Keller D S, Ague J J
Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, CT 06520-8109, USA.
Yale Peabody Museum of Natural History, Yale University, New Haven, CT 06511, USA.
Sci Adv. 2020 Mar 13;6(11):eaay5178. doi: 10.1126/sciadv.aay5178. eCollection 2020 Mar.
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.
金刚石和柯石英是超高压（UHP；深度≥100千米）变质作用的经典指示矿物，但它们在折返过程中很容易再结晶。石榴石中具有晶体取向的辉石和角闪石出溶片层记录了原本在金刚石级条件下稳定的分解超硅质UHP镁铁榴石，但仅在镁铁质和超镁铁质岩石中定量证明了其镁铁榴石前体。此外，关于镁铁榴石分解会产生哪些硅酸盐仍存在争议。我们提出了一种方法，用于重建阿巴拉契亚变质沉积片麻岩中前体镁铁榴石的化学组成，这些片麻岩中的石榴石保留了包括石英、角闪石和钠金云母在内的具有晶体取向片层的同心区域。我们将此与新的石英 - 石榴石晶体取向数据联系起来。结果揭示了在≥175千米深度稳定的镁铁榴石前体，并且石英和云母可能从石榴石中出溶。欧洲加里东造山带中的大型超高压地体是在波罗的古大陆和劳伦古大陆碰撞期间形成的；我们证明了超高压变质作用源自相邻且同期的阿巴拉契亚造山带所特有的微大陆 - 大陆汇聚作用。
