西太平洋卡罗琳海底高原裂解作用的岩浆过程响应

Magmatic response to the rifting process of the Caroline Plateau in the West Pacific

海底高原形成及其后的裂解事件是大洋板内地质作用的产物,因此研究海底高原可为大洋板内地球动力学过程提供重要线索。索罗尔海槽是卡罗琳海底高原初始裂解的重要产物,但到目前为止,其裂解过程的动力学机制仍然存在争议。本研究对采自索罗尔海槽基底的火山岩样品(玄武岩、英安岩)开展系统的单矿物(斜长石)原位主、微量元素地球化学研究,结果表明:玄武岩样品中斜长石种属为钙长石、培长石和少量拉长石;斜长石斑晶富集轻稀土和大离子亲石元素,亏损高场强元素,存在明显的Eu正异常;相比于斜长石斑晶,斜长石微晶具有更高的不相容元素含量。英安岩中斜长石种属为中长石,较玄武岩斜长石斑晶更富集轻稀土和大离子亲石元素,具有更明显的Eu正异常。玄武岩和英安岩中斜长石结晶温度和压力分别为1110~1171℃和961~966℃、0.27~0.42GPa和0.16~0.29GPa,且同一种岩石类型中的斜长石斑晶与微晶结晶温度无明显差异。基于以上结果,并对照前人所总结的双峰式岩浆系统的成因特征,本文提出卡罗琳海底高原裂解期间的岩浆作用具双层岩浆房模型,即地幔岩石部分熔融形成原始岩浆,聚集在深部岩浆房内结晶形成基性斜长石斑晶,随后被岩浆裹挟上涌喷出地表形成玄武岩;同时部分岩浆继续进入浅部壳层岩浆房,并结晶形成中酸性斜长石斑晶,后被演化岩浆携带上升并喷出地表形成英安岩。

The formation,evolution and subsequent rifting of the oceanic plateaus are products of oceanic intra-plate processes,and studies on these plateaus can provide important clues to oceanic intra-plate geodynamics.The Sorol Trough is a product of the initial rifting of the Caroline Oceanic Plateau,but the kinetic mechanism of its rifting process is still controversial.This study performed in-situ major and trace element geochemical analysis of single minerals(plagioclase)hosted in volcanic rocks(basalt,dacite)collected from the basement of the Sorol Trough.The results showed that the plagioclases in the basalts are mainly anorthite and bytownite,with a small amount of labradorite.Plagioclase phenocrysts in basalts are enriched in light rare earth elements(LREEs)and large ionic lithophile elements(LILEs),and depleted in high field strength elements,showing positive Eu anomalies.In contrast,the plagioclases in the dacites are andesine,and the plagioclase phenocrysts in the dacites are more enriched in LREEs and LILEs with stronger positive Eu anomalies compared to those in the basalts.Calculated crystallization temperatures and pressures of the plagioclase are 1110~1171℃(basalts)and 961~966℃(dacites),0.27~0.42GPa(basalts)and 0.16~0.29GPa(dacites),respectively,and there is no significant difference between plagioclase phenocrysts and microlites crystallization temperatures for the same type rocks.In combination with regional geological and geophysical characteristics of the study area and some genetic models for bimodal volcanism in previous studies,we proposed a two-layer magma chamber model to explain the rifting process of Caroline Oceanic Plateau.Firstly,primitive magmas formed through decompressional melting were aggregated in deep magma chambers and crystallized to form basic plagioclase phenocrysts.Then the crystals and most evolving magmas were together ascended and erupted on the surface.Meanwhile,the evolving magmas were partly migrated into the shallow crustal magma chamber and crystallized to form intermediate-acidic plagioclase phenocrysts,both of which were ascended together to the surface from the shallow magma chamber and then erupted to form dacite.