Helps, Paul A. (2009) Scales of heterogeneities and equilibrium volumes in granitoid magmas. (PhD thesis), Kingston University, .
Abstract
Chemical, textural, and isotopic heterogeneity within granitic magmas/rocks, and their extrusive equivalents, are common phenomena. This is to be expected since the source materials, from which the granitic magmas were created by partial melting, are themselves heterogeneous. Micro-sampling of feldspar crystals within granites has revealed complex internal Sr and Nd isotope variations thought to reveal subtle variations in the isotopic composition of the melt from which particular zones crystallised. These previous studies provide evidence of both macro-scale (regional) and micro-scale isotopic variation within granitic magmas. What is unknown is the scale and nature of chemical and isotopic variation occurring on the meso-scale (metre, decametre, hectometre). In this study, high-precision geochemical and isotopic (Sr, Nd, and O) data is obtained for spatially well-constrained samples, from three granitic intrusions from the UK, to study the scales over which isotope heterogeneities are preserved, and the maximum volumes of magma over which isotope and elemental homogenisation may have been achieved. This information provides important constraints on the physical and chemical characteristics of processes that occur during magma genesis, ascent, and emplacement. The regional-scale geochemical and isotopic heterogeneity in the Criffell pluton (SW Scotland) has been well documented and ascribed to 'incomplete hybridisation' between magmas derived from the melting of mantle and/or 'new' basic crust and metasediments. Sampling on the 10 to 100 m scale in this study demonstrates major-, trace- and rare earth element homogeneity between samples of similar petrography. Within the Dalbeattie Quarry granodiorite, subtle heterogeneity in initial ([sup]87Sr/[sup]86Sr)[sub]397 Ma compositions exist (0.70582 to 0.70615), between samples collected ~ 150 m apart. The small-scale heterogeneity is outside of analytical error (± 0.0001). [delta][sup]18O heterogeneity also occurs on similar small scales (~ 120 m), with values ranging from 8.9 to 10.7 %. The larger error on the [epsilon][sub]Nd(397 Ma) values do not allow any small-scale heterogeneity to be resolved. The geochemical and isotopic heterogeneity of the Cairnsmore of Fleet pluton, another Southern Upland granite, is also well known, and considered to be a result of similar processes to that of Criffell. This study demonstrates that the Cairnsmore granite is more petrographically variable in terms of grain-size, mafic/felsic mineral proportions, and accessory mineral assemblages over scales ranging between 100 and 350 m. The existence of more petrographic and geochemical heterogeneities on smaller scales suggests that homogenisation processes were less effective in this pluton compared to Criffell. On scales < 1 km, ([sup]87Sr/[sup]86Sr)[sub]392 Ma compositions and [epsilon][sub]Nd(397 Ma) values are essentially homogeneous. Forest Track samples PAHCF23 (0.70796) and PAHCF25A (0.70672) are ~ 1 km apart and can only just be distinguished outside of analytical error. However, [delta][sup]18O data demonstrate the existence of both regional-scale and locality-scale [delta][sup]18O heterogeneity on the 100 m, decametre and even cm-scale (8.3 to 9.7%0, error ± 0.1%0). The two main granite varieties of the Dartmoor pluton (SW England) are generally, homogeneous in terms of their petrography and geochemistry on 100 m, decametre, meter and even cm scale. However, isotope heterogeneity, outside of analytical error is shown to exist on even the cm-scale. The samples PAHD34A and P AHD34B, from Blackenstone Quarry, have ([sup]87Sr/[sup]86Sr)[sub]280Ma compositions of O.70949 and 0.71292 respectively, and were collected 50 cm apart. Samples PAHD35A and PAHD35B have ([sup]87Sr/[sup]86Sr)[sub]280 Ma compositions of 0.70960 and 0.71205, and were collected 100 cm apart. Variation in [delta][sup]18O and [epsilon][sub]Nd(280 Ma) values also exist. PAHD34A and PAHD34B have [delta][sup]180 compositions of 10.01 and 10.98 %0 (± 0.1%0), and SNd(280 Ma) values of -3.8 and -3.3 (± 0.2), respectively. Either the distinct magma batches, that coalesced to form the pluton, were much smaller in Dartmoor (compared to the Criffell and Cairnsmore plutons), or these heterogeneities may represent volumes of magma, within larger domains, that escaped homogenisation by mixing and diffusion. The existence of discrete, isotopically distinct batches of magma within the Criffell, Cairnsmore and Dartmoor plutons favours the dyke model for granitoid magma ascent through a pulsed magma delivery, in which separate magma batches (or pulses) coalesce to form plutons at or near their final emplacement levels. The scale over which such magma pulses can still be recognised varies within each pluton. The magma pulses are likely to have been larger in size, but subsequent homogenisation processes in the chambers led to a reduction in their size. However, homogenisation was not complete. In all three plutons, it is generally only the isotope ratios that display significant small-scale heterogeneity. Processes such as fractional crystallisation, are considered to be secondary, overprinting geochemical and isotopic heterogeneity from much deeper regions.
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