High-K calc-alkaline to shoshonitic intrusions in SE Tibet: implications for metasomatized lithospheric mantle beneath an active continental margin.

High-K calc-alkaline to shoshonitic suites are widespread and generally volumetrically small but provide key information on magmatic mantle-crust interactions. Limited work has addressed the multi-stage formation of relatively high-volume high-K to shoshonitic rocks. A newly identified, Late-Cretaceous to Early-Cenozoic high-volume high-K to shoshonitic association (ca. 28,000 km3) is described, from the southeastern Himalayan–Tibetan orogen. The mafic intrusions are shoshonitic (K2O contents, ~ 3.3 wt.%), have high-Mg contents (~ 5.1 wt.%) with high-Mg# (57), LREE and LILEs with low Ba/Th, Ba/La, Sm/La (< 0.3), Nb/Yb and 208Pb/206Pb ratios but high Hf/Sm (> 0.70), Th/Yb, Th/La (> 0.2) and La/Sm, and lack an Eu anomaly. Their mafic minerals are hydrous, dominated by magnesio-hornblende (Mg#, ~ 0.69–0.73) and Mg- and K-biotite (MgO, 7.27–9.26 wt.%; K2O, 9.65–10.1 wt.%). Such characteristics strongly suggest derivation from the sub-continental lithospheric mantle (SCLM), metasomatized by sediment-derived melts/fluids. The associated felsic intrusions have high SiO2, alkali content (Na2O + K2O contents up to 10.0 wt.%) and incompatible elements (e.g., K, Rb), with ferropargasite-hastingsite and ferrobiotite-siderophyllite as the mafic phases. These characteristics point to derivation from the continental crust enriched by fluids, likely those released from the contemporaneous mafic magmas crystallising at depth. In contrast to low-volume potassic magmatism from post-collision and earliest arc-rift settings, these high-volume high-K and shoshonitic intrusions define a mantle-to-upper crust pathway at an active continental margin. Alongside geophysical data, these observations are consistent with the contemporary subduction history, arising from subduction of the Neo-Tethyan ocean slab to initial collision of India-Asia, from steepening subduction to slab rollback and breakoff. [ABSTRACT FROM AUTHOR]