Thermodynamic constraints on the composition of orogenically thickened lower crust

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2022-06-09 DOI:10.1111/jmg.12679
Yi-Bing Li, Yu-Wen Wu, Bin Su, Si Chen, Qing-Hua Zhang, Yi Chen
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Abstract

Orogenically thickened lower crust is the key site of crustal differentiation, crustal deformation, and Moho modification. However, the composition of thickened lower crust is still highly debated. Here, we calculate a set of pseudosections with mafic lower crust compositions in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system. Our modelling results show that the maximum thickness of the mafic lower crust increases with the Moho temperature (TMoho). In addition, the lithologies of stable mafic crust are characterized by medium-pressure (MP) to high-pressure (HP) granulites at 40–50 km, HP granulites and garnet-omphacite granulites at 50–60 km, and garnet-omphacite granulites at 60–70 km. Under the Pamir geothermal conditions, mafic rocks with high SiO2 (>50.2 wt%), XMg (>0.70), XCa (>0.49), or low XAl (<0.11) could be stable at 70 km; however, only ~10% of global mafic granulite xenoliths lie within this compositional range. Further modelling indicates that if TMoho reaches 900–1000°C, neither the lower crust nor the upper mantle has significant strength relative to the upper crust and that only ~5–37% of mafic materials are gravitationally stable at 70 km. This implies that the base of doubly thickened (70 km) crust is dominated by intermediate-felsic rocks, consistent with the low Vp and Vp/Vs values seismically observed in young orogenic crustal roots. Thus, most mafic materials at >70 km could delaminate into the deep mantle. Our results provide insights on the formation of extremely thick crust with a predominantly intermediate-felsic base and the crustal thickness variation in continental collision zones.

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造山增厚下地壳组成的热力学约束
造山增厚下地壳是地壳分异、地壳变形和莫霍变质的关键部位。然而,增厚下地壳的组成仍有很大争议。在此,我们计算了na20 - cao - k20 - feo - mgo - al2o3 - sio2 - h2o - tio2 - o2 (NCKFMASHTO)体系中一组具有基性下地壳组成的伪截面。模拟结果表明,基性下地壳的最大厚度随着莫霍温度(TMoho)的升高而增大。稳定基性地壳岩性以40 ~ 50 km范围内的中压—高压麻粒岩、50 ~ 60 km范围内的高压麻粒岩和石榴石—辉石麻粒岩、60 ~ 70 km范围内的石榴石—辉石麻粒岩为特征。在帕米尔地热条件下,高SiO2 (>50.2 wt%)、XMg (>0.70)、XCa (>0.49)或低XAl (<0.11)的基性岩石在70 km处稳定存在;然而,全球基性麻粒岩捕虏体中只有约10%在这一成分范围内。进一步的模拟表明,当TMoho达到900-1000°C时,下地壳和上地幔相对于上地壳都没有明显的强度,只有~ 5-37%的基性物质在70 km处引力稳定。这表明双增厚(70 km)地壳基底以中长英质岩石为主,与地震观测到的年轻造山带地壳根的低Vp值和Vp/Vs值一致。因此,在70公里处的大多数镁质物质都可以分解到地幔深处。我们的研究结果提供了对以中长英质基底为主的极厚地壳的形成和大陆碰撞带地壳厚度变化的见解。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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