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Late Ordovician High Ba-Sr intrusion in the Eastern North Qilian Orogen: Implications for Crust-Mantle Interaction and Proto-Tethys Ocean Evolution

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10 May 2023

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Abstract
The petrogenesis of high Ba-Sr granitoids provide a great significance to penetrate the Proto-Tethys evolution in the North Qilian orogenic belt. This paper presents a combination of zircon U-Pb age, whole-rock major and trace element concentrations, and Sr-Nd-Hf isotopic data for Caowa high Ba-Sr dioritic intrusion from the eastern part of the North Qilian orogenic belt, aiming to decipher its petrogenesis and tectonic setting. LA-ICP-MS zircon U-Pb dating yields an emplacement age of 447±3 Ma for the Caowa intrusion, indicating a magmatic activity of the late Ordovician. The Caowa quartz diorites contain moderate contents of SiO2, MgO, Mg# and resultant high concentrations of Na2O+K2O, Fe2O3T and Al2O3, displaying calc-alkaline and metaluminous characteristics. Their relatively elevated Ba (up to 1165 ppm) and Sr (561 to 646 ppm) contents, with obvious enrichment in LILEs (e.g. Ba、Th、U) and depletion in HFSEs (e.g. Nb、Ta、Ti) resemble those of typical high Ba-Sr granitoids in subduction zone. Together with enriched Sr-Nd isotopic compositons[(87Sr/86Sr)i=0.7082−0.7086, εNd(t)= -5.1 to -4.9], and relatively extensive εHf(t) values (-13.2 to +8.5) of zircons, it suggests that these high Ba-Sr quartz diorites were derived from a mixture magma source between the ancient crust materials and the enriched lithospheric mantle metasomatised by fluid was released from subducted oceanic crust or sediment. Taking into account the ophiolites, high pressure metamorphic rocks and arc magmatic rocks in the region, we infer that affected by the northward subduction of the Qilian Proto-Tethys ocean, the Laohushan oceanic crust of the North Qilian back-arc basin was subducted during the Late Ordovician and resulted in extensive metasomatism of lithospheric mantle by fluids derived from oceanic crust or sediments, and the Caowa high Ba-Sr quartz diorites generated in the process of crust-mantle interaction during the Late Ordovician.
Keywords: 
Subject: Environmental and Earth Sciences  -   Geochemistry and Petrology

1. Introduction

The Qilian orogenic system, located in the northeastern Tibetan Plateau, has been considered as the northernmost orogenic collage of the Proto-Tethys Realm. It records a multistage tectonic processes from the Neo-proterozoic continental breakup, the Early Paleozoic oceanic subduction and accretion, finally resulting in arc-continent and continent collisions [1,2,3]. As a significant unit of the Qilian orogenic system, the North Qilian orogenic belt has been confirmed by a typical subduction-accretion suture with a complex trough-arc-basin system, marking the tectonic evolution of Proto-Tethys ocean [4,5,6]. Previous studies have been carried out on the distribution of Early Paleozoic mid-ocean-ridge type ophiolites, high-pressure metamorphic rocks (e.g., eclogites and blueschists), subduction/collision-related arc magmatic rocks and back-arc basin ophiolites in the North Qilian from south to north, and northward subduction of the Qilian Proto-Tethys ocean during the Ordovician was basically authenticated [5,7,8,9]. Nevertheless, the evolution process of Proto-Tethys ocean in the eastern part of North Qilian belt remains unclear. There are many Early Paleozoic adakitic granitoids have been recognized in the Eastern North Qilian orogenic belt, and some recent studies suggested that they generated during continent-continent collision or post-collision collapse [10,11,12,13,14]. However, their petrogenesis and tectonic settings are still debated and little research has focused on the oceanic subduction-related granitoids (e.g., I-type granite, high Ba-Sr granitic rocks) in the region, especially in the Nanhuashan area of Ningxia province, easternmost of the North Qilian belt.
High Ba-Sr granitoids, as a distinct group of magmatic rocks, are widespread in Phanerozoic orogenic systems and provide important insights into the deep geodynamic process of orogenic belts [15,16,17,18]. Compared to traditional I-, S- and A-type granitoids, the high Ba-Sr granitoids are characterized by alkali-rich, high Ba (> 500 ppm) and Sr (> 300 ppm) contents, and low Rb (< 200 ppm) and Y (< 30 ppm) contents with Rb/Ba ratios < 0.2. They also display high Sr/Y ratios, enrichment of light rare earth (LREE) and large-ion lithophile elements (LILE), depletion in heavy rare earth (HREE) and high field-strength elements (HFSE), with no significant negative Eu anomalies [15,19]. High Ba-Sr granitoids always carry geochemical and isotopic signatures of enriched mantle sources, which was linked to the oceanic subduction-related metasomatism [16,20,21,22,23,24]. However, the possible mechanisms for the generation of high Ba-Sr granitoids, such as partial melting of subducted ocean islands/ocean plateaus [15], melting of mafic lower crust [19,25], or magma mixing [17,26], have also been proposed. Therefore, the recognition of high Ba-Sr granitoids may provides particular information on the crust-mantle interactions and the growth of the continental crust in subduction zones.
In this contribution, we present zircon U-Pb geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic data for the Late Ordovician dioritic intrusion (mainly composed of quartz diorites) with high Ba-Sr signatures in the Nanhuashan area of Ningxia, Eastern North Qilian orogenic belt (Figure 1). These results, together with previously published data, are used to elucidate the petrogenesis of the high Ba-Sr quartz diorites, and further evaluate their geodynamic implications for the Proto-Tethys Ocean evolution.

2. Geological Background and Petrography

The Qilian orogenic belt, located in the northeastern margin of the Tibetan Plateau, has traditionally been regarded as an important part of the Central China orogenic belt (Figure 1a). It is sequentially divided into three units from north to south: the North Qilian orogenic belt (NQOB), the Central Qilian block, and the South Qilian accretionary complex belt [1,27]. The NQOB, extending NW-SE more than 1000 km, lies between the Alxa Block to the north and the Qilian Block to the south, and is separated from the Altyn-Tagh Fault to the west (Figure 1b). It is dominated by an outcrop of Early Paleozoic ophiolites, high-pressure metamorphic rocks and a series of subduction/collision-related magmatic rocks, and is considered to be a typical subduction-accretion orogenic belt with a complex trough-arc-basin system [5,6]. Two ophiolite sequences are distributed in the NQOB (Figure 1b). The southern ophiolite belt (550−496 Ma) is connected with Aoyougou, Yushigou and Dongcaohe ophiolites, mainly consists of mantle peridotite, cumulate gabbro and mid-ocean ridge basalt (MORB), which documented the oceanic crust fragments of the Qilian Proto-Tethys ocean [5]. The northern ophiolite belt (490−448 Ma) is a typical back-arc basin ophiolite (e.g., Jiugequan, Biandukou and Laohushan ophiolites), representing the extension of the North Qilian back-arc oceanic basin [5,28,29]. There is a volcanic-magma arc belt (520−440 Ma) outcrops between the two ophiolite belts, mainly including mafic and felsic volcanic rocks [8]. The high pressure-low temperature (HP-LT) metamorphic rocks in the NQOB are predominantly composed of blueschists and low-temperature eclogites, with the metamorphic ages ranging fom 490 to 440 Ma [2,30,31]. In addition, a large number of Early Paleozoic granitoids (520−420 Ma), including adakitic, I- and A-type granitoids, have been recognized in the NQOB from Changma-Dachadaban-Corridor Nanshan in the west to Leigongshan - Laohushan - Quwushan Mountain in the east, and their formation was linked to oceanic subduction, closure and post collision processes of the Qilian Proto-Tethys Ocean [11,13,14,32,33].
The Nanhuashan Mountain, connecting with Quwushan - Baojishan - Laohushan - Leigongshan Mountain to the west in the eastern part of NQOB, is developed many Early Paleozoic granitic intrusions. These intrusions, including Caiyuan, Shiwali, Youfangyuan, Luanduizi and Caowa intrusions, intrude into the Meso-proterozoic Haiyuan Group and they are closely related to regional Cu-Au mineralization. The high Ba-Sr signature rocks reported in this paper were collected from the Caowa intrusion (Figure 1c). The Caowa intrusion consists of quartz diorites, with an outcrop area of ~4 km2, which are medium- to coarse-grained rocks (Figure 2a). The quartz diorites are mainly composed of plagioclase (40–50 vol.%), hornblende (20–30 vol.%), quartz (5–10 vol.%), and K-feldspar (5–10 vol.%), with minor amounts of iron oxides, zircon, and apatite (Figure 2b–d). The plagioclase generally forms euhedral-subhedral laths with polysynthetic twinning. Some of these laths display concentric compositional zoning and have more sericitized (Figure 2b,d). Hornblende occurs as euhedral-subhedral grains, with length of 0.5–1 mm, and usually appears as a mafic polycrystalline agglomerate (Figure 2c,d). K-feldspar is subhedral-anhedral, mainly consists of orthoclase and occasional microcline. Quartz is anhedral and fills the interstices between amphibole and plagioclase crystals. In addition, acicular apatite commonly occurs nearby the hornblende polycrystalline agglomerate (Figure 2c,d).

3. Analytical Methods

3.1. Zircon U-Pb Dating and Hf Isotope Analyses

U-Pb dating and trace element analysis of zircon were simultaneously conducted by LA-ICP-MS at the Wuhan SampleSolution Analytical Technology (Wuhan, China). Laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7900 ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. A “wire” signal smoothing device is included in this laser ablation system [34]. The spot size and frequency of the laser were set to 32 µm and 8 Hz, respectively. Zircon 91,500 and glass NIST610 were used as external standards for U-Pb dating and trace element calibration, respectively. Each analysis incorporated a background acquisition of approximately 20-30 s followed by 50 s of data acquisition from the sample. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as description by Zong et al. (2017) [35]. An Excel-based software ICPMSDataCal was used to perform off-line selection and integration of background and analyzed signals, time-drift correction and quantitative calibration for trace element analysis and U-Pb dating [36]. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3 [37].
Zircon Hf isotope analyses were conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas HD excimer ArF laser ablation system (Coherent, Göttingen, Germany) that was hosted at the Wuhan SampleSolution Analytical Technology. The sampling spot size was 32 μm and the energy density was ~7.0 J cm−2 during the analyses. Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and analytical method are the same as description by Hu et al. (2012) [38]. In order to ensure the reliability of the analysis data, three international zircon standards of Plešovice, 91,500 and GJ-1 are analyzed simultaneously with the actual samples. The Hf isotope compositions of Plešovice, 91,500 and GJ-1 are 0.282478 ± 0.000008, 0.282300 ± 0.000011 and 0.282009 ± 0.000010, respectively [39].

3.2. Whole-Rock Major and Trace Elements Analyses

Major and trace elements analyses of the fresh whole-rock samples were carried out at Wuhan SampleSolution Analytical Technology. The major element analyses were conducted using Zsx Primus II wavelength dispersive X-ray fluorescence spectrometer (XRF) and the relative standard deviation is less than 2%. The trace elements were analysed on an Agilent 7700e ICP-MS. The sample powder was accurately weighed and placed in a Teflon bomb after drying for 12 h in an oven at 105 °C. The sample powders were then digested in an HF + HNO3 solution in Teflon bombs, which were then placed in a stainless-steel pressure jacket and heated to 190 °C in an oven for >24 h. The final solution was transferred to a polyethylene bottle and diluted to 100 g by the addition of 2% HNO3.

3.3. Whole-Rock Sr-Nd Isotope Analyses

Whole-rock Sr-Nd isotope analyses were measured on a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Dreieich, Germany) at Wuhan SampleSolution Analytical Technology. The Sr and Nd fractions were eluted using 2.5 and 0.3 M HCl, respectively, and gently evaporated to dryness prior to mass-spectrometric measurement. International standards of NBS987 and GSB were used as bracketing standards to monitor the instrument drift during the analysis of Sr and Nd isotopes, respectively. Repeated analysis for NBS987 gives an average 87Sr/86Sr = 0.710242 ± 14 (2σ). Repeated analysis for GSB gives an average 143Nd/144Nd = 0.512440 ± 1 (2σ).

4. Results

4.1. Zircon U–Pb Geochronology

LA-ICP-MS zircon U-Pb isotopic data and trace element results are listed in Table 1 and Table 2, respectively. Representative CL images and concordia diagrams are shown in Figure 3.
Zircon grains from the Caowa quartz diorite sample CW-6 are colorless to faint yellow, transparent and euhedral morphology. They have a size range of 50–150 μm, with a length/width ratio of 2:1 to 3:1, and show broad oscillatory growth zoning or are homogeneous in the CL images (Figure 3a). The analyzed zircons have U and Th contents of 1222–2859 and 342–1093 ppm, and show high Th/U ratios ranging from 0.25 to 0.40. They display enrichment in HREE and depletion in LREE, with obvious positive Ce anomalies, weak negative Eu anomalies and positive correlation between Th and U contents (Figure 3c), indicating their magmatic origin [40,41]. Nineteen spots were analyzed on 19 zircons and give 206Pb/238U ages ranging from 436 ± 5 to 459 ± 5 Ma. All of the analyses are nearly concordant, yielding a weighted age of 447 ± 3 Ma (MSWD = 1.5) (Figure 3b), which represents the emplacement age of the Caowa dioritic intrusion.

4.2. Whole-Rock Major and Trace Elements

Major and trace element data for the Caowa quartz diorites are listed in Table 3. Based on petrographic examination, all samples are not significantly affected by hydrothermally alternation, and their low loss on ignition (LOI) contents (1.36–2.45 wt.%, Table 3) further indicate that the samples are basically fresh.
The Caowa quartz diorites have a relatively variable SiO2 contents (57.53 to 62.87 wt.%) and are of intermediate diorite or monzonite compositions (Figure 4a). They have definite high-K calc-alkaline and metaluminous to weak peraluminous features, as indicated by their high total alkalis (Na2O + K2O = 6.16–6.89 wt.%), and relatively low A/CNK values (0.87–1.03) (Figure 4b-d). They have high Fe2O3T (5.22–7.88 wt.%), CaO (3.47–5.46 wt.%) contents, and moderate MgO (2.08–3.01 wt.%) contents and Mg# (42–44). The Caowa quartz diorites have higher contents of REE, ranging from 106.95 to 259.06 ppm (Table 3). They display slightly high La (18.27–60.86 ppm) and relatively low Yb (2.37–3.17 ppm), yielding the (La/Yb)N ratios of 4.56–17.47 (Table 3). All samples show similar chondrite-normalized REE patterns and have clearly fractionated LREE relative to HREE, with no significant negative Eu anomalies (Eu/Eu* = 0.77–0.93; Figure 5a). The Caowa quartz diorites display extremely high Ba (up to 1165 ppm) and Sr (561 to 646 ppm) contents, with obvious enrichment in LILEs (Ba, Th, U and K) and depletion in HFSEs (Nb, Ta, Ti and P; Figure 5b).

4.3. Whole Rock Sr-Nd Isotopes

Whole rock Sr-Nd isotopic data for selected samples are presented in Table 3 and shown in Figure 6. The initial Sr-Nd isotopic ratios were calculated at 447 Ma based on the zircon U-Pb age. The Caowa quartz diorites show heterogeneous Sr-Nd isotope compositions characterized by slight high (87Sr/86Sr)i ratios (0.70818–0.70860), and a little low εNd(t) values (-5.1 to -4.9), with two-stage Nd model ages (TDM2) of 1579–1607 Ma.

4.4. Zircon Hf Isotopes

In situ Hf isotope analyses were carried out on the same zircon domains where the U-Pb dating was done (Figure 3a), and the results of fifty isotopic analyses are listed in Table 4 and shown in Figure 7. The Caowa quartz diorite displays variable 176Hf/177Hf ratios (0.282135–0.282753) and εHf(t) values (-13.2 to +8.5), with two-stage Hf model ages (TDMc) of 830–2029 Ma.

5. Discussion

5.1. Petrogenesis

The whole-rock elemental signatures show that the Caowa quartz diorites are classified as high-K calc-alkaline series. The enrichments in LILEs (e.g. Ba, Th and U) and depletions in HFSEs (e.g. Nb, Ta and Ti), together with the enrichments of LREEs, demonstrate that they are similar to arc magmatic rocks in subduction zones [42]. In addition, the Caowa quartz diorites carry some geochemical characteristics of trace elements that distinguish them from traditional I-, S- and A-type granitoids. On the Rb-Ba-Sr diagram (Figure 8), the samples plot into the field of high Ba–Sr granitoids. The samples show high Ba (822–1165 ppm) and Sr (561–646 ppm) contents and low Rb (72.9–92.1 ppm) and U (1.39–2.17 ppm) contents, together with enrichment in LREEs and depletion in HREEs, and negligible Eu anomalies (Figure 5), which are the typical features for high Ba-Sr granitoids worldwide (e.g., Northern Scotland, Eastern China, ect) [15,17,18,21]. Several models have been proposed for the origin of high Ba-Sr granitoids: (1) partial melting of subducted ocean islands/ocean plateaus [15]; (2) partial melting of thickened mafic lower crust with or without mantle input [19,25,43]; (3) partial melting of enriched lithospheric mantle metasomatized by subduction-related fluids or melts [20,22,23]; (4) mixing of enriched mantle-derived mafic magmas with crustal felsic melts [17,18,26].
Tarney and Jones (1994) first proposed that partial melting of subducted oceanic islands or plateaus can generate high Ba-Sr initial magmas, which can also explain their strong fractionated REE patterns and negligible Eu anomalies [15]. However, based on the findings from experimental petrology, partial melting of basaltic oceanic crust (MORB- or OIB) usually produces Na-rich magmas (Na2O > 5 wt.%) [44], which is evidently inconsistent with the high-K calc-alkaline characteristics of the Caowa quartz diorites (Figure 4b). In addition, the Caowa quartz diorites have enriched Sr-Nd isotope compositons with initial 87Sr/86Sr ratios = 0.70818–0.70860 and εNd(t) = -5.1 to -4.9 that are obviously distinct from those of depleted mantle-derived Early Paleozoic ophiolites and adakites in NQOB [45,46,47] (Figure 6a). Since the partial melting of basaltic oceanic crust cannot generate the Caowa high Ba-Sr quartz diorites.
It is generally accepted that partial melting of thickened mafic lower continental crust is a key mechanism to produce high Ba-Sr granitoids [19,25,43], which is mainly due to their affinities with adakitic granitoids, such as: high alkali, Sr and LREE contents, low Rb, Y and HREE contents, and high Sr/Y and La/Yb ratios [20,48]. Although the rocks studied here display high Sr contents, enrichment in LREE and negligible Eu anomalies (some adakitic features), they have high Y (22.93–33.24 ppm), Yb (2.37–3.17 ppm) contents and low Sr/Y (19–27), La/Yb (4.56–17.47) ratios significantly different to typical adakites (Figure 9), and more evolved Sr-Nd isotope compositons than the adakitic granitoids that are proposed to have been originated from partial melting of thickened lower crust in the eastern part of NQOB [10,11,14] (Figure 6a). Furthermore, magmas, derived from high-pressure partial melting of the thickened lower crust, generally possess extinct fractioned HREE (e.g., Gd/Yb ratios >8) [49]. All samples studied here display low Gd/Yb ratios (1.43–1.72) and flat HREE distribution patterns (Figure 5a), and thus rule out a significant involvement of garnet during the magmatic generation. Consequently, the thickened crust model for the petrogenesis of the high Ba-Sr granitoids is maybe not applicable to the Caowa quartz diorites.
Previous studies have shown that subducted oceanic slab and sediments-derived melts or fluids have a high capacity to carry significant amounts of Ba and Sr [50,51], and transfer of these elements would result in enrichment of the overling lithospheric mantle through metasomatism. Thus, low-degree partial melting of enriched lithospheric mantle metasomatized by subduction-related fluids or melts can generate initial magmas with high Ba-Sr signatures [20,22,23]. The Caowa quartz diorites exhibit higher Nd (21.20–45.27 ppm), Nb (12.20–13.84 ppm) contents and Nb/Ta (14.46–20.22) values than those derived from the continent crust (Nd = 11–27, average Nb/Ta = 11) [52], indicating the significant contribution of mantle components. Besides, the Caowa quartz diorites have relatively enriched Sr-Nd-Hf isotopic compositions, which are consistent with those of the enriched mantle-derived high Ba-Sr granitoids from the southern margin of Alxa block (Figure 6a, Figure 7) [23]. As shown in plots of Ba/Th vs. Th/Zr and Rb/Y vs. Nb/Y (Figure 10), magma sources for the Caowa quartz diorites were probably related to the lithospheric mantle metasomatized by subduction-related fluids. This is also confirmed by the Nd-Hf isotope decoupling (Figure 6b), owing to the discrepant elemental behavior between Nd and Hf. Normally, Nd is much more mobile than Hf in subduction zone, it is difficult to cause Hf isotope enrichment when metasomatism occurs with the overlying lithospheric mantle [53,54], which is consistent with the occurrence of positive εHf(t) values for the Caowa dioritic intrusion (e.g., +6.6 and +8.5). However, it should be noted that partial melting of enriched mantle model provides a certain degree of support for the high Ba-Sr signatures of Caowa quartz diorites, but the relatively low MgO (2.08–3.01 wt.%), Mg# (42–44), Cr (4.58–6.43 ppm) and Ni (3.29–5.12 ppm) contents of these rocks are distinct from the high-Mg diorites and clearly argue against a single, common mantle evolution by partial melting process [55,56]. Since, such mechanism is not feasible here, we interpret that the magma source of these rocks may also have the addition of continental crustal components.
Here, we propose that the studied Caowa high Ba-Sr quartz diorites probably generated through mixing of enriched lithospheric mantle-derived basaltic and crustal felsic magmas. Their moderate SiO2 (57.53–62.87 wt.%), MgO (2.08–3.01 wt.%) contents and Mg# values (42–44), as well as the wide ranges of zircon Hf isotopes, support the mechanism of crust-mantle interaction (Figure 7). The studied rocks exhibit more radiogenic Sr and Nd isotopes when compared to the enriched mantle-derived mafic rocks from Central and North Qilian, and which are also different from those of the Precambrian metamorphic basement and associated granites (Figure 6a) [57,58]. A simple isotope model was adopted to evaluate the possible mixing proportion of mantle and crustal components, it is suggested that the Caowa high Ba-Sr quartz diorites might be products of mixing of 40% enriched mantle and 60% ancient crustal melts (Figure 6a). Besides, the Caowa quartz diorites are associated with the contemporary mafic rocks, dioritic enclaves, and felsic rocks in the surrounding areas, Such as Laohushan-Quwushan Mountain (Figure 1b), which were considered to be derived from partial melting of the metasomatized enriched lithosphere mantle, magma mixing, and partial melting of lower continental crust, respectively [11,13,59,60]. Successive variation in major elemental compositions between them (Figure 11), further substantiates a petrogenetic model of crust-mantle interaction [61,62]. Such a crust-mantle interaction mechanism can be further testified by the hyperbolic curves in diagrams involving the incompatible elements and its ratios [61,63]. In the Th vs. Th/Nd and Th/La vs. Zr/Sm diagrams, the studied rocks and associated contemporary mafic rocks, dioritic enclaves, and felsic rocks in the surrounding areas composed a characteristic hyperbolic mixing line (Figure 12), which confirms the major role of two-component mixing process. In order to further evaluate the possibility of magma mixing, both Laohushan hornblendite xenoliths (represented by enriched mantle source) and Quwushan granodiorites (represented by lower crustal source) were chosen as mixing end-members in the geochemical simulation. The modeling results show that the Caowa high Ba-Sr quartz diorites accords with formation of crustal and mantle melts mixing and the mixture proportion was approximately 6:4 (Figure 12b), which was consistent with the Sr-Nd isotope simulation results (Figure 6a).
In summary, the petrogenesis of Caowa high Ba-Sr intrusion can be explained by a two-stage model: firstly, a low-degree partial melting of enriched lithospheric mantle metasomatized by subduction-related fluids generated initial magmas with high Ba-Sr signatures, and then underplating of this high Ba-Sr basaltic melts triggered partial melting of the ancient lower continental crust and subsequent crust-mantle interaction.

5.2. Tectonic Implications

It is generally accepted that the Early Paleozoic NQOB is a typical subduction-accretionary orogenic belt, marking the tectonic evolution of Proto-Tethys Ocean [1,2,9,27]. Although both the Proto-Tethys Ocean subduction-related and the syn-collision/post-collision tectonic settings have been proposed, the subduction polarity and final closure of the Proto-Tethys Ocean are still controversial. The models of the subduction polarity issue include southward subduction [64], northward subduction [5,8,9], or bidirectional subduction [32,65]. However, considering the current tectonic geographical pattern and the distribution of Early Paleozoic mid-ocean-ridge type ophiolites, high-pressure metamorphic rocks, arc magmatic rocks and back-arc basin ophiolites in the NQOB from south to north, northward subduction of the Qilian Proto-Tethys ocean was basically authenticated, and the closure time of the Qilian Proto-Tethys ocean was no later than 440 Ma [5,7,8,9]. It should be noted that these previous studies concerning the Proto-Tethys evolution have focused on the western part of the NQOB. As for the eastern part of the NQOB, the former evolution process may be suffered “acclimatized” [14].
Yu et al. (2015) believed that the generation of crustal-derived low-Mg adakitic granitoids (461–440 Ma) from the Eastern NQOB reflects crustal thickening in response to an continent-continent collision [11]. As mentioned above, the Caowa high Ba-Sr dioritic intrusion examined in the Nanhuashan area of the Eastern NQOB was not correlate with crustal thickening which should display high Gd/Yb ratios and leave a residue with garnet. In addition, the distributions of the ca. 448 Ma Laohushan ophiolite (the Northern ophiolite belt, Figure 1b) and ca. 446 Ma Baiyin arc volcanic rocks indicate that the Eastern NQOB was probably undergone a new oceanic crust development and subduction during the Late Ordovician [5,8,59]. Combined with the new discovery of boninitic blueschists and associated greenschists from Laohushan area in Eastern NQOB by Fu et al. (2022) [29], which record the intra-oceanic subduction initiation at ca. 492–488 Ma, we assume that there may be no continent-continent event occurring there before Late Ordovician.
The Late Ordovician Caowa high-Ba-Sr quartz diorites studied here are located in the eastern part of the NQOB (Figure 1). As discussed above, these rocks generated through interaction of partial melts derived from subduction-related metasomatized lithospheric mantle and ancient lower continental crust. Therefore, we propose that they are most probably formed in a Late Ordovician oceanic crust subduction-related arc setting. The distribution of these rocks occurring to the northeast of Baiyin arc and Laohushan ophiolite belt and the southern margin of Alxa block (Figure 1b) indicates that they were related to the northward subduction of the North Qilian oceanic crust formed in a back-arc basin rather than the paleo-Qilian ocean (maybe a main ocean of Proto-Tethys in the Qilian orogenic system). Consequently, we suggest that affected by the northward subduction of the Qilian Proto-Tethys ocean, the Laohushan oceanic crust of the North Qilian back-arc basin was subducted during the Late Ordovician, which induced extensive metasomatism of lithospheric mantle by subduction-related fluids and subsequent crust-mantle interaction during the Late Ordovician (Figure 13).

6. Conclusions

(1) Zircon U-Pb dating suggests that the Caowa dioritic intrusion from Nanhuashan area have magma crystallization age of 447 Ma, representing Late Ordovician magmatism in the eastern part of North Qilian orogenic belt.
(2) The petrographic, geochemical and Sr-Nd-Hf isotopic characteristics indicate that the Caowa quartz diorites, classified as high Ba-Sr granitoids, were produced through a crust-mantle interaction between partial melt derived from subduction-related fluids metasomatized lithospheric mantle and ancient lower crust-derived magma.
(3) The petrogenesis of the Caowa high Ba-Sr dioritic intrusion in Nanhuashan area provides support for an existence of northward subduction of the North Qilian oceanic crust (Laohushan back-arc oceanic basin) in the eastern part of the North Qilian orogenic belt during the Late Ordovician.

Author Contributions

Conceptualization, S.Z. and L.H.; methodology, B.L.; software, H.D. and Q.X.; formal analysis, S.Z.; investigation, S.Z., L.H. and C.M.; resources, X.W. and C.X.M.; writing-original draft preparation, S.Z.; writing-review and editing, L.H. and B.L.; project administration, L.H. and X.W.; funding acquisition, B.L., H.D. and C.X.M. All authors have read and agreed to the published version of the manuscript.

Funding

This study is co-supported by the National Natural Science Foundation of China (Grant No. 42130309, 41802085), the Provincial Key Research & Development Program of Ningxia Hui Autonomous Region (Grant No. 2021BEG03003), and the Provincial Natural Science Foundation of Ningxia Hui Autonomous Region (Grant No. 2021AAC03447).

Data Availability Statement

The original contributions presented in the study are included in the article.

Acknowledgments

We thank Jinwei Guo for scientific discussions which are helpful for this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. (a) Tectonic sketch map of China (modified from [5]); (b) Simplified geological map of the North Qilian Orogenic Belt showing distributions of the main tectonic units (modified from [29]), and (c) simplified distribution map of the Caowa dioritic intrusion, showing the studied sample’s location.
Figure 1. (a) Tectonic sketch map of China (modified from [5]); (b) Simplified geological map of the North Qilian Orogenic Belt showing distributions of the main tectonic units (modified from [29]), and (c) simplified distribution map of the Caowa dioritic intrusion, showing the studied sample’s location.
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Figure 2. Field and microscope photographs of the Caowa dioritic intrusion in the Eatern NQOB. (a) Field photograph of the dioritic intrusion; (b–d) Photomicrographs of the quartz diorites. Abbreviations: Hb = hornblende, Pl = plagioclase, Kfs = K-feldspar, Qtz = quartz, Ap = apatite.
Figure 2. Field and microscope photographs of the Caowa dioritic intrusion in the Eatern NQOB. (a) Field photograph of the dioritic intrusion; (b–d) Photomicrographs of the quartz diorites. Abbreviations: Hb = hornblende, Pl = plagioclase, Kfs = K-feldspar, Qtz = quartz, Ap = apatite.
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Figure 3. Cathodoluminescence images (a), LA-ICP-MS U–Pb concordia diagram (b), and chondrite-normalized REE patterns (c) of representative zircons from Caowa quartz diorite (a). Solid and dashed circles indicate the location of U–Pb analysis and Hf analysis, respectively.
Figure 3. Cathodoluminescence images (a), LA-ICP-MS U–Pb concordia diagram (b), and chondrite-normalized REE patterns (c) of representative zircons from Caowa quartz diorite (a). Solid and dashed circles indicate the location of U–Pb analysis and Hf analysis, respectively.
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Figure 4. Geochemical classification of the Caowa quartz diorites. (a) Total alkali vs. silica (TAS) diagram (after [66]); (b) K2O vs. SiO2 diagram (after [67]); (c) (Na2O + K2O − CaO) vs. SiO2 diagram (after [68]); and (d) A/NK [molar ratio Al2O3/(Na2O+K2O)] vs. A/CNK [molar ratio Al2O3/(CaO+Na2O+K2O)] diagram (after [69]). The data for typical high Ba-Sr granitoids are from [16,20].
Figure 4. Geochemical classification of the Caowa quartz diorites. (a) Total alkali vs. silica (TAS) diagram (after [66]); (b) K2O vs. SiO2 diagram (after [67]); (c) (Na2O + K2O − CaO) vs. SiO2 diagram (after [68]); and (d) A/NK [molar ratio Al2O3/(Na2O+K2O)] vs. A/CNK [molar ratio Al2O3/(CaO+Na2O+K2O)] diagram (after [69]). The data for typical high Ba-Sr granitoids are from [16,20].
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Figure 5. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) for the Caowa quartz diorites. Normalizing values are from [52]. The compositions for typical high Ba-Sr granitoids are from [16,20].
Figure 5. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) for the Caowa quartz diorites. Normalizing values are from [52]. The compositions for typical high Ba-Sr granitoids are from [16,20].
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Figure 6. Initial 87Sr/86Sr vs. εNd(t) (a) and zircon εHf(t) vs. εNd(t) diagrams (b) for the Caowa quartz diorites. Data sources: Early Paleozoic ophiolites and slab-derived adakites in the NQOB are from [45,46,47]; enriched mantle-derived mafic rocks in the Central Qilian are from [70]; thickened lower crust-derived adakitic rocks, high Ba-Sr granitoids and old crust-derived granites in the NQOB are from [10,11,23,58]; Laohushan mafic rocks and Baojishan intermediate-acid rocks in the Eastern NQOB are from [12,59]; Qilian basement from [57]. Fields of MORB, OIB and marine sediments, and Terrestrial array after [71].
Figure 6. Initial 87Sr/86Sr vs. εNd(t) (a) and zircon εHf(t) vs. εNd(t) diagrams (b) for the Caowa quartz diorites. Data sources: Early Paleozoic ophiolites and slab-derived adakites in the NQOB are from [45,46,47]; enriched mantle-derived mafic rocks in the Central Qilian are from [70]; thickened lower crust-derived adakitic rocks, high Ba-Sr granitoids and old crust-derived granites in the NQOB are from [10,11,23,58]; Laohushan mafic rocks and Baojishan intermediate-acid rocks in the Eastern NQOB are from [12,59]; Qilian basement from [57]. Fields of MORB, OIB and marine sediments, and Terrestrial array after [71].
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Figure 7. Zircon εHf(t) vs. age (t) diagram for the Caowa quartz diorites. All εHf(t) values were calculated at the ages given by the U–Pb data. Data sources: Jingtieshan adakitic granites and felsic rocks of crust-mantle mixed source in the western part of NQOB are from [72,73]. See Figure 6 for other data sources.
Figure 7. Zircon εHf(t) vs. age (t) diagram for the Caowa quartz diorites. All εHf(t) values were calculated at the ages given by the U–Pb data. Data sources: Jingtieshan adakitic granites and felsic rocks of crust-mantle mixed source in the western part of NQOB are from [72,73]. See Figure 6 for other data sources.
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Preprints 73273 g008
Figure 8. Rb-Ba-Sr ternary diagram for the Caowa quartz diorites, modified after [15].
Figure 8. Rb-Ba-Sr ternary diagram for the Caowa quartz diorites, modified after [15].
Preprints 73273 g008
Preprints 73273 g009
Figure 9. Discrimination diagrams of Sr/Y vs. Y (a) and (La/Yb)N vs. YbN (b) for the Caowa quartz diorites, modified after [48]. See Figure 4 and 6 for the data sources.
Figure 9. Discrimination diagrams of Sr/Y vs. Y (a) and (La/Yb)N vs. YbN (b) for the Caowa quartz diorites, modified after [48]. See Figure 4 and 6 for the data sources.
Preprints 73273 g009
Preprints 73273 g010
Figure 10. Ba/Th vs. Th/Zr(a) and Rb/Yb vs. Nb/Yb (b) diagrams for the Caowa quartz diorites, modified after [22]. See Figure 4 and 6 for the data sources.
Figure 10. Ba/Th vs. Th/Zr(a) and Rb/Yb vs. Nb/Yb (b) diagrams for the Caowa quartz diorites, modified after [22]. See Figure 4 and 6 for the data sources.
Preprints 73273 g010
Preprints 73273 g011
Figure 11. Hark diagrams for the Caowa quartz diorites. Data sources: Quwushan-Laohushan mafic rocks, dioritic enclaves and intermediate-acid rocks are from [13,59,60]. See Figure 4 for other data sources.
Figure 11. Hark diagrams for the Caowa quartz diorites. Data sources: Quwushan-Laohushan mafic rocks, dioritic enclaves and intermediate-acid rocks are from [13,59,60]. See Figure 4 for other data sources.
Preprints 73273 g011
Preprints 73273 g012
Figure 12. Th/Nd vs. Th (a) and Zr/Sm vs. Th/La (b) diagrams for the Caowa quartz diorites, modified after [63]. See Figure 11 for the symbols and data sources.
Figure 12. Th/Nd vs. Th (a) and Zr/Sm vs. Th/La (b) diagrams for the Caowa quartz diorites, modified after [63]. See Figure 11 for the symbols and data sources.
Preprints 73273 g012
Preprints 73273 g013
Figure 13. Schematic diagram of the possible tectonic setting and petrogenesis of the Caowa high Ba-Sr dioritic intrusion, and the Late Ordovician tectonic evolution of the NQOB.
Figure 13. Schematic diagram of the possible tectonic setting and petrogenesis of the Caowa high Ba-Sr dioritic intrusion, and the Late Ordovician tectonic evolution of the NQOB.
Preprints 73273 g013
Table 1. LA-ICP-MS zircon U-Pb dating results of the Caowa quartz diorite.
Table 1. LA-ICP-MS zircon U-Pb dating results of the Caowa quartz diorite.
Spot No. Contents (ppm) Th/U Isotopic Ratios Isotopic Ages (Ma)
232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 208Pb/232Th 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ
CW-6-01 744 1889 0.39 0.0546 0.0018 0.5629 0.0146 0.0730 0.0012 0.0224 0.0005 394 79 453 10 454 7
CW-6-02 1093 2859 0.38 0.0602 0.0013 0.5984 0.0127 0.0721 0.0008 0.0247 0.0006 613 46 476 8 449 5
CW-6-03 781 2046 0.38 0.0559 0.0013 0.5572 0.0121 0.0709 0.0007 0.0215 0.0004 456 52 450 8 442 4
CW-6-04 744 1860 0.40 0.0559 0.0013 0.5516 0.0114 0.0712 0.0008 0.0219 0.0004 456 52 446 7 444 5
CW-6-05 370 1424 0.26 0.0538 0.0013 0.5396 0.0132 0.0725 0.0007 0.0213 0.0004 365 54 438 9 451 4
CW-6-06 619 1916 0.32 0.0561 0.0013 0.5718 0.0168 0.0734 0.0009 0.0225 0.0004 457 52 459 11 457 5
CW-6-07 368 1498 0.25 0.0599 0.0012 0.5898 0.0120 0.0712 0.0006 0.0251 0.0004 611 38 471 8 444 3
CW-6-08 465 1687 0.28 0.0557 0.0010 0.5507 0.0110 0.0715 0.0007 0.0218 0.0004 443 43 445 7 445 4
CW-6-09 342 1222 0.28 0.0560 0.0012 0.5480 0.0117 0.0708 0.0006 0.0220 0.0004 450 46 444 8 441 4
CW-6-10 516 1633 0.32 0.0562 0.0018 0.5645 0.0180 0.0728 0.0008 0.0228 0.0005 457 72 454 12 453 5
CW-6-11 393 1455 0.27 0.0577 0.0020 0.5787 0.0200 0.0723 0.0008 0.0272 0.0007 520 74 464 13 450 5
CW-6-12 761 2264 0.34 0.0559 0.0013 0.5637 0.0135 0.0727 0.0006 0.0212 0.0004 456 50 454 9 452 4
CW-6-13 494 1690 0.29 0.0553 0.0012 0.5519 0.0128 0.0719 0.0007 0.0230 0.0005 433 48 446 8 448 4
CW-6-14 428 1386 0.31 0.0561 0.0014 0.5554 0.0135 0.0716 0.0007 0.0221 0.0005 457 21 449 9 446 4
CW-6-15 615 1777 0.35 0.0556 0.0014 0.5521 0.0159 0.0714 0.0006 0.0226 0.0005 435 56 446 10 444 4
CW-6-16 451 1584 0.28 0.0558 0.0012 0.5587 0.0122 0.0722 0.0006 0.0216 0.0004 443 51 451 8 449 4
CW-6-17 495 1491 0.33 0.0571 0.0019 0.5540 0.0185 0.0700 0.0009 0.0233 0.0006 498 74 448 12 436 5
CW-6-18 608 1693 0.36 0.0621 0.0020 0.6192 0.0221 0.0718 0.0009 0.0242 0.0012 676 69 489 14 447 5
CW-6-19 487 1678 0.29 0.0563 0.0014 0.5778 0.0141 0.0737 0.0008 0.0233 0.0006 465 54 463 9 459 5
Table 2. Zircon trace element data of the Caowa quartz diorite.
Table 2. Zircon trace element data of the Caowa quartz diorite.
Spot No. La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Eu/Eu* Hf Ta Y Ti Nb
CW-6-01 0.16 65.71 0.30 2.50 5.64 3.43 41.79 15.29 207 88.11 442 106.56 1095 257 0.49 28295 6.09 2803 13.32 18.18
CW-6-02 2.71 95.28 2.51 16.76 13.22 6.67 59.00 21.00 273 113.41 556 132.00 1356 306 0.62 27149 8.94 3634 22.20 33.09
CW-6-03 0.01 66.60 0.08 1.92 6.56 3.44 42.17 16.44 217 91.56 472 113.50 1184 277 0.48 28530 6.55 2957 11.24 19.62
CW-6-04 0.20 76.26 0.21 3.53 6.55 4.17 44.84 17.58 227 97.89 498 118.97 1248 292 0.55 27469 6.58 3153 12.14 22.23
CW-6-05 0.02 50.15 0.11 1.65 4.07 2.80 30.96 13.94 189 80.95 418 102.28 1088 261 0.54 27647 5.66 2607 9.11 15.53
CW-6-06 13.94 82.46 4.87 22.53 7.96 3.02 38.18 13.71 181 76.11 395 94.46 1003 239 0.44 29534 5.88 2473 7.24 17.51
CW-6-07 2.96 62.56 1.85 11.27 10.34 4.18 41.91 14.74 210 92.24 490 122.45 1296 311 0.53 29319 6.04 3019 13.23 18.31
CW-6-08 0.00 60.01 0.09 2.03 6.29 3.34 44.66 16.44 239 108.89 573 139.26 1488 355 0.45 28350 6.75 3530 10.76 23.00
CW-6-09 0.01 51.36 0.11 2.37 5.80 3.42 40.01 15.27 228 100.04 538 131.31 1376 327 0.51 27151 5.18 3246 11.26 18.19
CW-6-10 0.06 55.93 0.15 2.10 4.98 3.37 36.84 14.65 201 85.35 452 110.00 1156 273 0.55 28330 6.46 2800 11.83 19.52
CW-6-11 1.61 82.48 1.61 11.68 10.50 4.74 42.03 14.21 192 83.62 420 98.51 1029 252 0.60 29354 4.25 2626 22.40 13.69
CW-6-12 0.08 60.44 0.08 1.06 4.83 3.30 35.79 13.82 191 83.22 420 102.55 1069 252 0.55 29612 7.33 2661 9.38 19.93
CW-6-13 0.01 62.43 0.11 2.33 6.54 3.99 45.41 18.07 254 114.76 605 150.52 1592 380 0.52 27316 6.95 3712 13.38 22.66
CW-6-14 1.56 56.25 0.78 6.05 6.50 3.42 40.46 15.75 209 92.13 485 117.96 1243 297 0.49 27661 5.72 2963 30.72 17.65
CW-6-15 0.40 62.18 0.30 2.90 6.80 3.51 42.94 15.20 214 91.05 469 113.20 1174 280 0.48 27915 6.77 2907 10.23 19.48
CW-6-16 0.01 53.87 0.11 2.03 5.84 2.90 38.17 15.39 211 94.62 492 120.89 1268 304 0.45 28605 6.03 3029 11.04 20.50
CW-6-17 0.19 57.12 0.20 2.29 5.43 2.38 32.30 12.46 165 70.94 370 88.25 928 220 0.43 28248 5.33 2278 21.60 13.96
CW-6-18 4.17 127.01 4.87 31.21 23.26 12.35 68.83 21.36 245 98.51 473 107.22 1144 262 0.87 26455 6.09 3042 15.51 18.31
CW-6-19 0.03 55.17 0.09 1.80 5.20 3.05 39.80 15.83 224 99.35 531 132.58 1388 334 0.46 28114 6.50 3256 10.68 21.27
Table 3. Major (wt.%), trace element (ppm) and Sr-Nd isotopic compositions of the Caowa quartz diorites.
Table 3. Major (wt.%), trace element (ppm) and Sr-Nd isotopic compositions of the Caowa quartz diorites.
Sample CW-1 CW-2 CW-3 CW-4 CW-5 CW-6
SiO2 62.80 62.87 59.75 58.34 57.53 61.97
TiO2 0.61 0.59 0.74 0.77 0.90 0.63
Al2O3 16.32 16.85 16.99 16.66 17.16 16.90
Fe2O3T 5.39 5.22 6.58 6.63 7.88 5.54
MnO 0.13 0.12 0.14 0.14 0.17 0.12
MgO 2.13 2.10 2.43 2.63 3.01 2.08
CaO 3.47 3.71 4.65 5.46 5.26 3.81
Na2O 3.91 3.95 3.78 3.71 3.65 3.86
K2O 2.95 2.93 2.78 2.94 2.50 2.82
P2O5 0.19 0.19 0.23 0.23 0.26 0.20
LOI 2.13 1.36 1.42 2.453 1.43 1.62
Total 100.03 99.90 99.48 99.97 99.74 99.56
Mg# 44 44 42 44 43 43
A/CNK 1.02 1.02 0.96 0.87 0.94 1.03
Na2O+K2O 6.86 6.89 6.55 6.65 6.16 6.68
Sc 9.22 8.41 11.84 11.76 15.11 9.24
V 78.3 80.2 105.7 106.7 130.8 83.7
Cr 4.81 5.21 5.69 5.45 6.43 4.58
Co 8.92 9.08 11.92 11.89 14.65 9.57
Ni 3.29 3.46 4.53 4.61 5.12 3.71
Cu 9.87 8.70 11.77 14.24 23.66 4.63
Zn 70.0 69.0 80.5 78.6 93.0 67.9
Rb 92.1 90.0 81.6 80.2 76.6 72.9
Sr 561 612 623 601 646 646
Y 25.07 22.93 28.90 28.01 33.24 23.98
Zr 193.6 180.5 218.9 206.7 228.8 193.5
Nb 12.53 13.02 13.42 13.84 13.05 12.20
Ba 1165 916 860 1151 822 902
La 60.86 40.90 27.52 18.27 57.29 44.74
Ce 112.96 77.92 53.83 37.62 111.28 85.59
Pr 12.26 8.69 6.64 5.05 12.74 9.50
Nd 41.73 30.58 25.94 21.20 45.27 33.69
Sm 7.06 5.42 5.92 5.23 8.40 6.20
Eu 1.58 1.36 1.60 1.57 1.96 1.54
Gd 5.04 4.30 5.20 4.96 6.60 4.66
Tb 0.74 0.64 0.81 0.79 1.00 0.70
Dy 4.30 3.70 4.92 4.67 5.89 3.98
Ho 0.86 0.79 0.96 1.00 1.17 0.83
Er 2.59 2.32 2.93 2.87 3.32 2.50
Tm 0.37 0.34 0.44 0.42 0.48 0.37
Yb 2.50 2.37 2.86 2.87 3.17 2.38
Lu 0.39 0.36 0.44 0.43 0.49 0.37
Hf 5.02 4.60 5.44 5.11 5.71 4.76
Ta 0.76 0.90 0.80 0.81 0.65 0.73
Pb 21.10 18.48 15.75 18.52 17.27 19.10
Th 15.53 10.99 6.12 4.29 14.31 11.99
U 1.68 1.39 1.57 1.68 2.17 1.79
REE 253.22 179.69 140.01 106.95 259.06 197.05
Eu/Eu* 0.77 0.83 0.86 0.93 0.78 0.84
Sr/Y 22.38 26.68 21.57 21.44 19.44 26.93
Nb/Ta 16.38 14.46 16.70 16.99 20.22 16.70
(La/Yb)N 17.47 12.36 6.91 4.56 12.95 13.47
(Gd/Yb)N 1.67 1.50 1.51 1.43 1.72 1.62
87Rb/86Sr 0.4258 0.3863 0.3267
87Sr/86Sr 0.711129 0.710716 0.710747
(87Sr/86Sr)i 0.708338 0.708184 0.708606
147Sm/144Nd 0.1071 0.1493 0.1113
143Nd/144Nd 0.512124 0.512243 0.512119
εNd(t) -4.8 -4.9 -5.1
TDM2 (Ma) 　 1579 　 1591 　 1607
Table 4. Zircon Hf isotopic compositions of the Caowa quartz diorites.
Table 4. Zircon Hf isotopic compositions of the Caowa quartz diorites.
Spot No. Age (Ma) 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf 1σ εHf(0) εHf(t) TDM (Ma) TDMc (Ma) fLu/Hf
CW-6-01 454 0.047583 0.001043 0.282401 0.000028 -13.1 -3.5 1204 1499 -0.97
CW-6-03 442 0.060913 0.001249 0.282459 0.000102 -11.1 -1.7 1128 1393 -0.96
CW-6-04 444 0.053286 0.001496 0.282355 0.000036 -14.7 -5.4 1283 1599 -0.95
CW-6-05 451 0.041103 0.000944 0.282449 0.000048 -11.4 -1.8 1133 1404 -0.97
CW-6-06 457 0.051349 0.001116 0.282409 0.000073 -12.8 -3.1 1195 1483 -0.97
CW-6-07 444 0.074723 0.001580 0.282135 0.000366 -22.5 -13.2 1598 2029 -0.95
CW-6-08 445 0.067271 0.001497 0.282319 0.000104 -16.0 -6.7 1335 1670 -0.95
CW-6-09 441 0.043239 0.001363 0.282365 0.000032 -14.4 -5.1 1264 1578 -0.96
CW-6-10 453 0.053681 0.001471 0.282335 0.000066 -15.4 -5.9 1310 1634 -0.96
CW-6-12 452 0.067973 0.001496 0.282411 0.000057 -12.8 -3.3 1204 1487 -0.95
CW-6-13 448 0.062911 0.001953 0.282340 0.000028 -15.3 -6.0 1320 1634 -0.94
CW-6-14 446 0.079210 0.002148 0.282753 0.000114 -0.7 8.5 730 830 -0.94
CW-6-15 444 0.057545 0.001758 0.282422 0.000041 -12.4 -3.1 1197 1473 -0.95
CW-6-16 449 0.059503 0.001403 0.282364 0.000085 -14.4 -5.0 1268 1579 -0.96
CW-6-19 459 0.061641 0.001196 0.282682 0.000099 -3.2 6.6 812 948 -0.96
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