Generation Of Crust Research Articles

Nd-Sr-Hf isotopes and U-Pb ages of mesoproterozoic Três Estradas Alkaline-Carbonatite Complex, Brazil: Implications for Sul-Riograndense Shield evolution and rodinia break-up

The first occurrence of Mesoproterozoic carbonatite in the Rio de la Plata Craton – RDPC indicates extensional event prior to the opening of the Charrua Ocean. Nd-Sr-Hf Isotopic and U-Pb on zircon geochronological data are presented for the Três Estradas Alkaline-Carbonatite Complex – TEC, located in southernmost Brazil. The complex consists predominantly of ultramafic rocks, carbonatites and, subordinate, deformed fenitized syenites metamorphosed under greenschist to amphibolite facies conditions. U-Pb ages between 1110 ± 4.8 Ma and 1123 ± 15 Ma, respectively derived from a dolomite metacarbonatite and an associated metasyenite, constrain the TEC emplacement timing. The Nd-Sr-Hf isotopic signatures indicate that the TEC rocks were derived from a heterogeneous mantle, isotopically depleted in Sm-Nd and slightly depleted to enriched in Rb-Sr, with little influence of ancient crust and that these rocks are derived from the same source of parental magma probably submitted to intense metasomatism. The Nd-Sr isotopic signatures and crystallization ages of the TEC allow correlating it to the carbonatitic intrusions of the Alkaline provinces of Ontario and Grenville, Canada, in Laurentia, associated with the Keweenawan magmatic event (1.11–1.09 Ga), to which extension efforts would be related causing ruptures in RDPC edge and Laurentia, probably juxtaposed in that time interval. According to paleogeographic reconstructions of Rodinia, the Kalahari Craton would only have joined Laurentia and the RDPC at 1050 Ma. Around 920 Ma there is the generation of an oceanic crust in the Rio de la Plata Craton with the installation of the Charrua Ocean and the beginning of the formation of the Dom Feliciano Belt, concluded at 540 Ma, with peak deformation and metamorphism at 650–630 Ma.

Testing oceanic crust–mantle decoupling by Sr–Nd–Hf–Os isotopes of Neo-Tethyan ophiolites

Oceanic crust and the mantle are thought to have identical Sr–Nd–Hf–Os isotopic compositions. However, there is increasing evidence of isotopic differences between the two units, suggesting the existence of oceanic crust–mantle decoupling. Here we present updated whole-rock and mineral Sr–Nd–Hf–Os isotope data for the Kızıldağ (southeast Turkey) and Xigaze (south Tibet, China) Neo-Tethyan ophiolites to test if oceanic crust–mantle decoupling characterizes these ophiolites. Mantle peridotites from the Kızıldağ ophiolite have large variations in clinopyroxene and orthopyroxene Hf isotopic compositions (εHf (t) = −27.7 to +20.6) and unradiogenic Os isotope ratios (187Os/188Os = 0.11853–0.12720). These mantle peridotites have experienced ancient (up to ~1.6 Ga) melt depletion events, as evidenced by their old Re depletion ages (TRD = 1632–357 Ma), although Hf isotopes have been locally reset by recent melt re-fertilization, which likely occurred in a forearc basin setting. Mantle peridotites from the Xigaze ophiolite mainly have DMM (depleted mid-ocean ridge basalt mantle)-like to ultra-depleted clinopyroxene Hf isotopic compositions (εHf (t) = +9.1 to +29.8 for 17 out of 19 analyses), which were extensively overprinted by silicate melt percolation. However, the Os isotopic compositions of the Xigaze mantle peridotites are similar to those of the Kızıldağ ophiolite, again suggestive of ancient melt extraction long before the Early Cretaceous formation of this oceanic crust. These ancient and ultra-depleted mantle domains are not uncommon in modern oceanic lithosphere, forearc basins and ophiolites, and are less dense than the fertile mantle, and also are too refractory to contribute to the recent generation of oceanic crust. Sampling bias is unavoidable when comparing the oceanic crust and its underlying lithospheric mantle. As such, the oceanic crust alone cannot be used to estimate the isotopic composition of its entire mantle source.

Anatomy of a crustal-scale accretionary complex: Insights from deep seismic sounding of the onshore western Makran subduction zone, Iran

Abstract The Makran subduction zone has produced M 8+ earthquakes and subsequent tsunamis in historic times, hence indicating high risk for the coastal regions of southern Iran, Pakistan, and neighboring countries. Besides this, the Makran subduction zone is an end-member subduction zone featuring extreme properties, with one of the largest sediment inputs and the widest accretionary wedge on Earth. While surface geology and shallow structure of the offshore wedge have been relatively well studied, primary information on the deeper structure of the onshore part is largely absent. We present three crustal-scale, trench-perpendicular, deep seismic sounding profiles crossing the subaerial part of the accretionary wedge of the western Makran subduction zone in Iran. P-wave travel-time tomography based on a Monte Carlo Markov chain algorithm as well as the migration of automatic line drawings of wide-angle reflections reveal the crustal structure of the wedge and geometry of the subducting oceanic plate at high resolution. The images shed light on the accretionary processes, in particular the generation of continental crust by basal accretion, and provide vital basic information for hazard assessment and tsunami modeling.

The Siderian crust (2.47–2.3 Ga) of the Goiás Massif and its role as a building block of the São Francisco paleocontinent

The basement of the northern Brasília Belt, in Central Brazil, is dominated by large volumes of TTG magmatism generated during the Early Paleoproterozoic, and represents a continental block formed prior to the welding of the São Francisco paleocontinent during the Rhyacian, holding important information concerning the mechanisms and products of Paleoproterozoic crustal growth and amalgamation. New U-Pb zircon geochronological data was obtained from metagranitoids and mafic–ultramafic intrusions from the Almas-Conceição do Tocantins Domain (ACTD). This was coupled with geochemical data from the metagranitoids to better constrain the processes of continental crust evolution in this region during the Siderian period. Two main plutonic episodes were recognized in the mapped area: (i) an early suite (2.47 Ga) dominated by tonalites and trondhjemites with subordinate biotite granites (Ribeirão das Areias Complex - RAC); and (ii) a later suite (2.30 Ga) including tonalites and trondhjemites (Ribeirão Itaboca Suite - RIS). Most of the RAC trondhjemite-tonalite-granodiorite (TTG) rocks are marked by fractionated REE patterns with high (La/Yb)N = 15–107 and samples have medium to high Sr/Y ratio (28–257). These TTG include both high- and low-Al types and high- and medium-pressure groups. The RIS TTG suite displays higher HREE contents with high (La/Yb)N = 65–89 and high Sr/Y ratio (59–95) and contains the high-Al type and the medium pressure group. In spite of the wide age gap between them, both RAC and RIS are characterized by TTG rocks formed in a subduction-like tectonic setting. The biotite granites are related to a magmatic stage after the TTG formation. Mafic rocks of the Gameleira Suite have an abundance of zircon, interpreted as crustal contamination, with dominant 207Pb/206Pb age populations of 2.48 and 2.30 Ga, and are interpreted to be associated with an extensional magmatic event following the Siderian period. Regional geotectonic correlations within the São Francisco Craton based on data compiled from the literature indicate a significant volume of Siderian crust generation that has been overlooked in continental amalgamation models and that TTG magmatism is a more common geochemical signature in Paleoproterozoic rocks than has been previously suggested.

Crustal Evolution in the New England Orogen, Australia: Repeated Igneous Activity and Scale of Magmatism Govern the Composition and Isotopic Character of the Continental Crust

AbstractThe generation of continental crust, its bulk composition and temporal evolution provide important records of plate tectonics and associated magma-generating processes. However, the long-term integrated effects of repeated magmatic events on crustal growth, composition and differentiation and, therefore, on crustal evolution are rarely considered. Here, we examine long-term (∼350 Myr) temporal compositional trends of granitic magmatism within a limited (∼200 km × 100 km) area in the Northern New England Orogen of Queensland, Australia to avoid lateral crustal variations in order to understand how temporal–compositional variations of silicic igneous rocks record crustal evolution. Long-term temporal compositional variations are tracked using whole-rock chemistry, zircon chronochemistry and zircon Hf isotopic compositions. We particularly focus on whole-rock U, Th and K abundances and calculated heat-production values as proxies for crustal evolution, and tracking crustal sources involved in granitic magmatism. We identified two major compositional groupings within the study area that were repeatedly produced over time: compositional Group 1 comprises voluminous I-type igneous rocks emplaced during the Permo-Carboniferous and Early Cretaceous; Group 2 represents mainly lower volume A-type igneous rocks of Triassic, Middle Cretaceous and Tertiary age. Importantly, these compositional groupings alternate over the 350 Myr history of granitic magmatism within the study area. Heat-production values over time exhibit a zigzag pattern and mirror zircon Hf isotopic signatures where rocks with elevated heat-production values exhibit unradiogenic (crustal) Hf isotopic compositions. We identify the composition of crustal sources, level of the crust undergoing partial melting, scale of magmatism and source crustal volume as important factors in understanding the compositional diversity of silicic igneous rocks. We interpret the two chemical groupings to reflect the following magma-generating conditions: Group 1 igneous rocks record large-scale magmatic systems triggered by extensive crustal melting of multiple lower to middle crustal sources, which produce more compositionally and isotopically uniform magma compositions that approach bulk crustal compositions. In contrast, Group 2 igneous rocks reflect smaller-scale magmatic systems generated from smaller-scale partial melting events of the middle to upper crust that produced A-type magmas. Over the long term, the successive large-scale magmatic events (recorded by Group 1 igneous rocks) through their concomitant basaltic underplating make the Hf composition of the lower crust more radiogenic, and tend to homogenize the isotopic composition of the continental crust. We consider three important coupled controls: (1) promotion of extensive crustal melting by large-scale magmatic systems, potentially blending multiple crustal sources that can also include a significant juvenile source contribution; (2) melt depletion, whereby older, and potentially more unradiogenic crustal materials become more refractory; (3) ‘crustal jacking’, where mantle-derived magmas are added as underplate to the crust (i.e. basification) and can shift older crustal materials to more shallow levels (potentially in concert with erosion and exhumation) and away from zones of crustal melting. Our findings highlight the importance of integrating the geological and intrusive history with whole-rock geochemical data and isotopic information, and have direct implications for continental regions that exhibit protracted igneous histories and where isotopic compositions may trend towards more juvenile compositions such as circum-Pacific or retreating accretionary orogens.

Lu-Hf systematics of 4.0 – 2.3 Ga old zircons from the Turee Creek Group (Pilbara Craton, W. Australia): Implications on the rise of atmospheric oxygen and global glaciation during the Paleoproterozoic

We investigated the Hf isotopic systematics of detrital zircons in a succession of siliciclastic sediments and glacial diamictites from the early Paleoproterozoic sequence of the Boolgeeda Iron Formation (Hamersley Group) and overlying Turee Creek Group of the Pilbara Craton, Western Australia. About 400 detrital zircons yielding >95% concordant U-Pb ages were analyzed for Hf isotopes to constrain their magmatic sources. 70% of the analyzed zircons display super-chondritic initial Hf isotopic compositions, demonstrating crystallization in mantle-derived magmas. Most of the data are comprised between model age lines at ~2.5 and 3.2 Ga, which suggests a sub-continuous crust generation by extraction from the depleted mantle during this time period. A single grain yields a 4.0 Ga age, which represents the first Hadean age for a zircon from the Pilbara Craton. Our results are significantly different from zircon Hf isotope data of the Glenburgh Terrane, in the southern border of the Turee Creek Group, or older successions of the Pilbara, Kaapvaal and Superior cratons, but show overlap with some of the Yilgarn Craton. This together with the occurrence of a Hadean zircon crystal preserved in the Boolgeeda glacial diamictite with similar Hf isotopic signature than the Jack Hills zircons makes the Yilgarn Craton a possible source material for the Boolgeeda glacial horizon. Alternatively, the majority of the zircons analyzed show ages which are consistent with those of the underlying 2.45–2.78 Ga Hamersley and Fortescue groups, formed by sedimentary successions interleaved with thick subaerial volcanic sequences associated with the emplacement of Large Igneous Provinces. Such subaerial volcanic rocks could account for the relatively juvenile character of the zircon analyzed. A local provenance of the siliciclastic material delivered to the Turee Creek Basin would support the role of large subaerial magmatic provinces as triggers of the rise of atmospheric oxygen and the onset of glaciations at the beginning of the Proterozoic.

Construction of an island arc and back-arc basin system in eastern Central Asian Orogenic belt: Insights from contrasting Late Carboniferous intermediate intrusions in Central Inner Mongolia, North China

Characterizing island arc and back arc basin system through detailed magmatic profile remains a pivotal task in any regional paleotectonic and supercontinental reconstructions along ancient convergent plate margins, as is of paramount importance for the northern China–southern Mongolia (NCSM) tract along the eastern Central Asian Orogenic belt (CAOB). Here we document episodic Late Carboniferous intermediate to felsic intrusive suites in the West Ujimqin region of central Inner Mongolia, spanning from ca. 315 Ma granodiorites through ca. 310 Ma hornblende diorites to ca. 305–301 Ma diorites. The earlier granodiorites exhibit dominant magnesian-tholeiitic affinity, enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), and possess whole-rock ISr(t) ratios of 0.707387–0.708799, negative ƐNd(t) of −2.3 to −2.4 and zircon ƐHf(t) values of −0.69 to +4.87 and δ18O of +6.33 to +8.40‰. These features are consistent with the magmatic derivation from dehydration melting of tholeiitic mafic to intermediate lower crustal underplates with obvious high δ18O supracrustal inputs. The ca. 310 Ma pod-like hornblende diorites display high-Ca boninite-like elemental traits and MORB-like isotopic compositions (ISr(t) = 0.704190 to 0.704203, ƐNd(t) = +7.9 to +8.3, zircon ƐHf(t) = +14.7 to +17.5 and δ18O = +4.88 to +6.16‰), indicative of a prior melt-depleted then fluid-enriched mantle source. By contrast, subsequent (301–305 Ma) diorites show arc-like elemental hallmarks, with notable enriched LILEs and depleted HFSEs as well as variable isotopic values (ISr(t) = 0.703690–0.704065, ƐNd(t) = +3.5 to +8.4, zircon ƐHf(t) = +7.10 to +16.4 and δ18O = +3.75 to +6.83‰). They are interpreted to represent the evolved products of hornblende-dominated fractional crystallization from depleted basaltic parental magmas. Given their coincidence with regional back-arc-basin type ophiolitic stratigraphy and detrital zircon age spectra, these intermediate to felsic intrusive suites tend to document the timeline of a back arc basin formation from initial rifting to maturation. In conjunction with Early Carboniferous forearc-type ophiolitic complexes and forearc magmatic records at West Ujimqin and the adjacent region, a Carboniferous island arc and back arc system can be reconstructed from Xilinhot to West Ujimqin. The characterization of this northward (present day coordinates) subduction system in terms of its temporal evolution not only provides overwhelming evidence that modern Pacific-like, archipelago-type arc configuration had prevailed in central Inner Mongolia during the Carboniferous, but also offers vivid insights into the diversity of crustal generation mechanism in a subduction factory that ushered in seminal continental crustal construction in the eastern CAOB.

Detrital zircon U-Pb age and Hf isotopic composition and whole-rock geochemical characteristics of the Statherian Huangqikou Formation, western margin of the North China Craton: Implications for provenance and tectonic evolution

Proterozoic sedimentary covers in the Helan-Qianli Mountain area, western margin of the North China Craton (NCC), include the Huangqikou, Wangquankou and Zhengmuguan formations from bottom to top. The Huangqikou Formation, mainly distributed in the Middle Helan Mountain (MHM), North Helan Mountain (NHM) and Qianli Mountain (QM), is composed of conglomerate, quartz sandstone and silty-argillaceous shale that are locally metamorphosed into quartzite and slate. Detrital zircon U-Pb dating and stratigraphic correlation bracket that the Huangqikou Formation deposited during 1.75–1.60 Ga. Petrologic and geochemical features suggest that the provenance was sourced from felsic rocks and pre-existed sedimentary rocks. U-Pb ages of detrital zircons from the Huangqikou Formation in the MHM area yield age ranges at 2.5–2.35 Ga, 2.2–2.0 Ga and 1.95–1.8 Ga, while those in the NHM-QM areas yield age ranges at 2.5–2.4 Ga, 2.0–1.95 Ga and 1.85–1.78 Ga. The remarkable geochronological and isotopic similarity suggests that the early Paleoproterozoic granitic gneiss in the Ordos Block and Zhaochigou complex in the Khondalite Belt are the dominant source rocks for the Huangqikou Formation in the MHM. However, the provenances of the Huangqikou Formation in the NHM-QM are dominated by the granitoids in Guyang area of the Yinshan Block, the S-type granites and paragneisses in the Khondalite Belt and the TTG gneisses, Bayanwulashan complex and Boluositanmiao complex in the Alxa Block, as well as a small amount basement rocks in Alxa Block. The characteristics of geochemistry, provenance and regional geological data indicate that the Huangqikou Formation deposited in a rift basin. Besides, detritus sourced from the Alxa Block in the clastic sedimentary rocks of the Huangqikou Formation imply that the Alxa Block was already a part of the NCC before ca.1.75 Ga. The U-Pb age and Lu-Hf isotopic characteristics of detrital zircons from the Statherian show that rapid generation of juvenile crust in the western Block was at 2.9–2.7 Ga, yet the ca.2.5 Ga magmatic activity strongly reworked pre-existed crust with negligible juvenile crust generation. The magmatic activities quite possibly developed in the Western Block during the global magmatic shutdown (2.45–2.2 Ga).

A new concept for the genesis of felsic magma: the separation of slab-derived supercritical liquid

Felsic magmas produced at subduction zones have played an important role in the generation and evolution of the continental crust. For the origin of felsic magmas, processes such as fractional crystallization of mafic magmas, partial melting of crustal materials, partial melting of subducting slabs, and partial melting of pyroxenitic mantle wedge components have been proposed. Recent experimental studies have predicted that felsic melt can also be produced in the mantle wedge by the separation of slab-derived supercritical liquid beyond depths corresponding to the critical point. To date, however, the presence of felsic magma of this origin has not yet been reported. In this study, we investigated dacitic lavas and preceding calc-alkaline andesite lavas from the Rishiri Volcano, located at the rear of the Kuril arc. We show that hydrous felsic melt and aqueous fluid were separated from slab-derived supercritical liquid in the mantle wedge. The former erupted as dacitic magma whilst the aqueous fluid induced the generation of primary basaltic magma involved in creating calc-alkaline andesite magma. We infer that slab-derived supercritical liquid is an efficient transport medium for moving silicate-rich components from subducting slabs to the Earth’s surface, and that this process may have contributed to the growth of the continental crust.

Compositional changes of granitoids from the Menglian Batholith in SW China at ca. 122 Ma: Implications for the origin of decoupled Nd-Hf isotopic compositions and crust generation in collision zones

Determining the origin and related mantle dynamics responsible for the generation of giant granitic batholiths in convergent margins is critical to unravel the formation mechanism of the silica–rich continental crust. This paper reports the whole–rock major element, trace element, Sr-Nd isotopic, and zircon U-Pb age and Hf isotope data of samples from the Menglian Batholith in the eastern Tengchong Terrane of southwest Yunnan. These samples consist of two groups, including the Group I rocks that are mostly granitic with zircon U-Pb ages of 132–122 Ma and the Group II rocks that display varying compositions from gabbroic to granitic with zircon U-Pb ages of 122–112 Ma. The Group I samples have zircon εHf(t) of −11.7 to −4.2 and whole–rock εNd(t) of −10.1 to −8.9, whereas the Group II samples show elevated zircon εHf(t) of −7.8 to +7.1 and whole–rock εNd(t) of −9.0 to −6.9 and εHf(t) of −3.1 to −1.0. The Nd-Hf isotopic compositions of the Group I rocks are coupled, likely resulting from the partial melting of an ancient lower crust. However, the Nd-Hf isotopic compositions of the Group II rocks are decoupled, likely inheriting from the mantle–derived material with decoupled Nd-Hf isotopic compositions. Regional geological correlation and compositional transition documented by the Group I and Group II samples collectively indicate that the slab rollback and subsequent breakoff of the subducting Bangong–Nujiang ocean lithosphere during the Tengchong–Baoshan collision can be invoked to explain the generation of the Menglian Batholith. The similarity of average compositions between the Group II samples and the bulk continental crust suggests that the addition of mantle–derived material to the ancient crust–derived melts seems to be an efficient process for crustal growth during continental collisions.

Generation of crustal movement correction parameter considering valid period and accuracy guarantee

In recent years, due to the development of precise point positioning (PPP), the inconsistency of map and satellite positioning results due to crustal deformation has come to the surface. In order to solve this problem, a parameter for correcting the crustal deformation is necessary, and if it is provided as a service, the validity period and the accuracy guarantee are also necessary. In this research, we considered the validity period of parameters and the estimation method of the coordinates used for calculating the amount of crustal deformation and actually constructed the crustal movement correction parameter generation system and accuracy evaluation system. The parameters generated by this method are called semi-dynamic reduction (SD/R) parameters. As a result of evaluating the accuracy of SD/R parameters, we achieved a target accuracy of 4 cm at 98.3% verification points in the horizontal direction. Therefore, the effectiveness of the proposed method was shown.

Generation of Eoarchean continental crust from altered mafic rocks derived from a chondritic mantle: The ∼3.72 Ga Aktash gneisses, Tarim Craton (NW China)

Our understanding of crust – mantle evolution and geodynamics of the early Earth has been hampered by the scarcity of Hadean to Eoarchean rocks and by the almost ubiquitous isotopic disturbance of the oldest rocks. Here we present in-situ zircon U-Pb ages, O-OH and Lu-Hf isotopes, as well as whole-rock Sr-Nd-Hf-Pb isotopes for the recently identified Eoarchean Aktash gneisses in the Tarim Craton, NW China. The new age data confirm that the Aktash gneisses were formed at least by ∼3.72 Ga and were affected by two metamorphic events at ∼3.56 and ∼2.0 Ga, respectively. Whole-rock isotopic data show that the Sr-Nd-Hf-Pb isotopic systematics have all been variously disturbed by ∼2.0 Ga regional granulite facies metamorphism, resulting in decoupling of Nd-Hf isotopic systematics that is, in some samples, similar to that observed in SW Greenland. The least disturbed sample (16ALT67), however, yielded chondritic to slightly sub-chondritic whole-rock εNd (−0.3) and εHf (−0.9). This is consistent with the in-situ magmatic zircon εHf (weighted mean −0.2 to −1.8) for most samples of the Aktash gneisses, suggesting a mafic source derived from chondritic mantle with a short crustal residence time. Our data thus provide no evidence for the presence of an Eoarchean depleted mantle or a Hadean enriched crust, implying that early Earth crust – mantle differentiation was either limited or balanced by large-scale crustal recycling. In-situ zircon O–OH data show that metamorphism and later alteration tend to decrease the zircon δ18O of the Aktash gneisses, and the weighed mean δ18O of pristine magmatic zircons (6.2–6.3‰) from the least disturbed samples indicate that the mafic source was mildly altered at low temperature by the early hydrosphere, facilitating partial melting and formation of some of the earliest preserved continental crust known on Earth.

The geodynamic history of the Famatinian arc, Argentina: A record of exposed geology over the type section (latitudes 27°- 33° south)

The Famatinian arc, over central-western Argentina, is one of the few examples of exposed crustal arc cross-section in the world. This Paleozoic magmatic arc and orogenic system on the Earth's surface offers unique insights into the nature of whole-arc processes, continental crust generation, and preservation. A detailed account of birth, growth, and closure is integrated throughout the region, and the entire geodynamic history of the Famatinian arc is presented. The Famatinian magmatic arc grew during a single-cycle episode that spanned a few tens of million years. Upon waning of the Cambrian Pampean orogen, an Upper Cambrian – Lower Ordovician marginal and open sea basin developed mostly on the recently stabilized Cambrian crystalline crust and was flooded by a turbidite wedge. Typical subduction zone magmatism resumed outboard of the Pampean orogen in the Lower Ordovician. After about 20 My of magmatism driven by subduction zone dynamics at plate scale, magmatism waned and stopped with the entry of a Laurentia-rifted continental microplate in the subduction zone. A full phase of a mountain-building process accompanied the continent-arc collision. The orogenic collapse occurred during the Devonian, ending the Famatinian system after nearly 150 My of magmatic arc, plate convergence, and continent-arc collision development on the proto-Andean Gondwana margin.

Origin of the Triassic Lincang granites in the southeastern Tibetan Plateau: Crystallization from crystal mush

The crustal generation of the Paleo-Tethys orogenic belt in the southeastern Tibetan Plateau is a major unresolved geological problem. The Changning-Menglian suture in the southeastern Tibetan Plateau represents a remnant of the Paleo-Tethys Ocean. The Lincang batholith and associated volcanic rocks (the Manghuai rhyolites and Xiaodingxi basalts) are located to the east of the Changning-Menglian suture, and represent important constraints for the abovementioned geological problem. New zircon UPb ages, together with previously published results, indicate that the Manghuai rhyolites were emplaced at 229–239 Ma and the Lincang batholith yielded longer age span (211–239 Ma). Broad age span of zircons (to 28 million years) from these rocks record a long-lived crustal magmatic system in the Changning-Menglian Paleo-Tethys orogenic belt. These intrusive and associated volcanic rocks have significant differences between them in terms of geochemistry. In this study, these rocks are geochemically classified into three groups: (1) Group 1 comprises biotite granite and granodiorite rocks; (2) Group 2 includes alkali feldspar granite, two-mica granite, and rhyolite, which are high-silica (generally >70 wt%) rocks that have low TiO2, Al2O3, FeOtot, MgO, CaO, P2O5, Eu, and Sr contents, compared to those of Group 1. Based on their geochemical relationships and mineral textures, Groups 1 and 2 include cumulative- and fractionated-type granitoids, respectively; (3) while Group 3 is comprised of dioritic enclaves and the Xiaodingxi basalts. Based on more robust evidence, dioritic enclaves are derived from a mantle source magma. The Lincang granites have high and variable initial 87Sr/86Sr ratios (0.703122–0.728476), significantly negative εNd (t = 220 Ma) values (−13.4 to −7.5), negative εHf (t) values (−17.7 to −6.8), and old TDM2 ages. The Manghuai rhyolites and Xiaodingxi basalts have lower initial 87Sr/86Sr ratios (0.695970–0.709398), and higher εNd (t) values (−3.4 to −0.6) and εHf (t) values (−1.2 to +3.7) than those of the Lincang granites. Geochemical data suggest that the Manghuai rhyolites could be seen as fractionated and erupted melts from crystal mush reservoirs, which later crystallized to form the Lincang granites. The Manghuai rhyolites provide an instantaneous snapshot of the state of the crystal mush at the time of eruption. The Lincang granites are interpreted to record prolonged histories of open-system magmatic evolution, involving magma mixing, wall-rock assimilation, and multiple cycles of crystallization from the crystal mush reservoirs. Magmatic evolution was likely related to mantle upwelling in a post-collisional setting, forming large-volume granites and volcanics in the Changning-Menglian Paleo-Tethys orogenic belt. Thus, this crystal mush model provides an integrated picture of the Triassic silicic magmatism, linking the evolution of the Lincang batholith and Manghuai rhyolites before storage in the Changning-Menglian Paleo-Tethys orogenic belt.

Granitic magmas with enriched isotopic compositions, from enriched mantle source: Implications for continental generation

Granitoids with depleted isotopic compositions are generally considered beneficial to continental generation. However, most granitoids exhibit enriched isotopic characteristics (high initial Sr isotopic ratios, negative ƐNd and ƐHf values) and their role in continental generation is scantly explored. In this contribution, we report petrological and geochemical data of the Permian (ca. 270 Ma) Yabulai granites and hornblende gabbros in the Alxa Block of western North China Craton. Compared to initial mantle-derived basalts, the hornblende gabbros show low MgO (3.74 to 5.68 wt%), Cr (34.9 to 66.2 ppm), Ni (16.6 to 45.6 ppm) contents and Mg# (0.46 to 0.54), with zircon ƐHf values of +3.5 to +6.6 and −8.4 to −4.8. Relative to heavy rare earth elements (HREEs), the hornblende gabbros are enriched in light rare earth elements (LREEs) with the absence of Eu anomalies. In primitive mantle-normalized spidergrams, the hornblende gabbros exhibit notably negative NbTa and positive Pb anomalies. Elemental and isotopic data indicate that they might have originated from a shallow and heterogeneous lithospheric mantle source modified by subducted materials. The granites are metaluminous to weakly peraluminous and can be classified as high-K calc-alkaline I-type, with negative zircon ƐHf values of −8.3 to −5.3. These granitic rocks were likely to be derived from a newly underplated mafic crustal source. Using our data and published data from the granitic rocks in the Alxa Block, we suggest that a Permian crustal generation event has occurred in the Nuru-Langshan belt of the Alxa Block. Our current results reveal that the crustal generation rates at the convergent plate margins might have been underestimated due to the unintended omission of the mantle contribution to the granitoids with enriched isotopic compositions.

Effects of reactive dissolution of orthopyroxene in producing incompatible element depleted melts and refractory mantle residues during early fore-arc spreading: constraints from ophiolites in eastern Mediterranean

Compositional variations of peridotites from the Cretaceous ophiolites in southern Turkey and Northern Cyprus are presented to document the nature of partial melting and possible effects of reactive dissolution of primary mantle phases during fore-arc spreading. The peridotites overall exhibit a range of 187Os/188Os ratios from 0.1171 to 0.1266 and appear to represent a mantle region that preserves a record of ancient melt depletion. The samples are depleted in 187Os/188Os compared to the ambient oceanic upper mantle (187Os/188Os ~0.127), suggesting that they are representatives of a shallow fore-arc mantle where transport of radiogenic 187Os during slab dehydration was limited. Chemical variations of primary mantle minerals indicate that the peridotites are the residues of moderate to high degrees (>16%) of partial melting and have experienced significant modal and chemical compositional modification through interaction with oxidizing hydrous basaltic melts. Interacting melts, which appear to be similar in composition to primitive arc tholeiite, are likely to have originated from sub-lithospheric lower part of the mantle wedge during early stages of fore-arc spreading and migrated upward to react with variably depleted harzburgites to induce further melting in the overlying lithospheric mantle through open-system reactive flow. This second stage melting resulted in (1) common occurrence of reactive harzburgites and dunites by incongruent melting of orthopyroxene and crystallization of olivine through interaction with olivine saturated melt; and (2) local development of refertilized peridotites by shallower melt impregnation that involves interaction with olivine + clinopyroxene saturated melt. The dissolution of orthopyroxene caused the reacting melt to be enriched in silica and diluted in incompatible elements which led to the production of the final melts similar in composition to fore arc basalt and boninite. Involvement of compositionally variable mantle and melt components with different rates of melt influx therefore appears to explain the generation of fore-arc crust with a range of diverse rock suites including temporally and spatially associated arc tholeiites and boninites with significant depletion in incompatible elements.

Petrogenesis of the Ulungur Intrusive Complex, NW China, and Implications for Crustal Generation and Reworking in Accretionary Orogens

Abstract Accretionary orogens are characterized by voluminous juvenile components (recently derived from the mantle) and knowing the origin(s) of such components is vital for understanding crustal generation. Here we present field and petrological observations, along with mineral chemistry, zircon U–Pb age and Hf–O isotope data, and whole rock geochemical and Sr–Nd isotopic data for the c.320 Ma Ulungur intrusive complex from the Central Asian Orogenic Belt. The complex consists of two different magmatic series: one is characterized by medium- to high-K calc-alkaline gabbro to monzogranite; the other is defined by peralkaline aegirine–arfvedsonite granitoids. The calc-alkaline and peralkaline series granitoids have similar depleted mantle-like Sr–Nd–Hf isotopic compositions, but they have different zircon δ18O values: the calc-alkaline series have mantle-like δ18O values with mean compositions ranging from 5·2 ± 0·5‰ to 6·0 ± 0·9‰ (2SD), and the peralkaline granitoids have low δ18O values ranging from 3·3 ± 0·5‰ to 3·9 ± 0·4‰ (2SD). The calc-alkaline series were derived from a hydrous sub-arc mantle wedge, based on the isotope and geochemical compositions, under garnet peridotite facies conditions. This study suggests that the magmas underwent substantial differentiation, ranging from high pressure crystallization of ultramafic cumulates in the lower crust to lower pressure crystallization dominated by amphibole, plagioclase and minor biotite in the upper crust. The peralkaline series rocks are characterized by δ18O values lower than the mantle and enrichment of high field strength elements (HFSEs) and heavy rare earth elements (HREEs). They likely originated from melting of preexisting hydrothermally altered residual oceanic crust in the lower crust of the Junggar intra-oceanic arc. Early crystallization of clinopyroxene and amphibole was inhibited owing to their low melting temperature, leading to HFSEs and HREEs enrichment in residual peralkaline melts during crystallization of a feldspar-dominated mineral assemblage. Thus, the calc-alkaline and peralkaline series represent episodes of crust generation and reworking, respectively, demonstrating that the juvenile isotopic signature in accretionary orogens can be derived from diverse source rocks. Our results show that reworking of residual oceanic crust also plays an important role in continental crust formation for accretionary orogens, which has not previously been widely recognized.

Generation and maturation of Mesoarchean continental crust in the Anshan Complex, North China Craton

The mechanisms underlying the generation of tonalite–trondhjemite–granodiorite suites and the transition to K-rich granitoids, which marks the most profound stage of Archean intracrustal differentiation, remain controversial. The Archean Anshan Complex (AAC) of the North China Craton (NCC) preserves diverse 3.8–2.5 Ga granitoids, providing an excellent opportunity to address these problems. We conducted a detailed field, petrological, geochemical, and zircon isotopic study on AAC units, demonstrating that: (1) the Li–Dong–Ying trondhjemite intrudes and encloses ca. 3.3 Ga gneiss–migmatite complex across the Anshan, and crystallized at ca. 3.14 Ga; and (2) the porphyritic and fine-grained Chentaigou monzogranites were emplaced contemporaneously with the trondhjemite, both at ca. 3.14 Ga.The Li–Dong–Ying trondhjemites show high SiO2, Na2O, low MgO, significant variations in Sr, Y, and Yb, REE fractionation (LaN/YbN = 9–140), and variable Eu anomalies (Eu/Eu* = 0.4–1.3). The magmatic zircons yield δ18O of +4.8 ± 0.2‰ to +6.9 ± 0.4‰ and initial εHf of −1.1 ± 0.7 to +4.4 ± 0.9, which translate to TDM ages of 3.6–3.3 Ga. The Chentaigou monzogranites have higher SiO2 and K2O, lower Na2O and CaO, similar δ18O of +5.1 ± 0.3‰ to +7.0 ± 0.3‰, and more negative Eu anomalies as compared with the trondhjemites. The Hf–O isotopes, geochemistry, particularly considerable variations in pressure-sensitive proxies of the trondhjemites, suggest that they were derived by melting of low-temperature altered basaltic rocks at 1.0–1.5 GPa and 900–950 °C, leaving a garnet amphibolite residue with variable amounts of plagioclase but no rutile; the most plausible source is ca. 3.3 Ga basaltic rocks. Polybaric melting of basaltic sources and fractionation of hornblende and plagioclase from trondhjemitic magmas account for the chemical variability of AAC granitoids, providing a mechanism for the coeval emplacement of diverse Archean granitoids. Secular compositional changes in AAC trondhjemites imply that the depth and interval of melting of basaltic sources increased from the Eoarchean to Mesoarchean. Changes in the nature of the source and melting depth characterized the early stage of Archean intracrustal differentiation in the NCC.

Integrated ophiolite and arc evolution, southern Brasiliano Orogen

Integrated study of Tonian ophiolite mélanges and intra-oceanic arc of the São Gabriel juvenile terrane exposes a key geotectonic unit in the evolutionary history of Gondwana during Brasiliano Orogeny. LA-ICP-MS U-Pb geochronology in rutile and zircon were combined with Lu-Hf and rare earth elements in zircon. We report the first rutile dating of ultramafic rock in the Brazilian Shield of 787.6 ± 2.6 Ma for the Cambaizinho ophiolite magnesian schist. Dating of rutile corresponds to the metamorphic age of the mantle section of the ophiolite. São Gabriel arc infrastructure is represented by the Cambaí Complex, including the Vila Nova Suite metatonalite with zircon U-Pb age of 724.6 ± 3.2 Ma and late Sanga do Jobim Suite monzogranite intrusions with two main pulses at 698.9 ± 4.2 Ma and 673.9 ± 6.8 Ma. Zircon εHf(t) values are positive for both rocks and range between +10.1 and +7.4 in metatonalite and +9.2 to +2.5 in monzogranite. Detrital zircon from metasedimentary rocks support superstructure arc investigation. Ages range from 817 to 650 Ma, with strong concentration between 750 and 730 Ma. Positive εHf(t) values and Rare Earth Elements (REE) signature point to main source of metasedimentary rocks in the arc, with ophiolitic contribution. Therefore, geotectonic processes in the Brasiliano Orogeny include the construction of São Gabriel terrane with Tonian oceanic crust evolving to intra-oceanic subduction until 650 Ma to final development along the margin of continental crust. Overthrusting of the São Gabriel terrane onto the Rio de La Plata Craton occurred at 650–620 Ma. Our data correlate with Tonian intra-oceanic arcs from Brasilia belt (Goiás arc ca. 862–630 Ma) and Ribeira belt (Serra da Prata arc – 856–838 Ma; Rio Negro arc – 790–620 Ma). Oceanic crust generation combined with subduction to form Tonian arcs along the Brasiliano Orogen were most significant for construction of West Gondwana.

Orosirian magmatism in the Tapajós Mineral Province (Amazonian Craton): The missing link to understand the onset of Paleoproterozoic tectonics

The Tapajós Mineral Province (TMP) located at south-central Amazonian Craton is dominated by igneous suites that show geochemical and petrologic evidence of a long-lived subduction process accounting for continental crust generation. The process lasted for approximately 100 Ma and yielded in different intrusive and extrusive manifestations within the 1.99–1.89 Ga interval. At its initial stage, arc magmatism is characterized by a strong crustal component on the magmas possibly due to subcrustal-erosion upon the onset of subduction. After this phase, metaluminous primary melts provided by the subduction zone gradually differentiate to peraluminous compositions, the case for the ca. 1.98–1.95 Ga Creporizão Intrusive Suite (CRP) and its extrusive correspondences. Geochemical modelling reveals that the first bursts of magmatism at the TMP (at ca. 1.99 Ga) evolved through pyroxene fractionation, whereas the following pulses are amphibole-bearing. As the arc evolves, the mantle wedge gets progressively metasomatized and hydrated, eventually melting to generate the high-K metaluminous to mildly peraluminous granitoids of the ca. 1.89 Ga Parauari Intrusive Suite (PAR), accompanied by more evolved εNd(1.88). The first pulses of the Iriri Group volcanism are synchronous with PAR, nevertheless, Nd isotopes point to different sources. Around 1.87 Ga, slab break off provokes astenospheric upwelling, which is responsible for destabilizing and melting of the subcontinental-lithospheric-mantle severely metasomatized by the previous magmatic events, generating shoshonitic and metaluminous melts with variable degrees of crust assimilation, translated into mildly negative εNd(1.86) of the Maloquinha Intrusive Suite (MLQ) and Iriri volcanic rocks at an early-anorogenic tectonic setting. Geochemical modelling and the depletion in Ba, Sr and Eu of MLQ and part of Iriri are compatible with early extraction of plagioclase and support the relatively dry evolution of these units. In addition, fractional crystallization alone cannot account for the geochemistry variability of MLQ and Iriri, reinforcing crust assimilation on their petrogenesis. In terms of metallogenetic potential, CRP and PAR have petrologic evolutions consistent with amphibole and magnetite crystallization, pointing to hydrated and oxidized magmas that might be considered as favorable for the formation of magmatic-hydrothermal mineral deposits (porphyry AuCu (Mo) and epithermal systems). Lastly, the magmatic events younger than 1.89 Ga might act incorporating residual Au-rich sulfide from the mantle, explaining the Au-only characteristic of many mineral deposits described at the TMP.