Geology of the Khanom Gneissic Complexes, Amphoe khanom, Changwat Nakhon Si Thammarat

Abstract

Khanom Gneissic Complex (approximately 225 sq km) is situated in the northern part of Changwat Nakhon Si Thammarat, southern Thailand. Various formations of the Complex have been recognized and are formalized as the Haad Nai Phalo Gneiss, occurring in the central of main range, is equigranular biotite+_sillimanite gneiss and calc-silicate occasionally. The Khao Yoi Schist consists mainly of mica+_garnet schist, quartzite and lenses of calc-silicate and marble. It covers the western rim of the Khao Dat Fa mountain. The Laem Thong Yang Gneiss occurs in eastern and southern parts of main range, consists principally of porphyroblastic biotite gneiss with augen texture, and is later cut by fine-grained biotite gneiss. The Khao Dat Fa Granite, which is well outcropped in the peak main mountain, is generally characterized by fine- to medium-grained, equigranular, biotite granite with well-defined orientation of flaky minerals. The youngest rocks in the complex area are assigned as the Khao Pret Granite, which consist of biottte granite, medium grained, equigranular. It can be geographically subdivided into 2 terranes, as the western and eastern terranes. Geochemically, both gneissic and granitic rocks are made up chiefly of calc-alkaline series. The Laem Thong Yang Gneiss and Khao Dat Fa Granite can be clearly divided by the difference in SiO2 composition, which have a relatively narrow range of SiO2 contents, whereas, the Haad Nai Phlao Gneiss and Khao Pret Granite show relatively wide range of SiO2 contents. These rocks have Al2O3/ (Na2O+K2O+CaO) ratio greater than 1.05 and high K2O/Na2O ratio as well as high normative corundum. It is possibly to have been derived from metasedimentary sources. The Haad Nai Phlao Gneiss is regarded to be paragneiss, and Laem Thong Yang Gneiss is possibly orthogneiss. Field evidences show at least three tectonic events. The superposition of these tectonic events has resulted in a structural style characterized by multidirectional foliation and fault patterns. The predominant regional foliation is the northwest trend with steep dip. Metamorphic conditions reached upper greenschist to lower amphibolite facies with the development of biotite-muscovite, biotite-garnet and sillimanite zones. The age of the Khanom Gneissic Complex is uncertain at present due to lack of geochronological data. However, it is assumed to be Lower Paleozoic or Precambrian age