The microstructures in the Erro-Tobbio peridotite indicate several stages of recrystallization of olivine + titanian clinohumite-bearing assemblages. The development of these assemblages is closely associated with serpentinite mylonites, in which they occur in shear bands and foliations and are inferred to have grown synkinematically, in veins, and as post-kinematic radial aggregates. In the peridotite wall-rock adjacent to these mylonites, the same assemblages have recrystallized statically at the expense of original olivine and pyroxenes, mesh-textured chrysolite and antigorite veins. In addition, the olivine-bearing assemblage occurs in widespread vein systems. The brittle deformation of the peridotite resulting in the development of these vein systems is closely related to ductile deformation of metagabbroic dykes in the peridotite. Although early metasomatism resulted in extensive rodingitization of the gabbros, some dykes show an eclogitic assemblage of Na-clinopyroxene + garnet + chloritoid + chlorite ± talc. These observations, the microstructures and the mineral chemistry all suggest that the assemblages in the ultramafic rocks and metagabbros developed during a prograde evolution towards high pressures (>13–16 kbar, 450–550° C), and during subsequent decompression. This metamorphic evolution is considered to be related to Late Cretaceous intraoceanic subduction in the Alps-Apennine system and closure of the Piedmont-Ligurian basin. 相似文献
Major-element compositions of minerals in peridotite xenoliths from the Lac de Gras kimberlites provide constraints on the
mode of lithosphere formation beneath the central Slave Craton, Canada. Magnesia contents of reconstructed whole rocks correlate
positively with NiO and negatively with CaO contents, consistent with variable partial melt extraction. Alumina and Cr2O3 contents are broadly positively correlated, suggestive of melt depletion in the absence of a Cr–Al phase. Garnet modes are
high at a given Al2O3 content (a proxy for melt depletion), falling about a 7 GPa melt depletion model. These observations, combined with high
olivine Mg# and major-element relationships of FeO-poor peridotites (<7.5 wt%) indicative of melt loss at pressures >3 GPa
(residual FeO content being a sensitive indicator of melt extraction pressure), and similar high pressures of last equilibration
(∼4.2 to 5.8 GPa), provide multiple lines of evidence that the mantle beneath the central Slave Craton has originated as a
residue from high-pressure melting, possibly during plume subcretion. Apparent low melt depletion pressures for high-FeO peridotites
(>7.5 wt%) could suggest formation in an oceanic setting, followed by subduction to their depth of entrainment. However, these
rocks, which are characterised by low SiO2 contents (<43 wt%), are more likely to be the result of post-melting FeO-addition, leading to spuriously low estimates of
melt extraction pressures. They may have reacted with a silica-undersaturated melt that dissolved orthopyroxene, or experienced
olivine injection by crystallising melts. A secular FeO-enrichment of parts of the deep mantle lithosphere is supported by
lower average Mg# in xenolithic olivine (91.7) compared to olivine inclusions in diamond (92.6). 相似文献
The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in
two laboratories of USTC, China and CRPG, France, respectively. The187Os/188Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC) and from 0.117 to 0.131 (17 samples, CRPG). The Os
isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good
correlation with the major element compositions. With the187Os/188Os-Al2O3 alumichron, the samples yield a model age of 2.5 ± 0.1 Ga (data of USTC) and 1.9 ± 0.1 Ga (data of CRPG), late Archean to
early Pro-terozoic. The two samples with the lowest187Os/188Os ratio (0.119 and 0.117) have the TRD (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite
xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They
are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic
mantle. 相似文献
Abstract: The Antique ophiolite, located in Panay island (west‐central Philippines), corresponds to several tectonic slices within the suture zone between the Philippine Mobile Belt (PMB) and the North Palawan Block (NPB). It includes dismembered fragments of a basaltic sequence, dominantly pillow‐lavas with minor sheet flows, rare exposures of sheeted dikes, isotropic gabbros, subordinate layered mafic and ultramafic rock sequences and serpentinites. Most of the ophiolite units commonly occur as clasts and blocks within the serpentinites, which intrude the whole ophiolitic body, as well as, the basal conglomerate of the overlying Middle Miocene sedimentary formation. The volcanic rock sequence is characterized by chemical compositions ranging from transitional (T)‐MORB, normal (N)‐MORB and to chemistry intermediate between those of MORB and island arc basalt (IAB). The residual upper mantle sequence is harzburgitic and generally more depleted than the upper mantle underlying modern mid‐oceanic ridges. Calculations using whole‐rock and mineral compositions show that they can represent the residue of a fertile mantle source, which have undergone degrees of partial melting ranging from 9‐22.5 %. Some of the mantle samples display chondrite‐nor‐malized REE and extended multi‐element patterns suggesting enrichments in LREE, Rb, Sr and Zr, which are comparable to those found in fore‐arc peridotites from the Izu‐Bonin‐Mariana (IBM) arc system. The Antique ultramafic rocks also record relatively oxidizing mantle conditions (Δlog fO2 (FMQ)=0.9‐3.5). As a whole, the ophiolite probably represents an agglomeration of oceanic ridge and fore‐arc crust fragments, which were juxtaposed during the Miocene collision of the PMB and the NPB. The intrusion of the serpentinites might be either coeval or subsequent to the accretion of the oceanic crust onto the fore‐arc. Volcanogenic massive sulfide (VMS) deposits occur either in or near the contact between the pillow basalts and the overlying sediments or interbedded with the sediments. The morphology of the deposits, type of metals, ore texture and the nature of the host rocks suggest that the formation of the VMS bodies was similar to the accumulation of metals around and in the subsurface of hydrothermal vents observed in modern mid‐oceanic ridge and back‐arc basin rift settings. The podiform chromitites occur as pods and subordinate layers within totally serpentinized dunite in the residual upper mantle sequence. No large coherent chromitite deposit was found since the host dunitic rocks often occur as blocks within the serpentinites. It is difficult to evaluate the original geodynamic setting of the mineralized bodies since the chemistry of the host rocks were considerably modified by alteration during their tectonic emplacement. A preliminary conclusion for Antique is that the VMS is apparently associated with a primitive tholeiitic intermediate MORB‐IAB volcanic suite, the chemistry of which is close to the calculated composition of the liquid that coexisted with the podiform chromitites. 相似文献
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
Mantle-derived xenoliths and xenocrysts in Pale-ozoic diamondiferous ki mberlites in Mengyin (Shan-dong Province) and Fuxian (Liaoning Province) showthe presence of a cold,thick lithospheric mantle be-neath the North China craton ( NCC) in the MiddleOrdovician ( Griffin et al ., 1998 ; Menzies et al .,1993 ;Fan and Menzies ,1992) . However ,studies onmantle peridotites captured in the Tertiary to Neo-gene basalts of the NCC have revealed the existenceof a thin, hot and fertile lithosph… 相似文献
Spinel lherzolite xenoliths from Tertiary basaltic host magmas at Allyn River, eastern Australia reveal two distinct petrographic and geochemical types. One group is distinguished by xenoliths with undeformed, equilibrated microstructures and interstitial melt patches; The second group shows deformation and contains abundant fluid inclusions but no melt patches. Trace-element signatures of clinopyroxene in these xenoliths provide evidence for metasomatism by a silicate agent with hydrous component and by a carbonate-rich agent respectively.
Melt patches in the undeformed xenoliths contain secondary minerals including clinopyroxene, olivine, feldspar, Mg- and Ca-rich carbonate, apatite, ilmenite and spinel. They are interpreted to represent volatile-rich melt captured shortly prior to entrainment in the host basalt. Sulfide globules, now recrystallised to discrete sulfide phases but inferred to be molten at lithospheric mantle T and P, are closely associated with the melt patches. The close association between sulfide and highly mobile, volatile-bearing fluid has important implications for the mobility of Re and Os, the use of their isotopes in dating mantle events, and the possible effect of volatile-bearing metasomatic agents on their composition. 相似文献