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Divergent behaviour of Th and U during anatexis: Implications for the thermal evolution of orogenic crust
Journal of Metamorphic Geology ( IF 3.4 ) Pub Date : 2019-02-07 , DOI: 10.1111/jmg.12469
Chris Yakymchuk 1 , Michael Brown 2
Affiliation  

Mobilization and migration of the heat‐producing elements (HPE) during anatexis is a critical process in the development of orogenic systems, the evolution of continental crust and the stabilization of cratons. In many crustal rocks the accessory minerals are the dominant hosts of Th and U, and the behaviour of these minerals during partial melting controls the concentrations of these elements in draining melt and residue. We use phase equilibrium modelling to evaluate if loss of melt saturated in the essential structural constituents of the accessory minerals can explain the concentrations of Th and U in residual metasedimentary migmatites and granulites along two well‐characterized crustal transects in the Ivrea zone, Italy and at Mt Stafford, Australia. While an equilibrium model of accessory mineral breakdown and melt loss approximates the depletion of U in the residual crust along both transects, it does not explain the relative enrichment of Th. We propose that the high Th concentrations in residual crust may be explained by either inhibition of monazite dissolution by kinetic factors or near‐peak growth of new high Th grains and overgrowth rims on undissolved monazite due to migration of melt through the orogenic crust. Retention of the HPE in the middle and deep orogenic crust may allow metasedimentary granulites to overcome the enthalpy barrier of melting to achieve ultrahigh temperature conditions and may be partly responsible for the slow cooling of many granulite terranes. Lastly, although the mantle was warmer and crustal heat production was higher in the past, peak temperatures and apparent thermal gradients of high‐temperature (HT)–ultrahigh temperature (UHT) granulite terranes have not decreased significantly since the Neoarchean. However, the pressure of HP granulite facies metamorphism has increased gradually from the Archean to the Phanerozoic, which suggests that the lithosphere became stronger as secular cooling of the mantle enabled plate collisions to form thicker orogens. Thus, as the lithosphere became stronger, the proportion of HT–UHT metamorphism associated with thin lithosphere and mantle heat has decreased, whereas the proportion associated with the formation of thick crust and radiogenic heat has increased.

中文翻译:

Th和U在麻醉期间的发散行为:对造山壳热演化的影响

厌食期间生热元素(HPE)的动员和迁移是造山系统发展,大陆壳演化和克拉通稳定的关键过程。在许多地壳岩石中,辅助矿物是Th和U的主要主体,这些矿物在部分熔融过程中的行为控制着这些元素在排出熔体和残渣中的浓度。我们使用相平衡模型来评估辅助矿物基本结构成分中熔体的饱和损失是否可以解释意大利伊夫雷亚地区和意大利伊夫雷亚地区两个特征明确的地壳断面残留的沉积沉积物的迁移辉石和花岗石中的Th和U的浓度。澳大利亚斯塔福德山。虽然辅助矿物分解和熔体流失的平衡模型近似于沿两个样带的残余地壳中U的消耗,但它不能解释Th的相对富集。我们认为,残余壳中的高Th浓度可以用动力学因素抑制独居石溶解或由于熔体通过造山壳的迁移而使新的高Th晶粒和未溶解独居石上的过度生长边缘接近峰值生长来解释。将HPE保留在中部和深部造山壳中,可能会使准沉积的粒岩克服熔融的焓垒,从而达到超高温条件,并且可能部分负责许多粒岩地层的缓慢冷却。最后,尽管过去地幔变暖,地壳热量产生较高,自新archarean以来,高温(HT)-超高温(UHT)花岗石地层的峰值温度和表观热梯度并未显着降低。但是,HP花岗岩相变质的压力从太古宙到生代地逐渐增加,这表明随着地幔的长期冷却使板块碰撞形成较厚的造山带,岩石圈变得更坚固。因此,随着岩石圈的变强,与岩石圈薄层和地幔热有关的HT-UHT变质比例降低,而与厚壳和放射源热形成相关的比例增加。高压麻粒岩相变质的压力从太古宙到生代界逐渐增加,这表明随着地幔的长期冷却使板块碰撞形成较厚的造山带,岩石圈变得更强。因此,随着岩石圈的变强,与岩石圈薄层和地幔热有关的HT-UHT变质比例降低,而与厚壳和放射源热形成相关的比例增加。高压麻粒岩相变质的压力从太古宙到生代界逐渐增加,这表明随着地幔的长期冷却使板块碰撞形成较厚的造山带,岩石圈变得更强。因此,随着岩石圈的变强,与岩石圈薄层和地幔热有关的HT-UHT变质比例降低,而与厚壳和放射源热形成相关的比例增加。
更新日期:2019-02-07
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