Metamorphic Zircon Research Articles

New 40Ar/39Ar geochronology data of the Fuping and Wutai Complexes: Further constraints on the thermal evolution of the Trans-North China Orogen

The Trans-North China Orogen has witnessed the assembly of the Western and Eastern blocks of the North China Craton during Late Paleoproterozoic. Numerous studies have focused on the subduction/collision timing and metamorphic evolution of the Trans-North China Orogen, whereas the thermal evolution and exhumation history are still poorly understood. The Fuping and Wutai Complexes are widely exposed of middle- to high-grade metamorphic rocks, which are excellent regions to investigate the thermal evolution and exhumation history of the Trans-North China Orogen. Previous U-Pb geochronological data of metamorphic zircon and monazite suggest that the peak granulite facies metamorphism in the Fuping Complex occurred during 1.92–1.80 Ga, and peak amphibolite facies metamorphism in the northeastern part of the Wutai Complex happened at 1.85–1.82 Ga. Here we report newly obtained hornblende 40Ar/39Ar age of 1813–1806 Ma in the Fuping Complex, and biotite 40Ar/39Ar age of 1802–1786 Ma in the northeastern part of the Wutai Complex, which representing the cooling ages. Combined with previous thermochronological data, the cooling rate of 2.0–8.5 °C/Ma was obtained for the middle to southern segments of the Trans-North China Orogen, possibly suggesting a moderate to fast exhumation relative to other Proterozoic orogens.

Integrated garnet and zircon petrochronology reveals the timing and duration of orogenic events in the North China Craton

Radiometric data from accessory zircon in long-lived polymetamorphic granulite facies rocks frequently show a large spread, with no clear relationship to zircon textures. For example, the granulites and associated rocks of the North China Craton which have an apparently continuous spread of metamorphic zircon dates through the Palaeoproterozoic from c. 1.95–1.80 Ga. The data are commonly interpreted to reflect long-lived metamorphism or discrete metamorphic events, with intermediate data reflecting variable Pb loss or a mixture of multiple growth domains. Herein, we present an approach that integrates direct dating of the P–T indexed mineral garnet with accessory zircon petrochronology which allows diagnostic time constraints for polymetamorphic processes in high-temperature rocks. We present high-precision LuHf and SmNd dates for two garnet-bearing mafic granulites and the adjoining granite and quartzite from the Gushan area between the Khondalite Belt and the Trans-North China Orogen, North China Craton. Statistically robust LuHf dates of 1815 ± 4 Ma and 1808 ± 3 Ma and corresponding SmNd isochron dates of 1790 ± 6 Ma and 1800 ± 9 Ma were obtained from the two mafic granulites, overlapping the zircon UPb date range of c. 1959–1785 Ma. Samarium–Nd dates from the accompanying garnet-bearing granite and quartzite are indistinguishable at 1794 ± 6 Ma and 1780 ± 6 Ma. The corresponding bulk garnet LuHf isochrons are poorly constrained—two-point garnet–matrix dates range from 1945 ± 10 Ma to 1849 ± 10 Ma for the granite and from 1936 ± 10 Ma to 1840 ± 9 Ma for the quartzite. Upper intercept UPb dates of 1950 ± 24 Ma (granite) and 1936 ± 17 Ma (quartzite) are obtained from the metamorphic zircons in equilibrium with garnet. The close agreement of SmNd isochron-dates (c. 1.80 Ga), the overlapping clusters of zircon UPb dates (c. 1.96 Ga), the identical LuHf and UPb dates of c. 1.80 Ga within uncertainty in the two granulites, and the older LuHf dates and comparable zircon UPb dates (c. 1.96 Ga) obtained in the garnet-bearing granite and quartzite than in those of the mafic granulites, strongly indicate that these rocks experienced coeval metamorphism from c. 1.95–1.80 Ga. These data require a prolonged episodic tectonometamorphic event in the North China Craton, which is probably related to the amalgamation of the Columbia supercontinent.

Early Devonian tectonic conversion from contraction to extension in the Chinese Western Tianshan: A response to slab rollback

Abstract Extensive angular unconformities that occur across the Tianshan Orogen provide insight into the Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt. This study presents new geochronological and geochemical data to unravel the age and origin of a pronounced angular unconformity in the Baluntai domain of the Chinese Western Tianshan. The angular unconformity separates a ductilely deformed metamorphic complex from overlying unmetamorphosed sequences. Detrital zircon age spectra suggest that the sedimentary samples received detritus from both the Precambrian basement and contemporaneous igneous rocks in the Central Tianshan and Yili blocks. The youngest detrital and metamorphic zircons in metasediments from the metamorphic complex reveal that their protoliths were deposited at ca. 419 Ma, and subsequently overprinted by metamorphism and ductile deformation at ca. 409 Ma. Along with ca. 532–407 Ma metavolcanics and foliated intrusions, the metamorphic complex constitutes a diachronous stratigraphic unit deposited during early Cambrian to late Silurian times, rather than a Precambrian basement as previously thought. The youngest detrital zircon age data from siliciclastic samples immediately above the angular unconformity constrain their deposition to ca. 407–397 Ma. In conjunction with a ca. 406 Ma rhyolitic tuff in the lower terrestrial siliciclastic unit and an early Carboniferous paleontological age of carbonates in the upper shallow-marine unit, it is suggested that the overlying unmetamorphosed sequences were accumulated during the early Devonian to the early Carboniferous. Thus, the angular unconformity in the Baluntai domain was formed between ca. 409 and 407 Ma. New and already published geochemical data reveal that the early Paleozoic (ca. 530–410 Ma) and late Paleozoic (ca. 410–320 Ma) igneous rocks are mainly medium- to high-K calc-alkaline I-type granitoids, which exhibit typical subduction-related geochemical signatures despite contrasting structural features. These geochronological and geochemical results are in accordance with those of similar early Devonian angular unconformities and Paleozoic igneous rocks in the southern Yili Block. The early Paleozoic igneous rocks display highly scattered zircon εHf(t) values, while the late Paleozoic rocks show a progressively increasing trend toward more positive values, attributed to slab rollback of the northward subducting South Tianshan Ocean under the Yili and Central Tianshan blocks. Thus, we propose that the formation of the angular unconformity was caused by tectonic conversion from contraction to extension due to slab rollback during the early Devonian. This study highlights the significance of geological and geochronological investigations of angular unconformities in the context of associated episodic magmatism and slab behavior in order to unravel distinct tectonic processes in the long-lived accretionary evolution of the Chinese Western Tianshan.

Unraveling the origins and P-T-t evolution of the allochthonous Sobrado unit (Órdenes Complex, NW Spain) using combined U–Pb titanite, monazite and zircon geochronology and rare-earth element (REE) geochemistry

Abstract. The Sobrado unit, within the upper part of the Órdenes Complex (NW Spain) represents an allochthonous tectonic slice of exhumed high-grade metamorphic rocks formed during a complex sequence of orogenic processes in the middle to lower crust. In order to constrain those processes, U–Pb geochronology and rare-earth element (REE) analyses of accessory minerals in migmatitic paragneiss (monazite, zircon) and mylonitic amphibolites (titanite) were conducted using laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS). The youngest metamorphic zircon age obtained coincides with a Middle Devonian concordia monazite age (∼380 Ma) and is interpreted to represent the minimum age of the Sobrado high-P granulite facies metamorphism that occurred during the early stages of the Variscan orogeny. Metamorphic titanite from the mylonitic amphibolites yield a Late Devonian age (∼365 Ma) and track the progressive exhumation of the Sobrado unit. In zircon, cathodoluminescence images and REE analyses allow two aliquots with different origins in the paragneiss to be distinguished. An Early Ordovician age (∼490 Ma) was obtained for metamorphic zircons, although with a large dispersion, related to the evolution of the rock. This age is considered to mark the onset of granulite facies metamorphism in the Sobrado unit under intermediate-P conditions, and related to intrusive magmatism and coeval burial in a magmatic arc setting. A maximum depositional age for the Sobrado unit is established in the late Cambrian (∼511 Ma). The zircon dataset also record several inherited populations. The youngest cogenetic set of zircons yields crystallization ages of 546 and 526 Ma which are thought to be related to the peri-Gondwanan magmatic arc. The additional presence of inherited zircons older than 1000 Ma is interpreted as suggesting a West African Craton provenance.

A buried Paleoarchean core of the Eastern Sarmatia, Kursk block: U-Pb, Lu-Hf and Sm-Nd isotope mapping and paleotectonic application

Zircon U-Pb and Lu-Hf isotope analyses along with whole-rock Sm-Nd isotope studies shed a light on distribution of the Paleoarchean crust in the Kursk block, Eastern Sarmatia. It has been revealed that this domain consists of two provinces with different crust ages: the Paleoarchean Kursk-Besedino granulite-gneiss domain (KBd) and the Mesoarchaean Mikhailovsky granite-greenstone area (Mgga). The KBd encompasses TTG orthogneisses and pelite, BIF, and mafic volcanics (sills) metamorphosed in the granulite facies. The orthogneiss protolith was formed ca 3.55 Ga (U-Pb SHRIMP data) from a juvenile source. In the metapelites, U-Pb detrital zircon ages cluster in three groups, 3535–3556, 3464–3472, and 3334–3384 Ma. The latter one suggests post-Middle Paleoarchean age of the protoliths deposition. Sm-Nd and Lu-Hf isotope characteristics of the volcanic-sedimentary rocks suggest the leading role of Paleoarchean crustal material in their protoliths. The age of metamorphic zircon is ca 2.8 Ga in all the KBd rocks.The Mgga consists of TTG and volcanic-sedimentary units. The ages of the TTG, felsic volcanics and metasediments fall in a narrow range of 2.96–3.12 Ga. They have juvenile isotope-geochemical characteristics (εNd(t) = +0.3 – + 4.0; εHf(t) = -0.3 – + 5.8) without any contribution of Paleoarchean crust. High-temperature 2.8 Ga-old metamorphism is not recorded in the Mgga.Pronounced differences in the age of crust-forming events in the KBd and Mgga indicate that these continental blocks had a different geological history, hence were distant from each other during most of the Archean Eon, probably up to 2.8 Ga. The Paleoproterozoic Trosnyanskaya intrusion in the Mgga bears signs of the KBd Paleoarchean crust. This suggests a collision of these Archean blocks with the Mesoarchean being thrust over the Paleoarchean rocks possibly at ca 2.8 Ga.

Garnet and zircon geochronology of the Paleoproterozoic Kuru-Vaara eclogites, northern Belomorian Province, Fennoscandian Shield

The Belomorian Province of the Fennoscandian Shield exposes numerous Precambrian eclogites, which makes it significant for the study of early tectonic processes. The age of these eclogites has been discussed for more than 15 years and regarded as either Archean or Paleoproterozoic. In the Kuru-Vaara quarry within the northern Belomorian Province, the eclogitic assemblage is preserved in concordant mafic boudins in felsic gneisses and a partially eclogitized gabbro-norite dike cutting discordantly through the gneiss fabric. Both eclogite types preserve zircon cores with a magmatic geochemical signature that yield protolith ages of ca. 2.88 Ga for a mafic boudin and ca. 2.44 Ga for the eclogitized gabbro-norite. Ca. 1.9 Ga zircon rims and grains from the eclogites show low Th/U ratio and HREE depletion, reflecting the growth of metamorphic zircon in equilibrium with garnet. In the eclogite boudin, the Archean zircon cores yield δ18O = 5.1–5.9‰ typical of mantle melts; the oxygen isotope composition of garnet (δ18O = 4.0–5.0‰) is in equilibrium with that of the 1.9 Ga zircon (δ18O = 4.5–5.4‰). Garnet Lu-Hf geochronology coupled with U-Pb zircon geochronology constrains prograde metamorphism for the Kuru-Vaara eclogites at 1.92–1.89 Ga. Mineral inclusions of garnet, zoisite, plagioclase, kyanite, amphibole, quartz, and low-Na clinopyroxene in ca. 1.9 Ga zircon from the eclogite boudin imply epidote–amphibolite/amphibolite facies conditions for the prograde metamorphism. All data point to a Paleoproterozoic age of the eclogite facies metamorphism.

Ediacaran ophiolite relics in the SE Brazilian coast: Field, geochemical and geochronological evidence from metabasites and paragneisses

Metabasites and paragneisses from the high-grade Ediacaran-Cambrian Cabo Frio Tectonic Domain, SE Brazilian coast, are investigated with field, geochronological (U-Pb combined with Lu-Hf), isotopic (Sm-Nd) and geochemical analyses. These units, tectonically interleaved with a Paleoproterozoic basement, record the pre-collisional setting within the eastern Ribeira Orogen. The four analyzed metabasite bodies indicate tholeiitic basalts/gabbros with OIB affinity as protoliths tending to high-Mg terms, also corroborated by their higher LILE, LREE and HFSE contents, in comparison to the E-MORB and N-MORB basalts. The Nb and Ta (Nb/Yb – alkaline proxy) and Ti/Yb (plume-melting proxy) resemble present-day OIBs values. One metabasite provided mainly metamorphic zircon crystals (n = 20) with a U-Pb age of 530 Ma. Only three crystals show oscillatory zoning, revealing ages of 1890 Ma, 676 Ma and 580 Ma. The Paleoproterozoic zircon, interpreted as a xenocryst, and the Epsilon Nd values (+3 and −6) for whole rock indicate crustal contamination. U-Pb data from detrital zircon of two paragneiss layers, structurally below and above the metabasite, constrain maximum depositional ages of 626 Ma and 549 Ma, respectively. The older has a major 750-620 Ma zircon source, similar to the age of Cryogenian magmatic units in the African terranes. The sediments of this layer are here interpreted as deposited in a distal continental passive margin, coeval with the OIB magmatism (metabasites) crystallized near or at the Ocean-Continent Transitional zone. The upper paragneiss layer shows a major juvenile Ediacaran detrital zircon source between 620 and 550 Ma, with mainly positive Epsilon Hf values (0 to +13). It also presents disrupted blocks of the metabasites, forming a sedimentary breccia. This latter evidence, in addition to its medium P-high T metamorphic paragenesis, supports the hypothesis of an accretionary prism system. The metabasites are here interpreted as ophiolite relics from an Ediacaran Ocean that predated the Gondwana amalgamation. The subsequent ca. 550-530 Ma continental collision preserved this tectonic sliver between the Paleoproterozoic continental crust, below, and the Ediacaran forearc sedimentary sequence, above (Palmital Succession). The formed Cambrian suture was later reactivated as one of the Cretaceous rift branches that evolved to the South Atlantic.

Paleoproterozoic Metamorphism of the Archean Tuntsa Suite, Northern Fennoscandian Shield

The Tuntsa Suite is a polymetamorphic Archean complex mainly consisting of metasedimentary gneisses. At least two strong metamorphic events can be distinguished in the area. The first took place at high temperatures in the Neoarchean at around 2.70–2.64 Ga, indicated by migmatisation and U-Pb ages of metamorphic zircon. During the Paleoproterozoic, metasedimentary gneisses were penetratively deformed and recrystallized under medium pressures producing staurolite, kyanite and garnet-bearing mineral assemblages. The suggested Paleoproterozoic PT path was clockwise where the temperature and pressure first increased to 540–550 °C and 6 kbar, crystallizing high Ca/low Mg garnet cores. The mineral compositions show that commonly garnet core was not in chemical equilibrium with staurolite but crystallized earlier, although garnet-staurolite-kyanite assemblages are common. The temperature and pressure increased to c. 650 °C and 8 kbars where staurolite and kyanite coexist. This was followed by decompression down to c. 550–600 °C and 3–4 kbars, shown by andalusite crystallization and cordierite formed in the breakdown of staurolite and biotite + kyanite. The observed garnet zoning where Mg increases and Ca decreases from the core to the rim was developed with both increasing and decreasing pressure, depending on the effective bulk composition. The U-Pb and Sm-Nd age determinations for monazite and garnet show that the Paleoproterozoic metamorphic cycle took place at 1.84–1.79 Ga, related with thrusting of the Lapland granulites onto the adjacent terranes and subsequent exhumation.

Time lag between metamorphism and crystallization of anatectic granites (Córdoba, Argentina)

SHRIMP and LA-ICP-MS analyses carried out on zircons from the Río de los Sauces granite revealed their metamorphic and igneous nature. The metamorphic zircons yielded an age of 537±4.8 (2σ)Ma that probably predates the onset of the anatexis during the Pampean orogeny. By contrast, the igneous zircons yielded a younger age of 529±6 (2σ)Ma and reflected its crystallization age. These data point to a short time lag of ca. 8Myr between the High Temperature (HT) metamorphic peak and the subsequent crystallization age of the granite. Concordia age of 534±3.8 (2σ)Ma, for both types of zircon populations, can be considered as the mean age of the Pampean HT metamorphism in the Sierras de Córdoba.

Carmacks Copper Cu-Au-Ag Deposit: Mineralization and Postore Migmatization of a Stikine Arc Porphyry Copper System in Yukon, Canada

Abstract Late Triassic to Early Jurassic porphyry Cu mineralization is common in British Columbia, yet there are few age-equivalent porphyry occurrences in Yukon. This study presents new data for the enigmatic Carmacks Copper Cu-Au-Ag deposit in south-central Yukon, Canada, which is hosted in amphibolite facies metamorphic inliers within the Early Jurassic Granite Mountain batholith. Sulfide mineralization occurs mainly as net-textured bornite and chalcopyrite in leucosome, and as chalcopyrite ± pyrite blebs and disseminations in amphibolite and quartz-plagioclase-biotite schist. Several studies suggest that the Carmacks Copper deposit and the nearby Minto deposit are related to porphyry belts in British Columbia, but constraining the timing of alteration, mineralization, and metamorphism has been difficult. This study establishes a geologic and high-precision geochronologic framework for sulfide mineralization and its host rocks at the Carmacks Copper deposit, using Re-Os dating of molybdenite, and chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) analysis of both whole zircon grains and laser-cut fragments of complexly zoned zircon grains. Our data indicate that the igneous protolith of the metamorphic inliers formed at 217.53 ± 0.16 Ma, followed by peak metamorphism at amphibolite facies at 205.82 ± 0.23 Ma, which occurred prior to Granite Mountain batholith emplacement but subsequent to Cu-Au-Ag mineralization of the protolith. An early phase of the Granite Mountain batholith was emplaced at 199.84 ± 0.14 Ma, followed by the main phase at 195 to 194 Ma. A second generation of metamorphic zircon in migmatite at 196.01 ± 0.12 Ma represents a partial melting event associated with Granite Mountain batholith emplacement. Two petrographically distinct populations of molybdenite are present in unstrained, net-textured copper sulfides. A sample dominated by strained molybdenite yielded an 187Re/187Os age of 212.5 ± 1.0 Ma, which represents the minimum mineralization age of the protolith. A sample dominated by euhedral grains yielded an 187Re/187Os age of 198.5 ± 0.9 Ma, constraining the maximum age of sulfide remobilization. These results indicate that primary mineralization is >212.5 Ma and potentially coeval with the ~217.5 Ma generation of Late Triassic magmatism. The mineralized protolith, best interpreted as the potassic alteration zone of a Late Triassic (~217–213 Ma) porphyry Cu-Au system, was metamorphosed to amphibolite facies at ~206 Ma, and subsequently migmatized during 200 to 194 Ma intrusion of the Granite Mountain batholith. The chalcopyrite-bornite-dominant assemblage in neosome precipitated from an immiscible Cu-Fe-S melt phase that partly consumed xenocrystic molybdenite and reprecipitated new molybdenite grains. The Carmacks Copper deposit and the related Minto deposit are remnants of a Late Triassic porphyry belt, where a significant fraction of the original metal endowment was likely lost through digestion of mineralized rocks by midcrustal magma in the Early Jurassic. These Yukon deposits are rare examples of metamorphosed porphyry Cu systems in the global geologic record, where rapid tectonic burial following mineralization was the principal factor in their preservation.

Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans–North China orogen

AbstractIn worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans–North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite-trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) <001> and (110) <001> slip systems, whereas quartz grains are dominated by prism <a> and prism <c> slip systems, suggesting an approximate shear condition of 650–700 °C. This result is consistent with traditional thermobarometry pressure-temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans–North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ∼30 km depth, where high differential stress triggered the large-scale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans–North China orogen at 1.9–1.8 Ga.

New U–Pb zircon ages of Nyong Complex meta‐plutonites: Implications for the Eburnean/Trans‐Amazonian Orogeny in southwestern Cameroon (Central Africa)

New LA–ICP–MS U–Pb zircon ages from the Nyong Complex of southwestern Cameroon—a part of the West Central African Fold Belt—trace Late Mesoarchean (∼2,850 Ma), Middle Palaeoproterozoic (∼2,080 Ma), and Neoproterozoic (∼605 Ma) events: Two meta‐syenites and the protolith of an amphibolite are Late Mesoarchean; two meta‐granodiorites are Middle Palaeoproterozoic; the amphibolite may have recrystallized in the Middle Palaeoproterozoic; all rocks are overprinted by the Neoproterozoic event. Integration with published data shows that our amphibolite sample has one of the oldest amphibolite‐protolith ages (∼2,810 Ma) reported so far. It shares the Middle Palaeoproterozoic metamorphism/recrystallization with other, previously dated amphibolites. An earlier reported metamorphic zircon age (∼2,090 Ma) from eclogite is somewhat older than the regional Middle Palaeoproterozoic metamorphism/recrystallization ages (∼2,040 Ma) reported from amphibolites. Thus, the eclogite–amphibolite ages may date an exhumation process. A published charnockite age, interpreted as an Early Mesoarchean crystallization age, is older than the Late Mesoarchean meta‐syenite and amphibolite‐protolith dates; its Middle Palaeoproterozoic metamorphism/recrystallization age, however, is identical with the meta‐granodiorites and amphibolites. The Neoproterozoic ages demonstrate the regional overprint of the Nyong Complex during this period. Integration of the Nyong Complex ages with published ones from the entire West Central African Fold Belt, and comparison with those from West Africa and South America, support their common origin from the Palaeoproterozoic collision between the Archean Congo and São Francisco shields.

The Mesoarchaean Akia terrane, West Greenland, revisited: New insights based on spatial integration of geophysics, field observation, geochemistry and geochronology

The northern part of the North Atlantic Craton (NAC) in southern West Greenland comprises a large tract of exposed Meso-Neoarchaean continental crust, divided into the ca 3300–2900 Ma Akia and ca 2900–2500 Ma Tuno terranes. We combine aeromagnetic, stream sediment geochemical, new litho-chemical and zircon geochronological data with previously published data to re-evaluate the crustal architecture and evolution of the Akia terrane and its boundary towards the Tuno terrane.The previously recognised, but overlooked, Alanngua complex, situated between the Akia and Tuno terranes is bounded by aeromagnetic lineaments interpreted as Neoarchaean shear zones and has a distinct spectrum of Neoarchaean magmatic and metamorphic zircon ages that are rare in the Akia terrane. The Alanngua complex comprises components derived from both the Akia and Tuno terranes and is interpreted as a tectonic melange created during the Neoarchaean assembly of the NAC.Within the Akia terrane, the chemistry of orthogneiss samples indicate that a large percentage is too mafic to classify as TTG s.s., implying that not only partial melting of mafic crust, but also some yet unaddressed mantle involvement is necessary in their formation. Previous models for the generation of the ca. 3015–2990 Ma quartz-dioritic Finnefjeld and Taserssuaq complexes conflict with their geochemical variation. We argue that the complexes may be genetically linked to buried gabbro-dioritic rocks indicated by their strong aeromagnetic response. Formed at same time are carbonatite, high-Mg gabbro and tonalite-trondhjemite, and we propose that this wide spectrum of rocks was formed by lithospheric and crustal melting in response to asthenospheric upwelling possibly in an extensional setting.Periods of extensive magmatism in the Akia terrane were previously recognised at ca. 3220–3180 Ma and 3070–2970 Ma. We now subdivide the latter period into three episodes: juvenile basaltic-andesitic volcanism at 3070–3050 Ma; tonalitic and dioritic plutonism at 3050–3020 Ma, and gabbroic-dioritic plus tonalitic-trondhjemitic plutonism at 3020–2985 Ma. This last episode was immediately followed by crustal reworking during collision at 2980–2950 Ma.

Extensive magmatism and metamorphism at ca. 3.2 Ga in the eastern Kaapvaal Craton

The timing of the emergence of modern-style plate tectonics on Earth is of fundamental importance in understanding the thermal and compositional history of the planet. Although the magmatism and metamorphism in the eastern Kaapvaal Craton was thought by some as geological records for mid-Archean subduction at 3.2 Ga, their petrogenesis was fiercely debated. To reveal the nature of the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton, here we provide a comprehensive zircon and monazite U-Pb geochronological investigation, coupled with in situ trace element and Hf-O isotopic analyses, for the magmatic and metamorphic rocks from the Barberton granitoid-greenstone terrane in South Africa and Ancient Gneiss Complex in Swaziland. In spite of deriving from potentially different sources as revealed by different lithology, nine magmatic rocks sampled from different plutons have consistent zircon/monazite SIMS 207Pb/206Pb ages of 3240–3220 Ma. Distinct zircon grains, including inherited magmatic and metamorphic, are identified in the metamorphic rocks by detailed CL images and U-Pb geochronological, trace elemental and Hf-O isotopic analyses. The metamorphic zircons give consistent 207Pb/206Pb ages of 3240–3220 Ma. One granitic gneiss and a metapelite give monazite SIMS 207Pb/206Pb dates of 3230.6 ± 2.7 Ma and 3223.1 ± 2.4 Ma, respectively, which are consistent with the results of metamorphic zircon and interpreted as the age of metamorphism. Most metamorphic samples give Ti-in-zircon temperatures of 650–740 °C and one sample yields Y-in-monazite temperatures of 918–966 °C, which imply that the rocks of AGC were universally metamorphosed to upper amphibolite facies, and locally reaching high to ultrahigh temperature (HT-UHT) granulite facies (>900 °C) at 3240–3220 Ma. This study, in combination with previous results, suggests widespread distribution for the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton without asymmetry in the thermal structure, which is quite different from those of the modern subduction zones. The synchronous occurrence of the metamorphism and magmatism in the eastern Kaapvaal Craton at 3.2 Ga was interpreted as the results of partial convective overturn of the crust.

Zircon U-Pb geochronology and Nd-Pb isotope geochemistry of Blue Ridge basement in the eastern Great Smoky Mountains, U.S.A.: Implications for the Proterozoic tectonic evolution of the southeastern Laurentian margin

The Mesoproterozoic to Paleozoic history of the southeastern Laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events. The Great Smoky Mountains Basement Complex (GSMBC) within the southern Appalachian Blue Ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of Laurentian margin evolution from ca. 1.9 Ga to 450 Ma. The GSMBC consists primarily of: (1) 1.34 to 1.31 Ga (pre-Elzevirian) granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (correlated with pre-Elzevirian crustal components in the Adirondack, Green Mountains, New Jersey Highlands, and French Broad massifs), (2) ca. l.15 to 1.05 Ga augen and granitic orthogneiss produced during Shawinigan and Ottawan phases of Grenville-age magmatism and metamorphism, respectively, and (3) paragneiss derived from protoliths with either Grenville-age (1.1–1.0 Ga) or post-Grenville (Neoproterozoic) depositional ages based on presence/absence of ca. 1.0 Ga metamorphic zircon and 1.9 to 1.1 Ga detrital zircon. All lithologies experienced Taconian metamorphism and variable migmatization. Pre-Ottawan paragneiss exhibits major detrital zircon ages modes at 2.0 to 1.6 and 1.4 to 1.3 Ga that require a component of older Proterozoic crust in the sediment source region. Detrital zircon grains in post-Ottawan paragneiss exhibits the full spectrum of Grenville-age modes that correlate with the five phases of Grenville magmatic/metamorphic events in eastern Laurentia. These paragneiss samples also contain scattered 750 to 600 Ma detrital zircon grains that constrain their maximum depositional age to late Neoproterozoic. The sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all Grenville crustal age components during post-orogenic exhumation and crustal extension leading up to Late Neoproterozoic breakup of Rodinia. Most zircon U-Pb age systematics exhibit variable discordance that can be attributed to disturbance of the U-Pb system and/or new zircon growth during either high-grade Ottawan (ca. 1.04 Ga) or Taconian (ca. 0.46 Ga) regional metamorphism and migmatization. Neodymium TDM model ages for granodioritic orthogneiss and paragneiss range from 1.8 to 1.6 Ga, indicating that the rocks were derived from recycling of Proterozoic crust (that is, they are not juvenile), consistent with the 1.9 to 1.6 Ga detrital zircon grains in pre-Ottawan paragneiss and with 1.8 to 1.7 Ga inherited zircon in some 1.33 Ga orthogneisses and a 1.35 Ga xenolith. Whole rock Pb isotope compositions of GSMBC rocks overlap the field of compositions characteristic of Amazonian crust and of other basement rocks from the south-central Appalachians. The Pb isotopes and geochronology in orthogneiss, mafic xenoliths, and pre-Ottawan paragneiss are consistent with a correlation of the GSMBC with the Mars Hill terrane within the French Broad massif and with the greater Grenvillian south-central Appalachian basement (SCAB) that is considered exotic to Laurentia, and transferred during Rodinian collision prior to ca. 1.2 Ga. Similarities in protolith ages and Pb isotopes point to the Paragua terrane of Amazonia (southwestern Brazil and eastern Bolivia) as a likely match for SCAB. Initial Amazonia-Laurentia collision occurred at ca. 1.35 to 1.32 Ga with final transfer of SCAB to Laurentia occurring after 1.20 Ga within the sinistral oblique collision zone between Laurentia and Amazonia defined by previous workers.

An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA)

AbstractWe investigated a suite of metabasite blocks from serpentinite matrix and shale matrix mélanges of the California Coast Ranges. Our new data set consists of 40Ar/39Ar dates of amphibole and phengite and U‐Pb dates of metamorphic zircon. Combined with published geochronology, including prograde Lu‐Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. In particular we find that exhumation from amphibole‐eclogite facies conditions occurred as a single episode at 165–157 Ma, with an apparent southward younging trend. The rate and timing of exhumation were initially uniform (when comparing individual blocks) and fast (with cooling rates up to ~140°C/Ma). In the cooler and shallower blueschist facies, exhumation slowed and became less uniform among blocks. Considering the subduction zone system, the high‐grade exhumation temporally correlates with a magmatic arc pulse (Sierra Nevada) and the termination of forearc spreading (Coast Range Ophiolite). Our findings suggest that a geodynamic one‐time event led to exhumation of amphibole‐eclogite facies rocks. We propose that interaction of the Franciscan subduction zone with a spreading ridge led to extraction of the forearc mantle wedge from its position between forearc crust and subducting crust. The extraction led to fast and uniform exhumation of subducted rocks into the blueschist facies. We also show that the Franciscan subduction zone did not undergo significant cooling over time and that its initiation was not coeval with blueschist‐facies metamorphism of the Red Ant schist of the Sierra Nevada foothills.

Exploring laser ablation U–Pb dating of regional metamorphic garnet – The Straits Schist, Connecticut, USA

Garnet is a prominent recorder of metamorphic processes and plays a pivotal role in deciphering the pressure-temperature evolution of metamorphic rocks. However, the potential of regional metamorphic garnet for laser ablation U–Pb dating remains unexplored, as variable and low U (<0.1 μg/g) coupled with elevated initial Pb contents make analyses challenging, and the presence of U-rich inclusions can affect results.We applied LA-ICP-MS U–Pb dating to garnet and zircon, monazite, and xenotime from The Straits Schist, western Connecticut, USA, to constrain the timing of garnet growth within the overall Acadian-age metamorphic history recorded by this unit. We assessed the effects of U-rich inclusions on garnet U–Pb data using high-resolution microbeam sampling, grain size analysis, and calculations assessing how inclusion size and abundance affect U concentrations measured in garnet.When analysing garnet, our results demonstrate that by scrutinizing the time-resolved U-signal, U-rich inclusions can be identified and effectively removed during data evaluation. If U-rich inclusions are unaccounted for, ‘garnet’ analyses have elevated U contents compared to pure garnet and in a Tera-Wasserburg diagram, data points are shifted to the left, right, or along the garnet regression line, for inclusions older, younger or of the same age as the garnet, respectively.Garnet from two metapelitic samples yielded lower intercept ages of 409 ± 10 Ma and 406 ± 15 Ma, and a combined U–Pb age of 406.1 ± 8.3 Ma. Metamorphic zircon yielded age clusters at 446.2 ± 4.2 Ma, 413.3 ± 3.0 Ma, and 382.9 ± 2.8 Ma, monazite ages range between 390 and 355 Ma, and xenotime yielded 370.1 ± 5.6 Ma. The ∼446 Ma age peak is attributed to detrital zircon formed during the waning stages of the Taconian orogeny. Early Acadian metamorphism is recorded by garnet and ∼413 Ma zircon, whereas monazite and zircon ages of 390 to 383 Ma, coincide with peak metamorphic conditions. Early Carboniferous monazite and xenotime ages could reflect the incursion of fluids into The Straits Schist during Neo-Acadian metamorphism, coeval plutonism in New England, or breakdown of garnet during regional exhumation.

Neoproterozoic extension and the Central Iapetus Magmatic Province in southern Mexico – New U-Pb ages, Hf-O isotopes and trace element data of zircon from the Chiapas Massif Complex

Final fragmentation of Rodinia occurred during the Ediacaran Period as Amazonia, Baltica and Laurentia drifted apart to form the Iapetus Ocean. Accompanying rift-related mafic dyke swarms of the Central Iapetus Magmatic Province (CIMP) were emplaced between 0.62 and 0.55 Ga, which are preserved in Laurentia and Baltica, whereas no coeval mafic rocks are known from Amazonia. First evidence for the CIMP extending into Oaxaquia (Rodinia-type basement of Mexico) was reported as tholeiitic dykes that intruded the Novillo gneiss, NE Mexico, at 619 ± 9 Ma. In Chiapas, SE Mexico, amphibolite dykes that are chemically similar to the Novillo dykes intruded anorthosite and gneiss. In this paper, a new dating approach to obtain mafic dyke intrusion ages is presented by targeting contact metamorphic zircon with the UPb method, employing Secondary Ion Mass Spectrometry. Zircon that crystallized in anorthosite at intrusive contacts to mafic dykes and at temperatures exceeding 700 °C (Ti-in-zircon thermometry) yields ages between 615 ± 7 Ma and 608 ± 12 Ma, reflecting the time of dyke intrusion. Zircon chemical and isotopic (Hf, O) characteristics suggest a diachronous sequence of metamorphic processes involving Zr release from FeTi oxides, breakdown and recrystallization of other phases, and fluid-mitigated reactions during Ordovician metamorphism. Zircon δ18O values of granulites from Oaxaquia range from +6.2‰ to +9.8‰, whereas Tonian (~0.92 Ga) metamorphic zircon from SE Chiapas yielded low δ18O values from +2.0‰ to +2.8‰ that are explained by the reactivation of major tectonic boundaries during Tonian gravitational collapse. The observations increase the known extent of the CIMP in Mexico, suggesting that a Neoproterozoic superplume was still active during the Early Ediacaran producing a Large Igneous Province that extended over Amazonia, Baltica and Laurentia. The results further suggest that Oaxaquia at the northern edge of Amazonia formed the conjugate margin of Baltica during rifting.

The earliest Cambrian UHT metamorphism in the Qaidam block, western China: A record of the final assembly of Greater Gondwana?

The final assembly of Gondwana, known as the late Pan-African orogeny, is characterized by Ediacaran–early Cambrian ultrahigh temperature (UHT) metamorphism, which is widely identified within reconstructed East Gondwana. This distinctive feature likely provides a reliable criterion for identifying new Gondwanan terranes that lack paleo-geomagnetic data. Here we present zircon U–Pb geochronology and phase equilibria calculations for a variety of granulite types newly recognized from western Qaidam, China, which provide the first evidence that the Qaidam block, at least western Qaidam, experienced high-grade metamorphism in excess of 900 °C before/at 540–520 Ma. These UHT metamorphic rocks, similar to many well-known Pan-African UHT metamorphic terranes, is inferred to evolve along a clockwise P–T path that is usually related to collisional orogens. Comparison between new metamorphic zircon U–Pb ages from western Qaidam and the published age data from the UHT metamorphic terranes within East Gondwana suggests that the UHT metamorphic rocks found in western Qaidam similarly records the final assembly of Gondwana. Although the exact paleo-geographical location of the Qaidam block during the Gondwana period is unknown yet because of lacking paleo-geomagnetic data, new Pan-African UHT metamorphic record found in western Qaidam indicates, for the first time, that the Qaidam block is a Gondwanan terrane that split from this semi-supercontinent after the Pan-African orogeny.

Ordovician metamorphism and magmatism preserved in the Ossa Morena Complex: SHRIMP geochronology, geochemistry and Sr[sbnd]Nd isotopic signatures of the Sierra Albarrana Domain (SW Iberian Massif)

This study integrates SHRIMP UPb zircon geochronology and whole-rock and SrNd isotope geochemistry data from migmatites and granitic rocks of the Sierra Albarrana Domain. We provide new information on the magmatic and metamorphic events of this domain of the Ossa-Morena Complex. The metasedimentary succession, nowadays corresponding to a migmatite dome, was probably deposited during the Ediacaran and Cambrian. SHRIMP UPb ages of magmatic and zoned metamorphic zircon grains indicate that the main tectono-magmatic event in the Sierra Albarrana Domain is Late Cambrian (granite emplacement; 481.2 ± 2.3 Ma) to early Ordovician (migmatization; 477.8 ± 2.4 Ma), supported by the intrusion of La Cardenchosa Pluton, by the low-pressure partial melting and by the ductile deformation of the host metasedimentary rocks. We suggest that the granitic magmatism, the migmatization and the main regional fabrics are part of a single Ordovician event, only slightly modified by Variscan reworking.Geochemical and isotopic data are compatible with an extensional environment in the middle-upper crustal levels, related to a continental rift development. This setting and timing are compatible with the latest magmatic event of the Early Paleozoic rifting (535–460 Ma), well documented in the Ossa Morena Complex. The geochemical characteristics of the subalkaline rift-related rocks of the Ossa Morena Complex and of the Central Iberian Zone partly coincide. As a result, it is not possible to discriminate between them and favor a correlation between both terranes based on the geochemical affinities of magmatic rocks of similar age.