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1.
Beishan Terrane, located in the northeast of the Tarim Block, in northwest China, has developed a 500-km long and 100-km wide belt of Permian mafic–ultramafic intrusions One of these mafic–ultramafic intrusions, the Xuanwoling Intrusion, is composed of dunite, troctolite, olivine gabbros and gabbros, with cumulate texture and rhythmic layering The crystallization sequence is olivine ? spinel ? plagio clase ? pyroxene, indicating that the crystallization pressure is lower than 0.5–0.8 GPa and that the intrusion has undergone variable degrees of crustal contamination, increasing from dunite to gabbros. The olivines found in the Xuanwoling Intrusion have high Fo values(up to 90), suggesting a primary magma with a high composition of mg. It is likely that this high-mg magma was produced at extremely high temperatures(1,330–1,350 °C), and as a result, Nd–Sr isotopic compositions similar to oceanic island basalts are found in the Xuanwoling Intrusion, which we propose arose from the mantle plume.  相似文献   

2.
The Kabr El-Bonaya mafic–ultramafic intrusion is exposed along the southeastern border of the Sinai Peninsula and the northernmost segment of the Arabian-Nubian Shield(ANS). It occurs as an elliptical intrusive body that is located along the major NE–SW trending fracture zones that prevail in the Kid metamorphic complex. The ultramafic rocks in the complex comprise ultramafic cumulates of peridotites(dunite, harzburgite and wehrlite) and pyroxenite. These rocks are generally unmetamorphosed and have intrusive contacts with the country rock. Mineral chemistry and whole-rock chemical compositions of these ultramafic rocks are mostly consistent with those of residual mantle peridotites from refractory suprasubduction tectonic settings. Based on the variations of the major elements, the studied ultramafic rocks are consistent with those of a supra-subduction zone mantle, as it seems to have melted at 1–2 GPa and 1300–1450°C. Linear variations of Al2 O3, CaO, V and Ni with MgO, coupled with incompatible and rare-earth-element depletion and mineral compositions,suggest prior events of partial melting in both wehrlites and harzburgites. The LREE enrichment in the harzburgite, as well as the development of Cr-rich spinel, is consistent with a history of melt–peridotite interaction. The calculated(Sm/Yb)N variations for the studied peridotites indicate a general increase in the addition of fluids with an increasing degree of melting from the wehrlite(～13–15 wt% of fluid) in the source, after initial spinel peridotite melting to the harzburgite(～20–25 wt% of fluid) in the same source, which is contrary to normal abyssal peridotites. The estimated equilibration temperature ranges from 1214 to 1321°C for the studied wehrlites and from 1297 to 1374°C for harzburgites. The Mg-rich nature of the analysed olivines from the studied ultramafic rocks(Fo = 81.41 to 91.77) reflect their primary composition and are similar to olivines in Alaskan-type ultramafic rocks. The Fo content of the analyzed olivines decrease slightly from the dunite to the harzburgite to the wehrlite and to pyroxenite, reflecting a fractional crystallization trend. The high Cr# and very low TiO2 contents(0.03–0.12 wt%) of the Cr-spinels from the studied peridotites are mostly consistent with modern highly refractory fore-arc peridotites, indicating that these peridotites developed in a supra-subduction zone environment.  相似文献   

3.
The Kekekete mafic-ultramafic rocks are exposed in the Kekesha-Kekekete-Dawate area,which are in the eastern part of the East Kunlun Orogenic Belt.It outcrops as tectonic slices intruding tectonically in the Paleoproterozoic Baishahe Group and the Paleozoic Nachitai Group.The Kekekete mafic and ultramafic rocks is located near the central fault in East Kunlun and lithologically mainly consists of serpentinite,augite peridotite,and gabbro.The LA-ICP-MS zircon U-Pb age of the gabbro is 501±7 Ma,indicating that Kekekete mafic-ultramafic rocks formed in the Middle Cambrian.This rock assemblage is relatively poor in SiO2 and(Na2 O+K2 O) but rich in MgO and SFeO.The chondrite-normalized REE patterns of the gabbro dip slightly to the right;the primitive mantle and MORBnormalized spidergrams of trace elements show enrichment of large-ion lithophile elements(Cs,Rb,Ba,etc.) and no differentiation of high field strength elements.The general dominance of E-MORB features and the geochemical characteristics of OIB suggest that the Kekekete mafic-ultramafic rocks formed in an initial oceanic basin with slightly enriched mantle being featured by varying degrees of mixing of N-MORB depleted mantle and a similar-OIB-type source.From a comprehensive study of the previous data,the author believes that the tectonic history of the East Kunlun region was controlled by a geodynamic system of rifting and extension in the late stages of the Neoproterozoic to early stages of the Early Paleozoic and this formed the paleo-oceanic basin or rift system now represented by the ophiolites along the central fault in East Kunlun,the Kekekete mafic-ultramafic rocks and Delisitan ophiolite.  相似文献   

4.
Located in the northern part of the Xinlin–Xiguitu suture zone,geochemistry and geochronology of the Xinlin ophiolite provide a unique opportunity to determine the the evolution of the eastern Xing’an–Mongolian Orogenic Belt.The Xinlin ophiolite was initially constrained roughly 21.5 km southeast of the Xinlin town by the First Regional Geological Survey Party of Heilongjiang Province.Subsequent work has shown that the mafic and ultramafic rocks in the adjacent Tayuan town was congenetic with the Xinlin ophiolite(Fig.1).Over the past three years we have conducted a series of studies to the Xinlin ophiolite with the aim to better understand its characteristic and tectonic implications.The present work is is to provide our preliminary geochemical data of the mafic rocks of the Xinlin ophiolite and possible"congenetic"mafic rocks in the Tayuan town.The mafic rocks of the Xinlin ophiolite including the gabbro,diabase and metabasalt show flat REE pattern[(La/Yb)N=0.68~1.58]and no Eu anomalies,which are transitional between normal and enriched mid-ocean ridge basalts(N-MORB and E-MORB).They also exhibit flat patterns from Ba to Yb in the trace elements spider diagram,which lie between those of typical E-MORB and N-MORB but closer to the former.The mafic rocks in the Tayuan town consist mainly of hornblende gabbro with alkaline affinity and are characterized by enriched in light rare earth elements and large ion lithophile elements,depleted in heavy rare earthelements and high field strength elements(Fig.2).The obvious differences in the geochemical characteristics indicate that the mafic rocks in the Tayuan town may not be cogenetic with those of the Xinlin ophiolite.This was further corroborated by their different formation time.Our zircon U–Pb dating indicates that the gabbro in the Tayuan town was emplaced during the late Carboniferous(~310 Ma;Fig.3),significantly younger than the recently reported U–Pb ages for the mafic rocks of the Xinlin ophiolite(~510 Ma;Feng,2015).Therefore,the two units appear as independent bodies and their origin and tectonic implication need to be further examined.  相似文献   

5.
Mineral chemistry,whole-rock major oxide,and trace element compositions have been determined for the Tuerkubantao mafic-ultramafic intrusion,in order to understand the early Paleozoic tectonic evolution of the West Junggar orogenic belt at the southern margin of the Central Asian orogenic belt.The Tuerkubantao mafic-ultramafic intrusion is a well-differentiated complex comprising peridotite,olivine pyroxenite,gabbro,and diorite.The ultramafic rocks are mostly seen in the central part of the intrusion and surrounded by mafic rocks.The Tuerkubantao intrusive rocks are characterized by enrichment of large ion lithophile elements and depleted high field strength elements relative to N-MORB.In addition,the Tuerkubantao intrusion displays relatively low Th/U and Nb/U(1.13—2.98 and 2.53—7.02,respectively) and high La/Nb and Ba/Nb(1.15—4.19 and 37.7—79.82,respectively).These features indicate that the primary magma of the intrusion was derived from partial melting of a previously metasomatized mantle source in a subduction setting.The trace element patterns of peridotites,gabbros,and diorite in the Tuerkubantao intrusion have sub-parallel trends,suggesting that the different rock types are related to each other by differentiation of the same primary magma.The intrusive contact between peridotite and gabbro clearly suggest that the Tuerkubantao is not a fragment of an ophiolite.However,the Tuerkubantao intrusion displays many similarities with Alaskan-type mafic-ultramafic intrusions along major sutures of Phanerozoic orogenic belts.Common features include their geodynamic setting,internal lithological zoning,and geochemistry.The striking similarities indicate that the middle Devonian Tuerkubantao intrusion likely formed in a subduction-related setting similar to that of the Alaskan-type intrusions.In combination with the Devonian magmatism and porphyry mineralization,we propose that subduction of the oceanic slab has widely existed in the expansive oceans during the Devonian around the Junggar block.  相似文献   

6.
There is a general consensus that most ophiolites formed above subduction zones(Pearce,2003),particularly during forearc extension at subduction initiation(Shervais,2001;Stern,2004;Whattam and Stern,2011)."Supra-Subduction zone"(SSZ)ophiolites such as the well-studied Tethyan ophiolites,generally display a characteristic sequential evolution from mid-oceanic ridge basalts(MORBs)to island arc tholeiities(IATs)or bonites(BONs)(Pearce,2003;Dilek and Furnes,2009,2011),which were generated in sequence from the decompression melting of asthenospheric mantle and partial melting of subduction-metasomatized depleted mantle(Stern and Bloomer,1992;Dilek and Furnes,2009;Whattam and Stern,2011).However,ophiolites with MORB and/or oceanic-island basalt(OIB)affinities are rare,and their origin and tectonic nature are poorly understood(Boedo et al.,2013;Saccani et al.,2013).It is interesting that the composition of these ophiolites from the central Tibetan Plateau(CTP)is dominated by MORBs and minor OIBs and a distinct lack of IATs and BONs,which is inconsistent with most ophiolites worldwide(Robinson and Zhou,2008;Zhang et al.,2008).But the generation and tectonic nature of these ophiolites are still controversial.*In this study,we present new geochronological,mineralogical and Sr-Nd isotopic data for the Chayong and Xiewu mafic complexes in the western Garzê-Litang suture zone(GLS),a typical Paleo-Tethyan suture crossing the CTP(Fig.1).The Triassic ophiolite in the western GLS has been described by Li et al.(2009),who foundthat it mainly consists of gabbros,diabases,pillow basalts and a few metamorphic peridotites.The ophiolite has been tectonically dismembered and crops out in Triassic clastic rocks and limestones as tectonic blocks.The Chayong and Xiewu mafic complexes are generally regarded as important fragments of the Triassic ophiolites(e.g.,Jin,2006;Li et al.,2009).Zircon LA-ICP-MS U-Pb ages of234±3 Ma and 236±2 Ma can be interpreted as formation times of the Chayong and Xiewu mafic complexes,respectively.The basalts and gabbros of the Chayong complexexhibitenrichedMORB(E-MORB)compositional affinities except for a weak depletion of Nb,Ta and Ti relative to the primitive mantle,whereas the basalts and gabbros of the Xiewu complex display distinct E-MORB and OIB affinities.The geochemical features suggest a probable fractionation of olivine±clinopyroxene±plagioclase as well as insignificant crustal contamination.The geochemical and Sr-Nd isotopic data reveal that the Chayong mafic rocks may have been derived from depleted MORB-type mantle metasomatized by crustal components and Xiewu mafic rocks from enriched lithosphericmantlemetasomatizedbyOIB-like components.The ratios of Zn/Fet,La/Yb and Sm/Yb indicate that these mafic melts were produced by the partial melting of garnet+minor spinel-bearing peridotite or spinel±minor garnet-bearing peridotite.We propose thatback-arcbasinspreadingassociated with OIB/seamount recycling had occurred in the western GLS at least since the Middle Triassic times,and the decompression melting of the depleted MORB-type asthenospheremantleandpartialmeltingof sub-continental lithosphere were metasomatized by plume-related melts,such as OIBs,which led to the generation of the Chayong and Xiewu mafic melts.  相似文献   

7.
Discontinuous chains of ultramafic rock bodies form part of the 3800–3700 Ma Isua Supracrustal Belt(ISB),hosted in the Itsaq Gneiss Complex of southwestern Greenland.These bodies are among the world’s oldest outcrops of ultramafic rocks and hence an invaluable geologic record.Ultramafic rocks from Lens B in the northwestern limb of ISB show characteristics of several stages of serpentinization and deserpentinization forming prograde and retrograde mineral assemblages.Ti-rich humite-group minerals such as titanian chondrodite(Ti-Chn)and titanian clinohumite(Ti-Chu)often occur as accessory phases in the metamorphosed ultramafic rocks.The Ti-rich humite minerals are associated with metamorphic olivine.The host olivine is highly forsteritic(Fo96-98)with variable Mn O and Ni O contents.The concentrations of the rare-earth elements(REE)and high-field strength elements(HFSE)of the metamorphic olivine are higher than typical mantle olivine.The textural and chemical characteristics of the olivine indicate metamorphic origin as a result of deserpentinization of a serpentinized ultramafic protolith rather than primary assemblage reflecting mantle residues from high-degrees of partial melting.The close association of olivine,antigorite and intergrown Ti-Chn and Ti-Chu suggests pressure condition between$1.3–2.6 GPa within the antigorite stability field(<700°C).The overall petrological and geochemical features of Lens B ultramafic body within the Eoarchean ISB indicate that these are allochthonous ultramafic rocks that recorded serpentine dehydration at relatively lower temperature and reached eclogite facies condition during their complex metamorphic history similar to exhumed UHP ultramafic rocks in modern subduction zone channels.  相似文献   

8.
The Changning–Menglian Belt represents the main Paleo-Tethyan Suture in the southeastern Tibetan Plateau, which divides Gondwana- and Eurasia-derived continental fragments from each other. The belt contains ultramafic–mafic volcanic rocks that provide evidence of the tectonic processes which operated during the evolution of the Paleo-Tethyan Ocean. New geochemical data for Early Carboniferous volcanics in the southern Changning–Menglian Belt show that wehrlites have cumulate and poikilitic textures, and contain low-Fo (84.2–87.2) olivine, clinopyroxene with low Mg# values (79.4–85.6), and spinel with high Cr# values (67.8–72.4). Estimated equilibrium temperatures obtained using olivine-spinel Fe-Mg exchange geothermometry range from 978°C to 1373°C (mean = 1205°C; n = 3). These observations combined with a lack of reaction or melt impregnation textures indicate that these units represent magmatic cumulates rather than having formed as a result of mantle-melt reactions. Both wehrlites and basalts in the belt have subparallel rare earth element (REE)-and primitive-mantle-normalized multi-element patterns with slightly positive Nb-Ta anomalies, but negligible Eu and Zr-Hf anomalies. The volcanics have similar Sr-Nd-Pb isotopic compositions with εNd(t) values of 4.2–4.5 (mean = 4.3; n = 3) and 4.0–4.4 (mean 4.2; n = 4), respectively, and also have similar immobile element ratios, such as Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr/Y. These characteristics indicate both units have ocean island basalt (OIB)-like geochemical affinities, consistent with the fact that the clinopyroxene in the wehrlites is compositionally similar to OIB-related cumulus clinopyroxene. This suggests that both the wehrlites and basalts were derived from similar parental magmas that underwent generally closed-system magmatic differentiation dominated by fractionation of the olivine and clinopyroxene. This parental magma was likely generated in an oceanic seamount setting from an OIB-type mantle source (i.e., asthenospheric mantle) containing garnet-spinel lherzolite material. Combing this new data with that from oceanic seamount volcano-sedimentary suites derived from previous research enables the identification of a mature late Paleozoic ocean basin between the passive northeastern Gondwanan margin and the northward-migrating microcontinent of Lanping–Simao.  相似文献   

9.
The Xuhe mafic rocks, located in Ziyang county of Shaanxi Province, are dominated by diabase-porphyrite, gabbro–diabase, diabase, and pyroxene diorite. Primitive mantle-normalized multi-element patterns show that, the Xuhe mafic rocks are enriched in large ion lithophile elements(LITE), such as Ba and Pb, depleted in K and Sr for basic rocks, and are depleted in Sr, P and Ti for pyroxene diorite. Chondrite-normalized REE patterns display LREE enrichment(LaN/YbN = 9.34–13.99) and have normalized patterns for trace element and REE similar to that of typical OIB. Detailed SIMS zircon U–Pb dating yields emplacement ages of 438.4 ± 3.1 Ma for Xuhe mafic rocks. The relatively low Mg O(basic rock: 3.11–7.21 wt%; pyroxene diorite: 0.89–1.21 wt%) and Mg#(0.20–0.49) for Xuhe mafic rocks suggest that they were possibly originated from an extremely evolved magma. The rising parental mafic magmas underwent pyroxene and plagioclase fractionation. Crustal contamination of pyroxene diorite before emplacement occurred at a higher crustal level compared to other lithology in Xuhe mafic rocks. The degree of partial melt was low(5%–10%) and in garnetspinel transition facies. Sr-Nd isotope of pyroxene diorite and enrichment mantle characteristics for Xuhe mafic rocks suggest that mafic rocks in the North Daba Mountains were derived from a mixture of HIMU, EMII and small amount of EMI components. Furthermore, this study discusses mantle geodynamic significance of Xuhe mafic rocks in the Silurian, which indicates subduction and uplift of magma caused back-arc extension.  相似文献   

10.
The Yarlung Zangbo suture zone extends more than2000 km along southern Tibet and marks the boundary between the Indian subcontinent and Eurasia.The Zedong terrane has been not suggested to represent the vestige of such an intra-oceanic arc developed within the Neo-Tethys Ocean,as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic.In this study,we present detailed geochemical and geochronological data of various types of magmatic rocks widely exposed in the Zedong terrane to constrain the formation age and tectonic setting of the Zedong terrane.We found that the Zedong volcanic rocks belong to high K2O calc-alkaline series,whereas the diabase and gabbro plotted in the low-K calcalkline.The basalt rocks are highly enriched in LREE and LILE,but strongly depleted in HFSE,indicating they were derived from a metasomatized mantle.Both gabbros and diabase have similar N-MORB geochemistry indicates that the cumulates were produced from MOR setting.Zircons from four samples,including the basalt rocks(158-161Ma)are older than the gabbro(131 Ma),certificate the gabbro are as the vein intrude into the basalt rocks.This suggests that the volcanic eruption and plutonic emplacement were coevally developed in the Zedonghave similar positiveεHf(t)values(+2.0 to+15.6)and(+8.6 to+18.4),indicating they were stemmed from similarly depleted mantle sources,same with the gabbro and granitic rocks from the Gangdese arc.Therefore,we proposed that the basalt rocks in the Zedong terrane were formed through partial melting of the mantle wedge metasomatized by slab-released fluids/melts.A part of hydrous basalts were underplated in the thickened lower crust beneath the Zedong terrane,which gave rise to the cumulate and granitic rocks.This suggests that the Zedong terrane represents a slice of the active continental margin developed on the southern margin of the Lhasa terrane as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic,although a possible intra-oceanic arc setting cannot be excluded.  相似文献   

11.
The Baishiquan and Pobei Early Permian mafic-ultramafic intrusions were emplaced into Proterozoic metamorphic rocks in the Central Tianshan and the Beishan Fold Belt,northern Xinjiang,NW China.The Baishiquan intrusion comprises mainly gabbro,and mela-gabbro sills occur within and along the margins of the gabbro body.In the Pobei intrusion,two distinct gabbroic packages,a lower gabbro and the main gabbro,are intruded and overlain by small cumulate wehrlite bodies.  相似文献   

12.
This paper investigates the petrogenesis of the Seqi Ultramafic Complex, which covers a total area of approximately 0.5 km~2. The ultramafic rocks are hosted by tonalitic orthogneiss of the ca. 3000 Ma Akia terrane with crosscutting granitoid sheets providing an absolute minimum age of 2978 ± 8 Ma for the Seqi Ultramafic Complex. The Seqi rocks represent a broad range of olivine-dominated plutonic rocks with varying modal amounts of chromite, orthopyroxene and amphibole, i.e. various types of dunite(s.s.),peridotite(s.l.), as well as chromitite. The Seqi Ultramafic Complex is characterised primarily by refractory dunite, with highly forsteritic olivine with core compositions having Mg# ranging from about 91 to 93. The overall high modal contents, as well as the specific compositions, of chromite rule out that these rocks represent a fragment of Earth's mantle. The occurrence of stratiform chromitite bands in peridotite, thin chromite layers in dunite and poikilitic orthopyroxene in peridotite instead supports the interpretation that the Seqi Ultramafic Complex represents the remnant of a fragmented layered complex or a magma conduit, which was subsequently broken up and entrained during the formation of the regional continental crust.Integrating all of the characteristics of the Seqi Ultramafic Complex points to formation of these highly refractory peridotites from an extremely magnesian(Mg# ~ 80), near-anhydrous magma, as olivinedominated cumulates with high modal contents of chromite. It is noted that the Seqi cumulates were derived from a mantle source by extreme degrees of partial melting(40%). This mantle source could potentially represent the precursor for the sub-continental lithospheric mantle(SCLM) in this region,which has previously been shown to be ultra-depleted. The Seqi Ultramafic Complex, as well as similar peridotite bodies in the Fiskefjord region, may thus constitute the earliest cumulates that formed during the large-scale melting event(s), which resulted in the ultra-depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth's first continents and the corresponding SCLM.  相似文献   

13.
The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.  相似文献   

14.
Mid-ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO–Al2O3 and negative MgO–Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium-bearing MORB sample E13-3B (MgO=9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13-3B only by fractional crystallization at pressure >4 ±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure ~1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at >4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at >4±1 kbar to mainly olivine+plagioclase crystallization at <1 kbar, which contributes to the explanation of the “clinopyroxene paradox”.  相似文献   

15.
The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The ε_(Nd)(t) values in the rock units vary from +6.70 to +9.64, and initial ~(87)Sr/~(86)Sr ratios range between 0.7035 and0.7042. Initial ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)_(PM) values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)_(PM)ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.  相似文献   

16.
<正>The rocks form as amphibolite±garnet±epidote and orthogneisses in the Pan-African basement of the Bitlis Massif.The petrochemical data of the studied metamorphic rocks suggest different igneous protoliths ranging from calcalkaline basalt to andesite in composition. Petrochemically,the rocks can be classified as group 1(low Zr and La) and group 2(high Zr and La), all showing various enrichments in large ion lithophiles and light rare earth elements,and a depletion in high-field strength elements,suggestive of a destructive plate margin setting.The protoliths of the all samples might have formed mostly by the partial melting of an enriched source,possibly coupled with the fractional crystallization of plagioclase,apatite,and titaniferous magnetite±olivine±clinopyroxene±amphibole in relation with subduction-related magmatism neighboring the Andeantype active margins of Gondwana.The group 2 samples could,however,be generated by a relatively lower degree of the partial melting of an inhomogeneous source with a preponderance of a high-level, fractional crystallization process in comparison to group 1.The protoliths of the samples were metamorphosed up to amphibolite facies conditions,which destroys original igneous texture and mineral assemblages.Geothermobarometric calculations show that the metamorphic rocks are finally equilibrated between 540 and 610℃and～5 kbars,following a clockwise P-T-t path.  相似文献   

17.
The Neotethys plays an important role in shaping the Gangdese magmatic belt,southern Tibet.However,the initial time of spreading and subduction of the Neotethys remains contentious.In this study,a suite of late Triassic cumulate hornblende gabbro was identified in the southern margin of the Gangdese magmatic belt.The gabbro exhibits cumulate structure,with hornblende and plagioclase as the primary mineral phases.Isotopic data indicate a hydrous magma source derived from a depleted mantle wedge that has been modified by slab dehydration.Geochemical discriminations suggest that the gabbro was formed in an intraoceanic arc setting,with crystallization ages of ca.220-213 Ma.Hornblende,hornblendelagioclase and ilmenite thermometers reveal that the crystallization temperature of 900-750°C for the gabbro.Hornblende and hornblende-plagioclase geobarometers yield an emplacement depth at ca.14.5-19.5 km.This gabbro constitutes a line of evidence for an intraoceanic arc magmatism that is coeval with the counterparts in the southern Turkey,revealing an intraoceanic subduction system within the Neotethys from west to east in the Late Triassic and that the oceanization of the Neotethys was much earlier than previous expectation.  相似文献   

18.
The Precambrian basement rocks of the Eastern Granite-Rhyolite Province(EGRP)in central Illinois(midcontinent region of North America)exhibit a complex history of early volcanism,granite emplacement,and intrusion of mafic rocks.A comprehensive suite of dedicated petrographic analyses,geophysical logs,and drill core from four basement-penetrating wells,two-dimensional and three-dimensional seismic reflection data,and U-Pb age data from the Illinois Basin–Decatur Project(IBDP)and Illinois Carbon Capture Storage(ICCS)Project site provide new constraints for interpreting the Precambrian basement of the Illinois Basin.These new data reveal the basement to be compositionally and structurally complex,having typical EGRP felsic volcanic rocks intruded by the first reported gabbro in the Precambrian basement in Illinois.Zircons(n?29)from rhyolite give a U-Pb weighted mean average age of 14679 Ma.Zircons(n?3)from a gabbro dike that intrudes the rhyolite yield a concordia age of 107312 Ma,which corresponds to Grenville-age extension and represents the first Grenville-age rock in Illinois and in the EGRP.A high-resolution three-dimensional seismic reflection volume,coincident with the four wells,provides a context for interpreting the petrological data and implies a high degree of heterogeneity for basement rocks at the IBDP–ICCS site,as also shown by the drill cores.The occurrence of Grenville-age gabbro is related to a prominent bowl-like structure observed on local two-dimensional seismic reflection profiles and the three-dimensional volume that is interpreted as a deep-seated mafic sill complex.Furthermore,heterogeneities such as the brecciated EGRP rhyolite and later gabbro intrusion observed in the basement lithology at the IBDP–ICCS may reflect previously unknown distal elements of the 1.1 Ga Midcontinent Rift in the EGRP and more likely Grenville-age extension.  相似文献   

19.
Fresh rocks sampled from the 14.0°S hydrothermal field of the South Atlantic Ridge can be divided into two categories： olivine-gabbro and basalt. The olivine-gabbro is composed mainly of three types of minerals： olivine, clinopyroxene and plagioclase, while a multitude of melt inclusions occur in the plagioclase phenocrysts of the basalts. We analyzed the whole-rock, major and trace elements contents of the basaks, the mineral chemistry of phenocrysts and melt inclusions in the basalts, and the mineral chemistry of olivine-clinopyroxene-plagioclase in the olivine-gabbro, then simulated magma evolution within the crust using the COMAGMAT program. The whole-rock geochemistry shows that all the basalts exhibit typical N-MORB characteristics. In addition, the mineral chemistry characteristics of the olivine-gabbro （low-Fo olivine, low-Mg# clinopyroxene, high-TiO2 clinopyroxene, low-An plagioclase）, show that strong magma differentiation occurred within the crust. Nevertheless, significant discrepancies between those minerals and phenocrysts in the basalts （high-Fo olivine, high-An plagioclase） reflect the heterogeneity of magma differentiation. High Mg# （-~0.72） melt inclusions isobaric partial crystallization simulations suggest that the magma differentiation occurred at the depth shallower than 13.03 km below the seafloor, and both the vertical differentiation column shows distinct discrepancies from that of a steady-state magma chamber. Instead, a series of independent magma intrusions probably occurred within the crust, and their corresponding crystallized bodies, as the primary high-temperature thermal anomalies within the off-axis crust, probably act as the heat source for the development of the 14.0°S hydrothermal system.  相似文献   

20.
In this contribution we use VMS mineral systems in the Bryah rift-basin to constrain the tectonic setting of the widespread mafic and ultramafic magmatism that characterises the rift-basin in question.Two distinct,but temporally closely associated,lithostratigraphic sequences,Narracoota and Karalundi Formations,are discussed.The Karalundi Formation is the main host of VMS mineral systems in the region.The Karalundi Formation consists of turbiditic and immature clastic sediments,which are locally intercalated with basaltic hyaloclastites,dolerites and banded jaspilites.We propose that the basaltic hyaloclastites,dolerites and elastics and jaspilites rocks,form a distinct unit of the Karalundi Formation,named Noonyereena Member.The VMS mineral systems occur near the north-east trendingJenkin Fault and comprise the giant and world-class DeCrussa and the Red Bore deposits.The nature of these deposits and their intimate association with terrigenous clastic rocks and dominantly marine mafic volcanic and subvolcanic rocks,as well as the common development of peperitic margins,are considered indicative of a Besshi-type environment,similar to that of present-day Gulf of California.Our Re-Os age data from a primary pyrite yielded a mean model age of 2012 ± 48 Ma,which coincides(within error) with recent published Re-Os data(Hawke et al.,2015) and confirms the timing of the proposed geodynamic evolution.We propose a geodynamic model that attempts to explain the presence of the Narracoota and Karalundi Formations as the result of mantle plume activity,which began with early uplift of continental crust with intraplate volcanism,followed by early stages of rifting with the deposition of the Karalundi Formation(and Noonyereena Member),which led to the formation of Besshi-type VMS deposits.With on-going mantle plume activity and early stages of continental separation,an oceanic plateau was formed and is now represented by mafic-ultramafic rocks of the Narracoota Formation.  相似文献   

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