泰國晚新生代火成岩之地球化學特徵與岩石成因

Abstract

中南半島各國均受到不同程度晚新生代板內岩漿活動之影響，泰國的新生代火成岩相比中南半島東岸的大規模噴發與出露，空間上分布較為零星並且是少數有前人研究報導出露中酸性火山岩的地方。本研究以地球化學方法探討泰國新生代火成岩之成因，藉此對板內岩漿系統的形成與噴發機制，以及整個東南亞地區的晚新生代地體構造有更深入的了解。本研究分析60件採集自泰國不同地區的新生代火山岩，其中包含前人研究的26個樣本（Barr and MacDonald, 1981和Barr and James, 1990），進行系統的岩象描述與地球化學分析。 岩象觀察表明，大多樣本受到蝕變的影響有限，保留相對新鮮的斑晶與基質，多數玄武質岩石樣本具有微斑晶結構，少數為無斑隱晶質。根據CIPW Norm及全岩主要元素分類，將基性樣本分為橄欖石鹼性玄武岩、橄欖石矽質玄武岩、石英矽質玄武岩及中酸性岩四組。基性樣本具有與洋島玄武岩相似的微量元素特徵，富集大離子親石元素（原始地函的20到150倍不等）與輕稀土元素（CI隕石的70到300倍不等），重稀土元素（CI隕石的6到17倍不等）相對輕稀土元素虧損；大多樣本的Nb、Ta與Ti均沒有呈現負異常，表明岩漿受到的地殼混染很輕微。根據基性樣本的主量、微量元素和Sr-Nd-Pb同位素等特徵，岩漿來源可能是含有橄欖岩和輝石岩的碳酸鹽化地函。中酸性岩之地球化學特徵無法通過基性岩漿結晶分異或地殼混染產生，相反地，特提斯洋隱沒相關之新生地殼的熔融提供較好的解釋。 本研究獲得的40Ar/39Ar年代證實前人研究結果，表明泰國晚新生代板內岩漿活動沒有明確的時空趨勢。另外，本研究未發現能代表高溫岩漿之岩石（例如苦橄岩），故結果不支持地函熱柱模式。相反地，岩漿的形成更可能與岩石圈被動擴張導致軟流圈上湧而熔融相關。倘若此推論正確，火山地區可能標示擴張的軌跡或所在地，其規模或許與擴張程度有關。此外，呵叻高原火成岩的重稀土元素含量比泰國其他區域的火成岩更低，可能與不同岩石圈結構所導致的石榴石結晶分異有關。

Late Cenozoic intraplate magmatism affected a vast area in various countries of Southeast Asia. Occurrences in Thailand are generally scattered and of limited sizes, whereas those in eastern Cambodia, southern Laos, and southern Vietnam are generally more extensive. In order to address this spatial difference, its probable link to magma source, magmatic processes and regional tectonics, we study the petrology and geochemistry of 64 samples, including 26 samples collected in previous studies (Barr and MacDonald, 1981 and Barr and James, 1990). Representative samples were chosen for 40Ar/39Ar dating and Sr-Nd-Pb isotopic analysis. Most samples are affected in a limited way by alteration, retaining relatively fresh phenocrysts and matrix. They are geochemically classified into olivine alkali basalt, olivine tholeiite, quartz tholeiite, and intermediate-felsic rocks. Most samples in the first three groups display ocean island basalt-like trace element patterns without negative anomalies in Nb, Ta and Ti, suggesting insignificant contamination of the ascending magma by continental crust. The elemental and Sr-Nd-Pb isotopic variations of these samples can be explained by melting of a carbonated mantle source containing both peridotites and pyroxenites, followed by variable degrees of fractional crystallization. The intermediate-felsic rocks have geochemical features that cannot be produced by closed system fractional crystallization or crustal contamination of the associated basaltic magmas. Instead, those features are best explained by derivation from isotopically juvenile, underplated mafic rocks produced by an earlier episode of Neotethyan subduction. New 40Ar/39Ar ages corroborate a previous suggestion that Late Cenozoic intraplate magmatism, Thailand has no clear spatial-temporal patterns. This result, together with the lack of high-temperature magmas like picrites, is hard to reconcile with magma generation by a mantle plume. Passive extension is a more likely magma-forming mechanism and was supported by independent lines of geological evidence. If that is correct, then locations of the volcanic fields might mark areas where extension was dominant, but their limited sizes might reflect relative degree of extension was low compared with occurrences in eastern Cambodia, southern Laos, and southern Vietnam. Further, volcanic fields in the Khorat plateau show lower heavy REE concentrations than those elsewhere in Thailand, which might be related to garnet fractionation as a result of different lithospheric structure.