Metamorphic evolution of sapphirine‐ and orthoamphibole‐cordierite‐bearing gneiss,Okanogan dome,Washington, USA

Gneiss domes are commonly cored by quartzofeldspathic rocks that provide little information about the pressure–temperature–fluid history of the domes. Three northern Cordilleran migmatite domes (Thor‐Odin and Valhalla/Passmore, British Columbia, Canada; Okanogan, Washington, USA), however, contain Mg–Al‐rich orthoamphibole‐cordierite gneiss as layers and lenses that record metamorphic conditions and pressure–temperature (P–T) path information not preserved in the host migmatite. These Mg–Al‐rich rocks are therefore a valuable archive of metamorphic conditions during dome evolution, although refractory rocks such as these commonly contain reaction textures that may complicate the calculation of metamorphic conditions. In the Okanogan dome, Mg–Al‐rich layers are part of the Tunk Creek unit, which occurs at the periphery of an underlying migmatite domain. Bulk compositional layers (mm‐ to m‐scale) consist of gedrite‐dominated, hornblende‐dominated and biotite‐bearing layers that contain variable amounts of gedrite, hornblende, anorthite, cordierite, spinel, sapphirine, corundum, kyanite, biotite and/or staurolite. The presence of different compositional layers (some with reaction textures, some without) allows systematic analysis of metamorphic history by a combined petrographic and phase equilibrium analysis. Gedrite‐dominated layers containing relict kyanite preserve evidence of the highest‐P conditions; symplectitic and coronal reaction textures around kyanite indicate decompression at high temperature. Gedrite‐dominated layers lacking these reaction textures contain layers of sapphirine and spinel in apparent textural equilibrium and record a later high‐T–low‐P part of the path. Phase equilibria (pseudosection) analysis for layers that lack reaction textures indicates metamorphic conditions of 720–750 °C at a range of pressures (>8 to <4 kbar) following decompression. Elevated crustal temperatures and concordant structural fabrics in the Tunk Creek unit and underlying migmatite domain suggest that the calculated P–T conditions recorded in Tunk Creek rocks were coeval with anatexis, extension, and dome formation in Palaeocene–Eocene time. In contrast to orthoamphibole‐cordierite gneiss in the other Cordilleran domes, the Tunk Creek unit occurs as a discontinuous km‐scale layer rather than as smaller (m‐scale) pods, is more calcic, and lacks garnet. In addition, kyanite did not transform to sillimanite, and spinel commonly occurs as a blocky matrix phase in addition to vermicules in symplectite. These differences, along with the compositional layering, allow an analysis of bulk composition v. tectonic (P–T path) controls on mineral assemblages and textures. Pseudosection modelling of different layers in the Tunk Creek unit provides a basis for understanding the metamorphic history of these texturally complex, refractory rocks and their host gneiss domes, and other such rocks in similar tectonic settings.     

Prograde reactions and garnet zoning reversals in staurolite schist, British Columbia: significance for thermobarometric interpretations

Abstract An outcrop of staurolite-bearing pelitic schist from the Solitude Range in the south-western Rocky Mountains, British Columbia, was examined in order to determine the nature of prograde garnet- and staurolite-producing reactions using information from garnet zoning and inclusion mineralogy. Although not present as a matrix phase, chloritoid is present as inclusions in garnet and is interpreted to have participated in the simultaneous growth of garnet and staurolite by a reaction such as chloritoid + quartz = garnet + staurolite + H₂O.
A garnet zoning trend reversal, which is most pronounced with respect to almandine and grossular components, is present in the outer core of garnets. The location of the zoning reversal corresponds to the outer limit of chloritoid inclusions in garnet. As there is no evidence for polymetamorphism, the zoning reversal is interpreted to indicate continued garnet growth by prograde reaction(s) during a single metamorphic event after the exhaustion of chloritoid as a matrix phase.
Metamorphic conditions recorded by mineral rim compositions are 550–600° C at 6–7 kbar. Because there is no evidence for partial resorption of garnet during production of staurolite, we interpret these results to represent peak conditions.     

Origin of Mafic Enclaves in the Dinkey Creek Pluton, Central Sierra Nevada Batholith, California

Two types of mafic enclaves occur in the Dinkey Creek pluton:ubiquitous microgranular enclaves, and rare gabbroic enclaves.Common petrographic features of the microgranular enclaves are:(1) fine grain-size, (2) abundant acicular apatite, and (3)plagioclase zoned from bytownitic cores to andesine-labradoriterims, with sharp boundaries between these main zones. Subordinateoscillatory variations are commonly superimposed on both coresand rims. It has been found by secondary ion mass spectrometrythat the rims are identical in major and trace element compositionto plagioclase in the tonalite, which suggests crystallizationfrom the same or similar magmas. The gabbroic enclaves are composedpredominantly of hornblende (50–85%) and appear to bemagmatic segregations. The microgranular enclaves and host rocks display two convergingtrends on silica variation diagrams for Fe₂O₃, TiO₂, Al₂O₃,Zn, and Zr. The dominant trend is defined by small microgranularenclaves, by samples from a large (20 m?30 m) microgranularenclave, and by the Dinkey Creek tonalites and granodiorites.The subordinate trend covers tholeiltic dikes and tonalitich and converges with the Dinkey Creek host rocks at 61 wt.%SiO₂ Alkali and alkaline earth elements exhibit greater variabilitythan the above constituents and appear to be either enrichedor depleted as required for equilibrium with the host rocks.Low CaO and Sr concentrations in small enclaves (<30 cm)apparently reflect a lower modal abundance of calcic plagioclaseand more sericitization of this feldspar as compared with theplagioclase of the large microgranular enclave. The large enclaveis also richer in MgO than the small enclaves. With the exceptionof the alkali elements, the major element compositions of themicrogranular enclaves approach high-Al basaltic to andesiticcom positions. In one analyzed microgranular enclave, low La/Cerelative to chondrites and more abundant HREE than in othermicrogranular samples suggest that it may also contain minorcumulus hornblende. The petrographic and whole-rock geochemical relations, and theplagioclase compositions in the microgranular enclaves and theirhost rocks, indicate that the microgranular enclaves representmixtures of quenched basalts and Dinkey Creek tonalites. Itappears that dikes of high-alumina basalt were intruded intothe lower, tonalitic portions of the Dinkey Creek pluton, wherethey were partially quenched along an interface with overlyingtonalitic magma. Large portions of residual liquid in the partiallyquenched basalts permitted mixing with the overlying magma toform a hybrid zone. This zone was then disaggregated, yieldingthe enclaves, and they were dispersed throughout the upper partof the Dinkey Creek magma chamber. Subsequent crystallizationof tonalitic melt within the enclaves produced the zoned plagioclaseand re-equilibrated hornblende and biotite in the enclaves tothe Dinkey Creek magmatic conditions. Scouring disrupted hornblende-richmagmatic segregations and produced the gabbroic enclaves.     

Metamorphic and tectonic evolution of a structurally continuous blueschist‐to‐Barrovian terrane,Sivrihisar Massif,Turkey

Metamorphic terranes comprised of blueschist facies and regional metamorphic (Barrovian) rocks in apparent structural continuity may represent subduction complexes that were partially overprinted during syn‐ to post‐subduction heating or may be comprised of unrelated tectonic slices. An excellent example of a composite blueschist‐to‐Barrovian terrane is the southern Sivrihisar Massif, Turkey. Late Cretaceous blueschist facies rocks are dominated by marble characterized by rod‐shaped calcite pseudomorphs after aragonite and interlayered with blueschist that contains eclogite and quartzite pods. Barrovian rocks, which have ⁴⁰Ar/³⁹Ar white mica ages that are >20 Myr younger than those of the blueschists, are also dominated by marble, but rod‐shaped calcite has been progressively recrystallized into massive marble within a ～200‐m transition zone. Barrovian marble is interlayered with quartzite and schist in which isograds are closely spaced and metamorphic conditions range from chlorite to sillimanite zone over ～1 km present‐day structural thickness. Andalusite, kyanite and prismatic sillimanite are present in muscovite‐rich quartzite; in one location, all three are in the same rock. Andalusite pre‐dates Barrovian metamorphism, kyanite is both pre‐ and syn‐Barrovian and sillimanite is entirely Barrovian. Muscovite with phengitic cores and relict kyanite in quartzite below the staurolite‐in isograd are evidence for pre‐Barrovian subduction metamorphism preserved at the low‐T end of the Barrovian domain; above the staurolite isograd, all evidence for subduction metamorphism has been erased. Some regional metamorphism may have occurred during exhumation, as indicated by syn‐kinematic high‐T minerals defining the fabric of L‐tectonite. Quartz microstructures in lineated quartzite reveal a strong constrictional fabric that may have formed in a transtensional bend in the plate boundary. Transtension accounts for the closely spaced isograds and development of a strong constrictional fabric during exhumation.     

Petrogenesis of lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, Turkey

The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre‐ to centimetre‐scale eclogite pods occur in blueschist facies metabasalt, marble and quartz‐rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P–T conditions: ～26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15–16 kbar, 480–500 °C (lawsonite‐epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P–T conditions and paths. For example, in some lawsonite‐bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P–T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P–T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P–T conditions were tectonically juxtaposed as kilometre‐scale slices and associated high‐P pods, although all shared the same exhumation path from ～9–11 kbar, 300–400 °C. Within the tectonic slices, alternating millimetre–centimetre‐scale layers of eclogite and blueschist formed together at the same P–T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO₂ or other chemical factors.     

Origin of CO2-rich fluid inclusions in leucosomes from the Skagit migmatites, North Cascades, Washington, USA

CO₂–CH₄ fluid inclusions are present in anatectic layer-parallel leucosomes from graphite-bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water-rich fluid during high-temperature metamorphism and migmatization. CO₂-rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta-igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO₂-rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post-entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO₂-rich fluid is attributed to water-saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al₂SiO₅+ water-rich fluid = garnet + melt + CO₂–CH₄. The presence of CO₂-rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis. Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high-temperature metamorphism. The infiltrating fluid was a water-rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.     

Progressive metamorphism of pelitic rocks from protolith to granulite facies, Dutchess County, New York, USA: constraints on the timing of fluid infiltration during regional metamorphism

A complete Barrovian sequence ranging from unmetamorphosed shales to sillimanite–K-feldspar zone metapelitic gneisses crops out in a region extending from the Hudson River in south-eastern New York state, USA, to the high-grade core of the Taconic range in western Connecticut. NNE-trending subparallel biotite, garnet, staurolite, kyanite, sillimanite and sillimanite–K-feldspar isograds have been identified, although the assignment of Barrovian zones in the high-grade rocks is complicated by the appearance of fibrolitic sillimanite at the kyanite isograd. Thermobarometric results and reaction textures are used to characterize the metamorphic history of the sequence. Pressure–temperature estimates indicate maximum metamorphic conditions of 475 °C, c. 3–4 kbar in the garnet zone to >720 °C, c. 5–6 kbar in the highest grade rocks exposed. Some samples in the kyanite zone record anomalous (low) peak conditions because garnet composition has been modified by fluid-assisted reactions. There is abundant petrographic and mineral chemical information indicating that the sequence (with the possible exception of the granulite facies zone) was infiltrated by a water-rich fluid after garnet growth was nearly completed. The truncation of fluid inclusion trails in garnet by rim growth or recrystallization, however, indicates that metamorphic reactions involving garnet continued subsequent to initial infiltration. The presence of these textures in some zones of a well-constrained Barrovian sequence allows determination of the timing of fluid infiltration relative to the P–T paths. Thermobarometric results obtained using garnet compositions at the boundary between fluid–inclusion-rich and inclusion-free regions of the garnet are interpreted to represent peak metamorphic conditions, whereas rim compositions record slightly lower pressures and temperatures. Assuming that garnet grew during a single metamorphic event, infiltration must have occurred at or slightly after the peak of metamorphism, i.e. 4–5 kbar and a temperature of c. 525–550 °C for staurolite and kyanite zone rocks.     

Formation of garnet polycrystals during metamorphic crystallization

Garnet polycrystals may form throughout the metamorphic history of a rock, starting at the earliest stages of garnet growth when closely spaced nuclei coalesce. In mica schist from Townshend Dam, VT, electron back-scattered diffraction (EBSD) analysis shows that garnet polycrystals possess two or more distinct lattice orientations separated by high-angle boundaries (28–61°). The minimum rotational displacements required to bring these lattice orientations into concordance with each other are commonly normal to the same low-energy planes that occur as crystal faces of euhedral garnet. There is no evidence for intracrystalline deformation, and the polycrystals therefore probably represent individual garnet crystals that coalesced during growth. The boundaries cross-cut growth zoning and inclusion trails of the polycrystals, indicating that early-formed polycrystals, once coalesced, behave chemically and physically as single crystals. Statistical analysis of a 3D, high-resolution X-ray computed tomographic data set of a large sample (912 cm³) of a Townshend Dam schist, combined with microprobe and EBSD analyses of garnet, are consistent with a high degree of clustering at all stages of garnet growth. The formation and prevalence of polycrystals implies that garnet nuclei impinged on each other and coalesced, and that coalescence was a common feature throughout garnet growth in the rock.     

MEASUREMENT OF PRIMARY PRODUCTION IN AN ENCLOSED WATER COLUMN

A CEE (2.5 m by 16 m) has been used to study the budget of organic production by measuring the changes in POC/N, oxygen and 14CO, uptake through three-week period. 14C primary production measurements were conducted with 4-hours and 24-hours incubation periods, and with size fractionation. Different types and sizes of bottle effects were examined. Results showed that the oxygen method production was highest, followed by the 14CO2 uptake method, and POC gain showed the lowest. The value of PQ should be more than 1.7. The ratio of 4-hour incubation to 24-hour incubation was 2.42±0.22, indicating that net daily production is equal to 9.7±0.9 h of illuminated growth. Different types and sizes of bottles had little effect on primary production.