Homogenising the upper continental crust : the Si isotope evolution of the crust recorded by ancient glacial diamictites

This work was supported by PhD funding to MM by the University of St Andrews School of Earth and Environmental Sciences and the Handsel scheme, as well as by NERC grant NE/R002134/1 to PS and NSF grant EAR-1321954 to RR and RG.Twenty-four composite samples of the fine-grained matrix of glacial diamictites deposited from the Mesoarchaean to Palaeozoic have been analysed for their silicon isotope composition and used to establish, for the first time, the long-term secular Si isotope record of the compositional evolution of upper continental crust (UCC). Diamictites with Archaean and Palaeoproterozoic Nd model ages show greater silicon isotope heterogeneity than those with younger model ages (irrespective of depositional age). We attribute the anomalously light Si isotope compositions of some diamictites with Archaean model ages to the presence of glacially milled banded iron formation (BIF), substantiated by the high iron content and Ge/Si in these samples. We infer that relatively heavy Si isotope signatures in some Palaeoproterozoic diamictites (all of which have Archaean Nd model ages) are due to contribution from tonalite-trondhjemite-granodiorites (TTGs), evidenced by the abundance of TTG clasts. By the Neoproterozoic (with model ages ranging from 2.3 to 1.8 Ga), diamictite Si isotope compositions exhibit a range comparable to modern UCC. This reduced variability through time is interpreted as reflecting the decreasing importance of BIF and TTG in post-Archaean continental crust. The secular evolution of Si isotopes in the diamictites offers an independent test of models for the emergence of stable cratons and the onset of horizontal mobile-lid tectonism. The early Archaean UCC was heterogeneous and incorporated significant amounts of isotopically light BIF, but following the late Archaean stabilisation of cratons, coupled with the oxygenation of the atmosphere that led to the reduced neoformation of BIF and diminishing quantities of TTGs, the UCC became increasingly homogeneous. This homogenisation likely occurred via reworking of preexisting crust, as evidenced by Archaean Nd model ages recorded in younger diamictites.Publisher PDFPeer reviewe

Titanium isotope evidence for the high topography of Nuna and Gondwana - Implications for Earth’s redox and biological evolution

Titanium isotopes recorded in glacial diamictites with depositional ages between 2.9 and 0.3 Ga show that the upper continental crust became significantly more felsic relative to the present-day crust during the amalgamation of the Paleoproterozoic Nuna and the Neoproterozoic Gondwana supercontinents. This can be attributed to the continental collisions involved in the assembly of Nuna and Gondwana. The resulting high topographic relief of Nuna and Gondwana orogens must have resulted in an enhanced erosional supply from the continents to oceans. The step changes in the development of organismal complexity from prokaryotes to eukaryotes, and eventually metazoans, appear to be temporally correlated to instances where collisional mountain-building sustained an elevated nutrient supply from the continents to oceans. The nutrient surge associated with the rise of the Gondwana mountains likely provided the necessary impetus for the Neoproterozoic ecological expansion of eukaryotes and the eventual radiation of metazoans. A similar link between the enhanced nutrient supply from Nuna mountains and the radiation of early eukaryotes is plausible, although its mechanistic underpinnings remain unclear. The termination of Nuna orogeny and its transition to Rodinia without significant breakup and subsequent collisional orogenesis corresponds to the long lull in Earth's redox and biological evolution in its middle age

Age and petrogenesis of the Idaho batholith and implications for basement architecture in the Northern Rockies

New in situ U-Pb zircon geochronology delineates a series of rock suites with distinct ages and compositions making up the Idaho batholith. The early metaluminous suite (98 to 87 Ma) and border zone suite (~90 Ma) are volumetrically minor in their present exposure but compositionally diverse and form the eastern and western margins of much of the batholith. The Atlanta peraluminous suite (83 to 67 Ma) represents the largest single component of the batholith and is compositionally homogeneous. The late metaluminous suite (~70 Ma) is geographically restricted but compositionally diverse. The Bitterroot peraluminous suite (66 to 54 Ma) is quite similar to the Atlanta peraluminous suite in terms of lithology but is smaller and temporally distinct. All of these units are cut by plutons of the Challis intrusive province (51 to 43), which show a great diversity of compositions. New Sr, Nd, Pb, and Hf (whole-rock and zircon) isotopic data suggest an increase and then decrease in crustal melting with time. The early metaluminous and border zone suite are hybrids of mantle-derived arc basalts and supracrustal rocks. Crustal thickening led to a shift to almost pure crustal melting, leading to the formation of the Atlanta peraluminous suite from Neoproterozoic and Archean components. Limited sampling of the late metaluminous suite makes its origin enigmatic. The Bitterroot peraluminous suite, although lithologically similar to the Atlanta peraluminous suite, is isotopically distinct and the result of melting of Paleoproterozoic basement and assimilation of small amounts of Mesoproterozoic sediments. The Challis intrusions formed after a switch to extensional tectonics, allowing mantle melts to enter and interact with the crust, leading to a wide range of magmatic products. The ages of inherited zircon components are fundamentally different between the southern Atlanta lobe and northern Bitterroot lobe of the batholith. The Atlanta lobe is dominated by two sharp age peaks at ~670 and 2.55 Ga, which are consistent with known local metaigneous sources, whereas the Bitterroot lobe shows a continuum of the ages, ranging from ~1.4 to 1.9 Ga and remarkably similar to the detrital zircon spectrum of large portions of the Belt Supergroup

Isotopic evolution of the Idaho batholith and Challis intrusive province, northern US cordillera

The Idaho batholith and spatially overlapping Challis intrusive province in the North American Cordillera have a history of magma-tism spanning some 55 Myr. New isotopic data from the 98Ma to 54Ma Idaho batholith and51Ma to 43Ma Challis intrusions, coupled with recent geochronological work, provide insights into the evolution of magmatism in the Idaho segment of the Cordillera. Nd and Hf isotopes show clear shifts towards more evolved compositions through the batholith’s history and Pb isotopes define distinct fields correlative with the different age and compositionally defined suites of the batholith, whereas the Sr isotopic compositions of the various suites largely overlap.The subsequent Challis magmatism shows the full range of isotopic compositions seen in the batholith.These data suggest that the early suites of metaluminous magmatism (98^87 Ma) represent crust^mantle hybrids. Subsequent voluminous Atlanta peraluminous suite magmatism (83^67 Ma) results pri-marily from melting of different crustal components. This can be attributed to crustal thickening, resulting from either subduction pro-cesses or an outboard terrane collision. A later, smaller crustal melt-ing episode, in the northern Idaho batholith, resulted in the Bitterroot peraluminous suite (66^54 Ma) and tapped different crustal sources. Subsequent Challis magmatism was derived from both crust and mantle sources and corresponds to extensional collapse of the over-thickened crust