Journal of Earth Science  2019, Vol. 30 Issue (3): 571-584   PDF    
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Post-Collisional, Potassic Volcanism in the Saga Area, Western Tibet: Implications for the Nature of the Mantle Source and Geodynamic Setting
Hui Zhao 1,2,3, Jingsui Yang 3, Fei Liu 3, Jian Huang 4, Li Zhang 5     
1. Harbin Institute of Technology, Southern University of Science and Technology, Harbin 150001, China;
2. Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen 518055, China;
3. Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
4. Yunnan Tuocheng Industrial Co., Kunming 650051, China;
5. Key Laboratory of Geochemical Cycling of Carbon and Mercury in the Earth's Critical Zone, Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences, Langfang 065000, China
ABSTRACT: Post-collisional potassic and ultrapotassic volcanic rocks are widely distributed across the Tibetan Plateau, and they are considered to be indicators of evolving mantle dynamics. A suite of potassic basalts younger than 55 Ma from the Saga area of western Tibet has been reported. The geochemical characteristics of these rocks distinguish themselves from other potassic-ultrapotassic volcanic rocks in Tibet, such as positive Nb, Ta, and Ti anomalies and strong enrichment in large ion lithophile elements (LILE), suggesting that phlogopite, rutile and/or sphene might have originated from the mantle source. These basalts are also characterized by a very wide range of 87Sr/86Sr ratios (0.709 043-0.711 915) and relatively high 143Nd/144Nd ratios (0.512 426-0.512 470, εNd=-4.60 to -3.87). We propose a petrogenetic model for the Saga potassic rocks in which the lithospheric mantle source was infiltrated by a volatilerich (H2O, CO2) and low-degree silicate melt derived from the asthenosphere in the Middle to Late Proterozoic. After the initial Indo-Asian collision, Neo-Tethyan slab breakoff resulted in the partial melting of the previously metasomatized lithospheric mantle and the formation of the Saga potassic rocks. It is likely that the eruption of these volcanic rocks lasted at least 10 Ma.
KEY WORDS: potassic volcanic rocks    basalt    Sr-Nd isotopes    Saga area    western Tibet    
0 INTRODUCTION

Both of the immense Tibetan Plateau and the surrounding mountain ranges are formed during the Indo-Asian collision in the Early Cenozoic. Critical information for understanding the tectonic evolution of orogenic belts and the mechanism of Cenozoic climate changes have been preserved (Molnar et al., 1993; Allégre et al., 1984). The evolving tectonic regimes of mountain belts are typically marked by changes in the composition of associated magmatism. The characteristics of extensive magmatic activities make Tibet a natural laboratory for studying such tectonomagmatic activities (Zhu et al., 2015; Mo, 2011; Chung et al., 2009, 2005; Mo et al., 2009, 2006; Mo and Pan, 2006; Turner et al., 1996, 1993). North-dipping subduction of Tethyan ocean crust during the Mesozoic gives rise to an Andean-type magmatic arc, which is now exposed as the Gangdese batholith associated with calc-alkaline volcanic rocks in the Lhasa terrane (Ma et al., 2017; Li et al., 2016; Coulon et al., 1986; Debon et al., 1986; Dewey and Bird, 1970). Paleocene–Eocene magmatism (65–41 Ma), which is represented by Linzizong volcanic rocks, is considered to be formed in a syn-collisional setting (Mo et al., 2009, 2003). However, other scholars thought that it was the product of slab rollback followed by slab breakoff (Wang et al., 2015; Zhu et al., 2015; Chung et al., 2009, 2005; Wen et al., 2008). Continuous east-west extension and uplift of the Tibetan Plateau in the Miocene were associated with ultrapotassic and adakitic magmatism (Guo et al., 2015; Chung et al., 2005, 2001; Hou et al., 2004; Williams et al., 2004; Blisniuk et al., 2001; Miller et al., 1999; Turner et al., 1996, 1993).

Potassic-ultrapotassic volcanic rocks, with ages ranging from 50 Ma to present (Fig. 1a; Ding et al., 2003), are widely distributed across the Tibetan Plateau. They provide a potentially important window into the deep thermal and compositional structure of the India-Asia Orogen. These rocks generally exhibit depletions in Nb, Ta and Ti, strong enrichments in incompatible elements, high Sr isotopic ratios and low Nd isotopic ratios. Potassium-rich melts are generally thought to originate from the mantle source with the presence of phlogopite in the melting assemblages (Foley, 1993, 1992). However, recent modeling experiments show that phlogopite-free mantle is an alternative source for potassium-rich volcanic rocks (Wang et al., 2015). To date, two popular models have been proposed for the potassic-ultrapotassic rocks: (ⅰ) mantle melting following convective removal of the lower part of thickened lithospheric mantle (Chung et al., 2009, 2005, 1998; Hou et al., 2004; Houseman and Molnar, 1997; Turner et al., 1996, 1993; England and Houseman, 1988; Houseman et al., 1981); and (ⅱ) melting related to intercontinental subduction (Guo et al., 2015; Wang et al., 2015; Zhao et al., 2009; Ding et al., 2003; Mahéo et al., 2002; Tapponnier et al., 2001; Deng, 1998; Meyer et al., 1998; Harrison et al., 1993; Arnaud et al., 1992). Williams et al. (2004) suggested that magma generation in the northern Tibet was triggered by convective removal of the lower part of the lithospheric mantle, whereas in southern Tibet mantle melting is related to lithospheric erosion associated with slab breakoff.

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Figure 1. (a) Index map showing the location of the Tibet Plateau in Asia; (b) simplified geological map of the Tibetan Plateau and the surrounding areas (modified from Chung et al., 2005; Yin and Harrison, 2000). MBT. Main boundary thrust; MCT. main central thrust; STDS. South Tibet detachment system; ITS. Indus-Tsangpo suture; BNS. Bangong-Nujiang suture; JS. Jinsha suture; AKMS. Ayimaqin-Kunlun-Muztagh suture; ATF. Altyn Tagh fault; KF. Kunlun fault; KJFZ. Karakorum-Jiali fault zone; RRF. Red River fault; WCF. Wang-Chao fault; GCT. great counter thrust; ZGT. Zhongba-Gyangze thrust

Zhao et al. (2015) reported potassic volcanic rocks younger than 55 Ma in the Saga area of western Tibet, the geochemical characteristics distinguished them from other potassic- ultrapotassic volcanic rocks in Tibet, such as positive Nb, Ta, and Ti anomalies and strong enrichment in large ion lithophile elements (LILE). However, the genesis and geodynamic setting of these potassic rocks are unclear. In this study, we provide new Sr-Nd isotopic data and zircon U-Pb ages for Saga potassic rocks, and then discuss their origin in combination with previously reported data.

1 GEOLOGICAL BACKGROUND

The Tibetan Plateau is a collage of continental fragments with intervening suture zones (Fig. 1). The Yarlung Zangbo suture zone (YZSZ), lying south of the Tibetan Plateau, separates the Indian Block from the Asian continent in the north. The Himalaya, which lies south of the suture zone, contains three tectonic slices (Tethyan Himalaya, the Greater Himalaya, and the Lesser Himalaya) bounded by three north-dipping, Late Cenozoic fault systems (Yin and Harrison, 2000; Allégre et al., 1984). The Tethyan Himalaya, consisting of Late Precambrian to Eocene metasedimentary and sedimentary rocks deposited along the passive continental margin of Northern India, lies between the YZSZ and the South Tibetan detachment system. Cretaceous to Lower Tertiary marine sedimentary sequences are only locally preserved in this region (Zhu et al., 2005; Ding et al., 2003; Willems et al., 1996). The Greater Himalaya is bounded by the main central thrust and the South Tibetan detachment system, and consists of Late Proterozoic to Early Cambrian metasedimentary rocks. The Lesser Himalaya, which is the structurally lowest slice, is bounded by the main boundary thrust and the main central thrust (Yin and Harrison, 2000). North of the YZSZ lies the Lhasa terrane, which represents the southernmost part of the Asian continent. The Jurassic to Early Tertiary trans- Himalayan batholiths, which are associated with the volcanic sequences of the Linzizong Formation, are exposed along the southern margin of the Lhasa terrane (Ji et al., 2009; Mo et al., 2008; Wen et al., 2008; Lee et al., 2007; Chung et al., 2005; Dong et al., 2005; Zhou et al., 2004; Mo et al., 2003; Coulon et al., 1986; Schrer et al., 1984a, b). Between the YZSZ and the Gangdese arc is the Xigaze forearc basin, which mainly contains shallow to bathyal flysch deposits.

The Yarlung Zangbo ophiolitic belt, which is the main component of the suture zone, consists of nearly continuous, but tectonically disrupted E-W-trending mafic-ultramafic massifs (Hébert et al., 2012; Aitchison et al., 2000). Early studies have interpreted these ophiolites as relics of mid-ocean ridge oceanic lithosphere (Nicolas et al., 1981), but recent investigations have shown that most of them have suprasubduction zone affinity (Dilek and Furnes, 2011; Bédard et al., 2009; Guilmette et al., 2008; Aitchison et al., 2000). Minor amounts of ocean island basalt (OIB) are associated with some of the ophiolites, particularly in the western part of the suture zone (Liu et al., 2015, 2013; Yang and Dilek, 2015; Dai et al., 2012; Geng et al., 2010; Xia et al., 2008; Zhu et al., 2008; Zhang et al., 2005; Xia and He, 1995; Xia and Cao, 1992; Xia, 1991). The western segment of the YZSZ (west of Saga) can be divided into two sub-belts, namely the Dajiweng-Saga sub-belt in the north, and the Daba-Xiugugabu sub-belt in the south. These two sub-belts are separated by the 60-km-wide Zhada-Zhongba micro-continental block, which mainly consists of discontinuous Precambrian basement rocks and Paleozoic and Triassic strata (Sun and Hu, 2012; Huang et al., 2006). The Zhada-Zhongba micro-continental block has a Tethyan-Himalayan affinity as demonstrated by detrital zircon and fossil evidence (Sun and Hu, 2012).

2 SAMPLE DETAILS

The Saga potassic rocks are located ~40 km northwest of Saga City and form a NW-trending belt 1–5 km wide and ~30 km long. They overlie the Eocene Jialazi Formation to the northeast and are in fault contact with the Permian Quga Formation to the southwest (Fig. 2). The Quga Formation is a part of the cover sequence of the Zhada-Zhongba micro-continental block and consists mainly of schist, slate, metasandstone and marble. The Jialazi Formation is a part of the Xigaze forearc basin sequence and mainly consists of mudstone and limestone.

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Figure 2. Detailed geological map of the Saga area in western Tibet (modified after Hebei Bureau of Geology and Mineral Resources Exploration, 2015)

The Saga potassic rocks are porphyritic with less than 20% phenocrysts (mainly as clinopyroxene and minor plagioclase) setting in microcrystalline matrix composed of plagioclase, pyroxene and Fe-Ti oxides. The clinopyroxene is titaniferous diopside (TiO2: 1 wt.%–2 wt.%) partly replaced by chlorite. And the plagioclase is mostly replaced by albite (Fig. 3).

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Figure 3. Photomicrographs showing the microstructures of the Saga potassic rocks

All the Saga potassic rocks are characterized by low SiO2 (40.86 wt.%–50.08 wt.%), but high K2O (1.76 wt.%–2.61 wt.%), Al2O3 (14.81 wt.%–18.43 wt.%) and TiO2 (2.44 wt.%–2.93 wt.%)(Fig. 4; Zhao et al., 2015). The relatively high TiO2 contents may reflect small degrees of partial melting that would have concentrated incompatible elements such as Ti in melts (Schaefer et al., 2000). Mg-numbers [Mg#=100×Mg2+/(Mg2++Fe2+)] range between 41 and 66, significantly lower than values of primitive magmas (68–72) (Frey et al., 1978), suggesting substantial fractionation of clinopyroxenes and olivines (Fig. 5).

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Figure 4. (a) K2O+Na2O vs. SiO2 and (b) K2O vs. Na2O plots for Saga volcanic rocks. Also plotted are potassic-ultrapotassic volcanic rocks from the Lhasa terrane and north Tibet. The data for Saga volcanic rocks are from Zhao et al. (2015); the data for the potassic-ultrapotassic volcanic rocks in the Lhasa terrane and north Tibet are from Zhao et al. (2009), Guo et al. (2006) and Ding et al. (2003). Classification scheme according to Le Maitre (1989)
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Figure 5. Harker diagrams showing the variations of selected major oxides (wt.%) and trace elements (ppm) against Mg# [=100×Mg2+/(Mg2++Fe2+)] in the Saga samples. The data are from Zhao et al. (2015)

The Saga potassic rocks are enriched in large ion lithophile elements (LILE) (Rb, Ba, Th) and high field strength elements (HFSE) (Zr, Hf, Nb, Ta, Th) with chondrite normalized (Ce/Yb)N ratios of 7.3–14.2 (Zhao et al., 2015). Despite their potassium-rich nature, none of the analysed samples display marked enrichment of K relative to other incompatible elements, such as La, Nb and Ta. In addition, Saga volcanic rocks, which show little or no depletion of HFSE relative to LILE, completely differ from other potassic-ultrapotassic rocks in the Lhasa terrane and northern Tibet (Fig. 6).

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Figure 6. (a) Chondrite-normalized rare earth element diagrams; (b) primitive mantle-normalized trace element diagrams. The OIB and normalizing values are from Sun and McDonough (1989). The data for the Saga potassic volcanic rocks are from Zhao et al. (2015). The data for the potassic-ultrapotassic volcanic rocks in the Lhasa terrane and North Tibet are from Zhao et al. (2009) and Guo et al. (2006), respectively
3 ANALYTICAL METHODS 3.1 Zircon U-Pb Dating

Zircons for U-Pb dating were separated from a fresh sample of potassic lava (14YL-16-21) with no veins. The sample was rinsed repeatedly to remove dirt, dried, crushed and passed through an 80-mesh sieve. Water elutriation, magnetic separation, and heavy-liquid separation were used to separate the zircons. Selected zircon grains were mounted in an epoxy resin disk and polished to about half their thickness. Cathodoluminescence, as well as reflected and transmitted images, were used to observe the textures of each zircon grain, and to select suitable positions for analysis. Simultaneous analysis of U-Pb ages and in-situ trace element concentrations by LA-ICP-MS were carried out in the Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China, Hefei. The standard silicate glass NIST (610, 612 and 614) was used to optimize the system and NIST 610 glass was selected as a standard for the trace element analyses. Zircon 91500 was used as the external standard for the U-Pb dating. The final isotopic ratios and ages of the zircons were calculated using the program by Andersen (2002). The weighted mean 206Pb/238U ages were calculated using the ISOPLOT program (Ludwig, 2003). The data are shown in Table 1.

Table 1 Zircon LA-ICP-MS U-Pb data of the potassic volcanic rocks in Saga area (14YL-16-21)
3.2 Sr-Nd Isotopes

We selected eight fresh samples for Sr-Nd isotope analysis. Samples were cut into chips from the fresh interiors of the samples, cleaned ultrasonically in ultrapure water three times, dried and then crushed into pieces ~2–3 mm across. These were leached in purified 0.1 N HNO3 at room temperature for 24 hours to minimize the influence of surface alteration or weathering, especially for the Sr isotope analyses. The dried samples were ground in an agate mortar to pass through a 200 mesh sieve. Both the agate mortar and pestle were cleaned with high purity clean sand (SiO2) powder and 6 N pure HCl in an ultrasonic bath, and then rinsed with ultrapure water. In order to prevent contamination between samples, the cleaning process was repeated prior to grinding each sample.

The powdered samples (~0.1 g) were dissolved in mixed acid (HF-HNO3-HClO4) in teflon capsules for 7 days at room temperature. Rb, Sr and REE fractions were separated from the solutions using AG50W×8(H+) cationic ion-exchange resin columns. Sm and Nd were separated from the solutions using AG50W× 8(H+) cationic ion-exchange resin columns and ɑ-HIBA leachate. The isotope measurements were carried out on a VG 354 mass spectrometer with 5 collectors at the Center of Modern Analysis, Nanjing University. Reproducibility and accuracy of the Sr and Nd isotope analyses were determined by running the Standard Reference Material NBS 987 for Sr and Laboratory Standard La Jolla for Nd. Repeated analyses of NBS 987 yielded a mean 87Sr/86Sr value of 0.710 233±0.000 004 (the recommended value is 0.710 340±0.000 006), whereas the La Jolla standard yielded a mean 143Nd/144Nd value of 0.511 863±0.000 006 (the recommended value is 0.511 860±0.000 020). Analytical errors are shown as two sigma of the mean in Table 2. The single analytical blank was less than 3×10-9 for Sr and less than 6×10-11 for Nd. Details of the analytical procedures are given in Wang et al. (2007). εNd is calculated relative to 143Nd/144NdCHUR=0.512 638 as shown below

Table 2 Sr-Nd isotope compositions of the Saga potassic volcanic rocks
$ {\varepsilon _{{\rm{Nd}}}} = \left[ {\frac{{{{\left({\frac{{{}^{143}{\rm{Nd}}}}{{{}^{144}{\rm{Nd}}}}} \right)}_{{\rm{Sample}}}}}}{{{{\left({\frac{{{}^{143}{\rm{Nd}}}}{{{}^{144}{\rm{Nd}}}}} \right)}_{{\rm{CHUR}}}}}} - 1} \right] \times {10^4} $ (1)

Nd model ages relative to the depleted mantle (TDM) are calculated from

$ {T_{{\rm{DM}}}}{\rm{ = }}\frac{1}{\lambda } \cdot \ln \left\{ {1 + \frac{{\left[ {{{\left({\frac{{{}^{143}{\rm{Nd}}}}{{{}^{144}{\rm{Nd}}}}} \right)}_{{\rm{DM}}}} - {{\left({\frac{{{}^{143}{\rm{Nd}}}}{{{}^{144}{\rm{Nd}}}}} \right)}_{{\rm{Sample}}}}} \right]}}{{{{\left({\frac{{{}^{147}{\rm{Sm}}}}{{{}^{144}{\rm{Nd}}}}} \right)}_{{\rm{DM}}}} - {{\left({\frac{{{}^{147}{\rm{Sm}}}}{{{}^{144}{\rm{Nd}}}}} \right)}_{{\rm{Sample}}}}}}} \right\} $ (2)

where the decay constant λ=6.54×10-12, 143Nd/144NdDM=0.513 15 and 147Sm/144NdDM=0.213 5.

The 147Sm/144Nd and 87Rb/86Sr ratios for the samples are from measured isotope-dilution abundances of Sm, Nd, Rb and Sr.

4 RESULTS 4.1 Zircon U-Pb Dating

Zhao et al. (2015) reported U-Pb ages of zircons separated from one sample of the Saga potassic rocks (13YL-94-10). This sample yielded a mean zircon U-Pb age of 55.1±1.5 Ma.

As part of the current investigation, we selected another fresh sample (14YL-16-21) for additional zircon separation and U-Pb dating (Fig. 7). These zircons are mainly subhedral to anhedral, short-prismatic grains with length-width ratios of 2 : 1. Besides, they have well-developed oscillatory zoning with Th/U ratios of 0.48 to 1.46, indicating they are typical magmatic zircons (Fig. 7). Although precautions were taken to exclude contamination, the dating results on 25 grains also span a wide range from 2 005 to 43 Ma (Table 1; Fig. 8). Thirteen of the youngest analysed grains yield an age range from 66 to 43 Ma with a mean age of 45.6±1 Ma. Therefore, the age of Saga potassic rocks spans a range of 55 to 45 Ma.

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Figure 7. The CL images and ages of 25 zircons from the Saga potassic volcanic rocks (14YL-16-21)
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Figure 8. (a) U-Pb concordia diagram of the potassic volcanic rocks in Saga; (b) relative probability and distribution plot of 206Pb/238U ages; (c), (d) U-Pb concordia and weighted average diagrams of the zircons ranging in age from 43 to 66 Ma (sample 14YL-16-21)
4.2 Radiogenic Isotope Ratios

Because the investigated sample (14YL-16-21) has a mean age of 45.6 Ma, the time-corrected Sr, Nd and Pb ratios are similar to the present-day values. Therefore, we have used uncorrected values in the following discussion (Zhao et al., 2009).

The investigated samples show a very wide range of 87Sr/86Sr values (0.709 043–0.711 915) that are lower than those of the potassic-ultrapotassic volcanic rocks in the Lhasa terrane, but slightly higher than those of potassic lavas in northern Tibet (Table 2; Fig. 9). Besides, the samples have higher 143Nd/144Nd ratios (0.512 426–0.512 470) and εNd values of -4.60 to -3.87 than most of potassic rocks in Tibet (Table 2; Fig. 9). In the plot of 143Nd/144Nd vs. 87Sr/86Sr (Fig. 9a), the samples lie far from any lines connecting BSE and EMII and extend to high 87Sr/86Sr values. In addition, Sr isotopic ratios of the samples show a positive correlation with Th contents (Fig. 10). Therefore, we infer that these samples are contaminated by continental crust, resulting in high 87Sr/86Sr. Nd single-stage model ages (TDM) relative to the depleted mantle are from 1.25 to 0.86 Ga.

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Figure 9. (a) Correlation diagram of 87Sr/86Sr and 143Nd/144Nd; (b) correlation diagram of 87Sr/86Sr and εNd. Data source: the fields for BSE, DM, EMI, EMII, HIMU are after Zindler and Hart (1986). The three major geochemical end-members are from Mo et al. (2006). Fields for other related volcanic rocks are based on Zhao et al. (2009). The data for the Lhasa terrane and North Tibet are from Zhao et al. (2009), Guo et al. (2006) and Ding et al. (2003). The data for the Saga lavas are from this study. DM. Depleted mantle; EM. enriched mantle; BSE. bulk silicate earth; PREMA. prevalent mantle; HIMU. high U mantle
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Figure 10. Correlation diagram of 87Sr/86Sr with Th content for the Saga potassic rocks. The data are from Zhao et al. (2015)
5 DISCUSSION 5.1 Comparison with Potassic-Ultrapotassic Volcanic Rocks in the Lhasa Terrane and North Tibet

The high LILE/HFSE ratios and low concentrations of Nb, Ta and Ti of the potassic-ultrapotassic rocks in the Lhasa terrane and

North Tibet indicate that they originate from a highly enriched mantle source (Guo et al., 2015, 2006; Chung et al., 2009, 2005; Zhao et al., 2009, 2001; Luo et al., 2006; Mo and Pan, 2006; Mo et al., 2006, 2003; Williams et al., 2004; Yu et al., 2004; Ding et al., 2003; Lai et al., 2003; Turner et al., 1993). Turner et al. (1996) suggested that the potential metasomatic agents were melts and fluids derived from subducted sedimentary rocks. Guo et al. (2006) proposed that subducted sedimentary rocks played an important role in the enrichment of asthenospheric mantle beneath the northern part of the plateau. Ding et al. (2003) advocated that Paleocene–Eocene alkaline volcanism in the Qiangtang terrane and calc-alkaline volcanism in the Lhasa terrane were attributable to Early Tertiary rollback of an initially shallow slab of northward-subducting Tethyan oceanic lithosphere. In this model, the Tibetan mantle lithosphere is metasomatized and melted during Cenozoic subduction of ancient continental lithosphere. Zhao et al. (2009) demonstrated that the Indian Plate is subducted underneath the Lhasa terrane leading to a highly contaminated lithospheric mantle source.

Compared with potassic-ultrapotassic volcanic rocks in the Lhasa terrane and North Tibet, the Saga potassic rocks show low LILE/HFSE ratios and high Ti abundances. These significant differences reflect different enrichment mechanisms of the mantle sources.

5.2 The Nature of the Mantle Source

The abundances of MgO, Ni and Cr in the Saga potassic rocks are 4.96 wt.%–7.95 wt.%, 86 ppm–154 ppm, and 80 ppm–140 ppm, respectively, implying that they were potentially derived from a primitive magma generated from mantle peridotite. Thus, the chemical characteristics of the potassic lavas could provide information about the deep mantle source. Even though high Sr isotopic ratios of the Saga potassic rocks may be attributed to crustal contamination as mentioned above, the high contents of LILE and HFSE imply that the Saga potassic rocks originate from an enriched mantle source.

Because of similar bulk solid/melt partition coefficients (Sun and McDonough, 1989; Hofmann, 1988), Nb and U cannot be significantly fractionated during partial melting. However, dehydration of subducted slabs usually causes significant fractionation between Nb and U. The Saga potassic rocks have markedly higher Nb/U ratios (Nb/U≈67) compared to those of subduction zone fluids (Nb/U≈0.15–0.3; Ayers, 1998; Keppler, 1996), indicating little involvement of slab-derived, hydrous fluids in the source mantle of the Saga lavas. The LILE are preferentially transported by hydrous fluids, whereas the HFSE are more likely to remain in phases such as rutile or ilmenite, which are preserved in the subducted slab (Ryerson and Watson, 1987). Thus the enrichment of LILE and HFSE of Saga potassic basalts in spidergrams (Fig. 6b) further confirms little contribution of hydrous fluids derived from subducted slabs to the mantle source.

Niu et al. (2011) advocated that volatile-rich (H2O, CO2) and low-degree silicate melts originating from the asthenosphere can rise upward and metasomatize the bottom of lithospheric mantle, leading to the formation of an enriched mantle. This metasomatism can occur beneath both continental and oceanic lithosphere (Donnelly et al., 2004). Such metasomatized mantle is usually thought to be the main source of enriched components in ocean island basalts (OIB) (Niu et al., 2011; Niu, 2008; Pilet et al., 2008). Ocean island basalts are commonly characterized by enrichment of HFSE and LILE (Sun and McDonough, 1989). Except for high K content, Saga potassic rocks share the similar features with them (Fig. 6). Accordingly, we propose that volatile-rich and low-degree silicate melts may be the major enriching agent of the mantle source for the Saga potassic rocks.

Nd isotopic ratios can be used to estimate the approximate age of source enrichment which is the time since the enriched metasomatized mantle was isolated from the underlying convecting asthenosphere (Gibson et al., 1995). The 143Nd/144Nd ratios of the Saga potassic basalts give TDM model ages of 0.86–1.25 Ga (Table 2). These calculations assume that Sm/Nd fractionation occurred predominantly during the precursor enrichment event, rather than during the melt formation. Thus, we propose that the enrichment of the magma source region occurred in the Middle to Late Proterozoic.

Infiltration of a fertile silicate melt into a depleted lithospheric mantle host would have affected the mineralogy of the mantle source. Even though it is difficult to determine the mineral assemblage in the source, some indication of the presence or absence of phases can be obtained from geochemical data. The relatively high heavy rare earth elements (HREE) abundances and flat HREE distribution patterns in chondrite-normalized diagrams of the Saga potassic rocks and the Yb contents (greater than 10 times chondritic abundances) preclude garnet as a residual mineral in the mantle source region. Thus, melting is inferred to have taken place in the spinel stability field at 65–80 km in the lithospheric mantle (Williams et al., 2004). The K-rich nature suggests that a potassic phase existed in the source region. Rb/Ba ratios of all analysed samples are lower than those of the depleted mantle (0.09, Sun and McDonough, 1989), which requires a mineral phase that has preferentially retained Rb relative to Ba. The depletion of Rb relative to Ba in the primitive mantle-normalized incompatible element patterns of the Saga rocks also supports this inference. These characteristics are consistent with the existence of phlogopite, which has a higher partition coefficient for Rb than Ba, although a K-rich amphibole (richterite) cannot be excluded (Foley et al., 1996; Green, 1994). The K2O/TiO2 ratio of phlogopite is greater than 1, whereas the ratios for the Saga alkaline basalts equal to 1. This implies that the Ti-bearing minerals such as rutile or sphene should be residual. Therefore, we infer that phlogopite, rutile and/or sphene possibly existed in the mantle source.

5.3 Geodynamic Setting of the Saga Potassic Rocks

Most studies suggest that the initial Indo-Asian collision occurred at about 50–55 Ma (Ding et al., 2016; Zhu et al., 2015; Meng et al., 2012; van Hinsbergen et al., 2012). Based on the observations of spatial, temporal, and compositional changes of magmatic activities in the Lhasa terrane, Zhu et al. (2015) inferred that at ~53 Ma the Neo-Tethyan lithosphere broke off. This inference is also supported by other evidences, such as the occurrence of abundant 52–47 Ma mafic enclaves and dykes, and the presence of ca. 52.5 Ma bimodal volcanic rocks (Zhu et al., 2015). As discussed above, the age of the Saga potassic volcanic rocks ranges from 55 to 45 Ma, which is coeval with, or slightly earlier than that of the Tethyan slab breakoff. Based on the available data, we propose a simplified petrogenetic model for the potassic rocks in Saga within the framework of subducted slab breakoff (Fig. 11).

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Figure 11. Simplified geodynamic model for the Saga post-collisional potassic volcanic rocks (modified from Chung et al., 2005). The volatile-rich (H2O, CO2) and low-degree silicate melts released from the asthenosphere rose and metasomatized the bottom of lithospheric mantle. After the initial Indo-Asian collision, the Neo-Tethyan slab broke off, resulting in partial melting of the early metasomatized lithospheric mantle and formation of the Saga potassic lavas

In the Middle to Late Proterozoic, the volatile-rich (H2O, CO2) and low-degree silicate melts released from the asthenosphere rose and metasomatized the bottom of lithospheric mantle. After the initial Indo-Asian collision, the breakoff of Neo- Tethyan slab resulted in partial melting of the early metasomatized lithospheric mantle and formation of the Saga potassic volcanic rocks. The ages of Saga potassic rocks span a range of 55 to 45 Ma. It is likely that the eruption of these volcanic rocks lasted at least 10 Ma.

Post-collisional potassic-ultrapotassic volcanic rocks are widely distributed across the Tibetan Plateau. However, the occurrence of mildly potassic rocks with significant enrichment of HFSE and LILE in the post-collisional epoch in Tibet is not common. Some of the remaining challenges need to be addressed in future studies.

6 CONCLUSIONS

(1) The ages of Saga potassic lavas span a range of 55 to 45 Ma. It is likely that the eruption of these lavas lasted at least 10 Ma.

(2) Saga potassic rocks have a wide range of 87Sr/86Sr ratios (0.709 043–0.710 915) and relatively high 143Nd/144Nd (0.512 426–0.512 470, εNd= -4.60 to -3.83).

(3) Volatile-rich and low-degree silicate melt is the most appropriate enrichment agent for the mantle source of the Saga potassic rocks. The mantle source may contain phlogopite, rutile, and/or sphene.

(4) Geochemical, geochronological and Sr-Nd isotopic data support a petrogenetic model for Saga potassic rocks in which the enrichment of the mantle source is caused by the infiltration of a volatile-rich (H2O, CO2) and low-degree silicate melt released from the asthenosphere during the Middle to Late Proterozoic. After the initial Indo-Asian collision, the Neo-Tethyan slab breakoff resulted in partial melting of the early metasomatized lithospheric mantle and formation of the Saga potassic rocks.

ACKNOWLEDGMENTS

We thank Profs. Paul Robinson, Sunlin Chung, Sandro Conticelli, Zhidan Zhao and an anonymous reviewer for comments and suggestions on the manuscript. This study was supported by the Ministry of Science and Technology of China (No. 2014DFR21270), China Geological Survey (Nos. DD20160023-01, DD20160022-01), and the National Natural Science Foundation of China (Nos. 41720104009, 41672063, 41773029). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1228-7.


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