Structure and Deformation of Metamorphic Rocks in Doi Phuk Sung Area, Wiang Sa District, Nan Province

Abstract

The Nan – Uttaradit suture is the most complex suture in Thailand. It has been interpreted to mark the boundary between the Sibumasu block located in the west of Thailand (Sukhothai Zone) and the Indochina block located in the east of Thailand. The mafic and ultramafic and metamorphic rocks are exhumed along the Nan river and are evidenced by the subduction zone. The amalgamation of these two blocks may have occurred in the Permian – Triassic. The study area is located in the south of Nan province and covers an area of approximately 22 km2. The purpose of this study is to characterize deformation event of metamorphism and define structural geology by using petrography and structural analysis. Geological setting of the study area can be divided into four rocks units; (1) the Mélange blocks unit (A), (2) the Limestone unit (B), (3) gray sedimentary rocks unit (C), and (4) the Red bed sedimentary rocks unit (D). The unit A comprises mafic – ultramafic igneous and metamorphic rocks including peridotite, gabbronorite, hyaloclastite, metabasite, metachert and deformed limestone. The unit B is demonstrated micritic limestone. The unit C consists of sedimentary rocks including polymictic conglomerate and sandstone lithic arenites. The unit D contains sedimentary rocks including sandstone lithic wacke, sandstone lithic arenite, sandstone feldspathic wacke, and monomictic conglomerate. The provenance analysis by using the ternary diagrams of Dickinson and Suczek (1979). The sandstone modal analysis of unit C indicated transitional arc provenance as the main source area with minor undissected arc and lithic recycled orogent that indicated back – arc tectonic setting. The sandstone modal analysis of unit D suggested undissected arc provenance as the main source area with minor transitional are and craton interior that indicated island arc or foreland arc basin and rifted continental margin tectonic setting, respectively. The structural study of metasedimentary rocks shows the direction of principal stress (σ1, σ2, and σ3). Their direction can be divided into two different directions. The first direction shows in metachert of unit A. The principal stress directions are 57/255, 24/028, and 22/128 respectively. The second direction shows in metapelite located in between the boundary of unit A and unit C. The principal stress directions are 19/100, 8/192, and 70/304 respectively. The first and second directions are nearly opposite sides of each other (180° difference). This relationship signifies by fault boundary in the study area. The deformation evolution can be separated into two events. The first deformation (D1) is shown by foliation (S1) of the mineral alinement and the second deformation (D2) develops open to close fold and zonal crenulation cleavage (S2). The mineralogy and P – T conditions of the studied metamorphic rocks are from metachert (quartz schist) and metabastie. Their mineral components compose of albite + quartz + piemontite + epidote + muscovite + hematite + magnetite ± stilpnomelane and magnesite + quartz + H2O. Based on P – T estimate, the metachert gives P – T condition temperature of 363 – 422 °C and 6.4 ± 0.4 kbars whereas the metabasite temperature is around 200 – 325 °C and 1 kbar. The P – T conditions relatively indicate high pressure and low to medium temperature metamorphism of greenschist facies.