Migmatitic Garnet Research Articles

Petrogenesis of meta-sedimentary rocks in the deep crust of the eastern Gangdese arc

The production of new crustal material at magmatic arcs is dominated by juvenile continental crust growth through the emplacement of voluminous mantle-derived magmatic rocks. There is evidence, however, that such settings can also accommodate the burial of supracrustal material to mid/lower crustal depths resulting in the production of the new crust through metamorphism and anatexis. Investigation of migmatitic garnet- and sillimanite-bearing schists and paragneisses demonstrate that rocks exposed within the exhumed, former middle to lower crust of eastern Gangdese arc, southern Tibet, underwent sillimanite-grade metamorphism and partial melting. Monazite in the studies rocks yield 208 Pb/ 232 Th ages of ca. 72–57 Ma, which overlaps with 206 Pb/ 238 U dates from zircon rim overgrowths of ca. 73–61 Ma. Inherited zircon cores in paragneisses and adjacent low-metamorphic grade Carboniferous sedimentary rocks of the Lhasa terrane yield similar U Pb dates, with main populations at 1220–1070 Ma, 570–470 Ma, and 370–320 Ma. When combined with previously published studies, these new data demonstrate that meta-sedimentary rocks are common in the middle/lower crust of the eastern Gangdese arc and were mainly derived from Carboniferous strata of the Lhasa terrane. The Carboniferous protolith rocks were buried to deep crustal levels via thrusting during crustal shortening during Late Cretaceous (ca. 90–70 Ma) flat subduction of the Neo-Tethyan slab. Subsequent (ca. 70–60 Ma) mantle-derived magmatism induced by slab rollback, likely facilitated metamorphism and anatexis of the sedimentary rocks in the arc. This study provides important new information on the origin of the meta-sedimentary rocks and on how they affected the evolution of the eastern Gangdese arc during the Late Cretaceous to Early Cenozoic. • 73–57 Ma sillimanite-grade meta-sedimentary rocks occur in the eastern Gangdese arc. • The meta-sedimentary rocks derived from the Carboniferous strata of the Lhasa terrane. • Crustal shortening and thrusting delivered the sedimentary rocks to the deep arc crust. • Melting of sedimentary rocks contribute to contemporaneous magmatism.

Shock-induced kelyphite formation in the core of a complex impact crater

We present a compositional and textural analysis of shock-induced microtextures in garnet porphyroblasts in migmatitic garnet–cordierite–biotite paragneisses from the centre of the Vredefort impact structure, South Africa. Detailed imaging and major element analysis of deformation features in, and adjacent to, the garnet porphyroblasts record a complex, heterogeneous distribution of shock effects at the microscale. As the most competent silicate mineral in the assemblage, with the highest Hugoniot Elastic Limit and a wide pressure–temperature stability field, the porphyroblastic garnet preserves a more diverse shock deformation response compared to minerals such as quartz and feldspar, which underwent more comprehensive shock metamorphism and subsequent annealing. The garnet porphyroblasts display pre-impact fractures that are overprinted by later intra-granular Hertzian and distinctive planar fractures associated with the impact event. Shock-induced strain localization occurred along internal slip planes and defects, including pre-existing fractures and inclusion boundaries in the garnet. Symplectitic (kelyphitic) coronas commonly enclose the garnet porphyroblasts, and inhabit intra-granular fractures. The kelyphite assemblage in fractures with open communication beyond garnet grain boundaries is characterized by orthopyroxene—cordierite—sapphirine. Conversely, the kelyphite assemblage in closed-off intra-granular fractures is highly variable, comprising spatially restricted combinations of a secondary garnet phase with a majoritic component, Al-rich orthopyroxene, sapphirine and cordierite. The impedance contrast between garnet porphyroblasts and their inclusions further facilitated the formation of shock-induced features (Al-rich orthopyroxene coronas). Together, the textural and mineralogical data suggest that these features provide a record of oscillatory shock perturbations initiated under confining pressure beneath the transient crater floor. This occurred as the shocked rock volume underwent post-shock expansion, forming the core of the central uplift, and was followed by variable textural re-equilibration. This study thus provides a microtextural and mineralogical perspective of the shock regime within confined crust immediately prior to and during central uplift formation.

Nanogranite inclusions in migmatitic garnet: behavior during piston‐cylinder remelting experiments

AbstractNanogranites represent totally crystallized inclusions of anatectic melt trapped within peritectic minerals of migmatites and granulites. They have recently been discovered in several locations. This discovery opens new possibilities for investigating crustal melting processes, provided that an appropriate method for retrieving the information contained within nanogranite inclusions is available. Here, we describe a series of remelting experiments that have been performed at different temperatures and under dry, andH2O‐added, conditions on nanogranite inclusions hosted in migmatitic garnet, using a piston‐cylinder apparatus. The glasses obtained by quenching the sample from temperature that approaches the trapping temperature have compositions very similar to those of preserved glassy inclusions coexisting with nanogranites in the same cluster. No significant differences inH2O contents were observed for nanogranites rehomogenized under dry and wet conditions. Higher (50–100°C) experimental temperatures resulted in dissolution of the host into the melt and inclusion decrepitation with the loss of volatiles. Therefore, piston‐cylinder remelting experiments may eliminate inclusion decrepitation, maintaining the primary fluid contents in the originally trapped melt. These volatiles would otherwise be lost during remelting experiments at ambient pressure. By preventing volatile loss, the inclusion does not have to be overheated to achieve homogenization, and the compositions of quenched glasses so obtained can be assumed to be those of melts produced (and trapped as inclusions) during crustal anatexis. The experimental approach described here represents a promising technique for the successful rehomogenization of crystallized melt inclusions from high‐pressure environments, such as the mafic continental crust.

Sveconorwegian Mid-crustal Ultrahigh-temperature Metamorphism in Rogaland, Norway: U-Pb LA-ICP-MS Geochronology and Pseudosections of Sapphirine Granulites and Associated Paragneisses

MgAl-rich sapphirine granulites (bulk X Mg 0·71–0·75) occur as boudinaged layers in migmatitic garnet–orthopyroxene–cordierite–spinel gneisses and migmatitic garnet–sillimanite metapelites in the vicinity of the c. 930–920 Ma Rogaland anorthosite–mangerite–charnockite complex, SW Norway. Investigation of the mineral reaction history of the sapphirine granulites and the surrounding paragneisses, combined with geothermobarometric calculations and constraints from pseudosections calculated in the Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 (NCKFMASHT) system, indicates a clockwise P–T path that reached peak-metamorphic ultrahigh-temperature (UHT) conditions of c. 1000°C at c. 7·5 kbar by prograde heating. UHT peak metamorphism is followed by near-isothermal (ultra)high-temperature decompression to P 850–940°C as estimated from Ti-in-zircon thermometry, suggesting that UHT metamorphism and the deduced clockwise P–T evolution is linked to regional Sveconorwegian metamorphism at c. 1010 Ma. Most of the metamorphic zircon surrounds largely resorbed inherited oscillatory zoned zircon cores ( 207 Pb/ 206 Pb apparent ages 1220–1841 Ma), testifying to the sedimentary origin of the sapphirine granulites. Epitactic growth of xenotime on metamorphic zircon at 933 ± 5 Ma is suggested to be related to crystallization of anatectic melt during post-decompressional cooling. The clockwise P–T path culminating at mid-crustal UHT conditions at c. 1010 Ma followed by (U)HT decompression is interpreted to result from collisional tectonics during the early stages of the Sveconorwegian Orogeny, followed by gravitational collapse of the mountain plateau.

Low-pressure regional amphibolite-facies to granulite-facies metamorphism of the Paleoproterozoic Thompson Nickel Belt, Manitoba

The Thompson Nickel Belt is a ca. 35 km × 400 km northeast-trending segment of the northwest margin of the Archean Superior craton in Manitoba, bounded to the west by the Paleoproterozoic Reindeer Zone. The belt was metamorphosed and deformed during the Trans-Hudson orogeny (ca. 1.9–1.7 Ga). Mineral assemblages in metamorphosed pelite, aluminous greywacke, mafic igneous rock, iron formation, and ferruginous wacke define regional metamorphic domains, separated by mineral isograds, that are subparallel to the strike of the belt and to regional-scale D3 structures. An elongate, ca. 5 km × 73 km, central zone of middle amphibolite-facies rocks is characterized by the following: muscovite-bearing mineral assemblages in pelites containing combinations of staurolite, andalusite, and sillimanite; muscovite-free, staurolite + cordierite + garnet-bearing mineral assemblages in greywackes; hornblende-bearing mineral assemblages in mafic metaigneous rocks; and grunerite-bearing mineral assemblages in iron formation. Pressure–temperature (P–T) conditions of the middle amphibolite-facies zone are ca. 550–620 °C and 3.0–5.0 kbar (1 kbar = 100 MPa), with pressure increasing to the northeast. The middle amphibolite-facies zone is bordered to the east and west by an upper amphibolite-facies zone, ca. 5 km wide on the east and ca. 3–5 km on the west. The upper amphibolite-facies zone is characterized by variably migmatitic K-feldspar + sillimanite-bearing mineral assemblages in pelites; migmatitic, garnet + cordierite + sillimanite-bearing mineral assemblages in greywackes; orthopyroxene-free, hornblende-bearing mineral assemblages in mafic rocks; and orthopyroxene-bearing mineral assemblages in iron formations. Pressure–temperature conditions of the upper amphibolite-facies zone are ca. 640–710 °C and 3.0–5.5 kbar in the southeast, and 675–755 °C and 4.5–6.0 kbar in the northwest. The outermost metamorphic zone is of the granulite facies, characterized by migmatitic garnet + cordierite + K-feldspar-bearing assemblages in pelites and greywackes, orthopyroxene + clinopyroxene ± garnet-bearing mineral assemblages in mafic rocks, and orthopyroxene + K-feldspar-bearing mineral assemblages in iron formation in which biotite is unstable. Pressure–temperature conditions of the granulite-facies zone are ca. 775–830 °C and 5.0–7.0 kbar. The P–T paths in the Thompson Nickel Belt appear to be broadly clockwise, except for some domains where they are close to isobaric. The peak P–T conditions, combined with local but widespread development of andalusite, imply relatively steep geothermal gradients of ca. 33–51 °C/km during metamorphism. Regional bathozones (domains of uniform peak-metamorphic pressure) correspond in general but not in detail with the metamorphic-facies zones. They reveal an increase in pressure towards the northeast, suggesting greater degrees of postmetamorphic exhumation in that region. Microstructural analysis suggests that peak metamorphism coincided with, and possibly outlasted, the D2 deformation event. Metamorphic isograds were deformed by D3–D4 structures. These features are consistent with a tectonic model in which the Superior craton moved in a northwest or west-northwest direction relative to the Reindeer Zone, with greatest convergence and tectonic burial occurring at the Thompson promontory.

Eocene high grade metamorphism and crustal anatexis in the North Himalaya Gneiss Domes, Southern Tibet

Determination of the timing and geochemical nature of early metamorphic and anatectic events in the Himalayan orogen may provide key insights into the physical and chemical behavior of lower crustal materials during the early stage of tectonic evolution in large-scale collisional belts. The Yardoi gneiss dome is the easternmost dome of the North Himalayan Gneiss Domes (NHGD), and contains three types of amphibolites with distinct mineral assemblage, elemental and radiogenic isotope geochemistry, as well as various types of gneisses. SHRIMP zircon U/Pb analyses on the garnet amphibolite and garnet-bearing biotite granitic gneiss yield ages of nearly peak metamorphism at 45.0±1.0 Ma and 47.±1.8 Ma, respectively, which are 2 to 4 Ma older than the age for partial melting in migmatitic garnet amphibolite (43.5±1.3 Ma). Available data have demonstrated that ultra-high pressure metamorphism in the Tethyan Himalaya occurred at ∼55 Ma, and high amphibolite facies to granulite facies metamorphism at 45 to 47 Ma. In addition, partial melting at thickened crustal conditions occurred at 43.5±1.3 Ma, which led to the formation of high Sr/Y ratios two-mica granites. The high-grade metamorphic rocks in the NHGD may represent the subducted front of the Indian continental lithosphere. In large collisional belts, fertile components in crustal materials could melt and form granitic melts with relatively high Na/K and Sr/Y ratios under thickened crustal conditions, significantly different from those formed by decompressional melting during rapid exhumation.

Application of Lu–Hf garnet dating to unravel the relationships between deformation, metamorphism and plutonism: An example from the Prince Rupert area, British Columbia

The tectonic history of the Prince Rupert area is marked by profound crustal thickening from thrusting that produced inverted metamorphic field gradients and transpression in crustal-scale shear zones. Syn-tectonic garnet in the Prince Rupert area has Lu–Hf ages of 102.6 ± 3.7 Ma and 108.3 ± 4.1 Ma (2 σ). Porphyroblast–matrix relationships in these samples, and samples from the same outcrops, indicate syn-tectonic garnet growth. These relationships imply that the garnet ages directly date the development of the metamorphic foliations. A third sample of migmatitic garnet amphibolite from the contact aureole of the 94–90 Ma Ecstall pluton had complex isotope systematics interpreted to indicate a garnet growth episode ∼ 105 Ma, similar to the ages obtained from the other samples and a growth or equilibration event at ∼ 94 Ma during pluton emplacement. The data show that the older Lu–Hf garnet ages date prograde metamorphism during foliation development and modification during pluton emplacement. The Ecstall pluton was emplaced 10 to 15 m.y. after regional metamorphism and thrust stacking in the Prince Rupert area. In order to place our samples in a regional tectonic context we compare our results to patterns of regional deformation, metamorphism and plutonism throughout the North American Cordillera in the time period between 110 and 85 Ma. Contractional and transpressional deformation occurred throughout much of the North American Cordillera at this time, from southeastern Alaska to the Baja Peninsula in Mexico. Left-lateral transpression dominated the Canadian Cordillera, whereas, right-lateral transpression affected areas south of the Idaho–Salmon River suture zone, including the Sierra Nevada batholith. This reversal in kinematics in the northern and southern cordillera within coeval magmatic belts appears to be a first-order feature of the geology of the North American Cordillera during the Cretaceous.

Evidence for 930 Ma metamorphism in the Shetland Islands, Scottish Caledonides: implications for Neoproterozoic tectonics in the Laurentia–Baltica sector of Rodinia

Abstract: Zircon and monazite laser-ablation inductively coupled plasma mass spectrometry U–Pb geochronological data for two metasediment samples from the Westing Group, northern Shetland Islands, Scottish Caledonides yield ages between 938 ± 8 and 925 ± 10 Ma (Tonian) for upper amphibolites-facies metamorphism. Texturally early metamorphism is recorded by a migmatitic garnet + sillimanite + plagioclase + muscovite + biotite assemblage, which formed at c . 650–700 °C and 7 kbar. Subsequent reworking resulted in the growth of a secondary garnet + kyanite + plagioclase + muscovite + biotite assemblage at c . 650 °C and 8–9 kbar. In situ electron probe microanalysis (EPMA) U–Th–Pb chemical dating of monazite hosted within garnet grains and the matrix of one sample also give Tonian ages, apparently indicating that all the metamorphism occurred during the Neoproterozoic. However, the dominant structural fabrics appear to have formed during the Ordovician–Silurian Caledonian orogeny, suggesting that the reworking was substantially younger despite the apparent absence of Caledonian monazite or zircon ages. Detrital zircons are consistent with Laurentia–Baltica provenance. Deposition of the Westing Group is constrained to between c . 1030 and 930 Ma. The timing of Tonian metamorphism suggests possible correlations with sequences elsewhere in the northern Caledonides, including the Krummedal Succession of East Greenland and Laurentian-derived successions in Svalbard and northern Norway. Supplementary material: U–Pb LA-ICPMS and EPMA data are available at http://www.geolsoc.org/SUP18379 .

Magmatic loading in the proterozoic Epupa Complex, NW Namibia, as evidenced by ultrahigh-temperature sapphirine-bearing orthopyroxene–sillimanite–quartz granulites

MgAl-rich sapphirine-bearing orthopyroxene–sillimanite–quartz granulites in the Proterozoic Epupa Complex, NW Namibia, occur as restitic schlieren in migmatitic garnet–orthopyroxene granulites. Sapphirine is present as (i) prograde inclusion in peak-metamorphic phases, (ii) peak-metamorphic porphyroblasts and, (iii) in retrograde symplectites. Reaction textures could be used to infer a detailed sequence of mineral reactions. Results from conventional geothermobarometry and P– T pseudosections point to an anticlockwise P– T path that reached ultrahigh-temperature (UHT) conditions. During near isobaric heating biotite was consumed by orthopyroxene-producing melting reactions. Inclusions of cordierite and spinel in the peak phases suggest that heating proceeded at moderate pressures of ca. 5–7 kbar. During subsequent loading orthopyroxene–sillimanite (±quartz (±garnet)) assemblages were formed at peak temperatures of ca. 1000 °C and 9.5 kbar. In addition, porphyroblastic sapphirine was formed in silica-undersaturated very MgAl-rich samples and domains. The inferred ultrahigh-temperatures are consistent with the high Al 2O 3 content of orthopyroxene (up to 11.9 wt%). During subsequent cooling high-Al orthopyroxene recrystallised to lower Al-orthopyroxene, which coexisted with garnet or sapphirine at ca. 970 °C. Sapphirine-bearing symplectites are interpreted to indicate ultrahigh-temperature near isothermal decompression to P < 6.5 kbar. During continued high-temperature decompression to ca. 5.5 kbar/800 °C sapphirine was replaced by spinel-bearing symplectites. Subsequent re-growth of biotite probably reflects mineral–melt interactions during late cooling. The anticlockwise P– T path and the subsidence of the granulites to crustal levels of ca. 30 km is interpreted to result from the syn-UHT magmatic accretion of anorthositic melts onto the granulite layer (i.e. magmatic loading), which led to heating of the granulites from above.

Unpairing metamorphic belts: P–T paths and a tectonic model for the Ryoke Belt, southwest Japan

Southwest Japan is divided into Outer and Inner Zones by the Median Tectonic Line (MTL), a major transcurrent fault. The Outer Zone is composed of the Sambagawa (high‐pressure intermediate or high P/T type metamorphism), Chichibu and Shimanto Belts. In the Inner Zone, the Ryoke Belt (andalusite– sillimanite or low P/T type metamorphism) was developed mainly within a Jurassic accretionary complex. This spatial relationship between high P/T type and low P/T type metamorphic belts led Miyashiro to the idea that metamorphic belts were developed as ‘paired’ systems. Textural relationships and petrogenetically significant mineral assemblages in pelites from the Ryoke Belt imply peak P–T conditions of ≈5 kbar and up to 850 °C in migmatitic garnet–cordierite rocks from the highest‐grade metamorphic zone. It is likely that the thermal anomaly responsible for metamorphism of the Ryoke Belt was related to a segment of the Farallon–Izanagi Ridge as it subducted under the eastern margin of the Asian continent during the Cretaceous. The sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dP/dT slope, inferred to have been generated by a nested set of hairpin‐like ‘clockwise’P–T paths. These P–T paths are characterized by limited prograde thickening, minor decompression at peak‐T , and near‐isobaric cooling, features that may be typical of P–T paths in low P/T type metamorphic belts caused by ridge subduction. A ridge subduction model for the Ryoke Belt implies that juxtaposition of the high‐P/T metamorphic rocks of the Sambagawa Belt against it was a result of terrane amalgamation. Belt‐parallel ductile stretching, recorded as syn‐metamorphic, predominantly constrictional strain in both Ryoke and Sambagawa Belt rocks, and substantial sinistral displacement on the MTL are consistent with left‐lateral oblique convergence. Diachroneity in fast cooling of the Ryoke Belt is implied by extant thermochronological data, and is inferred to relate to progressive SW to NE docking of the Sambagawa Belt. Thus, an alternative interpretation of ‘paired’ metamorphic belts in Japan is that they represent laterally contemporaneous terranes, rather than outboard and inboard components of a trench/arc ‘paired’ system. Amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of ‘paired’ metamorphic belts in the geological record.

Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California

The abundance, P- T stability, solubility, and element-partitioning behavior of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600°C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high- T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM. In order of increasing alteration from a probable metabasalt protolith, three common types of garnet amphibolite blocks in the Catalina Schist are: (1) non-migmatitic, clinopyroxene-bearing blocks, which are compositionally similar to MORB that has lost an albite component; (2) garnet-amphibolite blocks, which have rinds that reflect local interaction between metabasite, metaperidotite, and fluid; and (3) migmatites that are extremely enriched in Th, HFSE, LREE, and other trace elements. These trace-element enrichments are mineralogically controlled by rutile, garnet, sphene, apatite, zircon, zoisite, and allanite. Alkali and alkaline earth elements are much less enriched in the solid assemblage, and thus appear to be decoupled from the other elements in the inferred metasomatic process(es). The compositions of migmatitic garnet amphibolite blocks seem to complement that of “average” island-arc tholeiite. Trace-element metasomatism reflects fluid-solid, rather than melt-solid, interaction. The metasomatic effects indicate that H 2O-rich fluid, perhaps with a significant component of Na-Al silicate and alkalis, carried Th, U, Sr, REE, and HFSE. Fractionations of LREE in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex. “Exotic” LREE deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment. If the paleo-subduction zone represented by the mafic and ultramafic complex of the Catalina Schist had continued its thermal and fluid evolution, a selvage of similarly enriched rocks might have been generated along the slab-mantle wedge contact between ~30 and 85 km depth. Rocks affected by “subduction-zone metasomatism,” although rarely recognized at the surface, could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments.

Conditions of high grade metamorphism in the Dalradian of Connemara, Ireland

In E Connemara, Ireland, Dalradian metasediments were initially metamorphosed to kyanite-staurolite grade at pressures above 7kbar. The subsequent emplacement of basic to intermediate magmas, accompanied by uplift and erosion, produce a steepening of the geothermal gradient and led to the formation of migmatitic garnet–cordierite–sillimanite-gneisses in the highest grade regions in the S. Cation exchange equilibria between coexisting mineral pairs suggest that partial melting took place under high temperatures (≃ 750°C) and moderate pressures (4.5–6 kbar). Fluid inclusion studies and andalusite occurrences from migmatite leucosomes suggest that crystallization of the melts occurred at lower pressures (&lt;3.5 kbar) than melting, implying that uplift and erosion occurred while the rocks were hot.