Coexisting Sodic and Calcic Amphiboles from High-Pressure Metamorphic Belts and the Stability of Barroisitic Amphibole*

Content Provider	Semantic Scholar
Author	Ernst, W. Gary
Copyright Year	1979
Abstract	Compositions 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 AI i~ per formula unit, whereas AI V1 is very high; a situation reversed among calcic amphiboles. The Na + Ca contents ofglaucophanes are strongly clustered around the sum of 2.o (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 ~ MESOZOIC and younger blueschist terranes discontinuously border the Pacific Ocean and form portions of the Alp ine-Tethyan-Himalayan orogen. Dense, H20-bear ing metamorphic mineral assemblages developed in such belts require geothermal gradients about 5 I5 ~ (Ernst, 1973a; Newton and Fyfe, I976. ). Such high-pressure, lowtemperature regimes are associated with downgoing lithospheric slabs (Hasebe et al., I97O; Oxburgh and Turcotte, 197o, 1971 ; Toksrz et al., 197 L i973; Griggs, r97z; Turcotte and Schubert, I973). Hence glaucophane-schist belts are commonly regarded as products o f subduction zone metamorphism and may delimit the ancient convergent plate boundaries (Dewey and Bird, I97O; Ernst, 197o, I97Ia, I975). Although metasedimentary rocks predominate in many subduction zone complexes, metabasalts are widespread. The high-P, low-T recrystallization of the latter has given calcic, sodic calcic, and sodic amphibole-bearing assemblages. This study focuses on the compositions of these coexisting ctinoamphiboles in metabasaltic units from four different blueschist terranes: east-central Shikoku, Japan; W. California, USA: Valtournanche, W. Alps; and W. Liguria, Italy. Prograde and retrograde metamorphic parageneses and the chemistries o f the coexisting amphiboles in the different areas are examined and estimates made of the physical conditions attending the formation of sodic calcic amphiboles in metabasaltic rocks.
Starting Page	269
Ending Page	278
Page Count	10
File Format	PDF HTM / HTML
DOI	10.1180/minmag.1979.043.326.09
Volume Number	43
Alternate Webpage(s)	http://www.minersoc.org/pages/Archive-MM/Volume_43/43-326-269.pdf
Alternate Webpage(s)	https://doi.org/10.1180/minmag.1979.043.326.09
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article