Winchite and the actinolite–glaucophane miscibility gap
Leake et al.1 have recently emphasized the apparent scarcity of winchite s.l., NaCa(Mg, Fe2+)4(Al, Fe3+) [Si8O22/(OH)2] (ref. 2), an amphibole intermediate in composition between the glaucophane–riebeckite series of high-pressure blueschists and the tremolite–actinolite series of lower-pressure greenschists. The rarity of winchite is considered consistent with the existence of a miscibility gap3,4 between these two amphibole series at the metamorphic P–T conditions normally attained in nature. Here we present optical and X-ray properties of Mg- and Al-rich winchite from Venezuela and draw attention to several other recent winchite analyses. In fact, as Schliestedt's5 new data on co-existing sodic and calcic amphiboles in blueschists show, only the present Mg–Al-rich winchite definitely lies within the above miscibility gap.
Metamorphic evolution of blueschists, greenschists, and metagreywackes in the Cretaceous Mt. Hibernia Complex (SE Jamaica)
A suite of samples collected mainly from river cobbles in the Mt. Hibernia Complex in SE Jamaica includes metamorphosed basaltic and clastic sedimentary rocks, which are interpreted here as part of a Cretaceous accretionary prism. The metabasic rocks occur as blueschists and greenschists, whereas the metasedimentary rocks are former greywackes s.l ., or psammopelites, and can now be characterized as phyllites. Eight characteristic mineral assemblages can be defined within the metabasic sample suite, involving amphibole (Na amphibole and/or actinolite), pumpellyite, lawsonite, epidote, chlorite, titanite, phengite, albite and quartz. Augite is an important relict phase of the magmatic source rocks. White mica, chlorite and quartz predominate in the phyllites in variable proportions, but Na amphibole, epidote, albite and titanite may also be important. Calculated P–T pseudosections show that the assemblage glaucophane–pumpellyite–chlorite occurs only in a restricted field along the lower P–T limit of the blueschist facies. Pseudosections also indicate that peak P–T conditions for the blueschist samples studied here lie in the range 6.5–7.2 kbar and 310–340°C, that is at the upper stability limit of pumpellyite and lawsonite, and in the range 6.4–7.8 kbar and 340–380°C for greenschist samples. The P–T range for phyllites overlaps with both. Thus, blueschists and greenschists represent comparable pressure but different temperature conditions of formation. Literature data on the mapped distribution of metabasic rock types suggests an isobaric metamorphic field gradient in the Mt. Hibernia Complex, with temperatures increasing slightly but significantly from blueschist to greenschist zones. We interpret this gradient as reflecting either increasing temperatures in the accretionary complex away from the subducting slab or thermal re-equilibration in a cooling subduction zone. Sodium-Ca amphiboles, predominantly winchite, are commonly observed as rims around Na amphibole and formed together with albite porphyroblasts during decompression. Comparison with published information on the miscibility gap between Na and Ca amphiboles shows that many of these compositions fall within this gap for temperatures around 350°C. Thus, these amphiboles either grew metastably below the critical temperature of the gap, or some as yet undefined parameter must be responsible for drastically altering the shape of the gap. Published descriptions of deformation in the Mt. Hibernia Complex show that coaxial pure shear initially predominates, as is common in accretionary complexes; however, a late shearing event, postulated here to be caused by the collision between the Great Arc of the Caribbean and the continental margin of North America at 75–70 Ma, is held responsible for the rapid exhumation of the combined blueschist–greenschist sequence.
Petrochemical study of coexisting minerals from low-grade schists, Eastern Shikoku, Japan☆
Ten isofacial blueschist facies rocks were investigated from the Kōtu-Bizan District, eastern Shikoku, Japan, previously mapped by Iwasaki (1963). Twenty of the constituent minerals have been newly analysed, 17 by conventional chemical methods, and 3 by electron microprobe techniques. Combined with 15 previously determined wet chemical analyses of minerals from the same rocks, mineralogical data include a total of: 8 sodic amphiboles; 2 sodic pyroxenes; 9 white micas; 4 chlorites; 6 garnets; and 6 epidotes. Unit cell dimensions and optical properties for the newly analysed minerals have been determined, and available constants for all 35 minerals are tabulated. 
 
Chemographic phase relations indicate that the investigated rocks crystallized under physical conditions transitional between greenschist and blueschist facies, but within the latter facies. Intercalations of actinolite-bearing and sodic amphibole-bearing rocks probably result from differences in oxidation state, as demonstrated by graphic analysis; this conclusion agrees with an hypothesis previously proposed by Iwasaki. 
 
Element partitioning among coexisting phases has been investigated; these are the lowest-grade metamorphic rocks for which such a study has been undertaken up to the present time. Aluminum, iron, magnesium and manganese appear to show systematic distributions between the mineral pairs sodie amphibole-white mica, sodic amphibole-epidote, white mica-chlorite and white mica-epidote. Non-systematic partitioning of these elements between garnets and the other minerals may be explained in any of three ways: 
1. 
(1) at least some of the garnets are metastable in the present phase assemblages 
 
2. 
(2) manganese in garnet exerts a profound influence on the partitioning of ferrous iron and magnesium 
 
3. 
(3) some of the garnets have reached their Mg solubility limit. Petrographic relations support but do not prove the hypothesis of garnet metastability. Mn+++ appears to be diadochic with aluminum and ferric iron, Mn++ with ferrous iron and magnesium in garnets and epidotes, but not in the other minerals studied. Distributions of manganese, calcium and alkalis show apparently systematic equilibrium relations, and pronounced fractionation. Chemical distributions of the elements are explicable on the basis of valencies, ionic radii and sizes of the various structural sites. 
 
 
 
 
The observed systematic partitioning of elements among coexisting minerals implies that Kōtu-Bizan blueschists achieved a close approach to chemical equilibrium during the metamorphism, at least in hand specimen sized volumes of rock.