Formation of discordant chromitite at the initiation of sub-arc mantle processes: Observations from the northern Oman ophiolite
Formation of discordant chromitite at the initiation of sub-arc mantle processes: Observations from the northern Oman ophiolite
复制标题
DOI：
10.2465/jmps.131006
复制
发表时间：
2014
影响因子：
0.7
通讯作者：
M. Miura;S. Arai;A. Tamura
中科院分区：
文献类型：
--
作者：
M. Miura;S. Arai;A. Tamura
在阿曼北部蛇绿岩沿 Wadi Hilti 的地幔部分检查了不协调铬铁矿（蛇绿岩铬铁矿的结构类型之一）和周围的橄榄岩，以限制其起源。铬铁矿和邻近橄榄岩中的尖晶石显示出较高的 Cr#（= Cr/(Cr + Al) 原子比），约为 0.65 至 0.75，表明具有与电弧相关的特征。来自邻近方辉橄榄岩的单斜辉石显示出低重稀土含量，表明其具有高度贫化特征。计算得出的熔体成分与纯橄榄岩包层中的单斜辉石达到平衡，表明铬铁矿和周围的橄榄岩源自与弧相关的岩浆，例如博尼岩。从纯橄榄岩和方辉橄榄岩中不相容元素富集来看，与弧相关的岩浆可能是俯冲板片上贫化地幔方辉石的熔剂熔融产生的。不和谐纯橄榄岩和铬铁矿周围高度贫化的方辉橄榄岩显示出尖晶石和稀土元素化学向周围海洋方辉橄榄岩的过渡变化。这一特征表明，先前存在的海洋方辉橄榄岩被与弧相关的岩浆部分地改变了。不和谐的铬铁矿、纯橄榄岩和高度贫化的方辉橄榄岩很可能是在蛇绿岩仰冲之前的超俯冲带环境中形成的。地幔橄榄岩中的这种与弧相关的特征可能是在年轻的弧下地幔中产生的。
Discordant chromitite, one of the structural types of ophiolitic chromitite, and surrounding peridotites, were examined in the mantle section of the northern Oman ophiolite along Wadi Hilti, to constrain its origin. Spinels from chromitite and adjacent peridotites show high Cr# (= Cr/(Cr + Al) atomic ratio), around 0.65 to 0.75, indicating an arc-related feature. Clinopyroxenes from adjacent harzburgites show low HREE contents, indicating a highly depleted character. The calculated melt composition in equilibrium with clinopyroxene in the dunite envelope indicates that the chromitite and surrounding peridotites were derived from arc-related magma, such as boninite. The arc-related magma was probably generated by the flux melting of depleted mantle harzburgite on the subducted slab, in terms of incompatible element enrichment in clinopyroxene from dunite and harzburgite. The highly depleted harzburgite surrounding discordant dunite and chromitite shows transitional changes in spinel and REE chemistry toward the surrounding oceanic harzburgites. This feature indicates that pre-existing oceanic harzburgite was partially modified by arc-related magma. Discordant chromitite, dunite, and highly depleted harzburgites are probably formed at the supra-subduction zone environment just before ophiolite obduction. Such arc-related features in mantle peridotite may be produced at a young sub-arc mantle.