatures between 610 and 870 °C, after apatite and relatively simultaneously with amphibole and zircon. Titanite from lamprophyre exhibits compositional features typical of titanites formed in mafic rocks: low Al and F, high Ti4+/(Al + Fe3+), LREE (light rare earth elemet)-enriched chondrite-normalized REE patterns, low Y/Zr, Nb/Zr, Lu/Hf, high (Ce + Nd)/Y, Th/U and Zr. Titanite from hybrid rocks inherited these characteristics, indicating major contribution of the mantle-derived magma especially during early stages of magmatic evolution. Titanite compositional variations, as well as a wide range of crystallization temperatures in hybrid granodiorites point to prolonged crystallization from distinct magma domains of variable mafic versus felsic melt proportions. The extent of compositional variations decreases through subsequent stages of magmatic evolution, and titanite with the least contribution of the mafic component is characterized by higher total REE, Al and F contents, lower Ti4+/(Al + Fe3+), (Ce + Nd)/Y and Th/U ratios, LREE-depleted chondrite-normalized REE patterns and higher Y/Zr, Nb/Zr and Lu/Hf ratios. Titanite composition from the intermediate and late stage hybrids bears signature of decreasing amount of the mafic melt and higher degree of its evolution, however, the exact distinction between the former and the latter is very limited.

Highlights

The magmatic titanite constitutes the majority of LAM2, FOJ, RUD, MIL, MICH and SOK populations, is abundant in the BUK enclave, but very rare in the ENK enclave
The lateto post-magmatic titanite is rare in LAM2, FOJ, RUD, MIL, MICH and SOK samples, common in the BUK enclave and abundant in the ENK enclave
Several important conclusions can be drawn from our study. (i) Textural and chemical characteristics of titanite allowed the distinction between magmatic and late- to post-magmatic titanite

Summary

Introduction

Titanite (CaTiSiO5) is one of the main accessory minerals in granitoid rocks and provides a robust record of magma evolution processes It contains two groups of elements – REE and HFSE (high-field-strength elements) –that are important tracers of a wide range of petrogenetic processes and magma sources. Along with other accessory minerals, titanite controls REE and HFSE budget in the entire magmatic system It may crystallize over a wide range of compositions – from mafic to felsic and is sensitive to changes in oxygen fugacity (fO2), temperature, pressure, crystal composition and melt composition Trace element ratios in titanite commonly used in these studies as magma composition/source discriminators are (Ce + Nd)/ Y, Y/Zr, Nb/Zr, Th/U, Nb/Ta, La/Ce and Lu/Hf. Zr content is used as a geothemometer as Zr incorporation is temperature sensitive (Hayden et al 2008). Ti4+/(Al + Fe3+) ratio is indicative of mantle versus crustal origin (Piuzana et al 2008), while Al/Fe ratio can distinguish magmatic from hydrothermal/metamorphic titanite (Cao et al 2015; Rasmussen et al 2013)

Results

Discussion

Conclusion