Gneiss and Schist (Pre-Permian)
Gneissic and schistose rocks occur in several areas of Thailand. Along the western side of the Kingdom such rocks outcrop in an elongated band to the west of Chiangmai, west of Rahaeng, in a small area northwest of Kančhanaburi, in the Hua Hin area to the south of Phetchaburi, and to the west of Bandon. These areas are aligned in a general north-northwest direction, coinciding with the gneissic trend. There are also large outcrop areas in Chonburi and Rayong čhangwat and in the northwest corner of Khorat, northeast of Mu'ang Loei.
Mafic and Ultramafic Rocks (Triassic)
The mafic and ultramafic rock group consists of diorite, gabbro, and pyroxenite. Locally some of these rocks are serpentinized. Occurrences of the group are apparently limited to Uttaradit and Chiangrai in the North. Along the lowlands of the Nan River to the northeast of Uttaradit the rocks appear as diorite, gabbro, and pyroxenite dikes, small plugs and stocks intrusive into metasediments. In the hills on the south margin of the Maečhan floodplain near old Chiangsaen and along the upper reaches of the Maečhan and the Maekok in Chiangrai čhangwat are outcrop areas of coarse-textured gabbro associated with dioritic and granitic intrusives.
Granite (Triassic and Late Cretaceous)
Granite is by far the most extensive igneous rock that crops out in Thailand. It occurs in stocks and batholiths intruding Paleozoic and Mesozoic sedimentary rocks and has been exposed at the surface by their erosion. Granite is thought to form the rocks of most of the western mountain ranges. Where cupolas are developed, concentrations of minerals of economic importance may be present.
In most places the rocks are biotite-muscovite gneiss. Mica schist, associated with the gneiss, is mostly of biotite type along the western side of the kingdom, and muscovite in the east.
In the north and west several granite ranges trend in a general north-south direction. The great range west of Chiangmai shows exposures of granite in several places extending from Pai far south to Maesot amphoe (district). Another granite range begins about 15° north latitude and continues south through Kančhanaburi and Pračhuapkhirikhan čhangwat. A broad and continuous belt of granite starts at the Shan States frontier to the northeast of Mu'ang Fang and follows a sinuous course south at least to the Khuntan tunnel in the latitude of Lampang. It is likely that this belt continues much father south through Tak čhangwat to the northern edge of Kančhanaburi čhangwat.
In the Peninsula six main granite ranges lie en echelon. The most important of them, which extends, with minor breaks, from Ranong south to the island of Phuket, is associated with the richest and most productive tin fields of the Kingdom. The range second in importance begins in the islands of Phangan and Samui and continues in a southerly direction on the mainland from Sichon to Satun. A third belt of granite crops out east of Hatyai and south of Songkhla and extends south into Malayan Kedah. South of Napradu, granite is exposed for about 10 miles and can be traced in a fourth belt through Betong and into Malayan Perak. A fifth granite range begins in the Saiburi area and extends south into Kelantan. Last of the six is the granite range of Thomo along the Taluban River.
In Southeast Thailand granite is exposed in several areas, occurring most extensively near Chonburi and Čhanthaburi. The granite in the Chonburi region lies in the mountain range which is near the eastern coastline of the Gulf and extends from Chonburi to Rayong. East of this range is the granite of the Čhanthaburi region which can be followed north probably as far as Pračhinburi čhangwat.
There is considerable evidence that the granitic rocks of Thailand were intruded at two distinct times--early and late Mesozoic. They are slightly different from one another mineralogically, but are characteristically a hornblende-biotite granite. A notable exception is a younger granite stock with which are associated the important tungsten and tin deposits of Maesariang, Maehongson čhangwat. In the Peninsula only two granite bodies which can be considered as older intrusions have been recognized. One is north of Krabi, the other is south of Bandara, and neither is apparently tin-bearing.
Diorite and Quartz Diorite (Tertiary)
Diorite and quartz diorite in bosses, stocks, and small batholiths intrude the Khorat series and older rocks to the east of Pričhinburi, north of Lopburi, northwest of Phetchabun, and northeast of Chiangrai. Gold lodes and placers are associated with the dioritic intrusives to the northeast of Lopburi and the east of Pračhinburi.
In Thailand basalt occurs in many widely scattered areas as lava flows, dikes, and plugs. What is perhaps the largest basalt flow extends for about 12.5 miles along the Thai State Railroad to the east of Lampang. It has been little touched by erosion and appears to be relatively young.
Elsewhere in Thailand basalt crops out southwest of Sukhothai, northwest of Lopburi, to the south of Sisaket, and as an extensive sheet south of Buriram. The sapphire and ruby placers of the Čhanthaburi region are associated with several scattered plugs and local flows of nepheline-olivine basalt, as are the sapphire placers to the north of Kančhanaburi.
Andesite and Rhyolite Porphyries (Tertiary)
Andesite porphyry in stocks, dikes, or flows, cut by younger rhyolite porphyry in dikes and small flows, crops out near Saraburi, northeast of Lopburi, west of Phetchabun, and cast of Uttaradit. The andesite is generally green or purple and the rhyolite is buff or yellow. The andesite porphyry has intruded the Khorat series and older rocks.