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Tectonic Setting of the Permo-Triassic Chiang Khong Volcanic Rocks, Northern Thailand Based on Petrochemical Characteristics 总被引：1，自引：0，他引：1
The inferred Permo-Triassic Chiang Khong volcanic belt is composed of felsic to mafic volcanic rocks and their pyroclastic equivalents. Almost all the least-altered mafic volcanic rocks are lava flows; a few might have occurred as dykes. These mafic volcanic rocks are non-foliated to weakly foliated, and mostly have porphyritic textures. The phenocrysts/microphenocrysts in porphyritic samples are commonly plagioclase, and may include clinopyroxene, olivine, Fe-Ti oxide, apatite and amphibole. The matrix of lava flows ranges texturally from felty to trachytic but a few samples show felty to ophitic/subophitic, and glassy textures, whereas that of possible dyke samples is holocrystalline. The primary matrix constituents are largely plagioclase and variable proportions of clinopyroxene, Fe-Ti oxide, amphibole, olivine, apatite, quartz, alkali feldspar and/or glass. All the studied samples have been subjected to greenschist-facies regional metamorphism. Chemically, the samples show narrow ranges of least-mobile incompatible-element ratios and range compositionally from dacite to basalt of tholeiitic series. These samples are chemically analogous to those of the Tertiary andesite from Sardinian Rift, Sardinia, Italy, particularly in terms of least-mobile incompatible-element ratios. Accordingly, the studied mafic volcanic rocks are interpreted to have formed in a continental volcanic arc. However, the problem related to the geometry of plate convergence, giving rise to the continental volcanic arc, still exists. 相似文献
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Preliminary geochemical study of volcanic rocks in the Pang Mayao area, Phrao, Chiang Mai, northern Thailand: tectonic setting of formation 总被引：1，自引：0，他引：1
Burapha Phajuy Yuenyong Panjasawatwong Pukpong Osataporn 《Journal of Asian Earth Sciences》2005,24(6):765-776
The least-altered, Permian mafic volcanic rocks from the Pang Mayao area, Phrao District, Chiang Mai Province, part of Chiang Rai–Chiang Mai volcanic belt, have been analyzed and are found to be mid-ocean ridge and ocean–island basalts. The mid-ocean ridge basalts occur as lava flows or dike rocks. They are equigranular, fine- to medium-grained and consist largely of plagioclase, clinopyroxene and olivine. These basalt samples are tholeiitic, and have compositions very similar to T-MORB from the region where the Du Toit Fracture Zone intersects the Southwest Indian Ridge. The ocean–island basalt occurs as pillow breccia, and lava flows or dike rocks. They are slightly to moderately porphyritic, with phenocrysts/microphenocrysts of clinopyroxene, olivine, plagioclase and/or Fe–Ti oxide. The groundmass is very fine-grained, and made up largely of felty plagioclase laths with subordinate clinopyroxene. These basalt samples are alkalic, and chemically analogous to those from Haleakala Volcano, Maui, Hawaiian Chain. These mafic volcanic rocks may have been formed in a major ocean basin rather than in a mature back-arc basin. 相似文献
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The mafic volcanic rocks and hypabyssal rocks in the Chon Dean‐Wang Pong area are possibly the southern extension of the western Loei Volcanic Sub‐belt, Northeast Thailand. They are least‐altered, and might have been formed in Permian–Triassic times. The rocks are commonly porphyritic, with different amounts of plagioclase, clinopyroxene, orthopyroxene, amphibole, Fe–Ti oxide, unknown mafic mineral, and apatite phenocrysts or microphenocrysts, and are uncommonly seriate textured. The groundmass mainly shows an intergranular texture, with occasionally hyalophitic, intersertal and ophitic–subophitic textures. The groundmass constituents have the same minerals as the phenocrysts or microphenocrysts and may contain altered glass. The groundmass plagioclase laths may show a preferred orientation. Chemically, the studied rock samples can be separated into three magmatic groups: Group I, Group II, and Group III. These magmatic groups are different in values for Ti/Zr ratios. The averaged Ti/Zr values for Group I, Group II, and Group III rocks are 83 ± 6, 46 ± 12, and 29 ± 5, respectively. In addition, the Group I rocks have higher P/Zr, but lower Zr/Nb relative to Group II and Group III rocks. The Group I and Group II rocks comprise tholeiitic andesite–basalt and microdiorite–microgabbro, while the Group III rocks are calc‐alkalic andesite and microdiorite. According to the magmatic affinities and the negative Nb anomalies on normal mid‐oceanic ridge basalt (N‐MORB) normalized multi‐element plot, arc‐related lavas are persuasive. The similarity between the studied lavas and the Quaternary lavas from the northern Kyukyu Arc, in terms of chondrite‐normalized rare earth element (REE) patterns and N‐MORB normalized multi‐element patterns, leads to a conclusion that the mafic volcanic rocks and hypabyssal rocks in the Chon Daen–Wang Pong area have been formed in a volcanic arc environment. 相似文献
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Yuenyong Panjasawatwong Leonid V. Danyushevsky Anthony J. Crawford Keith L. Harris 《Contributions to Mineralogy and Petrology》1995,118(4):420-432
An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100)
and Al# (Al/(Al+Si)*100) values and H2O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H2O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR
spectroscopic technique. This study has shown that melt Ca# and Al#, H2O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase
An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role
played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase
An content, but melt Al# also exerts a significant control. H2O content can notably increase the An content of plagioclase, up to 10 mol % for H2O-undersaturated melts, and 20 mol % for H2O-saturated melts. Exceptionally calcic plagioclases (up to An100) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated
amounts of H2O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na2O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in
equilibrium with the most calcic plagioclase (An94) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na2O = 12–15, or from melts with exceptionally high Al2O3(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions
in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such
melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically
dominant, less refractory magmas. The high H2O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An95 in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na2O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks.
Received: 31 August 1993 / Accepted: 20 May 1994 相似文献
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