Petrogenesis of plutonic complex in Wang Nam Khiao area, Changwat nakhon Ratchasima

Abstract

Plutonic complex with a variety of rock has been discovered in Wang Nam Khiao Area, Changwat Nakhon Ratchasima. These rocks can be classified, based on petrographic features as mafic-ultramafic rocks (i.e., hornblendite, hornblende gabbro, and hornblende microgabbro) and granitic rocks (i.e., biotite granite, hornblende granite, and biotite-hornblende granite). Mineral chemistry and whole-rock geochemistry indicate that these mafic-ultramafic rocks and granitic rocks, significantly I-type granite, appear to have been formed by crystallization from the hydrous calc-alkaline magma. Both of the mafic-ultramafic and granitic rocks show similar enrichment of LILE (e.g. Ba, K, Sr) and depletion of HFSE (e.g. Nb, Ta, Zr). Moreover, mineral chemistries of particular minerals (e.g., clinopyroxene, hornblende and biotite) indicate arc-related subduction forming the continental arc. Amphibole-plagioclase thermometry and Al in hornblende barometry are selected appropriately to estimate PT conditions of crystallization. The mafic-ultramafic rocks yield 5.3-9.8 kbar and 670-1,000 oC whereas the granitic rocks are estimated at 2.0-5.8 kbar and 600 - 820 oC. Therefore, the mafic-ultramafic rocks appear to be stored and crystallized at 28-31 km depth of the lower crust while the granitic rocks may have intruded into the middle to upper crust at about 10-15 km depth. Zircon U-Pb age dating yields 257 Ma of mafic-ultramafic rocks, 314.6 Ma and 284.9 Ma of Carboniferous biotite granite, 253.4 Ma of Late Permian hornblende granite, and 237.8 Ma of Triassic biotite-hornblende granite. These data introduce the genetic model of multiple arc-related magmatism in association with Palaeo-Tethys subduction beneath the Indochina Terrane during Late Carboniferous/Early Permian, Late Permian, and Middle Triassic.