Contemporaneous Trachyandesitic and Calc-alkaline Volcanism of the Huerto Andesite, San Juan Volcanic Field, Colorado, USA

Locally, voluminous andesitic volcanism both preceded and followed large eruptions of silicic ash-flow tuff from many calderas in the San Juan volcanic field. The most voluminous post-collapse lava suite of the central San Juan caldera cluster is the 28 Ma Huerto Andesite, a diverse assemblage erupted from at least 5-6 volcanic centres that were active around the southern margins of the La Garita caldera shortly after eruption of the Fish Canyon Tuff. These andesitic centres are inferred, in part, to represent eruptions of magma that ponded and differentiated within the crust below the La Garita caldera, thereby providing the thermal energy necessary for rejuvenation and remobilization of the Fish Canyon magma body. The multiple Huerto eruptive centres produced two magmatic series that differ in phenocryst mineralogy (hydrous vs anhydrous assemblages), whole-rock major and trace element chemistry and isotopic compositions. Hornblende-bearing lavas from three volcanic centres located close to the southeastern margin of the La Garita caldera (Eagle Mountain-Fourmile Creek, West Fork of the San Juan River, Table Mountain) define a high-K calc-alkaline series (57-65 wt % SiO2) that is oxidized, hydrous and sulphur rich. Trachyandesitic lavas from widely separated centres at Baldy Mountain-Red Lake (western margin), Sugarloaf Mountain (southern margin) and Ribbon Mesa (20 km east of the La Garita caldera) are mutually indistinguishable (55-61 wt % SiO2); they are characterized by higher and more variable concentrations of alkalis and many incompatible trace elements (e.g. Zr, Nb, heavy rare earth elements), and they contain anhydrous phenocryst assemblages (including olivine). These mildly alkaline magmas were less water rich and oxidized than the hornblende-bearing calc-alkaline suite. The same distinctions characterize the voluminous precaldera andesitic lavas of the Conejos Formation, indicating that these contrasting suites are long-term manifestations of San Juan volcanism. The favoured model for their origin involves contrasting ascent paths and differentiation histories through crustal columns with different thermal and density gradients. Magmas ascending into the main focus of the La Garita caldera were impeded, and they evolved at greater depths, retaining more of their primary volatile load. This model is supported by systematic differences in isotopic compositions suggestive of crust-magma interactions with contrasting lithologie

Geological mapping of carbonatites and related ores from the Oulad Dlim massif (Dakhla Province, Morocco) using remote sensing, portable X-ray fluorescence, and mineralogical data

Abstract and Poste

Contemporaneous magmatic differentiation of S-rich trachyandesitic and High-K calc-alkaline andesite in an intracontinental setting, San Juan volcanic field, Colorado, U.S.A.

L'étude minéralogique, pétrographique et géochimique des andésites de la Formation de Huerto (Champ Volcanique de San Juan, Colorado) a permis de mettre en évidence deux lignées magmatiques contemporaines datées à 28 Ma : (1) les laves trachytiques à olivine, se différentiant à faible profondeur, en bordure de la caldera La Garita, par les processus de cristallisation fractionnée et d'assimilation d'une croûte supérieure granitique ; et (2) les laves andésitiques calco-alcalines à hornblende, riches en eau et en soufre, évoluant sous la partie centrale de la caldera à travers une croûte hybride et dense, avec une importante assimilation de la croûte inférieure, de la cristallisation fractionnée et de la cristallisation "in situ" (présence de xénolites syngénétiques de bordure de chambre). Ces deux séries semblent évoluer à partir d'un même magma parent tandis que leur caractère minéralogique et géochimique s'acquièrent par dégazage et rééquilibration à différents niveau crustaux

Phlogopite-Olivine Nephelinites Erupted During Early Stage Rifting, North Tanzanian Divergence

International audienceThe North Tanzanian Divergence (NTD, eastern branch of the East African Rift) corresponds to an early stage of continental breakup. In the southern NTD, two quaternary volcanoes of the Manyara-Balangida rift (Labait, Kwaraha) have erupted primary nephelinite lavas (Mg# = 79–57) that allow characterization of their deep mantle source and the alkaline magmas that percolated through the lithosphere during rift initiation. Nephelinites are olivine- and clinopyroxene-rich, and contain up to 4 vol% magmatic phlogopite that crystallized as a liquidus phase with olivine and clinopyroxene. The presence of hydrous mineral (phlogopite) phenocrysts in Kwaraha and Labait lavas strongly suggests that the alkaline melts were H2O-bearing at the time of phlogopite crystallization (1.57–2.12 wt% H2O in phlogopite), demonstrating that phlogopite may have influenced the partitioning of water between the silicate melt and anhydrous silicate minerals (<1 ppm wt H2O clinopyroxene, 1–6 ppm wt H2O in olivine). Geochemical modeling indicates that the nephelinite magmas resulted from a low degree of partial melting (0.2–1%) of a carbonate-rich (0.3%) garnet peridotite containing ∼2 vol% phlogopite. We estimate the depth of partial melting based on primary melt compositions and empirical relations, and suggest that melting occurred at depths of 110–130 km (4 GPa) for craton-edge lavas (Kwaraha volcano) and 150 km (5 GPa) for on-craton lavas (Labait volcano), close to or below the lithosphere-asthenosphere boundary in agreement with the presence of deep refractory mantle xenoliths in Labait lavas. The depth of melting becomes gradually deeper toward the southern NTD: highly alkaline magmas in the north (Engaruka-Natron Basin) are sourced from amphibole- and CO2-rich peridotite at 75–90 km depth, whereas magmatism in the south (south Manyara Basin) is sourced from deep phlogopite- and CO2-rich garnet-peridotite beneath the Tanzania craton (e.g., at the on-craton Labait volcano). Percolation of deep asthenospheric CO2-rich alkaline magmas during their ascent may have produced strong heterogeneities in the thick sub-continental lithospheric mantle by inducing metasomatism and phlogopite crystallization in glimmerite lithologies

Role of volatiles (S, Cl, H2O) and silica activity on the crystallization of hauyne and nosean in phonolitic magmas (Eifel, Germany and Saghro, Morocco)

International audienceTo constrain the crystallization of alkaline and volatile-rich lavas present in intraplate settings, we studied the petrological features and the geochemical composition of major, trace, and volatile elements of mineral and bulk-rock of two sodalite-bearing phonolites: (1) haüyne-plagioclase-bearing Si-K-rich phonolite from Laacher See (Germany) and (2) nosean-nepheline-bearing Si-poor phonolite from Saghro (Morocco). In haüyne-bearing phonolites (55–59 wt% SiO2, K > Na, Na+K/Al = 0.96–1.08), we found that the low silica and low sodium activity promoted the early crystallization of S-rich haüyne (13.7–13.9 wt% SO3, 0.4 wt% Cl) + S-rich apatite (0.7–0.9 wt% SO3), titanite, and rare pyrrhotite followed by clinopyroxene-plagioclase-sanidine at relatively low pressure and temperature (P = 250 MPa and T = 850 °C) and oxidized condition (ΔNNO-NNO+1, where NNO is nickel-nickel oxide buffer). The crystallization of haüyne occurred at fluid-undersaturated conditions from a silicate melt with 6 wt% H2O, 0.17–0.23 wt% Cl, 0.11–0.4 wt% S, and 0.07–0.14 wt% F. Nosean-bearing phonolites from Saghro are silica-poor and peralkaline (52–54 wt% SiO2, Na > K, Na+K/Al = 1.2) and crystallized at higher P and T (300 MPa and 950 °C) and more reduced conditions (NNO) compared to haüyne-bearing phonolites. The incongruent reaction to form nosean requires high silica and Na2O activity. The mineral assemblage and composition suggest early crystallization of nepheline followed by nosean (7.8–8.8 wt% SO3; 1–1.1 wt% Cl). The sequence of crystallization is: clinopyroxene + nepheline + S-poor apatite (<0.04 wt% SO3) + pyrrhotite followed by nosean and titanite. Nosean-bearing magmas are fluid-undersaturated with relatively low volatile content (4 wt% H2O, <0.25 wt% Cl, <0.056 wt% S, 0.08–0.1 wt% F), although Cl may have exsolved during ascent and formed a fluid phase (NaCl-bearing).Both haüyne- and nosean-bearing phonolites are last equilibrated at relatively low pressure and high temperature. Haüyne and nosean crystallized at oxidized and volatile-rich pre-eruptive conditions. They record the volatile concentrations at depth and may be used as oxybarometer. The incongruent reactions involved to form haüyne and nosean suggest that phonolitic magmas became more oxidized during crystallization. The initial volatile concentrations in basanite/nephelinite magmas, from partial melting of volatile-bearing K2O-rich mantle rock, should have been one important factor influencing the crystallization of haüyne-bearing Si-K-rich phonolite and nosean-bearing Si-poor phonolite in intracontinental setting

S-rich apatite-hosted glass inclusions in xenoliths from La Palma: constraints on the volatile partitioning in evolved alkaline magmas

International audienceThe composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO3 (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce2O3. Sulfur is incorporated in apatite via several independent exchange reactions involving (P5+, Ca2+) vs. (S6+, Si4+, Na+, and Ce3+). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from 0.2 wt% SO3). D (S) (apatite/glass) is only slightly dependent on the melt composition and can be expressed as: SO3 apatite (wt%) = 0.157 * ln SO3 glass (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and hauyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of hauyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock

CO2-rich phonolitic melt and carbonatite immiscibility in early stage of rifting: Melt inclusions from Hanang volcano (Tanzania)

Hanang volcano is the southern volcano of, the southern area of the east part of the East African Rift (the North Tanzanian Divergence) and represents volcanic activity of the first stage of continental break-up. In this study, we investigate glassy melt inclusions in nepheline phenocrysts to constrain the late stage of Mg-poor nephelinite evolution and the behaviour of volatiles (C02, H20, S, F, Cl) during magma storage and ascent during early stage rifting. The melt inclusions have a green silicate glass, a carbonate phase and a shrinkage bubble free of gas phase indicating that carbonatite:silicate (18:82) liquid immiscibility occurred during nephelinite magmatic evolution. The silicate glasses have trachytic composition (Na + K/Al = 1.6-7.2, Si02 = 54-65.5 wt%) with high C02 (0.43 wt% C02), sulfur (0.21-0.92 wt% S) and halogens (0.28-0.84 wt% Cl; 0.35-2.54 wt% F) contents and very low H20 content ( <0.1 wt%). The carbonate phase is an anhydrous Ca-Na-K-S carbonate with 33 wt% Cao, 20 wt% Na20, 3 wt% K20, and 3 wt% S. The entrapped melt in nepheline corresponds to evolved interstitial COrrich phonolitic composition (Na + K/Al = 6.2-6.9) with 6 ± 1.5 wt% C02 at pressure of 800 ± 200 MP a after crystallization of cpx ( 17%), nepheline ( 40%) garnet (6.5%) and apatite (1.7%) from Mg-rich nephelinitic magma. During ascent, immiscibility in phonolitic melt inclusions leads to Ca-Na carbonate melt with composition within the range of carbonate melt from Oldoinyo Lengai and Kerimasi, in equilibrium with trachytic silicate melt (closed-system, P < 500 MPa). The COrrich phonolitic melt inclusions from Hanang volcano may represent an early stage of differentiation before Na carbonatitic magmatism observed at Oldoinyo Lengai

Sulfur-bearing Magmatic Accessory Minerals

International audienceAlmost all magmas contain sulfide- or sulfate-bearing phases. In most natural samples the sulfur-bearing phase is a sulfide, which typically is pyrrhotite (Fe1−xS) or pyrite (FeS2) although chalcopyrite (CuFeS2), pentlandite ((Fe,Ni)9S8), sphalerite (ZnS) or molybdenite (MoS2) may be present as well. Sulfate minerals are rare at magmatic conditions, and anhydrite (CaSO4) is the most common. Other magmatic SO4-bearing minerals include the sodalite group minerals (haüyne simplified formula: (Na,Ca)4-8(Al6Si6(O,S)24)(SO4,Cl)1-2), scapolite minerals (silvialite: (Ca,Na)4Al6Si6O24(SO4,CO3)), and S-bearing apatite (Ca5(PO4)3(F, Cl, OH)). Barite (BaSO4) has been mentioned in rare cases (Marchev 1991). Sulfur-bearing minerals usually constitute a negligible fraction of the mineral assemblage in magmatic rocks and thus can be classified as accessory minerals. The crystallization of sulfide, sulfate, and S-bearing minerals strongly depends on melt composition, temperature and pressure, and the S speciation in melt which, in turn, is strongly dependent on the prevailing oxygen fugacity (Baker and Moretti 2011, this volume; Wilke et al. 2011, this volume). Irrespectively of the low abundance of S-bearing minerals, the evolution of sulfur in magmas may be evaluated from the occurrence of sulfides and sulfates. These minerals are critical for estimating the activity of various sulfur-bearing species in magmas and can be used to constrain the oxygen fugacity and the S concentration in the melt (e.g., pre-eruptive sulfur concentration in melts). The presence of either sulfide or sulfate in silicate melt indicates that the predominant dissolved sulfur species in the melt are S2− or S6+, respectively. Typically one of these species is dominant, but there are conditions where S2− ..

Experimental constraints on ultrapotassic magmatism from the Bohemian Massif (durbachite series, Czech Republic)

International audienceThe equilibrium phase relations of a mafic durbachite (53 wt.% SiO2) from the TA (TM) ebi pluton, representative of the Variscan ultrapotassic magmatism of the Bohemian Massif (338-335 Ma), have been determined as a function of temperature (900-1,100A degrees C), pressure (100-200 MPa), and H2O activity (1.1-6.1 wt.% H2O in the melt). Two oxygen fugacity ranges were investigated: close to the Ni-NiO (NNO) buffer and 2.6 log unit above NNO buffer (a dagger NNO + 2.6). At 1,100A degrees C, olivine is the liquidus phase and co-crystallized with phlogopite and augite at 1,000A degrees C for the whole range of investigated pressure and water content in the melt. At 900A degrees C, the mineral assemblage consists of augite and phlogopite, whereas olivine is not stable. The stability field of both alkali feldspar and plagioclase is restricted to low pressure (100 MPa) at nearly water-saturated conditions (< 3-4 wt.% H2O) and T < 900A degrees C. A comparison between experimental products and natural minerals indicates that mafic durbachites have a near-liquidus assemblage of olivine, augite, Ti-rich phlogopite, apatite and zircon, followed by alkali feldspar and plagioclase, similar to the mineral assemblage of minette magma. Natural amphibole, diopside and orthopyroxene were not reproduced experimentally and probably result from sub-solidus reactions, whereas biotite re-equilibrated at low temperature. The crystallization sequence olivine followed by phlogopite and augite reproduces the sequence inferred in many mica-lamprophyre rocks. The similar fractionation trends observed for durbachites and minettes indicate that mafic durbachites are probably the plutonic equivalents of minettes and that K- and Mg-rich magmas in the Bohemian Massif may have been generated from partial melting of a phlogopite-clinopyroxene-bearing metasomatized peridotite. Experimental melt compositions also suggest that felsic durbachites can be generated by simple fractionation of a more mafic parent and mixing with mantle-derived components at mid crustal pressures

Seismological and petrophysical properties of the lithospheric mantle in a nascent rift

International audience&lt;p&gt;The North Tanzanian Divergence (NTD) is a rift initiation zone situated at the southern tip of the Eastern Branch of the East African Rift. This zone is a unique continental open-air laboratory to study the beginning of the continental break-up. The rift surface expression results from the interaction between tectonic and magmatic processes. However, the role of each process on the observed surface activity is still debated, as their respective signal is difficult to differentiate. In order to consider the various factors that may interact in this complex zone, a multi-disciplinary study was carried out, combining seismological and petrophysical approaches.&lt;/p&gt;&lt;p&gt;First, our recent development of a new hybrid tomographic method for both P and S-body waves permitted to image at depth the main suture zones between the inherited structures (Archean craton and Proterozoic orogenic belts) and the mantle plume extension (Clutier &lt;em&gt;et al.&lt;/em&gt; 2021). We also inferred zones of fluid (melt or gas) presence from the Vp/Vs ratio maps deduced from these P and S independent inversions. Then, to quantify the proportion of fluid from the tomographic images, we carried out a petrophysical study on mantle xenoliths from the Pello Hills volcano, situated in the rift axis. The clinopyroxene-amphibole-phlogopite vein-bearing xenoliths allowed to compute, at a sample scale, the seismic properties of the mantle with and without crystallised or fluid-filled veins. By varying the composition and increasing the proportion veins in the samples, the P and S-wave maximum velocities can decrease from 9.2 down to 5.3 km/s and from 5.1 down to 3.1 km/s, respectively. Those velocity models point out anisotropy in the mantle below the NTD, and particularly in highly metasomatized zones. Finally, despite the difference in spatial and temporal scales between the petrological and geophysical studies, we managed to combine the tomographic velocity anomalies and the xenolith&amp;#8217;s seismic properties to infer a maximum volume of fluid in the lithospheric mantle below Pello Hills volcano. This volume may be intermediate between 20% of clinopyroxene-phlogopite-amphibole crystallised vein and 10% melt/fluid-filled vein.&lt;/p&gt