How do granitoid magmas mix with each other? Insights from textures, trace element and Sr–Nd isotopic composition of apatite and titanite from the Matok pluton (South Africa)

Abstract

International audienceIn plutonic systems, magma mixing is often modelled by mass balance based on whole-rock geochemistry. However, magma mixing is a chaotic process and chemical equilibration is controlled by non-linear diffusive–advective processes unresolved by the study of bulk samples. Here we present textural observations, LA-(MC-)ICP-MS trace element and Sr–Nd isotopic data of accessory apatites and titanites from a hybrid granodiorite of the Neoarchean Matok pluton (South Africa), collected in a zone of conspicuous mixing between mafic and felsic magmas. Apatite grains mostly show a pronounced zoning in CL images, corresponding to abrupt changes in REE and HFSE concentrations recording their transfer through compositionally different melt domains during mixing. These grains crystallized early, at temperatures of 950–1000 °C. Titanite grains crystallized at temperatures of 820–900 °C (Zr-in-sphene thermometry). They show limited intra-grain chemical variations but huge inter-grain compositional scatter in REE and HFSE, pinpointing crystallization within a crystal mush, from isolated melt pockets having different composition from one another owing to incomplete chemical homogenization and variable Rayleigh fractionation. These chemical–textural characteristics, in combination with partitioning models and Polytopic Vector Analysis, point to “self-mixing” between co-genetic dioritic and granodioritic/granitic magmas. Both resulted from differentiation of mantle-derived mafic melts, showing that mixing does not necessarily involve magmas from contrasted (crust vs. mantle) sources. Systematic variations in εNdt (−4.5 to −2.5) and 87Sr/86Sr(i) (0.703–0.707) of titanite and apatite grains/domains crystallized from the two magmas point to an isotopically inhomogeneous mantle source, which is not resolved by bulk-rock isotopic data. Interaction between the two magmas must have occurred at relatively high temperatures (ca. 900°C) so that their viscosity contrast remained low, allowing efficient mechanical mixing. Despite this, chemical homogenization was incomplete, as recorded by diffusive fractionation between REE–HFSE and Sr. Modelling thereof reveals that chemical exchange between the liquid phases of the two mixed magmas did not last more than a few tens to hundreds of years. The chemical equilibration between mixed magmas thus strongly depends on the considered elements and observational length scales

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

“…The populations of more mafic affinity could have crystallized earlier, when the mafic melt was not yet mixed with granitic magma, whereas the titanite of more uniform composition grew in more or less homogenized melt pockets (which could still slightly differ in chemical composition) located between already crystallized mineral assemblages. Similar scenario for titanite crystallization was documented by Laurent et al (2017). Additionally, in comparison to apatite and feldspar, titanite demonstrates decreased heterogeneity and almost exclusively mixed signature (more or less homogenized), whereas apatite and feldspar from granodiorites bear both mixed and pure mantle-or crust-related signature (Słaby et al 2007a(Słaby et al , 2007bLisowiec et al 2015).…”

“…Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00710-020-00715-x) contains supplementary material, which is available to authorized users. elemental variations in titanite may serve as useful petrogenetic indicators, unravelling otherwise hidden details of the magmatic evolution (Tiepolo et al 2002;Bruand et al 2014;Piccoli et al 2000;Frost et al 2000;Xu et al 2015;Hu et al 2017;Jiang et al 2016;Laurent et al 2017;Xie et al 2010Xie et al , 2018. Crystal zoning usually exhibited by titanite in various formsoscillatory, core-mantle-rim, patchy, sector, can be used to decipher complex crystallization histories (e.g.…”

“…Due to the capability of recording even subtle changes in magmatic environment, titanite has been widely used in the studies of magmatic systems formed by mixing of melts of various origin and composition (e.g. McLeod et al 2011;Jiang et al 2016;Xie et al 2018;Hu et al 2017;Laurent et al 2017). 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.…”

Ambiguous isotopic and geochemical signatures resulting from limited melt interactions in a seemingly composite pluton: a case study from the Strzegom–Sobótka Massif (Sudetes, Poland)

“…Whole-rock analyses provide average information from signals (partly overlapping) of all subsequent processes and are mostly used as a base for geochemical modelling of the mechanisms of magma differentiation. Single domains of minerals may preserve evidence of different differentiation mechanisms (Słaby et al 2007a(Słaby et al , b, 2008(Słaby et al , 2012McLeod et al 2010;Miles et al 2013;Michel et al 2016;Laurent et al 2017). The combination of both sets of data may be helpful in resolving a problem regarding the real nature of a pluton that is characterized as an apparently composite pluton.…”

Numerical simulations of magmatic enclave deformation

“…More recently, deformation studies have used mixtures with bulk non-Newtonian rheologies (e.g., Treagus, 2002;Jessell et al, 2009;Marques et al, 2014, and references therein). In hypersolidus conditions, which are relevant to magmatic systems, modeling enclave deformations by using the analogy of two fluids has also been used (e.g., Williams and Tobisch, 1994;Caricchi et al, 2012;Laurent et al, 2017). Magmas, however, are inherently multiphase mixtures of melt, crystals and possibly gas bubbles.…”