Andean-type Active Continental Margin Research Articles

Development of a Modern-Style Trench-Arc-Backarc System in the Proto-Tethys Ocean (Qilian Orogenic Belt, NW China)

Abstract The development of trench-arc-backarc (TABA) systems is uniquely associated with modern-style plate tectonics on Earth. The Qilian orogenic belt in NW China records the evolution history of the Proto-Tethys Ocean at the transition time from the Proterozoic to Phanerozoic. This paper presents systematic studies of petrography, U–Pb chronology and geochemistry on various rocks from a middle-ocean ridge (MOR)-type ophiolite belt, active continental margin and back-arc basin in the Qilian orogenic belt to address the development of a modern-style TABA system. Arc magmas include felsic intrusions with ages of 531 to 477 Ma and felsic-mafic arc volcanic rocks with ages of 506 to 439 Ma, showing distinctive features of typical magmatic rocks formed at an Andean-type continental margin. The back-arc basin is recorded by a 490- to 448-Ma suprasubduction zone (SSZ)-type ophiolite with boninite, and Silurian turbidite flysch formation. We establish a three-stage tectonic history from the initiation of subduction to the formation of a mature Japan-Sea-type back-arc basin at the active continental margin in the Early Paleozoic era. (1) Northward subduction of Proto-Tethys Ocean initiated and the Andean-type continental arc developed at ~530 to 500 Ma with continual crustal thickening; (2) a tectonic transition occurred from an Andean-type active continental margin to a West Pacific-type active continental margin at ~500 to 490 Ma with rapid thinning of crust to ~35 km; and (3) mature ocean basins and back-arc-basin (BAB) ophiolites were formed in the back-arc extensional environment at ~490 to 450 Ma.

“Where, when and why” for the arc-trench gap from Mesozoic Paleo-Pacific subduction zone: Sabah Triassic-Cretaceous igneous records in East Borneo

Establishing the location (where), timing (when) and reasons for the development (why) for the Mesozoic subduction along the western Paleo-Pacific that extended from Coastal South China to SE Vietnam has proven problematic due to the difficulty in the identification of the fore-arc igneous rocks. This paper firstly presents a set of zircon U-Pb and whole-rock 40Ar/39Ar geochronological, zircon in-situ Lu-Hf isotopes, and whole-rock elemental and Sr-Nd-Pb isotopic data for the Mesozoic igneous rocks from the Sabah ophiolites and Segama non-ophiolitic basement in NE Borneo. Our work documents major phases of the formation of these rock units at ∼185–140 Ma, ∼135–112 Ma and ∼130–85 Ma for the Telupid, Kudat and Darvel Bay ophiolites in Sabah, respectively. The mafic rocks within the ophiolites are classified into the MORB-, high-Nb, Nb-enriched and arc-like rocks, with the similar Sr-Nd (εNd(t) = +6.4–+10.2) and Atlantic-Pacific Ocean MORB-like Pb isotopic compositions. Their generation is related to input of slab-derived melt in the MORB mantle wedge source in an arc-trench gap setting. The Segama non-ophiolitic igneous rocks were dated at 251–178 Ma, with zircon in-situ εHf(t) = +9.5–+17.5, and are classified as high-Si and low-Si adakite, and high-mg andesite. They are the fractional products of the wedge-derived magma with the source being modified by slab-derived fluids. The magmatic flare-ups at ∼251–203, ∼185–153 Ma, ∼135–112 Ma and ∼95–85 Ma in Sabah are comparable with those in Coastal South China and West Borneo, indicating the Jurassic-Cretaceous accretionary orogenesis in Sabah. Sabah was tectonically located at the East Cathaysia margin of South China in the Mesozoic. It was a long-lived (>150 Ma) Andean-type active continental margin with multi-staged pulsed subduction and rollback in East Asia. Such a subduction system initiated no later than the earliest Triassic (∼251 Ma) and continued till the Late Cretaceous (∼85 Ma).

Nature of Paleozoic Basement of the Catalan Coastal Ranges (Spain) and Tectonic Setting of the Priorat DOQ Wine Terroir: Evidence from Volcanic and Sedimentary Rocks

The Paleozoic volcano-sedimentary rocks within the structural basement of the horst-and-graben system of the Catalan Coastal Ranges (CCR) are composed of black shale, volcaniclastic sediments, lava flows, sills and lithocrystalloclastic tuffs. Paleozoic sediments are depleted in high-field strength elements (HFSE) such as Nb, Ta, Zr, Hf and Ti, suggesting their accumulation within the Andean-type active continental margin environment. Volcanic rocks within the Paleozoic CCR sequence belong to shoshonitic and high-K volcanic series and are enriched in Cs, Rb and Ba and depleted in HFSE, which is consistent with their derivation from metasomatized (possibly through deep recycling of pelagic sediments) subduction-related mantle source. The presence of sills (sill-sediment complex) suggests that Paleozoic basement of the CCR was formed within the rifted active continental margin or an arc-back-arc basin system akin to the modern Western Pacific subduction configuration. This complex volcanic terroir hosts world-class wines of the Priorat DOQ region. The presence of framboidal pyrite and magnetite, siderite, sphalerite xenotime, (La–Ce–Nd)-monazite, zircon and baddeleyite, as well as cuprite, tenorite and cupriferous and native silver in volcanic-derived black shale (and consequently in the world-famous “llicorella” soil overlying it) may have had dramatic effects on wine quality and sensory characteristics. These mineral features, together with strong enrichment of Priorat shale in Au, Ag and, to a lesser extent, Pt could have pronounced effects on (1) rates and specific types of chemical reactions; (2) plant metabolism; (3) response to nutrient components and (4) determination of grape flavor. Volcanic wine terroirs, such as the Priorat DOQ region, are special geologic environments for wine growth, capable of producing unique wine aromatic and gustatory characteristics.

Geochemical evidence for a widespread Paleoproterozoic continental arc-back-arc magmatism in the Lesser Himalaya during the Columbia supercontinent assembly

The geodynamic evolution of the proto-northern Indian continental margin during Paleoproterozoic is widely debated. The proposed models include (i) a mantle plume triggered rift-related tectonic setting at a passive continental margin and (ii) an Andean type active continental margin. A widespread quasi-continuous belt of Paleoproterozoic (∼1.9–1.8 Ga) granitoids and orthogneisses, as well as mafic volcanic and intrusive rocks intercalated with metasedimentary rocks, occur all along the strike of Lesser Himalaya. A geochemical re-investigation is presented here using the existing geochemical data on mafic volcanic and intrusive rocks from the Lesser Himalayan Sequence of Himachal, Garhwal, and Nepal sectors to address the tectonic affiliation of these rocks and to infer the geodynamic setting of the proto-northern continental margin of the Indian Shield. These mafic magmatic rocks are classified as subalkaline basalt and basaltic andesite with calc-alkaline magmatic affinity to transitional character between calc-alkaline and tholeiites. Broad initial ɛNd range reveals a heterogeneous mantle source developed by mixing between a depleted mantle and a recycled component. Mantle source enrichment of these rocks is attributed to the interaction between the siliceous partial melt from the subducted sediment-laden oceanic crust and the mantle wedge, which is further supported by compositional similarity with experimental hybrid partial melts of eclogite-peridotite reaction. Estimated mantle potential temperature from the composition of these mafic rocks is comparable to that of mid-oceanic ridge basalts and does not invoke a mantle plume in their genesis. Geochemical proxies based on the major oxide abundances reveal the generation of these rocks from olivine-poor lithologies. It is suggested that these rocks were generated in a back-arc setting during the assembly of the Columbia supercontinent at the proto-northern continental margin of the Indian Shield. Distinct Nd isotopic systematics in these rocks compared to the Paleoproterozoic metamafic rocks of Cathaysia Block of South China Craton questions their purported linkage in the Columbia configuration.

A Mesozoic Andean-type active continental margin along coastal South China: New geological records from the basement of the northern South China Sea

Integrated analyses of in situ zircon U-Pb ages and geochemistry were conducted on both granitic and metasedimentary borehole samples from immediately below the Cenozoic sedimentary cover of the northern South China Sea basin. Our work suggests that basement rocks of the South China Sea basin are not Proterozoic or Paleozoic rocks, as previously speculated based on gravity-magnetic inversion work. The analyzed metavolcaniclastic and metasedimentary rocks were likely deposited at or soon after ca. 145 Ma but prior to ca. 85 Ma. A nearby source in coastal South China is proposed based on the generally immature nature of the metasedimentary rocks and the lack of detritus from older basement rocks farther inland. The vast majority of the metavolcaniclastic samples have sparse ca. 250–190 Ma (particularly 200–190 Ma) zircons. This magmatic quiescence is consistent with the existence of a coastal magmatic gap to the east of Hainan Island during this time, previously interpreted as reflecting a period of flat-slab subduction. Metaclastic samples mostly exhibit a dominant ca. 180–120 Ma zircon population, indicating extensive magmatism along the coastal areas. This finding, together with previously reported ca. 190–180 Ma I-type coastal granites and our newly dated ca. 115–98 Ma volcanic and granitic rocks under the western part of the South China Sea basin can best be interpreted by the former presence of an Andean-type continental arc during the Middle Jurassic – Early Cretaceous. Magmatism along this arc ceased soon after 90 Ma or before ca. 85 Ma, with its last pulse represented by a small group of ca. 100–85 Ma granitic intrusions in the eastern part of the northern South China Sea basin. The arc appears to have jumped toward the Western Pacific Ocean at or soon after 90 Ma, starting the development of the Western Pacific-type plate margin.

Origin of arc-like intraplate volcanism by melting of lithospheric mantle pyroxenite of the South China continental margin

Volcanic arcs usually produce silica-rich magmas that may modify the peridotitic sub-arc mantle, but direct observations on the lithology and geochemistry of the sub-arc lithospheric mantle are rare. An Andean-type arc system is inferred on the continental margin of southeastern China during the Mesozoic time; however, direct petrologic evidence for such an ancient arc is lacking. In this study, we have analyzed whole-rock major and trace elements, Sr–Nd–Pb–Hf isotope, and high-precision olivine geochemistry of the basaltic rocks of the Leizhou Peninsula and Hainan Island, on the South China continental margin. Unlike Hainan Island with only typical ocean island basalt (OIB)-type basalts, the Leizhou Peninsula has volcanic rocks ranging from typical OIB-type basalts to island arc basalt (IAB)-type with enrichment in large-ion lithophile elements and depletion in high-field-strength elements. The IAB-like rocks of the Leizhou Peninsula have less radiogenic Nd and Hf isotopes than the Hainan OIB-type basalts and the sub-continental lithospheric mantle. The IAB-like rocks can be explained by interaction of the Hainan plume with a subduction-derived component based on the plots of Nb/Nb* vs. Ce/Pb, Ba/Nb, Pb/Pb⁎, and SiO2. Olivine phenocrysts with anomalously high Ni and low Ca and Mn contents are consistent with a pyroxenite-rich mantle source for the IAB-like rocks. We suggest that the pyroxenite-rich mantle component of the IAB-like rocks formed by metasomatic reaction between silica-rich arc melts and the sub-continental lithospheric mantle during an ancient plate subduction event. We suggest that these arc-like volcanic rocks record melting of lithospheric mantle pyroxenite triggered by the Hainan plume. Our results are consistent with an Andean-type active continental margin related to subduction of the paleo-Pacific Plate beneath the southeastern China during the Mesozoic.

Late Palaeozoic magmatism of Northern Taimyr: new insights into the tectonic evolution of the Russian High Arctic

ABSTRACT We report new results of a comprehensive study into Carboniferous – Early Permian granite magmatism of the northern Taimyr Peninsula, Russia (southern part of the Kara Terrane). U-Pb zircon ages (SHRIMP and LA–ICP–MS) represent the crystallization ages of these intrusions and range from ca. 344.5 to 288.4 Ma, defining the onset of granite magmatism during the Early Carboniferous (Visean; ca. 344 Ma) and its termination in the Early Permian (Artinskian; ca. 288 Ma). Ar-Ar dating of micas and amphiboles indicates that Late Palaeozoic tectonic activity in northern Taimyr ceased during the Middle Permian (Roadian; ca. 272 Ma). Rb/Sr and Sm/Nd isotopic data from the same intrusions, show initial (87Sr/86Sr)i ratios of between 0.70288 and 0.71027, with εNd(t) values range between −3.3 and +3.1. The combined geochemical and isotopic compositions of the granites describe an affinity to peraluminous I-type granites, probably formed in an Andean-type active continental margin setting. Our new geochemical, isotopic, and geochronological data indicate the existence of a long-lived (ca. 56 Myr) Andean-type active continental margin along the southern edge of the Kara Terrane during the Late Palaeozoic. Furthermore, our study reveals a causal relationship between Late Palaeozoic magmatism across northern Taimyr and closure of the Uralian Ocean.

Composition of Pancarli Magmatic Ni-Cu±(PGE) Sulfide Deposit in the Cadomian-Avalonian Belt, Eastern Turkey

Pancarli Ni-Cu±(PGE) sulfide deposit occurs in the Neoproterozoic basement complex of the Bitlis massif, which is one of the Andean-type active continental margin fragments with arc-type assemblages represented by the Cadomian orogenic belt. Pancarli sulfides are associated with quartzo-feldspathic gneisses (country rock) and mafic intrusions (host rock). Composed of only semi-massive ore, the Ni-Cu±(PGE) sulfide deposit is a small-scale deposit, and it does not contain net-textured and disseminated ore. The mineral assemblage comprises pyrrhotite, pentlandite, and chalcopyrite. The semi-massive ore samples contain 2.2 wt.%-2.9 wt.% Ni, 0.8 wt.%-2.2 wt.% Cu (Cu/(Cu+Ni)=0.2-0.5) and 0.13 wt.%-0.17 wt.% Co. The Cu/Ni ratios (average 0.57) are consistent with the segregation of sulfides from a basaltic magma. Low Pt+Pd100%S values of 0.08 ppm-0.89 ppm, relatively low Pt/Pd ratios of 0.2-1.4, and Pd/Ir ratios of 4.5-39 have also been revealed. These values demonstrate that the magma reached S saturation before its emplacement and the mineralization with high Cu/Pd ratios formed by sulfides segregated from a PGE-depleted magma. δ34S isotope values (average -3.1‰) of Pancarli sulfides are lower than mantle source. Negative δ34S value indicates contamination from surrounding rocks. Concerning the composition, remobilization style and magma type, the Pancarli Ni-Cu±(PGE) sulfide deposit is similar to the deposits associated with Andean-type magmatic arcs located in the convergent plate margin settings.

Grenvillian orogeny in the Oulongbuluke Block, NW China: Constraints from an ∼1.1 Ga Andean-type arc magmatism and metamorphism

A systematic petrologic, geochemical, and zircon U-Pb and Lu-Hf isotopic investigation of an ∼1.1 Ga arc magmatism and metamorphism was studied to evaluate the Grenvillian orogeny and assembly of Rodinia in the Oulongbuluke Block in NW China. The peak metamorphic P-T conditions of the paragneiss were restricted to 4.8–5.8 kbar and 652–700 °C based on P-T pseudosection and conventional geothermobarometry. Zircon U-Pb ages of 1132 ± 33 Ma, 1118 ± 9 Ma, and 1124 ± 12 Ma were determined for the metabasite, augen granitic gneiss, and paragneiss, respectively, indicating magmatic intrusion and a metamorphism event in the late Mesoproterozoic. The metabasites exhibited enrichment of light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and depletion of heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs), resembling the geochemical characteristics of Andean arc magmatism. The whole-rock geochemistry and in-situ zircon εHf(t) values of 4.3 and 11.2 suggested that the protolithic mafic magma originated from a subduction-modified lithospheric mantle. The low zircon εHf(t) values of −5.2 and −0.5 also suggested that the calc-alkaline augen granitic gneisses were derived from the partial melting of a late Paleoproterozoic crustal source. The ∼1.1 Ga arc-related magmatism and metamorphism in the Oulongbuluke Block indicated the occurence of a tectonic-thermal event in an Andean-type active continental margin in the late Mesoproterozoic. In this regard, the Oulongbuluke Block was likely temporarily connected to the Qilian Block and Tarim Craton, forming a single continuous block with similar Grenville age during the late Mesoproterozoic to early Neoproterozoic (1.1–0.9 Ga).

Mid-Cretaceous aeolian desert systems in the Yunlong area of the Lanping Basin, China: Implications for palaeoatmosphere dynamics and paleoclimatic change in East Asia

The mid-Cretaceous constitutes a period of worldwide atmospheric and oceanic change associated with slower thermohaline circulation and ocean anoxic events, possible polar glaciations and by a changing climate pattern becoming controlled by a zonal planetary wind system and an equatorial humid belt. During the mid-Cretaceous, the subtropical high-pressure arid climate belt of the planetary wind system controlled the palaeolatitude distribution of humid belts in Asia as well as the spatial distribution of rain belts over the massive continental blocks at mid-low latitudes in the southern and northern hemispheres. Additionally, the orographic effect of the Andean-type active continental margin in East Asia hindered the transportation of ocean moisture to inland regions. With rising temperatures and palaeoatmospheric conditions dominated by high pressure systems, desert climate environments expanded at the inland areas of East Asia including those accumulated in the mid-Cretaceous of the Simao Basin, the Sichuan Basin, and the Thailand's Khorat Basin, and leading the Late Cretaceous erg systems in the Xinjiang Basin and Jianghan Basin. This manuscript presents evidences that allow to reinterpret previously considered water-laid sediments to be accumulated as windblown deposits forming part of extensive erg (sandy desert) systems. Using a multidisciplinary approach including petrological, sedimentological and architectural observations, the mid-Cretaceous (Albian-Turonian) Nanxin Formation from the Yunlong region of Lanping Basin, formerly considered to aqueous deposits is here interpreted as representing aeolian deposits, showing local aeolian-fluvial interaction deposits. The palaeowind directions obtained from the analysis of aeolian dune cross-beddings indicates that inland deserts were compatible with a high-pressure cell (HPC) existing in the mid-low latitudes of East Asia during the mid-Cretaceous. Compared with the Early Cretaceous, the mid-Cretaceous had extremely lower temperatures and pressure gradients, more arid climate, which is in accordance with the existing morphology of HPC, and the HPC was stable with little movement. Simultaneously, the deserts controlled by the mid-Cretaceous HPC were closer to the equator, indicating the shrinkage of the Hadley Cell relative to the Early Cretaceous.

Mid-Neoproterozoic amphibolite facies metamorphism at the northern margin of the Yangtze craton

The tectonic setting of the continental margin of the Yangtze craton during the Neoproterozoic has long been a subject of debate. The Douling Complex is a small segment of crystalline basement in the northern Yangtze craton; the study of which may provide useful insights into the tectonic evolution of this area. In this paper we report medium-grade regional metamorphism that occur in the Douling Complex. Petrographical textures and mineral compositions suggest that amphibolite facies rocks from the Douling Complex underwent a progressive metamorphic evolution from staurolite–kyanite grade to sillimanite grade. Conventional geothermobarometry defines P–T conditions to be 490–560 °C and 6–7.5 kbar for prograde metamorphic stage and 630–720 °C and 8–11 kbar for peak metamorphic stage. The post-peak evolution is characterized by decompression and thermal relaxation. Metamorphic zircons from garnet amphibolites and metapelites and magmatic zircons from a syn-deformed pegmatite pod yield U–Pb ages between 820 ± 5 Ma and 779 ± 3 Ma, with two populations at ca. 820–815 Ma and ca. 790–780 Ma, respectively. Biotites from metapelites also give two 40Ar/39Ar plateau ages of 814 ± 4 Ma and 790 ± 5 Ma. It is inferred that peak metamorphism occurred at ca. 820–815 Ma and subsequent cooling took place at ca. 790–780 Ma. Considering the distinct Precambrian crustal history of the Douling Complex, together with medium-grade amphibolite facies metamorphism accompanied by a clockwise P–T path, we propose that these observations provide evidence for the collision of the small Douling block with the northern margin of the Yangtze craton during the Mid-Neoproterozoic. Immediately after this collision, a newly accreted Andean-type active continental margin may have formed along the northern Yangtze craton.

Neoproterozoic backarc basin on the southeastern margin of the Yangtze block during Rodinia assembly: New evidence from provenance of detrital zircons and geochemistry of mafic rocks

Several tectonic models have been proposed for the Neoproterozoic amalgamation of the South China block during the assembly of Rodinia. However, the timing of the end of arc magmatism between the two subblocks in the South China block (i.e., the Yangtze and Cathaysia blocks) remains controversial because it is unclear whether the 860−820 Ma magmatic rocks and coeval sedimentary basins in this area are related to subduction or plume activity. Here, we present new detrital zircon U-Pb and Hf isotopic data for the sediments directly overlying early Neoproterozoic arc volcanic rocks in this region. These data reveal a rhythmic change in source coincident with a progressive increase in the amount of juvenile and old crustal detritus within these sediments. This result, combined with the presence of a fining-upward grain-size trend and horizontal bedding within these sediments, provides evidence of bidirectional sources that are consistent with a backarc setting. The juvenile crustal material within these sediments was sourced from adjacent arc terranes to the east, whereas the old crustal detritus was derived from the Yangtze block to the west. In addition, sensitive high-resolution ion microprobe zircon U-Pb dating of mafic rocks within equivalent sedimentary sequences yielded ages of ca. 860−840 Ma, and these mafic rocks have arc- or mid-ocean-ridge basalt−like geochemical features that indicate the initiation of backarc spreading associated with Neoproterozoic NW-directed subduction. The data from the sediments and mafic rocks suggest the presence of a backarc basin system at ca. 860−820 Ma within the southeastern margin of the Yangtze block. This in turn indicates that Rodinia assembly was not completed until ca. 820 Ma, with the South China block possibly acting as a connection between a Neoproterozoic Andean-type active continental margin and Grenvillian belts on the paleo−western margin of the Rodinia supercontinent.

Neoarchean arc magmatism and crustal growth in the north-eastern North China Craton: Evidence from granitoid gneisses in the Southern Jilin Province

Late Neoarchean granitoid gneisses are widespread in the Jiapigou–Jingyu area of the Southern Jilin Province (SJP) of the North China Craton (NCC). These rocks are composed of tonalitic–trondhjemitic–granodioritic–dioritic (TTGD) gneisses and preserve important information on the geodynamic evolution and Archean crustal growth along the northeastern margin of the NCC. Here we present LA–ICP–MS zircon U–Pb isotopic ages which reveal that the magmatic precursors of the tonalitic–trondhjemitic gneisses were emplaced at ∼2578–2574Ma, and the granodioritic–quartz dioritic magmas crystallized at ∼2546–2524Ma. The rocks were subjected to regional amphibolite facies metamorphism at ∼2510–2490Ma.Geochemically, the tonalitic–trondhjemitic gneisses can be subdivided into two subgroups. The magmas of high-Yb subgroup were derived by partial melting of dominantly ancient basaltic materials at the crustal level in the stable field of amphibole, clinopyroxene and significant amounts of garnet, and subsequently subjected to varying degrees of fractionation. Magmas of the low-Yb subgroup were generated by partial melting of dominantly juvenile basaltic materials with minor greywackes at the lower crustal level in the stability field of garnet and rutile, with a possible eclogitic residue. The magmatic precursors of granodioritic gneisses were produced by partial melting of subducted oceanic slabs and minor sediments, with the melts contaminated by the mantle wedge materials during their ascent. The parental magmas for the quartz dioritic gneisses were possibly generated by partial melting of a depleted mantle source that was significantly metasomatized by slab-derived fluids or melts.Combined with their petrogenesis, ∼2578–2574Ma tonalitic–trondhjemitic gneisses display positive zircon εHf(t2) values (+0.4 to +6.2) with TDM(Hf) ages of 2856–2643Ma, suggesting a crustal reworking process. On the other hand, ∼2546–2524Ma granodioritic to quartz dioritic gneisses have highly depleted zircon εHf(t2) values (+4.5 to +7.7) and TDM(Hf) ages of 2682–2550Ma that are close to their magmatic crystallization ages, indicating a significant crustal growth. Integrated with our recent studies on ∼2683–2654Ma metavolcanic rocks in Helong granite-greenstone belt and voluminous ∼2588–2536Ma metavolcanics in Jiapigou granite-greenstone belt, we infer that the Southern Jilin Province experienced Neoarchean subduction-related evolution history, with an Andean-type active continental margin along the northern margin of the NCC, and witnessed two major episodes of subduction-related crustal growth at ∼2.7Ga and ∼2.6–2.5Ga, with the latter period coupled with reworking of old crustal materials.

Geochemical and Hf isotopic compositions of Late Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China: petrogenesis and tectonic implications

ABSTRACTLate Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China, are located along the southern margin of the Mongol–Okhotsk orogenic belt. They comprise granodiorite, monzogranite, syenogranite, and lesser gabbro–diorite, of adakitic and calc­alkaline affinity. The adakite-like and calc­alkaline granites share similar light rare earth elements (LREE) characteristics; however, their heavy rare earth elements (HREE) trends differ from one another. The relative abundances of HREE in the calc­alkaline granites are relatively consistent and are similar to those of intrusive rocks formed from dehydration melting of garnet-free amphibolitic source rocks at relatively low pressures. In contrast, the adakite-like granites show more prominent HREE fractionation trends, indicating that they crystallized at higher pressures, where garnet in the source rocks was stable. At least two isotopically distinct sources were involved in the petrogenesis of the granites, but the extent to which they contributed varies between plutons. Most intrusions have incorporated an isotopically primitive component, possibly juvenile mafic crust. The other sources include a small proportion of old continental crustal material and isotopically evolved wall rocks. The gabbro–diorites have high MgO contents (>7 wt.%), a high Mg# (>0.6), and show moderate LREE and HREE fractionation, indicating they formed from the melting of subducted metasomatized lithospheric mantle. All of the intrusions in the study area are characterized by a relative enrichment in large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), indicating they were emplaced in an Andean-type active continental margin setting related to southward subduction of the Mongol–Okhotsk oceanic plate.

Early Cretaceous bimodal volcanism in the Duolong Cu mining district, western Tibet: Record of slab breakoff that triggered ca. 108–113 Ma magmatism in the western Qiangtang terrane

We report new zircon U–Pb ages and Hf isotope compositions, and whole-rock major and trace element and Sr–Nd isotope data for the Meiriqiecuo Formation (MF) bimodal volcanic rocks collected from the Duolong Cu mining district (DCMD) in the western Qiangtang terrane (QT), western Tibet. These data provide important constraints on the petrogenetic evolution and geodynamic setting of Early Cretaceous magmatism in the DCMD. The MF bimodal volcanic rocks are mainly basaltic andesite and andesite, with subordinate rhyolite. Four mafic samples yielded zircon U–Pb ages of ca. 108.2–113.0Ma, and one silicic sample has an age of 109.3±2.2Ma, indicating that the mafic and silicic eruptions were contemporaneous. The MF bimodal volcanic rocks belong to the medium-K calc-alkaline to shoshonite series. The rocks show arc-type affinities characterized by significant enrichment in light rare earth (LaN/YbN=7.74–12.60) and large-ion lithophile elements (Rb, Cs, K, and Pb), but depletions in the high-field-strength elements (Nb, Ta, and Ti), which geochemically resemble Andean arc basalts. Therefore, the MF bimodal volcanic rocks were likely emplaced at an Andean-type active continental margin and represent an Early Cretaceous magmatic arc that was located at the western QT margin. Moreover, the mafic volcanic rocks have high initial Sr isotopic ratios (0.705269–0.705413) and negative εNd(t) values of −1.5 to −0.6 compared with the silicic volcanic rocks ((87Sr/86Sr)i=0.704770–0.704903; εNd(t)=+1.2 to +1.3). Zircons from silicic samples have significantly higher εHf(t) values (+11.6 to +15.5) and predominantly lower Paleoproterozoic Hf crustal model ages (TDMC=180–428Ma) than the mafic samples, which have variable εHf(t) values of +3.4 to +13.0 and TDMC ages of 346–952Ma. These results indicate that the mafic and silicic end-members of the MF bimodal suite were generated from mantle and crustal sources, respectively. The basaltic andesite and andesite may have been derived from mantle enriched by the metasomatism of subducted fluids, whereas the rhyolite could have been derived by partial melting of mafic juvenile crust that originated from an older and more depleted mantle. In light of the geochemical characteristics and field relationships, we propose that breakoff of the Bangong–Nujiang oceanic lithosphere was responsible for the generation and emplacement of the MF bimodal volcanic rocks. The fact that the MF bimodal volcanic arc magmatism was active at ca. 108–113Ma indicates that it was associated with closure of the Bangong–Nujiang Ocean via an arc-arc “soft” collision during the Early Cretaceous.

Neoarchean Andean-type active continental margin in the northeastern North China Craton: Geochemical and geochronological evidence from metavolcanic rocks in the Jiapigou granite-greenstone belt, Southern Jilin Province

The Jiapigou granite-greenstone belt of Southern Jilin Province is located at the northeastern margin of the North China Craton (NCC). The Neoarchean metavolcanic rocks of this region provide strong lines of evidence in petrology and geochemistry for Neoarchean crust–mantle geodynamic evolution and significant crustal growth events. The metavolcanic rock assemblage is dominated by amphibolite, hornblende/garnet biotite plagioclase gneiss, and minor felsic gneiss. Zircon LA–ICP–MS U–Pb isotopic analyses reveal that these supracrustal rocks may be formed by two episodes of magmatism. The first magmatic episode occurred at ∼2688Ma and was recorded by minor felsic gneiss. The second episode occurred at ∼2588–2536Ma and was marked by a major lithological assemblage of metavolcanic rocks in this area. All the supracrustal rocks experienced amphibolite facies metamorphism at ∼2510–2480Ma.Except for the felsic gneiss, seventeen metavolcanic rock samples can be geochemically classified into three groups. Group #1 has tholeiitic basaltic chemical composition, and is characterized by slightly LREE-enriched chondrite-normalized REE pattern with low (La/Yb)N ratio and weakly negative Nb anomalies, similar to those of primitive arc tholeiite. Their magmatic precursor was most likely generated by partial melting of upper sub-arc mantle wedge slightly metasomatized by slab-derived fluids or melts in a subduction initiation environment. Group #2 with basaltic composition is akin to typical calc-alkaline basalt (CAB), showing fractionated chondrite-normalized REE patterns and evidently negative Nb, Ta and Ti anomalies. The magmatic precursor of the Group #2 was derived from partial melting of a sub-arc mantle wedge at relatively deeper levels, significantly metasomatized by slab-derived fluids or melts. Group #3 show chemical composition of andesite to dacite, and is distinguished by wide variations in SiO2, MgO and Mg# values, strongly fractionated chondrite-normalized REE patterns with higher (La/Yb)N ratios, low Yb and Y contents, and negative Nb, Ta, Ti and P anomalies, analogous to the Phanerozoic adakites. They might be derived from magma mixing between the mantle wedge melts metasomatized by slab-derived melts with perhaps an ancient crustal contribution.The felsic gneiss shows positive zircon εHf(t2) values (+2.2 to +4.5), together with ∼2.7Ga metamorphic volcanic rocks in the neighboring area, indicating a ∼2.7Ga crustal growth episode. Most of the metavolcanic rocks display positive εHf(t2) values of +0.4 to +7.4, the highest of which is close to the value of contemporaneous depleted mantle, suggesting a significant crustal growth event at ∼2.59–2.54Ga. Combined with numerous ∼2.83–2.65Ga inherited zircons and ∼2688Ma felsic gneiss, the Jiapigou granite-greenstone belt evolved at an Andean-type active continental margin, recording the transition from primitive arc to mature arc.

Carboniferous deposits in the basement of the southwestern West Siberian geosyneclise (Kurgan Region)

The 40Ar/39Ar age of volcanics from the sections of core boreholes in the northern Tyumen’-Kostanai trough (Kurgan Region) has been determined. The geologic structure of the northern part of the trough has been refined. The paleogeographic environments and geodynamic settings of formation of Carboniferous deposits have been established by sedimentological, petrological, and geochemical studies of the sections. In the Visean, marine environments with maximum basin depths were predominant. In the Kachar epoch (C1v3-s), the submarine relief of the basin became more complicated; blocks of older rocks appeared in the deposits. The geochemical indicator characteristics of the volcanics—contents of HFSE (Ti, Nb, Ta, Th, and Hf)—are close to those of andesitoids of island arcs and Andean-type active continental margins, as well as present-day andesites from the Kurile-Kamchatka zone. They are an order of magnitude lower than those in similar continental-rift volcanics, among which are volcanics of the Valer’yanovka zone (according to some researchers). The contents of Ni, Co, and V are similar to those in island-arc andesites, including rocks of the Kurile-Kamchatka zone. The Ba/Ta and Ba/La ratios and the proportions of Th, Hf, and Ta in andesitoids of the Valer’yanovka and Kachar Groups are close to the indicator characteristics of island-arc and active-continental-margin volcanics. The Kachar Group siliceous rocks are similar in the Rb/Zr-Nb, Fe-Nb, and Rb-(Y + Nb) correlations to rocks of mature island arcs and active continental margins. The incompatible-element and REE patterns for the Valer’yanovka and Kachar volcanics are typical of island-arc volcanics, including rocks from the Kurile-Kamchatka zone. The indicator geochemical features and petrology of the volcanics and the sedimentologic features of the deposits testify to their formation on an Andean-type active continental margin (western margin of the Kazakhstan paleocontinent) in the Early-Middle Carboniferous.

Zircon U–Pb–Hf isotope systematics and geochemistry of Helong granite-greenstone belt in Southern Jilin Province, China: Implications for Neoarchean crustal evolution of the northeastern margin of North China Craton

As one of the major Neoarchean granite-greenstone belts in China, the Helong terrane is located at the northeastern margin (Southern Jilin Province) of the North China Craton (NCC). It is dominated by metamorphic supracrustal rock assemblage and granitoid gneiss. The former contains amphibolite, hornblende plagioclase gneiss, biotite plagioclase gneiss and banded iron formation (BIF), whereas the latter is dominated by the Tonalite–Trondhjemite–Granodiorite (TTG) gneisses. Zircon LA–ICP–MS U–Pb isotopic analyses indicate that the supracrustal rock assemblage was formed at 2683–2654Ma and underwent regional metamorphism (lower amphibolite facies) at 2614–2553Ma before the widespread emplacement of plutonic granitoids at 2551–2532Ma. Both greenstone sequence and plutonic granitoid rocks experienced the subsequent regional metamorphism (greenschist to lower amphibolite facies) at 2508–2400Ma. The supracrustal rocks can be divided into three groups based on their lithological and geochemical characteristics. Detailed petrogenetic and geochemical studies suggest that the Group #1 amphibolite samples were derived from partial melting of depleted upper mantle that had been metasomatized by slab-derived fluids; Group #2 amphibolite samples were derived by partial melting of a depleted mantle wedge that was strongly metasomatized by subduction-derived fluids or melts; and Group #3 hornblende/biotite plagioclase gneiss samples were derived from partial melting of juvenile basalt and minor amounts of older sediments in the lower crust level. The granitoid gneisses in the Helong granite-greenstone belt can be divided into relatively high magnesium group (HMG) and low magnesium group (LMG). The HMG samples were derived from parental magmas generated by partial melting of juvenile basalt and minor amounts of older sediments within the lower crust level, whereas magmatic precursors of the LMG samples were generated by partial melting of a metapelite-dominated source under lower temperature melting conditions.U–Pb dating of inherited or captured zircons, combined with zircon Lu–Hf isotope data, indicates a significant ∼2.9–2.7Ga crustal growth episode, whereas the ∼2.55–2.53Ga magmatism involved crustal recycling in the Helong terrane. Consequently, the Helong terrane preserves a record of Neoarchean tectonic environment of Andean-type active continental margin.

Early-Cretaceous highly fractionated I-type granites from the northern Tengchong block, western Yunnan, SW China: Petrogenesis and tectonic implications

Western Yunnan, an important constituent of the southeastern segment of the East Tethyan tectonic domain, lies along a transformed orientation from the NWW trending Himalayan–Tethyan segment to the northerly trending Southeast Asian segment. However, the geodynamical setting of the Early Cretaceous tectonothermal magmatism along the Bangong–Nujiang–Lushui–Luxi–Ruili belt as the Tethyan branch in western Yunnan (SW China) remains controversial. The Donghe granitoid, which is located between the Gaoligong and Tengliang belts in the northern Tengchong block, reveals its petrogenesis and its tectonics, both of which play a vital role in resolving previous disputes. Our zircon laser ablation inductively coupled plasma mass spectrometry U–Pb dating of granites from the Donghe batholith yields ages of 119.9±0.9–130.6±2.5Ma. These granites display features typical of highly fractionated I-type granites: high SiO2 contents (>71wt.%), high K contents (K2O=3.88–5.66wt.%), calc-alkaline character, slight peraluminosity (A/CNK=1.02–1.16), and a highly differentiated index ranging from 83.6 to 95.6. In addition, as SiO2 contents increase, the rare earth element (REE) abundances, especially heavy REE abundances, and REE pattern slopes change gradually, but the negative Eu anomalies increase sharply, while the degree of enrichment in Rb, Th, U, and Pb and depletion in Ba, Nb, Sr, P, and Ti are enhanced. These features indicate that K-feldspar, ±plagioclase, ±biotite, ±amphibole, ±apatite, ±sphene/garnet, and ±Fe–Ti oxides such as ilmenite play the major role in the fractional crystallization process. The high initial 87Sr/86Sr (0.7067 and 0.7079) and negative εNd(t) values (−8.6 to −10.1), with T2DM ranging from 1.39 to 1.49Ga, indicate that the sources were mainly derived from the mature ancient middle to lower crust and minor mantle-derived materials. The initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.462–18.646, 15.717–15.735, and 38.699–39.007, respectively, signify that some subduction-related material such as ocean island volcanic rocks and mature arc primitive rocks may be involved as sources. Based on an analysis of similar zircon saturated temperature and geochemical characteristics of typical highly fractionated I-type granites in SE and SW China, and consideration of the regional geological setting, we suggest that the parent magma may be derived from the ancient middle to lower continental crust and mantle-derived basaltic magma. These were generated in the setting of a westward subducted Lushui–Luxi–Ruili (LLR) Tethyan oceanic slab, where mantle-wedge-derived sources provided enough heat and material to melt the middle to lower ancient crust. Taking into account the temporal–spatial distributions of Early Cretaceous magmatic rocks in the region, we further suggest the existence of an Andean-type active continental margin from the Lhasa block to the Tengchong block along the Bangong–Nujiang–LLR Tethys Ocean during the Early Cretaceous.

Neoproterozoic continental arc volcanism at the northern edge of the Arabian Plate, SE Turkey

New geochemical, Sr/Nd isotope and zircon U–Pb LA-ICP-MS data from the Derik Volcanics in the Southeast Anatolian Autochthone Belt of Turkey are consistent with an Andean-type Cadomian arc that developed along the northern edge of the Arabian Plate during the Late Neoproterozoic. The Derik Volcanics represent a volcanic complex including andesites, rhyolites and basalts, with volcanoclastic and fluvial sediments. They are unconformably overlain by playa sediments with Early Cambrian ichno-fossils, followed by Middle Cambrian-Silurian shallow marine deposits.Geochemically, the Derik Volcanics (DV) display transitional-calcalkaline affinities. On Th/Nb–Ti/Zr diagram, DV display negative trends from rhyolites to late to early-stage andesites and mafic dykes, respectively. The REE patterns are highly to moderately fractionated, with [(La/Yb)N=5.20–6.77 in late-stage andesites, 4.38–10.51 in rhyolites, 2.58–4.65 in the early-stage andesites, and 2.51–4.21 in mafic dykes]. Normalized trace element and REE diagrams display Th, La, Ce, Sm enrichment and depletion of Nb, Ti and Eu as is typical for Andean-type active continental margin igneous rocks. The enrichment of LILE and LREE, combined with depletion of HREE also suggests that the Derik Volcanic rocks were formed in relation with a subduction zone. Negative Eu anomalies (Eu/Eu)N, range from 0.50 to 1.81, indicates fractional crystallization of feldspar. The DV have positive ɛNd (+0.15 to +4.20) and mean model ages of 1.28Ga for the early-stage andesites, 1.34Ga for rhyolites, 1.35 for late-stage andesites, and 1.36Ga for mafic dykes. The positive ɛNd(T) data show that DV are product of mantle-derived magmas. Lower 143Nd/144Nd isotope ratios than the depleted MORB mantle (DMM) source indicate that the DV may have been contaminated by crustal material during magma genesis. Geochemical modeling shows that all four volcanic rock types may have been formed by different percentages of batch melting of DMM and subducting slab sources.Zircon LA-ICP-MS data give crystallization ages of 581.4±3.5Ma (n=7) and 559.2±3.2Ma (n=3) for the early and late-stage andesitic rocks, as well as ages of 569.6±1.6Ma (n=17), 571.6±1.9Ma (n=18), 575.4±4.3Ma (n=6) for the rhyolites.The geological and geochemical features together with the new age data suggest that the Derik Volcanics formed along a continental arc in the course of the southward subduction of Prototethys oceanic lithosphere along the northern margin of the Arabian Plate, which is attributed to a late-stage phase of the Cadomian Orogeny of the Pan-African cycle.