Diverse source materials contributed to a secular increase in δ18O for the Paleo-Mesoproterozoic A2-type granites

• Published in: Earth and planetary science letters Vol. 642; p. 118885
• Main Authors: Lu, Gui-Mei, Xu, Yi-Gang, Wang, Wei, Spencer, Christopher J., Roberts, Nick M.W., Condie, Kent C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.09.2024
• Subjects: A2-type granites; Collisional orogen; Great Proterozoic Accretionary Orogen; Paleo-Mesoproterozoic; Yangtze Block; Paleo-Mesoproterozoic; A2-type granites; Great Proterozoic Accretionary Orogen; Collisional orogen; Yangtze Block
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2024.118885

•2.25 Ga A2-type granites are identified in the SW Yangtze Block.•The Paleo-Mesoproterozoic A2-type granites yield a secular increase in δ18O.•Diverse high-δ18O materials contribute to this secular increase. The elevation of δ18O in zircon through Earth history has been previously identified as being coeval with the assembly of post-Archean supercontinents. This was interpreted to result from the increasing incorporation of high-δ18O sediments into magmas due to enhanced crustal thickening and reworking during supercontinent assembly. However, little attention has been paid to the variations of δ18O in igneous rocks during the extensional phases of the supercontinent cycle. Given the scarcity of early Paleoproterozoic granites due to the ca. 2.3–2.2 Ga tectono-magmatic lull, we first report a suite of ca. 2.25 Ga granites from the SW Yangtze Block by providing zircon U-Pb-Hf-O isotopes, whole-rock geochemical, and Sm-Nd isotopic analyses to constrain their petrogenesis. The results demonstrate that they are A2-type granite and originated from the partial melting of Meso-Neoarchean igneous crust in a post-collision extensional setting. Integrating these data with a global compilation of Paleo-Mesoproterozoic A2-type granites allows us to further explore secular changes in elemental and isotopic compositions of these granite types. The A2-type granites in this study exhibit mantle-like zircon δ18O values, in accord with those from the compiled pre-1.85 Ga A2-type granites. Additionally, the data show a secular increase in zircon δ18O for the compiled Paleo-Mesoproterozoic A2-type granites. Similar increasing trends are also observed in various compositional proxies that imply the increasing addition of mafic igneous rocks into magma sources that are likely associated with the back-arc extensional settings of the Great Proterozoic Accretionary Orogen (GPAO). Considering that the compiled post-1.8 Ga A2-type granites have depleted Nd-Hf isotopes, we posit that they were mainly derived from mafic rocks with arc-like geochemical compositions. In this context, the secular increase in zircon δ18O for the post-1.8 Ga A2-type granites might reflect the increasing assimilation of altered oceanic crust into the back-arc and inboard magmatic systems of retreating accretionary orogens. In contrast, the compiled ca. 1.85 Ga A2-type granites possess non-radiogenic Hf and elevated O isotopes, indicative of the incorporation of ancient sediments in their magma sources. We suggest that they are most likely generated in a post-collision extensional setting associated with the very early stages of assembly of the Columbia/Nuna supercontinent.