Two phases of post-onset collision adakitic magmatism in the southern Lhasa subterrane, Tibet, and their tectonic implications

Abstract

Abundant Neogene adakitic magmatism occurred in the southern Lhasa subterrane after the onset of the India–Asia collision while convergence continued. However, the tectonic setting and magmatic evolution of the adakitic rocks are still under discussion. This study includes new mineral chemical and whole-rock geochemical data as well as zircon U-Pb and Lu-Hf isotopes of adakitic intrusive rocks from the Gyaca and Nyemo locations in the southern Lhasa subterrane. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U-Pb dating yielded crystallization ages of ca. 30 Ma for the Gyaca and Nyemo monzogranite and ca. 15 Ma for the Nyemo granodiorite. Both have common chemical signatures such as low MgO and heavy rare earth element contents as well as low compatible element abundances, indicating that these rocks result from partial melting of thickened lower crust with residual eclogite and garnet amphibolite. Furthermore, these rocks are characterized by variable positive zircon εHf(t) values, suggesting a juvenile magma source with variable ancient crustal contributions. Taking previous data into account, the adakitic magmatism concurs with an early late Eocene to Oligocene (ca. 38–25 Ma) and a late Miocene (ca. 20–10 Ma) phase. The adakitic rocks of the two phases are characterized by different fractionation evolutions of light and medium rare earth elements. We propose that the early-phase adakitic rocks were generated by the anatexis of Lhasa terrane lower crust owing to crustal shortening and thickening subsequent to the onset of the India–Asia collision and the upwelling of hot asthenosphere beneath the Lhasa terrane caused by the break-off of the Neo-Tethyan oceanic slab. The latest phase of adakitic rocks, however, relates to upwelling asthenosphere following the delamination and/or break-off of the subducting Indian continental slab.

Along‐Strike Variation in the Initiation Timing of the North‐Trending Rifts in Southern Tibet as Revealed From the Yadong‐Gulu Rift

“…(b) Longitudinal variations in post‐collisional adakitic and ultrapotassic rocks and rift initiation timing (Bian, Gong, Zuza, et al., 2020; Z. F. Guo et al., 2015; L. Y. Zhang et al., 2014, and references therein). Adakitic rocks can be divide into two groups, characterized by different fractionation evolutions of light and medium rare earth elements (Lu et al., 2020). The eastern cluster of adakitic rocks was triggered by the breakoff of Neo‐Tethyan oceanic slab, whereas others by lateral detachment of the Indian continental slab (Lin et al., 2021; Lu et al., 2020).…”

“…Adakitic rocks can be divide into two groups, characterized by different fractionation evolutions of light and medium rare earth elements (Lu et al., 2020). The eastern cluster of adakitic rocks was triggered by the breakoff of Neo‐Tethyan oceanic slab, whereas others by lateral detachment of the Indian continental slab (Lin et al., 2021; Lu et al., 2020). The eastward‐younging trend of post‐collisional magmatism slowed down crossing the Yadong rift, which is consistent with the deceleration of the eastward propagation of rifting.…”

Origin of the Miocene Adakitic Rocks and Implication for Tectonic Transition in the Himalayan Orogen: Constraints from Kuday Granitoid Porphyry in Southern Tibet

“…However, the Himalayan Orogen was a passive continental margin. Previous studies have shown that closure of the Neo-Tethys Ocean and initial of Indo-Asia collision occurred at 65-55 Ma [5][6][7], and breakoff of the subducted Neo-Tethys Ocean plate occurred at 37,39,40,47]. With comprehensive consideration of the regional geological setting, subducted-related adakitic rocks cannot be formed in the Himalayan Orogen during the Miocene, and geological signatures also disprove the hypothesis that the Kuday adakitic rocks originated from subducted ocean slab melting.…”

“…(1) delamination of thickened lithospheric root [26][27][28][29][41][42][43][44], (2) subducted Indian continental slab breakoff [19,20,[28][29][30][31][45][46][47] or rollback [38,48], and (3) slab-tearing of subducted Indian Plate [29,46,49,50].…”

Emplacement of Young Island Arc Crust Over Older Mantle Along a Cratonic Foreland: Constraints From Multiple Isotopes and Elemental Geochemistry

“…Meanwhile, increasing studies have revealed that mantle‐derived mafic rocks are sometimes more isotopically enriched relative to the coexisting crustal felsic rocks, contrary to “standard” behavior (e.g., Jiang et al, 2018, and references therein; Shen et al, 2018). Furthermore, magmatic systems with such features commonly occur in regions associated with island arc accretion, such as the Central Asian Orogenic Belt (CAOB) (e.g., Huang et al, 2013; Shen et al, 2009, 2012) and Gangdese Belt (South Tibet) (e.g., Lu et al, 2019; Meng et al, 2019; R. Wang, Richards, Zhou, et al, 2015; Wang et al, 2016; Wang, Collins, et al, 2017; Wang, Weinberg, et al, 2018), but the relationship between the two processes (isotope inversion and island arc accretion) has not been clarified. Recently, Jiang et al (2018) have recognized an “reversed isotope” case in Gan‐Hang Belt (GHB) (South China), where the Neoproterozoic‐aged island arc rocks were locally exposed (Shu et al, 2019; Xia et al, 2018; Yao et al, 2019).…”