Modern Adakites Research Articles

Neoarchean high-Mg basalt and rhyolite bimodal complex in the Yinshan Block, North China Craton: Petrogenesis and tectonic implications

Plate tectonics plays an important role in Earth’s evolution, but it is a difficult and controversial issue to trace the operation of plate tectonics in the Archean. Calc-alkaline magmas are a diagnostic feature of modern subduction-related magmatism and thus identification of Archean subduction-related calc-alkaline magmas is crucial for understanding when Archean plate tectonics operated. Here we present zircon U-Pb, whole-rock geochemical and Sr-Nd isotopic data for a metamorphosed Neoarchean calc-alkaline bimodal volcanic complex from the Yinshan Block, the North China Craton. Zircon U-Pb dating results indicate that these meta-volcanic rocks were erupted in the late Neoarchean (∼2570 Ma). Its mafic unit is characterized by high contents of K2O (0.68–4.20 wt%), MgO (Mg# = 62.0-73.4), Cr (332-904 ppm), Ni (105-269 ppm), and is enriched in light rare earth elements (REEs), but depleted in heavy REEs and Nb, Ta and Ti. These geochemical features are similar to those of high-Mg basalts in the Phanerozoic arc setting. They have positive εNd(t) values (+1.3 to + 2.4), indicating that they were derived from the depleted mantle wedge. The felsic unit has variable K2O content (1.42–4.95 wt%) and K2O/Na2O ratios of 0.23–1.36 with positive εNd(t) values (+0.7 to + 3.1). They show characteristics of modern adakites with high Sr/Y and LaN/YbN ratios, suggesting that they were generated by partial melting of the thickened juvenile mafic lower crust. We conclude that the studied Neeoarchean bimodal meta-volcanic complex was formed during subduction-zone retreating and root delamination in a Neoarchean arc. Our results further indicate that plate tectonics had already operated in the Neoarchean in the Yinshan Block.

Early-Middle Devonian adakitic magmatism generated by slab retreat in southern West Junggar, NW China: implications for tectonic correlation with central and East Kazakhstan

ABSTRACT The West Junggar terrane (WJT) is known to tectonically correlate with the Central and East Kazakhstan terrane; however, the Middle Palaeozoic magmatism has rarely been reported in the southern WJT, leading to significant uncertainties on the tectonic evolution and correlation of the southwestern Central Asian Orogenic Belt (CAOB). Here, we present the adakitic quartz diorites with zircon U-Pb ages of 390–394 Ma in the Alashankou region to demonstrate the presence of Early-Middle Devonian magmatism in the southern WJT for the first time. They are characterized by low Yb (1.00–1.41 ppm) and Y (8.9–14.4 ppm) but high Sr (301.5–475.9 ppm), Sr/Y (23–41), and (La/Yb)N (8.1–11) values, typical of modern adakites. As suggested by the relatively high SiO2 (64.96–67.71 wt.%), moderate K2O (1.53–3.19 wt.%), but low MgO (1.56–2.31 wt.%), TiO2 (0.28–0.42 wt.%), Cr (≤13.7 ppm), Co (2.7–9.9 ppm), Ni (8.0–18.7 ppm), and Th (3.7–4.8 ppm) concentrations as well as the high positive ɛHf(t) values (+5.6 to +10.3) and low Nb/Ta ratios, the adakitic quartz diorites were probably generated by the partial melting of a slightly thickened lower crust, which consisted mainly of the Ediacaran-Middle Palaeozoic intra-oceanic arc rocks in the southern WJT. Combined with regional data, our new results support that the southern WJT probably evolved as the eastern extension of the Kazakhstan Early-Middle Devonian outer volcanic belt during the Middle Palaeozoic. The Early-Middle Devonian tectonic extension and resultant lower crust melting may thus be related to the slab retreat of the Junggar-Balkhash Ocean.

Geochronological, Geochemical and Sr-Nd-Hf Isotopic Studies of the A-type Granites and Adakitic Granodiorites in Western Junggar: Petrogenesis and Tectonic Implications

Late Carboniferous magmatism in the Western Junggar region of the Central Asian Orogenic Belt (CAOB) provides a critical geological record of regional tectonic and geodynamic history. In this study, we determined the zircon U-Pb isotopic compositions, bulk-rock Sr-Nd-Hf isotopic compositions, and major and trace element geochemistry of two granitic bodies in the Western Junggar, with the aim of constraining their emplacement ages, magmatic origin, and geodynamic significance. Radiometric ages indicate that the plutons were emplaced during the Late Carboniferous (322–307 Ma). Plutons in the North Karamay region are characterized by high Sr content (347–362 ppm) and low Y content (15.3–16.7 ppm), yielding relatively high Sr/Y ratios (20.8–23.7). They show consistent Yb (1.68–1.85 ppm), Cr (16–19 ppm), Co (7.5–8.1 ppm) and Ni (5.9–6.6 ppm) content, similar to that of modern adakites. The Hongshan plutons are characterized by high SiO2 (69.95–74.66 wt%), Na2O (3.26–3.64 wt%), and K2O (4.84–5.16 wt%) content, low Al2O3 (12.02–12.84 wt%;) and MgO (0.13–0 18 wt%) content, and low Mg# values (0.16–0.22). This group shows a clear geochemical affinity with A-type granites. All of the studied granitoids have positive εNd(t) (+4.89 to +7.21) and εHf(t) (+7.70 to +13.00) values, with young TDM(Nd) 806–526 Ma) and TDM(Hf) (656–383 Ma) ages, indicating a substantial addition of juvenile material. The adakitic granodiorites in the North Karamay region were likely generated via partial melting of thickened lower crust, while the A-type granites in the Hongshan area may have been derived from the melting of lower-middle crust in an intra-oceanic arc, which consists mainly of oceanic crust. The emplacement of these granitoids represents a regional magmatic “flare up”, which can be explained by the rollback of a subducting slab.

Slab roll-back and crustal growth in the Eastern Junggar terrane, NW China: evidence from Carboniferous A-type granitoids and adakitic rocks

ABSTRACT Revealing the formation mechanism of granitic plutons is vital for understanding crustal growth of Earth. Here, we present an integrated study of geochronological, elemental, whole rock Nd and zircon Hf isotopic data for a granitic pluton system that formed in the Eastern Junggar, NW China. Zircon U–Pb dating suggests the seven granitic plutons were emplaced at 356–312 Ma. The monzogranites have high alkaline and Zr contents, and high Ga/Al and TFe2O3/MgO ratios, which are geochemically similar to A-type granitoids. They display positive ɛNd(t) (+4.3 to +5.4) and ɛHf(t) (+7.6 to +12.4) values, and young Nd and Hf model ages, suggesting a lower arc crust source. The quartz monzodiorites have high Sr, low Y and Yb contents, yielding relatively high Sr/Y ratios that are similar to modern adakites. These rocks also have high positive ɛNd(t) (+7.6 to +8.0) and ɛHf(t) (+13.9 to +15.5) values, and young Nd and Hf model ages, indicating a subducted oceanic crustal source. In combination with data from other plutons in this area, we suggest that they were likely generated by roll-back of the subducted Junggar oceanic slab, which resulted in back-arc extension, asthenosphere upwelling, and partial melting of oceanic crust and lower arc crust.

Accessory mineral constraints on crustal evolution: elemental fingerprints for magma discrimination

Underexplored accessory minerals such as titanite and apatite have the potential to give insights into the nature and the petrogenesis of their host rock. Their trace element and REE-rich compositions carry a record of crystallisation history and chemical characteristics of their source. Moreover, titanite and, to a certain extent, apatite are resistant to erosion during sedimentary cycles which makes them ideal to reconstruct the history of long-eroded continental land-masses. Here we report new trace element data on apatite and titanite from granitoids of different Archean cratons and comparative granitoids from the Phanerozoic. Trace element signatures of both minerals reveal systematic chemical trends in Y, LREE and Sr contents related to the nature of their host magma, which are used to construct discrimination diagrams delineating Archean TTGs from sanukitoids, and modern adakites from S/I-type granites. By comparing Archean granitoids (TTG and sanukitoids) and their Phanerozoic counterparts (adakite and high Ba-Sr granites), we show that the robust nature of these phases makes them reliable recorders of petrogenetic information from Archean rocks, that usually have been affected by secondary processes (meta-morphism, deformation, hydrothermal activity). Applied to the rock record, both phases potentially provide detailed archives of magmatic evolution through time.

An oceanic subduction origin for Archaean granitoids revealed by silicon isotopes

Modern oceanic crust is constantly produced at oceanic ridges and recycled back into the mantle at subduction zones via plate tectonics. An outstanding question in geology is whether the Earth started in a non-plate tectonic regime, and if it did, when the transition to the modern regime occurred. This is a complicated question to address because Archaean rocks lack modern equivalents to anchor interpretations. Here, we present a silicon isotopic study of 4.0–2.8-Gyr-old tonalite–trondhjemite–granodiorites, as well as Palaeozoic granites and modern adakites. We show that Archaean granitoids have heavier silicon isotopic compositions than granites and adakites, regardless of melting pressure. This is best explained if Archaean granitoids were formed by melting of subducted basaltic crust enriched in sedimentary silica through interaction with seawater. Before the appearance of silica-forming organisms 0.5–0.6 billion years ago, the oceans were close to silicon saturation, which led to extensive precipitation of cherts on the seafloor. This is in contrast to modern oceans, where silica biomineralization maintains dissolved silicon at low concentration. The unique heavy silicon isotope signature of cherts has been transferred to Archaean granitoids during an oceanic subduction process, which was probably responsible for the formation of felsic rocks on Archaean emerged lands. Archaean granitic rocks formed by melting of silica-enriched subducted basaltic crust through interaction with seawater, according to heavy silicon isotopes measured in Archaean samples.

Early Silurian to Early Carboniferous ridge subduction in NW Junggar: Evidence from geochronological, geochemical, and Sr-Nd-Hf isotopic data on alkali granites and adakites

The Central Asian Orogenic Belt (CAOB) evolved through a long-lived orogeny involving multiple episodes of subduction and accretion marking a major phase of continental growth during the Paleozoic. The northern part of the Western Junggar region (NW Junggar) offers a window into these processes, particularly to constrain the timing of closure of the Paleo-Asian Ocean. Here we report geochemical, geochronological, and isotopic data from K-feldspar granites and adakitic rocks from the NW Junggar region. Zircon U-Pb ages suggest that the granites were emplaced during Early Silurian to the Early Carboniferous (434–328Ma). The granites show geochemical characteristics similar to those of A-type granites, with high SiO2 (71.13–76.72wt%), Na2O+K2O (8.00–9.59wt%), and Al2O3 (12.28–14.08wt%), but depleted Sr, Nb, Ta and Eu. They display moderate to high positive εNd(t) and εHf(t) values (4.26–8.21 and 7.69–14.60, respectively) and young Nd and Hf model ages (T2DM-Nd=489–740 Ma and T2DM-Hf=471–845 Ma), suggesting magma derivation through partial melting of lower crust in the Boshchekul-Chingiz and Zharma-Saur arcs. The adakites are characterized by high Sr content (406.5–751.6ppm), and low Y (13.8–16.4ppm) and Yb (1.5–1.8ppm) content, yielding relatively high Sr/Y ratios (25.38–49.41) similar to those of modern adakites. They have high positive εNd(t) and εHf(t) values (7.85–8.25 and 13.23–15.97, respectively) and young Nd and Hf model ages (T2DM-Nd=429–535Ma and T2DM-Hf=355–550Ma), indicating that their source magmas were likely derived from partial melting of the oceanic crust beneath the Boshchekul-Chingiz arc. The petrogenesis and distribution of the A-type granites and adakites, as well as the tectonic architecture of the region, suggest that a ridge subduction event might have occurred during the Early Silurian to Early Carboniferous. In combination with previous studies in the Chinese Altai, we suggest a two-sided ridge subduction model for the Junggar-Altai region.

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

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

Nature of three episodes of Paleoproterozoic magmatism (2180 Ma, 2115 Ma and 1890 Ma) in the Liaoji belt, North China with implications for tectonic evolution

Three episodes of Paleoproterozoic magmatism with distinctively different nature in the Liaoji belt of the Eastern North China Craton are discussed here: the 2200–2140Ma Liaoji A-type granites, the 2160–2110Ma Haicheng mafic sills and the ∼1890Ma Qingchengzi I-type granites. The Mafeng monzogranitic pluton, representative of the Liaoji A-type granites, gives a SIMS U-Pb zircon age of 2181±6Ma (n=20, MSWD=4.3). The Qingchengzi plagiogranitic pluton, representative of the Qingchengzi I-type granites, gives a SIMS U-Pb zircon age of 1891±10Ma (n=8, MSWD=1.8). The Mafeng monzogranites are high in Fe, Ti, K but low in Mg, Al and Ca. They have high abundance of total REEs (ΣREE=213–346ppm). They are relatively depleted in feldspar-compatible elements (e.g., Eu and Sr) and HFSEs (e.g., Nb, Ta, P and Ti). They have high 10000×Ga/Al ratios of 3.19–3.61 and Zr+Nb+Ce+Y concentrations of 472–656ppm, which are typical for A-type granite. The Qingchengzi plagiogranites have relatively high Al, Ca, Na but low Fe, Ti and K contents. They have low abundance of total REEs (ΣREE=17.6–21.6ppm). They are enriched in LILEs (e.g., Ba, K and Sr) but depleted in HFSEs (e.g., Nb, Ta, P, Ti and Y). These features, combined with extremely high Sr/Y ratios (327–413), are comparable with those of typical modern adakites. The Liaoji A-type granites show a source same to that of the Haicheng mafic sills and they originated from the Archean subcontinental lithospheric mantle plus with contamination from the Archean granites. However, the Qingchengzi I-type granites are interpreted to originate from subducted oceanic crust with significant contributions from sediments. The Liaoji A-type granites and the coeval bimodal volcanism in the Liaohe Group as well as the Haicheng mafic sills may represent episodic magmatism which was related to a protracted intra-continental rifting caused by lithospheric extension. The ca. 1890Ma Qingchengzi I-type granites and coexisting S-type granites as well as the coeval regional metamorphism are favored to represent an active continental magmatism linked to Paleoproterozoic subduction.

Carboniferous magmatism and mineralization in the area of the Fuxing Cu deposit, Eastern Tianshan, China: Evidence from zircon U–Pb ages, petrogeochemistry, and Sr–Nd–Hf–O isotopic compositions

The newly discovered Fuxing porphyry Cu deposit is located in the Dananhu–Tousuquan arc, adjacent to the Tuwu–Yandong Cu deposits of Eastern Tianshan, in the southern Central Asian Orogenic Belt. The Fuxing deposit is hosted by volcanic rocks (basalt and dacite) in the Early Carboniferous Qi'eshan Group and Carboniferous felsic intrusions (plagiogranite porphyry, monzogranite, and quartz diorite). New SIMS zircon U–Pb dating indicates that the plagiogranite porphyry and monzogranite emplaced at 332.1±2.2Ma and 328.4±3.4Ma, respectively. The basalts are characterized by low SiO2 contents (47.47–54.90wt.%), a lack of Eu anomalies, strong depletion of Na, Ta, and Ti elements but positive Sr, U, and Pb anomalies, high Y (20.8–28.2ppm) and HREE concentrations (Yb=2.23–3.06ppm), and relatively low (La/Yb)N (2.20–3.92) values; the dacite samples have high SiO2 contents (66.13–76.93wt.%), clearly negative Eu anomalies, high Mg# values (36–51), and high Y (41.8–54.9ppm) and Yb (5.76–8.98ppm) concentrations. The basalts and dacites exhibit similar signatures as normal arc rocks, and were considered to be derived from partial melting of mantle-wedge peridotite that was previously metasomatized by slab melts. In contrast, the plagiogranite porphyry, monzogranite, and quartz diorite show the same geochemical affinity with modern adakites, which are characterized by high SiO2 contents (67.55–79.00wt.%), minor negative to positive Eu anomalies, strong depletion of heavy rare earth elements (Yb=0.17–1.19ppm) and Y (1.86–10.1ppm), positive K, Rb, Sr, and Ba but negative Nb, Ta, Th, and Ti anomalies, and high (La/Yb)N ratios and Mg# values. Moreover, these adakitic felsic intrusions display relatively high positive zircon εHf(t) values (+11.4 to +18.3), low 87Sr/86Sr (0.706080–0.711239), high 143Nd/144Nd (0.512692–0.512922) ratios, and consistent zircon δ18O values (4.41‰–5.48‰), suggesting that their parental magma were most likely derived from partial melting of the subducted oceanic crust followed by mantle peridotite interaction. Based on the whole-rock geochemical and Sr–Nd–Hf–O isotopic data, as well as detailed petrographic analyses, we further suggest that the Fuxing igneous rocks and associated porphyry Cu mineralization were generated by the northward subduction of the paleo-Tianshan oceanic plate beneath the Dananhu–Tousuquan island arc during the Early Carboniferous.

Late Silurian–early Devonian adakitic granodiorite, A-type and I-type granites in NW Junggar, NW China: Partial melting of mafic lower crust and implications for slab roll-back

Late Silurian–early Devonian magmatism of the NW Junggar region in the Central Asian Orogenic Belt provides a critical geological record that is important for unraveling regional tectonic history and constraining geodynamic processes. In this study, we report results of Zircon U–Pb ages and systematic geochemical data for late Silurian–early Devonian largely granitic rocks in NW Junggar, aiming to constrain their emplacement ages, origin and geodynamic significance. The magmatism consists of a variety of mafic to felsic intrusions and volcanic rocks, e.g. adakitic granodiorite, K-feldspar granite, syenitic granite, gabbro and rhyrolite. U–Pb zircon ages suggest that the granitoids and gabbros were emplaced in the late Silurian–early Devonian (420–405Ma). Adakitic granodiorites are calc-alkaline, characterized by high Sr (407–532ppm), low Y (12.2–14.7ppm), Yb (1.53–1.77ppm), Cr (mostly <8.00ppm), Co (mostly <11.0ppm) and Ni (mostly <4.10ppm) and relatively high Sr/Y (31–42) ratios, analogous to those of modern adakites. K-feldspar granites and rhyolites are characterized by alkali- and Fe-enriched, with high Zr, Nb and Ga/Al ratios, geochemically similar to those of A-type granites. Syenitic granites show high alkaline (Na2O+K2O=8.39–9.34wt.%) contents, low Fe# values (0.73–0.80) and are weakly peraluminous (A/CNK=1.00–1.07). Gabbros are characterized by low MgO (6.86–7.15wt.%), Mg# (52–53), Cr (124–133ppm) and Ni (84.7–86.6ppm) contents. The geochemical characteristics of the gabbroic samples show affinity to both MORB- and arc-like sources. All granitoids have positive εNd(t) (+3.9 to +6.9) and zircon εHf(t) (+9.8 to +15.2) values and low initial 87Sr/86Sr ratios (0.7035–0.7043), with young TDM(Nd) (605–791Ma) and TDM(Hf) (425–773Ma) ages, suggesting significant addition of juvenile material. The adakitic granodiorites probably resulted from partial melting of mafic lower crust, leaving an amphibolite and garnet residue. The K-feldspar granites, rhyolites and syenitic granites probably formed from partial melting of the Xiemisitai mid-lower crust, while the gabbroic intrusion was probably generated by interactions between asthenospheric and metasomatized lithospheric mantle. Voluminous plutons of various types (adakites, A-type granites, I-type granites, and gabbros) formed during 420–405Ma, and their isotopic data suggest significant additions of juvenile material. We propose that a slab roll-back model can account for the 420–405Ma magmatic “flare up” in NW Junggar as well as an extensional setting.

Geochemical modelling of early Eocene adakitic magmatism in the Eastern Pontides, NE Anatolia: continental crust or subducted oceanic slab origin?

Early Eocene adakitic volcanic and granitoid rocks are widespread in the Eastern Pontides of NE Turkey, providing significant constraints for the early Cenozoic tectonomagmatic evolution of the region. These adakitic rock units exhibit relatively high Sr/Y and La/Yb ratios, but low Y and Yb values, similar to modern adakites generated by partial fusion of a subducted oceanic slab. They also have high K2O and low MgO contents, and show moderately enriched ISr and low ϵNd(t) isotopic signatures. Our trace element modelling suggests that these adakitic magmas were generated from partial melting at low pressures of a garnet-bearing amphibolitic source in the continental lower crust. This lower crustal melting resulted from slab break off-induced asthenospheric upwelling and related magmatic underplating beneath the Eastern Pontides. We interpret this melting event and the adakitic magmatic activity as a syn- to post-collisional process involving early Cenozoic collision of the Pontide and Anatolide–Tauride continental blocks. The geochemical and tectonic constraints presented here indicate that early Eocene adakitic magmatism in the Eastern Pontides did not result from partial fusion of a subducted oceanic slab, but instead represent continental-type adakite formation.

Geochemistry and petrogenesis of an adakitic quartz-monzonitic porphyry stock and related cross-cutting dike suites, Kighal, northwest Iran

A late Oligocene–early Miocene quartz-monzonitic porphyry stock intrudes upper Eocene andesitic-latitic lava flows, producing Cu–Mo mineralization and hydrothermal alteration zones in the Kighal area, northwest Iran. Numerous cross-cutting dikes of various compositions, ranging from quartz-diorite to granodiorite and microdiorite, branch off from three later-stage barren sub-volcanic bodies, and intrude the porphyry stock. Both the stock and comagmatic dikes display I-type, magnesian, calc-alkaline to high-K calc-alkaline signatures and metaluminous to peraluminous geochemical characters, being emplaced within an active continental margin setting. Analysed samples display the typical geochemical characteristics of adakites. Moreover, rare earth elements exhibit a fractionated pattern with low heavy rare earth element concentrations. The general trend in an La/Yb–Yb diagram shows that partial melting, rather than fractional crystallization, was the dominant process of magma generation. Based on the compositional classification of modern adakites, the studied rocks are of high-SiO2 adakite type. They were generated through low degrees of high-pressure partial melting of subducted rutile- and amphibole-bearing eclogitic Neo-Tethyan oceanic slab. The descending slab underwent break-off, leaving behind a residue rich in garnet + amphibole ± rutile, lacking significant plagioclase. Interaction of the slab-derived magma with the mantle-wedge peridotite is also evident.

PETROLOGY AND GEOCHEMISTRY OF THE GRANITOID ROCKS OF THE JOHANNESBURG DOME, CENTRAL KAAPVAAL CRATON, SOUTH AFRICA

The Johannesburg Dome (JD) in the central Kaapvaal Craton (KC) is dominated by granitoid rocks of the tonalite-trondhjemite-granodiorite (TTG) series. Based on modal analysis as well as a major and trace element investigation the JD granitoids could be subdivided into three main suites, i.e. a Tonalitic gneiss suite (TG) around the southern boundary, a Granodiorite to Adamellite Gneiss suite (GAG) across the northern part, and a Granodiorite/adamellite to Granodiorite suite (GG) occurring between the TG and GAG suites. These rocks are dominantly I-type and peraluminous with the tonalites (TG and partly the GAG suites) falling in the metaluminous field. TTGs of the JD are high-K calc-alkaline to calc-alkaline and are dominantly high silica rocks (~70 weight %), aluminous (Al2O3 >15wt%) with low Yb ( 30), high Na2O/K2O (>1), and have Na2O contents of between 3wt% and 5wt%, comparable to that of the average TTG. The JD tonalities (TG suite) have higher Al2O3, Sr, Na2O/K2O, Mg#, Ni, Cr and LILE contents compared to the more calc-alkaline granitoids (GG suite and trondhjemites of the GAG suite), which are typically richer in HREE (lower REE fractionation), Y and show a negative Sr and Eu anomaly. Other characteristic features of the JD TTG’s include HFSE depletion and distinct enrichment of fluid sensitive elements such as Pb. The strongly fractionated REE pattern, high (La/Yb)N ratio and depletion in HREE (Yb) of the JD TTGs are characteristics shared with modern adakites. The TG suite most probably formed through melting of a subducted oceanic slab with the melt interacting with mantle peridotite during its accent through a thin mantle wedge. The remaining JD granitoids (GAG and GG) most probably formed through the remelting of a TTG protolith, which has a subducted slab and mantle wedge signature (similar to the TG suite).

Geochronology, geochemistry and petrogenesis of rhyodacite lavas in eastern Jamaica: A new adakite subgroup analogous to early Archaean continental crust?

Rhyodacite lavas (Newcastle Volcanic Formation) from the Wagwater Basin in eastern Jamaica dated at 52.74 ± 0.34 Ma (2σ) have adakitic-like major element compositions, low Y and heavy rare Earth element (REE) concentrations and negative Nb and Ta anomalies on a normal mid-ocean ridge basalt normalised multi-element diagram. They also have lower Sr (< 400 ppm), MgO (≤ 2.0 wt.%), Ni (mostly ≤ 30 ppm) and Cr (mostly ≤ 40 ppm) concentrations compared to other modern adakites and middle-late Archaean (3.5–2.5 Ga) trondhjemite, tonalite and granodiorite/dacites (TTG/Ds). ε Nd(i) and ε Hf(i) values indicate that the Newcastle adakite-like lavas cannot be formed by assimilation and fractional crystallisation processes involving any other igneous rock in the area and so the composition of the lavas is largely the result of the residual mineralogy in the source region. Low Sr and Al 2O 3 contents indicate a fluid/vapour-absent source with residual plagioclase and REE systematics point to residual amphibole and garnet in the source region. Similarly, high silica values and constant Zr and P 2O 5 concentrations suggest residual quartz and accessory zircon and apatite. The plagioclase and garnet residue implies that the Newcastle magmas were derived from partially melting a metabasic protolith at 1.0–1.6 GPa, which would intersect the amphibole dehydration partial melt solidus at ~ 850–900 °C. Radiogenic isotopes along with the low MgO, Ni and Cr concentrations in the Newcastle lavas demonstrate that the garnet amphibolite source region cannot be part of (1) the lower Jamaican arc crust, (2) delaminated lower crust or (3) subducted Proto-Caribbean “normal” oceanic crust that may, or may not, have detached. This data, in addition to partial melting models involving a theoretical garnet-amphibolite source region for the Newcastle lavas, shows that the adakite-like rocks are derived from metamorphosed Caribbean oceanic plateau crust that underthrust Jamaica in the early Tertiary. The underplated oceanic plateau crust partially melted by either (1) influx of basaltic magma during lithospheric extension in the early Tertiary or (2) direct partial melting of the underthrusting (subducting) plateau crust. The Newcastle magmas ascended and erupted without coming into contact with a mantle wedge thus forming the low MgO, Ni and Cr contents. Most Cenozoic adakites have compositions similar to the middle-late Archaean TTG/D suite of igneous rocks. In contrast, early (> 3.5 Ga) Archaean TTG/D crustal rocks have lower Sr, MgO, Ni and Cr concentrations and prior to this study had no modern adakite analogue. However, the Newcastle adakites have similar compositions to the, early Archaean TTG/D. The discovery of these rocks has important implications for our understanding of the formation of the Earth's earliest continental crust and so it is proposed that the Newcastle lavas be classified as a unique subgroup of adakites: Jamaican-type adakite.

In situ U–Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: New insights to making old crust

The sources and petrogenetic processes that generated some of the Earth’s oldest continental crust have been more tightly constrained via an integrated, in situ (U–Pb, O and Hf) isotopic approach. The minerals analysed were representative zircon from four Eoarchaean TTG tonalites and two felsic volcanic rocks, and olivine from one harzburgite/dunite of the Itsaq Gneiss Complex (IGC), southern West Greenland. The samples were carefully chosen from localities with least migmatisation, metasomatism and strain. Zircon was thoroughly characterized prior to analysis using cathodoluminescence, scanning electron, reflected and transmitted light imaging. The zircon from all but one sample showed only minor post-magmatic recrystallisation. 207Pb/ 206Pb dating of oscillatory-zoned zircon using SHRIMP RG ( n = 142) indicates derivation of the felsic igneous rocks from different batches of magma at 3.88, 3.85, 3.81, 3.80 and 3.69 Ga. Analyses of 18O/ 16O compositions of olivine from a harzburgite/dunite ( n = 8) using SHRIMP II in multi-collector mode, indicate that the oxygen isotopic composition of this sample of Eoarchaean mantle ( δ 18O Ol = 6.0 ± 0.4‰) was slightly enriched in 18O, but not significantly different from that of the modern mantle. Zircon δ 18O measurements from the six felsic rocks ( n = 93) record mean or weighted mean compositions ranging from 4.9 ± 0.7‰ to 5.1 ± 0.4‰, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events. δ 18O Zr compositions indicate that the primary magmas were largely in equilibrium with the mantle or mantle-derived melts generated at similar high temperatures, while calculated tonalite δ 18O WR compositions (6.7–6.9‰) resemble those of modern adakites. LA-MC-ICPMS zircon 176Hf/ 177Hf analyses were obtained from six samples ( n = 122). Five samples record weighted mean initial ε Hf compositions ranging from to 0.5 ± 0.6 to −0.1 ± 0.7 (calculated using λ 176Lu = 1.867 × 10 −11 yr −1), while one sample records a composition of 1.3 ± 0.7, indicating the magmas were generated from a reservoir with a time averaged, near chondritic Lu/Hf. The derivation of TTG magmas from a chondritic Lu/Hf source implies either that there was not voluminous continental crustal growth nor major mantle differentiation leading to Lu/Hf fractionation during the Hadean or Eoarchaean, or alternatively that rapid recycling of an early formed crust allowed the early mantle to maintain a chondritic Lu/Hf. Previous studies have demonstrated that ancient TTG rocks were mostly produced by dehydration melting of mafic rocks within the stability field of garnet, probably in flatly-subducted or buried oceanic crust. The oxygen isotopic signatures measured here at high spatial resolution allow the source materials to be better defined. Melting of a mixed mafic source consisting of ∼80% unaltered gabbro ( δ 18O WR = 5.5‰) with ∼20% hydrothermally altered gabbro/basalt ( δ 18O WR = 4.0‰) would produce tonalite magmas within the average compositional range observed. 18O-enriched components such as altered shallow basaltic oceanic crust and pelagic or continental sediments were not present in the sources of these TTG melts. The absence of high 18O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab.

Geochemistry of adakites from Neoarchaean active continental margin of Shimoga schist belt, Western Dharwar Craton, India: Implications for the genesis of TTG

Adakites are identified for the first time from a Neoarchaean active continental margin (ACM) preserved in the eastern part of the Shimoga schist belt, near Ranibennur, Western Dharwar Craton, India. These adakites consists mainly of quartz, plagioclase and minor amphiboles and are interlayered with the turbidites. They exhibit typical adakitic low Y (8.35–14.70 ppm) and Yb (0.85–1.47 ppm), may be classified as high SiO 2 and low Mg#, Ni and Cr type adakites. Other constituents such as Na 2O, K 2O, Nb, Zr, LREE and HREE and their ratios such as Na 2O/K 2O, Zr/Sm, Nb/La, (La/Yb) n , (Gd/Yb) n and Th/U also closely resemble with the high silica adakites (HSA) of Martin et al. [Martin, H., Smithies, R.H., Rapp, R., Moyen, J.-F., Champion, D., 2005. An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79, 1–24]. Sr/Y values are moderate, generally lower than the high Sr–low silica Cenozoic adakites, but within the range of Archaean adakites. The negative Nb, Ta, and Ti anomalies characteristic of Cenozoic adakites are found in Ranibennur adakites also. Zr/Zr* and Hf/Hf* ratios are also similar to HSA. These adakites further depict compositional similarities with Meso and Neoarchaean tonalite–trondhjemite–granodiorites (TTGs). Such types of adakites have been experimentally generated by the partial melting of hydrous basalts, that are similar to adakitic glass veins (low MgO, CaO, Cr and Ni) found in peridotite xenoliths of Kamchatka arc [Xiong, X.L., Xia, B., Xu, J.F., Niu, H.C., Xiao, W.S., 2006. Na depletion in modern adakites via melt/rock reaction within the sub-arc mantle. Chem. Geol. 229, 273–292]. Since Archaean had higher geothermal regime, subduction of the basaltic slab was most probably shallow and significant mantle wedge was not available to react with adakitic melt. Hence Archaean adakites in general, and particularly from Ranibennur are relatively depleted in the peridotiitic mantle wedge components. Their identification from an Archaean ACM and their compositional similarities with the Paleo-Meso Archaean TTGs suggest the possibility of identical origin, i.e. the melting of a shallow hydrous basaltic slab. This mechanism during Archaean was widespread and rapid, resulted in faster continental crustal growth during the early history of the Earth. Identification of Archaean adakites in association with island arc turbidites from a greenstone belt sequence supports the rapid subduction mechanism for the crustal growth that coincides with 2.7 Ga peak (episode) of almost all geological processes.

Derivation of Mesozoic adakitic magmas from ancient lower crust in the North China craton

Three Mesozoic plutons were intruded into the Archean granulite terrains in northern margin of the North China craton adjacent to the granulite xenolith-bearing Tertiary Hannuoba basalts. They are Triassic Guzuizi and Honghualiang granites and Early Cretaceous Zhuanzhilian diorite. Rocks of the three intrusions are characterized by high Sr (576–1216 ppm) and Na 2O (⩾4.0%), low Y (⩽18 ppm) and heavy rare-earth elements (Yb < 1.8 ppm). These features are similar to modern adakites (siliceous rocks with high sodium, aluminum and strontium and low heavy rare-earth element and yttrium contents) from island arcs and Archean high-Al tonalite–trondhjemite–granodiorite (TTG). However, they are distinctly potassium-rich and their evolved strontium, neodymium, and hafnium isotopic compositions and inherited zircon ages coincide with those of the Archean granulites and the Hannuoba granulite xenoliths. Such features cannot readily be explained with previous models of the origin of adakites and TTGs. We propose that these adakitic rocks are formed by partial melting of ancient lower crust, and that the restites are represented by some of the Hannuoba granulite xenoliths; we further argue that this could be one major mechanism to generate voluminous Mesozoic adakitic magmas in the eastern North China craton. Our hypothesis can also explain the discrepancies in ages and compositions between the granulite terrains and xenoliths, and implies that crustal anatexis may be one of the major processes controlling the chemical differentiation of the continental crust.

Neoproterozoic ultramafic–mafic-carbonatite complex and granitoids in Quruqtagh of northeastern Tarim Block, western China: Geochronology, geochemistry and tectonic implications

U–Pb zircon and baddeleyite ages, and geochemical and Nd isotopic data, are reported for a ultramafic–mafic-carbonatite complex and granites in Quruqtagh of northeastern Tarim Block, NW China. The carbonatite and plagioclase-bearing pyroxenite from the Qiganbulake mafic–ultramafic-carbonatite ring complex (QMC), the Xingdi granodiorite and the Taiyangdao granite were emplaced at 810 ± 6, 818 ± 11, 820 ± 10 and 795 ± 10 Ma (95% confidence level), respectively. The QMC is composed of dunite, apatite- and/or feldspar-bearing pyroxenite, pyroxenite, phlogopitelite and carbonatite. Petrography, geochemistry and mineral chemistry suggest that the QMC rocks were generated by partial melting of a CO 2-metasomatized mantle in a rifting environment. The Xingdi and Taiyangdao granitoids possess high LREE, Na 2O/K 2O, Sr/Y, (La/Yb) N ratios and low HREE and HFSE contents, similar to modern adakites. However, they have lower MgO (or Mg #), Cr and Ni contents and unradiogenic Nd isotopes (pronounced negative ɛNd( t) value of −12.7 to −17.3 and Neoarchaean Nd model ages) than slab-derived adakites. Thus, they were likely formed by partial melting of Neoarchaean mafic protoliths in the lower crust, leaving behind a granulite residue. The QMC and the granitoids in Quruqtagh constitute a bimodal intrusive suite in a Neoproterozoic continental rift setting, possibly related to mantle plume activities beneath the Rodinian supercontinent.

Na depletion in modern adakites via melt / rock reaction within the sub-arc mantle

Interaction between slab-derived melt and mantle peridotite and the role of slab melt as a metasomatizing agent in the sub-arc mantle is being increasingly recognized. Adakite, the slab melt erupted on the surface, usually exhibits anomalously high MgO, CaO, Cr and Ni contents that indicate interaction with mantle peridotitite. Here we note that Cenozoic adakites have Na 2O contents below 5.8 wt.% with ∼95% samples lower than 5.0 wt.%, and are generally depleted in this component relative to experimental basalt partial melts (mostly beyond 5.0 wt.% and up to 9.0 wt.% Na 2O) produced under 1.5–3.0 GPa conditions that are most relevant to adakite production. We interpret the adakite Na depletion to be also a consequence of the melt / rock reaction that takes place within the hot mantle wedge. During ascent and reaction with mantle peridotite, primary adakite melts gain mantle components MgO, CaO, Cr and Ni but lose Na 2O, SiO 2 and perhaps K 2O to the mantle, leading to Na-rich mantle metasomatism. Selective assimilation of predominately mantle clinopyroxene, some spinel and minor olivine at high T/P has been considered to be an important process in producing high-Mg adakites from primary low-Mg slab melts [Killian, R., Stern, C. R., 2002. Constraints on the interaction between slab melts and the mantle wedge from adakitic glass in peridotite xenoliths. Eur. J. Mineral. 14, 25–36]. In such a process, Na depletion in the assimilated melt is the result of dilution due to the increase in melt mass. Phase relationships in the reaction system siliceous melt + peridotite and quantitative calculation suggest that assimilation of mantle clinopyroxene, olivine and spinel and fractional crystallization of sodic amphibole and orthopyroxene, under conditions of moderate T/P and increasing melt mass, is also an important process that modifies the composition of adakites and causes the Na depletion.