共查询到20条相似文献，搜索用时 46 毫秒
1.
Abstract The mantle-derived xenoliths entrained in the Pliocene basanite from Baengnyeong Island, South Korea, are spinel lherzolites and spinel harzburgites. The overall compositional range of the Baengnyeong xenoliths matches that of the post-Archean xenoliths of lithospheric mantle origin from eastern China, but without any compositional evidence for a refractory Archean mantle root. Mineral compositions of the xenoliths have been used to estimate the equilibrium temperatures and pressures, and to construct a paleogeothermal gradient of the source region. The xenolith-derived paleogeotherm is constrained from about 820°C at 7.3 kbar to 1000°C at 20.6 kbar. Like those from the post-Archean Chinese xenoliths of lithospheric mantle origin, the Baengnyeong geotherm is considerably elevated relative to the conductive models at the depth of the crust–mantle boundary, reflecting a thermal perturbation probably related to lithospheric thinning. There is no significant P / T difference between harzburgite and lherzolite, which suggests that the harzburgites are interlayered with lherzolites within the depth interval beneath Baengnyeong Island. 相似文献
2.
Abstract Geochemical data on Baegryeong Island spinel peridotites found in Miocene alkali basalt provide the information for lithosphere composition, chemical processes, equilibrium pressure and temperature conditions. Spinel peridotite xenoliths, showing transitional textures between protogranular and porpyroclastic textures, were accidentally trapped by the ascending alkali basalt magma. The xenoliths originate at depths from 50 to 70 km with a temperature range from 800 to 1100°C. The variations of modal and mineral compositions of the spinel peridotite xenoliths indicate that the xenoliths have undergone 1–10% fractional melting. The spinel peridotites from Baegryeong Island have undergone cryptic mantle metasomatism subsequent to melt extraction. Metasomatic agent of enriched spinel peridotite xenoliths was carbonatite melt. 相似文献
3.
Abstract Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB–oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history. 相似文献
4.
Abstract Spinel lherzolite is a minor component of the deep-seated xenolith suite in the Oki-Dogo alkaline basalts, whereas other types of ultramafic (e.g. pyroxenite and dunite) and mafic (e.g. granulite and gabbro) xenoliths are abundant. All spinel lherzolite xenoliths have spinel with a low Cr number (Cr#; < 0.26). They are anhydrous and are free of modal metasomatism. Their mineral assemblages and microtextures, combined with the high NiO content in olivine, suggest that they are of residual origin. But the Mg numbers of silicate minerals are lower (e.g. down to Fo86 ) in some spinel lherzolites than in typical upper mantle residual peridotites. The clinopyroxene in the spinel lherzolite shows U-shaped chondrite-normalized rare-earth element (REE) patterns. The abundance of Fe-rich ultramafic and mafic cumulate xenoliths in Oki-Dogo alkali basalts suggests that the later formation of those Fe-rich cumulates from alkaline magma was the cause of Fe- and light REE (LREE)-enrichment in residual peridotite. The similar REE patterns are observed in spinel peridotite xenoliths from Kurose and also in those from the South-west Japan arc, which are non-metasomatized in terms of major-element chemistry (e.g. Fo > 89), and are rarely associated with Fe-rich cumulus mafic and ultramafic xenoliths. This indicates that the LREE-enrichment in mantle rocks has been more prominent and prevalent than Fe and other major-element enrichment during the metasomatism. 相似文献
5.
Abstract Peridotite xenoliths from the subarc mantle, which have been rarely documented, are described from Iraya volcano of the Luzon arc, the Philippines, and are discussed in the context of wedge-mantle processes. They are mainly harzburgite, with subordinate dunite, and show various textures from weakly porphyroclastic (C-type) to extremely fine-grained equigranular (F-type). Textural characteristics indicate a transition from the former to the latter by recrystallization. The F-type peridotite has inclusion-rich fine-grained olivine and radially aggregated orthopyroxene, being quite different in texture from ordinary mantle-derived peridotites previously documented. Despite their strong textural contrast, the two types do not show any systematic difference in modal composition. The harzburgite of C-type has ordinary mantle peridotite mineralogy; olivine is mostly Fo91–92 and chromian spinel mostly has Cr#s (= Cr/[Cr + Al] atomic ratios) from 0.3 to 0.6. Olivine is slightly more Fe-rich (Fo89–91) and spinel is more enriched in Cr (the Cr#, 0.4–0.8) and Fe3+ in F-type peridotites than in C-type harzburgite. Orthopyroxene in F-type peridotites is relatively low in CaO (<1 wt%), Al2 O3 (<2 wt%) and Cr2 O3 (<0.4 wt%). The F-type peridotite was possibly formed from the C-type one by recrystallization including local dissolution and precipitation of orthopyroxene assisted by fluid (or melt) of subduction origin. Textural characteristics, however, indicate a deserpentinization origin from abyssal serpentinite of which protolith was a C-type peridotite. In this scenario the initial abyssal serpentinite was possibly dehydrated due to an initiation of magmatic activity beneath an incipient oceanic arc like Batan Island. The F-type peridotite is characteristic of the upper mantle of island arc, especially of incipient arc. 相似文献
6.
《Journal of Geodynamics》2011,51(5):424-440
This study presents geochemical and fabric analysis of coarse-grained, porphyroclastic and mylonitic spinel peridotite xenoliths derived from the shallow subcontinental lithospheric mantle of Jeju Island (South Korea). Fabric analysis of olivines in the studied peridotites shows activation of the high temperature (0 1 0)[1 0 0] slip system; however, in the porphyroclastic and mylonitic peridotites, activation of (0 k l)[1 0 0] slip system results in a weaker fabric. Formation of porphyroclastic and mylonitic fabrics are thought to form in a shear-dominated environment. The results of the trace element analysis reveal that the smaller the grain size and weaker the fabric, the more enriched in LREE and HFSE are the peridotites, which indicates a strong relationship between metasomatic agents and mantle shear zones. 相似文献
7.
Metasomatized harzburgite xenoliths from Avacha volcano as fragments of mantle wedge of the Kamchatka arc: Implication for the metasomatic agent 总被引：6，自引：0，他引：6
Abstract Abundant peridotite xenoliths have been found in pyroclasitics of Avacha (Avachinsky) volcano, the south Kamchatka arc, Russia. They are mostly refractory harzburgite with or without clinopyroxene: the Fo of olivine and Cr/(Cr + Al) atomic ratio of spinel range from 91 to 92 and from 0.5 to 0.7, respectively. They are metasomatized to various extents, and the metasomatic orthopyroxene has been formed at the expense of olivine. The metasomatic orthopyroxene, free of deformation and exsolution, is characterized by low contents of CaO and Cr2 O3 . The complicated way of replacement possibly indicates low viscosity of the metasomatic agent, namely hydrous fluids released from the relatively cool slab beneath the south Kamchatka arc. This is a good contrast to the north Kamchatka arc, where the slab has been hot enough to provide slab-derived melts. High content of total orthopyroxene, 40 vol% on average, in metasomatized harzburgite from Avacha suggests silica enrichment of the mantle wedge, and is equivalent to some subcratonic harzburgite. Some subcratonic harzburgites therefore could have been formed by transportation of subarc metasomatized peridotites to a deeper part of the upper mantle. 相似文献
8.
Abstract The major element and compatible trace element compositions of the northwest Kyushu basalts (NWKBs) collected from Saga-Futagoyama were analyzed to examine the petrogenesis of these basalts. Although nepheline-normative alkaline basalts are not found in the basalts from Saga-Futagoyama, the Saga-Futagoyama basalts almost cover the major element variations of NWKBs. The basalts can be chemically divided into two groups: an Fe-poor group (IPG) and an Fe-rich group (IRG). The compositional variation of IPG basalts is essentially controlled by the partial melting of the source as suggested by the following: (i) bulk rock MgO, FeO and NiO compositions indicate that some IPG samples were equilibrated with mantle olivine; and (ii) correlations between Al2 O3 , CaO and MgO are consistent with those of experimental partial melts of peridotites. The inconsistent behaviors of the elements compatible with clinopyroxene (Cpx), such as V (Sc and Cu), preclude the significant role of the fractional crystallization of Cpx and spinel in IPG variation. IPG basalts have low Al and high Fe concentrations compared to the products of melting experiments involving peridotites and pyroxenites, suggesting that the IPG source would have a lithology and bulk rock composition different from those of typical peridotites and pyroxenites. IRG basalts have negative correlations between Fe2 O3 * and MgO, and between V and Fe2 O3 */MgO, indicating that IRG basalts would have fractionated Cpx. However, the anomalously Fe-rich feature of IRG basalts compared with NWKBs collected from other areas suggests that the role of Cpx fractionation in NWKBs is minor. Relatively low melting temperatures would have principally caused the large chemical variation of NWKBs. 相似文献
9.
Subduction of mantle wedge peridotites: Evidence from the Higashi-akaishi ultramafic body in the Sanbagawa metamorphic belt 总被引：1，自引：0，他引：1
The Higashi-akaishi garnet-bearing ultramafic body in the Sanbagawa metamorphic belt, Southwest Japan, represents a rare example of oceanic-type ultrahigh-pressure metamorphism. The body of 2 km × 5 km is composed mostly of anhydrous dunite with volumetrically minor lenses of clinopyroxene-rich rocks. Dunite samples contain high Ir-type platinum group elements (PGE) and Cr in bulk rocks, high Mg and Ni in olivine, and high Cr in spinel. On the other hand, clinopyroxene-rich rocks contain low concentrations of Ir-type PGE and Cr, high concentrations of fluid-mobile elements in bulk rocks, and low Ni and Mg in olivine. Clinopyroxene is diopsidic with low Al2 O3 . The compositions of bulk rocks and mineral chemistry of spinel, olivine, and clinopyroxene suggest that the olivine-dominated rocks are residual mantle peridotites after high degrees of influx partial melting, and that the clinopyroxene-rich rocks are cumulates of subduction-related melts. Thus, the Higashi-akaishi ultramafic body originated from the interior of the mantle wedge, most likely the forearc upper mantle. It was then incorporated into the Sanbagawa subduction channel by a mantle flow, and underwent high pressure metamorphism to a depth greater than 100 km. Such a strong active flow in the mantle wedge is likely facilitated by the lack of serpentinites along the interface between the slab and the overlying mantle, as it was too hot for serpentine. These unusually hot conditions and strong active mantle flow may reflect conditions in the earliest stage of development of subduction, and may have been maintained by massive upwelling and subsequent eastward flow of asthenospheric mantle in the northeastern Asian continent in Cretaceous time when the Sanbagawa belt began to form. 相似文献
10.
D.C. Presnall 《Physics of the Earth and Planetary Interiors》1980,23(2):103-111
For a lherzolite mantle with about 0.1 wt.-percent CO2 or less, and a CO2/H2O mole ratio greater than about one, the mantle solidus curve in P-T space will have two important low-temperature regions, one centered at about 9 kbar (30 km depth) and another beginning at about 28 kbar (90 km depth). It is argued that the depth of generation of primary tholeiitic magmas beneath ridge crests is about 9 kbar, and that the geotherm changes from an adiabatic gradient at greater pressures to a strongly superadiabatic gradient at lesser pressures. Such a ridge geotherm would intersect the solidus at two separate depth intervals corresponding to the two low-temperature regions on the solidus. With increasing age and cooling of the lithosphere, the shallow partial melt zone would pinch out and the thickness of the deep partial melt zone would decrease. With increasing depth in a mature oceanic lithosphere, the rock types would consist of depleted harzburgite from directly beneath the crust to about 30 km depth, fertile spinel lherzolite from about 30 km to 50–60 km, and fertile garnet lherzolite from about 50–60 km to the top of the deep partial melt zone at about 90 km. 相似文献
11.
Csaba Szabó Károly Hidas Enik&#; Bali Zoltán Zajacz István Kovács Kyounghee Yang Tibor Guzmics Kálmán Török 《Island Arc》2009,18(2):375-400
In this paper we present a detailed textural and geochemical study of two equigranular textured amphibole-bearing spinel lherzolite xenoliths from Szigliget, Bakony–Balaton Highland Volcanic Field (BBHVF, western Hungary) containing abundant primary silicate melt inclusions (SMIs) in clinopyroxene rims and secondary SMIs in orthopyroxene (and rarely spinel) along healed fractures. The SMIs are dominantly composed of silicate glass and CO2 -rich bubbles. Clinopyroxene and orthopyroxene are zoned in both studied xenoliths, especially with respect to Fe, Mg, Na, and Al contents. Cores of clinopyroxenes in both xenoliths show trace element distribution close to primitive mantle. Rims of clinopyroxenes are enriched in Th, U, light rare earth elements (LREEs) and medium REEs (MREEs). Amphiboles in the Szg08 xenolith exhibit elevated Rb, Ba, Nb, Ta, LREE, and MREE contents. The composition of silicate glass in the SMIs covers a wide range from the basaltic trachyandesite and andesite to phonolitic compositions. The glasses are particularly rich in P2 O5 . Both primary and secondary SMIs are strongly enriched in incompatible trace elements (mostly U, Th, La, Zr) and display a slight negative Hf anomaly. The development of zoned pyroxenes, as well as the entrapment of primary SMIs in the clinopyroxene rims, happened after partial melting and subsequent crystallization of clinopyroxenes, most probably due to an interaction between hot volatile-bearing evolved melt and mantle wall-rocks. This silicate melt filled microfractures in orthopyroxenes (and rarely spinels) resulting in secondary SMIs. 相似文献
12.
Melting relations of a glassy magnesian olivine tholeiite from the FAMOUS area have been studied within the pressure range 1 atm to 15 kbar. From 1 atm to 10 kbar, olivine is the liquidus phase, followed by plagioclase and Ca-rich clinopyroxene. Above 10 kbar, Ca-rich clinopyroxene appears on the liquidus, followed by orthopyroxene and spinel. Near 10 kbar, olivine, orthopyroxene, clinopyroxene, spinel and plagioclase crystallize within 10°C of the liquidus. This indicates that a liquid of this magnesian olivine tholeiite composition could coexist with mantle peridotite at about 10 kbar. This result is in agreement with the geochemistry of Ni; the Ni concentration of the studied sample corresponds to the theoretical concentration in a primary magma [14,15].These data suggest that at least some magnesian mid-oceanic ridge basalts (MORBs) could be primary melts segregated from the mantle at depths near the transition zone between plagioclase lherzolite and spinel lherzolite (about 10 kbar). Based on this model, the residual mantle after extraction of MORBs should be lherzolite, not harzburgite.High-pressure (7–10 kbar) fractionation models involving olivine, plagioclase and clinopyroxene, which have been proposed by several workers (e.g. [36]) to explain the varieties of MORBs, were re-emphasized based on this melting study. The rare occurrence of clinopyroxene as a phenocryst phase in MORBs is explained by precipitation in a magma chamber at high pressure, or by dissolution of clinopyroxene formed earlier at high pressure. 相似文献
13.
Arnold Kadik 《Physics of the Earth and Planetary Interiors》1997,100(1-4):157-166
The oxygen fugacity (f(O2)) values recorded by diamondiferous peridotite and eclogite xenoliths from Siberia indicate that the redox state of the ancient lithosphere is heterogeneous on a scale of at least four log units, mainly in the range between the wüstite-magnetite (WM) and iron-wüstite (IW) oxygen buffers. Highly reduced peridotites can be interpreted as relict from earlier lower f(O2). The f(O2) values recorded by ‘fertile’ and less modified spinel peridotites from Mongolia, Baikal and Tien-Shan show that the redox state of the lithosphere beneath central Asia and Tien-Shan is heterogeneous on a scale of 2–3 log units, mainly in the range between the WM and IW + 1 oxygen buffers. These data provide evidence for the presence of a lower-f(O2) regime of carbon-bearing mantle beneath the Baikal rift zone and Tien-Shan, and the oxidation of diapirs ascending from the asthenosphere. The ‘dry’ xenoliths from Mongolia primarily reflect closed system behavior in the upper mantle, the f(O2) of which is buffered by ferric-ferrous redox equilibrium. The observed evolution of f(O2) values is closely linked to the distribution of volatile species in the mantle. H2O and CO2 are the dominant volatiles for the more depleted and oxidized part of peridotites, and CH4 for the more reduced and less modified part. It is proposed that the upper mantle was originally more reduced and has become progressively more oxidized, resulting perhaps largely from the preferential loss of hydrogen and carbon during melting. The oxygen budget of the upper mantle results from the opposing contributions of crustal recycling and transfer of carbon-bearing material from the deep mantle. 相似文献
14.
Pierre Schiano Bernard Bourdon Robert Clocchiatti Dominique Massare Maria E. Varela Yan Bottinga 《Earth and Planetary Science Letters》1998,160(3-4):537-550
The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Minerals in a spinel lherzolite nodule from Grande Comore island contain glass inclusions which, after homogenization by heating, exhibit a continuous suite of chemical compositions clearly distinct from that of the host basanitic lava. The compositions range from silicic, with nepheline–olivine normative, 64 wt% SiO2 and 11 wt% alkali oxides, to almost basaltic, with quartz normative, 50 wt% SiO2 and 1–2 wt% alkali oxides. Within a single mineral phase, olivine, the inferred primary melt composition varies from 54 to 64 wt% SiO2 for MgO content ranging from 8 to 0.8 wt%. An experimental study of the glass and fluid inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at moderate temperature and pressure (T≈1230°C and P≈1.0 Gpa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr2O3 and poorer in SiO2, K2O, Na2O, Al2O3 and Cl with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts, and indicate that primary melts with SiO2 >60 wt%, alkali oxides >11%, FeO <1 wt% and MgO <1 wt% are generated during incipient melting of spinel peridotite. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO2 and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO2 and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated. 相似文献
15.
Jin-Hai Yu Suzanne Y. O&#x;Reilly W. L. Griffin Xisheng Xu Ming Zhang Xinmin Zhou 《Journal of Volcanology and Geothermal Research》2003,122(3-4):165-189
Two localities on the Leizhou Peninsula, southern China (Yingfengling and Tianyang basaltic volcanoes) yield a wide variety of mantle-derived xenoliths including Cr-diopside series mantle wall rocks and two distinct types of Al-augite series pyroxenites. Metapyroxenites have re-equilibrated granoblastic microstructures whereas pyroxenites with igneous microstructures have not thermally equilibrated to the mantle conditions. An abundant suite of megacrysts and megacrystic aggregates (including garnet, plagioclase, clinopyroxene, ilmenite and apatite) is interpreted as the pegmatitic equivalents of the igneous pyroxenite suite. Layered spinel lherzolite/spinel websterite xenoliths were formed by metamorphic differentiation caused by mantle deformation, inferred to be related to lithospheric thinning. Some metapyroxenites have garnet websterite assemblages that allow calculation of their mantle equilibration temperatures and pressures and the construction of the first xenolith geotherm for the southernmost China lithosphere. Heat flow data measured at the surface in this region yield model conductive geotherms (using average crustal conductivity values) that are consistent with the xenolith geotherm for the mantle. The calculated mean surface heat flux is 110 mW/m2. This high heat flux and the high geotherm are consistent with young lithospheric thinning in southern China, and with recent tomography results showing shallow low-velocity zones in this region. The xenolith geotherm allows the construction of a lithospheric rock type section for the Leizhou region; it shows that the crust–mantle boundary lies at about 30 km, consistent with seismic data, and that the lithosphere–asthenosphere boundary lies at about 100 km. 相似文献
16.
A series of water-deficient partial melting experiments on a low-K tholeiite were carried out under lower crustal P–T–H2 O conditions (900–1200 °C, 0.7–1.5 GPa, 2 and 5 wt% H2 O added) using a piston-cylinder apparatus. With increasing temperature at 1.0 GPa, supersolidus mineral assemblages vary from amphibolitic to pyroxenitic. Garnet crystallizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low-K calc-alkalic trends, and are classified as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu–Bonin – Mariana (IBM) arc, for example Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) characteristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust (∼ 50% melting at 1050 °C and below 1.2 GPa). Estimated densities of pyroxenitic restites (∼ 3.9 g/cm3 ) after extraction of andesitic melts are higher than that of mantle peridotite beneath the island arc (3.3 g/cm3 ). The high density of the restite could cause delamination of the IBM arc lower crust. Rhyolitic magmas in the IBM arc (e.g. Niijima) could be formed by low degrees of partial melting of the amphibolitic crust at a temperature just above the solidus (10% melting at or below 900 °C). 相似文献
17.
Vladica Cvetkovi&#x; Hilary Downes Dejan Prelevi&#x; Marina Lazarov Kristina Resimi&#x;-&#x;ari&#x; 《Journal of Geodynamics》2007,43(4-5):504-527
A suite of highly depleted peridotite xenoliths in East Serbian Palaeogene basanites represents the lithospheric mantle beneath the Balkan Peninsula. The xenoliths are harzburgites, clinopyroxene-poor lherzolites and rare dunites. They contain mostly <5 vol.% of modal clinopyroxene and are characterized by high Mg# in silicates (>91), high Cr# in spinel (mostly 0.5–0.7), and by distinctively low Al2O3 contents in orthopyroxene (mostly 1–2 wt.%). They have experienced some mantle metasomatism which has slightly obscured their original composition. Nevertheless, the general characteristics of the xenoliths imply a composition which is significantly more depleted than most non-cratonic sub-continental mantle xenolith suites, as well as orogenic peridotites and abyssal peridotites. Geological and compositional evidence suggests that the xenoliths do not represent Archean mantle. The existence of Proterozoic mantle cannot be entirely excluded, although it is in disagreement with geological evidence. On the other hand, the studied xenoliths are compositionally very similar to peridotites of modern oceanic sub-arc settings. The existence of such a depleted lithospheric mantle segment is also inferred from the presence of rare orthopyroxene-rich xenoliths in the same suite. These are interpreted to have originated as lithospheric precipitates of high-Mg, SiO2-saturated magmas that require a highly depleted mantle source. Such source is typically required by boninitic-like magmas of intraoceanic suprasubduction settings. A proposed geodynamic model to explain these observations involves accretion or underplating of the lower parts of the Tethyan oceanic lithosphere during the Upper Jurassic closure of the eastern branch of the Vardar ocean. 相似文献
18.
Complete mantle section of a slow-spreading ridge-derived ophiolite: An example from the Isabela ophiolite in the Philippines 总被引：2，自引：0，他引：2
Abstract The Isabela ophiolite shows a complete ophiolite sequence exposed along the eastern coast of northern Luzon, the Philippines. It forms the Cretaceous basement complex for the northeastern Luzon block. This ophiolite is located at the northern end of a trail of ophiolites and ophiolitic bodies along the eastern margin of the Philippine Mobile Belt. This paper presents new findings regarding the nature and characteristics of the Isabela ophiolite. Peridotites from the Isabela ophiolite are relatively fresh and are composed of spinel lherzolites, clinopyroxene-rich harzburgites, depleted harzburgites and dunites. The modal composition, especially the pyroxene content, defines a northward depletion trend from fertile lherzolite to clinopyroxene-rich harzburgites and more refractory harzburgites. Variation in modal composition is accompanied by petrographic textural variations. The chromium number of spinel, an indicator of the degree of partial melting, concurs with petrographic observations. Furthermore, the Isabela ophiolite peridotites are similar in spinel and olivine major-element geochemistry and clinopyroxene rare earth-element composition to abyssal peridotites from modern mid-oceanic ridges. Petrological and mineral compositions suggest that the Isabela ophiolite is a transitional ophiolite subtype, with the fertile lherzolites representing lower sections of the mantle column that are usually absent in most ophiolitic massifs. The occurrence of the fertile peridotite presents a rare opportunity to document the lower sections of the ophiolitic mantle. The variability in composition of the peridotites in one continuous mantle section may also represent a good analogy of the melting column in the present-day mid-oceanic ridges. 相似文献
19.
The Ronda peridotite massif in southern Spain originated from the upper mantle, evidently as a rapidly rising diapir. Major and trace element abundance trends of the peridotites reflect their origin as residues from partial melting of garnet lherzolite. About 5% of the massif consists of mafic rocks, mainly pyroxenites and gabbros. They occur as concordant layers amidst the peridotites, and these layers do not cross-cut each other. However, major and trace element data show that the mafic layers do not have the geochemical characteristics of primary melts. We conclude that crystal/liquid fractionation occurred at high pressures ( > 19 kbar) as melts migrated through magma conduits towards the cooler exterior portion of the diapir. This process generated a sequence of “cumulates” (mainly clinopyroxene + orthopyroxene + spinel and clinopyroxene + garnet) along the walls of the conduits which are now represented by the mafic layers. 相似文献
20.
Trace elements and isotopic compositions of whole rocks and mineral separates are reported for 15 spinel-bearing harzburgite and lherzolite xenoliths from southeastern Australia. These samples have an exceedingly large range in isotopic compositions, with87Sr/86Sr ranging from 0.70248 to 0.70834 and εNd values ranging from +12.7 to −6.3. This range in isotopic compositions can be found in xenoliths from a single locality. The isotopic compositions of clinopyroxene separates and their whole rocks were found to be different in some xenoliths. Samples containing small glass pockets, which replace pre-existing hydrous minerals, generally show only small differences in isotopic composition between clinopyroxene and whole rock. In a modally metasomatized peridotite, significant differences in the Sr and Nd isotopic compositions of a coexisting phlogopite-clinopyroxene pair are present. Coexisting clinopyroxenes and orthopyroxenes from an anhydrous lherzolite have Sr isotopic compositions that are significantly different (0.70248 versus 0.70314), and yield an apparent age of 625 Ma, similar to that found previously by Dasch and Green [1]. However, the Nd isotopic compositions of the clinopyroxene and orthopyroxene are identical indicating recent (within 40 Ma) re-equilibration of Nd.Sr and Nd concentrations in the whole rocks and clinopyroxenes show an excellent positive correlation, and have an average Sr/Nd ratio of 15. This ratio is similar to the primitive mantle value, as well as that found in primitive MORBs and OIBs, but is much lower than that measured in island arc basalts and what might be predicted for a subduction zone-derived fluid. This indicates that a significant proportion of the Sr and Nd in these peridotites is introduced as a basaltic melt with intraplate chemical characteristics.The isotopic compositions of the peridotites reflect long-term, small-scale heterogeneities in the continental lithospheric mantle, and are in marked contrast to the near uniform isotopic compositions of the host alkali basalts (87Sr/86Sr= 0.7038–0.7041andεNd = +3.6 to +2.9). A minimum of three evolutionary stages are identified in the growth of the continental lithospheric mantle: an early basalt depletion event, recording the initial development and stabilization of the lithospheric mantle, followed by at least two enrichment episodes. These observations are consistent with continental lithospheric mantle growth involving the underplating of refractory peridotite diapirs. 相似文献