浙江龙游石榴角闪岩的岩石学特征与构造意义

Petrological properties and tectonic significance for Longyou garnet amphibolite

鉴于国内部分学者最近提出的＂浙江龙游石榴角闪岩乃华夏地块加里东期碰撞榴辉岩退变而成＂这一问题,在野外地质调查的基础上,我们对龙游地区的石榴角闪岩开展了岩相学、矿物化学、年代学及全岩地球化学等方面的研究.结果表明,少数以包裹体状存在于石榴子石和单斜辉石内部的斜长石为钙长石,而分布于单斜辉石和石榴子石周围的斜长石其An值在44~66之间,没有发现指示绿辉石分解形成的富钠斜长石（钠长石或更长石）;锆石CL图像显示,除存在变质成因的小颗粒锆石外,还存在较多具有明显核环构造的大颗粒锆石.经U-Pb年代学测试,其锆石核部年龄的加权平均值为879±11 Ma,和邻区陈蔡地区变角闪岩的原岩年龄一致;而其变质环及小颗粒锆石的加权平均年龄为446±5 Ma.全岩地球化学分析显示,龙游石榴角闪岩的原岩属于基性岩浆岩（Si O2含量为46.8%~48.2%）,微量元素特征具有E-MORB的地球化学亲缘性.依据龙游石榴角闪岩的有效全岩成分和EPMA分析结果,并结合观测到的矿物组合,建立了P-T视剖面图,该图揭示龙游石榴角闪岩至少经历过3期次变质阶段,分别为温压条件为0.67~0.75 GPa/685~719℃的进变质作用阶段,温压条件为0.91~0.97 GPa/766~782℃的峰期变质阶段和温压条件为~0.68 GPa/~767℃的退变质作用阶段（同时用传统温压计获得退变温压条件为0.66~0.67 GPa/720~740℃）.无论哪一个变质阶段,其变质压力都只是中压而没有达到高压.结合区域背景综合分析,我们认为龙游石榴角闪岩的成因演化过程为：新元古代古华南洋朝扬子陆块俯冲→引发弧后拉张→导致幔源岩浆上涌结晶,形成E-MORB型基性岩→志留纪时遭受强烈的区域中高级变质作用,遂形成龙游石榴角闪岩.

In view of the recent issue proposed by some researchers that ＂the garnet amphibolite in Longyou county of Zhejiang was retrograded from Caledonian collision-related eclogite＂.Based on detailed field geological investigation,we carried out a systematic study including petrography,mineral chemistry,zircon geochronology and bulk rock geochemistry.Our results indicate that,the relatively small amounts of plagioclase which only found in interior of garnet or clinopyroxene metacryst are anorthite,while plagioclase that largely distribute in the outer part of garnet or clinopyroxene metacryst are andesine and labradorite（with their An value within 44–66）,and no sodium-rich plagioclase（albite or oligoclase） which imply omphacite decomposition structure are found.The CL images of zircon exhibit obvious core-rim structure.U-Pb dating results indicate that the weighted average age of the zircon core part is 879±11 Ma,with Th/U ratio values ranging from 0.20–1.23 which suggest their magmatic origin;while the age of the rim is 446±5 Ma,and lots of their corresponding Th/U ratio show low values（？0.1）,suggesting their metamorphic origin.Moreover,the bulk rock geochemistry feature of garnet amphibolite suggest that its protolith is basic（the Si O2 content within 46.8%–48.2%）,and its trace element exhibit E-MORB geochemical affinity.Based on the effective bulk rock composition and EPMA analysis results,as well as the mineral association we found,the P-T pseudosection for Longyou garnet amphibolite was worked out.And this figure reveals that the Longyou garnet amphibolite at least experienced three stages of metamorphism：the prograde metamorphism stage with P-T condition of 0.67–0.75 GPa/685–719°C;the peak-metamorphism with P-T condition of 0.91–0.97 GPa/766–782°C;and the retrograde metamorphism stage with P-T condition of ~0.68 GPa/~767°C（by traditional P-T thermobarometer,we also get a P-T condition of 0.66–0.67 GPa/720–740°C for retrograde metamorphism stage）.Whichever stage,its pressure is medium-pressure but not highpressure.In combination with regional geological setting,we prefer the following genetic model for Longyou garnet amphibolite：the paleo-South China Ocean subduction under Yantze plate in Neoproterozoic→trigger back-arc extension→mantle-derived magma upwelling and forming E-MORB type basic rocks in Neoproterozoic→medium to high-grade metamorphism in Silurian.