Intercept Age Research Articles

Constraining timing and tectonic implications of Neoproterozoic metamorphic event in the Cathaysia Block, South China

The Cathaysia Block of the South China Craton includes a Proterozoic basement that experienced a prolonged Precambrian crustal evolution but to date lacks evidence of Proterozoic metamorphic ages. At Lichuan and Jianning, in the Wuyi Domain of the eastern Cathaysia Block, Proterozoic rock units include migmatized paragneiss of the Wanyuan Group and minor amphibolite of the Tianjingping Formation, which are enveloped by schist of Mayuan Group, and all are intruded by Paleozoic and Mesozoic igneous rocks. Detrital zircon grains from the Wanyuan paragneiss display metamorphic rims that yield concordant weighted average 206Pb/238U ages of 860±6Ma and 435±5Ma, along with variably disconcordant ages with lower intercept ages of 442±41Ma. The zircon core ages range from 3015Ma to 851Ma, with three major age populations at 930–865Ma, 1850–1200Ma and 2650–2400Ma. Detrital zircon grains from Mayuan schist samples at Jianning generally lack core-rim structures and yield three main age populations at 860–736Ma, 1835–1775Ma and 2720–2500Ma. Metamorphic ages of ca. 860Ma and ca. 435Ma for the Wanyuan paragneiss along with the youngest detrital zircon constrain the depositional age of the protolith to ca. 865–860Ma, whereas the Mayuan Group is younger and probably deposited after ca. 736Ma. Characteristics of detrital zircon age populations along with regional geological data suggest accumulation of the Wanyuan Group in a convergent and/or collisional setting. Metamorphism and a possible subduction-collision process within the Cathaysia Block at around 860Ma suggest it was not a unified block in early Neoproterozoic. The growth of ca. 440Ma metamorphic rims is likely related to granitic magmatism, such as that exposed in the Lichuan region. The sparse evidence for early Neoproterozoic metamorphism likely reflects widespread overprinting by the Paleozoic tectonothermal event at around 440Ma.

Geochronology and phase equilibria modelling of ultra‐high temperature sapphirine + quartz‐bearing granulite at Usilampatti, Madurai Block, Southern India

Sapphirine‐bearing granulite from Usilampatti in the Madurai block of southern India preserves a variety of mineral textures and reactions that help in reconstructing a three‐stage metamorphic evolution. Corroded biotite, sillimanite and quartz inclusions within garnet represent relics from the prograde history. Peak metamorphic conditions were attained with the development of sapphirine + quartz in textural equilibrium (Stage 1). This was followed by nearly isothermal decompression, leading to the formation of sapphirine + cordierite at Stage 2. Subsequent retrograde hydration (Stage 3) is only locally evident. Using the Perple_X software and the model system NCKFMASH, the peak P‐T conditions were estimated from core compositions, and the retrograde evolution was deduced from rim or symplectite compositions of different minerals as computed by isopleths of XMg garnet, XCa garnet, XMg orthopyroxene, XMg sapphirine and XMg biotite. The P‐T conditions for Stage 1 thus obtained, and supported by thermodynamic modelling using the winTWQ programme, is approximately 9 kbar and 940°C. Stage 2 conditions were constrained as 6.7 kbar and 900°C. Dating of zircon and monazite in the sapphirine‐bearing granulite and associated gneisses by the U‐Pb method using LA‐ICP‐MS indicates metamorphic overprint of zircon (lower intercept ages of discordant data arrays) at 546 ± 8 and 547 ± 11 Ma and metamorphic growth of monazite between 542 ± 3 and 551 ± 2 Ma. Upper intercept ages for zircon point to zircon growth at approximately 2514 ± 66 Ma. Although it remains unclear whether the metamorphic age data refer to Stage 1 or Stage 2 or, most likely, a continuum between both, they clearly document a late Ediacaran age for ultra‐high temperature (UHT) metamorphism in the area, which, based on the obtained P‐T path, was most likely the result of crustal thickening followed by uplift and erosion. Thus, it is concluded that the sapphirine‐bearing granulites formed in response to Pan‐African orogeny that led to the collision of the western and eastern Madurai domains, whereas initial zircon growth probably took place during late Neoarchaean arc magmatism that formed much of the western domain. Copyright © 2017 John Wiley & Sons, Ltd.

Palaeo‐Mesoproterozoic magmatic and metamorphic events from the Kuluketage block, northeast Tarim Craton: geochronology, geochemistry and implications for evolution of Columbia

Banded iron formations (BIFs), granites and diabases are extensively distributed in the Kuluketage block of the northeast Tarim Craton. Here we report laser ablation–inductively coupled plasma–mass spectrometry (LA‐ICP‐MS) zircon U–Pb ages and whole‐rock elemental data for these rocks. The detrital zircons from the BIFs show a peak age of 2.0–1.8 Ga with a weighted mean age of 1945 ± 10 Ma (MSWD = 0.77), and the zircons from the granite give an upper intercept age of 1974 ± 27 Ma (MSWD = 1.05). The trace element features suggest that the Asitingbulake granite, which belongs to the I‐type granite, may originate from the re‐melting of continental crust in a ~1.95 Ga collisional orogenic setting. Recently, the 2.0–1.8 Ga continuous magmatic and metamorphic events which are contemporaneous with the global orogenic event have been reported in the Kuluketage block and other massifs around the Tarim Craton (e.g. Central Tianshan, Dunhuang, Quanji, Altyn Tagh, West Kunlun). Based on the previous geochronological data, two 2.0–1.8 Ga orogens associated with the assembly of the Columbia supercontinent can be identified along the north and south margins of Tarim Craton: (1) the north Tarim Orogen, Central Tianshan–Kuluketage–Dunhuang orogenic belt and (2) the south Tarim Orogen, West Kunlun–Altyn Tagh–Aketashtage–Quanji orogenic belt. Additionally, this paper reports a new zircon U–Pb age of 1497 ± 21 Ma (MSWD = 0.96) from the Baowenbulake diabase dykes where the trace element features suggest that the parental magma of Baowenbulake diabases is derived from the mantle in an intra‐plate rifting or extensional setting. The ~1.5 Ga diabases from northwest Tarim corresponded to a major episode of mafic magmatism during the early Mesoproterozoic period identified in other crustal fragments of Laurentia, Siberia, Greater Congo, South China and North China Craton and probably belong to one of the three major large igneous provinces associated with the breakup of the Mesoproterozoic Columbia supercontinent. The data from this paper provide important constraints on the configuration of the Tarim Craton during the assembly and breakup of the Columbia supercontinent. Copyright © 2017 John Wiley & Sons, Ltd.

Petrology and Geochemistry of the Banded Iron Formation of the Kuluketage Block, Xinjiang, NW China: Implication for BIF Depositional Setting

AbstractThe Kuluketage block, located in the northeast Tarim craton, is one of the largest Precambrian blocks in the Xinjiang province. Recently, many banded iron formation (BIF)‐type (Superior‐type) deposits have been discovered in the western part of the Kuluketage block. These deposits occurred in the Paleoproterozoic Shayiti Formation, Xingditage Group, which has a nearly E–W distribution in the southern Xinger and Xingdi faults. Tremolite biotite schist and quartzite are the main wall rocks. The geochemical characteristics of schist indicate that the BIFs occurred in a passive continental margin environment. The LA–ICP–MS zircon 206Pb/238U ages of BIF and late syenite are 1945 ± 10 Ma(MSWD = 0.77) (weighted average age) and 1974 ± 27 Ma(MSWD = 1.05) (upper intercept age), respectively, indicating that the BIFs occurred in the Paleoproterozoic. In addition, the approximately 1.9 Ga magmatic and metamorphic events are consistent with the global‐scale 2.1–1.8 Ga collisional orogen events which are associated with the assembly of the Columbia supercontinent. The geochemical characteristics show that magnetite and quartz are dominant components (total content, 91.65–98.22 wt.%), and the Zr(Nb) and TiO2, Zr(Nb) and Al2O3 and Zr and Y/Ho display strongly positive correlations, illustrating the addition of crustal materials into the chemical precipitate of the original BIFs. The higher Zr, Nb and Al2O3 contents and a lower Y/Ho ratio of the Kuluketage BIFs indicate a higher terrigenous detrital component contaminant compared to BIFs of North China Craton (NCC). The rare earth and yttrium (REY) distribution patterns show a slight LREE enrichment and weak Eu positive anomaly features, indicating that the source of Fe and Si of the Kuluketage BIFs is mainly from the contribution of low‐temperature hydrothermal alteration of the oceanic crust. In addition, along with the decreasing BIF depositional age, the declining of Eu anomaly values reflects the increasing importance of low‐temperature hydrothermal solutions relative to high‐temperature hydrothermal solutions. Moreover, no Ce anomalies in studied BIFs, NCC and Xinyu BIFs are attributed to relative reducing environmental condition when the original BIFs precipitated.

K‐Ar Dating of Fault Gouges from the Red River Fault Zone of Vietnam

AbstractConstraining the timing of fault zone formation is fundamentally important in terms of geotectonics to understand structural evolution and brittle fault processes. This paper presents the first authigenic illite K‐Ar age data from fault gouge samples collected from the Red River Shear Zone at Lao Cai province, Vietnam. The fault gouge samples were separated into three grain‐size fractions (<0.1 µm, 0.1–0.4 µm and 0.4–1.0 µm). The results show that the K‐Ar age values decrease from coarser to finer grain fractions (24.1 to 19.2 Ma), suggesting enrichment in finer fraction of more‐recently grown authigenic illites. The timing of the fault movement are the lower intercept ages at 0% detrital illite (19.2 ± 0.92 Ma and 19.4 ± 0.49 Ma). In combination with previous geochronological data, this result indicates that the metamorphism of the Day Nui Con Voi (DNCV) metamorphic complex took place before ca. 26.8 Ma. At about 26.8 Ma‐25 Ma, the fault strongly acted to cause the rapid exhumation of the rocks along the Red River‐Ailoa Shan Fault Zone (RR‐ASFZ). During brittle deformation, the DNCV slowly uplifted, implying weak movement of the fault. This brittle deformation might have lasted for ca. 5 Ma.

U–Pb dating of zircon and cassiterite from the Early Cretaceous Jiaojiguan iron-tin polymetallic deposit, implications for magmatism and metallogeny of the Tengchong area, western Yunnan, China

ABSTRACTThe newly discovered Jiaojiguan deposit, a medium-scale skarn iron-tin polymetallic deposit on the Sino-Burma boundary of Yunnan Province (SW China), is spatially associated with the biotite monzonitic granite. Here, we report new in situ zircon LA-MC-ICP-MS U–Pb ages, trace element and Hf isotope data from the granite, and U–Pb dating ages of cassiterite from the ore bodies. In this study, we obtain a weighted mean 206Pb/238U age of 124.1 ± 1.4 Ma for the zircon and a 207Pb/206Pb-238U/206Pb intercept age of 123.8 ± 2.2 Ma for the cassiterite. The granite crystallized during the Early Cretaceous, with zircons exhibiting εHf(t) values from −5.8 to −0.6 and two-stage Hf model ages (TDM2) of 1.21–1.54 Ga. The close temporal and spatial links between pluton emplacement and ore-forming events suggest that magmatic-hydrothermal events were the key factors that triggered the genesis of the iron-tin polymetallic deposits in the area. Regional geochronological data show that tin mineralization took place three times during the Cretaceous–Palaeogene in the Tengchong block due to re-melting of the underlying supposed Proterozoic (1.5 ± 0.5 Ga) Sn-rich strata/materials. Compared with those in the Bangong–Nujiang metallogenic belt (BNMB), we propose that the Cretaceous iron-tin polymetallic mineralization events in Tengchong–Baoshan closely resemble those of the Bangong–Nujiang belt in northern Tibet, both of which have experienced similar tectono-magmatic-metallogenic histories since the Mesozoic.

Matrix-Matched Iron-Oxide Laser Ablation ICP-MS U–Pb Geochronology Using Mixed Solution Standards

U–Pb dating of the common iron-oxide hematite (α-Fe2O3), using laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS), provides unparalleled insight into the timing and processes of mineral deposit formation. Until now, the full potential of this method has been negatively impacted by the lack of suitable matrix-matched standards. To achieve matrix-matching, we report an approach in which a U–Pb solution and ablated material from 99.99% synthetic hematite are simultaneously mixed in a nebulizer chamber and introduced to the ICP-MS. The standard solution contains fixed U- and Pb-isotope ratios, calibrated independently, and aspiration of the isotopically homogeneous solution negates the need for a matrix-matched, isotopically homogenous natural iron-oxide standard. An additional advantage of using the solution is that the individual U–Pb concentrations and isotope ratios can be adjusted to approximate that in the unknown, making the method efficient for dating hematite containing low (~10 ppm) to high (>1 wt %) U concentrations. The above-mentioned advantage to this solution method results in reliable datasets, with arguably-better accuracy in measuring U–Pb ratios than using GJ-1 Zircon as the primary standard, which cannot be employed for such low U concentrations. Statistical overlaps between 207Pb/206Pb weighted average ages (using GJ-1 Zircon) and U–Pb upper intercept ages (using the U–Pb mixed solution method) of two samples from iron-oxide copper-gold (IOCG) deposits in South Australia demonstrate that, although fractionation associated with a non-matrix matched standard does occur when using GJ-1 Zircon as the primary standard, it does not impact the 207Pb/206Pb or upper intercept age. Thus, GJ-1 Zircon can be considered reliable for dating hematite using LA-ICP-MS. Downhole fractionation of 206Pb/238U is observed to occur in spot analyses of hematite. The use of rasters in future studies will hopefully minimize this problem, allowing for matrix-matched data. Using the mixed-solution method in this study, we have validated a published hematite Pb–Pb age for Olympic Dam, and provide a new age (1604 ± 11 Ma) for a second deposit in the same province. These ages are further evidence that the IOCG mineralizing event is tied to large igneous province (LIP) magmatism in the region at ~1.6 Ga.

The 1.0 Ga S–type granite in the East Kunlun Orogen, Northern Tibetan Plateau: Implications for the Meso– to Neoproterozoic tectonic evolution

The East Kunlun Orogen (EKO) is characterized by widely distributed granitoids with different ages, which are keys to understanding the tectonic evolution of the Central China Orogenic Belt. Zircon U–Pb ages and Hf isotopic compositions, as well as the whole rock geochemistry of the gneissic granite from the basement rock, are carried out to elucidate the Meso– to Neoproterozoic tectonics of the EKO. The Al–rich minerals, including muscovite and tourmaline, and the A/CNK ratios (1.07–1.18) indicate S–type affinity of the granite. The granite displays high SiO2 and K2O contents, and slightly enrichment in LREE in chondrite normalized REE distribution pattern with strong negative Eu anomalies (δEu=0.10–0.15). The samples exhibit positive anomalies of Rb, Th, U and Pb, and depletion of Ba, Nb, Ta, Sr and Ti. Meanwhile, the high Rb/Ba and Rb/Sr ratios and low (CaO+FeO+MgO+TiO2) contents indicate that they were derived from pelitic material. Together with the negative εHf(t) values ranging from −5.97 to −2.34 and two–stage Hf model ages varying from 1968 to 1786Ma, the gneissic granite is suggested being originally derived from partial melting of the metasedimentary rocks of the Paleoproterozoic Jinshuikou Group in the central EKO due to the crust thickening. Most zircon grains from the gneissic granite show typical magmatic zircon morphology, and yield an U–Pb upper intercept age of 1006±20Ma (MSWD=1.5), representing the crystallization age. Integrated with the regional geology, our results suggest that the EKO has been probably involved into a Meso– to Neoproterozoic plate collisional event related to the assembly of the Rodinia supercontinent.

New LA-ICPMS U–Pb ages of detrital zircons from the Highland Complex: insights into late Cryogenian to early Cambrian (ca. 665–535 Ma) linkage between Sri Lanka and India

ABSTRACTHere we report new LA-ICPMS U–Pb zircon geochronology of ultrahigh temperature (UHT) metasedimentary rocks and associated crystallized melt patches, from the central Highland Complex (HC), Sri Lanka. The detrital zircon 206Pb/238U age spectra range between 2834 ± 12 and 722 ± 14 Ma, evidencing new and younger depositional ages of sedimentary protoliths than those known so far in the HC. The overgrowth domains of zircons in these UHT granulites yield weighted mean 206Pb/238U age clusters from 665.5 ± 5.9 to 534 ± 10 Ma, identified as new metamorphic ages of the metasediments in the HC. The zircon ages of crystallized in situ melt patches associated with UHT granulites yield tight clusters of weighted mean 206Pb/238U ages from 558 ± 1.6 to 534 ± 2.4 Ma. Thus, using our results coupled with recently published geochronological data, we suggest a new geochronological framework for the evolutionary history of the metasedimentary package of the HC. The Neoarchean to Neoproterozoic ages of detrital zircons indicate that the metasedimentary package of the HC has derived from ancient multiple age provenances and deposited during the Neoproterozoic Era. Hence, previously reported upper intercept ages of ca. 2000–1800 Ma from metaigneous rocks should be considered as geochronological evidence for existence of a Palaeoproterozoic igneous basement which possibly served as a platform for the deposition of younger supracrustal rocks, rather than timing of magmatic intrusions into the already deposited ancient sediments, as has been conventionally interpreted. The intense reworking of entire Palaeoproterozoic basement rocks in the Gondwana Supercontinent assembly may have caused sediments of multiple ages and provenances to incorporate within supra-crustal sequences of the HC. Further, our data supports a convincing geochronological correlation between the HC of Sri Lanka and the Trivandrum Block of Southern India, disclosing the Gondwanian linkage between the HC of Sri Lanka and Southern Granulite Terrain of India.

Melt-fluid infiltration in Archean suprasubduction zone mantle wedge: Evidence from geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes from Wynad, southern India

The southern Peninsular India preserves a number of Archean cratonic nuclei among which the Coorg Block is considered to represent a Mesoarchean microcontinent. Here we investigate the southern margin of this block where remnants of rare ultramafic rocks in association with arc magmatic rocks and accreted metasediments occur in the Wynad region. We present petrology, mineral chemistry, whole rock geochemistry and zircon U–Pb and Lu–Hf data from a suite of meta ultramafics and associated diorite, orthopyroxene-bearing felsic charnockite, and quartz-mica schist. The peak P–T conditions of metamorphism of these rocks are estimated at 720–840°C and 6.5–8.7kbar. The geochemical features of the ultramafic suite provide evidence for melting and metasomatism of lithospheric mantle in a suprasubduction zone (SSZ) setting with the involvement of both silicate melts and slab-dehydrated fluids. The parent melts were generated at varying depth levels extending from spinel to garnet stability fields. A subduction-modified, LREE replenished, metasomatically enriched mantle source endorses arc-like chemical signatures in the melts that were acquired through interaction between the depleted sub-arc lithospheric mantle and subduction components.We report the common presence of magmatic zircons in the meta ultramafic rocks with crystallization ages of 3.3–3.0Ga. Some of these rocks also carry another younger group of magmatic zircons with crystallization ages down to ca. 2.5Ga. Zircons in the orthopyroxene-bearing felsic charnockite also formed at 3.2–3.0Ga whereas those in the metadiorite crystallized at around 2.6–2.5Ga. The detrital zircons in the quartz mica schist show upper intercept age of 3.3Ga, and lower intercept at 2.5Ga. The Mesoarchean ages from the Wynad suite are consistent with the ages reported from the Coorg Block, whereas the Neoarchean ages correspond with those in the Nilgiri Block to the south. The Lu–Hf data from zircon grains in the various samples show both negative and positive εHf(t) values ranging from −18.23 to 13.98. The data yield Hf depleted mantle model ages (TDM) of 2562–3837Ma and Hf crustal residence model ages (TDMC) ranging from 2576Ma to 4009Ma, indicating magma derivation from both juvenile and reworked components of Neohadean to early-mid Archean age. Our results reveal a suprasubduction zone mantle wedge that has been repeatedly replenished through multiple events of subduction zone magmatism in the Mesoarchean and Neoarchean.

New evidence for Palaeoproterozoic high grade metamorphism in the Trivandrum Block, Southern India

In-situ zircon U–Pb isotopic and monazite Th–U–Pb chemical ages from leucosome in migmatitic paragneiss at Kanjampara in the Trivandrum Block, India, demonstrate the occurrence of high-T metamorphism and anatexis at 1.92Ga. This Palaeoproterozoic tectonothermal event was strongly overprinted by the main Neoproterozoic-Cambrian granulite facies tectonism which itself involved significant partial melting and formation of garnet-bearing assemblages at 0.65GPa and >820–860°C. Monazite grains, which enclose zircon grains and locally preserve Palaeoproterozoic chemical age domains, were extensively reset and recrystallized at 565±6Ma. These monazite grains were further modified to form high-Th cuspate rims at 517±15Ma, equivalent to the lower intercept age for extensive Pb-loss from the highly discordant 1.92Ga zircon, 528±18Ma. These results confirm that at least some of the metasedimentary paragneisses in the Trivandrum Block are polymetamorphic, initially metamorphosed in the Palaeoproterozoic in an event older than the 1.89–1.85Ga granitic orthogneisses recognised from the area. The nature of the relationship between the paragneisses and older (2.0–2.1Ga) granitic and charnockitic orthogneisses of the Trivandrum and Nagercoil Blocks, in particular whether they share the same Palaeoproterozoic history or were interleaved after 1.92Ga, requires further investigation focussed on the presence or absence of the isotopic imprint of this event in both rock suites throughout the area.The growth or recrystallisation of monazite at ca. 565Ma and its further modification at ca. 520Ma indicates that the high-T metamorphism in the Pan-African was long-lived, with a duration of at least 45Myr. This adds further weight to recent proposals, based on zircon-monazite isotopic studies from other localities in the Trivandrum Block, that the region formed the mid- to deep-crust of a long-lived collisional hot orogenic belt that welded Gondwana from ca. 580–510Ma.

A-type stratoid granites of Madagascar revisited: Age, source and links with the breakup of Rodinia

The stratoid granites of Madagascar are sheet-like granite sills emplaced conformably to the schistosity of the Antananarivo and Ikalamavony domains of the Precambrian basement. Most of them are fine-grained rocks with A-type affinities. They belong to two different plutonic suites: the Kiangara Suite north of Antananarivo and the Imorona–Itsindro Suite recognized in the whole central Madagascar. Stratoid granites are everywhere characterized by the same structural pattern evidencing two deformation events. The first event (D1 characterized by foliations mildly dipping to the west and lineations trending WSW) was coeval with synkinematic magma emplacement. The second one, responsible for local reworking in ductile conditions, is regarded as the result of late Pan-African transcurrent tectonics along N–S striking shear zones. New in situ U–Pb zircon dating from both suites yields similar results evidencing two group of ages: upper intercept ages of 790–780Ma regarded as crystallization ages, and younger ages of ca. 555Ma corresponding to a metamorphic overprint. These new data suggest that both plutonic suites in central Madagascar were emplaced at 790–780Ma. New Sm–Nd isotopic data from the Kiangara Suite support the existence of two granite groups. The first group has εNd(t) at −15 and is likely derived from partial melting of the late Archaean crust. The second group made of granites with a more pronounced A-type nature and comagmatic syenites has εNd(t) at −12 evidencing the contribution of mantle melts. Mafic enclaves with εNd(t) ranging from −5 to −11 encompass the Nd isotopic signature of coeval Itsindro gabbros, which are regarded as derived from a subcontinental lithospheric mantle source. The interpretation of both structural and isotopic data is consistent with crustal thinning and extension during a Cryogenian Rodinia rifting stage at 790–780Ma, that left a major imprint in central Madagascar. Separation from a conjugage margin represented by the North China craton is suggested. The Pan-African influence is limited to reheating, tectonic reworking and fluid transfer in the vicinity of late-Neoproterozoic (∼550Ma) shear zones.

Floresta and Bodocó Mafic–Ultramafic Complexes, western Borborema Province, Brazil: Geochemical and isotope constraints for evolution of a Neoproterozoic arc environment and retro-eclogitic hosted Ti-mineralization

The retrograde high-pressure metamorphosed Floresta and Bodocó mafic–ultramafic complexes (FMUC and BMUC, respectively) occur as clusters of small bodies, lenses and boudins with close association with granitic, gneissic rocks and metasediments in the southern limits of the Alto Pajeú and Piancó–Alto Brígida domains, Borborema Province, northeast Brazil. Ultramafic lithologies including metadunites, olivine cumulate rocks, metapyroxenites, chromitites and massive ilmenomagnetitites showing evidence of cumulus textures and mafic members include garnet amphibolites and coarse metahornblendites. BMUC rocks include Cr–Fe–Ti chromitites in the Fazenda Esperança deposit, and FMUC rocks show important Fe–Ti–V economic mineralization in the mines of the Serrote das Pedras Pretas and Lagoa dos Angicos and the Riacho da Posse deposit in development (54 Mt @13% Ti). The whole-rock and mineral geochemistry of these complexes are akin to arc cumulate rocks, particularly Alaskan-type complexes, which are widely considered to have formed above subduction zones.A coarse metahornblendite from the Serrote das Pedras Pretas mine was dated by U–Pb zircon through laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). Information obtained from the cores of igneous zircons represent an upper intercept age of crystallization at 1024±18Ma with 4.2 mean square weighted deviation (MSWD), whereas zircon exhibiting coarse rims with low Th/U at 0.008–0.038 have concordia ages of 685±17 and 625±6Ma. According to their zircon textures, these ages may be representative of a high-grade eclogitic or progressive granulitic/retro-eclogitic stage during the Pan-African–Brasiliano event. Additionally, the age of the igneous protolith, 1028±63Ma, is well marked by a Sm–Nd isochron based on 12 samples.Positive ɛNd(t) values between 0.01 and 4.84 suggest a depleted mantle or a source close to the chondritic reservoir for the mafic–ultramafic bodies studied. The TDM model ages between 1.1 and 1.6Ga are consistent with a juvenile Mesoproterozoic source or mixing between Paleoproterozoic and Neoproterozoic sources. The dispersion of initial ratios of 87Sr/86Sr at 0.7016–0.7090 and the high fractionation of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) suggest a depleted mantle source metasomatized by subduction-related melts/fluids.The mafic–ultramafic complexes, volcanic and sedimentary rocks, and some granitic intrusions with post-collisional signature after 975Ma constitute a proto to marginal arc-back-arc basin system that evolved between 1025 and 975Ma. However, FMUC and BMUC which are currently arc cumulate rocks were originally picrite melts in a suprasubduction ophiolite zone.The economic deposits of Fe–Ti associated with retro-eclogitic and ultramafic members of the FMUC and enclosed within the late Ediacaran granite can be linked in part by early accumulation under magmatic conditions, secondary enrichment of rutile up to 12% in the high-pressure (HP) stage, and hydrothermal interactions during granite emplacement.

Neoarchean–Early Paleoproterozoic and Early Neoproterozoic arc magmatism in the Lützow–Holm Complex, East Antarctica: Insights from petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes

The Lützow–Holm Complex (LHC) of East Antarctica forms part of the Neoproterozoic–Cambrian high-grade metamorphic segment of the East African–Antarctic Orogen. Here we present new petrological, geochemical, and zircon U–Pb and Lu–Hf isotopic data for meta-igneous rocks including charnockite, felsic gneiss, metagabbro, and mafic granulite from the LHC and evaluate the Neoarchean to Early Paleoproterozoic (ca. 2.5Ga) and Early Neoproterozoic (ca. 1.0Ga) arc magmatic events. The trace element geochemical signatures reveal a volcanic arc affinity for the charnockites from Sudare Rocks and Vesleknausen and felsic gneiss from Rundvågshetta, suggesting that the protoliths of these rocks were derived from felsic arc magmas. In contrast, metagabbros from Skallevikshalsen and Austhovde, occurring as boudins in metasediments, show non-arc signatures (within-plate basalt or mid-oceanic ridge basalt). The upper intercept ages of magmatic zircons in charnockite plotted on concordia diagrams yielded 2508±14Ma (Sudare Rocks) and 2490±18Ma (Vesleknausen), clearly suggesting a Neoarchean to Early Paleoproterozoic arc magmatic event. A subsequent thermal event during Early Neoproterozoic traced by 206Pb/238U age of oscillatory-zoned core of zircon in mafic granulite from Langhovde (973±10Ma) is consistent with a similar Early Neoproterozoic magmatic event reported from the LHC, suggesting a second stage of arc magmatism. The timing of peak metamorphism has been inferred from 206Pb/238U mean ages of structureless zircons in metagabbros from Skallevikshalsen and Austhovde, mafic granulite from Langhovde, and felsic gneiss from Rundvågshetta in the range of 551±5.4 to 584±5.0Ma. Zircon Lu–Hf data of Neoarchean charnockites from Sudare Rocks and Vesleknausen indicate that the protolith magma was sourced from Paleo- to Neoarchean juvenile components mixed with reworked ancient crustal materials. Protolith magmatic rock of the felsic gneiss from Rundvågshetta might have been sourced from reworked Paleoarchean crust. In contrast, metagabbros from Skallevikshalsen and Austhovde were derived from the Paleoproterozoic reworked crust. The results of this study suggest the occurrence of several discrete crustal fragments of ca. 2.5Ga and ca. 1.0Ga juvenile magmatic arcs in the southwestern part of the LHC, which amalgamated during the latest Neoproterozoic collisional event related to the final stage of assembly of the Gondwana supercontinent.

Mesoproterozoic suturing of Archean crustal blocks in western peninsular India: Implications for India–Madagascar correlations

The Kumta and Mercara suture zones welding together Archean crustal blocks in western peninsular India offer critical insights into Precambrian continental juxtapositions and the crustal evolution of eastern Gondwana. Here we present the results from an integrated study of the structure, geology, petrology, mineral chemistry, metamorphic P–T conditions, zircon U–Pb ages and Lu–Hf isotopes of metasedimentary rocks from the two sutures. The dominant rocks in the Kumta suture are greenschist- to amphibolite-facies quartz–phengite schist, garnet–biotite schist, chlorite schist, fuchsite schist and marble. The textural relations, mineral chemistry and thermodynamic modelling of garnet–biotite schist from the Kumta suture indicate peak metamorphic P–T conditions of ca. 11kbar at 790°C, with detrital SHRIMP U–Pb zircon ages ranging from 3420 to 2547Ma, εHf (t) values from −9.2 to 5.6, and TDMc model ages from 3747 to 2792Ma. The K–Ar age of phengite from quartz–phengite schist is ca. 1326Ma and that of biotite from garnet–biotite schist is ca. 1385Ma, which are interpreted to broadly constrain the timing of metamorphism related to the suturing event. The Mercara suture contains amphibolite- to granulite-facies mylonitic quartzo-feldspathic gneiss, garnet–kyanite–sillimanite gneiss, garnet–biotite–kyanite–gedrite–cordierite gneiss, garnet–biotite–hornblende gneiss, calc-silicate granulite and metagabbro. The textural relations, mineral chemistry and thermodynamic modelling of garnet–biotite–kyanite–gedrite–cordierite gneiss from the Mercara suture indicate peak metamorphic P–T conditions of ca. 13kbar at 825°C, followed by isothermal decompression and cooling. For pelitic gneisses from the Mercara suture, LA-ICP-MS U–Pb zircon ages vary from 3249 to 3045Ma, εHf (t) values range from −18.9 to 4.2, and TDMc model ages vary from 4094 to 3314Ma. The lower intercept age of detrital zircons in the pelitic gneisses from the Mercara suture ranges from 1464 to 1106Ma, indicating the approximate timing of a major lead-loss event, possibly corresponding to metamorphism, and is broadly coeval with events in the Kumta suture. Synthesis of the above results indicates that the Kumta and Mercara suture zones incorporated sediments from Palaeoarchean to Mesoproterozoic sources and underwent high-pressure metamorphism in the late Mesoproterozoic. The protolith sediments were derived from regions containing juvenile Palaeoarchean crust, together with detritus from the recycling of older continental crust. Integration of the above results with published data suggests that the Mesoproterozoic (1460–1100Ma) Kumta and Mercara suture zones separate the Archean (3400–2500Ma) Karwar–Coorg block and Dharwar Craton in western peninsular India. Based on regional structural and other geological data we interpret the Kumta and Mercara suture zones as extensions of the Betsimisaraka suture of eastern Madagascar into western India.

Zircon U-Pb geochronology, Lu-Hf isotope systematics, and geochemistry of bimodal volcanic rocks and associated granitoids from Kotri Belt, Central India: Implications for Neoarchean–Paleoproterozoic crustal growth

The Bastar Craton of Central India has a thick sequence of volcano–sedimentary rocks preserved in Kotri–Dongargarh belt that developed on a tonalite-trondhjemite-granodiorite (TTG) basement followed upwards by the Amgaon, Bengpal, Bailadila, and Nandgaon Groups of rocks. Here, we report the U-Pb geochronology and Lu-Hf isotope systematics and whole rock geochemistry of volcanic rocks and associated granitoids belonging to the Pitepani basalts, Bijli rhyolites, and Dongargarh granite in the Nandgaon Group of the Kotri belt. The volcanic rocks of the Nandgaon Group are bimodal in nature in which the basalts exhibit intergranular, porphyritic to spherulitic texture composed of pyroxenes, plagioclase, tremolite, actinolite, and chlorite±Fe oxides. The rhyolites display porphyritic texture consisting of K-feldspar, quartz, and plagioclase as phenocrysts. The associated porphyritic granitoids have K-feldspar, microcline, plagioclase, and biotite phenocrysts within a groundmass of similar composition. The bimodal suite displays LILE, LREE enrichment, and HFSE depletion with significant negative Nb-Ta anomalies combined with slightly fractionated REE patterns in the basalts and highly fractionated patterns and prominent negative Eu anomalies in the rhyolites endorsing their generation in an island-arc/back-arc tectonic setting. The geochemical features of the associated granitoids indicate that these are potassic and classify as within-plate A-type granites. Zircons from the basalts show clear oscillatory zoning in their CL images. They cluster as a coherent group with 207Pb/206Pb spot ages ranging from 2446 to 2522Ma and weighted mean age of 2471±7Ma. Zircons from the rhyolite samples are subhedral to euhedral and show simple oscillatory zoning with some heterogeneous fractured domains. The data from two samples define upper intercept ages of 2479±13Ma and 2463±14Ma. Zircon grains in the granite show clear oscillatory zoning and their U-Pb data define an upper intercept age of 2506±50Ma. The Lu-Hf isotopic data on the zircons from the basalts show initial 176Hf/177Hf ratios from 0.280925 to 0.281018. Their εHf(t) values are in the range of −10.0 to −6.7. The Hf-depleted model ages (TDM) are between 3038Ma and 3171Ma, and Hf crustal model ages (TDMC) vary from 3387–3589Ma. The zircons from the rhyolites show initial 176Hf/177Hf ratios from 0.280919 to 0.281020 and from 0.281000 to 0.281103, respectively, with εHf(t) values varying from −10 to −6.4 and from −7.5 to −3.9. Among these, one sample shows TDM between 3038Ma and 3182Ma, and TDMC varies from 3377 to 3596Ma, whereas the other sample shows ages of 2925Ma and 3072Ma with TDMC varying from 3208 to 3432Ma. The initial 176Hf/177Hf ratios of the granites range from 0.280937 to 0.281062 with εHf(t) values of −8.8 to −4.3. The TDM shows a range of 2979Ma and 3170Ma, and TDMC varies from 3269 to 3541Ma. The predominant negative εHf(t) values of zircons from these rocks suggest that the source material was evolved from the Paleoarchean crust. The geological, geochemical, and geochronological evidence suggests coeval tectonic and magmatic episodes of volcanic and plutonic activity in an island-arc setting where the arc migrated toward the continental margin and played a significant role in the Neoarchean–Paleoproterozoic crustal growth of the Kotri belt of Central India.

Age, tectonic evolution and origin of the Aswa Shear Zone in Uganda: Activation of an oblique ramp during convergence in the East African Orogen

The Aswa Shear Zone (ASZ) is a major NW–SE trending structure of over 1000 km length in East Africa. In Uganda, the ASZ is a steeply NE-dipping, up to 11 km wide mylonitic shear zone that shows multiple stage brittle reactivation. On outcrop-scale, the fabric in the ASZ is characterized by a well-developed NW–SE striking and subvertical or steeply NE or SW dipping mylonitic foliation and a subhorizontal to moderately NW- or SE-plunging stretching lineation. Sinistral kinematics and fabric are very consistent along strike. The strain is heterogeneously distributed and partitioned into lens-shaped lower strain zones dominated by folding and characterized by pure shear, which are surrounded by high strain zones, some of them thick ultramylonites, with intense simple shear combined with flattening and strong transposition of pre-existing fabrics. Ductile shearing occurred during bulk E–W shortening, commenced at amphibolite facies conditions and continued with similar kinematics at greenschist and even lower grade conditions. A number of (sub-)parallel shear zones occur to the NE and SW of the main zone at a distance of up to 20–45 km. They show similar fabrics and kinematics and are thus related to activity along ASZ reflecting strain partitioning into simple shear and pure shear domains on a regional scale.Samples of mylonitic gneisses from the shear zone have been analyzed with U–Pb LA-MC-ICPMS and show Neoarchaean crystallisation ages between 2.66 and 2.61 Ga. Timing of ductile sinistral shearing is poorly constrained by lower intercept ages of 686 ± 62 and 640 ± 44 Ma. The fabric and structural relationship of the ca. 660 Ma Adjumani Granite exposed in the northern segment of ASZ suggest that the age of shear activity can be further limited to ca. 685 and 655 Ma.The Aswa Shear Zone is interpreted as an intra-cratonic, crustal-scale structure close to the northeastern margin of the Congo Craton, possibly inherited from previous continental extension. Early Aswa Shear Zone activation is linked to underthrusting of the Congo Craton and coeval high-grade metamorphism and intense deformation in the orogen interior. During E–W convergence between ca. 690 and 650 Ma, the NE-dipping ASZ was activated as an oblique ramp leading to deflection of the transport direction and concentration of non-coaxial strain and sinistral shear along the shear zone system. During progressive convergence, between ca. 645 and 620 Ma, sinistral shearing along ASZ changed to ductile–brittle deformation mechanisms, while thrusting took place in Pan-African belts in eastern and western Uganda. Late-orogenic brittle sinistral reactivation of the ASZ can be regarded as the result of continent collision and closure of the Mozambique ocean further to the east, that potentially caused lateral escape manifested in NW–SE striking sinistral shear zones in Kenya and the southern Arabina-Nubian Shield between 620 and 570 Ma.

Further evidence of 777 Ma subduction-related continental arc magmatism in Eastern Dom Feliciano Belt, southern Brazil: The Chácara das Pedras Orthogneiss

In this study new SHRIMP U–Pb zircon data for the Chácara das Pedras Gneiss in Porto Alegre, southern Brazil are presented. They represent a small exposure of the crust which was intruded by a large volume of orogenic to anorogenic granitoids at ca. 618-562 m.y. in the Eastern Domain of the Dom Feliciano Belt. The Chácara das Pedras tonalitic orthogneiss has geochemical similarities with subduction-related magmatic rocks of continental arcs. They present high Sr initial ratios (∼0.712), negative ɛNd(t = 777) values (∼−6), TDM varying from 1.8 to 2.0 Ga. The igneous protoliths of these orthogneisses were previously considered to be Paleoproterozoic based on an upper intercept age of discordant zircon analyses. In the present study these orthogneisses were re-sampled and re-analyzed in an attempt to obtain more concordant analytical data. The U–Pb zircon analyses were carried out using the SHRIMP IIe at the Laboratório de Geocronologia de Alta Resolução of the Universidade de São Paulo. The U–Pb concordia age obtained for igneous textural domains of the zircon grains is 777 ± 4 Ma. A few analyses on zircon overgrowths give poorly defined late Cryogenian ages of ca. 650 Ma. Older ages, mostly discordant, were obtained in a few zircon cores, showing an upper intercept age of ca. 1.9 Ga. One sample of the Três Figueiras Granodiorite, which crosscut the orthogneiss in the same outcrop, was also investigated. The zircons of this granodiorite are, however, mostly metamitic, preventing the determination of a reliable age. Some concordant analyses from a few grains define ages ranging in the interval between ca. 603 and 1022 Ma. The youngest (ca. 603 Ma) may represent a maximum age for the granodiorite crystallization. Older ages, with discordance <10%, are of 745, 777, 836 and 1022 Ma. The 777 ± 4 Ma age obtained for the Chácara das Pedras orthogneiss is the first Early Cryogenian magmatic age determined for granitoids in the Porto Alegre region, although records of similar ages (780–800 Ma) are recognized within the Eastern Domain of the Dom Feliciano Belt. Examples are the Piratini Gneisses (Piratini region) and in the Cerro Bori Orthogneisses (Rocha region, Uruguay). All these units show subduction-related continental arc affinities. The identification of a magmatic event of ca. 780–800 Ma in the Porto Alegre region implies that: (i) the collisional magmatism (650 Ma) is ca. 130 Ma younger than the subduction-related magmatism; (ii) the younger orogenic (arc related to post-collisional) and anorogenic granitoids (<618 Ma) of Eastern Domain were emplaced in a hot/weak lithosphere due to the presence of an earlier ca. 780–800 Ma magmatic arc rocks; (iii) the tectonic evolution of this region in West Gondwana suggests a more complex geotectonic scenario than those put forward by previous models involving the amalgamation of the Río de La Plata and Kalahari cratons.

High-grade metamorphism during Archean–Paleoproterozoic transition associated with microblock amalgamation in the North China Craton: Mineral phase equilibria and zircon geochronology

Metamorphic regimes in Archean terranes provide important keys to the plate tectonic processes in early Earth. The North China Craton (NCC) is one of the ancient continental nuclei in Asia and recent models propose that the cratonic architecture was built through the assembly of several Archean microcontinental blocks into larger crustal blocks. Here we investigate garnet- and pyroxene-bearing granulite facies rocks along the periphery of the Jiaoliao microcontinental block in the NCC. The garnet-bearing granulites contain peak mineral assemblage of garnet+clinopyroxene+orthopyroxene+magnetite+plagioclase+quartz±biotite±ilmenite. Mineral phase equilibria computations using pseudosection and geothermobarometry suggest peak P–T condition of 800–830°C and 7–8kbar for metamorphism. Isopleths using XMg of orthopyroxene and XCa of garnet in another sample containing the peak mineral assemblage of garnet+orthopyroxene+quartz+magnetite±fluid yield peak P–T conditions of 860–920°C and 11–14kbar. Geochemical data show tonalitic to granodioritic composition and arc-related tectonic setting for the magmatic protoliths of these rocks. Zircon LA-ICP-MS analyses yield well-defined discordia with upper intercept ages of 2562±20Ma (MSWD=0.94) and 2539±21Ma (MSWD=0.59) which is correlated with the timing of emplacement of the magmatic protolith. A younger group of zircons with upper intercept ages of 2449±41Ma (MSWD=0.83); N=6 as 2449±41Ma (MSWD=0.83; N=6) and 2480±44Ma (MSWD=1.2; N=9) constrains the timing of metamorphism. Zircon Lu–Hf data show dominantly positive εHf(t) values (up to 8.5), and yield crustal residence ages (TDMC) in the range of 2529 to 2884Ma, suggesting magma sources from Meso-Neoarchean juvenile components. The high temperature and medium to high pressure metamorphism is considered to have resulted from the subduction–collision tectonics associated with microblock amalgamation in the NCC at the end of Archean. Together with the evidence for high pressure metamorphism from an adjacent locality, our results correlate with models that predict paired metamorphism at the Archean–Proterozoic transition with the onset of modern style plate tectonics.

Discovery of Neoarchean suprasubduction zone ophiolite suite from Yishui Complex in the North China Craton

The Archean tectonic realm of the North China Craton (NCC) is considered in recent models as a collage of several microblocks which were amalgamated along zones of ocean closure during late Neoarchean. Here we report the finding of a dismembered ophiolite suite from the southern margin of the Jiaoliao microblock in the interior of the unified Eastern Block of the NCC. The suite is composed of lherzolite, pyroxenite, noritic and hornblende gabbro, and hornblendite intruded by veins and sheets of leuco granite. Together with transposed layers and bands of metavolcanics and amphibolites, banded iron formation (BIF), and diabase dykes in the adjacent locations, the Yishui complex corresponds well with a dismembered suprasubduction zone ophiolite suite. Clinopyroxene in the pyroxenite and gabbroic rocks is Mg rich and range in composition from augite to diopside. Among orthopyroxenes, those in lherzolite show the highest XMg of 0.84–0.85. Plagioclase in hornblende gabbro shows high anorthite content (An50–64). Calcic amphiboles in the gabbroic rocks range in composition from ferropargasite to ferro-edenite, edenite and pargasite. Spinel inclusions in lherzolite are Cr-rich magnesiospinel. Geochemically, the mafic rocks from Yishui complex show subalkaline basaltic source, whereas the granitoids show volcanic arc affinity. The hornblende gabbro and gabbro, lherzolite and hornblendite show compositional similarity to E-MORB and N-MORB respectively. The lherzolite and hornblendite possess arc-related ultramafic cumulate nature, with overall features straddling the fields of IAT and IAT-MORB. The geochemical features are consistent with evolution in a suprasubduction regime with no significant crustal contamination. The majority of zircon grains in the Yishui suite exhibit magmatic texture and high Th/U ratios. Zircon grains from hornblendite define 207Pb/206Pb upper intercept age of 2538±30Ma. Zircons from the granite show ages of 2538±16Ma and 2503±21Ma, and those from the gabbros yield ages of 2503±16Ma and 2495±10Ma. The well defined major age peak at 2500Ma is broadly coeval with Neoarchean ages reported from the microblocks in the North China Craton. The zircon Lu–Hf data from the Yishui suite display εHf(t) values between −2.5 and 5.0, with corresponding model ages suggesting magma derivation from Neoarchean juvenile sources together with limited reworked Paleo-Mesoarchean crustal components.Our study is the first report of Neoarchean suprasubduction-type ophiolites from a locality far from the margins of the major crustal blocks and suture zones in the NCC and strengthens the concept that the craton is a mosaic of several microblocks with intervening oceans that closed along multiple subduction zones. We envisage that the amalgamation between the Xuhuai and the Jiaoliao microblocks resulted in the accretion of the Yishui suprasubduction zone ophiolitic assemblages onto the southern margin of the Jiaoliao microblock. The Neoarchean microblock amalgamation in the North China Craton provides new insights into continental growth in the early Earth and confirms that modern style plate tectonics might have been initiated early in the history of our planet.