Geology and Geochronology of the Line Islands
Geology and Geochronology of the Line Islands
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莱恩群岛的地质和地质年代学
DOI：
10.1029/jb089ib13p11261
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发表时间：
1984
影响因子：
--
通讯作者：
R. Duncan
中科院分区：
文献类型：
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作者：
S. Schlanger;Michael O. Garcia;B. Keating;J. Naughton;W. Sager;J. Haggerty;J. A. Philpotts;R. Duncan
沿着莱恩群岛的整个长度进行了地质和地球物理研究，以检验这一主要线性岛链起源的热点模型。共打捞出火山岩21艘，沉积岩化石19艘。来自莱恩群岛的火山岩是碱性玄武岩和夏威夷岩。此外，在Line链的中部还发现了一种拉斑玄武岩和一种phonolite。碱性玄武岩中辉长岩的显微分析表明，它们含有高TiO2 （1.0-4.0 wt %）和Al2O3 (3.4-9.1 wt %)，具有碱性和过碱性亲和关系。莱恩群岛火山岩的主要元素组成与夏威夷火山岩非常相似。微量元素和稀土元素分析表明，岩石为典型的洋海岛碱性熔岩；莱恩群岛的熔岩与典型的洋中脊或断裂带玄武岩非常不同。利用40Ar-39Ar、K-Ar和古生物学方法对这些岩石进行测年，结合深海钻探项目中165、315和316地点的数据以及先前确定日期的挖泥船岩石，提供了从18°N到9°S的链条上20个地点的火山事件年龄，距离近4000公里。所有这些日期都确定了白垩纪中期至始新世晚期的火山构造或造岭事件。始新世火山事件发生在15°N ~ 9°S，晚白垩世火山事件发生在18°N ~ 9°S。在南莱恩群岛，白垩纪和始新世的火山活动都发生在同一座大厦或山脊上，这表明火山活动在一个地方反复发生。环礁在岛链中的不规则分布、晚白垩世岛礁在中线和南线岛链约2500 km的范围内发育，以及空间上密切相关的海山表现出不同的沉降历史，说明岛链的大部分并没有遵循t1/2相关的沉降路径。对11座海山的磁测结果表明，从中线链上看，其中4座海山的虚地磁极落在白垩纪海山虚地磁极的北面。这四极与太平洋地区始新世中晚期至渐新世早期的古地磁资料一致。其中一座海山的40Ar-39Ar总聚变年龄为39ma。由于这4座海山的两极属于一个紧密的群体，我们推断它们可能是始新世中晚期至渐新世早期。之前分析的另外三座海山以及位于莱恩群岛的一座海山都给出了与太平洋晚白垩纪古地磁数据一致的虚拟地磁极。在这4座海山中，3座海山给出了71 ~ 85 Ma的晚白垩世40Ar-39Ar年龄；另一个在南莱恩群岛，根据古生物学证据，被解释为白垩纪晚期。以前的工作者把莱恩群岛的起源归因于一个热点的影响和四个热点的作用。单一热点模型不能解释莱恩群岛上所有的火山大厦，尽管它确实解释了从北向南沿着火山链的9.6±0.4厘米/年的年龄变化，这是由一些年代久远的大厦得出的。四个热点模型解释了更多年代久远的火山大厦，但仍然不能解释所有可用的数据。岩石学资料反驳了早期工作者提出的洋中脊或转换断层的成因。以莱恩群岛为代表的复杂火山省对太平洋中板块火山活动起源的现有模式仍然是一个挑战。需要一个环礁钻探计划，以确定莱恩群岛的大厦建筑历史、海底山形成的古纬度以及下沉速度和模式。
Geological and geophysical studies along the entire length of the Line Islands were undertaken in order to test the hot spot model for the origin of this major linear island chain. Volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls. Volcanic rocks from the Line Islands are alkalic basalts and hawaiites. In addition, a tholeiitic basalt and a phonolite have been recovered from the central part of the Line chain. Microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high TiO2 (1.0–4.0 wt %) and Al2O3 (3.4–9.1 wt %) and are of alkaline to peralkaline affinities. Major element compositions of the Line Islands volcanic rocks are very comparable to Hawaiian volcanic rocks. Trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas; the Line Islands lavas are very much unlike typical mid-ocean ridge or fracture zone basalts. Dating of these rocks by 40Ar-39Ar, K-Ar, and paleontological methods, combined with Deep Sea Drilling Project data from sites 165, 315, and 316 and previously dated dredged rocks, provide ages of volcanic events at 20 localities along the chain from 18°N to 9°S, a distance of almost 4000 km. All of these dates define mid-Cretaceous to late Eocene edifice or ridge-building volcanic events. Eocene volcanic events took place from 15°N to 9°S, and Late Cretaceous events took place from 18°N to 9°S. In the southern Line Islands both Cretaceous and Eocene events took place on the same edifice or ridge, indicating recurrent volcanism at a single locality. The irregular distribution of atolls in the chain, the fact that Late Cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern Line Islands, and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t1/2 related subsidence path. Magnetic surveys of 11 seamounts show that four of the seamounts, from the central Line chain, give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of Cretaceous seamounts. These four poles agree with other paleomagnetic data of middle-late Eocene-early Oligocene age from the Pacific. One of these four seamounts yielded a 40Ar-39Ar total fusion age of 39 Ma. Because the poles of all four seamounts fall into a tight group we infer that they are probably of middle-late Eocene age to early Oligocene age. The other three seamounts as well as one seamont from the Line Islands analyzed previously all give virtual geomagnetic poles which agree with Late Cretaceous paleomagnetic data from the Pacific. Of these four seamounts, three give Late Cretaceous 40Ar-39Ar ages ranging from 71 to 85 Ma; another, in the southern Line Islands, is interpreted, on paleontological evidence, to be of Late Cretaceous age. The origin of the Line Islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots. The single hot spot model cannot account for all of the volcanic edifices in the Line Islands, although it does explain a general age progression of 9.6±0.4 cm/yr from north to south along the chain derived from a number of dated edifices. The four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data. The petrologic data argue against a mid-ocean ridge or transform fault origin proposed by earlier workers. The complex volcanic province represented by the Line Islands remains a challenge to existing models for the origin of midplate volcanism in the Pacific. An atoll drilling program which could determine edifice building histories, paleolatitudes of seamount formation, and subsidence rates and patterns in the Line Islands is needed.