Glaucophane

Abstract: Multichronometric studies of the low-temperature eclogitic Tso Morari unit (Ladakh, India) place timing constraints on the early evolution of the northwest Himalayan belt. Several isotopic systems have been used to date the eclogitization and the exhumation of the Tso Morari unit: Lu-Hf, Sm-Nd, Rb-Sr, and Ar-Ar. A ca. 55 Ma age for the eclogitization has been obtained by Lu-Hf on garnet, omphacite, and whole rock from mafic eclogite and by Sm-Nd on garnet, glaucophane, and whole rock from high-pressure metapelites. These results agree with a previously reported U-Pb age on allanite, and together these ages constrain the subduction of the Indian continental margin at the Paleocene-Eocene boundary. During exhumation, the Tso Morari rocks underwent thermal relaxation at about 9 ± 3 kbar, characterized by partial recrystallization under amphibolite facies conditions ca. 47 Ma, as dated by Sm-Nd on garnet, calcic amphibole, and whole rock from metabasalt, Rb-Sr on phengite, apatite, and whole rock, and Ar-Ar on medium-Si phengite from metapelites. Ar-Ar analyses of biotite and low-Si muscovite from metapelites, which recrystallized at <5 kbar toward the end of the exhumation, show that the Tso Morari unit was at upper crustal levels ca. 30 Ma. These results indicate variable exhumation rates for the Tso Morari unit, beginning with rapid exhumation while the Indian margin subduction was still active, and later proceeding at a slower pace during the crustal thickening associated with the Himalayan collision.

Abstract: Mineral chemistry and overgrowth relationships in mafic schist from Vermont are shown to be sensitive indicators of pressure, temperature, and relative time and to be good chronicles of the Paleozoic history of this polymetamorphic terrane. Within the common assemblage, amphibole + chlorite + epidote + plagioclase +quartz+ Ti-phase ± carbonate ± K-mica ± Fe^(3+)-oxide, electron microprobe analyses show that increasing metamorphic grade (as defined by intercalated pelitic schist) is recorded by an increase in the edenite, glaucophane, and tschermakite contents of amphibole, in the anorthite content of plagioclase, and in the substitution of (Al^(VI),F^(2+), Ti), Al^(IV) for (Fe^(2+), Mg, Mn), Si in biotite, chlorite, and muscovite. With increasing pressure the glaucophane component of amphibole increases. For medium-pressure metamorphism the albite-oligoclase gap is in the garnet zone, where amphibole has between 1.2 and 1.8 formula proportion Al^(IV) and (Al^(VI) + Fe^(3+) + Ti + Cr). This gap is below the garnet isograd in low-pressure mafic schist where Al^(IV) and (Al^(VI) + Fe^(3+) + Ti + Cr) in the amphibole are both less than 0.6. Mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two Ordovician (Taconic) and two Devonian (Acadian) events. Silurian-Devonian mafic schist in northeastern Vermont records two periods of low-pressure, Devonian metamorphism. These events are expressed by medium-pressure and low-pressure metamorphism in the Cambrian and Ordovician rocks to the west and south. In the pre-Silurian rocks high-pressure and medium-pressure metamorphism is assigned to the Ordovician. The observed high-pressure metamorphism is confined to a 110 by 40 km area along the Green Mountain anticlinorium axis in north-central Vermont.