Coexisting sodic and calcic amphiboles from high-pressure metamorphic belts and the stability of barroisitic amphibole

Abstract

SummaryCompositions of glaucophanes and actinolite-hornblende solid solutions occurring in chemically similar metabasaltic rocks from blueschist terranes in east-central Shikoku, W. California, Valtournanche (W. Alps), and W. Liguria are compared. Chemical contrasts among coexisting Na and Ca amphibole pairs, which reflect disparate P-T histories under the presumed attendance of local equilibrium, include: Na contents are rather high among barroisitic hornblendes from the western and Ligurian Alps, as well as among high-grade tectonic blocks from California; in contrast, actinolitic amphiboles from both lower-grade Franciscan tectonic blocks and in situ schists and the blueschists of Shikoku are impoverished in Na relative to blue-green hornblendes. Sodic amphiboles contain less than 0. 5 Aliv per formula unit, whereas Alvi is very high; a situation reversed among calcic amphiboles. The Na + Ca contents ofglaucophanes are strongly clustered around the sum of 2.0 (i.e. A site vacant) whereas calcic amphiboles have a wider range with the A site variably occupied. No solvus has been detected within either sodic or calcic amphiboles under blueschist facies conditions. For low-grade metabasaltic parageneses, a miscibility gap separates these two amphibole groups; at relatively high grade such compositions have sodic calcic amphiboles of barroisitic type; this may mean that glaucophane + hornblende assemblages are metastable, accounting for textural relations indicating that the sodic amphibole typically did not grow at the same time as the barroisite. Ti, Mn, and K appear to be concentrated in calcic amphibole compared to coexisting glaucophane, probably in the M2, M4, and A sites, respectively.Contrasts in coexisting amphibole tie lines are thought to be a consequence of the fact that the parageneses of Shikoku and California reflect high P and very high P prograde P-T paths respectively, whereas those from Valtournache and W. Liguria show evidence of decompression recrystallization (or back-reaction) of high P (i.e. eclogitic) protoliths. Comparison of the inferred physical conditions operating during the production of these four contrasting paragenetic sequences allows the provisional assignment of a P-T stability region for barroisitic amphibole in metabasaltic rocks as: P 4–5 kb at c. 350°; P 5–7 kb at c. 450 °C.

Triassic blueschists and eclogites from northwest Turkey: vestiges of the Paleo-Tethyan subduction

“…Because of the immiscibility between the sodic and sodic -calcic amphiboles (e.g., Toriumi, 1974;Brown, 1977;Ernst, 1979), the X Ca (Ca/(Ca + Na)) of the amphibole jumps from less than 0.06 in the sodic amphiboles to f 0.5 in the sodiccalcic amphiboles with a concomitant sharp increase in the tetrahedral aluminium. The stable mineral assemblage in the epidote -amphibolites is barroisite + ch- lorite + epidote + albite + phengite + quartz F garnet.…”

Alkaline intrusions in a near-trench setting, Franciscan Complex, California: Constraints from geochemistry, petrology, and 40Ar/39Ar chronology

“…Toward the rims of many kaersutites Ti, Al, Ca decrease whereas Na, Fe, Si increase. This reflects a continous transition from calcic to sodic-calcic barroisitic amphibole and is attributed to deuteric alteration caused by water-rich fluids from the crystallizing magma, although barroisitic amphibole has also been considered a high-pressure meatorphic phase (Ernst, 1979;Evans, 1990). …”

Interplay between deformation and metamorphism during eclogitization of amphibolites in the Sesia–Lanzo Zone of the Western Alps

“…P-T diagram showing the P-T conditions inferred by mineral compatibilities in hornblendites, amphibole-bearing eclogites, coronitic eclogitized amphibolites, and eclogites from Sesia-Lanzo Zone. Legend: Bar/Act/Hbl transitions after Ernst (1979); amphibole (Amp-out) and plagioclase (Pl-out) breakdown, garnet appearance (Grt-in), and wet solidus (solidus) on Ol-normative basalt-H 2 O system are after Liu et al (1996); omphacite (Omp), chlorite (Chl), and lawsonite (Lws) appearance and amphibole breakdown (Amp) on synthetic basalt are after Poli and Schmidt (1995) and coesite/quartz transition after Chopin (1984). Dotted lines (at 550° and 650°C) represent the T Max range for Sesia-Lanzo mafic rocks taken from the wide literature (Compagnoni 1977;Hy 1984;Lardeaux et al 1982;Castelli 1991;Zucali et al 2002Zucali et al , 2004.…”