摘要
苏鲁超高压变质地体中产出的与柯石英榴辉岩共生的大理岩发生了不同程度的退变,根据退变程度差异识别出三类大理岩:菱镁矿大理岩(弱退变)、滑石大理岩(中等退变)和透闪石大理岩(强烈退变)。菱镁矿大理岩的稳定矿物组合是单斜辉石+白云石+方解石,菱镁矿仅仅在方解石中与方解石集合体镶嵌共存。单斜辉石发育出溶结构且局部形成由角闪石和钠长石组成的后成合晶。另外两类大理岩不含菱镁矿,但含滑石和/或透闪石。在菱镁矿大理岩中观察到白云石的分解反应:(Mg,Ca)(CO3)2=MgCO3+CaCO3,说明这些大理岩是大陆地壳物质俯冲到地幔>180km(甚至超过210km)后返回到地表的产物。所观察到的白云石分解反应表明,苏鲁地区大陆地壳物质在所谓的“地幔禁区”俯冲,俯冲的温度增加梯度可能为-4.2℃/km。大陆地壳物质以如此低的温度梯度俯冲意味着非常高的俯冲速度。如此深循环的大陆地壳物质同时也经历了非常快的折返过程。这是白云石分解结构这种标志超高压条件下进变质反应阶段的证据得到保存的重要原因。
Marble coexists with coesite-bearing eclogites in Sulu ultrahigh pressure ( UHP) metamorphic terrane has undergone retrograde metamorphism. Based on the retrograde metamorphic degrees, the marble could be classified in three groups: magnesitebearing marble (slightly retrograded), talc marble (moderately retrograded), and tremolite marble ( strongly retrograded). The magnesite-bearing marble is mainly composed by clinopyroxene, dolomite and calcite. Magnesite only occurs as relics in calcite and coexists with calcite showing inlayed texture. Clinopyroxene shows exsolution texture with quartz rods in clinopyroxene. Clinopyroxerie rim was replaced by symplectites of plagioclase and amphibole locally. The moderately and strongly retrograded marbles do not contain any clinopyroxene and magnesite. However, they contain talc or/and tremolite. We report here the first discovery in nature of dolomite decomposition texture in metamorphic marble from Sulu UHP terrane(Mg, Ca) (Co-3)(2) = MgCO3 + CaCO3), which demonstrates that the continental materials recycled down to > 180 km (possibly > 210 km) and came back to surface quickly. This study firstly, suggests that continental materials recycled into the forbidden zone in the deep mantle with very, low geothermal of similar to 4.2 degrees C/km. The observation of this deep subducted continental materials means very high speed of exhumation, which is probably the major reason for presenation of dolomite decomposition texture showing progressive metamorphism at UHP conditions.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2005年第2期347-354,共8页
Acta Petrologica Sinica
基金
国家重大基础研究项目(973项目
编号2003CB716502)国家自然科学基金项目(编号40342018)的资助
关键词
苏鲁超高压地体
菱镁矿大理岩
白云石分解
大陆物质深循环
地幔禁区
Sulu UHP terrane
magnesite marble
dolomite decomposition
deep recycling of continental materials
forbidden zone
