Age Of Ultrahigh-pressure Metamorphism Research Articles

First finding of continental deep subduction in the Sesia Zone of the Western Alps and implications for subduction dynamics.

Continental deep subduction after the closure of large oceanic basins is commonly ascribed to the gravitational pull of the subducting oceanic slab. However, it is not clear how continental lithosphere adjacent to small oceanic basins was subducted to mantle depths. The Sesia Zone in the Western Alps provides an excellent target for exploration of subduction dynamics in such a tectonic setting. Here we report the first finding of coesite in a jadeite-bearing orthogneiss from the Sesia Zone, providing the first evidence for deep subduction of the continental crust to mantle depths for ultrahigh-pressure (UHP) metamorphism in this zone. Three coesite inclusions were identified by laser Raman spectroscopy in two garnet grains. Based on zircon U-Pb dating and trace element analysis, the UHP metamorphic age was constrained to be 76.0±1.0 Ma. The phase equilibrium modeling yields peak metamorphic pressures of 2.8-3.3GPa, demonstrating the continental deep subduction to mantle depths of >80km. The subducted continental crust was a rifted hyperextended continental margin, which was converted to the passive continental margin during seafloor spreading and then deeply subducted during the oblique convergence between the Adria microplate and Eurasian plate in the Late Cretaceous. Because the slab pull could only play a limited role in closing small oceanic basins for continental collision, the distal push of either continental breakup or seafloor spreading is suggested as the major driving force for the deep subduction of continental crust in the Western Alps. Therefore, deep subduction of the continental crust bordering small oceanic basins would have been induced by the far-field stress of compression, whereas that bordering large oceanic basins was spontaneous due to the oceanic slab pull. This provides a new insight into the geodynamic mechanism of continental deep subduction.

Melting of subducted continental crust during collision and exhumation: Insights from granitic rocks from the North Qaidam UHP metamorphic belt, NW China

The formation and reworking of continental crust have close connections with orogeny. Syn- and post-collisional magmatism is key to understanding crustal growth and reworking during continental collision and post-collisional extension. Here we report a series of syn-collisional felsic magmatic rocks intruded into migmatized gneiss and amphibolite/eclogite in the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, to provide insights into melting/reworking of UHP metamorphosed continental crust during continental collision and exhumation. The syn-collisional magmas are mainly tonalite-trondhjemite and mica granite that occur as plutons and dykes, and high potassium granite veins in migmatites. They were formed by decompression melting of subducted oceanic and continental crust at different levels during exhumation, without any discernible mantle contribution. The tonalite-trondhjemites show adakitic signatures of both oceanic crust and mafic continental crust with varying Nd isotopic compositions. The mica granite plutons have high silica and alkali contents, are weakly to strongly peraluminous, and have crustal SrNd isotope compositions. These mica granites were formed by partial melting of previously subducted continental crust, and formed mostly in high-pressure conditions, in the garnet stability field. The protolith is predominantly granitic gneiss, with minor meta-sediment and metabasite. A compilation of published ages and our results show that these syn-collisional magmatic rocks formed between 437 and 397 Ma, overlapping with UHP metamorphic ages (440–420 Ma), and suggests that exhumation occurred synchronously with continental subduction. Individual zircon grains show metamorphic cores and magmatic rims with a ≤ 20 Myr time interval, interpreted as the time between peak UHP metamorphism and melting during exhumation. Such long-lived and episodic melting of UHP metamorphic continental crust should result from periodic processes, involving melt weakening and decompression by upward movement within the subduction channel, rather than coherent motion of the entire UHP belt. This process represents crustal reworking during collision and exhumation, without input of mantle melts.

Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8–1.3GPa/485–569°C was followed by peak UHP metamorphism at 2.5–3.3GPa/567–754°C and cooling to amphibolite facies conditions at 0.6–0.7GPa/571–589°C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458Ma (peak at ~450Ma), a smaller group ranging from 770 to 1000Ma (peak at 780Ma), and minor >1.8Ga zircon aged ca. 430Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442Ma and a burial rate of ca. 1.0–1.1cm/yr when combined with P–T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely deposited in a terrigenous-dominated forearc marine basin rather than an intracontinental basin environment, further evidence that some continental arc volcanic rock may have been the source of eclogite in the North Qaidam. These sediments, formed in a forearc basin close to the Qaidam Block to the north, were transported in the subduction zone to 100–110km depth with UHP metamorphism prior to exhumation. Meanwhile, the new results suggest that subduction erosion occurred along the active continental margin during the Qaidam Block with north-dipping subduction, indicating that the North Qaidam UHP metamorphic belt may have formed during continental-arc collision.

Origin of allanite in gneiss and granite in the Dabie orogenic belt, Central East China

Allanite is a common accessory mineral phase, representing an important carrier of rare earth elements, Th, U, Sr and other trace elements in most continental rocks. As Th and U can be incorporated into the allanite lattice, the mineral is a good geochronological tool for constraining geological events. Moreover, the trace element features δEu, Th/U ratio and common lead content of allanite are indicators of the forming conditions. Allanite and coexisting epidote-group minerals are abundant in ultrahigh-pressure (UHP) metamorphic rocks from the Dabie-Sulu orogen in central East China. However, if these minerals formed in the Neoproterozoic as magmatic phases, or in the Triassic as metamorphic phases is a matter of long-standing controversy. We report major and trace element analyses of whole rocks, allanite and coexisting epidote-group minerals, together with U-Th-Pb isotopic compositions of allanite in UHP gneiss from the Dabie-Sulu orogen, and allanite in the adjacent Jingshan granite. The granite is emplaced along the southeastern margin of the North China Craton and considered a product of partial melting of the subducted Dabie-Sulu gneiss. Trace elements (low Th/U and La/Sm, high δEu and high Sr) and high common lead concentrations indicate a metamorphic origin of allanite-epidote in the UHP gneiss. On the other hand, coarse-grained allanite from the Jingshan granite shows a corrosion core and a magmatic rim with common 208Pb up to 70% in the core and less than 30% in the rim. The allanite cores are of peritectic and the rims of magmatic origin with ages of ∼160Ma, consistent with the granite crystallization age. In combination with previous studies, we conclude that the allanite of the Jingshan granite has form from the subducted and remolten Dabie-Sulu gneiss. Allanite records Triassic UHP metamorphic ages as well as Jurassic peritectic-magmatic ages as a part of the evolution of the Dabie-Sulu orogen.

Post-collisional magmatism: Consequences of UHPM terrane exhumation and orogen collapse, N. Qaidam UHPM belt, NW China

Exhumation of subducted slabs and extensional collapse of orogens are the main tectonic processes in ancient and modern continental collisional zones. Magmatism during these two processes may play important roles in understanding reworking and growth of the continental crust We report here that a series of plutonic magmas, including intrusions of two-mica granite, tonalite, granodiorite, biotite monzogranite, porphyritic biotite granite and diorite, as well as contemporaneous mafic dykes, have been recognized in Dulan edogite-bearing terrane, the North Qaidam ultra-high pressure metamorphic (UHPM) belt The magmatism represented by these plutons is temporally similar to 20-30 million years (Mys) younger than the UHPM age, lasting for similar to 40 Mys and derived from different sources with different mechanisms. The magmatism was initiated by exhumation of UHPM terranes during which strongly-peraluminous two-mica granite and metaluminous tonalite were produced respectively by decompression melting of the exhumed UHPM upper and lower continental crust, respectively. The genesis of mafic magmatic enclave (MME)-hosting granodiorite with a clear hybrid signature and coeval biotite monzogranite reflected the upwelling of asthenospheric mantle by extension of lithosphere during the orogen collapse. It was induced by detachment of the subducted lithospheric mantle, which then brought heat and mantle material into continental crust and triggered the partial melting of the exhumed UHPM continental crust, and gave rise to mixing of crustal and mantle melts. Porphyritic biotite granite reflects a late melting event of continental crust Diorite marked by high magnesium content represents mantle melts with slight crustal contamination, which implies that the orogen has been unrooted and collapsed completely. The post-collisional magmatism of the North Qaidam belt provides an improved understanding for the late thermal and tectonic evolution of a UHPM continental collision zone. (C) 2014 Elsevier B.V. All rights reserved.

SHRIMP zircon chronology of HP-UHP rocks of the Makbal metamorphic complex in the Northern Tien Shan, Kyrgyzstan

In this paper we present the first results of sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dating obtained for the ultra-high pressure (UHP) metamorphic rocks of the Makbal metamorphic complex in the Northern Tien Shan, Kyrgyzstan. Zircon grains from two HP eclogite samples and one sample of UHP garnet-talk-chloritoid schist have similar bright rims identified on cathodoluminescence (CL) images. The rims are interpreted as zircon overgrowth during UHP metamorphism. Dating of zircon rims from the three samples yielded similar ages of 509±7Ma, 502±10Ma and 498±7Ma. The time span of 510–500Ma is considered as the “minimum” age of UHP metamorphism of the Makbal complex. This age is in accordance with Middle Cambrian faunal age of the Kara-Archa ophiolite in the Makbal area. It is suggested that the Makbal rocks were metamorphosed in the Middle Cambrian subduction zone dipping north under the Northern Tien Shan microcontinent. A deformed granodiorite associated with the Makbal metamorphic rocks yielded an age of 514±5Ma, indicating development of a magmatic arc simultaneously with the UHP metamorphism. Cores of zircon grains from the garnet-talk-chloritoid schist yielded a wide spectrum of ages (1850–800Ma) characteristic for detrital zircons of continental origin. A depositional age <800Ma points to a possible derivation from a Riphean passive margin of the Terskey Ocean. Analyses of zircon grains from two eclogite samples fit well-defined discordia lines with upper intercepts at 1446±25Ma and 1447±29Ma, indicating a homogeneous Mezoproterozoic protolith of these rocks. The possible presence of such protoliths in Makbal is illustrated by a similar age of inherited zircon recovered from the granodiorite (1443±25Ma) and by Mesoproterozoic detrital zircons from the garnet-talk-chloritoid schist. Ages of Kokchetav, Makbal, Aktyuz and Anrakhai HP-UHP complexes in the Kokchetav–Northern Tien Shan belt vary from 537 to 475Ma and register subduction–accretion events during the Caledonian orogeny which finally formed the Paleo-Kazakhstan continent prior to Middle Ordovician.

Petrological and geochronological constraints on the origin of HP and UHP kyanite-quartzites from the Sulu orogen, Eastern China

Kyanite (Ky)-quartzites occur in both the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic belts in the southern Sulu orogen. The HP Ky-quartzites consist of quartz, kyanite and minor rutile with or without topaz and phengite, whereas those from the UHP unit consist of quartz, kyanite, phengite and rutile. The HP Ky-quartzites are characterized by high Al 2O 3 (up to 32.9 wt.%) and low SiO 2 (down to 60.4 wt.%) with very low other oxides contents (<1 wt.%); whereas the UHP Ky-quartzites are characterized by high SiO 2 (77.7–78.4 wt.%) and relatively low Al 2O 3 (19.1–19.5 wt.%). Both HP and UHP quartzites have variable trace element concentrations, but show distinctly negative Nb and Ta anomalies, and positive Zr anomalies. Rare earth element (REE) patterns for both types of quartzites show LREE enrichment and HREE depletion without Eu anomalies. Zircons from HP quartzites have inherited magmatic cores and metamorphic rims; most zircon cores show oscillatory zoning, contain low-pressure (LP) mineral inclusions of Kfs, Pl and Qz, and have relatively high REE contents (av. 1236.6 ppm) and high Th/U ratios (av. 1.57). The metamorphic rims show patchy zoning, contain HP inclusions of Ky and Tpz and have relatively low REE contents (av. 199.1 ppm) and Th/U ratios (av. 0.015). Most zircons from the UHP quartzites show irregular or patchy zoning, have mineral inclusions of Coe, Ph, and Rt, low REE contents (av. 332.8 ppm) and low Th/U values (av. 0.027). Only a few zircons from the UHP quartzite have inherited magmatic cores. LA-ICP-MS zircon U–Pb dating yields Neoproterozoic protolithic ages (790–764 Ma) for magmatic zircon cores of the HP quartzites, whereas the rims give Triassic (222–218 Ma) ages for HP metamorphism. Similarly, zircons from the UHP quartzites yield a Neoproterozoic protolithic age (819 Ma) and Triassic UHP metamorphic ages (223 and 221 Ma). Based on the comparable petrochemical compositions, and consistent protolithic and metamorphic ages between the quartzites and their country rock gneisses, we conclude that these quartzites were metasomatic products of granitic gneisses. Thus, they were interpreted as a part of the coherent Sulu terrane that was subjected to the coeval Triassic HP and UHP metamorphism at different subduction depths of the Yangtze plate.

Melting-induced fluid flow during exhumation of gneisses of the Sulu ultrahigh-pressure terrane

Hydrothermally altered rocks are products of fluid–rock interactions, and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks. Thus, quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure. In order to decipher the fluid activity in the Sulu ultrahigh-pressure (UHP) terrane in eastern China, quartz veins together with an adjacent eclogite lens and the host gneiss were studied. In one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens. The amphibolite-facies overprinting of the eclogite lens decreases from the rim to the core of the lens, with fresh eclogite preserved in the core. The foliated biotite gneiss contains felsic veins and residual phengites. Zircon rims from the gneiss are characterized by melt-related signatures with steep HREE patterns, high Hf contents and negative Eu anomalies, and a pool of weighted average 206Pb/ 238U analyses reveal an age of 219 ± 3 Ma (2σ), which is younger than the UHP metamorphic age (236 ± 2 Ma, 2σ) recorded by zircons from the eclogite lens. This suggests that the gneiss in the Sulu UHP terrane could have suffered from partial melting due to phengite dehydration during the “hot” exhumation stage. The formation age of the quartz vein (219 ± 2 Ma, 2σ) defined by zircon rims agrees well with the partial melting time (219 ± 3 Ma, 2σ) of the host gneiss. The initial 176Hf/ 177Hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens, but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss. These observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss. On the other hand, amphibole-bearing and HREE-rich zircon rims from the amphibolite pool an amphibolite-facies metamorphic age of 217 ± 5 Ma (2σ), overlap with the formation age of the quartz vein. This implies that retrogression of the eclogite lens could have been caused by melting-induced fluid flow. Based on the above observations, we speculate that partial melting of the gneiss in the continental subduction-related UHP belt could have induced a significant fluid flow during the exhumation stage, and thus contributed significantly to the extensive retrogression of eclogites in the Sulu UHP terrane.

Genesis of garnet peridotites in the Sulu UHP belt: Examples from the Chinese continental scientific drilling project-main hole, PP1 and PP3 drillholes

The main hole (MH), and pre-pilot holes PP1, and PP3 of the Chinese Continental Scientific Drilling Project (CCSD) penetrated three different garnet peridotite bodies in the Sulu ultrahigh pressure (UHP) metamorphic belt, which are 80 m, 120 m, and 430 m thick, respectively. The bodies occur as tectonic blocks hosted in eclogite (MH peridotite) and gneisses (PP1 and PP3 peridotites). The peridotites in the MH are garnet wehrlites, whose protoliths were ultramafic cumulates based on olivine compositions (Fo 79–89) and other geochemical features. Zoned garnet and omphacite (with 4–5 wt.% Na 2O) are typical metamorphic minerals in these rocks, and, along with P– T estimates based on mineral pairs, suggest that the rocks have undergone UHP metamorphism. SHRIMP U–Pb isotope dating of zircon from the garnet wehrlite yielded a Paleozoic protolith age (ca. 346–461 Ma), and a Mesozoic UHP metamorphic age (ca. 220–240 Ma). The peridotites in PP1 consist of interlayered garnet (Grt)-bearing and garnet-free (GF) peridotite. Both types of peridotite have depleted mantle compositions (Mg# = 90–92) and they display transitional geochemical features. The intercalated layers probably reflect variations in partial melting rather than pressure variations during metamorphism, and the garnets may have been formed by exsolution from orthopyroxene during exhumation. These peridotites were probably part of the mantle wedge above the subduction zone that produced the UHP metamorphism and thus belonged to the North China Block before its tectonic emplacement. The exhumation of the subducted Yangtze Block brought these mantle fragments to shallow crustal levels. The ultramafic rocks in PP3 are dominantly dunite with minor garnet dunite. Their high Mg# (92–93) and relatively uniform chemical compositions indicate that they are part of a depleted mantle sequence. The presence of garnet replacing spinel and enclosing pre-metamorphic minerals such as olivine, clinopyroxene and spinel suggests that these rocks have undergone progressive metamorphism. SHRIMP U–Pb isotope dating of zircon from these rocks yielded two age groups: 726 ± 56 Ma for relic magmatic zircon grains and 240 ± 2.7 Ma for the newly formed metamorphic zircon. The older group is similar in age to granitic intrusions within the Dabie–Sulu belt, suggesting that the PP3 garnet peridotite may record the early emplacement of the peridotite into the crust. The younger dates coincide with the age of UHP metamorphism during continent–continent collision between the Yangtze and North China Blocks, suggesting that these peridotites were subducted to depths equivalent to the coesite facies and later exhumed. Thus, the garnet peridotites in the CCSD cores include both ultramafic rocks that existed originally in the subducted plate and rocks from the mantle wedge above the subducted plate, i.e., part of the North China Block.

UHP metamorphic evolution and SHRIMP geochronology of a coesite-bearing meta-ophiolitic gabbro in the North Qaidam, NW China

We have for the first-time observed the well-preserved coesite inclusions in a kyanite-eclogite from the Shaliuhe terrane of the North Qaidam ultra-high pressure (UHP) metamorphic belt, NW China. This provides convincing evidence for UHP metamorphism for the North Qaidam eclogite. Detailed petrography indicates that the kyanite-eclogite records complex metamorphic histories that can be broadly described in terms of three stages: (1) a pre-peak metamorphic stage characterized by the mineral assemblage of Grt + Amp + Pl + Qtz at P = 0.49–0.67 GPa and T = 410–490 °C; (2) a peak eclogite-facies stage with a mineral assemblage of Grt + Omp + Ky + Phn + Rt ± Qtz/Coe at P = 2.7–3.4 GPa and T = 610–700 °C; (3) a retrograde “stage” represented by reaction texture which includes the garnet reaction rims, the breakdown of omphacite, the symplectitic breakdown of kyanite, the retrograded garnet amphibolite assemblage and the latest greenschist-facies overprint. Zircon U–Pb SHRIMP dating shows the peak UHP metamorphic age of the kyanite-eclogite to be ∼440 Ma. These data indicate that the kyanite-eclogite had ever been subducted to mantle depth of ∼100 km in the end of the Ordovician. All these observations allow the construction of a P-T path for the kyanite-eclogite, suggesting a cold geothermal gradient and rapid subduction, ultimately followed by rapid exhumation in response to continental collision in Shaliuhe terrane and throughout the North Qaidam UHP metamorphic belt.

Contrasting Pressure – Temperature Evolution of Pelitic Schists, Gneisses and Eclogites in Kaghan-Naran Valley, Pakistan Himalaya

A classical example of Barrovian type metamorphic sequence is observed in pelitic schists, gneisses and eclogites from Balakot in the southwest to Babusar Pass in the northeast, Kaghan – Naran Valley, Pakistan Himalaya. This sequence comprises the first appearance of chlorite followed by biotite, garnet, staurolite, kyanite and sillimanite. Based on mineral chemistry, pressure – temperature conditions of garnet, staurolite, kyanite zones and eclogites in kyanite zone were estimated as 6.2 – 6.9 kbar and 420 – 478 ° C; 7.1 – 8.3 kbar and 558 – 605 ° C; 12.8 – 14.6 kbar and 658 – 700 ° C; and 27 – 32 kbar and 727 – 799 ° C respectively (Figure 1). Based on detailed field survey in the study area and petrography, the previously called basement and cover sequence of Higher Himalayan sequence from the structural bottom to top is recently classified into three tectonic units referred herein as unit I, II and III (this study). Unit I mainly comprises the basement sequence and has a tectonic contact with the Main Central Thrust to the southwest. This unit mainly consists of pelitic schists and gneisses. Unit II representing ultrahigh-pressure (UHP) metamorphism, is sandwiched in between units I and III. It constitutes the lower cover having pelitic schists; gneisses; felsic gneisses, calcareous gneisses/marbles and eclogites, while unit III is uppermost part of the cover sequence and has a tectonic juxtaposition with low grade Tethyan metasediments locally and Main Mantle Thrust in particular to the northeast. It is also comprised of low grade pelitic gneisses. Presence of coesite relics in clinopyroxene from eclogites and as inclusions in zircon in gneisses from Higher Himalayan crystalline rocks (unit II) gives evidence of deep continental subduction. Geothermal interpolations from petrological data and presence of coesite proves the hypothesis that deep continental subduction occurred when Indian plate collided with Asian plate sandwiching Kohistan Arc approximately at 53 Ma with the closure of Tethys. At the collision boundary marked by Main Mantle Thrust, continental rocks along with oceanic crust subducted beneath Kohistan Arc reaching about 100 ± 10 km depth. Tectonic setup and relative P – T conditions (Figure 2a, b) interprets that the grade of metamorphism in Higher Himalayan sequence increased towards north close to the subduction front. Ultrahigh-pressure metamorphism took place in unit II when it reached to a considerable depth sufficient for the development of coesite. At this event the felsic/pelitic rocks metamorphosed to UHP gneisses and basaltic sills and flows metamorphosed to eclogites. The UHP rocks underwent medium-pressure Barrovian metamorphism during their exhumation stage. SHRIMP data for zircon core and rims from the felsic gneisses of unit II close to eclogite body yields the protolith age as of 253-170 Ma and UHP metamorphic age as 46.2 ± 0.7 Ma (Kaneko et al. 2003). Petrologic and P – T data indicate that these rocks exhumed to earth surface from depths of up to about 90~110 km evidenced by coesite retrogression to quartz and omphacite to amphibole in eclogites.

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.

In situ LA-ICP-MS zircon U-Pb age of ultrahigh-pressure eclogites in the Yukahe area, northern Qaidam Basin

Zircon grains were selected from two types of ultrahigh-pressure (UHP) eclogites, coarse-grained phengite eclogite and fine-grained massive eclogite, in the Yukahe area, the western part of the North Qaidam UHP metamorphic belt. Most zircon grains show typical metamorphic origin with residual cores in some irregular grains and sector, planar or misty internal textures on the cathodoluminescence (CL) images. The contents of REE and HREE of the core parts of grains range from 173 to 1680 μg/g and 170 to 1634 μg/g, respectively, in phengite eclogite, and from 37 to 2640 μg/g and 25.7 to 1824 μg/g, respectively, in massive eclogite. The core parts exhibit HREE-enriched patterns, representing the residual zircons of protolith of the Yukahe eclogite. The contents of REE and HREE of the rim parts and the grains free of residual cores are much lower than those for the core parts. They vary from 13.1 to 89.5 μg/g and 12.5 to 85.7 μg/g, respectively, in phengite eclogite, and from 9.92 to 45.8 μg/g and 9.18 to 43.8 μg/g, respectively, in massive eclogite. Negative Eu anomalies and Th/U ratios decrease from core to rim. Positive Eu anomalies are shown in some grains. These indicate that the presence of garnet and the absence of plagioclase in the peak metamorphic mineral assemblage, and the zircons formed under eclogite facies conditions. LA-ICP-MS zircon U-Pb age data indicate that phengite eclogite and massive eclogite have similar metamorphic age of 436±3Ma and 431±4Ma in the early Paleozoic and magmatic protolith age of 783–793 Ma and 748–759 Ma in the Neo-proterozoic. The weighted mean age of the metamorphic ages (434±2 Ma) may represent the UHP metamorphic age of the Yukahe eclogites. The metamorphic age is well consistent with their direct country rocks of gneisses (431±3 Ma and 432±19 Ma) and coesite-bearing pelitic schist in the Yematan UHP eclogite section (423–440 Ma). These age data together with field observation and lithology, allow us to conclude that the Yukahe eclogites were Neo-proterozoic igneous rocks and may have experienced subduction and UHP metamorphism with continental crust at deep mantle during the early Paleozoic, therefore the metamorphic age of 434±2 Ma of the Yukahe eclogites probably represents the continental deep subduction time in this area.

Comment on “Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from 40Ar/ 39Ar dating by stepwise crushing” by Qiu and Wijbrans (2006)

Qiu and Wijbrans [Qiu H.-N. and Wijbrans J. R. (2006) Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/ 39Ar dating by stepwise crushing. Geochim. Cosmochim. Acta 70, 2354–2370] present three Ar–Ar age spectra for fluid inclusions in garnet from eclogite at Bixiling in the Dabie orogen, east-central China. These Paleozoic ages of 427 ± 20 to 444 ± 10 Ma are interpreted to represent the first formation of Dabie ultrahigh-pressure (UHP) eclogite and thus require subduction of Yangtze crust to have started much earlier than previously accepted. However, no petrographic evidence, such as mineral inclusions in the garnet relating to the particular metamorphic conditions, is presented to substantiate the proposed UHP metamorphic event. Because garnet growth is not uniquely responsible for UHP eclogite-facies metamorphism, a distinction between UHP and high-pressure (HP) metamorphic events must be made in the interpretation of geochronological results. Available data from mineral Sm–Nd and zircon U–Pb dating of eclogites from the same area have firmly established that the UHP eclogite-facies metamorphism took place at Triassic. Neither the age of UHP metamorphism nor the timing of continental collision is reliably constrained by their presented data; the fluid inclusions in garnet must contain inherited 40Ar from UHP eclogite precursor, without considerable resetting of the Ar–Ar isotopic system during Triassic UHP metamorphism. Therefore, their data are either meaningless, or at best viewed as the age of garnet growth by low-T/HP blueschist/eclogite-facies metamorphism of the UHP eclogite precursor during arc-continent collision in the early Paleozoic. Furthermore, it is critical for metamorphic geochronology to substantiate the timing of UHP metamorphic event by means of zircon U–Pb in situ dating on coesite-bearing domains of metamorphically grown zircon.

Pressure–temperature–deformation paths of closely associated ultra-high-pressure (diamond-bearing) crustal and mantle rocks of the Kimi complex: implications for the tectonic history of the Rhodope Mountains, northern Greece

The ultra-high-pressure (UHP) Kimi complex (uppermost eastern Rhodope Mountains) is a tectonic mixture of crustal and mantle derived associations. Pressure–temperature (P–T) paths and microtextural and geochronological data reveal that crustal and mantle parts juxtaposed against each other at a depth corresponding to ~15 kbar (1 kbar = 100 MPa) had separate ascend histories. The crustal rocks comprise amphibolitised eclogites, orthogneisses, marbles, and migmatitic pelitic gneisses. The latter document UHP metamorphism within the dehydration-melting range of pelitic gneisses, with maximum P–T conditions of &gt;45 kbar at ~1000 °C, as determined by diamond inclusions in garnet and rutile needle exsolutions in Na-bearing garnet. Decompression was combined with only little cooling before 15 kbar, followed by more significant cooling between 15 and 10 kbar. This P–T path probably reflects ascent of UHP rocks within a subduction channel, followed by accretion in the lower crust of a thickened wedge. Although the first ascend phase was probably rapid, the overall time span for UHP metamorphism and final exhumation may have extended over more than 70 Ma. A U–Pb sensitive high-resolution ion microprobe (SHRIMP) age on zircons of ±149 Ma was suggested to date the UHP metamorphism, whereas Rb–Sr white mica and U–Pb zircon ages from syn-shearing pegmatites of ±65 Ma constrain medium- to low-grade shearing and final exhumation of UHP rocks. Mantle parts consisting of spinel–garnet metaperidotites and garnet pyroxenites reached maximum P–T conditions in the garnet-peridotite field at T &gt; 1200 °C and P &gt; 25 kbar. This was associated with plastic flow and followed by severe near isothermal cooling to T &lt; 800 °C at 15 kbar and static annealing. A garnet–clinopyroxene whole-rock Sm–Nd age from a garnet pyroxenite of ±119 Ma probably reflects the age of metamorphic mantle processes (static annealing following the high P/high T strain episode), rather than constraining the age of UHP metamorphism.

Isotopic constraints on age and duration of fluid‐assisted high‐pressure eclogite‐facies recrystallization during exhumation of deeply subducted continental crust in the Sulu orogen

AbstractFluid availability during high‐grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid‐absent metamorphism commonly resulting in neither zircon growth/recrystallization for U‐Pb dating nor Sm‐Nd isotopic resetting for isochron dating. While peak ultra‐high pressure (UHP) metamorphism is characterized by fluid immobility, high‐pressure (HP) eclogite‐facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm‐Nd and Rb‐Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U‐Pb method for coesite‐bearing domains of zircon. O isotope reequilibration was achieved between the Sm‐Nd and Rb‐Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite‐facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U‐Pb, Sm‐Nd, Rb‐Sr and O isotope systems in the eclogite minerals demonstrates that time‐scale for the HP eclogite‐facies recrystallization is c. 1.9–9.3 Myr or less. This provides a maximum estimate for duration of the fluid‐facilitated process in the HP eclogite‐facies regime during the exhumation of deeply subducted continental crust.

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.

U-Pb SHRIMP ages recorded in the coesite-bearing zircon domains of paragneisses in the southwestern Sulu terrane, eastern China: New interpretation

Laser Raman spectroscopy and cathodoluminescence (CL) images reveal that abundant and complicated mineral inclusions occur in zoned zircon separates from paragneisses in drill holes CCSDPP2, ZK-2304, and CCSD-MH, at Donghai, southwestern Sulu terrane, China. Many low-P mineral inclusions are preserved in inherited zircon cores. Coesite + quartz within a single inclusion, and some low-P minerals (e.g., quartz and albite) are identified in zircon mantles. Textures showing the quartz to coesite transformation in zircon indicate that both core and mantle of the zircon are detrital. Inclusions of coesite, together with garnet, jadeite, and phengite, are common in zircon rims formed during ultrahigh-pressure (UHP) metamorphism. SHRIMP U-Pb analyses of these zoned zircon grains identify three discrete and meaningful age groups. Proterozoic ages of 840 to 658 Ma for zircon cores indicate that the detrital zircons have a variety of sources. An early Paleozoic group of 505 to 455 Ma ages for zircon mantles constrain neither the timing of a discrete UHP metamorphic event, nor the transformation age from quartz to coesite. The 227 ± 9 Ma (the weighted mean age) obtained from coesite-bearing zircon rims represents the UHP metamorphic age. Thus, one Late Triassic UHP metamorphic event, rather than two UHP metamorphic events, is well established in Sulu UHP terrane by the method of U-Pb SHRIMP dating for coesite-bearing zircon rims in garnet-biotite-amphibole-albite gneisses.

The oldest UHP eclogites of the World: age of UHP metamorphism, nature of protoliths and tectonic implications

Coesite-bearing eclogitic rocks from the Pan-African belt in northern Mali were dated by multichronological methods (Rb–Sr, Sm–Nd and Ar–Ar). Rb–Sr and Sm–Nd isotope analyses on whole rock and mineral separates of an omphacite–kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 Ma. This is interpreted as the time of the eclogitisation, which represents the oldest ultrahigh-pressure (UHP) metamorphic event so far recorded in the continental crust. The nearly identical Rb–Sr and Sm–Nd ages suggest very rapid exhumation as observed in many Phanerozoic UHP rocks. 40Ar– 39Ar dating of phengites from two UHP micaschists gave much older ages of 1045 and 760 Ma. Phengite from a quartzite collected from the same site yielded a plateau date of 623±3 Ma, which is in agreement with the Rb–Sr and Sm–Nd dates. The two 40Ar– 39Ar ages, older than 625 Ma, testify once more the recurrent problem of excess Ar in ultrahigh-pressure phengites. The occurrence of the Mali UHP eclogites indicates that by the late Precambrian, the Earth has cooled enough to sustain the formation and preservation of deeply subducted UHP metamorphic rocks. The new ages of ca. 620 Ma also constrain the amalgamation in NW Gondwana to have occurred in the latest Proterozoic, but not in the Middle Cambrian. This is supported by several recent chronological data for the southern segment of the major suture zone in the Dahomeyide orogenic belt.

35 Ma old ultrahigh-pressure metamorphism and evidence for very rapid exhumation in the Dora Maira Massif, Western Alps

Ion microprobe (SHRIMP) data on zircons from various rock types of an ultrahigh-pressure (UHP) metamorphic whiteschist-type pyrope quartzite lens of the Dora Maira Massif (DMM) consistently show domains giving a Late Eocene age of 35.4 ± 1.0 Ma which is taken as the age of UHP metamorphism. These domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 Ma. Zircons of a metagranitic country rock next to the UHP metamorphic lens have these primary features best preserved. All zircons measured also yield intermediate ‘ages’ between 275 and 35 Ma with a statistical concentration between 260 and 210 Ma. Thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith, that is a granite intruded during the Late Herynian. While the intermediate ages are at least partly due to incomplete resetting of the zircons during UHP metamorphism, those in the 260–210 Ma range may be related to rifting episodes in the Permotriassic. The Mg-rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite, perhaps by fluids derived from evaporitic sediments dating as early as the Permotriassic as well. The more pervasive resetting of zircon ages during UHP metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and/or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous Mg-rich protoliths and not in their drier granitic neighborhood. Fission track ages determined partly on the same zircon samples yielding 29.9 ± 1.4 Ma mark the time when the UHPM unit had reached about 290°C at a shallow location within the crust. Thus exhumation over a vertical distance of about 120 km must have occurred within about 5–6 Ma indicating an average uplift rate of about 20–24 km/Ma and an average cooling rate of about 85–100°C/Ma. The radiometric data obtained do not lend any support to an Eo-Alpine Cretaceous subduction event so that deep subduction and immediately following exhumation all took place during Early to Mid Tertiary time. This scenario seems to apply to large portions of the Western and Central Alps as well calling for drastic geodynamic reinterpretations of these parts of the Alps.