Fractionation Of Basaltic Magma Research Articles

Geochemical signals of coexisting magma mixing and fractional crystallization processes in the arc setting: Case study of Wulan intrusive suite in the NE Tibet Plateau

To understand the fundamental processes for the generation of granitic plutons and compositional diversity of continental crust, we investigate the Wulan intrusive suite in the North Wulan terrane (NWT) along the NE Tibet Plateau. The intrusive suite consists of three-stage igneous rocks including stage I gabbro and granite emplaced at 258–254 Ma, stage II gabbro, diorite, and tonalite emplaced at 250–245 Ma, and stage III gabbro, granodiorite, and leucogranite emplaced at 242–241 Ma. Geochemical analyses and thermodynamic modeling suggest that gabbros were derived from partial melting of heterogeneous lithospheric mantle domains, and they were involved in the formation of coeval intermediate to granitic pluton/batholith. MMEs with variable compositions hosted in granitic pluton do not always represent the products that were generated by magma mixing between injected mafic melts and host granite. In contrast, some low-Si varieties may be generated by crystal fractionation of injected mafic magmas. The hybrid compositions of diorite also indicate a complex petrogenetic process characterized by fractionation of basaltic magmas followed by replenishment of mantle-derived mafic melts. These lines of evidence suggest that both fractional crystallization and magma mixing may concurrently occur in the construction of hybrid intermediate to granitic rocks. In addition, the injected mafic melts, together with newly formed MMEs, would induce compaction and convection of granitic crystal mush, thereby promoting melt extraction to form highly evolved leucogranites. These complex petrogenetic processes and their different combinations play an important role in the construction of the hybrid continental crust in the arc setting.

Crustal melting vs. fractionation of basaltic magmas: Part 2, Attempting to quantify mantle and crustal contributions in granitoids

Granitic melts may form either directly, by melting of pre-existing crustal rocks, or by fractionation of mafic to intermediate magmas, typically mantle-derived. Each model is applicable to distinct portions of the Earth at different times. Whenever there is an important flux of mafic magmas from the mantle, differentiation of basaltic magmas dominates. In contrast, locations with a lower mafic magma flux, elevated thermal gradients and/or a fertile, thick crust are dominated by crustal melting and reworking. This only partly overlaps with the dichotomy between magmatic arcs and collision zones, as places like e.g. inverted back-arcs also feature large-scale crustal melting, whereas post-collisional domains include a sizeable mantle-derived component. Petrogenesis of individual granitic suites probably cannot be accounted for by pure, end-member models, as these suites typically feature a fair proportion of hybrid or ambiguous granitoids. Thus, we explore various petrogenetic and geological scenarios leading to the formation of hybrid granitoids at various scales. Finally, we outline possibilities to quantify the respective contributions of crust and mantle involved in their formation – hopefully, paving the way for a better understanding and more rigorous discussion of the mechanisms of crustal growth and recycling.

Do arc silicic magmas form by fluid-fluxed melting of older arc crust or fractionation of basaltic magmas?

Do arc silicic magmas form by fluid-fluxed melting of older arc crust or fractionation of basaltic magmas?

An early Devonian intra-plate bimodal volcanic suite in the Kyrgyz North Tianshan belt, the central Asian orogenic belt

A suite of bimodal volcanic rocks in the Kyrgyz North Tianshan belt is composed of totally ∼100-m-thick basalt and basaltic andesite layers of the Karakastek Formation and totally ∼700-m-thick rhyolite layers of the Kastek Formation, with minor amounts of interlayered felsic to intermediate tuff/ignimbrite. Zircon grains from a rhyolite sample have a weighted mean 206Pb/238U age of 416 ± 8 Ma, which is considered to be the crystallization age of the rhyolite. The zircon grains have δ18O ranging from +6.5 to +7.3‰. The calculated zircon saturation temperatures (TZr) range from 850 to 900 °C. Rhyolites have high total alkaline, Zr, Y and Nb contents and high Ga/Al similar to A1-type granitoids formed in an intra-plate setting. Mafic volcanic rocks have SiO2 ranging from 49.4 to 55.3 wt% and FeOt/MgO from 1.4 to 2.4, showing a tholeiitic trend. They have negative Nb and Ta anomalies on the primitive mantle-normalized trace element patterns and have εNd(t) of −1.5 to 0. There is a positive correlation between εNd(t) and ΔεHf(t) of the mafic volcanic rocks, indicative of crustal contamination. However, one least evolved basalt sample has Mg# of 57 with low Zr and Hf concentrations, indicating insignificant crustal contamination. Modeling results indicate that the mafic volcanic rocks may have been derived from a dry, garnet-bearing lherzolite mantle source. They have εHf(t) and εNd(t) varying along the “Island arc array”, which is interpreted as the mantle source was metasomatized by slab-derived melts rather than fluids. In addition, the rhyolites have εNd(t) of −2.4 to −2.9, much lower than the mafic volcanic rocks, and were thus unlikely produced by the fractionation of basaltic magmas. The rhyolites show Nd-Hf isotope decoupling, which is interpreted as a result of the anataxis of Precambrian lower crust associated with the breakdown of garnet. Therefore, the early Devonian bimodal volcanic suite in the Kyrgyz North Tianshan belt was considered to have formed in an intra-plate setting.

Early Jurassic A-Type Granites in Southeast China: Shallow Dehydration Melting of Early Paleozoic Granitoids by Basaltic Magma Intraplating

AbstractA-type granites are widespread in most orogenic belts, and their petrogenesis has played an important role in reconstructing orogenic events. However, their sources, even today, are a subject of considerable controversy. At least four models have been proposed for their formation, namely, melting of a previously melted source, fractionation of basaltic magma, anatexis of crustal igneous sources, and magma mixing. Here we present Early Jurassic A-type granites in southeast (SE) China as an example of a shallow (ca. 15–20-km depth), igneous, crust-derived A-type granite. We present detailed laser ablation ICP-MS zircon U-Pb dating and major- and trace-element and Sr-Nd isotopic data for these granites and assess their origin and relationship to the geodynamics of the late Mesozoic extensive magmatism in SE China. The associated granites include the Zhaibei, Keshubei, Zhulanbu, and Guangze plutons, which were emplaced during the interval 191–188 Ma and occur in a northeast-trending belt, parallel to ...

The fate of mafic and ultramafic intrusions in the continental crust

Geochemical and petrological data indicate that the bulk continental crust results from the fractionation of basaltic magmas followed by the foundering of residual mafic cumulates. Structural and geological evidence for foundering has been elusive and it is argued that it lies in the shapes of mafic intrusions that have been preserved in the crust. Numerical calculations of visco-elasto-plastic deformation induced by a dense intrusive body in continental crust have been carried out for a wide range of physical conditions. Three regimes are defined on the basis of the amount of dense material that remains at the original emplacement level as well as on the shape of the residual body. With strong encasing rocks, the intrusion deforms weakly in a sagging regime characterized by downwarping of the floor. At the other extreme, the intrusion sinks through weak surroundings, leaving behind a very small volume of material. In an intermediate regime, the intrusion does not sink wholesale and undergoes a dramatic change of shape. A residual body is preserved with a shape that depends on the aspect ratio of the initial intrusion. For aspect ratios of order one, the residual body is funnel-shaped above a thin and deep vertical extension. For the small aspect ratios that typify large igneous complexes such as the Bushveld, South Africa, the residual body is characterized by thick peripheral lobes with inward-dipping igneous layers and a thinner central area that has lost some of the basal cumulates. The transitions between these regimes depend on the rheology and temperature of encasing rocks.

In situ fractionation and inward migration of the solidification front in the Skaergaard intrusion, East Greenland

For more than 80 years the Skaergaard intrusion, 68°N in southern East Greenland, has been a foremost natural laboratory for the study of the crystallisation and fractionation of basaltic magma. This process has been of prime importance in the evolution of the Earth and other stony planets. Models that have been developed and refined during numerous studies of this particular intrusion have been part of the foundation for petrogenetic modelling for decades. In later years, vast amounts of new data have been added, due to systematic sampling in the field and from analysis of exploration drill cores. Methods for the study on grain-size scale have advanced, and the quest for a wellsupported genetic model for the PGE-Au mineralisation of the intrusion has intensified. The new data and insight question the applicability of conventional petrogenetic modelling, and as a consequence, increasing importance is placed on in situ crystallisation and fractionation in mush zones at the roof, walls and floor of the intrusion.

Geochronological, geochemical, and Sr–Nd–Hf isotopic characteristics of Cretaceous monzonitic plutons in western Zhejiang Province, Southeast China: New insights into the petrogenesis of intermediate rocks

We present comprehensive petrological, geochemical, and Sr–Nd–Hf isotopic data for the Matou and Dalai plutons in western Zhejiang Province, Southeast China, with the aim of constraining the petrogenesis of monzonites and to offer new insights into the deep processes of interaction between crustal- and mantle-derived magmas beneath SE China. The Matou pluton comprises quartz monzonite, whereas the Dalai pluton consists of quartz monzodiorite. Zircon U–Pb ages obtained by laser ablation-inductively coupled plasma-mass spectrometry show that both plutons were emplaced at 99–101Ma. Rocks of both plutons are intermediate to silicic, metaluminous to weakly peraluminous, subalkaline, and K-rich in composition. Samples of the plutons are enriched in large ion lithophile (e.g., Rb, K, and Pb) and light rare earth elements, depleted in high-field strength elements (e.g., Nb, Ta, and Ti), and have small negative or no Eu anomalies. In addition, the rocks have high Mg# values (up to 53.9), high zircon εHf(t) values (up to −1.4), and low Nb/U and Ta/U ratios. Geochemical evidence suggests that both depleted asthenospheric and metasomatically enriched mantle components were involved in the formation of these monzonitic rocks. The presence of inherited zircons with Palaeoproterozoic ages and zircons with unusually low εHf(t) values (−12.9) in the Matou quartz monzonites indicates that ancient crustal materials were also involved in their petrogenesis. In combination with the presence of abundant mafic microgranular enclaves (MMEs) with spheroidal to ellipsoidal–ovoidal shapes and xenocrysts within the more diffused enclaves, and the results of trace element modelling, we suggest that the Matou quartz monzonites were generated by mixing between mantle-derived mafic magmas and crustally derived silicic magmas. The Dalai pluton is relatively homogeneous and contains fewer MMEs than the Matou pluton. Zircons from the Dalai pluton show no inherited components, indicating that crustal materials have played a limited role in the petrogenesis of the quartz monzodiorites. The Dalai quartz monzodiorites have lower SiO2 contents, higher Mg# values, and considerably higher and variable Cr, Co, and Ni concentrations than the Matou quartz monzonites. Zircon Hf isotopic compositions of the Dalai pluton are relatively homogeneous (εHf(t)=−5.2 to −3.2). The combined petrological, geochemical, and isotopic features indicate that the Dalai monzodiorites were generated by olivine- and pyroxene-dominated fractional crystallisation from basaltic magmas, which were in turn produced by mixing between melts from depleted asthenosphere and subduction-enriched mantle. Our interpretation implies that Late Mesozoic monzonitic rocks in Southeast China require a significant input of mantle melts, and some may have been generated solely by fractionation of basaltic magmas. This petrogenetic model may be applicable to other monzonitic rocks in Southeast China, and to similar tectonic settings and sites of monzonitic magma generation worldwide.

Precursory activity of the 161 ka Kos Plateau Tuff eruption, Aegean Sea (Greece)

The Kos Plateau Tuff (KPT) eruption of 161 ka was the largest explosive Quaternary eruption in the eastern Mediterranean. We have discovered an uplifted beach deposit of abraded pumice cobbles, directly overlain by the KPT. The pumice cobbles resemble pumice from the KPT in petrography and composition and differ from Plio-Pleistocene rhyolites on the nearby Kefalos Peninsula. The pumice contains enclaves of basaltic andesite showing chilled lobate margins, suggesting co-existence of two magmas. The deposit provides evidence that the precursory phase of the KPT eruption produced pumice rafts, and defines the paleoshoreline for the KPT, which elsewhere was deposited on land. The beach deposit has been uplifted about 120 m since the KPT eruption, whereas the present marine area south of Kos has subsided several hundred metres, as a result of regional neotectonics. The basaltic andesite is more primitive than other mafic rocks known from the Kos–Nisyros volcanic centre and contains phenocrysts of Fo89 olivine, bytownite, enstatite and diopside. Groundmass amphibole suggests availability of water in the final stages of magma evolution. Geochemical and mineralogical variation in the mafic products of the KPT eruption indicate that fractionation of basaltic magma in a base-of-crust magma chamber was followed by mixing with rhyolitic magma during eruption. Low eruption rates during the precursory activity may have minimised the extent of mixing and preserved the end-member magma types.

The central Kenya peralkaline province: Insights into the evolution of peralkaline salic magmas

The central Kenya peralkaline province comprises five young (< 1 Ma) volcanic complexes dominated by peralkaline trachytes and rhyolites. The geological and geochemical evolution of each complex is described and issues related to the development of peralkalinity in salic magmas are highlighted. The peralkaline trachytes may have formed by fractionation of basaltic magma via metaluminous trachyte and in turn generated pantellerite by the same mechanism. Comenditic rhyolites are thought to have formed by volatile-induced crustal anatexis and may themselves have been parental to pantelleritic melts by crystal fractionation. The rhyolites record very low temperatures of equilibration (≤ 700 °C) at low fO 2 (≤ FMQ). The development of compositional zonation within the magma reservoirs has been ubiquitous, involving up to tens of cubic km of magma at timescales of 10 3–10 4 years. Magma mixing has also been commonplace, sometimes between adjacent centres. Isotopic evidence relating to rates and timescales of pre-eruptive residence times and crystal fractionation processes is summarized.

Origin of Fe-rich lherzolites and wehrlites from Tok, SE Siberia by reactive melt percolation in refractory mantle peridotites

Lherzolite–wehrlite (LW) series xenoliths from the quaternary Tok volcanic field in the southeastern Siberian craton are distinguished from the more common lherzolite–harzburgite (LH) series by (a) low Mg numbers (0.84–0.89) at high modal olivine (66–84%) and (b) widespread replacement of orthopyroxene (0–12%) and spinel by clinopyroxene (7–22%). The LW series peridotites are typically enriched in Ca, Fe, Mn and Ti, and depleted in Si, Ni and Cr relative to refractory LH series rocks (Mg number ≥0.89), which are metasomatised partial melting residues. Numerical modelling of Fe–Mg solid/liquid exchange during melt percolation demonstrates that LW series rocks can form by reaction of host refractory peridotites with evolved (Mg numbers 0.6–0.7), silica-undersaturated silicate melts at high melt/rock ratios, which replace orthopyroxene with clinopyroxene and decrease Mg numbers. This process is most likely related to underplating and fractionation of basaltic magma in the shallow mantle, which also produced olivine–clinopyroxene cumulates found among the Tok xenoliths.

Structure and performance of PM ultrahard high speed steel: A.L.Wojcieszynski, W.B.Eisen. (Crucible Research, USA.)

Four distinct granitic magma series developed systematically in space and time during the evolution of the Southern Volcanic Zone of the Archean Abitibi granite-greenstone belt, Canada (∼ 2.75–2.67 Ga).Minor synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, such as the Lac Flavrian and Bourlamaque plutons, were emplaced at ∼ 2700 Ma at pressures of ∼ 1 kbar. Synvolcanic TTG's are chemically similar to Barker's low-Al type Archean TTG, and are characterised by extremely low K2O (< 1.5 wt%), Rb (< 50 ppm) and Sr (< 200 ppm) concentrations, enhanced TiO2 (0.5 wt%), Fe2O3 (4.4 wt%), Nb, Y and Sc, flat REE patterns, and negative Eu anomalies that increase with HREE abundance. The TTG's are interpreted to have originated by fractionation of basaltic magma from partial melting of a depleted mantle source in a supra-subduction zone environment.Voluminous tonalite-granodiorite-granite and quartz monzonite (TGGM series), exemplified by the Round Lake and Lac Abitibi batholiths, developed syntectonically at ∼ 2690 Ma. Compositionally, the TGGM-series granites resemble Archean and Phanerozoic high-Al type TTG; they feature low K2O (1–3 wt%), Rb (10–50 ppm), Ba (< 1000 ppm) and U contents, enhanced Sr, low Ba/Sr (0.5–1.25) ratios, normalised depletion of Ta, Nb and Ti, and strongly fractionated REE patterns [(LaYb)n = 49-21] with minor positive or no Eu anomalies. Syntectonic TGGM's formed by partial melting of an amphibolite and/or quartz eclogite basaltic protolith from a subducting or subcreted slab.Late-tectonic quartz syenite-quartz monzonite-granite (SMG) series range from batholithic dimensions (e.g., Watebeag) to stocks (e.g., Garrison), and were emplaced at shallow levels (⩽ 1 kbar) between 2681 to 2676 Ma. In composition they are similar to the TGGM series, including REE distributions, but feature lower CaO(K2O + Na2O) and greater concentrations of Rb, Ba, Th and U. This granitic series may represent mixtures of magmas derived from a “metasomatised” mantle and TGGM-type sources.Late- to post-tectonic alkali feldspar syenite-alkali feldspar quartz syenite (SS) series (2680-2670 Ma) are one expression of shoshonitic magmatism most prevalent along regional translithospheric strike-slip structures. Comagmatic trachytes unconformably overlie the deformed and metamorphosed greenstone belt. The most primitive magmas (SiO2 ⩽ 65 wt%) are characterised by the conjunction of extreme large-ion lithophile element (LILE) and light REE (LREE) enrichment, strongly fractionated REE patterns, and relative depletions of Ta, Nb and Ti, with enhanced Cr, Co and Ni abundances; evolved phases (SiO2 ⩾ 65 wt%) possess lower contents of compatible and incompatible elements stemming from differentiation. Primitive magmas of the SS series may have been derived from low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab.In the adjacent Pontiac subprovince, two granitic series predominate: a syn- to late-tectonic monzodiorite-monzonite-granodiorite-syenite (MMGS) series (2690-2670 Ma), and a garnet-muscovite granite (GMG) series (∼ 2645 Ma). The MMGS's have comparable major-element, LILE and REE contents to Abitibi SMG and evolved SS granitic series, but are distinguished by strong correlations of U, Th, Zr and Hf with SiO2, extremely high Ba/Th ratios, and by co-enrichment of Cr, Co and Ni with LREE and Li and Cs in some MMGS phases. GMG series, which are associated with migmatites and Mo,U,Be,Li-pegmatites are characterised by K2ONa2O ⩾ 1, restricted SiO2 range (69–75 wt%), strong enrichments of Rb, Li, Cs, Ta, Nb, Th and U, and moderately fractionated REE's [(LaYb)n = 0.9–16], with pronounced negative Eu anomalies: they are similar to collisional S-type granites and represent infracrustal melting of mature metasediments during a continent-arc collision involving thrusting of the Pontiac subprovince beneath the Abitibi Southern Volcanic Zone.Collectively, the six granitic series reflect a transition from extensional supra-subduction zone magmatism with depleted mantle sources, to volcanic arc magmatism involving subducting slab during early collision, to alkaline granites derived from metasomatised mantle sources beneath a mature arc undergoing extension, and finally to magmatism resulting from terrane-terrane collision over a period of ∼ 70 Ma.

Adhesive wear of hardmetals and cermets: H.Reshetnyak, J.Kubarsepp. (Tallinn Technical University, Estonia.)

Four distinct granitic magma series developed systematically in space and time during the evolution of the Southern Volcanic Zone of the Archean Abitibi granite-greenstone belt, Canada (∼ 2.75–2.67 Ga).Minor synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, such as the Lac Flavrian and Bourlamaque plutons, were emplaced at ∼ 2700 Ma at pressures of ∼ 1 kbar. Synvolcanic TTG's are chemically similar to Barker's low-Al type Archean TTG, and are characterised by extremely low K2O (< 1.5 wt%), Rb (< 50 ppm) and Sr (< 200 ppm) concentrations, enhanced TiO2 (0.5 wt%), Fe2O3 (4.4 wt%), Nb, Y and Sc, flat REE patterns, and negative Eu anomalies that increase with HREE abundance. The TTG's are interpreted to have originated by fractionation of basaltic magma from partial melting of a depleted mantle source in a supra-subduction zone environment.Voluminous tonalite-granodiorite-granite and quartz monzonite (TGGM series), exemplified by the Round Lake and Lac Abitibi batholiths, developed syntectonically at ∼ 2690 Ma. Compositionally, the TGGM-series granites resemble Archean and Phanerozoic high-Al type TTG; they feature low K2O (1–3 wt%), Rb (10–50 ppm), Ba (< 1000 ppm) and U contents, enhanced Sr, low Ba/Sr (0.5–1.25) ratios, normalised depletion of Ta, Nb and Ti, and strongly fractionated REE patterns [(LaYb)n = 49-21] with minor positive or no Eu anomalies. Syntectonic TGGM's formed by partial melting of an amphibolite and/or quartz eclogite basaltic protolith from a subducting or subcreted slab.Late-tectonic quartz syenite-quartz monzonite-granite (SMG) series range from batholithic dimensions (e.g., Watebeag) to stocks (e.g., Garrison), and were emplaced at shallow levels (⩽ 1 kbar) between 2681 to 2676 Ma. In composition they are similar to the TGGM series, including REE distributions, but feature lower CaO(K2O + Na2O) and greater concentrations of Rb, Ba, Th and U. This granitic series may represent mixtures of magmas derived from a “metasomatised” mantle and TGGM-type sources.Late- to post-tectonic alkali feldspar syenite-alkali feldspar quartz syenite (SS) series (2680-2670 Ma) are one expression of shoshonitic magmatism most prevalent along regional translithospheric strike-slip structures. Comagmatic trachytes unconformably overlie the deformed and metamorphosed greenstone belt. The most primitive magmas (SiO2 ⩽ 65 wt%) are characterised by the conjunction of extreme large-ion lithophile element (LILE) and light REE (LREE) enrichment, strongly fractionated REE patterns, and relative depletions of Ta, Nb and Ti, with enhanced Cr, Co and Ni abundances; evolved phases (SiO2 ⩾ 65 wt%) possess lower contents of compatible and incompatible elements stemming from differentiation. Primitive magmas of the SS series may have been derived from low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab.In the adjacent Pontiac subprovince, two granitic series predominate: a syn- to late-tectonic monzodiorite-monzonite-granodiorite-syenite (MMGS) series (2690-2670 Ma), and a garnet-muscovite granite (GMG) series (∼ 2645 Ma). The MMGS's have comparable major-element, LILE and REE contents to Abitibi SMG and evolved SS granitic series, but are distinguished by strong correlations of U, Th, Zr and Hf with SiO2, extremely high Ba/Th ratios, and by co-enrichment of Cr, Co and Ni with LREE and Li and Cs in some MMGS phases. GMG series, which are associated with migmatites and Mo,U,Be,Li-pegmatites are characterised by K2ONa2O ⩾ 1, restricted SiO2 range (69–75 wt%), strong enrichments of Rb, Li, Cs, Ta, Nb, Th and U, and moderately fractionated REE's [(LaYb)n = 0.9–16], with pronounced negative Eu anomalies: they are similar to collisional S-type granites and represent infracrustal melting of mature metasediments during a continent-arc collision involving thrusting of the Pontiac subprovince beneath the Abitibi Southern Volcanic Zone.Collectively, the six granitic series reflect a transition from extensional supra-subduction zone magmatism with depleted mantle sources, to volcanic arc magmatism involving subducting slab during early collision, to alkaline granites derived from metasomatised mantle sources beneath a mature arc undergoing extension, and finally to magmatism resulting from terrane-terrane collision over a period of ∼ 70 Ma.

Geochemical evolution of granitoids from the Archean Abitibi Southern Volcanic Zone and the Pontiac subprovince, Superior Province, Canada: Implications for tectonic history and source regions

Four distinct granitic magma series developed systematically in space and time during the evolution of the Southern Volcanic Zone of the Archean Abitibi granite-greenstone belt, Canada (∼ 2.75–2.67 Ga). Minor synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, such as the Lac Flavrian and Bourlamaque plutons, were emplaced at ∼ 2700 Ma at pressures of ∼ 1 kbar. Synvolcanic TTG's are chemically similar to Barker's low-Al type Archean TTG, and are characterised by extremely low K 2O (< 1.5 wt%), Rb (< 50 ppm) and Sr (< 200 ppm) concentrations, enhanced TiO 2 (0.5 wt%), Fe 2O 3 (4.4 wt%), Nb, Y and Sc, flat REE patterns, and negative Eu anomalies that increase with HREE abundance. The TTG's are interpreted to have originated by fractionation of basaltic magma from partial melting of a depleted mantle source in a supra-subduction zone environment. Voluminous tonalite-granodiorite-granite and quartz monzonite (TGGM series), exemplified by the Round Lake and Lac Abitibi batholiths, developed syntectonically at ∼ 2690 Ma. Compositionally, the TGGM-series granites resemble Archean and Phanerozoic high-Al type TTG; they feature low K 2O (1–3 wt%), Rb (10–50 ppm), Ba (< 1000 ppm) and U contents, enhanced Sr, low Ba/Sr (0.5–1.25) ratios, normalised depletion of Ta, Nb and Ti, and strongly fractionated REE patterns [ ( La Yb ) n = 49-21 ] with minor positive or no Eu anomalies. Syntectonic TGGM's formed by partial melting of an amphibolite and/or quartz eclogite basaltic protolith from a subducting or subcreted slab. Late-tectonic quartz syenite-quartz monzonite-granite (SMG) series range from batholithic dimensions (e.g., Watebeag) to stocks (e.g., Garrison), and were emplaced at shallow levels (⩽ 1 kbar) between 2681 to 2676 Ma. In composition they are similar to the TGGM series, including REE distributions, but feature lower CaO ( K 2 O + Na 2 O) and greater concentrations of Rb, Ba, Th and U. This granitic series may represent mixtures of magmas derived from a “metasomatised” mantle and TGGM-type sources. Late- to post-tectonic alkali feldspar syenite-alkali feldspar quartz syenite (SS) series (2680-2670 Ma) are one expression of shoshonitic magmatism most prevalent along regional translithospheric strike-slip structures. Comagmatic trachytes unconformably overlie the deformed and metamorphosed greenstone belt. The most primitive magmas (SiO 2 ⩽ 65 wt%) are characterised by the conjunction of extreme large-ion lithophile element (LILE) and light REE (LREE) enrichment, strongly fractionated REE patterns, and relative depletions of Ta, Nb and Ti, with enhanced Cr, Co and Ni abundances; evolved phases (SiO 2 ⩾ 65 wt%) possess lower contents of compatible and incompatible elements stemming from differentiation. Primitive magmas of the SS series may have been derived from low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab. In the adjacent Pontiac subprovince, two granitic series predominate: a syn- to late-tectonic monzodiorite-monzonite-granodiorite-syenite (MMGS) series (2690-2670 Ma), and a garnet-muscovite granite (GMG) series (∼ 2645 Ma). The MMGS's have comparable major-element, LILE and REE contents to Abitibi SMG and evolved SS granitic series, but are distinguished by strong correlations of U, Th, Zr and Hf with SiO 2, extremely high Ba/Th ratios, and by co-enrichment of Cr, Co and Ni with LREE and Li and Cs in some MMGS phases. GMG series, which are associated with migmatites and Mo,U,Be,Li-pegmatites are characterised by K 2 O Na 2 O ⩾ 1 , restricted SiO 2 range (69–75 wt%), strong enrichments of Rb, Li, Cs, Ta, Nb, Th and U, and moderately fractionated REE's [ ( La Yb ) n = 0.9–16 ], with pronounced negative Eu anomalies: they are similar to collisional S-type granites and represent infracrustal melting of mature metasediments during a continent-arc collision involving thrusting of the Pontiac subprovince beneath the Abitibi Southern Volcanic Zone. Collectively, the six granitic series reflect a transition from extensional supra-subduction zone magmatism with depleted mantle sources, to volcanic arc magmatism involving subducting slab during early collision, to alkaline granites derived from metasomatised mantle sources beneath a mature arc undergoing extension, and finally to magmatism resulting from terrane-terrane collision over a period of ∼ 70 Ma.

Derivation of some A-type magmas by fractionation of basaltic magma: An example from the Padthaway Ridge, South Australia

Various petrogenetic schemes have been proposed for A-type granitic and volcanic rocks, many involving melting of I-type granites or their residual sources. These models can be shown to have several short-comings and whilst there may be various kinds of A-type magmas their high temperatures suggest that mantle melts be invoked in any petrogenetic model. In addition, we suggest that the bimodal association with mafic rocks, the chemical characteristics and typically low initial 87Sr/ 86Sr ratios of many A-type rocks are entirely consistent with an origin by extended fractionation of bsaltic magmas. Interstitial felsic material from layered mafic intrusions also provides evidence for this origin, as do new data presented here for suite of A-type granites and volcanics from the Padthaway Ridge in South Australia. Mineralogical, chemical and isotopic arguments show that the Padthaway suite evolved from the basaltic magma that also formed contemporaneous gabbroic plutons which themselves contain A-type felsic fractionates. Olivine and pyroxene assemblages of these hypersolvus granites and volcanics attest to temperatures of 900–1000°C and water undersaturated conditions with final H 2O< 3 wt.%. Curvilinear geochemical trends and negative Eu anomalies indicate a history of protracted fractionation involving pyroxene and feldspar. Consistent with their high ϵNd (+ 2 to −3) and low initial 87Sr/ 86Sr (0.703-0.706), these A-type magmas are well modelled as the products of ∼90% crystallization of contemporary, mantle-derived, parental magma. Our calculations, supported by gravity data, suggest that A-type suites, which typically intrude in non-compressional settings, may mark episodes of crustal growth during which considerable mantle material is added to the crust.

Restites, Eu anomalies and the lower continental crust

Dehydration partial melting of the lower continental crust is the main process responsible for granite genesis in the post-Archean. If this melting occurs to undifferentiated mantle-derived rocks (i.e., basaltic protoliths), it should result in a granitic upper crust having a negative Eu anomaly and a mafic restitic lower crust having a positive Eu anomaly. However, most post-Archean high-pressure granulite facies terranes are not chemically complementary to granites because they themselves have bulk granitic compositions. If one restricts the discussion to mafic granulites from terranes, it is apparent that although their bulk composition is complementary to granite, most do not have positive Eu anomalies. Granite residues may, however, be present in granulite facies terranes if the parent rock giving rise to the granite had a negative Eu anomaly and a significant melt fraction were left behind with the residue. Such restitic granulites will have either no Eu anomaly or a negative one and “normal” LILE ratios due to the dominance of the incompatible element-rich melt. Granulite xenoliths do have the requisite compositions to balance the composition of the upper crust (i.e., mafic bulk composition and positive Eu anomalies); however, these samples are generally interpreted as melts or cumulates derived from underplated basaltic magmas rather than restites. Intracrustal differentiation may therefore proceed in several steps: intrusion and fractionation of basaltic magmas in the lower crust serves to 1. (1) create mafic and ultramafic cumulates (having positive Eu anomalies), 2. (2) produce evolved magmas (having negative Eu anomalies) 3. (3) melt the preexisting (evolved) crust to produce granites and evolved restitic granulites. The magmas produced by differentiation of the basalt may remain in the lower crust where they may be subsequently melted in a later underplating event or may mix with the crustal melts and intrude the upper crust.

A Model for Flood Basalt Vulcanism

The question of whether basaltic rocks in continental flood basalt provinces are primary magmas or whether they are descended in general from picritic parent magmas is reviewed. It is suggested that the latter is more likely to be correct on the evidence of phase relations and the relative rareness of mantle materials with appropriate Fe/Mg ratios. Major element variations in the residual liquids of fractional and equilibrium crystallization of basaltic magmas are modelled for a variety of crystallizing assemblages. It is concluded that crystallization of olivine, clinopyroxene, and plagioclase has a marked effect on buffering chemical change in many important elements. It is this effect which accounts for the apparent uniformity of large volumes of flood basalts, not, as has sometimes been supposed, a series of implausible coincidences in the amount of material fractionated from each magma batch. It is further argued that much of the variation seen in basalts may be imposed by polybaric fractionation operating throughout crustal depths, that is at pressures up to at least 12 kb. Parental picritic magmas rising from the mantle reach the surface in exceptional areas of crustal thinning. More usually, however, it is suggested that they intrude the base of the crust as a series of sills which differentiate into upper gabbroic and lower ultramafic portions. Much of the 'low pressure' fractionation of basaltic magmas may take place in this deep crustal sill complex and evolved liquids are transmitted to the surface as their density becomes sufficiently low. This implies that in areas of flood vulcanism a potentially large new contribution to the crust is made by underplating, the volumes of concealed cumulates being at least as large as the amount of erupted surface lava.

Petrochemistry and origin of pleistocene volcanic rocks from northern Taiwan

The Pleistocene volcanic rocks from northern Taiwan include the Tatun volcano group and the Chilung volcano group. Three rock types occur in this area: Tatun volcano group yield high-alumina basalt and andesites, whereas the chilung volcano group mainly consists of dacites. In addition, amphibole-rich nodules have also been found in different cruptive units of the former volcano group. Around seventy sample of various rock types have been conducted for geochemical studies, including analyses of major elements and trace elements such as Co, Cr, Cu, Li, Ni, Zn, Zr, V, Rb and Sr. Results of Al2O3, MnO, TiO2 total alkali content, MgO/ΣFeO and K2O/Na2O ratios and AMF diagram indicate that these Pleistocene volcanic rocks belong to typical calalkaline rock series. Detailed study of the trace elements reveals that these volcanic rocks are closely correlated with rocks of continental margin type with respect to Rb, Cu, Co, Ni, V and Cr contents, and K/Rb and Ni/Co ratios. These rocks are most probably derived from the fractionation of basaltic magma controlled mainly by the crystallization of amphibole and plagioclase with magnetite playing a minor role.

Late Cenozoic Transitional Alkali Olivine–Tholeiitic Basalt and Andesite from the Margin of the Great Basin, Southwest Utah

Late Cenozoic lava flows of southwest Utah comprise olivine basalt, with subordinate hypersthene-bearing basalt and andesite. Some basic flows containing normative nepheline may be classified chemically and mineralogically as alkali olivine basalt, others as tholeiitic on the basis of the presence of a reaction relation between olivine and Ca-poor pyroxene. Most of the basic lava flows, however, are transitional between typical alkaline and typical tholeiitic basalt and contain both olivine and hypersthene in the norm. The andesites are mineralogically similar to calc-alkaline andesites but are chemically richer in K 2 O. Most of the rocks are only mildly alkaline yet are lacking any clear trend of iron enrichment. Sr and Ba are the most abundant trace elements and are strongly enriched in andesite relative to basalt. Feldspar compositions vary from plagioclase of An 70 to potash feldspar of Or 70 , and olivine shows a considerable compositional range, except in basalt which contains groundmass hypersthene. Clinopyroxene is only weakly zoned and the behavior of its Al z is different according to whether the host rock is chemically alkaline or tholeiitic. Coexisting titano-magnetite and ilmenite in the groundmass of some of the basalts indicate consolidation temperatures in the range of 1040° to 800°C and oxygen fugacities ( f o 2 ) between 10 −10.3 and 10 −13.6 . The basaltic rocks probably originated by partial melting in the mantle, and their diversity is thought to reflect events which took place in the mantle rather than near the Earth9s surface. It is unlikely that the andesitic magmas were produced by crystal fractionation of basaltic magma, particularly if this involved removal of plagioclase. Sr 87 /Sr 86 ratios ranging from 0.7037 to 0.7054 place a limit on the possible amount of contamination of basalt by old, radiogenic crustal material. Thus the andesite may be the result of contamination of basaltic magma by young or nonradiogenic crustal material, or it may represent liquid produced by partial melting in the mantle. In either case, material rich in Sr and Ba seems to have been involved.

Nature of the clinopyroxene and iron enrichment in alkalic and transitional basaltic magmas

Iron enrichment and clinopyroxene composition are considered to be good indicators of the magmatic parentage of volcanic series produced by fractionation of basaltic magma. Tholeiitic series are in fact believed to be constantly more iron enriched than alkalic series and systematic differences in the clinopyroxene mineralogy of the two suites seem also to be well established. The aim of this paper is to demonstrate that such generalization cannot be accepted, as natural conditions can be realized under which the fractionation of alkalic or mildly alkalic basalts may produce series with strong iron enrichment and with clinopyroxene mineralogy “atypical” for basalts of alkaline affinity.