High-Mg Adakitic Rocks Formed by Lower-crustal Magma Differentiation: Mineralogical and Geochemical Evidence from Garnet-bearing Diorite Porphyries in Central Tibet
High-Mg Adakitic Rocks Formed by Lower-crustal Magma Differentiation: Mineralogical and Geochemical Evidence from Garnet-bearing Diorite Porphyries in Central Tibet
复制标题
下地壳岩浆分异作用形成的高镁埃达克岩：西藏中部含石榴石闪长斑岩的矿物学和地球化学证据
DOI：
10.1093/petrology/egaa099
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发表时间：
2020
影响因子：
3.9
通讯作者：
Gong-Jian Tang
中科院分区：
文献类型：
--
作者：
Jun Wang;Wei Dan;Qiang Wang;Gong-Jian Tang
摘要。具有高Mg值（摩尔100 × Mg/(Mg + Fe)比）的现代弧埃达克岩通常被认为是俯冲洋壳和/或侵蚀弧前壳的熔体与地幔楔中的橄榄岩相互作用的产物。基于全岩地球化学变化，提出了一种替代模式，即幔源岩浆的石榴石分选产生了高mg的阿达克岩；然而，在高镁埃达克岩中尚未发现岩浆石榴石，对这种岩浆分异所需的物理条件也知之甚少。富钙石榴石晶体与寄主岩和锆石自晶的成分对比一致，表明石榴石晶体生长于寄主岩浆中。闪孔、石榴石、锆石及寄主岩的Dy/Yb比值随岩浆分异程度的增加而增大。在石榴石之前结晶的与角闪洞平衡的原始岩浆不是角闪质的。含石榴石的高mg阿达奇岩可能是由辉石、角闪洞和石榴石在约1 GPa的原始安山岩母岩中分选而成，其地球化学特征与无石榴石闪长斑岩相似。原始安山岩岩浆具有丰富的同位素组成（[87Sr/86Sr]i 0·709,[Nd[t]−3·4），可能是在洋板断裂的碰撞后伸展环境中俯冲富集的岩石圈地幔的浅部熔融形成的。石榴石结晶最可能的情况是，地幔源含水（H2O 5 wt %）岩浆在地壳底部停滞、等压冷却并分化。该研究为高镁埃达质岩石的形成提供了直接的矿物学证据，表明高镁埃达质岩石是由石榴石晶体分馏作用形成的，而不是由壳源熔体与地幔相互作用形成的，尽管后者可能经常发生在地幔楔中。
Abstract. Modern arc adakites with high Mg values (molar 100 × Mg/(Mg + Fe) ratio) are generally considered products of interaction between melts derived from subducted oceanic crust and/or eroded forearc crust and peridotite in the mantle wedge. An alternative model, in which high-Mg adakitic rocks are produced by garnet fractionation of mantle-derived magmas, has been proposed based on whole-rock geochemical variations; however, magmatic garnet has not been found in high-Mg adakitic rocks, and little is known about the physical conditions required for this magmatic differentiation. Here we report geochronological, mineralogical and geochemical data for Late Triassic garnet-bearing high-Mg (Mg = 45–56) adakitic diorite porphyries and garnet-free non-adakitic diorite porphyries with Mg 62 from central Tibet. Consistent compositional correlation between Ca-rich garnet crystals, their host rocks and zircon autocrysts suggests that the garnet crystals grew in their host magmas. Amphibole, garnet, zircon and the host rocks display increasing Dy/Yb ratios with increasing magma differentiation. Pristine magmas in equilibrium with amphibole that crystallized prior to garnet are not adakitic. The garnet-bearing high-Mg adakitic rocks were probably generated by the fractionation of pyroxene, amphibole and garnet at ∼1 GPa from a primitive andesitic parent that was geochemically similar to the garnet-free diorite porphyries. The primitive andesitic magmas with enriched isotope compositions ([87Sr/86Sr]i 0·709, ɛNd[t] −3·4) may be derived from shallow melting of subduction-enriched lithospheric mantle in a post-collisional, extensional setting resulting from oceanic slab breakoff. The most likely scenario for garnet crystallization is that mantle-derived hydrous (H2O 5 wt %) magmas stalled, cooled isobarically and differentiated at the base of the crust. This study provides direct mineralogical evidence for the generation of high-Mg adakitic rocks by crystal fractionation involving garnet, rather than by interaction between crust-derived melt and the mantle, although the latter is potentially a frequent occurrence in the mantle wedge.