โครเมี่ยนสปิเนลจากหินอัคนีอัลตราเมฟิกบางส่วนในภาคเหนือและภาคตะวันออกของประเทศไทย / วีระศักดิ์ ลันวงษา = Chromian spinel from some ultramafic igneous rocks in northern and eastern Thailand / Weerasak Lunwongsa
This thesis is aimed at studying petrochemistry of chromian spinels from some selected ultramafic igneous rocks in Thailand. Comparison is also made for results on Thai spinel geochemistry with those of the well know tectonic setting oversea. A total of 120 chromian spinel grains were selected to represent 4 inferred major sutures in Thailand. Additional 106 grains of other countries were determined for geochemical analysis. All of chromian spinels were analysed by an electron microprobe. Major and minor elements to be analysed were composed of Mg, Fe, Cr, Al, Si, Na, K, Ca, Ti, Mn and Zn. All Fe, Mn and Ti. Spinels found in the Chiang Rai ultramafic rocks are anhedral, orange, and brown to reddish brown. The ultramafic host rock of this area is residual harzburgite and formed by slightly low degree of partial melting within the fore arc setting. All spinels found in ultramafic rocks of the Nan-Uttaradit area are equigranular with subhedral-to-euhedral single crystals. Their color varies from reddish brown to brownish black. This ultramafic host rock should be dunite and wehrite in part of a cumulate layer. The first group represents ultramafic masses that have formed by spreading above subduction zones. The second group indicates higher degree of partial melting that formed in island arc setting. Spinels found in Loei can be divided into two groups. The first group occurs in catablastic host rock. Spinels of this group are mostly subhedral to euhedral, black to orange brown. The ultramafic host rock of this area is considered as cumulate dunite. The other spinel group relates to porphyroblastic host rock. The spinel is subvermicular and multicrystal with reddish brown color. The ultramafic host rock of this group should be residual harzburgite. Their chemical compositions represent arc peridotite. An ultramafic source was generated by partial melting at subduction. Most spinel found in the Sra Kaeo ultramafic rocks are subhedral to euhedral single. Their color varies from moderate to dark reddish brown. The ultramafic host rock may be cumulate orthopyroxenite or residual harzburgite. A tectonic setting of ultramafic origin in this area was defined by a part of fore arc setting