引用本文: 樊银龙, 张立飞, 李忠海, 焦淑娟, 王杨, 邹雷. 2024. 显生宙碰撞造山带超高温变质作用的加热机制：来自二维数值模拟的约束. 岩石学报, 40(4): 1044-1060. doi: 10.18654/1000-0569/2024.04.02

大量的岩石学证据表明: 碰撞造山带中常发育900~1100℃的超高温变质作用。然而, 碰撞造山带中如何出现如此极端的超高温条件仍然存在争议。为了更好地理解超高温变质作用加热机制和碰撞造山带中主要热源的相对贡献, 我们建立了一系列高分辨率二维热-动力学模型, 借此探讨了俯冲大陆岩石圈密度亏损程度、大陆地壳放射性生热率和大陆汇聚速率等因素对碰撞造山过程中超高温变质主要热源的影响。当大陆岩石圈密度亏损(Δ ρ = ρ _{软流圈地幔} - ρ _{岩石圈地幔} )大于50kg/m ³ 时, 有利于发生大陆平板俯冲, 软流圈地幔无法上涌为地壳物质提供热源; 此时, 具有较高放射性生热率(>3μW/m ³ )的地壳可以发生"浅俯冲-折返"型超高温变质作用。而当大陆岩石圈密度亏损小于10kg/m ³ 时, 大陆上地壳在深俯冲阶段首先发生超高压榴辉岩相变质作用, 随后伴随着大陆岩石圈地幔后撤和软流圈上涌, 进而出现以异常高的地幔热流加热为主的"深俯冲-折返"型超高温变质作用。此外, 较低的大陆汇聚速率(< 1cm/yr)更有利于"深俯冲-折返"型超高温变质作用的产生。将数值模拟结果与特提斯构造域的变质岩石数据和地球物理观测进行对比, 我们认为在现今板块构造体制下, 由具有密度亏损程度较高的大陆岩石圈平俯冲有利于"浅俯冲-折返"型超高温变质作用的发生, 而由密度亏损程度较低的大陆岩石圈俯冲可能导致"深俯冲-折返"型超高温变质作用的产生。

超高温变质作用 碰撞造山带 大陆岩石圈密度 特提斯构造域 Abstract:

Many lines of petrological evidence show that ultra-high temperature (UHT) metamorphism of 900~1100℃ often developed in collisional orogens. However, how such extreme ultra-high temperature conditions arose in collision orogens is still a matter of debate. In order to better understand the heating mechanism of UHT metamorphism and the relative contribution of the main heat sources in collision orogens, we have established a series of high-resolution 2-D numerical models to investigate the influence of several key factors such as the degree of density depletion of the continental lithosphere, the radioactive heat production (RHP) of the continental upper crust and the rate of continental convergence on the main heat sources of ultrahigh temperature metamorphism. When the density depletion of a continental lithosphere (Δ ρ = ρ _{asthenosphere} - ρ _lithosphere ) is greater than 50kg/m ³ , the flat subduction of the continental lithosphere is generally resulting, and the asthenosphere mantle cannot upwell to provide heat source for the continental crust. With a higher RHP (>3μW/m ³ ), the continental upper crust can undergo "shallow subduction-exhumation" type UHT metamorphism. However, when the density depletion of the continental lithosphere is less than 10kg/m ³ , the continental upper crust undergoes ultra-high pressure eclogite facies metamorphism during the deep subduction stage, followed by mantle retreat and asthenosphere upwelling, and then "deep subduction-exhumation" type ultra-high temperature metamorphism dominated by abnormally high mantle heat flow heating. In addition, the lower continental convergence rate (< 1cm/yr) is more favorable to the generation of "deep subduction-exhumation" type UHT metamorphism. By comparing the numerical modelling results with the metamorphic rock data and geophysical observations in the Tethyan tectonic domain, we conclude that, under the present plate tectonic system, the flat subduction of the continental lithosphere with higher density depletion is favorable to the occurrence of "shallow subduction-exhumation" type UHT metamorphism; while the subduction of a continental lithosphere with lower density depletion may lead to "deep subduction-exhumation" type UHT metamorphism.

Key words: Ultra-high temperature metamorphism Collisional orogens Density of continental lithospheric mantle Numerical modelling Tethyan tectonic domain 注： ρ ₀ 、 k 、 T _solidus 、 T _liquidus 、 H _L 、 H _r 、 C _p 、 φ ₀ 分别代表：物质参考密度、热传导系数、固相线温度、液相线温度、熔融潜热、放射性生热、等压热容、塑性流变的有效内摩擦角；
^a 粘滞性流变参数见 表 2 ；
^b T S1=889+17900/( P +54)+20200/( P +54) ² , at P ＜1200MPa; or 831+0.06 P , at P ＞1200MPa. T L1=1262+0.09 P ；
^b T S2=973-70400/( P +354)+778×105/( P +354) ² , at P ＜1600MPa; or 935+0.0035 P +0.0000062 P ² , at P ＞1600MPa. TL 2=1423+0.105 P ；
^c k =[ k ₀ + k ₁ /( T +77)]exp(0.00004 P )；
^d 参考文献1-5分别为 Turcotte and Schubert (2002) , Bittner and Schmeling (1995) , Clauser and Huenges (1995) , Ranalli (1995) , Schmidt and Poli (1998) Kaneko Y, Katayama I, Yamamoto H, Misawa K, Ishikawa M, Rehman HU, Kausar AB and Shiraishi K. 2003. Timing of Himalayan ultrahigh-pressure metamorphism: Sinking rate and subduction angle of the Indian continental crust beneath Asia. Journal of Metamorphic Geology, 21(6): 589-599 doi: 10.1046/j.1525-1314.2003.00466.x Figure 1.

Age and depletion architecture of present continental lithospheric mantle formed by different mechanisms (modified after Wang et al. , 2022b )

Figure 2.

Initial model configuration and boundary conditions

Figure 3.

Model results of "deep subduction-exhumation" type UHT metamorphism

Figure 4.

Model results of "shallow subduction-exhumation" type UHT metamorphism

Figure 5.

Model results with different degrees of continental lithospheric mantle density depletion (Δ ρ = ρ _{asthenosphere} - ρ _lithosphere )

Figure 6.

Model results with different density structures of lithospheric mantle

Figure 7.

Model results with different radiogenic heat production of felsic crust

Figure 8.

Model results with different thickness of felsic crust

Figure 9.

Models evolution of continental convergence termination with density depletion of 10kg/m ³

Figure 10.

Models evolution of continental convergence termination with density depletion of 70kg/m ³

Figure 11.

Summary of numerical model results

Figure 12.

The relationship between temperature and time during the formation of different types of ultra-high temperature metamorphism

Figure 13.

Mechanical analysis of subducted continental lithosphere

Figure 14.

Dynamic models of different types of ultra-high temperature metamorphism