TTG的Nb/Ta比值以及Nb、Ta相对于La(代表LILE)的亏损取决于部分熔融体系中金红石、角闪石作为残留相矿物存在与否.本研究采用金红石和低Mg#角闪石的微量元素分配系数模拟部分熔融过程中Nb-Ta-La的分馏.模拟结果表明:如果与TTG熔体平衡的残留相是不含金红石的石榴角闪岩,熔体Nb/Ta比值低于源岩但Ta含量不低于La;若残留相是含金红石的榴辉岩,熔体Nb、Ta的含量低于La但Nb/Ta比值高于源岩.所以,TTG熔体的残留相应为含有金红石和相对低Mg#的钙质角闪石的角闪榴辉岩.前者使熔体的Nb、Ta相对于LILE亏损,后者平衡金红石导致的熔体Nb-Ta分馏效应.满足形成常见的低Nb/Ta比值且Nb、Ta相对于LILE亏损的TTG的成岩机制是:俯冲洋壳玄武岩含水条件下的部分熔融,或科马提质玄武岩在增厚地壳下部的失水熔融.对应的地球动力学环境分别是俯冲带或水平运动导致的地壳块体碰撞叠复增厚带,暗示在早太古时期地球表层已经出现板块构造.与俯冲过程伴生的岩浆作用以及与碰撞叠复作用相关的科马提质玄武岩失水熔融作用是太古宙大陆地壳的重要生长方式.太古宙TTG岩系的Nb/Ta比值具有重要的成岩指示意义.%The Nb and Ta concentrations as well as Nb/Ta ratios of TTG melt are controlled by the behaviors of futile and amphibole in partial melting system. In this study, trace element partition coefficients of rutile and low-Mg#amphibole from recent experiments are used to model Nb-Ta-La fractionation during partial melting of basaltic rocks. Our geochemical modeling shows that: the melt is characterized by i ) lower Nb/Ta ratio (relative to that of primitive mantle) without Ta depletion (relative to La) for partial melting system with rutile-free garnet amphibolite residuum; 2) Nb and Ta depletions as well as higher Nb/Ta ratio for partial melting system with rutile-bearing eclogite residuum. Thus, we suggest that the residuum of TTG melt is amphibole eclogite bearing rutile and low-Mg#amphibole. The existence of rutile in residuum results in Nb and Ta depletions, while low-Mg# amphibole offsets the Nb-Ta fractionation caused by rutile. For common basaltic source, low-Mg# amphibole co-exist with rutile only in the water-bearing system. For dehydration melting, only komatiitic basalt source can produce amphibole residuum within rutile stable pressure regime. Therefore, common TTG with low Nb/Ta ratio and Na and Ta depletions may be resulted from ( 1 ) the water-bearing melting of subducted oceanic crust, and (2) the dehydration melting of komatiitic basalt within thickened lower crust under geodynamic environments of subduction zone or the stacking belt related to horizontal movement of crustal terrains, respectively. It may imply that plate tectonics operated in Early Archean, and the Archean continental crust represented by TTG suite was produced via subduction magmatism or dehydration melting of komatiitic basalt in the stacking belts. Nb-Ta-La fractionation is an important indicator for TTG suite petrogenesis.
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