High-grade Metasedimentary Rocks Research Articles

Geochemical signatures of metasedimentary rocks of high-pressure granulite facies and their relation with partial melting: Carvalhos Klippe, Southern Brasília Belt, Brazil

Abstract High-grade metasedimentary rocks can preserve geochemical signatures of their sedimentary protolith if significant melt extraction did not occur. Retrograde reaction textures provide the main evidence for trapped melt in the rock fabrics. Carvalhos Klippe rocks in Southern Brasilia Orogen, Brazil, present a typical high-pressure granulite assemblage with evidence of mica breakdown partial melting (Ky + Grt + Kfs ± Bt ± Rt). The metamorphic peak temperatures obtained by Zr-in-Rt and ternary feldspar geothermometers are between 850 °C and 900 °C. The GASP baric peak pressure obtained using grossular rich garnet core is 16 kbar. Retrograde reaction textures in which the garnet crystals are partially to totally replaced by Bt + Qtz ± Fsp intergrowths are very common in the Carvalhos Klippe rocks. These reactions are interpreted as a result of interactions between residual phases and trapped melt during the retrograde path. In the present study the geochemical signatures of three groups of Carvalhos Klippe metasedimentary rocks are analysed. Despite the high metamorphic grade these three groups show well-defined geochemical features and their REE patterns are similar to average compositions of post-Archean sedimentary rocks (PAAS, NASC). The high-pressure granulite facies Grt-Bt-Pl gneisses with immature arenite (wacke, arkose or lithic-arenite) geochemical signatures present in the Carvalhos Klippe are compared to similar rocks in amphibolite facies from the same tectonic framework (Andrelândia Nappe System). The similar geochemical signatures between Grt-Bt-Pl gneisses metamorphosed in high-pressure granulite facies and Grt-Bt-Pl-Qtz schists from the Andrelândia and Liberdade Nappes, with minimal to absent melting conditions, are suggestive of low rates of melt extraction in these high-grade rocks. The rocks with pelitic compositions most likely had higher melt extraction and even under such circumstances nevertheless tend to show REE patterns similar to average compositions of post-Archean sedimentary rocks (PAAS, NASC).

Zircon ages and Hf isotopic constraints on sources of clastic metasediments of the Slyudyansky high-grade complex, southeastern Siberia: Implication for continental growth and evolution of the Central Asian Orogenic Belt

We present results of combined in situ U–Pb dating of detrital zircons and zircon Hf and whole-rock Nd isotopic compositions for high-grade clastic metasedimentary rocks of the Slyudyansky Complex in eastern Siberia. This complex is located southwest of Lake Baikal and is part of an early Paleozoic metamorphic terrane in the eastern part of the Central Asian Orogenic Belt (CAOB). Our new zircon ages and Hf isotopic data as well as whole-rock Nd isotopic compositions provide important constraints on the time of deposition and provenance of early Paleozoic high-grade metasedimentary rocks as well as models of crustal growth in Central Asia. Ages of 0.49–0.90Ga for detrital zircons from early Paleozoic high-grade clastic sediments indicate that deposition occurred in the late Neoproterozoic and early Paleozoic, between ca. 0.62–0.69 and 0.49–0.54Ga. Hf isotopic data of 0.82–0.69Ga zircons suggest Archean and Paleoproterozoic (ca. 2.7–2.8 and 2.2–2.3Ga; Hfc=2.5–3.9Ga) sources that were affected by juvenile 0.69–0.82Ga Neoproterozoic magmatism. An additional protolith was also identified. Its zircons yielded ages of 2.6–2.7Ga, and showed high positive εHf(t) values of +4.1 to +8.0, and Hf model ages tHf(DM)=tHfc=2.6–2.8Ga, which is nearly identical to the crystallization ages. These isotopic characteristics suggest that the protolith was quite juvenile. The whole-rock Nd isotopic data indicate that at least part of the Slyudyansky Complex metasediments was derived from “non-Siberian” provenances. The crustal development in the eastern CAOB was characterized by reworking of the early Precambrian continental crust in the early Neoproterozoic and the late Neoproterozoic–early Paleozoic juvenile crust formation.

Constraints on the subduction erosion/extrusion cycle in the Paleozoic Acatlán Complex of southern Mexico: Geochemistry and geochronology of the type Piaxtla Suite

The type high-pressure (HP) Piaxtla Suite in the Acatlán Complex of southern Mexico consists of retrogressed eclogite (amphibolite), megacrystic granitoids and high-grade meta-sedimentary rocks. Exhumation of these HP rocks has recently been interpreted as the result of extrusion into the upper plate, rather than by return flow up the subduction zone. Geochemical analyses of the retrograde eclogites indicate that they have a rift tholeiitic-transitional alkalic composition. These are closely associated with a megacrystic meta-granitoid that has yielded an intrusive age of 452±6Ma (concordant U–Pb zircon analyses) with inherited zircon populations at ca. 800–950Ma and 1000–1200Ma derived from the underlying basement, probably the Oaxacan Complex which borders the Acatlán Complex to the east. The bimodal nature of these igneous rocks and their spatial and temporal close association with continentally-derived sedimentary rocks is similar to most HP rocks in the Acatlán Complex derived from a rifted passive margin. The youngest detrital zircon population in a meta-psammite sample yielded a U–Pb age of 365±15Ma with older analyses distributed along a chord with an upper intercept of 1287±29Ma. The ca. 365Ma age provides a maximum age for the time of deposition of this sample. 40Ar/39Ar ages from the retrogressed eclogites provided hornblende plateau ages of 342±2Ma and 344±2Ma, whereas muscovite from the granitoid and meta-psammite yielded 334±2Ma plateau ages. These data constrain the subduction erosion–extrusion cycle to ≤35Ma during which the rocks were taken to a depth of ca. 40km at a rate of 2.7km/Ma and back to the surface at 2.4km/Ma. Such exhumation rates are slower than those in continent–continent collision zones, but similar to those in the Iberia–Czech Variscan belt where tectonic interpretation also suggests extrusion into the upper plate.

Precambrian detrital zircons in the Early Paleozoic Chinese Altai: Their provenance and implications for the crustal growth of central Asia

The basement characteristic of the Chinese Altai is not firmly constrained, which has strictly hindered our understanding of the early crustal growth in the region. The presence of Precambrian detrital zircons in metasedimentary rocks and xenocrystic zircons in granitoids may be regarded as evidence for the existence of unexposed old basement rocks, but can also be explained as detritus originally derived from a nearby cratonic fragment. This paper reports detrital zircon U–Pb and Hf isotopic data for both low- and high-grade metasedimentary rocks from the Chinese Altai. Zircons from these samples have similar age patterns and Hf isotopic compositions. They are characterized by an overwhelming dominance of Neoproterozoic-Early Paleozoic ages, with prominent peaks around 465–542 Ma and ɛ Hf( t) values varying from −25 to +15. Besides, a few zircons cluster at 1.8–2.0 Ga and sparse ones yield discordant ages around 2.3–2.6 Ga, and both give negative ɛ Hf( t) values between 0 and −28. Combined with previous results in the region, our data suggest that low- and high-grade metasedimentary rocks most likely derived from similar provenances. These data, plus some xenocrystic zircon ages from the granitoid rocks, are quite comparable with the age patterns of the micro-continents and arc terranes in western Mongolia. The zircon population of 440–580 Ma is dominant for gneissic granitoids within the Neoproterozoic–Early Paleozoic terranes in western Mongolian and the Chinese Altai. The Precambrian zircons (>540 Ma) resemble those from old rocks preserved in the Tuva-Mongolian (TM) block and its adjacent arc terranes. Therefore, we suggest that old zircons in the Chinese Altai more likely represent the detritus recycled from western Mongolia. Moreover, information from the TM-derived Precambrian zircons, combined with previous palaeomagnetic constraints, favors northern India as the potential origin for the TM block, from which a small micro-continent drifted northwards and incorporated into the CAOB in a time interval from the Neoproterozoic to Early Paleozoic. Accordingly, the crustal growth of the CAOB in western Mongolia and the Chinese Altai could be outlined by a single accretionary system, and secular amalgamation of magmatic arcs around a Precambrian micro-continent can account for the accretionary history of the region in the Early Paleozoic time.

Sources of monazite sand in southern Orissa beach placer, eastern India

This paper intends to explore whether there is an important source for monazite beach placer of the Gopalpur-Chhatrapur-Rushikulya coast, Orissa, in the adjacent Eastern Ghat Mobile Belt (EGMB). Petrographic and mineralogical studies were conducted on all the rock types constituting the EGMB exposed over a stretch extended up to ∼20 km landward from the estuary of the River Rushikulya that is believed to transport the major bulk of sand to the Gopalpur-Chhatrapur-Rushikulya beach. Heavy mineral population was concentrated using bromoform and percentages of each heavy mineral constituting the population were estimated for all the potential source rock types. Isodynamic separation and XRD techniques were deployed for precision identification of every heavy mineral present. The study identified the granitoid (or migmatite) basement rock as by far the major contributor of monazite to the Chhatrapur beach sand. The study also reveals that charnockite is an important contributor of orthopyroxene as well as garnet, although the sillimanite-garnet-quartz schist (khondalites) is also an important source for the latter. On the other hand, garnet-quartz schist and garnet-biotite-quartz schist may also contribute substantial quantity of pyroxene and garnet. The high grade metasedimentary rocks, in general, could be the major sources for rutile, while ilmenite, magnetite and zircon in the beach sand have their sources perhaps in all the varieties of rocks constituting the EGMB.

Significance of Provenance Ages from the Chiapas Massif Complex (Southeastern Mexico): Redefining the Paleozoic Basement of the Maya Block and Its Evolution in a Peri-Gondwanan Realm

Medium- to high-grade metasedimentary rocks are exposed as isolated domains in mostly metaigneous crystalline rocks of the Chiapas Massif Complex (CMC), which forms the basement of the southern Maya Block, southeastern Mexico. Laser ablation multicollector inductively coupled plasma mass spectrometry U-Pb isotope analyses on inherited detrital zircon cores show two principal age distributions: (a) Group I, with its highest peak at 500–650 Ma and smaller peaks at ∼380–400 Ma, 1.0–1.2 Ga, 1.5–2.0 Ga, and 2.6–3.1 Ga, and (b) Group II, with its highest peak at 1.0–1.2 Ga, a minor peak at 1.5–1.6 Ga (or ∼1.0-Ga zircon cores only), and a lack of 500–650-Ma zircons. The cores are commonly surrounded by metamorphic or anatectic overgrowths, and some zircon ages were reset by a tectonothermal event at ∼250 Ma. The age distribution of zircon cores from Group I metasediments are similar to detrital zircon ages in Carboniferous-Permian sediments (Santa Rosa Formation) in southeastern Chiapas, which were shed mostly from continental crust dominated by Pan-African–Brasiliano orogenic cycles. Group II metasediments have different sources, principally from Grenville orogens, such as the Oaxacan Complex, and early mid-Proterozoic sources, such as the Rio Negro–Juruena Province of western Amazonia. The lowest stratigraphic unit in the CMC (the Jocote Unit) is intruded by lower Ordovician S-type granite and contains only 1.5–1.6-Ga and older detrital zircons. The protoliths of the CMC can be correlated with Paleozoic strata across the southern Maya Block, and they show parallels with the geologic history and provenance patterns of protoliths from the Acatlán Complex (Mixteca Terrane) of central southern Mexico. The geologic history of the CMC and the Maya Block can be interpreted in terms of tectonothermal events accompanying accretion of the CMC and polarity reversal during formation of the western Pangea margin.

Protolith age and timing of Precambrian magmatic and metamorphic events in the Priest River complex, northern Rockies

U–Pb dating of detrital zircons, magmatic zircons, and metamorphic zircon overgrowths and titanites from the Priest River complex, USA, reveal the ages of high-grade metasedimentary rocks, intrusive ages of associated igneous rocks, and the timing of Precambrian metamorphic events. Sensitive high-mass resolution ion microprobe (SHRIMP) U–Pb dates of detrital zircons from the Hauser Lake gneiss and Gold Cup Quartzite are predominately Paleoproterozoic with some Archean grains. The Hauser Lake gneiss contains detrital zircons with nearly concordant ages as young as 1511 Ma. Isotope dilution – thermal ionization mass spectrometry (ID–TIMS) and chemical abrasion – thermal ionization mass spectrometry (CA–TIMS) dating of magmatic zircon from amphibolites in the Hauser Lake Gneiss yield 1470–1430 Ma crystallization ages based on discordant data, with Cretaceous lower intercepts. The U–Pb zircon systematics are very complex and reflect multiple periods of magmatic and metamorphic growth. A combination of mechanical abrasion, chemical abrasion (CA–TIMS), and SHRIMP methods were required to identify the major geochronological components. These data, in addition to one Nd model age, strengthen correlations between the Hauser Lake Gneiss and the lower Belt–Purcell Supergroup and the Gold Cup Quartzite with the Neihart Quartzite, which underlies the Belt–Purcell Supergroup. Four SHRIMP-dated metamorphic zircon overgrowths give concordant Grenville dates with a weighted average of 1127 ± 110 Ma. This Grenville-aged metamorphic event is recorded by many isotopic systems in the Belt–Purcell basin and reflects a static thermal event, possibly driven by magmatism at depth.

Deciphering polymetamorphic episodes in high-grade metamorphic orogens: Constraints from PbSL, Sm/Nd and Lu/Hf garnet dating of low- to high-grade metasedimentary rocks from the Kaoko belt (Namibia)

Three dating techniques for metamorphic minerals using the Sm–Nd, Lu–Hf and Pb isotope systems are combined and interpreted in context with detailed petrologic data from crustal segments in NW Namibia. The combination of isochron ages using these different approaches is a valuable tool to testify for the validity of metamorphic mineral dating. Here, PbSL, Lu–Hf and Sm–Nd garnet ages obtained on low- to medium-grade metasedimentary rocks from the Central Kaoko Zone of the Neoproterozoic Kaoko belt (NW Namibia) indicate that these samples were metamorphosed at around 550–560 Ma. On the other hand, granulite facies metasedimentary rocks from the Western Kaoko Zone underwent two phases of high-grade metamorphism, one at ca. 660–625 Ma and another at ca. 550 Ma providing substantial evidence that the 660–625 Ma-event was indeed a major tectonothermal episode in the Kaoko belt. Our age data suggest that interpreting metamorphic ages by applying a single dating method only is not reliable enough when studying complex metamorphic systems. However, a combination of all three dating techniques used here provides a reliable basis for geochronological age interpretation.

Lithostratigraphy and Geochemistry of the High-Grade Metasedimentary Rocks in the Thompson Nickel Belt and Adjacent Kisseynew Domain, Manitoba: Implications for Nickel Exploration

Exploration for nickel in the Thompson nickel belt is focused on specific paragneiss units within the Paleoproterozoic sedimentary and volcanic cover succession (Ospwagan Group) on the northwestern margin of the Archean Superior craton. These units host ultramafic bodies and all significant nickel deposits in the Thompson nickel belt. Knowledge of the lithostratigraphy is therefore essential for finding the units with the highest potential for the discovery of new nickel deposits. This is made practical by a distinctive stratigraphic succession with numerous marker units, including metamorphosed quartzite, carbonate, pelite, iron formation, and graywacke, as summarized in this paper. However, the sedimentary rocks are generally highly deformed and metamorphosed, making the recognition of some units difficult and their sedimentary origin obscure. To better recognize the sedimentary protoliths of orehosting and related units, a simple plot of SiO 2 versus Al 2 O 3 is used to estimate the approximate proportions of siliciclastic and chemical sedimentary components. This demonstrates that much of the succession, which was previously considered to be derived from calcareous and ferruginous chemical sediments, is a hybrid containing a substantial siliciclastic component, thus explaining the present mineralogy and geochemistry and assisting in the recognition of units at various metamorphic grades. Extended-element plots, using the most abundant metapelite unit in the Ospwagan Group as the normalizing factor, show a common provenance for the entire group. They also illustrate a clear difference between the Ospwagan Group and the underlying basements gneiss and allow the Ospwagan Group to be distinguished from petrographically similar but economically barren, younger paragneiss units of the Burntwood and Grass River groups. Unlike the Ospwagan Group, the younger units, which predominate in the Kisseynew Domain adjoining the Thompson nickel belt, were derived mainly from an exotic Paleoproterozoic arc-related source and therefore have a different geochemical signature. This geochemical difference can be utilized in areas of sparse geologic information, such as beneath Phanerozoic or glacial cover, where initial exploration drilling tests new geophysical targets. Combined litho- and chemostratigraphy improve insight into basin evolution, which probably occurred on a ca. 2 Ga passive continental margin or shelf. The ca. 1880 Ma maficultramafic magmatism related to the nickel deposits may have formed in a different tectonic environment, possibly in a continental back arc associated with a recently proposed episode of felsic calc-alkaline plutonism.

High-K, calc-alkaline I-type granitoids from the composite Yozgat batholith generated in a post-collisional setting following continent-oceanic island arc collision in central Anatolia, Turkey

The composite Yozgat batholith consists of a S-I-A-type granitoid association intruding the supra-subduction zone-type (SSZ-type) central Anatolian ophiolite and medium- to high-grade metasedimentary rocks of the central Anatolian crystalline complex. These rocks are unconformably covered by Palaeocene to Early Eocene sedimentary rocks. The I-type granitoids are the most common rock association of this huge batholith. In an area between the towns of Şefaatli and Yerkoy, the southwestern part of the batholith can be subdivided into five mappable units: the Akcakoyunlu quartz monzodiorite (mafic; hornblende K-Ar cooling ages of 77.6–79.3 Ma); the Cankili monzodiorite (mafic; hornblende K-Ar cooling age of 71.1 Ma); the Adatepe quartz monzonite (mafic; hornblende K-Ar cooling age of 68.0 Ma); the Yassiagil monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 69.9–79.8 Ma) and the Karakaya monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 71.3–77.0 Ma). All the lithological units, except the Karakaya monzogranite, include large K-feldspar megacrysts and various types of mafic microgranular enclaves in field outcrops, indicating mingling and mixing. In addition, microscopic textures showing the hybridization between the coeval mafic and felsic magma sources are present. Whole-rock major element geochemistry shows a high-K calc-alkaline, metaluminous, I-type composition with an aluminium saturation index (ASI) less than 1.10 and with CIPW diopside content in all the lithological units. Large ion lithophile elements (LILE), light rare earth elements (LREE), some high field strength elements (HFSE) (except Nb) enrichments and significant crustal contribution revealed by the oxygen and sulphur stable isotope compositions in the mafic and felsic I-type granitoid units are consistent with mafic lower crustal and metasomatized mantle sources the latter of which were metasomatized by earlier supra subduction zone (SSZ)-derived fluids during the development of the SSZ-type central Anatolian ophiolite.

Multistage growth and reworking of the Palaeoproterozoic crust in the Bergslagen area, southern Sweden: evidence from U–Pb geochronology

The Svecofennian Domain of the Fennoscandian Shield constitutes a considerable volume of Palaeoproterozoic crustal growth, 2.1–1.86 Ga ago, in between the Archaean craton in the NE and the 1.85–1.65 Ga Transscandinavian Igneous Belt (TIB) in the south and west. The Bergslagen area is a classical ore province located in the southwestern part of the Svecofennian Domain of south-central Sweden. Its northern part is dominated by volcanic and plutonic rocks of a magmatic arc with continental affinity, while the SE part is made up by a sedimentary basin. The Bergslagen area shows a metamorphic zonation from lower to middle amphibolite facies in the north to upper amphibolite facies and locally granulite facies in the south; a small greenschist area exists in the west. Identifying the age spectra of inherited components, magmatic crystallization, as well as metamorphic episodes, provide important constraints on the geodynamic evolution of this centrally located piece of the Shield.U–Pb zircon SIMS data presented in this paper complement the previous, regionally scattered TIMS data from this area. Magmatic zircons from two felsic metavolcanic rocks and two amphibolites (metagabbros) yield 1888±12, 1892±7 and 1887±5, 1895±5 Ma, respectively; i.e. within the 1.91–1.86 Ga range previously obtained for Early Svecofennian magmatism in Bergslagen. An augen gneiss from southern Bergslagen, assigned to the earliest TIB generation, yield an intrusive age of 1855±6 Ma. Metamorphic monazites from the same rock indicate that deformation and elevated thermal activity prevailed 1.83–1.82 Ga ago (TIMS). Metamorphic zircons in high-grade metasedimentary rocks from the south and west yield ages of 1793±5 and 1804±10 Ma, in accordance with ages for regional peak metamorphism and migmatite formation found elsewhere in the southern Svecofennian province of Sweden. More importantly, a few zircon crystals and overgrowths in rocks from the north indicate an early metamorphic episode at c. 1.87 Ga, indicating that Bergslagen has experienced two major metamorphic events. Detrital and inherited zircons span the range 2.78–1.90 Ga, with an apparent gap at 2.45–2.1 Ga, which further emphasize previous observations of a major juvenile (<2.1 Ga) and a minor Archaean provenance. This, and in particular the 1.94–1.91 Ga crystals present in the c. 1.89 Ga amphibolites, support the suggestion of a former Palaeoproterozoic pre-1.91 Ga crust in the Bergslagen area.

Isotope constraints on the origin of Pan-African granitoid rocks in the Kaoko belt, NW Namibia

The geodynamic evolution of the Kaoko belt, northwest Namibia, which is part of the late Neoproterozoic mobile belt system of western Gondwana, is considered to be the result of collision between the Congo (Africa) and Rio de la Plata (South America) cratons. In the coastal area of this belt, Pan-African dioritic to granitic melts intruded at ~650 Ma and between ~580 and ~550 Ma into sedimentary sequences of the Kuiseb Formation. Field observations suggest that the later intrusive episode coincided with the peak of high-grade metamorphism in this region, which is characterised by granulite facies conditions and anatexis. Franz et al. (1999) state that both intrusive episodes can be connected with high-grade metamorphic events. The δ18O values of the Pan-African granitoids range from +8.5 to +10.3‰ and tend to be slightly higher than those of the Palaeoproterozoic and late Archaean basement gneisses (+7.6 to +9.9‰ and +7.4 to +9.0‰, respectively). The Pan-African granitoids have low to moderate initial 87Sr/86Sr ratios between ~0.7036 and ~0.7121. Sm-Nd isotopic data of the Pan-African granitoids have moderate initial ϵNd values ranging from −1.2 to −6.8. They are distinct from Palaeoproterozoic as well as late Archaean orthogneisses from the adjacent Congo Craton, which reveal much lower ϵNdvalues between −18 and −28 during the time of emplacement of the Pan-African granitoids. Thus, the derivation of these granitoid melts exclusively from much older crust can be precluded. Although the Pan-African granitoids and the Pan-African high-grade metasedimentary rocks have similar initial ϵNd values of −1.2 to −6.8 and also similar mean crustal residence ages between 1.2 and 1.6 Ga, the derivation of the granitoids from these metasedimentary rocks alone seems also improbable since the granitoids reveal significantly lower initial 87Sr/86Sr ratios than the metasediments. It is here suggested that the Nd, Sr and O isotopic features of the Pan-African granitoids reflect mixing between upper crustal material (metasediments) and remelted basement gneisses from lower crustal levels.

Late Mesoproterozoic to early Neoproterozoic history of the East Greenland Caledonides: evidence for Grenvillian orogenesis?

Detrital zircons from high-grade metasedimentary rocks (Krummedal supracrustal sequence) in the East Greenland Caledonian orogen yield ion-microprobe U–Pb ages mainly in the range 1100–1800 Ma but with a few grains of c . 1000 Ma, different from zircon ages (mainly 1800–2800 Ma) obtained from the crystalline basement that underlies the metasedimentary rocks. These results indicate that original deposition of the Krummedal sequence took place after 1000–1100 Ma ago, and that the sediment was not derived from the underlying crystalline basement, but from younger, at present unknown sources. High-grade metamorphism of the Krummedal sequence and formation of anatectic granites took place around 930 Ma ago. Caledonian granites are also present in the region, but cannot be distinguished on visual criteria in the field from the older granites, unless emplaced into a younger (900–600 Ma) sequence of sedimentary rocks, the Eleonore Bay Supergroup. It is not yet certain whether the high-grade metamorphism and granite formation at c . 930 Ma are related to a ‘Grenvillian’ or slightly younger collisional event, or to an episode of rifting and crustal thinning. If present at all, a ‘Grenvillian’ orogen in East Greenland would be of very different character than that in North America and southern Scandinavia.

Origin of S-type granites coeval with I-type granites in the Hellenic subduction system, Miocene of Naxos, Greece

The island of Naxos in the Cyclades preserves a core complex developed by Miocene detachment faulting associated with extension of continental crust inboard of the Hellenic subduction zone. The extension has unroofed Miocene hornblende-biotite granite and leucogranite, previously defined as I-type and S-type granite respectively. Crystalline basement exposed by extension includes partially migmatised ortho- and paragneisses principally with Hercynian protolith ages and intercalated high-grade metasedimentary and metavolcanic rocks with Mesozoic protoliths. Most leucogranites, in addition to quartz, feldspar and biotite contain muscovite, tourmaline and garnet (almandine-spessartine). Mineral assemblages suggest crystallisation pressures < 2.5 kbar, with stability of muscovite and tourmaline resulting from high B, Mn and possibly F in the magma. Two types of leucogranite can be distinguished on the basis of rock chemistry, mineral chemistry and Nd/Sm isotopes. Earlier leucogranites (type II) have high LREE:HREE ratio, high LILE, low Y and Nb and ϵNd ∼-10 and appear to be a partial melting product of Hercynian paragneiss. Leucogranite dykes within the I-type granite appear similar. Later leucogranites (types Ia, Ib) have low LREE:HREE ratio and ϵNd ∼-7 and appear to be derived from melting of both Hercynian paragneisses and younger metasediments. The major element composition of rare leucogranite type Ic suggests derivation from partial melting of orthogneiss. These varied sources result in significant variation in mineral assemblages in the leucogranites.

SM/ND DATA OF METASEDIMENTARY ROCKS FROM THE CENTRAL SEGMENT OF RIBEIRA BELT, SOUTHEASTERN BRAZIL

The central segment of Ribeira Belt, southeastern Brazil, comprises several tectonic domains where, during the Proterozoic, extensive detritic material was deposited over pre-1.8 Ga basement rocks. This paper presents Sm-Nd isotopic data on the high-grade metasedimentary rocks of this belt and discusses some possible implications on their sedimentary provenance. The pre-1.8 Ga basement rocks display different Nd isotopic evolution and thus can be promptly discriminated as sources of different Nd model ages (T DM ) and isotopic characteristics such as: Mantiqueira Complex, 3.0 and 2.6 Ga T DM isochrons ages; Juiz de Fora Complex, 2.2 Ga T DM isochron age and Quirino Complex, 2.0 Ga and 3.2 Ga T ages. At the Juiz de Fora Domain, metapelites from the Andrelândia Depositional Cycle display a very narrow range of Sm/Nd values (0.15 to l).17) and T ages between 1.8 and 2.0 Ga. The corresponding measured e Nd ,. values [ e Nd (0)] vary between - 20.8 and -29.1. The metapsammopelites of the Jardim G1oria Unit, in the same geologic domain, display very different isotopic characteristics: higher Sm/Nd ratios (0.22 and 0.24), lower e Nd (0) values (-15.3 and -18.3) and Neoarchean T DM ages (2.5 and 2.6 Ga). At the Paraiba do Sul Klippe, the analyzed metapelites from the middle unit of Paraiba do Sul Group show T DM ages close to 1.7 Ga with e Nd (0) values of -14.10 and -17.34. The metapsammopelites from the basal and uppermost units plot along the T.v Ga T isochron and display e Nd (0) values varying between -21 and -32. An isolated T DM of an amphibolitic body, probably intrusive in the metaseaimentary rocks of Paraiba do Sul Group, yields 1.0 Ga T DM age, with a 147 Sm/ 144 Ndvalue of 0.1428.

SM/ND DATA OF METASEDIMENTARY ROCKS FROM THE CENTRAL SEGMENT OF RIBEIRA BELT, SOUTHEASTERN BRAZIL

The central segment of Ribeira Belt, southeastern Brazil, comprises several tectonic domains where, during the Proterozoic, extensive detritic material was deposited over pre-1.8 Ga basement rocks. This paper presents Sm-Nd isotopic data on the high-grade metasedimentary rocks of this belt and discusses some possible implications on their sedimentary provenance. The pre-1.8 Ga basement rocks display different Nd isotopic evolution and thus can be promptly discriminated as sources of different Nd model ages (T DM ) and isotopic characteristics such as: Mantiqueira Complex, 3.0 and 2.6 Ga T DM isochrons ages; Juiz de Fora Complex, 2.2 Ga T DM isochron age and Quirino Complex, 2.0 Ga and 3.2 Ga T DM ages. At the Juiz de Fora Domain, metapelites from the Andrelândia Depositional Cycle display a very narrow range of Sm/Nd values (0.15 to 0.17) and T DM ages between 1.8 and 2.0 Ga. The corresponding measured ε Nd values [ε Nd (0)] vary between – 20.8 and –29.1. The metapsammopelites of the Jardim Glória Unit, in the same geologic domain, display very different isotopic characteristics: higher Sm/Nd ratios (0.22 and 0.24), lower ε Nd (0) values (–15.3 and –18.3) and Neoarchean T DM ages (2.5 and 2.6 Ga). At the Paraíba do Sul Klippe, the analyzed metapelites from the middle unit of Paraíba do Sul Group show T DM ages close to 1.7 Ga with e Nd (0) values of -14.10 and -17.34. The metapsammopelites from the basal and uppermost units plot along the 2.0 Ga T DM isochron and display ε Nd (0) values varying between -21 and –32. An isolated T DM of an amphibolitic body, probably intrusive in the metasedimentary rocks of Paraíba do Sul Group, yields 1.0 Ga T DM age, with a 147 Sm/ 144 Nd value of 0.1428.

Identifying granite sources by SHRIMP U-Pb zircon geochronology: an application to the Lachlan foldbelt

The potential genetic link between granites and their host sediments can be assessed using zircon age inheritance patterns. In the Lachlan fold belt, southeastern Australia, granites and associated high-grade metasedimentary rocks intrude low-grade Ordovician country rock. This relationship is well-exposed in the Tallangatta region, northeast Victoria (part of the Wagga-Omeo Metamorphic Complex). In this region granites (two I-types and two S-types) have intruded during the mid-late Silurian between approximately 410–430 Ma based on the ages of magmatic zircons. The age spectra for inherited zircons from the granites have been compared with those of detrital zircons from the enclosing low- and high-grade metasediments. In broad terms, both for detrital zircons in all four sediments and for inherited zircons in three of the four granites, the dominant ages are early Paleozoic and Late Precambrian, with sporadic older Precambrian ages extending up to 3.5 Ga. The ages of the youngest detrital zircons from the low-grade Lockhart and Talgarno terranes limit the time of sedimentation to ca. 466 Ma or younger. The youngest detrital zircons from two samples of the high-grade Gundowring terrane are 473 Ma, making these sediments Ordovician or younger, not Cambrian as originally suggested. However, the individual age spectra for the four selected metasediments are not well matched when closely examined. The age spectra of the inherited zircons in the granites also do not adequately match those in any of the metasediments. Thus, the metasediments might not be representative of the actual source rocks of the granites. While the exact source of the granites cannot be identified from the analysed samples, the existence of a large population of ca. 495 Ma inherited zircon grains in the S-type granites requires that the granite source contains a significant proportion of Cambrian or younger material. This does not preclude the existence of a Precambrian basement to the Lachlan fold belt but indicates that at the level of S-type magma generation, a Cambrian and/or younger protolith is required.

Depositional age and provenance of the Marboo Formation from SHRIMP U–Pb zircon geochronology: Implications for the early Palaeoproterozoic tectonic evolution of the Kimberley region, Western Australia

The Marboo Formation is the oldest rock unit in the Hooper Complex in the West Kimberley region of Western Australia. The complex forms an elongate belt, 350 km long and at least 100 km wide, of deformed and metamorphosed Palaeoproterozoic low- to high-grade metasedimentary rocks, and mafic and felsic igneous rocks. The Marboo Formation consists of a sequence of turbiditic metasedimentary rocks derived from the northwest, an area of crust now buried beneath the unconformably overlying Speewah and Kimberley Basins. SHRIMP U–Pb dating of detrital zircons from a low-grade quartz wacke indicate that the provenance terrain contributed zircons dated at around 2500 to 2400 Ma, 2300 Ma, 2200 to 2050 Ma, 1970 Ma and 1910 Ma. The Mount Joseph Migmatite formed by in situ melting of the Marboo Formation, and zircons interpreted as detrital in origin, give ages of 2500 to 2000 Ma. A maximum depositional age is provided by a population of zircon cores dated at 1872±9 Ma. Growth of rims on zircons is dated at 1861±5 Ma, and is interpreted as the age of high-grade metamorphism. A minimum age for deposition is provided by the Richenda Microgranodiorite, which forms a high-level intrusive stock into the Marboo Formation, and is dated at 1858±5 Ma. Comparison with recent SHRIMP ages from the Halls Creek Group in the East Kimberley indicates that the turbidites of the Marboo Formation were deposited, deformed, metamorphosed, and intruded by granitoid before the majority of the turbiditic metasedimentary rocks in the Olympio Formation were deposited. This is not consistent with deposition of the turbiditic rocks in the same basin. The assumption of contemporaneous turbidite deposition throughout the Kimberley region has been taken as evidence of intracratonic Palaeoproterozoic orogeny. However, these results provide support for plate tectonic models and the recognition of tectonostratigraphic terranes within the Kimberley region. Detrital zircons from the Marboo Formation suggest that unexposed crust beneath the Speewah and Kimberley Basins is largely early Palaeoproterozoic in age rather than Archaean as previously assumed. The spread in ages indicates that the time period between formation of late Archaean basement in northern Australia and the formation of Palaeoproterozoic orogens was marked by ongoing crust formation and was not a period of quiessence.

Regional granulite facies metamorphism in the Ivrea zone: Is the Mafic Complex the smoking gun or a red herring?

One widely accepted paradigm for the development of continental lower crust is that regional granulite facies metamorphism is caused by intrusion of mafic magma beneath or into the crust (magmatic accretion). The amphibolite to granulite facies supracrustal section exposed in the Ivrea zone (southern Alps, northern Italy) is commonly cited as a classic example establishing this postulated genetic relationship. Our interpretation of the pattern of metamorphic isograds, compositional trends in high-grade metasedimentary rocks, and textural evidence in metapelite, however, indicates that final emplacement of the mafic plutonic rocks (the Mafic Complex) occurred subsequent to the regional thermal maximum. Field and petrographic relations suggest that a spatially restricted contact-melting event in crustal rocks accompanied the emplacement of the Mafic Complex. This inference is consistent with leucosome compositions in migmatites and a low-pressure, high-temperature metamorphic overprint recorded by mineral assemblages in wall rocks proximal to the intrusion. Therefore, evidence of anatexis and metamorphism of crustal rocks associated unequivocally with emplacement of the Mafic Complex is found only within an ~2-km-wide contact aureole overlying the intrusion. The narrow aureole associated with emplacement of the Mafic Complex demonstrates that, in some cases, emplacement of large volumes of mafic magma within the crust does not inexorably lead to regional-scale granulite facies metamorphism and large ion lithophile element depletion by melt loss.

Lithostratigraphic correlations in the western Himalaya—An isotopic approach

We present the results of a whole-rock Nd isotopic study of two contrasting regions of the western Himalaya, using the neodymium model age approach on the scale of a single orogen. High-grade metasedimentary rocks from Zanskar yield model ages ( T DM ) that are similar to those of the High Himalayan Crystalline Series ( T DM = 1.2–2.0 Ga; ϵ Nd = −6 to −16) and distinct from values from the Lesser Himalaya ( T DM = 2.3–3.4 Ga; ϵ Nd = −18 to −27). Hence these two lithological sequences can be recognized for 2000 km along the strike of the orogen. Data for the basement of the Nanga Parbat massif at the western extremity of the Himalaya ( T DM = 2.3–2.8 Ga; ϵ Nd = −18 to −30) suggest that these rocks are not equivalent to the High Himalaya, as previously supposed, but have affinities with the Lesser Himalaya. A thin metasedimentary cover sequence on the margins of the Nanga Parbat massif is isotopically indistinguishable from the High Himalaya ( T DM = 1.6–1.8 Ga; ϵ Nd = −10 to −14). The prior misidentification of the provenance of the massif stems from its high metamorphic grade, characteristic of the High Himalaya, but in this case related to the unique Neogene history of the Nanga Parbat massif, which has exhumed a higher-grade equivalent of the Lesser Himalaya that is not seen elsewhere.