超高壓變質作用

超高壓變質作用（）是指在足以穩定柯石英(coesite)（二氧化硅的高壓多晶型物）的壓力下的變質過程。 研究超高壓 (UHP) 變質岩的形成和出露過程，對板塊構造、地殼的組成和演化是很重要的綫索。1984 年超高壓變質岩的發現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. 。徹底改變了我們對板塊構造的理解。在1984年之前，幾乎沒有人懷疑在大陸上的岩石會達到如此高的壓力。許多超高壓地區的形成歸因於微大陸或大陸邊緣的俯衝，所有超高壓地區的出露主要歸因於由低密度引起的浮力。雖然俯沖在低於10°C/km的低熱梯度下進行，但出露在10-30°C/km的高熱梯度下進行。

超高壓變質指在壓力≥27kbar (2.7GPa)，而能穩定柯石英的高壓環境的變質作用。根據指標礦物例如柯石英或金剛石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. 、或指標礦物組合，例如菱鎂礦+文石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 來判定。

超高壓變質作用在岩石學的指標礦物通常保存在榴輝岩中，包括柯石英、鑽石或鎂久石質（majoritic）石榴石。礦物組合也可用於識別超高壓岩石；這些組合包括菱鎂礦+文石。由於礦物隨壓力和溫度的變化而改變成分，因此礦物成分也可用於計算變質壓力和溫度；對於UHP榴輝岩，最好的地球氣壓計包括石榴石+斜輝石+K-白雲母和石榴石+斜輝石+藍晶石+柯石英/石英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. 。大多數超高壓岩石在800 °C和3GPa的峰值條件下變質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. 。大多數長英質超高壓岩石經歷了廣泛的退化变质作用，幾乎沒有或沒有超高壓記錄。通常，只有少數榴輝岩能保存已俯衝到地幔深處的咨詢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. 。

目前在全球20多処找到超高壓岩石，大多數都在歐亞大陸造山帶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. 。柯石英分佈較廣，鑽石較少，鎂久石質石榴石更少。僅產於稀有地區。最古老的超高壓地區為620MaJahn, 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] 。最年輕的為8MaBaldwin, 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. 。所有都以石英長石片麻岩為主，其中含有少量基性岩（榴輝岩）或超基性岩（含石榴石橄欖岩）。有些原岩是被動大陸邊緣的沉積物或裂谷火山序列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. 。

參考文獻

 分類:變質岩學