南太平洋社会群岛塔希提岛(TAHITI)火山岩——一种典型的大洋岛屿玄武岩

塔希提岛火山岩属典型的大洋岛屿玄武岩(OIB)。该岛是法属波利尼西亚群岛中社会群岛链之一部分,火山活动可分三期:早期(1.7—1.3Ma),中期(1.3—0.6Ma)和晚期(0.6—0.3Ma)。早期火山岩兼有碱性和拉斑系列岩石,包括苦橄玄武岩、碱性玄武岩、拉斑玄武岩及少量玄武安山岩;中期火山岩主要有粗面玄武岩—粗面岩、碱性玄武岩和少量碧玄岩;晚期则以碧玄岩为主,并有部分碱玄岩出现。火山岩的这种岩性变化表明其岩浆由早到晚从富镁、硅弱不饱和向富碱和硅强烈不饱和演化。 社会群岛火山链的火山活动以平均11cm/a的速率从西北向东南迁移,与MORB相比,所有塔希提的岩石皆富大离子亲石元素并有较高的~(87)Sr/~(66)Sr比值,这一特征可能与其特殊的源区成分有关,即富集的地幔热柱或大洋岩石圈。早期岩石是地幔热柱和少量洋壳的部分熔融产物的混合体,故既有拉斑系列又有碱性系列。随着火山活动远离存在热柱的热点区域,洋壳部分熔融的程度逐渐降低,因而其产生的熔融体也越来越富碱,~(87)Sr/~(66)Sr比值也相应有所降低。

VOLCANIC ROCKS FROM TAHITI ISLAND OF SOCIETY ISLANDS, SOUTH PACIFIC- A TYPICAL OCEANIC ISLAND BASALTS(OIB)

Volcanic rocks on Tahiti Island belong to the cypical oceanic island basalts (OIB). This island is one of Society Islands , French Polynesian Archipelago. Its volcanic activities are divided into three phases: early phase (1. 7- 1. 3Ma), middle phase(l. 3-0. 6Ma) and late phase (0. 6 - 0. 3Ma). In the early phase, there are both alkalic andtholeiitic series rocks, including oceanites ,alkali basalts , theleiites and occasionally basaltic andesites ; the middle phase consists of mainly trachybasalt - trachytes, alkali basalts and a few basan-ites; afterwards basanites and secgndarily tephrites are only rock types of the late phase. This petrological variation indicates that the igneous magmas evolved from rich -Mg and weakly Si -unsaturated sorts to rich -alkali and strongly Si - unsaturated ones. In the Society Islands volcanic chain, the mean rate of migration of volcanic activities is 11cm/a in direction from NW toward SE. All of Tahiti rocks are rich in LILE and have higher 87Sr/86Sr ratios compared to MORB, which inferred their specific sources,i. e. , en-riched mantle plume and/or oceanic lithosphere. Rocks of the early phase derived from partial meltings of both mantle plume and partly oceanic crust, thus existing both tholei-itic and aklalic series. With migration of the oceanic crust far from the hotspot, its degrees of partial meltings decreased gradually, and as a result, their products became more alkalic and had slightly lower 87Sr/86Sr ratios.