Initial Rifting of the Lhasa Terrane from Gondwana: Insights From the Permian (~262 Ma) Amphibole‐Rich Lithospheric Mantle‐Derived Yawa Basanitic Intrusions in Southern Tibet

Abstract

The Permian tectonic setting of the Lhasa Terrane in southern Tibet remains controversial (i.e., continental rift vs. subduction‐collision) and is crucial to palinspastic reconstructions of the eastern Tethys during the breakup of Gondwana. In this study, we present new geochronological, geochemical, and mineralogical data for the Permian (~262 Ma) Yawa intrusions in the southern Lhasa Terrane. These rocks are silica‐undersaturated and alkaline, with high TiO2 and moderate MgO, and exhibit enrichments in Th, light rare earth elements, and Nb‐Ta, and depletions in K. These chemical compositions, combined with uniform whole‐rock (87Sr/86Sr)i (0.7039–0.7044), εNd(t) (1.85–2.81), and εHf(t) (4.21–6.90) values, and zircon εHf(t) (4.53–9.97) and δ18O (5.04‰–5.76‰) values, indicate the magmas were derived by partial melting of amphibole‐rich lithospheric mantle. The magmas subsequently underwent fractionation of clinopyroxene, amphibole, and Fe‐Ti oxides. The amphibole in the lithospheric mantle likely formed as cumulates from low‐degree asthenospheric melts during incipient extension. Given that the amphibole‐rich metasomatic veins have a lower melting temperature than the surrounding peridotite, they were susceptible to melting during the early stages of thermal perturbation of the mantle. Because there is no evidence of Permian continental subsidence in the Lhasa Terrane, we suggest the Yawa intrusions were formed at the onset of lithospheric extension associated with initial rifting of the Lhasa Terrane from the Indian Plate during Gondwana breakup, which was a precursor to the opening of the Neo‐Tethys Ocean.

Early Carboniferous Back‐Arc Rifting‐Related Magmatism in Southern Tibet: Implications for the History of the Lhasa Terrane Separation From Gondwana

“…Further evidence for the microcontinent hypothesis includes Permian arc‐type magmatism on the southern rim of the NL, which indicates a northward subduction system beneath the NL (Zhu et al, 2010), and paleomagnetic data for the lower Permian strata in the NL, which show that the Lhasa terrane drifted from northern Gondwana into middle‐lower latitudes rather than staying attached to the northern margin of Gondwana during the early Permian (Ran et al, 2012). Although emerging evidence does not support the traditional understanding, some authors still hold this view that the Lhasa terrane was positioned adjacent to the northern margin of Gondwana in the Permian and that the separation of the Lhasa terrane from Gondwana occurred from the late Permian to Triassic (e.g., Li et al, 2016; Zeng et al, 2019). For example, high‐precision paleomagnetic data for the late Triassic‐early Jurassic Sangri Group volcano‐sedimentary rocks in the SL suggest that the Lhasa terrane drifted from Gondwana in the late Triassic (Li et al, 2016, and references therein).…”

“…Extension‐type magmatism of middle Permian age is observed in the SL (ca. 265 Ma; Zeng et al, 2019), the oldest early Mesozoic magmatic rocks related to the northward subduction of the Neo‐Tethys oceanic slab have ages of ca. 245 Ma in the SL (X. X. Ma et al, 2020, and references therein; C. Wang et al, 2016), and a middle Triassic‐early Jurassic island arc system is within the Neo‐Tethys (Lang, Liu, et al, 2019; X. X. Ma, Meert, et al, 2019; X. X. Ma et al, 2020).…”

“…Tectonics and references therein). In addition, Permian Yawa basanitic intrusions from the Dajiacuo area in the western segment of the SL show geochemical characteristics of alkaline and OIB-type rocks, which also hint that a continental extensional setting may have corresponded to the onset of the lithospheric extension associated with the initial rifting of the Lhasa terrane from Gondwana (Zeng et al, 2019).…”

“…Extension-type magmatism of middle Permian age is observed in the SL (ca. 265 Zeng et al, 2019), the oldest early Mesozoic magmatic rocks related to the northward subduction of the Neo-Tethys oceanic slab have ages of ca. 245 Ma in the SL (X. X.…”

Geochronology and Geochemistry of Ore-Hosting Rhyolitic Tuff in the Kengdenongshe Polymetallic Deposit in the Eastern Segment of the East Kunlun Orogen

“…Although Kengdenongshe rhyolitic tuffs have high SiO 2 contents (>73 wt. %) that resemble highly fractionated granites [42,[46][47][48][49], they also exhibit relatively high Zr contents (73-100 ppm) and high Zr/Hf ratios (28.0-29.5). Moreover, in the plot of Rb-Ba-Sr, samples in this study fell into the field of normal granites rather than highly differentiated granites (Figure 7b).…”