Anhydrous Granulites Research Articles

Hydration, melt production and rheological weakening within an intracontinental gneiss dome

The Entia Gneiss Complex (EGC) in central Australia represents a deeply exhumed Paleoproterozoic basement terrane that underwent fluid-catalysed transformation culminating in the formation of a migmatitic gneiss dome during the 450–300 Ma intracontinental Alice Springs Orogeny (ASO). Foremost amongst the changes the EGC experienced during this event was the regional-scale conversion of nominally anhydrous granulite and igneous crust to hydrous amphibolite facies migmatite, together with the emplacement of locally voluminous mica-bearing pegmatite swarms. LA–ICP–MS U–Pb dating of zircon, monazite and xenotime demonstrates that pegmatite emplacement dominantly occurred late in the Alice Springs Orogeny (c. 360–300 Ma). The oldest generation of pegmatites are transposed into the regional Palaeozoic gneissic fabric and formed at c. 1770 Ma, indicating they belong to protolith rocks of the EGC. Three subsequent generations of pegmatite have ages that range between 360 and 300 Ma. The oldest of these is parallel to the migmatitic gneissic fabric and dated at c. 360 Ma, whereas the younger two generations are largely undeformed, discordant and intruded throughout the period c. 350–300 Ma. The capability for a comparatively dry intracratonic orogenic core to repeatedly produce melt over an interval of c. 60 Myr requires explanation. We suggest crustal-scale duplex development during the latter stages of the ASO resulted in the juxtaposition of dry and mechanically strong Paleoproterozoic basement over deeply buried Neoproterozoic–Cambrian sedimentary sequences. Fluid derived from the metamorphic dehydration of these sequences led to hydration, partial melting and rheological weakening of the basement slab. Prior to its hydration at c. 360 Ma, the EGC was metamorphically unresponsive, accounting for its comparatively late timing of deformation and partial melting compared to the overall history of the Alice Springs Orogeny. The development of the crustal-scale duplex system, combined with hydrous melting of the low-density basement slab beneath high-density supracrustal rocks, resulted in a thermomechanically unstable architecture that facilitated the formation of an orogen-scale gneiss dome during the waning stages of intracontinental orogenesis.

Global type area charnockites in southern India revisited: Implications for Earth's oldest supercontinent

The global ‘type area’ charnockites and those in the surrounding localities within Madras block of the Southern Granulite Terrane in India dominantly comprise felsic to intermediate, coarse to medium grained, orthopyroxene-bearing anhydrous granulite facies rocks with sporadic garnet. In several localities, the charnockitic suite contains mafic magmatic enclaves of gabbroic to dioritic composition showing calcic and peraluminous composition. The charnockite suite shows compositional range from monzonite through granodiorite to granite, and are calcic and calc-alkalic, peraluminous, including both magnesian and ferroan types. Their major and trace element variations are consistent with progressive magmatic differentiation. The charnockites show high Ba-Sr content with a trend from normal arc-related rocks to adakites. The geochemical features of the charnockites and mafic enclaves are consistent with subduction-related arc setting, and slab-derived magmas interacting with mantle wedge peridotite. The P-T conditions as estimated through mineral phase equilibria modelling and pseudosection computations of representative charnockite and mafic enclave samples show a range of ca. 7 to 9 kbar and 870 to 960 °C, suggesting high- to ultra-high temperature granulite facies conditions during the peak metamorphism.The zircon grains from the charnockites show magmatic features with oscillatory or banded zoning, and in many cases display core-rim structure indicating dissolution and metamorphic overgrowth. The magmatic grains/domains show typical steep LREE to HREE pattern, whereas the metamorphic domains show relatively flat HREE. Magmatic zircon U-Pb data indicate crystallization ages of ca. 2.53 Ga to 2.57 Ga. The identical age from magmatic zircon grains in the mafic enclaves suggests bimodal magmatism with underplated mafic magmas intruding into the felsic magma chamber. The U-Pb data from metamorphic zircon and monazite indicate that all the rocks were metamorphosed coevally at ca. 2.47 Ga to 2.49 Ga, soon after their emplacement. The close timing between magmatism and metamorphism of ca. 40 Myr also suggests the formation of the magmatic suite along an active convergent margin, followed by collisional metamorphism during the termination of subduction and ocean closure. The Lu-Hf analyses of magmatic domains in zircon show mostly positive ɛHf(t) values up to +8.7, with only a few spots showing slightly negative values up to −0.8. Zircon grains in the mafic enclaves also show mostly positive ɛHf(t) values up to +4.3. The U-Pb-Hf data are consistent with juvenile arc building during late Neoarchean, with no significant older components. Together with the Hf model ages, the data indicate that the magmas were derived from depleted mantle components of Meso- to Neoarchean age, which would suggest an active subduction regime that continued until the ocean closure during end Neoarchean-earliest Paleoproterozoic. The granulite blocks surrounding the southern margin of the Dharwar craton including the Madras block are interpreted as multiple arcs that coalesced and accreted onto the craton during the Neoarchean-Paleoproterozoic transition. These blocks, which are dominated by charnockites and ranging in age from Mesoarchean to late Neoarchean, and their equivalents in other cratonic fragments over the globe, can be correlated to the ‘expanded Ur’, in building the oldest supercontinent on Earth.

High Ba–Sr adakitic charnockite suite from the Nagercoil Block, southern India: Vestiges of Paleoproterozoic arc and implications for Columbia to Gondwana

The Nagercoil block is the southernmost crustal segment of the Southern Granulite Terrane (SGT) in India and is mainly composed of charnockitic rocks and felsic gneisses (charnockite suite). In this study, we present petrologic, geochemical, zircon U–Pb, REE, and Hf isotopic studies on the charnockites and leucogneiss from the Nagercoil block. Based on field investigations and petrologic studies, the charnockites can be divided into garnet-bearing and garnet-absent anhydrous granulite facies rocks with orthopyroxene. The charnockites and leucogneiss show transition from adakites to non-adakitic magmatic rocks, with enrichment in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and depletion in HREEs (heavy rare earth elements) and HFSEs (high field strength elements). Some of the charnockites and the leucogneiss show typical HSA (high silica adakite) characters, (high SiO2, Al2O3, Ba–Sr, La/Yb, and Sr/Y). The HSA is considered to have formed from the interaction of slab derived melts and peridotitic mantle wedge. The high Ba–Sr features were possibly inherited from subducted oceanic crust melting under high thermal gradient during Precambrian. The magmas were underplated and subjected to fractional crystallization. Zircon grains from the charnockite and leucogneiss show zoned magmatic cores surrounded by structureless metamorphic rims. Magmatic zircon grains from the charnockites show ages ranging from 1983 ± 8.8 Ma to 2046 ± 14 Ma, and the metamorphic domains show an age range of 502 ± 14 Ma to 547 ± 8.7 Ma. Zircon from the leucogneiss yielded magmatic and metamorphic ages of 1860 ± 20 Ma and 575.6 ± 8.8 Ma. Both charnockites and leucogneiss show two prominent age peaks at 1987 Ma and 568 Ma. The REE data of the zircon grains show LREE depletion and HREE enrichment, with the metamorphic grains showing more depletion in HREE. Zircon Hf isotopic data of the magmatic cores of zircon grains from the charnockite yielded εHf(t) values from −1.17 to 0.46 with TDM and TDMC and age peaks at 2392 Ma and 2638 Ma, suggesting Neoarchean to Paleoproterozoic juvenile sources. We suggest that the high Ba–Sr adakitic charnockite suite from the Nagercoil block formed in a Paleoproterozoic magmatic arc setting during the assembly of the Columbia supercontinent, and underwent high-grade metamorphism associated with the amalgamation of the Gondwana supercontinent during the late Neoproterozoic–Cambrian. Our study provides new insights into the vestiges of Columbia fragments within the Gondwana assembly with two distinct cycles of crustal evolution.

Element Mobility and Behaviour of Zircon during HT Metasomatism of Ferroan Basic Granulite at Ayyarmalai, South India: Evidence for Polyphase Neoarchaean Crustal Growth and Multiple Metamorphism in the Northeastern Madurai Province

An exposure of polyphase, Neoarchaean–early Palaeoproterozoic basement rocks has been identified in the northeastern part (10°53·44'N, 78°22·88'E) of the Madurai Province. The dominant migmatitic charnockite contains older enclaves of isoclinally folded syenitic biotite gneiss and boudinaged mafic dykes. A highly evolved (Mg# 10) and high field strength element (HFSE) + rare earth element (REE)-enriched ferroan basic dyke body exhibits a distinct mineralogical zonation produced during intense syn-boudinage infiltration metasomatism, coeval with high-grade metamorphism and anatexis of the host charnockite. Core domains that preserve an anhydrous granulite assemblage (domain A: garnet, clinopyroxene and minor plagioclase) grade through a narrow mineralogical transition zone (domain B) into a broad amphibole-rich rind (domain C: ferropargasite + K-feldspar ± plagioclase, quartz). Pseudosection modelling and thermobarometry yields P–T estimates of ∼800 °C, 8 kbar for the granulite-facies metamorphism (M1) and ∼730 °C, 7 kbar for the high-grade metasomatism (M2). The changes in fabric, mineral assemblage and whole-rock chemistry across domains A to C reveal near-isovolumetric–isochoric conditions of infiltration-driven metasomatism and an unusual enrichment in large ion lithophile elements (LILE), REE, and HFSE, causing prolific neoblastesis of apatite and zircon, and attest attainment of chemical equilibrium. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analyses for major constituents and accessory phases in the metasomatic domains provide a new set of equilibrium distribution coefficients for the REE. LA-ICP-MS spot analyses of zircon in well-documented microstructural settings identified the following events: ∼2·67 Ga: intrusions of foid-bearing syenite and ferroan basic dykes during crustal extension; ∼2·6 Ga: high-grade metamorphism M1; ∼2·6 Ga: emplacement of voluminous arc-type intrusions (the protolith of charnockite); ∼2·48 Ga: high-grade metamorphism and anatexis M2 of the charnockite protolith and metasomatism of the ferroan metabasite. A distinct Pan-African overprint (M3, ∼610 Ma, 520 Ma) of the Palaeoproterozoic high-pressure rocks is manifested by hydration related to the exhumation of the deep-seated granulites to mid-crustal levels. This study confirms the continuation of Neoarchaean crust farther south beyond the perceived Palghat Cauvery shear zone system and contradicts the view that this shear zone system represents the Neoproterozoic–Cambrian suture zone along which the Mozambique Ocean was closed.

Contrasts in sillimanite deformation in felsic tectonites from anhydrous granulite- and hydrous amphibolite-facies shear zones, western Canadian Shield

The deformation behavior of crustal materials in variably hydrated metamorphic environments can significantly influence the rheological and seismic properties of continental crust. Optical observations and electron backscatter diffraction (EBSD) analyses are used to characterize sillimanite deformation behavior in felsic tectonites from two deformation settings in the Athabasca granulite terrane, western Canadian Shield. Under estimated conditions of 0.8–1.0 GPa, 725–850 °C in the Cora Lake shear zone, the data suggest that sillimanite deformed by dislocation creep with slip in the [001] direction accompanied by subgrain rotation recrystallization. Where sillimanite locally remained undeformed, strain was concentrated in surrounding weaker phases. Under hydrated conditions of 0.4–0.6 GPa, 550–650 °C in the Grease River shear zone, textures and cathodoluminescence imaging point to dissolution-precipitation creep as the major deformation mechanism for sillimanite, resulting in synkinematic growth of foliation-parallel euhedral sillimanite in a preferred orientation with [001] parallel to the lineation. The results suggest that temperature, fluid content, and modal mineralogy of the surrounding phases may all have significant influence on sillimanite deformation but that preferential alignment of sillimanite [001] parallel to the lineation persists regardless of contrasts in the conditions or mechanisms of deformation.

Polymetamorphism in the mainland Lewisian complex, NW Scotland – phase equilibria and geochronological constraints from the Cnoc an t’Sidhean suite

AbstractThe metamorphic evolution of rocks cropping out near Stoer, within the Assynt terrane of the central region of the mainland Lewisian complex of NW Scotland, is investigated using phase equilibria modelling in the NCKFMASHTO and MnNCKFMASHTO model systems. The focus is on the Cnoc an t’Sidhean suite, garnet‐bearing biotite‐rich rocks (brown gneiss) with rare layers of white mica gneiss, which have been interpreted as sedimentary in origin. The results show that these rocks are polymetamorphic and experienced granulite facies peak metamorphism (Badcallian) followed by retrograde fluid‐driven metamorphism (Inverian) under amphibolite facies conditions. The brown gneisses are inferred to have contained an essentially anhydrous granulite facies peak metamorphic assemblage of garnet, quartz, plagioclase and ilmenite (±rutile, K‐feldspar and pyroxene) with biotite, hornblende, muscovite, chlorite and/or epidote as hydrous retrograde minerals. P–T constraints imposed by phase equilibria modelling imply conditions of 13–16 kbar at >900 °C for the Badcallian granulite facies metamorphic peak, consistent with the field evidence for partial melting in most lithologies. The white mica gneiss comprises a muscovite‐dominated matrix containing porphyroblasts of staurolite, corundum, kyanite and rare garnet. Previous studies have suggested that staurolite, corundum, kyanite and muscovite all grew at the granulite facies peak, with partial melting and melt loss producing a highly aluminous residue. However, at the inferred peak P–T conditions, staurolite and muscovite are not predicted to be stable, suggesting they are retrograde phases that grew during amphibolite facies retrograde metamorphism. The large proportion of mica suggests extensive H2O‐rich fluid‐influx, consistent with the retrograde growth of hornblende, biotite, epidote and chlorite in the brown gneisses. P–T conditions of 5.0–6.5 kbar at 520–550 °C are derived for the Inverian event. In situ dating of zircon from samples of the white mica gneiss yield apparent ages that are difficult to interpret. However, the data are permissive of granulite facies (Badcallian) metamorphism having occurred at c. 2.7–2.8 Ga with subsequent fluid driven (Inverian) retrogression at c. 2.5–2.6 Ga, consistent with previous interpretations.

Genesis of aluminous and intermediate granulites: A case study in the eastern Sierras Pampeanas, Argentina

We report petrologic and geochemical features of anhydrous granulites from the eastern Sierras Pampeanas, Argentina. Aluminous Grt–Crd-bearing granulite is the most abundant rock type in the study region, and hosts intermediate Opx–Grt bearing granulites as interlayered lenses. Both granulite types crystallized under similar P– T conditions of T = 850–900 °C and P c. 7.5 kbar. The formation of Grt–Crd granulites by subsolidus dehydration conflicts with the absence of K-feldspar in them. Moreover, the aluminous granulites have an REE pattern that precludes postulating they derived by close-system metamorphism of an extensively weathered protoliths. Comparing mineral and rock chemistry with experimental results in simple systems (ACFM, AKF, and AFM) permits the mineralogical changes and the bulk composition evolution experienced by granulites during melting to be identified. This work demonstrates that orthopyroxene is a primary phase in Opx–Grt granulites, however never crystallized within aluminous granulites. The aluminous granulites have both mineral and chemical compositions matching those expected for a residuum derived from the partial melting of a greywacke-like sedimentary precursor. Compared with the composition of the average continental crust, aluminous granulites are depleted in most lithophile trace elements and LREE, have somewhat low K / Rb ratios, and high contents of elements having high partition coefficients for garnet and/or ilmenite (HREE—Y, Cr, Ni). However, aluminous granulites have initial Sr isotope ratios (< 0.708) significantly lower than those measured in the rest of the metasedimentary rocks of the region (> 0.713). We discuss the possibility that an intense but short-lived melting event led to isotopic disequilibrium during melting of biotite-bearing protoliths, leaving a granulitic residuum with lower initial Sr isotope ratios. In contrast, Opx–Grt granulites have trace element features (e.g., high Sr contents and REE patterns with marked positive Eu anomalies) that do not support the idea that they are simply residues left after biotite-dehydration melting and melt extraction. Rather, these rocks display several petrological, major and trace element and isotopic features that suggest they formed through the interaction of mafic magmas and aluminous granulites.

Metamorphic evolution of the Río Santa Rosa Granulites, northern Sierra de Comechingones, Argentina

Highly anhydrous granulites from Río Santa Rosa in the eastern Sierras Pampeanas of Argentina occur as a thick lens surrounded by melt-depleted migmatites. Grt–Crd granulite composed of Qtz+Pl+Grt+Crd+Ilm±Spl±Ath±Phl is the dominant rock, whereas Opx–Grt granulite appears as discontinuous lenses in the center of the granulite body. Grt–Crd granulite includes blocks of metabasite that are relics of refractory lithologic beds interlayered in the supracrustal sequence. A distinct assemblage composed of Qtz, Pl, Grt, Crd, Opx, Spl, Crn, Sil, Bt, Phl, Ath, and Fe–Ti oxides in different combinations was generated in a reaction zone between Grt–Crd granulites and metabasites at peak metamorphism (850–900 °C and 7.6±0.5 kbar). The P– T trajectory of Grt–Crd granulites suggests an early prograde garnet-forming stage followed by nearly isothermal decompression that caused garnet breakdown. Melting and melt draining accompanying garnet growth was active during heating (to 900 °C) at intermediate pressures (∼7.6 kbar). Peak P– T estimates for Opx–Grt granulites are similar to those obtained with Grt–Crd granulites, which indicates that both granulites passed through the highest thermal stage. These results constrain the late evolution of Opx–Grt granulite to a garnet-consuming stage. Furthermore, they imply that garnet formation in Opx–Grt granulite happened at an early prograde P– T trajectory. Garnet growth in Opx–Grt granulite cannot result from heating at high pressure, which would lead to an apparent contradiction in the prograde P– T paths of the two granulites. This discrepancy may be solved by demonstrating that Opx–Grt granulite is the product of synmetamorphic mafic magmatism that was contaminated while cooling. The Río Santa Rosa granulites are inferred to have formed in a thickened crust in which mafic magmatic activity providing a local heat input.

Clockwise P– T paths for crustal melting and H 2O recycling in granite source regions and migmatite terrains

Clockwise pressure–temperature ( P– T) paths of metamorphism pass through two kinds of melting reactions with increasing temperature. The first generates H 2O-saturated partial melts at the H 2O-saturated solidus, only if excess H 2O is present. Usually, this accounts for less than 3% melt. The second generates H 2O-undersaturated melts through dehydration-melting reactions (DMRs) of micas, or other hydrous minerals, and occurs at temperatures higher than the H 2O-saturated solidus. If H 2O-undersaturated melts remain in situ, and the source regions are not exhumed, then the crystallisation reactions encountered along the near-reverse P– T path may produce the same mineralogy as in the source rock. In regions of exhumation, along the decompression segments of clockwise P– T paths, two other types of crystallisation reactions occur. The better-known type occurs at the recrossing of the pelite H 2O-saturated solidus at shallower depth. This can occur at mid-crustal depths or much shallower (at pressures lower than the appropriate invariant point which relates all of these reactions), and will result in granitic pegmatites. At the other type, remaining mica can react with hydrous melt to generate anhydrous granulite with free H 2O fluid. Such suprasolidus decompression–dehydration reactions (SDDRs) result in freezing of pelitic melts at mid-crustal depth to K-feldspar+quartz assemblages, and release of H 2O which can induce H 2O-saturated melting in adjacent fertile lithologies lacking H 2O (e.g., psammite, older granite). Distinct mineral proportions and textures will result from each reaction. The actual reactions occurring are related through specific invariant points whose locations are known in the P– T space. The displacement of these invariant points due to additional components in the solid, melt and fluid phases is reasonably well known. The flat-lying SDDR (very low d P/d T) will extend to much higher pressures if a low-aH 2O fluid is present. SDDR involving amphibole or epidote will occur in mafic to intermediate compositions, over a large depth range depending on Fe/Mg, Ca/Na, etc., of the rock compositions.

Petrology and Ar-Ar dating of granulites from the Galicia Bank (Spain): African craton relics in Western Europe

Intermediate orthogranulites were collected on the western flank of the Galicia bank during the Galinaute II cruise in 1995. The petrography of these rocks reveals two types of granulites. The first type is hydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + hornblende + garnet + biotite + opaque + zircon + apatite assemblage. Both hornblende and orthopyroxene define a weak foliation plane. A late deformation event is expressed by some fractures cross-cutting the foliation. The second is anhydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + clinopyroxene + opaque + zircon + apatite assemblage. The rocks display a granoblastic texture and are affected by brittle deformation as testified by the development of numerous microfractures. The P-T conditions (7 ± 1 Kbar, 750 ± 50 °C) calculated from two representative samples demonstrate that the rocks equilibrated under granulite facies conditions. Ar-Ar dating gives Precambrian ages ranging between ca. 2500–2000 Ma for the amphibole from the hydrous granulite and 1600–1500 Ma for the core of the K-feldspar from the anhydrous and hydrous granulites. A younger age of 900 Ma is obtained from the recrystallized rims of the K-feldspar from the two samples. These data indicate that the granulitic rocks in the Galicia Bank had already been exhumed and cooled below ca. 140–400 °C (blocking T° for K-feldspar) in Precambrian times (900 Ma). Given the very well preserved granulitic minerals assemblage of the rocks, the granulites behaved as competent and metastable boudins during their exhumation. The granulitic samples were previously interpreted as fragments of the lower continental crust sampled by the main detachment fault during Cretaceous rifting, but they were part of an upper continental crust from the Precambrian. Geochronological data and petrological assemblages suggest that the granulite blocks in the Galicia Bank probably were derived from the North Armorican Domain (northern part of France) where a Precambrian terrain outcrops. The opening of the Bay of Biscay could be responsible for the scattering of the Precambrian terrain and may explain the presence of the granulitic blocks on both sides of the Bay of Biscay. During the subduction of Europe below the Iberian peninsula the granulite blocks were transported southward and incorporated into a Cretaceous conglomerate forming the accrecionary prism on the Northern Iberia Margin. The granulite facies blocks found on the Galicia Bank represent another example of Gondwanian relics supporting the idea that the West European plate belonged to the West African craton during the Proterozoic. © 2000 Éditions scientifiques et médicales Elsevier SAS

Petrology and Ar-Ar dating of granulites from the Galicia Bank (Spain): African craton relics in Western Europe

Intermediate orthogranulites were collected on the western flank of the Galicia bank during the Galinaute II cruise in 1995. The petrography of these rocks reveals two types of granulites. The first type is hydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + hornblende + garnet + biotite + opaque + zircon + apatite assemblage. Both hornblende and orthopyroxene define a weak foliation plane. A late deformation event is expressed by some fractures cross-cutting the foliation. The second is anhydrous granulites with K-feldspar + plagioclase + quartz + orthopyroxene + clinopyroxene + opaque + zircon + apatite assemblage. The rocks display a granoblastic texture and are affected by brittle deformation as testified by the development of numerous microfractures. The P–T conditions (7 ± 1 Kbar, 750 ± 50 °C) calculated from two representative samples demonstrate that the rocks equilibrated under granulite facies conditions. Ar-Ar dating gives Precambrian ages ranging between ca. 2500–2000 Ma for the amphibole from the hydrous granulite and 1600–1500 Ma for the core of the K-feldspar from the anhydrous and hydrous granulites. A younger age of 900 Ma is obtained from the recrystallized rims of the K-feldspar from the two samples. These data indicate that the granulitic rocks in the Galicia Bank had already been exhumed and cooled below ca. 140–400 °C (blocking T° for K-feldspar) in Precambrian times (900 Ma). Given the very well preserved granulitic minerals assemblage of the rocks, the granulites behaved as competent and metastable boudins during their exhumation. The granulitic samples were previously interpreted as fragments of the lower continental crust sampled by the main detachment fault during Cretaceous rifting, but they were part of an upper continental crust from the Precambrian. Geochronological data and petrological assemblages suggest that the granulite blocks in the Galicia Bank probably were derived from the North Armorican Domain (northern part of France) where a Precambrian terrain outcrops. The opening of the Bay of Biscay could be responsible for the scattering of the Precambrian terrain and may explain the presence of the granulitic blocks on both sides of the Bay of Biscay. During the subduction of Europe below the Iberian peninsula the granulite blocks were transported southward and incorporated into a Cretaceous conglomerate forming the accrecionary prism on the Northern Iberia Margin. The granulite facies blocks found on the Galicia Bank represent another example of Gondwanian relics supporting the idea that the West European plate belonged to the West African craton during the Proterozoic.

Formation of saline brines and salt in the lower crust by hydration reactions in partially retrogressed granulites from the Lofoten Islands, Norway

Anorthosites, gneisses and mangeritic rocks from the Lofoten Islands in northern Norway contain Cl-rich amphibole (up to 3.5 wt percent Cl) and biotite (up to 1.5 wt percent Cl) that developed mostly during regional metamorphism at ~600°C and ~9 kb by replacement of older anhydrous granulite facies mineral assemblages. (Na,K)Cl halide solid solutions are closely associated spatially with amphiboles that have the highest measured Cl-contents. Rocks with Cl-rich amphiboles and biotites always have minerals that exhibit a wide range in OH/Cl; one sample from a shear zone contains virtually Cl-free amphibole within a few millimeters of Cl-rich amphibole coexisting with halide crystals. Textural evidence indicates that amphibole and biotite evolved from Cl-poor to Cl-rich compositions within individual samples; OH-rich, Cl-poor compositions are always much more abundant than Cl-rich ones. Cl-rich amphiboles and biotites exclusively occur in rocks that contain relict granulite facies minerals (pyroxene and/or olivine); samples in which no granulite facies minerals remain contain amphiboles and biotites without elevated Cl-contents. In one particular case, the restricted development of Clrich amphiboles and biotites within a shear zone and the absence of these minerals from undeformed lithologic equivalents outside the shear zone demomstrate that the amphibole and biotite developed by reaction between a preexisting granulite facies mineral assemblage and an aqueous fluid rather than by fluid-ansent metamorphism or by magmatic processes. We infer from this example that Cl-rich amphibole nad biotite with the same grani size and texture as in the shear zone developed throughout the Lofoten Islands by fluid-rock reaction. Mineralfluid equilibria indicate that the aqueous fluid was variable in composition ranging from XH2O (=[H2O/(H2O+Cl)] of 0.99 for equilibrium with the most Cl-poor amphiboles and biotites to 0.4-0.6 for equilibrium with the most Cl-rich minerals that coexist with salt. The most saline fluids correspond to NaCl equivalent concentrations of over 25 mol/l. The range in OH/Cl of amphiboles and biotite in individual samples and the restricted occurrence of Cl-rich minerals in rocks with relict pyroxene and/or olivine can be explained by a desiccation model in which small amounts of normal low-Cl crustal fluid infiltrates rocks with granulite facies minerals and is subsequently enriched in Cl by preferential consumption of H2O during the resulting hydration reaction. Saline fluids developed at the extremes of reaction progress and then produced the Cl-rich amphibole and biotite compositions coexisting with halide crystals. The desiccation model further explains variations in the whole-rock concentration of Cl in rocks from the Lofoten Islands, the absence of Cl-rich minerals from completely retrogressed granulites that lack olivine or pyroxene, the measured frequency distribution of amphibole and biotite compositions in individual samples, and the calculated evolution of fluid composition with reaction progress. Our results have implications for interpreting measurements of elevated electrical conductivity in the lower crust and for models of granulite genesis that appeal to fluids with low H2O activity. Amer. J. Science (1998), 298, 705-757

Fine-scale isotopic heterogeneities and fluids in the deep crust: a 40Ar/ 39Ar laser ablation and TEM study of muscovites from a granulite-eclogite transition zone

Spectacular exposures on Holsnøy Island, western Norway, reveal that eclogites formed in situ from adjacent anhydrous granulites as a result of the localized infiltration of fluids. Stepwise and laser 40Ar/ 39Ar experiments on muscovite from the eclogites have been used in conjunction with electron microprobe and transmission electron microscopy (TEM) analysis to examine the behavior of argon under high pressure metamorphism and subsequent cooling. Muscovites with 0.10–0.17Na/(Na + K) yield 40Ar/ 39Ar plateau and integrated ages in the range 450–465 Ma, 20–35 Myr older than 40Ar/ 39Ar muscovite plateau ages from adjacent eclogite samples. Laser 40Ar/ 39Ar analyses on ∼ 75 μm spots across two single muscovite grains from these samples are approximately uniform at 444.2 ± 1.9 (21 spots) and 443.3 ± 1.7 Ma (26 spots). However, incremental heating of high sodic (0.18–0.32 (Na/(Na + K)) muscovite failed to yield a plateau. Laser 40Ar/ 39Ar analyses of 100 areas from two single grains from this sample reveal complex two-dimensional patterns with variable apparent ages in the range of 445–625 Ma. Electron microprobe measurements obtained from one of the grains reveal that the paragonite content is also highly variable both parallel and perpendicular to (001). All the TEM bright field images obtained from this mica yield information consistent with a single homogeneous phyllosilicate phase, yet several of the selected area electron diffraction (SAED) analyses document locally intergrown muscovite and sodic muscovite, possibly in an exsolution relationship on a unit cell scale. Analytical electron microscopy (AEM) analyses obtained over a 5–100 nm scale on the same sample reveal great range and heterogeneity inNa/K, including pure muscovite, although no separate paragonite domains were found. In contrast, muscovite grains with consistent laser spot fusion and plateau ages show little heterogeneity inNa/K at the microprobe level. These data provide evidence of variable fluid-mediated incorporation of excess 40Ar under high pressure eclogite facies conditions at the time of the development of the sodic muscovite compositions. The data indicate that fine-scale heterogeneities in argon isotopic compositions were generated during eclogite facies equilibration and were preserved as the rocks were cycled back to the surface. Despite the influx of free fluids in the rocks at high pressure, the heterogeneities are best reconciled with local buffering and limited mobility of the fluids, at least during the final stages of equilibration.

Tectonics, alteration and the fractal distribution of hydrothermal veins in the lower ocean crust

Gabbros from Hole 735B preserve the igneous, structural and metamorphic history beneath the Southwest Indian Ridge down to middle amphibolite facies conditions. The gabbro was isolated from the zone of lithospheric necking by detachment faulting, unroofing and block uplift at the inside-corner high of the Atlantis II Transform. Lower crustal accretion, as preserved in the core, is a complex integration of igneous, hydrothermal and tectonic processes. Alteration began at anhydrous granulite conditions as surface faulting penetrated the brittle-ductile transition to form brittle-ductile shear zones. In the amphibolite facies, hydrous alteration around these zones was extensive, while undeformed sections remained virtually unaltered. Amphibole veins formed during the brittle-ductile deformation have a high fractal dimension, reflecting an unclustered distribution, consistent with high strain and cracking rates in the zone of lithospheric necking beneath the ridge. However, the fractal dimensions of the two major gneissic amphibolite zones in Hole 735B are different and suggest that they represent discrete fault zones formed at different times. Below middle amphibolite facies a dramatic drop in the extent of alteration reflects cooling, cracking and alteration under static conditions, similar to layered intrusions. Initially, fracture and vein formation exploited undeformed sections, where elastic strain accumulated during cooling from high temperatures was unrelieved by recrystallization. Plagioclase-diopside veins formed at this time have the same orientation as higher temperature veins, but a mineralogy reflecting much lower rock permeabilities, and a low fractal dimension, reflecting a tight clustering of veins, as would be anticipated for crack formation at low strain rates. Late zeolite and carbonate veins and irregular, smectite-lined, near-vertical cracks also have a very low fractal dimension, reflecting clustering appropriate to low strain, near static conditions. These, however, show no preference for undeformed sections of the core and are oriented with subvertical and subhorizontal maxima. This indicates crack formation under the present day stress field. Thus, elastic strain, due to cooling from high temperatures and extension during lithospheric necking, dissipated after plagioclase-diopside vein formation.

Elastic wave velocities in isochemical granulite and amphibolite: Origin of a low‐velocity layer at the slab/mantle wedge interface

Simultaneous measurements of compressional and shear wave velocities, Vp and Vs, in granulite and amphibolite with identical chemistry were carried out from room temperature to 850°C at 1 GPa. Both Vp and Vs in granulite and amphibolite decreased linearly with increasing temperature. The values of Vp and Vs at 700°C are 7.75 km/s and 4.37 km/s for granulite, and 7.40 km/s and 4.21 km/s for amphibolite, respectively. 3–4 % lowering of elastic wave velocities in hydrous amphibolite compared with anhydrous granulites may be consistent with seismological observations indicating the presence of a low‐velocity layer at the slab/mantle wedge interface in subduction zones.

A petrologic and fluid inclusion study of charnockites from the Lützow-Holm Bay region, East Antarctica: Evidence for fluid-rich metamorphism in the lower crust

The Proterozoic terrain of Lützow-Holm Bay in East Antarctica comprises an amphibolite- to granulite-facies progression from north to south, exposing an oblique section of the lower crust. The granulite terrain is represented dominantly by orthopyroxene-bearing anhydrous granulites (charnockites), garnet- and biotite-bearing gneisses (leptynites), and garnet, sillimanite and graphite bearing aluminous metapelites (khondalites). Thus it is petrographically identical with its Gondwanian counterparts in Sri Lanka and southern India. Charnockite formation is characterized by the consumption of garnet, biotite and quartz to produce orthopyroxene, alkali feldspar and plagioclase under isothermal conditions and at reduced water activities. A number of mineral-reaction textures, including orthopyroxene-plagioclase symplectites around clinopyroxene relics in intermediate charnockites and spinel rims between adjacent garnet and sillimanite in khondalites, suggest near-isothermal conditions of equilibration. Mineral phase equilibria barometry traverse across the granulite terrain indicates an increase in pressure from c. 5 kbar in the north to c. 8–9 kbar in the south. The temperature gradient of c. 100°C is less pronounced. Fluid inclusion studies indicate abundant CO 2 trapped within the charnockite minerals. The systematic variation in fluid densities with a garnet→feldspar→quartz succession within individual samples argues against post-metamorphic fluid infiltration and fits a scenario of mineral equilibration during near-isothermal uplift following a clockwise pressure-temperature-time path. Along the pressure gradient, the fluid densities show an increase from c. 0.98 to c. 1.10 g/cm 3. This strengthens other evidence of a synmetamorphic entrapment of CO 2. Preliminary carbon isotope measurements of CO 2 extracted from inclusions yield δ 13C values of c. −5 to −6‰, suggesting a sub-continental mantle source. The Gondwanan Lützow-Holm Bay terrain had undergone substantial extension during the late Proterozoic, with displacement along megafaults and emplacement of several alkaline-subalkaline granite and syenite plutons, and also K-basalts. It is therefore envisaged that the abundant CO 2 in the charnockites could have been derived from an attenuated continental lithosphere, and transferred through magmatic conduits.

Bootstrap inversion for vertical and lateral variations of the S wave structure and the υ p/υ s-ratio from shallow earthquakes in the Rhinegraben seismic zone, Germany

Using S wave arrival times of ~ 400 earthquakes in the southern Rhinegraben, 1-D and 3-D inversions for the shear (S)-wave crustal structure of that region are performed. These models are then combined with previously obtained 1-D and 3-D P wave models to compute vertical and lateral variations of the υ p / υ s (Poisson's)-ratio. Such an independent P and S wave inversion approach circumvents many of the limitations of the classical Wadati-diagram method usually used for the determination of crustal υ p/υ s-ratios. For the statistical evaluation of the 1-D models we have used a new Monte-Carlo bootstrap resampling technique which has been found useful in many statistical estimation problems where the underlying statistics of the data is poorly known. The 1-D bootstrap υ p / υ s crustal models exhibit significant vertical variations. Whereas the upper sedimentary and crystalline layer of the crust is characterized by a significant increase of υ p / υ s over the reference value, υ p / υ s in the upper and middle crust is lowered. This correlates well with several findings from Wadati-diagrams for υ p / υ s in other seismically active zones and might be explained through the gradual closing of saturated microcracks in that depth range. However, the υ p/υ s-reduction can also be interpreted in terms of a particular petrological composition such as orthoclase feldspar without the indispensable need for fluids. A lowering of υ p / gu s is also observed for the lower crust, which might reflect the strong lamination found here from previous deep seismic sounding experiments. For the upper mantle a strong increase of υ p / υ s is observed which is primarily triggered by a decrease of υ s , and is best explained by an eclogite petrological assemblage. For the 3-D S wave models significant reductions of more than 100% were obtained for the unexplained travel-time residuals. Variations of υ p / υ s in the upper and middle crust are mostly induced by changes in υ p rather than υ s . For the upper crust slight decreases of υ p / υ s are observed for the Rhinegraben, but increases are detected for the adjacent Vosges and Black Forest areas. In the middle crust the refraction-seismic deduced reduction of υ p / υ s under parts of the Black Forest can neither be corroborated nor rejected. However, because of some experimental ambiguity on the effects of petrological composition and interstitial pore fluids, this does not rule out the presence of the latter. In the lower crust a local reduction of υ p / υ s is observed under the Kaiserstuhl and graben-Black Forest shoulder and has been explained in terms of a metamorphic, dehydrating phase change of the hydrous amphibolite facies to the anhydrous granulite facies. Depending on the pore-pressure of the dehydrated water and the fractional amount of interstitial CO 2, granulite facies metamorphism occurs in the 650–800°C temperature range. Temperatures at the upper end of this interval can be generated by the mechanism of accretionary magmatic underplating as it has been proposed for some extensional rift zones where asthenospheric upwelling, as in the Rhinegraben, occurs.