Abstract

To reveal the Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block, a petrological and geochemical study was carried out for the mafic intrusions and associated granitoids from the Hannan and Xiaomoling Complexes. The ca. 900–880 Ma Xiaomoling diorite and granodiorite have high K2O contents (2.29–3.57 wt%) and belong to the high-K calc-alkaline series. They display depleted whole-rock Sr–Nd–Hf isotope compositions (0.70151–0.70398, +0.6 to +2.5 and +4.4 to +11.2, respectively) and positive zircon εHf(t) (+3.6 to +9.9) with slightly elevated bulk zircon δ18O values (6.4–6.8 ‰) and minor Paleoproterozoic (∼2.2 Ga) inherited zircons, indicative of derivation from remelting of juvenile mafic crust with crustal contamination in a continental arc setting. The ca. 866 Ma Hannan gabbro-diorites show enrichments in LILEs and LREEs and depletions in HFSEs and display depleted Sr–Nd–Hf isotope compositions (0.70267, +5.4 and +17.8, respectively), positive zircon εHf(t) values (+8.6 to +12.0) and mantle-like zircon δ18O values (5.0–6.2 ‰), suggesting that they were generated through flux melting of a depleted mantle wedge. The ca. 850–840 Ma Xiaomoling hornblende gabbro and diabase display depleted Sr–Nd–Hf isotope compositions (0.70279–0.70288, +2.7 to +3.2 and +9.8 to +10.5, respectively) and are characterized by high Nb/U ratios and less enrichment in LREEs and LILEs, indicating that they were formed by decompression melting of a depleted mantle wedge in a back-arc setting. However, the coeval high-Mg diorite displays enriched whole-rock Sr–Nd–Hf isotopes (0.70477–0.70701, −4.0 to −6.0 and −7.2 to −7.4, respectively), negative zircon εHf(t) values (−1.6 to −9.2) and elevated zircon δ18O values (5.3–7.5 ‰), reflecting that the mantle-derived melts have underwent various degrees of crustal contamination. The ca. 780 Ma Hannan gabbro is characterized by earlier crystallization of plagioclase than clinopyroxene, implying that their parental magma is anhydrous. Additionally, the relatively shallow emplacement depth and tholeiitic affinities imply that they were generated by mantle decompression melting probably associated with a continental rift environment in response to Rodinia breakup. Taken together, the secular changes in the geochemical and isotopic compositions of these mafic rocks in the northwestern margin of the Yangtze Block record the evolution from a continental arc at ca. 900–880 Ma to continental rifting at ca. 780 Ma.

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