A self-consistent top–down model for differentiation in bimodal suites: application to the Sonju Lake Intrusion–Finland granite system (MN)

The Sonju Lake Intrusion (SLI) is a 1200-meter thick layered mafic intrusion that directly underlies an equally large silicic pluton, the Finland granophyre (FG) within the Beaver Bay Complex of the Mid-Continent Rift (MN, USA). The SLI, with a simple mineralogical and compositional stratigraphy, provides an excellent case study for examining the changes in iron isotope ratios (δ56Fe). Here new Fe isotope data along with 87Sr/86Sr for a set of stratigraphically controlled samples from the SLI and FG are presented. The Fe isotope data show systematic changes within two differentiation sequences found in the lowermost FG as well as the upper portion of the SLI. Specifically, δ56Fe is observed to start at low values and increase to heavy values going stratigraphically up through each differentiation sequence. Within the middle portion of the SLI, δ56Fe varies between 0 and 0.1. Two samples from the SLI bottom are isotopically lighter than the middle SLI. The origin of the Fe isotope variations is discussed in terms of recently proposed explanations. A quantitative model shows that the observed spatial variation is consistent with the prediction of a temperature gradient model. Using present constraints on equilibrium phase partitioning, the iron isotope variations do not appear consistent with production by fractional crystallization. Based on these observations, a top–down sill emplacement process coupled with in situ differentiation remains a viable alternative model for forming this layered intrusion.

松朱湖侵入体（Sonju Lake Intrusion, SLI）是一套厚度达1200米的层状镁铁质侵入体，直接伏于规模相当的长英质深成岩体——芬兰花斑岩（Finland granophyre, FG）之下，二者均赋存于美国明尼苏达州中大陆裂谷（Mid-Continent Rift, MN, USA）的比弗湾杂岩体（Beaver Bay Complex）内部。该侵入体具备简洁的矿物学与地球化学层序特征，为探究铁同位素比值（δ56Fe）的演化规律提供了极佳的研究实例。本文报道了采自SLI与FG的一系列层序受控样品的全新铁同位素数据，以及对应的87Sr/86Sr同位素比值数据。铁同位素数据显示，在最下部FG的两个分异序列以及SLI的上部层位中，均存在系统性的同位素组成变化。具体而言，随着每个分异序列沿地层向上延伸，δ56Fe值从低水平逐步升高至重同位素富集的水平。在SLI的中部层位，δ56Fe值介于0与0.1之间；而两件采自SLI底部的样品，其同位素组成较中部层位更为轻同位素富集。本文结合新近提出的相关解释，对铁同位素组成变异的成因展开了讨论。定量模型结果表明，本次观测到的空间分布特征与温度梯度模型的预测结果相符。结合当前对平衡相分配的约束条件，铁同位素的组成变异似乎并不支持分离结晶作用的成因机制。基于上述观测结果，结合原地分异作用的自上而下岩床侵位过程，仍是阐释该层状侵入体形成机制的可行替代模型。