High Na2O Research Articles

The rheological, mineralogical and chemical characteristic of the original and the Na2CO3-activated Tunisian swelling clay (Aleg Formation) and their utilization as drilling mud

Clay deposits of the Aleg formation (Jebel Romana of the northern Chotts range, southern-east Tunisia) were sampled and investigated for their, mineralogical, chemical, rheological, and physical characteristics to develop their potential use as drilling mud. X-ray diffraction revealed a predominance of smectite (≥90%) with minor amounts of kaolinite and illite. Calcite, quartz and gypsum were also detected. Results show that the main oxides in the samples were SiO2, Al2O3, and Fe2O3, whereas the other oxides were present only in small quantities. However, the relatively high Na2O and MgO content is also one of the favorable points considered in terms of swelling and viscosity features. Standard tests show that the rheological behavior of the clay suspensions is unsuitable for preparing drilling muds. Therefore, the activation of these clays was performed using the most popular Na2CO3 in a wet condition. The drilling muds qualities of the Upper Cretaceous smectite-rich clay from Jebel Romana, significantly improved with additive sodium carbonate. The Ca-montmorillonite was completely converted into Na-montmorillonite after addition of 3% Na2CO3. The activation of the Aleg clay with soda influenced the viscosities, swelling indices and specific surface area. The optimum conditions for the preparation of high quality materials for drilling mud were investigated by studying the effects of clay minerals percentage, Na2CO3 concentration, activation time and temperature. Commercial bentonite samples imported from Algeria (A.G) were also included as reference materials. The optimum parameters for activation can be achieved under the operating conditions of 3% Na2CO3 concentration, 60min activation time, 75°C heating temperature and clay concentration of 75g/l. Under these conditions, the rheological and physical properties including apparent viscosity, surface area and swelling index showed a maximum values. Therefore, by alkali activation, it is possible to upgrade smectite from Jebel Romana to meet the minimum API viscosity (15cp); mud yield value (90barrels/sh.ton) and swelling index (22ml) standards. In addition, the apparent viscosity and mud yield efficiency was obtained around pH10, as 18cp, 101.93 (barrels/sh.ton), respectively. So, to achieve higher quality, optimization needs to be carried out in the formulation of batches or in the processing conditions. The utilization of these clays, after their upgrading and activation, can represent a value-added to the Tunisian economy by preventing the importation of bentonite.

Petrogenesis of Late Devonian–Early Carboniferous volcanic rocks in northern Tibet: New constraints on the Paleozoic tectonic evolution of the Tethyan Ocean

Paleozoic arc-type volcanic rocks in northern Tibet, provide important information on the Paleozoic tectonic evolution of the Longmu Co–Shuanghu Tethyan Ocean (LSTO). In this study, we obtained zircon U–Pb and Hf isotopic data, and major and trace element data for volcanic rocks from the Riwanchaka and Laxiongco areas in northern Tibet. LA-ICP-MS U–Pb analyses of zircons show that volcanic rocks in the Riwanchaka and Laxiongco areas formed at 374–372 and 356–353 Ma, respectively. These calc-alkaline volcanic rocks consist of basalt, andesite, dacite, and rhyolite. The basalt and andesite in the Riwanchaka area exhibit low TiO2 and high MgO and Na2O concentrations, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs). The basalt probably originated from partial melting of a lithospheric mantle source, metasomatized by subduction-related fluids, whereas the andesite was probably derived from hydrous basalts after fractional crystallization. The Riwanchaka and Laxiongco rhyolites are characterized by high Na2O/K2O ratios and low Al2O3 concentrations, are weakly metaluminous to peraluminous (A/CNK=0.97–1.11), and exhibit typical arc-like geochemical signatures (e.g., enrichment in LILEs and strong depletion in HFSEs and Sr). In addition, they show high zircon εHf(t) values (+7.61 to +4.45). The geochemical characteristics and zircon εHf(t) compositions of the rhyolites indicate that they were derived from partial melting of juvenile basaltic lower crust. These results, combined with previous data, indicate that the LSTO lithosphere was subducted northward beneath the North Qiangtang subterrane (NQT) during the Late Devonian–Early Carboniferous. We suggest that the NQT represents a slice of an active continental margin developed on the southern margin of the Eurasian continent during the Late Devonian–Early Carboniferous.

Magmatic evolution of the Tuwu–Yandong porphyry Cu belt, NW China: Constraints from geochronology, geochemistry and Sr–Nd–Hf isotopes

The Tuwu–Yandong porphyry Cu belt is located in the Eastern Tianshan mountains in the eastern Central Asian Orogenic Belt. Petrochemical and geochronological data for intrusive and volcanic rocks from the Tuwu and Yandong deposits are combined with previous studies to provide constraints on their petrogenesis and tectonic affinity. New LA–ICP–MS zircon U–Pb ages of 348.3±6.0Ma, 339.3±2.2Ma, 323.6±2.5Ma and 324.1±2.3Ma have been attained from intrusive units associated with the deposits, including diorite, plagiogranite porphyry, quartz albite porphyry and quartz porphyry, respectively. The basalt and andesite, which host part of the Cu mineralization, are tholeiitic with high Al2O3, Cr, Ni and low TiO2 contents, enriched LREEs and negative HFSE (Nb, Ta, Zr, Ti) anomalies consistent with arc magmas. Diorites are characterized by low SiO2 content but high MgO, Cr and Ni contents, similar to those of high-Mg andesites. The parental magma of the basalt, andesite and diorite is interpreted to have been derived from partial melting of mantle-wedge peridotite that was previously metasomatized by slab melts. The ore-bearing plagiogranite porphyry is characterized by high Na2O, Sr, Cr and Ni contents, low Y and Yb contents, low Na2O/K2O ratios and high Sr/Y ratios and high Mg#, suggesting an adakitic affinity. The high εNd(t) (5.02–9.16), low ISr (0.703219–0.704281) and high εHf(t) (8.55–12.99) of the plagiogranite porphyry suggest they were derived by a partial melting of the subducted oceanic crust followed by adakitic melt-mantle peridotite interaction. The quartz albite porphyry and quartz porphyry are characterized by similar Sr–Nd–Hf isotope but lower Mg# and whole-rock (La/Yb)N ratios to the plagiogranite porphyry, suggesting they were derived from juvenile lower crust, and negative Eu anomalies suggest fractionation of plagioclase. We propose that a flat subduction that started ca. 340Ma and resulted in formation of the adakitic plagiogranite porphyry after a period of “steady” subduction, and experienced slab rollback at around 323Ma.

Role of volatiles (S, Cl, H2O) and silica activity on the crystallization of haüyne and nosean in phonolitic magmas (Eifel, Germany and Saghro, Morocco)

To constrain the crystallization of alkaline and volatile-rich lavas present in intraplate settings, we studied the petrological features and the geochemical composition of major, trace, and volatile elements of mineral and bulk-rock of two sodalite-bearing phonolites: (1) hauyne-plagioclase-bearing Si-K-rich phonolite from Laacher See (Germany) and (2) nosean-nepheline-bearing Si-poor phonolite from Saghro (Morocco). In hauyne-bearing phonolites (55–59 wt% SiO2, K > Na, Na+K/Al = 0.96–1.08), we found that the low silica and low sodium activity promoted the early crystallization of S-rich hauyne (13.7–13.9 wt% SO3, 0.4 wt% Cl) + S-rich apatite (0.7–0.9 wt% SO3), titanite, and rare pyrrhotite followed by clinopyroxene-plagioclase-sanidine at relatively low pressure and temperature (P = 250 MPa and T = 850 °C) and oxidized condition (ΔNNO-NNO+1, where NNO is nickel-nickel oxide buffer). The crystallization of hauyne occurred at fluid-undersaturated conditions from a silicate melt with 6 wt% H2O, 0.17–0.23 wt% Cl, 0.11–0.4 wt% S, and 0.07–0.14 wt% F. Nosean-bearing phonolites from Saghro are silica-poor and peralkaline (52–54 wt% SiO2, Na > K, Na+K/Al = 1.2) and crystallized at higher P and T (300 MPa and 950 °C) and more reduced conditions (NNO) compared to hauyne-bearing phonolites. The incongruent reaction to form nosean requires high silica and Na2O activity. The mineral assemblage and composition suggest early crystallization of nepheline followed by nosean (7.8–8.8 wt% SO3; 1–1.1 wt% Cl). The sequence of crystallization is: clinopyroxene + nepheline + S-poor apatite (<0.04 wt% SO3) + pyrrhotite followed by nosean and titanite. Nosean-bearing magmas are fluid-undersaturated with relatively low volatile content (4 wt% H2O, <0.25 wt% Cl, <0.056 wt% S, 0.08–0.1 wt% F), although Cl may have exsolved during ascent and formed a fluid phase (NaCl-bearing). Both hauyne- and nosean-bearing phonolites are last equilibrated at relatively low pressure and high temperature. Hauyne and nosean crystallized at oxidized and volatile-rich pre-eruptive conditions. They record the volatile concentrations at depth and may be used as oxybarometer. The incongruent reactions involved to form hauyne and nosean suggest that phonolitic magmas became more oxidized during crystallization. The initial volatile concentrations in basanite/nephelinite magmas, from partial melting of volatile-bearing K2O-rich mantle rock, should have been one important factor influencing the crystallization of hauyne-bearing Si-K-rich phonolite and nosean-bearing Si-poor phonolite in intracontinental setting.

U–Pb geochronology, geochemistry, and H–O–S–Pb isotopic compositions of the Leqingla and Xin'gaguo skarn Pb–Zn polymetallic deposits, Tibet, China

The Leqingla and Xin'gaguo deposits are two representative skarn Pb–Zn polymetallic deposits of the Gangdese Pb–Zn polymetallic belt, Tibet, China. LA-ICP-MS zircon U–Pb dating of the mineralization-related biotite granites from both the Leqingla and Xin'gaguo deposits yielded weighted mean ages of 60.8Ma and 56.5Ma, respectively, which can be inferred as their mineralization ages. The Leqingla biotite granite is characterized by high Al2O3, total Fe, Na2O, and low K2O. In comparison, the Xin'gaguo biotite granite is characterized by relative higher K2O but lower Al2O3, total Fe, and Na2O. Geochemical and mineralogical characteristics indicate that the Leqingla and Xin'gaguo biotite granites are calc-alkaline I-type granite and High K calc-alkaline I-type granite, respectively. Both the Leqingla and Xin'gaguo biotite granites are enrichment in LREE and LILEs and depletion in HFSEs, and they were formed at the India–Asia collision stage. δ18O and δD values for the Leqingla and Xin'gaguo deposits are −8.8‰ to 5.3‰ and −140.4‰ to −90.1‰, −4.5‰ to 7.0‰ and −117.3‰ to −81.0‰, respectively, indicating magma fluids mixed with meteoric water in ore-forming fluids. δ34S values (−11.6‰ to −0.3‰) of ore sulfides from the Leqingla deposit show characteristics of biogenetic sulfur isotope compositions, suggesting sulfur for the Leqingla deposit were sourced from wall rocks of the Mengla and Luobadui Formation, which are rich in organic materials. δ34S values of ore sulfides from the Xin'gaguo deposits show bimodal distribution (−5.0‰ to −1.6‰ and 1.6–2.1‰), indicating sulfur in the Xin'gaguo deposit were derived from both wall rocks and magma. In the Leqingla deposit, most ore sulfides have the similar Pb isotopic compositions with that of the mineralization-related biotite granite, suggesting the biotite granite supplied most of the ore-forming metals. Pb isotopic compositions of ore sulfides and Hf isotopic compositions of biotite granite show that the majority of ore-forming metals are derived from mantle components of partial melting of the Neo-Tethys Ocean slab, with some upper crust materials of the Lhasa terrane. Pb isotopic compositions of ore sulfides from the Xin'gaguo deposit are similar to that of the Leqingla deposit, indicating they have the similar sources of ore-forming metals.

The Permian Dongfanghong island-arc gabbro of the Wandashan Orogen, NE China: Implications for Paleo-Pacific subduction

The Dongfanghong hornblende gabbro is located in the western part of the Wandashan Orogen and to the east of the Jiamusi Block in NE China. It was emplaced into Early Paleozoic oceanic crust (i.e. Dongfanghong ophiolite) at ~275Ma and both later collided with the eastern margin of the Jiamusi Block. The Dongfanghong gabbro is sub-alkaline with high Na2O contents and is characterized by enrichment in light rare earth elements (LREE), large ion lithosphile elements (LILE), Sr, Eu, and Ba, and depletion in high field strength elements (HFSE). The enriched isotopic signatures (87Sr/86Sri=~0.7065, εNd(t)=~−0.5, 208Pb/204Pbi=~38.05, 207Pb/204Pbi=~15.56, 206Pb/204Pbi=~18.20 and zircon εHf(t)=~+5.8) indicate an enriched mantle (EM2) source, with some addition of continental material. It has arc geochemical affinities similar to Permian arc igneous rocks in the eastern margin of the Jiamusi Block, the Yakuno Ophiolite in SW Japan, arc rocks along the western margin of the North America Craton, and also the Gympie Group in eastern Australia. All these features, together with information from tectonic discrimination diagrams, suggest that the Dongfanghong gabbro formed in an immature island arc. The spatial configuration of ~290Ma immature continental arc rocks in the eastern part of the Jiamusi Block and the ~275Ma immature island arc Dongfanghong gabbro in the Wandashan Orogen to the east is best explained by eastward arc retreat and slab roll-back of the Paleo-Pacific Plate. This model is also supported by the Carboniferous–Permian stratigraphic transition in the Jiamusi Block from oceanic carbonate rocks to coal-bearing terrestrial clastic rocks and andesites. We thus suggest that both Paleo-Pacific subduction and roll-back occurred in the Early Permian along the eastern margin of Asia.

Structural Study of Glass and Glass Ceramics Prepared with Egyptian Basalt

Egyptian basalt rock was used to prepare some glasses with different additives of Na2O by the melt quenching method. The prepared glasses were characterized applying FTIR, XRD and Mossbauer Effect (ME) spectroscopy. XRD indicated that all the samples showed a homogeneous glassy phase except the sample containing 35 mol % of Na2O which showed a very small crystalline phase of less than 5 %. FTIR measurements indicated the presence of various structural units including SiO4, FeO4 and AlO4. ME hyperfine parameters indicated the presence of both Fe2+and Fe3+in the low Na2O content glasses (the first three samples), while the iron in the high Na2O content glasses (the last three samples) showed only the Fe3+ state. All the glass samples were heated at 900°C for 6 h, of which XRD analysis showed different crystallites of nanometer size. ME spectra of the heat-treated (HT) samples indicated the presence of magnesium ferrites as confirmed by XRD study.

Postmortem Study of a Magnesia-Chromite Brick from a Lead Recycling Furnace

This study provides an example of a detailed postmortem analysis carried out on a used silicate-bonded magnesia-chromite brick out of a lead recycling furnace. The magnesia-chromite brick suffered from a high chemical attack due to the process slag. The high CaO, BaO, and sulfur-bearing silicate slag, as well as a high Na2O supply from soda resulted not only in a deep-reaching infiltration of the brick microstructure but also in a severe corrosion of the brick components. Both the sintered magnesia and chromite were attacked chemically. The FactSage calculations showed the formation of high amounts of liquid phase in the infiltrated microstructure and the formation of various Na-Ca-Al-silicates. A detailed investigation of the wear mechanisms through “postmortem studies” is a crucial prerequisite for every refractory producer to understand the interactions between slag and refractory materials. The obtained information and insights serve as a basis for improving refractory materials (i.e., choice of refractories for individual process and new developments) and consequently furnace operations (i.e., prolonged furnace campaigns).

The Neoarchean transition between medium- and high-K granitoids: Clues from the Southern São Francisco Craton (Brazil)

Field observations, geochemical data and LA-ICP-MS U–Pb ages for orthogneisses and granitoids from the Southern São Francisco Craton (SE, Brazil) indicate a major change in the composition of the continental crust during the Neoarchean. The crystalline basement comprises fine-grained banded gneisses that are intruded by leucogranitic sheets and dikes, by large granitoid batholiths and by small granitic bodies closely associated with the host banded gneisses. Granites and gneisses have high silica content (70–76wt.%) and, based on their major and trace element composition are subdivided in medium- and high-K granitoids. The distinctive geochemistry between these rock-types reflects melting of different sources. Medium-K rocks have chemical affinity similar to rocks of the tonalite–trondhjemite–granodiorite series (TTGs): high Na2O, Al2O3 and Sr content, low heavy-rare earth content. However, their overall chemical features suggest mixing between two end-members: a magma generated by partial melting of metamafic rocks and a component derived by recycling TTG rocks of older continental crust. The composition of high-K granites is similar to that of late-Archean biotite and two-mica granites. These rocks cannot be generated by melting of TTGs, but rather require melting of Archean metasediments capable of producing large volumes of K2O-rich magmas. Medium-K rocks were volumetrically dominant during two earlier periods of magmatism and associated greenstone belt deposition (2920–2850 and 2800–2760Ma), while high-K rocks mark the final cratonization event and consolidation of the granitic crust of the craton between 2750 and 2720Ma. The relatively sharp transition between the two rock types reflects the onset of basin deposition during the Neoarchean followed by melting of the neo-formed basins at depth.

Timing and nature of the Xinlin–Xiguitu Ocean: constraints from ophiolitic gabbros in the northern Great Xing’an Range, eastern Central Asian Orogenic Belt

Jifeng ophiolitic melange (ultramafic rocks, meta-basalts and gabbros) crops out in the northern segment of the Great Xing’an Range, the eastern segment of the Central Asian Orogenic Belt, which marks the closure of the Xinlin–Xiguitu Ocean associated with the collision between the Erguna block and Xing’an block. In order to investigate the formation age and magma source of the Jifeng ophiolitic melange, the gabbros from newly discovered the Jifeng ophiolitic melange are studied with zircon U–Pb ages, whole-rock geochemistry and zircon Hf isotopes. Zircon U–Pb dating from the ophiolitic gabbros yields U–Pb age of 647 ± 5.3 Ma, which may represent the formation age of the ophiolitic melange. The gabbros display low SiO2, TiO2, K2O contents, high Na2O, LREE contents and indistinctive REE fractionation [(La/Yb)N = 1.97–2.98]. It shows an E-MORB-like affinity, while the element concentrations of the Jifeng samples are lower than that of E-MORB. More importantly, Nb displays negative anomaly in comparison with Th, which shows a transitional SSZ-type ophiolite signature. Moreover, the e Hf (t) values of ~647 Ma zircons in the gabbros range from +8.4 to +13.4, and the corresponding Hf single-stage ages (T DM1) are between 687 and 902 Ma, which is obviously older than the crystallization age of 647 Ma. These geochemical features can be explained as melts from the partial melting of a depleted mantle source meta-somatized by fluids derived from a subducted slab. Accordingly, we conclude that the Jifeng ophiolitic melange is probably related to transitional SSZ-type ophiolite and developed in an intra-oceanic subduction, which indicates that an ocean (the Xinlin–Xiguitu Ocean) existed between the Erguna block and Xing’an block. The Ocean’s formation might be no later than the Neoproterozoic (647 Ma), and it was closed in the Late Cambrian because of the collision between the Erguna block and Xing’an block.

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.

Nature and Origin of the Amphibolites in the Precambrian Basement Complex of Iseyin and Ilesha Schist Belts, Southwestern Nigeria

Petrographic and geochemical studies of amphibolites from Wonu-Apomu area in Iseyin-Oyan schist belt and Ife-Ilesa area in Ilesha schist belt, southwestern Nigeria, were carried out with a view to unravel their nature and petrogenesis. The amphibolites in Wonu-Apomu are associated with quartzite, quartz schist, talc-schist, granite and pegmatites, while those of Ife-Ilesha area are associated with anthophyllite schist, talc tremolite and talc chlorite schists. Chemical data of the amphibolites showed high Al2O3 (>14%), Fe2O3 (>12.40%), CaO (>8.10), Na2O (>3.10%) and K2O (>5.0%) contents. Trace element data showed that the amphibolites are enriched in Ba (>137ppm), Sr (>122ppm), Rb (>33ppm), Zr (55pm) and Y (>37ppm), but impoverished in Cr (<62ppm), Ni (<57ppm) and Zn (<83ppm). Petrogenetic indices and discriminant plots of Na2O + K2O vs SiO2, Ga vs Y, Zr/Y vs Zr, Cr vs Ni, Fe2O3 –Na2O + K2O – MgO, indicated tholeiitic basalt affinity of the amphibolites and emplacements of the precursor basaltic rocks within continental crust.

Genetic relationship of high-Mg dioritic pluton to iron mineralization: A case study from the Jinling skarn-type iron deposit in the North China Craton

The Jinling complex is spatially and temporally associated with the Jinling skarn-type iron deposit. The complex is composed of biotite diorite, hornblende diorite, monzonite and quartz diorite. U–Pb dating of zircons from the biotite diorite and monzonite using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields ages of 126±1.9Ma and 128±1.4Ma, respectively. The unaltered rocks in the complex are characterized by variable contents of SiO2 (54.6–65.3wt.%), MgO (2.7–9.2wt.%), total FeO (3.5–8.8wt.%), Na2O+K2O (5.2–8.9wt.%), high Mg# values (73–88), Cr (103–452ppm) and Ni (49–212ppm) contents. The altered monzonite has lower MgO (2.1–3.7wt.%), total FeO (1.2–2.6wt.%) and higher Na2O+K2O (8.5–9.9wt.%) contents. The initial (87Sr/86Sr)t ranges from 0.70450 to 0.70555 and εNd(t) shows a range of −3.0 to −8.0. The geochemical characteristics suggest that the primary magma witnessed the interaction between the partial melts of relatively oxidized delaminated ancient crust and mantle peridotite. Fractional crystallization and crustal contamination during the magmatic ascent and emplacement are also indicated. The Jinling skarn-type Fe deposit is of hydrothermal origin and the Fe enrichment can be ascribed to multiple factors. The delaminated ancient crustal source contributed to the high oxygen fugacity of the primary magma. Two-stage Fe-enrichment process involving fractional crystallization of the primary magma giving rise to high Cl and Fe contents in the magmatic hydrothermal fluid and later Fe-leaching process, accounts for the high-grade ore bodies.

Formation and emplacement of two contrasting late-Mesoproterozoic magma types in the central Namaqua Metamorphic Complex (South Africa, Namibia): Evidence from geochemistry and geochronology

The Namaqua Metamorphic Complex is a Mesoproterozoic low-pressure, granulite facies belt along the southern and western margin of the Kaapvaal Craton. The NMC has formed between ~1.3 and 1.0Ga and its central part consists essentially of different types of granitoids intercalated with metapelites and calc-silicate rocks. The granitoids can be subdivided into three major groups: (i) mesocratic granitoids, (ii) leucocratic granitoids and (iii) leucogranites. The high-K, ferroan mesocratic granitoids (54–75wt% SiO2) have a variable composition ranging from granitic to tonalitic, and contain biotite and/or hornblende or orthopyroxene. They are strongly enriched in REE and LILE, indicating A-type chemical characteristics, and are depleted in Ba, Sr, Eu, Nb, Ta and Ti. The leucocratic granitoids and leucogranites (68–76wt% SiO2) differ from the other group in having a granitic or slightly syenitic composition containing biotite and/or garnet/sillimanite. They have lower REE and MgO, FeOt, CaO, TiO2, MnO concentrations, but higher Na2O and K2O contents. Compositional variations in mesocratic granitoids indicate their formation by fractional crystallization of a mafic parental magma. Leucocratic granitoids and leucogranites lack such trends, which suggests melting of a felsic crustal source without subsequent further evolution of the generated magmas. The mineralogical and geochemical characteristics of the mesocratic granitoids are consistent magmatic differentiation of a mantle derived, hot (>900°C) parental magma. The leucocratic granitoids and leucogranites granites were formed from low-temperature magmas (<730°C), generated during fluid-present melting from metasedimentary sources.New U-Pb zircon ages reveal that both magma types were emplaced into the lower crust within a 30–40 million years interval between 1220–1180Ma. In this time period the crust reached its thermal peak, which led to the formation of the leucocratic granitoids and leucogranites. A prolonged period of relatively high crustal temperatures is followed by a second heat pulse at ~1100Ma, that was intense enough to facilitate zircon growth in the older plutons and it produced a younger granite suite. The crust cools down below amphibolite facies conditions after a further 100 million years. The prolonged high-temperature history is best compatible with steady and long-lasting heat transfer from mantle sources, suggesting a continental back-arc situation as the most likely setting of the NMC in the late Mesoproterozoic.

Formation of the Jurassic South China Large Granitic Province: Insights from the genesis of the Jiufeng pluton

The magma source and thermal regime for the formation of Large Granitic Provinces (LGPs) have been the subjects of much debate. As mineralogical, geochemical and isotopic variations of plutonic rocks preserve records of crustal thermal evolution, the Jiufeng pluton was comprehensively studied to gain insights into material and heat sources and tectonic process in the formation of the South China LGP. SIMS zircon U-Pb analyses of a biotite granite (158.9±1.3Ma), three two-mica granites (stage I: 159.7±1.2Ma, stage II: 158.6±1.2Ma and 156.8±1.2Ma) and a muscovite granodiorite (159.0±4.4Ma) of the Jiufeng pluton suggest that they are coeval and were emplaced in a short period of no more than 5.3m.y. Limited variations in whole-rock Nd isotope (εNd(t)=−12.6 to −9.5) and zircon Hf-O isotopes (εHf(t)=−12.0 to −6.3, δ18O=6.8‰−9.4‰) suggest that the Jiufeng pluton was mainly derived from melting of a common metasedimentary source, possibly with a minor basaltic contribution. We consider the geochemical variations of the Jiufeng pluton are primarily a result of incremental assembly of magma batches produced from rapid step-like transition from fluid-saturated to fluid-absent melting of the source. The muscovite granodiorite, with high Na2O (>3.80wt.%; K2O/Na2O=~1), is interpreted to have been produced by fluid-saturated melting at low temperature (~650°C). High P2O5 (0.09–0.17wt.%), zircon saturation temperature (TZr=769–816°C) and La/Yb ratios (8.4–55.8) of the stage II two-mica granite support its formation from high-temperature (>800°C) biotite-dehydration melting, whereas lower P2O5 (<0.02wt.%), TZr (685–742°C) and La/Yb (<3) of the stage I two-mica granite suggest its generation at lower temperature, likely by muscovite-dehydration melting. We propose that extensive emplacement of basaltic melts in the lower crust most likely drove the rapid increase of mid-crustal (~20km) temperature (~50°C/m.y.) and widespread crustal melting for the formation of the Jurassic South China LGP. Therefore, formation of the LGP signifies prominent crustal growth as well as crustal reworking in an intraplate setting and was likely a response to flat-slab delamination and foundering.

Geochemistry, U‐Pb Geochronology and Sr‐Nd‐Hf Isotopes of the Early Cretaceous Volcanic Rocks in the Northern Da Hinggan Mountains

AbstractWhole‐rock geochemical, zircon U‐Pb geochronological and Sr‐Nd‐Hf isotopic data are presented for the Early Cretaceous volcanic rocks from the northern Da Hinggan Mountains. The volcanic rocks generally display high SiO2 (73.19–77.68 wt%) and Na2O+K2O (6.53–8.98 wt%) contents, with enrichment in Rb, Th, U, Pb and LREE, and depletion in Nb, Ta, P and Ti. Three rhyolite samples, one rhyolite porphyry sample, and one volcanic breccia sample yield weighted mean 206pb/238U ages of 135.1±1.2 Ma, 116.5±1.1 Ma, 121.9±1.0 Ma, 118.1±0.9 Ma and 116.9±1.4 Ma, respectively. All these rocks have moderate (87Sr/86Sr)i values of 0.704912 to 0.705896, slightly negative εNd(t) values of –1.4 to –0.1, and positive εHf(t) values of 3.7 to 8. Their zircon Hf and whole‐rock Nd isotopic model ages range from 594 to 1024 Ma. These results suggest that the Early Cretaceous volcanic rocks were originated from melting of subducted oceanic crust and associated sediments during the closure of the Mongol‐Okhotsk Ocean.

Petrography and Geochemistry (Trace, Ree and Pge) of Pedda Cherlo Palle Gabbro-Diorite Pluton, Prakasam Igneous Province, Andhra Pradesh, India

Abstract Prakasam Igneous Province (PIP) is an important geological domain in the Eastern Dharwar Craton (EDC), found in the junction zone between the EDC and Eastern Ghat Mobile Belt (EGMB). The Pedda Cherlo Palle (PCP) gabbros are massive, leucocratic-mesocractic, and show cumulus textures with minerals plagioclase, cpx, and amphiboles. Compositionally, plagioclase is a labradorite-bytownite, cpx is diopside to augite, olivines are hyalosiderites and amphiboles are magnesiohornblendes. PCP gabbros have normal SiO2, high Al2O3, moderate to high TiO2, Na2O and medium Fe2O3, so, classified as subalkaline tholeiitic gabbros. Fractionated rare earth element (REE) patterns, high abundance of large ion lithofile elements (LILE) and transitional metals coupled with light REE (LREE) relative enrichment over heavy REE (HREE) and Nb are characteristics of partial melting of depleted mantle and melts that have undergone fractional crystalisation. These partial melts are enriched in LREE and LILE, due to the addition of slab derived sediment and fluids. PCP gabbros contain low abundance (5.1 to 24.6 ng/g) of platinum group elements (PGE), and show an increase in the order Ir&gt;Os&gt;Pt&gt;Ru»Pd&gt;Rh. We propose that the subduction related intraoceanic island arc might have accreted to the southeastern margin of India to the east of Cuddapah basin in a collisional regime that took place during Ur to Rodinia amalgamations.

Transition from I‐type to A‐type magmatism in the Sanandaj–Sirjan Zone, NW Iran: an extensional intra‐continental arc

The South Dehgolan pluton, in NW Iran was emplaced into the Sanandaj–Sirjan magmatic–metamorphic zone. This composite intrusion comprises three main groups: (1) monzogabbro–monzodiorite rocks, (2) quartz monzonite–syenite rocks, and (3) a granite suite which crops out in most of the area. The granites generally show high SiO2 content from 72.1%–77.6 wt.% with diagnostic mineralogy consisting of biotite and amphibole along the boundaries of feldspar–quartz crystals which implies anhydrous primary magma compositions. The granite suite is metaluminous and distinguished by high FeOt/MgO ratios (av. 9.6 wt.%), typical of ferroan compositions with a pronounced A‐type affinity with high Na2O + K2O contents, high Ga/Al ratios, enrichment in Zr, Nb, REE, and depletion in Eu. The quartz monzonite–syenites show intermediate SiO2 levels (59.8%–64.5 wt.%) with metaluminous, magnesian to ferroan characteristics, intermediate Na2O + K2O contents, enrichment in Zr, Nb, REE, Ga/Al, and depletion in Eu. The monzogabbro–monzodiorites show overall lower SiO2 content (48.5%–55.9 wt.%) with metaluminous and calc‐alkaline compositions, relatively lower Na2O + K2O contents, low Ga/Al ratios, and FeOt/MgO (av. 1.6 wt.%) ratios, low abundances of Zr, Nb, and lower REE element concentrations relative to the granites and quartz monzonite–syenites. These geochemical differences among the three different rocks suites are likely to indicate different melt origins. We suggest that the South Dehgolan pluton resulted from a change in the geodynamic regime, from compression to extension in the Sanandaj–Sirjan zone during Mesozoic subduction of the Neo‐Tethys oceanic crust beneath the Central Iranian microcontinent. Copyright © 2015 John Wiley &amp; Sons, Ltd.

U–Pb dating, geochemistry, and Sr–Nd isotopic composition of a granodiorite porphyry from the Jiadanggen Cu–(Mo) deposit in the Eastern Kunlun metallogenic belt, Qinghai Province, China

The Jiadanggen porphyry Cu–(Mo) deposit is newly discovered and located in the Eastern Kunlun metallogenic belt of Qinghai Province, China. Here, we present a detailed study of the petrogenesis, magma source, and tectonic setting of the mineralization causative granodiorite porphyry. The new data indicate that the granodiorite porphyry is characterized by high SiO2 (68.21–70.41wt.%) and Al2O3, relatively high K2O, low Na2O, and low MgO and CaO concentrations, and is high-K calc-alkaline and peraluminous. The granodiorite porphyry has low Mg# (38–46) values that are indicative of no interaction between the magmas and the mantle. The samples that we have examined have low Nb/Ta (9.17–10.3) and Rb/Sr (0.28–0.39) ratios, which are indicative of crustal-derived magmas. Source region discrimination diagrams indicate that the magmas that formed the granodiorite porphyry were derived from melting of a mixed amphibolite source in the lower crust. The samples have ISr values of 0.70954–0.70979, εNd(t) values of −8.3 to −7.9, and t2DM ages ranging from 1644 to 1677Ma. These indicate that the magmas that formed this intrusion were generated by melting of Mesoproterozoic lower crustal material. Higher K(Rb) contents of the samples indicate that the magma source is high potassium basaltic material in the lower crust, which could be derived from an enriched mantle source. LA-ICP-MS zircon U–Pb dating of the granodiorite porphyry yields a late Indosinian age (concordia age of 227±1Ma; MSWD=0.31), which is close to the molybdenite Re–Os isochron age (227.2±1.9Ma), indicating further the close relationship between the granodiorite porphyry and the Cu–(Mo) mineralization. These samples are LREE and LILE (e.g., Rb, K, Ba, and Sr) enriched, and HFSE (e.g., Nb, Ta, P, and Ti) depleted, especially in P and Ti, similar to the characteristics of volcanic arc magmas. This intrusion most likely formed during the later stage of Indosinian deep subduction of oceanic slab. This was associated with underplating of mantle-derived magmas, which provided heat for crustal melting. Similar to the Jiadanggen granodiorite porphyry, Indosinian hypabyssal intermediate-felsic intrusive rocks, formed under subduction tectonism or a transitional regime from subduction to syn-collision, make up the most important targets for porphyry Cu(Mo) deposits in the Eastern Kunlun metallogenic belt.

Early Cretaceous adakitic magmatism in the Dachagou area, northern Lhasa terrane, Tibet: Implications for slab roll-back and subsequent slab break-off of the lithosphere of the Bangong–Nujiang Ocean

The late Mesozoic geodynamic evolution of the Lhasa terrane (Tibet) remains controversial due to a lack of systematic geochemical and chronological data. Here we present the results of geochemical and zircon U–Pb geochronological studies of two granodiorite plutons in the Dachagou area of the northern Lhasa terrane. The zircon U–Pb dating yielded magmatic crystallization ages of 104 and 117Ma, indicating pluton emplacement in the Early Cretaceous. On the basis of their K2O contents, the granodiorites can be divided into low-K calc-alkaline granodiorites (LKG, 104Ma) and high-K calc-alkaline granodiorites (HKG, 117Ma), and both types are characterized by the adakitic signatures of high Na2O and Sr contents, low Yb and Y abundances, and high Sr/Y ratios. The fact that the LKG and HKG formed at different times and have different geochemical characteristics suggests different petrogenetic mechanisms. Specifically, the LKG were generated by the interaction of an oceanic-slab-derived melt with enriched mantle, while the HKG were derived via the partial melting of the lower crust followed by hybridization with a significant amount of sediment and a minor amount of mantle. Based on these data and the regional tectonic setting, we propose that the LKG and HKG resulted from different geodynamic mechanisms: the HKG were the product of slab roll-back, while the LKG resulted from the subsequent slab break-off during the southwards subduction of the Bangong–Nujiang Ocean seafloor.