Partial Melting Of Oceanic Crust Research Articles

Petrogenesis of the late Neoproterozoic Carmo stock, northeastern Brazil: Implications for partial melting of oceanic crust and sediments in a syn-collisional setting

Granitic magmatism is undoubtedly the main geological feature of the Brasiliano Orogeny in the Borborema Province, northeastern Brazil. However, the petrogenesis and tectonic setting of the oldest magmatic pulse (640–620 Ma; Conceição-type granites) in the Central Subprovince is still hotly debated. The Carmo stock consists of calc-alkalic magmatic epidote-bearing Conceição-type porphyritic granodiorite and monzogranites that carry abundant mafic microgranular enclaves (MMEs). LA-ICP-MS zircon U–Pb data indicate crystallization around 615 Ma for these granitoids. Granitoids and MMEs exhibit similar chemical characteristics, both of which show high-K calc-alkalic, metaluminous and magnesian compositions. All studied rocks are enriched in large-ion lithophile elements (LILE) and light rare-earth elements (LREE), and depleted in high-field-strength elements (HFSE) and heavy rare-earth elements (HREE). Host granitoids are characterized by high initial 87Sr/86Sr ratios (0.707863–0.709677), weakly negative ε Nd(t) values (−2.1 to −3.1), and TDM model ages of 1.31–1.36 Ga. These geochemical and isotopic signatures, together with the low Ba/Th and Sm/La ratios and high La/Sm and Th/La ratios, suggest that granodiorites and monzogranites were generated from the partial melting of altered oceanic crust and subducted sediments. The MMEs have Sr–Nd isotopic compositions almost indistinguishable from their host granitoids, with initial 87Sr/86Sr ratios of 0.708895, ε Nd(t) values of −2.8 and TDM model ages of 1.40 Ga. The similarity in isotopic compositions and the petrographic evidence indicate that magma mixing and mingling played an important role in the formation of MMEs. Based on the new geochemical and isotopic data, together with regional geological features, we infer that the Carmo stock rocks, as well as Conceição-type granites, were most likely formed in a syn-collisional tectonic setting following the early break-off of a relatively weak oceanic slab.

Possible partial melting and production of felsic melt in a Jurassic oceanic plateau of the Izanagi Plate: Insights from 159 Ma plagiogranites from northern Japan

ABSTRACT A small tonalite – dacite body has been discovered in the Nakanogawa Group, Hidaka Belt, northern Japan, which is a Palaeogene subduction complex formed in the palaeo-Japan trench along the northeastern margin of Eurasia. The tonalites are characterized by extremely low K2O (0.2–0.3 wt.%) and relatively flat chondrite-normalized rare earth element patterns (La/Yb[N] ~2.3), similar to plagiogranites in ophiolites. The major-and trace-element characteristics are consistent with low-pressure partial melting of oceanic crust outside the garnet stability field. Zircon U – Pb dating of the tonalite yielded a weighted-mean 206Pb/238U age of 159.1 ± 1.6 Ma. Late Jurassic oceanic plateau-type basaltic rocks with a small amount of plagiogranitic rocks are distributed sporadically in subduction complexes along the palaeo-Japan arc – trench system. A similar zircon U – Pb age (151.6 ± 1.8 Ma) has been reported for greenstone in the Cretaceous subduction complex along the palaeo-Kuril arc – trench system. The tonalite – dacite block was probably derived from the subduction complex at the junction of the palaeo-Japan and -Kuril arc – trench systems. Thus, the greenstones and the tonalite body were likely part of a large oceanic plateau, which formed at the Izanagi – Pacific–Farallon ridge triple junction during the Late Jurassic – Early Cretaceous.

Contrasting tectonic regimes between late Jurassic and Early Cretaceous porphyry-epithermal Cu-Mo-Au mineralization in NE China: A perspective from the petrogenesis of the adakitic rocks in the Sishanlinchang porphyry Cu-Mo deposit

The Mongol–Okhotsk and Paleo-Pacific tectonic regimes both have an influence in NE China during the late Mesozoic. The Sishanlinchang and several newly reported late Mesozoic porphyry Cu–Mo deposits in NE China are typical products of this interaction. A systematic study of these deposits provides new insights into the genesis of porphyry Cu–Mo systems in the region. Here we present petrological, geochronological, whole–rock geochemical and isotopic data from the ore-forming rocks in the Sishanlinchang deposit, together with a compilation of data from late Mesozoic porphyry Cu–Mo deposits in NE China, to reveal their magma source, and corresponding geodynamic settings. Zircon U–Pb dating shows that the studied granites were emplaced at 111 Ma, which contrasts with previously reported occurrences of late Jurassic porphyry Cu–Mo deposits in the northern NE China, indicating at least two episodes of porphyry Cu–Mo mineralization during the late Mesozoic. Whole-rock geochemical data show that all of the late Mesozoic ore-forming granites have low Y and Yb, high Sr contents, and high Sr/Y ratios (35.70–71.59), which exhibit an adakitic affinity. High Na2O, Cr, and Ni contents, and Mg# (47–58) values, a low proportion of garnet in the source, positive whole-rock εNd(t) values (1.21–2.27) and pronounced positive zircon εHf(t) values (6.85–9.37) indicate that the adakitic rocks were derived from partial melting of oceanic crust with assimilation of enriched mantle materials. Combined with our studies and previous studies, we conclude that the Early Cretaceous porphyry–epithermal Cu–Mo–Au deposits in the eastern NE China were controlled by the Paleo-Pacific tectonic regime, which is different from the Late Jurassic Cu–Mo deposits in the northern NE China controlled by the Mongol–Okhotsk tectonic regime.

Melting of phengite-bearing eclogite at pressures of 4 and 9 GPa relevant to deep regions of a subduction zone

Melting experiments undertaken with finely ground powder of phengite-bearing eclogite yielded solidus temperatures of about 970°C at 4 GPa and 1250°C at 9 GPa. Additional experiments with a rock powder of psammopelitic composition established a solidus at 9 GPa at a temperature of 1350°C. Initial melts produced from both rocks are rich in potassium. The melts generated from eclogite tend to become richer in Na and Ca with rising temperature due to increasing decomposition of clinopyroxene. At the maximum temperatures of the experiments with eclogite, up to 450°C above the solidus at 4 GPa, this phase is still present in the restite together with abundant garnet. In the temperature interval of 1100-1300°C, when 22-30% of the studied eclogite was melted, the melts are quartz monzonitic in composition. According to the reported experimental results, we suggest that partial melting of oceanic crust is unlikely in a subduction zone. However, ascending melange diapirs, composed of material from the upper portion of a deep-seated subducted oceanic slab, can partially melt in the hot mantle wedge. The thus generated melts further ascend to contribute to lavas of magmatic arc systems.

Geochronology and geochemistry of an early Silurian plagiogranite from Xiarihamu area, East Kunlun, northwest China: implications for Proto-Tethyan evolution

Oceanic plagiogranites are directly or indirectly related to ophiolites. A small plagiogranite vein has been discovered in the No. III mafic–ultramafic complex of the Xiarihamu ophiolite from the eastern margin of Qiman Tagh, East Kunlun Orogen, northwest China. This plagiogranite yielded a weighted mean zircon U–Pb age of 435.7 ± 4.3 Ma (MSWD = 0.073) and is characterized by high SiO2 (73.08–76.24 wt.%) and Na2O (6.42–8.34 wt.%) contents and high Na2O/K2O ratios (3.06–12.36) with metaluminous and calc-alkaline affinity. It is enriched in Th and U, depleted in Nb, Sr, Ti, and Ba, and has low ΣREE contents (7.64–11.35 ppm), negative Eu anomalies (δEu = 0.31–0.71), and light REE enrichment. These features are typical of oceanic plagiogranites. Whole-rock Sr–Nd isotopic data for the plagiogranite show obvious EMII mantle characteristics. The petrographic, geochemical, and Sr–Nd isotopic characteristics suggest the plagiogranite is part of an ophiolite formed by partial melting of oceanic crust in the presence of hydrothermal fluids. Given this, other parts of the Xiarihamu ophiolite and other ophiolites in Qiman Tagh area, we propose the plagiogranite formed in a back-arc basin setting in the early Silurian, probably the youngest known intrusion related to the Proto-Tethys Ocean.

Zircon U–Pb geochronology and geochemistry of plagiogranites within a Paleozoic oceanic arc, the Erlangping unit of the Qinling accretionary orogenic belt: Petrogenesis and geological implications

Plagiogranites are a volumetrically minor component of oceanic crust and ophiolites, but offer an opportunity to probe the processes and mechanisms by which felsic crusts were produced from the mafic oceanic crusts. The Qinling-Tongbai orogenic belt in central China is a typical composite orogenic belt that records the amalgamation processes between the North China Block and the South China Block. The Erlangping unit represents an accretionary oceanic unit in the Qinling-Tonbai orogenic belt and contains minor plagiogranites, which have received little attention. In this study, we carried out an integrated study of U–Pb geochronology and Hf–O isotopes in zircon, as well as whole-rock isotope and geochemical analyses for felsic rocks in the Erlangping unit. These rocks occur as dikes to stocks intruding ambient basalts. They have relatively high Na2O and Al2O3, moderate MgO and low K2O contents, and are classified as oceanic plagiogranites. Zircon U–Pb geochronology yielded formation ages of ca. 465 Ma. Zircon δ18O compositions (4.4–5.1‰) are lower than or similar to those of zircons from normal mantle. The low TiO2 content, elevated Zr/Hf, and light rare-earth element (LREE) enrichments in these rocks indicate that these plagiogranites were generated by partial melting of hydrothermally altered oceanic crust. These samples have more depleted whole-rock εNd(t) (+4.4 to +5.1) and zircon εHf(t) (+10.5 to +12.8) values than the basaltic wall-rocks, implying they were derived from a newly accreted oceanic arc. Taking into account the tectonic framework of the broader Qinling Orogenic belt, a subduction-induced magma flare-up in the Ordovician could have triggered the anatexis of the existing Erlangping oceanic arc, and in turn generated these plagiogranites. Partial melting of oceanic crust that has been enriched in a subduction system may be an important mechanism for the formation and maturation of continental crust in oceanic arc settings.

Subduction of the Izanagi-Pacific Ridge–transform intersection at the northeastern end of the Eurasian plate

Recent reconstructions of global plate motions suggest that the Izanagi-Pacific Ridge was subducted along the eastern margin of Eurasia at ca. 50 Ma. In the Hidaka magmatic zone (HMZ), which was located at the northeastern end of the Eurasian plate, three magmatic pulses occurred (46–45, 40–36, and 19–18 Ma). We report whole-rock geochemical and Sr-Nd-Pb isotopic data for 36 Ma high-Sr/Y (adakitic) rocks from the HMZ and show that these rocks formed by partial melting of oceanic crust and were emplaced as near-trench intrusions during ridge subduction. We reevaluate the nature of plutonic rocks in the HMZ and show that both the 46–45 and 40–36 Ma granitoids have essentially identical geochemical features. The distribution of plutons and magmatic cessation between 45 and 40 Ma are best explained by subduction of a ridge-transform intersection with a large offset of the ridge axis. The boundary between the Eocene granitoids corresponds to the position of a paleo–transform fault, and adakitic magmatism was caused by partial melting triggered by slab tearing at an overlapping spreading center. The paleoridge-transform configuration coincides with the locations of later large faults and a peridotite body.

Petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites of the Songnen Block, NE China: Implications for the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans

The petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites in NE China offers new insights into the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans. This study reports new LA–ICP–MS zircon UPb ages, geochemical data, and zircon Hf isotopic compositions of syenogranites and quartz diorite porphyries exposed on the northern margin of the Songnen Block, NE China. Zircon UPb age data indicate that the syenogranites and quartz diorite porphyries were emplaced at ca. 319 and 120 Ma, respectively. The syenogranites have high K2O contents of 4.15–4.79 wt% and (K2O + Na2O)/CaO ratios of 33–102, and low MgO (0.06–0.20 wt%) and P2O5 (0.01–0.02 wt%) contents. Their remarkably high HREE contents and Y/Nb ratios (2.07–2.40), significantly negative Eu anomalies (Eu/Eu⁎ = 0.15–0.28), and low Sr contents (29.7–85.8 ppm) indicate that the syenogranites are A-type granites. They have positive zircon εHf(t) values of 8.4–12.1 and Neoproterozoic Hf TDM2 ages of 795–561 Ma, suggesting a juvenile lower crustal source. The quartz diorite porphyries have relatively high Na2O and MgO, and moderate K2O contents. Their A/CNK values (0.95–1.18) indicate a transition from being metaluminous to weakly peraluminous. They are enriched in LILEs (e.g., Rb, K, and Ba) and depleted in HFSEs (e.g., Nb, Ta, Ti, and P). Their low Y (11.1–14.4 ppm) and Yb (0.98–1.34 ppm) contents and high Sr (589–1131 ppm), Cr, and Ni contents and Sr/Y ratios indicate that they were generated by partial melting of oceanic crust. Based on these data and regional geological investigations, we propose that the syenogranites on the northern margin of the Songnen Block were formed in a post-collision extensional setting, likely due to closure of the Paleo-Asian Ocean between the Xing'an and Songnen blocks. Formation of the Early Cretaceous quartz diorite porphyries may have been triggered by partial melting of the Paleo-Pacific flat slab. Apart from rollback of the Paleo-Pacific oceanic slab, the main reason for the eastward migration of magmatism during the Cretaceous was the trench-ward dehydration, eclogitization, and sinking of the residual Paleo-Pacific flat 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.

Partial melting of subduction zones

俯冲带是地幔对流环的下沉翼，是地球内部的重要物理与化学系统。俯冲带具有比周围地幔更低的温度，因此，一般认为俯冲板片并不会发生部分熔融，而是脱水导致上覆地幔楔发生部分熔融。但是，也有研究认为，在水化的洋壳俯冲过程中可以发生部分熔融。特别是在下列情况下，俯冲洋壳的部分熔融是俯冲带岩浆作用的重要方式。年轻的大洋岩石圈发生低角度缓慢俯冲时，洋壳物质可以发生饱和水或脱水熔融，基性岩部分熔融形成埃达克岩。太古代的俯冲带很可能具有与年轻大洋岩石圈俯冲带类似的热结构，俯冲的洋壳板片部分熔融可以形成英云闪长岩-奥长花岗岩-花岗闪长岩。平俯冲大洋高原中的基性岩可以发生部分熔融产生埃达克岩。扩张洋中脊俯冲可以导致板片窗边缘的洋壳部分熔融形成埃达克岩。与俯冲洋壳相比，俯冲的大陆地壳具有很低的水含量，较难发生部分熔融，但在超高压变质陆壳岩石的折返过程中可以经历广泛的脱水熔融。超高压变质岩在地幔深部熔融形成的熔体与地幔相互作用是碰撞造山带富钾岩浆岩的可能成因机制。碰撞造山带的加厚下地壳可经历长期的高温与高压变质和脱水熔融，形成S型花岗岩和埃达克质岩石。;Subduction zones are descending limbs of mantle convection cells, and the major physical and chemical system of Earth's interior. Subduction zones have lower temperature than the adjacent mantle, and therefore it is proposed that subducted materials including oceanic crust and continental crust do not undergo partial melting, whereas dehydration of the subducted oceanic crust and underlying mantle causes partial melting of the overriding mantle wedge to generate arc magmatic rocks. However, some studies concluded that the hydrated oceanic crust could be partly molten during the subduction. In particular, partial melting of subducted oceanic slabs is probably main mechanism of arc magmatism in the following circumstances. The water-saturated and dehydration melting of subducted oceanic materials can generate the adakitic rocks during the shallow and slow subduction of the younger oceanic lithosphere. The Archean subduction zones have similar features to those of the younger oceanic lithosphere, and therefore the partial melting of the subducted Archean oceanic crust resulted in the formation of tonalite-trondhjemite-granodiorite (TTG). The partial melting of mafic rocks in the shallowly subducted oceanic plateau generated the adakitic rocks. The partial melting of oceanic crust near the margins of slab window during the subduction of spreading mid-oceanic ridge produced the adakitic rocks. Contrary to the subducted oceanic crust, partial melting of deeply subducted continental crusts should be rare due to very low water content. However, the ultrahigh-pressure rocks underwent intensive dehydration melting during the exhumation from the mantle to crust. Interaction between the melt derived from the ultrahigh-pressure metamorphic rocks and the mantle is a potential formation mechanism of the potassic or ultrapotassic magmatic rocks in the collisional orogens. The thickened lower crust of the collisional orogens experienced prolonged high-temperature and high-pressure metamorphism and associated partial melting, and in turn resulted in the formation of S-type granitoids and adakitic rocks.

Petrogenesis and geodynamic evolution of Neoproterozoic Abu Dabbab Albite Granite, Central Eastern Desert of Egypt: Petrological and geochemical constraints

Gebel Abu Dabbab, which is located at 50 km NW of Marsa Alam city, represents an unique and specialized stock-like felsic intrusion of rare metals and Sn bearing albite granite. This granitic stock was emplaced along two intersected major faults and/or tensional shear zones trending NE and NW-SE relevant to the Najd Fault system. Petrographically, the albite granite is composed mainly of albite, quartz and microcline-microperthite and muscovite, in association with accessories; cassiterite, tantalite-columbite, zircon, allanite and apatite. The studied albite granite is enriched in Na2O, HFSEs, Ta and Nb, but it is poor in MgO, CaO, Fe2O3, TiO2, Ba, Sr, rare earth elements and radioelements (U, Th). Geodynamic modelling reveals that Abu Dabbab albite granite was generated as a result of partial melting of oceanic crust due to the interaction of primitive basaltic melts-derived from the mantle source. The resultant hybride melts were subsequently fractionated to tonalitic melts giving rise to the A1-subtype granite, which was emplaced within the intraplate environment at the later phase of the Pan African Orogeny. Moreover, the fractional crystallization and reworking of plagioclase were suffered from alkali mtasomatism after extraction of alkali feldspar granite melts.

Early Carboniferous adakite-like and I-type granites in central Qiangtang, northern Tibet: Implications for intra-oceanic subduction and back-arc basin formation within the Paleo-Tethys Ocean

Recent studies have proposed that the Late Devonian ophiolites in the central Qiangtang region of northern Tibet were formed in an oceanic back-arc basin setting, which has led to controversy over the subduction setting of the Longmucuo–Shuanghu–Lancangjiang Suture Zone (LSLSZ) during the Late Devonian to Early Carboniferous. In this paper we present new data about a suite of granite plutons that intrude into ophiolite in central Qiangtang. Our aim was to identify the type of subduction and to clarify the existence of an intra-oceanic back-arc basin in the LSLSZ during the Late Devonian to Early Carboniferous. The suite of granites consists of monzogranites, syenogranites, and granodiorites. Our laser ablation–inductively coupled plasma–mass spectrometry zircon U–Pb data yielded Early Carboniferous crystallization ages of 357.2Ma, 357.4Ma and 351.1Ma. We subsequently investigated the petrogenesis and tectonic setting of these granites based on their geochemical and Hf isotopic characteristics. First, we divided the granites into high Sr/Y (HSG) and low Sr/Y granites (LSG). The HSG group contains monzogranites and granodiorites that have similar geochemical characteristics to adakites (i.e., high Sr/Y and La/Yb ratios, low MgO, Y, and Yb contents, and no pronounced negative Eu anomaly), although they have slightly lower Sr and Al2O3 contents, caused by crystal fractionation during late magmatic evolution. Therefore, we define the HSG group as adakite-like granites. The study of the HSG shows that they are adakite-like granites formed by partial melting of oceanic crust and experience fractional crystallization process during late evolution. However, some differences between the monzogranites and granodiorites indicate that there are varying degree contributions of subducted sediments during diagenesis. The LSG group contains syenogranites that have distinct negative correlations between their P2O5 and SiO2 contents, and Y and Th contents have significant positive correlations with Rb. The above characteristics indicate that the syenogranites are typical I-type granites. The results of this study also show that the LSG were produced by magma mixing between the mantle and juvenile oceanic crust. The field study found that the Early Carboniferous suite of granites intruded into contemporaneous ophiolites that formed in an intra-oceanic back-arc basin, and were associated with coeval A-type granites in this region. Based on the geochemical and isotopic data presented in this paper and regional geological data, we consider that the HSG were formed during intra-oceanic subduction of the Paleo-Tethys Ocean in the Early Carboniferous. The LSG and A-type granites were formed in an intra-oceanic back-arc basin setting caused by roll-back of the Paleo-Tethys Ocean slab. This confirms that the subduction of the Paleo-Tethys Ocean in the Early Carboniferous was intra-oceanic subduction, and provides important evidence for the existence of an intra-oceanic back-arc basin during the Late Devonian to Early Carboniferous.

Hot subduction in the middle Jurassic and partial melting of oceanic crust in Chilean Patagonia

Rare remnants of a Mesozoic subduction high pressure (HP) accretionary complex are exposed on Diego de Almagro Island in Chilean Patagonia. We herein focus on the Lazaro unit, a coherent slice of oceanic crust exposed on this island that has been first affected by high temperature (HT) metamorphism followed by a lower temperature deformation event (LT). Its Pressure-Temperature-time (P-T-t) evolution is reconstructed using field and petrographic observations, phase relations, thermobarometry and geochronology. Remnants of a primary amphibolite to HP granulite-facies event in mafic rocks comprising garnet (with ilmenite exsolutions), diopside, trondhjemitic melt, pargasite, plagioclase±epidote are reported for the first time in neosomes, indicating peak P-T conditions of 1.1–1.3GPa and c. 750°C. This peak T paragenesis has been thoroughly overprinted by a phengite-chlorite-actinolite assemblage during isobaric cooling down to c. 450°C. U-Pb dating of zircon metamorphic rims from a metasedimentary rock yielded a homogeneous age population of 162±2Ma for the HT event. Sm-Nd dating of two peritectic garnet-bearing samples yield ages of 163±2Ma and 163±18Ma for the HT event. Multi-mineral Rb-Sr dating of a metasedimentary rock overprinted by LT deformation suggests retrograde shearing between 120 and 80Ma. Our results show that the HT event in the Lazaro unit took place at around 160–165Ma, shortly before the onset of Patagonian Batholith emplacement. Partial melting of subducted oceanic crust reported in the Lazaro unit is related to the early stages of hot subduction along the Gondwana western margin. The Lazaro unit remained at c. 40km depth along the subduction interface for >80Ma, recording the deformation and long-term cooling of the subduction channel environment until the upper Cretaceous.

Petrogenesis of Neoarchean TTG rocks in the Yangtze Craton and its implication for the formation of Archean TTGs

The Yangtze Craton is one of the most important cratonic blocks in eastern Asia. However, its early evolution has not been well constrained yet, because of the poor exposure of Archean basement rocks. In this contribution, a combined study of zircon U–Pb age, Hf–O isotope compositions, as well as whole rock element compositions was carried out for gray gneiss rocks from the Douling complex in the northwestern part of the Yangtze Craton. Zircon U–Pb dating for two gneiss samples yielded weighted mean ages of 2509±21 and 2496±15Ma. The gray gneisses have high SiO2 (63.64–73.74%), Na2O (3.66–6.21%), but low K2O (0.76–2.93%) contents, and belong to the trondhjemite, tonalite and granodiorite series rocks. They are characterized by enrichments of LILEs and LREEs, but notable depletions in Nb–Ta and Ti, as well as heavy REEs and Y. All the samples plot into the fields of typical Archean TTG rocks in the (La/Yb)N vs. (Yb)N and Sr/Y vs. Y diagrams. Therefore, the gray gneisses in the Douling complex are Neoarchean TTG rocks. The relatively high Nb/Ta and Zr/Sm ratios of 19.6–46.0 and 14.5–143.4 suggest their formation under eclogite-facies conditions. They have low ɛHf(t) values of −3.21 and −3.39 and Hf model ages of 3194±26 and 3205±24Ma, indicating their derivation from ca. 3.2Ga juvenile mafic rocks. Their δ18O values of 6.41±0.12 and 5.44±0.12‰ suggest that surface rocks were differently recycled into their sources. Collectively, the TTG rocks in the Douling complex were generated by partial melting of thickened mafic crust rather than direct partial melting of oceanic crust in a convergent plate boundary. A compilation of available zircon U–Pb and Hf isotope compositions of the Yangtze Craton reveals that there are quite differences of Hf isotope compositions in Archean. The ca. 3.8–3.4Ga zircons exhibit ɛHf(t) values of −5.8 to 0.0. These chondritic to slightly enriched hafnium isotope compositions imply that their host rocks were formed by protracted intra-crustal reworking of ancient protoliths without juvenile input. By contrast, zircons with ages younger than ca. 3.2Ga register both negative and positive ɛHf(t) values up to the depleted mantle, providing clear evidence for the addition of juvenile materials. These imply that the subduction-driven plate tectonics might have been operated since ca. 3.2Ga in the Yangtze Craton.

Effect of time-evolving age and convergence rate of the subducting plate on the Cenozoic adakites and boninites

Partial melting of subducting oceanic crust expressed as high-Mg volcanic rocks such as adakites and boninites has been actively studied for decades, and Lee and King (2010) reported that time-evolving subduction parameters such as the age and the subduction rate of the converging oceanic plate play important roles in transient partial melting of the subducting oceanic crust (e.g., Aleutians). However, few subduction model experiments have considered time-evolving subduction parameters, posing problems for studies of transient partial melting of subducting oceanic crust in many subduction zones. Therefore, we constructed two-dimensional kinematic–dynamic subduction models for the Izu–Bonin, Mariana, Northeast Japan, Kuril, Tonga, Java–Sunda, and Aleutian subduction zones that account for the last 50Myr of their evolution. The models include the time-evolving age and convergence rate of the incoming oceanic plate, so the effect of time-evolving subduction parameters on transient partial melting of oceanic crust can be evaluated. Our model calculations revealed that adakites and boninites in the Izu–Bonin and Aleutian subduction zones resulted from transient partial melting of oceanic crust. However, the steady-state subduction model using current subduction parameters did not produce any partial melting of oceanic crust in the aforementioned subduction zones, indicating that time-evolving subduction parameters are crucial for modeling transient eruption of adakites and boninites. Our model calculations confirm that other geological processes such as forearc extension, back-arc opening, mantle plumes and ridge subduction are required for partial melting of the oceanic crust in the Mariana, Northeast Japan, Tonga, and southeastern Java–Sunda subduction zones.

Recycling of oceanic crust and the origin of intraplate volcanism

Source models for intraplate volcanism (IPV) include vertical introduction of material from deep in the mantle (plume model), contamination of the shallow mantle (perisphere and continental mantle delamination models) and derivation by selective partial melting of oceanic crust recycled into the depleted mantle (SUMA/streaky mantle models). The plume hypothesis became the ruling model after a flawed interpretation of helium isotope data in the mid 1980s that led to plumes being imposed on models for crustal recycling into the depleted mantle. This incorporation of otherwise competing concepts, is the cause of unnecessary complexity in modern geodynamic models. The plume model cannot explain all manifestations of IPV and a comprehensive explanation can only be found by invoking the alternative options, combined with their tapping by plate tectonic processes.

Geochemical characteristics and origin of the HIMU reservoir: A possible mantle plume source in the lower mantle

Combined Pb-Sr-Nd-Hf-Os isotopes, together with major and trace element compositions, were determined from clinopyroxene and olivine phenocrysts, along with whole rocks, for ocean island basalts with high μ (μ = 238U/204Pb) (HIMU) and enriched mantle isotopic characteristics from Cook-Austral Islands. Clinopyroxene and olivine separates record reliable isotopic information of the sources because of minimized in situ radiogenic ingrowth and their lower susceptibility to crustal contamination. Coherent isotopic systematics in multi-isotope spaces defined by the HIMU samples are best explained by recent mixing of melts derived from the HIMU reservoir and the local shallow mantle. The isotopic compositions of the HIMU reservoir are constrained to be low ɛNd (≤+4), low ɛHf (≤+3), and moderately radiogenic 187Os/188Os (0.14–0.15) in association with radiogenic Pb isotopes (206Pb/204Pb ≥ 21.5). Since ancient oceanic crust would have had exceptionally radiogenic 187Os/188Os, moderately high 187Os/188Os precludes recycled oceanic crust as the only contributor to the HIMU reservoir. Instead, mantle metasomatized with partial melts from subducted oceanic crust is a likely candidate for the HIMU reservoir. Moreover, partial melting of oceanic crust in equilibrium with Mg perovskite would fractionate U/Pb, Sm/Nd, and Lu/Hf, which are in accordance with the time-integrated U/Pb, Sm/Nd, and Lu/Hf deduced from Pb, Nd, and Hf isotopic compositions of the HIMU reservoir, respectively, with a formation age of 2–3 Ga. We thus propose that the HIMU reservoir was formed by hybridization of a subducted oceanic crust-derived melt with the ambient mantle and then stored for several billion years in the lower mantle.

Chronology and Geochemistry of the Nadingcuo Volcanic Rocks in the Southern Qiangtang Region of the Tibetan Plateau: Partial Melting of Remnant Ocean Crust along the Bangong‐Nujiang Suture

Abstract:The Nadingcuo high‐K calc‐alkaline rocks mainly composed of trachyte and trachyandesite are the largest outcrop area of volcanic rocks in southern Qiangtang terrane in the Tibetan plateau. However, their exact source and peterogenesis are still debated. 40Ar‐39Ar and LAM‐ICPMS zircon U‐Pb isotopic dating confirm that these rocks erupted in Eocene. In addition, the Nadingcuo volcanic rocks are characterized by high Sr/Y content ratios, similar with the adakite derived from partial melting of oceanic crust. They can be further classified as high Mg# (Mg#= 48–57) and low Mg# (Mg#= 33–42) subtypes. The Nadingcuo adakitic rocks have relatively low (87Sr/86Sr)i and high εNd(t), showing a trend of similarity to the Dongcuo ophiolite present in the Bangong‐Nujiang oceanic crust. Simple modeling indicates that the Nadingcuo adakitic rocks are a mix resulting from the basalt of Bangong‐Nujiang Ocean with 10%–20% crustal material of Lhasa terrane. On these bases we suggest that the low Mg# Nadingcuo adakitic rocks are the product of partial melting of remnant oceanic crust with small sediment, and the high Mg# rocks are the result of reaction between rising melt of remnant oceanic crust with subducted sediment and mantle wedge. Therefore, the origin of Nadingcuo adakitic rocks may be related to intracontinental subduction triggered by collision of India‐Asia during Cenozoic.

Origin of TTG-like rocks from anatexis of ancient lower crust: Geochemical evidence from Neoproterozoic granitoids in South China

TTG rocks are commonly assumed to form by partial melting of oceanic crust during slab subduction or anatexis of thickened lower crust. The two models are tested by an integrated study of geochronology and geochemistry for a composite batholith of Neoproterozoic granitoids (mainly TTG-like) from the Yangtze Gorge in South China. Zircon U–Pb dating indicates that all granitoids crystallized in a period of 820 to 800 Ma. Zircon δ18O values are 4.85 to 6.84‰, suggesting limited contributions from supracrustal materials to magma sources. Zircon εHf(t) values for syn-magmatic domains range from −3.6 to −29.7, concordant with whole-rock εNd(t) values of −2.1 to −21.3. Correspondingly, zircon Hf model ages are 2.09 to 3.16 Ga, suggesting their derivation from ancient crust of Paleoproterozoic to Archean ages. TTG-like rocks contain inherited zircon cores that yield two groups of U–Pb ages at Paleoproterozoic (1.8 to 2.0 Ga) and Archean (~2.9 Ga), respectively. They show variable εHf(t) values from −48.4 to −33.4. Because the Neoproterozoic magmatism leads to variable increases of εHf(t) values for some inherited cores, only the lowest εHf(t) values for the inherited cores are assumed to represent the original Hf isotopes in source rocks. The screened results are consistent with those for Archean TTG gneiss and migmatite in this region. In addition, the TTG-like rocks have lower εNd(t) and εHf(t) values, higher Sr/Y and (La/Yb)N ratios and stronger depletion in Nb, Ta and Ti than the other granitoids. Two alternative scenarios are proposed to account for their petrogenesis: (1) anatexis of Archean and Paleoproterozoic mafic crust in the stable depths of garnet and amphibole, respectively; or (2) anatexis of Archean TTG and Paleoproterozoic mafic rocks in amphibole-stable depths. Nevertheless, the second one is preferred because the inherited zircons with Paleoproterozoic to Archean U–Pb ages are preserved in the TTG-like rocks, which is incompatible with high temperatures to generate the primary TTG magmas. Therefore, the Neoproterozoic TTG-like rocks were derived from the anatexis of ancient non-thickened lower crust, with the TTG features inheriting from their source.

Dacitic ocelli in mafic lavas, 3.8–3.7 Ga Isua greenstone belt, West Greenland: Geochemical evidence for partial melting of oceanic crust and magma mixing

Mafic volcanic rocks in the 3.8–3.7 Ga Isua greenstone belt, southern West Greenland, contain randomly distributed 1 to 10-centimeters long white spheroidal structures. In this study, these structures are called ‘ocelli’. In the western part of the belt, ocelli-bearing lavas are enclosed in basaltic to picritic flows (MgO = 9–21 wt.%) with a subduction zone geochemical signature. The ocelli are composed predominantly of polycrystalline Na-plagioclase and quartz, with minor hornblende and biotite, whereas the surrounding amphibolite matrix (basaltic host) is composed mainly of hornblende, Ca-plagioclase, and quartz. The ocelli are devoid of radial or concentric internal structure, and display all stages of coalescence. Contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational. Compositionally, the ocelli are calc-alkaline dacites (SiO 2 = 62.9–72.0 wt.%; MgO = 0.60–3.50 wt.%; Ni = 58–143 ppm; Cr = 250–510 ppm), whereas the surrounding matrix is tholeiitic basalt (SiO 2 = 46.6–50.6 wt.%; MgO = 8.70–12.30 wt.%; Ni = 119–175 ppm; Cr = 330–600 ppm). In terms of major element composition, the Isua ocelli closely resemble plagiogranites in Phanerozoic supra-subduction zone ophiolites. Field and petrographic observations, and geochemical data (SiO 2 = 54.2–60.7 wt.%; MgO = 3.95–7.72 wt.%; Ni = 127–158 ppm; Cr = 500–570 ppm) on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts. On a chondrite-normalized diagram, the basaltic host is characterized by variably depleted LREE patterns (La/Sm cn = 0.30–0.94; Gd/Yb cn = 1.03–1.45), whereas the dacitic ocelli display LREE-enriched patterns (La/Sm cn = 1.30–2.60; Gd/Yb cn = 1.32–2.58). The strongly depleted REE patterns in the basaltic host are attributed to LREE loss during carbonate alteration. Partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host. The geochemical characteristic of the ocelli cannot be explained by post-magmatic alteration, slab melting, fractional crystallization of tholeiitic melts, or liquid immiscibility. We suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust (high-Mg volcanic rocks) that fell into the magma chamber, suggesting magma–crust interaction in the early Earth. Formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early Archean.