Hf Isotopic Signatures Research Articles

A new 3.59 Ga magmatic suite and a chondritic source to the east Pilbara Craton

The Pilbara Craton, Western Australia hosts one of the best-preserved Paleoarchean granite-greenstone terrains on Earth, and is inferred to have developed on an older (>3.8 Ga), possibly Hadean, continental substrate. Such ancient crust has, however, never been identified in outcrop. Here, we show that metamorphosed gabbroic, leucogabbroic and anorthositic rocks of the South Daltons area, in the western part of the Shaw Granitic Complex, formed at 3.59–3.58 Ga and were intruded by granitic magma at 3.44 Ga. The 3.59–3.58 Ga gabbroic rocks, here named the Mount Webber Gabbro, represent the oldest, unambiguous igneous rock emplacement in the Pilbara Craton and significantly predate the oldest volcanic activity of the 3.53–3.23 Ga Pilbara Supergroup within the East Pilbara Terrane. We interpret the Mount Webber Gabbro samples to represent fragments of a dismembered layered mafic intrusion. Mantle-like zircon δ18O and Hf isotope signatures indicate derivation from a chondritic to near chondritic mantle at ~3.59 Ga, and do not support the existence of a >3.8 Ga basement to the East Pilbara Terrane. These results strengthen the notion of an approximately chondritic >3.5 Ga mantle beneath the Pilbara Craton, and provide further evidence that recent estimates of Archean stabilised continental volumes, based on the assumption of crust extraction from a global, convecting depleted mantle reservoir, may be overestimated.

Termination of the Hegenshan Orogen in the Xing'an–Mongolian Orogenic Belt, North China: Geochemical and zircon U–Pb geochronological constraints from Early Permian mafic dykes

Based mainly on field geological observation, we identified the Dong Ujimqin mafic dykes in Inner Mongolia, north of Hegenshan ophiolite accretionary belt. The dykes are mainly dolerite, and their ages and source were determined using geochronological, geochemical, zircon Hf, and whole‐rock Sr–Nd isotopic data. Photomicrographs reveal ophitic texture and amygdale structures of the dolerite dykes. Laser ablation multiple collector inductively coupled plasma mass spectrometry U–Pb analysis of zircons yielded ages of 277.9 ± 1.8 and 275.9 ± 1.2 Ma for two of these mafic dykes, placing them within the Early Permian. The dykes are classified as alkaline and tholeiitic series according to their major elements and characterized by high Na2O (2.62–4.06 wt.%), high TiO2 (1.63–2.71 wt.%), and low K2O (0.74–2.06 wt.%). The mafic dykes have similar geochemical characteristics that exhibit large‐ion lithophile element enrichment relative to high‐field‐strength elements, depletion in Ta and Nb, and an asthenosphere‐like Sr–Nd–Hf isotopic signature. They have high Sm/Nd (0.131–0.141) and LaN/YbN (3.75–9.17), near‐chondritic heavy rare earth elements ([Gd/Yb]N = 1.21–2.39), and low Rb/Sr (0.03–0.08) ratios with negligible Eu anomalies (Eu/Eu* = 0.85–1.00). The dykes contain relatively low amounts of radiogenic Sr, as shown by (87Sr/86Sr)i values (0.7035–0.7042), and have positive values of εNd(t; 4.31–6.96) and εHf (t; 6.0–14.5) and negative values of εSr(t; −0.28 to −10.31), suggesting that the mafic dykes have been derived from partial melts of the depleted lithospheric mantle contaminated by crust in the source region and formed in post‐orogenic settings. All the above indicate an extensional event, which was related to the post‐subduction of the Paleo‐Asian Ocean, at ~277–275 Ma, and significant vertical continental growth of the crustal continuum occurred in the continental interior during post‐collisional extension.

Geochronology, mineralogy, and geochemistry of the Late Triassic Xiuwacu biotite granite in the southern Yidun Terrane, southwest China: Insights into the petrogenesis and magmatic fertility

The southern Yidun Terrane formed in response to the westward subduction of the Garzê–Litang oceanic lithosphere. The Xiuwacu pluton, consisting of the Late Triassic biotite granite and Late Cretaceous monzogranite porphyry, is located in the northern portion of the southern Yidun Terrane. It hosts the Late Cretaceous quartz‐vein‐type Mo–W deposit genetically related to the intraplate monzogranite porphyry. However, the petrogenesis and magmatic fertility of the biotite granite remain unclear. Sixteen zircons from the biotite granite yield a weighted mean 206Pb/238U age of 217 ± 1.2 Ma, indicating that the biotite granite emplaced synchronously with the arc‐related andesites and porphyries in the southern Yidun Terrane. Unlike these slab melt‐derived adakitic rocks, the Xiuwacu biotite granites show typical I‐type granite affinities, with relatively high SiO2 contents, low Sr/Y ratios, and high Y contents. These granites display depleted zircon Hf isotopic signatures (ƐHf(t) = 0.6–3.8) but enriched whole‐rock Sr–Nd and zircon O isotopic signatures, suggesting that the mixing between juvenile materials and external enriched melts might have contributed to the generation of biotite granite. Moreover, they show relatively high Al2O3/(FeOT + MgO + TiO2), Th/Yb, and Th/Nb ratios, which are collectively indicative of the input of sediment melts extracted from subducting slab into the melt source of biotite granite. The high MgO, TiO2, F, and Cl contents of biotite and the excess Al of plagioclase suggest that the Xiuwacu biotite granites formed under high oxygen fugacities, magmatic water contents, and volatiles components and they might have Cu prospecting potentials.

Age and composition of the subcontinental lithospheric mantle beneath the Xing'an–Mongolia Orogenic Belt: Implications for the construction of microcontinents during accretionary orogenesis

We present major- and trace-element concentrations, Sr–Nd–Hf isotopic compositions, ReOs isotopes, and highly siderophile element (HSE) abundances of 17 xenolithic mantle peridotites from the Nuomin volcanic field to constrain the assembly and evolution of the Xing'an–Mongolia Orogenic Belt (XMOB), Northeast China. The Nuomin peridotites are divided into five groups (harzburgite groups Haz-1, Haz-2, and Haz-3, and lherzolite groups Lhz-1 and Lhz-2). The Haz-1 peridotites are characterized by refractory chemical compositions (Al2O3 contents of 1.03–2.30 wt% and olivine forsterite (Fo) numbers of 90.7–92.5) that are best interpreted as residues after high degrees of mantle partial melting. The Haz-2 and Haz-3 peridotites exhibit slightly more fertile compositions (Al2O3 of 1.72–1.80 wt% and Fo of 89.0–89.9 for the Haz-2, and Al2O3 of 2.86 wt% and Fo of 91.2 for the Haz-3), resulting from refertilization by silicate melts. The Lhz-1 samples are more fertile (Al2O3 of 2.90–3.64 wt% and Fo of 89.1–90.3) than the Lhz-2 samples (Al2O3 of 1.64–2.16 wt% and Fo of 90.9–91.3). The combination of trace-element data (Ti/Eu ratio) and Sr–Nd–Hf isotopic compositions indicates the presence of at least three peridotite end-members (Haz-1, Haz-2, and Lhz-1–Lhz-2) beneath the Nuomin area, which can be explained by the infiltration of two distinct metasomatic agents (MA1: composition similar to potassic basaltic rocks in the Wudalianchi–Erkeshan–Keluo area, and MA2: Fe–Ti-enriched melt). HSE abundances and Os isotopic data show that the Os isotopic compositions of the Haz-1, Lhz-1, and Lhz-2 peridotites were not significantly affected by lithophile-element-dominated metasomatism and can be used to constrain the melt extraction ages of these peridotites. The lower 187Os/188Os ratios of Haz-1 peridotites define a prominent rhenium depletion age (TRD) of ~1.6 Ga and represent fragments of the Paleo–Mesoproterozoic lithospheric mantle rather than reflecting the heterogeneous characteristics of the asthenosphere or extraneous emplacement. In contrast, the Lhz-1 and Lhz-2 lherzolite groups together have a narrow range of 187Os/188Os ratios that give younger TRD ages (~0.5 to 1.0 Ga). Our data demonstrate that both the Paleo–Mesoproterozoic and Neoproterozoic subcontinental lithospheric mantle coexist beneath the XMOB and that they correspond respectively to the bimodal age peaks of crustal growth of ~1.6 Ga and ~0.8 Ga defined by the XMOB granitoids. The XMOB was assembled primarily during the Phanerozoic by multiple blocks that had formed from the Paleoproterozoic to Neoproterozoic. The similar chemical and Sr–Nd–Hf isotopic signatures of both young and old lithospheric mantle indicate that the large-scale metasomatism event involving potassic melts most likely occurred during the evolution of the Paleo–Asian Ocean. The crustal materials carried to the mantle by multiple subduction events could be an effective source of potassium for the origin of potassic basaltic rocks.

Decoding Earth’s rhythms: Modulation of supercontinent cycles by longer superocean episodes

The supercontinent cycle of episodic assembly and breakup of almost all continents on Earth is commonly considered the longest period variation to affect mantle convection. However, global zircon Hf isotopic signatures and seawater Sr isotope ratios suggest the existence of a longer-term variation trend that is twice the duration of the supercontinent cycle. Here we propose that since ∼2 billion years ago the superocean surrounding a supercontinent, as well as the circum-supercontinent subduction girdle, survive every second supercontinent cycle. This interpretation is in agreement with global palaeogeography and is supported by variations in passive margin, orogen, and mineral deposit records that each exhibits both ∼500–700 million years periodic signal and a 1000–1500 million years variation trend. We suggest that the supercontinent cycle is modulated by an assembly that alternates between dominantly extroversion after a more complete breakup, and dominantly introversion after an incomplete breakup of the previous supercontinent.

Experimental evidence for mineral-controlled release of radiogenic Nd, Hf and Pb isotopes from granitic rocks during progressive chemical weathering

The role of accessory minerals in the incongruent release of Hf and Pb during continental weathering and its implications for the generation of distinct seawater isotope compositions is subject of debate. While it has been suggested that radiogenic Hf and Pb isotope signatures released during the dissolution of rocks are controlled by the relative abundances of minerals with distinct isotope compositions and differences in their resistance to dissolution there has not been a comprehensive experimental investigation of these processes to date. We carried out systematic sequential leaching experiments on fresh and partly weathered granitic rock samples as well as separated zircons from the Central Aar Granite in Switzerland. Combined with major and rare earth element concentrations our new quantitative experimental data reveal systematic preferential release of radiogenic Nd, Hf and Pb isotopes primarily controlled by dissolution characteristics of the host rock's easily dissolvable accessory and major minerals, in particular apatite and sphene, during weak chemical weathering. Moreover, Pb isotope signatures of incipient weathering conditions, contrary to expectations, indicate initial congruent release of Pb from freshly exposed mineral surfaces that becomes subsequently incongruent. During more advanced chemical weathering stages, as well as enhanced physical weathering conditions, the dissolution of major minerals (i.e. feldspars) becomes dominant for Nd and Pb isotope signatures, whereas Hf isotopes are still dominated by contributions from highly radiogenic accessories.Additional leaching experiments of zircon separates were performed to test the specific role of zircons for Hf isotope compositions of riverine runoff. It is demonstrated that zircon is more efficiently dissolved when physical weathering is enhanced. This increased Hf release originating from partial dissolution of zircons, however, is quantitatively not sufficient to explain less radiogenic Hf isotope signatures in seawater during episodes of enhanced mechanical erosion alone. Moreover, the observed addition of Hf from the more congruent dissolution of the zircon-free fractions of the parent rock due to enhanced physical weathering indicates that these minerals also play an important role in controlling Hf isotope signatures released under deglacial conditions.

Compositional variations of granitic rocks in continental margin arc: Constraints from the petrogenesis of Eocene granitic rocks in the Tengchong Block, SW China

In order to address the causes of compositional variations of crust-derived granitic rocks in continental margin arc, we focused on the petrogenesis of Eocene granitic host rocks with enclosed MMEs, associated metagabbro, fine-grained quartz-diorite in Xima-Tongbiguan area, the Tengchong Block, which is a continental margin arc related to the subduction of Neo-Tethys. Both the Eocene granitic and mafic rocks are characterized by gradual increment in enriched compositions, such as K, Rb, Th, LREE, initial 87Sr/86Sr, εNd(t), and εHf(t), but decreasing Pb isotopic ratios from the west to the east of the Tengchong Block.Zircon UPb age data of granitic host rocks, MMEs, metagabbro and fine-grained quartz-diorite reveal a similar age range of 55–50 Ma. The lithologies, zircon Hf compositions, geochemical and isotopic signatures of host granitic rocks show that the origin of these host rocks are mainly controlled by magma mixing between felsic and mafic magma. The products of the mafic magma are dominated by metagabbro, fine-grained quartz-diorite and MMEs enclosed in granitic rocks, which are all resulted from the reaction and mixture between different mafic magmatic products and felsic magma. The metagabbros could be the products of felsic magma mixing with early crystallized mafic magma, and the fine-grained quartz-diorites could be the products of felsic magma mixing with crystal-rich mafic magma, and the MMEs could be the products of felsic magma mixing with crystal-poor mafic magma or melt. The metagabbro has positive εHf(t) values of 1.8 to 10.9, which could provide the relative primitive LuHf isotopic compositions of mafic magma, indicating that the mafic magma is derived from a depleted mantle. The host granitic rocks have negative εNd(t) values of −5.2 to −6.5, indicating that the felsic magma is derived from partial melting of an ancient mafic lower crust. Furthermore, there are excess reactions after magma mixing between the mafic and felsic magma, such as multi-stage reactions between solidified MMEs and felsic magma, and the disaggregation of MMEs, finally resulting in the basification of felsic magma. Magma mixing is therefore the main mechanism of the formation of granitic rocks in the Tengchong Block. The compositional variation of intermediate to acid igneous rock in a continental margin arc is likely dominated by the characteristics of mafic magma because the intermediate to acid igneous rock is derived from mixing of crust-derived granitic and different mantle sources-derived mafic magma.

Detrital zircon U-Pb and Hf isotope characteristics of the Early Neoproterozoic successions in the central-western Korean Peninsula: Implication for the Precambrian tectonic history of East Asia

The Early Neoproterozoic successions in the central–western Korean Peninsula preserve important geological records of the Late Paleoproterozoic to Mesoproterozoic events associated with the disruption of the Columbia supercontinent, and the amalgamation and disruption of the Rodinia supercontinent. In this study, we present SHRIMP and LA–MC–ICPMS U-Pb ages and Hf isotope signatures of detrital zircon grains from metasedimentary rocks of the Jangbong Formation in the Hongseong-Imjingang Belt and the Sangwon System in the Nangnim Massif. Most zircon grains from all the samples are magmatic in origin, yielding similar U-Pb age spectra ranging from Neoarchean to Late Mesoproterozoic or Early Neoproterozoic, with multiple age peaks within the Late Paleoproterozoic to Mesoproterozoic range (ca. 1876 Ma, ca. 1744 Ma, ca. 1578 Ma, and ca. 1168 Ma in the Jangbong Formation and ca. 1754 Ma, ca. 1588 Ma, ca. 1450 Ma, and ca. 1160 Ma in the Sangwon System). The maximum ages of deposition estimated from the youngest concordant detrital zircons are ca. 1.0 – 0.96 Ga from the Jangbong Formation and ca. 0.98 Ga from the Sangwon System. Ages and Hf isotope signatures of dominant Late Paleoproterozoic to Mesoproterozoic detrital zircon populations reflect magmatic history involving juvenile input and crustal reworking. In contrast, the minor magmatic detrital zircon populations with Neoarchean to Middle Paleoproterozoic ages show negligible contribution from basement source materials of the adjacent Gyeonggi and Nangnim massifs. The dominant Late Paleoproterozoic to Mesoproterozoic zircon populations examined in this study in conjunction with available data from the Korean Peninsula correlate with the geological and geochronological evidence from the North China Craton associated with rifting along the northeastern margin of the southeastern domain of the Columbia during the Late Paleoproterozoic to Early Neoproterozoic.

Episodic crustal growth and reworking of the Yudongzi terrane, South China: Constraints from the Archean TTGs and potassic granites and Paleoproterozoic amphibolites

The Yudongzi terrane is featured by well-preserved lithologic outcrops of basement rocks, including the ~2.7 Ga tonalite-trondhjemite-granodiorite (TTGs), ~2.5 Ga potassic granites, Neoarchean supracrustal rocks and ~1.85 Ga amphibolites. The TTG rocks contain ~2.8 Ga inherited/xenocrystic zircons, which indicates the existence of ancient rocks in the magma source. Most zircon grains in the TTG rocks have chondrite-like Hf isotopic compositions, indicative of derivation of magmas from the Paleo-Mesoarchean (3.3–3.0 Ga) crustal sources, consistent with their whole-rock Nd model ages (3.3–2.9 Ga). Zircon oxygen isotopic compositions (5.4 to 6.1‰) are mainly in equilibrium with the mantle, indicating insignificant surficial processes for the source rocks. Thus, the Archean TTGs likely originated from melting of an overthickened oceanic island arc where shallow subduction occurred. The compositional diversity of the TTG rocks can be ascribed to varying residual minerals after melting of mafic precursors at different crustal depths. The subsequent intracrustal melting of the TTG-dominated proto-crusts formed more potassic granites at ~2.5 Ga, probably associated with dense residue delamination and concomitant mantle upwelling. Detrital zircon age pattern and geochemical features of the meta-sediments from the Yudongzi Group demonstrate that the adjacent meta-volcanic components are the major sedimentary source, and thus they were proximal erosion and deposition products of an arc system within a proto-subduction zone. Protolith Hf isotope signature in metamorphic zircons of the amphibolites provides evidence for the existence of early Archean basaltic proto-arc crusts which were metamorphosed at ~1.85 Ga. Collectively, the above-stated Neoarchean to Paleoproterozoic tectonothermal events imply significant crustal growth and reworking, which were coincident with global-scale continental lithospheric development and stabilization during assembly and dispersal of supercontinents Kenorland and Columbia. Similar tectonostratigraphic, paleomagnetic and geochronologic records support a Neoarchean-Paleoproterozoic connection of the Yangtze Block with the circum-N Laurentia and southern Siberia. Furthermore, the late Archean geochemical transition from sodic TTG to potassic crust record, as a sign of cratonic maturation, is collectively related to the kinematic adjustment of supercontinent process and thermal evolution of the Earth system, therefore sheding lights on subduction tectonic operation and varied crustal growth mechanism.

Neoproterozoic magmatic and metamorphic events in the Cuchilla Dionisio Terrane, Uruguay, and possible correlations across the South Atlantic

The Cuchilla Dionisio (or Punta del Este) Terrane of the southern Dom Feliciano Belt, eastern Uruguay, experienced a prolonged Neoproterozoic to Cambrian tectonothermal evolution. Zircon and monazite age data from orthogneisses of the basement of the terrane (Cerro Olivo Complex) provide evidence of widespread magmatism at c. 780 Ma and high-grade, up to granulite-facies metamorphism between 655 and 632 Ma. Isotope data imply that the orthogneiss protolithic melts formed in a within-plate, rift-related setting by Neoproterozoic (re-) melting of lower crustal material that was extracted from the mantle during the Palaeoproterozoic. We show that the terrane extends further north than previously thought. The presence of inherited Meso- and Palaeoproterozoic zircon grains indicates the existence of older crustal components in the terrane. The ages of the detrital zircon grains and their Hf isotope signatures cannot be explained by a Río de la Plata Craton provenance but are consistent with a southern African Kalahari Craton origin. Ar-Ar ages of 628 and 625 Ma were obtained on magnesiohornblende in amphibolite from the northern Cuchilla Dionisio Terrane, whose protolith formed in an Ediacaran back-arc basin. These ages are interpreted as the time of amphibolite-facies metamorphism. Thus, crustal convergence between the Kalahari (or Congo?) and the Río de la Plata Craton and subduction of the intervening oceanic basin must have been underway between 655 and 625 Ma.The early Neoproterozoic basement of the Cuchilla Dionisio Terrane is part of the southern African Namaqua Province that was separated from the Kalahari Craton during Rodinia break-up and opening of the southern Adamastor Ocean in the Tonian. Closure of this ocean in the late Cryogenian to early Ediacaran was associated with arc magmatism and high-temperature metamorphism of the Cuchilla Dionisio Terrane basement. A back-arc basin formed and was closed between this arc and the western African margin. Relics of this basin were obducted onto the Cuchilla Dionisio Terrane at c. 625 Ma. Late Neoproterozoic intrusive igneous rocks from the Cuchilla Dionisio and the Coastal terranes were emplaced in a supra-subduction setting (at least in Uruguay) and have the same Hf isotope signature.

The zircon Hf isotope archive of rapidly changing mantle sources in the south Patagonian retro-arc

Rapid changes in geochemical and isotopic signatures of arc-related magmatic products can be used to trace magmatic processes in subduction zones across many scales, from the regional response of magmatism to large-scale geodynamic changes in the subduction system, to the emplacement of single intrusions. In this contribution, we use the south Patagonian subduction system as a natural laboratory to investigate magmatic processes in continental arcs. We use diverse intrusions and dikes from the retro-arc region at 49−51°S to investigate these processes both at the subduction zone scale, and within the exceptionally well-exposed Torres del Paine sheeted intrusion. We present Hf isotope data for zircon from 30 to 12 Ma magmatic units that were emplaced ∼50 km inboard of the main subduction-related batholith. These samples record an ∼18 m.y. period during which the region experienced profound geodynamic changes, resulting in transient migration of arc magmatism into the retro-arc, which then vanished to be replaced by more alkaline retro-arc magmatism. Integrating published whole rock geochemistry, we show that the Hf isotope signatures of these magmatic units directly record their mantle sources, with negligible assimilation of continental crust into magmas during transport through and storage in the crust. This allows us to trace the appearance and disappearance of the subduction component in the retro-arc mantle. Our data show that migration of calc-alkaline magmatism into the retro-arc produced magmas with a more enriched Hf isotope composition that was remarkably consistent over ∼200 km and >4 m.y. (eHf(i) of −1 to +2.5). These signatures record addition of subducted continental crust to the mantle wedge during a period of subduction erosion that was associated with arc migration, and show that only a few m.y. of fluxing by a subduction component is sufficient to leave a distinct Hf isotope imprint on a mantle wedge previously unmodified by subduction. The Torres del Paine laccolith was built up by discrete pulses of magmatism in <200 k.y. Our data show isotopic differences between magmatic batches, and an abrupt shift to more juvenile Hf isotope compositions during the buildup of the youngest part of this magmatic complex, recording the rapid input of new mantle-derived melts during its formation. This rapid rejuvenation occurred within 20 ± 10 k.y., demonstrating that different batches of magmatism within a single intrusive complex can tap geochemically distinct mantle reservoirs on very short timescales of <200 k.y.

The evolution of the Arabian-Nubian Shield and survival of its zircon U-Pb-Hf-O isotopic signature: A tale from the Um Had Conglomerate, central Eastern Desert, Egypt

In this study, we present in situ U-Pb, Hf and O isotopic analyses of detrital zircon grains from the Um Had Conglomerate from the Wadi Hammamat area in the central Eastern Desert of Egypt. The youngest detrital age population is 625 Ma, which gives an estimate to the maximum depositional age of the Um Had Conglomerate. Based on their Hf isotopic signature and U-Pb ages, detrital zircons fall into two distinct groups: a 625–680 Ma group and a 720–840 Ma group, with minor early Tonian-Stenian and Paleoproterozoic-Archean populations. These groups are consistent with derivation from local sources in the central Eastern Desert of Egypt. The 720–840 Ma group is characterized by exclusively positive εHf(t) values and it reflects the main juvenile Tonian intra-oceanic island arc stage characterizing the formation of the Arabian-Nubian Shield. On the contrary, εHf(t) values of the 625–680 Ma zircon group range between −17.5 and +12.8, which indicates involvement of Archean-Paleoproterozoic crust during the Cryogenian-Ediacaran magmatism associated with the accretion and assembly of the arc materials. The transition from juvenile intra-oceanic island arc stage to mixed juvenile magma and older crustal materials during the arc assembly stage in the central Eastern Desert is supported by the increase in δ18O values of detrital zircons from dominant mantle-like values in the Tonian to higher values in the Cryogenian-Ediacaran group. A wide variation in the εHf(t) values (−25.8 to +10) is recorded also in the early Tonian-Stenian population, which results in the double plunge U-Pb-Hf patterns of the detrital zircons. This pattern is similar to the Nile sediments and to the Paleozoic and Mesozoic Nubian sandstone, which may indicate substantial recycling of ANS crustal material by these sediments and survival of its original zircon U-Pb-Hf-O signature despite repeated recycling episodes.

Hydrothermally-altered mafic crust as source for early Earth TTG: Pb/Hf/O isotope and trace element evidence in zircon from TTG of the Eoarchean Saglek Block, N. Labrador

The North Atlantic craton hosts extensive exposures of Eoarchean crust, spread through areas of Western Greenland and Northern Labrador (Canada). Of these two areas, the crust of the Saglek Block of Northern Labrador has received far less attention from the scrutiny of modern analytical methods than its better documented Western Greenland equivalent, the Itsaq Gneiss Complex. Here, we present the first coupled trace element and U–Pb/Hf/O isotope dataset for zircon from an early TTG component of the Saglek Block.The combination of textural, elemental and isotopic in-situ analyses enables selection of the least disturbed zircon domains. From these it is demonstrated that the oldest felsic remnants exposed in the Saglek Block were emplaced 3.86 ± 0.01 billion yr (Ga) ago through partial melting of basaltic protoliths. The Hf isotope signature of the oldest zircon domains from the Saglek Block TTG indicates derivation from sources that did not undergo substantial Lu/Hf fractionation, resulting in initial Hf isotope compositions that are chondritic within uncertainty. The oxygen isotope ratios of the least disturbed zircon portions vary from 5.38 ± 0.16‰ to 6.64 ± 0.19‰ and document the interaction of the TTG protoliths with Earth's early hydrosphere at low temperature (≤150–200 °C) prior partial melting in the Eoarchean. The results support TTG production in the Eoarchean from variably hydrated basaltic protoliths.

No plate tectonic shutdown in the early Paleoproterozoic: Constraints from the ca. 2.4 Ga granitoids in the Quanji Massif, NW China

The global plate tectonic regime in the early Paleoproterozoic period is highly debated. Granitoids bear key information to address such a debate. Petrological, geochemical and geochronological studies are conducted on two post-collisional granitoid plutons in the Quanji Massif, northwestern China, to investigate the tectonic regime during this period. The granitoids are composed of syenogranite, monzogranite, and granodiorite, with minor tonalite, which intruded into the Delingha and Hudesheng regions at ca. 2.39–2.37 Ga. These plutons are high-K I-type granitoids with variable Ga/Al ratios, showing some characteristics of A2-type granitoids. They are characterized by enrichment of LILEs and LREEs and depletion of Sr, P, Ti, and Eu. They show depleted Nd and Hf isotope signatures with whole rock εNd(t) = +0.7 to +4.8 and zircon εHf(t) = −1.0 to +7.8, indicating a juvenile crustal growth event at ca. 2.44–2.37 Ga. Our new results together with other coeval post-collisional granitoids in the Quanji Massif suggest that the protracted post-collisional magmatism at ca. 2.39–2.34 Ga occurred just after a short interval of subduction and generation of juvenile magmas before or around ca. 2.4 Ga. Collectively, the formation of these ca. 2.4–2.3 Ga granitoids in the Quanji Massif may correlate with coeval granitoids in the Tarim and North China cratons, and is broadly coeval with magmatism in several other cratons in the West African and Canadian Shields. Thus, the globally well-documented magmatism at early Paleoproterozoic, or Siderian, provides further information for filling up the age gap of the so-called plate tectonic “shutdown” in the early Paleoproterozoic period worldwide. The geologic record therefore suggests no Siderian shutdown of plate tectonics, but instead, continuous global subduction and generation of juvenile magmas from the Archean through the Paleoproterozoic.

Reconstruction of the Cryogenian palaeogeography in the Yangtze Domain: constraints from detrital age patterns

AbstractDetrital zircons are often used to constrain the maximum sedimentary age of strata and sedimentary provenance. This study aimed at reconstructing the Cryogenian palaeogeography of the Yangtze Domain based on U–Pb ages and Lu–Hf isotopic signatures of detrital zircons from sandstones in the southeastern part of the Yangtze Domain. U–Pb ages of the youngest detrital zircon grains from the Niuguping, Gucheng and Datangpo formations yielded average ages of 712±24 Ma, 679.2±6.2 Ma and 665.1±7.4 Ma, respectively, which are close to the depositional ages of their respective formations. An integrated study of detrital zircon Lu–Hf isotopes and U–Pb ages from three samples revealed six main peak ages in the samples from the Anhua section atc. 680 Ma,c. 780 Ma,c. 820 Ma,c. 940 Ma,c. 2000 Ma andc. 2500 Ma. The characteristics of the U–Pb ages and Hf isotopes indicate a link between the north and southeast margins of the Yangtze Domain as early asc. 680 Ma, and the provenance of the coeval sedimentary sequences in the SE Yangtze Domain was the South Qinling Block on the northern margin of the Yangtze Domain. The provenance analysis on thec. 680 Ma detritus composing upper Neoproterozoic strata in the Yangtze Domain revealed that the detritus was transported southward from South Qinling to the southeast margin of the Yangtze Domain through the Exi Strait, but was hindered by the Jiangnan Orogenic Belt.

Zircon as a metamorphic timekeeper: A case study from the Caledonides of central Norway

During metamorphism, inherited or detrital zircon grains commonly serve as a template onto which new generations of metamorphic zircon precipitates, charting successive episodes in the thermal history of rocks. In addition, chemical modification of pre-metamorphic zircon can leave an imprint in the U–Pb systematics. We present a case study from a sample of migmatitic amphibole–biotite gneiss from central Norway that contains zircon grains that chart much of the history of an entire orogenic episode, not only through generation of new zircon, but also resetting of the U–Pb system via radiogenic-Pb loss. The investigated gneiss, from the Helgeland Nappe Complex, within the Uppermost Allochthon of the Scandinavian Caledonides records a prolonged metamorphic evolution. Phase equilibria modelling implies peak metamorphic conditions of 7.5–8.0 kbar and 650–675 °C, consistent with low-degree partial melting and the presence of small volumes of leucosome. Laser ablation split stream analysis that simultaneously measures U–Pb, REE and Lu–Hf in the same volume of zircon, yields a range of analyses that spread along concordia. A zircon population between 485 and 443 Ma is characterized by faded and blurred oscillatory-zoned domains that have gained nonformula elements. These analyses are interpreted to indicate the presence of reactive solutions during this time period that led to leaching and mobility of Pb. Another texturally distinct zircon component yields an age of 478 ± 4 Ma, has typical igneous REE zircon profiles, and records low apparent alpha doses, interpreted to reflect new zircon growth during migmitization. A further population of zircon rims and discrete grains yields an age of 453 ± 2 Ma, interpreted as a later episode of zircon growth during high-grade metamorphism. A final set of zircon overgrowths yield an age of 439 ± 3 Ma, also interpreted to reflect new metamorphic growth. Many zircon growth phases in this rock have similar REE patterns, implying broadly isochemical metamorphism, consistent with the evidence for only limited partial melting. Furthermore, the Hf isotopic signature in the analyzed zircon grains suggests that there was no release of radiogenic Hf during metamorphism. The metamorphic zircon in the studied rock records several episodes of growth and/or alteration over a period of >40 Ma that was related to protracted destruction of the Iapetus Ocean. In the context of regional data, the new results suggest that the Helgeland Nappe Complex, which is generally considered to be exotic to Baltica, underwent geographically widespread, long-lived Ordovician high-temperature metamorphism. This record of metamorphism is distinct from that recorded by tectonostratigraphically lower units, which are conventionally regarded to have been derived from Baltica. The results shed light on the assembly of the Scandinavian Caledonides and show that zircon from individual samples may be used to unravel complex orogenic histories.

Crustal and thermal structure of the Thomson Orogen: constraints from the geochemistry, zircon U–Pb age, and Hf and O isotopes of subsurface granitic rocks

The origin of elevated geothermal gradients in the subsurface Thomson Orogen and the nature of the crustal basement beneath it, whether oceanic or continental, remain enigmatic. Previous studies have demonstrated that a higher crustal radiogenic input is required to explain these anomalous thermal gradients. In this study, we have investigated the nature and age of this crustal input by undertaking geochemical, geochronological and Hf and O isotope analyses of buried granitic rocks as well as evaluating the heat-producing potential of metasedimentary rocks. The mineralogy, composition and Neoproterozoic/Cambrian to Devonian age of the low to moderate heat-producing I- and S-type granitic rocks strongly contrast with the Carboniferous A-type high-heat-producing granites of the Big Lake Suite, which have been suggested to be an important contributor to the elevated geothermal gradients, near the southwest corner of the Thomson Orogen. These differences suggest the Big Lake Suite rocks do not extend into the Queensland part of the temperature anomaly. Heat production of the metasedimentary rocks is also low to moderate. Based on Hf isotope compositions of zircons characterised by mantle-like oxygen signature (ϵHf(t) = –12 to +2), we propose the temperature anomaly results from the occurrence of Mesoproterozoic and/or Paleoproterozoic high-heat-producing rocks beneath the Thomson Orogen. Precambrian crust, therefore, lies well east of the Tasman line. The results do not support a Neoproterozoic to Cambrian oceanic crust, as previously suggested, but instead point to a continental substrate for the Thomson Orogen. Hf isotopes indicate an overall trend towards more isotopically juvenile compositions with a progressive reduction in the contribution of older crustal sources to granitic magmas towards the present time. Different Hf isotopic signatures for the Lachlan (ϵHf(t) = –13 to +15), Thomson (ϵHf(t) = –14 to +5) and Delamerian (ϵHf(t) = –7 to +4) orogens highlight lateral variations in the age structures of crustal basement beneath these orogens.

Detrital zircon U–Pb–Hf isotopes study of the Lower Carboniferous Anjihai Formation from the northern margin of the Yili Block, NW China

The North Tianshan Accretionary Complex represents a Late Palaeozoic suture between the Junggar Terrane and the Yili Block. We report detrital zircon U–Pb dating and Hf isotope results for a sandstone from the Lower Carboniferous Anjihai Formation in the northern margin of the Yili Block, to reveal the geochronological framework of the northern Yili Block, and discuss the subduction process of the North Tianshan Ocean (NTO). The detrital zircon ages were divided into 2 clusters, namely, the Precambrian and Palaeozoic ones. The Palaeozoic age group can be further divided into 3 subgroups, that is, 472–461 Ma, 448–423 Ma, and 406–335 Ma, with peaks at 465, 436, and 346 Ma, respectively. All the Palaeozoic zircons have oscillatory zoning textures and high Th/U ratios, showing an igneous provenance. The zircon Ti‐temperatures are between 584 and 809 °C, suggesting that their source rocks were I‐ or S‐type granitic rocks. In addition, most of the Palaeozoic zircons have positive εHf(t) values, indicating that the source rocks of the Palaeozoic zircons are derived from not only Mesoproterozoic continental crust but also involved depleted mantle components, which matched well with the Hf isotopic signatures of magmatic rocks from the northern Yili Block. Based on the regional geology, these zircons were thought to be sourced from magmatic rocks that were related to the subduction of the NTO. Taking into consideration the Middle–Late Ordovician arc‐type granitoids in the Wenquan area, we conclude that the northern margin of the Yili Block was a unified active continental margin in the Palaeozoic, and the subduction of the NTO started at least from the Middle Ordovician.

Zircon U–Pb–Hf constraints from Gongga Shan granites on young crustal melting in eastern Tibet

The Gongga Shan batholith is a complex granitoid batholith on the eastern margin of the Tibetan Plateau with a long history of magmatism spanning from the Triassic to the Pliocene. Late Miocene–Pliocene units are the youngest exposed crustal melts within the entire Asian plate of the Tibetan Plateau. Here, we present in-situ zircon Hf isotope constraints on their magmatic source, to aid the understanding of how these young melts were formed and how they were exhumed to the surface. Hf isotope signatures of Eocene to Pliocene zircon rims (ɛHf(t) = −4 to +4), interpreted to have grown during localised crustal melting, are indicative of melting of a Neoproterozoic source region, equivalent to the nearby exposed Kangding Complex. Therefore, we suggest that Neoproterozoic crust underlies this region of the Songpan–Ganze terrane, and sourced the intrusive granites that form the Gongga Shan batholith. Localised young melting of Neoproterozoic lower or middle crust requires localised melt-fertile lithologies. We suggest that such melts may be equivalent to seismic and magnetotelluric low-velocity and high-conductivity zones or “bright spots” imaged across much of the Tibetan Plateau. The lack of widespread exposed melts this age is due either to the lack of melt-fertile rocks in the middle crust, the very low erosion level of the Tibetan plateau, or to a lack of mechanism for exhuming such melts. For Gongga Shan, where some melting is younger than nearby thermochronological ages of low temperature cooling, the exact process and timing of exhumation remains enigmatic, but their location away from the Xianshuihe fault precludes the fault acting as a conduit for the young melts. We suggest that underthrusting of dry granulites of the lower Indian crust (Archaean shield) this far northeast is a plausible mechanism to explain the uplift and exhumation of the eastern Tibetan Plateau.

Deconvolving the pre-Himalayan Indian margin – Tales of crustal growth and destruction

The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence (LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence (GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 km along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800–2500 Ma, 1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonianage population (∼860–800 Ma) with a variably enriched radiogenic Hf isotope signature (εHf = 10 to −20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation bias and destruction of continental crust informs geologic models.