Eocene–Oligocene Crustal Thickening-Collapse of the Eastern Tibetan Plateau: Evidence from the Potassic Granitoids in SW China

Abstract

The eastern Tibetan Plateau is a key part of the eastern India–Asia collisional zone, a region that records multiple overprinting tectonic and magmatic events. This study presents new geochronological, geochemical, and Sr–Nd–Hf–O isotopic data for Cenozoic potassic granitoids in eastern Tibet, southwestern China, which recorded the tectonic evolution of the eastern Tibetan Plateau. These potassic granitoids are formed between 37.6 and 32.9 Ma and are geochemically subdivided into the following: Group 1, adakite-like granites; Group 2, syenites; and Group 3, low-εNdt granitoids. The Group 1 samples are similar to high-silica adakites in that they have variable SiO2 contents (63.31–73.62 wt.%) and high Sr/Y and La/Yb ratios. These samples have εNdt and εHft values that range from −5.8 to −0.6 and from −4.3 to +5.2, respectively, with δ18O values of 6.78‰–7.36‰. The Group 2 samples are syenites, contain 56.36–63.86 wt.% SiO2 and high concentrations of Y and Yb, and have εNdt values from −8.4 to −2.4, εHft values from −6.1 to +1.1, and δ18O values of 6.37‰–6.89‰. The Group 3 samples have a narrow range of SiO2 concentrations (62.27–64.59 wt.%), high Sr/Y and La/Yb ratios, δ18O values of 6.31‰–6.82‰, and low εNdt and εHft values (−12.6 to −10.9 and −11.4 to −6.6, respectively) that are similar to the values obtained for the contemporaneous Yao’an lamprophyres. These data indicate that the Group 1 samples are formed from magmas sourced from a heterogeneous and thickened region of the lower crust containing an enriched mantle component. Group 2 magmas were most likely derived from contemporaneous mafic melts sourced from an ancient region of the lithospheric mantle previously modified by the incorporation of recycled components. The Group 3 samples have distinct Sr–Nd–Hf isotopic compositions that are indicative of derivation from magmas generated by the fractional crystallization of lower crustal melts sourced from ancient enriched mantle of the Yangtze Block. Combining these new data with the results of previous research suggests that the Cenozoic potassic igneous rocks of eastern Tibet were formed as a result of the thinning of the lithospheric mantle and an associated crustal collapse event, potentially representing a regional late Eocene to early Oligocene transition from compression to transtension in the eastern Tibetan Plateau. These potassic igneous rocks are contemporaneous with or are younger than igneous rocks in the Qiangtang Block, suggesting that the magmatic response to the India–Asia collisional event was initiated in the central Tibetan Plateau before propagating towards the eastern margin of this region.

Extrusion tectonism of Indochina reassessed: constraints from 40Ar/39Ar geochronology from the Day Nui Con Voi metamorphic massif, Vietnam

“…The formation of the protolith of our D 1 gneiss fabrics, which formed under granulite facies prior to 32 Ma, can be linked to crustal thinning due to back arc extension and subduction of Neotethys as the SCS opened (e.g., Karig, 1971;Mai et al, 2018). These granulite facies metamorphism and wide-spread regional leucogranitic magmatism played a critical role in crustal weakening before the strike-slip movement (e.g., Chung et al, 1997;Junlai et al, 2020;Gou et al, 2021;Li et al, 2021). As the SCS further rifted, the continental crust was further thinned.…”

“…On the contrary, these magmatism events corresponded to our D 1 granulite facies metamorphism as the thickened crust started to stretch and extend. Geochemical data of Chung et al (1997) and Gou et al (2021) revealed these magmas originated from collapsing of the thickened crust within plate following the India-Eurasia collision. However, the proposed transtension shearing was not the forming mechanism for our D 3 event as a transtensional basin rather than a dome would have been expected in such a case.…”

“…Zhang and Schärer (1999) and Leloup et al (2001) constrained the timing for the onset of left-lateral shearing along the RRSZ to be around 35 Ma by dating the undeformed granitic rocks from the Fansipan massif, 10 km away from the RRSZ. A wide distribution of undeformed Late Eocene granitic rocks with ages ranging from around 39 to 30 Ma was reported across the Tibetan Plateau, Yunnan, and Vietnam regions (e.g., Chung et al, 1997;Junlai et al, 2020;Gou et al, 2021;Li et al, 2021). This geographical distribution further contradicted the hypothesis that these magmatism events were syn-transtensional shearing.…”

“…However, later studies by Wang et al (2000), Searle (2006), and Searle et al (2010) pointed out that most of the granitic rocks are pre-kinematics and so U/ Pb zircon ages provide only a maximum age for the initiation of left-lateral motion on the RRSZ. Additionally, recent studies (Junlai et al, 2020;Gou et al, 2021;Li et al, 2021) concluded that these granitic melts were formed as a product of the crustal collapse of the thickened crustal lithosphere and that this magmatism may have helped to weaken the lithosphere, facilitating the initiation of movement along the RRSZ. In other words, these dated granitic rocks were emplaced prior to the tectonic extrusion of the Indochina block.…”

Thermal and Physical Properties of Barrovian Metamorphic Sequence Rocks in the Ailao Shan‐Red River Shear Zone, and Implications for Crustal Channel Flow

“…A regional NE–SW compression and crustal thickening event (D1) prior to sinistral shear is also evidenced by: (a) granitic mylonite adjacent to both the ALS and the Red River fault shows early Eocene mica or amphibole Ar/Ar cooling ages (Zhang et al., 2006, Figure 2d); (b) the Ludian‐Zhonghejiang fold‐and‐thrust system, which lies parallel and is situated close to the ARSZ, was active during the period 50‐39 Ma (Cao et al., 2020); (c) a thin‐skinned fold‐thrust stratigraphic succession and the unconformity exists between Paleocene and Lower Eocene strata (45 ± 5 Ma) in Indochina (Liang et al., 2022; Wang & Burchfiel, 1997); (d) Mesozoic granitic and sedimentary rocks situated on the interior of the ARSZ and its flanks record Eocene zircon/apatite fission track and (U–Th)/He ages (Liu‐Zeng et al., 2018; Wang et al., 2020); (e) gneissose folding with sillimanite‐ and garnet‐bearing fabrics that formed due to pure shear deformation in the ARSZ, and which were pervasively developed prior to left‐lateral shearing initiation (Jolivet et al., 2001; Liu et al., 2012; Zhang et al., 2014, 2017a, 2017b); (f) the chronostratigraphic age of formation of the Jianchuan and Lühe syncontractional basins (Eocene: 37–35 Ma), which are interpreted to document synchronous E–W shortening of the ARSZ (Gourbet et al., 2017; Li et al., 2020); and (g) Eocene‐Oligocene potassic granitoids are interpreted as having formed in response to crustal thickening and collapse in the southeastern Tibetan Plateau (Gou et al., 2021). These lines of evidence lead us to propose that micaschists on the southwest side of the ALS massif recorded an episode of crustal stacking above the brittle‐ductile transition zone (∼25 km) during the soft collision stage between the Indian plate and the Eurasian plate (Lee & Lawver, 1995).…”