Diamondiferous and barren eclogites and pyroxenites from the western Kaapvaal craton record subduction processes and mantle metasomatism, respectively
Diamondiferous and barren eclogites and pyroxenites from the western Kaapvaal craton record subduction processes and mantle metasomatism, respectively
复制标题
DOI：
10.1016/j.lithos.2020.105588
复制
发表时间：
2020-09-01
期刊：
影响因子：
3.5
通讯作者：
Gerdes, Axel
中科院分区：
文献类型：
--
作者：
Aulbach, Sonja;Viljoen, K. S.;Gerdes, Axel
利用矿物主量元素和微量元素，结合单斜辉石的锶同位素，揭示了多恩科洛夫-索弗橙色岩脉(西卡帕瓦尔克拉通)中地幔榴辉岩和辉石岩包体的成因和演化，并研究了这些岩石中金刚石的生成和破坏。榴辉岩有两种不同的类型：(1)贫镁榴辉岩(MgO=7.3~14.5wt%；n=43)，伴生金刚石+/-刚玉和蓝晶石，石榴石粗粒含量高(中位数Ca#=0.25)，单斜辉石翡翠含量(0.39)。改造后的块状岩石轻稀土元素亏损(La中位数为0.29ppm)，中位Cr2O3值低(0.06wt%)，微量元素不相容(如锶、锆、钡、铅、钍)，锂和过渡金属丰度较高。有些具有阶梯状稀土配分模式或MREE中陡峭的坡度，类似于与俯冲带产生的脱水流体相互作用而影响的榴辉岩。这些流体也可能在钻石稳定的压力下典型地减少榴辉岩组合中沉积了钻石。(2)富镁榴辉岩和辉石岩(MgO=14.0~20.0wt%；n=29)，贫LREE(中位数La 1.39ppm)、富Cr2O3(0.25wt%)和不相容的微量元素，Li和过渡金属丰度低于贫镁组。这些都是地幔岩石圈碳酸盐化超镁铁质熔体交代作用的典型标志。锶同位素组成在两组中差异很大，但高Cr2O3值和高Ba值主要与锶-87/锶-86&0.7055有关。这反映了与古代岩石圈交代作用的相互作用，最终导致区域橙岩岩浆作用。这两个群中都存在强烈的Eu正异常，包括两个辉石岩，这需要低压火成岩原岩，推测来自大约3Ga的扩张脊，就像报道的来自西部Kaapvaal克拉通的其他榴辉岩物质一样。根据贫镁和富镁榴辉岩和辉石岩的比例，大约40%的钻石库存被集中在135+/-15公里深度的地幔交代破坏，并与低速异常(岩石圈中部不连续)重叠。两个含钻石的橙色岩石
Mineral major and trace elements combined with Sr isotopes of clinopyroxene are used to unravel the origins and evolution of mantle eclogite and pyroxenite xenoliths from the Doornkloof-Sover orangeite dike (western Kaapvaal craton), and to investigate the generation and destruction of diamond in these rocks. Two different eclogite types are present: (1) MgO-poor eclogites (MgO = 7.3 to 14.5 wt%; n = 43) with accessory diamond +/- corundum and kyanite, high garnet grossular content (median Ca# = 0.25) and clinopyroxene jadeite content (0.39). Reconstructed bulk rocks are LREE-depleted (median La 0.29 ppm) and have low median Cr2O3 (0.06 wt%) and incompatible trace-element contents (e.g. Sr, Zr, Ba, Pb, Th), and high Li and transition metal abundances. Some are characterised by stepped REE patterns or steep slopes in the MREE, similar to eclogites affected by interaction with dehydration fluids generated in subduction zones. These fluids may also have deposited diamond in typically reducing eclogite assemblages at diamond-stable pressures. (2) MgO-rich eclogites and pyroxenites (MgO = 14.0 to 20.0 wt%; n = 29), which are barren and enriched in LREE (median La 1.39 ppm), Cr2O3 (0.25 wt%) and incompatible trace elements, with lower Li and transition metal abundances than the MgO-poor group. These are typical signatures of carbonated ultramafic melt metasomatism in the mantle lithosphere. Strontium isotopic compositions vary widely in both groups, but high Cr2O3 and Ba contents are dominantly associated with Sr-87/Sr-86 > 0.7055. This reflects interaction with metasomatic agents remobilised from ancient lithospheric metasomes, which eventually gave rise to regional orangeite magmatism. The presence of strong positive Eu anomalies in both groups, including two pyroxenites, requires low-pressure igneous protoliths, presumably derived from a ca. 3 Ga spreading ridge, as reported for other eclogite materials from the western Kaapvaal craton. Based on the proportions of MgO-poor and -rich eclogites and pyroxenites, approximately 40% of the diamond inventory were destroyed by mantle metasomatism centred at similar to 135 +/- 15 km depth, overlapping a low-velocity anomaly (mid-lithospheric discontinuity). Two diamondiferous orangeites