超高压变质作用
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超高压变质作用（英语：ultra-high-pressure metamorphism）是指在足以稳定柯石英(coesite)（二氧化硅的高压多晶型物）的压力下的变质过程。 研究超高压 (UHP) 变质岩的形成和出露过程，对板块构造、地壳的组成和演化是很重要的线索。1984 年超高压变质岩的发现[1][2]。彻底改变了我们对板块构造的理解。在1984年之前，几乎没有人怀疑在大陆上的岩石会达到如此高的压力。许多超高压地区的形成归因于微大陆或大陆边缘的俯冲，所有超高压地区的出露主要归因于由低密度引起的浮力。虽然俯冲在低于10°C/km的低热梯度下进行，但出露在10-30°C/km的高热梯度下进行。
超高压变质指在压力≥27kbar (2.7GPa)，而能稳定柯石英的高压环境的变质作用。根据指标矿物例如柯石英或金刚石[3]、或指标矿物组合，例如菱镁矿+文石[4] 来判定。
超高压变质作用在岩石学的指标矿物通常保存在榴辉岩中，包括柯石英、钻石或镁久石质（majoritic）石榴石。矿物组合也可用于识别超高压岩石；这些组合包括菱镁矿+文石[4]。由于矿物随压力和温度的变化而改变成分，因此矿物成分也可用于计算变质压力和温度；对于UHP榴辉岩，最好的地球气压计包括石榴石+斜辉石+K-白云母和石榴石+斜辉石+蓝晶石+柯石英/石英[5]。大多数超高压岩石在800 °C和3GPa的峰值条件下变质[6]。大多数长英质超高压岩石经历了广泛的退化变质作用，几乎没有或没有超高压记录。通常，只有少数榴辉岩能保存已俯冲到地幔深处的咨询[7][8]。
目前在全球20多処找到超高压岩石，大多数都在欧亚大陆造山带[9]。柯石英分布较广，钻石较少，镁久石质石榴石更少。仅产于稀有地区。最古老的超高压地区为620Ma[10]。最年轻的为8Ma[11]。所有都以石英长石片麻岩为主，其中含有少量基性岩（榴辉岩）或超基性岩（含石榴石橄榄岩）。有些原岩是被动大陆边缘的沉积物或裂谷火山序列[12][13]。
参考文献
[编辑]- ^ Chopin, C., 1984, Coesite and pure pyrope in high-grade blueschists of the western Alps: a first record and some consequences: Contributions to Mineralogy and Petrology, v. 86, p. 107–118.
- ^ Smith, D. C., 1984, Coesite in clinopyroxene in the Caledonides and its implications for geodynamics: Nature, v. 310, p. 641–644.
- ^ Massonne, H. J., and Nasdala, L., 2000, Microdiamonds from the Saxonian Erzgebirge, Germany: in situ micro-Raman characterisation: European Journal of Mineralogy, v. 12, p. 495-498.
- ^ 4.0 4.1 Klemd, R., Lifei, Z., Ellis, D., Williams, S., and Wenbo, J., 2003, Ultrahigh-pressure metamorphism in eclogites from the western Tianshan high-pressure belt (Xinjiang, western China); discussion and reply: American Mineralogist, v. 88, p. 1153-1160
- ^ Ravna, E. J. K., and Terry, M. P., 2004, Geothermobarometry of phengite-kyanite-quartz/coesite eclogites: Journal of Metamorphic Geology, v. 22, p. 579-592.
- ^ Hacker, B. R., 2006, Pressures and temperatures of ultrahigh-pressure metamorphism: Implications for UHP tectonics and H2O in subducting slabs.: International Geology Review, v. 48, p. 1053-1066.
- ^ Hacker, B. R., Kelemen, P. B., and Behn, M. D., 2011, Differentiation of the continental crust by relamination: Earth and Planetary Science Letters, v. 307, p. 501-516.
- ^ Walsh, E. O., and Hacker, B. R., 2004, The fate of subducted continental margins: Two-stage exhumation of the high-pressure to ultrahigh-pressure Western Gneiss complex, Norway: Journal of Metamorphic Geology, v. 22, p. 671-689.
- ^ Liou, J. G., Tsujimori, T., Zhang, R. Y., Katayama, I., and Maruyama, S., 2004, Global UHP metamorphism and continental subduction/collision: The Himalayan model: International Geology Review, v. 46, p. 1-27.
- ^ Jahn, B. M., Caby, R., and Monie, P., 2001, The oldest UHP eclogites of the World: age of UHP metamorphism, nature of protoliths and tectonic implications: Chemical Geology, v. 178, p. 143-158.13]
- ^ Baldwin, S. L., Webb, L. E., and Monteleone, B. D., 2008, Late Miocene coesite-eclogite exhumed in the Woodlark Rift: Geology, v. 36, p. 735-738.
- ^ Oberhänsli, R., Martinotti, G., Schmid, R., and Liu, X., 2002, Preservation of primary volcanic textures in the ultrahigh-pressure terrain of Dabie Shan: Geology, v. 30, p. 609–702.
- ^ Hollocher, K., Robinson, P., Walsh, E., and Terry, M., 2007, The Neoproterozoic Ottfjället dike swarm of the Middle Allochthon, traced geochemically into the Scandian hinterland, Western Gneiss region, Norway: American Journal of Science, v. 307, p. 901-953.