Garnet Clinopyroxenite Research Articles

Origin and tectonic implication of an UHP metamorphic mafic–ultramafic complex from the Sulu UHP terrane, eastern China: Evidence from petrological and geochemical studies of CCSD-Main Hole core samples

Study of the Maobei garnet peridotite and pyroxenite are significant, because these rocks were subjected to subduction-zone UHP metamorphism that is indicative of deep continental subduction. In order to better understand the origin of the Maobei peridotite and pyroxenite and the processes of continental subduction, we performed geochemical analyses on whole-rock and mineral samples. The studied samples were recovered in the main-hole of the Chinese Continental Scientific Drilling in the Sulu UHP terrane, east-central China. The garnet peridotites occur as lenses or layers in pyroxenite; they comprise garnet wehrlite and garnet dunite with minor interlayers of garnet websterite and garnet. Mini-bands, nodules or veins of eclogite and garnet clinopyroxenite (“internal”) also irregularly occur in garnet wehrlite. The peridotites are composed of olivine (Fo 83–84), garnet (Prp 51–56), ± diopside ± enstatite (En 86–87) ± titanoclinochumite, and have lower Mg/(Mg + Fe total) values of 0.79–0.83 and higher Al 2O 3 and CaO contents in comparison with the gneiss-hosted Sulu mantle peridotites. Garnet wehrlite displays variable REE patterns, from LREE-enriched to slightly LREE depleted relative to MREE and HREE; whereas, other ultramafic rocks show roughly flat REE patterns. In general, Rb, Ba and LREE increase, and compatible element (e.g. Cr, Co and Ni) abundances decrease from peridotite, pyroxenite to “internal eclogite” and “internal Grt-clinopyroxenite”. The calculated REE partition coefficients (D REE) between Cpx and Grt of the Maobei peridotite and pyroxenite decrease regularly from LREE (D Ce = 32–708) to HREE (D Yb = 0.01–0.10), which is comparable with natural mantle peridotite and eclogite crystallized at 800–1100 °C. The geochemical data and occurrence of these rocks suggest that the protoliths of the Maobei peridotites are cumulates derived by differentiation of basaltic magma at the lower continental crust. Thus, the “Internal eclogite and Grt-clinopyroxenite” result from post-cumulus crystallization of the trapped melt. Ti-clinohumite orthopyroxenite may have formed by reaction between peridotitic olivine and indigenous melt or country rock-derived (“exotic”) TiO 2-bearing fluids at high-P condition during continental subduction to mantle depths. The peak P−T estimates of 795–840 °C and 5.3–6.8 GPa of the Maobei peridotites and the available petrological data of country rocks indicate that the continental crust was subducted to a depth of ∼ 200 km.

Insights into the origin of mantle graphite and sulphides in garnet pyroxenites from the External Liguride peridotites (Northern Apennine, Italy)

Abstract This paper describes a rare occurrence of graphite in non-cratonic mantle rocks. Graphite has been found in garnet clinopyroxenite layers from the External Liguride peridotites that represent slices of subcontinental lithospheric mantle exhumed at the ocean floor in Mesozoic times. The high-pressure assemblage of the pyroxenites is characterized by garnet+Al–Na-rich clinopyroxene, and testifies to an early stage of equilibration at approximately 2.8 GPa and 1100 °C. Graphite occurs as small dispersed euhedral flakes and stacks of flakes. Structural characterization by microRaman spectrometry indicates a highly ordered structure, compatible with a high-temperature mantle origin. C isotope composition of graphite has a typical mantle signature. Fe–Ni–Cu sulphides occur as accessory phases, both as blebs enclosed in silicates (E-Type) and interstitial grains (I-Type). The sulphide assemblage (Ni-free pyrrhotite, pentlandite, Cu–Fe sulphides) mainly reflects subsolidus exsolution from high-temperature Fe–Ni–Cu monosulphide solid solutions with variable Ni (up to 18 wt%) and Cu content (up to 7 wt%). The origin of E- and I-Type sulphides requires the existence of an immiscible Fe–Ni–Cu sulphide liquid, which segregated from the partial melt of the garnet pyroxenite. Graphite precipitation in the pyroxenite was presumably related to the reduction of a more oxidized carbon species interacting with the sulphide liquid as a reducing agent.

Strain coupling between upper mantle and lower crust: natural example from the Běstvina granulite body, Bohemian Massif

AbstractStrain patterns within mantle rocks and surrounding coarse‐grained felsic granulites from the Kutná Hora Crystalline Complex in the Variscan Bohemian Massif have been studied in order to assess their strain coupling. The studied rock association occurs within low‐strain domains surrounded by fine‐grained granulite and migmatite. The Doubrava peridotite contains closely spaced and steeply dipping layers of garnet clinopyroxenite, which are parallel to the NE–SW‐striking, high‐temperature foliation in nearby granulites, while the Úhrov peridotite lacks such layering. The Spačice eclogite is not associated with peridotite and shows upright folds of alternating coarse‐ and fine‐grained varieties bearing NE–SW‐striking axial planes. Electron back‐scattered diffraction measurements revealed full strain coupling between clinopyroxenites and coarse‐grained granulites in the S1 fabric that is superposed on the S0 fabric preserved in peridotites. The B‐type olivine lattice preferred orientation (LPO) characterizes the S0 fabric in peridotites and its reworking is strongly controlled by the presence of macroscopic clinopyroxenite layering. The S1 in clinopyroxenites and coarse‐grained granulites is associated with the LS‐type clinopyroxene LPO and prism <c> slip in quartz respectively. While the S1 fabric in these rock types is accompanied invariably by a sub‐vertical stretching lineation, the S1 fabric developed in reworked Úhrov peridotite is associated with strongly planar axial (010) type of olivine LPO. The peridotites with the S0 fabric are interpreted to be relicts of a fore‐arc mantle wedge hydrated to a various extent above the Saxothuringian subduction zone. The prograde metamorphism recorded in peridotites and eclogites occurred presumably during mantle wedge flow and was reaching UHP conditions. Strain coupling in the S1 fabric between clinopyroxenites and granulites at Doubrava and upright folding of eclogites at Spačice document a link between tectonic and magmatic processes during orogenic thickening, coeval with intrusions of the arc‐related calcalkaline suites of the Central Bohemian Plutonic Complex (c. 360–345 Ma). Juxtaposition of peridotites and granulites could be explained by a rheological heterogeneity connected to the development of clinopyroxenite layering in the upper mantle and a previously published model of a lithospheric‐scale transpressional arc system. It invokes vertical shearing along NE–SW trending, sub‐vertical foliations in the upper mantle that could have led to an emplacement of mantle bodies into the granulitized, orogenic root in the sub‐arc region. Clearly, such a transpressional arc system could represent an important pathway for an emplacement of deep‐seated rocks in the orogenic lower crust.

Petrology and geochronology of HP–UHP rocks from the South Altyn Tagh, northwestern China

Detailed field and petrological studies on the South Altyn Tagh in the last decade show that high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks are mainly exposed at Jianggalesayi and Qingshuiquan in the west, and at Yinggelisayi in the middle of the terrane. The South Altyn Tagh UHP rocks occur as lensoid blocks in granitic gneiss, paragneiss and marble and include eclogite, kyanite-garnet-bearing pelitic gneiss, magnesite-bearing garnet lherzolite, K-feldspar-bearing garnet clinopyroxenite and garnet-bearing granitic gneiss. Zircon U–Pb SHRIMP and LA-ICP-MS dating, combined with CL and trace element analysis yield geochronological data for different South Altyn Tagh HP/UHP rocks as follows: metamorphic ages of 493 ± 4 and 499 ± 27 Ma for the Jianggalesayi eclogite and its country rock respectively, 509 ± 12, 489 ± 8 and 487 ± 10 Ma for Yinggelisayi magnesite-bearing garnet lherzolite, K-feldspar-bearing garnet clinopyroxenite and garnet-bearing granitic gneiss, and 475 ± 4 and 486 ± 5 Ma for Qingshuiquan garnet clinopyroxenite and kyanite-bearing garnet-biotite gneiss. Magmatic protolith ages of eclogite, garnet lherzolite, garnet clinopyroxenite and garnet-bearing granitic gneiss are 754 ± 9, 844 ± 9, 821 ± 27 and 842 ± 18 Ma, respectively. Metamorphic ages of these rocks are consistent within errors, indicating that the South Altyn Tagh HP/UHP rocks were formed during 475–509 Ma; these are 20–80 Ma older than dated North Qaidam HP/UHP rocks (420–457 Ma). The occurrence of possible stishovite in pelitic gneiss and the older HP/UHP metamorphic ages suggest that the South Altyn Tagh belt may not necessarily be a western extension of the North Qaidam HP/UHP metamorphic belt, offset along the Altyn Tagh strike-slip fault.

Determination of field-independent and field-dependent components of anisotropy of susceptibility through standard AMS measurement in variable low fields II: An example from the ultramafic body and host granulitic rocks at Bory in the Moldanubian Zone of Western Moravia, Czech Republic

A large ultramafic body (garnet clinopyroxenite and garnet peridotite) embedded in felsic granulite at Bory (Moldanubian Zone of Western Moravia, Czech Republic) contains multiple carrier of susceptibility. Low-field variation of anisotropy of magnetic susceptibility (AMS) enables the separation of the field independent and low-field dependent AMS components to be made. In granulite, the former is carried by paramagnetic minerals and a subordinate admixture of magnetite, while the latter is due to pyrrhotite. In the ultramafite, the field-independent component is carried by pure magnetite formed by serpentinization, while the field-dependent component is due to pyrrhotite and subordinate paramagnetic minerals. Despite small differences, all AMS components are roughly coaxial both within granulite and within ultramafite, but very different between granulite and ultramafite. The difference indicates that the forces that imposed the later granulite fabric were not strong enough to obliterate the original magnetic fabric of the ultramafites.

Mantle peridotites from the Dinaridic ophiolite belt and the Vardar zone western belt, central Balkan: A petrological comparison

The nearly parallel Dinaridic ophiolite belt and the Vardar zone western belt are assumed to be the remnants of two distinct oceanic basins, constituting different parts of the Tethys Ocean that separated the Gondwana and Eurasia continents during Mesozoic time. These belts comprise numerous large peridotite massifs and small bodies whose petrology was poorly known. This paper presents a large set of internally consistent analytical data for peridotites, including primary mineralogy, major-element chemistry and clinopyroxene geochemistry for massifs of both the ophiolitic belts. We propose, discuss and apply a set of mineralogical, geochemical and petrologic criteria that allow a recognition of the probable geodynamic setting of formation of the ultramafic massifs. Ultramafic massifs of the Vardar zone western belt gradually change in composition northwards from depleted spinel lherzolites (Banjska massif) to depleted harzburgites (Maljen massif); these bodies originated in the same geodynamic setting, probably a back-arc spreading center. By contrast, the Dinaridic belt ultramafic massifs include several different types that formed in different geodynamic environments. Orogenic lherzolites, interpreted as subcontinental peridotites (Kozara, Čavka, Borja, Sjenički Ozren and Bistrica massifs) are dominated by fertile spinel and plagioclase lherzolites with subordinate amounts of depleted spinel lherzolite, spinel harzburgite, rare dunite and very rare vein garnet clinopyroxenite. The inferred subcontinental peridotites of these massifs are not co-magmatic with neighboring basalts and cannot be considered as members of a single ophiolitic assemblage. Massifs of two other types are less common in the Dinaridic ophiolite belt. These are composed of spinel lherzolite–harzburgite (Zlatibor and possibly Bosanski Ozren massifs) and depleted harzburgite (Tuzinje and Brezovica massifs); both probably originated in a suprasubduction environment. The available data suggest that the studied ultramafic rocks formed in two different oceanic basins, probably marginal seas. We also suggest that the orogenic lherzolites that dominate the central-northern part of the Dinaridic ophiolite belt formed as a result of continental lithosphere extension after closure of the Dinaride oceanic basin and were later tectonically incorporated into the ophiolitic mélange.

Eclogite and pyroxenite xenoliths from off-craton kimberlites near the Kaapvaal Craton, South Africa

Mantle xenoliths brought to the surface by kimberlite magmas along the south-western margin of the Kaapvaal craton in South Africa can be subdivided into eclogites sensu stricto, kyanite eclogites and orthopyroxene eclogites, all containing omphacite, and garnet clinopyroxenites and garnet websterites characterised by diopside. Texturally, chemically (major elements) and thermally, we observe an evolution from garnet websterites (T EG = 742–781 °C) towards garnet clinopyroxenites (T EG = 715–830 °C) and to eclogites (T EG = 707–1056 °C, mean value of 913 °C). Pressures calculated for orthopyroxene-bearing samples suggest upper mantle conditions of equilibration ( P = 16–33 kb for the garnet websterites, 18 kb for a garnet clinopyroxenite and 23 kb for an opx-bearing eclogite). The overall geochemical similarity between the two groups of xenoliths (omphacite-bearing and diopside-bearing) as well as the similar trace element patterns of clinopyroxenes and garnet suggest a common origin for these rocks. Recently acquired oxygen isotope data on garnet (δ 18O gnt = 5.25–6.78 ‰ for eclogites, δ 18O gnt = 5.24–7.03 ‰ for garnet clinopyroxenites) yield values ranging from typical mantle values to other interpreted as resulting from low-temperature alteration or precursors sea-floor basalts and associated rocks. These rocks could then represent former magmatic oceanic rocks that crystallised from a same parental magma as plagioclase free diopside-bearing and plagioclase-bearing crustal rocks. During subduction, these oceanic rock protoliths equilibrated at mantle depth, with the plagioclase-bearing rocks converting to omphacite and garnet-bearing lithologies (eclogites sensu largo), whereas the plagioclase-free diopside-bearing rocks converted to diopside and garnet-bearing lithologies (garnet websterites and garnet clinopyroxenites).

Petrogenesis of eclogites enclosed in mantle-derived peridotites from the Sulu UHP terrane: constraints from trace elements in minerals and Hf isotopes in zircon

Garnet clinopyroxenite and eclogite lenses are widespread in mantle-derived garnet peridotites from the Sulu UHP terrane, eastern China. In order to constrain their origin, major and trace element compositions of minerals from 6 eclogites and one garnet clinopyroxenite, and Hf isotopes of zircons from one eclogite, were analyzed by electron probe, LA-ICP-MS and LA- (MC) ICP-MS, respectively. Eclogitic garnets show a large variation in composition (Alm 20–48Prp 24–50Grs 15–38Sps 1), weakly correlated with garnets in their host peridotites. Garnets show LREE depletion and flat HREE patterns, with La n of 0.02–0.52 and Lu n of 5–75. Some garnets show zoning in HREE and Y with a U-shaped, concave-downward profile, but locally exhibit positive sharp variations in HREE and Y contents; such feature reflects disturbance in chemical–physical conditions during garnet growth. Clinopyroxene exhibits humped (mostly) or sinusoidal REE patterns, with La n of 0.3–6.0, Nd n of 2–14 and (La/Lu) n of 1.8–113 with a few exceptions. Partition coefficients of REE ( D) between coexisting clinopyroxene and garnet in eclogite show a large variation: D LREE and D HREE ranging from 10 to100 and from 0.1 to 0.01, respectively, similar to D REE values of the natural systems at 800–1100 °C. The D values (0.1–1) of HREE between Cpx and Grt from Grt clinopyroxenite and one eclogite differ distinctly from the other eclogites, which are consistent with high- T experimental data on garnet lherzolites. Zircons from the Xugou eclogite have no inherent core, which have low U/Th (most < 0.20) and 176Lu/ 177Hf (0.000008–0.0000637), and relative high 176Hf/ 177Hf (0.28241–0.28262) ratios. These features indicate a metamorphic signature, and document that metasomatism affected the eclogite protolith. The model ages ( T DM1) of zircons mostly cluster around 1.0–1.1 Ga, which probably reflects the metasomatic source. Bulk compositions of eclogite and Grt clinopyroxenite differ from those of Sulu eclogites in gneiss and MORB. The protoliths of these rocks may reflect the intrusion of asthenosphere-derived melts in Proterozoic. Eclogites with convex REE pattern and garnet clinopyroxenite were formed by high-pressure crystal accumulation whereas the LREE-enriched eclogites may represent crystallized melt; both eclogite and clinopyroxenite together with their host peridotite were metamorphosed at 700–965 °C and ≥ 4 GPa during the Triassic northward subduction of the Yangtze craton.

The Kutná Hora Complex (Moldanubian zone, Bohemian Massif): A composite of crustal and mantle rocks subducted to HP/UHP conditions

Two tectonic lenses of garnet peridotites and one of eclogite, all occurring in high-pressure granulites from the Kutná Hora Complex, were investigated. These rocks are thought to be correlative with the high-grade metamorphic Gföhl unit of the Moldanubian zone. The garnet peridotites contain small bodies of eclogites and thin layers of garnet clinopyroxenite. Also present are two varieties of eclogites (kyanite-bearing and kyanite-rfree) and one variety of garnetite, which form blocks and boudins in the garnet peridotites. The eclogite occurring in granulite without garnet peridotite belongs to the kyanite-bearing variety. Textural relations and mineral compositional zoning of garnet in peridotites and in eclogites were used to constrain PT evolution of these rocks and to determine whether they passed a PT path similar to that in the host peridotite or represent fragments with different metamorphic histories. The results of thermobarometric calculations show that the peridotites were cooled during an increase of pressure at 4.5/950 °C, and the pressure and temperature obtained for garnet clinopyroxenite are also close to these PT values. The eclogites seem to be derived from different rock compositions and to have passed different thermal evolutions prior to their emplacement into garnet peridotites. Temperatures calculated for the eclogites hosted in peridotites range between 850 and 1000 °C, but pressure obtained for kyanite-bearing variety is about 3.4 GPa at 960 °C. The eclogite present in granulite contains prograde zoned garnet and gives temperature of 900 °C at 3.4 GPa. All these rocks seem to represent tectonic fragments that were subducted to different depths and tectonically assembled during their exhumation.

Geochemical Characteristics, UHP Metamorphic Age, and Genesis of the Hujialing Garnet Clinopyroxenite, Sulu Terrane, China

The bulk composition of the Hujialing garnet clinopyroxenite from Rizhao, Sulu ultrahighpressure (UHP) metamorphic belt, is rich in fertile and low in refractory elements compared with primary mantle. This composition is characterized by high MgO (12.77-18.73 wt%) and CaO/Al2O3 (1.6-8.0), and low alkali and Mg# (74.8-86.1), suggesting that the rock has an affinity to aluminadepleted komatiite. The REE pattern resembles that of clinopyroxene with relative HREE depletion of (Gd/Yb)N = 3.51-5.87. The PGE concentrations lie in the range of 19.18-59.44 ppb, higher than that of mantle xenolith and Alpine type peridotites, with (Pd/Ir)N of 8.58-11.07. These chemical data indicate that the magma that recrystallized as garnet clinopyroxenite was formed mainly by melting of clinopyroxene from a mantle source containing residual garnet. The high concentration and positive correlation between PGE abundances and atomic number show that their distribution was controlled by a high degree of partial melting in the absence of sulfide segregation. A zircon U-Pb age of 226.3 ± 1.8 Ma represents the time of UHP metamorphism of the garnet clinopyroxenite, which is coincident with 220-240 Ma regional UHP events for the Dabie-Sulu terrane.

Petrology and U-Pb SHRIMP Geochronology of a Garnet Peridotite, Sulu UHP Terrane, East-Central China

Garnet clinopyroxenite at Rizhao occurs as a lens 100 x 225 m2 within a serpentinized peridotite body, in fault contact with felsic gneiss. Minor dunite also occurs within the same peridotite. The clinopyroxenite consists of porphyroblastic Grt-bearing (type 1), megacrystic Grt-bearing (type 2), porphyroblastic Cpx-bearing (type 3), and equigranular (type 4) garnet clinopyroxenite. Lamellaerich, coarse-grained Cpx occurs as inclusions in megacrystic Grt from type 2 clinopyroxenite, and as porphyroblasts in a matrix of Cpx + Grt + Ilm from type 3 clinopyroxenite; these clinopyroxenes contain abundant exsolution lamellae of Grt (Prp16-19Grs62Alm19-21Sps1) + Ilm ± Mag ± Amp (Mg#: 0.88-0.98; Na2O: 0.5-3.3 wt%; K2O: 0.7-1.0 wt%; TiO2: 0.1-0.2 wt%). Megacrystic and porphyroblastic garnet (Prp35-42Grs34-45Alm19-24Sps1) also occurs in a matrix of fine-grained Cpx + Grt + Ilm; spinel (Mg#: 0.60-0.62) is present as inclusions in both porphyroblastic Grt and lamellae-rich Cpx. Petrochemical data support a mantle origin involving crystallization of majoritic garnet. We propose that the protolith of the Rizhao clinopyroxenite initially was a spinel clinopyroxenite, and was convected to great depths (>450 km) to form a majoritic garnet that contains high CaO and considerable amounts of FeO and TiO2. During later decompression, the majoritic garnet broke down to form intergrowths of Cpx + Grt + Ilm ± Mag ± Amp at T = 700-800°C, P ≤ 3 GPa. It was then emplaced into a subduction zone and experienced UHP metamorphism at 620-880°C, 3 GPa during Triassic continental collision. Megacrystic/porphyroblastic garnet formed during this stage, and lamellaerich Cpx recrystallized to the matrix assemblage of Cpx + Grt + Ilm subsequently. The clinopyroxenite also experienced later retrogression during exhumation. Zircon U-Pb geochronology yields 215 ± 2 Ma for the Rizhao garnet clinopyroxenite; occurrence of garnet and clinopyroxene inclusions in zircon suggests that this age reflects the UHP metamorphism.

Ancient recycled crust beneath the Ontong Java Plateau: Isotopic evidence from the garnet clinopyroxenite xenoliths, Malaita, Solomon Islands

We present a Sr, Nd, Hf and Pb isotope investigation of a set of garnet clinopyroxenite xenoliths from Malaita, Solomon Islands in order to constrain crustal recycling in the Pacific mantle. Geological, thermobarometric and petrochemical evidence from previous studies strongly support an origin as a series of high-pressure (> 3 GPa) melting residues of basaltic material incorporated in peridotite, which was derived from Pacific convective mantle related to the Ontong Java Plateau magmatism. The present study reveals isotopic variations in the pyroxenites that are best explained by different extents of chemical reaction with ambient peridotite in the context of a melting of composite source mantle. Isotopic compositions of bimineralic garnet clinopyroxenites affected by ambient peridotite fall within the oceanic basalt array, similar to those of Ontong Java Plateau lavas. In contrast, a quartz-garnet clinopyroxenite, whose major element compositions remain intact, has lower 206Pb/ 204Pb– 143Nd/ 144Nd and higher 87Sr/ 86Sr– 207Pb/ 204Pb ratios than most oceanic basalts. These isotopic signatures show some affinity with proposed recycled sources such as the so-called EM-1 or DUPAL types. Constraints from major and trace element characteristics of the quartz-garnet clinopyroxenite, the large extent of Hf–Nd isotopic decoupling and the good coincidence of Pb isotopes to the Stacey–Kramers curve, all indicate that pollution of southern Pacific mantle occurred by the subduction or delamination of Neoproterozoic granulitic lower crust (0.5–1 Ga). This crustal recycling could have taken place around the suture of Rodinia supercontinent, a part of which resurfaced during mantle upwelling responsible for creating the Cretaceous Ontong Java Plateau.

Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy)

Garnet clinopyroxenite and garnet websterite layers occur locally within mantle peridotite bodies from the External Liguride Jurassic ophiolites (Northern Apennines, Italy). These ophiolites were derived from an ocean–continent transition similar to the presentday western Iberian margin. The garnet clinopyroxenites are mafic rocks with a primary mineral assemblage of pyrope-rich garnet þ sodic Al-augite (Na2O 2.5 wt %, Al2O3 12.5 wt %), with accessory graphite, Fe–Ni sulphides and rutile. Decompression caused Na-rich plagioclase (An50–45) exsolution in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene þ plagioclase (An85–72) þ Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine þ plagioclase-bearing assemblage, locally under syn-kinematic conditions, at the expense of two-pyroxenes þ Al-spinel. Mg-rich garnet has been also found in the websterite layers, which are commonly characterized by the occurrence of symplectites made of orthopyroxene þ Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu–Hf and Sm–Nd mineral isochrons (220 ± 13 Ma and 186.0 ± 1.8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T 1100 C and P 2.8 GPa. The early decompression was associated with moderate cooling, corresponding to T 950 , and development of a spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase–olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution occurred under a brittle regime and led to the superposition of hornblende to serpentine veining stages. The garnet pyroxenitebearing mantle from the External Liguride ophiolites represents a rare tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. The exhumation was probably accomplished through a two-step process that started during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, probably including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence became involved in an extensional detachment process that resulted in sea-floor denudation.

Mesozoic adakitic rocks from the Xuzhou-Suzhou area, eastern China: Evidence for partial melting of delaminated lower continental crust

Adakitic rocks in the Xuzhou-Suzhou area, eastern China, consist of dioritic and monzodioritic porphyries and were dated at 131–132 Ma by the SHRIMP U–Pb zircon method. These rocks have high MgO content (1.47–5.73%), high Mg # values (0.49–0.61), and high La/Yb and Sr/Y ratios. These features are similar to rocks derived from partial melting of a subducted oceanic slab. However, their high initial 87Sr/ 86Sr (0.7053–0.7075) and low ε Nd( t) values (−4.43∼−13.14) are inconsistent with the origin from slab melting. These rocks often contain garnet residual crystals and eclogite, garnet clinopyroxenite, and garnet amphibolite xenoliths. Petrographical characteristics and estimated P– T conditions of these xenoliths indicate that they were once deeply subducted and subsequently underwent rapid exhumation in the early Mesozoic. Garnet residual crystals from the porphyries show similar chemical compositions to garnets from garnet clinopyroxenite and garnet amphibolite xenoliths. Ages of the inherited zircons of the xenoliths and their host rocks likely indicate that sources for the adakitic magma and protoliths of the eclogite and garnet clinopyroxenite xenoliths in the study area were from Precambrian basement of the North China Craton. The data also suggest that the lower continental crust in the eastern North China Craton was thickened during the early Mesozoic and delaminated in the early Cretaceous. The high-Mg adakitic magma resulted from partial melting of this delaminated lower continental crust and its subsequent interaction with the mantle during upward transport, leaving garnet as the residual phase.

Comment on “High-pressure melting experiments on garnet clinopyroxenite and the alkalic to tholeiitic transition in ocean-island basalts” by Keshav et al. [Earth Planet. Sci. Lett. 223 (2004) 365–379

Comment on “High-pressure melting experiments on garnet clinopyroxenite and the alkalic to tholeiitic transition in ocean-island basalts” by Keshav et al. [Earth Planet. Sci. Lett. 223 (2004) 365–379

Mesozoic crustal thickening of the eastern North China craton: Evidence from eclogite xenoliths and petrologic implications

A suite of xenoliths of eclogite, garnet clinopyroxenite, and felsic gneiss is found in Early Cretaceous high-Mg [Mg # >45, where Mg # = molar 100 × Mg/(Mg + Fe total )] adakitic intrusions from the Xuzhou-Huaibei (Xu-Huai) region along the southeastern margin of the North China craton. The primary mineral assemblage of garnet + omphacite/augite + quartz + rutile ± pargasite of the eclogite and garnet clinopyroxenite xenoliths defines a minimum pressure of >1.5 GPa, while the estimated peak metamorphic temperatures range from 800 to 1060 °C. An Sm-Nd whole-rock–garnet isochron and zircon U-Pb dates show that timing of the eclogite facies metamorphism took place ca. 220 Ma. This Triassic age agrees with the age of eclogites from the Dabie-Sulu ultrahigh-pressure metamorphic (UHPM) belt. The ages of abundant Late Archean to early Paleoproterozoic (2.3–2.6Ga) inherited zircons correspond to the most prominent crustal growth event in the North China craton. In addition, these xenoliths and their host high-Mg adakitic intrusions have complementary major and trace element compositions, suggesting that the adakites formed by partial melting of Archean metabasalts that were the protoliths of the Xu-Huai eclogite and garnet clinopyroxenite xenoliths. Trace element and Sr-Nd isotopic modeling shows that the high-Mg adakitic intrusions can be modeled as melts from ∼40% partial melting of the metabasalts in the eclogite facies, followed by interaction with the convecting mantle and variable degrees of crustal assimilation. Together with the similar zircon age populations between the xenoliths and the host rocks, these lines of evidence strongly suggest their genetic link via thickening, foundering, and partial melting of the Archean North China craton mafic lower crust, followed by adakitic melt-mantle interaction. The crustal thickening resulted from Triassic collision between the Yangtze craton and the North China craton, which produced the Dabie-Sulu UHPM belt in the subducting Yangtze plate and eclogitization of the basaltic crustal root of the overriding North China craton plate. Such processes may have played an important role in generating the high-Mg character of the continental crust.

Response to the comment by M. Lustrino on “High-pressure melting experiments on garnet clinopyroxenite and the alkalic–tholeiitic transition in ocean-island basalts” by Keshav et al. [Earth Planet. Sci. Lett. 223, 365–379 (2004)

Abstract We welcome the comments made by Lustrino [M. Lustrino, Pyroxenites everywhere, Earth Planet. Sci. Lett., this issue.] on the high-pressure experimental melting phase relations of anhydrous garnet pyroxenite reported by Keshav et al. [S. Keshav, G.H. Gudfinnsson, G. Sen, Y. Fei, High-pressure melting experiments on garnet clinopyroxenite and the alkalic-to-tholeiitic transition in ocean-island basalts, Earth Planet. Sci. Lett. 223 (2004) 365–379.]. Such healthy debates, in our view, are needed to promote our understanding on the origin of oceanic-island basalts (OIB).

Low-degree melting of a metasomatized lithospheric mantle for the origin of Cenozoic Yulong monzogranite-porphyry, east Tibet: Geochemical and Sr–Nd–Pb–Hf isotopic constraints

Abstract SHRIMP zircon U–Pb dating, mineral chemical, element geochemical and Sr–Nd–Pb–Hf isotopic data have been determined for the Yulong monzogranite-porphyry in the eastern Tibet, China. The Yulong porphyry was emplaced into Triassic strata at about 39 Ma. The rocks are weakly peraluminous and show shoshonitic affinity, i.e., alkalis-rich, high K 2 O contents with high K 2 O / Na 2 O ratios, enrichment in LREE and LILE. They also show some affinities with the adakite, e.g., high SiO 2 and Al 2 O 3 , and low MgO contents, depleted in Y and Yb, and enrichment in Sr with high Sr / Y and La / Yb ratios, and no Eu anomalies. The Yulong porphyry has radiogenic 87 Sr / 86 Sr (0.7063–0.7070) and unradiogenic 143 Nd / 144 Nd ( e Nd = − 2.0 to − 3.0) ratios. The Pb isotopic compositions of feldspar phenocrysts separated from the Yulong porphyry show a narrow range of 206 Pb / 204 Pb ratios (18.71–18.82) and unusually radiogenic 207 Pb / 204 Pb (15.65–15.67) and 208 Pb / 204 Pb (38.87–39.00) ratios. In situ Hf isotopic composition of zircons that have been SHRIMP U–Pb dated is characterized by clearly positive initial e Hf values, ranging from + 3.1 to + 5.9, most between + 4 and + 5. Phenocryst clinopyroxene geothermometry of the Yulong porphyry indicates that the primary magmas had anomalously high temperature (> 1200 °C). The source depth for the Yulong porphyry is at least 100 km inferred by the metasomatic volatile phase (phlogopite–carbonate) relations. Detailed geochemical and Sr–Nd–Pb–Hf isotopic compositions not only rule out fractional crystallization or assimilation-fractional crystallization processes, but also deny the possibility of partial melting of subducted oceanic crust or basaltic lower crust. Instead, low degree (1–5%) partial melting of a metasomatized lithosphere (phlogopite–garnet clinopyroxenite) is compatible with the data. This example gives a case study that granite can be derived directly by partial melting of an enriched lithospheric mantle, which is important to understand the source and origin of diverse granites.

Mesozoic adakitic rocks from the Xuzhou–Suzhou area, eastern China: Evidence for partial melting of delaminated lower continental crust

Adakitic rocks in the Xuzhou–Suzhou area, eastern China, consist of dioritic and monzodioritic porphyries and were dated at 131–132 Ma by the SHRIMP U–Pb zircon method. These rocks have high MgO content (1.47–5.73%), high Mg # values (0.49–0.61), and high La/Yb and Sr/Y ratios. These features are similar to rocks derived from partial melting of a subducted oceanic slab. However, their high initial 87Sr/ 86Sr (0.7053–0.7075) and low ε Nd( t) values (−4.43 to −13.14) are inconsistent with the origin from slab melting. These rocks often contain garnet residual crystals and eclogite, garnet clinopyroxenite, and garnet amphibolite xenoliths. Petrographical characteristics and estimated P–T conditions of these xenoliths indicate that they were once deeply subducted and subsequently underwent rapid exhumation in the early Mesozoic. Garnet residual crystals from the porphyries show similar chemical compositions to garnets from garnet clinopyroxenite and garnet amphibolite xenoliths. Ages of the inherited zircons of the xenoliths and their host rocks likely indicate that sources for the adakitic magma and protoliths of the eclogite and garnet clinopyroxenite xenoliths in the study area were from Precambrian basement of the North China Craton. The data also suggest that the lower continental crust in the eastern North China Craton was thickened during the early Mesozoic and delaminated in the early Cretaceous. The high-Mg adakitic magma resulted from partial melting of this delaminated lower continental crust and its subsequent interaction with the mantle during upward transport, leaving garnet as the residual phase.

SHRIMP U-Pb Dating of Zircon from the Xugou UHP Eclogite, Sulu Terrane, Eastern China

Eclogites, together with garnet clinopyroxenites, occur as lenses within the Xugou garnet peridotite body in the southern Sulu ultrahigh-pressure (UHP) terrane. Combined cathodoluminescence (CL) imaging and SHRIMP U-Pb dating of zircon from two Xugou mafic eclogites provide added constraints on the timing of UHP metamorphism in this area. Zircons from both samples show subrounded to rounded shapes and patchy CL patterns without inherited igneous cores, indicating that they are metamorphic zircons. SHRIMP U-Pb analyses of these zircons yielded apparent U-Pb ages of 214-280 Ma, with a weighted mean age of 237 ± 8 Ma, which is consistent with previous reported UHP metamorphic ages from eclogite pods and country-rock gneisses. The Xugou mafic lenses may have formed by partial melting of the enclosing peridotites in the mantle before subduction (Zhang et al., 2003); then these eclogites, together with the host peridotites, were tectonically emplaced into the subduction zone and subjected to UHP metamorphism at 237 ± 8 Ma.