Late Jurassic plutonic suites with diverse geochemical compositions are widespread in central Tibet, recording the evolution of the Bangong-Nujiang Tethyan Ocean. This paper presents new zircon U-Pb ages, whole-rock major- and trace-element compositions, and zircon Hf isotopic data for the intrusive plagiogranite body formed within the Labuco ophiolite complex of the Bangong-Nujiang suture zone. The Labuco plagiogranites yield a laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon age of ca. 167 Ma, indicating a Late Jurassic age of crystallization. They are characterized by high Al2O3 and Na2O, and low K2O, TiO2, and MgO contents, with positive zircon εHf(t) values similar to those of high-Al plagiogranites. These features lead us to propose that partial melting of arc-type basalts at the base of the accretionary complex (at depths of >40 km) could be another petrogenetic explanation for the high-Al plagiogranites within the ophiolite complex. This study and earlier publications indicate that the Late Jurassic plutonic suites are characterized by diverse geochemical compositions and were formed in a subduction setting controlled by northward subduction of Bangong-Nujiang Tethyan oceanic lithosphere. Magmatic underplating, triggered by this subduction, played a key role in the vertical crustal growth in central Tibet.
Leucocratic intrusions are commonly present in ophiolite complexes, forming a volumetrically minor but ubiquitous constituent of the ophiolitic sequence. Their compositions include diorites, tonalites, trondhjemites, and granodiorites, and they have been collectively referred to as “oceanic plagiogranite” (Coleman and Peterman, 1975; Coleman and Donato, 1979). All plagiogranites are generally similar in essential primary mineralogy (predominantly plagioclase with quartz) and geochemical characteristics, with very low K, Rb, and Sr contents, high Na contents, and high K/Rb and Rb/Sr ratios. Knowledge of the processes whereby silicic melts are generated from mafic crust helps us to understand the genesis of continental crust (Rollinson, 2009). The study of plagiogranite petrogenesis thus provides an opportunity to examine evolutionary processes pertaining to both continental and oceanic crust. Furthermore, despite their minor volume, plagiogranites commonly contain zircons, which can be used to constrain the formation ages and origins of magma through the results of U-Pb and Hf isotopic analyses (e.g., Samson et al., 2004; Jiang et al., 2008).
A discontinuous belt of ophiolite extends east-west for more than 2000 km in central Tibet, marking the remnants of the Bangong-Nujiang Tethyan Ocean (Coulon et al., 1986; Pearce and Deng, 1988; Yin and Harrison, 2000; Guynn et al., 2006; Kapp et al., 2007; Zhu et al., 2011, 2016; Pan et al., 2012). Over the last three decades, these ophiolitic rocks have been intensively studied because their petrogenesis is closely associated with the tectonic evolution of the Tibetan Plateau (Girardeau et al., 1984; Zhang and Yang, 1985; Zhou et al., 1997; Shi et al., 2004, 2008; Wang et al., 2016; Liu et al., 2016). Plagiogranites have recently been found within the Bangong-Nujiang ophiolite belt. Previous research has suggested that Jurassic plagiogranites, which are characterized by low Al2O3 contents (<15 wt%), were generated by partial melting of amphibolitized gabbroic rocks at high temperatures, in ductile shear zones (Fan et al., 2010; Sun et al., 2011; Yin et al., 2015).
This study focuses on plagiogranites within the Labuco ophiolite complex in the Bangong-Nujiang ophiolitic belt. New zircon U-Pb ages, whole-rock major- and trace-element compositions, and zircon Hf isotopic data are presented for plagiogranites of the Labuco area. Their petrogenesis, tectonic setting, and source regions are considered, and the implications of these findings for the geodynamic evolution and crustal growth of central Tibet are discussed.
REGIONAL GEOLOGY AND SAMPLE DESCRIPTIONS
The Tibetan Plateau, located in SW China, is an amalgamation of terranes separated by major suture zones marked by ophiolitic fragments and mélanges (Yin and Harrison, 2000). From north to south, the plateau is divided into the Kunlun, Qiangtang, Lhasa, and Himalaya continental terranes (blocks; Pan et al., 2012; Wang et al., 2013; Zhu et al., 2011, 2013, 2016). The Bangong-Nujiang suture zone, which separates the Lhasa and Qiangtang terranes in central Tibet, is marked by ophiolite fragments and thick sequences of Jurassic flysch, mélange, and volcanic rocks (Yin and Harrison, 2000; Zhu et al., 2016). The Bangong-Nujiang ophiolite belt consists of blocks that occur within the suture zone. Much work had been done on the geochemical and isotopic compositions of these blocks to improve our understanding of their genesis and the origin and evolution of the oceanic basin (Zhang and Yang, 1985; Shi et al., 2004, 2008; Wang et al., 2016). The occurrence of ophiolitic rocks, combined with existing geochemical and geochronological data, indicates that the Bangong-Nujiang suture zone represents a Mesozoic ocean basin.
The Labuco ophiolite complex, ∼80 km northwest of Gerze County, is located in the middle segment of the Bangong-Nujiang suture zone. It includes three unnamed NW-SE–trending ophiolite blocks ∼10 × 1 km in surface area (Fig. 1). Ophiolitic members observed in the field are composed mainly of serpentinized peridotites and massive basalts, with lesser amounts of radiolarian cherts. There has, however, been relatively little research on the Labuco ophiolite complex. The formation and emplacement ages of the ophiolite complex remain unclear. The age of granitoids that intruded into the peridotites indicate that the Labuco ophiolite complex was emplaced before the Late Jurassic.
The plagiogranites investigated in the present study were the first to be found within the Labuco ophiolite complex. They intruded the surrounding metamorphic peridotite unit (Fig. 2A) as a pluton over an area of ∼100 × 20 m (Fig. 2B). The samples are uniform and broken at the surface within the plagiogranite pluton. Samples were collected in order to determine the ages and geochemical characteristics of the plagiogranites. The locations of the samples are indicated in Figure 1C. One sample was selected for zircon U-Pb dating analysis, and 10 samples were collected for geochemical analysis. The plagiogranites have a massive texture and hypidiomorphic structure, and they are medium to fine grained (Figs. 2C and 2D). They are composed mainly of plagioclase and quartz, with accessory zircon and apatite. The grain size of the main mineral phases is 0.2–0.6 mm.
ANALYTICAL METHODS AND RESULTS
Cathodoluminescence (CL) images were produced at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China. Zircon U-Pb dating was performed by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) at the Geological Laboratory Centre, China University of Geosciences, Beijing, using procedures detailed by Yuan et al. (2004). Major-element compositions and trace-element analyses were determined at the Geological Laboratory Centre, China University of Geosciences, Beijing, using ICP–optical emission spectrometry (ICP-OES) for major-element compositions and ICP-MS for trace-element analyses, respectively. Hf isotopic compositions were determined using multicollector ICP-MS (MC-ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, following the method of Hu et al. (2012).
U-Pb Zircon Geochronological Data
Zircons (23) were selected from sample DT16 of the Labuco plagiogranite for U-Pb analysis by LA-ICP-MS. Most zircon grains exhibited oscillatory zoning under CL imaging (Fig. 3). U-Pb results for this sample are given in Table 1 and plotted in a concordia diagram in Figure 3. One analysis yielded a discordant older 206Pb/238U age of 179 Ma, which is interpreted as representing an inherited zircon. Six other discordant analyses clustered within an age range of 183–168 Ma. The remaining 16 analyses yielded 206Pb/238U ages of 174–162 Ma. These zircons are generally subhedral grains or crystal fragments with average lengths of 60–100 μm and an aspect ratio of 2:1. In situ analyses revealed significant variations in Th and U concentrations (46–321 ppm and 116–1501 ppm, respectively), with high and variable Th/U ratios of 0.14–0.49 (with one value of 0.08), indicating a magmatic origin. They form a single cluster on the concordia curve, with a weighted-mean 206Pb/238U age of 167.4 Ma (mean square of weighted deviates [MSWD] = 2.7; 95% confidence), which is considered the best estimate of the timing of zircon crystallization within the Labuco plagiogranites.
Whole-Rock Geochemical Data
Ten samples from the Labuco area were analyzed for whole-rock major- and trace-element compositions, with results provided in Table 2. Well-preserved igneous textures and relatively low loss on ignition (LOI) values indicate that the samples have undergone only weak alteration, and therefore the geochemical data largely reflect the primary composition of the rocks.
The Labuco plagiogranites exhibit high Al2O3 contents (16.9–20.5 wt%) and Mg# values (68–76), and relatively low TiO2 (0.03–0.16 wt%), MgO (0.53–1.09 wt%), and Fe2O3 (0.39–1.15 wt%) contents. All samples have very low K2O (0.70–1.45 wt%) but high Na2O (9.40–11.5 wt%), typical of oceanic plagiogranites (e.g., Coleman and Donato, 1979). The samples plot as trondhjemite in the K2O-Na2O-CaO and albite-anorthite-orthoclase (Ab-An-Or) geochemical classification diagrams (Figs. 4A and 4B). They are alkaline rocks (Fig. 4C) with high Na2O + K2O contents, and they are metaluminous, with aluminum saturation index (A/CNK = Al2O3/[CaO + Na2O + K2O]) values of 0.80–1.07 (Fig. 4D).
Chondrite-normalized rare earth element (REE) plots (Fig. 5A) indicate marked depletions in heavy REEs (HREEs; e.g., Yb = 0.50–2.61 ppm; [La/Yb]N = 3–18) and positive Eu anomalies (Eu/Eu* = 1.71–5.52). Primitive-mantle–normalized multi-element patterns indicate enrichment in large ion lithophile elements (LILEs; e.g., Rb, Th, and U) and depletion in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti; Fig. 5B).
Zircon Hf Isotope Data
Hf isotopic measurements were performed on the same LA spots or age domains as those used for U-Pb dating, guided by CL images. Ten zircons from sample DT16 were selected for Hf isotope analysis, with the results presented in Table 3 and Figure 6.
The zircons yielded positive initial εHf(t) values of +0.22 to +1.86, corresponding to single-stage model ages (TDM1) of 847–760 Ma and crustal model ages (TDMC) of 1074–978 Ma.
Comparison with Coeval Granitic Rocks in Central Tibet
Previous studies have indicated that the extensive Late Jurassic plutonic suites in central Tibet are compositionally diverse and include normal I-type granites (J.X. Li et al., 2014; S.M. Li et al., 2014; Liu et al., 2014; Wu et al., 2016), highly fractionated I-type granites (S.M. Li et al., 2014; Liu et al., 2014), K-rich adakitic rocks (Hao et al., 2016; Wu et al., 2016), Na-rich adakitic rocks (Fan et al., 2016; Y.L. Li et al., 2016; S.M. Li et al., 2016), and low-Al plagiogranites (Fan et al., 2010; Yin et al., 2015). Furthermore, coeval dioritic enclaves are widespread in these Late Jurassic granitoids. Their various geochemical and isotopic characteristics, origins, and petrogeneses are compared in Table 4 and Figures 5, 6, and 7. Normal I-type granites, highly fractionated I-type granites, and K-rich adakitic rocks are characterized by high K2O contents and negative εHf(t) values, reflecting a magmatic source of ancient mafic lower crust, with or without thickening (J.X. Li et al., 2014; S.M. Li et al., 2014; Liu et al., 2014; Hao et al., 2016; Wu et al., 2016). Na-rich adakitic rocks are considered to be derived from subducted oceanic crust (Fan et al., 2016; Y.L. Li et al., 2016; S.M. Li et al., 2016), while low-Al plagiogranites are generated by partial melting of amphibolitized gabbroic rocks in ductile shear zones (Yin et al., 2015); the enclaves are believed to be derived from partial melting of mantle (J.X. Li et al., 2014; S.M. Li et al., 2014).
As shown in Table 4, the Labuco plagiogranites are different from the coeval granites and enclaves. The Labuco plagiogranites are obviously enriched in aluminum, which is reported in central Tibet for the first time here. On the basis of Al2O3 contents, the plagiogranites in central Tibet are classified into two types: low- and high-Al plagiogranites. The Labuco plagiogranites appear similar in outcrop to low-Al plagiogranites, with geochemical compositions similar to those of Na-rich adakitic rocks and zircon εHf(t) values similar to enclaves. Hence, we believe that the Labuco high-Al plagiogranites had a distinctive diagenetic process and magma source.
It is generally agreed that oceanic plagiogranites are produced by two very different petrogenetic processes: fractional crystallization from a hydrous gabbroic source (Coleman and Peterman, 1975; Coleman and Donato, 1979; Lippard et al., 1986; Jiang et al., 2008), and partial melting of metasomatized gabbros or amphibolites (Malpas, 1979; Gerlach et al., 1981; Flagler and Spray, 1991; Koepke et al., 2004; Grimes et al., 2013). These processes are considered to reflect different relationships between plagiogranites and accretionary complexes. In general, plagiogranites that form by partial melting should be younger than the ophiolite complex itself. As discussed already, the Labuco high-Al plagiogranites were formed after the emplacement of the surrounding Labuco ophiolite complex. Furthermore, the plagiogranite samples show a partial melting trend in the La/Sm-La diagram (Fig. 8). In summary, we believe that the petrogenesis of the Labuco plagiogranites was dominated by partial melting, rather than differentiation.
The Labuco plagiogranites are characterized by high Al2O3 and Sr contents, high Sr/Y and (La/Yb)N ratios, positive Eu anomalies, and depletion in HREEs, and they demonstrate a geochemical affinity with typical adakites or high-Al tonalite-trondhjemite-granodiorites (TTGs; Defant and Drummond, 1990; Condie, 2005; Martin et al., 2005; Castillo, 2012). An oceanic subduction setting, combined with the existence of coeval slab-derived adakitic rocks in the adjacent Kangqiong and Dongco areas (Fan et al., 2016; Y.L. Li et al., 2016), indicates that the partial melting of a subducted slab (eclogite) is the likely explanation for the generation of Labuco high-Al plagiogranites. Previous studies have indicated, however, that slab melts generally undergo metasomatic reactions with peridotite during their ascent through the mantle wedge, resulting in marked increases in the MgO, Ni, and Cr contents of adakitic melts (Drummond et al., 1996; Rapp et al., 1999). The low contents of compatible elements (e.g., MgO, Ni, and Cr) in the Labuco high-Al plagiogranites therefore preclude interaction between felsic melts and mantle ultramafic rocks, with this being considered an inevitable outcome of oceanic subduction–induced partial melting and ascent of felsic melt through the mantle wedge. Further evidence for a nonslab origin is provided by the discrimination plots of Th/Nb versus Zr/Nb, Rb/Sr versus Ba/Rb, and the ternary CaO-Na2O-MgO diagram, in which the samples plot away from coeval slab-derived adakites from central Tibet (Fig. 9). Furthermore, the relatively low zircon εHf(t) values indicate a magma source of enriched rather than depleted mantle, in contrast to that of mantle-derived oceanic slabs, which have high εHf(t) values (Fig. 6; Y.L. Li et al., 2016). The Labuco high-Al plagiogranites also show significant differences in geochemical and isotopic compositions compared to coeval slab-derived adakitic rocks from central Tibet (Y.L. Li et al., 2016; S.M. Li et al., 2016). Therefore, we conclude that the high-Al plagiogranites did not originate from slab-derived melts.
Experimental modeling of partial melting indicates that dehydration melting of enriched mid-ocean-ridge basalt (E-MORB)–like basalts at 12 kbar and 900 °C may generate ∼10 wt% felsic melt with high Al2O3 and low MgO contents and moderately high La/Yb and Sr/Y ratios, similar to high-Al TTGs (Zhang et al., 2013). In addition, negative Nb and Ta anomalies and elevated Nb/Ta ratios (11–17; Table 2) likely reflect the presence of rutile as a high-pressure phase (Green, 1995). Therefore, we suggest that the high-Al plagiogranites were formed through partial melting of metabasalts under high pressure at depths of >40 km, in equilibrium with an amphibole-garnet-clinopyroxene-plagioclase assemblage (Martin, 1994; Drummond et al., 1996; Turkina, 2000). Melts of this type can be also generated by the melting of an accretionary complex containing recycled crustal material eroded from adjacent arcs (Rapp and Watson, 1995; Winther, 1996; Smithies, 2000; Rapp et al., 2003).
Hf isotopic characteristics are unaffected by partial melting or fractional crystallization because of the high closure temperature of the zircon Hf isotopic system (Cherniak et al., 1997; Wu et al., 2007). The low positive zircon εHf(t) values of the high-Al plagiogranites are similar to those of mantle wedge–derived enclaves (Fig. 6), indicating that the protoliths were most likely arc-type basalts derived from an enriched mantle wedge. Recently, the identification of K-rich adakitic granites in the region suggested that crustal thickening had occurred in central Tibet during the subduction of Bangong-Nujiang oceanic crust (Wu et al., 2016; Hao et al., 2016). Therefore, a petrogenesis model was proposed that the high-Al plagiogranites could have been generated from underplated arc-type basalts at the base of the accretionary complex, at depths of >40 km.
The geochemical features of the Labuco plagiogranites, such as HFSE depletion, LILE enrichment, and negative Nb and Ta anomalies, indicate that they originated in a subduction setting. In the tectonic discrimination plots of Nb versus Y and Rb versus Nb + Y (Pearce et al., 1984), the Labuco samples plot within the field of volcanic arc granite (Figs. 10A and 10B), as also indicated by the Hf–(Rb/10)–(3Ta) discrimination diagram (Fig. 10C; Harris et al., 1986). This result is consistent with the geochemistry of coeval plagiogranites from central Tibet.
Late Jurassic slab-derived adakites and mantle-wedge–derived arc-type volcanic rocks have recently been identified in central Tibet (Y.L. Li et al., 2016; S.M. Li et al., 2016), and studies of Jurassic strata support an oceanic subduction setting for these rocks (e.g., Yan et al., 2016; Huang et al., 2017; S. Li et al., 2017). Therefore, it is inferred that central Tibet was the site of subduction of the Bangong-Nujiang Tethyan Ocean plate during the Late Jurassic. Extensive Middle–Late Jurassic igneous rocks, which show arc affinities and which developed in an active continental margin setting, are widely exposed at the southern margin of the Qiangtang terrane and in the Lhasa terrane, parallel to the Bangong-Nujiang suture zone, indicating that the subduction of the Bangong-Nujiang Tethyan Ocean plate was double sided, i.e., under both the Qiangtang and Lhasa active margins (e.g., Du et al., 2011; Pan et al., 2012; Deng et al., 2014; Wu et al., 2016; Zhu et al., 2016).
The Labuco plagiogranites are located on the northern margin of the Bangong-Nujiang suture zone, close to coeval granitic rocks from the Qiangtang terrane rather than Lhasa terrane. We therefore suggest that the Labuco plagiogranites of central Tibet were formed during the Late Jurassic in a subduction setting controlled by northward subduction of the Bangong-Nujiang Tethyan oceanic lithosphere.
Implications for Crustal Growth
It has long been recognized that modern continental crust is formed mainly at convergent margins, with subduction zones being important locations of Phanerozoic continental crustal growth (e.g., Rudnick, 1995; Castro et al., 2013). The Labuco plagiogranites and coeval thickened-crust–derived adakitic rocks are considered to have been generated at depths of >40 km under high pressures (>15 kbar), suggesting that vertical crustal growth in the subduction zone in central Tibet occurred during the Late Jurassic. Tectonic shortening and magmatic underplating are considered to be the two main causes of crustal thickening (e.g., Gill, 1981; Sheffels, 1990), with both processes apparently having occurred not only in central Tibet, but also in modern subduction zones. However, recent studies have suggested that the thickening process in central Tibet involved mainly tectonic shortening, as a result of northward subduction of the Bangong-Nujiang Tethyan oceanic lithosphere (Wu et al., 2016). The occurrence of high-Al plagiogranites, considered to have originated from arc-type basalts (Fig. 11), suggests that large volumes of basalt (representing mainly young continental crust) were underplated in central Tibet. In summary, the occurrence of high-Al plagiogranites leads us to consider that emplacement of mafic magma during the oceanic subduction played an important role in controlling the thickness of continental crust in central Tibet.
Our study of high-Al plagiogranites in the Labuco area leads to the following conclusions.
The Labuco plagiogranites have a zircon U-Pb age of ca. 167 Ma, indicating emplacement during the Late Jurassic.
They are characterized by high Al2O3 and Na2O contents, low MgO, TiO2, and Fe2O3 contents, and enrichment in light rare earth elements (LREEs), with positive Eu anomalies and εHf(t) values. These characteristics lead us to propose that partial melting of arc-type basalts from an accretionary complex at depth >40 km is a reasonable explanation for the generation of these high-Al plagiogranites.
Coeval Late Jurassic plutonic suites with diverse geochemical compositions are widespread in central Tibet and were formed in a subduction setting controlled by northward subduction of the Bangong-Nujiang Tethyan oceanic lithosphere during the Late Jurassic.
The present results, combined with those of earlier publications, indicate that mantle-derived arc basaltic magma was an important crustal component at the base of central Tibet. Finally, magmatic underplating likely played a key role in the vertical crustal growth of continental crust within the Bangong-Nujiang subduction zones in the Late Jurassic.
This research was funded by the China Postdoctoral Science Foundation (2016M600353 and 2017T100321), the Fundamental Research Funds for the Central Universities (2017B11614), and the Natural Science Foundation of Jiangsu Province (grant BK20170877). We also thank Jianjun Fan and Yiming Liu for their help in analyses and field work.