【作者】 陈涛；

【导师】 许德如；

【作者基本信息】 中国科学院研究生院（广州地球化学研究所） ， 地球化学， 2005， 硕士

【摘要】 本文主要论述了海南岛中东部屯昌地区变基性岩体的变质作用特征及与古特提斯构造演化的关系。岩石学、矿物学及显微结构观察表明,研究区变基性岩体原岩可能由辉长岩(粗粒的角闪岩)、辉长-辉绿岩(中-细粒和粗粒的角闪岩)和枕状熔岩(细粒或块状的角闪岩)组成;主要矿物组合为角闪石+斜长石(An=0∽91)+绿泥石+绿帘石,副矿物为:钛铁矿+榍石±金红石±锆石±电气石,次要矿物为白云母、葡萄石、钾长石和富铁(铜)-镍硫化物。大部分矿物(如角闪石)至少表现两个或两个以上的世代。岩体本身则发育良好的脆-韧性剪切变形,表现为褶皱、剪切叶理(或劈理)、S-C组构、北北东-北北西走向的拉伸线理和糜棱状显微结构。变基性岩角闪石环带的矿物化学分析结果表明:从核部到边部,钛、铝、(钠+钾)含量增长,反映了从绿片岩相到角闪岩相或角闪岩-麻粒岩相的进变质演化过程;相反,角闪石环带从核部到边部钛、铝和(钾+钠)含量减少,则暗示了角闪岩-麻粒岩相或角闪岩相到(亚)绿片岩相的退变质演化。测得Sm-Nd全岩及角闪石单矿物内部等时线年龄为128±12 Ma、Rb-Sr全岩等时线年龄为131.8±6.2 Ma和311.1±7.9 Ma。根据角闪石环带以及矿物共生组合、岩石微结构特征、Sr-Nd同位素年代学和变质温压条件的半定量计算,并结合区域地质、岩浆作用和华南大地构造发展特征,屯昌地区古生代变基性岩体可能经历了三个变质演化阶段:第一阶段(约330 Ma)记录了与洋底变质作用有关的(亚)绿片岩相至角闪岩相变质条件,早期的阳起石、Ca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(约310-260 Ma)进变质作用达到高峰的压力(P)约9 kbar、温度(T)约700℃,显示了过渡性的上角闪岩相-麻粒岩相变质条件。广泛的剪切变形、岩层堆叠及韭闪质普通角闪石和韭闪石边是该阶段的产物;第三阶段(约130 Ma)显示一个绿片岩相变质条件(P约1-3 kbar,T约300-400℃),可能是区域/热退变质作用的反映。研究结果综合表明,屯昌地区变基性岩体在进变质作用阶段压力随着温度升高而升高,随后温度和压力都明显降低而进入退变质作用阶段,显示一个逆时针P-T-t轨迹,暗示了这些变基性岩可能起源于类似于洋内岛弧/俯冲带之上的构造背景。因此,该变基性岩体可能代表了一个不完整的、或被肢解的蛇绿岩套上部组合。更多还原

【Abstract】 This paper examines the metamorphic characteristics of the Paleozoic metabasites in the Tunchang area, East-central Hainan Island, China with respect to the tectonic processes in connection with the evolution of Paleo-Tethys. The petrology, the mineral paragenese and the microstructure suggested that the protoliths for these metabasites are probably composed of gabbroic rocks (coarse-grained amphibolites), gabbroic-diabasic rocks (medium to fine-grained and coarse-grained amphibolites), and pillowed lava (fine-grained and/or massive amphibolites). The overall mineral assemblages for these rocks are amphibole+ plagioclase(An=:0∽91) + chlorite + epidote, accessory phases: ilmenite + titanite ± rutile ± zircon ± tourmaline. Minor muscovite, prehnite and K-feldspar, and abundant Fe (Cu) -Ni sulphide commonly occur as secondary minerals. Most minerals, e.g., amphiboles, are characterized by at least two generations, and the well-developed ductile-brittle shear deformations are shown by the development of folds, shear foliations or cleavages, S-C fabrics, NNE to NNW-striking stretching lineations and mylonitic microstructures. The mineral chemistry of zoned amphiboles in the metabasites indicates a progressive evolution from greenschist, amphibolite facies to amphibolite-granulite facies, being consistent with increasing Ti, Al and (K + Na) from core to rim. However, Ti, Al and (K+Na) decreasing likely indicate another retrogressive evolution from amphibolite-granulite or amphibolite to low greenschist facies. The whole-rock and amphibole separate yield a Sm-Nd isochronic age of 128 ± 12 Ma, whereas the Rb-Sr whole-rock ages is 131.8 ± 6.2 Ma and 311.1 ± 7.9 Ma, repsectively. Three successive and contrasting tectono-thermal-metamorphic events are clearly identified, based on the zoned amphiboles, the mineral paragenese, the microstructure and the Sr-Nd isochronic age, as well as on the regional geology, the granitic magmatism and the tectonic development of South China. The first stage of metamorphism(~330 Ma) records (sub) greenschist-amphibolite facies conditions, probably due to sea-floor metamorphism shown by the first generation of actinolite,hornblende and orthoamphibole cores. The second stage(~310-260 Ma) of "peak" metamorphic pressure and temperature conditions cluster around values of ∽ 9 kbar and ∽ 700 ℃, which show a transitional amphibolite-granulite facies metamorphism, and are marked by the pervasive ductile-shear deformation, the nappe stacking and the pargasitic horblende-pargasite rims. The third stage(~130 Ma) relates to the regional/thermal retrograde metamorphism at P/T= ∽ 1-3 kbar/ ∽ 300-400 ℃ conditions. 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 these metabasites as an origin of intraoceanic arc/suprasubduction zone-like tectonic setting. Thus the Tunchang area metabasites most likely represent the upper layers of an incomplete, dismembered ophiolite body, owing to the northerly subduction of the eastern Paleo-Tethys.A possible model to delineate the petrogenesises of these metabasites and the Paleozoic tectonic evolution in the Hainan Island has been proposed in this paper. The model further supports that the NE-SW-trending Baisha fault of the Hainan Island is a Late Paleozoic-Earliest Mesozoic suture. The Hainan Island Block had been composed of the northwest Island (separated from South China) and the northeast Island (separeated from Indochina), which have been sutured along the Baisha fault during Late Paleozoic-Earliest Mesozoic.更多还原