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 and ultrapotassic Weikai Li1,2,3, Zhiming Yang1,4*, Massimo Chiaradia3, Yong Lai2, Chao Yu1 and Jiayu Zhang1 1Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 2School of Earth and Space Sciences, Peking University, Beijing 100871, China 3Department of Earth Sciences, University of Geneva, Geneva 1205, Switzerland 4University of Science and Technology Beijing, Beijing 100083, China

ABSTRACT Fig. 1), lithospheric mantle 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 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 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 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

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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 - 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 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 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 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. S2D), suggesting their simultane- The oxybarometer of Ballhaus et al. (1991) of 64–79 km), and +0.5 to +1.2, respectively

ous crystallization. The proportion of olivine was used to calculate the fO2 of lherzolite­ (Fig. 2; Tables S1 and S2). phenocrysts gradually decreases from the most mafic trachyandesite to the most silicic tra- 4.0 4.0 chyte (Figs. S2A–S2C), indicating a fractional A B 19 38 57 76 Depth crystallization trend. The groundmass of these 3.5 3.5 (km) 3.0 3.0 samples mainly consists of clinopyroxene, sani- 2.5 2.5

dine, phlogopite, magnetite, ilmenite, and sul- Q 2.0 2.0 MQ

fides (Figs. S2D–S2F). From trachyandesites FM 1.5 F 1.5 to trachytes, the magnetite content gradually 1.0 1.0 increases, whereas that of ilmenite and sulfides 0.5 0.5 decreases (Figs. S2D–S2F). Abundant ilmen- 0.0 FMQ 0.0 FMQ -0.5 -0.5 ite and sulfide inclusions occur in olivine and 1000 1050 1100 1150 1200 1250 1300 13501400 0 0.5 11.5 2 2.5 clinopyroxene phenocrysts (Figs. S2G–S2I), T(oC) P (GPa) 4.0 C suggesting they are early fractionated phases. 3.5 Olivine-Spinel pair of lherzolite xenoliths Many apatite inclusions coexisting with gas 3.0 bubbles grew in the core or along the middle 2.5

zone of host phenocrysts (Figs. S3A, S3C, and Q 2.0 Olivine phenocryst of Chazi trachyandesite S3E). Raman spectra show that those bubbles FM 1.5 1.0 Olivine phenocryst of Sailipu trachyandesite have different gas assemblages, comprising CO2 0.5 N and pure CO (Figs. S3B, S3D, and S3F). + 2 2 0.0 FMQ Olivine phenocryst of Mibale trachyte Mantle xenoliths (<1.5 cm in size), mainly -0.5 consisting of lherzolite, were collected from tra- 0.5 0.6 0.7 0.8 Whole-rock Mg#

Figure 2. (A) ΔFMQ versus temperature (T), (B) ΔFMQ versus pressure (P), and (C) ΔFMQ 1Supplemental Material. Methods, Tables S1–S7, versus whole-rock Mg#, where FMQ is the deviation of log from the fayalite-magnetite- Δ fO2 and Figures S1–S8. Please visit https://doi​ quartz buffer. Error bars of lherzolite xenoliths are calculated errors of ΔFMQ, T, and P. For .org/10.1130/GEOL.26213S.12114525 to access the results of olivines in ultrapotassic volcanic rocks (UVRs; unfilled circles), only one crossed supplemental material, and contact editing@geoso- error bar was added to average-value position of each data group to represent average error. ciety.org with any questions. Data sources are given in Tables S1–S3 and S5 (see footnote 1).

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/7/733/5073406/733.pdf by guest on 28 September 2021 Several factors could influence the calculated The metasomatism of oxidizing melts and Fig. 2B), and it has a similar oxidization state

fO2 results, such as crystal-chemistry, T-P condi- fluids released from a subducted seawater-­ (ΔFMQ = +0.8 to +1.3) to that of the lherzo- tion, and hydrothermal alteration. For example, altered slab is commonly considered to be the lite xenoliths (ΔFMQ = +0.5 to +1.2; Fig. 2B), Fe3+-Cr substitutions are more favorable than most likely agent that could oxidize the mantle implying inheritance of the oxidized signature 3+ Al -Cr substitutions in spinel, which could lead wedge (e.g., Bénard et al., 2018). The upper from the mantle source. However, the fO2 of the 3+ to a positive correlation between Fe /∑Fespinel mantle beneath the Himalayan-Tibetan orogen ultrapotassic volcanic magmas shows a shift to- and Cr/(Cr + Al) ratios in spinel (e.g., Bénard experienced a continuous slab-related metaso- ward more oxidized values above 48 km (i.e., 3+ et al., 2018). However, a plot of Fe /∑Fespinel matic process from subduction to collision dur- the average formation depth of the most maf- versus Cr/(Cr + Al) showed no positive correla- ing ca. 210–65 Ma (Zhu et al., 2011). Although ic trachyandesitic magma; Table S2; Figs. 2B

tion (Fig. S6), which means no crystal-chemical oceanic subduction was not occurring during the and 2C), implying that fO2 was modified dur- 3+ effects influenced the Fe /∑Fespinel ratios and Miocene in this area, the former subduction of ing crustal evolution of the magma. Common

fO2 calculation. the Neo-Tethyan oceanic slab could have modi- factors that could influence the fO2 of mantle- Based on the expression of Ballhaus’s oxy- fied the redox state of the southern Tibetan litho- derived magmas include changing T-P condi- barometer (Equation 5 in the Supplemental spheric mantle. The low Mg/(Mg + Fe) ratios tions, assimilation, crystal fractionation, and ­Material), a variation of ±1 GPa and ±100 °C (0.35–0.49; Table S1) in spinel are indicative of degassing (e.g., Lee et al., 2005). Variations in can result in a maximum ΔFMQ error of ±0.25 iron enrichment, a trend commonly associated T-P conditions would not exert significant con- ol/melt and ±0.13, respectively (see similar errors in with metasomatic effects (Perinelli et al., 2006). trols on the magmatic fO2 because the DV Woodland et al., 1996). Therefore, the narrow T-P The porphyroclastic and disequilibrium mineral oxybarometer is not P-sensitive (Equation 6 in variation of lherzolite xenoliths (1316–1355 °C; textures in lherzolite xenoliths, especially the the Supplemental Material), and a variation of 1.7–2.1 GPa) cannot significantly influence fO reaction texture of spinel with clinopyroxene 100 °C only causes a maximum variation 2 ± fO2 calculation. (Fig. S5B), indicate metasomatism by Si- and of ±0.25 log units. As olivine-spinel pairs used for calculation alkali-rich melt/fluid derived from a subducted The similar Sr-Nd-Pb-Os isotopes and trace- were selected in the core of olivines without slab (e.g., Shaw et al., 2006). In addition, oliv- element ratios of the trachyandesite and trachyte zoning (Fig. S5C), and samples used here were ines in the lherzolite xenoliths are rich in highly (Tables S4 and S5; data are from Zhao et al., sufficiently fresh (Figs. S4 and S5), late-stage incompatible elements, such as Ba, K, and Th 2009; Liu et al., 2014; Guo et al., 2015) indicate hydrothermal alteration is also unlikely to have (Fig. S7). Since Ba and K are water-mobile ele- that these ultrapotassic volcanic magmas have affected the calculation. ments, and Th appears only to be mobilized in experienced similar assimilation degrees during Among UVRs, the most mafic Sailipu trachy- sediment melts (Woodhead et al., 2001), their ascent. Therefore, the lack of obvious correla-

andesite recorded the highest T of 1191–1236 °C enrichments imply the metasomatic agent was tion between fO2 and assimilation proxies sug- and P of 1.1–1.5 GPa (43–56 km paleodepth; 10 a water-rich and sediment melt–derived source, gests that assimilation is not a significant factor. EMPA spots; Fig. 2; Table S2) and the lowest respectively. Melt extractions from the mantle Degassing of volatile species has long been FMQ values of 0.8 to 1.3 (10 laser-ablation– lherzolite would result in the depletion of these suggested to affect magmatic , particularly Δ + + fO2 inductively coupled plasma–mass spectrometry elements rather than in such enrichments. Later degassing of reduced-type volatile species (e.g.,

[LA-ICP-MS] spots; Fig. 2; Table S3). interactions between lherzolite xenoliths and the H2, CH4, H2S; Moussallam et al., 2016). How- Calculated T and P values of the Chazi trachy- host magma during ascent are unlikely to have ever, the fact that mineral inclusions coexist

andesite were 1124–1137 °C and 0.7–0.8 GPa, caused the observed incompatible element en- with CO2 and N2 gas bubbles (Figs. S3A–S3F) respectively (25–31 km ­paleodepth; 5 EMPA richments because contents of these elements in suggests that gases were trapped during crystal- spots; Fig. 2; Table S2), and ΔFMQ values olivine phenocrysts of host trachyandesite are lization of host phenocrysts at magmatic tem- ranged from +1.3 to +2.0 (5 LA-ICP-MS spots; much lower than those in olivines of the lher- perature. Thus, degassing of these oxidized type

Fig. 2; Table S3). zolite (Fig. S7; Table S6). Hence, the gases cannot account for the elevated fO2 . By contrast, the most silicic Mibale trachyte interaction between lherzolite and slab-derived In contrast, the oxidized trend of the ultrapo- yielded the lowest T (1058–1061 °C) and P melt/fluid during metasomatism probably re- tassic volcanic magmas matches the decreasing (0.40–0.43 GPa, 15–16 km paleodepth; 4 EMPA sulted in the enrichment of incompatible ele- proportions of olivine, ilmenite, and sulfide, and spots; Fig. 2; Table S2) and the highest ΔFMQ ments in the mantle source. the increasing proportions of magnetite from

values of +1.9 to +3.0 (6 LA-ICP-MS spots; Furthermore, the Fe2O3/FeO ratios of the Ce- trachyandesite to trachyte (Figs. S2D–S2F). The Fig. 2; Table S3). nozoic lower-crustal rocks display roughly posi- early segregation of reduced minerals, such as tive correlations with Ba/La and Th/Yb ratios ilmenite and sulfide inclusions enclosed in early DISCUSSION (Zhang et al., 2019; Figs. S8B and S8C; Table crystallized phenocrysts (Figs. S2G–S2I), could Causes of the Oxidized State of Southern S7). This implies that water-rich fluids and sedi- have caused oxidation of the residual magma Tibetan Lithospheric Mantle ment melts liberated from the Indian continental by consuming reduced ions (e.g., Fe2+ and S2–).

The mantle xenoliths reveal that the plate since ca. 65 Ma (Yang et al., 2016) could Thus, the fO2 conditions of the evolved southern Tibetan lithospheric mantle was have also oxidized the southern Tibetan lower ­ultrapotassic volcanic magmas do not simply oxidized (ΔFMQ = +0.5 to +1.2) in the Mio- crust and the underlying lithospheric mantle. reflect their mantle source but were affected by cene. It is markedly more oxidized than cra- subsequent crustal evolution processes (e.g., tonic (ΔFMQ = −4 to −2) and oceanic mantle Redox Variation of Ultrapotassic Volcanic fractionation). (ΔFMQ = −1.2 to −0.4) and falls within the Magmas

typical fO2 range of oxidized mantle wedge There is a long-standing debate on whether ACKNOWLEDGMENTS (ΔFMQ = 0 to +3). This oxidized state of the the redox state of mantle-derived magmas rep- We thank C. Ballhaus and C. Perinelli for guid- southern Tibetan lithospheric mantle is consis- resents that of their mantle sources or not (e.g., ance with the calculations, and L. Danyushevsky and tent with the occurrence of oxidized volatile spe- Kelley and Cottrell, 2012). In this study, we M. Harlaux for experimental analyses. This work was funded by the National Natural Science Foundation cies of CO2 and N2 (Figs. S3G–S3J), which are found that the most mafic host trachyandesitic of China (91955207, 41825005) and the National thought to indicate oxidized mantle conditions magma formed at a paleodepth (43–56 km) close Key Research and Development Project of China (above FMQ; Green et al., 1987; Li et al., 2013). to that of the lherzolite xenoliths (64–79 km; (2016YFC0600305). We acknowledge editor Gerald

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