. Title : "Petrology and Tectonic Setting of Eruption of Basaltic Rocks Penetrated in Well GTE-1, San Kamphaeng Geothermal Field, Chiang Mai, Northern Thailand"

Researcher : Yuenyong Panjasawatwong

Office of Researcher : GEOLOGY


A 500-meter deep hole, GTE-1, in the San Kamphaeng geothermal field was drilled into the basaltic pile that erupted sometime in Permian-Early Triassic time. The basaltic cores from this hole included flows, pillow breccias, and hyaloclastites. Only samples from the least altered of these basaltic rocks were selected for petrochemical studies.

Under the petrographic microscope, the 10 salected basaltic rock sample show phyric texture, with abundant altered olivine (largely replaced by chlorite/serpentine) and clinopyroxene, and rare albitized plagioclase and chromian spinel phenocrysts/microphenocrysts. The matrix is essentially albitized plagroclase, clinopyroxene, Fe-Ti oxides(magnetite and ilmenite), and devitirified glass in variable proportions. Secondary mineral assemblages suggest that all samples have undergone low-grade regional metamorphism.

Mineral analyses were carried out on clinopyroxene and chromian spinel phenocrysts and chromian spinel phenocrysts and microphenocrysts. The clinopyroxenes are diopside and magnesian augite, with Mg# ranging from 0.77 to 0.86, and are compositionally comparable to intraplate tholeiitic and alkalic clinopyroxenes in non-orogenic basalt. Chromian spinels have Fe2+# , Cr# , and Fe3# that vary from 0.55 to 0.78, 0.57 to 0.68, and 0.12 to 0.21, respectively. Compositionally, The chromian spinels resemble those in Hawaiian tholeiites.

In terms of relatively immobile elements, the basaltic rocks are characterized by moderately high Tio2 (2.1-2.8 wt.%), P2O5 (0.2-0.4 wt.%0, Zr (133-194 ppm), Nb (14-30 ppm) and Y (21-27 ppm), high Ni (445-864 ppm) and Cr (619-1404 ppm), and ratherunifrom Ti/Zr (92 3), Zr/Nb (8 1), and Zr/Y (6.6 0.5) values. They have V contents that vary from 230 to 315 ppm, and Nb/Y ratios. (0.6-1.1) intermediate between tholeiitic and alkalic basalt. Their REE patterns have relatively flat LREE (La/Sm)cn = 1.34-1.55) with HREE depletion ((Sm/Yb)cn = 3.21-3.61). The N-MORB normalized multi-element patterns for the analyzed samples are step-like, similar to those for intra-plate basalt. The REE and N-MORB normalized patterns are analogous to the patterns for tholeiites and transitional tholeiites from the Loihi Seamount in the Hawaiian-Emperor Chain. As a consequence, the study basaltic rocks are inferred to have tholeiitic affinities and to have erupted in an ocean-island environment. These oceanic intra-plate basalts might have formed as a chain of island or seamounts in either a major ocean basin or in mature backarc basin.

Presented at International Symposium Shallow Tethys (ST) 5, Chiang Mai, 1-5 February 1999.