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GEOLOGY OF THAILAND(METAMORPHIC ROCKS)

GEOLOGY OF THAILAND(METAMORPHIC ROCKS)

Low-Grade Metamorphic Rocks (Lower Paleozoic)

High-Grade Metamorphic Rocks (Precambrian?)

THAILANDTHAILAND

EXPLANATIONEXPLANATION

Lower Paleozoic RocksLower Paleozoic Rocks(Low Grade)(Low Grade)Precambrian (?) RocksPrecambrian (?) Rocks(High Grade)(High Grade)

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BASEMENT COMPLEXBASEMENT COMPLEXBasement Complex: The crust of the earth below sedimentary strata extending downward to the “mohorovicicdiscontinuity”. In many places, the rocks of the basement complexex are igneous and metamorphic rocks of Precambrianage but in some places they are Paleozoic or Mesozoic or even Cenozoicin age.

Basement Complex: The crust of the earth below sedimentary strata extending downward to the “mohorovicicdiscontinuity”. In many places, the rocks of the basement complexex are igneous and metamorphic rocks of Precambrianage but in some places they are Paleozoic or Mesozoic or even Cenozoicin age.

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SHIELDSHIELDShield: A large area of exposed “basement complex” in a craton, commonly with a gently convex surface surrounded by sediment-covered platform, e.g. Canadian shield, Baltic shield. The rocks of almost all shield areas are Precambrian in age.

Shield: A large area of exposed “basement complex” in a craton, commonly with a gently convex surface surrounded by sediment-covered platform, e.g. Canadian shield, Baltic shield. The rocks of almost all shield areas are Precambrian in age.

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“Basement Complexes in Thailand”“Basement Complexes in Thailand”Metamorphic rocks of basement complexes are believed to be Precambrian in age based on:• They commonly underlie Ordovician limestone or Lower Paleozoic rocks (but usually separated from the cover sequence by low-angle faults).

• They were metamorphosed to amphibolite facies(high grade) while the overlying Lower Paleozoic rocks are of sub-greenschist to greenschist facies(very low-low grade).• They were more strongly deformed than the overlying strata (at least three phases of deformation were recognized).

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“Basement Complexes in Thailand”“Basement Complexes in Thailand”The sequence of metamorphic rocks of basement complexes are rather uniform throughout the country, from bottom to top as follows:

• Paragneiss-orthogneiss unit• Schist unit• Calc-silicate rock-marble unit• Quartzite unit• Marble unit

High grade rocks in Chonburi area also include amphibolite (metamorphosed basaltic rocks)

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“Basement Complexes in Thailand”“Basement Complexes in Thailand”

Lithostratigraphic Units:

• Chiang Saen massif• Wiang Pa Pao schist• Chiang Mai-Tak gneiss belt

(Chiang Mai gneiss)• Ban Rai complex• Kanchanaburi gneisses• Hua Hin gneiss• Chonburi massif

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Low Grade Metamorphic Rocks(Lower Paleozoic)Low Grade Metamorphic RocksLow Grade Metamorphic Rocks(Lower Paleozoic)(Lower Paleozoic)

High Grade Metamorphic Rocks(Precambrian?)High Grade Metamorphic RocksHigh Grade Metamorphic Rocks(Precambrian?)(Precambrian?)

THAILANDTHAILAND

Gulf of ThailandGulf of Thailand

CSmWPPs

CMg

BRc

KCg

HHgCm

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“Chiang Saen massif”“Chiang Saen massif”

The rocks occur on the south side of Mekhong river in the vicinity of Chiang Saen and probably extend across the river into Laos.

Lithology: In Thailand, mainly granite and granodiorite. In Laos, coarse leucocratic granite without foliation that includes areas of gneiss with bands of schists, calcareous schist, marble, and biotite-amphibole schists (Workman, 1972).

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“Wiang Pa Pao schist”“Wiang Pa Pao schist”

Outcrops of schists occur along Nam Mae Ko, approx. 16 km NNW of Wiang Pa Pao (P. Nutalaya, pers comm., 1973). These schists are unconformably overlain by Paleozoic formations.

Lithology: Medium to coarse-grained, dark gray biotite-sillimanite schist and biotite schist that contain porphyroblasts of sillimanite (> 5 cm in length). Other constituents are quartz, plagioclase, and muscovite.

Chiang SaenMassifChiang SaenMassif

Wiang Pa PaoSchistWiang Pa PaoSchist

Chaing Mai-TakGneiss BeltChaing Mai-TakGneiss Belt

Ban RaiComplexBan RaiComplex

GranitesGranitesGneissesGneisses

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“Chiang Mai-Tak gneiss belt”“Chiang Mai-Tak gneiss belt”

This is probably the most extensive exposure of the basement complexes in Thailand. The exposures formed the core of a N-S trending anticlinorium, some 120 km long and up to 50 km wide.

Lithology: The gneisses (paragneisses) and schistswere regarded by Baum et al. (1970) to have been sedimentary rocks affected by anatexis and granitization processes. The rocks are unconformably overlain by less metamorphosed Cambrian and Ordovician formations.

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“Chiang Mai-Tak gneiss belt”“Chiang Mai-Tak gneiss belt”

NW of Chiang Mai, the rocks consist of intensely folded series of gneisses, marbles, calc-silicate rocks, and biotite gneisses.

South of Chiang Mai, the sequence of gneissic rocks consist of (Baum et al., 1970):Uppermost - biotite marble (20m)

- thin bedded calcareous and quartzitic rocks (20m)

- schistose augen gneisses (500m)- biotite schists with marble

intercalations, locally transformed into calc-silicate rocks (unknown

thickness).

Doi Inthanon Geological Map ofDoi Inthanon Area

ChomThong

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“Chiang Mai-Tak gneiss belt”“Chiang Mai-Tak gneiss belt”

Bhumipol Dam site, Piyasin (1972) reported the presence of schist and gneiss. Schists contain quartz, sillimanite, and cordierite, with pinite and chlorite. Schists overlie marble which is coarse-grained, granoblastic to foliated with intercalated calc-silicate rocks. Calc-silicate rocks contain quartz, diopside, biotite, plagioclase, muscovite, garnet, tourmaline with hornblende, tremolite, and epidote. Gneiss is strongly-lineated and composed mainly of biotite and microcline with subordinate muscovite, plagioclase and quartz and minor sillimanite.

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“Chiang Mai-Tak gneiss belt”“Chiang Mai-Tak gneiss belt”

Tak-Mae Sot Highway and Lan Sang Park, high-grade metamorphic rocks are well exposed. These rocks are mainly paragneisses, including biotite gneiss, banded quartzo-feldspathic gneiss, biotite-microcline gneis, augen gneiss calc-silicate rocks and marble, with migmatite and orthogneiss. Structurally, the gneissic rocks overlie less metamorphosed Lower Paleozoic cover of quartzite, marble, limestone, and fine-grained clastic rock to the west.

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“Ban Rai complex”“Ban Rai complex”

This is the southward extension of the Chiang Mai gneiss. The rocks consist of biotite gneiss, schist, calc-silicate rock and biotite marble.Sillimanite and staurolite are locally present.

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“Kanchanaburi gneisses”“Kanchanaburi gneisses”

Exposures of the gneisses occur in Mae Klong river valley, NW of Kanchanaburi (Brown et al., 1951). The rocks are unconformably overlain by Ratburi limestone of Upper Carboniferous-Permian in age.

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“Hua Hin gneiss”“Hua Hin gneiss”Also known as “Pranburi-Hua Hin Complex”(Pongsapich et al., 1980)

Exposures of the Hua Hin gneiss are found in a N-S trending elongate belt of ca. 50 km long in Amphoe Pranburi and Hua Hin.

Lithology: Mainly gneisses, schists, calc-silicate rocks, quartzites, and marble. They are unconformably overlain by Devonian -Carboniferous Tanaosri Group and Permian-Carboniferous Ratburi limestone.

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“Hua Hin gneiss”“Hua Hin gneiss”Gneises: Foliated-lineated biotite gneiss containing garnet, sillimanite (fibrolite), and cordierite.

Quartzites: Fine to medium-grained granoblastictexture. Mainly quartz with K-feldspar, muscovite, biotite, graphite (?).

Schists: Mica schist containing sillimanite.Calc-silicate Rocks: Medium-grained granoblastictexture. Mineral assemblage include diopside, plagioclase, calcite, and calcic-amphibole.

Marble: Fine-grained granoblastic texture. Mainly calcite with dolomite.

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“Chonburi Massif”“Chonburi Massif”

Exposures of the gneisses occur extensively in Amphoe Panas Nikhom, Chonburi.

Lithology: Mainly gneisses of varied composition, including biotite gneiss, biotite-hornblende gneiss, sillimanite-biotite gneiss and schist, biotite-hornblende-diopside gneiss, calc-silcate rock and marble. Also present are migmatite gneiss and amphibolite (meta-basaltic rocks)

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“Low Grade Metamorphic Rocks”(Lower Paleozoic: Cambrian-Devonian)“Low Grade Metamorphic Rocks”

(Lower Paleozoic: Cambrian-Devonian)

Exposures of the regional low grade metamorphic rocks are in close spatial relationship with their high grade counterparts.

Not all Lower Paleozoic rocks were metamorphosed, some still retain their sedimentary characters, especially in the south.

Mineral assemblages indicate that the Lower Paleozoic rocks were metamorphosed, to sub-greenschist to greenschist facies.

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“Low Grade Metamorphic Rocks”“Low Grade Metamorphic Rocks”Cambrian metamorphic rocks, which are

restricted to the western mountain range, mainly include quartzite, quartz schist, and quartzo-feldspathic schists. They commonly grade upwards into well-bedded to massive recrystallizedlimestone of Ordovician age.

Koh Si Chang marble (Ordovician?) overlies quartzite (Cambrian?)

Silurian-Devonian metamorphic rocks include slate, phyllite, schist, and quartzite.

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The EndThe End