Crustal thickening and endogenic oxidation of magmatic sulfur

Abstract

Porphyry ore deposits, Earth’s most important resources of copper, molybdenum, and rhenium, are strongly associated with felsic magmas showing signs of high-pressure differentiation and are usually found in places with thickened crust (>45 kilometers). This pattern is well-known, but unexplained, and remains an outstanding problem in our understanding of porphyry ore deposit formation. We approach this problem by investigating the oxidation state of magmatic sulfur, which controls the behavior of ore-forming metals during magma differentiation and magmatic-hydrothermal transition. We use sulfur in apatite to reconstruct the sulfur oxidation state in the Gangdese batholith, southern Tibet. We find that magma sulfate content increased abruptly after India-Eurasia collision. Apatite sulfur content and the calculated magma S6+/ΣS ratio correlate with whole-rock dysprosium/ytterbium ratio, suggesting that residual garnet, favored in thickened crust, exerts a first-order control on sulfur oxidation in magmatic orogens. Our findings link sulfur oxidation to internal petrogenic processes and imply an intrinsic relationship of magma oxidation with synmagmatic crustal thickening.

Diapir Melting of Subducted Mélange Generating Alkaline Arc Magmatism and Its Implications for Material Recycling at Subduction Zone Settings

“…This may be a favorable condition to produce intermediate melts because thick crusts will suppress mantle wedge melting and enhance intra‐crustal differentiation, corresponding to statistical results that the SiO 2 content for arcs of crustal thicknesses less than 50 km is mafic‐intermediate (48–60 wt.% SiO 2 ; Figure S3 in Supporting Information ; Chen et al., 2020). Furthermore, a low oxygen fugacity of alkaline arc magmas (ΔFMQ < 0; Figure S3 in Supporting Information ) also supports our speculation because thick crusts particularly enhance the differentiation of garnets (absorb much Fe 2+ ), which will result in high oxygen fugacity in melts (Tang et al., 2020). As a result, we suggest that this tectonic transition scenario (oceanic slab roll‐back and later break‐off) may account for both the rapid exhumation of the mélange blocks and buoyant diapirs of the mélange matrix in the final stage of the subduction zone system.…”

Molybdenum and Boron Isotopic Compositions of Porphyry Cu Mineralization‐Related Adakitic Rocks in Central‐Eastern China: New Insights Into their Petrogenesis and Crust‐Mantle Interaction

“…Alternatively, there is a growing evidence suggesting that the high Sr/Y magmas associated with porphyry systems were derived via magma evolution at mid to deep crustal levels, where magmas fractionate amphibole ± garnet but not plagioclase, leading to relative Y depletion and Sr enrichment in the residual melt (Chiaradia et al, 2012; Richards, 2011; Richards & Kerrich, 2007). Recently, Tang et al (2020) proposed that increasing the proportion of residual of garnet in thickened crust (>45 km) can increase sulfur oxidation in magmas, which prevents chalcophile element depletion due to extensive sulfide segregation during crystallization and hence favors mineralization in the shallow crust.…”

Deconstructing plate tectonic reconstructions

“…More subtly, geochemical anomalies preserve information about the thickness of the volcanic arc crust [68][69][70] that can reflect advancing or retreating trenches. Typically, advancing trenches lead to overriding plate compression and thick arc crust and vice versa for retreating trenches.…”