The EW-trending Kuching zone in Borneo is a target region for exploring the southern continuation of Paleo-Pacific subduction from South China, Vietnam to SE Asia. Previous studies mainly focused on mafic igneous rocks, and poor attention has been paid to the contemporaneous granitoids in this zone. This study presented detailed zircon U-Pb geochronology and Lu-Hf isotope and whole-rock geochemistry analyses for late Cretaceous granitoids (granodiorites and granites) in the northern Kuching zone. These granitoids are dated at ~77.5-83.6 Ma with younger ages than the igneous rocks in the southern Kuching zone (~130-144 Ma). The granitoids have variable SiO2 (64.86-77.37 wt.%) and A/CNK (0.7-1.5) and are strongly enriched in LILE and depleted in HFSE with significant Ba, Nb, Sr, and Ti negative anomalies. They have variable (87Sr/86Sr)i (from 0.70656 to 0.71208), εNd(t) (from -4.4 to +0.9), and zircon εHf(t) (from -1.2 to +12.4) with high (206Pb/204Pb)i ratio of 18.78-19.74, suggesting derivation from a hybrid source involving meta-sedimentary and meta-igneous rocks. Combined with previously-published data, two episodes of Cretaceous (~77-98 Ma and ~130-144 Ma) magmatic activities are identified in the Kuching zone, showing a younging age trend from south to north. These episodes of Cretaceous igneous rocks and their spatial distribution in the Kuching zone can be totally comparable to those in South China and Vietnam. Thus, the Kuching zone was likely a part of the Paleo-Pacific subduction system during the Cretaceous, northerly linking to Vietnam and South China.
The Asia continental margin was mainly influenced by the successive consumption of Paleo-Pacific subducted slab since the Late Mesozoic, preserving abundant geological records associated with the slab subduction [1–3]. The Eastern Asia, as an important segment of Asia continent, has been a part of Circum-Pacific subduction system since the Cretaceous, developing massive Cretaceous island arc igneous rocks along the eastern North China, Japan, and eastern South China ([1, 2, 4–7]). Toward to south, few Cretaceous igneous rocks that were considered to be associated with the Paleo-Pacific subduction have sporadically been identified in Vietnam and Philippines of Southeastern (SE) Asia [8–13]. These Cretaceous igneous rocks thus reveal a magmatic belt that was related to the Paleo-Pacific subduction along the North China, South China, and SE Asia from north to south. The southern continuation of this magmatic belt and the potential preservation of geological records have been attractive and significant hotspots for comprehensively uncovering the evolution of the Paleo-Pacific subduction system in SE Asia. The Borneo is located at the south of Vietnam and Philippines (Figure 1(a)) and becomes a target region to trace the southern continuation of the magmatic belt.
The Borneo is the largest island in SE Asia and being squeezed by three plates (Figure 1(a); Eurasian, Philippines, and India-Australia plates). This island has undergone complicated tectonic histories and been welded together by several fragments with Australian/Indochina/Cathaysia affinities ([12, 14–21]). The Kuching zone, as one of the five major segments (SW Borneo, Kuching zone, Sibu zone, Miri zone, and East Borneo) in Borneo (Figure 1(b)), extends from West Sarawak and NW Kalimantan into Central Kalimantan and is a significant EW-trending tectonic-magmatic belt (Figure 1(b); [12, 16, 18, 20, 22, 23]). The general geological investigation and tectonic nature of this zone remain unclear due to the tectonic overprinting and rainforest cover [3, 16, 20, 23–28]. One group of scholars regarded the Kuching zone as the eastern continuation of the Paleo-Tetheys subduction system, connecting Sibumasu Block to the west [25, 27]. The others considered that the Kuching zone was a part of the Paleo-Pacific subduction system [14, 16, 19, 23, 24, 29]. As excellent indicators for exploring the tectonic nature of the Kuching zone, Cretaceous mafic igneous rocks and granitoids are sporadically exposed along the zone [16, 20, 22–24, 29, 30]. Recently, Fang et al.  and Wang et al.  provided detailed zircon U-Pb and whole-rock Ar-Ar geochronology constraints on late Cretaceous (~77-98 Ma) mafic igneous rocks in the Kuching zone (Figure 1(c) and Table 1), offering significant clues for tracing the trails of the Paleo-Pacific subduction system in Borneo. However, little attention has been paid to the contemporaneous granitoids, impeding our better understanding for the evolution of the Paleo-Pacific subduction in Borneo. The geochronological and geochemical data of the granitoids were poorly constrained and reported by previous studies [18, 31–33]. Thus, we presented detailed petrological, zircon U-Pb geochronological and Hf isotopic, and whole-rock geochemical analyses for late Cretaceous granitoids in the Kuching zone. Combined with recently-published data of mafic igneous rocks, our studies revealed an important episode of late Cretaceous magmatic activity in the Kuching zone which can be compared to that in South China and Vietnam and provide insights into the nature of the Kuching zone in Borneo which was a part of the Circum-Pacific subduction system during the late Cretaceous.
2. Geological Background
The Borneo is bounded to the north by South China Sea and Philippines, to the east by Sulu Sea, Celebes Sea and Sulawesi, to the south by Java Sea, and to the west by Malaysia and Sumatra (Figure 1(a)). This island is usually divided into five segments, including SW Borneo, Kuching zone, Sibu zone, Miri zone, and East Borneo (Figure 1(b); [15, 16, 18, 22, 31, 34–36]). The SW Borneo is commonly considered as a microcontinent that was rifted from Gondwana and accreted to Eurasia continent [17, 31, 37–39]. Recent studies on Permo-Triassic granitoids from the SW Borneo suggested that the SW Borneo was in an active continental margin due to the eastward subduction of the Paleo-Tethyan Ocean . The Sibu zone consists of a sequence of deep marine sediments (represented by the Rajang Group), and the Miri zone was dominated by continental clastics, carbornate, and molasses which were underlain by the Rajang Group [15, 16, 18, 24, 31, 35, 40–44]. The East Borneo was mostly covered by Cenozoic sedimentary basins with only late Cretaceous Meratus Mélange occurring in its southern part (Figure 1(b)). The tectonic nature of the Meratus Mélange remains debated [23, 45, 46]. The Kuching zone is separated from the Sibu zone by the Lupar Line to the north and from the SW Borneo by the Pinoh Metamorphics and Schwaner Mountain to the south (Figure 1(b); [18, 22]). This zone is geographically subdivided into two units in Sarawak (Malaysia) and Kalimantan (Indonesia) , henceforth named as the northern and southern Kuching zone, respectively. This zone contains many metamorphic units, ophiolite mélanges/complexes, and Mesozoic-Cenozoic volcano-sedimentary rocks and related intrusions [14, 16, 18–20, 23, 31–33].
The metamorphic rocks in the Kuching zone are represented by Kerait Schist, and outcrop as tectonic windows within the Mesozoic sedimentary strata [28, 44]. They consist of phyllite, slate, schist, hornfel, meta-sandstone, meta-graywacke, chert, and meta-volcanic rocks and show an unconformable contact with the overlying Sadong Formation. The Kerait Schist and Tuang Formation were previously considered to be the oldest rocks in the Kuching zone, equivalent to Pinoh Metamorphics in SW Borneo [14, 16, 18, 28, 32, 44]. They were usually assumed to be Paleozoic to Triassic and regarded as the basement rocks in the region [31, 44]. However, recent new geochronological results revealed that the basement rocks of Pinoh Metamorphics in SW Borneo were formed at ~130 Ma and metamorphosed at ~120-80 Ma [14, 37], arguing against a part of an ancient core in SW Borneo.
The mélanges/complexes in the Kuching zone are represented by Lubok Antu Mélange and Serabang Complex in the north and Boyan Mélange in the south (Figure 1(b); [18, 20, 47–49]). The Boyan Mélange shows an EW-trending distribution extending over 200 km and consists of multiply deformed tectonic breccia (fragments and blocks of sedimentary and igneous rocks) and pervasively sheared pelitic matrix . Based on the presence of the Orbitolina (Cenomanian) in limestone blocks, the Boyan Mélange was previously considered to be formed at the late Cretaceous . The Serabang Complex is composed of mylonitized and brecciated slate and greywacke, lentoid chert, cleaved mudstone and pelitic hornfels, meta-basalt, meta-gabbro, and minor conglomerate, and the foraminifera from this complex shows a Cretaceous age [18, 32, 47, 48, 50]. The meta-basalt and meta-gabbro from the Serabang Complex yield the zircon U-Pb ages of ~93 Ma and ~90 Ma, respectively (Figure 1(c) and Table 1), and the meta-sandstones have four detrital zircon U-Pb age-peaks of ~124 Ma, ~156 Ma, ~247 Ma, and ~1820 Ma . The Lubok Antu Mélange consists of a disrupted succession of conglomerate, mudstone, sandstone, shale, chert, limestone, hornfels, basalt, gabbro, and minor serpentinite [18, 33, 50]. The radiolarian and foraminifera from the Lubok Antu Mélange show a broad variation of age ranging from the late Jurassic to late Cretaceous [32, 47, 48, 50]. Wang et al.  recently reported the zircon U-Pb ages of ~96-97 Ma for meta-gabbro, ~98 Ma for meta-basalt, and an Ar-Ar plateau age of ~85 Ma for basalt from the Lubok Antu Mélange (Figure 1(c) and Table 1). The meta-sandstones from this mélange have three detrital zircon U-Pb age-peaks of ~122 Ma, ~260 Ma, and ~1866 Ma .
The Triassic volcano-sedimentary units in the Kuching zone are represented by Sadong Formation in the south and Kuching Formation in the north . The Sadong Formation is composed of shale, mudstone, siltstone, sandstone, conglomerate, limestone, marl, and coal with volcanic rocks, representing an estuarine to neritic deposit setting influenced by periodically brackish water . The Kuching Formation consists of sandstone, siltstone, and mudstone, usually regarded as marine turbidite successions [16, 51]. The Jurassic-Cretaceous volcano-sedimentary units are represented by Bau Limestone, Kedadom, and Pedawan formations [18, 28]. The Bau Limestone and Kedadom formations were previously considered to be deposited in a continental shelf setting, evidenced by the presence of corals . They have an unconformable contact with the underlying Sadong Formation and a conformable contact with the overlying Pedawan Formation . The Pedawan Formation consists of sandstone, mudstone, and contemporaneous volcanic rocks, indicating a switch from a shallow marine reef to a clastic-dominated deep marine environment in a fore-arc setting [16, 18, 28, 52, 53]. The basalt and andesite from Serian Volcanics in Pedawan Formation show zircon U-Pb ages of ~80-89 Ma and an Ar-Ar plateau age of ~77 Ma (Figure 1(c); ). Many contemporaneous Mesozoic intrusions are also exposed in the Kuching zone (Figure 1(c)). Wang et al.  reported the zircon U-Pb ages of ~133-134 Ma for gabbros and ~130-144 Ma for granodiorites in the southern Kuching zone. The monzogranite, granodiorite, and granite from Jagoi Mountain of the northern Kuching zone were dated at ~207-214 Ma [16, 19]. However, the precise ages of Cretaceous granotoids in the northern Kuching zone are not well constrained.
3. Sample and Analytical Methods
The intermediate-felsic igneous rocks along the northern Kuching zone are sporadically exposed in Sematan, Lundu, Serian, Sri Aman, and Lubok Antu (Figure 1(c)). They mainly intruded in the Triassic and Jurassic-Cretaceous strata (Figure 1(c)) and are composed of diorites, granodiorites, and granites. Our studied samples were collected at Sematan, Lundu, and Sri Aman (Figure 1(c)) and consist of granodiorites and granites. The granodiorites are gray-white and medium-grained with massive structure (Figures 2(a) and 2(b)). The mineral assemblages of the granodiorites consist of amphibole (~11-18%), plagioclase (~36-45%), K-feldspar (~10-15%), and quartz (~15-19%) (Figure 2(d)). The biotite granites are gray-white and medium- to coarse-grained with massive structure (Figure 2(c)). The mineral assemblages of the biotite granites are mainly characterized by quartz (~24-33%), plagioclase (~34-42%), K-feldspar (~14-22%), and biotite (~6-10%) (Figures 2(e)–2(i)). Plagioclase grains from biotite granites are euhedral with well-developed polysynthetic twinning, and K-feldspar grains occur as anhedral (Figures 2(f), 2(h), and 2(i)).
Aiming to acquire precise geochronological constraints and explore the origin of the granodiorites and granites in the northern Kuching zone, this study presented detailed zircon U-Pb geochronological and Lu-Hf isotope analyses and whole-rock geochemical analyses. The detailed analytical methods for sample preparations and analyses are described in Appendix, and corresponding results are presented in Supplementary Table S1–S3.
4.1. Zircon U-Pb Geochronology and Lu-Hf Isotope
Six samples were collected for zircon U-Pb geochronological analyses, and five of them for Lu-Hf isotope analyses (Table S1-S2). The zircon grains from the samples display identical morphological and structural characteristics, and they are prismatic and 100-150 μm in length (Figure 3(a)). The zircons have high Th/U ratios ranging from 0.2 to 2.6 (Figure 3(b)) and concentric oscillatory zoning in cathodoluminescence (CL) images (Figure 3(a)).
Twenty zircon grains from sample 17MY-46A1 show 206Pb/238U apparent ages of 78.1-81.5 Ma with a weighted mean age of (Figure 4(a)). The corresponding εHf(t) and TDMC values range from -1.2 to +6.9 and from 704 Ma to 1225 Ma, respectively (Table S2). Sample 17MY-47B1 yields a weighted mean 206Pb/238U age of (Figure 4(b)) with εHf(t) values ranging from +6.7 to +12.4 (Table S2). Six zircon grains from sample 17MY-47C1 have high Th/U (0.2-1.2) and yield a weighted mean age of (Figure 4(c)). For sample 17MY-48A, ten out of twenty-one zircon grains have Th/U ratios of 0.3-0.8 and old apparent 206Pb/238U ages of 120-2391 Ma (Table S1). The remaining eleven zircon grains give a weighted mean age of (Figure 4(d)) with Th/U ratios of 0.4-0.7, interpreted as the crystallization age. The εHf(t) values of zircon with crystallization age range from -0.9 to +8.0 (Table S2). Fourteen and twenty-four analytical spots defined a weighted mean age of for sample 17MY-50A1 (Figure 4(e)) and for sample 17MY-67A1 (Figure 4(f)). The corresponding zircon εHf(t) values range -0.9 to +3.4 for sample 17MY-50A1 and from +1.9 to +9.5 for sample 17MY-67A1 (Table S2).
4.2. Geochemical Results
The whole-rock major oxide, trace element, and Sr-Nd-Pb isotope analytical results for twenty-five samples are presented in Supplementary Table S3. These samples have low loss on ignition (LOI) (0.61-1.64 wt.%) (Table S3) and show no remarkable correlations of LOI with major oxides and trace elements (not shown), suggesting the negligible effects of alteration on the samples. The studied samples are granodioritic and granitic in composition (Figure 5(a)), with variable SiO2 (64.86-77.37 wt.%), Al2O3 (11.99-15.36 wt.%), CaO (0.53-6.26 wt.%), CaO/Al2O3 (0.04-0.47), FeOt (1.16-9.25 wt.%), and TiO2 (0.14-1.59 wt.%). They are characterized by high contents (4.21-7.97 wt.%) and K2O/Na2O ratios (0.1-1.8) with variable A/CNK (0.7-1.5) and A/NK (1.2-2.2) values, belonging to metaluminous to peraluminous series (Figures 5(b) and 5(c)). Besides, the samples have significant negative correlations of SiO2 with CaO/Al2O3, CaO, MgO, FeOt, TiO2, and P2O5 (Figure 6).
The studied samples are strongly enriched in large-ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) with significant Ba, Nb, Sr, and Ti negative anomalies (Figure 7(a)). They show enriched light rare earth elements (LREE) and flat heavy REE (HREE) patterns with significant Eu negative anomalies (Figure 7(b)). Their (La/Yb)n (where n means chondrite normalized), (Dy/Yb)n, and Eu/Eu are in the range of 1.94-11.1, 0.80-2.14, and 0.17-0.78, respectively. The studied samples have low Sr (20.2-198 ppm) and Sr/Y (0.3-7.2) and high Y (25.7-73.8 ppm) and Yb (2.09-8.60 ppm), distinct from the adakites [54, 55].
The measured 87Sr/86Sr and 143Nd/144Nd ratios of the studied samples are in the range of 0.708799-0.749139 and 0.512372-0.512667, respectively. The samples have large variations of initial 87Sr/86Sr ratios (from 0.70656 to 0.71208) and εNd(t) values (from -4.4 to +0.9). The samples have much lower εNd(t) values than late Cretaceous MORB-like mafic igneous rocks, early Cretaceous mafic-intermediate-felsic igneous rocks, and higher εNd(t) values than arc-like mafic igneous rocks in the Kuching zone (Figure 8(a)). They have similar εNd(t) values to Triassic felsic igneous rocks in the Kuching zone and Cretaceous arc mafic igneous rocks in South China (Figure 8(a)). The samples have high initial 206Pb/204Pb ratios of 18.78-19.74, 207Pb/204Pb ratios of 15.66-15.71, and 208Pb/204Pb ratios of 38.93-39.62, similar to those of Indian oceanic turbidites (Figure 8(b)).
5.1. An Episode of Late Cretaceous Magmatic Event in the Kuching Zone
The late Cretaceous igneous rocks are sporadically exposed along the Kuching zone (Figure 1(c)). Recently, increasing precise zircon U-Pb and Ar-Ar geochronology analyses have been conducted on the mafic igneous rocks in the Kuching zone (Figure 1(c) and Table 1; [20, 23, 29]). The meta-basalt in Lubok Antu and basalt-andesite in Serian show whole-rock Ar-Ar ages of ~77-98 Ma and zircon U-Pb ages of ~80-89 Ma (Figure 1(c) and Table 1; ). The meta-gabbro in Lubok Antu shows zircon U-Pb ages of ~96-97 Ma (Figure 1(c) and Table 1; ). The meta-basalt and gabbro in Serabang Complex and mebasite in Lundu have crystallization ages of , , and , respectively (Figure 1(c) and Table 1; [20, 29]). Apart from the mafic igneous rocks, the corresponding felsic igneous rocks were not well constrained previously. This study firstly offers precise zircon U-Pb geochronological analyses for the felsic igneous rocks in the Kuching zone (Figure 4). Our results suggest that the granodiorites in Sematan (17MY-50A1) and Lundu (17MY-46A1, -47B1 and -48A1) were dated at , , , and (Table 1). The granites in Lundu (17MY-47C1) and western Sri Aman (17MY-67A1) have the crystallization ages of and (Table 1). These precise geochronological analyses suggest an episode of late Cretaceous magmatic event (~77-98 Ma) along the Kuching zone. The igneous rocks in the northern Kuching zone have remarkable younger ages than those in the southern Kuching zone (~130-144 Ma; ), showing a younging age trend from south to north in the Kuching zone.
5.2. Petrogenesis of Late Cretaceous Granodiorites and Granites
The late Cretaceous granodiorites and granites have low Nb/La, Nb/U, and Ce/Pb ratios (Figure 9(a)). They show distinct Sr-Nd isotopes from the contemporaneous mafic igneous rocks in the Kuching zone (Figure 8(a)). They have low Sr and Sr/Y and high Y and Yb, different from adakites [54, 55]. Such signatures argue against the direct derivation from partial melting of mantle or subducted slab, or simple fractionation from the parent magmas of contemporaneous mafic igneous rocks for the origin of the granodiorites and granites.
The granodiorites and granites have a large variation of zircon εHf(t) values (>6 units; Figure 8(c)), leading to the speculation that they were probably the products of mixing of mantle- and crust-derived magmas. However, such scenario is not consistent with the following observations. First, there is currently no remarkable evidence (e.g., mafic microgranular enclaves, disequilibrium mineral pairs, acicular apatite, and accompanying mafic intrusions) for magma mixing (Figure 2). Second, the zircon εHf(t) variation, which is independent from the zircon Th/U ratios (Figure 8(d)), fails to resolve the magma mixing of mantle-derived components . Third, the samples show a nonnegative correlation between SiO2 and εNd(t) (Figure 9(b)) and a linear correlation between Th and Th/Nd (Figure 9(b)), arguing against a magma mixing trend. More importantly, the two end-members modeling results suggest a large amount (up to ~70%) involvement of mantle-derived components (Figures 9(a) and 9(d)), but such high proportional involvement cannot be in accord with other major oxide and trace element compositions (e.g., low MgO, Cr, and Ni). Thus, the heterogeneous zircon Hf isotopes were likely inherited from the crustal source of the studied samples.
The late Cretaceous granodiorites and granites have variable SiO2 (64.86-77.37 wt.%), low MgO (0.21-1.96 wt.%), and high (4.21-7.97 wt.%), suggesting that they likely originated from partial melting of continental crustal rocks (e.g., metapelites, greywackes, and metaigneous rocks; [57–63]). Previous experimental studies have demonstrated that partial melting of metasedimentary rocks can induce potassium-rich and strongly peraluminous granitic magmas, whereas partial melting of mafic igneous rocks will produce low-K to medium-K metaluminous felsic melts (e.g., [64–66]). Most of the studied samples have major oxide compositions similar to the partial melt of amphibolite (Figures 10(a) and 10(b)), suggesting a predominant derivation from the mafic crustal rocks. However, some samples have high A/CNK (>1.1) and plot in the field of partial melt of greywacke (Figures 10(a) and 10(b)). The studied samples have large variations of CaO/Na2O (0.17-1.91) and Al2O3/TiO2 (7.66-87.6) ratios, bridging between the pelite- and basalt-derived melts in Figure 10(a) [59, 60, 67, 68]. The samples have high initial 206Pb/204Pb and variable εNd(t) and zircon εHf(t) values (Figures 8(a)–8(c)) with Nd-Hf isotopic decoupling. A few zircon grains in the samples have a zircon core (Figure 3(a)). Such signatures suggest the involvement of another sedimentary component in the genesis of the studied samples [69, 70]. Thus, the late Cretaceous granodiorites and granites in the Kuching zone were likely derived from a hybrid magma source of meta-sedimentary and meta-igneous rocks.
The studied samples have large variations of major oxide (Figure 6), suggesting the fractional crystallization of some minerals during the magma evolution. The negative correlations of SiO2 with CaO/Al2O3, CaO, and MgO are consistent with the feldspar fractionation (Figure 6). This interpretation is further documented by the significant Sr, Ba, and Eu negative anomalies of the samples (Figure 7). The Fe-Ti oxide fractionation (amphibole and biotite) was evidenced by the negative correlations of SiO2 with FeOt and Ti2O (Figure 6). The removal of feldspar and Fe-Ti oxides is further supported by the relationship of Sr with Rb, Ba, and Eu/Eu (Figure 11). Thus, the studied samples mainly underwent the fractional crystallization of feldspar, amphibole, and biotite.
5.3. Implications for the Paleo-Pacific Subduction along the Kuching Zone
The paleogeographic position of the Kuching zone during the Mesozoic is the key to unravel the tectonic nature of the Kuching zone [12, 16, 20, 23, 24, 26, 28]. Paleontological studies have suggested that Carboniferous-Permian and late Triassic faunas in the Kuching zone are identical to those in East Peninsular Malaysia, East Peninsular Thailand, and SE Vietnam of Indochina Block, but different from those in West Peninsular Thailand of Sibumasu Block [16, 18, 28, 31, 44]. The Triassic Sadong Formation in the Kuching zone can be comparable to the Nongson Formation (containing Norian-Rhaetian fossils) in SE Vietnam [13, 20]. Furthermore, the Permo-Triassic granitoids from the SW Borneo and Kuching zone show geochemical signatures identical to those in East Peninsular Malaysia, suggesting their affinities with the Paleo-Tethyan Ocean subduction . Such signatures suggest that the Kuching zone has been connected or adjacent to East Peninsular Malaysia-SE Vietnam during the Triassic, influenced by the Paleo-Tethyan Ocean in SE Asia [12, 18–20, 28, 71, 72]. The Cretaceous greywacke from the Lubok Antu Mélange and Serabang Complex in the Kuching zone contains many chert, volcanic, siltstone, mudstone, and sandstone fragments with poor sorting and angular-subangular features . The detrital zircons in the greywacke and captured zircon grains in the mafic rocks have age-spectra similar to those in East Peninsular Malaysia and SE Vietnam [16, 20, 73–75]. The paleocurrent data collected from the Pedawan Formation also reveal a material provenance transport from southwest to northeast [15, 18, 32, 33, 35]. Thus, the Kuching zone was likely connected or adjacent to East Peninsular Malaysia and SE Vietnam during the Mesozoic [14–16, 20, 24].
The late Cretaceous Serian Volcanics in the northern Kuching zone display similar geochemical signatures to island arc basalts and andesites, and they were derived from a mantle wedge which was modified by recycled sediment-derived components . The late Cretaceous Pakong and Serabang mafic igneous rocks in the northern Kuching zone are coeval with deep-marine turbidites and shallow marine sedimentary rocks and considered as derivation from a MORB source with the input of slab-derived fluids, indicative of a fore-arc setting [18, 20]. The Cretaceous Pedawan Formation contains sandstone, mudstone, and contemporaneous volcanic rocks, deposited in a fore-arc sedimentary setting [14, 16, 26]. Thus, these geochemical and sedimentary signatures suggest that the late Cretaceous igneous rocks in the Kuching zone were formed in an active subduction setting.
Available data has demonstrated that the closure of East Paleo-Tethyan Ocean in SE Asia occurred at ~237 Ma . The Cretaceous igneous rocks in the Kuching zone cannot be the products of Paleo-Tethyan Ocean subdcution. Instead, the Cretaceous igneous rocks in the southern Kuching zone were dated at ~130-144 Ma, whereas those in the northern Kuching zone have crystallization ages of ~77-98 Ma (Table 1), suggesting a younging age trend from south to north. Such trend should actually be described as a trend from west to east during the Cretaceous, when considering an anticlockwise rotation of ~90° for Borneo since the Cretaceous, compared to its present position [14, 16–18, 20, 24, 41]. Such magmatic trend can be comparable to that in South China where the Cretaceous igneous rocks fall into two age-peaks of ~90 Ma and ~130 Ma and show a younging age trend from interior (west) to coastal province (east) [1, 6, 76]. The Cretaceous high-K calc-alkaline granitoids (~112-88 Ma) have also been identified in Vietnam . These features suggest that the Kuching zone in Borneo has Cretaceous magmatic activities identical to those in Vietnam and South China. Thus, the Kuching zone in Borneo was likely a part of Circum-Pacific subduction system during the Cretaceous (Figure 12), northerly linking to Vietnam and South China.
We presented detailed petrological, zircon U-Pb geochronological and Lu-Hf isotopic, and whole-rock geochemical analyses for late Cretaceous granodiorites and granites in the northern Kuching zone. Our new results, along with previous studies, allow us to draw the following conclusions:
The late Cretaceous granodiorites and granites in the northern Kuching zone were dated at ~77.5-83.6 Ma. Combined with previously-published data, two episodes of Cretaceous magmatic activities (~77-97 Ma and~130-144 Ma) are identified along the Kuching zone with a younging age trend from south to north
The late Cretaceous granodiorites and granites were derived from a hybrid magma source involving meta-sedimentary and meta-igneous rocks, forming in an active continental margin
The Cretaceous magmatism in the Kuching zone can be totally comparable to that in Vietnam and South China, suggesting that the Kuching zone was a part of Circum-Pacific subduction system during the Cretaceous
A. Analytical Techniques
A.1. Zircon U-Pb Geochronology
Zircon U-Pb geochronology analyses were conducted on six samples to constrain the crystallization ages of the granodiorites and granites in the northern Kuching zone of Borneo. Zircon grains were sorted from fresh rock samples by using standard density and magnetic separation techniques. Theses grains were fixed in epoxy resin, polished to about half thickness and vacuum-coated with high purity gold, and were checked by cathodeluminescence (CL) images to obtain their internal textures. The CL images were obtained using a Carl ZEISS ΣIGMA scanning electron microscope at the Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University (SYSU).
The zircon U-Pb geochronology analyses for all samples were performed at the SYSU. The instruments are composed of an iCAP-RQ inductively-coupled-plasma mass-spectrometer (ICP-MS) and an ArF 193 nm laser-ablation system. Zircon 91500 and Plešovice are used as standards for calibrating the U-Th-Pb absolute abundances and ratios and monitoring instrument conditions. Repeated measurements of 91500 and Plešovice yield weighted mean 206Pb/238U ages of (2 s; ) and (2 s; ), consistent with previous published results [78, 79]. The detailed sample preparations and analytical procedures refer to descriptions by Wang et al. . Off-line raw data and concordia diagrams were processed by GLITTER software and Isoplot program, respectively [81, 82].
B. In Situ Zircon Hf Isotope Analyses
Zircon Hf isotope analyses were carried out by a Neptune Plus multicollector ICP-MS (MC-ICP-MS) coupled with an ArF 193 nm laser-ablation system at the SYSU. The zircon grains were ablated using a spot size of 44 μm and laser repetition rate of 6 Hz during the analyses. 91500 and Plešovice were used as standard zircons to monitor instrument conditions, following identical analytical procedures in Hu et al. . The εHf(t) values were calculated using 176Hf/177Hf (0.282772) and 176Lu/177Hf (0.0332) ratios of the chondrule . Single-stage Hf model ages (TDM) were calculated by 176Hf/177Hf (0.283250) and 176Lu/177Hf (0.0384) ratios of depleted mantle , and two-stage Hf model ages (TDMC) were calculated by 176Lu/177Hf ratio (0.015) of the average continental crust .
C. Whole-Rock Major Oxide, Trace Element, and Sr-Nd Isotope Analyses
Fresh rock samples were crushed, washed, and powdered into 200-mesh size for whole-rock geochemical analyses. All whole-rock geochemical analyses were carried out at the SYSU, following identical sample preparations and analytical procedures in Wang et al. . Major oxide contents and trace element concentrations were measured at an X-ray fluorescence spectrometer and ICP-MS, respectively. Strontium (Sr), neodymium (Nd), and lead (Pb) isotopes were analyzed using a Neptune Plus MC-ICP-MS.
All whole-rock geochemistry data and zircon U-Pb dating and Lu-Hf isotope data support the findings of this study are included within the supplementary information files.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Drs. A-M Zhang, H-Y He, X Yang, and Y-K Wang are acknowledged for field investigations and formal analyses. This study was jointly supported by National Natural Science Foundation of China (41830211, 42072256, and 42002236) and Guangdong Basic and Applied Basic Research Foundation (2018B030312007, 2019B1515120019, and 2021A151501189).