St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure-temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent <i>instantaneous solid compositions</i> from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An_≤100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An₉₇ and An₉₅, respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (<i>X</i>H₂O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950-1025 °C, and <i>f</i>O₂ = NNO - 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt Kd_Ca-Na and dissolved H₂O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H₂O) basaltic andesites that crystallised over a wide pressure range (1.5-6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.

The Fawakhir area consists of an ophiolite sequence surrounded by an ophiolitic mélange. In the mélange, serpentinized ultramafic rock, gabbro, gabbroic diorite, diabase, andesite, and basalt occur as tectonic blocks within the metasediments. The gabbro gives a zircon U–Pb age of ~816 Ma, and the trace element composition of the zircon suggests its generation under a continental-arc tectonic setting. The geochemistry of gabbro and other tectonic blocks in the ophiolitic mélange indicates their formation from a backarc basin in a continental island arc tectonic setting. The ophiolite sequence consists of serpentinized ultramafic rock, gabbro, and basaltic rocks and was intruded by felsic dikes. The gabbro from the ophiolite sequence and felsic dikes give zircon U–Pb ages of 742 Ma and 723 Ma, respectively. Trace elements composition of this zircon refers to their formation in a continental-arc tectonic setting. The geochemistry of rocks in the ophiolitic sequence indicates their formation in a forearc basin. Together with previous studies, this study suggests that the tectonic blocks in the mélange formed in a backarc during the early-stage northwards subduction event, which may have started at ~816 Ma or earlier. On the other hand, the rocks in the ophiolite sequence can be considered to have formed in a forearc by the later eastwards subduction event at ~742–723 Ma.

Abstract Calc-alkaline andesite is a typical rock series in many island and continental arcs, and it commonly coexists with tholeiitic series rocks in individual volcanoes. In this study, the main controlling factors on the occurrence of calc-alkaline magmatism are examined for andesitic lavas from Rishiri Volcano, northern Japan. The whole-rock SiO2 contents of lavas range from 57.9 wt% to 61.9 wt%, and they are classified as calc-alkaline series. The lavas have petrographical and mineralogical features that show evidence of magma mixing. The whole-rock compositional variations of the lavas indicate that three end-member magmas were involved in their generation, a primitive mafic end-member magma, an evolved mafic end-member magma, and a felsic end-member magma. We suggest that the primitive mafic end-member magma represents a near primary magma generated in the upper mantle, the evolved mafic end-member magma was derived from the primitive magma by olivine fractionation, and the felsic end-member magma was produced primarily by partial melting of the crust. The primitive end-member magma is estimated to contain ~5 wt% water, which is much higher than those of the primary basaltic magmas of other volcanic stages of Rishiri that do not include calc-alkaline magmas. Based on this observation, we suggest that the water content of the primary magma was the main factor controlling the genesis of calc-alkaline magmatism at Rishiri. The more hydrous the primary magma and its deriva

The location and distribution of metal sources for felsic-magma related ore deposits in continental and island arcs (porphyry Cu-Au-Mo, epithermal Au-Ag, and skarn) is contentious. Crustal and mantle sources may both contribute to the metal budgets of these types of deposits, with a chemical signature that is set early. Mixing between mantle-sourced magmas and crustal partial melts in Mixing, Assimilation, Storage, and Homogenisation (MASH) zones, at or near the base of the Earth’s crust, is a possible mechanism to generate fertile magmas for ore development. Magma mixing of this sort may have an important role in sulfur solubility and thus sulfide mineral stability. Sulfide minerals partition Cu, Au, Ag, Mo, platinum group elements, and other chalcophile metals, which are key components in felsic magma-related ore deposits. This study investigates the role of magma mixing in the production of sulfide melts in the lower crust and the role that these sulfide melts have in ore generation in the upper crust. Geochemical data from samples collected in the lower crustal Opirarukaomappu Gabbroic Complex (OGC), southeastern Hokkaido, Japan suggest that sulfide occurrences are associated with magma compositions produced by mixing ~80% gabbro with ~20% tonalite. High temperature, high pressure piston cylinder experiments are used to simulate this mixing and the consequent saturation and exsolution of sulfide melt. A new, redox-controlled, model for sulfide saturation, called the “sulf

Abstract Even though no basalts have erupted at Asama volcano, its large, felsic magma chamber standing beneath the summit is frequently replenished by cryptic injections of mafic magma of basalt‐basaltic andesite composition. This mafic magma is preserved within melt inclusions trapped in olivine phenocrysts collected from the old Itabana pumice of the Kurofu stage and the Tenmei pumice of the current Maekake stage. The trapped mafic melts provide a detailed and vivid record of cryptic mafic magma injection into the felsic chamber where they admix to erupt andesite magma. The main phenocrysts of plagioclase, orthopyroxene, and clinopyroxene, commonly seen in the erupted products, are derived from the felsic magmas and trapped felsic melts of dacite‐rhyolite compositions, containing low sulfur contents (0.3 wt% of SO 3 or less; most are lower than 0.10 wt%). In contrast, the mafic melts, entrapped by olivine phenocrysts, show high concentrations of SO 3 , up to 1.06 wt% for Itabana and SO 3 0.65 wt% for Tenmei. In addition, the olivine phenocryst commonly encloses early crystalline phases precipitated from the sulfur‐rich mafic magma, such as Cr‐spinel, Mg‐rich orthopyroxene, Fe, Cu, and Ni‐bearing sulfides, and often Al‐rich clinopyroxene. The olivine‐hosted mafic melt inclusions always contain numerous vesicles. Furthermore, they are often included as hourglass‐shaped trapped melts, exhibiting snapshots of intense foaming and gas phase exsolution during the olivine growth and probably in the process of the cryptic injection into the felsic magma beneath the Asama summit. Our new data contributes to a better understanding of the magmatic system of the Asama volcano but also highlight the importance of the “Excess sulfur problem”, especially concerning the 1783 Tenmai eruption, which was contemporary to the Laki Fires.

masses occur with granite. Granite forms from silica-rich (felsic) magmas. Felsic magmas are thought to form by addition of heat or water vapor to rock Granite (, GRAN-it or , GRAN-eye-t) is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, mica and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes on Granite forms from silica-rich (felsic) magmas. Felsic magmas are thought to form by addition of heat or water vapor to rock of the lower crust, rather than by decompression of mantle rock, as is the case with basaltic magmas. It has also been suggested that some granites found at convergent boundaries between tectonic plates, where oceanic crust subducts below continental crust, were formed from sediments subducted with the oceanic plate. The melted sediments would have produced magma intermediate in its silica content, which became further enriched in silica as it rose through the overlying crust. Early fractional crystallisation serves to reduce a melt in magnesium and chromium, and enrich the melt in iron, sodium, potassium, aluminum, and silicon. Further fractionation reduces the content of iron, calcium, and titanium. This is reflected in the high content of alkali feldspar and quartz in granite. The presence of granitic rock in island arcs shows that fractional crystallization alone can convert a basaltic magma to a granitic magma, but the quantities produced are small. For example, granitic rock makes up just 4% of the exposures in the South Sandwich Islands. In continental arc settings, granitic rocks are the most common plutonic rocks, and batholiths composed of these rock types extend the entire length of the arc. There are no indication of magma chambers where basaltic magmas differentiate into granites, or of cumulates produced by mafic crystals settling out of the magma. Other processes must produce these great volumes of felsic magma. One such process is injection of basaltic magma into the lower crust, followed by differentiation, which leaves any cumulates in the mantle. Another is heating of the lower crust by underplating basaltic magma, which produces felsic magma directly from crustal rock. The two processes produce different kinds of granites, which may be reflected in the division between S-type (produced by underplating) and I-type (produced by injection and differentiation) granites, discussed below. The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's…

Magma Magma is the melted rock under the ground.[1] The liquid rock may have solid bits mixed in with it, and volcanic gas. There are different types of magma. One is called felsic magma. Felsic magma is thick and has silica minerals. It mostly makes light-coloured rocks. Another type is called mafic magma, which is runny and has less silica. It usually makes dark-coloured rocks. Magmas can be intermediate between both types. Magma may become solid by cooling slowly below the surface. This makes "plutonic" rocks such as granite. When magma comes out from the ground in a volcano it is called lava. Lava cools quickly, and forms other kinds of rock such as basalt. Magma is produced by melting of the Earth's mantle or the crust. It happens at tectonic settings, such as subduction zones, continental rift zones,[2] mid-ocean ridges and hotspots. Melted material from the mantle and crust moves up through the crust and collects in magma chambers.[3][4] When they come up through the crust, magmas may feed a volcano and come out as lava.