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Timing of Syenite‐Charnockite Magmatism and Ruby
and Sapphire Metamorphism in the Mogok
Valley Region, Myanmar
M. P. Searle
1
, J. M. Garber
2,3
, B. R. Hacker
2
, Kyi Htun
4
, N. J. Gardiner
1,5
, D. J. Waters
1
,
and L. J. Robb
1
1
Department of Earth Sciences, Oxford University, Oxford, UK,
2
Earth Research Institute, University of California, Santa
Barbara, CA, USA,
3
Now at Department of Geosciences, Penn State University, University Park, PA, USA,
4
Geological
Consultant, Yangon, Myanmar,
5
Now at School of Earth and Environmental Sciences, University of St‐Andrews, Scotland,
UK
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.
1. Introduction
High‐grade granulite‐and upper amphibolite‐facies marbles form a major part of the Mogok metamorphic
belt (MMB), Myanmar (Burma), stretching the length of northern Myanmar from the East Himalayan syn-
taxis south through the eastern Kachin state and the Mogok region to Mandalay (Figure 1). The Mogok val-
ley in northern Burma (Myanmar) contains some of world's best examples of gem‐quality spinel, ruby and
sapphire, extracted from upper amphibolite‐to granulite‐facies marbles in the MMB (Chhibber, 1934a,
1934b; Fermor, 1931; Gordon, 1888; LaTouche, 1913; Middlemiss, 1899‐1900; Myanmar Geosciences
Society, 2012; O'Connor, 1888; Searle & Haq, 1964; Searle et al., 2007, 2017). The marbles are white,
coarse‐grained, and contain calcite, phlogopite, graphite, red spinel, diopside, and forsterite. Rare metapeli-
tic rocks contain sillimanite and garnet, and some leucogneisses of possible metavolcanic origin are also pre-
sent. The marbles have been intruded by a series of charnockite and syenite intrusions that have sill‐like
structures and calc‐silicate skarns around the margins. Rare garnet‐and tourmaline‐bearing leucogranites
intrude the marbles and are probably related to the 16.8 Ma Kabaing granite intrusion west of Mogok
(Gardiner et al., 2016, 2014; Gardiner, Searle, Robb, et al., 2015).
The age of metamorphism and formation of the rubies and sapphires in Mogok has long been debated. Prior
to any geochronology, the Mogok granulite‐facies rocks were thought to be part of the Precambrian base-
ment (Fermor, 1931; Chhibber, 1934a, 1934b). Mitchell et al. (2007) proposed two metamorphic events,
one early Permian and the second Early Jurassic, based on field relationships in the region south of
©2020. American Geophysical Union.
All Rights Reserved.
RESEARCH ARTICLE
10.1029/2019TC005998
Key Points:
•Rubies and sapphires in
granulite‐facies marbles from the
Mogok metamorphic belt,
Myanmar, are spatially associated
with charnockite‐syenite sill‐like
intrusions and surrounding skarns
•U‐Th‐Pb LA‐ICPMS dating of
zircon, monazite, and titanite shows
that there were two groups of
charnockite‐syenite dates, one
Jurassic in age (170–168 Ma) and
one latest Cretaceous to early
Miocene (~68–21 Ma)
•Regional granulite‐facies
metamorphism along the Mogok
metamorphic belt is Late Cretaceous
to Oligocene or early Miocene in age
(~68‐21 Ma), peaking with
garnet‐present melting between 45
and 21 Ma
Supporting Information:
•Supporting Information S1
•Table S1
Correspondence to:
M. P. Searle,
mike.searle@earth.ox.ac.uk
Citation:
Searle, M. P., Garber, J. M., Hacker, B.
R., Htun, K., Gardiner, N. J., Waters, D.
J., & Robb, L. J. (2020). Timing of
syenite‐charnockite magmatism and
ruby and sapphire metamorphism in
the Mogok valley region, Myanmar.
Tectonics,39, e2019TC005998. https://
doi.org/10.1029/2019TC005998
Received 22 NOV 2019
Accepted 28 FEB 2020
Accepted article online 1 MAR 2020
SEARLE ET AL. 1of21
Mandalay. Mitchell et al. (2012) published U‐Pb zircon ages from a variety of igneous rocks along the MMB
south of Mandalay and proposed an Early Cretaceous age of metamorphism, with the main fabric‐forming
metamorphic event predating the India‐Asia collision. However, these ages were mainly from diorites and
granites and may not date timing of regional metamorphism. The MMB has a range of biotite‐and
hornblende‐bearing granites, granodiorites, and diorites that are thought to be related to the precollision,
subduction‐related Gangdese‐type plutons along the southern margin of Asia, which range in age from
Late Jurassic to early Eocene (Lhasa block; Chung et al., 2005; Gardiner, Searle, Morley, et al., 2015).
D.L. Searle and Haq (1964) first suggested that metamorphism along the MMB was related to the Cenozoic
Himalayan orogeny. A Himalayan connection was confirmed by preliminary U‐Th‐Pb ID‐TIMS and
LA‐ICPMS dating of metamorphic monazite, zircon, xenotime, and thorite by M.P. Searle et al. (2007).
These data suggested two main periods of high‐grade metamorphism in the MMB around Mandalay: (a) a
Late Cretaceous–Paleocene event that ended with intrusion of 59 Ma biotite granite dykes, which cut meta-
morphic fabrics at Belin quarry and (b) a late Eocene–Oligocene main event (at least 37–29 Ma, possibly
extending from 47 Ma to 25 Ma), when monazite grew at high temperature, sillimanite + muscovite replaced
andalusite, zircon rims grew at 47–43 Ma, and tourmaline‐bearing leucogranites formed at 45.5 ± 0.6 Ma and
25.5 ± 0.7 Ma (Searle et al., 2007). Metamorphic monazites from rare sillimanite‐and andalusite‐bearing
pelites from Kyaushe (600–650 °C; 4.4–4.9 kbar) revealed a peak‐metamorphic age of 29.3 ± 0.5 Ma, and gar-
net + tourmaline leucogranite dykes cutting all earlier fabrics were dated at 24.5 ± 0.3 Ma (Searle et al., 2007).
Until recently, the age of crystallization of the spinel‐, ruby‐, and sapphire‐bearing marbles around the
Figure 1. Regional geological map of Myanmar from Searle et al. (2007).
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Mogok valley has not been directly dated. These gems are common in the Mogok valley and hills to the north
but rarely occur along the MMB south and north of Mogok. Thus, the two metamorphic episodes, pre‐57 Ma
(Paleocene) and 47–29 Ma (late Eocene–Oligocene), proposed by Searle et al. (2007, 2017) apply to the MMB
around Mandalay but not necessarily to the ruby‐and sapphire‐bearing marbles in the gems fields of the
Mogok valley farther north. Another question of direct relevance to the genesis of the MMB is why gem qual-
ity ruby and sapphire are localized in the Mogok Valley north of Mandalay and not widely distributed
throughout the belt.
Recently Win et al. (2016) and Thu and Zaw (2017) and Sutherland et al. (2019) reported some preliminary
U‐Pb age data from Mogok rubies. A titanite inclusion in ruby from Thurein Taung gave a U‐Pb date of 32.4
Ma, and subordinate nepheline was also noted as inclusions in the ruby. The adjacent syenite gave a U‐Pb
zircon date of 25 Ma (Thu, 2007; quoted in Sutherland et al., 2019). Also reported is a U‐Pb date of 16.1
Ma in an extremely rare painite (CaZrAl
9
O
15
(BO
3
)) overgrowth on ruby from Wet Loo mine (Thu, 2007).
A zircon inclusion from a ruby collected at Mong Hsu gave a U‐Pb date of 23.9 Ma (Sutherland et al., 2019)
and is interpreted as the age of regional metamorphism. The geological and geochronological evidence
seems to suggest that the MMB was the site of Jurassic–Paleocene Andean‐type granitoid‐diorite intrusions
and localized low‐pressure metamorphism, but the main high‐temperature metamorphic event was
Eocene‐Oligocene and related to the Himalayan orogenic event.
The major ruby and sapphire mines are located mainly in the Mogok valley and the hills to the north
(Fermor, 1934; Chhibber, 1934a, 1934b; Clegg & Iyer, 1953; Searle & Haq, 1964; Themelis, 2008) and are
uncommon to the south of Mogok (Figure 2). Over 1,000 mines in the Mogok valley region produce spinel,
ruby, sapphire, and numerous other gems. The Mogok valley has been closed to foreigners for decades but
reopened in 2012. It remains sporadically closed due to local insurgencies, now from the TNLA
(Palaung‐armed ethnic group). Many of the larger ruby and sapphire mines remain off‐limits to foreigners,
but smaller locally owned mines are accessible. Recent changes in mining law in Myanmar have released the
significant acreage to local artisanal miners, resulting in a gem rush of exploration in the Mogok valley area.
Since 2014, we have carried out extensive field investigations around the Mogok valley and surrounding hills
and visited many of the ruby and sapphire mines in the region. A new geological map of the Mogok valley
area is presented in Figure 3. Dense tropical jungle and thick red laterite soil make geological mapping dif-
ficult, and almost all critical contacts are not exposed. We were able to access underground shafts in several
ruby and sapphire mines where continuous exposures made key structural observations possible. This paper
presents new mapping and structural relationships together with new U‐Th‐Pb LA‐ICPMS dating of zircon,
monazite, and titanite that constrain timing of intrusion of the syenite‐charnockite intrusions and the age of
metamorphism in the Mogok valley. Field relationships clearly suggest that the distribution of ruby and sap-
phire in the Mogok valley is spatially related to the syenite and charnockite intrusions, but our data suggest a
complex relationship between pluton emplacement, skarn formation, and ruby and sapphire crystallization.
2. Geology of the Mogok Region
The geology of the Mogok valley region was first mapped and studied by Gordon (1888), O'Connor (1888),
LaTouche (1913), Fermor (1931, Fermor, 1934), Barrington‐Brown (1933), Barrington‐Brown & Judd (1896),
Kane & Kammerling (1992), and Keller (1983). The most detailed descriptions were made by Chhibber
(1934a, 1934b), and a geological map was published by Clegg and Iyer (1953) and Iyer (1953). In the days
before radiometric age dating, these authors presumedthat the high‐grade metamorphic rocks in Mogok were
Archean in age. They also suggested that the rubies in Mogok were formed by contact metamorphism around
large granites such as the Kabaing pluton. However, metamorphism in the Mogok region is of regional extent
(Searle et al., 2007, 2017; Searle & Haq, 1964) and not restricted to contact aureoles around granites.
The Mogok valley area is dominated by an ~10 km wide central zone of thick, coarse‐grained calcite + phlo-
gopite + graphite + spinel ± apatite‐bearing marble that hosts many spinel, ruby, and sapphire mines
(Figure 3). Marbles also contain scapolite, wollastonite, clinopyroxene (diopside), and olivine (forsterite)
indicating granulite‐facies conditions of formation. The marbles have been intruded by a large
syenite‐charnockite intrusion (Taung‐met syenite) with several offshoots, mainly aligned as large sills
(Figure 4a). On Taung‐met mountain, the syenites commonly show interlayered felsic and mafic bands
(Figure 4b) with small veins of felsic syenite cross‐cutting the igneous layering (Figure 4c,d).
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Coarse‐grained orthopyroxene‐bearing charnockites have igneous textures but are usually interpreted as
granulite‐facies rocks (Figure 4e). The Taung‐met syenite intrusion is approximately 400 meters thick and
may extend west as far as the Baw‐lon‐gyi area north of Kyat‐pyin (Figure 5f), making this one of the
largest alkaline igneous intrusions in Myanmar.
In the western part of the Mogok region around the Htay‐pying ruby mine, small dykes of orthopyroxene
charnockite intrude Mogok marbles (Figure 5a,b). The surrounding marbles are rich in high‐quality rubies
and have sapphire‐bearing skarns around the dyke margins. Further west at Yadarnar kaday kadar mine
next to Kyauk‐pya‐that pagoda hill, a variety of clinopyroxene syenites and orthopyroxene‐bearing char-
nockites intrude Mogok marble (Figure 5c). The marbles are laden with both rubies and sapphires, and
the margins have extensive metasomatic veining with large biotites (Figure 5d). East of Mogok the
syenite‐marble contact is well exposed at Nga Yant (Figure 5e). The syenite shows strong magmatic layering
with interbanded felsic and mafic syenites (Figure 5f).
The Le Oo mine site east of Mogok town shows clear field relationships with a sharp contact between the
Mogok marbles and syenite‐charnockite intrusions (Figure 6a). The intrusions vary compositionally
between two pyroxene‐charnockites and quartz syenites (Figure 6b). Some show perthitic textures with
Figure 2. (a) Digital elevation model for the Mogok area and (b) geological overlay showing the main structures. Box
shows outline of the Mogok valley region in Figure 3.
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K‐feldspar and quartz and a mafic constituent consisting of both clinopyroxene and amphibole. Calc‐silicate
skarns around the intrusion margins are rich in ruby (Figure 6c,d) and sapphire (Figure 6e). Individual ruby
crystals can reach up to 4–5 cm (Figure 6f). Pale blue apatites are common across the Mogok marbles and can
reach up to 5 cm length (Figure 6g). Extensive metasomatism along the marble contacts is evident from large
biotites and hydrothermal minerals such as sodalite (Figure 5h).
High‐grade regional metamorphic rocks comprising garnet + sillimanite gneisses and migmatites crop out
to the SE of the Mogok valley but are poorly mapped. The surrounding rocks to the east comprise the
Neoproterozoic‐Cambrian Chaung Magyi Group (Dew et al., 2019), although the nature of its contact with
the Mogok metamorphic rocks is not known. Overlying the Chaung Magyi Group are the Cambrian‐Early
Ordovician Pangyun Formation quartzites, sandstones, and siltstones and the Bawdwin volcanic series,
which host the large Pb‐Zn‐(Cu‐Ag‐Ni) deposits at Bawdwin mine (Gardiner et al., 2017). To the west of
the Mogok valley, a large leucogranite intrusion, the Kabaing granite, dated at 16.8 ± 0.5 Ma (U‐Pb zircon)
intrudes the Mogok marbles and syenites (Gardiner et al., 2016). To the north in the Pyang‐yuang area
(Bernardmyo; Figures 2 and 3), a large mass of peridotite, including dunite, harzburgite, and
hornblende‐bearing peridotite, was thought to represent an ophiolitic mantle rock (Searle et al., 2017) but
may instead be a layered ultramafic intrusion, possibly related to the adjacent syenite intrusion at
Taung‐met (Figure 2). No gabbros, sheeted dykes, or pillow lavas are present at Pyang‐yuang.
Gem‐quality peridot (olivine) and enstatite were mined from this massif. Marbles containing abundant
rubies and sapphires crop out in the hills immediately north of the peridotite ridge at Ah Chauk and
Htin‐Shu Taung mines (Figure 3). The northern boundary of the Mogok region is a prominent
north‐dipping fault showing both normal and dextral fabrics, along the Momeik valley.
The structure of the Mogok valley is difficult to ascertain in detail due to the heavy jungle cover, but regional
marble layers seem to dip consistently at steep angles, 45–60° SE. Thus, the garnet + sillimanite gneisses to
the SE are structurally higher, above the Mogok marbles, and the Pyang‐yuang ultramafic intrusion in the
north is structurally beneath the Mogok marbles (Figure 2). It is possible that the entire Mogok massif repre-
sents a giant metamorphic core complex structurally below overlying Neoproterozoic and Paleozoic sedi-
mentary rocks (Figure 2), although this is speculative since the whole area was covered in dense forest
and in need of detailed mapping.
Figure 3. Simplified geological map of the Mogok valley region, also showing locations of samples dated in this study.
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2.1. Spinel‐, Ruby‐, and Sapphire‐Bearing Marbles
Red spinel (MgAl
2
O
4
) is the most common gem mineral in Mogok marbles, frequently forming euhedral
octahedra within coarse‐grained marble. Ruby and sapphire (corundum Al
2
O
3
) differ in color only as
defined by their trace contents of iron, titanium, vanadium, and chromium; these phases both occur in
Mogok marbles, with sapphires possibly more abundant in skarns. The presence of diopside and forsterite
in marble requires amphibolite or granulite‐facies metamorphic conditions (Bowen, 1940). The highest
grade olivine (forsterite)‐bearing marbles have the paragenesis Cal + Dol + Fo + Spl + Phl + Amp + Grt
and contain gem‐quality rubies and sapphires; spinel is not present with ruby or sapphire. Impure marbles
may contain micas (phengite, biotite, and fuchsite), graphite from organic debris, feldspars, and garnet. Pale
blue apatite crystals that reach 5 cm or more in size are present in many Mogok marbles. Calc‐silicate rocks
contain Scp + Di + Cal + Qtz + Kfs + Grt + Ttn. Clinohumite‐and scapolite‐bearing assemblages yield high
metamorphic grades at >780–810° C and 0.8 GPa (Thu & Enami, 2018). The calcite‐dolomite
Figure 4. Field relations of the Chaungyi, Taung‐met region. (a) View of Mogok town and hills to the north showing
the Taung‐met syenite sill and Pyang‐yuang peridotites in distance. (b) Layered clinopyroxene‐bearing syenite from
Chaungyi (sample MY 215). (c) Layered mafic syenites at Chaungyi (sample MY216). (d) Perthitic feldspathic vein
intruding mafic syenite, Chaungyi. (e) Coarse‐grained enstatite crystals in charnockite, Chaungyi. (f). Outcrops of syenite
at Baw‐lon gyi, north of Kyat‐pyin.
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geothermometer also suggests a minimum temperature of 720–765° C and a possible equilibrium
temperature up to 780–860° C (Thu & Enami, 2018). These temperatures are consistent with
granulite‐facies metamorphic conditions, in which decarbonation of buried carbonate‐rich rocks released
CO
2
‐rich fluids, while some H
2
Ofluids may also have been associated with acidic igneous melts (Yui
et al., 2008). The presence of uncommon wollastonite (CaSiO
3
) in some skarns in Mogok indicates high
temperature–low pressure breakdown of calcite + quartz with the release of CO
2
.
Potential protoliths of the Mogok marbles are the thick Permian Fusulinid‐bearing limestones
(Chhibber, 1934a), which are widespread across SE Asia or the mid‐Cretaceous Orbitolina‐bearing lime-
stones (Clegg, 1941). The Neoproterozoic‐Early Cambrian Chaung Magyi Group and the
Ordovician‐Devonian argillaceous limestones of the Shan Plateau are probably too muddy to be suitable pro-
toliths for the purer marbles in Mogok. Elevated aluminum content could be a result of specific
aluminum‐rich, silica‐poor source rocks, such as laterites or evaporites. MgO combined with Al
2
O
3
to
Figure 5. (a) Syenite dyke intruding Mogok marble, Htay‐pying quarry, north of Kyat‐pyin. (b) Sharp intrusive contact of
syenite with Mogok marble, Htay‐pying. (c) Coarse‐grained clinopyroxene + amphibole syenite, Yadaney kadey
kadar. (d) Hydrothermal metasomatic biotites along the intrusive contact of syenite in marble, Yadarnar kaday kadar.
(e) Sharp intrusive contact of layered syenite with Mogok marble, Nga Yant, east of Mogok. (f) Layered felsic and mafic
syenites, Nga Yant, east of Mogok.
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form spinel at low SiO
2
and CaO activities and corundum formed locally (Sutherland et al., 2019). Spinels are
generally not found together with ruby or sapphire in the field, suggesting that variable Mg activity in the
marbles may have controlled the occurrence of corundum. High‐temperature fluids (CO
2
and H
2
O) were
driven off from surrounding skarns and infiltrated the marbles possibly during intrusion of the hot, dry
charnockitic magmas. Metasomatic phases such as sodalite and lapis lazuli are common in several Mogok
localities. The close association of charnockites with gem minerals (notably ruby, sapphire, spinel,
cordierite, and sapphirine) has also been noted in the Proterozoic Highland Group of Sri Lanka, where
Figure 6. (a) Syenite‐marble contact above Le Oo mine, east of Mogok. (b) Clinopyroxene‐bearing syenite at Le Oo
ruby mine. (c) Ruby‐bearing calc‐silicate skarn, Le Oo mine. (d) Rubies mined from Le Oo marbles. (e) Large
euhedral sapphire crystals from Le Oo mine. (f) Large ruby hosted in Mogok marble. (g) Large pale blue apatite and small
red spinel in Mogok marble. (h) Blue sodalite, a metasomatic product of fluids along the syenite‐marble contact.
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the gems have been related to contact metamorphism around ultra‐high temperature charnockite intrusions
(Rupasinghe & Dissanayake, 1985).
2.2. Pelites
Pelitic rocks containing the assemblage Grt + Bt + Pl + Sil + Qtz ± Crd ± Spl are present but uncommon
along the MMB, making precise PT conditions difficult to ascertain. The metamorphosed pelites, now garnet
+ sillimanite gneisses, are called khondalites in the older literature (Chhibber, 1934a, 1934b). Rare pelites
containing garnet and sillimanite at Kyaushe, south of Mandalay, revealed PT conditions of 600–650 °C
and 4.4–4.9 kbar, and a peak‐metamorphic U‐Pb monazite age of 29.3 ± 0.5 Ma (Searle et al., 2007).
Kyanite is uncommon but occurs in the Kyaushe region. Yonemura et al. (2013) obtained PT conditions
of 6.5–8.7 kbar and 800–950 °C from granulite‐facies gneisses and also published imprecise U‐Th‐Pb ages
of <50 Ma. Core samples from the Letpanhla–Kyitauk Pauk gold mine in the western part of the Mogok
area, adjacent to the Sagaing fault, show extensive garnet + sillimanite migmatites with late cordierite, rock
types that are not exposed in the Mogok valley or in the many quarries south of Mandalay along the MMB.
2.3. Charnockites and Syenites
2.3.1. Definitions
Syenites are defined as coarse‐grained, intrusive igneous rocks with essential K‐feldspar, frequently with
perthitic textures and ferromagnesian minerals (biotite, hornblende, clinopyroxene, and orthopyroxene).
A few syenite bodies have the full range from ultramafic jacupirangites (amphibole, pyroxene, and biotite
rocks) to quartz syenites. Syenites are generally associated with anorogenic intraplate intrusions or along
rifted continental margins such as along the East Africa rift system. It is unusual that the differentiated
sequence from alkaline ultramafic to quartz syenite occurs in one intrusion, but one example, the
Borralan intrusion in the Moine thrust zone, NW Scotland (Parsons, 1999; Searle, Law, et al., 2010), shows
a similar compositional range. The Borralan intrusion includes an early suite of pyroxenites, nepheline sye-
nites, and pseudoleucite‐bearing syenites, and a later suite of feldspathic and quartz syenites. It was intruded
into Cambrian sedimentary rocks and Ordovician marbles forming a high‐grade contact metamorphic aur-
eole of yellowish brucite and pyroxene marble. The dolomites were metasomatized and intruded as carbo-
natite sheets. The Borralan syenite intrusion thus has many similarities with the Mogok syenites.
Charnockites were originally defined as orthopyroxene‐bearing (A‐type) granite‐granodiorites (opdalites),
tonalites (enderbites), or monzogranites (mangerites; Howie, 1955; LeMaitre et al., 2005). They were named
from the tombstone rock on Col. Job Charnock's grave in Calcutta, India (Holland, 1893). Charnockites are
hot (~1,000 °C), dry, granulite‐facies quartzofeldspathic rocks with orthopyroxene that can be either igneous
or metamorphic in origin (Frost et al., 2000; Frost & Frost, 2008, 2000; Bhattacharya, 2010). Frost and Frost
(2008) defined charnockite as “an Opx‐(or Fay‐) bearing granitic rock that is clearly of igneous origin or that
is present as an orthogneiss within a granulite terrane”(Frost & Frost, 1987). Charnockites can form by dif-
ferentiation from anhydrous tholeiitic rocks with low water activity and enriched levels of CO
2
. The process
of charnockitization may be associated with several processes including high‐temperature dehydration melt-
ing of mafic to intermediate protoliths, infiltration of CO
2
from deep crust levels, and magmatic differentia-
tion above mantle anomalies such as rift zones or continental hot spots. Charnockites, despite having an
igneous origin in many cases, are invariably associated with high‐temperature granulite‐facies terranes
(Bhattacharya, 2010). Most charnockites are Precambrian in age, with classic examples from south India
and Sri Lanka (Highland Group). Although granulites are widespread in Phanerozoic rocks from a variety
of lower crust tectonic settings (deep levels of calc‐alkaline batholiths, island arcs, or thickened continental
plateau regions like Tibet), there are only a few possible examples of Phanerozoic charnockites, for example,
in the Coast Ranges of British Columbia, the Variscan Bohemian massif, Czech Republic, and the Variscan
Aracena belt, Spain (Windley, 1981).
Skarns are calcium‐magnesium‐iron‐manganese aluminum silicate rocks that may also be referred to as
calc‐silicate rocks. They are typically formed by metasomatic or hydrothermal alteration of country rocks
by fluids, usually only locally around igneous intrusions (contact metamorphism), but in Mogok, the skarn
effects could be of more regional extent.
2.3.2. Mogok Charnockites and Syenites
The Mogok valley shows three or four major charnockite‐syenite sill‐like intrusions into the marble; the lar-
gest, the Taung‐met intrusion, forms the high mountain ridge north of Mogok town (Figure 4a). At least two
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further sills to the south have been mapped around Dattaw, and at least three sills have been mapped in the
west, north of Kyat‐pyin (Figure 3). It is not clear from the limited exposures whether these sills are all part of
one major intrusion or whether separate sills have intruded in the same general area. A wide range of alkali
granites, syenites, and charnockites has intruded the MMB. These include hornblende‐rich melanocratic
syenite, orthopyroxene‐bearing charnockite, clinopyroxene + hornblende‐bearing K‐feldspar + plagioclase
syenite, and quartz syenite. Some show compositional layering that may be an original igneous texture;
others also show a tectonic foliation with aligned hornblende indicating a high‐temperature fabric superim-
posed on the original igneous rock. In this study, most of the samples from larger sheets are charnockitic,
whereas syenites are dominant in associations with marble and skarn at the mine localities. Rare alkaline
ultramafic rocks consisting of amphibole + clinopyroxene + biotite jacupirangites (called “urtite”in older
literature; Chhibber, 1934a, 1934b) with no feldspar are associated with the deepest levels of the syenite
intrusions. Chhibber (1934a, p. 123) described nepheline in syenites from Chaunggyi; silica undersaturated
melting could result in feldspathoids, such as nepheline replacing albite and leucite replacing orthoclase, but
none of our samples contain nepheline. The map of Clegg and Iyer (1953) also shows several outcrops of
nepheline syenites, but we were unable to confirm this at several localities around Pingu Taung and
Kyauk pya‐that (Figure 3).
2.4. Granites and pegmatites
Uncommon leucogranites containing garnet, sillimanite, and tourmaline intrude the marbles and may be
related to the large Kabaing granite intrusion, west of the Mogok valley. The Kabaing leucogranite appears
to be a large‐scale midcrustal intrusion and differs from the Himalayan leucogranites that are sill‐like intru-
sions emanating from a thick sillimanite ± cordierite migmatite terrane (Searle, Cottle, et al., 2010). One pos-
sible source could be the sillimanite migmatites known from drill cores in the Letpanhla–Kyitauk Pauk gold
mine in the western part of the Mogok area (Figure 2), but other pelitic rocks are extremely rare along the
MMB. A suite of pegmatites associated with the Kabaing granite contains gem‐quality topaz, quartz, tourma-
line, lepidolite, and aquamarine (e.g., at Sakangyi mine). K‐feldspar augen gneisses are present in the area
south of Mogok (e.g., Kyanikan and Nattaung quarries) and are associated with in situ partial melting to
form tourmaline ± garnet leucogranites (Searle et al., 2007, 2017).
3. Field Relations and U‐Th‐Pb Geochronology
We present U‐Th‐Pb zircon, monazite, and titanite dates and trace elements for 13 samples tied to structural
mapping from across the Mogok valley. These samples are described from three main regions (Figure 3): (a)
the Mogok valley and region to the east, including the Le Oo, Dattaw, and Pein Pyit (Gorkha, Nepali)
ruby‐sapphire mines; (b) the Chaunggyi valley and Taung‐met hill, north of the Mogok valley; and (c) the
western part of Mogok, including Baw Lon Ley, Baw Mar, and Yadarnar kaday kadar ruby‐sapphire mines
around Kyat‐pyin village. Figure 7 is a summary diagram showing the full range of dates and minerals dated
(titanite, zircon, and monazite) from 13 samples across the Mogok region.
3.1. Methodology
We conducted laser‐ablation split‐stream inductively coupled plasma mass spectrometry (LASS) measure-
ments on zircon, monazite, and titanite. Mineral zoning was qualitatively assessed in select samples with
cathodoluminescence or X‐ray maps (measured by EPMA), and U‐Pb dates and trace elements (Table S1
in the supporting information) were measured quantitatively on all samples during LASS. The LASS analy-
tical protocols and data‐reduction strategies have been presented in earlier papers (e.g., Kylander‐Clark
et al., 2013). In summary, a Photon Machines 193 nm excimer laser and HelEx sample cell were used, and
data were collected on a Nu Plasma or Plasma 3D multicollector ICP‐MS coupled to an Agilent 7700S quad-
rupole ICP‐MS. Analyses of NIST 612 glass and basalt standard BHVO‐2 (Jochum et al., 2005) were inter-
leaved with the unknowns as trace‐element reference materials, and well‐characterized zircon, monazite,
and titanite were interleaved as U‐Th‐Pb reference materials. Data were processed using Iolite (Paton
et al., 2011), which corrects for machine drift and downhole interelement fractionation using reference
material. Most analyses were standard LASS analyses in which all data from a single hole were interpreted
as a single date‐trace element pair. Two samples, MY83 and MY164, were also evaluated using depth profil-
ing, in which the downhole variation in date‐element data was treated as spatially significant.
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We interpret changes in mineral trace‐element signatures in the following ways: (a) an increase/decrease in
Lu/Gd ratio indicates garnet breakdown/growth; (b) an increase/decrease in Eu/Eu* ratio is compatible
with plagioclase‐rich melt injection/extraction or plagioclase breakdown/growth; (c) an increase/decrease
in Th/U reflects recrystallization in the presence of a silicate melt/hydrous fluid. Unless otherwise noted,
all dates quoted here for zircon are
207
Pb/
206
Pb‐corrected
238
U/
206
Pb intercept dates, and all monazite dates
are concordant
232
Th/
208
Pb–
238
U/
206
Pb dates. Many of the monazite analyses have a concordant spread in
U/Pb–Th/Pb ratios that could be the result of Pb loss, mixing during laser ablation, long‐term recrystalliza-
tion, or spatially heterogeneous short‐term recrystallization.
3.2. Mogok Valley and Region to the East
The Mogok valley runs NE–SW between high mountain ridges of Taung‐met to the north and the more sub-
dued jungle‐clad hills toward Pyin Oo Lyin to the south. The valley shows at least 2 km thickness of
coarse‐grained, upper amphibolite, or granulite‐facies marble that is host to at least six major
ruby‐sapphire mining districts extracting gems from bedrock as well as alluvial deposits (Bawpadan,
Yebu, Le Oo, Dattaw, Onbin, and Pein Pyit). The marbles are intruded by several charnockite‐syenite intru-
sions that appear to form sill‐like structures. Skarns are present around the margins with a concentration of
sapphires as well as large biotite flakes. Sample MY‐Le Oo is from a ruby‐bearing marble from the Le Oo
mine, MY‐228 is from a coarse‐grained clinopyroxene‐bearing syenite collected in situ immediately above
the alluvial washing pits. Sample MY‐229 is from a skarn along the syenite‐marble contact from the same
mine; it consists of 50% clinopyroxene with K‐feldspar and around 5% quartz. Two further samples
(MY‐142 and MY‐144) were collected from the Pein Pyit (also called Nepali Gorkha) ruby‐sapphire mine east
of Le Oo and Mogok. MY‐142 is a leucocratic rock interbedded within marbles and contains lilac‐colored
garnet + plagioclase + K‐feldspar + biotite + apatite. MY‐144 is a graphitic garnet + biotite + plagioclase
rock, inferred to represent a residual assemblage after loss of partial melt from a metapelite, interbanded
within marbles, one of the more aluminous samples we found in the Mogok valley.
We dated zircon, monazite, and titanite from these samples (Figure 8). Zircon in MY142 comprise tiny euhe-
dral grains, of which the oldest are 111 Ma; this sample could be a tuff that erupted at 111 Ma or a metasedi-
ment deposited after that time. Monazite from this rock is mostly younger and shows a distinct drop in
Eu/Eu* and Lu/Gd from ~78 to 24 Ma, compatible with melt extraction and the crystallization of garnet.
Zircon from MY144 has a large range in concordant dates—1.2 Ga to Cretaceous—typical of a
Figure 7. Summary and geologic interpretation of zircon, monazite, and titanite dates presented in this study. Gray bands
emphasize geological events recorded in multiple samples and/or known from elsewhere in the orogen. “mmm,”meta-
morphism; “xlzn,”crystallization; “w.,”with; “w/o,”without. Solid colors indicate clusters of dates, and faded colors
indicate ranges of concordant dates.
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Figure 8. Concordia diagrams for U‐Pb and Th‐Pb data, REE‐date data, and trace‐element‐date data from the central and eastern Mogok valley. (left column) cited
dates are either concordia ages (sensu Ludwig) or “
207
Pb/
206
Pb‐corrected”
238
U
/206
Pb intercepts. (center column) REE changes are shown as a function of date.
(right column) Th/U, Eu/Eu*, and Lu/Gd changes over time may result from recrystallization in the presence of a silicate melt/hydrous fluid, melt injection/
extraction or plagioclase breakdown/growth, and garnet breakdown/growth, respectively.
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metasedimentary rock that might have a Cretaceous depositional age. Most of the zircon analyses are 74–24
Ma, compatible with metamorphism ending by 24 Ma. Monazite from MY144 mirrors the younger zircon,
with a range from 80 Ma (high‐Y and low‐Th cores) to 35 Ma (low‐Y and high‐Th rims; Figure S1). Both zir-
con and monazite have weak decreases in Eu/Eu* and increases in Th/U compatible with melting; the
reduction seen in zircon Lu/Gd is suggestive of the growth of garnet, but this signature is absent from mon-
azite. Zircon from MY‐228 is entirely Cenozoic and of constant composition, compatible with either igneous
or metamorphic parentage. The youngest titanite in MY229 and MY‐Le Oo is ~22 Ma and certainly meta-
morphic in origin.
3.3. Chaunggyi Valley and Taung‐Met Hill
This region lies north of the Mogok valley and is dominated by a long, high ridge leading up to
Taung‐met summit. The high ridge is composed of a large charnockite intrusion nearly concordant with
the foliation in the surrounding marbles. The southwestern (Injauk valley) and southeastern (Chaunggyi
valley) margins of the charnockite‐syenite are intrusive into marbles, with numerous mines along the
contact. The northern contact of the Taung‐met charnockite‐syenite intrusion is close to the layered ultra-
mafic rocks of the Pyang‐yaung (Bernard‐myo) region. Samples MY‐215, MY‐216, and MY‐164 were all
collected from the Taung‐met intrusion north of the Chaunggyi valley (Figure 3). MY‐164 is a clinopyr-
oxene charnockite with perthite and titanite. MY‐215 is a felsic syenite consisting of K‐feldspar + quartz
+ clinopyroxene. MY‐216 is a mafic syenite (Figure 4b,c). Sample MY‐83 was collected from a layered
charnockite containing K‐feldspar + plagioclase + quartz + orthopyroxene at the coffee plantation estate
NE of the Injauk road junction (Figure 3). Sample MY‐138 is a diopside + plagioclase + K‐feldspar +
phlogopite calc‐silicate skarn from Chaunggyi collected from the southern margin of the Taung‐met
charnockite‐syenite intrusion.
The simplest zircon sample is MY138, which gave a range of U‐Pb dates from 44 to 28 Ma and shows mini-
mal variation in Th/U, Lu/Gd, and Eu/Eu* over that time span (Figure 9). These dates are definitively meta-
morphic because the rock is a skarn. The zircon data from the rest of the samples are not as simple to
interpret. All four (MY83, MY164, MY215, and MY216) give broad ranges in U‐Pb dates from ~170 to 22
Ma, with possible clusters of intermediate dates from 68 to 20 Ma (Figures 7 and 9). Most of the dates are
concordant, or nearly so, indicating either small amounts of common Pb, partial resetting, or mechanical
(laser) mixing of zones with distinct dates. That all four samples have a cluster of oldest analyses of ~170
Ma—observed exclusively in oscillatory to sector‐zoned zircon cores (Figures S2 and S3)—indicates that
the dated rocks are of roughly the same age and crystallized at the same time; there might be a range of crys-
tallization ages from 170 to 163 Ma, but dispersion in the data makes this difficult to assess. Th/U decreases
monotonically until ~60 Ma in MY83 and MY164, compatible with metamorphism at that time and then
increases markedly after ~40 Ma, compatible with melting. Some U‐Pb dates arrayed between ~170 and
60 Ma occur in obviously metamict zircon cores (Figures S2 and S3), although many do not; in contrast,
Late Cretaceous (~68 Ma) to Miocene dates (~20 Ma) typically occur in distinct zircon overgrowths. The
youngest dates from each sample always occur in cathodoluminescence‐bright rims, and the range of mea-
sured dates from these rims (~40–20 Ma) are reproducible between spot and depth‐profiling methods applied
to the same samples. Eu/Eu* is relatively invariant in all samples except for MY215, in which a peak at 60–40
Ma is compatible with plagioclase‐rich melt injection. Titanite from MY216 gives a simple intercept date of
19.8 ± 0.4 Ma. In summary, and when considered in light of the geochronology data presented below, this
data set is compatible with ~170 Ma igneous crystallization of the charnockite‐syenite intrusions, meta-
morphism beginning at 65–40 Ma, and sustained high temperatures through to 20 Ma.
3.4. Kyat‐Pyin and Western Mogok Region
This region lies west of Mogok and is centered around the town of Kyat‐pyin and the Kyauk‐pya‐that golden
pagodas (Figure 3). Numerous ruby and sapphire mines in this region are either within the marble or along
skarn contacts with several syenite sills. We found no corundum gems in the syenite itself, although sap-
phires have been found in syenite elsewhere in the Mogok region (Themelis, 2008). Three main
charnockite‐syenite sills are mapped along the transect from Kyat‐pyin north to Baw‐mar. The main
ruby‐sapphire mines are at Kyat‐pyin, Wet Loo, Baw‐lon‐lay, Baw‐lon‐gyi, and Bawmar. Farther west, the
mines at Thurein Taung and Yadarnar kaday kadar (Burmese for “millions and billions”) produce gem star
sapphires and rubies. Sample MY‐227 is a syenite from a dyke intruding marble, north of Kyat‐pyin
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(Figure 5a). Sample MY‐122 is a garnet + biotite leucogneiss adjacent to a clinopyroxene‐olivine bearing
calc‐silicate or skarn, collected from Baw‐lon‐lay mine. Sample MY‐94 is a garnet + sillimanite + biotite
paragneiss that contains K‐feldspar‐rich leucosomes attributable to partial melting and is one of the few
metapelitic rocks seen in the Mogok valley.
Figure 9. Concordia diagrams for U‐Pb and Th‐Pb data, REE‐date data, and trace‐element‐date data from the Chaunggyi valley and Taung‐met hill (cf. Figure 8
caption for additional figure details).
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We analyzed zircon from MY‐227 and monazite and zircon from MY‐94 and MY‐122 (Figure 10). Zircon in
MY‐227 is 66.6 ± 1.2 Ma and invariant in REE composition. The Cretaceous date could be an intrusion
age, and the few younger analyses are compatible with subsequent Pb mobilization. Zircon in MY‐122
gave chiefly concordant dates from 151 to ~90 Ma, with a cluster at 124 Ma and little variation in REE
composition. The Jurassic date is likely an intrusion age, and the few younger analyses are compatible
with subsequent Pb mobilization. Monazite from the same sample is significantly younger, with concor-
dant dates from ~101 to 21 Ma and a clear decrease in Th/U, Eu/Eu*, and Lu/Gd until ~60 Ma. These data
are compatible with metamorphism without garnet prior to 60 Ma and metamorphism in the presence of
garnet since. Monazite from MY‐94 is similar, with high‐Y cores mantled by low‐Y rims (Figure S4), and
warrants the same interpretation; however, thin, young, Th‐rich rims on several grains suggest a late
(~25 Ma) melting event. For this sample, the outermost zircon rim dates have U‐Pb dates similar to the
youngest monazite, revealing that the zircon underwent late metamorphism‐related recrystallization or
rim growth.
Figure 10. Concordia diagrams for U‐Pb and Th‐Pb data, REE‐date data, and trace‐element‐date data from Kyat‐pyin and the western Mogok region (cf. Figure 8
caption for additional figure details).
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4. Model for Formation of Spinel, Ruby, and Sapphires in Mogok
Figure 11 is a simplified model showing the structural relationships of the lower and middle crustal rocks
exposed in the Mogok region. Abundant charnockite‐syenite magmas intruded from a hot lower crustal
source. Alkaline ultramafic rocks (jacupirangites) associated with mafic syenites reflect an unusual and
extreme alkaline source from the upper mantle. The charnockite‐syenites intruded to midcrust levels where
they became sill‐like intrusions. Heat from the intrusion of hot (>1,000–1,200 °C) charnockite‐syenites
would likely have produced a contact metamorphic aureole and skarns around the margins.
Magmatic‐metasomatic fluids desilicified the surrounding country rock forming first spinel and then corun-
dum. Both rubies and sapphires are regionally distributed and are almost always found close to (within 1–
2 km structural thickness) the major syenite intrusions north of the Mogok valley. Sapphire mines appear
to correlate mainly with skarns around the margins of the intrusions.
Although the large Taung‐met charnockite‐syenite is probably Jurassic in age (170–168 Ma), other
charnockite‐syenite intrusions around Thurein Taung, Kyauk Pya‐that, and Bawmar in the west may be lat-
est Cretaceous to early Miocene (~68 Ma and 44–21 Ma). One syenite along the northern margin of the Le Oo
mine, east of Mogok has a U‐Pb zircon crystallization age of 37 Ma (MY228), and a titanite crystallization age
of 21.6 Ma (MY229), similar to the calc‐silicate ruby‐bearing skarns at Le Oo mine (22 Ma). U‐Pb geochro-
nology shows that all the metamorphic ages are latest Cretaceous through to Oligocene—earliest
Miocene. The ages from the syenites and charnockites in Mogok are more difficult to interpret, and several
possible scenarios are proposed (see section 5).
Garnet‐and melt‐present metamorphism occurred between ~45 and 24 Ma in Mogok and is coeval with pre-
vious U‐Th‐Pb metamorphic ages from MMB rocks near Mandalay (Searle et al., 2007, 2017). Systematic
Figure 11. Model for the crustal structure of the Mogok metamorphic belt. Mantle rocks are the Pyang‐yuang peridotites
(dunites, harzburgites, etc.). Lower crustal rocks comprise the large layered syenite intrusion of Taung‐met, Chaungyi,
and several other sills exposed around Dattaw, Le Oo, and Bawpadan, intrusive into Mogok marbles. Ruby (R) and
sapphire (S) mines are spatially associated with the syenite sills and their surrounding skarns. The garnet + sillimanite
gneisses exposed in the mountains south of Mogok structurally overlie the marbles. West of Mogok, a large intrusive
leucogranite, the Kabaing granite, intrudes the ruby marbles and gneisses. Gem‐bearing pegmatites (e.g., Sakangyi
topaz‐quartz pegmatite) emanate from the roof of the Kabaing granite.
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thermobarometry on the high‐variance assemblages sampled in the
Mogok valley area is problematic and has not been attempted.
Moreover, there is very little evidence in the metamorphic assemblages
for multiple events, despite the apparently complex history revealed in
the zircon and monazite ages. Nevertheless, broad constraints can be
placed on the maximum conditions (Figures 12) by phase equilibrium
modelling of bulk compositions for Grt‐Bt leucogneiss (e.g., MY‐122 and
MY‐142) and Grt‐Bt‐Sil paragneiss (e.g., MY‐94), based on observed
mineral proportions in the samples. The location of the solidus places
an important constraint on the peak temperature, given that MY‐94 is a
migmatitic gneiss and that MY‐142 shows textural evidence for dehydra-
tion melting of biotite. The solidus curve is calculated for a bulk H
2
O con-
tent that is defined by the volume proportion of biotite, the only hydrous
mineral in the samples. A further constraint on the Grt‐Bt leucogneiss is
the absence of orthopyroxene, which is predicted to occur at lower pres-
sure and also in melt‐bearing assemblages through further dehydration
melting of biotite. For the Grt‐Bt‐Sil paragneiss, cordierite is predicted to
occur at lower pressure, and the high‐P limit is given by the Ky‐Sil curve.
The extent of each field also takes into account uncertainty on the esti-
mated bulk composition (cf. Palin et al., 2016). The area of overlap for
the two rock compositions is centered on 7.5 kbar and 750–800 °C condi-
tions that are comparable to those determined in granulite‐facies
paragneisses by Thu et al. (2017) at localities c. 45 km WSW of the
Mogok valley.
Late garnet + tourmaline leucogranites resulted from widespread
Miocene midcrustal anatexis and intrusion of the Miocene Kabaing
granite with its gem‐bearing pegmatites, and appear to cut fabrics in all
surrounding metamorphic rocks. The age of the Kabaing granite (16.8 ±
0.5 Ma; Gardiner et al., 2016) reflects the final phase of metamorphism
and melting in the Mogok region. The relative lack of large‐scale fold‐nappe structures and absence of evi-
dence for Himalayan‐scale crustal thickening in the MMB may suggest a heat source other than orogenic
thickening and radiogenic heating for the observed upper amphibolite‐granulite metamorphism and the for-
mation of corundum‐bearing marbles. It could be argued that the close spatial association between ruby and
sapphire‐bearing marbles and the syenite‐charnockite intrusions suggest a temporal connection, but the
geochronological data presented here are more complicated and suggest that the alkaline magmatic intru-
sions are only indirectly related to the ruby and sapphire formation.
5. Discussion
Field structural relationships, metamorphism, and U‐Pb geochronology from the Mogok area suggest three
possible tectonic scenarios:
Model 1. All charnockite‐syenites in the Mogok region are Jurassic, but only the large Taung‐met–
Chaunggyi intrusion preserves the original intrusion ages (170–163 Ma). The Kyat‐pyin and Le Oo syenites
and meta‐skarns do not have Jurassic ages from our data, but later regional granulite‐facies metamorphism
during Late Cretaceous–Paleocene time could have overprinted an earlier intrusion event. Skarns formed
around the syenite intrusions at ~170 Ma but did not necessarily contain ruby or sapphire at that time.
Regional Mogok metamorphism was a Late Cretaceous–Miocene granulite‐amphibolite facies event, and
rubies and sapphires were formed from burial and metamorphism of meta‐skarns and surrounding marbles.
Model 2. Two or three episodes of charnockite‐syenite intrusion could be interpreted from our new U‐Th‐Pb
geochronology data, the Jurassic Taung‐met intrusion (170–163 Ma), the Kyat‐pyin syenite (MY227; 67 Ma),
and the Le Oo syenite (MY228; 37–28 Ma) with adjacent meta‐skarns (MY229; 22 Ma). It is quite likely that
there was some chemical‐metasomatic effect of intrusion of these hot magmas into a regional, long‐lasting
Figure 12. Metamorphic peak P–Tconditions for gneisses in the Mogok val-
ley area based on calculated phase diagrams (pseudosections) showing sta-
bility fields for assemblages in Grt‐Bt leucogneiss (Kfs‐Pl‐Qz‐Bt‐Grt‐Ilm)
and felsic Grt‐Bt‐Sil paragneiss (Kfs‐Qz‐Pl‐Grt‐Bt‐Sil‐Ilm). Area of overlap is
centered on 775 ± 50 °C, 7.5 ± 1 kbar. Diagram calculated with Theriak/
Domino software (De Capitani & Petrakakis, 2010), using the thermoche-
mical database DS6 of Holland and Powell (2011) and activity models from
White et al. (2007), White, Powell, Holland, et al., 2014, White, Powell, &
Johnson, 2014). Mineral abbreviations follow Whitney and Evans (2010).
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(ca. 45 m.y.) granulite‐facies terrane, such that the occurrence of rubies and sapphires was directly related to
multiple, distinct episodes of skarn formation.
Model 3. The charnockite‐syenites were all intruded during the latest Cretaceous‐Oligocene–early Miocene
(between ~68 and 22 Ma) concomitant with regional metamorphic ages from zircon, monazite, and titanite
dates. In this model, the older Jurassic zircon ages from Taung‐met to Chaunggyi (170–163 Ma) are inter-
preted as inherited from the source and escaped overprinting during later granulite‐facies metamorphism.
The exclusively young zircon ages reported by Thu (2007) and Sutherland et al. (2019) support this model.
The Jurassic dates appear to be restricted to the large intrusion at Taung‐met and Chaunggyi (MY 83,
MY‐164, MY‐215, and MY‐216). One sample, the Taung‐met charnockite (MY 216), has zircon ages at
168, 63, and 26 Ma. The Cretaceous igneous rocks may be part of the trans‐Himalayan Gangdese batholith
in south Tibet and its proposed extension south into Myanmar (Lin et al., 2019; Zhang et al., 2017). All these
samples have younger zircons, reflecting subsequent high‐temperature metamorphism. One syenite sample
from a dyke intruding marble (Figure 5a) at Kyat‐pyin (MY‐227) has only younger zircon dates ranging of
44–38 Ma. Zircon has a high closure temperature, but in charnockites and syenites, it may not preserve older
inheritance ages due to a combination of diffusive Pb resetting and zircon resorption. If so, the oldest zircon
ages should reflect the timing of intrusion.
The main pulse of metamorphism recorded by rocks in the Mogok valley appears to have begun around 60
Ma and extended until titanite crystallization or diffusive Pb closure at 22 Ma. This timing is similar to that
from the MMB to the south around Mandalay (Searle et al., 2007, 2017). The oldest dates may reflect regional
contact metamorphism around the trans‐Himalayan batholith. Most of the metamorphic dates and particu-
larly those with trace‐element characteristics suggestive of melting, garnet growth, and granulite‐facies con-
ditions in the ruby‐and sapphire‐bearing skarns around the margins of the charnockite intrusions begin
around ~45 Ma and extend to ~24 Ma. We suggest that this timing records the crystallization of the gem spi-
nels, rubies, and sapphires in Mogok. The thick marble bands around the Dattaw mine north of Mogok have
produced some of the best quality gem rubies and are located between the upper Taung‐met
charnockite‐syenite intrusion and the lower Le Oo syenite intrusion (Figure 3).
High temperatures were maintained until as late as 16.8 Ma when the Kabaing leucogranite was intruded
into the MMB to the west (Gardiner et al., 2016). Abundant garnet + sillimanite ± cordierite migmatites
are known from drill core samples in the Kyi Tauk Pauk gold mine west of Mogok. At this location, numer-
ous mesothermal gold‐bearing quartz veins intrude the garnet‐sillimanite migmatites. These migmatitic
rocks must reflect a different, more pelitic source than the Mogok valley region and are of unknown age.
Similar migmatites may be present SE of the Mogok valley (on the road to Mandalay and Pyin Oo Lyin),
but the ages of these rocks are also unknown. Further research combining field structural mapping with
detailed U‐Th‐Pb geochronology is required to unravel the complex metamorphic and magmatic history
of the region.
6. Conclusions
Granulite‐and upper amphibolite‐facies marbles occur throughout the MMB that runs for >700 km north–
south through central Burma. Spinel, ruby, and sapphire gemstones are common in the Mogok valley region
but are rare outside this area. The ruby and sapphire in the Mogok valley are spatially related to a series of
charnockite‐syenite sill‐like intrusions around the Mogok valley. Gem‐quality sapphires are related to the
metasomatic calc‐silicate skarns around the margins of these charnockite‐syenites. The composition range
of charnockites and syenites is broad with both mafic and felsic varieties, ranging from ultramafic
hornblende‐pyroxene‐biotite rocks through orthopyroxene and clinopyroxene‐bearing charnockites to
quartz syenites. Four charnockites and syenites from Taung‐met and the Chaunggyi valley have U‐Pb zircon
dates spanning 170–163 Ma, indicating an earlier Jurassic phase of alkali igneous intrusion in the protolith
rocks. U‐Th‐Pb zircon dates on six charnockite‐syenites span 67–22 Ma, including four samples from the
Taung‐met charnockites that have Jurassic dates, and one (MY‐227), together with a skarn rock (MY‐138),
that do not have any Jurassic dates. A single U‐Pb titanite date from a syenite at Le Oo mine in Mogok is
22 Ma similar to a U‐Pb titanite date of 21 Ma from an adjacent ruby‐bearing calc‐silicate skarn‐marble.
These are broadly coeval with monazite and zircon dates from metasedimentary rocks along the MMB in
the Mandalay‐Kyaushe area to the south. A cluster of U‐Pb monazite ages from 97 to 75 Ma are thought
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to reflect the thermal influence of precollision subduction‐related granite‐granodiorite intrusions along the
MMB. U‐Th‐Pb ages of the Mogok metamorphic rocks are all latest Cretaceous to early Miocene, related to
the India‐Asia (Sibumasu) collision. The MMB continues northward, east of the Putao region along the
Myanmar‐China border, toward the East Himalayan syntaxis and beyond, possibly to the basement units
of the Northern Lhasa terrane in SE Tibet (Palin et al., 2014; Searle et al., 2011).
The unusual mineralogy and richness of ruby and sapphire gems in the Mogok area are spatially related to
the ultrahigh temperature dry charnockites, alkali granites, and syenites in the lower crust. U‐Pb geochro-
nology, however, suggests three possible models, (a) that all the charnockites and syenites were Jurassic,
but only the Taung‐met‐Chaunggyi intrusion has preserved Jurassic zircons; Late Cretaceous–Oligocene zir-
con and monazite ages reflect a regional metamorphic overprint that was synchronous with ruby and sap-
phire formation. (b) Three phases of charnockite‐syenite intrusion are recorded in the Middle Jurassic
(170–163 Ma), latest Cretaceous–Paleocene (68–63 Ma), and early Eocene–early Miocene (~47–22 Ma), each
of which was associated with ruby and sapphire formation in adjacent skarns. However, only the second and
third phases of intrusion during Late Cretaceous–Oligocene or earliest Miocene time were concomitant with
granulite‐facies metamorphism during the later period of intrusion. (c) The charnockites and syenites were
Late Cretaceous–early Miocene in age and related to regional granulite‐facies metamorphism. Older Jurassic
zircon ages in the Taung‐met syenite would have been inherited from the source or from contamination dur-
ing magmatic transport.
Phanerozoic granulite terranes with charnockites are not common in the world, and multiple phases of
charnockite‐syenite intrusion in the same locality seem improbable. Further, because of the high tempera-
tures of intrusion of charnockites (~1,000–1,200 °C), zircons are unlikely to preserve any older ages; thus, the
oldest zircon ages from a given sample are probably either intrusive or metamorphic dates but are unlikely to
be inherited. We suggest that the most likely model is that of Jurassic (~170–163 Ma) intrusion of syenites
and charnockites that were affected by a regional granulite‐facies metamorphism lasting from ~68 to 21
Ma. Rubies and sapphires were formed during this regional metamorphic episode by granulite‐facies meta-
morphism of meta‐skarns and thick marbles.
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Acknowledgments
We thank the Oxford–Burma Aung San
Suu Kyi trust for funding research and
fieldwork visits to Myanmar for MS,
NG, and LR. We thank Than Than Nu
and Ney Lin for hospitality in the
University of Mandalay and discussion
on Mogok gems. Thanks to U Than
Naing, mine manager, for access to Le
Oo ruby‐sapphire mine in Mogok;
Aung Moe, mine manager of
Htay‐pying, for access to Baw lon‐lay
ruby mine; Htun Lynn Shein
(Myanmar Precious Resources Group)
for permission to access Kyi Tauk Pauk
gold mine; Tin Aung Myint for logistic
help in Mandalay; Thu Htet Aung for
expert off‐road driving in Mogok; and
Sam Weatherly and John Cottle for
discussions on syenite petrology and
U‐Th‐Pb geochronology.
Geochronology was funded by UCSB
and NSF grants EAR‐1348003 and
EAR‐1551054. We thank Shuguang
Song (Peking University) and an anon-
ymous reviewer for helpful reviews. All
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