Alkalic Magmas Research Articles

Archean lithospheric mantle beneath Arkansas: Continental growth by microcontinent accretion

The Cretaceous Prairie Creek lamproites of southern Arkansas intrude Proterozoic crust near the boundary between the 1.5–1.3 Ga Granite-Rhyolite Province and the 1.3–1.0 Ga Grenville orogen. They carry xenocrysts and rare xenoliths derived from the subcontinental lithospheric mantle (SCLM) and the deep crust. U-Pb age dating of groundmass perovskite in the Prairie Creek lamproites gives a poorly constrained Cretaceous age. U-Pb dating and in situ Sr and Nd isotope data show that perovskite micronodules in the Twin Knobs #2 lamprophyre are ca. 600 Ma old, and may represent samples of rift-related alkalic magmas derived from a juvenile mantle. A lithologic section constructed from the mantle-derived xenocrysts shows a moderately depleted SCLM that has experienced a high degree of melt-related metasomatism, especially in the depth range 150 to 140 km. In situ Re-Os analysis of sulfide grains in the xenoliths yields model ages ranging up to 3.4 Ga, with major peaks at 1.4–1.5 Ga and 200–300 Ma. Early Paleoproterozoic model ages appear to reflect mixing between residual Archean high-Os sulfides and later low-Os sulfide melts. These data suggest that the SCLM beneath the Prairie Creek area formed in Archean time, and has been progressively refertilized by a series of magmatic events, which appear to correlate in time with events in the overlying crust. The Archean SCLM sampled by the lamproites may represent the mantle root of the Sabine microcontinent, which lies mainly to the south of the lamproite field and is recognizable in seismic tomography as a feature with higher shear-wave velocity (Vs) (100–175 km depth). Seismic tomography also shows several blocks with high Vs beneath the Grenville province to the east, which may represent other microcontinental blocks. These findings suggest that the growth of individual continents is significantly affected by the accretion of older microcontinental blocks, and that the extent of early continental crust therefore may be greater than generally estimated.

Oceanic Volcanism from the Low-velocity Zone – without Mantle Plumes

We develop a model for oceanic volcanism that involves fracturing of the seismic lithosphere to access melts from the partly melted seismic low-velocity zone. Data on global seismic shear-wave velocities are combined with major-element compositions of global mid-ocean ridge basalt glasses, Hawaiian basalt glasses, and phase relations in the CaO-MgO-Al2O3-SiO2-CO2 and CaO-MgO-Al2O3-SiO2-Na2O-FeO systems at pressures from 1 atm to 6 GPa. We use these data to constrain the pressure–temperature conditions for melt extraction at Hawaii and mid-ocean ridges (including Iceland), and to evaluate the existence of hot mantle plumes. In the low-velocity zone, the maximum reduction and maximum anisotropy of seismic shear-wave velocity (maximum melt fraction) occurs at a depth of ∼140–150 km for crustal ages >∼50 Ma, and a depth of ∼65 km at the East Pacific Rise axis. Seismic data indicate a smooth depth transition between these extremes. Experimental phase-equilibrium data, when combined with natural glass compositions, show that pressure–temperature conditions of tholeiitic melt extraction at Hawaii (∼4–5 GPa, 1450–1500°C) and the global oceanic ridge system (∼1·2–1·5 GPa, 1250–1280°C) are an excellent match for the two depth ranges of maximum melting indicated by seismic shear-wave data. At Hawaii, magmas escape to the surface along a fracture system that extends through the lithosphere into the low-velocity zone. This allows eruption of progressively deeper melts from the low-velocity zone, which exist at equilibrium along a normal geotherm. No significant decompression melting occurs. This produces the characteristic sequence at each volcano of initial low-volume alkalic magmas, then voluminous tholeiitic magmas that show low-pressure olivine-controlled crystallization, and final low-volume alkalic magmas from extreme depths. At the East Pacific Rise, the more shallow depth of magma extraction is caused by a perturbed ridge geotherm that grazes the lherzolite solidus within the thermal boundary layer. This results in an absence of olivine-controlled crystallization. Hawaii is not a hot plume. Instead, it shows magmas characteristic of normal mature-ocean thermal conditions in the low-velocity zone. We find no evidence of anomalously high temperatures of magma extraction and no role for hot mantle plumes anywhere in the ocean basins.

Mafic alkalic magmatism in central Kachchh, India: a monogenetic volcanic field in the northwestern Deccan Traps

Magmatism in Kachchh, in the northwestern Deccan continental flood basalt province, is represented not only by typical tholeiitic flows and dikes, but also plug-like bodies, in Mesozoic sandstone, of alkali basalt, basanite, melanephelinite and nephelinite, containing mantle nodules. They form the base of the local Deccan stratigraphy and their volcanological context was poorly understood. Based on new and published field, petrographic and geochemical data, we identify this suite as an eroded monogenetic volcanic field. The plugs are shallow-level intrusions (necks, sills, dikes, sheets, laccoliths); one of them is known to have fed a lava flow. We have found local peperites reflecting mingling between magmas and soft sediment, and the remains of a pyroclastic vent composed of non-bedded lapilli tuff breccia, injected by mafic alkalic dikes. The lapilli tuff matrix contains basaltic fragments, glass shards, and detrital quartz and microcline, with secondary zeolites, and there are abundant lithic blocks of mafic alkalic rocks. We interpret this deposit as a maar-diatreme, formed due to phreatomagmatic explosions and associated wall rock fragmentation and collapse. This is one of few known hydrovolcanic vents in the Deccan Traps. The central Kachchh monogenetic volcanic field has >30 individual structures exposed over an area of ∼1,800 km2 and possibly many more if compositionally identical igneous intrusions in northern Kachchh are proven by future dating work to be contemporaneous. The central Kachchh monogenetic volcanic field implies low-degree mantle melting and limited, periodic magma supply. Regional directed extension was absent or at best insignificant during its formation, in contrast to the contemporaneous significant directed extension and vigorous mantle melting under the main area of the Deccan flood basalts. The central Kachchh field demonstrates regional-scale volcanological, compositional, and tectonic variability within flood basalt provinces, and adds the Deccan Traps to the list of such provinces containing monogenetic- and/or hydrovolcanism, namely the Karoo-Ferrar and Emeishan flood basalts, and plateau basalts in Saudi Arabia, Libya, and Patagonia.

SILICA-UNDERSATURATED PORTION OF THE SYSTEM NEPHELINE - KALSILITE - SiO2 AT 2 GPa [P(H2O) = P(Total)

The nepheline (Ne) – kalsilite (Kls) – sanidine (Sa) – albite (Ab) quadrilateral representing petrogeny’s residua system for silica-undersaturated alkalic magmas has been studied at 2 GPa [P(H 2 O) = P(Total)] and various temperatures. The composition of the eutectics, their assemblages and the equilibrated temperatures of the various binary joins studied at 2 GPa in presence of excess H 2 O are as follows. Along the Kls–Sa join, the eutectic occurs at Kls 13 Sa 87 (all compositions are expressed in wt% unless specified) and 700°C, where Kls, Sa ss , liquid and vapor are in equilibrium. Along the Kls–Ne binary join, the eutectic is located at Kls 44 Ne 56 and 650°C, where Kls and Ne ss coexist with liquid and vapor. The eutectic point of the binary join Ne–Ab occurs at Ne 87 Ab 13 and 670°C, where Ne ss , Jd ss , liquid and vapor are in equilibrium. In the case of the Ab–Kfs binary join, the eutectic lies at Ab 73 Kfs 27 and 630°C, where Jd ss , Sa ss , quartz, liquid and vapor are the equilibrium assemblage. There are two eutectics in the silica-undersaturated portion of the Ne–Kls–SiO 2 system, studied at 2 GPa in presence of excess H 2 O: 1) E 1 occurs at Kls 18 Ne 52 Qtz 30 and 620°C, where Jd ss , Sa ss , Ne ss , liquid and vapor are in equilibrium, and 2) E 2 is located at Kls 49 Ne 25 Qtz 26 and 615°C, where Ne ss , Kls, Sa ss coexist with the liquid. In this system, the maximum (M) produced by the intersection of the join nepheline–sanidine with the nepheline–feldspar cotectic line occurs at Kls 35 Ne 38 Qtz 27 and 635°C, where Ne ss + Sa ss + Gl + V are in equilibrium. In contrast to the phase-equilibria studies made at 1 atm, 0.1, 0.2 and 0.5 GPa in presence of excess H 2 O, there is no field of leucite ss at 2 GPa. The join Ab–Sa no longer acts as a thermal barrier as at low pressure. Instead, Jd (jadeite)–Sa becomes the stable thermal barrier; because of this, all compositions within the triangle Ab–Sa–Jd yield Sa ss , Jd ss and quartz at 2 GPa isobaric polythermal conditions in presence of excess H 2 O. The same compositions at low pressure produce feldspathoid– feldspar-bearing assemblages. The occurrence of jadeite along with feldspathoid and feldspar in nepheline syenites arises because the bulk composition of the magma lies in the silica-deficient portion of the system Jd–Sa–Ne, whereas the occurrence of Jd–Sa– SiO 2 observed in association with granitic rocks is related to crystallization of the magma at a high pressure.

Kahoolawe Island, Hawaii: The role of an ‘inaccessible’ shield volcano in the petrology of the Hawaiian islands and plume

Kahoolawe volcano (∼10×17 km) forms one of the eight major Hawaiian islands. Access for geologic sampling has long been restricted due to military and preservation policies. However, limited visits to Kahoolawe in the 1980s yielded >200 samples, many of which have since been used to study the volcano within the framework of Hawaiian shield and mantle source geochemistry, petrology, mineralogy, and igneous processes. Kahoolawe is a tholeiitic shield with tholeiitic caldera-filling lavas, and at least seven postshield vents that erupted tholeiitic and (sparse) alkalic lavas. On smaller scales are a gabbro intrusion and ultramafic and gabbroic xenoliths in some postshield lavas. There is no evidence for rejuvenated volcanism. In its structural setting, Kahoolawe lies along the “Loa” trend of Hawaiian shields. Major element compositions of shield and caldera-filling lavas are similar and cluster at ∼6–7 wt% MgO, range from ∼5.5 to 16 wt% MgO, and include ∼9 wt% MgO examples that can be modeled as parental to the evolved lavas. For example, least squares mass balancing demonstrates that from ∼15% to 30% crystallization of olivine (±orthopyroxene), clinopyroxene, and plagioclase accounts for the ∼5.5–6 wt% MgO range of tholeiitic lavas. Greater differentiation occurred in the gabbro (diabasic) intrusive body as a segregation vein with ∼2.9 wt% MgO, and extreme differentiation produced local, small-volume rhyolitic melts that segregated into lava vesicles. Postshield lavas are mainly tholeiitic, have ∼5–7 wt% MgO, and overlap shield compositions. Some are alkalic, as low as ∼3.9 wt% MgO (hawaiite), and can be modeled as liquids after a ∼9 wt% MgO alkalic magma crystallized ∼30% olivine, clinopyroxene, plagioclase, and magnetite. Important aspects of Sr, Nd, Hf, and Pb isotopic ratios in Kahoolawe shield and caldera-filling lavas are slightly higher 87Sr/ 86Sr than in Koolau shield lavas (Oahu island; Makapuu-stage; e.g., Koolau mantle ‘endmember’) when compared at a given 143Nd/ 144Nd (e.g., ∼0.7042 at 0.5128), 206Pb/ 204Pb largely at the low end of the range for Hawaiian shields (e.g., ∼18), and ε Hf generally comparable to the values of other Hawaiian shields and ocean islands (e.g., ε Hf 8 at ε Nd 4). The isotopic ratios overall suggest small-scale source heterogeneity, considering the island size, and that Kahoolawe shield and caldera lavas were derived from a Hawaiian plume source containing recycled oceanic crust of gabbro and sediments. Based on certain geochemical indicators, however, such as Ce/Sr and La/Nb vs. 87Sr/ 86Sr, the source contained slightly less gabbro component than other shield sources (e.g., Koolau). Isotopic data for Kahoolawe postshield lavas are scarce, but those available generally overlap the shield data. However, ratios among certain alteration-resistant incompatible trace elements (e.g., Zr/Nb) discriminate some postshield alkalic from shield lavas. But because the different ratios for those postshield lavas can be explained by smaller partial-melting percentages of the shield source and by differentiation, neither isotopes nor trace elements identify postshield magmas as originating in a source unlike that for the shield lavas.

SHRIMP U-Pb dating of recurrent Cryogenian and Late Cambrian-Early Ordovician alkalic magmatism in central Idaho: Implications for Rodinian rift tectonics

Composite alkalic plutonic suites and tuffaceous diamictite, although discontinuously exposed across central Idaho in roof pendants and inliers within the Idaho batholith and Challis volcanic-plutonic complex, define the >200-km-long northwest-aligned Big Creek-Beaverhead belt. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dates on these igneous rocks provide direct evidence for the orientation and location of the Neoproterozoic–Paleozoic western Laurentian rift margin in the northern U.S. Cordillera. Dating delimits two discrete magmatic pulses at ca. 665–650 Ma and 500–485 Ma at the western and eastern ends, respectively, of this belt. Together with the nearby 685 Ma volcanic rocks of the Edwardsburg Formation, there is a 200 Ma history of recurrent extensional magmatic pulses along the belt. A similar history of recurrent uplift is reflected in the stratigraphic record of the associated miogeoclinal and cratonal platform basins, suggesting that the Big Creek–Beaverhead belt originated as a border fault during continental rift events. The magmatic belt is paired with the recurrently emergent Lemhi Arch and narrow miogeoclinal facies belts and it lies inboard of a northwest-striking narrow zone of thinned continental crust. These features define a northeast-extending upper-plate extensional system between southeast Washington and southeast Idaho that formed a segment of the Neoproterozoic–Paleozoic miogeocline. This segment was flanked on the north by the St. Mary–Moyie transform zone (south of a narrow southern Canadian upper-plate margin) and on the south by the Snake River transfer zone (north of a broad Great Basin lower-plate margin). These are the central segments of a zigzag-shaped Cordilleran rift system of alternating northwest-striking extensional zones offset by northeast-striking transfers and transforms. The data substantiate polyphase rift and continental separation events that included (1) pre- and syn-Windermere rifting, (2) Windermere margin subsidence, (3) late Ediacaran-Cambrain rifting, and (4) well-developed late Ediacaran-Devonian passive margin subsidence and deposition. Timing and geometries support synchronous but opposing divergence along Cordilleran and Atlantic rifts with a junction in Southern California–Sonora.

Composition and provenance of the Snowcap assemblage, basement to the Yukon-Tanana terrane, northern Cordillera: Implications for Cordilleran crustal growth

The Yukon-Tanana terrane of the northern Cordillera comprises a basement of metamorphosed continental margin sedimentary rocks of pre–Late Devonian age (Snowcap assemblage) and overlying subduction-generated Late Devonian to Permian arc and backarc facies igneous rocks. While preliminary analytical data have suggested that the Yukon-Tanana terrane originally formed as part of the western peri-Laurentian margin, its position outboard of an upper Paleozoic oceanic terrane (Slide Mountain) and the lack of information from its basement, the Snowcap assemblage, continue to raise questions about its original paleogeographic location along the margin. We describe here the geological relationships, geochemical and Nd-Hf isotopic compositions, and the detrital zircon signature of the Snowcap assemblage. Geochemical and Nd-Hf isotopic data for most siliciclastic rocks suggest derivation from evolved, upper crustal material, with Paleoproterozoic Nd-Hf depleted mantle model ages and detrital zircon data with major peaks in age ca. 1870 Ma and ca. 2720 Ma, and secondary peaks ca. 2080 Ma and ca. 2380 Ma. Minor juvenile contributions to some metaclastic rocks are more likely related to coeval, rift-related mafic alkalic magmatism than to younger arc magmatism in the terrane, as previously suggested. The detrital zircon signature of the Snowcap assemblage confirms a northwestern Laurentian cratonic source, similar to that of the adjacent Cordilleran miogeocline, and provides a local source in the Yukon-Tanana terrane for evolved signatures and Paleoproterozoic-Archean zircons (both detrital grains and xenocrystic cores) in younger mid- to late Paleozoic rocks of the terrane, at times when the Laurentian craton was probably not available as a direct source. Mafic alkalic rocks of the Snowcap assemblage were the products of low degree partial melting of incompatible element–enriched lithospheric mantle sources, most likely related to one of several Neoproterozoic–early Paleozoic rifting events recorded along the western margin of Laurentia. Marble and calc-silicate rocks have trace element compositions similar to modern seawater and juvenile Nd-Hf isotopic signatures similar to the mafic rocks, implying coeval carbonate sedimentation and magmatism. The overall character and composition of the Yukon-Tanana terrane suggest that crustal recycling processes dominated its evolution. Its accretion to the western margin of North America in early Mesozoic time contributed only a limited amount of juvenile crustal material to the Cordillera.

High-Ti amphibole as a petrogenetic indicator of magma chemistry: evidence for mildly alkalic-hybrid melts during evolution of Variscan basic–ultrabasic magmatism of Central Iberia

Central Iberian Variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous, high-K calc-alkaline, ultrabasic to intermediate rock series. Despite their overall calc-alkaline affinity, the mafic–ultramafic members contain high-Ti amphibole oikocrysts rimmed by lower-Ti amphibole ± cummingtonite and high-Ti amphibole replacing early phlogopite. To understand the factors controlling the saturation of high-Ti amphibole in the parental magmas, clinopyroxene-melt, phlogopite-melt and amphibole-melt relationships are reviewed. This analysis reveals that for melts with intermediate compositions, the affinity of TiO2 for amphibole rises in alkalic magmas. Accordingly, mildly alkalic trachytoid to subalkaline medium- to high-K andesite and dacite compositions are estimated for interstitial high-Ti amphibole-saturated melts. Amphibole Ce/Pb ratios reveal a mantle–crust hybrid nature for interstitial melts with subalkaline trachytoid compositions. The hydrous character of the Variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of Na2O with respect to silica during amphibole crystallization.

Deccan plume, lithosphere rifting, and volcanism in Kutch, India

Kutch (northwest India) experienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-like bodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinel wehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transient carbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg# = 64–72) relative to the tholeiites (Mg# = 38–54), and are enriched in incompatible trace elements. Isotope and trace element compositions of the tholeiites are similar to what are believed to be the crustally contaminated Deccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalic basalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field of Reunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed with Reunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinity toward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. We present a new model for the metasomatism and rifting of the Kutch lithosphere, and magma generation from a CO 2-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatized the lithosphere. Further partial melting of CO 2-rich lherzolite at about 2–2.5 GPa from a mixed source of asthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deep lithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiites may have been generated from the main plume head further south of Kutch.

Black Hills–Alberta carbonatite–kimberlite linear trend: Slab edge at depth?

Carbonatites and kimberlites (~ 48 to ≤ 46 Ma) occur along a 700-km-long, N40°W linear trend of alkalic magmatism (~ 55 to ≤ 46 Ma) that extends from the Black Hills of South Dakota and Wyoming, across eastern Montana to southern Alberta. Isotopic and trace-element concentrations point to a genetic linkage among alkalic igneous rocks, carbonatites, and kimberlites along the trend (group 1), with Sr–Nd isotopic ratios flanking bulk silicate earth. In contrast, alkalic rocks southwest of the N40°W trend (groups 2 and 3) have significantly lower initial epsilon-Nd values and different trace-element contents (e.g. higher Ba, but lower REE, U, and Th). A tectonomagmatic model is proposed here to account for placement of the genetically related group 1 magmas along the N40°W linear zone; the model suggests that the edge of the Kula plate was lodged in the mantle transition zone directly below the N40°W trend by ~ 50 Ma. Mantle material rising through the Kula–Farallon slab window may have been concentrated at the southwest edge of the Kula slab, focusing melts upward along the trend. Geochemical attributes of group 1 magmas along the trend can be explained by an extremely small partial melt of carbonated peridotitic mantle (0–1%). Initiation of extension at ~ 53–49 Ma is important because it may have stimulated mantle upwelling and decompressional melting, as well as facilitated intrusion of asthenospheric melts into the Wyoming Archean craton. Although groups 2 and 3 are thought to be largely lithospheric melts, their Sr–Nd isotopic compositions reflect a greater proportion of asthenospheric component after ~ 50 Ma, following onset of extension in the west.

Phase Relationships of Hydrous Alkalic Magmas at High Pressures: Production of Nepheline Hawaiitic to Mugearitic Liquids by Amphibole-Dominated Fractional Crystallization Within the Lithospheric Mantle

Experimental melting studies were conducted on a nepheline mugearite composition to pressures of 31 kbar in the presence of 0-30% added water. A temperature maximum in the near-liquidus stability of amphibole (with olivine) was found for a water content of 3·5 wt % at a pressure of 14 kbar. This is interpreted to have petrogenetic significance for the derivation of nepheline mugearite magmas from nepheline hawaiite by amphibole-dominated fractional crystallization at depth within the lithospheric mantle. Synthetic liquids at progressively lower temperatures range to nepheline benmoreite compositions very similar to those of natural xenolith-bearing high-pressure lavas elsewhere, and support the hypothesis that continued fractional crystallization could lead to high-pressure phonolite liquids. Independent experimental data for a basanite composition modeled on a lava from the same igneous province (the Newer Basalts of Victoria) permit the inference that primary asthenospheric basanite magmas undergo polybaric fractional crystallization during ascent, and may evolve to liquids ranging from nepheline hawaiite to phonolite upon encountering cooler lithospheric mantle at depths of 42-50 km. Such a model is consistent with the presence in some evolved alkalic lavas of both lithospheric peridotite xenoliths indicative of similar depths and of megacryst suites that probably represent disrupted pegmatitic segregations precipitated from precursor alkalic magmas in conduit systems within lithospheric mantle.

Homer's chimaera fires (SW of Antalya/Turkey); burning abiogenic methane gases; are they generated by a serpentinization process related to alkalic magmatism?

Homer's chimaera fires (SW of Antalya/Turkey); burning abiogenic methane gases; are they generated by a serpentinization process related to alkalic magmatism?

Provenance of Proterozoic Basal Aravalli mafic volcanic rocks from Rajasthan, Northwestern India: Nd isotopes evidence for enriched mantle reservoirs

Komatiites, komatiitic basalts and tholeiites from Nathdwara, Rajasthan, have been investigated for their Nd isotopic characteristics. 143Nd/ 144Nd vary broadly in the komatiites (0.51131–0.51252) and show a restricted range in komatiitic basalts and tholeiites (0.51120–0.51179 and 0.51141–0.51202, respectively). The corresponding epsilon Nd (ɛ Nd) values indicate an enriched source for these rocks. The model ages of komatiitic basalts and tholeiites cluster between 2.2 and 3.0 Ga. The komatiites show older model ages between 3.5 and 3.8 Ga. If the correlations found between Nd and Sm isotopic ratios can be interpreted as isochrones, then the komatiites point to the age of ∼2300 Ma and the komatiitic basalts and tholeiites constrain the age of ∼1800–2000 Ma. The age of 2300 Ma for the komatiite samples is similar to the range estimated for the komatiitic basalt and tholeiite samples (1800–2000 Ma), considering the errors involved. Collectively the data suggest an age of 2300–1800 Ma for the Basal Aravalli volcanism. The variation of Nd isotopic ratios between the komatiites and the associated basalts is too large to reflect analytical bias or open system behavior for Sm and Nd. It may be due to significant mantle heterogeneity in the source region. Isotopically distinct sources for the komatiites and komatiitic basalts are possible, but simple mixing between a depleted mantle component and a crustal component in the manner of magmatic assimilation of wall-rock material is inconsistent with the observed isotopic ratios. The inference of an enriched mantle component appears inevitable, as the alternative model of mixing would have generated more siliceous high magnesian (boninitic) rocks with much lower Fe and Ti than are observed. This enriched material may have been part of the sub-continental lithospheric mantle (SCLM) under the Aravalli craton, which could have generated the Basal Aravalli magmatic rocks. We suggest that during early middle Proterozoic the SCLM under cratonic regions consisted of relatively fertile mantle, capable of generating komatiites, tholeiites and alkalic magmas.

Temporal variations in crustal assimilation of magma suites in the East Greenland flood basalt province: Tracking the evolution of magmatic plumbing systems

We review published radiogenic isotope data (> 350 samples in total) on various suites of magmatic rocks within the Palaeogene central East Greenland flood basalt province to evaluate the types of crustal assimilants and the extent of crustal assimilation involved in each suite. We use these observations to build a regional picture of how magmatic plumbing systems changed with time and location during the sequential development of the province as magmatism responded to the development of a volcanic rifted margin and eventual plate separation. The earliest phase of magmatic activity (c. 62–57 Ma) is characterised by highly contaminated magmas that show a temporal change in assimilant type from amphibolite to granulite. This transition has been linked to the effects of an increasing magma supply rate which allows the more refractory granulite lithologies to be melted. The voluminous break-up phase of magmatism (c. 56–54 Ma) saw a significant decrease in the extent of assimilation because of the decreasing availability of assimilant material in the mature feeder systems, and many samples have Sr–Nd–Pb isotope compositions that overlap with those of asthenospheric melts (as represented by recent Icelandic basalts and North Atlantic MORB). Detailed study has allowed us to recognise packets of lavas that ponded at different levels in the crust and assimilated material of different compositions. The later stages of break-up magmatism show more diverse and more contaminated compositions that indicate a shift from a few large robust feeder systems to numerous small new conduits as the rifting continued. The post-break-up magmatism (c. 54–13 Ma) is characterised by a return to more highly contaminated magmas, which reflects a change in the style of magmatism: the eruption of small-volume alkalic lava flows from newly established conduits through the thicker inland crust, and the intrusion of mafic and silicic alkalic magmas at shallow levels in the Archaean basement along the present coast.

Large-scale silicic alkalic magmatism associated with the Buckhorn Caldera, Trans-Pecos Texas, USA: comparison with Pantelleria, Italy

Three major rhyolite systems in the northeastern Davis and adjacent Barrilla Mountains include lava units that bracketed a large pantelleritic ignimbrite (Gomez Tuff) in rapid eruptions spanning 300,000 years. Extensive silicic lavas formed the shields of the Star Mountain Formation (37.2 Ma-K/Ar; 36.84 Ma 39Ar/40Ar), and the Adobe Canyon Formation (37.1 Ma-K/Ar; 36.51-39Ar/40Ar). The Gomez Tuff (36.6 Ma-K/Ar; 36.74-39Ar/40Ar) blanketed a large region around the 18×24 km diameter Buckhorn caldera, within which it ponded, forming sections up to 500 m thick. Gomez eruption was preceded by pantelleritic rhyolite domes (36.87, 36.91 Ma-39Ar/40Ar), some of which blocked movement of Star Mountain lava flows. Following collapse, the Buckhorn caldera was filled by trachyte lava. Adobe Canyon rhyolite lavas then covered much of the region. Star Mountain Formation (~220 km3) is composed of multiple flows ranging from quartz trachyte to mildly peralkalic rhyolite; three major types form a total of at least six major flows in the northeastern Davis Mountains. Adobe Canyon Formation (~125 km3) contains fewer flows, some up to 180 m thick, of chemically homogenous, mildly peralkalic comendite, extending up to 40 km. Gomez Tuff (~220 km3) may represent the largest known pantellerite. It is typically less than 100 m thick in extra-caldera sections, where it shows a pyroclastic base and top, although interiors are commonly rheomorphic, containing flow banding and ramp structures. Most sections contain one cooling unit; two sections contain a smaller, upper cooling unit. Chemically, the tuff is fairly homogeneous, but is more evolved than early pantelleritic domes. Overall, although Davis Mountains silicic units were generated through open system processes, the pantellerites appear to have evolved by processes dominated by extensive fractional crystallization from parental trachytes similar to that erupted in pre- and post-caldera lavas. Comparison with the Pantelleria volcano suggests that the most likely parental magma for the Buckhorn series is transitional basalt, similar to that erupted in minor, younger Basin and Range volcanism after about 24 Ma. Roughly contemporaneous mafic lavas associated with the Buckhorn caldera appear to have assimilated or mixed with crustal melts, and, generally, may not be regarded as mafic precursors of the Buckhorn silicic rocks, They thus form a false Daly Gap as opposed to the true basalt/trachyte Daly gap of Pantelleria.

Chemical and isotopic relationships between peridotite xenoliths and mafic–ultrapotassic rocks from Southern Brazil

Peridotite xenoliths from the late-Cretaceous Alto Paranaíba and Goiás mafic–alkalic magmatic provinces of central and southeast Brazil reveal the existence of compositionally and temporally distinct lithospheric mantle beneath these areas. Garnet and spinel–lherzolites and spinel–harzburgites from the Alto Paranaíba province are generally depleted in Ca, Al and Re, which indicates that they are residues of melt extraction. Old Re-depletion model ages (average 2.4 Ga) for these peridotites show that this area is underlain by the early-Proterozoic to late-Archean melt-depleted lithospheric mantle of the São Francisco Craton. In contrast, spinel–lherzolites from the Goiás alkalic province, located 500–600 km northwest of the Alto Paranaiba province, have major- and trace-element compositions similar to modern fertile mantle. Most Goiás peridotites have fertile mantle 187Os/ 188Os (0.1261–0.1292), but two samples with lower 187Os/ 188Os define Re-depletion model ages of ∼ 1.2 Ga. The compositional distinction between the lithospheric mantle samples is mirrored in the kamafugitic rocks of these two provinces, which suggests that the composition of these magmas was strongly influenced, or determined entirely, by the lithospheric mantle. Most of the kamafugitic rocks have 187Os/ 188Os higher than observed for peridotite, which suggests that the source of these magmas is a mixture of lithospheric peridotite of varying age, veined and/or interlayered with various olivine-poor components. The relatively limited range in Sr, Nd, and Hf isotopic composition of the mafic–alkalic magmas indicates that the olivine-poor component was added regionally, overprinting the incompatible-element characteristics of the lithospheric mantle beneath these provinces. The age of metasomatism in the lithospheric mantle of this area is poorly constrained, but the isotopic characteristics of the mafic–alkalic magmatism suggest that this event may have occurred during the mid- to late-Proterozoic assembly of the Brasília mobile belt.

The Chimakurti, Errakonda, and Uppalapadu plutons, Eastern Ghats Belt, India: An unusual association of tholeiitic and alkaline magmatism

Mesoproterozoic tholeiitic and alkalic magmatism is widespread in the southern segment of the Eastern Ghats Belt, India, and is particularly well recorded in three cospatial plutons: Chimakurti, a pluton consisting of clinopyroxenite, anorthosite, and gabbro; Errakonda, a ferrosyenite pluton; and the Uppalapadu nepheline syenite. U–Pb zircon ages of ferrosyenite and nepheline syenite suggest that these cospatial plutons were intruded at approximately 1352 Ma, and Nd and Sr isotope compositions suggest that each pluton experienced varying degrees of crustal assimilation. Petrography, mineral chemistry, geochemistry and isotopic compositions suggest that the clinopyroxenite, anorthosite, and gabbro of the Chimakurti suite were derived from a high-Al tholeiitic parental magma. The ferrosyenite exhibits A-type characteristics and are interpreted as a differentiate or partial melt of a tholeiitic source. The Uppalapadu nepheline syenites were derived from an alkaline (basanitic) parental magma. These three plutons illustrate the spectrum of basaltic magmas that may be involved in producing intraplate intrusive complexes. Tholeiitic magmas emplaced at depth in the crust may differentiate to produce massif anorthosites and related A-type differentiates, represented in the Eastern Ghats Belt by the Chimakurti and Errakonda plutons. More alkalic basalts, such as basanites, will evolve to nepheline syenites like those of the Uppalapadu pluton. The 1352 Ma magmatism in the Eastern Ghats is unusual in that it documents a spatial and temporal association of both tholeiitic and alkaline endmembers of continental rift magmatism.

Plutonic xenoliths reveal the timing of magma evolution at Hualalai and Mauna Kea, Hawaii

238 U- 230 Th and U-Pb dating of zircons from leucocratic plutonic xenoliths indicating that lava stratigraphy is an incomplete monitor of magmatic evolution within subsurface res- ervoirs. Our results indicate that diorites from Mauna Kea record postshield evolution over tens of thousands of years when the depth of magma storage increased and highly evolved lavas began erupting. In contrast, diorites and syenogabbros from Hualalai record generation of evolved alkalic magma during the shield stage, and growth of a deep composite pluton ABSTRACT Hawaiian volcanoes evolve through stages that have been delimited by the compositions of their erupted lavas. New in situ 238 U- 230 Th and U-Pb dating of single zircons from leucocratic plutonic xenoliths erupted at Mauna Kea and Hualalai volcanoes, Hawaii, reveals that impor- tant episodes of magmatic evolution are not necessarily refl ected in the stratigraphy or com- positions of erupted lavas. Zircons from Mauna Kea diorites form a heterogeneous population with apparently bimodal ages of ca. 125 ka and ca. 65 ka, suggesting fractionation, intrusion, and crystal recycling about the time of transition of postshield volcanism from basaltic to hawaiitic compositions. Hualalai syenogabbros and diorites record extreme fractionation and generation of alkalic magma at 41 ± 9 ka and 261 ± 28 ka, indicating that alkalic magma was generated ~130,000 yr before the shield to postshield transition inferred from lava stra- tigraphy, and that coeval evolution of chemically distinct magma reservoirs at shallow and deep levels may have characterized the shield stage. These episodes at Hualalai did not cause eruption of evolved lavas, indicating that extreme differentiation in Hawaiian volcanoes is not necessarily followed by eruption of highly evolved magma.

A comparison of Siberian meimechites and kimberlites: Implications for the source of high‐Mg alkalic magmas and flood basalts

Chemical and radiogenic isotope (Sr, Nd, Hf, Os, and Pb) data are presented for a variety of mafic‐alkalic rocks from the Maymecha‐Kotuy section of the Siberian flood‐volcanic province. These data are compared to a similar data set for Siberian kimberlites that were emplaced both before and after the flood‐volcanic event in order to examine the spatial‐temporal evolution of Paleozoic magma sources in the mantle beneath this site of voluminous magmatic activity. As shown in previous studies, the high‐Mg, meimechitic composition rocks extend the range in Sr and Nd isotopic composition seen in the flood basalts in the direction of more “depleted” compositions, i.e., higher 143Nd/144Nd and lower 87Sr/86Sr, overlapping values typically observed in intraplate ocean‐island basalts. Sr, Nd, Hf, and Pb isotopic compositions show little correlation with major‐ and trace‐element compositions in the Maymecha‐Kotuy rocks. Os isotopic compositions, on the other hand, show rough correlations with a number of major‐element characteristics of the magmas. The Os data suggest that the magma sources range from peridotite for the meimechitic magmas to a mixture of peridotite and pyroxenite for the nephelinitic, melilititic, and trachybasaltic compositions. The isotopic overlap of both old and young kimberlites with the Maymecha‐Kotuy rocks is consistent with all these magmas being derived from mantle sources that were present beneath Siberia long before, and long after, the flood‐volcanic event. The isotopic characteristics of the mantle source of these magmas best match the FOZO component observed in ocean‐island basalts, which suggests that this mantle composition may be prevalent in the upper mantle outside of ocean basins. The long‐lived presence of this source beneath Siberia makes it unnecessary to appeal to a mechanism, such as a plume, to bring this type of mantle into play only during the flood‐volcanic episode.

Short-lived mafic magmatism at 560–570 Ma in the northern Norwegian Caledonides: U–Pb zircon ages from the Seiland Igneous Province

The Seiland Igneous Province (SIP) of northern Norway comprises a suite of mainly gabbroic plutons, with subordinate ultramafic, syenitic and felsic intrusions. Several intrusions from the Seiland Igneous Province have been dated by ID-TIMS U–Pb zircon and monazite analyses. The Hasvik Gabbro on the island of Sørøy, previously assigned an age of 700±33 Ma by Sm–Nd, yields a U–Pb zircon age of 562±6 Ma, within error of the Storelv Gabbro (569±5 Ma) and a diorite associated with the Breivikbotn Gabbro (571±4 Ma). Various intrusions on the Øksfjord peninsula give nearly identical ages of 565±9 Ma (gabbro), 566±4 Ma (monzonite), 565±5 Ma (monzodiorite), 570±9 Ma (norite), and 566±1 Ma (orthopyroxenite). These ages overlap with those from Sørøy, and define a single and short-lived period of gabbroic (to felsic) magmatism for the region between 570 and 560 Ma, pre-dating a subordinate episode of alkalic magmatism at 530–520 Ma. The U–Pb ages contradict the previous geochronological interpretation for the Finnmark area, which implied a period of 250 m.y. for the emplacement of the SIP intrusions. The new age data also clearly distinguish the Seiland intrusions, emplaced into the Sørøy Group metasediments of the Kalak Nappe Complex, from several older granitic intrusions (c. 850 to 600 Ma) that cut the Sørøy Group farther east and south. The coincident ages of the different Seiland intrusive bodies also contradict the previous structural model for the area, which posits that the different gabbro bodies were emplaced at intervals, with compressional deformation affecting the gabbros between periods of intrusion. The short time span between the main plutonic phases strongly suggests that the mechanism for the emplacement of mafic magma operated in a single, probably extensional, tectonic regime. The mafic intrusions were later deformed and metamorphosed to at least amphibolite facies, most likely by the Scandian (420 Ma) phase of the Caledonian Orogeny.