Late Cretaceous Nature of SW Borneo and Paleo-Pacific Subduction: New Insights from the Granitoids in the Schwaner Mountains | Lithosphere (2024)

Xin Qian;

Yongqi Yu;

Yuejun Wang;

Chengshi Gan;

Yuzhi Zhang;

Junaidi Bin Asis

Yongqi Yu

Yuejun Wang

Chengshi Gan

Yuzhi Zhang

Article history

Sundaland continent, located at the eastern margin of Southeast Asia, is known to have been created by a series of continental blocks/fragments with basem*nt rocks during the Late Paleozoic to Mesozoic, which were derived from the Gondwana northern margin. The eastern margin of Sundaland has been suggested to be an Andean-type continental margin, which records abundant magmatic and metamorphic activities and can extend from the South China Coastal Provinces through eastern Indochina into western Kalimantan (Borneo), forming the giant Western Circum-Paleo-Pacific subduction system (e.g., [1–12]). Previous studies have put emphasis on the Late Mesozoic igneous rocks along the South China Coastal Provinces and southern Vietnam and revealed an active continental margin setting in response to the Paleo-Pacific subduction (e.g., [8, 13–20]). Besides, the Late Cretaceous granitoids have also been identified from Eastern Peninsular Malaysia and Singapore to the east of Sundaland (e.g., [21–23]). In contrast, few petrological, zircon geochronological, and systematically geochemical studies have been focused on the equivalent igneous rocks in western Borneo, and limited reported ages were mainly focused on the K–Ar isochron (e.g., [24, 25]).

Borneo Island is located in the joint position of the India-Australia, Eurasian, and Philippines plates and consists of northwestern Sarawak and northern Sabah states of Malaysia, Brunei, and southern and central parts of Indonesian Kalimantan (Figure 1(a)). Most of southern Borneo has been suggested to be formed by the NW Borneo and Schwaner Mountains, which are surrounded by the Kuching and Meratus zones/complexes (Figure 1(b); e.g., [7, 9–11, 26–31]). The ultramafic-mafic and sedimentary rocks along the Kuching zone in NW Borneo have been suggested to be formed in an accretionary orogenesis related to the Paleo-Pacific evolution (e.g., [1, 9–11, 32–35]), and the Cretaceous Meratus Complex in SE Borneo might represent the eastern extension of the Tethyan Ocean (Figure 1(b); e.g., [6, 36–38]). Besides, abundant Mesozoic igneous and metamorphic rocks are distributed to the south of the Kuching zone and formed the largest igneous province of Schwaner Mountains in the Borneo Island, which can be further subdivided into the NW Schwaner (NWSZ), North Schwaner (NSZ) and South Schwaner (SSZ) zones (Figure 1(b); e.g., [7, 11, 26, 27, 30, 32]). Wang et al. [39] recently named the Kuching zone and NWSZ as NW Borneo and combined the NSZ and SSZ as SW Borneo (Figure 1(b)). However, the tectonic affinity of SW Borneo is still not clear. The SSZ has been suggested to represent the main body of SW Borneo that separated and originated from the northern margin of Gondwana since the Late Triassic and was subsequently accreted to eastern Sundaland during the Early Cretaceous (e.g., [6, 7, 26, 27]). In contrast, Metcalfe [34], Metcalfe [40], proposed the SW Borneo as a pre-Mesozoic continental fragment of the South China/Indochina origin.

In SW Borneo, previous studies on the igneous and metamorphic rocks have revealed ~144–85 Ma arc-related and~85–72 Ma collision-related tectonic events, which have been suggested to be related to the Paleo-Pacific evolution (e.g., [7, 9–11, 26, 27, 41, 42]). However, these studies mainly put emphasis on the dating and regional geological background study (e.g., [7, 26, 27, 41]), lacking systematically petrological and geochemical researches on the Cretaceous igneous rocks in the Schwaner Mountains. In this study, new zircon in situ U–Pb–Hf–O isotopic data, systematic whole-rock geochemical, and Sr–Nd–Pb isotopic data are presented for twenty-eight fresh granodiorite, biotite granite, and monzogranite samples collected in the Sepauk and Sukadana plutons of Schwaner Mountains to constrain their formation age, geochemical characteristic, petrogenesis, and tectonic setting (Figure 2). According to our new data, in conjunction with previous studies, we provide new evidence for understanding the Late Cretaceous tectonic affinities in SW Borneo and an updated interpretation of the Late Cretaceous Paleo-Pacific westward-subduction, and further discuss their connection and relationship with those Cretaceous igneous rocks in the Kuching zone and eastern Sundaland.

NW Borneo has been suggested to be the Sundaland core, which mainly consists of Triassic metagranitoids and previously mapped Triassic Sadong and Kuching formations (Figure 1(b); e.g., [7, 26, 27, 42]). Recent study suggests that the granitoids in NW Borneo with zircon ages of ~256–207 Ma represent the southern extension of the Eastern Paleotethyan Domain, which was formed by the Eastern Paleotethyan subduction and subsequent collision between the Sibumasu and Indochina blocks (e.g., [10]). During the Triassic, NW Borneo was located at the southeastern margin of Sundaland (Figure 1(b); e.g., [7, 26, 27, 43]).

The Kuching zone in NW Borneo is traditionally bounded by the Lupar Line to the northeast and the Pinoh metamorphic Complex (PMC) to the southwest and contains Jurassic to Cretaceous Lupar-Serabang complexes/ophiolitic mélanges, Serian volcanic rocks, and northern granitoids (Figure 1(b); e.g., [5, 9, 27, 32, 35, 41, 44, 45]). Wang et al. [9] recently reported that the Lupar-Serabang mafic rocks and Serian volcanic rocks along the northern Kuching zone in Malaysia Sarawak were formed at ~97–77 Ma and suggested a Late Cretaceous accretionary orogenesis related to the Paleo-Pacific subduction in NW Borneo. Besides, according to the sedimentary records in the Kuching zone, Wang et al. [9] proposed that NW Borneo to the southwest of Sarawak Kuching zone was a part of the Indochina/Eastern Peninsular Malaysia fragment in SE Asia prior to the Jurassic. Gan et al. [5] suggested that the northern granitoids along the Kuching zone with I-type affinities were formed at the Late Cretaceous (~84–78 Ma).

The Jurassic to Cenozoic granitoid is the main lithology in the Schwaner Mountains, which also includes the Cretaceous Biwa gabbros, Kerabai and Menunuk volcanic rocks, and Pinoh metamorphic rocks (Figure 2; e.g., [7, 25, 26, 30, 32, 42, 46, 47]). The sedimentary rocks are mainly distributed in the southwestern Schwaner Mountains and contain the Jurassic Ketapang siltstones, sandstones, shales, and tuffaceous rocks (Figure 2; e.g., [26, 48]). In this paper, we use the name of Schwaner Batholith for the granitoid plutons in the Schwaner Mountains. The Cretaceous Sukadana, Sepauk, Lau, Mandahan, and Sangiyang granitoid plutons constitute the main body of the Schwaner Batholith with the width of >200 km and the length of >500 km (Figure 2). The Lau pluton is equivalent to the Sepauk pluton, and consists of tonalite, granodiorite, syenogranite, monzogranite, and a minor amount of mafic-intermediate intrusions, and the granitoids with I-type affinities in the Lau and Sepauk plutons were intruded by the Late Cretaceous Biwa gabbros in the NWSZ (Figure 2; e.g., [26, 30, 49]). Hennig et al. [7] and Breitfeld et al. [26] recently reported zircon ages of ~101–81 Ma for the granitoids in the Lau and Sepauk plutons. The Sukadana pluton is dominated by the potassic granite and syneogranite, which were intruded by the Sangiyang alkali granite, and the Mandahan alkaline granite is mainly distributed in the southern Schwaner Batholith and might be the equivalence to the Sangiyang pluton (Figure 2; e.g., [26, 50]). ~85–79 formation ages and one zircon age of ~72 Ma have been reported from the Sukadana and Sangiyang plutons, respectively (Figure 2; e.g., [24, 26, 50]). The Kerabai volcanic rocks are mainly distributed in the southwestern Schwaner Mountains and consist of basalt, andesite, dacite, rhyolite, and breccia, which have been suggested to be formed at the latest Cretaceous and might represent the volcanic equivalences to the Sukadana granitoids (e.g., [26, 48, 49]). The Early Cretaceous Menunuk felsic volcanic rocks and related clastic rocks have a limited distribution in the northern Schwaner Mountains (Figure 2). The Cenozoic igneous rocks include the Singtang intermediate-felsic intrusions and minor mafic-felsic volcanic rocks, which are mainly distributed in the southern and eastern of Schwaner Mountains (Figure 2; [26]).

The Pinoh metamorphic Complex is mainly distributed in NSZ (Figures 1(b) and 2) and contains muscovite-quartz schist, quartzite, phyllite, slate, gneiss, and metatuff, which have been previously assumed to represent the Paleozoic basem*nt of Borneo [48, 49]. Davies et al. [41] and Breitfeld et al. [26] suggested that the metamorphic rocks from the PMC contain the Early Cretaceous zircon ages of ~135–110 Ma, which are contemporaneous with those subduction-related granitoids and associated volcanic rocks in the NSZ. Besides, the original sources of PMC were suggested to be derived from volcaniclastic sediments, which were metamorphosed during the Cretaceous (e.g., [26]).

Sampling locations of the analyzed granitoid samples are present in Figures 2 and 3 and Table 1. Granodiorite and biotite granite samples (18JV-70A, 18JV-72, 18JV-74A, and 18JV-74B) and monzogranite samples (18JV-98 and 18JV-99) from the Sepauk granitoids were collected from the southeastern of Pontianak and northwestern of Palangkarayan, respectively. Granodiorite samples from the Sukadana granitoids (18JV-86 and 18JV-88) were collected from the western of Ketapang area. The granodiorite samples show granitic textures and have a mineral assemblage of plagioclase (35–45 vol%), K-feldspar (10–20 vol%), quartz (15–20 vol%), hornblende (10–15 vol%), and biotite (10–15 vol%) with minor amounts of accessory minerals of apatite, zircon, and Fe-Ti oxides (Figures 4(a)–4(d)). Plagioclase phenocrysts are mainly subhedral to anhedral crystal with grain size varying from 0.5 mm to 2.0 mm in diameter, and partial phenocrysts show a low degree of sericitization in the microscopic photographs (Figures 4(a)–4(d)). The biotite granites are characterized by a coarse- to medium-grained or porphyritic texture and composed of 25–30 vol% K-feldspar, 20–30 vol% plagioclase, 20–25 vol% quartz, and 10–20 vol% biotite with minor accessory minerals (Figures 4(e) and 4(f)). The monzogranites are granitic textures with subhedral to anhedral plagioclase (20–30 vol%), K-feldspar (20–30 vol%), perthite (10–15 vol%), quartz (10–15 vol%), and minor amounts of biotite and hornblende (Figures 4(g) and 4(h)). The surfaces of some plagioclase phenocrysts show a low degree of sericitization (Figures 4(g) and 4(h)).

The detailed analytical methods for zircon in situ U–Pb–Hf–O isotopic, whole-rock geochemical, and Sr–Nd–Pb isotopic analyses are described in Appendix A. Analytical methods and corresponding results are presented in Supplementary Table S1–S3.

Eight granitoid samples from the Schwaner Batholith in SW Borneo were chosen for zircon in situ U–Pb geochronological and Hf–O isotopic analyses (Figure 2 and Table 1). Zircon in situ Hf–O isotopic analyses were conducted on these grains that have been previously analyzed for the U–Pb analyses. In cathodoluminescence (CL) images (Figure 5), most zircon grains from the representative samples are 150–250 μm long with elongation ratios of 2 : 1–3 : 1. Grains from the biotite granite and granodiorite samples (18JV-70A-1, 18JV-72-2, 18JV-74A-2, 18JV-74B-2, 18JV-86-1, and 18JV-88-1) display typical characteristics of concentric oscillatory zoning (Figures 5(a)–5(f)). In contrast, zircon grains from the monzogranite samples (18JV-98-5 and 18JV-99-4) show obvious to weak internal oscillatory zoning (Figures 5(g) and 5(h)).

The Th/U ratios of the selected spots on zircon grains from the biotite granite and granodiorite samples range from 0.17 to 1.75 (mostly >0.4). Eight analyses on 8 grains and twenty-two analyses on 22 grains from samples 18JV-70A-4 and 18JV-72-2 yield similar weighted mean ages of $86±1$ Ma (⁠$MSWD=0.31$⁠; Figure 5(a)) and $87±1$ Ma (⁠$MSWD=0.62$⁠; Figure 5(b)), respectively. The remaining analytical spots from these two samples give older ages of 104–1720 Ma, interpreted as inherited zircon ages (Table 1). Eight and fourteen grains from samples 18JV-70A-4 and 18JV-72-2 have positive $εHft$ values of +4.8–+10.2 and+7.6–+14.5 and $TDM2$ ages of 0.50–0.85 Ga and 0.22–0.67 Ga, respectively (Figure 6(a) and Table 1). Zircon grains from biotite granite sample 18JV-74A-2 and granodiorite samples 18JV-74B-2 define same zircon ages of $85±1$ Ma (⁠$n=20$⁠, $MSWD=0.73$⁠; Figure 5(c)) and $85±1$ Ma (⁠$n=23$⁠,$MSWD=2.10$⁠; Figure 5(d)), respectively. The $εHft$ values and $TDM2$ ages for samples 18JV-74A-2 and 18JV-74B-2 range from +7.3 to +11.0 and+6.6 to +10.1 and 0.45 Ga to 0.68 Ga and 0.50 Ga to 0.73 Ga, respectively (Figure 6(a) and Table 1). Twelve spots from sample 18JV-74A-2 have $δ18O$ values ranging from 5.3‰ to 6.0‰ (Figure 6(b) and Table 1). Sixteen zircon grains from 18JV-86-1 yield a weighted mean age of $92±1$ Ma (⁠$MSWD=2.40$⁠; Figure 5(e)), and have $εHft$ values of +1.2–+8.0 and $TDM2$ ages of 0.65–1.08 Ga (Figure 6(a) and Table 1). Fifteen spots from this sample have $δ$¹⁸O values of 5.9–6.6‰ (Figure 6(b) and Table 1). Twenty-three spots on 23 zircon grains from sample 18JV-88-1 give a weighted mean age of $85±1$ Ma (⁠$n=23$⁠, $MSWD=2.50$⁠; Figure 5(f)). $εHft$⁠, $δ$¹⁸O values, and $TDM2$ ages on fourteen analyses from sample 18JV-88-1 range from +1.9 to +6.4, 5.2‰ to 6.1‰, and 0.74 Ga to 1.03 Ga, respectively (Figure 6 and Table 1).

The Th and U concentrations from the monzogranite samples range from 42 ppm to 2582 ppm and 63 ppm to 2491 ppm, respectively, with Th/U ratios of 0.24–1.78. Most Th/U ratios are higher than 0.4. Fourteen and twenty-three zircon analyses from both samples (18JV-98-5 and 18JV-99-4) yield similar weighted mean ages of $80±1$ Ma (⁠$MSWD=1.30$⁠; Figure 5(g)) and $81±1$ Ma (⁠$MSWD=1.90$⁠; Figure 5(h)), respectively. Thirteen and fifteen grains from the two samples have similar $εHft$ values of −1.5–+3.0 and−0.8–+5.9 and corresponding $TDM2$ ages of 0.96–1.24 Ga and 0.77–1.20 Ga, respectively (Figure 6(a) and Table 1). Thirteen spots from sample 18JV-98-5 yield $δ$¹⁸O values of 6.1–6.6‰ (Figure 6(b) and Table 1).

The major oxides reported below are volatile-free normalized to 100%. According to their zircon ages, elemental, and isotopic characteristics, the studied samples can be geochemically subdivided into the Group 1 biotite granite and granodiorite samples with zircon ages of 92–85 Ma and Group 2 monzogranite samples with zircon ages of 82–81 Ma. Group 1 samples contain various SiO₂ of 64.36–75.86 wt.%, Al₂O₃ of 14.35–17.78 wt.%, Fe₂O₃t of 2.52–5.75 wt.%, and MgO of 0.70–2.51 wt.%, with Mg# values $molarMg×100/Mg+Fe$ ranging from 31 to 47. In Harker diagrams (Figure 7), these samples show negative correlations between Al₂O₃, Fe₂O₃t, MgO, CaO, TiO₂, P₂O₅, and SiO₂, while no clear correlations between K₂O, Na₂O, and SiO₂ can be observed. In Figures 8(a) and 8(b), Group 1 samples plot in the fields of granodiorite and monzogranite. In Figure 8(c), these samples fall within the medium-K to high-K calc-alkaline series. The A/CNK [molar Al₂O₃/(CaO+Na₂O+K₂O)] and A/NK [molar Al₂O₃/(Na₂O+K₂O)] values for Group 1 samples varying from 0.92 to 1.64 and 1.37 to 2.23, respectively, and mainly plot along the boundary between metalumions and peraluminous with three samples plotting as strongly peraluminous (Figure 8(d)). Group 2 monzogranite samples have similar SiO₂ (75.67–77.41 wt.%), Al₂O₃ (12.39–13.17 wt.%), Fe₂O₃t (1.12–1.68 wt.%), and MgO (0.06–0.18 wt.%) contents, and their Mg# values range from 7 to 18. In Harker diagram (Figure 7), their Al₂O₃ and Fe₂O₃t show slightly negative correlations with SiO₂. Their differentiation index (DI) (⁠$DI=Q+Or+Ab+Ne+Lc+Kp$⁠) values range from 95 to 97, suggesting that their magma was highly evolved. These Group 2 samples fall in the fields of monzogranite and granite and can be classified as the high-K calc-alkaline monzogranites (Figures 8(a)–8(c)). Their A/CNK and A/NK values range from 1.02 to 1.07 and 1.07 to 1.14, respectively, and plot in the peralumunous field (Figure 8(d)).

Group 1 biotite granite and granodiorite samples have relative enrichment of light rare earth elements (LREEs) with variable (La/Yb)_N (⁠$N$ herein refers to chondrite-normalized value; e.g., [51]) ratios of 2.82–21.99 and (Gd/Yb)_N ratios of 0.70–4.41 (Figure 9(a)). They have Eu/Eu$∗$ anomalies of 0.57–1.39 (Figure 9(a)). Group 1 samples are enriched in large ion lithophile elements (LILEs) (e.g., Ba and U) and depletion in high-field-strength elements (HFSEs) (e.g., Nb, Ta, and Ti) and show slight Sr anomalies (Figure 9(b)). Group 2 samples exhibit obvious LREEs enrichment in comparison with the HREEs contents (Figure 9(a)). Their (La/Yb)_N and (Gd/Yb)_N ratios range from 4.56 to 11.41 and 1.08 to 1.96, respectively. They have obvious negative Eu/Eu$∗$ anomalies of 0.22–0.31 (Figure 9(a)). The enrichment in Rb, Ba, U, and depletion in Nb, Ta, Sr, and Ti can also be observed from Group 2 samples in Figure 9(b), resembling the Group 1. Both groups show similarities with the Late Cretaceous I-type granites in the Kuching zone (Figure 9; [5]).

Whole-rock Sr–Nd–Pb isotopic compositions for the studied samples are shown in Figure 10 and Table 1. The calculated initial isotopic ratios were based on their formation ages of 92–80 Ma from Section 4. Group 1 samples display (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.704672–0.706338 and $εNdt$ values of −1.9–+1.7. The (⁸⁷Sr/⁸⁶Sr)_i ratios and $εNdt$ values for Group 2 samples are similar to the Group 1 with ranging from 0.701153 to 0.707693 and from −1.5 to −1.1, respectively. Both Group 1 and 2 samples plot in the fields of Eastern Paleotethyan arc volcanic rocks and I-type granites in the Lachlan Fold Belt (Figure 10(a); e.g., [52, 53] and references therein) and are similar to the Late Cretaceous I-type granites in the Kuching zone (Figure 10(a); e.g., [5]). Group 1 samples have variable (²⁰⁶Pb/²⁰⁴Pb)_i ratios of 18.63–19.29, (²⁰⁷Pb/²⁰⁴Pb)_i ratios of 15.62–15.69, and (²⁰⁸Pb/²⁰⁴Pb)_i ratios of 38.67–39.49 and are roughly similar to the Late Cretaceous I-type granites in the Kuching zone and fall in the field of Jurassic-Early Cretaceous igneous rocks in western Borneo (Figures 10(b) and 10(c); e.g., [5, 10, 11]). All these Pb isotopic data also plot to the left side of the north hemisphere reference line (NHRL) defined by Hart [54] and fall in the fields of Gangdese potassic-ultrapotassic rocks and Indian oceanic turbidites (Figures 10(b) and 10(c); e.g., [55–59]).

The correlations between SiO₂ and Nb/La and $εNdt$ in Figures 11(a) and 11(b) indicate that the both Group 1 and 2 samples were controlled by the fractional crystallization rather than the crustal contamination. The Al₂O₃, Fe₂O₃t, MgO, CaO, TiO₂, and P₂O₅ of analyzed Group 1 samples show negative correlations with SiO₂ (Figure 7), suggesting the fractional crystallization of plagioclase, K-feldspar, biotite, Fe-Ti oxides, and apatite. These observations can be further supported by the correlations between Ba and Eu/Eu$∗$ and Sr in Figures 11(c) and 11(d). However, some samples with slightly positive Eu and Sr anomalies from Group 1 in Figure 9 might indicate the partial plagioclase accumulation during the magma process. The slightly negative correlations between Al₂O₃ and Fe₂O₃t and SiO₂, along with the Eu, Sr, and Ti negative anomalies of Group 2 samples indicate the fractional crystallization of plagioclase and Fe-Ti oxides (Figures 7 and 9). The correlations between Ba and Eu/Eu$∗$ and Sr for Group 2 samples in Figures 11(c) and 11(d) suggest the fractional crystallization of K-feldspar.

In general, a small number of felsic magma can be derived from a large amount of basaltic magma through differentiation (e.g., [60, 61]). In fact, in SW Borneo, felsic igneous rocks (mainly granitoids) are widely distributed as stocks and batholiths, while the synchronous mafic-intermediate igneous rocks are distributed sporadically (Figure 2). Moreover, no mafic magmatic enclaves (MMEs) can be observed from our outcrops and microscopic photographs (Figures 3 and 4). These signatures suggest that the fractional crystallization of mantle-derived basaltic magma is unlikely for our samples. In contrast, the partial melting of low to middle crust can produce granitic magma via fractional crystallization (e.g., [62–64]).

Our Group 1 samples with low Rb/Sr (0.05–0.96) and Rb/Ba (0.05–0.24) ratios mainly plot in the clay-poor field, and partial samples are close to the basalt-derived melt in Figure 11(e). In Figures 11(f) and 11(g), most Group 1 samples plot along the line of basalt-derived melt or fall in the field of partial melts of amphibolites. These signatures, along with their positive $εHft$ values of +1.9–+14.5 further indicate a juvenile mafic lower crust for the Group 1 granitoid samples. Besides, their $δ$¹⁸O values (5.2‰–6.6‰) of selected granitoid samples are close to those of mantle-derived magmas, but significantly lower than those of S-type granites, which have high $δ$¹⁸O values of >8‰ (Figures 6(b) and 6(c); e.g., [65–68]). Their calculated whole-rock $δ$¹⁸O values of 7.0‰–8.3‰ are consistent with those of lavas and mantle xenoliths from convergent margin settings (Figure 6(b); e.g., [69]), further suggesting that they were mainly derived from the newly underplated mafic rocks during the subduction. It is notable that some samples with high (²⁰⁶Pb/²⁰⁴Pb)_i, (²⁰⁷Pb/²⁰⁴Pb)_i, and (²⁰⁸Pb/²⁰⁴Pb)_i ratios plot close to the field of Indian oceanic turbidites (Figures 10(b) and 10(c)), indicating the involvement of crustal rocks. This can be further evidenced by their slightly negative $εNdt$ values of −1.9–−1.1 in Figure 10(a), linear correlation in Figure 11(h), and high A/CNK values of 1.35–1.64 in Figure 8(d). Therefore, we propose that the Group 1 granitoid samples were derived from partial melting of newly underplated mafic rocks with a component of crustal rocks, and fractional crystallization of plagioclase, K-feldspar, biotite, Fe-Ti oxides, and apatite took place after their initial melt formation.

Group 2 peraluminous monzogranites with negative $εNdt$ values of −1.5–−1.1 have higher Rb/Sr and Rb/Ba and lower CaO/Na₂O and Al₂O₃/TiO₂ ratios than Group 1 samples, and plot in the fields of clay-rich or pelite sources in Figures 11(e) and 11(f). In addition, these samples also pot in the field of partial melt of greywackes (Figure 11(g)), and their $δ$¹⁸O values of 6.1–6.6‰ with corresponding whole-rock $δ$¹⁸O values of 8.3–8.8‰ are higher than Group 1 samples (Figures 6(b) and 6(c) and Table 1). All these signatures indicate that these peraluminous granites might be derived from the partial melting of metasedimentary rocks. However, their zircon grains have negative to positive $εHft$ values of −1.5–+5.9, suggesting a juvenile mafic crust has been involved into their source region. Such a consideration can be supported by the following observations. (1) Their Hf/Sm and Zr/Y ratios show a positive correlation in Figure 11(h), and (2) their Sr–Nd–Hf isotopic data and elemental patterns are similar to the I-type granites in the Kuching zone and Lachlan belt (Figures 6(a), 9, and 10; e.g., [5, 52]), which were suggested to be derived from a mixed source of crustal rocks with a juvenile mafic component. As a result, the monzogranite samples might have been originated from partial melting of metasedimentary rocks with a juvenile mafic crust component.

The Borneo Island has underwent a ~35° counterclockwise rotation constrained to the Late Eocene and an additional ~10° counterclockwise rotation since the Early Miocene according to the paleomagnetic data (e.g., [70]). The SSZ of SW Borneo formerly has been interpreted to accrete to the Sundaland and record the Paleo-Pacific subduction and collision events since the Early Cretaceous (e.g., [3, 6, 7, 26, 71]). Wang et al. [11] recently suggested that the detrital age patterns of the Jurassic sedimentary sequences in the Kuching zone and western SSZ are comparable to those from the East Malaysia-Indochina and Sibu zone, but distinctive from the Meratus zone in SE Borneo, which represents the eastern extension of the Neotethyan Ocean (e.g., [37]). Although Deng et al. [72] proposed that the regional tectonic regime transformation from Pacific plate subduction to Neotethyan plate subduction occurred at ~110 Ma according to the Pb isotopic transition from the Pacific type to India type, our Pb isotopes are similar to those Mesozoic Paleo-Pacific subduction-related igneous rocks along the eastern margin of Sundaland (Figures 10(b) and 10(c); e.g., [5, 10, 11]). Besides, studies on the detrital zircon, igneous, and metamorphic rocks have constrained that the Neotethyan arc system can extend to the Sumatra and further to the Java, SE Borneo, and SW Sulawesi (e.g., [12, 37, 73–75]). Moreover, the Sr–Nd–Hf–Pb isotopic data and elemental patterns of our granitoid samples in SW Borneo are similar to the Late Cretaceous I-type granites in the Kuching zone of NW Borneo, further support that the crust of NW Borneo and SW Borneo might have similar provenance characteristics. Thus, the SSZ of SW Borneo might be the southern extension of the Sundaland and controlled by the Paleo-Pacific Domain prior to the Jurassic.

Our zircon dating reveals that the granitoids from the Sepauk and Sukadana plutons in the Schwaner Mountains were formed at the Late Cretaceous with ages of 92–80 Ma (Figure 5 and Table 1). These new zircon ages fall in the range of those previously reported age data (~102–72 Ma) in the Schwaner Mountains (Figure 2 and Table 2; e.g., [7, 24, 26, 50]). Henning et al. [7] suggested that the I-type granitoids with ages of ~102–81 Ma in the Schwanner Mountains recorded the Cretaceous Paleo-Pacific subduction, and the post-collisional magmatism formed the Pueh (⁠$77.2±0.8$ Ma) and Gading granitoids (⁠$79.7±1.0$ Ma) in West Sarawak. Breitfeld et al. [26] recently studied and reviewed the Cretaceous igneous and metamorphic rocks in the Schwanner Mountains and proposed two phases of postcollisional magmatism after the ~90–85 Ma Paleo-Pacific subduction, including ~85–79 Ma Sukadana and ~72 Ma Sangiyang plutons in the Schwanner Mountains and Late Cretaceous granitoids along the Kuching zone of West Sarawak. In contrast, Wang et al. [9] suggested that the Late Cretaceous (~98–77 Ma) Lupar-Serabang mafic igneous rocks, Serian volcanic rocks, and related sediments along the Kuching zone of Sarawak were formed by the Paleo-Pacific subduction-related accretionary orogenesis, which likely initiated at or before the Early Cretaceous and ended no earlier than the latest Cretaceous. Besides, Gan et al. [5] studied the Late Cretaceous (~84–78) I-type granites along the Kuching zone of West Sarawak and suggested that these granites were likely formed in a continental arc setting rather than postcollisional magmatism. Our Group 1 samples with ages of 92–85 Ma are consistent with the time of arc magmatism proposed by Henning et al. [7], Breitfeld et al. [26], and Wang et al. [9]. Moreover, Group 1 samples mainly plot in the field of volcanic-arc granite (Figure 12), suggesting a typical arc setting in response to the Paleo-Pacific subduction. Group 2 samples with ages of ~81–80 Ma are geochemically similar to those Late Cretaceous I-type granites in the Kuching zone (Figures 6, 9, and 10(a)), indicating that both of them might share a similar source region under a same tectonic setting. In Figure 12, Group 2 samples also plot in the field of volcanic-arc granite. In addition, the Mesozoic arc-related igneous rocks in western Borneo define two prominent Late Cretaceous age peaks of ~85 Ma and~80 Ma (Figure 13 and Table 2), which are consistent with the formation ages of our Group 1 and 2 samples. It is notable that the geochemical data and petrogenesis indicate that the Group 2 monzogranites contain more crustal materials than the Group 1 granites and granodiorites. This signature might indicate that the crustal thickening occurred at ~80 Ma, and the recycled sediments and sediments/fluids-modified magmas have been involved into the crust as proposed by Wang et al. [9]. Therefore, we propose that our Group 1 and 2 samples might be formed in the subduction stage of the Paleo-Pacific Ocean and subsequent tectonic stage from subduction to continental collision, respectively.

Available data show that the abundant Cretaceous igneous rocks are distributed from the South China Coastal Provinces through eastern Indochina of southern Vietnam into western Borneo (e.g., [1, 2, 4, 6–8, 13, 17, 18, 26, 76]). However, the Cretaceous igneous rocks from the South China Coastal Provinces and eastern Indochina are dominantly characterized by >100 Ma mafic igneous rocks and I-type granitoids and <100 Ma A- and I-type granitoids, whereas most Late Cretaceous granitoids with less radiogenic Nd–Hf isotopic compositions were formed in response to the slab roll-back of Paleo-Pacific subduction beneath the South China Block (e.g., [8, 17, 76]). Our zircon in situ Hf–O isotopic data show that the Late Cretaceous granitoid samples from the Schwaner Mountains mainly contain positive $εHft$ and mantle-like $δ$¹⁸O values (Figure 5), indicating their source region contains more juvenile or newly underplated mafic components than those of Late Cretaceous granitoids with $εHft$ values of −9.9–+4.5 and $εNdt$ values of −5.5–+0.6 from the South China Coastal Provinces and eastern Indochina Block (e.g., [8, 17, 76]). Besides, our samples have geochemical affinities to the contemporaneous I-type granites in the Kuching zone, indicating these Late Cretaceous Schwaner and Kuching plutons were formed in an Andean-type continental margin, where the mafic igneous rocks and sediments along the Kuching zone can represent an accretionary wedge of Paleo-Pacific Ocean during the Early to Late Cretaceous (Figure 14(a); e.g., [5, 9]). During the Late Cretaceous, the Paleo-Pacific westward subduction formed the Lupar-Serabang and Serian accretionary igneous rocks, and the mafic magmas derived from the mantle wedge newly underplated under the low crust and mixed with recycled crustal rocks to form the source of our granitoids (Figure 14(b)).

The Late Cretaceous granitoids have also been identified from the Stong-Noring area and Tioman Island of Eastern Peninsular Malaysia and Singapore to the east of Sundaland, with formation ages of ~95–76 Ma (e.g., [21–23]). However, the origin and tectonic setting of these Late Cretaceous granitoids are still unclear. Searle et al. [23] proposed that the Tioman Late Cretaceous granites might represent the western extension of the arc-related granites in southern Borneo, which could be formed by the west-dipping subduction of Palaeo-Pacific Ocean according to their similar formation ages. In contrast, Hutchison [77] suggested that these Late Cretaceous granitoids from Peninsular Malaysia are rift-related, and their geochemical characteristics have some A-type affinities (e.g., [21, 22]). Therefore, we propose that the Late Cretaceous granitoids in the Schwaner Mountains can extend to the Kuching zone, and those Late Cretaceous granitoids with A-type affinities from Eastern Peninsular Malaysia and Singapore to the east of Sundaland might be formed in a back-arc extensional setting in response to the Paleo-Pacific subduction (Figure 14(a)).

All whole-rock geochemistry data and zircon U-Pb dating and Lu-Hf isotope that data support the findings of this study are included within the supplementary information files.

The authors declare that they have no conflicts of interest.

We would like to thank Dr. Limin Zhang and Mr. Joni from Indonesia for their help during the fieldwork, and Drs. Xue Yang, Qiyu Gou, and Yukun Wang for their help during the zircon and geochemical analyses. This work was jointly supported by the National Natural Science Foundation of China (41830211 and 42072256), the Guangdong Basic and Applied Basic Research Foundation (2018B030312007 and 2019B1515120019), and the Fundamental Research Funds for the Central Universities, Sun Yat-sen University (22lgqb14).