Petrogenesis of Massif Anorthosites

块状斜长岩是一种几乎完全由斜长石组成的深成火成岩。在对这些神秘岩石的研究中，反复出现的问题包括：导致在岩基规模上如此不寻常的斜长石浓度的机制，它们的母岩浆的性质和来源，它们与伴生的铁钛氧化物矿石的关系，它们与伴生的亚辉岩-绿辉岩-锰矿岩套装的花岗质火成岩的关系，以及将部分或全部这些岩石连接在一起的结晶-分异过程，这些岩石是单一或重复岩浆活动的一部分。块状斜长岩的侵位在空间和时间上都局限于中元古代的较窄的侵位带。因此，块状斜长岩及其伴生岩的形成代表了大陆地壳生长发育的一个重要阶段，其时间跨度仅为地球历史的一个短暂时期。我们将通过对魁北克St. Urbain和Labrieville斜长岩的综合野外、岩石学和地球化学研究，调查块状斜长岩的起源和伴生岩石类型。我们对圣厄本的研究是几年前开始的工作的延续，而对拉布里维尔的研究则是一项新的倡议。每个地块都包含一系列岩石类型，这些岩石类型与对岩体内斜长岩问题的关键评估有关，这些岩体很小，可以进行详细的研究。我们的工作将包括通过电子探针方法分析矿物成分，以及通过x射线荧光（XRF）和仪器中子活化（INA）方法的组合分析整个岩石的主要和选定的微量元素。我们将建立一个全面的岩石学和地球化学数据库，这将使我们能够为斜长岩的化学组合开发定量模型，评估其岩浆分化的过程，并提供对其母岩浆成分的良好约束估计。

Massif anorthosites are plutonic igneous rocks consisting almost entirely of plagioclase feldspar. Recurring problems in the study of these enigmatic rocks include the mechanisms that lead to such unusual concentrations of plagioclase on the scale of batholiths, the nature and source of their parental magmas, their relationship to associated iron-titanium oxide ores, their relationship to associated granitic igneous rocks of the admellite-charnockite-mangerite suite, and the crystallization - differentiation processes that link some or all of these rocks together as parts of single or repeated episodes of magmatism. The emplacement of massif anorthosites appears restricted in space and time to relatively narrow belts of mid-Proterozoic age. Accordingly, the formation of massif anorthosites and their associated rocks represents an important stage in the growth and development of the continental crust spanning only a short interval of Earth history. We will investigate the origin of massif anorthosite and associated rock types through an integrated field, petrological and geochemical study on the St. Urbain and Labrieville anorthosites, Quebec. Our study of St. Urbain is a continuation of work begun a number of years ago, whereas the study of Labrieville represents a new initiative. Each massif contains a range of rock types pertinent to a critical evaluation of the anorthosite problem within plutons that are small enough to be investigated in great detail. Our work will include the analysis of mineral compositions by electron microprobe methods, and the analysis of whole-rocks for major and selected trace elements by a combination of X-ray fluorescence (XRF) and instrumental neutron activation (INA) methods. We will establish a comprehensive petrological and geochemical data base that will allow us to develop quantitative models for the chemical assembly of the anorthositic rocks, to evaluate processes responsible for their magmatic differentiation, and to provide well-constrained estimates of their parental magma compositions.