Pure sediment-derived granites in a subduction zone

Abstract

The Earth is unique in the Solar System due to significant volumes of granite in the lithosphere. However, the origins of granites are still highly debated, especially sediment-derived granites, which are often treated as a geochemical end-member of the continental crust. In the Yunnan region of South China, we identify the occurrence of pure sediment-derived granite in a subduction system. The suite of strongly peraluminous granite reported herein is interpreted to represent pure metasedimentary melts based on their whole-rock elemental and Sr-Nd-B and zircon Hf-O isotopic compositions. These Late Permian−Early Triassic (ca. 254−248 Ma) granites are characterized by radiogenically enriched Sr, Nd, and Hf isotopic signatures. They show δ11B and δ18O signatures akin to those of continental shales. Geochemical modeling indicates no contributions from the mantle that can be detected. Considering the regional tectonic evolution, these granites are suggested to be formed in a subduction zone by decompression melting of rapidly exhumed back-arc sediments. We posit that decompression melting was triggered by widespread extension and thinning of the crust prompted by rollback of the subducting oceanic crust. These granites thus provide evidence that granite formation in subduction zones does not necessarily contribute to crustal growth. These subduction-related pure sediment-derived granites have different elemental ratios and contents (e.g., Al2O3/TiO2 and Yb) from the Himalayan leucogranites. Considering their source compositions (e.g., pelitic rocks), which are similar to those of the Himalayan leucogranites, these differences are likely due to their higher formation temperature and lower pressure despite a great similarity in isotopic compositions. Identification of pure sediment-derived, strongly peraluminous granites (SPGs) in subduction systems provides an important geodynamic mechanism for crustal anatexis, which can both geochemically and tectonically complement their collisional counterparts identified in the Himalayas.

Identification of High δ18O Adakite‐Like Granites in SE Tibet: Implication for Diapiric Relamination of Subducted Sediments

“…There is a possibility that the varying Sr‐Nd‐Pb isotopes of the two plutons were originated from heterogeneous source regions. Permian‐Triassic and Cenozoic S‐type granites outcropped along the ASRR shear zone were interpreted to be derived from partial melting of dominantly supracrustal materials (J. Xu et al., 2021; Zhang & Schärer, 1999), which possibly have the same source with our studied samples. Combined with their close space relation with our newly identified granitic plutons, we consider that they could be the reasonable candidate for a sediment endmember at present.…”

“…Data source: subducted oceanic crust‐derived and subducted continental crust‐derived adakitic rocks (Q. Wang et al., 2008 and references therein), ASRR leucogranites (Zhang & Schärer, 1999); Jamaica adakite‐like lavas (Hastie et al., 2010); Alpin‐Himalayan lavas, OIB (oceanic island basalt), MORB (mid‐ocean‐ridge basalt) and arc magmas (compiled by Y. Wang et al., 2021); Paleogene Yunnan shoshonitic rocks (Huang et al., 2010; Lu et al., 2013; L. Xu et al., 2011; L.‐L. Xu et al., 2019) and thickened continental crust‐derived adakitic rocks (Lu et al., 2013; Zhou et al., 2019), Permian‐Triassic S‐type granites (J. Xu et al., 2021). Model parameters used in mixing calculations are listed in Table S4 in Supporting Information .…”

“…Chondrite and primitive mantle normalizing values are from Sun and McDonough (1989). Data source: Hf‐O isotope sedimentary endmember (δ 18 O = 11.7‰; εHf (t) = −11.5) from Permian‐Triassic S‐type granites (J. Xu et al., 2021), depleted mantle‐like O (δ 18 O = 5.3 ± 0.6‰) and Hf (εHf (t) = +12) isotope are from Valley et al. (1998) and Chauvel et al.…”

“…Xu et al, 2011;L.-L. Xu et al, 2019) and thickened continental crust-derived adakitic rocks (Lu et al, 2013;Zhou et al, 2019), Permian-Triassic S-type granites (J. Xu et al, 2021). Model parameters used in mixing calculations are listed in Table S4 in Supporting Information S1.…”

“…; Paleogene Yunnan shoshonitic rocks(Huang et al, 2010;Lu et al, 2013;L. Xu et al, 2011;L.-L. Xu et al, 2019) and thickened continental crust-derived adakitic rocks(Lu et al, 2013;Zhou et al, 2019), Permian-Triassic S-type granites (J Xu et al, 2021)…”