Oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahighpressure (UHP) metamorphism. The presence of apparently exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca.Eskola component (Ca0.5⃞0.5AlSi2O6, CaEs) that is presumed to be stable only at UHP conditions. Eclogite from the Eastern Blue Ridge, North Carolina, contains clinopyroxene (Jd20CaTs5Ac5CaEs0Di65Hd5) with oriented needles of quartz and calcic amphibole that appear to have exsolved together. The quartz + amphibole intergrowths are surrounded by 1.5 µm haloes of neoformed pyroxene (Jd10CaTs10Ac5CaEs0Di70Hd5). The modes of quartz, amphibole, and clinopyroxene haloes were determined using BSE images, and reintegrated with the host clinopyroxene. Viewing the quartz and amphibole needles down the c-axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections. Reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no CaEs component. Clinopyroxene with CaEs solid solution has been repeatedly synthesized at UHP conditions. However, examination of the phase equilibria usually cited as evidence for CaEs stability at conditions of ≥25 kbar shows that clinopyroxene with 10 mol% CaEs is stable well within the quartz field, and provides a pressure minimum similar to the albite = jadeite + quartz barometer. Exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the Blue Ridge eclogite that lacks coesite or other evidence for UHP metamorphism. The presence of a diluted (5.10%) CaEs component in clinopyroxene does not require UHP conditions.
The Adirondack Mountains, NY are ideal for studying melting and migmatites in conformable granitic leucosomes cutting melanocratic metasediments. Leucocratic bands range in thickness from mm- to dm-scale, and have average quartz:K-spar:plagioclase norms of 46:16:38 in the upper amphibolite facies NW Lowlands and 26:51:23 in the granulite facies SE Highlands. To determine whether these leucocratic bands were local melts of surrounding melanosomes or externally-derived intrusions, we have correlated cathodoluminescence (CL) imaging, in situ U-Pb geochronology by SHRIMP, in situ zircon δ18O measurements by CAMECA IMS 1280 ion microprobe, and metamorphic garnet δ18O measurements by laser fluorination at 9 locations. CL imaging indicates three populations of zircons in both regions: 1. relatively featureless rounded 'soccer balls' (metamorphic), and rhythmically zoned (igneous) cores truncated by either 2. discordantly zoned (igneous) or 3. unzoned (metamorphic) rims. The U-Pb ages confirm CL classification as either 'igneous' or 'metamorphic' and determine the timing of different events. Typical δ18O ion microprobe spot-to-spot reproducibility of zircon standards is ±0.14‰ (1SD). For zircons from 3 leucosomes from the NW, igneous cores average 7.7±2.2‰ (1SD, VSMOW, n=5), a single igneous rim is 8.2‰, and metamorphic rims and whole grains average 10.1±1.9‰ (n=14). In corresponding melanosomes, igneous zircon rims average 8.4±1.4‰ (n=3) and garnets average 10.5±0.5‰. Average zircon rim age is 1208±33Ma, while average metamorphic age is 1165±58Ma. For zircons from 6 leucosomes from the SE, igneous cores average 8.2±2.1‰ (n=7) and igneous rims average 11.6±0.6‰ (n=4); metamorphic rims and grains average 10.1±1.4‰ (n=36). In corresponding melanosomes, igneous zircon rims average 11.7±0.04‰ (n=2), while garnets average 11.5±1.4‰. Average zircon rim age is 1103±63Ma, while average metamorphic age is 1132±80Ma. Values of 'igneous' δ18O in leucocratic layers are unusually high for plutonic rocks, especially in the SE. These high δ18O values (>10‰) cannot represent nearby magmas and indicate melting of surrounding metapelites. Metamorphism and anatexis occurred concurrently, and dehydration by melting at 1.2-1.1Ga lead to low water activity during Ottawan granulite metamorphism at ~1050Ma.
We identify olivine grains with compositions up to Fo99·8, which are found in multiple primitive basaltic lava flows from a monogenetic volcano in the Big Pine Volcanic Field, California, USA. In this study, we show that the forsterite in these basalts formed by subsolidus recrystallization in a high-fO2 environment. Olivine compositions are bimodal, with flows having either all normal compositions (Fo74·9–94·4) or highly forsteritic (Fo97·2–99·8) compositions. In many grains, the subhedral forsteritic olivine has a hematite and clinopyroxene rim, and internal parallel-oriented planes of hematite, clinopyroxene and orthopyroxene. Results of isotopic, chemical, crystallographic, petrographic and mineralogical analyses show that the forsterite formed through subsolidus oxidation of olivine phenocrysts. The forsteritic olivines generally occur in the thinner flows. We infer that a rapidly emplaced sequence of thin, vesicular, spatter-fed flows allowed the original olivine phenocrysts to become repeatedly reheated while exposed to air. Our study required sampling each flow, so it was difficult to avoid the altered portions of the thinner flows. Other studies would tend to avoid such flows, which may account for why such forsteritic olivines have not been more widely recognized.
The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet-clinopyroxene-rutile-hornblende-plagioclase-quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite ¼ grossular + rutile + quartz + H2O) yield pressures (6-10 kbar, 400-600 � C and 8-12 kbar 450-680 � C) at or below the minimum peak conditions from matrix phases (10-13 kbar at 600-800 � C). Inclusions of hornblende, biotite and quartz give higher pressures (13-16 kbar and 630-660 � C). Early matrix pyroxene is partially or fully broken down to a diopside-plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two-pyroxene thermometry of this assemblage gives a temperature of c. 750 � C. Pairing the most Mg-rich garnet composition with the assemblage plagioclase-diopside-hypersthene-quartz gives pressures of 14-16 kbar at this temperature. The hornblende-plagioclase-garnet rim-quartz assemblage yields 9-12 kbar and 500-550 � C. The combined P-T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P-T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.
