地中海东部地幔演化过程:来自希腊Lesvos岛中新世钾质岩浆作用的启示

Mantle evolution of the eastern Mediterranean Sea:Implications from the Miocene potassic rocks in Lesvos Island,Greece

希腊Lesvos岛广泛出露的中新世钾质岩浆岩为深入探讨地中海东部地区新生代构造演化和深部过程提供了宝贵机会。本文对Lesvos岛碰撞后钾质岩石开展了系统的矿物电子探针分析、锆石U-Pb定年和Hf同位素分析及全岩地球化学分析。研究结果表明:Lesvos岛出露的碰撞后火山岩主要由玄武粗安岩、玄武安山岩和粗面岩组成,锆石U-Pb年龄为17.2Ma。三类岩石均表现出相对富集大离子亲石元素以及相对亏损高场强元素的地球化学特征。玄武粗安岩和玄武安山岩样品具有相似且富集的Sr-Nd-Pb同位素组成,其较低的Dy/Yb和整体较高且变化范围较大的Ba/La、Zr/Hf比值表明这两类钾质基性岩石起源于受到俯冲陆源沉积物起源流体/熔体交代的尖晶石相岩石圈地幔的低度部分熔融。相比之下,同时代粗面岩具有更亏损的Sr-Nd同位素组成,其均匀的同岩浆锆石Hf同位素组成排除了在岩浆演化过程中发生显著地壳混染或者岩浆混合作用的可能。而较高的Y和Yb含量、相对较低的Sr/Y和(La/Yb)_(N)比值则表明这种粗面质岩浆起源于中-上地壳的部分熔融。考虑到地震层析结果和Pelagonia陆块与Apulian陆块初始碰撞的时间,研究区各类碰撞后钾质岩浆活动可能反映了深俯冲的Pindos洋壳在中新世发生了板片断离过程。

The widespread Miocene potassic rocks in the Lesvos Island,Greee,offer a prime opportunity to probe into the Cenozoic tectonic evolution and deep processes of the eastern Mediterranean Sea.Here we carried out a combined study of mineral electron probe anlaysis,in situ zircon U-Pb dating and Hf isotopic compositions,and whole-rock geochemistry analyses on the postcollisional potassic rocks in the Lesvos Island.It can be found that these postcollisional volcanic rocks are mainly composed of basaltic trachyandesite,basaltic andesite,and trachyte,with a zircon U-Pb age of 17.2Ma.These three kinds of potassic rocks are characterized by relative enrichments of large ion lithophile elements and depletions in high field strength elements.Basaltic trachyandesite and basaltic andesite have simialr and enriched Sr-Nd-Pb isotpic compositions.The relatively low Dy/Yb and variably high Ba/La and Zr/Hf indicate that these basaltic rocks were derived from low-degree partial melting of spinel-facies lithospheric mantle that had been metasomatized by fluids/melts orignating from subducted terrigeneous sediments.In contrast,the coeval trachyte samples exhibit more depleted Sr-Nd isotopes,and their homogeneous Hf isotopic compositions in co-magmatic zircon preclude the occurrence of significant crustal assimilation and magma mixing during magmatic evolution.The relatively high Y and Yb concentrations and low Sr/Y and(La/Yb)_(N) ratios in these felsic rocks suggest that they were derived from partial melting of upper-middle crust.With respect to the seismic profiling studies and the initiation of continental collision between Pelagonia block and Apulian block,the postcollisional potassic volcanism in the Lesvos Island may result from the break-off of the subduced Pindos oceanic lithosphere in the Miocene.