Quartz Syenite Research Articles

Sediment-derived origin of the putative Munnar carbonatite, South India

Metacarbonate assemblages in high-grade metamorphic terranes often pose challenges when trying to distinguish between mantle-derived carbonatite and sedimentary carbonate protoliths. We present a study of granulite-facies metacarbonate samples of the putative Munnar carbonatite described as decimeter-thick dikes and veins, and layers of a meter-thick metacarbonate and calc-silicate assemblage, respectively. Thin sections of the metacarbonate dike samples show absence of pyrochlore and ubiquitous scapolite, titanite, wollastonite, and detrital zircons are compatible with impure limestone protoliths. Nd and Sr isotope compositions indicate protoliths with Paleoproterozoic crustal residence times which contrast the mantle sources of Indian and global carbonatites. Trace-element patterns display the characteristics of upper crust, and Ce- and Y-anomalies in a number of samples suggest protolith formation under marine conditions. Carbon and oxygen isotope compositions of the metacarbonate samples interlayered with calc-silicate rocks are similar to those in marine limestone. The metacarbonate dikes, however, show mantle-like compositions which are interpreted as reflecting equilibration with mantle-derived CO2 during granulite-facies metamorphism. The dikes yielded a U-Pb zircon crystallization age of 1020 ± 70 Ma and a cross-cutting quartz syenite, thought to be cogenetic, a magmatic age of 620 ± 35 Ma; the hosting gneiss provided a magmatic age of 2452 ± 14 Ma. We conclude that the layered metacarbonate and calc-silicate rocks represent a former marine limestone and marl sequence and the metacarbonate dikes and veins small-volume melts of crust-derived carbonate-rich sediment.

Genesis of the Yujiadian F-Pb-Zn-Ag deposit, Inner Mongolia, NE China: Constraints from geochemistry, fluid inclusion, zircon geochronology and stable isotopes

The Yujiadian F-Pb-Zn-Ag deposit in the South Great Xing’an Range (SGXR) consists of vein-type fluorite and F-Pb-Zn-Ag mineralization associated with granitoid. The orebodies occur in NS-trending extensional faults. Wallrock alterations are characterized by a series of medium-low temperature hydrothermal alteration, including silification, kaolinization, chloritization, sericitization, pyritic and carbonate alteration. Four stages of primary paragenetic sequences are recognized: polymetallic sulfide stage (stage I), sulfide fluorite sub-stage (stage II-1), silver-fluorite sub-stage (stage II-2), pure fluorite stage (stage III) and late carbonate stage (stage IV). Silver mineralization primarily belongs to the stage II. The 139–135 Ma quartz syenite porphyry is metaluminous and peralkalic, and is classified as A-type granite that formed in a post-orogenic extensional setting. The δ34S (1.8‰) of hydrothermal fluid calculated from coeval minerals (barite and galena) is comparable with the majority of adjacent magmatic hydrothermal deposits in the SGXR, implying a magmatic source. Microthermometric measurements show that ore minerals were deposited at intermediate temperatures (109.7–320.1 °C) and had low salinities (0.18–5.62 wt% NaCl equive). Ore-forming fluid originated from post-magmatic hydrothermal fluid and mixed with heated meteoric water. Successive precipitation of base metals occurred with changing physicochemical conditions and boiling of ore fluid. The main precipitation mechanism for fluorite in the late stage is related to fluid-rock reaction. We propose that the A-type granite related to the fracture-controlled hydrothermal vein-type Yujiadian F-Pb-Zn-Ag deposit forms part of the magmatic response to Paleo-Pacific subduction and associated lithospheric thinning, extension and delamination processes. Furthermore, the deposit represents the terminal and distal member of the regional Mesozoic large-scale mineralization events.

Geological and geochemical constraints on the genesis of the Bengge gold deposit in the Yidun Terrane, SE Tibet

The Bengge gold deposit is one of a small number of deposits in the Zhongdian-Muli region, which is part of the southern segment of the Yidun Terrane in the eastern Tibetan Plateau. The deposit is hosted in a Late Triassic alkaline igneous complex comprising mainly biotite–pyroxene syenite and quartz syenite, emplaced into Triassic slate. These syenites show phyllic alteration with mineral association of quartz–sericite–pyrite–carbonate. The main gold mineralization occurs in fault zones as gold–sulfide–quartz veins, in altered syenite as telluride in veinlets, and as fissure-filling gold in disseminated pyrite. Four stages of veins are recognized: sheeted K-feldspar–quartz–magnetite veins (dikes) (stage I), quartz–carbonate–K-feldspar veins with sericite–quartz–pyrite alteration haloes (stage II), polymetallic sulfide–telluride–sericite–quartz veins with pervasive phyllic alteration (stage III), and quartz–carbonate–stibnite veins (stage Ⅳ). The stage II and III veins are closely related to gold mineralization.40Ar/39Ar dating of hydrothermal sericites from the phyllic alteration zone, which are intergrown with gold-bearing pyrite and telluride, yielded an age of ~126 Ma, indicating the mineralization event occurred in the Cretaceous extensional setting. Raman spectroscopy and microthermometry study for fluid inclusions in quartz from stage II revealed heterogeneous trapping of low-salinity (0.14–6.63 wt% NaCl equivalent) and mesothermal (270–300 °C) CO2–H2O–NaCl ore-forming hydrothermal fluids. The calculated fluid-trapping pressures range mainly from 750 to 850 bar. δ18O values of quartz from the gold mineralization stage (stage II) vary between 15.21‰ and 19.35‰. The δ18O values for waters in equilibrium with quartz vary from 6.96‰ to 10.73‰, which are consistent with values for magmatic waters. We propose that the auriferous fluids were derived from an underlying intrusion during the Cretaceous. The geological characteristics, low salinity, and mesothermal CO2–H2O–NaCl fluids, and the magmatic source of the ore-forming fluids, all support the hypothesis that the Bengge deposit is an intrusion-related gold deposit.

Origin of late Neoarchean granitoid diversity in the Western Shandong province, North China Craton

Diverse late Neoarchean granitoids are widely exposed in the Western Shandong province (WSP). According to geological relationships, lithological features, magmatic zircon U–Pb–Lu–Hf isotopes and whole-rock chemical characteristics, eight main lithological assemblages are identified, and their petrogenesis is determined. (1) The ~2.54 Ga and ~2.51 Ga potassic granites show the highest SiO2 contents (67.06–75.84 wt%) and were sourced from partial melting of metamorphic greywackes. (2) The ~2.53 Ga quartz syenites exhibit the highest alkali contents (10.32–11.23 wt%) and were sourced from mantle-derived basaltic rocks and recycled sediments. (3) The ~2.53–2.52 Ga quartz diorites (less-differentiated sanukitoids) formed from partial melting of metasomatized mantle. (4) The ~2.53 Ga hornblendites have crystallization ages and Lu–Hf isotopes similar to those of the quartz diorites, implying that they are cogenetic cumulates from the quartz dioritic magmas. (5) The ~2.56–2.52 Ga tonalite-trondhjemite-granodiorite (TTG) gneisses were formed by melting of thickened crust. (6) The ~2.53 Ga high-Mg granodiorites (differentiated sanukitoids) were formed by mixing of crust-mantle melts. (7) The ~2.51–2.49 Ga gabbros originated from partial melting of fluid-metasomatized mantle. (8) The ~2.54–2.53 Ga high-Mg andesites (boninites) originated from partial melting of refractory mantle that experienced the extraction of basaltic magma at earlier stage and was then enriched in LILEs and LREEs by slab-derived melts.Integrating the petrogenesis of these late Neoarchean diversified granitoids and associated volcanic rocks, we consider that the late Neoarchean was a crucial period of crust-mantle interaction in the WSP, and partial melting of the metasomatized mantle produced intermediate-mafic melts, then underplating of the mantle-derived magmas caused the partial melting of crustal materials, further led to the mixing of crust-mantle magmas. The above complex crust-mantle interactions triggered the transfer of energy and materials from the Earth’s interior to the surface, forming modern-like continental crust thickness and geothermal gradient. Moreover, combining late Neoarchean intense granitic magmatism and ~2.49 Ga mafic dike swarms implies that the North China Craton experienced cratonization processes at the end of the Neoarchean.

Petrogenesis and tectonic significance of the Fenshuiling and Mengha granitic plutons, SW China: insights from bulk elements, zircon U-Pb ages and Sr-Nd-Hf isotopes

ABSTRACT The Ailaoshan shear zone developed plenty of Cenozoic granitic plutons, located between the Yangtze block in the east and the Simao block in the west. Although a mass of previous studies have enriched the understanding of petrogenesis of these plutons, there are few reports on the plutons within the Mojiang-Lvchun terrane, a suspect domain between the Yangtze and Simao blocks. Among two plutons investigated in this paper, the Fenshuiling pluton is characterized by strongly shoshonitic (K20 = 5.8–6.4 wt.%) nature, high Y (6.5–30.0 ppm) and Yb (0.88–2.97 ppm) contents, and has limited variations in La/Yb (8.6–20.5) and Dy/Yb (1.6–1.9) with increasing SiO2. In Mengha, the pluton is also strongly shoshonitic (K20 = 5.7–6.1 wt.%), high Y (18.7–32.3 ppm) and Yb (2.03–3.50 ppm) contents, and shares limited variations in La/Yb (17.0–17.6) and Dy/Yb (1.5–1.7) with increasing SiO2. These tendencies correspond with the differentiation from a shoshonitic mafic melt by olivine, clinopyroxene and plagioclase (gabbroic) dominated fractional crystallization. These plutons mainly composed of quartz syenites are characterized by island-arc basalt (IAB) patterns, which are consistent with those evolved by shoshonitic mafic melt derived from metasomatic lithospheric mantle within the Yangtze block. Zircon εHf(t) values of Fenshuiling and Mengha plutons range from −4.1 to −0.7 and −2.8 to −1.1, respectively, and their whole-rock εNd(t) values are −5.0 to −4.6 and −4.7 to −4.6, respectively. These Hf isotopes are similar to those from Neoproterozoic granitoids caused by a persistent Neoproterozoic subduction around the southwestern margin of the Yangtze block. This similarity supports a scenario that the enriched lithospheric mantle source area of these plutons caused by the Neoproterozoic subduction, also means that the suspect terrane belongs to the Yangtze block. The location of the Fenshuiling and Mengha plutons separated from those within the western margin of the Yangtze block is attributed to Cenozoic sinistral shearing events.

Genesis of rhyolitic melts in the upper crust: Fractionation and remobilization of an intermediate cumulate at Lake City caldera, Colorado, USA

Lake City Caldera (22.93 ± 0.02 Ma) is the youngest of 25 Tertiary calderas within the Southern Rocky Mountain Volcanic Field (SRMVF) and offers an opportunity to study the relationship between plutonic rocks and their volcanic equivalents. Extreme topographical relief of the area reveals the three-dimensional exposure of a complex, high-K calc-alkaline, magmatic system. Lake City caldera is comprised of two principal units: 1) a resurgent quartz syenite intrusion, and 2) the Sunshine Peak Tuff (Lower, Middle, and Upper). The Lower and Middle Sunshine Peak Tuff (LSPT and MSPT) are crystal-poor rhyolite, while the Upper Sunshine Peak Tuff (USPT) is a crystal-rich trachyte. Rhyolite-MELTS modeling and geochemical analyses show that Lake City rhyolites were formed by melt extraction from a long-lived quartz syenitic magma reservoir at intermediate crystallinity (~50–70% crystals). A slight compositional zoning from the Lower Sunshine Peak Tuff (~76 wt% SiO2) to the Middle Sunshine Peak Tuff (~74 wt% SiO2) ignimbrites indicates that further modification by either mixing or additional crystal fractionation might have occurred following melt extraction from the mushy syenitic reservoir. Geochemical and textural analyses show that the Upper Sunshine Peak trachyte was formed by later re-melting of a portion of the left-over syenite cumulate. Quartz syenite crystal size distributions of potassium feldspar show the characteristic shape of crystal accumulation, and both volcanic and plutonic units contain abundant glomerocrysts. The presence of mafic enclaves within both the syenite and USPT trachyte, coupled with large increases of Ba content in rims of the K-feldspars from the trachyte suggest that the injection of less evolved magma into the host reservoir caused the re-melting of the syenite cumulate mush, allowing for the eruption of the full SPT sequence. Titanium-in-quartz thermobarometry shows that the majority of grains in all units formed below ~800 °C, further providing evidence for a petrogenic relationship for all the rocks comprising Lake City Caldera.

Age, origin and tectonic setting of Dulaankhan granitic pluton in northern Mongolia

Dulaankhan granitic pluton, which is situated in northern Mongolia, the southern portion of the Mongolian-Transbaikalian belt (MTB), is petrographically composed of fine to medium-grained peralkaline granite and is intruded by a small body of quartz syenite. Geochemical data show the Dulaankhan granite and the intruding quartz syenite are both slightly peraluminous and high-K calc-alkaline, and are enriched in LREEs relative to the HREEs, with negative Eu anomaly, and in large ion lithophile elements (LILEs; such as K, Cs and Rb) with respect to high field strength elements (HFSEs; e.g., Nb, Ta and Ti). In terms of relations of Nb, Zr and Y to Ga/Al, however, the Dulaankhan granite and quartz syenite show geochemical features of A-type granites and can be classified into the A2-sub type granite, implying that the pluton formed in an post-collision extensional environment. LA-ICPMS zircon U-Pb dating results suggest that the Dulaankhan granite crystallized at 198±1 Ma, whereas the intruding quartz syenite at 180±1 Ma, consistent with our field observation that the quartz syenite intrudes the granite, attesting that the two granitic bodies were emplaced at different times although both of them formed during the Early Jurassic period. According to these new data, as well as regional ones, we propose that the Dulaankhan granitic pluton was likely generated in the post-collision setting related to the orogenesis of the Mongol-Okhotsk belt that seems to occur prior to Early Jurassic in the northern Mongolian segment.

Petrogenesis of the Neoarchean granitoids and crustal oxidation states in the Western Shandong Province, North China Craton

Na-rich and K-rich granitoid rocks contain geological signatures that are crucial for understanding the global evolution and oxidation states of the Archean crust. Here we present an integrated investigation of the lithology, geochronology, and geochemistry of Neoarchean plutonic granitoid gneisses and supracrustal rocks from Xintai to Feixian within the Western Shandong Province (WSP) granite-greenstone belt of the North China Craton (NCC). Six main lithological assemblages are identified within the WSP granite-greenstone belt: (1) The ∼2.69–2.68 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses, which were derived from the partial melting of low-K mafic rocks at lower crust, have low MgO contents (0.68–2.11 wt%) and high (La/Yb)N ratios (16.7–92). (2) The ∼2.53 Ga granodioritic gneisses exhibit a low MgO content (0.87–1.91 wt%) but high K2O content (3.06–4.66 wt%), their magma originated from the partial melting of high-K mafic rocks. (3) The ∼2.54–2.49 Ga monzogranitic gneisses show the highest SiO2 content (71.67–75.26 wt%), and their magma was formed from the partial melting of metagreywackes. (4) The ∼2.56–2.52 Ga TTG gneisses also have low MgO content (1.42–1.69 wt%) and relatively low (La/Yb)N ratios (17.1–23); their magma was produced by the partial melting of low-K mafic rocks at lower crustal levels. (5) The ∼2.53–2.52 Ga quartz syenites have the highest alkali content (9.66–11.23 wt%) but relatively low MgO content (0.71–1.50 wt%), and their magma was generated by the partial melting of juvenile basaltic rocks associated with recycled sediments. (6) An ∼2.73 Ga xenolith of plagioclase amphibolite is characterized by high positive εHf(t2) values from +5.2 to +8.8, and fractionated chondrite-normalized REE patterns, indicating it may have been derived from a depleted mantle source enriched either by the addition of melt or by fluid metasomatism.The oxygen fugacity values of the crust-derived Neoarchean granitoid magmas are calculated based on a zircon trace element oxybarometer, the results show that the log (f O2) values for the ∼2.7 Ga TTG gneisses that range from FMQ-7.4 to FMQ+5.5 with an average value of FMQ+0.5, whereas the various ∼2.5 Ga granitoid gneisses have log (f O2) values ranging from FMQ-4.3 to FMQ+8.9 with an average value of FMQ+3.3. In conjunction with mineralogical results, we consider that the ∼2.7 Ga continental crust was in a relatively more reduced state than the ∼2.5 Ga continental crust in the WSP, which may be associated with differences in their source regions and compositions.

PETROLOGY AND GEOCHEMISTRY OF INTRUSIVE ROCKS FROM THE SOUTH OF NAQADEH, WEST AZERBAIJAN, IRAN

The intrusive rocks of Naqadeh are features of Laramide magmatism in the Sanandaj–Sirjan zone. According to petrographic studies, the composition of intrusions consist of acidic, intermediate, and basic rocks. including syenogranites, monzogranite, granodiorites, quartz diorites, quartz monzonites, quartz monzonite, quartz syenite, and olivine gabbro—in order of abundance. Minerals forming intrusive rocks include alkali feldspar (orthoclase and microcline), quartz, plagioclase (andesine to anorthite), amphibole (magnesiohornblende to tschermakite hornblende), pyroxene (diopside), olivine (hyalosiderite to hortonlite), apatite, titanite, zircon, muscovite, and opaque minerals. The chemical analysis of the granitoids revealed their potassium-rich calc-alkaline nature, falling within the meta-alumin to per-alumin range in terms of alumin saturation. Tectonomagmatic diagrams for the studied rocks suggest that they are compatible with granitoids of the continental-subduction zone. Primitive-mantle- and chondrite-normalized spider diagrams indicated no depletion in any elements, exhibiting a negative overall slope which is consistent with subduction patterns (negative Ta, Ti, Zr anomalies, and positive K anomaly). Overall, a review of field observations, mineralogy, geochemistry, and diagrams illustrating the sources of granites showed that the intrusive rocks in the study region are, in fact, I-type granite and that the magmatism is a result of the subduction of the oceanic crust, Neotethys, under the Iranian continental crust.

Geochemical constraints on the origin and tectonic setting of Chargar intrusions in the Alborz orogenic belt, NW Iran

Chargar area is located in the southern border of Tarom subzone within the Alborz magmatic belt of NW Iran. Two types of intrusions, mainly present in the southern part of the area, have been identified. These bodies intruded into the Karaj Formation volcanic and volcaniclastic rocks. Type-I intrusions occur in the south and include two magmatic bodies: (a) Gabbro-pyroxene quartz monzodiorite-quartz monzodiorite and (b) quartz syenite. Type-II crops out in the west and has a gabbro – gabbro-diorite composition. Geochemically, Chargar intrusive rocks belong to the high-K calc-alkaline to shoshonitic series and are classified as I-type and metaluminous granitoids. These intrusive rocks show insignificant different distribution patterns of trace elements and REEs, but generally they are characterized by highly enriched large ion lithophile elements (e.g., K and Ba) and depleted Nb, similar to that of continental arc magmatism. Type-I intrusions and Type-II mafic intrusion were originated from mantle source with amphibole-bearing spinel peridotite composition. These intrusions formed in a continental arc to post-collisional tectonic setting.

Origin of Late Permian syenite and gabbro from the Panxi rift, SW China: The fractionation process of mafic magma in the inner zone of the Emeishan mantle plume

Contemporaneous alkaline and gabbroic intrusions are commonly observed around high-Ti basalt in the inner zone of Emeishan large igneous province (ELIP), the distribution and origin of these igneous rocks are significant to understand the process associated with a head of Emeishan mantle plume activity. In this study, we present detailed petrology, chronology, major and trace element geochemistry, whole rock Sr-Nd isotopes and zircon Hf isotopes for quartz syenite and hornblende gabbro from Miyi area, middle-southern Panxi rift. The Hengshan quartz syenite closely lies to the west of the Hepingcun hornblende gabbro pluton, and both units have identical U-Pb ages (260 Ma). The quartz syenite has high SiO2 (66.74 to 70.63 wt%), Na2O (4.94 to 5.64 wt%) and K2O (4.95 to 5.35 wt%) contents, with significantly negative Eu and Sr anomalies, while the hornblende gabbro displays lower SiO2 (47.80 to 48.85 wt%) and higher MgO (8.09 to 9.06 wt%), TFe2O3 (10.57 to 11.16 wt%) contents with slightly positive Eu and Sr anomalies. The highest εHf(t) values from quartz syenites (QS-1) is up to +10, which is similar to the values of hornblende gabbro (+6.6 to +9.4), indicating the both magmas were predominantly from a mantle-derived component. The negative εNd(t) values (−3.9 to −5.2) and variations in overall εHf(t) values (−5.4 to +1.3) for quartz syenite indicate the rocks had undergone certain extent of contamination by crustal material, combined with the reciprocal normalized trace element patterns, suggesting a common source of both two units. The syenitic and gabbroic units exhibit well-developed correlation of SiO2 with various major oxides, indicating the fractional crystallization plays a major role in magma evolution. We use the MELTS modeling to evaluate the detail fractional crystallization process and the results show that magma composition similar to the quartz syenite can be produced by fractional crystallization of ~4% olivine, ~11% clinopyroxene, ~9% orthopyroxene, ~40% plagioclase and ~8% oxide from a high-Ti basalt magma at ~900 °C. We propose the syenite-gabbro association in Panxi rift coincides with final activity of the Emeishan mantle plume, and the lower part formed the hornblende gabbroic unit in a closed system, the upper part of the intrusion formed the syenitic unit and mixed with the crust material.

Zircon U-Pb age and Hf-O isotope insights into genesis of Permian Tarim felsic rocks, NW China: Implications for crustal melting in response to a mantle plume

Deciphering the contribution of crustal materials to A-type granites is critical to understanding their petrogenesis. Abundant alkaline syenitic and granitic intrusions distributed in Tarim Large Igneous Province, NW China, offer a good opportunity to address relevant issues. This paper presents new zircon Hf-O isotopic data and U-Pb dates on these intrusions, together with whole-rock geochemical compositions, to constrain crustal melting processes associated with a mantle plume. The ∼280 Ma Xiaohaizi quartz syenite porphyry and syenite exhibit identical zircon δ18O values of 4.40 ± 0.34‰ (2σ) and 4.48 ± 0.28‰ (2σ), respectively, corresponding to whole-rock δ18O values of 5.6‰ and 6.0‰, respectively. These values are similar to mantle value and suggest an origin of closed-system fractional crystallization from Tarim plume-derived melts. In contrast, the ∼275 Ma Halajun A-type granites have higher δ18O values (8.82–9.26‰) than the mantle. Together with their whole-rock εNd(t) (−2.0–+0.6) and zircon εHf(t) (−0.6–+1.5) values, they were derived from mixing between crust- and mantle-derived melts. These felsic rocks thus record crustal melting above the Tarim mantle plume. At ∼280–275 Ma, melts derived from decompression melting of Tarim mantle plume were emplaced into the crust, where fractional crystallization of a common parental magma generated mafic-ultramafic complex, syenite, and quartz syenite porphyry as exemplified in the Xiaohaizi region. Meanwhile, partial melting of upper crustal materials would occur in response to basaltic magma underplating. The resultant partial melts mixed with Tarim plume-derived basaltic magmas coupled with fractional crystallization led to formation of the Halajun A-type granites.

Petrogenesis and tectonic implications of Early Cretaceous shoshonitic syenites in the northern Wuyi Mt Range, Southeast China

Potassic igneous rocks, such as shoshonitic rocks, are an important constituent of young volcanic-intrusive suites in varied tectonic settings. The genesis of these rocks is related to tectonic environments and is thus important for tectonic reconstruction of ancient terranes. Here we report zircon U–Pb and Hf isotopes and whole-rock geochemical compositions for shoshonitic quartz syenites from Madi and Chashan in the northern Wuyi Mt Range, southeast China. LA-ICP-MS zircon U-Pb dating shows that both the Madi and Chashan quartz syenites were formed at 134.8 Ma and 135.0 Ma, respectively. They also have similar geochemical compositions and are characterized by high K2O contents and enriched in LILEs and LREEs but depleted in HFSEs, suggesting a shoshonitic affinity. Elemental and isotopic compositions indicate that they were likely formed by partial melting of metasomatised lithospheric mantle with the presence of phlogopite and potassic amphibole. It is also suggested that silicate melts derived from subducted sediments played a dominant role in mantle metasomatism. An integration of our new results with previous ones allows us to propose a modified tectonic model about the evolution of SE China in which a continental arc related to the subduction of the Palaeo-Pacific plate during the Middle Jurassic was gradually changed to southeastward-migrating, extension-related magmatic arc and back-arc basins formed in a continental arc-rifting setting as a consequence of slab rollback since the Early Cretaceous.

Two-pyroxene syenitoids from the Moldanubian Zone of the Bohemian Massif: Peculiar magmas derived from a strongly enriched lithospheric mantle source

In the Variscan Bohemian Massif, (ultra-)potassic plutons are conspicuously associated with the high-grade, lower crustal/upper mantle Gföhl Unit (Moldanubian Zone). They can be subdivided into two contrasting groups: (1) coarse amphibole–biotite melagranite to quartz syenite with conspicuous K-feldspar phenocrysts (the ‘durbachite suite’), and (2) essentially equigranular biotite–two-pyroxene quartz syenites to melagranites (the ‘syenitoid suite’).The latter suite, represented by the Tábor and Jihlava plutons, is characterized by an originally ‘drier’ mineral assemblage orthopyroxene + clinopyroxene + Mg-biotite + plagioclase + K-feldspar + quartz, with accessory zircon, apatite, ilmenite, monazite and/or rutile + chromite. The rich assemblage of both rock-forming and accessory minerals allows testing of numerous geothermobarometers. The resulting P–T data are mutually consistent; they document emplacement of the parental magmas at mid-crustal levels (~5 kbar/19 km for Tábor and 7–8 kbar/26–30 km for Jihlava) and record near-isobaric crystallization from at least 1170 °C to the solidus.New Isotope-Dilution Thermal Ionization Mass-Spectrometry (ID-TIMS) U–Pb ages for zircon (336.9 + 0.6 Ma) and rutile (336.8 + 0.8 Ma) from the Tábor Pluton, together with previously published ages from the Jihlava Pluton, provide temporal constraints for the emplacement and rapid cooling of the syenitoids below c. 600 °C. This supports the hypothesis of post-tectonic emplacement of a hot and dry melt (>1200 °C?) into an essentially consolidated orogenic crust.The two syenitoid plutons have comparable, crust-like radiogenic isotope signatures (87Sr/86Sr337 = 0.7114–0.7133, ε337Nd = −6.8 to −8.0). This, in context of whole-rock geochemical variation and K-feldspar Pb isotopic compositions, is consistent with generation from a strongly enriched lithospheric mantle source. Shortly before melting, the local orogenic mantle was most likely modified by deep subduction and relamination of felsic crustal material of the Saxothuringian provenance, transformed to the felsic high-pressure granulites common in the Moldanubian Zone. In the subsequent evolution of the two-pyroxene syenitoid plutons, crystal fractionation and accumulation played a key role, unlike in the durbachite suite itself, where magma mixing with leucogranitic melts was much more important.Structural relations inside and around the (ultra-)potassic plutons (~343–335 Ma) suggest that – besides different depths and specific processes of magma emplacement – these plutons track different kinematic histories and evolutions of regional strain fields in their high-grade country rocks in this crucial period of the Variscan Orogeny.

Kumystin granosyenites complex of the late Cryogenian in Bolshoi Karatau (South Kazakhstan), age substantiation

The results of U–Pb geochronological studies of quartz syenites of the Kumystin complex of the Bolshoi Karatau ridge in southern Kazakhstan are presented and their late Neoproterozoic (717 ± 4 Ma) age is substantiated. Kumystin syenites complex together with rhyolites and basalts of the Kainar Formation are the youngest formations taking part in the basement of Karatau-Dzhebagly precambrian massif and formed in the second half of Cryogenian. The data set about the ages of the youngest complexes taking part in the basement of precambrian massifs of the Western part of the Central Asian belt indicates that the ending of magmatic activity within various massifs occurred asynchronously during the Neoproterozoic.

Geochemistry, Zircon U–Pb Geochronology, and Lu–Hf Isotopes of the Chishan Alkaline Complex, Western Shandong, China

Mass alkaline magmatic activities in Western Shandong during the late Mesozoic controlled the mineralization processes of gold and rare earth element (REE) polymetallic deposits in the region. The Chishan alkaline complex is closely associated with the mineralization of the Chishan REE deposit, which, as the third largest light REE deposit in China following the Baiyenebo (Inner Mongolia) and Mianning (Sichuan) deposits, is considered a typical example of alkaline rock mineralization throughout the North China Craton. To determine how the Chishan alkaline complex and REE deposit interact with each other, a systematic study was conducted on the petrology, rock geochemistry, zircon U–Pb geochronology, Lu–Hf isotopes of the quartz syenite, and alkali granite contained in the Chishan alkaline complex. The results reveal that the deposits feature similar geochemical characteristics typical of an alkaline rock series—both are rich in alkali, high in potassium, metaluminous, and poor in Ti, Fe, Mg, and Mn. In terms of REEs, the deposits are strongly rich in light REEs but poor in heavy REEs, with weak negative Eu anomalies. In terms of trace elements, they are rich in large ion lithophile elements Ba, Sr, and Rb but poor in high field-strength elements Nb, Ta, and Hf. Zircon LA-ICP-MS U–Pb dating indicated that the quartz syenite and alkali granite formed in Early Cretaceous at 125.8 ± 1.2 Ma and 127.3 ± 1.0 Ma, respectively; their εHf(t) values are −22.67 to −13.19, with depleted model ages (TDM) ranging from 1296 Ma to 1675 Ma and crustal model ages (TDMC) of 2036–2617 Ma. The Chishan alkaline complex originated from partial of the EM I-type (enriched mantle I) lithospheric mantle with assimilation of ancient crustal materials. The complex is of the same origin as the REE deposit, and developed in an extensional setting that resulted from plate subduction and lithospheric thinning and upwelling in the eastern area of the North China Craton.

Tracing magma mixing and crystal–melt segregation in the genesis of syenite with mafic enclaves: Evidence from in situ zircon Hf–O and apatite Sr–Nd isotopes

Alkaline rocks contain essential information on post-collisional/intraplate extensional magmatic processes within the continental lithosphere. The Erhulai syenitic pluton in the Liaodong Peninsula, North China Craton, consists of quartz syenite, mafic enclaves, and porphyritic K-feldspar granite. Field observations and zircon U–Pb dating reveal that the quartz syenite and the hosted mafic enclaves were coeval (with identical emplacement ages of 129–126 Ma), and slightly older than the porphyritic K-feldspar granite (ca. 123 Ma). The mafic enclaves have relatively low SiO2 and high MgO contents (up to 3.0%), and lack Ba or Eu anomalies. Whole-rock analyses of the mafic enclaves and accessory minerals in the enclaves show relatively low initial 87Sr/86Sr ratios (0.7056 to 0.7078) and variable εNd(t) (−11.2 to −2.0) and εHf(t) (−15.8 to −5.5) values. Large proportion of zircons from the mafic enclaves possess mantle-like δ18O values. These geochemical features suggest an enriched mantle source, with the addition of crustal material. The quartz syenites have relatively high SiO2 and low CaO and MgO contents, as well as significantly positive K, Ba, and Eu anomalies. All the quartz syenites show variable (87Sr/86Sr)i ratios (0.7073 to 0.7081) and εNd(t) values (−10.4 to −8.6) for the whole-rocks and apatites, εHf(t) values of −12.4 to −3.6 for the zircons, and zircon δ18O values of +4.4‰ to +7.0‰ (mainly greater than +5.3‰). All these features can be isotopically produced by the partial melting of ancient crustal material, with the involvement of mantle-derived magma. Crystal fractionation process, such as K-feldspar crystallization and accumulation, played an important role in the formation of the quartz syenite. The porphyritic K-feldspar granites possess relatively evolved geochemical compositions with high silica contents (SiO2 > 69 wt%) and remarkable negative Ba and Eu anomalies, which are chemically complementary to the quartz syenite. A crystal mush model was employed to establish the genetic link between the porphyritic K-feldspar granite and the quartz syenite, with the granite representing the interstitial melt extracted from the crystal mush and the quartz syenite representing the residual accumulation of crystals. The variable isotopic compositions of the quartz syenite and the porphyritic granite were mainly the result of extensive interaction of isotopes between mantle- and crust-derived magmas during magma mixing.In situ analyses of the Sr, Nd, Hf, and O isotopes of accessory minerals, coupled with field observations and whole-rock geochemical data for the Erhulai syenitic pluton, indicate a complex multi-stage magmatic process involving magma mixing, crystal–melt segregation, and wall-rock assimilation. Our study also shows that in situ isotopic analyses of accessory minerals provide unique information on, and are powerful geochemical tracers of, mantle–crust interactions and magmatic processes.

The Late Cryogenian Age of the Kumysty Granosyenite Complex, Greater Karatau, Southern Kazakhstan

The results of geochronological U–Pb studies of quartz syenite of the Kumysty Сomplex of the Greater Karatau Ridge (Southern Kazakhstan) revealed its Late Neoproterozoic age of 717 ± 4 Ma. It is shown that the Kumysty Complex syenite along with the Kainar Formation, rhyolite and basalt are the youngest rocks composing the basement of the Precambrian Karatau–Dzhebagly massif: the formation was completed in the second half of the Cryogenian. The data on the ages of the youngest basement complexes of the Precambrian massifs in the western part of the Central Asian Belt indicate that the completion of magmatic activity within different massifs occurred diachronously during the Neoproterozoic.

Zircon U–Pb age, Hf isotope, and geochemistry of Late Permian to Triassic igneous rocks from the Jiapigou gold ore belt, NE China: Petrogenesis and tectonic implications

The Jiapigou gold ore belt is located in the northeastern corner of the wedge‐shaped North China Craton and is hosted by Neoarchean basement gneiss and amphibolite, along with late Palaeozoic and Mesozoic granitoids. In this paper, whole‐rock major and trace element geochemistry together with zircon U–Pb ages and Lu–Hf isotope compositions are reported for Late Permian to Triassic igneous rocks from the gold belt. The dominant population of zircons in all these rocks shows oscillatory growth zoning in CL images, and relatively high Th/U ratios (0.10–5.23) that are indicative of a magmatic origin. Zircon U–Pb data indicate that the igneous rocks formed during two major phases: ca. 252 Ma and ca. 230–213 Ma. Geochemically, the Late Permian (ca. 252 Ma) diorites are high‐K calc‐alkaline in composition and low total Fe2O3, MgO, and CaO contents, and represented by enrichment in LILEs relative to HFSEs and HREEs, all of which are indicative of a postcollision high‐K calc‐alkaline granite affinity. In contrast, the Late Triassic (ca. 230–213 Ma) igneous rocks are mainly quartz syenite porphyries, felsites, and granodiorites, and are variable medium‐ to high‐K calc‐alkaline series and exhibit a negative Eu anomaly with A‐type affinities, indicating an extensional environment. In addition, in situ Hf isotopic analyses of zircons from four dated samples reveal that they have εHf(t) values of −3.0 to +2.2, with two‐stage model (TDM2) ages range from 964 to 1,210 Ma, indicating that they probably originated from the partial melting of a dominantly Mesoproterozoic to Neoproterozoic crustal source. On the basis of the geochemical data and regional geological investigations, we further propose that the Jiapigou gold ore belt in the Late Permian was formed in a postcollisional tectonic setting, whereas an extensional tectonic setting in the Late Triassic that caused lithospheric thinning during a postorogenic event.

A Pragmatic Approach to Coping with Matrix Effects during ICP-MS Analysis of Trace Elements in Silicate Rocks and Calibration of REE Interferences

Operating an Agilent 7700X ICP-MS spectrometer under robust plasma conditions (1550 W) with a He-filled octopole collision cell and analysing solutions (−1 total dissolved solids) still suffered analyte peak suppression due to matrix effects. International reference rocks BCR-1, BHVO-1, AGV-1, G-2 and BCR-2 all showed count rate reductions for 36 elements (mass range 7Li to 238U) averaging ~10% but with no dependence on isotope mass. Use of an internal standard (103Rh) and/or using a ten-fold dilution of sample solutions reduced these effects but problems with reduced count rates combined with larger errors for some elements introduced other problems. The best approach was to normalise the count rates for each element in the other samples against those for BCR-1 as an external standard; thus the count suppression due to the matrix effect is corrected for each individual element. This approach provides standardization “traceability” in line with the ERM ISO/IEC requirement. Experiments are also reported on quantifying the proportions of Ba and selected REE oxide/hydroxide components versus parent isotopes (XO/X and XOH/X). This information is essential for correcting peak interferences on higher mass number REE for the rock samples, and equations are developed to use measured CeO/Ce and CeOH/Ce ratios to predict such values for any other member of the REE suite. Concentrations obtained show excellent agreement with recommended values for the international reference materials especially for the REE. Robust data are also provided for two other standard rocks: nepheline syenite STM-1 and quartz syenite CAAS-1; the latter shows exceptional enrichments of Zr, REE, Th, and U.