The Role of Siliceous High-Magnesium Basalts during the Formation of a Neoproterozoic Mafic-Ultramafic Intrusion in the Tarim Craton (China)

The Xingdi No.2 intrusion in the Kuluktag Block of northeastern Tarim Craton, which intrudes into the Palaeoproterozoic basement with an exposed area of ca. 12 km2, is an orthopyroxene-rich mafic-ultramafic intrusion in a continental rift setting. It consists of gabbros, pyroxenites, and peridotites, and exhibits a crystallization sequence of the principal rock-forming minerals from olivine, orthopyroxene, clinopyroxene, to plagioclase. The gabbros show a concordant SHRIMP U-Pb zircon age of 752 ± 5.4 Ma. In addition, the olivine grains have forsterite content values of 78–85 mole% and mostly contain low NiO, MnO, CaO, and Cr2O3. The rocks are relatively enriched in large ion lithophile elements and LREE, and depleted in HSFE, have non-radiogenic Pb, low εNd (t) values (−2.8 to −23), initial 87Sr/86Sr ratios (0.7059–0.7130). It could be concluded that the rocks represent an analogue of siliceous high magnesium basaltic magma originated by the partial melting of a hydrous and enriched subcontinental lithospheric mantle and contaminated by the continental crust.


Petrography
The Xingdi No.2 intrusion consists of gabbro, pyroxenite, and peridotite units (Figure 1b). The gabbro unit in the west and on the margin accounts for about 70-75% of the outcropped area of the intrusion. Several rock types can be recognised, including olivine norite, olivine gabbronorite, olivine gabbro, gabbronorite, gabbro, along with a lesser amount of pyroxene diorite. The rocks mainly show a typical gabbro and poikilitic textures (Figure 2a), and have a
The peridotite unit exposed in the northeast of the intrusion has mainly lherzolite-containing olivine (40-70%), clinopyroxene (10-30%), orthopyroxene (10-20%), along with minor plagioclase, amphibole, phlogopite, spinel, and sulphide. Olivine grains are subhedral or rounded, with a variable diameter of 0.5-4 mm. They are generally enclosed in clinopyroxene or orthopyroxene, and have a poikilitic texture (Figure 2c). Locally, serpentinization and talcization are visible around the olivine grain margins (Figure 2d). Clinopyroxene and orthopyroxene occur as anhedral crystals with diameters ranging from 1 to 6 mm and are interstitial to olivine. Some pyroxene grains have been altered to tremolite or actinolite. The plagioclase exhibits an irregular distribution in the olivine and pyroxene grains, and similarly does the phlogopite in the olivine grains.
The peridotite unit exposed in the northeast of the intrusion has mainly lherzolite-containing olivine (40-70%), clinopyroxene (10-30%), orthopyroxene (10-20%), along with minor plagioclase, amphibole, phlogopite, spinel, and sulphide. Olivine grains are subhedral or rounded, with a variable diameter of 0.5-4 mm. They are generally enclosed in clinopyroxene or orthopyroxene, and have a poikilitic texture (Figure 2c). Locally, serpentinization and talcization are visible around the olivine grain margins (Figure 2d). Clinopyroxene and orthopyroxene occur as anhedral crystals with diameters ranging from 1 to 6 mm and are interstitial to olivine. Some pyroxene grains have been altered to tremolite or actinolite. The plagioclase exhibits an irregular distribution in the olivine and pyroxene grains, and similarly does the phlogopite in the olivine grains.

Analytical Methods
One coarse-grained gabbro sample (41 • 07 33.7" N, 87 • 49 52.7" E) is collected from the Xingdi No.2 intrusion for U-Pb zircon dating. Zircons are extracted from crushed rock (5-10 kg) using heavy liquids and a Frantz magnetic separator, and then handpicked, mounted on epoxy resin, and polished until the grain centers are exposed. Before isotopic analysis, photographs are obtained using an optical microscope, and cathodoluminesce (CL) images are recorded under a scan electron microprobe (SEM), to identify the morphology and internal textures. Isotope analyses are performed using a SHRIMP II ion microprobe at the Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences. The analytical procedures are similar to those described in [37]. Mass resolution during the analytical sessions is 5000 (1% definition), and the intensity of the primary ion beam is 5-8 nA. Primary beam size is 25-30 µm normally and 10-15 µm when analyzing very narrow overgrowth of zircons. Each analytical site is rastered for 120-200 s prior to analysis, and five scans through the mass stations are made. Standards SL13 (U = 238ppm) and TEMORA ( 206 Pb/ 238 U age = 417 Ma) are used for calibrating the U abundance and isotope ratios, respectively. The decay constants used for age calculation are those recommended in [38]. Common lead correction is applied, and data processing is carried out using the SQUID and ISOPLOT programs [39]. The uncertainties for individual analyses are quoted at the 1σconfidence level, whereas errors for pooled ages are quoted at 95% confidence. Due to the small amount of 207Pb in young (i.e., <1000 Ma) zircons, which resulted in low count rates and high analytical uncertainties, the ages have to be determined primarily based on their 206 Pb/ 238 U ratios, whereas the ages of older zircon are determined from 207 Pb/ 206 Pb ratios.
Mineral compositions are determined by wavelength-dispersive X-ray analysis using an EPMA-8100 electron microprobe (JEOL, Tokyo, Japan) at Chang'an University, Xi'an, China, with 15 kV acceleration voltage, 20 nA beam current, 1 µm beam size, and a peak counting time of 20 s for major elements. The Ni in olivine is analyzed using a beam current of 80 nA and a peak counting time of 50 s. The detection limit for Ni is 100 ppm. Analytical error is within 2%, which ensured test reproducibility. The accuracy of olivine analysis is monitored using the San Carlos olivine standard.
Rock samples for whole-rock analyses are crushed and then pulverized in an agate mill. The whole-rock major elements are analyzed using X-ray fluorescence (RIX2100, SHIMADZU, Kyoto, Japan) on fused glass beads at the Key Laboratory of Western China's Mineral Resources and Geological Engineering of Ministry of Education at Chang'an University. The analytical error is between 1% and 5%.
A Thermo-X7 ICP-MS is used to analyze trace elements at Chang'an University. Sample powders (50 mg) are dissolved in high-pressure Teflon vessels in a mixture of HF: HNO 3 . Rh is used as an internal standard to monitor signal drift during ion counting. The analytical error for most elements is less than 5%.
Whole-rock Sr-Nd-Pb isotope analyses are carried out at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. Rb-Sr and Sm-Nd isotopes are determined using MC-ICP-MS. Total procedural blanks are 10 pg for Sm and Nd, and 20 pg for Rb and Sr. The values for the La Jolla Nd standard and the SRM987 Sr standard measured during data acquisition are 143 Nd/ 144 Nd = 0.511861 ± 10 and 87 Sr/ 86 Sr = 0.710263 ± 10, respectively. Pb isotopes are determined using a VG354 thermal ionization mass spectrometer. The total procedural blank is~100 pg for Pb. Values for the SRM981 Pb standard measured during data acquisition are 206 Pb/ 204 Pb = 16.934 ± 0.007, 207 Pb/ 204 Pb = 15.486 ± 0.017, and 208 Pb/ 204 Pb = 36.673 ± 0.033 (n = 20).

Olivine Compositions
All olivine grains in the Xingdi No.2 intrusion have also low CaO (<0.05 wt %), MnO (<0.40 wt %), and Cr 2 O 3 (<0.50 wt %). Most olivine grains have low Ni content (<0.20 wt %) ( Table S2). The Fo value varies from 85 to 71 mole% and decreases gradually from lherzolite to gabbro. The results show that all peridotites along with some pyroxenites and gabbros belong to the tholeiitic series, whereas most pyroxenites and gabbros belong to the calc-alkaline series ( Figure 5). The CIPW calculation shows that all rocks contain significant hypersthene (1.85-69.37%) and variable quartz (0.03-12.10%). The results show that all peridotites along with some pyroxenites and gabbros belong to the tholeiitic series, whereas most pyroxenites and gabbros belong to the calc-alkaline series ( Figure  5). The CIPW calculation shows that all rocks contain significant hypersthene (1.85-69.37%) and variable quartz (0.03-12.10%). Most rocks of the Xingdi No.2 mafic-ultramafic intrusion have a ΣREE of 10.8-159 ppm, and ΣREE gradually increasing from peridotite to gabbro (Table S4) (Table S4)

The Compositions of the Parental Magmas
All rocks contained variable amounts of orthopyroxene, indicating that the parental magmas for the Xingdi No.2 intrusion must have been rich in Si and Mg [3,7,45]. No chilled margins or coeval small dikes that may represent magma compositions have been found in the Xingdi area, the compositions of the parental magmas for the mafic-ultramafic intrusion could be estimated by whole-rock average composition, the composition of most primitive olivine in the rocks, and olivine-liquid compositional relationships [46]. To reduce the uncertainty due to the occurrence of other cumulus phases such as clinopyroxene (CaO-rich phase) and plagioclase (Al2O3-rich phase) in the Xingdi No.2 olivine cumulates, we selected four samples of Al2O3 < 4 wt % and CaO < 3.5 wt %. The contents of MgO in the parental magma for the Xingdi No.2 intrusion are then estimated using their average whole-rock composition, the highest Fo content in olivine (85), the olivine-liquid

The Compositions of the Parental Magmas
All rocks contained variable amounts of orthopyroxene, indicating that the parental magmas for the Xingdi No.2 intrusion must have been rich in Si and Mg [3,7,45]. No chilled margins or coeval small dikes that may represent magma compositions have been found in the Xingdi area, the compositions of the parental magmas for the mafic-ultramafic intrusion could be estimated by whole-rock average composition, the composition of most primitive olivine in the rocks, and olivine-liquid compositional relationships [46]. To reduce the uncertainty due to the occurrence of other cumulus phases such as clinopyroxene (CaO-rich phase) and plagioclase (Al2O3-rich phase) in the Xingdi No.2 olivine cumulates, we selected four samples of Al2O3 < 4 wt % and CaO < 3.5 wt %. The contents of MgO in the parental magma for the Xingdi No.2 intrusion are then estimated using their average whole-rock composition, the highest Fo content in olivine (85), the olivine-liquid

The Compositions of the Parental Magmas
All rocks contained variable amounts of orthopyroxene, indicating that the parental magmas for the Xingdi No.2 intrusion must have been rich in Si and Mg [3,7,45]. No chilled margins or coeval small dikes that may represent magma compositions have been found in the Xingdi area, the compositions of the parental magmas for the mafic-ultramafic intrusion could be estimated by whole-rock average composition, the composition of most primitive olivine in the rocks, and olivine-liquid compositional relationships [46]. To reduce the uncertainty due to the occurrence of other cumulus phases such as clinopyroxene (CaO-rich phase) and plagioclase (Al 2 O 3 -rich phase) in the Xingdi No.2 olivine cumulates, we selected four samples of Al 2 O 3 < 4 wt % and CaO < 3.5 wt %. The contents of MgO in the parental magma for the Xingdi No.2 intrusion are then estimated using their average whole-rock composition, the highest Fo content in olivine (85), the olivine-liquid Fe-Mg exchange coefficient [Kd = FeO/MgO) Ol /(FeO/MgO) liq ] of 0.3 [47], and an assumed FeO/Fe 2 O 3 of 0.9 for the liquid. The estimated MgO contents in the parental magma are 11.6 wt %. Although the estimated MgO content of the Xingdi No.2 intrusion is likely to be a minimum because the observed olivine composition may have been affected by subsequent re-equilibration with trapped silicate liquid, it also suggested that the parental magma was high magnesium basaltic magma.
Experiments have demonstrated that the CaO content of olivine depends on both the Fo contents in the olivine and the composition of coexisting melt [46]. Many komatiitic or high-Mg basalts contain olivine with <0.2 wt % CaO, whereas alkaline basalts, ferro-basalts, and MORBs have olivine with CaO ranging at 0.2-1.0 wt % [48]. Thus, Ca-poor olivine is believed either to represent xenocrysts or to have formed from Si-rich and Ca-poor mafic magmas (e.g., SHMB) [49][50][51][52]. Most olivine grains of the Xingdi No.2 intrusion have very low CaO content (<0.06 wt %). Since their Fo content are lower than 85 mol%, they could not represent mantle xenocrysts. In addition, the Xingdi magmas have an initial CaO content of 5.5 wt % according to the equation published in [49]. Therefore, the Ca-poor olivine from the Xingdi No.2 intrusion was likely derived from low-Ca SHMB-like magmas.
Six samples of the Xingdi No.2 intrusion have a La/Sm ratio of 4.5-5.8, suggesting limited crustal contamination. In addition, the Nb/U ratio of seven samples is within the range of 9.69-11.48, and three samples have a Ce/Pb ratio of 13.05-16.26, which also indicate some crustal contamination. Although the Sr-and Nd-isotopic compositions belong to the enrichment type, the wide ranges of Sr-and Ndisotope ratios are also reported crustal contamination. Initial 143 Nd/ 144 Nd values plotted against the initial 87 Sr/ 86 Sr ratios plot on the mixing line between DMM and crustal rocks (Figure 7), suggesting variable degrees of the crustal contamination during magma ascent and differentiation. Hence, samples with low εNd(t) values may have undergone variable degrees (up to 10-20%) of contamination by the Tarim Proterozoic paragneiss (Figure 7).
Fractional crystallization is likewise no less important than assimilation in magmatic evolution. Petrological studies on the Xingdi No.2 intrusion indicate the crystallization sequence from olivine, orthopyroxene, clinopyroxene, to plagioclase. The whole-rock geochemical characteristics of the intrusion also reflect these features. Crystal fractionation of olivine and orthopyroxene is the primary factor that controls the abundance of Fe, Ni, and Co in the magma. Since SiO 2 , FeO*, Ni, and Co show significant correlations with MgO (Figure 9b,g,h), their presence is an indication of the fractionation of olivine and orthopyroxene. The positive Eu and Sr anomalies in several samples could be attributed to the cumulation of plagioclase ( Figure 6). In the presence of plagioclase, a pronounced trend of decreasing CaO/Al 2 O 3 along with decreasing Mg-number (Figure 9f) could corroborate the importance of clinopyroxene in the crystal fractionation [57]. Therefore, the magmas of the Xingdi No.2 intrusion might have undergone assimilation and fractionation of olivine, pyroxene, and plagioclase.

The Melting of a Hydrous and Enriched Lithospheric Mantle Source
Rocks from the Xingdi No.2 intrusion have high initial 87 Sr/ 86 Sr ratio, low initial 143 Nd/ 144 Nd ratio, and a common isotopic composition indicative of nonradiogenic lead. Figure 8 shows that compositions of the Xingdi No.2 mafic-ultramafic rocks fell in the fields of basalts from the Parana Basin of southwestern Brazil (PBB). Figure 9 shows that their projection points agreed with the basalt of the Sky island in the North Atlantic, the Mull island, and ODDleg152 (SMOB) [58]. The Pb isotope ratio of the mafic-ultramafic rocks varies within a narrow range and is devoid of radiogenic lead, thus belonging to normal lead with minimal U(Th)/Pb. Non-altered olivine pyroxenite sample (XDII-1) has no obvious hydrothermal alteration, with a εNd(t) value of −4.91 and an initial 87 Sr/ 86 Sr

The Melting of a Hydrous and Enriched Lithospheric Mantle Source
Rocks from the Xingdi No.2 intrusion have high initial 87 Sr/ 86 Sr ratio, low initial 143 Nd/ 144 Nd ratio, and a common isotopic composition indicative of nonradiogenic lead. Figure 8 shows that compositions of the Xingdi No.2 mafic-ultramafic rocks fell in the fields of basalts from the Parana Basin of southwestern Brazil (PBB). Figure 9 shows that their projection points agreed with the basalt of the Sky island in the North Atlantic, the Mull island, and ODDleg152 (SMOB) [58]. The Pb isotope ratio of the mafic-ultramafic rocks varies within a narrow range and is devoid of radiogenic lead, thus belonging to normal lead with minimal U(Th)/Pb. Non-altered olivine pyroxenite sample (XDII-1) has no obvious hydrothermal alteration, with a ε Nd (t) value of −4.91 and an initial 87 Sr/ 86 Sr ratio of 0.7068. This composition could basically represent the mantle source, indicating that the Xingdi No.2 intrusion originated by the partial melting of an enriched lithospheric mantle.
Although crustal contamination is evident in the Xingdi No.2 intrusion, hydrous melting of mantle peridotites is also important for the generation of SHMB-like magmas. In the Xingdi No.2 intrusion, biotite and amphibole in gabbro, norite, olivine gabbro, pyroxenite, and peridotite show the hydrous nature of the parental magma [59]. Meanwhile, high Ba/Zr ratios (1.11-25.72) also suggest the presence of fluids in the parental magma [7]. Hence, the SHMB-like parental magma might have originated by the partial melting of hydrous and enriched lithospheric mantle.

Implications for the Genesis of SHMB-Like Magmas in Continental Rifts
Extensive Neoproterozoic magmatism has produced abundant volcanic rocks and mafic-ultramafic intrusions in the Kuluktag Block of the Tarim Craton. Representative Beiyixi volcanic rocks that erupted at 755 ± 15 Ma represent a bimodal suite of basalt and dacite-rhyolite with a SiO 2 gap between 55% and 65%. According to their petrological and geochemical features, these volcanic rocks formed in a continental rift related to mantle plume activity beneath the Rodinia supercontinent [25]. Analogously, the Xingdi No.2 intrusion displays similar negative ε Nd (t) values (−23 to −2.8), suggesting the partial melting of enriched lithospheric mantle source. According to olivine geo-thermometer by [47], olivine starts to crystallize alone from the parental magma at~1300 • C, indicating that the parental magma should be high temperature basaltic magma. The high temperature characteristic for the parental magma may be related to mantle plume.
According to the paleomagnetic data, the Tarim Craton is believed to have been located along the northern margin of Rodinia adjacent to northwestern Australia during the early Neoproterozoic [60][61][62]. Some previous studies have suggested that the Rodinia supercontinent rifting began at ca. 780 Ma, which led to the final breakup of the supercontinent between 750 and 725 Ma [63,64]. The mantle plume activity at ca. 780-745 Ma has been reported in northern Namibia [65], North America [66,67], northwestern India [68], Australia [69], India [70] and South China [71] (Figure 10). As discussed above, the ca. 752 Ma Xingdi No.2 mafic-ultramafic intrusion in northeastern Tarim Craton could be genetically related to the mantle plume activity. In addition, a coeval magmatism has also been documented in the Tarim Craton, including the ca. 783 Ma basalts in the Sugetbtak, NW Tarim [72], ca. 773 Ma mafic dyke swarms [73], ca. 760 Ma bimodal volcanic rocks [25] in Kuluktag, NE Tarim, ca. 759 Ma mafic dyke swarms in Aksu, NW Tarim [73]. Thus, the ca. 780-750 Ma igneous events in Tarim are a part of the global magmatism during the breakup of the Rodinia supercontinent and related to the mantle plume activity. Although crustal contamination is evident in the Xingdi No.2 intrusion, hydrous melting of mantle peridotites is also important for the generation of SHMB-like magmas. In the Xingdi No.2 intrusion, biotite and amphibole in gabbro, norite, olivine gabbro, pyroxenite, and peridotite show the hydrous nature of the parental magma [59]. Meanwhile, high Ba/Zr ratios (1.11-25.72) also suggest the presence of fluids in the parental magma [7]. Hence, the SHMB-like parental magma might have originated by the partial melting of hydrous and enriched lithospheric mantle.

Implications for the Genesis of SHMB-Like Magmas in Continental Rifts
Extensive Neoproterozoic magmatism has produced abundant volcanic rocks and mafic-ultramafic intrusions in the Kuluktag Block of the Tarim Craton. Representative Beiyixi volcanic rocks that erupted at 755 ± 15 Ma represent a bimodal suite of basalt and dacite-rhyolite with a SiO2 gap between 55% and 65%. According to their petrological and geochemical features, these volcanic rocks formed in a continental rift related to mantle plume activity beneath the Rodinia supercontinent [25]. Analogously, the Xingdi No.2 intrusion displays similar negative εNd(t) values (−23 to −2.8), suggesting the partial melting of enriched lithospheric mantle source. According to olivine geo-thermometer by [47], olivine starts to crystallize alone from the parental magma at ~1300 °C, indicating that the parental magma should be high temperature basaltic magma. The high temperature characteristic for the parental magma may be related to mantle plume.
According to the paleomagnetic data, the Tarim Craton is believed to have been located along the northern margin of Rodinia adjacent to northwestern Australia during the early Neoproterozoic [60][61][62]. Some previous studies have suggested that the Rodinia supercontinent rifting began at ca. 780 Ma, which led to the final breakup of the supercontinent between 750 and 725 Ma [63,64]. The mantle plume activity at ca. 780-745 Ma has been reported in northern Namibia [65], North America [66,67], northwestern India [68], Australia [69], India [70] and South China [71] (Figure 10). As discussed above, the ca. 752 Ma Xingdi No.2 mafic-ultramafic intrusion in northeastern Tarim Craton could be genetically related to the mantle plume activity. In addition, a coeval magmatism has also been documented in the Tarim Craton, including the ca. 783 Ma basalts in the Sugetbtak, NW Tarim [72], ca. 773 Ma mafic dyke swarms [73], ca. 760 Ma bimodal volcanic rocks [25] in Kuluktag, NE Tarim, ca. 759 Ma mafic dyke swarms in Aksu, NW Tarim [73]. Thus, the ca. 780-750 Ma igneous events in Tarim are a part of the global magmatism during the breakup of the Rodinia supercontinent and related to the mantle plume activity. In summary, the current identification of SHMB-like magmas in a Neoproterozoic craton suggests that SHMB-like magmas could also be generated by AFC and partial melting of hydrated lithospheric mantle in continental rift settings related to the mantle plume activity. In summary, the current identification of SHMB-like magmas in a Neoproterozoic craton suggests that SHMB-like magmas could also be generated by AFC and partial melting of hydrated lithospheric mantle in continental rift settings related to the mantle plume activity.

Conclusions
It could be inferred that the Xingdi No.2 mafic-ultramafic intrusion in the Kuluktag Block in northeastern Tarim Craton was emplaced at ca. 752 Ma. Whole-rock major and trace element geochemistry data suggested that the intrusion was derived from magma enriched in LREE and LILE and depleted in HFSE. The intrusion has initial 87 Sr/ 86 Sr ratios in 0.7059-0.7130 and ε Nd (t) values from −23 to −2.8, which suggests an enriched lithospheric mantle source. It could be additionally argued that the Xingdi No.2 mafic-ultramafic intrusion was derived from low-Ca parental magmas that resembled Siliceous high-magnesium basalt (SHMB). Hence, SHMB-like mafic-ultramafic magmatism could also occur in continental rift settings in the presence of hydrous and enriched mantle sources.