Research Article Identification of Forearc Sediments in the Milin-Zedong Region and Their Constraints on Tectonomagmatic Evolution of the Gangdese Arc, Southern Tibet

Home , Gangdese batholith, Nang County, Qiangtang terrane

GeoScienceWorld Lithosphere Volume 2020, Article ID 8835259, 20 pages https://doi.org/10.2113/2020/8835259

1,2 1 1 1 1 Shao-Hua Zhang, Wei-Qiang Ji , Hao Zhang, Guo-Hui Chen, Jian-Gang Wang, 1,2 1 Zhong-Yu Meng, and Fu-Yuan Wu

1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

Correspondence should be addressed to Wei-Qiang Ji; [email protected]

Received 12 May 2020; Accepted 19 September 2020; Published 2 November 2020

Academic Editor: Songjian Ao

The Xigaze forearc sediments revealed the part of the tectonomagmatic history of the Gangdese arc that the bedrocks did not record. However, the sediments’ development is restricted to the region around and west of Xigaze City. Whether the eastern segment of the arc had a corresponding forearc basin is yet to be resolved. In this study, a field-based stratigraphic study, detrital zircon U-Pb geochronology (15 samples), and Hf isotopic analyses (11 of the 15 samples) were carried out on four sections in the Milin-Zedong area, southeast Tibet. The analytical results revealed the existence of three distinct provenances. The lower sequence is characterized by fine-grained sandstone, interbedded mudstone, and some metamorphic rocks (e.g., gneiss and schist). The detrital zircon U-Pb age distribution of this sequence is analogous to those of the Carboniferous-Permian strata and metasediments of the Nyingtri group in the Lhasa terrane. The middle and upper sequences are predominantly composed of medium- to coarse-grained volcaniclastic/quartzose sandstones, which are generally interbedded with mudstone. The detrital zircon U-Pb ages and Hf isotope signatures indicate that the middle sequences are Jurassic to Early Cretaceous in ~ – ffi ε ðtÞ age ( 200 100 Ma) and show clear a nity with the Gangdese arc rocks, that is, positive Hf values. In contrast, the upper – ε ðtÞ sequences are characterized by Mesozoic detrital zircons (150 100 Ma) and negative Hf values, indicative of derivation from the central Lhasa terrane. The overall compositions of the detrital zircon U-Pb ages and Hf isotopes of the middle to upper sequences resemble those of the Xigaze forearc sediments, implying that related forearc sediments may have been developed in the eastern part of the Gangdese arc. It is possible that the forearc equivalents were eroded or destroyed during the later orogenesis. Additionally, the detrital zircons from these forearc sediments indicate that this segment of the Gangdese arc experienced more active and continuous magmatism from the Early Jurassic to Early Cretaceous than its bedrock records indicate.

1. Introduction of the Xigaze forearc sediments have been conducted. These studies have revealed its tectonic, erosional, and sedimentary Forearc basins develop along convergent plate margins and evolution and have eﬀectively constrained the magmatic his- receive detritus from the adjacent magmatic arc, and thus, tory of the Gangdese arc [2, 3, 6, 7]. However, only restricted they are important for studying the evolution of arcs [1–5]. exposures of the forearc sediments have been documented The Xigaze forearc basin, located in the southern margin of along the huge Gangdese plutonic belt (>1500 km long) [8], the Lhasa terrane, southern Tibet, is one of the best exposed that is, mainly outcropping near and to the west of Xigaze. forearc basins in the world (e.g., [4, 6]). Numerous studies of In contrast, no forearc sediments have been reported in the the detrital zircon U-Pb ages and Hf isotopic characteristics eastern part of the Gangdese arc (e.g., [4, 5]). There are two

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possible explanations for this: either forearc sediments did tectonic units (from north to south): the widely distributed not develop along this section of the arc or they developed Cretaceous to Miocene Gangdese batholith (100–16 Ma) but were destroyed during later orogenesis. (e.g., [9, 26, 27, 30, 31, 34–39]) and the Yarlung Zangpo In this study, sporadic occurrences of potential forearc mélange zone (Figure 1(a)). There are exposures of sedimen- sedimentary records were verified in the Milin-Zedong tary strata distributed sporadically along the Yarlung Zangpo region with a discontinuous distribution in the suture zone River near Milin-Baga, but they are distributed more contin- mélange (Figure 1 and Fig. S1). We conducted a detailed field uously near Nang-Dongga (Figures 1(c) and 1(d)). An addi- investigation of four exposures in this region (Figure 2) and tional, well-developed exposure of these sediments was also performed detrital zircon U-Pb geochronology and Hf found in Jinlu in the Zedong area (Figure 1(e)). isotope geochemistry on the samples collected from these In this study, a total of 23 samples were collected, of exposures. Our study emphasized provenance tracing and which 21 samples were collected from the four sections (see comparative analysis with the detrital zircons in the Xigaze Figure 2 for detailed lithology and sample positions), and forearc basin with the goal of further constraining the two samples were collected from Nang County magmatic evolution of the Gangdese arc. (Figures 1(d) and 3(m)–3(o)). Samples Ld01–07 were collected from section 1 in the Milin area. They are mainly com- 2. Geologic Setting and Samples posed of sandstones (Ld01–04 and Ld07), but a small proportion is composed of gneiss (Ld05–06). Samples The Tibetan Plateau is composed of several east–west- Bg01–04 were collected from section 2, which predominantly trending blocks, including the Himalayas, Lhasa, Qiangtang, comprises metamorphic rocks. Samples Dzg01–07 were col- Songpan-Ganzi, and Kunlun terranes from south to north lected from section 3, including five sandstone samples [10] (Figure 1(a)). The Lhasa terrane is separated from the (Dzg01–02 and Dzg04–06) and two marble samples (Dzg03 Himalayas by the Indus-Yarlung Zangpo suture zone to the and Dzg07). Samples Jl01–03 are medium- to coarse- south and is separated from the Qiangtang terrane by the grained volcaniclastic sandstones collected from section 4. Bangong-Nujiang suture to the north, which is interpreted to A relatively fresh sandstone (Lx01) and an intrusive vein be the southernmost part of the Asian Plate [9] (Figure 1(a)). sample (Lx02) were collected from Nang County. Their The Lhasa terrane consists of the locally developed Pre- detailed stratigraphy and sedimentology are described in cambrian metamorphic basement (i.e., the Nyainqentanglha the next section. In addition, eight samples were collected group and Nyingtri group), the Paleozoic to Cenozoic sedi- from several other sporadic exposures of the suture zone mentary strata, and various types of magmatic rocks [10]. mélange in Milin-Nang (see supplementary materials and The Luobadui-Milashan fault zone in the south and the Figures S1-S3 for more details). Shiquan River-Nam Tso ophiolitic mélange belt in the north separate the Lhasa terrane into its southern, central, and 3. Stratigraphy and Sedimentology northern parts [9] (Figure 1(b)). The south Lhasa terrane mainly consists of the juvenile Gangdese magmatic arc, Exposures and remnants of potential forearc sediments were including the Gangdese batholith and Mesozoic-Tertiary vol- observed and described as follows. Additionally, detailed field canic rocks such as the Middle-Lower Jurassic Yeba Forma- and petrographic observations of several other outcrops are tion [11, 12], the Upper Jurassic-Lower Cretaceous Sangri listed and illustrated in the Supplementary materials. group [13], and the Paleocene Linzizong group [14–16]. ° ′ The Gangdese batholith, which is an important part of the 3.1. Section 1. Section 1 (GPS location: 29 13.1177 N, ° Trans-Himalayan batholith, occurs as a huge plutonic belt 94 11.3320′ E) outcrops on a hillside near Lidi Village, (>1500 km) along the southern margin of the Lhasa terrane approximately 5 km northwest of Milin (Figure 1(c)), where [8], which extends from the Kailas in the west to the Namche a well-preserved sedimentary stratum is exposed Barwa in the east. It is widely considered to have been a (Figures 3(a)–3(d)). It is ~10 m thick but has good continuity typical Andean-type convergent continental margin prior to (>100 m). The lower part of the succession is characterized the Paleocene India-Eurasia collision [10, 17–20]. Studies of by medium-grained sandstone and interbedded mudstone. the Gangdese batholith have revealed its long magmatic his- Above this, the strata transition to relatively intact units of tory from the Middle Triassic to the Miocene, with peaks in medium-grained sandstone (~2 m thick), which overlies the activity at 109–80 Ma and 55–45 Ma [18, 21–29]. Specifically, lower sequence conformably (Figure 2(a)). Further up, the the Gangdese batholith has a juvenile isotopic composition, section is composed of ~2 m thick gneiss, with preferentially for example, positive εHf ðtÞ values [18, 19, 25, 26, 28, 30, 31]. oriented biotite and muscovite (Figure 4(a)). Thin-bedded The north Lhasa terrane consists of Early Cretaceous volca- sandstone and gneiss interbedded with weakly bedded mud- nic rocks and granites (130–100 Ma) [9, 32]. In contrast, stone occur near the upper part of the succession. There are the central Lhasa terrane consists of ancient crystalline base- no faults developed in this section area. Samples Ld01–07 ment (Nyainqentanglha group) and Early Cretaceous (130– were collected from the bottom to the top (Figure 2(a)).

110 Ma) granites that were produced by melting the ancient ° basement materials [9, 18, 32, 33]. 3.2. Section 2. Section 2 (GPS location: 29 20.9195′ N, ° The sampling sites are located in the mélange unit in the 94 24.3924′ E) is located approximately 2.5 km northwest Milin-Zedong area on the southeastern margin of the Lhasa of Baga Town (Figure 1(c)) and is characterized by medium- terrane (Figure 1(b)). The study area mainly consists of two to thick-bedded sandstones and highly metamorphosed

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Granitoids: Volcano-sedimentary rocks: Songpan-Ganzi Kunlun Tarim JSSZ Gangdese batholith Upper Jurassic-Lower Cretaceous Sangri Group Qiangtang Mesozoic granites in Lower Cretaceous volcano- BNSZ central Lhasa sedimentary rocks

Lhasa Terrane IYZSZ Others: BNSZ Xigaze forearc basin Himalayas Fig.b Research area 250 km (a) Yanhu

SNMZ BNSZ LMF Nyima Nagqu Northern Lhasa

Central Lhasa SNMZ IYZSZ Luoza Damxung

N LMF Bomi W E Milin S 100 km Section 4 Nang Sections 1, 2, 3 Nanga bawa group Dazhuka conglomerate Late Triassic Himalaya sediments Nyingchi group Nyingchi group Gangdese batholith Qudegong group Gangdese batholith Plate boundary Unconformable contact Section 2 Fault Ductile shear zone Section 3 L Sample point Pt1

Pt L K 1 1

K 1 T N 3 Section 1 Pt1

5 km KL Pz q 1 (c) (d)

Quaternary sediments Q Q Dazhuka conglomerate KL.rom MRF Late Triassic Himalaya sediments Jinlu Section 4 Kagyu group

T 3 KG (e)

Figure 1: (a) Tectonic framework of the Tibetan Plateau. (b) Simpliﬁed tectonic units of the Lhasa terrane showing the spatial and temporal distribution of the magmatic rocks in the Gangdese arc and the central Lhasa terrane (modiﬁed from ref. 9). (c–e) Geologic maps showing the sample collection locations in the Milin region (sections 1–3), Nang, and Jinlu (section 4), respectively. JSSZ: JinShajiang suture zone; BNSZ: Bangong-Nujiang suture zone; SNMZ: Shiquan River-Nam Tso mélange zone; LMF: Luobadui-Milashan fault; IYZSZ: Indus-Yarlung Zangpo suture zone.

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(m) (m) (m) (m) Bg04 10 20 50 500~2500Ma 390~570Ma Ld07 101~242Ma Ld06 97~188Ma 60 Dzg07 Jl03 Ld05 106~170Ma Dzg06 Ld04 136~179Ma 93~207Ma Ld03 105~174Ma

Ld02 0 Ld01 391~2826Ma 10 ms f mc 40 50 Legend Bg03 Sandstone

Jl02 Bg02 Schist 99~199Ma Bg01 0 30 ms f mc 40 Mudstone

Dzg05 Marble

20 30 Dzg04 Amphibolite

Gneiss (medium grain) Jl01

98~125Ma

10 Gneiss (fne grain) 20

Dzg03 Ophiolitic conglomerate Dzg02 500~1900Ma

Conglomerate Dzg01 0 510~2200Ma 10 ms fmc

Ophiolite

c sfmc c: clay s: siltstone

f: fnestone Samples m: medium sandstone c: coarse sandstone 0 csfmc

Figure 2: Lithostratigraphy of the studied sections. The GPS locations of the samples and the prominent detrital zircon U-Pb age populations are illustrated.

rocks (Figure 2(b)). Grayish-green amphibolite (~5 m thick) thick) (Figure 3(e)). The succession continues upwards into is locally present in the lower part of the succession and is intercalated thin-bedded schist and ﬁne- to medium- conformably overlain by thickly bedded sandstones (~2m grained gneisses (~10 m thick). The succession is terminated

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Top Downward fner

Bottom Bottom

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Interbedded sandstone and shale

(j) (k) (l)

(m) (n) (o)

Figure 3: Field photographs: (a) photograph of section 1; (b) interbedded sandstone and mudstone at the bottom of section 1; (c) contact between the sandstone and gneiss in section 1; (d) gneiss in section 1; (e) photograph of section 2; (f) bottom of section 3; (g) middle of section 3; (h) top of section 3; (i) the boundary between section 4 and the ophiolitic conglomerate; (j, k) interbedded sandstone and mudstone in section 4; (l) conglomerate at the top of section 4; (m) photograph of the sedimentary rocks near Nang County; (n) sandstone sample Lx01 near Nang County; (o) intrusive vein in the sedimentary rocks near Nang County.

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(a) (b)

200 �m 500 �m

500 �m 500 �m

(e) (f)

500 �m 200 �m

Figure 4: Representative microphotographs of samples from the investigated sedimentary strata: (a) gneiss from section 1 (Ld05); (b–d) volcaniclastic sandstones from section 4 (Jl01–03); (e) sandstone from Nang County (Lx01); (f) intrusive granitic vein near Nang County (Lx02). Bt: biotite; Ms: muscovite; Qtz: quartz; Pl: plagioclase; Hbl: hornblende; Lv: volcanic rock fragment; VG: volcanic glass.

by conformably overlying ﬁne-grained sandstones, which are 3(h)). The lower part of the succession includes thinly bedded up to 2 m thick. There are no faults developed in this section. mudstone and thickly bedded sandstone intercalations We collected samples Bg01–04 from the bottom to the top. (~3.2 m). The middle part of the succession has increasing The sandstone is composed of abundant monocrystalline occurrences of thinly to thickly bedded mudstone, sandstone, quartz gains (~80%), which are likely cemented by quartz and schist (up to 6 m thick), which are conformably overlain overgrowth, and dark detrital grains (e.g., biotite) are locally by thickly bedded mudstone (Figure 2(c)). The succession present. passes upwards into thickly bedded caesious, calcareous sandstones (up to 1 m thick), thickly bedded mudstones 3.3. Section 3. A comparable sedimentary sequence (section (~1.5 m thick), and schist. Thinly bedded marbles occur 3) is present on the hillside, approximately 2 km north of sporadically near the base and top of the succession ° ° Danzugang (GPS location: 29 19.395′ N, 94 21.2385′ E) (Figure 2(c)). There are no obvious faults developed in this (Figure 1(c)). This section is composed of units of sandstone, section. We then collected samples Dzg01–07 from the mudstone, and schist, which are up to 50 m thick (Figures 3(f)– bottom to the top of the succession.

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° 3.4. Section 4. The succession (GPS location: 29 12.9050′ N, working voltage during the CL imaging was at 10.0 kV. The ° 91 37.5318′ E) outcrops on a hill slope southeast of Jinlu zircon U-Pb isotopic measurements were conducted using Town, Zedong, and is up to 60 m thick (Figure 1(e)). It begins an Agilent 7500a quadrupole inductively coupled plasma with ophiolitic conglomerates at the bottom and is termi- mass spectrometer (Q-ICPMS) and a 193 nm excimer ArF nated by an ophiolitic mélange at the top, which is likely part laser-ablation system (Geolas Plus) at the IGGCAS using 207 206 of the Robuza ophiolitic mélange (Figures 3(i) and 3(j)). The the methods detailed by Xie et al. [44]. The Pb/ Pb and 206 238 lower part of the succession is characterized by alternating Pb/ U ratios were calculated using GLITTER4.0 [45] thinly bedded mudstone, siltstone, and sandstone (~20 m and were calibrated using measurements of Harvard zircon thick). Upwards, the occurrence of medium- to coarse- 91500 [46]. The weighted mean U-Pb age and concordia grained and medium- to thickly bedded sandstone layers plots were obtained using ISOPLOT v. 3.0 [47]. The in situ increases. These units were deposited directly on top of the zircon Hf isotopic analysis was performed using a Thermo- lower sequence (Figures 2(d) and 3(k) and 3(l)). A thickly Finnigan Neptune multi-collector (MC)-ICPMS connected bedded conglomerate layer (~3 m thick) is locally present, to a 193 nm Excimer ArF laser-ablation system (Geolas plus) and it is in turn unconformably overlain by the ophiolitic at the IGGCAS. The detailed analytical procedures can be mélange. Samples Jl01–03 were collected from the bottom found in Wu et al. [48]. to the top of this succession, and they consist of moderately sorted grains of quartz (~25%), hornblende, and volcanic 5. Results lithic fragments (~70%). Occasional, well-preserved undevi- trified volcanic glass was observed (Figures 4(b)–4(d)). The petrographic analyses of the 11 sandstone samples indicate that most of the sandstone compositions fall into the 3.5. Nang. Additional exposures in the Nang-Dongga were lithic, quartzo-lithic, and litho-quartzose descriptive petro- ° studied for comparison (GPS location: 29 1.0737′ N, graphic classification fields on the QFL ternary diagram ° 93 10.7954′ E). The stratum dips steeply to the northeast at (Figure 5(a)) [42]. However, almost all the samples are ° angles of up to 60 . Large blocks of thickly bedded, gray- located in the arc basement setting field on the LmLvLs dia- grayish green quartz sandstone are present (Figures 3(m) gram except sample Jl01 from section 1 (Figure 5(a)) [43]. and 3(n)). This is likely the allochthonous forearc sedimen- The U-Pb zircon dating of 15 samples and the in situ tary component, and therefore, only a relatively fresh sand- Hf isotopic analysis of 11 samples were carried out at the stone sample (Lx01) was collected for comparison same position or in the same zircon domain. Not all of the (Figure 3(n)). The grains of sample Lx01 are subangular to zircon grains have combined U-Pb and Hf analyses due to angular and moderately sorted with sutured contacts. The either the small grain size or the presence of late meta- quartz grains predominate and exhibit a preferred oriented morphism and metamictization. The petrographic data (Figure 4(e)). In particular, we found a celadon intrusive vein and isotopic results are given in the supplementary mate- ° ° near Nang Bridge (GPS location: 29 3.2679′ N, 93 4.1637′ E) rial (Tables S1, S2, and S3). Representative detrital zircon (Figure 3(o)). The vein mainly consists of deformed plagio- cathodoluminescence (CL) images for all of the samples clase, quartz, and a small amount of biotite (Figure 4(f)). are shown in Figure 6.

4. Analytical Methods 5.1. Section 1. A total of seven samples were collected from section 1 (Figure 2), and six fine- to medium-grained sand- 4.1. Sandstone Modal Analysis. Eleven samples of medium- stone samples underwent detailed U-Pb dating and Hf iso- to coarse-grained sandstone with well-preserved textures tope analysis. were cut for standard thin sections. An optical microscope Sample Ld01 contained abundant zircons that were was used to determine their minerology and petrology. At mostly subhedral and variable in size (100–150 μm). Surface least 300 points were counted in each sample following the cracks were locally present (Figure 6(a)). The zircons gener- Gazzi-Dickinson methods [40, 41]. Standard ternary dia- ally had relatively high Th/U ratios (0.13–2.02). Nineteen zir- grams (quartz-feldspar-lithic (QFL) fragment ternary dia- con grains with extremely high radiogenic Pb values (up to gram and metamorphic-volcanic-sedimentary (LmLvLs) 100 ppm) had typical ages of >1000 Ma. Of the 99 detrital zir- ternary plot) were plotted after Garzanti [42, 43]. cons dated, 86 had concordant ages (Figure 7(a)). The oldest age was 2827 ± 8 Ma, and the youngest age was 391 ± 5 Ma. 4.2. Zircon U-Pb Dating and Hf Isotopic Analysis. Zircon The main age population ranged from 1741 ± 5 to 502 ± 4 crystals were separated from the crushed rocks using heavy- Ma, with peaks at 500, 1100, and 1700 Ma (Figure 7(b); liquid and magnetic separation techniques, and individual Table S1). crystals were handpicked. The detrital zircons were ran- The detrital zircons from samples Ld03–07 exhibited domly picked, mounted in epoxy resin, and polished to similar characteristics, such as oscillatory zoning, slightly remove the upper one-third of the grain. The cathodolumi- elongated prisms (Figures 6(b)–6(f)), and high Th/U ratios nescence (CL) analysis was conducted at the Institute of (0.17–1.35). In contrast, the concordance-filtered (90– Geology and Geophysics, Chinese Academy of Sciences 110%) zircon populations of these five samples all clustered (IGGCAS), and the images obtained were used to observe in the Mesozoic (Table S1). The prominent age populations the internal structures of the zircons and to select appropriate of these five samples can be simply described as follows. (1) spots for in situ U-Pb dating and Hf isotopic analysis. The In sample Ld03, 100 detrital zircons were dated, and 93 of

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Q Lm Arc basement

Litho-quartzose

lFQ fLQ Dissected Feldapatho-quartzose arc

Quartzo-lithic

lQF fQF

qLF qFL Transitional Quartzo-feldspathic arc Litho-feldspathic Feldspatho-lithic Undissected arc F L Lv Ls (a) (b)

Sections Section 1 Section 4 Section 2 Nang Section 3

Figure 5: (a) Quartz-feldspar-lithic (QFL) fragment ternary diagram for the lithological classiﬁcation (after ref. [42]). (b) Metamorphic- volcanic-sedimentary (LmLvLs) ternary plot for the discrimination of the tectonic setting (after ref. [43]).

the ages were usable. The concordia diagram is shown in observed (Figure 6(g)). The U-Pb dating results of seventy Figure 7(d). The sample yielded ages are ranging from 174 detrital zircons from sample Bg04 had concordant ages, and ±18 to 105 ± 5 Ma, with peaks at 130 and 140 Ma the concordia diagram is shown in Figure 8(a). Except for (Figure 7(e); Table S1). (2) Ninety-seven detrital zircons two Cretaceous zircon ages of 93 ± 4 and 100 ± 5 Ma, the from sample Ld04 were dated, and 96 of the zircons were other zircon ages (90%–110% concordant; 64 out of 70) Mesozoic in age, except for zircon no. 18 (1685 ± 7 Ma). ranged from 566 ± 17 to 339 ± 28 Ma, with a peak at Among the 96 Mesozoic zircons, 90 zircons had concordant ~480 Ma (Figure 8(b); Table S1). ages (179 ± 6 to 135 ± 6 Ma; with a peak at 150 Ma). The other six zircons had discordant ages (nos. 49, 54, 71, 86, 5.3. Section 3. Representative samples from section 3 were 88, and 96) (Figures 7(g) and 7(h); Table S1). (3) Samples analyzed to provide basic coverage of the succession Ld05–07 had similar age distributions (~150–100 Ma), with (Dzg01, Dzg02, and Dzg07). These samples contained abun- a peak at 130 Ma (Figures 7(j), 7(k), 7(m), 7(n), 7(p), and dant zircons that were mostly subrounded and variable in 7(q)). sized (50–200 μm) (Figures 6(h)–6(j)). Mesoproterozoic A total 464 Hf isotope analyses were obtained for the detrital zircons (>1000 Ma) with relatively high radiogenic dated zircons from section 1. The Proterozoic to Cambrian Pb values (up to 100 ppm) were present (up to 28, 14, and detrital zircons (~2500–500 Ma) had variable 176Hf/177Hf 27 grains, respectively) in the three samples. One hundred isotopic ratios, with εHf ðtÞ values of -26.13 to +4.13 detrital zircons from sample Dzg01 were dated, providing (Figure 7(c); Table S2). In marked contrast, the Mesozoic 75 usable ages. The concordia diagram is shown in detrital zircons had two distinct groups of 176Hf/177Hf Figure 9(a). Sample Dzg01 was characterized by a wide age isotopic ratios. Samples Ld03–04 were characterized by range of 2761 ± 9 to 603 ± 12 Ma, with peaks at 950, 1650, strongly positive εHf ðtÞ values of +4.80 to +23.99 and 1750 Ma (Figure 9(b); Table S1). Ninety-nine detrital (Figures 7(f) and 7(i); Table S2), whereas samples Ld05–07 zircons from sample Dzg02 were dated and all, but six had negative εHf ðtÞ values of -18.51 to -2.38 (Figures 7(l), zircons (nos. 19, 33, 35, 71, 97, and 99) yielded good 7(o), and 7(r); Table S2). concordant ages (Figure 9(c)). The concordant zircons from sample Dzg02 had ages of 2235 ± 7 to 427 ± 15 Ma, with the 5.2. Section 2. The low-grade metamorphosed sandstone dominant fraction having ages of 1862 ± 8 to 489 ± 8 Ma, (Bg04), near the top of the succession, was sampled and ana- and the main peaks occurred at 530, 870, 1030, and lyzed. The detrital zircons from this sample were mostly sub- 1750 Ma (Figure 9(d); Table S1). For sample Dzg07, 70 of hedral to euhedral, 100–200 μm long, and had length/width the 91 detrital zircons yielded usable ages. The concordia ratios of 2–3 (Figure 6(g)). Oscillatory zoning was commonly diagram is shown in Figure 9(e). The concordance-ﬁltered

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50 �m 50 �m

(a) (b) 50 �m 50 �m

(e) (f) 50 �m 50 �m

(g) (h) 50 �m 50 �m

(i) (j) 50 �m 50 �m

(k) (l) � 50 �m 50 m

(m) (n) 50 �m

(o)

Figure 6: Selected cathodoluminescence (CL) images of the zircons analyzed. The analytical spots of the in situ U-Pb isotope dating and Hf isotopic analysis are highlighted by white solid circles and yellow dashed circles, respectively. Age values are written in white, and εHf ðtÞ values are written in yellow.

(90%–110%) zircon populations of sample Dzg07 mainly lower uranium contents or to Pb loss. The concordia cluster from 1903 ± 11 to 545 ± 14 Ma, with peaks at 900 diagram for the 53 usable ages is shown in Figure 10(d). and 1750 Ma (Figure 9(f); Table S1). The concordant (90%–110%; n =53) data had a relatively restricted age range of 199 ± 6 to 99 ± 8 Ma, with a peak at 5.4. Section 4. Three sandstone samples (Jl01–03) from the 110 Ma (Figure 10(e); Table S1). In contrast, sample Jl03 is well-preserved sedimentary stratum in Jinlu, Zedong, were characterized by a predominantly older age population analyzed (Figure 2). The detrital zircons of the three samples (~190 Ma). Twenty-six out of the 90 detrital zircons had had similar characteristics, that is, 50–100 μm long and sub- discordant ages, exhibiting a greater 207Pb/235U age error hedral to rounded. Some of the grains exhibited igneous- (Table S1). The usable ages all had good concordance related oscillatory zoning (Figures 6(k)–6(m)) and had high (Figure 10(g)), with ages mainly ranging from 207 ± 9 to 93 Th/U ratios (0.29–1.95). For sample Jl01, 100 detrital zircons ±3Ma(Figure 10(h); Table S1). were dated and all, but three zircons (nos. 37, 41, and 50) In terms of the Hf isotope compositions of the zircons, all yielded good concordant ages (Figure 10(a); Table S1). The of the detrital zircons from sample Jl01 had strongly positive 97 usable ages (90%–110% concordant) ranged from 125 ± εHf ðtÞ values of +4.22 to +14.82 (Figure 10(c); Table S2). In 2 to 98 ± 3 Ma, with a peak at 107 Ma (Figure 10(b)). contrast, the detrital zircons from sample Jl02 yielded Unfortunately, only 71 detrital zircons were obtained from predominantly positive εHf ðtÞ values of +5.29 to +19.94, sample Jl02, and a quarter of these grains showed and a minor fraction had low negative εHf ðtÞ values of signiﬁcant discordance (Table S1). This is likely due to their -17.47 to -5.28 (n =11; Figure 10(f); Table S2). The

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25 Ld07 n = (85/101) 0.045 35 20 Ld07 Depleted mantle Quartz sandstone 260 Ld07 30 15 T C (n = 85) DM = 0.5 Ga 10 0.035 C

U 25 T ) = 1.0 Ga

t 5 DM ( 238 CHUR

20 Hf 0

� C

Pb/ 0.025 T = 1.6 Ga Number 15 –5 DM 206 –10 10 0.015 –15 T C = 2.5 Ga 5 –20 DM 0.005 0 –25 0.10.0 0.2 0.3 0.4 0 50 100 150 200 250 50 100 150 200 250 25 (p) (q) (r0) n 40 Ld06 = (93/100) 20 Ld06 Depleted mantle 0.035 35 Ld06 Mica gneiss 15 C (n = 93) T = 0.5 Ga 30 10 DM C U

) T 25 t 5 DM = 1.0 Ga

0.025 ( 238

Hf 0 CHUR 20 � Pb/ T C Number –5 DM = 1.6 Ga 206 15 0.015 –10 10 –15 T C = 2.5 Ga 5 –20 DM 0.005 0 –25 0.00.1 0.2 0.3 0.4 0 50 100 150 200 250 80 100 120 140 160 180 200 (m) (n) (o)

0.032 n Ld05 = (100/102) 60 20 Ld05 Depleted mantle Ld05 Mica gneiss 15 T C 0.028 50 (n = 100) DM = 0.5 Ga 10 C

U T 0.024 ) = 1.0 Ga

40 t 5 DM ( 238 CHUR Hf 0 0.020 � C Pb/ 30 T = 1.6 Ga

Number –5 DM 206 0.016 20 –10 –15 T C 0.012 10 DM = 2.5 Ga –20 0.008 0 –25 0.00 0.04 0.160.120.08 0.240.200.28 0 50 100 150 200 250 80 100 120 140 160 180 200 (j) (k) (l) Up-section 25 Ld04 n = (90/97) 50 0.032 20 Ld04 Depleted mantle Quartz sandstone Ld04 15 40 n T C = 0.5 Ga ( = 90) 10 DM 0.028 C U T ) = 1.0 Ga

t 5 DM ( 238 30 CHUR Hf 0 � C Pb/ 0.024 T = 1.6 Ga Number –5 DM 206 20 –10 0.020 –15 10 T C = 2.5 Ga –20 DM 0.016 0 –25 0.02 0.06 0.180.140.10 0.250.220.30 0 50 100 150 200 250 80 100 120 140 160 180 200 (g) (h) (i) 25 Ld03 (n = 93/100) 20 0.035 25 Ld03 Depleted mantle Sandstone Ld03 15 T C = 0.5 Ga n DM 20 ( = 93) 10 T C U ) = 1.0 Ga

t 5 DM ( 238 0.025 CHUR 15 Hf 0 � C

Pb/ T DM = 1.6 Ga Number –5 206 10 0.015 –10 –15 C 5 T = 2.5 Ga –20 DM 0.005 0 –25 0.0 0.1 0.2 0.3 0.4 0 50 100 150 200 250 80 100 120 140 160 180 200 (d) (e) (f) 25 n 25 Ld03 ( = 86/99) 20 Depleted mantle 0.6 Biotite quartz sandstone Ld01 Ld01 15 T C = 0.5 Ga 20 DM (n = 86) 10 C

U T ) DM = 1.0 Ga t 5 ( 238 0.4 15 CHUR

Hf 0 � C Pb/ T = 1.6 Ga Number –5 DM 206 10 0.2 –10 –15 C 5 T = 2.5 Ga –20 DM 0.0 0 –25 04 2016128 0 5001000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 207Pb/235U Zircon U-Pb age (Ma) Zircon U-Pb age (Ma) (a) (b) (c)

Figure 7: Concordia diagrams, frequency distribution histograms, and Hf isotope analytical results for the detrital zircons from samples Ld01 and Ld03–07 from section 1 (see Figure 2 for sample locations). n = total number of zircon analyses.

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0.12 35 Bg04Bg04 n = (66/70)(66/70) Bg04 QuartzQuartz feldsparfeldspar sandstone (n = 66) 0.10 30

0.08 25 U ber

238 20

0.06 m Pb/ NumberNu

206 15 0.04 10 0.02 5

0.00 0 0.0 0.2 0.4 0.6 0.8 1.0 0 200 400 600 800 1000 Zircon U-Pb age (Ma) 207Pb/235U (a) (b)

Figure 8: Concordia diagrams and frequency distribution histograms for sample Bg04 from section 2 (see Figure 2 for sample location). n = total number of analyses.

majority of the detrital zircons from sample Jl03 had segment of the Indus-Yarlung Zangpo suture zone where superchondritic εHf ðtÞ values of +3.35 to +20.61, but three the suture belt has been destroyed by the continuous conver- zircons had negative εHf ðtÞ values (nos. 10, 32, and 89) gence of India and Eurasia. Based on studies of other seg- (Figure 10(i); Table S2). ments of the suture belt, there contains various rock units, including the Xigaze forearc sediments, the Yarlung- 5.5. Nang. One representative sandstone sample (Lx01) and Zangpo ophiolite, syn- to postcollisional molasse deposits an extra vein sample (Lx02) were analyzed for comparison. (e.g., the Liuqu conglomerate and Kailas/Gangriboche con- Most of the zircons from sample Lx01 were subhedral, glomerate or the Kailas Formation), and mélange and India around 100 μm long, pale gray in color, with localized occur- passive margin sediments ([8] and reference therein). There- rences of tiny inclusions (Figure 6(n)), and high Th/U ratios fore, the sediments collected from Milin-Zedong are proba- (0.20–1.01). Ninety-nine detrital zircons from sample Lx01 bly dismembered blocks (e.g., [49]) derived/recycled from were dated. Ninety-eight zircons yielded Mesozoic ages, these units, that is, forearc sediments and passive margin sed- and one zircon yielded a Paleoproterozoic age, i.e., zircon iment from the Himalayas or the Lhasa terrane. The charac- no. 18 (2370 ± 5 Ma) (Table S1). The concordant ages are teristic zircon U-Pb-Hf features of these units in the predominantly group at 140 ± 7 to 102 ± 4 Ma, with a peak Himalayan-Tibetan orogenic belt can undoubtedly shed light at ~120 Ma (Figures 11(a) and 11(b); Table S1). The zircons on the evolution of the Gangdese arc (e.g., [18, 28]) and can separated from sample Lx02 were characterized by euhedral provide information for the paleogeographic reconstruction to subhedral shapes, oscillatory zoning, pale gray rims of the Lhasa terrane [50] and for sedimentary provenance- (Figure 6(o)), and high Th/U ratios (0.45–1.07). Apart from related studies of the surrounding basins [4, 5, 51]. two grains with relatively older ages, that is, zircon no. 15 (107:4±3:9Ma) and zircon no. 19 (111:4±2:6Ma), the concordant data (97%–101%) within the dominant age 6.2. Section 1. This section is characterized by medium- distribution yielded a weighted mean 206Pb/238U age of grained sandstone and interbedded mudstone, which is con- 91:59 ± 0:97 Ma (MSWD = 1:3, n =17) (Figures 11(d) and formably overlain by relatively intact units of sandstone. The 11(e)). modal petrographic data indicate that the sandstones from The corresponding εHf ðtÞ values of the zircons from sam- section 1 primarily plot within the litho-quartzose and ple Lx01 were generally between +7.83 and +19.93 quartzo-lithic descriptive petrographic classification fields (Figure 11(c)). The Late Cretaceous zircon population from (Figure 5(a)) [42]. An arc basement setting is indicated by sample Lx02 yielded a more restricted range of positive εHf ðtÞ the LmLvLs diagram (Figure 5(b)) [43]. values of +9.81 to +13.06 (Figure 11(f)), with resulting The detrital zircons from the samples collected from dif- Early Triassic to Cambrian first and second Hf model ages ferent levels of section 1 exhibit variable characteristics. Sam- of 375–232 Ma and 539–318 Ma, respectively (Table S2). ple Ld01, which is from the lowest level of this section, consists of Archean to Paleozoic zircons (2827–391 Ma), with 6. Discussion three peaks at 539–502, 1170–920, and 1741–1602 Ma, which is similar to the ages of the Nyingtri group and the 6.1. Provenance of the Investigated Sediments in Milin- Carboniferous-Permian strata of the Lhasa terrane Zedong. The samples investigated in this study were collected (Figure 12). Thus, the lower part of section 1 formed before from the locally exposed sedimentary rocks in the eastern the formation or exposure of the Mesozoic magmatic rocks.

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0.8 DzDzg07g07 n = ((70/91)70/91) 2020 Dzg070 SanSandstonedstone (n = 770) 0.6 3000 15 2600 U ber 238 22220000

0.4 m

Pb/ 10

18001800 NumberNu 206 14001400 0.2 10001001 00 5

0.0 0 0468100 2 4 6 8 10 0 5001 1000000 1 1500500 2000 22500500 300 3000 (e)(e) (f) 0.5 1166 DzDzg02g02 n = (93(93/99)/99) 14 DzgDzg02022 Quartz sandstone 2200 (n = 93)3 0.4 12 18018000 10 U 0.3 ber 238 11400400

m 8 Pb/ NumberNu Up-section 206 0.2 10010000 6

660000 4 0.1 2

0.0 0 046810 2 4 6 8 1000 500500 10001000 1500 2000 25003 300000 ((c)c) (d)(d)

n 14 DDzg01zg01 = ((75/100)75/100) 14 DzgDzg01011 FeFeldsparldspar quartz sandstonesandstone n 0.6 12 ( = 75)5 26002600 10

U 22002200 ber 238 0.4 8

18001800 m Pb/

NumberNu 6 206 14001400 0.2 10001000 4

0.0 0 06814 20 500 1000 1500 2000 2500 3000 207Pb/235U Zircon U-Pb age (Ma) (a) (b)

Figure 9: Concordia diagrams and frequency distribution histograms for samples Dzg01, 02, and 07 from section 3 (see Figure 2 for sample locations). n = total number of analyses.

The sandstone samples from the middle part of the sec- Ld05–07 were collected from the upper level of section 1. tion (Ld03 and Ld04) are dominated by Jurassic to Early Cre- Although these samples consist of Mesozoic zircons with taceous zircon U-Pb ages with predominantly high, positive similar ages, most of the zircons have low Hf isotopic values εHf ðtÞ values, which are similar to the Hf isotopic character- and negative εHf ðtÞ values, which is distinctly different from istics of the Gangdese arc (Figure 13) (e.g., [18, 28, 38, 52]). the samples in the middle part of this section. Similar changes The characteristics of these sediments are consistent with a were also documented in the middle to upper Ngamring derivation from the Gangdese arc. From sample Ld03 to Formation in the Xigaze forearc basin and were interpreted Ld04, the youngest zircon age increased significantly from as the contribution from provenances in the central Lhasa 105 ± 5 Ma to 135 ± 6 Ma, indicating that the earlier volcanic terrane [4, 5], in which the magmatic rocks are characterized sequences in the sediment source area were being rapidly by negative zircon εHf ðtÞ values (Figure 13) (e.g., [9, 52, 53]). eroded, that is, the unroofing of the Gangdese arc. Samples Furthermore, the youngest detrital U-Pb zircon population

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0.045 25 30 Jl03 n = (64/90) 20 Jl03 Depleted mantle Jl03 15 C 0.035 Sandstone 25 n T = 0.5 Ga ( = 64) 10 DM C

U T 20 5 DM = 1.0 Ga 238 0.025 ( t ) CHUR

Hf 0 C

Pb/ 15 � T = 1.6 Ga Number –5 DM 206 10 –10 0.015 –15 5 T C = 2.5 Ga –20 DM 0.005 0 –25 0.0 0.1 0.2 0.3 0.4 0.5 0 50 100 150 200 250 50 100 150 200 250 (g) (h) (i) 25 n 25 Jl02 = (53/71) 20 Jl02 Depleted mantle 0.035 Sandstone Jl02 15 T C 20 (n = 53) DM = 0.5 Ga 180 10 C U 5 T 0.025 15 DM = 1.0 Ga 238 ( t ) CHUR

Hf 0 C � Pb/ T = 1.6 Ga Number –5 DM

206 10

0.015 –10 Up-section –15 5 T C = 2.5 Ga –20 DM 0.005 0 –25 0.0 0.1 0.2 0.3 0.4 0.5 0 50 100 150 200 250 80 100 120 140 160 180 200 (d) (e) (f) 0.022 25 50 20 Depleted mantle Jl02 n = (97/100) Jl01 Jl01 0.020 15 C Sandstone 40 (n = 97) T = 0.5 Ga 10 DM C

U T = 1.0 Ga 0.020 30 5 DM 238 ( t ) CHUR

Hf 0 C � Pb/ T 0.016 Number –5 DM = 1.6 Ga

206 20 –10 0.014 10 –15 T C = 2.5 Ga –20 DM 0.012 0 –25 0.00 0.04 0.08 0.12 0.16 0.20 0.24 0 50 100 150 200 250 80 100 120 140 160 180 200 207Pb/235U Zircon U-Pb age (Ma) Zircon U-Pb age (Ma) (a) (b) (c)

Figure 10: Concordia diagrams, frequency distribution histogram, and Hf isotope analytical results for the detrital zircons from samples JL01–03, section 4 (see Figure 2 for sample locations). n = total number of analyses.

35 25 0.034 220 20 Depleted mantle 30 0.030 15 T C DM = 0.5 Ga 0.026 25 10 T C U ) DM = 1.0 Ga t 5

20 ( 238 0.022 CHUR Hf 0

� C Pb/ T 15 DM = 1.6 Ga 0.018 Number –5 206 0.014 10 –10 –15 0.010 5 T C = 2.5 Ga –20 DM 0.006 0 –25 0.0 0.1 0.2 0.3 0.4 0 50 100 150 200 250 80 100 120 140 160 180 200 (a) (b) (c) 0.016 100 25 20 Depleted mantle 15 T C = 0.5 Ga 0.015 95 DM 10 T C

U = 1.0 Ga

) DM

t 5 ( 238 CHUR 0.014 90 Hf 0 C

� T Pb/ DM = 1.6 Ga Age (Ma) Age

206 –5 –10 0.013 85 –15 T C DM = 2.5 Ga –20 0.012 80 –25 0.06 0.08 0.10 0.12 80 100 120 140 160 180 200 207Pb/235U Zircon U-Pb age (Ma) (d) (e) (f)

Figure 11: (a, b) Concordia diagrams and frequency distribution histogram for sample Lx01. (d, e) Concordia diagrams and weighted average U-Pb age of sample Lx02. (c, f) Hf isotope analytical results for samples Lx01 and Lx02, respectively. n = total number of analyses.

of the samples from this section is between 97 and 108 Ma. agreement with the evidence from foraminiferal assemblages This is consistent with the maximum depositional age of [54]. However, the change in the provenance of the sedi- the oldest strata of the Ngamring Formation [6, 7] and is in ments in the study area occurred much faster than in the

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6.3. Sections 2 and 3. Sections 2 and 3 are mainly composed of 20 Section 1 (n = 86) medium-thickly bedded sandstone, mudstone, and meta- 15 morphic rocks, e.g., gneiss and schist. The ternary diagrams show that the samples from these two sections are quartzo- 10

Number lithic and feldspatho-lithic, with more lithics on the QFL dia- 5 gram (Figure 5(a)) [42]. Furthermore, the sandstone samples (a) from this section have an arc basement setting (Figure 5(b)) 0 [43], which also explains the metamorphism of these 45 40 Section 3 samples. 35 (n = 238) The samples collected from sections 2 and 3 are domi- 30 25 nated by pre-Mesozoic detrital zircons (Figures 8 and 9). 20 The sample from section 2 (Bg04) is characterized by unim- Number 15 odal ages of ~480 Ma, whereas the samples from section 3 10 5 (b) exhibit multiple peaks from the Early Paleozoic to the Paleo- proterozoic, that is, at 1800–1500, 1200–900, and 600– 160 500 Ma. The three age peaks of the detrital zircons from 140 Nyingtri group 120 (n = 870) section 3 are similar to those of the samples from the lower 100 level of section 1 (Figures 12(a) and 12(b)), indicating that 80 section 3 belongs to an earlier sedimentary sequence devel- Number 60 oped before the uplift and erosion of the Gangdese arc. These 40 20 age spectra were found in the Carboniferous-Permian strata (c) 0 of the Lhasa terrane (e.g., [61, 64]) and in the Metasedimen- 300 tary rocks of the Nyingtri group in the Milin-Nyingtri area 250 Carboniferous-permian strata (e.g., [56, 58–60]) (Figure 12). However, the Neoarchean 250 of the Lhasa terrane peak at ~2500 Ma was not identified there (Figure 12). Thus, (n = 1333) 150 the pre-Mesozoic detrital zircons from the lower level of the Number 100 section were derived from the Lhasa terrane before the uplift 50 and erosion of the Gangdese arc. (d) 0 180 6.4. Section 4. In section 4, alternating conglomerate, sand- 160 stone, and shale sediments were deposited. In addition, the 140 Tethyan Himalaya thickening- and coarsening-upward sequences of the 120 (n = 721) 100 restored units (up to 60 m thick) (Figure 2(d)) are also well 80 Number documented in the Ngamring Formation in the Xigaze 60 40 forearc basin [3, 7], which is probably part of the preexisting 20 (e) forearc sedimentary succession. The microphotographs of 0 the three samples (Jl01–03) from section 4 indicate that most 160 of the minerals have undergone significant metamorphism, 140 High Himalaya n 120 ( = 749) and the boundaries of the minerals are unclear 100 (Figures 4(b)–4(d)). Therefore, they were not ideal for petro- 80 graphic point-counting, and only one sample (Jl01) was Number 60 selected for petrographic analysis. The samples are plotted 40 within the lithics area on the QFL diagram [42] and within 20 (f) 0 the undissected arc setting field on the LmLvLs diagram 0 500 1000 1500 2000 2500 3000 3500 [43], which is quite different from the samples from the other Zircon U-Pb age (Ma) sections (Figure 5(b)). Figure 12: Summary of the detrital zircon ages obtained in this The U-Pb age spectra of the detrital zircons from samples study and in previous studies: (a) data for sample Ld01 in section Jl01 and Jl02 are similar with peaks at ~110 Ma (Figures 10(b) 1; (b) data for section 3; (c) data from [53, 56–60]; (d) data from and 10(e)). However, the in situ Hf isotopic characteristics of [50, 61–64]; (e) data from [9, 65–68]; (f) data from [69, 70]. n = these two samples are different. All of the detrital zircons total number of analyses . in sample Jl01 record a Gangdese arc origin, with positive εHf ðtÞ values. In contrast, the detrital zircons from sample Jl02 predominantly have positive εHf ðtÞ values, but a minor fraction has negative εHf ðtÞ values (n =11) indicating the Xigaze forearc basin, which is demonstrated by the fact that contribution from a provenance of the central Lhasa terrane. the samples from the upper level are dominated by detrital This provenance shift is similar to that reported for the zircons from the central Lhasa terrane. This is because of Ngamring Formation in the Xigaze forearc basin [6, 7]. Fur- the diachronous topographic growth in the Lhasa terrane thermore, the age distribution of sample Jl03 is characterized occurred earlier in the eastern part [55]. by older ages with a peak at ~190 Ma, which is quite different

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Ld01 Ld03 Ld04 Jl01 Jl02 Section 1 Section 3 Nang Ld05 Ld06 Ld07 Jl03 25

20 Depleted mantle 15 T C DM = 0.5 Ga Depleted mantle 10 C = 0.1 Ga C = 1.6 Ga T C T DM 5 DM = 1.0 Ga T DM C = 2.0 Ga

( t ) CHUR CHUR 0 T DM Hf � T C –5 DM = 1.6 Ga –10 C = 3.0 Ga Central Lhasa terrane T DM –15 Damxung T C = 2.5 Ga –20 DM –25 50 100 150 200 250 1250 2250 3000 Zircon U-Pb age (Ma)

Figure 13: Hf isotopic characteristics of the analyzed samples. Hf isotopic compositions of the magmatic rocks from the Gangdese arc and the central Lhasa terrane. Detrital zircons from the Damxung conglomerates are presented for comparison (data sources: [9, 18, 28, 38, 52, 72, 73], and references therein).

from the ages of samples Jl01 and Jl02 (Figures 10(b), 10(e), contain Paleozoic and even older detrital zircons. These and 10(h)). However, the Hf isotopic characteristics of zircons have similar age spectra, including peaks at 1800– sample Jl03 are similar to those of sample Jl02. That is, they 1500, 1200–900, and 600–500 Ma (Figures 12(a) and 12(b)). predominantly record a Gangdese arc source with positive These spectra are more similar to that of the Nyingtri group εHf ðtÞ values, except for a small number of detrital zircons and the Carboniferous-Permian strata of the Lhasa terrane (n =3) with negative εHf ðtÞ values, which indicates that they than to the Tethyan and High Himalaya terranes are from the central Lhasa terrane. The U-Pb age spectra of (Figure 12). Thus, these sediments were mainly derived from the detrital zircons changed from unimodal in sample Jl01 the Lhasa terrane. Based on the zircon U-Pb age and Hf iso- (peak at 108 Ma) to bimodal in samples Jl02 and Jl03 tope data for the detrital zircons from section 1, section 4, (Figure 10). This change was accompanied by a decrease in and the Nang area, we tentatively proposed that these sedi- the number of Early Cretaceous zircons and an increase in ments were deposited in a forearc setting. Although section the number of Jurassic zircons. Similar evolution patterns 1 is thin, its continuous profile indicates a long-term evolu- were also found in the lower to middle parts of the Ngamring tion of provenances on the southern margin of the Lhasa ter- Formation in the Xigaze forearc basin [6, 7]. In addition, the rane. The detrital zircons from the bottom-most sample youngest U-Pb age population of the detrital zircons from (Ld01) have the source characteristics that indicate they were samples Jl01–03 is similar to that of the Ngamring Forma- eroded before the uplift of the Gangdese arc, and in the tion, which is also shown in section 1. Namco and Damxung areas, the strata contain detrital zircons with only pre-Mesozoic ages (cf. [61]). The zircon U- 6.5. Nang. Sample Lx01 was collected from Dongga in the Pb age and Hf isotope data and the transition in provenances Nang area (Figure 1(d)). This sample plots in the fQF field exhibited by the samples in the middle and upper levels of on the QFL diagram (Figure 5(a)) and in the arc basement section 1 are analogous to the characteristics of the Xigaze setting field on the LmLvLs diagram (Figure 5(b)) [43]. All forearc sediments [4, 5]. The middle and upper levels of sec- of the detrital zircons have Jurassic to Cretaceous ages tion 1 may correspond to the middle to upper parts of the (185–83 Ma) and positive εHf ðtÞ values (most >10, Ngamring Formation, respectively, while section 4 corre- Figure 11), which is similar to the detrital zircons from the sponds to the lower to middle parts of the Ngamring Forma- Xigaze forearc sediments and the zircons from the Gangdese tion. One potential explanation for the small thickness of arc. Therefore, there are locally preserved forearc sediments section 1 is that it developed in the interior margin of the in the Nang area. In the western part of Nang County, the forearc basin close to the arc. The inland parts of the forearc locally preserved stratum was intruded by a Late Cretaceous sediments were prone to being preserved during the later granitic dike (91:59 ± 0:97 Ma), which formed during the orogenesis, and this is consistent with the development of flare-up of the east Gangdese batholith [38]. We suggest that the Cretaceous granitic dike (sample Lx02) in the sediments the preserved sediments are located in an interior zone of the in the western part of Nang County. The large thickness of forearc area close to the volcanic arc, which guaranteed their section 4 indicates that it was located in an outer part of the survival during the later orogenesis. forearc basin, so it was preserved on top of the ophiolites. Sample Ld01 from the lower part of section 1 and all of Therefore, the most likely scenario is that these sediments the analyzed samples from section 3 (Dzg01, 02 and 07) only were deposited in a forearc setting (e.g., SE Tibet) but have

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120

100 Xigaze detrital zircons 80 (n = 340)

60 Number 40

20 (a) 0 120

100 Milin-Zedong 80 detrital zircons (n = 465)

60 Number 40

20 (b) 0 70 60 Milin-Zedong batholith 50 (n = 302) 40

Number 30 20 10 (c) 0 140

120 Gangdese batholith 100 (n = 682)

Number 60 40

20 (d) 0 0 50 100 150 200 250 Zircon U-Pb age (Ma)

Figure 14: (a) Relative probability of the U-Pb ages of the detrital zircons in the Xigaze sediments [4]. (b) Relative probability diagram of the U-Pb ages of detrital zircons in the Milin-Nang sediments. (c) Relative probability diagram of the U-Pb zircon age of the Milin-Zedong batholith (e.g., [9, 26, 27, 30, 31, 34, 36–39]). (d) Relative probability diagram of the U-Pb zircon age of the Gangdese batholith (e.g., [28, 73]). n = total number of analyses.

been tectonically dismembered (Figures 1(c)–1(e)) as a result with a Gangdese arc origin (n = 465) in the sandstones are of the Paleocene India-Eurasia collision. This is also consis- characterized by predominantly Early Jurassic to Early tent with the pattern of the Indus-Yarlung Zangpo suture Cretaceous (~200–100 Ma) age populations, with peaks at zone, which is locally distributed in the southeast but is rela- 110, 155, and 190 Ma (Figure 13(b)). However, the geo- tively continuous in the west [71] (Figure 1(b)). chronological evidence from the batholith exposed in the Milin-Zedong region suggests that the related magmatism 6.6. Constraints on the Early Jurassic to Early Cretaceous was mainly concentrated in the Cretaceous to Miocene Magmatic Evolution of the Gangdese Arc. The detrital zircons (108–16 Ma) (e.g., [9, 26–28, 30, 31, 34–39, 73]). In

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addition, the Middle-Lower Yeba Formation [11, 12] and Data Availability the Upper Jurassic-Lower Cretaceous Sangri group [13] fi developed along the batholith. The age distribution of The data used to support the ndings of this study, including the detrital zircons differs significantly from that of the the supporting materials, zircon U-Pb ages (Table S1), Hf Gangdese arc in the research area where Early Jurassic to isotopic compositions (Table S2), and modal petrographic Early Cretaceous magmatic rocks are scarce (Figures 14(b) data (Table S3), are included within the supplementary fi and 14(c)). A potential explanation for this is that the information les. magmatic rocks from this period have been eroded and con- tributed to the forearc sediments, that is, the Gangdese arc in Conflicts of Interest the Milin-Zedong region was already active and widespread in the Early Cretaceous and even as far back as the Early The authors declare that there is no conflict of interest Jurassic. regarding the publication of this article. Based on the above discussion (Section 6.1), the forearc sediments in the eastern segment (sections 1 and 4) of the Gangdese arc are consistent with Xigaze forearc Acknowledgments basin to the west. Furthermore, the Early Jurassic to Early This work was funded by the National Key R&D Program of Cretaceous detrital zircons in the sandstones have an age China (2016YFC0600407) and the National Science Founda- spectrum similar to that of the Xigaze forearc sediments tion of China (grants 41888101 and 41572055). We thank the (Figures 14(a) and 14(b)). This similarity may indicate a staff of the MC–ICPMS lab at the Institute of Geology and synchronous magmatic and erosion history for the entire Geophysics, Chinese Academy of Sciences, for their help with Gangdese arc from the eastern section to the western sec- the zircon U-Pb dating and Hf isotopic analysis. We thank tion, that is, the Gangdese arc was characterized by more LetPub (http://www.letpub.com) for its linguistic assistance active and continuous magmatism during the Early Juras- during the preparation of this manuscript. sic to Early Cretaceous than its bedrock recorded, which was accompanied by significant uplift and erosion during the deposition of the studied forearc sediments. Notably, Supplementary Materials the age spectra of the forearc sediments are complemen- tary to the age of the magmatism of the Gangdese arc Supplementary 1. Figure S1 Detailed geological maps show- (Figure 14). ing locations of the samples we collected in Milin, Nang, and Jinlu areas. Figure S2 Field photographs of outcrops in Milin-Nang area where no well-exposed potential forearc 7. Conclusions sediments (sample position can also be found in Figure S1c, d). Figure S3 Microphotographs of samples collected from The field investigations, petrographic data, and detrital Milin-Nang area where no well-exposed forearc sediments. zircon U-Pb geochronology and Hf isotopic geochemistry The numbers (a) to (h) in Figure S3 correspond to the (a) of the locally preserved forearc sediments in Milin-Zedong, to (h) in Figure S2, respectively. southern Tibet, led to the following conclusions: Supplementary 2. Table S1: Zircon U-Pb age. (1) The detrital zircon U-Pb ages and Hf isotope values Supplementary 3. Table S2: Hf isotope. of the sediments from the middle to upper parts of section 1, section 4, and the Nang area are similar Supplementary 4. Table S3: Recalculated modal petrographic to those of the Xigaze forearc sediments. 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