The West of the Longmenshan orogenic belt, the triangular-shaped Songpan-Ganzi turbidite fold belt (Figure 1), which represents a Paleo-Tethyan accretionary complex (Chen et al., 1995), is dominated by a thick succession of Middle-Upper Triassic flysch turbidites that settled mainly during Ladinian through Norain (230 Ma–203 Ma) (Huang and Chen, 1987; Brugier et al., 1997; Weislogel et al., 2006; Enkelmann et al., 2007; Zhang et al., 2008; Zhang et al., 2012). Moreover, the only deep oil exploratory well HC1 of 7,012.4 m located in the area shows at least six times tectonic repetitions, resulting in more than ∼46% thickening of the Triassic sequence (Liu et al., 2013b). It indicates that the true thickness of the Songpan–Ganzi Triassic flysch is not 10 km–15 km as previously assumed (BGMRSP, 1991), but not more than 3 km–5 km (Liu et al., 2013b). From the end of the Late Triassic to the Early Jurassic, most flysches were intensely folded and uplifted (Burchfiel et al., 1995), which is associated with the amalgamation involving the Qiangtang block and accompanied by synchronous east-directed emplacement of the Longmenshan orogenic belt (Chen et al., 1995). The Songpan–Ganzi Triassic flysch has a multi-provenance origin with peak zircon ages of 265 Ma, 440 Ma, 740 Ma, 1,860, and ca.2,450 Ma (Weislogel et al., 2006, 2010; Enkelmann et al., 2007; Deng et al., 2008; Ding et al., 2013; Zhang et al., 2014; Tang et al., 2017; Jian et al., 2019; Gong et al., 2021), and experienced low-to medium-grade greenschist facies metamorphism during the Late Triassic–Early Jurassic (BGMRSP, 1991; Dirks et al., 1994; Brugier et al., 1997; Huang et al., 2003; Wilson et al., 2006; Roger et al., 2010). In contrast, the underlying upper Sinian–Palaeozoic sequences, predominantly composed of carbonates, experienced higher-grade, medium-pressure metamorphism (Huang et al., 2003). Heavy mineral assemblages in the Triassic flysch contain abundant zircon, apatite, garnet, leucoxene, haematite-limonite, tourmaline, epidote, and rutile (Zhang et al., 2008; Yan et al., 2014; Tang et al., 2017). The geochemical analysis of clastic garnet shows that the type of garnet is mainly Bi (70%) type and Type A (18%) (Zhang et al., 2008; Yan et al., 2014). During the Mesozoic, large amounts of granodiorite to monzonite with ages of 228 Ma–185 Ma were emplaced in the flysch turbidites (Roger et al., 2010; Ding et al., 2013; Zhang et al., 2014).

The Yidun terrane is located to the southwest of the SPGZ, bounded by the Jinsha and Ganzi-Litang suture zone in the south and north respectively (Figure 1). From west to east, it includes the Precambrian–Paleozoic Zhongza massif, Middle Triassic-Early Jurassic eastern Yidun felsic plutons, and middle Cretaceous West Yidun felsic plutons (120 Ma) (Jian et al., 2019). The Zhongza massif is composed of Neoproterozoic basement rocks consisting of granitic gneisses and metavolcanic rocks (Bureau of Geology and Mineral Resources of Sichuan Province (BGMRSP, 1991) and a cover sequence of Paleozoic greenschist facies metasedimentary rocks, shallow-to deep-marine carbonates, and clastic rocks intercalated with sporadic mafic volcanic rocks (BGMRSP, 1991; Chang, 2000). The eastern Yidun Terrane includes widespread exposure of the Triassic Yidun Group flysch that is intruded by large Late Triassic dioritic - granitic plutons (230 Ma-210 Ma) (Reid et al., 2007; Weislogel, 2008; Jackson et al., 2018a; Jackson et al., 2018b). Detrital zircon ages from the Yidun Group in the northern part of the Yidun terrane show prominent bimodal characteristics of 400 Ma–480 Ma and 880 Ma–980 Ma, with a small number of ages of 2,450 Ma–2,500 Ma (Wang et al., 2013). The detrital zircons from the southern Yidun Group are mainly ∼220 Ma–240 Ma, ∼400 Ma–480 Ma, ∼720 Ma–1,000 Ma, ∼1,700 Ma–1,900 Ma, and ∼2,400 Ma–2,500 Ma (Ding et al., 2013; Wang et al., 2013; Liu et al., 2021).

The Longmenshan orogenic belt, separating the Songpan-Ganzi turbidite fold belt to the west from the Western Sichuan foreland basin to the east (Figure 1), evolved into an intracontinental transpressional orogen in the Late Triassic (Burchfiel et al., 1995; Chen et al., 1995; Worley and Wilson, 1996). It is NE-SW trending, approximately 500 km long by 30 km–50 km wide (Lin et al., 1996; Jia et al., 2006; Lin, 2008). This belt is characterized by numerous large-scale nappes separated by several NW-dipping thrusts (Jia et al., 2006), in which deformed Paleozoic passive margin carbonate rocks of South China Block and Precambrian crystalline basement massifs are involved (Ministry of Geology and Mineral Resources, 1991; Burchfiel et al., 1995). The massifs consist of high-grade metamorphic quartzofeld-spathic gneisses and associated granitoids with an age of 699 Ma–864 Ma (Zhou et al., 2002a; Zhou et al., 2002b; Zhou et al., 2006a; Druschke et al., 2006; Sun et al., 2007; Zhao and Zhou, 2007; Yan et al., 2008). The passive margin sequence itself consists primarily of shallow water marine rocks of Neoproterozoic (Sinian) to Permian age (BGMRSP, 1991; Guo et al., 1996; Yan et al., 2008). Directly northwest of Longmenshan, Paleozoic metasedimentary units of the Maoxian Group are exposed within the Maoxian-Wenchuan shear zone (BGMRSP, 1991; Worley and Wilson, 1996; Yan et al., 2011). The detrital zircons of Paleozoic sedimentary rocks in the Longmenshan orogenic belt are aged 490 Ma–670 Ma, 730 Ma–850 Ma, 900 Ma–1,000 Ma, and 2,450 Ma–2,550 Ma. The peak ages were ∼554 Ma, ∼800 Ma, ∼940 Ma, and ∼2,500 Ma, respectively (Duan et al., 2011; Chen et al., 2016, 2018). Heavy mineral assemblages in the Longmenshan region contain abundant garnet, sillimanite, staurolite, and kyanite (BGMRSP, 1991; Worley and Wilson, 1996). Airaghi et al. (2017b) showed that there was obvious uplift and denudation in the lower Cretaceous (137 Ma) of Longmenshan orogenic belt.

Mesozoic sedimentary basins are widespread along the eastern Tibetan Plateau. As the largest Mesozoic sedimentary basin of the Upper Yangtze plate (Figure 1), the Sichuan Basin is located to the east of the Tibetan Plateau with a total area of 18 ×10⁴ km² (Tong, 1992). It is a multi-stage, superimposed basin characterized by three main basin evolution processes: marine carbonate platform (Ediacaran to Late Triassic), Indosinian-Yanshanian orogeny foreland basin (Late Triassic to Late Cretaceous), and uplift and tectonic modification (Late Cretaceous to Quaternary) (Chen et al., 1995; Guo et al., 1996; Liu et al., 2018; Deng et al., 2013a, b; Li Z W et al., 2012; Li J Z et al., 2009; Li W et al., 2009; Shen et al., 2009; Liu et al., 2008; Richardson et al., 2008). Since the Late Triassic, the Sichuan Basin had become a typical continental sedimentary basin (Liu, 1993; Li Y et al., 2003).

The exposed Cretaceous terrestrial sediments sequence consists entirely of non-marine synorogenic deposits, including lacustrine, alluvial fan and fluvial sandstone and conglomerate, siltstone, and mudstone [Bureau of Geology and Mineral Resources of Sichuan Province (BGMRSP), 1991]. Cretaceous strata exist only in the southeast, west, and northwest of the Sichuan Basin, with the central basin dominated by Jurassic red beds (Bureau of Geology and Mineral Resources of Sichuan Province (Compiling Group of Continental Mesozoic Stratigraphy and Palaeontology in Sichuan Basin of China, 1982; Zhuang et al., 1988; BGMRSP, 1991).

In the study area (Figure 2), the Cretaceous strata are composed of (from oldest to youngest) the Tianmashan (K₁ t), Jiaguan (K_1-2 j), and Guankou(K₂ g) Formations (Figure 3). These strata locally exceed 3,000 m in total thickness in the Southwest corner of the Sichuan Basin (Figures 1, 2). Sedimentary rocks interpreted to have been deposited in the foreland basin thin eastward toward the interior of the Sichuan Basin. This Cretaceous succession in the southwestern corner of the Sichuan Basin is mainly composed of brown red sandstone interbedded with red mudstone and conglomerate, which exhibit decreasing compositional and textural maturity with time (BGMRSP, 1991; SBGMR, 1997; Gou, 1998).

The sedimentary age of the Cretaceous foreland deposits is well constrained by magnetostratigraphic and fossil data (Wei, 1979; Su, 1983; Li, 1987; Ye, 1983; Chen, 1983; Zhuang et al., 1988; BGMRSP, 1991; Gou, 1998; Wang et al., 2006) (Figure 3). Paleomagnetic and paleontological data show that the depositional ages of Tianmashan, Jianguan, and Guanguou Formations are, in turn, early Cretaceous Berriasian-Barremian, middle Cretaceous Aptian-Santonian and late Cretaceous Campanian-Maastrichtian stage (Figure 3) (Zhuang et al., 1988; Li and Ji, 1993; Gou, 1998).

The Early Cretaceous Tianmashan Formation is 350 m thick and parallel unconformably overlies the Upper Jurassic Penglaizheng Formation (Chengdu Institute of Geology and Mineral Resources (CGMR), 1979; BGMRSP, 1991; Wang et al., 2006) (Figure 3). It is composed of brown-red quartz-lithic sandstone interbedded with mudstone and conglomerate lenses, with thick-bedded massive conglomerate at the base (Figure 4A). The depositional environment is interpreted as alluvial fan and fluvial deposits (CGMR, 1979; BGMRSP, 1991; Gou, 1998; Li et al., 2016b). The conglomerate bed is commonly present in the basal portion of the Tianmashan Formation, approximately 10 m thick and consists of massive, light brown-red, matrix to clast supported, calcium cemented. The matrix is sandy to siltly and normally is purple to reddish-purple in color. The clasts are ranging from 2 mm to 15 mm in size (average 5 mm), and are mainly composed of sub-rounded to rounded limestone, quartz sandstone, quartzite, and vein quartz. They often show fining-upward sequence and occasional gravel shingle imbricate structures, and we interpret this conglomerate bed as a lag deposit of a meandering river. Non-marine Cretaceous ostracod assemblages of the Tianmashan Formation are characterized by Jingguella—Pinnocypridea—Damonella—Minheella (Wei, 1979; Ye, 1983; Chen, 1983; Li, 1987; Wang et al., 2006), combined with paleomagnetic data (Zhuang et al., 1988), the sedimentary age corresponding to ca. 145 Ma–125 Ma (Figure 3).

The ∼420-m-thick Jiaguan Formation was parallel unconformity above the Tianmashan Formation (Figure 3) and consists of a fining-upwards unit of red, conglomerate, feldspar-quartz sandstone, siltstone, and mudstone (Figures 4C, D). Whereas the Jiaguan Formation corresponds to a braided river environment (SBGMR, 1993; Gou, 1998; Li J. et al., 2016). Non-marine Cretaceous ostracod assemblages of the Jiaguan Formation are characterized by Cypridea—Monosulcocypris (Wei, 1979; Ye, 1983; Chen, 1983; Li, 1987; Wang et al., 2006), combined with paleomagnetic data (Zhuang et al., 1988), corresponding to ca. 125 Ma–83.6 Ma (Figure 3). The conglomerate beds are commonly present at the bottom of the Jiaguan Formation. Their composition and texture are varied. Pebbles and cobbles are generally rounded to subangular and are composed of metamorphic rocks and volcanic rocks, quartzite, vein quartz, sandstone, and carbonate clasts (Figure 4B).

The Upper Cretaceous Guankou Formation overlies the Jiaguan Formation and has an age of ca. 83.6 Ma–66 Ma (Zhuang et al., 1988), with non-marine Cretaceous ostracod assemblages characterized by Cristocypridea—Sinocypris (Quadracypris)—Lunicypris (Wei, 1979; Ye, 1983; Chen, 1983; Li, 1983; Li, 1987; Wang et al., 2006). This formation consists predominantly of 800 m brown-red mudstone with unequal siltstone and comparatively thick marlstone, fine conglomerates, gypsums, and halite (BGMRSP, 1991; Wang et al., 2006) (Figures 4E, F), which were deposited in fluvial to closed lacustrine sedimentary environment (BGMRSP, 1991; SBGMR, 1993; Gou, 1998) (Figure 3).

To determine the paleotransport directions, paleocurrent indicators (trough-cross, conglomerate clast imbrications, and oblique bedding) were measured wherever possible in the stratigraphic sections to infer the input direction of the detrital material. A total number of 112 directional data were collected from the Late Jurassic Penglaizheng to Late Cretaceous Guankou Formation (Figure 3). All the paleocurrent data were corrected to horizontal by standard stereonet techniques (DeCelles et al., 1983), and the corrected data are then plus or minus 180°C (Wu et al., 2012), and plotted the average value represented by a black arrow (Figure 3).

The lithology, size, sorting and roundness of conglomerate clasts were assessed by conducting in situ clast counts at bottom of Tianmashan and Jiaguan Formations, respectively (Figure 3). Locations of the clast counts and various lithological types at each site are shown in the stratigraphic column (Figure 3). In addition, we collected clasts composition data from 20 locations on the western margin of the Sichuan Basin from published literature to compare regional provenance changes in clast types and identify specific sediment sources (Figure 5).

Sandstone framework compositions were collected from 27 standard petrographic thin sections of samples obtained from Cretaceous sandstones (Figures 3, 6). These medium-grained sandstone samples were systematically collected to represent the variations in stratigraphy, lithology, and composition through the Tianmashan, Jiagaun, and Guankou Formations along the log of the SW of the Sichuan Foreland Basin. The logged section and the position of selected samples for different analyses are given in the stratigraphic column (Figures 3, 6). Thin sections were stained for potassium and calcium feldspar, and at least 300 relevant grains of quartz, feldspar, and lithic rock fragments per thin section were counted according to the Gazzi-Dickinson method to reduce the influence of grain size (Dickinson, 1970; Dickinson and Suczek, 1979; Dickinson et al., 1983; Ingersoll et al., 1984). This allows us to examine the detrital framework and understand the composition of sandstones within the Cretaceous strata, which ultimately provides additional information on the composition of source rocks from which the sediment was derived (e.g., Dickinson and Suczek, 1979; Dickinson et al., 1983; Ingersoll et al., 1984). Provenance data from the light mineral assemblage is displayed on the QFL (quartz–feldspar–lithics) and QmFLt (quartz monocrystalline–feldspar–lithic total) diagrams to indicate possible derivation from “continental block”, “recycled orogen” or “magmatic arc” settings (Dickinson et al., 1983). Recent studies have highlighted issues with over-simplified interpretation of these plots when applied in tropical settings (van Hattum et al., 2006; Garzanti et al., 2007; Smyth et al., 2008; Sevastjanova et al., 2012). Therefore, the two triangular diagrams are used to show the composition differences between different Formations without much discussion about the tectonic background of the potential source (Zimmermann and Hall, 2016).

Textural categories from 1 to 4 were assigned for sorting and roundness of grains (Zimmermann and Hall, 2016, 2019; Löwen et al., 2018). Sorting categories are 1) poorly sorted, 2) moderately sorted, 3) well sorted, and 4) very well sorted. Rounding categories are 1) angular, 2) sub-angular, 3) subrounded, and 4) rounded. Typical examples of Cretaceous rocks are shown in Figure 3. The collection of conglomerate compositional data was conducted at two outcrop localities, they come from the bottom of the Tianmashan Formation and the Jiaguan Formation, respectively (Figure 3).

There were 21 heavy mineral separates analyzed, comprising three samples from the Tianmshan Formation sandstones, eight samples from the Jiaguan Sandstone, and 10 samples from the Guankou, respectively (Figure 3). To make sure the samples were representative, each sandstone sample is fresh fine-to coarse-grained with an average grain size between 0.2 mm–2 mm, weighing at least ∼1.5 kg–2 kg. Heavy mineral separation was disaggregated using a ceramic mortar and pestle, and wet-sieved into four grain-size categories: > 1 mm, 0.25 mm–1 mm, 0.0625 mm–0.25 mm, and < 0.0625 mm, and the 0.0625 mm–0.25 mm sand fraction was then processed by density separation using sodium polytungstate with a density of 2.89 g cm⁻³ (Morton, 1985; Mange and Maurer, 1992; Morton & Hallsworth 1994). 300 heavy mineral grains per sample were classified.

Heavy mineral analysis is one of the most sensitive and widely used methods to determine possible provenance areas of sandstones (e.g., Mange and Maurer, 1992; Morton and Hallsworth, 1994, 1999). Hence, they often do not reflect the original composition of the source area (Morton and Hallsworth, 1994). Common ultra-stable heavy minerals were categorized into groups corresponding to their most likely protoliths, based on suggested source rocks (Feo-Codecido, 1956; Mange, 2002; Nichols, 2009). Based on the types of heavy minerals isolated from the samples and the background information of potential provenance, common heavy minerals were grouped by their most likely protoliths, based on suggested source rock associations (Mange, 2002; Nichols, 2009; Zimmermann and Hall, 2016, 2019) zircon, tourmaline, anatase, monazite, and apatite are considered to indicate acid igneous (granitic) sources. Magnetite, ilmenite, and chromium spinel represent basic igneous and ultrabasic sources. Rutile and garnet are interpreted to indicate metamorphic sources, mainly of continental character. Other minerals, such as barite, haematite-limonite, and leucoxene are thought to be sedimentary rock sources.

The provenance-sensitive heavy mineral ratios ATi (apatite: tourmaline index), GZi (garnet: zircon index), RuZi (rutile: zircon index), MZi (monazite: zircon index) and CZi (chrome spinel: zircon index) were determined following Morton and Hallsworth (1994) and Morton et al. (2005). In addition, the ZTR index (zircon, tourmaline, rutile) was calculated as defined by Hubert (1962) (Figure 8).

Garnet is a very common heavy mineral that occurs in a wide range of metamorphic and igneous rocks and is relatively stable during sedimentary transport and burial diagenetic conditions (Wright, 1938; Morton, 1985; Deer et al., 1992). Its comparatively wide range of major and trace element composition is primarily controlled by host rock composition and reflects changes in pressure and temperature conditions during mineral growth as well (Mange and Morton, 2007; Krippner et al., 2014). Therefore, the mineral chemistry of detrital garnet is widely used as a provenance indicator in studies of sedimentary rocks (Mange and Morton, 2007; Krippner et al., 2014, 2015, 2016; Huang et al., 2019b).

In order to characterize the detrital garnets in the Western Sichuan foreland, three samples were taken from the Tianmashan, Jiaguan, and Guankou Formation, fine-to medium-grained fluvial sandstones, respectively. Garnet major element geochemistry analysis was performed with a JEOL JXA8230 RL EMP equipped with five wavelength dispersive spectrometers at the Sample Solution, Wuhan, Hebei Province. Measuring conditions for garnet include a beam current of 20 nA and an accelerating voltage of 15 kV. The counting times per spot were 15 s for Si, Mg, Ca, Fe, and Al, and 30 s for Ti, Cr, and Mn. Garnet assemblages have been compared by determining the relative abundances of garnet types A (low -Ca, high-Mg), Bi (low-Mg, Ca < 10%), Bii (low-Mg, Ca > 10%), and C (high-Mg, high-Ca), as defined by Mange and Morton (2007). Previous studies have shown that garnets from felsic crystalline rocks (e.g., intermediate–acidic igneous rocks and metapelites) often have similar major element composition (Mange and Morton, 2007), while there are significant differences in trace elements, which can be effectively used to distinguish garnets from medium-acidic igneous rocks and different metapelite rocks (Lenaza et al., 2018; Hong et al., 2020a, 2020b).

It is possible to compare potential source compositions to the detrital ones because the potential source rocks were already narrowed down to particular regions based on other provenance proxies and because many of these rocks are still preserved in the western provenance of the basin today. For comparison we used data from published source rocks data from different units including Triassic flysch from the southern Songpan-Ganzi (Zhang et al., 2008), Early and Middle Triassic sandstones in the West Qinling (Yan et al., 2014), Triassic granite within the Ganzi-Litang suture zone (Li et al., 2017), Metamorphic crystalline basement of Yidun terrane (Tian et al., 2018), Metamorphic massifs in Longmenshan orogenic belt (He et al., 1988), Metamorphic rocks in Danba area of the southeast corner of Tibetan Plateau (Huang et al., 2003; Billerot et al., 2017), and garnet from modern river sands in the upper Reaches of the Yangtze River (Wang et al., 2018).

Three middle-coarse sandstone samples (FX-1G, from Tianmashan Formation; FX-3G, from Jiaguan Formation, and FX-14G from Guankou Formation) were used for zircon U-Pb dating analysis. At least 250 zircon grains were randomly selected and mounted in epoxy resin and polished to nearly half their original width to expose the internal structures. The internal texture of the zircons was observed with transmitted and reflected light micrographs and cathodoluminescence (CL) images. U–Pb dating was generated from detrital zircons using laser-ablation inductively coupled plasma mass spectroscopy (LA–ICP–MS) at the State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an, China, following a procedure similar to that reported by Yuan et al. (2007). Eighty detrital zircons from each sample were analyzed with a spot size of 32 µm using an Agilent 7,500a ICP–MS coupled to a Lambda Physik Geolas 200 M excimer laser ablation system (193 nm). U–Th–Pb isotopic data were simultaneously acquired on the same spot.

Isotopic ratios and element concentrations of zircon grains were calculated by GLITTER (ver. 4.0, Macquarie University), and were corrected for both instrumental mass bias and depth-dependent elemental and isotopic fractionation using Harvard zircon 91,500 as an external standard with a recommended ²⁰⁶Pb/²³⁸U age of 1,065.4 Ma ± 0.6 Ma (Wiedenbeck et al., 2004). Concordia ages and diagrams were obtained using Isoplot/Ex (ver. 3.0; Ludwig, 2003). Common lead was corrected by the LA-ICP-MS common lead correction (ver. 3.15) method, following Andersen (2002). Interpreted ages were based on ²⁰⁶Pb/²³⁸U for younger than 1,000 Ma grains and on ²⁰⁶Pb/²⁰⁷Pb for older than 1,000 Ma grains. Only zircon ages with 90%–110% concordance were used to estimate percentages and build a probability plot of each sample, and considered in the following discussions. The analytical data are reported in Supplementary data (See Supplementary Material for details).

We also collected detrital zircon geochronological data of four published samples to compare with the new data (Figure 2), including sample G124 (Jiaguan Formation), G136 (Jiaguan Formation), G130 (Guankou Formation), and G178 (Guankou Formation) (Li et al., 2018).

To reconstruct the organization of flow direction during the foreland basin evolution in the study area, 86 new palaeocurrent data (23 in Tianmashan Formation, 41 in Jiaguan Formation, 22 in Guankou Formation) were measured from the gravel slant plane and the direction of inclination of the oblique bedding and combined with 124 palaeocurrent measurements previously published from the Jiaguan and Guankou Formations (Li and Ji, 1993; SBGMR, 1993). Measurements indicate a clear SE-directed transport direction (Figure 3). There is no significant change that occurs in paleocurrent patterns. Paleocurrent data show that from the Early Cretaceous Tianmashan Formation to the Late Cretaceous Guankou Formation in the southwestern Sichuan Basin, the palaeoflow pattern was inherited, and the main body was SE 110°C–220°C with an average of 130°C (Figures 3, 5). It is suggested that the source area of sediments is located at the West and Northwest of the basin from Longmenshan orogenic belt and the Songpan-Ganzi fold belt.

Figure 8 summarises heavy mineral assemblages, ZTR index, heavy mineral ratios, protoliths, and zircon morphology types of the Lower Cretaceous to Upper Cretaceous sediments. Hematite-limonite and ilmenite are the dominant phase in the Feixianguan section sandstones (average of 72%) and together with zircon and rutile comprises 73%–99% of the heavy mineral fraction. The main heavy minerals detected in the studied samples are haematite–limonite (granular), ilmenite (granular), zircon (euhedral, subhedral, and subrounded), rutile (brown red, columnar, and granular), leucoxene (irregular granular), garnet (pink, irregular granular), apatite (columnar, subhedral, and rounded granular), and barite (granular). However, their relative percentages vary widely, and zircon and apatite exhibit different crystal shapes, indicating the coexistence of two provenances of proximal and distant sources, which might be related to sediment recycling. Other minerals, such as tourmaline (dark brown and columnar), anatase (granular), monazite (granular), and magnetite (granular) are present, either in very low percentages. Chrome spinel (granular) occurs only in a few samples.

Samples 11-1G, LJ-1G, and FX-1G collected from the Early Cretaceous Tianmashan Formation contain the highest abundances of Hematite-limonite (average 46.5%), zircon (average 25.2%), rutile (average 6.9%), leucoxene (average 5%), tourmaline (average 1.8%), and anatase (average 1.7%) and the lowest abundances of ilmenite, apatite, magnetite, and monazite. Sample 11-1G also yields abundant garnet (11.36%), and sample LJ-1G contains abundant barite (15%). These two samples also contain a small amount of chrome-spinel (0.2%). The highest ZTR index value (49.7) is measured in this formation, along with the lowest average ATi (2) and RuZi (20) indices. While intermediate-basic volcanic accessory minerals such as hornblende, epidote, and pyroxene, and aluminosilicate accessory minerals characteristic of medium-to high-grade metamorphic sources, such as andalusite, kyanite, and sillimanite, are absent. Their heavy mineral assemblage reflects their mineralogical maturity and is dominated by the ultra-stable phases with high ZTR value. Less stable accessories are apatite and garnet, and there is a lack of ilmenite and magnetite. The high ZTR index indicates enhanced recycling or chemical weathering.

The Tianmashan Formation in Yaan has a significant acidic igneous (9%–43%) and metamorphic signal (9%–14%) that reflects the abundance of mainly zircon, rutile, and garnet (Figure 8). Sandstones contain on average 59.3% sedimentary rocks and 0.2% basic igneous grains. There is a mixture of angular (39%), subangular (23%), and subround-rounded zircon grains (38%).

In contrast, there is a significant change in provenance in the overlying Jiaguan and Guankou Formations, which have similar heavy mineral assemblage characteristics. Hematite–limonite (average 60%), ilmenite (average 16%), magnetite (average 7.5%), garnet (average 2.2%), apatite (average 1%) and monazite (average 0.1%) abundances are higher, while zircon (average 4.8%), rutile (average 1.6%), leucoxene (average 1.4%) and tourmaline (average 0.1%) abundances are lower. This stratigraphic interval has the highest mean ATi (56), GZi (11.6), RuZi (26), and MZi (1.32) indices, and the lowest mean ZTR index (6.7). Hematite–limonite and leucoxene represent strong oxidation conditions, are widely distributed in the study area, possibly reflecting the dry oxidation of the climate environment or the decreasing water scale at that time (An et al., 2011; Zhang et al., 2016; Tang et al., 2017), and a very low percentage of zircon, which is interpreted to indicate basic igneous rock sources. The Jiaguan and Guankou Formation sandstones contain a basic igneous (24.5%) and acidic igneous (7.2%) signal indicated by a high percentage of ilmenite, magnetite, and apatite (Figure 8). Zircon grains are a mixture of subround-rounded (38%–44%), subangular (18%), and angualr (43%). At the same time, the rutile index also slightly increased (Figure 8). Thus, we infer an increased source input from basic igneous rocks and medium-to high-grade metapelite during the deposition of the Jiaguan and Guankou Formations. Compared with all the samples from the Guankou Formation, the zircon and rutile of the top two samples MD-6G and LC-1G increased, which may be related to the enhanced recycling in the Late Cretaceous (Li et al., 2018).

A total of 239 single grains of detrital garnet were analyzed from three samples of the Lower Cretaceous Tianmashan Formation (Sample 11-1G), the Mid-Cretaceous Jiaguan Formation (Samples 12-2G), and the Late Cretaceous Guankou Formation (Sample LC-1G). Results of the garnet single-grain geochemical analysis are shown in the classification scheme after Mange and Morton (2007) (Figure 9). Garnets from the three samples predominantly scatter in fields A and B of the diagram but, non-etheless, show distinct characteristics. Garnet type distribution for the three studied formations is displayed as pie charts in Figure 9.

The most diverse garnet population is present in the Tianmashan Formation with dominant input of type A (granulite-facies metasediments-70%), Bi (intermediate to felsic igneous rocks-28%), and small amounts of type Bii (amphibolite-facies metasediments -2%) garnet (Figure 9A). In contrast, the Jiaguan Formation exhibits considerably higher amounts of Bi (39%) and Bii (23%), but a lower percentage of A type (36%), grains with negligible amounts of garnet from high-grade metamafic igneous rocks (Ci type) (2%) (Figure 9B). Compositional data of garnets from sample LC-1G of the Guankou Formation suggest mainly amphibolite-facies metasediments (45%) and granulite-facies metasediments (45%) as host lithologies with only minor input of Bi (9%) and Ci (1%) type garnets (Figure 9C). Up section, type Bii garent increased significantly.

Using the scheme of Teraoka et al. (1997, 1998) we see a wide range in compositions but with some coherent differences between the different samples (Figures 9D–F). The sample 11-1G has a strong preference for garnets plotting in the lg1 + lg2 (intermediate P–T, 87%) field. In contrast, sample 12-2G has some grains of that composition but some grains are biased towards common L (low P-T, 28%) and H (high P-T, 15%) garnets. More obviously, more garnet grains appeared in the la (intermediate P–T, 14%) and H (high P-T, 31%) regions of the sample LC-1G. Almost all the samples have a significant number of grains plotting within the lg2 (intermediate-granulite facies) field. Grains plotting in the H (high P–T) field are quite common in the Guankou Formation. Such grains are rare in the Tianmashan Formation.

Three sandstone samples yield a total of 235 U–Pb ages with concordance within 90%–110% (Figure 10). More than 90% of the zircons are short prisms, with small cores, well-developed crystal faces, and well-preserved magmatic oscillatory zoning visible in the CL images. These grains are identified as magmatic in origin and are supported by a high Th/U (> 0.2). U–Pb results (Figure 10) from three stratigraphic units are preliminarily presented in the light of stratigraphic order.

Sample FX-1G from the Early Cretaceous Tianmashan Formation. Seventy-eight grains yield six age populations of 192 Ma–240 Ma (n=7, 8.9%), 252 Ma–425 Ma (n=13, 16.7%), 700 Ma–860 Ma (n=14, 17.9%), 1,473 Ma–1,644 Ma (n=9, 11.5%), 1731 Ma–2098 Ma (n=21, 26.9%), and 2,405 Ma–2,528 Ma (n=11, 14.1%). The main peaks are at 740 Ma, 1840 Ma, and 2,498 Ma (Figure 10A), with three weaker peaks at age 1,620, 260, and 228 Ma. Only two zircons (grains FX-1G-68 and FX-1G-75) of 742 Ma and 2,526 Ma have low Th/U values of <0.2 and blastopsammitic textures in the CL images, suggesting metamorphic origin. All other grains have relatively high Th/U values and well-developed crystal faces and are identified as magmatic sources. 22% of the zircon grains have a good roundness, 26% of the zircon grains are sub-angular, and 52% of the zircon grains are angular (Figures 8, 10B).

Sample FX-3G from the Mid-Cretaceous Jiaguan Formation. Eighty zircon grains with five major age populations at 201 Ma–250 Ma (n=13, 16.25%), 300 Ma–461 Ma (n=14, 17.5%), 679 Ma–945 Ma (n=27, 33.75%), 1,804 Ma–1,900 Ma (n=8, 10%) and 161 Ma–192 Ma (n=5, 6.25%). Six zircon grains with ages ranging 1,006 Ma–1,521 Ma (n=6, 7.5%). Relative major peaks are at 164 Ma, 215 Ma, 440 Ma, 780 Ma, and 1,860 Ma (Figure 10C). Five zircon grains (6%) have round shape, blastopsammitic textures in the CL images, and relatively low Th/U values (0.11–0.18), and they are likely metamorphism origin. The remaining grains are clearly of magmatic origin, with high Th/U values of > 0.2 and well-preserved oscillatory zoning. 9% of the zircon grains have a good roundness, 19% of the zircon grains are sub-angular, and 72% of the zircon grains are angular (Figures 8, 10D).

Sample FX-14G from the Late Cretaceous Guankou Formation. Seventy-seven grains with five major age populations at 207 Ma–242 Ma (n = 20, 25.97%), 1,800 Ma–1900 Ma (n = 12, 15.6%), 720 Ma–860 Ma (n = 9, 11%), 400 Ma–476 Ma (n = 8, 10%) and 2,509 Ma–2,674 Ma (n = 5, 6.49%), and three subordinate age populations of 165 Ma–190 Ma (n = 3, 3.89%), 120 Ma–135 Ma (n = 3, 3.89%), and 627–690 Ma (n = 3, 3.89%) (Figure 10E). Seven zircon grains with ages ranging 1,360 Ma–1,700 Ma. The sample had two dominant peak ages at 213 Ma, and 1849 Ma. Only two zircon grains (3%) have round shape, blastopsammitic textures in the CL images, and relatively low Th/U values (0.11–0.17), and they are likely metamorphism origin. The remaining grains are clearly of magmatic origin, with high Th/U values of > 0.2 and well-preserved oscillatory zoning. 5% of the zircon grains have a good roundness, 11% of the zircon grains are sub-angular, and 84% of the zircon grains are angular (Figures 8, 10F).

Previous studies showed that hydrodynamic has a certain effect on the separation of garnet geochemistry, thus affecting the geochemistry-based provenance interpretation (Huang et al., 2019a). Specifically, the smaller size of grains (63–125 microns) are more abundant in Fe²⁺ and Mn²⁺, and indicate that the provenance is mainly low-level metamorphic rocks of amphibolite facies. Other larger grains (125–250 microns) have higher Ca²⁺ and Mg²⁺, and their source rocks are more diverse including low-grade metamorphic rocks, intermediate-acid igneous rocks, and eclogite, etc. In this study, the grains size of detrital garnet is mainly distributed in the range of 110–230 microns, and most of the grains are angular and a few are subangular-subround (Figure 11). Our analysis result shows that there was no significant difference in grain size between each type of garnet, suggesting that hydrodynamic sorting has less effect on the analysis results. It may be due to the sample itself having a similar sedimentary environment, implying the result is reliable. Therefore, the geochemical differences of garnet between samples are directly related to their provenance.

Garnets typically occur in a wide variety of metamorphic and igneous rocks, as well as in peraluminous rocks. According to the classification of Mange and Morton (2007) (Figure 9), detrital garnets from the Cretaceous sandstones are composed of type A, Bi, and Bii (Figure 9), which account for about 50%, 25.3%, and 23.4% of the analyzed garnet grains, respectively. Thus, they probably derived from granulite-facies metasedimentary rocks, intermediate to acidic igneous rocks, and amphibolite-facies metasedimentary rocks, rather than from high-grade metabasic rocks and skarns and low-grade metabasic rocks. Moreover, some low- Ca and high- Mg garnets (type A) can also be derived from charnockites and intermediate-acidic igneous rocks (Sabeen et al., 2002; Morton et al., 2004). Type Bii grains are Mg- and Ca-poor but Mn-rich (> 5%) garnets that most likely originated from amphibolite-facies metasedimentary rocks (Deer et al., 1997). This suggests that ultrahigh-temperature metamorphic calcsilicate granulites, amphibolite-facies metasedimentary rocks, and intermediate-acidic igneous rocks are the major source of detrital garnets in the Cretaceous sandstones from the southwest corner of the Sichuan Basin.

There are three possible sources of garnet in Cretaceous sandstones in the Sichuan Basin: 1) Songpan-Ganzi Triassic flysch sediments; 2) Metamorphic basement, massifs, and amphibolite -facies metasedimentary rocks of the Longmenshan orogenic belt; 3) Recycling of older pre- Cretaceous sedimentary rocks in the Western margin of the Sichuan Basin. The possibilities and contributions of these three sources are discussed in more detail below.

Previous studies have shown that Songpan-Ganzi flysch has a multi-provenance origin, such as the east Kunlun high-grade metamorphic rock used to be one of its main source (Weislogel et al., 2006, 2010; Enkelmann et al., 2007). The detrital garents in Triassic flysch sediments in the West Qinling and Songpan-Ganzi fold belt are mainly type Bi (>76%) and type A (18%) (Figure 12) (Zhang et al., 2008; Yan et al., 2014). At the same time, the metamorphic strata of the Triassic in the Songpan-Ganzi metamorphic belt generally do not exceed the low green schist facies (BGMRSP, 1991; Dirks et al., 1994; Brugier et al., 1997; Huang et al., 2003; Roger et al., 2010). Therefore, type Bi garnet should be the main type of garnet supplied by the Songpan-Ganzi fold belt. Our provenance analysis shows that the material supply by the Songpan-Ganzi fold belt is gradually stronger from the Lower Cretaceous Tianmashan to Upper Cretaceous Guankou Formation. Based on this, Bi type garnet should take up a higher proportion in the sediments of the Late Cretaceous, which is inconsistent with the actual analysis results.

Li et al. (2018) showed that the continental clastic rocks deposition in the Sichuan Basin had increased recirculation since the Middle Jurassic period. Although there are abundant detrital garnets in the Upper Triassic Xujiahe Formation and the Middle Jurassic Shaximiao Formation sandstones in the Sichuan Basin (An et al., 2011; Shi et al., 2011), the morphological characteristics of garnets in the Cretaceous sandstones do not support the strong recirculation deposition (Figure 11). All the heavy mineral analysis samples showed that the content of garnet in the Early Cretaceous sandstone samples was relatively low (average 3.8%), and the most obvious rich garnet sediments in the region appeared in the Upper Cretaceous Guankou Formation (this study) and Gaokanba Formation (Deng et al., 2018; Jiang et al., 2019), whose geochemical analysis indication takes type Bii as the absolute advantage. The presence of Type A garnets in all formations is interesting. Type A garnet accounts for a major proportion (70%) in Early Cretaceous and has a similar proportion in Middle-Late Cretaceous, still maintaining at about 40% (Figure 9). They are most likely derived from high-grade granulite-facies metasedimentary rocks or charnockite and intermediate felsic igneous rock (Sabeen et al., 2002; Morton et al., 2004; Mange and Morton, 2007; Krippner et al., 2014). There was no charnockites reported from the potential source area of the Sichuan Basin.

Regional geological research shows that high-grade metamorphic complexes exist in the widely outcropping metamorphic massifs and Kangdian grey gneisses in the Longmenshan orogenic belt (SBGMR, 1982; He et al., 1988; Zhou et al., 2006b; Liu, 2006; Xu et al., 2016). Granitic gneisses, locally accompanied by variable amounts of amphibolite and garnet or amphibole two-pyroxene granulite, are the most voluminous rocks in these complexes (Zhou et al., 2006a). In the mixed gneiss of these metamorphic complexes, mainly garnet assemblage consists of pyrope, almandine, grossularite, and andradite (He et al., 1988). Moreover, the posterior mountain metamorphic belt in the Longmenshan orogenic belt is an important garnet-rich metamorphic belt in the East margin of the Tibetan Plateau (Dirks et al., 1994; Worley and Wilson, 1996; Wallis et al., 2003) (Figure 1). It is especially represented by the kyanite-sillimanite-garnet belt in Danba area and the garnet belt in the Maoxian-Wenchuan ductile shear zone (SBGMR, 1982; Worley et al., 1995). The garnet in the Damba metamorphic complex and garnet zone is mainly almandine and a small amount of grossular (Huang et al., 2003; Billerot et al., 2017).

More importantly, the garnet geochemistry of modern river sand samples in the upper reaches of the Yangtze River reveals that the garnet produced by numerous parent rocks (magmatic rocks + metamorphic rocks) in the Yangtze River catchment is mainly type B, while type A garnet is significantly lacking (Figure 12), indicating that garnet came from the medium-low grade metamorphic rocks (Wang et al., 2018). This further confirms that the garnet supplied by the Songpan-Ganzi fold belt is mainly of type B. All of the garnets from the Triassic granite plot are close to the discrimination line between the Ci and Bii fields (Figure 12), but the garnet in the Cretaceous sandstones in the Sichuan Basin is rarely near this boundary. It is suggested that there may be few garnets from granite, while metamorphic rocks are the main provenance. The Cretaceous sandstones in the Sichuan Basin obviously lack Type C garnet, which is significantly different from the garnet type in Jurassic sandstones in the Hefei Basin, which was provided by ultrahigh pressure eclogite from the Qinling-Dabieshan orogenic belt (Li R. W. et al., 2000; Wang Q. C., 2013), indicating that the provenance areas of the Sichuan Basin are lack of high-grade basic-ultrabasic metamorphic rocks, which is also consistent with the actual geological background. In conclusion, the most likely explanation for the type A garnet is that it comes from the metamorphic complexes or relatively high-grade metamorphic rocks in the Longmenshan orogenic belt, and kyanite was detected in the Tianmashan Formation (Cao et al., 2008), which supports the interpretation of proximal high metamorphic rocks provenance. Due to its special structure-geographical location, the Longmenshan orogenic belt is not only the main provenance of the basin sediments, but also the only way for the detrital materials from western source areas to flow into the basin, so the type A garnet can be continuously reflected in the sediments. The type B garnet mainly comes from the joint contribution of the Songpan-Ganzi fold belt and the metamorphic belt of the Longmenshan orogenic belt (Figure 12). The garnet in the Late Cretaceous sediments is mainly composed of intermediate P-T metamorphic rocks, which is more suitable for the Longmenshan orogenic belt.

U-Pb detrital-zircon age signatures of the Cretaceous strata of the southwest corner of the Sichuan Basin (SWCSB) contain distinct age distributions that can be linked to particular source regions (Figure 13). To assess the provenance of the Cretaceous strata preserved in the SWCSB, we include previously reported DZ results from the Jiaguan and Guankou Formation sandstones (Li et al., 2018) (Figure 2). The results obtained from the U–Pb geochronology provide a wide range of ages. Age populations occur at ∼2,569 Ma–2,200 Ma, ∼1,976 Ma–1,464 Ma, ∼865 Ma–610 Ma, ∼485 Ma–322 Ma, ∼242 Ma–200 Ma, and ∼167 Ma–120 Ma (Figures 10, 13). By comparing the age populations of zircons in seven sediment samples and potential source areas, the results show that the age spectra of zircons have significant temporal and spatial differences (Figure 13).

In terms of time, the Early Cretaceous Tianmashan Formation shows 2,498 Ma, 1,840 Ma, 740 Ma, and 230 Ma peaks, but the 440 Ma population is significantly lower. In the Mid-Cretaceous Jiaguan Formation, the peak of the oldest age component (2,569 Ma–2,200 Ma) decreased or even disappeared, but the 440 Ma peak became one of the dominant age peaks. The most striking feature in the Late Cretaceous Guankou Formation is the abundance of late Triassic zircons (with a peak at 213 Ma) (Zhang et al., 2015) that are missing in Lower Cretaceous Tianmashan Formation sandstone. Spatially, the samples (G124 and 130) at the west edge of the basin were enriched to 1,840 Ma (48%–56%) at the peak, while those at 740 Ma, 440 Ma, and 230 Ma peak ages were poorer than samples at the interior of the basin (Figure 13A).

Figure 13C shows zircon age normalized probability density for the eastern margin of Tibetan Plateau units that contain ca. 2,480, 1,840 Ma, 750 Ma, 441 Ma, 260 Ma, and 230 Ma zircons, such as the Upper Triassic-Jurassic sediments in the southwestern and western Sichuan Basin (Deng et al., 2008; Chen et al., 2011; Zhang et al., 2015; Shao et al., 2016; Li et al., 2018), Triassic flysch sediments in Songpan-Ganzi fold belt (Weislogel et al., 2006, 2010; Enkelmann et al., 2007; Tang et al., 2017; Jian et al., 2019), Yidun group in the Yiduan terrane (Wang B. Q. et al., 2013), and the Longmenshan orogenic belt and Kangdian rift (Li X. H. et al., 2002; Li Z. X. et al., 2002; Li Z. X. et al., 2003; Zhou et al., 2006b; Zhao and Zhou, 2007; Zhao J. H. et al., 2008; Zhao X. F. et al., 2008; Huang et al., 2009; Sun et al., 2009; Duan et al., 2011; Wang L. J. et al., 2012; Chen et al., 2016, 2018), including the Emeishan large igneous province (Shellnutt, 2014 and references there in). It can be observed that the Yidun group has a significant peak at 970 Ma, while the early Paleozoic of Longmen shan orogenic belt includes more ages of 944 Ma and 513 Ma (Figure 13C). These ages are not obviously reflected in Cretaceous sediments in the Sichuan Basin. Meanwhile, MDS analysis shows that the above two potential sources are far away from the Cretaceous sandstone samples, indicating that they are less likely to provide clastic materials to the sediments in the Sichuan Basin (Figure 13D).

The oldest age population within the sample FX-1G from the Lower Cretaceous Tianmashan Formation illustrates Palaeoproterozoic ages, and shows bimodal characteristics (with a peak at 1,840 Ma and 2,498 Ma) (Figure 13C). The two age populations make up 52% of 78 concordant zircon ages. Although the sandstones were from the Jurassic and Upper Triassic Xujiahe Formations in the western margin of the Sichuan Basin, and the Songpan-Ganzi Triassic flysch may provide zircons of 1,850 Ma and 2,500 Ma ages for the Tianmashan Formation, but the detrital materials from Songpan-Ganzi are obviously enriched with zircons of 440 Ma ages. Significant Paleoproterozoic, Neoproterozoic, and Triassic populations show similarities to Upper Triassic-Jurassic sandstones in the southwestern and western Sichuan Basin (Figure 13C), indicating that their provenance is mainly from recycling sediments, while in Early Cretaceous period Songpan-Ganzi fold belt was not the main provenance area. This also explains the high ZTR index and higher percentage of rounded zircons in the Lower Cretaceous Tianmashan Formation than in overlying strata (Figure 8).

Samples from the Jiaguan and Guankou Formations, the detrital U-Pb zircon age spectra display four major age peaks at ca. 1,840 Ma, 780 Ma–830 Ma, 440 Ma, and ca. 207 Ma–230 Ma, with two minor U-Pb age peaks at ca. 167 Ma and 2,450 Ma. The peak at ca. 240 Ma–188 Ma is consistent with the occurrence of widespread granites in the Songpan-Ganzi fold belt (Zhang et al., 2007). The major age peak encountered at ca. 860 Ma–760 Ma can be linked to the Kangdian rift. The detrital U-Pb age spectra indicate that the dominant provenance of the Jiaguan Formation sediments during the Mid-Cretaceous was the Songpan-Ganzi fold belt and Longmenshan orogenic belt (Figure 13D).

Using the multi-proxy provenance data, we were able to determine the provenance evolution of a thick stratigraphic succession within the southwest corner of the Sichuan Basin and two significant provenance shifts (Figure 14). The two shifts are characterized by different compositions attributed to changes in denudation areas.

Our analysis indicates that the first significant provenance change occurs at the bottom of the Mid-Cretaceous Jiaguan Formation (ca. 125 Ma) (Figure 14). This initial transition is interpreted to reflect the introduction of sediment from the Longmenshan orogenic belt and decreasing sediment input from the Pre-Cretaceous clastic rocks in the Western margin of the Sichuan Basin. Sandstones modal analysis indicates that the lithology of the Early Cretaceous Tianmashan Formation sandstone is mainly lithic-quartz sandstone with high quartz and low plagioclase. This composition strongly indicates recycled orogen in a collision orogen as the sediment source for Tianmashan sandstones (Figure 6) (Dickinson, 1985). However, in the Jiaguan Formation sandstone, the content of microcline, plagioclase, and mica increased significantly, while the content of quartz decreased, and granite lithic fragments appeared, and few samples of Jiaguan Formation are shown in the mixed field in the Qm-F-Lt diagram, which clearly shows the composition difference with that of the Tianmashan Formation (Figure 6). Coincidentally, conglomerate clast compositions indicate from the Tianmashan Formation to the upper Jiaguan Formation, the amount of carbonate and sandstone clasts decreases while igneous and metamorphic rocks clasts increase (Figures 5, 14).

Our garnet geochemical data, based on provenance fields of idealized compositions, show that the Tianmashn Formation has more type A and less type Bii garnets than the younger Jiaguan Formation sediments (Figure 9). This provenance shift is also indicated in the detrital zircons age distribution diagrams (Figures 10A,C). The zircon U-Pb data presented here record a dramatic change in sediment provenance from the Tianmashan Formation to the upper Jiaguan Formation, and they refine existing constraints on sediment sources for both formations, which were deposited during the transition from fluvial system to alluvial fan-braided river system in the Southwest corner of Sichuan Basin. The Tianmashan Formation zircon age population is dominated by three peaks at 740 Ma, 1,840 Ma, and 2,498 Ma (Figure 10A). By contrast, the Jiaguan Formation is dominated by ca. 164 Ma, 213 Ma, 440 Ma, ca. 800 Ma, and 1,840 Ma zircons, whereas ca. 2,440 Ma ages are minor (Figure 10B). By comparing the previous studies on Late Triassic—Late Jurassic detrital rocks in the basin and the potential provenance data (Figures 13C,D), the detrital zircon ages of the Tianmashan Formation are very consistent with the late Triassic Xujiahe Formation in the western margin of the Sichuan Basin, reflecting the bimodal characteristics of 1,840 Ma and 2,500 Ma. This indicates the migration of the erosion center from the Late Triassic Xujiahe Formation ranges to the Longmenshan orogenic belt and Songpan-Ganzi fold belt. Heavy mineral compositions support this migration: samples of Tianmashan Formation have higher ZTR values (42.58%–52%) and leucoxene with low garnet compared to that of the Jiaguan Formation. Moreover, ilmenite, monazite, and biotite with unstable physical properties are increased in the Jiaguan Formation. In addition, chromium spinel and ilmenite, which represent the basic-ultrabasic parent rocks, began to increase.

A new significant compositional change occurred in the Guankou Formation, with a Late Cretaceous age indicated by provenance shift 2. This compositional change is characterized by the increase of fossiliferous carbonate clasts and the substantial decrease of sandstone, quartzite, and vein quartz clasts (Figure 5). The content of carbonate clasts in the conglomerate can reach up to 92% (average 82%). The microfossil assemblages in the Guankou Formation carbonate clasts are similar to the reported microfauna from the Devonian Carbonates (Gou, 2001). The unstable carbonate clasts had a shorter transport distance and were probably derived from proximal sources such as the southern section of the Longmenshan orogenic belt. This provenance change is also evidenced by the slight decrease lithic fragments of metamorphic rocks. The heavy mineral is reflected in the increase of garnet content, and the geochemical characteristics show that garnet is mainly of Type Bii, indicating that the main source is low-grade metasediments (up to amphibolite metamorphic facies). The Guankou Formation zircon age population is dominated by two peaks at 213 Ma and 1,840 Ma, with only minor Mid-Cretaceous (120 Ma–135 Ma) and Paleozoic ages (358 Ma–476 Ma), but lacking ages of 720 Ma–860 Ma. The provenance change is also reflected in the Liujia area of Yibin in southeastern Sichuan, which shows that the garnet content in the Late Cretaceous Sanhe and Gaokangba Formation is suddenly increased compared with that in the underlying strata (Deng et al., 2018; Jiang et al., 2019).

In summary, the above analyses based on multi-proxy provenance data reveal that two marked changes of sedimentary provenances occurred in the Cretaceous: a local source region for the Lower Cretaceous clastic rocks and SPGZ and LMS source regions for the Middle-Late Cretaceous successions.

Cretaceous strata of the southwestern corner of the Sichuan Basin provide an excellent opportunity to decipher changes in sediment provenance during the transition from Longmenshan thrusting through the formation of regional Cretaceous Plateau uplifts.

Our detailed provenance analysis revealed that two provenance shifts were located at the bottom of the Jiangaun Formation and within the Guankou Formation, respectively (Figure 14). The two provenance changes were accompanied by the change of sedimentary environment from fluvial facies to alluvial fan and alluvial fan-lacustrine sedimentary systems (Gou, 1998; Gou, 2001).

Abrupt shifts in sediment provenance at geologic timescales can be an indication of tectonic reconfiguration of upland source areas (Horton et al., 2004). A Cretaceous palaeogeographic reconstruction with major sediment transport directions (Figure 15) shows suggested sources for Cretaceous in the Sichuan Basin that are SPGZ, LMS, and pre-Cretacepous sediments in the west edge of the Sichuan Basin. The changes in sedimentary facies and provenance indicate that the eastern margin of the Tibetan Plateau has become the main provenance area of Cretaceous clastic rocks, and rapid uplift and denudation began to occur during the sedimentary period of the Jiaguan Formation. In other words, intense tectonic activity began in the eastern margin of the Tibetan Plateau began from Middle Cretaceous (125 Ma).

Tectonic reconstructions indicate that the Mountain Frontal Fault (Beichaun Fault), involving crystalline basement at depth, would have been most reactive during the Lower Cretaceous Tianmashan and Jiaguan syntectonic depositions, as observed at Pengguan massif (Airaghi et al., 2017b). There is a good agreement between the uplift denudation events of sedimentary records and cooling events of the Cretaceous at the eastern margin of the Tibetan Plateau revealed by low-temperature thermochronology (Liu et al., 2018).

Reviewing the history of regional tectonic evolution, the most important tectonic transformation occurred in the late Early Cretaceous, when the tectonic environment of the Sichuan Basin changed from the Qinling tectonic domain to the Tibetan Plateau tectonic domain (Liu et al., 2013a; Liu et al., 2018). On the one hand, the tectonic nature of the Qinling orogenic belt has undergone a major transformation and the structural deformation of the northwestern boundary of the basin and the northern part of the basin was basically finalized in this event, and the deposition has since ended and entered a state of uplift and denudation. Since then, the tectonic activities of the Qinling orogenic belt have no significant influence on the western Sichuan foreland basin. At the same time, affected by the long-range effects of the collision between the Lhasa and Qiangtang terranes and the collision between the Indian plate and the Eurasian plate, the central and southern section of Longmen mountains, became active again, forming a new foredeep on its front edge. The subsidence center migrated from early northwestern Sichuan to southwestern Sichuan, and then entered the Late Cretaceous to Paleogene foreland basin evolution stage under the control of the Tibetan Plateau tectonic domain. The regional uplift in the late Early Cretaceous made the Lower Cretaceous in the northern part of the basin and the Tianmashan Formation in the southern part of the basin suffered different degrees of denudation. During the deposition of the Jiaguan Formation, the depositional range was reduced, and the basin turned into a differential evolution stage of north uplift-south down. The provenance transition of the Guankou Formation responded to the late Cretaceous period (∼80 Ma) with another significant uplift and denudation of Longmen Mountain at the eastern edge of the Tibetan Plateau.