Origin of some calc-alkalic andesites in the Japanese Islands

Origin of calc-alkalic andesite in the Japanese Islands is reviewed on the basis of the recent trace element data and new experimental results. It is suggested that calc-alkalic andesites in the Japanese Islands have at least four different origins; (1) fractional crystallization with separation of magnetite of high-alumina basalt magma, (2) partial melting of hydrous upper mantle peridotite (for magnesian andesite), (3) fractional crystallization with separation of olivine and/or orthopyroxene of magnesian andesite magma and (4) mixing of dacitic and basaltic magmas. Emphasis is placed on the possible generation of primary magnesian calc-alkalic andesite magmas by direct partial melting of the upper mantle peridotite under hydrous conditions at depths between 40 and 60 km.     

Origin of high-magnesian andesites in the Setouchi volcanic belt,southwest Japan,II. Melting phase relations at high pressures

Melting phase relations of an augite-olivine high-magnesian andesite and an augite-olivine basalt from the Miocene Setouchi volcanic belt in southwest Japan have been studied under water-saturated, water-undersaturated and under anhydrous conditions. Both the andesite and the basalt are characterized by low FeO*/MgO ratios (0.86 and 0.76 in weight, respectively) and qualify as primary magmas derived from the upper mantle.The andesite melt coexists with olivine, orthopyroxene and clinopyroxene at 15 kbar and 1030°C under water-saturated conditions, and at 10 kbar and 1070°C under water-undersaturated conditions (7 wt.% H₂O in the melt). The basalt-melt also coexists with the above three phases at 11 kbar and 1305°C under anhydrous conditions, and at 15 kbar and 1205°C in the presence of 4 wt.% water.Present studies indicate that high-magnesian andesite magmas may be produced even under water-undersaturated conditions by partial melting of mantle peridotite. It is suggested that two types of high-magnesian andesites in the Setouchi volcanic belt (augite-olivine and bronzite-olivine andesites) were produced by different degrees of partial melting; augite-olivine andesite magmas, whose mantle residual is lherzolite, were formed by lower degrees of partial melting than bronzite-olivine andesite magmas, which coexist with harzburgite. The basalt magmas, which were often extruded in close proximity to the high-magnesian andesite magmas, are not partial melting products of a mantle peridotite which had previously melted to yield high-magnesian andesite magmas.     

Aleutian magnesian andesites: Melts from subducted Pacific ocean crust

Several diagnostic chemical characteristics of an uncommon Aleutian magma type support a proposed origin that involves a small amount of partial melting of subducted Pacific ocean crust (basalt) consisting mainly of garnet and clinopyroxene (eclogite or garnet websterite). Among the characteristics are high La/Yb ratios and Sr contents and low ratios of radiogenic to non-radiogenic Sr and Pb. The major element composition of the andesites resembles that of hydrous melts in equilibrium with peridotite: a low ratio of total Fe to Mg is distinctive. These disparate observations can be reconciled if large ion lithophile (LIL)- element-rich hydrous melt from the subducted oceanic crust equilibrates with olivine and orthopyroxene in overlying LIL-element-depleted mantle and then erupts without interacting with the island are crust. The compositional dissimilarity of the magnesian andesites and most other andesites from the Aleutian island arc precludes application of this model to island are magmatism in general.     

Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America)

A geochemical and isotopic study of lavas from Pichincha, Antisana and Sumaco volcanoes in the Northern Volcanic Zone (NVZ) in Ecuador shows their magma genesis to be strongly influenced by slab melts. Pichincha lavas (in fore arc position) display all the characteristics of adakites (or slab melts) and were found in association with magnesian andesites. In the main arc, adakite-like lavas from Antisana volcano could be produced by the destabilization of pargasite in a garnet-rich mantle. In the back arc, high-niobium basalts found at Sumaco volcano could be produced in a phlogopite-rich mantle. The strikingly homogeneous isotopic signatures of all the lavas suggest that continental crust assimilation is limited and confirm that magmas from the three volcanic centers are closely related. The following magma genesis model is proposed in the NVZ in Ecuador: in fore arc position beneath Pichincha volcano, oceanic crust is able to melt and produces adakites. En route to the surface, part of these magmas metasomatize the mantle wedge inducing the crystallization of pargasite, phlogopite and garnet. In counterpart, they are enriched in magnesium and are placed at the surface as magnesian andesites. Dragged down by convection, the modified mantle undergoes a first partial melting event by the destabilization of pargasite and produces the adakite-like lavas from Antisana volcano. Lastly, dragged down deeper beneath the Sumaco volcano, the mantle melts a second time by the destabilization of phlogopite and produces high-niobium basalts. The obvious variation in spatial distribution (and geochemical characteristics) of the volcanism in the NVZ between Colombia and Ecuador clearly indicates that the subduction of the Carnegie Ridge beneath the Ecuadorian margin strongly influences the subduction-related volcanism. It is proposed that the flattening of the subducted slab induced by the recent subduction (<5 Ma?) of the Carnegie Ridge has permitted the progressive warming of the oceanic crust and its partial melting since ca. 1.5 Ma. Since then, the production of adakites in fore arc position has deeply transformed the magma genesis in the overall arc changing from ‘typical’ calc-alkaline magmatism induced by hydrous fluid metasomatism, to the space- and time-associated lithology adakite/high-Mg andesite/adakite-like andesite/high-Nb basalts characteristic of slab melt metasomatism.     

An outline of the geology and geochemistry,and the possible petrogenetic evolution of the volcanic rocks of the Tonga-Kermadec-New Zealand island arc

The Pleistocene-Recent volcanism of this arc extends nearly linearly NNE from northern New Zealand for some 2800 km. Along its western margin lies an active marginal basin (Lau Basin and Havre Trough) which has its southern termination in the Taupo volcanic zone (TVZ, New Zealand). The New Zealand arc segment is developed within a continental crust, whereas the Tonga-Kermadec segments are developed on a ridge system within the oceanic basin. Submarine morphology suggests that the Kermadec volcanoes represent a less advanced stage of evolution relative to those of Tonga.Magmas erupted within the TVZ are dominantly rhyolitic (≈16,000 km³) with subordinate andesites and rare high-alumina tholeiites and dacites. The Kermadec Islands are dominated by tholeiites and basaltic andesites, with subordinate andesites and dacites. The Tongan Islands are dominated by basaltic andesites, with locally developed andesites and dacites. These Tonga-Kermadec lavas are characterised by subcalcic groundmass clinopyroxenes, whereas the younger group of TVZ andesites contain groundmass hypersthene and augite.Geochemically, the TVZ andesites are systematically enriched (relative to those of Tonga-Kermadec) in “incompatible” elements (e.g. K, Rb, Cs, Ba, light REE, U, Th, Zr, Pb), are less Fe-enriched, and contain more radiogenic Sr and Pb (excepting certain ²⁰⁷Pb/²⁰⁴Pb compositions). The evidence points to crustal equilibration of the TVZ andesites prior to eruption.A complete overlap of major and trace element chemistry (including TiO₂) is observed between the Kermadec-TVZ tholeiites and basaltic andesites, and the ocean floor tholeiites of the Lau Basin. Compared to the Tongan lavas, those of the Kermadecs exhibit a greater degree of chemical variability, also reflected in the greater heterogeneity in their Pb isotopic compositions. Moreover, many of the Tonga-Kermadec basaltic andesites exhibit more depleted “incompatible” trace element abundances than the Kermadec and TVZ tholeiites.The “primary” magmas of this arc are interpreted to be of basaltic andesite type, derived from Benioff zone melting (essentially anhydrous), but extensively modified by low-pressure crystal fractionation processes. The Kermadec tholeiites are explained as products of relatively shallow upper mantle partial fusion induced during the earlier stages of diapiric rise of Benioff zone-derived magmas, which are sufficiently hot to intersect the peridotite solidus. This should result in the production and intermixing of a series of magmas extending from olivine tholeiite to basaltic andesite composition. The voluminous rhyolites of TVZ are interpreted as the products of crustal fusion involving Mesozoic sediments.     

Evidence for crustal assimilation by turbulently convecting,mafic alkaline magmas: Geochemistry of mantle xenolith-bearing lavas from northern Sardinia

Alkali basalt, trachybasalt and basanite magmas, containing abundant xenoliths of upper mantle origin, were erupted during the Plio-Pleistocene (2.4-0.14 Ma) in northern Sardinia. The magmas are enriched in K, Rb, Th and Ba relative to mid-ocean ridge basalts (MORB) and most ocean island basalts (OIB), resulting in high K/Nb, Th/Nb, Ba/Nb and Rb/Nb ratios. The large number of spinel peridotite inclusions in these lavas suggests that these chemical features cannot be explained by combined assimilation and fractional crystallization within the continental crust. However, volcanic rock chemistry can be explained by the assimilation of sialic rocks by turbulently convecting, mafic magmas during their ascent to the surface. Fractionation of Ba and K from the light rare earth elements (LREE) is required to explain the positive correlation of K/La and Ba/La with ⁸⁷Sr/⁸⁶Sr_(i). Consequently, bulk assimilation of crystalline basement rocks by rising, hot basaltic magmas cannot explain the observed chemical trends, and preferential melting of a low melting quartzo-feldspathic crustal component probably occurred, leaving the REE in residual phases such as apatite, zircon, sphene and amphibole. Alternatively, large ion lithophile element (LILE) enrichment may have been related to interaction of rising mafic lavas with metasomatized lithospheric mantle or enriched asthenosphere.     

Petrogenesis of orthopyroxene- and amphibole-bearing andesites, Mustique, Grenadine Islands, Lesser Antilles Arc: Isotope, trace element and physical constraints

Abstract On the island of Mustique, fresh and propylitized olivine–plagioclase–clinopyroxene basalt, plagioclase–clinopyroxene–orthopyroxene and plagioclase–clinopyroxene–amphibole andesite lavas and minor intrusions are interbedded with Oligocene pyroclastic and epiclastic rocks. Chemical data show that two isotopically identical, but chemically different, suites of lava are present: (i) the OPXS (⁸⁷Sr/⁸⁶Sr 0.70403–0.70454; ¹⁴³Nd/¹⁴⁴Nd 0.512952–0.512986; δ¹⁸O_cpx 5.49 and 5.61), comprising basalts and orthopyroxene‐bearing andesites; and (ii) the AMPHS (⁸⁷Sr/⁸⁶Sr 0.70401–0.70457; ¹⁴³Nd/¹⁴⁴Nd 0.512981–0.513037; δ¹⁸O_cpx 5.54), made up of basalts and amphibole‐bearing andesites. The OPXS has higher contents of TiO₂, P₂O₅, light rare earth elements, Sm, Pb, Th, U, Zr, Y and Nb, and higher La/Yb ratios than the AMPHS. The isotopic data suggest that both suites formed from melts derived from the same subduction‐modified depleted mantle source as the volcanic rocks of nearby St Vincent and Bequia, and the northern islands of the Lesser Antilles Arc. The immobile trace element contents, and La/Yb ratios, of the OPXS are indicative of ～10% partial melting of the source, whereas those of the AMPHS are indicative of ～25% partial melting. The within‐suite chemical variation of the OPXS is consistent with ～45% fractional crystallization of its intratelluric mineral assemblages, and that of the AMPHS is consistent with the removal of ～65% of its intratelluric assemblages. Experimental evidence suggests that both suites of basalt crystallized at pressures <8 kbar from melts containing 1–2 wt% water. After extensive fractional crystallization, the andesites crystallized at pressures between approximately 5 and 2 kbar. The OPXS magmas appear to have lost more of their water content than the AMPHS magmas. Thus, the OPXS andesites formed from melts with an estimated water content of 2–3 wt%, whereas the AMPHS andesites formed from melts containing at least 4.5 wt% water.     

Cenozoic volcanism and lithospheric tectonic evolution in Qiangtang area, northern Qinghai-Tibet Plateau

Accompanying with the shortening,thickening and uplifting of the lithosphere,a series of Cenozoic potassic volcanic rock zones are developed in the northern Qinghai-Tibet Plateau.From south to north,the volcanic rocks can be divided into three volcanicrock belts:Qiangtang-Nangqian volcanic belt,Middle Kunlun-Hoh Xil volcanic belt and Western Kunlun-Eastern Kunlun volcanic belt[1].Spatiotemporal evolu-tion of the volcanism and the origins of magmas con-strains on the pulsing uplifting and …     

Genesis of high-magnesium andesites and associated basalts from Saga–Futagoyama, northwest Kyushu, southwest Japan

High-magnesium andesites associated with basalts erupted after the opening of the Sea of Japan are present at Saga–Futagoyama in northwest Kyushu, southwest Japan. High Mg/(Mg + Fe) [=0.84] of orthopyroxene phenocrysts and bulk rock Mg–Fe–Ni compositions suggest that these high-magnesium andesites were originally primitive melts insignificantly modified in crustal magma chambers. K_D^Ca–Na [= (Ca/Na)_pl/(Ca/Na)_{bulk rock}] ranges from 1.21 to 0.97 and suggests that the high-magnesium andesite magmas would originally have contained H₂O less than 1.8 wt.%. Nb/La does not show a negative correlation with respect to SiO₂. These lines of evidence indicate that hydrous components derived from the subducting slab would not have played a significant role in the genesis of the high-magnesium andesite magmas. Instead, the normative olivine − quartz − [CaTs + Jd] compositions and a negative correlation between Sr/Nd and SiO₂ indicate that the basalt-high-magnesium andesite association would have been formed by multi-stage partial melting of relatively anhydrous source at pressure ranging from 1.5 to 0.5 GPa.     

Major- and trace-element systematics and isotope geochemistry of Cenozoic mafic volcanic rocks from the Vogelsberg (central Germany): Constraints on the origin of continental alkaline and tholeiitic basalts and their mantle sources

The Cenozoic basaltic province of the Vogelsberg area (central Germany) is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar Ba/Nb, Ba/La and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, Zr/Nb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher Ba/Nb, Zr/Nb, La/Nb, but lower Ce/Pb ratios and lower Nd isotope compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower (granulite facies) crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites.     

Flux versus decompression melting at stratovolcanoes in southeastern Guatemala

The major and trace element geochemistry of lavas erupted from four volcanic front (VF) stratovolcanoes in southeastern Guatemala show differences in the relative importance of flux and decompression melting in a continental arc setting. The VF stratovolcanoes exhibit a wide compositional range from basalt to dacite, although modern Pacaya erupts basaltic lavas. The VF basalts have relatively low MgO contents and plot outside the field of primary arc magmas defined by melting experiments on hydrous peridotite. After subtracting the effects of the fractionation, assimilation, and alteration of some VF lavas, separate partial melting and mixing trends were identified for Agua–Pacaya and Tecuamburro–Moyuta.The distinct chemical signatures of the hemipelagic and carbonate sediments subducted off Guatemala provide constraints on material transfer processes that occurred between the slab and mantle wedge. Model fluids and melts from the subducted slab were calculated using recently published mineral–aqueous fluid partition coefficients. Wide separation of the model fluid and melt compositions on a U/La versus Ba/Th diagram creates diagnostic mixing curves with an enriched mid-ocean ridge basalt source. Fluid from mature ocean crust has high U/La, fluid from carbonate sediment has high Ba/Th, and fluid and melt from hemipelagic sediments have both high U/La and Ba/Th. In a simple single-stage model, a mantle metasomatized by fluid originating largely from the oceanic crust with only minor sediment fluid contributions best explains the overall large ion lithophile element composition of the VF lavas. (Th/Rb)_N ratios of ∼1 in the VF lavas from southeastern Guatemala require a component of sediment melting. Therefore, a more realistic two-stage model to describe the Guatemalan arc data involves an initial hemipelagic sediment melt input to the wedge followed by minor fluid additions from the oceanic crust or sediments. Correlation between measures of slab input and extent of melting in the older VF lavas from Tecuamburro and Moyuta favors flux-dominated melting near the base of the mantle wedge. In sharp contrast, the lack of a relationship between slab additions and melting in younger lavas from Agua and Pacaya volcanoes implies a significant role for decompression melting closer to the top of the wedge. In this melting scenario, the rate of crustal extension determines the extent of melting.     

Deep melting of old subducted oceanic crust recorded by superchondritic Nb/Ta in modern island arc lavas

Niobium–tantalum systematics of slab-derived melts are powerful tracers that discriminate residual high-pressure rutile-bearing eclogite from low-pressure garnet-bearing amphibolite in subducting plates. Previously reported low Nb–Ta ratios in modern slab melts suggested a predominance of shallow melting in the presence of residual amphibole and that deep melting of rutile-bearing eclogitic slabs, devoid of residual amphibole, is volumetrically insignificant. This study evaluates Nb/Ta in combination with other trace element systematics of modern intra-oceanic and slab melt-related arc lavas from the south-western volcanic chain of the Solomon Islands that cover over 1000 km of the SW Pacific plate border. After a change of subduction polarity, an old subducted Pacific slab and a recently subducting Indian–Australian slab are both present beneath the arc. Solomon arc lavas show sub- to superchondritic Nb–Ta ratios (ca. 10 to 27) which is the largest range ever reported in modern island arc lavas. The large range of Nb/Ta likely results from enrichment of the depleted sub-arc mantle by two distinct slab-derived melts in addition to fluids. One minor slab melt component is derived from the shallow and recent subducting Indian–Australian plate where amphibole is still a significant residual phase. The second slab melt component is predominant in Solomon arc lavas and can be attributed to deep rutile–eclogite-controlled melting of old subducted Jurassic Pacific oceanic crust where residual amphibole is entirely absent or insignificant. The deep Pacific slab melt component is the most likely origin of the extremely high and superchondritic Nb/Ta signatures that produce the upper half of the observed range of Nb/Ta in Solomon arc lavas. The slab melt component that enriched the sub-arc mantle with an unusually high Nb/Ta signature is derived from an initially intact Pacific plate that was probably subject to a slab break-off event and subsequent melting at depths exceeding 100 km. The geochemical evidence presented here shows that old and cold subducted oceanic crust, which is initially not torn, may resist shallow melting but can melt at greater depths instead. The resulting slab melts are generated in the presence of residual rutile-bearing eclogite and significantly fractionate Nb–Ta ratios which may be of relevance at a global scale.     

Mesozoic mafic magmatism in North China: Implications for thinning and destruction of cratonic lithosphere

The North China Craton (NCC) has been thinned from >200 km to <100 km in its eastern part. The ancient subcontinental lithospheric mantle (SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction. This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series, manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts (OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast, mafic igneous rocks emplaced before and after this age exhibit both island arc basalts (IAB)-like trace element distribution patterns and enriched Sr-Nd isotope compositions. This difference indicates a geochemical mutation in the SCLM of North China at ~121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite not only with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at ~144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative ε_Nd(t) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled asthenospheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying asthenospheric mantle peridotite to generate the ultramafic metasomatites that show positive ε_Nd(t) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at ~121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by modern seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.     

Transverse geochemical variations across the Antarctic Peninsula: Implications for the genesis of calc-alkaline magmas

Magnetic activity throughout the Antarctic Peninsula and the South Shetland Islands has been dominantly of a calc-alkaline nature for the last 200 Ma. Chemically, the plutonic and volcanic products are typical of a continental margin magmatic arc, similar to those from western South America. Within any one area, volcanic and plutonic rocks are compositionally indistinguishable, and all magmatic products show increasing SiO₂, and increasing K/Si, Rb/Si, Th/Si and to a lesser extent Ce/Si and La/Si ratios away from the proposed trench axis. The calc-alkaline basaltic compositions also have high large ion lithophile (LIL; e.g. K, Rb, Th)/high field strength (HFS; e.g. Zr, Nb, Ti) ratios relative to non-orogenic counterparts, and increasing LIL/HFS element ratios with increasing fractionation. It is proposed that the high LIL/HFS element ratios in basaltic and andesitic melts are primary features due to dehydration processes with the subducted slab and to fractionation of minor mineral phases from the melt. The increasing LIL/HFS element ratios in more acid rocks are probably due to removal of minor mineral phases from the melt. Although zone refining may contribute to the spatial variations across the peninsula, we have proposed that an enriched subcontinental mantle provides a viable alternative source for the observed K-h variations and for the increased LIL-element contents found in continental margin calc-alkaline magmas.     

Interaction of adakitic melt-peridotite: Implications for the high-Mg# signature of Mesozoic adakitic rocks in the eastern North China Craton

Abundant dunite and harzbugite xenoliths are preserved in Early Cretaceous high-Mg# [63–67, where Mg# = molar 100 × Mg/(Mg + Fe_tot)] diorite intrusions from western Shandong in the North China Craton (NCC). Dunite and some harzburgite xenoliths typically preserve areas of orthopyroxenite (sometimes accompanied by phlogopite) either as veins or as zones surrounding chromite grains. Harzburgite is chiefly composed of olivine, orthopyroxene, minor clinopyroxene and chromian-spinel. High Mg#'s (averaging 91.4) and depletions in Al₂O₃ and CaO (averaging 0.52 wt.% and 0.29 wt.%, respectively) in harzburgite and dunite xenoliths suggest that they are residues formed by large degrees of polybaric melting. However, olivines and orthopyroxenes from dunite xenoliths spatially associated with orthopyroxenite display lower Mg#'s (i.e., 82–87 and 83–89, respectively), suggesting that an adakitic melt–peridotite reaction has taken place. This is consistent with the production of veined orthopyroxene or orthopyroxene + phlogopite in dunite and some harzburgite xenoliths in response to the introduction of adakitic melt into the previously depleted lithospheric mantle (i.e., harzburgite and dunite xenoliths). The presence of orthopyroxene in veins or as a zones surrounding chromite in peridotite xenoliths is thought to be representative of adakitic melt metasomatism. The dunite and harzbugite xenoliths are relatively rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), poor in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), and lack Eu anomalies on chondrite normalized trace element diagrams. The initial ⁸⁷Sr/⁸⁶Sr ratios and εNd(t) values for the xenoliths range from 0.7058 to 0.7212 and + 0.18 to ? 19.59, respectively. Taken together, these features, combined with the strong depletion in HFSE and the existence of Archean inherited zircons in the host rocks, suggest that the adakitic melt was derived from the partial melting of early Mesozoic delaminated lower continental crust. The interaction of the adakitic melt with peridotite is responsible for the high-Mg# character of the early Cretaceous diorites in western Shandong.     

Late Oligocene post-obduction granitoids of New Caledonia: A case for reactivated subduction and slab break-off

Abstract   In southern New Caledonia, Late Oligocene granodiorite and adamellite are intruded into an ultramafic allochthon emplaced in the Late Eocene period. Previous studies of these granitoids proposed an origin associated with the melting of the underlying continental crust, but our new data show that these high-K to medium-K calc-alkaline granitoids display the geochemical and isotopic features of volcanic arc magmas uncontaminated by crust-derived melts. These magmas were probably generated in a post-Eocene and pre-Miocene subduction, the geophysical traces of which have been detected along the western coast of New Caledonia. Sr, Nd and Pb isotopic ratios indicate derivation from an almost isotopically homogeneous mantle wedge, but in contrast, some variation in trace element ratios uncorrelated to differentiation is indicative of source heterogeneity. Prominent heavy rare earth element (HREE) depletion of some of the younger granitoids may be the result of an equilibrium achieved with garnet-bearing subcrustal material (granulite) found as xenoliths, while a relative Nb, Ta and Hf enrichment, irrespective of crystal fractionation, may be related to either a modest contamination by previously underplated mafic material, heterogeneous hydration of the mantle wedge, or mixing with uplifted Nb-rich mantle. Post-obduction slab break-off can be proposed to have played a role in sublithospheric mantle mixing and the subsequent heterogeneity. The Late Oligocene subduction described here may be tentatively extended southward into northern New Zealand allochthons.     

Adakitic magmas in the Ecuadorian Volcanic Front: Petrogenesis of the Iliniza Volcanic Complex (Ecuador)

The Iliniza Volcanic Complex (IVC) is a poorly known volcanic complex located 60 km SSW of Quito in the Western Cordillera of Ecuador. It comprises twin peaks, North Iliniza and South Iliniza, and two satellite domes, Pilongo and Tishigcuchi. The study of the IVC was undertaken in order to better constrain the role of adakitic magmas in the Ecuadorian arc evolution. The presence of volcanic rocks with an adakitic imprint or even pristine adakites in the Ecuadorian volcanic arc is known since the late 1990s. Adakitic magmas are produced by the partial melting of a basaltic source leaving a garnet rich residue. This process can be related to the melting of an overthickened crust or a subducting oceanic crust. For the last case a special geodynamic context is required, like the subduction of a young lithosphere or when the subduction angle is not very steep; both cases are possible in Ecuador. The products of the IVC, made up of medium-K basaltic andesites, andesites and dacites, have been divided in different geochemical series whose origin requires various interactions between the different magma sources involved in this subduction zone. North Iliniza is a classic calc-alkaline series that we interpret as resulting from the partial melting of the mantle wedge. For South Iliniza, a simple evolution with fractional crystallization of amphibole, plagioclase, clinopyroxene, magnetite, apatite and zircon from a parental magma, being itself the product of the mixing of 36% adakitic and 64% calc-alkaline magma, has been quantified. For the Santa Rosa rhyolites, a slab melting origin with little mantle interactions during the ascent of magmas has been established. The Pilongo series magma is the product of a moderate to high degree (26%) of partial melting of the subducting oceanic crust, which reached the surface without interaction with the mantle wedge. The Tishigcuchi series shows two stages of evolution: (1) metasomatism of the mantle wedge peridotite by slab melts, and (2) partial melting (10%) of this metasomatized source. Therefore, the relative ages of the edifices show a geochemical evolution from calc-alkaline to adakitic magmas, as is observed for several volcanoes of the Ecuadorian arc.     

Petrology, geochemistry and tectonic implications of volcanics dredged from the intersection of the Yap and Mariana trenches

Rocks dredged from the forearc very close to the intersection of the Yap and Mariana trenches include a suite of highly depleted arc tholeiites, and several samples of transitional to slightly alkaline basalt. The tholeiites range from magnesian quartz tholeiites with 0.46–0.6% TiO₂, to andesites with up to 62% SiO₂ and 8.2% FeO^*. All show pronounced LREE depletion and have very low contents of Ba and Sr. They are postulated to have been produced by partial melting of upper mantle peridotite residual after MORB extraction, following influx of hydrous fluids from the subducted slab. While these fluids were responsible for small enrichments in Ba, K, Rb and Sr in melts generated, LREE were not involved in the metasomatism, and the strong LREE depletion probably reflects the unmodified, depleted source peridotite.

The second lava suite includes slightly Ne-normative, Ti-augite-bearing basalts with convex-upward REE patterns, showing slight LREE depletion ((La/Sm)_N = 0.76). The chemical features of these basalts support affinities with basalts erupted during the earliest stages of backarc basin opening. A KAr age on one sample(7.8 ± 1.3m.y.) is in good agreement with the initial opening of the Mariana Trough.

The tectonic significance of the dredged arc tholeiite suite is less obvious. A KAr age of10.8 ± 0.4 My on one andesite, and the occurrence of similar lavas in dredges from at least 300 km along the length of the Yap arc, suggest that subduction was occurring beneath the Yap arc in the Late Miocene, after overthrusting of the Yap greenschist allochthon, and while calc-alkaline arc magmatism was occurring further north on the West Mariana Ridge. We suggest that the depleted arc tholeiites in dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the Yap forearc following subduction beneath this area of young, hot Sorol Trough crust. These arc tholeiites represent a magma type transitional between more typical arc tholeiites (e.g. Tongan) and high-Mg andesites and boninites.     

Low water contents in pyroxenes from spinel-peridotites of the oxidized, sub-arc mantle wedge

Pyroxene water contents measured by Fourier transform infrared spectrometry for Mexican and Simcoe (WA, USA) spinel-peridotite xenoliths range from 140 to 528 ppm in clinopyroxenes and 39 to 265 ppm in orthopyroxenes. Correlations between these water contents and major-element compositional data for the pyroxenes, associated spinels, and whole-rock xenoliths demonstrate that these water contents record mantle values that have not been perturbed since the xenoliths were brought to the surface by their host magmas. Broad positive correlations of pyroxene water contents with whole-rock Al₂O₃ are consistent with water behaving as an incompatible element during peridotite melting. The main control on the range of pyroxene water contents, however, appears to be the redox state of the peridotite, because estimates of oxygen fugacity from Mössbauer (Simcoe) and microprobe data (Mexico) on spinels are negatively correlated with water contents. This is consistent with the dominant mechanism of H incorporation into pyroxene, which is dependent on the oxidation-reduction of iron. Metasomatism of sub-arc mantle-wedge peridotites by oxidized fluids or melts rising from the slab raises the oxygen fugacity of the peridotites, and where temperature is high enough, induces them to partially melt. The oxidation, in turn, lowers the solubility of water in the peridotite minerals, causing more than half of the original water to be expelled. That water enters the hydrous partial melts and these ascend through the lithosphere to feed the arc magmatic system in the upper crust. Low water contents in pyroxenes from sub-arc mantle-wedge peridotites, such as those from Simcoe and some western Mexican sites, therefore appear to be complementary to the high water contents that characterize subduction-zone magmas and fuel their explosive eruptions. An estimate of water budget in subduction zones, however, indicates that the amount of water coming from the dehydration of mantle-wedge anhydrous minerals probably accounts for less than 5% of the total water present in subduction-related magmas. The high water contents of arc magmas thus are mainly attributed to fluids or melts from the slab proper. The relatively dry sub-arc mantle wedge appears to be an effective medium through which subducted water is transported from slabs toward the surface.     

The Zealandia Volcanic Complex: Further evidence of a lower crustal “hot zone” beneath the Mariana Intra‐oceanic Arc,Western Pacific

This paper addresses formation of felsic magmas in an intra‐oceanic magmatic arc. New bathymetric, petrologic, geochemical, and isotopic data for Zealandia Bank and two related volcanoes in the south‐central Mariana arc is presented and interpreted. These three volcanoes are remnants of an older andesitic volcano that evolved for some time and became dormant long enough for a carbonate platform to grow on its summit before reawakening as a rhyodacitic volcano. Zealandia lavas are transitional between low‐ and medium‐K and tholeiitic and calc‐alkaline suites. They define a bimodal suite with a gap of 56–58 wt% SiO₂; this suggests that mafic and felsic magmas have different origins. The magmatic system is powered by mantle‐derived basalts having low Zr/Y and flat rare earth element patterns. Two‐pyroxene thermometry yields equilibration temperatures of 1000–1100 °C for andesites and 900–1000 °C for dacites. Porphyritic basalts and andesites show textures expected for fractionating magmas but mostly fine‐grained felsic lavas do not. All lavas show trace element signatures expected for mantle and crustal sources that were strongly melt‐depleted and enriched by subduction‐related fluids and sediment melts. Sr and Nd isotopic compositions fall in the normal range of Mariana arc lavas. Felsic lavas show petrographic evidence of mixing with mafic magma. Zealandia Bank felsic magmatism supports the idea that a large mid‐ to lower‐crustal felsic magma body exists beneath the south‐central Mariana arc, indicating that MASH (mixing, assimilation, storage, and homogenization) zones can form beneath intra‐oceanic as well as continental arcs.