Plagioclase Amphibolite Research Articles

Cu-Au-Platinum Group Element Mineralization in the Mbesa Prospect, Southern Tanzania: Unconventional Magmatic Sulfides

Abstract High-grade Cu mineralization was recently discovered in the Mbesa prospect in southern Tanzania, but its origin is poorly constrained. Herein, we present new major element, trace element, rare earth element (REE), and platinum group element (PGE) geochemistry and petrographic data to constrain the origin of the mineralization. The sulfide mineralization is dominated by chalcopyrite and bornite and characterized by high Cu/Ni over 100 in the massive and disseminated ores. The mineralization lacks pyrrhotite and pentlandite, which are found commonly in magmatic Ni-Cu-PGE sulfide deposits. Platinum group minerals present are michenerite (PdBiTe) and sudburyite (PdSb). PGE concentrations expressed as Pd + Pt + Au are up to 1.31 and 1.04 ppm in the massive and disseminated ores, respectively. The δ34S of chalcopyrite separates range from –3.9 to –0.6‰, bornite separates vary from –1.8 to 1.1‰, and bornite-chalcopyrite mixtures yield –1.3 and 0.6‰, mostly consistent with a magmatic sulfur origin. The sulfides are hosted by coarse-grained plagioclase amphibolite and hornblende gneiss with recrystallization textures suggestive of metamorphism. Both rocks are characterized by enrichment in large ion lithophile elements (LILEs), with negative anomalies of Zr and Nb, consistent with crustal contamination of magmas derived from partial melting of subcontinental lithospheric mantle (SCLM). An SCLM origin is further suggested by the close association of Cu sulfides with carbonates and apatite. It is suggested that the sulfides crystallized from the fractionated Cu-rich sulfide melt enriched in intermediate solid solution (iss) at shallow crustal levels. Sulfide were likely mobilized as droplets attached to CO2 vapor phase as expressed by close association between carbonates and apatite with the Cu-rich sulfides. Nickel-rich sulfides may remain undiscovered at deeper portions around the Mbesa prospect.

Thermophysical properties of plagioclase amphibolite at high temperatures and pressures and its geophysical implications

The thermal conductivity (κ) and thermal diffusivity (D) of plagioclase amphibolite were measured via the transient heat-source method at 298–773 K and 0.5–3.0 GPa, respectively. The experimental results show that the κ and D of the plagioclase amphibolite decrease with increasing temperature but slightly increase with pressure. Combining geothermal parameters with the present results, we calculated the average geothermal gradient curve under North China. Our estimation showed the average depth of the thermal lithosphere in North China might be 95 km.

Experimental investigation on multiscale damage characteristics of plagioclase amphibolite under water immersion

Water immersion significantly influences the mechanical characteristics of rock materials, with the degradation of these properties often leading to engineering hazards. Understanding the micro mechanism of rock-water interaction is crucial when evaluating the mechanical behavior of a rock mass. A series of multiscale tests were conducted to investigate the micro and macro behaviors of Plagioclase amphibolite, sourced from Gansu, China, under water immersion conditions. The results indicated that the micro particles transitioned from a flake-dispersed structure in a dry state to floc clusters after 20 days of immersion. The surface micro morphology was analyzed using a laser scanning machine, revealing a transformation from a smooth state to roughness, attributed to the emergence and swelling of numerous micro humps. The evolution of parameters such as the standard deviation of the height distribution (Sq), profile area ratio(Sdr), and the root mean square of the first derivative of surface (Sdq) demonstrated that the rock expanded uniformly, with the micro grains swelling to varying degrees. NMR-based porosity testing suggested an exponential increase in damage, leading to the interconnectedness of newly formed micro-pores with neighboring ones due to water immersion. Ultimately, the water-induced porosity doubled compared to the original porosity. Additionally, the axial swelling rate exhibited a corresponding increase in tandem with the growth in porosity, with an average value of 0.141 %. Finally, linear functions were established between uniaxial compressive strength, elastic modulus, and axial swelling rate. These findings offer a comprehensive understanding of the multiscale evolution characteristics of Plagioclase amphibolite under water intrusion.

Geodynamic regime of the southeastern North China Craton before the Tectono-Magmatic Lull: evidence from ca. 2510 Ma and 2430 Ma igneous rocks in the Xuhuai micro-block

ABSTRACT The Tectono-Magmatic Lull (TML) refers to the period between 2.37 and 2.24 Ga when magmatic activity declined globally, the cause of which remains debated. The North China Craton (NCC), which was formed by assembling several micro-blocks in Neoarchean to Paleoproterozoic, preserves the most records of magmatism across the TML. In this contribution, we report petrological, geochemical, and zircon U-Pb-Hf isotopic data of Paleoproterozoic basement rocks from the Xuhuai (XH) micro-block in the southeastern part of the NCC, and use them to constrain the geodynamic regime at the onset of the TML. Our data reveal that the central part of the XH micro-block is mainly composed of biotite granodioritic gneiss and plagioclase amphibolite, which is divided into a ca. 2510 Ma suite of low- and medium-pressure TTGs and a ca. 2430 Ma bimodal igneous suite including high-pressure TTG and plagioclase amphibolite. Variable magmatic zircon εHf(t) values (1.9 to 7.2), Nb, Ta and Ti anomalies for the 2510 Ma TTGs indicate that the parental magma, which was involved with mantle materials, was generated in an active subduction environment in this pre-TML era. The felsic component of bimodal igneous rocks with high Na/Ta ratios (27 to 30) and stable Mg# (41 to 44) suggests their magma source region was as deep as the rutile stability field, and the delamination-derived asthenosphere mantle ascending. The influx of hot mantle induces thermal weakening and mechanical stretching of the overlying crust, leading to back-arc extension. The results of this study support the hypotheses that the NCC formed from micro-block amalgamation and the trigger of the TML could be the large-scale delamination of the lower crust into the mantle.

In Situ Trace Element and Sulfur Isotope Composition of Pyrite from the Beiwagou Pb-Zn Deposit, Liaodong Peninsula, Northeast China: Implications for Ore Genesis

The Beiwagou Pb-Zn deposit, located in the western part of the Liaodong Peninsula, is a carbonate-hosted stratiform deposit with a Pb + Zn reserve of 0.08 Mt @ 4.14% (Pb + Zn). The orebodies occur as conformable layers and lenses and are strictly controlled by strata (the Paleoproterozoic Gaojiayu and Dashiqiao Formations) and lithology (plagioclase amphibolite and dolomitic marble). Given that previous studies have focused only on the mineralization features and mineralogy of deposits, herein, we report in situ trace element analyses of pyrite using LA-ICP-MS, together with in situ sulfur isotopes of pyrite, to constrain the composition, substitution mechanisms, source of sulfur, and sulfate reduction pathways of pyrite in the Beiwagou deposit. Based on pyrite morphology, texture, and chemistry, four pyrite types were identified: subhedral, porous-to-massive pyrite (Py1) related to chalcopyrite; subhedral, porous crushed pyrite (Py2) associated with fine-grained sphalerite; rounded and porous pyrite (Py3) related to the Zn-rich part of the laminated ore; and anhedral, porous-to-massive pyrite (Py4) associated with pyrrhotite, arsenopyrite, sphalerite, and galena. Py1 is characterized by high As, Ag, Cd, In, Au, Cu, and Zn concentrations and low Te, Bi, and Mo concentrations, whereas Py2 has high concentrations of Co and Ni and low concentrations of other trace elements, such as Cu, Zn, Bi, and Te. Py3 is characterized by elevated As concentrations, low Co, Ni, In, W, Te, and Tl concentrations, and varying Pb concentrations, whereas Py4 has low Ag, Cd, In, Zn, Cu, and Mn concentrations and varying W, Co, Ni, Pb, Sb, and As concentrations. Significant correlations between some elements in each pyrite type suggest substitution mechanisms, such as (Zn2+ + Cu2+ + Mn2+ + Cd2+) ↔ 2Fe2+, Ag+ + (Sb)3+ ↔ 2Fe2+, and (Te+ + Ag+) + Sb3+ ↔ 2Fe2+, and the existence of a negative correlation between Co and Ni implies competition between both elements. The strongly positive δ34S values (12.11‰–23.54‰) are similar to that of seawater sulfates and likely result from thermochemical sulfate reduction (TSR). In conclusion, the Beiwagou Pb-Zn deposit is a typical SEDEX deposit and mineralization likely occurred during diagenesis.

The isotope geochemistry of host rocks of the late Archean Guandi and Banshigou banded iron formations, southern Jilin Province: temporal and tectonic significance

The Guandi and Banshigou iron deposits of southern Jilin Province, China, are Algoma-type banded iron formations (BIFs) located in the Eastern Block of the North China Craton (NCC). LA–ICP–MS U–Pb dating shows the magmatic zircons in plagioclase amphibolite from the Guandi BIF (PA–GD) and plagioclase hornblende gneiss from the Banshigou BIF (PHG–BSG) were crystallised at 2556 ± 17 Ma and 2545 ± 27 Ma, respectively, representing the formation age of the BIFs. Metamorphic overgrowth zircons from both samples gave two U–Pb ages of 2452 ± 23 Ma and 2446 ± 70 Ma, indicating the age of metamorphism. The zircon εHf(t) values of PA–GD (–1.3 to +2.8) and PHG–BSG (–1.5 to +2.7) indicate that the magma source was derived from depleted mantle with contamination of crustal material. According to our study, the PA–GD and PHG–BSG were formed in an island arc setting and support previously proposed subduction zone tectonic models for the northeastern NCC during the Neoarchean.

Rheological behavior of amphibolite facies migmatites during orogenesis: A case study from the North Qinling Belt, China

The rheological behavior of amphibolite-facies migmatites are key to understanding the evolution of middle–lower crust during orogenesis. The amphibolite-facies metamorphosed migmatites from the North Qinling Terrane are investigated to elucidate high temperature melt-present deformation and shear localization. The metatexite (plagioclase amphibolite and biotite plagiogneiss), diatexites, and deformed residuum (mylonitic amphibolite and layered amphibolite) are well-preserved in outcrops. The SW dipping and E-W striking foliation, and ptygmatic, asymmetrical folds indicate compressional deformation with top-to-the-WNW shearing. Zircon UPb geochronology indicates that crustal anatexis mainly occurred at ca. 400–386 Ma, associated with the collision between the North and South China Blocks. The occurrence of peritectic amphibole, the estimated anatectic conditions of 2.32–4.29 kbar and 595–691 °C and variation of mineral major compositions jointly demonstrate incongruent water-present melting reactions, which involved biotite- or amphibole-breakdown, while the deformation temperature is constrained at 605–644 °C. In the plagioclase amphibolite, the amphibole is dominated by (100)[001] dislocation slip, while plagioclase doesn't display a clear fabric. In the biotite plagiogneiss, plagioclase is dominated by strain-induced (010)[100] slip and quartz shows prism <a > slip. Rheological processes (brittle fracturing, melt(fluid)-assisted partial mylonitization and layering process) indicate that melt migration has an effect on the residuum. The formation of amphibole type-II fabric is controlled by cataclastic flow and rigid rotation during progressive deformation process and dissolution-precipitation processes lead to further weakening and strain localization.

The ca. 1.13–0.92 Ga magmatism in the western Yangtze Block, South China: Implications for tectonic evolution and paleogeographic reconstruction

The tectonic regime of the Yangtze Block in the South China and its palaeogeographic location in the Rodinia supercontinent during the late Mesoproterozoic to early Neoproterozoic remain enigmatic due to limited geological records. In this study, we report late Mesoproterozoic basalts and granodiorites and early Neoproterozoic plagioclase amphibolites in the western Yangtze Block. Zircon LA-ICP-MS U-Pb dating yield concordant ages of 1130–1128 Ma for basalts, 1026 ± 1.6 Ma for granodiorites, and 917 ± 1.4 Ma for plagioclase amphibolites. The basalts are tholeiitic in chemical composition and are slightly enriched in LREEs with (La/Yb)N ratios of 2.87–3.99 and exhibit E-MORB-like trace element patterns. They are depleted in whole-rock Nd (+3.3 to +5.9) and zircon Hf isotopes (+11.1 to +12.8). Geochemical compositions showed that the basalts were derived from high-degree (7%–20%) partial melting of spinel lherzolite mantle. The granodiorites belong to the A-type granites and have negative whole rock εNd(t) (−5.5 to −7.8) and zircon εHf(t) (−15.3 to +0.6) values. We suggested that these A-type granites were mainly generated by the partial melting of felsic crustal rocks under low-pressure conditions. The plagioclase amphibolites are tholeiitic and exhibit arc-like geochemical affinities. They show depleted isotopic compositions (zircon εHf(t) = +11.0 to +13.6; whole rock εNd(t) = +0.6 to +2.5) and were interpreted as derived from a lithospheric mantle source that was metasomatized by subduction-related fluids. Geochemical compositions show that basalts were formed in an intracontinental rift setting, whereas granodiorites and plagioclase amphibolites were emplaced into a subduction background. Integrated with published work, we suggest that the late Mesoproterozoic and early Neoproterozoic magmatism in the western Yangtze Block record a tectonic switch from a passive margin to a convergence setting. In addition, we also proposed that the South China may be located at the western periphery of the Rodinia supercontinent.

Archean records in the Kontum Complex (Central Vietnam) and implications for Precambrian crustal evolution in Indochina

Zircon U-Pb age-clusters of ∼3.5 Ga and ∼3.2 Ga with εHf(t) = +1.0 ∼−13.5 and THfDM = ∼3.4–4.0 Ga are documented from plagioclase amphibolites in the Kham Duc rock-unit of the Kontum Complex, Indochina. These data provide evidence for a Paleoarchean crustal nucleus that is in part synchronous with, as well as older than, the Yangtze basement of the South China Block. In addition, the presence of ∼2.2 Ga, ∼1.85 Ga and ∼1.42 Ga age-records in the complex correlate with those in the southern Yangtze, suggesting their connection at the time of Nuna assembly and breakup. These data reveal a similar Paleoarchean-Mesoproterozoic crustal evolution among Indochina, Yangtze, Australia and NW Laurentia. The Ordovician-Silurian and Triassic tectonothermal events in the Kontum Complex link to the breakup of the Indochina Block from the northern margin of Gondwana prior to accretion to Asia.

Geochronology and Geochemistry of Mesoproterozoic Mafic Rocks in the Kontum Complex and Its Implication for the Columbia Reconstruction

Abstract The assembly–breakup of the Columbia/Nuna supercontinent is one of the most important issues in the Precambrian geology. The reconstruction of the Indochina Block in the Columbia supercontinent is poorly constrained by far, due to the deficiency of available geochronological and geochemical data for the exposed Precambrian rocks. The Mesoproterozoic plagioclase amphibolites in the Kontum Complex have significant implication for the reconstruction of the Indochina Block within the Columbia supercontinent. This study presents detailed petrological, zircon U–Pb geochronological and Lu–Hf isotopic, and whole-rock geochemical analyses for the plagioclase amphibolites. The plagioclase amphibolite protoliths were formed at ~1432–1403 Ma and experienced metamorphism at 486–457 Ma, suggesting the preservation of a Mesoproterozoic basement in the Kontum Complex. The samples are strongly enriched in LREEs and LILEs but depleted in Nb, Ta, and Ti. They have relatively low (87Sr/86Sr)i ratios (from 0.705055 to 0.708728), positive εNd (t) values (from +2.6 to +2.8), and positive zircon εHf (t) values (from +9.9 to +17.1). Such signatures suggest that they were derived from a mantle wedge that has been metasomatized by sediment-derived melts in an arc setting and caused the breakup of the Columbia supercontinent. The Kontum Complex from the Indochina Block, along with Laurentia and East Antarctica, was distributed at the Columbian periphery.

Late Neoarchean geodynamic regime of the northeastern North China Craton: Constraints from metamorphosed volcanic rocks of the Anshan-Benxi greenstone belt

The Anshan-Benxi greenstone belt in the northeastern North China Craton preserves ∼ 3.88 Ga to ∼ 2.5 Ga lithologies that exhibit based secular trends in their geological signatures and hosts large amounts of valuable Neoarchean banded iron formations. However, whether plume or subduction mechanism responsible for those iron deposits remains confusing. This contribution focuses on the metamorphosed volcanic rocks and aims to constrain their geodynamic regime, based on temporal and spatial distributions, petrogenesises and tectonic characteristics. The late Neoarchean metamorphosed volcanic rocks of the Anshan-Benxi greenstone belt are prominently exposed in the Waitoushan-Gongchangling-Benxi area and consist of plagioclase amphibolites, amphibole/mica plagioclase gneisses and amphibole/mica two-feldspar gneisses. All 107 metamorphosed volcanic rock samples may be geochemically and petrographically classified as tholeiites, tholeiitic to calc-alkaline basalts, andesite, and dacite-rhyolite. The tholeiites are characterized by approximately flat REE patterns with negligible Eu anomalies and show negative Nb anomalies on primitive normalized multiple element diagrams, consistent with a back-arc basin basalt origin. The tholeiitic to calc-alkaline basalts have slightly heavy REE-depleted patterns and obvious negative Nb anomalies, which were likely produced by partial melting of mantle peridotites that were metasomatized by subducted slab-released fluids and melts. They may be comparable to island arc basalts. The andesites exhibit notably high MgO, Mg# and transition metal element contents, low FeOT/MgO ratios, fractionated REE patterns and evident negative Nb, Ta and Ti anomalies, which are akin to Phanerozoic high magnesian andesites and are melts of mantle peridotites that were previously metasomatized by subducted slab melts/fluids. The dacite-rhyolites have the highest SiO2 and lowest MgO, Cr and Ni contents and were derived from intracrustal remelting.The zircon U-Pb dating results reveal that the arc tholeiitic to calc-alkaline basalt-andesite-dacite-rhyolite assemblages erupted at ∼ 2.57 Ga – ∼2.52 Ga and back-arc basin basalt lithological association at ∼ 2.55–2.52 Ga. Furthermore, both the arc basalt-andesite-dacite-rhyolite and back-arc basin basalt associations are predominantly exposed within the Waitoushan-Gongchangling-Benxi range of the northeastern Anshan-Benxi greenstone belt. Therefore, we suggest that the Waitoushan-Gongchangling-Benxi area represents a rear-arc to back-arc zone, which was subsequently subjected to consistent NE-SW compression (σ1) and top-to-northeast kinematic features during back arc closure and underwent up to high amphibolite facies metamorphism. However, the Anshan area in the southwestern range of the Anshan-Benxi greenstone belt preserves large amounts of thick-bedded metasedimentary rocks instead of back-arc and arc volcanic rocks, which indicates a stable depositional environment in a typical back arc basin depositional zone. The supracrustal rocks within the back arc basin depositional domain underwent NE-SW compression (σ1) and top-to-southwest kinematic features and mostly experienced greenschist facies metamorphism.These crust-mantle interaction features inferred by metamorphosed volcanic rocks in the Anshan-Benxi greenstone belt, spatial and temporal variations in the lithological assemblages, together with the style of structural deformations, indicate a late Neoarchaean geodynamic evolution from SW-dipping subduction to arc-back-arc development and collisions between ancient continental blocks and arcs.

Contribution of Precambrian basements to the Mesozoic ore-fluid system: An illustration using the Majiayao gold deposit, Jiaodong, China

In contrast to most Mesozoic granite-hosted gold deposits in the giant Jiaodong gold province (China), the Majiayao deposit represents a significant example hosted in the Archean high-grade metamorphic terrane. The mineralization is characterized by auriferous quartz-sulfide veins together with notable Fe-bearing carbonates (ankerite and siderite), iron oxides and barite. In this study, the comprehensive in-situ analyses of vein-hosted hydrothermal minerals (such as: xenotime U-Pb dating, ankerite Rb-Sr isotopes and monazite Sm-Nd isotopes) and sulfur isotopic analyses of gold-bearing sulfides are used to constrain the timing of mineralization and to better understand the ore genesis. The syn-ore hydrothermal xenotime yielded a U-Pb age of 120.0 ± 1.2 Ma marking timing of gold mineralization, which is much younger than wallrocks’ latest metamorphism at 1950 to 1850 Ma. The relatively higher and variable initial 87Sr/86Sr ratios (0.715809–0.718753) of ore-related ankerite may be leached from ore-hosting Archean metamorphic rocks and/or regional Neoproterozoic Penglai Group low-grade metasedimentary rocks via extensive fluid-rock interactions. This inference is also supported by the newly reported near-zero δ34S values (–1.2 to 1.4‰) of pyrite from altered wallrocks, which are significantly comparable to the surrounding Archean plagioclase amphibolite. The monazite has relatively homogenous εNd(t=120Ma) values ranging from –19.3 to –17.6, which are much higher than the Nd isotope compositions of the ore-hosting Archean high-grade metamorphic rocks. Combining the widespread mafic dykes and previous relevant geochemical studies, it is proposed that the ore-forming fluids were probably derived from deeply mantle-related contents, followed by intense interaction with Archean basements at the shallow crustal level. In addition, the progressive decrease of δ34S values of pyrite and more appearances of iron oxides and barite from early to late mineralization stages, are ascribed to gradual fluid oxidation by extensive carbonatization of Fe-rich Archean wallrocks.

An Early Paleoproterozoic back-arc system along the southern margin of the Yinshan Block: Evidence from a newly-defined bimodal volcanic sequence in the Daqingshan Complex, Khondalite Belt

As one of the 2.1 to 1.9 Ga orogenic belts that welded the Columbia supercontinent, the Khondalite Belt in the North China Craton is a typical continent-continent collisional orogen that formed through the collision between the Yinshan and Ordos Blocks. Previous studies mostly focused on the collisional event in the Khondalite Belt but paid little attention to how the subduction system operated before the final closure of the ocean. To address this issue, we identified a series of interlayered meta-mafic and felsic rock assemblages in the Daqingshan Complex and implemented geochemical and geochronological analyses. Petrological and geochemical studies revealed that these rocks are bimodal and include plagioclase amphibolite (Group 1) and biotite plagiogneiss (Group 2). Geochemically, Group 1 samples show tholeiitic affinity, whereas Group 2 samples belong to the high-K calc-alkaline series. Geochemical data indicate that the protolith magma of Group 1 was most likely derived from the partial melting of lithospheric mantle with minor crustal contamination, whereas Group 2 rocks represent highly differentiated magma derived from the partial melting of ancient crustal materials. All the samples show depletion of HFSEs and enrichment of LILEs, indicative of a subduction-related magmatic arc environment. Zircon U-Pb dating results show that the protoliths of Group 1 samples yield crystallization ages of ∼2.47 Ga and metamorphic ages of 1.95 to 1.85 Ga, whereas the protoliths of Group 2 samples yield crystallization ages of ∼2.40 Ga and metamorphic ages of ∼1.85 Ga. Our new results and available geochemical, petrological, and isotopic data demonstrate that the bimodal volcanic sequence of the Daqingshan Complex was developed in a 2.47 to 2.40 Ga back-arc system along the southern margin of Yinshan Block. Subsequent collision between the Ordos and Yinshan Blocks resulted in the formation of the Khondalite Belt and final amalgamation of the Western Block between 1.95 and 1.85 Ga.

Petrogenesis of (meta-) basalts from the North Qilian Orogenic Belt, NW China: implications for the Palaeoproterozoic–Mesoproterozoic tectonic evolution of the North Qilian Block

AbstractThe North Qilian Orogenic Belt is surrounded by the Tarim Craton to the NW and the North China Craton to the NE. The Precambrian continental crust remnants that are distributed in the North Qilian Orogenic Belt are termed the North Qilian Block (NQB), and their tectonic evolution has profound implications for the evolution of the Columbia Supercontinent. Here we present major- and trace-element and Sr–Nd–Hf isotope data for (meta-) basalts from the Beidahe Group (BDHG) and Zhulongguan Group (ZLGG) in the western North Qilian Orogenic Belt, to investigate the tectonic evolution of the NQB during the Proterozoic Eon. The protoliths of Palaeoproterozoic amphibole gneisses and plagioclase amphibolites from the BDHG are calc-alkaline series basalts. These metabasalts show island-arc-basalt affinities with variable Nd and Hf isotopes (ϵNd(t) = −5.0–0.6 and 2.7–4.3;ϵHf(t) = −14.2–2.0 and 6.9–8.8) and were generated by partial melting of the asthenospheric mantle that was metasomatized by aqueous fluid and sediment melt in a continental-arc setting. The early Mesoproterozoic ZLGG basalts show features of shoshonite-series basalts and are geochemically similar to ocean-island basalts. These basalts show variable (87Sr/86Sr)i,ϵNd(t) andϵHf(t) values of 0.70464–0.70699, −1–2.6 and −1.5–5.7, and are products of mantle plume magmatism that participated with subducted oceanic crust in an intracontinental rift setting. This study suggests that the NQB underwent tectonic evolution from palaeo-oceanic subduction to intracontinental rifting during the Palaeoproterozoic–Mesoproterozoic eras. Furthermore, the above tectonomagmatic events were in response to convergence–splitting events of the Columbia Supercontinent during the Palaeoproterozoic–Mesoproterozoic eras.

Geochronological and geochemical constraints on the petrogenesis of late Mesoproterozoic mafic and granitic rocks in the southwestern Yangtze Block

Late Mesoproterozoic igneous rocks in the SW Yangtze Block are important for understanding the role of it in reconstruction of the Rodinia supercontinent. In the present study, we report new geochronological, geochemical, and Nd–Hf isotopic data for the Cuoke plagioclase amphibolites and granites in the SW Yangtze Block. Geochronological results show that the plagioclase amphibolites and granites have similar late Mesoproterozoic zircon U–Pb ages of 1168–1162 ​Ma, constituting a bimodal igneous assemblage. The plagioclase amphibolites have high and variable TiO2 contents (1.15–4.30 ​wt.%) and Mg# (34–66) values, similar to the tholeiitic series. They are characterized by enrichment in LREEs and LILEs, and have OIB-like affinities with positive Nb and Ta anomalies. The plagioclase amphibolites have positive whole-rock εNd(t) (+3.2 to +4.3) and zircon εHf(t) (+4.3 to +10.7) values, indicating that they were derived from an OIB-like asthenospheric mantle source. The granites belong to the reduced peralkaline A-type series and have negative εNd(t) value of −6.0 and εHf(t) values of −5.8 to −13.8, indicating a derivation from the partial melting of ancient mafic lower crust. In combination with the ∼1.05–1.02 ​Ga bimodal igneous assemblage in the SW Yangtze Block, we propose that the Cuoke 1168–1162 ​Ma igneous rocks were likely formed in a continental rift basin and argue against the existance of Grenvillian Orogen in the SW Yangtze Block during the late Mesoproterozoic.

Rifting and subduction records of the Paleo–Tethys in North Laos: Constraints from Late Paleozoic mafic and plagiogranitic magmatism along the Song Ma tectonic zone

Abstract In Southeast Asia, the Jinshajiang–Ailaoshan suture in Southwest China and the Song Ma zone in Vietnam were generally accepted as the amalgamation boundary of the South China and Simao/Indochina blocks. However, the tectonic location of the Paleotethyan suture in North Laos and its tectonic affinity remain ambiguous. To address this issue, we present the geochemical and geochronological data of a set of mafic and plagiogranitic rocks along the Song Ma suture zone in North Laos. Three groups of rocks with distinctive petrological, geochronological, and geochemical signatures were identified. The ca. 370 Ma Group 1 plagioclase amphibolite was preserved as xenoliths in the ca. 260 Ma granitoids. It exhibits a normal mid–oceanic–ridge basalt (N–MORB)-like, multi-elemental pattern with highly positive εNd(t) (+3.3 to +10.7) and εHf(t) (+6.68 to +7.41) values and low δ18O values (5.3‰−5.5‰), that are interpreted as products of the Late Devonian − earliest Carboniferous continental rift setting. The ca. 270 Ma Group 2 gabbro/diabase–diorite exhibits arc-like geochemical affinity with the εNd(t), εHf(t), and δ18O values of –5.85 to –3.76, –3.8 to +3.2, and 6.6‰–7.6‰, respectively. It is derived from the metasomatized wedge. The ca. 260 Ma Group 3 plagiogranite shows Nb-Ta and Ti negative anomalies and positive εNd(t) (+3.21 to +4.21), εHf(t) (+8.4 to +13.6), and δ18O (5.9‰−6.8‰) values, which are indicative of its derivation from mafic oceanic crust in a subduction setting. In combination with the published data, these results support the development of the Carboniferous–Permian Paleotethyan branch in North Laos, which represents the tectonic location of the Song Ma suture zone that spatially connects the Jinshajiang–Ailaoshan suture zone in Southwest China and the Chenxing–Bangxi suture zone in Hainan Island, South China.

The early Paleozoic oceanic island seamount in the Chencai area, Zhejiang Province: Implication of the Yangtze–Cathaysia amalgamation

Details of the collision between the Cathaysia and Yangtze blocks have long been debated. Detailed geological, geochemical, and C and O isotopic studies were carried out on the plagioclase amphibolite and marble in the Chencai area, Zhuji city, Zhejiang Province. The plagioclase amphibolite yields U–Pb age of 435 ± 11 Ma, which represents the metamorphic age, and the youngest age indicates that the plagioclase amphibolite formed after 463 Ma. The marble in the Chencai area yields U–Pb age of 431 ± 7 Ma and represents the metamorphic age, and the youngest inherited zircon indicates that the marble formed after 457 Ma. Combined with the two ages, we believed that they were both formed at early Paleozoic (after 457 Ma) and suffered metamorphism during the Caledonian (431–435 Ma). Meanwhile, REE and trace‐element characteristics show that the plagioclase amphibolite originated from the mantle source with the characteristics of OIB. The marble C and O isotopic characteristics indicated that the protolith of marble may be marine carbonate. The occurrence of the oceanic island seamount indicates that the Chencai Complex should be a subduction–accretion complex (CSAC), which formed at early Paleozoic. Given the presence of Caledonian granulite and garnet amphibolite in the Longyou area, amalgamation of the Cathaysia and Yangtze blocks was completed after 457 Ma. The CSAC is the product of the Yangtze–Cathaysia collision, with the collision causing widespread amphibolite‐facies metamorphism.

Petrology and Metamorphic P-T Paths of Metamorphic Zones in the Huangyuan Group, Central Qilian Block, NW China

The Central Qilian Block is a Precambrian block in the Qilian Orogen, which has long drawn international attention for the study of orogeny and continental dynamics. The Huangyuan Group in the Datong area is one of the Precambrian metamorphic basement units in the Central Qilian Block and reflects metamorphism in the Barrovian garnet zone and sillimanite zone from south to north. Based on detailed fieldwork, this study presents a systematic study of petrography, mineral chemistry and phase equilibria of schists and gneisses from the two metamorphic zones. The garnet metamorphic zone is composed of micaschist, garnet-bearing micaschist and felsic leptynite, with interlayered plagioclase amphibolite. The sillimanite metamorphic zone consists of garnet-bearing biotite micaschist, sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite. Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartine from core to rim. Garnet from the sillimanite metamorphic zone is almost homogeneous. Towards the outer rim, the contents of almandine and pyrope slightly decrease and grossular slightly increase. Biotite in both metamorphic zones is ferro-biotite. Plagioclase is oligoclase in garnet metamorphic zone and andesine in sillimanite metamorphic zone. Phase equilibrium modeling of a sample from garnet metamorphic zone resulted in a clockwise P-T path with a prograde stage (4.5–5.0 kbar, 520–530 °C), a peak P stage (9.8–10.2 kbar, 560–570 °C), a stage of thermal relaxation (8.0–8.5 kbar, 580–590 °C) and finally a retrograde stage (6.8–7.0 kbar, 560–580 °C). Thermodynamic modeling of a sample from the sillimanite metamorphic zone indicates a prograde stage (5.5–6.0 kbar, 540–550 °C) and a peak stage (7.8–8.5 kbar, 660–690 °C). The results indicate that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism, which resulted from continental-continental collision between the Qaidam Block and the Central Qilian Block.

Formation age and geodynamic setting of the Neoproterozoic Shalong iron formation in the Central Tianshan, NW China: Constraints from zircon U–Pb dating, geochemistry, and Hf–Nd isotopes of the host rocks

The recently discovered Shalong iron deposit in the Central Tianshan with a reserve of 14 Mt of iron at 25–30 wt.% Fe shows the typical iron formation geological characteristics: The host rocks are mainly metamorphosed volcanic rocks including quartz hornblende schist, mica quartz schist, and plagioclase amphibolite; the iron orebodies are stratiform and bedded, indicative of strata‐controlled deposits; metallic minerals are dominated by magnetite with haematite and ilmenite in minor quantities, whereas gangue minerals are quartz, amphibole, chloritoid, and plagioclase; and ore textures are banded and laminated. Here, we present LA‐ICP‐MS zircon U–Pb ages, whole‐rock geochemistry, Sm–Nd isotope, and zircon Lu–Hf isotope analyses on the host rocks in order to constrain the timing and geodynamic setting of the deposit. The geochemical characteristics, Sm–Nd isotope, and zircon Lu–Hf isotope compositions of the host rocks indicate that their protoliths were likely basic and felsic volcanic rocks. The bimodal kind rock types, the juvenile Nd–Hf isotopic characters, and the within‐plate trace element geochemical features of the host rocks together indicate that these rocks formed in a rift‐like extensional setting. Zircon U–Pb dating of the host rock interlayer suggests that the Shalong iron formation was formed in the Neoproterozoic (ca. 760 Ma). Combined with geological characteristics, close association with meta‐volcanic rocks, absence of glacial deposits, and within‐plate geochemical and juvenile isotopic signatures of the host rocks, the Shalong iron deposit is interpreted to be an Algoma‐type Neoproterozoic iron formation. The formation of this deposit can be linked to rift‐like volcanic activities related to the breakup of the Rodinia supercontinent. Our results in conjunction with previous studies suggest that the Central Tianshan should be related to the breakup of Rodinia, as indicated by the Neoproterozoic rift‐related igneous events of the Central Tianshan.

Geochemistry of metamorphosed volcanic rocks in the Neoarchean Qingyuan greenstone belt, North China Craton: Implications for geodynamic evolution and VMS mineralization

The Qingyuan greenstone belt (QGB) situated in the northeastern section of the North China Craton (NCC) is considered to be the unique metallogenic province in China that hosts both banded iron formations (BIFs) and volcanogenic massive sulfide (VMS) deposits. Traditionally, the QGB is divided into the Hunbei and Hunnan terranes separated by the Hunhe fault and grouped into the lower Shipengzi, middle Hongtoushan, and upper Nantianmen formations, which have undergone amphibolite- to granulite-facies metamorphism. Exposed supracrustal rocks are composed of plagioclase amphibolite and biotite plagioclase gneiss with minor amounts of BIF, marble, and schist. In this study, detailed field and petrographic observations, lithogeochemical, U-Pb geochronological, as well as whole-rock Sm-Nd isotope analyses were conducted on the metamorphosed mafic and felsic volcanic rocks (i.e. plagioclase amphibolite and biotite plagioclase gneiss, respectively) in the QGB in order to discuss implications for tectonic evolution and comitant VMS mineralization.The supracrustal rocks of both Hunbei and Hunnan regions were formed synchronously from 2.57 to 2.53 Ga and then had undergone 2.52–2.48 Ga regional metamorphism. Specifically, the lower section of the Shipengzi Formation is composed dominantly of transitional-tholeiitic basalts with negative Nb anomalies, suggesting that they were likely generated by partial melting of a mantle source that was previously altered by variable amounts of slab-derived fluids or melts. In contrast, the lithologic assemblages grade upward into interlayered bimodal volcanic rocks (i.e. N-MORB-type tholeiitic basalts and FI- to FII-type dacites of transitional to calc-alkalic affinities), representing a change in tectonic regime related to subduction from compression to extension. Among them all the basalts are characterized by consistently positive initial Nd compositions (εNd(t) = +2.7 to +3.4) further indicate a depleted mantle source; whereas those dacites possess negative to positive initial Nd values (εNd(t) = −4.6 to +3.0), suggesting they were formed by partial melting of depleted mafic source (likely lower mafic crust) and accompanied with variable addition of old crustal material. Significantly, VMS deposits are genetically linked to this extensional event and closely associated with the FII-type dacites that were generated by higher melting degree at high-temperature conditions and shallow crustal level (likely 10–15 km). In all, geochemical features (e.g., negative Nb anomalies and εNd value), together with contamination from continental crusts for tholeiitic mafic rocks and inherited zircons from these volcanic rocks, imply a continental arc setting where the QGB was evolved.