Forty years of TTG research Moyen, Jean-François; Martin, Hervé
Lithos,
09/2012, Volume:
148
Journal Article
Peer reviewed
TTGs (tonalite–trondhjemite–granodiorite) are one of the archetypical lithologies of Archaean cratons. Since their original description in the 1970s, they have been the subject of many studies and ...discussions relating to Archaean geology. In this paper, we review the ideas, concepts and arguments brought forward in these 40years, and try to address some open questions — both old and new.
The late 1960s and the 1970s mark the appearance of “grey gneisses” (TTG) in the scientific literature. During this period, most work was focused on the identification and description of this suite, and the recognition that it is a typical Archaean lithology. TTGs were already recognised as generated by melting of mafic rocks. This was corroborated during the next decade, when detailed geochemical TTG studies allowed us to constrain their petrogenesis (melting of garnet-bearing metamafic rocks), and to conclude that they must have been generated by Archaean geodynamic processes distinct from their modern counterparts. However, the geodynamic debate raged for the following 30years, as many distinct tectonic scenarios can be imagined, all resulting in the melting of mafic rocks in the garnet stability field. The 1990s were dominated by experimental petrology work. A wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to TTG liquids; whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate. From 1990s onwards, one of the key questions became the comparison with modern adakites. As originally defined these arc lavas are reasonably close equivalents to Archaean TTGs.
Pending issues largely revolve around definitions, as the name TTG has now been applied to most Archaean plutonic rocks, whether sodic or potassic, irrespective of their HREE contents. This leads to a large range of petrogenetic and tectonic scenarios; a fair number of which may well have operated concurrently, but are applicable only to some of the rocks lumped together in the ever-broadening TTG “bin”.
Combined whole-rock and zircon MC-ICP-MS Lu–Hf isotope data are reported for a large collection of Archean granitoids belonging to typical tonalite–trondhjemite–granodiorite (TTG) suites. Our data ...demonstrate that the time-integrated Lu/Hf of the mantle source of TTGs has not significantly changed over the last 4Gy. Continents therefore most likely grew from nearly primordial unfractionated material extracted from the deep mantle via rising plumes that left a depleted melt residue in the upper mantle. The deep mantle could retain its primitive relative element abundances over time because sinking plates are largely stripped barren of their oceanic and continental crust components at subduction zones; this process results in only small proportions (<15–25%) of present-day continental mass getting recycled to great depths. Zircon populations extracted from the analyzed TTGs have Hf isotopic compositions broadly consistent with those of their host whole-rocks, whereas the U–Pb system in the same grains is often disturbed, causing a discrepancy that creates spurious initial εHf values. This problem is endemic to the Archean detrital zircon record and consistent with experimental results bearing on the relative retentivity of Hf vs. U and Pb in zircon. We argue that this behavior biases the Archean zircon record toward negative εHf values, which are at odds with the present TTG data set. If Hadean Jack Hills zircons are considered in light of these results, the mantle source of continents has remained unchanged for the last 4.3Gy.
► Zircons and their host whole-rocks have consistent Hf isotope compositions. ► The U–Pb system in old zircons is often disturbed, leading to spurious initial εHf. ► Time-integrated source Lu/Hf for TTGs and zircons has been chondritic for 4Gy. ► Continental crust derives from the deep rather than the depleted upper mantle. ► Oceanic plateau recycling was the major crustal growth process in the past.
Both geochemical and experimental petrological research indicate that Archaean continental crust was generated by partial melting of an Archaean tholeiite transformed into a garnet-bearing ...amphibolite or eclogite. The geodynamic context of tholeiite melting is the subject of controversy. It is assumed to be either (1) subduction (melting of a hot subducting slab), or (2) hot spot (melting of underplated basalts). These hypotheses are considered in the light of modern adakite genesis. Adakites are intermediate to felsic volcanic rocks, andesitic to rhyolitic in composition (basaltic members are lacking). They have trondhjemitic affinities (high-Na
2O contents and K
2O/Na
2O∼0.5) and their Mg no. (0.5), Ni (20–40 ppm) and Cr (30–50 ppm) contents are higher than in typical calc-alkaline magmas. Sr contents are high (>300 ppm, until 2000 ppm) and REE show strongly fractionated patterns with very low heavy REE (HREE) contents (Yb≤1.8 ppm, Y≤18 ppm). Consequently, high Sr/Y and La/Yb ratios are typical and discriminating features of adakitic magmas, indicative of melting of a mafic source where garnet and/or hornblende are residual phases. Adakitic magmas are only found in subduction zone environments, exclusively where the subduction and/or the subducted slab are young (<20 Ma). This situation is well-exemplified in Southern Chile where the Chile ridge is subducted and where the adakitic character of the lavas correlates well with the young age of the subducting oceanic lithosphere. In typical subduction zones, the subducted lithosphere is older than 20 Ma, it is cool and the geothermal gradient along the Benioff plane is low such that the oceanic crust dehydrates before it reaches the solidus temperature of hydrated tholeiite. Consequently, the basaltic slab cannot melt. The released large ion lithophile element (LILE)-rich fluids rise up into the mantle wedge, inducing both its metasomatism and partial melting. Afterwards, the residue is made up of olivine+clinopyroxene+orthopyroxene, such that the partial melts are HREE-rich (low La/Yb and Sr/Y). Contrarily, when a young (<20 Ma) and hot oceanic lithosphere is subducted, the geothermal gradient along the Benioff plane is high, so the temperature of hydrated tholeiite solidus is reached before dehydration occurs. Under these conditions, garnet and/or hornblende are the main residual phases giving rise to HREE-depleted magmas (high La/Yb). The lack of residual plagioclase accounts for the Sr enrichment (high Sr/Y) of the magma. Experimental petrologic data show that the liquids produced by melting of tholeiite in subduction-like
P–
T conditions are adakitic in composition. However, natural adakites systematically have higher Mg no., Ni and Cr contents, which are interpreted as reflecting interactions between the ascending adakitic magma generated in the subducted slab and the overlying mantle wedge. This interpretation has been recently corroborated by studies on ultramafic enclaves in Batan lavas where olivine crystals contain glass inclusions with adakitic compositions Schiano, P., Clochiatti, R., Shimizu, N., Maury, R., Jochum, K.P., Hofmann, A.W., 1995. Hydrous, silica-rich melts in the sub-arc mantle and their relationships with erupted arc lavas. Nature 377 595–600.. This is interpreted as demonstrating that adakitic magmas passed through the mantle wedge and interacted with it. Sajona Sajona, F.G., 1995. Fusion de la croûte océanique en contexte de subduction collision: géochimie, géochronologie et pétrologie du magmatisme plioquaternaire de Mindanao (Philippines). Unpublished thesis, Brest University, France, 223 pp. also considers that the high-Nb basalts, which are associated with adakites, reflect mantle–adakite interactions. Recent structural studies have demonstrated that plate tectonics operated during the first half of Earth history. The very strong similarities that exist between modern adakites and Archaean tonalite, trondhjemite and granodiorite (TTG) attest that both have the same source and petrogenesis. Consequently, when Archaean-like
P–
T conditions are exceptionally realised in modern subduction zones, Archaean-like magmas are generated. Contrarily, hot spots never produce TTG-like magmas, thus, strongly supporting the hypothesis of the generation of the Archaean continental crust within a subduction environment. However, Archaean TTG are poorer in Mg, Ni and Cr than adakites, indicating that mantle–magma interactions were less efficient, probably due to the shallower depth of slab melting. In this case, the slab-derived magmas rise through a thinner mantle wedge, thus, reducing the efficiency of the interactions. This is corroborated by the absence of a positive Sr anomaly in TTG, which indicates that plagioclase could have been a residual phase during their genesis.
Until recently it was assumed that the Archaean continental crust (made of TTGs: tonalites, trondhjemites, and granodiorites) was generated through partial melting of MORB-like basalts in hot ...subduction environments, where the subducted oceanic crust melted at high pressure, leaving a garnet-bearing amphibolitic or eclogitic residue. However, recent geochemical models as well as basalt melting experiments have precluded MORB as a plausible source for TTGs. Rather, geochemical and experimental evidences indicate that formation of TTG required a LILE-enriched source, similar to oceanic plateau basalts. Moreover, subduction is a continuous process, while continental growth is episodic. Several “super-growth events” have been identified at ~4.2, ~3.8, ~3.2, ~2.7, ~1.8, ~1.1, and ~0.5Ga, which is inconsistent with the regular pattern that would be expected from a subduction-driven process. In order to account for this periodicity, it has been proposed that, as subduction proceeds, descending residual slabs accumulate at the 660-km seismic discontinuity. When stored oceanic crust exceeds a certain mass threshold, it rapidly sinks into the mantle as a cold avalanche, which induces the ascent of mantle plumes that in turn produce large amounts of magmas resulting in oceanic plateaus.
However, melting at the base of thick oceanic plateaus does not appear to be a realistic process that can account for TTG genesis. Modern oceanic plateaus contain only small volumes (≤5%) of felsic magmas generally formed by high degrees of fractional crystallization of basaltic magmas. The composition of these felsic magmas drastically differs from that of TTGs. In Iceland, the interaction between a mantle plume and the mid-Atlantic ridge gives rise to an anomalously (Archaean-like) high geothermal gradient resulting in thick basaltic crust able to melt at shallow depth. Even in this favorable context though, the characteristic Archaean TTG trace element signature is not being produced. Consequently, internal recycling of oceanic plateaus does not appear to be a suitable process for the genesis of Archaean continental crust.
A possible alternative to this scenario is the subduction of oceanic plateaus. This hypothesis is supported by a present-day analog. In Ecuador, the Carnegie ridge, which is an oceanic plateau resulting from the Galapagos hot spot activity, is being subducted beneath the South American plate. Not only are the resulting magmas adakitic (TTG-like) in composition, but the volcanic productivity is several times greater than in other parts of the Andean volcanic arc. Above the location where the plateau is subducted, the arc is wide and the quaternary volcanoes numerous (about 80 active edifices). The volcanic productivity of each individual volcano also is more intense than away from the subduction focal point with an average output rate of about 0.4–0.5km3·ka−1 compared with only about 0.05–0.2km3·ka−1 for production rates at volcanoes erupting in the rest of the arc. Consequently, we infer that occasional subduction of oceanic plateaus throughout Earth's history can account for the episodic nature of crustal growth. Additionally, the generation by this mechanism of huge volumes of TTG-like magmas would readily dominate the crustal growth record.
•Geochemistry and experimental petrology preclude MORB as source for Archaean TTGs.•Episodic crustal growth is inconsistent with continuous subduction mechanisms.•TTGs require an enriched basaltic source deriving from an enriched mantle.•Melting of the base of oceanic plateaus fails in generating TTG-like magmas.•Melting subducted oceanic plateau generates TTG magmas that emplace episodically.
•We performed zircon U–Pb dating by LA-ICP-MS for 15 granitoids in the northern Kaapvaal craton.•This terrane comprises gneiss units (3350–2840Ma) and late-stage granites (2780–2690Ma).•Our age data ...support that the Pietersburg block accreted to the Kaapvaal craton ~2950Ma-ago.•Ongoing deformation and crust reworking took place between 2950 and 2780Ma.•Collision with the Central Zone of the Limpopo Belt is linked with granitoid magmatism at 2690Ma.
In order to unravel the late-Archean magmatic evolution of the northern Kaapvaal craton, we performed LA-ICP-MS U–Pb isotopic analyses on zircon grains from 15 granitoid samples of the Pietersburg block, including tonalitic and granodioritic gneisses (TTG) as well as weakly to not deformed high-K monzogranites, granodiorites and diorites.
Our new age data, coupled to already published results, show that the Pietersburg block is schematically made up of two lithologically and chronologically distinct domains, namely (1) gneiss units that cover most of the surface of this terrane and comprising the formerly defined Goudplaats, Hout River, Groot Letaba gneisses as well as the so-called “Duiwelskloof batholith”, where crust formation and reworking took place between 3350 and 2840Ma; and (2) late-stage high-K plutons and batholiths that emplaced at ~2780Ma and ~2690Ma.
In details, the tectono-magmatic evolution of the Pietersburg block can be divided into five successive episodes: (1) The first crust-forming event is related to the emplacement of juvenile TTG magmas in the range 3150–3350Ma. In our dataset, it is only represented by inherited zircon cores from younger granitoids; (2) Two TTG samples from the Goudplaats–Hout River gneiss unit and the Duiwelskloof area yielded emplacement ages of ~2950Ma. This magmatic event is very widespread in the whole Pietersburg block, and we propose that it represents an important period of crustal growth linked with the accretion of the Pietersburg block to the northern edge of the older nucleus (ca. 3600–3100Ma) of the Kaapvaal craton; (3) We obtained an intrusion age of ~2840Ma for two samples of high-K biotite-bearing granites that are intimately associated with, and probably formed by melting of, the ~2950Ma-old TTGs. Thus, this episode represents an important event of crust reworking that is likely associated with a long-lasting deformation event; (4) Further intracrustal melting led to the development of large batholiths of high-K biotite granites (e.g. Turfloop batholith) and their satellite intrusions, the latter being represented by one of our samples dated at ~2780Ma; (5) The intrusion of high-K calc-alkaline granitoids of possible mixed crust-mantle origin (Mashashane, Matlala, Moletsi and Matok plutons) took place at ~2690Ma. They are likely associated with collision between the Pietersburg block and the Central Zone of the Limpopo Belt, together with localized strain along shear zones (e.g. Hout River shear zone) and granulite-facies metamorphism that both occurred in the same time.
•A high-level data model for representing trajectory episodes and context is proposed.•Spatial and temporal extents as well as granularity levels can be easily expressed.•Quantitative and qualitative ...semantic descriptions can be used to annotate episodes.•A framework for trajectory annotation shows how the model can be instantiated.•A spatial annotation method that uses OpenStreetMap data is shown.
We are witnessing an increasing usage of location data by a variety of applications. Consequently, information systems are required to deal with large datasets containing raw data to build high level abstractions. Semantic Web technologies offer powerful representation tools for pervasive applications. The convergence of location-based services and Semantic Web standards allows an easier interlinking and annotation of trajectories. However, due to the wide range of requirements on modeling mobile object trajectories, it is important to define a high-level data model for representing trajectory episodes and contextual elements with multiple levels of granularity and different options to represent spatial and temporal extents, as well as to express quantitative and qualitative semantic descriptions. In this article, we focus on modeling mobile object trajectories in the context of Semantic Web. First, we introduce a new version of the Semantic Trajectory Episodes (STEP) ontology to represent generic spatiotemporal episodes. Then, we present FrameSTEP as a new framework for annotating semantic trajectories based on episodes. As a result, we combine our ontology, which can represent spatiotemporal phenomena at different levels of granularity, with annotation algorithms, which allow to create instances of our model. The proposed spatial annotation algorithm explores the Linked Open Data cloud and OpenStreetMap tags to find relevant types of spatial features in order to describe the environment where the trajectory took place. Our framework can guide the development of future expert systems in trajectory analysis. It enables reasoning about knowledge gathered from large trajectory data and linked datasets in order to create several intelligent services.
This paper aggregates the main basic data acquired along the Chile Triple Junction (CTJ) area (45°–48°S), where an active spreading center is presently subducting beneath the Andean continental ...margin. Updated sea-floor kinematics associated with a comprehensive review of geologic, geochemical, and geophysical data provide new constraints on the geodynamics of this puzzling area. We discuss: (1) the emplacement mode for the Pleistocene Taitao Ridge and the Pliocene Taitao Peninsula ophiolite bodies. (2) The occurrence of these ophiolitic complexes in association with five adakite-like plutonic and volcanic centers of similar ages at the same restricted locations. (3) The inferences from the co-occurrence of these sub-coeval rocks originating from the same subducting oceanic lithosphere evolving through drastically different temperature–pressure (
P
–
T
) path: low-grade greenschist facies overprint and amphibolite-eclogite transition, respectively. (4) The evidences that document ridge-jump events and associated microplate individualization during subduction of the SCR1 and SCR-1 segments: the Chonos and Cabo Elena microplates, respectively. The ridge-jump process associated with the occurrence of several closely spaced transform faults entering subduction is controlling slab fragmentation, ophiolite emplacement, and adakite-like production and location in the CTJ area. Kinematic inconsistencies in the development of the Patagonia slab window document an 11- km westward jump for the SCR-1 spreading segment at ~6.5-to-6.8 Ma. The SCR-1 spreading center is relocated beneath the North Patagonia Icefield (NPI). We argue that the deep-seated difference in the dynamically sustained origin of the high reliefs of the North and South Patagonia Icefield (NPI and SPI) is asthenospheric convection and slab melting, respectively. The Chile Triple Junction area provides the basic constraints to define the basic signatures for spreading-ridge subduction beneath an Andean-type margin.
► High-K calc-alkaline granitoids of the Bulai pluton belong to the sanukitoid series. ► They result from differentiation of coeval mantle-derived monzodiorites. ► Differentiation occurred either as ...crystallization and mixing or dehydration melting. ► Most sanukitoid and high-K calc-alkaline series likely result from similar processes. ► High-K calc-alkaline magmatism contributes to crustal growth and differentiation.
The late-Archaean (∼2.59Ga) Bulai pluton from North-Eastern South Africa mostly consists in granodioritic rocks and subordinate amounts of mafic lithologies, mainly monzodiorites, both belonging to the sanukitoid series. Many lines of evidence point to a genetic link between these two components, such as similar mineralogical assemblages, parallel trace-element patterns and isotopic homogeneity. The nature of this genetic link has been investigated using semi-quantitative geochemical modelling.
Several models reproduce the geochemistry of the Bulai granodiorites, including magma mixing, crystallization from a primary magma and partial melting of mafic rocks. Pure mixing between two magmas that are highly contrasted in terms of geochemistry and physical properties is of doubtful reliability in generating massive volumes of homogeneous hybrid magma. Thus, the models that best fit the data involve either equilibrium or fractional crystallization from a mafic mantle-derived magma, coupled with increasing amounts of mixing with felsic melt in course of differentiation. Alternatively, melts computed by dehydration melting of monzodiorites (incongruent melting of biotite) are also similar in composition to the most differentiated Bulai granodiorites (SiO2>65wt.%). However, formation of the least differentiated granodiorites (SiO2<65wt.%) by such a process requires unrealistic melting conditions. Rather, they would result from entrainment of the (mafic) residual solid after melting.
The widespread differentiated rocks of the sanukitoid suite were probably derived from differentiation of mafic mantle-derived melts (sanukitoids s.s.), either as residual liquids (crystallization and mixing) or partial melts. Thus, the presence of pre-existing differentiated crust is not a requirement to the generation of these granitoids that therefore can be considered as a juvenile addition to the crust. In addition, the composition of the sanukitoid series matches the one of some post-Archaean, high-K calc-alkaline suites of late-orogenic affinity, as well as the average composition of the continental crust. As a result, growth and differentiation of the crust since the end of the Archaean were likely accommodated by similar processes that gave rise to the sanukitoid series and post-Archaean high-K calc-alkaline granitoids.
The large (~700km2) composite Karkonosze (Krkonoše) pluton in the West Sudetes, on the border between Poland and the Czech Republic, consists mainly of porphyritic and equigranular granitoids, but ...contains a range of lithologies from lamprophyre to leucogranite. The absolute age and duration of the plutonism have proved difficult to determine. Previous age measurements by Rb–Sr, Ar–Ar and U–Pb range from ~330 to 290Ma, with more recent results converging to ~320–300Ma. Dating of zircon and monazite from samples of a variety of major and minor lithologies by SIMS U–Th–Pb, several from the geochemical study of Słaby and Martin (2008), has narrowed the possible age range further. U–Pb ages measured on eight of ten zircon and monazite samples are in the range ~314–311Ma. Zircon ages measured on the two major types of porphyritic granitoid are 313±3 and 311±4Ma, and monazite ages are 312±2, 313±3 and 311±3Ma. Monazite from one hybrid granitoid has an age of 314±3Ma, and zircon from another an age of 314±4Ma. Zircon from a composite dyke has an age of 311±6Ma. The monazite U–Pb age of an equigranular granite, at 318±6Ma, is consistent with geological evidence that it is older than the porphyritic granitoids but, because of the relatively large uncertainties, is not conclusive. Zircon from one microgranular enclave is anomalously young, 302±4Ma. Evidence is mounting that the main porphyritic granitoids, hybrid granitoids and composite dykes were emplaced within a short time interval between 314±4 and 311±3Ma. Given the uncertainties, emplacement of these units could have been effectively simultaneous. The larger difference between the ages from the equigranular granite and microgranular enclave, however, indicates that the whole Karkonosze thermal episode possibly lasted as long as 15Ma.
•This is the largest geochronological dataset obtained yet for Karkonosze pluton.•Monazite dating assist with the interpretation of complex zircon data.•Consistent ages demonstrate rapid evolution of magma produced by mixing.•Improved geochronology assists in the understanding of the stage of Variscan orogenesis.
