摘要
海南岛屯昌晨星地区变基性岩体原岩可能包括辉长岩(粗粒的斜长角闪岩)、辉长-辉绿岩(中细粒和中粗粒的斜长角闪岩)和枕状熔岩(细粒或块状斜长角闪岩)。由于受多期变质和蚀变作用的影响,组成这些岩石的多数矿物如角闪石、斜长石和绿泥石等常表现两个或两个以上的世代。角闪石环带矿物化学分析表明,从核到边,钛、铝、(钠+钾)含量增长,反映了压力和温度同时升高的进变质演化过程;相反,角闪石环带从核部到边部钛、铝和钾+钠含量降低,则暗示温度和压力同时降低而进入另一个退变质演化阶段。所获得的 Sm-Nd 全岩及角闪石的内部等时线年龄为128±12 Ma、Rb-Sr 全岩等时线年龄分别为131.8±6.2 Ma和344±11Ma。根据角闪石环带、矿物共生组合、微结构以及变质温压条件的半定量计算和同位素定年,屯昌晨星地区变基性岩体的变质作用可能经历了三个演化阶段。第一阶段(～527～450 Ma)可能记录了与洋底变质作用有关的(亚)绿片岩相至角闪岩相条件,早期的阳起石、钙质角闪石及镁铁质角闪石残余核为该阶段的产物;第二阶段(～330～240 Ma)进变质作用达到高峰的P=～0.9 GPa、T=～700℃,显示了过渡的高角闪石相-麻粒岩相变质条件。广泛的剪切变形及韭闪质普通角闪石-韭闪石边是该阶段的产物;第三阶段(～130 Ma～90 Ma)显示一个绿片岩相变质条件(P=～0.1～0.3 GPa、7=～300～400℃),可能与区域/热接触变质有关。因而屯昌晨星地区变基性岩体的变质作用演化显示了一个逆时针 P-T-t 轨迹,最可能与古太平洋板块俯冲导致的大陆边缘裂解及洋壳化和随后的年轻洋壳消亡、陆(华南大陆)-陆(印支板块)碰撞事件有关。
The petrology, mineral paragenese and microstructure suggest that the protoliths for the Chenxing metabasite massif in Tunchang area, Hainan Island probably are composed of gabbroic rocks （coarse-grained amphibolites）, gabbroic to diabasic rocks （medium to fine-grained and medium to coarse-grained amphibolites） , and pillowed lava （fine-grained and/or massive amphibofites）. These rocks are characterized not only by main mineral assemblages of amphiboles, plagioclase and chlorite which show at least two or more than two generations, but also by well-developed ductile-brittle shear deformations, being related to regional/thermal metamorphism and/or sea-floor metamorphism. The mineral chemistry of zoned amphiboles in the metabasites indicates a progressive evolution at early stage, being consistent with increasing Ti, A1 and （ K ＋ Na） from core to rim. By contrary, the reverse trend for Ti, Al and K ＋ Na contents reflects another retrogressive evolution. The whole rock and amphibole separate yield a Sm-Nd isochronic age of 128 ± 12 Ma, whereas the Rb-Sr whole-rock ages are 131.8±6.2 Ma and 344± 11 Ma, respectively.
Three successive and contrasting tectono-thermal-metamorphic events are clearly identified. The first stage of metamorphism （～527 -450 Ma） records （sub） greenschist-amphibolite facies conditions, due probably to sea-floor metamorphism, being shown by the first generation of actinolite, hornblende and orthoamphibole cores. ＂ Peak＂ metamorphic pressure and temperature conditions clustering around values of - 9 GPa and - 700℃ indicate a transitional amphibolite-granulite facies metamorphism, being marked by the pervasive ductile-shear deformation and the pargasitic borblende-pargasite rims related to the subduction and collision event of ～330～240 Ma age. The third stage is associated with the regional/ther Mal retrograde metamorphism at P/T=～0.1 ～0.3 GPa/～ 300 ～400℃ conditions, due to emplacement of the Yanshanian granites （～ 130 ～90 Ma）. All the study point to pressure increasing with increasing temperature on the prograde metamorphic path, followed by retrograde metamorphism （i. e. an anticlockwise P-T-t path）, which characterized the Chenxing metabasite massif most likely as the relics of an Ordovician oceanic crust due to rifting and oceanization from subduction of the Paleo-Pacific ocean, and to closure of the young ocean and subsequent collision of the South China （including Hainan Island） with the Indochina.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2006年第12期2987-3006,共20页
Acta Petrologica Sinica
基金
国家自然科学基金(40173017)
中国科学院广州地球化学研究所所级创新项目(GIGCX-04-04)联合资助
