The Fate of Accessory Minerals and Key Trace Elements During Anatexis and Magma Extraction

Abstract

Granite genesis and crustal evolution are closely associated with partial melting in the lower or middle crust and extraction of granite magmas to upper crustal levels. This is generally thought to be the leading mechanism by which the upper continental crust became enriched in incompatible components such as the heat-producing elements U and Th through time. However, field evidence from anatectic terrains, the source rocks of granite magmas, raises doubt about the efficiency of this process. Leucosomes and associated leucogranites, representative of melts in such terrains, are often depleted in U, Th and REE compared to their source and therefore unable to enrich the upper crust in these elements. This paper demonstrates using anatectic turbidites exposed on Kangaroo Island that accessory minerals, the main hosts of U, Th and REE, become preferentially concentrated in the melanosomes, effectively removing these elements from the melt. Whole rock geochemistry and detailed petrography suggests that (1) peraluminous melts dissolve only small fractions of monazite and xenotime, because efficient apatite dissolution saturates melt early in phosphorous; and (2) local melt–host reaction emerging from melt migration may cause melt to crystallize in the magma extraction channelways in or close to the magma source region. Crystallization causes oversaturation of the magma triggering crystallization and capture of accessory minerals in the growing biotite-rich selvedge rather than in the melt channel itself. Crystallization of accessory minerals away from the leucosome explains the apparent under-saturation of elements hosted by these accessory minerals in the leucosome and leucogranites. While intense reworking of thick piles of turbidites, common in accretionary orogens, reflect important processes of crustal formation, the fate of accessory phases and the key elements they control, such as the heat producing elements U and Th, are strongly dependent on the interaction between melt and surrounding solids during segregation and extraction.

Significance of selective crystal entrainment and differential crystal‐melt separation in petrogenesis of granites from the Tongbai orogen

“…The compositions of typical leucogranites, experimental melts, and natural melt inclusions produced by biotite dehydration melting based on metasedimentary rocks and gneisses at similar pressures are also plot for comparison. Data sources for leucogranite: LI 2017–Li et al, 2017; SO 2013–Sola et al, 2013; SC 2020–Schwindinger & Weinberg, 2020; YA 2019–Yang et al, 2019; LIU 2020–Liu et al, 2020; for experimental melt: PB 1995–Patiňo Douce & Beard, 1995; PB 1996–Patiňo Douce & Beard, 1996; CA 2000–Castro et al, 2000; PJ 1991–Patiňo Douce & Johnston, 1991; and for natural melts: BA 2016– From the collected data of Bartoli et al, 2016; AC 2016–Acosta‐Vigil et al, 2016.…”

“…Dyck et al, 2019; Sawyer, 2014; Taylor et al, 2014); (2) in‐source melt (melt segregated from its forming site but still in the source; e.g. Fancello et al, 2018; Schwindinger & Weinberg, 2020; Zhou et al, 2019) and (3) exotic melt or magma (melt or magma produced by other source and moved to the present location; e.g. Jung et al, 1998; Sawyer, 2020).…”

“…The occurrences of cumulates composed of different minerals are widely reported in different migmatite terranes (e.g. Carvalho et al, 2016; Mengel et al, 2001; Sawyer, 2020; Schwindinger & Weinberg, 2020; Sola et al, 2013), suggesting that magma evolution induced by differential melt‐crystal separation is a common in‐source process in the generation of granites.…”

Melting and melt segregation processes controlling granitic melt composition

“…10a), because of the low content of these elements in major mineral phases (Table A6). We therefore focus on the fate of accessory phases, such as zircon (Y and HREE), monazite (Th and LREE), xenotime (Y and HREE) and apatite (REE) (Bea, 1996;Ayres and Harris, 1997;Zeng et al, 2005;Schwindinger et al, 2020).…”

“…Continuous dissolution of apatite in the peraluminous melts may cause oversaturation of monazite and xenotime with respect to phosphorus (Wolf and London, 1995;Wolfram et al, 2017;Schwindinger et al, 2020). In the melanosome samples, the rounded apatite grains are commonly surrounded by irregular rims of monazite and xenotime (Fig.…”