Late Triassic post-collisional slab break-off along the Ailaoshan suture: insights from OIB-like amphibolites and associated felsic rocks

What dynamic processes the South China-Indochina collision had gone through is still a pending problem. Our recent investigations identified a Late Triassic gabbroic intrusion in Mengdong village and several granitic plutons near Wana village, Yunnan province (SW China). Both have underwent strong metamorphism and been altered to amphibolite and granitic gneiss, respectively. We carried out SIMS and LA-ICPMS zircon U-Pb dating for the amphibolites and granitic gneisses, respectively. Dating results yield weighted mean Pb-206/U-238 ages of 221.5 +/- 5.3 Ma for the amphibolites and 224.0 +/- 1.8 and 235.4 +/- 0.6 Ma for the granitic gneisses. The amphibolite samples have low Mg-# (41.7-42.8), high TiO2 (similar to 3.7 wt%) and Na2O (Na2O/K2O = 1.89-2.68) contents and depleted Sr-Nd isotopic compositions [(Sr-87/(8)6Sr)i = 0.707647 and epsilon (Nd)(t) = +1.17]. They exhibit OIB-like REE patterns and spidergrams, with strongly enriched LREE contents, insignificant Eu anomalies (Eu* = 0.99-1.03) and moderate negative Sr anomalies (Sr* = 0.46-0.56). The protolith gabbro of the Mengdong amphibolites is derived from low-degree partial melting of a homogeneous OIB-type mantle source in the garnet stability field and experienced significant fractionation of olivine, clinopyroxene and plagioclase during magma evolution. The Wana samples are strongly peraluminous (A/CNK > 1.1) and K-enriched (K2O/Na2O = 2.93-3.63). They show enriched Sr-Nd isotopic compositions with (Sr-87/Sr-86)i = 0.718589-0.719754 and epsilon (Nd)(t) = -11.34 to -10.92. The Wana plutons are typical S-type granite and product of the dehydration melting of meta-sedimentary rocks. We summarized Late Triassic age data in the literature along the South China-Indochina suture zone and recognized a significant Late Triassic magmatic flare-up. In combination with previous studies on the Late Triassic HP-UHP metamorphic rocks, we proposed that during the Late Triassic the South China-Indochina suture zone had transformed into post-collisional setting, and accompanied slab break-off triggered the OIB-type asthenospheric mantle upwelling and provided the heat source for the Late Triassic magmatic flare-up.