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1.
Summary The oxygen and strontium isotope compositions of the Cambro-Ordovician granitoids cropping out in the Wilson Terrane (Granite Harbour Intrusives–GHI) constrain the petrological evolution of the magmatism in Antarctica, related to the Ross Orogeny. The measured δ18OWR values of these intrusives define three different compositional groups: the metaluminous rocks (MAG), with δ18OWR ranging from 6.9 (olivine gabbro) to 11.4‰ (monzogranite); the unaltered peraluminous granites (PAG), having δ18OWR values ranging from 10.6 to 13.2‰, and the foliated peraluminous leucogranites (SKG), characterised by δ18OWR values above 14‰. The analysis of equilibrium mineral assemblages indicates that the high δ18OWR values are magmatic and unaffected by low-temperature processes. A few peraluminous granites sampled in the vicinity of Cenozoic intrusions show anomalously low δ18OWR, due to meteoric-hydrothermal alteration. The isotopic data indicate that the coeval and spatially related metaluminous mafic and felsic intrusives forming the GHI were not comagmatic: the mafic and intermediate rocks were likely derived from lower crustal contamination of a pristine basaltic magma; their δ18OWR values were also increased during emplacement, due to the interaction with the adjacent 18O-rich hydrous felsic magmas (mixing). Oxygen isotope data indicate that the crustal sources producing the Granite Harbour intrusives were not homogeneous: the felsic metaluminous intrusives were produced by partial melting of fertile rock with possible igneous origin, whereas partial melting of a metapelitic source rock is claimed for the genesis of the peraluminous granites. Received February 9, 2001; revised version accepted August 10, 2001  相似文献   

2.
《Precambrian Research》1986,32(4):343-368
Late Archaean granites have been identified within the northern part of the Napier Complex of Enderby Land, Antarctica, although intrusives of this age are not common elsewhere in the complex. The oldest intrusive (3070 ± 34 Ma), synorogenic granite at Proclamation Island, was probably derived from melting of felsic crustal rocks of igneous origin. Two younger granites were emplaced at about 2840 Ma and 2481 ± 3 Ma, and the latter has geochemical similarities with other late and post-orogenic intrusives elsewhere in the East Antarctic Shield. Because of their fractionated character, it is difficult to discern whether the granites were derived by melting of sedimentary or igneous protoliths, but a granulite-facies source is probable.TNdCHUR model ages of the three analysed granites suggest that the Napier Complex is the product of at least two temporally discrete episodes of continental crust formation. The granite precursors probably formed at about 3100 Ma, whereas most of the exposed Napier Complex is much older (back to almost 4000 Ma). Two of the granites appear to record a significant hiatus (in one case of about 600 Ma) between the formation of this crustal precursor and final emplacement. The 3100 Ma old episode of crustal formation was roughly synchronous with a widespread, intense, and high-grade tectonothermal event (D1-M1), which produced metamorphic fabrics and mineral assemblages that are widely preserved within the Napier Complex. It is therefore likely that this new crust formation, deformation and metamorphism are all attributable to a single tectonic episode, possibly related to emplacement of magma into the lower crust. The two younger granites were emplaced at about the time of two subsequent and less intense tectonothermal events (D2-M2 and D3-M3).Concordant Rb-Sr total-rock and U-Pb zircon ages, which are interpreted as emplacement ages, have been obtained for two of the intrusions. However, the third exhibits the unusual relationship of a Rb-Sr isochron age older than the U-Pb zircon age. In this case the Rb-Sr age is believed to date magmatic emplacement. The complex interpretation required for the zircon data is forewarned by a non-perfect alignment of analytical points, in contrast to the perfect analytical alignments produced by the isotopically coherent granites. Two of the three granites yield similar and relatively precise lower intercepts with the U-Pb concordia but these do not appear to have direct geological significance.The new isotopic data are combined with earlier results to derive an integrated tectonothermal evolution for the Napier Complex.  相似文献   

3.
In contrast to I-type granites, which commonly comprise infracrustal and supracrustal sources, S-type granites typically incorporate predominantly supracrustal sources. The initial aim of this study was to identify the sources of three Scottish Caledonian (~460 Ma) S-type granites (Kemnay, Cove and Nigg Bay) by conducting oxygen, U–Pb and Hf isotope analyses in zircon in order to characterise one potential end-member magma involved in the genesis of the voluminous late Caledonian (~430–400 Ma) I-type granites. Field, whole-rock geochemical and isotopic data are consistent with the generation of the S-type granites by melting their Dalradian Supergroup country rocks. While Hf isotope compositions of magmatic zircon, U–Pb data of inherited zircons, and high mean zircon δ18O values of 9.0 ± 2.7‰ (2SD) and 9.8 ± 2.0‰ for the Kemnay and Cove granites support this model, the Nigg Bay Granite contains zircons with much lower δ18O values (6.8 ± 2.1‰), similar to those found in Scottish I-type granites. This suggests that the Nigg Bay Granite contains low-δ18O material representing either altered supracrustal material, or more likely, an infracrustal source component with mantle-like δ18O. Mixing trends in plots of δ18O vs. εHf for S-type granite zircons indicate involvement of at least two sources in all three granites. This pilot study of Scottish Caledonian S-type granites demonstrates that, while field and whole-rock geochemical data are consistent with local melting of only supracrustal sources, the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic evolution involving two or more magma sources.  相似文献   

4.
锡矿往往与长英质岩浆岩伴生,然而锡矿形成的热能源区尚不清楚,其可能与地幔物质相关。我国云南锡矿带中出露的中-酸性岩石及碱性岩杂岩体为研究锡矿及其周围岩浆成因提供了良好的物质条件。本文报道了云南个旧地区代表性的花岗岩、辉长-闪长岩和碱性岩类新的全岩地球化学、锆石U-Pb年代学和Hf同位素数据。LA-ICP-MS锆石U-Pb定年表明上述岩石分别形成于81.43±0.46Ma(82.89±0.58M)、81.35±0.22Ma和80.35±0.72Ma,指示它们为晚白垩世近同期岩浆活动的产物。其中闪长岩、碱性岩和花岗岩中锆石的Hf同位素组成不均一,ε_(Hf)(t)分别为-4.2~+0.8、-7.5~-1.9和-8.4~+0.4。尽管这些岩体的侵入时代一致,但它们的地球化学特征和同位素特征存在差异,表明这三类火成岩来自不同的岩浆源区,三者不是同一母岩浆相互演化的关系。个旧杂岩体中花岗岩为弱过铝质岩石,SiO_2与P_2O_5含量呈负相关的关系,排除S型花岗岩的可能。亏损Zr、Nb、Sr、Eu等大离子亲石元素的特征可能为锆石、磷灰石、长石类造岩矿物分离结晶作用的结果;Zr、Nb、Ce和Y总量较低,低的FeOT/MgO比值和低的锆石饱和温度表明,指示出个旧地区的花岗岩应为高分异I型花岗质岩石而非A型花岗岩。个旧地区形成于晚白垩世时期的中基性、碱性岩石可能为不同的幔源岩浆近同时侵入的产物,底侵的幔源熔体带来热量诱发中、下地壳岩石发生部分熔融形成含矿的花岗岩,幔源岩浆对于成矿至少在能量也可能在成矿物质上有重要的贡献。  相似文献   

5.
The dominantly Triassic Main Range Granite of Peninsular Malaysia that occurs west of the Paleo-Tethyan Bentong–Raub suture zone was regarded exclusively as an S-type granite. The Main Range dominantly biotite granites are of batholithic proportion and host one of the world’s largest tin provinces. The S-type characteristics include high initial 87Sr/86Sr isotope ratios (>0.710), a narrow range in silica, presence of ilmenite and occasional cordierite and andalusite, and the presence of pelitic or quartzose meta-sedimentary xenoliths. However, the present review shows that the Main Range Granites also have many features that are more characteristic of I-type granites such as the very large scale of plutonism, the presence of primary titanite and amphibole, occurrence of hornblende-bearing mafic enclaves, increasing peraluminosity towards the more differentiated end-members of the suite and decreasing P2O5 with increasing SiO2 contents. The moderately peraluminous nature of the bulk Main Range Granite, without containing cordierite, Fe–Mg garnet or sillimanite, is consistent with derivation from a meta-sedimentary protolith that was itself undersaturated with respect to Al2SiO5.  相似文献   

6.
Comprehensive geochemical and geochronological studies were carried out for two-mica granites of the Biryusa block of the Siberian craton basement. U-Pb zircon dating of the granites yielded an age of 1874 ± 14 Ma. The rocks of the Biryusa massif correspond in chemical composition to normally alkaline and moderately alkaline high-alumina leucogranites. By mineral and petrogeochemical compositions, they are assigned to S-type granites. The low CaO/Na2O ratios (< 0.3), K2O - 5 wt.%, CaO < 1 wt.%, and high Rb/Ba (0.7-1.9) and Rb/Sr (3.9-6.8) ratios indicate that the two-mica granites resulted from the melting of a metapelitic source (possibly, the Archean metasedimentary rocks of the Biryusa block, similar to the granites in £Nd(t) value) in the absence of an additional fluid phase. The granite formation proceeded at 740-800 °C (zircon saturation temperature). The age of the S-type two-mica granites agrees with the estimated ages of I- and A-type granitoids present in the Biryusa block. Altogether, these granitoids form a magmatic belt stretching along the zone of junction of the Biryusa block with the Paleoproterozoic Urik-Iya terrane and Tunguska superterrane. The granitoids are high-temperature rocks, which evidences that they formed within a high-temperature collision structure. It is admitted that the intrusion of granitoids took place within the thickened crust in collision setting at the stage of postcollisional extension in the Paleoproterozoic. This geodynamic setting was the result of the unification of the Neoarchean Biryusa continental block, Paleoproterozoic Urik-Iya terrane, and Archean Tunguska superterrane into the Siberian craton.  相似文献   

7.
Whole rock elemental and Sr–Nd isotope geochemistry and in situ K-feldspar Pb isotope geochemistry were used to identify the sources involved in the genesis of Neoproterozoic granites from the Embu Terrane, Ribeira Belt, SE Brazil. Granite magmatism spanned over 200 Ma (810–580 Ma), and is dominated by crust-derived relatively low-T (850–750 °C, zircon saturation) biotite granites to biotite-muscovite granites. Two Cryogenian plutons show the least negative εNdt (−8 to −10) and highest mg# (30–40) of the whole set. Their compositions are strongly contrasted, implying distinct sources for the peraluminous (ASI ∼ 1.2) ∼660 Ma Serra do Quebra-Cangalha batholith (metasedimentary rocks from relatively young upper crust with high Rb/Sr and low Th/U) and the metaluminous (ASI = 0.96–1.00) ∼ 630 Ma Santa Catarina Granite. Although not typical, the geochemical signature of these granites may reflect a continental margin arc environment, and they could be products of a prolonged period of oceanic plate consumption started at ∼810 Ma. The predominant Ediacaran (595–580 Ma) plutons have a spread of compositions from biotite granites with SiO2 as low as ∼65% (e.g., Itapeti, Mauá, Sabaúna and Lagoinha granites) to fractionated muscovite granites (Mogi das Cruzes, Santa Branca and Guacuri granites; up to ∼75% SiO2). εNdT are characteristically negative (−12 to −18), with corresponding Nd TDM indicating sources with Paleoproterozoic mean crustal ages (2.0–2.5 Ga). The Guacuri and Santa Branca muscovite granites have the more negative εNdt, highest 87Sr/86Srt (0.714–0.717) and lowest 208Pb/206Pb and 207Pb/206Pb, consistent with an old metasedimentary source with low time-integrated Rb/Sr. However, a positive Nd–Sr isotope correlation is suggested by data from the other granites, and would be consistent with mixing between an older source predominant in the Mauá granite and a younger, high Rb/Sr source that is more abundant in the Lagoinha granite sample. The Ediacaran granites are coeval with profuse granite magmatism attributed to continental arc magmatism in northern Ribeira and Araçuaí belts. However, their evolved compositions with low mg# and dominantly peraluminous character are unlike those of magmatic arc granites, and they are more likely products of post-collisional magmatism or correspond to an inner belt of crust-derived granites.  相似文献   

8.
The Mangalwar Complex of the Aravalli craton is marked by the presence of late Paleoproterozoic granites referred to as Anjana Granite and Amet Granite. These granites occur as 1.64 Ga old plutons intruding greenstone sequences and migmatitic gneisses of Mangalwar Complex which comprises parts of BGC of the Aravalli craton. In the present contribution major, trace and REE data of these granites along with associated microgranular mafic enclaves (MMEs) are presented and discussed. Geochemically these granites are quartz monzonite, metaluminous, sub-alkaline and high-K calc-alkaline rocks. The most important characteristics of Anjana and Amet granites are low SiO2, high MgO, Mg#, K2O, Ba, and low Na2O/K2O ratios. In addition, the REEs show moderate to high fractionation, with (La/Yb) ratios up to 22 and 23 of the Anjana and Amet granites respectively, with no or positive europium anomalies. In the primitive mantle-normalized trace element diagrams both granites show depletion in high-field strength elements (HFSE) such as Nb, Ta, P, Ti and enrichment in LILEs. Most of these features are comparable to those of sanukitoid series rocks. Geochemically both granites are distinguished as high-Ti sanukitoids. Geochemical characteristics of MMEs suggest that they are similar to Anjana and Amet granites and in turn to sanukitoids with lower SiO2 content. They display LREE enriched patterns with low values (avg. 13) of (La/Yb)N, negative Eu anomalies and high HREE contents (58 ppm). It is suggested that the parental magma of Anjana and Amet granitic plutons originated through a four stage process (1) Generation of magmatic melts produced by partial melting of terrigeneous sediments of subducting slab in an arc setting; (2) interaction of those melts with the overlying mantle wedge, and total consumption of slab-derived melts during the reaction resulting in production of a metasomatized mantle; (3) tectonothermal event, possibly related to the slab break-off, causing asthenospheric mantle upwelling. This may have induced the melting of the metasomatized mantle and the generation of sanukitoid magmas. The parental magmas of Anjana and Amet granites and their mafic enclaves were generated at lower and higher lithospheric levels respectively (4) Granitic magma ascended due to viscosity and gravity instabilities and interacted with enclave magma at higher mantle level. Both magmas ascended towards upper crust and evolved through fractional crystallisation. Existing data suggest that in the Mangalwar Complex, the formation of sanukitoid magma started even during Mesoarchaean times and continued till late Paleoproterozoic. Formation of sanukitoid magma during this time indicates that in northern Indian shield the multi-stage subduction- accretionary orogenic processes continued for a protracted geological period and played a major role in the origin and evolution of early continental crust.  相似文献   

9.
In the northeastern Dom Feliciano Belt, Santa Catarina/Brazil, Paleoproterozoic rocks (mainly the Camboriú Complex) and Neoproterozoic granitoids – with the older Itapema Granite and the younger Corre-mar, Rio Pequeno and Serra dos Macacos granites – experienced a deformation history from magmatic to greenschist facies temperatures, under different rheological conditions. The concordance of flat amphibolite facies structures of the Camboriú Complex and magmatic and subsolidus structures in the Itapema Granite indicate the late-tectonic character of the latter. Based on tectonic features, the Corre-mar Granite is interpreted as older than the Rio Pequeno Granite and as related to transcurrent tectonics of the Southern Brazilian Shear Belt.In all granites, microstructures point to widespread magmatic alignment, followed by weak subsolidus and, locally, amphibolite to greenschist facies deformation. Magmatic foliations are progressively weaker in the younger granites. Synmagmatic shear zones in the Rio Pequeno Granite are possibly concentrated at the intrusive contact. The weak solid-state deformation at late-magmatic conditions argues for magmatism within a low-strain zone, which is compatible with the location of the area relative to the Major Gercino and Itajaí shear zones. The amphibolite to greenschist facies deformation structures are attributed to continuous deformation within the same low-strain zone during decreasing temperatures.  相似文献   

10.
《International Geology Review》2012,54(10):1121-1149
Peraluminous intrusives of the Akum-Bamenda Massif, Pan-African Belt, Central Cameroon, were synkinematically emplaced in a Pan-African sinistral strike-slip shear zone. The rock sequences consist of medium-grained leucogranites, fine-grained leucogranites, and orthogneisses of biotite granite composition; in aggregate, they cover a range from about 65 to 74 wt.% SiO2, defining a continuous chemical evolutionary trend and displaying characteristics of the high-K and medium calc-alkaline series. Leucogranites are strongly peraluminous (A/CNK > 1.1) and plot in the field of S-type granites, whereas orthogneisses are metaluminous and plot in the field of I-type granitoids. Major and trace element compositions and the Rb/Sr isotopes of the leucogranites indicate crustal derivation by remelting of a composite metapelite?+?metagreywacke protolith similar to the metasedimentary rocks of the central domain of the Cameroon Pan-African North-Equatorial fold belt.  相似文献   

11.
Abstract The St Malo region in north-west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis at c. 540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range from c. 550 to c. 300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation. 40Ar/39Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting. Eleven muscovite concentrates and one whole-rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record 40Ar/39Ar plateau ages of c. 527 and 521 Ma. An internally discordant 40Ar/39Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain-Deux Evailles-Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north-western sector of the St Malo region exhibit internally discordant 40Ar/39Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south-eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole-rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan-Bobital Granite Complex, yields a 40Ar/39Ar plateau age of c. 307 Ma. The 40Ar/39Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to underlie the St Malo region.  相似文献   

12.
Proterozoic silicic magmatic activity in the Tennant Creek area includes a suite of pre- to syn-orogenic granitoids (i.e. the Tennant Creek Granite and compositionally similar porphyries) predominantly intruded during the period 1870–1830 Ma, a group of silicic volcanics and volcaniclastics (Flynn Subgroup) erupted shortly after the main D1 deformation, and apparently anorogenic silicic intrusives represented by the Warrego Granite. Chemical and petrographic data indicate that the majority of the pre- to syn-orogenic granitoids and porphyries are I-type or infracrustal-derived, whereas the peraluminous muscovite-rich Warrego Granite has many characteristics of supracrustal-derived (S-type) granitoids. The Warrego Granite also appears to be moderately fractionated with relatively low Ba, Sr, Zr, K/Rb, and high Th, Nb, Be, Bi, Rb/Sr compared with the associated I-type granitoids, porphyries and volcanics. Fractionation of feldspar and minor zircon has also resulted in substantial relative enrichment in LREE together with a pronounced negative Eu anomaly in the Warrego Granite. Previous studies suggest that the Cu-Au-Bi mineralisation at Tennant Creek post-dates formation of the host ironstones, but the absolute timing is imprecisely known. Mass-balance calculations utilising background Au concentrations for potential source rocks suggest it is feasible for the Au in some of the deposits to have been leached by relatively high-temperature (250 to 300 °C) deep basinal brines, and deposited by reaction with the ironstones. However, the inferred hydrothermal leaching cells would need to have been very large unless the leaching process was very effective. An alternative preferred model is that the mineralising fluids were exsolved from incompatible element-enriched, fractionated granitic magma, mixed with ground water, and reacted with the ironstones to deposit the base and precious metals. On the basis of chemical similarity to other granitoids closely associated with mineralisation, the Warrego Granite seems the most likely source of the fluids. However, problematic constraints imposed by the apparent age of the mineralisation (1810 Ma), and an emplacement age (Rb-Sr) of 1670 Ma for the Warrego Granite indicate a need for additional geochronological studies.  相似文献   

13.
This study presents the geochemical characteristics of granitic rocks located on the northern margin of Chotanagpur Gneissic Complex (CGC), exposed in parts of Gaya district, Bihar and discusses the possible petrogenetic process and source characteristics. These granites are associated with Barabar Anorthosite Complex and Neo-proterozoic Munger–Rajgir group of rocks. The granitic litho-units identified in the field are grey, pink and porphyritic granites. On the basis of geochemical and petrographic characteristics, the grey and pink granites were grouped together as GPG while the porphyritic granites were named as PG. Both GPG and PG are enriched in SiO2, K2O, Na2O, REE (except Eu), Rb, Ba, HFSE (Nb, Y, Zr), depleted in MgO, CaO, Sr and are characterised by high Fe* values, Ga/Al ratios and high Zr saturation temperatures (GPGavg～ 861 °C and PGavg～ 835 °C). The REE patterns for GPG are moderately fractionated with an average (La/Yb)N～ 4.55 and Eu/Eu* ～ 0.58, than PG which are strongly fractionated with an average (La/Yb)N～ 31.86 and Eu/Eu* ～ 0.75. These features indicate that the granites have an A-type character. On the basis of geochemical data, we conclude that the granites are probably derived from a predominant crustal source with variable mantle involvement in a post-collisional setting.  相似文献   

14.
The 1.1 Ga Concordia Granite Gneiss (CGG) is part of the late to postorogenic Spektakel Suite in the western Namaqualand Metamorphic Complex, South Africa. It intruded synkinematically, with respect to the main (D2) deformation event, into lower to middle crustal rocks and granite emplacement was more or less coeval with the peak of granulite-facies metamorphism ( > 800°C, 5 kbar). Several genetically related rock types, megacrystic garnet-bearing granite, minor aplitic leucogranites and pegmatites are distinguished. All varieties are SiO2-rich (69–79 wt.%) peraluminous granites and show subalkaline-monzonitic magma characteristics. Geochemical differences in whole-rock chemistry between megacrystic granite and aplitic leucogranites (e.g., lower Al2O3, MgO, CaO, Ba, Zr; higher K2O, Rb, Nb, W, Rb/Sr, Ga/Al) and the decrease of e.g., CaO, MgO, Fe2O3, Ba, Zr, Th/U with increasing SiO2 in the megacrystic granites as well as the variation in Fe/Mn of magmatic garnets are best explained with crystal fractionation processes. Fractional crystallization of plagioclase produced potassium- and silica-rich residual melts characterised by very high Rb/Sr, Rb/Ba, U/Th, Mn/Fe ratios and higher concentration of W, Cu and Zn. Crystal fractionation processes also resulted in a relative LREE depletion and HREE enrichment (megacrystic granite: La/Lu)cn = 8.87−31.67; aplitic leucogranite and pegmatites: La/Lu)cn = 0.71−1.44) and evolution of pronounced negative Eu-anomalies. The crystallization sequence (near-solidus crystallization of biotite prior to alkali feldspar) suggests that the CGG magmas were H2O-undersaturated over a long period of their evolution. Water saturation during late-stage crystallization is, however, indicated by coarse late-stage eutectic mineral textures, pegmatites and WMo-bearing siliceous rocks. Furthermore the economic potential of the CGG is supported by its geochemical signature (e.g., high U, Th contents) which is similar to evolved high heat production (HHP) granites. The granitic magmas are attributed to partial melting of peraluminous crustal source rocks and are tentatively interpreted as fractionated S-type granites. The WMo deposits represent vein-type and pegmatitic deposits genetically related to a deep-seated granitic system.  相似文献   

15.
Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton (Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO2 and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals. Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years.  相似文献   

16.
Heterogeneous magnesium isotopic composition of the upper continental crust   总被引：3，自引：0，他引：3  
High-precision Mg isotopic data are reported for ∼100 well-characterized samples (granites, loess, shales and upper crustal composites) that were previously used to estimate the upper continental crust composition. Magnesium isotopic compositions display limited variation in eight I-type granites from southeastern Australia (δ26Mg = −0.25 to −0.15) and in 15 granitoid composites from eastern China (δ26Mg = −0.35 to −0.16) and do not correlate with SiO2 contents, indicating the absence of significant Mg isotope fractionation during differentiation of granitic magma. Similarly, the two S-type granites, which represent the two end-members of the S-type granite spectrum from southeastern Australia, have Mg isotopic composition (δ26Mg = −0.23 and −0.14) within the range of their potential source rocks (δ26Mg = −0.20 and +0.15) and I-type granites, suggesting that Mg isotope fractionation during crustal anatexis is also insignificant. By contrast, δ26Mg varies significantly in 19 A-type granites from northeastern China (−0.28 to +0.34) and may reflect source heterogeneity.Compared to I-type and S-type granites, sedimentary rocks have highly heterogeneous and, in most cases, heavier Mg isotopic compositions, with δ26Mg ranging from −0.32 to +0.05 in nine loess from New Zealand and the USA, from −0.27 to +0.49 in 20 post-Archean Australian shales (PAAS), and from −0.52 to +0.92 in 20 sedimentary composites from eastern China. With increasing chemical weathering, as measured by the chemical index of alternation (CIA), δ26Mg values show a larger dispersion in shales than loess. Furthermore, δ26Mg correlates negatively with δ7Li in loess. These characteristics suggest that chemical weathering significantly fractionates Mg isotopes and plays an important role in producing the highly variable Mg isotopic composition of sedimentary rocks.Based on the estimated proportions of major rock units within the upper continental crust and their average MgO contents, a weighted average δ26Mg value of −0.22 is derived for the average upper continental crust. Our studies indicate that Mg isotopic composition of the upper crust is, on average, mantle-like but highly heterogeneous, with δ26Mg ranging from −0.52 to +0.92. Such large isotopic variation mainly results from chemical weathering, during which light Mg isotopes are lost to the hydrosphere, leaving weathered products (e.g., sedimentary rocks) with heavy Mg isotopes.  相似文献   

17.
Preliminary investigations in the Younger Granite Province of Nigeria have revealed that precious and semi-precious minerals like rubies, sapphires, emeralds, aquamarine, zircon and fluorite can be found in the region. The gem minerals are shown to have been produced either by direct deposition along fissures, veins and greisens by hydrothermal fluids or as a result of hydrothermal fluids reacting with wall-rocks. These wall rocks are either biotite granites from which the hydrothermal fluids originated or basement rocks or any other rocks which the biotite granites intrude and their residual hydrothermal fluids have invaded. The hydrothermal fluids appear to have been rich in alkalis (Na+, K+, etc.), rare elements (Be, Zr, F, REE, etc.) and siliceous. As these fluids rose through fractures and channel ways through the rocks, they either deposited the gem minerals in the fractures at the appropriate stability conditions or reacted with the wall-rocks producing the gem minerals at the expense of elements like Ca and A1 in the minerals of these rocks.  相似文献   

18.
The 2,700-Ma Vermilion Granitic Complex of northern Minnesota is a granite-migmatite terrane composed of supracrustal metasedimentary rocks, mafic rocks, tonalitic and granodioritic plutonic rocks, and granite. The metasedimentary rocks are predominantly graywacke, which has been regionally metamorphosed to garnet-sillimanite-muscovite-bearing biotite schist, and has locally undergone anatexis. The mafic rocks form early phases within the complex and are of two types: (1) basaltic amphibolite, and (2) monzodiorite and essexite rich in large ion lithophile elements (LILE). The members of the early plutonic suite form small bodies that intrude the metasedimentary rocks and mafic rocks, producing an early migmatite. The granite is of two distinct varieties: (1) white garnet-muscovite-biotite leucogranite (S-type; Chappell and White 1974) and (2) grayish-pink biotite-magnetite Lac La Croix Granite (I-type). The leucogranite occurs in the early migmatite and in paragneissic portions of the complex, whereas the Lac La Croix Granite is a late-stage intrusive phase that invades the early migmatite and metasediment (producing a late migmatite) and forms a batholith. This study focuses specifically on the origin of granite in the Vermilion Granitic Complex. Chemical mass-balance calculations suggest that the S-type two-mica leucogranite had a metagraywacke source, and that the I-type Lac La Croix Granite formed via partial fusion of calc-alkaline tonalitic material, which may have been similar to rocks of the early plutonic suite. This model is satisfactory for petrogenesis of similar Late Archean post-kinematic granites throughout the Canadian Shield.  相似文献   

19.
Exploration of Zn-rich sulphide deposits at Leadville, northern Lachlan Fold Belt, New South Wales, for over two decades has been largely on the premise that the mineralisation represents felsic volcanic-hosted massive sulphides (VHMS). Deposits are hosted by ?Silurian felsic metavolcanic, psammopelitic and calcareous metasedimentary rocks which have been intruded by the late Carboniferous I-type Gulgong Granite. Evidence for an epigenetic replacement (skarn) origin of the deposits, rather than representing metamorphosed volcanogenic massive sulphides, includes the proximity of evolved granitic intrusives and reactive carbonate rocks, a skarn mineral assemblage (with characteristic prograde and retrograde stages), lack of textural or lithological indications of an exhalative origin, and gossan and sulphide compositions consistent with Zn-Pb skarns and atypical of Lachlan Fold Belt VHMS deposits. Furthermore, sulphide lead isotope ratios are significantly more radiogenic than signatures for VHMS deposits in the Lachlan Fold Belt. Carbonate δ13C and δ18O and sulphide δ34S values are consistent with the interaction of magmatic hydrothermal fluids with Palaeozoic carbonate rocks and a largely magmatic source of sulphur. It is concluded that the Leadville deposits are of skarn type, genetically related to the Gulgong Granite.  相似文献   

20.
《China Geology》2021,4(4):658-672
The Paleoproterozoic tectonic evolution of the Bangweulu Block has long been controversial. Paleoproterozoic granites consisting of the basement complex of the Bangweulu Block are widely exposed in northeastern Zambia, and they are the critical media for studying the tectonic evolution of the Bangweulu Block. This study systematically investigated the petrography, zircon U-Pb chronology, and petrogeochemistry of the granitoid extensively exposed in the Lunte area, northeastern Zambia. The results show that the granitoid in the area formed during 2051±13–2009±20 Ma as a result of Paleoproterozoic magmatic events. Geochemical data show that the granites in the area mainly include syenogranites and monzogranites of high-K calc-alkaline series and are characterized by high SiO2 content (72.68% –73.78%) and K2O/Na2O ratio (1.82–2.29). The presence of garnets, the high aluminum saturation index (A/CNK is 1.13–1.21), and the 1.27%–1.95% of corundum molecules jointly indicate that granites in the Lunte area are S-type granites. Rare earth elements in all samples show a rightward inclination and noticeably negative Eu-anomalies (δEu = 0.16–0.40) and are relatively rich in light rare earth elements. Furthermore, the granites are rich in large ion lithophile elements such as Rb, Th, U, and K and are depleted in Ba, Sr, and high field strength elements such as Ta and Nb. In addition, they bear low contents of Cr (6.31×10−6–10.8×10−6), Ni (2.87×10−6–4.76×10−6), and Co (2.62×10−6–3.96×10−6). These data lead to the conclusion that the source rocks are meta-sedimentary rocks. Combining the above results and the study of regional tectonic evolution, the authors suggest that granitoid in the Lunte area were formed in a tectonic environment corresponding to the collision between the Tanzania Craton and the Bangweulu Block. The magmatic activities in this period may be related to the assembly of the Columbia supercontinent.©2021 China Geology Editorial Office.  相似文献   

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