Abstract

One highly debated topic related to the nature of the Cretaceous mantle and geodynamic processes in SE Asia is whether they were dominated by the Tethyan or Pacific Ocean subductions. The main reason for this puzzle is the lack of geochemical studies on mantle-derived mafic rocks in SE Asia. In this paper, we report Early Cretaceous Nb-enriched diabases (ca. 139 Ma) and Late Cretaceous Nb-enriched (ca. 68 Ma) and oceanic-island basalt (OIB)–like (ca. 73 Ma) amphibolites from the Ailaoshan tectonic zone in the Yunnan-Burma region. The ca. 139 Ma diabases and ca. 68 Ma amphibolites display similar geochemical features with typical Nb-enriched basalts, e.g., high Na2O, TiO2, and Nb (>7 ppm) contents, and high (Nb/Th)PM, (Nb/La)PM, and Nb/U ratios. Unlike adakite-associated Nb-enriched basalts, our Nb-enriched diabases and ampibolites were derived through mixing between OIB-like and arc-like mantle source components in the spinel stability field (<70 km), and the Nb-enriched diabases involved an additional high degree (∼18%) of partial melting of the mixed source. In comparison with the Nb-enriched mafic rocks, the ca. 73 Ma OIB-like amphibolites have lower Mg# values, higher TiO2 and Nb (20.0–26.0 ppm) contents, and OIB-like rare earth element patterns and trace-element spidergrams. The OIB-like rocks were generated by a low degree of partial melting of garnet-facies peridotite (>80 km). In combination with previous Cretaceous to early Eocene age data from granitoids, three significant magmatic flare-ups are identified at 140–115 Ma, 75–64 Ma, and 55–50 Ma, with two magmatic gaps at 115–75 Ma and 64–55 Ma. Given the distances of the studied area from the Pacific and Neotethyan trenches (2400–5000 km and 320 km, respectively), these Cretaceous igneous rocks more likely formed in a Neotethyan subduction-related tectonic setting, rather than a Pacific Ocean–related setting. We propose a tectonic model involving episodic flat slab subduction, slab rollback, and intracontinental back-arc extension for Neotethyan subduction in the Yunnan-Burma region.

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