Trondhjemite And Granodiorite Research Articles

Constraining provenance and age of the siliciclastic rocks from the south-western Bundelkhand Craton, Central India

The lithology of the Bundelkhand craton, central India, includes highly deformed tonalite, trondhjemite and granodiorite (TTG) gneisses (3.55–2.7 Ga), followed by volcano-sedimentary greenstone belts and a suite of undeformed granitoids ranging in age from 2.52 to 2.49 Ga. The granitoids, which are by far the most dominant lithology of the craton, have intruded into the TTG gneiss-greenstone assemblage. In addition to huge granitic bodies, rhyolitic rocks of 2.54 Ga have also been observed in the Bundelkhand craton. In this study, we report the first occurrence of a small isolated outcrop of siliciclastic sedimentary rocks within the Bundelkhand granitoid suites in and around the Panchwara village, in the southwestern part of the craton. These siliciclastic sedimentary rocks are intruded by the youngest granitic phase of the Bundelkhand granitoid suite, dated as 2.49 Ga old. Thus, their age is determined to be older than 2.49 Ga. Petrographic studies suggest that these rocks are arkose in nature and geochemical composition indicates that they were derived from the older gneiss-greenstone successions and the older granitic phase (2.52 Ga) of the Bundelkhand granitoid suite. The REE modelling suggests that the sediment contribution from different sources is: 50% greenstone belt (15% basalt + 35% sedimentary rocks), 35% gneisses and 15% older granitoids. Detrital zircons from these sedimentary rocks reveal two age populations: one group of zircons is clustering around 2.52 Ga and the other group is ranging from 3.0 to 3.3 Ga indicating at least two protoliths for these sediments. Our field, petrographical and geochemical data, coupled with previously studied zircon geochronological data, is best explained by a model involving deposition of sediments derived from TTG gneiss, greenstone belt and also from the older phase of the granitoid suite. It is interesting to note that the basin received sediments from the older granitic phase of 2.52 Ga age and was closed before the emplacement of the youngest granitic phase at 2.49 Ga. This study, thus, provides for the first time, conclusive evidence for the presence of a late Archean sedimentary basin within the Bundelkhand craton. It is proposed that the sediments were deposited penecontemporaneously with the pulses of the granitoid magmatism in the Bundelkhand Craton that took place ~2.5 Ga.

Archean trondhjemitic crust at depth in Yangtze Craton: Evidence from TTG xenolith in mafic dyke and apatite inclusion pressure in zircon

Granitic rocks are the most abundant plutonic rocks in the continental crust, and their Archean counterparts include tonalite, trondhjemite and granodiorite (TTG), which dominate the early Earth’s rock archive. Understanding Archean crustal structure have been impeded not only by the rarely exposed deep Archean crustal sections but also by the lack of universally applicable barometers for granitic rocks. Here, we focus on ~2.9 Ga trondhjemite xenoliths brought from depth by mafic dykes that intrude shallower country rocks with the same age and chemistry as the xenolith, in the Kongling Complex, South China. The mafic dykes were emplaced at or after 2.0 Ga and were hybridized by the xenoliths showing signs of partial melting. Exploring the apatite-in-zircon system, for the first time, we establish maximum range of remnant pressure for the inclusions inside zircon hosts. The results yield systematically higher inclusion pressures for the apatites fully encapsulated in xenolith zircons than in the country rock. Based on this data, we envisage a widespread and at least locally thick trondhjemitic crust in the Yangtze Craton by the Mesoarchean.

No evidence for high-pressure melting of Earth\u2019s crust in the Archean

Much of the present-day volume of Earth’s continental crust had formed by the end of the Archean Eon, 2.5 billion years ago, through the conversion of basaltic (mafic) crust into sodic granite of tonalite, trondhjemite and granodiorite (TTG) composition. Distinctive chemical signatures in a small proportion of these rocks, the so-called high-pressure TTG, are interpreted to indicate partial melting of hydrated crust at pressures above 1.5 GPa (>50 km depth), pressures typically not reached in post-Archean continental crust. These interpretations significantly influence views on early crustal evolution and the onset of plate tectonics. Here we show that high-pressure TTG did not form through melting of crust, but through fractionation of melts derived from metasomatically enriched lithospheric mantle. Although the remaining, and dominant, group of Archean TTG did form through melting of hydrated mafic crust, there is no evidence that this occurred at depths significantly greater than the ~40 km average thickness of modern continental crust.

TTG‐Adakitic‐Like (Tonalitic‐Trondhjemitic) Magmas Resulting From Partial Melting of Metagabbro Under High‐Pressure Condition During Continental Collision in the North Qaidam UHP Terrane, Western China

AbstractThe tectonic regime and melting conditions of adakitic tonalitic‐trondhjemitic leucosomes and tonalite plutons from the North Qaidam terrane are evaluated through petrology, whole‐rock major, trace element and Sr‐Nd isotopic data and zircon U‐Pb data. Zircons from leucosomes and metabasites preserve distinct U‐Pb age populations: early Ordovician inherited zircon cores record a protolith age of ~470 Ma, whereas zircon rims preserve subsequent HP granulite facies metamorphism, anatexis, and magmatism between 437 and 433 Ma. Whole‐rock geochemistry suggests that three types of leucosomes and tonalite plutons have geochemical features similar to adakites. Type 1 leucosomes record high Sr contents and positive Eu anomalies representing an earlier period of plagioclase accumulation. The flat fractionated rare earth element patterns of type 2 leucosomes correspond to residues of HP mafic granulites, and the steep fractionated rare earth element patterns of type 3 leucosomes correspond to garnet‐ and clinopyroxene‐rich residues. Petrological observations with P‐T paths and experimentally constrained melting curves together infer that melting of the metagabbro were triggered by the breakdown of amphibole ± zoisite under high‐pressure conditions. Therefore, tonalite, trondhjemite, and granodiorite (TTG)‐adakitic magmas in the North Qaidam terrane were produced through partial melting of newly emplaced gabbroic rocks with an arc affinity under high‐pressure, granulite facies conditions in a thickened lower crust during continental collision. This model explains the origin of TTG‐adakitic rocks and related continental growth and crustal differentiation genetically associated with continental collision.

Influence of water on granite generation: Modeling and perspective

Water influences the physics and chemistry of magmatic differentiation because it reduces the melting point, decreases melt viscosity, modifies phase equilibria, and controls how latent heat is released. Here, we explore how compositional trends of fractionating magmas can be used to evaluate the amount of water in the crystallizing system. Of interest is the role of water in generating silicic magmas such as granites and rhyolites. Water expands the crystallization window over which silicic melts are stable, such that granites are limited to small residual melt fractions (F < 0.2) under dry conditions but stable over a wider range of residual melt fractions (F < 0.6) at high water contents. Variations of SiO2 versus F are relatively insensitive to oxygen fugacity and can be used as a hygrometer for granitoids, where F is estimated through relative enrichments in incompatible elements, such as Th and K. We also show that generation of granitoids in arc environments follow distinctly different Mg#-SiO2 trends compared with anorogenic or intra-plate granites. We confirm that arc magmas differentiate at higher water contents and possibly higher oxygen fugacities than anorogenic/intra-plate magmas. Finally, Archean TTGs (tonalite, trondhjemite, and granodiorite) show similar Mg#-SiO2 systematics as Phanerozoic arc-related granitoids, suggesting similar petrogenetic physical and chemical conditions.

Paleoproterozoic granitoids of the Losevo terrane, East European Craton: Age, magma source and tectonic implications

The Losevo terrane lies between the Sarmatia and Volgo-Uralia segments of the East European Craton and is considered to be part of the East-Sarmatian Orogen (ESO). Here we report the results from field, petrologic, geochemical, Sm–Nd isotopic and zircon U–Pb geochronological studies on a suite of granitoids from the Losevo terrane (LT). The LT granitoids are divided into four types: (1) migmatite leucosome (from quartz diorite to granodiorite); (2) tonalite, trondhjemite and granodiorite (TTG) with high Si, Na, Sr contents, high Sr/Y, La/Yb ratios and low Mg, K, Y, Yb contents; (3) trondhjemite, granodiorite, granite (TGG) spatially related to the TTG suite, but showing higher K, negative Eu anomalies and flat REE patterns; and (4) high-K calc-alkaline monzogranite and granodiorite of I-type. The early (2115Ma) migmatites occur as lensoid and stromatic layers. The TTG and TGG (2100–2075Ma) constitute about 20% of the LT and form batholiths (150–540km2) and stocks. Small massifs of the I-type granites (2081±12Ma) intrude the migmatites. The migmatites, TTG and TGG contain inherited zircon cores that yield ages between 2130 and 2172Ma. The granitic suite shows positive εNd values ranging from +2.1 to +5.7 for the TTG and TGG and +2.1 and +2.2 for the migmatites, suggesting magma derivation from juvenile Paleoproterozoic sources (model ages=2255–2450Ma). In contrast, the I-type granite shows slightly negative εNd value of −0.5, suggesting that the magma sources involved a mixture of Archean and Paleoproterozoic components. All the granitoids from LT record varying degrees of fractional crystallization with or without crustal contamination. The temperature of parental melts of the granites are estimated to be in the range of 793±33°C to 878±16°C. The migmatites were derived mainly from metaigneous-sedimentary protoliths under P∼5–6kbar; whereas the TTGs were derived from lower crustal metabasites under P∼15kbar. The TGG were derived from mid crustal metagreywackes under P<10kbar, and the I-type granites were mainly derived from ancient lower crustal components involving amphibolites, gneisses and metagreywackes under P⩾12kbar. Our new data suggest that the anatexis and formation of migmatites occurred during the peak collisional event, and the other members of the granitoid suite including the TTGs formed during the post-collisional orogenic collapse of the ESO. The estimated average exhumation rate of the granitic suite during collision to post-collisional collapse is about 235m/Ma.

The diversity and evolution of late-Archean granitoids: Evidence for the onset of “modern-style” plate tectonics between 3.0 and 2.5Ga

The end of the Archean aeon (3.0–2.5 Ga) was a period of fundamental change in many aspects of the geological record. In Archean cratons, this timespan is marked by a considerable diversification in both the nature and petrogenesis of granitoid rocks. In this article, we review the nature, petrogenesis and global evolution of late-Archean granitoids and discuss their geodynamic significance. Late-Archean granitoids can be classified into four groups: (1) volumetrically-dominant and juvenile tonalites, trondhjemites and granodiorites (TTGs), whose geochemistry is consistent with an origin through partial melting of meta-igneous mafic rocks at various pressures; (2) Mg-, Fe- and K-rich, metaluminous (monzo)diorites and granodiorites, referred to as sanukitoids s.l., which derive primarily from hybridization between mantle peridotite and a component rich in incompatible elements; (3) peraluminous and K-rich biotite- and two-mica granites, formed through melting of older crustal lithologies (TTGs and meta-sediments, respectively); and (4) hybrid high-K granites with mixed characteristics from the first three groups. The chronology of granitoid emplacement in late-Archean times is different from one craton to another but, in general, follows a very specific two-stage sequence: (1) a long period (0.2–0.5 Ga) of TTG emplacement; (2) a shorter period (0.02–0.15 Ga) during which all other granitoid types were generated. We propose that this sequence represents the first global subduction–collision cycle in the Earth's history. Although possibly present in the geological record prior to 3.0 Ga, such mechanisms became progressively prevalent on a planetary scale only between 3.0 and 2.5 Ga, indicating that the late-Archean geodynamic changes resulted from the global initiation of “modern-style” plate tectonics. The Archean–Proterozoic transition thus represents a major change in the mechanisms of the Earth's heat loss: before 3.0–2.5 Ga, it took place by large-scale magmatic differentiation characterized by generation of proto-continents that underwent crustal maturation locally, but without obvious cyclic activity on a planetary scale. After this, heat loss became accommodated by plate tectonics and global Wilson subduction–collision cycles. These changes were the consequence of the Earth's cooling, which in turn controlled a number of different parameters locally (thickness, temperature, volume and rheology of the crust). This explains why the changes took place over a short timespan (~ 0.5 Ga) relative to the Earth's history, but at different times and with different characteristics from one craton to another.

The end of continental growth by TTG magmatism

Terra Nova, 25, 130–136, 2013AbstractHigh‐Al2O3 tonalite, trondhjemite and granodiorite (TTG) magmas characterise felsic Archaean crust, yet are uncommon in the post‐Archaean rock record. Consequently, understanding the petrogenesis of these rocks provides valuable insights into early Earth processes. Fluid‐absent slab melting represents the dominant hypothesis for the origin of these rocks; however, the absence of voluminous magmas of intermediate composition formed concurrently with these TTGs is incompatible with expectations of slab water loss prior to slab melting. This study demonstrates that for reasonable Archaean mantle temperatures, slab‐derived water is captured by an anatectic zone near the slab surface, which melts via reactions that consume quartz, clinopyroxene and water to produce high‐Al2O3 Archaean trondhjemite. Late in the Archaean, the mantle cooled sufficiently to prevent wet melting of the slab, allowing slab water to migrate into the wedge and produce intermediate composition magmatism, which has since been associated with subduction zones.

The continental record and the generation of continental crust

Continental crust is the archive of Earth history. The spatial and temporal distribution of Earth’s record of rock units and events is heterogeneous; for example, ages of igneous crystallization, metamorphism, continental margins, mineralization, and sea water and atmospheric proxies are distributed about a series of peaks and troughs. This distribution refl ects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. The physiochemical resilience of zircons and their derivation largely from felsic igneous rocks means that they are important indicators of the crustal record. Furthermore, detrital zircons, which sample a range of source rocks, provide a more representative record than direct analysis of grains in igneous rocks. Analysis of detrital zircons suggests that at least ~60%–70% of the present volume of the continental crust had been generated by 3 Ga. Such estimates seek to take account of the extent to which the old crustal material is underrepresented in the sedimentary record , and they imply that there were greater volumes of continental crust in the Archean than might be inferred from the compositions of detrital zircons and sediments. The growth of continental crust was a continuous rather than an episodic process, but there was a marked decrease in the rate of crustal growth at ca. 3 Ga, which may have been linked to the onset of signifi cant crustal recycling, probably through subduction at convergent plate margins. The Hadean and Early Archean continental record is poorly preserved and characterized by a bimodal TTG (tonalites, trondhjemites, and granodiorites) and greenstone association that differs from the younger record that can be more directly related to a plate-tectonic regime. The paucity of this early record has led to competing and equivocal models invoking plate-tectonic– and mantle-plume–dominated processes. The 60%–70% of the present volume of the continental crust estimated to have been present at 3 Ga contrasts markedly with the <10% of crust of that age apparently still preserved and requires on going destruction (recycling) of crust and subconti nental mantle lithosphere back into the mantle through processes such as subduction and delamination.

Geochemistry of the Mesoarchean Fiskenæsset Complex at Majorqap qâva, SW Greenland: Evidence for two different magma compositions

The Mesoarchean Fiskenæsset Complex was emplaced as multiple sills and dykes of magma and crystal mush into oceanic crust, forming an association of ca. 550-meter-thick layered anorthosites, leucogabbros, gabbros, hornblendites, pyroxenites, dunites, and peridotites. The complex was intruded by tonalites, trondhjemites and granodiorites (TTG), and underwent multiple phases of deformation. The Fiskenæsset Complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions.This study presents new field, petrographic, and major and trace element geochemical data for the six major stratigraphic units of the Fiskenæsset Complex at Majorqap qâva, SW Greenland. In addition, new high-precision trace element data are reported for hornblendes from three stratigraphic units. The sampled major lithological units from bottom to top are: Lower Gabbro, Ultramafic, Lower Leucogabbro, Middle Gabbro, Upper Leucogabbro, and Anorthosite. Both whole-rock and hornblende trace element data are consistent with crystallization of the complex from two different parental magmas, originating from different mantle sources. Subsequent differentiation of these parental magmas in the oceanic arc crust resulted in two evolved geochemical rock suites (Suite A and Suite B), which are both characterized by depletions of HFSE (Nb, Ta, Zr and Hf) relative to Th, U, LREE and LILE (Rb, Ba), and enrichments of Sr and Pb relative to MREE; these data are consistent with a magmatic arc geodynamic setting.

Geochemistry of ultramafic rocks and hornblendite veins in the Fiskenæsset layered anorthosite complex, SW Greenland: Evidence for hydrous upper mantle in the Archean

The Fiskenæsset Complex, SW Greenland, contains the world's best preserved Archean (∼2970Ma) layered anorthosite, leucogabbro, gabbro, and ultramafic association. The complex was emplaced into Archean oceanic crust distal from continental lithosphere and later intruded by tonalites, trondhjemites and granodiorites (TTG) constituting Archean continental crust. The complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions.This study presents new whole-rock major and trace element, petrographic, and SEM (Scanning Electron Microscope)-BSE (backscatter electron) image data for a 45–60m-thick ultramafic sill and cross-cutting hornblendite veins. The ultramafic sill is composed of ∼75% olivine–pyroxene hornblendite, ∼15% pyroxene hornblendite, and ∼10% hornblende pyroxenite. Despite granulite facies metamorphism and multiple phases of deformation, the Fiskenæsset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and Alaskan-type ultramafic complexes. Hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism. Formation of orthopyroxene–magnetite symplectitic (vermicular) intergrowths, mainly at the expense of olivine, is attributed to chemical reactions between late stage, residual hydrous melts and olivine.Field, geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion. High MgO (16–31wt.%) contents suggest that the sill was emplaced as a crystal mush, rather than as melt only. Petrographic observations and BSE images indicate that olivine, orthopyroxene, and clinopyroxene were the major crystal phases, whereas hornblende represents the main interstitial liquid phase in the crystal mush. The sill exhibits three different REE patterns, corresponding to three cryptic layers. Although the origin of these REE patterns is not fully understood, they may reflect the intrusion of three batches of magma, containing different hornblende-forming melt/crystal ratios. Geochemical data indicate that the cross-cutting hornblendite veins likely originated from evolved, late-stage melts but were not related to the melts of the ultramafic sill.Large negative Nb anomalies (Nb/Nb*=0.04–0.66) suggest that the Fiskenæsset hornblendites, hornblende pyroxenites, and hornblendite veins were derived from a hydrous sub-arc mantle peridotite. Phanerozoic hornblendites are typically associated with supra-subduction zone ophiolites and magmatic arcs. Accordingly, it is suggested that water was recycled to the source of the Fiskenæsset hornblendites through subduction of altered oceanic crust. Recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites, but also played an important role in the formation of TTG-dominated Archean continental crust. Therefore, the field, petrographic and geochemical data presented in this study suggest that the origin of the Mesoarchean to Neoarchean terrane in the Fiskenæsset region, SW Greenland, is consistent with subduction zone geodynamic processes, rather than non-uniformitarian, density-driven, vertical, crustal overturn, and diapiric processes.

Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids

We report d 7 Li, Li abundance ((Li)), and other trace elements measured by ion probe in igneous zircons from TTG (tonalite, trondhjemite, and granodiorite) and sanukitoid plutons from the Superior Province (Canada) in order to characterize Li in zircons from typical Archean continental crust. These data are compared with detrital zircons from the Jack Hills (Western Australia) with U-Pb ages greater than 3.9 Ga for which parent rock type is not known. Most of the TTG and sanukitoid zircon domains preserve typical igneous REE patterns and CL zoning. (Li) ranges from 0.5 to 79 ppm, typical of (Li) in continental zircons. Atomic ratios of (Y ? REE)/(Li ? P) average 1.0 ± 0.7 (2SD) for zircons with magmatic composition preserved, supporting the hypothesis that Li is interstitial and charge compensates substitution of trivalent cations. This substitution results in a relatively slow rate of Li diffusion. The d 7 Li and trace element data constrain the genesis of TTGs and sanukitoids. (Li) in zircons from granitoids is significantly higher than from zircons in primitive magmas in oceanic crust. TTG zircons have d 7 Li (3 ± 8%) and d 18 O in the range of primitive mantle- derived magmas. Sanukitoid zircons have average d 7 Li (7 ± 8%) and d 18 O higher than those of TTGs supporting genesis by melting of fluid-metasomatized mantle wedge. The Li systematics in sanukitoid and TTG zircons indicate that high (Li) in pre-3.9-Ga Jack Hills detrital zircons is a primary igneous composition and suggests the growth in proto-continental crust in magmas similar to Archean granitoids.

Plate tectonics began in Neoproterozoic time, and plumes from deep mantle have never operated

Archean, Paleoproterozoic, and Mesoproterozoic rocks, assemblages, and structures differ greatly both from each other and from modern ones, and lack evidence for subduction and seafloor spreading such as is widespread in Phanerozoic terrains. Most specialists nevertheless apply non-actualistic plate-tectonic explanations to the ancient terrains and do not consider alternatives. This report evaluates popular concepts with multidisciplinary information, and proposes options. The key is fractionation by ca. 4.45 Ga of the hot young Earth into core, severely depleted mantle, and thick mafic protocrust, followed by still-continuing re-enrichment of upper mantle from the top. This is opposite to the popular assumption that silicate Earth is still slowly and unidirectionally fractionating. The protocrust contained most material from which all subsequent crust was derived, either directly, or indirectly after downward recycling. Tonalite, trondhjemite, and granodiorite (TTG), dominant components of Archean crust, were derived mostly by partial melting of protocrust. Dense restitic protocrust delaminated and sank into hot, weak dunite mantle, which, displaced upward, enabled further partial melting of protocrust. Sinkers enriched the upper mantle, in part maintaining coherence as distinct dense rocks, and in part yielding melts that metasomatized depleted-mantle dunite to more pyroxenic and garnetiferous rocks. Not until ca. 3.6 Ga was TTG crust cool enough to allow mafic and ultramafic lavas, from both protocrust and re-enriched mantle, to erupt to the surface, and then to sag as synclinal keels between rising diapiric batholiths; simultaneously upper crust deformed ductily, then brittly, above slowly flowing hot lower TTG crust. Paleoproterozoic and Mesoproterozoic orogens appear to be largely ensialic, developed from very thick basin-filling sedimentary and volcanic rocks on thinned Archean or Paleoproterozoic crust and remaining mafic protocrust, above moderately re-enriched mantle. Subduction, and perhaps the continent/ocean lithospheric dichotomy, began ca. 850 Ma – although fully modern plate-tectonic processes began only in Ordovician time – and continued to enrich the cooling mantle in excess of partial melts that contributed to new crust. “Plumes” from deep mantle do not operate in the modern Earth and did not operate in Precambrian time.

New age (ca. 2970 Ma), mantle source composition and geodynamic constraints on the Archean Fiskenæsset anorthosite complex, SW Greenland

The Archean Fiskenæsset Complex, SW Greenland, consists of an association of ca. 550-meter-thick layered anorthosite, leucogabbro, gabbro, and ultramafic rocks (peridotite, pyroxenite, dunite, hornblendite). The complex was intruded by tonalite, trondhjemite, and granodiorite (TTG) sheets (now orthogneisses) during thrusting that was followed by several phases of isoclinal folding. The trace element systematics of the Fiskenæsset Complex and associated volcanic rocks are consistent with a supra-subduction zone geodynamic setting. The Fiskenæsset anorthosites, leucogabbros, gabbros and ultramafic rocks collectively yield an Sm–Nd errorchron age of 2973 ± 28 Ma (MSWD = 33), with an average initial ε Nd = + 3.3 ± 0.7, consistent with a long-term depleted mantle source. Regression of Pb isotope data define an age of 2945 ± 36 Ma (MSWD = 44); and the regression line intersects the average growth curve at 3036 Ma. Slightly lower Pb–Pb errorchron age is interpreted as reflecting partial disturbance of the U–Pb system in gabbros, leucogabbros and ultramafic rocks during intrusion of TTGs. Complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca. 3200 and 2650 Ma. Zircon ages peak at about 3200, 3100, 3000, 2950, 2820, and 2750 Ma. The 3200–3000 Ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks. 2950 Ma is considered as an approximate intrusion age of sampled TTGs. The 2940–2650 Ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the Fiskenæsset region. On the basis of recently published trace element data, and new Nd and Pb isotope and U–Pb zircon age data, a three-stage geodynamic model is proposed to explain the evolution of the Fiskenæsset Complex. Stage 1 represents the formation of depleted shallow mantle source > 3000 Ma (ε Nd = + 3.3 ± 0.7) for the complex. Stage 2 corresponds to the development of an intra-oceanic island arc between 3000–2950 Ma. Stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950–2940 Ma.

Trace element systematics of the Neoarchean Fiskenæsset anorthosite complex and associated meta-volcanic rocks, SW Greenland: Evidence for a magmatic arc origin

New major and high-precision ICP-MS trace element data on the Neoarchean (ca. 2970 Ma) layered Fiskenæsset Complex and associated volcanic rocks, southern West Greenland, provide new constraints on the petrogenesis and geodynamic setting of the complex. The complex appears to have been emplaced as multiple sills of magma and crystal mush into oceanic crust (tholeiitic basalts and gabbros), forming an association of ca. 550 m thick anorthosite, leucogabbro, gabbro, and peridotite layers. The Fiskenæsset Complex and the associated volcanic rocks were intruded by Neoarchean tonalite, trondhjemite, and granodiorite (TTG) sheets during thrusting that was followed by several phases of isoclinal folding. Despite the intense deformation and amphibolite to granulite facies metamorphism, primary cumulate textures and igneous layering are locally well preserved throughout the complex. The presence of calcic-plagioclase (An 75-95) and igneous amphibole in anorthosites, gabbros and leucogabbros, and hornblendite veins in peridotites suggests a hydrous magma source(s). The major and trace element compositions of tholeiitic basalts (amphibolites) suggest that they are petrogenetically related to the Fiskenæsset Complex by fractional crystallization. The trace element systematics of the least-altered anorthosites, gabbros, leucogabbros, peridotites, tholeiitic basalts, and calc-alkaline high-magnesian andesites (HMA) is collectively consistent with a supra-subduction zone geodynamic setting. On the log-transformed tectonic discrimination diagram, including La/Th, Sm/Th, Yb/Th, and Nb/Th ratios, tholeiitic basalts display a trend projecting from mid-ocean ridge basalt (MORB) field to island arc basalt (IAB) field. This trend is interpreted as reflecting a transition from the Neoarchean depleted upper mantle to a subarc mantle wedge following the initiation of intra-oceanic subduction and arc migration. Collectively, on the basis of field relationships, petrographic features, and geochemical characteristics, the Fiskenæsset Complex is interpreted as a fragment of a Neoarchean oceanic island arc. On the basis of REE patterns, anorthosites are divisible into four major groups: (1) Group 1 displays moderately depleted to slightly enriched LREE and HREE patterns; (2) Group 2 possesses strongly enriched LREE and moderately depleted HREE patterns; (3) Group 3 has strongly enriched LREE and depleted HREE patterns; and (4) Group 4 (garnet anorthosite) exhibits concave-upward REE patterns; that appear to have resulted from contamination of anorthositic magma by basaltic rocks or magma. The different REE patterns in Group 1, 2, and 3 anorthosites are interpreted to reflect various depths of partial melting and variably enriched (in incompatible elements) source compositions. The majority of anorthosites and leucogabbros were derived from depths above the garnet stability field.

Simultaneous generation of Archean crust and subcratonic roots by vertical tectonics

Archean cratons are characterized by granite-greenstone belts, by abundant tonalites, trondhjemites, and granodiorites (TTGs), and by the presence of strong and buoyant subcratonic lithospheric mantle (SCLM). Individually, mechanisms for their formation remain controversial, and together they provide no clear picture of the tectonic style prevalent during the late Hadean and Early Archean. Following earlier observation that the dome-and-keel structure of granite-greenstone terranes resembles salt diapirs, we present numerical calculations that show that in pre-Archean to Middle Archean thermal regimes, mafic to ultramafic volcanics overlying a felsic basement will overturn diapirically in as few as 10 Ma, displacing as much as 60% of the volcanics to the lower crust. This suggests that diapirism may have dominated Hadean and Early Archean crustal tectonics. Furthermore, repeated cycles of bimodal volcanism and crustal recycling by diapirism provide a mechanism for making TTGs, leaving a restite that contributes to the SCLM from above. This diapiric tectonism later declined as the Earth cooled and plate tectonics became the dominant paradigm.

Geochemistry of adakitic quartz diorite in the Yamizo Mountains, central Japan: Implications for Early Cretaceous adakitic magmatism in the inner zone of southwest Japan

Abstract Small‐volume plutons of Early to Late Cretaceous ages are widely distributed in the Yamizo Mountains, central Japan. These plutons consist predominantly of granitoids, classified into hornblende gabbro, quartz diorite, hornblende–biotite granodiorite and coarse‐grained biotite granite. The quartz diorite (52–64 wt% of SiO2) is characterized by a high Sr content (606–769 p.p.m.) associated with a low Y (13–27 p.p.m.) and heavy rare earth element content (Yb content of 1.19–2.13 p.p.m.). On the Sr/Y versus Y diagram, this rock type mainly plots in the adakite and Archean high‐Al tonalite, trondhjemite and granodiorite (TTG) field. Together with its initial Sr isotopic ratios, which range from 0.7038 to 0.7046, these data suggest that quartz diorite originated as slab melts. However, geochemical calculations assuming either eclogite or garnet amphibolite as the source material do not support this suggestion. Instead, the chemical compositions of quartz diorite are better explained by the fractional crystallization of hornblende, plagioclase and biotite from a primitive, basaltic melt in a magma chamber. In this case, the formation of the associated hornblende gabbro can also be explained by the accumulation of hornblende and plagioclase. Adakitic rocks of Early Cretaceous ages have also been reported in the Tamba Belt of the inner zone of southwest Japan, located ca 500 km west of the Yamizo Mountains. These rocks can be correlated to the adakitic rocks in the Yamizo Mountains based on the geology, petrography, geochemistry and radiometric ages. Therefore, we propose the possibility that the Early Cretaceous adakitic rocks in the inner zone of southwest Japan were produced by fractional crystallization from basaltic arc magmas generated by a partial melting of metasomatized wedge mantle peridotite.

Geology, geochronology, and tectonic setting of the Jiapigou gold deposits, southern Jilin Province, China

The Jiapigou area, at the northeastern corner of the North China Craton (NCC), is one of the most important gold-producing areas in China. It was responsible for nearly half of China's gold production in the 1960s and at present has >100 t Au reserves. Although numerous studies, mostly not published in international literature, have been undertaken on these deposits, their genesis, age, and tectonic setting are still controversial, mainly because of the lack of reliable geochronological data. Gold deposits at Jiapigou are hosted by several ductile shear zones within late Archean gneissic tonalite, trondhjemite, and granodiorite (TTG) and amphibolite (2523±6 Ma), which have been retrogressed to greenschist facies. The deposits occur as massive auriferous quartz veins that contain sulfides (<10 vol.%) and less common disseminated- and stockwork-style mineralization. The lodes are characterized by high Au/Ag ratios and relatively higher Cu (up to 1.3%) and Pb (as much as 10.3%) contents than most other gold deposits in the NCC. Although phyllic alteration is most common, potassic and chloritic alterations characterize felsic and mafic host rocks, respectively. Compiled stable isotope and fluid inclusion data show δ 34S, δ 13C, δ 18O and δD values ranging from −0.2‰ to 12.6‰, −4.2‰ to −5.0‰, 5‰ to 12‰, and −70‰ to −94‰, respectively. These data likely indicate consistent sources of sulfur, carbon, and hydrothermal fluids, but do not actually indicate any specific reservoir. Fluid inclusions from the deposits show variable homogenization temperatures between 150 and 350 °C, and are characterized by low salinity (0.7 to 6.5 wt.% NaCl equiv.) and H 2O–CO 2±CH 4, N 2 solutions. The initial lead isotope compositions of the deposits have considerable variations, even within individual deposits, possibly indicating multiple lead sources for the deposits. In the Jiapigou gold orefield, there are no major intrusions, but felsic, alkaline, and mafic dikes are common. These dikes usually occupy the same fault system as the gold lodes, with dikes locally hosting gold mineralization. Zircon SHRIMP U–Pb dates on pre-ore dikes yield ages of ca. 220 Ma, which constrains the maximum age of gold mineralization, whereas 40Ar/ 39Ar dates on hydrothermal sericite give an age of ca. 204 Ma. The timing of major gold mineralization in the Jiapigou area is therefore constrained between 220 and 204 Ma; this suggests a late Triassic gold-forming event. This episode of gold mineralization is coeval with, or slightly later than, the post-orogenic magmatism that followed the final orogenesis of the Inner Mongolian-Daxinganling orogenic belt (IMDOB), suggesting that a post-orogenic tectonic setting is appropriate for the late Triassic gold mineralization. Zircon SHRIMP dating of a granitic batholith adjacent to the Jiapigou region and also hosting gold lodes that have been previously constrained by Ar–Ar geochronology between 144 and 120 Ma yielded an age of 166±2 Ma, indicating that a mid-Jurassic (Yanshanian) gold-forming episode occurred at Jiapigou and in neighboring regions. This Yanshanian gold mineralization was contemporaneous with subduction of the Pacific plate. On the basis of the nature, timing, and tectonic setting, we favor a classification of ‘orogenic gold’ for the Jiapigou deposits.

Geochemical, Sr and Nd isotopic characteristics and tectonic implications for three stages of igneous rock in the Late Yanshanian (Cretaceous) orogeny, SE China

ABSTRACTLate Yanshanian magmatism in SE China includes three stages of thermal event induced by the interaction between the continental margin of Eurasia and the paleo-Pacific plate during the Cretaceous period. Products of syn-orogenic magmatism (130–110 Ma) include high-Al gabbros (HAG), and gneissic tonalite, trondhjemite and granodiorite (TTG), which intruded into the deep basement (18–24 km). Rocks of the post- and an-orogenic magmatism are shallow-level (6–8 km) I-type granitoids (110–99 Ma), and miarolitic A-type granites plus rhyolite-dominate bimodal volcanics (94–81 Ma), respectively. Geochemically, HAG and TTG belong to the medium-K calc-alkaline affinity with high Sr/Y, whereas other granitoids are mainly high-K calc-alkaline to shoshonitic rocks with low Sr/Y. Sr and Nd isotope compositions suggest different sources of HAG and TTG from other rocks. Progressive depletions of Ba, Sr, Eu and P from I- to A-type granites reflect partial melting of felsic granulites from hydrous to dry conditions, whereas high Sr/Y in HAG and TTG are compatible with dehydration melting of amphibolites. Tectonic models which accommodate HAG and TTG may involve thickening of the lithosphere to convert the pre-existing lower-crust basic rocks into amphibolites. It was followed by basaltic underplating which is attributed to delamination of the thickened lithosphere and led to triggering of crust melting under exten

Middle Archean continent formation by crustal delamination

The processes that created the first large cratonic areas such as the Pilbara and the Kaapvaal remain poorly understood. Models based on the uniformitarian extrapolation of present-day arc volcanic processes to a hotter early Earth have not adequately explained the observations in these terranes. Here we propose an alternative mechanism for the formation of the earliest continental crust. The formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau–like protocrust. Such delamination results in uplift, extension, and the production of tonalite, trondhjemite, and granodiorite (TTG) suites as recorded in Middle Archean cratons. The available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction-based hypotheses.