Detrital rutile petrochronology: Assessing metamorphism through time at the global scale

碎屑金红石岩石年代学：在全球范围内评估随时间变化的变质作用

• 批准号: 509465124
• 负责人: Dr. Jan Schoenig
• 金额: --
• 依托单位: Abteilung Petrologie und Geochemie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/509465124?language=en
• 关键词: Detrital; rutile; petrochronology; Assessing; metamorphism

Reconstructing metamorphic conditions throughout Earth history is of fundamental importance to understand the long-term evolution of thermal, dynamic, and chemical regimes. A major and apparently insuperable obstacle is marked by the fragmentarily preserved crystalline rock record. Interpretations of whether plate tectonics globally initiated early or late, whether the transition from a preceding non-plate-tectonic regime was abrupt or long lasting, and whether thermal regimes secularly evolved from warm to cold conditions suffer in particular from extrapolating local-scale observations to the global scale. By contrast, detrital mineral grains have remarkable potential to efficiently acquire large datasets from a wide range of sources, including preserved sediments shed from orogens that are lost by erosion, and thus provide a much better representation of the global-scale picture. However, up-to-date, no tool exists which is able to reliably combine age and P–T (pressure–temperature) information from a detrital perspective. Based on a preliminary application of advanced statistical methods on a compiled literature dataset, a multivariate consideration of trace elements in rutile demonstrates high potential to close this gap. The proposed project is subdivided into two phases. In the first phase, a rutile trace-element database will be compiled by acquiring new data from well-defined crystalline rock samples and integration with published data. This will include 17 trace-element concentrations from rutiles of metasomatic, igneous, mantle, and metamorphic origin. The latter will be additionally analyzed for 8 element concentrations. Rutile-bearing samples from metamorphic rocks cover felsic and mafic bulk-rock compositions as well as a broad range of P–T conditions. A multivariate discrimination model will be developed that (i) separates metasomatic, igneous, mantle and metamorphic rutile, and (ii) predicts T/P gradients for metamorphic rutile. In the second phase, the developed discrimination scheme will be combined with U–Pb dating and applied to ~10,000 detrital rutile grains from 33 modern-sand samples of rivers draining considerable parts of S Africa, S America, NE Canada, S India, and W Australia. These rivers mainly sample material sourced from Archean and Proterozoic units as well as reworked sediments from basins recording earlier stages of erosion, which are inaccessible via the crystalline rock record. For samples indicating a contribution from low T/P metamorphic rocks, inclusion assemblages of detrital rutiles will be screened for characteristic low T/P minerals in order to confirm geochemical predictions by mineralogical evidence. A successful project will shed new light on the evolution of metamorphic gradients through time and support or falsify our central hypothesis that “modern-style low T/P gradients were established globally in the Paleoproterozoic”.

重建整个地球历史上的变质条件对于理解热、动力和化学状态的长期演化具有重要意义。一个主要的和显然不可逾越的障碍是零碎保存下来的水晶岩石记录。关于全球板块构造是早还是晚开始的解释，从先前的非板块构造制度的转变是突然的还是长期的，以及热制度是否从温暖条件长期演变为寒冷条件，特别是由于将局部尺度的观测外推到全球尺度而受到影响。相比之下，碎屑矿物颗粒具有显著的潜力，可以有效地从各种来源获取大型数据集，包括因侵蚀而丧失的造山带流失的保存沉积物，从而更好地反映全球范围的图景。然而，到目前为止，还没有工具能够从碎屑的角度可靠地结合年龄和P-T(压力-温度)信息。基于对汇编的文献数据集的高级统计方法的初步应用，对金红石中的微量元素的多元考虑显示出弥合这一差距的巨大潜力。拟议的项目分为两个阶段。在第一阶段，金红石痕量元素数据库将通过从定义明确的结晶岩石样品中获取新数据并与已公布的数据相结合来汇编。这将包括来自交代成因、火成岩成因、地幔成因和变质成因的金红石的17种微量元素。后者将另外分析8种元素的浓度。变质岩中含金红石的样品包括长英质和镁铁质块状岩石成分以及广泛的温压条件。建立了(1)分离交代、火成岩、地幔和变质金红石的多元判别模型，(2)预测了变质金红石的T/P梯度。在第二阶段，所开发的判别方法将与U-Pb测年相结合，并应用于33个现代砂样中的约10,000个碎屑金红石颗粒，这些现代砂样排放了非洲S、美国S、加拿大东北部、印度S和澳大利亚西部的大部分河流。这些河流主要是来自太古宙和元古界单元的样品，以及记录了早期侵蚀阶段的盆地重新加工的沉积物，这些沉积物无法通过结晶岩石记录获得。对于来自低T/P变质岩的样品，将筛选碎屑金红石包裹体组合中的特征低T/P矿物，以矿物学证据证实地球化学预测。一个成功的项目将为变质梯度随时间的演化提供新的线索，并支持或证伪我们的中心假设，即现代风格的低T/P梯度是在古元古代全球范围内建立的。