Petrography and geochemistry of Permian basalts of the Cam Thuy formation and their relation to Song Da and Emeishan magmatic rocks

as low- and high-Ti basalt types. The low-Ti basalt is characterized by low Ti/Y (<500) ratio, low total FeO (<12 wt%), high SiO2 (48-53 wt%) and high Mg# (52-64); whereas the high-Ti basalt type has high Ti/Y (>500) ratio, high FeO (>12.7-16.4 wt%), low SiO2 (45-50 wt%) and high Mg# (51-61) (Xu et al., 2001). 2.2. Mafic, ultramafic magmatic rocks in the Song Da rift zone Permian mafic and ultramafic magmatic rocks are distributed widely in the Song Da rift zone. On the basis of their geochemical characteristics, the mafic and ultramafic Song Da magmatic rocks may be classified into four associations belonging to high- and low- Ti magmatic types. The high-Ti andesite- basalt association outcrops in the Cam Thuy and Son La areas; the high-Ti picrite-basalt- andesite association occurs in the Nam So area, and the trachyandesite, trachydacite and trachybasalt association appears in the Doi Nguyen Hoang, et al./Vietnam Journal of Earth Sciences 38 (2016) 375 Bu, Vien Nam and Nam Muoi areas. The low- Ti rock type, including mafic and ultramafic volcanic rocks belongs to the picrite (komatiite?)-basalt association in the Nam Muoi, Pa Uon and Deo Chen areas (Fig. 1a) (Polyakov et al, 1991; Balykin et al, 1996, 2010; Chung et al, 1997; Tran Trong Hoa et al., 1998, 2004, 2008, 2015; Hanski et al., 2004; Nguyen Hoang et al., 2004, 2016a). Low MgO, high FeO, CaO and Na2O (mafic components) contents in the high-Ti basalt in the Song Da rift zone, together with oceanic island basalt (OIB)-like trace elemental (and rare earth element) and Sr, Nd isotopic characteristics, suggest that the high- Ti basalt is derived from an enriched and fertile (asthenosphere) mantle source (Hofmann, 1997; Nguyen Hoang et al., 2016a). The low-Ti basalt type, in contrast, shows high MgO, low in the mafic component, and various trace element compositions, which reflect various geochemical features including those of island arc, mid-oceanic ridge basalt (N-MORB) and oceanic island basalt (OIB)-like type. The initial (255 Ma) Sr and εNd of the low-Ti volcanic rocks are highly variable, most certainly produced from highly heterogeneous lithospheric mantle source. In general, the Sr- Nd isotopic compositions of Song Da Permian magmatic rocks are anomalously enriched, suggesting the melt may have interacted with crustal materials (Nguyen Hoang et al., 2016a). 2.3. Formation ages of Emeishan and Song Da magmatism The radiometric age range of Song Da magmatism is controversial. Age dates obtained for Song Da magmas range from 257 ±24 Ma by Rb-Sr (Polyakov et al., 1996), of 258.5±1 by 40Ar/39Ar (Tran Trong Hoa et al., 2008), and 270 ± 21 Ma by Re-Os (for 12 komatiite samples) (Hanski et al., 2004), closely matching those of Emeishan basalts (Lo et al., 2002; Zhou et al., 2002) which suggest the bulk of activity occurred between c. 251 and 259 Ma. Balykin et al. (1996) reported Rb-Sr ages of 257 ±7.2 Ma for Song Da komatiitic clinopyroxene separates from the northwestern side of the rift zone. More recently, Tang et al. (2015) reported U-Pb ages of 256.2 ±1.4 Ma for zircons from a volcanic sequence in Binchuan, southern part of Emeishan LIP, and of 258.5 ±3.5 Ma for a Baimazhai picrite, northwest of Jinping, immediate northern tip of the Song Da volcanic zone. Therefore, ages between 255 and 258 Ma (late Permian) are currently taken for mafic and ultramafic magmatic rocks in the Song Da rift zone and several other nearby regional magmatic formations (e.g., Tran Trong Hoa et al., 2015; Usuki et al., 2015; Nguyen Hoang et al., 2016b). Until recently, no reliable age has been determined for the Cam Thuy volcanic formation. While expecting new radiometric ages, we temporarily adopt ages of 258-255 Ma for Cam Thuy formation for the following reasons: (1) its geological proximity to the Song Da magmatic rift zone, (2) high concentration of late Permian age (258 to 255 Ma) of magmatic formations in NW Vietnam (e.g. Tang Q. et al., 2015; Tran Trong Hoa et al., 2015; Usuki et al., 2015), and (3) the availability of stratigraphic correlation-based late Permian (P3) age for Cam Thuy volcanic formation (Tong Zuy Thanh and Vu Khuc, 2005, and references therein). 3. Sampling and analytical procedures Basalt sampling was conducted in the Lam Son area (Tho Xuan district, Thanh Hoa province), where massive basaltic lavas occur as large blocks along the Ho Chi Minh trail (Figs.1b-c; 2a-d), with thickness up to 600 m (Le Duy Bach et al., 1995). Layers of pyroclastic flows associated with the basaltic volcanism are exposed widely along the road Vietnam Journal of Earth Sciences, 38(4), 372-392 376 connecting Lam Son and Sao Vang airport (Tho Xuan town) (Fig. 1b, 2c-d). The pyroclastic products include welded, medium- grained tuff, intercalated with layers of coarse- to fine grained volcanic ash (Fig. 2d). Individual thickness varies from a few centimeters to approximately 30 cm, making the total visible thickness of the pyroclastic flows reach about 50 m (Fig. 2c-d). Samples were processed for microscopic study (Figs. 3a-f) and selection for geochemical and isotopic composition analysis. Figure 1. (b) Distribution scheme of Cam Thuy late Permian basalts in the Lam Son area (Tho Xuan, Thanh Hoa province), showing sampling sites (dashed rectangles). Simplified from 1:200,000 Geological Map of Viet Nam (after Le and Dang, 1995) The major element compositions were acquired from fused glass beads made by mixture of sample and lithium tetraborate (Li2B4O7) using a Bruker Pioneer X-Ray Fluorescence (XRF) analyzer at the Institute of Geological Sciences (VAST). Another set of samples was prepared for analysis using a Panalytical XRF at the Geological Survey of Nguyen Hoang, et al./Vietnam Journal of Earth Sciences 38 (2016) 377 Japan (GSJ) for comparison. A set of 12 GSJ geological standards were used as external data calibration and accuracy evaluation. The trace element and rare earth element compositions were acquired at the Geological Survey of Japan using an Agilent 8800 ICP-MS following procedures described in Ishizuka et al. (2003). The analytical accuracy as estimated from repeated measurements of GSJ standards is ±1% to ±6% (for Nd and Nb). Figure 2. Outcrops Cam Thuy basalts in the Lam Son area, Tho Xuan, Thanh Hoa province: a- b: massive basaltic lava flows; c-d: volcanic pyroclastic layers Sr, Nd and Pb isotopic ratios were measured at the GSJ using a VG-54 thermal ionization mass spectrometry (TIMS, GSJ) and at the University of the Ryukyus (Okinawa, Japan) using a Neptune multi-collector (MC)-ICP-MS. The element extraction chemistry was performed at the Geological Survey of Japan. The extraction procedures and TIMS Sr, Nd, Pb isotopic running parameters and analytical accuracy were described in Hoang and Uto (2006) and Nguyen Hoang et al. (2013). The data are shown in Table 1. Vietnam Journal of Earth Sciences, 38(4), 372-392 378 Figure 3. a, b: phyric olivine basalt (sample 040213/1) with subhedral olivine phenocrysts and needle-shaped plagioclase microlitic groundmass; c, d: aphyric alkaline basalt (sample 040213/6) containing microlites of olivine, clinopyroxene and plagioclase in the groundmass; e, f: phyric olivine basalt (sample 040213/8) with doleritic texture on the clinopyroxene and plagioclase microlitic groundmass; a, c, e: nichol (+); b, d, f: nichol (-); ruler is 0.5 mm Nguyen Hoang, et al./Vietnam Journal of Earth Sciences 38 (2016) 379 Table 1. Major, trace element and Sr-Nd-Pb isotopic compositions of the Cam Thuy late Permian basalts in the Lam Son area (Tho Xuan District, Thanh Hoa Province) Sample 040213-1 040213-2 040213-3 040213-4 040213-5 040213-6 040213-7A 040213-7B 040213-8 Rock type olivine basalt olivine basalt olivine basalt olivine basalt olivine basalt alkaline basalt olivine basalt olivine basalt tholeiitic basalt SiO2 47.70 47.92 48.84 48.33 48.58 47.99 48.64 49.89 50.33TiO2 2.81 2.68 2.74 2.70 2.70 2.67 2.69 2.61 2.66Al2O3 15.92 15.31 14.91 14.76 15.04 15.36 15.31 14.92 15.16FeO* 12.43 12.98 12.86 13.26 12.76 12.70 12.41 12.14 11.87 MnO 0.19 0.20 0.22 0.20 0.19 0.22 0.20 0.20 0.19 MgO 6.17 5.78 5.63 6.00 6.09 5.63 6.31 6.15 5.65 CaO 11.50 11.60 11.20 11.26 11.47 10.95 11.63 11.17 11.16 Na2O 1.76 2.31 2.41 2.27 2.31 3.62 1.70 1.73 1.93K2O 1.12 0.83 0.79 0.82 0.49 0.48 0.72 0.81 0.65P2O5 0.41 0.39 0.40 0.40 0.40 0.39 0.39 0.38 0.39Mg# 52.5 49.8 49.4 50.2 51.5 49.7 53.1 53.0 51.5 Ti (ppm) 16830 16065 16419 16190 16178 16029 16137 15662 15949 K (ppm) 9306 6909 6555 6776 4029 3967 5947 6742 5361 Na2O+K2O 2.88 3.14 3.20 3.09 2.79 4.10 2.42 2.54 2.58 CIPW Quartz 2.05 Orthoclase 6.62 4.92 4.67 4.82 2.87 2.82 4.23 4.80 3.82 Albite 14.85 19.53 20.35 19.25 19.54 24.38 14.39 14.61 16.34 Anorthite 32.24 28.95 27.57 27.64 29.23 24.26 32.02 30.56 30.80 Nepheline 3.38 Diopside 18.45 21.81 21.22 21.41 20.93 23.02 19.26 18.64 18.38 Hypersthene 12.60 5.99 11.90 10.31 13.91 21.96 24.38 22.67 Olivine 8.98 12.83 8.17 10.53 7.50 16.18 2.13 Magnetite Ilmenite 5.33 5.09 5.20 5.13 5.12 5.08 5.11 4.96 5.05 Apatite 0.94 0.90 0.94 0.93 0.92 0.91 0.91 0.89 0.90 Table 1. (continued)  Sample 040213-1 040213-3 040213-5 040213-6 040213-7A CT150616-C CT150616-G CT150616-J CT150616-L Rock type olivine basalt olivine basalt olivine basalt alkaline basalt alkaline basalt olivine basalt tholeiite alkaline basalt olivine basalt Rb 23.26 22.66 9.72 9.43 12.95 21.01 10.26 14.67 6.61 Sr 322.14 370.49 292.18 739.52 360.62 327.80 256.51 450.26 517.99 Y 33.23 31.26 32.61 30.99 32.54 27.29 29.16 28.91 33.58 Zr 177.22 161.86 176.63 167.93 172.07 155.53 164.68 185.71 191.21 Nb 22.94 20.84 21.76 21.32 22.18 18.95 19.10 27.98 28.38 Cs 0.57 0.86 1.17 0.57 0.53 0.50 0.08 0.26 0.11 Ba 261.04 302.32 130.16 149.33 153.96 285.94 124.37 601.53 530.67 La 25.09 25.26 27.15 23.88 26.52 20.80 25.04 31.95 34.49 Ce 58.15 54.61 57.73 53.57 58.42 48.15 53.99 76.70 82.07 Pr 7.10 6.64 6.88 6.65 7.09 5.81 6.75 9.75 10.18 Nd 31.54 29.58 31.07 29.51 31.82 25.28 30.13 43.50 45.50 Sm 7.01 6.70 6.83 6.55 7.02 5.73 6.66 8.72 9.32 Eu 2.40 2.25 2.22 2.31 2.33 2.11 2.44 3.43 3.74 Gd 6.94 6.55 6.55 6.42 6.90 5.60 6.45 7.78 8.66 Tb 1.11 1.04 1.06 1.04 1.09 0.89 1.00 1.12 1.24 Dy 6.26 5.73 6.01 5.86 6.19 5.11 5.68 6.17 6.55 Ho 1.21 1.15 1.19 1.16 1.23 1.00 1.08 1.12 1.25 Er 3.31 3.19 3.08 3.15 3.12 2.69 2.86 2.93 3.25 Tm 0.47 0.44 0.45 0.44 0.44 0.40 0.40 0.40 0.47 Yb 2.90 2.71 2.77 2.68 2.81 2.35 2.43 2.51 2.65 Vietnam Journal of Earth Sciences, 38(4), 372-392 380 Lu 0.40 0.41 0.39 0.39 0.39 0.33 0.35 0.36 0.39 Hf 4.82 4.38 4.55 4.46 4.64 4.12 4.44 4.94 5.01 Ta 1.79 1.61 1.66 1.64 1.69 1.46 1.47 2.20 2.19 Pb 38.67 2.86 2.89 3.05 3.27 3.38 2.92 4.64 2.95 Th 3.64 3.35 3.39 3.38 3.45 2.80 2.79 3.47 3.54 U 0.90 0.74 0.80 0.70 0.90 0.55 0.63 0.25 0.87 V 324.98 326.07 330.74 336.07 357.86 337.43 321.15 406.91 401.09 Cr 150.18 148.35 161.59 152.94 157.01 261.50 208.18 95.69 102.91 Ni 95.44 92.71 95.19 91.88 92.33 147.54 110.99 75.38 80.74 Table 1. (continued)  Sample 040213-1 040213-3 040213-5 040213-6 040213-7A CT150616-C CT150616-G CT150616-J CT150616-L Rock type olivine basalt olivine basalt olivine basalt alkaline basalt alkaline basalt olivine basalt tholeiite alkaline basalt alkaline basalt 87Rb/86Sr 0.228 0.202 0.107 0.042 0.115 0.179 0.112 0.091 0.0356 87Sr/86Srm 0.706105 0.706269 0.705815 0.705986 0.705926 0.705989 0.706274 0.705655 0.70546287Sr/86Sr(255Ma) 0.705279 0.705536 0.705429 0.705836 0.705507 0.705340 0.705869 0.705325 0.7053327147Sm/144Ndm 0.142 0.140 0.136 0.137 0.136 0.140 0.137 0.124 0.126143Nd/144Ndm 0.512568 0.512578 0.512464 0.512570 0.512573 0.512615 0.512598 0.512572 0.512584 Nd -1.36 -1.17 -3.40 -1.32 -1.27 -0.45 -0.78 -1.29 -1.05 Nd(255Ma) 0.43 0.67 -1.42 0.61 0.69 1.40 1.17 1.08 1.23 143Nd/144Nd(255Ma) 0.512332 0.512345 0.512237 0.512341 0.512345 0.512382 0.512370 0.512365 0.512373 T(DM) 1.23E+09 1.18E+09 1.34E+09 1.16E+09 1.14E+09 1.1E+09 1.1E+09 9.8E+08 9.9E+08U (ppm) 0.90 0.74 0.80 0.70 0.90 0.548 0.627 0.247 0.865 Th (ppm) 3.65 3.35 3.39 3.38 3.45 2.802 2.794 3.470 3.542 Pb (ppm) 5.3 4.8 3.6 4 1.2 3.382 2.916 4.638 2.945 238U/204Pb 1.39 15.72 16.83 13.94 16.68 9.847 13.061 3.240 17.848 235U/204Pb 0.01 0.12 0.12 0.10 0.12 0.072 0.096 0.024 0.131 232Th/204Pb 5.84 73.70 73.56 69.56 66.26 52.027 60.165 46.976 75.520 206Pb/204Pbm 18.309 19.291 19.439 19.348 19.436 19.189 19.271 18.674 19.282207Pb/204Pbm 15.606 15.647 15.638 15.626 15.647 15.632 15.631 15.594 15.622208Pb/204Pbm 38.357 39.725 39.813 39.787 39.791 39.583 39.625 39.424 39.681 206Pb/204Pb(255Ma) 18.253 18.670 18.774 18.797 18.777 18.792 18.744 18.543 18.562 207Pb/204Pb(255Ma) 15.603 15.615 15.604 15.597 15.613 15.611 15.604 15.587 15.585 208Pb/204Pb(255Ma) 38.284 38.808 38.897 38.921 38.966 38.922 38.861 38.827 38.722 4. Analytical results 4.1. Petrographic characteristics The analyzed samples are mostly phyric olivine basalts and subsidiary alkaline basalts with major phenocrysts of olivine constituting about 3 to 5 vol.% (Figs. 3a-f). The groundmass is intersertal, micro-doleritic, containing microlites of clinopyroxene, plagioclase, ore minerals, (rare) olivine and volcanic glass (Figs. 3c-d). Olivine phenocrysts are euhedral or subhedral, tablet- or irregular shaped, with sizes ranging from 0.1 by 0.3 mm to 0.3 by 0.5 mm (Fig. 2a-c). Some alkaline basalts contain iddingsite, a product of altered olivine. Some basalts are aphyric with the groundmass comprising microlites of clinopyroxene and plagioclase (Fig. 3c-d, e-f). The welded tuff contains fragments of altered lavas cemented by volcanic ash (Figs. 4c-d). The volcanic ash is coarse- or fine grained (Fig. 2d), disoriented (Fig. 5a) or layered and oriented (Fig. 5b). 4.2. Major element compositions Cam Thuy basalts (in the Lam Son area) with SiO2, varying from 47.70 to 50.33 wt.% and total alkaline oxides (Na2O+K2O) from 2.42 to 4.10 wt.%, are distributed in the subalkaline field, while only a few samples plot in the alkaline field (Fig. 6). This features Nguyen Hoang, et al./Vietnam Journal of Earth Sciences 38 (2016) 381 are expressed in terms of CIPW normative mineralogical compositions showing that only one sample (040213/6) contains nepheline (Ne)-normative of 3.38 wt.% (Table 1), and all remaining samples are subakaline rock type (containing olivine (Ol)-normative) or tholeiitic basalt, containing quartz (Q)- and hypersthene (Hy)-normative (Table 1). Figure 4. a-b (sample CT150616-E) and c-d (samplCT150616-F) tuff composed of basaltic fragments of variable sizes, cemented by volcanic ash; layered and oriented; fragments are partially chloritized, carbonatized and albitized lava Figure 5. a: (sample CT150616-G); b: (CT150616-H) volcanic ash, coarse- or fine-grained, with or without orientation and zonation Vietnam Journal of Earth Sciences, 38(4), 372-392 382 MgO contents of the Cam Thuy basalts vary from 5.8 wt.% to 7.2 wt.%, plotted between MgO values of the Song Da and Emeishan basalts (Fig. 7). In a similar way, except for having much higher CaO contents than those of the Song Da and Emeishan basalts, the other oxides such as SiO2, TiO2, FeOt, Na2O and K2O of the Cam Thuy basalts plot between fields of the Song Da and Emeishan magmatic rocks and almost overlap those of high-Ti basalt type (Fig. 7). Figure 6. Basaltic TAS (total alkalis vs. SiO2) classification (after Le Bas et al., 1986) showing samples of Cam Thuy basalt (this study); Song Da basalts (Hoang et al., 2016) and Emeishan samples (Xu et al., 2001, 2004). Cam Thuy basalts mostly overlap those of Song Da high-Ti basalt series and plot between high- and low-Ti of Emeishan magmas Figure 7. Correlation between MgO (wt.%) and major silicate oxides showing Emeishan and Song Da basalts with CaO values being lower compared with the Cam Thuy basalts, while TiO2 contents of Cam Thuy basalts being comparable to Song Da high-Ti basalts (empty triangle) but lower than Emeishan high-Ti basalts (cross); symbols for Song Da low-Ti: empty diamond, Emeishan low-Ti magma: x 4.3. Trace element compositions Primitive mantle normalized trace element (Hofmann, 1988) and chondrite normalized rare earth element (Anders and Grevesse, 1989) distribution patterns of the Cam Thuy basalts are shown (Figs. 8a, 9a, respectively) along with Song Da high-Ti basalts (Son La Pass and nearby areas) (Figs. 8b, 9b) and geological standards made from volcanic rocks of various tectonic settings (Figs. 8c, 9c) for comparison (N. Hoang et al., 2016a). The trace and rare earth element distribution patterns of Cam Thuy basalts show smooth decrease from left to right (Figs.8b, 7b), almost overlapping trace and rare earth element distribution curve of the Song Da high-Ti basalts (Figs. 8b, 9b) (data from Hoang et al., 2016a). The basalt samples both of Song Da and Cam Thuy show patterns, which are closely analogous to oceanic island basalts (e.g. JB-1a and BHVO- 2 in Figures 8c and 9c). Nguyen Hoang, et al./Vietnam Journal of Earth Sciences 38 (2016) 383 Figure 8. Primitive mantle normalized trace element patterns of Cam Thuy basalts (a) as compared to Song Da high-Ti basalts (b); geological standards (BIR-1: Indian MORB; BHVO-2: Hawaiian OIB; JB-1a: continental intraplate basalt; JA-2: arc andesite) are shown for comparison (c). Normalizing data are after Hofmann (1988). Note that the trace element distribution patterns of Cam Thuy basalts being closely analogous to Song Da high-Ti basalts are essentially oceanic island basalt-like Figure 9  Figure 9. Chondrite normalized rare earth element (after Anders and Grevesse, 1989) distribution patterns of Cam Thuy basalts (a); shown are Song Da high-Ti basalts (b) and geological standards (c) for comparison (see Figure 8 caption). The rare earth element configuration curves of Cam Thuy basalts are comparable to Song Da high-Ti basalts while vastly different from the low-Ti basalt type, having a N-MORB shape (BIR-1) Vietnam Journal of Earth Sciences, 38(4), 372-392 384 4.4. Isotopic compositions The initial 87Sr/86Sr ratios of the Cam Thuy basalts calculated for 255 Ma after Song Da basaltic eruption age (e.g. Balykin et al., 1996; Tran Trong Hoa et al., 2008, after Tang Q. et al., 2015) range from 0.70528 to 0.70584. These initial isotopic ratios accompanied by εNd(255Ma) varying from 0.69 to -1.41, are plotted between two fields of depleted mantle (DM) and enriched continental crust (Fig. 10), overlapping the field of the Song Da high-Ti basalt and covering partially that of Emeishan high-Ti basalt. The initial87Sr/86Sr ratios of the Cam Thuy basalts are lower as compared with Emeishan basalts and much lower compared with Song Da low-Ti basalts having 255 Ma initial 87Sr/86Sr ratios from 0.7055 to 0.7115 accompanied by εNd(255Ma) changing between 7.5 and -9 (Xu et al., 2001; Nguyen Hoang et al., 2004, 2016a). Figure 10. Plots of initial (255 Ma) 87Sr/86Sr isotopic ratios versus Nd(t) of Cam Thuy magmatic rocks. Emeishan and Song Da basalts are shown for comparison. Fields of depleted mantle (DM) and enriched mantle type 1 and 2 (EM1, EM2) and Hawaiian OIB (data from Norman and Garcia, 1999) are shown for reference. Note Cam Thuy basaltic distribution field includes that of Song Da high-Ti basalts away from Song Da low-Ti and Emeishan magmatic rocks The initial 206Pb/204Pb ratios (255 Ma) of the Cam Thuy basalts plotted against207Pb/204Pbi and 208Pb/204Pbi ratios are shown in Fig. 11a-b. The lead isotopic ratios plot close to the depleted mantle (DM) field (represented by Pacific MORB), overlapping the field of the Song Da high-Ti basalt and separating from the field of the low-Ti basalt. The latter, with higher 206Pb/204Pbi, 207Pb/204Pbi and 208Pb/204Pbi, trend toward enriched fields. Figure 11. Plots of initial (255Ma) 206Pb/204Pb isotopic ratios versus (a) 207Pb/204Pb and (b) 208Pb/204Pb of Cam Thuy basalts as compared to Song Da basalts; EM2, EM1 (enriched mantle type 1 and 2), Hawaiian OIB (Norman and Garcia, 1999) and (depleted) Mid-Ocean Ridge Basalt (N-MORB) are shown for reference. Symbols as in Figure 10. Northern Hemisphere Reference Line (NHRL) illustrates enriched (above) and depleted mantle domains 5. Discussion 5.1. Tectonic setting of Cam Thuy basalts Igneous rocks formed in different tectonic settings may be affected by in situ materials at different levels. For example, at a subduction zone, magmatic melts may be affected by oceanic crustal material, brought to the mantle by the subducting slab, in the form of hydrous Nguyen Hoang, et al./Vietnam Journal of Earth Sciences 38 (2016) 385 fluid, which appears to be one of the causes of lowering melting temperature of the mantle wedge. Using Zr/Y plotted against Zr in the tectonic setting discrimination diagram (after Pearce and Norry, 1979), the Cam Thuy basalts, Emeishan and most of Song Da magmatic rocks plot in the field of intraplate magmas (Fig. 12). Some Song Da low-Ti basalts plot in the subduction field, making them different from the Cam Thuy lavas. Figure 12. Tectonic discrimination diagram (after Pearce and Norry, 1979) showing Cam Thuy basalts plotting in intraplate basalt field comparable to Emeishan basalts and many of the Song Da magmatic rocks; some Song Da low-Ti basalts plot in field of island arc. Symbols as in Figure 10 Basaltic melts on the way to the surface may interact with crustal rocks. Crustal material interaction may result in increasing Ba, Rb, Th, etc., contents relative to Nb, Ta, Zr, in basaltic melts, forming positive correlation between (for example) Ba/Nb against SiO2 and negative correlation with MgO (or Mg#). Cam Thuy basalts have low SiO2 and K2O contents and show oceanic island basalt-like trace element distribution patterns (Figs. 8, 9), indicating minimal crustal involvement. Besides, correlation between Ti/Zr against Ba/Zr and Rb/Zr shows that most of the Cam Thuy basalts plot along the mantle array separating the mantle source and continental crust fields (Figs. 13a, b, 14) (after Hoang and Uto, 2003). Note that some of the Song Da low-Ti basalts plot in the crustal field, suggesting, to some extent, crustal contamination. Taken together with the Sr and Nd chondritic isotopic compositions (Fig. 10) and OIB-like trace element distribution patterns (Figs. 8 and 9), the Cam Thuy basalts reported here are certainly free from crustal contamination. These geochemical features are also observed for Song Da high-Ti basalts reported elsewhere (e.g. Hoang et al., 2016a), suggesting close similarity in source of origin and melt generation parameters between these two basaltic magmas. Figure 13. Correlation between (a) Ti/Zr and Ba/Zr and (b) Ti/Zr and Rb/Zr for Cam Thuy basalts as compared to Song Da basalts relative to depleted mid-ocean ridge basalt mantle (N-MORB), the mantle array (representing by oceanic island basalt: OIB, after Kogiso et al. (1997) and Frey et al. (2000), and primitive mantle after Hofmann (1988). Modified after Hoang and Uto (2003). See text for explanations Vietnam Journal of Earth Sciences, 38(4), 372-392 386 Figure 14. Plots of Zr/Y against Nb/Y of Cam Thuy basalt along with Song Da and Emeishan magmatic rocks; fields of OIB (North Arch, data from Frey et al., 2000), Hawaiian OIB (after Norman and Garcia, 1999), N-MORB and Arc magmas are shown for reference. Dashed lines signifying ranges of mantle-derived magmas are compiled from world literature Figure 15. Plots of Olivine (Ol) – Plagioclase (Pl) – Quartz (Qz) for representative computed Cam Thuy primitive melt compositions (after Walker et al., 1979) compared with experimental isobaric liquidi from Hirose and Kushiro (1993) and Kushiro (1996). Accordingly, Cam Thuy basalts segregated from their magma sources between 22.5 and 28 Kb with potential temperatures of about 1400ºC to 1450ºC (after Hoang and Flower, 1998) 5.2. Mantle sources and melt forming conditions Geochemical compositions of lithospheric mantle-derived rocks commonly have high MgO and SiO2 contents but especially low FeO, CaO, Na2O and K2O (termed as mafic component), as a result of previous partial melting events (Turner and Hawkesworth, 1995). Generally, the lithospheric mantle- derived mafic melts are low in trace element compositions, especially highly incompatible elements such as Rb, Ba, K and light rare earth elements such as La, Ce and Nd. However, depending on the timing of melting events (long enough for radioactive decay to form sizable daughter products), cumulating melts from deeper mantle or extracting melts due to local melting could lead to enrichment or depletion of trace element or isotopic composition that may be different (Menzies et al., 1987; Carlson and Irving, 1994; Ionov and Hofmann, 2007). Several studies have suggested that lithospheric mantle is refractory in mafic component and “dry”, therefore it is difficult for partial melting to occur. However, Gallagher and Hawkesworth (1994) showed that water liberation from water-rich minerals (such as amphibole) (Ionov and Hofmann, 2007) along with hot mantle upwelling following a lithospheric extension event may facilitate melting processes to occur easily (Turner and Hawkesworth, 1995; Nguyen Hoang and Flower, 1998). Cam Thuy basalt are relatively low in MgO and SiO2 contents, high FeO, CaO and Al2O3 although moderate in Na2O and K2O (Figs. 6, 7; Table 1). As mentioned above the trace element and rare earth element distribution patterns of Cam Thuy basalts are essentially oceanic island basalt (OIB)-like (Figs. 8, 9) in terms of BHVO-2, a Hawaiian OIB standard and JB-1a, a continental intraplate basalt, whose melts are viewed as asthenosphere- derived. There are several approaches to estimating magma segregation depths. These include: (1) mathematical inversion of melt compositions of specified source and its sub-solidus residua, Nguyen Hoang, et al./Vietnam Journal of Earth Sciences 38 (2016) 387 assuming fractional or batch melting within a polybaric melt column (e.g. McKenzie and O’Nions, 1991; Scarrow and Cox, 1995; Turner and Hawkesworth, 1995), (2) interpolation from H2O-saturated and unsaturated experimental studies of fertile and refractory peridotite (e.g. Hoang and Flower, 1998). Assuming mantle H2O contents beneath NW Vietnam to be minimal we have made best estimates of pressure, temperature and melt fraction by comparing primitive melt compositions with anhydrous or near- anhydrous experimental studies (e.g. Hirose and Kushiro, 1993; Kushiro, 1990, 1996), the approach adopted by Hoang and Flower (1998). The primitive melt compositions used were interpolated from high MgO basalts, with possible effects of olivine fractionation minimized. Assuming that realistic Mg/(Mg + Fe2+) ratios of olivine in mantle residua approximate 0.70, having equilibrated with segregated partial melts. This was achieved by adding small (0.1%) increments of olivine to eruptive composition with MgO> 6wt%, assuming olivine being the sole liquidus phase observed (Yamashita et al., 1996; after Walker et al., 1979). It was likewise assumed that magmas with mafic phenocrysts of around Fo89–90, match residual mantle olivine, according to an olivine-melt Kd (FeO/MgO) value of 0.30 and Fe2O3 to be 0.15 of FeO* (Roeder and Emslie, 1970). Plotted in the pseudo-ternary (normative) Ol-Pl-Qz system (Fig. 15; after Walker et al., 1979) estimated primitive Cam Thuy melt compositions are shown for comparison with experimental isobaric, partial melts of spinel/garnet lherzolite (Hirose and Kushiro, 1993; Kushiro, 1996). In the case of Cam Thuy, the calculated melts may reflect a decrease in melt fraction with increasing pressure (22.5 → 28 Kbar) suggesting a polybaric partial melting column between depths of 65 and 85 km. According to the approach adopted, comparison of primitive magmas to experimental studies of primitive basalts (Kushiro, 1996) and relatively fertile peridotites (HK-66 in Hirose and Kushiro, 1993), potential temperatures of about 1400ºC →1450ºC appear to be reasonable with primitive melts segregating at pressures between 22.5 and 28 Kbar (after Hoang and Flower, 1998), and the most plausible melting mechanism being adiabatic decompression of ductile asthenosphere. 5.3. Geochemical - petrographic comparison between Cam Thuy, Song Da and Emeishan magmatic rocks Song Da magmatic rocks are categorized into high-Ti and low-Ti types. Low-Ti rock type is lower in Nb, Ta, lighter rare earth elements and higher in MgO and SiO2 relative to the high-Ti magma type (Balykin et al., 2010; Xu et al., 2001, 2004). Trace element and isotopic characteristics of the Cam Thuy basalts are essentially oceanic island basalt (OIB)-like, approximated to Song Da high-Ti basalts (Figs. 8-9). Their initial (255 Ma) Pb, and especially Sr and Nd isotopic ratios are chondritic (Figs. 10, 11). The above geochemical and isotopic characteristics along with the computed melting pressures and temperatures Fig. 15), for Cam Thuy basalts and Song Da (in the Son La area) high-Ti basalt indicate that they are most likely derived from an enriched and possibly fertile mantle source (Fig. 14). These features also differentiate the above magmatic rocks from the Song Da low-Ti rock type, which has been considered as heterogeneous lithospheric mantle- derived melts (Figs. 10-14). An analogous type has yet to be discovered among the Cam Thuy volcanic rocks. The Emeishan high- and low-Ti magmatic types are geochemically heterogeneous, showing highly variable major and trace element contents, covering both Cam Thuy and Song Da magmatic distribution fields (this study; Xu et al., 2001; Nguyen Hoang et al., 2016a). Their Sr-Nd isotopic compositions Vietnam Journal of Earth Sciences, 38(4), 372-392 388 are also highly variable (Figs. 10, 12-14), trending from depleted to enriched field; although the enrichment, is not comparable to Song Da low-Ti magmatic type, leaving the latter most enriched magma type among the three basaltic regions (Figs. 10-11). A study of picrites in the Emeishan large igneous province suggests a secular change from melting of a peridotite to a garnet pyroxenite mantle source produced, respectively, from the low- and high-Ti magma end-members. Moreover, the similarity in Sr and Nd isotopic compositions (87Sr/86Sri~ 0.7045 and Nd(t)~ 1.7) of the two magma types may reflect a source in the sub-continental lithospheric mantle rather than the convective asthenosphere or a deep mantle plume (Kamenetsky et al., 2012). It has been long believed that Song Da (and Cam Thuy) magmatic association is part of the Emeishan LIP having been extruded southeastward about 700 km from its SW Chinese site along the Red River Shear zone or an extruding channel between the Red River and Song Ma Fault zone (Fig. 1a) following the India - Asia collision about 30 Ma (Chung et al., 1997; Wang et al., 1997; Lan et al., 2000; Tran et al., 2008; after Tapponnier et al., 1982, 1986; Le Loup et al., 1995; Gilder et al., 1996). While there is not much physical evidence supporting the extrusion mechanism (see Flower et al., 1998; Cung and Geissman, 2013); the fact that the Song Da high- and low-Ti magma types may not be genetically related (Nguyen Hoang et al., 2016a; after Kamenetsky et al., 2012), and that the distribution of Cam Thuy magmatic formation on both sides of the Song Ma fault zone needs further investigation. 6. Concluding remarks Cam Thuy Permian volcanic rocks exposed in the Lam Son area (Tho Xuan, Thanh Hoa province) comprise thick basaltic lava flows and pyroclastic layers, with a total thickness exceeding a few hundred meters. The rock types are mostly olivine-phyric basalt and a minor amount of alkaline basalt and (quartz-normative) tholeiitic basalt. The computed melt compositions of the Cam Thuy basalts compared with experimental partial melting of a relatively fertile and enriched spinel lherzolite (HK-66, Hirose and Kushiro, 1993) show their melt segregation pressures from 22.5 to 28 Kb (ca. 65 to 85 km) and corresponding temperatures from 1400ºC to 1450ºC. Their trace element characteristics are enriched oceanic island basalt (OIB)-like. The major and trace element features along with their chondritic Sr-Nd (and Pb) isotopic compositions match those of the Song Da (and some of Emeishan) high-Ti magma type, and in conjunction with their geographical proximity, suggesting that they may share a same fertile and thermally anomalous mantle source. Various low-Ti basaltic and picritic rock types, viewed as melts generated from heterogeneously depleted and refractory sources in the sub-continental lithospheric mantle, appear popular in the Song Da and Emeishan magmatic associations (e.g. Chung and Jahn, 1995; Xu et al., 2001, 2004; Kamenetsky et al., 2012; Tran Trong Hoa et al., 2015) but have yet to be discovered in the Cam Thuy magmatic formation. This difference may reflect discrete source heterogeneity and melting mechanism among the three Permian volcanic fields. Acknowledgments This study is supported by (Vietnam) National Foundation for Science and Technology Development (NAFOSTED) under grant number 105.05-2012-22 (NH). The Institute of Geological Sciences, VAST, is thanked for financing the major and trace element analysis. Tran Thanh Hai, Hanoi University of Mining and Geology, and Phan Van Hung (IGS) are thanked for providing information on the outcrops. Critical Nguyen Hoang, et al./Vietnam Journal of Earth Sciences 38 (2016) 389 comments by two anonymous reviewers help improve the manuscript significantly from an earlier version. Proof reading and suggestions by Rolando Peña (University of the Philippines, Quezon) are gratefully acknowledged. References Anders, E., Grevesse, N., 1989. Abundances of the elements: meteorite and solar. Geochimica et Cosmochimica Acta 53, 197-214. Balykin, P.A, Polykov G.V., Petrova T.E., Hoang Huu Thanh, Tran Trong Hoa., Ngo Thi Phuong, Tran Quoc Hung, 1996. 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