Owing to the lack of persistent magma chamber and the complex interplay between magmatism and tectonism at slow- to ultraslow-spreading ridges, the likeliness of fractional crystallization being the predominant process of evolution of crustal magmas is weak. Here, we report a detailed petro-geochemical investigation from the lower crustal gabbroic rocks recovered from Hole U1473A in Atlantis Bank oceanic core complex, Southwest Indian Ridge that may potentially provide new insights on the dynamic accretion process at ultraslow-spreading ridge. Sampling the entire lithological spectrum encompassing a single geochemical cycle in this study permit us to probe the underlying magmatic processes in ultraslow-spreading lower crust. Grain-size variation is ubiquitously recorded in the gabbroic lithologies of Hole U1473A, where fine-grained intervals represent relatively evolved chemistry than the coarser domains. This suggests that, they were crystallized from a common genetic melt during different degrees of evolution. The forward geochemical modelling approach have not only affirmed the long-established theory of assimilation (of early mushes’ crystals)-fractional crystallization to be the key mechanism in lower crust formation, but also provided explanation for the restricted whole rock geochemical cluster observed throughout all the rock types. We demonstrate for the first time the relationship between the progressive modal mineralogical evolution with respect to the different assimilation/fractionation ratios (r). The observed trend has been successfully replicated for the experiments with high r values = 0.8–0.9, previously predicted from this region. Integrating all results, our study supports the model of lower crustal formation via reactive porous flow of the squeezed-out melt percolating throughout the cumulate pile and segregated into different horizons.

由于缺乏持久的岩浆房，以及在慢到超慢蔓延的脊上岩浆作用与构造作用之间的复杂相互作用，分馏结晶成为地壳岩浆演化的主要过程的可能性较弱。在这里，我们报告了从西南印度洋海脊亚特兰蒂斯银行海洋核心复合体的 U1473A 孔中回收的下地壳辉长岩的详细石油地球化学调查，这可能为超慢扩张海脊的动态吸积过程提供新的见解。在本研究中对包含单个地球化学循环的整个岩性光谱进行采样，使我们能够探索超慢扩展下地壳中潜在的岩浆过程。在孔 U1473A 的辉长岩岩性中普遍记录了粒度变化，其中细粒度区间代表相对较粗的域进化的化学。这表明，它们是在不同程度的进化过程中从共同的基因熔体中结晶出来的。正向地球化学建模方法不仅肯定了长期以来建立的同化（早期 mushes 晶体）-分次结晶理论是下地壳形成的关键机制，而且为整个岩石地球化学簇在整个过程中观察到的受限提供了解释。岩石类型。我们首次证明了渐进式矿物学演化与不同同化/分馏比 (r) 之间的关系。对于先前从该区域预测的高 r 值 = 0.8-0.9 的实验，已成功复制观察到的趋势。