Carbon Isotope Characteristics as Evidence of an External Source of High-Temperature Granitoids in Granulite Complexes

Doklady Earth Sciences - Data on the carbon isotope composition of graphite and CO2 from inclusions in quartz of granitoids of the Southern Marginal Zone (SMZ) of the Limpopo granulite belt, South...     

Fluids in granulites of the Southern Marginal Zone of the Limpopo Belt, South Africa

The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO₂(-CH₄) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H₂O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H₂O activity (0.2-0.3). The absence of water in CO₂-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO₂ inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H₂O and CO₂-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.     

The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex,Barberton Greenstone Belt,South Africa

Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins et al. in Bull Volcanol 72:579–592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims. Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like, skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile TiO₂, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows.     

SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt,South Africa

SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt yields a mean ²⁰⁷Pb/²⁰⁶Pb age of 2010.3 ±4.5 Ma calculated from 23 analyses. This age, combined with petrographic and field observations, suggests the metamorphism in the syenites occurred during Palaeoproterozoic event.     

Melting, carbonic fluids and water recycling in the deep crust: an example from the Limpopo Belt, South Africa

The proposed retrograde orthoamphibole isograd in the Southern Marginal Zone of the Limpopo Belt separates hydrated, amphibolite grade metapelites from their granulite grade precursors and provides an intriguing geological dilemma. Widespread rehydration of metapelitic granulites under conditions of 660–600 °C and ≥0.6 GPa, and CO₂-dominated fluid-inclusion populations appear to suggest thorough flushing of the high-grade crust with an externally derived carbonic fluid. However, past studies of the carbon and oxygen isotope geochemistry of the hydrated rocks have not demonstrated the involvement of any voluminous out of equilibrium’ fluid in the evolution of the rocks. This contribution proposes a model wherein the hydrating fluids are derived from crystallizing anatectic leucosomes, generated by in situ fluid-absent biotite melting along the prograde path. Model equilibrium fluid compositions suggest that reaction between this melt-derived H₂O and biogenic graphite produced CO₂-rich fluid compositions and potentially high fluid:rock ratios at the wet granite solidus. Declining temperature resulted in fluid compositions shifting to higher XH₂O, with the precipitation of graphite essentially at the sites of initial fluid generation, thereby preserving original (pre-metamorphic) isotopic heterogeneities. The hydration pattern of the Southern Marginal Zone appears to be a function of melt migration. In the hydrated zone, leucosomes generally approximate minimum melt compositions and in this zone H₂O was effectively recycled between the prograde and retrograde assemblages. In contrast, leucosomes in the granulite grade portion of the terrane have lost a K₂O- and H₂O-rich melt fraction, and although some hydration has occurred in this zone, orthopyroxene is generally preserved in metapelites. In a general context, in situ crystallization of graphitic partially melted source rocks has the potential to produce high fluid-rock ratios at temperatures close to the wet granite solidus. This single process holds the potential for widespread retrogression of formerly high-grade assemblages, at a variety of aH₂O values, without external fluid input.     

对北祁连造山带前寒武纪基底物性、火山岩的 源区性质及找矿问题的分析和综述

[摘 要]本文主要在先前一些研究者对北祁连山加里东造山带的构造、海相火山岩和块状硫化矿床研究所获成果和认识的基础上,重点从造山带的前寒武纪基底的物性特征,来分析和讨论所见不同时代地层中的变火山岩类的物源区及形成机制,进而提出一些区域找矿思路。所得结论主要包括:(1) 北祁连山加里东造山的真正基底应是从阿拉善或华北地块分裂出来的一套绿岩类型火山沉积建造,并以古元古宙的北大河群和龙首山群为代表;(2)北祁连造山带的基底从克拉通向大陆裂谷构造体制转换大致发生在早-中元古宙的分界时限(约1777Ma),并普遍以一套大陆裂谷型双峰式火山岩组合为典型特征;(3)不同时代或不同构造体制下形成的火山岩,各有其不同的物源区和成岩模式。具体说,在元古宙 绿岩类型的火山岩的源区可能是克拉通早期由原始玄武质下地壳转变来榴辉岩或基性麻粒岩相,进入大陆裂谷构造体阶段,便转换为一种幔-壳双层岩源区模型,而晚期的次火山岩类的源岩明显是已经过先期大量改造或改变了中上地壳物质作为物源;(4) 按大陆裂谷成矿体系并结合以往一些区域调查资料,提出托莱山西段的小龙口-九个青羊地段可能是个与海相火山岩有关的块状硫化物矿床潜在远景找矿地段。     

Pro- and retrograde P–T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa): constraints from quantitative phase diagrams and geotectonic implications

Interpretations based on quantitative phase diagrams in the system CaO–Na₂O–K₂O–TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂–H₂O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P–T path. That path displays a prograde P–T increase from 600 °C/7.0 kbar to 780 °C/9–10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P–T decrease to 600 °C/4 kbar. The data used to construct the P–T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al‐rich garnet–biotite–cordierite–sillimanite–K‐feldspar–plagioclase–quartz–graphite gneiss (2) a K‐poor, Al‐rich garnet–biotite–staurolite–cordierite–kyanite–sillimanite–plagioclase–quartz–rutile gneiss, and (3) a K, Al‐poor, Fe‐rich garnet–orthopyroxene–biotite–chlorite–plagioclase–quartz–rutile–ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro‐ and retrograde P–T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983 ).     

Neoarchaean high-grade metamorphism in the Central Zone of the Limpopo Belt (South Africa): Combined petrological and geochronological evidence from the Bulai pluton

The Bulai pluton represents a calc-alkaline magmatic complex of variable deformed charnockites, enderbites and granites, and contains xenoliths of highly deformed metamorphic country rocks. Petrological investigations show that these xenoliths underwent a high-grade metamorphic overprint at peak P–T conditions of 830–860 °C/8–9 kbar followed by a pressure–temperature decrease to 750 °C/5–6 kbar. This P–T path is inferred from the application of P–T pseudosections to six rock samples of distinct bulk composition: three metapelitic garnet–biotite–sillimanite–cordierite–plagioclase–(K-feldspar)–quartz gneisses, two charnoenderbitic garnet–orthopyroxene–biotite–K-feldspar–plagioclase–quartz gneisses and an enderbitic orthopyroxene–biotite–plagioclase–quartz gneiss. The petrological data show that the metapelitic and charnoenderbitic gneisses underwent uplift, cooling and deformation before they were intruded by the Bulai Granite. This relationship is supported by geochronological results obtained by in situ LA-ICP-MS age dating. U–Pb analyses of monazite enclosed in garnet of a charnoenderbite gneiss provide evidence for a high-grade structural-metamorphic–magmatic event at 2644 ± 8 Ma. This age is significantly older than an U–Pb zircon crystallisation age of 2612 ± 7 Ma previously obtained from the surrounding, late-tectonic Bulai Granite. The new dataset indicates that parts of the Limpopo's Central Zone were affected by a Neoarchaean high-grade metamorphic overprint, which was caused by magmatic heat transfer into the lower crust in a ‘dynamic regional contact metamorphic milieu’, which perhaps took place in a magmatic arc setting.     

Archaean to Proterozoic Crustal Evolution in the Central Zone of the Limpopo Belt (South Africa-Botswana): Constraints from Combined U-Pb and Lu-Hf Isotope Analyses of Zircon

A combined set of U–Pb and Lu–Hf in situ laser ablationICP-(MC)-MS zircon analyses were obtained from orthogneissesand granitoids in the Central Zone of the Limpopo Belt, whichcomprises the Beit Bridge and Mahalapye complexes. The resultsindicate that by combining the two isotope systems primary magmaticzircon domains can be distinguished from those formed duringlater metamorphic events, even if the distinct zircon domainsunderwent multiple Pb loss and the texture–age relationships,as obtained by cathodoluminescence images and U–Pb analyses,are ambiguous. Furthermore, the applied technique allows distinctionof zircon grains formed in juvenile magmas from those generatedby melting of older continental crust or affected by substantialcrustal contamination. The combined U–Pb and Lu–Hfdata reveal that the Sand River gneiss suite of the Beit BridgeComplex was emplaced at 3283 ± 8 Ma and formed from meltingof an older Archaean crust, which was derived from a depletedmantle source at around 3·65 Ga. The hafnium model age(T_DM^Hf) is significantly older than those obtained from zirconsfrom numerous Neoarchaean granitoids of the Beit Bridge Complex,comprising the Singelele gneiss (2647 ± 12 Ma), the Bulaigranite (2612 ± 7 Ma), the Regina gneiss (2649 ±9 Ma) and two samples of the Zanzibar gneiss (2613 ±6 Ma). These granitoids show initial Hf(t) values between +0·5 and –7·1, which correspond to initialT_DM^Hf between 3·46 and 3·01 Ga. These variableT_DM^Hf_initial and Hf(t)_initial values are interpreted to be theresult of different mixtures of reworked 3·65 Ga Palaeoarchaeancrust with juvenile magmas extracted from the depleted mantleduring the Neoarchaean at 2·65 Ga. This conclusion issupported by results obtained from the Mahalapye Complex, whichwas affected by migmatization and granite intrusions duringthe Palaeoproterozoic at 2·02–2·06 Ga. TheMokgware granite (2019 ± 9 Ma) contains zircon xenocrystswith Pb–Pb ages of 2·52–2·65 Ga and2·93 Ga and hafnium model ages of 3·0–3·4Ga, indicating that this granite is derived from remelting ofArchaean crust. In contrast, uniform T_DM^Hf_initial ages of 2·61–2·67Ga obtained from a diorite gneiss (2061 ± 6 Ma) of theMahalapye Complex indicate that its protolith may have beenformed from remelting of a Neoarchaean juvenile crust. VariableHf(t)_initial values from –3·7 to +6·3 ofzircon cores (2711 ± 11 Ma) in an adjacent leucosomealso support a model of mixing of juvenile mantle derived matterwith older crust in the Neoarchaean. KEY WORDS: Archaean; Palaeoproterozoic; Limpopo Belt; zircon, U–Pb dating; Lu–Hf isotopes; LA-ICP-MS     

Mineral Chemistry of Scheelite as An Indicator of Multi-stage Fluid Evolution Associated with the Riviera W-REE-Mo Deposit South Africa

The Neoproterozoic Riviera W-REE-Mo deposit is one of the largest unmined tungsten resources in the world and is associated with pervasively altered A-type granites of the Neoproterozoic Cape Granite Suite Western Cape, South Africa. The present study investigated the mineral chemistry of scheelite, the principal ore mineral with the aim to record the variation of solid solution molybdenum for geometallurgical purposes and also as an indicator of changing redox conditions of the mineralizing hydrothermal fluids. Methodology included UV-fluorescence studies and micro-analyses by LA-ICP-MS. Results have shown that at least four phases of scheelite are hosted by the endoskarn zone, potassic alteration zone and various quartz-carbonate veins. This reflects hydrothermal fluid evolution from early stage reduced to late stage, vein associated and more oxidized. The molybdenum content of the dominant early phase scheelite is low and renders the deposit amenable to low penalty mineral recovery.     

Rapid evolution from sediment to anatectic granulite in an Archean continental collision zone: the example of the Bandelierkop Formation metapelites,South Marginal Zone,Limpopo Belt,South Africa

The metamorphic history of the Southern Marginal Zone (SMZ) of the Limpopo Belt, South Africa, possibly provides insight into one of the oldest preserved continental collision zones. The SMZ consists of granitoid gneisses (the Baviaanskloof Gneiss) and subordinate, infolded metasedimentary, metamafic and meta‐ultramafic lithologies (the Bandelierkop Formation) and is regarded as the c. 2700 Ma granulite facies reworked equivalent of the Kaapvaal craton basement. The granulite facies metamorphism is proposed to have occurred in response to collision between the Kaapvaal and Zimbabwe cratons. Previous studies have proposed a wide variety of P–T loops for the granulites, with considerable discrepancy in both the shapes of the retrograde paths and the magnitude of the peak P–T conditions. To date, the form of the prograde path and the timing of the onset of metamorphism remain unknown. This study has used a range of different metasedimentary rocks from a large migmatitic quarry outcrop to better constrain the metamorphic history and the timing of metamorphism in the SMZ. Detrital zircon ages reveal that the protoliths to the metasedimentary rocks were deposited subsequent to 2733 ± 13 Ma. Peak metamorphic conditions of 852.5 ± 7.5 °C and 11.1 ± 1.3 kbar were attained at 2713 ± 8 Ma. The clockwise P–T path is characterized by heating in the sillimanite field along a P–T trajectory which approximately parallels the kyanite to sillimanite transition, followed by near‐isothermal decompression at peak temperature and near‐isobaric cooling at ~6.0 kbar. These results support several important conclusions. First, the sedimentary rocks from the Bandelierkop Formation are not the equivalent of any of the greenstone belt sedimentary successions on the Kaapvaal craton, as has been previously proposed. Rather, they post‐date the formation of the Dominion and Witwatersrand successions on the Kaapvaal craton. From the age distribution of detrital zircon, they appear to have received significant input from various origins. Consequently, at c. 2730 Ma, the Baviaanskloof Gneiss most likely acted as basement onto which the sedimentary succession represented by the Bandelierkop Formation metapelites was deposited. Second, the rocks of the SMZ underwent rapid evolution from sediment to granulite facies anatexis, with a burial rate of ~0.17 cm yr^?1. Peak metamorphism was followed by an isothermal decompression to 787.5 ± 32.5 °C and 6.7 ± 0.5 kbar and isobaric cooling to amphibolite facies conditions, below 640 °C prior to 2680 ± 6 Ma. This age for the end of the high‐grade metamorphic event is marked by the intrusion of crosscutting, undeformed pegmatites that are within error the same age as the crosscutting Matok intrusion (2686 ± 7 Ma). Collectively, the burial rate of the sedimentary rocks, the shape of the P–T path, the burial of the rocks to in excess of 30 km depth and the post‐peak metamorphic rapid decompression argue strongly that the SMZ contains sediments deposited along an active margin during lateral convergence, and that the SMZ was metamorphosed as a consequence of continental collision along the northern margin of the Kaapvaal craton at c. 2700 Ma.     

Phase relationships in grunerite–garnet-bearing amphibolites in the system CFMASH, with applications to metamorphic rocks from the Central Zone of the Limpopo Belt, South Africa

A petrogenetic grid in the model system CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O is presented, illustrating the phase relationships among the minerals grunerite, hornblende, garnet, clinopyroxene, chlorite, olivine, anorthite, zoisite and aluminosilicates, with quartz and H₂O in excess. The grid was calculated with the computer software thermocalc , using an upgraded version of the internally consistent thermodynamic dataset HP98 and non‐ideal mixing activity models for all solid solutions. From this grid, quantitative phase diagrams (P–T pseudosections) are derived and employed to infer a P–T path for grunerite–garnet‐bearing amphibolites from the Endora Klippe, part of the Venetia Klippen Complex within the Central Zone of the Limpopo Belt. Agreement between calculated and observed mineral assemblages and garnet zonation indicates that this part of the Central Zone underwent a prograde temperature and pressure increase from c. 540 °C/4.5 kbar to 650 °C/6.5 kbar, followed by a post‐peak metamorphic pressure decrease. The inferred P–T path supports a geotectonic model suggesting that the area surrounding the Venetia kimberlite pipes represents the amphibolite‐facies roof zone of migmatitic gneisses and granulites that occur widely within the Central Zone. In addition, the P–T path conforms to an interpretation that the Proterozoic evolution of the Central Zone was controlled by horizontal tectonics, causing stacking and differential heating at c. 2.0 Ga.     

东濮凹陷盐湖相烃源岩有机硫同位素分布特征及其地球化学意义

盐湖相烃源岩单体烃硫同位素分布特征和影响因素研究薄弱.采用气相色谱-电感耦合等离子体质谱,结合色谱/质谱、微量元素以及碳氧同位素技术,对东濮凹陷北部盐湖相烃源岩进行了分析.结果 表明,不同单体含硫化合物的δ34S值有较大差异,以δ34S=25.00‰为界,将烃源岩分成了两类,第一类:δ34S＞25.00‰,主要为卫城地...     

Nature of ore-forming fluid and formation conditions of BIF-hosted gold mineralization in the Archean Amalia Greenstone Belt,South Africa: Constraints from fluid inclusion and stable isotope studies

Orogenic gold mineralization in the Amalia greenstone belt is hosted by oxide facies banded iron-formation (BIF). Hydrothermal alteration of the BIF layers is characterized by chloritization, carbonatization, hematization and pyritization, and quartz-carbonate veins that cut across the layers. The alteration mineral assemblages consist of ankerite-ferroan dolomite minerals, siderite, chlorite, hematite, pyrite and subordinate amounts of arsenopyrite and chalcopyrite. Information on the physico-chemical properties of the ore-forming fluids and ambient conditions that promoted gold mineralization at Amalia were deduced from sulfur, oxygen and carbon isotopic ratios, and fluid inclusions from quartz-carbonate samples associated with the gold mineralization.Microthermometric and laser Raman analyses indicated that the ore-forming fluid was composed of low salinity H₂O-CO₂ composition (~3 wt% NaCl equiv.). The combination of microthermometric data and arsenopyrite-pyrite geothermometry suggest that quartz-carbonate vein formation, gold mineralization and associated alteration of the proximal BIF wall rock occurred at temperature-pressure conditions of 300 ± 30 °C and ∼2 kbar. Thermodynamic calculations at 300 °C suggest an increase in fO₂ (10⁻³²–10⁻³⁰ bars) and corresponding decrease in total sulfur concentration (0.002–0.001 m) that overlapped the pyrite-hematite-magnetite boundary during gold mineralization. Although hematite in the alteration assemblage indicate oxidizing conditions at the deposit site, the calculated low fO₂ values are consistent with previously determined high Fe/Fe + Mg ratios (>0.7) in associated chlorite, absence of sulfates and restricted positive δ³⁴S values in associated pyrite. Based on the fluid composition, metal association and physico-chemical conditions reported in the current study, it is confirmed that gold in the Amalia fluid was transported as reduced bisulfide complexes (e.g., Au(HS)₂⁻). At Amalia, gold deposition was most likely a combined effect of increase in fO₂ corresponding to the magnetite-hematite buffer, and reduction in total sulfur contents due to sulfide precipitation during progressive fluid-rock interaction.The epigenetic features coupled with the isotopic compositions of the ore-forming fluid (δ³⁴S_ΣS = +1.8 to +2.3‰, δ¹⁸O_H2O = +6.6 to +7.9‰, and δ¹³C_ΣC = −6.0 to −7.7‰ at 300–330 °C) are consistent with an externally deep-sourced fluid of igneous signature or/and prograde metamorphism of mantle-derived rocks.     

前寒武纪细粒碎屑岩中纺锤状裂缝的成因分析:以燕山东部长城群为例

燕山东部天津蓟县至河北兴隆一带中元古界长城群细粒碎屑岩中,普遍发育有在层面上表现为纺锤状裂缝的沉积构造,并曾经被解释为后生动物遗迹化石或地震震荡液化脉等。这种沉积构造常与变余波痕、皱饰构造等相伴产出。根据初步观察与研究的结果推断,纺锤状裂缝可能是由于微生物席对沉积面的封闭作用,导致沉积物在成岩作用早期发生脱气与脱水等作用的产物,因此可以将其归入微生物参与形成的原生沉积构造(席底构造)的范畴。纺锤状裂缝与其他的席底构造一样,为前寒武纪的沉积环境重塑提供了一个重要的证据,同时也说明在前寒武纪的地球表层环境中,微生物以其新陈代谢的多样性及巨大的生物量,在沉积纪录中留下了重要的烙印。     

Insights into the complexity of crustal differentiation: K2O‐poor leucosomes within metasedimentary migmatites from the Southern Marginal Zone of the Limpopo Belt,South Africa

Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well‐exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm‐ to m‐scale, K₂O‐poor garnet‐bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), also known as future leucosome; (3a) Re‐equilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H₂O, K₂O), which triggers the co‐precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt‐dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction, and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H₂O and K₂O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K₂O‐poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling, is <5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis.     

The timing of sub-solidus hydrothermal alteration in the Central Zone, Limpopo Belt (South Africa): Constraints from titanite U–Pb geochronology and REE partitioning

In the Central Zone of the Limpopo Belt (South Africa), Palaeoproterozoic granulite-facies metamorphism was superimposed on an earlier Archaean orogenic history. Previously determined ages of  2030–2020 Ma obtained from high-temperature chronometers (zircon, garnet, monazite) are generally thought to provide the best estimate of the peak of Palaeoproterozoic granulite-facies metamorphism in the Central Zone, whereas ages as young as  2006 Ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period. We present a new MC-ICP-MS ²⁰⁷Pb–²⁰⁶Pb age of 2030.9 ± 1.5 Ma for titanite found in amphibolite- to greenschist-facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed Central Zone metabasite. This age could potentially date cooling of rocks at this locality to temperatures below the wet solidus. Alternatively, the titanite could be inherited from the metabasite host, and the age determined from it date the peak of metamorphism. Integration of the geochronology with LA-ICP-MS trace element data for minerals from the metabasite, the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite-facies hydrothermal alteration, not at the metamorphic peak or during the greenschist-facies phase of veining. This suggests that high-grade rocks in the Central Zone have cooled differentially through the wet solidus, and provides timing constraints on when Palaeoproterozoic reworking in the Central Zone began. This study illustrates the potential of combined geochronological and high-resolution geochemical studies to accurately match mineral ages to distinct crustal processes.     

湖相烃源岩生烃动力学及排油效率——以松辽盆地青山口组为例

为准确评价湖相烃源岩的排油量及致密油资源潜力,结合湖相I型有机质的黄金管热模拟实验和青山口组烃源岩的有机地球化学分析,对松辽盆地湖相烃源岩生烃特征、动力学参数和排油效率进行了研究。热模拟实验的产物定量结果表明,松辽盆地青山口组湖相烃源岩具有可观的生油潜力,最大生油量约为600 mg/g·TOC,主要的生油阶段在Easy Ro=0.5%~1.2%。动力学计算结果表明,该烃源岩生油的平均活化能为218.5 k J/mol,重质组分生成活化能要低于轻质组分。结合青山口组烃源岩的生油动力学参数和英X58井热史的地质推演,证实该井湖相烃源岩的当前生油转化率为40%~60%,生油量为240~360 mg/g·TOC;通过残留烃的定量,计算得到该烃源岩的排油量为150~200mg/g·TOC,相对排油效率约为60%。生油量及排油效率结果表明,松辽盆地致密油资源潜力大。     

Carbonate-silicate inclusions in garnet as evidence for a carbonate-bearing source for fluids in leucocratic granitoids associated with granulites of the Southern Marginal Zone,Limpopo Complex,South Africa

We present a study of carbonate-bearing polyphase inclusions in garnets from leucocratic granitoids intruding metapelitic granulites of the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo high-grade complex, South Africa, during the post-peak stage (2710–2650 Ma; U-Pb ages for zircons and monazites). Ternary feldspar thermometry suggests that the granitoid magma cooled from temperatures 800–900 °C at a pressure of ca. 6.5 kbar. Abundant CO₂ fluid inclusions in quartz and T-X_CO2 phase equilibria modeling via PERPLE_X imply action of an essentially carbonic fluid in the granitoids. Cores of almandine-rich garnet grains from the granitoids contain polyphase carbonate-bearing inclusions with a distinct negative crystal shape. The major carbonate in the inclusions is a strongly zoned magnesite-siderite variety, whereas pyrophyllite is the predominant silicate phase. Raman spectra of unexposed inclusions revealed a presence of CO₂, as well as CH₄ and H₂O. The carbonate-bearing inclusions coexist with larger polyphase inclusions composed of biotite, quartz, K-feldspar, plagioclase, sillimanite, which are interpreted as relics of granitic melts. Modeling the mineral assemblage preserved within the carbonate-bearing inclusions shows that their present mineral and chemical compositions are a product of interaction of the trapped aqueous‑carbonic fluid with host garnet during cooling below 400 °C. Despite strong modifications, the inclusions bear evidence for initial saturation of the fluid with Mg‑carbonate. This is taken as an evidence for an origin of the fluids by devolatilization of the Mg-rich carbonate-bearing ultrabasic greenstone rocks of the Kaapvaal Craton that were buried under the SMZ. Being generated at temperatures between 650 and 700 °C, the fluid subsequently participated in anatexis and coexisted with the granite magma during exhumation and interaction of the SMZ granulites with cratonic rocks.