【摘要】 和Fe一样,V和Sc都是过渡族金属元素,它们在地幔部分熔融和岩浆分异中的地球化学行为是近年来行星地质和岩石地球化学的研究热点之一。通过分析V和Sc在部分熔融和岩浆分异过程中的地球化学行为,发现V／Sc是可以透视岩浆成岩后所经历的变质、交代或流体作用的新的地球化学工具。本文通过系统分析中国大陆科学钻探工程主孔100～600米榴辉岩,尤其是540—600米钛铁矿-赤铁矿榴辉岩的V／Sc比值的变化特征,来进一步限定榴辉岩原岩可能的成岩过程。在CCSD主孔岩心中,和100～540m榴辉岩相比,540～600m段榴辉岩表现出以下十分特别的地球化学特征:(1)相当高的V,但十分低的Nb和Ta;(2)较高的TFeO,Fe2O3／FeO比值,高度变化且较高的Eu正异常(Eu／Eu*高达2．9);(3)异常高的V／Sc比值(8．39—43．23,平均15．07),V／Sc比值和MgO之间呈负相关关系,但和Fe2O3／FeO之间呈正相关关系;(4)近乎正相同步变化的V和Fe2O3／FeO,但反相变化的V和Eu／Eu*。这些地球化学特征反映了原岩形成过程中,高度动态的斜长石分离结晶作用与岩浆氧逸度之间的负耦合关系,即在原始基性岩浆中,斜长石的分离结晶作用导致岩浆氧逸度升高,诱发钛磁铁矿和斜长石及单斜辉石的同步分离结晶作用,优先猎取大量的V和Fe,Sc主要分布在单斜辉石,导致所成堆晶的V和Fe含量升高及高度变化且增高的V／Sc比值,同时MgO,SiO2相对降低。这种过程导致上述的V和Fe2O3／FeO之间近乎同相变化关系,但反相变化的V和Eu／Eu*。CCSD主孔100-600m榴辉岩的V／Sc和MgO系统关系实际上透视了上述的过程,即除橄榄石堆晶作用外,含钛铁矿物相的分离结晶作用对形成的堆晶及相应岩浆的V、Sc和V／Sc比值的强烈控制作用。CCSD及区域地质所揭示的南北长约2000m,宽约500m的反S型、层序较完整的基性-超基性杂岩体所经历的岩浆过程有可能代表着大别-苏鲁超高压带中榴辉岩原岩经历了类似的过程。通过对较大规模榴辉岩及其伴生的超基性岩的V、Sc和V／Sc系统关系的系统研究可能进一步揭示更多类似的基性-超基性杂岩体,为厘定大别-苏鲁超高压带的碰撞前的物理和化学状态提供更好的限定。更多还原

【Abstract】 V and Sc, same as Fe, are transitional metals. The behavior and systematics of V and Sc in partial melting of upper mantle and magma differentiation has been a focus of recent advanced planetary and petro-geochemical studies. These studies have greatly advanced our knowledge of the geochemical behavior of V and Sc in mantle melting and magma differentiation, and show that V/Sc ratio can see through metamorphism, metasomatism, and fluid-rock reaction. Therefore, V/Sc systematics offers us another powerful tool to unravel magmatic processes that were responsible for the formation of protolith for high-grade metabasites. The 540 -600 m eclogites in the CCSD main borehole are extraordinary in that they have anomalous high magnetic susceptibility, high P-wave velocity, and geochemistry. Petrography and mineral chemistry on these rocks show that opaque phases in these rocks are dominantly of ilmenite-haematite solid solution with very fine intergrowth structures. Such intergrowths can amount up to 30 vol% of mineral compositions of these eclogites. Major and trace element geochemical data show that as compared to the other types of eclogites, they have: (1) relatively high V with an average of 606 ppm, but very low Nb and Ta; (2) highest TFeO, Fe2O3/FeO ratio, and highly variable but strong positive Eu anomalies with Eu/Eu * up to 2. 9; (3) anomalously high V/Sc ratios ranging from 8. 39 to 43. 23, average 15. 03. V/Sc ratios are correlated negatively with MgO, but positively with Fe2O3/FeO ratios; (4) nearly in-phase correlation of V with Fe2O3/FeO ratios, but anti-phase with Eu/Eu*. These data suggest a highly negative feedback between dynamic fractional crystallization of plagioclase and fO2 in the fractionated magma during the formation of the protolith of 540 - 600m eclogite. Plagioclase fractional crystallization from a basaltic magma elevated the fO2 in the magma to a level at which crystallization of Ti-Fe-bearing oxides together with plagioclase and clinopyroxene started. In turn, crystallization of Ti-Fe-bearing oxides reduced fO2 in the magma to a level at which Ti-Fe-bearing oxide crystallization stopped. Repetition of such a cycle could explain the observed in phase correlation of V with Fe2O3/FeO, as well as anti-phase correlation between V and Eu/Eu*. V/Sc and MgO systematics in the 100 -600 m eclogites also support such a mechanism. The suggested mechanism is extremely similar to processes responsible for the formation of large-scale layered mafic-ultramafic intrusions such as Skaergaard, Bushveld, and Stillwater. The layered intrusion identified in the CCSD project may represent a least destructed of many unidentified layered intrusions in the Dabie-Sulu UHP metamorphic belt.更多还原