Adakite-like Rocks Research Articles

Petrogenesis and Geological Implications of the Oligocene Mingze monzodiorites, Southern Lhasa

The Mingze Cu-Mo deposit is located in the southern margin of the Lhasa block of the Himalayan Tibetan Plateau. Here, we report the geochronological and geochemical data from Mingze monzodiorites, which hosts the Mingze deposit. Zircon dating indicates that the Mingze monzodiorites were emplaced at ca. 31 Ma (i.e., the Oligocene). The monzodiorites have variable SiO2 and MgO contents, strongly negative high field-strength element (HFSE, such as Ta, Nb, Zr and Hf) anomalies on the normalized trace element diagram and show uniform (87Sr/86Sr)i (0.7066–0.7076), εNd(t) (−2.50 to −4.04) and εHf(t) (+1.50 to +7.50). Their geochemical compositions are different from coeval (40–30 Ma) adakite-like rocks but comparable to coeval mafic enclaves and gabbros. We propose that Mingze monzodiorites were derived from partial melting of the lithospheric mantle, which previously metasomatized by the subducted Indian continental plate that probably subducted into the overlying mantle. The concurrency of the genetically related mafic enclaves and associated intermediate to mafic rocks implies the heterogeneity of the Lhasa lower crust.

Genesis and tectonic setting of Shenshan Fe–Cu deposit in Inner Mongolia, Northeast China: Constraints from geochemistry, U–Pb and Re–Os geochronology, and Hf isotopes

The newly-discovered Shenshan Fe–Cu skarn deposit is located in the southern Great Xing’an Range of northeastern China. The mineralization occurs mainly in the exoskarn or marble wallrocks of the Permian Zhesi Formation, which were intruded by the Shenshan granodiorite and granite porphyry dykes. Molybdenite Re–Os dating yielded a weighted mean age of 160.2 ± 2.4 Ma (MSWD = 0.38), consistent with the LA–ICP–MS zircon U–Pb age of the granodiorite (161.6 ± 2.2 Ma; MSWD = 0.75), but significantly older than the granite porphyry dykes (132.8 ± 1.9 Ma; MSWD = 0.87). This suggests a coeval and causative relationship between the granodiorite and Fe–Cu mineralization. The Shenshan granodiorite has an adakite-like geochemical signature with high Sr contents (704–871 ppm), and high Sr/Y (46.79–80.21) and (La/Yb)N (17.54–31.13) ratios, but low Y (9.7–13.1 ppm) and Yb (0.70–1.18 ppm) contents. The granodiorite has relatively low contents of Mg (1.71–1.94 wt%), Cr (18–23 ppm), and Ni (4.1–9.0 ppm), and positive εHf(t) (−0.3 to 3.0, average 1.51), yielding two-stage model ages (TDM2) of 1.38–1.68 Ga (average 1.51 Ga). The results indicate that the parental magmas of the granodiorite were derived from partial melting of juvenile thickened lower crust with minor mantle-derived input. Meanwhile, the Shenshan granite porphyry dykes are high-K calc-alkalic and weakly peraluminous, with low MgO (0.21–0.24 wt%) and Mg# (19–21). The dykes exhibit HFSE (e.g., Nb, Ta) depletions and LILEs (e.g., Rb, K) enrichments, and strong to moderate negative Eu anomalies, indicating major continental crust-derived input in the magma source. Geochemical and Hf isotopic data compilation for granitoids across the Great Xing’an Range indicate that the mineralization at Shenshan was influenced by the south-dipping subduction of the Mongol–Okhotsk oceanic plate. The discovery of adakite-like rocks in the Shenshan Fe–Cu deposit, and the voluminous adakite-like rocks in the region, suggest that the Mesozoic Great Xing’an Range has considerable prospecting potential for Cu–Au–Mo deposits.

The multiple granitic magmatism in the giant Huayangchuan uranium polymetallic ore district: Implications for tectonic evolution of the southern margin of North China Craton in the Qinling Orogen

The giant Huayangchuan U–Nb–Pb deposit in the Qinling Orogen is a newly verified carbonatite-hosted deposit in the southern margin of the North China Craton (NCC), central China. Granitic magmatism is extensively developed in the Huayangchuan deposit area, but their ages and petrogenesis are not well constrained. The exposed granitic rocks are mainly biotite monzogranite porphyry, granite pegmatite, granodiorite, and biotite monzogranite with zircon U–Pb ages of 1808 ± 10 Ma, 1807 ± 14 Ma, 233 ± 1.4 Ma, and 132 ± 0.6 Ma, respectively. The Paleoproterozoic biotite monzogranite porphyry belongs to shoshonite and metaluminous series, showing enrichment of LREE and LILE and depletion of HREE and HFSE, with high Zr + Y + Nb + Ce values and Ga/Al ratios, which are consistent with A-type granite. Whereas, the contemporaneous granite pegmatite dykes with weak mineralization are also cala-alkaline to shoshonite and peraluminous series, enriched in Rb, Ba, and LREE, and depleted in Nb, Ta, Ti and HREE. The shoshonite and weakly peraluminous Triassic granodiorite is slightly enriched in LREE with flat HREE patterns, enriched in Ba and Sr and depleted in Nb, Ta, P, and Ti, with similar geochemical characteristics to adakite-like rocks. The Early Cretaceous biotite monzogranite is characterized by LREE enrichment and flat HREE patterns, belonging to shoshonite and metaluminous to weakly peraluminous I-type granite, with U and LILE enrichment, and HFSE-depleted. The high initial 87Sr/86Sr ratios and enriched Nd (εNd(t) = −17.5 to −17.1) and Hf (εHf(t) = −33.2 to −14.6) isotopes reveal that the Huayangchuan granitic rocks are obviously sourced from crustal-derived magmas. Combined with regional geology and this study, we proposed that: (1) the Paleoproterozoic biotite monzogranite porphyry and granite pegmatite were generated from ancient lower crust during post-collisional extension setting; (2) the granodiorite was likely sourced from partial melting of thickened lower crust with pelagic sediments materials addition during the Triassic although the mineralization-related carbonatite having similar age with granodiorite may derive from mantle; and (3) the Early Cretaceous biotite monzogranite was mainly sourced from the partial melting of lower crust induced by the underplating of the mafic magma. We suggest that the first phase of magma occurred in the Huayangchuan district during the Paleoproterozoic following the amalgamation of the Eastern and Western blocks along the Trans-North China Orogen. Since the Mesozoic, the ongoing northward subduction of the Yangtze Craton (YZC) resulted in the crust thickening, developing the Triassic granitic magmatism and major mineralization associated with carbonatite. The widespread Cretaceous granite and deposit in the southern margin of NCC indicate that intracontinental extension and lithospheric thinning occurred in response to the tectonic regime transition from NS-trending to EW-trending subduction.

Eocene tonalite–granodiorite from the Havza (Samsun) area, northern Turkey: adakite-like melts of lithospheric mantle and crust generated in a post-collisional setting

ABSTRACT Eocene intermediate to felsic plutons of different sizes and compositions are widespread in the Eastern Pontides Orogenic Belt in northern Turkey. Of these, the Taşlık Tepe pluton in the Havza (Samsun) area is fine-to-medium-grained, with granular, porphyritic, and micrographic textures, and include mafic microgranular enclaves (MMEs). LA-ICP-MS U-Pb zircon dating yielded emplacement ages of 42.9 (± 1.4) and 40.5 (± 1.3) Ma for the host granodioritic pluton and the dioritic MMEs, respectively. Petrochemically, the host pluton has I-type, high-K calc-alkaline, and metaluminous-to-slightly peraluminous features (A/CNK = 0.95–1.06). The host pluton also shows geochemical features of adakite-like rocks with high SiO2 (67–68 wt%) and Al2O3 (15.5–16.0 wt%) content and Ba/La (17–23), Sr/Y (40.7–61.6), and LaN/YbN (14.4–23.7) ratios and low Y (8.2–9.9 ppm) and YbN (3.1–4.4) contents. Whole-rock major and trace element variations suggest that fractional crystallisation played a significant role in the pluton evolution. The N-MORB normalised trace element patterns of the pluton are similar to those of MMEs with enrichment in large-ion lithophile elements, Th and Ce, and negative Nb and Ti anomalies. Chondrite-normalised rare earth element plots show moderate-to-highly enriched concave patterns (LaN/LuN = 14.2–21.6) with insignificant negative Eu anomalies (EuN/Eu* = 0.86–1.14), all of which imply hornblende fractionation during magmatic evolution. The pluton samples have 87Sr/86Sr ratios of 0.704767 to 0.704927, 143Nd/144Nd ratios of 0.512767–0.512774, εNd values of (+2.52) – (+2.65), and δ18O values of 7.9–9.7‰. The isotopic compositions of the host pluton and MMEs are similar to I-type granitoids derived from mantle sources. The MMEs show incomplete magma mixing/mingling, representing small bodies of mafic parental magma. Combined with regional studies, these new data suggest that the parental magma of the studied adakite-like pluton was generated from the lithospheric mantle and then modified by fractional crystallisation and assimilation in a post-collisional setting.

Postcollisional transition from subduction- to intraplate-type magmatism in the eastern Sakarya zone, Turkey: Indicators of northern Neotethyan slab breakoff

AbstractPostcollisional magmatism in the eastern Sakarya zone was recorded by voluminous basic volcanism and repeated plutonism during the early Cenozoic. The temporal and geochemical evolution of these magmatic rocks is important for understanding the possible geodynamic history of the Sakarya zone. Here, we investigated three representative plutons lying between the towns of Çamlıhemşin (Rize) and İspir (Erzurum), Turkey. These are largely composed of medium-K gabbroic diorites (Marselavat Pluton), shoshonitic monzonites (Güllübağ Pluton), and high-K granites (Ayder Pluton). We present whole-rock geochemistry, 40Ar/39Ar geochronology, and Sr, Nd, and Pb isotope analyses from the plutons to constrain the timing of variations in magmatism and source characteristics, and we provide a new approach to the proposed geodynamic models, which are still heavily debated. The 40Ar/39Ar geochronology reveals a cooling sequence from ca. 45 Ma for the Marselavat Pluton through ca. 41 Ma for the Güllübağ Pluton to ca. 40 Ma for the Ayder Pluton. Whole-rock geochemistry and Sr, Nd, Pb isotopes suggest that crustal contamination was not an important factor affecting magma compositions. Although there was no arc-related tectonic setting in the region during the middle Eocene, the Marselavat Pluton shows some subduction affinities, such as moderately negative Nb and Ta anomalies, and slightly positive Pb anomalies. These signatures were possibly inherited from a depleted mantle source that was modified by hydrous fluids released from the oceanic slab during Late Cretaceous subduction. Geochemical traces of the earlier subduction become uncertain in the Güllübağ samples. They display ocean-island basalt–like multi-element profiles and Nb/Ta, Ce/Pb, and La/Ba ratios. All these point to a mantle source in which earlier subduction signatures were hybridized by the addition of asthenospheric melts. Melting of calc-alkaline crustal material, probably emplaced during the first phase of middle Eocene magmatism (Marselavat), led to the formation of granitic plutonism (Ayder Pluton). Our data in conjunction with early Eocene adakite-like rocks show that melt generation, as in the given sequence, was most probably triggered by breakoff of the northern Neotethyan oceanic slab, ∼13 m.y. after the early Maastrichtian collision between the Sakarya zone and Anatolide-Tauride block, and continued until the end of the middle Eocene. A shallow-marine transgression occurred contemporaneously with the middle Eocene magmatism throughout the Sakarya zone. An extension in this magnitude seems unlikely to be the result of orogenic collapse processes only. The main cause of this extension was most probably related to the northward subduction of the southern Neotethys Ocean beneath the Anatolide-Tauride block. The result is a volumetrically larger amount of middle Eocene magmatism than that expected in response to slab breakoff.

Late Carboniferous to Early Permian magmatic pulses in the Uliastai continental margin linked to slab rollback: Implications for evolution of the Central Asian Orogenic Belt

The Paleo Asian Ocean underwent a protracted closure history during Late Paleozoic. Here we investigate the magmatic evolution during this process based on a detailed study in the Baiyinwula region along the Uliastai continental margin. The major rock types in this area are Late Carboniferous-Early Permian volcanic sequences and coeval intrusions. We identified four stages of magmatic evolution based on the diverse assemblages and their precise isotopic ages. The first stage is represented by andesites with a zircon 206Pb/238U age of ca. 326±12Ma. These rocks are metaluminous to weakly peraluminous, high-K calc-alkaline, and possess high Na2O/K2O ratios in the range of 1.23 to 2.45. They also display enrichment of large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE), with markedly positive zircon εHf (t) varying from 8.1 to 15.6.The geochemical features of these andesites are similar to those of typical arc volcanic rocks. The second stage includes granodiorites emplaced at 318.6+1.8Ma. The rocks are high-K calc-alkaline with A/CNK values ranging from 0.95 to 1.06, and show enrichment in LILE and depletion in HFSE. They show geochemical affinities to adakites, with high Sr and low Y and Yb contents, indicating magma derivation from thickened lower crust. Zircon grains from these rocks display positive initial εHf (t) values ranging from 11.1 to 14.6 with corresponding two-stage Hf model ages (TDM2) of 394–622Ma. The third stage consists of syenogranite together with a volcanic suite ranging in composition from rhyolite todacite, which formed during 303.4±1.2 to 285.1±2.2Ma. They possess elevated silica and alkali contents, high FeOt/MgO and Ga/Al ratios, low Al2O3, MgO and CaO contents, and high Rb, Y, Nb, Ce, Zr, Y, and Ga contents, strong negative Ba, Sr and Eu anomalies, showing I- to A-type granitic affinities. Zircons in these rocks show elevated Hf isotopic compositions (εHf (t)=9.9 to 14.6) with TDM2 varying from 324 to 673Ma. The fourth magmatic pulse is represented by K-feldspar granite with zircon U-Pb ages from 283.2±1.9Ma to 280.0±1.4Ma, and typical alkalic A-type granite geochemistry. These rocks possess positive εHf (t) values in the range of 9.7 to15.2, and a restricted range of Hf model age from 327 to 684Ma. The magmatic rocks from the four stages show comparable εHf (t) and T2DM, suggesting that the magmas were derived from the same evolving mantle-derived source. We propose a tectonic model linking the evolution of the magmatism with the closure of the Paleo Asian Ocean that involved the following stages. The andesites were formed during the initial oceanic subduction stage with magma sourced from the metasomatized lithospheric mantle. Stage 2 adakite-like rocks were derived from subduction-induced thickened crust. Subsequent slab rollback resulted in asthenospheric upwelling and melting of residual juvenile crust to generate the I- and A- type syenogranite, rhyolite and dacite suite, finally followed by the A-type K-feldspar granite.

Miocene Ultrapotassic, High-Mg Dioritic, and Adakite-like Rocks from Zhunuo in Southern Tibet: Implications for Mantle Metasomatism and Porphyry Copper Mineralization in Collisional Orogens

Miocene Ultrapotassic, High-Mg Dioritic, and Adakite-like Rocks from Zhunuo in Southern Tibet: Implications for Mantle Metasomatism and Porphyry Copper Mineralization in Collisional Orogens

Two different types of granitoids in the Suyunhe large porphyry Mo deposit, NW China and their genetic relationships with molybdenum mineralization

The Suyunhe large porphyry Mo deposit (∼0.57Mt molybdenum), located in the West Junggar, NW China, is the largest known porphyry Mo deposit in Xinjiang. Granitoids in this deposit are mainly characterized by three closely spaced intrusive centers (known as stocks I, II and III respectively). The stocks I and III mainly consist of barren granodiorite porphyry and tonalite porphyry, whereas the stock II is mainly composed of fertile monzonitic granite porphyry and granite porphyry. Based on detailed major and trace element, and Sr–Nd isotopic analyses, two distinct compositional groups can be identified. The first group of high-silica end-members (HSE) is characterized by high SiO2 (mostly >75wt%), low MgO (0.07–0.69wt%) and Mg# (0.19–0.36), significant Eu depletion in the chondrite-normalized diagram, and low Sr/Y and La/Yb, as well as noticeably negative anomalies of Ba, Sr, P and Ti in the primitive mantle-normalized diagram. The second group of low-silica end-members (LSE), however, displays adakite-like features with lower SiO2 (<75wt%), higher MgO (0.52–1.32wt%) and Mg# (0.32–0.52; mostly >0.4), and higher Sr/Y (mostly >20) and La/Yb (>8). The depleted Sr–Nd isotopic characteristics (εNd(T)=3.5–6.4 and Isr=0.7026–0.7055) and young two-stage model ages of HSE and LSE indicate that they were both derived from partial melting of juvenile lower crust that might be triggered by asthenosphere upwelling subsequent to a slab rollback event. However, the depths of initial melting might be different. The current evidence demonstrates that HSE in the Suyunhe deposit formed by partial melting of juvenile crust at depths of less than ∼33km with a plagioclase residue, whereas that for LSE occurred at depths of >40km where a garnet residue existed and the crust was thickened. The lower source depth, as well as subsequently strong plagioclase fractionation, results in the absence of adakite-like characteristics in HSE.The Ce4+/Ce3+and EuN/EuN∗ ratios in zircons of HSE are much lower than ore-forming intrusions from porphyry Cu deposits in the Central Asian Orogenic Belt, but noticeably higher than barren intrusions from the Lachlan fold belt and ore-bearing intrusions from small-intermediate porphyry Mo deposits from the East Qinling–Dabie and the Nanling metallogenic belts, China, indicating that neither too high nor too low oxygen fugacities are favorable for large porphyry Mo deposits. Based on previous studies of adakitic rocks in the world, adakite-like LSE in the Suyunhe deposit are believed to have higher oxygen fugacities, and thus be less fertile than HSE. We finally suggest that adakites and adakite-like rocks are unproductive for porphyry Mo deposits.

“Normal” to adakite-like arc magmatism associated with the El Abra porphyry copper deposit, Central Andes, Northern Chile

The El Abra porphyry copper deposit belongs to the Late Eocene—Early Oligocene metallogenic belt of northern Chile, which host several world-class porphyry copper deposits. Our previous geochronological work done on this deposit provides the temporal framework for petrological data interpretation. The magmatic history of the El Abra deposit lasts for ~8.6 Ma and can be divided into two stages. An early period, from about 45 to ~38.7 Ma, dominated by diorites and quartz monzodiorites with “normal” (non-adakite) arc geochemistry and a late period, with rocks younger than ~38.7 Ma that developed adakite-like geochemistry, where equigranular granodiorites are the volumetrically dominant rock type (e.g., Clara granodiorite ~38 Ma). These granodiorites are then intruded by leucocratic porphyry dikes and aplites. Most copper mineralization is associated with multiple intrusions of these younger porphyritic rocks, described as the El Abra porphyry unit, and emplaced over a ~1.4 Ma period, from ~37.5 to 36.1 Ma. The adakite-like geochemistry of the younger rock units (<38.7 Ma) is attested by a significant depletion in REE contents, particularly MREE and HREE (concave MREE distribution patterns), high La/Yb and Sr/Y ratios, and Na2O and Al2O3 contents, along with the absence of the Eu anomaly in normalized REE distribution patterns. The evolution of this large, long-lived magmatic system from “normal” to adakite-like arc magmatism is discussed in a tectonic context of crust overthickening due to a major orogenic episode (Incaic compressive phase). This tectonic setting may have promoted higher pressure conditions at the lower crust “hot zone” and increased the crustal residence time of derivative melts favoring extensive differentiation leading to water-rich (and oxidized?) felsic melts, where amphibole fractionation played an important role. Strontium, Nd, and Pb isotope data suggest a common mantle source for both the non-adakite and adakite-like rocks. This implies that these two groups of rocks from the El Abra porphyry copper deposit may have largely resulted from the different crustal conditions under which they have evolved.

Petrogenesis and Metallogenesis of Malek Siah Kuh Adakite-Like Rocks and Associated Hydrothermal Mineralization (Sistan Area, Iran)

The granitoids-diroretic complex and associated volcanic suits of Malek Siah Kuh (eastern Iran) were examined by RS, XRD, XRF, AAS and ICP-MS methods. The Multispectral data were used for differentiating of lithological units, main alterations, structural trends, and hydrothermally altered minerals. Based on the results of remote sensing, field studies and petrographic data, the complex classified into three main groups: 1) mafic unit (gabbro-basalt), 2) intermediate rocks (diorite-andesite suite) and 3) felsic unit (granitoide rocks). The complex is characterized by high Al2O3 and Sr contents as well as high Sr/Y and La/Yb ratios, while low in Y and Yb contents. The geochemical characteristics of these rocks indicate that they are similar to adakite like rocks. The complex is high calc-alkaline to shoshonitic nature. The results indicated derivation from similar silicate melt source followed by differentiation and assimilation of crustal rocks. Based on set of petrological and geochemical relations, 3 distinct stages of mineralization process can be recognized. 1) It is differentiation during magma evolution as well as metal enrichment and subsolidus leaching by hot fluids. 2) It occurred by ascending of the gas-rich magma and release of S and As. 3) It was accompanied by hydrothermal activities leading to the generation of sulfidation and associated polymetallic (Cu, Au, Fe) mineralization.

Geochemistry and petrogenesis of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan marginal seas

The results of ICP-MS trace-element (LILE, HFSE, REE) study of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and geochronological K-Ar dating of the Eocene volcanic rocks are presented. Specifics of volcanism developed on submarine rises of these seas was characterized for the first time, and magma sources and geodynamic settings of the volcanic complexes predating the formation of the deep-water basins were determined. It is established that the Late Mesozoic magmas were formed in a subduction setting from spinel peridotites of suprasubduction mantle wedge, which was metasomatically reworked by aqueous fluids that were released by dehydration of sedimentary layer of subducting oceanic plate. This follows from the elevated concentrations of H2O, alkalis, potassium, LILE and LREE, and lowered HFSE (including Ta-Nb minimum) and HREE contents, at lowered Sm/Yb, Nb/Ta, Nb/Y and elevated La/Nb, Ba/La, and Zr/Y ratios. Eocene adakite-like volcanic rocks were identified for the first time in the Sea of Okhotsk. They vary from andesitic to more felsic compositions with elevated MgO (>4%) and elevated La/Yb (>14) and Sr/Y (50–60) ratios. Identification of adakite-like volcanic rocks serves as evidence in support of the transform continental-margin (or plate sliding) setting, which is characterized by breaking apart of subduction slab and formation of slab “windows” acting as pathways for the transfer of asthenospheric mantle into continental lithosphere. New geochemical data on the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and analysis of literature data were used to distinguish two geodynamic settings within these seas: subduction and transform margin. Similar settings operated at that time in the adjacent continental- margin volcanic belts (Akinin and Miller, 2011; Martynov and Khanchuk, 2013; et al.).

Multiple rifting and alkaline magmatism in southern India during Paleoproterozoic and Neoproterozoic

The Southern Granulite Terrane (SGT) in India preserves the history of tectonothermal events ranging from Paleoarchean to latest Neoproterozoic–Cambrian. Here we investigate alkaline magmatism possibly associated with rifting events in Paleoproterozoic and Neoproterozoic based on petrological, geochemical and zircon U–Pb and Lu–Hf isotopic studies on the alkaline complexes of Korangani (KGAC) and Kambamettu (KAC) in the Madurai Block of SGT. The mica pyroxenite which represents the first intrusive phase at KGAC crystallized from a mildly alkaline hydrous magma derived from a metasomatized mantle. The younger shoshonitic syenite was emplaced at 2533±16Ma, carries mafic microgranular enclaves, and shows trace-elements ratios consistent with magma mixing trend, and zircon εHf(t) values display mixed positive and negative values −2.6 to 3.6 suggesting the mixing of adakite-like felsic crustal melt and non-adakitic mantle derived melt. In KAC, four distinct magmatic intrusions are identified: i) quartz-monzonite (emplaced at 2498±16Ma), an ultrapotassic adakitic rock derived from a carbonated alkali-rich lower crustal source with negative zircons εHf(t) values in zircon (−8.0 to −0.8); Y/Nb (>1.2) and Th/Ce (0.03–0.8) ratios; lower Ni (<30ppm) and Cr (<14ppm) contents; ii) phlogopite-rich pyroxenite, crystallized from an alkali-rich basaltic parental magma derived from carbonate metasomatized mantle; iii) mantle derived high Ba–Sr carbonatite (emplaced at 2470±15Ma); and iv) shoshonitic peralkaline syenite rock (emplaced at 608±6Ma) with strong adakitic signature, low MgO (<1wt.%), Ni (12–5ppm) and Cr (49–35ppm) contents and negative zircon εHf(t) values (−30.3 to −27.3) and trough of Zr–Hf in spidergrams suggesting a carbonated alkali-rich garnet-bearing crustal source.The geochemical features and petrogenetic considerations of the felsic shoshonitic-ultrapotassic adakite-like rocks (syenite, quartz monzonite), mica-pyroxenites and carbonatite reveal rift-related extensional tectonic settings with magma sources involving both mantle and crustal components during Paleoproterozoic. The late Neoproterozoic shoshonitic adakite-like syenite from KAC also formed in a rift-related setting. In both cases, intracontinental rifting associated with post-collisional extension is proposed.

Geochemical characteristics of the Shujiadian Cu deposit related intrusion in Tongling: Petrogenesis and implications for the formation of porphyry Cu systems in the Middle–Lower Yangtze River Valley metallogenic belt, eastern China

Porphyry Cu deposits can form in intracontinental or post-collision settings; however, both the genesis of fertile magmas and the mechanism of metal enrichment remain controversial. The Shujiadian porphyry Cu deposit is located in the Tongling area of the Middle–Lower Yangtze River Valley metallogenic belt. It is hosted by the Shujiadian complex, which mainly consists of quartz diorite porphyry (143.7±1.7Ma) and pyroxene diorite (139.8±1.6Ma). They both belong to the calc-alkaline series, with enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field-strength elements (HFSE) and heavy rare earth elements (HREE), and slightly negative Eu anomalies. Both quartz diorite porphyry and pyroxene diorite have geochemical affinities with adakite, and their low MgO (1.5–3.7wt%), and Ni (3.7–6.9ppm), Cr (2.0–44ppm), and Th/Ce contents (0.06–0.11) indicate that the intrusive rocks have some characteristics of adakite-like rocks derived from thickened lower crust and melts from metabasaltic rocks and eclogites. Plagioclases from the quartz diorite porphyry are andesine (An value=31.8–40.5) and from the pyroxene diorite are felsic albite and oligoclase with large-scale zones and variable An value (An value=8.9–18.3), Fe and Sr contents, which indicate that mixing of mafic and felsic magma may have occurred in the shallow magma chamber. Compared to the barren quartz diorite porphyry, relatively lower SiO2 contents (49.5–55.2wt.%), higher εNd(t) values (−7.4 to −6.9), εHf(t) values (−11.0 to −9.1) compositions, Ti-in-zircon temperatures (714–785°C), and variations of HREE contents of the mineralization-related pyroxene diorite suggest mixing with high-temperature mafic magma. Calculated Ce4+/Ce3+ values of pyroxene diorite plot between the Ni–NiO buffer (NNO) and magnetite–hematite buffer (MH), and barren quartz diorite porphyry samples plot below the Ni–NiO buffer (NNO). Geochemical features of relatively enriched LILEs, depleted HFSEs, and markedly negative Nb–Ta anomalies, imply that the mafic magma was generated by partial melting of enriched lithospheric mantle, which had been metasomatized by slab-derived fluids from Neoproterozoic-subducted oceanic lithosphere; as a result, the parental magma was rich in H2O, S, and metals (e.g. Cu), with high fO2.

Zircon U–Pb age, Hf isotope and geochemistry of Carboniferous intrusions from the Langshan area, Inner Mongolia: Petrogenesis and tectonic implications

Late Paleozoic was a critical period for the tectonic evolution of the northern margin of the Alxa-North China craton, but the evolutionary history is not well constrained. The Carboniferous intrusions in the Langshan area in the western part of the northern margin of the Alxa-North China craton are mainly composed of tonalite, quartz diorite, olivine gabbro and pyroxene peridotite. Zircon LA-ICP-MS U–Pb dating indicates that the Langshan Carboniferous intrusions were emplaced at ca. 338–324Ma. The quartz diorites are characterized by high amounts of compatible trace elements (Cr, Ni and V) and high Mg# values, which may suggest a significant mantle source. The positive Pb and negative Nb–Ta–Ti anomalies, the variable εHf(t) (−6.9 to 2.0) values and the old Hf model ages (1218–1783Ma) imply some involvement of ancient continental materials in its petrogenesis. The tonalite has relatively high Sr/Y ratios, low Mg#, Yb and Y contents, features of adakite-like rocks, negative εHf(t) values (−9.8 to −0.1) and older Hf model ages (1344–1953Ma), which suggest significant involvement of ancient crust materials and mantle-derived basaltic component in its petrogenesis. The high Mg# values, high Cr and Ni contents, and low Zr and Hf contents of the mafic–ultramafic rocks show evidence of a mantle source, and the relatively low zircon εHf(t) values (−5.9 to 3.2) might point to an enriched mantle. The trace element characteristics indicate the influence of subducted sediments and slab-derived fluids. In the tectonic discrimination diagrams, all the rocks plot in subduction-related environment, such as volcanic arc and continental arc. Considering the regional geology, we suggest that the Carboniferous intrusions in the Langshan area were likely emplaced during the late stage of the southward subduction of the Paleo-Asian Ocean plate, which formed a continental arc along the northern margin of the Alxa-North China craton.

Petrogenesis and tectonic significance of the Eocene adakite-like rocks in western Yunnan, southeastern Tibetan Plateau

Eocene magmatic rocks are widespread in western Yunnan, southeastern Tibetan Plateau. However, their petrogenesis and tectonic significance remain controversial. In this paper, we report geochemical and geochronological data of adakite-like rocks from the eastern part of western Yunnan. Zircon U–Pb dating reveals that they were emplaced at ca. 35Ma. A geochemical study shows that these rocks have high SiO2 (68.97–72.44wt.%), K2O (4.35–5.87wt.%) and low MgO (0.61–1.16wt.%), Y (6.65–12.6ppm) and Yb (0.58–1.02ppm) contents as well as high Sr/Y (74–228) and La/Yb (59–131) values, belonging to high-K calc-alkaline to shoshonitic adakite-like rocks. These rocks have high (87Sr/86Sr)i (0.70623–0.70653), low εNd(t) (−4.5 to −7.5) and slightly low radiogenic Pb isotopic compositions with (206Pb/204Pb)i=18.042–18.179. Zircons of these rocks show εHf(t) values ranging from −6.3 to +1.2 and model ages (TDM2) of 1513–1035Ma. The geochemical characteristics indicate that the Eocene adakite-like rocks from the eastern part of western Yunnan were derived from the partial melting of Neoproterozoic mafic rocks underplated in the lower crust of the western margin of the Yangtze Plate with input of mantle-derived potassic–ultrapotassic melt. Integration of published geochemical data clearly shows that zircon εHf(t) and whole-rock εNd(t) values of Eocene adakite-like rocks from the whole western Yunnan exhibit a trend of westward increase. We infer that the adakite-like rocks from the western part of western Yunnan were derived from the partial melting of late Paleozoic–Mesozoic mafic rocks formed as the lower crust of a continental magmatic arc during eastward subduction of the Paleo-Tethyan Ocean, and the Eocene magmatic rocks in western Yunnan were generated during the removal of thickened continental lithosphere triggered by the India and Asia collision.

Origin of Miocene Cu-bearing porphyries in the Zhunuo region of the southern Lhasa subterrane: Constraints from geochronology and geochemistry

The Zhunuo Cu-bearing porphyries are located in the westernmost part of the Miocene Gangdese porphyry Cu (Mo–Au) deposit belt. Zircon U–Pb dating of the diorite porphyry, K-feldspar granite porphyry, and monzonitic granite porphyry in Zhunuo yielded crystallization ages of 12.5±0.4Ma, 12.3±0.3Ma, and 12.4±0.3Ma, respectively. The diorite porphyry is characterized by low SiO2 (58.61–61.14wt.%) and Th (0.30–0.76ppm) concentrations, low Th/La (0.05–0.1) ratios, and high Mg# (>49) values coupled with low (87Sr/86Sr)i (0.703777–0.703783) and high εNd(t) (+4.07 to +4.90) values. They also have adakite-like affinities, such as low Y (10.5–12.0ppm), and high Sr/Y ratios (61–65). They were probably derived from a thickened juvenile lower continental crust. The K-feldspar granite porphyry probably originated in the middle–upper continental crust because of their high SiO2 (73.59–74.98wt.%) and Th (50.1–52.1ppm) concentrations, high Th/La (1.67–2.10), and low Sr/Y (20.2–20.7) ratios and Mg# (32–38) values, combined with high (87Sr/86Sr)i (0.710921–0.712008), low εNd(t) (−8.47 to −9.26) isotopic compositions and old Nd model ages (1.16–1.25Ga). Their magmas were most likely partial melts of the preserved ancient crust similar to the central Lhasa subterrane. The geochemical characteristics and Sr–Nd isotopic compositions of the monzonitic granite porphyry display trends that lie between those of the diorite porphyry and K-feldspar granite porphyry, and they are therefore likely to be production of hybridization between the above two melts. The ore-bearing diorite porphyry and monzonitic granite porphyry have higher zircon Ce4+/Ce3+ ratios than the ore-barren K-feldspar granite porphyry, indicating a higher oxygen fugacity in the ore-bearing magmas. We suggest that metals were released from the re-melting of arc-related cumulates which formed during lower crustal growth and thickening. This mechanism provides a reasonable explanation for the significant flare-up of mineralization during the Miocene in the Gangdese region. The lower continental crust beneath southern Lhasa subterrane probably was uniformly juvenile but the region to the west of Zhunuo was not mineralized due to input of large ancient crustal materials in the source of these ore-barren adakite-like rocks.

Adakite-like geochemical signature produced by amphibole-dominated fractionation of arc magmas: An example from the Late Cretaceous magmatism in Gangdese belt, south Tibet

Late Cretaceous (~106–76Ma) adakite-like intrusive rocks in the middle-eastern Gangdese belt occur in an E–W trending belt paralleling the Indus–Yarlung suture, south Tibet. Their petrogenesis and geodynamic processes have been a subject of debate. We report here U–Pb zircon ages, geochemical and Sr–Nd–Hf isotopic data for adakite-like intrusive rocks as well as the normal arc rocks (gabbros and gabbroic diorites) in the middle Gangdese belt. LA-ICPMS U–Pb zircon analyses yielded an identical age of ~88Ma for two adakite-like rocks, which are slightly younger than the gabbro and gabbroic diorite (ca. 94–90Ma). Both the adakite-like rocks and the normal arc rocks have similar whole-rock Sr–Nd and zircon Hf isotope compositions, indicating that they have been derived from a common source. Similarly, the adakite-like and normal arc intrusive rocks in the eastern Gangdese belt also show similar Sr–Nd–Hf isotope compositions. In the middle-eastern Gangdese belt, the >85Ma Late Cretaceous intrusive rocks consist of a magma series from gabbro to granodiorite, including both normal arc rocks and adakite-like rocks. These rocks overlap in space and time that conform to a normal arc differentiation trend. In terms of major and trace elements, they also show a clear evolution from the normal arc magmatic into adakitic field. Thus, we suggest that these >85Ma Late Cretaceous intrusive rocks were ultimately derived from melting of the hydrated mantle wedge and the adakite-like rocks can be generated in normal arc magmas by amphibole-dominated fractionation. Taking into accounting for the spatial and temporal distribution of the Cretaceous magmatic rocks in the Lhasa terrane, we prefer a model of early Late Cretaceous rollback following Early Cretaceous low-angle oceanic slab subduction. At intermediate pressure and H2O-rich conditions, fractionation of amphibole changes the major and trace element compositions of arc magmas, and will efficiently drives basaltic composition to andesitic composition in arc magmas.

Zircon U–Pb geochronology and Hf isotopic constraints on the petrogenesis of Early Triassic granites in the Wulonggou area of the Eastern Kunlun Orogen, Northwest China

Widespread granitic intrusions in the northeast part of the Wulonggou area were previously thought to be emplaced into the Palaeoproterozoic Jinshuikou Group during the Neoproterozoic. This contribution presents detailed LA-ICP-MS zircon U–Pb geochronology, major and trace element geochemistry, and zircon Hf isotope systematic on the Wulonggou Granodiorite and Xiaoyakou Granite from the Wulonggou area. Three granodiorite samples yielded U–Pb zircon ages of 247 ± 2, 248 ± 1, and 249 ± 1 Ma, and one granite sample yielded U–Pb zircon age of 246 ± 3 Ma. The granodiorite samples are metaluminous with an alumina saturation index of 0.90–0.96, as well as intermediate- to high-alkali contents of 5.49–6.14 wt.%, and low Zr+Nb+Ce+Y contents, and low Fe2O3T/MgO ratios, which suggest an I-type classical island arc magmatic source. The granite samples are peraluminous with an alumina saturation index of 1.02–1.03, Sr content of 305.00–374.00 ppm, Sr/Y ratios of between 17.68 and 28.77, (La/Yb)N values of 16.98–25.07, low HREEs (Yb = 1.10–2.00 ppm), and low Y (13.00–21.10 ppm), which suggest adakite-like rocks. All granodiorite samples have zircons εHf(t) values ranging from −2.9 to +3.9, and granite samples have zircon εHf(t) values ranging from −7.8 to +3.2. These Hf isotopic data suggest that the Early Triassic granites were derived from the partial melting of a mafic Mesoproterozoic lower crust, although the degree of ancient crustal assimilation may be higher for the Xiaoyakou Granite. It is suggested here that the ca. 246–248 Ma magma was generated during the northward subduction of the Palaeo-Tethys oceanic plate.

Petrogenesis of OIB-like basaltic volcanic rocks in a continental collision zone: Late Cenozoic magmatism of Eastern Iran

Hundreds of compositionally diverse volcanoes have erupted on the Turkish–Iranian Plateau in the last 15Myr, attributed to one or more of the processes of Middle Miocene Tethyan slab break-off, localised lithospheric delamination and small-scale convection. Magmatism post-dates the initial Late Eocene or Early Oligocene Arabia–Eurasia collision by >10Myr. By contrast, in the east of the plateau in Eastern Iran there has been intermittent magmatism from the Late Oligocene to the Quaternary. Magma types include alkali basalt flows and scoria cones and adakite-like intermediate–felsic lavas and domes. New elemental and Nd–Sr–Pb–Hf isotope data from Quaternary mafic alkaline flows near Quchan in the Meshkan area in the NE of the country are combined with existing data from Miocene–Quaternary volcanic centres in the Binalud Range and the Nayband/Neh Fault zones. These mafic, incompatible element-enriched rocks, with positive Nb–Ta anomalies, are OIB-like and are argued to have formed by low-degree melting of a heterogeneous mantle source which contained DMM- and EMII-like components. At least some of the melting took place in the garnet stability field. Significant crustal contamination during magma ascent is recognised in the Nayband/Neh samples. Penecontemporaneous adakite-like rocks are argued to be high-pressure fractionates of basaltic melts which may have a separate source to the OIB-like basalts. Late Cenozoic mantle melting was aided by (1) localised extension and (2) enhanced small-scale asthenospheric convection related to Makran subduction, low mantle viscosity owing to the prior Tethyan subduction history of the region, and possibly edge-driven convection on the margin of the thickened Zagros lithosphere.

High-Mg Diorite from Qulong in Southern Tibet: Implications for the Genesis of Adakite-like Intrusions and Associated Porphyry Cu Deposits in Collisional Orogens

We have investigated a suite of Miocene high-Mg diorite porphyries from Qulong in southern Tibet, the largest porphyry Cu–Mo deposit in China. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating shows that the high-Mg diorite porphyry was emplaced at 15.7±0.2 Ma, which is slightly younger than the Qulong adakite-like Rongmucuola pluton (c. 19.5–16.4 Ma). The Qulong high-Mg diorites have phenocrysts showing disequilibrium textures and include high-Mg# clinopyroxene (0.91–0.97). These porphyry rocks exhibit both ultrapotassic and adakite-like features, and are characterized by high contents of MgO (4.2–5.1 wt %), K2O (3.2–3.6 wt %), and compatible trace elements (e.g. Ni: 115–142 ppm; Cr: 214–291 ppm), as well as by high Sr/Y and La/Yb ratios. The rocks have highly radiogenic isotopic compositions with (87Sr/86Sr)i = 0.707004–0.707198, eNd(t)=-5.1 to -5.5, 207Pb/204Pb = 15.697–15.704, and 208Pb/204Pb = 39.082 – 39.116, as well as variable zircon eHf values of -3.0 to +5.9. Petrographic, elemental, and isotopic evidence suggests that the Qulong high-Mg diorites were formed by mixing between ultrapotassic and adakite-like melts, derived from metasomatized Tibetan lithospheric mantle and juvenile lower crust, respectively. In contrast, the Qulong pre-ore Rongmucuola pluton is characterized by high SiO2 (66.3–68.9 wt %) and Al2O3 (16.4–17.0 wt %) contents, high Sr/Y ratios (121–151), low compatible element contents (e.g. Ni = 16.0 – 17.4 ppm; Cr=14.5–20.2 ppm), low Mg# values (0.44–0.52), positive large-ion lithophile element (LILE) anomalies, marked negative high field strength element (HFSE) anomalies, positive eNd(t) values (+0.4 to +2.5), and low (87Sr/86Sr)i values (0.704847–0.705237). These features indicate that the Rongmucuola pluton was formed by partial melting of subduction-modified juvenile lower crust within the garnet stability field. The newly identified Qulong high-Mg diorite allows us to propose a mixing model for the origin of the Gangdese high-K, adakite-like rocks. In this model, the formation of these rocks occurred in two stages: (1) partial melting of highly metasomatized lithospheric mantle that generated ultrapotassic mafic melts; (2) underplating of such melts beneath thickened juvenile lower crust, which resulted in melting of the lower crust and the generation of adakite-like magmas. Mixing of the adakite-like melt with ultrapotassic magmas elevated the K2O, MgO, and other LILE (e.g. Rb and Ba) contents of the adakite-like melt. Exogenous water necessary for formation of the Gangdese porphyry Cu systems was mainly added during mixing of ultrapotassic magma with adakite-like melt at lower and/ or upper-crustal depths, reflecting the large decrease in the H2O solubility of the ultrapotassic mafic melt upon ascent and decompression. Upper-crustal fractionation of the Rongmucuola magma could also possibly increase the water content of mineralization-related, adakite-like porphyry intrusions at Qulong. Fluid exsolution from the ultrapotassic magma is likely to have been a key process in the generation of the Gangdese porphyry Cu deposits, as well as other porphyry Cu deposits in the Tibetan collisional orogens.