Amphibole Inclusions Research Articles

Inclusion study in zircon from ultrahigh-pressure metamorphic rocks in the Kokchetav massif: an excellent tracer of metamorphic history

Abstract: Zircon is an excellent material to preserve the complex history of ultrahigh-pressure (UHP) metamorphic rocks, whereas mineralogical evidence of UHP conditions is mostly obliterated in matrix assemblages as a result of extensive retrograde overprinting during exhumation. Zircons from the Kokchetav UHP–HP massif contain numerous inclusions of graphite, quartz, garnet, omphacite, jadeite, phengite, phlogopite, rutile, albite, K-feldspar, amphibole, zoisite, kyanite, calcite, dolomite, apatite and monazite, as well as the diagnostic UHP minerals, such as microdiamond and coesite, which were identified by laser Raman spectroscopy. The internal structure of zircon displays a distinct zonation, which comprises an inherited core, a wide mantle and an outer rim, each with distinctive inclusion micro-assemblages. The low-pressure mineral inclusions, such as graphite, quartz and albite, are common in the inherited core and thin outer rim, whereas diamond, coesite and jadeite occupy the mantle domain. The zircon core and outer rim are of detrital and relatively low-grade metamorphic origin, whereas the mantle domain is of HP to UHP metamorphic origin. The mineral assemblages and chemistry of inclusions preserved in zircon have been used to constrain the metamorphic P – T path of the Kokchetav UHP–HP rocks, and indicate peak metamorphism at 60–80 kbar and 970–1100 °C followed by nearly isothermal decompression at 10 kbar and c . 800 °C. Sensitive high-resolution ion microprobe U–Pb spot analyses of the zoned zircon indicate four discrete ages of the Kokchetav metamorphic evolution: (1) a Middle Proterozoic protolith age; (2) 537 ± 9 Ma for UHP metamorphism; (3) 507 ± 8 Ma for the late-stage amphibolite-facies overprint; (4) 456–461 Ma for post-orogenic thermal events. This indicates that Middle Proterozoic supracrustal protoliths of the Kokchetav UHP–HP rocks were subducted to mantle depths in the Middle Cambrian, and exhumed to mid-crustal levels in the Late Cambrian. The zonal arrangement of inclusions and the presence of coesite and diamond without back reaction imply that aqueous fluids were low to absent within zircon, and that zircon is capable of retaining minerals of each metamorphic stage. We suggest that the study of inclusions in zircon is a powerful method to clarify the multiple stages and timing of metamorphic evolution of UHP–HP rocks, the evidence for which has been more or less obliterated in the host rock.

Oriented quartz + calcic amphibole inclusions in omphacite from the Saualpe and Pohorje Mountain eclogites, Eastern Alps—An assessment of possible formation mechanisms based on IR- and mineral chemical data and water storage in Eastern Alpine eclogites

The composition of mineral phases and their modal proportions have been determined for three representative Eoalpine eclogites from the Saualpe type locality/Eastern Austria (sample SKP31) and the Pohorje Massif/Slovenia (CM31/03 and CM15/01) using electron microprobe, laser ICP-MS, IR spectroscopy and modal analysis to evaluate possible mechanisms for the formation of composite oriented calcic amphibole + quartz inclusions (COIs) in omphacite and to assess the relative importance of hydrous and nominally anhydrous phases as H 2O carriers in these eclogites. For omphacites in CM31/03 with a zonal distribution of COIs, a comparison of water and trace element concentrations of areas containing COIs and those free of COIs and a comparison with the trace element concentration of calcic amphibole indicate that COIs have formed through an open-system alteration of clinopyroxene and not through a closed system exsolution process. In sample SKP31, both textural and mineral chemical evidence suggests that COIs did not form by exsolution involving a Ca-Eskola component in clinopyroxene but formed by progressive growth under eclogite-facies P-T conditions and prior to the onset of retrogressive symplectite formation analogous to the formation of poikiloblastic quartz–calcic amphibole grains in the matrix. Bulk H 2O contents of the eclogites are between ca. 750 and 2150 ppm with 6–25% of the total water contributed by nominally anhydrous minerals (NAMs). Because of high modal amounts of 37–65%, omphacite is the major nominally anhydrous water carrier, containing 145–580 ppm H 2O with significant concentration variations on a thin section scale. Due to their very low H 2O concentrations of < 5–10 ppm (garnet, kyanite) or insignificant modal amounts ≤ 3% (rutile) the remaining NAMs contribute less than 1.5% to the bulk eclogite H 2O content. Calcic amphibole forming part of COIs may be a major carrier of H 2O as evidenced by CM31/03 containing both COIs and texturally primary calcic amphibole. In this sample calcic amphibole of the COIs contributes 63% whereas primary calcic amphibole only accounts for 13% of the bulk water. The relative order of H 2O concentrations in NAMs is H 2O rutile > H 2O omphacite ≫ H 2O garnet for CM31/03 and CM15/01 and H 2O rutile ≈ H 2O omphacite ≫ H 2O garnet in SKP31.

Multistage metamorphism of orogenic garnet–lherzolite from Zhimafang, Sulu UHP terrane, E. China: Implications for mantle wedge convection during progressive oceanic and continental subduction

Seven stages of mineral assemblage were recognized in the Zhimafang orogenic garnet lherzolite from the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China, which suggest that it experienced progressive mantle wedge convection during subduction of previous oceanic and subsequent continental slabs. M 1 is recorded by inclusions of high-Mg olivine, high-Mg–Al–Cr–Ca orthopyroxene and high-Mg clinopyroxene in the high Ca–Cr cores of the garnet porphyroclasts, suggesting that the Zhimafang peridotite was probably a garnet lherzolite originated from a deep and hot mantle wedge above the previously subducted oceanic slab. M 2 is represented by inclusions of high-Mg–Al chromite, high-Mg olivine and high-Mg orthopyroxene in the low-Cr mantles of the garnet porphyroclasts and core of the matrix garnet, suggesting that the rock was convected to shallow and hot mantle wedge, and was transformed to refractory spinel harzburgite or spinel dunite due to high degree of decompressional partial melting. M 3 is manifested by the origin of metasomatic clinopyroxene and orthopyroxene porphyroblasts with inclusions of high-Fe chromite, tremolitic or edenitic amphibole, phlogopite, barite and pyrite, indicating that the rock was convected to shallow, cold and wet corner of the mantle wedge, and was transformed to a fertile amphibole-bearing spinel lherzolite during continental subduction. The lherzolites was subsequently transformed to high-pressure (HP) amphibole-bearing garnet lherzolite ( M 4) and UHP garnet lherzolite ( M 5), indicating its convection to deep mantle wedge. Finally, the UHP garnet–lherzolite exhumed to the Earth’s surface, together with the UHP terrane, and subsequently retrograded to amphibole-bearing garnet lherzolite ( M 6) and chlorite-amphibole-bearing spinel lherzolite ( M 7). Detailed analyses of reaction textures and mineral compositions revealed several stages of metasomatism related to oceanic and continental subduction and exhumation.

Ultrahigh‐pressure metamorphism and exhumation of garnet‐bearing ultramafic rocks from the Lanterman Range (northern Victoria Land, Antarctica)

AbstractNorthern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo‐Pacific margin of Gondwana. A narrow and discontinuous high‐ to ultrahigh‐pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet‐bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium‐grained garnet + olivine + low‐Al orthopyroxene + clinopyroxene, whereas finer‐grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg‐chlorite ± serpentine ± talc. A high‐temperature (∼950 °C), spinel‐bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet‐bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity‐driven exhumation.

Sub-solidus Oligocene zircon formation in garnet peridotite during fast decompression and fluid infiltration (Duria, Central Alps)

A garnet peridotite lens from Monte Duria (Adula nappe, Central Alps, Northern Italy) contains porphyroblastic garnet and pargasitic amphibole and reached peak metamorphic conditions of ∼830 C, ∼2.8 GPa. A first stage of near isothermal decompression to pressures <2.0 GPa is characterised by domains where fine grained spinel, clinopyroxene, orthopyroxene and amphibole form. The newly formed amphibole contains elevated levels of fluid mobile elements such as Rb, Ba and Pb indicating that recrystallization was assisted by infiltration of a crustal-derived fluid. Further decompression and cooling to ∼720 °C, 0.7–1.0 GPa associated with limited fluid influx is documented by the formation of orthopyroxene-spinel-amphibole symplectites around garnet. Zircon separated from this garnet peridotite exhibits two distinct zones. Domain 1 displays polygonal oscillatory zoning and high trace element contents. It contains clinopyroxene and amphibole inclusions with the same composition as the same minerals formed during the spinel peridotite equilibration, indicating that this domain formed under sub-solidus conditions during decompression and influx of crustal fluids. Domain 2 has no zoning and much lower trace element contents. It replaces domain 1 and is likely related to zircon recrystallization during the formation of the symplectites. SHRIMP dating of the two domains yielded ages of 34.2 ± 0.2 and 32.9 ± 0.3 Ma, respectively, indicating fast exhumation of the peridotite within the spinel stability field. We suggest that the Duria garnet peridotite originates from the mantle wedge above the tertiary subduction of the European continental margin and that it was assembled to the country rock gneisses between 34 and 33 Ma.

Flow kinematics in the deeper part of a subduction channel, as inferred from inclusion trails in plagioclase porphyroblasts from the Sambagawa metamorphic rocks, south‐west Japan

AbstractThe subduction and exhumation of accretionary prism metasedimentary rocks are accompanied by large‐strain ductile deformations which may be recorded in microstructures. Porphyroblast microstructures have been a key to unravel the kinematics in such deformed belts. Shape‐preferred orientation (SPO) of epidote and amphibole inclusions that define S‐shaped trails in prograde cores of plagioclase porphyroblasts were analysed from the high‐P/T Sambagawa metamorphic rocks. Inclusions are found to be elongate parallel to the [010] and [001] directions, respectively, and their long‐axis orientations define an internal foliation Si (best‐fit great circle) and lineation Li (maximum on the Si). S‐shaped inclusion trails in the orthogonal sections do not exhibit the same geometries, but rather are grouped into two types, where the foliation intersection axes (FIAs) are nearly perpendicular and parallel to Li, respectively. These two types of S‐shaped inclusion trails are seen in the sections inclined at low and high angles to the Li, respectively. However, the latter type commonly consists of composite trails, where the Si is first rotated about an FIA perpendicular to the Li (i.e. unique axis), and then about an FIA parallel to the Li. The S‐shaped inclusion trails are interpreted to have formed by the successive overgrowth of matrix minerals and rotation of the plagioclase porphyroblast cores about a unique axis in non‐coaxial deformation. The rotation of Si about an FIA nearly parallel to the Li is perhaps an apparent rotation, caused by the deflection of foliation around the growing prismatic plagioclase grain prior to inclusion into the porphyroblast. This study has for the first time documented the 3‐D geometry of S‐shaped inclusion trails in porphyroblasts from accretionary prism metasedimentary rocks and identified their origin, which helps to understand the flow kinematics in the deeper part of a subduction channel.

The origin, evolution and provenance of the Northern Belt (Ordovician) of the Southern Uplands Terrane, Scotland: a heavy mineral perspective

The heavy mineral components of Ordovician greywacke turbidites of the Southern Uplands (subduction-accretion prism) Terrane were studied using high-resolution heavy mineral analysis. Diagnostic heavy minerals and the discovery of hitherto unreported species provide a new insight into the interplay of sediment fluxes, tectonic controls and provenance. Each formation is characterized by distinctive heavy mineral suites, indicating that the sediments of successive or laterally-equivalent formations were sourced from different and rapidly-changing lithologies. Appreciable variations were detected even in greywacke packages within an individual formation, indicating that these changes were taking place at a very fast rate. Detritus came from either an eroding Dalradian or a similar Barrovian metamorphic complex and its superjacent obducted ophiolite complex to the northeast, and from oceanic island arcs and collapsing seamounts to the south. The rapid appearance of metamorphic detritus, following a short period of ophiolite stripping, indicates that post-subduction-flip exposure of and erosion into the metamorphic complex was fast, suggesting extensional unroofing. The chemical similarity of detrital garnet with Scottish Grampian garnet may suggest derivation from that complex and, therefore, that the Southern Uplands Terrane has not travelled, sinistrally, more than about 200 km with respect to the Scottish Highlands. The absence of detrital staurolite in the greywackes raises the possibility that stripping either did not reach abundant staurolite levels or that staurolite was a relatively uncommon mineral in the source area. The former explanation is unlikely because, in western Ireland, a similar Dalradian stripping sequence involves the appearance of staurolite in the first arrival of metamorphic detritus. However, possibly, in western Ireland, the Dalradian source was extensionally unroofed to a much deeper structural level than was Scotland in the same time interval. Only the Shinnel Formation shows spatial detrital consistency, indicating that, by Shinnel times, the Dalradian hinterland had achieved a relatively uniform level of erosion. The discovery of lawsonite, as part of a ferroglaucophane-glaucophane blueschist assemblage in the Portpatrick Formation, is important yet enigmatic. Ordovician crosstie-winchite-ferroglaucophane-phengite assemblages are known throughout the northwestern Appalachian/Caledonian belt and may have provided the source for the Portpatrick blueschist-facies detritus. However, a possible source was from the Cadomian blueschist belt of Anglesey, then on the southern margin of the Iapetus Ocean. This is supported by garnets with sodic/calcic amphibole inclusions, unknown in the Dalradian. Perhaps Cadomian ‘terrane fragments’ were rifted from the southern margin of Iapetus to drift towards and collide with Laurentia to shed detritus into the Laurentian margin trench.

Origin of placer laurite from Borneo: Se and As contents, and S isotopic compositions

AbstractWe examined grains of the platinum-group mineral, laurite (RuS2), from the type locality, Pontyn River, Tanah Laut, Borneo, and from the Tambanio River, southeast Borneo. The grains show a variety of morphologies, including euhedral grains with conchoidal fractures and pits, and spherical grains with no crystal faces, probably because of abrasion. Inclusions are rare, but one grain contains Ca-Al amphibole inclusions, and another contains an inclusion of chalcopyrite+bornite+pentlandite+heazlewoodite (Ni3S2) that is considered to have formed by a two-stage process of exsolution and crystallization from a once homogeneous Fe-Cu-Ni sulphide melt.All grains examined are solid solutions of Ru and Os with Ir (2.71 –11.76 wt.%) and Pd (0.31–0.66 wt.%). Their compositions are similar to laurite from ophiolitic rocks. The compositions show broad negative correlations between Os and Ir, between As and Ir, and between As (0.4 –0.74 wt.%) and Se (140 to 240 ppm). Laurite with higher Os contains more Se and less Ir and As. The negative correlations between Se and As may be attributed to their occupancy of the S site, but the compositional variations of Os, Ir and As probably reflect the compositional variation of rocks where the crystals grew.Ratios of S/Se in laurite show a narrow spread from 1380 to 2300, which are similar to ratios for sulphides from the refractory sub-arc mantle. Sulphur isotopic compositions of laurite are independent of chemical compositions and morphologies and are similar to the chondritic value of 0%. The data suggest that S in laurite has not undergone redox changes and originated from the refractory mantle. The data support the formation of laurite in the residual mantle or in a magma generated from such a refractory mantle, followed by erosion after the obduction of the host ultramafic rocks.

A coherent lithostratigraphic unit in the coesite–eclogite complex of Dabie Shan, China: geologic and petrologic evidence

As a contribution to the discussion about the “in situ” vs. “exotic” origin of the ultrahigh-pressure metamorphic (UHPM) rocks, and to the real size of the exhumed tectonic units, the Wuhe–Pailou Unit (WPU) in eastern Dabie Shan was studied. The WPU typically consists of two micas—epidote gneiss, marble with minor eclogite, and jadeite-bearing granofels with rare garnetite and eclogite nodules; these lithologies are enclosed within leucocratic granitic gneiss. Jadeite-bearing granofels is the most peculiar lithology of the WPU, being the only felsic rock in Dabie Shan still preserving extensive relics of the UHPM mineral assemblage. The UHP peak assemblage is coesite+jadeite+garnet±epidote±phengite±kyanite, with accessory rutile, apatite, and zircon. All eclogites consist of garnet, omphacite, quartz/coesite, rutile or high-Al titanite±phengite±clinozoisite±kyanite. Two micas—epidote gneiss associated with the jadeite-bearing granofels typically consists of epidote, muscovite, biotite, plagioclase, K-feldspar, rare garnet, and accessory rutile, titanite, apatite, and zircon. Locally, the WPU paragneiss contains garnet with inclusions of phengite, kyanite, rutile, and amphibole+Na plagioclase symplectite after omphacite. A clockwise P– T path was inferred for the WPU, which consists of an early prograde part up to coesite–eclogite facies conditions ( P∼3.3 GPa and T∼720 °C), an early decompression stage and a late part characterised by a near-adiabatic decompression down to medium pressure ( P∼1.1 GPa, T∼670 °C) and to low-pressure amphibolite-facies conditions, followed by a final cooling to greenschist-facies conditions. Both field and laboratory data consistently suggest that the WPU, characterised by the close association of paragneiss, marble, eclogite, and granofels, is a supracrustal metasedimentary sequence, which underwent subduction, UHPM, and exhumation as a coherent tectonic unit. Therefore, within collisional orogens, large coherent slices of continental crust may be subducted at depths greater than 100 km.

P-T-time Path of Ultrahigh-Pressure Metamorphism Induced by Zircon Inclusion Method

Zircon is an excellent capsule to protect primary evidences of the ultrahigh-pressure (UHP) metamorphism, whereas mineralogic evidences of the UHP conditions were mostly obliterated in the matrix minerals of the rock due to the extensive retrograde overprinting related to exhumation. Zircons from the Kokchetav UHP-HP massif contain numerous inclusions of graphite, quartz, garnet, clinopyroxene, phengite, phlogopite, rutile, albite, K-feldspar, amphibole, zoisite, kyanite, calcite, dolomite, apatite and monazite, as well as diagnostic UHP minerals, microdiamond and coesite, which are identified by laser Raman spectroscopy. The diamond inclusions coexist with coesite and garnet, and appear also with low-pressure minerals, especially graphite in single zircon grains with zonal arrangements. Internal structure of zircon displays distinct zonal fabrics, which comprise inherited core, wide mantle and outer rim, each with distinctive inclusion micro-assemblages. These observations indicat-ed that zircon has been grown at several distinct stages including UHP mineral-bearing mantle, low-pressure mineral-bearing rim and inherited core containing low-pressure minerals. Sensitive high resolution ion micro-probe (SHRIMP) analyses of the zoned zircons resulted four discrete ages of the Kokchetav ultrahigh-pressure metamorphic rocks ; (1) Middle Proterozoic protolith age, (2) 537 ± 9 Ma for the UHP metamorphism, (3) 57 ± 8 Ma for the late-stage amphibolite facies overprint, and (4) 456-461 Ma for the post-orogenic thermal events. The zircon inclusion method presented here is a comprehensive technique to demon-strate the history of ultrahigh-pressure metamorphic rocks, and can be applied to understand P-T-time history of other UHP-HP metamorphic terranes.

四国中央部・三波川帯，東赤石超苦鉄質岩体のＰ‐Ｔ‐Ｄ履歴―ウェッジ・マントルカンラン石の沈み込み過程―

Garnet-bearing ultramafic rocks including clinopyroxenite, wehrlite, and websterite locally crop out in the Higashi-akaishi peridotite of the Besshi region in the Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins, or layers with a thickness ranging from a few centimeters to 1 meter. The wide and systematic variations of bulk-rock composition and overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite, and magnetite, implying equilibrium at relatively low T and hydrous conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell-shaped Al zoning with a continuous decrease of Al from the core towards the rim, and its rim records peak metamorphic conditions. Estimated P-T conditions imply a high P/T gradient (> 3.1 GPa/100 °C) from 1.5-2.4 GPa/700-800 °C to 2.9-3.8 GPa/700-810°C during prograde metamorphism. Olivines recrystallized at the high P/T prograde metamorphic stage show a B-type lattice preferred orientation (LPO) with a-axis concentrations normal to the stretching lineation. The presence of B-type LPO indicates that deformation of the prograde metamorphic stage possibly progressed under hydrous and high-stress conditions at the wedge mantle adjacent to the subducted slab. The Higashi-akaishi peridotite is a unique example that well records the prograde evolution of subducted ultramafic rocks.

Metamorphic evolution of garnet‐bearing ultramafic rocks from the Gongen area, Sanbagawa belt, Japan

AbstractGarnet‐bearing ultramafic rocks including clinopyroxenite, wehrlite and websterite locally crop out in the Higashi‐akaishi peridotite of the Besshi region in the Cretaceous Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins or layers with a thickness ranging from a few centimetres to 1 metre. The wide and systematic variation of bulk‐rock composition and the overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite and magnetite, implying equilibrium at relatively low P–T conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell‐shaped Al zoning with a continuous decrease of Al from the core towards the rim, consistent with rims recording peak metamorphic conditions. Estimated P–T conditions using core and rim compositions of orthopyroxene are 1.5–2.4 GPa/700–800 °C and 2.9–3.8 GPa/700–810 °C, respectively, implying a high P/T gradient (&gt; 3.1 GPa/100 °C) during prograde metamorphism. The presence of relatively low P–T conditions at an early stage of metamorphism and the steep P/T gradient together trace a concave upwards P–T path that shows increasing P/T with higher T, similar to P–T paths reported from other UHP metamorphic terranes. These results suggest either (1) down dragging of hydrated mantle cumulate parallel to the slab–wedge interface in the subduction zone by mechanical coupling with the subducting slab or (2) ocean floor metamorphism and/or serpentinization at early stage of subduction of oceanic lithosphere and ensuing HP–UHP prograde metamorphism.

Evolution of Caledonian deformation fabrics under eclogite and amphibolite facies at Vårdalsneset, Western Gneiss Region, Norway

The Vårdalsneset eclogite situated in the Western Gneiss Region, SW Norway, is a well preserved tectonite giving information about the deformation regimes active in the lower crust during crustal thickening and subsequent exhumation. The eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts of barroisite, actinolite, clinozoisite, kyanite, quartz, paragonite, phengite and rutile. The rocks record a five‐stage evolution connected to Caledonian burial and subsequent exhumation. (1) A prograde evolution through amphibolite facies (T =490±63 °C) is inferred from garnet cores with amphibole inclusions and bell‐shaped Mn profile. (2) Formation of L&gt;S‐tectonite eclogite (T =680±20 °C, P=16±2 kbar) related to the subduction of continental crust during the Caledonian orogeny. Lack of asymmetrical fabrics and orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the L&gt;S fabric was coaxial. (3) Formation of sub‐horizontal eclogite facies foliation in which the finite stretching direction had changed by approximately 90°. Disruption of eclogite lenses and layers between symmetric shear zones characterizes the dominantly coaxial deformation regime of stage 3. Locally occurring mylonitic eclogites (T =690±20 °C, P=15±1.5 kbar) with top‐W kinematics may indicate, however, that non‐coaxial deformation was also active at eclogite facies conditions. (4) Development of a widespread regional amphibolite facies foliation (T =564±44 °C, P&lt;10.3–8.1 kbar), quartz veins and development of conjugate shear zones indicate that coaxial vertical shortening and sub‐horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions. (5) Amphibolite facies mylonites mainly formed under non‐coaxial top‐W movement are related to large‐scale movement on the extensional detachments active during the late‐orogenic extension of the Caledonides. The structural and metamorphic evolution of the Vårdalsneset eclogite and related areas support the exhumation model, including an extensional detachment in the upper crust and overall coaxial deformation in the lower crust.

HIGH-Mg ARC-ANKARAMITIC DIKES, GREENHILLS COMPLEX, SOUTHLAND, NEW ZEALAND

Minor dikes 8–70 cm thick of ankaramitic composition were emplaced late in the crystallization of the Greenhills Complex, a layered dunite – wehrlite – hornblende gabbronorite intrusion of Permian age in the volcanic arc-derived Brook Street Terrane of southern New Zealand. Mg contents range from 10 to 28 wt% MgO. The interior of a hornblende ankaramite dike at Steep Head contains phenocrysts of olivine Fo 93–82 , in some cases skeletal, and scattered phenocrysts of diopside, set in a groundmass of magnesiohornblende. The chilled margins of this dike consist almost wholly of amphibole (91%) and anorthite, An 99–94 (9%). In other dikes, crystallization of amphibole followed that of olivine, diopside, enstatite, and overlapped with plagioclase. Olivine in these dikes varies in composition from Fo 90 to Fo 72 . Diopside contains oriented blebs of magnesiohornblende and edenite. We believe that the components of these amphibole inclusions were in part introduced and in part derived from the host pyroxene by a process of reactive exsolution. Spinel varies from chromite containing 53 wt% Cr 2 O 3 through compositions at first increasingly Al-rich, and then through chromian magnetite to virtually Cr-free magnetite. Amphiboles range from magnesiohornblende to edenite, pargasite, and tschermakite, with minor late-stage high-Si magnesiohornblende. The aphyric margin of the Steep Head dike contains 17.4 wt% MgO on a volatile-free basis. Some of the melts may have exceeded this MgO content. Dike centers are more Mg-rich than margins, reaching 28.3% in one case, in part an effect of concentration of phenocrysts by flow differentiation, and in part a result of changing composition of the melt. The presence of abundant magmatic amphibole, especially in the most magnesian rocks, indicates a relatively hydrous magma. Various geothermometers suggest extensive low-temperature re-equilibration. Even where the dikes consist of almost monomineralic amphibole, chemically they are arc-ankaramites with CaO/Al 2 O 3 >1, REE contents less than twice primitive mantle, low abundances of high-field-strength elements and of incompatible elements, highly calcic plagioclase, and Cr-rich spinels with Ti, Mg, and Cr contents typical of low-K, high-Mg ankaramitic arc-tholeiitic suites. They have some chemical similarities to komatiites, but have lower Ni/MgO, Cr/MgO, and Ti/V values. The most highly magnesian dike-rocks have olivine in excess of diopside, whereas in dikes with lower Mg contents, the proportion of diopside phenocrysts exceeds that of olivine. Progressive fractionation of these two minerals lowered the bulk CaO/Al 2 O 3 ratio. Below about 10% MgO, CaO/Al 2 O 3 was no longer greater than one, the ankaramitic nature was lost, and evolution continued along an arc-tholeiitic trend. The dike compositions support the hypothesis that the Greenhills Complex is the remnant of an upper-crust magma chamber from which arc-ankaramites, tholeiitic basalts, and more evolved effusive volcanic products in the Brook Street Terrane were erupted. The highly magnesian ankaramitic dikes and their effusive analogues provide another example of an association increasingly recognized in Phanerozoic island-arc systems.

Glaucophane and barroisite eclogites from the Upper Kaghan nappe: implications for the metamorphic history of the NW Himalaya

Abstract This paper presents the results of a petrographical and mineral chemical study of glaucophane- and barroisite-bearing eclogites from the Upper Kaghan nappe in the Higher Himalayan Crystallines of the Pakistan Himalaya, and discusses the implications of the P-T path recorded in such rocks for the tectonic and metamorphic history of the NW Himalaya. The eclogites described here come from a previously undescribed outcrop at Gittidas, but belong to the same suite as the garnet-omphacite-phengite-quartz-rutile eclogites previously described elsewhere in the Upper Kaghan nappe. The metamorphic peak assemblage is garnet-omphacite-rutile-quartz in glaucophane eclogite and garnet-omphacite-zoisite-rutile±kyanite±phengite±ankerite in barroisite eclogite. Most samples contain a significant amount of amphibole, white mica and quartz. White mica may be present either as part of the peak assemblage (phengite) or as a retrogressive phase after kyanite (paragonite). Amphibole is later than the metamorphic peak assemblage and is barroisite in most samples, but in relatively Fe-rich eclogite it is glaucophane with significant Na in the A site, Ca in the M4 site and tetrahedral A1. Garnet displays strong prograde zoning in the barroisite eclogites, with Mg increasing and Fe decreasing from core to rim. The iron-rich core is crowded with mineral inclusions of the peak assemblage, but inclusions of earlier paragonite, green and blue-green amphibole were also found. Peak metamorphic conditions in the barroisite eclogites have been estimated at T = 610 ± 30 °C and P = 24 ± 2 kbar from Fe/Mg partition in garnet-omphacite pairs, and from the garnet-omphacite-phengite barometer. These values are close to the equilibration conditions estimated for the eclogites of the North Himalayan Tso-Morari Dome. Strong similarities between metamorphic evolution of the Upper Kaghan nappe and metamorphic evolution of the eclogite-bearing units of the Neelum valley just to the east of the Kaghan valley indicate that the eclogite occurrences of the Kaghan-Neelum area define an eclogite-bearing terrane of regional extent, which was subjected to high pressure metamorphism in middle Eocene times. A comparison of the metamorphic evolution recorded in the eclogites of the NW Himalaya with that of the granulitized eclogites recently discovered in the E Himalaya suggests the possibility of a Himalaya-wide eclogitic metamorphism of pre-Miocene age. Therefore, the main difference between the Higher Himalayan Crystalline nappes of the NW Himalaya and those of the E Himalaya appears to lie less in the early part of their metamorphic evolution than in the different P-T paths they followed during exhumation.

Cathodoluminescence and trace element zoning in quartz phenocrysts and xenocrysts

Quartz phenocrysts and xenocrysts are a common component of volcanic rocks but rarely contribute to our understanding of magmatic or eruptive processes due to an apparent internal chemical and structural homogeneity. Scanning electron microscopy cathodoluminescence imaging (SEM-CL) and secondary ion mass spectroscopy (ion microprobe) analysis (SIMS) are used here to demonstrate that quartz crystals in volcanic rocks show hitherto unrecognised internal structural and chemical zoning. Two contrasting crystal morphologies have been studied. Porphyritic euhedral bipyramidal quartz crystals in dacite from Bonne Nuit Bay, Jersey, UK show fine-scale growth zoning in SEM-CL. Zones are narrow (1–40 μm wide), planar, parallel to crystal faces, and overgrow more broadly zoned, rounded cores. These crystals are interpreted to have formed at small degrees of undercooling by diffusion-controlled growth. Quiescent magma conditions are indicated by the preservation of an Al-rich diffusive boundary layer which has not been removed by melt shearing during convection. In contrast, rounded and embayed porphyritic quartz grains in a quartz dacite from the Esperanza Mine, E1 Salvador, northern Chile show broad internal SEM-CL zones with irregular boundaries, some of which are truncated by grain margins indicating dissolution. Crystal rounding may occur by dissolution or melting due to thermal or thermal-chemical disequilibrium with surrounding melt, and embayments by gas bubble enhanced convective dissolution are consistent with the development of incipient ocelli textures (strings of amphibole inclusions adjacent to quartz rims). Several episodes of rounding can be recognised in some xenocrysts. Ion microprobe (SIMS) analysis of quartz from both samples shows that A1 contents are higher in areas of bright CL emission. Previous studies of CL in quartz have suggested that several factors control luminescence. Although A1 substitution for Si in SiO4 tetrahedra may be partly responsible for SEM-CL zoning recorded in volcanic quartz, additional mechanisms (including the effect of growth rate on defect density) probably also play a major role.

Structural and metamorphic evolution of the Orocopia Schist and related rocks, southern California: Evidence for late movement on the Orocopia fault

The Pelona, Orocopia, and Rand Schists (POR schists) of southern California and southwesternmost Arizona are late Mesozoic or early Tertiary subduction complexes that underlie Precambrian to Mesozoic continental basement along the low‐angle Vincent‐Chocolate Mountains (VCM) fault system. The VCM faults are often considered to be remnants of the original subduction zone, but recent work indicates that many have undergone substantial postsubduction reactivation. In the Orocopia Mountains, for example, the Orocopia Schist exhibits an exceptionally complex structural and metamorphic history due to multiple periods of movement along the Orocopia fault. Structures in the schist include isoclinal folds with axial‐planar schistosity, open‐to‐tight folds that fold schistosity, penetrative stretching lineations, and crenulation lineations, all of which show a nearly 360° range in trend. Folds and lineations that trend approximately NE–SW occur throughout the schist and are thought to be part of an early phase of deformation related to subduction. Folds of this orientation show no consistent vergence. Folds and lineations that trend approximately NW–SE are concentrated near the Orocopia fault and are interpreted to have formed during exhumation of the schist. The NW–SE trending folds, and shear indicators in late‐stage mylonite at the top of the schist, consistently verge NE. The exhumation event culminated in emplacement of the schist against brittlely deformed upper plate. Exhumation of the Orocopia Schist was accompanied by retrograde replacement of garnet, biotite, epidote, and calcic amphibole by chlorite, calcite, and sericite. Matrix amphibole has a lower Na/Al ratio than amphibole inclusions in albite, consistent with a late‐stage decrease in pressure. As NE vergence in the Orocopia Mountains is associated with exhumation of the schist, the NE movement along other segments of the VCM fault may also be late and therefore have no bearing on the facing direction of the VCM subduction zone, contrary to past interpretations.

Eclogites and Blueschists from Northeastern Oman: Petrology and P-T Evolution

Eclogites and blueschists occur in the basement and continental shelf deposits in the Saih Hatat tectonic window of northeastern Oman, where they structurally underlie the Semail ophiolite. These eclogites and blueschists constitute part of a coherent high-P/T metamorphic terrain comprised of interlayered metabasites, metapelites, and quartzofeldspathic and calcareous mica schists. The metabasites record a continuous regional trend of increasing P and T from west to east, where crossite-epidote schists in central and western Saih Hatat grade into blueschists and eclogites at As-Sifah, allowing for the subdivision of this terrain into three metamorphic zones. P-T conditions range from 4·5 to 5·5 kb atT <340°C for zone A to >10·2 kb at 500–580°C for zone C. Zoning patterns of garnets, pyroxenes, and phengite, and the compositions of pyroxene and amphibole inclusions in garnet, indicate that the eclogites and enclosing high-grade blueschists of zone C followed a ‘clockwise’ P-T rpath of increasing P and T followed by increasing T and decreasing P and a final stage of retrogression. Garnets and pyroxenes crystallized at different stages of P-T evolution of the terrain in the various eclogite pods and boudins. This conclusion and the contrasting mineralogies of the eclogites and the enclosing blueschists are attributed to differences in bulk-rock chemistry, fluid availability, or fluid compositions within the terrain. The blueschists and eclogites of this terrain formed as a result of A-type subduction and crustal thickening of the Oman continental margin along an east-northeast-dipping thrust or ‘subduction zone’ which was initiated by the change in plate motion between Africa and Eurasia at 131 Ma. High P/T metamorphism continued until the final stages of ophiolite emplacement in the late Cretaceous, as indicated by the metamorphism of the thrust sheets overlying the coherent terrain.

Magmatic and metamorphic controls on chemical variations within the Eiksunddal eclogite complex, Sunnmøre, western Norway

The Eiksunddal eclogite complex is one of the larger group B (“country-rock”) eclogites within the Western Gneiss Region (WGR) of Norway. Whole-rock chemical data suggest a magmatic origin for the rocks within the eclogite complex. The chemical variations among the eclogitic rocks were controlled by accumulation of olivine (Fo 80) and plagioclase (An 70) and probably magnetite. The composition of the interstitial (“trapped”) liquid is derived by linear regression methods and is a high-Al orthopyroxene normative basalt. The most likely conditions of crystallization are: T ≈ 1150°C, P < 10 kbar. The Al-rich nature of the interstitial liquid may have implications for the formation of anorthosites and leucogabbros associated with similar mafic rocks elsewhere in the WGR. Trace-element characteristics are compatible with a continental origin of these rocks. During the subsequent metamorphic history, the marginal parts of the complex have suffered variable degrees of contamination. Abundant amphibole inclusions in large garnets in garnet websterites suggest local hydration at pre-eclogitic stages. This may be associated with introduction of radiogenic Sr as previously reported by W.L. Griffin and H.K. Brueckner. During prograde metamorphism, alkali exchange occurred during growth of garnet, clinopyroxene and biotite at the expense of pargasitic amphibole and orthopyroxene. The most extensive metasomatism however, was probably associated with the introduction of fluid during amphibolitization (retrogression) of the eclogitic rocks. Three model reactions — eqs. 5–7 — qualitatively explain the relation between degree of retrogression and the observed metasomatic effects. Whether Si and Ca are gained or lost during retrogression seems to be controlled by the extent of amphibole growth in diopside + plagioclase symplectites initially formed by breakdown of omphacite. Alumina has behaved in a more or less immobile way during metasomatism. Low mobility of Al is also reflected by the formation of subsilic (Al-rich) amphiboles and Al-rich plagioclase during garnet breakdown. This demonstrates the importance of kinetic factors in controlling mineral chemistry.

Garnet peridotite from Colorado Plateau ultramafic diatremes: Hydrates, carbonates, and comparative geothermometry

Crystal fragments of pyrope from diatremes of ultramafic microbreccia in the Navajo Province of the Colorado Plateau contain inclusions of olivine, pyroxene, spinel, chlorite, amphibole, chlorapatite, and dolomite. The included suite supports earlier hypotheses that hydrous phases and carbonates were primary parts of some garnet peridotite assemblages in the Plateau lithosphere. Garnets with spinel and orthopyroxene inclusions likely all were sampled at pressures less than 36 kb and perhaps as low as 15–20 kb; no evidence was found for inclusions from greater depths. Temperature estimates are 800°–900° C for garnet-clinopyroxene equilibration, but only 500°–700° C for garnetolivine equilibration. Inherent differences between geothermometry methods account for only part of the discrepancy. Pronounced Fe-Mg zoning in garnet at olivine contacts and the lack of such zoning at clinopyroxene contacts are evidence that the difference in part relates to relative reequilibration rates with cooling. The garnet-olivine temperature estimates may be the best approximations to mantle temperatures before eruption. Our data are compatible both with the hypothesis that the garnet peridotite was emplaced in the mantle by large-scale, horizontal transport in the lithosphere and with the hypothesis that rocks were sampled from Precambrian lithosphere cooled to temperatures like those along a low heat flow geotherm. Discordances between the geothermometers here and in other lherzolite localities may be keys to evaluating tectonic histories of lherzolite masses.