Cogenetic Dykes the Key to Identifying Diverse Magma Batches in the Assembly of Granitic Plutons

This study demonstrates that dykes that are coeval and cogenetic with plutons can provide an important tool for recognizing discrete batches of magma with similar overall chemical compositions and physical attributes, but different isotopic characteristics, and which contributed to pluton formation. The Qianlishan granitic pluton, located in the Qin–Hang fault zone separating the Yangtze block from the Cathaysia block in South China, was emplaced at 155 Ma to 152 Ma in the Late Jurassic. It consists of a central zone of strongly differentiated zinnwaldite-bearing equigranular granite surrounded by a less differentiated porphyritic granite. The pluton is spatially associated with an extensive granitic dyke swarm dated here at 153–152 Ma, demonstrating a coeval relationship. Amongst the dykes, two discrete end-member sources can be identified from the bimodal nature of their zircon hafnium and oxygen systematics, with one group showing a range in εHf(t) of − 11.9 to − 8.0 and in δ18O of 9.0–10.4‰, whereas in the other group the ranges are from −7.3 to − 4.1 and 8.4–9.4‰, respectively. This contrasts with the two phases of the Qianlishan pluton, which record wide ranges in εHf(t) of − 11.1 to − 5.1 and in δ18O of 8.3‰ to 10.4‰, but without bimodality. Hence, the overlapping Hf–O isotopic profiling shows the dykes and pluton to be cogenetic. Small-volume magma batches, with their rapid transport through the crust and quick cooling, are all typical features of dyke generation, thus preserving the original heterogeneous Hf–O isotopic signatures that are characteristic of two distinct crustal sources. However, although the pluton was formed from similar sources to the dykes, the bimodal source identity was lost during its assembly through mixing of the magma batches. These findings also provide a potential explanation for the wide range of zircon hafnium isotopic systematics typical of granitic plutons, as shown by sampling at all scales.

Contrasting garnet parageneses in a composite Grenvillian granitoid pluton, Newfoundland

Almandine- and grossular-rich garnet occurs as both a coronitic and non-coronitic phase in ferruginous, Grenvillian (c. 1050 Ma) granitoid rocks of the composite Potato Hill pluton of the Long Range Inlier, Newfoundland. The country rock includes garnetiferous gneiss, but garnets in the pluton are compositionally distinct (higher Ca and Mn, lower Mg), so none are interpreted as xenocrysts from the Long Range gneiss complex.Coronal garnet, quartz and hornblende separate primary pyroxene, ilmenite and hornblende from feldspar in two-pyroxene charnockite. Balanced mass-transfer reactions based on microprobe data and modes for the pyroxene-centred corona structures suggest that corona sites gained Fe and lost Na. The flux of Fe apparently controlled corona growth in the charnockite. The corona structures are attributed to subsolidus cooling of the pluton rather than to a metamorphic overprint because the coronas are obliterated in high strain zones cutting the charnockite. Temperature of formation is constrained at c. 775–630°C by two-pyroxene and garnet-hornblende thermometry. Compositionally-similar coronas in rare, Fe-rich enderbite of the Long Range gneiss complex probably formed during cooling after early, high-grade metamorphism or following the regional emplacement of the Grenvillian plutons.Non-coronitic garnets occur in equigranular and megacrystic hornblende-biotite granite. Garnets in the equigranular granite are large, well-formed, and in some instances are associated with compositional layering of probable igneous origin. These garnets are enriched in grossular (XCa = 0.28), and are therefore interpreted as phenocrysts crystallized at high pressure (>9 kbar?). They would nevertheless have been stabilized to lower pressures by their moderate spessartine content (XMn = 0.13).Garnets in foliated megacrystic granite form tiny crystals depleted in Fe and Mg, and enriched in Mn and Ca, and in these respects are similar to garnet in deformed charnockite. These garnets are therefore interpreted to have formed (or re-equilibrated) during the late Grenvillian deformation. Garnet is absent in relatively magnesian Grenvillian granites (bulk XFe2+ = 0.50–0.85) elsewhere in the inlier. The restriction of garnet to the Potato Hill pluton (bulk XFe2+ = 0.88–0.94) therefore testifies to bulk compositional controls on the formation of both magmatic and subsolidus garnet in this intrusion.

Sur trois granites des sondages de Marchenoir, Boissy-sur-Saint-Yon et Courgent de la partie occidentale du bassin de Paris

Three cores from the western part of the Paris basin (France) were taken in the equigranular granite of the basement complex. The three phases in the development of this rock are characterized by microclinization of albite and pneumatolysis, possibly occuring contemporaneously. A cataclastic granite was retrieved from the Boissy-sur-Saint-Yon core. The formation of this rock occurred in two phases corresponding to the original paragenesis of quartz, orthoclase, and albite, and subsequent low-temperature paragenesis of quartz and albite. The Courgent core yielded a fine-grained granite containing mica, albite with polysynthetic twinning, perthitic microcline, quartz and accessory minerals. Evidence of secondary silicification is present in the zones of orthogneissic planar texture, where neomineralization has been especially strong. The origins of the three granites, all of which are alkalic, are determined using the method proposed by M. Gorai (1951), and the results are plotted on a triangular diagram. Results of chemical analyses of the rocks are presented in tabular form.