Alkalic Magmas Research Articles

Early extension and associated mafic alkalic volcanism from the southern Basin and Range Province: Geology and petrology of the Rodeo and Nazas volcanic fields, Durango, México

Research Article| June 01, 2001 Early extension and associated mafic alkalic volcanism from the southern Basin and Range Province: Geology and petrology of the Rodeo and Nazas volcanic fields, Durango, México James F. Luhr; James F. Luhr 1Department of Mineral Sciences, NHB-119, Smithsonian Institution, Washington, DC 20560, USA Search for other works by this author on: GSW Google Scholar Christopher D. Henry; Christopher D. Henry 2Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar Todd B. Housh; Todd B. Housh 3Department of Geological Sciences, University of Texas, Austin, Texas 78712, USA Search for other works by this author on: GSW Google Scholar J. Jorge Aranda-Gómez; J. Jorge Aranda-Gómez 4Unidad de Investigación en Ciencias de la Tierra, Instituto de Geología, Universidad Nacional Autonoma de México, Campus Juriquilla, A.P. 1-742, Querétaro, QRO 76001, Mexico Search for other works by this author on: GSW Google Scholar William C. McIntosh William C. McIntosh 5New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, P.O. Box 3398 Campus Station, Socorro, New Mexico 87801, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2001) 113 (6): 760–773. https://doi.org/10.1130/0016-7606(2001)113<0760:EEAAMA>2.0.CO;2 Article history received: 10 Aug 1999 rev-recd: 01 May 2000 accepted: 10 Jul 2000 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation James F. Luhr, Christopher D. Henry, Todd B. Housh, J. Jorge Aranda-Gómez, William C. McIntosh; Early extension and associated mafic alkalic volcanism from the southern Basin and Range Province: Geology and petrology of the Rodeo and Nazas volcanic fields, Durango, México. GSA Bulletin 2001;; 113 (6): 760–773. doi: https://doi.org/10.1130/0016-7606(2001)113<0760:EEAAMA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract East-northeast extension ca. 24 Ma at Rodeo and Nazas, México, was accompanied by eruption of hawaiites, marking some of the earliest intraplate-type mafic alkalic volcanism associated with development of the southern Basin and Range Province. An earlier extensional pulse, 32.3–30.6 Ma, concurrent with subduction- related rhyolitic volcanism of the Sierra Madre Occidental, is the earliest established extension in the southern Basin and Range Province. The Rodeo hawaiites are mostly along or just west of the north- northwest−striking, west-dipping Rodeo fault, a major breakaway fault that separates moderately extended terrane to the west from less extended terrane to the east. Hawaiites and interbedded conglomerates in the Rodeo half graben are flat lying to gently tilted and cut by the Rodeo fault. Underlying Eocene−Oligocene ash-flow tuffs to the west are cut by numerous west- dipping faults and tilted as much as 40°. Nazas hawaiites are also along north-northwest−striking faults but are generally unfaulted.Like other Miocene mafic alkalic volcanic rocks from the southern Basin and Range Province, those from Rodeo and Nazas are characterized by (1) moderately evolved hawaiitic compositions, (2) abundant megacrysts, including sodic plagioclase (An26–51), olivine (∼Fo55), Al-augite (7– 9 wt% Al2O3), and a wide variety of spinels, (3) lack of granulitic or peridotitic xenoliths, (4) variable Cs enrichments, and (5) isotopic compositions that indicate interaction with crust (87Sr/86Sri = 0.7037−0.7041; ϵNd = 4.8−2.8; 206Pb/204Pbi = 18.91−18.77; 207Pb/204Pbi = 15.57−15.60). These observations are consistent with a model whereby Miocene intraplate-type magmas rose slowly through the lithosphere, differentiating and interacting with the crust. The megacrysts are interpreted as disrupted gabbroic bodies, formed by slow cooling of mafic alkalic magmas that stagnated in the lower crust, possibly during the earlier extensional episode. Many of the elemental and isotopic parameters used to probe mantle sources of mafic volcanic rocks have been seriously obscured by crystallization, crustal interaction, and megacryst incorporation, complicating efforts to identify temporal changes in mantle source regions during development of the southern Basin and Range Province. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Submarine landslides around the Canary Islands

The morphology and structure of the submarine flanks of the Canary Islands were mapped using the GLORIA long‐range side‐scan sonar system, bathymetric multibeam systems, and sediment echosounders. Twelve young (&lt;2 Ma) giant landslides have been identified on the submarine flanks of the Canary Islands up to now. Older landslide events are long buried under a thick sediment cover due to high sedimentation rates around the Canary Islands. Most slides were found on the flanks of the youngest and most active islands of La Palma, El Hierro, and Tenerife, but young giant landslides were also identified on the flanks of the older (15–20 Ma) but still active eastern islands. Large‐scale mass wasting is an important process during all periods of major magmatic activity. The long‐lived volcanic constructive history of the islands of the Canary Archipelago is balanced by a correspondingly long history of destruction, resulting in a higher landslide frequency for the Canary Islands compared to the Hawaiian Islands, where giant landslides only occur late in the period of active shield growth. The lower stability of the flanks of the Canaries is probably due to the much steeper slopes of the islands, a result of the abundance of highly evolved intrusive and extrusive rocks. Another reason for the enhanced slope instability is the abundance of pyroclastic deposits on Canary Islands resulting from frequent explosive eruptions due to the elevated volatile contents in the highly alkalic magmas. Dike‐induced rifting is most likely the main trigger mechanism for destabilization of the flanks. Flank collapses are a major geological hazard for the Canary Islands due to the sector collapses themselves as well as triggering of tsunamis. In at least one case, a giant lateral blast occurred when an active magmatic or hydrothermal system became unroofed during flank collapse.

The Tanlu Fault Zone and Gold Ore Metallogenesis in Eastern China

The Tanlu Fault Zone (TFZ) is a large NE-trending fault system in eastern China that is the locus of several significant gold deposits. At different periods of its evolution and in different zones along its length, the TFZ has distinct geological features that control gold mineralization. In the northeastern part of the TFZ, early-stage faulting activity (from the Jurassic to Cretaceous) is associated with abundant calc-alkaline volcanic rocks, reflecting the compressive structural setting in the Jurassic and Cretaceous. However, activities in the late stage (Cenozoic) produced alkali basalts, indicating a mainly extensional tectonic regime. In the middle and southern segments of the TFZ, early-stage (Jurassic) activity was characterized by calc-alkalic granite intrusions, followed later (Cretaceous) by partial alkalic and alkalic volcanic-intrusive complexes, and in the latest stage (Cenozoic) by alkalic volcanic eruptions. The TFZ system controls the distribution of gold metallogenic provinces in eastern China, and periods of mineralization of gold ore deposits coincide with the major stages of faulting. Gold ore deposits in eastern China are products of the evolution of the TFZ. During the early evolution of the TFZ, gold ore deposits related to calc-alkalic granite intrusions were formed—e.g., of the quartz-vein and altered-rock types. Gold deposits of the interlayer–sliding fault breccia type were formed along the margin of the extensional basin during the middle period of TFZ evolution. Finally, epithermal gold deposits related to alkalic magmatism were formed during the latest stage of TFZ evolution.

Evidence for fluid phase separation in high-grade ore zones at the Porgera gold deposit, Papua New Guinea

Coexisting, liquid-rich and vapor-rich primary fluid inclusions in quartz provide direct evidence for fluid phase separation in high-grade quartz–roscoelite–gold veins and breccias from the Porgera alkalic-type gold deposit. Vapor-rich fluid inclusions are CO2-rich, and sometimes contain liquid CO2 at room temperature. The close spatial and paragenetic relationship between these “boiling assemblage” fluid inclusions and gold suggests that gold was precipitated by phase separation, at least locally. Additionally, the occurrence of carbonate and sulfate minerals in high-grade veins (reflecting pH increase and oxidation of the boiled fluid) and the appearance of hydrothermal breccias, are consistent with the process of fluid phase separation. Liquid CO2-bearing fluid inclusions are rare in near-surface epithermal deposits, and indicate that the Porgera vein system was formed at greater depths and pressures (our estimates suggest pressures between 250 and 340 bars). It is suggested that alkalic-type gold deposits may be distinguished from other epithermal deposit types by the more gaseous nature of the ore-forming fluids, in addition to their association with alkalic magmas.

Plagioclase of Hawaiian tholeiitic and alkalic magma parentages: distinctions based on REE, Sr, Ba, Hf, and Ta

¶To enhance the ability to distinguish tholeiitic from alkalic magma parentages by mineral compositions, I determined trace-element abundances in plagioclase separated from xenolithic gabbros of Mauna Kea volcano. These gabbros have origins in tholeiitic and alkalic magmas of the Hamakua postshield stage of Mauna Kea volcanism. Chondrite-normalized rare-earth element (REE) patterns for plagioclase show that highly calcic plagioclase, ≥ An78, from alkalic magma has greater light-REE/heavy-REE (LREE/HREE) ratios than less calcic plagioclase, An64–75, from tholeiitic magma (ratios, 22–33 vs < 20), suggesting that higher LREE/HREE ratios are inherent to plagioclase of alkalic magmas. However, with compositional evolution (i.e., to lower An), plagioclase REE patterns are of limited use for distinguishing tholeiitic from alkalic parentage because LREE/HREE ratios within each group increase and overlap in the range of ∼ 20–90. Sr, Ba, Hf, and Ta can also discern parentages as their abundances in plagioclase largely reflect abundances inherent to their parental magmas. The best expressions for identifying parentage use Sr abundances (Sr vs An; vs Ce/Yb; vs Sr/Ce), although Hf, Ba, and Ta abundances vs An and vs Ce/Yb are also useful – the distinctions due to tholeiitic plagioclase having relatively low Sr (∼ 500–1000 ppm), Ba (< 100 ppm), Hf (< 0.10 ppm), and Ta (< 0.05 ppm). These relationships help to distinguish between the effects of differentiation on trace-element abundances in plagioclase and their abundances owed to intrinsic concentrations in their magmas. They create compositional fields for tholeiitic and alkalic parentages that remain graphically separated even though differentiation may have enriched the plagioclase in incompatible elements.

A newly dated Cretaceous hydrothermal event in the Iberian Ranges (Eastern Spain) and its significance within the Mesozoic thermal history in the Iberian Peninsula

The Iberian Peninsula hosts the world-class Hg mining district of Almadén. Besides pre-Hercynian ore bodies, alpine-cycle Hg-bearing veins are also present in the eastern Iberian Ranges (Espadán deposits). We present both the first absolute ages (84±4 and 85±3 Ma) for a post-Hercynian Hg deposit in Spain, obtained from primary muscovites, and a complete compilation of published Mesozoic radiometric absolute ages of the Iberian Peninsula. We deduce that there are three main thermal episodes that affected the Iberian Peninsula, which have been revealed after the comparison among the magmatic, metamorphic and hydrothermal ages. Moreover, the Espadán hydrothermal system was active when both the Cretaceous alkalic magmatism took place in Southern Portugal and in the Pyrenees, and during the opening of the Bay of Biscay.

Petrogenesis and Stratigraphy of the High-Ti/Y Urubici Magma Type in the Parana Flood Basalt Province and Implications for the Nature of 'Dupal'-Type Mantle in the South Atlantic Region

The high-Ti/Y Urubici (or Khumib) magma type of the Paraná–Etendeka large igneous province has a restricted spatial extent, near the southeast Brazilian coast and in the northern Etendeka (Namibia). Urubici flows are interbedded with low-Ti/Y Gramado flows. Flow correlations indicate that local topographic relief was important in controlling emplacement of flows, and that lavas near the coast have undergone up to 1 km of post-magmatic uplift relative to inland areas. Urubici magmas have undergone extensive fractional crystallization (MgO <5.5 wt %). Stratigraphic variations highlight complexities of mixing and minor crustal assimilation indicative of open-system magmatic plumbing. The least contaminated samples have high La/Nb (∼1.5) and (Tb/Yb)N (∼2.5), Sr–Nd isotopes close to Bulk Earth (87Sr/86Sri ∼0.7050; εNdi −2.7), and Dupal Pb isotopes with unradiogenic 206Pb/204Pb (∼17.6). These features are similar to those of the Walvis Ridge DSDP (Deep Sea Drilling Project) Site 525A basalts that define the EM1 oceanic mantle component, and many are also shared with local Cretaceous alkalic magmas that are inferred to be lithospheric mantle melts. Low 206Pb/204Pb material found in the Urubici and Site 525A basalts is not seen as a mixing end-member within the modern Tristan plume system or in South Atlantic mid-ocean ridge basalt. An origin from lithospheric mantle material, delaminated and dispersed within the asthenosphere following continental break-up, is preferred. Thus the South Atlantic Dupal mantle anomaly cannot be considered as a single entity: Urubici flood basalts and Walvis Ridge Site 525A basalts have a relatively shallow origin within originally lithospheric mantle, whereas the Tristan plume is a deep mantle upwelling.

Geochemical evolution of Devonian-Carboniferous igneous rocks of the Magdalen basin, Eastern Canada: Pb- and Nd-isotope evidence for mantle and lower crustal sources

Magmatism associated with the extensional Magdalen basin includes voluminous tholeiitic gabbro and basalt and local granite and rhyolite. Pb- and (or) Nd-isotope determinations have been made on 70 igneous rocks from throughout the basin, and a further 15 samples of Avalonian basement from the southern margin of the basin, to characterize the contribution of lower crustal blocks and mantle sources to the magmatism and to constrain tectonic models for the basin. Five phases of magmatic evolution are distinguished in the Magdalen basin. (1) Middle to Late Devonian partial melting of lithospheric mantle, producing principally tholeiites and minor alkalic basalt. Tholeiites have Pb isotopic compositions similar to that of younger Triassic tholeiites generated from the same mantle, but experienced less crustal contamination. Regional variations in trace element composition of the mantle can be recognized. (2) The mafic magma triggered anhydrous base-of-crust melting, principally along the transpressive Cobequid and Rockland Brook faults, producing A-type granites in which radiogenic Pb increases northeastward. (3) In the latest Devonian, a large base-of-crust fractionating magma chamber evolved. It contained immiscible mafic and minor felsic magma, with uniform Nd isotopes, and high Ti in the mafic magma. (4) Although late Tournaisian dykes are not strongly fractionated, their evolution involved more crustal assimilation than earlier mafic rocks. (5) Local Viséan-Westphalian alkalic magmas, which ascended along crustal-scale faults, have Pb and Nd isotopic compositions resembling mantle plumes or their mixtures with lithospheric mantle sources. Only these youngest rocks show any isotopic evidence for input from an asthenospheric plume source, suggesting that regional extension was responsible for most of the magmatism.

Alkalic magma modified by incorporation of diverse tholeiitic components: ?Complex? hybridization on Kahoolawe Island, Hawaii

Kahoolawe Island is a ∼ 1.4 to 1 Ma shield volcano composed of shield, caldera-fill, and postshield tholeiitic lavas, and postshield alkalic basalt and hawaiite lavas. One postshield vent erupted alkalic lavas (K2O 1.1–1.6 wt.%) with resorbed olivines of wide compositional range (cores, Fo86–71), resorbed Na-plagioclase (An57–30; largely andesine) and clinopyroxene (evolved; Mg#s 76–71), and groundmass orthopyroxene (Mg# ≤62). They also contain tholeiitic gabbro xenoliths, which, as a suite, have a continuum of mineral compositions — clinopyroxene Mg#s 83–74, orthopyroxene Mg#s 83–76, and plagioclase An69–35 (e.g., includes andesine gabbro). Lava compositions do not fall on expected ‘Hawaiian’ fractionation trends due to MgO ‘enrichment’ (e.g., CaO ∼ 7 wt.% @ MgO ∼6 wt.%). This assortment of mineral and rock components within alkalic lavas with apparent Mg enrichment is owed to a complex history that began with protracted mixing among primitive and differentiated tholeiitic magmas, probably near the end of shield building. These hybrid magmas crystallized a compositional variety of olivines that were resorbed during reservoir replenishments, and also crystallizedin situ to form orthopyroxene-bearing gabbro on reservoir walls. When magma production rates declined during the shield to postshield transition of tholeiitic to alkalic magmatism, the tholeiitic hybrids in reservoirs fractionated to yield highly evolved phases such as andesine and clinopyroxene with Mg# < 75. When postshield hawaiite magmas subsequently entered reservoirs, alkalic-tholeiitic hybridization occurred; the resulting `complex' mixture of hawaiite+tholeiitic hybrids resorbed andesine and clinopyroxene crystals and, upon eruption, entrained xenoliths of gabbro. Mass balancing suggests that the alkalic-tholeiitic hybridization involved ∼44% hawaiite mixed with a nearly equal amount of tholeiitic hybrid (MgO ∼9.5 wt.%) plus olivine and andesine. This type of complex hybridization is a logical process for magmatism associated with tholeiitic to alkalic transitions and waning magma production, and this Kahoolawe example is the first to document such mixing in Hawaiian reservoirs.

Highly precise and accurate Re-Os ages for molybdenite from the East Qinling molybdenum belt, Shaanxi Province, China

Isotope dilution with a modified alkali fusion procedure and negative thermal ion mass spectrometry yields highly precise and accurate Re-Os ages for molybdenite from two well-studied molybdenite deposits in the East Qinling molybdenum belt, China. Individual Re-Os ages carry a 2Sigma precision of + or - 0.40 to 0.57 percent which includes a 0.31 percent uncertainty in the 187 Re decay constant. For the unusual carbonatite-hosted Mo-Pb deposit at Huanglongpu, the weighted average of seven analyses yields an age of 221.5 + or - 0.3 (0.15%) Ma. The weighted average of two analyses of molybdenite from a porphyry Mo deposit at Jinduicheng, about 10 km to the southwest, yields an age of 138.4 + or - 0.5 (0.39%) Ma. These data provide uncertainties an order of magnitude less than previous Re-Os ages. Molybdenite Re-Os ages are slightly older than ages obtained by other isotopic methods for genetically related host-rock and vein material. It appears that the direct dating of sulfide, rather than altered host and vein material, may be critical to acquiring the correct age for mineralization.The East Qinling molybdenum belt is part of a larger east-west-trending zone that marks the suture between two major cratonic blocks. Consequently, the belt was a site for Early-Middle Triassic compression (Indosinian orogeny) followed by Jurassic-Cretaceous extension (Yenshanian orogeny). We suggest that the Huanglongpu and Jinduicheng deposits provide an analogue for processes that may have been important in generating major molybdenum deposits in the Colorado mineral belt. In Colorado, Late Cretaceous (Laramide) compression-related, alkalic magmatism was followed by Tertiary (Rio Grande) extension-related, granitic magmatism and the development of major Climax-type porphyry Mo deposits. In particular, the Jinduicheng deposit appears to be a nearly perfect match for Climax-type mineralization in Colorado. In contrast, the older Huanglongpu deposit may record a mechanism whereby molybdenum is concentrated in the lower crust. In both the Qinling molybdenum belt and the Colorado mineral belt, a time gap of about 50 to 80 m.y. separates alkalic magmatism and exceptionally evolved granitic magmatism.

Degassing of alkalic basalts

In order to model quantitatively exsolution of volatiles over the range of basaltic melt compositions found on oceanic islands, I present compositional parameterizations of H 2 O and CO 2 solubilities and use these parameterizations to develop vapor saturation and degassing models for alkalic basaltic liquids. Vapor-saturation diagrams generated as a function of melt composition are used to determine the pressure at which the melt was last in equilibrium with a vapor and the composition of the vapor phase based on measured H 2 O and CO 2 contents in basaltic glasses. These models allow the calculation of the pressure at which a magma of known initial volatile content reaches vapor saturation and begins to exsolve a vapor phase. The higher solubility of CO 2 in alkalic magmas causes vapor saturation in CO 2 -bearing alkalic magmas to be reached at lower pressures than in CO 2 -bearing tholeiitic magmas having identical volatile contents. However, if variations in major element and volatile concentrations were linked by variations in the extent of melting, then volatile-rich, strongly alkalic magmas would begin to exsolve a vapor at slightly higher pressures than volatile-poor alkali olivine basalts or tholeiites. Partitioning of H 2 O and CO 2 into the vapor during volatile exsolution is controlled by the difference between H 2 O and CO 2 solubilities. As melts become more alkalic, the relative difference between H 2 O and CO 2 solubilities decreases, thus diminishing the preferential partitioning of CO 2 into the vapor. Exsolution of volatiles from tholeiites is characterized by strong partitioning of CO 2 into the vapor such that most or all CO 2 is lost before any significant loss of H 2 O. In contrast, the combination of higher CO 2 solubility and higher volatile contents (and perhaps higher CO 2 /H 2 O ratio) in alkalic melts results in less fractionation between CO 2 and H 2 O during volatile exsolution.

Liquid immiscibility between nephelinite and carbonatite from 1.0 to 2.5 GPa compared with mantle melt compositions

In order to define the conditions for the formation of immiscible carbonatite magmas in the lithosphere and in the crust, we have conducted phase equilibrium experiments to determine the effect of pressure and temperature on the silicate-carbonate liquid miscibility gap in bulk compositions appropriate for magmas in the upper mantle. A primitive (magnesian) nephelinite (NEPH) was used as a starting material, mixed with carbonates. Experiments were made with mixtures in the joins NEPH-dolomite-Na2CO3 (NEPH-Dol-NC) at 1.0 to 2.5 GPa, and NEPH-calcite (NEPH-CC) at 1.0 GPa. The miscibility gap was intersected by the join NEPH-Dol-NC (liquids with olivine), but not by NEPH-CC. Together with previous results for the Mg-free system (Na2O-CaO-Al2O3-SiO2-CO2), it was established that the size of the miscibility gap for magnesian compositions increases with decreasing pressures from depths of ˜100 km to ˜ 35 km; it increases further as compositions are changed by decreasing Mg/Ca. The maximum CaCO3 in liquids associated with the miscibility gap is 50 wt % for Mg-bearing liquids, and 80 wt % for Mg-free liquids. There is no experimental evidence for nearly pure-CaCO3 immiscible liquids, but abundant evidence for the precipitation of rounded calcite crystals from carbonate-rich liquids. The join NEPH-CC locates a piercing point on the liquidus field boundary for coprecipitation of olivine and calcite at NEPH50CC50 (wt %), part of the silicate-carbonate liquidus field boundary which defines the locus of liquids formed from carbonate-peridotites. The miscibility gap results are compared with magmas formed during partial fusion of CO2-bearing mantle peridotites, and during fractional crystallization of mantle-derived magmas. None of the probable magma paths in mantle processes intersects the miscibility gap. CO2-bearing mantle-derived alkalic magmas such as nephelinites and melilitites may fractionate during uprise through the mantle and crystallization within the crust. The compositions of these evolved nephelinites and phonolites approach the silicate side of the miscibility gap, confirming the probable generation of immiscible, alkalic carbonate-rich liquids at crustal pressures.

Plume-rift interaction in the Deccan volcanic province

It is widely accepted that the grand volcanic episode of the Deccan in India was a consequence of the passage of the northerly drifting Indian subcontinent over the Réunion starting plume in the Late Cretaceous. This plume also produced the three-rift Cambay triple junction, the three arms being the West Coast graben belt, the Narmada-Tapi rift zone and the Cambay rift. Deccan-related alkalic magmatism both preceded and followed the main tholeiitic phase of ∼ 65 Ma ago by about 3 MY. The uniformly tholeiitic thick basalt sequence of the Western Ghats was derived from direct melting of the plume head, but the tholeiitic-alkalic magmatism of the three rift belts was due to melting of the hydrous lithospheric mantle due to variable amounts of rifting. Any lithospheric loading mechanism for generating post-tholeiite alkalic lavas does not seem feasible for the Deccan; nor can it explain the pre-tholeiite alkaline magmatism in the Deccan. Similarly, conductive heating of the lithosphere by the plume cannot have generated the alkalic melts. Some amount of actual rifting did occur. Direct melt supply from the plume is not necessary for magmatism in the rifts, though it seems to have occurred in some cases. Our documentation of indigenous magmatism for the three rift belts calls into question any attempts at inferring the size of the plume head from synchroneity in geochronologic data alone, and we negate the possibility or necessity of large-distance transport of magmas from the plume through the lithosphere, to feed the rifts. Recent evidence indicates that alkalic magmatism north of the main Deccan outcrop preceded the main tholeiite phase by ∼ 3 MY. The Cambay graben was active 3.5 MY before the main tholeiite phase. As the alkalic complexes at the northern end of this graben are 68.5 Ma in age (so far the oldest well-proven Deccan-related rocks), they are the first expression of the Réunion plume, and the borehole picrites from the triple junction area, until now conventionally regarded as the oldest, can no longer be considered so. Accurate radiometric dating of these picrites is therefore much needed.

Eocene potassic magmatism at Two Buttes, Colorado, with implications for Cenozoic tectonics and magma generation in the western United States

Potassic mafic intrusions at Two Buttes, Colorado, enable examination of melt generation and mantle evolution at the eastern extent of Oligocene–Eocene magmatism in the western United States, and comparison of these processes to those in similar tectonic settings but within mantle lithosphere of contrasting age. Average K-Ar age determinations from four intrusions yield a 36.8 ± 0.4 Ma (1-σ) age for the Two Buttes complex. This late Eocene age precludes a genetic connection with inception of the Rio Grande rift, documented to have begun ca. 26 Ma, as advocated by others. The magmatism may have been related to Eocene subduction, although no isotopic component from Cenozoic subduction was recognized. Initial 87 Sr/ 86 Sr of whole-rock samples and clinopyroxene separates ranges from 0.7061 to 0.7073 and ϵNd (37 Ma) from −3.5 to −5.1. 207 Pb/ 204 Pb ranges from 15.56 to 15.60, 206 Pb/ 204 Pb ranges from 18.69 to 19.00, and 208 Pb/ 204 Pb ranges from 38.13 to 38.73. These isotopic ratios primarily reflect processes within heterogeneous, enriched mantle sources. The Two Buttes minettes have depletions in high field strength elements similar to those characteristic of subduction-related magmas, but the subduction event giving rise to these compositions cannot be Cenozoic because Nd depleted-mantle model ages are near 1.0 Ga. The Two Buttes magmas appear to have been produced from mantle enriched by Proterozoic subduction-related processes, as were some of the similar Cenozoic alkalic rocks intrusive into Proterozoic and younger crust in the Rocky Mountains region. The regional pattern of model Nd ages for similar alkalic rocks near the Rocky Mountain front may document successive periods of Precambrian crust formation from north to south. Two Buttes magmatism may have resulted from slab collision with the western edge of a keel of thick continental lithosphere beneath the Great Plains. A similar slab-lithosphere interaction may have immediately preceded small-volume alkalic magmatism along the Rocky Mountain front in Montana and Wyoming ca. 48–54 Ma and in Colorado and New Mexico ca. 37 Ma. These small-volume alkalic igneous rocks may mark the locations of the western edge of the keel in mid-Cenozoic time, the times of slab-keel collision, and the times of regional uplift consequent to the thermal event responsible for mantle melting.

Chemical and Os isotopic study of Cretaceous potassic rocks from Southern Brazil

During the late Mesozoic, an unusually broad range of alkalic magma compositions was erupted along the southern border of the Sao Francisco craton of Brazil. This magmatic activity includes carbonatite, kimberlite, lamprophyre, lamproite, syenite and the largest known example of extrusive kamafugite, the Mata da Corda formation. To determine the nature of the sources of this magmatism, and their geochemical history, an Os isotope study along with major and trace element and Sr, Nd and Pb isotope analyses of kimberlitic, lamproitic and kamafugitic rocks from the Alto Paranaiba province of Brazil was undertaken. This complements recent geochemical and isotopic studies of these magmas. The Os isotope data for Alto Paranaiba samples point to a peridotitic lithospheric mantle source for the kimberlites and lamproites that was variably depleted in Re, presumably by melt removal at some time between the late Archean and mid-Proterozoic. These lithospheric peridotites experienced LIL-element enrichment by fluid/melt metasomatism at roughly 1 Ga, most likely during mobile belt formation along the western border of the Sao Francisco craton. Kamafugitic samples have very radiogenic Os, suggestive of mafic (e.g. pyroxenite, websterite, eclogite) source materials that again appear to have been stabilized in the lithospheric mantle of Brazil in the mid to late Proterozoic. The Os isotope evidence for lithospheric sources for the Alto Paranaiba activity, coupled with Sr, Nd and Pb isotopic characteristics that overlap those of the Walvis Ridge hot-spot trace indicate that the EM1 component in South Atlantic ocean island basalts most likely represents the influence of delaminated Brazilian lithospheric mantle mixed into mantle circulation beneath the South Atlantic and is not related to the plume(s) commonly associated with this ocean island magmatism.

Spatial and temporal variations in the compositions of Upper Miocene to Recent basic lavas in the northern main Ethiopian rift: implications for the causes of Cenozoic magmatism in Ethiopia

The rate of lithospheric extension has previously been suggested as the most important factor governing the compositions of magmas generated in the Cenozoic Ethiopian volcanic province (CEVP). However, the distribution and chemistry of volcanic rocks extending from the western plateau margin at Addis Ababa to the rift floor in Nazret, northern sector of the main Ethiopian rift (MER), suggest that transitional magmatism in the region may have been triggered by an increase in the amount of lithospheric extension in the Early Pliocene. The rocks occur across an area of variable crustal thickness and show a general age progression from Upper Miocene (≤9 Ma) to Recent toward the rift. Alkalic basalts are extensive in the western part of the rift and along its margin but are found only locally within the rift, whereas transitional basalts are found within the rift only. Both types of basalts appear to have been derived from a common mantle source. In contrast, alkalic and transitional basalts on the Ethiopian plateau are mutually exclusive in terms of their spatial distribution, but exhibit a compositional contiguity which suggests that transitional magmas on the plateau formed at the expense of alkalic magmas, i.e. by equilibration of alkalic magmas at relatively shallow depth. The alkalic basalts bear clear record of a decrease in the degree of partial melting with time, suggesting that magmatism on the plateau was possibly triggered by a transient thermal anomaly.

Spatial and temporal variations in the compositions of Upper Miocene to Recent basic lavas in the northern main Ethiopian rift: Implications for the causes of Cenozoic magmatism in Ethiopia

The rate of lithospheric extension has previously been suggested as the most important factor governing the compositions of magmas generated in the Cenozoic Ethiopian volcanic province (CEVP). However, the distribution and chemistry of volcanic rocks extending from the western plateau margin at Addis Ababa to the rift floor in Nazret, northern sector of the main Ethiopian rift (MER), suggest that transitional magmatism in the region may have been triggered by an increase in the amount of lithospheric extension in the Early Pliocene. The rocks occur across an area of variable crustal thickness and show a general age progression from Upper Miocene (≤9 Ma) to Recent toward the rift. Alkalic basalts are extensive in the western part of the rift and along its margin but are found only locally within the rift, whereas transitional basalts are found within the rift only. Both types of basalts appear to have been derived from a common mantle source. In contrast, alkalic and transitional basalts on the Ethiopian plateau are mutually exclusive in terms of their spatial distribution, but exhibit a compositional contiguity which suggests that transitional magmas on the plateau formed at the expense of alkalic magmas, i.e. by equilibration of alkalic magmas at relatively shallow depth. The alkalic basalts bear clear record of a decrease in the degree of partial melting with time, suggesting that magmatism on the plateau was possibly triggered by a transient thermal anomaly.

Tholeiitic‐alkalic transition at subglacial volcanoes, Tuya region, British Columbia, Canada

Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volcanoes in the Stikine volcanic belt of northern British Columbia. The volcanoes rise 700, 500, and 400 m above their bases and are about 3.2, 1.6, and 2.6 km3 in volume, respectively. They began eruptive activity under several hundred meters of overlying glacial ice, or water in an ice‐impounded lake, and undegassed pillow lava was erupted and forms the bases of all three. Later, as the vents grew into shallow water, explosive phreatomagmatic activity erupted partly degassed glassy tuffs. Finally, when the volcano emerged through the surface of the ice or water (or the water was drained), degassed subaerial lava flows were erupted and were converted to assemblages of foreset‐bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes. The undegassed subglacial pillow base of Ash Mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes; at South Tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava; at Tuya Butte the pillow base is overlain by foreset‐bedded pillow lava, pillow breccias, and hyaloclastite tuffs, which in turn are overlain by subaerial lava flows composing a small shield volcano. The undegassed basal subglacial pillow lava of the three volcanoes contain 0.10 ± 0.01 wt % sulfur and ∼0.5 wt % H2O. The overlying partly degassed assemblages contain 0.06 ± 0.02% sulfur and ∼0.2% H2O at Ash Mountain, 0.07±0.01% sulfur at South Tuya, and 0.03±0.01% sulfur at Tuya Butte. The differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, as shown by change to more explosive activity and then to subaerial lava flows (and by a marked reduction of sulfur in volcanic glass), the magma shifted from tholeiitic to alkalic composition. This transition occurs at each of the three volcanoes. The tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source. Prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under subglacial conditions. As the volcano grew through the ice (or ice‐impounded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts, particularly of the more volatile‐rich alkalic melts, causing them to rise to the top of the chamber and erupt.

The Cerro Mencenares volcanic center, Baja California Sur: Source and tectonic control on postsubduction magmatism within the Gulf Rift

At the end of active subduction along the North American plate (12.5 Ma), in the Baja California peninsula arc magmatism was replaced mainly by alkali-rich high-Mg basaltic andesites and andesites with peculiar and unusually high K/Rb, Sr/Rb, and LREE/HREE ratios defined “bajaite” by Rogers et al. (1985) and “Baja alkalic suite” by Sawlan (1991). Two interesting features of this magmatic change are represented by (i) emplacement of transitional, high-K calc-alkaline andesites that preceded, in places, the Baja alkalic suite magmatism and (ii) subduction unrelated medium-K calc-alkaline magmatism in Baja California Sur, active in areas within the Gulf of California Rift, for example, Tres Virgenes–Reforma caldera, Isla Coronado. A previously unknown example of this calc-alkaline activity was studied in the area of Cerro Mencenares located north of Loreto, Baja California Sur. The Mencenares volcanic complex is located along the Gulf coast, east of the main Gulf escarpment. Activity of the Mencenares volcanic complex occurred in three stages: (i) an initial phase of rhyolitic dome emplacement; (ii) andesitic to dacitic volcanism that produced the Mencenares stratovolcano; (iii) late-stage emplacement of silicic domes and flows, mostly dacite and rhyolite, along north-south–trending normal faults. Tephra and lava flows interfinger with marine sediments of the Pliocene Loreto Basin and the youngest volcanic products were probably emplaced during Quaternary times. This volcanism is coeval with the Baja alkalic suite magmatism west of the Gulf escarpment. In the latter areas, Baja alkalic suite magmas ascended rapidly with only limited differentiation. In the Mencenares volcanic complex, a complex tectonic framework favored the formation of crustal magma chambers where more complex phenomena such as extensive differentiation and mixing could take place. A change from medium-K calc-alkaline toward Baja alkalic suite–like chemical characteristics is present in the younger products of the Mencenares volcanic complex and mixing possibly occurred between medium-K calc-alkaline and Baja alkalic suite magmas. Magma evolution in the Loreto area suggests that (i) Baja alkalic suite and medium-K calc-alkaline magmas can coexist in time and interact in the same volcanic field; (ii) if a relic, subduction-related component is the main cause of medium-K or high-K calc-alkaline volcanism then this component appears to decrease from east to west; and (iii) although prior calc-alkaline magmatism may not be unquestionably considered a necessary condition for the generation of Baja alkalic suite magmas, episodes of medium-K or high-K calc-alkaline melt extraction from mantle sources precede or accompany Baja alkalic suite magmatism in many instances.

The potential role of magmatic gases in the genesis of Illinois-Kentucky fluorspar deposits; implications from chemical reaction path modeling

In this paper, we present results of reaction path calculations using the chemical speciation and reaction path programs SOLVEQ and CHILLER to model possible fluorite deposition mechanisms in the Illinois- Kentucky fluorspar district. Input to the reaction path calculations were the temperatures, salinities, and concentrations of major cation and anions (Na, K, CI, SO 4 , etc.) and dissolved gases (H 2 S, CO 2 , etc.) in the hydrothermal fluids, based on published fluid inclusion data and unpublished fluid inclusion gas compositions. Fluid pH values and concentrations of fluorine and various metals were then calculated assuming saturation with observed hydrothermal minerals. We assumed saturation with fluorite, dolomite, quartz, muscovite (approximating illite), anhydrite, sphalerite, galena, and pyrite. Fluorite ore deposition mechanisms which we initially modeled include (1) simple cooling (from 150 degrees - 140 degrees C, based on data and conclusions of Richardson and Pinckney, 1984), (2) reactions of the hydrothermal fluids with limestones (based upon the observed extensive replacement of limestones by fluorite), (3) combinations of cooling and limestone reaction, (4) isothermal or near-isothermal boiling of the fluids in response to pressure drops, and (5) mixing of two fluids with different Ca and F contents. Using the assumed initial fluid composition discussed above, the modeling predicts that none of these mechanisms accurately reproduces the observed fluorite-dominant mineralization stages and limestone replacement features. We then modeled how interactions of the hydrothermal fluids at 300 degrees C with HF and CO 2 (approximating a CO (sub 2 and HF-bearing gas phase expelled from a crystallizing alkalic magma) might affect fluid chemistries and results of the above deposition mechanisms. The influence of alkalic magmatism on Illinois-Kentucky fluorspar district mineralization is probable based on the geologic relations of the district. Titration of 0.5 g HF and 0.5 g CO 2 into 1 kg of a fluorspar fluid composition produces extensive fluorite precipitation and acidic (pH 2.5 or lower), F-rich, Ca-poor fluids. Cooling of these fluids to 150 degrees C leads to further fluorite precipitation. Other possible magmatic gas compositions were modeled and yielded generally comparable...