Initial Sr Ratios Research Articles

Geochronology, Geochemical Characterization and Tectonic Background of Volcanic Rocks of the Longjiang Formation in the Lengjimanda Plate Area, Middle Da Hinggan Mountains

The Lengjimanda plate is situated in the middle section of the Da Hinggan mountains, in the eastern section of the Tianshan Xingmeng orogenic belt. To determine the formation age of the volcanic rocks in the Longjiang formation in this area, to explore their origin and tectonic background, and to reconstruct the geodynamic evolution of the region, this study conducted petrological, zircon U–Pb geochronological, geochemical, and isotopic analyses of the volcanic rocks in the Longjiang formation. The Longjiang formation’s volcanic rocks are primarily composed of trachyandesite, trachyte trachydacite, and andesite, which are intermediate basic volcanic rocks. They are enriched in large-ion lithophile elements, are depleted in high-field-strength elements, are significantly fractionated between light and heavy rare earth elements, and exhibit a moderate negative Eu anomaly in most samples. The results of the LA–ICP–MS zircon U–Pb dating indicate that the volcanic rocks in this group were formed in the Early Cretaceous period at 129.1 ± 0.82 Ma. The zircon εHf(t) ranges from +1.13 to +43.77, the tDM2 ranges from +655 to +1427 Ma, the initial Sr ratio (87Sr/86Sr)i ranges from 0.7030 to 0.7036, and the εNd(t) ranges from +2.1 to +6.6. Based on the geochemical compositions and isotopic characteristics of the rocks, the initial magma of the volcanic rocks in the Longjiang formation originated from the partial melting of basaltic crustal materials, with a source material inferred to be depleted mantle-derived young crustal. These rocks were formed in a superimposed post-collisional and continental arc environment, possibly associated with the Mongol-Okhotsk Ocean closure and the oblique subduction of the Pacific plate. This study addresses a research gap regarding the volcanic rocks of the Longjiang formation in this area. Its findings can be applied to exploration and prospecting in the region.

Petrogenesis and Rb-Sr Isotopic Characteristics of PaleoMesoproterozoic Mirgarani Granite Sonbhadra Uttar Pradesh India: Geodynamics Implication for Supercontinent Cycle

The Rb-Sr whole-rock isochron, age 1636 ± 66 Ma of Mirgarani granite, is the one of the oldest granite dated in the northwestern part of the Chhotanagpur Granite Gneiss Complex (CGGC). The initial Sr ratio is 0.715 ± 0.012 (MSWD = 0.11), showing an S-type affinity. The Mirgarani granite has intruded the migmatite complex of the Dudhi Group and forms the Mirgarani formation comparable to the granites of the Bihar Mica Belt around Hazaribagh (1590 ± 30 Ma). The present studies have established the chronostratigraphy of the Dudhi Group and adjoining areas in CGGC. Petro graphic and geochemical studies revealed that the granite is enriched in Rb (271 ppm), Pb (77 ppm), Th (25 ppm), and U (33 ppm) and depleted in Sr (95 ppm), Nb (16 ppm), Ba (399 ppm) and Zr (143 ppm) contents as compared to the normal granite. The Mirgarani granite is a peraluminous (A/CNK = 1.23), high potassic (K2O 6.42%), Calc-Alkalic to Alkali-Calcic {(Na2O+K2O)-CaO = 6.29} S-Type granite, a feature supported by the presence of modal garnet and normative corundum (2.68%). The Mirgarani granite is considered to have been formed by the anatexis of a crustal sedimentary protolith at a depth of approximately 30 km with temperatures ranging from 685-700 °C during the Co lumbian - Nuna Supercontinent.

Geochronology and Petrogeochemistry of Metallogenic Intrusions in the Shanmen Large‐scale Silver Deposit in the Siping Area of Jilin Province, NE China

AbstractThe Shanmen deposit, located in the Siping area of Jilin Province, is one of large‐scale silver deposits in Northeast (NE) China. Due to its high Ag grade, associated gold resources and special tectonic location, this deposit has important theoretical and ore‐prospecting significance. To present new data on the ore genesis, mineralization time and tectonic settings, the relationship between silver mineralization and intrusions in this deposit has been studied and some analyses have been carried out, including LA‐ICP‐MS zircon U‐Pb dating, whole‐rock major and trace element analysis and Sr‐Nd‐Pb isotope analysis of the granitoids associated with silver mineralization. Studies on deposit geology indicate that the ore‐hosting granodiorite and monzogranite have intimate genetic relationships with silver mineralization. LA‐ICP‐MS zircon U‐Pb dating results of the both intrusions are 167.6 ± 1.9 Ma and 167.0 ± 1.5 Ma, respectively, implying that the two intrusions and associated silver mineralization in the Shanmen deposit formed during the Middle Jurassic. Major element analytical data suggest that the two intrusions are high‐K calc–alkaline series I‐type granites and belong to metaluminous and peraluminous rocks. Both intrusions have similar REE characteristics with regard to the relative enrichment of LREEs, depletion of HREEs, obvious fractionation of LREEs and HREEs (ΣLREE/ΣHREE = 8.68–14.09, (La/Yb)N = 12.51–21.96), moderately negative Eu anomalies as well as weakly negative Ce anomalies (δEu = 0.56–0.71, δCe = 0.93–1.09). Moreover, the samples are generally enriched in LILEs and depleted in HFSEs. The rock assemblages and geochemical characteristics of granitoids in the Shanmen deposit suggest that they formed in an active continental margin associated with the Paleo‐Pacific plate subduction. The Sr‐Nd‐Pb isotope compositions show that both the granodiorite and monzogranite have low Sr initial ratios and high Sm/Nd initial ratios. The εNd(t) values and young Nd‐model ages are –3.2 to –1.7 and 990 Ma to 1110 Ma, respectively. Considering the Sr‐Nd isotope compositions, in conjunction with the relatively high 206Pb, 207Pb and 208Pb, alongside the relatively low 204Pb, it can be concluded that the Middle Jurassic intermediate‐acidic magma represented by the studied monzogranite and granodiorite in the Shanmen deposit, was derived from the partial melting of juvenile lower crust enriched in mantle‐derived materials and affected by the mixing of ancient crustal materials during the ascension and intrusion processes.

Tracking the Tectonic Evolution of the Junggar-Balkhash Ocean: A Case Study from the Post-Collisional Takergan Pluton in the West Junggar, Xinjiang

The Late Carboniferous and Early Permian igneous rocks are widely developed in the West Junggar, Xinjiang, which are considered to be related to the evolution of the Junggar-Balkhash Ocean. However, their tectonic settings have been controversial for a long time. With the aim of providing new evidence for the Late Paleozoic tectonic evolution of the West Junggar, we present petrology, zircon U-Pb chronology, whole-rock major and trace elemental and Sr-Nd isotopic data, to discuss the petrogenesis and tectonic setting of Takergan pluton from the Barleik Mountains in the West Junggar. The Takergan pluton is mainly composed of quartz diorite porphyry and quartz monzonite. The quartz diorite porphyry has low SiO2 (57.76 wt.%–57.81 wt.%), high total alkali contents (Na2O+K2O=6.29 wt.%–6.56 wt.%), and high Mg# values (45–46), with a zircon U-Pb age of 304±5 Ma. The quartz monzonite shows relatively high SiO2 (58.71 wt.%–64.71 wt.%), total alkali contents (7.73 wt.%–9.70 wt.%), and Mg# values (34–47), with the A/CNK values of 0.91–0.98, which belongs to shoshonitic and metaluminous I-type granite series. The quartz monzonite yields zircon U-Pb ages of 302±2 and 296±3 Ma, and is characterized by low initial Sr ratios of 0.703 97–0.704 09, high eNd(t) values of +6.8–+7.0, and young Nd model ages of 551–587 Ma. Both the quartz diorite porphyry and quartz monzonite are enriched in light rare earth elements and Rb, Th, U, K, and depleted in Nb, Ta, Ti, with different degrees of negative Eu anomalies. These features indicate that the Takergan pluton was most likely formed in a post-collisional setting by partial melting of a depleted mantle source that had been metasomatized by subduction-related fluids, with significant fractional crystallization and slightly contaminated by crustal materials. Combined with the widespread distribution of the coeval stitching plutons, the occurrences of terrestrial Late Carboniferous to Permian volcano-sedimentary formations, and the absence of subduction-related rocks later than Early Carboniferous, it is believed that the Junggar-Balkhash Ocean was closed at about 320 Ma, and the central West Junggar has transformed to a post-collisional environment during the Late Carboniferous and Early Permian.

Zircon U–Pb geochronology of Paleoproterozoic Statherian intraplate A-Type magmatic associations of the Lagoa Real Uranium Province, São Francisco Craton (Bahia, Brazil)

The Lagoa Real Complex, in the Lagoa Real Uranium Province, is a manifestation of extensive intraplate A-type magmatism that occurred in the São Francisco Craton during the Paleoproterozoic. U–Pb dating of zircon by LA-MC-ICP-MS shows that granites were emplaced during a Statherian, rift-related, 1.76–1.72 Ga magmatic episode, coeval with emplacement of the A-type Borrachudos suite and felsic metavolcanic rocks from the southeastern, central and northern Espinhaço Igneous Province of the São Francisco Craton. The Lagoa Real Complex involves at least three co-genetic pulses, starting with the initial emplacement of the alkali-feldspar-bearing Lagoa do Barro granite, which crystallized at 1762 ± 9 Ma. This was followed by emplacement of the Juazeirinho granite at 1755 ± 6 Ma, and finally the intrusion of the São Timóteo granite, which represents the main magmatic peak of the Lagoa Real Uranium Province, at 1741 ± 4 Ma. Although the São Timóteo granite has some characteristics that suggest it may be a rapakivi granite, there is no classical cogenetic association with basic rocks. This study confirms that the charnockites of the Lagoa Real Complex have an age of 2047 ± 5 Ma. They are therefore not part of the evolution of the 1.76–1.74 Ga magmatic episode. Also, rocks within the Lagoa Real Uranium Province that were previously mapped as leucodiorites are, in fact, albite-gneiss with ages of 1743–1740 Ma, which originated through the gneissification processes that affected the São Timóteo granite and can also be classified as albitites. The highly negative values of ϵNd(0) from −27 to −33 and high initial Sr ratio are compatible with the Lu–Hf isotopic data of previous studies. These results suggest the Lagoa Real Complex has a crustal derivation with minimum mantle involvement. The formation of the Lagoa Real Complex granites is inferred to be a result of fractional crystallization. All studied granites have a similar geochemical signature in terms of trace elements including REEs, which indicates that the granites come from the same magmatic source and are related to rifting and the initial stages of the Espinhaço Supergroup evolution. The source of the Lagoa Real Complex would be reworked Archean crustal material, probably in the mid to deep crust. Finally, the studied granites, although predominantly metaluminous, have U contents (maximum 12 ppm) below the world average for A-type granites (20 ppm). The fractionation values of Th in relation to U, and the absence of metamictization of the minerals (mostly allanite) that potentially host uranium confirms this observation. Therefore, the granites of the Lagoa Real Complex, despite being the protolith of bodies that host U mineralization, should not be considered as the principal source of U in the U mineralization found in the Lagoa Real albitites. However, the provenance of an important part of the uranium could be associated with Statherian meta-rhyolites and meta-rhyodacites, contemporaneous with the São Timóteo granite, and occurring to the west of the Lagoa Real Complex.

The Middle Permian to Triassic tectono-magmatic system in the southern Korean Peninsula

The Permo-Triassic adakitic magmatism in the southern Korean Peninsula preserves important tectonomagmatic records along the East Asian continental margin. In this study, we present SHRIMP/LA–MC–ICPMS U-Pb zircon ages and Hf isotope signature with whole-rock geochemical compositions of the Middle Permian to Middle Triassic granitoid gneiss-granitoid suites and the Middle to Late Triassic plutonic rocks in the southeastern Korean Peninsula. The former yields three age groups of ca. 272–263 Ma, ca. 258–251 Ma and ca. 243–240 Ma, respectively. The latter group yields ca. 237–216 Ma ages, indicating a long term magmatism that lasted from the Middle Permian to late Triassic. The granitoid gneiss-granitoid suites have chemical compositions similar to high silica adakitic rocks, showing high SiO2 (59.55–73.97 wt.%), Na2O (3.55–5.30 wt.%), Sr/Y (30–401), and La/YbN (11–173), and low MgO (0.18–1.82 wt.%), Y (2–14 ppm) and YbN (2–7). These rocks are peraluminous with high Ba and Sr, and show relatively high K2O contents and K2O/Na2O ratios (> 0.7), with positive correlation between La/YbN vs. Sr/Y ratios and high initial Sr ratios. These features reflect different contribution of the slab-derived adakite melts that interacted with peridotitic mantle in the metasomatized mantle wedge and the Middle Paleoproterozoic lower crustal basement of the Yeongnam Massif. On the contrary, the Triassic alkaline magmatism in the Yeongnam Massif is alternatively interpreted either by a tectonic switch to the extension-dominated arc system or by a delamination of an overthickened arc. The results from this study offer important evidence for a common subduction-accretion system existed between the North and South China Cratons and microcontinents between them along margins of the East Asian continental blocks, with different plate interaction in different locations generating distinct magmatic suites.

Petrogenesis of Early Paleozoic lamprophyre in North Qinling Orogenic Belt and their implications

Early Paleozoic tectonic evolution of Qinling Orogenic Belt and Shangdan Ocean remains an argument because of the different understandings of Early Paleozoic geological records, including the petrogenesis of magmatic and metamorphic rocks and formation setting of Liuling Groups. In this paper, we present zircon U–Pb data and geochemical and isotopic compositions of the lamprophyres in the southern part of North Qinling Orogenic Belt. The lamprophyres yield zircon U–Pb ages of ca. 470 Ma and are coeval with the Early Paleozoic potassium–rich mafic rocks (Fushui complex). The lamprophyres belong to calc-alkaline series and are characterized by enrichment in large-ion lithophile elements and light rare earth elements and depletion in high field strength elements. Lamprophyre and mafic rocks, etc, the crust-like geochemical signatures represent a source feature. The lamprophyres also have high initial Sr ratios of 0.706859 to 0.713288 and enrich Nd isotopic composition with eNd(t) values of − 6.8 to − 8.3, implying that they are derived from the enriched subcontinental lithospheric mantle. In addition, the lamprophyres intrude into the supracrustal Luohansi Formation, representing their shallow intrusive level, and cross through the crust and carry the key information. In the lamprophyres, there are many Neoproterozoic zircon xenocrysts with ages of 813.3 to 825.6 Ma, which is affinity with South Qinling Orogenic Belt. This is an implication of the closure of Shangdan Ocean and collision between North Qinling Orogenic Belt and South Qinling Orogenic Belt during the formation of lamprophyre.

Eocene arc magmatism and related Cu-Au (Mo) mineralization in the Shangalon-Kyungalon district, Wuntho-Popa Arc, northern Myanmar

The Shangalon-Kyungalon Cu-Au (Mo) ore district is located on the south-eastern margin of the Kanzachaung batholith in the Wuntho-Popa Arc, Myanmar. Porphyry and epithermal Cu-Au mineralization are associated with the Late-Eocene magmatic rocks. In this study, zircon U-Pb dating of gabbroic diorite, diorite porphyry, tonalite porphyry, and granodiorite yielded ages of 38.0 ± 0.35 Ma, 37.8 ± 0.35 Ma, 37.4 ± 0.27 Ma, and 36.9 ± 0.26 Ma, respectively. These rocks show sub-alkaline characteristics, LREE-enriched patterns with weakly positive/negative Eu anomalies, enrichment of LILEs and depletion of HFSEs. They have obviously higher initial Sr ratios and lower εNd(t) and εHf(t) values (Isr = 0.7066; εNd(t) = −0.5 to −4.8; εHf(t) = 0.7–9.7) than the mid-Cretaceous Kanzachaung batholith (Isr = 0.7046; εNd(t) = 2.4–4.5; εHf(t) = 10.9–15.1), indicating crustal materials were most likely added to the arc magma source. In particular, the gabbroic diorite has higher initial Sr, more enriched Nd-Hf isotopic compositions (Isr = 0.7081, εNd(t) = −4.6 to −4.8, εHf(t) = 0.7–2.3) and lower SiO2 content relative to the other magmatic rocks studied (Isr = 0.7068–0.7059; εNd(t) = −0.5 to −2.8, εHf(t) = 5.0–9.7). The results suggest that the gabbroic diorite was derived from melting of a mantle wedge, metasomatized by subducted oceanic sediment. Underplating on juvenile lower crust followed, accompanied by MASH processes, which led to increased εNd(t) (and SiO2 content) and decreased initial Sr ratios in later formed rocks. Moreover, the adakite-like features (Sr/Y = 19.3–53.4, Y = 7.54–18.8,) suggest fractional crystallization of hornblende in a hydrous magma, potentially leading to formation of the Shangalon-Kyungalon porphyry-epithermal Cu-Au system. In summary, the Late-Eocene magmas in Shangalon-Kyungalon formed from a mantle source metasomatized by subducted sediments during Neo-Tethyan subduction beneath Western Burma terrane, with the involvement of juvenile crustal materials.

Non-Sequential Injection of PGE-rich Ultramafic Sills in the Platreef Unit at Akanani, Northern Limb of the Bushveld Complex: Evidence from Sr and Nd Isotopic Systematics

Abstract The Platreef Unit is a deceptively complex sequence of layered cumulates located in the northern limb of the 2·055 Ga-old Bushveld Complex. The unit contains the Platreef, a thick, richly mineralized stratabound PGE orebody which differs markedly from the comparatively thin, predominantly stratiform Merensky Reef found in the Upper Critical Zone of the eastern and western limbs. The Platreef Unit is interpreted as a localized facies of the Upper Critical Zone, despite layering being neither as systematic nor as clearly defined as in the equivalent stratigraphy found in the other limbs. The Platreef Unit in the Akanani project area includes well-defined layers of feldspathic harzburgite and norite, in addition to the ubiquitous feldspathic orthopyroxenite–melanorite that characterizes other sections. The paucity of floor-rock xenoliths is an additional feature. The relatively well-developed nature of the layering and paucity of xenoliths in the Platreef Unit at Akanani is explained by separation of the unit from the floor of the intrusion by a thick succession of ultramafics assigned to the Lower Critical Zone. We identify three lithological subgroups in the Platreef Unit at Akanani. They do not define an upward-younging stratigraphy. The primary stratigraphy, or PU1 subunit, is dominated by multiple layers of feldspathic orthopyroxenite, melanorite, and norite. This subunit built up from incremental addition of relatively small magma pulses. Repeated magma replenishment induced concomitant partial melting of earlier-formed layers. The PU1 subunit includes thin chromite stringers that contain Cr-spinels with unusual, amoeboidal textures consistent with several stages of growth and re-equilibration. The feldspathic harzburgite of the younger PU2 subunit was emplaced non-sequentially into the already complexly-layered PU1 subunit as a series of sinuous lenses or syn-intrusive sills. One of the PU2 sills contains the richest and most consistent of the mineralized sections at Akanani, i.e., the Main Mineralized Reef (MMR). The irregularly-developed pegmatoidal lithologies of the PU3 subunit are ascribed to recrystallization of earlier-formed cumulates (PU1 and PU2). Whole rock isotopic data for a section of the Platreef Unit, together with the overlying Lower Main Zone and underlying Lower Critical Zone, mostly from drill-hole ZF-1, demonstrate a complex pattern in both Sr87/Sr86 initial ratios and ϵNd values. These patterns are consistent with multiple lineages of parental magmas. The Lower Main Zone and the majority of the Platreef Unit are characterized by anomalously high Sr initial ratios (with a large degree of scatter) and low ϵNd values (relatively tightly constrained). Harzburgite layers from the Lower Critical Zone have a low Sr initial ratio and a relatively high ϵNd value. The new isotopic data suggest these sequences crystallized from multiple magma batches, broadly constrained within the U-type (ultramafic) and A-type (tholeiitic) lineages, derived from mantle sources and/or staging chambers which experienced varying degrees of crustal contamination. The MMR crystallized from a specific pulse of the U-type magma lineage characterized by a high Sr87/Sr86 initial ratio (average of 0·71113) and a markedly low ϵNd value (average of -11·35). The olivine-saturated magmas associated with the MMR were derived from a localized mantle source and yet underwent an unusually high degree of crustal contamination. Some layered PGE orebodies in the Bushveld Complex, including the Platreef and Merensky Reef, were emplaced as syn-magmatic sills which crystallized from anomalously PGE-rich parental magmas with an unique isotopic fingerprint.

Relationship of F-Be mineralization to granites and syenites at the Ermakovka deposit (Western Transbaikalia)

The paper presents the mineralogical and geochemical characteristics of two groups of hydrothermal rocks and their relation with subalkaline granites of the Ermakovskoe deposit. The first group includes fluorite-phenakite-bertrandite ore bodies, occurring outside the granite massif. The second group is presented by silicification bodies with sulfates, phosphates, kaolinite, muscovite and hematite. It bears REE (rare earth elements) mineralization (monazite, florencite, xenotime) and occurs within the massif. Our research included isotopic analyses of Sr, Nd and O, studies of trace, including rare-earth element compositions and age determination (U-Pb) of apatite from F-Be ores. Geochemical and isotopic studies are not according with relation between F-Be ores and granites. This is proven by the absence of Be-mineralization in granites and schlieren pegmatites, and a sharp difference in composition of their fluid phases. A reductive fluid specification forming F-Be ores (containing CH4, H2, N2, CO2 and H2S), contrasts sharply with fluid specification of granites. The granites are characterized by high oxygen fugacity, due to ferrous iron, sulfates and phosphates. Besides isotopic composition of oxygen in quartz (7.4 and 5.1‰ δ18О V-SMOW respectively), initial Sr ratios (0.7056-0.7065 and 0.707-0.709 respectively) and REE compositions are different.

Initial 87Sr/86Sr as a sensitive tracer of Archaean crust-mantle evolution: Constraints from igneous and sedimentary rocks in the western Dharwar Craton, India

Among the rocky planets of the solar system only the Earth has “granitic” continental crust. The timing and processes involved in the formation of Earth’s first extensive crust is still enigmatic. The chemical and isotope compositions of ancient crustal rocks preserve a record of their genesis. The Rb-Sr system proves to be an efficient proxy for the reconstruction of crust-mantle evolution since it can bring together information from seawater as preserved in chemical sedimentary rocks and information from magmatic rocks that can trace the time and extent of crust formation and concomitant mantle depletion during the Archaean eon. The Dharwar Craton in India preserves a suite of metamorphosed igneous and sedimentary rocks that record its early crustal evolution. To overcome the susceptibility for resetting and the difficulty in determining initial 87Sr/86Sr, the minerals barite and apatite are used to obtain precise and accurate 87Sr/86Sr, because these minerals preferentially incorporate Sr and exclude Rb and preserve the initial Sr isotope compositions at the time of their formation. Initial 87Sr/86Sr of apatite were obtained in situ using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry. The robustness and only minor dispersion of the isotope ratios demonstrate the capability of matrix apatites in preserving initial Sr ratios. The least radiogenic value is used as the best estimate for the initial 87Sr/86Sr. The 87Sr/86Sr ratios of apatite from igneous rocks that formed from 3.5 Ga to 2.6 Ga constrain the Rb/Sr of the source over the whole time-span. A comparison of the Sr isotopes between seawater-derived barite and initial Sr isotope ratios in apatite from igneous rocks reveals that significant mafic to intermediate crust had formed by 3.2 Ga. Studying the entire Archaean time window, a dominantly mafic crust was the main source for the granitoid rocks in the Dharwar Craton from 3.5 to 3.1 Ga, whereas the rocks from 2.9 to 2.7 Ga were extracted dominantly from the depleted mantle.

Geochemistry, age and origin of the Mons Claudianus TTG batholith (Egypt): insight into the role of Pan-African magmatism in uniting plates of Gondwana

AbstractThe tonalite–trondhjemite–granodiorite (TTG) Mons Claudianus Batholith (MCB) of Egypt is subsolvus, metaluminous to mildly peraluminous, exhibits wide ranges of SiO2, Al2O3, Sr, Rb, Zr, shows large Ba enrichment, is moderately enriched in rare earth elements (REE) and is depleted in K, Ti, Nb, Y, Hf and heavy REE (HREE), reflecting strong arc geochemical signatures. Moderate fractionation of REE and lack of Eu anomaly characterize the MCB. It is typical of high-Al TTGs of volcanic-arc affinities. U–Pb–zircon dating produced a Pan-African age of 664.12 ± 0.38 Ma. The MCB exhibits 87Sr/86Sr isotopic compositions of 0.70352–0.70626 (initial Sr ratio of 0.70259) and 143Nd/144Nd ratios of 0.51261–0.51276 (εNd; –0.5 to +2.4), suggestive of a mantle source. Anatexis of a basaltic slab under eclogitic conditions leaving garnet in the residue produces high-Al TTG rocks characterized by low Yb values (<1.8 ppm). Values (in ppm) of Yb (0.65–1.8), Y (2.2–19), Nb (1.2–6.4), and ratios of Nb/Ta (7–17), (La/Yb)N of 11.7, Sr/Y and Zr/Sm (58 and 45, respectively) are all consistent with anatexis of a basaltic slab under eclogitic conditions, leaving garnet in the residue to produce this high-Al TTG suite. The data conform to magma generation via partial melting (F = 0.25–0.50) leaving 15–25 % garnet in the residue. Voluminous synorogenic magmatic pulses, resulting from slab melting during the closure of the Mozambique Ocean via convergence of east and west Gondwana, produced the MCB and similar large batholiths forming the core of Pan-African belts. These belts welded together vestiges of fragmented Rodinia, assembling them into a united Gondwana.

The Keimoes Suite redefined: The geochronological and geochemical characteristics of the ferroan granites of the eastern Namaqua Sector, Mesoproterozoic Namaqua-Natal Metamorphic Province, southern Africa

Voluminous granite gneisses and granites straddle the boundary between the Kakamas and Areachap Terranes in the eastern Namaqua Sector (NS) of the Mesoproterozoic Namaqua-Natal Metamorphic Province (NNMP). These rocks have been previously poorly defined and loosely grouped into the Keimoes Suite, but a recent U-Pb age study has suggested the suite be subdivided into syn-tectonic and post-tectonic groups relative to the main phase of the Namaqua Orogeny. This study adds new whole rock geochemical, isotopic and age data for these granites that confirms the subdivision is appropriate. The older group of syn-tectonic granite gneisses, dated between 1175 and 1146 Ma, have penetrative foliations and are largely derived from fractionated, leucogranitic metaluminous to peraluminous magmas with low maficity, low Ti, Mn and Ca. They were derived from mildly depleted sources (εNd(t): −1.47 to 1.78), with Meso-to Paleoproterozoic Nd model ages (1.57–1.91 Ga), and high initial Sr ratios (0.71970–0.75567) suggesting mixing between younger depleted and older, arc-like sources imparting an arc-like signature to the magmas. High initial Sr ratios appear to be an intrinsic character of these granites reflecting those of granites in the region and the highly radiogenic nature of the NS.The weakly to unfoliated late-to post-tectonic megacrystic granodiorites and monzogranites, including charnockites, intruded between 1110 and 1078 Ma and constitute the Keimoes Suite proper. They have I-type characteristics, being strongly metaluminous and locally hornblende- and orthopyroxene-bearing with moderate SiO2 and with arc-type affinities (LILE enrichment relative to the HFSE, Ta-Nb, Ti and P anomalies). However, the granitoids also have high Fe/Mg ratios, along with high HFSE, LILE and REE contents more indicative of A-type granites. They show an increasing maficity, metaluminous character, and general decreasing degree of fractionation with decreasing age. They are similar to the syn-tectonic granites in having εNd(t) values close to zero (−2.95 to 2.83) and Meso-to Paleoproterozoic model ages (TDM: 1.38–1.99 Ga) but lower initial Sr ratios (<0.723 in general) suggesting derivation from relatively depleted sources with a variable enriched and/or crustal component.The timing of emplacement of the post-tectonic granites places peak D2 deformation in the eastern NS predominantly at ∼1.13–1.10 Ga, with peak D2 varying from ∼1.18–1.13 Ga. There was more voluminous granitic magmatism in the Areachap Terrane to the east during the late-to post-tectonic magmatic episode, whereas the earlier, ∼1.18–1.14 Ga syn-tectonic magmatic episode is more concentrated to the west in the Kakamas Terrane. The broad, protracted period of magmatism in the eastern NS attests to a long-lived duration of high-heat flow in this portion of the southern African crust at this time. Nd model ages of Meso-to Paleoproterozoic age reflect those in other granites throughout the NS suggesting extensive reworking of Paleoproterozoic crust during the 1.2–1.0 Ga Namaquan Orogeny.

Geochemical and geochronological characteristics of Late Cretaceous to Early Paleocene granitoids in the Tengchong Block, Southwestern China: Implications for crustal anatexis and thickness variations along the eastern Neo-Tethys subduction zone

The Tengchong Block of Southwestern China is key to tracing the eastward subduction of Neo-Tethys and collision between Indian and Asian continents. The block contains a magmatic belt that represents the southeastward continuation of the Gangdese belt, produced by the eastward subduction of eastern Neo-Tethyan oceanic lithosphere. In this paper we present geochemical and geochronological data of Late Cretaceous to Early Paleocene (~64Ma) granitic rocks of the Guyong and Husa batholiths in the Tengchong Block. These can be subdivided into high-silica peraluminous granites and low-silica metaluminous granodiorites, and all belong to the high-K calc-alkaline series, are enriched in LILE, and depleted in HFSE. The Guyong granitoids have high initial Sr ratios of 0.706511–0.711753, negative εNd(t) values of −9.2 to −11.6, two-stage model ages of 1.39–1.55Ga, and Pb isotopic compositions that indicate a crustal affinity. The Husa granodiorites also have high initial Sr ratios of 0.716496, negative εNd(t) value of −16.5, two-stage model age of 1.89Ga, variable εHf(t) values of 3.4 to −18.1 and Pb isotopic compositions similar to lower crustal values. These geochemical and isotopic data indicate that the Guyong granitoids were likely derived from partial melting of ancient crustal metapelite or mixed pelite-greywacke sources, while the Husa granodiorites were derived from the partial melting of lower crustal mixed sources involving metasedimentary and metaigneous rocks. To understand the thermal state and architecture of the Late Cretaceous to Early Paleocene magmatic arc crust, the crust-derived intermediate to acidic igneous rocks of the southern-central Lhasa and Tengchong blocks and eastern Himalayan syntaxis are compared. We infer that partial melting of crust occurred at great depth in the southern Lhasa Block, intermediate depths in the eastern Himalayan syntaxis, and shallow depths in the central Lhasa and Tengchong Block. Sr/Y ratios indicate that the Tengchong Block was characterized by thin crust, while the eastern Himalayan syntaxis and northernmost part of southern Lhasa Block contained crust of normal thickness, and the southernmost part of the southern Lhasa Block contained thickened crust.

Further evidence of 777 Ma subduction-related continental arc magmatism in Eastern Dom Feliciano Belt, southern Brazil: The Chácara das Pedras Orthogneiss

In this study new SHRIMP U–Pb zircon data for the Chácara das Pedras Gneiss in Porto Alegre, southern Brazil are presented. They represent a small exposure of the crust which was intruded by a large volume of orogenic to anorogenic granitoids at ca. 618-562 m.y. in the Eastern Domain of the Dom Feliciano Belt. The Chácara das Pedras tonalitic orthogneiss has geochemical similarities with subduction-related magmatic rocks of continental arcs. They present high Sr initial ratios (∼0.712), negative ɛNd(t = 777) values (∼−6), TDM varying from 1.8 to 2.0 Ga. The igneous protoliths of these orthogneisses were previously considered to be Paleoproterozoic based on an upper intercept age of discordant zircon analyses. In the present study these orthogneisses were re-sampled and re-analyzed in an attempt to obtain more concordant analytical data. The U–Pb zircon analyses were carried out using the SHRIMP IIe at the Laboratório de Geocronologia de Alta Resolução of the Universidade de São Paulo. The U–Pb concordia age obtained for igneous textural domains of the zircon grains is 777 ± 4 Ma. A few analyses on zircon overgrowths give poorly defined late Cryogenian ages of ca. 650 Ma. Older ages, mostly discordant, were obtained in a few zircon cores, showing an upper intercept age of ca. 1.9 Ga. One sample of the Três Figueiras Granodiorite, which crosscut the orthogneiss in the same outcrop, was also investigated. The zircons of this granodiorite are, however, mostly metamitic, preventing the determination of a reliable age. Some concordant analyses from a few grains define ages ranging in the interval between ca. 603 and 1022 Ma. The youngest (ca. 603 Ma) may represent a maximum age for the granodiorite crystallization. Older ages, with discordance <10%, are of 745, 777, 836 and 1022 Ma. The 777 ± 4 Ma age obtained for the Chácara das Pedras orthogneiss is the first Early Cryogenian magmatic age determined for granitoids in the Porto Alegre region, although records of similar ages (780–800 Ma) are recognized within the Eastern Domain of the Dom Feliciano Belt. Examples are the Piratini Gneisses (Piratini region) and in the Cerro Bori Orthogneisses (Rocha region, Uruguay). All these units show subduction-related continental arc affinities. The identification of a magmatic event of ca. 780–800 Ma in the Porto Alegre region implies that: (i) the collisional magmatism (650 Ma) is ca. 130 Ma younger than the subduction-related magmatism; (ii) the younger orogenic (arc related to post-collisional) and anorogenic granitoids (<618 Ma) of Eastern Domain were emplaced in a hot/weak lithosphere due to the presence of an earlier ca. 780–800 Ma magmatic arc rocks; (iii) the tectonic evolution of this region in West Gondwana suggests a more complex geotectonic scenario than those put forward by previous models involving the amalgamation of the Río de La Plata and Kalahari cratons.

Geochronological and geochemical implications of Early to Middle Jurassic continental adakitic arc magmatism in the Korean Peninsula

Sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb ages and whole-rock chemical compositions of Early to Middle Jurassic plutons from the central to southern Korean Peninsula are reported to investigate the effect of paleo-Pacific plate subduction preserved along the continental margin. Twenty-one plutonic rocks from the Yeongnam massif (i.e., Sunchang and Namwon plutons), the Okcheon belt (Jeongup, Boeun, and Mungyeong plutons), the northeast (Gangreung pluton), and the Gyeonggi massif (Gonam, Anheung, and Ganghwa plutons) have age ranges from ca. 189–186Ma to 177Ma, 177–166Ma, and 177–173Ma, respectively. Most plutonic rocks have chemical compositions similar to adakites, showing high SiO2 (45.62–74.96wt.%), low MgO (0.01–2.84wt.%), high Na2O (2.65–4.83wt.%), high Sr/Y and La/Yb, low Y and Yb, as well as low HFSEs (Nb and Ta), but initial Sr ratios (0.7048–0.7262), K2O (0.50–5.88wt.%), and K2O/Na2O (0.34–2.1) were unlikely to have been formed by melting of either a thickened and/or delaminated lower continental crust, or an altered oceanic crust. These suggest that the “adakitic” plutonic rocks in this region resulted from partial melting of an enriched mantle source metasomatized by dewatering from a delaminated flat-slab. The spatial distributions of this continental adakitic plutonic belt, based on the present study, together with previously reported geochronological results, indicate that magmatic pulses gradually migrated toward the continent across the Korean Peninsula as a result of slab shallowing caused by periodic slab buckling. The similar geochronological and geochemical characteristics, petrogenesis and tectonic setting of the plutonic belt spanning the Korean Peninsula, Japan, eastern China, and eastern Russia indicate a possible link to an active subduction system that existed during the Early to Middle Jurassic, although the products of the plate subduction might differ in different locations along the East Asian continental margin.

Oxygen Isotopic Compositions of the Cape Ashizuri Granitoids in Southwest Japan

AbstractOxygen isotopic composition was determined on representative samples of the Cape Ashizuri plutonic rocks, in order to estimate the genetic background of the biotite granite and alkaline granitoids. The biotite granite (70.1–76.1% SiO2) ranged from 8.61 to 9.30‰ δ18O and averaged as 8.9‰ δ18O (n = 3), which is much smaller than the same Miocene granitoids of the Okueyama (avg. 10.1‰) and Takakumayama (11.6‰) granitic bodies, which are associated with tin mineralization. Among the alkaline granitoids, quartz syenites also have values as low as 7.14–8.70‰, with an average of 8.0‰ (n = 3), and monzonite and gabbro vary from 6.14 to 7.86‰, with an average of 7.0‰ (n = 3). These alkaline granitoids may be lower crustal in origin. The gabbroids containing 12.5% MgO at the maximum with low Sr initial ratio, are considered to be derived from the upper mantle through the fore‐arc tectonic break‐up on the subducting slab.

Petrogenesis of Cretaceous igneous rocks from the Duolong porphyry Cu–Au deposit, central Tibet: evidence from zircon U–Pb geochronology, petrochemistry and Sr–Nd–Pb–Hf isotope characteristics

The Duolong porphyry Cu–Au deposit (5.4 Mt at 0.72% Cu, 41 t at 0.23 g/t Au) was recently discovered in the southern Qiangtang terrane, central Tibet. Here, new whole‐rock elemental and Sr–Nd–Pb isotope and zircon Hf isotopic data of syn‐ and post‐ore volcanic rocks and barren and ore‐bearing granodiorite porphyries are presented for a reconstruction of magmas associated with Cu–Au mineralization. LA–ICP–MS zircon U–Pb dating yields mean ages of 117.0 ± 2.0 and 120.9 ± 1.7 Ma for ore‐bearing granodiorite porphyry and 105.2 ± 1.3 Ma for post‐ore basaltic andesite. All the samples show high‐K calc‐alkaline compositions, with enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE: Cs and Rb) and depletion of high field strength elements (HFSE: Nb and Ti), consistent with the geochemical characteristics of arc‐type magmas. Syn‐ and post‐ore volcanic rocks show initial Sr ratios of 0.7045–0.7055, εNd(t) values of −0.8 to 3.6, (206Pb/204Pb)t ratios of 18.408–18.642, (207Pb/204Pb)t of 15.584–15.672 and positive zircon εHf(t) values of 1.3–10.5, likely suggesting they dominantly were derived from metasomatized mantle wedge and contaminated by southern Qiangtang crust. Compared to mafic volcanic rocks, barren and ore‐bearing granodiorite porphyries have relatively high initial Sr isotopic ratios (0.7054–0.7072), low εNd(t) values (−1.7 to −4.0), similar Pb and enriched zircon Hf isotopic compositions [εHf(t) of 1.5–9.7], possibly suggesting more contribution from southern Qiangtang crust. Duolong volcanic rocks and granodiorite porphyries likely formed in a continental arc setting during northward subduction of the Bangong–Nujiang ocean and evolved at the base of the lower crust by MASH (melting, assimilation, storage and homogenization) processes. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Tchabal Gangdaba massif in the Cameroon Volcanic Line: a bimodal association

Tchabal Gangdaba (TG) volcanic massif, which is a part of the continental sector of the Cameroon Volcanic Line (CVL), is dated between 34.4 and 25.1 Ma. It displays mafic lavas (picrobasalt and basanite, 41–43 wt % SiO2) and felsic lavas (rhyolite, 68–73 wt % SiO2). The lack of intermediate rocks evidences a pronounced Daly gap between 43 and 68 wt % SiO2, which corresponds to an important time span of 3.4 Ma. It is interpreted as due to extensive fractional crystallization under peculiar thermodynamical conditions. Felsic lavas yield strong negative anomalies in Ba, Sr and Eu (0.1 < Eu/Eu* < 0.3) reflecting alkali feldspar and plagioclase fractionation and in Ti reflecting titanomagnetite and ilmenite fractionation. All TG rocks are enriched in LREE and mildly fractionated, which suggests enriched mantle sources. Sr initial isotopic ratios ranging from 0.7033 to 0.7059, eNd from +2.89 to +4.64 and Pb isotopic ratios of 18.988 < 206Pb/204Pb < 19.998, 15.592 < 207Pb/204Pb < 15.673 and 38.948 < 208Pb/204Pb < 39.648 are typical of an enriched mantle source close to HIMU and FOZO. A discrete Sr crustal leaching is evidenced.

Petrogenesis and tectonics of late Permian felsic volcanic rocks, eastern Qiangtang block, north-central Tibet: Sr and Nd isotopic evidence

The Palaeo-Tethyan tectonic evolution of central Tibet remains a topic of controversy. Two Permian to Late Triassic arc-like volcanic suites have been identified in the eastern Qiangtang (EQ) block of north-central Tibet. Three competing models have been proposed to explain the formation of these volcanic suites, with two models involving a single stage of long-lived subduction but with opposing subduction polarities, while the other model involves a two-stage subduction process. Here, we present new whole-rock geochemistry, including Sr–Nd isotope data, for late Permian felsic volcanics of the Zaduo area. These volcanics are mainly low to middle K calc-alkaline felsic tuffs and rhyolites with SiO2 concentrations up to 73 wt.%. In primitive mantle-normalized diagrams, the volcanics are typified by large ion lithophile element enrichment and high-field-strength element (e.g. Nb, Ta, P, and Ti) depletion, with slightly negative Eu anomalies. They have initial Sr ratios (87Sr/86Sr) i of 0.70319–0.70547, and ϵNd(t) values of +3.4 to +3.5, suggesting derivation by the partial melting of a depleted mantle wedge, followed by assimilation of crustal material. The available geochemical data indicate the presence of two distinctive igneous evolution trends within the Permian to Late Triassic volcanics of the EQ block, consistent with a two-stage subduction model. Permian to Early Triassic arc-like volcanics are formed during northward (present-day orientation) subduction, whereas the Late Triassic volcanics are related to southward (present-day orientation) subduction of mafic crust of the Garze–Litang Ocean.