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Discovery of clinoenstatite in garnet pyroxenites from the Dabie-Sulu ultrahigh-pressure terrane, east-central China

¹ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.
² Department of Marine Resources, National Sun Yat-sen University, Kaohsiung 80424, Taiwan, R.O.C.
³ Department of Geosciences, National Taiwan University, Taipei 106, Taiwan, R.O.C.

Correspondence: ^* E-mail: zhang{at}pangea.stanford.edu

Transmission electron microscopy (TEM) reveals that some enstatites from garnet-pyroxenites in the Chinese Dabie-Sulu ultrahigh-pressure (UHP) terrane consist of intimate intergrowths of orthoenstatite (OREN) and clinoenstatite (CLEN), and some grains of pyroxene (MgFe)₂Si₂O₆ exhibit inclined extinction. These garnet pyroxenites are enclosed in coesite-bearing eclogite or interlayered with harzburgite and omphacitite. They contain garnet (Prp₅₄ to Prp₇₃), "enstatite" (En₈₅ to En₉₂), magnesite, and titanomagnetite (Sulu sample) or clinohumite (Dabie sample), and recrystallized at about 4.0–6.5 ± 0.2 GPa, ~750 ± 50 °C. All clinoenstatite lamellae in the host orthoenstatite have even numbers of 9 Å (100) fringes without twins, and are oriented parallel to (100) of the host. The lamellae and host phases have nearly identical (MgFe)₂Si₂O₆ compositions. The thickest observed CLEN lamellae are ~0.4 µm. Clinoenstatites from Sulu have cell parameters: a = 9.67(7) Å, b = 8.88(6) Å, c = 5.22(3) Å, ß = 107.7(2)°, and V = 427(8) ų, and those from Dabie: a = 9.53(6) Å, b = 8.88(6) Å, c = 5.24(3) Å, ß = 107.0(2)°, and V = 424(8) ų. The TEM electron-diffraction patterns indicate that both clinoenstatites possess P2₁/c structures and are the low-P polymorph of clinoenstatite. The host orthoenstatites have cell dimension: a = 18.32(12) Å, b = 8.88(6) Å, c = 5.20(3) Å for the Sulu sample, and a = 18.24 (12) Å, b = 8.88(6) Å, c = 5.18(3) Å for the Dabie sample. Clinoenstatites in garnet pyroxenites from the Dabie-Sulu UHP terrane may have formed either by inversion from OREN, or by the displacive transformation from high-P clinoenstatite during decompression.

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