Petrogenesis and emplacement depths of the Petite Pluton during the closure of the Rocas Verdes basin, southern Patagonia - preliminary results

<p>The Petite Pluton is a Cretaceous intrusion covering an area of nearly 136 km<sup>2 </sup>located in Isla Capitán Aracena, southernmost Patagonia, Chile. This pluton and other stocks are located outside of the margins the Early Cretaceous-Paleogene Fuegian Batholith. The Petite Pluton intrudes the Capitán Aracena ophiolitic complex, interpreted as supracrustal remnants generated during the rifting stage of the Rocas Verdes marginal basin (Late Jurassic- Early Cretaceous; cf. Calderón et al., 2013, Geochem. J.) overlain by hemi-pelagic sedimentary basin infill (Yahgan Formation). These units are locally deformed and exposed in the southern limit of the NW-SE-trending Magallanes fold-and-thrust belt. The satellite plutons consist of amphibole-bearing diorites and quartzdiorites (48-55 wt.% SiO<sub>2</sub>) with calc-alkaline compositional trends consistent with their generation in a subduction environment. On N-MORB normalized incompatible elements pattern, the rocks show peaks in LILE (Rb, Ba, Sr) and subtle throughs in Ti, Zr, Nb, Ta and Y. Chondrite-normalized REE pattern is concave upwards with enrichment of LREE relative to HREE without Eu anomaly. The mineral compositions of diorites of Petite pluton consist of amphibole (magnesio-hornblende and tschermakitic hornblende), plagioclase is labradorite and andesine (An<sub>44-59</sub>), with Ca-rich composition in small grains included within poikilitic amphibole, biotite (annite), quartz, minor contents of K-feldspar, titanite, magnetite-ilmenite pairs and traces of apatite and zircon. Amphibole composition can be used as a proxy of the amount of H<sub>2</sub>O-rich fluids involved in magma evolution and could potentially be used to constrain the crustal depths of pluton emplacement in magmatic plumbing systems (Yavuz & Döner, 2017, P. di Mineralogia; Torres García et al., 2020, Lithos). The calculated pressure and temperature of 3 kbar and 800-850°C, indicate the emplacement and crystallization of magma batches in the upper crust. Oxygen fugacity [log (ƒO<sub>2</sub>)] varies between -9.9 and -10.7 (NNO), indicating amphibole crystallization from basaltic-andesitic melts under moderately oxidizing conditions. The moderately Mg# (60-72) of amphibole is consistent with their crystallization from mafic-intermediate melt-dominated crystal mushes with residual melts generated after the fractionational crystallization of olivine and clinopyroxene at deeper crustal depths. The amphibole composition constraint an amount of 6 wt% of H<sub>2</sub>O in the residual melts. The subtle negative Eu anomaly in amphibole indicates its partially simultaneous fractionation with plagioclase, suggesting rapid undercooling. The emplacement of the Petite Pluton at ~10 km depth occurred during and/or lately after the tectonic emplacement of ophiolitic complexes within an accretionary wedge, governed by a NE tectonic transport (Muller et al., 2021, Tectonophysics). Late Cretaceous satellite plutons suggest a continentward migration of the magmatic arc, related to the flattening of the subducted oceanic lithosphere of the proto-Pacific Ocean.</p><p>Acknowledgements. The study is supported by  Fondecyt grant 1161818.</p>

Identifying the components of Milos island subvolcanic plumbing system (South Aegean Volcanic Arc, Greece): An amphibole perspective

<p>Over the last ~3 Ma, the volcanic complex of Milos Island has evolved from a shallow submarine into a subaerial edifice. It has erupted almost the entire range of calc-alkaline series compositions, but silicic units are volumetrically dominant (Fytikas et al., 1986; Stewart & McPhie, 2006). Although numerous studies have been published, data on the mineral record of the magmatic processes are absent. We examined amphiboles from 3 explosive and 4 effusive units, ranging from andesite to rhyolite, to gain insights into the structure and evolution of the plumbing system. Like many arc volcanoes worldwide, Milos products contain bimodal amphibole populations, often present within the same unit. Mg-hornblende (6.79-7.22 a.p.f.u. Si) forms macro-crysts (>600 μm; often partly decomposed) and crystal clots with plagioclase (An<sub>47-51</sub>), orthopyroxene (Wo<sub>1-2</sub>En<sub>61-62</sub>Fs<sub>37-38</sub>), and magnetite in the effusive units and phenocrysts (300-600 μm) in more evolved pumices. Mg-hastingsite occurs in effusive units as: (1) pristine micro-phenocrysts (<300 μm; 6.22-6.58 a.p.f.u. Si); (2) relics (6.22-6.46 a.p.f.u. Si) in the inner domains of pseudomorphs mostly replaced by coarse-grained orthopyroxene (Wo<sub>2</sub>En<sub>68</sub>Fs<sub>30</sub>) rimmed by clinopyroxene (Wo<sub>43</sub>En<sub>47</sub>Fs<sub>10</sub>), plagioclase (An<sub>47</sub>), and magnetite; and (3) framework-forming crystals in quenched enclaves; and (4) the only amphibole (6.29-6.59 a.p.f.u. Si) phenocrysts in andesitic scoria.</p><p>Temperature (T) and pressure (P) conditions were calculated by applying hornblende-plagioclase (Holland and Blundy, 1994) and amphibole composition (Ridolfi and Renzulli, 2012) thermo-barometers. Amphibole compositions and calculated P-T conditions are in good agreement with experimentally grown amphiboles. Mg-hornblende compositions and their petrographic context are consistent with cold storage (780±24°C) in a near-solidus, upper crustal (1.7-2.8 kbar) silicic mush. This scenario is further supported by the rhyolitic (74±3.6 wt.% SiO<sub>2</sub>) compositions of calculated melts in equilibrium with Mg-hornblende, in contrast with the less evolved bulk compositions of the host effusive units. Although the explosive eruptions likely originated from differentiated, crystal-poor melt pockets in the mush, the more common effusions of hybrid andesite-dacite magmas resulted from interaction between mafic recharge magma and the silicic mush. This interaction is preserved in the disequilibrium textures affecting both Mg-hornblendes and Mg-hastingsites, coupled with the growth of high-T (960-885°C) post-recharge Mg-hastingsite. Most of the recharge magmas in Milos are effectively dispersed, trapped, and hybridized in the upper crust, although in rare cases magmas from a deeper crustal storage region (T~960-885°C;P~3.8-5.1 kbar) erupted after limited interaction with the upper crustal storage system.</p><p>The mineral chemistry reveals that a large, shallow, silicic reservoir has been the dominant component of the Pliocene plumbing system beneath Milos. Magma inputs from deeper crustal sources are preserved in enclaves and volumetrically minor explosive products. The plumbing system of Milos shares similarities with other Aegean arc volcanoes, where magmas experience storage, differentiation, and assimilation in different crustal levels, like Methana (Popa et al., 2020).</p><p><strong>Acknowledgements</strong></p><p>The research work was supported by the Hellenic Foundation for Research and Innovation (HFRI) under the HFRI PhD Fellowship grant (Fellowship Number: 364).</p><p><strong>References</strong></p><p>Fytikas, M. et al. (1986). JVGR,28(3-4),297-317.</p><p>Holland, T., & Blundy, J. (1994).CTMP,116(4),433-447.</p><p>Popa, R.G. et al. (2020).JVGR, 106884.</p><p>Ridolfi, F., &Renzulli, A. (2012).CTMP,163(5),877-895.</p><p>Stewart, A.L., & McPhie, J. (2006).BulV,68(7-8),703-726.</p>

Analysis of Baseline and Perimeter Air Monitoring Data from the Calaveras Dam Replacement Project (CDRP), Fremont, California

<p>This paper presents the results of 810 pre-project baseline samples collected over four years (2010-2011), and 7,210 offsite (ambient) and 14,314 perimeter samples collected over 7 years (2012-2018) during the CDRP project. The principal asbestos particles were chrysotile from serpentinite, and glaucophane-winchite amphibole from blueschist. The baseline data showed that asbestos concentrations measured at each station are not representative of a regional average background, rather, they reflect contributions from several variables such as: location on or near NOA-containing units, wind direction, intensity of localized soil disturbance, and time of year. The data shows that baseline sampling prior to a project cannot be used as a measure of “background” during the project. The analysis of amphibole composition in air and rock/soil samples was applied to differentiate local source impacts from the primary CDRP asbestos emissions. Of particular value was the application of the calcic-amphibole to total amphibole ratio (Ca index) measured during ABS sampling and comparison with the ratios measured in the samples. This analysis delineated three primary amphibole sources: 1) alluvium in the Sunol Valley with a high Ca index, 2) imported road surfacing material with a moderate Ca index, and 3) blueschist with a low Ca index. When the data was sorted by wind direction, the analysis showed that the contribution of CDRP-generated asbestos to monitoring stations was significant near the point of disturbance only, and did not significantly impact offsite stations that were located at or near sensitive receptors. The asbestos measured at the offsite stations were correlated with local geologic units. The analysis verified that the CDRP emissions were well below the project-specific risk-based thresholds established for the CDRP project, documenting that the offsite receptors were not exposed to an adverse risk by CDRP activities.</p>

Minerals as key markers of melt/fluid percolation in the lithospheric mantle from the French Massif Central

<p>Peridotite xenoliths from the French Massif Central (FMC) have undergone a complex mantle metasomatic history by percolation of melts/fluids of variable composition. The two main points are: How do the minerals react with the percolating agent? What information can be extracted from these interactions? We present a detailed investigation of major/trace element and Li isotopic composition in fresh spinel lherzolites from the FMC (Devès area). We discuss 1) the variations in the amphibole composition with focus on the Ti behaviour; and 2) the distribution of Li and Li isotopic composition in co-existing phases.</p><p>1) <strong>Amphibole </strong>occurs as disseminated crystals generally developed at the expense of spinel ± cpx, fills cross-cutting veinlets, and forms bands with variably abundant relict spinels. Some samples are surrounded by an amphibole selvage (3 mm thick) with sharp contact with the peridotite. The amphibole composition varies from the selvage to the peridotite part. In the selvage outer part amphibole is a cumulative Cr-free Al-rich kaersutite, which shows a decrease in Ti and Al, while mg* increases towards the contact. The outer part of the selvage is the remnant of a dyke, while in the selvage inner part amphibole has reacted with the peridotite. Disseminated amphibole farther from the selvage-peridotite contact is a Cr-rich pargasite. The distinct Ti-Al trends observed in amphibole from the selvage (positive) and the peridotite (negative) are linked to distinct Ti-incorporation mechanisms in the octahedral sites of the amphibole structure: a) (Ti<sup>4+</sup><sup>6</sup>Al<sup>3+</sup><sub>2)</sub> (M<sup>2+</sup><sub>-1</sub> Si<sup>4+</sup><sub>-2</sub>) for amphibole in the selvage and b) (Ti<sup>4+</sup> M<sup>2+</sup>) (<sup>6</sup>Al<sup>3+</sup><sub>-2</sub>) for disseminated amphibole in the peridotite. Mechanism (a) is likely to result from the crystallization of a percolating silicate melt in the mantle, whereas mechanism (b) results from hydration of the peridotite reacting with a percolating fluid emanating from the silicate melt.</p><p>  2) <strong>Li</strong> is preferentially incorporated into olivine compared to pyroxenes (1.1-1.4 ppm/0.2-0.9 ppm, average values) in the anhydrous xenoliths. Metasomatic processes increase Li abundances in all phases of the amphibole-bearing xenoliths, which deviate from the trend of equilibrium partitioning between phases, showing a preferential enrichment in cpx (2.4-5.4 ppm). In the hydrous xenoliths, the correlation between Li and REE elements in cpx and between Li in cpx and amp suggests that the carrier of the Li was a silicate melt. The  d<sup>7</sup>Li (‰) average values range (+5 to +15) in the anhydrous samples extend up to +35 in the amphibole-bearing xenoliths with large intra-grain variations (up to 18 ‰). These variations do not provide evidence for different sources but likely result from high temperature diffusion-related Li fractionation during metasomatism. The absence of correlation between the Li concentration and the isotopic composition in the anhydrous phases is linked to the pervasive character of the metasomatism, which allows strong Li exchanges as the melt interacts with the peridotite minerals. The preservation of the Li isotope kinetic fractionation in minerals and the sample isotopic heterogeneities implies that the Li exchange event occurs just before the extraction of the xenoliths from the mantle.</p>

Pargasite-bearing vein in spinel lherzolite from the mantle lithosphere of the North America Cordillera

Basanite lavas near Craven Lake, British Columbia, host a spinel lherzolite xenolith containing cross-cutting veins with pargasitic amphibole (plus minor apatite). The occurrence of vein amphibole in spinel lherzolite is singular for the Canadian Cordillera. The vein crosscuts foliated peridotite and is itself cut by the basanite host. The amphibole is pargasite, which is the most common amphibole composition in mantle peridotite. Rare earth element concentrations in the pargasite are similar to those for mafic alkaline rocks across the northern Cordilleran volcanic province (light rare earth elements ∼50× chondrite and heavy rare earth elements ∼5× chondrite). Two-pyroxene geothermometry suggests that the vein and host peridotite were thermally equilibrated prior to sampling by the basanite magma. Calculated temperature conditions for the sample, assuming equilibration along a model steady-state geotherm, are between 990 and 1050 °C and correspond to a pressure of 0.15 GPa (∼52 ± 2 km depth). These conditions are consistent with the stability limits of mantle pargasite in the presence of a fluid having XH2O < ∼0.1. The pargasite vein and associated apatite provide direct evidence for postaccretion fracture infiltration of CO2–F–H2O-bearing silicate fluids into the Cordilleran mantle lithosphere. Pargasite with low aH2O is in equilibrium with parts per million concentrations of H2O in mantle olivine, potentially lowering the mechanical strength of the lithospheric mantle underlying the Cordillera and making it more susceptible to processes such as lithospheric delamination. Remelting of Cordilleran mantle lithosphere containing amphibole veins may be involved in the formation of sporadic nephelinite found in the Canadian Cordillera.