Pb Geochronological Data Research Articles

The tectonic significance of lower Permian successions in the Texas Orocline (Eastern Australia)

The Texas Orocline is a prominent orogenic curvature that developed during the early Permian in the southern New England Orogen. Outliers preserving lower Permian sedimentary successions (Bondonga, Silver Spur, Pikedale, Terrica, Alum Rock and Ashford beds) approximately outline the oroclinal structure, but the tectonic processes responsible for the development of these basinal successions, and their relationships to the Texas Orocline, are unclear. Here we address this shortcoming by providing new U–Pb detrital and primary zircon ages from these successions, as well as detailed stratigraphic and structural data from the largest exposed succession (Bondonga beds). Field observations and U–Pb geochronological data suggest that the lower Permian successions in the Texas Orocline are remnants of a single, formerly larger basin that was deposited after ca 302 Ma. Time constraints for formation of this basin are correlative with constraints from the lower Permian Nambucca Block, which was likely deposited in response to regional back-arc extension during and/or after the development of the Texas Orocline. The conclusion that the lower Permian sedimentary basins in the Texas Orocline belong to this back-arc extensional system supports the suggestion that oroclinal bending in the New England Orogen was primarily controlled by trench retreat and associated overriding-plate extension.

Age and sources of Precambrian zircon–rutile deposits in the Kokchetav sialic massif (northern Kazakhstan)

The U–Pb geochronological data on detrital zircons from placers confined to Neoproterozoic quartzite–schist sequences, which are widespread in the Kokchetav massif of northern Kazakhstan, are discussed. Detrital zircons (332 grains in total) originate from the ore occurrences in the central, northern, and western parts of the massif. The concordant ages of detrital zircons from all the examined occurrences largely correspond to intervals of 1017–1528, 1628–1946, and 2653–2739 Ma. The obtained data imply that material of quartzite–schist sequences of the Kokchetav massif was provided by Mesoproterozoic, Paleoproterozoic, and Neoarchean rock complexes. The lower age limit determined for these sequences is approximately 1.06 Ga. The dates obtained for detrital zircons are most consistent with events that took place in Laurentia. They correspond to the formation and breakup of the Columbia/Nuna supercontinent (approximately 1650–1580 and 1450–1380 Ma ago, respectively) and formation of the Rodinia supercontinent in the period of 1300–900 Ma ago.

Granitoids of the Olekma Complex in the Selenga–Stanovoi superterrane of the central Asian mobile belt: Age and tectonic position

Granitoid massifs of the Olekma Complex form a large synonymous magmatic belt, which is located in the southern part of the Selenga–Stanovoi superterrane and is traceable in the northeasterly direction for over 700 km. According to U–Pb geochronological data, they are 358 ± 2 Ma old. The geochemical data indicate their collisional nature. The massifs were most likely formed during collision between the Selenga–Stanovoi superterrane and structures of the Baikal mountainous system and its terminal formation stage as a single structural block.

Evolution of magmatic-hydrothermal system of the Kalaxiange’er porphyry copper belt and implications for ore formation (Xinjiang, China)

The Kalaxiange’er porphyry copper ore belt is situated in the eastern part of the southern Altai of the Central Asian Orogenic Belt and forms part of a broad zone of Cu porphyry mineralization in southern Mongolia, which includes the Oyu Tolgoi ore district and other copper–gold deposits. The copper ore bodies are spatially associated with porphyry intrusions of granodiorite, quartz diorite, quartz syenite, and quartz monzonite and have a polygenetic (polychromous) origin (magmatic porphyry, hydrothermal, and supergene). The mineralized porphyries are characterized by almost identical REE and trace element patterns. The Zr/Hf and Nb/Ta ratios are similar to those of normal granite produced through the evolution of mantle magma. The low initial Sr isotope ratio ISr, varying within a narrow range of values (0.703790–0.704218), corresponds to that of primitive mantle, whereas the εNd(T) value of porphyry varies from 5.8 to 8.4 and is similar to that of MORB. These data testify to the upper-mantle genesis of the parental magmas of ore-bearing porphyry, which were then contaminated with crustal material in an island-arc environment. The isotopic composition of sulfur (unimodal distribution of δ34S with peak values of −2 to −4‰) evidences its deep magmatic origin; the few lower negative δ34S values suggest that part of S was extracted from volcanic deposits later. The isotopic characteristics of Pb testify to its mixed crust–upper-mantle origin. According to SHRIMP U–Pb geochronological data for zircon from granite porphyry and granodiorite porphyry, mineralization at the Xiletekehalasu porphyry Cu deposit formed in two stages: (1) Hercynian “porphyry” stage (375.2 ± 8.7 Ma), expressed as the formation of porphyry with disseminated and vein–disseminated mineralization, and (2) Indosinian stage (217.9 ± 4.2 Ma), expressed as superposed hydrothermal mineralization. The Re–Os isotope data on molybdenite (376.9 ± 2.2 Ma) are the most consistent with the age of primary mineralization at the Xiletekehalasu porphyry Cu deposit, whereas the Ar–Ar isotopic age (230 ± 5 Ma) of K-feldspar–quartz vein corresponds to the stage of hydrothermal mineralization. The results show that mineralization at the Xiletekehalasu porphyry Cu deposit was a multistage process which resulted in the superposition of the Indosinian hydrothermal mineralization on the Hercynian porphyry Cu mineralization.

Petrology and zircon U–Pb geochronology of metagabbro from the Highland Complex, Sri Lanka: Implications for the correlation of Gondwana suture zones

The basement in the Sri Lankan fragment of Gondwana is composed of upper amphibolite- to granulite-facies rocks that underwent latest Neoproterozoic – Cambrian high-grade metamorphism. A recent tectonic model suggests that the Highland Complex in the central part of the island corresponds to a suture zone formed by the closure of a Neoproterozoic ocean between the Wanni and Vijayan Complexes. In order to test the model and further unravel the complex tectonic evolution of this region, we present new petrological, pressure–temperature, and zircon U–Pb geochronological data on garnet-bearing metagabbros from the Highland Complex. The metagabbros, which occur as blocks or lenses within metasediments of the Highland Complex, contain granulite-facies mineral assemblages of garnet+clinopyroxene+orthopyroxene+ilmenite+hornblende+plagioclase+quartz (sample HC7) and garnet+plagioclase+clinopyroxene+orthopyroxene+pargasite+quartz+rutile+ilmenite (sample HC8). Application of phase equilibria modeling in the system NCFMASHTO indicates peak P–T conditions of 900–950°C/10.5–11kbar and 800–830°C/8.5–9kbar, respectively, and clockwise P–T paths as inferred from decompression textures and mineral compositional isopleths. These conditions are significantly lower than the high-pressure (>17kbar) and ultrahigh-temperature (>1000°C) conditions obtained previously for similar metagabbros from the Highland Complex, whereas our results are consistent with those of similar rocks from the Palghat-Cauvery Suture Zone in India and the Lützow-Holm Complex in East Antarctica. The latest Neoproterozoic to Cambrian metamorphic ages (574±28Ma and 534±14Ma) as well as the available petrological data suggest that the metagabbros in the Highland Complex could be correlated with those in the Lützow-Holm Complex in East Antarctica. The results of this study confirm that the Highland Complex is an extension of the Lützow-Holm Complex, and that both these terranes involve a package of accreted trench sediments and remnant oceanic lithosphere metamorphosed during the final collision related to the Gondwana assembly.

The relative and absolute chronology of strato-tectonic events in the Gorom-Gorom granitoid terrane and Oudalan-Gorouol belt, northeast Burkina Faso

The integration of field mapping and analytical studies of magmatic units of the northeast of Burkina Faso has provided new information on the tectonic and magmatic evolution of the Oudalan-Gorouol belt and the Gorom-Gorom granitoid terrane. Structural, geochemical and geochronological analyses have helped to clarify the geological evolution of the region during the Tangaean Event (D1) and Eburnean Orogeny (D2) through to the Wabo Tampelse Event (D3). Further to this, zircon U–Pb geochronology data have demonstrated that the Oudalan-Gorouol belt and the Gorom-Gorom granitoid terrane represents some of the oldest outcropping geology in the Palaeoproterozoic Baoulé-Mossi domain recognised to date, with the oldest age at 2253±9Ma. The geochronology and geology suggests that the basement or a pre-Birimian crust to the Birimian Supergroup may be found in the northeast of Burkina Faso.The Eburnean Orogeny in the northeastern Burkina is preceded by the two phases of deformation (D1−x and D1), and two phases of magmatism. D1−x is associated with the emplacement of the Dori Batholith at the onset of D1 (2164–2141Ma). D1 ductile–brittle deformation formed F1 folds and discrete high-strained mylonite zones that deformed the Oudalan-Gorouol belt and the Gorom-Gorom granitoid terrane during a southwest-directed palaeo-principal transport direction. The pre-Birimian to Birimian supracrustal rocks and intrusions were regionally metamorphosed during D1 to greenschist to amphibolite facies.The Eburnean Orogeny (2130–1980Ma) is characterised by northwest–southeast shortening; it was followed by north–northwest to south–southeast shortening with development of sinistral strike-slip faults and shears. D2 brittle-(ductile) deformation is manifested by refolding of F1 by northeast-trending F2, and development of a pervasive northeast-trending S2 to S2-C foliation. Metamorphic grade attained greenschist facies during D2 with development of the mineral assemblage of quartz-chlorite–muscovite±actinolite.The Wabo Tampelse (D3) deformation event is brittle in character and does not significantly affect regional geological architecture in the study area.

Microblock amalgamation in the North China Craton: Evidence from Neoarchaean magmatic suite in the western margin of the Jiaoliao Block

The Archaean Earth is considered to have been characterized by microcontinents that formed, dominantly, through the accretion of oceanic arcs and plateaus. The North China Craton (NCC) provides a typical case where at least seven ancient microcontinental nuclei with distinct lithological features and independent tectonic histories were amalgamated into the cratonic framework at the end of the Archaean. Here we investigate a suite of magmatic rocks developed at the periphery of one of these microblocks, the Jiaoliao Block that forms part of the composite Eastern Block of the NCC. We present petrological, geochemical and zircon U–Pb geochronological data from the Taipingzhai charnockite suite, and associated amphibolites, metagabbros and orthogneisses from the Qianxi Complex. Geochemically the rocks show a wide range of SiO2 (charnockite suite: 52.57–75.50wt.%; metagabbro: 43.71wt.%; amphibolite: 50.24wt.%; garnet-bearing biotite: 63.73wt.%), and MgO (charnockite suite: 0.89–5.01wt.%; metagabbro: 3.99wt.%; amphibolite: 6.23wt.%; garnet-bearing biotite: 2.08wt.%). The composition of the felsic units straddle from diorite through syeno-diorite to granite with both alkalic and subalkalic affinity, with dominantly magnesian composition and arc-related features. Their immobile trace element relationships suggest calc-alkaline affinity. They show positive Pb, Ba, La, Nd, and Gd and negative Nb, Ta, Sr, Th and Ti anomalies with slightly negative anomalies of Ce and Y, attesting to arc-related features. In tectonic classification diagrams, the rocks plot in the VAG+syn-COLG field or the VAG area suggesting subduction-related origin.The dominant population of zircons in all these rocks displays magmatic crystallization features including high Th/U values with core-rims textures indicating subsequent thermal events. The zircon U–Pb data yield upper intercept ages of 2587±10Ma to 2543±17Ma and 207Pb/206Pb mean ages of 2578±7.3Ma to 2536±8Ma for the charnockite suite, marking the timing of emplacement of the arc magmas. The overgrowth rims as well as discrete neoformed grains are interpreted as dating subsequent metamorphism and yield 207Pb/206Pb ages between 2533Ma to 2490Ma. Zircons in the metagabbro preserve upper intercept ages of 2556±20Ma representing the crystallization age of this rock. The younger ages of 2449±58Ma (upper intercept age) and 1845±25Ma (207Pb/206Pb spot age) are interpreted to represent subsequent multiple thermal events in this area. Zircons in the amphibolite preserve the 207Pb/206Pb mean age of 2539±9Ma, representing the crystallization age of this rock. The garnet-bearing biotite gneiss shows an upper intercept age of 2562±10Ma (MSWD=0.66; N=36) and the 207Pb/206Pb mean age of 2561±9Ma (MSWD=0.63; N=33) which is taken to represent the crystallization age of this rock. Some inherited zircons are also identified with 207Pb/206Pb ages of 2664±26Ma and 2628±26Ma. Zircon Lu–Hf data show dominantly positive εHf(t) values and combined with crustal residence ages, the results suggest Mesoarchean to Neoarchean juvenile crust formation in the NCC. We interpret the data presented here to represent a phase of major late Neoarchaean arc magmatism along the western margin of the Jiaoliao Block related to the birth of microcontinental nuclei within the NCC. Our data suggest that the Western and Eastern Blocks might not have existed as discrete crustal blocks, and that the construction of the NCC is a result of the assembly of several microblocks or terranes at the end of Archaean. Similar Archean cratonic nuclei in other regions of the world might have formed part of a primitive supercontinent in the early Earth.

Provenance of metasedimentary rocks from the Ceará Central Domain of Borborema Province, NE Brazil: implications for the significance of associated retrograded eclogites

In the Forquilha area (NE Brazil), in NW Borborema Province, high to ultra-high pressure rocks are an important geological key to understanding West Gondwana amalgamation. U–Pb geochronological data for a retrograded eclogite sample yielded an upper intercept age of ca. 1520 Ma and a lower intercept age of ca. 620 Ma. These ages most likely represent the crystallization age of the basaltic protolith and the regional metamorphism, respectively. The retrograded eclogites are enclosed in migmatized quartz-feldspathic gneiss and sillimanite (after kyanite)-garnet-biotite gneiss. Detrital U–Pb zircon data for these paragneisses show only Paleoproterozoic zircon grains with ages clustering from ca. 1800 Ma (the maximum depositional age) to ca. 2480 Ma, and frequency peaks at 2.2–2.0 Ga. Combined with Nd isotopic data from the Forquilha paragneisses, one can assume a single Paleoproterozoic source. Basement rocks of the Ceará Central and the Rio Grande do Norte domains are the most likely candidates. The absence of Meso- and Neoproterozoic zircon grains suggest that the retrograded eclogite bodies possibly do not represent slivers of oceanic rocks captured in active margin sequences during subduction. It was identified that the high-pressure rocks of the Forquilha area are in tectonic contact with high-pressure granulite facies rocks of the Ceará Complex (Independência unit) that present detrital zircon records of an active margin setting, with ages ranging from ca. 660 Ma to 2200 Ma. Metamorphism of this sequence occurred at ca. 650 Ma. Considering previous studies, field relationships, and metamorphic paragenesis, a tectonic scenario is inferred, in which the Forquilha retrograded eclogites represent Mesoproterozoic basaltic rocks of an extensional event that were metamorphosed under eclogite facies conditions during Late Neoproterozoic continental subduction/collision, and juxtaposed to an active margin sequence during the exhumation process.

The 2.31 Ga mafic dykes in the Karelian Craton, eastern Fennoscandian shield: U–Pb age, source characteristics and implications for continental break-up processes

Major- and trace-element geochemistry, Sm–Nd isotopic and U–Pb geochronological (ID TIMS, baddeleyite) data are presented for Paleoproterozoic mafic dykes in the Karelian Craton, eastern Fennoscandian shield. Mafic dyke swarms recognized in the Lake Upper Kuito area in the Western Karelian terrane of the Karelian Craton include (i) NE-trending ca. 2450Ma gabbronorite dykes, (ii) NW-trending ca. 2310Ma dolerite dykes, and (iii) NNW-trending ca. 2130Ma continental MORB-type tholeiitic dykes. Each swarm belongs to a distinct intraplate igneous event and indicates specific mantle melting and extensional processes.The ca. 2.31Ga doleritic dykes have high-Fe, high-Ti tholeiitic series affinity and characterized by high concentrations of lithophile trace elements. The enrichment of LREE and unfractionated HREE patterns together with depletion of Nb on multi-element diagrams and initial ɛNd values of +0.5 to +0.8 are characteristic for ca. 2.31Ga dolerites. The geochemical modelling results suggest that these features could be originated via combination of fractional crystallization and crustal contamination processes, and do not indicative of a mantle source enrichment. It also indicates long-term storage of melts in an intracrustal magma chamber and a relatively slow ascent to the surface. The primary melts of ca. 2.31Ga dolerites possibly originated by partial melting of a DM-type mantle source in the spinel peridotite stability field.In contrast to previous ca. 2.45Ga event studied ca. 2.31Ga dolerites do not indicate high-temperature mantle-plume induced mantle melting. On the other hand, the ca. 2.31Ga dykes have some similarities with syn-breakup ca. 2.13Ga continental MORB-type tholeiites, although there is no evidence for the break-up of the continental lithosphere of the Karelian Craton at ca. 2.31Ga. Thus ca. 2.31Ga dykes could indicate the existence of relatively stable extensional tectonic environment and an unsuccessful attempt of break-up of the continent.The ca. 2.31Ga dykes together with previously studied Runkaus basalts and Taivalkoski dykes could be considered as a component of distinct relatively wide-spread Kuito-Taivalkoski igneous event within the ca. 2.2–2.4Ga “crustal age gap” period in the eastern Fennoscandian shield. This probably indicate that the number and intensity of endogenous events of age 2.4–2.2Ga are greatly undervalued. This also argues against a decrease of endogenic activity and magmatic “slowdown” during ca. 2.4–2.2Ga period.

The case for a distinct Taltson orogeny: Evidence from northwest Saskatchewan, Canada

The Taltson magmatic and Thelon tectonic zones have historically been considered parts of a single orogen recording collision between the Slave craton and Buffalo Head terrane to the Rae craton between 2.0 and 1.9Ga. The Taltson–Thelon connection stemmed from recognition that the Taltson magmatic zone coincides with the Taltson aeromagnetic high that extends northward and merges with the Thelon aeromagnetic high. Analysis of geophysical data in this study, however, coupled with recent mapping, indicates that this magnetic high is underlain in large part by rocks of Taltson basement complex. In addition, both basement rocks and Taltson-age plutons can be traced eastward into northern Saskatchewan, below Athabasca Basin cover, and reappear/re-emerge to the south where they are truncated by the Virgin River shear zone (Snowbird tectonic zone). This paper presents new field, geochronological and geochemical data for the Lloyd domain of Saskatchewan bearing on the extent and nature of the Taltson orogen. The Lloyd domain is underlain mainly by magnetic dioritic–granodioritic orthogneisses and subordinate supracrustal rocks. It also hosts the Clearwater anorthosite complex, a ∼300km2, northeast-elongate gabbro-anorthosite intrusion about 15km northwest of the Virgin River shear zone. While previous geochronology established that some Lloyd domain orthogneisses are of Taltson (i.e., 1.98Ga) age, geochronology presented herein has identified an appreciably older (ca. 2460Ma) component. Geochemical study indicates that these older metaplutonic gneisses are distinguishable from younger (Taltson-age) ones based on trace element characteristics, but the true extent of basement rocks is impossible to evaluate. U–Pb geochronological data from the Clearwater anorthosite complex are complicated but allow interpretation that it was emplaced into this older basement complex at ca. 2.1Ga and was subsequently metamorphosed during Snowbird orogenesis at ca. 1.9Ga. Geochemical study also indicates that the Taltson-age quartz dioritic to granodiorite suite in Lloyd domain is chemically related but less evolved than the younger dominantly granitic, Taltson magmatic zone plutons of Alberta and N.W.T. The two are interpreted to be related and reflect gradual evolution of a subduction-related continental magmatic arc, challenging previous models invoking intra-plate magmatism. Collectively, these data, along with regional geological relationships, indicate that the Taltson orogen originally trended northwest–southeast and is a unique tectonic entity, distinct from the Thelon tectonic zone. Two alternative models are provided for Taltson orogeny. In the first, the Thelon orogeny develops along the margin of the Slave craton due to accretion of the Buffalo Head terrane between 2.0 and 1.95Ga. In the meantime, the Taltson arc was built on the southwest margin of the Rae province as the now combined Slave craton and Buffalo Head terrane approached. Terminal collision of the Rae and Slave culminated with high-grade metamorphism between 1.94 and 1.93Ga. In the second, the Slave craton was attached to the Rae craton far in advance of the Taltson orogeny, perhaps during the ca. 2.5–2.3Ga Arrowsmith orogeny. In this model, the Taltson orogeny developed on the southwest margin of the amalgamated Rae-Slave craton due to accretion of the Buffalo Head terrane and the Thelon tectonic zone represents an intracontinental zone of reworking.

Petrogenesis of mid-Carboniferous volcanics and granitic intrusions from western Junggar Basin boreholes: geodynamic implications for the Central Asian Orogenic Belt in Northwest China

The western Junggar Basin is located on the southeastern margin of the West Junggar terrane, Northwest China. Its sedimentary fill, magma petrogenesis, tectonic setting, and formation ages are important for understanding the Carboniferous tectonic evolution and continental growth of the Junggar terrane and the Central Asian Orogenic Belt. This paper documents a set of new zircon secondary ion mass spectrometry U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the Carboniferous strata and associated intrusions obtained from boreholes in the western Junggar Basin. The Carboniferous strata comprise basaltic andesite, andesite, and dacite with minor pyroclastic rocks, intruded by granitic intrusions with zircon secondary ion mass spectrometry U–Pb ages of 327–324 Ma. The volcanic rocks are calc-alkaline and show low high εNd(t) values (5.3–5.6) and initial 87Sr/86Sr (0.703561–0.703931), strong enrichment in LREEs, and some LILEs and depletion in Nb, Ta, and Ti. Furthermore, they also display high (La/Sm)N (1.36–1.63), Zr/Nb, and La/Yb, variable Ba/La and Ba/Th and constant Th/Yb ratios. These geochemical data, together with low Sm/Yb (1.18–1.38) and La/Sm (2.11–2.53) ratios, suggest that these volcanic rocks were derived from a 5–8% partial melting of a mainly spinel Iherzolite-depleted mantle metasomatized by slab-derived fluids and melts of some sediments in an island-arc setting. In contrast, the granitic intrusions represent typical adakite geochemical features of high Sr and low Y and Yb contents, with no significant Eu anomalies, high Mg#, and depleted εNd(t) (5.6–6.4) and εHf(t) (13.7–16.2) isotopic compositions, suggesting their derivation from partial melting of hot subducted oceanic crust. In combination with the previous work, the West Junggar terrane and adjacent western Junggar Basin are interpreted as a Mariana-type arc system driven by northwestward subduction of the Junggar Ocean, possibly with a tectonic transition from normal to ridge subduction commencing ca. at 331–327 Ma.

Role of crustal contribution in the early stage of the Damara Orogen, Namibia: New constraints from combined U–Pb and Lu–Hf isotopes from the Goas Magmatic Complex

The tholeiitic to calc-alkaline Goas Complex represents the earliest magmatic activity of the inland branch of the Damara Orogen, and reflects the Pan-African convergence between the Congo and Kalahari Cratons. New results from laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb geochronology coupled with single-zircon Lu–Hf isotopic data constrain the age of emplacement and allow the identification of a crustal contribution to the magma sources. No temporal gap is registered between the first tholeiitic intrusion at ca. 575Ma and the main diorite to granite episodes. Invariably sub-chondritic εHft values (−3.8 to −34.4) attest to a significant crustal residence time with long-term reworking of multiple and mixed crustal components. No significant juvenile magma was involved in the magmatogenesis. Hf model ages coupled with an exhaustive dataset of U–Pb geochronological data suggest that the crustal components which fed the Goas sources might have formed during specific orogenic events with a major contribution of material derived from the central-western African Paleoproterozoic Eburnean Orogen. Geochemistry, U–Pb geochronology and Hf isotope systematics however, do not unequivocally clarify the long debate regarding whether or not the Pan-African orogenic cycle included ocean closure and subduction, as the involvement of crustal melting processes during the early stage of the Damaran event seems to have played a major role.

Paleoproterozoic arc magmatism in the North China Craton: No Siderian global plate tectonic shutdown

Arc magmatism in convergent plate margins has been a major contributor to continental growth. Following arc–arc and arc–continent collisions in the Archean leading to the amalgamation of micro-blocks, the North China Craton (NCC) witnessed major pulses of continental arc magmatism during the Paleoproterozoic. In this study, we present geochemistry, zircon U–Pb geochronology and Lu–Hf isotope data from a suite of magmatic rocks sampled from the region of confluence of two major Paleoproterozoic suture zones in the NCC — the Inner Mongolia Suture Zone (IMSZ) and the Trans-North China Orogen (TNCO). Our zircon U–Pb geochronological data indicate new zircon growth during multiple tectonothermal events as displayed in the 207Pb/206Pb weighted mean ages of 2410±41Ma for metagranite, 2480±12Ma, 2125±18Ma, 1946±8Ma, 1900±15Ma and 1879±12Ma from metagabbros, 2446±11Ma from charnockite, and 1904±6Ma and 1901±9Ma from metatuffs. The 207Pb/206Pb upper intercept age of zircons in the khondalite shows 2102±76Ma which is identical to the age obtained from the magmatic zircons in one of the metagabbros. The khondalites also carry a group of concordant metamorphic zircons with 207Pb/206Pb mean age of 1881±20Ma. Metamorphic zircons in the gabbros and charnockites also yield similar ages of 1890±14Ma and 1852±19Ma respectively. The age data suggest prolonged arc magmatism in a convergent margin setting during ca. 2.48 to 1.9Ga, followed by metamorphism at ca. 1.89–1.85Ga associated with the final collision. Lu–Hf analyses reveal that the dominant populations of zircons from all the rock types are characterized by positive εHf values (−1.9 to 6.8; mean 1.8). The εHf and TDMC data suggest that the magmas were mostly derived from Neoarchean and Paleoproterozoic juvenile components. The salient geochemical features of these rocks attest to magma generation from heterogeneous sources involving subduction-derived arc components with minor input from continental crust. The results presented in this study, together with those from previous investigations in different domains of the IMSZ and TNCO suggest major Paleoproterozoic arc magmatic events in the NCC lasting for nearly 600millionyears associated with the final assembly of the crustal blocks into a coherent craton. Construction of the final cratonic architecture of the NCC thus witnessed not only the arc–continent amalgamations at 2.7–2.5Ga, but also major crust building events in the Paleoproterozoic through melts generated from juvenile and recycled components in continental magmatic arc systems along an active convergent margin, followed by intense deformation and metamorphism during the final collision at 1.85–1.80Ga. The prominent Paleoproterozoic magmatic records in the NCC do not support the proposal of global plate tectonic shutdown in the Siderian and confirm vigorous convergent margin magmatism and crust building processes throughout the Paleoproterozoic.

The Paleoproterozoic Campinorte Arc: Tectonic evolution of a Central Brazil pre-Columbia orogeny

The Campinorte Arc is a poorly exposed 2.19–2.07Ga Paleoproterozoic terrane in contact with the Neoproterozoic Goiás Magmatic Arc by the Rio dos Bois Fault in the northern Brasília Belt, Central Brazil. The Campinorte Arc is divided into Pau de Mel Suite metatonalites to metamonzogranites and the Campinorte volcano-sedimentary Sequence. Pau de Mel Suite whole rock geochemistry indicates at least three separate coeval parental magmas compatible with arc signatures. U–Pb geochronology data of paragranulite and mafic granulite exposed in the region as part of the Campinorte Arc provide additional information on this Paleoproterozoic orogenic cycle. The formation and preservation of these granulites was due to tectonic switching of the back arc basin and consequent lithospheric thinning from 2.14 to 2.09Ga with metamorphic peak from 2.11 to 2.08Ga. A Pau de Mel Suite granodiorite sample dated at ca. 2.08Ga indicates post-peak magmatism. The arc was thereafter rapidly contracted preserving Paleoproterozoic high metamorphic grade mineral assemblages. Additionally, the Campinorte Arc and the neighboring Crixás/Guarinos/Faina greenstone belts may have shared the same source of sediments as attested by geographic proximity, coeval maximum sedimentation, felsic volcanism and the occurrence of similar rock types. Gravimetric and seismic data also support a common basin hypothesis. The formation of the Campinorte Arc is contemporaneous to other northern Brasília Belt basement terranes and, along with similar arcs within and at the São Francisco Craton edges, indicate a continental crust formation event that eventually led to the assemblage of Columbia.

Petrology and zircon U–Pb geochronology of metagabbros from a mafic–ultramafic suite at Aniyapuram: Neoarchean to Early Paleoproterozoic convergent margin magmatism and Middle Neoproterozoic high-grade metamorphism in southern India

Several mafic–ultramafic complexes occur within the Palghat-Cauvery Suture Zone (PCSZ) in southern India. The PCSZ is regarded in recent models as the zone along which crustal blocks were amalgamated during the Late Neoproterozoic–Cambrian (550–530Ma) Gondwana assembly. Here we report petrologic and zircon U–Pb geochronologic data from gabbros associated with the Aniyapuram mafic–ultramafic suite in the central domain of the PCSZ. Geothermobarometry and pseudosection approach in the system NCFMASHTO for the metagabbro (Grt+Cpx+Opx+Hbl+Pl+Qtz+Ilm+Rt) yield peak P–T condition of 9.8–10.6kbar and 730–790°C, which was followed by decompression to 6.5–8.0kbar and ca. 750°C as inferred from the formation of Opx+Pl symplectite around garnet, probably along a clockwise P–T path. Zircon U–Pb data analyzed by LA-ICP-MS plot along a well-defined discordia with upper and lower intercepts in the concordia at 2436±22Ma and 731±11Ma respectively, suggesting Neoarchean–Early Paleoproterozoic magmatic emplacement of the protolith and progressive Pb loss related to the Middle Neoproterozoic (Cryogenian) thermal event (or high-grade metamorphism). These results closely compare with the available Neoarchean magmatic ages of mafic–ultramafic complexes (e.g., Sittampundi, Devanur, Agali Hills, and Kanja Malai) and Middle Neoproterozoic magmatic event (e.g., Manamedu and Kadavur) in the PCSZ and adjacent granulite blocks. The 650Ma concordia ages obtained from unzoned zircons might indicate the timing of high-grade metamorphism or post-peak hydration event. The P–T conditions obtained from Aniyapuram are significantly lower than the high-pressure and ultrahigh-temperature conditions of the 550–530Ma final collisional event (P>14 kbar and T>950°C). The Middle Neoproterozoic (ca. 730Ma or 650Ma) high-grade metamorphism in Aniyapuram reported for the first time from the PCSZ is possibly associated with magmatism in arc tectonic setting.

U–Pb and Lu–Hf isotopes applied to the evolution of the late to post-orogenic transtensional basins of the dom feliciano belt, Brazil

The Santa Bárbara and Guaritas Groups, located in southern Brazil, represent the last episode of deposition in the Camaquã Basin, which included deposition of sandstone, siltstone and conglomerates during the late Neoproterozoic and Eopaleozoic as part of the collisional period of the Brasiliano/Pan-African Orogeny. Combined U–Pb and Lu–Hf analyses of detrital zircon grains from outcrops and drill holes in both groups together with an analysis of paleocurrent data provide information regarding the sedimentary provenance and tectonic evolution. The U–Pb geochronological data from both groups include concordant ages that are predominantly Paleoproterozoic and Neoproterozoic, with an increasing contribution by Early Neoproterozoic and Paleoproterozoic zircon grains in the upper unit, the Guaritas Group. The maximum depositional age of the basal siltstone units of the Santa Bárbara Group is 567.9±5.9Ma, and the maximum depositional age of the Guaritas Group is 540–535Ma. The Lu–Hf model ages (TDM) indicate that the zircon grains in both groups display a predominantly Archean and Paleoproterozoic inheritance, with predominantly negative ɛHf values, which indicates that the source rocks originated from the subcontinental lithosphere.

Neoproterozoic subduction along the Ailaoshan zone, South China: Geochronological and geochemical evidence from amphibolite

Lenses of amphibolites occur along the Ailaoshan suture zone at the southwestern margin of the Yangtze Block, South China. Petrological, geochemical and zircon U–Pb geochronological data indicate that they are divisible into two coeval groups. Group 1, represented by the Jinping amphibolite, has mg-number of 71–76 and (La/Yb)cn ratios of 7.2–7.7, and displays a geochemical affinity to island arc volcanic rocks. Group 2 amphibolites occur at Yuanyang and are characterized by high Nb contents (14.3–18.4ppm), resembling Nb-enriched basalts. The ɛNd(t) values for Group 1 range from −3.45 to −2.04 and for Group 2 from +4.08 to +4.39. A representative sample for Group 1 yields a U–Pb zircon age of 803±7Ma, whereas two samples for Group 2 give U–Pb zircon ages of 813±11Ma and 814±12Ma. Petrogenetic analysis suggests that Group 1 originated from an orthopyroxene-rich source and Group 2 from a mantle wedge modified by slab-derived melt. In combination with other geological observations, these amphibolites are inferred to constitute part of an early Neoproterozoic (∼815–800Ma) arc-back-arc basin system. The Neoproterozoic amphibolites and related rocks along the Ailaoshan zone may be the southward extension of the Neoproterozoic supra-subduction zone that developed along the western margin of the Yangtze Block.

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c . 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an ϵ Nd(0) signature of −18.3 to −12.1, and is intruded by c . 800 Ma and c . 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c . 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an ϵ Nd(0) signature of −27.7 to −23.4, intruded by c . 1830 Ma granites. The data reveal a c . 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones. Supplementary material: Zircon U–Pb geochronological data, whole-rock Sm–Nd isotopic data, sample locations, photomicrographs of sample thin sections, zircon CL images, and detailed analytical conditions are available at www.geolsoc.org.uk/SUP18704 .

The Eastern Black Sea-type volcanogenic massive sulfide deposits: Geochemistry, zircon U–Pb geochronology and an overview of the geodynamics of ore genesis

The Meso-Cenozoic geodynamic evolution of the eastern Pontides orogenic belt provides a key to evaluate the volcanogenic massive sulfide (VMS) deposits associated with convergent margin tectonics in a Cordilleran-type orogenic belt. Here we present new geological, geochemical and zircon U–Pb geochronological data, and attempt to characterize the metallogeny through a comprehensive overview of the important VMS mineralizations in the belt. The VMS deposits in the northern part of the eastern Pontides orogenic belt occur in two different stratigraphic horizons consisting mainly of felsic volcanic rocks within the late Cretaceous sequence. SHRIMP zircon U–Pb analyses from ore-bearing dacites yield weighted mean 206Pb/238U ages ranging between 91.1±1.3 and 82.6±1Ma. The felsic rocks of first and second horizons reveal geochemical characteristics of subduction-related calc-alkaline and shoshonitic magmas, respectively, in continental arcs and represent the immature and mature stages of a late Cretaceous magmatic arc. The nature of the late Cretaceous magmatism in the northern part of the eastern Pontides orogenic belt and the various lithological associations including volcaniclastics, mudstones and sedimentary facies indicate a rift-related environment where dacitic volcanism was predominant. The eastern Pontides VMS deposits are located within the caldera-like depressions and are closely associated with dome-like structures of felsic magmas, with their distribution controlled by fracture systems. Based on a detailed analyses of the geological, geophysical and geodynamic information, we propose that the VMS deposits were generated either in intra arc or near arc region of the eastern Pontides orogenic belt during the southward subduction of the Tethys oceanic lithosphere.

Early Mesoproterozoic metamorphism in the Barossa Complex, South Australia: links with the eastern margin of Proterozoic Australia

LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870°C and peak pressures of 8–9 kbar, followed by decompression to pressures of ∼6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.