Tectono-magmatic Evolution Research Articles

Precambrian and Early Palaeozoic metamorphic complexes in the SW part of the Central Asian Orogenic Belt: Ages, compositions, regional correlations and tectonic affinities

Precambrian metamagmatic complexes of the Zhingeldy and Kendyktas blocks of the Zheltau and Chu-Kendyktas terranes of the SW part of the Central Asian Orogenic Belt (CAOB) consist of predominant Neoproterozoic orthogneisses with the estimated protolith ages of ca. 790–800 Ma, and subordinate epidote and garnet amphibolites, metabasalts and metadolerites of the lower metamorphic grades. The orthogneisses are characterized by negative εNd(t) values of −12 to −4 with Nd model ages of 2.3–1.7 Ga, indicating their protoliths could have formed by a reworking of the Palaeoproterozoic crustal materials. Precambrian metavolcanic rocks in the studied region are rhyolites with the estimated crystallization ages of ca. 800–830 Ma, and are associated with metabasalts. Metasedimentary complexes in the studied region consist of intensively retrogressed garnet-mica schists interlayered with marmorized limestones, metasilicic rocks, epidote and garnet amphibolites and strongly altered eclogites. The inferred protoliths of the garnet-mica schists are terrigenous rocks accumulated during the latest Ediacaran – middle Cambrian and formed from Mesoproterozoic (ca. 1.0 Ga) and to a lesser extent Palaeoproterozoic felsic source rocks (ca. 1.6–1.7 Ga and ca. 2.5 Ga). The garnet-mica schists of the Kendyktas block have negative εNd(t) of −13.4 and a model age tNd(DM) of 2.2 Ga, implying formation of their protolith from Palaeoproterozoic crustal source. The studied Zheltau and Chu-Kendyktas terranes from the SW part of the CAOB have much in common in terms of Precambrian and Early Palaeozoic evolution and chemical compositions of the key rock types. The presence of the Early Precambrian rocks (Zheltau terrane), an abundance of the Late Mesoproterozoic (1000–1100 Ma) detrital zircons in the Ediacaran-Cambrian metasedimentary complexes and a wide distribution of the Late Neoproterozoic magmatism, are distinctive features of the studied terranes. We refer such Precambrian terranes of the western CAOB to the Ulutau-Moyunkum group where tectono-magmatic evolution is consistent with that observed from the northern part of the Tarim craton. Our results also aid comprehensive correlation of metamorphic complexes of the Precambrian terranes from the SW CAOB.

A Comparison between the ∼1.08–1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa

A Comparison between the ∼1.08–1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa

Late Carboniferous – Early Permian mafic dikes and granitoids in the heart of the Western Tianshan Orogen, NW China: Implications for a tectonic transition from a syn- to post-collisional setting

Syn- to post-collisional magmatism in the Chinese western Tianshan Orogen is recorded in the Late Carboniferous to Early Permian magmatic rocks of the Awulale Mountains. However, the orogen's tectono-magmatic evolution remains controversial. In particular, two contrasting genetic models have been proposed, i.e., post-collisional extension vs. mantle plume. Here, we report zircon U-Pb ages, whole-rock geochemical data and Nd-Hf isotopic data from the mafic dikes and granitoids of the western Awulale Mountains. These data better constrain the rock's petrogenesis and geodynamic processes following the amalgamation of the Tianshan Orogen. In combination with previous data, we have identified two magmatic episodes in the Awulale Mountains. The Late Carboniferous (ca. 319–307 Ma) episode emplaced the mafic gabbro dikes and intermediate-acidic granitoids with I-type and adakitic affinity. The latest Carboniferous-Permian (ca. 302–286 Ma) episode emplaced bimodal intrusions containing mafic diabase dikes, acidic adakite and I- and A-type granites. Mafic dikes and granitoids of both episodes show depleted whole-rock Nd and zircon Hf isotopic compositions (εNd(t): +2.21 ~ +7.54, εHf(t): +6.48 ~ +15.97), indicating a juvenile magmatic source. However, the mafic dikes have variable geochemical features that clearly suggest their derivation from two distinct mantle sources during the two magmatic episodes. The earlier gabbro dikes have lower Th/Yb (0.08–0.12) and Zr/Yb ratios (15.32–25.45) that resemble a normal mid-ocean ridge basalt (N-MORB) from a depleted mantle source. In contrast, the later diabase dikes show enrichment in the light rare earth elements (LREE) with relatively higher Th/Yb (0.22–0.86) and Zr/Yb ratios (38.05–68.97) that are similar to an enriched mid-ocean ridge basalt (E-MORB) mantle source. Also, the Nb/Ta ratios from the earlier adakitic granitoids (21.26–23.28) differ markedly from the later acidic adakites (0.41–19.50), which implies the transition from a thicker to a thinner crust. Integrating our results with the previous geological records, leads us to propose a two-stage tectonic model for the Chinese western Tianshan Orogen's transition from syn- to post-collision. The Late Carboniferous stage of magmatism was triggered by the break-off of the North Tianshan Oceanic slab as the Yili Block and Junggar Terrane collided. The subsequent latest Carboniferous-Permian stage followed the switch to a post-collisional setting with delamination of the thickened lower crust and the intrusion of voluminous lithospheric mantle-induced bimodal magmatism.

A model involving amphibolite lower crust melting and subsequent melt extraction for leucogranite generation

Abstract In the southern Tibetan Plateau, leucogranites are dominantly distributed in the Himalayan orogenic belt with minor occurrences in the southern Lhasa subterrane. In this paper, we report the first Miocene Anglonggangri leucogranites in the northern Lhasa subterrane. This finding provides important constraints on both leucogranite petrogenesis and the tectono-magmatic evolution of the Lhasa terrane. The Anglonggangri leucogranites include biotite-muscovite granite and slightly younger garnet-muscovite granite and pegmatite. Zircon U-Pb and muscovite 40Ar-39Ar dating of these leucogranites yields Miocene ages of 11.1–10.2 Ma. The biotite-muscovite and garnet-muscovite granites are characterized by high SiO2 (72.3–74.4 wt.%) and Al2O3 contents (14.4–15.4 wt.%) and are peraluminous. The biotite-muscovite granite displays geochemical signatures with high Sr/Y (29.2–81.0) and (La/Yb)N (37.5–98.9) ratios, low Y (4.30–7.22 ppm) and Yb contents (0.26–0.47 ppm), low to moderate initial (87Sr/86Sr)i ratios (0.7085–0.7192), and moderate εNd(t) values (−10.17 to −6.94). Furthermore, they also exhibit radiogenic Pb isotope and variable zircon εHf(t) values (−9.6 to +4.4) with Proterozoic Nd (1.1–1.4 Ga) and Hf model ages (0.8–1.7 Ga). By comparison, the garnet-muscovite granite has lower CaO, MgO, TiO2, and total FeO contents and is enriched in Rb (380–466 ppm) and depleted in Sr (24.1–38.5 ppm) and Ba (30.7–58.6 ppm) and further characterized by a significant rare earth element (REE) tetrad effect and non-charge and radius-controlled (CHARAC) trace element behaviors. The garnet-muscovite granite shows a negative Eu anomaly and positive correlations among Sr and Eu, Sr and Ba, and Th and light rare earth elements (LREEs). Pegmatite comprising Nb-Ta oxides and cassiterite occurs in the garnet-muscovite granite. Geochronological and geochemical characteristics of the Anglonggangri leucogranites indicate that the magma of the biotite-muscovite granite was derived from partial melting of amphibolite lower crust contaminated with Proterozoic-Archean upper crustal materials. The garnet-muscovite granite was generated through melt extraction from the biotite-muscovite granite crystal mush. These results confirm that partial melting of the amphibolite lower crust not only occurred in the southern and central Lhasa subterranes but also in the northern Lhasa subterrane.

Transition from subduction to Post-Collisional Paleoproterozoic magmatism in the Correntina erosive window of the north São Francisco craton: Evidence for mantle source and slab breakoff from alkaline magmatism by U-Pb, Nd-Sr radiometric and geochemistry data

The interaction of multiple sources during convergent setting generates a wide compositional diversity of magmas, sometimes with initial mafic components. Subduction zones produce extensive magmatism, derived from several reservoirs at different depths. New and published geologic data were used here to constrain the tectonomagmatic evolution in the northwestern São Francisco craton. The study area comprises the Correntina Window and Guanambi batholith, where a suite of calc-alkaline rocks was dated at 2.18–2.11 Ga. This suite has metaluminous to peraluminous compositions and chemical characteristics typical of a calc-alkaline series produced through slab melting. The age-corrected Sr-Nd isotope compositions are similar to those of MORB (ƐNd(T): −4 to +4; Sr/Sri: 0.701–0.706), consistent with an origin in a moderately juvenile subduction setting and variable crustal input. This convergent process progressively enriched the subcontinental mantle. The available geologic data indicate that peak metamorphism interpreted to reflect crustal thickening in a convergent regime occurred between 2.11 and 2.05 Ga, and was associated with regional-scale granulitization and migmatization. At the end of this period, a second suite of alkaline rocks was emplaced at ~2.05 Ga in the northwestern São Francisco craton. The petrochemical features indicate a shoshonitic affinity with high compatible element contents, metaluminous to peralkaline compositions, and enriched isotopic compositions (ƐNd(T): −7 to −6 and Sr/Sri: 0.706–0.707), indicating derivation from enriched ultramafic sources. This suite is derived from enriched lithospheric mantle components, with contributions from slab-derived fluids, and possibly asthenospheric mantle. This voluminous magmatism was likely triggered by slab breakoff, which can cause asthenospheric upwelling and accelerated the tectonic transition from convergence to decompression. These processes induced melting of mantle components that were emplaced along transtensional zones and generated the present-day tectonic architecture in the NW São Francisco craton.

Hafnium isotopic constraints on crustal assimilation in response to the tectono–magmatic evolution of the Okinawa Trough

Crustal assimilation and the input of subducted sediment in a subduction zone play vital roles in generating arc and back-arc lavas along a continental margin. However, distinguishing the impacts of these two processes on lava geochemistry is not straightforward or well understood. In this paper, we present new Hf–Nd–Sr–Pb isotopic data for 20 igneous rocks ranging in composition from basalt to rhyolite, which when combined with data from the literature allow us to fingerprint the contributions of crustal and slab-derived materials along the Okinawa Trough (OT). Consistent with previous results, the basaltic lavas were generated by melting of an Indian-type mantle source metasomatised by slab-derived fluids and subducted pelagic/detrital sediments. We confirm that Hf behaves as a non-conservative element during subduction, constrained by Hf-Nd isotopic systematics. Our new data suggest that crustal assimilation processes worked independently of the extent of magma differentiation but were determined by the stage of tectonic development in individual regions. In the southern OT (seafloor spreading stage), crustal assimilation is insignificant, and the addition of subducted sediment is 0.4%–1.5%. In the middle OT (crustal rifting stage), the basaltic and rhyolitic suites do not show distinctive Hf–Nd–Sr–Pb isotopic compositions, and both record moderate crustal assimilation (<10%) and < 0.6% addition of subducted sediment. In contrast, in the southwestern OT (crustal extension stage), the basaltic and dacitic lavas record the highest involvement of 10%–40% crustal material. We conclude that the various stages of back-arc development exerted first-order controls on the extent of crustal assimilation in the OT and had marked effects on the geochemical and isotopic compositions of OT lavas.

Early Paleozoic tectono–magmatic evolution in the South Altun orogenic belt, northwest China: Insights from zircon U–Pb geochronology, Hf isotope and geochemistry of the granitoids

ABSTRACT Precise timing of granitoids and constraints of their magma sources are critical to understanding the subduction–collision tectonic evolution of the Altun orogenic belt (AOB), a vital part of the Qinghai-Tibet plateau in northwestern China. We provide in-situ zircon U–Pb age, Hf isotopic composition and whole-rock geochemistry of the Yusupualeke granitic pluton in the South Altun orogenic belt (SAOB), an integral unit of the AOB, to determine the tectono-magmatic evolution of the SAOB at early Palaeozoic. The Yusupualeke granitic pluton comprises the medium-coarse grained porphyritic monzogranite and medium-fine grained granodiorite. The monzogranite sample yielded a weighted mean 206Pb/238U age of 476.8 ± 3.6 Ma (MSWD = 0.59), while the granodiorite yielded a weighted mean 206Pb/238U age of 453.2 ± 4.7 Ma (MSWD = 0.013). Zircon U–Pb ages suggest that the subduction–collision process of the SAOB remained from the middle stage of the early Palaeozoic to the end. Both of the studied granitoids belong to metaluminous to weakly peraluminous series and show typical I-type granite characteristics with depletions in Ba, Nb, Sr, P, and Ti, and enrichments in light rare earth elements (LREEs), Rb, Th, K, and slightly negative Eu anomalies. Based on the geochronological data and regional geological background, we believe that the porphyritic monzogranites were formed during the northward subduction process of the South Altun ocean (SAO), while the granodiorites were formed during the tectonic regime transition stage. Besides, combing the previous research achievements with our newly obtained data, we put forward a new division and geodynamic model of the early Palaeozoic tectono-magmatic evolution for the South Altun orogenic belt.

Rhaetian tectono-magmatic evolution of the Central Atlantic Magmatic Province volcanism in the Betic Cordillera, South Iberia

Volcanic rocks of the Zamoranos Formation exposed in the Betic Cordillera of southern Spain reveal Late Triassic CAMP volcanism extending from southwest to southeast of the Iberian massif. Comparison with volcanic rocks from the Upper Triassic of the Algarve region in Portugal enables correlation between these different magmatic provinces based on the striking geochemical similarity. The studied mafic magmatic rocks in the Betic Cordillera reveal three volcanic episodes during the deposition of the Zamoranos Formation with two types of geochemical affinity: alkaline (Group 1) and sub-alkaline (Group 2 and 3). The sub-alkaline rocks resemble the studied Triassic rocks of Portugal and the magmatism described as CAMP on a global scale. In the present study, the identification of alkaline rocks preceding the main CAMP sub-alkaline phase by ca. 15 Ma, is the first evidence of alkaline precursors prior to CAMP magmatism and indicates a change from a prior deeper enriched mantle source to a subsequent, shallower one. The temporal evolution of alkaline magmatism followed by a CAMP-type sub-alkaline magmatism is best explained in a context of initial rifting during Norian-Rhaetian times. It is inferred that rifting gradually developed from west to east in the southern area of Iberia.

Petrological and geochemical characteristics of the diabase and metasomatised dikes from the Tekirova ophiolite (SW Anatolia, Turkey): Tectonomagmatic evolution of the southern Neotethys

Ophiolites are widespread across the Anatolian plate and formed during the termination of different branches of the Neotethys ocean. The Tekirova ophiolite, situated in the Antalya ophiolite nappes in Southwest Anatolia, originated in the southern branch of the Neotethys. The former ophiolite consists of mantle and crustal sequences, in which, it is predominantly crosscut by the isolated diabasic dikes. Here, we have distinguished two dike types on the basis of their petrographic features and also degree of alteration. The relatively fresh types are generally named as diabase dikes. While the altered types are called as metasomatised dikes, and these dikes display ophitic to subophitic textures. The diabase and metasomatised dikes show enrichment in LILEs and depletion in HFSEs, indicating characteristics of suprasubduction zone ophiolites. This depletion of mobile elements may have been caused by metasomatism. The HFSEs versus HREEs diagrams show that the dikes were formed from 10 to 30% melting of extracted melts from previous mantle melting in which modified by subduction-derived fluids.

Late Jurassic–Early Cretaceous magmatic activity in the Central Lhasa Terrane: Petrogenesis and implications for the initial subduction of the Slainajap oceanic lithosphere

Models for the magmatic and tectonic evolution of the late Mesozoic Lhasa Terrane remain controversial. This study presents new zircon U-Pb age and Hf isotopic, whole-rock major and trace element, and Sr-Nd isotopic data for silicic igneous rocks within the Shiquanhe and Guchang areas. The Shiquanhe rhyolites yield zircon U-Pb ages of ca. 148 Ma, which are consistent with the 144 Ma age of the Guchang granites. All of these silicic igneous rocks are peraluminous, high-K calc-alkaline, and have negative εHf(t) and εNd(t) values and low (87Sr/86Sr)t values. These characteristics are indicative of derivation from magmas formed by the partial melting of ancient crustal metagraywackes. Combining our new data with the results of previous research suggests that a Late Jurassic–Early Cretaceous (165–135 Ma) magmatic belt is present along the Shiquan River–Nam Tso mélange zone within northern Tibet. These rocks collectively record the magmatic evolution from forearc to continental arc settings during the initial southward subduction of the Slainajap Ocean. Our results provide critical constraints on the tectonic evolution of the Slainajap Ocean and the tectono-magmatic evolution of the Lhasa Terrane.

Rift structures and magmatism focus VMS and gold mineralisation in the Paleoproterozoic Bryah Rift Basin, Australia

• Geophysical interpretation reveals syn -rift structures and magmatic centres. • Magmatic centres control syn -rift VMS and post-rift orogenic gold mineralisation . • Long-lived structures favour high endowment mineralisation. The interaction of structure development and magmatism in rift-settings provides systemic controls on the emplacement of ore deposits both during rifting and subsequently, and a knowledge of these may help to predict better the likely locations of major deposits. The Paleoproterozoic Bryah Rift Basin includes substantial mafic magmatism and deep-crust penetrating structures and possesses syn -rift volcanogenic massive sulphide (VMS) and later gold mineralisation, providing a good opportunity to study interactions between magmatism, structures, and mineralisation. The volcano-sedimentary Bryah Group was deposited in a continental rift developed within the broader Yerrida Basin on the northern margin of the Yilgarn Craton at ca. 2030 Ma. Multi-scale interpretation and forward modelling of gravity and magnetic data were applied to characterise the magmatic and structural patterns of the Bryah Group, enabling a better understanding of its tectono-magmatic development and controls on VMS and gold mineralisation. The interpretation highlights the bounding extensional faults and the inward-deepening structural pattern of the Bryah Rift Basin that reflects the initial rift geometry. Deep-rooted gravity sources show magmatism focused in the southern-central part of the basin, where mafic rocks' thickness can reach more than 10 km. At shallower levels, the rift magmatism extends in three east to east-northeast trending magmatic corridors with an en-echelon arrangement along the rift, oblique to major structures. These magmatic corridors have an intrinsic relationship with the internal structure of the rift. VMS mineralisation associated with mafic magmatism shows a spatial connection with magmatic corridors and major syn -rift faults, whereas VMS mineralisation associated with felsic volcanic rocks is related with off-axis volcanism and regional pre-rift faults. Later orogenic gold mineralisation is also spatially associated with the borders of the magmatic centres and rift faults, despite occurring ~200 Ma later. These associations suggest that primary rift architecture has substantially focused syn -rift VMS mineralisation and also later gold mineralisation, as a consequence of rheological and compositional contrasts.

Origin, petrogenesis and geodynamic implications of the early Eocene Altınpınar adakitic andesites in the eastern Sakarya Zone, northeastern Turkey

Early Cenozoic magmatism in the eastern Sakarya Zone (NE Turkey) provides an important constraint on the regional tectono-magmatic evolution of the region. Early Eocene syn-collisional adakitic rocks are observed as small stocks with outcropping areas commonly less than 10 km2. This study presents petrography, whole-rock geochemistry and Sr-Nd-Pb isotope data, as well as in-situ 40Ar/39Ar age constraints on one of these adakitic andesites in the Altınpınar area of Gümüşhane, and discusses source region, petrological processes and geodynamic setting prevailed during their genesis. Andesites commonly show microlitic porphyric and vitrophyric porphyric textures, and include significant amounts of mafic microgranular enclaves (MMEs). Plagioclase, hornblende, Fe-Ti oxides and minor pyroxene are the main mineral phases. In-situ 40Ar-39Ar amphibole dating constrains the cooling age of andesites into a time span from 52.8 ± 1.3–48.8 ± 1.9 Ma. Andesites are medium to high-K calc-alkaline and display most of the signatures typical of those of the adakites. They are characterized by moderate MgO (1.7–4.1 wt%), low Y (9−14 ppm), Yb (0.9–1.5 ppm), and HREE and high Sr (325−964 ppm) contents, and high Sr/Y (36–76) ratios. 87Sr/86Sr(t) (0.704948−0.705100) and 143Nd/144Nd(t) (0.512588−0.512628) ratios are in the isotopic range of the adakites. All these geochemical and isotopic data suggest that the parental magma of adakitic andesites has been produced by partial melting of oceanic basalts under amphibole-eclogite facies conditions during the breakoff of the northern Neotethyan oceanic slab.

Compositional signatures of dolerite dykes from the Purang ultramafic massif, Tibet: Implications for garnet-bearing components in the Neo-Tethyan mantle

We present geological, whole-rock geochemical, Sr–Nd isotopic and U–Pb geochronological data for the mafic dykes from the Purang ultramafic massif, South Tibet (China). The mafic dykes are composed of dolerite within a mylonitized harzburgite body. These mafic dykes are dated at 138.0–138.5 Ma using zircon and apatite U–Pb methods, and are characterized by enrichments in large-ion lithophile elements (LILEs) and high-field-strength elements (HFSEs), resembling those of typical ocean island basalts (OIB). They have initial 87Sr/86Sr ratios of 0.70612 to 0.70670, initial 143Nd/144Nd ratios of 0.51231 to 0.51233 and high εNd(t) values of −3.1 to −2.6. Geochemical modeling suggests that the parental melts of dolerites were derived by ~50% melting of garnet pyroxenite or eclogite veins in the lithospheric mantle. The formation of the dyke-filling melts can thus be linked to melting of a thin mantle layer containing old low buoyancy plume (LPB)-components in the form of garnet-bearing veins. Our results provide a scenario for the tectono-magmatic evolution of the oceanic lithosphere beneath the Neo-Tethys in the South Tibet. We conclude that the existence of a thermal plume might not be essential to the petrogenesis of the Yarlung-Zangbo Suture Zone (YZSZ) ophiolites.

In situ Reaction-replacement Origin of Hornblendites in the Early Cretaceous Laiyuan Complex, North China Craton, and Implications for its Tectono-magmatic Evolution

Abstract Two suites of amphibole-rich mafic–ultramafic rocks associated with the voluminous intermediate to felsic rocks in the Early Cretaceous Laiyuan intrusive–volcanic complex (North China Craton) are studied here by detailed petrography, mineral and melt inclusion chemistry, and thermobarometry to demonstrate an in situ reaction-replacement origin of the hornblendites. Moreover, a large set of compiled and newly obtained geochronological and whole-rock elemental and Sr–Nd isotopic data are used to constrain the tectono-magmatic evolution of the Laiyuan complex. Early mafic–ultramafic rocks occur mainly as amphibole-rich mafic–ultramafic intrusions situated at the edge of the Laiyuan complex. These intrusions comprise complex lithologies of olivine-, pyroxene- and phlogopite-bearing hornblendites and various types of gabbroic rocks, which largely formed by in situ crystallization of hydrous mafic magmas that experienced gravitational settling of early crystallized olivine and clinopyroxene at low pressures of 0·10–0·20 GPa (∼4–8 km crustal depth); the hornblendites formed in cumulate zones by cooling-driven crystallization of 55–75 vol% hornblende, 10–20 vol% orthopyroxene and 3–10 vol% phlogopite at the expense of olivine and clinopyroxene. A later suite of mafic rocks occurs as mafic lamprophyre dikes throughout the Laiyuan complex. These dikes occasionally contain some pure hornblendite xenoliths, which formed by reaction-replacement of clinopyroxene at high pressures of up to 0·97–1·25 GPa (∼37–47 km crustal depth). Mass-balance calculations suggest that the olivine-, pyroxene- and phlogopite-bearing hornblendites in the early mafic–ultramafic intrusions formed almost without melt extraction, whereas the pure hornblendites brought up by lamprophyre dikes required extraction of ≥20–30 wt% residual andesitic to dacitic melts. The latter suggests that fractionation of amphibole in the middle to lower crust through the formation of reaction-replacement hornblendites is a viable way to produce adakite-like magmas. New age constraints suggest that the early mafic–ultramafic intrusions formed during ∼132–138 Ma, which overlaps with the timespan of ∼126–145 Ma recorded by the much more voluminous intermediate to felsic rocks of the Laiyuan complex. By contrast, the late mafic and intermediate lamprophyre dikes were emplaced during ∼110–125 Ma. Therefore, the voluminous early magmatism in the Laiyuan complex was probably triggered by the retreat of the flat-subducting Paleo-Pacific slab, whereas the minor later, mafic to intermediate magmas may have formed in response to further slab sinking-induced mantle thermal perturbations. Whole-rock geochemical data suggest that the early mafic magmas formed by partial melting of subduction-related metasomatized lithospheric mantle, and that the early intermediate to felsic magmas with adakite-like signatures formed from mafic magmas through strong amphibole fractionation without perceptible plagioclase in the lower crust. The late mafic magmas seem to be derived from a slightly different metasomatized lithospheric mantle by lower degrees of partial melting.

Late Jurassic–Early Cretaceous irregular slab rollback of paleo-Pacific plate beneath southeastern China: Insights from the petrogenesis of volcanic rocks of Moshishan Group in Dazhou volcanic basin, Gan-Hang Belt

The geodynamic model of the paleo-Pacific plate subduction beneath southeastern China during the Late Mesozoic, though generally accepted, remains controversial with respect to the exact plate subduction model. The Moshishan Group volcanic strata are well developed in the Dazhou volcanic basin in the Gan-Hang Belt (GHB). In this study, detailed investigations on geochronology, elemental geochemistry, and isotopic compositions of these volcanic rocks were performed to constrain their formation ages and petrogenesis as well as the tectonomagmatic evolution of the GHB. LA-ICP-MS zircon UPb dating results show that the volcanic eruptions in the Dazhou volcanic basin began approximately 150.9 ± 2.7 Ma to 128.8 ± 1.9 Ma and lasted for ~20 Myr from the Late Jurassic to Early Cretaceous. These volcanic rocks show high Ga/Al ratio, high K2O + Na2O and Zr + Nb + Ce + Y contents, and high formation temperature (~812–1022 °C), indicating A-type granite affinity. Such types of A-type volcanic rocks formed in an extensional tectonic environment, likely corresponding to a back-arc extension due to the slab rollback of the paleo-Pacific plate. The εNd(t) values of the volcanic rocks of Moshishan Group vary from −11.60 to −5.41, and the zircon εHf(t) values vary from −18.30 to −3.84, indicating that these volcanic rocks were mainly derived from the partial melting of para-metamorphic rocks. Different proportions of mantle-derived materials were also involved in their sources, such as little or no addition of mantle-derived material in the early Dashuang Formation. However, a significantly increased proportion of mantle-derived material addition was found in the later Gaowu Formation, Xishantou Formation, and Jiuliping Formation. We found that the volcanic rocks in the southwestern GHB were formed during the Early Cretaceous (137–132 Ma), while those in the northeastern GHB formed during the Late Jurassic to Early Cretaceous (150–120 Ma). Jurassic calc-alkaline granites (181–151 Ma) that formed in a continental arc environment related to the paleo-Pacific plate subduction are also distributed along the GHB. This indicates that the shift from an arc environment dominated by the subduction of the paleo-Pacific plate to a back-arc extensional environment characterized by slab rollback began at ~150 Ma in the GHB. We propose an irregular slab rollback model beneath southeastern China to interpret the distribution pattern of the volcanic basins in the GHB. The slab rollback in the GHB did not initiate simultaneously, or with homogeneous velocity, throughout the region. Slab rollback in the southwestern part of the GHB occurred between 137 and 132 Ma, whereas in the northeastern part, rollback occurred between 150 and 120 Ma. In addition, the mantle-crust interaction that produced A-type granitic magmas along the northeastern part of the GHB gradually intensified from the early to late stage between ~150 and ~ 120 Ma.

Carboniferous tectono-magmatic evolution of the northern Luliang arc: evidence from geochemistry and petrography of Carboniferous volcanic rocks in the northern Luliang Uplift, NW China

Carboniferous tectono-magmatic evolution of the northern Luliang arc: evidence from geochemistry and petrography of Carboniferous volcanic rocks in the northern Luliang Uplift, NW China

Petrogenesis of Proterozoic alkaline ultramafic rocks in the Yilgarn Craton, Western Australia

The Yilgarn Craton and its northern margin contain a variety of petrogenetically poorly defined small-volume alkaline ultramafic rocks of Proterozoic age. This study documents the petrography, mineral and bulk-rock geochemistry and Nd-Hf-Sr-Pb isotope compositions of a selected suite of these rocks. They comprise ~2.03–2.06 Ga ultramafic lamprophyres (UML) and carbonatites from the Eastern Goldfields Superterrane (EGS), ~0.86 Ga UML from Norseman, and orangeites from the Earaheedy Basin, including samples from Jewill (~1.3 Ga), Bulljah (~1.4 Ga) and Nabberu (~1.8–1.9 Ga). The Proterozoic UML and carbonatites from the EGS and Norseman display very consistent chondritic to superchondritic Nd-Hf isotope compositions and trace-element ratios similar to modern OIBs, which are indicative of a common mantle source across this wide alkaline province. These Nd-Hf isotope compositions overlap with the evolution trends of global kimberlites through time, thus suggesting that this mantle source could be deep and ancient as that proposed for kimberlites. Conversely, the orangeites located in the Earaheedy Basin along the northern margin of the Yilgarn Craton display trace element signatures similar to subduction-related calc-alkaline magmas. Taken together with their highly enriched Sr-Nd-Hf isotope compositions, these characteristics indicate an ancient lithospheric mantle source, which was probably metasomatised by subduction-related fluids. As the ages of the Bulljah and Jewill orangeites overlap with the breakup of the Columbia supercontinent, it is proposed that orangeite magmatism was triggered by changes in plate stress conditions associated with this event. This study provides a comprehensive picture of the genesis of Proterozoic alkaline magmatism in the Yilgarn Craton, highlighting the complex tectono-magmatic evolution of this lithospheric block after its assembly in the Archean.

Reinterpretation of the northern South China Sea pre-Cenozoic basement and geodynamic implications of the South China continent: constraints from combined geological and geophysical records

The pre-Cenozoic northern South China Sea (SCS) Basin basement was supposed to exist as a complex of heterogeneous segments, divided by dozens of N–S faulting. Unfortunately, only the Hainan Island and the northeastern SCS region were modestly dated while the extensive basement remains roughly postulated by limited geophysical data. This study presents a systematic analysis including U-Pb geochronology, elemental geochemistry and petrographic identification on granite and meta-clastic borehole samples from several key areas. Constrained from gravity-magnetic joint inversion, this interpretation will be of great significance revealing the tectono-magmatic evolution along the southeastern margin of the Eurasian Plate. Beneath the thick Cenozoic sediments, the northern SCS is composed of a uniform Mesozoic basement while the Precambrian rocks are only constricted along the Red River Fault Zone. Further eastern part of the northern SCS below the Cenozoic succession was widely intruded by granites with Jurassic-to-early Cretaceous ages. Further western part, on the other hand, is represented by meta-sedimentary rocks with relatively sporadic granite complexes. To be noted, the western areas derived higher-degree and wider metamorphic zones, which is in contrast with the lower-degree and narrower metamorphic belt developed in the eastern region. Drastic collisions between the Indochina Block and South China continent took place since at least late Triassic, resulting in large-scale suturing and deformation zones. At the westernmost part of the northern SCS, the intracontinental amalgamation with closure of the Meso-Tethys has caused fairly stronger and broader metamorphism. One metamorphic biotite granite is located on the suturing belt and yields a Precambrian U-Pb age. It likely represents the relict from the ancient Gondwana supercontinent or its fringes. Arc-continental collision between the Paleo-Pacific and the southeast China Block, on the other hand, results in a relatively narrow NE-SW trending metamorphic belt during the late Mesozoic. Within the overall geological setting, the Cenozoic SCS oceanic basin was subsequently generated from a series of rifting and faulting processes along the collisional-accretionary continental margin.

Petrology, geochemistry, stratigraphy, zircon U–Pb geochronology and Hf isotopic compositions of subsurface lithologies in northwestern Mesa Central, Durango, Mexico: Implications for the tectonomagmatic evolution of northwestern Mexico

We present whole-rock geochemistry, LA-ICP-MS zircon U–Pb ages and Hf isotopic compositions of subsurface lithologies in the northwestern corner of Mesa Central, Mexico. Zircon U–Pb ages identify five magmatic events: (1) ca. 162 Ma Nazas volcano-sedimentary province; (2) 88–61 Ma early stage Laramide age magmatism; (3) 57–48 Ma late stage Laramide age rhyodacitic/dacitic magmatism; (4) 46–41 Ma transition period andesitic magmatism; and (5) 32–28 Ma silicic volcanism of the Sierra Madre Occidental silicic large igneous province. A ca. 162 Ma zircon U–Pb crystallization age obtained for a dacitic flow interbedded within the Guanacevi conglomerate yielded generally negative εHf (t) composition varying from −3.3 to +0.8 suggesting magmas derived from crustal source(s). The new geochronological data, lithological features, and stratigraphic position indicate deposition of the conglomerate was synchronous with intermittent volcanism of the Nazas volcano-sedimentary province. Late stage Laramide age (ca. 51 Ma) rhyodacites/dacites yielded largely positive and heterogeneous εHf (t) compositions ranging from −1.6 to +6.1 indicating the dominant role of mantle-derived sources. These ca. 51 Ma rhyodacites/dacites have radiogenic Hf isotopic compositions comparable to those of Late Cretaceous – Early Tertiary Laramide plutons of the southern granite domain suggesting that they share similar subcontinental mantle sources. The 46–41 Ma andesite samples yielded dominantly negative and heterogeneous εHf (t) compositions varying from −4.8 to +4.0. Zircon U–Pb ages, and geochemical and Hf isotopic compositions indicate a rapid shift (~ 2 Ma) from predominantly mantle-derived magmas during the 57–48 Ma late stage Laramide age magmatism to more crustal dominated sources during the 46–41 Ma transition period magmatism. Therefore, we propose the development of a tear in the subducting slab underneath central Durango. The proposed tear in the subducting slab would have facilitated asthenospheric upwelling, providing the necessary heat to induce crustal melting during the 46–41 Ma transition period magmatism.

U–Pb geochronology of the meta-volcanic rocks from Sierra de Calcatapul: Implications for the Middle Triassic syn-rift volcanism and tectonic evolution of northern extra-Andean Patagonia

This contribution deals first new ICP-MS U–Pb zircon ages that indicate Middle- and Late Triassic magmatic episode and tectonometamorphic events, respectively, for the central Patagonia region, instead of a Paleozoic history as previously estimated. The Calcatapul Formation consists of a (meta-) volcano-sedimentary syn-rift sequence deposited unconformably, shortly after the extensional Gondwanide orogenic collapse, over a basement composed of Permian granites. Two 206Pb/238U ages yielded 245.1 ± 2.8 and 244.8 ± 4.1 Ma to magmatic crystallization of ignimbrites, along with another weighted mean of 226.7 ± 4.4 Ma interpreted as the age of the tectonometamorphic event D1-M1 affecting the Calcatapul sequence. U–Pb ages open new insights into the tectonomagmatic evolution of the area because the syn-rift wedge deposited in the Uribe and Yancamil half-grabens under the extensional tectonic regime was then inverted along the basin-bounding faults like the Yancamil fault. Inversion tectonics involved reverse fault-related uplift against a buttress of the rigid granitic block, and under dynamic greenschist-to-amphibolite facies metamorphic conditions, before the Late-Triassic granitic magmatism of the Lipetrén Suite took place. The pre-rift granitic basement represents the southern extension of the main magmatic phase of Choiyoi Province in Patagonia, whereas the Middle Triassic recorded the extensional, post-Choiyoi magmatism. The local, compressive D1 event punctuated an overall widespread lithospheric extension that leads to Gondwana breakup since the earliest Jurassic times.