Intraplate Extensional Setting Research Articles

Implications of Late Triassic–Middle Jurassic detrital zircons from the southeastern margin of the South China Block for the Paleo-Pacific Plate subduction

There existed an early Mesozoic tectonic regime transformation in the South China Block (SCB). Although it was believed that this transformation was closely related to the subduction of Paleo-Pacific Plate, the process and initial time of the subduction of the Paleo-Pacific Plate have been controversial for a long time. Based on the published Upper Triassic and Middle Jurassic succession detrital zircons geochronology from the southeastern margin of the SCB, the newly obtained Late Triassic to Middle Jurassic detrital zircons Hf isotope data, and the available magmatic age data and Hf isotope data, this study discussed the subduction of the Paleo-Pacific Plate. The Hf isotope composition of zircon grains selected from the Early Mesozoic strata of the SCB, spanning from the Late Triassic to the Middle Jurassic, exhibits a consistent temporal trend with that of the Triassic–Middle Jurassic magmatic rocks. Both display a transition from negative to positive εHf(t) values, indicative of a gradually increasing contribution of mantle-derived materials from the Triassic to the Middle Jurassic. Zircon trace elements indicate that a magmatic arc appeared outside the southeastern margin of the SCB at 200–190 Ma and continued to develop into the Middle Jurassic, which may have been generated by the Paleo-Pacific Plate subduction. This study proposed that the Paleo-Pacific Plate subduction was confined to the southeastern coast of the SCB in the Late Triassic, and the subduction of the flat slab was halted by obstruction at the end of the Late Triassic. In the Early Jurassic, the Paleo-Pacific Plate began to subduction again, and arc-related magmatic rocks were formed along the coast of SCB. At the same time, due to the remote effect of the subduction of the Paleo-Pacific Plate, the weak tectonic belt existing in the Nanling area was reactivated, resulting in the Nanling area in the intraplate extensional setting. Subsequently, the continuous subduction of the Paleo-Pacific Plate led to the thickening of the crust along the SCB in the Middle Jurassic, and the Nanling area was still under in the extensional setting.

Petrogenesis and Tectonic Implication of Jurassic Granites in Central Guangdong, SE China: Constraints from Zircon U-Pb-Hf-O and Whole-Rock Geochemical and Sr-Nd Isotopic Data

Abstract The origin and tectonic regime responsible for the inland Jurassic granites in Southeast (SE) China remain controversial. This study presents zircon secondary ion mass spectrometry (SIMS) U-Pb ages, in situ zircon Hf-O isotopes, and whole-rock geochemical and Sr-Nd isotopic data for the Fogang and Xinxing Batholiths in central Guangdong. Mineralogical and geochemical features indicate that these granites are high-K (>4.8 wt% K2O at 72 wt% SiO2), calc-alkaline I-type granites. SIMS U-Pb analyses on magmatic zircons yield consistent ages ranging from 158 to 163 Ma, suggesting that the Fogang and Xinxing granites were emplaced in the period of 163–158 Ma. In addition, these granites have whole-rock initial Sr87/Sr86 ratios of 0.6802–0.7072 and negative εNd(t) values of −9.5 to −8.2, zircon negative εHf(t) values of −12.34 to −0.56, and high δ18O values of 7.64‰–10.08‰. The above features imply that the granites were most likely generated through the mixture of supracrustal sedimentary components with minor addition of mantle-derived magmas. Granites from the Fogang and Xinxing Batholiths in SE China should be derived from the Proterozoic crustal reworking due to asthenosphere upwelling or underplating and intrusion of mafic magmas. These Jurassic granites reflect anorogenic magmatism probably formed in an intraplate extensional setting resulted from the foundering of the flat slab beneath SE China.

Ar-Ar ages and geochemistry of Late Cretaceous basalts in the Nanxiong Basin, SE China: Constraints on the subduction and rollback of the Paleo-Pacific Plate

The Cretaceous arc-related and intraplate mafic rocks are widely distributed across Southeast (SE) China, and provide important information for investigating the Cretaceous dynamical processes of the Paleo-Pacific Plate. This study presents new petrological, geochronological, and geochemical analyses for the Late Cretaceous basalts from the Nanxiong Basin in SE China. The Nanxiong basalts yield the 40Ar/39Ar plateaus ages of 76.2-69.9 Ma, they belong to alkaline-calc-alkaline basalts and have OIB-like geochemical patterns with enrichment on LILEs and LREEs. These basalts have initial 87Sr/86Sr ratios of 0.7055-0.7062 and εNd (t) values of +2.5-+3.4. Such signatures suggest that the Nanxiong basalts were the products of partial melting of asthenospheric mantle, which might be metasomatized by slab-derived melts with a proportion of sediments. Our new results, along with the regional reported ages and geological observations, suggest that the Nanxiong OIB-like basalts were formed in an intraplate extensional setting, in response to the rollback of the subducted Paleo-Pacific slab during the Late Cretaceous.

Neoproterozoic tectonic shift from collisional orogenesis to intraplate extension in the Yili Block, southern Central Asian Orogenic Belt

Understanding the tectonic evolution of the many micro-continents in the Central Asian Orogenic Belt is a complex and controversial issue. One such micro-continent, the Yili Block is characterized by Neoproterozoic magmatic rocks, which can provide important constraints on the tectono-magmatic history of the region. Herein, we present a systematic investigation of the petrology, whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes on newly identified leucogranite intrusions and gabbroic dikes in the Wenquan district of the northern Yili Block. Two leucogranite samples yielded zircon U-Pb ages of 983 ± 9 Ma and 905 ± 9 Ma, respectively. Both of muscovite-bearing leucogranites have S-type affinities and show distinctive geochemical compositions similar to peraluminous granites. Furthermore, the leucogranites exhibit low Rb/Sr ratios (0.10–0.49) and elevated zircon εHf(t) values (+3.04 - +13.59), indicating that they were derived from partial melting of supracrustal pelitic and mafic rocks. In contrast, one gabbroic dike sample yielded a zircon U-Pb age of 779 ± 5 Ma. The gabbroic samples have distinctly high Th/Yb (1.10–1.68) and low Nb/La (0.37–0.46) ratios, with enrichment in large ion lithophile elements and depletion in Nb, Ta and Ti, implying that they were sourced from a metasomatic subcontinental lithospheric mantle. Integrated with other available data, we propose that the Yili Block underwent continuously collisional orogenesis during the early Neoproterozoic (ca. 983–780 Ma), with a subsequent tectonic shift from collisional to an intraplate extensional setting at ca. 779–776 Ma. A comparable magmatic evolution also occurred in adjacent micro-continents of the Kazakhstan region and northern Tarim Craton, indicating that these micro-continents may have shared a common Neoproterozoic geodynamic evolution.

Petrogenesis of the granite related to the Baishaziling Sn deposit, Dayishan ore field, Southern China

The Baishaziling greisen-type tin deposit is located in the Dayishan ore field, Nanling Range, Southern China. In this study, for the first time, we present both zircon and cassiterite UPb dating, whole-rock elements, zircon LuHf and apatite Nd isotopic compositions to better constrain the petrogenesis of granite and its genetic link with Sn mineralization. Zircon UPb ages of fine-grained granite and coarse-grained granite are 154 ± 1.8 Ma and 153 ± 2.1 Ma, respectively, which are consistent with the cassiterite UPb dating of 154 ± 5.4 Ma, implying genetic relationship between the Baishaziling granite and tin metallogenesis. The Baishaziling granites exhibit high SiO2, K2O + Na2O, Zr + Nb + Ce + Y contents, low P2O5 and Sr contents, and high ratios of Ga/Al, TFeO/(TFeO + MgO), implying A-type granite affinity with characteristics of high-K calc-alkaline and weakly peraluminous. The zircon εHf(t) values and apatite εNd(t) values of the granite vary from −4.46 to −1.81 and −8.37 to −7.10, with two-stage Hf and Nd model ages of 1.40 to 1.50 Ga and 1.52 to 1.64 Ga, indicating that they were generated by the partial melt of the Proterozoic basement with the involvement of mantle magma. In addition, formation of Dayishan granite was likely associated with an intraplate extensional setting caused by the subduction of the Palaeo-Pacific plate. The Baishaziling reduced granites have high stannum and boron contents, which are in favor of the tin mineralization.

Partial melting of thickened lower crust in the intraplate setting: constraints from triassic postectonic baishandong granitic pluton in Eastern Tianshan

ABSTRACT Thickened crust is of great significance for tectonic evolution and mineralization and generally occurs in collisional orogenic belts. Whether accretionary orogenic belts, such as the Central Asian Orogenic Belt (CAOB), are able to create remarkably thickened crusts and what the mechanism is, remain to be determined. In the southwestern of CAOB, the Eastern Tianshan with broad magmatism and integrated arc-continental system is an excellent area for understanding the crustal evolution. Here, we report geochronological, geochemical and Nd-Hf-O isotope data for Middle Triassic Baishandong granites in the Eastern Tianshan, which exhibit three stages: Group 1 (ca. 237 Ma) granodiorites, Group 2 (ca. 234 Ma) monzogranites, and Group 3 (ca. 229 Ma) highly fractionated I-type granites. The Group 1 and 2 granites exhibit high-K, calc-alkaline, and metaluminous characteristics, weak negative Eu anomalies, and low (K2O+Na2O)/CaO and FeOT/MgO ratios with high Na2O/K2O, Sr/Y, and (La/Yb)N values and low MgO, Y, and Yb contents, which are indicative of adakitic affinity. The Group 3 rocks have extremely high SiO2 contents, notable Eu and Sr depletions, lower Mg# values, and the ‘tetrad effect’ of REEs, indicating that they are highly fractionated I-type granites. All the granites have similar depletion in Sr, Nd, Hf, and O isotope compositions (εNd(t) = 3.1–5.5, (87Sr/86Sr)i = 0.7036–0.7051, εHf(t) = 11.3–13.4, δ18O = 6.61–7.21) with young Nd, Hf second-stage model age (565–755 Ma, 425–541 Ma). It indicates that they are all the remelting of thickened juvenile lower crust that was initially derived from Neoproterozoic crust-mantle differentiation. Based on the discussion, we proposed that the granitic pluton intruded in an intraplate extensional setting, which indicates that thickened crust had existed before the Triassic and that the final amalgamation of the Paleo-Asian Ocean occurred in late Palaeozoic. Geochemical features indicate tectonic compression played a more important role in significant lower crust thickening.

Isotopic and geochemical record of the active to passive margin transition in NW Iberia during the Cambrian-Ordovician: vestiges of a waning continental arc

Time-resolved geochemical data from the orthogneisses of the basal units of the Allochthonous complexes of NW Iberian Massif allow us to track the changes in composition of the magmas generated in a subduction-related setting that affects a continental margin, which evolves from the building of a volcanic arc to its dismantlement during a rifting episode. The basal units are interpreted as a peripheral section of the North African margin of Gondwana and record Neoproterozoic to Cambrian arc-related magmatism, followed by an episode of Cambro-Ordovician rifting. The intrusive rocks of the basal units, preserved as orthogneisses, can be grouped into two magmatic suites: calc-alkaline (c. 493–475 Ma) and alkaline to per-alkaline (c. 475–470 Ma). The calc-alkaline association has a chemical composition ranging from magnesian calcic metaluminous types to ferroan calc-alkalic and alkali-calcic peraluminous types. The moderately alkaline association and per-alkaline metagranites have ferroan alkalic and per-alkaline to metaluminous composition. The chemical characteristics of the metaigneous rocks from the basal units indicate that the older members of the calc-alkaline suite (tonalites and granodiorites) were generated in a continental arc environment, while the alkaline and per-alkaline rocks were formed in an intraplate extensional setting. The negative values in eNd (− 7.8 to − 1.5) of the calc-alkaline suite suggest a crustal influence in its generation. The eNd values in the alkaline orthogneisses range between − 2.0 and 2.2, and are positive in the per-alkaline group, suggesting a greater participation of a mantle component. The transition from the calc-alkaline suite to the alkaline suite is represented by alkali-granites, which are coeval with the intrusion of basic magmas, and likely derived from a mantle magma underplating that triggered the rifting episode after the cessation of continental arc magmatism during the Ordovician.

Protolith-controlled superposition of Sn-(W) and Cu-Mo mineralization: Examples from the Jurassic and Cretaceous granite-related mineralization in the coastal region of southeastern China

Sn (W) and Cu-Mo have distinct geochemical behaviors during magmatic evolution, thus these two group deposits are seldom superposed. Specially, the Mesozoic granite-related Sn-W/W-Sn and Mo-Cu/Cu-Mo deposits in south China are spatial overlapped, with some are even temporal-coincided. To reveal the factors controlling the distribution and the locally coeval occurrence of different mineralization types, here we compared the ages, sources compositions, and tectonic settings of the Jurassic Zijinshan Sn-rich granite, the Jurassic Gutian porphyry Mo-Cu deposit, and the Cretaceous Luoboling porphyry Cu-Mo deposit from the southeastern (SE) China. The results indicate that the Jurassic Sn-W and Mo-Cu rich magmas were derived from Paleoproterozoic meta-sedimentary and meta-basaltic rocks, respectively, through partial melting (>850 °C) along the suture zones triggered by asthenospheric upwelling in the intra-plate extensional setting. The Cretaceous porphyry Cu-Mo deposit was formed by water-flux melting (<800 °C) of the mantle wedge metasomatized by the fluids derived from the subducted Paleo-Pacific plate. A protoliths-controlled distribution model was proposed, namely that during Jurassic, the suture zones, where were the ancient continental margins containing both meta-sedimentary and meta-basaltic rocks, favored heat input from upwelling mantle and high temperature melting, which consumed the both protoliths to form W-Sn and Mo-Cu rich magmas in the same region; shift in tectonic setting from intra-plate extension to subduction during Cretaceous, introduced mantle or juvenile crust-derived materials to source, therefore formed Cu-Mo deposits superposing the Jurassic W-Sn and Mo-Cu ores in SE China.

New insights into the genesis of Neoproterozoic low-δ18O granitoids in the Seychelles: crustal cannibalization within an intra-plate extensional setting

New insights into the genesis of Neoproterozoic low-δ18O granitoids in the Seychelles: crustal cannibalization within an intra-plate extensional setting

Early Cretaceous bimodal volcanic rocks in the Yinshan belt, North China Craton: age, petrogenesis, and geological significance

Determining the age and petrogenesis of the Mesozoic magmatic rocks may provide crucial evidence regarding the tectonic setting and evolution of the North China Craton (NCC). In this paper, we present new zircon U–Pb ages and geochemical data for bimodal volcanic rocks from Zhuozi area in the eastern Yinshan belt, NCC, to discuss their petrogenesis and tectonic implications. The volcanic rock suite consists of basalts, rhyolites, and trachytes. According to the results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the rhyolitic and trachytic rocks have zircon U–Pb ages of 136.9 ± 0.8 Ma and 134.0 ± 1.7 Ma, respectively, representing their eruption ages. The basalts are enriched in large-ion lithophile elements (LILE; Ba, Rb, Sr, and Th) and light rare-earth elements (LREE), and are relatively depleted in high field strength elements (HFSE; Nb, Ta, and Ti). In addition, the basalts are characterized by high fractionations of rare-earth elements (REE), with (La/Yb)N = 24.87–42.85. These features suggest that they were derived from low-degree partial melting of enriched lithospheric mantle metasomatized by subduction-related components. The rhyolitic rocks have low mean Al2O3 (12.26 wt%) and TiO2 (0.14 wt%) contents and Mg# (14.92) values, with relatively low total REE abundances. They have high alkali and Zr, Ce, Y, and Nb contents and high Ga/Al ratios, with notably positive Pb and negative Sr, Ba, and Eu anomalies observed in the primitive mantle-normalized trace element diagram. These features indicate that the rocks exhibit characteristics of A1-type granite and likely derived from the partial melting of a crustal source. The trachytic rocks are show moderate trace elements and SiO2, MgO, Fe2O3T, and TiO2 contents. They were possibly generated by fractional crystallization of the underplating basaltic magmas and assimilated minor crustal materials. The geochemical characteristics, in combination with the regional geology, suggest that the bimodal volcanic rocks formed in an intraplate extensional setting likely associated with the retreat of the subducted Paleo-Pacific plate.

Geology, geochemistry, and geochronology of the paleoproterozoic Donggouzi mafic-ultramafic complex: Implications for the evolution of the North China craton

The Paleoproterozoic tectonic setting and spatial-temporal evolution of the northern North China and Central Orogenic belt constrain the assembly of the North China craton. Here we describe a new Paleoproterozoic mafic-ultramafic unit and intermediate-mafic intrusions located between the Khondalite belt and Central Orogenic belt, which we refer to as Donggouzi mafic-ultramafic complex. This complex is structurally complicated, and comprises a mixture of peridotite, amphibole pyroxenite, and websterite blocks, with associated alkali granitoid and gabbroic dikes, which were intruded into Paleoproterozoic granulites. Zircon UPb geochronology of the granulite, intruding syenite and alkali granites together with the field observations constrain the formation of the Donggouzi mafic-ultramafic complex to between 1849 and 1845 Ma, i.e. 1.85 Ga. Geochemical and SrNd isotopic data indicate that the Paleoproterozoic mafic-ultramafic rocks were sourced from enriched metasomatized mantle by the fluid from a subduction slab. Emplacement of the complex occurred in a post-orogenic extensional setting, possibly associated with the collision along the northern margin of the North China craton at ca. 1.9–1.88 Ga. The ca. 1767 Ma gabbroic dike was emplaced in an intraplate extensional setting and experienced crustal contamination, which suggests that an upwelling mantle plume produced magmas that interacted with the lithosphere during the late Paleoproterozoic.

An Early Cretaceous Ag-Pb-Zn mineralization at Halasheng in the South Erguna Block, NE China: Constraints from U-Pb and Rb-Sr geochronology, geochemistry and Sr-Nd-Hf isotopes

The Halasheng deposit, located in the southern Erguna Block, is a vein-type Ag-Pb-Zn deposit in the Derbugan metallogenic belt. The orebodies, hosted in Middle-Late Jurassic volcanic rocks, show a close spatial and temporal relationship with Early Cretaceous granites. Zircon U-Pb dating suggests that the granites were emplaced at 134–128 Ma. Rb-Sr dating of ore-related sphalerite gives an imprecise age of 122 ± 18 Ma, indicating that the Halasheng deposit formed in the Early Cretaceous. Whole-rock major and trace element analyses show that the Cretaceous granites belong to the metaluminous to peraluminous, shoshonite to high-K calc-alkaline series, and can be classified as A-type granites. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field-strength elements (HFSE) and heavy rare earth elements (HREE). The quartz monzonite, which has a spatial relationship with mineralization, has low initial 87Sr/86Sr (0.7058–0.7069), and positive εNd (t) (0.42–0.61) and εHf (t) (3.47–6.45), with Nd and Hf two-stage model ages of 0.81–0.85 Ga and 0.69–0.86 Ga, respectively. These geochemical features are similar to those of major Mesozoic granite intrusions in the Erguna Block. We propose that these graintes originated from partial melting of juvenile crust in an intraplate extensional setting. Taken together, the Halasheng Ag-Pb-Zn deposit formed in the Early Cretaceous after the orogeny related to closure of the Mongol-Okhotsk Ocean. The relatively low-temperature of Ag-Pb-Zn ore and alteration mineralogy suggest that there is only a distal relationship, if any, to nearby potentially-causative granite intrusions.

Geochemical and Sr–Nd–Pb isotopic constraints on the origin and petrogenesis of Paleozoic lamproites in the southern Yangtze Block, South China

Lamproites and kimberlites are natural probes of the subcontinental lithospheric mantle providing insights into the Earth’s continental lithosphere. Whole-rock major-, trace-element and Sr–Nd–Pb isotopic compositions of the Paleozoic (~ 253 Ma) lamproite dikes from the Baifen zone of the Zhenyuan area in southeastern Guizhou Province (in the southern Yangtze Block, South China) are presented. The Baifen lamproites are characterized by high MgO (7.84–14.1 wt%), K2O (3.94–5.07 wt%) and TiO2 (2.69–3.23 wt%) contents, low SiO2 (41.3–45.7 wt%), Na2O (0.21–0.28 wt%) and Al2O3 (6.10–7.20 wt%) contents. All lamproites have elevated Cr (452–599 ppm) and Ni (485–549 ppm) abundances, as well as high Ba (1884–3589 ppm), La (160–186 ppm), Sr (898–1152 ppm) and Zr (532–632 ppm) concentrations. They show uniform REE distribution patterns that are strongly enriched in light REEs relative to heavy REEs [(La/Yb)N = 71.1–87.6], and exhibit OIB-like geochemical features with obvious enrichment of both LILEs and HFSEs in the primitive mantle-normalized multi-element distribution diagram. Moderately radiogenic Sr (87Sr/86Sri = 0.706336–0.707439), unradiogenic Nd (143Nd/144Ndi = 0.511687–0.511704 and eNd(t) = − 12.2 to − 11.9), and low initial Pb (206Pb/204Pbi = 16.80–16.90, 207Pb/204Pbi = 15.34–15.35 and 208Pb/204Pbi = 37.43–37.70) isotopic compositions are obtained from the rocks. They yield old model ages of TDM(Nd) = 1.48–1.54 Ga. These signatures suggest that the Baifen lamproite magmas are alkaline, ultrapotassic and ultramafic in character and mainly represent mantle-derived primary melts, which have undergone insignificant crustal contamination and negligible fractional crystallization. The Baifen lamproites originated from a veined metasomatized lithospheric mantle source. We envisage that they were derived by partial melting of old, mineralogically complex metasomatic vein assemblages in the subcontinental lithospheric mantle beneath the southern Yangtze Block. The source region experienced ancient mantle metasomatism with complex modification by enriched fluids and melts. The metasomatic agents are most likely to originate from pre-existing slab subduction beneath the southeastern margin of the Yangtze Block. Tectonically, the Baifen lamproites were emplaced at the southern margin of the Yangtze Block, and they formed in an intraplate extensional setting, showing an anorogenic affinity. In terms of time and space, the genesis of Baifen lamproites is presumably related to the Emeishan large igneous province. The Emeishan mantle plume is suggested as an effective mechanism for rapid extension and thinning of the lithosphere, followed by decompression melting of the subcontinental lithospheric mantle. Combined with the thermal perturbation from asthenospheric upwelling induced by the Emeishan mantle plume, the lamproite magmas, representing small volume and limited partial melts of ancient enriched mantle lithosphere, arose. We propose that the generation of the Baifen lamproite dikes probably was a consequence of the far-field effects of the Emeishan mantle plume.

Indosinian magmatism and rare metal mineralization in East Tianshan orogenic belt: An example study of Jingerquan Li-Be-Nb-Ta pegmatite deposit

The East Tianshan orogenic belt, a part of the Central Asian Orogenic Belt like Altai orogenic belt, has shown great potential for rare metal mineralization. Jingerquan Li-Be-Nb-Ta deposit is the largest pegmatite deposit in this region. Muscovite granite, quartz monzonite and gabbro have developed in the Jingerquan Li-Be-Nb-Ta pegmatite deposit in the eastern part of East Tianshan orogenic belt. Based on the content of elemental Li and the presence of Li-bearing minerals, we recognize two sets of intrusive assemblages, namely, class I, including muscovite granite and quartz monzonite, which are rich in Li with presence of spodumene and/or lepidolite, and class II mainly gabbro, which are poor in Li and absent of Li-bearing minerals. Class I rocks are rich in Si, Al, Na, K, calc-alkaline to high-K calc-alkaline series, and poor in Ca, P, Fe, and Mg, with A/CNK ≥ 1.1, which are similar to S-type peraluminous granites. Class I rocks are characterized by Rb, Ta, Nb, and Hf enrichments and significant Ti, Ba, and Sr depletions. The chondrite-normalized rare earth element (REE) patterns exhibit significant Eu anomalies (δEu = 0.01–0.20), “tetrad effect,” and increased fractionation of rare earth elements with relatively low ΣREE. Class II rocks are calc-alkaline rocks that are rich in Ca, Al, and P and poor in Si, K, with A/CNK < 1; they are characterized by Ba and Sr enrichments and Th, Nb, and Ta depletions. The chondrite-normalized REE patterns exhibit no significant Eu anomalies (δEu = 0.97–1.40) and a smooth curve that slopes to the right. The monazite SIMS U-Th-Pb ages of class I (246–252.9 Ma) and the zircon LA-ICP-MS U-Pb ages of class II (247–250.7 Ma) indicate that both types of intrusions were formed in intraplate environment in Indosinian period. The εHf(t) values (12.0–15.2) and the depleted mantle model ages TDM (279–411 Ma) of class II rocks indicate that the magmas were mainly derived from partial melting of the depleted mantle. In the intraplate extensional setting, the mantle magmas represented by class II basaltic magmas intraplated into the crust and induced partial melting of the middle and upper crust to produce class I S-type granitic magmas. After that, the granitic magmas underwent continuous differential evolution and volatile enrichment, and finally form Li-Be-Nb-Ta mineralized granitic pegmatite. The geological setting, diagenetic and metallogenic chronology, and mineral assemblages of the East Tianshan orogenic belt and the Altai orogenic belt can be compared, and rare metal mineralization of both belts were controlled by the Indosinian tectonic-magmatic event.

The Paleoproterozoic bimodal magmatism in the SW Yangtze block: Implications for initial breakup of the Columbia supercontinent

The fragmentation of the Columbia supercontinent is thought to have begun as early as ~1.7 Ga; however, the breakup of its core domain occurred only at ~1.5–1.25 Ga. This study presents the integrated petrological and chemical characteristics of gabbro and granite porphyry intrusions in the Haizi region of China, which record the oldest magmatism associated with the initial breakup of Columbia in the southwestern Yangtze Block. The dating of gabbro and granite porphyry samples using laser ablation–inductively coupled plasma–mass spectrometry zircon UPb geochronology gives crystallization ages of 1754 ± 14 Ma and 1743 ± 4 Ma, respectively. The gabbro is characterized by low La/Ta, Th/Y, (La/Yb)N, and (Dy/Yb)N ratios, negligible NbTa anomalies, moderately depleted Nd isotope signatures (εNd (t) = +2.4 to +5.3), and enriched zircon Hf isotopes (εHf(t) = −1.8 to −7.8), indicating derivation of the primary magma from ambient asthenospheric mantle and complex petrogenetic processes before emplacement. Geochemical modeling indicates a variable but low degree of partial melting (1%–15%) and minor crustal contamination during magma ascent. The gabbro samples have relatively high TiO2 (1.94–3.73 wt%) and Zr (95–271 ppm) contents, typical of rocks formed in an intraplate tectonic setting. The granite porphyry samples display high FeOT/MgO and Ga/Al ratios, high contents of high-field-strength elements, and low CaO–Sr contents, resembling A-type granites. Compared with the gabbros, these porphyries show wider geochemical gaps, more variable ratios between incompatible elements, and lower radiogenic Nd isotopic compositions (εNd(t) = +0.3 to +2.7 and TDM2(Nd) = 2226– 2028 Ma). However, the porphyries have more radiogenic zircon Hf isotopes (εHf(t) = 0 to +3.7 and TDM2(Hf) = 2421–2189 Ma) than those of the gabbros, suggesting their derivation from the partial melting of a Paleoproterozoic crustal source triggered by an upwelling basaltic magma in an intraplate extensional setting. The ~1.75 Ga bimodal magmatism reported in this study, together with the ~1.75–1.65 Ga mafic magmatism documented in other cratonic blocks, suggests a possible spatial linkage between the Yangtze Block, northern Australia, northwestern Laurentia, and southern Siberia in the late Paleoproterozoic Columbia supercontinent.

Petrogenetic and tectonic implications of Triassic granitoids in the Chinese Altay: the Alaer granite example

Petrogenesis of the Triassic Alaer granite in the Chinese Altay is important to understand the regional crustal evolution. This study found that the biotite granite is metaluminous and high-K calc-alkaline to shoshonitic. The rocks contain low SiO2 (64.3–68.7 wt.%), high Al2O3 (13.46–16.56%) and K2O (mostly 3.65–7.57 wt.%), and are featured by distinctly higher concentrations of HFSE (e.g., Th, Hf) and ΣREE than those of the nearby Devonian granites. REE patterns show right-inclining trend and weak negative Eu anomalies (δEu = mostly 0.68–0.94), whilst the spidergrams show consistent negative anomalies of Ba, Sr, P, Ti, Nb and Ta and positive anomalies of Th, K, La, Ce, Nd, Zr and Hf. Compared with the local Devonian granites, the Triassic granites have higher calculated initial 87Sr/86Sr ratios (0.70601–0.70920) and εNd(t) values −(−1.24 to −0.68). Their Nd model ages (average of 1.08 Ga for one-stage and 1.07 Ga for two-stage) are early Neoproterozoic to late Mesoproterozoic.We infer that the Triassic Alaer granite belongs to I-type granite and likely formed in an intraplate extensional setting under high temperature (829–885 °C) and pressure (depths of 30–50 km, corresponding to the middle-lower crust in the Chinese Altay region). Granitic magma was produced through the melting of old continental basement with substantial mantle input, triggered by extensional upwelling of mantle-derived magmas. Mixing equation of Nd isotope compositions yielded a 40% of mantle-derived juvenile components for the Alaer granite petrogenesis.

Paleoproterozoic Multiple Tectonothermal Events in the Longshoushan Area, Western North China Craton and Their Geological Implication: Evidence from Geochemistry, Zircon U–Pb Geochronology and Hf Isotopes

The Alxa block is located in the southwestern margin of the North China Craton. The Paleoproterozoic tectonic evolution, crustal growth and tectonic affinity of the block remain unknown or controversial. The Longshoushan (LS) area is one of the few areas that outcrop Paleoproterozoic to crystalline basement rocks in the Alxa Block. In this study, we preset whole-rock geochemistry, zircon U–Pb geochronology and Lu–Hf isotope data from metagabbro, metadiorite, quartz syenite, granitic leucosome and pegmatoid leucosome in the LS area. These rocks all are enriched in LREE and LILE, and depleted in HREE and HFSE. Eight new LA-ICP-MS zircon U–Pb ages yielded three magmatic ages of 2044 Ma, 2029 Ma and 1940 Ma, and three metamorphic ages of 1891 Ma, 1848 Ma and 1812 Ma. Lu–Hf analyses reveal that the magmatic zircons and anatectic/metamorphic zircons from all the rock types are characterized by positive εHf(t) (−0.16 to 10.89) and variable εHf(t) (−11.21 to 6.24), respectively. Based on the previous studies and our new data, we conclude that the LS area experienced three magmatic events (2.5–2.45 Ga, ~2.1–2.0 Ga and ~1.95–1.91 Ga) and three regional metamorphism/anataxis events (~1.93–1.89 Ga, ~1.86–1.84 Ga and ~1.81 Ga) in Paleoproterozoic. The age–Hf isotope data establishes two main crustal growth events at ~2.9–2.5 Ga and ~2.2–2.0 Ga in the LS area. These data indicate that the LS area experienced intraplate extensional setting in the middle Paleoproterozoic, and continental subduction, collision and exhumation in the late Paleoproterozoic. Combining the geochronological framework and tectonic evolution, we suggest that the Alxa Block is part of the Khondalite Belt.

Nature and origin of Mesozoic granitoids and associated mineralization in the Sanjiang Tethys Orogeny, SW China: the Xiuwacu complex example

ABSTRACTThe magmatic generation for the Late Triassic–Early Jurassic (~215–200 Ma) and Early Cretaceous–Late Cretaceous (~108–79 Ma) post-collisional granites in the Sanjiang Tethys orogeny remain enigmatic. The Xiuwacu complex, located in the southern Yidun Terrane, consists of biotite granite with a weight mean 206Pb/238U age of 199.8 ± 2.5 Ma, aplite granite of 108.2 ± 2.3 Ma, monzogranite porphyry of 80.8 ± 1.0 Ma, and diorite enclaves of 79.2 ± 0.9 Ma and 77.9 ± 0.8 Ma. The Late Triassic biotite granites show I-type granite affinities, with high SiO2 contents, high Mg# values, high zircon δ18O values, and negative whole-rock ƐNd(t) values, indicating a predominant ancient crustal source with the input of juvenile materials. Their fractionated REE patterns and concave-upward middle-to-heavy REE patterns require garnet-bearing amphibolite as the melt source. The Cretaceous highly fractionated aplite granites and monzogranite porphyries have relatively high SiO2 contents, high (Na2O + K2O)/CaO ratios, high zircon δ18O values, and enriched whole-rock Sr–Nd isotopic signatures, suggesting that their parent magmas were likely originated from the ancient middle- to lower crust. Their significant negative Eu anomalies and obvious depletions in Nb, Sr, and Ti demonstrate that the Cretaceous granitic magmas had experienced more fractionation than the Late Triassic felsic magmas. The Late Cretaceous diorite enclaves show low SiO2 contents, high Mg# values, and high zircon δ18O values, suggesting that they were probably derived from the partial melting of subcontinental lithospheric mantle enriched by the Late Triassic subduction. The Late Triassic–Early Jurassic and Early Cretaceous–Late Cretaceous magmatism witnessed the post-collisional setting and intraplate extensional setting in response to the slab break-off and lithospheric-scale transtensional faulting, respectively. The partial melting of subduction-modified lithospheric mantle or/and residual sulphide cumulates within the lower crust during the origination of Late Cretaceous magmas could have provided metals for the formation of Xiuwacu deposit.

Geochronology and geochemistry of Late Devonian-Carboniferous igneous rocks in the Songnen-Zhangguangcai Range Massif, NE China: Constraints on the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt

This paper presents new zircon U–Pb ages and Hf isotopic data, and whole–rock geochemical data for Late Devonian–Carboniferous igneous rocks from the Songnen–Zhangguangcai Range Massif (SRM), NE China, to constrain the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt and the nature of the crust beneath the SRM. LA–ICP–MS zircon U–Pb dating reveals two stages of magmatism in the Late Devonian (366Ma) and Carboniferous (325–313Ma). The Late Devonian magmatic event produced alkali feldspar granites with high SiO2 and total alkali contents, as well as pronounced negative Eu anomalies, similar to A-type granites. Based on these features, their high zircon εHf(t) values (+9.6 to +13.0), and the early Paleozoic tectonic evolution of the eastern SRM, we conclude that the primary magma for the Late Devonian granites was generated by partial melting of juvenile lower crust in a post-collisional extensional environment. The Carboniferous magmatic event produced a suite of intermediate–felsic igneous rocks. The intermediate rocks, including basaltic andesite, andesite, and quartz monzodiorites, have low SiO2 contents, high Mg#, high Cr, Co, and Ni contents, and display arc-like trace element characteristics. Based on these observations and their high zircon εHf(t) values (+5.8 to +10.7), we suggest their primary magma was derived from partial melting of depleted mantle, metasomatized by subduction-related fluids. The felsic rhyolites and monzogranites have high SiO2 contents, low Mg#, extremely low Cr, Co, and Ni contents, and zircon εHf(t) values of +10.5 to +13.7 and +4.2 to +6.9, respectively. These results indicate that the primary magma for the felsic rocks was derived from partial melting of heterogeneous juvenile crust. The Carboniferous igneous rock assemblages, together with coeval carbonate sedimentary formations, record an intra-plate extensional setting. The zircon εHf(t) data of Paleozoic igneous rocks from the eastern SRM define a temporal trend that is consistent with tectonic evolution. The increase in εHf(t) values during the early late Paleozoic age mirrors changes in magma source, corresponding to a switch from subduction–collision to post-collision tectonic regimes in the SRM. The zircon Hf isotopic compositions of the studied Late Devonian–Carboniferous igneous rocks record crustal accretion in the eastern SRM during the Neoproterozoic–early Paleozoic. Reworking of these crustal materials occurred in a post-collisional extensional environment during the late Paleozoic.

Geochemical evidence for Paleozoic crustal growth and tectonic conversion in the Northern Beishan Orogenic Belt, southern Central Asian Orogenic Belt

The Beishan Orogenic Belt is located in the central southernmost part of the Central Asian Orogenic Belt (CAOB), which plays a key role in understanding the formation and evolution of the CAOB. Granitoids are the documents of crustal and tectonic evolution in orogenic belts. However, little is known regarding the petrogenesis and geodynamic setting of the widely distributed Paleozoic granitoids in the Northern Beishan Orogenic Belt (NBOB). The present study reveals significant differences concerning the petrogenesis and tectonic setting of early and late Paleozoic granitoids from the NBOB. The early Paleozoic granitoids from the 446–430Ma Hongliuxia granite complex of the Mazongshan unit and the 466–428Ma Shibanjing complex of the Hanshan unit show classic I-type granite affinities as revealed by the relative enrichment of LILEs and LREEs, pronounced depletions of Nb, Ta and Ti and the abundant presence of hornblende. Furthermore, they are characterized by strongly variable zircon εHf(t) values between −16.7 and +12.8 and evolved plagioclase Sr isotopic compositions of 0.7145–0.7253, indicating the involvement of both juvenile and ancient continental crust in the magma source. Thus, we propose that the early Paleozoic granitoids in the NBOB were generated in a subduction-related continental arc setting. In contrast, the late Paleozoic 330–281Ma granitoids from the Shuangjingzi complex of the Hanshan unit exhibit positive zircon εHf(t) values between +5.8 and +13.2 and relatively depleted plagioclase Sr isotopic compositions of 0.7037–0.7072, indicating that they were mainly formed by remelting of juvenile crust. Thus, an intra-plate extensional setting is proposed to have occurred during formation of the late Paleozoic granitoids. Therefore, between the early and late Paleozoic, the magma sources of the NBOB granitoids converted from the reworking of both juvenile and ancient crusts during a subduction-induced compressional setting to the remelting of juvenile crust during an intra-plate extensional setting, respectively. The corresponding crustal growth in the southern CAOB is dominated by early Paleozoic lateral accretion of arc complexes and late Paleozoic vertical addition of juvenile material from the mantle.