A-type Granites Research Articles

Late Paleozoic tectonic transition in East Junggar, NW China: Insights from I- and A-type granitic magmatism in the Karamaili region

The closure time of the Karamaili Ocean is the key to understand the accretionary history of the East Junggar at the southern Central Asian Orogenic Belt in the Paleozoic. In this study, we report geochronology, whole-rock elemental and Sr-Nd isotopic data and zircon Lu-Hf isotopic data of four Late Carboniferous to Early Permian I- and A-type granitic plutons in the Karamaili region in East Junggar, NW China. The Carboniferous Hongliuxia gneissic granite and the Hongliugou granodiorite yield zircon 206Pb/238U ages of 324 ± 1 Ma and 306 ± 1 Ma, respectively, and both of them are high-K calc-alkaline and belong to I-type granitoids. These granites show (87Sr/86Sr)i values from 0.7034 to 0.7046, ɛNd(t) values from 4.4 to 6.3, with Nd TDM2 model ages of 559–727 Ma; and ɛHf(t) values from 9.4 to 12.8, with Hf TDM2 model ages of 481–710 Ma. The variable positive Nd-Hf isotopic signature and young TDM2 model ages suggest that the I-type granitoids were derived from partial melting of juvenile crustal source, probably earlier arc intrusion. The Early Permian Daliugou and Mawangmiao plutons have 206Pb/238U ages of 297 ± 1 Ma and 284 ± 2 Ma, respectively, show high-K calc-alkaline to shoshonitic characteristics and belong to A2-type granite. They exhibit (87Sr/86Sr)i values from 0.7040 to 0.7065, ɛNd(t) values from 2.1 to 5.4, with Nd TDM2 model ages of 612–887 Ma; and ɛHf(t) values from 5.2 to 11.5, with Hf TDM2 model ages ranging from 548 Ma to 955 Ma, and the Nd-Hf isotope data also suggest that they were derived from juvenile crustal source. Mafic microgranular enclaves in the Daliugou pluton yield zircon 206Pb/238U age of 299 ± 2 Ma, and show similar geochemistry affinity with the granite host. The presence of enclaves and reverse zoning K-feldspars indicate that the magma mixing may have played an important role in the evolution of the Daliugou pluton. In this study, only the Hongliuxia gneissic granite was deformed, and a Nd-Hf isotopic transition occurred at ~300 Ma. Furthermore, the Late Carboniferous I-type granitoids from the Hongliuxia and the Hongliugou plutons show an arc affinity, and the Early Permian A-type granitoids from the Mawangmiao and the Daliugou plutons show post-collisional affinity. These lines of evidence enable us to argue that the Karamaili Ocean probably closed at a period between 324 Ma and 306 Ma. The post-collisional Early Permian magmatism in East Junggar was independent from the Tarim mantle plume.

Petrogenesis of the granite related to the Dashunlong Sn polymetallic deposit, Dayishan ore field, South China

The roles of the mantle in generating granites and the extent of crust-mantle interaction are fundamental topics to our understanding of Sn mineralization. In this study, we report geochronological, geochemical, and Sr-Nd–Hf isotopic data for the granites, and muscovite 40Ar/39A isotopic data of the greisen type Sn ores, in the Dashunlong Sn deposit, Dayishan orefield, middle Qinzhou-Hangzhou metallogenic belt. LA–ICP–MS zircon U–Pb dating for the medium-fine grained- and coarse grained-biotite monzogranite of Dayishan pluton yields emplacement ages of 154.6 ± 1.2 Ma (MSWD = 0.73) and 154.7 ± 1.6 Ma (MSWD = 1.9), respectively, which are consistent with the muscovite 40Ar/39Ar plateau age of 151.1 ± 0.9 Ma (MSWD = 0.09) for the greisen-type Sn ores. It is indicated that the Sn mineralization in the Dashunlong deposit is related to the Late Jurassic Dayishan granite. The granite shows the geochemical features of highly fractionated A-type granite: 1) high 10,000 Ga/Al ratios of 2.95–3.88; 2) high Zr + Nb + Ce + Y contents of 292–637 ppm; 3) high TFeO/(TFeO* + MgO) ratios of 0.86–0.97; and 4) high Rb/Sr ratios of 2.80–18.8. Whole rocks isotopes show that this granite has variable initial 87Sr/86Sr ratios (0.71607–0.72437), negative εNd(t) values (−7.20 to −5.46), and two-stage Nd model ages of 1530–1388 Ma. LA–MC–ICP–MS zircon Lu-Hf isotopes show that the medium-fine-grained monzogranite has εHf(t) values of −0.94–2.20 and two-stage Hf model ages of 1298–1059 Ma, whereas the coarse-grained biotite monzogranite has relatively high εHf(t) values of −0.84–5.95 and two-stage Hf model ages of 1257–823 Ma. Different Hf and Nd isotopes indicate that the Nd-Hf isotopic decoupling occurs in the Dayishan granite, which was likely caused by the partial melting of the rocks containing abundant high Lu/Hf minerals. The magma of the Dayishan granite is reduced, highly fractionated, and enriched in Sn and F, resulting in large-scale Sn mineralization in the Dayishan orefield. It is proposed that these granites are originated from melts mixed by crustal- and mantle constituents and are formed in an extensional setting caused by the subduction of the Palaeo-pacific plate. A two end-member modeling indicates that ca. 10–20 percent of mantle-derived melts were likely involved in the formation of the Dayishan granites.

The Early Cretaceous tectonic evolution of the southern Great Xing’an Range, northeastern China: new constraints from A2-type granite and monzodiorite

The widespread Early Cretaceous plutons intruding along the southern Great Xing’an Range (SGXR) provide evidence for tectonic evolution of the region. Petrological, geochemical, zircon U–Pb geochronology, and zircon Hf isotopic studies are conducted on intrusions from Bianjiadayuan and Hongling areas. These suites classify as A2-type granites and monzodiorites, respectively. The 138–133 Ma A2-type granites originated from partial melting of continental crustal materials at high temperatures and shallow depths with significant addition of juvenile mafic lower crust sourced from a metasomatized mantle. The 136–134 Ma monzodiorites originated from the partial melting of an enriched mantle that was modified by melts of a previously subducted slab coupled with crustal contamination. The Early Cretaceous magmatism in the SGXR occurred in two periods: ∼145–136 Ma (peak at ∼139 Ma; εHf (t) = 5 to 10) and ∼136–130 Ma (peak at ∼131 Ma; εHf (t) = –10 to 15). The Early Cretaceous granite–monzodiorite suite in the SGXR suggests a bimodal magmatism in an extensional setting. The ∼145–130 Ma magmatism may have been triggered by asthenospheric upwelling induced by the Mongol–Okhotsk oceanic slab breakoff and large-scale lithospheric delamination resulting from post-orogenic extension. The variation of subduction direction of the Paleo-Pacific Ocean likely triggered a change in stress regime at ca. 136 Ma and likely promoted the lithospheric delamination beneath the SGXR resulting in intense magmatism originating from various sources. As such, the Paleo-Pacific Oceanic subduction likely played an important role in the Early Cretaceous magmatism in the SGXR.

Late Paleozoic multi-stage subduction accretion of the southwestern Central Asian Orogenic Belt: insights from the Late Carboniferous-Early Permian granites in the southern West Junggar, NW China

ABSTRACT The Late Carboniferous to Early Permian tectonic evolution of the South West Junggar (SWJ), northwest China, a crucial constituent in the southwestern domain of the Central Asian Orogenic Belt (CAOB), has long been the subject of debate. Here, detailed petrographic studies, new nine LA-ICP-MS zircon U-Pb ages and forty-nine whole-rock geochemical results from six granitic plutons in the SWJ were combined with previously available data to elucidate their petrogenesis and re-decipher Late Palaeozoic tectonic evolution and accretionary process of the southwestern CAOB. The studied granites can be geochemically classified as I- and A-type granites, and the former is further subdivided into I-type and highly fractionated I-type granites. The calc-alkaline I-type amphibole-bearing granites (Group I) yield crystallization ages of ca. 317–315 Ma, and exhibit depletions in HFSEs and enrichments in LILEs with no to slight Eu anomalies and low zircon saturation temperatures, indicating that the formation of these granites may be closely related to the subduction of the Junggar oceanic slab. The Group II granite porphyries (ca. 312 Ma) contain negligible amphibole, and are characterized by evolved SiO2 but low zircon saturation temperatures with sharply negative Eu, Ba, Nb, Ta, Sr, P and Ti anomalies, belonging to highly fractionated I-type granites, and were probably produced in a tectonic transition from compressional to extensional setting. In contrast, the latest Carboniferous (ca. 304–298 Ma) alkaline A-type granites (Group III) are amphibole-absent, and are similar to Group II granites in major element compositions, but exhibit higher zircon saturation temperatures, indicating that they probably resulted in a back-arc extensional setting triggered by roll-back of the subducting Junggar oceanic slab since ca. 310 Ma. In combination with regional geology and re-interpretation of previous data, a multi-stage subduction-accretion model is suggested to account for the Late Palaeozoic tectonic evolution of the southwestern CAOB. This model can plausibly interpret the differences in duration of subduction between the northern and southern domains of West Junggar and the coexistence of A-type granites with subduction-related sanukites in the SWJ during Carboniferous to Permian.

Origin of the Low δ18O Signals in Zircons from the Early Cretaceous A-Type Granites in Eastern China: Evidence from the Kulongshan Pluton

Origin of the Low δ18O Signals in Zircons from the Early Cretaceous A-Type Granites in Eastern China: Evidence from the Kulongshan Pluton

Unravelling the hidden evidences of magma mixing processes via combination of in situ Sr isotopes and trace elements analyses on plagioclase crystals

Granites from the ~580 Ma oxidized A-type Salto Rapakivi Granite (Itu Granitic Province, SE Brazil) bear abundant felsic microgranular enclaves (FMEs) and scarce cm-sized mafic microgranular enclaves (MMEs) with whole-rock Sr and Nd isotope signatures identical to those of their host rapakivi granites. In order to unravel the sources involved in the formation of both enclave types we conducted in situ trace elements and Sr isotopic analyses on plagioclase megacrysts from representative rocks of rapakivi granites, porphyry granites and both microgranular enclave varieties. Plagioclase from host rapakivi granites have 87Sr/86Sr ~0.7068, and are relatively more radiogenic than those from the enclaves. Plagioclase xenocrysts from the FMEs typically register a rimward decrease in 87Sr/86Sr ratios, with core 87Sr/86Sr up to 0.7069, whereas rims are usually in equilibrium with whole rock (~0.7060–0.7063). Crystals with U-shaped Sr isotopes profiles (less radiogenic cores with rimward increase in 87Sr/86Sr ratios) are rare, as are crystals with strongly radiogenic signatures observed in partially digested xenoliths found within the enclaves (87Sr/86Sr ~0.709–0.710). Plagioclase from MMEs show more primitive signatures (87Sr/86Sr 0.7055–0.7058). Crystals from the MMEs are distinguished by higher Sr (~1000 ppm) and lower Eu (< 0.5 ppm) and light rare earth elements (LREE, 6–12 ppm) compared to crystals from FMEs, porphyry and granite, which have Sr = 200–600 ppm, and LREE = 20–80 ppm. Our results allowed us to infer that FMEs were formed by mixing between a resident melt similar to that of rapakivi granites with a less radiogenic melt similar to that which originated the MMEs. Therefore the combined use of Sr isotopes and trace element contents in plagioclase crystals from A-type granites might shed light on the sources of rock varieties that have similar whole-rock isotope signatures.

The ca. 1.18–1.14 Ga A-type granites in the southwestern Yangtze Block, South China: New evidence for late Mesoproterozoic continental rifting

The late Mesoproterozoic tectonic history of the Yangtze Block, South China in the context of the supercontinent cycles remains poorly understood due to the scarce preservation of geological records of this age. In this study, we report petrology, whole-rock geochemistry, zircon U-Pb ages and Hf-Nd isotopes for newly identified late Mesoproterozoic monzogranites from the southwestern Yangtze Block. SHRIMP and LA-ICP-MS zircon U-Pb dating results reveal that these monzogranites crystallized at ca. 1.18–1.14 Ga. All the studied samples possess typical geochemical signatures of A1-type granite, such as relatively high SiO2 (73.31–77.50 wt.%), Na2O + K2O (7.14–8.09 wt.%) and Zr + Nb + Ce + Y (362–445 ppm) contents, zircon saturation temperatures (825–851 °C), and low Y/Nb ratios (0.78–1.04). They have negative whole-rock εNd(t) values from −6.9 to −5.7 and zircon εHf(t) values from −7.4 to −2.5, with consistent Nd and Hf model ages of 2.4–2.1 Ga. The parental magmas for these monzogranites were generated by partial melting of the early Paleoproterozoic crustal materials at low pressure and high temperature. Taking into account of the coeval within-plate mafic magmatism in the southwestern Yangtze Block, these ca. 1.18–1.14 Ga A1-type monzogranites were most likely emplaced in a continental rift setting. Available data indicate continental rifting may have occurred extensively in the Yangtze Block at 1.2–1.1 Ga, followed by oceanic plate subduction starting at 1.1 Ga. We propose the Yangtze Block was an isolated plate during 1.2–1.1 Ga and started to drift toward the Rodinia supercontinent at ca. 1.1 Ga, favoring the proposal that a coherent supercontinent did not yet exist in the late Mesoproterozoic.

Foulwind Suite magmatism in the Buller Terrane, New Zealand: geochemistry of the Carboniferous Foulwind and Windy Point Granites

ABSTRACT The peraluminous Foulwind and Windy Point Granites are important components of the Foulwind Suite – a diverse group of high field strength element-enriched Carboniferous granitoids in New Zealand’s Western Province. Four phases are identified in the Foulwind Granite (Seal Point, Siberia Bay, Tauranga Bay, Lighthouse). A new age of 324.0 ± 4.1 Ma has been obtained for the relatively undeformed Siberia Bay phase. The rocks are generally enriched in high-field strength elements and the Tauranga Bay and Lighthouse phases can be classified as ferroan A-type granites, whereas classification of the other phases is ambiguous. Sr isotope compositions were disturbed by the formation of the Paparoa Metamorphic Core Complex. Nd isotopes vary considerably and indicate involvement of multiple sources. The Nd isotope and A-type signatures can be explained by melting of mid-Paleozoic I-type granites with variable involvement of Greenland Group metasediments. The tectonic setting of the Foulwind Suite is unclear, but may be related to processes subsequent to the generation of voluminous mid-Paleozoic granitoids and amalgamation of the Buller and Takaka Terranes.

Late Carboniferous – Early Permian mafic dikes and granitoids in the heart of the Western Tianshan Orogen, NW China: Implications for a tectonic transition from a syn- to post-collisional setting

Syn- to post-collisional magmatism in the Chinese western Tianshan Orogen is recorded in the Late Carboniferous to Early Permian magmatic rocks of the Awulale Mountains. However, the orogen's tectono-magmatic evolution remains controversial. In particular, two contrasting genetic models have been proposed, i.e., post-collisional extension vs. mantle plume. Here, we report zircon U-Pb ages, whole-rock geochemical data and Nd-Hf isotopic data from the mafic dikes and granitoids of the western Awulale Mountains. These data better constrain the rock's petrogenesis and geodynamic processes following the amalgamation of the Tianshan Orogen. In combination with previous data, we have identified two magmatic episodes in the Awulale Mountains. The Late Carboniferous (ca. 319–307 Ma) episode emplaced the mafic gabbro dikes and intermediate-acidic granitoids with I-type and adakitic affinity. The latest Carboniferous-Permian (ca. 302–286 Ma) episode emplaced bimodal intrusions containing mafic diabase dikes, acidic adakite and I- and A-type granites. Mafic dikes and granitoids of both episodes show depleted whole-rock Nd and zircon Hf isotopic compositions (εNd(t): +2.21 ~ +7.54, εHf(t): +6.48 ~ +15.97), indicating a juvenile magmatic source. However, the mafic dikes have variable geochemical features that clearly suggest their derivation from two distinct mantle sources during the two magmatic episodes. The earlier gabbro dikes have lower Th/Yb (0.08–0.12) and Zr/Yb ratios (15.32–25.45) that resemble a normal mid-ocean ridge basalt (N-MORB) from a depleted mantle source. In contrast, the later diabase dikes show enrichment in the light rare earth elements (LREE) with relatively higher Th/Yb (0.22–0.86) and Zr/Yb ratios (38.05–68.97) that are similar to an enriched mid-ocean ridge basalt (E-MORB) mantle source. Also, the Nb/Ta ratios from the earlier adakitic granitoids (21.26–23.28) differ markedly from the later acidic adakites (0.41–19.50), which implies the transition from a thicker to a thinner crust. Integrating our results with the previous geological records, leads us to propose a two-stage tectonic model for the Chinese western Tianshan Orogen's transition from syn- to post-collision. The Late Carboniferous stage of magmatism was triggered by the break-off of the North Tianshan Oceanic slab as the Yili Block and Junggar Terrane collided. The subsequent latest Carboniferous-Permian stage followed the switch to a post-collisional setting with delamination of the thickened lower crust and the intrusion of voluminous lithospheric mantle-induced bimodal magmatism.

GEOLOGIA, GEOQUIMICA E QUIMICA MINERAL DOS STOCKS GRANÍTICOS RAPAKIVI CAMPINA DO VEADO E SANTA BLANDINA, SUDOESTE DO ESTADO DE SÃO PAULO

Os stocks graníticos Campina do Veado e Santa Blandina localizam-se na região sudoeste do estado de São Paulo, no município de Nova Campina, intrusivos em rochas da sequência metavulcanossedimentares do Grupo Itaiacoca. Os corpos encontram-se relacionados à evolução da granitogênese rapakivi neoproterozoica da parte norte do Terreno Apiaí e da Província Mantiqueira Central. As rochas do Campina do Veado, abrangem uma área de aproximadamente 20 km² e as suas rochas ocorrem no leito do rio Taquari Guaçu, na interface dos contrafortes erosivos da Unidade Geomorfológica da Depressão Periférica, a partir de rochas dos grupos Furnas e Itararé da sequência sedimentar da Bacia do Paraná. O corpo de Santa Blandina de aproximadamente 4 km2 é identificado por pequenas ocorrências isoladas de rochas graníticas. As rochas são representadas por variedades graníticas de coloração rósea a vermelha, rara ocorrência cinza, isotrópicos e porfiríticas a equi- a inequigranulares, com variedades viborgíticas e apresentando como mineral máfico preservado somente a biotita. As rochas podem ser distintas por composições monzograníticas, sienograníticas e álcali granitos, além das rochas das fases tardias aplíticas e pegmatíticas. O magmatismo dos stocks graníticos demonstra uma forte relação de semelhanças entre as suas rochas, sugerindo constituir em um mesmo corpo. Os granitos rapakivi são peraluminosos e podem ser correlacionados à séries cálcio-alcalina alto potássio a shoshonítica, do Tipo I Caledonianos, pós-colisional a anorogênico de intraplaca, altamente diferenciado e da fase final da construção do Arco Magmático Ribeira ou associado aos granitos peraluminosos alcalinos do tipo A. Estão associados às estruturas transtensivas das zonas de cisalhamento do final do evento colisional da Orogênese Ribeira e reflete o arranjo final geotectônico de estabilização do Terreno Apiaí.

The relationship between gold mineralization, high K calc-alkaline to alkaline volcanic rocks, and A-type granite: Formation of the Daxiyingzi gold deposit in northern North China Craton

The Daxiyingzi gold deposit is located in the Chifeng-Chaoyang gold ore concentration area, toward the northern margin of the North China Craton. The ore bodies of the Daxiyingzi gold deposit are mainly hosted in the Manitu volcanic rocks, which are intruded on by the Mesozoic granite porphyry. The ore formation consists of four main stages: magnetite-hematite (Ⅰ), quartz-metal sulfide (II), gold-bearing polymetallic sulfide-quartz (III), and pyrite-carbonate (IV). The homogenization temperatures of the fluid inclusion range from 189.5 to 310.0 °C, while the salinity ranges from 0.53 to 13.55 wt% NaCl equiv., corresponding to entrapment depths of 0.1–1.0 km below the paleowater table. The quartz from the stage II and III veins from Daxiyingzi have δDV-SMOW values range from −62.1 to −82.8‰, while the δ18OH2O values range from 0.46 to 1.26‰. The samples for galena, sphalerite, and pyrite from the Daxiyingzi veins have approximate S isotopic compositions, with δ34S values ranging from 3.5 to 3.9‰, 3.9 to 4.2‰ and 3.5 to 5.0‰, respectively. The S-H-O isotopic data suggest that the ore-forming fluid had originated from magmatic sources. The zircon U-Pb ages of the volcanic rocks and granite porphyry are ca. 157–158 and 149 Ma, respectively, which suggests that the gold mineralization took place from 149 to 158 Ma. The volcanic rocks are belong to the high-K calc-alkaline to alkaline series, while the granite porphyry is belongs to A-type granite, which suggests an extensional tectonic setting. Our new data indicates that the Daxiyingzi gold deposit is a typical epithermal-mesothermal deposit formed by a Late Jurassic magmatic-hydrothermal system.

Genesis, metallogenetic and tectonic significance of the A-type granites in Hashitu Mo deposit, southern Great Hinggan Range, NE China

The Hashitu Mo deposit located in the southern part of the Great Hinggan Range (GHR) is a newly-discovered Mo deposit. The orebodies mainly occur as quartz-sulfide veins in monzogranite and granite porphyry. Besides, minor quartz porphyry is located in the orefield. The zircon U-Pb ages, ore-forming elements, and whole-rock geochemical and Sr-Nd-Pb isotopic data were obtained from granitic rocks associated with Mo mineralization to constrain their age, petrogenesis, metallogenic and tectonic significance. The ore-bearing monzogranite and granite porphyry have zircon U-Pb ages of 149.3 ± 0.7 Ma and 149.5 ± 1.1 Ma, respectively, and the quartz porphyry has an age of 153.2 ± 1.1 Ma. All rocks have similar geochemical characteristics, with high contents of K2O and SiO2, enrichment in Rb, Th, U, K, Nd, Zr, and Hf, depletion in Ba, Nb, Sr, and Ti, and strong negative Eu anomalies. The rocks have low initial 87Sr/86Sr (0.7049–0.7069) and εNd(t) values (2.29–3.47) and young two-stage Nd model ages (TDM2 = 656–752 Ma). The whole-rock Pb isotopic compositions show a wide range, i.e., (206Pb/204Pb)t = 18.93–19.79, (207Pb/204Pb)t = 15.51–15.59, and (208Pb/204Pb)t = 37.21–38.18. The granites are significantly enriched in Mo (concentration coefficient (K) = 19.94). The factor analysis result shows that the factor F1(As-Bi-Zn-Pb-Ag-Sb-B) represents regional background information of Pb-Zn-Ag polymetallic mineralization in the southern GHR, whereas the factor F3(Mo-F) represents the local metallogenic factor. The geochemical and isotopic data suggest that the Hashitu granitic rocks were derived from partial melting of the juvenile lower crust, possibly the Neoproterozoic crust. Based on the geology and geochemistry, we propose that the Hashitu Mo deposit formed in a post-orogenic extensional tectonic setting associated with the southward subduction of the Mongol-Okhotsk oceanic plate.

Petrogenesis of the Jurassic Guiping Complex in the Southwestern South China Block: Insights into the Subduction Processes of the Paleo-Pacific Slab

Late Jurassic NE-trending A-type granitoids are widespread in the Shihang belt, South China, though their petrogenesis and geodynamic settings remain controversial. The Guiping complex is located on the southwest margin of the Shihang belt. In this study, the petrography, major and trace element geochemistry, whole-rock Sr-Nd isotopes, and zircon U-Pb geochronology of the Guiping complex were investigated. The Guiping complex is composed of the Fenghuangling and Xishan plutons; both plutons yielded zircon U-Pb ages of ca. 160 Ma. The Fenghuangling pluton has low SiO2 content of 54.26% to 60.31%, whereas the Xishan pluton exhibits high SiO2 content of 65.19% to 71.18%. Both of them are metaluminous and belong to the high-K calc-alkaline series and are enriched in large-ion lithophile elements (LILEs) such as Rb, Th, U, and Pb. The Fenghuangling and Xishan plutons showed enrichment in light rare earth elements (LREEs) and high-field strength elements (Nb, Ta, Zr, and Hf) and depletion in heavy rare earth elements (HREEs). Marked Nb and Ta negative anomalies were not observed. Due to the high contents of Zr + Ce + Nb + Y and high Ga/Al ratios, all the samples belonged to the group of A-type granites. The Fenghuangling and Xishan plutons had low ISr (mainly in the range of 0.7046–0.7058) and high εNd(t) (−0.60 to 1.94) values, though obviously different from those of the Precambrian basement in South China. Furthermore, they lie between the ocean island basalt (OIB) of the asthenosphere and the arc basaltic rocks of the enriched lithospheric mantle. Therefore, we proposed that the basaltic parental magma of the Guiping complex originated from partial melting of the enriched lithospheric mantle, which was metasomatized by asthenosphere-related OIB-type basaltic magma. Mafic microgranular enclaves in the Xishan pluton displayed positive Nb and Ta anomalies, which is consistent with OIB-type basalts. The enclaves also had similar Sr-Nd isotopic compositions to the Xishan pluton. That indicated that the enclaves were probably formed by mixing of the OIB-type basaltic magma and the Xishan pluton. In conclusion, the formation of the Late Jurassic NE-trending A-type granite belt was attributed to back-arc extension as a result of the rollback of the Paleo-Pacific Plate.

Geochemistry and mineralogy of the Jebel Aja Igneous Intrusion and the associated exotic pegmatites, Arabian Shield, Saudi Arabia

We describe the Jebel Aja igneous intrusion (JAII), which sits at the farthest northeastern edge of the exposed Arabian Shield in Saudi Arabia. The JAII consists of late Neoproterozoic post-collisional A-type granites, and was emplaced during tectonic extension. It is a composite pluton consisting of a granitic core (monzogranite, syenogranite and alkali feldspar granite) surrounded by a later phase of alkaline and peralkaline granites. Pegmatites occur as pockets and dykes at the contact between the alkaline/peralkaline granites and syenogranite. The pegmatites have sharp contacts with their host granites and show variable colors (principally pink and white), grain sizes, textures, and mineral compositions. We report here that the white pegmatites host pockets with a unique mineralogy that includes the first observation of pyroxmangite, thortveitite and thalénite-(Y) in the whole Arabian Shield as well as a novel (Sc, Y, vacancy)-rich pyroxenoid and a vacancy-rich bustamite with excess octahedral Si. The pyroxmangite-dominated lithology also features accessory quartz, albite, spessartine, pyrochroite, muscovite, and fluorite. The presence of pyroxmangite rather than rhodonite, together with vacancy-rich and Si-excess pyroxenes, suggests an elevated pressure of emplacement. The granites of the JAII are highly evolved and have undergone extreme fractional crystallization of feldspars, leading to increases in the concentration of volatile and incompatible elements in the residual melt, which resulted eventually in injection of coeval pegmatites into the mostly solidified host granite. The syenogranite and alkaline/peralkaline granite of the JAII contain sodic amphibole and have geochemical characteristics common among within-plate rare-metal bearing A-type rocks. They have positive Nb-Ta anomalies that increase from syenogranite to alkaline/peralkaline granite. The pink pegmatite is highly mineralized and contains high concentrations of Nb (1540–1769 μg/g), Ta (103–136 μg/g), Y (1116–1616 μg/g), Zr (6362–9707 μg/g), Hf (215–264 μg/g), Th (278–384 μg/g), U (110–147 μg/g), and ∑REE (2334–3251 μg/g).

Rapid transition from oceanic subduction to postcollisional extension revealed by Carboniferous magmatism in East Junggar (NW China), southwestern Central Asian orogenic belt

AbstractKnowledge of the subduction to postcollision tectonic transition in response to oceanic closure is crucial for tracking the final stage of orogenic evolution. Here, we report new geochronology, geochemistry, and isotopic data for Carboniferous magmatism in East Junggar (NW China), southwestern Central Asian orogenic belt, which may record such processes following the closure of the Kalamaili Ocean (a branch of the Paleo-Asian Ocean). The early Carboniferous calc-alkaline volcanic rocks (dominated by basalt and basaltic andesite) yielded zircon U-Pb ages of ca.340–330 Ma and are characterized by arc-like trace-element patterns showing enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Pb) but depletion of high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). Combined with their variable Ba/Nb (9.80–454) and lowNb/La (0.21–0.54) and Sm/Yb (1.77–3.08) ratios as well as depleted mantle–like Sr-Nd-Pb-Hf (whole-rock 87Sr/86Sri = 0.7037–0.7040; εNd[t] = +3.5 to +5.9; 206Pb/204Pbi = 17.728–17.996; zircon εHf[t] = +11.8 to +18.8) isotopic values, we suggest that they were produced by melting of a lithospheric mantle wedge fluxed by slab-derived fluids under spinel-facies conditions. With whole-rock 40Ar/39Ar dating of ca. 320 Ma, the late Carboniferous mafic dikes have geochemical features and Sr-Nd-Pb (87Sr/86Sri = 0.7039–0.7041; εNd[t] = +6.6 to +6.8; 206Pb/204Pbi = 17.905–17.933) isotopic compositions similar to those of the early Carboniferous volcanics, but they show less pronounced Pb anomalies and negative Nb and Ta anomalies. They are interpreted to have formed by partial melting of a spinel-bearing lithospheric mantle metasomatized by limited influx of subduction-related fluids. The late Carboniferous felsic volcanic rocks (dacite and rhyolite) yielded zircon U-Pb ages of ca. 305 Ma and are geochemically equivalent to those of A2-type granites in East Junggar. They have juvenile isotopic compositions (εNd[t] = +4.5 to +6.8; εHf[t] = +13.3 to +18.7) and relatively young Nd and Hf model ages that roughly coincide with the ages of the ophiolites in the area, suggesting that they could have originated from melting of a juvenile basaltic lower crust. Whole-rock geochemistry, assimilation–fractional crystallization (AFC), and isotopic mixing modeling argue for insignificant crustal contamination for the Carboniferous magmatism. We suggest that the early Carboniferous lavas erupted in an island-arc setting related to the northward subduction of the Kalamaili oceanic crust, whereas the late Carboniferous magmatism formed in a postcollisional extensional regime in response to slab breakoff or lithospheric delamination. Combined with regional geological information, we propose that a rapid tectonic transition from oceanic subduction to postcollisional extension may have occurred in East Junggar during the Carboniferous, marking the final closure of the Kalamaili Ocean, which most likely took place ca. 330–320 Ma. This study provides overall geochronological and petrogeochemical evidence to better constrain the amalgamation of the southwestern Central Asian orogenic belt and may be of great importance for understanding the final stage of orogenic evolution elsewhere.

Water availability controls crustal melting temperatures

Although the capacity for water to lower the solidus during crustal melting is well recognised, a modern consensus is that most granitic magmas are generated under fluid-absent melting conditions at temperatures ≥850 °C, and that water has little influence on crustal melting temperatures. By contrast, we show that granite magmas produced in different tectonic settings equilibrated in the crust at significantly different temperatures, ranging between 700 and 1000 °C, depending on water availability during melting. On a molecular scale, the process that lowers the solidus is similar to hydroxylation, i.e. the depolymerization of the aluminosilicate framework melt by complexing of OH− ions with the melt network formers, Si4+ and Al3+. On a larger scale, the process is “water fluxing”, which is consistent with the growing realisation that many granitic magma reservoirs form under protracted, cool-storage conditions, and consistent with the typically low (100–200 ppm) Zr content (hence low zircon saturation temperatures) of granitic magmas. Global compilations of common igneous and sedimentary rocks also show that Zr contents are consistently in the range of 100–200 ppm, indicating that average Zr contents do not change significantly during crustal differentiation. Such low average contents are most easily reconciled with models of low-T silicic melt generation and entrainment of refractory or antecrystic zircon in the magma. Low-K, I-type (Cordilleran) granitic magmas are cooler and much more voluminous than A-type magmas because they are hydrous, although transiently heated by mafic magma infusions. In turn, water availability is tectonically controlled, occurring in most suprasubduction (SSZ) environments where mantle-derived water drives melting of orogenic crust already elevated in heat flux (>75 mWm−2), typical of upper amphibolite facies conditions. However, within intraplate environments, where melting proceeds by high-T, hydrate-breakdown melting, crustal temperatures are not buffered during water fluxing and rise to granulite facies and ultrahigh temperature (UHT) conditions, particularly for refractory protoliths, producing ferroan, alkalic A-type granites and charnockites in deep crustal hot zones (DCHZ). The combination of tectonic setting, contrasting source rocks, and variation in water availability, controls the compositional diversity of granitic magmas.

The Carboniferous-Permian tectonic setting for the southernmost Central Asian Orogenic Belt: Constraint from magmatic and sedimentary records in the Alxa area, NW China

The northern Alxa orogenic belt, a junction between the western and eastern Central Asian Orogenic Belt (CAOB), is not well understood, especially in its Carboniferous to Permian tectonic history. In this paper, an integrated study incorporating the geochronology, geochemistry, and isotopes of magmatic rocks and provenance analysis of clastic rocks is conducted to address the above issue. The zircon UPb age data, both in this study and in previous studies, indicates that the Permian magmatic activities in the Alxa Block were intensive and peaked at ca. 276 Ma. The magmatic zircons yield negative εHf(t) values with a wide range, indicating their dominant derivation from the crustal reworking of the Alxa Block with the addition of juvenile materials. The Permian high-K to shoshonitic, I-type granitoids combined with the A-type granitoids/rhyolites were most likely formed in extensional settings. The late Carboniferous sandstones collected along the Badain Jaran fault yield a high proportion of early Paleozoic zircons peaked at ca. 429 Ma with both positive to negative Hf(t) values and a low proportion of Precambrian zircons with positive to negative Hf(t) values. Thus, their provenance were likely the Precambrian to Paleozoic rocks in the Alxa Block and in the terranes in the CAOB. The zircon age spectra with no indicators of Carboniferous zircons approximating their depositional age supports an extensional setting. The Permian A-type granites and volcanic rocks in the Alxa area have some geochemical affinity with the Permian alkaline granitoids in the eastern Tianshan–Beishan and central-north Inner Mongolia regions. These findings support the extension of the Gobi–Tianshan belt in the Alxa area.

Late Triassic dacites from Well NK-1 in the Nansha Block: Constraints on the Mesozoic tectonic evolution of the southern South China Sea margin

Identifying whether and when the southern continental margin of the South China Sea (SCS) was part of the South China Block (SCB) is important for Mesozoic–Cenozoic paleogeographic reconstructions of Southeast Asia. In this paper, we present geochronological and geochemical data for dacites from Well NK-1 in the Nansha Block (Dangerous Grounds) of the southern SCS margin to investigate their petrogenesis and tectonic affinity. Zircon UPb and 40Ar/39Ar datings show that these dacites formed during the Late Triassic (218–217 Ma). The dacites have high total alkali (K2O + Na2O = 7.39–8.42 wt%) and relatively low CaO contents (1.87–3.38 wt%), and show geochemical affinities with A-type granites, with high Zr saturation temperatures (806–855 °C), FeOTot/MgO ratios (5.22–7.48), Zr + Nb + Ce + Y contents (530–680 ppm) and 10,000 × Ga/Al values (2.40–2.94), and relatively low A/CNK values (0.86–1.04). The primitive magma for the dacites was likely generated by partial melting of ancient crustal materials. Late Triassic dacites in the Nansha Block and Triassic A-type granites in the SCB formed in an extensional setting at the continental margin of the SCB. This setting was probably related to the Paleo-Pacific plate subduction that restarted prior to the Early Triassic. Therefore, the southern SCS margin was part of the SCB continental margin during the early Mesozoic, which provides more detailed insights into the paleogeography of SE Asia since the Mesozoic.

Rare earth element enrichment in the ion-adsorption deposits associated granites at Mesozoic extensional tectonic setting in South China

Ion-adsorption type rare earth element mineralization occurs in weathered profile of three Mesozoic granites in the Bachi area, Guangdong Province, South China. In this study, we investigate the REE enrichment processes in these granites using geochemical and geochronological data. The three granites yield zircon U-Pb ages of ca. 189 Ma, 153 Ma and 94 Ma, respectively, and could be divided into Early Jurassic, Late Jurassic and Cretaceous granites. The Early Jurassic and Cretaceous granites show similar geochemical characteristics, i.e. high K2O and low CaO, Fe2O3, and MgO contents, low K2O/Na2O ratios, and depletion of Eu, Sr, Ba, Ti, and P. They are characterized by high HFSE (high field strength elements) contents and enriched isotopic compositions with εNd(t) values ranging from −5.4 to-2.7, and εHf(t) values ranging from −4.5 to 1.7. The Late Jurassic granite also shows Eu, Sr, Ba, Ti, and P depletion and high HFSE contents. It has, however, higher CaO, Fe2O3, MgO and lower K2O contents than the Early Jurassic and Cretaceous granites. The Late Jurassic granite also shows more enriched isotopic compositions with εNd(t) values ranging from −9.0 to −8.9, and εHf(t) values ranging from −11.1 to −6.1. In addition, the Late Jurassic granite comprises, in contrast to the Early Jurassic and Cretaceous granites, abundant inherited/captured zircons cores and accessory REE-rich minerals of allanite, titanite, apatite and fluorite. These Mesozoic granites have the geochemical affinity of A-type granite. The Zr saturation temperature is >820 °C for the Early Jurassic and Cretaceous granites, and ~900 °C for the Late Jurassic granite. The Early Jurassic and Cretaceous granites were derived from partial melting of felsic igneous rocks under high-temperature, low-pressure, and anhydrous conditions. The felsic igneous source could provide relatively high REE contents for the melts, and the high-temperature, low-pressure, and anhydrous conditions resulted in residual plagioclase and melting of REE-rich minerals in the source area, which favour to REE enrichment in the melt. The Late Jurassic granite, on the other hand, was derived from a more mafic source and associate with fast crustal magma generation as indicated by its abundant presence of inherited/captured zircon cores in the high-temperature condition. The fast magma generation of the Late Jurassic granite prevented the separation of REE-rich minerals from the melt resulting in REE enrichment. Further, the high-temperature, low-pressure, and anhydrous physicochemical conditions and fast magma generation, which favour to REE enrichment in the granites, are tightly associated with large-scale Mesozoic extension in South China.

Origin of S-, A- and I-Type Granites: Petrogenetic Evidence from Whole Rock Th/U Ratio Variations

The origin and evolution of granites remain a matter of debate and several approaches have been made to distinguish between different granite types. Overall, granite classification schemes based on element concentrations and ratios, tectonic settings or the source rocks (I-, A-, S-type) are widely used, but so far, no systematic large-scale study on Th/U ratio variations in granites based on their source or tectonic setting has been carried out, even though these elements show very similar behavior during melting and subsequent processes. We therefore present a compiled study, demonstrating an easy approach to differentiate between S-, A- and I-type granites using Th and U concentrations and ratios measured with a portable gamma ray spectrometer. Th and U concentrations from 472 measurements in S- and I-type granites from the Variscan West-Bohemian Massif, Germany, and 78 measurements from Neoproterozoic A-type Malani granites, India, are evaluated. Our compendium shows significant differences in the average Th/U ratios of A-, I- and S-type granites and thus gives information about the source rock and can be used as an easy classification scheme. Considering all data from the studied A-, I- and S-type granites, Th/U ratios increase with rising Th concentrations. A-type granites have the highest Th/U ratios and high Th concentrations, followed by I-type granites. Th/U ratios in S- to I-type granites are lower than in A-type and I-type granites, but higher than in S-type granites. The variation of Th/U ratios in all three types of granite cannot be explained by fractional crystallization of monazite, zircon and other Th and U bearing minerals alone, but are mainly due to source heterogeneities and uranium mobilization processes.