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1.
A high‐P granulite facies gneiss complex occurs in north‐west Payer Land (74°28′?74°47′N) in the central part of the East Greenland Caledonian (Ordovician–Devonian) orogen. High‐P metamorphism of the Payer Land gneiss complex resulted in formation of the assemblages Grt + Cpx + Amp + Qtz + Ru ± Pl in mafic rocks, and Grt + Ol + Cpx + Opx + Spl in rare ultramafic pods. Associated metapelites experienced anatexis in the kyanite stability field. Peak metamorphic assemblages formed around 800–850 °C at pressures of c. 1.4–1.7 GPa, corresponding to crustal depths of c. 50 km. Mafic granulites contain abundant reaction textures, including the replacement of garnet by symplectites of Opx + Spl + Pl, indicating that the high‐P event was followed by decompression while the granulites remained at elevated temperatures. Charnockitic gneisses from Payer Land show evidence of late Archean (c. 2.8–2.4 Ga) crustal growth and subsequent Palaeoproterozoic (c. 1.85 Ga) metamorphism. The gneiss complex experienced intense reworking during the Caledonian continental collision. On the basis of Caledonian monazite ages recorded from the high‐P anatectic metapelites, the clockwise P–T evolution and formation of the high‐P granulite facies assemblages is related to Caledonian crustal thickening, which resulted in formation of eclogites approximately 300 km north of Payer Land. The Payer Land granulites comprise a metamorphic core complex, which is separated from the overlying low‐grade supracrustal rocks (the Neoproterozoic Eleonore Bay Supergroup) by a late Caledonian extensional fault zone, the Payer Land Detachment. The steep, nearly isothermal, unloading P–T path recorded by the granulites can be explained by erosional and tectonic unroofing along the Payer Land Detachment.  相似文献   

2.
Ultramafic blocks that themselves contain eclogite lenses in the Triassic Su-Lu ultrahigh-P terrane of eastern China range in size from hundreds of metres to kilometres. The ultramafic blocks are enclosed in quartzofeldspathic gneiss of early Proterozoic age. Ultramafic rocks include garnetiferous lherzolite, wehrlite, pyroxenite, and hornblende peridotite. Garnet lherzolites are relatively depleted in Al2O3 (<3.8wt%), CaO (<3.2%) and TiO2 (<0.11 wt%), and are low in total REE contents (several p.p.m.), suggesting that the rocks are residual mantle material that was subjected to low degrees of partial melting. The eclogite lenses or layers within the ultramafic rocks are characterized by higher MgO and CaO, lower Al2O3 and TiO2 contents, and a higher CaO/Al2O3 ratio compared to eclogites enclosed in the quartzofeldspathic gneiss. Scatter in the plots of major and trace elements vs. MgO, REE patterns and La, Sm and Lu contents suggest that some eclogites were derived from melts formed by various degrees (0.05–0.20) of partial melting of peridotite, and that other eclogites formed by accumulation of garnet and clinopyroxene ± trapped melt in the upper mantle. Both ultramafic and eclogitic rocks have experienced a complex metamorphic history. At least six stages of recrystallization occurred in the ultramafic rocks based on an analysis of reaction textures and mineral compositions. Stage I is a high temperature protolith assemblage of Ol + Opx + Cpx + Spl. Stage II consists of the ultrahigh-pressure assemblage Ol + Cpx + Opx + Grt. Stage III is manifested by the appearance of fine-grained garnet after coarse-grained garnet. Stage IV is characterized by formation of kelyphitic rims of fibrous Opx and Cpx around garnet, and replacement of garnet by spinel and pargasitic-hornblende. Stage V is represented by the assemblage Ol + Opx + Prg-Hbl + Spl. The mineral assemblages of stages VIA and VIB are Ol + Tr-Amp + Chl and Serp + Chl ± talc, respectively. Garnet and orthopyroxene all show a decrease in MgO with retrogressive recrystallization and Na2O in clinopyroxene also decreases throughout this history. Eclogites enclosed within ultramafic blocks consist of Grt + Omp + Rt ± Qtz ± Phn. A few quartz-bearing eclogites contain rounded and oval inclusion of polycrystalline quartz aggregates after coesite in garnet and omphacite. Minor retrograde features include thin symplectic rims or secondary amphiboles after Cpx, and ilmenite after rutile. P-T estimates indicate that the ultrahigh-metamorphism (stage II) of ultramafic rocks occurred at 820-900d? C and 36-41 kbar and that peak metamorphism of eclogites occurred at 730-900d? C and >28 kbar. Consonant with earlier plate tectonic models, we suggest that these rocks were underplated at the base of the continental crust. The rocks then underwent ultrahigh-pressure metamorphism and were tectonically emplaced into thickened continental crust during the Triassic collision between the Sino-Korean and Yangtze cratons.  相似文献   

3.
We report here the detailed microstructure and chemistry of pyroxene exsolution from a polycrystalline garnet porphyroblast of the Western Gneiss Region (WGR) garnet peridotite, Otrøy, Norway. For both clinopyroxene (Cpx) and orthopyroxene (Opx), the same basic crystallographic relationship is found with the host garnet: (100)py//{112}grt, (010)px//{110}grt and (001)px//{111}grt for the majority (>90%) of its intracrystalline pyroxene rods. In addition, this pattern is also exhibited by some interstitial Opx and a subpopulation of both pyroxenes shows a different pattern or no discernible pattern. The results provide quantitative microstructural evidence demonstrating an exsolution (precipitation) origin of both the intracrystalline Cpx and Opx and the small interstitial Opx crystals. The reconstructed precursor majoritic garnet, taking into account both the intracrystalline pyroxenes and interstitial Opx, was characterized by Si = ～3.07 cation per formula unit that corresponds to a minimum pressure of 7.7 GPa (～250 km depth). We also deduce from the observation of Opx being the majority of intracrystalline precipitates and 100% of the interstitial ones that the precursor majoritic garnet probably originated from a pressure less than ～10 GPa (～300 km depth). A multistage decomposition hypothesis is proposed for this WGR majoritic garnet during exhumation of the peridotite from 250 to 300 km depth to explain the topotaxy and chemistry of the exsolved pyroxenes.  相似文献   

4.
ABSTRACT

We investigated lherzolitic peridotites in the Cretaceous Purang ophiolite along the Yarlung Zhangbo suture zone (YZSZ) in SW Tibet to constrain their mantle–melt evolution history. Coarse-grained Purang lherzolites contain orthopyroxene (Opx) and olivine (Ol) porphyroclasts with embayments filled by small olivine (Ol) neoblasts. Both clinopyroxene (Cpx) and Opx display exsolution textures represented by lamellae structures. Opx exsolution (Opx1) in clinopyroxene (Cpx1) is made of enstatite, whose compositions (Al2O3 = 3.85–4.90 wt%, CaO = <3.77 wt%, Cr2O3 = 0.85–3.82 wt%) are characteristic of abyssal peridotites. Host clinopyroxenes (Cpx1) have higher Mg#s and Na2O, with lower TiO2 and Al2O3 contents than Cpx2 exsolution lamellae in Opx, and show variable LREE patterns. Pyroxene compositions of the lherzolites indicate 10–15% partial melting of a fertile mantle protolith. P–T estimates (1.3–2.3 GPa, 745–1067°C) and the trace element chemistry of pyroxenes with exsolution textures suggest crystallization depths of ~75 km in the upper mantle, where the original pyroxenes became decomposed, forming exsolved structures. Further upwelling of lherzolites into shallow depths in the mantle resulted in crystal–plastic deformation of the exsolved pyroxenes. Combined with the occurrence of microdiamond and ultrahigh-pressure (UHP) mineral inclusions in chromites of the Purang peridotites, the pyroxene exsolution textures reported here confirm a multi-stage partial melting history of the Purang lherzolites and at least three discrete stages of P-T conditions in the course of their upwelling through the mantle during their intra-oceanic evolution.  相似文献   

5.
The upper deck of the East Athabasca mylonite triangle (EAmt), northern Saskatchewan, Canada, contains mafic granulites that have undergone high P–T metamorphism at conditions ranging from 1.3 to 1.9 GPa, 890–960 °C. Coronitic textures in these mafic granulites indicate a near‐isothermal decompression path to 0.9 GPa, 800 °C. The Godfrey granite occurs to the north adjacent to the upper deck high P–T domain. Well‐preserved corona textures in the Godfrey granite constrain igneous crystallization and early metamorphism in the intermediate‐pressure granulite field (Opx + Pl) at 1.0 GPa, 775 °C followed by metamorphism in the high pressure granulite field (Grt + Cpx + Pl) at 1.2 GPa, 860 °C. U–Pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c. 2.5 Ga and c. 1.9 Ga. The oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith. A population of 1.9 Ga zircon in one mafic granulite is interpreted to constrain the timing of high P–T metamorphism. Titanite from the mafic granulites yields dates ranging from 1900 to 1894 Ma, and is interpreted to have grown along the decompression path, but still above its closure temperature, indicating cooling following the high P–T metamorphism from c. 960–650 °C in 4–10 Myr. Zircon dates from the Godfrey granite indicate a minimum crystallization age of 2.61 Ga, without any evidence for 1.9 Ga overgrowths. The data indicate that an early granulite facies event occurred at c. 2.55–2.52 Ga in the lower crust (c. 1.0 GPa), but at 1.9 Ga the upper deck underwent high P–T metamorphism, then decompressed to 0.9–1.0 GPa. Juxtaposition of the upper deck and Godfrey granite would have occurred after or been related to this decompression. In this model, the high P–T rocks are exhumed quickly following the high pressure metamorphism. This type of metamorphism is typically associated with collisional orogenesis, which has important implications for the Snowbird tectonic zone as a fundamental boundary in the Canadian Shield.  相似文献   

6.
Abstract End-member, continuous and degenerate reactions are derived for the multisystem with the six components Na2O, CaO, (Mg/Fe)O, Al2O3, SiO2, H2O among the phases plagioclasess, garnetss, amphiboless, cpx, opx, olivine, spinel, quartz and an aqueous fluid. The chemography of this system is degenerate due to the co-linearity 2Opx = Ol + Qtz. This co-linearity has its implications both on reaction space and phase equilibria. From a total of 28 reaction systems, reaction space is derived for nine subsystems (phases in parentheses are absent): Case A1: (Cpx,Ol) (Cpx,Opx) and (Cpx,Qtz), Case A2: (Spl,Ol) (Spl,Opx) and (Spl,Qtz), Case B: (Ol,Opx) (Ol,Qtz) and (Opx,Qtz). In the absence of either cpx or spl (case A), three reactions form an invariant point, either [Cpx] or [Spl], where the co-linear phases olivine, opx and quartz coexist on the transformation line 2Opx = Ol + Qtz. Changing mineral compositions force invariant points to move along the line with the different reaction curves changing their relative position according to Schreinemakers’rules. Zero contours, i.e. the location where (a) phase(s) disappear(s) in reaction space correspond to singular points in phase diagrams. Two types are distinguished; singular points of indispensable and of substitutable phases. In the first case the phase disappears from the entire bundle while in the second it disappears from a single reaction. In the specific case where the substitutable phases are also the co-linear ones, two of the three co-linear phases disappear simultaneously. Two of the three reaction curves coincide. In the system including Cpx and Spl (Case B) three reactions, (Ol,Opx) (Ol,Qtz) and (Opx,Qtz), oppose three invariant points, [Ol], [Opx] and [Qtz]. Invariant points no longer move along the line 2Opx = Ol + Qtz. The coincidence of the zero contours of all three co-linear phases in reaction space-the result of the chemographic degeneracy-causes the respective singular points to coincide in the phase diagrams. This is the location where curves must be rearranged in a bundle to conform Schreinemakers’rules. The reaction Grs1Prp2= 2 Ol + An is fourth order degenerate and part of all nine subsystems (cases A and B). It can be used to relate the different phase diagrams to one another.  相似文献   

7.
胶北莱西古元古代的高压基性麻粒岩和钙硅酸盐岩的基本矿物组合分别为以铁铝榴石为主的石榴石-普通辉石-铁紫苏辉石和钙铝榴石-黝帘石-葡萄石-钠长石.矿物岩石学研究表明钙硅酸盐岩是由含石榴石高压基性麻粒岩经退变质和钙质交代作用形成.南山口高压基性麻粒岩记录了麻粒岩相变质作用前、麻粒岩相变质作用、退变质和钙硅酸盐岩化共同作用以及完全钙硅酸盐岩化的四个阶段的地质作用,其矿物组合分别为Cpx+ Pl+ Qtz(M1),Grt+ Cpx+ Rt+ Qtz(M2),Cpx+Pl+ Opx+ Ilm+ Mgt+ Ep(M3)和Grs+ Zo+ Prh+ Ab+ Cal(M4).微量元素研究表明,高压基性麻粒岩中大离子亲石元素Ba、Rb、K、Rb、Th富集,而高场强元素Nb、Zr、Ti、Y亏损,具有轻稀土富集的右倾型稀土配分曲线.稀土元素和微量元素配分图解显示了岛孤拉斑玄武岩的特征.主元素、微量元素的构造判别图解进一步分析表明高压基性麻粒岩及其钙硅酸盐岩的原岩形成于大陆边缘的岛弧环境.综合高压基性麻粒岩岩石学、元素地球化学特征认为,莱西高压基性麻粒岩的原岩是拉斑玄武岩质岩石,可能是形成于孤后扩张背景下基性的侵入岩或喷出岩.岩石形成以后,在胶-辽-吉带碰撞闭合过程中,经历了麻粒岩相变质作用,又在后来的抬升过程中经历退变质和钙硅酸盐岩化作用.  相似文献   

8.
Archaean banded iron formation (BIF) of the Voronezh Crystalline Massif (VCM) contains coexisting clino‐ and orthopyroxenes with exsolution textures. The pyroxene in the VCM BIF is found in two generations, with only the first generation containing such textures. Clinopyroxene contains large (up to 5–10 μm) (0 0 1) orthopyroxene (Opx1L) lamellae in a host clinopyroxene (Cpx1H). This host Cpx, in turn, exsolves into thin (～1 μm) (1 0 0) lamellae of orthopyroxene (Opx2L) and clinopyroxene (Cpx2H). Orthopyroxene exhibits similar exsolution textures with large (up to 50 μm) (0 0 1) clinopyroxene (Cpx1L) lamellae developed in a host orthopyroxene (Opx1H). This clinopyroxene Cpx1L shows further exsolution of thin (1 0 0) Opx2LL lamellae and clinopyroxene (Cpx2LH). Point microprobe analysis, raster‐mode microprobe analysis, and microprobe element mapping of mineral grains with a large number of point analysis were used to determine the composition of the exsolution products and the primary chemistry of the coexisting clinopyroxene (CaO = 14.86–17.26 wt%) and pigeonite (CaO = 4.45–6.23 wt%). These pyroxenes crystallized during the peak of metamorphism, and application of the Lindsley geothermometer suggested that they formed at extremely high temperatures of about 1000 °C. Primary very dense CO2‐rich fluid inclusions (ρ = 1.152 g cm?3, Th = ?49.2 °C) were discovered for the first time in these BIF. With these data, the metamorphic pressure was estimated as 10–11 kbar (depth 36–40 km). Such ultrahigh temperature–high pressure (UHT–HP) conditions for the regional metamorphism of the Precambrian BIF have previously been reported only for Archaean meta‐ironstone from the Napier Complex (Enderby Land, Antarctica). They give an insight into the peak metamorphic conditions of the BIF of the VCM, their burial under thickened continental crust during this period of Earth evolution and suggest a more complicated multistage metamorphic and tectono‐thermal history for the region than has previously been postulated.  相似文献   

9.
To elaborate physicochemical models for the origin of crystalline rocks, experimental studies of the field of high-alumina assemblages of the system CaO–MgO–Al2O3–SiO2 were carried out at 10–30 kbar and 1250–1535 °C. We have determined the phase relations between the melt (L) and An, Sp, Cpx, Cor, and Ga, the slope of the rays of the monovariant reactions An + Sp = Cpx + Cor + (Ga) and L = Cpx +Ga + Cor + Sp, the position of the nonvariant point (An, Sp, Cpx, Cor, Ga, L), and the compositions of phases participating in these reactions. Based on a topological analysis of the studied segment of the system CaO–MgO–Al2O3–SiO2, we have substantiated that “eclogitization” must follow the reaction Opx + An + Sp = Cpx + Ga. A fundamental continuous series of eutectic monovariant equilibria was observed: L = Cpx + Opx + Fo + An, L = Cpx + Opx + An + Sp, L = Cpx (+ Ga) + An + Sp, and L = Cpx + Cor (+ Ga) + An. A change in the melt composition in this series of eutectic reactions depending on pressure must reflect the most likely magma genesis trend in nature. Comparision of the composition fields in which the above series of reactions is observed with the composition fields of the rocks of magmatic formations showed that this series is most similar to the alkali-earth series of rocks. The mineralogical compositions of cumulates and phenocrysts found in the effusive and dike varieties of these rocks correspond to unique sets of subsolidus phase associations and individual subsolidus phases crystallizing in this fundamental eutectic series.  相似文献   

10.
Two Rongcheng eclogite‐bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta‐dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE‐enriched pattern and trace‐element contents indicate that the bulk‐rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen‐isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2‰ and +6.2‰ (δ18O), in the range of typical mantle values (+5.1 to +6.6‰). The eclogite enclosed within the Chijiadian lherzolite shows an LREE‐enriched pattern and was formed by melts derived from variable degrees (0.005–0.05) of partial melting of peridotite. It has higher δ18O values (+7.6‰ for garnet and +7.7‰ for omphacite) than those of lherzolite. Small O‐isotope fractionations (ΔCpx‐Ol: 0.4‰, ΔCpx‐Grt: 0.1‰, ΔGrt‐Ol: 0.3–0.4‰) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P–T estimates suggest that these rocks experienced subduction‐zone ultrahigh‐pressure (UHP) metamorphism at ～700–800 °C, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 ± 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction‐related UHP metamorphism for the Rongcheng mafic–ultramafic bodies.  相似文献   

11.
The Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite–trondhjemite–granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium‐pressure granulite facies conditions during M1A in the Neoarchean, resulting in the development of two‐pyroxene melanosome assemblages in mafic granulites containing garnet‐bearing leucosome. Reworking of these rocks during retrogression introduced garnet to the melanosome in the form of overgrowths, coronas and grain necklaces that separate the mafic minerals from plagioclase. NCFMASHTO pseudosection modelling constrains the peak metamorphism during M1A to ～850 °C and 7.5 kbar at fluid‐saturated conditions. Following M1A, the rocks retained their M1A H2O content and became fluid‐undersaturated as they underwent near‐isobaric cooling to ～700 °C and 6.5–7 kbar, prior to reworking during M1B. These low H2O contents allowed for the formation of garnet overgrowths and coronas during M1B. The stability of garnet is greatly increased to lower pressure and temperature in fluid‐absent, fluid‐undersaturated mafic rocks, indicating that fluid and melt loss during initial granulite facies metamorphism is essential for the introduction of garnet, and the formation of garnet coronas, during retrogression. The occurrence of garnet coronas is consistent with, but not unique to, near‐isobaric cooling paths.  相似文献   

12.
The largest mafic pluton in Europe (area = 630 km2, thickness = 5 to 7 km) is the early Proterozoic (2445 ± 4 Ma; Pb-Pb) Burakovsky Layered Intrusion (BLI). It is located in the southern part of Russian Karelia, in the SE part of the Baltic Shield, within an Archean granite-greenstone terrain. The BLI is overlain by Quaternary deposits, and our present understanding of its character, composition, and internal structure is based on geophysical surveys and the nature of the rocks at depth, as sampled by diamond drill core. In order to better understand the petrogenesis of the BLI, we present geologic, petrographic, mineral-chemical, and whole-rock chemical analyses from throughout the stratigraphic sequence.

The BLI is a lopolith-like body and is divided into two major units: the Layered Series (LS), which exhibits layering that is discordant to the contact, and the Border Group (BG), with layering that conforms to the contact surface. The LS constitutes most of the BLI and consists of, from bottom to top: the ultramafic zone (UZ), 3 to 3.5 km thick, with a lower dunite (01 ± Chr) and an upper peridotite subzone (01 ± Chr, 01 + Opx ± Chr, and rare Chr cumulates); the pyroxenite zone (PZ) (Opx ± Chr ± 01, Opx + Cpx ± Chr ± 01), 0.2 km thick; the gabbronorite zone (GZ), 1.1 km thick and composed of a lower banded subzone (Opx, Opx + Cpx ± Chr, Opx + Pig ± Cpx cumulates) and an upper massive subzone (Opx + Cpx + Pig and Pig cumulates); the pigeonite-gabbronorite zone (PGZ) (Pig + inverted Pig, Pig-Aug + Pig cumulates), 1.2 km in thickness; and the magnetite-gabbrodiorite zone (MGZ) (Ti-Mt + Pig + Pig + Cpx), 0.8 km thick.

A quite unusual feature of the BLI is an irregular conformable body of clinopyroxenite, in the lower part of the PZ, which consists of inverted Pig-Aug with a non-cumulate texture. The pyroxenites contain occasional relict cumulate structures. The pyroxenes contain small oval-shaped quartz-carbonate inclusions. We speculate that the clinopyroxenites may be the result of subsolidus metasomatism by fluids likely generated within the intrusion.

Economic mineralization is represented by chromite, Ti-V-Mt, and platinum-group minerals (PGMs). Chromite mineralization is well developed in the upper peridotite subzone of the UZ. The largest chromite seam—the main chromite horizon (MCH)—is 3 to 4 m thick and is situated at the top of the UZ, in contact with the PZ. Potential reserves of Ni, in both sulfides and olivine, are present in the UZ. Syngenetic platinum-group-element (PGE) mineralization is observed in chromitite layers in the layered subzone of the GZ and in the rocks of the Border Group. In the MCH, Os, Ir, and Rh occur as small inclusions of sulfides in chromite; these inclusions have compositions that would place them within the aurite-erlichmanite and isoferroplatinum-awaruite series. Low-sulfur PGE mineralization (mainly Pd and Pt) occurs in the banded subzone of the GZ and in sulfide-bearing rocks of the BG. PGMs consist of Pd- and Pt-bearing tellurides and bismuthides: moncheite, merenskyite, froodite, sobolevskite, and kotulskite. Epigenetic PGE mineralization also occurs in tectonic breccias.

The BLI is similar in many respects to other early Proterozoic layered mafic intrusions (i.e., the Bush veld, Monchetundra, and Koilismaa intrusions), especially in terms of the general rock sequence and economic mineralization. The main differences are the very thick dunite cumulates (comprising half of the intrusion) and the relatively minor proportion of pyroxenerich and plagioclaserich cumulates. Another contrasting feature is the predominance of refractory PGE (Os, Ir, and Rh), over Pt and Pd, in the MCH. This observation leads to the interpretation that the MCH has a non-cumulate (deuteric hydrothermal?) origin.  相似文献   

13.
在高喜马拉雅带的定日县曲当—扎乡一带出露的高喜马拉雅结晶岩系中, 发现了高压变质的石榴辉石岩及其降压变质的镁铁质麻粒岩组合, 早期高压条件下形成的石榴辉石岩矿物组合为Grt+Cpx (富铝) +Ru+Q, 斜长石已完全消失, 形成温度为845~896℃, 压力大于1.2GPa, 已达到榴辉岩相的压力条件.中期的麻粒岩相组合为Opx+Pl±Cpx±Ga, 其中Opx、Cpx和Pl为石榴石的后成合晶, 形成温度为993~776℃, 压力为0.90~1.21GPa, 为中压麻粒岩相产物, 晚期矿物仅见普通角闪石、斜长石和石英, 是角闪岩相退变质的产物, 表明HHC经历了降压升温-降压降温的快速抬升过程, 证明其抬升作用与地幔热源的参与有关.   相似文献   

14.
对来自滇东南马关木厂的尖晶石二辉橄榄岩中名义上无水矿物(NAMs)进行显微傅里叶变换红外光谱(Micro-FTIR)分析,结果显示,单斜辉石、斜方辉石和橄榄石中均含有以羟基形式存在的结构水,单斜辉石的水含量为160×10~(-6)~557×10~(-6)(质量分数,下同),斜方辉石的水含量为85×10~(-6)~207×10~(-6),橄榄石的水含量为5×10~(-6)~12×10~(-6),根据矿物百分比含量估算的全岩水含量为46×10~(-6)~137×10~(-6);元素地球化学特征表明,本次研究的橄榄岩包体是岩石圈地幔经历较低程度部分熔融的残余;低(La/Yb)N值(0.22~0.57)以及高Ti/Eu比值(4 076~6 772)暗示橄榄岩可能经历了以硅酸盐熔体为交代介质的微弱地幔交代作用;单斜辉石的微量元素组成比较单一,整体表现出高场强元素、大离子亲石元素以及轻稀土元素的同步亏损;结合中国东部地区已经发表的橄榄岩包体含水量数据来看,滇东南马关木厂岩石圈地幔具有明显富水的特征,可能与该区自中生代以来遭受的新特提斯洋壳大规模俯冲流体交代作用有关;与华北克拉通含水量的明显差异有可能反映的是两地岩石圈地幔正处于不同的演化阶段。  相似文献   

15.
Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to ~500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite-harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of ca. 3,650 Ma, but with an age of ca. 3,800 Ma most likely. The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at ca. 3,800 Ma. This is interpreted to have occurred during the development of a 3,810-3,790-Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. This northern part of the Itsaq Gneiss Complex is the most favourable for the geochemical study of early Archaean protoliths because it experienced peak metamorphism only within the amphibolite facies with little or no in-situ melt segregation, and contains some areas that have undergone little deformation since ca. 3,800 Ma. Most of the ultramafic enclaves are thoroughly altered, and largely comprise secondary, hydrous phases. However, the centres of some enclaves have escaped alteration and comprise dunite and harzburgite with >95% olivine (Fo89-91) + orthopyroxene (En89) + Al-spinel (Cr8-20) assemblages. The dunites and harzburgites are massive or irregularly layered and are olivine-veined on 5-10-m to 10-cm scales. Their whole rock major and rare earth element, and olivine and spinel compositions differ significantly from xenoliths representing the Archaean cratonic lithospheric mantle, but are typical of some modern abyssal peridotites. The harzburgites and dunites show both LREE depleted and enriched patterns; however, none show the massive REE depletion associated with the modelled removal of a komatiite. They are interpreted as being the products of small degrees of melt extraction, with some showing evidence of refertilisation. These Greenland dunites and harzburgites described here are currently the best characterised 'sample' of the early Archaean upper mantle.  相似文献   

16.
Within the Belomorian eclogite province, near Gridino Village, rocks of different compositions (tonalite-trondhjemite-granodioritic gneisses, granites, mafic and ultramafic rocks) were metamorphosed. The metamorphism included subsidence with increasing pressure and temperature, an eclogite stage, decompression in the granulitic facies, and a retrograde stage in the amphibolitic facies. We attempted to characterize the succession and to date igneous and metamorphic events in the evolution of the Gridino eclogite association. For this purpose, we conducted the following studies: U–Pb isotope dating of zircon (conventional and SHRIMP II methods) from gneisses, a mafic dike, and a high-pressure granitic leucosome; U–Pb dating of rutile from mafic dikes; 40Ar/39Ar dating of amphibole and mica; and Sm–Nd studies of rocks and minerals. The Sm–Nd model ages of felsic (2.9–3.1 Ga) and mafic (3.0–3.4 Ga) rocks from the Gridino eclogite association and individual magmatic zircon grains with an age of ca. 3.0 Ga indicate the Mesoarchean age of the metamorphic-rock protoliths. The most reliable result is the upper age bound of eclogitic metamorphism (2.71 Ga), which reflects the time of the posteclogitic decompression melting of eclogitized rocks under high-pressure retrograde granulitic metamorphism. The mafic dikes formed from 2.82 Ga to 2.72 Ga, most probably, at 2.82 Ga, in accordance with the crystallization age of magmatic zircon from metagabbro. Superimposed amphibolitic metamorphism and the “final” exhumation of metamorphic complexes at 2.0–1.9 Ga are associated with the later Svecofennian tectonometamorphic stage. Successive cooling of the metamorphic associations to 300 °C at 1.9–1.7 Ga is shown by U–Pb rutile dating and 40Ar/39Ar mica dating.  相似文献   

17.
Rare Archaean light rare earth element (LREE)-enriched mafic rocks derived from a strongly refractory mantle source show a range of features in common with modern boninites. These Archaean second-stage melts are divided into at least two distinct groups—Whundo-type and Whitney-type. Whundo-type rocks are most like modern boninites in terms of their composition and association with tholeiitic to calc-alkaline mafic to intermediate volcanics. Small compositional differences compared to modern boninites, including higher Al2O3 and heavy REE (HREE), probably reflect secular changes in mantle temperatures and a more garnet-rich residual source. Whundo-type rocks are known from 3.12 and 2.8 Ga assemblages and are true Archaean analogues of modern boninites. Whitney-type rocks occur throughout the Archaean, as far back as ca. 3.8 Ga, and are closely associated with ultramafic magmatism including komatiites, in an affiliation unlike that of modern subduction zones. They are characterised by very high Al2O3 and HREE concentrations, and their extremely depleted compositions require a source which at some stage was more garnet-rich than the source for either modern boninites or Whundo-type second-stage melts. Low La/Yb and La/Gd ratios compared to Whundo-type rocks and modern boninites either reflect very weak subduction-related metasomatism of the mantle source or very limited crustal assimilation by a refractory-mantle derived melt. Regardless, the petrogenesis of the Whitney-type rocks appears either directly or indirectly related to plume magmatism. If Whitney-type rocks have a boninitic petrogenesis then a plume related model similar to that proposed for the modern Tongan high-Ca boninites might apply, but with uniquely Archaean source compositions and source enrichment processes. Second-stage melts from Barberton (S. Africa –3.5 Ga) and ca. 3.0 Ga rocks from the central Pilbara (Australia) have features in common with both Whundo- and Whitney-types, but appear more closely related to the Whitney-type. Subduction zone processes essentially the same as those that produce modern boninites have operated since at least ~3.12 Ga, while a uniquely Archaean boninite-forming process, involving more buoyant oceanic plates and very inefficient mantle-source enrichment, may have occurred before then.  相似文献   

18.
Garnet growth in high‐pressure, mafic garnet granulites formed by dehydration melting of hornblende‐gabbronorite protoliths in the Jijal complex (Kohistan palaeo‐island arc complex, north Pakistan) was investigated through a microstructural EBSD‐SEM and HRTEM study. Composite samples preserve a sharp transition in which the low‐pressure precursor is replaced by garnet through a millimetre‐sized reaction front. A magmatic foliation in the gabbronorite is defined by mafic‐rich layering, with an associated magmatic lineation defined by the shape‐preferred orientation (SPO) of mafic clusters composed of orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and oxides. The shape of the reaction front is convoluted and oblique to the magmatic layering. Opx, Amp and, to a lesser extent, Cpx show a strong lattice‐preferred orientation (LPO) characterized by an alignment of [001] axes parallel to the magmatic lineation in the precursor hornblende‐gabbronorite. Product garnet (Grt) also displays a strong LPO. Two of the four 〈111〉 axes are within the magmatic foliation plane and the density maximum is subparallel to the precursor magmatic lineation. The crystallographic relationship 〈111〉Grt // [001]Opx,Cpx,Amp deduced from the LPO was confirmed by TEM observations. The sharp and discontinuous modal and compositional variations observed at the reaction front attest to the kinetic inhibition of prograde solid‐state reactions predicted by equilibrium‐phase diagrams. The P–T field for the equilibration of Jijal garnet granulites shows that the reaction affinities are 5–10 kJ mol.?1 for the Grt‐in reaction and 0–5 kJ mol.?1 for the Opx‐out reaction. Petrographic and textural observations indicate that garnet first nucleated on amphibole at the rims of mafic clusters; this topotactic replacement resulted in a strong LPO of garnet. Once the amphibole was consumed in the reaction, the parallelism of [001] axes of the mafic‐phase reactants favoured the growth of garnet crystals with similar orientations over a pyroxene substrate. These aggregates eventually sintered into single‐crystal garnet. In the absence of deformation, the orientation of mafic precursor phases conditioned the nucleation site and the crystallographic orientation of garnet because of topotaxial transformation reactions and homoepitaxial growth of garnet during the formation of high‐pressure, mafic garnet‐granulite after low‐pressure mafic protoliths.  相似文献   

19.
The Xolapa Complex (XC) is the largest plutonic and metamorphic mid‐crustal basement unit in Mexico and represents an ancient continental magmatic‐arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high‐grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre‐migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre‐, syn‐ and post‐migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2–D3 phases. Late post‐migmatitic ductile‐brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E–W left‐lateral mylonitic shear zones (D5). Biotite‐breakdown melting in felsic rocks and amphibole‐breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 °C and pressures ranging from ≥6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P–T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time‐marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north‐east‐verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous – Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic‐arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.  相似文献   

20.
Taking for example the Luobosa chromite deposit in Tibet combined with other deposits of the same type in the world, the paper discusses the genetic mechanism of podiform chromite deposits. The study indicates that chromite and dunite-harzburgite are both the products of different degrees of partial melting of the same primary pyrolite(spinel lherzolite) and that chromite and dunite are the end product of higher degree of partial melting.The melting mechanism lies in the convcrsion of the two subgroups of pyroxenes(Cpx and Opx) into olivine and spinel respectively as a result of their incongruent melting, accompanied by regulation and reformation of accessory chrome spinel and such rock-forming minerals as Ol, Opx and Cpx, thus resulting in the close association of chromite deposits and dunite. The results of this study are of general significance for understanding the genesis of chromite deposits of the same type in the world.  相似文献   

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