The Phanerozoic: reconciling modern plate tectonics with ancient orogenic systems and crustal growth

[Extract] Phanerozoic Earth history affords us the previous opportunity of understanding the links between active tectonic processes and crustal growth, because we have the oceanic and continental record to combine into a coherent, whole-Earth geodynamic model

Zirconolite from högbomite-bearing metamorphic rocks from the Koyubi-Ridge, Sør Rondane Mountains, East Antarctica

第3回極域科学シンポジウム/第32回極域地学シンポジウム 11月30日（金） 国立極地研究所 ３階ラウン

Granite

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Petrology of high-Mg, low-Ti igneous rocks of the Glenelg River Complex (SE Australia) and the nature of their interaction with crustal melts

Although commonly ascribed a mantle derivation, the nature and origin of the parental magmas of the microgranular enclaves encountered in granitic plutons is typically a matter of deduction. This inevitably imparts uncertainty in petrogenetic models for the host silicic magmas, and, in the extreme, can cast doubt over a mingling origin for the enclaves. In contrast, this paper describes a rare example from southeastern Australia where the near-pristine mafic and felsic end-members involved in magma mingling and microgranular enclave formation are identified in outcrop, and their crustal and mantle heritages, respectively, can be demonstrated. The mafic magmas are represented by unusual, Mg-rich, Ti-depleted quartz melagabbros, quartz meladiorites and melatonalites that have affinity with the boninitic lavas erupted above subduction zones. The distinctive chemical signature of these allows the physiochemical processes culminating in microgranular enclave formation to be traced. The participating felsic end-member is muscovite leucogranite, formed by anatexis of metasedimentary rocks near the current exposure level. Interpenetration between these magmas produced mafic cumulates, evolved, variably hybridised derivatives and dispersed microgranular enclave swarms throughout the leucogranite. Localised mixing generated hornblende tonalite, though the widespread occurrence of dykes of this material indicate that mixing was extensive at depth. These relationships suggest that high-Mg magmas may have a key role in the formation of granitic bodies in subduction environments, and in explaining the geochemical variation shown by these plutons

The two-component model for the genesis of granitic rocks in southeastern Australia — Nature of the metasedimentary-derived and basaltic end members

Granitic magmas in three areas of southeastern Australia are the product of melting of quartzofeldspathic\ud metasedimentary rock (high-silica end member) combined with varying degrees of incorporation of mafic\ud (dominantly basaltic) magma (low-silica end member). The link between granitic magma and local\ud metasedimentary rock is established by identifying shared geochemical signatures in Sr, Na2O and K2O\ud contents. High-silica end members have compositions formed in a genetic spectrum from total source\ud mobilisation to partial melt segregation from metasedimentary rock. The position of a granitic end member\ud in this spectrum is monitored by titanium contents varying from normal metasedimentary to the strongly\ud depleted induced by partial melting. The titanium signature of partial melting is established in a type area\ud with unequivocal links between migmatite mesosomes and leucosomes and the high-silica end members of\ud granitic variation systems that extend to lower silica. The low-silica end members approximate high-alumina\ud basalts on the basis of observed gabbros and projection of granitic variation lines to equivalent gabbroic\ud compositions. High-alumina basalt is postulated to be a fundamental ingredient in the petrogenesis of\ud orogenic granitic rocks

On the difficulty of assigning crustal residence, magmatic protolith and metamorphic ages to Lewisian granulites: constraints from combined in situ U–Pb and Lu–Hf isotopes

Zircons from two granulite facies gneisses from the central region of the Lewisian Complex have been investigated by high spatial resolution ion-microprobe U–Pb dating and laser ablation combined Pb–Hf isotope methods. The ion-microprobe data reveal a complex pattern of zircon ages distributed along the concordia curve between the time of granulite facies metamorphism at c. 2.5 Ga and the oldest zircon in each sample (respectively 2.89 Ga and 3.04 Ga). This Pb-loss pattern complicates assignment of an unambiguous magmatic protolith age to the zircon although cathodoluminescence (CL) imaging is used to suggest a preferred age of c. 2.85 Ga for both samples, with older grains being inherited. In situ Hf isotopes show a larger spread in the sample containing older grains which is also consistent with inheritance and further suggests that several crust extraction events are represented in the inherited population. Comparison of Hf isotope compositions with plausible model evolution curves suggests that crustal precursors to the Lewisian granulites were derived from their mantle source at c. 3.05–3.2 Ga

The differentiation and rates of generation of the continental crust

A new approach is developed to evaluate the rates of crust generation and hence the quantities of incompatible elements processed through the continental crust over the last 4 Ga. This relies more on minor and trace elements, and residence times in the upper crust and less on radiogenic isotopes since the latter constrain the stabilisation of continental crust rather than the rates of crust generation. In this model, the composition of new material added to the continental crust is similar to estimates of the average lower continental crust. The median composition of granitic magmas with Eu/Eu* = 0.7 is strikingly similar to that of the average upper crust and, in the simplest model, this represents not, vert, similar 14% melting or 86% fractional crystallisation of new crust. For an upper crust of 12.5 km thickness, there would be 77 km of complementary residue, for which there is scant geological evidence. It is therefore inferred that the residence times of elements in the lower crust is much less than in the upper crust. The annual flux of material into the upper crust can be inferred from its volume and the residence times of elements in the upper crust. A maximum value of the latter is provided by the model Nd age of the upper crust of 2 Ga, indicating that the average rates of crust generation are in excess of six times those in the recent geologic past and two to three times greater than the rates inferred from radiogenic isotopes. Over 4 Ga more than half the K, and one quarter of the Li, in the silicate Earth may therefore have been processed through the continental crust

Response of zircon to melting and metamorphism in deep arc crust, Fiordland (New Zealand): implications for zircon inheritance in cordilleran granites

The Cretaceous Mount Daniel Complex (MDC) in northern Fiordland, New Zealand was emplaced as a 50 m-thick dyke and sheet complex into an active shear zone at the base of a Cordilleran magmatic arc. It was emplaced below the 20-25 km-thick, 125.3 +/- 1.3 Ma old Western Fiordland Orthogneiss (WFO) and is characterized by metre-scale sheets of sodic, low and high Sr/Y diorites and granites. 119.3 +/- 1.2 Ma old, pre-MDC lattice dykes and 117.4 +/- 3.1 Ma late-MDC lattice dykes constrain the age of the MDC itself. Most dykes were isoclinally folded as they intruded, but crystallised within this deepcrustal, magma-transfer zone as the terrain cooled and was buried from 25 to 50 km (9-14 kbar), based on published P-T estimated from the surrounding country rocks. Zircon grains formed under these magmatic/ granulite facies metamorphic conditions were initially characterized by conservatively assigning zircons with oscillatory zoning as igneous and featureless rims as metamorphic, representing 54% of the analysed grains. Further petrological assignment involved additional parameters such as age, morphology, Th/U ratios, REE patterns and Ti-in-zircon temperature estimates. Using this integrative approach, assignment of analysed grains to metamorphic or igneous groupings improved to 98%. A striking feature of the MDC is that only similar to 2% of all igneous zircon grains reflect emplacement, so that the zircon cargo was almost entirely inherited, even in dioritic magmas. Metamorphic zircons of MDC show a cooler temperature range of 740-640 degrees C, reflects the moderate ambient temperature of the lower crust during MDC emplacement. The MDC also provides a cautionary tale: in the absence of robust field and microstructural relations, the igneous-zoned zircon population at 122.1 +/- 1.3 Ma, derived mostly from inherited zircons of the WFO, would be meaningless in terms of actual magmatic emplacement age of MDC, where the latter is further obscured by younger (ca. 114 Ma) metamorphic overgrowths. Thus, our integrative approach provides the opportunity to discriminate between igneous and metamorphic zircon within deep-crustal complexes. Also, without the tight field relations at Mt Daniel, the scatter beyond a statistically coherent group might be ascribed to the presence of "antecrysts", but it is clear that the WFO solidified before the MDC was emplaced, and these older "igneous" grains are inherited. The bimodal age range of inherited igneous grains, dominated by similar to 125 Ma and 350-320 Ma age clusters, indicate that the adjacent WFO and a Carboniferous metaigneous basement were the main sources of the MDC magmas. Mafic lenses, stretched and highly attenuated into wisps within the MDC and dominated by similar to 124 Ma inherited zircons, are considered to be entrained restitic material from the WFO. A comparison with lower- and upper-crustal, high Sr/Y metaluminous granites elsewhere in Fiordland shows that zircon inheritance is common in the deep crust, near the source region, but generally much less so in coeval, shallow magma chambers (plutons). This is consistent with previous modelling on rapid zircon dissolution rates and high Zr saturation concentrations in metaluminous magmas. Accordingly, unless unusual circumstances exist, such as MDC preservation in the deep crust, low temperatures of magma generation, or rapid emplacement and crystallization at higher structural levels, information on zircon inheritance in upper crustal, Cordilleran plutons is lost during zircon dissolution, along with information on the age, nature and variety of the source material. The observation that dioritic magmas can form at these low temperatures (<750 degrees C) also suggests that the petrogenesis of mafic rocks in the arc root might need to be re-assessed

Neodymium isotope equilibration during crustal metamorphism revealed by in situ microanalysis of REE-rich accessory minerals

Radiogenic isotopes are widely used to investigate crustal evolutionary processes, however recent claims of Nd and Sr isotope disequilibrium during anatexis question the reliability of such information. We have conducted an in situ Sm–Nd isotope study of apatite, allanite, titanite, xenotime and monazite in metasedimentary rocks of different metamorphic grade to test Nd isotope equilibrium during metamorphism. Our results show that apatite retains an original, probably detrital, highly variable Nd isotopic signature until at least 500 °C before being isotopically homogenised, irrespective of textural context within the rock. Once equilibrated, apatite retains its Nd isotope signature throughout anatexis. In contrast, allanite and titanite are equilibrated at temperatures as low as 350–400 °C. REE-rich accessory minerals in high-grade rocks (∼600°C) show very similar initial εNd values at the time of metamorphism. We conclude that under these metamorphic conditions Nd isotope disequilibrium between crustal melts and metasedimentary sources is unlikely. Intra-grain Nd isotope zoning of monazite indicates that partial melting was open system, involving the injection of externally-derived melt into migmatites. This process, likely to be common in anatectic terranes but not always obvious at hand-specimen scale or from bulk rock geochemical data, can produce isotope variation that could potentially be misinterpreted as disequilibrium between the melt and its protolith

Hf isotopes in zircon reveal contrasting sources and crystallisation histories for alkaline to peralkaline granites of Temora, southeastern Australia

Peralkaline granites exhibit the hallmark features of A-type igneous rocks, but strongly differentiated chemistry and intense hydrothermal alteration camouflage their ultimate origins. We present the first in situ Hf isotope data from zircons of peralkaline granites, aimed at clarifying the protoliths of these plutons and their genetic relationship to associated metaluminous/weakly peraluminous granites. This study used rocks of the Devonian Narraburra Complex in southeastern Australia, and found that correlations between Hf isotopes and trace element ratios reveal fundamentally different origins for the nonperalkaline and peralkaline granites. The latter have a depleted mantle-like ancestry, whereas a weakly peraluminous rock formed from melts of older arc crust that were modified by interaction with juvenile, probably alkaline magmas. Juxtaposition of crust- and mantle-derived magmas reflects the high heat flow and lithosphere-scale faults associated with continental extension, and explains the diversity of A-type granites