Low MgO Contents Research Articles

Genesis of Upper Cambrian-Lower Ordovician dolomites in the Tahe Oilfield, Tarim Basin, NW China: Several limitations from petrology, geochemistry, and fluid inclusions

Thick Upper Cambrian-Lower Ordovician carbonate units in the Tahe Oilfield of northern Tarim Basin were extensively dolomitized during their diagenetic history. Petrographic examination identified three types of matrix dolomites, one type of cement dolomite, and one type of saddle dolomite in these units: micritic to very finely crystalline, nonplanar matrix dolomite (D1); finely crystalline, planar-s(e) matrix dolomite (D2); medium to coarsely crystalline, nonplanar-a matrix dolomite (D3); finely to medium crystalline, planar-e cement dolomite (Cd); and coarsely crystalline, planar-s to nonplanar saddle dolomite (Sd). Isotopic studies (C, O, and Sr), elements measurements, and fluid-inclusion analysis were combined to constrain the origins of dolomitizing fluids. The occurrence of near-micritic dolomite crystals, as well as isotopic compositions which are consistent with the values and ratios of Upper Cambrian-Lower Ordovician marine limestones, coupled with the rare earth elements (REE) patterns with negative Ce anomalies, collectively suggest precipitation of D1 dolomite at low temperatures of near-surface settings from mesosaline to penesaline seawater. D1 dolomite, characterized by high Mg/Ca ratios and MgO values, moderate Sr concentrations, and low Mn and Fe values, was formed in the early diagenetic stage and scarcely experienced recrystallization. The dolomitizing fluid of D2 dolomite was derived from connate seawaters resulting from mechanical compaction and burial dissolution under shallow burial conditions, which is indicated by textures, isotopic compositions, and REE patterns. The relatively low MgO contents and Mg/Ca ratios constrain the formation of the D2 dolomite to a closed diagenetic system with a limited local supply of Mg2+ ions. The large overlaps of isotopic compositions between D3 dolomite and earlier dolomite, as well as the similar REE patterns and depleted Sr contents, indicate that D3 dolomite was likely the result of intense recrystallization upon previous matrix dolomites in a deeply buried formation-fluid system. The petrographic features and geochemical attributes of Cd dolomite strongly indicate progressive growth or recrystallization in the burial environment. Inclusions with high temperatures developed only in the outer edges of Cd crystals demonstrate diffusionally mediated recrystallization during the later hydrothermal activity. Sd dolomite is interpreted to directly precipitate from hydrothermal fluids, as evidenced by high homogenization temperature measured from fluid inclusions and high Sr contents. High salinity levels suggesting the existence of a very saline fluid (deep-basinal brine). The enriched radiogenic strontium isotope coupled with positive Eu anomalies indicate that hydrothermal fluids are likely to be crustal magmatic. Since Early Hercynian orogeny, long period tectonic activities were responsible for the hydrothermal alteration. High-angle faulting, which appeared as strike-slip faulting initiated along basement-rooted faults, likely acted as the important pathway that channeled the hydrothermal fluids from the depths. Initially, hydrothermal fluids with elevated pressure, high temperature, and low fluid/rock conditions were likely responsible for hydrofracturing and brecciation. When distal fluids of cooler temperatures flowed into permeable formations, the open fluid-dominated system with higher fluid/rock conditions facilitated the occurrence of dissolution vugs and precipitation of Sd dolomite. This study provides a basis for understanding the deep burial carbonate reservoirs in Tarim Basin by clarifying the process of dolomitization.

Early Carboniferous adakite-like and I-type granites in central Qiangtang, northern Tibet: Implications for intra-oceanic subduction and back-arc basin formation within the Paleo-Tethys Ocean

Recent studies have proposed that the Late Devonian ophiolites in the central Qiangtang region of northern Tibet were formed in an oceanic back-arc basin setting, which has led to controversy over the subduction setting of the Longmucuo–Shuanghu–Lancangjiang Suture Zone (LSLSZ) during the Late Devonian to Early Carboniferous. In this paper we present new data about a suite of granite plutons that intrude into ophiolite in central Qiangtang. Our aim was to identify the type of subduction and to clarify the existence of an intra-oceanic back-arc basin in the LSLSZ during the Late Devonian to Early Carboniferous. The suite of granites consists of monzogranites, syenogranites, and granodiorites. Our laser ablation–inductively coupled plasma–mass spectrometry zircon U–Pb data yielded Early Carboniferous crystallization ages of 357.2Ma, 357.4Ma and 351.1Ma. We subsequently investigated the petrogenesis and tectonic setting of these granites based on their geochemical and Hf isotopic characteristics. First, we divided the granites into high Sr/Y (HSG) and low Sr/Y granites (LSG). The HSG group contains monzogranites and granodiorites that have similar geochemical characteristics to adakites (i.e., high Sr/Y and La/Yb ratios, low MgO, Y, and Yb contents, and no pronounced negative Eu anomaly), although they have slightly lower Sr and Al2O3 contents, caused by crystal fractionation during late magmatic evolution. Therefore, we define the HSG group as adakite-like granites. The study of the HSG shows that they are adakite-like granites formed by partial melting of oceanic crust and experience fractional crystallization process during late evolution. However, some differences between the monzogranites and granodiorites indicate that there are varying degree contributions of subducted sediments during diagenesis. The LSG group contains syenogranites that have distinct negative correlations between their P2O5 and SiO2 contents, and Y and Th contents have significant positive correlations with Rb. The above characteristics indicate that the syenogranites are typical I-type granites. The results of this study also show that the LSG were produced by magma mixing between the mantle and juvenile oceanic crust. The field study found that the Early Carboniferous suite of granites intruded into contemporaneous ophiolites that formed in an intra-oceanic back-arc basin, and were associated with coeval A-type granites in this region. Based on the geochemical and isotopic data presented in this paper and regional geological data, we consider that the HSG were formed during intra-oceanic subduction of the Paleo-Tethys Ocean in the Early Carboniferous. The LSG and A-type granites were formed in an intra-oceanic back-arc basin setting caused by roll-back of the Paleo-Tethys Ocean slab. This confirms that the subduction of the Paleo-Tethys Ocean in the Early Carboniferous was intra-oceanic subduction, and provides important evidence for the existence of an intra-oceanic back-arc basin during the Late Devonian to Early Carboniferous.

Petrogenesis of Cretaceous (133–84 Ma) intermediate dykes and host granites in southeastern China: Implications for lithospheric extension, continental crustal growth, and geodynamics of Palaeo-Pacific subduction

This paper presents U-Pb zircon geochronology, petrology, and major and trace element, Sr-Nd and zircon Hf isotopic geochemistry of Cretaceous granites and intermediate dykes in the Quanzhou and Xiamen regions of southeastern China. These data are used to investigate igneous petrogenesis and Cretaceous tectonic evolution, and interpret the geodynamics of Palaeo-Pacific slab subduction. Granites in Quanzhou and Xiamen range in age from 133Ma to 87Ma, have high SiO2 and K2O contents, low abundances in P2O5, and an A/CNK index that ranges from 0.97 to 1.09, indicating that they are high-K calc-alkaline metaluminous I-type rocks. Slightly negative ɛ Nd (t) values (−1.2 to −4.4), young Nd model ages (0.87Ga to 1.20Ga) and positive ɛ Hf (t) values (−0.5 to +9.9) of zircon grains indicate that the granites were derived from magmas that melted amphibolite in the middle-lower crust, and which may have assimilated country rocks during emplacement in shallow chambers. The intermediate dykes have no genetic link to the granites and magma mixing was negligible. Eight dyke samples have low SiO2 and high MgO, Ni and Cr contents. Negative ε Nd (t) values (−1.5 to −2.7) and positive ε Hf (t) values (2.7 to 7.6) suggest that the dykes were derived from residual basic lower crust after mafic-crystal accumulation. Two samples of adakite-like dykes are characterised by high Sr/Y ratios (89 to 100) and high SiO2, low K2O, Ni, Cr contents. In combination with slightly negative ε Nd (t) values (−1.7 to −1.8) and positive ε Hf (t) values (2.9 to 4.3), the adakite-like dykes were derived from cumulate basic lower crust which had a mixed source between depleted mantle- and crust-derived melts. Based on our data, combined with previously published work, we suggest that extension-induced middle-lower crustal melting and underplating of mantle-derived basaltic melts were the principal driving mechanisms for Cretaceous granitic magmatism in coastal Fujian Province. Extension was related to subduction retreat whereas steep slab subduction caused underplating of mantle-derived basaltic melts. These processes were coupled and mainly responsible for the tectonic transition during the Cretaceous from compression to extension in the coastal belt of the Cathaysia Plate.

Paleocene (c. 62 Ma) Leucogranites in Southern Lhasa, Tibet: Products of Syn-collisional Crustal Anatexis during Slab Roll-back?

Voluminous peraluminous leucogranites are common in large-scale orogenic belts and are crucial in gaining a fuller understanding of the related geodynamic process. However, the origin of such syn-collisional leucogranites remains highly controversial. In this contribution, we report petrological and geochemical data for Paleocene (c. 63 Ma) garnet-bearing, two-mica granites and associated biotite granites from the Gangdese batholith in southern Tibet. The Zhengga biotite granites have high SiO2 (70–73 wt %) and low MgO (0·4–0·7 wt %) contents with initial 87Sr/86Sr ratios of 0·7049–0·7050, eNd(t) values of +0·5 to +1·2 and zircon δ18O values of 5·6–6·9‰, similar to most early Paleocene granitoids in southern Lhasa. These geochemical characteristics suggest that the Zhengga biotite granites were derived from a crustal source that mixed with variable amounts of Gangdese juvenile lower crust and minor ancient crust-derived melts. The Zhengga peraluminous, garnet-bearing, two-mica granites have similar Sr–Nd–O isotope compositions to the biotite granites (0·7037–0·7050, +0·4 to +0·8, 5·5–7·3‰, respectively) as well as higher SiO2 (73–76 wt %) and lower TiO2 (<0·06 wt %), MgO (<0·3 wt %), Fe2 OT3 (<2 wt %) and CaO (<0·7 wt %) contents. These most probably represent highly evolved biotite granite magmas that differentiated in the mid-crust. The first contact of India with Asia appears to have occurred in central Lhasa during the early Paleocene (65–63 Ma) and led to crustal thickening and cessation of magmatism. Early Paleocene slab roll-back would have significantly enhanced asthenospheric corner flow and supplied a long-lived heat source for coeval crustal anatexis and metamorphism in southern Lhasa during the early phase of continental collision. Similar interaction between continental collision and oceanic subduction may also occur in other large-scale convergence zones in which the lithosphere and crust are anomalously hot.

Leucogranites in Lhozag, southern Tibet: Implications for the tectonic evolution of the eastern Himalaya

Petrogenesis of the Himalayan leucogranite is strongly influenced by conditions which are associated with the tectonic evolution of Himalayan orogen. In this article, we present petrological, geochronological and geochemical results of the Lhozag leucogranites that crop out alongside the South Tibetan Detachment System (STDS) in the east of Himalaya. Zircon U-Pb dating revealed three episodes of leucogranitic magmatism in Lhozag at 17.8±0.1Ma, 15.1±0.1Ma, and 12.0±0.1Ma, respectively. The Lhozag leucogranites show relatively low εNd(t), low zircon εHf(t) and high initial 87Sr/86Sr ratios, which are similar to the High Himalayan Crystalline Series (HHCS), indicating that they were derived from the HHCS. The characteristics of relatively high Na2O and Rb contents, high Rb/Sr ratios and low CaO, MgO, TFe2O3, TiO2, and Sr contents indicate that both the ca. 18Ma Lhozag tourmaline leucogranites and the ca. 15Ma Lhozag two-mica granites were derived from fluid-absent muscovite-dehydration melting of metasediments. The opposite geochemistry characteristics of the ca. 12Ma Khula Kangri two-mica granites imply that these granites are derived from fluid-present melting of metasediments. Four Khula Kangri two-mica granite samples with relatively lower TiO2, TFe2O3, MgO, and CaO contents, higher Rb concentrations and Rb/Sr ratios could be evolved from the Khula Kangri two-mica granites with relatively lower Rb/Sr ratios. The melting behaviors of the Lhozag leucogranites varied from fluid-absent melting to fluid-present melting, implying that there were P-T-XH2O variations in the deep crust. The tectonic evolution would give rise to variation of P-T-XH2O variation, and subsequent transformation of melting behavior. Our new results display the transformation of melting behavior of the Lhozag leucogranites, which implies the tectonic evolution from earlier N–S extension to later E–W extension in the eastern Himalaya at ca. 12Ma.

Characteristics and reactivity of ferronickel slag powder

In this study, the hydration properties of a blended cement containing 30% ferronickel slag powder are investigated. The activities of electric arc furnace ferronickel slag (EFS) powder and blast furnace ferronickel slag (BFS) powder are both very low under non-alkaline conditions. Due to its higher CaO and Al2O3 contents and lower MgO content, the BFS powder exhibits higher pozzolanic activity than the EFS powder. The reaction of ferronickel slag in the cement decreases the Ca/Si ratio of the calcium silicate hydrate (C-S-H) gel and increases the degree of polymerization of the silicate chains. EFS powder tends to coarsen the pore structure of the hardened paste and decreases the compressive strength of concrete at any age; on the other hand, concrete containing the BFS powder exhibits a late-age microstructure and strength close to or even superior to that of plain cement concrete. Ferronickel slag powder improves the resistance of concrete to chloride ion penetration and the effect of BFS powder is more substantial.

Effect of magnesium content on bioactivity of near invert phosphate‐based glasses

AbstractA series of phosphate glasses 40P2O5‐(40−x)CaO‐xMgO‐(20−y)Na2O‐yTiO2 (where 0 ≤ x ≤ 24 and y = 0 or 1) with varying MgO contents were investigated for their in vitro calcium phosphate (CaP) formation. Thermal analysis of these glass compositions was conducted and a significant decrease in glass transition temperature from 448°C to 430°C was seen with reducing MgO content from 24 to 8 mol%. Degradation studies were performed in phosphate buffered saline (PBS) at 37°C, where the 8 mol% MgO glass showed the highest mass loss of around 3.4% after 28 days of immersion. Cation release studies were conducted via ion chromatography, using ultrapure water at 37°C as the degradation medium. The highest release of Ca2+ and Na+ ions was observed with the 8 mol% MgO glass. In vitro CaP formation studies were conducted using glass discs immersed in simulated body fluid (SBF) at 37°C for up to 28 days. The amorphous phase and chemical composition of deposited CaP layers on the glass discs were confirmed via X‐ray diffraction (XRD) and energy‐dispersive X‐ray spectroscopy (EDX) analysis, respectively. CaP layers with Ca/P ratio 0.8‐1.1 were found to be deposited on the lower MgO content (8 to 2 mol%) glass surface after 28 days of SBF study.

Early Jurassic porphyry copper mineralization in NE China: A case study of the Yanghuidongzi deposit

The Yanghuidongzi Cu deposit is located in the southeastern part of Heilongjiang Province in NE China and represents one of the few known Early Jurassic porphyry Cu deposits within the eastern edge of the Xing’an–Mongolia Orogenic Belt. Here we present new whole-rock major and trace element and Sr–Nd isotopic data, zircon U–Pb and Hf isotopic data, and molybdenite Re–Os data for units associated with the deposit. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) dating of zircons from the granodiorite porphyry in the study area yielded ages of 191.0±1.1 and 191.2±1.0Ma that are interpreted to be the emplacement age of this intrusion. The molybdenite Re–Os dating undertaken during this study yielded model ages of 192.7–187.0Ma, an isochron age of 188.2±3.8Ma, and a weighted mean model age of 190.2±2.6Ma, representing the timing of formation of the Cu mineralization in this area. The Yanghuidongzi granodiorite porphyry contains relatively high concentrations of SiO2 (68.88–72.49wt%), Al2O3 (15.37–16.31wt%), and Sr (230–479ppm), but relatively low concentrations of Y (7.97–9.87ppm) and Yb (0.72–0.85ppm), suggesting that this intrusion has an adakitic affinity. Combining this with the positive εNd (t) (0.7–1.1) and εHf(t) values (4.0–6.9), low Mg# values (41–50), and the low concentrations of MgO (0.86–1.22wt%), Cr (3.17–4.37ppm), and Ni (2.11–4.20ppm) within this intrusion suggests it formed from a magma generated by the partial melting of juvenile lower crustal material during the Early Jurassic subduction of the Paleo-Pacific Plate.

Mafic dikes at Kahel Tabelbala (Daoura, Ougarta Range, south-western Algeria): New insights into the petrology, geochemistry and mantle source characteristics

New petrological, geochemical and Sr–Nd isotopic data of the Late Triassic and Early Jurassic Kahel Tabelbala (KT) mafic dikes (south-western Algeria) offer a unique opportunity to examine the nature of their mantle sources and their geodynamic significance. An alkaline potassic Group 1 of basaltic dikes displaying relatively high MgO, TiO2, Cr and Ni, La/YbN ∼ 15, coupled with low 87Sr/86Sri ∼ 0.7037 and relatively high ɛNd(t) ∼ +3, indicates minor olivine and clinopyroxene fractionation and the existence of a depleted mantle OIB source. Their parental magma was generated from partial melting in the garnet–lherzolite stability field. A tholeiitic Group 2 of doleritic dikes displaying low MgO, Cr and Ni contents, La/YbN ∼ 5, positive Ba, Sr and Pb anomalies, the absence of a negative Nb anomaly coupled with moderate 87Sr/86Sri ∼ 0.7044 and low ɛNd(t) ∼ 0 (BSE-like), indicates a contamination of a mantle-derived magma that experienced crystal fractionation of plagioclase and clinopyroxene. This second group, similar to the low-Ti tholeiitic basalts of the Central Atlantic Magmatic Province (CAMP), was derived from partial melting in the peridotite source within the spinel stability field. Lower Mesozoic continental rifting could have been initiated by a heterogeneous mantle plume that supplied source components beneath Daoura, in the Ougarta Range.

An evolving tectonic environment of Late Carboniferous to Early Permian granitic plutons in the Chinese Altai and Eastern Junggar terranes, Central Asian Orogenic Belt, NW China

The Central Asian Orogenic Belt (CAOB) represents one of the most important sites of juvenile crustal growth during the Phanerozoic. Located in the central part of the CAOB, the Chinese Altai and Eastern Junggar terranes record the collisional processes between the peri-Siberian and Kazakhstan orogenic systems. However, the precise timing of collision between the two terranes remains controversial. The Wukuli and Kadelat plutons in the Chinese Altai belt are dated at ∼305 and ∼280Ma respectively, whereas the Aketas pluton in the Eastern Junggar terrane is dated at ∼308Ma. Granites from the Wukuli and Kadelat plutons are strongly peraluminous (A/CNK>1.1), and are characterized by low Al2O3, Na2O, MnO, MgO, CaO and heavy rare earth element (HREE) contents, but with high SiO2, K2O and Rb contents as well as high Rb/Sr ratios. Granites from the Wukuli pluton have low εNd(t) and εHf(t) values of −3.7 to −3.4 and −9.7 to +4.9, whereas those from the Kadelat pluton have values of −3.6 to −3.4 and −8.0 to +2.6. These features suggest S-type affinity for the Wukuli and Kadelat plutons with magma derivation through partial melting of Mesoproterozoic metasediments. The Aketas pluton is composed of weakly peraluminous quartz monzonites that have A/CNK values ranging from 0.92 to 1.08, with high Na2O, Sr, and Sr/Y, and low Y, Yb, Nb, and Ta. These rocks display positive εNd(t) (+4.8 to +6.4) and εHf(t) (+9.7 to +14.6) values, and low initial 87Sr/86Sr ratios (0.703357–0.703868), similar to modern adakites, suggesting that the quartz monzonites were derived from the partial melting of lower crustal material. The geochemical characteristics suggest that the Aketas pluton was formed in a subduction-related setting, the Wukuli pluton in a syn-collisional setting, and the Kadelat pluton in the subsequent post-orogenic strike-slip-related setting. In combination with data from other granitoids in these two terranes, the Aketas pluton represents the youngest record of subduction-related environments, suggesting that the final collision between the Chinese Altai and Eastern Junggar terranes might have occurred between 308 and 304Ma.

Early Cretaceous diabases, lamprophyres and andesites-dacites in western Shandong, North China Craton: Implications for local delamination and Paleo-Pacific slab rollback

Early Cretaceous tectono-magmatic activity in the eastern North China Craton has long been linked to cratonic destruction, but no systematic investigations about the geodynamic processes have been performed. Early Cretaceous diabases, lamprophyres, andesites and dacites are distributed in the western Shandong Peninsula, and these rocks have a close relationship as they formed synchronously. Zircon U-Pb dating results show that the diabases and lamprophyres formed dykes or sills at 133–127Ma and that the andesites erupted at ∼125Ma. The diabases could be divided into high- and low-Mg# value groups. The high-Mg# (>60) diabases have lower SiO2 (48.44–54.68wt%) and higher MgO (7.41–19.30wt%) contents than the low-Mg# (<60) ones (SiO2: 52.90–56.41wt%; MgO: 2.70–4.37wt%). The lamprophyres (Mg# values of 58–70) exhibit low SiO2 (45.04–49.99wt%) and moderate MgO (4.77–9.87wt%) contents. The diabases and lamprophyres are enriched in large ion lithophile elements (e.g., Ba, Th and U) and light rare earth elements, are depleted in Nb and Ta, and feature negligible Eu anomalies. They have lower zircon εHf(t) values (−12 to −19) and whole-rock εNd(t) values (2 to −12), with (87Sr/86Sr)i ratios of 0.7066–0.7097. The andesites and dacites have intermediate SiO2 contents (60.10–64.95wt%), high Mg# values (56–65), high Sr (1005–1300ppm), and low Y (11.3–12.9ppm) contents, with high Sr/Y ratios (78–115), showing adakitic affinity. They have lower zircon εHf(t) (−12 to −26) and whole-rock εNd(t) (−9 to −13) values and high (87Sr/86Sr)i ratios (0.7109–0.7111). We suggest that the diabases and lamprophyres were derived from the partial melting of enriched lithospheric mantle, and the parental magmas of some of them had more contribution from the asthenosphere. The andesites and dacites were derived from the partial melting of delaminated thickened lower crust with addition of mantle material. Since the late Mesozoic, local delamination may have occurred beneath the Shandong Peninsula. During the Early Cretaceous, the rollback process of the Paleo-Pacific slab beneath the Shandong Peninsula could have facilitated delamination of the lithospheric mantle, and enhances the activity of trans-lithospheric faults.

Petrogenesis and tectonic implications of early Palaeozoic igneous rocks of the western South Qilian Belt, central China

ABSTRACTEarly Palaeozoic granitoids in the South Qilian Belt, central China, record details of the tectonic evolution and crustal growth of the Qilian orogenic belt. Five representative granitoids from the western South Qilian Belt were sampled for zircon LA-ICPMS U–Pb dating, Lu–Hf isotopes, and whole-rock geochemical analyses. Zircon U–Pb dating of two porphyritic granodiorites and a porphyritic monzogranite yielded ages of 442.7 ± 3.5, 441.8 ± 4.3, and 435.4 ± 3.5 Ma, respectively. These granitoids exhibit a geochemical affinity to I-type granite, are metaluminous with a low aluminium saturation index (A/CNK = 0.75–1.15), have moderate Al2O3 and low MgO contents, high La/Yb and low Sr/Y ratios, and are depleted in Nb, Ta, P, and Ti, which suggests a subduction zone magmatic arc affinity, with mixing between a primary mantle-derived magma with lesser continental crustal material. The syenogranite and monzogranite from the South Qilian Belt, which yield U–Pb zircon ages of 440.4 ± 9.0 and 442.3 ± 1.2 Ma, respectively, have pronounced S-type geochemical affinities, are peraluminous with A/CNK values of 1.07–1.16, have relatively high SiO2, Al2O3, K2O, and Rb contents, low Y and Yb, low Sr/Y and La/Yb ratios, positive Th, U, and light Rare Earth Element (REE) anomalies, and depletions in Nb, Ta, Sr, and Ti. Their geochemical signature suggests derivation from partial melting of continental crust in a syn-collisional setting. The Hf isotopic data of zircons from the granitoids show a significant input of Paleoproterozoic crust in the crustal formation of the western South Qilian Belt in Palaeozoic. Compare the εHf(t) value of S-type granite with that of I-type granite, the former may have a comparatively homogeneous source. Together with regional evidence, it is proposed that a collisional event occurred between the South Qilian Belt and the Central Qilian Belt at ca. 442–435 Ma.

Geochemistry and origin of alkaline granites at Wadi Umm Adawi-Yahmid area, south Sinai- Egypt

The alkaline granites of the study area are characterized by the predominance of alkali feldspars, quartz minor amounts of riebeckite, plagioclase and biotite. Accessories are represented by zircon, apatite, titanite, allanite, chevkinite with secondary chlorite. They have peralkaline to slightly metalumminous character and were emplaced during post orogenic stages. The chondrite-normalized REE patterns display enrichment of light REE against depletion of heavy REE with significant Eu anomaly indicating extreme feldspar fractionation. They belong to A-type granites and have mantle derived source characterized by high K2O, extremely Low MgO and CaO contents, the pronounced Eu anomaly and depleted in Ba, Sr and Ta against enrichment of (HFSE) Zr, Y, Rb,Nb and ƩREE. They have formed under low oxygen fugacity (fO2) conditions with high magmatic temperature up to 896 °C with fairly low water pressures. Intrusions of alkali feldspar granite followed by riebeckite granite (alkaline granite) are consistent with fractional crystallization model. This was supported by the presence of syenitic microgranular enclaves with felsic alkaline granite. The most plausible source of these alkaline granites is mantle derived potassium rich source, that may accompanied by further magma mixing or assimilations.

S-type granite from the Gongpoquan arc in the Beishan Orogenic Collage, southern Altaids: Implications for the tectonic transition

Voluminous Paleozoic intrusions occur in the Beishan Orogenic Collage (BOC) and their genesis and tectonic background are important to reconstruct the accretion-collision processes in the southernmost Altaids. Paleozoic is an important period for arc development in the BOC, where the Gongpoquan and Huaniushan arcs are located. There are two pulses of magmatism in the Huaniushan and Gongpoquan arcs, i.e., the ca. 470–423Ma I-type and ca. 424–395MaS- and A-type granitoids. In this study, we focus on two peraluminous granitic plutons in the Gongpoquan arc, i.e., the Baitoushan muscovite granite and Haergen two-mica granite, aiming at unraveling their petrogenesis and tectonic background. Zircon LA-ICP-MS U-Pb dating yields emplacement ages of ca. 409–395Ma and ca. 409Ma for the Baitoushan and Haergen plutons, respectively. Both the granitic plutons are strongly peraluminous with A/CNK ratios of 1.10–1.20, indicative of S-type affinities. The rocks are characterized by high SiO2 and K2O contents with high CaO/Na2O ratios. Moreover, the rocks possess low MgO contents, Rb/Sr and Rb/Ba ratios, together with their relatively high initial 87Sr/86Sr ratios (0.7139–0.7152) and less radiogenic εNd(t) values (−3.15 to −5.17), implying a clay-poor and plagioclase-rich crustal source.Compared with earlier pulse of arc-related magmatism (ca. 470–423Ma), the latter pulse of magmatism (ca. 424–395Ma) consists mainly of “normal granite” characterized by higher SiO2 (>66%) and K2O contents, weaker fractionated REE patterns and lower δEu values, and gabbroic to dioritic intrusions are only sporadic. Moreover, the granitoids of the latter pulse show variable but more crust-like Sr-Nd isotopic compositions ((87Sr/86Sr)0=0.7038–0.7327; εNd(t)=−6.70 to +0.33) than the earlier ones ((87Sr/86Sr)0=0.7024–0.7080; εNd(t)=−2.56 to +8.86), indicating that the Early Devonian (ca. 424–395Ma) experienced extensive crustal melting with minor involvement of mantle materials. Considering Early Devonian geological evidence, the transition from I- to S- and A-type magmatism probably reflects the Early Devonian amalgamation between the Gongpoquan and Huaniushan arcs, which caused not only regional unconformity and strong deformation-metamorphism, but also extensive melting of the accreted crustal materials in the BOC.

A mixed source for the Late Triassic Garzê-Daocheng granitic belt and its implications for the tectonic evolution of the Yidun arc belt, eastern Tibetan Plateau

Many Late Triassic granitic plutons are present in the eastern Yidun arc belt (EYAB), and seven have been investigated in this study. From west to east, they are: the Sucuoma (235±2Ma), Ajisenduo (224±2Ma), Jiaduocuo (218±1Ma), Cuojiaoma (219±1Ma), Maxionggou (225±2Ma), Dongcuo (222±3Ma) and Daocheng (220±2Ma) plutons. Most of the plutons have granitic compositions and contain high SiO2, Al2O3 and K2O+Na2O, but low MgO, FeO* and CaO contents. They have similar trace element patterns, with depletion in high field strength elements (HFSE, e.g., Nb and Ta) and enrichment in large-ion lithophile elements (LILE, e.g., Rb, Th and U). Samples from the Sucuoma, Jiaduocuo and Dongcuo plutons have similar zircon Hf isotopic compositions (εHf(t)=−1.8 to −0.3, −3.1 to 0.4 and −4.6 to −1.4, respectively), whereas those from the Ajisenduo pluton exhibit more unradiogenic Hf (εHf(t)=−11.9 to −4.8). Additionally, the Sucuoma pluton has the lowest initial 87Sr/86Sr values (0.7060–0.7090) but the least negative εNd(t) (−4.9 to −3.3) values, whereas samples from the Ajisenduo pluton have the highest initial 87Sr/86Sr values (0.7111–0.7160), but the most negative εNd(t) values (−7.9 and −11.3). All the samples have similar high radiogenic Pb isotopic compositions (206Pb/204Pb=18.6–19.4; 207Pb/204Pb=15.7–15.8; 208Pb/204Pb=39.1–40.4). Based on the new geochemical data, it is determined that most of the granitoids are I-type granites and the magma source was a mixture of the metamorphic Kangding Complex, which is considered to be the basement of the western Yangtze Craton, and metasediments from the basement. However, the samples from the Ajisenduo pluton are S-type granites that were derived from partial melting of basement metasediments with only limited components from the Kangding Complex. Together with an evaluation of previously-published work, these new data indicate a dominant magmatic peak at ca. 216Ma for the Late Triassic granitoids of the YAB and that the ages of Late Triassic magmatism become younger eastward towards the Garzê-Litang suture zone. We consider that slab roll-back, with subsequent slab break-off, best explains the origin of these granitic plutons.

Petrogenesis of carbonatitic lamproitic dykes from Sidhi gneissic complex, Central India

Petrographic, mineral chemical and whole-rock geochemical characteristics of two newly discovered lamproitic dykes (Dyke 1 and Dyke 2) from the Sidhi Gneissic Complex (SGC), Central India are presented here. Both these dykes have almost similar sequence of mineral-textural patterns indicative of: (1) an early cumulate forming event in a deeper magma chamber where megacrystic/large size phenocrysts of phlogopites have crystallized along with subordinate amount of olivine and clinopyroxene; (2) crystallization at shallow crustal levels promoted fine-grained phlogopite, K-feldspar, calcite and Fe-Ti oxides in the groundmass; (3) dyke emplacement related quench texture (plumose K-feldspar, acicular phlogopites) and finally (4) post emplacement autometasomatism by hydrothermal fluids which percolated as micro-veins and altered the mafic phases. Phlogopite phenocrysts often display resorption textures together with growth zoning indicating that during their crystallization equilibrium at the crystal-melt interface fluctuated multiple times probably due to incremental addition or chaotic dynamic self mixing of the lamproitic magma. Carbonate aggregates as late stage melt segregation are common in both these dykes, however their micro-xenolithic forms suggest that assimilation with a plutonic carbonatite body also played a key role in enhancing the carbonatitic nature of these dykes. Geochemically both dykes are ultrapotassic (K2O/Na2O: 3.0–9.4) with low CaO, Al2O3 and Na2O content and high SiO2 (53.3–55.6 wt.%) and K2O/Al2O3 ratio (0.51–0.89) characterizing them as high-silica lamproites. Inspite of these similarities, many other features indicate that both these dykes have evolved independently from two distinct magmas. In dyke 1, phlogopite composition has evolved towards the minette trend (Al-enrichment) from a differentiated parental magma having low MgO, Ni and Cr content; whereas in dyke 2, phlogopite composition shows an evolutionary affinity towards the lamproite trend (Al-depletion) and crystallized from a more primitive magma having high MgO, Ni and Cr content. Whole-rock trace-elements signatures like enriched LREE, LILE, negative Nb-Ta and positive Pb anomalies; high Rb/Sr, Th/La, Ba/Nb, and low Ba/Rb, Sm/La, Nb/U ratios in both dykes indicate that their parental magmas were sourced from a subduction modified garnet facies mantle containing phlogopite. From various evidences it is proposed that the petrogenesis of studied lamproitic dykes stand out to be an example for the lamproite magma which attained a carbonatitic character and undergone diverse chemical evolution in response to parental melt composition, storage at deep crustal level and autometasomatism.

Geochemistry of Martian basalts with constraints on magma genesis

Over the past decade, our knowledge of the geochemical and mineralogical diversity of Martian rocks has greatly increased. Rocks on the surface of Mars are older than most Martian meteorites and have a wider range in bulk chemistry (higher SiO2, higher total alkalis, and lower MgO contents). Recent Martian meteorite discoveries (NWA 7034, NWA 7635, and NWA 8159) are also significantly older than the shergottites and NWA 7034 contains clasts which have a similar range in chemistry of the surface basalts. In this invited review, I summarize what is known about the bulk chemistry (major elements) of Martian igneous rocks and use the chemistry to constrain the formation conditions in the interior, and how these conditions have changed through time.The basalts at Gale Crater are consistent with forming from a metasomatized mantle source which affected not only the chemistry of the basalts, but also the formation conditions. Basalts at Gusev Crater, Bounce Rock at Meridiani Planum, and those basalts not affected by metasomatism like those at Gale Crater have a mantle potential temperature of ~1450°C for the Noachian. Conversely, Martian shergottites have a much higher mantle potential temperature (~1745°C), which has always been problematic to explain for such young rocks. New formation conditions calculations are consistent with previous models of formation by a hot mantle plume, but solve the pressure of formation paradox from the previous model. Combining the calculations for mantle potential temperature with previous estimates for GRS measurements of surface volcanics from the Hesperian and Amazonian, I calculate a baseline cooling curve and mantle melting conditions for the Martian interior through time.

Zircon U–Pb–Hf isotopic and geochemical characteristics of the Xierzi biotite monzogranite pluton, Linxi, Inner Mongolia and its tectonic implications

The opening, subduction and final closure of the Paleo-Asian Ocean led to the formation of the Central Asian Orogenic Belt. Controversy has long surrounded the timing of final closure of the Paleo-Asian Ocean. Here we present zircon U–Pb ages and petrological, geochemical and in situ Hf isotope data for the Xierzi biotite monzogranite pluton, Linxi, SE Inner Mongolia. U–Pb dating of zircon by LA-ICP-MS yields a middle Permian emplacement age (268.7 ± 2.3 Ma) for the Xierzi pluton that is dominated by biotite monzogranites with high SiO2 (71.2–72.8 wt.%), alkali (Na2O + K2O = 8.05–8.44 wt.%), Al2O3 (14.4–15.2 wt.%) and Fe2O3T relative to low MgO contents, yielding Fe2O3T/MgO ratios of 2.87–3.44, and plotting within the high-K calc-alkaline field on a SiO2 vs. K2O diagram. The aluminum saturation indexes (A/CNK) of the biotite monzogranites range from 1.06 to 1.19, corresponding to weakly to strongly peraluminous. They are enriched in rare earth elements (REE), high field strength elements (HFSEs; Zr, Hf), and large ion lithophile elements (LILEs; Rb, U, Th). The LREEs are enriched relative to the HREEs, with a distinct negative Eu anomaly in a chondrite–normalized REE diagram. Geochemically, the Xierzi biotite monzogranite is classified as an aluminous A-type granite, with all samples plotting within the A2-type granite field on a Y/Nb vs. Rb/Nb diagram. Zircon εHf(t) values and two-stage modal ages of the zircons within the pluton range from +4.80 to +13.65 and from 983 to 418 Ma, respectively, indicating that the primary magma was generated through partial melting of felsic rocks from juvenile crust. Consequently, these results demonstrate that the Xierzi pluton formed under the post-orogenic extensional setting after arc–continent collision in the middle Permian.

Combined spectroscopic analysis of stratified glass eye beads from China dated to the Warring States Period

We herein report the application of a combined spectroscopic approach for the analysis of eight stratified glass eye beads unearthed from China, which was dated back to the Warring States Period (475 bc to 221 bc). Portable X‐ray fluorescence spectrometry (pXRF) was used to determine that seven samples were typical natron type soda–lime–silicate glass beads with low contents of MgO and K2O except for one plant‐ash type soda–lime–silicate sample. In addition, combined with micro‐Raman spectroscopy, two different types of colorants were present in these beads, namely oxidized transition metals such as Co, Cu and Fe, and antimony‐based compound opacifiers/colorants. Furthermore, the distribution of their glassy and scattering phases could be determined by the optical coherence tomography (OCT) images, in which it exhibits that their morphologies were related to their Sb contents. According to the determined typology, chemical composition, micro‐crystal, and eyeball structural characteristics within the samples, the possible relations among them were speculated. It is the most interesting finding that seven natron type soda–lime glasses could be imported from the eastern Mediterranean area and one plant‐ash type glass could be imported from the eastern region of the Euphrates River such as Iran area. Finally, through comparison with similar contemporary beads excavated from the famous tomb of Marquis Yi of the Zeng State (~433 bc) in the Hubei Province, provenance studies of these beads were carried out, and their origins and relationships are discussed briefly. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Ultra-refractory mantle domains in the Luqu ophiolite (Tibet): Petrology and tectonic setting

Fresh mantle peridotites occur in the Luqu ophiolite, which is located at the central segment of the Yarlung-Tsangpo Suture Zone in the Tibetan Plateau. This study presents major and trace element compositions of clinopyroxene from harzburgites and dunites of the Luqu ophiolite, with the aim to study melt depletion and late-stage melt–rock interaction processes. The studied harzburgites have refractory to ultra-refractory compositions, as indicated by low whole-rock Al2O3 and high MgO contents, as well as high spinel Cr# values. Clinopyroxenes in the Luqu harzburgites are variably depleted in light rare earth elements (LREEs) relative to the middle and heavy rare earth elements (MREEs and HREEs), with Yb contents of 1.4–3.4 times of CI chondrites. Modeling results suggest that the Luqu harzburgites have experienced up to 8% garnet-facies melting followed by 10–18% fractional melting in the stability field of spinel. In both cases melting occurred under anhydrous conditions. This suggests that the Luqu ophiolite originated from a mid-ocean ridge setting rather than a supra-subduction zone environment. Nevertheless, the high degrees of partial melting are inconsistent with the thin oceanic crustal sequence preserved in the Luqu ophiolite, implying that the Luqu peridotites might have experienced melt extraction prior to entering into the spreading centers of the Neo-Tethyan Ocean during the Early Cretaceous. Clinopyroxenes in the dunites are interstitial among olivine grains. They display slightly LREE-enriched patterns and have higher REE contents than those in the harzburgites. They also show markedly positive anomalies in Sr, Zr and Hf. These features suggest that the dunites were generated through reaction between percolating melts and the harzburgites, during which the interstitial clinopyroxenes formed.