Phase equilibria modeling of anatexis during ultra-high temperature metamorphism of the crust
Phase equilibria modeling of anatexis during ultra-high temperature metamorphism of the crust
复制标题
地壳超高温变质作用过程中深熔相平衡模拟
作者：
Guangyu Huang;Jinghui Guo;Richard Palin
超高温麻粒岩相变质作用常见于富镁铝岩石中。这种岩性被认为是以碎屑锆石的存在为基础的变质沉积岩。然而，是否需要变质沉积原岩来形成诊断矿物组合，以及在埋藏和加热过程中损失多少熔体以达到超高温条件，根据前进压力-温度路径的不同有很大不同。本研究采用热力学模拟、副矿物模拟和痕量元素模拟相结合的方法，对开放系统条件下变质岩平均组成和大洋中脊玄武岩(MORB)平均组成进行了模拟。结果表明，在初始饱和水的泥质岩和玄武岩中，在达到超高温条件之前分别发生了至少三次和两次熔体损失事件。在这些演化过程中，将分别产生体积分数为22-27体积%的S型花岗岩和体积分数为12-17%的I型花岗岩，熔体提取作用导致变质岩残渣中铝和镁的含量增加。然后，矿物组合蓝宝石+石英被允许在较高的品位形成；然而，无论熔体损失多少，变质玄武岩都不会形成这些超高温诊断组合。在平衡熔融过程中，超高温变质作用引起的地壳分异作用将显著减少熔体贫化残渣中的生热元素(HPE)的数量，从而逐渐降低岩石的产热能力。然而，实际上，在峰期变质过程中可能会形成新的独居石颗粒，一些独居石颗粒会被斑晶遮挡，从而导致加热过程中残留物中Th的增加。尽管如此，我们认为，鉴于Th的产热率远低于美国，产热效率仍将下降。考虑到一些产热率较高的花岗岩侵位到麻粒岩地体中，我们认为放射性热产可能是超高温变质作用的贡献动力，但这是不够的。产生超高温麻粒岩所需的大部分热量必须来自混合来源，例如来自地幔的平流热量、来自附近岩浆侵入的传导热量、放射性热产生和导管剪切变形。
Ultra-high temperature (UHT) granulite-facies metamorphism is commonly identified in Mg–Al-rich rocks. Many.such lithologies are thought to be metasedimentary rocks based on the presence of detrital zircon. However,.whether a metasedimentary protolith is required to form diagnostic mineral assemblages and how much melt is.lost during burial and heating to reach UHT conditions varies significantly according to the prograde pressure–.temperature path. In this study, integrated thermodynamic modeling, accessory mineral modeling, and trace.element modeling has been conducted on an average metapelite composition and average mid-ocean ridge basalt.(MORB) composition under open-system conditions. The results show at least three and two melt loss events.occur before reaching UHT conditions in initially water-saturated pelitic and basaltic rocks, respectively. Around.22–27 vol% S-type granite and 12–17 vol% I-type granite would be produced during these evolutions, respectively..Melt extraction leads to an increase of Al and Mg in metapelite residua. Mineral assemblage sapphirine +.quartz is then allowed to form at higher grades; however, metabasalt does not form these UHT diagnostic assemblages,.regardless of the amount of melt lost. During equilibrium melting, crustal differentiation induced by.UHT metamorphism would significantly reduce the amount of heat-producing elements (HPEs) in the meltdepleted.residuum, which progressively decreases a rock's heat production capacity. In reality, however, new.monazite grains likely form during peak metamorphism and some monazite grains would be shielded by porphyroblasts,.resulting in an increase of Th in the residua during heating. Even so, we suggest the efficiency of.heat production would still decrease, given the heat production rate of Th is much lower than U. Considering.some granitic rocks with relatively high heat production are emplaced into the granulite terrane, we suggest that.radioactive heat production may be a contributing driving force for UHT metamorphism; however, it is not.sufficient. Most heat required to generate UHT granulites must come from hybrid sources, such as advected heat.from the mantle, conducted heat from nearby magmatic intrusions, radioactive heat production, and ductile.shear deformation.
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