Lithium (Li) elemental and isotopic compositions of the Jurassic Jingshan leucogranites, including garnet-rich mafic enclaves and wall rock Wuhe gneisses from the southeast margin of North China ...Craton (NCC) were investigated to understand the behavior of Li isotopes during post-collisional magmatism. The Jingshan leucogranites have distinct U-shape REE patterns with Y and REE concentrations significantly lower yet Sr/Y ratios higher than their presumed source rocks, i.e., the Dabie-Sulu gneisses. Trace element modeling of REE and Sr/Y suggests these elemental signatures of the Jingshan leucogranites can be consistently explained by a fluid-present crustal incongruent partial melting: Bt+Qz+Pl+H2O=Grt+melt, leaving mainly Grt+Bt with minor allanite in the residuum. The mafic enclaves show identical Sr-Nd isotopic compositions with their host leucogranites, contrasting with the Wuhe gneiss and the exposed regional lower crust. The garnet-rich mafic enclaves are thus interpreted as entrained residual phases formed by this incongruent partial melting.
The Jingshan leucogranites show relatively high δ7Li values (+4.0‰ to +9.0‰) and low Li concentrations (4.7–11.3ppm) in comparison to published data for worldwide granites. In contrast, the residual enclaves show low δ7Li values (as low as +0.6‰) and high Li concentrations (as high as 118ppm). Garnet separated from residual enclaves is characterized by a narrow range of low δ7Li values (−1.5‰ to −0.1‰) with high Li concentrations from 32.9 to 81.7ppm. By contrast, coexisting quartz shows relatively high δ7Li values (+15.0‰ to +16.6‰) with very low Li concentrations (~1ppm). Biotite from both leucogranite and residual enclaves shows high Li concentrations (195–382ppm) and relatively heavy Li isotope compositions (+3.2‰ to +7.5‰). The Li elemental and isotopic signatures of the residual enclaves can be modeled as a Grt-Bt rich residuum mixed with leucogranite melt in various proportions. This work indicates that the Li isotopic compositions for magmatic rocks that are derived from anatexis of mid to lower crustal gneisses may not be a faithful source indicator as commonly suggested.
•Trace element modelling (REE and Sr/Y) suggests these elemental signatures of the Jingshan leucogranites can be consistently explained by a fluid-present crustal incongruent partial melting: Bt+Qz+Pl+H2O=Grt+melt, leaving mainly Grt+Bt with minor allanite in the residuum.•The Jingshan leucogranites show relatively heavy δ7Li values (+4.0‰ to +9.0‰) and low Li concentrations (4.7-11.3 ppm) in comparison to that for worldwide granites. In contrast, the residual enclaves show light δ7Li values (as low as +0.6‰) and high Li concentrations (as high as 118 ppm).•Garnet separated from residual enclaves spans a narrow range of low δ7Li values (-1.5‰ to -0.1‰) with high Li concentrations from 32.9 to 81.7 ppm.•This work indicates that the Li isotopic compositions for magmatic rocks that are derived from anatexis of mid to lower crust may not be a faithful source indicator as commonly suggested.
Pegmatite-type Li deposit is a critical metal, which has been attracting extensive attention. However, its petrogenesis and Li-mineralization mechanism remain under debate. Recently, four types of ...Late Triassic (222−212 Ma) pegmatites are recognized in the Chakabeishan area of Markam–Yajiang–Karakoram (MYK) giant Li ore belt in northern Tibet: tourmaline-, garnet-. spodumene-, and lepidolite-bearing pegmatites. The tourmaline-bearing pegmatites show uniform Nd isotopic compositions (εNd(t) = −13.8 to −14.5), closed to those of the metasedimentary wall rocks (εNd(t) = −15.2 to −17.8), but significantly lower than those of the surrounding Middle Triassic (∼ 244 Ma) syenogranites (εNd(t) = −4.7 to −5.5) that are typical I-type granites. These data, together with their higher Sm/Nd ratios, lower Y and LREE contents relative to the metasedimentary wall rocks, and lack of biotite, demonstrate that the tourmaline-bearing pegmatites were formed by the disequilibrium melting of the metasedimentary wall rocks involving water-absent muscovite dehydration melting with monazite in the residue. From the tourmaline-bearing pegmatites through the garnet-bearing pegmatites to the spodumene- and lepidolite-bearing pegmatites, gradually increasing whole-rock LiBe contents as well as decreasing K/Rb ratios and increasing Cs contents in the K-feldspar and muscovite suggest that they were generated by continuous evolution of magmas similar to the tourmaline-bearing pegmatites. In addition, mineralogical observation and variable Nb and Ta contents in muscovite indicate that the Chakabeishan pegmatites experienced a three-stage evolution process, i.e., magmatic stage for tourmaline- and garnet-bearing pegmatites, coexisting aqueous and silicic magma to hydrothermal transition stage for spodumene-bearing pegmatites, and hydrothermal stage for lepidolite-bearing pegmatites. Taking into account regional geological constraints and our data, we suggest that the low-degree partial melting of clay-rich sedimentary rocks initially control the Li enrichment. The subsequent exsolution and accumulation of a fluid phase during magmatic evolution process occurred in the Li-poor and Li-rich pegmatites, respectively, which is responsible for the further Li mineralization.
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•The barren pegmatites were formed by the muscovite dehydration melting of wall rocks.•The mineralization pegmatites were formed by the extreme differentiation.•Low-degree partial melting of Li-rich source fundamentally control Li mineralization.•A fluid phase dominated by H2O plays a role in Li mineralization.
The Cretaceous tectonomagmatism of the Korean Peninsula was examined based on geochemical and geochronological data of the Cretaceous plutonic rocks, along with distribution of volcano-sedimentary ...nonmarine N- to NE-trending fault bounded sedimentary basins. We conducted sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb ages and whole-rock geochemical compositions of 21 Cretaceous plutonic rocks, together with previously published data, from the central to southern Korean Peninsula. Four age groups of plutonic rocks were identified: Group I (ca. 119–106Ma) in the northern to central area, Group II (ca. 99–87Ma) in the central southern area, Group III (ca. 85–82Ma) in the central to southern area, and Group IV (ca. 76–67Ma) in the southernmost area. These results indicate a sporadic trenchward-younging trend of the Cretaceous magmatism in the Korean Peninsula. The Group I, II, and III rocks are dominated by high-K calc-alkaline I-type rocks with rift-related A-type granitoids. In contrast, the Group IV rocks are high-K calc-alkaline I-type plutonic rocks with no A-type rocks. The geochemical signatures of the entire groups indicated LREEs (light rare earth elements) enrichments and negative Nb, Ta, and Ti anomalies, indicating normal arc magmatism. A new tectonic model of the Cretaceous Korean Peninsula was proposed based on temporal and spatial distribution of the Cretaceous plutons represented by four age groups; 1) magmatic quiescence throughout the Korean Peninsula from ca. 160 to 120Ma, 2) intrusions of the I- and A-type granitoids in the northern and central Korean Peninsula (Group I plutonic rocks from ca. 120 to 100Ma) resulted from the partial melting of the lower continental crust due to the rollback of the Izanagi plate expressed as the conversion from flat-lying subduction to normal subduction. The Gyeongsang nonmarine sedimentary rift basin in the Korean Peninsula and adakite magmatism preserved in the present-day Japanese Islands supported the slab rollback followed by steepening of the Izanagi plate with an injection of upwelling of the hot asthenosphere into the mantle wedge. 3) Alternating shallow (from ~100 to 85Ma) to steep (from ~85 to 65Ma) subduction resulted in the migration of the normal arc magmatism in the southern Korean Peninsula, expressed as the intruded I- and A-type (Group III) and I-type granitoids (Group IV), respectively. The tectonomagmatism of the Korean Peninsula showed the unique style of evolution, different from those of South China and Japanese Islands.
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•Cretaceous plutonism of the Korean Peninsula was geochemically and geochronologically analyzed.•Four age groups of the Cretaceous plutonism were identified through temporal and spatial distributions of the plutonic rocks.•Conversion of flat to normal subduction and subsequent steepening and shallowing slab resulted in the Cretaceous plutonism.
The Alxa Terrane is situated in a key area between the North China and Tarim cratons. Paleozoic magmatic records in this terrane place important constraints on the subduction processes of the ...southern Paleo-Asian Ocean. New data of zircon U-Pb ages and whole-rock elemental and isotopic data reveal two groups of intermediate to felsic plutons in the Alxa Terrane. One group consists of diorites and granitoids that were emplaced at ca. 460–440Ma and characterized by lower Al2O3/TiO2 ratios and higher TiO2 contents, implying high temperature–low pressure crystallization conditions and a shallow source region. The second group is dominated by granitoids aged at ca. 420–407Ma and displays high Sr and Ba, low Y and high rare earth elements, with very high Sr/Y ratios and mostly positive Eu anomalies. These characteristics imply low temperature–high pressure crystallization conditions and source regions at deep crustal levels where garnet is stable in the residual phase. Both of the two groups are mostly calc-alkaline to high-K calc-alkaline, depleted in Nb, Ta and Ti and enriched in Ba, K and Sr, indicative of an arc affinity most likely related to the southward subduction of the Paleo-Asian Ocean. Zircon εHf(t) and whole-rock εNd(t) values of these magmatic rocks decrease from 458Ma to 440Ma and increase from 417Ma to 407Ma, whereas whole-rock initial 87Sr/86Sr ratios display an opposite trend. Such an isotopic change suggests a tectonic switch from an advancing to a retreating subduction regime at ~407Ma. Synthesized data from this and previous studies suggest that the 460–400Ma magmatic arc in the Alxa Terrane represented the western extension of the Paleozoic arc belt on the northern margin of the North China Craton.
•458–407Ma diorites and granitoids were recognized in the Alxa Terrane.•458–440Ma and 417–407Ma groups have different geochemistry and petrogeneses.•Southward subduction of the Paleo-Asian Ocean commenced in the early Paleozoic.•A tectonic switch from advancing to retreating subduction occurred at ~407Ma.•The Alxa had a close link with the North China Craton since the early Paleozoic.
Stable sulfur isotope ratios of mid-ocean ridge and ocean island basalts (MORBs and OIBs) preserve unique information about early Earth processes and the long-term volatile cycles between Earth's ...mantle and the surface. Icelandic basalts present ideal material to examine the oldest known terrestrial mantle reservoir, accessed through a deep-rooted mantle plume, but their multiple sulfur isotope systematics have not been explored previously. Here, we present new sulfur concentration (30–1570 ppm) and isotope data (δ34S = −2.5 to +3.8‰ and Δ33S = −0.045 to +0.016‰; vs. Canyon Diablo Troilite) from a sample suite (n = 62) focused on subglacially erupted basaltic glasses obtained from Iceland's neovolcanic zones. Using these data along with trace element systematics to account for the effects of magmatic processes (degassing and immiscible sulfide melt formation) on δ34S, we show that primitive (MgO > 6 wt.%), least degassed glasses accurately record the δ34S signatures of their mantle sources. Compared to the depleted MORB source mantle (DMM; δ34S = −1.3±0.3‰), the Iceland mantle is shown to have a greater range of δ34S values between −2.5 and −0.1%. Similarly, Icelandic basalts are characterized by more variable and negatively shifted Δ33S values (−0.035 to +0.013‰) relative to DMM (0.004±006‰). Negative δ34S-Δ33S signatures are most prominent in basalts from the Snæfellsnes Volcanic Zone and the Kverkfjöll volcanic system, which also have the lowest, most MORB-like 3He/4He (8–9 R/RA, where RA is the 3He/4He of air) and the highest Ba/La (up to 12) in Iceland. We propose that subduction fluid-enriched, mantle wedge type material, possibly present in the North Atlantic upper mantle, constitutes a low-δ34S-Δ33S component in the Icelandic mantle. This suggests that volatile heterogeneity in Iceland, and potentially at other OIBs, may originate not only from diverse plume-associated mantle components, but also from a heterogeneous ambient upper mantle. By contrast, a set of samples with high 3He/4He (up to 25.9 R/RA) and negative μ182W anomalies define an ancient lower mantle reservoir with a near-chondritic Δ33S and δ34S signature of ∼0‰. The difference between DMM and the high high-3He/4He mantle may reflect separate conditions during core-mantle differentiation, or a previously unidentified flux of sulfur from the core to the high-3He/4He reservoir.
•Multiple sulfur isotope data presented for 62 Icelandic lavas.•Effects of degassing and sulfide immiscibility on δ34S evaluated.•Undegassed Icelandic basalts show greater S isotopic variability than MORBs.•Negative δ34S-Δ33S values tied to a subduction fluid-modified mantle component.•A near-chondritic S isotopic signature suggested for the primordial mantle.
We report the first study of rodingite in the Ess ophiolite, exposed at the northeast end of the Yanbu suture of the Arabian Shield. The rodingite forms thin cross-cutting dykes in serpentinite and ...irregular blocks in ophiolitic mélange. Both dykes and blocks of rodingite are bounded by greenish chloritite blackwall zones and may contain relics of ophiolitic metagabbro. The mineral assemblage in rodingite is (hydro)garnet+vesuvianite+diopside+chlorite+Mn-ilmenite±titanite. The bulk chemical change from gabbro to rodingite involves loss of SiO2, K2O, Na2O, Al2O3, Fe2O3, and MgO compensated by a strong increase in CaO contents. Petrographic evidence indicates a two-stage process that formed diopside, hydrogarnet and chlorite first, followed by vesuvianite. We show that the major episode of rodingitization was contemporaneous with serpentinization, proceeding from temperatures near 400 °C down to about 250 °C, and predated the obduction of the Ess ophiolite section onto the Arabian Shield. This sequence of events is indicated by formation of chloritite blackwall, by shear planes cutting rodingite dikes, and by fragments of rodingite in fault breccias. We conclude that gabbroic protoliths were transformed to rodingite by Ca-metasomatism related to serpentinization of adjacent ultramafic rocks in an intraoceanic supra-subduction spreading environment. The calcium-rich hydrothermal solutions that drove rodingitization may have been derived from CO2-bearing slab fluids released by subducted carbonate sediments, modified by reaction with and serpentinization of ultramafic mantle lithologies. The presence of rodingite and associated chloritite blackwall therefore indicates a distinctive feature of serpentinization in suprasubduction zone oceanic lithosphere, namely Ca-rich fluid fluxes during hot serpentinization on the seafloor. Because such features predate and are not associated with obduction, it is likely that subducted suprasubduction zone oceanic lithosphere would contain similar assemblages, able to modulate the transport of volatiles, Ca, and trace elements into the deep mantle.
•First study of Rodingites from the Arabian Shield.•Rodingites are bounded by greenish chloritite blackwall zones.•Two stages of rodingitization are recognized in Ess rodingites.•The major episode of rodingitization was contemporaneous with serpentinization.•Gabbroic protoliths were transformed to rodingite by Ca-metasomatism.
New U-Pb geochronological, petrologic, elemental and Sr-Nd-Hf-O isotopic data for the granites from the Inthanon and Sukhothai zones in NW Thailand in conjunction with correlations with SW China are ...presented to constrain the age and position of the Paleotethys Ocean in this region and the associated assembly of Southeast Asia. The geochronological data show that the granitic rocks in the Inthanon and Sukhothai zones, herein named Group 1 and Group 2 granites, respectively, yield similar crystallization ages of 230–200Ma. Group 1 samples are characterized by monzogranite and granite with I- and S-type geochemical affinity and Group 2 samples by I-type monzogranite and granodiorite. They have generally similar chondrite-normalized REE and PM-normalized multi-element patterns but distinct Sr-Nd-Hf-O isotopic compositions. Group 1 samples have slightly higher initial 87Sr/86Sr ratios (0.7111–0.7293) but lower εNd(t) values (−11.1 to −14.1) than those of Group 2 samples (87Sr/86Sr(i)=0.7073–0.7278 and εNd(t)=−8.3 to −11.0). Group 1 samples show the lower εHf(t) values (−5.4 to −18.2), older TDM (1.62–2.40Ga) and higher δ18O values (+7.95 to +9.94) than those of Group 2 samples (εHf(t) of −11.1 to +4.80, TDM of 0.96–1.95Ga and δ18O of +4.95 to +7.98) for the Triassic crystallization zircons. These geochemical signatures are similar to the Kwangsian and Indosinian granites in the South China and Indochina blocks but distinct from those of the Gangdese I-type granite and Sibumasu Paleozoic granite. Our data suggest that Group 1 samples mainly originated from the early Paleozoic supracrustal rocks containing metapelite and metavolcanic components, which had previously experienced the surface weathering. Group 2 samples were derived from a hybridized source of an old metamorphic and a newly underplated mafic component. Synthesis of our data with available regional observations indicates that the Inthanon zone represents the main suture zone of the eastern Paleotethyan Ocean in NW Thailand and links with the Changning-Menglian suture zone in SW Yunnan (SW China). In NW Thailand, a switch from the eastward subduction of the Paleotethyan oceanic plate to the collision of the Sibumasu with Indochina blocks occurred at ~237Ma, and syn- and post-collisional time being at ~237–230Ma and ~200–230Ma, respectively. The late Triassic granites in the Inthanon and Sukhothai zones are representative of the post-collisional magmatic products.
•The granitic rocks in the Inthanon and Sukhothai zones yield the zircon U-Pb crystallization ages of 230–200Ma.•The Inthanon and Sukhothai granites show similar elemental but distinct Sr-Nd-Hf-O isotopic compositions, respectively.•The Inthanon and Sukhothai granites mainly originated from the supracrustal and underplating rocks, respectively.•The Inthanon zone represents the main suture zone of the eastern Paleotethyan Ocean in NW Thailand.•The syn- and post-collisional events in NW Thailand occurred at ~237–230Ma and ~200–230Ma, respectively.
We studied the texture of 256 chondrules in thin sections of 16 different carbonaceous (CV, CR, CO, CM, CH) and Rumuruti chondrites. In a conservative count ∼75% of all chondrules are mineralogically ...zoned, i.e. these chondrules have an olivine core, surrounded by a low-Ca pyroxene rim. A realistic estimate pushes the fraction of zoned chondrules to >90% of all chondrules. Mineralogically zoned chondrules are the dominant and typical chondrule type in carbonaceous and Rumuruti chondrites. The formation of the mineralogical zonation represents a fundamentally important process of chondrule formation. The classic typification of chondrules into PO, POP and PP might in fact represent different sections through mineralogically zoned chondrules. On average, the low-Ca pyroxene rims occupy 30vol.% of the entire chondrule. The low-Ca pyroxene most probably formed by reaction of an olivine rich chondrule with SiO from the surrounding gas. This reaction adds 3–15wt.% of material, mainly SiO2, to the chondrule. Chondrules were open systems and interacted substantially with the surrounding gas. This is in agreement with many previous studies on chondrule formation. This open system behaviour and the exchange of material with the surrounding gas can explain bulk chondrule compositional variations in a single meteorite and supports the findings from complementarity that chondrules and matrix formed from the same chemical reservoir.
There is a long-standing controversy regarding the tectonic division, composition and structure of the continental crust in the Da Hinggan Mountains and adjacent areas, which are mainly part of the ...southeastern Central Asian Orogenic Belt (CAOB). This paper approaches these issues via neodymium isotopic mapping of Paleozoic–Mesozoic (480 to 100Ma) granitoids. On the basis of 943 published and 8 new whole-rock Nd isotopic data, the study area can be divided into four Nd isotopic provinces (I, II, III and IV). Province I (the youngest crust, Nd model ages (TDM)=0.8–0.2Ga) is a remarkable region of Phanerozoic crustal growth, which may reflect a major zone for closures of the Paleo-Asian Ocean. Province II (slightly juvenile crust, TDM=1.0–0.8Ga), the largest Nd isotopic province in the southeastern CAOB, is considered to reflect the recycling of the initial crustal material produced during the early stage (Early Neoproterozoic) evolution of the Paleo-Asian Ocean. Province III (slightly old crust, TDM=1.6–1.1Ga) is characterized by ancient crustal blocks, such as the central Mongolian, Erguna, Dariganga and Hutag Uul–Xilinhot blocks, which represent micro-continents and Precambrian basements in the southeastern CAOB. Several parts of Province III are located along the northern margin of the North China Craton (NCC), which is interpreted as a destroyed cratonic margin during the Paleozoic and Mesozoic. Province IV (the oldest crust, TDM=2.9–1.6Ga) mainly occurs within the NCC and reflects its typical Precambrian nature. These mapping results indicate that the boundary between Provinces II and III (the northern margin of the NCC) along the Solonker–Xar Moron Fault can be regarded as the lithospheric boundary between the CAOB and NCC. Provinces I and II account for 20% and 44% of the area of the southeastern CAOB, respectively, and therefore the ratio of continental growth is 64% from the Neoproterozoic to the Mesozoic, which is typical for this part of the CAOB and distinguishes the CAOB from other Phanerozoic orogens in the world.
•Four provinces are determined by mapping of 951 Nd isotopic data in the study area.•These provinces provide evidence for division of tectonic units.•The juvenile crust (1000 to 200Ma) is account for ca. 64% area in the SE-CAOB.
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