Timing of the granulite facies metamorphism in the Sanggan area, North China craton: zircon U-Pb geochronology

Zircon U-Pb ages are reported for three samples of intrusive rocks in Khondalite series in the Sanggan area, North China craton. The age of meta-granite is dated as 2005∓9 Ma, implying that the sedimentary sequences in Khondalites series formed before 2.0Ga. The age of 1921 ∓ 1Ma for the meta-diorite constrain the age of granulite facies metamorphism younger than this date. The age of 1892 ∓ 10 Ma for garnet granite is obtained, but the granite crystallization age seems a little younger than the date considering the morphology of zircons. On the basis of these dates and of a concise review of previous age data, it is inferred that the Khondalite series was subjected to granulite facies metamorphism at about 1.87Ga together with tonalitic granulites and HP basic granulites in the Sanggan area.     

U–Pb ages of zircons from metamorphic rocks in the upper sequence of the Hidaka Metamorphic Belt,Hokkaido, Japan: Identification of two metamorphic events and implications for regional tectonics

The Hidaka Metamorphic Belt is a well-known example of island-arc crustal section, in which metamorphic grade increases westwards from unmetamorphosed sediment up to granulite facies. It is divided into lower (granulite to amphibolite facies) and upper (amphibolite to greenschist facies) metamorphic sequences. The metamorphic age of the belt was considered to be ~55 Ma, based on Rb – Sr whole-rock isochron ages for granulites and related S-type tonalities. However, zircons from the granulites in the lower sequence yield U – Pb ages of ~21 – 19 Ma, and a preliminary report on zircons from pelitic gneiss in the upper sequence gives a U – Pb age of ~40 Ma. In this paper we provide new zircon U – Pb ages from two pelitic gneisses in the upper sequence to assess the metamorphic age and also the maximum depositional age of the sedimentary protolith. The weighted mean ²⁰⁶Pb/²³⁸U ages from a biotite gneiss in the central area of the belt yield 39.6 ± 0.9 Ma for newly grown metamorphic rims and 53.1 ± 0.9 Ma for the youngest detrital cores. The ages of zircons from a cordierite–biotite gneiss in the southern area are 35.9 ± 0.7 Ma for metamorphic rims and 46.5 ± 2.8 Ma for the youngest detrital cores. These results indicate that metamorphism of the upper sequence took place at ~40 – 36 Ma, and that the sedimentary protolith was deposited after ~53 – 47 Ma. These metamorphic ages are consistent with the reported ages of ~37–36 Ma plutonic rocks in the upper sequence, but contrast with the ~21–19 Ma ages of metamorphic and plutonic rocks in the lower sequence. Therefore, we conclude that the upper and lower metamorphic sequences developed independently but coupled with each other before ~19 Ma as a result of dextral reverse tectonic movement.     

A Pan African age for the HP-HT granulite gneisses of Zabargad island: implications for the early stages of the Red Sea rifting

Up to now the age of granulite gneisses intruded by the Zabargad mantle diapir has been an unsolved problem. These gneisses may represent either a part of the adjacent continental crust primarily differentiated during the Pan African orogeny, or new crust composed of Miocene clastic sediments deposited in a developing rift, crosscut by a diabase dike swarm and gabbroic intrusions, and finally metamorphosed and deformed by the mantle diapir. Previous geochronological results obtained on Zabargad island and Al Lith and Tihama-Asir complexes (Saudi Arabia) suggest an Early Miocene age of emplacement for the Zabargad mantle diapir during the early opening of the Red Sea rift. In contrast, SmNd and RbSr internal isochrons yield Pan African dates for felsic and basic granulites collected 500–600 m from the contact zone with the peridotites. Devoid of evidence for retrograde metamorphic, minerals from a felsic granulite provide well-defined RbSr and SmNd dates of 655 ± 8 and 699 ± 34 Ma for the HP-HT metamorphic event (10 kbar, 850°C). The thermal event related to the diapir emplacement is not recorded in the SmNd and RbSr systems of the studied gneisses; in contrast, the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the RbSr isotopic system of the mafic granulite. The initial¹⁴³Nd/¹⁴⁴Nd ratio of the felsic granulite is higher than the contemporaneous value for CHUR and is in agreement with other Nd isotopic data for samples of upper crust from the Arabian shield. This result suggests that source rocks of the felsic granulite were derived at 1.0 to 1.2 Ga from either an average MORB-type mantle or a local 2.2 Ga LREE-depleted mantle. Zabargad gneisses represent a part of the disrupted lower continental crust of the Pan African Afro-Arabian shield. During early stages of the Red Sea rifting in the Miocene, these Precambrian granulites were intruded and dragged upwards by a rising peridotite diapir.     

Sm-Nd and zircon SHRIMP U-Pb dating of Huilanshan mafic granulite in the Dabie Mountains and its zircon trace element geochemistry

Granulites in the Dabie Mountains are mainly ob-served in northern Dabie complex zone. Huangtuling intermediate-acid granulites and Huilanshan mafic granulites in the Luotian dome are two famous out-crops (Fig. 1)[1]. It is important to know the genesis and metamorphic age of these granulites for under-standing tectonic evolution and exhumation history of the Dabie Mountains. Previous geochemical and geo-chronological work[2―8]1) on the Huangtuling granu-lites indicates that their protoli…     

Evolutionary mega-cycles of the early precambrian proto-north China platform

The rocks of the early Precambrian proto-north China platform in general have an isotopic age of more than 1800–1900 Ma and thus belong to the Archaean and early Proterozoic.Based on a study of six representative regions, four evolutionary stages have been recognized in terms of volcano-sedimentary and/or sedimentary mega-cycles, two for the Archaean and two for the early Proterozoic. The geochronological division between the first and second mega-cycles is ca. 2800—3000 Ma, that between the second and third ca. 2500–2600 Ma, and that between the third and fourth ca. 2200–2300 Ma.Prior to ca. 2800–3000 Ma ago the area probably already possessed characteritics of a proto-crustal basement on which basaltic, dacitic and neritic clastic rocks had been deposited. This was followed during the second stage (cycle) by the formation of basaltic and intermediate volcanics, silty graywackes and even carbonate rocks. Most of these rocks were metamorphosed to granulite facies and amphibolite facies respectively during Archaean time. They were also influenced by an earlier sodic migmatization and a later potassic one and related magmatic activity and at least two episodes of folding. They probably produced an elevated, rather widespread sialic basement.The third stage was characterized by the accumulation of volcano-sedimentary rocks in local basins, troughs and rift-depressions which differ in actual setting from place to place. These rocks were later affected by varying degrees of tectonism and insignificant magmatism and were metamorphosed to amphibolite to greenschist facies.The last stage was one of deposition, near the earlier basins or troughs, of dominant clastic and dolomitic carbonate rocks with locally developed minor volcanics. This stage reflects comparatively stable tectonic conditions, seemingly transitional into those of the more stable middle to late Proterozoic. Most of the rocks were metamorphosed to greenschist facies before 1800–1900 Ma ago.     

Miocene emplacement and deformation of the Konga Shan granite (Xianshui He fault zone, west Sichuan, China): Geodynamic implications

The presently active sinistral Xianshui He strike-slip fault (XSH) is a lithospheric scale strike-slip fault in the eastern Himalaya. In the study area this fault affects the eastern edge of the Konga Shan granitic massif, where it has caused both brittle and ductile deformation. A RbSr isochron and Nd and Pb isotope study of three samples, and a UPb zircon study of a single sample, were completed on the granite.

UPb data indicate a granite emplacement age of 12.8 ± 1.4 Ma. The RbSr isochrons show that the granite emplacement and the deformational event were synchronous, at around 12-10 Ma (minimum age for deformation). The Nd and Pb isotope compositions of whole rocks and K-feldspars indicate the involvement of Proterozoic continental crust, which is confirmed by UPb systematics indicating inherited zircons.

Sinistral faulting along the XSH began at the latest at 12 Ma and marks the extrusion toward the east of the West Sichuan and South China blocks, following their extrusion along the Red River fault zone between 50 and 21 Ma.     

The oldest basement rock in the Yangtze Craton revealed by zircon U-Pb age and Hf isotope composition

Here we report an integrated study of zircon U-Pb age and Hf isotope composition for a gneiss sample from the Kongling terrain in the Yangtze Craton. CL imaging reveals that most zircons are magmatic, and a few of them have thin metamorphic rims. The magmatic zircons gave a weighted mean U-Pb age of 3218±13 Ma, indicating the gneiss is the oldest basement rock in the Yangtze Craton found to date. They have εHf(t) value of -2.33±0.51,and two-stage Hf model age of 3679±49 Ma,indicating that the gneiss was der...     

Dating the lower crust by ion microprobe

Ion microprobe UThPb ages of zircons from granulite facies lower crustal xenoliths from north Queensland, Australia, correlate well with the ages of major orogenic episodes manifest at the earth's surface. About half of the xenoliths contain Proterozoic zircons which are similar in age to the episodes of high-grade metamorphism of the older surface rocks. All the xenoliths contain late Paleozoic zircons which show a real 100 Ma range in²⁰⁶Pb²³⁸/U ages (from 320 to 220 Ma), which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust. The Paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash-flows and intrusion of high-level granites in this region (320-270 Ma). Mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths, and coupled with the age information, can be used to infer the geological processes operating in the lower crust. The zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two-stage history for these rocks, involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt. Some of the zircons from the felsic xenoliths contain CO₂-rich fluid inclusions, indicating that those zircons grew during high-grade metamorphism. The isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic, mantle-derived, lithologies. The major orogenic event responsible for the formation of that crust occurred in the late Paleozoic, when Proterozoic supracrustal rocks were emplaced into the lower crust, possibly along thin-skinned thrust slices. This was accompanied by intrusion of high-temperature, mantle-derived melts which caused partial melting of pre-existing crust. The most likely setting for such tectonism is a continental margin subduction zone.     

Nature of the Precambrian metamorphic blocks in the eastern segment of Central Tianshan: Constraint from geochronology and Nd isotopic geochemistry

The Central Tianshan Tectonic Zone (CTTZ) is anarrow domain between an early Paleozoic southernTianshan passive continental margin and a late Paleo-zoic northern Tianshan arc zone, which is character-ized by the presence of numerous Precambrian meta-morphic basement blocks. Proterozoic granitoidgneisses and metamorphic sedimentary rocks,namely Xingxingxia and Kawabulag and Tianhugroups, are the most important lithological assem-blages in these metamorphic basement blocks, and alittle of …     

Sensitive high-resolution ion microprobe U-Pb ages of the Latest Oligocene-Early Miocene rift-related Hidaka high-temperature metamorphism in Hokkaido, northern Japan

Abstract   In order to define the timing of granulite facies metamorphism, sensitive high-resolution ion microprobe (SHRIMP) U-Pb analyses were performed on zircons of three pelitic granulites from the lower metamorphic sequence of the Hidaka Metamorphic Belt, southern central Hokkaido, Japan. Both rounded and prismatic zircons were found in the granulite samples. The rounded zircons had thin (10–20 µm) concentric overgrowth rims on detrital cores, while the prismatic zircons did not have detrital cores. Both the overgrowth rims on the rounded zircons and the entire prismatic zircons were formed under granulite facies metamorphism and consistently yield Latest Oligocene–Early Miocene ages (23.7 ± 0.4 Ma to 17.2 ± 0.5 Ma; ²⁰⁶Pb/ ²³⁸U ages ( n  = 31) with low Th/U ratios, mostly <0.1). The internal structure of zircons and their SHRIMP U-Pb ages provide strong evidence in support of the granulite facies event occurring during the Latest Oligocene-Early Miocene. The detrital cores of rounded zircons show a huge variety of ages; Mesoarchean to Paleoproterozoic, Paleozoic to Mesozoic and Paleogene. The interior and marginal portions of the Eurasian continent including cratonic areas are suggested for their source provenances. These wide variations in age suggest that the protolith of the granulites of the lower metamorphic sequence were deposited near the trench of the Eurasian continental margin during Paleogene. The protolith of the lower metamorphic sequence of the Hidaka metamorphic belt was thrust under the upper metamorphic sequence, which had already been metamorphosed in early Paleogene. The Latest Oligocene-Early Miocene Hidaka high-temperature metamorphic event is presumed to have been caused by asthenospheric upwelling during back-arc rifting of the Kuril and Japan basins.     

Nagssugtoqidian mobile belt of West Greenland: a cryptic 1850 Ma suture between two Archaean continents—chemical and isotopic evidence

New chemical and isotopic data permit the recognition of a cryptic suture zone between two Archaean continental masses within the Nagssugtoqidian mobile belt of West Greenland. This discovery has important implications for Precambrian crustal evolution: suture zones may not always be identifiable from geological field observations, with the consequence that mobile belts in which undetected sutures exist may be mis-identified as ensialic, and thought to require special non-plate tectonic models to account for their development.The Nagssugtoqidian belt consists mainly of Archaean gneisses reworked during the Proterozoic, with metamorphic grade and degree of isotopic disturbance increasing towards the centre of the belt. At the centre of the belt the Nagssugtoqidian includes metasediments and calc-alkaline volcanic and plutonic rocks of Proterozoic age, almost always strongly deformed and metamorphosed. From isotopic evidence (Sr_i ca. 0.703; model μ₁ values ca. 8.0; initial ε_Nd ca. 0) it is clear that the Proterozoic igneous rocks do not include any significant contributions derived from the Archaean crust, and the chemistry of the rocks, together with the isotope data, suggests that they were formed at a destructive plate margin. The Proterozoic rocks are found in a narrow zone (up to 30 km wide) between the Archaean gneisses to the north and south of Nordre Strømfjord, and are interpreted as reflecting the existence of a suture between two Archaean continental blocks. Zircon UPb data and other isotope evidence show that subduction started before ca. 1920 Ma ago, and lasted until ca. 1850 Ma when collision occurred, with consequent crustal thickening, high-grade metamorphism and local anatexis. Given the time-span for the operation of subduction, the existence of a wide Nagssugtoqidian ocean can be inferred, even for slow rates of plate motion.The Proterozoic and Archaean gneisses in the Nagssugtoqidian belt are very similar lithologically and chemically, and it has only been possible to distinguish between them using isotopic criteria. Suture zones of this kind are very difficult to detect, and may be present elsewhere within the reworked Archaean terrains of northern Greenland and Canada.     

Uranium-lead zircon and titanite ages from the northern portion of the Western Gneiss Region, south-central Norway

The geochronologic history of the northern portion of the Western Gneiss Region, south-central Norway has been investigated by UPb analyses of zircon and titanite from various basement units and one supracrustal gneiss. A six-point (four zircon and two titanite) discordia line from a migmatite melt-pod and its host-gneiss (Åstfjord migmatite gneiss) defines upper- and lower-intercept ages of 1659.1 ± 1.8 Ma and 393.6 ± 3.6 Ma, respectively. The upper-intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region.A seven-point (three zircon and four titanite) discordia line from the Ingdal granite gneiss has similar upper- and lower-intercept ages (1652.9 ± 1.7 Ma and 396.1 ± 4.9 Ma, respectively) and the upper-intercept age is interpreted as the time of granite crystallization. The lower-intercept age from both theÅstfjord migmatite gneiss and the Ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for Caledonian influence in the region.Titanite from these two gneiss units, as well as from other tonalite, granite, and supracrustal gneisses throughout the Western Gneiss Region displays a remarkably uniform pattern of discordance. Titanite discordance is variable from 6% to 100% and, in general, the degree of discordance is proportional to the Caledonian metamorphic grade of nearby supracrustal schists. Episodic, diffusional lead loss from titanite during Caledonian metamorphism probably caused the discordance pattern, but a combined mechanism of mixing and diffusional lead loss cannot be ruled out. In any case, the seventeen-point titanite and zircon discordia line clearly indicates that: (1) the bulk of the granitoid terrane in this portion of the Western Gneiss Region was emplaced, migmatized, and cooled in a short time interval about 1657 Ma ago; (2) a second, short-lived thermal event that exceeded the blocking temperature of titanite occurred about 395 Ma ago; and (3) titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 Ma, or at any time after 395 Ma.     

<Emphasis Type="Italic">In-situ</Emphasis> trace element analyses and Pb-Pb dating of zircons in granulite from Huangtuling,Dabieshan by LAM-ICP-MS

It is revealed by CL images that there are multi-stage growth internal structures of zircons in the Huangtuling granulite, including the inherited zircons, protolith zircons, sector and planar zone zircons and retrograde zircons. In-situ trace element compositions and Pb-Pb ages have been analyzed by LAM-ICP-MS. The sector and the planar zone domains show typical trace element characteristics of granulite zircon (low Th, U, Th/U, total REEs, clear negative Eu anomalies, relatively depleted HREE and small differential degree between MREE and HREE, etc.), indicating that they formed during granulite-facies metamorphism. The protolith zircons have trace element characteristics of crustal zircon (high Th, U, Th/U, total REEs and enriched HREEs, etc.). 12 analyzed spots on granulite-facies domains give a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of (2154±26) Ma (MSWD = 3.8), which is the best estimated age of granulite-facies metamorphism of this sample. The weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 5 analyzed spots on protolith zircon domains is (2714 ± 22) Ma (MSWD = 1.4), which represents the protolith forming time. The discovery of ca. 3.4 Ga inherited zircon indicates that there are Palaeoarchean continental materials in this area. The interpretation of formation conditions and the ages of zircons can be constrained by simultaneous in-situ analysis of trace elements and ages.     

The discovery of the Neoarchean mafic dyke swarm in Hengshan and reinterpretation of the previous “Wutai greenstone belt”

The structural mapping and section study indicate that the “greenstone belts” in the southern to central parts of Hengshan were intensively sheared and transposed mafic dyke swarm, which originally intruded into the Neoarchean grey gneiss and high-pressure granulite terrain (HPGT). The HPGT is characterized by flat-dipping structures, to the south it became steep and was cut by the Dianmen mafic dyke swarm. After high-pressure granulite-facies metamorphic event, the mafic dyke swarm occurred, and was associated with the extensional setting and reworked by the late strike-slip shearing. The zircon age dating proves that the Dianmen mafic dyke swarm was emplaced during the period between 2499±4 Ma and 2512±3 Ma, followed by late tectonothermal reworking. The Dianmen mafic dyke swarm further documents the extensional episode in the central to northern parts of North China Craton (NCC), providing the important constraint for the limit between Archean and Proterozoic and correlation between NCC and other cratonic blocks of the world.     

Geology of the Grove Mountains in East Antarctica

Grove Mountains consists mainly of a series of high-grade (upper amphibolite to granulite facies) metamorphic rocks, including felsic granulite, granitic gneiss, mafic granulite lenses and charnockite, intruded by late tectonic gneissic granite and post-tectonic granodioritic veins. Geochemical analysis demonstrates that the charnockite, granitic gneiss and granite belonged to aluminous A type plutonic rocks, whereas the felsic and mafic granulite were from supracrustal materials as island-arc, oceanic island and middle oceanic ridge basalt. A few high-strained shear zones disperse in regional stable sub-horizontal foliated metamorphic rocks. Three generations of ductile deformation were identified, in which D₁ is related to the event before Pan-African age, D₂ corresponds to the regional granulite peak metamorphism, whereas D₃ reflects ductile extension in late Pan-African orogenic period. The metamorphic reactions from granitic gneiss indicate a single granulite facies event, but 3 steps from mafic granulite, with P-T condition of M1 800°C, 9.3×10⁵ Pa; M₂ 800–810°C, 6.4 × 10⁵ Pa; and M₃ 650°C have been recognized. The U-Pb age data from representative granitic gneiss indicate (529±14) Ma of peak metamorphism, (534±5) Ma of granite emplacement, and (501±7) Ma of post-tectonic granodioritic veins. All these evidences suggest that a huge Pan-African aged mobile belt exists in the East Antarctic Shield extending from Prydz Bay via Grove Mountains to the southern Prince Charles Mountains. This orogenic belt could be the final suture during the Gondwana Land assemblage.     

Discovery of khondalite series from the western segment of Altyn Tagh and their petrological and geochronological studies

The khondalite series, which are characterized by aluminum-rich gneisses (schists) consisting of sillimanite-garnet-biotite-monzonite gneiss, garnet-biotite-monzonite gneiss, graphite-sillimanite-biotite schist, and garnet-amphibole two-pyroxene granulites occurring as lenses and layers within gneisses (schists), were discovered in Tula area of western segment of Altyn Tagh. The petrology and geochemistry indicate that the protoliths of aluminum-rich gneisses (schists) are aluminum-rich pelitic and pelitic arenaceous sedimentary rocks, the protoliths of basic granulites are continental tholeiitic basalts. Therefore, the khondalite series may be produced at continental margin. They had suffered granulitic facies metamorphism with peak temperatures of 700-850℃ and pressures of 0.8-1.2 GPa. The U-Pb and Pb-Pb isotopic dating of zircons provided the ages of 447-462 Ma representing the ages of peak granulitic metamorphism. The U-Pb dating of detrital zircons from aluminum-rich gneisses yielded older upper intercept ages which reflect the times of older materials derived from source rocks of the gneiss protoliths.     

Mineral inclusions in zircon domains and geological significance of SHRIMP U-Pb dating for coesite-bearing zircons of paragneiss in Sulu terrane,eastern China

Laser Raman spectroscopy and cathodoluminescence (CL) image reveal that zircons separated from paragneisses in the southwestern Sulu terrane (eastern China) preserve multi-stage mineral assemblages in different zircon domains. In the same paragneiss zircon sample, some zircon grains retain inherited (detrital) cores with abundant low-pressure mineral inclusions of Qtz + Phe + Ap + impurities and Qtz + Phe + impurities. The ultrahigh-pressure (UHP) metamorphic overgrowths mantles of these zircons preserve Coe, Coe + Phe and other UHP mineral inclusions, indicating that these inherited (detrital) zircons from protoliths experienced metamorphic recrystallization during the Sulu UHP metamorphic event. However, other zircon grains preserve UHP mineral inclusions of Coe, Coe + Ap and Coe + Phe in the cores and mantles, whereas the outmost rims contain quartz (Qtz) and other low-pressure mineral inclusions. These phenomena prove that the second group zircons were crystallized at UHP metamorphic stage and overpr     

SHRIMP U–Pb zircon chronology of ultrahigh‐temperature spinel–orthopyroxene–garnet granulite from South Altay orogenic belt,northwestern China

Diagnostic mineral assemblages, mineral compositions and zircon SHRIMP U–Pb ages are reported from an ultrahigh‐temperature (UHT) spinel–orthopyroxene–garnet granulite (UHT rock) from the South Altay orogenic belt of northwestern China. This Altay orogenic belt defines an accretionary belt between the Siberian and Kazakhstan–Junggar Plates that formed during the Paleozoic. The UHT rock examined in this study preserves both peak and retrograde metamorphic assemblages and microstructures including equilibrium spinel + quartz, and intergrowth of orthopyroxene, spinel, sillimanite, and cordierite formed during decompression. Mineral chemistry shows that the spinel coexisting with quartz has low ZnO contents, and the orthopyroxene is of high alumina type with Al₂O₃ contents up to 9.3 wt%. The peak temperatures of metamorphism were >950°C, consistent with UHT conditions, and the rocks were exhumed along a clockwise P–T path. The zircons in this UHT rock display a zonal structure with a relict core and metamorphic rim. The cores yield bimodal ages of 499 ± 8 Ma (7 spots), and 855 Ma (2 spots), with the rounded clastic zircons having ages with 490–500 Ma. Since the granulite was metamorphosed at temperatures >900°C, exceeding the closure temperature of U–Pb system in zircon, a possible interpretation is that the 499 ± 8 Ma age obtained from the largest population of zircons in the rock marks the timing of formation of the protolith of the rock, with the zircons sourced from a ～500 Ma magmatic provenance, in a continental margin setting. We correlate the UHT metamorphism with the northward subduction of the Paleo‐Asian Ocean and associated accretion‐collision tectonics of the Siberian and Kazakhstan–Junggar Plates followed by rapid exhumation leading to decompression.     

Nd isotopic systematics and chemistry of Central Australian sapphirine granulites: an example of rare earth element mobility

Lower Proterozoic sapphirine-bearing and associated granulites from Central Australia exhibit the greatest range of present-day¹⁴³Nd/¹⁴⁴Nd ratios (∈_Nd(O)= ?26.5 to +112.3) yet reported for rocks believed to be cogenetic. The Nd isotopic data and REE abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform CuPbZn sulfide deposits with which they are closely associated. Field relationships, petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite-rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism; calculations employing REE abundances of the sapphirine granulites and associated rocks, combined with bulk solid-fluid distribution coefficient data yield high fluid/rock ratios, consistent with a pre-metamorphic hydrothermal origin for the unusual REE patterns. The SmNd data for these rocks define an age of 1760±75Ma, which is significantly younger than the crust formation age of the terrain ( 2070±125Ma) but indistinguishable from the RbSr whole rock age for granulite facies metamorphism (1790±35Ma). These data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly (perhaps tens of millions of years) before granulite facies metamorphism, followed by redistribution of Nd isotopes or small fractionations of the Sm/Nd ratio during the granulite facies event, and possibly also during intense retrogression which reset RbSr whole rock and UPb zircon and monazite systematics at about 1700 Ma.     

Zircon geochronology and geochemistry of mafic xenoliths from Liaoning kimberlites:Track the early evolution of the lower crust,North China Craton

Since the formation of the primal nucleus at 3.8Ga[1], the North China had underwent the multi-cycle geologic processes such as arc accretion[2—4] andrift[5—7], and finally accomplished the craton[8] aroundat 1.8 Ga[2,3,9]. Paleozoic Fuxian kimberlites (LiaoningProvince) contain not only abundant peridotitic butalso mafic xenoliths. The investigation on peridotiticxenoliths indicted that the North China Craton hadcold and thick lithospheric root in the middle Ordovi-cian. The deep part o…