The nature of the continental crust of North-Central Africa has been a subject of debate for decades. Assessments of the geology, geochronology, and isotope geochemistry of igneous and metamorphic basement rocks have led to two broad-based models that offer very different views on the crustal evolution of the region. The Saharan Metacraton model suggests that the continental crust between the Tuareg Shield, Arabian-Nubian Shield, and the Central African Orogenic Belt was a single craton that was collectively reworked during the Late Neoproterozoic. In contrast, the Central Sahara Shield model proposes that there was a Great Central Gondwana Arc that extended similar to 2400 km from the Tibesti Massif to Cameroon. It was along the arc system that the continental crust of North-Central Africa was sutured during the Late Neoproterozoic. The remnant collisional belt structure is identified as an arcuate gravity anomaly within the crust of central Chad and is known as the Chad Lineament. However, new observations from central, eastern, and southern Chad indicate that neither model is correct, but rather that the continental crust is composed of three or more distinct lithotectonic terranes that were sutured together. In southern Chad, Neoproterozoic granitic rocks of the Adamawa-Yad & eacute; domain, Gu & eacute;ra Massif, and Ouadda & iuml; Massif have enriched Nd isotopic values (epsilon(Nd)(t) < 0) and Archean to Mesoproterozoic (3400-1000 Ma) depleted mantle model ages that indicate they were derived from or interacted with ancient cratonic crust that was affected by a Mesoproterozoic tectonothermal event. In central Chad, the Neoproterozoic granitoids of the Poli domain, Mayo Kebbi Massif, and the Late Fitri Inlier are isotopically juvenile (epsilon(Nd)(t) > +1) and have Mesoproterozoic to Neoproterozoic (1600-600 Ma) depleted mantle model ages indicating that this region is a sliver of juvenile continental crust in the middle of the Saharan region. In northern Chad, the Neoproterozoic granitic rocks of the Tibesti Massif represents a third, but poorly defined terrane. The granitic rocks in the Tibesti Massif have enriched Nd isotopic values (epsilon(Nd)(t) < -0.5) and Paleoproterozoic to Mesoproterozoic (2170-1100 Ma) depleted mantle model ages. The amalgamation of the southern and central terranes occurred over a protracted period from the Early Cryogenian to Late Ediacaran with final suturing with the northern terrane during the latest Ediacaran to early Cambrian. The nature and origin of the North-Central Africa continental crust is best viewed from the perspective of Ediacaran 'terrane amalgamation' and that it likely resembles an accretion-collision belt similar to the Central Asian Orogenic Belt.

Zircon U-Pb dating of the rhyolitic Panjal Traps yielded Early Permian 206Pb/238U crystallization ages (287.2 ± 2.5 Ma; 287.1 ± 2.8 Ma; 290.5 ± 3.3 Ma; 287.5 ± 3.3 Ma; 287.0 ± 3.7 Ma). A minor amount of Carboniferous xenocrystic zircons (309.2 ± 2.8 Ma) were identified and are likely representative of the earliest rift-related volcaniclastic rocks of the Panjal Traps (i.e., Agglomeratic slate). The zircons from all rhyolites have enriched εHf(t) isotopic values (εHf(t) = -10.4 to −4.2) indicating that they were derived by partial melting of continental crust. The lone dacite of the study did not yield a definitive crystallization age, but contains abundant xenocrystic zircons with Neoarchean (2.6 − 2.5 Ga), Paleoproterozoic (2.4 − 1.8 Ga), Mesoproterozoic (1.4 − 1.0 Ga), Neoproterozoic (0.9 − 0.6 Ga), and Early Paleozoic (0.58 − 0.43 Ga) groups. The xenocryst groupings match the detrital zircon age populations from the Indus River system, Indus Group sedimentary rocks, and major tectonomagmatic periods within the Archean-Paleoproterozoic Aravalli-Bundelkhand Craton. Furthermore, the xenocrystic zircons show secular variability in their Hf isotopic values (εHf(t) = -18.9 to + 10.2). The isotopic variability indicates that isotopically juvenile magmas were injected into the Aravalli-Bundelkhand Craton during the Paleoproterozoic (1900 − 1700 Ma) suggesting that there was an isotopic re-fertilization event.

The Meguma Terrane of the Northern Appalachians is primarily composed of Cambro-Ordovician metapelites and metapsammites that were intruded by Late Devonian (380-360 Ma) granitic plutons. The Port Mouton Pluton (373 +/- 1 Ma) is composed of tonalite and monzogranite, contains silicic microgranular enclaves and is cross-cut by lamprophyric dykes. The silicic enclaves are peraluminous, magnesian and calc-alkalic to calcic. The enclaves can be divided into two groups. The Group 1 enclaves are temporally, compositionally and isotopically (epsilon(373 )= -1.2 to +1.5) similar to the host Port Mouton Pluton (PMP). The Group 2 enclaves are Si-rich (SiO2 = 74.3-79.7 wt%) and Al-poor (Al2O3 = 10.0-12.2 wt%). The youngest zircons are of Early Cambrian age (529 +/- 6 Ma) and the rocks are isotopically different (epsilon(530 )= -6.7 to -6.4). MELTS modelling (P = 3 kbar; & fnof;O-2 = NNO = +1; H2O = 2.00 wt%) shows that fractional crystallization can generate the range of PMP rock compositions. The parental magma of the PMP was probably derived by partial melting of sub-Meguma Terrane rocks in a lower crustal hot zone over a 20-30 myr time period. The Group 2 enclaves are xenoliths derived by melting of metasedimentary upper crustal rocks of Avalonia.

Venus is a telluric planet with similar size, composition, and mass to that of the Earth. The Venusian crust is mainly divided into lowland (similar to 80%) and highland regions (similar to 10%) based on their surface elevation. The lowland regions are characterized by featureless lava plains, whereas the highlands consist of crustal plateaux, tesserae terrane, and volcanic troughs. The presence of evolved silicic igneous rocks in the highland regions of the Venus has been debated. In this study, phase equilibria modelling using THERMOCALC and the basaltic compositions obtained from the Venera 14 and Vega 2 lander missions are employed to estimate partial melt compositions in both hydrous and anhydrous conditions. Hydrous partial melting of the Venera 14 composition generated tonalitic-trondhjemite-granodiorite (TTG) melts at shallow crustal depths with 5% partial melting. The Vega 2 composition could also generate TTG-like melts in hydrous conditions, but at a slightly higher-pressure (similar to 5 kbar). However, anhydrous partial melting modelling results were unable to generate a TTG-like melts. The results of THERMOCALC modelling indicate that TTG-like melts can be generated in the crust from the basaltic compositions of Venera 14 and Vega 2 by hydrous partial melting. The implication is that the highland regions of Venus may be an ideal location to search for silicic rocks that are typical of terrestrial Archean crust.

The Panjal, Rajmahal–Sylhet, and Deccan Traps in India constitute voluminous flood basalt provinces emplaced under distinct tectono-magmatic regimes. The formation of Panjal Traps (∼289 Ma) is attributed to extensional tectonic processes—specifically lithospheric thinning and decompression melting of the lithospheric mantle domains associated with rifting of the Cimmerian terranes and opening of the Neo-Tethys Ocean, whereas the Rajmahal–Sylhet (∼117 Ma) and Deccan (∼66 Ma) Traps are considered to represent mantle plume-related large igneous provinces (LIPs) associated with the Kerguelen and Réunion hotspots, respectively. Using the most primitive basalt compositions, we constrained mantle potential temperatures (Tp), source lithologies, and melting depths of these spatially and temporally distinct provinces. The Panjal basalts yield Tp of 1379–1470 °C, with onset of melting at ∼81 km depth. The Rajmahal–Sylhet and Deccan basalts exhibit elevated Tp (1545–1639 °C and 1519–1556 °C), with melting initiated at depths of ∼177 km (Rajmahal–Sylhet) and ∼ 132 km (Deccan), respectively. Peridotite versus pyroxenite melting models suggest that plume-derived LIPs are best explained by the melting of hydrous peridotite sources. Pyroxenites are likely restricted to lithospheric mantle domains and are not intrinsic to deep mantle plume sources. The rapid increase in root mean square (RMS) plate velocities of India during the post-Pangean period are correlated with the spatio-temporally associated Mesozoic LIP events. This suggests that mantle plumes likely played a key role in accelerating India's motion by enhancing the influence of far-field plate boundary forces, possibly through thermomechanical erosion of the cratonic keel beneath the Indian plate.

The Doba Basin is one of four oil producing basins of southern Chad that developed within the West and Central Africa Rift System during the Late Cretaceous opening of the Central Atlantic Ocean. The Cretaceous basins were built upon older Ediacaran basins that formed after the collision between the Congo-Sao Francisco Craton and the continental crust of North-Central Africa. Oil exploration drill wells in the Doba Basin (Mouroumar-1, Benoy-W2, Kiagor-1, Djabi-1, Bebalem-1) encountered granitic rocks at depths from 2250 m to 3230 m. The granitic rocks were dated by zircon UPb methods and yielded ages of 594 +/- 4.4 Ma (Benoy), 594 +/- 4.2 Ma (Kiagor), 595 +/- 4.3 Ma (Bebalem), and 579 +/- 4.1 Ma (Mouroumar). The older granitoids are magnesian and metaluminous to weakly peraluminous, and compositionally similar to volcanic-arc granites. The younger granites are magnesian to ferroan and classify as within-plate or post-collisional granite. The magmatic zircon Hf isotopes (epsilon(Hf)(t) = -12.5 to 0.0) and whole rock Nd isotopes (epsilon(Nd)(t) = -3.3 to -8.2) show that all granitoids were derived from an isotopically enriched source. Crystallization pressure estimates from biotite and titanite indicate that they were emplaced in the upper crust (1-4 kbar). The volcanic-arc granitoids likely had lower parental magma temperatures (similar to 800 degrees C) than the post-collisional granitoid (similar to 900 degrees C). The results from this study along with previously published results demonstrate that contemporaneous (620-590 Ma) volcanic-arc magmatism stretched 1000-1500 km from the Doba Basin through the Gu & eacute;ra Massif to the Ouadda & iuml; Massif and was followed by post-collisional magmatism at <= 580 Ma. The implication is that the southeastern and eastern portions of the Saharan Metacraton were not structurally contiguous with the western and northwestern portions until after similar to 590 Ma. Consequently, the continental crust of North-Central Africa is not a coherent craton, but rather it is a composite shield terrane similar to the Arabian Nubian Shield or Central Asian Orogenic Belt.

The Galápagos archipelago, a chain of islands formed by hotspot volcanism on the Nazca tectonic plate, exhibits a pronounced rock age gradient with distance from the volcanic hotspot from west to east. Here, we investigate chemical weathering along a soil chronosequence (1.5 to 1070 ka) under humid conditions. Our results show considerable loss of base cations already in the early to intermediate phases of weathering (e.g. 95% of Na and 78% of Mg lost from the topsoil after 26 ka) and almost complete loss from the entire profile in soils older than 800 ka. Depletion of Si was less pronounced, with topsoil losses of 24% and 63-68% after 26 ka and >800 ka, respectively. Total weathering flux and associated CO2 consumption rates estimated from profile-scale element losses in this study exceeded catchment-scale estimates reported for other volcanic islands or global averages during the early weathering phase, but were much lower in the intermediate and late phases. Nevertheless, total C drawdown was dominated by soil organic C sequestration (70-90% share) rather than inorganic, weathering-induced CO2 consumption during early pedogenesis (≤4.3 ka), and the relative importance switched in the intermediate and late phases (90-95% share of weathering-induced C drawdown at ≥166 ka). Dust deposition derived from a nearby ocean sediment core was 800 ka). Our results suggest that (1) young volcanic surfaces are very efficient (inorganic and organic) C sinks, (2) the development of thick soil covers at advanced pedogenic stages effectively shields the underlying rocks from further weathering, and (3) dust inputs become an increasingly important biogeochemical factor in such highly weathered environments.

Abstract The formation of large igneous provinces (LIPs) has been widely believed to be linked to mantle plume activity. However, how the plume modifies the overlying lithosphere, particularly its compositional structure, remains uncertain. Here, we characterize the deep thermochemical structure beneath the Emeishan LIP (ELIP), which is a well‐known Permian plume‐related LIP in China, by taking a multi‐observable probabilistic inversion. Our results find a clear correlation between the lithospheric composition with the ELIP's concentric zones. We infer that the fertile feature of the lithospheric mantle in the ELIP's inner zone was caused by the plume‐derived fertile magmas which infiltrated into and chemically refertilized the ambient depleted lithosphere. This plume‐modified lithospheric compositional structure is likely to be preserved after the plume event, while the present lithospheric thermal structure has been mainly influenced by the subsequent thermal‐tectonic activity. Our results improve our understanding of the physicochemical interactions between the lithosphere and ancient plume.

The Silhouette alkaline volcano-plutonic complex is the largest exposed Early Paleogene (62−64 Ma) igneous complex of the Seychelles microcontinent. The rocks of this study were collected from Baie Cipailles, Pte. Vareur, and Pte. Ramasse Tout and include syenite (SiO 2 = 60−63 wt%), microgranite (SiO 2 = 70−74 wt%), tuffaceous trachyte (SiO 2 = 64−65 wt%), and a basaltic xenolith (SiO 2 = 44 wt%; MgO = 6.4 wt%; Mg# = 47.6). The silicic rocks of this study are ferroan, metaluminous to weakly peralkaline, and classify as A1-type granitoids. The whole rock Sr-Nd ( 87 Sr/ 86 Sr i = 0.703894−0.706534; ε Nd ( t ) = +0.5−+1.8) isotopes of the silicic rocks are similar to the basalt xenolith ( 87 Sr/ 86 Sr i = 0.703576; ε Nd ( t ) = +1.9). There is limited to no geochemical evidence for crustal contamination in any of the rocks. The syenitic rocks were likely derived by fractional crystallization of an alkaline basaltic parental magma in the upper crust and under reducing conditions (ΔFMQ = -1). The Sr-Nd isotopes and zircon Hf (ε Hf ( t ) = +3.3−+9.1) isotopes of the silicic rocks are indicative of a moderately depleted source. We found distinct textural, mineralogical, and compositional differences between the syenitic rocks suggesting that the Silhouette complex is composed of at least two syenitic magma pulses.

Surface geologic features form a detailed record of Venus’ evolution. Venus displays a profusion of volcanic and tectonics features, including both familiar and exotic forms. One challenge to assessing the role of these features in Venus’ evolution is that there are too few impact craters to permit age dates for specific features or regions. Similarly, without surface water, erosion is limited and cannot be used to evaluate age. These same observations indicate Venus has, on average, a very young surface (150–1000 Ma), with the most recent surface deformation and volcanism largely preserved on the surface except where covered by limited impact ejecta. In contrast, most geologic activity on Mars, the Moon, and Mercury occurred in the 1st billion years. Earth’s geologic processes are almost all a result of plate tectonics. Venus’ lacks such a network of connected, large scale plates, leaving the nature of Venus’ dominant geodynamic process up for debate. In this review article, we describe Venus’ key volcanic and tectonic features, models for their origin, and possible links to evolution. We also present current knowledge of the composition and thickness of the crust, lithospheric thickness, and heat flow given their critical role in shaping surface geology and interior evolution. Given Venus’ hot lithosphere, abundant activity and potential analogues of continents, roll-back subduction, and microplates, it may provide insights into early Earth, prior to the onset of true plate tectonics. We explore similarities and differences between Venus and the Proterozoic or Archean Earth. Finally, we describe the future measurements needed to advance our understanding of volcanism, tectonism, and the evolution of Venus.

We report new petrology, mineral chemistry, P-T conditions, and fluid inclusion data on mafic granulites from the Mettupalayam region along the Bhavani Suture Zone, Southern Granulite Terrane, India. Phase equilibria modelling of mafic granulites yielded peak P-T conditions of 780-860 degrees C and 7.6-10.1 kbar followed by a near isothermal decompression along a clockwise P-T path. The trapped fluid inclusions in the peak metamorphic minerals display a melting temperature range from -57.4 degrees C to -56.6 degrees C, close to the triple-point temperature of pure CO2. The primary inclusions homogenized at -18.9 degrees C to +0.2 degrees C, corresponding to density values of 0.93-1.03 g/cm(3). Homogenization of the secondary inclusions occurred within the range from -6.3 to +18.1 degrees C, corresponding to low CO2 densities of 0.79-0.96 g/cm(3). From the textural characteristics of the high-density primary carbonic fluid inclusions, we interpret these inclusions as the CO2-rich syn-metamorphic fluid present during the high-grade metamorphism. The secondary fluids characterised by lower densities have undergone re-equilibration during the exhumation stage (decompression) from the peak granulite-facies metamorphism along a clockwise P-T trajectory. This interpretation is consistent with the occurrence of hornblende + plagioclase symplectite around the porphyroblastic garnet, suggesting decompression. We infer that the high-density CO2 was the dominant syn-metamorphic fluid components present during the granulite-facies metamorphism in the Mettupalayam region. Such carbonic fluids, possibly derived by degassing from carbonates or mantle sources, probably played a significant role in stabilizing high-grade mineral assemblages along this collisional suture zone.

The Poularies igneous complex is a Neoarchean (2728 Ma) intrusion composed of diorite, quartz diorite, hornblende tonalite, and biotite tonalite. It is was emplaced into the shallow crust of the Abitibi granite-greenstone belt during volcanic cycle 1 (2730-2725 Ma) and is contemporaneous with the eruption of mafic (Stoughton-Roquemaure Group) and silicic volcanic rocks (Hunter Mine Group). The petrogenetic relationship between the silicic rocks of the Poularies igneous complex is not constrained. In this study we test the petrological association between the different rock types of the Poularies complex using fractional crystallization modeling. Hydrous (H2O = 3 wt%) fractional crystallization modeling using a 'primitive' intermediate starting composition demonstrates that all rock types of the Poularies complex can be generated from a common parental magma in the upper crust (1 kbar) under mildly oxidizing conditions (Delta FMQ = 0). Moreover, it is demonstrated that the parental magma of the Poularies complex was likely derived by partial melting of mafic rocks from the Abitibi granite-greenstone belt. We conclude that the Poularies complex is representative of a magma chamber that generated the silicic lavas of the spatially associated Hunter Mine Group in a rifting or tensional plate stress environment. Our model may be applicable to other shallow syn-volcanic plutons of the Abitibi granite-greenstone belt.

The Galápagos archipelago, a chain of islands formed by hotspot-induced volcanism on the Nazca tectonic plate, exhibits pronounced gradients of rock age and climate. Here, we investigate chemical weathering along a soil chronosequence (1.5 to 1070 ka) and under humid vs. dry conditions. Our results show considerable loss of base cations already in the early to intermediate phases of weathering under humid conditions (e.g. 95 % of Na and 78 % of Mg lost from the topsoil after 26 ka) and almost complete loss from the entire profile in soils older than 800 ka. Depletion of Si was less pronounced, with topsoil losses of 24 % and 63–68 % after 26 ka and >800 ka, respectively. Under dry conditions, weathering rates were much lower, e.g. 33 % of Na and 1.4 % of Mg lost from the topsoil after 26 ka. Indices of chemical weathering, e.g. the Chemical Index of Alteration and the Ruxton Ratio, correlated well with indicators of pedogenic development, such as solum thickness, soil pH, or the ratio of oxalate- to dithionite-extractable Fe. Total weathering flux and associated CO2 consumption rates estimated from profile-scale element losses in this study exceeded catchment-scale estimates reported for other volcanic islands or global averages during the early weathering phase, but were much lower in the intermediate and late phases. Nevertheless, total C drawdown was dominated by soil organic C sequestration (70–90 % share) rather than inorganic, weathering-induced CO2 consumption during early pedogenesis (≤4.3 ka), and the relative importance switched in the intermediate and late phases (90–95 % share of weathering-induced C drawdown at ≥ 166 ka). Dust deposition derived from a nearby ocean sediment core was <20 % of total basalt mass loss at the young and intermediate-aged sites, but reached 40–60 % at the older sites (>800 ka). Our results suggest that (1) young volcanic surfaces are very efficient (inorganic and organic) C sinks, (2) the development of thick soil covers at advanced pedogenic stages effectively shields the underlying rocks and decelerates weathering, and (3) dust inputs become an increasingly important biogeochemical factor in such highly weathered environments.

Mantle xenoliths hosted in volcanic rocks from the island of Lutao offer a glimpse into the nature of the mantle beneath the northern Luzon volcanic arc. The xenoliths are spinel-bearing and composed mostly of harzburgite with one lherzolite and one olivine orthopyroxenite. The olivine (Fo(92.5-88.9)), orthopyroxene (Mg# = 94.6-89.2), and clinopyroxene (Wo(49.1-38.1)En(57.0-45.4)Fs(3.0-11.0)) compositions are similar to those of abyssal peridotites. The spinel compositions are variable and can be principally divided into high-Al (Cr# < 45) and low-Al (Cr# > 45) groupings. The whole rock compositions are similar to abyssal peridotite (Al2O3 = 0.95-2.07 wt %; Mg# = 88.5-90.9) and have U-shaped chondrite normalized rare earth element patterns. The Sr-Nd isotopes of the xenoliths are broadly chondritic (Sr-87/Sr-86(i) = 0.704400-0.707908; epsilon(Nd)(t) = 0.0 - +1.5). The two-pyroxene equilibrium temperatures range from 900 to 1200 degrees C with the majority of temperature estimates >1000 degrees C. The olivine-orthopyroxene-spinel oxygen barometry estimates yielded Delta FMQ values from 0 to +2 and correspond to moderately oxidizing to oxidizing conditions. The xenoliths are likely derived from the Philippine Sea Plate lithospheric mantle that was modified by melt extraction and/or fluid enrichment processes. Trace element and isotopic mixing modeling indicate that 1-2% contamination by subducted South China Sea sediment can explain the Sr-Nd isotopic enrichment and Th and U elemental variability within the xenoliths assuming an initial composition similar to enriched depleted mid-ocean ridge mantle (E-DMM). The anomalously high two-pyroxene equilibrium temperatures of the Lutao xenoliths relative to other regions of the northern Luzon volcanic arc (Iraya <1000 degrees C) indicate that they were affected by a high-temperature event that was likely a consequence of recent intra-arc rifting that occurred after collision (<6 Ma) between the Luzon arc and the Eurasian margin.

The Early Permian (c. 290 Ma) Panjal Traps is the most extensive sequence of flood basalts within the Tethyan domains of the Himalaya. The Panjal Traps erupted during a period of tensional plate stress related to the rifting of Cimmerian terranes from the Tethyan margin of Gondwana. The majority of the mafic Panjal Traps were affected by postemplacement low-temperature deuteric alteration and/or regional deformation. Consequently, constraining the magmatic conditions of the rocks is difficult. The least altered Panjal Traps are located within the Guryal Ravine section of the western Zanskar Range and the southern Pir Panjal Range. The rock and mineral textures are preserved and they have a primary mineralogy of clinopyroxene (Wo(30.3-42.5)En(29.4-49.7 )Fs(13.8-34.2)) and plagioclase (An(61.0-43.5)) phenocrysts within an aphanitic matrix. Secondary minerals include chlorite, epidote, actinolite, quartz, albite, orthoclase, rutile, and titanite and indicate that some of the rocks underwent greenschist to sub-greenschist facies metamorphism. Clinopyroxene-liquid geothermobarometers were used to assess the equilibrium crystallization temperature and pressure of the least altered Panjal Traps. The clinopyroxene-liquid saturation conditions yielded temperatures of 1104-1184 degrees C and pressures of 1.8-7.0 kbar which are within the uncertainty of the jadeite-diopside-hedenbergite exchange thermometer (1064-1167 degrees C) and the Al exchange barometer (1.1-6.8 kbar). The equilibrium temperatures are not anomalously high and similar to lavas that erupt at a passive rift setting rather than from a mantle plume. The whole rock V/Sc and V/Ga ratios suggest that the oxygen fugacity of the lavas was likely at or below the fayalite-magnetite-quartz buffer (triangle FMQ triangle FMQ = 0 to-1). Rhyolite-MELTS modeling (triangle FMQ triangle FMQ = -1; P = 2 kbar) indicates the water content within the basalts was variable and probably did not exceed 2.25 wt% prior to eruption. The post-emplacement metamorphic conditions are constrained using Perple_X modeling and demonstrate that the rocks from both Guryal Ravine and southern Pir Panjal Range (P = < 3.0 kbar and T = 390 - 415 degrees C) underwent greenschist facies metamorphism at similar temperature. However, the pressure is not well constrained. We attribute the metamorphism to regional deformation associated with terrane accretion to the Indian plate during the Mesozoic and Cenozoic that occurred after the deposition of the Late Permian to Early Triassic Pangea megasequence but before the Oligocene.

Since the 17th century, the geological sciences have transitioned from a mostly qualitative "order of operations" discipline to a fully quantitative scientific discipline. In order to properly contextualize the origin and evolution of the solid Earth, a firm understanding of physical time and how it can be measured is required. Geological philosophy primarily advanced through rock and mineral observations, experimentation, and the study of the fossil record, but it was the development of analytical geochronology that quantified the age of geological and extraterrestrial materials and constrained the rates of past geological processes. The ability to measure individual isotopes and isotopic ratios by fission track methods and mass spectrometry revealed the vastness of geological time and permitted the robust correlations between rock formations across the globe, the reconstruction of supercontinents, and provided temporal constraints on biological evolution and the longevity of ancient ecosystems. The development of whole rock, single crystal, and in situ methodologies enhanced the ability of geoscientists to push scientific boundaries in the investigation of deep time. The application of geochronological methods can now offer accurate and precise results at various scales that range from crystal growth rates to crustal growth rates. In this chapter, we provide a perspective on the concept of geological time, the development of analytical geochronology, and the application of instruments. It is intended that this chapter acts as the historic foundation to the subsequent chapters that provide greater insight and the scientific basis for methods and applications of analytical geochronology.

The Early Paleogene (63.65 ± 0.52 Ma, 63.11 ± 0.45 Ma) North Island syenitic complex of the Seychelles microcontinent is composed principally of diorite (SiO2 ≈ 57 wt%), syenite (SiO2 = 61–65 wt%), and microsyenite (SiO2 = ∼70 wt%). The rocks are metaluminous, ferroan, and alkalic, and are compositionally similar to the A1-type granitoids. The trace element compositions of the syenitic rocks show minor spatial variability between the eastern (Congoment, Bernica) and western portions (Grand’Anse, Mt. Des Cèdres) of the island. The whole rock Sr-Nd (87Sr/86Sri = 0.704095–0.707533; εNd(t) = +1.2–+1.9) and zircon Hf ( εHf(t) = +2.1–+8.4) isotopes are indicative of a juvenile magma source. The low Th/NbPM (0.3–1.5) and high Nb/U (30.9–109) ratios do not indicate a crustal origin of the rocks nor do they suggest crustal contamination was significant. Hydrous fractional crystallization modeling shows that a mafic alkaline parental magma can yield residual liquid compositions similar to the diorites and syenites under reducing conditions ( ΔFMQ = −1) at a pressure of 0.3 GPa. However, feldspar accumulation likely occurred as some rocks have elevated Eu/Eu* (>1.1) values. The emplacement of the North Island complex is contemporaneous with the eruption of the Deccan Traps and rifting of the Seychelles microcontinent from India. Rifting and magmatism was likely related to the passage of the Indian plate over the Réunion hotspot. The modeling results of the study demonstrate that crystallization pressure has an influence on whether basalt-derived A-type granitoids will evolve to metaluminous or peralkaline compositions.