Quartz, mica, and amphibole exsolution from majoritic garnet reveals ultra-deep sediment subduction, Appalachian orogen
Quartz, mica, and amphibole exsolution from majoritic garnet reveals ultra-deep sediment subduction, Appalachian orogen
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DOI：
10.1126/sciadv.aay5178
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发表时间：
2020-03-01
期刊：
影响因子：
13.6
通讯作者：
Ague, J. J.
中科院分区：
文献类型：
--
作者：
Keller, D. S.;Ague, J. J.
金刚石和柯石英是超高压（UHP; >100公里深度）变质作用的经典指标，但它们在折返过程中容易重结晶。晶体取向的辉石和角闪石出溶lamicum在石榴石文件分解超高压majoritic石榴石原本稳定在金刚石级的条件下，但majoritic前体只被定量证明在镁铁质和超镁铁质岩石。此外，争论仍然存在，关于硅酸盐多晶石榴石击穿产生。我们提出了一种方法重建前体majoritic石榴石化学变沉积阿巴拉契亚片麻岩中含有石榴石保存同心区的晶体取向的层状，包括石英，角闪石，钠金云母。我们将此链接到新的石英石榴石晶体取向数据。结果表明，在>175公里的深度，石英和云母可以从石榴石中溶出，而多角体的前体是稳定的。大型超高压变质作用在欧洲加里东期形成的碰撞过程中的古大陆波罗的海和劳伦特;我们展示了超高压变质作用的特点，从微大陆-大陆收敛的连续和同时代的阿巴拉契亚造山带。
Diamond and coesite are classic indicators of ultrahigh-pressure (UHP; >100-kilometer depth) metamorphism, but they readily recrystallize during exhumation. Crystallographically oriented pyroxene and amphibole exsolution lamellae in garnet document decomposed supersilicic UHP majoritic garnet originally stable at diamond-grade conditions, but majoritic precursors have only been quantitatively demonstrated in mafic and ultramafic rocks. Moreover, controversy persists regarding which silicates majoritic garnet breakdown produces. We present a method for reconstructing precursor majoritic garnet chemistry in metasedimentary Appalachian gneisses containing garnets preserving concentric zones of crystallographically oriented lamellae including quartz, amphibole, and sodium phlogopite. We link this to novel quartz-garnet crystallographic orientation data. The results reveal majoritic precursors stable at >175-kilometer depth and that quartz and mica may exsolve from garnet. Large UHP terranes in the European Caledonides formed during collision of the paleocontinents Baltica and Laurentia; we demonstrate UHP metamorphism from the microcontinent-continent convergence characterizing the contiguous and coeval Appalachian orogen.