Secular Evolution of Continents and the Earth System

Understanding of secular evolution of the Earth system is based largely on the rock and mineral archive preserved in the continental lithosphere. Based on the frequency and range of accessible data preserved in this record, we divide the secular evolution into seven phases: (a) “Proto-Earth” (ca. 4.57–4.45 Ga); (b) “Primordial Earth” (ca. 4.45–3.80 Ga); (c) “Primitive Earth” (ca. 3.8–3.2 Ga); (d) “Juvenile Earth” (ca. 3.2–2.5 Ga); (e) “Youthful Earth” (ca. 2.5–1.8 Ga); (f) “Middle Earth” (ca. 1.8–0.8 Ga); and (g) “Contemporary Earth” (since ca. 0.8 Ga). Integrating this record with knowledge of secular cooling of the mantle and lithospheric rheology constrains the changes in the tectonic modes that operated through Earth history. Initial accretion and the Moon forming impact during the Proto-Earth phase likely resulted in a magma ocean. The solidification of this magma ocean produced the Primordial Earth lithosphere, which preserves evidence for intra-lithospheric reworking of a rigid lid, but which also likely experienced partial recycling through mantle overturn and meteorite impacts. Evidence for craton formation and stabilization from ca. 3.8 to 2.5 Ga, during the Primitive and Juvenile Earth phases, likely reflects some degree of coupling between the convecting mantle and a lithosphere initially weak enough to favor an internally deformable, squishy-lid behavior, which led to a transition to more rigid, plate like, behavior by the end of the early Earth phases. The Youthful to Contemporary phases of Earth, all occurred within a plate tectonic framework with changes between phases linked to lithospheric behavior and the supercontinent cycle.Peter A. Cawood, Priyadarshi Chowdhury, Jacob A. Mulder, Chris J. Hawkesworth, Fabio A. Capitanio, Prasanna M. Gunawardana, and Oliver Nebe

Tectonic controls on post-subduction granite genesis and emplacement : the late Caledonian suite of Britain and Ireland

Rates of magma emplacement commonly vary as a function of tectonic setting. The late Caledonian granites of Britain and Ireland are associated with closure of the Iapetus Ocean and were emplaced into a varying regime of transpression and transtension throughout the Silurian and into the early Devonian. Here we evaluate a new approach for examining how magma volumes vary as a function of tectonic setting. Available radiometric ages from the late Caledonian granites are used to calculate probability density functions (age spectra), with each pluton weighted by outcrop area as a proxy for its volume. These spectra confirm an absence of magmatic activity during Iapetus subduction between c. 455 Ma and 425 Ma and a dominance of post-subduction magmas between c. 425 Ma and 380 Ma. We review possible reasons why, despite the widespread outcrop of the late Caledonian granites, magmatism appears absent during Iapetus subduction. These include shallow angle subduction or extensive erosion and tectonic removal of the arc. In contrast to previous work we find no strong difference in the age or major element chemistry of post-subduction granites across all terranes. We propose a common causal mechanism in which the down-going Iapetus oceanic slab peeled back and detached beneath the suture following final Iapetus closure. The lithospheric mantle was delaminated beneath the suture and for about 100 km back beneath the Avalonian margin. While magma generation is largely a function of gravitationally driven lithosphere delamination, strike-slip dominated kinematics in the overlying continental crust is what modulated granitic magma emplacement. Early Devonian (419–404 Ma) transtension permitted large volumes of granite emplacement, whereas the subsequent Acadian (late Early Devonian, 404–394 Ma) transpression reduced and eventually suppressed magma emplacement

The genesis of gold mineralisation hosted by orogenic belts: A lead isotope investigation of Irish gold deposits

Lead isotope analyses have been performed on 109 gold and 23 sulphide samples from 34 Irish gold occurrences, including 27 placers, and used to shed light on the sources of mineralising fluids and metals associated with gold mineralisation hosted by orogenic belts. The Pb isotope ratios of lode and placer gold range from 206Pb/204Pb=17.287-18.679, 207Pb/204Pb=15.382-15.661, and 208Pb/204Pb=37.517-38.635, consistent with the Pb isotopic data on previously reported Irish sulphide mineralisation. There is no evidence that gold mineralisation is associated with distinctive source regions, and it appears to have been derived from similar sources to those responsible for the widespread sulphide mineralisation in Ireland. It is inferred that the principal controls on the Au mineralisation are structural and not related to the distribution of Au in their source rocks. The range of Pb isotope ratios favours the interaction of multiple source reservoirs predominantly during the Caledonian Orogeny (c. 475-380Ma). Underlying basement was the primary control on two key sources of Pb. Gold occurrences located to the south-east of the Iapetus Suture are characterised by Pb compositions that derive predominantly from the Late Proterozoic crustal basement or overlying Lower Palaeozoic sediments, whilst those located north-west of the Iapetus Suture are characterised by less radiogenic Pb signatures derived predominantly from Late Proterozoic or older crustal basement. A third source, relatively enriched in radiogenic Pb, also played a role in the formation of a number of Irish gold occurrences, and may have been associated with syn- to post-orogenic intrusives. Magmatic processes may therefore have played an important role in the formation of some orogenic gold occurrences

Detrital zircon record and tectonic setting

Funding: University of St AndrewsDetrital zircon spectra reflect the tectonic setting of the basin in which they are deposited. Convergent plate margins are characterized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional and intracratonic settings contain greater proportions with older ages that reflect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sediment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the tectonic setting in which the sediment was deposited.Publisher PDFPeer reviewe

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

The denudation history of the onshore continental margin of SE Brazil inferred from apatite fission track data

A suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast Brazil are presented. Some of the results reflect resetting related to the eruption of the Parana continental flood basalt (similar to 130 Ma), and four others are directly related to Late Cretaceous magmatism (similar to 80 Ma). The majority of the results were obtained from Precambrian basement rocks and sediments from the Parana Basin. The fission track ages broadly increase inland from 60-90 Ma on the coastal plain to &gt; 300 Ma in the continental hinterland. The track length distributions show a characteristic trend, being negatively skewed at the coast and showing increasing bimodality inland, except for the oldest samples which are more unimodal. These trends do not reflect rift-related heating and resetting but are the result of protracted denudation since the opening of the South Atlantic. The results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland. The topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology. For the region further south, model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long-wavelength topography, unless the flexural rigidity is more than 10(25) N m. In this case the initial elevation of the rift margin would need to be &gt;2.5 km in order to accommodate the amount of denudation inferred from the fission track data. However, it is possible that the effective flexural rigidity varies across the margin, becoming greater in the continental interior. Alternatively, the broad scale wavelength topography may be the result of magmatic underplating related to the Parana continental flood basalt event. If so, estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5.</p

Felsic lavas or rheomorphic ignimbrites: is there a chemical distinction?

In the environment of felsic magma generation associated with continental flood basalt (CFB) provinces there is a close association between magma composition, phenocryst assemblage, temperature and eruption mechanism. In this paper we propose that by examining the chemical composition and, in particular the high field strength element (Zr and Nb) contents of the high silica rocks, we can identify those that contained halogen-rich volatiles and which degassed at different levels (deep versus shallow). The degassing depth has a direct influence on the type of eruption, with shallow degassing promoting explosive rather than effusive behaviour and consequentially the former is more likely to give rise to ignimbrites. Thus, we can infer likely eruption mechanisms, as high concentrations of F and Cl dramatically decrease magma viscosities and should favour deep degassing and hence promote lava effusion rather than explosive pyroclastic eruption. This hypothesis is tested by constraining the other possible variables which have an effect on the mode of eruption, and by examining the nature and composition of melt and fluid inclusions in quartz from CFB-associated felsic volcanic rocks.</p

Emergence of modern continental crust about 3 billion years ago

The continental crust is the principal record of conditions on the Earth during the past 4.4 billion years1,2. However, how the continental crust formed and evolved through time remains highly controversial3,4. In particular, the composition and thickness of juvenile continental crust are unknown. Here we show that Rb/Sr ratios can be used as a proxy for both the silica content and the thickness of the continental crust. We calculate Rb/Sr ratios of the juvenile crust for over 13,000 samples, with Nd model ages ranging from the Hadean to Phanerozoic. The ratios were calculated based on the evolution of Sr isotopes in the period between the TDM Nd model age and the crystallization of the samples analysed. We find that the juvenile crust had a low silica content and was largely mafic in composition during the first 1.5 billion years of Earth’s evolution, consistent with magmatism on a pre-plate tectonics planet. About 3 billion years ago, the Rb/Sr ratios of the juvenile continental crust increased, indicating that the newly formed crust became more silica-rich and probably thicker. This transition is in turn linked to the onset of plate tectonics5 and an increase of continental detritus into the oceans6