秦岭杂岩清油河斜长角闪岩多期变质的证据--来自锆石微量元素和包裹体的启示

Multi-metamorphism of amphibolite in the Qinling complex,Qingyouhe area: Revelation from trace elements and mineral inclusions in zircons

北秦岭清油河地区广泛发育以透镜状、条带状或岩墙状赋存于秦岭杂岩石英二长片麻岩中的斜长角闪岩,矿物组合为角闪石、斜长石以及少量的石榴子石、单斜辉石等。锆石阴极发光图像、不同微区矿物包裹体和稀土元素分析,以及不同性质锆石LA-ICP-MS U-Pb定年和微量元素分析等综合研究结果表明,该斜长角闪岩锆石成因十分复杂,可进一步划分为三种类型。第一类锆石呈半自形-他形晶,阴极发光显示条带状分带,锆石稀土总量较高,球粒陨石标准化稀土配分模式中重稀土富集,Eu负异常明显,记录的206Pb/238U加权平均值为774±13Ma,代表了该岩石原岩形成时代;第二类锆石呈他形晶,为典型的变质锆石,阴极发光图像为扇状或面状分带,锆石稀土总量较低,球粒陨石标准化稀土配分模式中重稀土相对平坦,没有Eu的负异常,表明该类锆石结晶时岩石中含有石榴子石,而没有长石,该类锆石矿物包裹体主要为Grt+Cpx+Q,可能代表榴辉岩相(榴辉岩或者石榴子石辉石岩)变质矿物组合,记录的^(206)Pb/^(238)U加权平均值为493±5Ma;第三类锆石呈他形晶或构成第二类锆石的薄边,亦为典型变质锆石,阴极发光为面状分带,锆石稀土总量较低,球粒陨石标准化稀土配分模式中重稀土平坦,具有Eu的异常,锆石包裹体矿物组合为Grt+Cpx+Pl+Amp+Q,应代表退变质矿物组合,记录的206Pb/238U加权平均值为448±4Ma。该斜长角闪岩的原岩年龄(774±13Ma)、峰期变质年龄(493±5Ma)和后期退变质年龄(448±4Ma)与北秦岭已厘定的高压-超高压岩石的原岩年龄、峰期变质和退变质年龄非常吻合,说明该岩石可能是北秦岭高压-超高压变质岩带的一部分。据此,并结合清油河斜长角闪岩中发现残存金刚石的研究成果,可以推测现今秦岭杂岩中出露的一些低级变质相岩石峰期也可能经历了高压-超高压变质,只是由于后期强烈的退变质作用的改造而难于识别和辨认,北秦岭高压-超高压岩石的分布可能远比目前观察到的丰富。此外,本次研究还显示清油河斜长角闪岩原岩具有大陆拉斑玄武岩特征,是陆壳岩石的一部分。这些资料为进一步论证北秦岭早古生代高压-超高压岩石(包括榴辉岩、长英质片麻岩、石榴子石辉岩、榴闪岩和部分斜长角闪岩等)是陆壳俯冲-深俯冲作用的产物提供了新的证据。

Amphibolites as lense and dyke are commonly identified within quartz-gneiss rocks in Qingyouhe area,North Qinling.Zircon grains separated from amphibolite in Qingyouhe area of North Qinling can be subdivided into three types based on cathodoluminescence（ CL） images,mineral inclusions assemblage,rare earth element,trace element analyses and in situ U-Pb LAICP-MS dates：（ 1） TypeⅠis inherited（ magmatic） zircons with weakly zoned,the chondrite-normalized REE patterns show a steep slope HREE pattern,high rare earth element（ REE） contents,and negative Eu anomalies. This zircon domain yield a206 Pb /238 U weight mean age of 774 ± 13Ma;（ 2） Type Ⅱ is metamorphic zircon with homogeneous CL images,characterized by low REE and HREE,a remarkably flat HREE profile without a significant Eu anomaly. This type of zircon preserved mineral inclusion assemblage Grt ＋ Cpx ＋ Q,and give a206 Pb /238 U age with weight mean of 493 ± 5Ma,interpreted as the age of eclogite-facies metamorphism;（ 3）Type Ⅲ is metamorphic zircon rim. Comparing with the type Ⅱ zircons,metamorphic rim show a remarkably flat HREE profile and a significant Eu anomaly,contains granulite-facies or high grade amphibolite facies metamorphic mineral inclusion assemblage of Grt ＋Cpx ＋ Amp ＋ Pl ＋ Q. These metamorphic zircons or metamorphic rim yield an age of 448 ± 4Ma. The protolith age（ 774 ± 13Ma）,eclogite-facies age（ 493 ± 5Ma）,and amphibolite-facies retrogression age（ 448 ± 4Ma） from amphibolite in Qingyouhe area are coincide with other HP-UHP rocks of North Qinling complex,suggesting that the amphibolites also a part of HP / UHP unit in Qinling complex. These studies indicate that the present amphibolite facies metamorphic rocks in the Qinling complex might have undergone a former HP-UHP metamorphism,but it is difficult to identify due to the succedently strong overprinted retrogress metamorphism.Therefore,the HP-UHP rocks are volumetrically abundant in the Qinling complex than previously recognized. Furthermore,this investigation obtain the protolith age of 774 ± 13 Ma from Qinyouhe amphibolites,and it has the geochemical characteristics of continental tholeiite,which is interpreted as the protolith as a part of the continental crustal rocks. Therefore,these data provides new evidence for further demonstrate that the North Qinling Early Palaeozoic HP-UHP rocks（ including eclogite,felsic gneiss,garnet pyroxenite,garnet amphibolite and part of amphibolite,etc.） are the product of continental crust subduction-deep subduction.