Record of Tethyan ocean closure and Indosinian collision along the Ailaoshan suture zone (SW China)

Zircon U–Pb and Lu–Hf isotopic data along with whole-rock elemental and Sr–Nd isotopic analytical results for the Xin'anzhai and Tongtiange granitic plutons in the Ailaoshan suture zone record the transition from subduction to collision associated with the accretion of Indochina to Yangtze Blocks. The Xin'anzhai monzogranite yields zircon U–Pb age of 251.6 ± 2.0 Ma and εHf(t) values of − 6.2– − 9.8. The Tongtiange leucogranite gives zircon U–Pb age of 247.5 ± 2.2 Ma and εHf(t) values ranging from − 3.1 to − 11.1. The Tongtiange leucogranites have lower MgO, Na2O, CaO, FeOt and TiO2 contents but higher A/CNK values than those of the Xin'anzhai monzogranites. The εNd(t) values for Xin'anzhai and Tongtiange plutons are in the range of − 8.5 to − 8.8 and − 10.6 to − 11.4, respectively, similar to those of the Ailaoshan metamorphic basement. The Tongtiange leucogranites are the product of dehydration melting of mica-rich metasedimentary rocks whereas the primary source of the Xin'anzhai monzogranites is probably Proterozoic gneiss with an addition of 35–45% Proterozoic amphibolite. Our geochronological results, together with other published data, indicate the presence of Permo-Triassic magmatism associated with the Indosinian Orogeny along the Ailaoshan suture zone. This zone links with the Jinshajiang suture toward the northwest and the Song Ma–Hainan suture to the southeast. It is herein proposed that latest Permian convergent margin magmatism represented by the Xin'anzhai granitoid pluton (~ 252 Ma) terminated through the accretion of the Simao-Indochina to the South China Blocks, which marks the commencement of the Indosinian Orogeny resulting in the generation of the ~ 247 Ma Tongtiange S-type leucogranite.PostprintPeer reviewe

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

Felsic crust development in the Kaapvaal Craton, South Africa: A reference sample collection to investigate a billion years of geological history

The crust of the Kaapvaal craton accreted throughout the Archaean over nearly 1 billion years. It provides a unique example of the various geological processes that shape Earth's continental crust, and is illustrated by a reference collection of granitoids and mafic rocks (SWASA collection). This sample collection is fully characterised in term of age, major and trace elements, and documents the following multistage history of the craton. In the Barberton area, the initial stages of accretion (stage B·I, > 3.33 Ga and B.II, 3.28—3.21 Ga) correspond to the formation of a sodic (TTG) crust extracted from a near-chondritic reservoir. Stage B.III (ca. 3.1 Ga) corresponds to reworking of this crust, either through intracrustal melting, or via recycling of some material into the mantle and melting of this enriched mantle. Stage B.IV (2.85—2.7 Ga) corresponds to the emplacement of small, discrete plutons involving limited intracrustal reworking. The Northern Kaapvaal craton corresponds to a mobile belt flanking the Barberton cratonic core to the North. Stage NK·I (> 3.1 Ga) resembles stages B·I and B.II: formation of a TTG crust from a chondritic reservoir. In contrast, stage NK.II. (2.97–2.88 Ga) witnesses probable rifting of a cratonic fragment and formation of greenstone basins as well as a new generation of TTGs with both the mafic and felsic magmatism extracted from an isotopically depleted mantle (super-chondritic) reservoir. Intra-crustal reworking dominates stage NK.III (2.88–2.71 Ga), whereas sanukitoids and related granites, involving a mantle contaminated by recycled crustal material, are common during stage NK.IV (ca. 2.67 Ga)

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

Terminal suturing of Gondwana along the southern margin of South China Craton : evidence from detrital zircon U-Pb ages and Hf isotopes in Cambrian and Ordovician strata, Hainan Island

This work was supported by the National Natural Science Foundation of China (grants 41472086 and 41272120), “111” Project (B08030), the fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG2012019240 and CUG2013019137). The first author also acknowledges China Scholarship Council (grant 201208420001) for supporting his research in the University of St. Andrews. Date of Acceptance: 20/11/2014Hainan Island, located near the southern end of mainland South China, consists of the Qiongzhong Block to the north and the Sanya Block to the south. In the Cambrian, these blocks were separated by an intervening ocean. U-Pb ages and Hf isotope compositions of detrital zircons from the Cambrian succession in the Sanya Block suggest that the unit contains detritus derived from late Paleoproterozoic and Mesoproterozoic units along the western margin of the West Australia Craton (e.g., Northampton Complex) or the Albany-Fraser-Wilkes orogen, which separates the West Australia and Mawson cratons. Thus, in the Cambrian the Sanya Block was not part of the South China Craton but rather part of the West Australian Craton and its environs. In contrast, overlying Late Ordovician strata display evidence for input of detritus from the Qiongzhong Block, which constituted part of the southeastern convergent plate margin of the South China Craton in the early Paleozoic. The evolving provenance record of the Cambrian and Ordovician strata suggests that the juxtaposition of South China and West Australian cratons occurred during the early to mid-Ordovician. The event was linked with the northern continuation of Kuungan Orogeny, with South China providing a record of final assembly of Gondwana.Publisher PDFPeer reviewe

Late paleozoic to early mesozoic provenance record of paleo-pacific subduction beneath South China

Northeast trending Yong'an Basin, southeast South China Craton, preserves a Permian-Jurassic, marine to continental, siliciclastic-dominated, retroarc foreland basin succession. Modal and detrital zircon data, along with published paleocurrent data, sedimentary facies, and euhedral to subhedral detrital zircon shapes, indicate derivation from multicomponent, nearby sources with input from both the interior of the craton to the northwest and from an inferred arc accretionary complex to the southeast. The detrital zircon U-Pb age spectra range in age from Archean to early Mesozoic, with major age groups at 2000-1700-Ma, 1200-900-Ma, 400-340-Ma, and 300-240-Ma. In addition, Early Jurassic strata include zircon detritus with ages of 200-170-Ma. Regional geological relations suggest that Precambrian and Early Paleozoic detritus was derived from the inland Wuyi Mountain region and Yunkai Massif of the South China Craton. Sources for Middle Paleozoic to early Mesozoic detrital zircons include input from beyond the currently exposed China mainland. Paleogeographic reconstruction in East Asia suggests derivation from an active convergent plate margin along the southeastern rim of the craton that incorporated part of Southwest Japan and is related to the subduction of the Paleo-Pacific Ocean. Integration of the geologic and provenance records of the Yong'an Basin with the time equivalent Yongjiang and Shiwandashan basins that lie to the southwest and south, respectively, provides an integrated record of the subduction of the Paleo-Pacific Ocean along the southeast margin of the South China Craton and termination of subduction of the Paleo-Tethys beneath its southwest margin in Permo-Triassic. Key Points Yong'an Basin, southeast China was a retroarc foreland basin in Permian-Jurassic Accretionary orogenic belt developed along SE margin of SCC in Permian-Jurassic Both Paleo-Tethys and Paleo-Pacific affect the Permian-Triassic event in the SCCPublisher PDFPeer reviewe

Early tectonic dewatering and brecciation on the overturned sequence at Marble Bar, Pilbara Craton, Western Australia: dome-related or not?

Cataclastic breccias and hydrothermal fault arrays of likely c. 3400 Ma timing are well developed and exceptionally well exposed in the Marble Bar Chert Member of the Pilbara Craton. Brecciation involved centimetre- to metre-scale clast transport distances, in breccia zones up to 5 m wide, cutting the c. 60 m thick chert in a series of right-lateral fault zones. Our observations of downward facing pillow basalts, the geometry of the breccias, and oxygen isotope data for rocks and the breccia matrix suggest the rocks were at least steeply overturned on this flank of the Mt Edgar Dome prior to brecciation. The breccias are inferred to represent steep conjugate fault zones developed by local transtension. The history of overturning and brecciation predates the formation of dome-related regional foliation and metamorphism, and therefore occurred between 3460 and 3320 Ma, the established ages for deposition of the underlying Duffer Formation and intrusion of the Mt Edgar Batholith respectively. Local overturning of the Marble Bar sequence prior to both brecciation. and the main phase of dome formation suggests a protracted deformation history for this segment of the Pilbara Craton. The transtensional movement along the breccias may be representative of strain accommodation accompanying an early doming phase, or could be a deformation event that developed independently of doming. Fluids involved in brecciation were most likely formation waters expelled from the cherts and basalts in response to overpressuring induced by the overturning and progressive burial

Not all supercontinents are created equal: Gondwana-Rodinia case study

The geologic records associated with the formation of the supercontinents Rodinia and Gondwana have markedly different seawater Sr and zircon Hf isotopic signatures. Rodinia-related (Grenville-Sveconorwegian-Sunsas) orogens display significantly less enriched crustal signatures than Gondwana-related (Pan-African) orogens. Seawater Sr isotope ratios also exhibit a more pronounced crustal signal during the span of the Gondwana supercontinent than at the time of Rodinia. Such isotopic differences are attributed to the age and nature of the continental margins involved in the collisional assembly, and specifically to the depleted mantle model ages, and hence the isotope ratios of the material weathered into the oceans. In our preferred model the isotopic signatures of Rodinia-suturing orogens reflect the closure of ocean basins with dual subduction zones verging in opposite directions, analogous to the modern Pacific basin. This would have resulted in the juxtaposition of juvenile continental and island arc terrains on both margins of the colliding plates, thus further reworking juvenile crust. Conversely, the assembly of Gondwana was accomplished primarily via a number of single-sided subduction zones that involved greater reworking of ancient cratonic lithologies within the collisional sutures. The proposed geodynamic models of the assembly of Rodinia and Gondwana provide a connection between the geodynamic configuration of supercontinent assembly and its resulting isotopic signature