Adakitic Granodiorite Research Articles

Late Carboniferous adakitic granodiorites in the Qiongkusitai area, western Tianshan, NW China: Implications for partial melting of lower crust in the southern Central Asian Orogenic Belt

Carboniferous granitic rocks are wide spread in the western Tianshan, but their tectonic contexts remain controversial. We present zircon U–Pb age, major element, trace element, and in situ zircon Hf isotopic data for the Qiongkusitai granodiorites from the southern part of the Yili block in the western Tianshan. The granodiorites were emplaced in the Late Carboniferous (ca. 314Ma) and are characterized by high SiO2 (62.1–65.7wt.%), high Sr (376–485ppm), low Y (13.7–17.7ppm) and low Yb (1.34–1.81ppm) with Sr/Y ratios of 23–34. Such characteristics imply adakitic affinities. Their low concentrations of Cr (3.85–6.75ppm), Co (7.63–10.7ppm) and Ni (2.94–6.80ppm) suggest little, if any, interaction with a mantle source. Compared to the thickened lower crustal-derived adakitic rocks, the relatively low Nb/Ta ratios of the Qiongkusitai adakitic granodiorites indicate that they were probably generated by partial melting of amphibolite facies rocks, but not eclogite facies rocks. The Qiongkusitai adakitic granodiorites have positive zircon εHf(t) values of +0.7 to +5.1 and TDM2 of 1.28–1.68Ga, and they are interpreted as being derived from remelting of amphibolite facies Mesoproterozoic basement with an input of juvenile material. Late Carboniferous magmatism in the western Tianshan was possibly triggered by asthenospheric upwelling as a result of the roll-back of the subducted southern Tianshan oceanic lithosphere.

Syn-collisional adakitic granodiorites formed by fractional crystallization: Insights from their enclosed mafic magmatic enclaves (MMEs) in the Qumushan pluton, North Qilian Orogen at the northern margin of the Tibetan Plateau

The Qumushan (QMS) syn-collisional granodiorite, which is located in the eastern section of the North Qilian Orogen at the northern margin of the Greater Tibetan Plateau, has typical adakitic characteristics and also contains abundant mafic magmatic enclaves (MMEs). This recognition offers an unprecedented insight into the petrogenesis of both the adakitic host granodiorite and the enclosed MMEs. The MMEs and their host granodiorites share many characteristics in common, including identical crystallization age (~430Ma), same mineralogy, similar mineral chemistry and whole-rock isotopic compositions, indicating their genetic link. The MMEs are most consistent with being of cumulate origin formed at earlier stages of the same magmatic system that produced the QMS adakitic granodiorite. Subsequent replenishment of adakitic magmas could have disturbed the cumulate piles as “MMEs” dispersed in the adakitic granodiorite host during emplacement. The geochemical data and petrogenetic modeling of trace elements suggest that the QMS adakitic host granodiorite is most consistent with fractional crystallization dominated by the mineral assemblage of the MMEs. The parental magma for the QMS granodiorite is best explained as resulting from partial melting of the ocean crust together with recycled terrigenous sediments during continental collision, which may have also experienced interaction with mantle peridotite during ascent.

Late Jurassic sodium-rich adakitic intrusive rocks in the southern Qiangtang terrane, central Tibet, and their implications for the Bangong–Nujiang Ocean subduction

The lack of magmatic records with high-quality geochronological and geochemical data in the central segment of the southern Qiangtang subterrane in central Tibet inhibits a complete understanding of the subduction polarity of the Bangong–Nujiang Ocean lithosphere during the Mesozoic. In this study, we present the zircon U–Pb age as well as geochemical and Sr–Nd–Pb isotopic data for the Late Jurassic pluton from the Kangqiong area in the central segment of the southern Qiangtang subterrane. The Kangqiong pluton primarily consists of granodiorites (SiO2=62.87–65.17wt.%) and was emplaced in the Late Jurassic (147.6±2.4–149.9±2.1Ma). The granodiorites display high Na2O numbers (Na2O/K2O=1.75–2.24) as well as high MgO (2.21–3.14wt.%) and Mg-numbers (53–58), are characterized by a low abundance of heavy rare earth elements (e.g., Yb=1.05–1.92ppm) and Y (12.63–17.52ppm), and high Sr/Y (29–61) and La/Yb (14–18) ratios, which are comparable in composition to those of slab-derived adakitic rocks. The Kangqiong adakitic granodiorites have initial (87Sr/86Sr)i ratios of 0.70611 to 0.70669, negative εNd(t) values (−1.06 to −0.25), (206Pb/204Pb)t ratios of 18.42 to 18.47, (207Pb/204Pb)t ratios of 15.62 to 15.63, and (208Pb/204Pb)t ratios of 38.50 to 38.60. These geochemical signatures indicate that the magmas were most likely derived from the partial melting of the subducted Bangong–Nujiang oceanic crust and minor contaminants from the accretionary complex. Our results, in combination with the coeval magmatism in the western segment of the southern Qiangtang subterrane, indicate that the Bangong–Nujiang oceanic lithosphere was subducted northward beneath the Qiangtang Terrane, forming a west-east magmatic arc over 800km during the Late Jurassic.

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.

Geochemistry, geochronology and Sr–Nd–Pb–Hf isotopic compositions of Middle to Late Jurassic syenite–granodiorites–dacite in South China: Petrogenesis and tectonic implications

In situ zircon U–Pb ages and Hf isotope data, major and trace elements and Sr–Nd–Pb isotopic compositions are reported for coeval syenite–granodiorites–dacite association in South China. The shoshonitic syenites are characterized by high K2O contents (5.9–6.1wt.%) and K2O/Na2O ratios (1.1–1.2), negative Eu anomalies (Eu/Eu*=0.65 to 0.77), enrichments of Rb, K, Nb, Ta, Zr and Hf, but depletion of Sr, P and Ti. The adakitic granodiorite and granodiorite porphyry intrusions are characterized by high Al2O3 contents (15.0–16.8wt.%), enrichment in light rare earth elements (LREEs), strongly fractionated LREEs (light rare earth elements) to HREEs (heavy rare earth elements), high Sr (438–629ppm), Sr/Y (29.2–53.6), and low Y (11.7–16.8ppm) and HREE contents (e.g., Yb=1.29–1.64ppm). The calc-alkaline dacites are characterized by LREE enrichment, absence of negative Eu anomalies, and enrichment of LILEs such as Rb, Ba, Th, U and Pb, and depletion of HFSEs such as Nb, Ta, P and Ti.Geochemical and Sr–Nd–Hf isotopic compositions of the syenites suggest that the shoshonitic magmas were differentiated from parental shoshonitic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas may have originated from partial melting of the lithospheric mantle with small amount contribution from crustal materials. The adakitic granodiorite and granodiorite porphyry have Sr–Nd–Pb isotopic compositions that are comparable to that of the mafic lower crust. They have low Mg# and MgO, Ni and Cr contents, abundant inherited zircons, low εNd(t) and εHf(t) values as well as old whole-rock Nd and zircon Hf model ages. These granodiorites were likely generated by partial melting of Triassic underplated mafic lower crust. The Hf isotopic compositions of the dacites are relatively more depleted than the Cathaysia enriched mantle, suggesting those magmas were derived from the partial melting of subduction-modified mantle sources. The coeval shoshonitic, high-K calc-alkaline and calc-alkaline rocks in Middle to Late Jurassic appear to be associated with an Andean-type subduction. This subduction could have resulted in the upwelling of the asthenosphere beneath the Cathaysia Block, which induced partial melting of the mantle as well as the mafic lower crust, and formed an arc regime in the coastal South China during Middle to Late Jurassic.

Petrogenesis of the early Paleozoic low-Mg and high-Mg adakitic rocks in the North Qilian orogenic belt, NW China: Implications for transition from crustal thickening to extension thinning

The petrogenesis and geodynamic implications of the early Paleozoic adakitic rocks in the North Qilian orogenic belt remain topics of debate. In this study, petrology, zircon U–Pb age and Lu–Hf isotopic composition, whole-rock geochemistry and Sr–Nd isotopes were carried on both low-Mg and high-Mg adakitic rocks from the North Qilian orogenic belt. These results will provide significant constraints on the evolution of the North Qilian orogenic belt. In general, the low-Mg adakitic granites mainly consist of plagioclase (50–55%), alkali feldspar (20–25%), quartz (20–25%) and biotite (5%). In contrast, the high-Mg adakitic granodiorites are generally composed of alkali feldspar (30–35%), plagioclase (30–45%), quartz (20–25%) and amphibole (∼10%). The low-Mg adakitic granitoids (446–457Ma) are characterized by high SiO2 (69–72wt.%), low Mg# (39–42) and low Cr and Ni contents. However, the high-Mg adakitic granitoids (430–431Ma) have relatively lower SiO2 (65–66wt.%), higher Mg# (50–59) and higher Cr and Ni contents. The low-Mg adakitic rocks have high initial 87Sr/86Sr ratios (0.7068–0.7080), negative εNd(t) (−2.3 to −3.3) and εHf(t) values (−6.8 to 1.9) with old zircon Hf model ages (1.2–1.6Ga). However, the high-Mg adakitic rocks show lower initial 87Sr/86Sr ratios (0.7044–0.7066), higher εNd(t) (−1.8 to 3.0) and positive εHf(t) values (2.3–7.7) with younger zircon Hf model ages (0.9–1.1Ga). These results suggest that the low-Mg rocks were generated by partial melting of thickened crust, whereas the high-Mg rocks were derived from anatexis of delaminated lower crust, which subsequently interacted with mantle magma upon ascent. The data obtained in this study provide significant information about the geological and tectonic processes after the closure of the Qilian Ocean. The continent–continent collision and thickening probably occurred during 440–467Ma with formation of low-Mg adakitic rocks; and the transition of the tectonic regime from compression to extension probably occurred at ∼430Ma with formation of high-Mg adakitic rocks.

Elemental and Sr-Nd-Hf isotopic constraints on the origin of Late Jurassic adakitic granodiorite in central Fujian province, southeast China

This paper presents the first detailed SHRIMP or LA-ICP-MS zircon U–Pb dating, major and trace element geochemical and Sr–Nd–Hf isotopic data of an adakitic pluton (Tangquan pluton) and an I-type granitic pluton (Xiadao pluton) in central Fujian. SHRIMP and LA-ICP-MS zircon U–Pb dating indicates that the Tangquan and Xiadao plutons were emplaced in the Late Jurassic (~160 Ma) and Early Cretaceous (~143 Ma), respectively. The Tangquan pluton is mainly composed of high-K calc-alkaline granodiorite. The rocks show adakitic affinities, characterized by high Sr and low Y and Yb contents, with high Sr/Y ratios, and by high Mg#. They have initial 87Sr/86Sr of 0.7084–0.7087, eNd (T) of −8.8 to −9.0 and eHf (T) (in-situ zircon) of −11.2. Detailed elemental and isotopic data suggest that the Tangquan adakitic granodiorite was formed by partial melting of Paleoproterozoic metamorphic basement at a depth of ~40 km (P ~12.5 kbar) plus additional input from coeval basaltic magma. The Xiadao pluton consists of monzogranite, syenogranite and alkali-feldspar granite. These granites are high-K calc-alkaline and belong to I-type. They have relatively low Sr and high Y and Yb contents and show similar Mg# to pure crustal melts. The Xiadao granites have slightly higher initial 87Sr/86Sr (0.7095–0.7102) and lower eNd (T) (−9.0 to −9.4) and eHf (T) (−14.4; in-situ zircon) than the Tangquan granitoids. Detailed elemental and isotopic data suggest that the Xiadao I-type granites were formed by partial melting of Paleoproterozoic metamorphic basement at a depth of ~30 km (P ~10 kbar).

Petrogenesis and geodynamic significance of the Late Triassic Tadong adakitic pluton in West Qinling, central China

The tectonic setting of the West Qinling orogenic belt (QOB) during the Middle–Late Triassic remains a subject of debate. Petrogenesis of adakitic granodiorite plays a critical role in determining the nature of the lower continental crust and mantle dynamics during orogenic processes in the region. The Tadong adakitic granodiorite pluton in the western part of the West QOB is an important element of this system. Its petrogenesis can place severe constraints on the nature of the lower continental crust and mantle dynamics during the formation of the orogenic belt. U–Pb dates obtained through zircon laser-ablation inductively coupled mass spectrometry indicate that the Tadong pluton was emplaced at 220.2 ± 2.5 Ma, coeval with abundant magmatic rocks in the region. This indicates a prominent magmatic event in the western part of West Qinling during the Late Triassic. Geochemically the granodiorites are metaluminous to peraluminous high-K calc-alkalic and characterized by relatively high SiO2 (63.84–67.91 wt.%), Al2O3 (15.39–16.54 wt.%), and Sr (435.08–521.64 ppm), and low MgO (1.16–1.88 wt.%; Mg# = 38–46), Y (5.49–8.84 ppm) and Yb (0.34–0.91 ppm) contents, variable Eu anomalies (Eu/Eu* = 0.87–1.1), and high Sr/Y (51.72–84.45) ratios. These are compositional features of adakites that are commonly assumed to have been produced through partial melting of subducted oceanic basalt. In addition, the adakitic rocks are relatively enriched in light rare earth elements, large ion lithophile elements (Rb, Ba, Sr, Th, and K), and depleted in high field strength elements. However, petrological, geochronological, and geochemical characteristics indicate that the adakitic rocks were most likely formed by partial melting of a thickened mafic lower crust. Therefore, we suggest that the Tadong adakitic granodiorites were produced in a syn-collisional regime and associated with asthenospheric upwelling triggered by slab break-off or gravitational instability. This mechanism was responsible for generating the Late Triassic magmatism of West Qinling.

Some Pertinent Features of Mo‐Mineralized Granitoids in the Circum‐Pacific Region

AbstractPetrological characteristics of granitic rocks related to the world large molybdenum deposits are studied. The granitoids are evaluated by Fe2O3+TiO2‐FeO+MnO‐MgO diagrams, and found to all plot to the magnetite‐series field. They are all high silica and high‐K series, but not A‐type, except for the Climax‐type porphyries and some others in the Colorado mineral belt. By‐product molybdenum contained in porphyry copper deposits, lower grade but huge tonnage, occurs with calc‐alkaline I‐type magnetite‐series granodiorite and monzogranite.Felsic intrusive rocks of the Climax mine are A‐type and are exceptionally high in trace elements such as F and Rb, which are generally enriched with W and Sn‐related granitoids that originated in crustal source rocks. The by‐product molybdenites in porphyry copper deposits appear to originate in adakitic granodiorite or monzogranite, having deep origins with the subducted slab or thickened juvenile mafic lower crust. Therefore, there is no single magma type but the magnetite series, which concentrates a large volume of molybdenum in the ore deposits.

Transition from oceanic to continental lithosphere subduction in southern Tibet: Evidence from the Late Cretaceous–Early Oligocene (~ 91–30 Ma) intrusive rocks in the Chanang–Zedong area, southern Gangdese

Little is known about the detailed processes associated with the transition from oceanic to continental lithosphere subduction in the Gangdese Belt of southern Tibet (GBST). Here, we report zircon U–Pb age, major and trace element and Sr–Nd–Hf isotopic data for Late Cretaceous–Early Oligocene (~91–30Ma) intermediate-acid intrusive rocks in the Chanang–Zedong area immediately north of the Yarlung–Tsangpo suture zone. These rocks represent five magmatic episodes at ~91, ~77, ~62, ~48, and ~30Ma, respectively. The 91–48Ma rocks have slightly lower initial 87Sr/86Sr (0.7037 to 0.7047), and higher εNd(t) (+1.8 to +4.3) and εHf(t) (+3.5 to +14.7) values in comparison with those (0.7057 to 0.7062, −3.3 to −2.5 and +2.2 to +6.6) of the ~30Ma intrusive rocks. The ~91, ~62 and ~30Ma rocks are geochemically similar to slab-derived adakites. The ~91Ma Somka adakitic granodiorites were likely derived by partial melting of the subducting Neo-Tethyan oceanic crust with minor oceanic sediments, and the ~91Ma Somka dioritic rocks with a geochemical affinity of adakitic magnesian andesites likely resulted from interactions between adakitic magmas and overlying mantle wedge peridotite. The ~77Ma Luomu diorites were probably generated by partial melting of juvenile basaltic lower crust. The ~62Ma Naika and Zedong adakitic diorites and granodiorites were likely generated mainly by partial melting of thickened juvenile mafic lower crust but the source region of the Zedong adakitic rocks also contained enriched components corresponding to Indian continental crust. The ~48Ma Lamda granites were possibly generated by melting of a juvenile basaltic crust. The younger (~30Ma) Chongmuda adakitic quartz monzonites and minor granodiorites were most probably derived by partial melting of Early Oligocene northward-subducted Indian lower crust beneath the southern Lhasa Block. Taking into account the regional tectonic and magmatic data, we suggest that the Gangdese Belt of southern Tibet (GBST) underwent a tectonodynamic transition from oceanic subduction to continental subduction between 100 and 30Ma. It evolved through four stages: 100–65Ma roll-back of subducted Neo-Tethyan oceanic lithosphere; 65–60Ma initial collision between Indian and Asian continents; 60–40Ma breakoff of subducted Neo-Tethyan oceanic lithosphere; and ~30Ma northward subduction of the Indian continent.

Adakitic magmatism in post-collisional setting: An example from the Early–Middle Eocene Magmatic Belt in Southern Bulgaria and Northern Greece

Post-collisional (56.0–40.4Ma) adakitic magmatism in the Rhodope Massif and the Kraishte region, including W. Srednogorie, in South Bulgaria followed the collision of the Rhodope and Pelagonian Massifs. It forms a 250km NW trending belt which continues into the 1000km long belt of Eocene magmatism in northern Turkey and Iran. The rocks are represented by felsic subvolcanic dykes and sills in the Kraishte and plutons in the Rhodopes. Here, we synthesize new chemical (whole-rock major and trace elements, and Sr and Nd isotopes) and LA–ICP/MS mineral and U–Pb zircon age data along with published similar data in order to constrain the genesis of this magmatism and the early Cenozoic geodynamic evolution of the central Balkan Peninsula. The rocks display typical subduction-related characteristics with enrichment in LILE and LREE and depletion in HFSE (Nb, Ta and Ti). In the Kraishte and western Srednogorie Zones these are calc-alkaline to high-K calc-alkaline rhyolites, displaying a typical adakitic signature, i.e. high La/Yb and Sr/Y ratios. The studied Rhodope Massif rocks are predominantly high-K calc-alkaline and subordinate calc-alkaline granites and granodiorites with a minor amount of tonalites. Petrographically, they are H2O- and accessory-rich (allanite, epidote, titanite, apatite) rocks, showing geochemical affinities from non-adakitic tonalites and mafic granodiorites to adakitic granodiorites and granites. Similarity of Sr and Nd isotopic compositions of the Kraishte subvolcanic and Rhodope intrusive adakitic rocks with the neighboring and coeval NW Anatolian basaltic to dacitic volcanics and plutons suggests that the most likely source for the South Bulgarian adakitic rocks is the subduction-enriched depleted lithospheric mantle. The nearby and contemporaneous East Serbian alkaline basalts are isotopically and compositionally different and, probably, originate from an OIB-like mantle source. Subsequent fractionation within an isotopically similar lower or middle crust in the Kraishte and interaction with the mid- to lower part of collision- and underplating-induced thickened crust in the Rhodopes can explain their isotopic variations. Transition from non-adakitic tonalites and granodiorites into adakitic granodiorites and granites in the Rhodopes was developed in response to amphibole fractionation accompanied by trace-element rich accessory minerals. Data from the literature show that the adakitic signature of the Early–Middle Eocene rocks disappeared in the following Late Eocene–Early Oligocene (35–26Ma) magmatic episode. Our interpretation is that adakitic magmatism is related to a deep (>250km) slab break-off, followed by asthenospheric upwelling, heating, fast exhumation and formation of core complexes in the Rhodopes and Kraishte in the interval 42–35Ma. The process was followed by thinning of the crust, orogenic collapse, steep faulting and extensional magmatism.

Late Cretaceous magmatism in Mamba area, central Lhasa subterrane: Products of back-arc extension of Neo-Tethyan Ocean?

Cretaceous magmatism in southern Lhasa subterrane, Tibetan plateau has been investigated for many years and a series of models have been proposed to illustrate their petrogenesis and geodynamic implications. But rare work has been done on the Cretaceous magmatism in central Lhasa subterrane. Here we report the petrology, zircon in situ U–Pb geochronology, Hf isotopes, trace element, and whole-rock elements and Sr–Nd isotopic geochemical data of the host granodiorites, and gabbroic and dioritic enclaves in Mamba area, central Lhasa subterrane. Zircon U–Pb dating for a Mamba host granodiorite yields a crystallization age of ~84Ma, with in situ Hf isotopic analyses for 18 spots of the same zircons of εHf (t) ranging from −7.5 to −0.3. A dioritic enclave (85.2Ma) is coeval with the host granodiorite and shows similar zircon Hf isotopic compositions (εHf (t)=−4.0 to +0.2). Mamba granodiorites (SiO2=66.6–67.5wt.%) and dioritic enclaves (SiO2=53.9–57.6wt.%) are high-K calc-alkaline, and a gabbroic enclave is shoshonitic (K2O=2.81%). All these samples are metaluminous, and enriched in large ion lithophile elements (LILEs, such as Rb, Ba, K, U, Th) and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, and Zr). The host granodiorites are enriched in light rare earth elements (REEs), depleted in heavy REEs with weakly negative Eu anomalies (δEu=0.86–0.88), with high Al2O3 (15.0–15.7wt.%), high Sr/Y ratio (58.1–68.3) and Sr (680–755ppm), and low Y (10.8–13.0ppm) abundance, suggesting adakitic affinities. Mamba adakitic granodiorites, gabbroic and dioritic enclaves exhibit homogeneous Sr isotopes ((87Sr/86Sr)i=0.7066–0.7067, 0.7073, and 0.7067, respectively) and Nd isotopes (εNd(t)=−5.7 to −4.4, −4.0, and −3.6, respectively). These geochemical features allowed us to conclude that the adakitic host granodiorites and mafic (gabbroic–dioritic) enclaves were derived from magma mixing between ancient thickened lower crust and enriched fluid-metasomatized mantle. The distance between Mamba and the suture zone was more than 200km when the intrusives emplaced at ~85Ma, which implies that these rocks cannot be resulted from the mid-ocean ridge subduction. Combining of the intra-plate environment indicated by the gabbroic enclave of this study, the presence of the coeval bimodal igneous rocks in the similar latitude in central Lhasa subterrane, and other records in late Cretaceous sedimentary basin, the Mamba ~85Ma magmatism were attributed to the back-arc extension of Neo-Tethyan Ocean.

Errata: 40Ar/39Ar ages of granitoids from the Truong Son fold belt and Kontum massif in Laos

Geochemical and geochronological studies were conducted on granitoids in central-southern Laos. S-type ilmenite-series granitoids associated with Sn mineralization are distributed in the Truong Son fold belt of central Laos. Plateau ages determined by 40Ar/39Ar incremental heating method for biotite and muscovite of the granitoids range from 253 Ma to 247 Ma and from 244 Ma to 199 Ma in the southwest and northeast of the fold belt, respectively. During this period, the fractionated granites in the early stage are related to mineralization of medium-large Sn deposits. The granitoids of the Truong Son fold belt formed in the Indosinian orogeny are contemporaneous with the Late Permian-Early Jurassic Sn-bearing granites in the Southeast Asian Sn Belt. In contrast, I-type magnetite-series granitoids are distributed in the Kontum massif of southeasternmost Laos. Plateau ages for biotite and previously reported mineral ages of the magnetite-series granitoids range widely from 414 Ma to 252 Ma. The Carboniferous-Permian granitoids represents adakitic characteristics and the Permian adakitic granodiorites are associated with Cu ± Au mineralized quartz veins.

40Ar/39Ar ages of granitoids from the Truong Son fold belt and Kontum massif in Laos

Geochemical and geochronological studies were conducted on granitoids in central-southern Laos. S-type ilmenite-series granitoids associated with Sn mineralization are distributed in the Truong Son fold belt of central Laos. Plateau ages determined by 40Ar/39Ar incremental heating method for biotite and muscovite of the granitoids range from 253 Ma to 247 Ma and from 244 Ma to 199 Ma in the southwest and northeast of the fold belt, respectively. During this period, the fractionated granites in the early stage are related to mineralization of medium-large Sn deposits. The granitoids of the Truong Son fold belt formed in the Indosinian orogeny are contemporaneous with the Late Permian-Early Jurassic Sn-bearing granites in the Southeast Asian Sn Belt. In contrast, I-type magnetite-series granitoids are distributed in the Kontum massif of southeasternmost Laos. Plateau ages for biotite and previously reported mineral ages of the magnetite-series granitoids range widely from 414 Ma to 252 Ma. The Carboniferous-Permian granitoids represents adakitic characteristics and the Permian adakitic granodiorites are associated with Cu ± Au mineralized quartz veins.

A pseudo adakite derived from partial melting of tonalitic to granodioritic crust, Kyushu, southwest Japan arc

Cretaceous peraluminous granitic intrusions occur in Kyushu in the southwest Japan arc as batholiths or stocks accompanied by metaluminous tonalite to granodiorite. The peraluminous intrusions are classed as adakitic based on Sr/Y–Y (ppm) relationships, but they contain abundant K-feldspar, a phase not common in typical adakites. To consider their significance in the arc system, we investigated the Tsutsugatake stock as representative of the intrusions exposed in central Kyushu. The Tsutsugatake rocks are mainly garnet-bearing two-mica leuco-granitoids. Their Sr/Y ratios and rare earth element concentrations are appropriate for adakite, but K 2O/Na 2O ratios (0.53–1.43), and Al 2O 3 (13.7–15.0 wt.%), MgO (0.20–0.55 wt.%), Sr (207–462 ppm), Cr (≤ 12 ppm), and Ni (≤ 8 ppm) contents differ from those of typical adakite. Geochemical modeling shows that the petrogenesis of the Tsutsugatake stock can be explained by partial melting of arc-type tonalite or adakitic granodiorite under middle to lower crustal conditions, leaving amphibole and plagioclase as residual phases. The differences between the Tsutsugatake stock and classical adakites lie in their source rocks and melting conditions. The Tsutsugatake source was not basaltic, and the melting pressure was insufficient to stabilize garnet. The Tsutsugatake rocks are here designated “pseudo adakites”. Such “pseudo adakites” may be found in both modern and ancient arcs, and would play an important role in crustal evolution as a member of the recycled constituent.

Zircon LA‐ICPMS U‐Pb Age, Sr‐Nd‐Pb Isotopic Compositions and Geochemistry of the Triassic Post‐collisional Wulong Adakitic Granodiorite in the South Qinling, Central China, and Its Petrogenesis

Abstract: The Indosinian post‐collisional Wulong pluton intruded into the Mesoproterozoic Fuping Group, South Qinling, central China. In the southern part of the pluton, some mafic enclaves have sharp or gradational contact relationships with the host biotite granodiorite. Geochemistry, zircon LA‐ICP MS (laser ablation inductively‐coupled plasma mass spectrometry) U‐Pb chronology and Sr‐Nd‐Pb isotope geochemistry of the pluton are reported in this paper. The biotite granodiorite shows close compositional similarities to high‐silica adakite. Its chondrite‐normalized REE patterns are characterized by strong HREE depletion (Yb = 0.33‐0.96 10−6 and Y = 4.77‐11.19 times 10−6), enrichment of Ba (775–1386 times10−6) and Sr (643–1115 × 10−6) and high Sr/Y (57.83–159.99) and Y/Yb (10.99‐14.32) ratios, as well as insignificant Eu anomalies (δEu = 0.70‐0.83), suggesting a feldspar‐poor, garnet±amphibole‐rich residual mineral assemblage. The mafic enclaves have higher MgO (4.15‐8.13%), Cr (14.79–371.31 times 10−6), Ni (20.00–224.24 times 10−6) and Nb/Ta (15.42‐21.91) than the host granodiorite, implying that they are mantle‐derived and might represent underplated mafic magma. Zircon LA‐ICP MS dating of the granodiorite yields a 206Pb/238U weighted mean age of 208±2 Ma (MSWD=0.50, 1s̀), which is the age of emplacement of the host biotite granodiorite. This age indicates that the Wulong pluton formed during the late‐orogenic or post‐collisional stage (≤242±21 Ma) of the South Qinling belt. The host biotite granodiorite displays 87Sr/86Sr = 0.7059‐0.7062, ISr = 0.7044‐0.7050, 143Nd/144Nd = 0.51236‐0.51238, εNd(t) = −2.26 to −2.66, 206Pb/204Pb = 18.099‐18.209, 207Pb/204Pb = 15.873‐15.979 and 208Pb/204Pb = 38.973‐39.430. Those ratios are similar to those of the Mesoproterozoic Yaolinghe Group in the South Qinling. Furthermore, its Nd isotopic model age (˜1.02 Ga) is consistent with the age (˜1.1 Ga) of the Yaolinghe Group. Based on the integrated geological and geochemical studies, coupled with previous studies, the authors suggest that the Wulong adakitic biotite granodiorite was probably generated by dehydration melting of the Yaolinghe Group‐like thickened mafic crust, triggered by underplating of mafic magma at the boundary of the thickened mafic crust and hot lithospheric mantle, and that the Wulong adakitic biotite granodiorite may have resulted from thinning and delamination of the lower crust or breakoff of the subducting slab of the Mianlue ocean during the Indosinian post‐collisional orogenic stage of the Qinling orogenic belt.