Journal of Earth Science  2019, Vol. 30 Issue (1): 20-36   PDF    
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Petrogenesis of the Payangazu Complex in Southern Mandalay, Central Myanmar and Its Tectonic Implications
Kaixuan Li1,2, Huaying Liang1, Zhiwei Bao1, Wenting Huang1, Jian Zhang1,2, Long Ren1,2    
1. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China
ABSTRACT: The Payangazu complex in the central Myanmar is composed mainly of quartz diorite, granodiorite, and some synplutonic mafic dikes. The quartz diorite and granodiorite have zircon U-Pb ages of 130.5±4.0 (MSWD=3.5) and 118.4±2.5 Ma (MSWD=2.4), respectively. Rock samples of the quartz diorite and granodiorite are metaluminous, enriched in large-ion lithophile elements like LREE, Rb, Th, and U, and depleted in high field-strength elements such as HREE, Nb, Ta, P, and Ti, indicative of arc-type magmatic affinities. Whole rock samples of the quartz diorite have εHf(t) value of +0.6, initial 87Sr/86Sr ratios of 0.708 6 to 0.710 0, and εNd(t) values of -4.8 to -4.9; whereas rocks of the granodiorite are relatively isotopically enriched, with εHf(t) values of -5.1 to -7.2, initial 87Sr/86Sr ratios of 0.711 7 to 0.711 8, and εNd(t) values of -8.7 to -8.8. The isotopic data together with the high Mg# (both the quartz diorite and granodiorite have Mg# values of >40) suggest a strong involvement of mantle materials in the genesis of the parent magmas. The possible petrogenetic process may be that the ascending of melts from partial melting of metasomatized mantle wedge triggered by dehydration of subducted slab resulted in partial melting of the lower crust and mixed with the latter. These Early Cretaceous intrusions from the complex are older than those found in the eastern Wuntho-Popa arc in western Myanmar, eastern Himalaya, and western Yunnan which are interpreted to be related to the Neo-Tethyan subduction, and have εNd(t), εHf(t) values lower than the latter. On the contrary, the ages and geochemical characteristics of the Payangazu complex are consistent with some of the intrusions in the northern magmatic belt in Tibet, eastern Himalaya, and western Yunnan which are believed to be associated with the subduction of the Bangong-Nujiang Ocean crust. Thus, we propose that the Early Cretaceous intrusions in the central Myanmar are most likely related to the southward subduction of an ocean slab that was possibly an extension of the Bangong-Nujiang Ocean.
KEY WORDS: central Myanmar    zircon U-Pb age    isotope    Early Cretaceous subduction    

0 INTRODUCTION

Myanmar is tectonically located in the eastern section of the Tethyan tectonic system. The magmatic and tectonic activities in Myanmar are related to different stage subductions of the oceanic slabs and the subsequent collision between continents (Huangfu et al., 2016; Zaw et al., 2014; Metcalfe, 2013; Curray, 2005). These include the slab subductions of the Neo-Tethys and the Paleo-Tethys, and continent-continent collision subsequent to the closure of the Paleo-Tethyan Ocean. In addition, in central Myanmar, an ocean basin was suggested to have existed on the east of the Sagaing fault based on evidences from geochronology and sequence stratigraphy (Prosanta et al., 2017; Mitchell et al., 2012, 2007; Wang et al., 2012; Mitchell, 1993). However, petrogenetic and geochronological investigations on magmatic rocks in central Myanmar are rare whilst previous studies in the region are mostly focused on the metamorphic events of the well-known Mogok metamorphic belt and northern Myanmar because they host plentiful valuable jade and gem deposits (Yonemura et al., 2013; Waltham, 2010; Mitchell et al., 2007; Searle et al., 2007; Barley et al., 2003; Goffé et al., 2002; Bertrand et al., 2001).

A well exposed intrusive complex occurring near the town of Payangazu, in central Myanmar, provides an excellent locus to probe the magmatic origin and tectonic evolution of the region. This complex previously was regarded as diorite with reported age of 131 Ma (Chen et al., 2016) or granodiorite with reported ages of 128 Ma (Mitchell et al., 2012) and 121 Ma (Barley et al., 2003). These petrological inconformity and chronological discrepancy are vary confusing. Besides, their geochemical characteristics of the complex are still unclear. Thus, more detailed chronological results and geochemical data of the intrusions are needed to constrain their petrogenesis and possible tectonic setting.

In this paper, we report precise zircon U-Pb ages and bulk-rock major, trace elements and Sr-Nd-Hf isotopic results of the main rock types from the complex, and discuss their petrogenesis and possible tectonic setting.

1 GEOLOGICAL BACKGROUND 1.1 Tectonic Zones in Myanmar

Myanmar could be divided into three parts from the west to the east: the Indo-Burma range, the West Burma Block and the Shan-Thai Block (Fig. 1a).

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Figure 1. (a) Geological map of study area; (b) tectonic outlines of Myanmar. MMB. Mogok metamorphic belt.

The Indo-Burma range consists mainly of the Late Cretaceous–Paleogene strata and the overlying Cenozoic sediments. Its west boundary is the Andaman subduction zone. This presently active subduction zone extends northward through the Himalayan Syntaxis into India where it transformed into a northward thrust nappe structure. The east boundary is an Early Jurassic ophiolite zone which was regarded as extension of the Yarlung Tsangpo suture zone and was offset by the eastern Himalayan Syntaxis at the north part of Myanmar (Mitchell, 1993). This suture zone represents the subduction of the Neo-Tethyan Ocean and along which ultramafic rocks, pillow basalts, and cherts distribute intermittently.

The West Burma Block situates in-between the ophiolite zone and the Sagaing fault, and it has stratigraphic units similar to those of the Indo-Burma range. The Sagaing fault, an important dextral strike-slip fault accommodating the collision of India and Southeast Asian Plate in the Eocene, is about 1 200 km long and terminates near the eastern Himalayan Syntaxis (Maurin et al., 2010; Curray, 2005). In the central part of the block, there is a north-south magmatic belt called Wuntho-Popa magmatic arc which consists mainly of the Late Cretaceous granodiorite batholith, some small Late Cretaceous to Early Tertiary calc-alkaline intrusions and the Late Cretaceous to Quaternary volcanic strata.

The Shan-Thai Block in the east part of Myanmar is located to the east of the Sagaing fault and west of the Bentong- Chiang Rai suture. The suture is characterized by remnants of the Paleo-Tethys Ocean subduction and the subsequent collision between the Shan-Thai Block and Indochina Block in the Late Triassic (Metcalfe, 1988; Mitchell, 1977). The Shan-Thai Block together with Malay Peninsula, the eastern Sumatra, and the Baoshan Block in Yunnan Province of China was termed as the Sibumasu Block (Metcalfe, 1988). The strata of this block are mainly composed of the Cambrian to Triassic strata with overlying Upper Jurassic–Lower Cretaceous sediments (Barber et al., 2005; Mitchell, 1992). The Shan-Thai Block could be subdivided as Shan Plateau and Shan scarps. The Shan scarps on the east margin of the Shan-Thai Block consist of two stratigraphic units. One is the well-known Mogok metamorphic belt which stretches embowed and bends towards the eastern Himalayan Syntaxis in the north part of Myanmar. The other unit is the Mergui Group, which distributes in the east part of the Mogok metamorphic belt and absent in the north part of Myanmar. In Shan scarps, a Mogok-Mandalay-Mergui magmatic belt with granitoids of the Late Cretaceous and Paleocene– Eocene was identified (Gardiner et al., 2015; Mitchell et al., 2012; Barley et al., 2003). The Shan Plateau, making up the major part of Shan-Thai Block, is separated from the Shan scarps by an east-dipping fault and an ocean regarded as the extension of the Nujiang-Luxi Ocean situated in-between the Tengchong and Baoshan blocks in Southwest China (Mitchell et al., 2012, 2007; Wang et al., 2012; Mitchell, 1993).

1.2 The Intrusive Complex in Central Myanmar

The Payangazu complex is located about 130 km south of Mandalay (Fig. 1b). The complex consists of largely granodiorite in the east part and minor quartz diorite in the west part, and some synplutonic mafic dikes. The complex intruded the Mogok metamorphic belt and the Mergui Group. The biotite and garnet bearing augen gneiss, schists, and minor marble and pegmatite veins, which are believed to be the metamorphosed Lower Paleozoic–Mesozoic strata of the Mogok metamorphic belt, occur at the west side of the complex; whereas the mica schists of the Mogok metamorphic belt and the overlying diamictites of the Mergui Group occur at the east side of the complex. Far to the east, the diamictites are in turn overlain by the Late Jurassic limestone and clastic rocks, and some volcanic and pyroclastic rocks. Hornfels were observed near the contact of the intrusions and the diamictites.

The granodiorite has medium-coarse grained texture and composes of about 50%–60% plagioclase, 10%–15% K-feldspar, 5%–10% hornblende, 20%–25% quartz, 5%–10% biotite, and accessory minerals including titanite, apatite, zircon, and magnetite (Fig. 2). The quartz diorite is coarse-grained with plagioclase content of 50%–60%, K-feldspar content of 5%–10%, hornblende content of 10%–20%, biotite content of 5%–10% and quartz content of 5%–10%. The intrusive mafic dikes extend in north-south direction with width ranging from 40 cm to 2 m and length of tens to hundreds meters.

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Figure 2. Hand specimen photos and photomicrographs of the diorite and granodiorite from the central Myanmar. (a) Diorite; (b) granodiorite; (c) photo showing mineral assemblage of diorite under perpendicular polarized light; (d) photo showing mineral assemblage of granodiorite under perpendicular polarized light. Q. Quartz; Kf. K-feldspar; Pl. plagioclase; Pt. perthite; Hb. hornblende; Bt. biotite.
2 METHODS

Five least altered samples of each rock types were selected for major and trace elements analyses. They were crushed to grains of about 2–5 mm, leached by 3% HCl solution, then rinsed by deionized water. After dried at temperature of 100 C, the samples were ground to less 200 meshes for bulk major, trace element and isotopic analyses. Zircon crystals from the quartz diorite (TGK-25) and granodiorite (TKG-31) were separated for U-Pb geochronological study. All the analyses were carried out at the National Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.

2.1 Major and Trace Element Analyses

The major elements contents of bulk rock samples were analyzed by using X-ray fluorescent spectrometry following procedures described by Goto and Tatsumi (1996). The analytical precision was better than 5%. The loss on ignition (LOI) was calculated by weighing the difference before and after the burning at temperature of about 900 C. In order to proceed trace element analysis, about 50 mg powdered samples were dissolved in Teflon bombs using mixed acids of HF and HNO3 at about 200 C for two days. After the dissolution, the generated sample solutions were transferred into test tubes and diluted for analysis. The trace element concentrations of the rocks were determined by using inductively coupled plasma mass spectrometry method on a PerkinElmer Elan, 9000, details of the procedures are similar to those described by Liu et al. (1996). Rh was used as an internal standard to monitor signal drift during the analysis. The analytical precision of which is better than 10%.

2.2 Whole-Rock Sr-Nd-Hf Isotopic Analysis

Whole-rock Sr-Nd-Hf isotopic compositions were measured on a Neptune Plus multi-collector mass spectrometer of Isoprobe. The analytical procedures for the Sr, Nd isotopic compositions are similar to those described by Yang et al. (2011). The mass fractionation of Sr and Nd were corrected by using the values of 86Sr/88Sr=0.119 4 and 146Nd/144Nd=0.721 9 respectively. The analytical standards used during the measurement were the NIST SRM 987 with 87Sr/86Sr=0.710 290± 0.000 014 (2σ) and the Shin Etsu JNdi-1 with 143Nd/144Nd= 0.512 094±0.000 008 (2σ). The analytical procedure for Hf isotope follows Li et al. (2006). Briefly, 0.5 g powdered rock samples was mixed with 1.0 g Li2B4O7, and put into high-frequency furnace and melted in Pt crucibles at 1 250 C for 15 min. And then the samples were crushed and dissolved in the 2 mol/L HCl. The mass fractionation correction of Hf was based on 179Hf/177Hf=0.732 5. During the analysis, the values of 176Lu/175Lu=0.026 549 and 176Yb/173Yb=0.786 96 were used to calculate the concentrations of 176Lu and 176Yb, and then the influences of them were deducted.

2.3 Zircon U-Pb Chronology

Zircon crystals from the quartz diorite and granodiorite were separated using conventional heavy liquid and magnetic separation techniques, and handpicked under a binocular microscope. The zircon grains were mounted on a transparent epoxy resin disk and polished near the core to expose the internal structures. Cathodoluminescence (CL) images and microscopic observation were made to study the internal structure.

U-Pb dating of zircon was conducted on inductively coupled plasma mass spectrometry equipped with a laser ablation system (LA-ICP-MS). The epoxy resin disks with embedded zircons were placed in a special sample cell which flushed with He and Ar. The laser used for ablation was a pulsed resonetic 193 nm ArF excimer laser with constant energy of 80 mJ and repetition rate of 8 Hz. The diameter of ablating spot was 31 μm. The ablated aerosol was delivered to an Agilent 7500a ICP-MS by carrier gas of He and detected there (Tu et al., 2011). Standard of zircon 91500 was used as an external standard during U-Pb dating (Black et al., 2003; Wiedenbeck et al., 1995), and analyses of the standard were conducted twice for every five analyses of the samples. Off-line inspection, integration of background and analytical signals, time-drift correction and quantitative calibration for U-Pb dating were performed by using ICPMSDataCal (Liu et al., 2008). Concordia diagrams and weighted mean values calculation were made by Isoplot/Ex Version 3.0 (Ludwig, 2003).

3 RESULTS 3.1 Whole-Rock Major and Trace Elements

The major and trace elemental concentrations of the quartz diorite and granodiorite from the Payangazu complex are listed in Table 1. The quartz diorite has SiO2 of 61.1 wt.%–63.2 wt.%, Na2O of 3.18 wt.%–4.20 wt.%, K2O of 1.56 wt.%–2.47 wt.%, Al2O3 of 15.7 wt.%–17.1 wt.%. The granodiorite has higher SiO2 (65.7 wt.%–66.8 wt.%) and K2O (3.20 wt.%–3.80 wt.%) contents and relative lower Al2O3 (14.5 wt.%–15.3 wt.%) and Na2O (2.93 wt.%–3.18 wt.%). On the TAS diagram (Fig. 3a), the quartz diorite falls into the field of diorite and the granodiorite fall into the field of granodiorite, which are consistent with their mineral compositions. Both the quartz diorite and the granodiorite are largely metaluminous and belong to calc-alkaline and high-K calc-alkaline series, respectively (Figs. 3b3c).

Table 1 Major (wt.%) and trace element (ppm) concentrations of the diorite and granodiorite intrusions in the central Myanmar
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Figure 3. (a) TAS diagram (Middlemost, 1994); (b) K2O vs. SiO2 diagram (Rollinson, 1993; Peccerillo and Taylor 1976); (c) A/NK (molar ratio Al2O3/(Na2O+K2O)) vs. A/CNK (molar ratio Al2O3/(CaO+Na2O+K2O)) diagram (Maniar and Piccoli, 1989).

The two type rocks have similar trace elements and REE (rare earth element) patterns (Fig. 4). Rare earth element (REE) concentrations of the quartz diorite and granodiorite vary in ranges of 96.7 ppm–160.0 ppm and 161.0 ppm–271.1 ppm, respectively. As shown in the chondrite normalized REE patterns (Fig. 4a), both the quartz diorite and the granodiorite have moderate negative Eu anomalies, highly fractionated light rare element and weak- or non-fractionated heavy rare earth element, which are characterized by high (La/Yb)N ratios of 3.64–8.78 and 7.64–17.28, respectively. The primitive mantle normalized trace element spider diagrams of the rocks exhibit arc-type signatures such as enriched large ion lithophile elements (e.g., Rb, Th, U) and depleted high field strength elements (HFSE) (e.g., Nb, Ta, P, Ti) (Fig. 4b).

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Figure 4. Chondrite-normalized rare earth elements and primitive mantle-normalized trace element spiderdiagrams for the diorite and granodiorite. Normalized values of chondrite and primitive mantle are from Sun and McDonough (1989).
3.2 Zircon U-Pb Ages

Zircon grains from the quartz diorite (sample TKG-25) and granodiorite (sample TKG-31) are euhedral crystals with length of 150–200 m, width of 50–120 m and length of 100–200 m, width of 50–120 m, respectively. On the cathodoluminescence images (Fig. 5), all the zircon crystals show clear oscillatory zoning, indicative of a magmatic origin (Hoskin, 2003).

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Figure 5. The CL images of representative zircons analyzed for in situ ages. Small dots indicate the sites of ablation and the obtained ages were also labelled on the images. (a) Zircons from granodiorite; (b) zircons from diorite.

The analytical results for zircon grains from the quartz diorite (TKG-25) are listed in Table 2. The zircons have U and Th concentrations of 145 ppm–285 ppm and 114 ppm–340 ppm, respectively, with U/Th ratios of 0.63–1.19. Removing those with concordance of less than 90%, 14 qualified data set of the 20 tests yielded a weighed mean 206Pb/238U age of 130.5±4.0 Ma (1σ, MSWD=3.5) (Fig. 6a). U-Pb isotopic results for zircon grains from the granodiorite (TKG-31) are listed in Table 3. The zircon crystals have U and Th concentrations in ranges of 114 ppm–681 ppm and 104 ppm–690 ppm, respectively, with U/Th ratios of 0.61–1.50. Removing those with concordance of less than 90%, 15 of the 20 tests yielded a weighed mean 206Pb/238U age of 118.4±2.5 Ma (1σ, MSWD=2.4) (Fig. 6b).

Table 2 U-Pb dating results for zircons from the diorite
Table 3 U-Pb dating results for zircons from the granodiorite
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Figure 6. U-Pb concordia plots of zircons from the diorite (a) and the granodiorite (b).
3.3 Whole-Rock Sr-Nd-Hf Isotopes

The Sr-Nd-Hf isotopic compositions for the quartz diorite and granodiorite are listed in Table 4 and Table 5. The Rb, Sr, Sm, Nd, Lu and Hf concentrations of the whole-rock samples in Table 1 and zircon U-Pb ages were used to calculate the initial 87Sr/86Sr ratios, εHf(t) and εNd(t) values.

Table 4 Sr and Nd isotopic compositions for the diorite and granodiorite in the Payangazu complex
Table 5 Whole-rock Hf isotopic compositions of the diorite and granodiorite

Two quartz diorite samples have initial 87Sr/86Sr ratios of 0.708 6 and 0.710 0 and εNd(t) values of -4.8 and -4.9 with the calculated two stage model ages (TDM2) of 1 319 and 1 323 Ma. Two granodiorite samples show initial 87Sr/86Sr ratios of 0.711 8 and 0.711 7 and εNd(t) values of -8.7 and -8.8 with calculated TMD2 of 1 623 and 1 627 Ma. One quartz diorite sample and two granodiorite samples were selected for whole-rock Hf isotopic analysis. The quartz diorite sample has εHf(t) of 0.6 and two stage model age (TMDC) of 1 147 Ma the two granodiorite samples have εHf(t) values of -7.2 and -5.1 and TMDC of 1 127 and 1 498 Ma.

4 DISCUSSION 4.1 Zircon U-Pb Ages of Payangazu Complex

Zircon U-Pb ages of 130.5±4.0 and 118.4±2.5 Ma for the quartz diorite and granodiorite, respectively. Our new data are identical to the previously published ages of 128–131 Ma (Chen et al., 2016; Mitchell et al., 2012) and 121 Ma (Searle et al., 2007) correspondingly within errors. Thus, the complex was emplaced in the Early Cretaceous. However, there is a proximate 10 Ma temporal interval between the two kind rocks and this interval seems too short for magmas to originate in two episodes. The interval may indicate that the magmas originated in one episode and subsequently went through different evolutionary processes and finally emplaced and crystallized in two stages. These Early Cretaceous ages are earlier than those Late Cretaceous to Paleogene magmatic rocks in the west Wuntho- Popa arc (Wang et al., 2014; Mitchell et al., 2012) and the Late Cretaceous granitoids in the southern Myanmar (Jiang et al., 2017; Gardiner et al., 2015; Mitchell et al., 2012). It is interesting to note that the Early Cretaceous magmatism has rarely been reported in other areas of Myanmar, which may imply an unknown or largely ignored tectonic event.

4.2 Petrogenesis 4.2.1 Magmatic affinity and significance

Both of the granodiorite and quartz diorite are metaluminous and belong to calc-alkaline to high-K calc-alkaline series. The rocks are characterized by enriched LILE such Rb, Th, U and light REE and depleted HFSE including Nb, Ta, P, Ti and heavy REE, showing typical arc magmatic affinity (Ulmer, 2001; Kogiso et al., 1997; López-Escobar et al., 1993). The arc type affinity is obviously manifested by the diagrams of Rb vs. (Y+Nb) and Nb vs. Y (Pearce et al., 1984) (Fig. 7), as well as the Sr/Y vs. Y and La/Yb vs. Yb plots (Castillo et al., 1999; Defant and Drummond, 1993, Fig. 8). Furthermore, the abundant biotite and hornblende in the rocks imply high H2O-contents of the magma, which is also consistent with features of hydrous arc magma (Ulmer, 2001).

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Figure 7. Rb vs. (Y+Nb) and Nb vs. Y discrimination diagrams (Pearce et al., 1984).
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Figure 8. Discrimination diagrams for adakite and island-arc andesite-dacite-rhyolite. (a) Sr/Y vs. Y (Defant and Drummond, 1993); (b) La/Yb vs. Yb (Castillo et al., 1999). Fractionation trend of minerals are after Richards and Kerrich (2007).

The occurrence of granodiorite and quartz diorite with arc-type affinity suggests onset of an Early Cretaceous magmatism related to oceanic slab subduction, which is supported by chronological results of the sequence stratigraphy (Mitchell et al., 2012, 2007; Wang et al., 2012; Mitchell, 1993).

4.2.2 Magmatic source

The positive bulk rock εHf(t) value (0.6) and young TDMC (1 147 Ma) of the quartz diorite, as well as the highly varied zircon εHf(t) values of -0.08 to -12.77 (Chen et al., 2016), indicate that the quartz diorite could not be derived directly from a mantle or crust source alone (Fig. 9). On the contrary, a mixed source of mantle and crustal materials can be inferred. Indeed, the negative εNd(t) (-4.8 and -4.9) and relatively high (87Sr/86Sr)i (0.708 6 and 0.710 0) of the quartz diorite are projected near the bivariate hybrid line (Fig. 10) of the Early Cretaceous Tethyan basalt and lower crust with a lower crustal contribution of about 60%. Therefore, the quartz diorite was likely derived from a mixed source of the mantle and the lower crustal components. In comparison, the granodiorite has bulk rock εHf(t) values (-7.2 and -5.1) lower than that of the quartz diorite, which may reflect higher crustal contribution in the magma source. And consistently, the granodiorite samples are projected near the mixing line on the εNd(t) vs. initial Sr isotope ratio diagram (Fig. 10) and suggest that the lower crustal contribution is around 85%.

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Figure 9. εHf(t) vs. age diagram of granitoids in the central Myanmar and other areas of the Tethyan tectonic region. Data sources: Gangdese batholiths (Zhu et al., 2011, 2009a; Ji et al., 2009; Chu et al., 2006); Northern magmatic belt (Zhu et al., 2011, 2009a; Chu et al., 2006); Eastern Himalayan (Chiu et al., 2009; Zhu et al., 2009b; Liang et al., 2008); West Yunnan (Xu et al., 2012; Liang et al., 2008); Diorite (literature) (Chen et al., 2016). The lines of crustal extraction are calculated by current value of (176Lu/177Hf)CC=0.015 0 from average continental crust (Goodge and Vervoort, 2006).
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Figure 10. εNd(t) vs. (87Sr/86Sr)i diagrams of the Early Cretaceous granitoids in the central Myanmar and the other areas of the Tethyan tectonic system. Data sources: Gangdese batholiths (Wen et al., 2008); Northern magmatic belt (Zhu et al., 2009a); Eastern Himalayan (Lin et al., 2012; Zhu et al., 2009b); West Yunnan (Yang et al., 2006). UC. Upper crust; LC. lower crust.

The Mg# is an effective indicator in discriminating the mantle or crust source of magmas. Experimental petrological results have shown that melts from the lower crust have Mg# below 40 regardless of the degree of partial melting, whereas melts with Mg# higher than 40 require a contribution of mantle component (Rapp and Watson, 1995). Here, the Mg# values (Table 1) of the quartz diorite and granodiorite are 46.9–50.2 and 41.4–42.8 respective, both of which are higher than 40, suggestive of the mantle contribution. In addition, the contemporaneous mafic dikes (Chen et al., 2016) in the complex also support the involvement of mantle materials in magmas. Thus, the genesis of the quartz diorite and granodiorite is related to a mixed source of the mantle and crustal component.

4.2.3 Petrogenesis of the complex

As discussed above, the trace element concentrations of the quartz diorite and granodiorite show arc-type magmatic affinity while their Sr-Nd-Hf isotopes demonstrate a magma source of a mixture of mantle and crustal components; therefore, the complex was most likely formed in a subduction setting. In subduction setting, the generation of magmas is related to interactions among the subducted oceanic crust, the overlying mantle wedge and the crust. Partial melting of basaltic slab with plagioclase completely consumed would produce adakitic magmas (Martin et al., 2005; Peacock et al., 1994; Mcdonough, 1991; Defant and Drummond, 1990). The lower Sr contents (Table 1, Fig. 8) of the quartz diorite and granodiorite of this study suggest that melting of subducted slab was insignificant if any. Nevertheless, dehydration of subducted slab is necessary to metasomatize the overlying mantle wedge, a key process to generate arc magmas (Ulmer, 2001; López-Escobar et al., 1993). Thus, it seems that the fluid released by dehydration of slab with or without melting (maybe include melting of the subducted sediments) and its subsequent interaction with the overlying mantle wedge triggered the partial melting of the mantle and generated the initial arc magmas. However, the initial magmas generated in mantle wedge were generally basaltic (Hattori and Guillot, 2003; Schmidt and Poli, 1998; Peacock, 1993). The addition of the lower crustal component was most likely a scenario that underplating of the initially mantle-derived magmas at the base of the lower crust led to the partial melting of lower crust which formed the crustal melts that subsequently mixed with the mantle magmas (Wörner et al., 1992; Hildreth and Moorbath, 1988).

In addition to the higher contribution of the lower crust, deviations of the quartz diorite and granodiorite from the bivariate hybrid line may result from assimilation of mid-upper crust during the ascent and emplacement of the mixed magmas (Fig. 10). Besides, the granodiorite has decreasing Al2O3, CaO, Fe2O3T, MgO, Na2O contents and obviously increasing K2O content (Fig. 11), higher large-ion lithophile elements (LILE) (Fig. 4) in comparison with quartz diorite. This maybe resulted from various degrees of contribution of lower crust and/or assimilation of mid-upper crust. Thus, assimilation may play a role in magmatic evolution. On the other hand, the weakly negative Eu anomalies and low Sr contents of the quartz diorite and granodiorite may result from fractionation of plagioclase (Figs. 4 and 8, Table 2). However, the smooth MREE (middle REE) and HREE patterns (Fig. 4), and high Y and Yb contents (Fig. 8, Table 2) show little fractionation of garnet and hornblende. In summary, the possible scenario of magma origination and evolution for the Payangazu complex might be that ascent of mantle magmas at the base of the lower crust resulted in partial melting of the lower crust, where the resultant melts subsequently mixed with the mantle magmas; and during subsequent ascent and emplacement, the magmas experienced assimilation of mid-upper crust and fractionation of plagioclase. The near 10 Ma temporal interval between the quartz diorite and granodiorite may indicate a prolonged interaction between mantle-derived magma and lower crust or the mixed melt and the mid-upper crust which was responsible for the more silicic granodiorite with lower εHf(t) and εNd(t) values.

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Figure 11. Variations of major elements vs. SiO2 content (wt.%).
4.3 Tectonic Implications

The Tethyan tectonic domain in China could be divided as the Gangdese belt, the northern magmatic belt, the eastern Himalayan, and the western Yunnan (Fig. 12). The Gangdese belt is composed of the Early Jurassic to Paleogene intrusions with positive εHf(t), εNd(t), and low (87Sr/86Sr)i values, and was thought to be generated by subduction and subsequent roll-back of Neo-Tethyan slab (Ma et al., 2017; Zhu et al., 2011, 2009a; Ji et al., 2009; Wen et al., 2008; Chu et al., 2006; Figs. 9, 10).

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Figure 12. Simplified tectonic framework of Tethys showing the south extension of the Bangong-Nujiang suture (modified from Xu et al. 2012).

Intrusions with I-type affinities are widespread in the northern magmatic belt, the eastern Himalaya, and western Yunnan; however, those in the northern magmatic belt mainly emplaced in the Early Cretaceous (Zhu et al., 2011, 2009a; Chu et al., 2006; Fig. 9), whereas those in the eastern Himalaya and western Yunnan were formed in the Early Cretaceous and Late Cretaceous to Paleogene (Lin et al.; 2012; Xu et al., 2012; Chiu et al., 2009; Zhu et al., 2009b; Liang et al., 2008; Yang et al., 2006; Fig. 9). Formation of the I-type intrusions in the East Himalaya and the West Yunnan was suggested to have been formed under post-collisional setting subsequent to the closure of the Bagong-Nujiang Ocean (part of the Mesotethyan Ocean) in Late Jurassic–Early Cretaceous (Xu et al., 2012; Chiu et al., 2009). However, the occurrences of the contemporaneous volcanic rocks with obvious arc-type signatures in eastern Himalaya argue against a post-collisional region (Lin et al., 2012). A link between the Early Cretaceous I-type intrusions and volcanic rocks in the eastern Himalaya and subduction of Neo-Tethys was proposed by Lin et al. (2012). However, the ages of emplacement for these intrusions are significantly older than the Neo-Tethyan subduction related intrusions in Myanmar (the Wuntho-Popa arc), the Gangdese, the East Himalaya, and the West Yunnan (Wang et al., 2014; Mitchell et al., 2012; Xu et al., 2012; Chiu et al., 2009; Ji et al., 2009). In addition, Sr-Nd-Hf isotope compositions of these intrusions are distinct from those related to the Neo-Tethyan subduction which are characterized by positive εHf(t) and low (87Sr/86Sr)i, high εNd(t) values (Wang et al., 2014; Mitchell et al., 2012, Xu et al., 2012; Chiu et al., 2009; Ji et al., 2009; Figs. 9, 10). Therefore, the Early Cretaceous intrusions in central Myanmar, eastern Himalaya and western Yunnan have nothing to do with the subduction of Neo-Tethyan Ocean.

The relationship between the Early Cretaceous I-type magmatism and the subduction of the Bangong-Nujiang Ocean is also supported by previous works, for instance, the Early Cretaceous I-type intrusions in the northern belts, the eastern Himalaya, and the western Yunnan were speculated to be related to the subduction of the Bangong-Nujiang oceanic slab (Zhu et al., 2009a, b; Yang et al., 2006). The association of the Early Cretaceous I-type magmatism with the Bangong-Nujiang oceanic slab subduction can well explain their arc-type affinities and distribution as well, i.e., these Early Cretaceous intrusions occur closer to the Bangong-Nujiang suture in the eastern Himalaya and the western Yunnan (Xu et al., 2012; Chiu et al., 2009) rather thant the suture of the Neo-Tethyan Ocean.

The quartz diorite and granodiorite of the Payangazu complex in the central Myanmar belong to the Mogok- Mandalay-Mergui belt and have geochemical features of subduction related arc magmatism, and show geochemical characteristics similar to the Early Cretaceous counterparts in the north magmatic belt, the eastern Himalaya and the western Yunnan, and thus they were likely formed in similar tectonic setting. Considering the fact that these Early Cretaceous intrusions have Sr-Nd-Hf isotopic features similar to the contemporaneous I-type intrusions in the northern magmatic belt, eastern Himalaya and western Yunnan (Xu et al., 2012; Zhu et al., 2011, 2009a, b; Liang et al., 2008; Yang et al., 2006; Figs. 9, 10), formation of these Early Cretaceous intrusions is most likely related to the Bangong-Nujiang Ocean (part of the Tethyan System). Therefore, the Bangong-Nujiang Ocean might extend into the central Myanmar through the eastern Himalaya Syntaxis. In fact, the south extension of the Bangong-Nujiang Ocean to the western Yunnan and Myanmar was proposed by some researchers (Mitchell et al., 2012; Wang et al., 2012; Mo et al., 1994), while this study further demonstrated that the Bangong-Nujiang Ocean might extend far south to the central Myanmar.

5 CONCLUSIONS

(1) The quartz diorite and granodiorite in the Payangazu complex, have zircon ages of 130.5±4.0 (MSWD=3.5) and 118.4±2.5 Ma (MSWD=2.4), respectively.

(2) Both the quartz diorite and granodiorite are metaluminous, enriched in LILE like LREE, Rb, Th, and U, and depleted in HFSE such as HREE, Nb, Ta, P, and Ti, indicative of arc-type magmatic affinities.

(3) The isotopic data together with the high Mg# of the quartz diorite and granodiorite suggest a strong involvement of mantle materials in the genesis of the parent magmas. The possible petrogenetic process may be that ascent of mantle melts from the metasomatized mantle wedge at the base of the lower crust resulted in partial melting of the lower crust, where the resultant melts subsequently mixed with the mantle magmas, and late stage assimilation of mid-upper crust and fractionation of plagioclase may be also involved during their subsequent ascent and emplacement.

(4) The Early Cretaceous intrusions in the central Myanmar were most likely related to the southward subduction of the Bangong-Nujiang Ocean and the Bangong-Nujiang Ocean might extend far south to the central Myanmar.

ACKNOWLEDGMENTS

This Study was supported by the National Key R & D Program of China (No. 2016YFC0600407) and the Natural Science Foundation of China (Nos. 41772065, 41421062). We thank Dr. Swe Win, Mr. Kyaing Sein and Soe Thura Tun for their assistance during our fieldwork. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0862-9.


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