Amphibolite-facies Retrograde Metamorphism Research Articles

EVIDENCE OF EVAPORITES IN THE GENESIS OF THE VANADIAN GROSSULAR 'TSAVORITE' DEPOSIT IN NAMALULU, TANZANIA

The Namalulu tsavorite (vanadian grossular up to 0.80 wt.% V 2 O 3 ) deposit in northeastern Tanzania occurs in metasomatized graphitic gneiss overlain by a dolomitic marble unit. These rocks belong to the Neoproterozoic metamorphic Mozambique Belt. The graphitic gneiss is composed of quartz, V-bearing kyanite (up to 0.4 wt.% V 2 O 3 ), V-bearing muscovite (up to 1.0 wt.% V 2 O 3 ), V-bearing rutile (up to 1.9 wt.% V 2 O 3 ), and graphite. This rock is affected by calcic metasomatism precipitating calcite and mobilizing the elements for tsavorite crystallization in quartz-calcite veins. The dolomitic marble unit hosts an anhydrite-gypsum-dolomite lens (lens I) and a calcite-scapolite-diopside-sulfides-graphite lens (lens II). This last unit is characterized by the presence of F-bearing minerals (tremolite, phlogopite, tainiolite, titanite), with up to 9.4 wt.% F for tainiolite, and Ba-bearing minerals (feldspar, phlogopite), with up to 7.5 wt.% BaO for feldspar. Lithium (up to 2.0 wt.% Li 2 O) and boron (up to 110 ppm) are also present in tainiolite, as well as in F-bearing tremolite. This is the first description of metamorphic tainiolite. The protoliths of the Namalulu rocks correspond to organic-rich black shales, pure and magnesian carbonates, and evaporites. They were presumably deposited in a marine coastal sabkha located at the eastern margin of the Congo-Kalahari cratons bounded by the Mozambique Ocean during early Neoproterozoic. Tsavorite formed in the Ca-metasomatized gneiss during amphibolite facies retrograde metamorphism at 5.6–6.7 kbar and 630 ± 30 °C.

A polyphase metamorphic evolution for the Xitieshan paragneiss of the north Qaidam UHP metamorphic belt, western China: In-situ EMP monazite- and U–Pb zircon SHRIMP dating

In-situ electron microprobe (EMP) U–Th–Pb monazite-, sensitive high-resolution ion microprobe (SHRIMP) zircon analyses, metamorphic phase equilibrium (Domino/Theriak)- and geothermobarometric calculations are performed on kyanite/sillimanite-bearing garnet biotite gneisses forming part of the dominant rock association in the Xitieshan ultra-high pressure metamorphic belt, north Qaidam, western China. Results are consistent with the following complex polyphase tectono-metamorphic evolution.The kyanite/sillimanite bearing garnet biotite gneisses contain monazite ages of 938±23Ma and zircon SHRIMP ages of 945±7Ma, referring to a Neoproterozoic metamorphism, i.e. similar to the age of the Jinning orogeny in the Yangtze block of southern China. This correlation suggests that the paragneiss has affinities with the Yangtze block (South China block). The Neoproterozoic monazites were found inside coarse grained porphyroblastic garnets containing amphibolite facies mineral inclusion assemblages.The kyanite/sillimanite-bearing garnet biotite gneisses also contain early Paleozoic monazite ages of 422–425Ma and 455–460Ma, detected in amphibolite facies mineral assemblages associated with matrix minerals. Using phase equilibrium- and geothermobarometric calculations, PT conditions of 560–610°C/5.8–7.0kbar and 610–675°C/4.6–6.5kbar were calculated respectively for both amphibolite facies assemblages. The early Paleozoic ages of 422–425Ma and 455–460Ma were detected in 8 monazite grains from the investigated paragneiss samples. Based on the Y and Eu contents variation of the early Paleozoic monazite domains (measured by EMP), the 422–425Ma monazite ages are interpreted to have formed during an amphibolite facies tectono-metamorphic overprint that post-dates (U)HP metamorphism and can thus be related to exhumation of previously deeply subducted rocks. Alternatively, the 455–460Ma monazite ages are interpreted to represent the age of the prograde subduction zone metamorphism.We conclude therefore that the mineral assemblage of the kyanite/sillimanite-bearing garnet–biotite gneiss (and associated retro-eclogite) was formed during an early Paleozoic subduction/collision event, which involved late Proterozoic (938–945Ma) crystalline basement inliers of minimal mid amphibolite facies grade. Early Paleozoic deep subduction towards (U)HP depths occurred around 455–460Ma followed by retrograde amphibolite facies metamorphism at 422–425Ma during exhumation.

Exact timing of granulite metamorphism in the Namche-Barwa, eastern Himalayan syntaxis: new constrains from SIMS U–Pb zircon age

Zircon grains separated from 2 granulites from the eastern Himalaya were investigated by Raman spectroscopy, cathodoluminescence imaging, and secondary ion mass spectrometry. These grains have a thin homogeneous rim and an oscillatory inner zone domain with or without a relict inherited core. Garnet, kyanite, and rutile inclusions were identified within only the rim domain of zircon grains, indicating that the rim had formed during peak granulite-facies metamorphism. U–Pb zircon data record three distinct age populations: 1,805 Ma (for the inherited core), ca. 500 Ma (oscillatory inner zone), as well as 24–25 Ma and ca. 18 Ma (for the metamorphic rim). These new precision ages suggest that the peak metamorphic age for the HP granulite is at ca. 24–25 Ma, and subsequent amphibolite-facies retrograde metamorphism occurred at ca. 18 Ma.

Amphibolite facies retrograde metamorphism of the Zhujiachong eclogite, SE Dabieshan: 40Ar/39Ar age constraints from argon extraction using UV‐laser microprobe, in vacuo crushing and stepwise heating

AbstractThe Zhujiachong eclogite in the south‐eastern Dabieshan ultra‐high‐P terrane has been overprinted during retrograde metamorphism, with the development of garnet‐amphibolite mineral assemblages in most rocks in the outcrop. This study is focused on providing age constraints for the retrograde amphibolite facies and greenschist facies mineralogy by 40Ar/39Ar dating. By applying a novel approach of combining three different techniques for extracting argon: laser stepwise heating of single grains and small separates, a spot fusion technique by UV‐laser ablation microprobe on polished sections and an in vacuo crushing technique for liberating radiogenic argon from fluid inclusions, it is demonstrated that an internally consistent thermal history can be derived. The 40Ar/39Ar ages indicate that phengite formed before 265 Ma, probably during the ultra‐high‐P event. Ages associated with amphibolite facies retrograde metamorphism range from 242 to 217 Ma by the analyses of amphibole. Ages of c. 230 Ma were found for the symplectite matrix that formed during retrogression from eclogite pyroxene. Late stage hydrothermal activity leading to the formation of coarse‐grained paragonite and fluid inclusions in vein amphibole was dated at c. 200 Ma. These age results agree well with the mineral crystallization sequence observed from thin‐sections of the retrograded eclogite: phengite → paragonite and amphibole in matrix → amphibole in the corona.

Constraining ultrahigh-pressure (UHP) metamorphism and titanium ore formation from an infrared microthermometric study of fluid inclusions in rutile from Donghai UHP eclogites, eastern China

Eclogites in the Donghai area of the Sulu ultrahigh-pressure (UHP) metamorphic belt in eastern China contain both disseminated rutile and rutile deposits. The rutile occurs as small inclusions in garnet, omphacite, phengite, and quartz, as larger euhedral to subhedral grains in equilibrium with garnet, omphacite, phengite, and kyanite, and as discrete veinlets, aggregates, and megacrysts in cracks and along grain boundaries. To investigate the processes of rutile mineralization and its relationship to the evolution of the Sulu UHP metamorphic belt, we sampled the exposed Maobei deposit at Donghai and drill core between 100 and 2000 m in the nearby Chinese Continental Scientific Drilling Project (CCSD) main hole to carry out a detailed infrared microthermometric fluid inclusion study. Three types of fluid inclusions were identified: aqueous (Type I), CO 2 -H 2 O (Type II), and CH 4 (Type III). Type I and Type II inclusions are the most common. Primary and pseudosecondary Type I inclusions, which were trapped during recrystallization of the eclogite or early amphibolite facies retrograde metamorphism, have a narrow range of homogenization temperatures but a wide range of salinities. In contrast, secondary Type I inclusions, which formed entirely during retrograde amphibolite-facies metamorphism associated with exhumation of the UHP rocks, have higher and more varied homogenization temperatures but medium to low salinities. Type II inclusions formed contemporaneously with the Type I variety and are characterized by having small amounts of CO 2 and N 2 in addition to H 2 O. In addition, some of the rutile grains contain sparse inclusions composed of CH 4 (Type III). Our results combined with those of previous studies, suggest a great diversity of fluid composition in the peak metamorphic to early retrograde stages, implying very limited fluid-rock interaction during syn- to post-peak high-pressure (HP) and UHP metamorphism. This study shows that infrared microthermometric investigation of fluid inclusions can provide information not only on the composition of titanium ore-forming fluids but also on the relationship between mineralization and the evolution of UHP metamorphic belt.

Petrology of eclogites from north of Shahrekord, Sanandaj-Sirjan Zone, Iran

Metabasic rocks were recently found within a ductile shear zone in the north of Shahrekord, being a part of the structural zone of Sanandaj-Sirjan, SW Iran. The rocks give evidence of a so far unrecognized eclogite facies metamorphic event and testify to high pressure metamorphism in the Sanandaj-Sirjan Zone, near the Main Zagros Reverse Fault, which is the assumed suture zone between the Arabian plate and the Iranian block. The eclogites occur as lenses or blocks within ortho- and paragneisses. The petrographic features and reaction textures display at least two main metamorphic stages: (1) a peak eclogite facies stage, and (2) a subsequent amphibolite facies stage. The eclogite facies metamorphism is indicated by omphacite + garnet + sodic-calcic amphiboles (barroisite, magnesiokatophorite and magnesiotaramite) + phengite + rutile + (clino-)zoisite + quartz ± dolomite. The garnets are mainly almandine-rich, which fits with the C-type eclogite classification. Calcic amphiboles (hornblende, tschermakite and pargasite) + plagioclase are secondary phases formed during the retrograde amphibolite-facies metamorphism. P-T estimates for the eclogite facies give pressures of 21–24 kbar and temperatures of 590–630 °C (geothermometry) and 470–520 °C (THERMOCALC), respectively. Geothermobarometry for the amphibolite-facies metamorphism yields 10–11 kbar and 650–700 °C.

Ultrahigh-Pressure Mineral Assemblages in Zircons from the Surface to 5158 m Depth in Cores of the Main Drill Hole, Chinese Continental Scientific Drilling Project, Southwestern Sulu Belt, China

The 5158 m deep main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) at Maobei, southwestern Sulu Belt, penetrated five major lithologic units: eclogite, amphibolite ± retrograde eclogite, ultramafic rock, paragneiss, and orthogneiss. All analyzed samples are overprinted to various extents by amphibolite-facies retrograde metamorphism. Ultrahigh-pressure (UHP) index minerals are preserved as inclusions in zircons separated from 137 core samples. These index assemblages include: Coe + Phe + Grt from Grt-Ep-Bt paragneiss; Coe + Jd + Grt + Ap, Coe + Jd + Phe + Ap and Coe + Grt + Jd + Phe from Grt-Ep-Hbl paragneiss; Coe + Phe, Coe + Ky + Ap, and Coe + Ky + Ttn from orthogneiss; and Coe + Grt + Omp, Coe + Grt + Phe, Coe + Omp + Rt, and Coe + Grt + Omp + Phe from Bt-Ep amphibolite and other retrograde eclogites. The common occurrences of these UHP assemblages throughout the section indicate that the drill hole did not penetrate through the UHP tectonic slice. All of the UHP mineral assemblages, whether from retrograde ecologites or from the surrounding gneisses, record similar metamorphic conditions of 773-843°C and 3.2-4.0 GPa. Zircons separated from surface samples and cores from other shallow holes in the area also contain similar coesite-bearing UHP mineral inclusions. This widespread occurrence of UHP assemblages in the southwestern Sulu area suggests that voluminous continental materials were subducted and metamorphosed at mantle depths > 100 km, and then were rapidly exhumed to crustal levels.

Zoned Zircon from Eclogite Lenses in Marbles from the Dabie‐Sulu UHP Terrane, China: A Clear Record of Ultra‐deep Subduction and Fast Exhumation

Abstract: Eclogite lenses in marbles from the Dabie‐Sulu ultrahigh‐pressure (UHP) terrane are deeply subducted meta‐sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark‐luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high‐pressure (HP) metamorphic conditions of T = 571–668°C and P = 1.7‐2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white‐luminescent images. It contains coesite eclogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782–849°C and P > 5.5 GPa. Domain 3, with gray‐luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low‐pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600–710°C and P = 0.7‐1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245 ± 4 Ma for domain 1, 235 ± 3 Ma for domain 2 and 215 ± 6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244 ± 4 Ma, 233 ± 4 Ma and 214 ± 5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10–11 Ma and exhumation of the rocks occurred over a period of 19–20 Ma. Thus, subduction from ∼ 55 km to > 160 km deep mantle depth took place at rates of approximately 9.5–10.5 km/Ma and exhumation from depths >160 km to the base of the crust at −30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin Iithosphere followed by ultrafast exhumation driven by buoyancy forces after break‐off of the UHP slab deep within the mantle.

Oxygen isotope geochemistry of ultrahigh-pressure metamorphic rocks from 200–4000 m core samples of the Chinese Continental Scientific Drilling

Oxygen isotope studies were carried out for ultrahigh-pressure metamorphic rocks from the main hole of the Chinese Continental Scientific Drilling in the Sulu orogen. The samples of interest include various types of lithology (mainly eclogite and gneiss) in a depth of 200 to 4000 m, containing five continuous core segments between contrasting lithologies. The results show a large variation in δ 18O value from − 10.41 to 9.63‰ for constituent minerals. Distinct 18O depletions are observed in the frequently alternated layers of eclogite and gneiss, in which the variations of δ 18O values are gradual, regardless of lithologies. Both equilibrium and disequilibrium O isotope fractionations are observed between quartz and the other minerals. Special attention was paid to the relationship between distance, petrography and δ 18O value of adjacent samples. The results show O isotope heterogeneities between the different and same lithologies on scales of about 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental collision. Amphibolite-facies retrograde metamorphism during exhumation caused mineral reactions and O isotope disequilibria between some of the minerals. Considerable changes occur in δ 18O and petrography at the contact between eclogite and gneiss, suggesting that the contact between different lithologies is the most favorable place for fluid activity. Despite the widespread retrogression, retrograde fluid was internally buffered in the stable isotope compositions. The retrograde fluid is of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl. Although locally external fluids became available along fault zones and lithological layers, it still acted within the exhumed slabs with internal origins. Fluid flow also took place after the amphibolite-facies retrogression, but it only affects the O isotope compositions of feldspar and mica. Premetamorphic protoliths are deduced to have heterogeneous δ 18O values due to varying degrees of meteoric water–rock interaction before continental collision. Minimum depth of the 18O depletion is up to 3300 m. Together with areal 18O depletion in surface outcropped rocks along the Dabie–Sulu orogenic belt, at least 66,000 km 3 of supracrustal rocks were interacted with meteoric water along the northern margin of the South China Block during the Neoproterozoic.

Identification of UHP and Non-UHP Orthogneisses in the Sulu UHP Terrane, Eastern China: Evidence from SHRIMP U-Pb Dating of Mineral Inclusion-Bearing Zircons

Mineral inclusions in zoned zircons reveal that both ultrahigh-pressure (UHP) and non-UHP orthogneisses occur in cores from drill hole CCSD-PP1 at Donghai, southwestern Sulu terrane. Zircons from the UHP orthogneiss have inherited magmatic cores containing low-P mineral inclusions, narrow mantles with UHP mineral inclusions, and rims that contain low-P minerals or are inclusion free. In contrast, zircons from non-UHP orthogneiss have only magmatic cores and metamorphic rims, both of which are characterized by low-P mineral inclusions, or are inclusion free. SHRIMP U-Pb analyses of zircons from the UHP orthogneiss identify three discrete and meaningful age groups: Proterozoic magmatic protolith ages >753 Ma in the cores, a UHP metamorphic event at 227 ± 8 Ma in the coesite-bearing mantles, and an amphibolite-facies overprint at 213 ± 7 Ma in the overgrowth rims. Zircons from non-UHP orthogneiss record only two discrete age groups, inherited magmatic cores with Proterozoic protolith ages >748 Ma, and metamorphic rims with an age of about 211 ± 6 Ma. The metamorphic rims have the same Triassic age as the late amphibolite-facies overprint of the UHP orthogneiss within analytical uncertainty. These data, together with previous studies, suggest that Neoproterozoic supracrustal protoliths of the Yangtze craton experienced Late Triassic (227 ± 8 Ma) subduction to depths of at least 120 km, and were then rapidly exhumed to mid-crustal depths where both UHP rocks and a few non-UHP Neoproterozic granitic intrusives were subjected to coeval amphibolite-facies retrograde metamorphism at 211 ± 6 Ma. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P-T data is estimated to be 6.3-7.1 km/m.y. This rapid exhumation suggests that the Sulu UHP metamorphic rocks returned to crustal depths as part of a buoyant sliver, possibly produced by slab breakoff.

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P– T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites. Carbonic fluid (CO 2-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH 4 and/or N 2 as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N 2, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO 2 and mixed H 2O–CO 2(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO 2±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO 2, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.

Granulite-facies metamorphism in the Punggi area, northeastern Yeongnam Massif, Korea and its tectonic implications for east Asia

Abstract In the Punggi area of the northeastern part of the Yeongnam Massif, one of Precambrian basement terrains in Korean Peninsular, the metamorphic grade increases from the amphibolite- to the granulite-facies zones towards the southeast. The boundary of the granulite-facies zone is defined by the orthopyroxene (Opx)-In reaction in metabasites. As metamorphic grade increases across the Opx-In isograd, the mineral assemblage in calcic metabasites is changed from clinopyroxene-amphibole-plagioclase to orthopyroxene-clinopyroxene-amphibole-plagioclase, and that in alkali metabasites from garnet-orthoamphibole-plagioclase to garnet-orthopyroxene-plagioclase. The metamorphic rocks in the granulite-facies zone were overprinted by upper amphibolite-facies retrograde metamorphism. During retrograde metamorphism, cordierite and spinel formed in the migmatitic gneiss. The P–T conditions of granulite-facies metamorphism are 767–815 °C and 6.0–8.4 kb and the P–T conditions for the retrograde stage are 595–707 °C/4.7–6.4 kb. A U–Pb zircon age of 1904±9 Ma has been obtained from a garnet granitic gneiss and is inferred to date the emplacement of its precursor. This is consistent with the intrusion age of precursors to most orthogneisses in other parts of the Yeongnam Massif. The same sample has yielded a Sm–Nd whole rock-garnet age of 617±38 Ma, which is inferred to represent a late Proterozoic metamorphic event of upper amphibolite facies.

Evolution of the archean/proterozoic crust in the southern São Francisco craton near Perdões, Minas Gerais, Brazil: petrological and geochemical constraints

Detailed geochemical and petrographic analyses of a lower crustal bimodal sequence near Perdões/Minas Gerais contribute to the reconstruction of the Archean–Proterozoic evolution of the southern São Francisco craton. Trace element patterns of enderbites with tonalitic/trondhjemitic compositions (felsic members) indicate an origin by a two-phase partial melting process from a mantle source at depths, where garnet exists as a residual phase. Gabbroic rocks and mafic–ultramafic cumulates (mafic members) occur as lens-shaped elongated bodies exposed in the felsic complex. The gabbroic rocks are tholeiitic in composition, with rare earth element (REE) patterns indicating generation through relatively high percentages of partial melting from mantle material. Trace element ratios indicate a slightly depleted mantle source for the gabbroic rocks. Charnockites, which occur in the same area, are granitic/granodioritic in composition. The REE patterns indicate magma generation by intracrustal partial melting of tonalitic/trondhjemitic material, which must have occurred at depths of less than 40 km, where plagioclase is a stable residual phase. The whole-rock sequence shows a uniform PT path. The rocks were generated in granulite facies conditions. During exhumation, they underwent a strong retrograde amphibolite facies metamorphism, followed by a weak greenschist facies overprint. The granitic rocks of Perdões represent a rock association that is characteristic of the transition between Archean and Proterozoic times, during which the continental crust evolved from Archean tonalitetrondhjemitegranodiorite (TTG) suites towards an increased lithologic diversity in PostArchean time, with greater amounts of more evolved and more potassium-rich granitoids (granodiorite/granite/monzogranite (GGM) suites).

Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project

The pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quartzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe+Omp+Grt+Phe, Coe+Jd+Phe+Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814–852 °C and pressures of ≥28 kbar, presenting the P– T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OG1) and non-UHP (OG2). In OG1 orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz+Phe+Ab+Ksp+Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengite-bearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz+Phe+Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event.

Exhumation of UHP Terranes: A Case Study from the Dabie Mountains, Eastern China

Two processes are suggested to explain how UHP rocks are exhumed from mantle depths. One is removal of the overburden either by erosion or by extension, whereas the other involves the uplifting of the UHP rocks through the overburden. Application of either of these mechanisms to the Dabie Mountains, however, is fraught with difficulty. When combined with previously published data, new studies on metamorphic P-T paths, regional structures, and deep upper-mantle architecture revealed by seismic tomography lend support to a multi-stage exhumation process that operated in the Dabie Mountains. The first stage (230 to 200 Ma) is characterized by ductile deformation, produced during eclogite-facies recrystallization under a geothermal gradient as low as 10°C/km, implying a synsubduction exhumation. Some of the UHP rocks evidently were exhumed to a depth of ˜60 km, as indicated by petrological study of the Shuanghe eclogite. The second stage (200 to 170 Ma) attended ductile deformation and amphibolite-facies retrograde metamorphism. Subduction of the Yangtze block was halted by slab breakoff at a depth of ∼200 km. The resultant geothermal gradient recovered to ∼20″C/km. Slab breakoff permitted buoyancy-driven ascent of the UHP low-density melange to shallow crustal levels in a diapir structure. When the UHP portion of the mountain root rose, the shallow portion was heated to a temperature higher than that of the peak metamorphic pressure. The third stage (170 to 120 Ma) is characterized by extension and thermal uplift, as well as erosion. Sedimentary basins and volcanic rocks developed on both sides of the Dabie Mountains. Gab-bro-pyroxenite intruded the hanging wall of the UHP terrane, and granite, as well as migmatite, developed in that stage. Exhumation mechanisms might include corner flow for the first stage, buoyancy-driven squeezing-up for the second stage, and crustal extension, as well as erosion, for the third. Rupture and loss of the subducted lithospheric plate generated the gravity instability that resulted in exhumation of the subducted UHP section.

Deposition and imbrication of a 2670-2629 Ma supracrustal sequence in the Indin Lake area, southwestern Slave Province, Canada

New U-Pb age data from the southwestern Slave Province demonstrate that units of the Indin Lake supracrustal belt form an imbricated structural stack. The oldest rocks of the belt are undated mafic volcanic flows of the Hewitt Lake group that are crosscut by a 2670 Ma felsic sill, itself coeval with mafic through felsic volcanic rocks of the 2668-2671 Ma Leta Arm group. The youngest rocks of the belt are 2647-2629 Ma turbidites and felsic volcanic rocks of the unconformably overlying Chalco Lake group. Tonalite orthogneiss of the adjacent Cotterill gneiss complex is 2680 Ma, suggesting that it does not represent in situ basement to the supracrustal belt. Intercalation of the older Hewitt Lake and Leta Arm groups with the younger Chalco Lake group is interpreted to result from D1 imbrication and folding between 2629 and 2609 Ma, the age of a crosscutting tonalite intrusion. Subsequent D2 folding and regional low-pressure metamorphism occurred between 2609 Ma and ca. 2590 Ma. D3 normal faulting between the belt and Cotterill gneisses, ca. 2590 Ma, is interpreted to overlap with retrograde amphibolite-facies metamorphism and decompression of the gneiss complex. Comparisons between the tectonic history of the Indin Lake area and the central Slave Province show that turbidite deposition was regionally diachronous and overlapped with regional deformation elsewhere, supporting existing models favouring some form of accretionary orogenesis. The imbricated and intercalated 2670-2629 Ma supracrustal sequence may characterize a distinct crustal block in the southwestern Slave Province.

Dating synmagmatic folds: a case study of Schlingen structures in the Strona‐Ceneri Zone (Southern Alps, northern Italy)

The Strona‐Ceneri Zone (Southern Alps) contains folds with moderately to steeply inclined axial planes and fold axes, and amplitudes of up to several kilometres (so‐called ‘Schlingen’). These amphibolite facies folds deform the main schistosity of Late Ordovician metagranitoids and are discordantly overlain by unmetamorphic Permian sedimentary rocks. Mutually cross‐cutting relationships between these folds and garnet‐bearing leucotonalitic dykes indicate that these dykes were emplaced during folding. Sm–Nd systematics and the strongly peraluminous composition of these dykes point to an anatectic origin. Pb step leaching of magmatic garnet from a leucotonalitic dyke yielded a 321.3±2.3 Ma intrusive age. Rb–Sr ages on muscovites from leucotonalitic dykes range from 307 to 298 Ma, interpreted as cooling ages during retrograde amphibolite facies metamorphism. Conventional U–Pb data of zircons from an older granodioritic dyke that pre‐dates the Schlingen folds yielded discordant U–Pb ages ranging from 371 to 294 Ma. These ages reflect a more complicated multi‐episodic growth history which is consistent with the observed polyphase structural overprint of this dyke. Schlingen folding was accompanied by prograde amphibolite facies metamorphism, during the thermal peak of which the leucotonalitic dyke material was generated by partial melting in a deeper source region from where these S‐type magmas intruded the presently exposed level. Because partial melting may occur in a relatively late stage of a clockwise P–T–t path, or even during decompression on the retrograde path, we do not exclude the possibility that Schlingen folding had already started in Early Carboniferous time. Schlingen folds also occur in Penninic and Austroalpine basement units with a very similar pre‐Alpine history, indicating that Variscan folding affected large segments of the future Alpine realm.

Mafic granulites of the Schirmacher region, East Antarctica: fluid inclusion and geothermobarometric studies focusing on the Proterozoic evolution of the crust

AbstractIn the Proterozoic complex of the Schirmacher region of East Antarctica, a retrograde pressure–temperature (P–T) history has been inferred through quantitative geothermobarometry and fluid inclusion studies of the mafic granulites. Microthermometric investigations of the fluid phases trapped in quartz and garnet identified three types of inclusions, namely, earliest pure CO2 inclusions (0·987–1·057 g cm−3), CO2–H2O inclusions and aqueous inclusions.The temperature and pressure of metamorphism have been estimated through different calibrations of geothermometers and geobarometers. The mineral reactions and compositional zoning in the minerals record P–T conditions from nearly 837 ± 26°C, 7·1±0·2 kbar to 652 ± 33°C, 5·9 ± 0·3 kbar. A good correlation between the fluid and mineral data is observed. The isochores typical of highdensity CO2 fluids fall well within the P–T box estimated by mineral thermobarometry. The abundance of primary CO2 inclusions in early metamorphic minerals (notably quartz and primary garnet) and the general correspondence between fluid and mineral P–T data indicate a ‘fluid-present’ carbonic regime for the high-grade metamorpism; however, from the present data largescale CO2 advection could not be envisaged. The subsequent stages involved a decrease in CO2 density, a progressive influx of hydrous fluids and the generation of retrograde amphibolite facies metamorphism in the area.The estimated P–T conditions of the region suggest that the rocks were metamorphosed at a depth of 19–24 km, with a geothermal gradient of c. 3°5C km−1. The estimated P–T conditions of the rocks imply a clockwise P–T–t path with a gradual decrease in temperature of around 250°C and a decrease in pressure of around 1700 bar. They have a dP/dT gradient of ≈7 ± l bar °C−1, arguing for an isobaric cooling history of the terrane under normal thickened crust after the underplating of mantle-derived material.

Petrogenesis of a high‐temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China

ABSTRACT The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic‐ultramafic blocks, and abundant Jurassic‐Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high‐pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene‐bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near‐isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase‐amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite‐amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km‐1 at intermediate to low pressures, and may represent the Sino‐Korean hangingwall during Triassic subduction for formation of the ultrahigh‐ and high‐P units to the south. Post‐collisional intrusion of a mafic‐ultramafic cumulate complex occurred due to breakoff of the subducting slab.

Rare-earth element distribution in exhalites associated with Broken Hill-type mineralisation at the Pinnacles deposit, New South Wales, Australia

Rare-earth element (REE) concentrations in exhalites and pelites associated with strata-bound base-metal (Broken Hill-type) mineralisation, have been measured for the Early Proterozoic Pinnacles deposit in western New South Wales. Garnetites, garnet-rich rocks and sulphide-rich garnet-gahnite-bearing rocks exhibit a wide variety of REE concentrations, particularly LREE and Eu, which cannot be related entirely to changes in mineralogy, but show some correlation with stratigraphic position relative to the main orebody. Exhalites lying in the footwall to the mineralised rocks have negative Eu anomalies and steep, LREE-enriched patterns, whereas exhalites in the hanging wall to the ores and silicate-rich rocks from within the ore lenses have zero or positive Eu anomalies and less LREE-enriched patterns. HREE have more consistent values throughout the deposit, but in places are strongly depleted. Non-mineralised pelites are slightly enriched in the REE relative to shale composite values and have negative Eu anomalies. It is argued in this paper that, because the variations in data can be related to the stratigraphic location of samples relative to the orebodies, that the observed REE distributions are due to fluctuations in REE concentrations in the primary exhalative fluids rather than differential mobilisation of REE during a second, post-depositional episode of hydrothermal alteration, or mobilisation of the REE during prograde granulite-facies metamorphism or retrograde amphibolite-facies metamorphism which has affected the area. The lack of association between metamorphic mineral assemblages and REE distribution and the absence of any homogenisation of the REE concentrations, suggest that the data reflect the primary exhalative fluid composition and that the variations in REE data reflect changes in palaeoenvironmental conditions such as f O 2 and temperature. A corollary to this conclusion is that there was minimal mobilisation of the REE during subsequent metamorphism.