K-feldspar megacrysts (Kfm) are used to investigate the magmatic evolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba, Italy). Dissolution and regrowth of Kfm during magma mixing or mingling events produce indented resorption surfaces associated with high Ba contents. Diffusion calculations demonstrate that Kfm chemical zoning is primary. Core-to-rim variations in Ba, Rb, Sr, Li and P support magma mixing (i.e. high Ba and P and low Rb/Sr at rims), but more complex variations require other mechanisms. In particular, we show that disequilibrium growth (related to variations in diffusion rates in the melt) may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber. Initial 87Sr/86Sr ratios (ISr) (obtained by microdrilling) decrease from core to rim. Inner core analyses define a mixing trend extending towards a high ISr–Rb/Sr melt component, whereas the outer cores and rims display a more restricted range of ISr, but a larger range of Rb/Sr. Lower ISr at the rim of one megacryst suggests mixing with high-K calc-alkaline mantle-derived volcanics of similar age on Capraia. Trace element and isotopic profiles suggest (1) early megacryst growth in magmas contaminated by crust and refreshed by high ISr silicic melts (as seen in the inner cores) and (2) later recharge with mafic magmas (as seen in the outer cores) followed by (3) crystal fractionation, with possible interaction with hydrothermal fluids (as seen in the rim). The model is compatible with the field occurrence of mafic enclaves and xenoliths.
Abstract The presence of major crystalline basement provinces at depth in NW Ireland is inferred from in situ Hf isotope analysis of zircons from granitoid rocks that cut structurally overlying metasedimentary rocks. Granitoids in two of these units, the Slishwood Division and the Tyrone Central Inlier, contain complex zircons with core and rim structures. In both cases, cores have average ϵHf values that differ from the average ϵHf values of the rims at 470 Ma (the time of granitoid intrusion). The Hf data and similarity in U–Pb age between the inherited cores and detrital zircons from the host metasedimentary rocks suggests local contamination during intrusion rather than transport of the grains from the source region at depth. Rims from the Slishwood Division intrusions have average ϵHf 470 values of −7.7, consistent with a derivation from juvenile Palaeoproterozoic crust, such as the Annagh Gneiss Complex or Rhinns Complex of NW Ireland, implying that the deep crust underlying the Slishwood Division is made of similar material. Rims from the Tyrone Central Inlier have extremely negative ϵHf 470 values of approximately −39. This isotopic signature requires an Archaean source, suggesting rocks similar to the Lewisian Complex of Scotland, or sediment derived wholly from it, occurs at depth in NW Ireland.
Sedimentary rocks and modern sediments sample large volumes of the Earth’s crust, and preserve units that vary greatly in age and composition. Determining the provenance of component minerals is complicated by the ability of some minerals to be recycled through multiple sedimentary cycles, so minerals from completely unrelated sources may end up in the same sedimentary basin. To untangle these multi-stage signals, two or more chemical signatures measured in minerals with different stability are required. For instance, labile minerals, such as feldspar, can break down rapidly during sedimentary transport, while refractory minerals, such as zircon, can be much more resilient and survive repeated recycling. One sedimentary succession suitable for testing this hypothesis is the Upper Carboniferous Millstone Grit Group, a fluvio-deltaic, upward-coarsening sequence of mudstones, sandstones and conglomerates deposited in the Pennine Basin of northern England over c. 14 myr. New isotopic data have been measured in detrital K-feldspar and zircon from five of the seven stages, complementing previous work in the area [1,2,3]. Two K-feldspar Pb isotope peaks at 206Pb/204Pb = 12.5–15.5 and c. 18.4 indicate derivation from Archaean–Proterozoic basement and Caledonian granites, respectively. Zircon U–Pb age peaks at c. 2700, 1000–2000 and 430 Ma reflect a mixture of Archaean basement, Proterozoic sediments and Caledonian granites, while Hf model ages form two broad peaks at c. 4500–3000 and 2300–1500 Ma, indicating contributions from both juvenile and reworked crust. Strong similarities between potential sources in this complicated region mean no one mineral or isotopic system can provide a unique provenance determination. Instead, comparing first-cycle and multi-cycle minerals with different hydrodynamic properties is necessary to untangle the full story. Combining these results with published garnet, monazite and muscovite data demonstrates the power of multi-proxy provenance work, indicating a primary source area in the Greenland Caledonides, with minor contributions from Norway and Scot-land. Comparisons between zircon U–Pb distributions in Palaeozoic sediments suggest long-lived sedimentary systems recycled material around the North Atlantic over c. 100 myr, much of it ultimately derived along the Grenvillian margin of Laurentia. This consistency is interrupted only by regular variations in palaeoflow direction, reflecting tectonic evolution in the region.
Abstract In Scotland and Ireland, a Laurentian passive margin sequence, the Dalradian Supergroup, was deformed during the c. 470–460 Ma Grampian orogeny, resulting in the formation of crustal-scale recumbent nappes. In Ireland, this passive margin sequence is in general bounded to the SE by the Fair Head-Clew Bay Line (FHCBL), a segment of a major lineament within the Caledonides. Adjacent to the FHCBL, Dalradian metasediments in two separate inliers have undergone post-Grampian strike-slip movement, with the initially flat-lying Grampian nappe fabric acting as a décollement-like slip surface in both cases. As the orientation of these foliation slip surfaces was oblique to the local shear plane in both inliers, displacement along these pre-existing foliation surfaces was also accompanied by crenulation slip. However, the crenulation-slip morphologies produced imply the opposite sense of movement in the two inliers. 40 Ar- 39 Ar dating of muscovite defining the crenulation-slip surfaces indicates that post-Grampian dextral displacement took place along the FHCBL at 448 ± 3 Ma. A subsequent phase of sinistral movement along the FHCBL took place at c. 400 Ma, based on previously published Rb-Sr muscovite ages for synkinematic pegmatites. The kinematic information obtained from crenulationslip morphologies combined with geochronology can thus be used to constrain the reactivation history of a major crustal-scale shear zone.
The Palaeoproterozoic to early Neoproterozoic Annagh Gneiss Complex structurally underlies the Dalradian sequence in north Mayo, Ireland, and has been proposed as the depositional basement to Dalradian metasediments. The Annagh Gneiss Complex was deformed, metamorphosed and migmatized during the Grenville Orogeny and later reworked under amphibolite-facies conditions. This paper focuses on the timing of the post-Grenville events and particularly on the possible presence of post-Grenville, pre-Grampian deformation that could be attributed to the Knoydartian Orogeny. Seven U–Pb titanite analyses from Annagh Gneiss Complex gneisses have a weighted mean 207 Pb/ 206 Pb age of 963 ± 8 Ma, which dates cooling after the main Grenville metamorphism. Locally, a later phase of titanite growth at 943 ± 8 Ma post-dates the last phase of Grenville deformation. The weak discordance of the titanite data suggests that post-Grenville events had little effect on the U–Pb system in titanite. If the discordance was caused by a tectonic event, this is likely to have occurred during the early Ordovician Grampian Orogeny rather than in the Neoproterozoic. Within the Annagh Gneiss Complex, cross-cutting metadolerites provide a structural marker allowing post-Grenville deformation to be distinguished. In contrast, correlative metadolerites cutting the adjacent Dalradian metasediments share all Grampian deformation events affecting their host. Ar–Ar hornblende ages from the post-Grenville metadolerites indicate that reworking of the Annagh Gneiss Complex and the first episodes of Dalradian deformation occurred during the Grampian Orogeny in this part of Ireland. One sample yields a 475 ± 4 Ma Ar–Ar plateau age, which is interpreted to date Grampian deformation. Younger Ar–Ar hornblende and Rb–Sr mica ages record post-Grampian cooling. Neither field nor isotopic evidence for the Knoydartian Orogeny has been found in this part of Ireland.
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