Crustal Evolution History Research Articles

Sedimentary basins, hydrocarbons, graphite, coal, and Cu-Au deposits - from Mongolia to the Pacific margin: Interplay between the ubiquitous orthogonal fracture network and Global Wrench Tectonics

Mongolia is exceptionally rich in coal and copper-gold resources - with world-class deposits like Tavan Tolgoi, Oyu Tolgoi and Erdenet. Thus, the mining industry has a crucial importance for the national economy, yet most of the country remain very underexplored. Within today's global tectonics, an acceptable understanding of metal enrichments - including leaching, the internal hydrostatic-hydraulic pumping system, and surface emplacement mechanisms - has remained unresolved. However, a broader view of the structural situation in the Mongolia-China region shows a close link between orientation of elongate sedimentary basins, important mineral belts, and the fundamental orthogonal fracture/fault system. In the east the tectonic trend is dominantly northeast, while it is northwest in western areas. The main east Mongolian graphite deposits have northeast structural trends like numerous regional Cu and Au belts. A new theory of the earth, Global Wrench Tectonics, offers an exciting approach to better understanding the various facets of Earth's geological history and its surface resources. Earth’s degassing, dynamo-tectonic consequences, inertia-driven crustal wrench tectonics, as well as surface products such as water, hydrocarbons and ore deposits are given a coherent system explanation. Many hydrocarbons are products from the interior of our slowly degassing Earth, with massive hydrocarbon fields such as Songliao and the Yamal megaproject producing from the basement. Crustal thinning in the Songliao region is about the same as in southeast Mongolia, suggesting that they may have had similar degassing and crustal evolution histories. As such, it is not unlikely that the underexplored Mesozoic basins of southeast Mongolia - particularly at the deepest levels and/or in the adjacent crystalline basement - may have important hydrocarbon potential.

Trace elements in zircon record changing magmatic processes and the multi-stage build-up of Archean proto-continental crust

Zircon trace element geochemistry has become an increasingly popular tool to track crustal evolution through time. This has been especially important in early-Earth settings where most of the crust has been lost, but in some fortuitous instances detrital zircons derived from that lost crust have been preserved in younger sediments. To study the formation and geochemical evolution of continental crust from the Hadean to the Paleoarchean, the 3.6 to 3.2 Ga Barberton Greenstone Belt in southern Africa is an excellent target due to its outstanding preservation and presence of detrital zircons that span almost a billion years. Here, we use trace elements, in combination with hafnium and oxygen isotopes, of 3.65 to 3.22 Ga detrital and tuffaceous zircons of the Moodies and Fig Tree groups and compare their geochemistry to previously studied 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed of the Onverwacht Group. The major detrital zircon age clusters in the former at 3.55 Ga, 3.46 Ga, and 3.26–3.23 Ga overlap with episodes of TTG emplacement and felsic volcanism in the Barberton area, suggesting a local provenance. In contrast, age clusters at 3.65 Ga and 3.29 Ga of the Moodies and Fig Tree groups as well as 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed do not have known intrusive sources and were likely derived from outside the present-day Barberton belt. This indicates that more than half of the felsic igneous events in the detrital zircon record do not have a whole-rock representation that can be directly studied. The similar compositions and inferred crustal evolution histories recorded in zircons from the Fig Tree and Moodies groups, as well as from the Green Sandstone Bed, suggest that they were derived from connected terranes experiencing similar crustal processes diachronously. Together, they show three phases of felsic continent formation, reflecting different crustal processes: (1) long-lived protocrust formed in the Hadean from undepleted mantle sources. These zircons are vastly different from younger zircons and, hence, Barberton TTGs are not good analogues of Hadean crust formation. (2) At 3.8 Ga, onset of significant crustal growth though cyclic juvenile additions and hydrous melting, possibly within a volcanic plateau setting but an arc-like setting cannot be excluded based on this data. (3) Between 3.4 and 3.3 Ga, felsic crust is generated through a previously unrecognized episode of crustal growth by shallow melting of mafic, mantle-derived sources. This is immediately followed by the onset of crustal thickening through the transport of surface-altered, hydrated materials to deep crustal levels. Since there is geological evidence for extension and shortening at that time this may reflect the onset of horizontal movement. Whether this last geodynamic setting reflects modern-style plate tectonics or not, continent formation and the onset of plate tectonics in the Barberton area occurred through complex multi-stage processes spanning almost a billion years, most of which is only accessible through the detrital zircon record.

Trondhjemites from the Western Dharwar Craton, Southern India: Implications for Mesoarchean crustal growth

Trondhjemite, together with tonalite and granodiorite (TTG) are important building blocks of the Archean continental crust and their studies provide insights into the formation of the early continents. The Western Dharwar Craton (WDC) in southern India contains voluminous TTG rocks that record prolonged Archean crustal evolution history with evidence for multiple episodes of magmatic accretion and metamorphic events from the Paleoarchean (∼3600 Ma) to Archean-Proterozoic transition (∼2500 Ma ago). In this study, we investigate trondhjemites from different typical locations in the WDC. Petrological, geochemical and zircon UPb, REE and LuHf data are presented and evaluated to understand the timing of magma emplacement and petrogenetic history. The trondhjemites in our study were generated from partial melting of juvenile hydrous basalt with arc features. The melting depth is variable, mainly shallow, while some melts originated at greater depths with variable plagioclase, amphibole with or without garnet, ilmenite or rutile in source residue. The age data presented here suggest that the crust accretion of the Mesoarchean Western Dharwar Craton occurred dominantly at 3.13–3.27 Ma followed by reworking of lower crustal basement 3.0–2.9 Ga.

Detrital zircon U‐Pb geochronology and provenance of Silurian, Devonian and Permian sedimentary rocks, South Qinling: Constraints on late Palaeozoic tectonic evolution history of the Qinling Orogenic Belt

The Qinling Orogenic Belt (QOB), formed by the subduction and collision between the North China Block (NCB) and the South China Block (SCB), is a key area for exploring the tectonic process and timing of integration between the two blocks. A range of tectonic models have been proposed with regard to the subduction and collision of the QOB, but controversy in the timing and integration process still remain debatable, especially in the late Palaeozoic orogeny. The South Qinling Belt (SQB) is located between the Shangdan and Mianlue sutures, widely dispersed sediments outcropped in the SQB provide natural ground for the detrital zircon study, that could trace the sedimentary provenance and continental crustal evolution history, thus provide important evidence for the late Palaeozoic evolutionary processes in the QOB. In this study, we applied a detailed geochronological U‐Pb zircon dating of the Silurian, Devonian and Permian sedimentary rocks from the SQB. The result of the U‐Pb isotopic data shows distinct age populations at 439–474 Ma of the Silurian samples, ca. 400–490 Ma of the Devonian samples and 260–310 Ma of the Permian samples, respectively. By comparing the regional geochronological data, we sum up that the Silurian detrital zircons are mainly derived from the SQB and the northern margin of SCB (N‐SCB), the Devonian detrital zircon age populations are dominated by the North Qinling Belt (NQB) and SQB and the Permian sediments mainly stemmed from the southern margin of the North China Block (S‐NCB) and the NQB. A mutual age population of the Devonian sediments at ca. 400–490 Ma from the NQB constrains the closure of the Shangdan Ocean between the NCB and the SCB before the middle Devonian times. Ca. 260 ~ 380 Ma age population of the Permian sediments show similarities with the age data from the northern margin of the NCB, indicating an uplift event of NQB before the late Permian, and further constrains the closure of Mianlue ocean later than Permian times.

Tracing the crustal evolution of the Precambrian Southern Granulite terrane in East Gondwana: New insights from zircon U-Pb/Hf geochronology

The Precambrian Southern Granulite terrane of south India has a crustal evolution history broadly bracketed between the late Archean and Cambrian with records of polyphase deformation, metamorphism, and magmatism. The Southern Granulite terrane comprises distinct crustal blocks bounded by shear/suture zones that have been variably correlated with supercontinent fragments including Madagascar, Sri Lanka, Africa, Eastern Ghats, and Antarctica. However, the timing and mechanism of assembly of different crustal blocks within the Southern Granulite terrane and its linkages with counterparts in East Gondwana are highly debated. This study aimed to unravel the complex crustal evolutionary pattern of the terrane by generating robust zircon U-Pb/Hf isotopic data from basement charnockites, gneisses, granitoids, and alkaline intrusive units from the central part of Southern Granulite terrane and comparing these results with similar data from different East Gondwanan terranes. The study identified four distinct crustal growth episodes in the Madurai block: (1) Neoarchean−early Paleoproterozoic, (2) Rhyacian−Orosirian, (3) late Tonian, and (4) Ediacaran−Cambrian. Analysis of zircon Hf isotope data revealed that the first two events are marked by juvenile magmatic signatures, whereas the latter two are distinctly associated with intense reworking and remelting of older crust with no significant juvenile input. Our new results combined with existing data from other Gondwanan terranes suggest a common Paleoproterozoic ancestry for the Southern Granulite terrane and its corresponding Gondwanan fragments, proposing a revision to the existing geodynamic models.

Crustal evolution and tectonomagmatic history of the Indian Shield at the periphery of supercontinents

River sand detrital zircons from several rivers flowing through the Peninsular Indian cratons were analyzed for U-Pb, Lu-Hf, and O isotopes to characterize the Precambrian crustal evolution of the Indian Shield and to constrain its role in the early supercontinent cycles. Analyzed river sand zircon samples exhibit a prominent age grouping at 2.7–2.4 Ga and additional peaks at 1.8–1.7 Ga, 1.0–0.8 Ga, and 0.6–0.5 Ga. The time-related Hf and O isotope trends of the Indian zircons display a slight offset from the global trends. The age peaks and distinct Hf-O isotopic compositions of the Indian zircons also lack coherence with the global patterns associated with supercontinent cycles, implying a discrete crustal evolutionary history for the Indian Shield. Their contrasting εHf-age trajectories indicate that the Indian Shield was accreted to the Columbia/Nuna supercontinental framework through a collision event that postdated the 2.1–1.8 Ga global-scale orogeny, and was part of a long-lived subduction system along the margin of Rodinia. This implies that the Indian Shield occupied a peripheral paleo-position during the assembly of the two Precambrian supercontinents. The ca. 2.6–2.4 Ga Indian river sand zircons have remarkably low δ18O values that are distinct from the global zircon O isotopic record. This period coincides with the fragmentation of the Archean supercraton and the flaring-up of a subaerial Large Igneous Province that facilitated the generation of 18O-depleted magmas. Depending on whether the 2.6–2.4 Ga zircons with low δ18O values (<4.7 ‰) are considered or not, approximately 2.77 ‰ or 1.78 ‰ rise in the zircon δ18O values occurred between ca. 2.4 Ga and ca. 1.87 Ga (decoupled from zircon Lu-Hf isotopes), further suggesting that the fine-grained sediments were enriched in 18O after the Great Oxidation Event. These high δ18O sediments were subsequently incorporated into the magmatic systems resulting in elevated δ18O in the zircons crystallizing from such melts. Nevertheless, a ca. 0.9 Ga peak of >10 ‰ in δ18O system was followed by a pronounced δ18O drop at ca. 0.759 Ga, suggesting that δ18O of recycled sedimentary reservoirs was not the only controlling factor for zircon δ18O characteristics. Variations in the Hf-O data for Indian and global zircons could be attributed to the initiation of the supercontinent cycle at ca. 2.0 Ga. A significant volume of the juvenile crust was added during the 3.0–2.7 Ga mafic magmatism in the Indian Shield, as underlined by the Hf model ages of all the zircon grains and those with mantle-like δ18O signatures. A gradual addition of continental crust during ca. 3.6 Ga to 3.1 Ga can also be deduced, with the oldest crust being derived from the mantle at ca. 4.4 Ga.

Two kinematic transformations of the Pamir salient since the Mid-Cenozoic: Constraints from multi-timescale deformation analysis

The Pamir salient is a key part of the Himalayan–Tibetan Plateau orogenic system and has undergone intense tectonic deformation during the India–Asian collision. Delineating the Cenozoic kinematics and geodynamics of the Pamir salient requires a comprehensive understanding of the active arcuate structures along its frontal margin, from the perspective of the multi-spatiotemporal evolution of deformation patterns. Here, we reviewed the deformation rates of the major structures at different timescales, reanalyzed the published Global Positioning System velocities, and examined the present-day seismicity to constrain the kinematics of the Pamir salient since the Late Cenozoic. Integrated with the crustal evolution history during the Middle–Late Cenozoic and the deep structure, we proposed a new model to explain the multi-stage kinematics and associated geodynamics of the Pamir salient. During ∼37–24 Ma, the initial Pamir salient moved northward via radial thrusting that rotated the basins on both sides, which was driven by the continuous compression of the Indian slab after the breakoff of its oceanic part. During ∼23–12 Ma, the gravitational collapse of the Central and South Pamir crusts, which was induced by the breakoff of the continental part of the Indian slab, triggered the extension within the Pamir and foreland-ward movement of the upper crust. The upper crustal materials moved in varying directions due to the differential strength of the foreland areas, transforming the crustal kinematics from radial thrusting into a combination of radial thrusting and transfer faulting. Since the coupling of the Indian and Pamir slabs at ∼12–11 Ma, the deformation propagation towards the forelands accelerated, after which the kinematics of the Pamir salient exhibited asymmetric radial thrusting that has been sustained until the present. The asymmetric radial thrusting was likely driven by the compressive stress effect of the lithospheric basal shear generated by the underthrusting of the cratonic Indian lithosphere, which further led to the rollback of the Pamir slab and the consequent migratory extension in the South Pamir.

A PRELIMINARY INVESTIGATION OF GEMSTONES, MINES,MINERS AND PROBLEMS IN GILGIT-BALTISTAN, PAKISTAN

Gilgit-Baltistan possesses a complex history of crustal evolution and rich diversity of geological environments, and is favorable for various types of mineralization and production of gemstone all over GB. Two batholiths; Kohistan Batholith and Karakorum Batholith which are hosting gemstones mineralization. This investigation reveals that thirty-two precious and semi-precious stones are mined, 95% of the total gemstones produced in Pakistan come from GB. These varieties of gemstones come from 2150 mines which are in working conditions scattered all over GB and number of miners involved is 25000. Miners face problems like lighting, ventilation and drilling. They lack the basic safety equipment like helmets, mining shoes, gloves, googles and masks. Due to the fumes of explosives, miners are suffering from different diseases like Silicosis. The main gemstone-producing areas are Hunza valley, Sumayar (Nager Valley), Shengus and Haramosh (District Gilgit), Bulachi and Mir Malik (District Astore), Istak Nalah and Shiger Valley. The common gemstones mined are ruby, pargasite, spinel, corundum, sapphire, calcite, aquamarine, tourmaline, beryl, quartz, topaz, albite, microcline, apatite, fluorite, emerald, marganite, diopside, tanzanite, axinite, felsite, kunzite, emerald and garnet.

Detrital zircon in an active sedimentary recycling system: Challenging the ‘source‐to‐sink’ approach to zircon‐based provenance analysis

ABSTRACTThe age and properties of detrital zircon in a sediment reflect properties of the rocks in which the zircon crystallized, and not necessarily the immediate precursor of the host sediment. Where clastic sediments are recycled, these properties are preserved so that U‐Pb ages and Lu‐Hf data on zircon grains will no longer give information on the routing of detritus. Because the crustal evolution and sedimentary recycling history of Southern Africa is well‐established, Holocene sediments in the region provide a good test example for detrital zircon geochronology applied to sands or sandstones with a complex recycling history. Data from 16 samples of unconsolidated sand, including dunes from the southern part of the Kalahari Basin, beach sands at the Atlantic coast, and isolated inland dune occurrences, yield ranges of early Mesozoic to late Archaean ages. Uranium–lead and lutetium–hafnium isotope data provide no evidence of direct derivation of zircon from protosources in crystalline bedrock. The complex distribution patterns can be decomposed into seven ‘provenance components’ that have previously been encountered in Palaeoproterozoic to Jurassic sedimentary rocks in the region. These components occur in non‐random combinations, in proportions that suggest mixing of recycled material from different sedimentary precursors. Taking pre‐Cenozoic geology, Cretaceous to Pleistocene drainage history of southern Africa and Holocene wind and current patterns into account, it is possible to work out a consistent model for transport of detritus from sedimentary precursors into the basin, followed by aeolian remobilization within the basin itself. The detrital zircon distributions can thus be interpreted from prior knowledge of the basin filling history, regional geology, geomorphology, drainage evolution and wind pattern. This amounts to a line of reasoning from ‘sink back to (intermediate) ‘source’, which is the opposite of the ‘(proto)source‐to‐sink’ paradigm commonly invoked in detrital zircon studies.

Timing of UHT metamorphism and cooling in south Indian granulites: New P-T-t results from a sapphirine granulite

The Precambrian granulite terrane of south India is known for the preservation of regional-scale high to ultrahigh-temperature (HT-UHT) metamorphic rocks. In the Madurai block, the largest crustal block in the terrane, a significant proportion of these HT-UHT rocks occur along a narrow belt referred to as the Kambam ultrahigh-temperature belt. Understanding the P-T-t evolution of these HT-UHT granulites is vital in decoding the spatial and temporal evolution of the Madurai block in relation to other crustal blocks. Here we present the petrology, mineral chemistry, phase equilibrium modelling, and accessory mineral (zircon, monazite, rutile and apatite) geochronological and geochemical data for a hitherto unreported sapphirine granulite from the Kambam ultrahigh-temperature belt. Combined mineral reaction and phase equilibrium modelling indicate extreme P-T conditions up to 1130 °C at 11 kbar. The geometry of the P-T path is clockwise with initial isothermal decompression followed by near isobaric cooling. LA-ICPMS U-Pb geochronological studies on accessory phases report two distinct thermal events: (i) Paleoproterozoic high-T event and associated crustal anatexis at ∼2.5 Ga from zircons and (ii) Neoproterozoic UHT event at ∼550 Ma from monazites. The monazite ages suggest a 50–60 Ma prolonged thermal event, attributed to Ediacaran-Cambrian collisional orogenesis. Additional metamorphic pulses are also identified, demonstrating the polydeformed crustal evolution history of the terrane. U-Pb age dating of rutile and apatite in the sample provide ages of ∼458 Ma and ∼392 Ma and cooling rates of ∼6 °C/Ma and ∼0.3–2 °C/Ma respectively. This suggests a complex history of slow cooling followed by ultraslow cooling. The results bring forth the significance of the Kambam ultrahigh-temperature belt in understanding the tectonothermal evolution of Madurai block and provide additional evidence for long-lived Neoproterozoic UHT orogenesis in south India.

Comparative Sm-Nd isotope behavior of accessory minerals: Reconstructing the Sm-Nd isotope evolution of early Archean rocks

Crustal growth and mantle differentiation through Earth’s history are often traced using two radiogenic isotope systems - 176Lu-176Hf and 147Sm-143Nd. Unlike most post-Archean igneous rocks that show correlated initial Hf and Nd isotopic compositions, many ancient rocks have broadly chondritic zircon initial εHf values but highly variable whole-rock initial εNd values. These features have classically been interpreted as differences in the behavior of the Lu-Hf and Sm-Nd isotope systems during either deep magma ocean crystallization, subduction zone processes, or post-crystallization metamorphism. To clarify the cause of early Archean Hf-Nd isotope relationships, which are essential for understanding early Earth’s evolution, we investigated the in situ U-Th-Pb and Sm-Nd isotope systematics of co-existing titanite, apatite, and allanite—the major Sm-Nd carriers in early Archean felsic rocks—in a representative early Archean (3.5–3.4 Ga) tonalite-trondhjemite-granodiorite (TTG) suite from the Minnesota River Valley (MRV) terrane, northern USA. These rocks exhibit multiple generations of closed-system zircon growth with chondritic initial zircon Hf isotope signatures, and apparent decoupled zircon initial Hf and whole-rock Nd isotopic compositions, and thus serve as an useful test of the role of accessory minerals in controlling the whole rock isotopic signatures.Our new U-Pb results show that the REE-rich accessory phases were reset during the 2.60–2.58 Ga Sacred Heart Orogeny, representing the accretion of the MRV terrane to the southern margin of the Superior Craton. Additionally, two samples contain apatite that yields significantly younger U-Pb dates of 1.9–1.8 Ga, suggesting that a portion of the MRV basement was thermally overprinted by the ∼1.85 Ga Penokean Orogeny. The ∼2.6 Ga Sm-Nd array recorded by apatite and titanite in these rocks suggest terrane-wide Sm-Nd isotopic re-equilibration in the MRV during the Sacred Heart Orogeny, with no Penokean disturbance of the Sm-Nd isotope systematics. Nevertheless, allanite in one sample has survived the post-crystallization 147Sm-143Nd resetting event, and yielded a Sm-Nd isochron date broadly consistent with the crystallization age of the host rock (∼3.4 Ga), with chondritic initial εNd. Therefore, allanite appears to be an important target in order to obtain the primary Nd isotopic signature of early Archean rocks in ancient terranes. In contrast to the ambiguous whole-rock data, the Nd isotopic compositions of the coexisting apatite, titanite and allanite collectively reconstruct a clear crustal evolution history in which the crust was repeatedly re-melted in a closed-system since formation at 3.5 Ga, initially from a source evolving with broadly chondritic Lu-Hf and Sm-Nd systematics. This suggests a minimally differentiated, or already rehomogenized, mantle at 3.5 Ga. Our study highlights the importance of using REE-rich accessory phases to obtain a clear Nd isotopic record to constrain the history of continent formation on the early Earth.

Combined U-Pb/Hf isotopic studies and phase equilibrium modelling of HT-UHT metapelites from Kambam ultrahigh-temperature belt, south India: Constraints on tectonothermal history of the terrane

The Madurai block which forms the central crustal block in the Precambrian Southern Granulite Terrane, south India preserves crustal evolution history with record of multiple magmatic and metamorphic episodes ranging from early Archaean to late Neoproterozoic. Documenting the precise chronology and tectonothermal evolution of these events are of paramount importance to correlate with other crustal blocks in Southern Granulite Terrane as well as to link with other continental fragments of East Gondwana. The present study focuses on hitherto unreported location within a key section of Madurai block along the transcrustal Suruli shear zone known as the Kambam ultrahigh-temperature belt. This belt is noticeable for the proliferous occurrence of HT-UHT metapelites with an overall clockwise P-T trajectory. Samples of garnet-spinel granulite, cordierite-spinel granulite and associated granite gneiss were subjected to detailed petrographic, thermobarometric and geochronologic analysis. Combined petrography, mineral reaction, geothermobarometry and pseudosection modelling of the granulite samples reveal HT-UHT metamorphic conditions with clockwise P-T path demonstrating significant decompression and cooling. LA-ICPMS U-Pb dating of zircon, monazite and rutile phases within these samples constrain the timing of metamorphism as well as cooling. U-Pb zircon ages of ~2500 Ma from the granulites points to new evidences for Paleoproterozoic high-grade metamorphism in the area where monazite U-Pb ages at ~593 and ~557 Ma throw light into the imprints on ~36 Ma prolonged Neoproterozoic thermal event attributed to Pan African collisional orogeny. LA-MC-ICPMS Hf isotopic record from zircon cores point to the role of significant juvenile crustal melting/anatexis during Paleoproterozoic. In addition, several cryptic metamorphic pulses are also identified evincing the polydeformed evolutionary history of these rocks. The ca. ~432 Ma cooling age record from rutile estimates an average cooling rate of ~3.0 °C/Ma over a time period of ~125 Ma, classifying Madurai block as a classic slow cooled granulite terrane with long lived UHT orogenic history. The results also bring to the fore the importance of Kambam ultrahigh-temperature belt and Suruli shear zone which can be projected as a major terrane boundary within the Southern Granulite Terrane.

Multiple sources and magmatic evolution of the Late Triassic Daocheng batholith in the Yidun Terrane: Implications for evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau

Abstract Granitoids with diverse composition and tectonic settings provide important tools for exploring crustal evolution and regional geodynamic history. Here we present an integrated study using petrological, mineralogical, zircon U-Pb geochronological, whole-rock geochemical, and isotopic data on the Late Triassic Daocheng batholith in the Yidun Terrane with a view to understanding the petrogenesis of a compositionally diverse batholith and its implications for the evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau. The different lithological units of the batholith, including granodiorite, monzogranite, and quartz diorite, with abundant mafic microgranular enclaves in the granodiorite (MME I) and monzogranite (MME II), show identical crystallization ages of 218–215 Ma. The mineral assemblage and chemical composition of the granodiorite are identical to those of tonalitic-granodioritic melts generated under water-unsaturated conditions. The insignificant Eu anomalies and low magmatic temperatures indicate hydrous melting in the source. The relatively narrow range of whole-rock chemical and Sr-Nd isotopes, as well as the zircon trace element and Hf isotopic compositions of the granodiorite, suggest a homogeneous crustal source for the magma. Our modeling suggests that the rock was produced by 20–50% of lower crustal melting. The Daocheng monzogranites display more evolved compositions and larger variations in Sr-Nd-Hf isotopes than the granodiorite, which are attributed to assimilation and the fractional crystallization process. This is evidenced by the presence of metasedimentary enclave and inherited zircon grains with Neoproterozoic and Paleozoic ages, a non-cotectic trend in composition, and the trend shown by the modeling of initial 87Sr/86Sr ratios and Sr. The quartz diorites and MMEs showing composition similar to that of andesitic primary magma have high zircon εHf(t) values and are characterized by enrichment in LILEs and depletion of HFSEs. They were derived from the partial melting of lithospheric mantle that had been metasomatized by slab melts and fluids. The MMEs in both rocks display typical igneous texture and higher rare earth element (REE) and incompatible element concentrations than their host granites. The presence of fine-grained margins, acicular apatite, and plagioclase megacrysts suggests a magma mingling process. The overgrowth of amphibole around the pyroxene, quartz ocelli rimmed by biotite, and oscillatory zones of plagioclase are all indicative of chemical diffusion. Their enriched Sr-Nd isotopes imply isotopic equilibrium with the host granites. Based on a comparison with the coeval subduction-related magmatism, we propose that subduction and subsequent rollback of the Paleo-Tethys (Garzê-Litang Ocean) oceanic slab was the possible mechanism that triggered the diverse Triassic magmatism within the eastern Tibetan Plateau.

Late Neoarchean to Paleoproterozoic arc magmatism in the Shandong Peninsula, North China Craton and its tectonic implications

The transition in tectonic regime of our planet during the late Archean with the initiation broadly modern style subduction system is one of the major topics of interest in earth science. The North China Craton (NCC), one of the oldest cratonic nuclei in the world, preserves important records of Archean and Paleoproterozoic crustal evolution, cratonization and related geodynamic history that are important in understanding the continental growth of the early Earth. Here we investigate a suite of Archean to Paleoproterozoic rocks including hornblendite, amphibolite, gabbro, diorite, basalt, plagiogranite, granite porphyry and greenschist from Shandong Peninsula within the southwestern Jiaoliao block of the eastern NCC using zircon U-Pb geochronology, Lu-Hf isotopes, and whole-rock geochemistry. We aim to understand the timing and petrogenesis of the rock suite and its implications on the geodynamic history of the NCC. Zircon U-Pb data show 207Pb/206Pb ages in the range of 2.68–1.83 Ga, and is divided into three groups of 2.68–2.43, 2.38–1.96 and 1.95–1.83 Ga. Among these, the 2.68–2.43 Ga group confirms with the major magmatic phase during Neoarchean-Paleoproterozoic transition in the eastern NCC, whilst the other groups of 2.38–1.96 and 1.95–1.83 Ga ages might represent the Paleoproterozoic orogenic events in the NCC. Zircon Lu-Hf isotopic data display εHf(t) values ranging from −0.4 to 10.0 and TDMC of 3074–2080 Ma, which are subdivided into one group with U-Pb ages of 2672–2445 Ma characterized by εHf(t) values from −0.4 to 7.0 and TDMC of 3074–2588 Ma and another group (2134–2009 Ma) with εHf(t) values in the range of 2.8–10.0 and TDMC ranging from 2458 to 2080 Ma. These results indicate that the magmas (2672–2445 Ma group) were sourced mainly from Late Mesoarchean to Neoarchean juvenile components with minor input of reworked Mesoarchean crustal components, whereas the 2134–2009 Ma group was derived from Paleoproterozoic juvenile components. Whole-rock geochemical data of the granite porphyry and plagiogranites show typical subduction-related arc signature with the enrichment of LREEs and LILEs and depletion of HREEs and HFSEs (e.g. Nb, Ta, Ti), and mainly belong to I-type granites generated at arc-related setting. The ultramafic–mafic magmatic suite corresponds with tholeiitic to calc-alkaline series which were formed by partial melting of enriched mantle within an island arc setting in an active continental margin. In conjunction with the results from previous studies in the NCC, we correlate the 2.68–2.43 Ga ages with Late Neoarchean amalgamation of microblocks and cratonization of the NCC driven by late Neoarchean oceanic plate subduction, and the 2.38–1.83 Ga ages represent Paleoproterozoic orogenic events including the assembly of major crustal blocks in the NCC within the Paleoproterozoic supercontinent Columbia, and the final collision of these crustal blocks and cratonization.

Multi-stage Jurassic magmatism in the Liaodong Peninsula: Constraints on crustal evolution beneath the eastern North China Craton

Whole-rock geochemical and in situ zircon UPb and HfO isotope compositions have been measured for the Jurassic granitoids in the Liaodong Peninsula, in order to investigate their genesis and reveal the Jurassic crustal evolution of the eastern North China Craton. Our new data, combined with field and petrological observations, indicate multi-stage Jurassic magmatism, now represented by Early to Middle Jurassic biotite granites and monzogranites, and Late Jurassic two-mica monzogranites. The Early to Middle Jurassic granitoids have relatively low SiO2 contents (64.4–69.6 wt%) and K2O/Na2O ratios (0.4–1.0), adakitic characteristics with enriched LREE and depleted HREE contents, high Sr/Y ratios (52–189), positive Eu and Ba anomalies, and negative Nb and Ta anomalies. Together with their highly negative zircon ɛHf(t) (−26.6 to −16.1) and mantle-like δ18O (4.6‰ to 6.3‰) values, partial melting of a basaltic lower crust, which formed at ca. 2.5 Ga, can be proposed for the magma source of the Early to Middle Jurassic granitoids. These source rocks have not experienced significant water–rock alteration after formation. In contrary, the Late Jurassic two-mica granitoids exhibit relatively differentiated geochemical characteristics, with high SiO2 contents (>72 wt%) and K2O/Na2O ratios and variable Eu anomalies. Their parental magma was also enriched in LREEs and depleted in HREEs, with high Sr/Y (49.8–134) and (La/Yb)N (22.6–38.0) ratios. The magmatic zircons have extremely negative ɛHf(t) (−37.5 to −19.6) and relatively high δ18O (6.9‰ to 9.6‰) values, indicating the involvement of an ancient crustal component with high δ18O values, such as metasedimentary rocks. We propose, therefore, that the Late Jurassic two-mica monzogranites originated from partial melting of reworked ancient crustal materials at the depth of lower crust, and with the magma subsequently evolving as a result of plagioclase-dominated fractional crystallization. During Early to Middle Jurassic, westward subduction of the Paleo-Pacific plate, resulted in the partial melting of the mafic lower crust and the burial of ancient crustal materials in to deep crust, while crustal stretching in Late Jurassic might account for the decompression melting of the ancient crustal source. Thus, multi-stage Jurassic magmatism in the Liaodong Peninsula recorded a complex crustal evolution history, which closely related to the westward subduction of the Paleo-Pacific plate.

U–Pb systematics of the western Dharwar Craton - Glimpses of a billion year history of crustal evolution and relations to ancient supercratons

The Dharwar Craton developed progressively over a billion years, through two main stages of crustal growth separated by a few-hundred million year long period of relative quiescence. The first stage between 3.4 and 3.0 Ga developed a proto-craton, which was considerably amplified during the second main stage between 2.7 and 2.4 Ga, through extensive magmatism, tectonism, and crustal consolidation. This paper reports U–Pb dating results obtained in four specific areas of the craton, with the data encompassing key moments in this long development. Rocks formed during the proto-craton stage include a 3089 Ma augen gneiss and a 2973 Ma evolved granite, the latter of which marks the final cratonization event of the proto-craton. The beginning of the second main stage is recorded in this study by 2650 Ma tonalite and trondhjemite, a 2623 Ma granite dyke cutting augen gneiss, and 2614, 2602 and 2588 Ma volcanic rocks. Titanite responded differently to the long evolution, as a function of location and type of overprint. It preserved an original 2973 Ma magmatic age in the west, but was reset and/or crystallized during secondary events in central domains of the craton, yielding ages between 2590 and 2360 Ma. A diorite stock intruded at 2207 Ma in the consolidated crust. It is correlated with the Anantapur-Kunigal mafic dyke swarm, one of a series of such events in the Dharwar Craton between 2.35 and 1.79 Ma. In terms of its overall evolution the Dharwar Craton has an affinity with the Slave clan, which includes the Wyoming and Zimbabwe cratons. It also matches many features in the evolution of the São Francisco Craton, a probable other member of Sclavia. This is in contrast to the Amazonian Craton, which has more affinity with the Superior clan.

Archean crustal evolution of the Bundelkhand Craton: evidence from granitoid magmatism

Abstract This study presents petrological and geochemical data on Neoarchean granitoids from the northern and central parts of the Bundelkhand Craton to discuss its crustal evolution and tectonic history. The study deals with two granitoid suites, i.e. tonalites–trondhjemites–granodiorites (TTG) and sanukitoids. TTGs are characterized by high SiO 2 , Na 2 O and mostly low to moderate Mg#. They display enrichment in light rare earth elements, low to moderately fractionated heavy rare earth elements (HREE) and low Sr/Y ratios, suggesting their high-HREE character or low-pressure origin from melting of a mafic protolith. The sanukitoid samples show relatively low SiO 2 , high K 2 O (2.1–4.6 wt%), Pb, Sr and Ba, and moderate to low Mg#, Cr, Ni. These granitoids probably generated from partial melting of hydrous mafic rocks followed by interaction with a mantle peridotite. Geochemical characteristics, tectonic discrimination using ratios like (Ce/Pb) PM , (La/Nb) PM and (Th/Nb) PM and regional rock association suggest that the Neoarchean TTGs and sanukitoids were emplaced in a subduction setting. Combining the existing knowledge base, a schematic model for generation and evolution of crust from Paleoarchean to Neoarchean has been proposed for the Bundelkhand Craton.

Bimodal magmatism in the Eastern Dharwar Craton, southern India: Implications for Neoarchean crustal evolution

Archean cratons provide windows to the crustal evolution history in the early Earth. The Dharwar Craton is one of the major cratonic blocks in Peninsular India and is composed of several ancient microcontinents where multiple continental growth and recycling occurred from 3.8 to 2.5 Ga. Here we investigate a suite of magmatic rocks (including granitoids, mafic magmatic enclaves, ultramafic inclusions and synplutonic mafic dyke) from the western margin of the Eastern Dharwar Craton which provide insights on bimodal magmatism in a subduction-related arc setting and crustal growth during Neoarchean. Field evidence and geochemical features suggest mixing and mingling of mafic and felsic magmas. Geochemical features including incompatible and high field strength element features of the enclaves indicate heterogeneous sources involving dominantly enriched mantle reservoirs with minor depleted mantle whereas the host granitoids display depleted to enriched source. We present zircon UPb age data from the various rock types and the combined results display unimodal distribution with a single peak of 207Pb/206Pb age at 2531 Ma (n = 159) suggesting a major Neoarchean bimodal magmatic pulse. The zircon UPb ages of the mafic enclaves indicate either similar ages or slightly younger ages as compared to their host granitoids implying that enclaves represent late stage injections into crystallizing host granitoid magmas. The trace and rare earth element data on zircon from these rocks are consistent with magmatic crystallization. Zircons from most of the rocks in our study display uniformly positive εHf(t) (0.3–8.6), suggesting depleted mantle (juvenile) source for the Neoarchean arc building, and formation of new crust. Some of the mafic magmatic and ultramafic enclaves, as well as the mafic dyke and charnockite display mixed positive and negative εHf(t) values (−7.0 to 3.8), implying mixed depleted mantle and reworked crustal components of Neoarchean to Mesoarchean age. Most of the Hf depleted mantle model ages (TDM; 2520–3115 Ma) and crustal residence ages (TDMC; 2507 Ma to 3452 Ma) fall within the region bounding CHUR and depleted mantle lines, suggesting that the dominant magma source involved juvenile (depleted mantle) components. Thus, latest Neoarchean marks a major phase of continent building in the Eastern Dharwar Craton, and the results presented in our study suggest that bimodal magmatism in a subduction-related setting identical to those in modern convergent margins contributed to crustal growth.

Overprinting by episodic mineralization in the Dongyaozhuang gold deposit, Wutai Mountain, China: Constraints from geology, mineralogy, and fluid inclusions

The Archean volcanic‐sedimentary rocks (greenstones) hosting the Dongyaozhuang (DYZ) gold deposit are situated in the Wutai Mountain, north‐central China. Field geology and mineral assemblages show at least two episodes of mineralization. The first episode features deformed quartz‐pyrite veins filling in the regional foliation (MI, Paleoproterozoic) and can be divided into two stages (MIa and MIb) based on whether the mineralization is accompanied by tourmalinization. The mineral assemblages are mainly quartz + carbonate + pyrite + chalcopyrite (+ tourmaline for MIb); both are cut by deformed barren quartz veins, which represent the final stage of MI. The second episode features undeformed stockworks/veinlets with a mineral assemblage of quartz + carbonate + pyrite + chalcopyrite + arsenopyrite ± native gold (MIIa), crosscutting MI; these ore bodies are also cut through by undeformed barren carbonate and/or quartz veins (MIIb), which represent the end of metallogeny. By integrating the crustal evolution history with ore geology, we suggest that the DYZ gold deposit is a product by overprinting of episodic mineralization rather than a single metallogenic event. For further understanding and evidence in support of this speculation, fluid inclusions and hydrothermal chlorite and biotite were studied to reveal the physicochemical conditions of fluids from various stages, and C‐H‐O isotopes were used to trace the source(s) of fluids. Temperatures recorded in chlorite from various stages C1 (from the MIa), C2 (MIb), and C3 (MIIa) generally agrees with the homogenization temperatures of aqueous type fluid inclusions (FIs) of corresponding stages. Components of FIs from each stage are dominated by water with low salinities (0–12.6 wt% NaCl equiv). The Ib‐1, Ib‐2, and Ib‐s type CO2‐H2O FIs observed from the same deformed quartz vein of the MIb can be attributed to overprinting of hydrothermal fluids. The primary Ib‐1 and Ib‐2 FIs (209–404°C) were trapped at pressures of approximately 2.2–3.7 kbar, representing the formation pressure of the MI stage, whereas the secondary Ib‐s FIs (240–349°C) were trapped at pressures of approximately 0.8–1.5 kbar, which indicates overprinting induced by MII. The C‐H‐O isotope compositions of quartz and carbonates from MIa, MIb, and MIIa can be interpreted to indicate that the fluids were derived from the same source with multistage evolution or to reflect a homogeneous signature induced with overprinting mineralization. According to the geology, mineral assemblages, component of fluid inclusions, and C‐H‐O isotopes, the DYZ gold deposit is likely to be a product of overprinting of an orogenic gold deposit.

New insights into Neoarchean–Paleoproterozoic crustal evolution in the North China Craton: Evidence from zircon U–Pb geochronology, Lu–Hf isotopes and geochemistry of TTGs and greenstones from the Luxi Terrane

Tonalite–trondhjemite–granodiorite (TTG) suites constitute dominant components of Precambrian continental crust and provide important insights into the crustal evolution history of the early Earth. Here we investigate a suite of TTGs and associated greenstones from the Luxi Terrane in the south–eastern part of the North China Craton (NCC). We present data from petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes to characterize these rocks and understand their genesis. Mineral assemblage in the greenstones and TTGs suggests that the rocks have undergone amphibolite–facies metamorphism with regression under greenschist–facies. Zircon U–Pb data from the magmatic grains yield upper intercept ages of 2753 ± 37 and 2582 ± 40 Ma for greenstone, 2552 ± 13 Ma for TTG gneiss and 2476 ± 18 Ma for tonalite, and weighted mean 207Pb/206Pb age of 2507 ± 22 Ma for granodiorite. Lu–Hf isotope systematics show εHf(t) values ranging from −2.13 to 7.29 and crustal model ages (TCDM) in the range of 2603–3125 Ma for the TTGs, whereas the εHf(t) values are up to 8.53 with TCDM ages of 2644–3006 Ma for the greenstone. The results suggest that the protolith for the greenstone was derived from a Meso- to Neoarchean mantle contaminated by the crust source, whereas the TTGs were mostly sourced from juvenile crustal sources with reworking of older basement. The geochemical features of greenstone are similar to island arc basalt, with low Na2O, K2O and TiO2, low total rare earth elements (REEs) values, relative enrichment in light REEs and depletion in heavy REEs absence of obvious Ce anomaly, and negative Nb, Ta and Ti anomalies. The TTGs show similar features, except for the high K2O content and moderate total REEs values. We propose that the greenstone likely represent a part of the early and initial crustal growth at ca. 2750–2600 Ma, and the TTGs were generated subsequently at ca. 2550–2450 Ma. Our results are consistent with the previous models on the crustal growth models in the NCC and provide further insights into the Neoarchean to Paleoproterozoic tectonic history of the craton.