https://doi.org/10.1130/G47411.1 Manuscript received 10 January 2020 Revised manuscript received 11 March 2020 Manuscript accepted 15 March 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 29 April 2020 Redox state of southern Tibetan upper mantle and ultrapotassic magmas Weikai Li1,2,3, Zhiming Yang1,4*, Massimo Chiaradia3, Yong Lai2, Chao Yu1 and Jiayu Zhang1 1 Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 2 School of Earth and Space Sciences, Peking University, Beijing 100871, China 3 Department of Earth Sciences, University of Geneva, Geneva 1205, Switzerland 4 University of Science and Technology Beijing, Beijing 100083, China ABSTRACT Fig. 1), lithospheric mantle xenoliths have been The redox state of Earth’s upper mantle in several tectonic settings, such as cratonic mantle, entrained by postcollisional ultrapotassic vol- oceanic mantle, and mantle wedges beneath magmatic arcs, has been well documented. In canic rocks (UVRs) in the Sailipu area (Zhao contrast, oxygen fugacity ( fO2 ) data of upper mantle under orogens worldwide are rare, and et al., 2008; Liu et al., 2011). The UVRs were the mechanism responsible for the mantle fO2 condition under orogens is not well constrained. derived from low-degree partial melting of the In this study, we investigated the fO2 of mantle xenoliths derived from the southern Tibetan highly metasomatized Tibetan upper mantle lithospheric mantle beneath the Himalayan orogen, and that of postcollisional ultrapotassic during the Miocene (Turner et al., 1996; Miller volcanic rocks hosting the xenoliths. The fO2 of mantle xenoliths ranges from ΔFMQ = +0.5 et al., 1999; Zhao et al., 2009; Guo et al., 2015). to +1.2 (where ΔFMQ is the deviation of log fO2 from the fayalite-magnetite-quartz buffer), Volcanic rock–hosted mantle xenoliths are com- indicating that the southern Tibetan lithospheric mantle is more oxidized than cratonic and monly considered to preserve the physicochemi- oceanic mantle, and it falls within the typical range of mantle wedge fO2 values. Mineralogical cal features of their mantle source (Coltorti and evidence suggests that water-rich fluids and sediment melts liberated from both the subduct- Grégoire, 2008), and therefore the mantle xeno- ing Neo-Tethyan oceanic slab and perhaps the Indian continental plate could have oxidized liths in southern Tibet may provide insights into the southern Tibetan lithospheric mantle. The fO2 conditions of ultrapotassic magmas show the fO2 condition of the upper mantle beneath a shift toward more oxidized conditions during ascent (from ΔFMQ = +0.8 to +3.0). Crustal the Himalayan-Tibetan orogen. evolution processes (e.g., fractionation) could influence magmatic fO2 , and thus the redox Here, we report for the first time: (1) fO2 state of mantle-derived magma may not simply represent its mantle source. values of lithospheric mantle xenoliths from southern Tibet; and (2) fO2 values of ultrapo- INTRODUCTION references therein). In contrast, the fO2 data of tassic volcanic magmas and their variation dur- The redox state of Earth’s upper mantle upper mantle beneath orogens worldwide are ing ascent. The main objectives of this study controls partial melting, the valence state of rare and indicate a broader range of variabil- were to identify the redox state of the southern multivalent and chalcophile elements, and the ity (ΔFMQ = –4.5 to +2.6; Foley, 2010). The Tibetan upper mantle and reveal the fO2 evolu- speciation of volatiles, and therefore it strong- mechanism responsible for the orogenic mantle tion of mantle-derived magmas along the path ly affects subsequent magma generation, ore- fO2 condition is not well constrained, except for from mantle source to shallow crust. forming process, and atmospheric evolution a few studies based on orogenic peridotite mas- (e.g., Frost and McCammon, 2008). The oxy- sifs (e.g., Woodland et al., 2006, and references GEOLOGICAL SETTING gen fugacity ( fO2 ) of the upper mantle in several therein). However, orogenic peridotite massifs The Lhasa terrane in southern Tibet is bound- tectonic settings has been well documented by were tectonically emplaced into the crust at con- ed by the Indus–Yarlung Zangbo suture (IYS) mantle-derived magmas and mantle peridotites vergent margins. Compared to mantle xenoliths, to the south and the Bangong-Nujiang suture (e.g., xenoliths, orogenic massifs), particularly their original geodynamic setting cannot be ex- (BNS) to the north (Fig. 1A). The final closure using ΔFMQ, which is the deviation of log fO2 actly constrained (Menzies and Dupuy, 1991). of the Neo-Tethys Ocean and collision of India from the fayalite-magnetite-quartz buffer. For Moreover, mantle structures and fO2 conditions with Asia along the southernmost margin of the instance, typical cratonic mantle (ΔFMQ = −4 may be obscured by deformation and metamor- Lhasa terrane occurred during the early Cenozo- to −2; Frost and McCammon, 2008) and oceanic phic recrystallization during tectonic emplace- ic (ca. 65–55 Ma; Leech et al., 2005; Mo et al., mantle (ΔFMQ = −1.2 to −0.4; Bézos and Hum- ment (Bodinier and Godard, 2014). 2007). After the collision, the Lhasa terrane in ler, 2005) are generally reduced. Mantle wedges The Himalayan-Tibetan orogen, created by the Himalayan-Tibetan orogen became part of a beneath magmatic arcs are relatively oxidized the collision between India and Asia after sub- postcollisional setting. Recent geophysical stud- (e.g., ΔFMQ = 0 to +3; Bénard et al., 2018, and duction of the Neo-Tethyan oceanic slab (e.g., ies (Zhao et al., 2011) have suggested that the Mo et al., 2007), is arguably one of the young- Indian continental plate subducted northward est collisional belt on Earth. In the southern beneath the Tibetan Plateau after collision (e.g., *E-mail: [email protected] margin of the Tibetan Plateau (Lhasa terrane; since the Eocene; Leech et al., 2005). Since the CITATION: Li, W., et al., 2020, Redox state of southern Tibetan upper mantle and ultrapotassic magmas: Geology, v. 48, p. 733–736, https://doi.org/10.1130/G47411.1 Geological Society of America | GEOLOGY | Volume 48 | Number 7 | www.gsapubs.org 733 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/7/733/5073406/733.pdf by guest on 28 September 2021 o o xenoliths based on olivine-spinel pairs 80 E 90 E A Figure 1. (A) Sketch 100 km (Fig. S5C). The ferric iron content of spinel in Shiquanhe Qiangtang terrane map of tectonic units in Xiongba Tibet, and distributions lherzolite xenoliths was determined by electron Zabuye BNS of ultrapotassic volcanic microprobe analysis (EMPA), because the very Sailipu Garwa rocks (UVRs; red-filled Maiga small size and volume of the spinel (Fig. S5) Yaqian Mibale IYS Lhasa terrane o areas) and adakite-like Chazi 30 N rocks (blue-filled areas). were not suitable for the Mössbauer spectros- Lhasa Qulong copy analysis. Due to the disequilibrium texture Pabbai Stars represent sampling Zhunuo Bairong IYS Tibet Xigaze locations. STDS—South of the two pyroxenes in lherzolite xenoliths (Fig. STDS Tibetan detachment S5B), the single-clinopyroxene thermometer and system; IYS—Indus– barometer (Nimis and Taylor, 2000; Putirka, India Himalaya Yarlung Zangbo suture; BNS—Bangong-Nujiang 2008; see Equations 1 and 2 in the Supplemen- suture. (B) Tectonic frame- tal Material) were used to calculate temperature India 25oN B 400 km work of Tibetan Plateau. (T) and pressure (P), respectively. For compari- son, results calculated by other thermometers are mid-Oligocene, postcollisional magmatism has chyandesite in the Sailipu area (Fig. 1; Fig. S4). also provided in Table S2. Olivine phenocrysts been widespread in the Lhasa terrane, mainly They display a porphyroclastic texture, with larg- in all UVRs were used to calculate fO2 by the ol/melt consisting of ca. 25–8 Ma potassic to ultrapotas- er grains (0.1–1 mm) of olivine, orthopyroxene, DV oxybarometer of Mallmann and O’Neill sic volcanic rocks and ca. 21–12 Ma adakite-like clinopyroxene, and phlogopite, and a few fine- (2013). The whole-rock compositions were used intrusions (Fig. 1A; Yang et al., 2016). grained (<50 μm) spinel and apatite grains (Fig. as the melt compositions. The primary melt may The UVRs are mainly distributed along S5A). Clinopyroxenes show embayed texture have higher V content than whole rock because north-south–trending rifts within the Lhasa ter- filled by orthopyroxene (Fig. S5B), suggesting V is a compatible element, which may lead to rane west of 87°E (Fig. 1A), ranging in compo- disequilibrium between the two pyroxene types. a slight underestimation of the fO2 (Equation 6 sition from trachyandesite to trachyte (see Fig. Orthopyroxenes in lherzolite xenoliths host tiny in the Supplemental Material). Clinopyroxene S1 in the Supplemental Material1). two-phase melt inclusions, composed of silicate inclusions in olivine phenocrysts were used to glass (≤5 μm) and CO2 + N2 gas bubbles (Figs. calculate crystallization T and P of host olivines SAMPLES AND PETROGRAPHY S3G–S3J). These melt inclusions are isolated by the clinopyroxene–liquid thermobarometer The UVRs reported here were collected from without healed fracture trails (Figs. S3G and (Putirka et al., 2003; see Equations 3 and 4 in the Sailipu, Chazi, and Mibale areas (Fig. 1A). S3I). Phlogopite is the only hydrous mineral the Supplemental Material). Detailed methods of These samples show porphyritic textures, with phase. Spinel in the lherzolite is Cr-rich and often all calculations and error estimations are given the main phenocrysts (0.2–0.8 mm in size) be- occurs as inclusions in large crystals (Figs. S5A– in the Supplemental Material. ing olivine, clinopyroxene, phlogopite, and sani- S5C). Detailed descriptions of lherzolite xeno- On the basis of 60 sets of EMPA data, cal- dine (Figs. S2A–S2C). Some of the subhedral liths can be found in the Supplemental Material. culated T, P, and ΔFMQ values of lherzolite clinopyroxenes in these samples are enclosed xenoliths were 1316–1355 °C, 1.7–2.1 GPa or poikilitically enclosed in olivine pheno- METHODS AND RESULTS (corresponding to a continental paleodepth crysts (Fig.