柴北缘鱼卡河超高压变质金红石中Si含量及其地质意义

利用LA-ICP-MS方法对柴达木盆地北缘鱼卡河超高压变质的多硅白云母榴辉岩及共生的石榴角闪钠长岩中的金红石进行了详细的矿物学和LA-ICP-MS原位微量元素研究。结果表明:多硅白云母榴辉岩中金红石显示Si成分环带,且Si含量(质量分数,下同)随变质压力而发生规律性的变化;从进变质到峰期变质阶段,金红石中Si含量从核部向边部呈升高趋势,峰期金红石的边部Si含量达到最高,退变质阶段金红石中Si含量从核部向边部呈降低趋势;石榴角闪钠长岩中的金红石主要呈包体产于多硅白云母、石榴子石、角闪石和钠长石中;在相同的超高压变质条件下,石榴角闪钠长岩中多硅白云母包体金红石的Si含量((1018~2741)×10^-6,平均为1924×10^-6)明显高于多硅白云母榴辉岩中的峰期金红石((450~2397)×10^-6,平均为952×10^-6)。综合多硅白云母榴辉岩和石榴角闪钠长岩的产状、变质演化和全岩成分以及前人对大别—苏鲁超高压榴辉岩中富硅金红石的研究结果提出,超高压变质岩石中金红石的Si含量与变质压力成正相关关系,金红石中Si含量大于500×10^-6可以作为榴辉岩经历超高压变质作用的指示标志。超高压金红石中Si含量与全岩成分中SiO2和TiO2含量有关,随SiO2含量的升高和TiO2含量的降低而升高。

Si Contents and Their Geological Significance of Yukahe Ultrahigh-pressure Metamorphic(UHPM)Rutile in the Northern Qaidam Basin,NW China

An integrated study of mineralogy and in-situ LA-ICP-MS trace element analysis was conducted on rutiles from Yukahe ultrahigh-pressure metamorphic(UHPM)phengite eclogite and garnet amphibole albitite in the northern Qaidam Basin,NW China.The results show that rutiles from these two UHPM rocks have high Si contents,and during the metamorphic evolution of phengite eclogite,Si contents of rutile grains grown in different metamorphic stages as well as the core and rim parts of a single rutile grain change regularly with metamorphic pressure.The Si contents in prograde rutiles,which occur as inclusions in garnet,omphacite and phengite core,increase gradually from core((342-360)×10^-6)to rim;the Si contents in peak rutiles,which coexist with garnet,omphacite and phengite in matrix or occur as inclusions in garnet mantle and rim,have the highest Si contents with an average value of 952×10^-6 and a maximum value of 2397×10^-6;the Si contents in retrograde rutiles,which coexist symplectite with amphibole+plagioclase formed by omphacite decomposition or occur in phengite+quartz veins of eclogite,decrease gradually from core towards rim(the average Si content of symplectite is 726×10^-6,and that of microveins is 674×10^-6).Rutiles in garnet amphibole albitite mainly occur as inclusions in phengite,garnet,amphibole and albite,and the rutiles in different occurrences have obviously higher Si contents(1808×10^-6 on average)than those from phengite eclogite(762×10^-6 on average).In which,the rutiles included in phengite have Si contents of(1018-2741)×10^-6 with an average value of 1924×10^-6,whereas the rutiles hosted by albite have variable Si contents of(685-3910)×10^-6 with an average value of 1761×10^-6.Combined with the occurrences,metamorphic evolutions and whole rock compositons of phengite eclogite and garnet amphibole albitite as well as the previous results on Si-bearing rutiles of ultrahigh-pressure eclogites in Dabie-Sulu orogenic belt,it is proposed that Si contents of rutiles from UHPM rocks are positively correlated with metamorphic pressure,and the rutiles with high-Si content(>500×10^-6)may be a fingerprint for eclogite experiencing ultrahigh-pressure metamorphism;the whole rock SiO2 and TiO2 contents affect the Si contents of rutiles under the same ultrahigh metamorphic pressure,and the Si contents of rutiles increase as whole SiO2 rises and TiO2 decreases.