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1.
The volatile content of glassy pillow rims from East Scotia Sea back-arc basin (BAB) lavas are unlike those of mid-ocean ridge (MOR) pillow-rim glasses, although non-volatile compositions of the two rock groups overlap. The East Scotia Sea samples have three to ten times greater water contents and nearly twice the average CO2 and Cl contents of MOR samples; F contents are similar. S contents are only one-third those from MOR samples. H2O and CO2 contents of glassy pillow rims from Mariana island arc andesites are similar to those in the BAB lavas studied. Nevertheless, volatiles in the East Scotia Sea BAB magmas are probably not directly derived from the subducted slab, because there is no seismic evidence that the slab extends within 200 km of the spreading axis of the East Scotia Sea. Available data do not preclude the possibility that the magmas were contaminated by seawater prior to eruption or that the mantle under the East Scotia Sea spreading center is volatile-rich. The volatiles may have been added to the mantle during an earlier period of subduction, perhaps during the initial formation of the East Scotia Sea basin.  相似文献   

2.
Boninites are widely distributed along the western margin of the Pacific Plate extruded during the incipient stage of the subduction zone development in the early Paleogene period. This paper discusses the genetic relationships of boninite and antecedent protoarc basalt magmas and demonstrates their recycled ancient slab origin based on the T–P conditions and Pb–Hf–Nd–Os isotopic modeling. Primitive melt inclusions in chrome spinel from Ogasawara and Guam islands show severely depleted high‐SiO2, MgO (high‐silica) and less depleted low‐SiO2, MgO (low‐silica and ultralow‐silica) boninitic compositions. The genetic conditions of 1 346 °C at 0.58 GPa and 1 292 °C at 0.69 GPa for the low‐ and ultralow‐silica boninite magmas lie on adiabatic melting paths of depleted mid‐ocean ridge basalt mantle with a potential temperature of 1 430 °C in Ogasawara and of 1 370 °C in Guam, respectively. This is consistent with the model that the low‐ and ultralow‐silica boninites were produced by remelting of the residue of the protoarc basalt during the forearc spreading immediately following the subduction initiation. In contrast, the genetic conditions of 1 428 °C and 0.96 GPa for the high‐silica boninite magma is reconciled with the ascent of more depleted harzburgitic source which pre‐existed below the Izu–Ogasawara–Mariana forearc region before the subduction started. Mixing calculations based on the Pb–Nd–Hf isotopic data for the Mariana protoarc basalt and boninites support the above remelting model for the (ultra)low‐silica boninite and the discrete harzburgite source for the high‐silica boninite. Yb–Os isotopic modeling of the high‐Si boninite source indicates 18–30 wt% melting of the primitive upper mantle at 1.5–1.7 Ga, whereas the source mantle of the protoarc basalt, the residue of which became the source of the (ultra)low‐Si boninite, experienced only 3.5–4.0 wt% melt depletion at 3.6–3.1 Ga, much earlier than the average depleted mid‐ocean ridge basalt mantle with similar degrees of melt depletion at 2.6–2.2 Ga.  相似文献   

3.
Back-arc basin basalt systematics   总被引：7，自引：0，他引：7  
The Mariana, east Scotia, Lau, and Manus back-arc basins (BABs) have spreading rates that vary from slow (<50 mm/yr) to fast (>100 mm/yr) and extension axes located from 10 to 400 km behind their island arcs. Axial lava compositions from these BABs indicate melting of mid-ocean ridge basalt (MORB)-like sources in proportion to the amount added of previously depleted, water-rich, arc-like components. The arc-like end-members are characterized by low Na, Ti and Fe, and by high H2O and Ba/La; the MORB-like end-members have the opposite traits. Comparisons between basins show that the least hydrous compositions follow global MORB systematics and an inverse correlation between Na8 and Fe8. This is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures (Lau/Manus hotter than Mariana/Scotia). This interpretation accords with numerical model predictions that faster subduction-induced advection will maintain a hotter mantle wedge. The primary compositional trends within each BAB (a positive correlation between Fe8, Na8 and Ti8, and their inverse correlation with H2O(8) and Ba/La) are controlled by variations in water content, melt extraction, and enrichments imposed by slab and mantle wedge processes. Systematic axial depth (as a proxy for crustal production) variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate. Hydrous fluxing enhances decompression melting, allowing depleted mantle sources just behind the island arc to melt extensively, producing shallow spreading axes. Flow of enriched mantle components around the ends of slabs may augment this process in transform-bounded back-arcs such as the east Scotia Basin. The re-circulation (by mantle wedge corner flow) to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the East Lau Spreading Center, Manus Southern Rifts, and Mariana Trough and the very depleted lavas of east Scotia segments E8/E9. The crust becomes mid-ocean ridge (MOR)-like where the spreading axes, further away from the island arc and subducted slab, entrain dominantly fertile mantle.  相似文献   

4.
Fine-scale sampling with alvin and by dredging of the axial ridge in the Mariana Trough between 17°40′N and 18°30°N recovered basalts with isotopic compositions that span the range between N-type MORB and Mariana island arc basalts. There is a local tectonic-morphological control on basalt compositions; MORB-like basalts are found on the deeper ridge segment bounded by the Pagan transform and the ridge offset at 17°56′N, while basalts from the shallower ridge to the north are typical Mariana Trough basalts (MTB) having compositions intermediate between the two endmember rock types. Arc-like basalts were recovered from one site on the axial ridge.The discovery of basalts with such diverse isotopic characteristics from a short (100 km) section of this backarc spreading center constrains the chemical characteristics and distribution of mantle source variability in the Mariana Trough. SrNdPb isotopic variability suggests that the MTB source is heterogeneous on the scale of individual melt batches. The principal component in the MTB mantle source region is depleted peridotite similar to the source of MORB. The enriched component, most evident in the arc-like basalts and intimately mixed in MTB, has isotopic characteristics similar to those observed in the Mariana arc basalts. The isotopic data suggest that source variability for Mariana axial ridge basalts can be explained by mixed arc-like and MORB-like mantle. We hypothesize that there are fragments of old oceanic lithosphere in the backarc source region. This lithospheric component may reflect remnants of subducted seafloor or forearc-volcanic arc mantle that predate rifting in the backarc basin.  相似文献   

5.
Yasuhiko  Ohara 《Island Arc》2006,15(1):119-129
Abstract In order to obtain a general view of the mantle process beneath a back‐arc basin spreading ridge, the diversity of peridotite petrology and tectonic occurrences in two back‐arc basin spreading ridges from the Philippine Sea were examined: the Parece Vela Rift and the Mariana Trough. The Parece Vela Basin spreading ridge (Parece Vela Rift) was a physically fast/intermediate‐spreading ridge, although many tectono‐magmatic features resemble those of slow‐ to ultraslow‐spreading ridges. Two unusual features of the Parece Vela Rift further demonstrate the uniqueness of the ridge: full‐axial development of oceanic core complexes and exposure of mantle peridotite at segment midpoints. The Parece Vela Rift yields a lithological assemblage of residual but still fertile lherzolite/harzburgite, plagioclase‐bearing harzburgite and dunite; similar assemblages are reported from the equatorial Mid‐Atlantic Ridge at the Romanche Fracture Zone and the ultraslow‐spreading ridges from the Indian and Arctic Oceans. The tectono‐magmatic characteristics of the Parece Vela Rift suggest that diffuse porous melt flow and pervasive melt–mantle interaction were the important mantle processes there. Globally, this ‘porous melt flow‐type’ mantle process is likely to occur beneath a segment midpoint of the ridge having a thick lithosphere, typically an ultraslow‐spreading ridge. In contrast, the Mariana Trough is a typical slow‐spreading ridge, exposing mantle peridotite at segment ends. The Mariana Trough yields a lithological assemblage of residual harzburgite and veined harzburgite, a common assemblage among the global abyssal peridotite suite. The tectono‐magmatic characteristics of the Mariana Trough suggest that channeled melt/fluid flow and limited melt–mantle interaction are the important mantle processes there, because of the colder wall‐rock peridotite in the segment end. This ‘channeled melt flow‐type’ mantle process is likely to occur in the shallow lithospheric mantle at the segment ends of any spreading ridges.  相似文献   

6.
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.  相似文献   

7.
We studied nine samples of igneous rocks from the inner wall of the Mariana Trench above the Challenger Deep from 4150 to 6100 m depth recovered by manned submersible and ROV. Samples from two regions that bracket the Moho were studied: (i) 7 samples from a N‐S transect a few km to the west of the Shinkai Seep Field; and (ii) 2 samples from the Shinkai Seep Field. Transect samples include olivine‐2 pyroxene hornblendites, amphibole basalts, basaltic andesite, and hornblende andesite. We analyzed three transect samples for 40Ar/39Ar ages; two yielded good plateau ages of 46.5 ±0.5 Ma (hornblendite) and 46.60 ±0.15 Ma (hornblende andesite). These results combined with previously published results, indicate that this crust formed during an intense 46–47 Ma magmatic episode that occurred 5–6 my after subduction initiation. Hornblendites and hornblende basalts formed from primitive magmas, as shown by high MgO (11–21 wt%), Ni (222–885 ppm) and Cr (412–1145 ppm) contents. Electron microprobe analyses indicate that hornblende is Na‐rich (up to 3.0 wt% Na2O) and that many samples have an atypically large range in plagioclase composition (i.e. individual samples have An < 10 to An 90 plagioclase). Two subgroups can be identified: a mostly deeper depleted suite and a mostly shallower enriched suite. These results indicate that (i) the crust–mantle boundary in this region is transitional, occurring over a ~ 1.5 km interval, with interlayered peridotite and hornblendites between 5800 and 4300 m; and (b) extension to form the Challenger Deep forearc segment occurred by combined stretching of old crust and injection of young basaltic magmas. In contrast to the mostly fresh nature of transect samples, the two samples from the Shinaki Seep Field are intensely altered peridotite and basalt.  相似文献   

8.
Abstract The Mariana Trough is an active back-arc basin, with the rift propagating northward ahead of spreading. The northern part of the Trough is now rifting, with extension accommodated by combined stretching and igneous intrusion. Deep structural graben are found in a region of low heat flow, and we interpret these to manifest a low-angle normal fault system that defines the extension axis between 19°45' and 21°10'N. A single dredge haul from the deepest (∼5.5 km deep) of these graben recovered a heterogeneous suite of volcanic and plutonic crustal rocks and upper mantle peridotites, providing the first report of the deeper levels of back-arc basin lithosphere. Several lines of evidence indicate that these rocks are similar to typical back-arc basin lithosphere and are not fragments of rifted older arc lithosphere. Hornblende yielded an 40Ar/39Ar age of 1.8 ± 0.6 Ma, which is interpreted to approximate the time of crust formation. Harzburgite spinels have moderate Cr# (<40) and coexisting compositions of clinopyroxene (CPX) and plagioclase (PLAB) fall in the field of mid-ocean ridge basalt (MORB) gabbros. Crustal rocks include felsic rocks (70-80% SiO2) and plutonic rocks that are rich in amphibole. Chemical compositions of crustal rocks show little evidence for a 'subduction component', and radiogenic isotopic compositions correspond to that expected for back-arc basin crust of the Mariana Trough. These data indicate that mechanical extension in this part of the Mariana Trough involves lithosphere that originally formed magmatically. These unique exposures of back-arc basin lithosphere call for careful study using ROVs and manned submersibles, and consideration as an ocean drilling program (ODP) drilling site.  相似文献   

9.
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.  相似文献   


10.
Ascertaining the emplacement mechanism of oceanic basaltic lavas is important in understanding how ocean floor topography is produced and oceanic plates evolve, particularly during the early stages of crustal development of a supra-subduction zone. A detailed study of the volcanic stratigraphy at International Ocean Discovery Program (IODP) Site U1438 in the Amami Sankaku Basin, west of the Kyushu–Palau Ridge, has revealed the development of lava accretion and ridge topography on the Philippine Sea plate at about 49 Ma. Igneous basement rocks penetrated at Site U1438 are the uppermost 150 m of ~6 km-thick oceanic crust, and comprise, in a downhole direction, sheet flows (12.6 m), lobate sheet flows (61.3 m), pillow lavas (50.7 m), and thin sheet flows (25.3 m). The lowermost sheet flows are intercalated with layers of limestone and epiclastic tuff. Lithofacies analysis reveals that the lowermost sheet flows, limestone, and tuff formed on an axial rise, the pillow lavas were emplaced on a ridge slope, and the lobate sheet flows formed off ridge on an abyssal plain. The lithofacies of the basement basalt corresponds to the upper portions of fast-spreading oceanic crust, suggesting that subduction initiation was associated with intermediate to fast rates of seafloor spreading. The surface sheet flows are olivine–clinopyroxene-phyric basalt and differ from the lower basalt flows that contain phenocrysts of olivine and plagioclase, with or without clinopyroxene. The depleted chrome-spinel composition and olivine–clinopyroxene phenocryst assemblage in the surface sheet flows suggests a slight contribution of water for magma generation not present for the lower basalt flows. Considering the lithological difference between the backarc and forearc oceanic crust in the Izu–Bonin–Mariana arc, with sheet flow dominant in the former, seafloor spreading occurred faster in the later stage of subduction initiation.  相似文献   

11.
Abstract Volcanism in the back-arc side region of Central Luzon, Philippines, with respect to the Manila Trench is characterized by fewer and smaller volume volcanic centers compared to the adjacent forearc side-main volcanic arc igneous rocks. The back-arc side volcanic rocks which include basalts, basaltic andesites, andesites and dacites also contain more hydrous minerals (ie, hornblende and biotite). Adakite-like geochemical characteristics of these back-arc lavas, including elevated Sr, depleted heavy rare earth elements and high Sr/Y ratios, are unlikely to have formed by slab melting, be related to incipient subduction, slab window magmatism or plagioclase accumulation. Field and geochemical evidence show that these adakitic lavas were most probably formed by the partial melting of a garnet-bearing amphibolitic lower crust. Adakitic lavas are not necessarily arc–trench gap region slab melts.  相似文献   

12.
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.  相似文献   

13.
Middle Miocene to Quaternary lavas on Kunashir Island in the southern zone of the Kurile Arc were examined for major, trace, and Sr–Nd–Pb isotope compositions. The lavas range from basalt through to rhyolite and the mafic lavas show typical oceanic island arc signatures without significant crustal or sub-continental lithosphere contamination. The lavas exhibit across-arc variation, with increasingly greater fluid-immobile incompatible element contents from the volcanic front to the rear-arc; this pattern, however, does not apply to some other incompatible elements such as B, Sb, and halogens. All Sr–Nd–Pb isotope compositions reflect a depleted source with Indian Ocean mantle domain characteristics. The Nd and Pb isotope ratios are radiogenic in the volcanic front, whereas Sr isotope ratios are less radiogenic. These Nd isotope ratios covary with incompatible element ratios such as Th/Nd and Nb/Zr, indicating involvement of a slab-derived sediment component by addition of melt or supercritical fluid capable of mobilizing these high field-strength elements and rare earth elements from the slab. Fluid mobile elements, such as Ba, are also elevated in all basalt suites, suggesting involvement of slab fluid derived from altered oceanic crust. The Kurile Arc lavas are thus affected both by slab sediment and altered basaltic crust components. This magma plumbing system has been continuously active from the Middle Miocene to the present.  相似文献   

14.
A bathymetric overview of the Mariana forearc   总被引：1，自引：0，他引：1  
Bathymetric data at a 200-m contour interval for the entire Mariana forearc, from south of 13°N to 25°N, permits the first comprehensive overview of this feature. The Mariana forearc represents a sediment-starved end-member. The forearc in its southern and central sections is divisible into a structurally complex eastern province and a less-deformed western province. Despite the absence of an accretionary complex the Mariana forearc has a well-defined outer-arc high; this probably results from a greater concentration of low-density serpentinized mantle lithosphere beneath the outer forearc relative to the inner forearc. This serpentinization gradient is coupled with differing deformational styles of thinner and more brittle lithosphere beneath the outer forearc compared to thicker and more ductile lithosphere beneath the inner forearc. The bathymetric data also support models calling for extension along-strike of the forearc, reflecting an increase in arc length accompanying the crescent-shaped opening of the Mariana Trough back-arc basin. Both northeast and northwest ridges and grabens can be identified, with the latter restricted to the southern part of the forearc and the former widely distributed in the central and northern forearc. Northeast-oriented extensional structures are supplanted northward by long, linear northwest-trending structures that are interpreted as left-lateral strike–slip faults. These variations in deformation along-strike of the forearc manifest a transition from nearly orthogonal convergence in the south to highly oblique convergence in the north.  相似文献   

15.
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.  相似文献   

16.
Abstract Oxygen is the most abundant element in the earth, and isotopic analysis of this element in island arc lavas potentially provides sensitive constraints on the proportion of oxygen recycled from subducted material, relative to that extracted from the mantle. Here we report on 225 new oxygen isotopic analyses of whole‐rock and glass samples, and clinopyroxene separates, from lavas collected from the southernmost 1500 km of the Izu–Bonin–Mariana (IBM) convergent margin. Whole‐rock samples clustered around a mean of 6.11 ± 0.47‰, whereas Mariana Trough glasses and mafic melts, calculated to be in equilibrium with mafic phenocrysts, clustered narrowly around a mean of 5.7‰. These data demonstrate that unequivocal identification of magmatic oxygen requires analysis of fresh glass or mafic minerals, and that the source of southern IBM Arc melts is entirely, or almost entirely, in equilibrium with normal mantle oxygen. If the elemental enrichments characteristic of the subduction component originate in subducted materials, these oxygen isotopic data are most consistent with the interaction of a small amount of sediment melt (<4%; mostly less than 1%) with mantle peridotite to yield the hybrid mantle that melts to form IBM Arc magmas.  相似文献   

17.
The results of a controlled source seismic reflection–refraction experiment carried out in 1992 reveal the following characteristics of the northern Izu–Bonin (Ogasawara) oceanic island arc–trench system. (1) The crust rapidly thickens from the Shikoku back-arc basin to the arc, is thickest beneath the active rifts, and then gradually thins to the forearc. The thickness of the crust beneath the arc rift zone and the back-arc basin are ∼ 20 km and 8 km, respectively. (2) The Moho vanishes beneath the forearc. Velocities rapidly decrease eastwards beneath the inner trench wall. (3) The velocity of the lower crust of the arc and the back-arc basin is 7.1–7.3 km/s. This velocity is higher than the typical oceanic lower crust whose velocity is ∼ 6.7 km/s. (4) The velocity of the middle crust of the arc is ∼ 6 km/s. This layer does not exist beneath the back-arc basin. (5) A slight difference in the velocity gradient of the middle crust exists between the arc rift zone and the forearc. Based on these findings and previous studies, it is inferred that: (i) the middle crust is probably granitic rock and formed in more than two episodes; (ii) the lower crust formed by igneous underplating which may also have affected part of the back-arc basin; and (iii) the root of the serpentinite diapir on the inner trench wall is a low-velocity mantle wedge that was probably caused by large amounts of water released from the subducting Pacific plate at depths shallower than 30 km.  相似文献   

18.
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 εNd(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 εNd(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.  相似文献   

19.
A model is proposed for the origin of hot spots that depends on the existence of major-element heterogeneities in the mantle. Generation of basaltic crust at spreading centers produces a layer of residual peridotite ～20–25 km thick directly beneath the crust which is depleted in Fe/Mg, TiO2, CaO, Al2O3, Na2O and K2O, and which has a slightly lower density than undepleted peridotite beneath it. Upon recycling of this depleted peridotite back into the deep mantle at subduction zones, it becomes gravitationally unstable, and tends to rise as diapirs through undepleted peridotite. For a density contrast of 0.05 g cm?3, a diapir 60 km in diameter would rise at roughly 8 cm y?1, and could transport enough heat to the base of the lithosphere to cause melting and volcanism at the surface. Hot spots are thus viewed as a passive consequence of mantle convection and fractionation at spreading centers rather than a plate-driving force.It is suggested that depleted diapirs exist with varying amounts of depletion, diameters, upward velocities and source volumes. Such variations could explain the occurrence of hot spots with widely varying lifetimes and rates of lava production. For highly depleted diapirs with very low Fe/Mg, the diapir would act as a heat source and the asthenosphere and lower lithosphere drifting across the diapir would serve as the source region of magmas erupted at the surface. For mildly depleted diapirs with Fe/Mg only slightly less than in normal undepleted mantle, the diapir could provide not only the source of heat but also most or all of the source material for the erupted magmas. The model is consistent with isotopic data that require two separate and ancient source regions for mid-ocean ridge and oceanic island basalts. The source for mid-ocean ridge basalts is considered to be material upwelling at spreading centers from the deep mantle. This material forms the oceanic lithosphere. Oceanic island basalts are considered to be derived from varying mixtures of sublithospheric and lower lithospheric material and the rising diapir itself.  相似文献   

20.
Abstract Two new cases of association of adakites with ‘normal’ island arc lavas and transitional adakites are recognized in the islands of Batan and Negros in northern and central Philippines, respectively. The Batan lavas are related to the subduction of the middle Miocene portion of the South China Sea basin along the Manila trench; those of Negros come from the almost aseismic subduction of the middle Miocene Sulu Sea crust along the Negros trench. The occurrence of the Batan adakites is consistent with previous findings showing adakitic glass inclusions within minerals of mantle xenoliths associated with Batan arc lavas. The similarity of adakite ages (1.09 Ma) and that of the metasomatized xenoliths (1 Ma) suggests that both are linked to the same slab‐melting and metasomatic event. Earlier Sr, Pb and Nd‐isotopic studies, however, also reveal the presence of an important sediment contribution to the Batan lava geochemistry. Thus, the role played by slab melts, assumed to have mid‐ocean ridge basalts‐like (MORB) isotopic characteristics, in enriching the Batan subarc mantle is largely masked by the sediment input. The Negros adakites are present only in Mount Cuernos, the volcanic center nearest to the Negros trench. Batch partial melting calculations show that the Negros adakites could be derived from a garnet amphibolitic source with normal‐MORB (N‐MORB) geochemistry. This is supported by the MORB‐like isotopic characteristics of the Mount Cuernos lavas. The volcanic rocks from the other volcanoes consist of normal arc and transitional adakitic lavas that have slightly higher Sr‐ and Pb‐isotopic ratios, probably due to slight sediment input. Mixing of adakites and normal arc lavas to produce transitional adakites is only partly supported by trace element geochemistry and not by field evidence. The transitional adakites can be modeled as partial melts of an adakite‐enriched mantle. Trace element enrichment of non‐adakitic lavas could reflect the interaction of their mantle source with uprising slab melts, as metasomatic mantle minerals scavenge certain trace elements from the adakitic fluids. Therefore, in arcs beneath which thick (up to 2 km) continent‐derived detrital sediments are involved in subduction, like in Batan, the sediment signature can overwhelm the slab melt input. In arcs like Negros where slow subduction could cause a more efficient scraping of thinner (approximately 1 km) detrital sediments, the contribution of slab melts is easier to detect.  相似文献   

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