Late Ediacaran juvenile magmatism in the Monts-du-Lyonnais metamorphic complex: implications for the pre-Variscan evolution of the French Massif Central
Simon CouziniéOscar LaurentPierre BouilholCyril Chelle-MichouAnne-Céline GanzhornVéronique GardienJean‐François Moyen
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The identification of oceanic sutures is key for understanding the evolution of the Paleozoic Variscan belt and the structure of the West European lithosphere. In the French Massif Central, the suture of the “Mid-Variscan” ocean would be stamped by a distinctive lithological formation known as the “Leptynite–Amphibolite Complex”. This formation comprises a Cambrian–Ordovician bimodal (meta-)igneous association interpreted as rifted-margin magmatism and preserves evidence for Devonian high-pressure metamorphism. Our study provides new geochronological and geochemical data on mafic–felsic rocks from the Riverie belt, part of the Monts-du-Lyonnais Leptynite–Amphibolite Complex. There, metaluminous, amphibole-bearing felsic gneisses represent former tonalites originally intrusive within mafic rocks (now amphibolites). LA–ICP–MS zircon U–Pb dating reveals a latest Ediacaran (c. 545 Ma) crystallization age for the tonalitic magmas, and a latest Devonian (c. 360 Ma) very limited metamorphic overprint related to the Variscan orogeny. Whole-rock geochemistry (notably pronounced Nb negative anomalies) and the highly radiogenic zircon Hf isotope compositions with εHf(545Ma) of c. +11 (in the range expected for the Depleted Mantle reservoir) indicate that the parent tonalitic melt originated from a mafic precursor sourced in a mantle metasomatized by oceanic slab-derived fluids. The (meta-)mafic rocks share a similar “arc” signature and were possibly generated from the same mantle source. The mafic–felsic association of the Riverie belt bears no relation to the rifting that led to the opening of the Mid-Variscan ocean, in marked contrast with what is observed in other Leptynite–Amphibolite Complexes. Instead, they can be correlated to a discrete juvenile magmatic event identified in the southern Variscan realm (Maures and Iberian massifs, Corsica) and interpreted as reflecting Cadomian back-arc magmatism.Keywords:
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The Neoarchaean high amphibolite facies Fuping Group consists mainly of meta arkosic leuco leptite, semipelitic biotite leptite gneiss, hornblendic rocks, marbles and calcsilicate rocks, which have been partly subjected to differential anatexis. In the Xiaohuilonggou section, northwest of the Xiaojue town , the hornblendic rocks occur in a relatively high proportion and can be divided into two types. One is thick bedded amphibolite, consisting of plagioclase, hornblende, and locally small amounts of quartz, garnet and biotite, with some accessory minerals such as zircon and apatite. In which, there are some felsic patches of various shapes and scales as the products of the initial stage of anatexis. The anarectic derivatives are low in REE and HFS element contents and somewhat high in LREE/HREE ratio compared with their parent rock. The main reason is that in the anatectic process the accessory minerals , especially zircon, behaved as restitic crystals. The other is bedded or boudinage like amphibolite interbedded with biotite leptite gneiss. It is composed mainly of plagioclase and hornblende, without or only with very little accessory minerals. So being different from what mentioned above, the newly formed felsic derivatives are higher in REE contents and lower in LREE/HREE ratio than the amphibolite. In the both cases, the felsic derivatives are obviously lower in LREE/HREE ratio than the typical Archaean TTG rocks because of that the anatexis of the Fuping Group happened under middle low pressure conditions. It is also observed that the thick bedded amphibolite and its anatectic product are quite different in Nd isotopic compositions, showing the preservation of isotopic disequilibrium. Some felsic materials nearby the bedded amphibolite, being very high in LREE/HREE ratio and quite different in Nd isotopic composition from the amphibolite, are considered to be formed by the anatexis of the biotite leptite gneiss.
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Abstract: I– and S‐type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S‐type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I‐type analogues. Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S‐types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S‐type granites are likely to be closely related in origin, with the former comprising both restite‐rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation. We propose a subdivision of I‐type granites into two groups, formed at high and low temperatures. The high‐temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low‐temperature I–type granites, the compositions extend to lower SiO 2 contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO 2 in the more mafic rocks. While the high‐temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low‐temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low‐temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite. High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H 2 O.
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ABSTRACT Granites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earth's surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe 3+ /Fe 2+ , and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks. S-type granites have higher δ 18 O values and more evolved Sr and Nd isotopic compositions, although the radiogenic isotope compositions overlap with I-types. Although the isotopic compositions lie close to a mixing curve, it is thought that the amount of mixing in the source rocks was restricted, and occurred prior to partial melting. I-type granites are thought to have been derived from deep crust formed by underplating and thus are infracrustal, in contrast to the supracrustal S-type source rocks. Crystallisation of feldspars from felsic granite melts leads to distinctive changes in the trace element compositions of more evolved I- and S-type granites. Most notably, P increases in abundance with fractionation of crystals from the more strongly peraluminous S-type felsic melts, while it decreases in abundance in the analogous, but weakly peraluminous, I-type melts.
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Granites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earth's surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe3+/Fe2+, and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks.
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In monomagmatic chambers density differences between overlying country rock and magma should, following development of ring fractures, result in mafic ring dikes above mafic magma, and in foundering of the roof into felsic magma with the development of felsic stocks. The observed prevalence of felsic ring dikes may be due to shallow floors in felsic magma chambers, or to shallow constructed floors at the top of the mafic fraction in polymagmatic chambers.
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Petrographic studies on the mafic and felsic dykes from the Piratini region, RS, reveal a porphyritic texture with an aphanitic to fine-grained matriz, with glomeroporphyritic and spherulitic textures in the case of felsic, and ophitic to sub-ophitic with myrmekitic intergrowths in the case of mafic dykes. These dykes are intrusive in granitic rocks of the Pelotas Batholith along NW-SE and N-S trends associated with high angle shear zones. They are strongly metaluminous (mafic) and slighty metaluminous to peraluminous (felsic), with an alumina saturation index between 0.60 and 0,65 and 0,75 and 1,2, respectively. The Si02 content in the mafic dykes varies from 44 to 48 wt % and in the felsic varies from 67 to 75 wt %. The mafic dykes present higher Ti, Mg, Ca, Fe, Mn and P contents in comparison with the felsic dykes. The behavior of several major and trace elements (Fe, Mn, Mg, Ti, P, Ca and Sr, and, in minor degree, Ca and Mg show the importance of crystallization of iron-magnesium and feldspars in both dykes. The Nb/Ta e U/Th ratios of trace elements of these rocks show evidence of crustal contamination. The HREE display sub horizontallized patterns, showing Tb/Lu ratios from 1,5 to 2,7 in the samples of mafic dykes and from 1,6 to 2,5 in the felsic dykes. The samples have shown Lu concentration from 0,3 ppm to 0,6 ppm in mafic, and from 18 ppm to 41 ppm in the felsic dykes, respectively. The felsic dykes are more enriched in LREE, with La values from 73 ppm to 155 ppm, whereas mafic dykes present 18 ppm to 41 ppm of La and La/Sm ratios in the range of 3,0 to 5,0.
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