Pyroxene Granulite Research Articles

Granulite xenoliths and their zircons, Tuoyun, NW China: Insights into southwestern Tianshan lower crust

Granulite xenoliths from the Cretaceous-Paleogene Tuoyun basalts in the southwestern Tianshan area (NW China) are mainly pyroxene granulite and olivine granulite, with minor garnet granulite and quartz granulite. They can be divided into two types by geochemistry and petrography; both are interpreted as magmatic rocks that have undergone multistage metamorphism. Type I granulites are foliated, mafic to intermediate in composition and have negative Zr and Hf anomalies. Type II xenoliths are massive mafic rocks with high Ni contents, and little or no Zr and Hf anomaly. P– T estimates for garnet granulites suggest the xenoliths were derived from depths of ca. 40 km, near the base of the crust. LAM-ICPMS U–Pb data for zircons from four xenoliths lie on discordia with upper intercepts of 690–770 Ma and lower intercepts of 80–125 Ma. Oscillatory zoning in zircon cores suggests that the upper-intercept ages reflect magmatic crystallization and subsequent granulite-facies metamorphism, and the lower intercepts reflect a major thermal event in the lower crust, related to the eruption of the host basalts. Hf-isotope compositions show little variation with age, indicating that most zircons with young ages were thermally reset. Mean Hf model ages for the zircons from each xenolith range from 1.3 to 1.7 Ga. These data and whole-rock chemistry suggest that the protoliths of the granulites were generated by underplating of mafic magmas that assimilated variable proportions of pre-existing crust. Simple modeling of the Hf-isotope data indicates that this lower crust contained components ≥2.5 Ga, and probably >3.4 Ga, in age in Neoproterozoic time. These observations suggest that the southwestern Tianshan (e.g. in Tuoyun area) may represent the presence of a microcontinental block within the Paleozoic East Central Asian Orogenic Belt.

Low-pressure Granulites of the Lišov Massif, Southern Bohemia: Viséan Metamorphism of Late Devonian Plutonic Arc Rocks

The Lisov Granulite Massif differs from neighbouring granulite bodies in the Moldanubian Zone of southern Bohemia (Czech Republic) in including a higher proportion of intermediate–mafic and orthopyroxene-bearing rocks, associated with spinel peridotites but lacking eclogites. In addition to dominantly felsic garnet granulites, other major rock types include quartz dioritic two-pyroxene granulites, tonalitic granulites and charnockites. Minor bodies of high-pressure layered gabbroic garnet granulites and spinel peridotites represent tectonically incorporated foreign elements. The protoliths of the mafic–intermediate granulites (quartz-dioritic and tonalitic) crystallized 360–370 Ma ago, as indicated by laser ablation inductively coupled plasma mass spectrometry U–Pb ages of abundant zircons with well-preserved magmatic zoning. Strongly metamorphically recrystallized zircons give ages of 330–340 Ma, similar to those of other Moldanubian granulites. For the overwhelming majority of the Lisov granulites peak metamorphic conditions probably did not exceed 800–900 C at 4–5 kbar; the equilibration temperature of the pyroxene granulites was 670– 770 C. This is in sharp contrast to conditions of adjacent contemporaneous Moldanubian granulites, which are characterized by a distinct HP–HT signature. The mafic–intermediate Lisov granulites are thought to have originated during Visean metamorphic overprinting of metaluminous, medium-K calc-alkaline plutonic rocks that formed the mid-crustal root of a Late Devonian magmatic arc. The protolith resembled contemporaneous calc-alkaline intrusions in the European Variscan Belt.

40Ar– 39Ar geochronology of the charnockites and granulites of the Kan Nack complex, Kon Tum Massif, Vietnam

The Truong Son Belt forms the eastern rim of the Indochina Block in Southeast Asia. The age of the metamorphism, mainly along NW–SE mylonitic shear zones that affects this belt, has been formerly determined at about 240–250 Ma. This age corresponds to the Indosinian tectonometamorphic episode. The Kon Tum Massif, situated to the south of this belt, comprises high-temperature rocks, the Kan Nack Complex, including charnockites and granulites. The main charnockitic outcrops, restricted to the Song Ba Valley, establish the intrusive nature of these magmatic rocks within granulite facies material. Basic charnockitic rocks are mainly quartz enderbites to norites and hornblende–pyroxene granulite facies rocks. The 40Ar– 39Ar age of intrusion-cooling of charnockitic magmas is determined from primary magmatic biotites at about 245 Ma. In the east of the Kan Nack Complex some granulite facies rocks exhibit relicts of primary granulite facies parageneses, whereas others show evidence of overprinting by a retrogressive low-grade metamorphism. Ar–Ar dating confirm this evolution, giving ages of 400 Ma for primary relict granulite facies phases and 260–270 Ma from the most retrogressed samples establishing the youngest limit for the granulite facies metamorphism. Granulites intruded by charnockites in the Song Ba Valley yield ages of about 250 Ma, equivalent to the ages of the charnockites, and have evidently been completely reset by these high temperature intrusions. Therefore, the Kan Nack Complex of the Kon Tum Massif is not an independent unit with respect to the Indosinian orogen, but represents the deep-crustal part of this belt.

Discontinuous change in temperature derivative of Vp in lower crustal rocks

Compressional wave velocities (Vp) of gabbronorite, pyroxene granulite, wehrlite and websterite were measured up to 1000°C at 1 GPa during heating and cooling. The wehrlite and websterite, which contain secondary hydrous minerals, show irreversible change in temperature derivative of Vp (∂Vp/∂T). During heating, these rocks show sudden decrease in Vp when dehydration reactions took place. During cooling, the rocks show a linear increase in Vp. In contrast, the gabbronorite and pyroxene granulite, which are nearly dry crystalline assemblage, show reversible and discontinuous change in ∂Vp/∂T at around 400°C. It changes from −2.4 and −2.1 × 10−4 km s−1 °C−1 to −4.7 and −4.5 × 10−4 km s−1 °C−1. This reversible and discontinuous change in ∂Vp/∂T is difficult to explain by the previously proposed mechanisms, such as thermal cracking, dehydration reaction, and/or partial melting. The data suggest that this reversible and discontinuous change would be associated with phase transition in plagioclase.

Origins of the lower crustal reflectivity in the Lützow-Holm Complex, Enderby Land, East Antarctica

The combination of rock velocities with a seismic signature has been used as a clue to understanding the structure and evolution of the continental lithosphere. The lower crustal reflectivity beneath part of the Pan-African orogeny, the Lutzow-Holm Complex (LHC), Western Enderby Land, East Antarctica, has been imaged on single-fold seismic reflection profiles using active seismic studies on the continental ice sheet. The set of velocity layers at middle to lower crustal depths were obtained by modeling the later phases of teleseismic receiver functions observed at Syowa Station (39°E, 69°S), in the LHC. The later phases around 10–16 s from P onset in radial components are explained by assuming a layered lower crustal model with velocity changes of 0.3 km/s in shear waves for 0.5–1.0 km thick layers. The origin of lower crustal reflectivity is discussed in terms of high-pressure laboratory measurements on metamorphic rocks from Western Enderby Land. Lower crustal velocities of 6.9 km/s derived by seismic refraction surveys can be explained by a major composition of mafic pyroxene granulite, as occurs in the Archean Napier Complex. A tectonic model involving collision between the paleo East and West Gondwana blocks during the last stage of Pan-African orogeny is presented to explain the high velocities and reflectivity in the lower crust underlying the LHC. The Napier Complex is considered to have descended eastward under part of the Pan-African belt (LHC), generating a higher-pressure mafic granulite composition. The reflectivity of the lower crust of the LHC may have been enhanced subsequently by extensional stress during the breakup process.

The lithospheric mantle beneath the Kerguelen Islands (Indian Ocean): petrological and petrophysical characteristics of mantle mafic rock types and correlation with seismic profiles

Deep-seated meta-igneous xenoliths brought to the surface by alkali basaltic magmas from the Kerguelen Islands reveal that basaltic magmas have intruded the upper mantle throughout their geological evolution. These xenoliths record volcanic activity associated with their early South East Indian Ridge location and subsequent translation to an intraplate setting over the Kerguelen Plume. The meta-igneous xenoliths sample two distinctive geochemical episodes: one is tholeiitic transitional and one is alkali basaltic. Geothermobarometry calculations provide a spatial context for the rock type sequence sampled and for interpreting petrophysical data. The garnet granulites equilibrated over a pressure range of 1.15 to 1.35 GPa and the garnet pyroxenite at 1.8 GPa. Ultrasonic measurements of compressional wave speed VP have been carried out at pressures up to 1 GPa, and densities measured for representative samples of meta-igneous xenoliths and for a harzburgite that represents the peridotitic mantle. VP and density have also been calculated using modal proportions of minerals and appropriate elastic properties for the constituent minerals. Calculated and measured VP agree well for rock types with microstructures not complicated by kelyphitic breakdown of garnet and/or pervasive grain-boundary cracking. Pyroxene granulites have measured and calculated VP within the range 7.37–7.52 km/s; calculated velocities for the garnet granulites and pyroxenites range from 7.69 to 7.99 km/s, whereas measured and calculated VP for a mantle harzburgite are 8.45 and 8.29 km/s respectively. The seismic structure observed beneath the Kerguelen Islands can be explained by (1) a mixture of underplated pyroxene granulites and ultramafic rocks responsible for the 2–3 km low velocity transitional zone below the oceanic layer 3, (2) varying proportions of granulites and pyroxenites in different regions within the upper mantle producing the lateral heterogeneities, and (3) intercalation of the granulites and pyroxenites throughout the entire upper mantle column, along with elevated temperatures, accounting for the relatively low mantle velocities (7.70–7.95 km/s).

Generation of new continental crust and terrane accretion in Southeastern Alaska and Western British Columbia: constraints from P- and S-wave wide-angle seismic data (ACCRETE)

The ACCRETE study addresses the question of continental assemblage in southeastern Alaska and western British Columbia through accretion of exotic terranes and generation of new crust by magmatic addition in a former continental arc. We present results of wide-angle P- and S-wave seismic interpretation of a 300-km long marine-land seismic line across the contacts between accreted terranes and Coast Mountains. Additional constraints on the model are obtained from correlation with geologic mapping. Our results indicate that the Coast Shear Zone (CSZ) is a nearly vertical fault zone probably related to a transpressive regime. West of the CSZ, the mid-Cretaceous (90 Ma) thrust belt is rooted in the deep crust and is truncated by the CSZ. From the interpretation of the imaged sub-vertical reflecting zones, we infer the positions of the Alexander–Wrangellia terrane boundary (AWB) and of Tertiary extensional grabens within Dixon Entrance near its intersection with the profile. The observed values of Vp and Vp/Vs in the lower crust of the Alexander terrane are similar to those of oceanic crust and distinctly different from the Coast Mountains Batholith (CMB) to the northeast. The crust under the CMB (32 km) is thinner than the average continental crust, and the Moho is sharp (∼200 m) and highly reflective. The low-velocity mantle (7.9 km/s) suggests high temperature consistent with the stability of garnet in mafic rocks in the lower crust. The lower crustal velocity of 6.9 km/s supports a lower crust composed of interlayered garnet pyroxene granulite and quartzofeldspathic-restite related to batholith generation. The crustal section under the CMB is seismically identical to the lower two thirds of normal crust, heated and inflated by intrusions of tonalite, and gabbro interlayered with restites from batholith generation and uplifted during exhumation.

Experimental high pressure investigation of partial melting in natural rocks and their influence on Vp and Vs

Plutonic and high-grade metamorphic rocks from the crystalline complexes of the Saxonian Erzgebirge and Granulitgebirge in Germany were used for experiments up to 0.5 GPa at laboratory temperature to measure the elastic wave velocities (vp) and (vs), under the minimized influence of pore space and cracks in an oil pressure chamber. The obtained high pressure data were also used to select samples for further experiments under the conditions of mineral dehydration and partial melting. We also applied a high performance gas pressure apparatus to investigate vp and vs on cylindrical samples up to 2 GPa and temperatures up to 1200°C. This paper shows the results of the elastic wave velocities vp and vs, measured simultaneously with encapsulated samples of granite, pyroxene granulite and pyroxenite under different partial melting conditions. Post-experimental microscopic analyses of the quenched samples including digital image processing were undertaken to determine the material and/or structural cause of a change in the physical properties, resulting in empirical relationships between elastic wave velocities and mineral content, dissipation and orientation of melt along grain boundaries. In a granite melt starts to form at 650°C with an amount of about 4 % at 900°C and reaching massive melting at temperatures above 1000°C. For basic to ultrabasic rocks at a pressure of 2 GPa a much smaller amount of melt has formed in the temperature interval above 900°C. At lower pressures the Poisson's ratio increases in the range of partial melting dramatically from 0.25 to about 0.4. For basic to ultrabasic rocks at 2 GPa pressure it keeps nearly unchanged up to 1000°C.

Preface to thematic issue: Petrotectonic characteristics of the Kokchetav Massif, northern Kazakhstan

Preface to thematic issue: Petrotectonic characteristics of the Kokchetav Massif, northern Kazakhstan

Sm–Nd geochronology and petrography of garnet pyroxene granulites in the northern Sulu region of China and their geotectonic implication

Abundant garnet-bearing granulite lenses are widely distributed in the northern part of the Sulu region and adjacent areas. They are possibly re-metamorphosed high-pressure metamorphic rocks. On the basis of detailed petrographic study, samples WD01, WD04 and ML06 from Laixi and Wendeng were identified as high-pressure granulites, and WH1 from Weihai as an original coesite-bearing eclogite. Three high-pressure granulite samples give mineral-WR isochron ages of 1846±76, 1743±79 and 1752±30 Ma. TDM ages are 3.3, 3.0 and 2.8 Ga. The Sm–Nd mineral-WR isochron ages are interpreted to date as the metamorphic resetting within the medium-pressure granulite facies, representing an isotopic re-homogeneity during uplifting of the high-pressure granulites from deep continent crust. It is important that Sm–Nd chronological characteristics are the same as Archaean high-pressure granulites in the North China craton. However, sample WH1 from Weihai demonstrates abnormal Sm–Nd characteristics. Its whole rock ε Nd (0) value is +129. TDM age is 1.3 Ga, and constrains the minimum age of re-metamorphosed eclogite protolith formation to the mid-Proterozoic. This result is identical to those reported by Jahn (1994), showing complicated processes of metamorphism and metasomatism. The data in this paper provide further evidence to define the boundary between the North China craton and UHPM belt in eastern Shandong and to understand the geotectonic nature of the boundary.

Origin of Mg-Metatholeiites of the Schirmacher Region, East Antarctica: Constraints from Trace Elements and Nd-Sr Isotopic Systematics

The mafic granulites of Schirmacher region, East Antarctica, the rocks under study, occur more or less as concordant sills or as lenses or as boudinage structures within the felsic rocks, charnockites or metapelites of the region. They show variation from garnet bearing two-pyroxene granulites and garnet free pyroxene granulites to transitional amphibolite-pyroxene granulites. Their major, trace, REE and isotopic chemistry are not distinct from each other and they represent Mg-basalts with MgO >7% and Al 2O 3 <16%. The majority of the analyzed samples plot in the tholeiitic field or show tholeiitic trends, suggesting their metatholeiitic nature as well as general preservation of original composition. The rocks are characterized by enriched large-ion lithophile elemental concentrations than that of mid-oceanic ridge basalts. Their high-field strength elements and heavy rare-earth elemental concentrations, however, are as that of mid-oceanic ridge basalts, a feature which is also reflected in the ratios of their large-ion lithophile elements against high-field strength elements and heavy rare-earth elements, wherein we find these ratios are higher than N-type MORB. Further, the rocks show negative Nb anomaly, high Th/Ta ratio and low La/Nb ratio, which are also characteristics of subduction-related magmatism. The isotopic studies carried out on these samples show that, the Sm-Nd and Rb-Sr dating did not yield much spread, but suggested a Sm-Nd metamorphic age of ∼960 Ma. Rb-Sr dating gave ages ∼886 Ma, suggesting the reworking of the Rb and Sr elements during subsequent tectonothermal overprinting. The Nd model ages (T DM Nd) of these rocks show a relatively restricted range of 1120 to 1357 Ma, suggesting mafic magmatism ∼1200 Ma. The positive eNd values (+4.22 to +6.07) shown by these rocks, represent a juvenile crustal fragment derived from melting of mantle precursors, without significant reworking of older crustal material. It is proposed that these rocks were produced by partial melting of a mantle source, characterized by LILE enrichment, related principally to dehydration of subducted oceanic crust.

Relationships between magmatism, metamorphism and deformation in the Fraser Complex, Western Australia: Constraints from new SHRIMP U–Pb zircon geochronology

SHRIMP U–Pb zircon isotopic data have been obtained for four samples collected from granitoids and paragneisses in the Fraser Complex, a large composite metagabbroic body cropping out in the Mesoproterozoic Albany‐Fraser Orogen of Western Australia. The data are combined with the results of field mapping and petrographic analysis to revise a model for the geological evolution of the Fraser Complex. Three main phases of deformation are recognised in the Fraser Complex (D1–3) associated with two metamorphic events (M1–2), which involve four distinguishable episodes of recrystallisation. The first metamorphic event recognised (M1a/D1) reached granulite facies and is characterised by peak T ≥800°C and P = 600–700 MPa. A syn‐M1a/D1 charnockite has a U–Pb SHRIMP zircon age of 1301 ± 6 Ma, which also provides an estimate for the age of intrusion of Fraser Complex gabbroic rocks. Disequilibrium textures comprising randomly oriented minerals (M1b), consistent with approximately isobaric cooling, formed in various lithologies in the interval between D1 and D2. Post‐D1, pre‐D2 granites intruded at 1293 ± 8 Ma and were foliated during the D2 event, which culminated in the burial of the Fraser Complex to depths equivalent to 800–1000 MPa. Following burial, pyroxene granulites on the western boundary of the complex were pervasively retrogressed to garnet amphibolite (M2a). An igneous crystallisation age of 1288 ± 12 Ma from a syn‐M2a aplite dyke suggests that retrogression may have occurred only a few millions of years after the peak of granulite facies metamorphism. Exhumation to depths of less than ∼400 MPa occurred within ∼20–30 million years of the M2a pressure peak. Associated deformation (D3) is characterised by the development of mylonite and transitional greenschist/amphibolite facies disequilibrium textures (M2b).

Southern Granulite Terrain, South of the Palghat-Cauvery Shear Zone: Implications for India-Madagascar Connection

The present paper correlates the southern Madgascar terrain, south of the Ranotsara shear with the granulite terrain of southern India, occurring south of the Palghat-Cauvery (P-C) shear zone. Both the terrains have witnessed high temperature to ultra high temperature granulite metamorphism at 550 Ma and are traversed by shear zones and deep crustal faults. The 550 Ma old granulite terrains of Madagascar and southern India have similar lithologies, in particular, sapphirine bearing pelitic assemblages. Graphite deposits and gem occurrences are common to both these terrains. The 550 Ma old southern granulite terrain of southern India comprises of different blocks, the Madurai and the Kerala Khondalite belt, but all the blocks have similar lithologies with pelite—calc silicate rocks inter-banded with two pyroxene granulite bodies. These lithologies occur amidst an essentially charnockitic terrain. The protolith ages of the southern granulite terrain, south of the P-C shear zone ranges between 2400–2100 Ma. The terrain as a whole has witnessed the 550 Ma old granulite event. The granulite metamorphism took place under temperatures of 800–1000°C and at pressures of 9.5 to 5 Kbar. The source of heat for the high temperature granulite event of the southern Madagascar terrain has been linked to advective heat transfer along mantle deep faults. The source for the high temperature granulite metamorphism for the southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening. Many workers have linked Madagascar to southern India by connecting the Ranotsara shear either to the P-C shear zone or to the Achankovil shear zone, further south. The important factor is the lithologies of the Madagascar terrain, south of Ranotsara shear zone and the 550 Ma. old southern Indian granulite terrain are similar in many aspects. It will be more appropriate to link the Ranotsara shear to the curvilinear lineament bounding the Anaimalai-Kodaikanal ranges and which merges with the southern margin of the P-C shear zone. However, north of the Ranotsara shear/fault, the northern Madagascar terrain comprises of a dominant Itremo sequence (< 1850 Ma) and 780 Ma old calc-alkaline intrusives. The latter have similarities with that of Aravallis and the Sirohi, Malani sequences occurring further north east. The Rajasthan terrain has witnessed igneous intrusive activity at 1000–800 Ma. If we can broaden the area of investigations and include the above areas, the Madagascar-India connection can be better understood.

FT-IR Spectroscopic Investigation of Hydrous Components in Sillimanite from Eastern Ghat Granulite Belt, India

FT-IR spectra of sillimanite samples from high grade regionally metamorphosed rocks belonging to the granulite terrain (amphibolite to pyroxene granulite facies) deciphers prominent OH features. Heating experiments indicate growth of prominent band at 3161cm −1. Heating above 1000°C all OH features disappear in intensity into broad features with slight shift of bands towards lower energies. Complete dehydration requires temperatures above 1000°C. Coexistence of boron and OH features are also observed in all sillimanite samples. The high temperature behaviour of sillimanite from the granulite terrain discerns that the hydrous species in sillimanite were incorporated much below 700°C, however, secondary hydration due to pegmatite activity, retrograde metamorphism and migmatisation is not ruled out. Thus a near anhydrous condition were probably not achieved during the granulite facies metamorphism in Eastern ghat granulite terrain.

Garnet granulite facies xenoliths from Yingfengling Cenozoic basalt in Leizhou, Guangdong Province

The granulite xenoliths are first found in Yingfengling pyroclastic rocks of Leizhou region, Guangdong Province. Of them high-pressure garnet granulite xenolith found is very sparse in China. Garnet granulite is different from pyroxene granulite in mineral assemblage and composition.P-T calculation shows that garnet granulite was formed at 1130–1160°C and 1.4–1.7 GPa, and pyriclasite at about 800°C and 0.65–0.80 GPa. High xenolith-derived paleogeotherm indicates Cenozoic rifting in Leizhou area. Granulites with varied mineral assemblages were formed at different depths by the metamorphism of the underplated basaltic melt.

Crustal thickening of the lower crust of the Kohistan arc (N. Pakistan) deduced from Al zoning in clinopyroxene and plagioclase

The lower‐crustal rocks of the Kohistan complex (northern Pakistan) are mostly composed of metabasic rocks such as pyroxene granulites, garnet granulites and amphibolites. We have investigated P–T trajectories of the relic two‐pyroxene granulites, which are the protolith of the amphibolites within the Kamila amphibolite belt. Aluminous pyroxene retains igneous textures such as exsolution lamellae developed in the core. The significant amount of Al in clinopyroxene is buffered by breakdown reactions of plagioclase accompanied by film‐like quartz as a product at grain boundaries between plagioclase and clinopyroxene. Distinct Al zoning profiles are preserved in pyroxene with exsolution lamellae in the core and in plagioclase adjacent to clinopyroxene in pyroxene granulites. In the northern part of the Kamila amphibolite belt, Al in clinopyroxene increases towards the rim and abruptly decreases at the outer rim, and anorthite in plagioclase decreases towards the rim and abruptly increases near the grain boundary between plagioclase and clinopyroxene. In the southern part of the Kamila amphibolite belt, Al in clinopyroxene and anorthite in plagioclase simply increase towards the margins of the grains. The anorthite zoning in plagioclase is in agreement with the zoning profiles of Ca‐Tschermaks and jadeite components inferred from variations of Al, Na, Ti and Fe3+ in clinopyroxene. Assuming that the growth surface between them was in equilibrium, geothermobarometry based on Al zoning in clinopyroxene coexisting with plagioclase indicates that metamorphic pressures significantly increased with increasing temperature under granulite facies metamorphism. The peak of granulite facies metamorphism occurred at conditions of about 800 °C and 800–1100 MPa. These prograde P–T paths represent a crustal thickening process of the Kohistan arc during the Early to Middle Cretaceous. The crustal thickening of the Kohistan arc was caused by accretion of basaltic magma at mid‐crustal depths.

Lower crustal and possible shallow mantle samples from beneath the Hebrides: evidence from a xenolithic dyke at Gribun, western Mull

A Permian (268 ± 2 Ma) olivine-nephelinite dyke cutting Proterozoic (Moinian) psammites on the west coast of Mull, contains an abundance of xenoliths inferred to be from the lower crust and possibly uppermost mantle. The majority are pargasite pyroxenites grading to hornblendites. Next most abundant are pyroxene granulite orthogneisses. High-grade meta-arenites are also relatively common. Scarce xenoliths and related megacrysts are composed of anorthoclase, ferrosalite, apatite, magnetite and ilmenite (af–cpx–ap–mt–il suite). Scarce kaersutite megacrysts are thought to be derivative from pegmatitic hornblendites. The pargasite pyroxenites are derived from ultramafic protoliths that have experienced recrystallization, modal metasomatism (with introduction of amphibole) and deformation prior to entrainment. Olivines are wholly pseudomorphed but pyroxenes are diopsides (Fs 7.8–9.2 ) with up to 12.5% Al 2 O 3 . Plagioclase (An 45.6 ) is a rare accessory. Whether the pyroxenite protoliths were upper mantle peridotites or lower crustal ultramafic cumulates is indeterminate. The mesocratic xenoliths are granulite-facies orthogneisses comprising plagioclase (An 38–22 ), augite (Fs 16–22 ), pseudomorphed pigeonite (?), magnetite, ilmenite and apatite. Pb/U SHRIMP dating of zircons indicates a crystallization age of 1850 ± 50 Ma for the orthogneisses, suggesting that the Archaean/ Palaeoproterozoic Lewisian gneisses were magmatically underplated by younger Proterozoic (Rhinnian or Ketilidian age) rocks. The pyroxenite and granulite gneiss xenoliths may be coeval fragments of a lower crustal and possibly uppermost mantle sequence. The most evolved of the orthogneisses are two-feldspar quartz diorites containing subordinate sanidine (Or 64–74 ). The magmas parental to the postulated underplating intrusion are thought to have been mildly alkaline (transitional) basalts. The metasomatic introduction of amphibole that affected the ultramafic rocks is attributed to pervasive influx of Fe, Ti, K, LREE (etc.)-rich small fraction partial melts from the asthenosphere. The af–px–ap–mt–il megacryst suite, however, appears to represent a distinct younger event involving intrusion of a geochemically evolved vein system within the amphibole pyroxenites. Temperature estimates based on coexisting (a) feldspar and (b) oxide pairs indicate establishment of equilibrium at approximately 800°C, suggesting a geothermal radient of c . 27°C km −1 .

Fluid inclusions in high-pressure granulites of the Pan-African belt in Tanzania (Uluguru Mts): a record of prograde to retrograde fluid evolution

The high-pressure granulites of the Uluguru Mountains are part of the Pan-African belt of Tanzania, the metamorphic evolution of which is characterized by an anticlockwise P-T path. Mineral assemblages that represent distinct metamorphic stages are selected for fluid inclusion studies in order to deduce the fluid evolution in metapelites and pyroxene granulites from the prograde to the retrograde stage. Fluid inclusion data improve the petrologically derived P-T path and confirm the anticlockwise evolution. Fluid inclusions in quartz enclosed in garnet porphyroblasts in metapelites preserve prograde fluids of CO2–N2 composition and later-trapped pure CO2. During isochoric heating at temperatures near the peak of metamorphism, deformation and recrystallization led to fluid homogenization yielding N2-poor CO2 composition in the metapelites. Near-peak CO2–N2 fluid inclusions in quartz of metapelites and CO2 inclusions in garnet-pyroxene granulites are characterized by perfect negative crystal shape. Garnet formed in veins and as coronas around orthopyroxene represent the near-isochoric/isobaric cooling stage which is characterized by high-density CO2-rich fluid inclusions. Up to 15 mol% N2 in some primary CO2 inclusions in corona garnet indicate small-scale fluid heterogeneity during the static garnet growth. The fact that high-density fluid inclusions are preserved, suggests a shallow dP/dT slope of the uplift path. Nevertheless, some fluid inclusions decrepitated or re-equilibrated and low-density CO2 inclusions were trapped in the garnet-pyroxene granulite while N2–CH4 inclusions formed in the metapelites. Different fluid compositions in metapelite and metabasite argue for an internal control of the fluid composition by phase equilibria. In shear zones where the pyroxene granulite was transformed into scapolite-biotite schist, CO2–N2 and low-density N2–CH4 fluid inclusions indicate several stages of tectonic activity and suggest fluid influx from the nearby metapelites. High- and low-salinity aqueous inclusions observed beside CO2 inclusions in garnet-pyroxene granulites, in vein quartz and shear zones could be of high-grade origin but are mainly re-equilibrated or re-trapped along healed microfractures during lower-grade stages.

Structure and development of the lower crust and upper mantle of Southwestern Japan: Evidence from petrology of deep‐seated xenoliths

Abstract Basic and ultrabasic xenoliths included in Cenozoic alkali basalts from the Kibi and Sera plateaus, Southwest Japan, can be classified into five groups on the basis of mineral association and texture. Their equilibration P‐T conditions estimated from paragenesis and mineral chemistry indicate that the dominant rock type from the lower crust to upper mantle changes with increasing depth as follows: (i) pyroxene granulite (Group V) and meta‐sediments; (ii) garnet gabbro (Group 111) and corundum anorthosite (Group IV); (iii) spinel pyroxenite (Group 11); and (iv) spinel peridotite and pyroxenite (Group I). Groups I1 and I11 show a lower degree of recrystallization than Groups I and V, and have similarities in composition and mineral chemistry to host basalts. Based on these facts along with the P‐T conditions of equilibration, Groups I1 and I11 are interpreted as formed from basaltic magma that intruded beneath the crust‐mantle boundary at an early stage of the magmatism of the alkali basalts, where the lower crust and uppermost mantle had consisted of Group V and metasediments, and Group I, respectively. It follows that the crust has grown downward due to underplating of basaltic magma beneath the bottom of pre‐existing crust. Group IV has commonly the same mineral assemblage, corundum + calcic plagioclase + aluminous spinel, and shows locally, nearby kyanite crystals, almost the same texture as fine‐grained aggregates in a quartzite xenolith. The aggregates appear to have been formed by reaction between kyanite and host basalt, and accordingly Group IV is interpreted as formed by reaction between metasediments and basaltic magma at the time of the underplating. The Kibi, Sera and Tsuyama areas are distinguished from the areas nearby the Sea of Japan by the occurrence of the garnet gabbro and corundum anorthosite xenoliths, by the absence of the association of olivine + plagioclase in basic and ultrabasic xenoliths, and by the lower temperature of equilibration of basic xenoliths. From these facts it is stressed that in general the crust becomes thinner and geothermal gradient becomes higher towards the back‐arc side. Such a regional variation in crustal structure must reflect the tectonic situation of Southwest Japan at the time of the magmatism of the alkali basalts, namely rifting and shallow‐level magmatism at the back‐arc side.

Geochemistry and genesis of granitoid rocks from Southern Sri Lanka

Pyroxene granulites of intermediate to felsic composition comprise a major part of the Precambrian terrain exposed in southern Sri Lanka. Chemical characteristics and field relations reveal that there are two closely associated groups of granitoids both occurring as discrete layers or deformed bodies. Granitic-adamellitic rocks are more common and can be distinguished from tonalitic-trondhjemitic rocks. Granitoids of these two groups and minor metabasalt and metadiorite/meta-andesite have been subjected to high-grade metamorphism and were mapped as charnockite or charnockitic rocks. Rocks of granitic composition are mostly meta-aluminous and, in the AFM diagram, fall in the tholeiitic field. These rocks are mainly A-type granites characterized by high K2O/Na2O, FeO/MgO and high Zr, Nb and LREE contents, most belonging to the relatively Ce- and Y-enriched A2-type. Tonalitic rocks are meta-aluminous to per-aluminous and display a calc-alkaline trend. These rocks are classified as I-type, although some of them have very low K2O and REE contents.