山东荣成高钙石榴辉石岩的地球化学特征及其成因

在苏鲁超高压变质带荣成蓝晶石榴辉岩中, 发现一具有特殊结构和地球化学特征的石榴辉石岩.该石榴辉石岩以包裹体的形式赋存于强烈退变的蓝晶石榴辉岩中.岩相学观测表明该岩石的主要结构为石榴石呈薄带、项链状相互连接的网状结构, 分割由辉石和石榴石、钛铁氧化物等出溶片晶组成的区域.全岩地球化学分析表明, 和邻近的蓝晶石榴辉岩相比, 该岩石具有(1) 相对低的SiO₂ (42.5%~43.1%), 异常高的CaO (21.4%~21.9%)和CaO/Al₂O₃比值(1.46~1.64); (2) 较高的TiO₂ (1.77%~1.89%)、V (359~419μg/g)、Nb (~8μg/g)、Y (17.7~23.1μg/g)和Zr (~150μg/g); (3) LREE富集和微弱的Eu负异常; (4) 较低的Cr、Ni和Co.上述地球化学特征表明荣成辉石岩的原岩形成于经过橄榄石分离结晶作用的超基性岩浆或经历了Na和K亏损过程的高钙基性岩浆.这2种过程都要求较高的温压条件, 压力 > 15×105kPa, 温度 > 1300 ℃, 这和荣成辉石岩的原岩的初始稳定条件的温压估算相一致.该研究结果表明在苏鲁超高压变质带中, 一些超高压变质岩的母岩形成于高温部分熔融或岩浆演化作用, 明显不同于大多数榴辉岩或片麻岩的原岩形成环境. 

Rongcheng Ultra-Calcic Garnet Pyroxenite: Its Geochemistry, Origin and Implications for Pre-UHP Tectonics in the Sulu UHP Metamorphic Belt

We report an unusual garnet pyroxenite, as a nodule enclosed within kyanite eclogites from Rongcheng, northeastern Sulu UHP metamorphic belt. Apart from its unusual texture, high degrees of various minerals (garnet, titanite, and Fe-Ti oxides) and solid exsolution within garnet or clinopyroxene, this nodule has unusual major and trace element geochemistry as shown by (1) low SiO₂ (42.5%-43.1%), extremely high CaO (21.4%-21.9%), and high CaO/Al₂O₃ ratio (1.46-1.64); (2) high TiO₂ (1.77%-1.89%), Zr (to 150 μg/g), V (359-419 μg/g), Nb (to 8 μg/g), and Y (17.7-23.1 μg/g) contents; (3) LREE-enriched, and nepherine normative; and (4) low Cr, Ni, and Co. Previous study on the mineral chemistry and texture relationships between different generations of minerals has suggested that it was formed as a cumulative clinopyroxene magacryst with high Ca-tschermakite component (31%-34%) from a gabbroic magma at p= (15-22) ×105 kPa, and T > 1 100 ℃. Major and trace element geochemistry on this nodule and its host kyanite eclogite suggest that the precursor of the Rongcheng pyroxenite was derived either from an ultramafic magma highly fractionated by accumulation of olivines or from an ultra-calcic basic magma that had lost some K₂O and Na₂O. No matter which was responsible for the formation of its precursor, both require a melting event and magma differentiation to occur at a pressure and temperature greater than 15×10-5 kPa and 1 300 ℃, respectively. This contribution suggests that protoliths for some of the eclogites within the Sulu UHP metamorphic belt formed in a tectonic environment distinct from those of Neoproterozoic age.