The Precambrian Period

Transcription

1 The Precambrian Period Precambrian rocks in Thailand are metamorphic rocks found directly below the welldefined Cambrian sedimentary rocks containing distinctive Cambrian fossils. The rocks are mostly regional metamorphic rocks and are easy to distinguish from other types of rocks in Thailand. The typical grade of metamorphism is generally between medium to high especially for that in gneiss as we can see segregation bandings in fresh rock faces. Together with gneiss, schist, calcsilicate and marble are also the major species Precambrian rocks often found as layers in the gneiss body. In some areas such as Hod district of Chiang Mai province and Mae Sareang district of Mae Hong Son province, migmatite which is the evidence of partial melting taking place during the metamorphic process is observed. Granite can also be found associated with migmatite. The metamorphic rocks with this type of history is common in areas from the northern region to the southern region and in some areas in the eastern region of the country. For the convenience of presenting the Precambrian rocks of Thailand, we classify the rocks based on their locations and the result shows five major Precambrian rock regions. These locations are (i.) The Northen and Upper-Western region, (ii.) The Eastern region, and (iii.) The Lower-Western and Southern region (Figure 1.). Moreover, to avoid repetition and confusion, we choose to discuss other issues regarding the Precambrian rocks of Thailand as a whole. These discussions are about (iv.) Comparisons of rocks and their compositions, (v.) Precambrian rock structures, (vi.) Relations between high-grade Precambrian and low-grade Lower-Paleozoic metamorphic rocks, (vii.) Rocks derived from metamorphism, (viii.) The age of the rocks and the age of metamorphism, and (ix.) Types of rocks before metamorphism. (I.) Precambrian Rocks in the Northern and Upper-Western Region Among the layers of Precambrian rocks in this region, the base of the rock unit contains high-grade gneiss and schist. The calcareous substances such as calcsilicate and marble are also present especially towards the top of the section. These two calcareous rocks often contain their relict sedimentary structures, therefore they are called paragneiss. For those metamorphic rocks that have evidences of partial melting, they usually contain granite and are named migmatite. Because most of the Precambrian rocks in this region has undergone tectonic deformation, we observe the high-grade metamorphic rocks only in following limited areas. (I.i) Mae Klang Waterfalls Mt. Intanon Area between Hod District and Ob Luang National Park The rocks found in this area are among the oldest ages in Thailand, phologopite marble, quartzo-feldspathic gneiss, and schist. Along with these three rocks, veins of marble and calcsilicates, porphyroblastic biotite gneiss and migmatite are sometimes found intertwined in the rock bodies. 1

2 Figure 1. The distribution of Precambrian rocks in various areas of Thailand. Note: เช ยงใหม - Chiang Mai, ตาก Tak, ก าแพงเพชร Kampang Pet, อ ท ยธาน Utai Thani, กาญจนบ ร Kanchanaburi, กร งเทพมหานคร Bangkok, ชลบ ร Chonburi, ประจวบค ร ข นธ Prachuabkirikan, นครศร ธรรมราช Nakorn Sritammarat 2

3 (I.i.i) Phologopite marble This rock is generally m. thick and placed at the topmost layer of the Precambrian rocks. Its color is light grey to grey. The crystal grains are fine to coarse. In a closer look, it contains quartz, phologopite and small layers of calcsilicate minerals. The lower section of phologopite marble often contains more calcsilicate minerals. Major constituents of this rock are calcite, dolomite, phologopite and small quantities of epidote, quartz and actinolite. At the quarry west of Tan Dokmai in Jomtong district, we find very distinguished foliation and folds in the phologopite marble. Furthermore, we find rounded or ellipsoid pegmatite or leucosome oriented parallel with phologopite grains. Leucosome contains mostly feldspars with tiny amount of phologopite and quartz. However, leucosome from some adjacent areas is found to contain epidote, actinolite, diopside and garnets in the rim of leucosome adjacent to phologopite grains which are located just before the boundary between this rock and the surrounding marble. There is a report of black grey find-grain marble existing as thin layers along the Tak Mae Sot highway and at Lang Sang waterfalls (Campbell, 1975). This rock unit should be similar to the phologopite marble as discussed above. The mineral composition is 90% calcite and the rest consists of quartz, plagioclase-andesine, microcline, phologopite, sphene, clinozoisite, epidote, blue hornblended, actinolite and subcalcic augite. The plagioclase often shows oscillatory zoning. This marble contains two types of distinct rocks often found as pods or rounded masses with diameter up to one meter. The first type is calcsilicate rock with quartz and plagioclase making the rock look grey green to dark green. The other has the same composition as granite with white grey color, semi-course grain and its crystal grains have roughly similar size. There is no evidence of foliation in the granite. This type of granite is usually found on top of the marble unit. Consequently, it can be inferred that the top most layer of the marble was subjected to higher pressure than the lower part of the rock in the past. The calcsilicate and pegmatite portion contains clusters of high-order foliated area. The feldspars and pegmatite normally shows that they were experienced the force making their bodies tilted in particular directions. (I.i.ii) Quartzofeldspathic gneiss, biotite gneiss and schist This rock unit is found right below the previous rock unit, the phologopite marble. Its thickness ranges from 300 to 400 m. The top part of the unit consists of diobside marble, calcsilicate, and garnet-diopside calcsilicate. This unit is cataclastic or has experienced mylonitic deformation with a great magnitude. In general, the crystal grains are smaller than the lower units. Some calcsilicate layers show tight fold to recumbent fold. The bottom part of this unit is altered to layered schist and layered gneiss. The unit also includes porphyroblastic quartz biotite schist, banded gneiss and augen gneiss. Moreover, layers or lens of pegmatite rocks are sometimes present. The rock bodies are semi-coarse at the lower part and become fine-grained at the top part. They also contain white bands with garnet crystals. Large feldspar grains and lens of pegmatite are always rotated to particular directions (Figure 2. Figure 4.). 3

4 Figure 2. Gneiss found along the highway to mt. Intanon, the highest peak in Thailand. Figure 3. The close-up look of cataclastic gneiss in mt. Intanon area. Figure 4. Gneiss at Ob Luang national park in Chiang Mai province. 4

5 The mineral compostion of calcsilicate is quartz, calcite, diopside, phologopite, epidote, garnets and minute amount of other minerals. For the marble, there are only calcite and diopside. For gneiss rock, the major minerals are quartz, potash feldspar, plagioclase, biotite and the minor minerals which are found in some sample only are muscovite, garnets, sillimanite and cordierite. Other accessory minerals are apatite, monazite, zircon and dark minerals such as magnetite, ilmenite and sulfides. For pegmatite rock, it primarily consists of quartz, potash feldspar, plagioclase, and biotite. In the white layer, we often find red garnet, apatite, zircon and small amount of dark minerals. (I.i.iii) Biotite gneiss and migmatite The orientation of this rock unit is quite conformed with previous units. The thickness exceeds 600 m. The rock unit is usually found at the bottom of the metamorphic strata or found at the center of high-grade metamorphic rock body. The unit consists of biotite gneiss, biotite-garnet-sillimanite gneiss and migmatite (Figure 5.). In gneiss, we normally find quartz, potash feldspar, plagioclase and biotite, but sillimanite garnet and muscovite are only found in some samples. The texture of the rock coarse and irregular or heterogeneous. In some portion of the unit, we find orthogneiss. In migmatite especially in its granitic part, pegmatite and aplite, we often find muscovite and/or large sheeted biotite (diameter about 1-2 cm.). Furthermore, other minerals found in these rocks but only in small amount are plagioclase, quartz, orthoclase, microcline, sillimanite, and the easy-to-identify garnets. Figure 5. Migmatite and biotite gneiss in mt. Intanon area. (I.ii) Western Part of Chiang Mai Province Metamorphic rocks found in this area include gneiss, schist, marble and calcsilicate. The studies of Baum et al. (1970) and Braun (1969) indicate that this rock unit are mostly derived from sedimentary rocks. Some portion of the rock had undergone anatexis and were converted to granite by granitization process. They also believed that the metamorphic process responsible for creating high-grade paragneiss and orthogneiss occurred in the Precambrian. They argued that this rock unit was metamorphosed with a greater degree than the rock of the lower Paleozoic which located above the Precambrian rock in great quantity. The granitization and melting should take place in the Carboniferous. 5

6 Moreover, they added that migmatite were normally found in the lower part of biotite +/- garnet zone and biotite garnet +/- sillimanite zone because their observations showed that granite were present in larger quantity than the rest of the zone. (I.iii) Bhumibhol Dam Area High-grade metamorphic rocks found in Bhumibhol dam area were studied by Natalaya (1974), Piyasin (1974) and Bunopas (1980). The finding was several rock units from youngest to oldest as follows. (I.iii.i) Mica Schist Mica schist with thickness around 700 m., color grey to brown grey, are present. The rock contains mostly quartz, plagioclase, biotite and muscovite. Other minor minerals are garnets, andalusite, cordierite, penite and chlorite and accessory minerals are apatite, zircon, tourmaline, and dark minerals (Figure 6.) Figure 6. The illustration of crenalation in quartz-mica schist located at the Bhumibhol dam. (I.iii.ii) Marble and Calcsilicate This rock unit is about 500 m. thick. Its color varies from white to grayish green. The texture is dense, compact, coarse and granular. Minerals consisted in the rock, mostly calcite, are well-oriented. One may find plagioclase, biotite, phologopite, epidote, soilsite and amphibole. Accessory minerals, tremolite, diopside, garnets, tourmaline and dark minerals are sometimes found in this rock unit. (I.iii.iii) Biotite-microcline Gneiss This rock unit is rather thick, 1,200 m. It consists of mostly microcline and biotite. The less abundant minerals in this rock are quartz, and muscovite. Minute quantities of sillimanite, epidote and chlorite are also expected. Apatite, zircon, tourmaline and dark minerals are rarely present. 6

7 (I.iv) Lansang Area and Along the Tak Mae Sot Highway Metamorphic rocks in Lansang area are gneiss, schist, calcsilicate and marble. These rocks can be seen at several waterfalls in Lansang national park and along the highway between Tak city and Mae Sot district around the kilometer post 12 to the kilometer post In the studies of Campbell (1975) and Bunopas (1976), they discussed about this rock unit a follows. The metamorphic rock, quartz-feldspar-biotite gneiss, covers about % of all the rocks found in this area. The gneiss is often found associated with lime silicate (covers about 10 20%) and igneous rocks with some degree of metamorphism (covers about 10 2-%). Moreover, they reported gneiss that had undergone some erosion processes becoming cataclastic gneiss in the easternmost part of the national park (Figure 7. and Figure 8.). For some part of this rock unit that is leucocratic granite, it is suspected that the rock s origin is melting or anatexis. The granite is found penetrating along the mineral grain. Some portion developed to pegmatite and the rest was cutting through mineral grains of gneiss and granite gneiss. Figure 7. Illustration of folding and recrystallization of calcsilicate rocks of Precambrian age observed in Ban Tak district, Tak Province. Figure 8. Ptymatic fold and foliation patterns observed in gneiss in Tak province. 7

8 (I.v) Ban Rai District Area High-grade metamorphic rock unit located in the western part can be seen in the geological map of Thailand with 1:250,000 scale that the unit extends south of Tak province, Kampang Pet province and ends at Ban Rai district, Utai Thani province. This rock unit contains biotite schist, calcsilicate and biotite marble. Silimanite mineral can be found in schist as well (Campbell, 1975). (I.vi) Kanchanaburi Province and Western Supanburi Province Area The metamorphic rocks found at Tapsila village, River Kaew Yai and mt. Chon Kai belong to the same rock unit. They are significantly different from the mt. Chong Insee metamorphic rock unit which is located in Bo Ploy district, Kanchanaburi province. These two rock units contain slightly similar rock, quartz-feldspar-biotite gneiss. The major difference is that gneiss of mt. Chong Insee rock unit consists of silimanite mineral and also has some amount of calcsilicate rocks (Figure 9.) (Bunopas, 1976). Figure 9. The close-up look of quartz-feldspar-biotite gneiss in Saiyoke district, Kanchanaburi province. Metamorphic rocks found in Tamaka district, Kanchanaburi province (Figure 10.), is essentially the extended portion of the River Kaew Noi Lin Tin village in Tongpapoom district metamorphic rock unit. This rock unit consists of sillimanite schist, paragneiss, calcsilicate, granite gneiss containing fibrolitic sillimanite crystals that are not well-oriented. This feature is perhaps the result of granitic intrusion (Dheeradilok et al., 1985). Sillimanite minerals are also limited in the confined area parallel to the boundary of granite rocks. 8

9 Figure 10. Schist with calcsilicate-compositioned passive flows which contain welldeveloped fractures along the fold axes in Tamaka district, Kanchanaburi province. (II.) Precambrian Rocks in the Eastern Region Rocks of regional metamorphism origin found in the eastern part of Thailand are prominent in Chachengsao, Chonburi, and Rayong provinces. These areas were named Chonburi massif after Campbell (1975) and Areesiri (1982). The massif is oriented northwest southeast. Reports on metamorphic rocks in this area are limited to studies of Brown et al. (1951) and Nakinbordi et al. (1985). The detailed geology of mt. Chao, Nong Yai district, Chonburi province reported by Areesiri (1982) is as follows. (II.i) Mt. Chao Area, Panasnikom District Nong Yai District, Chonburi Province are; High-grade metamorphic rocks in the area ordering from the youngest to the oldest - Biotite-diopside gneiss - Biotite-sillimanite gneiss - Hornblend-diopside gneiss - Biotite gneiss - Biotite-hornblend gneiss - Biotite-feldspar-quartz gneiss. (II.i.i) Biotite-diopside gneiss This rock unit is usually fine-grained. In some areas, there are intrusions of biotitehornblend-diopside gneiss, amphibolite and biotite gneiss. The main constituents of the biotite-diopside gneiss are diopside, biotite, plagioclase, potash feldspar, quarts and minute quantities of apatite and zircon. (II.i.ii) Biotite-sillimanite gneiss The upper part of this rock unit is sillimanite-biotite-potash feldspar augen gneiss, while the lower part contains garnet-biotite-sillimanite-quartz schist. Cordierite minerals are expected to be in the lower part of the rock unit as well. 9

10 (II.i.iii) Hornblend-diopside gneiss The texture of this rock unit is fine to semi-coarse. Typical minerals found in the rock are hornblend and diopside in a large percentage. Potash feldspar, apatite and sphene are minor minerals in this rock. In some areas, biotite-hornblend-diopside gneiss, amphibolite and biotite-sillimanite schist are found accompanying the hornblend-diopside gneiss rock unit. (II.i.iv) Biotite gneiss The texture of this rock unit is also fine to semi-coarse and usually found right below the hornblend-diopside gneiss unit. Major minerals in this rock unit are quartz, potash feldspar, plagioclase and biotite. Minute quantities of zircon, magnetite and apatite are also present. (II.i.v) Biotite-hornblend gneiss The texture of this rock unit is semi-coarse to coarse. There are small amount of amphibolite, calcsilicate and biotite-sillimanite schist present with this biotite-hornblend gneiss rock unit. Minerals with great quantities in this rock unit are biotite, hornblend, plagioclase, quartz, potash feldspar and garnets. Minerals with lesser amount present are apatite, zircon and magnetite. (II.i.vi) Biotite-feldspar-quartz gneiss This type of gneiss are found in layers sandwiched between semi-coarse and coarse grained rocks. We often find thin amphibolite rock layers associated with this rock unit. In some areas, this rock was partially converted to migmatite, and veins of amphibolite were left in the rock in a discontinuous fashion (Figure 11). Figure 11. Biotite-feldspar-quart gneiss in the quarry in Nong Yai district, Chonburi province. Notice the presence of migmatite and the isoclinal fold with NE-direction axial plane. Beside these six high-grade metamorphic rocks, Areesiri (1982) also distinguished marble-calcsilicate unit and amphibolite unit from them. Descriptions of both rock units are as follows. Marble-calcsilicate: Marble is often formed into lens accompanying biotitehornblend gneiss. Major minerals in this marble are calcite, dolomite, and phologopite. For 10

11 calcsilicate, the nature of origin is similar. Its major minerals are diopside, calcite, and sphene. Amphibolite rock unit is found in layers whose thickness ranges from 2 cm. to 10 m. It is commonly found intertwined with all types of gneiss. Classification of amphibolite is based on its composition such that amphibolite contains hornblend, plagioclase, quartz, apatite, and sphene. Others are called using this classification rule as biotite amphibolite, diopside amphibolite, garnets amphibolite, and hornblend amphibolite. (II.ii) Area North of Chamao Mountains in Glang District, Rayong Province Rocks in this area are also high-grade Precambrian metamorphic rocks. This rock unit is located in the easternmost part of the country and is controlled by faulting making the outcrop narrow and long. Several rocks such as mica schist (Figure 12.), hornblend schist, calcsilicate, biotite-muscovite granite, pegmatite, aplite with large garnet crystals, belong to this rock unit. We often find gneiss and mica schist in small hills between mt. Chamao and the municipal center of Glang district. Figure 12. Porphyroblastic garnet schist in the mt. Chamao area, Glang district, Rayong province. (III.) Precambrian Rocks in the Lower-Western and Southern Region High-grade metamorphic rocks found in this region of Thai peninsula were reported in Hua Hin district and Pran Buri district of Prachuabkirikan province (Campbell, 1975, and Pongsapich et al., 1980), in Tab Sakae district of Prachuabkirikan province, and in Ranong province (Campbell, 1975). Another study (Lamjuan, 1977) also indicated their presence in Kanom district and Sichon district of Nakorn Sritammarat province. Classification of rock units and their major features are discussed in the following sections. (III.i) Area in Pran Buri District of Prachuabkirikan Province 11

12 The study by Pongsapich et al. (1980) showed metamorphic rock units in a stratigraphic sequence from youngest to oldest as; - Marble - Quartzite - Calcsilicate, quartzofeldsparthic and marble - Metapelite and gneiss. (III.i.i) Marble The typical color of this marble rock unit is light grey to white. Chert with diameter from 2-3 cm. up to 35 cm. can be occasionally found on top of quartzite, but there is no evidence for the boundary. This occurrence of rocks can be seen at mt. Karok and north of mt. Huatam. Major minerals in this marble rock unit are calcite and dolomite. (III.i.ii) Quartzite The quartzite rock unit of this area has fine-grained to semi-coarse-grained texture. The unit contains calcsilicate and quartzite. Major minerals composed in this unit are green hornblend, glossularite, sphene, and plagioclase. In some samples, small amount of zoizite and clinozoizite is expected. (III.i.iii) Calcsilicate, quartzofeldsparthic and marble These rocks generally occur as strata with semi-coarse textures. The calcsilicate is observed to gradually transform its compostion to be quartzofeldsparthic. Major minerals in this rock unit are diopside, plagioclase, microline, quartz, calcite, tremolite, hornblend, and sphene. Minute quantities of scapolite and biotite are also present. (III.i.i) Metapelite and gneiss This rock unit consists of mica-sillimanite schist and gneiss. Mineral bands showing the evidence of metamorphism are clearly observed. Schist contains quartz, biotite, sillimanite, and garnets. In some schist samples, there are orthoclase and/or cordierite. For gneiss, it usually contains quartz, biotite, microcline, garnets, sillimanite, orthoclase, and plagioclase (An 20 to An 30 ). Orthoclase feldspar grains are generally big with ellipsoid shapes. (III.i) Area in Kanom District and Sichon District of Nakorn Sritammarat Province High-grade metamorphic rocks in the Thai peninsula were also reported in Nakorn Sritammarat province (Nakinbordi et al., 1985). The late study of Kosuwan (1996) added more information about the rock unit of this area. These studies divided the rocks in to two rock units locating at the top and bottom; - Kao Yoi schist unit - Ni Plaw beach gneiss unit (Figure 13.). 12

13 Figure 13. Some features indicating that this gneiss rock belongs to the Ni Plaw beach gneiss rock group found in Kanom district, Nakon Sritammarat province. (III.ii.i) Kao Yoi schist unit This rock unit consists of schist, quartzite, small lens of calcsilicate and marble. The schist unit actually contains mica schist, muscovite-garnet schist, and quartz-mica schist. The quartzite unit also contains fine-grained quartzite, and mica quartzite. Minerals found in the schist unit are quartz, muscovite, orthoclase, biotite, garnets and stibnite. For calcsilicate rock, diopside, quartz, plagioclase, actinolite, tremolite, calcite, epidote and hematite are present. (III.ii.ii) Ni Plaw beach gneiss unit This gneiss rock unit actually contains intertwined layers of biotite gneiss and sillimanite gneiss. The texture of the rock unit is uneven. In the rock body, the coarsegrained portion is porphyroblastic while the fine-grained portion contains uniform mineral crystals with intrusions of calcsilicate. The latter aggregate causes white, green and brownish purple bands of colors. In gneiss, there are potash feldspar (orthoclase), and microcline minerals that often engulf mica, quartz, and sillimanite. Other minerals found in gneiss are quartz, biotite, plagioclase, muscovite, sillimanite and garnets. For sillimanite, we find that it is fibrolite with needle-like shapes embedded in quartz and biotite crystals. We also observed the rotation of garnet crystals which drag quartz and biotite crystals along. In calcsilicate, we find that it was altered from gneiss. Minerals contained in calcsilicate are diopside, quartz, plagioclase, calcite, sphene and actinolite-tremolite. There was a report on finding pegmatite and aplite with clear foliation occurring as dikes penetrating this Ni Plaw beach gneiss unit. These rocks consist of quartz, potash feldspar, muscovite and garnets. Kosuwan (1996) also observed that there are Tong Yang gneiss rock unit which composes of gneissic granite or orthogneiss rather than paragneiss, Mt. Datfah granite unit, and finally Mt. Pret granite unit intruding Kao Yoi schist and Ni Plaw beach gneiss rock units. (IV.) Comparison of Rocks and Their Compositions Pongsapich et al. (1983) reported that all high-grade metamorphic rocks in Thailand have the roughly the same features. The lowermost layer consists of orthogneiss and paragneiss and the subsequent layers all the way to the top are schist, calcsilicate, quartzite 13

14 and marble respectively. Quartzite and marble are only present in some particular portion of the rocks. Outcrops in the Northern and Upper-Western region contain very similar stratigraphic sequences of Percambrian rocks. The lowermost part is coarse-grained gneiss with embedded lens of marble. Granite, migmatite or orthogneiss are found in this same lowermost part of some outcrops. In the middle of the Precambrian strata, gneiss and schist with smaller mineral crystals are present. The upper strata contains higher quantities of calcsilicate and marble than the rest. The rocks of this section are cataclastic or show distinct evidences of mylonization. The feldspar crystals are often rotated, and in some portion are crushed forming mylonite rocks. Near the upper Precambrian boundary, they observed grey marble with phologopite and feldspar crystals emerging in clusters. Quartzite is only present in small amount usually in Hod and Jom Thong districts. In the Eastern region, areas around mt. Chao in Panasnikom district and Nong Yai district, Chonburi province have paragneiss outcrops. The composition of paragneiss is toward mafic because the contained minerals are mostly hornblend, diopside, biotite, red granet, and calcic plagioclase. This area contains amphibolite outcrops in the largest amount in the country (Areesiri, 1982). The lower portion of paragneiss often consists of migmatite. There are schist outcrops with sillimanite in the area south-west of mt. Chao. The schist is normally located right on top of paragneiss (Salyapongse et al., 1997). There was also a report of biotite-muscovite granite showing clear foliation of minerals in the same area. In the Lower-Western and Southern region specially the areas in Hua Hin and Pranburi districts of Prachuabkirikan province. Dheeradilok et al. (1985) reported quartzite beneath marble and calcsilicate rock starta. The lower strata contain schist and gneiss. For areas in Nakorn Sritammarat province, Lamjuan (1979), Sananseang et al. (1985) and Kosuwan (1996) agreeably reported that areas in Kanom and Sichon districts contain an outcrop of gneiss in the east, and an outcrop of schist in the west. Biotite-muscovite gneissic granite was found occasionally and in small quantity. The final picture of the Precambrian strata is as follows. - The topmost stratum: Marble and calcsilicate and sometimes quartzite are present. Major minerals of marble and calcsilicate of this layer are calcite, dolomite, phologopite, epidote, actinolite-tremolite, plagioclase, potash feldspar, zoizite, diopside, and garnets (Puttaphiban et al., 1987). - The middle stratum: This layer is generally called the paragneiss stratum. It consists of schist, calcsilicate, marble and mylonite. In paragneiss and schists, major minerals are quartz, potash feldspar, plagioclase, biotite, sillimanite, garnets, andalusite and cordierite. In calcsilicate and marble, the contained minerals are quartz, calcite, diopside, phologopite, epidote and garnets. There is a remark for the presence of cordierite in paragneiss. This mineral is always found embedded in paragneiss layers with many biotite crystals (Macdonald et al., 1992). Other minerals that are often observed with this type aggregate are muscovite, garnets, and sillimanite. Moreover, there was a report of discovering schist with andalusite and cordierite coexisting at Bhumibhol dam (Natalaya, 1973). This schist was suspected to be at the top of this stratum. - The lowermost stratum: Anatexite, migmatite or orthogneiss are terms used to describe the same high-grade metamorphic rock unit found in this section. This rock was undergone so severe metamorphism that it melted back to granite composition. However, paragneiss is commonly present. In orthogneiss, the contained minerals are quartz, plagioclase, microcline, orthoclase, biotite, muscovite and sillimanite (Macdonald, et al., 1992). For migmatite and mixtures between anatexite and paragneiss, it was found that quartz, potash feldspar, 14

15 plagioclase, biotite, muscovite, garnets, and sillimanite are normally present (Mantajit, 1975; Puttaphiban, et al., 1987). Pegmatite and aplite are coexisting and contain quartz, potash feldspar, muscovite, and biotite. These minerals are usually oriented in veins parallel with the strata. Another important feature of the Precambrian rock is the clear difference between dark mineral lens and light-colored mineral lens. Moreover, the difference in sizes of these two types of mineral lens is also observed. Precambrian rocks in areas beyond the above discussion rather have acid composition while those of the Northern region have mafic composition. This conclusion is reached after comparing contents of minerals. This rock has smaller amount of potash feldspar, quartz, and muscovite than the paragneiss unit does. Granite, actually biotite-muscovite granite, is also present in the Precambrian rock units. This granite usually contains different-sized minerals and often shows foliation. Tourmaline, muscovite, garnets, pegmatite and aplite generally coexist in this granite. Furthermore, contact metamorphic rocks are found because the intrusion of granite in the Precambrian rock layers. This contact metamorphism occurred after the great regional metamorphism of the area. Mineral composition of the rocks provides explanations of this conclusion. Some special minerals required pressures and temperatures caused only by contact metamorphism to be made such as orthopyroxene (enstatite), diopside, anorthite, hornblend, olivine, and potash feldspar (Mantajit, 1975). Other minerals with this special origin also are wollastonite, grossularite, plagioclase, calcite, and diopside (Macdonald et al., 1992). (V.) Precambrian Rock Structures Puttaphiban et al. (1987) concluded the nature of metamorphism and structures of the high-grade metamorphic Precambrian rocks in Jomtong and Hod districts of Chiang Mai province in chronological orders as the followings. 1.) The first regional metamorphism generated several high-grade metamorphic rocks at the same time as the fist schistosity. In the lowermost stratum, there was melting which generated granite with intrusions of pegmatite and aplite. Minerals in these intruded rocks are aligned with the direction of schistosity. 2.) The first cataclastic deformation was found mostly in the east of the area and it occurred together with the first schistosity. Evidences supporting this event were the rotation of feldspar crystals and the presence of overturned isoclinal fold. 3.) The second regional metamorphism also occurred because there were large garnet crystals penetrating the rock fabric resulting from the first schistosity. This can be found in feldsparthic schist. 4.) The second cataclastic deformation was found in small zones that have high dip angles. This second-time deformation is not parallel with the first one. 5.) Thrust faults also exist which can be seen from the westward movement of Ordovician limestone and this limestone is found covering the youngest Precambrian rock unit, the phologopite marble, at mt. Mokalan. They further discovered a similar configuration of thrust fault west of Pae village. In this area, the Ordovician limestone covers the red rock which is believed to be of Mesozoic- Triassic age. Therefore, the age of this latter observed thrust fault must be less than that of the red rock. For the high-grade metamorphic rocks of Eastern region,areesiri (1982) reported at least three events of rock transformations and two events of metamorphism. The first rock 15

16 transformation coincided with the first metamorphism in the late Carboniferous. This combined event was responsible for the formation of Hercynian orogeny. The second and last rock transformations also coincided with the second metamorphism during the Permian and Triassic generating Permo-Triassic orogeny. The mineral evidences of the first metamorphis were usually obscured by the following events. However, because veins of amphibolite found as penetrated veins are parallel with axial plane cleavage of isoclinal fold which was clearly derived from the second metamorphism, it can be inferred that the first metamorphism had previously occurred in the rock (Figure 14.). The second metamorphism caused the melting to crystallize granite and generations of pegmatite and aplite. Figure 14. The growth of sillimanite crystals penetrating through folded biotite and quartzitic-mica schist. Kosuwan (1996) reported the structure of high-grade metamorphic rocks in Kanom district of Nakorn Sritammarat province that there were foliation and schistosity with inclined angles to the beddings remaining which can be concluded from differences in composition between quartz-mica schist and mica schist. In the adjacent areas of foliation, slip cleavage or crenulation cleavage are present. In paragneiss, he observed overturned isoclinal fold, while in pegmatite-aplite, he found ptygmatic fold. Interesting structures of this Precambrian rock unit are the followings. 1.) The first regional metamorphism coincided with the formation of several foliated metamorphic rocks, foliated granite, and intrusions of pegmatite, and aplite. These rocks share the same direction of foliation that is parallel to the original sedimentary strata. Overturned isoclinal fold and cataclastic deformation were also observed to follow this regional metamorphic event. 2.) The existence of the second regional metamorphism is controversial. Evidences are unclear such that they only indicate small metamorphism events causing lowgrade metamorphic rocks. Moreover, mineral veins in amphibolite observed as intrusions in mica can not be used to support the second regional metamorphic event (Areesiri, 1982). 3.) The second cataclastic deformation clearly occurred. 4.) Thrust faults are suspected to form after the Triassic. (VI.) Relations between High-grade Precambrian and Low-grade Lower-Paleozoic Metamorphic Rocks 16

17 The reasons behind the uncertainty of the relations between these rocks are that the rocks are usually covered by Quarternary sediments, there are some faults cutting through the rock strata or the contact between these rock units is replaced totally by granite. With these, the interpretation finally leads to the presence of an unconformity (Braun, 1969; Baum et al., 1970; Natalaya, 1974; Campbell, 1975; Bunapas, 1976; Dheeradilok and Lamjuan, 1978; Puttaphiban et al., 1987) After investigating types of rocks in the high-grade Precambrian rock unit and the low-grade Paleozoic rock unit from reports of Baum et al. (1970) and Mantajit (1975), we find that the upper Precambrian rock consists of calcsilicate, marble, and chert-like quartezite. In the mean time, lower-paleozoic rock units (Cambrian unit and Ordovician unit) contain siliciclastic-pelitic-carbonate rocks with mineral content that looks just like that of the upper-precambrian rocks. There was a report of a suspected unconformity between the high-grade Precambrian metamorphic unit and the lower-paleozoic unit located west of Chiang Mai province because there was a layer of pebble sediments separating the two strata (Braun, 1969). Later, Putthapiban et al. (1987) argued that there was a movement of pebbles caused by block faulting mechanism instead. Order of metamorphism relation between these two rock units was discussed in the study by Macdonald et at. (1992). This study states that there is the lower-paleozoic rock unit found in mt. Inthanon area contains biotite schist covered by fine-grained marble. The marble layer is continuous and extends to the limestone unit with known Ordovician fossils. In lower-paleozoic rocks, garnets often coexist with andalusite crystals. In the marble layer, typical minerals found in amphibolite are absent. The biotite schist is derived from high temperature greenschist facies. In the mean time, there were many reports on this Precambrian metamorphic rock unit that overwhelmingly argued for the low-grade metamorphism occurring only in a limited amphibolite stratum (Mantajit, 1975; Putthapiban et al. 1987; and Macdonald et al., 1992). These studies extensively state that the calcsilicate unit, the topmost layer, contain calcite, dolomite, actinolite-tremolite, phologopite, epidote, plagioclase, potash feldspar, and diopside. In the Bhumibhol dam area, Tak province, there was a report of andalusite and cordierite in the upper-precambrian rocks (Natalaya, 1974). From these data, the orientation of the metamorphosed layers must be parallel to the original strata. The supporting evidences are the gradual increase of rock age from the Precambrian to the lower Paleozoic, the uniform types of metamorphic rocks, and the sameness in grades of metamorphism observed in nation-wide areas. The scenario where high and medium-grade metamorphic rocks created in the Precambrian extending to the lower Paleozoic strata is concluded. In these rock strata, there was perhaps a single event of regional metamorphism (Dheeradilok et al., 1985; Chanmee et al., 1981; Macdonald et al., 1992 and Salayaphong, 1996). *************************************************************************** (VII.) Rocks derived from metamorphism อาจารย คร บ section น ยากมากส าหร บคนท ม Petrology background ไม ค อยด อย างผม ผมต องขออน ญาตเว นไว นะคร บ ห วข อ 3.7 ห นแปรท เก ดจากกระบวนการแปรสภาพ เร มต งแต หน า 48 หมด 51 17

18 ส ท ศชา ว นจ นทร ท 19 ก มภาพ นธ พ.ศ เวลา 11:00 น. Figure 15. Graph showing the melting curves of several mixtures of minerals that are major constituents of the Precambrian metamorphic rocks. *************************************************************************** (VIII.) The Age of the Rocks and The Age of Metamorphism The age of the Precambrian rocks is defined as the age of the original sedimentary rocks before metamorphism occurred. To determine the age, grade of metamorphism, stratigraphic sequence of the rocks and radioactive isotope age data are considered. With the fact that Precambrian metamorphic rocks have the highest grade of metamorphism, it becomes easier to distinguish the Precambrian rocks from the adjacent rocks that are also metamorphic but at a much lower grade of metamorphism. Moreover these adjacent rocks often contain lower Plaeozoic fossils because the mild metamorphis did not destroy all of them. The lothology of these two types of rocks such as the contact between the younger and older rocks also helps. Occasionally, we observed granite dikes right at the contact. With these evidences and logical considerations, we conclude that the high-grade metamorphic rocks are older and are the original materials which later became sediments during the lower Paleozoic (Baum et al., 1970; Natalaya, 1974; Bunopas, 1983; Pongsapich et al., 1983 and Puttaphiban et al., 1987). The studies of Dheeradilok (1975) and Janmee (1981) on Precambrian rocks in Tamaka and Muang districts of Kanchanaburi province respectively conclude in agreement that the lithology observed in green schist indicates that this schist had continued eroding and making sedimentary rocks up to the Cambrian and Ordovician. The radioactive isotope age data and the mineral composition of the regional metamorphic rocks really help us understand the history and evolution of the rocks. Macdonald et al. (1992) and Dunning et al. (1995) reported the preliminary age of orthogneiss, the age of granite that evolved during metamorphism and the age of cataclastic deformation. The conclusion is that the regional metamorphism in Thailand only occurred 18

19 once in the late Cretaceous. They further hypothesized that the age metamorphic rocks in Thailand was perhaps late Precambrian early Cambrian. The geology section of the Department of Mineral Resources of Thailand has supported Institute for Geology and Dynamic Lithosphere of Gottingen, Germany to perform research on age determination of minerals contained in the previously described metamorphic rocks, and of minerals contained in the low-grade metamorphic cover units in various areas since The method used in the research is the measurement of radioactive isotope ratios of U/Pb, Sm/Nd, Rb/Sr and K/Ar for crystalline samples. For sediments or low-grade metamorphic rocks, only K/Ar is used to determine the age of the rock. Results (Mickein, 1997) indicate that there were two events of regional metamorphism occurred in the rock history. The first event was right at the Triassic/Jurassic boundary (approximately 200 Ma) which was very severe regional metamorphism event resulting as the gneiss forming event. This conclusion is drastically different from the previous results obtained by Mcdonald et al. (1992) and Dunning et al. (1995) which confirmed the single regional metamorphic event scenario only during the Cretaceous. The second regional metamorphic event took place during the Cretaceous (approximately Ma) from the U/Pb data of monazite minerals. This second event was a milder one that did not alter the pre-existing rock fabric. There was also an uplift event in the recent Oligocene/Miocene which indicates the existence of several large thrust faults in the region. Up to the present, there has been no evidence indicating the metamorphic event in the Precambrian. The age of minerals composed in original sedimentary rocks before undergoing highgrade regional metamorphism in the Triassic and Jurassic is found to be between 1,000 2,000 Ma which marks the lowermost part of the sediments. The age of the topmost sediments is perhaps Cambrian and Ordovician (approximately 500 Ma). These topmost sediments belong to Tarutao and Tung Song rock units which occur adjacent to the highgrade metamorphic rocks. Therefore, the age of the Precambrian rocks falls between 1, Ma. (IX.) Types of Rocks before Metamorphism Baum et al. (1970) argued that the original rocks before becoming the Precambrian metamorphic rocks in Thailand were sedimentary rocks such as limestone. Moreover, with the discovery of quartz crystals in pebbles with gneiss composition, he concluded that the original rocks must be very old and contain a lot of quartz for example gneissic granite. Mantajit (1975) proposed that paragneiss were derived from impure arenaceous sediments such as subgraywacke sandstone or subarkosic sandstone with shale. For schist, it was metamorphosed from shale with sandstone. For quartzite, it was derived from sandstone with high percentage of quartz minerals. Chert, calcsilicate and marble were transformed from limestone and limestone with small impurities. Puttaphiban et al. (1987) reported that the Precambrian rocks were derived from subgraywacke sandstone, shale, and pyroclastic rocks. The original rocks at the top of the sediment stratra were expected to be calcareous sediments and limestone with some degree of impurity. Macdonald et al. (1992) discussed about the Thai Precambrian rocks that because the major rock component of the strata is orthogneiss, the original rock type should be granite. He did not provide the origin of these granite rocks whether they came from melting or from the crystallization of magma in the upper mantle. 19