Triassic magmatism along both sides of the Simao terrane, SE Tibetan Plateau: Implications for the evolution of the Main Palaeo‐Tethyan Ocean and the Ailaoshan Ocean

Abstract

Subduction and closure of the Palaeo‐Tethyan ocean in the south‐eastern (SE) Tibetan Plateau accounted for the amalgamation of the Indochina Block with the Sibumasu and Yangtze blocks during the Triassic, however, the spatial distribution of the Main Palaeo‐Tethyan ocean and the nature of the Ailaoshan ocean in this region remain controversial. This is partially due to the absence of related magmatic records in the middle‐north segment of the Lancangjiang tectonic belt (LTZ) and consequently, the lack of regional comparison with the Ailaoshan tectonic belt (ATZ). In this contribution, we reported for the first time the presence of subduction‐related Triassic dioritic‐granitic rocks from the Chongshan metamorphic belt (CMB) and the Diancangshan metamorphic belt (DMB) along both sides of the Simao terrane. Zircon U–Pb‐Hf isotopic and whole‐rock geochemical data for these dioritic‐granitic rocks and 135 previously published magmatic rocks in the LTZ, ATZ and the Longmu Co‐Shuanghu suture (LSS) are compared and place important constraints on (1) the spatial distribution of the Palaeo‐Tethyan ocean in this region and (2) the nature of the Ailaoshan Ocean. All these dioritic‐granitic samples from the CMB and DMB yielded Triassic crystallization ages from 225 to 251 Ma, representing late‐stage magmatic records for the Palaeo‐Tethyan evolution in the middle‐north segment of both the LTZ and ATZ in this region. These Triassic dioritic‐granitic rocks in the CMB and DMB share similar geochemical features. Dioritic samples display positive εHf(t) values and relatively high MgO, Cr, Ni contents, moderately differentiated rare earth element (REE) patterns and depletion in Nb‐Ta‐Ti, suggesting their generation from partial melts of metasomatized mantle wedge. Granitic rocks are mostly S‐type with low MgO contents but high K2O/Na2O and A/CNK ratios, mainly generated by regional clastic rocks dominant by juvenile contributions, except for granodiorite samples CS1601 and DC1734‐2, exhibiting identical geological features subduction‐derived hybridized magma from regional clastic sediments and basalts. Our discovery of the Triassic dioritic‐granitic rocks in the CMB stands as the first direct evidence for the Main Palaeo‐Tethyan evolution in the middle‐north segment of the LTZ. This discovery bridges 135 contemporaneous reported magmatic records and supports the spatial distribution of the Main Palaeo‐Tethyan ocean from the LTZ to the LSS. Our Triassic dioritic‐granitic rocks in the CMB and DMB recorded comparable late‐stage evolution involving slab break‐off of the Main Palaeo‐Tethyan ocean and the Ailaoshan Ocean, respectively. Together with previous studies, the Ailaoshan ocean should be a wide oceanic Basin that shares similar time span and evolution processes as the Main Palaeo‐Tethyan ocean.