Divergent metamorphism within the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet, China: Insights from in situ U–Th–Pb dating of metamorphic monazite

Abstract

Geothermobarometry shows that metapelite samples from Namche Barwa Complex (NBC) reached upper amphibolite to near‐granulite facies during the peak metamorphic stage, with similar conditions of ~700–750°C/8–10 kbar, and then experienced retrograde metamorphism at ~630–700°C/4–7 kbar. In situ monazite laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U–Th–Pb dating suggests divergent metamorphism in the NBC: metapelite on the hanging wall of Namu–La thrust preserved a continuous metamorphic record of >19–3 Ma, whereas metapelite on the footwall yielded age ranges of >18–14 and 8–3 Ma, with a gap in recorded ages between 14 and 8 Ma. Monazite grains in the garnet porphyroblasts, more depleted in the heavy rare earth elements (HREE), yielded the youngest age of ~14 Ma. This is interpreted as the timing of upper amphibolite facies peak metamorphism in the metapelite from the NBC, with the NBC being exhumed coherently thereafter. Furthermore, the discrepancy between reported peak metamorphic ages of high‐pressure granulite (~40–30 Ma, ~25–20 Ma) and mid‐pressure metapelite (~14 Ma, this study) indicate asynchronous subduction–exhumation processes in the NBC. We suggest that crustal flow has played an essential role in exhumation since ~40 Ma, and recent surficial erosion (<8 Ma) intensified the exhumation of the NBC, with young leucogranite (<10 Ma) resulting from decompression melting. From ~3 Ma to the present, the interplay of erosion and tectonic movement caused ubiquitous rapid uplift, resulting in the concomitant exhumation of various types of rocks and the formation of the spectacular high relief between Yarlung Tsangpo Gorge and Namche Barwa Peak.

Tracing high-pressure metamorphism in the eastern Himalayan syntaxis using detrital zircon and monazite from modern stream sediments

“…Wang et al, 2017a;Warren et al, 2019), which is consistent with the previous studies that monazite recorded the exhumation events or anatexis during decompression in the EHS (Booth et al, 2009;Y. Liu et al, 2011;Peng et al, 2022).…”

“…Previous studies mainly focused on several outcrops of granulites in the Zhibai and Pai areas (e.g., Liu et al, 1997;Ding et al, 2001;Xu et al, 2010;Zhang et al, , 2021Su et al, 2012;Tian et al, 2020;Peng et al, 2022). However, the dense vegetation hinders us from fully understanding the metamorphic history of the EHS.…”

The origin and compositions of melt inclusions in an Al2SiO5‐free paragneiss from the Namche Barwa Complex in the Eastern Himalayan Syntaxis

“…500 Ma) (Zhang et al, 2012). According to previous studies, metapelite and metabasic rocks in the NBC experienced HP granulite‐facies metamorphism under peak P‐T conditions of 1.3–1.6 GPa and 825–900°C (e.g., Booth et al, 2009; Groppo et al, 2012; Guilmette et al, 2011; Iaccarino et al, 2015; Kang et al, 2020; Tian et al, 2016; Zhang et al, 2015), while some metapelites just reached upper amphibolite to near‐granulite facies at the peak stage of 0.8–1.0 GPa and 700–750°C (Peng et al, 2021). Such a divergent metamorphism was interpreted as reflecting asynchronous tectonic processes among different petrol‐tectonic slices in the EHS (Peng et al, 2021; Tian et al, 2016), and zircon U‐Pb ages showed that the HP granulite‐facies metamorphism and associated partial melting took place at ~40 to ~20 Ma in the EHS (e.g., Burg et al, 1998; Ding et al, 2021; Guilmette et al, 2011; Peng et al, 2018; Xu et al, 2010).…”

Intraoceanic Subduction System Within the Neo‐Tethys: Evidence From Late Cretaceous Arc Magmatic Rocks of the Eastern Himalaya

“…The upper GHS unit consists of gneiss, schist, amphibolite, marble, calc-silicate rock and high-pressure (HP) granulite and experienced an extended period of upper amphibolite-facies to granulite-facies metamorphism and partial melting from ∼50 to ∼3 Ma (Booth et al, 2004(Booth et al, , 2009L. Ding et al, 2001;Peng et al, 2018Peng et al, , 2022Tian et al, 2016Tian et al, , 2020; Z. M. Zhang et al, 2012Zhang et al, , 2015; Z. M. . The lower (southern) GHS unit is composed mainly of migmatitic amphibolite and orthogneiss and underwent upper amphibolite-facies metamorphic and partial melting.…”

“…The upper (northern) unit occurs as a NE‐SW striking belt within the northwestern part of the GHS and includes three tectonic slices that are separated by ductile shear zones or detachment and thrust faults (DF 1 and DF 2 ; Figures 1b and 1c). The upper GHS unit consists of gneiss, schist, amphibolite, marble, calc‐silicate rock and high‐pressure (HP) granulite and experienced an extended period of upper amphibolite‐facies to granulite‐facies metamorphism and partial melting from ∼50 to ∼3 Ma (Booth et al., 2004, 2009; L. Ding et al., 2001; Peng et al., 2018, 2022; Tian et al., 2016, 2020; Z. M. Zhang et al., 2012, 2015; Z. M. Zhang, Ding, Palin, et al., 2022). The lower (southern) GHS unit is composed mainly of migmatitic amphibolite and orthogneiss and underwent upper amphibolite‐facies metamorphic and partial melting.…”