We present new zircon U-Pb ages and Hf isotope compositions as well as whole-rock major- and trace-element geochemical and Sr-Nd isotopic data for silicic plutonic and volcanic rocks from the Duolong area of central Tibet. Combined with existing data, our new data indicate that these plutonic and volcanic rocks were formed in two stages ca. 120 Ma and ca. 110 Ma, respectively, in a postcollisional extensional setting that was triggered by slab breakoff. The similar geochemical compositions of granitoids and rhyolites, combined with their close spatial and temporal relationships, suggest that they were both derived from juvenile crustal material within a single magmatic system. We propose that the two inferred crustal melting events in the Duolong area were caused by two episodes of deep mantle activity triggered by the transition of the plate subduction angle from steep to shallow in response to the ascent of buoyant continental lithosphere during slab breakoff. Furthermore, rapid surface uplift during the late Early Cretaceous caused by slab breakoff made an important contribution to the formation of the proto–Tibetan Plateau. This study provides new insights into postcollisional tectonomagmatism and plateau uplift in central Tibet triggered by slab breakoff. We propose more generally that tectonic uplift during postcollisional processes (i.e., slab breakoff and lithospheric delamination) is a major contributor to plateau uplift in collision zones.
It has been proposed that the Tibetan Plateau was formed by the accretion of a series of blocks derived from the northern margin of Gondwana and the subsequent collision of the Indian and Asian continents (Figs. 1A and 1B; e.g., Murphy et al., 1997; Yin and Harrison, 2000; Chung et al., 2005, 2009). Uplift of the Tibetan Plateau has had a substantial effect on the topographic framework of Earth and on the global climate system during the Cenozoic (Dupont-Nivet et al., 2007; Royden et al., 2008; Hetzel et al., 2011). However, previous studies have indicated that the uplift of the Tibetan Plateau was progressive, and that the initial rapid surface uplift of central Tibet occurred during the late Mesozoic (e.g., Murphy et al., 1997; Wang et al., 2008, 2014a), which probably had an important influence on the topographic framework of the modern Tibetan Plateau. Abundant low-temperature thermochronologic data indicate that central Tibet underwent multistage uplift during the Cretaceous in response to the continental collision between the Lhasa and Qiangtang blocks, resulting in a proto–Tibetan Plateau prior to the India-Asia collision (Murphy et al., 1997; Kapp and DeCelles, 2019; Bi et al., 2020; Li et al., 2022). However, the spatialtemporal evolution and related geodynamics of this multistage uplift are uncertain due to a lack of constraints.
The Duolong area has received much research attention owing to the complex tectonism, magmatism, and mineralization that took place in the area during the Early Cretaceous, and the Duolong igneous association (a suite of Early Cretaceous plutonic and volcanic rocks) is present in this area of central Tibet. The Duolong igneous association consists of igneous rocks, including ocean-island basalt (OIB)– like gabbros, mineralization-associated granitoids, arc-like basalts and andesites, and rhyolites (Li et al., 2008, 2017a, 2017b; Geng et al., 2016; Zhang et al., 2017; Wu et al., 2019a). Studies have indicated that the Duolong area underwent a process of rapid surface uplift, resulting in the exhumation of mineralization-associated granitoids (ca. 120 Ma), which were then immediately covered by volcanic rocks (ca. 110 Ma; Yang et al., 2014, 2020; Song et al., 2018). The synexhumation magmatic rocks of the Duolong igneous association therefore provide an excellent opportunity to investigate the uplift and formation of the proto–Tibetan Plateau.
We present here new petrologic, geochemical, zircon U-Pb geochronologic, and isotopic data from volcanic and plutonic rocks in the Duolong area. These data provide better constraints on the age relationships and genetic links of the volcanic-plutonic rocks in this silicic magmatic system, and they also provide better constraints on the processes and geodynamic mechanisms of Early Cretaceous tectonic uplift in central Tibet.
The Tibetan Plateau records several continental collisional events since the early Paleozoic (Dewey et al., 1988; Zhu et al., 2011; Kapp and DeCelles, 2019). Central Tibet consists of two continental blocks separated by the Bangong-Nujiang suture zone, namely, the Lhasa block to the south and the Qiangtang block to the north (Fig. 1B). These two blocks have different geologic histories and were amalgamated along the Bangong-Nujiang suture zone, although the actual amalgamation process remains controversial (e.g., Zhu et al., 2011, 2016; Pan et al., 2012). The intense Mesozoic magmatic activity recorded within central Tibet is thought to have been associated with the geologic evolution of the Bangong-Nujiang Tethyan Ocean (e.g., Zhu et al., 2011, 2016; Wu et al., 2016a, 2019a, 2019b), whereas the late Early Cretaceous igneous rocks that are widely exposed on both flanks of the Bangong-Nujiang suture zone are generally thought to have been associated with postcollisional tectonism (e.g., Zhu et al., 2011, 2016; Wu et al., 2018, 2019a).
The Duolong Cu-Au mining district is located in the western Bangong-Nujiang suture zone, ~80 km northwest of Gerze County. The district contains extensive outcrops of Jurassic sandstone basement, as well as Early Cretaceous plutonic and volcanic rocks that vary in composition from mafic to silicic (Fig. 1C). The Jurassic strata contain turbidite deposits that are mixed with ophiolitic fragments, together forming an accretionary complex that is part of the Bangong-Nujiang suture zone. The mineralization-associated granitoids in this area are porphyritic, vary in composition from intermediate to silicic, and were emplaced into Jurassic sedimentary rocks and are locally covered by volcanic rocks (Li et al., 2008). Both the Jurassic basement and the volcanic rocks are overlain by terrestrial molasse deposits of the Upper Cretaceous Abushan Formation. In addition, an E-W–trending belt of mafic dikes of Early Cretaceous age has been identified in the central Duolong area (Xu et al., 2017; Wu et al., 2019a). Notably, unconformities occur beneath the Early Cretaceous volcanic rocks and the Late Cretaceous sedimentary rocks, indicating the occurrence of two uplift-denudation events.
Our study investigated plutonic and volcanic rocks in the Duolong area to establish their genesis and infer the geodynamic mechanism of the Early Cretaceous tectonomagmatic activity of central Tibet. Sample locations are shown in Figure 1C, and photo-micrographs of representative samples are shown in Figures 1D and 1E. Although there is no direct contact between the investigated plutonic and volcanic samples, previous studies have suggested that these rocks could represent the overlying and underlying members of the earlier unconformity contact (Song et al., 2018). Porphyritic granodiorites are intruded into the Jurassic sandstones and are covered by volcanic rocks. These granodiorites consist mainly of quartz and plagioclase phenocrysts (Fig. 1D), with accessory zircon, apatite, and monazite. In comparison, rhyolites from the Meiriqiecuo Formation are covered by Cenozoic deposits and are dominated by phenocrysts of plagioclase and quartz in a microlitic felsic groundmass (Fig. 1E).
U-Pb ZIRCON GEOCHRONOLOGIC DATA
U-Pb age data for zircons from two of our samples were obtained using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). The analytical methods and results are given in Table S11, and U-Pb concordia diagrams and cathodoluminescence (CL) images of the analyzed zircons are shown in Figure 2.
The analyzed zircons were generally long and prismatic, with aspect ratios of 2:1–3:1, and they exhibited clear concentric oscillatory zoning in CL images. In combination with their relatively high Th/U ratios (>0.1), this suggests that these zircons had a magmatic origin (Hoskin and Schaltegger, 2003). The zircons from the granodiorite porphyry yielded ages of 129–118 Ma and a weighted mean 206Pb/238U age of 124.9 ± 1.1 Ma (mean square of weighted deviates [MSWD] = 1.0; Fig. 2A). Zircons from the rhyolite yielded ages of 113–107 Ma and a weighted mean 206Pb/238U age of 109.7 ± 0.8 Ma (MSWD = 2.1), as well as three inherited zircon ages of 145–143 Ma (Fig. 2B). These new ages are contemporaneous with the ages of previously dated magmatic rocks in this area, and they represent the timing of emplacement of the volcanic and plutonic rocks in the Duolong area.
The analytical methods used to determine the whole-rock major- and trace-element compositions of samples and the associated results are given in Table S2. The granodiorite porphyries were metaluminous to weakly peraluminous and had medium- to high- K calc-alkaline affinities (Fig. 3). All samples were enriched in large ion lithophile elements (LILEs; e.g., Rb, Th, U, and Sr) but depleted in high field strength elements (HFSEs; e.g., Nb and Ta; Figs. 4A and 4B).
Compared with the granodiorite porphyries, the rhyolites were characterized by higher SiO2 (>70 wt%) but lower Fe2O3, MgO, CaO, and TiO2 contents. They were also high- K calc-alkaline and peraluminous rocks (Fig. 3), and they yielded low contents of Sr, negative Ba, Sr, and Eu anomalies (Figs. 4C and 4D), and high Rb/Ba and Rb/Sr ratios, all of which indicate that they are geochemically similar to high-silica rhyolites (Bachmann and Bergantz, 2004; Hildreth, 2004; Wu et al., 2017). Variations in selected major- and trace-element contents in these units are shown relative to SiO2 in the Harker diagrams of Figure 5. The rhyolites are more evolved than the granitoids, as evidenced by their plotting at the end of the evolutionary trend defined by the granitoids.
Sr-Nd-Hf ISOTOPE DATA
The Lu-Hf isotopic compositions of the dated zircons were determined at the same analytical sites used for U-Pb analyses, and the analytical approach and results are given in Table S3. Zircons from the porphyritic granodiorite yielded initial 176Hf/177Hf ratios of 0.282915–0.283108 and corresponding εHf(t) values and crustal Hf depleted mantle model ages (TDMC) of +7.58 to +14.46 and 625–239 Ma, respectively. Zircons from the rhyolites yielded initial 176Hf/177Hf ratios of 0.282959–0.283101 and corresponding εHf(t) values and TDMC ages of +8.75 to +13.79 and 547–265 Ma (Fig. 6A), respectively.
Whole-rock Sr-Nd isotopic data for the three rhyolite samples analyzed during this study are provided in Table S4. The rhyolites gave a narrow range of εNd(t) values (−1.03 to −1.31), with associated TDM2 ages of 1013–990 Ma and 87Sr/86Sr(i) values of 0.70466–0.70487 (Fig. 6B).
Spatial, Temporal, and Compositional Variation within the Duolong Igneous Association
The Duolong igneous association consists of different igneous rocks with diverse geochemical compositions that vary from mafic to silicic. This area represents one of the largest mining districts in Tibet and has been the focus of extensive research into the timing and evolution of magmatic activity. We combined our new data with previously published geochronologic and geochemical data to determine the spatial and temporal variations recorded within the different units of the Duolong igneous association.
Numerous geochronologic data indicate that almost all the magmatic samples from the Duolong igneous association have ages between 125 and 105 Ma, consistent with the late Early Cretaceous magmatic flare-up recorded in central Tibet (Zhu et al., 2011, 2016). The compiled ages are given in Table S5. However, our new age data for the study area indicate a gap in timing between the emplacement of the granodiorite porphyry (125 Ma) and eruption of the rhyolite (110 Ma). This gap is consistent with published ages for plutonic and volcanic rocks in the Duolong area, which suggests that the Duolong igneous association represents two major episodes of magmatic activity, where plutons (except the Dibao pluton) were emplaced mainly at 125–115 Ma (peak age ca. 120 Ma), and volcanic rocks were erupted predominantly at 115–105 Ma (peak age ca. 110 Ma; Fig. 7A).
The plutonic and volcanic rocks of the Duolong igneous association have diverse geochemical compositions, with the intrusive rocks being dominated by gabbros and granitoids and the volcanic rocks being dominated by basalts, andesites, and rhyolites. The gabbros are geochemically similar to alkaline OIB-like rocks and were dominantly derived from the asthenospheric mantle (Wu et al., 2019a). In contrast, the basalts are geochemically similar to calc-alkaline arc rocks, and they were derived from metasomatized lithospheric mantle (Wei et al., 2017; Zhang et al., 2017, 2022). The different ages and sources of the OIB-like gabbros and arc-like basalts suggest that the magmas within the Duolong igneous association were derived during two different instances of mantle activity, which resulted in two basaltic underplating events (Figs. 7B and 7C). In addition, the contemporaneous nature of the mafic and silicic magmatism suggests that the two basaltic underplating events associated with the formation of the gabbros and basalts induced two episodes of crustal anatexis, which respectively generated the mineralization-associated granitoids and rhyolites (Figs. 7D and 7E).
Granites can be subdivided into I-, S-, A-, and M-type granite according to magma source and tectonic setting (Chappell and White, 1974). The mineralization-associated granitoids in the Duolong area are all geochemically and isotopically similar (Figs. 5 and 6). Most of these intrusive rocks are high- K, calc-alkaline, and weakly peraluminous rocks, with low Zr + Nb + Y + Ce contents and Ga/Al ratios (Figs. 8A and 8B). The negative correlation between SiO2 and P2O5 contents (Fig. 5B), combined with their low A/CNK values (where A/CNK = molar ratio Al2O3/[CaO + Na2O + K2O]; Fig. 3D), indicates they are I-rather than S-type granites (Chappell and White, 1974; Chappell, 1999). I-type granites are generally thought to form from magmas generated by the partial melting of meta-igneous lower-crustal material (Petford and Atherton, 1996; Chappell and White, 2001). The low Al2O3 contents and Rb/Ba and Rb/Sr ratios and high CaO/Na2O ratios of our samples indicate that the rocks formed from magmas derived from basaltic magma (Figs. 8C and 8D). In addition, when compared with experimental melts from various sources (Patiño Douce, 1999), our granitoid samples mostly correspond to mafic amphibolite-derived melts (Figs. 8E and 8F).
The positive εHf(t) values of the Duolong granitoids indicate derivation from mantle-derived basaltic magmas rather than ancient crustal sources (Fig. 6A). Recent research has identified juvenile crustal material beneath the northern Lhasa block, which was accreted during the subduction of Bangong-Nujiang oceanic lithosphere (Zhu et al., 2011; Hou et al., 2015). However, the relatively wide range of εHf(t) values is indicative of the assimilation of some ancient crustal material during magma evolution (Fig. 6A), consistent with the more variable εNd(t) values and initial 87Sr/86Sr isotopic compositions of these mineralization-associated granitoids (Fig. 6B). The Southern Qiangtang block contains ancient lower-crustal material that represents the source of the igneous rocks with negative εHf(t) values and relatively enriched Sr-Nd isotopic compositions (Fig. 6B), and it probably provided a component for magma mixing. The linear trends between Sr-Nd isotopic compositions and SiO2 contents and A/CNK values within the Early Cretaceous granitoids of the Duolong igneous association and Middle–Late Jurassic granitoids of the Southern Qiangtang block indicate magma mixing (Fig. 9). The fact that the Duolong igneous association is located within the Bangong-Nujiang suture zone also supports a model whereby the magmas that generated the granitoids assimilated ancient crustal material from the Southern Qiangtang block. Moreover, the recently identified Nd-Hf isotopic decoupling within the granitoids further supports a process of magma mixing between juvenile and ancient lower-crust–derived magmas (Fig. 6C; Sun et al., 2021).
In summary, we favor the interpretation that the investigated granodiorite porphyries were derived largely from a thickened juvenile lower crust, and that subsequent contamination by ancient crustal material from the Southern Qiangtang block also played a crucial role in producing the diverse geo-chemical compositions of the coeval granitoids of the Duolong igneous association.
The rhyolites formed contemporaneously with arc-like basalts and andesites in the Duolong area, which suggests that the rhyolites formed by fractional crystallization of the basalts and andesites (or associated mantle-derived melts). However, the rhyolites could also have been derived by partial melting of the juvenile lower crust of the northern Lhasa block, as shown by the similar Sr-Nd-Hf isotopic compositions of the rhyolites and granitoids (Fig. 6). A compilation of geochemical data for the Duolong igneous association rocks shows an SiO2 compositional gap between the andesites and rhyolites (Fig. 5), which indicates that the rhyolites formed mostly by anatexis of juvenile crust rather than intensive fractionation of coeval arc-like magma. In addition, our new isotopic data for the rhyolites also reveal clear Nd-Hf isotopic decoupling (Fig. 6C). However, in contrast to the granitoids of the Duolong igneous association, the depleted Sr-Nd isotopic compositions of the rhyolites imply that their Nd-Hf isotopic decoupling was not caused by magma mixing with ancient crust of the Southern Qiangtang block (Fig. 9). Previous studies of arc rocks have argued that the Nd-Hf isotopic decoupling observed in such rocks could be produced by the involvement of subducted sediments and/or oceanic basalt (Nowell et al., 2004; Marini et al., 2005; Chauvel et al., 2009). Thus, with the recent identification of ophiolitic mélange in the Duolong area, we infer that the Nd-Hf isotopic decoupling of rhyolites probably reflects the involvement of mélange.
We observed that the rhyolites are more evolved than the regional granitoids and that they plot in the highly fractionated fields of Figures 8A and 8B, which indicates that the rhyolitic magmas underwent significant fractionation. However, in contrast to the high-pressure garnet differentiation indicated by the regional granitoids, the Sr/Y and Eu/Eu* ratios of the rhyolites decrease with increasing SiO2 content, indicating low-pressure fractional crystallization (Figs. 5H and 5I; Castillo et al., 1999). This is consistent with the negative Sr and Eu anomalies of these rocks (Figs. 4C and 4D), which record significant plagioclase and/or K-feldspar fractionation, as shown in Figures 8G and 8H. The fact that plagioclase does not fractionate at pressures greater than 1.2–1.5 GPa (Rapp et al., 2003) indicates that these high-silica rhyolites represent significant low-pressure fractional crystallization within shallow magma chambers (Ackerson et al., 2018). Furthermore, the correlations between SiO2 and P2O5, TiO2, and Fe2O3T indicate fractionation of apatite and Fe-Ti oxides (Fig. 5). The low rare earth element (REE) contents of these samples also indicate the fractionation of REE-enriched accessory minerals (e.g., titanite).
The rhyolites and granitoids within the Duolong igneous association were both derived from juvenile lower-crustal sources and have close spatial and temporal associations, suggesting they may be genetically related and were generated from a single silicic magmatic system. Previous studies have suggested that relatively hot and partially crystallized intermediate to silicic magma chambers can be easily reactivated when exposed to an influx of exotic magma, heat, fluid, and/or volatiles (Suzuki and Nakada, 2007; Pistone et al., 2017; Lewis et al., 2022). Here, we present a new petro-genetic model where early-formed granitic melts (ca. 120 Ma), derived from a juvenile region of the crust, ascended within silicic magma chambers and underwent further fractionation in the shallow crust. Subsequently, basaltic underplating at ca. 110 Ma promoted reworking of the preexisting shallow crystal mush reservoir, causing liquid-crystal segregation within a partially crystallized magma chamber. This generated silica-rich melts that were extracted and erupted to form the high-silica rhyolites of the Duolong igneous association.
Rapid Uplift of Central Tibet
Although there is no direct contact relationship between the investigated granitoids and rhyolites, recent geologic surveys have shown that the plutonic and volcanic rocks of the Duolong igneous association are separated by an unconformity (Fig. 10A; Song et al., 2018; Zhang et al., 2022). This unconformity, combined with the ~10 m.y. interval between the formation of the plutonic and volcanic rocks of the Duolong igneous association, indicates that a substantial amount of topographic uplift and uplift-related mechanical denudation occurred during the late Early Cretaceous. Furthermore, a late Early Cretaceous angular unconformity is widely developed throughout the Lhasa-Qiangtang collision zone between the nonmarine Lower Cretaceous Duoni Formation (ca. 114 Ma) and the underlying Jurassic ophiolites and marine flysch (Fig. 10B). The Duoni Formation was deposited mainly in a foredeep tectonic setting in terrestrial foreland basins adjacent to rapidly uplifting terrain and related denudation (Zhu et al., 2019, 2020).
Moreover, previous studies of the Duolong igneous association have revealed that the mineralization-associated granitoids were intruded and emplaced at depths of 1.4–2.5 km (Yang et al., 2014; Sun et al., 2017a; Song et al., 2018). However, drill-hole data show several meters of weakly weathered alluvial material between the underlying granitoids (ca. 120 Ma) and the overlying volcanic rocks (ca. 110 Ma) (Song et al., 2018). Thus, the probable exhumation of 1.4 km of rocks and a rapid exhumation rate of 0.14 km/m.y. can be inferred for central Tibet during the late Early Cretaceous. This inferred event of rapid exhumation is supported by low-temperature thermochronology, which suggests rapid cooling during 120–100 Ma (Bi et al., 2020; Li et al., 2022). The regional unconformity, together with the rapid exhumation, suggests the occurrence of a rapid surface uplift event in central Tibet during the late Early Cretaceous. Tectonic uplift is further evidenced by the widely distributed Duoni Formation, which was deposited mainly in a foredeep tectonic setting in terrestrial foreland basins adjacent to rapidly uplifting ranges (Zhu et al., 2019, 2020).
In summary, the relationships between plutonic and volcanic rocks within the Duolong igneous association, combined with tectono-stratigraphic observations, suggest that central Tibet underwent a rapid surface uplift event during the late Early Cretaceous, coeval with a regional magmatic flareup event. This uplift event caused rapid denudation, deposition, and exhumation in central Tibet.
Slab Breakoff Model
Mesozoic igneous rocks are widespread within central Tibet. Our new data, combined with other available geochronologic data, show that a late Early Cretaceous magmatic flareup event took place along the Bangong-Nujiang suture zone of central Tibet. Recently, studies of sedimentary rocks have suggested that closure of the Bangong-Nujiang Tethyan Ocean and initiation of the Lhasa-Qiangtang continental collision occurred no later than the early Cretaceous (Kapp et al., 2007; Zhu et al., 2016, 2019; Chen et al., 2020; Li et al., 2019). The occurrence and timing of this collision are supported by regional tectonic compression and crustal shorting in the Lhasa-Qiangtang collision zone (Murphy et al., 1997; Zhao et al., 2020). These features, together with the presence of bimodal magmatic rocks and A2-type silicic rocks (Qu et al., 2012; Wu et al., 2015, 2018; Wei et al., 2017), suggest that the late Early Cretaceous magmatic rocks of central Tibet formed in a postcollisional extensional setting.
Slab breakoff of subducted oceanic lithosphere is a typical feature of the terminal stage of subduction in response to the buoyancy contrast between oceanic (negatively buoyant) and continental (positively buoyant) crust (Davies and Blanckenburg, 1995; Wortel and Spakman, 2000). In general, slab breakoff occurs 10–25 m.y. after the initial continental collision, owing to gravitational settling (van Hunen and Allen, 2011; Wu et al., 2016b). A model involving postcollisional slab breakoff has been used to explain the late Early Cretaceous magmatic flareup in the Lhasa-Qiangtang collision zone (e.g., Zhu et al., 2011, 2016; Wu et al., 2019a). In addition, OIB-like alkaline gabbros (ca. 125 Ma), which were derived by the partial melting of upwelling asthenosphere, have recently been identified within the Duolong igneous association (Wu et al., 2019a). These gabbros provide direct petrologic evidence for the development of a slab window in response to slab breakoff beneath the Lhasa-Qiangtang collision zone. Furthermore, the upwelling of hot asthenosphere would have provided sufficient thermal energy for melting of the overlying lithosphere (including continental crust, oceanic crust, and lithospheric mantle), thus explaining the compositional diversity of coeval magmatic rocks in central Tibet (Zhu et al., 2011, 2016; Wu et al., 2019a). Therefore, we propose that a slab breakoff model explains the late Early Cretaceous magmatic flareup in the Lhasa-Qiangtang collision zone.
Slab breakoff, which results in the cessation of slab pull, would trigger topographic uplift through isostatic rebound (Molinaro et al., 2005; Duretz et al., 2011). Furthermore, the resultant mantle upwelling and continental lithospheric eduction in response to slab breakoff would also promote uplift of the surface (Guillaume et al., 2010; Fernández-García et al., 2019). As mentioned above, the widely distributed angular unconformity and the presence of the terrestrial Lower Cretaceous Duoni Formation imply a substantial amount of topographic uplift and uplift-related denudation in central Tibet. In turn, this uplift constitutes important evidence for the shallow/surficial topographic response to slab breakoff beneath the Lhasa-Qiangtang collision zone. In summary, we propose that the Duolong igneous association formed in a postcollision extensional setting during tectonic uplift and magmatic activity triggered by slab breakoff following Lhasa-Qiangtang collision.
We note that the diverse igneous rocks of the Duolong igneous association formed in two distinct stages as a result of two episodes of basaltic under-plating due to two different episodes of mantle activity (Fig. 7). According to the slab breakoff paradigm, the subducted continental lithosphere would finally rotate upward owing to its negative buoyancy in response to the initiation of slab breakoff, and this would result in the transition of the subduction angle from steep to shallow (Leech et al., 2005). In addition, the buoyant ascent of continental lithosphere would repel the previously injected asthenosphere from the mantle wedge, resulting in the temporary cessation of magmatic activity (Ji et al., 2016). Considering the two main evolutionary stages of slab breakoff alongside the spatial and temporal variations in geochemistry within the Duolong igneous association, we constructed the following new two-stage tectonomagmatic model for the Duolong area.
Stage 1 (ca. 125–115 Ma): At the initiation of slab breakoff, the slab pull would have resulted in steep subduction and the opening of a slab window beneath central Tibet. The upwelling asthenosphere would have produced the first basaltic underplating event, and, together with the related crustal anatexis, this would have induced the generation of OIBs and granitoids within the Duolong area. Magmas ascended into and were stored within the upper crust, where they continued to cool and crystallize to form highly silicic residual magmas. In addition, the isostatic surface uplift in response to slab breakoff resulted in mechanical erosion, finally exposing the plutons (Fig. 11A).
Stage 2 (ca. 115–105 Ma): As slab breakoff proceeded, the buoyant ascent of the continental lithosphere caused a shallow subduction regime. Together with asthenospheric corner flow, this induced partial melting of the metasomatized mantle wedge, generating arc-like magmas that formed a second basaltic underplate. This basaltic under-plating reworked the existing large silicic magma reservoirs and caused the eruption of high-silica rhyolites. The resultant widespread volcanic rocks covered the previously exposed plutons in the Duolong area (Fig. 11B).
Our study indicates that slab breakoff can satisfactorily explain the late Early Cretaceous tectonomagmatic activity in the Duolong area of central Tibet. The two stages of postcollisional mantle activity and the resultant two episodes of crustal anatexis can be attributed to the transition of the subduction angle from steep to shallow during slab breakoff.
Implications for Growth of the Tibetan Plateau
Previous studies have argued that the vertical growth and lateral expansion of the Tibetan Plateau were diachronous and asymmetric (e.g., Wang et al., 2008; Ding et al., 2014). The elevation of central Tibet was likely higher than 4 km during the Cretaceous, thus demonstrating the existence of a proto–Tibetan Plateau prior to India-Asia collision (Murphy et al., 1997; Kapp et al., 2003, 2007; Wu et al., 2019b). The increase in the elevation of central Tibet has been attributed to tectonic shortening and crustal thickening during continental collision between the Lhasa and Qiangtang blocks (Murphy et al., 1997; Kapp et al., 2007; Zhao et al., 2020). However, other studies have argued that surface uplift of the proto–Tibetan Plateau was mainly the result of isostatic rebound in response to lithospheric delamination during the Late Cretaceous (Wang et al., 2014b; Sun et al., 2015; Wu et al., 2019b). The occurrence of Late Cretaceous isostatic uplift is indicated by the existence of regionally distributed molasse deposits (i.e., the Abushan and Jingzhushan Formations) and an underlying angular unconformity (Fig. 10). The occurrence of isostatic uplift is further supported by low-temperature thermochronologic data, which suggest rapid rock exhumation and cooling during the Late Cretaceous (Ren et al., 2015; Li et al., 2022).
Our research provides comprehensive evidence for rapid surface uplift of central Tibet during the late Early Cretaceous, which can be explained by postcollisional slab breakoff of the Bangong-Nujiang Tethyan oceanic plate. Previous numerical modeling experiments have suggested that slab breakoff could trigger ~4 km of vertical topographic uplift (Buiter et al., 2002; Duretz et al., 2011). In addition, topographic uplift related to slab breakoff is considered to have made an important contribution to the increase in elevation of the modern Tibetan Plateau, the Anatolian Plateau, and the Cimmerian Mountains (Chung et al., 2005; Wilmsen et al., 2009; Schildgen et al., 2014).
In summary, slab breakoff triggered a topographic response involving rapid uplift and denudation in central Tibet during the late Early Cretaceous, which made a substantial contribution to the uplift of the proto–Tibetan Plateau. We propose that the surface uplift of the proto–Tibetan Plateau took place over multiple stages in response to syn- and postcollisional processes in the Lhasa-Qiangtang collision zone during the Cretaceous. The high topography of the proto–Tibetan Plateau persisted until the onset of India-Asian collision and enabled the growth of the modern Tibetan Plateau.
Our study of plutonic and volcanic rocks from the Duolong area of central Tibet revealed that late Early Cretaceous tectonomagmatic processes can be divided into two stages, an earlier plutonic stage (ca. 120 Ma) and a later volcanic stage (ca. 110 Ma). Integrated tectonomagmatic-stratigraphic results show that central Tibet was in a postcollision extensional setting during the late Early Cretaceous as a result of slab breakoff of the Bangong-Nujiang Tethyan oceanic plate.
Relationships between the late Early Cretaceous plutonic and volcanic rocks within the Duolong igneous association of central Tibet provide a precise geometric picture of a two-stage tectonomagmatic process in response to slab breakoff. The initial slab breakoff and resultant development of a slab window triggered asthenospheric upwelling and caused extensive lower-crustal anatexis, producing plutonic rocks (ca. 125–115 Ma). The late buoyant ascent of continental lithosphere and the resultant mantle flow caused partial melting of the mantle wedge and reworked crystal mushes present within the shallow crust to generate volcanic rocks (ca. 115–105 Ma).
The unconformity between the plutonic and volcanic rocks within the Duolong igneous association suggests rapid uplift and denudation of central Tibet and provides clear evidence for a topographic response to slab breakoff. We suggest that surface uplift resulting from slab breakoff made an important contribution to the formation of the proto–Tibetan Plateau during the Cretaceous and enabled the growth of the modern Tibetan Plateau. We propose that the uplift of plateaus in collision zones is generally episodic and that tectonic uplift during postcollisional processes (i.e., slab breakoff and lithospheric delamination) is probably a major contributor to such plateau uplift.
We thank Christopher Spencer for editorial handling, and two anonymous reviewers for constructive comments that improved the quality of this paper. This study was jointly supported by the National Natural Science Foundation of China (92055208 and 4177205), the Shandong Provincial Natural Science Foundation (ZR2020QD045), the Natural Science Foundation of Guangxi (GXNSFGA380004 and AD21220033), and the Talent-Introduction Program of Guilin University of Technology (GUTQDJJ2020126). This is a contribution to Guangxi Key Mineral Resources Deep Exploration Talent Highland.