Investigating the metamorphic evolution of the Bancroft Terrane (Grenville Province, Canada) using advanced equilibrium assemblage diagrams

1 Ma古加拿大Grenville省是一个中元古代造山带，发育于Rodinia超大陆形成时期。中央元沉积带是格伦维尔省的一部分，其演变在文献中备受争议。存在两种假说：a)是一个遗留的弧后盆地或b)是一个复合弧带，增生到元古界劳伦带南缘。自70年代初以来，许多岩石学、地球化学和地球物理研究对中央变质沉积带复杂的构造历史进行了探讨。然而，中央变质沉积带的某些部分，如Bancroft地体，在P-T条件方面研究不足。缺乏数据的原因是，以往的研究只使用传统的热气压测量方法（例如，使用石榴石-辉石温度计和石榴石-斜辉石-斜长石-石英温度计）。这种方法高度依赖于样品中现有的矿物组合。由于Bancroft地块主要由不含石榴石的岩石组成，很明显，大多数地温计都无法应用。这是Bancroft Terrane中缺乏数据的主要原因。该项目的主要建议是利用先进的平衡组合图（也称为伪剖面）来揭示Bancroft Terrane的多阶段变质历史。初步研究表明，应用平衡组合图可以对该地区的变质史有重要的认识。因此，有可能模拟一个假定的早期变质事件的P-T条件，这与该地区提出的P-T条件不同。为了破译影响Bancroft地体的早期变质事件的岩石学记录，在野外工作中需要结合先进的热力学模拟进行系统的采样。所取的样品将用于热力学计算，得到样品的P-T条件。获得的数据将在班克罗夫特地体的变质图中显示出来。因此，新的地图和数据将有助于了解中央元沉积带和整个格伦维尔省的复杂构造演化。

The 1 Ma old Canadian Grenville Province is a Mesoproterozoic orogen, which developed during the formation of the supercontinent Rodinia. The Central Metasedimentary Belt is a part of the Grenville Province and its evolution is highly debated in the literature. Two hypothesis exits: a) it is a relict back-arc-basin or b) it is a composite arc belt, accreted to the southern margin of Proterozoic Laurentia. Since the early 70ties, many petrological, geochemical and geophysical studies investigate the complex tectonic history of the Central Metasedimentary Belt. However, some parts of the Central Metasedimentary Belt, like the Bancroft Terrane, are understudied in terms of P-T conditions. The reason for the lack of data is that previous studies only used conventional thermobarometric methods (e.g., using garnet-pyroxene thermometers and garnet-clinopyroxene-plagioclase-quartz barometers). This approach highly depends on the existing mineral assemblage of a sample. Because the Bancroft Terrane is mainly composed of non-garnet-bearing rocks, it is quite obvious that most geothermobarometers cannot be applied. This is the main reason for the lack of data within the Bancroft Terrane. The principal propose of this project is to unravel the multi-stage metamorphic history of the Bancroft Terrane, applying advanced equilibrium assemblage diagrams (also called pseudosections). Preliminary work has shown, that applying equilibrium assemblage diagrams can give essential insights into metamorphic history of the region. Thus, it was possible to model the P-T conditions of a presumable early metamorphic event, which differs from the proposed P-T conditions of the region. In order to decipher this petrological record of an earlier metamorphic event, affecting the Bancroft Terrane, there is a crucial need for systematic sampling during fieldwork combined with advanced thermodynamic modeling. The samples taken will be prepared for thermodynamic calculations, yielding the P-T conditions of the samples. The gained data will be visualized in a metamorphic map of the Bancroft Terrane. Consequently, the new map and data will help to understand the complex tectonic evolution of the Central Metasedimentary Belt and the whole Grenville Province.