The Precambrian Period

The Precambrian Period Precambrian rocks in Thailand are metamorphic rocks found directly below the well- defined 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 Figure 1. The distribution of Precambrian rocks in various areas of Thailand. Note: เชียงใหม - Chiang Mai, ตาก – Tak, กําแพงเพชร – Kampang Pet, อุทัยธานี – Utai Thani, กาญจนบุรี – Kanchanaburi, กรุงเทพมหานคร – Bangkok, ชลบุรี – Chonburi, ประจวบคีรีขันธ – Prachuabkirikan, นครศรีธรรมราช – Nakorn Sritammarat 2 (I.i.i) Phologopite marble This rock is generally 50 – 100 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 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 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 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.

Load more