Arc-like Geochemical Features Research Articles

Petrogenesis and Tectonic Significance of Early Permian Intermediate–Felsic Rocks in the Southern Beishan Orogen, Northwest China: Geochronological and Geochemical Constraints

Permian intermediate–felsic igneous rocks, widely distributed in the southern Beishan orogen, provide crucial constraints on the geodynamic process of the late Paleozoic Paleo-Asian Ocean. New zircon U–Pb dating using LA–ICP–MS determines the age of the northern Qingshan diorites, the Heishantou quartz diorites, and the southern Qingshan biotite granodiorites at 300 Ma, 294 Ma, and 291–286 Ma, respectively. Their whole-rock compositions exhibit arc-like geochemical features. Moreover, their zircon trace elements show the characteristics of continental arc zircons. The diorites, characterized by low SiO2, high MgO with Mg# (50–52), and low Cr, Co, and Ni, display enrichment in Sr-Nd-Hf isotopes (87Sr/86Sr = 0.7060 to 0.7061; ℇNd(t) = −1.4 to −1.7; ℇHf(t) = −4.7 to −0.6), originating from the fractionation process of magma derived from the enriched mantle. The quartz diorites show moderate SiO2 and variable MgO (2.75–3.84 wt%) and exhibit enrichment in Sr-Nd (87Sr/86Sr = 0.7048–0.7050; ℇNd(t) = −1.5–+0.9) and depletion in zircon Hf isotopes (ℇHf(t) = 3.8 to 7.8). Combined with their high Y (20.0–21.0 ppm) and low (La/Yb)N (6.0 to 17.2), we conclude that they originated from the juvenile lower crust previously influenced by oceanic sediments, with the input of enriched mantle-derived materials. The biotite granodiorites display low A/CNK (0.91–0.97), 10000*Ga/Al (1.8–1.9), and Ti-in-zircon temperatures (average 711 °C), indicating that they are I-type granitoids. These rocks show enrichment in Sr-Nd isotopes (87Sr/86Sr = 0.7054 to 0.7061; ℇNd(t) = −2.0 to −1.6) and many variable zircon Hf isotopes (ℇHf(t) = −2.3 to +4.5). Geochemical studies indicate that they originate from the mixing of magmas derived from the enriched mantle and preexisting juvenile lower crust. All these data imply the existence of oceanic subduction in southern Beishan during the early Permian. Integrating these results with previous studies, it is inferred that the retreating subduction of the Liuyuan Ocean contributed to early Permian intermediate–felsic rocks becoming widespread in the Shibanshan unit, the southernmost part of the Beishan orogen, and also why the Paleo-Asian Ocean in southern Beishan did not close during the early Permian.

Two episodes of mid-Neoproterozoic mafic magmatism in the Hong'an Terrane, Central China: Records of continental rifting during the breakup of the Rodinia supercontinent

The mid-Neoproterozoic (800–635 Ma) tectonic regime along the northern margin of the Yangtze Block is still debated, particularly the timing and mode of transition from a subduction-related setting to a rifting-related setting. The new identification of two episodes of mafic magmatism, represented by (I) 736–710 Ma Daleishan and (II) 635 Ma Hong'an intrusions in the Hong'an Terrane, central China, provides new constraints on the nature of their mantle source and tectonic evolution of the northern Yangtze Block. (I) Daleishan mafic rocks are tholeiitic in composition and exhibit N-MORB- to arc-like trace element patterns. They have variable whole-rock initial 87Sr/86Sr (0.7041 to 0.7092), εNd(t) (−2.98 to +2.17), and εHf(t) (+3.37 to +10.25) values. Their magmatic zircons preserve relatively homogeneous δ18O (+4.50 to +6.01 ‰) and highly variable εHf(t) (+1.3 to +13.1) values. Chemical compositions and modeling results suggest that Daleishan rocks were derived from a depleted asthenospheric mantle source with involvement of recycled sediment component, and the magma subsequently underwent 5–15% contamination by the wall-rocks during its emplacement. (II) Hong'an mafic rocks mainly show E-MORB- to arc-like geochemical features with more enriched HfNd isotopes [zircon εHf(t) = −3.0 to +3.6; whole-rock εNd(t) = −6.87 to −0.75, and εHf(t) = −0.50 to +2.00], suggesting they were derived from the interactions between the asthenosphere and the previously-metasomatized lithospheric mantle. Integrating the new data with previous results for the Neoproterozoic rocks in the northern margin of the Yangtze Block (including the Qinling-Dabie orogen), we emphasize that episodic extension-related asthenosphere upwelling in a continental rifting setting, coupled with interactions between the asthenosphere and the lithosphere, played a prominent role in the formation of the 780–635 Ma bimodal-type magmatic rocks and diverse geochemical features in the mafic counterparts.

Diverse Late Triassic arc magmatism and porphyry Cu deposits in the southeastern Tibetan Plateau: Related to slab tear along a subducting passive margin?

The subduction of the ocean–continent transition (OCT) zone on a passive margin has received limited attention, despite its significant impact on tectono-magmatic processes. This study focuses on the Late Triassic basalts and andesites in the southeastern Tibetan Plateau, where the Yushu-Yidun Arc belt intersects the Longmen Shan Fault system. The goal is to understand the interplay between these linear systems. The basalts exhibit trace element and isotopic signatures resembling those of within-plate enriched mid-ocean ridge basalts or ocean island basalts (EOBs), such as positive NbTa anomalies, high Nb/Th (7.72 to 10.3) and εNd(t) (+2.23 to +6.34) values, and low La/Sm values (2.69 to 4.05), suggesting an asthenospheric mantle source. The contemporaneous andesites display typical arc-like geochemical characteristics (e.g., enrichment in Ba, Th, and U but depletion in Nb, Ta, and Ti), with negative εNd(t) values (−2.41 to −1.08), indicating a mantle wedge source affected by subducted components. Geological evidence indicates that the Longmen Shan Fault zone, previously defined by its magmatic rocks, was an active OCT belt during the Late Triassic, separating the South China Continent from the Garze–Litang Ocean. Consequently, we propose a model suggesting the tearing of a subducted oceanic slab from the continental lithosphere within the OCT belt under the southernmost segment of the Yushu–Yidun Arc belt, driven by strong slab pull in the Garze–Litang Ocean subduction zone. This model effectively explains the intriguing geology of the Songpan–Garze–Hoh Xil Terrane, in which Late Triassic EOBs and normal arc-related igneous rocks coexist and are accompanied by abundant slab-derived mineralized porphyries. Our findings have global implications for investigating geochemically diverse magmatism and associated mineralization on passive continental margins.

Metasomatized mantle under the Eastern Mexican Alkaline Province: evidence from the Oligocene Rincón Murillo Gabbro, Sierra de San Carlos-Cruillas, NE México

The Cenozoic Sierra de San Carlos-Cruillas (SSCC) magmatic complex belongs to the Eastern Mexican Alkaline Province (EMAP). The SSCC mainly consists of plutonic and subvolcanic bodies displaying diverse lithological compositions. These rocks exhibit both OIB- and arc-like geochemical characteristics. In this contribution, we present new petrographic, geochemical, isotopic, and geochronological data of the Rincón Murillo Gabbro (RMG), situated in the southern part of the SSCC. The RMG is nepheline normative and displays an OIB-like intraplate character. Laser ablation ICP-MS U-Pb analyses on titanites yielded Oligocene (Rupelian) crystallization ages of 31.8 ± 1.8 and 31.0 ± 1.7 Ma. The 87Sr/86Sr(i) (0.703400–0.704159) and 143Nd/144Nd(i) (0.512732–0.512815) ratios indicate a mantle origin. Trace element data indicate the participation of different enriched mantle components in the source. One component is related to previous subduction processes, generating the HFSE-depleted arc-like magmas of San José Gabbro in the northern part of SSCC. The second component is related to deep mantle melts with a slightly carbonatitic signal and HFSE-enriched fluids derived from the subducted Hess conjugate plateau generating different enriched sources in the upper mantle under the SSCC. The passage, eclogitization, detachment and foundering of the Hess plateau gave rise to an asthenospheric upwelling that triggered the low to moderate partial melting degrees (3–15%) of an amphibole-bearing garnet peridotite, generating the OIB-like parental magmas of RMG, located in the southern part of SSCC. Two-component mixing, and AFC models indicate that variable degrees of crystal fractionation and minor crustal assimilation are involved in the evolution of Rincón Murillo Gabbro.

Diverse metavolcanic sequences in the Cambrian accretionary complex at the Pamir Syntax: Implications for tectonic evolution from Proto-Tethys to Paleo-Tethys

The Pamir Plateau contains critical information on the tectonic evolution from the Proto- to Paleo-Tethys. In this contribution, we report geochronology, geochemistry and Sr-Nd isotopic compositions of diverse metavolcanic rocks in the Bulunkuole Group (BG) in the NE Pamir, to construct the architecture of the thick Cambrian volcanic-sedimentary sequence in the NE Pamir and to provide new insights into the tectonic evolution from Proto-Tethys to Paleo-Tethys. Our newly obtained zircon U-Pb ages combined with ages from literature reveal that these volcanic-sedimentary sequences were deposited during the Cambrian and metamorphosed during the Early Jurassic. The bimodal volcanic rocks (ca. 530 Ma) at the lowest BG show typical arc-like geochemical features, representing the initial arc produced by the southward subduction of the Proto-Tethys Ocean. Amphibolites (ca. 526 Ma) from the Maeryang area show an OIB-like affinity with high Nb/La ratios (0.75–1.01) and positive εNd(t) values (4.19–4.76), representing a seamount component in the Proto-Tethys Ocean. High-pressure mafic granulites to the north of Tashkorgan County exhibit E-MORB geochemical features. Combined with recent advances in the NE Pamir geology, we suggest that the southward subduction of the Proto-Tethys slab under the Mazar-Tianshuihai terrane initiated earlier than 530 Ma. Subsequent trench retreat led to the formation of the massive early Paleozoic accretionary wedge (BG) followed by the northward migration of arc magmatism to the BG. The Proto-Tethys Ocean at Pamir did not close in the early Paleozoic. Instead, it was finally closed during the early Mesozoic as revealed by the Early Jurassic amphibolite-facies metamorphism in the BG.

Miocene transition from an active continental margin to a trench–arc–basin system in Northeast Asia: Constraints from compositional variations of basaltic rocks in the Russian Far East

The timing and mantle processes of the Cenozoic transition from an active continental margin to a trench–arc–basin system in northeast Asia remain controversial. Here we present major and trace element, and Sr–Nd–Pb–Hf isotope data for Cenozoic basaltic rocks from the Russian Far East, which provide new constraints on the deep mantle processes during the transition. The basaltic rocks formed mainly in four stages. The results show that middle Eocene basaltic rocks have arc-like geochemical features, such as depletions in high-field-strength elements, indicating an origin by partial melting of a mantle wedge that had been modified by slab-derived fluids and recycled sediments. Early and middle Miocene basaltic rocks also have arc-like geochemical features, and have relatively low contents of fluid-mobile elements and high contents of fluid-immobile elements, as compared with the middle Eocene basaltic rocks. This suggests that these magmas were derived by the melting of previously melted lithospheric mantle that had been modified by slab-derived fluids. In contrast, late Miocene basalts have ocean island basalt-like geochemical features, such as enrichments in high-field-strength elements, indicating their derivation by partial melting of the asthenospheric mantle. The temporal variations in the mantle sources of the Cenozoic basaltic rocks in the Russian Far East, along with an abrupt change in Sr Nd isotopic compositions at ca. 20 Ma, demonstrate that the transition from an active continental margin to a trench–arc–basin system in northeast Asia occurred at ca. 20 Ma. • The basalts in the Russian Far East formed mainly at ca. 45, 20, 13, and 7 Ma. • The temporal variations in the sources of the Cenozoic basalts were determined. • Transition from active continental margin to trench-arc-basin occurred at 20 Ma. • The rollback of Pacific Plate and trench retreat control the tectonic transition.

Late Permian plume and Neoproterozoic subduction-modified mantle interaction: Insights from geochronology and Sr-Nd-O isotopes of mafic dikes of the western Emeishan large igneous province

Geochronological investigations of mafic dikes along the southwestern margin of the Emeishan large igneous province (ELIP) in the South China block display a restricted range of U-Pb zircon, baddeleyite, and apatite isotopic ages ranging from 263 to 257 Ma, which overlaps with that of previously studied ELIP basalts and mafic intrusions. The dikes are divided into high-Ti and low-Ti groups, whereby the latter is further divided into two subgroups (low-Ti group-1 and -2). The high-Ti group rocks (Ti/Y > 500) are characterized by ocean island basalt-like trace element patterns with mantle-like zircon δ¹⁸O of 5.0 ± 0.10‰ and slightly enriched ε_Nd(t) values of −1.0 to +1.0. The low-Ti group-1 rocks (Ti/Y < 500) have trace element patterns similar to those of the high-Ti group, yet generally with weak negative Nb-Ta anomalies, lower (Sm/Yb)_N ratios, elevated zircon δ¹⁸O (6.6 ± 0.33 ‰), and highly variable ε_Nd(t) values (−3.9 to +3.2). The low-Ti group-2 rocks (Ti/Y < 500) are characterized by pronounced negative Nb-Ta anomalies, more negative ε_Nd(t) (−8.4 to −6.6) values, and higher initial ⁸⁷Sr/⁸⁶Sr ratios than those of the other two groups. The compositional variations of the high-Ti group and the low-Ti group-1 rocks, in conjunction with the negative correlation between the ε_Nd(t) values and the (Th/Nb)_N ratios, suggest that the two groups were generated from an isotopically heterogeneous mantle plume at different depths, and experienced varying degrees of crustal contamination (but < 20 wt.%). The high-Ti group rocks are considered to have originated from a deeper garnet-stable source, and the low-Ti group-1 rocks from a shallower source. Mixing calculations indicate that the highly enriched Sr-Nd isotopes of the low-Ti group-2 rocks cannot be explained by crustal contamination. A subduction-modified mantle source is required to account for the arc-like geochemical characteristics of this group. This is consistent with the spatial overlap of the low-Ti group-2 rocks and previously studied geochemically similar samples with rocks from the Neoproterozoic subduction zone along the western margin of the South China block. Furthermore, a fertilized mantle is also consistent with the variable δ¹⁸O values of various mafic-ultramafic rocks of the western and central ELIP due to the involvement of recycled oceanic crustal materials. Our results are in accordance with the model that the western ELIP late Permian magmatism was generated by the interaction of two distinct sources, that is, an isotopically heterogenous mantle plume and a Neoproterozoic subduction-modified, Nd isotope-enriched lithospheric mantle with distinct heterogenous oxygen isotope characteristics.

Petrogenesis of the Permian Luotuoshan sulfide-bearing mafic-ultramafic intrusion, Beishan Orogenic Belt, NW China: evidence from whole-rock Sr–Nd–Pb and zircon Hf isotopic geochemistry

The Permian Luotuoshan mafic-ultramafic intrusion is one of the ~280 Ma mafic-ultramafic complexes located in the Beishan Orogenic Belt at the southern margin of the Central Asian Orogenic Belt, NW China. The intrusion is predominantly composed of wehrlite, olivine clinopyroxenite, troctolite, olivine gabbro, and gabbro. Disseminated sulfides occur in wehrlite and olivine clinopyroxenite at the center of the intrusion. SHRIMP zircon U–Pb dating of a gabbro sample yielded a concordia age of 282.6 ± 2.6 Ma. The Luotuoshan mafic-ultramafic rocks are characterized by elevated εNd (282.6Ma) values from +0.42 to +6.10 and εHf (282.6Ma) values from +7.9 to +14.1, relatively low initial 87Sr/86Sr (0.703919–0.705272) and Pb isotope composition, and low ratios of TiO2/Yb, Nb/Yb, etc., suggesting the Luotuoshan magma was derived from a depleted asthenosphere mantle source. Some arc-like geochemical characteristics such as enrichment of light rare earth elements and large ion lithophile elements (Ba, Sr, Rb, etc.), pronounced negative Nb–Ta–Zr–Hf anomalies, and La/Nb, Ce/Pb, and Th/Yb ratios indicate that the mantle source was previously modified by subduction-related metasomatism. Petrographic features and mixing calculations of Sr–Nd–Hf isotopes suggest that the parental magma of the Luotuoshan intrusion underwent a moderate degree of crustal contamination (5–15%) followed by fractional crystallization process. Petrological modeling reveals that both fractional crystallization and crustal contamination may have played the important role in triggering the sulfide saturation in the Luotuoshan magma. The comparable formation age, petrographic and mineralogical characteristics, geochemistry of the parental magma and evolution process compared with intrusions in the Pobei area, indicate that the Luotuoshan intrusion may also have similar mineralization potential. This also suggests that the mineralization prospection of Permian mafic-ultramafic intrusions in the Gansu Beishan area should not be neglected, and it is worth considering that whether further exploration is worthy of being implemented in this region.

Neoproterozoic protracted arc basaltic magmatism in the southern margin of the Yangtze Block, south China: New constraints from mafic-ultramafic intrusive rocks

Mantle plume activity and subduction are two popular geodynamic models that have been used to explain the widespread Neoproterozoic basaltic magmatism in the Jiangnan Orogenic Belt along the southern margin of the Yangtze Block, South China. We have evaluated the competing hypotheses using new geochronological, petrological and geochemical data of two mafic–ultramafic intrusions in the Yuanbaoshan (YBS) area of this orogenic belt, namely the YBS south and west intrusions. Zircons from the YBS south intrusion yielded a U-Pb age of 835 ± 3 Ma. This new age and the previously reported age for another mafic–ultramafic intrusion situated < 10 km to the northwest (the YBS west intrusion) reveal a protracted (12–20 Myr) episode of mafic magmatism in a very small area. Such temporal-spatial relationship is more consistent with the style of subduction-related magmatism than that of mantle plume-related magmatism. The YBS south and west intrusions are characterized by arc-like geochemical features such as light REE enrichments and pronounced negative Nb-Ta anomalies in whole rocks. Olivine Mn/Fe and Ca contents, clinopyroxene AlZ/TiO2 and Cr-spinel TiO2/Al2O3 of these intrusions are similar to those of some undisputed subduction-related mafic–ultramafic intrusions elsewhere in the world. Zircons from the YBS south intrusion have mantle-likeHf-O isotope compositions, with εHf(t)varying from + 3.3 to + 8.7 and δ18Oof5.32 ± 0.29‰. The whole rock samples yielded εNd(t) from + 5.2 to + 8.8 and (87Sr/86Sr)i from 0.7034 to 0.7051 for the YBS south intrusion, and εNd(t) from + 1.7 to + 5.2 and (87Sr/86Sr)i from 0.7036 to 0.7051 for the YBS west intrusion. The Sr-Nd isotope compositions of these intrusions are similar to those of global Cenozoic arc basalts, but are also consistent with mantle plume-derived magma with < 5% of crustal contamination. However, the lack of correlation between Th/Nb and εNd(t) or (87Sr/86Sr)i for the intrusive rocks is inconsistent with the contamination model. The results from this study support for a popular view that northward oceanic subduction beneath the southern margin of the Yangtze Block took place in the Neoproterozoic.

Zircon U-Pb age, trace element, and Hf isotopic constrains on the origin and evolution of the Emeishan Large Igneous Province

The late Permian to Early Triassic volcano-sedimentary succession associated with the Emeishan Large Igneous Province (LIP) is well preserved in the Binchuan area, Southwest China. This paper presents coupled zircon U-Pb age, trace element, and Hf isotope analyses from the succession, together with whole-rock geochemical compositions, to reveal the origin and evolution of volcanic rocks of the LIP. Zircons from the matrix and a rhyolite clast in the Lower Triassic conglomerate bed yield identical U-Pb ages of ca. 260 Ma and have geochemical affinities to those crystallized from within-plate magmas. These features, combined with the dominance of rhyolite clasts from this horizon, imply a source related to Emeishan silicic volcanic rocks. Zircons from silicic ignimbrite and rhyolite in the upper volcanic succession of the LIP display high εHf(t) values (+4.2 to +12.9), low Th/Nb and U/Yb ratios, and similar εNd(t) values (−0.17 to +0.47) to high-Ti basalts. These chemical and isotopic characteristics are consistent with the eroded silicic volcanic rocks, indicating that the late-stage silicic volcanic rocks were generated by fractionation of high-Ti basaltic magmas without significant crustal contamination. Although the late Permian zircons from the lower and upper low-Ti basalt successions all show arc-like geochemical characteristics with high Th/Nb and U/Yb ratios, the former has much lower εHf(t) values (−11.7 to −3.6) than the latter (+4.4 to +11.6), and the zircons from the upper low-Ti basalt succession show positive εHf(t) values similar to the studied rhyolites. Samples from the lower low-Ti basalt succession have relatively higher εNd(t) values (−2.66 to +3.38) than those from the upper low-Ti basalt succession (−4.48 to −0.82). These geochemical features indicate the early-stage low-Ti basalts may be mainly derived from a previously enriched subcontinental mantle lithosphere, whereas the late-stage low-Ti basalts may be generated from the increasing involvement of asthenospheric mantle source with crustal contamination.

Magmatic controls on the mineralization potential of a porphyry Cu system: The case of Jurassic Tongshan skarn Cu deposit in the Qin–Hang Belt, South China

The Qin–Hang suture belt in South China formed during the Neoproterozoic amalgamation between the Cathaysia and Yangtze blocks. There are several Jurassic porphyry–skarn Cu deposits in this region, and the factors controlling the mineralization potential remain unclear. This study reports geochemical data for the Tongshan skarn deposit and zircon trace element data for five porphyry- or skarn-type Cu deposits in this region, in order to identify geochemical indexes for the mineralization potential of the ore-related intrusions. A garnet U–Pb age (170 Ma) for the Tongshan skarn indicates that Cu mineralization occurred in the Middle Jurassic, which is consistent with the zircon U–Pb ages (173 Ma–171 Ma) of the host granodiorite porphyries. Granodiorite porphyry samples from the Tongshan deposit have moderately high SiO2 contents (64.4–65.0 wt%), low MgO contents (1.39–1.47 wt%), and high (La/Yb)N ratios (14.7–16.8), typical of low-Mg adakitic rocks. The whole-rock geochemistry, bulk Earth-like εNd (t) values (–1.5 to –1.6) and positive zircon εHf(t) values (+2.7 to +6.0), corresponding to two-stage Hf model ages of 1014–832 Ma, suggest the porphyries were derived from thickened juvenile lower crust initially generated during Neoproterozoic amalgamation. In addition, the porphyry samples have arc-like geochemical characteristics (Nb–Ta–Ti depletion), low Th/Yb ratios (average = 4.7), high Ba/Th ratios (average = 346), zircon Eu/Eu* values (average = 0.71) and 10000×(Eu/Eu*)/Y values (average = 6.80), and moderate zircon Ce/Ce* values (average = 705) and Ce/Nd ratios (average = 17.1). These features demonstrate that the parental magmas of the porphyries were formed by melting of a mantle wedge.That had been metasomatized by moderately oxidized slab-derived fluid near a Neoproterozoic subduction zone. Previous studies suggest porphyries from large to giant deposit (e.g., Dexing and Yinshan) have similar two-stage Hf model ages and were derived from Neoproterozoic juvenile crust. But these porphyries have higher whole–rock Th/Yb ratios and zircon Ce/Ce* and Ce/Nd values than those of intrusions from medium-sized deposits (e.g., Tongshan, Chuankeng and Jiande). This indicates that the magma source of the former may have been distant from the trench in the Neoproterozoic, and formed by melting of a mantle wedge that had been metasomatized by highly oxidized slab-derived melt. Our results and previously published data indicate that the Neoproterozoic two-stage Hf model ages of the intrusions are positive indicators of Jurassic Cu mineralization, and granitoids with relatively high Th/Yb ratios and zircon Ce/Ce* and Ce/Nd values likely have more potential to form large to giant Cu deposits.

Opening of the Proto-Tethys Ocean: Implications from the late Neoproterozoic mafic dike swarms in the South Qinling Belt, South China

Magmatism in the passive margin records continuum rift of continental lithosphere and onset of seafloor spreading. The ca. 650 Ma ultramafic-mafic dike swarms are the latest magmatic episode in the South Qinling Belt during the Neoproterozoic, and therefore provide a window to address their mantle sources, and the continental rift to drift transition. These mafic dikes have distinct geochemical signatures, and can be grouped as N-MORB-, E-MORB-, and Arc-affinities. The Shiziyan amphibole gabbros have low Rb/Y (<0.75), Nb/Y (<0.15), Nb/Zr (<0.05), Th/Zr (<0.01) ratios, positive εNd (+4.3 to +5.4), low Sri (0.70650–0.70759) and moderate 207Pb/204Pb (15.48) and 206Pb/204Pb (17.57), similar to N-MORB, suggesting that they were derived from an asthenosphere mantle. The Yindongshan pyroxenites display enrichment of LILEs and HFSEs, similar to the E-MORB characters. They also have low Sri (0.70384–0.70410), high εNd (+4.2 to +5.4), and 207Pb/204Pb (15.53–15.54) and 206Pb/204Pb (18.18–18.37). They have high zircon εHf (+13.1 to +15.0), but low δ18O values (+4.43 to +5.17‰). These chemical features indicate an enriched mantle source with involvement of recycled component. The Duchongshan olivine gabbros show arc-like geochemical features, with high Nb/Zr (0.07–0.16), Nb/Yb (3.52–20.1) ratios, and high Sri (0.70545–0.70586), negative εNd (−6.1 to −5.2), large variable zircon εHf (−0.5 to +4.2) and high δ18O (+5.41 to +6.41‰). These features favor that they were derived from a subduction-modified lithosphere mantle followed by significant crustal contamination. The northern Yangtze Block experienced secular crustal extension and developed into a volcanic rift margin during the middle to late Neoproterozoic. Mafic dike swarms emplaced into the volcanic successions and unconformably overlayed by late Ediacaran neritic to abysmal deposition. The magmatic termination and sedimentary facies transition indicate that the northern Yangtze Block broke up during the late Neoproterozoic and the Proto-Tethys Ocean opened at ca. 650 Ma.

Early Cretaceous basalts record the modification of the North China Craton lithospheric mantle: implications for lithospheric thinning

ABSTRACT The modification of the Archaean lithospheric mantle root beneath the eastern North China Craton (NCC) has been noticed. However, the degree of modification and the characteristics of metasomatic agents for the NCC lithospheric mantle are still unclear. Here, we compile the geochemistry and 87Sr/86Sr and 143Nd/144Nd isotope ratios of Cretaceous primitive basalts (Yixian, Sihetun, Fangcheng and Feixian) from the eastern NCC and estimate the inputs of subduction-related elements into the Archaean lithospheric mantle beneath the NCC. These basalts show primitive (high MgO, Cr and Ni contents) and arc-like geochemical features (enrichments in LILEs and depletions in HFSEs) that indicate that the mantle sources were modified by fluids related to subducted crustal materials. Substantial proportions of subduction-mobile elements (e.g. ~95% Ba and ~90% Th) transferred to the basalt sources via hydrous melts. Thus, a large volume of fluids transferred into the lithospheric mantle. The data support the model that the NCC Archaean lithospheric mantle was weakened by hydrous melts, which resulted in a fusible weakened lithospheric mantle. Preliminary lithospheric thinning was induced by the extension of the NCC resulting from trench retreat of the Paleo-Pacific plate at ~140-120 Ma. Then, decompression melting of the lithospheric mantle caused pervasive melts in the weakened lithospheric mantle, resulting in the lithospheric mantle having a low viscosity comparable to the asthenospheric mantle, which principally reduced the lithospheric mantle.

Magnesium and zinc isotopic anomaly of Cenozoic lavas in central Myanmar: Origins and implications for deep carbon recycling

Carbon in the form of carbonate in sediments and altered oceanic crust can be transported into the mantle by subducted slabs, but how to identify recycled carbonate and decipher its storage depths in the mantle still requires to be investigated for typical subduction zones. Here we investigate possible carbonate recycling related to subduction of the Neo-Tethyan oceanic slab, by a combined study of Mg and Zn isotopes on three types of Cenozoic mantle-derived lavas from central Myanmar. The calc-alkaline lavas have arc-like geochemical characteristics and originated from the mantle wedge. These rocks have high δ26Mg (−0.20‰ to +0.03‰), interpreted to be caused by the input of Mg-rich fluids from dehydration of altered subducted slab. The alkali basalts of suite I are depleted in Nb and Ta and were derived from partial melting of the lithospheric mantle, which have normal δ26Mg and δ66Zn. By contrast, the alkali basalts of suite II display OIB-like trace element patterns and originated from the asthenospheric mantle. They possess extremely low δ26Mg (−0.66‰ to −0.48‰) and high δ66Zn (0.57‰ to 0.77‰) that is typically characterized by marine carbonates. No known processes (e.g., magmatic processes and diffusion effects) can account for the coupling of low δ26Mg and high δ66Zn except source heterogeneity caused by recycled magnesian carbonates (magnesite ± dolomite). This is supported by Mg-Zn-Sr isotopic mixing modelling that suggests a “magnesian carbonate + pyroxenite” source. Since calcic carbonate transforms to magnesite at depths of >300 km during subduction, carbonates carried by the Neo-Tethyan oceanic slab may have been recycled into the mantle transition zone as revealed by seismic tomography observations on central Myanmar.

Slab roll-back triggered back-arc extension south of the Paleo-Asian Ocean: Insights from Devonian MORB-like diabase dykes from the Chinese Altai

To better understand the origin of mid-ocean ridge basalt (MORB)-like mafic magmas and its geodynamic implications for the subduction zone system, we present an integrated study of petrology, mineralogy, geochronology and geochemistry of newly-discovered diabase dykes from the Chinese Altai, southwestern Central Asian Orogenic Belt. The diabases have zircon UPb ages of ~386 Ma, and are mainly composed of clinopyroxene (Wo43–55En17–27Fs26–34) and albite (An0–1Ab92–99Or0–7). Although the clinopyroxenes have consistent Mg# values (up to 75) that appear to be in equilibrium with bulk compositions, they can be divided into two groups based on their differences in other geochemical variables: type ǀ has high SiO2 and CaO but low TiO2 and Al2O3 contents while type ǁ displays low SiO2 and CaO but high TiO2 and Al2O3 contents. This indicates that the two types of clinopyroxenes may share a common source but crystallized at different depths under different P-T conditions (e.g., type ǀ: 6.3 kbar (~20 km), 1227 °C; type ǁ: 15.5 kbar (~49 km), 1332 °C). The dykes have low SiO2 (44.5–48.2 wt%) and K2O (0.17–0.33 wt%), but high (Fe2O3)T (11.2–13.6 wt%) and MgO (7.16–9.90 wt%) contents, placing them within the low-K tholeiitic series. With both N-MORB- and arc-like geochemical characteristics, including weakly fractionated rare earth element (REE) patterns ((La/Yb)N 1.43–1.62), enrichment in large-ion lithophile elements (LILE) (e.g., Pb) and slightly depletion of high field strength elements (HFSE) (e.g., Nb, Ta and Ti), and highly depleted SrNd isotopic compositions (87Sr/86Sri 0.7039–0.7042, ɛNd(t) +8.3 to +8.6), the diabase dykes were probably derived by partial melting of a N-MORB-like asthenospheric mantle source metasomatized by subduction-related fluids under spinel-facies conditions. Melting temperatures and pressures for the primary magmas were estimated at 13711394 °C and 2.22.4 GPa, respectively. The dykes underwent insignificant crustal contamination, but trace-element modeling indicates that minor subcontinental lithospheric mantle (SCLM) components could have been involved in their source region. We suggest that the MORB-like magmatism was triggered by asthenospheric upwelling in a back-arc extensional setting, in response to the roll-back subduction of the Paleo-Asian oceanic slab. Combined with the coeval arc magmatism in the study area, we envisage that an active arc–back-arc basin system developed in the Chinese Altai during Devonian time, linked to the northward subduction of the Paleo-Asian Ocean. This study emphasizes that MORB-like mafic rocks in paleo-subduction zones could be an important tectonomagmatic process-indicator (e.g., back-arc extension), which offers a new perspective in reconstructing past tectonic regimes.

Transition from the lithospheric to asthenospheric mantle-derived magmatism in the Early Jurassic along eastern Bangong–Nujiang Suture, Tibet: Evidence for continental arc extension induced by slab rollback

Abstract The transition of the geochemical signature in mafic rocks along the eastern Bangong–Nujiang suture in Tibet contains important information about geodynamic processes in the upper mantle. This study recognized two episodes of Early Jurassic gabbros from the Kaqiong terrane, a microblock within the Bangong–Nujiang suture zone. Early gabbros (ca. 197–191 Ma) appear as lenses in the basement complex and were overprinted by amphibolite/granulite-facies metamorphism at ca. 180 Ma. Later undeformed hornblende gabbros (ca. 177–175 Ma) occur as dikes intruding into the basement complex. The early metagabbros are characterized by arc-like geochemical features and enriched Nd-Hf isotopic compositions (whole rock ∑Nd(t) = –0.7 to +0.3; zircon ∑Hf(t) = –5.7 to –2.2), which suggests formation by partial melting of an enriched lithospheric mantle source. In contrast, the later hornblende gabbros have depleted Nd-Hf isotopic compositions (whole rock ∑Nd(t) = +6.1 to +7.1; zircon ∑Hf(t) = +10.7 to +16.8) and normal mid–oceanic–ridge basalt (N–MORB)-type rare earth element (REE) features. They also show variable enrichments of fluid mobile elements (e.g., Rb, U, Pb), indicative of the input of slab-derived fluids in their mantle source. Thus, the hornblende gabbros were most likely originated from the asthenospheric mantle metasomatized by subducted oceanic slab-derived fluids. The transition in geochemical and isotopic compositions of these mantle-derived magmas reveals a long-lasting lithosphere extension and thinning along the southern margin of the Qiangtang terrane in the Early Jurassic. Combined with geological observations, we propose that this transition has resulted from the southward rollback of the subducting Bangong–Nujiang Tethyan oceanic slab. The slab rollback could have initiated the overriding plate extension and the asthenosphere upwelling. Wider implications of this study are that an onset of slab rollback could be an important trigger for the transition of magmatic geochemistry in subduction zones.

Remobilization of metasomatized mantle lithosphere: a new model for the Jiaodong gold province, eastern China

The ~ 120 Ma Jiaodong province in the North China Craton, composed of abundant Mesozoic granites that intrude Precambrian metamorphic basement, is bordered by the Sulu orogenic belt, which formed through the collision between the North China and Yangtze cratons in the Triassic. Insights on the geodynamic setting and source of gold and fluids in the Jiaodong gold province are based on a detailed study of the Sanshandao deposit. The δ13CPDB and δ18O values of hydrothermal calcite from this deposit range from − 4.3 to − 6.5‰ and 11.4 to 15.1‰, respectively, which are compatible with a mantle source. Pre-ore mafic dykes in the deposit show typical arc-like geochemical features, suggesting that the mantle source was metasomatized prior to the basic magmatism. The δ34S values of gold-related pyrite from Sanshandao and other large deposits in the province range from 10.9 to 11.5‰ and are higher than those of the country rocks, but consistent with the signature of orogenic gold deposits sourced from a Neoproterozoic sedimentary reservoir. These high positive δ34S values are interpreted to be inherited from auriferous Neoproterozoic sedimentary rocks of the northern Yangtze Craton, which were subducted into the mantle lithosphere beneath the North China Craton during the formation of eclogite in the adjacent Sulu orogenic belt in the Triassic. The syn-ore Lower Cretaceous mafic dykes display oceanic island basalt-like geochemical features, suggesting that asthenosphere upwelling triggered the release of gold and sulfur from an enriched and fertilized mantle lithosphere, contributing to the auriferous fluid which formed the widespread gold mineralization in the Jiaodong province at ca. 120 Ma.

Post-collisional ultramafic complex in the northern North China Craton: Implications for crust-mantle interaction

The post-collisional ultramafic intrusions within the northern margin of the North China Craton (NCC) preserve important imprints of crust-mantle interaction. Here we investigate ultramafic intrusions from the Luojianggou complex composed of pyroxenite and hornblendite associated with serpentine, with a view to gain insights into the nature of orogenic lithospheric mantle in this major continental collision zone. Zircon UPb data from the ultramafic suite define different age populations, with Paleoproterozoic ages (>2.2 Ga and 1.82 Ga) representing xenocrystic grains captured from the basement. The magmatic zircon grains range in age from 872 Ma to 458 Ma, and are possibly related to multiple magma emplacement associated with the Paleo-Asian Ocean closure. The youngest age of ca. 230 Ma is related to a period of post-collisional extension in the northern margin of the NCC, and this inference is further supported by apatite UPb ages of ∼207 Ma. Phase equilibrium modeling suggests temperatures of 700–800 °C and pressures of 11–13 Kbar. The pyroxenite and hornblendite show similar geochemical features and REE patterns, indicating the same magma source and formation through differentiation and accumulation. The arc-like geochemical features of the rocks with enrichment of LILE (Rb, Th and La) and LREE, but depletion of HFSE (Nb, Zr and Hf), possibly formed at the boundary of sub-continental lithospheric mantle (SCLM) and crust through metasomatic reaction of peridotite with felsic melts derived from subduction-related components. The arc-like features, zircon rare earth element patterns and obvious positive Pb anomaly in primitive mantle-normalized trace element spidergrams also indicate the mixing of continental materials in the magma source. The post-collisional extensional setting is correlated to the tectonics associated with the assembly of the Mongolian arc terranes within the northern NCC during the Triassic.

Geochronology, petrology and Sr-Nd-Hf-S isotope geochemistry of the newly-discovered Qixin magmatic Ni-Cu sulfide prospect, southern Central Asian Orogenic Belt, NW China

The Qixin mafic-ultramafic complex is present in the southern margin of the Central Asian Orogenic Belt in northern Xinjiang where many magmatic Ni-Cu sulfide deposits occur. To determine the temporal association of the newly-discovered Ni-Cu prospect associated with the Qixin complex with the known major magmatic Ni-Cu sulfide deposits in the region, as well as the origin of sulfide mineralization in the Qixin complex, we have carried out geochronological, petrological and geochemical studies of this complex. Gabbro, olivine gabbro, norite, olivine gabbronorite, troctolite, websterite and lherzolite are the major rock types of this complex. Two drill cores completed to date show two types of sulfide mineralization associated with the complex: (1) disseminated sulfides that occur near the base of bodies of lherzolite and troctolite; (2) blotchy-massive sulfides in the hornfels country rocks. The disseminated sulfides are composed of pyrrhotite, pentlandite and chalcopyrite, whereas the blotchy-massive sulfides in the country rocks are composed of pentlandite, violarite, pyrrhotite and chalcopyrite. Zircon crystals from the Qixin olivine gabbronorite yield a U-Pb age of 286.5 ± 2.5 Ma, which is among the oldest post-subduction magmatic Ni-Cu sulfide deposits in the region. The TiO2/Al2O3 ratios of Cr-spinel and clinopyroxene from the Qixin complex are similar to those from the older, subduction-related mafic-ultramafic intrusions in the region. This and other arc-like geochemical characteristics such as whole-rock negative Nb-Ta anomalies coupled with low initial 87Sr/86Sr (0.7036–0.7057), positive εNd(t) (+2.3 to +6.0) and high εHf(t) of zircon (+8.3 to +15), indicate that the parental magma of the Qixin complex was derived from a mantle source modified by pre-Permian metasomatic processes, similar to other contemporaneous mafic-ultramafic intrusions in the region. Mixing calculations using Sr-Nd-Hf isotopes indicate that the parental magma for the Qixin mafic-ultramafic rocks experienced no more than 15 wt% contamination with siliceous country rocks. The δ34S values of different sulfide assemblages of the Qixin Ni-Cu prospect are similar and vary from 1.1 to 1.7‰, which are within the range of typical mantle values as well as that of the country rocks, providing no direct evidence for addition of external sulfur. However, the role of external sulfur in sulfide mineralization at Qixin still cannot be ruled out, as forward modelling indicates that fractional crystallization plus crustal contamination involving siliceous country rocks free of S could not have induced sulfide saturation in the Qixin magma. This may also be true for the contemporaneous magmatic Ni-Cu sulfide deposits in the region. Based on the results from this study, we suggest that fractures in the solid country rocks nearby the Qixin complex and the ultramafic bodies of the complex are preferred targets if exploration continues.

Late Cryogenian magmatic activity in the North Lhasa terrane, Tibet: Implication of slab break-off process

The North Lhasa terrane in Tibet is generally interpreted to be paleotectonically unrelated to the East African Orogen (EAO) and is instead thought to have derived from northeastern India or northwestern Australia. In this study, we present petrogenetic and geochronological results pertaining to the analysis of gabbros (ca. 652 Ma), diorites (ca. 658 and 646 Ma), and tonalites (ca. 652 Ma) from the North Lhasa terrane. The gabbros are calc-alkaline and exhibit arc-like geochemical features. Low positive zircon εHf(t) values (+1.0 to +3.8), high zircon δ18O (6.25‰ to 7.94‰), and low negative whole-rock εNd(t) values (−3.5 to −1.4) indicate that the gabbros were derived from the lithospheric mantle, with geochemical modification by a subduction component. The diorite suite is characterized by a wide range of whole-rock chemistries (e.g., SiO2 = 51.33–61.98 wt%) and Hf–O–Sr isotopic compositions (εHf(t) = −10.8 to −0.1; δ18O = 5.17‰ to 7.11‰; ISr = 0.706 to 0.710), and negative whole-rock εNd(t) values (−7.0 to −4.7). These diorites are geochemically similar to OIB and are interpreted to be products of the partial melting of a relatively deep mantle source (>85 km) prior to extensive modification by continental crustal material. The tonalites are adakitic and have moderate Mg# values (47–54), low compatible element abundances, positive zircon εHf(t) values (+3.4 to +6.2), high ISr values (0.714 to 0.715), and small negative whole-rock εNd(t) values (−1.6 to −0.4). These tonalites most likely formed by the melting of thickened Mesoproterozoic continental crust. The generation of these ca. 650 Ma magmatic rocks was related to slab break-off in a collision zone. By integrating the findings of previous studies with the data of the present study, we suggest that the North Lhasa terrane was most likely located in the northern segment of the EAO in paleotectonic reconstructions of the Gondwana supercontinent.