Ba Anomalies Research Articles

New evidence for Jurassic continental rifting in the northern Sanandaj Sirjan Zone, western Iran: the Ghalaylan seamount, southwest Ghorveh

ABSTRACT We address the growing controversy about the tectonic setting in which Jurassic magmatism of Iran occurred: arc or continental rift. In the Ghorveh area of the northern Sanandaj Sirjan zone (SaSZ), the Ghalayan metabasites are interlayered with marble and schist and locally cut by acidic dikes. Zircon U-Pb dating of the metabasitic rocks shows that these crystallized at ca. 145–144 Ma ago in the Late Jurassic (Tithonian). This complex was metamorphosed in the lower greenschist facies, however, some protolithic structures such as pillow lava and primary minerals are preserved. The metabasites are tholeiites with low SiO2 (45.6–50.5 wt.%), moderate Al2O3 (11.3–17.0 wt.%), and high TiO2 (0.7–2.9 wt.%) and Fe2O3 (9.4–14.1 wt.%). The Ghalayan metabasites are enriched in Light rare earth elements (LREEs) without significant Nb, Ta, Pb, Sr and Ba anomalies, similar to modern continental intra-plate tholeiitic basalts such as Afar and East African rifts. The Ghalaylan metabasites show wide ranges for 87Sr/86Sr(i) (0.7039–0.7077) and positive εNd(t) values (+0.1 to +4.6). These isotopic compositions are similar to those expected for slightly depleted subcontinental lithospheric mantle sources. Independently built discrimination diagrams indicate an intra-continental rifting regime for the source of Jurassic metabasites in the northern SaSZ. Geochemical and tectonic evidence suggests that rifting or a mantle plume was responsible for volcanic activity in the Upper Jurassic SaSZ. Considering the variation of ages of basaltic volcanism along the SaSZ, we suggest that Ghalayan basaltic magmatism reflected a submarine volcano that formed as part of the late stage continental rift, similar to Afar in the East African Rift system. Our results indicate that an extensional tectonic regime dominated SaSZ tectonics in the Middle to Late Jurassic.

Early Neoproterozoic evolution of Southeast Pakistan: evidence from geochemistry, geochronology, and isotopic composition of the Nagarparkar Igneous Complex

ABSTRACTRocks of the early Neoproterozoic age of the world have remained in discussion for their unique identity and evolutionary history. The rocks are also present in Sindh province of Pakistan and have been in debate for a couple of years. Yet, these igneous rocks have been studied very poorly regarding U-Pb and Lu-Hf age dating. The early Neoproterozoic rocks located in Nagarparkar town of Sindh have been considered as shoulder of Malani Igneous Suite (MIS) discovered in Southwest of India. The Nagarparkar Igneous Complex (NPIC) rocks are low-grade metamorphosed, mafic and silicic rocks. These rocks are accompanied by felsic and mafic dikes. Two types of granite from NPIC have been identified as peraluminous I-type biotite granites (Bt-granites), of medium-K calc-alkaline rocks series and A-type potash granites (Kfs-granites) of high-K calc-alkaline rocks series. Geochemical study shows that these Kfs-granites are relatively high in K and Na contents and low MgO and CaO. The Bt-granites have positive Rb, Ba, and Sr with negative Eu anomalies rich with HFSEs Zr, Hf, and slightly depleted HREEs, whereas Kfs-granites have positive Rb with negative Ba, Sr, and Eu anomalies and have positive anomalies of Zr and Hf with HREEs. In addition, these rocks possess crustal material, which leads to the enrichment of some incompatible trace elements and depletion of Sr and Ba in Kfs-granites and relatively high Sr and Ba in Bt-granites, indicating a juvenile lower continental crust affinity. Zircon LA-ICP-MS U-Pb dating of these granites yielded weighted mean 206Pb/238U ages ranging from 812.3 ± 14.1 Ma (N = 18; MSWD = 3.7); and 810 ± 7.4 Ma (N = 16; MSDW = 0.36) for the Bt-granites, and 755.3 ± 7.1 Ma (N = 21; MSDW = 2.0); NP-GG-01 and 736.3 ± 4.3 Ma (N = 24; MSWD = 1.05) for Kfs-granites, respectively. The Bt-granites and Kfs-granites have positive zircon εHf(t) values, which specify that they are derived from a juvenile upper and lower continental crust. Based on the geochemical and geochronological data, we suggest that the Bt-granites were formed through lower continental crust earlier to the rifting time, whereas the Kfs-granites were formed via upper continental crust, during crustal thinning caused by Rodinia rifting. These zircon U-Pb ages 812 to 736 Ma, petrographic, and geochemical characteristics match with those of the adjacent Siwana, Jalore, Mount Abu, and Sirohi granites of MIS. Thus, we can suggest that NPIC granites and adjacent MIS can possibly be assumed as a missing link of the supercontinent Rodinia remnants.

Geochronology and geochemistry of the Carboniferous Ulann Tolgoi granite complex from northern Inner Mongolia, China: Petrogenesis and tectonic implications for the Uliastai continental margin

This study presents zircon U–Pb ages and geochemical and Sr–Nd isotopic data for acid granites from the Uliastai continental margin, which is located in the Ulaan Tolgoi area of the south‐eastern Central Asian Orogenic Belt (CAOB). Although most samples have similar calc‐alkaline (K2O/Na2O = 0.55–1.70, average of 1.35) and peraluminous (A/CNK = 1.02–1.20 and A/NK = 1.09–1.87) characteristics, the granodiorites, monzogranites, and alkali feldspar granites exhibit distinct light rare earth element (LREE)/heavy REE (HREE) anomalies, including negative Nb, P, Ti, Sr, and Ba anomalies and negative εNd(t) values. These characteristics indicate that this complex massif comprises lower Early Carboniferous subduction‐related calc‐alkaline granodiorites (348 ± 1 Ma); an upper Early Carboniferous syn‐collisional segment characterized by S‐type, high‐K, calc‐alkaline, and peraluminous monzonitic granites (334 ± 1 Ma); and Late Carboniferous post‐orogenic extensional A2‐type peraluminous alkali feldspar granites (317 ± 1 Ma). Previous studies performed on coeval volcanic rocks and Early Palaeozoic granites in the same area have demonstrated that the Early Carboniferous granodiorites have the same geochemical characteristics as volcanic arc granites (VAGs), thus indicating that the northward subduction of the Hegenshan Ocean may have continued through the Early Carboniferous. The monzogranite and alkali feldspar granites, in conjunction with the Late Carboniferous lagged arc volcanic rocks observed in this area, suggest a syn‐collisional to post‐collisional setting. The new data presented in this study provide constraints on the timing of the transition from island‐arc magmatism to post‐collisional extension‐related magmatism in this region during the later stage of the Early Carboniferous.

Petrology of Metaluminous A-Type Rhyolite Discovered from Hadjer el Hamis Volcanoes (Lake Chad Basin)

Metaluminous (P.I. > 1) rhyolite from Hadjer el Hamis consisted of quartz, alkali feldspar, clinopyroxene (hedenbergite), amphibole (F-arfvedsonite) and oxides-hydroxides (ilmenite, magnetite, limonite) phenocrysts is characterized by the negative Eu, Ba, P, Sr and Ti anomalies. This metaluminous rhyolite and the early discovered peralkaline rhyolites in Hadjer el Hamis volcanoes derive likely from the same source, according to their coexistence on the same sector and their similar Zr/Nb ratios. The causes of magma heterogeneity are likely linked to varying amounts of extraction of an earlier melt phase or tectonic juxtaposition or a sudden increasing of fO2 in silicic magmas, triggered from a hydrothermal process, associated with F- and alkali-bearing fluids influx, which promoted the enrichment of Na in the hedenbergite rims and the crystallization of arfvedsonite.

Origins of two types of serpentinites from the Qinling orogenic belt, central China and associated fluid/melt-rock interactions

Serpentinites are important volatile and fluid mobile element repositories in oceanic lithosphere and subduction zones, and thus provide significant constraints on global geochemical cycles and tectonic evolution at convergent margins. In this contribution, two types of serpentinites from the Mianlue suture zone in the Qinling orogenic belt, central China, are identified on the basis of detailed mineralogical and geochemical study. Serpentinites from the Jianchaling region (Group 1) are composed of lizardite/chrysotile+magnesite+magnetite. Most of these serpentinites (Group 1a), consist of pseudomorphic orthopyroxene and olivine, and are characterized by low Al2O3/SiO2, high MgO/SiO2 and Ir-type PGEs to Pt ratios, suggesting a residual mantle origin. Meanwhile, the U-shape REE pattern and positive Eu, Sr and Ba anomalies of these serpentinites indicate that serpentinization fluids have interacted with gabbroic cumulates at moderately high temperatures or associate with the chlorinity and redox conditions of the fluid. Considering the limited mobility of U in the hydrating fluids for the Group 1a serpentinites, hydrating fluids for these serpentinites are most likely derived from the dehydrated slab, and have been in equilibrium with subducting sediments. There are also some serpentinites with low-grade metamorphic recrystallization from the Jianchaling region (Group 1b), represented by recrystallized serpentine minerals (antigorite). The trace element compositions of these Group 1b serpentinites suggest that partial dehydration of serpentinites associated with the transformation from lizardite to antigorite in subduction zone is also likely to affect the geochemistry of serpentinites. Serpentinites from the Liangyazi region (Group 2) are composed of antigorite+dolomite+spinel+magnetite. The high Cr number (0.65–0.80) and low Ti concentrations of spinels in Group 2 serpentinites indicate a refractory mantle wedge origin. Fertile major element compositions (e.g., high Al2O3 content and Al2O3/SiO2) and conjoint enrichment in light rare earth elements and high field strength elements, however, suggest melt-rock interactions before serpentinization. Combined with their geochemical affinity to “subducted serpentinites”, we conclude that their protoliths (refractory mantle wedge peridotite) experienced melt-rock interactions and then were incorporated into the subduction channel before serpentinization. Studies on these two types of serpentinites indicate that serpentinites from the orogenic belt are most likely characterized by multi-source, multi-stage and multi-genesis, further providing important constraints on subduction channel processes.

Geochemical constraints on the link between volcanism and plutonism at the Yunshan caldera complex, SE China

The Yunshan caldera complex is part of a larger scale, ca. 2000-km-long volcanic–plutonic complex belt in the coastal region of SE China. The volcanic rocks in the caldera complex are characterized by high-silica peraluminous and peralkaline rhyolites associated with an intracaldera porphyritic quartz monzonite pluton. In this study, we present zircon U–Pb, Hf and stable O isotopes along with geochemical data of both volcanic and plutonic rocks to evaluate the potential petrogenetic link between volcanism and plutonism in the Yunshan caldera complex. SHRIMP zircon U–Pb geochronology of both volcanic and plutonic rocks yields almost identical ages ranging from 95.6 to 93.1 Ma. The peraluminous and peralkaline rhyolites show negative anomalies of Sr, P, Ti and Ba and to a lesser extent negative Nb and Ta anomalies, along with positive Rb anomalies and ‘seagull-like’ rare earth element (REE) patterns with negative Eu anomalies and low (La/Yb)N ratios. The intracaldera porphyritic quartz monzonite displays minor negative Rb, Nb, Ta, Sr, P and Ti anomalies and a positive Ba anomaly with REE patterns characterized by relatively high (La/Yb)N ratios and lack significant Eu anomalies. The peraluminous and peralkaline rhyolites and the porphyritic quartz monzonite exhibit consistent e Nd(t) of − 3.7 to − 2.2 and display zircon e Hf(t) values of − 2.1 to 3.7. They further have similar, mantle-like, zircon oxygen isotopic compositions (δ18OVSMOW mainly = 4.63 to 5.76‰). We interpret these observations to be in agreement with a crystal mush model in which the parental magma of the volcanic and plutonic rocks of the Yunshan caldera complex was likely produced by interaction of asthenosphere melts with subduction-influenced enriched mantle wedge. The peralkaline rhyolites are interpreted to represent the most differentiated magma that has subsequently experienced significant fluid–melt interactions, whereas the porphyritic quartz monzonite may be representative of the residual crystal mush. The Yunshan rhyolites typically match the geochemical characteristics of ‘hot-dry-reduced’ rhyolites indicating that, during the late Cretaceous, the tectonic setting of SE China changed from a compressional environment to an extensional environment, i.e., from an arc into a back-arc setting. Our results imply that volcanic and plutonic rocks in caldera systems may provide unique constraints on the evolution of the magmatic system in which both the erupting melt and the residual crystalline material are being preserved.

Geochemical constraints on the spatial distribution of recycled oceanic crust in the mantle source of late Cenozoic basalts, Vietnam

This study presents a comprehensive analysis of the major and trace element, mineral, and Sr, Nd, Pb and Mg isotopic compositions of late Cenozoic intraplate basaltic rocks from central and southern Vietnam. The Sr, Nd, and Pb isotopic compositions of these basalts define a tight linear array between Indian mid-ocean-ridge basalt (MORB)-like mantle and enriched mantle type 2 (EM2) components. These basaltic rocks contain low concentrations of CaO (6.4–9.7wt%) and have high Fe/Mn ratios (>60) and FeO/CaO–3MgO/SiO2 values (>0.54), similar to partial melts derived from pyroxenite/eclogite sources. This similarity is also supported by the composition of olivine within these samples, which contains low concentration of Ca and high concentrations of Ni, and shows high Fe/Mn ratios. The basaltic rocks have elevated Dy/Yb ratios that fall within the range of melts derived from garnet lherzolite material, although their Yb contents are much higher than those of modeled melts derived from only garnet lherzolite material and instead plot near the modeled composition of eclogite-derived melts. The Vietnamese basaltic rocks have lighter δ26Mg values (−0.38±0.06‰) than is expected for the normal mantle (−0.25±0.07‰), and these values decrease with decreasing Hf/Hf* and Ti/Ti* ratios, indicating that these basalts were derived from a source containing carbonate material. On primitive mantle-normalized multi-element variation diagrams, the central Vietnamese basalts are characterized by positive Sr, Eu, and Ba anomalies. These basalts also plot within the pelagic sediment field in PbPb isotopic space. This suggests that the mantle source of the basalts contained both garnet peridotite and recycled oceanic crust. A systematic analysis of variations in geochemical composition in basalts from southern to central Vietnam indicates that the recycled oceanic crust (possibly the paleo-Pacific slab) source material contains varying proportions of gabbro, basalt, and sediment. The basalts from south-central Vietnam (12°N–14°N) may be dominated by the lowest portion of the residual slab that contains rutile-bearing plagioclase-rich gabbroic eclogite, whereas the uppermost portion of the recycled slab, including sediment and basaltic material with small amounts of gabbro, may be a major constituent of the source for the basalts within the central region of Vietnam (14°N–16°N). Finally, the southern region (10°N–12°N) contains basalts sourced mainly from recycled upper oceanic crust that is basalt-rich and contains little or no sediment.

The Mesoarchean Tiejiashan-Gongchangling potassic granite in the Anshan-Benxi area, North China Craton: Origin by recycling of Paleo- to Eoarchean crust from U-Pb-Nd-Hf-O isotopic studies

Mesoarchean and older potassic granites are important indicators of recycling of ancient continental crust early in Earth's history. This study of integrated whole rock and zircon geochemistry and geochronology reports the age and identification of the source materials of the >200km2 Mesoarchean Tiejiashan-Gongchangling granite in the Anshan-Benxi area, North China Craton, the largest pre-Neoarchean granite domain in the craton. SHRIMP U-Pb zircon dating on 15 samples indicates the magmatic crystallization of the granites between 2.95 and 3.0Ga and reveals a superimposed tectonothermal event at ~2.91Ga. The granites are characterized by high SiO2 and K2O, low CaO, FeOt, MgO and TiO2 with peraluminuous features. They show large variations in (La/Yb)n and strong negative Eu and Ba anomalies and Nb, P and Ti depletions. Whole rock Nd and magmatic zircon Hf isotopic compositions show large variations, but with most having εNd(t) and εHf(t) values <0, with tDM(Nd) and tDM(Hf) values varying from 3.3 to 3.9Ga and 3.3 to 4.0Ga, respectively. Magmatic zircons without very strong lead loss (discordance ≤20%) have δ18O values of +3.14 to +8.39. 3.3–3.7Ga xenocrystic zircons occur in some samples. The granite formed as a result of recycling of Paleo- to Eoarchean continental material in an intracontinental environment, with little if any contribution from Mesoarchean mantle sources. The sources could be predominantly unaltered ancient gneisses, together with yet to be identified Paleo- to Eoarchean materials affected by early low temperature alteration (weathered rocks or clastic sediment).

Untangling biogeochemical processes from the impact of ocean circulation: First insight on the Mediterranean dissolved barium dynamics

Based on an unprecedented dissolved barium (D_Ba) data set collected in the Mediterranean Sea during a zonal transect between the Lebanon coast and Gibraltar (M84/3 cruise, April 2011), we decompose the D_Ba distribution to isolate the contribution of biogeochemical processes from the impact of the oceanic circulation. We have built a simple parametric water mass analysis (Parametric Optimum Multiparameter analysis) to reconstruct the contribution of the different Mediterranean water masses to the thermohaline structure. These water mass fractions have then been used to successfully reconstruct the background vertical gradient of D_Ba reflecting the balance between the large‐scale oceanic circulation and the biological activity over long time scales. Superimposed on the background field, several D_Ba anomalies have been identified. Positive anomalies are associated with topographic obstacles and may be explained by the dissolution of particulate biogenic barium (P_Ba barite) of material resuspended by the local currents. The derived dissolution rates range from 0.06 to 0.21 μmol m−2 d−1. Negative anomalies are present in the mesopelagic region of the western and eastern basins (except in the easternmost Levantine basin) as well as in the abyssal western basin. This represents the first quantification of the nonconservative component of the D_Ba signal. These mesopelagic anomalies could reflect the subtraction of D_Ba during P_Ba barite formation occurring during organic carbon remineralization. The deep anomalies may potentially reflect the transport of material toward the deep sea during winter deep convection and the subsequent remineralization. The D_Ba subtraction fluxes range from −0.07 to −1.28 μmol m−2 d−1. D_Ba‐derived fluxes of P_Ba barite (up to 0.21 μmol m−2 d−1) and organic carbon (13 to 29 mmol C m−2 d−1) are in good agreement with other independent measurements suggesting that D_Ba can help constrain remineralization horizons. This study highlights the importance of quantifying the impact of the large‐scale oceanic circulation in order to better understand the biogeochemical cycling of elements and to build reliable geochemical proxies.

The source and tectonic implications of late Carboniferous–early Permian A-type granites and dikes from the eastern Alataw Mountains, Xinjiang: geochemical and Sr–Nd–Hf isotopic constraints

ABSTRACTZircon U–Pb ages, and geochemical, Sr–Nd and zircon Hf isotopic compositions are reported for the A-type granites and dikes in the Alataw Mountains of the northwestern Tianshan Orogenic Belt (NTOB), with the aim of investigating the sources and genesis of A-type granites and dikes. The laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating of A-type granites yielded a concordant weighted mean 206Pb/238U age of 297.4 ± 1.5 and 300.6 ± 0.9 Ma, respectively, defining a late Carboniferous–early Permian magmatic event. Geochemically, the granitic intrusions and dikes are characterized by high SiO2 and total alkalies (K2O + Na2O), high Zr, Nb, Ta content, and Ga/Al ratio with prominent negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate that the granitic intrusions and dikes in the eastern Alataw Mountains are of an A-type affinity. The depleted Nd isotope compositions of the granitic intrusions and dikes are consistent with those of the Carboniferous volcanic rocks in the Alataw Mountains, especially Carboniferous adakites (εNd(t) = +3.6 to +6.6), suggesting that they were likely generated by partial melting of less evolved crustal materials, such as oceanic crust stored in the middle and/or lower crust or Carboniferous volcanic arc crust. The widespread late Carboniferous–early Permian magmatism in the NTOB may have been related to a ridge subduction accompanied by slab roll-back of the subducting plate of the North Tianshan Ocean.

Paleoproterozoic magmatism across the Archean-Proterozoic boundary in central Fennoscandia: Geochronology, geochemistry and isotopic data (Sm–Nd, Lu–Hf, O)

The central Fennoscandia is characterized by the Archean-Proterozoic (AP) boundary and the Central Finland Granitoid Complex (CFGC), a roundish area of approximately 40,000km2 surrounded by supracrustal belts. Deep seismic reflection profile FIRE 3A runs across these units, and we have re-interpreted the profile and crustal evolution along the profile using 1.92–1.85Ga plutonic rocks as lithospheric probes. The surface part of the profile has been divided into five subareas: Archean continent (AC) in the east, AP, CFGC, boundary zone (BZ) and the Bothnian Belt (BB) in the west. There are 12 key samples from which zircons were studied for inclusions and analyzed (core-rim) by ion probe for U–Pb dating and oxygen isotopes, followed by analyzes for Lu–Hf by LA–MC–ICP–MS.The AC plutonic rocks (1.87–1.85Ga) form a bimodal suite, where the proposed mantle source for the mafic rocks is 2.1–2.0Ga metasomatized lower part of the Archean subcontinental lithospheric mantle (SCLM) and the source for the felsic melts is related plume-derived underplated mafic material in the lower crust. Variable degrees of contamination of the Archean lower crust have produced “subduction-like” Nb–Ta anomalies in spidergrams and negative εNd (T) values in the mafic-intermediate rocks. The felsic AC granitoids originate from a low degree melting of eclogitic or garnet-bearing amphibolites with titanite±rutile partly prevailing in the residue (Nb–Ta fractionation) followed by variable degree of assimilation/melting of the Archean lower crust. The AP plutonic rocks (ca. 1.88Ga) can be divided into I-type and A-type granitoids (AP/A), where the latter follow the sediment assimilation trend in ASI diagram, have high δ18O values (up to 8‰) in zircons and exhibit negative Ba anomalies (Rb–Ba–Th in spidergram), as found in sedimentary rocks. A mixing/assimilation of enriched mantle-derived melts with melts from already migmatized sedimentary rocks±amphibolites is proposed. The CFGC is characterized by both I-type and A-type (CFGC/A) intermediate and felsic granitoids. The I-type granitoids are divided into two groups at ≥1885Ma and ≤1882Ma, where the latter overlap in age with the CFGC/A granitoids. Both I-type CFGC and CFGC/A granitoids are interpreted to have formed from mixing of Paleoproterozoic SCLM-derived melts with crustal melts from hydrous and dry intermediate-felsic igneous sources, respectively. The geochemistry, dominantly δ18O values below 6.5‰ in zircons and TDM (2.11–2.42Ga) of the CFGC granitoids favor the occurrence of older crust (ca. 2.1–2.0Ga) in their genesis. The BZ granitoids are similar in age but more juvenile with TDM ages between 1.94Ga and 2.16Ga. The 1.92Ga granodiorite in the BB is correlated with juvenile gneissic tonalites and granodiorites found from the AP boundary.We suggest that the present high-velocity lower crust under the CFGC is composed of melt-extracted granulites (crustal source age≥2.0Ga) and mafic cumulates which both formed during 1.90–1.88Ga arc magmatism. The ≤1.88Ga stage represents the end of compression/transpression and is followed by 1.87–1.86Ga buckling, forming the Bothnian Oroclines.

Dados aeromagnéticos e Propriedades físico-químicas de alguns complexos da Província Alcalina de Goiás

The Goias Alkaline Province (GAP), located on the north edge of the Parana Basin, has alkaline complexes with strong magnetic signatures. The 3D inversion of magnetic signal was performed on the aeromagnetic data. Since the intrusions present a strong remnant magnetization, as measured in the laboratory, a careful analysis of this component was also realized. The sum vector of the remnant and induced components were used as a virtual induced magnetic field during the 3D inversion process. The geometric parameters were obtained from the quantitative analysis of magnetic data, using Total Gradient Direction and Analytical Signal Phase, performed over magnetic signal reduced to the pole. Magnetic susceptibility and density were measured in laboratory. All this information were used as the initial model in the inversion. The results of 3D inversion show that the alkaline intrusions have roots up to 10-12 km depth. The magnetic susceptibility is distributed in almost spherical and cylindrical shapes. The possibility of spherical shapes arise the hypothesis that the GAP intrusions at the northern part of the province represent magmatic chambers. The alkaline magma ascends from the lithosphere, and used two main fault systems as space for emplacement. These faults systems appear in the magnetic signal analysis as linear magnetic features. Chemical analysis confirmed the alkaline and subalkaline character of the magmas. The high MgO content shows the primitive character of these intrusions but the Ba anomaly indicates a possible crustal contamination.

Devonian alkaline magmatism in the northern North China Craton: Geochemistry, SHRIMP zircon U-Pb geochronology and Sr-Nd-Hf isotopes

The Wulanhada pluton is among the rare suite of Devonian alkaline plutons occurring along the northern margin of the North China Craton (NCC). The intrusion is mainly composed of quartz-monzonite. Here we report zircon SHRIMP U-Pb data from this intrusion which shows emplacement age of ca. 381.5 Ma. The rock is metaluminous with high (Na2O + K2O) values ranging from 8.46 to 9.66 wt.%. The REE patterns of the rocks do not show any Eu anomaly whereas the primitive-mantle-normalized spider diagram shows strong positive Sr and Ba anomalies. The Wulanhada rocks exhibit high initial values of (87Sr/86Sr)t = 0.70762–0.70809, low ɛNd(t) = −12.76 to −12.15 values and negative values of ɛHf(t) = −23.49 to −17.02 with small variations in (176Hf/177Hf)t (0.281873–0.282049). These geochemical features and quantitative isotopic modeling results suggest that the rocks might have been formed through the partial melting of Neoarchean basic rocks in the lower crust of the NCC. The Wulanhada rocks, together with the Devonian alkaline rocks and mafic-ultramafic complex from neighboring regions, constitute a post-collisional magmatic belt along the northern NCC.

Physicochemical parameters of crystallization differentiation and Fe–Ti ore-forming processes in the magmatic system of the Elet’ozero massif (Northern Karelia)

The rocks of the Elet’ozero intrusion are characterized by high relative abundances of incompatible elements, with (La/Lu) N varying from 2.1 to 36.6, and a distinct positive Ba anomaly, which may record crystallization of Ba minerals during the formation of mafic rocks. The anomalous Ba enrichment is probably associated with a large-scale fluid-mediated mass transfer during subduction of a lithospheric slab within the Karelian craton which was synchronous with the generation of magmas parental to the Elet’ozero pluton. The presence of Al-rich spinel in gabbroic rocks as early-formed magmatic phases suggests crystallization at relatively high pressures (~0.5 GPa or ~15 km depth). The oxygen fugacity changed with time during the evolution of the magmatic system, approaching its maximum values (approximately 1.5 log units above the quartz–fayalite–magnetite buffer) during extensive crystallization of Fe–Тi oxides. High oxygen fugacities, Ti enrichment of mafic rocks, and lack of any positive correlation between Mg-numbers and Ni content in olivine can be useful indicators of potentially mineralized mafic intrusions.

Chemical evidence for differentiation, evaporation and recondensation from silicate clasts in Gujba

The silicate and metal clasts in CB chondrites have been inferred to form as condensates from an impact-generated vapor plume between a metal-rich body and a silicate body. A detailed study of the condensation of impact-generated vapor plumes showed that the range of CB silicate clast compositions could not be successfully explained without invoking a chemically differentiated target. Here, we report the most comprehensive elemental study yet performed on CB silicates with 32 silicate clasts from nine slices of Gujba analyzed by laser ablation inductively coupled plasma mass spectrometry for 53 elements. Like in other studies of CBs, the silicate clasts are either barred olivine (BO) or cryptocrystalline (CC) in texture. In major elements, the Gujba silicate clasts ranged from chondritic to refractory enriched. Refractory element abundances ranged from 2 to 10×CI, with notable anomalies in Ba, Ce, Eu, and U abundances. The two most refractory-enriched BO clasts exhibited negative Ce anomalies and were depleted in U relative to Th, characteristic of volatilization residues, while other BO clasts and the CC clasts exhibited positive Ce anomalies with excess U (1–3×CI), and Ba (1–6×CI) anomalies indicating re-condensation of ultra-refractory element depleted vapor. The Rare Earth Elements (REE) also exhibit light REE (LREE) enrichment or depletion in several clasts with a range of (La/Sm)CI of 0.9–1.8. This variation in the LREE is essentially impossible to accomplish by processes involving vapor–liquid or vapor–solid exchange of REE, and appears to have been inherited from a differentiated target. The most distinctive evidence for inherited chemical differentiation is observed in highly refractory element (Sc, Zr, Nb, Hf, Ta, Th) systematics. The Gujba clasts exhibit fractionations in Nb/Ta that correlate positively with Zr/Hf and span the range known from lunar and Martian basalts, and exceed the range in Zr/Hf variation known from eucrites. Variations of highly incompatible refractory elements (e.g., Th) against less incompatible elements (e.g., Zr, Sr, Sc) are not chondritic, but exhibit distinctly higher Th abundances requiring a differentiated crust to be admixed with depleted mantle in ratios that are biased to higher crust/mantle ratios than in a chondritic body. The possibility that these variations are due to admixture of refractory inclusion-debris into normal chondritic matter is raised but cannot be definitively tested because existing “bulk” analyses of CAIs carry artifacts of unrepresentative sampling. The inferences drawn from the compositions of Gujba silicate clasts, here, complement what has been inferred from the compositions of metallic clasts, but provide surprisingly detailed insight into the structure of the target. Evidence that metal and silicate in CB chondrites both formed from impact-generated vapor plumes, taken together with recent work on metallic nodules in E chondrites, and on ordinary chondrites, indicates that chondrule formation occurs by this mechanism quite widely. However, the nature of the impact on the CB body is quite different than the popular conceptions of impact of partially or wholly molten chondritic bodies and the younger (5Ma) age of CB chondrules is consistent with origin in a disk with more evolved targets and impactors gravitationally perturbed by nascent planets.

Cretaceous magmatism and metallogeny in the Bangong–Nujiang metallogenic belt, central Tibet: Evidence from petrogeochemistry, zircon U–Pb ages, and Hf–O isotopic compositions

Porphyry Cu–Au and porphyry–skarn Cu±Au±Mo deposits are widely distributed in the Bangong–Nujiang metallogenic belt, central Tibet. Zircon U–Pb dating has revealed that Cretaceous ore-bearing intrusions related to Cu±Au±Mo mineralization formed in two periods (118–115Ma and 90–88Ma). These primarily high-K calc-alkaline series intrusions show light rare earth element enrichment (LaN/YbN=6.06–20.2) and negative to no Eu anomaly (Eu*/Eu=0.39–1.02). On primitive mantle-normalized diagrams, all the intrusions show strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb, Ta, and Ti, and negative Ba anomalies characteristic of arc magma. These intrusions show a wide range of zircon εHf(t) values from –6.3 to +10.9 and δ18O values from 5.82 to 10.82 ‰, suggesting variable contributions from mantle and crustal sources. Considering the ~100Ma Lhasa–Qiangtang collision, the 118–115Ma magmas and related deposits were likely formed by melting of subduction metasomatized mantle wedge in a continental arc setting during northward subduction of the Bangong–Nujiang ocean, and then further evolved in the upper crust as a result of MASH processes. The younger 90–88Ma ore-bearing magmas were potentially derived from melting of previously metasomatized lithospheric mantle during slab tear and break-off after the Lhasa–Qiangtang collision.

Zircon U–Pb geochronology and geochemistry of Late Cretaceous–early Eocene granodiorites in the southern Gangdese batholith of Tibet: petrogenesis and implications for geodynamics and Cu ± Au ± Mo mineralization

Cu ± Au ± Mo mineralization is found in multiple intrusive suites in the Gangdese belt of southern Tibet (GBST). However, the petrogenesis of these ore-bearing intrusive rocks remains controversial. Here, we report on mineralization-related Late Cretaceous-early Eocene intrusive rocks in the Chikang–Jirong area, southern Gangdese. Zircon U–Pb analyses indicate that the mainly granodioritic Chikang and Jirong plutons were generated in the Late Cretaceous (ca. 92 Ma) and early Eocene (ca. 53 Ma), respectively. They are high-K calc-alkaline suites with high SiO2 (64.8–68.3 wt.%) and Al2O3 (15.1–15.7 wt.%) contents. Chikang granodiorites are characterized by high Sr (835–957 ppm), Sr/Y (118–140), Mg# (58–60), Cr (21.8–36.6 ppm), and Ni (14.3–22.9 ppm), and low Y (6.0–8.1 ppm), Yb (0.54–0.68 ppm) values with negligible Eu anomalies, which are similar to those of typical slab-derived adakites. The Jirong granodiorites have high SiO2 (64.8–65.3 wt.%) and Na2O + K2O (7.19–7.59 wt.%), and low CaO (2.45–3.69 wt.%) contents, Mg# (47–53) and Sr/Y (14–16) values, along with negative Eu and Ba anomalies. Both Chikang and Jirong granodiorites have similar εHf(t) (7.6–13.1) values. The Chikang granodiorites were most probably produced by partial melting of subducted Neo-Tethyan oceanic crust, and the Jirong granodiorites were possibly generated by partial melting of Gangdese juvenile basaltic crust. In combination with the two peak ages (100–80 and 65–41 Ma) of Gangdese magmatism, we suggest that upwelling asthenosphere, triggered by the rollback and subsequent break-off of subducted Neo-Tethyan oceanic lithosphere, provided the heat for partial melting of subducted slab and arc juvenile crust. Taking into account the contemporaneous occurrence of Gangdese magmatism and Cu ± Au ± Mo mineralization, we conclude that the Late Cretaceous–early Eocene magmatic rocks in the GBST may have a significant potential for Cu ± Au ± Mo mineralization.

Aeromagnetic and physical-chemical properties of some complexes from Goiás Alkaline Province

The Goiás Alkaline Province (GAP), located on the north edge of the Paraná Basin, has alkaline complexes with strong magnetic signatures. The 3D inversion of magnetic signal was performed on the aeromagnetic data. Since the intrusions present a strong remnant magnetization, as measured in the laboratory, a careful analysis of this component was also realized. The sum vector of the remnant and induced components were used as a virtual induced magnetic field during the 3D inversion process. The geometric parameters were obtained from the quantitative analysis of magnetic data, using Total Gradient Direction and Analytical Signal Phase, performed over magnetic signal reduced to the pole. Magnetic susceptibility and density were measured in laboratory. All this information were used as the initial model in the inversion. The results of 3D inversion show that the alkaline intrusions have roots up to 10-12 km depth. The magnetic susceptibility is distributed in almost spherical and cylindrical shapes. The possibility of spherical shapes arise the hypothesis that the GAP intrusions at the northern part of the province represent magmatic chambers. The alkaline magma ascends from the lithosphere, and used two main fault systems as space for emplacement. These faults systems appear in the magnetic signal analysis as linear magnetic features. Chemical analysis confirmed the alkaline and subalkaline character of the magmas. The high MgO content shows the primitive character of these intrusions but the Ba anomaly indicates a possible crustal contamination.

Aeromagnetic data analysis and physical-chemical properties of some complexes from Goiás Alkaline Province

The Goias Alkaline Province (GAP), located on the north edge of the Parana Basin, has alkaline complexes with strong gravity and magnetic signatures. The 3D inversion of magnetic signal was performed on the aeromagnetic data. Since the intrusions present a strong remnant magnetizations as measured in the laboratory a careful analysis of this component was also realized. The sum vector of the remnant and induced components were used as a virtual induced magnetic field during the inversion process. The geometric parameters were obtained form the quantitative analysis of magnetic data using Total Gradient Direction and Analytical Signal Phase performed over magnetic signal reduced to the pole. Magnetic susceptibility and density were measured in laboratory. All those information were used as the initial model in the inversion. The results of 3D inversion show that the alkaline intrusions have roots up to 10-12 km depth. The magnetic susceptibility is distributed in almost spherical and cylindrical shapes. The possibility of spherical shapes arise the hypothesis that the GAP intrusions at the north part of the province represent magmatic chambers. The alkaline magma ascends from the lithosphere, used two main fault systems as space for emplacement. These faults systems appear in the magnetic signal analysis as linear magnetic features. Chemical analysis confirmed the alkaline and subalkaline character of the magmas. The high MgO content shows the primitive character of these intrusions but the Ba anomaly indicates a possible crustal contamination.

Geochronology, geochemistry, and zircon Hf isotopic compositions of Mesozoic intermediate–felsic intrusions in central Tibet: Petrogenetic and tectonic implications

Abstract Mesozoic intermediate–felsic intrusions are widely distributed in the southern Qiangtang terrane, central Tibet. Zircon U–Pb dating shows that these intrusions formed in two periods in the Jurassic (169–150 Ma) and Cretaceous (127–113 Ma). They mostly belong to the high-K calc-alkaline series, and show strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb, Ta, and Ti, and negative Ba anomalies on primitive mantle-normalized diagrams. P2O5 contents decrease with increasing SiO2 content, and Th contents increase with increasing Rb content, consistent with the evolution trend of I-type magmas. These intrusions show a wide range of zircon eHf(t) values from − 19.4 to 11.2, including in rocks with similar SiO2 contents, suggesting variable contributions from mantle and Qiangtang crustal sources. Fine-grained mafic to intermediate igneous enclaves in Jurassic intrusions have similar zircon U–Pb ages and similar or slightly higher zircon eHf(t) values to the host rocks, suggesting that the enclave magmas were derived from mixed magmas at depth and injected into more evolved magmas in upper crustal magma chambers. Magma mixing is also supported by the wide range of zircon Hf isotopic compositions (eHf(t) = − 19.4 to 2.5) from within individual Jurassic and Cretaceous intrusions, and Jurassic enclaves. The Jurassic–Cretaceous magmas likely formed in a continental arc setting during subduction of the Bangong–Nujiang ocean between 170 and 110 Ma, and evolved in the upper plate crust by MASH processes.