Timing of Syenite‐Charnockite Magmatism and Ruby and Sapphire Metamorphism in the Mogok Valley Region, Myanmar

Abstract

The Mogok metamorphic belt (MMB) extends for over 1,000 km along central Burma from the Andaman Sea to the East Himalayan syntaxis and represents exhumed lower and middle crustal metamorphic rocks of the Sibumasu plate. In the Mogok valley region, the MMB consists of regional high‐grade marbles containing calcite + phlogopite + spinel + apatite ± diopside ± olivine and hosts world class ruby and sapphire gemstones. The coarse‐grained marbles have been intruded by orthopyroxene‐ and clinopyroxene‐bearing charnockite‐syenite sheet‐like intrusions that have skarns around the margins. Syenites range from hornblende‐ to quartz‐bearing and frequently show layering that could be a primary igneous texture or a later metamorphic overprint. Calc‐silicate skarns contain both rubies and blue sapphires with large biotites. Rubies occur in marbles with scapolite, phlogopite, graphite, occasional diopside, and blue apatite. Both marbles and syenites have been intruded by the Miocene Kabaing garnet‐muscovite‐biotite peraluminous leucogranite. New mapping and structural observations combined with U‐Th‐Pb zircon, monazite, and titanite geochronology from syenites, charnockites, leucogranites, meta‐rhyolite‐tuffs, and skarns have revealed a complex multiphase igneous and metamorphic history for the MMB. U‐Pb zircon ages of the charnockite‐syenites fall into three categories, Jurassic (170–168 Ma), latest Cretaceous to early Paleocene (~68‐63 Ma), and late Eocene–Oligocene (44–21 Ma). New ages from five samples suggest that metamorphism in the presence of garnet and melt occurred between ~45 and 24 Ma. U‐Pb titanite ages from the ruby marbles and meta‐skarns at Le Oo mine in the Mogok valley are 21 Ma, similar to titanite ages from an adjacent syenite (22 Ma). U‐Th‐Pb dating shows that all the metamorphic ages are Late Cretaceous–early Miocene and related to the India‐Sibumasu collision.

Full text

1
Timing of syenite-charnockite magmatism and ruby- and sapphire 1
metamorphism in the Mogok valley region, Myanmar 2
3
M.P. Searle1, J.M. Garber2,3, B.R. Hacker2, Kyi Htun4, N.J. Gardiner1,5, D.J. Waters1, 4
and L.J. Robb1 5
6
1. Department of Earth Sciences, Oxford University, South Parks Rd. Oxford OX13AN, UK 7
2. Earth Research Institute, University of California, Santa Barbara, CA, 93106, USA 8
3. Present address: Department of Geosciences, Penn State University, University Park, PA 9
16803 USA 10
4. Geological Consultant, B201, B14 Ward, Thanthumar Street, Okkalapa Township, Yangon, 11
Myanmar 12
5. Present address: School of Earth and Environmental Sciences, University of St-Andrews, 13
Scotland, UK 14
15
Abstract 16
The Mogok Metamorphic Belt (MMB) extends for over 1000 km along central Burma from 17
the Andaman Sea to the East Himalayan syntaxis and represents exhumed lower and middle 18
crustal metamorphic rocks of the Sibumasu plate. In the Mogok valley region the MMB 19
consists of regional high-grade marbles containing calcite+phlogopite+spinel+apatite 20
±diopside±olivine, and hosts world-class ruby and sapphire gemstones. The coarse-grained 21
marbles have been intruded by orthopyroxene- and clinopyroxene-bearing charnockite-22
syenite sheet-like intrusions that have skarns around the margins. Syenites range from 23
hornblende- to quartz-bearing and frequently show layering that could be a primary igneous 24
texture or a later metamorphic overprint. Calc-silicate skarns contain both rubies and blue 25
sapphires with large biotites. Rubies occur in marbles with scapolite, phlogopite, graphite, 26
occasional diopside and blue apatite. Both marbles and syenites have been intruded by the 27
Miocene Kabaing garnet-muscovite-biotite peraluminous leucogranite. New mapping and 28
structural observations combined with U-Th-Pb zircon, monazite and titanite geochronology 29
from syenites, charnockites, leucogranites, meta-rhyolite-tuffs, and skarns has revealed a 30
complex multi-phase igneous and metamorphic history for the MMB. U-Pb zircon ages of the 31
charnockite-syenites fall into three categories, Jurassic (170–168 Ma), latest Cretaceous to 32
early Paleocene (~68-63 Ma) and late Eocene – Oligocene (44–21Ma). New ages from five 33
samples suggest that metamorphism in the presence of garnet and melt occurred between 34

2
~45-24Ma. U-Pb titanite ages from the ruby marbles and meta-skarns at Le Oo mine in the 35
Mogok valley are 21 Ma, similar to titanite ages from an adjacent syenite (22Ma). U-Th-Pb 36
dating shows that all the metamorphic ages are Late Cretaceous–early Miocene, and related 37
to the India–Sibumasu collision. 38
39
Key points: 40
• Rubies and sapphires in granulite facies marbles from the Mogok Metamorphic belt, Myanmar are 41
spatially associated with charnockite-syenite sill-like intrusions and surrounding skarns. 42
• U-Th-Pb LA-ICPMS dating of zircon, monazite, and titanite shows that there were two groups of 43
charnockite-syenite dates, one Jurassic in age (170–168 Ma), and one latest Cretaceous to early 44
Miocene (~68–21 Ma). 45
• Regional granulite-facies metamorphism along the Mogok Metamorphic belt is Late Cretaceous to 46
Oligocene or Early Miocene in age (~68-21 Ma), peaking with garnet-present melting between 45–21 47
Ma. 48
49
Correspondence to: 50
M.P. Searle: mike.searle@earth.ox.ac.uk 51
52
Author ORCIDs 53
Mike Searle 0000-0001-6904-6398 54
Joshua Garber 0000-0001-5313-0982 55
Bradley Hacker 0000-0003-2732-1712 56
Nicholas Gardiner 0000-0003-3465-9295 57
David Waters 0000-0001-9105-9953 58
Laurence Robb 0000-0002-9032-1320 59
60
61
1. Introduction 62
High-grade granulite- and upper amphibolite-facies marbles form a major part of the 63
Mogok Metamorphic Belt (MMB), Myanmar (Burma), stretching the length of northern 64
Myanmar from the East Himalayan syntaxis south through the eastern Kachin state and the 65
Mogok region to Mandalay (Fig.1). The Mogok valley in northern Burma (Myanmar) 66
contains some of world’s best examples of gem-quality spinel, ruby and sapphire, extracted 67
from upper amphibolite- to granulite-facies marbles in the MMB (Gordon, 1888; O’Connor, 68
1888; LaTouche, 1913; Fermor, 1931; Chhibber, 1934; Searle & Haq, 1964; Searle et al., 69

3
2007, 2017). The marbles are white, coarse-grained and contain calcite, phlogopite, graphite, 70
red spinel, diopside and forsterite. Rare metapelitic rocks contain sillimanite and garnet, and 71
some leucogneisses of possible metavolcanic origin are also present. The marbles have been 72
intruded by a series of charnokite and syenite intrusions that have sill-like structures and calc-73
silicate skarns around the margins. Rare garnet- and tourmaline-bearing leucogranites intrude 74
the marbles and are probably related to the 16.8 Ma Kabaing granite intrusion west of Mogok 75
(Gardiner et al., 2016). 76
The age of metamorphism and formation of the rubies and sapphires in Mogok has 77
long been debated. Prior to any geochronology, the Mogok granulite-facies rocks were 78
thought to be part of the Precambrian basement (Fermor, 1931; Chhibber, 1934a, b). Mitchell 79
et al. (2007) proposed two metamorphic events, one Early Permian and the second Early 80
Jurassic, based on field relationships in the region south of Mandalay. Mitchell et al. (2012) 81
published U-Pb zircon ages from a variety of igneous rocks along the MMB south of 82
Mandalay, and proposed an Early Cretaceous age of metamorphism, with the main fabric-83
forming metamorphic event pre-dating the India – Asia collision. However, these ages were 84
mainly from diorites and granites, and may not date timing of regional metamorphism. The 85
MMB has a range of biotite- and hornblende-bearing granites, granodiorites and diorites that 86
are thought to be related to the pre-collision, subduction-related Gangdese-type plutons along 87
the southern margin of Asia which range in age from Late Jurassic to Early Eocene (Lhasa 88
block; Chung et al., 2005). 89
D.L. Searle and Haq (1964) first suggested that metamorphism along the MMB was 90
related to the Cenozoic Himalayan orogeny. A Himalayan connection was confirmed by 91
preliminary U-Th-Pb ID-TIMS and LA-ICPMS dating of metamorphic monazite, zircon, 92
xenotime and thorite by M.P. Searle et al. (2007). These data suggested two main periods of 93
high-grade metamorphism in the MMB around Mandalay: (1) a late Cretaceous–Paleocene 94
event that ended with intrusion of 59 Ma biotite granite dykes which cut metamorphic fabrics 95
at Belin quarry, and (2) a late Eocene–Oligocene main event (at least 37–29 Ma, possibly 96
extending from 47 Ma to 25 Ma), when monazite grew at high temperature, sillimanite + 97
muscovite replaced andalusite, zircon rims grew at 47–43 Ma, and tourmaline-bearing 98
leucogranites formed at 45.5 ± 0.6 Ma and 25.5 ± 0.7 Ma (Searle et al., 2007). Metamorphic 99
monazites from rare sillimanite- and andalusite-bearing pelites from Kyaushe (600–650ºC; 100
4.4–4.9 kbar) revealed a peak-metamorphic age of 29.3 ± 0.5 Ma, and garnet + tourmaline 101
leucogranite dykes cutting all earlier fabrics were dated at 24.5 ± 0.3 Ma (Searle et. al. 2007). 102
Until recently, the age of crystallisation of the spinel, ruby and sapphire-bearing marbles 103

4
around the Mogok valley has not been directly dated. These gems are common in the Mogok 104
valley and hills to the north, but rarely occur along the MMB south and north of Mogok. 105
Thus, the two metamorphic episodes, pre-57 Ma (Paleocene) and 47–29 Ma (late Eocene–106
Oligocene), proposed by Searle et al. (2007, 2017) apply to the MMB around Mandalay, but 107
not necessarily to the ruby- and sapphire-bearing marbles in the gems fields of the Mogok 108
valley farther north. Another question of direct relevance to the genesis of the MMB is why 109
gem quality ruby and sapphire are localized in the Mogok Valley north of Mandalay, and not 110
widely distributed throughout the belt. 111
Recently Thu & Zaw (2017) and Sutherland et al. (2019) reported some preliminary 112
U-Pb age data from Mogok rubies. A titanite inclusion in ruby from Thurein Taung gave a U-113
Pb date of 32.4 Ma, and subordinate nepheline was also noted as inclusions in the ruby. The 114
adjacent syenite gave a U-Pb zircon date of 25 Ma (Thu, 2007; quoted in Sutherland et al., 115
2019). Also reported is a U-Pb date of 16.1 Ma in an extremely rare painite 116
(CaZrAl9O15(BO3)) overgrowth on ruby from Wet Loo mine (Thu, 2007). A zircon inclusion 117
from a ruby collected at Mong Hsu gave a U-Pb date of 23.9 Ma (Sutherland et al., 2019), 118
and is interpreted as the age of regional metamorphism. The geological and geochronological 119
evidence seems to suggest that the Mogok Metamorphic Belt was the site of Jurassic – 120
Paleocene Andean-type granitoid-diorite intrusions and localised low-pressure 121
metamorphism, but the main high-temperature metamorphic event was Eocene-Oligocene, 122
and related to the Himalayan orogenic event. 123
The major ruby and sapphire mines are located mainly in the Mogok valley and the 124
hills to the north (Fermor, 1934; Chhibber, 1934; Clegg & Iyer, 1953; Searle & Haq, 1964; 125
Themelis, 2008), and are uncommon to the south of Mogok (Fig. 2). Over one thousand 126
mines in the Mogok valley region produce spinel, ruby and sapphire, and numerous other 127
gems. The Mogok valley has been closed to foreigners for decades but re-opened in 2012. It 128
remains sporadically closed due to local insurgencies, now from the TNLA (Palaung armed 129
ethnic group). Many of the larger ruby and sapphire mines remain off-limits to foreigners, but 130
smaller locally owned mines are accessible. Recent changes in mining law in Myanmar have 131
released the significant acreage to local artisanal miners, resulting in a gem rush of 132
exploration in the Mogok valley area. 133
Since 2014 we have carried out extensive field investigations around the Mogok 134
valley and surrounding hills, and visited many of the ruby and sapphire mines in the region. 135
A new geological map of the Mogok valley area is presented in Fig. 3. Dense tropical jungle 136
and thick red laterite soil make geological mapping difficult, and almost all critical contacts 137

5
are not exposed. We were able to access underground shafts in several ruby and sapphire 138
mines where continuous exposures made key structural observations possible. This paper 139
presents new mapping and structural relationships together with new U-Th-Pb LA-ICPMS 140
dating of zircon, monazite, and titanite that constrain timing of intrusion of the syenite–141
charnockite intrusions and the age of metamorphism in the Mogok valley. Field relationships 142
clearly suggest that the distribution of ruby and sapphire in the Mogok valley is spatially 143
related to the syenite and charnockite intrusions, but our data suggest a complex relationship 144
between pluton emplacement, skarn formation, and ruby and sapphire crystallization. 145
146
2. Geology of the Mogok region 147
The geology of the Mogok valley region was first mapped and studied by Gordon 148
(1888), O’Connor (1888), LaTouche (1913), Fermor (1931, 1934), and Barrington-Brown 149
(1933). The most detailed descriptions were made by Chhibber (1934), and a geological map 150
was published by Clegg and Iyer (1953). In the days before radiometric age dating, these 151
authors presumed that the high-grade metamorphic rocks in Mogok were Archean in age. 152
They also suggested that the rubies in Mogok were formed by contact metamorphism around 153
large granites such as the Kabaing pluton. However, metamorphism in the Mogok region is 154
of regional extent (Searle & Haq, 1964; Searle et al., 2007, 2017) and not restricted to contact 155
aureoles around granites. 156
The Mogok valley area is dominated by a ~10 km wide central zone of thick, coarse-157
grained calcite + phlogopite + graphite + spinel ± apatite-bearing marble that hosts many 158
spinel, ruby and sapphire mines (Fig. 3). Marbles also contain scapolite, wollastonite, 159
clinopyroxene (diopside) and olivine (forsterite) indicating granulite-facies conditions of 160
formation. The marbles have been intruded by a large syenite–charnockite intrusion (Taung-161
met syenite) with several offshoots, mainly aligned as large sills (Fig. 4a). On Taung-met 162
mountain the syenites commonly show inter-layered felsic and mafic bands (Fig. 4b) with 163
small veins of felsic syenite cross-cutting the igneous layering (Fig. 4c,d). Coarse-grained 164
orthopyroxene-bearing charnockites have igneous textures but are usually interpreted as 165
granulite facies rocks (Fig. 4e). The Taung-met syenite intrusion is approximately 400 meters 166
thick and may extend west as far as the Baw-lon gyi area north of Kyatpyin (Fig. 5f), making 167
this one of the largest alkaline igneous intrusions in Myanmar. 168
In the western part of the Mogok region around the Htay pying ruby mine, small 169
dykes of orthopyroxene charnockite intrude Mogok marbles (Fig. 5a,b). The surrounding 170
marbles are rich in high-quality rubies and have sapphire-bearing skarns around the dyke
171

6
margins. Further west at Yadarnay kaday kadar mine next to Kyauk-pya-that pagoda hill, a 172
variety of clinopyroxene syenites and orthopyroxene-bearing charnockites intrude Mogok 173
marble (Fig. 5c). The marbles are laden with both rubies and sapphires, and the margins have 174
extensive metasomatic veining with large biotites (Fig. 5d). East of Mogok the syenite-175
marble contact is well exposed at Nga Yant (Fig. 5e). The syenite shows strong magmatic 176
layering with interbanded felsic and mafic syenites (Fig. 5f). 177
The Le Oo mine site east of Mogok town shows clear field relationships with a sharp 178
contact between the Mogok marbles and syenite-charnockite intrusions (Fig. 6a). The 179
intrusions vary compositionally between two pyroxene-charnockites and quartz syenites (Fig. 180
6b). Some show perthitic textures with K-feldspar and quartz, and a mafic constituent 181
consisting of both clinopyroxene and amphibole. Calc-silicate skarns around the intrusion 182
margins are rich in ruby (Fig. 6c,d) and sapphire (Fig. 6e). Individual ruby crystals can reach 183
up to 4-5 cm (Fig. 6f). Pale blue apatites are common across the Mogok marbles and can 184
reach up to 5 cm length (Fig. 6g). Extensive metasomatism along the marble contacts is 185
evident from large biotites, and hydrothermal minerals such as sodalite (Fig. 5h). 186
High-grade regional metamorphic rocks comprising garnet + sillimanite gneisses and 187
migmatites crop out to the SE of the Mogok valley, but are poorly mapped. The surrounding 188
rocks to the east comprise the Neoproterozoic – Cambrian Chaung Magyi Group (Dew et al., 189
2019), although the nature of its contact with the Mogok metamorphic rocks is not known. 190
Overlying the Chaung Magyi Group are the Cambrian-Early Ordovician Pangyun Formation 191
quartzites, sandstones and siltstones, and the Bawdwin volcanic series which host the large 192
Pb-Zn-(Cu-Ag-Ni) deposits at Bawdwin mine (Gardiner et al., 2017). To the west of the 193
Mogok valley a large leucogranite intrusion, the Kabaing granite, dated at 16.8 ± 0.5 Ma (U-194
Pb zircon) intrudes the Mogok marbles and syenites (Gardiner et al., 2016). To the north in 195
the Pyang-yuang area (Bernard-myo; Figs. 2,3) a large mass of peridotite, including dunite, 196
harzburgite and hornblende-bearing peridotite, was thought to represent an ophiolitic mantle 197
rock (Searle et al., 2017), but may instead be a layered ultramafic intrusion, possibly related 198
to the adjacent syenite intrusion at Taung-met (Fig. 2). No gabbros, sheeted dykes or pillow 199
lavas are present at Pyang-yuang. Gem-quality peridot (olivine) and enstatite is mined from 200
this massif. Marbles containing abundant rubies and sapphires crop out in the hills 201
immediately north of the peridotite ridge, at Ah Chauk and Htin-Shu Taung mines (Fig. 3). 202
The northern boundary of the Mogok region is a prominent north-dipping fault showing both 203
normal and dextral fabrics, along the Momeik valley. 204

7
The structure of the Mogok valley is difficult to ascertain in detail due to the heavy 205
jungle cover, but regional marble layers seem to dip consistently at steep angles, 45–60o SE. 206
Thus, the garnet + sillimanite gneisses to the SE are structurally higher, above the Mogok 207
marbles, and the Pyang-yuang ultramafic intrusion in the north is structurally beneath the 208
Mogok marbles (Fig. 2). It is possible that the entire Mogok massif represents a giant 209
metamorphic core complex structurally below overlying Neoproterozoic and Paleozoic 210
sedimentary rocks (Fig. 2), although this is speculative since the whole area is covered in 211
dense forest and in need of detailed mapping. 212
213
2.1. Spinel, ruby and sapphire-bearing marbles 214
Red spinel (MgAl2O4) is the most common gem mineral in Mogok marbles, 215
frequently forming euhedral octahedra within coarse-grained marble. Ruby and sapphire 216
(corundum Al2O3) differ in colour only as defined by their trace contents of iron, titanium, 217
vanadium and chromium; these phases both occur in Mogok marbles, with sapphires possibly 218
more abundant in skarns. The presence of diopside and forsterite in marble requires 219
amphibolite or granulite facies metamorphic conditions (Bowen, 1940). The highest grade 220
olivine (forsterite)-bearing marbles have the paragenesis Cal + Dol + Fo + Spl + Phl + Amp + 221
Grt and contain gem-quality rubies and sapphires; spinel is not present with ruby or sapphire. 222
Impure marbles may contain micas (phengite, biotite, fuchsite), graphite from organic debris, 223
feldspars and garnet. Pale blue apatite crystals that reach 5 cm or more in size are present in 224
many Mogok marbles. Calc-silicate rocks contain Scp + Di + Cal + Qtz + Kfs + Grt + Ttn. 225
Clinohumite and scapolite-bearing assemblages yield high metamorphic grades at >780–226
810oC and 0.8 GPa (Thu & Enami, 2018). The calcite–dolomite geothermometer also 227
suggests a minimum temperature of 720–765oC, and a possible equilibrium temperature up to 228
780–860oC (Thu & Enami, 2018). These temperatures are consistent with granulite-facies 229
metamorphic conditions, in which decarbonation of buried carbonate-rich rocks released 230
CO2-rich fluids, while some H2O fluids may also have been associated with acidic igneous 231
melts (Yui et al., 2008). The presence of uncommon wollastonite (CaSiO3) in some skarns in 232
Mogok indicates high temperature – low pressure breakdown of calcite + quartz with the 233
release of CO2. 234
Potential protoliths of the Mogok marbles are the thick Permian Fusulinid-bearing 235
limestones (Chhibber, 1934a) which are widespread across SE Asia, or the mid-Cretaceous 236
Orbitolina-bearing limestones (Clegg, 1941). The Neoproterozoic - Early Cambrian Chaung 237
Magyi Group and the Ordovician – Devonian argillaceous limestones of the Shan Plateau are 238

8
probably too muddy to be suitable protoliths for the purer marbles in Mogok. Elevated 239
aluminium content could be a result of specific aluminium-rich, silica-poor source rocks, 240
such as laterites or evaporites. MgO combined with Al2O3 to form spinel at low SiO2 and 241
CaO activities, and corundum formed locally (Sutherland et al., 2019). Spinels are generally 242
not found together with ruby or sapphire in the field, suggesting that variable Mg activity in 243
the marbles may have controlled the occurrence of corundum. High-temperature fluids (CO2 244
and H2O) were driven off from surrounding skarns, and infiltrated the marbles possibly 245
during intrusion of the hot, dry charnockitic magmas. Metasomatic phases such as sodalite 246
and lapis lazuli are common in several Mogok localities. The close association of 247
charnockites with gem minerals (notably ruby, sapphire, spinel, cordierite, and sapphirine) 248
has also been noted in the Proterozoic Highland Group of Sri Lanka, where the gems have 249
been related to contact metamorphism around ultra-high temperature charnockite intrusions 250
(Rupasinghe and Dissanayake, 1985). 251
252
2.2. Pelites 253
Pelitic rocks containing the assemblage Grt + Bt + Pl + Sil + Qtz ± Crd ± Spl are 254
present but uncommon along the MMB, making precise PT conditions difficult to ascertain. 255
The metamorphosed pelites, now garnet + sillimanite gneisses, are called khondalites in the 256
older literature (Chhibber, 1934a,b). Rare pelites containing garnet and sillimanite at 257
Kyaushe, south of Mandalay, revealed PT conditions of 600–650ºC and 4.4–4.9 kbar, and a 258
peak-metamorphic U-Pb monazite age of 29.3 ± 0.5 Ma (Searle et al., 2007). Kyanite is 259
uncommon but occurs in the Kyaushe region. Yonemura et al. (2013) obtained PT conditions 260
of 6.5–8.7 kbar and 800–950ºC from granulite-facies gneisses, and also published imprecise 261
U-Th-Pb ages of <50 Ma. Core samples from the Letpanhla–Kyitauk Pauk gold mine in the 262
western part of the Mogok area, adjacent to the Sagaing fault, show extensive garnet + 263
sillimanite migmatites with late cordierite, rock types that are not exposed in the Mogok 264
valley or in the many quarries south of Mandalay along the MMB. 265
266
2.3. Charnockites and Syenites 267
2.3.1. Definitions. 268
Syenites are defined as coarse-grained, intrusive igneous rocks with essential K-269
feldspar, frequently with perthitic textures, and ferromagnesian minerals (biotite, hornblende, 270
clinopyroxene, orthopyroxene). A few syenite bodies have the full range from ultramafic 271
jacupirangites (amphibole, pyroxene, biotite rocks) to quartz syenites. Syenites are generally 272

9
associated with anorogenic intra-plate intrusions, or along rifted continental margins such as 273
along the East Africa rift system. It is unusual that the differentiated sequence from alkaline 274
ultramafic to quartz syenite occurs in one intrusion, but one example, the Borralan intrusion 275
in the Moine thrust zone, NW Scotland (Parsons, 1999; Searle et al., 2010), shows a similar 276
compositional range. The Borralan intrusion includes an early suite of pyroxenites, nepheline 277
syenites and pseudoleucite-bearing syenites, and a later suite of feldspathic and quartz 278
syenites. It was intruded into Cambrian sedimentary rocks and Ordovician marbles forming a 279
high-grade contact metamorphic aureole of yellowish brucite- and pyroxene marble. The 280
dolomites were metasomatised and intruded as carbonatite sheets. The Borralan syenite 281
intrusion thus has many similarities with the Mogok syenites. 282
Charnockites were originally defined as orthopyroxene-bearing (A-type) granite-283
granodiorites (opdalites), tonalites (enderbites), or monzogranites (mangerites) (Howie, 1955; 284
LeMaitre et al., 2005). They were named from the tombstone rock on Col. Job Charnock’s 285
grave in Calcutta, India (Holland, 1893). Charnockites are hot (~1000ºC), dry, granulite 286
facies quartzofeldspathic rocks with orthopyroxene, that can be either igneous or 287
metamorphic in origin (Frost et al., 2000; Frost and Frost, 2008; Bhattacharya, 2010). Frost 288
and Frost (2008) defined charnockite as “an Opx- (or Fay-) bearing granitic rock that is 289
clearly of igneous origin or that is present as an orthogneiss within a granulite terrane”. 290
Charnockites can form by differentiation from anhydrous tholeiitic rocks with low water 291
activity, and enriched levels of CO2. The process of charnockitisation may be associated with 292
several processes including high-temperature dehydration melting of mafic to intermediate 293
protoliths, infiltration of CO2 from deep crust levels, and magmatic differentiation above 294
mantle anomalies such as rift zones or continental hot-spots. Charnockites, despite having an 295
igneous origin in many cases, are invariably associated with high-temperature granulite facies 296
terranes (Bhattacharya 2010). Most charnockites are Precambrian in age, with classic 297
examples from south India and Sri Lanka (Highland Group). Although granulites are 298
widespread in Phanerozoic rocks from a variety of lower crust tectonic settings (deep levels 299
of calc-alkaline batholiths, island arcs, or thickened continental plateau regions like Tibet), 300
there are only a few possible examples of Phanerozoic charnockites, for example in the Coast 301
Ranges of British Columbia, the Variscan Bohemian massif, Czech Republic, and the 302
Variscan Aracena belt, Spain (Windley, 1981). 303
Skarns are calcium-magnesium-iron-manganese aluminium silicate rocks that may 304
also be referred to as calc-silicate rocks. They are typically formed by metasomatic or 305
hydrothermal alteration of country rocks by fluids, usually only locally around igneous 306

10
intrusions (contact metamorphism), but in Mogok the skarn effects could be of more regional 307
extent. 308
309
2.3.2. Mogok charnockites, and syenites 310
The Mogok valley shows three or four major charnockite-syenite sill-like intrusions 311
into the marble; the largest, the Taung-met intrusion, forms the high mountain ridge north of 312
Mogok town (Fig. 4a). At least two further sills to the south have been mapped around 313
Dattaw, and at least three sills have been mapped in the west, north of Kyat-pyin (Fig. 3). It is 314
not clear from the limited exposures whether these sills are all part of one major intrusion, or 315
whether separate sills have intruded in the same general area. A wide range of alkali granites, 316
syenites and charnockites have intruded the MMB. These include hornblende-rich 317
melanocratic syenite, orthopyroxene-bearing charnockite, clinopyroxene + hornblende-318
bearing K-feldspar + plagioclase syenite, and quartz syenite. Some show compositional 319
layering that may be an original igneous texture; others also show a tectonic foliation with 320
aligned hornblende indicating a high-temperature fabric superimposed on the original 321
igneous rock. In this study, most of the samples from larger sheets are charnockitic, whereas 322
syenites are dominant in associations with marble and skarn at the mine localities. Rare 323
alkaline ultramafic rocks consisting of amphibole + clinopyroxene + biotite jacupirangites 324
(called ‘urtite’ in older literature; Chhibber 1934a,b) with no feldspar are associated with the 325
deepest levels of the syenite intrusions. Chhibber (1934a, p.123) described nepheline in 326
syenites from Chaunggyi; silica undersaturated melting could result in feldspathoids, such as 327
nepheline replacing albite and leucite replacing orthoclase, but none of our samples contain 328
nepheline. The map of Clegg and Iyer (1953) also shows several outcrops of nepheline 329
syenites, but we were unable to confirm this at several localities around Pingu Taung and 330
Kyauk pya-that (Fig. 3). 331
332
2.4. Granites and pegmatites 333
Uncommon leucogranites containing garnet, sillimanite and tourmaline intrude the 334
marbles and may be related to the large Kabaing granite intrusion, west of the Mogok valley. 335
The Kabaing leucogranite appears to be a large-scale mid-crustal intrusion, and differs from 336
the Himalayan leucogranites that are sill-like intrusions emanating from a thick sillimanite ± 337
cordierite migmatite terrane (Searle et al., 2010b). One possible source could be the 338
sillimanite migmatites known from drill cores in the Letpanhla–Kyitauk Pauk gold mine in 339
the western part of the Mogok area (Fig. 2), but other pelitic rocks are extremely rare along 340

11
the Mogok metamorphic belt. A suite of pegmatites associated with the Kabaing granite 341
contains gem-quality topaz, quartz, tourmaline, lepidolite and aquamarine (e.g. at Sakangyi 342
mine). K-feldspar augen gneisses are present in the area south of Mogok (e.g. Kyanikan, 343
Nattaung quarries), and are associated with in situ partial melting to form tourmaline ± garnet 344
leucogranites (Searle et al., 2007, 2017). 345
346
3. Field relations and U-Th-Pb geochronology 347
We present U-Th-Pb zircon, monazite and titanite dates and trace elements for 13 348
samples tied to structural mapping from across the Mogok valley. These samples are 349
described from three main regions (Fig. 3): (1) the Mogok valley and region to the east, 350
including the Le Oo, Dattaw and Pein Pyit (Gorkha, Nepali) ruby–sapphire mines; (2) the 351
Chaunggyi valley and Taung-met hill, north of the Mogok valley; and (3) the western part of 352
Mogok, including Baw Lon Ley, Baw Mar and Yadarney kadey kadar ruby–sapphire mines 353
around Kyat-pyin village. Figure 7 is a summary diagram showing the full range of dates and 354
minerals dated (titanite, zircon, monazite) from 13 samples across the Mogok region. 355
356
3.1. Methodology 357
We conducted LASS (laser-ablation split-stream inductively coupled plasma mass 358
spectrometry) measurements on zircon, monazite, and titanite. Mineral zoning was 359
qualitatively assessed in select samples with cathodoluminescence (CL) or X-ray maps 360
(measured by EPMA) and U-Pb dates and trace elements (Table S1) were measured 361
quantitatively on all samples during LASS. The LASS analytical protocols and data-362
reduction strategies have been presented in earlier papers (e.g., Kylander-Clark et al., 2013). 363
In summary, a Photon Machines 193 nm excimer laser and HelEx sample cell were used, and 364
data were collected on a Nu Plasma or Plasma 3D multicollector ICP-MS coupled to an 365
Agilent 7700S quadrupole ICP-MS. Analyses of NIST 612 glass and basalt standard BHVO-366
2 (Jochum et al., 2005) were interleaved with the unknowns as trace-element reference 367
materials, and well-characterized zircon, monazite, and titanite were interleaved as U-Th-Pb 368
reference materials. Data were processed using Iolite (Paton et al., 2011), which corrects for 369
machine drift and downhole inter-element fractionation using reference material. Most 370
analyses were standard LASS analyses in which all data from a single hole were interpreted 371
as a single date–trace element pair. Two samples, MY83 and MY164, were also evaluated 372
using depth profiling, in which the downhole variation in date–element data were treated as 373
spatially significant. 374

12
We interpret changes in mineral trace-element signatures in the following ways: 1) an 375
increase/decrease in Lu/Gd ratio indicates garnet breakdown/growth; 2) an increase/decrease 376
in Eu/Eu* ratio is compatible with plagioclase-rich melt injection/extraction or plagioclase 377
breakdown/growth; 3) an increase/decrease in Th/U reflects recrystallization in the presence 378
of a silicate melt/hydrous fluid. Unless otherwise noted, all dates quoted here for zircon are 379
207Pb/206Pb-corrected 238U/206Pb intercept dates, and all monazite dates are concordant 380
232Th/208Pb–238U/206Pb dates. Many of the monazite analyses have a concordant spread in 381
U/Pb–Th/Pb ratios that could be the result of Pb loss, mixing during laser ablation, long-term 382
recrystallization, or spatially heterogeneous short-term recrystallization. 383
384
3.2. Mogok valley and region to the east 385
The Mogok valley runs NE–SW between high mountain ridges of Taung-met to the north 386
and the more subdued jungle-clad hills toward Pyin-Oo Lyin to the south. The valley shows 387
at least 2 km thickness of coarse-grained, upper amphibolite or granulite facies marble that is 388
host to at least six major ruby–sapphire mining districts extracting gems from bedrock as well 389
as alluvial deposits (Bawpadan, Yebu, Le Oo, Dattaw, Onbin, Pein Pyit). The marbles are 390
intruded by several charnockite-syenite intrusions that appear to form sill-like structures. 391
Skarns are present around the margins with a concentration of sapphires as well as large 392
biotite flakes. Sample MY-LeOo is from a ruby-bearing marble from the Le Oo mine, MY-393
228 is from a coarse-grained clinopyroxene-bearing syenite collected in situ immediately 394
above the alluvial washing pits. Sample MY-229 is from a skarn along the syenite–marble 395
contact from the same mine; it consists of 50% clinopyroxene with K-feldspar and around 396
5% quartz. Two further samples (MY-142, MY-144) were collected from the Pein Pyit (also 397
called Nepali Gorkha) ruby–sapphire mine east of Le Oo and Mogok. MY-142 is a 398
leucocratic rock interbedded within marbles and contains lilac-coloured garnet + plagioclase 399
+ K-feldspar + biotite + apatite. MY-144 is a graphitic garnet + biotite + plagioclase rock, 400
inferred to represent a residual assemblage after loss of partial melt from a metapelite, 401
interbanded within marbles, one of the more aluminous samples we found in the Mogok 402
valley. 403
We dated zircon, monazite, and titanite from these samples (Fig. 8). Zircon in MY142 404
comprise tiny euhedral grains, of which the oldest are 111 Ma; this sample could be a tuff 405
that erupted at 111 Ma or a metasediment deposited after that time. Monazite from this rock 406
is mostly younger and shows a distinct drop in Eu/Eu* and Lu/Gd from ~78 Ma to 24 Ma, 407
compatible with melt extraction and the crystalization of garnet. Zircon from MY144 has a 408

13
large range in concordant dates—1.2 Ga to Cretaceous—typical of a metasedimentary rock 409
that might have a Cretaceous depositional age. Most of the zircon analyses are 74–24 Ma, 410
compatible with metamorphism ending by 24 Ma. Monazite from MY144 mirrors the 411
younger zircon, with a range from 80 Ma (high-Y, low-Th cores) to 35 Ma (low-Y, high-Th 412
rims) (Fig. S1). Both zircon and monazite have weak decreases in Eu/Eu*, and increases in 413
Th/U, compatible with melting; the reduction seen in zircon Lu/Gd is suggestive of the 414
growth of garnet, but this signature is absent from monazite. Zircon from MY-228 is entirely 415
Cenozoic and of constant composition, compatible with either igneous or metamorphic 416
parentage. The youngest titanite in MY229 and MY-LeOo are ~22 Ma and certainly 417
metamorphic in origin. 418
419
3.3. Chaunggyi valley and Taung-met hill 420
This region lies north of the Mogok valley and is dominated by a long, high ridge leading 421
up to Taung-met summit. The high ridge is composed of a large charnockite intrusion nearly 422
concordant with the foliation in the surrounding marbles. The southwestern (Injauk valley) 423
and southeastern (Chaunggyi valley) margins of the charnockite-syenite are intrusive into 424
marbles, with numerous mines along the contact. The northern contact of the Taung-met 425
charnockite-syenite intrusion is close to the layered ultramafic rocks of the Pyang yaung 426
(Bernard-myo) region. Samples MY-215, MY-216 and MY-164 were all collected from the 427
Taung-met intrusion north of the Chaunggyi valley (Fig. 3). MY-164 is a clinopyroxene 428
charnockite with perthite and titanite. MY-215 is a felsic syenite consisting of K-feldspar + 429
quartz + clinopyroxene. MY-216 is a mafic syenite (Fig. 4b,c). Sample MY-83 was collected 430
from a layered charnockite containing K-feldspar + plagioclase + quartz + orthopyroxene at 431
the coffee plantation estate NE of the Injauk road junction (Fig. 3). Sample MY-138 is a 432
diopside + plagioclase + K-feldspar + phlogopite calc-silicate skarn from Chaunggyi 433
collected from the southern margin of the Taung-met charnockite-syenite intrusion. 434
The simplest zircon sample is MY138, which gave a range of U-Pb dates from 44 to 435
28 Ma and shows minimal variation in Th/U, Lu/Gd and Eu/Eu* over that time span (Fig. 9). 436
These dates are definitively metamorphic because the rock is a skarn. The zircon data from 437
the rest of the samples are not as simple to interpret. All four (MY83, MY164, MY215, and 438
MY216) give broad ranges in U-Pb dates from ~170 Ma to 22 Ma, with possible clusters of 439
intermediate dates from 68 Ma to 20 Ma (Figs. 7, 9). Most of the dates are concordant, or 440
nearly so, indicating either small amounts of common Pb, partial resetting, or mechanical
441
(laser) mixing of zones with distinct dates. That all four samples have a cluster of oldest 442

14
analyses of ~170 Ma – observed exclusively in oscillatory to sector-zoned zircon cores (Figs. 443
S2-S3) – indicates that the dated rocks are of roughly the same age and crystallized at the 444
same time; there might be a range of crystallization ages from 170 to 163 Ma, but dispersion 445
in the data makes this difficult to assess. Th/U decreases monotonically until ~60 Ma in 446
MY83 and MY164, compatible with metamorphism at that time, and then increases markedly 447
after ~40 Ma, compatible with melting. Some U-Pb dates arrayed between ~170–60 Ma 448
occur in obviously metamict zircon cores (Figs. S2-S3), although many do not; in contrast, 449
late Cretaceous (~68 Ma) to Miocene dates (~20 Ma) typically occur in distinct zircon 450
overgrowths. The youngest dates from each sample always occur in CL-bright rims, and the 451
range of measured dates from these rims (~40–20 Ma) are reproducible between spot and 452
depth-profiling methods applied to the same samples. Eu/Eu* is relatively invariant in all 453
samples except for MY215, in which a peak at 60–40 Ma is compatible with plagioclase-rich 454
melt injection. Titanite from MY216 gives a simple intercept date of 19.8 ± 0.4 Ma. In 455
summary, and when considered in light of the geochronology data presented below, this 456
dataset is compatible with ~170 Ma igneous crystallization of the charnockite-syenite 457
intrusions, metamorphism beginning at 65–40 Ma, and sustained high temperatures through 458
to 20 Ma. 459
460
3.4. Kyat-pyin and western Mogok region 461
This region lies west of Mogok and is centered around the town of Kyat-pyin and the 462
Kyauk-pya-that golden pagodas (Fig. 3). Numerous ruby and sapphire mines in this region 463
are either within the marble, or along skarn contacts with several syenite sills. We found no 464
corundum gems in the syenite itself, although sapphires have been found in syenite elsewhere 465
in the Mogok region (Themellis, 2008). Three main charnockite-syenite sills are mapped 466
along the transect from Kyat-pyin north to Baw-mar. The main ruby–sapphire mines are at 467
Kyat-pyin, Wet Loo, Baw-lon-lay, Baw-lon-gyi, and Bawmar. Farther west the mines at 468
Thurein Taung and Yadarnar kaday-kadar (Burmese for ‘millions and billions’) produce gem 469
star sapphires and rubies. Sample MY-227 is a syenite from a dyke intruding marble, north of 470
Kyat-pyin (Fig. 5a). Sample MY-122 is a garnet + biotite leucogneiss adjacent to a 471
clinopyroxene-olivine bearing calc-silicate or skarn, collected from Baw-lon-lay mine. 472
Sample MY-94 is a garnet + sillimanite + biotite paragneiss that contains K-feldspar-rich 473
leucosomes attributable to partial melting, and is one of the few metapelitic rocks seen in the 474
Mogok valley. 475

15
We analysed zircon from MY-227, and monazite and zircon from MY-94 and MY-476
122 (Fig. 10). Zircon in MY-227 is 66.6 ± 1.2 Ma and invariant in REE composition. The 477
Cretaceous date could be an intrusion age, and the few younger analyses are compatible with 478
subsequent Pb mobilization. Zircon in MY-122 gave chiefly concordant dates from 151 Ma 479
to ~90 Ma, with a cluster at 124 Ma and little variation in REE composition. The Jurassic 480
date is likely an intrusion age and the few younger analyses are compatible with subsequent 481
Pb mobilization. Monazite from the same sample is significantly younger, with concordant 482
dates from ~101 Ma to 21 Ma, and a clear decrease in Th/U, Eu/Eu*, and Lu/Gd until ~60 483
Ma. These data are compatible with metamorphism without garnet prior to 60 Ma, and 484
metamorphism in the presence of garnet since. Monazite from MY-94 is similar, with high-Y 485
cores mantled by low-Y rims (Fig. S4), and warrants the same interpretation; however, thin, 486
young, Th-rich rims on several grains suggest a late (~25 Ma) melting event. For this sample, 487
the outermost zircon rim dates have U-Pb dates similar to the youngest monazite, revealing 488
that the zircon underwent late metamorphism-related recrystallization or rim growth. 489
490
4. Model for formation of spinel, ruby and sapphires in Mogok 491
Figure 11 is a simplified model showing the structural relationships of the lower and 492
middle crustal rocks exposed in the Mogok region. Abundant charnockite-syenite magmas 493
intruded from a hot lower crustal source. Alkaline ultramafic rocks (jacupirangites) 494
associated with mafic syenites reflect an unusual and extreme alkaline source from the upper 495
mantle. The charnockite-syenites intruded to mid-crust levels where they became sill-like 496
intrusions. Heat from the intrusion of hot (>1000-1200ºC) charnockite-syenites would likely 497
have produced a contact metamorphic aureole and skarns around the margins. Magmatic–498
metasomatic fluids that desilicified the surrounding country rock forming first spinel, and 499
then corundum. Both rubies and sapphires are regionally distributed and are almost always 500
found close to (within 1-2 km structural thickness) the major syenite intrusions north of the 501
Mogok valley. Sapphire mines appear to correlate mainly with skarns around the margins of 502
the intrusions. 503
Although the large Taung-met charnockite-syenite is probably Jurassic in age (170–504
168 Ma), other charnockite-syenite intrusions around Thurein Taung, Kyauk Pya-that and 505
Bawmar in the west may be latest Cretaceous to early Miocene (~68 Ma and 44–21 Ma). One 506
syenite along the northern margin of the Le Oo mine, east of Mogok has a U-Pb zircon 507
crystallization age of 37 Ma (MY228), and a titanite crystallization age of 21.6 Ma (MY229), 508
similar to the calc-silicate ruby-bearing skarns at Le Oo mine (22 Ma). U-Pb geochronology 509

16
shows that all the metamorphic ages are Latest Cretaceous through to Oligocene – earliest 510
Miocene. The ages from the syenites and charnockites in Mogok are more difficult to 511
interpret and several possible scenarios are proposed (see Discussion). 512
Garnet- and melt-present metamorphism occurred between ~45-24 Ma in Mogok, and 513
is coeval with previous U-Th-Pb metamorphic ages from MMB rocks near Mandalay (Searle 514
et al. 2007, 2017). Systematic thermobarometry on the high-variance assemblages sampled in 515
the Mogok valley area is problematic, and has not been attempted. Moreover, there is very 516
little evidence in the metamorphic assemblages for multiple events, despite the apparently 517
complex history revealed in the zircon and monazite ages. Nevertheless, broad constraints 518
can be placed on the maximum conditions (Fig. 12) by phase equilibrium modelling of bulk 519
compositions for Grt–Bt leucogneiss (e.g. MY-122, MY-142) and Grt–Bt–Sil paragneiss (e.g. 520
MY-94), based on observed mineral proportions in the samples. The location of the solidus 521
places an important constraint on the peak temperature, given that MY-94 is a migmatitic 522
gneiss, and that MY-142 shows textural evidence for dehydration melting of biotite. The 523
solidus curve is calculated for a bulk H2O content that is defined by the volume proportion of 524
biotite, the only hydrous mineral in the samples. A further constraint on the Grt–Bt 525
leucogneiss is the absence of orthopyroxene, which is predicted to occur at lower pressure, 526
and also in melt-bearing assemblages through further dehydration melting of biotite. For the 527
Grt–Bt–Sil paragneiss, cordierite is predicted to occur at lower pressure, and the high-P limit 528
is given by the Ky–Sil curve. The extent of each field also takes into account uncertainty on 529
the estimated bulk composition (cf. Palin et al. 2016). The area of overlap for the two rock 530
compositions is centred on 7.5 kbar and 750–800°C, conditions that are comparable to those 531
determined in granulite-facies paragneisses by Thu et al. (2017) at localities c. 45 km WSW 532
of the Mogok valley. 533
Late garnet + tourmaline leucogranites resulted from widespread Miocene mid-crustal 534
anatexis and intrusion of the Miocene Kabaing granite with its gem-bearing pegmatites, and 535
appear to cut fabrics in all surrounding metamorphic rocks. The age of the Kabaing granite 536
(16.8 ± 0.5 Ma; Gardiner et al., 2016) reflects the final phase of metamorphism and melting 537
in the Mogok region. The relative lack of large-scale fold-nappe structures and absence of 538
evidence for Himalayan-scale crustal thickening in the MMB may suggest a heat source other 539
than orogenic thickening and radiogenic heating for the observed upper amphibolite-granulite 540
metamorphism and the formation of corundum-bearing marbles. It could be argued that the 541
close spatial association between ruby and sapphire-bearing marbles and the syenite-542
charnockite intrusions suggest a temporal connection, but the geochronological data 543

17
presented here are more complicated and suggest that the alkaline magmatic intrusions are 544
only indirectly related to the ruby and sapphire formation. 545
546
5. Discussion 547
Field structural relationships, metamorphism and U-Pb geochronology from the 548
Mogok area suggest three possible tectonic scenarios: 549
550
Model 1. All charnockite-syenites in the Mogok region are Jurassic, but only the large 551
Taung-met – Chaunggyi intrusion preserves the original intrusion ages (170-163 Ma). The 552
Kyat-pyin and Le Oo syenites and meta-skarns do not have Jurassic ages from our data, but 553
later regional granulite facies metamorphism during Late Cretaceous – Paleocene time could 554
have overprinted an earlier intrusion event. Skarns formed around the syenite intrusions at 555
~170 Ma, but did not necessarily contain ruby or sapphire at that time. Regional Mogok 556
metamorphism was a Late Cretaceous – Miocene granulite – amphibolite facies event and 557
rubies and sapphires were formed from burial and metamorphism of meta-skarns and 558
surrounding marbles. 559
560
Model 2. Two or three episodes of charnockite-syenite intrusion could be interpreted 561
from our new U-Th-Pb geochronology data, the Jurassic Taung-met intrusion (170-163 Ma), 562
the Kyat-pyin syenite (MY227; 67 Ma), and the Le Oo syenite (MY228; 37 – 28 Ma) with 563
adjacent meta-skarns (MY229; 22 Ma). It is quite likely that there was some chemical – 564
metasomatic effect of intrusion of these hot magmas into a regional, long-lasting (ca. 45 565
m.y.) granulite facies terrane, such that the occurrence of rubies and sapphires was directly 566
related to multiple, distinct episodes of skarn formation. 567
568
Model 3. The charnockite-syenites were all intruded during the Latest Cretaceous – 569
Oligocene-early Miocene (between ~68-22 Ma) concomitant with regional metamorphic ages 570
from zircon, monazite and titanite dates. In this model, the older Jurassic zircon ages from 571
Taung-met - Chaunggyi (170-163 Ma) are interpreted as inherited from the source, and 572
escaped overprinting during later granulite facies metamorphism. The exclusively young 573
zircon ages reported by Thu (2007) and Sutherland et al. (2019) support this model. 574
575
The Jurassic dates appear to be restricted to the large intrusion at Taung-met and 576
Chaunggyi (MY 83, MY-164, MY-215, MY-216). One sample, the Taung-met charnockite 577

18
(MY 216) has zircon ages at 168 Ma, 63 Ma and 26 Ma. The Cretaceous igneous rocks may 578
be part of the trans-Himalayan Gangdese batholith in south Tibet and its proposed extension 579
south into Myanmar (Zhang et al., 2017; Lin et al., 2019). All these samples have younger 580
zircons, reflecting subsequent high-temperature metamorphism. One syenite sample from a 581
dyke intruding marble (Fig. 5a) at Kyat-pyin (MY-227) has only younger zircon dates 582
ranging of 44–38 Ma. Zircon has a high closure temperature, but in charnockites and syenites 583
it may not preserve older inheritance ages due to a combination of diffusive Pb resetting and 584
zircon resorption. If so, the oldest zircon ages should reflect the timing of intrusion. 585
The main pulse of metamorphism recorded by rocks in the Mogok valley appears to 586
have begun around 60 Ma and extended until titanite crystallization or diffusive Pb closure at 587
22 Ma. This timing is similar to that from the MMB to the south around Mandalay (Searle et 588
al., 2007, 2017). The oldest dates may reflect regional contact metamorphism around the 589
trans-Himalayan batholith. Most of the metamorphic dates, and particularly those with trace-590
element characteristics suggestive of melting, garnet growth and granulite-facies conditions 591
in the ruby- and sapphire-bearing skarns around the margins of the charnockite intrusions, 592
begin around ~45 Ma and extend to ~24 Ma. We suggest that this timing records the 593
crystallization of the gem spinels, rubies and sapphires in Mogok. The thick marble bands 594
around the Dattaw mine north of Mogok have produced some of the best quality gem rubies, 595
and are located between the upper Taung-met charnockite-syenite intrusion and the lower Le 596
Oo syenite intrusion (Fig. 3). 597
High temperatures were maintained until as late as 16.8 Ma when the Kabaing 598
leucogranite was intruded into the Mogok metamorphic belt to the west (Gardiner et al., 599
2016). Abundant garnet + sillimanite ± cordierite migmatites are known from drill core 600
samples in the Kyi Tauk Pauk gold mine west of Mogok. At this location, numerous 601
mesothermal gold-bearing quartz veins intrude the garnet–sillimanite migmatites. These 602
migmatitic rocks must reflect a different, more pelitic source than the Mogok valley region 603
and are of unknown age. Similar migmatites may be present SE of the Mogok valley (on the 604
road to Mandalay and Pyin Oo Lyin), but the ages of these rocks are also unknown. Further 605
research combining field structural mapping with detailed U-Th-Pb geochronology is 606
required to unravel the complex metamorphic and magmatic history of the region. 607
608
6. Conclusions 609
Granulite and upper amphibolite facies marbles occur throughout the Mogok 610
Metamorphic Belt that runs for >700 km north–south through central Burma. Spinel, ruby 611

19
and sapphire gemstones are common in the Mogok valley region, but rare outside this area. 612
The ruby and sapphire in the Mogok valley are spatially related to a series of charnockite-613
syenite sill-like intrusions around the Mogok valley. Gem-quality sapphires are related to the 614
metasomatic calc-silicate skarns around the margins of these charnockite–syenites. The 615
composition range of charnockites and syenites is broad with both mafic and felsic varieties, 616
ranging from ultramafic hornblende-pyroxene-biotite rocks through orthopyroxene and 617
clinopyroxene-bearing charnockites to quartz syenites. Four charnockites and syenites from 618
Taung-met and the Chaunggyi valley have U-Pb zircon dates spanning 170–163 Ma, 619
indicating an earlier Jurassic phase of alkali igneous intrusion in the protolith rocks. U-Th-Pb 620
zircon dates on six charnockite-syenites span 67 – 22 Ma, including four samples from the 621
Taung-met charnockites that have Jurassic dates, and one (MY-227), together with a skarn 622
rock (MY-138), that do not have any Jurassic dates. A single U-Pb titanite date from a 623
syenite at Le Oo mine in Mogok is 22 Ma, similar to a U-Pb titanite date of 21 Ma from an 624
adjacent ruby-bearing calc-silicate skarn - marble. These are broadly coeval with monazite 625
and zircon dates from metasedimentary rocks along the MMB in the Mandalay – Kyaushe 626
area to the south. A cluster of U-Pb monazite ages from 97–75 Ma are thought to reflect the 627
thermal influence of pre-collision subduction-related granite–granodiorite intrusions along 628
the Mogok Metamorphic Belt. U-Th-Pb ages of the Mogok metamorphic rocks are all latest 629
Cretaceous to early Miocene, related to the India–Asia (Sibumasu) collision. The MMB 630
continues northward, east of the Putao region along the Myanmar–China border, toward the 631
East Himalayan syntaxis and beyond, possibly to the basement units of the Northern Lhasa 632
terrane on SE Tibet (Searle et al., 2011; Palin et al., 2014). 633
The unusual mineralogy and richness of ruby and sapphire gems in the Mogok area is 634
spatially related to the ultrahigh temperature dry charnockites, alkali granites and syenites in 635
the lower crust. U-Pb geochronology however, suggests three possible models, (1) that all the 636
charnockites and syenites were Jurassic, but only the Taung-met – Chaunggyi intrusion has 637
preserved Jurassic zircons; late Cretaceous-Oligocene zircon and monazite ages reflect a 638
regional metamorphic overprint that was synchronous with ruby and sapphire formation. (2) 639
Three phases of charnockite-syenite intrusion are recorded in the Middle Jurassic (170-163 640
Ma), Latest Cretaceous – Paleocene (68-63 Ma), and Early Eocene – Early Miocene (~47-22 641
Ma), each of which was associated with ruby and sapphire formation in adjacent skarns. 642
However, only the second and third phases of intrusion during Late Cretaceous – Oligocene 643
or earliest Miocene time were concomitant with granulite facies metamorphism during the 644
later period of intrusion. (3) The charnockites and syenites were Late Cretaceous – Early 645

20
Miocene in age and related to regional granulite facies metamorphism. Older Jurassic zircon 646
ages in the Taung-met syenite would have been inherited from the source, or from 647
contamination during magmatic transport. 648
Phanerozoic granulite terranes with charnockites are not common in the world, and 649
multiple phases of charnockite-syenite intrusion in the same locality seems improbable. 650
Further, because of the high temperatures of intrusion of charnockites (~1000-1200ºC) 651
zircons are unlikely to preserve any older ages; thus, the oldest zircon ages from a given 652
sample are probably either intrusive or metamorphic dates, but are unlikely to be inherited. 653
We suggest that the most likely model is that of Jurassic (~170-163 Ma) intrusion of syenites 654
and charnockites that were affected by a regional granulite facies metamorphism lasting from 655
~68-21 Ma. Rubies and sapphires were formed during this regional metamorphic episode by 656
granulite facies metamorphism of meta-skarns and thick marbles. 657
658
Acknowledgements 659
We thank the Oxford–Burma Aung San Suu Kyi trust for funding research and fieldwork 660
visits to Myanmar for MS, NG and LR. We thank Than Than Nu and Ney Lin for hospitality 661
in the University of Mandalay, and discussion on Mogok gems. Thanks to U Than Naing, 662
mine manager for access to Le Oo ruby–sapphire mine in Mogok, Aung Moe, mine manager 663
of Htay-pying for access to Baw lon-lay ruby mine, Htun Lynn Shein (Myanmar Precious 664
Resources Group) for permission to access Kyi Tauk Pauk gold mine, Tin Aung Myint for 665
logistic help in Mandalay, Thu Htet Aung for expert off-road driving in Mogok, and Sam 666
Weatherly and John Cottle for discussions on syenite petrology and U-Th-Pb geochronology. 667
Geochronology was funded by UCSB and NSF grants EAR-1348003 and EAR-1551054. We 668
thank Shuguang Song (Peking University) and an anonymous reviewer for helpful reviews. 669
All data supporting the conclusions in this paper are available online at 670
doi:10.17605/OSF.IO/FMC5S. 671
672
673
674
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26
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tectonic evolution of the western Myanmar continental margin. Gondwana Research, 52, 860
18-38. 861
862
863
864
865
Figure Captions: 866
867
Figure 1. Regional geological map of Myanmar, from Searle et al. (2007). 868
Figure 2. (a) Digital elevation model for the Mogok area and (b) geological overlay showing 869
the main structures. Box shows outline of the Mogok valley region in Figure 3. 870
Figure 3. Simplified geological map of the Mogok valley region, also showing locations of 871
samples dated in this study. 872
Figure 4. Field relations of the Chaungyi, Taung-met region. (a) View of Mogok town and 873
hills to the north showing the Taung-met syenite sill and Pyang yuang peridotites in 874
distance. (b) Layered clinopyroxene-bearing syenite from Chaungyi (sample MY 215). 875
(c) Layered mafic syenites at Chaungyi (sample MY216). (d). Perthitic feldspathic vein 876
intruding mafic syenite, Chaungyi. (e) Coarse-grained enstatite crystals in charnockite, 877
Chaungyi. (f). Outcrops of syenite at Baw-lon gyi, north of Kyatpyin. 878
Figure 5. (a) Syenite dyke intruding Mogok marble, Htay-pying quarry, north of Kyat-pyin. 879
(b) Sharp intrusive contact of syenite with Mogok marble, Htay-pying. (c) coarse-grained 880
clinopyroxene + amphibole syenite, Yadaney Kadey kadar. (d) Hydrothermal 881

27
metasomatic biotites along the intrusive contact of syenite in marble, Yadarney Kadey 882
Kadar. (e) Sharp intrusive contact of layered syenite with Mogok marble, Nga Yant, east 883
of Mogok. (f) Layered felsic and mafic syenites, Nga Yant, east of Mogok. 884
Figure 6. (a) Syenite-marble contact above Le Oo mine, east of Mogok. (b) Clinopyroxene-885
bearing syenite at Le Oo ruby mine. (c) Ruby-bearing calc-silicate skarn, Le Oo mine. (d) 886
Rubies mined from Le Oo marbles. (e) Large euhedral sapphire crystals from Le Oo 887
mine. (f) Large ruby hosted in Mogok marble. (g) Large pale blue apatite and small red 888
spinel in Mogok marble. (h) Blue sodalite, a metasomatic product of fluids along the 889
syenite-marble contact. 890
Figure 7. Summary and geologic interpretation of zircon, monazite, and titanite dates 891
presented in this study. Gray bands emphasize geological events recorded in multiple 892
samples and/or known from elsewhere in the orogen. “mmm”, metamorphism; “xlzn”, 893
crystallization; “w.”, with; “w/o”, without. Solid colors indicate clusters of dates, and 894
faded colors indicate ranges of concordant dates. 895
Figures 8–10. Concordia diagrams for U-Pb and Th-Pb data, REE–date data, and trace-896
element–date data. Left column: cited dates are either concordia ages (sensu Ludwig) or 897
“207Pb/206Pb-corrected” 238U/206Pb intercepts. Center column: REE changes are shown as 898
a function of date. Right column: Th/U, Eu/Eu*, and Lu/Gd changes over time may result 899
from recrystallization in the presence of a silicate melt/hydrous fluid, melt 900
injection/extraction or plagioclase breakdown/growth, and garnet breakdown/growth, 901
respectively. 902
Figure 11. Model for the crustal structure of the Mogok Metamorphic belt. Mantle rocks are 903
the Pyang-yuang peridotites (dunites, harzburgites etc). Lower crustal rocks comprise the 904
large layered syenite intrusion of Taung-met, Chaungyi, and several other sills exposed 905
around Dattaw, Le Oo, and Bawpadan, intrusive into Mogok marbles. Ruby (R) and 906
sapphire (S) mines are spatially associated with the syenite sills and their surrounding 907
skarns. The garnet + sillimanite gneisses exposed in the mountains south of Mogok 908
structurally overlie the marbles. West of Mogok a large intrusive leucogranite, the 909
Kabaing granite, intrudes the ruby marbles and gneisses. Gem-bearing pegmatites (e.g. 910
Sakyangyi topaz-quartz pegmatite) emanate from the roof of the Kabaing granite. 911
Figure 12. Metamorphic peak P–T conditions for gneisses in the Mogok valley area based on 912
calculated phase diagrams (pseudosections) showing stability fields for assemblages in 913
Grt–Bt leucogneiss (Kfs–Pl–Qz–Bt–Grt–Ilm) and felsic Grt–Bt–Sil paragneiss (Kfs–Qz–914
Pl–Grt–Bt–Sil–Ilm). Area of overlap is centred on 775 ±50°C, 7.5 ±1 kbar. Diagram 915

28
calculated with Theriak/Domino software (De Capitani and Petrakakis, 2010), using the 916
thermochemical database DS6 of Holland & Powell (2011) and activity models from 917
White et al. (2007, 2014a, b). Mineral abbreviations follow Whitney & Evans (2010). 918
919
920
Supplementary Data: 921
Figs S1, S4. Monazite chemical maps for a subset of the analyses presented in the paper. 922
Figs S2-S3. Zircon cathodoluminescence images for a subset of the analyses presented in the 923
paper. All data were collected at UC Santa Barbara using a FEI Quanta 400f Scanning 924
Electron Microscope with a CL detector (CL images) or a Cemeca SX-100 Electron 925
Microprobe (Chemical maps). 926
927
Table S1. LASS data (U-Pb isotopic + trace element) for Mogok-area zircon, monazite, and 928
titanite. 929
930
931





Cenozoic volcanic rocks
Chaung Magyi Group
and metamorphic rocks
Lower Paleozoic quartzites
and limestones
Syenite, Charnockite
Kabaing granite
Pyang yuang peridotite
Mogok metamorphic belt
Fig.3
96°30’E
22°30’N
23°00’N23°00’N
23°00’N
22°30’N
23°00’N
97°30’E
96°30’E 97°30’E
Mong Long
Hsipaw
Bawdwin
Letpanhla
Singu
Thabeikyin
Kyitauk-pauk
Mogok
Mong Mit
Kyit-
Pauk
Kabaing
Momeik fault
Sagaing fault
Irrawaddy
River
20 km
Nanting fault
N



Kabaing granite (16.8 Ma U-Pb zircon)
- bt ± ms ± crd leucogranite
Pyang yuang peridotite
- dunite, harzburgite, wehrlite
Mogok metamorphic rocks
- sil + grt + bt ± cord gneisses
Syenite, charnockite
Mogok marble
- marbles, calc-silicates, rare psammites
Mines (ruby, sapphire, spinel, peridot)
peaks, mountains
roads
rivers
Nwaibo Taung
Kabaing granite
Kabaing
Kin
Sakangyi
Gwebin
Baw-
Ion-
lay
Baw-Ion
gyi
Khauk
phar
G.B.
On-bin
Kathe
Kyatpyin
Htay
pying
Ye-U-Le
Pingu Taung
Kyauk
Pyathat Yadanar-kaday
kadar
Thurein
Taung
Yebu
MyLe Oo
Mintada
MOGOK
Myaw-pyet
Ho-Mine
-sho
LayBin Sin
Shive daing
Kyint
Taung
Pein Pyit
Dattaw
LeOo
50
40
40
MY215 MY138
MY144
MY142
MY229
MY228
MY216
MY122
MY83
MY164
MY227
MY94
45
Onbin
Ohn-gaing
Kyauk Bawpadan
Kolan
to
Momeik
Kyaukpya
Zalat Taung
Kyaukpan
Pyang yuang
Bernard myo
Ah Chauk
Le Taw
Injauk
Pandaw-Pey
Taung
Taung met
2296m
Htin-Shu
Taung
Gurkha
Mana Lisu Shan
Konzan
22°52’00”
Bawmar
mine
poke
22°52’00”
23°00’
96°33’30”
96°20’45”
96°33’30”









20
40
60
80
100
120
>130
date (Ma)
zircon
monazite
titanite
MY138
Taung-met
skarn
38 Ma
28 Ma
44 Ma
Gangdese-
type
batholith?
Jurassic
plutonism
plutonism/
mmm?
begin melting
garnet breakdown?
plutonism
MYLeOo
Le Oo
ruby-bearing
skarn
22 Ma
MY229
Le Oo
skarn
22 Ma
MY228
Le Oo
syenite
37 Ma
28 Ma
MY142
Mogok
meta-psammite
111 Ma
78 Ma
59-53 Ma
32-24 Ma
???
mmm
w. melt
inheritance
mmm
w/o
gar
MY144
Mogok
meta-psammite
1.2 Ga
0.6 Ga
mmm
w.
gar+
melt
84 Ma
74 Ma
24 Ma
33 Ma
34 Ma
45 Ma
mmm
w gar
mmm?
w/o
gar
MY94
Mogok
leucogneiss
107 Ma
0.5 Ga
75 Ma
46 Ma
60 Ma
26 Ma
28-24Ma
garnet-
bearing
protolith
melting?
w/o
gar
mmm w
gar
mmm
w.
gar+
melt
mmm
w.
gar
mmm
w.
gar
MY122
Baw-lon-lay
leucogneiss
150 Ma
124 Ma
100 Ma
80 Ma
60 Ma
25-21 Ma
mmm
w/o
gar
xlzn??
xlzn
mmm
w/o
gar
MY83
Injauk
charnockite
MY164
Taung-met
charnockite
MY215
Taung-met
syenite
MY216
Taung-met
charnockite
168 Ma 168 Ma
63 Ma
39 Ma
26 Ma
22 Ma
163 Ma
170 Ma
60 Ma
mmm
mmm
w.
melt
20 Ma
mmm
42 Ma
34 Ma
24-23 Ma 24 Ma
40 Ma
melting
Pb loss/
mixing?
68 Ma
22 Ma
xlznxlzn
xlzn
faded colors indicate concordant date ranges
solid colors indicate date clusters
Mio Oligo Eocene
MY227
Kyat-pyin
syenite
67 Ma
30 Ma
Mogok Valley and Eastern RegionChaunggyi Valley and Taung-met Hill Kyat-pyin and Western Region



100
80
60
40
20 Ma
MY228 zircon
34 32 30 28 26 24 22
0.0
0.1
0.2
0.3
0.4
0.5
180 200 220 240 260 280 300 320
238U/206Pb
7/6 intercept:
0.75 ± 0.07
21.6 ± 0.6 Ma
MSWD = 1.2
1
10
100
1000
10000
1
130 120 110 100 90 80 70
0.046
0.048
0.050
0.052
0.054
0.056
0.058
0.060
0.062
45 55 65 75 85 95
111.3 ± 2 Ma
MSWD = 1.5
45 Ma
84 Ma
34 Ma
78
24
300
100
0.04
0.05
0.06
0.07
0.08
0.09
0 100 200 300
concordia age
24.1 ± 0.5 Ma
1241-1205 Ma (projected from 70-24 Ma)
565-553 Ma (projected from 70-24 Ma)
0
0.0 7
20 30 40 50 60 70 80
0
0.06.
0
4
100
80
60
40
20 Ma
0.000
0.004
0.008
0.012
0.016
0.020
0.000 0.001 0.002 0.003 0.004 0.005 0.006
20
0.000
0.004
0.008
0.012
0.016
0.020
0.000 0.001 0.002 0.003 0.004 0.005 0.006
0
0.003
0
0.14
0
25
date (Ma)
20 30 40 50 60 70 80
date (Ma)
20 30 40 50 60 70 9080
date (Ma)
date (Ma)
0
0.5
0
2
0
0.25
32-24 Ma
59-53 Ma
78 Ma
MY229 titanite
MYLeOo titanite
0.2
0.3
0.4
0.5
110 130 150 170 190 210 230
238
U/
206
Pb
238
U/
206
Pb
238
U/
206
Pb
232
Th/
208
Pb
232
Th/
208
Pb
238
U/
206
Pb
22.6 ± 3.4 Ma
MSWD = 1.5
37.2 ± 0.3 Ma
MSWD = 1.2 28 Ma
44 40 36 32 28
0.040
0.042
0.044
0.046
0.048
0.050
0.052
0.054
140 160 180 200 220 240 260
0
30
0
1.40
0
0.25
25 27 29 31 33 35 37 39
date (Ma)
38
28
date (Ma)
date (Ma)
88
21
date (Ma)
84
27
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
207
Pb
206
Pb
207
Pb
206
Pb
207
Pb
206
Pb
207
Pb
206
Pb
207
Pb
206
Pb
1
10
100
1000
10000
0.1
sample/chondrite
1
10
100
1000
10000
0.1
sample/chondrite
1
10
100
1000
10000
0.1
sample/chondritesample/chondrite
1
10
100
1000
10000
sample/chondrite sample/chondrite
MY142 monazite
21.5 ± 0.6 Ma
MSWD = 1.2
MY142 zircon
MY144 zircon
MY144 monazite
206Pb/238U
206Pb/238U
10
100
1000
1e4
1e5
1e6
1
sample/chondrite
10
100
1000
1e4
1e5
1e6
1
no variation with date



140
100
60
20
0.043
0.045
0.047
0.049
0.051
0.053
0 100 200 300 400
238
U/
206
Pb
207
Pb
206
Pb
162.9 ± 2.0 Ma
MSWD = 0.6
68.2± 0.8 Ma
MSWD = 0.7
22 Ma
date (Ma)
22
174
date (Ma)
21
172
date (Ma)
166
22
date (Ma)
172
22
240
80
0.04
0.05
0.06
0.07
0 100 200 300 400
238U/206Pb
207Pb
206Pb
170.2 ± 1.9 Ma
24.2 ± 0.5 Ma
21 Ma
0
2.5
0
0.7
0
15
0
0
2
0
200
0
0.25
168 ± 3 Ma
44.7 ± 0.8 M a
33.6 ± 0.5 M a
24.0 ± 0.6 Ma
22.5 ± 0.6 Ma
240
80
0.04
0.05
0.06
0.07
0.08
0 100 200300
238U/206Pb
207Pb
206Pb
44-38 Ma 28 Ma
MY216 titanite
MY216 zircon
MY138 zircon
MY164 zircon
MY83 zircon
MY215 zircon
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
1
10
100
1000
10000
100000
0.25
0.35
0.45
0.55
0.65
0.75
0.85
0 40 80 120 160 200 240
238
U/
206
Pb
207
Pb
206
Pb
19.8 ± 0.4 Ma
MSWD = 1.3
140
100
60
20
0.043
0.045
0.047
0.049
0.051
0.053
0.055
0 100 200 300 400
238
U/
206
Pb
207
Pb
206
Pb
26.5 ± 1.1 Ma
MSWD = 3.7
168.5 ± 2.1 Ma
MSWD = 0.8
0
450
20 40 60 80 100 120 140 160 180
20 40 60 80 100 120 140 160 180
20 40 60 80 100 120 140 160 180
0
1.5
0
1.0
0
180
20 25 30 35 40 45 50
0
0.20
0
0.25
56 52 48 44 40 36 32 28
0.044
0.046
0.048
0.050
110 130 150 170 190 210 230 250
238
U/
206
Pb
207
Pb
206
Pb
0
160
20 40 60 80 100 120 140 160 180
0
0.50
0
1.20
22 Ma
39 Ma
sample/chondrite
1
10
100
1000
10000
0.1
sample/chondrite
1
10
100
1000
10000
0.1
sample/chondrite
1
10
100
1000
10000
0.1
sample/chondrite
1
10
100
1000
10000
0.1
sample/chondrite sample/chondrite
1
10
100
1000
10000
0.1
date (Ma)
date (Ma)
date (Ma)
date (Ma)
date (Ma)
no variation with date
no variation with date



10190 170 150 130 110 90
0.044
0.048
0.052
0.056
0.060
0.064
0.068
0.072
0.076
0.080
30 40 50 6 0 70 80
150.9 ± 4.8 Ma
MSWD = 2.0
80.0 ± 0.7 Ma
99.9 ± 0.4 Ma
107.0 ± 2.3 Ma
75.4 ± 1.6 Ma
46.5 ± 0.4 Ma
25.8 ± 0.3 Ma
124.2 ± 1.4 Ma
MSWD = 1.4
100
80
60
40
20 Ma
0
0.000
0.004
0.008
0.012
0.016
0.020
0.00 0.001 0.002 0.003 0.004 0.005 0.006
20
40
60
80
100
120
0
0.05
20 40 60 80
date (Ma)
date (Ma)
date (Ma)
date (Ma)
80 100 120 140 160
20 40 60 80 100 120
100 120
0
1.6
0
0.15
0.000
0.004
0.008
0.012
0.016
0.020
0.00 0.001 0.002 0.003 0.004 0.005 0.006
100
73 Ma
63 Ma
80
60
40
20 Ma
21 Ma
0
0.06
0
0.08
0
1.6
20
40
60
80
100
120
date (Ma)
date (Ma)
200
120
40
28-24 Ma 28-24 Ma
older
0.042
0.046
0.050
0.054
0.058
0.062
0 100 200 300
0
0.035
0
80
0
0.1
66.6 ± 0.3 [1.2] Ma
MSWD = 1.0
30 Ma
100
80
60
40
0.045
0.046
0.047
0.048
0.049
0.050
0.051
60 100 140 180 220 260
0
120
25 30 35 40 45 50 55 60 65 70
0
0.60
0
0.25
MY94 monazite
MY94 zircon
(depth
profiling)
MY122 zircon
MY122 monazite
MY227 zircon
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
Lu/Gd
Eu/Eu*
Th/U
238U/206Pb
207Pb
206Pb
1
10
100
1000
10000
0.1
sample/chondrite
1
10
100
1000
10000
0.1
sample/chondrite
1
10
100
1000
10000
0.1
sample/chondrite
238U/206Pb
207Pb
206Pb
238U/206Pb
207Pb
206Pb
232
Th/
208
Pb
206Pb/238U
232
Th/
208
Pb
206Pb/238U
date
106
20
sample/chondrite
10
100
1000
1e4
1e5
1e6
1
date
107
25
sample/chondrite
10
100
1000
1e4
1e5
1e6
1
no variation with date
no variation with date
101 Ma



layered syenite
charnokite
(ol + ch)
(ol + cpx + opx)
(ol + opx)
dunite
wehrlite
harzburgite
Moho
granulites
Mogok
ruby + spinel
marble
pegmatite
Kabaing
leucogranite
migmatite
sill ± crd gneisses
calc-silicate
silt + and schist
migmatite
syenite
syenite
layered
leucogranite
sill + crd gneisses
?
pelitic
source
alkaline
ultramafic
peridotite
skarn
skarn
ol
SS
S
RRR
RRR/S
R/S
R
R
R
R
R
R
R
R
S
ol
Ruby
Sapphire
olivine (Peridot)
Middle
crust
Lower
crust
Petrologic
Moho
Seismic Moho
Mantle
RR
R/S
S
ruby marbles / calc silicates
MOGOK METAMORPHIC BELT - MODEL
skarn
skarn
approximate scale
0
10
km



Sil
Ky
Field for Grt-Bt
leucogneiss
Solidus for Grt-Bt
leucogneiss
Solidus for
Grt-Bt-Sil
paragneiss
Field for
Grt-Bt-Sil
paragneiss
+ Crd in
paragneiss
+ Opx in
leucogneiss
600 700 800 900 1000
Temperature (°C)
4
5
6
7
8
9
10
Pressure (kbar)