Typical I-type Granites Research Articles

Geochemistry and Zircon LA–ICP–MS U–Pb Geochronology of the Shuangwang Au Deposit, Shaanxi Province: Implications for Tectonic Evolution and Metallogenic Age

The Shuangwang Au deposit (with a gold resource of approximately 70 t Au), is located in the Fenxian-Taibai fore-arc basin in the West Qinling Orogen of central China. Igneous intrusions in the region include the Xiba granitic pluton and granite porphyry and lamprophyre dykes. The Xiba pluton is composed of granodiorite and monzonite granite. The granodiorite is typical I-type granite, and it yields a crystallization age of 221.1 ± 1.2 Ma and a two-stage Hf model age of 1432–1634 Ma. The monzonite granite shows a transitional characteristic between I-type and A-type granite, and it yields a crystallization age of 214.8 ± 1.2 Ma and a two-stage Hf model age of 1443–1549 Ma. The granitoid was derived mainly from a crust–mantle mixed source. The ages indicate that the granodiorite and monzonite granite formed during two different stages. The REE distribution patterns of the Xiba granitoid exhibit significant fractionation between LREE and HREE, showing right-dipping curves, with an enrichment of LREE and a deficit of HREE. The granodiorite displays a light negative Eu anomaly, while the monzonite granite displays an obvious negative Eu anomaly. The granite porphyry dikes are distributed in the No. I breccia and Jiupinggou granite porphyry, and they yield crystallization ages of 219.9 ± 1.5 Ma and 213.1 ± 0.89 Ma, respectively, and two-stage Hf model ages of 1382–1501 Ma and 1373–1522 Ma, respectively. The lamprophyre dikes in the deposit yield a crystallization age of 214.4 ± 2.7 Ma. After the collision event between the Yangtze and the North China Plates along the Qinling orogenic belt, at approximately 220 Ma in the Late Triassic, the detachment of the slab produced the upwelling of the asthenosphere material. Under conditions of mantle heat and tectonic stress, widespread partial melting of the subducted continental crust and the upper lithosphere mantle occurred, forming granitoids with various degrees of adakite characteristics.

Mantle contribution to the generation of the giant Jinduicheng porphyry Mo deposit, Central China: New insights from combined in-situ element and isotope compositions of zircon and apatite

Identifying crust–mantle interaction is important for understanding the genesis of granite-hosted mineral deposits in post-collisional or intracontinental settings. We have used combined zircon-apatite trace element compositions, zircon HfO isotopes and apatite SrNd isotopes, which were all acquired by in-situ techniques, to evaluate the mantle contribution to the formation of a giant porphyry Mo deposit, the Jinduicheng deposit in the East Qinling–Dabie orogenic belt, Central China. Zircons from the host granites yielded a UPb age of ∼139 Ma, suggesting a post-collisional setting for the ore-related intrusion. Most magmatic zircons from the granites have Ti-in-zircon temperatures lower than 830 °C, moderate Zr/Hf ratios (25–55), and mantle-like δ18O values (peaking at ∼5.6‰), similar to typical I-type granites worldwide. The magmatic zircons have variable εHf(t) values (−26.7 to −7.2) and Ti-in-zircon temperatures (646–894 °C, peaking at ∼740 °C). The coexisting igneous apatites have initial 87Sr/86Sr ratios of 0.70694–0.71027, and variable εNd(t) values (−16.0 to −9.3), Sr contents (132–663 ppm), and Eu/Eu* values (0.12–0.47). The Sr–Nd–Hf isotopes of the apatites and zircons, along with the positive correlations between apatite Sr contents, Eu/Eu* values, and εNd(t) values, a positive correlation between Ti-in-zircon temperatures and zircon εHf(t) values, as well as mixing modeling using SrNd isotopes, suggest that at least two end-member melts were involved in the formation of the host granites: one is the ancient lower crust-derived melt, and the other is the metasomatized lithospheric mantle-derived melt, which have relatively enriched and depleted Sr–Nd–Hf isotopic compositions, respectively. We suggest that the post-collisional lithospheric thinning, asthenospheric upwelling and underplating of the mantle-derived mafic magmas may have triggered the crustal melting, magma mixing, and generation of the host granites of the Jinduicheng deposit. We postulate that, besides the heat, the metasomatized lithospheric mantle-derived magmas likely supplied additional volatiles (e.g., Cl, S), facilitating the porphyry Mo formation. Our study highlights that, combined in-situ element and isotope compositions of accessory minerals can provide multidimensional insights into petrogenetic tracing, and enhance the genetic interpretations of granites and related mineral deposits.

Petrogenesis and tectonic significance of Late Paleozoic magmatism in the Xilinhot micro-continent, central Xingmeng orogenic belt

ABSTRACT The evolution history of the Hegenshan Ocean is an important part of studying the evolution of the Xingmeng orogenic belt, but it is still controversial. Here we present an integrated study of petrology, zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd isotopes on a series of intrusive rocks from the Baiyinguole area, Xiwuqi, central Inner Mongolia, NE China. Chronological data indicate that the intrusive rocks can be divided into three stages: 326–317, 297–295, and 259–254 Ma. The 326–317-Ma gabbros and diorites belong to the calc-alkaline series, and are characterized by positive whole-rock εNd(t) values (+1.1–+6.7), relatively low La/Ba ratios (0.04–0.10), and high La/Nb ratios (1.82–3.18), indicating that the magma source is from a subduction-modified lithospheric mantle. These samples have arc features owing to the southward subduction of the Hegenshan oceanic crust. The 297–295-Ma granodiorites and monzogranites show positive εNd(t) values (+1.9–+2.4), low 87Sr/86Sri ratios (0.704185–0.705234), and young Nd two-stage model ages (TDM2), which is consistent with typical I-type granites. On various tectonic setting discrimination diagrams, the granodiorites and monzogranites are plotted in the syn-collisional field. The 259–254-Ma syenogranites have low abundances of mafic minerals, low contents of Zr/Hf (18.31–31.35) and Nb/Ta (6.80–11.80) values, high SiO2 (76.36–78.54 wt.%) content, A/CNK (1.08–1.76), and D.I. of 96.8–97.4 (average 97.1), and notable negative Eu anomalies (δEu* = 0.07–0.17) and weakly tetrad-effects in REE pattern, which is affinity to the characteristics of highly differentiated granites. Moreover, they have positive εNd(t) values (+2.0–+2.1) with scattered TDM2 of 730–1217 Ma and low Nb/U ratios. This study suggests that the syenogranites are formed in a post-collisional tectonic setting, which is related to the closure of the Hegenshan Ocean. In addition, based on a review of the pre-existing literature data, a model of southward ridge subduction is proposed to explain the tectonic evolution of the Hegenshan Ocean.

Geology and genesis of auriferous porphyritic monzogranite and its correlation with the Qiyugou porphyry-breccia system in East Qinling, Central China

The Qiyugou deposit in the Qinling Orogenic Belt includes breccia pipe hosted gold mineralization and newly discovered porphyry gold mineralization, whereas the genetic relationship between porphyry stocks, stockwork-disseminated mineralization and breccia pipe-hosted mineralization remains obscure. Here we present the results from an integrated field investigation, petrography, whole-rock geochemistry, zircon U-Pb geochronology, trace elements and Hf-O isotopes on a suite of porphyritic monzogranite from the newly discovered auriferous ‘191 pluton’, to elucidate the petrogenesis, magma source, timing of magmatic-hydrothermal event and the possible genetic link with the Qiyugou porphyry-breccia system. The porphyritic monzogranite shows varying degree of hydrothermal alterations including potassic alteration, phyllic alteration, silicification, sericitization and chloritization, resulting in the high loss on ignition values (LOI = 2.88–6.64 wt%) and linear relationships between LOI and large ion lithophile elements (LILEs) which suggest element mobility in fluids. The fluid immobile high field strength elements (HFSEs) and zircon trace elements indicate that the porphyritic monzogranite is typical I-type granite. Zircon U-Pb dating of the porphyritic monzogranites yield weighted mean ages of 136.4 ± 1.1 Ma, 134.6 ± 1.8 Ma and 135.2 ± 1.1 Ma, representing the emplacement age of the auriferous ‘191 pluton’. Zircon Hf-O isotopes show εHf(t) values from −25.6 to −20.0 with crustal model ages ranging from 2807 to 2458 Ma, and δ18O values from 4.75‰ to 5.66‰, indicating that the parental magma involved mixing between the partially melted Taihua Group lithologies and mantle-derived component. The available geochronological data constrain the timing and duration of magmatic-hydrothermal activities in the Qiyugou deposit, including pre-mineralization 158.7–150.1 Ma granitic magmatism, syn-mineralization 136.4–128.6 Ma porphyry magmatism and Au mineralization, and post-ore 128.9–124.7 Ma granitic dyke intrusion. The duration of the magmatic-hydrothermal activity in the Qiyugou deposit ranges between 136.4 Ma and 128.6 Ma, when the breccia pipes and granitoid plutons formed through multiple pulses of magma emplacement and fluid exsolution from a fertile magma chamber. The 136.4–131.3 Ma porphyry magmatism and gold mineralization in the ‘191 pluton’ represent an early stage magmatic-hydrothermal activity in the Qiyugou porphyry-breccia system. We predict that the deeper levels of the breccia pipes could be potential target for porphyry gold ore bodies in the Qiyugou deposit.

Evaluating Sn mineralization potential at the Haobugao skarn Zn–Pb deposit (NE China) using whole-rock and zircon geochemistry

The metal association in a skarn deposit is largely controlled by the magma redox state and fractionation degree. The Haobugao skarn deposit in northeastern China contains significant Zn–Pb sulfides, with subordinate by-products Cu and Fe ores. Recent explorations also found minor cassiterite occurring locally in the deposit, inspiring the speculation of Sn mineralization. To better evaluate the Sn mineralization potential at Haobugao, the geochemical compositions of the ore-related biotite granite porphyry and its magmatic zircons have been investigated to quantify the oxidation state and the degree of magmatic evolution, which provide constraints on the capability of the Haobugao intrusion to produce economic Sn mineralization.The causative biotite granite porphyry (~140 Ma) of the Haobugao skarn is a typical I-type granite that belongs to high-K calc-alkaline series. It contains 68.88–72.27 wt% SiO2 and 0.52–1.38 wt% MgO, and has a Rb/Sr ratio range of 0.82–2.07 and a Nb/Ta ratio range of 8.69–22.49, indicating that it is moderately fractionated. The zircon Ce and Eu anomalies have been used as proxies for magma redox state, and the results (CeN/CeN* = 36–213, EuN/EuN* = 0.17–0.52) suggest that the ore-forming magmas are relatively oxidized, with an estimated ΔFMQ range of −0.4 to +3.5 (+1.0 in average). This is also consistent with the high whole-rock Fe2O3/(FeO + Fe2O3) mass ratios (0.54–0.76) and the occurrence of abundant ferric garnet in the skarn (i.e., andradite). These results collectively show that the ore-related granitic intrusion at Haobugao is generally more oxidized but less evolved when compared with other intrusions responsible for typical skarn Sn mineralization worldwide. Therefore, it is unlikely for the Haobugao intrusion to produce Sn mineralization of economic significance. The present study demonstrates that the whole-rock and zircon geochemistry data are useful in evaluating the mineralization potential of metals like Sn in a given mineral deposit or prospect, which may help guide exploration.

Raman spectroscopy-based screening of zircon for reliable water content and oxygen isotope measurements

Abstract Water content and oxygen isotopes in zircon provide crucial constraints on magma source and process, yet they can be significantly modified by zircon metamictization, which causes secondary water absorption into the zircon crystal and the concomitant oxygen isotope changes. Therefore, it is imperative to develop a screening scheme to select the least-metamict zircons for the analyses. We propose a screening scheme based on our study on the Suzhou A-type granite (South China) through integrating Raman spectroscopy, water and trace element measurements, and oxygen isotope analysis. The results show that the primary water content is retained in zircon when the full-width at half maximum (FWHM) is <8 cm−1 or the Raman shift is >1007 cm−1 of ν3(SiO4) vibration band, while the primary δ18O is preserved at <10 cm−1 FWHM or >1005.5 cm−1 Raman shift. Changes in trace element concentrations in Suzhou zircons are different from previous observations in metamict zircons but most likely related to magma evolution, which implies that trace elements are insensitive to metamictization. Primary δ18O in Suzhou zircons (4.5–6.0‰) fall into the mantle range, indicating a dominant mantle contribution to Suzhou granites. Primary water content was estimated at ca. 650–1400 ppm, significantly higher than those of typical I-type granite (400–736 ppm) and upper mantle-derived zircons (81–177 ppm). The high primary zircon water content was not controlled by the sub-solidus process, temperature, pressure, and cation charge balance but considered to reflect the high-water content in melts. This suggests a hydrous origin for the Suzhou A-type granite, which challenges the conventional view of anhydrous petrogenesis for A-type granites.

Mixing of cogenetic magmas in the Cretaceous Zhangzhou calc-alkaline granite from southeast China recorded by in-situ apatite geochemistry

AbstractMixing of cogenetic magmas represents an important process in granite petrogenesis but is difficult to identify and is consequently often overlooked due to the absence of obvious isotopic distinctions between the mixed melts. We have conducted in situ elemental and O isotope analyses on apatite from Cretaceous Zhangzhou calc-alkaline granite in southeast China. We integrated these data with micro-analyses on other minerals (plagioclase, zircon, and titanite) as well as whole-rock geochemistry to decipher the mixing history of this granitic complex. The apatite occurs as an early crystallizing phase forming inclusions in biotite, plagioclase, and titanite, and is characterized by core-rim zonation textures with a dark core and bright rims in backscattered images. The core domains have remarkably higher SO3 and Li concentrations but much lower SiO2, REE, and Y concentrations than the rim domains. However, both the cores and rims show geochemical compositions similar to that from typical I-type granite and also have mantle-like O isotope compositions (the core has δ18O = 5.3–6.8‰ and the rim has δ18O = 5.2–6.4‰, respectively), indicating crystallization from granitic melts derived from newly accreted crust. The combined major and trace element and O isotope compositions of apatite and whole-rock geochemistry suggest that compositional evolution of the Zhangzhou granite involved mixing between two cogenetic magma batches, with variable degrees of subsequent differentiation. Batch I magma was a low-SiO2 and high-SO3 melt, whereas Batch II magma was a high-SiO2 and low-SO3 melt that experienced devolatilization. The high-S content in apatite cores further suggests the parental magma of the Zhangzhou granite likely originated from a sulfur-rich source comprising mainly newly accreted arc crust in response to subduction of the paleo-Pacific Ocean. The geochemical records of these magmatic processes are rarely observed in coeval zircon, titanite, and plagioclase. Our study, therefore, demonstrates that apatite geochemistry is potentially a more suitable monitor of complex magmatic evolution, including devolatilization and mixing of isotopically indistinguishable magmas.

Whole-rock geochemical and zircon Hf–O isotopic constraints on the origin of granitoids and their mafic enclaves from the Triassic Mishuling pluton in West Qinling, central China

The origin of mafic microgranular enclaves (MMEs) in granitoids bears significant information on their petrogenesis. An integrated study including petrology, mineralogy and geochemistry was carried out for monzogranites and the hosted MMEs as well as biotite granites from the Mishuling pluton in the West Qinling, central China. LA-ICPMS zircon U–Pb dating yields similar emplacement ages of 212 ± 2 Ma to 217 ± 3 Ma for the monzogranites, biotite granites and MMEs. All of them display similar whole-rock Sr–Nd–Hf–O isotope and in-situ zircon Hf–O isotope compositions, suggesting their origination from common crustal sources, most likely meta-igneous rocks.MMEs have igneous textures, and contain acicular apatite and amphibole. Plagioclase with distinct core-rim compositions and clinopyroxene with replacement texture by amphibole have also been observed in MMEs. These features suggest that MMEs were formed by mixing of an intermediate magma with a felsic magma. Biotite granites are weakly peraluminous, and have high zircon δ18O values of 7.6–8.8‰ and low zircon saturation temperatures, indicating their derivation from partial melting of crustal rocks at relatively low temperatures. Compared with biotite granites, monzogranites are metaluminous, have high zircon saturation temperatures, and contain amphibole, reminiscent of typical I-type granites. However, they also show high in-situ zircon δ18O values of 7.9–9.2‰. These geochemical features indicate that monzogranites were derived from crustal rocks at elevated temperatures. The sources themselves have experienced low-temperature interaction with surface waters. Therefore, partial melting of crustal rocks at different temperatures plays an important role in dictating the geochemical diversity of the resulted granitoids.

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.

Late Mesozoic magmatism and tectonic evolution in the Southern margin of the North China Craton

Late Mesozoic granitic magmatism (158–112 Ma) are widespread in the southern margin of the North China Craton (NCC), contemporary with many world-class Mo-Au-Ag-Pb-Zn polymetallic deposits. There are abrupt changes in the elements and isotopic compositions of these granites at about 127 Ma. The early stage (158–128 Ma) granites show slightly or no negative Eu anomalies, large ion lithophile elements enriched and heavy REE depleted (such as Y and Yb), belonging to typical I-type granite. The late stage (126–112 Ma) granites are characterized by A-type and/or highly fractionated I-type granite, with higher contents of SiO2, K2O, Y, Yb and Rb/Sr ratio and lower contents of Sr, δ Eu value and Sr/Y ratio than that of the early-stage granites. Moreover, the whole rock Nd and Hf isotopic compositions of the granites younger than 127 Ma show more depleted than those of the older one. The two stages of Late Mesozoic granites were derived from a source region of the ancient basement of the southern margin of the NCC incorporated the mantle material. The late stage (126–112 Ma) granites contain more fractions of mantle material with depleted isotopic composition than the early ones. The granites record evidence for a strong crust-mantle interaction. They formed in an intracontinental extensional setting which was related to lithospheric thinning and asthenospheric upwelling in this region, which was possibly caused by westward subduction of the Paleo-Pacific plate. 127 Ma is an critical period of the transformation of the tectonic regime.

Early to late Yanshanian I-type granites in Fujian Province, SE China: Implications for the tectonic setting and Mo mineralization

The Cathaysia Block is the southeastern part of the South China Block in Southeast (SE) China, and it hosts voluminous late Mesozoic I-, S-, and A-type granitoids, as well as minor highly fractionated granites. We present here zircon U–Pb age data and Nd–Hf isotopic data for the Dayang and Juzhou granites, together with new petrological and geochemical analyses. The Dayang pluton consists of fine-grained two-mica monzonitic granites in which the plagioclases exhibit zoning and poikilitic textures. In contrast, the Juzhou pluton consists of medium- to coarse-grained biotite K-feldspar granites that lack zoning and poikilitic textures. The emplacement ages are 143±2.3Ma for the Dayang pluton and 133±2.1Ma for the Juzhou pluton according to zircon U–Pb isotope analyses. The Dayang and Juzhou granites are both metaluminous and belong to the shoshonitic series. The Dayang granite exhibits very flat REE patterns, showing the tetrad effect, and the spidergrams show striking negative Ba, Sr, Nb, and Ti anomalies and a positive Ta anomaly. In contrast, the Juzhou granite has sloping REE patterns, but like the Dayang granite it also has striking negative Ba, Sr, Nb, Ta, and Ti anomalies. Petrographic and geochemical evidence indicates that the Dayang granite is a highly fractionated I-type granite and that the Juzhou granite is a typical I-type granite. The tetrad effect in the Dayang granite can be interpreted in terms of melt–rock interactions at a late stage of magma evolution, whereas the main mechanism during the evolution of the Juzhou magma was fractionation of plagioclase, biotite, hornblende, apatite, zircon, and allanite. Nd–Hf isotope data suggest that the Dayang and Juzhou granites were both formed partial melting of Paleoproterozoic basement rock and juvenile material (underplating basalts or Mayuan Group amphibolites), with the Juzhou granite having a greater contribution from juvenile material than the Dayang granite. Our new data, together with existing data, suggest that the tectonic setting of the early Yanshanian (∼143Ma) highly fractionated I-type Dayang granite was a back-arc that formed in response to the westward subduction of the Paleo-Pacific Plate, and that the late Yanshanian (∼133Ma) Juzhou granite formed in a continental arc setting in response to rollback of the Paleo-Pacific Plate toward the coastline. The Mo mineralization in the Makeng ore area was probably the result of the exsolution of Mo-bearing fluids from the Dayang granitic magmas due to extensive fractional crystallization.

Petrogenesis of the early Cretaceous Funiushan granites on the southern margin of the North China Craton: Implications for the Mesozoic geological evolution

Late Mesozoic granitoids are ubiquitous in the southern margin of the North China Craton and are keys to the understanding of the Mesozoic geological evolution. The early Cretaceous Funiushan granitic pluton in the southern margin of the North China Craton is composed of porphyritic biotite monzogranites. Rocks from the Funiushan pluton have high SiO2 (64.45–73.98wt.%), Na2O (3.19–4.67wt.%) and K2O (3.76–7.95wt.%) and low MgO (0.11–1.34wt.%). They are enriched in Rb, Ba, Th, U and LREE ((La/Yb)N=9.63–45.0), and depleted in Nb, Ta, Ti and P, and have negative Eu anomalies (Eu/Eu*=0.29–0.72). This geochemical feature is similar to those of typical I-type granites. Zircons from the granites were dated using LA-ICP-MS and SIMS, and yielded 206Pb/238U ages of 115–131Ma. They have εHf(t) values mainly vary from −17.7 to +0.9 and TDMC ages mainly from 2301 to 1118Ma. Whole rock εNd(t) values range from −20.3 to −9.6 and TDM ages from 1.49 to 2.29Ga, indicating that the magma was produced by partial melting of the Neoarchean to Paleoproterozoic crustal rocks, as represented by the Taihua Group basement rocks, with minor involvement of mantle-derived melts. The Funiushan pluton is considered to have been formed through a strong crust-mantle interaction process under a low pressure condition in an extensional setting, due to the lithospheric thinning caused by westward subduction of the Paleo-Pacific oceanic crust.

Intraplate crustal remelting as the genesis of Jurassic high-K granites in the coastal region of the Guangdong Province, SE China

The origin and tectonic significance of high-K granites (>3wt% K2O at 70wt% SiO2), calc-alkaline I-type granites in particular, remain controversial. This paper takes granitic plutons distributed in the coastal region of the Guangdong Province of southeastern China as examples to explore the genesis of such rocks. SIMS zircon U–Pb geochronological data show that the granites were emplaced at 166–159Ma. These granites can be subdivided generally into two groups on the basis of integrated mineralogical, geochemical, whole-rock Sr–Nd isotopic and in situ zircon Hf–O isotopic studies. The group A granites (SiO2=64–72wt%) are characterised by their common occurrence of amphibole (±titanite) and dominantly metaluminous feature (A/CNK=0.85–1.03). They are high in K2O (3.5–7.0wt%) and K2O/Na2O (>1), and have trace element concentrations (e.g., Nb, Y, Zr and Ga) similar to typical I-type granites in the Lachlan Fold Belt, southeastern Australia. Their whole-rock ISr (0.7057–0.7077) and εNd(t) (−6.46 to −3.13) are less evolved than many coeval granites in this region. As in situ zircon Hf–O isotopes show little evidence of magma mixing, these granites with low zircon δ18O (6.3−7.9‰) and high εHf(t) (−5.9 to −0.2) could have been generated from melting of oxidised high-K basaltic rocks. The group B granites, emplaced to the east of group A granites, are dominantly weakly peraluminous (A/CNK=1.00–1.05). They have higher SiO2 (70–76wt%), less common or absence of amphibole, higher zircon δ18O (6.6−9.0‰) and lower εHf(t) (−11.4 to −5.9) than the group A granites. Zircon Hf–O isotope data reveal that the group B granites contain higher percentage of supracrustal materials than those of the group A, but the variations of major and trace elements do not support an assimilation and fractional crystallization (AFC) model. Instead, the group B granites, with features transitional between typical I-type and S-type granites, were most likely formed in a region where there was physical juxtaposition between infracrustal metaluminous and supracrustal peraluminous source rocks. Thus, granites of both groups represent products of crustal reworking likely due to asthenosphere upwelling and/or underplating and intrusion of mafic magmas. The close association in time and space of these granites with OIB-like basaltic rocks and the secular compositional change of Jurassic basaltic rocks in the region suggest that these rocks probably formed in an intraplate extensional setting resulted from the delamination of a flat-subducted oceanic slab.

Ages and geochemistry of granites in the Pingtan–Dongshan Metamorphic Belt, Coastal South China: New constraints on Late Mesozoic magmatic evolution

Zircon U–Pb dating and Hf-isotope compositions and bulk geochemical analyses of granites and metasedimentary rocks from the NE-striking Pingtan–Dongshan Metamorphic Belt (PDMB), coastal South China, are used to constrain their formation ages and tectonic settings. LA-ICPMS zircon U–Pb dating results reveal four periods of magmatism/metamorphism along the PDMB, including ~187Ma, 133–130Ma and 108–100Ma granitic magmatism in the northern PDMB and 147–140Ma granites and metamorphic rocks in the southern belt. A low P/T metamorphic event at 144–140Ma and two deformation phases at 130–125Ma and 108–100Ma are also recognized. The Early Yanshanian high-K calc-alkaline granites (187–140Ma) are characterized by high contents of SiO2, alkali and low light rare earth element (LREE)/ heavy rare earth element (HREE) fractionation (LREE/HREE=4.82–11.54, (La/Yb)N=3.99–13.53) and weak Eu depletion (0.54–0.61), whereas the Late Yanshanian calc-alkaline granites (140–100Ma) have low A/CNK, middle LREE/HREE ratios (10.13–19.18) and positive to inconspicuous Eu anomalies (0.79–1.95). Granites in both periods are depleted in high field strength elements (HFSE, e.g. Nb, Ta, Ti) but enriched in LREE and large ionic lithophile elements (LILE), with low Rb/Ba and Rb/Sr ratios and A/CNK values, indicating that they are typical I-type granites in the arc-related tectonic setting due to subduction of the Paleo-Pacific plate. However, both the Early and Late Yanshanian granites have distinct Sr, Nb and K2O contents and La/Yb, Sr/Y and Eu/Eu* ratios, suggesting different P–T conditions. Zircon grains from the granites in the northern PDMB have higher Hf-isotope compositions with εHf(t)=+0.01 to +6.47 and Hf model ages (TCDM)=0.82–1.16Ga, whereas those from the granites in the southern PDMB show slightly lower Hf-isotope compositions with εHf(t)=−6.63 to +0.07 and older Hf model ages (TCDM)=1.19–1.61Ga. Compared with the metamorphic basement near the study area, the granites in the northern PDMB were probably derived from a Neoproterozoic crust, whereas those in the southern PDMB may have originated from a mixed source of Neoproterozoic and minor older crustal materials. We propose that the Early Yanshanian granites in the PDMB are the eastern extension of the E–W-striking granitoid–volcanic belt in the Nanling Range, whereas the Late Yanshanian igneous rocks mostly distribute in coastal Southeast China and strike NE. The distinct distribution and geochemical features between the Early and Late Yanshanian magmatic events can be related to the change of direction and dip angle of the Paleo-Pacific plate that was subducted underneath the Eastern Eurasian continent in the two episodes.

LA-ICP-MS zircon dating, geochemistry, petrogenesis and tectonic implications of the Dapingliang Neoproterozoic granites at Kuluketage block, NW China

The Kuluketage block, located in the northeast Tarim Craton, is one of the largest Precambrian blocks in Xinjiang Province, China. Recently the Dapingliang skarn Cu deposit was discovered at the eastern part of the Kuluketage block. The ore bodies are developed in skarn rocks along the NE fault. This deposit formed at the middle Neoproterozoic (826Ma) and the ore-related plagiogranite also formed at 826Ma. Plagiogranite, biotite monzogranite and K-feldspar granite are the major rock types intruded in sequence in the Dapingliang area. The LA-ICP-MS zircon 206Pb/238U age of the monzogranite is 816±5Ma. The emplacement age of the Dapingliang granites is then constrained between ca. 816 and 826Ma. Geochemical characteristics show that the rocks belong to the low-Ca, high-K calc alkaline and shoshonite series. They are typical I-type granites. These series have a negative slope from La to Lu. The plagiogranite and biotite monzogranite show no (or weak negative) Eu anomaly, low (La/Yb)N (9.87–23.39), (Dy/Yb)N (1.01–1.14), high Sr (531–976ppm) and Sr/Y (30.5–50.7) with no garnet and plagioclase residual. The K-feldspar granite shows weak to high negative Eu anomaly and relative lower LREE/HREE fractionation. The low La/Yb and Sr/Y show that the origin has plagioclase residual and no garnet residual. Trace elements of the plutons show a negative anomaly of Nb, Ta, P, Zr and Ti, and positive anomaly of Ba, K, Sm and Sr with respect to primary mantle. ɛNd(t) varies from −6.1 to −14.6 and initial 87Sr/86Sr ranges from 0.70235 to 0.70912 for all the samples show lower crust origin and are contaminated with Archean rocks. Pb isotope show low radiogenetic 208Pb/204Pb and 207Pb/204Pb. The initial 208Pb/204Pb and 207Pb/204Pb are well above the Northern Hemisphere Reference Line. The Pb isotopes also indicate lower crust origin. These rocks maybe formed by partial melting of K-rich basalt of the lower crust during a period of transition from crustal compression to extension. There are also K-rich granites formed at the same tectonic setting in the Kuluketage block which contributed to the post-collisional granite belt derived from lower crust.

The Late Precambrian Timna igneous complex, Southern Israel: Evidence for comagmatic-type sanukitoid monzodiorite and alkali granite magma

New data from a geochemical, geochronological and isotopic study of the Late Precambrian Timna igneous complex suggest the formation of alkali granites from a LIL-enriched, mantle derived, sanukitoid-type monzodiorite (a silica oversaturated rock with Mg# >60). These data also provide new insights into the petrology, timing and regional tectonic control of the transition from the calc-alkaline to the alkaline magmatic activity in the northern Arabian-Nubian Shield (ANS) during the Late Precambrian. The Timna alkali granite was formed by fractional crystallization from the monzodioritic magma in a quasi-stratified magmatic cell which formed 610 Ma ago in the 625 Ma old calc-alkaline, porphyritic granite crust. These monzodiorites are mantle-derived, as demonstrated by their high Mg# (63), Cr (230 ppm), and Ni (120 ppm). They are characterized by initial 87 Sr 86 Sr of 0.7034, ϵ-Nd (610 Ma) = +3.4, and are enriched in K 2O (2.9%), Sr (840 ppm), Ba (1290 ppm) and LREE [ ( La Lu ) n = 10–25 ]. The chemical characteristics and REE patterns of the monzodiorites and andesitic dykes of Timna are very similar to Dokhan andesites from northeastern Egypt and the Archean sanukitoids from Canada. The isotopic, geochemical and geochronologic data all indicate that Timna monzodiorites are comagmatic with the alkali granite. The alkali granite is a typical post-orogenic, borderline A-type granite. It is enriched in potassium (K 2O=4.68–6.64%), has a negative europium anomaly ( Eu Eu ∗ =0.058–0.38 ) and ϵ-Nd (610 Ma) of +3.9. The calc-alkaline granite is a typical I-type granite with a small positive europium anomaly ( Eu Eu ∗ =1.02–1.16 ). Its age and the Sr, Nd and Pb isotopic characteristics with ϵ-Nd (625 Ma) of +5.6 to +5.9 are significantly different from these of the alkali granite and monzodiorites, and indicate little interaction with the monzodiorite during the formation of the alkali granite. The alkali granites are correlative with the post-collisional extensional granites in Jordan and Egypt while the porphyritic granites can be correlated with the late orogenic types. Crustal thickening associated with orogenic compression resulted in crustal anatexis to form the I-type granitic rocks, whereas crustal thinning associated with extension allowed LIL-enriched mantle melts to rise very near to the surface, where space was available for these to pond and fractionate to alkali granite.