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 age (~200–100 Ma) and show clear affinity with the Gangdese arc rocks, that is, positive values. In contrast, the upper sequences are characterized by Mesozoic detrital zircons (150–100 Ma) and negative 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.
Forearc basins develop along convergent plate margins and receive detritus from the adjacent magmatic arc, and thus, they are important for studying the evolution of arcs [1–5]. The Xigaze forearc basin, located in the southern margin of the Lhasa terrane, southern Tibet, is one of the best exposed forearc basins in the world (e.g., [4, 6]). Numerous studies of the detrital zircon U-Pb ages and Hf isotopic characteristics of the Xigaze forearc sediments have been conducted. These studies have revealed its tectonic, erosional, and sedimentary evolution and have effectively constrained the magmatic history of the Gangdese arc [2, 3, 6, 7]. However, only restricted exposures of the forearc sediments have been documented along the huge Gangdese plutonic belt (>1500 km long) , that is, mainly outcropping near and to the west of Xigaze. In contrast, no forearc sediments have been reported in the eastern part of the Gangdese arc (e.g., [4, 5]). There are two possible explanations for this: either forearc sediments did not develop along this section of the arc or they developed but were destroyed during later orogenesis.
In this study, sporadic occurrences of potential forearc sedimentary records were verified in the Milin-Zedong region with a discontinuous distribution in the suture zone mélange (Figure 1 and Fig. S1). We conducted a detailed field investigation of four exposures in this region (Figure 2) and performed detrital zircon U-Pb geochronology and Hf isotope geochemistry on the samples collected from these exposures. Our study emphasized provenance tracing and comparative analysis with the detrital zircons in the Xigaze forearc basin with the goal of further constraining the magmatic evolution of the Gangdese arc.
2. Geologic Setting and Samples
The Tibetan Plateau is composed of several east–west-trending blocks, including the Himalayas, Lhasa, Qiangtang, Songpan-Ganzi, and Kunlun terranes from south to north  (Figure 1(a)). The Lhasa terrane is separated from the Himalayas by the Indus-Yarlung Zangpo suture zone to the south and is separated from the Qiangtang terrane by the Bangong-Nujiang suture to the north, which is interpreted to be the southernmost part of the Asian Plate  (Figure 1(a)).
The Lhasa terrane consists of the locally developed Precambrian metamorphic basement (i.e., the Nyainqentanglha group and Nyingtri group), the Paleozoic to Cenozoic sedimentary strata, and various types of magmatic rocks . The Luobadui-Milashan fault zone in the south and the Shiquan River-Nam Tso ophiolitic mélange belt in the north separate the Lhasa terrane into its southern, central, and northern parts  (Figure 1(b)). The south Lhasa terrane mainly consists of the juvenile Gangdese magmatic arc, including the Gangdese batholith and Mesozoic-Tertiary volcanic rocks such as the Middle-Lower Jurassic Yeba Formation [11, 12], the Upper Jurassic-Lower Cretaceous Sangri group , and the Paleocene Linzizong group [14–16]. The Gangdese batholith, which is an important part of the Trans-Himalayan batholith, occurs as a huge plutonic belt (>1500 km) along the southern margin of the Lhasa terrane , which extends from the Kailas in the west to the Namche Barwa in the east. It is widely considered to have been a typical Andean-type convergent continental margin prior to the Paleocene India-Eurasia collision [10, 17–20]. Studies of the Gangdese batholith have revealed its long magmatic history from the Middle Triassic to the Miocene, with peaks in activity at 109–80 Ma and 55–45 Ma [18, 21–29]. Specifically, the Gangdese batholith has a juvenile isotopic composition, for example, positive values [18, 19, 25, 26, 28, 30, 31]. The north Lhasa terrane consists of Early Cretaceous volcanic rocks and granites (130–100 Ma) [9, 32]. In contrast, the central Lhasa terrane consists of ancient crystalline basement (Nyainqentanglha group) and Early Cretaceous (130–110 Ma) granites that were produced by melting the ancient basement materials [9, 18, 32, 33].
The sampling sites are located in the mélange unit in the Milin-Zedong area on the southeastern margin of the Lhasa terrane (Figure 1(b)). The study area mainly consists of two tectonic units (from north to south): the widely distributed Cretaceous to Miocene Gangdese batholith (100–16 Ma) (e.g., [9, 26, 27, 30, 31, 34–39]) and the Yarlung Zangpo mélange zone (Figure 1(a)). There are exposures of sedimentary strata distributed sporadically along the Yarlung Zangpo River near Milin-Baga, but they are distributed more continuously near Nang-Dongga (Figures 1(c) and 1(d)). An additional, well-developed exposure of these sediments was also found in Jinlu in the Zedong area (Figure 1(e)).
In this study, a total of 23 samples were collected, of which 21 samples were collected from the four sections (see Figure 2 for detailed lithology and sample positions), and two samples were collected from Nang County (Figures 1(d) and 3(m)–3(o)). Samples Ld01–07 were collected from section 1 in the Milin area. They are mainly composed of sandstones (Ld01–04 and Ld07), but a small proportion is composed of gneiss (Ld05–06). Samples Bg01–04 were collected from section 2, which predominantly comprises metamorphic rocks. Samples Dzg01–07 were collected from section 3, including five sandstone samples (Dzg01–02 and Dzg04–06) and two marble samples (Dzg03 and Dzg07). Samples Jl01–03 are medium- to coarse-grained volcaniclastic sandstones collected from section 4. A relatively fresh sandstone (Lx01) and an intrusive vein sample (Lx02) were collected from Nang County. Their detailed stratigraphy and sedimentology are described in the next section. In addition, eight samples were collected from several other sporadic exposures of the suture zone mélange in Milin-Nang (see supplementary materials and Figures S1-S3 for more details).
3. Stratigraphy and Sedimentology
Exposures and remnants of potential forearc sediments were observed and described as follows. Additionally, detailed field and petrographic observations of several other outcrops are listed and illustrated in the Supplementary materials.
3.1. Section 1
Section 1 (GPS location: 29°13.1177 N, 94°11.3320 E) outcrops on a hillside near Lidi Village, approximately 5 km northwest of Milin (Figure 1(c)), where a well-preserved sedimentary stratum is exposed (Figures 3(a)–3(d)). It is ~10 m thick but has good continuity (>100 m). The lower part of the succession is characterized by medium-grained sandstone and interbedded mudstone. Above this, the strata transition to relatively intact units of medium-grained sandstone (~2 m thick), which overlies the lower sequence conformably (Figure 2(a)). Further up, the section is composed of ~2 m thick gneiss, with preferentially oriented biotite and muscovite (Figure 4(a)). Thin-bedded sandstone and gneiss interbedded with weakly bedded mudstone occur near the upper part of the succession. There are no faults developed in this section area. Samples Ld01–07 were collected from the bottom to the top (Figure 2(a)).
3.2. Section 2
Section 2 (GPS location: 29°20.9195 N, 94°24.3924 E) is located approximately 2.5 km northwest of Baga Town (Figure 1(c)) and is characterized by medium- to thick-bedded sandstones and highly metamorphosed rocks (Figure 2(b)). Grayish-green amphibolite (~5 m thick) is locally present in the lower part of the succession and is conformably overlain by thickly bedded sandstones (~2 m thick) (Figure 3(e)). The succession continues upwards into intercalated thin-bedded schist and fine- to medium-grained gneisses (~10 m thick). The succession is terminated by conformably overlying fine-grained sandstones, which are up to 2 m thick. There are no faults developed in this section. We collected samples Bg01–04 from the bottom to the top. The sandstone is composed of abundant monocrystalline quartz gains (~80%), which are likely cemented by quartz overgrowth, and dark detrital grains (e.g., biotite) are locally present.
3.3. Section 3
A comparable sedimentary sequence (section 3) is present on the hillside, approximately 2 km north of Danzugang (GPS location: 29°19.395 N, 94°21.2385 E) (Figure 1(c)). This section is composed of units of sandstone, mudstone, and schist, which are up to 50 m thick (Figures 3(f)–3(h)). The lower part of the succession includes thinly bedded mudstone and thickly bedded sandstone intercalations (~3.2 m). The middle part of the succession has increasing occurrences of thinly to thickly bedded mudstone, sandstone, and schist (up to 6 m thick), which are conformably overlain by thickly bedded mudstone (Figure 2(c)). The succession passes upwards into thickly bedded caesious, calcareous sandstones (up to 1 m thick), thickly bedded mudstones (~1.5 m thick), and schist. Thinly bedded marbles occur sporadically near the base and top of the succession (Figure 2(c)). There are no obvious faults developed in this section. We then collected samples Dzg01–07 from the bottom to the top of the succession.
3.4. Section 4
The succession (GPS location: 29°12.9050 N, 91°37.5318 E) outcrops on a hill slope southeast of Jinlu Town, Zedong, and is up to 60 m thick (Figure 1(e)). It begins with ophiolitic conglomerates at the bottom and is terminated by an ophiolitic mélange at the top, which is likely part of the Robuza ophiolitic mélange (Figures 3(i) and 3(j)). The lower part of the succession is characterized by alternating thinly bedded mudstone, siltstone, and sandstone (~20 m thick). Upwards, the occurrence of medium- to coarse-grained and medium- to thickly bedded sandstone layers increases. These units were deposited directly on top of the lower sequence (Figures 2(d) and 3(k) and 3(l)). A thickly bedded conglomerate layer (~3 m thick) is locally present, and it is in turn unconformably overlain by the ophiolitic mélange. Samples Jl01–03 were collected from the bottom to the top of this succession, and they consist of moderately sorted grains of quartz (~25%), hornblende, and volcanic lithic fragments (~70%). Occasional, well-preserved undevitrified volcanic glass was observed (Figures 4(b)–4(d)).
Additional exposures in the Nang-Dongga were studied for comparison (GPS location: 29°1.0737 N, 93°10.7954 E). The stratum dips steeply to the northeast at angles of up to 60°. Large blocks of thickly bedded, gray-grayish green quartz sandstone are present (Figures 3(m) and 3(n)). This is likely the allochthonous forearc sedimentary component, and therefore, only a relatively fresh sandstone sample (Lx01) was collected for comparison (Figure 3(n)). The grains of sample Lx01 are subangular to angular and moderately sorted with sutured contacts. The quartz grains predominate and exhibit a preferred oriented (Figure 4(e)). In particular, we found a celadon intrusive vein near Nang Bridge (GPS location: 29°3.2679 N, 93°4.1637 E) (Figure 3(o)). The vein mainly consists of deformed plagioclase, quartz, and a small amount of biotite (Figure 4(f)).
4. Analytical Methods
4.1. Sandstone Modal Analysis
Eleven samples of medium- to coarse-grained sandstone with well-preserved textures were cut for standard thin sections. An optical microscope was used to determine their minerology and petrology. At least 300 points were counted in each sample following the Gazzi-Dickinson methods [40, 41]. Standard ternary diagrams (quartz-feldspar-lithic (QFL) fragment ternary diagram and metamorphic-volcanic-sedimentary (LmLvLs) ternary plot) were plotted after Garzanti [42, 43].
4.2. Zircon U-Pb Dating and Hf Isotopic Analysis
Zircon crystals were separated from the crushed rocks using heavy-liquid and magnetic separation techniques, and individual crystals were handpicked. The detrital zircons were randomly picked, mounted in epoxy resin, and polished to remove the upper one-third of the grain. The cathodoluminescence (CL) analysis was conducted at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), and the images obtained were used to observe the internal structures of the zircons and to select appropriate spots for in situ U-Pb dating and Hf isotopic analysis. The working voltage during the CL imaging was at 10.0 kV. The zircon U-Pb isotopic measurements were conducted using an Agilent 7500a quadrupole inductively coupled plasma mass spectrometer (Q-ICPMS) and a 193 nm excimer ArF laser-ablation system (Geolas Plus) at the IGGCAS using the methods detailed by Xie et al. . The 207Pb/206Pb and 206Pb/238U ratios were calculated using GLITTER4.0  and were calibrated using measurements of Harvard zircon 91500 . The weighted mean U-Pb age and concordia plots were obtained using ISOPLOT v. 3.0 . The in situ zircon Hf isotopic analysis was performed using a Thermo-Finnigan Neptune multi-collector (MC)-ICPMS connected to a 193 nm Excimer ArF laser-ablation system (Geolas plus) at the IGGCAS. The detailed analytical procedures can be found in Wu et al. .
The petrographic analyses of the 11 sandstone samples indicate that most of the sandstone compositions fall into the lithic, quartzo-lithic, and litho-quartzose descriptive petrographic classification fields on the QFL ternary diagram (Figure 5(a)) . However, almost all the samples are located in the arc basement setting field on the LmLvLs diagram except sample Jl01 from section 1 (Figure 5(a)) .
The U-Pb zircon dating of 15 samples and the in situ Hf isotopic analysis of 11 samples were carried out at the same position or in the same zircon domain. Not all of the zircon grains have combined U-Pb and Hf analyses due to either the small grain size or the presence of late metamorphism and metamictization. The petrographic data and isotopic results are given in the supplementary material (Tables S1, S2, and S3). Representative detrital zircon cathodoluminescence (CL) images for all of the samples are shown in Figure 6.
5.1. Section 1
Sample Ld01 contained abundant zircons that were mostly subhedral and variable in size (100–150 μm). Surface cracks were locally present (Figure 6(a)). The zircons generally had relatively high Th/U ratios (0.13–2.02). Nineteen zircon grains with extremely high radiogenic Pb values (up to 100 ppm) had typical ages of >1000 Ma. Of the 99 detrital zircons dated, 86 had concordant ages (Figure 7(a)). The oldest age was , and the youngest age was . The main age population ranged from to , with peaks at 500, 1100, and 1700 Ma (Figure 7(b); Table S1).
The detrital zircons from samples Ld03–07 exhibited similar characteristics, such as oscillatory zoning, slightly elongated prisms (Figures 6(b)–6(f)), and high Th/U ratios (0.17–1.35). In contrast, the concordance-filtered (90–110%) zircon populations of these five samples all clustered in the Mesozoic (Table S1). The prominent age populations of these five samples can be simply described as follows. (1) In sample Ld03, 100 detrital zircons were dated, and 93 of the ages were usable. The concordia diagram is shown in Figure 7(d). The sample yielded ages are ranging from to , with peaks at 130 and 140 Ma (Figure 7(e); Table S1). (2) Ninety-seven detrital zircons from sample Ld04 were dated, and 96 of the zircons were Mesozoic in age, except for zircon no. 18 (). Among the 96 Mesozoic zircons, 90 zircons had concordant ages ( to ; with a peak at 150 Ma). The other six zircons had discordant ages (nos. 49, 54, 71, 86, 88, and 96) (Figures 7(g) and 7(h); Table S1). (3) Samples Ld05–07 had similar age distributions (~150–100 Ma), with a peak at 130 Ma (Figures 7(j), 7(k), 7(m), 7(n), 7(p), and 7(q)).
A total 464 Hf isotope analyses were obtained for the dated zircons from section 1. The Proterozoic to Cambrian detrital zircons (~2500–500 Ma) had variable 176Hf/177Hf isotopic ratios, with values of -26.13 to +4.13 (Figure 7(c); Table S2). In marked contrast, the Mesozoic detrital zircons had two distinct groups of 176Hf/177Hf isotopic ratios. Samples Ld03–04 were characterized by strongly positive values of +4.80 to +23.99 (Figures 7(f) and 7(i); Table S2), whereas samples Ld05–07 had negative values of -18.51 to -2.38 (Figures 7(l), 7(o), and 7(r); Table S2).
5.2. Section 2
The low-grade metamorphosed sandstone (Bg04), near the top of the succession, was sampled and analyzed. The detrital zircons from this sample were mostly subhedral to euhedral, 100–200 μm long, and had length/width ratios of 2–3 (Figure 6(g)). Oscillatory zoning was commonly observed (Figure 6(g)). The U-Pb dating results of seventy detrital zircons from sample Bg04 had concordant ages, and the concordia diagram is shown in Figure 8(a). Except for two Cretaceous zircon ages of and , the other zircon ages (90%–110% concordant; 64 out of 70) ranged from to , with a peak at ~480 Ma (Figure 8(b); Table S1).
5.3. Section 3
Representative samples from section 3 were analyzed to provide basic coverage of the succession (Dzg01, Dzg02, and Dzg07). These samples contained abundant zircons that were mostly subrounded and variable in sized (50–200 μm) (Figures 6(h)–6(j)). Mesoproterozoic detrital zircons (>1000 Ma) with relatively high radiogenic Pb values (up to 100 ppm) were present (up to 28, 14, and 27 grains, respectively) in the three samples. One hundred detrital zircons from sample Dzg01 were dated, providing 75 usable ages. The concordia diagram is shown in Figure 9(a). Sample Dzg01 was characterized by a wide age range of to , with peaks at 950, 1650, and 1750 Ma (Figure 9(b); Table S1). Ninety-nine detrital zircons from sample Dzg02 were dated and all, but six zircons (nos. 19, 33, 35, 71, 97, and 99) yielded good concordant ages (Figure 9(c)). The concordant zircons from sample Dzg02 had ages of to , with the dominant fraction having ages of to , and the main peaks occurred at 530, 870, 1030, and 1750 Ma (Figure 9(d); Table S1). For sample Dzg07, 70 of the 91 detrital zircons yielded usable ages. The concordia diagram is shown in Figure 9(e). The concordance-filtered (90%–110%) zircon populations of sample Dzg07 mainly cluster from to , with peaks at 900 and 1750 Ma (Figure 9(f); Table S1).
5.4. Section 4
Three sandstone samples (Jl01–03) from the well-preserved sedimentary stratum in Jinlu, Zedong, were analyzed (Figure 2). The detrital zircons of the three samples had similar characteristics, that is, 50–100 μm long and subhedral to rounded. Some of the grains exhibited igneous-related oscillatory zoning (Figures 6(k)–6(m)) and had high Th/U ratios (0.29–1.95). For sample Jl01, 100 detrital zircons were dated and all, but three zircons (nos. 37, 41, and 50) yielded good concordant ages (Figure 10(a); Table S1). The 97 usable ages (90%–110% concordant) ranged from to , with a peak at 107 Ma (Figure 10(b)). Unfortunately, only 71 detrital zircons were obtained from sample Jl02, and a quarter of these grains showed significant discordance (Table S1). This is likely due to their lower uranium contents or to Pb loss. The concordia diagram for the 53 usable ages is shown in Figure 10(d). The concordant (90%–110%; ) data had a relatively restricted age range of to , with a peak at 110 Ma (Figure 10(e); Table S1). In contrast, sample Jl03 is characterized by a predominantly older age population (~190 Ma). Twenty-six out of the 90 detrital zircons had discordant ages, exhibiting a greater 207Pb/235U age error (Table S1). The usable ages all had good concordance (Figure 10(g)), with ages mainly ranging from to (Figure 10(h); Table S1).
In terms of the Hf isotope compositions of the zircons, all of the detrital zircons from sample Jl01 had strongly positive values of +4.22 to +14.82 (Figure 10(c); Table S2). In contrast, the detrital zircons from sample Jl02 yielded predominantly positive values of +5.29 to +19.94, and a minor fraction had low negative values of -17.47 to -5.28 (; Figure 10(f); Table S2). The majority of the detrital zircons from sample Jl03 had superchondritic values of +3.35 to +20.61, but three zircons had negative values (nos. 10, 32, and 89) (Figure 10(i); Table S2).
One representative sandstone sample (Lx01) and an extra vein sample (Lx02) were analyzed for comparison. Most of the zircons from sample Lx01 were subhedral, around 100 μm long, pale gray in color, with localized occurrences of tiny inclusions (Figure 6(n)), and high Th/U ratios (0.20–1.01). Ninety-nine detrital zircons from sample Lx01 were dated. Ninety-eight zircons yielded Mesozoic ages, and one zircon yielded a Paleoproterozoic age, i.e., zircon no. 18 () (Table S1). The concordant ages are predominantly group at to , with a peak at ~120 Ma (Figures 11(a) and 11(b); Table S1). The zircons separated from sample Lx02 were characterized by euhedral to subhedral shapes, oscillatory zoning, pale gray rims (Figure 6(o)), and high Th/U ratios (0.45–1.07). Apart from two grains with relatively older ages, that is, zircon no. 15 () and zircon no. 19 (), the concordant data (97%–101%) within the dominant age distribution yielded a weighted mean 206Pb/238U age of (, ) (Figures 11(d) and 11(e)).
The corresponding values of the zircons from sample Lx01 were generally between +7.83 and +19.93 (Figure 11(c)). The Late Cretaceous zircon population from sample Lx02 yielded a more restricted range of positive values of +9.81 to +13.06 (Figure 11(f)), with resulting Early Triassic to Cambrian first and second Hf model ages of 375–232 Ma and 539–318 Ma, respectively (Table S2).
6.1. Provenance of the Investigated Sediments in Milin-Zedong
The samples investigated in this study were collected from the locally exposed sedimentary rocks in the eastern segment of the Indus-Yarlung Zangpo suture zone where the suture belt has been destroyed by the continuous convergence of India and Eurasia. Based on studies of other segments of the suture belt, there contains various rock units, including the Xigaze forearc sediments, the Yarlung-Zangpo ophiolite, syn- to postcollisional molasse deposits (e.g., the Liuqu conglomerate and Kailas/Gangriboche conglomerate or the Kailas Formation), and mélange and India passive margin sediments ( and reference therein). Therefore, the sediments collected from Milin-Zedong are probably dismembered blocks (e.g., ) derived/recycled from these units, that is, forearc sediments and passive margin sediment from the Himalayas or the Lhasa terrane. The characteristic zircon U-Pb-Hf features of these units in the Himalayan-Tibetan orogenic belt can undoubtedly shed light on the evolution of the Gangdese arc (e.g., [18, 28]) and can provide information for the paleogeographic reconstruction of the Lhasa terrane  and for sedimentary provenance-related studies of the surrounding basins [4, 5, 51].
6.2. Section 1
This section is characterized by medium-grained sandstone and interbedded mudstone, which is conformably overlain by relatively intact units of sandstone. The modal petrographic data indicate that the sandstones from section 1 primarily plot within the litho-quartzose and quartzo-lithic descriptive petrographic classification fields (Figure 5(a)) . An arc basement setting is indicated by the LmLvLs diagram (Figure 5(b)) .
The detrital zircons from the samples collected from different levels of section 1 exhibit variable characteristics. Sample Ld01, which is from the lowest level of this section, consists of Archean to Paleozoic zircons (2827–391 Ma), with three peaks at 539–502, 1170–920, and 1741–1602 Ma, which is similar to the ages of the Nyingtri group and the Carboniferous-Permian strata of the Lhasa terrane (Figure 12). Thus, the lower part of section 1 formed before the formation or exposure of the Mesozoic magmatic rocks.
The sandstone samples from the middle part of the section (Ld03 and Ld04) are dominated by Jurassic to Early Cretaceous zircon U-Pb ages with predominantly high, positive values, which are similar to the Hf isotopic characteristics of the Gangdese arc (Figure 13) (e.g., [18, 28, 38, 52]). The characteristics of these sediments are consistent with a derivation from the Gangdese arc. From sample Ld03 to Ld04, the youngest zircon age increased significantly from to , indicating that the earlier volcanic sequences in the sediment source area were being rapidly eroded, that is, the unroofing of the Gangdese arc. Samples Ld05–07 were collected from the upper level of section 1. Although these samples consist of Mesozoic zircons with similar ages, most of the zircons have low Hf isotopic values and negative values, which is distinctly different from the samples in the middle part of this section. Similar changes were also documented in the middle to upper Ngamring Formation in the Xigaze forearc basin and were interpreted as the contribution from provenances in the central Lhasa terrane [4, 5], in which the magmatic rocks are characterized by negative zircon values (Figure 13) (e.g., [9, 52, 53]). Furthermore, the youngest detrital U-Pb zircon population of the samples from this section is between 97 and 108 Ma. This is consistent with the maximum depositional age of the oldest strata of the Ngamring Formation [6, 7] and is in agreement with the evidence from foraminiferal assemblages . However, the change in the provenance of the sediments in the study area occurred much faster than in the Xigaze forearc basin, which is demonstrated by the fact that the samples from the upper level are dominated by detrital zircons from the central Lhasa terrane. This is because of the diachronous topographic growth in the Lhasa terrane occurred earlier in the eastern part .
Sections 2 and 3 are mainly composed of medium-thickly bedded sandstone, mudstone, and metamorphic rocks, e.g., gneiss and schist. The ternary diagrams show that the samples from these two sections are quartzo-lithic and feldspatho-lithic, with more lithics on the QFL diagram (Figure 5(a)) . Furthermore, the sandstone samples from this section have an arc basement setting (Figure 5(b)) , which also explains the metamorphism of these samples.
The samples collected from sections 2 and 3 are dominated by pre-Mesozoic detrital zircons (Figures 8 and 9). The sample from section 2 (Bg04) is characterized by unimodal ages of ~480 Ma, whereas the samples from section 3 exhibit multiple peaks from the Early Paleozoic to the Paleoproterozoic, that is, at 1800–1500, 1200–900, and 600–500 Ma. The three age peaks of the detrital zircons from section 3 are similar to those of the samples from the lower level of section 1 (Figures 12(a) and 12(b)), indicating that section 3 belongs to an earlier sedimentary sequence developed before the uplift and erosion of the Gangdese arc. These age spectra were found in the Carboniferous-Permian strata of the Lhasa terrane (e.g., [61, 64]) and in the Metasedimentary rocks of the Nyingtri group in the Milin-Nyingtri area (e.g., [56, 58–60]) (Figure 12). However, the Neoarchean peak at ~2500 Ma was not identified there (Figure 12). Thus, the pre-Mesozoic detrital zircons from the lower level of the section were derived from the Lhasa terrane before the uplift and erosion of the Gangdese arc.
6.4. Section 4
In section 4, alternating conglomerate, sandstone, and shale sediments were deposited. In addition, the thickening- and coarsening-upward sequences of the restored units (up to 60 m thick) (Figure 2(d)) are also well documented in the Ngamring Formation in the Xigaze forearc basin [3, 7], which is probably part of the preexisting forearc sedimentary succession. The microphotographs of the three samples (Jl01–03) from section 4 indicate that most of the minerals have undergone significant metamorphism, and the boundaries of the minerals are unclear (Figures 4(b)–4(d)). Therefore, they were not ideal for petrographic point-counting, and only one sample (Jl01) was selected for petrographic analysis. The samples are plotted within the lithics area on the QFL diagram  and within the undissected arc setting field on the LmLvLs diagram , which is quite different from the samples from the other sections (Figure 5(b)).
The U-Pb age spectra of the detrital zircons from samples Jl01 and Jl02 are similar with peaks at ~110 Ma (Figures 10(b) and 10(e)). However, the in situ Hf isotopic characteristics of these two samples are different. All of the detrital zircons in sample Jl01 record a Gangdese arc origin, with positive values. In contrast, the detrital zircons from sample Jl02 predominantly have positive values, but a minor fraction has negative values () indicating the contribution from a provenance of the central Lhasa terrane. This provenance shift is similar to that reported for the Ngamring Formation in the Xigaze forearc basin [6, 7]. Furthermore, the age distribution of sample Jl03 is characterized by older ages with a peak at ~190 Ma, which is quite different from the ages of samples Jl01 and Jl02 (Figures 10(b), 10(e), and 10(h)). However, the Hf isotopic characteristics of sample Jl03 are similar to those of sample Jl02. That is, they predominantly record a Gangdese arc source with positive values, except for a small number of detrital zircons () with negative values, which indicates that they are from the central Lhasa terrane. The U-Pb age spectra of the detrital zircons changed from unimodal in sample Jl01 (peak at 108 Ma) to bimodal in samples Jl02 and Jl03 (Figure 10). This change was accompanied by a decrease in the number of Early Cretaceous zircons and an increase in the number of Jurassic zircons. Similar evolution patterns were also found in the lower to middle parts of the Ngamring Formation in the Xigaze forearc basin [6, 7]. In addition, the youngest U-Pb age population of the detrital zircons from samples Jl01–03 is similar to that of the Ngamring Formation, which is also shown in section 1.
Sample Lx01 was collected from Dongga in the Nang area (Figure 1(d)). This sample plots in the fQF field on the QFL diagram (Figure 5(a)) and in the arc basement setting field on the LmLvLs diagram (Figure 5(b)) . All of the detrital zircons have Jurassic to Cretaceous ages (185–83 Ma) and positive values (most >10, Figure 11), which is similar to the detrital zircons from the Xigaze forearc sediments and the zircons from the Gangdese arc. Therefore, there are locally preserved forearc sediments in the Nang area. In the western part of Nang County, the locally preserved stratum was intruded by a Late Cretaceous granitic dike (), which formed during the flare-up of the east Gangdese batholith . We suggest that the preserved sediments are located in an interior zone of the forearc area close to the volcanic arc, which guaranteed their survival during the later orogenesis.
Sample Ld01 from the lower part of section 1 and all of the analyzed samples from section 3 (Dzg01, 02 and 07) only contain Paleozoic and even older detrital zircons. These zircons have similar age spectra, including peaks at 1800–1500, 1200–900, and 600–500 Ma (Figures 12(a) and 12(b)). These spectra are more similar to that of the Nyingtri group and the Carboniferous-Permian strata of the Lhasa terrane than to the Tethyan and High Himalaya terranes (Figure 12). Thus, these sediments were mainly derived from the Lhasa terrane. Based on the zircon U-Pb age and Hf isotope data for the detrital zircons from section 1, section 4, and the Nang area, we tentatively proposed that these sediments were deposited in a forearc setting. Although section 1 is thin, its continuous profile indicates a long-term evolution of provenances on the southern margin of the Lhasa terrane. The detrital zircons from the bottom-most sample (Ld01) have the source characteristics that indicate they were eroded before the uplift of the Gangdese arc, and in the Namco and Damxung areas, the strata contain detrital zircons with only pre-Mesozoic ages (cf. ). The zircon U-Pb age and Hf isotope data and the transition in provenances exhibited by the samples in the middle and upper levels of section 1 are analogous to the characteristics of the Xigaze forearc sediments [4, 5]. The middle and upper levels of section 1 may correspond to the middle to upper parts of the Ngamring Formation, respectively, while section 4 corresponds to the lower to middle parts of the Ngamring Formation. One potential explanation for the small thickness of section 1 is that it developed in the interior margin of the forearc basin close to the arc. The inland parts of the forearc sediments were prone to being preserved during the later orogenesis, and this is consistent with the development of the Cretaceous granitic dike (sample Lx02) in the sediments in the western part of Nang County. The large thickness of section 4 indicates that it was located in an outer part of the forearc basin, so it was preserved on top of the ophiolites. Therefore, the most likely scenario is that these sediments were deposited in a forearc setting (e.g., SE Tibet) but have been tectonically dismembered (Figures 1(c)–1(e)) as a result of the Paleocene India-Eurasia collision. This is also consistent with the pattern of the Indus-Yarlung Zangpo suture zone, which is locally distributed in the southeast but is relatively continuous in the west  (Figure 1(b)).
6.6. Constraints on the Early Jurassic to Early Cretaceous Magmatic Evolution of the Gangdese Arc
The detrital zircons with a Gangdese arc origin () in the sandstones are characterized by predominantly Early Jurassic to Early Cretaceous (~200–100 Ma) age populations, with peaks at 110, 155, and 190 Ma (Figure 13(b)). However, the geochronological evidence from the batholith exposed in the Milin-Zedong region suggests that the related magmatism was mainly concentrated in the Cretaceous to Miocene (108–16 Ma) (e.g., [9, 26–28, 30, 31, 34–39, 73]). In addition, the Middle-Lower Yeba Formation [11, 12] and the Upper Jurassic-Lower Cretaceous Sangri group  developed along the batholith. The age distribution of the detrital zircons differs significantly from that of the Gangdese arc in the research area where Early Jurassic to Early Cretaceous magmatic rocks are scarce (Figures 14(b) and 14(c)). A potential explanation for this is that the magmatic rocks from this period have been eroded and contributed to the forearc sediments, that is, the Gangdese arc in the Milin-Zedong region was already active and widespread in the Early Cretaceous and even as far back as the Early Jurassic.
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 basin to the west. Furthermore, the Early Jurassic to Early Cretaceous detrital zircons in the sandstones have an age spectrum similar to that of the Xigaze forearc sediments (Figures 14(a) and 14(b)). This similarity may indicate a synchronous magmatic and erosion history for the entire Gangdese arc from the eastern section to the western section, that is, the Gangdese arc was characterized by more active and continuous magmatism during the Early Jurassic to Early Cretaceous than its bedrock recorded, which was accompanied by significant uplift and erosion during the deposition of the studied forearc sediments. Notably, the age spectra of the forearc sediments are complementary to the age of the magmatism of the Gangdese arc (Figure 14).
The field investigations, petrographic data, and detrital zircon U-Pb geochronology and Hf isotopic geochemistry of the locally preserved forearc sediments in Milin-Zedong, southern Tibet, led to the following conclusions:
The detrital zircon U-Pb ages and Hf isotope values of the sediments from the middle to upper parts of section 1, section 4, and the Nang area are similar to those of the Xigaze forearc sediments. The Mesozoic zircons with high, positive values suggest that the provenance of these sediments was the adjacent Gangdese arc, while the Mesozoic zircons with negative values are from the central Lhasa terrane. The studied sediments in section 1 and the Nang area were deposited in a forearc setting close to the arc, whereas the sediments in section 4 were deposited in the outer part of the forearc basin. The forearc basin was located in the Milin-Zedong segment of the Gangdese arc, but it was dismembered during the later orogenesis
We propose that the eastern Gangdese arc was characterized by more continuous Jurassic to Early Cretaceous magmatism than its bedrock records. The eastern Gangdese arc had an uplift and erosion history similar to that of the Middle Cretaceous middle-western parts of the arc
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this article.
This work was funded by the National Key R&D Program of China (2016YFC0600407) and the National Science Foundation of China (grants 41888101 and 41572055). We thank the staff of the MC–ICPMS lab at the Institute of Geology and Geophysics, Chinese Academy of Sciences, for their help with the zircon U-Pb dating and Hf isotopic analysis. We thank LetPub (http://www.letpub.com) for its linguistic assistance during the preparation of this manuscript.