Source: https://mem.lyellcollection.org/content/48/1/385?ijkey=c3353fbcea69e5d102e5c6bef19caa15bbf65bb9&keytype2=tf_ipsecsha
Timestamp: 2019-04-20 01:42:30+00:00

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Cobbing et al. (1986, 1992) distinguished Granitic Provinces in the Southeast Asian Tin Belt; this concept has formed a widely accepted basis for the discussion of the granites in Southeast Asia. In Myanmar the Central Valley Granite Province (Cobbing et al. 1992) extends from Wuntho southwards into Salingyi and Popa. Most of the granitoids in this belt are metaluminous, calc-alkaline I-type granites; their composition ranges from diorite to granodiorite and their ages from Cretaceous to Cenozoic (Khin Zaw 1990; Cobbing et al. 1992; Barley et al. 2003; Mitchell et al. 2012).
Intrusive granitoid bodies of the Western Granite Province (Cobbing et al. 1992) were emplaced along the western margin of Sibumasu (Metcalfe 1988). In Myanmar, the Western Granite Province contains a mixture of S- and I-type granitoids with Cretaceous and Tertiary ages (Khin Zaw 1990; Cobbing et al. 1992; Barley et al. 2003; Searle et al. 2007; Mitchell et al. 2012). These granitoids are emplaced mainly in the metasedimentary and meta-intrusive rocks of the Mogok Metamorphic Belt (MMB; Searle & Ba Than Haq 1964; Mitchell et al. 2007; Searle et al. 2017) and the Palaeozoic sediments of the ‘Slate Belt’ (SB; Mitchell et al. 2004) (Lebyin, Mawchi and Mergui groups). The Cretaceous granitoids are metaluminous, sodic–potassic magnetite-series I-type granites (Cobbing et al. 1992). A granite–granodiorite association is prevalent in the southern part (Thanintharyi area) while a diorite–granodiorite association is more common in northern parts (Yebokeson area). Mildly peraluminous ilmenite-series monzogranite is found in Parker Island (Cobbing et al. 1992). Zircon U–Pb dating has yielded ages of 120 Ma for a diorite in the Yebokeson area, 90.8 ± 0.8 Ma for a quartz diorite from the Mokpalin area and 71.8 ± 0.5 Ma for a granite from the Nattaung area (Mitchell et al. 2012). A granodiorite near Yebokeson also has a zircon SHRIMP age of 121.12 ± 0.89 Ma (Barley et al. 2003). Tertiary granitoids consist of metaluminous to mildly peraluminous, high-K calc-alkaline rocks with both magnesian and ferroan characters (Cobbing et al. 1992; Aung Zaw Myint et al. 2013a). Miocene dyke rocks cut the older granitic rocks in some places (Payangazu area). These rocks overprint the Cretaceous plutonism along this province and Sn–W mineralization is associated with strongly fractionated, peraluminous granites.
The Eastern Belt of Khin Zaw (1990) represents the northern continuation of the Central Province granitoid rocks of Cobbing et al. (1992) in NW Thailand and these granitoids are generally Permo-Triassic–Triassic in age. Details of these granitoids and their associated tin mineralization in eastern Myanmar are discussed by Than Htun et al. (2017).
The Mawchi Granite forms a small elongated pluton in the western part of Shan Plateau, and is situated on the eastern margin of the Western Granite Province. The geology of the Mawchi area (Fig. 17.2) consists of metamorphic rocks of the Mogok Metamorphic Belt (MMB), low-grade metasediments of the Mawchi Group (SB) and the Plateau Limestone, which unconformably overlies the Mawchi Group (Mawchi Series of Hobson 1940).
Geological map of the Mawchi area showing the localities of Sn–W occurrences (modified after Ye Myint Thein & Aung Kyawe Oo 2000).
The low-grade metasediments of the Mawchi Group pass gradationally to the west into a sequence of gneisses, granite gneisses and schists (Hobson 1941) which are considered to be a southern component of the MMB, discussed by Searle et al. (2017). The Mawchi Group is very varied in lithology and includes shale, fine-grained siltstone and sandstone, slate, thin limestones, quartzite and metagreywacke (Hobson 1941). It can be correlated with the Mergui Group (United Nations 1978) and the Lebyin Group (Wolfart et al. 1984), which is of Early Permian–Carboniferous age according to the palaeontological evidence (Tun Soe 1985). The Mawchi Group is probably younger than Carboniferous on the basis of the U–Pb zircon age from a siltstone unit in the present study.
A wide outcrop of Plateau Limestone rests unconformably on the Mawchi Group in the mine area, whose lithology is similar to that of the Permian Thitsipin Limestone (Garson et al. 1976). This limestone unit is slightly metamorphosed, and scheelite-skarn mineralization occurs along the contact with the Mawchi Granite. A thin aureole of quartzite, spotted grit and indurated slate is developed at the contact of the granite and the Mawchi Group (Hobson 1941) (Fig. 17.3). NNW–SSE-striking faults in the mine area are parallel to the regional strike in the Mawchi Group (Fig. 17.2); later faults have also controlled mineralization (Hobson 1940).
(a) Geological map of the Mawchi Mine (modified after a Mawchi Mine project map) showing line of cross-section; and (b) geological cross-section (A–B–C) of the Mawchi Mine.
Away from the Sn–W mineralization, the Mawchi Granite is a medium- to coarse-grained biotite granite (BG) which is locally porphyritic, with large K-feldspar porphyroblasts. In the area of the Mawchi Mine, the BG is exposed as a narrow and elongated body below the limestone cap (Fig. 17.4a). It is an ilmenite-series granite (Ishihara 1979) and with a range of magnetic susceptibility from 0.020 to 0.039 × 10−3 SI (mean value of 0.029 × 10−3 SI). The upper part of the BG is decomposed due to the extensive weathering. Approaching the mineralized area, the granite is rapidly transformed into a tourmaline granite (TG) in which tabular tourmaline is dispersed in medium- to coarse-grained granitoids (Fig. 17.4b). The magnetic susceptibility of the TG is low and varies from 0.002 to 0.059 × 10−3 SI. The sharp boundary between the BG and the TG cannot be observed in the field, but a progressive increase of tourmaline from BG to TG can be traced. The highly altered granite (HAG) represents a highly altered facies of the TG, developed at the sediment contact zone associated with the mineralized veins. The HAG is very rich in both tabular and acicular tourmalines.
Photographs of rock exposures and photomicrographs of samples from the Mawchi area. (a) Exposure of biotite granite (BG) in contact with metamorphosed limestone (ML). (b) Tourmaline granite (TG) body in Mawchi Mine. (c) Photomicrograph showing mineral assemblage in biotite granite (BG) and zircon halos in biotite. (d) String perthite texture in a BG. (e) Early formed tourmaline in tourmaline granite (TG). (f) Hydrothermal muscovite crystallized in fractures in albite in TG (ab, albite; bt, biotite; mu, muscovite–sericite; tu, tourmaline).
A geological map including the Mawchi Mine workings is shown in Figure 17.3a, b. The main Sn–W mineralization style of the Mawchi deposit (Aung Zaw Myint et al. 2013b, 2014a, b; Than Htun et al. 2017) is a hydrothermal vein system in which cassiterite and wolframite occur in north–south-trending vertical or steeply dipping quartz veins, accompanied by subordinate amounts of sulphides, tourmaline, fluorite, white micas, chlorite, calcite and beryl (Dunn 1938; Aung Zaw Myint et al. 2013b). The quartz vein system is present both in the tourmaline granites and the metasediments. The veins have a maximum length of 570 m and a thickness of up to 2.5 m. Minor scheelite-skarn mineralization occurs along the granite–limestone contact in the upper levels of the mine. Greisens containing cassiterite–muscovite ± tourmaline ± chlorite masses are present in some parts of the granite body. The Sn:WO3 ratios vary in different veins, with a range mostly between 1.5: 1 and 2:1(unpublished Mawchi Mine Assay data).
The biotite granite (BG) is composed of quartz, plagioclase (albite), K-feldspar and biotite (Fig. 17.4c). Quartz is anhedral and more abundant in the groundmass than as phenocrysts. The K-feldspars are orthoclase and microcline, altered to sericite and kaolinite. Coarse-grained plagioclase is mostly albite. Biotite is the primary mafic mineral (>5 vol%) and trace amounts of tourmaline are present. Chlorite and sericite–(muscovite) are the most common secondary minerals, and replace biotite grains along the margins and cleavage planes. Accessory minerals are zircon and sphene. Myrmekitic and perthitic textures (Fig. 17.4d) are common in the BG.
The tourmaline granite (TG) is composed of quartz, orthoclase, plagioclase, black tourmaline (schorl) and muscovite with minor amounts of sericite and epidote. Biotite is a very rare accessory mineral. Sericite clusters or aggregates are commonly associated with tourmaline, and sometimes with illite. Muscovite–sericite clusters of hydrothermal in origin occasionally replace tourmaline (Fig. 17.4e). Most of the K-feldspar and albite are altered to kaolinite and sericite (Fig. 17.4f).
Highly altered granite (HAG) samples are composed of quartz and tourmaline (which are mostly post-magmatic hydrothermal tourmaline) with subordinate feldspars. In some parts of the HAG two generations of albite are present; the later generation of albite is smaller than the earlier, lacks sericite alteration and was formed during an episode of albitization. Most of the tourmalines in the HAG are of the late-generation tourmaline and their long, acicular form is quite different to that of the earlier generation. Minute pyrite cubes (<400 µm) are dispersed in some parts of the HAG.
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon geochronological dating was performed in the CODES Laboratories, University of Tasmania, and demonstrated that the biotite granite (BG) and the tourmaline granite (TG) of the Mawchi area are both of Middle Eocene age. Twelve zircon crystals from the BG gave an age of 42.72 ± 0.94 Ma (Fig. 17.5a, c), while two zircon grains had ages of 104 Ma and 106 Ma. The Late Eocene age can be regarded as the magmatic age of the tin granite, and the Cretaceous zircons are tentatively interpreted as being inherited from Cretaceous plutonism in the Central Belt. The TG has low zircon contents, and six zircon crystals gave a mean age of 43.71 ± 0.39 Ma (Fig. 17.5b). A histogram summarizing the spread of the zircon ages in the Mawchi granites is shown in Figure 17.5c. A siltstone unit in the Mawchi Group was found to contain three populations of zircons – (1) Carboniferous; (2) Ordovician; and (3) Precambrian (Fig. 17.5f) – pointing to the depositional age of the Mawchi Group as being equal to or younger than Carboniferous.
Diagram showing the CL images of zircon grains from (a) biotite granite (BG) and (b) tourmaline granite (TG). (c) LA-ICP-MS U–Pb zircon age determination of BG; (d) LA-ICP-MS U–Pb zircon age determination of TG; (e) age distribution of zircon grains in BG and the TG; and (f) the age distribution of zircon grains from a siltstone unit in the Mawchi Group.
X-ray fluorescence (XRF) determinations on the BG and TG samples were performed at the Economic Geology Laboratory at Kyushu University, and the trace elements and REE were determined by ICP-MS at ALS, Canada. Major- and trace-element abundances of the HAG samples were determined by XRF spectrometry; trace elements and the REE were determined by ICP-MS in the ARC Centre of Excellence in Ore Deposits CODES-SES, University of Tasmania.
There are no earlier petrological or geochemical data available for the western part of the Mawchi Pluton; this is the first reported geochemical data. XRF and ICP-MS geochemical data are listed in Table 17.1. In general, Sn–W-bearing granites of the Mawchi Mine have 19 ppm to >1 wt% Sn and 10 ppm to >1 wt% W. In the BG, the high values of SiO2 (76.82–78.51 wt%) are associated with TiO2 (0.04–0.06 wt%), Al2O3 (11.72–12.26 wt%), FeO (0.71–1.03 wt%), MnO (0.03–0.06 wt%), MgO (0.19–0.28 wt%), CaO (0.08–0.74 wt%), Na2O (2.98–3.75 wt%), K2O (4.20–5.1 wt%) and P2O5 (0.01–0.02 wt%). The tourmalinized granites (TG and HAG) have relatively high SiO2 (73.06–85.81 wt%) with low TiO2 (0.01–0.05 wt%) and P2O5 (0.01–0.03 wt%), Al2O3 (8.57–15.75 wt%), FeO (0.51–4.77 wt%), MnO (0.03–0.32 wt%), MgO (0.09–0.69 wt%), CaO (0.03–1.36 wt%), Na2O (0.10–3.91 wt%) and K2O (1.08–5.99 wt%).
The geochemical behaviour of the major oxides and trace elements is discussed with reference to the SiO2 contents. SiO2 shows a linear positive correlation with Fe2O3 (Fig. 17.6a) and negative linear correlations with Al2O3 and K2O (Fig. 17.6b, c). Rb and SiO2 correlate positively in the BG, but negatively in the TG and the HAG (Fig. 17.6d). Although there is no distinct correlation between Rb and Cs in the BG, they show positive correlations in both the TG and the HAG (Fig. 17.6e). The A/CNK (molar Al2O3/(CaO + Na2O + K2O)) value in fresh BG samples ranges from 1.08 to 1.14, and is higher in the TG and HAG. Loss on ignition (LOI) is very low in the BG (0.45–0.73%) suggesting low volatile contents, but the LOI of the TG (1.26–2.84%) is relatively high, with values probably reflecting the alteration.
Variation diagrams of (a) FeO, (b) Al2O3, (c) K2O, (d) Rb v. SiO2 for the Mawchi granites; and (e) Rb v. Cs variation diagram.
The Mawchi granites are peraluminous with magnesian and high-K calc-alkalic characters (Aung Zaw Myint et al. 2013a). They have high contents of Rb (685–961 ppm in BG; 557–1200 ppm in TG; 287–1025 ppm in HAG), Cs (21–5 ppm in BG; 16–36 ppm in TG; 20–29 ppm in HAG) and moderate concentrations of Y (135–173 ppm in BG; 104–316 ppm in TG; 99–167 ppm in HAG) with depletions of Zr (35–121 ppm in BG; 14–116 ppm in TG; 15–92 ppm in HAG) and Hf (2–6 ppm in BG; 2–9 ppm in TG; 2–6 ppm in HAG). They contain relatively high concentrations of U (11–63 ppm in BG; 5–88 ppm in TG; 287–1025 ppm in HAG) and Th (10–59 ppm) with the Th/U ratio ranging over 1.65–2.76 in the BG, 0.76–2.51 in the TG and 0.92–2.28 in the HAG. They exhibit low K/Rb ratios (31–50), Nb/Ta ratios (0.58–6.08) and Zr/Hf ratios (6.54–14.84) with high Rb/Sr ratios (27–308) and Y/Ho ratios (32–48).
REE abundances in the granites are listed in Table 17.2 and the variation in REE contents is illustrated in the chondrite-normalized REE patterns (Fig. 17.7). All samples are characterized by the tetrad REE pattern and have deep negative Eu anomalies (Eu/Eu* values range from 0.005 to 0.049). The total REE contents range over 143–206 ppm in the BG, 158–251 ppm in the TG and 93–197 ppm in the HAG.
Chondrite-normalized rare earth element abundances for (a) BG (fresh), (b) TG and (c) highly altered granite (HAG) (normalizing values from Sun & McDonough 1989).
REE patterns for the TG and HAG are nearly identical, although the HREE contents in the TG are slightly higher than those of the HAG. This is probably due to extremely high abundance of quartz, tourmaline and deficiency of feldspar in the HAG. The BG has relatively higher LREE contents (La/YbN = 0.56–1.42) than the TG and HAG tourmaline granites with (La/Yb)N = 0.22–0.52).
The charge- and radius-controlled (CHARAC) trace-element distribution predominates in typical magmatic systems, but the lanthanide tetrad effect is frequently accompanied by non-CHARAC behaviour of trace elements (Bau 1996; Jahn et al. 2001). The Mawchi granites also exhibit non-CHARAC character (Fig. 17.8). This phenomenon (Bau 1996) is an expression of the particular physicochemical properties of the high-silica magmatic systems, where transitional characters exist between pure silicate melts and hydrothermal fluids. Both the biotite and tourmaline granites exhibit REE tetrad patterns that are consistent with fluid-melt reactions during crystallization, the principal factor controlling their heavy rare earth element (HREE) enrichment (Irber 1999; Jahn et al. 2001; Zhenhua et al. 2002). BG, TG and HAG have the degree of the tetrad effect (TE 1,3) values of 1.09–1.15, 1.08–1.18 and 1.12–1.19, respectively, and all tetrad patterns are clearly visible (Aung Zaw Myint 2015).
Y/Ho and Zr/Hf variation diagram (Bau 1996) of Mawchi granites showing their non-CHARAC behaviour.
The Ti v. Zr/Hf variation diagram shows that Ti and Zr become progressively depleted between the BG and the TG, following the trend of magmatic differentiation and concurrent fluid–melt interaction (Fig. 17.9a, d). Nb also exhibits a depletion trend during this process (Fig. 17.9b). The curved positive correlation between Nb/Ta and Zr/Hf within the BG and TG illustrates the hydrothermal overprint (Fig. 17.9c). Although there is no direct field evidence that can pinpoint the relationship between the BG and the TG, their geochemical signatures suggest that the TG is more fractionated than the BG; the similar Nd isotope signatures of the BG and TG suggest that they were derived from the same parental magma. The decrease in the Zr/Hf ratio from the BG to the TG demonstrates their magmatic fractionation and their Zr/Hf ratio (<25) is consistent with their enrichment in Sn and W (Haapala & Lukkari 2005; Zaraisky et al. 2009). Differences in Rb and Cs contents (Fig. 17.6e) in the BG and tourmaline-rich HAG are related to the deficiency of K-feldspar in the latter.
(a) Ti v. Zr/Hf; (b) Zr/Hf v. Nb; and (c) Zr/Hf v. (Ta/Nb) variation diagrams describing the behaviour of HFSEs during fractionation of the Mawchi granites.
The progressive depletion of Zr and Hf contents between the BG and the TG suggests loss of magmatic zircons during magmatic differentiation. The low Zr concentrations reflect the felsic nature of the granite, consistent with the hypothesis that they were formed under low-temperature conditions and emplaced as high-level intrusions (Watson & Harrison 1983; Kalsbeek et al. 2001; Miller et al. 2003).
It is difficult to determine the genetic type of such highly differentiated and altered granites based on their geochemical signatures. In such cases, P2O5 contents can sometimes be used to distinguish I-type and S-type granites. In general, P2O5 increases slightly with increasing SiO2 content in S-type, and decreases with increasing SiO2 in I-type granite. P2O5 contents in highly fractionated I-type granites are therefore very low compared to S-type granites (Chappell &White 1974; Champion & Bultitude 2013). However, in highly fractionated magmatic systems, some S-type and A-type granites also show very low P2O5 contents (Jiang et al. 2011; Li et al. 2014); we therefore cannot rely on P2O5 in this case. However, 10 000 × Ga/Al ratios of BG and TG are high (>3.9) and their Zr + Nb + Ce + Y contents range over 250–430 ppm. These characteristics are typical of A-type granites and the Mawchi granites fall in the A-type regime of Whalen et al. (1987) (Fig. 17.10a–d). The above ratios are very useful to discriminate A-type granites from other granite types. In the triangular plot of Nb–Y–Ce (Eby 1992) both BG and TG have the geochemical character of the A2 group of A-type granites (Fig. 17.10d, e), consistent with a post-collisional setting and reactions with continental crust rocks.
(a–d) 10 000×Ga/Al v. Zr, Nb, Ce and Y variation diagrams of Whalen et al. (1987) which plot the Mawchi granites in the A-type field. (e) Nb–Y–Ce triangular plot (Eby 1992) showing that the Mawchi granites plot within the A2 group of A-type granites.
The origin of the peraluminous Mawchi granites is probably linked to the melting of calc-alkaline granites (Clemens et al. 1986; Creaser et al. 1991; Douce 1997) in an older magmatic arc in the shallow crust, as evidenced by the occurrence of Cretaceous inherited zircons. On the other hand, the primitive-mantle-normalized spider diagram of trace elements shows enrichment in U, Th and Ta, and of some of the large-ion lithophile elements (LILE) including Rb and Cs, but depletion in Ti, Nb, Ba and Sr (Fig. 17.11). Most of these features, such as the pronounced negative Ba, Sr, Nb and Ti and positive Rb, Pb and U anomalies, are consistent with the signatures of a crustal component (Champion & Bultitude 2013). In addition the abundance of Rb, Th and Y and low Nb/Ta and Zr/Hf values support a crustal derivation (Sun et al. 2005).
Chondrite-normalized trace-element abundances for biotite granite (lines) and tourmaline granite (shaded area) (normalizing values from McDonough et al. 1992).
Although granites from the Western Province have low initial 87Sr/86Sr ratios (ISr), which suggests a mantle origin (Cobbing 2011), some Middle Eocene granites from this belt (e.g. Sedo and MEC granites) show 143Nd/144Nd ratios εNd (T) and ISr values that suggest their derivation from old crustal material (Mitchell et al. 2012). Furthermore, our unpublished Nd isotopic data has found that the BG and the TG have low εNd(T) of −10.57 to −10.74 and −10.12 to −10.45, respectively, which suggests a crustal influence in the magmatism (Aung Zaw Myint 2015); the geochemical data of BG and TG fall partly in the field of experimental melts of felsic pelite (Fig. 17.12a, b). It is evident that the parental magma for the Mawchi granites underwent extensive magmatic differentiation, resulting from the melting of the crustal component which included metasediments and igneous rocks followed by the interaction with late-magmatic hydrothermal fluids, resulting in an HREE-enriched system.
(a, b) Link between the Mawchi granites and experimental melts derived from felsic pelites (Douce 1999).
As with most of the other Southeast Asian tin-bearing granites (Lehmann 1982), the Mawchi Granite is of transitional magmatic–hydrothermal origin. In the Sn and TiO2 variation diagram, the Sn values of the BG suggest that Sn increased along with the magma differentiation trend (Fig. 17.13a). However, during the intense fluid–melt interactions (e.g. for the TG), as Ti content decreases the Sn content remains nearly constant. In the Mawchi granites, Sn and W demonstrate opposite geochemical behaviours in the Rb/Sr system of BG (Fig. 17.13b, c) and a dramatic increase in W content is observed in the HAG. It is therefore evident that the W enrichment is controlled by late-stage hydrothermal activity.
(a) TiO2 v. Sn variation diagram; (b, c) Rb/Sr v. Sn and Rb/Sr v. W, demonstrating the opposite behaviour of Sn and W in the Mawchi magma.
Yttrium has a positive correlation with the HREE (Fig. 17.14a), and their coupled enrichment occurs throughout the co-existing fluid–melt reaction. The decrease of Y and HREE contents in the HAG is probably due to the absence of feldspars in the highly altered rock, which contains mostly quartz and late-generation tourmaline. On the other hand the relationship between Y and the LREE is more diffuse, but in the LREE-enriched BG there is a slight negative correlation (Fig. 17.14b). In addition it can be suggested that the depletion of LREE contents with the increase in Y content from BG to TG corresponds to mineral precipitation from a fluid that experienced prolonged interaction with the host rocks (Monecke et al. 2002). Although Y in both the BG and the tourmalinized granites show negative correlations with (La/Yb)N, the correlation trend in the BG is gentler, confirming that low (La/Yb)N ratios were coupled with high Y and that the HREE abundances increased more vigorously during the late stage of fluid–magma reaction (Fig. 17.14c).
Variation diagrams of (a) Y v. HREE; (b) Y v. LREE and (c) (La/Yb)N v, Y, illustrating the relationship between the rare earth elements and yttrium in the Mawchi magmatic–hydrothermal system.
According to the zircon U–Pb dating, the age of the Mawchi Sn–W granite is younger than other tin granites in the Central Belt of Myanmar. NW of Mawchi, zircon U–Pb dating has yielded an age of 48 ± 0.9 Ma (Middle Eocene) for the Sedo Granite of the Yamethin area; K–Ar dating of biotite (Brook & Snelling 1976) gave a Late Paleocene age of 55 ± 1 Ma for the Padatchaung Granite of the Pyinmana area (Fig. 17.15). In the Thanintharyi Region, SHRIMP zircon data yielded an age of 61.7 ± 1.3 Ma (Paleocene) for the Hermingyi Granite (Pickard & Barley, unpublished data, cited in Mitchell et al. 2012) and the Wagone Granite has an Ar–Ar age of 60.2 ± 0.6 Ma (unpublished data, location shown in Fig. 17.1). The Phuket Island Granite, which is located at the southern extremity of this province, has a U–Pb zircon age of 81.2 Ma with inherited ages of 212 Ma and 214 Ma in cores of zircon crystals (Searle et al. 2012). Late Triassic zircon cores of the Phuket Granite indicate that subduction-related calc-alkaline plutonism in the Western Province was probably initiated at the same time as the Permo-Triassic magmatism in the Eastern Province (Searle et al. 2012). Early Cretaceous magmatism in the Mogok Metamorphic Belt is represented partly by I-type hornblende and biotite granites, probably originating from subducted oceanic crust (Barley et al. 2003; Searle et al. 2007) and partly from crustal-derived diorite (Mitchell et al. 2012). These granites were followed by Late Cretaceous I-type arc magmatism (91–106 Ma) with a juvenile mantle origin, and these events can probably be correlated with Cretaceous plutonism in the eastern Transhimalayas (Mitchell et al. 2012). The youngest granites of the eastern Transhimalayas also are of Paleocene age (Chiu et al. 2009). The MMB and Slate Belt (SB) can be regarded as part of the Western Myanmar Block, and most magmatic and metamorphic events in and near the MMB were related to the westwards subduction of Neotethys I Ocean (Mitchell et al. 2012).
The age of tin granites in the Western Granite Province (1Mitchell et al. 2012; *Searle et al. 2012; aunpublished data).
In contrast, the granitoids of Tanintharyi Region are hosted by the SB and their geology is relatively simple, without overlapping of MMB with SB. Recent U–Pb zircon geochronological data have suggested a new concept concerning the emplacement of granites in Tanintharyi Region, by back-arc and island-arc magmatism related to westwards subduction. The results of Sanematsu et al. (2014) indicate that the ages of granites become younger gradually from the eastern back-arc side (85–59 Ma) to the western island-arc side (61–48 Ma). Granites on the back-arc side were contaminated by crustal matter and formed tin granites.
Late Cretaceous (73 Ma) to early Tertiary (52 Ma) A-type tin granite emplacement within the northern extension of the Western Province was recently recognized, and is associated with an extensional regime following microplate collision or back-arc extension related to Neotethyan subduction (Chen et al. 2015). Emplacement of granites at 42 Ma in the Mawchi area originated as the result of lithospheric thinning and decompressional melting in an extensional regime following collision, together with or before regional Late Eocene–Early Miocene extension (Mitchell et al. 2012; Morley 2012, 2014). Previous 31–40 Ma U–Th–Pb ages (Searle et al. 2007) and 43 Ma SHRIMP zircon rims (Barley et al. 2003), without deformation in the Eocene granites of the MMB, suggest that a high-temperature thermal event was the cause of the intrusion of these and other granites during regional extension (Mitchell et al. 2012).
Models of the geotectonic setting indicate that the biotite and tourmaline granites have geochemical affinities with both post-collisional and post-orogenic granites and within-plate granites (WPG) (Fig. 17.16a–c). Generally, the composition of A-type granites ranges from peralkaline to slightly peraluminous (Whalen et al. 1996) and develop in various geodynamic settings from within-plates to plate boundaries, including late- to post-collisional tectonic settings (e.g. Whalen & Currie 1990; Eby 1992; Whalen et al. 1994; Bonin 2007). Their components could be derived from both the crust and the mantle (Trumbull et al. 2004; Goodge & Vervoort 2006; Wei et al. 2008). Aluminous A-type granites (King et al. 1997) are prevalent in a post-orogenic or post-collisional setting (Jung et al. 1998; Konopelko et al. 2007; Chen et al. 2015), while alkaline to peralkaline A-type granites occur in within-plate settings (Li et al. 2014). The Mawchi biotite granite (BG) therefore has a post-collisional geochemical setting, suggesting a highly fractionated melt system but with an enrichment of HREE and Y which is explained here as due to fluid–magma reactions.
(a) Rb v. Y + Nb discrimination diagram (Pearce et al. 1984); (b) Rb/30 v. Hf–Tax3 triangular plot (Harris et al. 1986); (c) R1 v. R2 (Batchelor & Bowden 1985) showing the tectonic affinities of Mawchi granites.
The granites of the Mawchi area are of Middle Eocene age. Their origin has been attributed to a magma system which was generated by melting of crustal rocks, and their magmatic differentiation was followed by fluid–magma reactions. Lithospheric delamination due to regional extension provided a favourable setting for the emplacement of this tin granite. Magmatic-hydrothermal interaction and post-magmatic fluid–melt interaction resulted in enrichments in W, HREE and Y. Fractionation associated with hydrothermal inputs was responsible for the development of the mineralized tourmaline granite and the filling of the Sn–W quartz veins along cooling fractures. Trace-element geochemistry, distinct negative anomalies of primitive-mantle-normalized Ba, Sr, Ti and Nb and the positive Pb anomaly, and low εNd(T) values indicate that crustal rocks have been involved in the fractionation of peraluminous granitic rocks and their Sn–W enrichment. Cretaceous zircons affirm a possible contribution from older magmatic rocks. U–Pb zircon dating has shown that granite emplacement in the Mawchi area occurred later than the emplacement of the other tin granites in the Western Province.
Sincere thanks are due to the Japan International Cooperation Agency (JICA) for a PhD scholarship for the first author and to the Global COE Program of Kyushu University for support for the fieldwork. Special thanks are due to Dr Michael Crow and the reviewers for their critical reading of manuscript and constructive suggestions. The authors are deeply grateful to Professor Bo Min Jahn (National Taiwan University) and Professor Bernd Lehmann (Technical University of Clausthal) for their fruitful suggestions and comments on an earlier version of the manuscript. We thank Minn Chit Thu, Aung Aye Lynn, Kaung Myat Htun and Lazing Nawhtral for their valuable help during field investigation. Special thanks are also due to Kayah Metal Production Co. Ltd and Ye Htut Kyaw Mining Co. Ltd for generously providing office space for the first author to work in, and for providing logistical support.
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